FORMATION OF SILLIMANITE-QUARTZ-MAGNETITE VEINS VIA INCONGRUENT PRESSURE SOLUTION OF A HOST GRANITE. IMPLICATIONS FOR FLUID FLOW ALONG THE CARTHAGE COLTON SHEAR ZONE

A belt of nodular granitic gneiss (NOD) extends for several kilometers within the footwall of the Carthage-Colton Shear Zone (CCSZ) forms a portion of a late Ottawan (Grenville) aged pluton belonging to the Lyon Mountain Gneiss. The nodular granite gneiss contains veins and disrupted veins of quartz and masses of well-oriented sillimanite (+/- magnetite). The matrix of this gneiss consists of granular k-feldspar + hornblende+/- plagioclase. Directly to the east of the Nodular Gneiss Belt and in contact with it is a small weakly deformed granite body. Deformation in this body is restricted to a series of anastomosing shear zones ranging from sub-centimeter to meter widths. The mineralogy of the host granite is made up of perthitic k-feldspar, plagioclase, quartz, and hornblende. In the shear zones, feldspar and hornblende have been completely removed via incongruent pressure solution leaving the assemblage of fibrolitic sillimanite, quartz and magnetite. Sillimanite needles are oriented with their long axes parallel to the shear zone and many have been extended via brittle failure along basal partings in the sillimanite. The textural and mineralogical similarities between these shear zones and the disrupted veins and nodules in the adjacent NOD suggests that both formed as shear zones that in the case of the NOD were subsequently disrupted during more holistic deformation of the gniess. The formation of these sillimanite-rich zones via incongruent pressure solution requires high time integrated fluid/rock ratios along this portion of the CCSZ. The origin(s) for these fluids may be exhalative from the Lyon Mountain Granite or sourced from metasediments in the hanging wall of the CCSZ.