2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 20
Presentation Time: 1:30 PM-5:30 PM


MORGAN, Valerie L., Department of Geology, Oberlin College, Oberlin, OH 44074 and WHITNEY, Donna L., Department of Geology and Geophysics, Univ of Minnesota, Minneapolis, MN 55455, vmorgan@oberlin.edu

Garnets in micaschists in the southern Menderes Massif, Turkey, range widely in abundance and size within a single outcrop, allowing us to test ideas about physical and chemical controls on garnet crystallization during regional metamorphism. The micaschists were metamorphosed at 500-550°C, 6-8 kbar, during a single metamorphic episode, and contain Ms + Bt + Qz + Grt ± Pl, with retrograde Chl. In the outcrop studied, Grt distribution, mode, size, and shape vary in dm- to m-wide bands. We examined 6 schist samples that represent the range of observed Grt sizes, 1 to 10 mm. Schists with large garnets contain fewer garnets; schists with the smallest garnets contain large numbers of garnet. In all schists studied, garnets are euhedral to subhedral and have poikiloblastic cores (dominated by Qz inclusions) and relatively inclusion-free rims. We did not detect any systematic trends in Grt composition or zoning among the samples; all garnets are Fe-rich (core XAlm = 0.61-0.79, increasing to 0.71-0.82 at rims), and slightly zoned (from core to rim, Mn and Ca decrease). We used serial sectioning and image analysis to calculate the volume of Grt in each sample and determine the modal amounts of garnet. Our results show that the schists with many small Grts have a much higher modal volume of Grt: 40% compared to 10% in the schists with large Grts. In the schist with 40% Grt, at least 60% of the Grts are in contact with one or more other Grts; in the schists with larger but fewer Grts, clustering varies from 5-40%. The main difference between the schists with few large Grts and those with many small Grts is the relative abundance (but not grain size or texture) of biotite, muscovite, and quartz. Large Grts grew in the most biotite-rich schist (25% Bt) and the smallest garnets grew in the most Qz- and Pl-rich (50% Qz, 5% Pl), mica-poor schist (3% Bt, trace Ms only). These proportions may not reflect relative mineral abundances when the Grts nucleated, but in general, our results suggest that Grts in different layers of the schist grew from similar metamorphic reactions at the same P-T conditions, producing Grts with similar compositions and zoning patterns. However, slight differences in protolith heterogeneity that influenced the amount of mica in the metamorphic rocks had a dramatic effect on Grt nucleation density and therefore Grt size.