Bignold, Stella M., Treloar, Peter J. and Petford, Nick (2006) Changing sources of magma generation beneath intra-oceanic island arcs: an insight from the juvenile Kohistan island arc, Pakistan Himalaya. Chemical Geology, 233(1-2), pp. 46-74. ISSN (print) 0009-2541

Abstract

The Kohistan arc, situated in the northwestern Himalaya of North Pakistan, is a Cretaceous intra-oceanic island arc which was initiated during the northward movement of the Indian Plate. Having been uplifted and tilted northward when the Indian and Asian continents sutured, the arc offers a unique insight into the mechanisms of arc formation. The polarity of subduction beneath the arc is uncertain, as is the width of ocean crust which separated the arc from the Asian continent. Field relations, geochemical analysis, and rare earth element modelling are used to determine the main magma source types in the mantle beneath the juvenile arc. Field relations demonstrate that the Kamila Amphibolites grade upward into the Peshmal Formation of the Jaglot Group, and that the undeformed Mankial Metavolcanics, formerly assigned to the pre-suturing Kalam Group, represent the earliest volcanic rocks following the suturing of the arc with Asia. REE modelling demonstrates that the Kamila Amphibolites fall into two successions: the ‘E-type’ rocks, which have a MORB-type signature and may have been produced from 6% partial melting of a primitive mantle source type; and the ‘D-type’ rocks, which have an arc signature, represent the earliest arc volcanic rocks and may have originated from 15.5% partial melting of a primitive spinel-bearing, mantle source type. The Jaglot Group (Gashu Confluence Volcanic Formation and Peshmal Formation) and the Chalt Volcanic Group (Ghizar Formation) represent the main arc volcanic rocks which may have been generated by 7.5%, 13% and 2.5% partial melting, respectively, of a primitive, garnet-bearing, mantle source-type. The Hunza Formation of the Chalt Volcanic Group, which contains primitive, high-Mg rocks, has a MORB-type chemistry and a weak arc signature. These rocks may have been produced from 10-15% partial melting of a depleted, spinel-bearing mantle source type, which may have been the residuum of partial melting which produced the ‘E-type’ Kamila Amphibolites. That the Hunza Formation was generated from this source is consistent with the stratigraphy. The ‘E-type’ Kamila Amphibolites form the basement to the arc and the ‘D-type’ Kamila Amphibolites represent the earliest arc volcanic rocks above a north-dipping subduction zone. The Jaglot Group and Ghizar Formation were derived from partial melting of a garnet-bearing source at greater depth. The Hunza Formation, which represents the youngest pre-suturing rock suite was erupted behind the volcanic front in a spatially and temporally restricted back-arc basin. Results of REE modelling are consistent with models of intra-oceanic arc formation where arc volcanic rocks originate from sources which evolve from spinel-bearing to garnet-bearing mantle with increasing depth of the subducting slab.

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