Petrogenetic evolution of pegmatites of the Shigar Valley, Skardu, Gilgit-Baltistan, Pakistan
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ORIGINAL PAPER
Petrogenetic evolution of pegmatites of the Shigar Valley, Skardu, Gilgit-Baltistan, Pakistan Muhammad Hassan Agheem 1 & Mohammad Tahir Shah 2 & Tahseenullah Khan 3 & Mamoru Murata 4 & Humaira Dars 1 & Muhammad Zafar 3
Received: 21 December 2014 / Accepted: 24 March 2015 # Saudi Society for Geosciences 2015
Abstract Pegmatites of granitic composition intrude the Dassu orthogneiss of the Asian plate in the Shigar Valley of Pakistan. On the basis of field and petrographic studies, these pegmatites are distinguished into evolved and simple pegmatites. The evolved pegmatites are further distinguished as muscovite-tourmaline-beryl-garnet and muscovitetourmaline pegmatites whereas the simple pegmatites as biotite ± garnet ± muscovite and muscovite-biotite ± garnet pegmatites. In the evolved pegmatites, the predominant minerals are albite and muscovite while in the simple pegmatites orthoclase and biotite are distinctive. Both the pegmatite types seem to be associated genetically to one common magma source and the variation within the mineral constituents are due to fractional crystallization. In spider diagrams, all the pegmatite types illustrate similar negative sloping trends from LILEs to HFSEs with depletion in Ba, Sr, and Ti concentration. In simple pegmatites, P is strongly depleted as compared to the evolved pegmatites. REEs illustrate nearly smooth patterns in all the pegmatite types with contrasting negative Eu anomaly signifying plagioclase fractionation. Both the evolved and simple pegmatites contain aluminous mineral assemblages and show mainly peraluminous geochemistry, which suggest
* Muhammad Hassan Agheem [email protected]
their derivation from pelitic rocks of the Asian continental crust in syn-collision tectonic setting, ensued possibly by the collision of the Indian plate with Asian plate. Keywords Pegmatites . Shigar Valley . Peraluminous . Syn-collision
Introduction The Shigar Valley is located about 32 km north of Skardu, the gateway for most of the expeditions to K-2. This valley has plenty of pegmatite intrusions, hosting various types of gemstones such as aquamarine, goshenite, topaz, tourmaline, and fluorite, which are being excavated locally and marketed internationally (e.g., Agheem et al. 2011). Several studies have been conducted in the past for the classification of pegmatites. For example, Ćerný (1982a), on the basis of mineralogy, texture and internal structure, reported simple and complex pegmatites, which could be the last fractionated part of a granitic melt or separate magmatic pulses in any metamorphic terrain (Ćerný 1982a; 1991a; Baker 1998). Major studies, carried out in the past were mostly focused on the internal evolution, zoning, formation of cavities and vugs, and types or classes of pegmatites (e.g., Jahns 1982; Ćerný 1982a, b; 1991a, b; London 1986; Baker 1998; Nabelek and Liu 2004). In this paper, petrological and geochemical approach has been adopted for knowing the possible origin of the Shigar Valley pegmatites, which has not been done before.
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