Abstract
If the net fl ux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafi c keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo‐island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafi c crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display fl at or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafi c crust that have higher REE concentrations and are derived from light rare earth element (LREE)‐enriched melts similar to island arc basalt. This observation, along with the lower 208 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafi c rocks derive from a common melt via crystal fractionation. In the 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram, ultramafi c rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafi c rocks. These data are consistent with a scenario whereby the Jijal ultramafi c section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafi c cumulates complementary to the evolved mafi c plutonic rocks implies either (1) that a substantial volume of such ultramafi c cumulates was delaminated or torn out by subcrustal mantle fl ow from the base of the arc crust in extraordinarily short time scales (0.10‐0.35 cm/yr), or (2) that the net fl ux to the Kohistan arc crust was more evolved than primitive arc basalt.
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