Tahseenullah Khan

Geology of the Chalt-Babusar transect, Kohistan terrane, N. Pakistan: implications for the constitution and thickening of island-arc crust

The Kohistan island arc terrane is sandwiched between the collided Indian and Karakoram plates in the Himalaya of North Pakistan. Structures related to collision, during which the arc was thrust onto the leading edge of continental India along the Main Mantle Thrust, have resulted in exposure of an almost complete section of arc crust. Mapping along a transect across the east end of the arc terrane provides new data concerning the magmatic emplacement of several of the principal units. The base of the arc here is occupied by a major stratiform ultramafic–gabbroic complex, the Sapat complex. This was intruded into the base of a thick pile of meta-volcanic rocks which make up the Kamila amphibolite belt, and which comprise a varied sequence of basalts some with MORB-type tholeiitic affinities and some with island-arc tholeiitic affinities as well as calc-alkaline andesites. Ultramafic and gabbronorite rocks of the Chilas complex are intrusive into the top of the Kamila amphibolite belt. The upper part of the crust comprises meta-sediments and meta-volcanic rocks of the Jaglot and Yasin-Chalt Groups. These were formed in one or more arc-related basins, and host much of the Kohistan batholith. A three-stage history of crustal thickening can be documented for the Kohistan arc. From its initiation at ca. 125–120 Ma until ca. 90 Ma, the arc grew downward through magmatic emplacement, into its base, of stratiform ultramafic–gabbroic plutonic complexes, and upward through extrusion of volcanic sequences. In Stage 2, the focus of crustal growth shifted upwards from the base of the arc with emplacement of the Chilas complex along the interface between the Kamila amphibolite belt and the overlying volcano-sedimentary cover. This stage of crustal thickening was accompanied by shortening associated with the 90–80 Ma Kohistan–Karakoram collision. Finally, in Stage 3 (80–45 Ma), the Kohistan batholith was emplaced into deformed cover rocks of the uppermost part of the arc crust.

Back-arc basin assemblages in Kohistan, Northern Pakistan

AbstractThe east central part of the Kohistan magmatic arc is made up principally of the Jaglot Group. From bottom to top it consists of I) paragneisses and schists intercalated with amphibolites and calc-silicates (Gilgit Formation), II) Gashu-Confluence Volcanics (GCV) and III) the Thelichi Formation comprising a volcanic base (Majne volcanics) and turbidites, marble, volcanoclastic sediments and lava flows. Metamorphic grade varies up to the sillimanite zone. The GCV are correlated with the Chalt volcanics and the Thelichi Formation with the Yasin Group. Other lithologies include the Chilas Complex, the Kohistan Batholith and part of the Kamila Amphibolite. Metavolcanics show a broad range in chemical composition. Geochemical parameters used to specify the tecto-nomagmatic regime suggest affinities of both island arc and MORB-like back-arc basin basalts. Kohistan can be divided into three tectonic zones, I) the southern (Kamila) zone comprises amphibolitized basalts, and mafic and ultramafic rocks, II)...

Sm-Nd and Lu-Hf Isotope Geochemistry of the Himalayan High- and Ultrahigh-Pressure Eclogites, Kaghan Valley, Pakistan

Eclogites are generally considered as derived from basaltic or gabbroic rocks which have either been intensely metamorphosed during subduction-obduction related processes or, associated with continental crust and affected by major crustal thrusting. The advantage of petrological, geochemical, and geochronological study of eclogitic rocks is twofold. First, metabasic rocks are capable of preserving the original magmatic characteristics of igneous formations. Second, the study of eclogites enables us to appreciate the behaviour of isotopic tracers during high-grade metamorphism.

Petrogenetic Comparison of the Mafic Dykes in the Kohistan Paleo-Island Arc-Back-Arc System, Himalayas of North Pakistan

Kohistan paleo-island arc is considered a complex entity of island arc and back-arc rock assemblages. It contains two major magmatic complexes: the layered ultramafic and mafic Chilas complex at the base, and the Kohistan batholith in the middle part. Back-arc basin rock assemblages, the Jaglot group, occur as septum within the Kohistan batholith. Mafic dykes intrude the ultramafic and mafic rocks of the Chilas complex and the diorites, granodiorites and tonalities of the Kohistan batholith. Mafic dykes both basaltic and doleritic, intrude the metasediments and gabbros of the Jaglot group. Petrographic and geochemical variations are observed in all mafic dykes of the area. Mafic dykes of the Chilas complex are picrobasalt to basalt, tholeiitic and subalkaline. These dykes are mainly amphibolites that contain green hornblende, biotite, epidote and plagioclase. Mafic dykes of the Kohistan batholith are tholeiitic and calc-alkaline/alkaline basaltic andesite to trachyandesite, which preserved trachytic-type texture. Mafic dykes of the Jaglot group are tholeiitic basalt and basaltic andesite, which preserved ophitic to subophitic texture. Mafic dykes of the Chilas complex are high in Al2O3, MgO, CaO, and low in Na2O, K2O, P2O5, Zr, Rb, Sr, Ba and Nb. Mafic dykes of the Kohistan batholith contain diagnostically high Al2O3 and low TiO2, Zr, Rb, Sr, Ba and Nb. The calc-alkaline/alkali basaltic mafic dykes are distinctly high in Na2O, K2O and P2O5, Rb, Sr, Ba, Nb and low in MgO, CaO, Fe2O3 and Y. Mafic dykes of the Jaglot group are distinct to contain high TiO2, Fe2O3, Na2O, Y, Zr and low Al2O3, K2O, Rb, Sr and Ba. These dykes show flat pattern with slightly high HFSE/LILE ratios whereas the other dykes show opposite characteristics with marked Nb depletion and Sr enrichment, when compared with N-MORB and primitive mantle values. Mafic dykes of the Jaglot group show enriched MORB-type affinity whereas mafic dykes of the Chilas complex and the tholeiitic dykes of the Kohistan batholith give island arc type signatures. Calc-alkaline/alkali basaltic dykes give continental margin origin. All mafic dykes of the area are derived by the partial melting of depleted, heterogeneous mantle and enriched mantle sources during island arc, continental margin and back-arc tectonic settings.

Origin of the mafic dykes in Nagarparker area of Pakistan

The Nagarparker area of Pakistan comprises igneous and metamorphic rocks dissected by coarse-grained gabbro dykes and medium- to fine-grained dolerites at Karai, Dhedvero, and the central part of the area. The gabbro dykes contain plagioclase (An79), diopside, diopsidic-augite, and olivine with minor biotite, magnetite, ilmenite, titanite, and ilmenite-titanite composite grains. The dolerites consist of plagioclase, titaniferous augite, augite, hornblende, biotite, titanite, apatite, zircon, titanomagnetite, rutile, and ilmenite. Based on geochemistry, the gabbro dykes (group I) and dolerites (group II) are classified as subalkaline whereas group III dolerites as alkaline. In spidergram, Nb troughs and Sr crests are depicted in the group I and group II dykes whereas the group III dykes illustrate positive Nb and negative Sr with relatively smooth trace element pattern. The mineralogical composition, discriminant quadratic analysis, and trace element variations distinguish the mafic dykes of the study area into two magmatic groups. The first group comprises group I and group II dykes which depict island-arc type whereas the group III dykes resemble OIB-type compositions similar to Seychelles islands and the Tavidar region of Indian Rajasthan. The tentative timing of their emplacement may be Neoproterozoic and Cretaceous (?), respectively.

Petrogenetic evolution of pegmatites of the Shigar Valley, Skardu, Gilgit-Baltistan, Pakistan

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 muscovite-tourmaline 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 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.

Shigar valley gemstones, their chemical composition and origin, Skardu, Gilgit-Baltistan, Pakistan

A variety of gemstones is being mined in the Shigar valley, Skardu, Pakistan. These include beryl (goshenite and aquamarine), tourmaline (schorl), garnet (almandine–spessartine), apatite, topaz, fluorite, zoisite, clinozoisite, and axinite, mostly occurring in complex or zoned pegmatites and metamorphic rocks. These have been analyzed using electron probe micro-analyzer and X-ray diffractometer. The mineral chemistry of each gemstone is similar to its respective typical gemstone variety with homogenous chemical composition. Field and chemical characteristics suggest that beryl, tourmaline, garnet, apatite, topaz, and fluorite are occurring in zoned pegmatites which are largely formed by magmatic hydrothermal fluids in the cavities and vugs within the intermediate zone. However, zoisite, clinozoisite, and axinite may have a metamorphic and/or metasomatic origin.