M. Ebrahimi; M. Rafiei
Abstract
Eocene pyroclastic-volcanic rocks, ranging in composition from basic to intermediate and acidic rocks, are widely exposed at north of Zavieh, southwest of Karaj. The pyroclastic-volcanic sequence is composed of basalt, trachy-basalt, basaltic trachy-andesite, andesite, trachy-andesite, trachydacite, ...
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Eocene pyroclastic-volcanic rocks, ranging in composition from basic to intermediate and acidic rocks, are widely exposed at north of Zavieh, southwest of Karaj. The pyroclastic-volcanic sequence is composed of basalt, trachy-basalt, basaltic trachy-andesite, andesite, trachy-andesite, trachydacite, dacite and rhyolite along with various pyroclastic rocks including ignimbrite, welded tuff, crystal tuff, lithic tuff, lithic crystal tuff, ash tuff and lapillistone. The pyroclastic-volcanic sequence contains large amount of ignimbrite. Common textures found in the studied volcanic rocks are porphyritic, glomeroporphyritic, cumulophyric, amygdaloidal and trachytic. Rock fragments and broken crystals are usually found in the pyroclastic rocks. Eutaxitic texture is commonly observed in the ignimbrite. Flow banding is seen in the study rhyolitic rocks. Meanwhile, columnar structure is commonly observed in the lava flows and ignimbrites. Pyroxene, amphibole, biotite, plagioclase and quartz are the major minerals in the volcanic sequence along with K-feldspar, opaque minerals and altered olivine as minor minerals. Calcite, chlorite, epidote and sericite are also present as accessory minerals. According to geochemical data the studied volcanic rocks are mainly calc-alkaline, however few samples plot in the alkaline and shoshonitic fields in some diagrams. Negative anomalies of Nb and Ta, Pb positive anomaly, large ion lithophile elements enrichment, rare earth elements enrichments and depletion in high field strength elements all indicate that the parent magma of the studied rocks has originated from a metasomatized asthenospheric mantel over the Benioff-Waddatti zone. Common existence of sieve texture in plagioclase crystals, corroded crystals of plagioclase and pyroxene and dark rim of Fe-Ti oxides around biotite and amphibole crystals all indicate that a disequilibrium condition existed during crystallization which is most probably due to magma mixing. However, assimilation, decompression and rapid ascend of magma may also produce such textures.
