ORDERING OF ALKALI AND PLAGIOCLASE FELDSPARS IN META-IGNEOUS AND META-SEDIMENTARY ROCKS ACROSS METAMORPHIC GRADE IN RHODE ISLAND

This study was designed to evaluate the effects on Al-Si order in plagioclase and K-feldspars of (1) rapid regional metamorphism and cooling in igneous rocks; and (2) the effect of metamorphic fluids in first-cycle arkosic metasediments across the same metamorphic field gradient. The Barrovian metamorphic profile of Rhode Island exposes rocks from chlorite through staurolite + kyanite + sillimanite to migmatite grade in both Late-Pennsylvanian Narragansett basin arkosic metasediments and in metagranitoids below a nonconformity. Experiments show that K-feldspars in rocks exceeding garnet grade should have a disordered Al-Si distribution, and so a gradient in the degree of disorder should have existed during the metamorphism. Therefore, these rocks provide an opportunity to compare feldspars in rocks that have experienced the same metamorphic history but in very different protoliths. Samples from the metagranitoids and the Narragansett basin were analyzed by thin-section petrography and by X-ray powder diffraction to determine the mineralogy and state of Al-Si order of the feldspars. Granitoid samples contained major quartz, low-albite (a=8.1436(8); b=12.802(1); c=7.1588(4)Å), and maximum microcline (a=8.5846(6); b=12.9832(7); c=7.2253(3)Å), with low and variable amounts of biotite, chlorite, and amphibole. In contrast, the arkosic metasediments contained low albite, but the state of order in the small amount of K-feldspar (2-4%) was consistent with orthoclase (a=8.621(7); b=12.944(6); c=7.165(4)Å). Results showed no relationship between metamorphic grade and Al-Si order in the metagranitoids; all K-feldspars were full ordered. K-feldspar contents of the arkosic metasediments were variable and low as a result of the reaction: K-feldspar + chlorite Þ muscovite + biotite. The small amounts of K-feldspar present (generally <4%) were always partially disordered. We conclude that annealing times in the granitoids were sufficient to allow K-feldspars to reorder. Apparently, the fluid-rich environment in the arkosic metasediments retarded the diffusion necessary to order the K-feldspars on cooling.