Mineral Chemistry and Trace Element Composition of Clinopyroxenes from the Middle Cambrian Ust’-Sema Formation Ankaramites and Diopside Porphyry Basalts and the Related Barangol Complex Intrusions, Gorny Altai, Russia

Abstract

The origin and geodynamic settings of the Ust’-Sema Formation and the Barangol Complex are some of the most controversial issues in the Early Paleozoic history of the Altai–Sayan Fold Belt. The Ust’-Sema Formation volcanic rocks are enriched in high-Ca clinopyroxene phenocrysts and were classified as ankaramites and diopside porphyry basalts. In this work, we first present LA-ICP-MS analyses of the clinopyroxenes, along with studies of the petrography, mineral composition, and whole-rock chemistry of the Ust’-Sema Formation and related Barangol Complex rocks. An LA-ICP-MS clinopyroxene study showed a slight depletion of light rare-earth elements (La/Yb)N = 0.1–1.0 (on average 0.4); and strong depletion of the high-field-strength elements (Zr, Hf, and Nb) and large-ion lithophile (Rb) elements. An Sr anomaly showed a positive correlation with Mg#. Major- and trace-element composition of the clinopyroxene cores show that these clinopyroxene grains were not captured from the mantle rocks as previously assumed and that the Ust’-Sema Formation and the Barangol Complex rocks were formed from magma with island arc characteristics. The increased titanium and light rare-earth element contents in the phenocryst rims from Biyka volcano suggest an active interaction of the ankaramitic magma with rocks or melts of OIB type.