Timing and origin of migmatitic gneisses in south Karakoram: Insights from U–Pb, Hf and O isotopic record of zircons

Abstract

The timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between HT metamorphism and magmatism in over-thickened crust. In the present study, we used the in-situ isotopic (Hf, O and U–Pb) record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the Dassu dome in south Karakoram. The new U–Pb zircon dating identified the Proterozoic inherited cores (1.8–1.9Ga and 2.3–2.5Ga) surrounded by a Neogene overgrowth with ages ranging from ∼6 to ∼20Ma. These ages imply that the partial melting in the Karakoram Metamorphic Complex lasted from >20Ma to ∼6Ma and can be correlated with the Miocene magmatism in the adjacent Baltoro region. Oxygen isotopic data from Proterozoic inherited cores (1.8–1.9Ga) and Neogene overgrowths are indistinguishable and generally vary from 8‰ to 9.5‰. These values are slightly higher than the most igneous zircons (6.5–8‰, Valley et al., 2005) indicating an igneous precursor with heavy initial O composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely. However, a few low U/Th, relatively old inherited cores (2.3–2.5Ga) showed mantle-like (δ18O=5.3±0.6‰, Valley et al., 2005) values of δ18O=5.5±2.7‰. The present-day weighted mean εHf (0) of the Proterozoic inherited cores ranges from −50±1.0 to −44.3±1.2. In contrast, the Neogene rims are 15–20 ε-units higher than the inherited core with present-day εHf (0)=−30.6±0.9. This implies that the Hf composition of the Neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely. We suggest a contribution from the Neogene, less-evolved magmatism in the Baltoro region (εHf (0)=∼−4 to −10). The elevated oxygen composition is not consistent with the contribution from pristine mantle-derived magmas. The observed homogeneous and uniform Hf–O isotopic composition of the Proterozoic inherited cores suggest their derivation from mildly evolved infracrustal sources with minor input from supracrustal material. The older inherited zircons (2.3–2.5Ga) were precipitated from juvenile mantle derived magmas.