TEXTURAL AND PETROCHEMICAL VARIABILITY ACROSS A SHEARED PLUTON MARGIN, NORTHEASTERN APPALACHIANS, USA

Tectonic strain within the northeastern Appalachians Mountains, USA is heterogeneously distributed across and along strike during Paleozoic orogenesis. Much of the strain is localized along orogen-parallel shear zones, which commonly display strong penetrative fabrics that include textural and mineralogical transformations. A distinct textural, mineralogical, and petrochemical variation has been identified across a strain gradient within the sheared margin of the Lincoln Syenite in south-central Maine. The igneous assemblage of megacrystic alkali feldspar + clinopyroxene + orthopyroxene + biotite is transformed to a strongly foliated, amphibole + biotite + alkali feldspar + quartz + sphene assemblage near the contact with the Washington Shear Zone. A transitional, non-foliated zone has been identified that displays various dis-equilibrium microstructures and mineral associations. Microstructural evidence for plastic strain and early-stage mineralogical instability is observed in the igneous assemblage, whereas foliated samples exhibit a texturally stable mineral assemblage.

Electron microprobe data show that distinct petrochemical variations in biotite (and to a lesser degree amphibole) are in-step with the degree of foliation development (strain) in sheared marginal rocks. Data collected from amphibole and biotite within the transitional zone have complex petrochemical variations. Thus, chemical data and microstructural interpretations suggest that coupling of strain and reactions led to the observed mineralogical changes and resultant weakening of the bulk rock.

Numerical solutions have been obtained regarding the partitioning of strain across an active orogen as a function of changes in crustal strength. The 3-D solutions demonstrate that strain related weakening, which stabilizes two structures and strain partitioning, competes with thermal advection driven weakening, which stabilizes a single oblique structure. The transition from partitioning of strain to a single mode of strain accommodation is a partial function of recrystallization and asymmetry of surfacial processes. The results of these models provides a platform upon which the effects of mineralogical and textural changes on crustal strength can be addressed.