Petrology, geochemistry and tectonic setting of the Khoy ophiolite, northwest Iran: implications for Tethyan tectonics

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lit hosphere formed in the Mesozoic Neo-Tethys. This northwest-southeast trendi ng ophiolite complex consists from bottom to top (east to west) of a well-d efined basal metamorphic zone, peridotites (dunite, harzburgite) and serpen tinized peridotite, gabbros, sheeted dikes, pillow and massive lava flows, and pelagic sedimentary rocks, including radiolarian chert. The rocks of th e metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. The high-grade metamorphic rocks are immediately ad jacent to the peridotite and the gabbros and the low-grade rocks are in con tact with the Precambrian Kahar Formation. Based on mantle-normalized incom patible trace element diagrams there are two distinct types of basalt flows present at the Khoy ophiolite: (1) massive basalts that have patterns virt ually identical to E-MORB, and (2) pillow basalts that have more primitive chemical composition whose trace element patterns plot between E-MORB and N -MORB. The chondrite-normalized REE patterns for the pillow basalts are LRE E-depleted [(La-N/Sm-N)(ave)=0.70], similar to patterns for the mean diabas e composition for the Oman ophiolite and LREE-depleted basalts of the Band- e-Zeyarat ophiolite of southern Iran. The REE patterns for the massive basa lts are similar in general REE abundances to the pillow basalt patterns, bu t they are slightly LREE-enriched [(La-N/Sm-N)(ave)=1.09] and their pattern s cross those of the pillow basalts. The REE patterns for the gabbros and d iorites indicates that the crustal-suite rocks were most likely derived by a process of fractional crystallization from a common basaltic melt. This b asaltic melt was most likely generated by approx. 20-25% partial melting of a simple Iherzolite source and had REE concentrations of roughly 10x chond rite. A comparison between the results from the Khoy ophiolite and the data from other Iranian ophiolites reveals geochemical evidence to suggest a te ctonic link between the Khoy ophiolite and the rest of the Iranian ophiolit es. Our results suggest that Khoy ophiolite is equivalent to the inner grou p of Iranian ophiolites (e.g. Nain. Shahr-Babak, Sabzevar, Tchehel Kureh an d Band-e-Zeyarat) and was formed as a result of closure of the northwestern branch of a narrow Mesozoic seaway which once surrounded the Central Irani an microcontinent. (C) 1999 Elsevier Science Ltd. All rights reserved.