Hafiz Ur Rehman

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.