Early Paleozoic accretionary orogenesis along northern margin of Gondwana constrained by high-Mg metaigneous rocks, SW Yunnan.

SW Yunnan of China constituted part of the northern margin of Gondwana facing the proto-Tethys ocean in the early Paleozoic. However, the evolution of the region and its relationship with the accretionary orogenism have been poorly established. This paper reports a set of new zircon U-Pb age data and whole-rock major oxides, elemental and Sr-Nd isotopic data for early Paleozoic metavolcanic rocks from the previously defined Lancang Group and reveals the development of an Ordovician suprasubduction zone in SW Yunnan. Zircon U-Pb ages of 462 ± 6 and 454 ± 27 Ma for two representative samples indicate eruption of the volcanic rocks in the Late Ordovician. Geochemical data for the metavolcanic rocks together with other available data indicate a calc-alkaline affinity with high AlO (13.04-18.77 wt%) and low TiO (0.64-1.00 wt%). They have Mg-numbers ranging from 62 to 50 with SiO of 53.57-69.10 wt%, compositionally corresponding to the high-Mg andesitic rocks. They display enrichments in LREEs and LILEs with significant Eu negative anomalies ( δEu = 0.20-0.33), and depletions in HFSEs, similar to arc volcanic rocks. Their initial Sr/Sr ratios range from 0.721356 to 0.722521 and εNd( t) values from −7.63 to −7.62 with Nd model ages of 2.06-2.10 Ga. Integration of ages and geochemical data with available geological observations, we propose the presence of Ordovician magmatism related to proto-Tethyan evolution in SW Yunnan and the metaigneous rocks formed in an island-arc setting. They were part of a regional accretionary orogen that extended along the northern margin of Gondwana during Neoproterozoic to early Paleozoic period. [ABSTRACT FROM AUTHOR]