Ankaramites of Gorny Altai: Mineralogical, Petrographic, and Petrochemical Features of Diopside Porphyry Basalts of the Ust’-Sema Formation

Abstract

Abstract —The mineral composition, petrography, and petrochemistry of middle Cambrian diopside porphyry basalts of the Ust’-Sema Formation in Gorny Altai are considered in comparison with ankaramites of different geodynamic settings. The basalts of the Ust’-Sema Formation are enriched in phenocrysts of high-Mg clinopyroxene (Mg# ≤ 94) (20 to 40–50 vol.%) with a high content of Cr2O3 (up to 1.11 wt.%), enclosed in the clinopyroxene–plagioclase microlitic groundmass. In addition, there are minor phenocrysts of saussuritized plagioclase (An49–71), olivine replaced by secondary minerals, amphibole with Mg# = 55.7–68.2, and Cr-spinel with Cr# = 36.2–41.7. Inclusions of Cr-spinel in high-Mg clinopyroxene are richer in Cr (Cr# ≤ 72.8). The basalts of the Ust’-Sema Formation are chemically heterogeneous and are subdivided into two main groups: high-Ca (MgO = 7.98–14.77 wt.% and CaO/Al2O3 = 1.0–1.8) and low-Ca (MgO = = 2.84–9.89 wt.% and CaO/Al2O3 = 0.2–0.9). The obtained data on the high-Ca basalts of the Ust’-Sema Formation show that the rocks are similar to the reviewed ankaramites and thus can be assigned to this type of rocks. The low-Ca basalts of the Ust’-Sema Formation might have resulted from the fractionation of ankaramitic melt in intermediate magma chambers. The Gorny Altai ankaramites formed through the melting of the wehrlitized suprasubductional lithospheric mantle during the closure of the Paleoasian Ocean.