Geochronology and Geochemistry of Archean TTG and Tremolite Schist Xenoliths in Yemadong Complex: Evidence for ≥3.0 Ga Archean Continental Crust in Kongling High-Grade Metamorphic Terrane, Yangtze Craton, China

The origin and significance of the tonalite-trondhjemite-granodiorite (TTG) units and the familiar metabasite xenoliths they host in the Yangtze Craton, China, remain controversial, and resolving these issues is important if we are to understand the evolution of the early Yangtze Craton. We focused on biotite-tremolite schist xenoliths in the Archean TTG units of the Kongling high-grade metamorphic terrane, and U-Pb dating of their zircons yielded Pb-207/Pb-206 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 the initiation of plate tectonics during the Mesoarchean (<= 2.94 Ga).