Cretaceous magmatic migration and flare-up in Pamir–Karakoram

Abstract

The Pamir–Karakoram terrain contains abundant Cretaceous granitoids, and minor coeval volcanic rocks and mafic intrusions, which represent a wide (ca. 400 km) continental arc developed during northward subduction of Neo-Tethys oceanic lithosphere. However, little is known of the tectonic controls on the magmatic flare-ups and their implications for crustal thickening. In this paper, we report new zircon UPb age and Hf isotope data for Cretaceous biotite granites, two-mica granites and mafic microgranular enclaves (MMEs) from the large-scale granitic batholiths in Pamir. These and previously published results show there were four episodes of Cretaceous magmatism (124–108, 107–103, 102–92, and 80–70 Ma). The Cretaceous arc migrated progressively landward from Karakoram to Central Pamir with a maximum arc–trench distance of ca. 400 km during the first magmatic episode (124–108 Ma). The igneous rocks are mainly S-type granites with enriched zircon Hf isotopic compositions (ɛHf(t) = −19.8 to −5.4), indicating derivation by partial melting of metasedimentary units. A magmatic flare-up at 105 ± 2 Ma generated voluminous I-type granites and minor mafic igneous rocks that cover ca. 30% of the study area. The I-type granites have higher zircon ɛHf(t) values (−19.0 to −2.0) as compared with those of the earlier S-type granites, which reflect the addition of mantle-derived materials (ca. 30%). The third episode of magmatism (102–92 Ma) generated mainly intermediate–silicic volcanic rocks, which are geochemically similar to I-type granites and have ɛHf(t) values of −15.0 to −1.5. The final episode of magmatism (80–70 Ma) was dominated by alkaline and mafic igneous rocks formed in a post-collisional setting, which have variable ɛHf(t) values of −6.6 to 10.5. Our study highlights that the Early Cretaceous magmatic flare-up in the Pamir–Karakoram terrain was related to foundering of the flat-slab of Neo-Tethys oceanic lithosphere that was previously subducted, which induced voluminous underplating of mantle-derived magma in the lower crust. This process had a key role in crustal thickening in the Pamir–Karakoram terrain prior to the Cenozoic India–Asia collision.