Petrogenesis and tectonic implications of early Carboniferous alkaline volcanic rocks in Karamay region of West Junggar, Northwest China

摘要

Zircon U-Pb geochronological and geochemical analyses are reported for a suite of the early Carboniferous volcanic rocks from West Junggar (Northwest China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb analysis from an andesite yielded concordant weighted mean Pb-206/U-238 age of 345 +/- 3Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The early Carboniferous volcanic rocks consist of basalt, basaltic andesite, and andesite. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with minor variations in SiO2 compositions (45.13-53.05wt.%), high concentrations of Na2O+K2O (5.08-8.89wt.%) and TiO2 (1.71-3.35wt.%), and LREE enrichment and HREE depletion ((La/Yb)(N)=7.1-12.4), with weak Eu anomalies (Eu/Eu*=0.9-1.1) and no obvious Nb, Ta, and Ti negative anomalies. These features suggest that the early Carboniferous volcanic rocks were derived from an OIB-related source that consists of oceanic lithosphere with similar to 1-3% degree partial melting of garnet lherzolite. From these observations, in combination with previous work, we conclude that the early Carboniferous alkaline volcanic rocks in Karamay region formed by upwelling of asthenospheric mantle through a slab window in a forearc setting during consumption of the West Junggar Ocean. Meanwhile, seamounts, which formed in the Late Devonian and were accreted and subducted in Karamay arc, also brought geological effects in the subduction zone.