Petrogenesis And Metamorphism Of Paleoproterozoic Amphibolites From The Quanji Massif, Northwestern China And Tectonic Implications

This thesis carries a study on petrogenesis and metamorphism of the garnet amphibolites garnet-free amphibolites from the Quanji Massif, Northwestern China. The new data provide further constraints on evolution of the early history of the Quanji Massif and its relationship with the continental blocks in NW China and the global Columbia Supercontinent. The garnet amphibolites in the Wulan region occur as lenses mafic dykes within the paragneisses of the Lower Dakendaban sub-Group composed of the Cpx-bearing and the Cpx-free garnet amphibolites. These rocks are of ortho-amphibolite with protoliths of basaltic dykes; a whole-rock Sm-Nd dating yielded first time an age of 2364 ±165 Ma and suggested emplacement age for the precursor magmas of their protoliths. The precursor magmas show sub-alkaline basalt series derived from mixed sources composed of highly depleted young (Early Paleoproterozoic) mantle and late Neoarchean sub-continental lithospheric mantle that had been metasomatized by subduction-induced fluids, and were emplaced in an arc-related tectonic setting. The amphibolites in the Delingha region occur also as lenticular mass or dykes within the Upper Delingha sub-Group, and the precursor magmas can broadly be constrained to form during 1.92-1.84 Ga. Their magmas were sourced from subduction-induced fluid metasomatized Archean sub-continental lithospheric mantle, emplaced in volcanic arc-related tectonic setting.Comprehensive study revealed two episodes of regional metamorphism in Quanji Massif during the early Paleoproterozoic period. The first metamorphism occurred at ～1.96 Ga. It developed three mineral generations in the garnet amphibolites in the Wulan region, with P-T conditions of～600 C at 6 to 8 kbar,～634-731 C and 11.2-11.8 Kbar and ～663-689 C and ～7.1-8.5 Kbar for the first, second and third generations, respectively. These sequential P-T conditions for the mineral generations defined a suite of clockwise P-T paths and a remarkable near isothermal decompression (ITD) from～12 Kbar (Pmax) to～8.5 Kbar, followed by a retrograde process during cooling, indicating a metamorphism in continent-continent collision environment. The second metamorphism firstly occurred as a low-P/T type in arc-related tectonic setting at～1.84 Ga (1835 ±17 Ma) for the garnet-free amphibolites in Delingha region, and a medium-P/T type metamorphism for both amphibolites in the Wulan and Delingha regions at～1.82 Ga, with also a clockwise P-T path documented by the garnet amphibolites in Wulan region, which can be linked to arc-continent or continent-continent collisions.The new data of this thesis provide further evidences and supply new approval for a prolonged (～2.4 -1.8 Ga) subduction-accretion-collision process history for amalgamation of the Quanji Massif with the continental blocks in NW China, in response to the assembly of the global Columbia Supercontinent during the Paleoproterozoic period.