Thermobarometry of the Garnet-Bearing Rocks of the Jijal Complex (Western Himalayas, Northern Pakistan)
2013-01-15T15:02:59Z (GMT) by
The Kohistan terrane located in the western Himalayas is an oceanic arc of Early Cretaceous age accreted to the Asian plate and then abducted onto the Indian plate. It can be observed along a complete oblique section from ultramafic/mafic rocks to supracrustal volcanics. The 150 sq km Jijal complex constitutes the lowermost part of this arc and consists of rocks that probably crystallized near the crust/mantle boundary. Detailed petrological study of the garnet-bearing rocks of the Jijal complex indicates their magmatic origin because primary garnet, plagioclase (highly saussuritisized), clinopyroxene, and amphibole occur as relicts among metamorphic grains. Plagioclase, clinopyroxene and amphibole grains are usually homogeneous in composition, contrasting with zoned garnet grains that suggest complex pressure-temperature paths. However, plagioclase composition is heterogeneous at thin section scale (from albite to anorthite = 80). An average variation of 5-10% of A1VI, jadeite and Ca-tschermak content between grains of a sample indicates a major decompressional metamorphic event that post-dates the magmatic stage, hence probably related to the obduction of the arc onto the Indian plate. Phases like actinolitic hornblende, epidote, muscovite, paragonite, and chlorite are widely distributed within host rocks, veins and shear zones of the Jijal complex and result from retrogression associated with penetrative fluid circulation during late stage of the obduction. Geothermobarometry of the garnet-bearing rocks of the Jijal complex indicates disequilibrium among phases mainly including garnet, plagioclase, and clinopyroxene. Such disequilibrium is more likely to be attributed to the variability of plagioclase composition within a sample, but also to clinopyroxene A1VI, jadeite and Ca-tschermak variability. Considering the magmatic assemblages, temperature-pressure conditions of crystallization can be estimated between <1100-1300°C, 13-25 kbar. A minimum pressure calculated from a high-jadeite content ( 16%) metamorphic clinopyroxene, normative (An = 50%) plagioclase and garnet assemblage reaches 17 kbar at 950°C, and can be related to the burial of the Jijal complex during its accretion with the Asian plate that preceded its obduction onto the Indian plate.