Petrogenesis of mafic garnet granulite in the lower crust of the Kohistan paleo-arc complex (Northern Pakistan): Implications for intra-crustal differentiation of island arcs and generation of continental crust

We report the results of a geochemical study of the Jijal and Sarangar complexes, which constitute the lower crust of the Mesozoic Kohistan paleo-island arc (Northern Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite with minor lenses of garnet hornblendite and granite, grading up-section to hornblende gabbronorite. The Sarangar complex is composed of metagabbro. The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, light rare earth element (LREE)-enriched REE patterns similar to those of island arc basalts. Together with the Jijal garnet granulite, they define negative covariations of La-N, Yb-N and (La/Sm)(N) with Eu-* [Eu-* = 2 x Eu-N/(Sm-N + Gd-N), where N indicates chondrite normalized], and positive covariations of (Yb/Gd)(N) with Eu-*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of the garnet granulites as plagioclase-garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration-melting of hornblende gabbronorite, provided that the amount of segregated or restitic garnet was low (< 5 wt %). Field, petrographic, geochemical and experimental evidence is more consistent with formation of the Jijal garnet granulite by dehydration-melting of Jijal hornblende gabbronorite. Similarly, the Jijal garnet-bearing hornblendite lenses were probably generated by coeval dehydration-melting of hornblendites. Melting models and geochronological data point to intrusive leucogranites in the overlying metaplutonic complex as the melts generated by dehydration-melting of the plutonic protoliths of the Jijal garnet-bearing restites. Consistent with the metamorphic evolution of the Kohistan lower arc crust, dehydration-melting occurred at the mature stage of this island arc when shallower hornblende-bearing plutonic rocks were buried to depths exceeding 25-30 km and heated to temperatures above c. 900 degrees C. Available experimental data on dehydration-melting of amphibolitic sources imply that thickening of oceanic arcs to depths > 30 km (equivalent to c. 1.0 GPa), together with the hot geotherms now postulated for lower island arc crust, should cause dehydration-melting of amphibole-bearing plutonic rocks generating dense garnet granulitic roots in island arcs. Dehydration-melting of hornblende-bearing plutonic rocks may, hence, be a common intracrustal chemical and physical differentiation process in island arcs and a natural consequence of their maturation, leading to the addition of granitic partial melts to the middle-upper arc crust and formation of dense, unstable garnet granulite roots in the lower arc crust. Addition of LREE-enriched granitic melts produced by this process to the middle-upper island arc crust may drive its basaltic composition toward that of andesite, affording a plausible solution to the 'arc paradox' of formation of andesitic continental-like crust in island arc settings.