Crystallization Conditions and Mineral Chemistry in the East of Tafresh, Central Iran, with Insights into Magmatic Processes

Abstract

The Tafresh granitoids are located at the central part of the Urumieh‐Dokhtar Magmatic Arc (UDMA) in Iran. These rocks, mainly consisting of diorite and granodiorite, were emplaced during the Early Miocene. They are composed of varying proportions of plagioclase + K‐feldspar + hornblende ± quartz ± biotite. Discrimination diagrams and chemical indices of amphibole phases reveal a calc‐alkaline affinity and fall clearly in the crust‐mantle mixed source field. The estimated pressure, derived from Al in amphibole barometry, is approximately 3 Kb. The granitoids are I‐type, metaluminous and belong to the calc‐alkaline series. They are all enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements and display geochemical features typical of subduction‐related calc‐alkaline arc magmas. Most crystal size distribution (CSD) line patterns from the granitoids show a non‐straight trend which points to the effect of physical processes during petrogenesis. The presence of numerous mafic enclaves, sieve texture and oscillatory zoning along with the CSD results show that magma mixing in the magma chamber had an important role in the petrogenesis of Tafresh granitoids. Moreover, the CSD analysis suggests that the plagioclase crystals were crystallized in a time span of less than 1000 years, which is indicative of shallow depth magma crystallization.