Tectonic regime transition of the western South China Block in early Cambrian: Evidence from the Meishucun volcanic ash beds

Abstract

The tectonic evolution of the South China Block, especially its western part, is ambiguous in the early Paleozoic owing to the lack of contemporary tectono-magmatic records. Thick ash beds preserved in the early Cambrian successions in the eastern Yunnan Province of the western South China Block provide new insights into their magmatic origins and tectonic evolution. We report here integrated in-situ zircon U-Pb data, Hf-O isotopes, and whole-rock geochemistry of two early Cambrian ash beds (Bed 5 and Bed 9) from the Meishucun section in the eastern Yunnan Province. SIMS zircon U-Pb data indicate the two ash layers were deposited at 533.2 ± 3.8 Ma and 526.2 ± 4.1 Ma, respectively. Zircon Hf-O isotopes and whole-rock geochemistry of the two ash beds demonstrate crustal origins of their parental magma and their close affinities to A-type granites. These features suggest that the ash beds were formed in extensional settings and the tectonic regime of the study area switched from compression to extension in the early Cambrian. The Meishucun volcanic records, in combination with other late Ediacaran–early Paleozoic magmatism along the northern margin of East Gondwana, may represent the initial opening phase of the Paleo-Tethys Ocean.