Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya

Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ‹ 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ‹ 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced ascent'). Together with new ®eld observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif. Introduction Along the >2500 km Himalaya orogen, ®eld mapping and isotopic age data have revealed important spacetime patterns of plutonism associated with thrusting, metamorphism and, indirectly, extension. The apparent coincidence of these inherently linked tectonic processes has lead many authors to invoke several tectonomagmatic models over the past score or so of years (e.g. decompression melting, shear heating, dehydration melting, etc.: Le Fort 1975, 1981; Debon et al. 1985; Le Fort et al. 1987; Zeitler and Chamberlain 1991; Harris and Massey 1994; Scaillet et al. 1995; Harrison et al. 1998). The High Himalayan granites are considered to be the result of anatexis of the Tibetan Slab rocks and imbricated portions of the North Indian margin (e.g. Le Fort et al. 1987), and granites of the central section of the orogen have received the majority of attention (e.g. Gangotri, Manaslu, Rongbuk-Makalu, Khula Kangri). Intrusion ages are typically Early Miocene (e.g. Le Fort et al. 1987; Harrison et al. 1995) or Middle Miocene (Edwards and Harrison 1997) for the eastern Himalaya. Recent debates involving the relationship between plutonism, high topography and the Himalayan detachment system suggest a coupling between plutonism and tectonic deformation that is associated with orogenic collapse (Molnar et al. 1993; Fielding 1996, Murphy and Harrison 1998). Recently, in other orogenic belts (e.g. Seward Peninsula, Alaska; Coast Ranges, British Columbia; Papua New Guinea) several authors have emphasized the association of plutonic activity with the development of synto late-orogenic structures (e.g. Hollister and Crawford 1986; Hill et al. 1992; Baldwin et al. 1993; Amato et al. 1994). Such studies generally conclude that granitoids are emplaced and guided by shear zone geometry and that plutonism provides a rheological weakening mechanism that further enhances localized strain, accelerating deformation along principal shears. This typically creates a dynamic condition or `positive Contrib Mineral Petrol (1999) 136: 273 ± 284 Ó Springer-Verlag 1999 D.A. Schneider (&) á P.K. Zeitler Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania, 18105, USA e-mail: [email protected]; [email protected] M.A. Edwards Department of Earth and Atmospheric Sciences, State University of New York, Albany, New York, 12222, USA C.D. Coath Department of Earth and Space Sciences, University of California, Los Angeles, California, 90095, USA e-mail: [email protected] Present address: Institut fuer Geologie, Universtaet Wuerzburg, Wuerzburg, Germany email: [email protected] Editorial responsibility: T.L. Grove feedback' (Brown 1994). One result is a close temporal concordancy of pluton emplacement and of the cooling of the associated high-grade metamorphic terranes transported upward during uplift (e.g. Hollister and Crawford 1986). The northwest Himalaya syntaxis, where the main Himalayan chain terminates, includes the Nanga Parbat-Haramosh massif. Previous Himalayan leucogranite crystallization ages from northwestern Pakistan, determined by U-(Th)-Pb ion microprobe dating of zircons and monazites, are younger then those reported elsewhere in the Himalaya; undeformed dikes within the massif give accessory mineral ages between <2 and 7 Ma (Zeitler and Chamberlain 1991; Schneider et al. 1997) and the Tato pluton yields zircon rim ages of 1 Ma (Zeitler et al. 1993). We present here the youngest monazite ages yet reported for a granite intrusive into the westernmost Himalayan crystallines: the Mazeno Pass Pluton. We analyzed the Mazeno Pass pluton to constrain further the young magmatism around the Nanga Parbat summit. Also dated was the Jutial pluton, located in the northwest corner of the massif. Both the Mazeno Pass pluton and the Jutial pluton are adjacent to the principal uplift-related (Edwards et al. 1996, 1997; Edwards 1998; Schneider et al. in press) structures; subsequent to reporting our geochronology and ®eld relationships, we discuss the implications of our data with respect to granite intrusions into shear zones. It is through geochronologic and structural investigation of plutons situated near and within shear zones (e.g. Solar et al. 1998) that the association between deformation and magmatism can be understood.