Elsevier

Chemical Geology

Volume 233, Issues 1–2, 30 September 2006, Pages 46-74
Chemical Geology

Changing sources of magma generation beneath intra-oceanic island arcs: An insight from the juvenile Kohistan island arc, Pakistan Himalaya

https://doi.org/10.1016/j.chemgeo.2006.02.008Get rights and content

Abstract

The Kohistan arc, situated in the Pakistan Himalaya, is a Cretaceous intraoceanic island arc which was initiated during the northward movement of the Indian Plate. The arc was sutured to Asia at ca. 100 Ma. It was subsequently tilted northward when underplated by Indian continental crust during the early stages of India–Asia collision. Deep erosion of this tilted section provides a spectacular section through the whole arc sequence and offers a profound insight into the mechanisms of early stages of arc formation. Geochemical analysis and rare earth element modelling of basaltic sequences which date from the intraoceanic stages of arc development allow identification of three main magma source types in the mantle beneath the juvenile arc. The ‘E-type’ Kamila Amphibolites, with a MORB-type chemistry, form the intraoceanic basement to the arc. The ‘D-type’ Kamila Amphibolites are the earliest of the arc volcanic rocks. These were extracted from a primitive spinel-bearing mantle source, above a north-dipping subduction zone. The stratigraphically younger basalts of the Jaglot Group and Ghizar Formation of the Chalt Volcanic Group were derived from partial melting of a garnet-bearing source at greater depth. The Hunza Formation of the Chalt Volcanic Group contains the youngest mafic volcanic rocks of the intraoceanic arc. Although coeval with the Ghizar Formation of the Chalt Volcanic Group, they were generated by melting of a depleted, spinel-bearing mantle source rock and were erupted into a spatially and temporally restricted back-arc basin developed behind the volcanic front. The Chalt Volcanic Group was therefore formed from two different, adjacent, mantle source regions active at the same time. Results of REE modelling are consistent with models for intraoceanic arc formation in which the earliest volcanic rocks are derived from shallow level spinel-bearing peridotite, and later ones from a deeper garnet-bearing source. This is consistent with the melt region becoming deeper with time as subduction continues. A two-stage model is proposed for the back-arc basalts of the Hunza Formation in which a mantle source, depleted from a previous melting event, is underplated beneath the arc and later remelted during decompression as a consequence of extension and rifting of the arc.

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

References (74)

  • M.G. Petterson et al.

    Changing source regions of magmas and crustal growth in the trans-Himalayas: evidence from the Chalt volcanics and Kohistan Batholith, Kohistan, Northern Pakistan

    Earth Planet. Sci. Lett.

    (1991)
  • A.H.F. Robertson et al.

    Shyok Suture Zone, N Pakistan: late Mesozoic–Tertiary evolution of a critical suture separating the oceanic Ladakh Arc from the Asian continental margin

    J. Asian Earth Sci.

    (2002)
  • Y. Rolland et al.

    Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)

    Tectonophysics

    (2000)
  • Y. Rolland et al.

    The Cretaceous Ladakh arc of NW Himalaya: slab melting and melt-mantle interaction during fast northward drift of Indian Plate

    Chem. Geol.

    (2002)
  • Z. Ahmed et al.

    Petrology of the Babusar area, Diamir district, Gilgit, Pakistan

    Geol. Bull. Punjab Univ.

    (1976)
  • Z. Ahmed et al.

    Petrology of the Thelichi area, Gilgit Agency

    Geol. Bull. Punjab Univ.

    (1977)
  • R. Anczkiewicz et al.

    Isotopic constraints on the evolution of metamorphic conditions in the Jijal–Patan complex and the Kamila Belt of the Kohistan arc, Pakistan Himalaya

  • J.H. Bédard et al.

    Evidence for forearc seafloor-spreading from the Betts Cove ophiolite, Newfoundland: oceanic crust of boninitic affinity

    Tectonophysics

    (1998)
  • S.M. Bignold et al.

    Northward subduction of the Indian Plate beneath the Kohistan island arc, Pakistan Himalaya: new evidence from isotopic data

    J. Geol. Soc. (Lond.)

    (2003)
  • S.H. Bloomer et al.

    Early arc volcanism and the ophiolite problem: a perspective from drilling in the western Pacific

  • J.P. Burg et al.

    Infra-arc mantle-crust transition and intra-arc mantle diapirs in the Kohistan Complex (Pakistani Himalaya): petro-structural evidence

    Terra Nova

    (1998)
  • W.E. Cameron et al.

    Boninites, komatiites and ophiolitic basalts

    Nature

    (1979)
  • P.D. Clift

    Volcaniclastic sedimentation and volcanism during the rifting of western Pacific backarc basins

  • R.I. Corfield et al.

    Tectonic setting, origin, and subduction history of the Spontang ophiolite, Ladakh Himalaya, NW India

    J. Geol.

    (2001)
  • M.P. Coward et al.

    Geo-tectonic framework of the Himalaya of N. Pakistan

    J. Geol. Soc. (Lond.)

    (1982)
  • M.P. Coward et al.

    Collision tectonics in the NW Himalayas

  • M.P. Coward et al.

    The tectonic history of Kohistan and its implications for Himalayan structure

    J. Geol. Soc. (Lond.)

    (1987)
  • A.J. Crawford et al.

    Classification, petrogenesis and tectonic setting of boninites

  • T.J. Falloon et al.

    Petrology and geochemistry of back-arc basin basalts from Lau Basin spreading ridges at 15°, 18° and 19°S

    Min. Pet.

    (1992)
  • M.T. George et al.

    The tectonic implications of contrasting granite-magmatism between the Kohistan island arc and the Nanga Parbat–Haramosh Massif, Pakistan Himalaya

  • J.B. Gill

    Orogenic Andesites and Plate Tectonics

    (1981)
  • G.N. Hanson

    Rare earth elements in petrogenetic studies of igneous systems

    Annu. Rev. Earth Planet. Sci.

    (1980)
  • M.Q. Jan

    Petrography of the upper part of Kohistan and southwestern Gilgit agency along the Indus and Kandia rivers

    Geol. Bull. Univ. Peshawar

    (1970)
  • M.Q. Jan

    Petrography of pyroxene granulites from northern Swat and Kohistan

    Geol. Bull. Univ. Peshawar

    (1979)
  • M.Q. Jan

    Geochemistry of amphibolites from the southern part of the Kohistan arc, N. Pakistan

    Min. Mag.

    (1988)
  • M.Q. Jan et al.

    The mineralogy and geochemistry of the metamorphosed basic and ultrabasic rocks of the Jijal complex, Kohistan, NW Pakistan

    J. Petrol.

    (1981)
  • M.Q. Jan et al.

    Preliminary geology and Petrography of Swat Kohistan

    Geol. Bull. Univ. Peshawar

    (1971)
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