Late Cambrian calc-alkaline magmatism during transition from subduction to accretion: Insights from geochemistry of lamprophyre, dolerite and gabbro dikes in the Dzhida terrain, Central Asian orogenic belt

Abstract

The Central Asian orogenic belt (CAOB) is one of the largest orogens on Earth characterized by complex and long-term history resulted from the development of the Paleo-Asian Ocean. In Late Cambrian–Ordovician, closure of Paleo-Asian basins and accretion of microcontinents, island arcs and back-arc basins to the southern margin of the Siberian continent resulted in large-scale magmatism recorded in Southern Siberia and Mongolia. The timing of terminal stage of subduction and start of accretion-collision stage is a matter of discussion for different parts of this extensive collage of terrains. In this paper, we report new data on whole-rock and mineral composition and 40Ar/39Ar age of calc-alkaline dolerites, lamprophyres and gabbros of Bayangol river area (Northern Mongolia) from the Dzhida terrain of CAOB. The Bayangol calc-alkaline mafic rocks were previously considered as a member of boninite-basaltic sequence originated in a primitive island arc. We found that the Bayangol calc-alkaline mafic rocks compose late mafic dikes, cutting terrigenous sediments and serpentinite mélange with boninite blocks, and, therefore, calc-alkaline rocks are not related to boninites. We obtained 40Ar/39Ar age of 498.9 ± 7.7 Ma for amphibole from Bayangol lamprophyre. The calc-alkaline rocks preserve clinopyroxene (diopside, Mg# of 76–90) and amphibole (mainly pargasite and magnesiohastingsite, Mg# of 38–92), which crystallized at 1170–1210°С, 2.2–9.5 kbar and 760–1030°С, 2.0–7 kbar, respectively. Dolerites, lamprophyres and gabbros show variations in major oxides (3.3–14.9% MgO, 10.1–19.4% Al2O3, 0.09–0.92% P2O5) and immobile trace element systematics (e.g., Th and LREE enrichment, HFSE depletion) corresponding to island arc calc-alkaline series. Isotope compositions of Nd (εNd(T) = −0.7 to +2.5) and Sr (87Sr/86Sr(T) = 0.7049–0.7078) suggest 1–5% addition of recycled terrigenous sediment melts to mantle wedge source of the Bayangol mafic rocks. Relations of trace element and SiO2 abundances and NdSr isotope ratios are resulted from fractional crystallization and do not support in situ crustal contamination of Bayangol magmas. Within the Dzhida terrain, the Bayangol mafic rocks have Nd isotope compositions transitional between those of Late Cambrian island arc intrusions with juvenile signatures and Early Ordovician collisional granitoids with crustal characteristics. Therefore, the Bayangol calc-alkaline mafic rocks mark transition from subduction to the accretion-collision regime during accretion of Dzhida island arc to the Siberian continent as a result of Paleo-Asian Ocean closure in SW Transbaikalia and Northern Mongolia.