Abstract
Abstract Reconstructions of the tectonometamorphic architectures of orogenic systems rely on petrogenetic indicator minerals, such as garnet, to trace the transport of rocks through space and time. We show the results of new garnet Lu–Hf geochronology and inclusion U-(Th-)Pb geochronology from exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of the eastern Nepalese Himalaya. Garnet in three of the five specimens analyzed yielded pre-Himalayan Lu–Hf dates of ca. 292–230 Ma, which contrasts with Himalayan-aged inclusions therein. Garnet in these specimens either grew entirely during the Early Permian opening of the Neo-Tethys Ocean or grew partly during Cambrian–Ordovician orogenesis and partly during the Cenozoic. The remaining two specimens yield Lu–Hf ages of ca. 50 Ma, which are some of the oldest recorded dates for Himalayan metamorphism. The apparent discordance between the geochronological techniques highlights a potential issue with interpretations that rely on single-geochron-method inclusion records. These results further show that some pressure-temperature determinations used in Himalayan models may not be Himalayan in age.
Highlights
Major element geochemistry of garnet in thin sections were acquired using Cameca SXFiveFE electron probe microanalyzer (EPMA) housed in the Fipke Laboratory for Trace Element Research (FiLTER) at the University of British Columbia, Okanagan (UBCO)
Trace element geochemistry and mapping of garnet mounted in epoxy resin were acquired using Photon Machines Analyte 193 Excimer laser coupled with Agilent 8900 Triple Quadrupole Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) housed in FiLTER at UBCO
NIST SRM 610 was used as an external reference material, while for trace element spot analysis, NIST SRM 612 and 610 were used as external primary and secondary reference material respectively
Summary
Major element geochemistry of garnet in thin sections were acquired using Cameca SXFiveFE electron probe microanalyzer (EPMA) housed in the Fipke Laboratory for Trace Element Research (FiLTER) at the University of British Columbia, Okanagan (UBCO). Trace element geochemistry and mapping of garnet mounted in epoxy resin were acquired using Photon Machines Analyte 193 Excimer laser coupled with Agilent 8900 Triple Quadrupole Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) housed in FiLTER at UBCO. Operating conditions for garnet mapping and spot analysis were 100% laser energy, 8 Hz repetition rate and 6.78 Jcm-2 fluence.
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