Petrographical and geochemical constraints on the origin of the igneous intrusions in Gilgit Metamorphic Complex, northern Kohistan Island Arc (Pakistan): Implications for Petrogenesis, magma evolution and tectonic setting
Keywords:
Keywords: Fractionation; geochemistry; petrography; Syn-collision; Kohistan Island Arc.Abstract
The small granitic and volcanic intrusions in the Gilgit Metamorphic Complex in the northern Kohistan Island Arc are not studied till date. As a result, the petrogenesis, magma evolution and tectonic setting of these igneous intrusion are conjectural. In this contribution, we present petrography, major and trace elements data of these igneous intrusion from Sakar Koi to Chimis Dass area Gilgit with a view to describe their petrogenesis, magma evolution and geodynamic setting. The rocks mainly contain quartz, plagioclase, biotite, muscovite, olivine, pyroxene, biotite, and hornblende with some accessory minerals such as apatite, sphene and zircon. The SiO2 and Na2O+K2O values are in range of 53.60 to 73.19 wt.% and 4.55 to 9.129 wt.%, respectively. Geochemically, the rocks were recognized as granite, granodiorite, and gabbroic-diorite and basalt trachy andesite based on TAS diagrams for plutonic and volcanic rocks. The values of A/NK and A/CNK are in the range of 1.32 to 3.14 Wt.% and 0.67 to 1.50 wt. %, respectively indicating calc-alkaline metaluminous magma series except granite rock which is peraluminous. In tectonic discrimination diagram, the granite and granodiorite fall in the syn-collisional field and the gabbroic-diorite show volcanic arc environment. According to the ratios of k/Rb (112.54-970 wt.%) and Ba/Rb (0.1-2.9 wt.%), the granite and granodiorite were highly evolved, while the gabbro-diorite and basalt trachy andesite which show higher values of Fe2O3 (112.54 wt.% and 8.13 wt.%, respectively) and MgO 6.56 and 5.76 wt.%, respectively) with higher ratios of K/Rb (970 and 924.86 wt.%, respectively) and Ba/Rb (8.3 and 2.9 wt.%, respectively) indicating that these rocks were comparatively less evolved. Field observation, available geochemical and petrographic data indicate the source for rocks is upper mantle and magmatism was more acidic with time and formed granite and granodiorite.
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