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Mafic granulite, garnet peridotite, and garnet pyroxenite occurred as slices or lenses within dominant felsic granulite, and they together constitute a high‐pressure metamorphic terrane in the Bashiwake unit, South Altyn Tagh, Northern Tibet, China. Previous studies focused on the metamorphic evolution, and geothermobarometry results indicated that the mafic granulite has experienced high pressure/(ultra‐)high temperature (HP/(U)HT) metamorphism, followed by a medium pressure (MP) granulite‐facies overprint. However, the nature and petrogenesis of the mafic granulite in the dominant felsic granulite are poorly known. Combining the previous geothermobarometry results with the petrographic observations, mineral chemistry, and pseudosection modelling in this study, at least four stages were suggested for the metamorphic evolution of the mafic granulites in the South Altyn Tagh, including the eclogite‐facies stage (3–4 GPa, 910–1000°C), high pressure–ultrahigh temperature (HP–UHT) metamorphism, an isothermal decompression, and subsequent MP granulite‐facies overprint. The U–Pb dating of zircons yielded two age clusters: one age cluster at ca. 500 Ma, representing the retrograde age of HP–UHT metamorphism after the eclogite‐facies stage, and another age cluster of ca. 900 Ma that represented the age of the protolith for the mafic granulite. This indicated that the protolith of the mafic granulite was formed in the early Neoproterozoic and then was taken to extreme temperatures and pressures during the early Palaeozoic orogenic event. The elemental abundances of the mafic granulites in the Bashiwake area clearly indicated that they were higher in FeO and TiO2, but were significantly lower in MgO, Cr, and Ni than those of associated garnet peridotites/pyroxenites, and they showed LREE‐enriched patterns with slightly positive Eu anomalies. Sr–Nd isotopic data suggested a basaltic magmatic origin with crust contamination for the protolith of the mafic granulite. Integrating these results together with previous studies, we suggest that the mafic granulites were derived from the basaltic magma intrusion in the continental crust during the Neoproterozoic and subsequently suffered a common HP/UHT metamorphism with felsic crust rocks in the early Palaeozoic (ca. 500 Ma) after the eclogite‐facies metamorphism related to the continental collision (>500 Ma). [ABSTRACT FROM AUTHOR]

Abstract Carbonatitic magmatism in subduction zones provides extremely valuable information on the cycling, behavior and storage of deep carbon within the Earth. It may also shed light on insights into crust-mantle interaction and mantle metasomatism within subduction zones. Origin of carbonatite has long been debated: all hypotheses need to reflect the different mineral assemblages and geochemical compositions of carbonatites and their diverse tectonic settings. Here we present a petrological, geochronological, geochemical and isotopic study of carbonatite bodies associated with orogenic peridotites, which occur as stocks or dykes with widths of tens to hundreds of meters in the Luliangshan region, North Qaidam, northern Tibet, China. On the basis of modal olivine (Ol) content, the studied samples were subdivided into two groups: Ol-poor carbonatite and Ol-rich carbonatite. Zircon grains from the Ol-poor carbonatite show detrital features, and yield a wide age spectrum between 400 Ma and 1000 Ma with a pronounced peak at ca. 410–430 Ma. By contrast, oscillatory zoned zircons and inherited cores show two relatively small Neoproterozoic age peaks at ca. 920 and 830 Ma. Zircon grains from the Ol-rich carbonatite sample are also distributed in a wide spectrum between 400 and 1000 Ma, with a pronounced peak at ca. 440 Ma and a slightly inferior peak at ca. 410 Ma. The oscillatory zoned zircons and inherited cores exhibit a smaller Neoproterozoic age peak at ca. 740 Ma. The pronounced peaks ranging from 430 to 410 Ma are consistent with the deep subduction and mantle metasomatic events recorded in associated ultramafic rocks. Both groups of carbonatites are characterized by enrichment of light rare earth elements (LREEs) with high (La/Yb) N values and pronounced negative Eu anomalies. They show high 87Sr/86Sr values (0.708156–0.709004), low 143Nd/144Nd values (0.511932–0.512013) and high δ18O V-SMOW values (+17.9 to +21.3‰). This geochemical and isotopic evidence suggests that these carbonatites were derived from remobilized sedimentary carbonate rocks. We propose that the primary carbonatite magma was formed by partial melting of sedimentary carbonates with mantle contributions. Sedimentary carbonates were subducted into the shallow upper mantle where they melted and formed diapirs that moved upwards through the hot mantle wedge. The case presented provides a rare example of carbonatite originating from sedimentary carbonates with mantle contributions and relevant information on the mantle metasomatism within a subduction zone. Highlights • Geochemistry data indicate a sedimentary carbonate origin with mantle contribution. • Sedimentary carbonate formed diapirs that moved upwards through the mantle wedge. • The carbonatitic melt reacted with mantle peridotite to form the carbonatites. [ABSTRACT FROM AUTHOR]

In the northern West Qinling orogen (WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma (MSWD=0.88) and 433±4 Ma (MSWD=0.27), respectively. Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups: 424±3 Ma (MSWD=0.45) and 402±3 Ma (MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma (MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event. [ABSTRACT FROM AUTHOR]

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint. Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al 2 O 3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens. • The U-Pb dating of metamorphic zircons at ca. 500 Ma represented the HP-UHT stage. • The distinct metamorphic events were documented by Y+REEs concentrations in the garnet grain. • REE-based thermobarometer suggests the granulitized kyanite eclogite has recorded the UHP metamorphism. • A tectonic model of subduction-relamination is proposed for the geodynamic evolution of the granulite-peridotite unit. [ABSTRACT FROM AUTHOR]

Plagioclase-rich leucosomes and K-feldspar--rich veins are widely distributed within retrogressed eclogite in the Xitieshan area of the North Qaidam ultrahigh-pressure metamorphic terrane, western China. In this contribution, a combined study of petrology, zircon internal structure, zircon U-Pb age, zircon trace-element composition, whole-rock geochemistry, and Sr-Nd isotopes in plagioclase-rich leucosomes and K-feldspar--rich veins provides insight into the nature and timing of partial melting in these rocks. Petrological evidence for partial melting is provided by elongated, highly cuspate feldspars, quartz along grain boundaries (e.g., muscovite, garnet, and clinopyroxene), and felsic veinlets composed of quartz and feldspar along the boundaries of garnet, clinopyroxene, and clinopyroxene-plagioclase symplectite in the metabasite. Whole-rock geochemistry suggests that the plagioclase-rich leucosomes have lower contents of both rare earth elements (REEs) and high field strength elements (HFSEs) but higher large ion lithophile elements (LILEs; e.g., Rb, Ba, K, Sr, and Pb) than the metabasite hosts. The plagioclase-rich leucosomes may be divided into two subgroups according to their distinct REE patterns: (1) higher total REE content with or without weak negative Eu anomalies, and (2) lower total REE content with conspicuous positive Eu anomalies. The Eu-rich group also shows higher Sr content than those in the Eu-poor group. However, the K-feldspar--rich veins show higher REE, HFSE, and LILE concentrations than the plagioclase-rich leucosomes and metabasites. The inherited cores of K-feldspar--rich veins are of typical magmatic origin (oscillatory zoning with high Th/U ratios, enriched heavy [H] REEs, and negative Eu anomalies), yielding 206Pb/238U ages ranging from 904 ± 8 Ma to 915 ± 28 Ma. The zircon cores of plagioclase-rich leucosomes and mantles of K-feldspar--rich veins exhibit no zoning or weak zoning, with flat HREE patterns and no negative Eu anomalies, and they contain mineral inclusions of garnet and clinopyroxene, implying that these zircon domains were formed during an episode of eclogite-facies metamorphism with 206Pb/238U ages ranging from 444 ± 10 Ma to 452 ± 9 Ma. Internal textures and mineral inclusions (quartz, plagioclase, and K-feldspar) and trace-element systematics (steep HREE patterns with negative Eu anomalies and low Th/U ratios) of the zircon rims from plagioclase-rich leucosomes and K-feldspar--rich veins are similar to anatectic zircon. These anatectic zircon domains yielded 206Pb/238U ages ranging from 430 ± 10 Ma to 440 ± 12 Ma, and 406 ± 12 Ma to 430 ± 13 Ma, respectively. An integrated study of petrology, geochronology, and geochemistry demonstrated that the metabasites of the North Qaidam ultrahigh-pressure terrane likely experienced initial partial melting with plagioclase-rich leucosome formation under eclogite-facies conditions and partial melt crystallization during the granulite-facies stage, triggered by dehydration melting involving zoisite and rare muscovite. The K-feldspar--rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses. [ABSTRACT FROM AUTHOR]

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian. [ABSTRACT FROM AUTHOR]