شواهد كاني شناسي و ژيوشيميايي آميختگي ماگمايي در برونبوم هاي موجود در سنگ هاي گرانوديوريتي منطقه آستانه (جنوب غرب اراك)

Geochemical and mineralogical evidence for magma mixing in enclaves of Astaneh area (south western Arak)

Astaneh granodiorite intrusion, located at south west of Arak, includes an abundant rounded and rarely ellipsoid microgranular enclaves composed of gabbro, diorite and quartz diorite. The diameter of enclaves ranges from a few millimeters to 40 centimeters. These enclaves are very fine-grained and show chilled margins. The size of crystals decreases gardually from center toword margin. The chilled margin is an evidence of rapid magma cooling once the enclaves were surrounded by host felsic magma. Occurrence of the mafic microgranular enclaves within the felsic rocks with disequilibrium textures including: poikilitic texture of K-feldspar megacrystals, ocelli quartz rimmed by zones of fine-grained accumulations of early formed minerals, zoned plagioclase, mafic clots, acicular apatite, small lath-shaped plagioclase within large plagioclase, all indicates the mixing of two magmas. Based on microprobe analyses, the plagioclase crystals show a variable composition from oligoclase to andesine in granodiorite and from andesine to labradorite in enclaves. They have essentially normal zoning. The enclaves and their host rocks, geochemically, are not different from each other and enclaves are enriched in Cr, Ni, and Co. In addition, the enclaves display the HREE enrichment compared to their host granodiorites. Therefore, it seems that the enclaves and granodiorites have been originated from two various depths of magmas with different depths which were located adjacent to each other by magma mixing process.