Geochronology and geochemistry of Carboniferous metabasalts in eastern Tianshan, Central Asia: evidence of a back-arc basin

摘要

Carboniferous volcanic rocks are widely distributed in the Jueluotage belt of eastern Tianshan, located in the southern Central Asian Orogenic belt. The petrogenesis and tectonic history of this region have long been a matter of debate. These volcanic rocks erupted in two episodes, with zircon U-Pb ages of ca. 336Ma and ca. 320Ma. The ca. 336Ma volcanic rocks are dominated by arc-like igneous rocks. They show enrichment of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depletion of high field-strength elements (HFSEs) according to the N-MORB-normalized and chondrite-normalized diagrams. However, the ca. 320Ma volcanic rocks mainly consist of MORB-like igneous rocks that show nearly flat REE patterns. They are also slightly enriched in LILEs and LREEs according to the N-MORB-normalized and chondrite-normalized diagrams. These Carboniferous volcanic rocks are geochemically similar to the back-arc basin basalt (BABB) from the Mariana and Okinawa troughs. The inherited zircon ages of volcanic rocks from the Jueluotage belt show similar trends and features to those of the Central Tianshan Block, suggesting they shared the same basement, at least before the early Carboniferous. The evolution from arc-like volcanic rocks to MORB-like volcanic rocks reflects the development of the back-arc basin in the northern Central Tianshan Block during the Carboniferous. The Palaeo-Asian oceanic plate subducted beneath the Central Tianshan Block. The position of one of the major sutures of the Palaeo-Asian Ocean should be in the north of the Jueluotage belt. The Jueluotage belt may have the same basement as the Central Tianshan Block. The back-arc basin formed above the Central Tianshan whole' Block due to the subduction-exhumation of the Palaeo-Asian oceanic plate.