Title of Thesis
GEOCHEMISTRY OF PRINCIPAL SILICATE PHASES OF THE CHILASS MAFIC-ULTRAMAFIC COMPLEX, KOHISTAN, N. PAKISTAN
Author(s)
Rubina Bilqees
Institute/University/Department Details
University of Peshawar, Peshawar / National Center of Excellence Geology
Session
2005
Subject
Geology
Number of Pages
289
Keywords (Extracted from title, table of contents and abstract of thesis)
Silicate Phases, Mafic-Ultramafic Complex, Tectonic Settings, Geology, Oliv1ne, Plagioclase, Pyroxenes, Amphibole, MGN Rocks, UMA Rocks
Abstract
The Chilas complex is an important element of the Kohistan magmatic arc. It is thought to have been emplaced 85 Ma ago during the rifting of the arc after collision with the Karakoram plate. Covering 8000 km2 area, it stretches for 250 km from the Nanga Parbat to western Dir. Over 85 % of the complex comprises homogeneous gabbronorites (MGN) with local, minor layers of pyroxenites and anorthosite. Within these are emplaced small bodies (<10 km2) of ultramafic- mafic-anorthositic (UMA) rocks, mostly in the eastern part of the complex. The MGN rocks contain biotite, and Fe-Ti-oxides but no olivine and spinel while the UMA rocks contain olivine, Cr- and/or Al-spinel but no biotite. The UMA commonly show well-developed layering and several syndepositional structures (folding, faulting, brecciation, slumping) suggestive of emplacement in tectonically active areas. Thousands of microprobe analyses show that the UMA rocks are characterized by mineral phases that are more magnesian and more aluminian than those of the MGN rocks. In the MGN, plagioclase is An40 to An70, orthopyroxene En50 to En68, clinopyroxene is diopsidic to augitic and amphibole is generally hornblendic. In the UMA, plagioclase ranges from An75 to An100, olivine from FO7l to FO94, orthopyroxene from En58 to En84, whereas clinopyroxene is diopsidic and amphibole is tschermakitic-and pargasitic-hornblende. The range of olivine compositions (Fo < 96) and the coexistence of olivine and pyroxenes with highly calcic plagioclase in the UMA association are comparable with other island arc cumulates. High modal abundance of orthopyroxene, magnetite and ilmenite (in MGN) and Mg-Fe-Alk variation indicate a calc-alkaline origin for the complex. Projection of the pyroxene compositions of Chilas complex on Mg-Fe-Ca face and their Ti and Cr contents suggest an island arc affinity. The rather low Cr-number in the Cr-spinel from UMA and field evidence suggests the emplacement of the complex in extensional environment during arc rifting. The Al and Fe-Mg contents of the orthopyroxene indicate a transition between igneous and metamorphic domains. Reaction between calcic plagioclase and olivine has resulted in the development of coronas comprising shells of orthopyroxene, followed by clinopyroxene (in a few), hornblende, and hornblende + spinel symplectite in the UMA association. The occurrence of highly calcic plagioclase in the UMA may be a result of high PH20 or high Ca/Na ratio of the magma. But the scarcity of primary hydrous minerals in the complex is suggestive of rather anhydrous magma. Huge volume and localized petrographic and chemical variations suggest that the complex has been built by the emplacement of several batches of very similar magma. .
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