Title of Thesis
PETROLOGICAL AND STRUCTURAL STUDIES OF THE IGNEOUS ROCKS OF THE BARAN LAK AREA, BELA-KHUZDAR, DISTRICTS, BALOCHISTAN, PAKISTAN
Author(s)
Mehrab Khan
Institute/University/Department Details
University of Balochistan, Quetta, Pakistan/Department of Mineralogy
Session
1998
Subject
Mineralogy
Number of Pages
147
Keywords (Extracted from title, table of contents and abstract of thesis)
igneous rocks, baran lak area, bela ophiolite, neoptethysian, ophiolites, mylonitic zone, sepentinzed harzburgitic mantle, gabbros, dioritic
Abstract
The Bela ophiolite is a piece of the NeopTethysian oceanic lithosphere, obducted on the western Indo-Pakistan continental margin, during the late Maastrichtian to Early Eocene. It has not yet been crushed by continental collision, as the northern ophiolites of Indo-Pakistan plate were involved in continent-continent collision. The Bela ophiolite has preserved the structures imprinting during its formation and emplacement. It is the largest outcrop of oceanic lithosphere in this western belt and forms a 10-60 km wide, N-S oriented, 300 km long belt.
The mantle peritonitis of the Bela ophiolite have recorded two successive plastic deformation events, the first one related to the accretion of the lithosphere (asthenopheric shear flow) and the second one is imprinted during the first step of its emplacement (intra-oceanic thrusting). During the intra oceanic thrusting, the mantle peridotites were plastically deformed in high stress conditions responsible fro making metamorphic sole. This deformation grades upward into the mylonitic zone
Shear sense determinations in the peridotite mylonite in the ophiolite foot wall and imbrication structures in the underlying accretionary wedge indicate emplacement toward ESE.
New Ar39 /Ar40 dating and paleontological studies show that Bela ophiolite consists of two oceanic lithosphere of different age. The upper unit (true ophiolite) formed around 70 Ma and the lower unit (Accretionary wedge-trench sediments) formed Aptian / Albian-Maasttrichtian.
The upper unit shows a classical ophiolite sequence (in ascending order) : a metamorphic sole, sepentinzed harzburgitic mantle sequence, layered peridotite/gabbros, layered/foliated and isotropic gabbros sheeted dikes and <100m an extrusive sequence. The lower unit contains ocean ridge related pillow lavas, covered by a Mn-enriched horizons, a sedimentary sequence (radiolarian cherts, shales and calci turbidites) and covered or intruded by doleritic to micro dioritic/gabbroic Fe-rich tholeiitic intrusions and alkaline rocks generated due to the passage of Indian Continental margin over the reunion hotspot
Post-Nal limestone (Miocene) faulting and folding structures developed during the post-emplacement tectonic activities. These deformations are related to the evolution of the sinistral Ghazaband and Ornach-Nal fault systems, which developed in response to the Himalayan collision. By this process parts of the upper ophiolitic unit were juxtaposed with the lithologies of the acceretionary prism and with post-emplacement “exotic” Nal limestone blocks, creating the impression of a mélange
At the time of obduction, the Indo-Pakistan plate was not its present position, the Bela ophiolites thus traveled to the margin of the Indian continental plat in the Tertiary where the ophiolite reached to its present position.
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