Petrogenesis and tectonic implications of the high-K Alamas calc-alkaline granitoids at the northwestern margin of the Tibetan Plateau: Geochemical and Sr–Nd–Hf–O isotope constraints

• SIMS zircon U–Pb dating shows that the Alamas granitoids crystallized at ∼446 Ma.
• The granitic rocks were derived from partial melting of the mafic lower crust, with a component of mantle-derived magma.
• The Alamas pluton was the product of early stage post-collisional magmatism.
• The NKT and the SKT may be amalgamated prior to ca. 446 Ma along the Kudi-Qimanyute suture zone.

The Alamas granitoid pluton in the eastern part of the Western Kunlun Orogen, the northwestern margin of the Tibetan Plateau, is composed of quartz diorite. Zircon separates from the pluton has SIMS U–Pb age of ∼446 Ma. Rocks from the pluton have a narrow range of SiO2 (56.84–62.57 wt%), MgO (1.76–2.94 wt%), and total alkalis (Na2O + K2O = 5.14–9.59 wt%), and are metaluminous and high-K calc-alkaline to shoshonitic in composition. They are enriched in light rare earth elements (LREEs), with (La/Yb)N = 14–25, and show weakly negative Eu anomalies. These rocks are relatively enriched in Sr (472–676 ppm) and Ba (435–2388 ppm), and depleted in Nb, Ta, Th, and Ti. Their εNd(t) values range from −6.4 to −8.4, and (87Sr/86Sr)i = 0.7184–0.7200. Zircons from the pluton show εHf(t) values of −1.4 to −8.8, and δ18O = 6.4–9.0‰. Geochemical data indicate that the granitoids were likely derived from the reworking of an ancient, deep crustal source, influenced by a minor mantle-derived component. Magmatic differentiation was dominated by the fractional crystallization of hornblende, biotite, and accessory minerals such as apatite, allanite, and Fe–Ti oxides. In summary, the Late Ordovician Alamas pluton is an I-type granitoid that was emplaced in a post-collisional environment, suggesting that this tectonic stage had already initiated prior to ∼445 Ma.

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