At present, geochemical data are widely used for reconstructing geodynamic settings, especially, volcanic rocks of mafic composition, i.e., basalts, because they are widespread in many orogenic belts and are indicative of different geodynamic environments. In general, we propose the reconstruction of the tectonic settings of basalts according to their relationships with associated ocean plate stratigraphy (OPS) sediments, their petrogenesis and their geochemical features.In this paper, we present new data on geochemistry of basalts from the Tsoroidog and Bumbattolgoi formations in the Tsetserleg accretionary wedge terrane in southwestern part of the Khangai-Khentey orogenic system [Tomurtogoo, 2012], which have been identified as the key region in the Central Asian Orogenic Belt (CAOB), the world largest accretionary orogeny. Detailed study of their geochemistry of volcanic rocks allowed us to recognize two types of chert-basalt association.Middle Silurian -Lower Devonian Tsoroidog formation is mainly composed of basaltic rocks with thin layers/lenses of thin-bedded, red-whitish color chertquartzite and slates. Basaltic rocks are characterized by medium TiO2 and high Zr/Nb ratio, Mg#=36-47, which is depleted in REE, Nb and Ti. Concentrations of TiO 2 , Fe 2 O 3 and P 2 O 5 decrease when MgO is increased in the binary diagram, whereas Al 2 O 3 is normal, furthermore, Nb, La and Sm decrease when MgO increases to compare with the rare elements. Nb is relatively less than La and Th. (Nb/La) PM value less than 1 is generally indicated in the MORB [Haase, 2002].Lower Devonian Bumbattolgoi formation mainly consists of basaltic rocks associated with shale, sandstone and red color cherts, which is represented by
The paper presents new data on geochemistry (major and trace elements, isotopes) and petrology (composition of phenocrysts and mineral thermometry) of basalts from the Early Cambrian Katun’ accretionary complex of the Kuznetsk–Altai island arc (Gorny Altai). We also discuss the geodynamic settings of the formation of basalts based on petrologic, geochemical and isotope data taking into account their relationships with associated sedimentary rocks of oceanic origin and with terrigenous sedimentary rocks of the accretionary complex. The Late Neoproterozoic basalts associated with siliceous sediments are characterized by medium TiO2 and Zr/Nb, flat REE patterns and Nb/Lapm < 1 (MORB-type oceanic floor basalts). The oceanic floor basalts are represented by high- and low-magnesium varieties. The Early Cambrian basalts (Manzherok Formation) were formed in an oceanic island setting (OIB-type). They occur in sections consisting of paleo-oceanic island basalts alternated with siliceous-terrigenous-carbonate sediments of slope facies. The Manzherok basalts are characterized by high crystallization temperatures (1175 °C), and enriched incompatible elements (LREE, Ti, Nb), whose variations suggest a mantle plume source and variable degrees of partial melting in the spinel and garnet stability fields. The low to high-Mg Middle Cambrian basalts of the Ust’-Sema Formation occur as subparallel dikes and lava flows, which cut and overlap the accretionary complex. The basalts are characterized by low TiO2, Nb, LREE, high Zr/Nb and, compared to the OIB-type basalts, lower temperatures of crystallization, 1047 °C and 1138 °C for the low- and high-Mg varieties, respectively. Geochemically, the low-Mg basalts of the Ust’-Sema Formation are close to MORB, and the high-Mg basalts—to island-arc tholeiites. Such a combination of geochemical and geological data suggest that the low-Mg basalts of the Ust’-Sema Formation formed during ridge subduction, and the high-Mg basalts formed in a suprasubduction setting, at the crust-mantle boundary.
The paper presents new petrographic, geochemical, and petrologic data from volcanic rocks of suprasubduction origin of the Char shear zone in eastern Kazakhstan. We discuss bulk rock composition (concentrations of major and trace elements), types of mantle sources and parameters of their melting, conditions of crystallization of mafic magma, and geodynamic settings of basalt eruption. According to the major element composition, the volcanic rocks are basalt, andesibasalt, and andesite of tholeiitic and transitional, from tholeiitic to calc-alkaline, series. They are characterized by low TiO2 (0.85 wt.% on average) and crystallization trends in MgO–major elements plots. In terms of trace element composition, the volcanic rocks possess moderately LREE-enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/Lapm = 0.14–0.47; Nb/Thpm = 0.7–1.6). The distribution of rare-earth elements (La/Smn = 0.8–2.3, Gd/Ybn = 0.7–1.9) and the results of geochemical modeling in the Nb–Yb system suggest high degrees of melting of a depleted mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase, and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry calculations suggest that the melts crystallized within a range of 1020–1180 °C. We think that this volcanic complex formed at a western active margin of the Paleo-Asian Ocean.
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