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The South Qinling orogen in central China hosts carbonatites occurring as stocks associated with syenites and collectively regarded as the Miaoya intrusive complex. The complex hosts economic resources of rare-earth elements (REE) and Nb. The Miaoya syenites are strongly metasomatized at the contact with the carbonatites and cross-cut by carbonate and felsic veinlets. Small oscillatory-zoned crystals of zircons from the syenites give a concordant U–Pb age of 147 ± 0.5 Ma, which differs significantly from the ages of both large magmatic zircon grains from the syenites and primary monazite from the carbonatites (766 Ma and 234 Ma, respectively). To account for the possibility that the Miaoya syenites are coeval and cogenetic with the carbonatites, the trace-element budget of both rock types was examined in detail. The Miaoya carbonatite contains primary REE-rich fluorapatite and monazite, which precipitated earlier than the rock-forming REE-poor calcite, indicating that the primary carbonatitic magma was rich in REE. The compositions of the parental syenitic and carbonatitic magmas, calculated on the basis of the trace-element composition of primary fluorapatite in the two rock suites, show that the carbonatitic magma contained higher Sr and REE (La–Tb), but lower Ba, Pb, Th, U, Nb and Ta levels in comparison with the syenitic melt. These differences are inconsistent with derivation of the Miaoya rocks from a homogeneous carbonate–silicate melt by immiscibility or crystal fractionation. It is therefore concluded that the carbonatitic magma at Miaoya was generated directly in the mantle. Emplacement of the carbonatites in the South Qinling orogen marked transition to a postorogenic regime, and was preceded by oceanic crust subduction and closure of the Mianlue Ocean in the Triassic. Our models show that melting of the Mianlue crust and up to 10 wt.% of sediments cannot produce the levels of REE enrichment observed in the Miaoya carbonatites. More complex models, involving recycling of the Mianlue oceanic crust and a REE-rich carbonate liquid from an old deep-seated mantle source are required to explain the observed trace-element characteristics of the Miaoya carbonatites. [ABSTRACT FROM AUTHOR]

Abstract: Quartz-rich veins, together with their host high-pressure (HP) to ultra-high-pressure (UHP) rocks, are ideal for deciphering the fluid–rock interaction history in subduction zones. Previous studies of samples from Dabie–Sulu have shown that most quartz-rich veins within UHP/HP eclogite were formed during the prograde or early exhumation stage of host rocks. In contrast, a combined study of petrology and U–Pb, Sm–Nd and Lu–Hf geochronologies for a quartz-rich vein and its host retrogressed eclogite from North Qinling, which is associated with a granite intrusion and obliterated with amphibolite-facies overprint, reveals that the amphibolite-facies overprint is likely due to the late regional magmatism and the quartz-rich vein might derive from the nearby granite. A pool of U–Pb analyses of zircon grains from the retrogressed eclogite reveals zircons grew over an extended period of time but with two caveats at ca. 490Ma and ca. 473Ma. Analyses of zircon grains from the quartz vein yield a weighted mean 207Pb/206Pb ages of 428±5Ma (1σ), which is indistinguishable from that of formation of the adjacent granite. Combined elemental zoning patterns in the garnet with the spherical geometry effect, the Lu–Hf age of 416±5Ma (2σ) of the retrogressed eclogite is likely a mix age of eclogite-facies and amphibolite-facies metamorphism while the garnet–amphibole Sm–Nd age of 400±8Ma (2σ) approximates amphibolite-facies retrogression metamorphism. The latter is consistent with the garnet–biotite Sm–Nd age of 393±4Ma (2σ) of the adjacent granite, suggesting contact metamorphism. The >70Myr gap between the eclogite-facies metamorphism and the amphibole-facies overprint and the consistency of the timing the amphibolite-facies overprint for the eclogite and the cooling of the magma intrusion in this area indicate that the eclogite-facies metamorphism and amphibolite-facies metamorphism might be distinct metamorphic events, and the latter might be trigged by the granite intrusion. [Copyright &y& Elsevier]