Two contrasting late Paleozoic magmatic episodes in the northwestern Chinese Tianshan Belt, NW China: Implication for tectonic transition from plate convergence to intra-plate adjustment during accretionary orogenesis

Abstract

Late Carboniferous to Early Permian is a critical period for the final amalgamation of the Central Asian Orogenic Belt (CAOB). However, as most of the accreted terranes of the CAOB are unclear in tectonic nature and origin, the timing and processes of their mutual amalgamation have been poorly constrained. To understand assembly of the West Junggar Terrane with the Yili Block, a suite of the late Paleozoic magmatic rocks, including ignimbrite, rhyolite and granite, in northwestern Chinese Tianshan Belt were studied for their petrogenesis and tectonic implications. Our new results of secondary ion mass spectrometry (SIMS) zircon U-Pb dating reveal two separate magmatic episodes, ca. 300Ma volcanism (ignimbrite and rhyolite) and ca. 288Ma plutonsim (biotite granite). Geochemically, for the ca. 300Ma volcanism, the ignimbrites have low SiO2 (65.8–71.5wt.%) and Mg# (6–13) values, and exhibit arc affinity with significantly enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE) such as Nb, Ta and Ti. The whole-rock εNd(t) and zircon εHf(t) values range from +6.9 to +7.0 and +9.9 to +14.1 respectively, indicating a juvenile basaltic lower crustal origin. Rhyolites have slightly high SiO2 (72.7–74.0wt.%) and K2O (3.86–4.53wt.%) contents, high zircon δ18O (11.67–13.23‰) values, and low whole-rock εNd(t) (+2.9 to +3.8) and zircon εHf(t) (+2.8 to +10.0) values, which may suggest sediment involvements during magma generation. In contrast, for the ca. 288Ma plutonism, the biotite granites have obviously higher SiO2 (74.7–75.5wt.%) contents and whole-rock εNd(t) (+7.7 to +8.8), zircon εHf(t) (+9.8 to +12.7), and lower zircon δ18O (5.99–6.84‰) values, than those of the ca. 300Ma volcanic rocks, which are consistent with signatures of juvenile magma source. According to our estimates of zircon saturation temperatures, together with their contrasting genesis, we attribute the formation of ca. 300Ma high temperature (815–938°C) volcanism to oceanic slab break-off during assembly of the West Junggar Terrane with the Yili Block, and relate the generation of ca. 288Ma low temperature (723–735°C) plutonism to subsequent strike-slipping of North Tianshan Fault that facilitated introduction of water-fluxes triggering hydrous partial melting of juvenile lower crust. The sequential magmatic episodes in the northwestern Chinese Tianshan Belt may provide a crucial clue to a tectonic transition from plate convergence to intra-plate adjustment during the formation of the Kazakhstan Orocline in the late Paleozoic.