CONTRASTING ACCESSORY MINERAL BEHAVIOR IN MINIMUM-TEMPERATURE MELTS: EMPIRICAL CONSTRAINTS FROM THE HIMALAYAN METAMORPHIC CORE

Medium-grained leucogranite in the Tama Kosi region of the Nepalese Himalayan Metamorphic Core yields a relatively narrow range of monazite ²⁰⁸Pb/²³²Th dates with a dominant population at ~ 21.0 Ma, inferred to represent crystallization of an early plutonic phase. In contrast, the pegmatitic portion of the same intrusive complex, that cross-cuts the medium-grained leucogranite, contains zircon, monazite and xenotime that each display near-identical age spectra, recording semi-continuous (re-)crystallization from 27.5 Ma to 21.0 Ma, followed by a ~2 m.y. hiatus then further (re-)crystallization. The ‘gap’ in pegmatite dates corresponds well to the crystallization age of the older leucogranite, whereas the end of accessory phase growth in the pegmatite coincides with the onset of regional-scale cooling as inferred from ⁴⁰Ar/³⁹Ar biotite ages. Detailed textural, trace element and thermochronologic (i.e. petrochronologic) data indicate that the range of zircon, monazite and xenotime dates recorded in the pegmatite reflect inherited components that underwent semi-continuous (re-)crystallization during metamorphism and/or anatexis in the source region(s), whereas dates younger than the hiatus indicate accessory phase recrystallization, related to both fluid influx and a concomitant increase in temperature during pegmatite emplacement. In contrast, the lack of an inherited component(s) in the medium-grained leucogranite phase is inferred to be a result of complete dissolution during partial melting. A model is proposed in which the H₂O-rich nature of the pegmatite system promoted partial monazite dissolution and delayed further (re-)crystallization during initial cooling. In contrast, the stability of zircon and xenotime are not as affected by the presence of H₂O and dissolution of these phases occurred in response to a concomitant increase in temperature accompanying pegmatite emplacement. These data highlight the importance of measuring spatially resolved dates, trace elements and textural patterns from multiple accessory minerals to understand the often-complex crystallization history of anatectic melts in collisional orogens.