229-8 Petrogenesis, geochemistry, and geochronology of plutonic rocks from the Kohistan Batholith and Khunjerab Pluton, NW Pakistan

Session: Crustal Petrology (Posters)

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Abstract:

The subduction of the Neo-Tethys oceanic crust beneath the Eurasian continent had created a sequence of magmatic arcs in the northwest of Pakistan composed of different plutonic and volcanic rocks. This study examines the Khunjerab Pluton (KP) and the granitoid rocks of the Kohistan Batholith (KB), a component of the Kohistan-Ladakh Island Arc (KLIA). Petrology, zircon U-Pb geochronology, mineral chemistry, and the major and trace element geochemistry of whole-rock samples are among the primary domains of examination. Conferring to zircon U-Pb dating, the KP formed at ~106.4 ± 0.4 Ma ago, while the KB was developed at ~91.7 ± 0.3 Ma ago. Both granitoids have metaluminous to peraluminous compositions (ASI: 0.9–1.2) and exhibit calcic to calc-alkaline properties (Na₂O + K₂O: ~3.6–10.6 wt.%; SiO₂: ~60–73 wt.%). Depletion in Ti, Ta, and Nb, as well as enrichment in big ion lithophile elements like Cs, Rb, and K, are notable geochemical markers. Furthermore, the hypothesis of I-type arc magmatism is supported by a direct correlation between Th and Rb and a negative correlation between SiO₂ and P₂O₅. TEM observations, thermal modelling, and zircon U-Pb data all indicates the ongoing magmatic activity in the Kohistan arc from the Late-Jurassic to the Late-Cretaceous (150–91 Ma). On the other hand, major magmatic events occurred between 120 and 106 Ma on the Khunjerab pluton, which is part of the Karakoram terrain of the Eurasian plate. According to geochemical data, both locations have acidic compositions, with relatively low total alkali level (~3.6–10.6 wt.%; KB and ~5.1–7.4 wt.%; KP) and SiO₂ concentrations ranging from ~67.5–73.3 wt.% for the KB and ~60–71.4 wt.% for the KP. For both units, mineral chemistry, notably that of amphibole and biotite, indicates water-rich magma sources (8–10 weight percent and 3.1–3.5 weight percent, respectively), suggesting partial melting of a hydrous, metasomatized mantle wedge. This work infers that the two magmatic units are different in terms of both age and tectonic setting: island arc magmatism happened in Kohistan during the late Cretaceous (~91 Ma), while continental-arc magmatism happened in the Khunjerab area during the ~106 Ma (mid-Cretaceous). The study also emphasizes how crucial transmission electron microscopy (TEM) together with zircon U-Pb dating is for understanding intricate magmatic histories.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9672

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