Episodic crustal growth and reworking of the Yudongzi terrane, South China: Constraints from the Archean TTGs and potassic granites and Paleoproterozoic amphibolites

Abstract

The Yudongzi terrane is featured by well-preserved lithologic outcrops of basement rocks, including the ~2.7 Ga tonalite-trondhjemite-granodiorite (TTGs), ~2.5 Ga potassic granites, Neoarchean supracrustal rocks and ~1.85 Ga amphibolites. The TTG rocks contain ~2.8 Ga inherited/xenocrystic zircons, which indicates the existence of ancient rocks in the magma source. Most zircon grains in the TTG rocks have chondrite-like Hf isotopic compositions, indicative of derivation of magmas from the Paleo-Mesoarchean (3.3–3.0 Ga) crustal sources, consistent with their whole-rock Nd model ages (3.3–2.9 Ga). Zircon oxygen isotopic compositions (5.4 to 6.1‰) are mainly in equilibrium with the mantle, indicating insignificant surficial processes for the source rocks. Thus, the Archean TTGs likely originated from melting of an overthickened oceanic island arc where shallow subduction occurred. The compositional diversity of the TTG rocks can be ascribed to varying residual minerals after melting of mafic precursors at different crustal depths. The subsequent intracrustal melting of the TTG-dominated proto-crusts formed more potassic granites at ~2.5 Ga, probably associated with dense residue delamination and concomitant mantle upwelling. Detrital zircon age pattern and geochemical features of the meta-sediments from the Yudongzi Group demonstrate that the adjacent meta-volcanic components are the major sedimentary source, and thus they were proximal erosion and deposition products of an arc system within a proto-subduction zone. Protolith Hf isotope signature in metamorphic zircons of the amphibolites provides evidence for the existence of early Archean basaltic proto-arc crusts which were metamorphosed at ~1.85 Ga. Collectively, the above-stated Neoarchean to Paleoproterozoic tectonothermal events imply significant crustal growth and reworking, which were coincident with global-scale continental lithospheric development and stabilization during assembly and dispersal of supercontinents Kenorland and Columbia. Similar tectonostratigraphic, paleomagnetic and geochronologic records support a Neoarchean-Paleoproterozoic connection of the Yangtze Block with the circum-N Laurentia and southern Siberia. Furthermore, the late Archean geochemical transition from sodic TTG to potassic crust record, as a sign of cratonic maturation, is collectively related to the kinematic adjustment of supercontinent process and thermal evolution of the Earth system, therefore sheding lights on subduction tectonic operation and varied crustal growth mechanism. © 2018 Elsevier B.V.