Petrographic/geochemical studies of primary and alteration-weathering minerals in garnetiferous ultramafic xenoliths-basanite, Tartous Province, NW Syria

摘要

Kimberlitic-pyropic peridotite-xenolites, probably of Jurassic-Cretaceous age, were found mixed with a younger Upper Tertiary basanitic diabase, as flow texture of plagioclase laths and ilmenite rods around those xenoliths indicated. Mafic-ultramafic rocks were crushed, sheared, and cropped along a creek about 15-18 km NE of the town of Dreikeesh, NW Syria. K-40-Ar-40 isotopic dating of a pure fresh black cpx sample, collected from the peridotite xenoliths, yielded an age of about 70 Ma. This age is concurrent to the time when Africa, Eurasia, and America were part of the super continent Pangaea. It also suggests that kimberlite-pyropic peridotitic rocks were located within a cratonic pipe prior to their 2000-km eastward journey (starting from the Mid-Atlantic Rift). Sampled outcrops were located within a ring of about 40 km diameter, considered to be a dome consisting of one or more clusters of kimberlitic pipes. The dome structure, mostly covered with Upper Jurassic-Cretaceous carbonate beds, was tilted westward, and rifted and sheared along its eastern edge. Tilting and crushing were accomplished after the opening of the Red Sea in the Miocene, and the counterclockwise movements of the Arabian plate, which folded the carbonate beds to form the N-S-trending Alawite mountain range along the Syrian coast. Olivine, cpx, and pyrope were the major phases in peridotite. Pyrope, including its Cr-Ni-contents, was found to be the best indicator to decipher the temperature-pressure (T-P) conditions for the system. The inferred temperature was found to be about 1460 degreesC, at a pressure of 62 kb (around 207 km-depth). Petrographic studies revealed many zoned, resorbed, octahedral and non-metasomatized tiny grains, associated with partly to wholly metasomatized and iddingsitized olivines. (C) 2004 Elsevier B.V. All rights reserved.