查看更多>>摘要：Whether the voluminous Early Cretaceous magmatism in the Tengchong and Baoshan blocks, western Yunnan, China, was related to subduction of the Meso-Tethys or the Neo-Tethys is controversial. Here, we report SHRIMP and LA-ICP-MS U—Pb zircon ages, bulk-rock major and trace elements, and Sr—Nd isotope data from volcanic rocks in the Lameng area of the Baoshan block, SW China, investigate their genesis, and discuss the evolution of the Tengchong and Baoshan blocks. These rocks yield U—Pb zircon ages of 120.3 ± 1.7 and 120.8 ± 1.4 Ma and constitute a bimodal volcanic suite (group 1 and group 2). Group 1 samples show alkaline basaltic signatures (SiO2 = 46.2-53.1 wt.%), enrichments in Zr, Hf, Ti, and light rare earth elements, and slight depletions in Nb, Ta, and heavy rare earth elements. They have ε_(Nd)(t) =-1.0-+3.7 and initial 87Sr/86Sr = 0.70503-0.70709. Group 2 samples display higher silica contents (SiO2 = 61.7-65.0 wt.%) and evolved ε_(Nd)(t) (-4.3 to-3.6) and initial 87Sr/86Sr (0.70653-0.70678) values. These features suggest that the group 1 melts were derived from an asthenospheric mantle source mixed with subcontinental lithospheric mantle components and that the group 2 samples were produced by partial melting of the lower crust in a continental back-arc setting. From an affinity with widespread coeval magmatic rocks in the Tengchong and Lhasa blocks, we suggest that the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks had closed by 120 Ma and that the Early Cretaceous magmatism in the Tengchong and Baoshan blocks formed in an arc-back-arc system associated with Neo-Tethyan slab rollback.
查看更多>>摘要：Fluid inclusions trapped in high-grade metamorphic minerals offer direct evidence for the nature and role of fluids involved in lower crustal processes. Here, we report extremely high-density (—1.155 g/cm3) carbonic fluid inclusions preserved within garnet grains in metagabbros from the central segment of the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The metagabbros are composed of coarse-grained garnet, clinopyroxene, orthopyroxene, plagioclase, quartz, and retrograde calcic amphibole. Garnet occurs either as porphyroblastic grains formed during prograde to peak stage, or as fine-grained aggregates around plagioclase possibly formed by post-peak decompressional (near-isobaric) cooling. The peak and retrograde P-T conditions have been estimated as —940 °C/~11.5 kbar and — 840-850 °C/~9.3-10.2 kbar, respectively, based on phase equilibria modelling using pseudosection computation. Fluid inclusions occur as primary and secondary phases within the porphyroblastic garnet and matrix plagioclase. The melting temperatures of these fluid inclusions are in the range of-57.4 to-56.6 °C, indicating nearly pure CO2 composition. The lowest homogenization temperature from the primary inclusions in garnet (-50.1 °C) translates to extremely high density of 1.155 g/cm3. The calculated isochores for the inclusions (—11.3 kbar at 950 °C) are broadly consistent with the peak P-T conditions, suggesting that the fluids are trapped near the peak metamorphic stage under a dry condition. The preservation of ultrahigh-density carbonic inclusions is probably due to isochore-parallel decompressional cooling after the peak metamorphism. The secondary inclusions show slightly lower densities of 1.014-1.053 g/cm3 (in garnet) and 0.955-0.976 g/cm3 (in plagioclase), the isochores of which suggest a lower-pressure range of 4.8-5.8 kbar at 800 °C. The high-density pure CO2 fluids reported in this study as well as in previous studies from the PCSZ, and the isochoric evolution provide robust evidence for the presence of supercritical carbonic fluids in the lower continental crust.
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