Author name: Matthew J. Kohn

Himalayan Leucogranites: A Minimal Role in Deformation

A popular model of Himalayan metamorphic and structural evolution argues that partial melting of deeply buried rocks triggered crustal weakening, ductile flow, orogenic collapse, and genesis of leucogranites. Here, we review the origins and evolution of partial melts and leucogranites to demonstrate that they are largely incidental to deformation. Although a pulse of orogenic collapse and leucogranite crystallization occurred at 15–25 Ma, pervasive partial melts formed as much as 20 My earlier. Thus, leucogranites date extraction and transport, not necessarily melting onset. Extensional structures and distributed extensional strain occur in many rocks that lack partial melt and leucogranites, indicating these are not prerequisite to facilitate orogenic collapse. Most mass transfer appears to occur via thrusting, even in partially molten rocks.

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Garnet: Witness to the Evolution of Destructive Plate Boundaries

Thanks to its unique chemical and mechanical properties, garnet records evidence of rocks’ paths through the crust at tectonic plate boundaries. The compositions of garnet and coexisting mineral phases permit metamorphic pressure and temperature to be determined, while garnet’s compositional zoning allows the evolution of these parameters to be constrained. But careful study of garnet reveals far more, including the dehydration history of subducted oceanic crust, the depths reached during the earliest stages of continental collision, and the mechanisms driving heat and mass flow as orogens develop. Overall, chemical and textural characterization of garnet can be coupled with thermodynamic, thermoelastic, geochronologic, diffusion, and geodynamic models to constrain the evolution of rocks in a wide variety of settings.

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Applications of Raman Spectroscopy in Metamorphic Petrology and Tectonics

Raman spectroscopy is widely applied in metamorphic petrology and offers many opportunities for geological and tectonic research. Minimal sample preparation preserves sample integrity and microtextural information, while use with confocal microscopes allows spatial resolution down to the micrometer level. Raman spectroscopy clearly distinguishes mineral polymorphs, providing crucial constraints on metamorphic conditions, particularly ultrahigh-pressure conditions. Raman spectroscopy can also be used to monitor the structure of carbonaceous material in metamorphic rocks. Changes in structure are temperature-sensitive, so Raman spectroscopy of carbonaceous material is widely used for thermometry. Raman spectroscopy can also detect and quantify strain in micro-inclusions, offering new barometers that can be applied to understand metamorphic and tectonic processes without any assumptions about chemical equilibrium.

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