Delineation of multiple metamorphic events in the Himalayan Kathmandu Complex, central Nepal

Abstract

Quartz-inclusion geothermobarometry of texturally and chemically zoned garnet, together with U-(Th)-Pb accessory phase petrochronology, phase equilibria modelling, and major element thermobarometry delineates two metamorphic events withinthe Kathmandu Complex in central Nepal. Combined titanium-in-quartz thermometry (TitaniQ) and quartz-in-garnet barometry (QuiG) of quartz inclusions in garnet cores yields temperatures and pressures (555-580°C, 0.89-0.91 GPa) statistically higher than those estimated for both garnet rims and secondary garnet neoblasts (535-545°C, 0.81-0.84 GPa). Comparison of these temperatures and pressures with the results from phase equilibria modelling indicates that the garnet cores nucleated after overstepping the equilibrium garnet isograd. Estimation of the degree to which overstepping may have occurred, however, is complicated by uncertainty in the estimated reactive bulk compositions. Major element thermobarometry of the garnet rims returns temperatures and pressures lower than those preserved by the garnet cores but are consistent with previous estimates of Cenozoic metamorphism in the frontal portion of Kathmandu thrust sheet. Two generations of titanite are recognized on the basis of mineral morphology, chemical zoning, and U-Pb date (c. 500 Ma versus 29 ± 5 Ma). Trace-element systematics of the two titanite populations indicate varying co-stability with allanite and are consistent with Th-Pb dates of allanite growth (c. 489 Ma) and breakdown to clinozoisite (c. 30 Ma). Inclusion patterns within titanite indicate that the younger group formed following breakdown of rutile that originally coexisted with the older generation of titanite at pressures and temperatures similar to those determined from the garnet cores. Application of the Zr-in-titanite and Zr-in-rutile thermometers yields temperatures 100-150°C in excess of those indicated by both TitaniQ-QuiG and major element thermobarometry. This discrepancy is interpreted to reflect disequilibrium partitioning of Zr and limited zircon reactivity during metamorphism. Collectively, these observations are consistent with previous estimates of the timing and conditions of metamorphism associated with the Palaeozoic Bhimphedian and Cenozoic Himalayan orogenies in central Nepal. The estimated temperatures and pressures for growth of the garnet cores are consistent with rapid burial during thrust stacking.

Equation of State for Natural Almandine, Spessartine, Pyrope Garnet: Implications for Quartz-In-Garnet Elastic Geobarometry

“…Inclusion pressure uncertainties reported by EntraPT [44] are 0.201 GPa, producing an entrapment pressure uncertainty of 0.276 GPa at 559 • C. Inclusion pressures that are referred to as 'P stress ' were calculated using Grunëisen tensors in EntraPT [44]. These are calculated using only two modes following [45], rather than the preferred three, as the 128 cm −1 mode was not used due to noise from the incident laser line causing a high signal-to-noise ratio and high background at the 128 position, which significantly increased the fit uncertainty on this mode. Only four inclusions were suitable for calculating pressures using three modes, these data are provided in the supplement (Table S2).…”

Strengths and limitations of in situ U-Pb titanite petrochronology in polymetamorphic rocks: An example from western Maine, USA.

“…295 Ma. Johnson, Cottle, & Larson (2020) suggested limited equilibrium between titanite and zircon in their samples resulted in Zr-in-titanite temperatures that are 100 to 150 °C higher than temperatures calculated from other techniques and inconsistent with the observed mineral assemblages. Here we also suggest that quartz and/or zircon cannot be in equilibrium with titanite at ca.…”

“…Additionally, if radiogenic Pb released during the breakdown of U-bearing phases does not reach isotopic equilibrium with Pb in other phases then the 207 Pb/ 206 Pb ratio of low U/Pb phases cannot be used to determine the initial 207 Pb/ 206 Pb ratio of titanite. Often the 207 Pb/ 206 Pb ratios measured on U-poor phases, such as plagioclase and alkali feldspar, fall within the value of initial 207 Pb/ 206 Pb defined by regression through titanite U-Pb data (e.g., Johnson, Cottle, & Larson, 2020;Kohn & Corrie, 2011;Walters & Kohn, 2017). For Sample SSP18-1B, we observe that isochrons for pooled dates at ca.…”

“…Titanite U-Pb dates have been interpreted to reflect a variety of processes, including neocrystallization and growth (Castelli & Rubatto, 2002;Corfu, 1996;Kohn & Corrie, 2011;Rapa et al, 2017;Scott & St-Onge, 1995;Spencer et al, 2013;Stearns et al, 2015;2016;Verts, Chamberlain, & Frost, 1995;Walters & Kohn, 2017), fluid driven recrystallization or alteration (Corfu, 1996;Garber et al, 2017;Marsh & Smye, 2017), Pb volume diffusion and cooling (Cherniak, 1993;Kirkland et al, 2016;Mattinson, 1978;Mezger et al, 1991;Spear & Parish, 1996;Tucker et al, 1987), and deformation-induced recrystallization (Bonamici et al, 2015;Gordon et al, 2021;Papapavlou et al, 2018;Spencer et al, 2013;Timms et al, 2020). Additionally, disequilibrium between titanite and zircon (or quartz) may result in overestimated Zr-in-titanite temperatures (Cruz-Uribe et al, 2018;Johnson, Cottle, & Larson, 2020). Combined, these factors greatly complicate linking titanite zoning textures, compositions, and U-Pb dates to metamorphic, metasomatic, and tectonic processes.…”

“…In the absence of Pb diffusion, disturbance of titanite U-Pb is anticipated to occur through fluid-or deformation-induced recrystallization. Consequently, multi-generational titanite with both recrystallized and secondary domains are commonly observed (e.g., Castelli & Rubatto, 2002;Corfu, 1996;Corfu & Ayers, 1991;Corfu & Muir, 1989: Garber et al, 2017Johnson, Cottle, & Larson, 2020;Spencer et al, 2013;Tucker et al, 2004;Verts, Chamberlain, & Frost, 1995). Recrystallized zones are typically patchy to lobate and often oriented along fractures and cleavage planes, consistent with interface-coupled dissolution-reprecipitation reactions (ICDR) (Bonamici et al, 2015;Garber et al, 2017;Marsh & Smye, 2017;Walters & Kohn, 2017).…”