| Popis: |
An alkaline volcanic activity with a relative Plio-Quaternary age (based on the succession of layers on the field) occurred in the northern part of Uromieh Dokhatar magmatic belt, Iran. Hornblendite xenoliths mostly displaying cumulitic texture occur in the trachyandesites from this magmatic episode. The thermobarometric results indicate that these xenoliths formed within the crust. Based on mineralogical and chemical characteristics, these xenoliths are divided into two groups. In Group 1, plagioclase (andesine) modal content is less than 10% while amphibole (magnesiohastingsite) and biotite are the main minerals. In Group 2, plagioclase (labradorite) modal content is higher than 20% while amphibole (pargasite) and biotite are the main minerals. Positive anomalies of U, Ba, Ti, and K in hornblendite xenoliths are probably related to the accumulation process of amphibole and biotite. The study of the chemical composition of amphiboles indicates crystallization of amphibole from Group 1 and Group 2 hornblendite xenoliths and host volcanic rock under high oxidation conditions. Hornblendite xenoliths display high LREE/HREE and LREE/MREE ratios. The $^{86}$Sr/$^{87}$Sr ratios in xenoliths of Groups 1 and 2 and host volcanic rocks are 0.706291, 0.704685, and 0.709545, respectively, while $^{143}$Nd/$^{144}$Nd ratios are 0.512580, 0.512736, and 0.512561, respectively. Group 1 xenoliths are characterized by a negative Eu anomaly while this anomaly is positive for Group 2. Considering the REE concentrations and distribution patterns, the major element compositions of amphibole, biotite, and plagioclase as well as the Sr and Nd isotopic ratios, it appears that the parental melts of Group 1 and Group 2 xenoliths differ in composition. Those two theoretical parental melts also have different compositions when compared to that of the host volcanic rock. The studied alkaline rocks are commonly located along the main faults. Therefore it seems that the movement of the main faults (especially North Tabriz Fault, North Misho and Tasuj faults) has provided a path for the lavas carrying the investigated xenoliths to penetrate the continental crust. Taken together the geochemical characteristics of the studied rocks indicate a mantle source variably metasomatized by a subduction event prior to the collision between Eurasian and Arabian continents. |
