Petrogenesis of the Early Permian volcanic rocks in the Chinese South Tianshan: Implications for crustal growth in the Central Asian Orogenic Belt

• Eruption of volcanic flows dated as Early Permian (ca. 285 Ma).
• Their genesis is unrelated to the Permian Tarim plume.
• Difference in lithospheric thickness related to oblique collision
• The Late Paleozoic magmatic suite is associated with post-collisional extension.
• Magmatism contributed to vertical continental growth in CAOB.

The Paleozoic and Early Mesozoic magmatic suites in the Central Asian Orogenic Belt (CAOB) provide important insights on the crustal growth and reworking process associated with the construction of the largest Phanerozoic orogen on the Earth. Among the tectonic blocks of the CAOB, the South Tianshan Terrane (STT) occupies the southwestern margin and is located adjacent to the Tarim Craton. Here we investigate the Early Permian Xiaotikanlike Formation in the central part of the Chinese STT in Xinjiang in Northwest China. The formation is composed of a series of terrestrial volcanic lava flows and volcanic breccia, interbedded with siltstones, sandstones and sandy conglomerates. Zircon U–Pb and Lu–Hf isotopic analysis, whole-rock major oxide, trace element and Sr–Nd isotopic data are presented for the volcanic lava flows of the Xiaotikanlike Formation exposed in the Boziguo'er, Laohutai and Wensu regions. The new zircon ages from our study, together with those reported in previous investigations on the rhyolitic lava flow from the Wensu region, suggest that the volcanic rocks of the Xiaotikanlike Formation simultaneously erupted at ca. 285 Ma. The lavas of the formation show a wide range of SiO2 (49.88 to 78.56 wt.%). The basaltic rocks show SiO2 from 49.88 to 53.78 wt.%, MgO from 3.73 to 7.01 wt.% and Mg# from 41 to 61. They possess slightly enriched Sr–Nd isotope signature [(87Sr/86Sr)t = 0.70495–0.70624 and εNd(t) = − 0.5 to + 0.6], and have trace and rare earth element patterns similar to those of oceanic island basalts (OIBs). Petrographic and whole-rock chemical characteristics indicate that the basaltic lava flows are dominantly tholeiitic, and were likely derived from a spinel-dominated peridotite asthenospheric mantle source. The felsic lavas of the Xiaotikanlike Formation show SiO2 in the range of 60.71 to 78.56 wt.% and display overall similar immobile element pattern characterized by notable troughs at Nb–Ta, P and Ti and gently sloping REEs. Zircon Lu–Hf analysis yields εHf(t) values of − 8.7 to − 0.3 for the felsic lavas from the Boziguo'er region. Geochemical and isotopic data suggest that the felsic lava flows were likely derived from an ancient crustal source(s). Our study suggests that the Xiaotikanlike volcanic lava flows erupted after the collision between the Tarim Craton and the Kazakhstan–Yili–Central Tianshan Terrane, and that the South Tianshan Terrane was not affected by Permian Tarim mantle plume activities. Furthermore, our data also suggest that the lithosphere under the eastern part of the STT at ~ 285 Ma had been considerably thinned, whereas the lithosphere under the western part of STT was still thick enough to allow the presence of a garnet-bearing source in the lower lithospheric mantle. Such distinction in the lithospheric thickness may be attributed to the oblique collisional tectonics. The widespread Late Paleozoic igneous rocks from the Western Tianshan were genetically associated with regional-scale post-collisional extension. These rocks provide robust evidence for net vertical continental growth at the final stage of evolution of the CAOB.

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