THE NOT-SO-DRY OTTAWAN OROGENY: FLUID-DRIVEN REACTIONS DURING LATE-STAGE DEFORMATION ALONG THE CARTHAGE-COLTON SHEAR ZONE

The introduction of an aqueous-rich (XCO2=~5%) fluid into developing shear zones in an anorthositic to gabbroic body (Dana Hill Metagabbro (DHMG)) is marked by a progressive sequence of mineral reactions that result in a variety of assemblages in individual shear zones. Various reaction paths are controlled by the failure of one or more solid reactant phases during recrystallization but all involve the growth of hornblende + scapolite + titanite +/- quartz at the expense of ilmenite (ILM) clinopyroxene (CPX) and plagioclase (PLAG). Deformation in the DHMG took place during late stage extension (post 1020Ma) along the Carthage Colton Shear Zone at conditions 680-750°C (Hbl-Plag geothermometry) P=60-70 MPa . The DHMG locally preserves orthocumulate igneous textures that grade abruptly into mylonite to ultramylonite textures across a series of shear zones that cut the body. Knowledge of the original (igneous) mineral assemblage along with the variety of assemblages present in the shear zones provides the opportunity to model reactions attending deformation. Mass balance calculations clearly show that subtle variations in original mineralogy control the range of assemblages observed in the shear zones and that fluid infiltration/availability was not a limiting factor. Minor variations in modal abundances of clinopyroxene (CPX) – ilmenite (ILM) – plagioclase(PLAG) lead to very different points on a reaction path. During deformation this path initially drives the rock to the complete consumption of ilmenite (all cases) followed by partial to complete loss of CPX from the original assemblage and finally to the complete loss of CPX with re-growth of ILM (path A) or complete loss of CPX with the appearance of quartz (path B) in the final assemblage. Mass-balance calculations of the volume of fluid needed to drive these reactions to completion represents a significant addition requiring high time-integrated fluid rock ratios during deformation.