Changing sources of magma generation beneath intra-oceanic island arcs: An insight from the juvenile Kohistan island arc, Pakistan Himalaya
Introduction
The Kohistan arc, located in NW Pakistan, was initiated in the Neotethys Ocean during the Cretaceous as an intraoceanic island arc developed above a N-dipping subduction zone (Tahirkheli et al., 1979, Coward et al., 1987, Khan et al., 1993, Treloar et al., 1996, Burg et al., 1998, Bignold and Treloar, 2003). The arc was subsequently sutured to Asia between 104 Ma (Petterson and Windley, 1985) and 85 Ma (Treloar et al., 1996), when it became an Andean-type volcanic margin. The arc was structurally telescoped along N-dipping thrusts during suturing and subsequent underthrusting by the leading edge of continental India. As a result, a full stratigraphic succession from the base of the arc to its stratigraphic top can now be traversed along accessible valleys. The opportunity therefore arises to trace temporal and spatial changes in volcanic style, chemistry and magma source regions through the complete life of the arc from its initiation as a juvenile intraoceanic island arc through its evolution and eventual suturing with Asia to become a continental margin arc.
Much geochemical data have been published from rock suites throughout the accessible regions of Kohistan (Jan and Howie, 1981, Petterson and Windley, 1985, Jan, 1988, Khan et al., 1989, Treloar et al., 1989, Jan and Windley, 1990, Petterson et al., 1990, Petterson and Windley, 1991, Petterson and Windley, 1992, Sullivan, 1992, George et al., 1993, Khan et al., 1993, Petterson et al., 1993, Sullivan et al., 1993, Khan et al., 1996, Khan et al., 1997, Bignold and Treloar, 2003). This paper presents new stratigraphic and geochemical data for volcanic successions in both the eastern and western parts of the arc. The new geochemical data supplement previously published data and include complete rare earth element datasets.
Rare earth element modelling is used to identify potential magma sources and suggest the degree of partial melting in the mantle wedge beneath the arc, with the aim of determining changes in magma source regions as the juvenile arc evolved. The results of modelling each volcanic succession across the arc are combined with stratigraphic and geochemical analysis to formulate a model for magma generation beneath the arc from its initiation until suturing with Eurasia.
Section snippets
Outline geology of the Kohistan arc
The rocks of the Kohistan island arc trend generally east–west (Fig. 1), and dip northward. The arc is bounded to the north by the Shyok Suture, along which it is sutured to Asia, and to the south by the Main Mantle Thrust (MMT), the western continuation of the Indus–Tzangpo Suture Zone along which it was thrust southward over continental India in the early Tertiary (Coward et al., 1982, Corfield et al., 2001.). It is bounded to the east by the Raikot–Sassi fault zone (Coward et al., 1986),
Field relations and geochemistry
In this study, samples of basaltic and andesitic volcanic rocks from all of the volcano–sedimentary groups have been analysed for major, trace and rare earth elements. Some samples from the Chalt Volcanic Group, previously analysed by Petterson and Windley (1991) and from the Kamila Amphibolites previously analysed by Khan et al. (1997), were re-analysed in order to generate complete rare earth element datasets. Sample locations are plotted in Fig. 2, Fig. 3, Fig. 4.
Samples were ground to fine
Rare earth element modelling of rocks of the juvenile arc
The rare earth elements have similar chemical and physical properties but, because of small differences in ionic radius, they may become fractionated relative to each other. As a result, they are particularly useful for modelling mantle melting in order to try to identify appropriate mantle sources for the rock suites and the types and amounts of partial melting that may have been involved.
Rare earth element modelling in this study was carried out using computer software ‘DW’, developed by
Discussion
REE modelling presented here strongly suggests that, with the exception of the Hunza Formation, each of the discrete volcanic sequences within the Kohistan Island Arc was derived from melting of a primitive mantle source beneath Kohistan. Modelling of the chemical data suggests significant variations in both amount and depth of melting. The latter is essentially characterised by the presence of garnet or spinel in the source. Fig. 12 shows the geographical distribution of the melt products and
Conclusions
The Kohistan arc offers almost unique access to a complete stratigraphic succession of an intraoceanic island arc. Geochemistry, isotopic data and REE modelling in this study, despite the fact that the rocks have been metamorphosed, offer the opportunity to identify sources of magma generation beneath the arc which may be used as a model for other intraoceanic island arc volcanoes. Despite the assumptions that had to be made, the results are consistent with models of oceanic island arc
Acknowledgements
SMB wishes to thank Professor Brian Windley at the University of Leicester for the loan of rock samples and Professor M. Asif Khan at the National Centre of Excellence, Peshawar University, Pakistan, for providing assistance in the field and for supplying powders for geochemical analysis. Staff at the NERC ICP-MS Facility are thanked for their assistance. with the geochemical analyses under grant No. ICP/89/1295, as are staff at NIGL, Keyworth, UK, for the use of their facilities for isotope
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2022, LithosCitation Excerpt :These rocks have almost comparable HREE concentrations but variable LREE concentrations (Fig. 11a). The Chalt boninites display nearly flat to somewhat depleted LREE-patterns which can be attributed to the subduction initiation magmatism with least contribution from the subducting slab (Bignold et al., 2006; Thanh et al., 2012). On the contrary, Shyok and Nubra volcanics display LREE enrichment with moderate fractionation of MREE and HREE (Fig. 11a) corresponding to fluid metasomatism in a subduction zone setting.
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2022, Geoscience FrontiersCitation Excerpt :However, it is worth noting that the Spontang samples fall within the most radiogenic reported samples for εNd(t) along the Indus Suture Zone and display typical shift towards radiogenic 87Sr/86Sr(t) classically attributed to sea water contamination (Lanphere et al., 1981; McCulloch et al., 1981; Benoit et al., 1999). Finally, despite the fact that they display εNd(t) values close to the most radiogenic data obtained by Bignold et al. (2006) and Dhuime et al. (2007, 2009) for the Kohistan island-arc formations, they also fall within the radiogenic part of Indian MORB domain (Mahoney et al., 1998, 2002), indicating juvenile characteristics of their source. Olivine from the Spontang peridotites is forsteritic and show limited compositional range from Fo87.2-91.5, avg.