Triassic collision in the Paleo-Tethys Ocean constrained by volcanic activity in SW China

Abstract

A suite of collision-related Triassic volcanic rocks cropping out within the Jinshajiang–Ailaoshan orogenic belt in SW China offers insights into closure of the Paleo-Tethys and associated terrane/continent collision. Combined geochemical and Sr–Nd isotopic data for samples from the Jinshajiang segment demonstrate that the lower part of the volcanic suite is dominated by high-Si rhyolites (the Pantiange Formation) derived through crustal anatexis; whereas the upper units (the Cuiyibi Formation) are characterized by basalts alternating with intermediate-felsic lavas that share similar Sr–Nd isotopic signatures, and may have originated from partial melting of subduction-enriched lithospheric mantle. SHRIMP U–Pb analyses on zircons yield ages of 247–246 Ma for the rhyolites of the Pantiange Formation, and ages ranging from ca. 245 Ma to 237 Ma for the basaltic and intermediate-felsic samples from the overlying Cuiyibi Formation. These data, integrated with other geologic evidence (e.g., stratigraphic record), suggest an Early-Middle Triassic timing of terrane–continent collision along the Jinshajiang and Ailaoshan sutures with the 247–246 Ma Pantiange high-Si rhyolites representing the early magmatic products generated by this event. The development of the 245–237 Ma bimodal volcanism was associated with subsidence and deep-marine sedimentation.We interpret this as reflective of an extensional setting within an evolving collisional orogen, probably related to oblique convergence along the collisional zone. Triassic volcanic rocks with similar geochemical signatures and ages have been identified along the > 1000 km length of the Jinshajiang–Ailaoshan suture zone. The Indochina block to the south of the suture records 250–240 Ma tectonothermal activity. Taken together, these relationships suggest that initial collision and amalgamation of the Qamdo–Simao terrane (Indochina) with the Yangtze Block (South China) along the Jinshajiang–Ailaoshan and Song Ma sutures probably took place in the Early Triassic following consumption of the Paleo-Tethys Ocean.