Tectonometamorphic evolution of the Likhu Khola study area, east-central Nepal

Abstract

Recent models for the evolution of the Himalaya and adjacent regions have changed our understanding of how large mountain belts form. The key data that govern these models has largely been extracted from the exhumed mid-crustal core of the orogen, the Greater Himalayan sequence, and its bounding structures. Targeted mapping in the Likhu Khola region was carried out across the Greater Himalayan sequence as the initial phase of a project aimed to evaluate the viability of those models. The exhumed mid-crustal core in the study area exposes upper greenschist to upper amphibolite grade metamorphic rocks that have been pervasively deformed by ductile shearing. Mantled porphyroclasts and c, c’ and s fabrics record top-to-the-south directed shear. As with most transects across the Himalaya, metamorphic grade increases up structural section. Pressure and temperature estimates using THERMOCALC v.3.26 in average-PT mode with the internally consistent data set of Holland & Powell (1998) were conducted on eleven specimens at different structural positions. Temperatures increase slightly up structural section but become constant within error for the upper portion of the study area. Pressure estimates increase up structural section followed by an abrupt pressure decrease once partial-melting increases to form migmatitic rocks. This may indicate a potential tectonometamorphic discontinuity that separates two distinct domains that have different structural, thermal and metamorphic histories (e.g., Larson et al, 2010a; Yakymchuk and Godin, 2012). In situ U-Th-Pb Monazite geochronology was utilized on six of the specimens used for P-T analyses to constrain the P-T data. Multiple domains of ages were obtained ranging from 27.2 Ma to 15.1 Ma and are interpreted to represent several recorded metamorphic events. These metamorphic events are best interpreted in ii conjunction with the relative concentration of trace elements present at each U-Th-Pb data point that was collected using a split stream LA-MC-ICP-MS. More specifically, certain rare earth elements provide insight into whether garnet was growing, resorbing into melt or being homogenized at high temperatures during specific U-Th-Pb age dates. The relationships between metamorphism, crustal melting, P-T conditions and monazite/garnet growth and resorption are critical to evaluating if current models proposed for the evolution of the Himalaya are applicable in east-central Nepal.