Genetic classification of magmatic rocks from the Alvand plutonic complex, Hamedan, western Iran, based on zircon crystal morphology

Abstract

The Alvand plutonic complex consists of gabbroic and felsic rocks, the latter can be divided into (1) porphyritic, fine-grained and mylonitic granites and (2) leucocratic granitoids. We investigated the external zircon morphology and their internal structures from all major granitoids of the pluton employing the classic Pupin method supplemented by electron microscope analyses. Zircons of gabbroic rocks are free of visible cores or inclusions and are commonly characterized by {101} pyramids and {100} prisms and show mainly zircon types P5 and D typical for mantel-derived rocks. The zircon population from the porphyritic granite is characterized by the predominance of the pyramidal {211} and prism {110} forms and mainly composed of the subtypes S1, S2, S6 and S7 typical for peraluminous granites of crustal origin. Melt inclusions, recrystallization patches and low-CL intensity rims are typical features in these grains. Zircons from the fine-grained granites are characterized by the predominance of the pyramidal {211} and the prism face {110} and by a preponderance of the subtypes S3, S4, S7 and especially S12 and occasionally S2, L2, L3 and L4, typical for aluminous monzogranites and granodiorites of crustal origin. Some grains have pre-magmatic inherited domains with overgrow rims. The mylonitized granites contain zircons with {101} pyramids and {110} prisms and include subtypes G1, P1, P2, S5 whereas P3, S4, L5 are rarely present, typical for I-type granites. Metamictization, radial cracks and partial overgrowths are prevalent in these zircons. Zircons from the leucocratic granitoids have well-developed magmatic oscillatory zonation and pre-magmatic zircon cores. They are characterized by {101} pyramids and {110} prisms and are mainly composed of subtypes L5, S5, S10 and rarely P1, P2, S2, S3, S4, S7, G1 typical for hybrid calc-alkaline granites.