Petrogenesis of Carboniferous ultradeep volcanic rocks in the central Junggar Basin: Constraints from zircon U-Pb geochronology and geochemical compositions

As a key geologic archive on the southwestern margin of the Central Asian orogenic belt, the Junggar Basin offers critical insights into the evolution of the Paleo-Asian Ocean and associated crustal growth processes. It also represents a primary target for China’s deep to ultradeep hydrocarbon exploration. This study presents an integrated investigation of Carboniferous volcanic rocks from the newly drilled ultradeep exploration well JG1 in the central Junggar Basin, combining zircon U-Pb geochronology, petrology, and geochemistry. The primary objective was to place robust constraints on the Carboniferous tectonic evolution of the Junggar Basin and to advance the exploration and development of ultradeep hydrocarbon reservoirs hosted in volcanic successions. Well−seismic correlation revealed two distinct shield volcanic edifices with multiple eruptive vents. Basalt and basaltic andesite dominate the early and late Carboniferous volcanic suites cored in well JG1. The early Carboniferous basalt samples yielded zircon U-Pb ages of 335 ± 3 Ma, while the late Carboniferous basaltic andesite samples yielded ages of 305 ± 2 Ma. Both volcanic rocks exhibit relative enrichment in light rare earth elements and Th, and significant depletion in Nb, Ta, and Ti, similar to those of mafic rocks derived from subduction-modified lithospheric mantle. The εNd(t) values of the early Carboniferous and late Carboniferous volcanic rocks are +6.9 to +8.0 and +6.7 to +7.0, respectively, indicating that they originated from a relatively depleted mantle. Both suites of samples have relatively constant Th/Nb and Th/Yb ratios and large variations in Ba/Th and Ba/La ratios, suggesting the involvement of aqueous fluids. Zircon Hf isotopic compositions and binary mixing modeling further indicate that the mantle sources for the late Carboniferous volcanic rocks experienced a greater degree of subduction-related metasomatism than those for early Carboniferous volcanics. In addition, both suites of volcanic rocks have relatively low Zn/Fet and Sm/Yb ratios, further indicating a mantle source dominated by spinel + minor garnet lherzolite. Synthesizing regional multidisciplinary datasets, we propose two distinct tectonic regimes for Carboniferous volcanic activity in the Junggar region: (1) Early Carboniferous magmatism developed in a continental arc environment driven by the northward subduction of the North Tianshan Ocean and (2) late Carboniferous volcanism formed in a continental intra-arc extensional environment triggered by slab rollback.