Petrogenesis and tectonic implications of peridotites of the Shangla Complex Ophiolite along Main Mantle Thrust, Northern Pakistan

Abstract

The Shangla Complex ophiolite represents a relic of the Neo-Tethyan oceanic lithosphere along the Indus Suture Zone (also known as the Main Mantle Thrust) in northern Pakistan. This section, thrust onto the continental margin between the Indian and Karakoram (Asian) plates, is predominantly composed of depleted harzburgites, dunites and chromitites. In this study, we conducted a thorough analysis of mineralogy, whole-rock geochemistry (major oxides, trace elements, PGE), and integrated Re-Os isotopic data from mantle-derived peridotites to understand their petrogenesis and melt evolution. These peridotites exhibit a depleted nature, characterized by a low modal composition of clinopyroxene, a wide forsterite content range in olivine (86.5 to 95.2), and a large variation in Cr# values (25.1–91.4). Their diverse whole-rock geochemistry further suggests varying degrees of partial melting. The Cpx-harzburgites show high average Al₂O₃ (1.83 wt.%), CaO (2.27 wt.%), ΣREE (12.9 ppb), and ¹⁸⁷Os/¹⁸⁸Os values between 0.13095 and 0.12571. On the other hand, the depleted harzburgites and dunites exhibit lower average Al₂O₃ (0.57 wt.% and 0.14 wt.%, respectively), CaO concentration (0.59 wt.% and 0.21 wt.%, respectively), and ∑REE concentrations, measured at 12.7 ppb and 8.9 ppb, respectively. The ¹⁸⁷Os/¹⁸⁸Os ratios in the depleted harzburgites and dunites range from 0.12643 to 0.11777, indicating they are less radiogenic compared to the Cpx-harzburgites. The spoon-shaped rare earth elements (REE) patterns suggest that the Cpx-harzburgites underwent low degrees of partial melting (∼10%–15%), whereas the depleted harzburgites and dunites indicate somewhat higher degrees of partial melting (additional melting of the Cpx-harzburgites). The PGE abundances in these depleted harzburgites and dunites are linked to the partial melting of Cpx-harzburgites, resulting in a boninitic-like melt. Their low degree of melting and melt extraction suggests that Cpx-harzburgites initially formed at a mid-ocean ridge (MOR) spreading center or a distal fore-arc basin. In contrast, the depleted harzburgites and dunites were formed during a second phase of melting, followed by refertilization, closely associated with a supra-subduction zone (SSZ) setting. The Re-Os isotopic systematics of the Shangla Complex peridotites reveal model age clusters of ca. 250 Ma and ca. 450 Ma, potentially corresponding to significant tectonic events in the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto- Tethyan oceans.