Sayed Zia Hosseini; Hadi Shahriari
Abstract
IntroductionThe Urumieh-Dokhtar Magmatic assemblage includes various volcanic and intrusive rocks from Eocene to the Pleistocene. The dominant theory in describing the tectonomagmatic ...
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IntroductionThe Urumieh-Dokhtar Magmatic assemblage includes various volcanic and intrusive rocks from Eocene to the Pleistocene. The dominant theory in describing the tectonomagmatic environment of the Urumieh-Dokhtar Magmatic assemblage emphasizes the formation of calc-alkaline rocks from Eocene to Miocene as a result of subduction in an active continental margin tectonic environment. Volcanic activities after the Miocene are observed to a smaller extent in some parts of the Urumieh-Dokhtar Magmatic assemblage, including in Bijar, Shahrbabak, and Dehaj (Stern et al., 2021). These activities are characterized by the formation of Pliocene andesite-dacite rocks and Pleistocene andesite-basalt rocks in the Dehaj area. Post-collision magmatism in the Dehaj region is calc-alkaline and the subduction process influences its origin before the Miocene (Pang et al., 2016). In this research, the origin and petrogenetic evolution of Pliocene hyalo-andesitic rocks are studied using field investigation, geochemical data of rare elements, and isotopic ratios (87Sr/86Sr and 143Nd/144Nd) in the Dehaj area. This knowledge can help to understand the tectonic events of the Urumieh-Dukhtar Magmatic assemblageGeology of the areaHyallo-andesites of Dehaj, which are located in the northwest of Kerman province, SE part of the Urumieh-Dukhtar Magmatic assemblage. Tectonically, this area is very active and is located between Anar and Dehshir faults. The Pliocene volcanic phase is the most important magmatic activity has caused the formation of andesitic to dacite volcanic and semi-volcanic rocks in the region. Pliocene hyalo-andesites of Dehaj have also been exposed in the form of several outcrops in the vicinity of the Aj volcanic peaks. Pliocene hyalo-andesites of Dehaj are also exposed in the form of several outcrops near the Aj volcanic peaks. Hyallo-andesites are composed of a lava unit with a thickness of fewer than 10 meters on the underlying pyroclastic unit.Research MethodForty rock samples of Dehaj hyalo-andesites were collected during the field visit. After petrographic studies, thirty samples were selected for XRF and ICP-MS chemical analysis. The Middlemost (1989) method was performed to separate total iron into FeO and Fe2O3. Five samples were used to calculate isotopic ratios of strontium and neodymium.PetrographyPliocene hyalo-andesites of Dehaj are dark gray to black with a fine grain texture. Petrographically, the rocks under study, are formed from plagioclase microlites and very fine brown hornblende crystals in a cryptocrystalline and glassy matrix, and quartz crystal is not visible. The hyallo-andesitic rocks of Dehaj have disequilibrium textures, including sieve texture and oscillatory zoning in plagioclase microphenocrysts.GeochemistryPliocene hyallo-andesitic rocks of Dehaj are classified in the dacite range and the andesite-dacite boundary based on the TAS classification (Le Bas et al. 1986). Based on SiO2 versus Na2O+K2O, AFM, and K2O versus SiO2 diagrams, these rocks are calc-alkaline with moderate potassium nature. LILE elements such as Sr, K, Rb, Ba, and Th showed enrichment, and HFSE, especially Nb, Ta, and Ti presented depletion. The primitive mantle-normalized multi-element diagram showed the positive anomaly of Sr and the negative anomaly of Nb. Chondrite-normalized REE patterns showed enrichment of LREE compared to HREE with La/Yb>9 and Sm/Yb>1.8 ratios without any positive or negative Eu anomalies. The initial ratios calculated for 87Sr/86Sr varied from 0.704498 to 0.704967 and for 143Nd/144Nd from 0.512821 to 0.512842. The calculated values of ℇNd also vary between +3.55 and +3.98.Discussion Geochemical features including high amounts of strontium (Sr>750 ppm), low amounts of yttrium (Y<8ppm) and HREE, high Sr/Y ratio, and the strongly fractionated pattern of REE without Eu anomalies, define the adakitic nature of Dehaj hyallo-andesites. Enrichment in LILE elements and depletion in HFSE elements (i.e., Nb, Ta, Ti) in Dehaj adakites show the connection of these rocks with the pre-Pliocene subduction environment. High values of Sr and Rb/Sr ratios between 0.02 and 0.04 and La/Yb>10 along with isotopic ratios of 87Sr/86Sr and 143Nd/144Nd as well as positive values of ℇNd (+3.55 to +3.98) demonstrate the hyalo-andesitic rocks of Dehaj were formed as a result of the melting of the oceanic slab. Several reasons indicate the presence of garnet in the origin of the Pliocene adakites of Dehaj and the remaining of this mineral in the slab resulting from partial melting, which also represents the eclogite composition of the source rock. These reasons include values of Y<8ppm and Yb<0.76ppm and the sloping pattern of REE elements with La/Yb>9 and Sm/Yb>1.8 ratios. The melting process of the eclogite oceanic plate occurred at the same time or following the tectonic collision between the Central Iranian and Arabian tectonic plates during the Miocene-Pliocene.ConclusionPliocene hyallo-andesitic rocks of Dehaj were created as a monogenetic magmatic activity. Geochemical evidence indicates that the partial melting of subducting oceanic crust is the origin of silica-rich Dehaj adakites. The Pliocene Dehaj adakites formed after the cessation of Neotethys subduction and in a tectonic environment following the collision. Thinning and separation of the subducting oceanic crust and sinking into the asthenosphere gave rise to melting conditions. These events have occurred in an active tectonic environment and simultaneously with the formation of numerous faults. The occurrence of numerous outcrops of Pliocene volcanic rocks along with Pliocene hyallo-andesitic lavas and Pleistocene basaltic lavas in the study area compared to nearby areas indicates the presence of multiple fractures in the crust of this area. These fractures have caused the rapid ascent of adakite melt from the subducting oceanic crust. During the ascent, the adakite melt has been contaminated by the upper metasomatized mantle and crust to a small extent.
