Geochronology, geochemistry, and origin of plagiogranitic rocks and related granitic dikes in the Dar Gaz district, Kahnouj ophiolite complex, SE Iran: Analysis of their petrogenesis in a back-arc tectonic setting

Abstract

The Dar Gaz district is part of the Kahnouj ophiolitic complex in southeastern Iran. Major intrusive rocks exposed in the Dar Gaz district are gabbro, diabase dikes, aplitic granitic dikes, and plagiogranites (plagiogranitic dikes and quartz diorite to tonalite intrusions). Plagiogranites are mainly composed of quartz, plagioclase, and ferromagnesian minerals, and are typically low in K2O, high in Na2O and enriched in REE showing geochemical features of oceanic plagiogranites. LA-ICP-MS U-Pb analysis on zircons extracted from two plagiogranitic dike samples yields weighted mean ages of 126.4 ± 1.7 Ma and 126.0 ± 1.3 Ma, and 130.8 ± 1.8 Ma and 128.5 ± 1.3 Ma from quartz diorite to tonalite intrusions, respectively. The plagiogranitic intrusions were formed by partial melting of altered gabbroic rocks. Plagiogranite dikes are characterized by greater fractionation of felsic melts produced by partial melting. The aplitic granitic rocks yielded LA-ICPMS U-Pb ages of 123.0 ± 1.7 Ma and 123.3 ± 1.9 Ma, and have typical features of high-K alkali-calcic to calc-alkalic rocks and are metaluminous to peraluminous formed by partial melting of sedimentary material. All members of the plagiogranitic rocks have moderately high LILE/HFSE ratios and are depleted of Nb, Ti, and P similar to those of a subduction-related tectonic settings. Field data and geochemical signatures of these plagiogranites suggest a supra-subduction origin compatible with the tectonic environment of the Kahnouj ophiolitic complex. These are considered to have formed in a late Jurassic to early Cretaceous back-arc basin.