ORIGINAL PAPER Karen Viskupic Æ Kip V Hodges Æ Samuel A Bowring Timescales of melt generation and the thermal evolution of the Himalayan metamorphic core, Everest region, eastern Nepal Received: 12 February 2004 / Accepted: 6 October 2004 / Published online: 29 January 2005 Ó Springer-Verlag 2005 Abstract In the Everest region of the Nepalese Hima- laya, 40 Ar/ 39 Ar and U-Pb geochronology provide evi- dence for a complex thermal history marked by multiple episodes of granite intrusion. The oldest mobilized melt formed syn-deformational granitic sills that have U-Pb crystallization ages of 21.33±0.03 and 21.80±0.05 Ma. Preserved in these same granites is a record of earlier magmatic crystallization of xenotime, zircon and mon- azite between ca. 26 Ma and ca. 23 Ma. This pattern of accessory phase crystallization is interpreted to reflect incremental melting and crystallization in the source region of the sills before ultimate melt migration, and provides the earliest evidence for anatexis in the Everest region. The beginning of crustal melting in the Everest region predates the earliest known movement on both the Main Central Thrust and the South Tibetan fault systems, but is temporally associated with the implied pressure decrease between ‘‘Eohimalayan’’ and ‘‘Neo- himalayan’’ metamorphism. Introduction The spatial distribution of granitic melts in a collisional orogenic system provides a general indication of the thermal structure of the system at a given time. This structure is dynamic in active orogens; its spatial and temporal evolution reflecting the tectonic displacement of crustal geotherms as well as the accretion of addi- tional crust enriched in radiogenic heat producing ele- ments (Huerta et al. 1998). In addition, the generation and subsequent mobilization of granitic melts them- selves also influences the thermal and presumably structural evolution of an orogen. The Himalaya offer a remarkable setting in which to investigate the changing thermal structure of a colli- sional orogen through study of the temporal and spatial pattern of anatexis. Spectacular exposures through the Himalayan metamorphic core (hereafter referred to as the ‘‘Greater Himalayan Sequence (GHS)’’; Hodges 2000) reveal leucogranitic melts at all scales, ranging from centimeter-scale leucosomes in anatexites to plu- tons and batholiths that crop out over tens of square kilometers. Models of the thermal structure of the Himalayan orogen have implied that the upper GHS experienced a relatively simple thermal history charac- terized by one or two pulses of granite emplacement (Harris and Massey 1994; Harrison et al. 1998, 1999). Field relationships, however, indicate multiple phases of leucogranite intrusion. These relationships are sup- ported by geochronologic studies that report a wide range of crystallization ages (ca. 24–12 Ma) for granites in the GHS throughout the orogen (e.g. Scha¨rer 1984; Scha¨rer et al. 1986; Edwards and Harrison 1997; Searle et al. 1997; Harrison et al. 1999). However, important questions about the temporal and spatial generation of granitic melts remain unanswered. How frequent were episodes of anatexis and intrusion? Was melt generation continuous or episodic? What were the durations of intrusive events? What was the thermal evolution of the country rocks during the intervals between intrusive events? Such questions can best be addressed with de- tailed geologic, thermochronologic, and geochronologic studies of multiple generations of granitic melts and their country rocks at the outcrop scale. In this paper, we contribute to this effort by focusing on the thermal and intrusive history of the upper GHS in one small area, the Everest region of eastern Nepal (Fig. 1). This is an attractive study area because it con- Editorial Responsibility: T. L. Grove K. Viskupic Æ K. V. Hodges Æ S. A. Bowring Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA K. Viskupic (&) Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725-1535, USA E-mail: karenviskupic@boisestate.edu Tel.: +1-208-4263658 Fax: +1-208-4264061 Contrib Mineral Petrol (2005) 149: 1–21 DOI 10.1007/s00410-004-0628-5