Abstract

The age of the Kalamaili orogenic belt, marking the final amalgamation in East Junggar, North Xinjiang, is significant for the reconstruction of Paleozoic evolution of the southern Central Asian orogenic belt. The Tamugang and Songkarsu Formations of terrestrial molasse in the southeastern part of the Kalamaili belt, shed from the rising Kalamaili orogen, record the orogenic history. The strata consist of proximal conglomerate thinning to distal fine-grained sandstone and mudstone. Poorly sorted conglomerate is composed of dominant pyroclastic rocks with lesser andesitic, granitic, and ophiolitic clasts. Imbricated clasts indicate that the paleocurrents were directed to the present-day southwest to west-southwest. Laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) detrital zircon U-Pb dating of sandstones from both formations confirms that the Yemaquan arc northeast of the Kalamaili orogenic belt was the main source. Two granitic cobbles with zircon sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of 432.2 ± 7.8 Ma and 428.1 ± 6.8 Ma indicate the presence of Silurian magmatism in the Yemaquan arc.

The SHRIMP U-Pb age of volcanic rocks from the Batamayineishan Formation, which overlies the molasse on both sides of the Kalamaili belt, is 349.5 ± 6.0 Ma. The depositional age of molasse is confined to between 343.5 Ma and 345 Ma, based on the 2σ range of possible ages for the youngest detrital zircons and the overlying volcanic rocks. Combined with the previously dated plagiogranite and biostratigraphic ages on chert in the Kalamaili ophiolite as the lower age limit, the Kalamaili collision is restricted to 373.8–343.5 Ma, taking into account 2σ error, suggesting that the termination of Kalamaili paleo-ocean subduction and the final amalgamation in East Junggar occurred before the Visean.

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