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This study reports zirconium-in-rutile thermometry data from the Himalayan ultrahigh-pressure eclogites and their retrogressed counterparts. The Zr contents of three textural varieties of rutile i.e. matrix rutile, inclusion phase, and overgrown by ilmenite/titanite from coesite-bearing eclogites, slightly retrogressed eclogites, highly retrogressed eclogites, and amphibolites were analyzed for Zr contents using the electron probe micro-analyser. Majority of the rutile grains exhibited homogeneous chemical compositions in all the aforementioned textural varieties; however, some of grains were relatively heterogeneous. Average temperature values of 729 °C, 727 °C, and 706 °C were obtained for the matrix, inclusion, and overgrown rutiles from UHP eclogites and of 705 °C, 699 °C, and 707 °C for those from HP eclogites, respectively. Slightly retrogressed eclogites yielded similar average T values of 703 °C, 706 °C, and 706 °C, respectively, whereas highly retrogressed eclogites resulted in relatively low average temperatures, ca. 690 °C, 691 °C, and 679 °C, respectively. When compared with these eclogites, lower average T values of 672 °C, 630 °C, and 656 °C were obtained from garnetites, and of 665 °C, 658 °C, and 640 °C from garnet amphibolites, respectively. The relatively homogeneous Zr contents and higher temperature values from the matrix rutile in HP and UHP eclogites suggest peak or near-peak metamorphic crystallization, whereas grains with low Zr contents, yielding low temperature values, were likely to be affected by the late-stage retrogression.

Interactions of hydrous fluid and melt with dry mantle rocks are known to result in metasomatic alteration of the lithospheric mantle. Here we investigate such interactions that occurred beneath the Hangay Dome, Mongolia, in 22 mantle xenoliths, which were recovered from Cenozoic basalts at the Tsagan, Zala, Horgo, and Shavaryn-Tsaram localities near the village of Tariat. The xenoliths are medium- to coarse-grained spinel lherzolites that exhibit variable degrees of reaction with silicate melt and fluid (indicated by their Fe- and LREE-enrichment and the presence of secondary clinopyroxene, amphibole, phlogopite, apatite, and sulfide). According to their normalized REE patterns and microstructures, the spinel lherzolites were divided into three groups. Group 1 lherzolites contain LREE-depleted clinopyroxene and whole-rock compositions, exhibit a greater number of preserved deformation textures, and are the least affected by metasomatism. These lherzolites are interpreted to represent the sub-continental lithosphere before the rejuvenation processes that occurred during the Cenozoic. Group 3 lherzolites are characterized by partial annealing of pre-existing textures, and LREE-enrichment in clinopyroxenes and whole-rock compositions compared to Group 1 lherzolites. Group 3 lherzolites are interpreted to be the result of the interaction of depleted lithospheric mantle with a basaltic melt during the Cenozoic. The lack of correlation between the intensity of metasomatism, the degree of annealing, and the calculated temperatures of the lherzolites suggests that the Cenozoic basaltic melt percolation postdates the static recrystallization. Group 2 lherzolites exhibit characteristics from both Groups 1 and 3. Numerical modelling of the interaction between depleted lherzolites and basaltic melts evidences that: (i) a single initial liquid may fractionate to produce a range of element patterns: the observed spectrum of REE compositions of the Tariat lherzolites cannot have resulted from simple mixing of basaltic melt with a depleted mantle rocks; instead, it can be explained by chromatographic fractionation during reactive porous melt flow; (ii) highly fractionated element patterns are derived from conventional initial melt compositions and do not imply the existence of exotic melts; and (iii) strong element fractionation can be produced along short distances even at the thin section and mineral scale; this opposes the view that long percolation distances are required to produce significant chromatographic effects: the clinopyroxene core-rim disequilibrium demonstrates that REE variations in clinopyroxene rims were acquired in response to interactions with a more evolved REE-rich melt.

The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying 40Ar/39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S–C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ 40Ar/39Ar dating on muscovite yield ages mainly within 42–38 Ma with calculated closure temperatures of 435–330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset 40Ar/39Ar ages. The K-feldspar 40Ar/39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42–30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology.

High-grade metasedimentary rocks of the South Muya block (Baikal-Muya Foldbelt) have been studied with respect to whole-rock elemental and Nd isotopic compositions, as well as the U-Pb and Lu-Hf systematics of detrital zircon and in-situ grown metamorphic grains in order to reveal the initial source provenance for sedimentation and the age of major metamorphic modification. Based predominantly on trace-elemental compositions, the sedimentary protolith is most likely a poorly sorted and immature greywacke, such as those formed in island arc or active continental margin settings. The island arc-related chemical signature coupled with zircon U-Pb age data supports a Neoproterozoic (>760 Ma) sedimentation age, whereas Nd-Hf isotope data of precursor sediments are linked to a relatively immature Tonian (∼940–780 Ma) continental arc involving Paleoproterozoic crustal basement. The limited provenance information and metamorphic history of the Muya block suggests it was exotic to Siberia and originally related to another continent, e.g. – subduction-accretion complexes of South China. The polyphase metasediments experienced high-grade metamorphism in the Tonian during the 764–754 Ma arc accretion to an enigmatic continental block and then in the Ediacaran during the 622–608 Ma accretion of the Baikal-Muya belt to the arc situated at the southern Siberia margin. Metamorphic overprinting during the introduction of supracrustal rocks to deeper-crustal levels led to protracted recrystallization of primary igneous zircon population and growth of new granulite-facies grains that did not involve notable juvenile input. Tonian sedimentary provenance and two metamorphic events suggest a protracted evolution for the Baikal-Muya belt associated with the formation of precursor complexes beyond the southern margin of Siberia and their later accretion to Late Cryogenian – Ediacarian arc complexes of the southern Siberian, which proceeded only after the opening of the Paleoasian Ocean (670–630 Ma).

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed.

To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years bp to ad 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (>30 µm) display complex growth zoning, and only the smallest crystals (60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of

A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na2O/K2O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions (87Sr/86Sr = 0.7037–0.7048 and 143Nd/144Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.

Mafic magmatism of the rifting Taupo Volcanic Zone (TVZ) of the North Island, New Zealand, is volumetrically minor, but is thought to tap the material that provides the heat source for voluminous rhyolite production through partial melting of the crust, which ultimately results in very large volume explosive eruptions. We have studied the major and trace element chemistry of 14 mafic samples from across the entire TVZ, and the U isotopic composition of whole-rocks, groundmasses and separates of mafic mineral phases from a selection of nine samples (with the remaining five too sparsely phyric for mineral separation). Some minerals yield significant 234U enrichments despite groundmass and whole-rock close to 238U–234U secular equilibrium, pointing to uptake of variably hydrothermally altered antecrystic minerals prior to the eruption of originally sparsely phyric to aphyric mafic magmas. However, incompatible trace element patterns indicate that there are three chemically distinct groups of samples, and that samples may be used to derive primary melt compositions. We employ the latest version of the Arc Basalt Simulator (ABS5) to forward model these compositions, deriving mantle source parameters including mantle fertility, slab liquid flux, mantle volatile content, degree of melting, and P–T conditions of melt segregation. We show that mafic rocks erupted in areas of old, now inactive calderas constitute low-degree, deep melts, whereas those in areas of active caldera-volcanism are high-degree partial melts segregated from a less depleted source at an intermediate depth. Finally, high-Mg basaltic andesites erupted in the SW and NE of the TVZ point to a fertile, shallow mantle source. Our data are consistent with a petrogenetic model in which mantle melting is dominated by decompression, rather than fluid fluxing, and progresses from shallow to deeper levels with time. Melt volumes initially increase to a tipping point, at which large-scale crustal melting and caldera volcanism become prominent, and then decrease owing to progressive depletion of the mantle wedge by melting, resulting in the dearth of heat provided and eventual cessation of very large volume rhyolitic volcanism. ABS5 modelling therefore supports the notion of a direct link between the chemistry of recently erupted mafic magmas and the long-term activity and evolution of rhyolitic volcanism in the TVZ.

The southern Saharan Metacraton is one of the least geologically constrained regions in the world as bedrock exposures are rare. To the west of Lake Fitri near the communities of Ngoura and Moyto in south-central Chad there are two granitic inliers that form a series of lenticular to ellipsoidal low laying hills. Very little is known about the Lake Fitri inliers and their regional correlation to larger massifs in Chad is undetermined. The granites yielded weighted-mean zircon 206Pb/238U ages of 554 ± 8 Ma and 546 ± 8 Ma indicating they were emplaced ~45 million years after the cessation of arc-related magmatism and the subsequent collision between the Congo-Sao Francisco Craton and the Saharan Metacraton. The rocks have distinct groupings of inherited zircons with ages of ~580 Ma and ~635 Ma suggesting they are at least in part derived by recycling of older crustal rocks. The biotite mineral chemistry, whole rock compositions and petrological modeling indicate the granites were derived by melting of crustal lithologies but the whole rock Nd isotopes (eNd(t) = +1.3–+2.9) are characteristic of a mantle source. The contrasting inheritance-rich nature of the granites with a juvenile Nd isotopic signature is likely due to mixing between magmas derived from juvenile (Neoproterozoic) arc-related crust and asthenospheric magmas. Asthenospheric upwelling was probably a response to post-orogenic lithospheric delamination related to fault movement along the Chad Lineament, a possible extension of the Tchollire-Banyo shear zone that extended to the interior of the Saharan Metacraton. The implications are that lithospheric delamination may not have occurred immediately after collision but rather propagated along a narrow belt that extended well into the central regions of the Saharan Metacraton.

An explosive eruption occurred at Nakadake first crater, Aso Volcano in central Kyushu, southwestern Japan, on September 14, 2015. The sequence and causes of the eruption were reconstructed from the distribution, textures, grain-size, component and chemical characteristics of the related deposits, and video record. The eruptive deposits are divided into ballistics, pyroclastic density current and ash-fall deposits. A large number of ballistic clasts (mostly

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides, and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasites entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950–1010 °C at mid- to deep crustal levels). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from 50 μm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent as well as small increases in temperature. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, identified as irregular and aligned. The former is indicative of slow reaction of amphibole with melt entrapped in fractures and cleavages during decompression-induced degassing, while the other is indicative of heating-induced breakdown of amphibole triggered by uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such crystals may be useful in constraining volcanic pre- and syn-eruptive processes and, when appropriate experimental data become available, their rates, which may be of the order of days to weeks.

The Trans-Mexican Volcanic Belt (TMVB) is known for the chemical diversity in its erupted products. We have analyzed the olivine, pyroxene, and plagioclase mineral chemistry of 30 geochemically well-characterized mafic eruptives from Isla Maria at the western end of the arc to Palma Sola in the east. The mineral major oxide data indicate the dominance of open system processes such as antecryst uptake, and the scarcity of mineral-mineral and mineral-melt equilibria suggests that apart from forming microlites, erupted melts do not significantly crystallize during ascent. A combination of plagioclase antecryst chemistry and MELTS thermodynamic modeling of H2O-saturated isobaric fractional crystallization was employed to develop a pressure sensor aimed at determining the ponding depths of the co-genetic magmas from which the erupted plagioclase crystal assemblage originates. We show that the depth of magma-mush reservoirs increase eastward along the TMVB. We suggest that magma ponding is triggered by degassing-induced crystallization during magma ascent, and that the pressure sensor can also be regarded as a degassing sensor, with more hydrous melts beginning to degas at greater depths. Modeled initial magma H2O contents at the Moho range from ~4 to ~9 wt%. Magma-mush ponding depth variations fully explain the observed westward increase of average surface heat flux along the TMVB, supporting a new model of mafic arc magma ascent, where rapidly rising, initially aphyric melts pick up their antecrystic crystal cargo from a restricted crustal depth range, in which small unerupted batches of previously risen co-genetic magmas typically stall and solidify. This implies that, globally, mafic arc magmas may be used to constrain the depths of degassing and mush zone formation, as well as the amount of H2O in the primary melts.

Tsangpoite, ideally Ca5(PO4)2(SiO4), the hexagonal polymorph of silicocarnotite, and matyhite, ideally Ca9(Ca0.5□0.5)Fe(PO4)7, the Fe-analogue of Ca-merrillite, were identified from the D'Orbigny angrite meteorite by electron probe microanalysis, electron microscopy and micro-Raman spectroscopy. On the basis of electron diffraction, the symmetry of tsangpoite was shown to be hexagonal, P63/m or P63, with a = 9.489(4) Å, c = 6.991(6) Å, V = 545.1(6) Å3 and Z = 2 for 12 oxygen atoms per formula unit, and that of matyhite was shown to be trigonal, R3c, with a = 10.456 (7) Å, c = 37.408(34) Å, V = 3541.6 (4.8) Å3 and Z = 6 for 28 oxygen atoms per formula unit. On the basis of their constant association with the grain-boundary assemblage: Fe sulfide + ulvöspinel + Al-Ti-bearing hedenbergite + fayalite-kirschsteinite intergrowth, the formation of tsangpoite and matyhite, along with kuratite (the Fe-analogue of rhönite), can be readily rationalised as crystallisation from residue magmas at the final stage of the D'Orbigny meteorite formation. Alternatively, the close petrographic relations between tsangpoite/matyhite and the resorbed Fe sulfide rimmed by fayalite + kirschsteinite symplectite, such as the nucleation of tsangpoite in association with magnetite ± other phases within Fe sulfide and the common outward growth of needle-like tsangpoite or plate-like matyhite from the fayalite-kirschsteinite symplectic rim of Fe sulfide into hedenbergite, infer that these new minerals and the grain-boundary assemblage might represent metasomatic products resulting from reactions between an intruding metasomatic agent and the porous olivine-plagioclase plate + fayalite-kirschsteinite overgrowth + augite + Fe sulfide aggregates. Still further thermochemical and kinetics evidence is required to clarify the exact formation mechanisms/conditions of the euhedral tsangpoite, matyhite and kuratite at the grain boundary of the D'Orbigny angrite. Fil: Hwang, Shyh Lung. National Dong Hwa University; República de China Fil: Shen, Pouyan. National Sun Yat-sen Universit; República de China Fil: Chu, Hao-Tsu. Central Geological Survey; República de China Fil: Yui, Tzen-Fu. Institute of Earth Sciences; República de China Fil: Varela, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina Fil: Iizuka, Yoshiyuki. Institute of Earth Sciences; República de China

The initiation of subduction is associated with sequential magmatic responses that lead to the formation of the forearc lithosphere, yet the detailed characteristics of these magmatic activities are not well constrained. Here we use mineral chemistries and bulk-rock trace-element contents of highly-depleted harzburgites from the Guleman and Kizildag ophiolites in Southeast Turkey to examine mantle wedge melting dynamics during subduction initiation. We focus on how different components from the subducting slabs potentially contribute to various stages of magmatism throughout the process. Mineral and bulk-rock compositions of these harzburgites are significantly different from those of abyssal peridotites, suggesting that our harzburgites cannot be explained as residues of anhydrous adiabatic melting and melt-rock interaction at mid-ocean ridges alone. This implies that the petrogenesis of SE Turkey harzburgites involves additional processes and components. Harzburgites with the most depleted heavy-rare earth element (HREE) contents are the ones with the highest abundance of strongly incompatible elements, which can be explained by open-system processes where the peridotites in the mantle wedge experienced melting and infiltration of enriched components simultaneously. Open-system dynamic melting models with continuous flux of sediment-derived melts can account for the observed correlation, but are numerically too low compared to the measured values. Based on the observed fractionation between Zr, Hf, and elements with similar incompatibility (middle REEs), we hypothesized the involvement of amphibolite-derived melt and modeled its numerical trace-element contents. Binary mixing between this hypothetical melt and residues of the former open-system model can coherently account for the majority of the obtained trace-element data. This indicates that magmatic events during subduction initiation likely involve multiple components and occur in multiple stages, and that melt-mantle interaction plays a significant role in oceanic forearc lithosphere formation. Based on our model, we suggest the high (Zr/MREE)N signatures in some boninites and depleted harzburgites found in modern forearcs and ophiolites could be inherited from amphibolite-derived melts. Moreover, the existence of slab melts agrees with current constraints on the reconstructed geothermal gradients during subduction initiation based on the petrology and geochemistry of metamorphic soles.

Kuratite, ideally Ca4(Fe2+10Ti2)O4[Si8Al4O36], the Fe2+-analogue of rhönite and a new member of the sapphirine supergroup, was identified from the D'Orbigny angrite meteorite by electron microscopy and micro-Raman spectroscopy. Based on the least-squares refinement of 25 d-spacings measured from selected-area electron diffraction patterns of 11 zone axes, the symmetry of kuratite was shown to be triclinic (space group by analogy to rhönite) with a = 10.513(7), b = 10.887(7), c = 9.004(18) Å, α = 105.97(13), β = 96.00(12), γ = 124.82(04)°, V = 767 ± 2 Å3 and Z = 1 for the 40 oxygen formula. The empirical formula based on eight electron microprobe analyses is (Ca3.88Na0.02REE3+0.03Mn0.03Mg0.01Ni0.02Zn0.01Sr0.01)∑4.01 (Fe2+9.989.9Ti2.00)∑11.98(Si7.80Al3.52Fe3+0.64P0.05S0.02)∑12.03O39.98F0.01Cl0.01. The simplified formula is Ca4(Fe2+10Ti2)O4[Si8Al4O36]. Micro-Raman spectroscopy showed four main bands resembling those of lunar rhönite but with higher frequencies due to different chemical composition. Analogous to the occurrence of kuratite in terrestrial basaltic rocks, kuratite coexisting with Al, Ti-bearing hedenbergite, ulvöspinel, iron-sulfide, tsangpoite, Ca-rich fayalite and kirschsteinite in D'Orbigny angrite most probably was formed at >1000°C by rapid cooling of an interstitial melt, which is subsilicic, almost Mg-free but enriched in Al-P-Ca-Ti-Fe.

New pseudosection modeling was applied to better constrain the P-T conditions and evolution of glaucophane-bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane-bearing high-pressure rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type-I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type-II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type-III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type-I), barroisite → Mg-katophorite/taramite → Fe-glaucophane (Type-II), glaucophane → winchite (Type-III). Using petrographic data, mineral compositions, and Perple_X modeling (pseudosections and superimposed isopleths), peak P-T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type-I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type-II (thermal peak), and 11 ± 1 kbar and 530 ± 30 °C for Type-III. The calculations yield higher pressures and temperatures than previously thought; the difference is about 1-6 kbar and 50-200 °C. The three rock types record similar P-T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near-isothermal conditions (Type-I and Type-III), with the probable exception of Type-II where decompression followed colder geotherms. The P-T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11-14 ˚C km-1. This article is protected by copyright. All rights reserved.

Using a high resolution ion microprobe with SCAPS imaging, the peritectic reaction of forsterite + silica to enstatite was studied down to submicron level in a natural andesitic tephra from the Central Plateau of North Island, New Zealand. The fayalitic component of natural olivines is stable in high-silica melts, and therefore the reaction is in fact a two-step progress: 1. Dissolution of Mg-rich olivine, rate-limited by Fe–Mg interdiffusion at the crystal rim, results in enrichment of Fe in the crystal rim and of Mg in the c . 1 μm wide melt boundary layer around the crystal. 2. Magnesian pyroxenes preferentially but not exclusively nucleate in the melt boundary layer and grow; as soon as these microlites touch the rim of the dissolving olivine, they shield the crystals from the silica-rich melt, thereby preventing further olivine dissolution. At this point, Fe–Mg interdiffusion begins to destroy the Fe-enrichment of the olivine rim. The reaction is completed when the dissolving olivine crystal is completely mantled by magnesian pyroxene microlites. Thick pyroxene mantles are likely the result of pyroxene overgrowth rather than due to peritectic transformation. The morphology of the olivine rim preserves information about the reaction history of the grain. Modeling of Fe–Mg interdiffusion in the olivine rim following its shielding from the melt by pyroxene overgrowth may yield the rates of olivine dissolution and the rates of pyroxene growth if temperature is known. For the tephra we have studied, microlite thermometry yields a temperature of 1137 (±41) °C, indicating an olivine dissolution rate of 3–6 × 10 −11 ms −1 and an initially volumetric pyroxene growth rate of 2.2–5.6 × 10 −21 m 3 s −1 . This points to timescales between olivine crystal uptake into the SiO 2 -rich melt and explosive eruption at the surface of a few hours to at most a day.

[][1] Previous research on super-reducing ultrahigh-pressure (SuR UHP) phases from the Tibetan ophiolitic chromitites were mainly conducted on isolated grains extracted from extremely large samples. This approach has been questioned because of possible contamination. To elucidate the occurrence and origin of these SuR UHP minerals, we studied 33 thin sections and rock chips of three ophiolitic chromitites from the Yarlung Zangbo suture zone. Here we report and analyze unambiguously in situ SuR UHP assemblages from the ophiolitic chromitites by electron probe micro-analyzer, scanning microscope and Laser Raman spectroscope. The SuR UHP and associated phases include: (1) blue moissanite as inclusions in olivine (Fo96–98), and in olivine domains between disseminated chromite grains; (2) multiple inclusions of moissanite + wustite + native Fe in olivine; (3) FeNi and FeCr alloys in olivine and chromite; and (4) native Fe and Si in chromite. Crustal asphaltum and h-BN also occur as inclusions in chromite. Our documented in situ SuR UHP phases, combined with the previously inferred existence of ringwoodite + stishovite, all indicate that these assemblages formed under a highly reducing environment (oxygen fugacities several orders of magnitude lower than that of the iron-wustite buffer) in the mantle transition zone (MTZ) and in the deep upper mantle. Diamond + moissanite with distinct 13C-depleted compositions from chromitites have a metasedimentary carbon source. Associations with existing crustal minerals in chromitites demonstrate that carbon-bearing metasedimentary rocks were recycled into the mantle through subduction, and locally modified its composition. Finally we propose a three-stage model to explain the formation of SuR UHP phase-bearing chromitite. Discoveries of SuR UHP phases in Luobusa and other ophiolitic podiform chromitites from the polar Ural Mountains and from Myanmar imply existence of a new type of ophiolitic chromitite. Such occurrences provide an additional window to explore the physical-chemical conditions of the MTZ, mantle dynamics, and the profound recycling of crustal materials. [1]: /embed/graphic-1.gif

Olivinites, together with olivine megacrysts, are the most magnesian phases found in angrites. Their chemical composition (mg# 90) is out of equilibrium with the groundmass and far away from that of possible precipitates from angrite parent melts. Therefore olivinites, as well as olivine megacrysts, were considered as xenoliths and xenocrysts. We report here a detailed study of five olivinites from the angrite D'Orbigny. Our results indicate that D'Orbigny experienced metasomatic alteration processes, which led to enrichments in FeO and MnO (relative to the original composition), changing the initial Mg-rich composition of the olivines to the one seen now. As this process took place in equilibrium with a chondritic reservoir (e.g., Fe/Mn ratios spreading around primitive values), the primitive (Mg-rich) olivine chemical composition was changed towards a more fayalitic one while preserving a chondritic signature. This chondritic signature was preserved in the Fe/Mn ratio of the olivinites, olivine megacrysts, augite grains in olivinites and groundmass olivine of D'Orbigny. Therefore the fayalite content of about 35 mol.% that characterizes the groundmass olivine of this rock – as well as other angrites-does not correspond to its original composition but may be the result of a late metasomatic process that affected these rocks. If so, olivinites and Mg-rich olivines might not be compositionally exotic phases but are an early constituent phase that retained the pristine more reducing conditions that have been preserved in some angrites, where they form either a small part of the rock (e.g., Asuka 881371 and D'Orbigny) or the majority of it (NWA 8535). Fil: Varela, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentina Fil: Hwang, S. L.. National Dong Hwa University; República de China Fil: Shen, P.. National Sun Yat-sen University; República de China Fil: Chu, H. T.. Central Geological Survey; República de China Fil: Yui, T. F.. Academia Sinica; República de China Fil: Iizuka, Y.. Academia Sinica; República de China Fil: Brandstätter, F.. Naturhistorisches Museum; Austria Fil: Abdu, Y. A.. University of Sharjah; Emiratos Arabes Unidos

The asterism effect of star garnet has been attributed to the oriented distribution of needle-like rutile inclusions. Rutile needles occur in garnet from a wide range of metamorphic settings and rock bulk compositions, and their origin has been ascribed to different mechanisms, such as exsolution, and used to interpret petrological and tectonic processes. Results from an optical and transmission electron microscopy of Idaho star garnet indicate a co-precipitation origin. It was found that rutile needles are predominantly oriented along the rt// grt and rt// grt directions following multiple crystallographic orientation relationships (CORs); i.e. COR-1, 2, 2′, 3, 4 and 5 in 6-ray star garnet, and are oriented solely along the rt// grt directions following exclusively COR-2 in 4-ray star garnet. The sole presence of COR-2 grt needles in the common 4-ray star garnet, in contrast to the presence of both grt and grt needles with multiple CORs in the rare 6-ray star garnet, suggests that the COR-2 grt needle probably is the energetically most favoured variant, as is also supported by the coincidence site lattice considerations. The unique crystallography-controlled microstructures of 4-ray star garnet, including the cloudy domains behind the {111}grt or {100}grt fronts with abundant inclusions of rutile needle, rutile compound needle and multiple-phase-inclusion, as well as the clear domains behind the {110}grt fronts with only a few above inclusions concentrated exclusively within the linear, grt-oriented, continuous tube-like domains, further suggest that the COR-2 grt needles in 4-ray star garnet most likely have a growth-in origin, co-precipitating with garnet at its growth fronts close to thermodynamic equilibrium conditions. The 6-ray star garnet, on the other hand, most likely formed under far-from equilibrium conditions, thereby yielding a maximum of 99 crystallographic variants of rutile needles with multiple CORs in a single crystal. In the light of these findings, along with the common occurrences of the sole COR in many inclusion-host systems owing to the requirement to minimize the energy barrier in an exsolution process, the presence of both rt// grt and rt// grt needles with multiple CORs in garnet of Sulu eclogite and Erzegebirge quartzofeldspathic rock would therefore cast doubt on the assertion of an exsolution origin of rutile needles in garnet from these ultrahigh-pressure rocks.

[1] The role silicate-liquid immiscibility plays in the formation of macro-scale, bimodal volcanic/plutonic igneous complexes, and Fe-Ti oxide deposits is debated as the rock compositions produced by immiscibility are similar to those produced by other petrological processes. Within the flows of the North Mountain basalt of the Central Atlantic Magmatic Province are centimeter-thick granophyre layers. The granophyre layers are a mixture of mafic (i.e., ilmenite, magnetite, ferroaugite, plagioclase, stilpnomelane, ferrorichterite) and felsic (i.e., sanidine, quartz) minerals and highly siliceous (>75 wt% SiO2) mesostases. Petrological modeling indicates that the siliceous mesostasis + sanidine + quartz ± ferrorichterite represents a Si-rich silicate immiscible melt whereas the ferroaugite + plagioclase + stilpnomelane represent the Fe-rich silicate immiscible liquid. The identification of naturally occurring silicate-liquid immiscibility at scales greater than micron level is an important observation which may be useful in identifying volcanic and plutonic rocks which formed by macro-scale silicate-liquid immiscibility.

This study reports zircon LA-ICPMS U–Pb ages of 50 igneous rock samples from the Urumieh–Dokhtar magmatic arc (UDMA) and Sanandaj–Sirjan structural zone (SSZ) in Iran. These results, together with literatures and our unpublished age data, better delineate the magmatic evolution related to the Neotethyan subduction and subsequent Zagros orogeny that resulted from the Arabia–Eurasia collision. Subduction-related magmatism was active during Jurassic time, as evidenced by the presence of widespread I-type granitoids from the Middle to Late Jurassic (176–144 Ma) in the SSZ. After a protracted magmatic quiescence in the Early Cretaceous, igneous activity renewed inland in the UDMA from which we identify Late Cretaceous granitoids (81–72 Ma) in Jiroft and Bazman areas, the southeastern segment of the UDMA. The UDMA volcanism was most active and widespread during the Eocene and Oligocene (55–25 Ma), much longer lasting than previously thought as just an Eocene pulse. Such a prolonged igneous “flare-up” event in the UDMA can be correlated to Armenia where coeval calc-alkaline rocks are common. The UDMA magmatism ceased progressively from northwest to southeast, with magmatic activities ending the Early Miocene (ca. 22 Ma) in Meghri, the Middle Miocene (ca. 16 Ma) in Kashan and the Late Miocene (ca. 10–6 Ma) in Anar, respectively. The southeastward magmatic cessation is consistent with the notion of oblique and diachronous collision between Arabia and Eurasia. Post-collisional volcanism started ca. 11 Ma in Saray, east off the Urumieh Lake, which, along with later eruptions in Sahand (6.5–4.2 Ma) and Sabalan (≤ 0.4 Ma) volcanoes, forms a compositionally unique component of the vast volcanic field covering much of the Lesser Caucasus, NW Iran and eastern Anatolia regions.

Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U–Pb dating and trace element composition analyses. Three types of zircons, with respect to U–Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from 153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic replacement processes. Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures, with low Th/U ratio ( The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India–Asia continental collision at Paleocene. Previously proposed Late Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely recrystallized inherited zircons.

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1 Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122 ± 15 Ma and 2181 ± 7 Ma for the Birimian Na-rich granitoids and 2151 ± 10 Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition ( I Sr ( T ) ≤ 0.7007), positive epsilon neodymium values ( ɛ Nd ( T ) = +0.7 to +2.5) and Sm–Nd model ages ( T DM ) of 2363–2558 Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: ( I Sr ( T ) = 0.7003–0.7015, ɛ Nd ( T ) = +3.2 to +4.1, T DM = 2336–2552 Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15 Ga) of a large scale crustal growth event (2.2–2.0 Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

The source of intraplate basalts has long been a controversial topic, particularly in continental settings where ambiguity increases because both crustal contamination and crystal fractionation may mask important source characteristics. We present geochemical data to constrain the source and the chemical evolution of the continental intraplate magmas from the Aleppo Plateau and vicinity, NW Syria. New 40Ar/39Ar ages, coupled with published 40Ar/39Ar and K–Ar ages, reveal two discrete Miocene volcanic phases, ~ 19–18 Ma (Phase 1) and ~ 13.5–12 Ma (Phase 2), in the studied area. New chemical and isotopic compositions [87Sr/86Sr = 0.7036–0.7051, eNd = + 4.5 to + 1.1 and (187Os/188Os)t = 0.151–0.453] of the lavas reflect the unequivocal influence of crustal assimilation and fractional crystallisation (AFC). Despite the effects of the AFC processes, there still appear to be some differences between the most-primitive, least contaminated magmas of the two volcanic phases, interpreted as a result of source heterogeneity. Whereas the Phase 1 lavas, with relatively high Si, low Ti and trace-element contents, are consistent with partial melting of a largely peridotitic mantle source, the origin of the Phase 2 lavas is more complicated. The latter are characterised by a source component depleted in Si and enriched in Ti, Fe, Ca, P, alkalis, light and middle rare earth elements (REEs) relative to heavy REEs and with sub-chondritic Th–(U)/Nb, Pb/Ce and Zr/Sm. They approach compositions of experimental melts of amphibole-rich metasomatic veins. The compositional variations among the most primitive Phase 2 lavas are difficult to reconcile with varying degrees of partial melting of either the metasomatic veins or peridotite, but could be explained if partial melts of both lithologies were variably mixed, a scenario that could be sensibly envisioned as ascending (peridotitic) plume/asthenosphere derived melts assimilating highly fusible metasomatic veins during their traverse through the lithosphere. This process can be loosely quantified by trace-element forward partial melting modelling that suggests mixing of up to 80% metasomatic melts derived from ~ 40% melting of amphibole-rich metasomatic veins (which themselves were inevitably compositionally and mineralogically heterogeneous) with 20% plume/asthenospheric melts derived from ~ 7% melting of a garnet peridotite. Within the compressional framework of northern Arabia, invocation of diapiric material reasonably accounts for the generation of the intraplate basalts in Syria. Derivation of the Phase 2 hybrid melts was probably triggered by lateral flow of this diapiric material beneath the lithosphere subsequent to its arrival, with the migrating flow-front controlling the locus of volcanism. The increase in degree of Si-undersaturation with time for the Phase 1 and Phase 2 lavas is best explained by decreasing temperatures of this flow-front that resulted in less melt contribution from the diapiric mantle while the amphibole-rich veins within the lithosphere continued to be easily fusible, although we cannot totally exclude the possibility that the Phase 2 volcanism tapped a vein-richer domain which formed subsequent to the Phase 1 volcanism.

Devonian evaporites and associated sedimen- tary rocks in the Norilsk region were contact metamor- phosed during emplacement of mafic sills that form part of the end-Permian (*252 Ma) Siberian Traps. We present mineralogical, geochemical and Sr-Nd isotopic data on sedimentary rocks unaffected by metamorphism, and meta- sedimentary rocks from selected contact aureoles at Norilsk, to examine the mechanisms responsible for magma-evaporite interaction and its relation to the end- Permian environmental crisis. The sedimentary rocks include massive anhydrite, rock salt, dolostone, calcareous siltstones and shale, and the meta-sedimentary rocks comprise calcareous hornfels, siliceous hornfels and minor meta-anhydrite and meta-sandstone. Contact metamor- phism took place at low pressure and at maximum temperatures corresponding to the phlogopite-diopside stability field. Calcareous hornfels have high CaO, MgO, CO2 ,S O3, low SiO2 and initial Sr isotopic ratios of 0.7079-0.7092, features indicative of calcareous siltstone protoliths. Siliceous hornfels, in contrast, have high SiO2, Al2O3 ,N a 2O, low in other major element oxides and initial Sr isotopic ratios of 0.7083-0.7152, consistent with pelitic or shaley protoliths. Loss of CO2 in a subset of calcareous hornfels can be explained by decarbonation reactions dur- ing metamorphism, but release of SO2 from evaporites cannot be accounted for by a similar mechanism. Occur- rences of wollastonite and a variety of hydrous minerals in the calcareous hornfels are consistent with equilibration with hydrous fluid, which was capable of leaching large quantities of anhydrite in the presence of dissolved NaCl. In this way, substantial sediment-derived sulfur could have been mobilized, incorporated into the magmatic system and released to the atmosphere. The release of CO2 and SO2 from Siberian evaporites added to the variety of toxic gases generated during metamorphism of organic matter, coal and rock salt, contributing to the end-Permian envi- ronmental crisis.

Southeast Asia consists of several microcontinents that detached from the northeastern margin of Gondwanaland. The Song Ma belt in northern Vietnam consists of ophiolite, metabasite, metasedi- mentary rocks and eclogite, and it is thought to be a suture zone between the Indochina and South China blocks. However, the nature and boundaries of the Song Ma belt and the collision age of the two blocks have long been debated. In this article, petrological and geochemical studies on the Song Ma ophiolite and eclogite and first sensitive high-resolution ion microprobe (SHRIMP) age dating of eclogite provide new light to resolve such debate. Eclogite consisting of garnet, omphacite, phengite, quartz, barroisite and rutile is closely associated with garnet-phengite-quartz schist in the Nam Co antiform, a northern subunit of the Song Ma belt. The eclogite experienced a three-stage metamorphic evolution: (I) pre-eclogite stage (amphibolite facies) defined by inclusions of taramite, barroisite, quartz, zoisite ⁄ epidote, mica, rutile & rare chlorite in garnet, (II) eclogite stage and (III) retrograde stage of amphibolite to greenschist facies. The P-T conditions of the three stages are of 14-16 kbar and 520- 550 � C (I), 24-27 kbar and 650-750 � C (II), and 3-7 kbar and 430-510 � C (III), and show a clockwise P-T path based on their mineral assemblages and stability fields in the P-T pseudosection. Thermobarometric results yield similar peak pressure and temperature (26-28 kbar and 650-710 � C). These data suggest that the Song Ma eclogite underwent high-pressure metamorphism in subduction zone with a low thermal gradient 8 � Ck m )1 . The Song Ma ophiolite is composed of serpentinized peridotite, gabbro, basalt, mafic dyke and chert, and experienced ocean-floor metamorphism. Metabasalt and gabbro of ophiolite suite and eclogite all have MORB-type geochemical affinities. Zircon separates from eclogite have very low Th ⁄ U ratios of 0.01-0.05, indicating a metamorphic origin. SHRIMP U-Pb isotopic analyses of this zircon yield a 206 Pb ⁄ 238 U weighted mean age of 230.5 ± 8.2 Ma. This age is interpreted as the closure age of the Paleotethys Ocean that separated the South China and Indochina blocks, and the subsequent collision of the two blocks that took place at the Middle Triassic corresponding to the major episode of the Indosinian Orogeny.

The Ailao Shan–Red River (ASRR) shear zone, one of the most prominent geologic strike-slip shear zones in Southeast Asia, consists of high-grade metamorphic complexes that provide a rare opportunity to sample the mid-crustal rocks along the western margin of the Yangtze Block of South China. Here we report combined, in-situ analyses of zircon U–Pb and Lu–Hf isotopes of eight gneisses from the Diancang Shan and Ailao Shan segments of the ASRR shear zone. Our zircon U–Pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 Ma, with peak ages at ca. 770, 350 and 240 Ma, suggesting three major periods of protolith formation. The 350-Ma zircons, observed only in an orthogneiss from the Diancang Shan, show uniform initial Hf isotopic ratios marked by high and positive e Hf (T) values from + 16 to + 10. This is in contrast with the other two zircon populations that are more common, occurring in both the Diancang Shan and Ailao Shan, and overall delineate very heterogeneous Hf isotopic compositions, with e Hf (T) values ranging from + 15 to − 16. Many zircons reveal distinctive core-rim age variations. Zircon rims, formed between ca. 34 and 26 Ma, show significant variations in Th/U ratios (5–0.01) and e Hf (T) values (+ 14 to − 10) that suggest complicated magmatic and metamorphic zircon overgrowth during the Oligocene. The presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies, corresponding to mid-crustal level P–T conditions, as also suggested by structural and petrologic data. Our data furthermore suggest that the ASRR gneisses were not produced solely by shear heating during the Tertiary strike-slip faulting, but are uplifted, mid-crustal basement rocks that formed essentially during two major stages of magmatism, represented by the Neoproterozoic Kangding Complex and Late Permian Emeishan large igneous province. As a result of the India–Asia collision, these basement rocks underwent regional magmatic and metamorphic overprinting in the Oligocene that, based on our relevant work (Searle et al., 2010, Geosphere, 6, 1–23), predated the initial left-lateral movement along the ASRR shear zone. The 350-Ma protolith, which requires a dominant depleted-mantle input in the petrogenesis, cannot be linked to any major magmatic events in South China but may be interpreted as part of the Paleotethys remnants that were later incorporated into the ASRR shear zone.

International audience; Magmas play a key role in the genesis of epithermal and porphyry ore deposits, notably by providing the bulk of ore metals to the hydrothermal fluid phase. It has been long shown that the formation of major deposits requires a multi-stage process, including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments. Both aspects have been intensively studied for most of noble metals in subsurface conditions, whereas the effect of pressure on the concentration (i.e., solubility) of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected. Here, we present new experimental data aiming to constrain the processes of gold (Au) dissolution in subduction-linked magmas along a range of depth. We have conducted hydrous melting experiments on two dacitic/adakitic magmas at 0.9 and 1.4 GPa and ∼1000°C in an end-loaded piston cylinder apparatus, under fO2 conditions close to NNO as measured by solid Co-Pd-O sensors. Experimental charges were carried out in pure Au containers, the latter serving as the source of gold, in presence of variable amounts of H2O and, for half of the charges, with elemental sulfur (S) so as to reach sulfide saturation. Au concentrations in melt quenched to glass were determined by LA-ICPMS. When compared to previous data obtained at lower pressures and variable redox conditions, our results show that in both S-free and sulfide-saturated systems pressure has no direct, detectable effect on melt Au solubility. Nevertheless, pressure has a strong, negative effect on sulfur solubility. Since gold dissolution is closely related to the behaviour of sulfur in reducing and moderately oxidizing conditions, pressure has therefore a significant but indirect effect on Au solubility. The present study confirms that Au dissolution is mainly controlled by fO2 in S-free melts and by a complex interplay of fO2 and melt S2- concentration in sulfide-saturated melts, at given temperature. In addition, we propose that the transition from sulfide (S2-) to sulfate (S6+) species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase. If this is true, this may have important consequences during mantle melting and magma ascent. In particular, if mantle melting occurred in moderately oxidizing conditions, a small degree of partial melting would allow the primary melts to become Au-enriched but the relatively high pressure would move the sulfide-sulfate transition to more oxidizing conditions, making the primary melts saturated with sulfide phases that would sequester gold from the melt. During magma ascent, the decreasing pressure would favour the destabilization of sulfides and the release of gold to the silicate melt. However, at shallow levels, decreasing pressure, magma evolution, and varying redox conditions would be continuously competing to concentrate or fractionate gold.

Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otroy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR-I: grt ⁄⁄ opx and {110}grt ⁄⁄ {100}opx (13� off) and COR-II: grt ⁄⁄ opx and {110}grt ⁄⁄ {100}opx (14� off), in four garnet grains analysed. Both variants of orthopyroxene have a blade-like habit with one pair of broad crystal faces parallel ⁄ sub-parallel to {110}grt plane and the long axis of the crystal, opx for COR-I and opx for COR-II, along grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails ⁄ veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving-infiltrating-precipitating- healing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed ⁄ clarified. The exsolved ortho- pyroxene in a relict majoritic garnet follows COR-III: {112}grt ⁄⁄ {100}opx and grt ⁄⁄ opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112}grt ⁄⁄ {100}opx and the long crystal axis along grt ⁄⁄ opx. Such a crystal habit can be rationalized by the differences in oxygen sub-lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid-state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid-state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains ⁄ trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving-infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid-state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.

The Kulet eclogite in the Kokchetav Massif, northern Kazakhstan, is identified as recording a prograde transformation from the amphibolite facies through transitional coronal eclogite to fully recrystallized eclogite (normal eclogite). In addition to minor bodies of normal eclogite with an assemblage of Grt + Omp + Qz + Rt ± Ph and fine-grained granoblastic texture (type A), most are pale greyish green bodies consisting of both coronal and normal eclogites (type B). The coronal eclogite is characterized by coarse-grained amphibole and zoisite of amphibolite facies, and the growth of garnet corona along phase boundaries between amphibole and other minerals as well as the presence of eclogitic domains. The Kulet eclogites experienced a four-stage metamorphic evolution: (I) pre-eclogite stage, (II) transition from amphibolite to eclogite, (III) a peak eclogite stage with prograde transformation from coronal eclogite to UHP eclogite and (IV) retrograde metamorphism. Previous studies made no mention of the presence of amphibole or zoisite in either the pre-eclogite stage or coronal eclogite, and so did not identify the four-stage evolution recognized here. P–T estimates using thermobarometry and Xprp and Xgrs isopleths of eclogitic garnet yield a clockwise P–T path and peak conditions of 27–33 kbar and 610–720 °C, and 27–35 kbar and 560–720 °C, respectively. P–T pseudosection calculations indicate that the coexistence of coronal and normal eclogites in a single body is chiefly due to different bulk compositions of eclogite. All eclogites have tholeiitic composition, and show flat or slightly LREE-enriched patterns [(La/Lu)N = 1.1–9.6] and negative Ba, Sr and Sc and positive Th, U and Ti anomalies. However, normal eclogite has higher TiO2 (1.35–2.65 wt%) and FeO (12.11–16.72 wt%) and REE contents than those of coronal eclogite (TiO2

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

Products of contrasting mingled magmas are widespread in volcanoes and intrusions. Subvolcanic trachyte intrusions hosting mafic enclaves crop out in the Manori-Gorai area of Mumbai in the Deccan Traps. The petrogenetic processes that produced these rocks are investigated here with field data, petrography, mineral chemistry, and whole rock major, trace, and Pb isotope chemistry. Local hybridization has occurred and has produced intermediate rocks such as a trachyandesitic dyke. Feldspar crystals have complex textures and an unusually wide range in chemical composition. Crystals from the trachytes cover the alkali feldspar compositional range and include plagioclase crystals with anorthite contents up to An47. Crystals from the mafic enclaves are dominated by plagioclase An72-90, but contain inclusions of orthoclase and other feldspars covering the entire compositional range sampled in the trachytes. Feldspars from the hybridized trachyandesitic dyke yield mineral compositions of An80-86, An47-54 ,A b94-99 ,O r45-60 ,a nd Or96-98, all sampled within individual phenocrysts. We show that these compositional features are consistent with partial melting of granitoid rocks by influx of mafic magmas, followed by magma mixing and hybridization of the partial melts with the mafic melts, which broadly explains the observed bulk rock major and trace element variations. However, heterogeneities in Pb isotopic compositions of trachytes are observed on the scale of individual outcrops, likely reflecting initial variations in the isotopic compositions of the involved source rocks. The combined data point to one or more shallow-level trachytic

Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~ 50 km thick continental crust. We propose that the central MVB arc magmas are hybrids of high-Mg# > 70 basaltic and dacitic initial mantle melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation between major element oxides SiO2, MgO and FeO which is not reproduced by mantle–crust mixing models. Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying crust.

The Emeishan large igneous province contains a diverse assemblage of igneous rocks including mildly peralkaline granitic rocks of A-type affinity. The granitic rocks from the Panzhihua, Baima and Taihe plutons are temporally, spatially and chemically associated with layered mafic-ultramafic intrusions. Electron microprobe analyses of the major and accessory minerals along with major and trace element data were used to document the magmatic conditions of the three peralkaline plutons. The amphiboles show magmatic/subsolidus trends and are primarily sodic-calcic in composition ( i.e ., ferrorichterite or richterite). Sodic ( i.e ., riebeckite-arfvedsonite) amphiboles are restricted to the Panzhihua and Taihe plutons. The amphiboles from the Panzhihua and Taihe granites are very similar in composition whereas amphiboles from the Baima syenites have higher MgO wt% and lower FeOt wt% and TiO2 wt%. Whole-rock Zr saturation temperature estimates indicate the initial average magma temperatures were ~940 ± 21 °C for the Panzhihua pluton, ~860 ± 17 °C for the Baima pluton, and ~897 ±14 °C for the Taihe pluton. The initial Fmelt(wt%) values were calculated to be 1.1 ± 0.1, 0.8±0.1 and 1.1±0.1 wt% for the Panzhihua, Baima and Taihe plutons, respectively. The estimated Fmelt(wt%) values are higher than what can be accounted for in the Panzhihua and Taihe plutons and indicate that they may have lost F during crystallization. In contrast the Fmelt(wt%) value for the Baima pluton can be accounted for. The presence of titanite +magnetite +quartz in the Baima syenites indicates oxidizing f O2 conditions whereas the presence of aenigmatite and ilmenite in the Panzhihua and Taihe granites indicate that they were relatively reducing. Although the Atype granitoids formed by the same processes ( i.e ., fractional crystallization of mafic magmas), their differences in major element and mineral chemistry are likely related to a combination of initial bulk magma composition and magmatic oxidation state.

Apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements. In this study, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of theTranshimalayan igneous plutonic suite: S-type granites, the I-type Gangdese batholith and postcollisional adakites. F, Mn, Sr and rare earth elements (REE) in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers. More specifically, the F and Mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index. Combined with Sr and REE data, which show significant variations in apatite from different rock types, these elements are useful for constructing ‘discrimination diagrams’. This study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution. Systematic variations of Sr and REE in apatite with bulk-rock aluminosity are the results of partition competition with pre-existing and coexisting major and accessory minerals in silicate melts, and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing, which is signaled by inconsistent Eu anomalies, Sr abundances and REE patterns relative to bulkrock compositions.

Inclusions of polycrystalline K-feldspar aggregates after K-cymrite (KAlSi 3 O 8 ·nH 2 O) were discovered in garnet from a Dulan eclogite in the Qaidam ultrahigh-pressure (UHP) terrane, western China. The eclogite consists of garnet (Alm 56 Grs 23 Prp 20 Sps 01 ), omphacite (Jd 35 Aeg 6 Aug 59 ), and minor rutile and apatite. The 20 to 200 μm inclusions vary in shape from prismatic, hexagonal to rounded, and exhibit palisade and mosaic textures. Host garnets show radial fractures, similar to those surrounding quartz pseudomorphs after coesite. Some inclusions consist of almost end-member K-feldspar (Or 99–100 Ab 0–1 ) polycrystalline aggregates, whereas others are composed of >90 vol% K-feldspar (Or 96–99 Ab 1–4 ) with minor secondary albite occurring along the margins of the inclusions. Raman spectra of K-feldspar crystalline aggregates vary slightly reflecting various degrees of Si-Al ordering, and show a Raman peak at ~390–395 cm −1 , typical for cymrite structure. These characteristics of the K-feldspar polycrystalline inclusions imply the presence of former K-cymrite in the Dulan eclogite formed at >3 GPa at ~720 °C. The occurrence of K-cymrite in UHP eclogite is significant because of its potential as an important carrier of crustal K and H 2 O to the upper mantle.

ARTICLE I NFO Complex rocks, consisting of different lithologic breccias and sediments in the Tungho area of the southern Coastal Range, eastern Taiwan, were formed by magmas and magma-sediment mingling. Based on field occurrences, petrography, and mineral and rock compositions, three components including mafic magma, felsic magma, and sediments can be identified. The black breccias and white breccias were consolidated from mafic and felsic magma, respectively. Isotopic composition shows these two magmas may be from the same source. Compared to the white breccias, the black breccias show clast-supported structures, higher An values in plagioclase, higher contents of MgO, CaO, and Fe2O3 and lower SiO2, greater enrichment in the light rare earth elements (LREE), and depletion in the heavy rare earth elements (HREE). The white breccias show matrix-supported blocks and mingling with tuffaceous sediments to form peperite. Physical and chemical evidence shows that the characteristics of these two components (mafic and felsic magmas) are still apparent in the mingled zone. According to their petrography, mafic and felsic magmas did not have much time for mingling. White intrusive structures and black flow structures show that mingling occurred before they solidified. Finally, the occurrence of mingling between magmas and sediments suggests that the mingling has taken place at the surface and not in the magma chamber.

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3–6.6 GPa; 853–957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101] Mt //[001] Ol , [110] Mt //[011] Ol , and [011] Mt //[011] Ol ; and [0001] Hm //[100] Ol and [1010] Hm //[001] Ol . These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe 2+ to Fe 3+ in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6–11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

Chronological, geochemical and Sr–Nd–Pb isotopic analyses have been carried out on the Mesozoic plutons in western Shandong with the aim of characterizing crustal–mantle evolution during the tectono-thermal reactivation of the craton. Detailed SHRIMP zircon U–Pb dating reveals two main periods of Mesozoic activity with contrasting compositions. The older magmatic pulse is manifested by monzonites and monzodiorites from Tongshi for which zircon rims yield a concordant age of 177±4 Ma and the cores have a discordant age of ca. 2.5 Ga. Low MgO and Cr, high Na2O contents and especially their isotopic compositions (87Sr/86Sr 40 km) of the late Mesozoic high Sr and low Y granitoids from the same region. Distinctively different depths of crustal melting suggest dynamic thickening of the crust by magmatic underplating during the Jurassic and Cretaceous. The younger dioritic plutons from Laiwu and Yinan were emplaced at 132–126 Ma and show relatively high MgO and Cr contents and large isotopic variability. They were likely derived from enriched lithospheric mantle source and were subjected to crustal contamination during magma evolution. Early Cretaceous mantle melting is coeval with the widespread late Yanshanian granitic magmatism in North China. Early Cretaceous time may correspond to a critical period when a temperature increase due to lithospheric thinning allowed the intersection of the local geotherm and the wet peridotite solidus. While some mantle-derived magmas were erupted, most were trapped at variable crustal depths, triggering large-scale concomitant melting of the crust. Lithospheric thinning must have continued until the late Cretaceous because of the change in the source of mafic magmas from lithospheric to asthenospheric at that time. It is proposed that removal of the lithospheric keel beneath the North China craton may have been initiated as early as the Jurassic, but with the most intense period in the Cretaceous between 130–75 Ma. Such a relatively long timescale (~100 Ma) emphasizes the role of thermomechanical erosion by convective mantle in lithospheric thinning beneath this region.

Volcanic glasses with minor mafic mineral fragments, such as biotite and hornblende, found in deep-sea sediments of the South China Sea Basin (SCSB) have been clearly identified as eruptive products of the Youngest Toba Tuff (YTT), northern Sumatra, Indonesia. The tephra layer occurs between marine oxygen isotopic event 5.1 (79.3 ka) and event 4.22 (64.1 ka), with an interpolated age of 74.0 ka, which is in good consistence with previous radiometric dating (73‐75 ka) and ice-core dating (71 ^ 5 ka) of the YTT. The tephra consists predominantly of bubble-wall shards with minor elongated vesicles of pumice fragments. Geochemical characteristics of the tephra, such as high total alkali content, high 87 Sr/ 86 Sr ratio and uniformity of their compositions, all suggest that the recovered tephra is of the Youngest Toba Tuff. This finding supports an extended dispersal of coarse (.63 mm) glass shards over 1500 km northeast of the Toba caldera, a direction opposite to what previously conceived. While providing a better documentation of the distribution extent of the Toba ash, this report points to the need to reestimating the eruptive volume of the YTT and re-evaluating its environmental impact. q 2000 Elsevier Science B.V. All rights reserved.

article i nfo Residence times of olivine and plagioclase phenocrysts and xenocrysts in mid-ocean ridge (MOR) basaltic melts have been studied since the mid 1980s using geospeedometric techniques (i.e. using diffusion of major and trace elements) in order to constrain the processes of melt ascent and differentiation in this impor- tant magmatic setting. Residence times range from a few hours to several years, but potential links between these timescales and specific tectonomagmatic variables such as spreading rate and relative locations of eruption site and ridge axis have remained elusive. Here we demonstrate how incomplete chemical diffusion of Sr within plagioclase crystals from MOR basalts erupted in on- and off-axis settings on a number of ridges with variable spreading rates provide geospeedometric constraints. We combine electron probe microanalyt- ical crystal maps with detailed laser ablation profiles of almost 70 plagioclase crystals from the fast spreading East Pacific Rise (EPR) at 9-10°N, the intermediate spreading Gorda and Juan de Fuca (JdF) ridges, and the ultraslow spreading Gakkel ridge to calculate crystal residence times. These range from a few days to several months. The scarcity of residence times exceeding years corroborates previous data indicating that most of the growth of plagioclase phenocrysts occurs within the conduit at the onset of and during eruption on the sea floor, and extends this result to the fast-spreading EPR. Further, statistical analysis is employed to show for the first time that residence times are systematically longer at slower spreading rates, in off-axis samples, and samples sourced from laterally distal axial melt lenses. Plagioclase textures and residence time variations appear to be linked to differences in the dynamics of late-stage, pre-eruptive magma storage and ascent in the different tectonomagmatic settings investigated. In the future, geospeedometric work on MOR samples will be required to assess if the effect of spreading rate on crystallization timescales are globally applicable, and to investigate potential variations in magma plumbing systems within individual ridge segments.

Profiles of a total of 23 plagioclase crystals erupted within the 1982-1991 and 1993 flows of the Coaxial segment of the Juan de Fuca ridge, the 1996 flow of the North Gorda ridge, and from the Western Volcanic Zone of the ultra-slow spreading Gakkel Ridge, have been studied for variations in major and trace element concentrations. We derive equilibration times for the relatively rapidly diffusing Sr in mid-ocean ridge basalt (MORB) plagioclase crystals of the order of months to a few years in each case. All crystals preserve diffusive disequilibria of strontium and barium. Crystal residence times at MORB magmatic temperatures are thus significantly shorter, of the order of days to a few months at most, precluding prolonged crystal storage in axial magma chambers and instead pointing to rapid crystal growth (up to similar to 10(-8) cm s(-1)) and cooling (up to similar to 1 degrees C h(-1)) shortly prior to eruption of these samples. Growth of these crystals is therefore inferred to occur almost entirely within oceanic layer 2 during dike injection. Crystals that grew at lower crustal levels or earlier in the differentiation sequence appear to have been excluded from the erupted magmas, as might occur if most of the gabbroic rocks in oceanic layer 3 formed an interlocking crystal framework, with viscosities that are too high to carry earlier formed crystals with the melt. The vertical extent of eruptible, crystal-poor melt lenses within the gabbroic zone is constrained to similar to 1 m or less by considering the width of local equilibrium growth zones, equilibration times, and crystal settling velocities. This lengthscale is consistent with field evidence from ophiolites. Finally, crystal aggregates within the Gakkel ridge sample studied here are the result of synneusis within the propagating dike during melt ascent.

[1] New chemical and isotopic analyses of the tephra layers plus deep-sea tephrostratigraphic record from two cores from either side of Luzon Island (Philippines) have allowed the identification of two periods of explosive volcanic activity originating from the Macolod Corridor in the southwestern part of the Luzon. The first period extended from prior to 1355 ka to 1977 ka, and the second period extended from 478 ka to the present, separated by a period of relative quiescence. The time intervals between large explosive eruption events in each period were 31 ± 15 ka and 156 ± 52 ka, respectively. Combined with published chronological and geochemical data from onshore volcanic deposits, the tephrostratigraphic record shows that the locus of large explosive eruptions has migrated southwestward from the northeastern section to the middle and southwestern sections of the Macolod Corridor. The period of relative quiescence is characterized by monogenetic volcanism in the central section of the corridor. The migration of active volcanism across the southwestern part of Luzon during the Quaternary is used to infer the evolution of the subducting South China Sea crust. The period of relative quiescence represents a period of adjustment of the subducted slab by steepening, which began around 1355 ka or shortly thereafter and finished at around 478 ka.