| چکیده انگلیسی مقاله |
Introduction
Volcanic rocks widely extended throughout the Earth's continental crust (Brilhante et al., 2024). Among the different tectonic setting, a group of igneous rocks is related to volcanic arc; these rocks, according to Tang et al. (2022), include a range of mafic-intermediate rocks with SiO2 (47.34-60 wt%), Fe2O3 (11.63-73.4 wt%) and Mg coefficient (26-66 wt%), remarkable chemical criteria are related to calc-alkaline series with medium to high potassium and LILE enrichment. Moreover, the Ce negative anomaly in these rocks is consistent with the subduction zone. In this paper, based on whole-rock chemistry data, the geochemical properties of volcanic rocks of the Ahovan area in northeast of Semnan (Central Iran) are investigated. The main purpose of the present paper is the study of tectonic setting as well as the tectonomagmatism of these rocks.
Analytical Methods
During the field studies, 80 samples were collected from the volcanic rocks for petrographic observations. Following the preparation of thin sections, their mineralogy and textural relationships were carefully considered, and 10 of them with least alteration were sent to the ALS-CHEMEX laboratory in Canada for identification of major elements by ICP-OES and trace and rare earth elements by ICP-MS (Table 1).
Furthermore, 11 samples of the Khalatbari-Jafari and Etesami (2018) were used in this study for comparison The measurement accuracy for all major elements was 0.01%w.t, the maximum measurement accuracy in the trace elements for Cr, Li, and Tl was 10 ppm and for Cs, Ho, Lu, Tb, and Tm was 0.01 ppm.
Petrography
According to field studies, the Eocene volcanic units, trending east-west, are much more widespread than the intrusive rocks of Ahovan area. These units have a mixed spectrum of basalt, trachyte and andesite-trachyandesite, which the latter rocks cover a large volume.
The basaltic rocks, with gray and black in color, are dominated by the presence of plagioclase and clinopyroxene as the main minerals. The white and grayish white andesite-trachyandesite are characterized by the occurrence of plagioclase, clinopyroxene, amphibole and biotite as well as porphyry and microlithic porphyry textures. Trachyte rocks are white and grayish white in hand spacemen. Plagioclase and sanidine as phenocrysts and microlites constitute the main minerals of the trachyte rocks. Amphibole and biotite are small amounts in these rocks (10 vol.%). Iron oxides, calcite, chlorite and epidote are the alteration products of these minerals. Dolerite dykes, trending N-S and a maximum thickness of 2 m crosscut all the volcanic rocks. under study.
Geochemistry
The overall petrographic and geochemical data point to the nature of calc-alkaline to high-potassium calc-alkaline of the rocks studied, which are originated by the low-pressure melting of spinel lherzolite and developed at a depth less than the garnet stability field. In the Y versus Zr diagram, the study rocks are in the volcanic arc setting. The REE pattern of the study rocks normalized to chondrite follow the sloping pattern of calc-alkane rocks related to island arcs. In spider diagram of incompatible elements normalized to N-MORBs, the study rocks show LILE enrichment and HFSE depletion indicating the crustal contamination process involved in the formation of these rocks. The depletion of Nb, Ti and Ta show characteristics of arc islands. Moreover, LILE's enrichment such as K and Sr indicate that the rocks under study originated from the mantle metasomatism resulting from hydrous fluids penetrated from the subducting crust into the mantle wedge at different depths.
Conclusion
The volcanic rocks of Ahovan area are dominated by the presence of basalt, andesite-trachyandesite and trachyte belonging to Middle-Late Eocene. A series of trachybasalt and basalt-trachyandesite dykes, trending north-south, and a maximum thickness of 2 meters, crosscut the rocks of the area. The essential minerals of basalts are plagioclase and clinopyroxene and those of andesite-trachyandesite are plagioclase, clinopyroxene, amphibole and biotite. Plagioclase and sanidine are the dominant minerals of trachytes. In magmatic series diagram, all the rocks studied are in the range of calc-alkaline to high-potassium calc-alkaline series, originated from a relatively low-pressure melting of spinel lherzolite at a depth of lower than the garnet melting stability field. The Chondrite-normalized rare earth element (REE) patterns are similar to calc-alkaline rocks from island arc. Also, in the spider diagram of incompatible elements normalized to N-MORB, these rocks show enrichment in LILE and depletion in HFSE indicating the involvement of the crustal contamination process in formation of these rocks. The LILE enrichment (i.e., K and Sr) points to mantle alteration resulting from aqueous fluids penetrated from the subducting crust of the Neotethys ocean into the mantle wedge at different depths played a role in the formation of these rocks. In tectonic setting diagrams, the rocks under investigation are in the range of calc-alkaline basalts related to volcanic arcs. |