Tectonic Implications of Partial Melting in the Ruby Mountain-East Humboldt Metamorphic Core Complex, Northeastern Nevada

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

The physical and chemical makeup of the deep crust is impossible to examine in situ. Thus, this part of the earth has remained enigmatic despite its significance for geochemical transport during episodes of mountain building as is important, for example, in the generation of ore deposits. Study of the deep crust can only be undertaken through examination of rocks that have been exhumed from great depths, and the focus of this research is to conduct such a study on rocks of a deeply exposed mountain core in Nevada, the Ruby-East Humboldt range. Rocks exposed in this range are metamorphic and, having originated at the Earth's surface, were subjected to elevated pressures and temperatures as a result of burial during the formation of the Rocky Mountains. In fact, these rocks were at sufficient depth that they began to melt and this melting caused major changes in the physical properties of the rock, similar to changes observed in a block of ice when it begins to melt and, subjected to forces of compression, begins to move rapidly. The hypothesis to be examined and tested in the present study is the degree to which this melting (the products of which can be observed in the present day rock outcroppings) contributed to the rapid upheaval, exhumation, and emplacement of these high-grade metamorphic rocks into the core of the mountain range. Many such partially-melted deep crustal metamorphic belts are exposed in the cores of mountain ranges, and the results of this study will be directly applicable to these other terranes, and may have profound implications for the evolution of mountain belts in general. Funding will allow the P.I. and his graduate student to gather the data necessary to test the hypothesis that partial melting in the Ruby Mountains-East Humboldt Range, Nevada, through related changes in rheology and strain rates, initiated emplacement of metamorphosed lower crustal rocks into the middle crust to set the stage for metamorphic core complex formation. This research will employ major and accessory mineral thermobarometry (including an application of the new Ti-in-quartz thermobarometer), in situ U/Th-Pb and Sm-Nd geochronology, and diffusion modeling to constrain the conditions and timing of partial melting of metapelitic rocks. What is unique about this study is that it will determine both the absolute and relative timing of not simply prograde/peak metamorphism, but of specific partial melting reactions, their progress, and how they relate to the cooling and exhumation history of a metamorphic core complex. Results of this study will have implications for present day lower crustal conditions in the cores of active orogenic belts that may be undergoing anatexis. Samples and data collected as part of this research, incorporated into MetPetDB (the NSF-funded database for metamorphic geochemistry), acting to broaden the existing public dataset and serve as an example to new users, demonstrating the use of MetPetDB as a tool for collaboration.