SUBDUCTION RELATED MAGMATISM OF THE SOUTHERN CRIMEA: OFFSHORE AND ONSHORE

Abstract

A geochemical comparison of igneous rocks of two regions of Mesozoic subduction related magmatism located on the opposite sides from the southern end of modern Crimea was carried out. The first region covers Lomonosov submarine massif (LSM) to the west offshore, and the second region includes a com plex of small intrusions of Mountainous Crimea and Ayu Dag (SI AD) to the northeast onshore. Contents and ratios of Rb, Sr, Zr, Y, Th, Yb, Ta, Nb and other rare earth and rare elements with petro chemical and geochronological data having been taking into account were used to determine the geody namic conditions for the formation of normal series igneous rocks of both regions. All studied rocks dis play subduction features more or less. Crustally contaminated LSM rocks were formed as a result of «anomalous» suprasubduction volcanism. Small intrusions show signs of typically subduction related «normal» island arc rocks with a marked influence of spreading processes; they are also contaminated by the continental crust, but the crust itself was different. Spreading features are most pronounced in Ayu Dag tholeiitic rocks, crustal input to the formation of which was minimal. It is concluded that igneous rocks of the studied regions were formed in consequence of two different subduction cycles: the first Middle Jurassic Early Cretaceous and the second — Late Cretaceous Early Paleogene. The first cycle was caused by the southward subduction, and the second one — by the northward but very gentle (flat slab) subduction; the continental fragments under which subduction occured were different. The course of both subduction zones supposed to be not sub latitudinal but diagonal, sub parallel to the modern structure of the Crimean Mountains. A succession of magmatic and geodynamic events was proposed for the Southern Crimea throughout the Jurassic — Paleogene, according to which in the SI AD magmatism manifested itself during the first subduction cycle when increasing back arc spreading, and the LSM was formed as a result of superposition of both subduction cycles with the second being dominant.

Keywords

• іgneous rocks
• crimea
• subduction
• geochemistry