چكيده لاتين
The Ashin Mesozoic ophiolitic mélange is located in the west of the Central-East Iranian Microcontinent, near to the drastic direction change of the Great Kavir fault. The main rock-units of this ophiolitic association are pelagic limestone, massive and pillow lavas, diabasic dikes, plagiogranite, gabbro, mantle peridotites (lherzolite, harzburgite and dunite), and chromitite. The metamorphic rocks are schist, marble, amphibolite, rodingite, skarn and liswaenite. Volcanic rocks present good exposures in all parts of this ophiolite. Volcanic rocks of the west of the Zavar Mountain and in the Kalut-e-Ghandehari area are studied in this research. In the field and petrography studies, three types of volcanic rocks have been identified, which are basalt, andesite, and dacite, from oldest to youngest. The textures in these rocks are porphyritic, glomeroporphyritic, amygdaloidal, poikilitic and corrosion gulf. The volcanic rocks of this ophiolite have low values of potassium, high amounts of sodium, and are metaluminous. They belong to the tholeiitic and calc-alkaline magmatic series. In the chondrite-normalized diagram, the studied samples show a nearly horizontal to ascending patterns, and in the primitive mantle –normalized spidergram they present negative anomalies of Ti and Nb, as well as highly variable patterns in LILEs. The source rock of the primary magma of the studied rocks has spinel peridotite composition. The volcanic rocks of the Ashin ophiolite possibly are formed in a suprasubduction zone (SSZ) geotectonic environment (Island arc).
The reason of the intermediate to acidic composition of the studied volcanic rocks, is incongruent melting of the orthopyroxene, after congruent melting of clinopyroxene. Incongruent melting of orthopyroxene will cause to the gradual increasing of the SiO2 content, from basic to intermediate and acidic magma. The reason of increasing of the degree of partial melting, is high values of the slab-derived fluids in the suprasubduction zone.
