Multistage evolution of the Jijal ultramafic-mafic complex (Kohistan, N Pakistan): Implications for building the roots of island arcs

摘要

The ultramafic-mafic complexes located at the base of obducted paleo-island arcs are commonly interpreted as evidence for intra-crustal fractionation of primitive (high-Mg#) mantle melts. The present study, realized on the Jijal mafic-ultramafic basal section of the Cretaceous Kohistan oceanic arc (N Pakistan), discards a crystal fractionation model from a single parental magma to take into account the geochemical and isotope variations observed between the plutonic crust and the underlying ultramafic section. The basal ultramafic rocks, i.e. dunites, wehrlites and clinopyroxenites, show high Mg#, extremely depleted REE patterns, no marked HFSE anomalies and high LILE enrichment. Cpx and leached whole rocks from this unit yield restricted epsilon(Nd) values (+5.5 to +6.8), a large epsilon(Sr) range (-5.9 to +8.8) and mainly radiogenic Pb-207/Pb-204 ratios. The overlying mafic section, i.e. gabbroic rocks, displays low Mg# (46