MULTIPLE GENERATIONS OF AL2SIO5 POLYMORPHS IN A LOW PRESSURE METAMORPHIC SETTING: THE RYE COMPLEX, NEW HAMPSHIRE

The occurrence of four generations of aluminosilicates in pelitic rocks of the Rye complex, identified with petrography and corroborated by micro-chemical analyses, demonstrates a series of incomplete crystallization events in a low pressure metamorphic setting. Stage I andalusite grains include undeformed biotite flakes and straight ilmenite inclusion trails; these andalusites with garnet replaced muscovite and biotite. Stage II andalusite grains include staurolite; these andalusites with biotite replaced staurolite and muscovite. Stage III sillimanite forms isolated prisms or oriented fibrolite clusters; these sillimanite needles replaced micas. Stage IV andalusite grains include undulating trails of kinked biotite (+/- muscovite), sillimanite, and include and overgrow quartz ribbons and ilmenite that connect to the external fabric these andalusites replace.

Microprobe analyses of these andalusites show Fe concentrations in Stage II andalusite replacing staurolite are up to 3.5x higher than in stages I and IV replacing micas. Muscovite rimming stage II andalusite is truncated by sillimanite suggesting sillimanite postdates staurolite and earlier stages of andalusite. Electron backscatter petrography shows the replacement of Fe-rich andalusite by Fe-poor sillimanite occurred through intermediate muscovite replacement (following Carmichael, 1969). Stage IV andalusite with low Fe concentrations and sillimanite inclusions replaced a fibrolite rich fabric and overprinted the previous Al2SiO5 stages. Foliations associated with each Al2SiO5 stage show strain accompanied these reactions.

These assemblages define a low pressure P-T-t path passing the andalusite+staurolite field, the andalusite without staurolite field, the sillimanite field, and back to the andalusite field. Zircon U-Pb geochronology of plutons and hornblende, muscovite, and K-feldspar Ar/Ar thermochronology on rocks from the Rye complex suggest a P-T-t-deformation path in which Al2SiO5 stages I, II, and III grew during a low pressure Cambrian-Ordovician heating event and were overprinted by stage IV andalusite in the Devonian. These overprinting reactions are preserved because reaction progress was apparently unable to keep pace with changing temperatures in this low pressure, dynamic setting.