Search

• Sedimentary rocks in the Baoshan Block contain dominantly Precambrian zircon. • This block might not be involved into orogenic events along the Gondwana margin. • Sedimentary sources were akin to old crustal rocks of Northwest India. The Baoshan Block in western Yunnan is one of the major micro-continents in the eastern Tethyan orogenic belt, but its provenance and crustal evolution remain enigmatic. Sedimentary rocks prior to an amalgamation of continental blocks recorded key information about crustal evolution and the origin of terranes and can provide vital data for understanding orogenic processes. Here, we present U–Pb ages and Hf isotopic data on detrital zircon and Nd isotopic data on whole rock for Paleozoic sedimentary rocks from the Baoshan Block to constrain their provenance and tectonic history. Crystallization ages of detrital zircon grains illustrate that the magmatic activities of the source areas occurred at ~2.5 Ga, ~1.9 Ga, and ~1.0–0.5 Ga, with a major peak occurring around 0.95 Ga. Several old zircon grains from >3.0 Ga up to ~3.8–3.7 Ga were observed. Zircon Hf isotopic data from one sample suggest that the major crustal growth occurred at ~2.1 Ga. Cambrian to Carboniferous sedimentary rocks showed similar Nd isotopic features with two–stage Nd model ages of approximately 2.1 Ga. The age distribution patterns and isotopic data suggest that detritus material in the Paleozoic sedimentary rocks was mainly derived from continental terrane similar to the Indian craton, implying that the Baoshan Block was likely adjacent to Northwest India before the collision of the Baoshan and Simao blocks. No significant inputs of the juvenile crust were documented in the Nd–Hf isotopic data, appearing to be incompatible with a magmatic arc setting for the Baoshan Block during the Paleozoic. This block might have been situated at the interior of Gondwana before rifting and drifting northwards to the Eurasia continent. [ABSTRACT FROM AUTHOR]

[Display omitted] • 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. [ABSTRACT FROM AUTHOR]