干旱区生态环境知识资源中心

Field evidence from the Baladiyah complex in the northern part of the Arabian Nubian Shield of Saudi Arabia indicates several erosional unconformities separating different high-and medium-grade metasedimentary sequences. This suggests that the collision between East and West Gondwana involved several cycles of exhumation and burial, providing a unique opportunity to study the multiple stages of this orogeny. A mineral equilibria approach and thermodynamic modeling are used to place constraints on the formation conditions of each of these cycles. It is shown that the complex is characterized by three regional metamorphic events followed by a fourth metamorphic event related to shear heating owing to the thrusting of post-tectonic granites. During the first metamorphic event peak metamorphism was at around 705 715 degrees C and 5.2-5.6 kbar followed by subsequent decompression to the Earth’s surface. Subsequently deposited sediments attained 635-670 degrees C and 4.2-5 kbar during a second metamorphic event, followed by exhumation, erosion and deposition of molasse sediments. The rocks were then buried for a third time and metamorphosed to greenschist-facies metamorphic conditions (330 +/- 30 degrees C and 3.6-4.6 kbar) under the load of the molasse sediments. Finally, post-tectonic granites were intruded and thrust during the final Pan-African exhumation, causing a fourth metamorphic event (700 +/- 25 degrees C). Correlation of this metamorphic evolution with the deformation history shows that the first and the second metamorphic events occurred in a compressional regime (D-1 and D-2), interpreted to be related to the first (750 Ma) and the second (676 Ma) collision stages between East and West Gondwana, respectively. The third deformation phase began with a compressional regime causing the third metamorphic event, and then turned into an oblique transpressive regime, which led to escape tectonics and the development of the large-scale Najd strike-slip shear zone system.