Phanerozoic Multistage Tectonic Rejuvenation of the Continental Crust of the Cathaysia Block: Insights from Structural Investigations and Combined Zircon U-Pb and Mica 40Ar/39Ar Geochronology of the Granitoids in Southern Jiangxi Province

摘要

Granitoids are the predominant geological constituents of South China and important for the understanding of the formation and evolution of continental crust. We describe field structural features of four small-scale granitic plutons and their country rocks in southern Jiangxi Province and present new zircon U-Pb and mica 40Ar/39Ar ages that are used to discuss the tectonic-magmatic evolution and thermal overprint events, that is, the tectonic rejuvenation of continental crust of the Cathaysia Block. The muscovite-bearing granite of Mashuang pluton shows sub-north-south mylonitic foliation and stretching/mineral lineation that shallowly plunges to the northeast, generally consistent with fabrics in the host rocks that are composed of widely migmatized metasediments. Zircons and muscovites yielded U-Pb and 40Ar/39Ar ages at 457 ± 4 and 199 ± 1 Ma, which are considered to represent the ages of emplacement and postmylonitic thermal overprint, respectively. The undeformed granites of Tianjiu and Junmenling plutons display massive structure, porphyritic texture bearing huge phenocrysts of feldspar, and a sharp contact with the host rocks that consist of greenschist- to amphibolite-facies metamorphic rocks and/or early Paleozoic granite. New dating results suggest that these granites formed during 235 ± 2 to 231 ± 2 Ma and cooled down at 224 ± 3 Ma. The biotite K-feldspar granite of Jitan pluton has an apparent compositional foliation in northeast-southwest direction defined by grain shape-preferred orientation of euhedral feldspar, biotite, and hornblende, such foliation is indicative of a syntectonic crystallization dominated by a southeast-northwest compressional stress field. The age of emplacement of this biotite K-feldspar granite is constrained at 158 ± 3 Ma, and the granite was reheated at 98 ± 1 Ma. On the basis of our observations and chronological data combined with previously published results, it is suggested that the continental crust of the Cathaysia Block was subjected to multistage rejuvenation during the Phanerozoic, including (1) reworking of crustal rocks induced by partial melting mainly in early Paleozoic and also in the Triassic to the Cretaceous by mixing of crustal melts with increasing involvement of juvenile materials and (2) tectonic and thermal overprints during early Paleozoic and Triassic intracontinental deformation events and Jurassic to Cretaceous magmatism.