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

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.

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

“…The rocks were subdivided into two main groups: ankaramites and diopside porphyry basalts. Criteria for the identification of ankaramites included an abundance of clinopyroxene phenocrysts and a high CaO/Al 2 O 3 > 1 ratio [10,24]. In the present study, according to these criteria, ankaramites were also identified among the volcanic rocks of the Ust'-Sema Formation in other areas (Table 1).…”

“…The mineral composition, petrography, and chemistry of the volcanic rocks from the Biyka volcanic edifice were studied in detail in previous work [10]. The rocks were subdivided into two main groups: ankaramites and diopside porphyry basalts.…”

“…The Choburak intrusions were represented by gabbro with a porphyritic texture generally consisting of large subhedral crystals of clinopyroxene and small grains of altered plagioclase (Figure 3e). [10]. (1) Sample from the Kuyus site.…”

“…The genesis of Early Paleozoic volcanic and intrusive complexes in Gorny Altai (in the western part of the Altai-Sayan Fold Belt) is a controversial issue. Previous researchers attributed them to either the magmatism of back-arc basins and island arcs or continental margin rifting, or a mid-ocean ridge subduction and suprasubduction setting combination [1][2][3][4][5][6][7][8][9][10]. These magmatic associations include Middle Cambrian volcanic rocks of the Ust'-Sema Formation and intrusions of the Barangol Complex.…”

“…The Ust'-Sema Formation volcanics are enriched in clinopyroxene phenocrysts and are classified into two groups: ankaramites and diopside porphyry basalts (Di-basalts) [10]. These rocks are spatially associated with ultramafic-mafic intrusions of the Barangol Complex considered to be comagmatic to volcanic rocks of the Ust'-Sema Formation [4][5][6].…”