Journal of Economic Geology (Dec 2022)
Mineral chemistry, Thermobarometry, Geochemistry and tectonic setting of Tertiary andesitic lavas in the Shourestan area (west of Sarbisheh), Southern Khorasan
Abstract
In the Shourestan area, 14 kilometers west of Sarbisheh city, in South Khorasan province, volcanic rocks with pyroxene-andesite composition belonging to Eocene-Oligocene are exposed. The constituent minerals of these rocks include plagioclase and pyroxene. The composition of plagioclases have range from Ab32, An68 to Ab58, An42 and are andesine-labradorite type. Clinopyroxene and orthopyroxene have diopside-like augite and enstatite composition, respectively. The crystallization temperature for clinopyroxene and orthopyroxene were about 1175 and 1200 °C respectively and the pressure (for both types) was 2 to 5 kb. The geochemical data of whole rocks show that the andesitic lavas of Shourestan have high potassium calc-alkaline nature and the amount of Mg# in them varies from 40.97 to 60.97, which indicates the role of mantle components in their formation. These rocks show signs of differentiation including LREE/HREE ((La/Yb)N) between 9.95 to 12.42, LREE/MREE ((La/Sm)N) between 3.53 to 6.55, MREE/HREE ((Sm/Yb)N) between 1.89 to 2.99. High ratios of Zr/Nb (9.81-22.10), Th/Nb (0.68-1.79), Th/Ta (7.29-24), and Nb/Ta (9.69-15.66) along with the pattern of LIL elements, support the possibility of different degrees of crustal contamination-assimilation of magma during its ascent to the earth's surface. The studied rocks have low ratios of Ce/Y (2.44-3.48), (Tb/Yb)N (1.17-1.39), Sm/Yb (1.92-2.78), and relatively flat MREE-HREE pattern that confirms the melting of the subcontinental lithospheric mantle in the field of spinel stability and at a depth of fewer than 75 kilometers. Introduction Calc-alkaline magmas are commonly active in convergent plate margins and their petrogenesis is crucial for understanding the origin and evolution of the andesitic continental crust. The generation of calc-alkaline magmatic series in oceanic subduction zones has been primarily attributed to the partial melting of enriched mantle sources with the involvement of fluids and/or melts from the subducted oceanic lithosphere or the partial melting of metasomatized sub-continental lithospheric mantle that had been modified by previous plate subduction (Cheng et al., 2020). Andesite is the second most common volcanic rock type on earth and provides abundant information about the interaction between the mantle and crust in the subduction zones. However, the petrogenesis of subduction-related andesite is being debated, since andesite can form via different processes, such as (1) magma mixing between felsic and mafic/ultramafic melt; (2) fractional melting or assimilation fractional crystallization from basaltic composition; (3) partial melting of the hydrated mantle wedge peridotite (Li et al., 2013). Experimental investigation demonstrates that plagioclase and clinopyroxene composition can be used to estimate the P-T condition of volcanic rock crystallization. The chemical composition of pyroxene in volcanic rocks shows the nature of the host lava and is used to determine the magmatic series, tectonic environment, and origin of the igneous rock (Putirka, 2008). In the Shourestan area, 14 kilometers west of Sarbisheh city in South Khorasan province, Eocene-Oligocene volcanic rocks with pyroxene-andesite composition are exposed. Based on the results of previous studies, the Tertiary lavas in the Sarbisheh area have calc-alkaline nature and are related to active continental margins (Mohammadi and Nakhaei, 2022). In this research, the chemical composition of minerals has been used to determine the nature of magma, tectonic setting and evaluation of the temperature and pressure conditions for the crystallization of andesitic lavas. Also, by using the geochemical data of the whole rock, the geochemical characteristics, tectonic setting, and origin of these rocks have been investigated. Materials and Methods Microprobe analysis of pyroxene and plagioclase minerals was done at the institute of Earth sciences in Academia Sinica, Taipei, Taiwan. A scanning electron microscope (JEOL SEM JSM-6360LV) was used to observe micro-scale texture. Identification of mineral phases was done by an energy dispersive spectrometer equipped with SEM, under the beam conditions of 15 kV, and 0.2 nA for the acceleration voltage, and beam current, respectively. Mineralogical investigation was carried out by an electron probe micro analyzer (JEOL EPMA JXA-8900R) equipped with four wave-length dispersive spectrometers. For geochemical investigations, 8 samples were analyzed in Acme laboratory in Canada by ICP method (for major elements) and ICP-MS (for trace and rare earth elements) and 3 samples in ZarAzma Company (Tehran, Iran) by alkaline melting method (for major elements) and ICP-MS (for trace and rare earth elements). Results The constituent minerals of these rocks include plagioclase and pyroxene. The composition of plagioclases changes from Ab32 An68 to Ab58 An42 and are andesine-labradorite type. Clinopyroxene and orthopyroxene have diopside-like augite and enstatite composition, respectively. The crystallization temperature for clinopyroxene and orthopyroxene were about 1175 and 1200 °C respectively and the pressure was 2 to 5 kb. The geochemical data of whole rocks show that the andesitic lavas of Shourestan have high potassium calc-alkaline nature and the amount of Mg# in them varies from 40.97 to 60.97, which indicates the role of mantle components in their formation. These rocks show signs of differentiation including LREE/HREE ((La/Yb)N) from 9.95 to 12.42, LREE/MREE ((La/Sm)N) from 3.53 to 6.55, MREE/HREE ((Sm/Yb)N) from 1.89 to 2.99. High ratios of Zr/Nb (9.81-22.10), Th/Nb (0.68-1.79), Th/Ta (7.29-24), and Nb/Ta (9.69-15.66) along with the pattern of LIL elements, support the possibility of different degrees of crustal contamination-assimilation of magma during its ascent to the earth's surface. The studied rocks have low ratios of Ce/Y (2.44-3.48), (Tb/Yb)N (1.17-1.39), Sm/Yb (1.92-2.78), and relatively flat MREE-HREE pattern that confirms the melting of the subcontinental lithospheric mantle in the field of spinel stability and at a depth of fewer than 75 kilometers. Discussion Petrographic studies show that the volcanic rocks of the Shourestan area have pyroxene-andesite composition. After plagioclase, pyroxene is the most abundant mineral in Shourestan andesitic lavas. The values of Mg# in clinopyroxene and orthopyroxene are 72-78 and 71-77, respectively. High values of Mg# in pyroxenes indicate the role of mantle components in the magma source. Based on the chemistry of clinopyroxene, andesitic lavas of the Shourestan area have sub-alkaline nature and are located in the field of volcanic arc basalts. The anorthite content of plagioclases in andesitic lavas of Shourestan (52-66%) and Mg# of clinopyroxenes (72-78) indicate the low amount of water during the formation of these minerals from primary magma. The formation temperature of investigated clinopyroxene and orthopyroxene was about 1200 °C and the calculated pressure at the time of their crystallization was determined 2 to 5 kb. The volcanic rocks of Shourestan were located in the range of andesite with high potassium calc-alkaline nature. The amount of Mg# in these rocks varies from 40.97 to 60.97, which indicates the role of mantle components in their formation. The presence of negative Ti, Nb, and P anomalies in trace element diagrams of the studied samples, confirms the formation of these rocks in subduction zones. Relatively low values of YbN in the samples (8.42 to 10.05 ppm) indicate low amounts of garnet in the source. Geochemical characteristics of the Shourestan andesitic rocks such as K2O/P2O5 ratio >2 along with high Al2O3 and Th enrichment can be related to crustal contamination or magma formation from a heterogeneous metasomatized mantle source. In addition, Th/Ta (7.29-24), Nb/Ta (9.69-15.66) and Ta/La (0.02-0.05) ratios also indicate different degrees of the crustal contamination-assimilation of magma during the ascent to the surface of the earth. Based on various element ratios, the Shourestan andesitic lavas originated from a subcontinental lithospheric mantle that evolved during subduction. The geochemical characteristics of the investigated rocks, such as the high ratio of LILE/HFSE and LREE/HREE, as well as the different tectonic discriminant diagrams, confirm active continental margin tectonic setting.
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