ORIGIN OF MYLONITES AND ULTRAMYLONITES IN THE KELLYLAND FAULT ZONE, WASHINGTON COUNTY, MAINE

In the middle and lower continental crust strain is often localized in discrete mylonitic shear zones. Possible origins for these shear zones include: reaction softening due to fluid mobility, grain size reduction by brittle processes, and nucleation along weaker areas in a heterogeneous protolith. This project is a case study of strain localization and mylonitization in the Kellyland Fault Zone (KFZ), Washington County, ME. The KFZ cuts megacrystic granite and is characterized by discrete mylonite/ultramylonite zones. The granite protolith contains large feldspar grains with Rapakivi rims, quartz, biotite and hornblende. Local quartz-rich and biotite-rich regions are also present. At the margins of the KFZ, discrete mylonite/ultramylonite zones with abrupt contacts appear within the granite. These zones coalesce over an across-strike distance of 2-10 m, and the central part of the KFZ is all mylonite and ultramylonite. Altered and deformed pseudotachylite and cataclasite are visible in many mylonites and ultramylonites.

Major and trace element compositions were measured using combined ICP, INAA and XRF analyses. The mineralogy of the samples was determined using x-ray diffractometry and traditional point counting. Preliminary results show no change in major element composition between undeformed granite, mylonites and ultramylonites, but there is some variability in trace element abundance. During deformation hornblende altered to green biotite, but other major phases were largely unaltered. Quartz, feldspar and biotite are largely dynamically recrystallized in mylonites and ultramylonites. Quartz underwent subgrain rotation dynamic recrystallization while feldspars exhibit deformation lamellae and record grain-boundary bulging recrystallization and ample brittle fracturing. Many ultramylonites contain fine-grained, mineralogically mixed domains interpreted as devitrified pseudotachylite or deformed cataclasite. These observations, coupled with a lack of evidence for solution transfer, indicate deformation under relatively fluid poor conditions. Our data and observations suggest that the localized high-strain zones of the KFZ stemmed from grain size reduction during episodic brittle deformation that catalyzed plastic deformation in the mylonites.