Alkaline basalts in the Karamay ophiolitic mélange, NW China: A geological, geochemical and geochronological study and implications for geodynamic setting

Abstract

Alkaline basalts are common constituents in many ophiolite belts and accretionary complexes. Here we report geological, geochronological, geochemical, and whole-rock Sr-Nd isotopic data for alkaline basalts from the Karamay ophiolitic mélange of West Junggar, southern Central Asian Orogenic Belt (CAOB). In the ophiolitic mélange, imbricate thrusts, duplexes, "web" structures, pinch-and-swell structures, tilted structures in pillow metabasalts, and shear band cleavages are widely developed. U-Pb analyses of zircon grains from a representative basalt by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded crystallization age of ca. 395 ± 3 Ma, suggesting formation during the Middle Devonian. Geochemically, all the samples bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of Na2O + K2O (4.22-7.28 wt.%) and TiO2 (1.72-2.89 wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N = 8.4-15.6), with very strong or no Eu anomalies (Eu/Eu* = 0.6-1.1), and no obvious Nb, Ta or Ti negative anomalies. The rocks display consistent Sr-Nd isotopic compositions (initial 87Sr/86Sr ratios = 0.70326-0.70453, εNd(t) = + 3.1-+7.6). The results suggest that all the alkaline basalts were derived from a mantle plume-related source in an intra-oceanic setting with ca. 1-3% degree partial melting of garnet lherzolite. These observations, in combination with previous work, indicate that the alkaline basalts from Karamay ophiolitic mélange can be correlated to a Middle Devonian mantle plume-related magmatism within Paleo-Asian Ocean. We further suggest that the plume-related activity in the CAOB was generally continuous during the development of the Paleo-Asian Ocean.