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Recent studies of the Cretaceous lower arc crust exposed in Fiordland, New Zealand, conclude that shear zones are sites of melt migration and mass transfer through the deep crust. Here, we investigate the 4–10 km‐wide George Sound Shear Zone, which cuts the Western Fiordland Orthogneiss, comprising two main rock types: two‐pyroxene gneiss and hornblende gneiss. Previous studies infer a predominantly igneous origin for the two types of gneiss. However, this study finds that melt‐rock interaction within the George Sound Shear Zone formed the hornblende gneiss from the precursor two‐pyroxene gneiss. Petrographic analyses of samples collected in transects across the shear zone show hydration reaction textures ranging from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases towards the higher strain rocks in the shear zone. Backscatter images and polarized light microscopy show microstructures indicative of former melt‐present deformation, including (a) interconnected mineral films of quartz and K‐feldspar along grain boundaries, (b) grains that terminate with low apparent dihedral angles, (c) interstitial grains, with some (d) undulose extinction in plagioclase and (e) serrated grain boundaries. In addition, zircon microstructures are consistent with Zr mobility, further supporting the former presence of melt; geochemical data show enrichment of Zr in the hornblende gneiss as compared to the two‐pyroxene gneiss. From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated through the George Sound Shear Zone reacting with the host two‐pyroxene gneiss of the Western Fiordland Orthogneiss. Melt migration along grain boundaries was deformation assisted, (i) causing hydration of pyroxene to hornblende + quartz, and plagioclase to clinozoisite, (ii) increasing proportions of biotite within the shear zone and (iii) causing depletion of Cr + Ni and Zr enrichment in the hydrated rock. Our interpretation is supported by published observations of pegmatite dyke swarms that intruded into the George Sound Shear Zone, the P‐T conditions of deformation and characterization of microstructures that contrast sharply with those typically found in mylonitic rocks formed under solid‐state metamorphic conditions. Our results confirm that hydrous shear zones within otherwise anhydrous country rock are retrogressive and may represent evidence of melt migration through zones of deformation. [ABSTRACT FROM AUTHOR]

Granulite facies metapelitic gneisses collected over a 200×120 km exposed area of the Kakamas Domain of the Namaqua–Natal Metamorphic Province in southern Namibia all contain similar garnet–sillimanite–cordierite–biotite–quartz–K‐feldspar–ilmenite ± plagioclase ± magnetite mineral assemblages. These assemblages are interpreted to have equilibrated at suprasolidus retrograde conditions, and most samples contain distinct biotite‐ or sillimanite‐free peak assemblages. Pseudosection modelling constrains extremely uniform residuum solidus conditions of 5.5±1 kbar and 790±30°C for the entire Kakamas Domain. Estimated peak metamorphic conditions overlap with these but are more smeared out at between 4 and 7 kbar at 760°C to potentially more than 900°C. The uniformity of residuum solidus conditions is not coincidental, but is a consequence of retrograde re‐equilibration due to minor melt retention after peak metamorphism. Re‐equilibration could only stop once all retained melt had crystallized, which required the concomitant growth of a hydrous mineral to account for its H2O component. Biotite is the most stable hydrous mineral in these rocks, such that the residuum P– T conditions in the Kakamas Domain reflect the upper‐ T stability of biotite, and also corresponds to the intersection of the well‐known biotite–sillimanite melting reaction that consumed all biotite during prograde metamorphism. The calculated melt fertility of the sample suite indicates that the variable amounts of heat consumed to overcome the latent heat of fusion could have caused a 25°C spread in the peak temperature achieved by the most and least fertile samples. Peak temperature in the Kakamas Domain may have been as much as 100°C higher than residuum solidus conditions for specific samples but cannot be confidently constrained as it is obscured by the effects of both thermal buffering during prograde metamorphism and melt retention during retrograde metamorphism. Both processes are an inescapable part of the evolution of all granulite facies rocks, but their effects are most pronounced in fertile rocks like metapelites that are traditionally the preferred lithology for quantifying the P– T history of exhumed terranes. [ABSTRACT FROM AUTHOR]

A sample of micaceous schist of the Cushamen Metamorphic Complex in the Cushamen area (northwestern North Patagonia, Argentina) preserves a complex mineral assemblage, including staurolite, andalusite, garnet, sillimanite, biotite, quartz, and plagioclase. This unit proves an opportunity to analyse a complex mineral association often related to disequilibrium stages or polymetamorphic contexts. Through detailed petrological analysis combining mineral chemistry, X-ray compositional maps, conventional thermobarometry, and phase equilibria analysis, we reconstructed the pressure–temperature (P–T) path of this schist. The schist unit preserves a polymetamorphic history characterized by M1, M2, and M3 events. The M1 event is represented by biotite, muscovite, quartz, and plagioclase. The M2 event, associated with local mid-Carboniferous pluton intrusion, is characterized by andalusite and garnet assemblages, with peak conditions at ~ 3.3 kbar and ~ 563 °C. The main M3 event, at the time of the Carboniferous–Permian boundary, is defined by garnet, staurolite, sillimanite, biotite, muscovite, plagioclase, and quartz. This event records a progressive P–T evolution from ~ 3.5 kbar and ~ 553 °C to ~ 4.9–5.6 kbar and ~ 620–635 °C, nearing peak conditions. This work highlights the importance of comprehensive approaches in P–T trajectory reconstructions and the critical role of selecting the reactive bulk composition, particularly in rocks with complex mineral assemblages. In addition, this study significantly contributes to understanding the metamorphic evolution of the Cushamen Complex, a unit for which there is limited knowledge regarding its structural and metamorphic evolution. This complex is part of the igneous-metamorphic basement of North Patagonia region (Argentina), which records the Paleozoic evolution of the southwestern margin of Gondwana. [ABSTRACT FROM AUTHOR]

Investigating the geochemical composition of bulk sediments stands as a crucial method for unraveling the complexities of various sedimentary processes. However, the intricacies arising from extensive datasets and alterations in sediment due to diverse factors often impede the clear identification of underlying patterns in geochemical fluctuations. In addressing these, employing multivariate statistical analyses has proven to be an invaluable tool for elucidating intricate patterns within large dataset. In this study, we focus on the utilization of Principal Component Analysis (PCA), a multivariate statistical technique, to uncover the underlying sedimentary processes influencing distinct geochemical dataset. Specifically, our attention is directed towards the examination of geochemical data from the previously published geochemical data of metasediments from Shimla and Chail group (referred to as SCM) and the mudflat sediments of Diu Island (referred to as DMS). Our PCA outcomes reveal that the initial three principal components (PC1, PC2, and PC3) account for 52.51% and 79.30% of the total variance within the SCM and DMS geochemical data, respectively. Notably, the negative loading of SiO2, alongside positive loadings of incompatible elements and those associated with mafic rocks on PC1 within the SCM dataset, indicates sediment origins ranging from felsic to intermediate sources. Additionally, the coexistence of Th, U, Zr, and Sc, exhibiting positive loadings in PC1 and PC2, suggests a significant influence of reworking and recycling from felsic to intermediate sources. In the context of the DMS dataset, PCA analysis highlights the dominant influence of in-situ productivity and mafic sediment sources along the positive axis of PC1. Conversely, the negative axis of PC1 is shaped by intermediate and potentially other sources. Further granularity in interpretation reveals the positive axis of PC2 being attributed to weathering proxies, while the dominance of plagioclase minerals in the clayey fraction controls the positive axis of PC3. Through this investigation, our study underscores the essential role of PCA-assisted geochemical data analysis in unraveling the intricate web of processes contributing to the variance observed within sedimentary systems. By effectively distilling the multifaceted factors driving geochemical variability, this approach emerges as a pivotal asset in enhancing our understanding of sedimentary dynamics. [ABSTRACT FROM AUTHOR]

K-feldspar megacrysts are common in silicic plutons, but there is a long-running debate around how they form and what their presence tells us about magmatic systems. Field, textural, and geochemical evidence supports growth in a melt-rich environment, but experimental evidence and phase-equilibria modeling indicate that K-feldspar grows late in the crystallization sequence, when the magma is highly crystalline. We provide a new perspective on this problem by examining the arrangement of plagioclase inclusions within megacrysts to test whether they exhibit the systematic low-energy crystallographic relationships expected from attachment by synneusis in melt-rich environments where crystals have space to rotate. We use electron backscatter diffraction to quantify the crystal orientations and find that the megacrysts’ plagioclase inclusions do occupy these preferred orientations and therefore were incorporated in a melt-rich environment. K-feldspar is also present as an interstitial network, but plagioclase crystals hosted within this network have non-systematic orientations. This transition from systematic to non-systematic plagioclase orientations marks the point at which the crystals formed a rigid, interconnected framework that impeded rotation into lowenergy orientations. Phase-equilibria modeling indicates that this transition occurred when the magma was ∼55% crystalline. The remaining ∼45% melt crystallized at the eutectic, forming the interstitial phases. Thus, we resolve the “megacryst paradox”; the megacrysts grew freely in melt, and the groundmass K-feldspar formed after crystal lock-up. Megacrysts therefore provide a detailed textural and chemical record of a critical period in the system’s evolution: the transition from a mobile and potentially eruptible magma to an immobile mush. [ABSTRACT FROM AUTHOR]

New U–Pb zircon geochronology identifies a latest Triassic (ca 214–201 Ma) igneous suite of tuff, hypabyssal dikes, and a pluton on the southern Kenai Peninsula, Alaska. The igneous suite was emplaced within Upper Triassic sedimentary rocks along the southern margin of Western Wrangellia, the western-most fragment of the Wrangellia composite terrane. The igneous rocks range from mafic (50.6% SiO2) to felsic (78.3% SiO2), characteristically have less than 1.55% K2O, and generally have low trace element abundances. The tonalitic and trondhjemitic magmas were largely sourced in mafic-rich lower crust and incompletely assimilated quartz and other mineral xenocrysts are common. Fractionation involving plagioclase and amphibole is indicated for some magmas and composite intrusions and igneous xenoliths indicate magma mixing was possible. Paleozoic and Precambrian inherited zircons and initial 87Sr/86Sr (0.704103–0.705609) and 143Nd/144Nd (0.512396–0.512777) ratios indicate that the Western Wrangellia crustal sources are heterogeneous and contain sialic components. The latest Triassic magmatism reflects processes that preceded Early Jurassic subduction along the Wrangellia composite terrane and Pacific Ocean plate boundary. These processes involved heating and melting of mantle lithosphere and lower crust as mantle instabilities accompanied the breaking of the plate boundary linkages. The Late Triassic transition to subduction along the Wrangellia composite terrane margin coincided with the transition to subduction cessation in the Late Triassic arcs of the western Intermontane terranes of Canada. The shift to subduction along the outboard Wrangellia composite terrane margin marks the beginning of the Pacific Ocean–Cordillera plate interactions that came to dominate the tectonic evolution of the northern Cordillera from the Early Jurassic to today. [ABSTRACT FROM AUTHOR]

Lunar basaltic meteorite Asuka‐881757 (A‐881757), a member of the source crater paired YAMM meteorites (Yamato‐793169, A‐881757, Miller Range 05035 and Meteorite Hills 01210), provides information on potassium‐rare earth element‐phosphorous (KREEP)‐free magmatic sources within the Moon. Asuka‐881757 is an unbrecciated and Fe‐rich (Mg# 36) gabbro with coarse pyroxene (2–8 mm) and plagioclase (1–3 mm). The coarse pyroxene preserves mm‐scale, near‐complete hour‐glass sector zoning with strong Ca and Fe partitioning, similar to some Fe‐rich Apollo basalts. In contrast to the most Mg‐rich Apollo basalts, A‐881757 contains various types of symplectites (~8 vol%) formed by the breakdown of pyroxferroite due to slow cooling, resembling a few extreme Fe‐rich (Mg# ≤$$ \le $$40) Apollo basalts. Petrographic observations and thermodynamic modeling suggest crystallizing in the order: Fe‐poor pyroxenes (Mg# 58–55) → co‐crystallized plagioclase and Fe‐rich pyroxenes (Mg# 49–20) → late‐stage assemblage including Fe‐augite, Fayalite, and Fe‐Ti oxides. Combining phase stability at variable P–T with petrographic observations, the minimum depth of formation of the A‐881757 parent magma can be constrained to between 60 and 100 km. KREEP‐free basalts (such as A‐881757 and the YAMM meteorites) originated from a relatively shallow mantle source and later underwent polybaric crystallization that occurred prior to eruption at the lunar surface. In contrast, the Apollo mare basalts mostly crystallized within lava flows from relatively deeper‐seated mantle sources. The crystallization of A‐881757 and other YAMM meteorites is unlike most Apollo basalts from the Procellarum KREEP terrane, and likely represent hidden cryptomare basalts close to lunar surface. [ABSTRACT FROM AUTHOR]

This work deals with the archaeometric investigation on 25 fragments of terra sigillata (red‐coated ceramic ware and moulds) found in the city of Arezzo, Tuscany (central Italy), and attributed to several important workshops from the first century BCE to the second century CE. Optical and spectroscopic techniques were used to analyse both the ceramic bodies and the red slips. All the potsherds showed a very fine‐grained ceramic body, sharing similar mineralogical compositions, mainly consisting in quartz, plagioclase, pyroxene, hematite, K‐feldspars, and illite/muscovite. The mineralogical data suggest that both the red‐coated wares and the moulds were produced using the same calcareous‐illitic clay and fired under oxidising conditions at temperatures between 850°C and 1000°C. A K‐rich illitic clay with a Fe content around 10–15% (in wt%) was used to elaborate the slips. Al‐substituted hematite was found in red slips by micro‐Raman spectroscopy. Comparison of the chemical data with terra sigillata from other important production areas in Italy and from other regions of the Meditteranean Sea, allowed to define that the studied samples, locally produced in Arezzo, differ systematically from all others, although they show similarities with nearby Pisan productions as well as those Puteolan. [ABSTRACT FROM AUTHOR]

The high-pressure (HP) migmatitic paragneiss of the Beni Bousera metamorphic unit (BBMU) from the Beni Bousera Massif underwent granulite-facies metamorphism. They mainly consist of garnet, biotite, kyanite, sillimanite, cordierite, spinel, quartz, plagioclase, K-feldspar, rutile, ilmenite and graphite. These minerals form a sequence of metamorphic assemblages corresponding to five metamorphic events (M1, M2, M3, M4, and M5). The first three events correspond to a latest Carboniferous-earliest Permian metamorphic evolution, while the last two are retrograde and Alpine in age. The p-T conditions of these metamorphic events were estimated using pseudosection calculations in the MnNCKFMASTHO system. Following prograde stage conditions of 9.3–10.9 kbar and 630–665°C (M1 stage), the p-T evolution culminates in melting of the paragneiss which occurred mainly at HP conditions, within the kyanite stability field, at 11.8–14.2 kbar and 820–845°C (M2 pressure-peak); anatexis then continued at high to medium pressure (HMP), within the sillimanite stability field, at 9.9–11 Kbar and 840–860°C (M3 thermal-peak), accompanied by the production of very small proportions of melt. The prograde p-T segment of the inferred clockwise path is characterized by a pressurization related to a post-Variscan collisional event triggered by the closure of the Paleotethys Ocean, and is followed by the exhumation of the granulitic unit to mid-crustal depth, with slight coeval heating. The retrograde Alpine evolution is coeval with the final exhumation of the migmatitic paragneiss and underlying peridotites during back-arc extension related to the westward retreat of the Alpine subduction. It occurred under sub-solidus conditions from 9.5–10.7 kbar and 818–848°C (M4) to 4.9–5.85 kbar and 710–785°C (M5) and corresponds to a near-isothermal and strongly decompressive metamorphic evolution (ΔP ≈ 5 Kbar). [ABSTRACT FROM AUTHOR]

The geochemical compositions of apatite are commonly used to provide reliable constraints on the intrinsic magmatic variables of the parental magma. There is a widespread distribution of Mesozoic granitic rocks in the eastern Jiangnan Orogen (EJNO), which include I-type and A-type granitoids. This study aims to investigate the geochemical composition of apatite in 11 representative I-type and A-type granitic intrusions from the EJNO to understand their petrogenesis, intrinsic magmatic variables, and polymetallic mineralization potential. The results show that apatites in these granitoids were characterized by oscillatory zonation and homogeneous texture, and exhibited high F, and low Cl contents, as well as high REE, Y, and Th contents, indicating magmatic origin. These apatites were divided into two groups depending on their geochemical characteristics and original rock types. In Group I apatites (in the I-type granitoids), the absence of an abrupt change in major and trace element concentrations from the core to the rim of the apatite crystals, combined with their low levels of MgO, Sr, and HREY (HREE+Y), serves as evidence to classify the I-type granitoids as low Sr-Y adakitic granitoids with a low Mg#, which can be attributed to the partial melting of the thickened lower crust. In Group A apatites (in the A-type granites), the normal increase in incompatible elements from the core to the rim, as well as the presence of a flat REE pattern with pronounced Eu anomalies, can be attributed to crystal fractionation of plagioclase, allanite, and zircon. Furthermore, Group I apatites have higher SO3, Cl, and molar OH/F and Cl/F ratios compared to Group A apatites. Quantitative calculation by apatite-melt partitioning and modelling suggests that the I-type granitic magmas were more volatile-rich and more oxidized than A-type granitic magmas. Taken together, based on the relationship between the intrinsic magmatic variables and the behaviour of ore-forming elements, our study demonstrates that early I-type granitoids of the EJNO have a strong W-Mo-Cu polymetallic mineralization potential, while late A-type granites may possess rare metal mineralization potential. [ABSTRACT FROM AUTHOR]

The Sakarya Zone (northern Turkey) is characterized by the emplacement of voluminous granitoids during the late Carboniferous. In this study, we present geological, U-Pb zircon age, elemental abundance and Sr-Nd-Pb isotopic data on two highly evolved Carboniferous granite bodies within the Early to Late Jurassic volcaniclastic rocks in the Şiran region. The largest body forms a roughly E-W trending ~30 km long and 0.5–2.5 km wide stripe the southern boundary of which is a southvergent thrust, and the other one a subcircular outcrop, ~5 by ~3 km, unconformably overlain by Lower Jurassic volcaniclastic rocks. Their emplacement ages are constrained by LA-ICP-MS U-Pb zircon dating to 310–313 ± 7 Ma (2σ, late Carboniferous). The granitic rocks consist of quartz, microperthitic K-feldspar, plagioclase, and minor biotite, and are characterized by highly evolved compositions with high concentrations of SiO2, Na2O, K2O and Ba, and low concentrations of TiO2, Fe2O3*, MgO, CaO, P2O5 and Sr. Geochemical characteristics indicate a highly fractionated high-K calc-alkaline peraluminous I-type affinity. Chondrite-normalized rare-earth element (REE) patterns are slightly fractionated, with light REEs enriched with respect to middle and heavy ones, and showing variable negative Eu anomalies. On the N-MORB-normalized multi-element variation diagrams, samples display negative anomalies of Ba, Nb, Sr, P, Zr, Eu, and Ti, and positive anomalies of K, and Pb. Initial 87Sr/86Sr and εNd values are 0.70432–0.70610 and −6 to −11, respectively, similar to other Carboniferous high-K calc-alkaline granites and mafic-ultramafic intrusions in the region. The compositional characteristics of the Şiran granites can be explained mainly by fractional crystallization involving hornblende, plagioclase, and biotite and without the involvement of significant amounts of garnet. There is no need to invoke magma mixing and/or melting of a heterogeneous source to account for the geochemical variation. We propose that the late Carboniferous granites formed by remelting of middle- to high-K calc-alkaline mafic rocks at lower crustal depths, followed by extensive fractionation at the upper crustal depths. A thorough review of data from the literature reveals that highly fractionated granites constitute a significant component of the late Carboniferous granites in the Eastern Pontides. [ABSTRACT FROM AUTHOR]

The Rare Earth Element (REE) geochemistry of siliciclastic sediment in the Ilaje area, eastern Dahomey basin, southwestern Nigeria was conducted to determine the provenance, geotectonic setting as well the REE economic potential in the sediments. Geochemical analysis results (ppm) revealed the concentration of La varies from 1.87–21.47 while Ce ranges from 1.16–45.98. Others include; Pr (1.07–39.56), Nd (0.71–26.02), Sm (0.42–12.51), Eu (0.20–4.66), Gd (0.27–8.79), Tb (0.29–3.97), Dy (0.28–3.62), Ho (0.22–2.76), Er (0.29–2.64), Tm (0.33–3.83), Yb (0.37–3.75) and Lu (0.29–3.03) respectively while the ∑REE ranges from 2.25 to 6686.75 ppm. The REE normalization plot is characterized by an inclined shape depletion indicating depletion trend. The patterns show an enriched Light Rare Earth Elements (LREE) and Heavy Rare Earth Element (HREE) pattern indicating upper continental crust sourced materials. The plot also revealed a negative Eu anomaly which may be associated with plagioclase fractionation in the acidic source rocks. The high ∑REE in the sediments has uncovered the economic potential of the sediments as a useful raw material resource for technological advancement. [ABSTRACT FROM AUTHOR]

George Barrow's maps (1893, 1912) of the metamorphic zones named after him in the south‐eastern Scottish Highlands, show a prominent staurolite zone running from the west of Glen Clova to the North Sea coast near Stonehaven in the east. Pelitic mica schists throughout this zone commonly show principal mineral assemblages of staurolite+biotite+garnet+muscovite +quartz (St + Bt + Grt), with sodic plagioclase, ilmenite and sometimes magnetite as common accessories; these schists have bulk compositions corresponding with those of common Dalradian metasediments. However, the mineral assemblages on the low‐ and high‐grade margins of the staurolite zone show evidence of changes from west to east. Lower grade assemblages near Stonehaven include chloritoid+biotite, which has not been found further west in the classic ‘Barrovian’ sequence; whilst the higher grade assemblages near Stonehaven lack kyanite, which is widely seen further west. The Stonehaven area has been suggested to have under gone metamorphism at lower pressure (P) than the classic region of the ‘Glens’ inland to the west.This paper describes the occurrence at middle grade in the broad staurolite zone, of unusual bulk rock compositions showing high ratios of Fe2O3/(Fe2O3 + FeO) and MgO/(MgO + FeO) (abbreviated to M/FM). The high M/FM assemblages are distinct from those of the common St‐Bt‐Grt schists. The high ratios of ferric to ferrous iron are thought to result from original sedimentary protolith compositions, that led to metamorphism at relatively high conditions of oxygen fugacity (ƒo2), and resulted in the formation of haematite‐bearing assemblages, rather than ones carrying accessory ilmenite and magnetite as in the common St + Bt + Grt rocks.In the west (Glen Lethnot), the haematite‐bearing pelites show the occurrence of kyanite at a distinctly lower grade than the common (haematite‐free) pelites, and a transition from staurolite‐biotite (St + Bt) through staurolite–kyanite‐biotite (St + Ky + Bt) to kyanite‐biotite (Ky‐Bt) assemblages occurs with increasing abundance of haematite and higher bulk M/FM values. In all bulk compositions, the upgrade formation of kyanite may be associated with the movement of the St + Ky + Bt assemblage to lower M/FM bulk compositions with increasing temperature (T). The difference in the temperature of lowest‐grade kyanite formation in haematite‐rich schists, compared with that in common haematite‐free schists may approach 50°C.In the east (Stonehaven section), the haematite‐bearing assemblages are much rarer than in Glen Lethnot, but those found so far are very distinctive in showing large porphyroblasts of probable cordierite (typically altered to chlorite and sericite). Occasional assemblages of andalusite‐cordierite‐biotite (And‐Crd‐Bt) also occur in the haematite‐bearing rocks and confirm lower pressures of metamorphism near Stonehaven than those seen further west in Glens Clova, Lethnot and Esk. [ABSTRACT FROM AUTHOR]

Mineralogical and geochemical studies of the Abakaliki Shale have been undertaken in order to constrain source rock composition and palaeoweathering conditions, as well as to deduce the provenance terrane types. The mineralogical analysis using X-ray diffraction has revealed an abundance of quartz and clay minerals (illite, chlorite and illite/smectite mixed layer) with a minor content of albite and gypsum. Geochemical data have demonstrated a depletion in SiO2, MnO, Y, Zr, Hf, U, Y, Cu, Ni, V, Ba and Cr and an enrichment in Rb, Sr, Th, Sc, Co, Zn and Pb in the Abakaliki Shale in comparison to the Post-Archean Australian Shale (PAAS; a proxy for the upper continental crust). Rare earth element contents generally show LREE fractionation and enrichment (LaN/YbN = 16.08–35.75 and LaN/SmN = 3.61–6.16, respectively) with a strong negative Europium anomaly (Eu/Eu* = 0.56–0.73). These geochemical characteristics suggest that the provenance of the Albian Abakaliki Shale lay in ancient upper continental crust composed mainly of felsic basement rocks with a minor contribution from mafic rocks. This is in contrast to earlier studies which suggested possible contribution of mafic components from Jurassic volcanigenic terrane as source rocks of the Albian shale in the southern Benue Trough. Moderate chemical weathering of the source rocks with mixed granite-granodiorite compositions is indicated by the value of the chemical index of alteration (average CIA 76.60); the plagioclase index of alteration (average PIA 85.20) and the index of compositional variability (average ICV 0.88), as well as by the Rb/Sr (0.32–0.92), K/Rb (125.36–193.55) and K/Na (1.13–2.70) ratios. [ABSTRACT FROM AUTHOR]

The Dahongshan Group, situated at the southwestern margin of the Yangtze Block, represents a geological unit characterized by relatively high-grade metamorphism in the region. This paper investigates the garnet-biotite schist from the Laochanghe Formation of the Dahongshan Group, employing an integrated approach that includes petrological analysis, phase equilibrium modeling, and zircon U–Pb dating. The schist is mainly composed of garnet, biotite, plagioclase, quartz, rutile, and ilmenite. Phase equilibrium modeling revealed the peak metamorphic conditions of 8–9 kbar and 635–675 °C. By further integrating the prograde metamorphic profile of garnet and geothermobarometric results, a clockwise P–T metamorphic evolution path is constructed, which includes an increase in temperature and pressure during the prograde stage. LA–ICP–MS zircon U–Pb dating and zircon Ti thermometry constrains the post-peak metamorphic age of 831.2 ± 7.2 Ma. Integrated with previously reported results, it is revealed that the southwestern margin of the Yangtze Block experienced a large-scale regional metamorphism during the Neoproterozoic (890–750 Ma), which is related to the collisional orogenic process. This may be associated with the late-stage assembly of the Rodinia supercontinent or with local compression and subduction processes during the breakup of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

Compaction and melt extraction are key processes in the generation of melt-dominated magma bodies. We analyze magmatic fabrics in the Aztec Wash pluton (15.7 Ma; Nevada, USA) to understand magma dynamics during emplacement of a large (2 m diameter) felsic enclave (LFE) in a silicic magma body. We hypothesize that the LFE moved through crystal-poor magma, settling into crystal-rich magma. We analyze four granite samples collected near the enclave: two 0.25 m below the enclave (E-5 and E-2); one 0.5 m below (E-6); and one 2.5 m to the side (E-3, far field). We use the scanning electron microscope (SEM) to gather data using backscattered electron (BSE) microscopy, energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) imaging. BSE maps show large (up to 10 mm) euhedral alkali feldspar grains in E-5 and E-2, while E-6 and E-3 have large (up to 6 mm) alkali feldspar with distinct irregular and wavy rims. Dark BSE alkali feldspar rims correlate with Ba depletion in EDS maps. Broken feldspars in E-5 and E-2 have mismatched zones in BSE and orientation differences in EBSD. Grain orientation spread (GOS) maps show that alkali feldspar has the highest degree of intragrain deformation, followed by plagioclase, then quartz. Histograms of the angle between the vertical plane and the pole to the (010) plane demonstrate clustering in E-5, suggesting an enhanced fabric. Differences in rim texture suggest melt extraction beneath the enclave and melt retained in the far field. EBSD data reveal an enhanced foliation beneath the LFE. We conclude that mush under the LFE was deformed during emplacement of the LFE, expelling melt and reorienting grains. [ABSTRACT FROM AUTHOR]

This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~ 0.7 GPa (in the southern massif) to <600°C and < 0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution–precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition. [ABSTRACT FROM AUTHOR]

For the first time, high-alumina mineral assemblages including rare minerals of the rhönite–kuratite series were found in postcaldera lava flows of the Kudryavy and Menshiy Brat volcanoes (Kuril Islands). These lava flows consist of olivine-bearing basalts and andesite-basalts of the calc-alkaline series. Partially crystallized olivine-hosted melt inclusions in these rocks contain high-alumina daughter phases. Residual glass with up to 25 wt % Al2O3, Al-pyroxenes, and spinel with minerals of the rhönite subgroup were identified. The rhönite–kuratite series minerals are characterized by strong variations in Al, Fe2+, Fe3+, Mg, Si, as well as very low Ti, which makes it possible to call it a low-Ti Fe2+ analogue of rhönite with the formula (Ca,Mg,Na)4(Mg,Fe,Fe, Al,Ti,Mn)12O4[Si8.2Al3.8O36]—(Ca,Na,Mg)4(Fe,Mg,Al,Fe,Ti)12O4[Si6.78Al5.22O36]. It is assumed that high-alumina daughter phases form in olivine-hosted inclusions via water accumulation in trapped melts during crystallization of the host olivine on the walls of melt inclusions. Water-rich melts can inhibit early crystallization of plagioclase, which promotes enrichment of the residual melt in Al2O3. [ABSTRACT FROM AUTHOR]

Innovative techniques are required for the in situ investigation of the surfaces of planetary bodies when landings are planned. Raman spectroscopy turned out as an excellent tool for fast mineralogical analyses on space missions. Contribution from a photoluminescence signal is not unexpected and is likely to be even more pronounced on celestial surfaces with a dilute or absent atmosphere exposed to strong space weathering, for example, micrometeorite bombardment. Such signals were found, for example, in Raman analysis of the probes from sample‐return missions. While photoluminescence is generally considered as an accompanying undesired product in the Raman spectral measurement, our studies show that some analytical information can be derived from this signal, and even more, due to the specific correlation of luminescence intensity with space weathering products. Therefore, we investigate the Raman spectra alteration of characteristic rock‐forming mineral mixtures (olivine, pyroxene and plagioclase) by micrometeorite bombardment, which is simulated by nanosecond‐pulse laser irradiation. The changes in the minerals are strongly dependent on the composition and structure. They range from disappearing changes in the minerals with simple chemistry and structure to complete amorphization of minerals with relatively low melting enthalpy. With Raman spectroscopy, we found out that the photoluminescence signals show resonant or anti‐resonant changes to specific mineral phases and amorphization. Furthermore, ablation‐induced iron nanoparticles of minerals containing Fe are detectable by Raman spectroscopy due to their alteration into iron oxides. Trapped volatiles in the matrices are analysed due to the formation of the compounds containing them. This broad spectrum of results indicating specific change phenomena due to space weathering can be effectively used for in situ Raman analysis in planetary missions. [ABSTRACT FROM AUTHOR]

The results of studying bottom sediments taken from the high-mountain lake Sagan-Nur located on the Oka plateau (East Sayan Mountains), which is insufficiently studied, are presented. The lake is freshwater and has a glacial origin. Comprehensive investigations of the mineral composition of the lake sediments accumulated over the past ~8600 cal. years have been carried out for the first time. The research methods include X-ray diffraction (XRD), infrared spectroscopy, laser granulometry, SiO2 bio determination, AMS, and 210Pb dating. The mineral composition of the bottom sediments is dominated by layered silicates, quartz, and plagioclase; furthermore, the X-ray amorphous component consisting of biogenic silica and organic matter is present. The method of mathematical modeling of their complex XRD patterns is used for correct identification of the layered silicates. It is established that the layered silicate assemblage is stable and is represented by muscovite, chlorite, illite, mixed-layer illite–smectite and chlorite–smectite, and kaolinite. At the same time, the structural characteristics and quantitative ratios of these minerals in the deposits changed significantly in response to changes in the natural environment of the region, which reveals the potential of a number of phyllosilicates (illite, mixed-layer minerals) as indicators of the paleoclimate in the bottom sediments of freshwater basins. The results of studying the sediment composition and, especially, the precision mineralogical–crystallochemical analysis of layered silicates have made it possible to reconstruct the paleolandscapes and climatic conditions of sediment accumulation and to assign them to the respective Holocene sub-stages. [ABSTRACT FROM AUTHOR]

To investigate the chemical variation during space weathering of young mare basalts, here we report elemental, radiogenic Sr-Nd and stable Fe-Mg-Ca isotopic data of Chang'E-5 sieved soils and breccias. From the coarse fraction to the fine one, the sieved soils display increasing Al 2 O 3 (10.34 wt%–13.36 wt%) and Sr (248 ppm–307 ppm) but decreasing FeO (23.50 wt%–20.22 wt%), MgO (6.88 wt%–5.78 wt%), FeO/Al 2 O 3 (2.27–1.51) and MgO/Al 2 O 3 (0.67–0.43). The contents of rare earth elements (except Eu) and high field strength trace elements do not vary with particle size but correlate with P 2 O 5 contents. Given the limited contribution from contamination by meteorites and exotic materials ejected far away from the landing site, these elemental variations can be explained by differential comminution and distribution behaviors of plagioclase and mesostasis phases. These sieved soils yield a Sm-Nd isochron age (1.84 ± 0.83 Ga) comparable to that of basaltic clasts obtained by U-Pb dating (∼2.0 Ga). However, their Rb-Sr isotopic system is disturbed as indicated by their relatively homogeneous 87Sr/86Sr (0.701425–0.701592) despite variable Rb/Sr (0.017–0.028). These results suggest the Sm-Nd isotopic system is more robust to impact disturbance during space weathering compared to the Rb-Sr isotopic system. Given that the bulk soil still plots on the 2.03 Ga Rb-Sr reference isochron from the pristine plagioclases in CE-5 basalts, this disturbance did not affect the Rb-Sr isotopic system on the bulk scale. The CE-5 bulk soil has higher Mg# (33.6), 87Rb/86Sr (0.06) and present-day 87Sr/86Sr (0.701542) than the mean composition of reported basaltic clasts (Mg#: ∼28; 87Rb/86Sr: ∼0.038; 87Sr/86Sr: ∼0.700941), possibly implying that the bedrocks in CE-5 landing site consist of multiple magma pulses. The δ56Fe (0.122 ± 0.002 ‰ to 0.199 ± 0.008 ‰) and δ26Mg (−0.204 ± 0.016 ‰ to −0.109 ± 0.006 ‰) of sieved CE-5 soils increase with decreasing particle sizes but their δ 44/42Ca (0.38 ± 0.04 ‰ to 0.44 ± 0.02 ‰) are relatively homogeneous. Mass balance modelling indicates that differential comminution has limited influence on the Fe-Mg-Ca stable isotopic compositions. We further dismiss the role of solar-wind sputtering, as Ca and Mg are more susceptible to sputtering and thus would be expected to show larger isotope fractionations compared to Fe, which is inconsistent with the observations. Free evaporation may explain the elevated δ56Fe and δ26Mg in fine fractions at given very limited depletion in FeO and MgO. The observed positive correlation between δ56Fe and δ26Mg, however, is much steeper than the slope expected for free evaporation, indicating also other mechanisms (e.g., Fe-Mg inter-diffusion). Since the CE-5 soil has a unique composition compared with Apollo and Luna soils, the chemical differentiation identified in this study provides new insights for establishing a connection between the chemistry and reflectance spectral properties of lunar soil. Our combined Fe-Mg-Ca isotopic study also provides a paradigm to distinguish the role of solar-wind sputtering and impact evaporation, and shows that the inter-particle diffusion process may be an important mechanism for the isotope fractionation among lunar soil components. [ABSTRACT FROM AUTHOR]

Rare amphibolite in the New Jersey Highlands containing plagioclase megacrysts ≤13 cm long forms bodies 0.5 to 2 m thick that preserve a penetrative metamorphic fabric and have sharp, conformable contacts against Mesoproterozoic country rocks. The megacrystic amphibolites were emplaced as thin dikes along extensional faults between 1160 and 1130 Ma. Amphibolites contain weakly zoned, subhedral andesine megacrysts (An29–44) in a matrix of plagioclase (An18–38), magnesio-hastingsite, biotite, diopside, Fe-Ti oxides, and apatite. The whole-rock major oxide composition of the megacrystic amphibolite matrix has high TiO2 (2.07 wt.% ± 2.0%), Al2O3 (17.03 wt.% ± 0.87%), and Fe2O3t (12.84 wt.% ± 3.2%) that represents an Al-Fe-rich mafic magma type that is characteristic of anorthosite associations globally. The whole-rock, rare earth element (REE) composition of the megacrystic amphibolite matrix is characterized by enrichments in light rare earth elements (LREEs) (La/YbN = 1.73–10.69) relative to middle (MREEs) and heavy (HREEs) rare earth elements (Gd/YbN = 1.30–1.85), and most samples have small positive Eu anomalies (Eu/Eu* = 0.95–1.25). The megacrystic amphibolite matrix is also enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) (e.g., Ba/Nb = 24–22). Megacrystic amphibolites formed through partial melting of subduction-modified lithospheric mantle that fractionated olivine and plagioclase, producing a high-Al-Fe mafic magma. Plagioclase megacrysts formed through extraction of a plagioclase-rich crystal-liquid mush from anorthosite that mixed with mafic magma and collected in the upper part of the mid-crustal magma (depth of ~20 km based on Al-in-hornblende geobarometry) chamber through flotation. Periodic tapping of this mixed source by extensional fractures led to emplacement of the amphibolites as dikes and may have interrupted the extensive fractionation and plagioclase separation necessary to form voluminous anorthosite intrusions. [ABSTRACT FROM AUTHOR]

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are constrained through the sequence and conditions of metamorphic reactions in major and accessory phases. Petrographic, isotopic and elemental data from metabasite samples in NW Bhutan, eastern Himalaya, suggest initial equilibration under high‐pressure (plagioclase‐absent and rutile‐present) conditions, followed by decompression to lower pressure conditions at high‐temperatures that stabilized plagioclase, orthopyroxene and ilmenite. Field observations and chemical indicators suggest equilibration under the lower pressure conditions is likely linked to the infiltration of melt from the host metasedimentary rocks. The metabasites preserve two metamorphic growth stages of chemically‐and petrographically distinct allanite that temporally overlap two stages of zircon growth. Allanite cores and zircon mantles grew at c. 19 ± 2 and 17–15.5 Ma respectively, linked texturally and chemically to the high‐pressure evolution. Symplectitic rims on embayed allanite cores, wholly symplectized Aln–Ilm and Aln–Cpx grains, and high U zircon rims grew at c. 15.5–14.5 Ma, linked chemically to the presence of melt and lower pressure, high‐temperature conditions. A single garnet Lu–Hf date is interpreted as geologically meaningless, with the bulk rock composition modified by melt infiltration after garnet formation. The open system evolution of these rocks precludes precise determination of the reactive bulk composition during metamorphic evolution and thus absolute conditions, especially during the early high‐pressure evolution. Despite these limitations, we show that combined geochemical and petrographic datasets are still able to provide insights into the rates and timescales of deep orogenic processes. The data suggest a younger and shallower evolution for the NW Bhutan metabasites compared to similar rocks in the central and eastern Himalayas. [ABSTRACT FROM AUTHOR]

In the present study, red slip wares excavated from the ancient city of Germanicia (modern Kahramanmaraş, Southeastern Anatolia) were subjected to an archaeometric analysis. A multi‐analytical approach was employed, including stereomicroscopy, optical microscopy, XRPD, SEM‐EDS, and XRF to explore the mineralogical and chemical compositions of the samples. Based on both archaeological and archaeometric criteria, the samples were classified into three main categories: African, Phocaean, and Sagalassos red slip wares. The African red slip wares were characterized by an abundance of coarse quartz inclusion, accompanied by lesser quantities of plagioclase and mica. The Phocaean red slip wares primarily consisted of quartz, feldspar, and mica. In contrast, the Sagalassos red slip wares featured a groundmass rich in quartz, mica/biotite, hematite, and opaque minerals. Thermoanalytical data indicated that all samples were fired at temperatures between 800 and 900°C. The results suggest that red slip wares found at Germanicia were not manufactured using local available clay sources but rather originated from different regions. This evidence further provides that Germanicia was actively involved in trade networks and interacted with various ancient urban centers, such as Africa, Phocaean, and Sagalassos, during the Late Roman Period, between the fourth and fifth centuries AD. [ABSTRACT FROM AUTHOR]

We present 230Th‐238U crystallization ages and trace element compositions for zircons spanning the late Pleistocene to Holocene rhyolite eruptive record at South Sister volcano in the central Oregon Cascade Range. Most zircon ages are between 100 and 20 ka, with very few in secular equilibrium (>350 ka). The weighted mean of zircon ages for the two oldest South Sister rhyolites, 31.5 ± 2.1 and 39.1 ± 2.4 ka, are significantly younger than the associated 40Ar/39Ar ages, 47.4 ± 9.7 and 51.4 ± 9.7 ka. We propose that these 40Ar/39Ar dates, performed on plagioclase separates, are compromised by a subtle amount of excess Ar and therefore the younger weighted mean zircon ages yield more reliable eruption ages. These results imply that the interval of rhyolite eruption at South Sister during the late Pleistocene was both shorter and more productive than previously thought and that eruption at South Sister initiated after Middle Sister. Compositionally, zircons from the Pleistocene rhyolites are broadly similar and show down‐temperature zircon and plagioclase crystallization trends. However, we argue that destabilized amphibole and titanite in a common mush also exert leverage on the Pleistocene zircon trace element compositions. Divergence in the Eu/Eu* ratio between the Pleistocene and Holocene lavas implies chemically distinct magma reservoirs originating from the Pleistocene rhyolite eruptive sequence and the Holocene eruptive sequence. This work suggests a higher flux of rhyolite volcanism than previously thought and characterizes magmatic storage distinctions between the Pleistocene and Holocene rhyolites, aiding in the assessment of future eruptive hazards at South Sister volcano. Key Points: Zircon ages shift inferred eruption ages for two South Sister rhyolites over 10 kyr youngerCrystallization of zircon and plagioclase as well as destabilized amphibole and titanite leveraged melt compositions of Pleistocene magmasCompositional distinctions between the Pleistocene and Holocene lavas suggest distinct melt sources for both eruption sequences [ABSTRACT FROM AUTHOR]

This study explores the capability of amphibole in tracing the physicochemical process of magma mixing through spatially associated gabbros, mafic microgranular enclaves (MMEs) and granodiorites from central Tibet. These rocks share similar zircon ages as well as zircon Hf‐O and plagioclase Sr isotopes. However, the amphiboles within the gabbros and granodiorites have different Sr and B isotope compositions, while amphiboles with both heterogeneous isotopic imprints occur in the MMEs. According to data and modeling, significant mixing of two isotopically distinct magmas is recorded by amphibole but not by zircon and plagioclase. Based on a synthesis of petrography, geochemistry and thermobarometry, we interpret this inconsistency by the crystallization order of minerals and propose that magma mixing occurred after the parent magma was emplaced at ∼10 km and cooled to ∼750°C. Our study highlights that amphibole may be a more sensitive tracer of magma mixing relative to other commonly used methods. Plain Language Summary: Magma mixing significantly shapes the composition of silicic igneous rocks representing the major constituent of the upper continental crust. Since bulk‐rock composition may only bear the average of mixed sources, in situ techniques such as Hf‐O isotopes in zircon and Sr‐Pb isotopes in plagioclase are widely used to investigate the details of magma mixing. However, these methods cannot constrain how magma mixing operates in the deep crust. This study novelly uses in situ Sr‐B isotopes and trace elements in amphibole to trace the physicochemical process of magma mixing through spatially associated gabbros, mafic microgranular enclaves and granodiorites from the central Tibetan Plateau. Our data show that significant mixing of two isotopically distinct magmas is recorded by amphibole but not by zircon and plagioclase. We demonstrate that the amphibole geochemistry (i.e., trace element, Sr‐B isotope compositions) may be more sensitive in tracing magma mixing relative to traditional isotopic tools and has the potential to unravel the physicochemical process(es) of magma mixing in the deep crust. In addition, our work reinforces the use of B isotopes in amphibole to discern the nature (fluids released from altered oceanic crust vs. residual slab) of the slab components that metasomatized the supra‐subduction mantle. Key Points: The amphiboles in central Tibetan magmatic complex have contrasting geochemical and Sr‐B isotopic compositionsSignificant magma mixing is well recorded by amphibole but not by zircon and plagioclaseAmphibole is a more sensitive tracer of magma mixing relative to zircon and plagioclase [ABSTRACT FROM AUTHOR]

The Poroshiri ophiolite (Hidaka metamorphic belt, Japan) occurs within a crustal‐scale network of high‐temperature, dextral shear zones that accommodated hundreds of kilometres of displacement due to the opening of the Japan Sea in the Neogene. The opholitic rocks comprise ultramafic, mafic and sedimentary protoliths that have been variably hydrated and metamorphosed. This work investigates the mechanisms and timing of fluid influx relative to viscous deformation in metagabbros and amphibolites deformed during exhumation from granulite‐ to amphibolite‐facies conditions. We consider a range of microstructures, from low strain domains and 1–2 mm thick shear bands to mylonites with a thickness of a few meters. Low strain domains of metagabbros exhibit corona textures with symplectites consisting of pargasitic amphibole (Ed0.7) ‐ anorthitic plagioclase (An80–92) ± orthopyroxene ± clinopyroxene forming around olivine and igneous pyroxene and with similar plagioclase–amphibole‐orthopyroxene ± clinopyroxene granoblastic aggregates in micrometric‐thick fractures. These textures result from hydration under low fluid‐rock ratio, with elevated H2O content only occurring locally (H2O > 1–1.2 wt%). Igneous mineral replacement leads to grain size reduction from 1 mm to ~10 μm. Amphibole exhibits a strong core‐rim zonation primarily controlled by the high diffusivity of Fe, Mg and OH and the low diffusivity of Al. Mineral compositional equilibrium is achieved at the scale of 100–200 μm. In mm‐thick localized shear bands and in metric‐scale mylonitic amphibolites, the heterogeneous mineral composition of amphibole (Ed0.2–0.5) and plagioclase (An40−An80) indicates only partial re‐equilibration (at the scale of 200–500 μm) despite higher fluid‐rock ratios and more pervasive fluid percolation than in metagabbros. Plagioclase–amphibole thermometry and equilibrium phase diagrams indicate that the initial fluid infiltration and corona formation occurred at 800–850°C by fracturing and percolation along grain boundaries. This was followed by the main fluid percolation and mylonitization event, which occurred during exhumation and cooling at conditions of 720–580°C, ~4 kbar. Continuous hydration during retrogression was achieved by the influx of dominantly seawater‐derived fluid, as attested by the high chlorine (Cl) contents (>300–400 ppm) of amphiboles in fractures. The heterogeneous distribution of fractures controls the distribution of fluid from the earliest stages of hydration, creating positive feedback where the growth of hydrous minerals as amphibole (up to +300 vol% of amphibole in high strain areas relative to the low‐strain ones) and the formation of fine‐grained mixed domains that led to further localization of viscous strain and mass transfer (variations of ± 30–40% in major elements). The Poroshiri ophiolite therefore represents a good fossil example of a former transpressional plate boundary where coupled metamorphic and deformation processes were triggered by seawater‐derived fluid that percolated to depths of ~15 km. [ABSTRACT FROM AUTHOR]

Garnet peridotite xenoliths have been rarely reported from suboceanic mantle. Petrographic and geochemical characteristics of garnet-bearing oceanic peridotite xenoliths provide precious information on dynamics of the suboceanic lithosphere and asthenosphere interaction. We examined a lherzolite xenolith included in olivine nephelinite lava from Aitutaki Island, a member of the Cook-Austral volcanic chain. The lherzolite xenolith contains reddish fine-grained (< 5 µm in size) mineral aggregates (FMAs) with size range of 0.5–6 mm, consisting of olivine, calcic and sodic plagioclases, aluminous spinel, native iron, and nepheline. Microstructural observations and chemical data corroborate that the FMA is a decomposed pyrope-rich garnet including chromian spinel grains with an irregular highly indented morphology in the center. The FMA is surrounded by pyroxene-poor and olivine-rich aureole. The spatial and morphological relationships of FMA and chromian spinel with pyroxene-depleted margin suggest a reaction of aluminous spinel + pyroxenes → pyrope-rich garnet + olivine, which requires a compression before decomposition of the garnet to FMA. An orthopyroxene grain shows slight but clear chemical zoning characterized by increase in Al, Ca, and Cr from the grain center to the rim. The zoning patterns of Al and Ca in the orthopyroxene grain can be modeled by diffusion-controlled solid-state reactions induced by pressure and temperature changes, keeping surface concentrations in equilibrium with the other coexisting mineral phases. The results indicate that the mantle, from which the lherzolite xenolith was derived, underwent isothermal decompression followed by a weak heating on a time scale of a few tenths of million years before the xenolith extraction. From the deduced compression and decompression histories, we hypothesize that the mantle beneath Aitutaki Island was once dragged down to a garnet-stable deep mantle region and brought up later by small-scale sublithospheric convection. [ABSTRACT FROM AUTHOR]

High-Ti basalts are commonly believed to represent parental magmas leading to the formation of mafic-ultramafic layered intrusions, such as Late Permian Panzhihua and Hongge in the Emeishan Large Igneous Province (SW China). Consequently, elucidation of the crystallisation and crustal differentiation of high-Ti basalts is critical for our understanding of the petrogenesis of these layered intrusions and the associated oxide ore mineralisation. Here, we present the results of crystallisation experiments carried out in internally heated pressure vessels using a primitive high-Ti basaltic composition. The experiments were conducted at 100 and 300 MPa, in the temperature interval of 950–1200 °C and with water activities (a H2O) from 0 to 1. The oxygen fugacity (f O2) was controlled and varied from FMQ −1 to FMQ +3.3 log units (FMQ corresponds to fayalite-quartz-magnetite buffer). The main mineral phases are olivine, clinopyroxene and plagioclase, accompanied by Cr-Fe-Ti-oxides, orthopyroxene, apatite and amphibole, depending on the conditions. Redox conditions primarily influence the stability fields of Cr-Fe-Ti oxides. Clinopyroxene, orthopyroxene and amphibole are pressure-dependent and have larger stability fields under high pressure conditions. The olivine→orthopyroxene and olivine→amphibole peritectic reactions are observed. Comparisons of phase equilibria between this study and experiments conducted with parental magma of Skaergaard layered intrusion demonstrate the effect of bulk system composition. For instance, ilmenite crystallisation is determined not only by intrinsic parameters such as f O2 but also by additional compositional parameters (e.g. melt Ti, Fe, Al and Mg content). Although COMAGMAT and MELTS modelling results generally reproduce the crystallisation sequence, only the stability field of clinopyroxene and its composition are perfectly modelled. The comparison of experimental results with the rocks from the lowest units of Panzhihua and Hongge layered intrusions are applied to constrain storage conditions in the magma reservoirs. Compared to Hongge, we conclude that the Panzhihua magma chamber was probably located at a shallower depth (~3–6 km), that magma crystallisation started at lower temperatures (~1125–1100 °C), higher f O2 (~FMQ + 1 to FMQ + 2) and that its initial melt H2O content was lower (~0.5–1 wt. %). [ABSTRACT FROM AUTHOR]

Magmatic systems below volcanoes are often dominated by partially crystalline magma over the long term. Rejuvenation of these systems during eruptive events can impact lava composition and eruption style—sometimes resulting in more violent or explosive activity than would be expected, as was the case at Fissure 17 during Kīlauea's 2018 eruption. Here, we explore how the crystallinity of unerupted intrusion magmas affect hybrid magma compositions and petrological signatures by constructing phase-equilibria models to evaluate mineral and melt compositions of low-MgO lavas erupted along the East Rift Zone of Kīlauea volcano on 30 to 31 January 1997 (Episode 54, Fissures A-F). We then compare calculated mixing proportions and petrologically derived magma volumes to GPS-based geodetic inversions of ground deformation and intrusion growth in an attempt to reconcile geodetic and petrologically estimated magma volumes. Open-system phase-equilibria thermodynamic models were used to constrain the composition, degree of differentiation, and thermodynamic state of a rift-stored, two pyroxene + plagioclase saturated low-MgO magma body immediately preceding its mixing with high-MgO recharge and degassed drainback (lava lake) magma from Pu'u'ō'ō', shortly before fissure activity within Nāpau Crater began on 29 January 1997. Mixing models constructed using the Magma Chamber Simulator reproduce the mineralogy and compositions of Episode 54 lavas within uncertainties and suggest that the identity of the low-MgO magma body may be either variably differentiated remnants of un-erupted magmas intruded into Nāpau Crater in October 1968, or another spatially and compositionally similar magma body. We find that magmas derived from a single, compositionally stratified magma emplaced beneath Nāpau Crater in 1968 can mix with mafic Kīlauea magmas to reproduce average Episode 54 bulk lava, mineralogy and mineral compositions without necessitating the interaction of multiple, low-MgO rift-stored magma bodies to produce Episode 54 lava compositions. Further, by constructing phase equilibria-based mixing models of Episode 54, we can better define the pre-eruptive state of the magmatic system. The resultant mineral assemblages and compositions are consistent with the possibility that the now-fractionated, rift-stored magma body was compositionally stratified and ~ 40% to 50% crystalline at the time of mixing. Finally, we estimate the volume of the low-MgO magma body to be ~7.51 Mm3. Phase-equilibria model results corroborate field and geochemical relationships demonstrating how shallow intrusions at intraplate shield volcanoes can crystallize, evolve, and then be remobilized by new, later batches of mafic magma. Most notably, our MCS models demonstrate that the pre-eruptive conditions of an intrusive body may be recovered by examining mineral compositions within mixed lavas. Discrepancies between the geodetic constraints on volumes of stored rift versus newly intruded (recharge) magma and our best-fit results produced by MCS mixing models (which respectively are mmafic:mlow-MgO ≈ 2 vs. mmafic:mlow-MgO ≈ 0.75) are interpreted to highlight the complex nature of incomplete mixing on more localized scales as reflected in erupted lavas, compared to geodetically constrained volumes that likely reflect large spatial scale contributions to a magmatic system. These dissimilar volume relationships may also help to constrain eruptive versus unerupted volumes in magmatic systems undergoing mixing. By demonstrating the usefulness of MCS in modeling past eruptions, we highlight the potential to use it as a tool to aid in petrologic monitoring of ongoing activity. [ABSTRACT FROM AUTHOR]

We investigate ten of the most recent tephra-fall deposits emplaced between ≤21 and 2 ka from the Pacheco stage of Misti volcano, Peru, to elucidate magma dynamics and explosive eruption triggers related to magma storage, recharge, and remobilization. Whole-rock, glass, and mineral textures and compositions indicate the presence of broadly felsic, intermediate, and mafic magmas in a chemically and thermally stratified magma storage system (Zones 1–3) that interact to differing extents prior to eruption. Intermediate magmas are defined by plagioclase + amphibole + two-pyroxenes + Fe-Ti oxides and phase equilibria indicate they formed at ~300 to 600 MPa and ~950°C to 1000°C. Intermediate magmas dominate the Pacheco stage and either erupted alone as hybridized magmas or mingled with minor volumes of cool felsic magmas (~800°C) in which only plagioclase + Fe-Ti oxides are stable. Felsic magmas do not exclusively comprise any tephra-fall deposit emplaced during the Pacheco stage but were remobilized by recharge and mixing with intermediate magmas in order to erupt. Furthermore, felsic-hosted amphibole cognate to the intermediate magmas are reacted despite the felsic magmas being water saturated, which suggests they are staged above the amphibole stability limit (≤200 MPa). The cryptic presence of mafic magmas is indicated by high-An plagioclase cores (An74–88), rare anhedral olivine (Fo77–80), and possibly high Mg# augite and amphibole (up to Mg# 84 and 77, respectively). The dearth of basalt to basaltic andesite melts recorded in erupted glasses and exclusivity of high-An plagioclase to crystal cores signals mafic magmas are staged deeper in the crust than the intermediate magmas. Periodic interactions between these magmas tracked via glass compositions and crystal exchange reveal an alternation between the production of mingled magmas and their eruption shortly after a recharge event, followed by a period of homogenization and eruption of hybridized magmas. As such, we identify magma recharge as a key mechanism by which half of the explosive eruptions were triggered in the Pacheco stage. A >100°C increase in Misti's fumarole temperatures from 1967 to 2018 coincident with changes in fumarolic gas compositions is consistent with degassing of a mafic recharge magma, signaling that Misti could produce similar explosive eruptions in the future. [ABSTRACT FROM AUTHOR]

Porphyry Cu, and porphyry Cu–Au deposits, are associated with arc magmatism and their ore-forming systems generally follow the magmatic evolution of typical arcs. However, most arc magmas are barren and giant economic porphyry Cu ± Au deposits are rare. In this study, we model variations in rare earth element concentrations in evolving arc magmas and giant porphyry Cu ± Au systems to quantify the percentage of the fractionating minerals required to produce the observed changes. We find that, during the andesitic stage of fractionation, ore-forming systems in thick crusts fractionate ~35% more amphibole than an average of thick arc magma systems (the thick-crust reference suite) and that ore-forming systems in thin crusts fractionate twice as much amphibole as their equivalent thin-arc magma reference suite. Thick-crust ore-forming suites also fractionate ~50% less plagioclase, and thin-crust ore systems ~40% less plagioclase, than their associated reference suites during the same andesitic stage of fractionation. Taken together, these observations imply that ore-producing magmas are appreciably wetter than their associated barren reference suites. Our modeling also shows that ~80% more amphibole is required to reproduce the andesite stage of fractionation in the thick-crust reference suite than in its thin-crust equivalent, suggesting that magmas produced under thick crusts are wetter than those produced under thin crusts. On the other hand, the chalcophile element contents of the thick- and thin-crust ore-forming systems are similar to and higher than those of the thick- and thin-crust reference suites, respectively. Therefore, we suggest that the high water content plays a critical role in the formation of giant porphyry Cu ore in thick crusts, whereas both high chalcophile contents and high water contents are required to form giant porphyry Cu–Au deposits in thin crusts. The high fraction of amphibole fractionation in giant economic porphyry suites, compared with their relevant reference suites, results in lower Y in the ore-associated suites and this difference increases with fractionation. As a consequence, plots of Y against MgO can be used to identify porphyries that have economic potential and are preferred to Sr/Y plots because they are less affected by the intense alteration associated with giant porphyry Cu ± Au deposits. [ABSTRACT FROM AUTHOR]

The Tepsi ultrabasic body is located in the northeastern Fennoscandian Shield close to the junction of the Serpentinite Belt–Tulppio Belt (SB–TB) with suites of the Lapland–Belomorian Belt (LBB) of Paleoproterozoic age. The body is a deformed laccolith that has tectonic contacts with Archean rocks. Its primary textures and magmatic parageneses are widely preserved. Fine-grained olivine varies continuously from Fo90.5 to Fo65.4. The whole-rock variations in MgO, Fe2O3, SiO2, and other geochemical data are also indicative of a significant extent of differentiation. Compositional variations were examined in the grains of calcic and Mg-Fe amphiboles, clinochlore, micas, plagioclase, members of the chromite–magnetite series, ilmenite, apatite, pentlandite, and a number of other minor mineral species. Low-sulfide disseminated Ni-Cu-Co mineralization occurred sporadically, with the presence of species enriched in As or Bi, submicrometric grains rich in Pt and Ir, or diffuse zones in pentlandite enriched in (Pd + Bi). We recognize two series: the pentlandite series (up to 2.5–3 wt.% Co) and the cobaltpentlandite series (~1 to ~8 apfu Co). The latter accompanied serpentinization. The two series display differences in their substitutions: Ni ↔ Fe and Co → (Ni + Fe), respectively. Relative enrichments in H2O, Cl, and F, observed in grains of apatite (plus high contents of Cl in hibbingite or parahibbingite), point to the abundance of volatiles accumulated during differentiation. We provide the first documentation of scheelite grains in ultrabasic rocks, found in evolved olivine-rich rocks (Fo77–72). We also describe unusual occurrences of hypermagnesian clinopyroxene associated with tremolite and serpentine. Abundant clusters of crystallites of diopside display a microspinifex texture. They likely predated serpentinization and formed owning to rapid crystallization in a differentiated portion of a supercooled oxidized melt or, less likely, fluid, after bulk crystallization of the olivine. We infer that the laccolithic Tepsi body crystallized rapidly, in a shallow setting, and could thus not form megacycles in a layered series or produce a well-organized structure. Our findings point to the existence of elevated PGE-Au-Ag potential in numerous ultrabasic–basic complexes of the SB–TB–LBB megastructure. [ABSTRACT FROM AUTHOR]

The Lianzigou deposit, which has an Au–Te paragenetic association, is hosted in plagioclase gneiss of the Qincanggou Formation in the Taihua Group in the Xiaoqinling region, central China. This quartz vein-type Au deposit comprises native Au and a variety of tellurides. The latter include calaverite (AuTe2), krennerite (Au3AgTe8), petzite (Au3AgTe2), hessite (Ag2Te), melonite (NiTe2), and altaite (PbTe). Four stages have been recognized in this deposit: stage I consists of K-feldspar and quartz; stage II is of milky quartz veins accompanied by coarse-grained disseminated and lumps of pyrite with weak Au mineralization; stage III is composed mainly of Au, tellurides, and sulfides; and stage IV is characterized by abundant carbonate and quartz. Based on mineral assemblage and thermodynamic data, we estimated the physicochemical conditions of the main metallogenic stages. Based on thermodynamic modelling, the physicochemical conditions of Au–Ag–Te mineral associations were estimated. The Au–Ag–Te minerals from stage III formed mainly under conditions of logƒO2 = −43.15 to −33.31, logƒH2S = ~−9.29, pH < 7, logfTe2 = −10.6 to −9.8 and logαAu+/αAg+ = −7.2 to −6.5. In contrast, the physicochemical conditions of stage II were higher, specifically pH (8.3–8.5) and logƒO2 (−34.90−31.96). In the ore-forming fluids of the Lianzigou deposit, the dominant Au species was Au(HS)2− while the dominant Te species were HTe−(aq) and Te22−(aq). Moreover, the Au–Ag–Te metal associations in the Lianzigou Au deposit were derived from mantle materials related to lithospheric thinning of the eastern North China craton in the Early Cretaceous under an extensional tectonic system. [ABSTRACT FROM AUTHOR]

The Kunduleng granite hosts one of several significant uranium anomalies within the southern Great Xing'an Range, NE China. Whole-rock geochemistry and mineral chemistry data, along with the zircon U-Pb-Hf isotope have been used to constrain the petrogenesis of this granitic intrusion and the origin of the uranium anomaly. Microscopically, quartz, alkali-feldspar, and plagioclase are the essential mineral constituents of the granite, with minor biotite, while monazite, apatite, xenotime, and zircon are accessory minerals. Geochemically, the silica- and alkali-rich granites show a highly fractionated character with "seagull-shaped" REE patterns and significant negative anomalies of Ba and Sr, along with low Zr/Hf and Nb/Ta ratios. The granite has positive zircon εHf(t) values ranging from +12.7 to +14.5 and crustal model ages (TDM2) of 259–376 Ma, indicating a Paleozoic juvenile crustal source. Uraninite and brannerite are the main radioactive minerals responsible for the uranium anomaly within the Kunduleng granite. Uraninite presents well-developed cubic crystals and occurs as tiny inclusions in quartz and K-feldspar with magmatic characteristics (e.g., elevated ThO2, Y2O3, and REE2O3 contents and low CaO, FeO, and SiO2 concentrations). The calculated U-Th-Pb chemical ages (135.4 Ma) are contemporaneous with the U-Pb zircon age (135.4–135.6 Ma) of the granite, indicating a magmatic genesis for uraninite. The granites are highly differentiated, and extreme magmatic fractionation might be the main mechanism for the initial uranium enrichment. Brannerite is relatively less abundant and typically forms crusts on ilmenite and rutile or it cements them, representing the local redistribution and accumulation of uranium. [ABSTRACT FROM AUTHOR]

We report whole-rock chemistry, mineral chemistry, and volatile content from Villarrica volcano's major recent paroxysms and background activity. Composition of the volcanic products are basalt to basaltic andesite with whole-rock SiO2 content between 50 and 56 wt%, and a mineralogy dominated by olivine (Fo71-80), clinopyroxene (Mg# ~ 50) and plagioclase (An60–80). Volatile contents in melt inclusions are up to 1.5 wt% H2O, 500 ppm CO2, 1230 ppm sulfur and 580 ppm chlorine. Regardless of the type of activity, there are no substantial variations in whole-rock composition or the volatile content when the activity switches from background activity to a major paroxysm, strongly suggesting that this shift does not just depend on the arrival of new magma in the shallow magmatic system. Geothermobarometry constrains crystallization of the major mineral phases at various depths between 3 and 12.7 km, suggesting that degassing of a volatile-rich recharge magma occurs deeper than 12 km, producing efficient mixing throughout the whole system, and sustaining the lava lake activity in Villarrica's summit crater. The occurrence of a permanent lava lake also suggests that the magma recharge must be close to continuous and therefore sudden changes between background and paroxysmal volcanic activity are likely controlled by relatively small changes in the rate of recharge and/or the volatile release rate in the magmatic system. This has important implications for the understanding of eruption triggers and the forecasting of volcanic eruptions. [ABSTRACT FROM AUTHOR]

Interpreting reservoir properties through log data and logging responses in complex strata is critical for efficient petroleum exploitation, particularly for metamorphic rocks. However, the unsatisfactory accuracy of such interpretations in complex reservoirs has hindered their widespread application, resulting in severe inconvenience. In this study, we proposed a multi-mineral model based on the least-square method and an optimal principle to interpret the logging responses and petrophysical properties of complex hydrocarbon reservoirs. We began by selecting the main minerals based on a comprehensive analysis of log data, X-ray diffraction, petrographic thin sections and scanning electron microscopy (SEM) for three wells in the Bozhong 19-6 structural zone. In combination of the physical properties of these minerals with logging responses, we constructed the multi-mineral model, which can predict the log curves, petrophysical properties and mineral profile. The predicted and measured log data are evaluated using a weighted average error, which shows that the multi-mineral model has satisfactory prediction performance with errors below 11% in most intervals. Finally, we apply the model to a new well "x" in the Bozhong 19-6 structural zone, and the predicted logging responses match well with measured data with the weighted average error below 11.8% for most intervals. Moreover, the lithology is dominated by plagioclase, K-feldspar, and quartz as shown by the mineral profile, which correlates with the lithology of the Archean metamorphic rocks in this region. It is concluded that the multi-mineral model presented in this study provides reasonable methods for interpreting log data in complex metamorphic hydrocarbon reservoirs and could assist in efficient development in the future. [ABSTRACT FROM AUTHOR]

Voluminous Neoproterozoic intermediate to felsic rocks intruded the Neoarchean Douling Complex in the South Qinling Belt of Central China, which may provide new constraints on the controversial issue about the nature of the continental margin of the Yangtze Block during the Neoproterozoic. This study presents zircon U–Pb geochronology, whole‐rock geochemistry and Sr–Nd isotope of the Douling dioritic–granodioritic–granitic intrusion, aiming to clarify its petrogenesis and tectonic significance. The dioritic–granodioritic–granitic rocks yield similar zircon U–Pb ages ranging from 735 to 705 Ma. The enrichment of light rare earth elements (LREEs) and large‐ion lithophile elements (LILEs), and depletion of high‐ field‐strength elements (HFSEs) (e.g., Nb, Ta) are typical features of arc magmatic rocks. The whole‐rock Sr–Nd isotopic compositions show initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7059, and εNd(t) values from −7.3 to −1.4. Elemental and isotopic data suggest that the Neoproterozoic dioritic–granodioritic–granitic rocks were co‐genetic and were generated by partial melting of lower crust materials combined with mantle‐derived melts. The granites were the derivative product of dioritic rocks by fractional crystallization of amphibolite, plagioclase, mica and zircon. Combined with literature data, we infer a subduction‐related setting for the northern margin of the Yangtze Block during the Middle Neoproterozoic, and a tectonic model of accretion along an Andean‐type active continental margin after the collision of the Douling micro‐block with the northern Yangtze Block is further proposed. [ABSTRACT FROM AUTHOR]

The Slate Islands (Ontario) is one of Canada's larger impact structures at 32 km in diameter and has been linked to the Ordovician meteorite event (OME). We report zircon U–Pb dates from two suevite and two syenite samples collected from the Slate Islands. Plagioclase 40Ar/39Ar dates were also obtained from one of the samples. The plagioclase and most zircon dates record pre‐impact ages with links to known tectonic events, including those associated with the assembly of the Superior Craton at approximately 2700 Ma. However, Neoarchean zircon grains appear to be reset at 456.1 ± 6.9 Ma (±2σ) based on the lower intercept of discordia for all dated samples. The date overlaps its previously accepted age of 450 Ma and would be 2–19 million years following the parent asteroid breakup if related to the OME. [ABSTRACT FROM AUTHOR]

Achondrites provide an opportunity to examine the igneous processes of differentiated bodies in our solar system. The recent discovery of several silica‐rich achondrites suggests that andesitic crusts were more common among planetesimals than previously thought, though the processes behind their emplacement are not well understood. Here, electron backscatter diffraction (EBSD) is used to investigate the igneous emplacement conditions of Erg Chech 002 (EC 002), a recently discovered ungrouped achondrite representing andesitic magmatism ~2 Myr after the formation of calcium–aluminum‐rich inclusions (CAIs). EBSD analyses of crystallographic preferred orientations (CPOs) for augite and plagioclase feldspar phenocrysts indicate that EC 002 exhibits a weak foliation CPO. Augite misorientation inverse pole figures (mIPF) indicate preferential slip along the (100)[001] system with a distinct shift toward the {0kl}[u0w] system in plastically deformed grains. Our findings support the hypothesis that EC 002 was likely emplaced in the lower regions of a magmatic intrusion. Augite slip signatures suggest that EC 002 crystallization and emplacement were restricted to high temperatures (>800°C) and experienced at least two strain regimes. The distinct shift from a dominant (100)[001] slip system, which corresponds to high temperatures (800–1050°C), to a [0kl][u0w] slip system indicates an increased strain rate due to shock deformation (1–5 GPa) attributed to ejection by hypervelocity impact. [ABSTRACT FROM AUTHOR]

High-silica granites are significant carriers of highly incompatible elements and are closely associated with mineralization of the rare metal beryllium. Thus, understanding their origin and evolution is of paramount importance for comprehending the evolution of the continental crust and enrichment processes of beryllium. This study presents zircon U-Pb ages, whole-rock major and trace element compositions, and Nd-Hf-O isotopic data for Early Cretaceous high-silica granite porphyries, monazite U-Pb ages and Nd isotopic data for beryllium-rich quartz veins, and whole-rock and apatite Nd isotopic data for Permian tuffs in the Dongshanwan WMo-Be deposit in the southern Great Xing'an Range, northeastern China. Our aim is to elucidate the link between mineralization and magmatism and to further our insight into the processes of crystal-melt separation and the mechanisms driving beryllium enrichment in granitic magmatic systems. Zircon U-Pb dating of the high-silica granite porphyry yields a weighted mean 206Pb/238U age of 141 ± 1 Ma, which is consistent with the age of hydrothermal monazite (ca. 140 Ma) from the beryllium-rich quartz veins. Hydrothermal monazite from the intra-granite porphyry quartz vein has positive eNd(t) values (+0.76 to +1.63) that overlap broadly with whole-rock Nd isotopic compositions of the host granite porphyry [eNd(t) = + 1.24 to +1.61]. These characteristics indicate that the Dongshanwan beryllium mineralization was temporally and genetically associated with magmatic-hydrothermal activity of the Dongshanwan high-silica granite porphyries. Our systematic studies suggest that both the Dongshanwan high-silica granite porphyries and coeval Be-barren high-silica granites in the study area are high-K calc-alkaline I-type granites. They originated from a shared magmatic system and formed by the partial melting of a predominantly juvenile medium- to high-K basaltic crustal source, with a minor addition of old crustal components. Rayleigh fractionation modeling indicates that the granitic magma underwent a two-stage crystal-melt separation process. During the first stage, the fractional crystallization of a large proportion of plagioclase, in which beryllium is compatible, not only effectively inhibited beryllium enrichment in the differentiated melt, but also removed a large amount of Ca. During the second stage, minerals in which beryllium is incompatible dominated the fractionating assemblage owing to the low Ca contents in the magma, resulting in a surge in beryllium concentration in the differentiated melt. Our findings reveal that beryllium cannot be appreciably enriched in calc-alkaline granitic magmatic systems until plagioclase fractionation has substantially removed Ca from the system. [ABSTRACT FROM AUTHOR]

The early Paleozoic tectono-magmatic activity within the South China block, which is well illustrated by Ordovician-Devonian granites in the western Qinhang belt, was the response to closure of the Proto-Tethys Ocean and convergence of continental blocks. The spatiotemporal distribution and source characteristics of the granites provide us the opportunity to understand the processes and driving mechanisms of intracontinental orogeny. As an example, the Miaoershan-Yuechengling granite batholith in northern Guangxi, located along the western margin of the Qinhang orogenic belt, is mainly composed of quartz monzonite and monzogranite. All the granitic rocks from Miaoershan-Yuechengling batholith are composed of K-feldspar, quartz, plagioclase, biotite, and hornblende. Geochronologic dating indicates that the Miaoershan-Yue-chengling batholith was emplaced during the late Silurian and Early Devonian, respectively. The rocks have high SiO2, with an average value of 73.29 wt%, and total alkalis (Na2O + K2O = 7.21-10.03 wt%), but low Al2O3 (12.96-15.51 wt%), showing characteristics of the high-potassium calc-alkaline series of S-type peraluminous granites (Al2O3/[CaO + Na2O + K2O] = 1.03-1.22). Trace elements in the Miaoershan-Yue-chengling granitic rocks are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements. Their rare earth element (REE) trends are characterized by relatively flat distribution patterns with weak light REE enrichment, weak heavy REE fractionation, and negative Eu anomalies. Zircons from the rocks have negative eHf(Z) values ranging from -13.24 to -5.1, with crustal model ages (THf2) of 2.2-1.7 Ga. These features indicate that they are S-type granites with parental magmas originating from partial melting of sandy argillaceous sources of Paleoproterozoic lower continental crust. The thermal budget for Ordovician to Early Devonian magmatism is attributed either to crustal thickening in relation to intracontinental orogenic compression or to crustal thinning due to postoro-genic tectonic extension during assembly and breakup of Greater Gondwana. This study reveals that the change in mantle convection systems during plate interactions acted as a major driving force for the early orogenic processes, late collapse of the orogenic belt, and massive syncollisional to postorogenic magmatism. [ABSTRACT FROM AUTHOR]

Hard Mode IR powder absorption spectroscopy has been used to characterize local strain relaxation associated with Al/Si ordering in a suite of synthetic anorthite samples with structural states that vary from a high degree of Al/Si order through a metastable incommensurate structure at intermediate states of order to long-range order with I 1 symmetry. The dominant feature accompanying the changing structural states is line broadening, which has been quantified by autocorrelation analysis and is attributed to local heterogeneous strain variations on a length scale of at least 1–5 unit cells. The autocorrelation results are consistent with contributions to the line broadening as being due to order parameters for both the C 1 → I 1 and I 1 → P 1 transitions, which couple biquadratically, λ Q od 2 Q displ 2. Close correlation with enthalpy variations from previously published calorimetric data indicates that the driving force for ordering can be understood in terms of elimination of strain fields arising from accommodating more or less rigid AlO4 and SiO4 tetrahedra in the feldspar framework. The metastable incommensurate structure of anorthite is closely analogous to the stable incommensurate structure that develops at intermediate compositions in the plagioclase solid solution, confirming that the same strain relaxation mechanism dominates the properties and behavior of all structural states across the solid solution. Elimination of strain heterogeneity by ordering on the basis of I 1 symmetry determines the form of non-ideal mixing shown by the solid solution at high temperatures, and changes in elastic properties may contribute to a break in the slope of partitioning of trace elements between crystals and melt. [ABSTRACT FROM AUTHOR]

A-type granites generally have much lower water, higher temperature, and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism, and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H2O and δD variations, i.e., 341–3892 ppm H2O and –325 to +336‰ δD in I-type granitoids; 67–1366 ppm H2O and –251 to +1439‰ δD in A-type granites. H2O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H2O increases up to hundreds of ppm, both rock types show a rapid decrease of H2O/Ce, an increase of F/Cl, and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na2O, K2O, F, and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating the addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H2 degassing via a reaction of 3Na3Fe5Si8O22(OH)2 + 2H2O = 9NaFeSi2O6 + 2Fe3O4 + 6SiO2+5H2. The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H2O content, and δD of apatite between the I- and A-type granites are likely attributed to varying degrees of differentiation, fluid metasomatism and magmatic degassing. The I-type granitoids experienced hornblende, biotite, plagioclase, K-feldspar, and apatite fractionation and close-system degassing. The A-type granite was likely formed from the I-type monzogranitic magma that was metasomatized by the mantle-derived F-HFSE-rich alkaline fluids to produce the peralkaline magma, which further experienced K-feldspar + plagioclase + biotite + apatite fractionation and open-system degassing. Further numerical estimation indicates that the primary magma of Fuzhou granitic complex contained ~3.0 wt% H2O, and the lower water content of A-type granite was likely attributed to strong degassing during its emplacement. Our results indicate that some peralkaline A-type granites can be generated from relatively water-poor I-type granitic magmas by fluid metasomatism and degassing. [ABSTRACT FROM AUTHOR]

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Copyright of Geology & Exploration is the property of Geology & Exploration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Garnet granulite xenoliths from the Nurbinskaya diatreme in the central part of the Archean Anabar province in Siberia are fragments of the local lower crust that experienced multiple metamorphic events in the Paleoproterozoic and reheating events in the Mesoproterozoic and later. This study addresses the timing of metamorphic transformations, and constrains the cooling rate and the time of stabilization of the lower crust. The observed metamorphic mineral assemblage of garnet, clinopyroxene, plagioclase, amphibole, rutile and ilmenite was formed at ~ 800 °C, 1.1–1.2 GPa under water-undersaturated conditions at ~ 1.88 Ga. However, the mineral assemblage is not well equilibrated and retains evidence of earlier and subsequent metamorphic stages. Late titanite formed in response to hydrous fluid influx according to phase equilibria modeling. U-Pb dating shows two events of titanite formation at 1850 ± 5 Ma and at 1788 ± 2 Ma. After deformation, which led to the porphyroclastic rock textures, the granulites underwent near-isobaric cooling. The cooling rate was higher than ~ 6 °C/Myr, to retain the garnet compositional zoning. Rutile ages are discordant, with 207Pb/206Pb dates ranging from 1.43 to 1.53 Ga. However, rutile may have responded to earlier thermal pulses, and was also reset later, so it does not record the stabilization of the crust. Crustal stabilization after Paleoproterozoic orogenic events may have occurred shortly after titanite formation. [ABSTRACT FROM AUTHOR]

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones. [ABSTRACT FROM AUTHOR]

Regional metamorphism is characterized by complicated mineral-mineral reactions and fluid-mineral interactions, which may promote elemental migration and ore genesis. A robust tracer of metamorphic reactions and fluid activities is a key for identifying material sources and quantifying mass flow along metamorphic pathways. Here, through examinations of not only bulk-rock nitrogen (N) isotopic variation along six prograde metamorphic zones but more importantly inter-mineral comparison of isotopic equilibration/disequilibration in the classic Barrovian metamorphic sequence in the Mica Creek area in the Canadian Cordillera, we show that N isotopes are a sensitive tracer for probing metamorphic devolatilization, mineral reaction and external fluid infiltration at various spatial scales from prograde to retrograde stages. In detail, our results show that, despite a relatively large variation in bulk-rock N/Al molar ratio, low-grade rocks (slates and phyllites) display a narrow δ15N range, which suggests relatively homogeneous N isotope compositions in their protoliths. Biotite-zone schists show reduced N/Al ratios with elevated δ15N values, which is consistent with devolatilization of 15N-depleted NH 3 from rocks during prograde metamorphism at T < 500 °C. In contrast, samples from high-grade rocks (kyanite zone and sillimanite–K-feldspar zone) do not follow the metamorphic devolatilization trend but decrease in both N/Al ratios and δ15N values, indicating overprinting by a relatively large-scale flow (over at least tens of kilometers) of a 15N-depleted external fluid during the high-grade stage. The fluid was likely derived from the granitic magmas that intruded into these rocks during or slightly post peak metamorphism. The δ15N values of minerals in kyanite and sillimanite–K-feldspar zones displays negative correlations between biotite and both muscovite and plagioclase, with high δ15N biotite values close to isotope equilibrium but low δ15N biotite values at isotope disequilibrium with muscovite and plagioclase. These negative correlations can be best explained by a kinetic isotopic effect associated with metamorphic reactions that consume muscovite and plagioclase to produce biotite. This suggests that plagioclase may be more involved in the metamorphic reactions in pelitic rocks than previously assumed. The associated kinetic isotopic effect resulted in a Δ15N plagioclase-biotite > 5 ‰, much larger than the theoretically predicted equilibrium N isotope fractionation factor (Δ15N plagioclase-biotite < 0.5 ‰ at T > 350 °C) but close to the large N isotope fractionation (Δ15N plagioclase-biotite ≈ 8 ‰) recently observed in field samples. Our data also imply that plagioclase is susceptible to retrograde alteration. Overall, combined N signatures of bulk rocks and coexisting minerals can provide multiple lines of constraints on mineral reactions and fluid activities from prograde to retrograde metamorphism. [ABSTRACT FROM AUTHOR]