Discovery And Zircon U-Pb Dating Of The Garnet Amphibolite In The Northern Margin Of The Central Qilian Massif And Its Geological Significance

The garnet amphibolite is discovered for the first time in the northern margin of the central Qilian massif. The metamorphic grade of garnet amphibolite is between amphibolite facies and eclogite facies. The reaserch of the garnet amphibolite in the northern margin of the central Qilian massif has important significance for the analysis of evolution model of north Qilian orogenic belt.The garnet amphibolite in the northern margin of the central Qilian massif is founded in Dayanglong and Daheicigou, which occur as lenses in fault contact with the schist of the Tuolai group complex. The wall rock of the Dayanglong garnet amphibolite consists of two-mica-quartz schist, garnet-bearing two-mica-quartz schist, amphibolite. Some light color felsic veins is founded in part of Dayanglong garnet amphibolite. The wall rock of Daheicigou garnet amphibolite consists of garnet-bearing muscovite quartz schist. The character of rock geochemistry shows that the garnet amphibolite is rich in the large-ion lithophile such as Rb, Ba, K, especially rich in Th, and poor in the contents of high field strength elements such as Nb, P, Ti, and characterized by negative Eu anomalies, and rich in LREE((La/Yb)N=1.45-2.62). The original rock of the garnet amphibolite is oceanic basalt. LA-ICP-MS zircon U Pb dating data suggest that the metamorphic age and the original rock age of the garnet amphibolite in the northern margin of the central Qilian massif are(485±22) Ma and(600-800) Ma, the original rock age range of the amphibolite between 1480 Ma and 1700 Ma, the zircon U-Pb age of the light color felsic veins which founded in part of Dayanglong garnet amphibolite is(481.6±8.7) Ma.Electron microprobe analysis shows that the schist which is the wall rock of the garnet amphibolite contains phengite. Lithological characteristics and petrography characteristics and regional contrast indicate that the possible peak metamorphic temperature-pressure condition of the garnet amphibolite in the northern margin of the central Qilian massif are T= 450-600℃, P= 0.3-0.7GPa. The character of rock geochemistry shows that the light color felsic veins which founded in part of Dayanglong garnet amphibolite is granodiorite, and it should be the production of partial melting of the garnet amphibolite during the exhumation.Based on regional geological background, this article puts forward a evolution model of the ancient Qilian ocean plate: The original rock of the garnet amphibolite in the northern margin of the central Qilian massif is oceanic crust of ancient Qilian ocean which is formed after the breakup of Rodinia. The central Qilian massif underthrusted southward beneath the Alxa massif in Caledonian period of Early Palaeozoic age, the original rock of the garnet amphibolite and part of continental crust were brought into deep crust, but not deep enough to eclogite facies metamorphism. When the underthrust stoped, the exhumation of the garnet amphibolite begin, and the prtial melting of the garnet amphibolite happened. The discovery of the garnet amphibolite in the northern margin of the central Qilian massif and the early Ordovician syn-collisional granite in the northern margin of the central Qilian massif demonstrates that the idea of the southward subduction of the tectonic polarity of the ancient Qilian ocean plate in in Caledonian period of Early Palaeozoic age.