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The Mad Gap Yards ultramafic lamprophyre (UML) dykes in the East Kimberley region of northern Western Australia form part of a widespread Neoproterozoic (~ 842–800 Ma) alkaline mafic–ultramafic magmatic province in the north, east and central regions of the Kimberley Craton of Western Australia. The NE-trending Mad Gap Yards dykes lie at the southeastern margin of the Kimberley Basin adjacent to the Greenvale Fault and intrude the Paleoproterozoic Elgee Siltstone. The dykes are classified as alnöite, and contain abundant macrocrystic olivine in a groundmass of phlogopite, perovskite, spinels, diopside, apatite, andradite–hydroandradite, serpentine, calcite, pseudomorphs after melilite and rare gittinsite. Mantle-derived olivine macrocrysts have compositions in the range Mg#91–92, similar to moderately refractory peridotite from other parts of the Kimberley Craton, whereas magmatic olivine phenocrysts have Mg#88–90. Olivine and chromian spinel were the earliest phenocrysts; they record equilibration temperatures of ~ 1030–920℃ under moderately reducing conditions with fO2 values below the fayalite-magnetite-quartz (FMQ) oxygen buffer (Δ FMQ = mostly − 0.8 to − 1.7 log units). Magnetite rims and groundmass grains crystallised at ~ 850–740℃ under more oxidising conditions with Δ FMQ ~ + 0.6 to − 0.75 log units. Perovskite is well preserved in parts of the dykes and indicates crystallisation inside this fO2 range. The perovskite yielded a SHRIMP 206Pb/238U age of 842 ± 8 Ma. The Mad Gap Yards dykes carry rare partially altered spinel-peridotite xenoliths containing olivine (Mg#86.3–90), Cr-diopside, enstatite and Al-Cr spinel, and well as mantle xenocrysts of Cr-Al spinel and Cr-diopside. Bulk rock trace-element geochemistry, Cr-diopside thermobarometry and Sr–Nd-isotopic compositions of perovskite suggest that the UML magma was derived from partial melting of a garnet-bearing asthenospheric mantle source at ~ 200 km depth. Nd depleted-mantle model ages (TDM) for perovskite range from 1106–865 Ma and broadly correlate with regional mantle metasomatism during the Yampi Orogeny (~ 1000–800 Ma). Rare evidence of mantle metasomatism by LILE and HFSE-enriched melts has been found in the form of priderite and loveringite replacing Mg-rich ilmenite in an olivine macrocryst. The timing of emplacement of the Mad Gap Yards UML dykes at ~ 842 Ma correlates with the early stages of the breakup of Rodinia. [ABSTRACT FROM AUTHOR]

Cadomian calc-alkaline I-type and within-plate A-type igneous rocks are widespread in the crust of Iran where they are ascribed to a convergent margin associated with the southward subduction of Prototethyan oceanic lithosphere beneath N Gondwana. These rocks are found as unmetamorphosed magmatic rocks and their metamorphic equivalents (mafic to felsic gneisses) could have been generated in both Cadomian arcs and associated rear-arcs. Nearly all these exposures also contain metamorphosed metasediments, whereas Cadomian igneous rocks of central Iran are associated with thick sequences of unmetamorphosed terrigenous rocks. In the Zagros Fold-Thrust belt of S Iran, salt diapirs contain abundant xenoliths of Cadomian igneous and sedimentary rocks in association with evaporites, dolomites, carbonates, and banded iron-salt deposits. This paper presents new zircon U-Pb and geochemical-isotopic data from igneous clasts in the Chah-Banu salt diapir in SE Zagros. Petrographic and geochemical data indicate two different types of rock clasts; calc-alkaline, I-type dacites-rhyolites, and arc-related to E-MORB-OIB-like gabbros, basalts, and dolerites. New zircon U-Pb ages show that dacites formed at 538.2 ± 2.2 Ma, whereas gabbros show ages of 539.0 ± 1.8 Ma. Zircons from dacites have negative εHf(t) values of – 1.1 to – 8.3. In contrast, zircons from gabbros have higher εHf(t) values of +4.5 to +8.5, indicating crystallization from mantle-derived juvenile magmas. Bulk rock Nd-Sr isotopic data (e.g. ɛNd(t) = +0.3 to +4.0 and 87Sr/86Sr(i) = 0.7059 to 0.70848) for gabbros, dolerites, and basalts confirm that these rocks originated from a mantle source similar to enriched parts of the subcontinental lithospheric mantle, whereas dacites and rhyolites (with εNd(t) = −3.4 to −4.1 and 87Sr/86Sr(i) = 0.70806 to 0.70907) show strong interaction with, and/or re-melting of older continental crust. We suggest that the bimodal calc-alkaline and E-MORB-OIB-like magmatic rocks in salt diapirs as well as associated evaporites and sedimentary rocks formed in a retro-arc rifted basin behind the Cadomian magmatic arc. [ABSTRACT FROM AUTHOR]

Silicate melt inclusions (SMI) containing several daughter minerals, residual glass, and a CO2 bubble were analyzed to constrain the composition and evolution of the metasomatic melt present in the upper mantle beneath the Nógrad-Gomor Volcanic Field (NGVF), northern Hungary to southern Slovakia. The SMI were analyzed with a combination of Raman spectroscopy, FIB-SEM, and LA-ICP-MS to identify phases and obtain their volume proportions and major-and trace-element geochemistry. Slicing through the entire volume of the inclusions and collecting geochemical information at each slice with FIB-SEM allowed us to model the 3D appearance of the phases within the SMI and to use this information to calculate bulk major-element compositions. The partially crystallized SMI are hosted in clinopyroxene in a lherzolite xenolith that shows evidence of a metasomatic event that altered the lherzolites to produce wehrlites. Based on bulk compositions, the SMI trapped the metasomatic melt linked to wehrlite formation in the NGVF. The melt is enriched in Fe and has an OIB-like trace-element pattern, which suggests an intraplate mafic melt similar to the host basalt, but with slightly different chemistry. Pre-entrapment evolution and reaction with the lherzolite wall rock produced an intermediate melt composition. Petrogenetic modeling indicates that the melt was generated as a result of a very small degree of partial melting of a garnet lherzolite source. Following entrapment, a volatile bubble exsolved from the residual melt during ascent to shallow depths as suggested by consistent densities of CO2 in vapor bubbles. Small crystals, including sulfates and mica, that formed at the boundary of the bubble and the glass indicate that the exsolved fluid originally contained S and H2O, in addition to CO2. [ABSTRACT FROM AUTHOR]

New geochronological and geochemical data for Late Neoproterozoic to Mesozoic intrusive rocks from NW Iran define major regional magmatic episodes and track the birth and growth of one of the Cimmerian microcontinents: the Persian block. After the final accretion of the Gondwanan terranes, the subduction of the Prototethyan Ocean beneath NW Gondwana during the Late Neoproterozoic was the trigger for high magmatic fluxes and the emplacement of isotopically diverse arc-related intrusions in NW Gondwana. The Late Neoproterozoic rocks of NW Iran belong to this magmatic event which includes intrusions with highly variable εHf(t) values. This magmatism continued until a magmatic lull during the Ordovician, which led to the erosion of the Neoproterozoic arc, and then was followed by a rifting event which controlled the opening of Paleotethys. In addition, it is supposed that a prolonged pulse of rift magmatism in Persia lasted from Devonian-Carboniferous to Early Permian time. These magmatic events are geographically restricted and are mostly recorded from NW Iran, although there is some evidence for these magmatic events in other segments of Iran. The Jurassic rocks of NW Iran are interpreted to be the along-strike equivalents of a Mesozoic magmatic belt (the Sanandaj-Sirjan Zone; SaSZ) toward the NW. Magmatic rocks from the SaSZ show pulsed magmatism, with high-flux events at both ~176–160 Ma and ~130 Ma. The SaSZ magmatic rocks are suggested to be formed along a continental arc but a rift setting is also considered for the formation of the SaSZ rocks based on the plume-related geochemical signatures. The arc signatures are represented by Nb-Ta depletion in the highly contaminated (by upper continental crust) plutonic rocks whereas the plume-related signature of less-contaminated melts is manifested by enrichment in Nb-Ta and high εHf(t) values, with peaks at +0.6 and +11.2. All these magmatic pulses led to pre-Cimmerian continental growth and reworking during the Late Neoproterozoic, rifting and detachment of the Cimmerian blocks from Gondwana in Mid-Late Paleozoic time and further crustal growth and reworking of Cimmeria during the Mesozoic. Unlabelled Image • U-Pb zircon ages define Late Neoproterozoic, Paleozoic and Mesozoic magmatic events in NW Iran. • Late Neoproterozoic magmatism is characterized by highly variable εHf(t) values. • Late Paleozoic magmatic pulses show rift-related geochemical signatures with radiogenic εHf(t) values. [ABSTRACT FROM AUTHOR]

Saindak is one of the typical porphyry Cu deposits (PCDs) in the Chagai magmatic arc in Pakistan. Ore-forming porphyries at Saindak PCD are mainly composed of tonalite. Here, we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization. Although apatite inclusions have homogeneous intra-grain S and Cl compositions, there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase. Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An, FeO and Mg contents of the hosting plagioclase phenocryst. It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber, rather than by one major injection or by addition of mafic melt derived from different source region. Each primitive melt injection introduced essential ore-forming materials such as S and Cl, which were rapidly and effectively released to the coexisting fluids, causing mineralization. Once primitive melt injection stops, signaling the end of growth of underlying magma chamber, mineralization will cease quickly although the hydrothermal system can still survive for a long time. However, the later released fluids are relatively depleted in ore-forming materials, and thus have lower capability to generate mineralization. Accordingly, predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system. [ABSTRACT FROM AUTHOR]

Despite diverse geochronological-geochemical studies on Cenozoic igneous rocks from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran, the nature of the Oligocene magmatic rocks from the farthermost end of the SE segment- where it is linked to the Makran magmatic belt- has been ignored due to the difficulty of access. In this study, we focus on syn -collisional mafic to felsic igneous rocks of calc-alkaline and high-K calc-alkaline affinities from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) near Nagisun, south of Bam. The Nagisun rocks have low Sr/Y and La (n) /Yb (n) , similar to igneous rocks from typical arcs. Zircon U Pb ages show comparable ages for plutonic (~ 34–25 Ma) and volcanic (~34–27 Ma) rocks. The εHf(t) values for zircons from plutonic rocks range from −0.3 to +12.8, whereas the εHf(t) values for the volcanic rocks vary from −2.6 to +13. Modelling of trace elements compositions using Nagisan basaltic samples indicate that an 87:2:11 mixture of the depleted MORB mantle, subducting (trench)-sediments and altered oceanic crust with 5% aggregated fractional melting closely matches the trace-element abundances of the Nagisun basaltic rocks. Indeed, the modelling of Sr and Nd isotopic data emphasizes that the Nagisun magmatic rocks could be products of bulk mixing between a depleted MORB mantle and/or a mixed, fertilized mantle with the Cadomian lower and upper continental crust. Furthermore, our compiled data display that the magmatism in the SE segment of the UDMB changed through time from normal calc-alkaline magmatism to adakitic magmatism at ~20 Ma, after the collision with Arabia began ca 27 Ma. • Collision-related, Oligocene magmatic rocks are abundant in the SE segment of the Urumieh-Dokhtar magmatic belt of Iran. • Zircon U-Pb data show ages of 34-25 Ma for plutonic rocks and 34-27 Ma for volcanic rocks from SE UDMB. • Isotope modelling suggests mixing between the mantle and Cadomian crust for the formation of these rocks. [ABSTRACT FROM AUTHOR]

Abstract: Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg#<90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg#>90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al2O3, CaO and TiO2), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high 143Nd/144Nd (0.513219–0.513331) and low 86Sr/87Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low 143Nd/144Nd (0.512499–0.512956) and high 86Sr/87Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time. [Copyright &y& Elsevier]

Abstract: The Carboniferous tectonic setting of the Junggar terrane, northern Xinjiang, NW China, has long been a matter of debate. Voluminous Carboniferous volcanic rocks are widely distributed in the Karamaili area, the southern part of the eastern Junggar terrane. Early Carboniferous rocks comprise basalts and basaltic andesites, with enrichment of LREE and LILE and depletion of HFSE, and uniformly high ε Nd(t) (+3.7 to +4.0). Late Carboniferous rocks consist of basalts, basaltic andesites, rhyolites and minor dacites, and can be subdivided into basic and felsic groups. The basic rocks are depleted in HFSE, and show variable high ε Nd(t) (+4.8 to +6.9). They have higher Cr and Ni and lower Na2O, U and Th contents than early Carboniferous basic rocks. The felsic rocks show A-type affinity, with typical enrichment of alkalis, LREE and HFSE and strong depletion in Ba, Sr, Eu and Ti. They have high values of ε Nd(t) and zircon ε Hf(t) (+11.6 to +17.9). New LA-ICPMS zircon U–Pb analyses constrain their emplacement to late Carboniferous time (306.5–314.3Ma). The Carboniferous basic rocks show negative Zr-Hf anomalies and low Th/Ce (<0.07) and Th/La (0.06–0.16), excluding significant crustal contamination during magma evolution. They have low La/Ba (0.03–0.12), Ce/Y (<3) and (Tb/Yb)N (<2) and variable Ba/Th (28–318) and Ba/La (3.1–34), suggesting that they were derived from a main spinel with minor garnet lherzolite mantle source metasomatized by slab-derived fluids. The late Carboniferous felsic rocks were produced when upwelling asthenosphere triggered partial melting of juvenile lower crust. The early Carboniferous volcanism occurred in an island-arc setting related to the southward subduction of the Paleo-Junggar Ocean plate, whereas the late Carboniferous rocks erupted in a post-collisional extensional setting. Thus, a rapid tectonic transition from arc to post-collisional extension may have occurred between early and late Carboniferous, and probably resulted from slab break-off or lithospheric delamination. [Copyright &y& Elsevier]

Abstract: Podiform chromite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone (SSZ) settings. We have measured the Re–Os isotopic compositions of the major chromite deposits and associated mantle peridotites of the Dongqiao Ophiolite in the Bangong–Nujiang suture, Tibet, to investigate the petrogenesis of these rocks and their genetic relationships. The 187Os/188Os ratios of the chromite separates define a narrow range from 0.12318 to 0.12354, less variable than those of the associated peridotites. Previously-reported 187Os/188Os ratios of the Os-rich alloys enclosed in the chromitites define two clusters: 0.12645±0.00004 (2s; n=145) and 0.12003 to 0.12194. The ultra-depleted dunites have much lower 187Os/188Os (0.11754, 0.11815), and the harzburgites show a wider range from 0.12107 to 0.12612. The average isotopic composition of the chromitites (187Os/188Os: 0.12337±0.00001) is low compared with the carbonaceous chondrite value (187Os/188Os: 0.1260±0.0013) and lower than the average value measured for podiform chromitites worldwide (0.12809±0.00085). In contrast, the basalts have higher 187Os/188Os, ranging from 0.20414 to 0.38067, while the plagioclase-bearing harzburgite and cumulates show intermediate values of 187Os/188Os (0.12979~0.14206). Correspondingly, the basalts have the highest 187Re/188Os ratios, up to 45.4±3.2, and the chromites have the lowest 187Re/188Os ratios, down to 0.00113±0.00008. We suggest that melts/fluids, derived from the subducting slab, triggered partial melting in the overlying mantle wedge and added significant amounts of radiogenic Os to the peridotites. Mass-balance calculations indicate that a melt/mantle ratio of approximately 15:1 (melt: 187Re/188Os: 45.4, 187Os/188Os: 0.34484; mantle peridotite: 187Re/188Os: 0.0029, 187Os/188Os: 0.11754) is necessary to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/mantle ratio during the formation of the chromitite deposits. The percolating melts probably were of variable isotopic composition. However, in the chromitite pods the Os from many melts was pooled and homogenized, which is why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/188Os ratios of chromitites may not be representative of the DMM, but only reflect an upper limit. Importantly, the Os-isotope compositions of chromitites strongly suggest that such deposits can be formed by melt/mantle mixing processes. [Copyright &y& Elsevier]

Primary sulphides occur both as micro-inclusions in major silicate and oxide phases and as individual grains in spinel lherzolite and pyroxenite xenoliths from Qilin, southeast China. Most of the lherzolite-hosted sulphide inclusions, typically 20-50 µm across, occur as isolated spheres or spheroids; host grains are olivine, clinopyroxene and orthopyroxene, but not spinel. In contrast, sulphide inclusions in pyroxenite are mostly 20-80 µm across and are almost exclusively hosted by clinopyroxene and spinel. These sulphide inclusions are typically multifaceted polygons, with their shapes epitaxially controlled by the host minerals. Isolated sulphide grains occur only in pyroxenite; they are up to 500 µm across, show evidence of deformation and are spatially associated with spinel. Lherzolite-hosted sulphide grains are polyphase assemblages that consist of pentlandite ± chalcopyrite ± Ni-poor monosulphide solid solution (mss1) ± Ni-rich monosulphide solid solution (mss2) ± cubanite ± heazlewoodite ± millerite ± bornite. Pyroxenite-hosted sulphide grains are pyrrhotite with minor chalcopyrite. All assemblages are likely to be the low-T ( <= 300°C) re-equilibrated products of high-T monosulphide solid solutions (MSS). The bulk compositions of these sulphide grains, estimated using proton microprobe analysis, show no consistent differences between the inclusion suites and intergranular sulphide grains, either in spinel lherzolite or pyroxenite samples. Average values of 111 proton microprobe analyses reveal that the lherzolite-hosted sulphides are rich in Ni (21%), Cu (<9%), Se (110 ppm) and platinum group elements (PGE) ( <= 30 ppm) but poor in Fe (37%) compared with the pyroxenite-hosted sulphides (Ni 1·4%, Cu <4%, Se 35 ppm, PGE absent, Fe 61·5%). Other trace elements (Co, Zn, As, Mo, Ag, Sn, Sb, Te and Pb) show no significant difference between the two suites. Lherzolite-hosted sulphides are inferred to be derived from immiscible sulphide melts trapped in residual mantle during partial melting. The sulphide melts had themselves undergone MSS fractionation before their incorporation into the depleted mantle rocks. In contrast, pyroxenite-hosted sulphides were produced by sulphur saturation during the crystallization of mafic magmas intruded into lherzolitic mantle. [ABSTRACT FROM PUBLISHER]

The age and composition of the lower crust are critical in understanding the processes of continental formation and evolution, and deep-seated granulite xenoliths can offer direct information on the lower crust. Here, we report mineral chemistry, whole-rock major and trace elements, Sr–Nd isotopes and zircon U–Pb–Hf results for a suite of deep-seated crustal xenoliths, recently discovered in the Cenozoic basalts of the Nangaoya area in the northern part of the North China Craton (NCC). Based on the P–T estimates, these xenoliths including mafic, intermediate and felsic granulites and hornblendites were sampled from different levels of the lower crust. While a hornblendite has a flat REE pattern, all other xenoliths display LREE enrichment and depletion of Nb, Ta, Th and Ti. The mafic granulite xenolith has relatively high whole-rock ε Nd (t) value of − 13.37, and yields Mesozoic (188–59 Ma) zircons ages with high ε Hf (t) values from − 15.3 to − 9.2. The garnet-bearing intermediate granulite-facies rocks show low ε Nd (t) values from − 16.92 to − 17.48, and reveal both Paleoproterozoic (1948 Ma) and Mesozoic (222–63 Ma) zircon U–Pb ages. Their Mesozoic zircons have lower ε Hf (t) values (from − 18.4 to − 13.8) than those from the mafic xenolith. The remaining intermediate to felsic xenoliths show Paleoproterozoic zircon ages, and the lowest ε Nd (t) values (from − 20.78 to − 24.03). The mafic–intermediate granulites with Mesozoic zircons originated from the interaction of lower crust-derived magmas with mantle melts, with higher proportions of mantle magmas involved in the generation of mafic granulite, whereas intermediate to felsic xenoliths without Mesozoic zircons represent ancient Paleoproterozoic to Neoarchean deep crust. These deep-seated xenoliths reveal complicated crustal evolution processes, including crustal growth during Neoarchean (2.5–2.7 Ga), middle Paleoproterozoic (2.2–2.1 Ga) and Mesozoic, and reworking during early Paleoproterozoic, late Paleoproterozoic and Mesozoic related to magmatic underplating. The integrated analyses of lithological, geochemical and age data for a suite of deep-seated xenoliths show that the lower crust in the Nangaoya area is temporally and compositionally zoned. The upper part of the lower crust mainly comprises Neoarchean to Paleoproterozoic intermediate–felsic rocks with intercalated hornblendites, the majority of which record ~ 1950 and ~ 1850 Ma metamorphism; the middle part is dominated by a Paleoproterozoic and Mesozoic intermediate garnet-bearing granulite-facies hybrid layer; and the lowermost crust is represented by a Mesozoic mafic granulite layer, which was significantly modified by episodic magmatic underplating. Such a modification induced by crust–mantle interaction can result in Mesozoic ages and more mafic components for xenolith granulites, and thus is an effective mechanism to explain the differences between exposed and xenolithic granulites. [ABSTRACT FROM AUTHOR]

The Precambrian tectonic evolution of the North China Craton (NCC) has long been a matter of debate. The Huai’an Complex, situated at the conjunction of the Khondalite Belt and Trans-North China Orogen (TNCO), undoubtedly can provide some key constraints on the geological evolution of the Khondalite Belt and the TNCO. We report zircon U–Pb ages and Hf-isotope results for four gneissic rocks and one Mesozoic mafic dyke in the Huai’an Complex to constrain the Precambrian tectonothermal evolution and crustal accretion in the northern margin of the NCC. The zircons from three dioritic to granitic gneisses and inherited zircons from the mafic dyke all yield Neoarchean to early Paleoproterozoic protolith ages of 2445–2533 Ma, and register three later tectonomagmatic events with ages of ∼2360, ∼2150 and ∼1850 Ma. The ∼2360 Ma magmatism is recorded by a dioritic gneiss (2358 ± 34 Ma) from the Dongyanghe area and a granodioritic gneiss (2370 ± 24 Ma) from the Hunyuanyao area. The ∼2150 Ma magmatism is recognized in a dioritic gneiss (2172 ± 7 Ma), granodioritic gneiss (2159 ± 63 Ma) and the Mesozoic mafic dyke (2144 ± 57 Ma). The ∼1850 Ma magmatism is represented by zircon populations with ages of 1877 ± 48 Ma in the dioritic gneiss, 1844 ± 69 Ma in the granodioritic gneiss and ∼1800 Ma in the mafic dyke. These Neoarchean to Paleoproterozoic ages reveal that the Huai’an Complex underwent a successive, complicated tectonomagmatic evolution, similar to the Hengshan–Wutai–Fuping Complexes, which represent a long-lived Neoarchean to Paleoproterozoic magmatic arc. In contrast, a sillimanite–garnet gneiss from the Kouzicun area yields completely different zircon U–Pb ages with peaks at 2056 ± 32 Ma, 1952 ± 25 Ma and 1901 ± 25 Ma. These three age groups suggest that the Kouzicun area experienced a tectonothermal evolution diagnostic of the Khondalite Belt, including protolith ages (2100–2000 Ma), peak metamorphism (∼1950 Ma) and post-orogenic exhumation (∼1900 Ma). Thus, the geological boundary between the Khondalite Belt and the TNCO can be constrained to a position between the Kouzicun and Hunyuanyao areas. Coupled with zircon U–Pb ages, zircon Hf isotopic results reveal that ∼2500 Ma mantle-derived mafic magmas not only provided enormous heat for partial melting of the 2.7–2.8 Ga juvenile crust to form widespread TTG rocks, but also contributed directly to the generation of dioritic rocks. The Huai’an Complex in the northern margin of the NCC witnessed two Neoarchean episodes of continental-crust growth at 2.7–2.8 Ga and at 2.5 Ga. [ABSTRACT FROM AUTHOR]

The geochemical compositions of minerals from the Moho transition zone of ophiolites potentially can help to understand the magmatic evolution of the ophiolites, and subsequent mantle-melt interactions. The Jurassic-Late Cretaceous Makran ophiolite of south Iran comprises one of the most extensive tracts of oceanic crust which were scraped off and preserved in an accretionary complex. The Makran ophiolite records traces of MORB-OIB-type magmatism during the Jurassic, but mostly supra-subduction zone magmatic activity during the Late Cretaceous. Despite a few geochemical studies on the crustal rocks, the nature and geochemical signatures of the mantle rocks from this ophiolite remain controversial. The Sorkhband mantle-crust transition zone underlying crustal cumulates in the western Makran consists of stratiform chromitites, harzburgites, chromite-rich dunites and dunites, with crosscutting dikes of olivine websterite and olivine clinopyroxenite. Major- and trace-element compositions of clinopyroxene grains in olivine websterite and clinopyroxenite dikes indicate crystallization from melts similar to boninites and low-Ti fore-arc basalts. Spinel compositions in olivine websterite and clinopyroxenite dikes suggest crystal fractionation from boninitic or high- Mg# magmas have played a major role in the genesis these rocks. We propose a two-stage model for the formation of the Sorkhband dunites including (1) supra-subduction zone-related melt infiltrates through harzburgites in the mantle-crust transition zone to dissolve peridotite orthopyroxene and leave dunites with high forsterite-NiO olivines, and (2) boninitic melts accumulate and react with surrounding harzburgites to crystallize cumulate dunites with low-Mg# olivine and high-Ti spinels. We conclude that there have been temporal changes in the composition of mantle melts in the fore-arc mantle section of the Makran ophiolite during the initial subduction of the Neotethyan ocean beneath the Lut block during the Late Cretaceous. [Display omitted] • Moho transition zone of Makran includes peridotites and stratiform chromitites. • Boninites and forearc basalts have generated Makran websterites and clinopyroxenites. • The geochemistry of mantle melts in the Makran mantle section changed through time. [ABSTRACT FROM AUTHOR]

Abstract: Type I and Type II eclogite xenoliths from the Roberts Victor kimberlite (South Africa) show marked differences in terms of microstructures, mineralogy, major- and trace-element compositions and oxygen-isotope compositions. The unequilibrated microstructures of Type I eclogites, their typical accessory assemblages (phologopite, diamond, sulphides, fluid inclusions) and the ubiquitous presence of “melt pockets” in garnets provide strong evidence of metasomatism. Type II eclogites systematically lack such features and are microstructurally more equilibrated. Type I eclogites are more magnesium-rich than most Type II (mean Mg#=0.56 vs. 0.46), while Type II eclogites are generally more Ca-rich (mean CaO=9 vs. 12wt%) and Fe-rich (mean FeO=10 vs. 12wt%). Type I eclogites are systematically enriched in LREE, Sr, Ba, alkali elements, HFSE, Th and U compared to the more depleted Type II eclogites. Calculated trace-element patterns of fluids in equilibrium with Type I eclogites are closely similar to those of volatile-rich small-volume mantle melts in the carbonatite–kimberlite spectrum commonly inferred to be responsible for mantle metasomatism. Although oxygen isotopes are often used to argue for a subduction origin of mantle eclogites, correlations between δ18O of garnet and typical metasomatic tracers suggest that the metasomatic process also has shifted the oxygen-isotope compositions of the Type I eclogites toward heavier values. Roberts Victor Type I eclogites thus carry the imprint of a metasomatic process that strongly modified their major-element, trace-element and isotopic compositions, while the more pristine Type II eclogites escaped this modification. Therefore, attempts to constrain the origin of Roberts Victor eclogites should not be based on the much more abundant Type I eclogites, which retain little geochemical memory of their protoliths. The most suitable materials for such investigations may be the less metasomatised, but more rare, Type II eclogites. [Copyright &y& Elsevier]

Abstract: High-resolution global seismic tomography (Vs) models reveal high-velocity domains beneath cratonic crust in Africa that extend to depths of 300–400 km. These high-velocity domains show a distinct contrast with the characteristics of “normal” asthenosphere and are interpreted as depleted, buoyant roots that formed in the Archean and have been metasomatised over time, but have remained attached to the overlying crust. Such deep roots are impediments to free horizontal convection in the upper mantle. The movement of magmas and other fluids in such regions may be more vertically constrained (a shallow lava lamp regime), creating a geodynamic environment conducive to interaction of such magmas with the boundaries of deep mantle domains that would carry old “crustal” geochemical signatures. The tomographic models and the new world magnetic-anomaly map show that these continental roots, overlain by thinned continental crust, locally extend well out under the deep Atlantic Ocean basin. However, such high-velocity domains are not confined to the basin margins, but are scattered randomly through the basin, some quite distant from the continental margins of South America and Africa. These high-velocity domains are interpreted to be remnant lithospheric fragments isolated by disruption of the ancient continental regions during rifting. This interpretation is supported by the old Os depletion ages of mantle peridotites from mid-ocean ridges and oceanic islands. Basaltic magmas near such high-velocity domains carry the geochemical signatures (EM1, EM2) of interaction with refertilised cratonic mantle. The interaction of rising magmas with fragments of ancient lithospheric mantle can explain such geochemical signatures and obviates the need for complex models in which these geochemical reservoirs are isolated and preserved in the convecting mantle. [Copyright &y& Elsevier]

Porphyry copper deposits (PCDs) represent the most important type of Cu reservoir on Earth, but the mechanism of Cu enrichment in PCDs is debated due to the lack of direct constraints. This issue may be resolved by the study of copper isotopes (δ65Cu), which are strongly fractionated during formation and/or precipitation of sulfides. Here we report high-precision Cu-isotope data on a large set of porphyries, mafic magmatic enclaves (MMEs), and sulfide ores from PCDs in southern Tibet. For comparison, barren intrusions from southern Tibet were also analyzed. The fertile porphyries and MMEs from PCDs have high Cu contents and elevated δ65Cu values compared with the barren intrusions and global average felsic rocks. These features are inconsistent with the known supergene processes after sulfide formation (e.g., leaching and weathering). Because sulfides formed from secondary Cu-rich fluids are enriched in heavy Cu isotopes, the elevated Cu contents and Cu-isotope ratios indicate that the Cu source for PCDs was a refertilized lithosphere enriched in sulfides. This suggests that initial Cu enrichment in magma sources could be a key step in the formation of giant PCDs in continental collision zones. [ABSTRACT FROM AUTHOR]

Abstract Eclogite lenses and boudins are volumetrically minor, but petrologically important, features of peridotite massifs worldwide. In the Western Gneiss Region of the Scandinavian Caledonides, eclogites in the Almklovdalen and Raubergvik peridotites originated as basaltic to picrobasaltic dikes, comprising both olivine–normative and nepheline–normative types, with a wide variation in Mg–number from 34 to 65. Positive anomalies for Pb and Sr and negative anomalies for Zr and Hf reflect a subduction signature in the basic melts, and rare–earth element modelling requires 20% to 70% fractional crystallization, combined with 20% to 70% assimilation of peridotite. Clinopyroxenes in eclogites have a wide variation in ε Nd (0) from +68 to −26, which is comparable to that for associated garnet peridotites and pyroxenites, +55 to −38, and a range in 87Sr/86Sr from 0.7021 to 0.7099, which is much larger than that in peridotites and pyroxenites, 0.7014 to 0.7033. Plagioclase and amphibole inclusions in eclogite garnet provide evidence for prograde metamorphism, which attained a maximum temperature of ~775 °C and pressure of ~25 kb. Such conditions are allofacial with those of associated garnet peridotites and pyroxenites, which equilibrated at ~825 °C and ~37 kb. Eclogites yield mixed Sm-Nd isochron ages, as do the peridotites and pyroxenites, but ages in eclogites are <1000 Ma, and those in peridotites and pyroxenites are >1000 Ma. Three eclogites yield Ordovician U-Pb ages for rutile at 440 ± 12, 445 ± 51, and 480 ± 29 Ma, which are coeval with the Taconic Orogeny and are consistent with a Laurentian provenance for the host peridotites. Eclogites in both Norwegian and Czech peridotites originated from melts passing through a mantle wedge above a subduction zone, and both suites exhibit subduction geochemical signatures, although they differ dramatically in petrogenesis. Eclogites in Norwegian peridotites initially crystallized as relatively low–pressure, plagioclase–bearing basaltic or gabbroic dikes and subsequently recrystallized to high–pressure eclogite, whereas most eclogites in Variscan Moldanubian peridotites crystallized directly from magmas at high pressure to produce eclogite facies assemblages. Highlights • Eclogite layers in Norwegian garnet peridotites originated as basalt or gabbro dikes • AFC modelling yields 20–70% crystallization and 20–70% assimilation of peridotite • The dikes recrystallized to eclogite at 775 °C and 25 kb during the Taconic Orogeny • Eclogite in Czech peridotites originated by HP crystallization of garnet and pyroxene [ABSTRACT FROM AUTHOR]

Abstract Tuyeh-Darvar granitoid, which outcrop ca 45 km Sw of Damghan city, in the Eastern Alborz zone, comprise mainly the pluton emplaced into the Barut Formation of Lower Cambrian ages. Zircon U-Pb ages show Carboniferous ages (325 ± 3 Ma) for the formation of this granitoid. The granitoid is mostly metaluminous, ferroan and alkalic monzonite to monzodiorite. "These rocks have high values of FeOT/MgO and Ga/Al, high concentrations of K2O+Na2O, low abundances of MgO and transitional elements. Plots normalized to chondrite and primitive mantle compositions show strong enrichments of LREE relative to HREE and of LILE relative to HFSE, accompannied by negative anomalies of Nb and Sr."They contain Fe-rich hydrous mafic minerals and magnetite. These features are typical of A-type granites. Sr-Nd isotopic geochemistry, with initial ɛNd values from −1.1 to −1.5 and initial 87Sr/86Sr ratios between 0.70562 and 0.70678, are consistent with magmatic differentiation from mafic melts produced from an enriched mantle source. However, other models such as melting of mafic crust or mixing of components from depleted mantle and continental crust cannot be discarded. On the basis of the U-Pb zircon age (325 ± 3 Ma) and the known magmatic tectonic regime in Iran during the Paleozoic, it is suggested that the pluton, formed in a rift environment related to extensional structures of the Alborz block in Early Carboniferous time. Highlights • New zircon U-Pb ages reveled a Carboniferous magmatism in N Iran. • Bulk rock geochemical data show A-type signature for Tuyeh- Darvar granitoid. • Tuyeh- Darvar granitoid is related to extensional activity of the Alborz block in Early Carboniferous time. [ABSTRACT FROM AUTHOR]

Pyroxenites exposed in ophiolites and orogenic peridotite massifs may record petrogenetic processes occurring in mantle domains generated and/or transferred in supra-subduction environments. However, the timing of their formation and the geochemical characteristics of their source region commonly are obscured by metamorphic and metasomatic overprints. This is especially critical in arc-related environments, where pyroxenites may be formed during the differentiation of primitive magmas. Our approach combines Sr- and Nd-isotope geochemistry and geochronology, and modelling of REE diffusion, to further constrain the origin of a well-characterized set of pyroxenites from the arc-related Cabo Ortegal Complex, Spain. In the light of petrological constraints, Sr- and Nd-isotope systematics consistently indicate that cpx and amphibole have acquired disequilibrium during two main episodes: (1) a magmatic/metasomatic episode that led to the formation of the pyroxenites, coeval with that of Cabo Ortegal granulites and corresponding to the incipient stage of a potential Cadomian arc (459–762 Ma; isochron and second-stage Nd model ages); (2) an episode of metamorphic amphibolitization upon the percolation of relatively unradiogenic and LREE-enriched hydrous fluids, subsequent to the delamination of the pyroxenites from their arc-root settings during Devonian subduction. Calculations of diffusional timescale for the re-equilibration of REE are consistent with this scenario but provide only poor additional constraints due to the sensitivity of this method to grain size and sub-solidus temperature. We thus emphasize the necessity to combine isochron ages and Nd model ages corrected for radiogenic ingrowth to put time constraints on the formation of subduction- and/or collision-related pyroxenites, along with petrological and geochemical constraints. Homogeneous age-corrected 143 Nd/ 144 Nd of 0.5121–0.5125 ( ε Nd between 0 and +7.5) and 87 Sr/ 86 Sr of 0.7037–0.7048 provide information on the sources of the metasomatic agents involved (and potentially the parental melts) and notably indicate the contributions from enriched mantle components (EM I and/or II). This suggests the involvement of an old crustal component, which is consistent with the derivation of the pyroxenites and granulites from an ensialic island arc, potentially built on the northern margin of either Gondwana or a pre-Gondwanan continental block. This case study thus documents the role of melt–rock reactions as major pyroxenite-forming processes in the sub-arc mantle, providing further constraints on their sources and timing in the Cabo Ortegal Complex. [ABSTRACT FROM AUTHOR]

The petrogenesis of kimberlites is commonly obscured by interaction with hydrothermal fluids, including deuteric (late-magmatic) and/or groundwater components. To provide new constraints on the modification of kimberlite rocks during fluid interaction and the fractionation of kimberlite magmas during crystallisation, we have undertaken a detailed petrographic and geochemical study of a hypabyssal sample (BK) from the Bultfontein kimberlite (Kimberley, South Africa). Sample BK consists of abundant macrocrysts (> 1 mm) and (micro-) phenocrysts of olivine and lesser phlogopite, smaller grains of apatite, serpentinised monticellite, spinel, perovskite, phlogopite and ilmenite in a matrix of calcite, serpentine and dolomite. As in kimberlites worldwide, BK olivine grains consist of cores with variable Mg/Fe ratios, overgrown by rims that host inclusions of groundmass phases (spinel, perovskite, phlogopite) and have constant Mg/Fe, but variable Ni, Mn and Ca concentrations. Primary multiphase inclusions in the outer rims of olivine and in Fe-Ti-rich (‘MUM’) spinel are dominated by dolomite, calcite and alkali carbonates with lesser silicate and oxide minerals. Secondary inclusions in olivine host an assemblage of Na-K carbonates and chlorides. The primary inclusions are interpreted as crystallised alkali-Si-bearing Ca-Mg-rich carbonate melts, whereas secondary inclusions host Na-K-rich C-O-H-Cl fluids. In situ Sr-isotope analyses of groundmass calcite and perovskite reveal similar 87 Sr/ 86 Sr ratios to perovskite in the Bultfontein and the other Kimberley kimberlites, i.e. magmatic values. The δ 18 O composition of the BK bulk carbonate fraction is above the mantle range, whereas the δ 13 C values are similar to those of mantle-derived magmas. The occurrence of different generations of serpentine and occasional groundmass calcite with high 87 Sr/ 86 Sr, and elevated bulk carbonate δ 18 O values indicate that the kimberlite was overprinted by hydrothermal fluids, which probably included a significant groundwater component. Before this alteration the groundmass included calcite, monticellite, apatite and minor dolomite, phlogopite, spinel, perovskite and ilmenite. Inclusions of groundmass minerals in olivine rims and phlogopite phenocrysts show that olivine and phlogopite also belong to the magmatic assemblage. We therefore suggest that the crystallised kimberlite was produced by an alkali-bearing, phosphorus-rich, silica-dolomitic melt. The alkali-Si-bearing Ca-Mg-rich carbonate compositions of primary melt inclusions in the outer rims of olivine and in spinel grains with evolved compositions (MUM spinel) support formation of these melts after fractionation of abundant olivine, and probably other phases (e.g., ilmenite and chromite). Finally, the similarity between secondary inclusions in kimberlite olivine of this and other worldwide kimberlites and secondary inclusions in minerals of carbonatitic, mafic and felsic magmatic rocks, suggests trapping of residual Na-K-rich C-O-H-Cl fluids after groundmass crystallisation. These residual fluids may have persisted in pore spaces within the largely crystalline BK groundmass and subsequently mixed with larger volumes of external fluids, which triggered serpentine formation and localised carbonate recrystallisation. [ABSTRACT FROM AUTHOR]

Perovskite (CaTiO 3 ) has become a very useful mineral for dating kimberlite eruptions, as well as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the first work of in situ U–Pb geochronology and Sr–Nd isotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskite were identified, differing in texture, major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb 2 O 5 ) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts. U–Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2 ± 6.5 Ma), Mulepe 2 (123.0 ± 3.6 Ma), Calonda (119.5 ± 4.3 Ma) and Cat115 (133 ± 10 Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (> 300 km) during the break-up of Gondwana. Sr–Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations. [ABSTRACT FROM AUTHOR]

Abstract: Recycling of oceanic crust into the deep mantle via subduction is a widely accepted mechanism for creating compositional heterogeneity in the upper mantle and for explaining the distinct geochemistry of mantle plumes. The oxygen isotope ratios (δ18O) of some ocean island basalts (OIB) span values both above and below that of unmetasomatised upper mantle (5.5±0.4‰) and provide support for this hypothesis, as it is widely assumed that most variations in δ18O are produced by near-surface low-temperature processes. Here we show a significant linear relationship between δ18O and stable iron isotope ratios (δ57Fe) in a suite of pristine eclogite xenoliths. The δ18O values of both bulk samples and garnets range from values within error of normal mantle to significantly lighter values. The observed range and correlation between δ18O and δ57Fe is unlikely to be inherited from oceanic crust, as δ57Fe values determined for samples of hydrothermally altered oceanic crust do not differ significantly from the mantle value and show no correlation with δ18O. It is proposed that the correlated δ57Fe and δ18O variations in this particular eclogite suite are predominantly related to isotopic fractionation by disequilibrium partial melting although modification by melt percolation processes cannot be ruled out. Fractionation of Fe and O isotopes by removal of partial melt enriched in isotopically heavy Fe and O is supported by negative correlations between bulk sample δ57Fe and Cr content and bulk sample and garnet δ18O and Sc contents, as Cr and Sc are elements that become enriched in garnet- and pyroxene-bearing melt residues. Melt extraction could take place either during subduction, where the eclogites represent the residues of melted oceanic lithosphere, or could take place during long-term residence within the lithospheric mantle, in which case the protoliths of the eclogites could be of either crustal or mantle origin. This modification of both δ57Fe and δ18O by melting processes and specifically the production of low-δ18O signatures in mafic rocks implies that some of the isotopically light δ18O values observed in OIB and eclogite xenoliths may not necessarily reflect near-surface processes or components. [Copyright &y& Elsevier]