Abstract
The Cenozoic volcanism of western Saudi Arabia extends from southern Yemen to Jordan northward. They cover an area of nearly 180,000 km (super 2) . The rocks are dominated by alkali olivine basalts and olivine basalts. Al-Wahbah crater, a part of Harrat Kishb, represents a model occurrence to study the gneisses of these rocks. New mineral chemistry and isotopic data are presented. It aims to follow the isotopic, mineralogical, and thermobarometry variations among these volcanics. Amphiboles of the studied volcanics belong to the monoclinic calcic group. The chemistry of the amphibole crystals shows two ranges of pressure. They are 3.6-5.6 and 0.38-0.78 kbar. The Al (sub iv) values of the amphiboles are in the range of 1.202 and 1.407, indicating corresponding temperature condition of 820-920 and 620-720 degrees C, respectively. The feldspar of the studied samples has the composition of plagioclase, though some grains have sanidine composition. They are formed in temperature range of 975 and 400 degrees C. The coexisting amphiboles and plagioclases indicate two sets of pressure and temperature. They are 540-575 degrees C (3.5-4 kbar) and 510-525 degrees C ( approximately 2 kbar), respectively. Rb-Sr isochron of the whole rock yields an age of 0.867+ or -0.160 Ma with initial Sr (super 87) /Sr (super 86) of 0.702+ or -0.00086. The low initial ratio of Sr (super 87) /Sr (super 86) together with positive values of epsilon Nd today implies that the studied volcanics have mantle source. Meanwhile, the present isotopic data suggest extraction of juvenile magma from asthenosphere source. The present study shows that the Al-Wahbah crater rocks belong to Cenozoic basalts and indicate EM-I-like signature. Copyright 2017 Saudi Society for Geosciences