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

摘要

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 × 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°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.