Tectono-magmatic evolution of the western Yangtze Block, South China

Abstract

The South China Block (SCB) consists of the Yangtze and Cathaysia blocks that amalgamated in the early-middle Neoproterozoic time. Crustal generation and reworking of South China are closely associated with the global supercontinent cycle and global orogenic activities, although the paleo-geographic position of South China with respect to Rodinia and Gondwana supercontinents are still issues of hot debate. This project attempts to address this issue by conducting an intergrated study of the basement nature, provenance analysis using detrital zircon data, orogenic history constrained by geological and geochronological data in order to unravel a spatial and temporal coupling between tectono-magmatic evolution of the western Yangtze Block and the dispersal and assembly processes of the Rodinia and Gondwana supercontinent. The widely distributed Neoproterozoic magmatism along the western periphery of the Yangtze Block is important in understanding the early tectonic evolution of South China. The petrogenesis of adakitic tonalites and high-K granites indicates an eastward subduction system with young and hot slab initially operated at ~860 Ma, and the local compressional regime switched to extensional setting at ~830 Ma. This is consistent with information from the interior and eastern Yangtze Block. Such tectonic transition was possibly caused by slab retreat. The Neoproterozoic subduction process along the west Yangtze Block of South China is much younger than the global Grenvillian orogeny, which supports a marginal position of South China with the Rodinia supercontinent at that period. Our zircon U-Pb-Hf isotopic data for the late Neoproterozoic (Sinian) siliciclastic-volcanic rocks of the western margin of the Yangtze Block show a dominant U-Pb age range of 720-800 Ma and positive εHf(t) values, which suggest proximal sedimentation with arc affinity. Geochemical data further constrain continental-arc dominated deposition in the Neoproterozoic along the western margin of the Yangtze Block. The age patterns of detrital zircons from Cambrian to Devonian sedimentary sequences record a provenance change in early Cambrian from a single age source (540-570 Ma) to Devonian from a multiple age source (2.4-2.6 Ga, 1.75-1.95 Ga, 0.9-1.0 Ga, 730-870 Ma, and 500-680 Ma), indicating increase of ancient materials in the source. Combined with regional unconformity between the Cambrian and Ordovician strata and the Hf isotopic evolution, we interpret that the abundant 540-570 Ma zircons in the Cambrian strata were sourced from an early Paleozoic subduction-accretionary prism, similar to the Bhimphedian orogeny along the north margin of Gondwana, whereas the zircon detritus of Grenvillian and Pan-African ages with more variable εHf(t) values in the upper Cambrian and Silurian-Devonian sediments came from an exotic input. Their provenance analysis suggests a long-term connection of South China-north India-Himalaya as well as a large similar-sourced drainage area until the dispersal of Gondwana in the late Devonian. Geochemical, isotopic and geochronological data constrain that the Tagong granitoids and their hosted microgranular mafic enclaves (MMEs), in the Songpan-Ganze terrane (SGT) to the west Yangtze Block, were coevally emplaced at ca. 208-209 Ma, and they were originated from different magma sources but underwent a mafic-felsic magma mixing process in a continental-arc setting. This mixing mechanism for its formation was associated with the slab break-off of subduction of the Paleo-Tethys, marking the last episode of orogenic magmatism in this region (522 words).