2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 343-5
Presentation Time: 2:00 PM


ACOSTA, Marisa D. and STELLING, Pete, Geology, Western Washington University, 516 High St, Bellingham, WA 98225, risa.acosta@gmail.com

Mafic enclaves are commonly found in granitic plutons, and the injection of mafic material into a silicic host is often interpreted to be a mechanism of magma hybridization. However, the exact mechanisms of mafic-felsic hybridization are difficult to constrain due to thermal and rheological contrasts between the two magmas. Optical petrography, LA-ICP-MS, SEM- EDS/BSE/CL, and Mossbauer Spectrometry were used to characterize populations of mafic material injected into the Shannon Falls pluton, British Columbia Canada. Mafic and felsic material present are not derived from the same source, but carry distinct REE signatures. There are two biotite crystallization events preserved in the pluton - the first occurred during the unperturbed evolution of the granitic host, and the second occurred late in the crystallization history as mafic enclaves were introduced into the semi-molten host. The latter biotite-forming event is characterized by mafic-felsic disequilibrium reactions contemporaneous with the crystallization of amphibole. REE patterns in the reaction biotite are controlled by proximal minerals. Reaction biotite at the rims of enclaves have similar REE patterns from sample to sample, while reaction biotite at the core of enclaves is variable from crystal to crystal. Whether or not biotite or amphibole crystallizes appears to be dictated by the amount of surface area the host and enclave share, and requires a late-stage potassic fluid.