CHARACTERIZING THE TRANSITION FROM PULVERIZED GRANITE TO ULTRAMYLONITE ACROSS AN ABRUPT STRAIN GRADIENT IN THE KELLYLAND FAULT ZONE, MAINE, USA

The Deblois granite pluton in eastern Maine is cut by the dextral Kellyland fault zone (KFZ). The KFZ contains three distinct strain facies that formed near the brittle-ductile transition: (1) a 2-3-km-wide belt of foliated granite, (2) a 100-300-m-wide belt of pulverized granite cut by mm- to cm-wide shear zones and associated brittle faults, and (3) a 200-400-m-wide belt of homogenous ultramylonite with lesser amounts of mylonite (Sullivan et al., 2013).

There is no significant chemical alteration between foliated granite and mylonitic rocks. Pulverization predates the formation of mylonitic rocks and mylonitic rocks cutting pulverized granite formed after pseudotachylyte and cataclasite (Sullivan et al., 2013; Sullivan and Peterman, 2017). Therefore, we hypothesize that formation of the 200-400-m-wide ultramylonite zone was catalyzed by brittle grain size reduction and phase mixing related to pulverization.

Site KL102 is an ideal location to test this hypothesis because it features a continuous exposure from pulverized granite into ultramylonite over a 10-m-wide zone. We will constrain the microstructural changes in 8 representative samples from KL102 by using optical microscopy, SEM imaging, SEM-EDS compositional mapping, and SEM-EDS-EBSD crystallographic mapping. Bulk mineralogical changes also will be assessed with whole-rock powder XRD.

Published analyses of pulverized-granite samples will provide a reference point for our study and additional samples from the interior of the ultramylonite zone will be compared to ultramylonite samples at KL102. Samples from KL102 transition zone will also be compared to samples from a 2-3-m-wide cataclasite body hosted in the ultramylonite 45 m across strike from KL102.

Preliminary optical microscopy and SEM imaging shows that pulverized granite is overprinted by mm- to cm-wide brittle shear zones and that cataclasitic granite is overprinted by mylonitic zones. Cataclastic shear zones cutting pulverized granite often nucleate on hornblende and biotite grains. The fractures are dilated and primarily filled with very fine grained biotite. Grain size reduction and phase mixing associated with deposition of biotite in these voids may have helped form the fine-grained mylonitic matrix of the 200-400-m-wide ultramylonite zone.