GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 4-13
Presentation Time: 11:35 AM


WRIGHT, Samuel1, HOISCH, Thomas2, WELLS, Michael3, CRADDOCK, Suzanne D.2 and MULLIGAN, Suzanne R.4, (1)School of Earth & Sustainability, Northern Arizona University, P.O. Box 4099, Flagstaff, AZ 86011, (2)School of Earth Sciences and Environmental Sustainability, Northern Arizona University, 625 S. Knoles Dr., Flagstaff, AZ 86011, (3)Department of Geoscience, University of Nevada, 4505 Maryland Parkway, Las Vegas, NV 89154-4010, (4)Department of Geoscience, University of Nevada, Las Vegas, 4505 S Maryland Parkway, Las Vegas, NV 89154-4010

Barrovian metamorphic rocks are exposed in the footwall of the Boundary Canyon detachment in the Funeral Mountains, California. The highest-grade rocks are found in lower Monarch Canyon in the NW part of the range. Lithologies include garnet amphibolite, quartzo-feldspathic paragneiss, and pelitic schist. In-situ trondhjemitic partial melts formed locally within pelitic schist. Melt consists of Qz + Grt + Pl + Bt ± Ms ± Ky ± Sil, and displays an allotriomorphic texture consistent with crystallization at near eutectic conditions. Resorbed xenocrystic garnet and skeletal muscovite suggests these phases were dissolving congruently into the melt. High-resolution BSE images of Ca zoning in melt-crystallized plagioclase indicate that in regions of high xenocryst concentration plagioclase nucleated on xenocryst grain boundaries and grew into interstitial spaces. P-T isochemical plots calculated for pelitic schist and garnet amphibolites constrain peak conditions to 670-700 °C at 9-10 kb. T-xH2O isochemical plots calculated at 10 kbar for pelitic schist show that fluid-present melting begins at ~660 °C, well below the conditions of the muscovite dehydration partial melting reaction (~780 °C). Partial melts are localized, suggesting that melts formed where an externally derived water-rich fluid was introduced. LASS-ICP-MS dating of monazite and xenotime yielded two age populations, Late Jurassic (164-157 Ma) and Late Cretaceous (94-72 Ma). Matrix monazite grains from this study all yielded Late Cretaceous ages. The Late Jurassic monazite ages are from one grain preserved in a garnet core, and are similar to ages obtained from garnet, monazite, and xenotime in the medium grade parts of the northern Funeral Mountains. Plate margin reconstructions place the shallowly subducting Farallon slab beneath the Death Valley region near the beginning of Late Cretaceous metamorphism. We propose that metamorphic dehydration reactions within the slab was the source of the fluids that caused localized partial melting of pelitic rocks in lower Monarch Canyon.