THREE STAGES OF ANDALUSITE GROWTH IN THE RYE COMPLEX, NEW HAMPSHIRE

New work on a poorly understood sliver of high grade mylonitic rocks faulted between low-grade Peri-Gondwana terrains in coastal New England, the Rye Complex, shows a complicated, low-pressure, P-T-t path. Rocks from a large outcrop (Fort Stark) near Portsmouth, NH are aluminous and quartzo-feldspathic and contain andalusite, staurolite, cordierite, and sillimanite in various assemblages and textures. Straight inclusion trails of ilmenite and quartz in subhedral garnets, and linear inclusion trails of ilmenite and biotite in andalusite porphyroblasts preserve an older (S1?) foliation. These stage 1 andalusite porphyroblasts are universally undulose. Later, stage 2, andalusite forms rims and replaces staurolite showing pressures < 350 Mpa during this prograde reaction. Sillimanite needles and fibers associated with oriented biotite (S2?) and migmatite indicate that peak temperatures were > ~650ºC and in the sillimanite field. Oriented biotite, ilmenite, and sillimanite inclusions in anhedral andalusite porphyroblasts associated with late cordierite porphyroblasts document a third stage of andalusite growth and a return to the andalusite field after peak metamorphic temperatures. Kinked biotite flakes in S2/S3 folia, muscovite and biotite fish, micro-shear zones, and boudinage of andalusite porphyroblasts, all show evidence for mylonitic ductile (stage 4) deformation during waning lower amphibolite facies metamorphic conditions. Undulose quartz ribbons, dynamically recrystallized quartz, and refoliated muscovite micro-breccia show evidence for continued (stage 5) deformation through green schist facies metamorphism. Together these observations converge to show a strongly dynamic, low pressure clockwise P-T-t path. Detrital zircon U-Pb geochronology and hornblende, muscovite, and K-feldspar thermochronology constrain this high-grade metamorphism to the Cambrian-Ordovician (Taconic Orogeny?) and the lower grade mylonitic deformations to the Acadian and Alleghanian orogenies. Thus this family of fabrics requires significant and sustained transpression through most of the Paleozoic.