Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for Mesoarchean Continental Crustal evolution in Kongling High-grade Metamorphic Terrane, Yangtze Craton, China

Abstract

The origin and significance of the tonalite–trondhjemite–granodiorite (TTG) units in the Yangtze Craton, China, and the metabasite xenoliths they host, remain controversial, and resolving these issues is important if we are to understand the geodynamics of the early Yangtze Craton. We have discovered many biotite–tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U–Pb dating of their zircons yielded 207Pb/206Pb ages of ca. 3.00 Ga, which provides a minimum age for the formation of the pre-metamorphic basic igneous rock. The host TTGs and late intrusive granitic dikes yield three groups of upper intercept ages at 2.87–2.88, 2.91–2.94, and 3.07 Ga, and a concordant age at 2.94 Ga, which suggest that the Yangtze continental nucleus underwent three important metamorphic–magmatic events in the Mesoarchean at ca. 3.00, 2.94, and 2.87 Ga. The biotite–tremolite schists have high ratios of K2O/Na2O and high contents of CaO, Cr, and Ni, thus showing the characteristics of high-K calc-alkaline island-arc volcanic rocks (basalt–andesite) that form by the partial melting of subducted oceanic crust. The data also provide further proof that a Mesoarchean metamorphic basement exists in the Yangtze Plate. Derivation of the magmatic protoliths of the biotite–tremolite schist enclaves from an oceanic crust during slab subduction, and the presence of these xenoliths within the TTG suite, indicate the existence of a Mesoarchean granite–greenstone belt in the Kongling area. The dikes of alkali granite might also be related to this oceanic plate subduction and the initiation of plate tectonics during the Mesoarchean (≤2.94 Ga).