Identification and Origin of Jurassic (~182 Ma) Zircon Grains from Chromitite within the Peridotite of the Jijal Complex, Kohistan Arc in North Pakistan

Abstract

The Jijal ultramafic–mafic complex in Pakistan probably preserves the most complete fragments of the petrological Moho. However, a few studies argue for multiple origins (including petrogenetic speculations and tectonic reconstructions) for different lithologies. One of the main reasons for this dispute is the lack of direct age information of the ultramafic rocks. Zircon grains, despite generally being exotic in ultramafic rocks, can provide significant insights into the petrogenetic process of the host ultramafic rocks. This study reports the first zircon U–Pb age and Lu–Hf and trace element data for zircon grains separated from chromitite lenses within the peridotite, which is commonly considered the lowermost part of the Jijal complex. These zircon grains yield concordant 206Pb/238U ages of ~182 ± 3 Ma, which is much older than the late Early Cretaceous age (<120 Ma) of the Jijal complex, and lying above it, the other complexes of the Kohistan paleo-arc. Furthermore, these Jurassic zircon grains present radiogenic εHf(t) values (+9.7 to +6.0) which are obviously lower than the values for the Cretaceous zircon grains of the Kohistan arc. From integrated analysis of the zircon trace element signatures (e.g., high Th, U, Th/U, and U/Yb ratios) and regional geology, we speculate that these zircon grains came from a ‘missing’ Early Jurassic arc akin to the Gangdese belt to the east, and entered the mantle by oceanic subduction processes. Although these Jurassic zircon grains cannot actually constrain the formation age of the chromitite as well as the peridotite, it reminds us that some cryptic pre-Cretaceous complexes and geodynamic processes were incorporated in building the oceanic crust of the Jijal intra-oceanic arc, or the mantle section (at least part of it) should probably belong to the Indus ophiolite mélange. Further research, particularly chronological studies on mantle (or ultramafic) rocks, as well as detailed geological mapping, should be carried out in the future for solving this issue.