Late Jurassic adakite˗like granodiorite along the southern Karakoram block, NE Pakistan: New evidence for subduction initiation of the Neo-Tethys Ocean

Abstract

The Neo-Tethys Ocean separated the India and the Karakoram block of southern Asia during the late Mesozoic. Most native records have been significantly obscured by subsequent deformation and metamorphism during the early Tertiary India-Asia collision, the consecutively opening and closure of the Neo-Tethys is least understood. This paper presents new petrologic, bulk rock major and trace elements, and zircon U-Pb age and Hf isotopic investigation on the recently identified Karimabad meta˗granodiorites in the southern Karakoram block, N Pakistan. The results indicated these meta˗granodiorites experienced one stage of crystallization (150 ± 0.8 Ma, n = 28, MSWD = 1.5) and at least one stage of metamorphism (59 to 63 Ma). The Late Jurassic meta˗granodiorites display some adakite-like geochemical features, such as high SiO2(67.8–69.3 wt%), Al2O3(16.4–17.7 wt%), Sr (386–476 ppm), low Y (6.5–13.8 ppm) and Yb (0.49–0.98 ppm) contents, and thus high Sr/Y (32.8–72.0) ratios. In addition, they have low MgO (0.67–1.12 wt%), Mg# (31.14–43.08) and K2O/Na2O (0.32–0.46) with positive zircons ԐHf(t) values from +7.1 to +14.0. Few zircon grains from the same meta-granodiorite yielded values around 59–63 Ma, respectively, that may be associated with the metamorphic events associated with the crustal thickening soon before or during the later continental collision. Combining with previous researches, we suggest that these igneous rocks are related to Neo-Tethys oceanic lithosphere break-off beneath the Karakoram block, and the northward subduction initiated at the Late Jurassic (~150 Ma).