 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; May 2001; v. 29; no. 5; p. 435–438; 4 figures; 1 table. 435 Coesite in Himalayan eclogite and implications for models of India-Asia collision P.J. O’Brien* N. Zotov    Bayerisches Geoinstitut, Universita ¨ t Bayreuth, D-95440 Bayreuth, Germany R. Law Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA M.A. Khan Government College, Sargodha, Pakistan M.Q. Jan National Centre of Excellence in Geology, University of Peshawar, Peshawar, Pakistan ABSTRACT Coesite, recognized petrographically and confirmed by in situ Raman microprobe spec- troscopy, is reported from an eclogite from the Kaghan valley, Pakistan, and represents the first record of ultrahigh-pressure metamorphism in the Himalaya. The formation con- ditions of 27 kbar implied by the presence of coesite are supported by garnet-pyroxene- phengite barometry (27–29 kbar, 690–750 C). If, as seems likely from previous field and geochronologic studies, the eclogites represent metamorphosed dikes, sills, and lava flows of Permian age within the granitic gneiss-metapelite-marble sequence of the Higher Hi- malayan crystalline nappes, then continental crust—the leading edge of the Indian plate— must also have been subducted to coesite-forming depths (90–100 km). This more than doubles previous depth estimates and, on the basis of available geochronological data for this area, requires average exhumation rates at least twice as fast (10 mm/yr) as pre- viously imagined. A further implication, based on interpretations of deep seismic data, is that the present-day shallow angle of subduction of Indian plate lithosphere beneath Tibet represents a significant change from an initially much steeper angle. Keywords: Himalaya, Pakistan, eclogite, coesite, Raman. Figure 1. Simplified geologic map of Upper Kaghan Valley, north Pakistan, showing location of coesite-bearing eclogite (modified after Greco and Spencer, 1993; Lombardo et al., 2000). Inset map shows regional location and eclogite-bearing Tso Morari area. INTRODUCTION The approach and collision of India with Asia and a variety of intervening arc and mi- croplate units caused crustal thickening, meta- morphism, magmatism, uplift, and extension, and produced the 2500-km-long Himalayan mountain belt (e.g., Treloar and Searle, 1993). In contrast to the collision belt of similar age represented by the European Alps, where high-pressure (P)–low- to moderate-tempera- ture (T) metamorphism plays a major role (Droop et al., 1990), blueschists and eclogites are remarkably scarce in the Himalaya. The first descriptions of Himalayan blueschists, of Cretaceous (80–100 Ma) age, from the me- lange units of the Indus suture zone, appeared in the 1970s (Shams, 1972) and were followed by later finds (e.g., Honegger et al., 1989) in the same tectonic situation. The first definite eclogites in the Himalayas were reported only in the late 1980s (Ghazanfar and Chaudhry, 1986, 1987) and there are still only very few areas where eclogites (or their retrograde or overprinted equivalents) have been identified (Pognante and Spencer, 1991; Guillot et al., 1997; de Sigoyer et al., 1997; Le Fort et al., 1997; Fontan, 1998; Lombardo et al., 2000). *Now at: Institut fu ¨r Geowissenschaften, Univ- ersita ¨t Potsdam, Postfach 601553, D-14415 Pots- dam, Germany. E-mail: obrien@geo.uni-potsdam. de. GEOLOGIC SETTING AND SAMPLE DESCRIPTION In the Kaghan Valley, northeast of Islama- bad, Pakistan, high-grade crystalline rocks of the Indian plate (Higher Himalayan crystalline nappes) are separated by the Indus suture (also called Main Mantle thrust) from mafic-ultra- mafic rocks of the Kohistan arc to the north and west, and are underthrust by lower grade, less intensely deformed series to the south (Fig. 1). The rocks of the Higher Himalayan crystalline nappes in the Upper Kaghan Valley have been subdivided (Greco et al., 1989; Spencer, 1993) into basement (metagranite and paragneiss) and an amphibolite, marble, dolomite, quartzite, and micaschist-bearing cover sequence thought to be of Paleozoic to early Mesozoic age. However, no protolith ages are available for basement gneisses or the cover rocks. Eclogites are preserved in the cores of large boudins in the basement gneiss- es, although Pognante and Spencer (1991) re- ported them also from the cover sequences. Mafic sheets that are 1 m thick and show a greenschist overprint on an earlier amphibolite assemblage are found in the marbles and me- tapelites, as well as in the granitic basement gneisses, and probably represent retrograded eclogites. The dominant metapelites and me- tagranites of the Higher Himalayan crystalline