SA Microfabric Study of Calcite, Dolomite, and Quartz around Papoose Flat Pluton, California

Abstract

Post-crystalline strain features were studied in quartzite and dolomite-bearing marble around the Papoose Flat pluton to compare various hypotheses regarding stress orientations obtained from deformation lamellae in quartz, calcite, and dolomite. Analyses of twin lamellae in calcite and dolomite yield maxima of “compression” (C) and “tension” (T) axes that are not symmetrically related to the foliation, lineation, or patterns of preferred orientations of minerals. The C-T maxima are oriented consistently with respect to geographic coordinates and record a post-crystalline state of stress considerably different than the para-crystalline stress previously inferred for the same rocks. The quartz deformation lamellae are unusual in their crystallographic orientation: most of them are inclined between 20° and 80° to the {0001} planes; no near-basal and few prismatic lamellae are present. Statistically, the lamellae define two planes in most specimens, and are believed to have formed in planes of high shear stress, consistent with experimental evidence. The acute and obtuse bisectrices of the two planes of lamellae are parallel to the maxima of C- and T-axes, respectively, determined from twinning in calcite and dolomite. Thus, these axes derived from the orientation of nonbasal quartz lamellae appear to reflect the orientation of the principal stresses during their formation, as other investigators have demonstrated for the more common near-basal lamellae. The maximum principal (compressive) stress was horizontal and NNE, and the least principal stress was horizontal and WNW. These orientations are consistent with those determined from a conjugate system of faults which also formed soon after emplacement of the pluton.