Petrological and geochemical characterization of metamorphic sole rocks beneath the RasKoh ophiolite, western Pakistan: Implication for a Late Cretaceous supra‐subduction zone‐type forearc

Abstract

Small, isolated outcrops of metamorphic sole rocks to the RasKoh ophiolite are exposed south‐east of the RasKoh Arc range, western Pakistan. These crop out as thin slices beneath the sheared serpentinites, and consist of amphibolites, epidote‐bearing amphibolites, and muscovite‐rich amphibolites with granoblastic, grano‐nematoblastic, and porphyroblastic textures. Geochemically, amphibolites present two compositional groups. Group‐I amphibolites are alkaline and enriched in light rare‐earth elements (LREE; LaN/YbN = 11.47–11.60) with Sr‐Nd isotopic compositions of (87Sr/86Sr)i = 0.705481–0.706128, (143Nd/144Nd)i = 0.512668–0.512672, and lower values of εNd(t) (+2.24 to +2.32). While, Group‐II amphibolites are tholeiitic with LREE‐depleted signatures (LaN/YbN = 0.88–1.31), (87Sr/86Sr)i = 0.706103–0.706303, (143Nd/144Nd)i = 0.512851–0.512936, and higher values of εNd(t) (+5.81 to +7.47). Primitive mantle‐normalized and other geochemical diagrams indicate that the protoliths for Group‐I amphibolites are oceanic island basalts (OIB). Conversely, Group‐II amphibolites are more akin to subduction‐related arc tholeiites with geochemical signatures, slightly enriched in large‐ion lithophile elements and with negative Nb‐Ta anomalies. The geothermobarometric results estimate that these rocks were metamorphosed at amphibolite facies at 690°C and 6.5 kbar. Alkaline volcanism is well documented from the other Mesozoic ophiolites of Neo‐Tethyan suture zones. We suggest that the OIB accreted to the base of the obducted plate at an early stage, whereas the island arc tholeiites (IAT) basalts metamorphosed and accreted to the base of the RasKoh ophiolite at a later stage. The OIB‐ and IAT‐type magmas as protolith of amphibolites suggest that the RasKoh ophiolite was emplaced in a supra‐subduction zone‐type forearc in Late Cretaceous (ca. 68–64 Ma).