Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean Pb-206/U-238 age of 110.6 +/- 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean Pb-206/U-238 age of 106.9 +/- 3.3 Ma (the top strata of section A) and 120.5 +/- 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1 degrees, I = 33.1 degrees with alpha(95) = 2.3., corresponding to a paleopole at 75.2 degrees N, 278.1 degrees E with dp/dm = 1.5 degrees /2.6 degrees and a paleolatitude of 18.1 degrees +/- 1.5 degrees N for the study area (32.41 degrees N, 83.39 degrees E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5 degrees, I = 48.5 degrees with alpha(95) = 2.0 degrees, corresponding to a paleopole at 38.7 degrees N, 159.6 degrees E with dp/dm = 1.7 degrees/2.6 degrees and a paleolatitude of 29.5 degrees +/- 1.7 degrees N for the study area (32.41 degrees N, 83.39 degrees E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7 degrees +/- 1.6 degrees and corresponding paleolatitude of 24.0 degrees +/- 1.2 degrees N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 +/- 3.1 degrees and corresponding paleolatitude of 22.5. +/- 2.2 degrees N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous.

Automated summary

By studying the Risong Formation redbeds and volcanic rocks in the western Lhasa terrane, it was determined that the Risong Formation redbeds were formed during the Early Cretaceous, contrary to previous assignments to the Late Jurassic. Through paleomagnetic records and geochronological analysis, it was confirmed that the Lhasa-Qiangtang collision occurred as early as the Early Cretaceous.