Mineralogy, geochemistry and geodynamic setting of the granitoids from NW Iran

Abstract

AbstractThe Mahneshan granitoids intruded the Neoproterozoic‐Lower Cambrian regional metamorphic rocks in northwestern Iran. These granitoids consisting mainly of K‐feldspar, plagioclase, quartz, muscovite, garnet and biotite display a number of subtypes in terms of structure, texture and mineralogy. Geochemically, they are peraluminous and (or) slightly peraluminous with variable normative corundum contents (0.28–4.50%), and medium to high potassic with calc‐alkaline affinity. Chondrite‐normalized REE patterns indicate that these granitoids can be divided into three distinct groups, supported by petrographic data. The REE patterns of the first group are shallow‐sloping in LREE relative to HREE ([La/Yb]n = 1.37–2.48), exhibiting pronounced negative Eu (Eu/Eu* = 0.23–0.35). The second group granitoids are characterized by strong LREE‐enrichment relative to HREE ([La/Yb]n = 3–6.20), with positive Eu anomalies (Eu/Eu* = 1.15–1.47). The third group of granitoids is depleted in the middle REE relative to other LREE and HREE. These REE patterns suggest the role of plagioclase and hornblende in their source of granitoids for group 1 and groups 2 and 3, respectively. The trends of Eu/Eu* ratio versus silica contents suggest mixing of mafic material with components formed by crustal melting with a plagioclase‐rich residue. Furthermore, thermobarometric estimations indicate that these rocks may have been formed at depths of 15–18 km at relatively low temperatures. The Mahneshan granitoids are S‐type and may have been emplaced in a syn‐ to post‐collisional tectonic setting, consistent with an origin of water‐saturated magmas with heterogeneous composition derived from different crustal rocks. Copyright © 2010 John Wiley & Sons, Ltd.