Petrological characteristics of the Middle Eocene Toveireh pluton (southwest of Jandaq, central Iran): implications for the eastern branch of the Neo-Tethys subduction

Abstract

The Middle Eocene Toveireh plutonic body is located in the western margin of the Central-East Iranian Microcontinent (CEIM). This plutonic body consists of granodiorite, syenogranite, and monzogranite compositions. Granodiorite is the most predominant rock unit, which is composed of quartz, plagioclase, K-feldspar, hornblende, and biotite main mineral phases. The Toveireh pluton is metaluminous to weakly peraluminous (A/CNK = 0.85-1.04) and shows a calc-alkaline I-type affinity. Primitive mantle-normalized spidergrams show enrichment of large ion lithophile elements (Rb, Ba, Th, U) and light rare earth elements (REEs) (La/Yb N = 6.8-8.24), as well as depletion of high-field strength elements (Nb, Ta, Ti, P). These rocks are characterized by unfractionated heavy REEs [(Gd/Yb) N = 1.02-1.80] and a moderate negative Eu anomaly (Eu/Eu* = 0.39-0.77) in the chondrite-normalized REE patterns. The geochemical data suggest that the Toveireh pluton was derived from a low degree of partial melting of a mixed source, primarily of mafic and metasedimentary rock, in the middle crust by underplating of mafic magma. Geochemical and petrological features of the studied samples, such as a wide range of Mg# values (21.3-62.2, average: 35.6) and low amounts of mafic microgranular enclaves, indicated minor involvement of the mantle-derived magma components in the source and about 10% mixing with a felsic melt. Magma chamber processes, including melting, assimilation, storage and homogenization, magma mixing, and assimilation and fractional crystallization, played an important role in the magmatic evolution. The hornblende thermobarometry yielded 720 °C to 840 °C ± 23.5 °C and 0.6-1.4 ± 0.16 kbar for the granodiorites, and the biotite thermobarometry revealed 700 °C to 750 °C and 0.77-0.78 kbar for the syenogranites. The combined results suggest that the studied rocks were crystallized in shallow crustal magma chambers. The Toveireh pluton was formed by the subduction of the eastern branch of Neo-Tethyan oceanic crust beneath the CEIM during the Late Triassic to Early Tertiary.