Early Permian seafloor to continental arc magmatism in the eastern Paleo-Tethys: U–Pb age and Nd–Sr isotope data from the southern Lancangjiang zone, Yunnan, China

Abstract

Southwestern Yunnan is a key region for the understanding of the complex geological evolution of the Paleo-Tethys and the Eurasia–Gondwana collision at the end of the Paleozoic. The southern Lancangjiang zone near the city of Jinghong at the Laotian border hosts MORB-like basaltic andesites and gabbros (Nanlianshan volcano–plutonic complex). Fine-grained gabbro with a U–Pb zircon age of 292±1 Ma and εNd(t) of 5.3 is indicative of early Permian sea-floor spreading. The Jinghong area also hosts small granodiorite intrusions which have U–Pb zircon ages of 284±1 Ma to 282±1 Ma, εNd(t) of −3.1 to −3.6, and initial Sr isotopic ratios of 0.709–0.710. The chemical and isotope signature of the granodiorite is characteristic for an arc setting and is similar to arc-like andesites from the same area. Both the earlier MORB-type magmatism and the granodiorite intrusions can be correlated in time and composition with ophiolites and trondhjemites several 100 km north and point to Permian subduction of oceanic crust between the Lincang Block and the Lanping–Simao Block. The regionally distributed peraluminous Triassic Lincang granite batholith marks the closure of the Paleo-Tethys. The northern Lincang granite gave a zircon U–Pb age of 239±1 Ma, similar to the Baimaxueshan granodioritic intrusion about 250 km north of the Lincang granite, which likely represents the northward extension of the Lincang granite. Nd-model ages range from 1.7 to 2.1 Ga and point to Paleoproterozoic basement which probably represents a fragment of the Yangtze Block. Our data suggest a complex Permo-Triassic history of continental rifting, subduction and collision at the southern border of the Yangtze Block.