جستجوی مقالات مرتبط با کلیدواژه « subducted oceanic crust » در نشریات گروه « زمین شناسی »

In the north of Torud and the eastern part of the Trood- Chah Shirin magmatic arc, the Late Eocene- Early Oligocene subvolcanic rocks with andesite, trachyandesite and dacite composition intruded the Eocene volcanic- pyroclastic sequence. Mineralogically, they are dominated by plagioclase, hornblende, biotite, quartz and magnetite with microlithic porphyry and hyallomicrolithic porphyric textures. Plagioclase with polysynthetic twinning and oscillatory zoning is the most abundant mineral that sometimes having sieve texture, dissolution, and corroded margins. Biotite and amphibole phenocrysts are idiomorphs to subidiomprph with opacitic margins. Quartz phenocrysts sometimes have corrosion gulf. Microscopic evidence, including non- equilibrium textures (oscillatory zoning and sieve texture of plagioclases), dissolution, and corrosion of crystals point to assimilation and magmatic contamination. The geochemical signatures of the rocks studied including enrichment in LILEs (especially Rb, K, Th) and LREEs relative to HREEs and HFSEs (Ta, Nb, Ti), a calc- alkaline affinity and their position on discrimination tectonomagmatic diagrams, display that these rocks derived from a subduction- related environment. These mentioned signatures with a differentiated pattern of rare earth elements without Eu anomaly, depletion in Y and Yb, and high ratio of Sr/Y and La/Yb show these rocks belong to high silica adakites. The parent magma of adakitic rocks in this region originated from partial melting of subducted oceanic crust (in garnet stability depth) associated with fractional crystallization and assimilation processes.

The Iju porphyry stock as a part of the Urumieh-Dokhtar magmatic belt is situated 72 km northwest of Shahr-e Babak city and 147 km west-northwest of Sarcheshmeh porphyry copper deposit. The stock, intruded as a hybabyssal Miocene granitoid (9.25±0.5 Ma) the Eocene volcanic-volcanoclastic rocks and induced mineralization and alteration in the region. The existing phenocrysts in the Iju intrusive rocks including tonalite and quartz diorite with plagioclase, alkali feldspar, biotite and hornblende, which reside in a groundmass consisting of quartz and minor potassium feldspar and secondary minerals. The dominant texture in the rocks is porphyry. Based on petrography and geochemical analyses, the tonalitic rocks in the Iju region are classified as calc-alkaline and arc-related continental margin magmatism. High ratios of Ba/Nb (average 101 ppm) and Ba/Ta (average 1326 ppm) as well as the distribution of mantle-normalized trace elements (enrichment in LREE and LILE relative to HREE and HFSE, negative anomalies in Nb and Ta) indicate formation of the Iju granitoid is related to subduction. Evaluation of geochemical evidence including the amounts of SiO2>56%, Al2O3≥15%, MgO3%, Sr>300 ppm, Y20, La/Yb>20, high enrichment in LREE and low enrichment in HREE show that the parent magma of the Iju tonalite has high silica adakitic affinity. The lack of negative anomalies in Eu and Ba in the rock samples point to the lack of influence of partial melting in the lower crust for this unit formation. Thus, the adakitic magma developed as the result of melting of subducting oceanic crust in the mantle and at the final stages of subduction or post-subduction.