TIMESCALES OF GRANITIC MELT PRODUCTION, SEGREGATION AND CRYSTALLIZATION IN THE HIMALAYAN OROGEN

Determining the timescales of granitic magmatism in the Himalaya is complicated by inheritance of xenocrystic accessory phases used for U-Pb dating. While xenocrystic monazite, xenotime and zircon with pre-Himalayan (usually Orodvician) dates are relatively easy to identify and interpret, a 2-10 Ma range in Oligocene-Miocene accessory mineral dates characterizes many granites. A combination of field, petrographic and U-Pb IDTIMS geochronologic techniques was used to investigate the timing and processes involved in leucogranite magmatism in the Everest region of the Himalayan orogen where in situ crustal melting and multiple generations of leucogranite intrusions can be observed. These studies suggest that, in some cases, the range of accessory mineral dates is best interpreted as a record of early anatexis prior to final melt segregation and emplacement.

For example, two samples of syn-deformational granitic sills from different structural levels in the Himalayan metamorphic core have U-Pb crystallization ages of 21.33 ± 0.03 and 21.80 ± 0.05 Ma. Preserved within these granites, however, is a record of earlier magmatic crystallization of xenotime, zircon and monazite between ca. 26 and ca. 23 Ma. Grains of xenotime, zircon, and monazite that formed during this time interval are interpreted to be magmatic based on analysis of internal structures and inclusion suites observed using backscattered electron imaging and energy dispersive analysis techniques. Grains of zircon and xenotime often contain cores with distinctive “flame-like” zoning that are rich in inclusions of quartz, alkali feldspar, sillimanite and biotite; the same phases involved in the muscovite dehydration reaction widely believed to have produced the granitic melts. Similar cores were observed in magmatic intergrowths of zircon and xenotime that also crystallized between ca. 26 and ca. 23 Ma. This pattern of accessory phase crystallization is interpreted to reflect incremental melting and crystallization in the source region of the granitic sills before ultimate melt migration. Because the granites that preserve this record of early anatexis are syn-deformational, final melt migration was likely deformation induced.