Petrology of the Shewa-Shabazgarhi Igneous Complex, district Mardan, North Pakistan
Abstract
The Shewa-Shabazgarhi complex is an isolated triangular outcrop, occurring about 12 km northeast of Mardan. The complex resides 60 km south of Main Mantle Thrust as part of the Indo-Pak plate and consists of basic and acidic rocks including microporphyry, metagabbro, metadolerite and local quartz monzonite, riebeckite gneiss, aegirine riebeckite porphyry and porphyritic microgranite, together with basic and acidic dykes and sills.
The common minerals in basic rocks include hornblende, clinopyroxene, magnetite, biotite, epidote and apatite. The acidic rocks contain orthoclase, perthite and palgioclase together with riebeckite and aegirine, indicating alkaline characters. The mineralogy reflects amphibolite facies metamorphism followed by retrogression down to at least the upper greenschist facies environment in basic as well as in acidic rocks. Following metamorphism a considerable degree of deformation (cataclasis and/or mylonisation) is indicated in the acidic rocks.
Assessment of the chemical data suggests two different parent magmas; a rift-related tholeiitic type for the basic rocks and an alkaline to peralkaline type for the acidic rocks; both being emplaced in continental environments. The chemistry also suggests the control of fractional crystallization on the initial distribution of elements among the minerals of the two groups. A plagioclase-magnetite-clinopyroxene dominant assemblage on the liquidus in basic rocks and a plagioclase-hornblende dominant assemblage on the liquidus in acidic rocks, during fractionation, are suggested. A rift valley origin proposed by previous workers is supported.
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