PETROLOGIC INSIGHT INTO THE P-T HISTORY OF HIGH GRADE METAPELITES OF THE INTERNAL ZONE OF THE PENOKEAN-YAVAPAI OROGENIC BELT, NORTHERN WISCONSIN

Fault bound terranes in the interior of the Penokean-Yavapai belt preserve the highest metamorphic grades in the region. Published monazite U-Pb geochronology from these rocks has indicated metamorphism in the Watersmeet Terrane consistent with the Penokean (~1.83 Ga) and Yavapai (~1.76 Ga) orogenies. A classic kyanite + garnet schist unit from this block has been subject of prior major phase thermobarometry studies indicating upper amphibolite facies metamorphism. This study presents new mineral chemistry data reflecting major element zoning in garnet. Notably, rutile is present as inclusions in kyanite and garnet, but does not appear in the matrix of this rock. Rutile inclusions in kyanite give Zr concentrations between 237 and 383 ppm, indicating temperatures of 592­–629 ºC, providing temperature constraints for the formation of significant large kyanite porphyroblasts. Additionally, kyanite porphyroblasts consistently show plagioclase corona textures surrounding them, some with biotite intergrown with plagioclase. The plagioclase coronas consistently separate kyanite from adjacent quartz. Similar textures in granulate facies rocks have been interpreted to reflect a decompression reaction, which is consistent with minor sillimanite needles present in some places in the schist. Kyanite is therefore interpreted as a relict phase recording an earlier, likely higher pressure, segment of the P-T path for this block. The slope or nature of the decompression segment of the P-T path is unclear, but it may represent exhumation of this block in a late phase of Yavapai tectonism.