GEOCHEMISTRY OF A GRANITIC PEGMATITE INCUBATOR AT STREAKED MOUNTAIN, WESTERN MAINE

Trace element concentrations of samples of feldspar and quartz from an evolved granitic pluton at Streaked Mountain, Hebron, ME were analyzed to investigate models of magmatic evolution of this texturally diverse body. Proximity of the mountain to major gem-bearing pegmatite localities (e.g., Mt. Mica, c. 3.6 km NW) focused attention on the site as a potential pegmatite source. Exposure of bedrock outcrops on the mountain (c. 0.2 km²) affords > 200 m of vertical relief. This study involved small-scale mapping and collection of samples along NE-SW and NW-SE transects. Coarse average grain size and heterogeneous textures challenge whole-rock approaches to understanding the geochemical evolution of Streaked Mountain; we chose feldspar and quartz as targets as they are the most widespread minerals. The major rock types that were mapped include aplite (fine-grained quartz and feldspar), graphic granite (smoky quartz intergrowths with albitic plagioclase), and pegmatite (quartz core rimmed with feldspar and accessory tourmaline). The pegmatite bodies occur as meter-sized pods at lower elevations, while larger masses of pegmatite occur at higher elevations. Hand picked mineral grains < 0.25 mm in diameter were digested and analyzed in the Interdisciplinary Elemental Measurement Facility at SUNY Oswego using solution quadrupole ICP-MS.

Ranges of the rare alkalis in albitic feldspars (n = 5) are as follows: Li 25-70 ppm, Rb 500-760 ppm, Cs 14-24 ppm. The consistently most abundant trace element measured in quartz was Li (30-52 ppm; n = 5). These values are comparable to quartz from, for example, Variscan felsic volcanic rocks in central Europe (Breiter et al., Chem. Geol. 2012). Feldspar shows no compelling correlations between sample elevation and Rb/Sr or Rb/Ba. Concentration ratios in quartz (Rb/Sr, K/Rb, Li/Be) from graphic and pegmatite zones show weak correlations with sample elevation. In general with increasing elevation Rb/Sr and Li/Be decrease and K/Rb increases. The lack of increase in fractionation indicators such as these with increasing elevation argues against a model by which the original Streaked Mountain magma underwent single-stage vertical (gravitationally-assisted) magmatic differentiation.