WHAT IS THE SIGNIFICANCE OF AMS FABRIC IN PLUTONS? INITIAL RESULTS FROM A STUDY OF MAGMATIC FABRIC IN THE CADILLAC MOUNTAIN INTRUSIVE COMPLEX, ACADIA NATIONAL PARK, MAINE

In light of new petrologic models that focus on granitic pluton construction by pulses of magma injection into an active magma chamber that is small relative to final pluton size, we are reassessing the interpretation of AMS data collected across large plutons. Our initial results call into question the significance of AMS fabric in plutons. In granites, AMS fabrics are commonly used as proxies for weak magmatic fabrics, under the assumption that AMS foliation and lineation mimic fabric defined by shape-preferred orientations (SPO) of major phases. We present the initial results of an AMS study and detailed fabric analysis of the Somesville granite, one of the latest intrusive phases in the Cadillac Mountain Intrusive Complex (CMIC) in Acadia National Park, Maine. No fabric is evident upon field observation of the Somesville granite, and there is no evidence for solid state deformation. Thus, we collected AMS data in order to detect any weak fabric present. High susceptibilities suggest that magnetite is responsible for AMS fabrics. Within the entire CMIC, AMS fabrics trend subparallel to intrusive contacts in the SW half of the pluton, and at large angles to the contacts in the NE half. AMS foliation dips moderately to steeply; AMS lineation plunges shallowly. At one site, we used a mean AMS ellipsoid to select three mutually orthogonal faces for fabric analysis by quantification of SPO of the major mafic (biotite) and felsic (K-feldspar) phases present. Preliminary results show disagreement between the orientations of AMS foliation/lineation and major mineral SPO. It is not feasible to construct a true ellipsoid from the SPO data, but shallow SPO long axis orientations are consistent with the subhorizontal long axis orientations of unstrained mafic enclaves. In addition, within each plane, the SPO of mafic and felsic minerals are in good agreement. Such a shallow magmatic fabric could be associated with subhorizontal flow or settling of crystals, consistent with the petrologic model of intrusion by magma pulses into an active magma chamber with a subhorizontal floor. Two goals of future work are to quantify K-feldspar and biotite SPO fabrics throughout the CMIC, and to assess the significance of AMS foliation and lineation.