Geochemical Characteristics of Pb Isotope of High~Pressure Metamorphic Rocks and Foliated Granites from HP Unit of Tongbai~Dabie Orogenic Belt
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The amphibolite facies deformation dominates in the deformation of the ultrahigh pressure metamorphic(UHPM) rocks in the Dabie Mountains,by which the structural elements both in mecroscopic and microscopic scales were mainly produced Rb-Sr dating of the granitic gneiss in a ductile shear zone of the UHPM rocks belt leads to a whole rock muscovite isochronic age for the deformed granitic gneiss,indicating that the amphibolite facies deformation developed at around 180Ma and that the major phase of deformation of the UHPM rocks is indeed synchronous with the amphibolite facies metamorphism The age is consistent with the second quick cooling ages of the UHPM rocks,confirming that the UHPM rocks quickly exhumed from the lower crust to the middle crust at around 180Ma
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The metamorphic rocks undergoing different grade metamorphism extensively spread in the Dabie orogen.The protolith's formation time and the time of metamorphism are very important for the studying of the evolution of the Dabie orogen.This paper summarizes the published zircon U- Pb ages for metamorphic rocks from the Dabie orogen and gives a preliminary geochronologic framework of the metamorphic rocks.
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Bixiling and Shuanghe are two typical outcrop areas in the Dabie ultrahigh-pressure (UHP) metamorphic terrane. Based on field outcrop observations and petrographic and petrochemical studies, the authors find that the rock association consists predominantly of eclogite, amphibolite, gneiss and foliated garnet-bearing granite. Eclogite has the characteristics of the crustal source and is considered to form by UHP metamorphism crustal rocks in the deep interior of the mantle; amphibolite is the product of retrogressive metamorphism of eclogites during exhumation of the plate; gneiss was produced by anatexis and progressive evolution of amphibolite in an ideal environment during exhumation; and foliated garnet-bearing granite is the result of partial melting of gneiss. In conclusion, there is a reworked in-situ relationship between eclogite and gneiss (and/or foliated garnet-bearing granite).
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The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.
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