Metamorphic Evolution of an Obducted Island Arc: Example of the Kohistan Sequence (Pakistan) in the Himalayan Collided Range
Abstract
In the northern Pakistan, the extraordinary 40km thick Kohistan sequence of ma fie, ultramafic and calc-alkaline layered plutonic and volcanic (mainly andesitic to rhyodacitic) rocks has been interpreted as the only complete vertical section of: an intraoceanic island arc presently exposed anywhere in the world. Plate-scale models have been suggested to explain the origin and the tectonic evolution of this arc in the Himalayan collided range. Despite some noticeable differences in the models, there is a general agreement that the Kohistan sequence represents the crust of an arc obducted onto the Indian plate before the Oligocene collision of India against Asia. The purpose of the present paper is to give new data on the tectono-metamorphic evolution of the Kohistan sequence and to try to estimate its changing metamorphic conditions on petrological, mineralogical and crystal chemistry grounds. The structural, textural and mineralogical data suggest two main tectono-metamorphic events that have affected the Kohistan arc before late lower grade overprints. From various geothermometry estimates as well as from field zonations, it is concluded that the major (Dl) phase developed with a thermal structure strongly controlled by thermal anomalies in the metamorphic pile. These anomalies seem centered around isolated bodies of two-pyroxenes and cpx-garnet granulites. Because the petrographical evidences indicate the latter are derived from large pre-tectonic layered calc-alkaline intrusion(s), the former anomalies are interpreted as remnant magmatic heat source(s) within the arc. This interpretation fits nicely with theoretical thermal models. Assuming Dl is contemporaneous with abduction of the arc and together with the geochronological data, it is suggested that the tectonic emplacement of the Kohistan arc was 2055 m.y. after the intrusion of enormous calc-alkaline pluton(s) within the arc crust. Such a relatively short elapsed time between a strong magmatic activity and a tectono. metamorphic event may explain why the T-P paths during DI produced sinuous geotherms in the petrogenetic grid, i.e. the dT/dP are varying from 35o C +- 5oC/km upto 100oC/km in the 40km thick pile.
The second (D2) phase is contemporaneous with synkinematic low- to mid-grade retrogressions up to the epidote-amphibolite facies conditions. The T-P curve during this phase is a classical one and the D geotherm approached 25oC +- 5oC/km within a re-equilibrated thermal structure. Scarce geochronological data suggest that D2 was contemporaneous with the Oligocene collision of India against Asia and possibly synchronous with the Oligocene Barrovian metamorphism which bas affected the Indian plate south of Kohistan. The origin of the rocks of the Kohistan granulites as well as the significance of a blueschist belt under the Kohistan metamorphic pile are briefly discussed.
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