GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 40-6
Presentation Time: 9:00 AM-5:30 PM


PIZARRO, Maxwell, Department of Geology and Environmental Science, Hartwick College, 1 Hartwick Drive, Oneonta, NY 13820 and JOHNSON, Eric L., Geology and Environmental Sciences, Hartwick College, Oneonta, NY 13820

This on-going study examines the mineralogic and geochemical (bulk trace and oxygen isotope) response to increasing amounts of strain at the boundary of a 30 m wide shear zone in the Grenville-aged Dana Hill Metagabbro Body (DHMG). Textures along this strain gradient range from undeformed ad-cumulate anorthositic gabbro to ultra-mylonitic gabbro. Undeformed DHMG samples contain the mineral assemblage: plagioclase (An 0.49-0.65) with exsolved rutile, augite (with rims of secondary hornblende), magnetite, ilmenite, and accessory apatite. The deformed samples of DHMG show: 1. widespread replacement of augite by Fe- rich hornblende, 2. lack of opaque oxides, 3. abundant titanite, 4. plagioclase (An 0.44) clear of rutile inclusions, 5. scapolite replacement of plagioclase in contact with amphibole and titanite and 6. accessory apatite. Plagioclase away from amphibole/titanite contacts remains intact and plagioclase/scapolite contacts are clean and sharp. Chemical response across the strain gradient show enrichment in Ca, Na, P, K, Na, LREE elements, U and Th and depletion in Ti, Fe, Mg and Mn. Iron and titanium from the loss of ilmenite and rutile is partially incorporated into the growth of Fe-rich hornblende and growth of titanite with the remainder lost to an infiltrating (possible halogen-rich) fluid attending deformation. Rare Earth Element (REE) data show enrichment in LREEs and little change for most of the HREEs across the strain gradient. Apatite and Titanite are sinks for REE during recrystallization of the DHMG yet these minerals cannot explain the LREE enrichment. The fractionation of HREE and LREE requires the presence of a LREE enriched fluid during recrystallization. Uranium is enriched by nearly 700% and Thorium is enriched by 54% across the deformation gradient. Enrichments in these elements are most likely driven by the infiltration of fluid or fluids during shear deformation of the DHMG. Titanite in the deformed DHMG is likely the major sink for U. Preliminary data on whole-rock oxygen isotope data from the DHMG show a small decrease in del18O from delO18=8.51 in the undeformed to delO18=7.73 in the mylonitic DHMG.