Bidirectional subduction of the Proto-Tethys Ocean: constraints from geochronology and geochemistry of S-type granites from Baoshan Block in western Yunnan (SW China)

Abstract

Understanding the tectonic evolution of the Proto-Tethys Ocean is important for exploring the initial assembly of the Gondwana supercontinent. The closure of the Proto-Tethys Ocean represents the end of convergence along the northern edge of Eastern Gondwana. However, the timing and processes of the closing of the ocean have different interpretations. This work focuses on the Early Paleozoic granitic rocks in the Baoshan Block, SW China to constrain the tectonic evolution of the Proto-Tethys Ocean. Zircons from the Pinghe and Zhen'an granitic plutons yield concordant U–Pb ages from 489.8 to 467.7 Ma. The bulk-rock geochemical features suggest that these samples are high-K calc-alkaline, S-type granites enriched in light rare earth elements and depleted in heavy rare earth elements. With obvious negative Eu anomalies and high K 2 O/Na 2 O ratios (1.01–2.57), they are enriched in large ion lithophile elements and depleted in high field strength elements. These rocks were therefore derived from partial crustal melting associated with subduction of the converging plate. Previous studies on the Changning–Menglian suture zone have suggested that the Early Paleozoic magmatic activity in the Baoshan Block was related to the westward subduction of the Proto-Tethys Ocean. Combined with the Early Paleozoic subduction-related magmatic activity to the east, it is therefore suggested that the Changning–Menglian Ocean experienced bidirectional subduction.