Geochemical Composition Variations and Tectonic Implications of the Baoligaomiao Formation Volcanic Rocks from the Uliastai Continental Margin, Southeast Central Asian Orogenic Belt

The Permo-Carboniferous tectonic evolution in the Uliastai continental margin (UCM), north of the southeast central Asian Orogenic Belt, remains controversial. This work examined the geochemical composition of the felsic volcanic rocks from the lower and upper part of the Baoligaomiao Formation in the UCM. Zircon U-Pb ages reveal that the Baoligaomiao Formation has a long-lived eruption duration, from ca. 285 to 328 Ma. The lower part (ca. 328-310 Ma) of the Baoligaomiao Formation is dominated by clastic and pyroclastic rocks with subordinate intermediate-felsic volcanic rocks, whereas the upper part (ca. 307-285 Ma) mainly consists of felsic volcanic rocks and pyroclastic rocks. Calculations reveal that the felsic volcanic rocks from the lower part have low zircon saturation temperatures (TZr = 747 degrees C-795 degrees C), whereas those from the upper part exhibit high TZr (ca. 793 degrees C-930 degrees C). Zircons from the lower part exhibit high epsilon Hf(t) values and 176Lu/177Hf ratios, in contrast to the low epsilon Hf(t) values and 176Lu/177Hf ratios of zircons from the upper part. Those petrogeological and geochemical shifts might support the tectonic switch model in the UCM at the end of the Carboniferous, providing new constraints on the Late Carboniferous closure of the Hegenshan Ocean.