Research Information System
Lithos, vol.526-527, 2026 (SCI-Expanded, Scopus)
Ophiolitic sequences obducted onto continental margins provide critical insights into the oceanic lithosphere's structure and evolution. This study integrates field observations and petrochemical data from Chitral Complex mantle peridotites along the Main Karakoram Thrust (MKT), northwest Pakistan, to study compositional heterogeneity and geodynamic evolution in the Late Cretaceous Neo-Tethyan domain. The peridotites, primarily spinel lherzolite and harzburgite, exhibit notable geochemical variability: spinel Cr# ranges from 0.21 to 0.74, Mg# from 0.40 to 0.71, and whole-rock Al2O3 (0.26–2.06 wt%) and CaO (0.34–2.13 wt%) show broad intervals. Clinopyroxenes are LREE-depleted, though some show LREE enrichment due to melt-rock interaction. Combined spinel and clinopyroxene data suggest ∼10–20% fractional melting from a depleted MORB mantle source, supported by Os isotope signatures (low Re/Os = 0.02–0.07; 187Os/188Os = 0.1219–0.1266; γOs = −3.73 to −0.10), indicating a highly depleted mantle. Trace element modeling of clinopyroxenes suggests refertilization by slab-derived fluids, reflecting a shift in melt-depletion mechanisms. These heterogeneous compositions deviate from typical abyssal or forearc lithologies, indicating a tectonic transition from anhydrous MORB-like conditions to fluid-rich subduction environments. This challenges the classical forearc-spreading model for the Karakoram ophiolites because it cannot account for the observed thin crust (<6 km) and low magmatic budgets, whereas subduction re-initiation along an ultraslow-spreading ridge, potentially facilitated by oceanic detachment faulting, naturally explains both features. The Chitral Complex, one of the most compositionally diverse mantle exposures in the northwest Pakistani ophiolitic belt, offers unique insights into Neo-Tethyan mantle dynamics and lithospheric transformation across the Cretaceous–Paleogene boundary.