Description
The Scove Canyon segment, a ductile shear zone within the Peninsular Ranges, California, was produced during D2 deformation about 105-94 Ma. In order to understand the petrological, chemical, and structural development of the Scove Canyon segment, a small area of the shear zone underlain entirely by the -118 Ma Pine Valley granodiorite was mapped and sampled for thin section, chemical, and density analysis. Within the study area protomylonite, exhibiting a composite planar fabric, is mineralogically like Pine Valley granodiorite outside the Scove Canyon segment. When the composite planar fabric is viewed looking NW and perpendicular to XZ, S2 surfaces dip -700NE and are deflected in a dextral sense by -80-85° NE-dipping C2 surfaces. A well developed stretching lineation plunges steeply down the dip of S2. Though these relationships suggest an east-side-down normal-sense of movement, postkinematic, Late Cretaceous to Tertiary anticlockwise rotations greater than -5-1 oo have been documented for the Peninsular Ranges, and, as a result, the Scove Canyon segment during the Late Cretaceous may have formed as a result of west-over-east reverse-sense movements.Microscopically, S2 surfaces are defined by ribboned to lensoid domains of quartz, alternating with similar shaped domains composed of K-feldspar+plagioclase± quartz± biotite. Bulbous grain boundaries suggest that grain boundary migration recrystallization was an important process during development of the Scove Canyon segment.In addition, new grains of quartz, plagioclase, and potassium feldspar indicate that temperatures were sufficiently high that dislocation climb was an effective process. Thus, temperatures were probably in excess of 5QOOC, whereas, strain rates were probably relatively low.The inferred high temperatures and low strain rates under which protomylonite of the Scove Canyon segment developed are conditions that commonly lead to significant mass and volume changes during mylonitization. Thus, the question of whether or not mass and/or volume changes occurred during mylonitization within the Scove Canyon segment was addressed. Six samples of the Pine Valley granodiorite and seven samples of protomylonite within the Scove Canyon segment were analyzed for major, trace, and rare earth elements. Mass balance arguments indicate that if no change in mass of some element i occurs during mylonitization, then c = (M° / Mm)C:, where c is the concentration of element i in protomylonite of the Scove Canyon segment, M0 is the mass of the Pine Valley granodiorite prior to mylonitization, Mm is the mass of Pine Valley granodiorite converted to protomylonite, and C;° is the concentration of element i in the Pine Valley granodiorite outside the Scove Canyon segment. In a plot of ct versus ct the above equation defines a line emanating from the origin with a slope equal to M0 / Mm . L.P.Baumgartner and S. N. Olsen developed a computerized x-yweighted least squares analysis routine that solves for M0 I Mm , and provides an estimate of the la uncertainty in this value. In addition, their program includes a subroutine, termed the overlapping cone technique, for evaluating which elements were probably immobile during mylonitization. The overlapping cone technique indicated that 38 out of 43 elements analyzed during this study were probably immobile. These 38 elements follow the relationship c = 0.993(±0.0lS)c;'. This result indicates that 0 mM / is not statistically distinguishable from 1 at the lcr level, and therefore implies that the conversion of granodiorite to protomylonite was not accompanied by a mass change. However, differences in the elemental masses of Ba, Sr, Eu, Cu, and Ni were indicated by the least squares analysis, and, though not well understood, may be the result of the heterogeneous distribution of these elements prior to mylonitization. Finally, estimates of volume strain showed that mylonitization was not accompanied by a statistically significant volume change. Thus, the Scove Canyon segment is interpreted to be an example of an isovolume shear zone which probably evolved within a relatively dry system through only minor, if any, changes in elemental mass.
