Provenance of the early Paleozoic sedimentary succession in the Lancang Block, SW China: Implications for the tectonic evolution of the northern margin of Gondwana

Abstract

• Sediments of the late Ordovician Lancang Group in the Lancang Block, SW China, were sourced mainly from the northern margin of Gondwana . • Provenance change of the sediments might reflect initial subduction of the Proto-Tethys in the Ordovician . • The Lancang Block had affinity to Gondwana during the early Paleozoic. In the present study, we report zircon U-Pb ages, Hf isotopic compositions, and geochemistry for sedimentary rocks of the early Paleozoic Lancang Group in the Lancang Block of the western Yunnan Tethyan orogenic belt, SW China, to reveal the nature of sedimentary provenance with further implication on the orogenic evolution. Detrital zircon grains yield two major U-Pb age populations at 1300–900 Ma and 600–500 Ma, while their ε Hf (t) values range from –28.7 to + 13.0 with Hf model ages mostly cluster at ∼ 2200–2100 Ma. These features on zircon U-Pb age patterns and Hf isotopic compositions are extremely similar to those of exposed rocks in North India. It is noteworthy that the sedimentary provenance had changed dramatically from the lower to the upper sequences of the Lancang Group, as highlighted by the increase of Pan-African and younger Grenvillian (1100–900 Ma) materials accompanied by the decrease of older Grenvillian (>1100 Ma) sediments. Besides, whole rock Nd model ages also increase from the basal sequences upwards. Combining these findings with those of previous studies, we propose that the Lancang Block was adjacent to the northern margin of Gondwana during the early Paleozoic with sediments of the Lancang Group dominantly sourcing from North India. The provenance change recorded in the Lancang Group might have resulted from a transition from passive to convergent continental margin initialed by the subduction of Proto-Tethys in the late Ordovician along the northern margin of Gondwana.