Geochronology and geochemistry of basalts from the Karamay ophiolitic mélange in West Junggar (NW China): Implications for Devonian–Carboniferous intra-oceanic accretionary tectonics of the southern Altaids

摘要

New laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon ages and geochemical data are reported for the Karamay ophiolitic mélange in the West Junggar area at the southwestern margin of the Altaids (or Central Asian orogenic belt), northwestern China. The data were used to identify the petrogenesis of the mélange, and to evaluate its tectonic significance. In addition, this study aimed to assess the possible presence of Devonian mantle plume in the region. The Karamay ophiolitic mélange crops out at the boundary of the Junggar Basin but is partly hidden by Mesozoic sediments, with an exposure area of ∼40 km2. The site contains ultramafic rocks, cumulates, gabbros, pillow lavas, abyssal radiolarian cherts, turbidites, and tuffs, which show typical block-in-matrix structures. Zircon U-Pb analyses from the basalt and gabbro by LA-ICP-MS yielded weighted mean ages of 395 ± 3 Ma and 387 ± 8 Ma, respectively. These ages suggest a Middle Devonian emplacement. All basalts bear the signature of ocean-island basalt (OIB) and are characterized by alkaline compositions with high concentrations of Na2O + K2O (3.7–8.5 wt%) and TiO2 (1.5–3.1 wt%); large ion lithophile element and light rare earth element enrichment and heavy rare earth element depletion; very weak or no Eu anomalies (Eu/Eu* = 0.9–1.0); and no obvious Nb, Ta, or Ti negative anomalies. We propose that these basaltic rocks were derived from mantle plume–related magmatism associated with the evolution of the Paleoasian oceanic system. The enriched mantle source could have contained 2%–5% garnet and ∼3% spinel. The rocks also display strong geochemical similarities with the Xigaze seamount basalts, which formed in intra-oceanic settings. Compared to the basalts, the gabbros display mid-ocean-ridge-basalt–type tholeiitic basalt features, derived from a depleted mantle source with the addition of fluids from a subducted slab within a suprasubduction-zone environment. These observations are supported by previous work, and they indicate Devonian mantle plume–related magmatism within the Paleoasian Ocean. However, as these rocks are mainly parts of accretionary complexes, whether the Karamay and Darbut ophiolitic mélange formed in a single belt remains equivocal, and further work is required to resolve this issue. Thus, there was a complex evolution by subduction-accretion processes from the Devonian to the Carboniferous before final amalgamation and docking to the northern Siberian block.