Petrogenesis and tectonic setting of the Cambrian Kharly alkaline–carbonatite complex (Sangilen Plateau, Southern Siberia): Implications for the Early Paleozoic evolution of magmatism in the western Central Asian Orogenic Belt

Abstract

The Cambrian Kharly alkaline plutonic complex composed mainly of foidolite and nepheline syenite makes up a small intrusive field in the Sangilen Plateau in Tuva (southern Siberia). The rocks show large ranges of major oxides (38–58 wt% SiO2; 1–18 wt% Na2O + K2O; 11–28 wt% Al2O3; 1.5–20 wt% CaO; 0.1–8 wt% MgO; 2–12 wt% Fe2O3) controlled by variable percentages of minerals: clinopyroxenes, calcic amphiboles, micas, nepheline and feldspars. Alkaline rocks are cut by carbonatite veins composed of predominant calcite coexisting with femic minerals (10–15% of aegirine-ferrosalite-hedenbergite, sodic-calcic amphiboles, ferrobiotite, Ti-garnet), Na–K feldspar and nepheline (up to 15–20%), fluorapatite (up to 20–25%), Sr-apatite, and accessory carbocernaite, titanite, Ti-magnetite and ilmenite. Carbonatites (4057–8859 ppm Sr, 426–1901 ppm Ba (Sr/Ba ≥ 2), 290–980 ppm REE + Y, 2 to 100 ppm Zr, and 0.5 to 15 ppm Nb) possibly originated at high (≥500–650 °C) temperatures as a result of liquid immiscibility. The isotope systematics of rocks and minerals (εNd(t) from ~2.9 to 6.5; 207Pb/206Pbin = 0.89; 208Pb/206Pbin = 2.15; 87Sr/86Sr(t) = 0.70567–0.70733, δ18OV-SMOW ≈ 7.2–19.5‰, and δ13CV-PDB from −6.0 to −1.4‰) suggest mixing of PREMA and EM 1 material during magma generation and crustal contamination of the evolving melts. The rocks bear signatures of interaction with “magmatic-equilibrated” fluids or heated meteoric waters. LILE/HFSE ratios indicate mixed magma sources that involved the material of IAB and OIB, as well as a crustal component, possibly, due to interaction of a mantle plume with rock complexes on the active continental margin.