Geochemical and isotopic constraints on the origin and evolution of the Matum-Das tonalite and mafic enclaves from the Kohistan Batholith, Northern Pakistan

• Published in: Lithos 2024-04, Vol.470-471, p.107535, Article 107535
• Main Authors: Ali, Mubasir, Zhao, Kui-Dong, Wang, Chao, Rehman, Hafiz U., Hussain, Amjad, Mahar, Munazzam Ali, Farhan, Muhammad, Lutfi, Wasiq
• Format: Article
• Language: English
• Subjects: Crystallization Fractionation; Kohistan Island Arc (KIA); Mafic microgranular enclaves (MMEs); Matum Das tonalite; Subduction zone
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2024.107535

Granitoids at subduction zones play a significant role in the generation and evolution of continental crust. The Kohistan Batholith in the Kohistan Island Arc is, in large part, comprised of granitoids with various sizes, amongst which the Matum-Das tonalitic pluton stands out as the most enigmatic due to its emplacement age and unique geochemical signatures. The genesis and formation mechanism of the Matum-Das tonalite is still unclear. In addition, the Matum-Das tonalite hosts abundant mafic microgranular enclaves (MMEs), which have rarely been mentioned in previous studies. Here, a combined investigation of the petrological characteristics, mineral chemistry, zircon UPb geochronology and Hf isotope, whole-rock major, trace element and SrNd isotope geochemistry has been conducted on the Matum-Das tonalite and the MMEs. Zircon UPb dating indicates that the Matum-Das tonalite was emplaced at ∼150 ± 2 Ma, comparable with the age of the MMEs (∼149 ± 1 Ma). The consistent isotopic signatures of the host tonalite (ISr = 0.7046∼–0.7053; εNd(t) = +6.0 ∼ +7.8; εHf(t) = +13.9 ∼ +18.0) with the MMEs (ISr = 0.7034∼–0.7048; εNd(t) = +6.6 ∼ +9.4; εHf(t) = +11.2 ∼ +20.4) indicate the same parental magma source, which was mainly derived from the depleted mantle. The tonalite displays highly acidic signatures with SiO2 contents ranging from 73.72 to 76.06 wt% and low alkali (Na2O + K2O) contents of from 3.62 to 5.04 wt%. It has low Sr and high Y contents with low Sr/Y ratios. Compared to the host tonalite, the MMEs are dioritic in composition and have metaluminous affinity. Combined with the non-adakitic traits and high-water contents (9.07–11.68 wt%) inferred from the amphibole chemistry, we propose that the Matum-Das tonalite was most likely formed by fractional crystallization of hydrous magma derived from the metasomatized asthenosphere mantle. The associated MMEs were formed in the earlier stage of magma evolution from the same parental magma. The onset of intra-oceanic subduction of the Neo-Tethys Ocean in the late Jurassic is responsible for the island arc volcanism. The studied tonalitic rocks yielded the oldest ages within the KIA, and they represented the magmatic products in the initial stage of the Neo-Tethys intra-oceanic subduction. •The Matum-Das tonalites were emplaced at ∼150 Ma.•They were formed by fractional crystallization of a basaltic magma.•The MMEs are cognate in origin with the host tonalite.•They represent the initial magmatic activities in the