THE SHAGGY PEAK SHEAR ZONE, A CA. 1.43-1.35 GA DUCTILE SHEAR ZONE: PRELIMINARY RESULTS FROM IN-SITU Rb/Sr LA-QQQ-ICP-MS MUSCOVITE AND BIOTITE ANALYSES REVEAL CONNECTION TO PICURIS OROGENY

The Shaggy Peak shear zone is a Proterozoic, east to northeast striking, moderately north-dipping, top-to-south, reverse displacement ductile shear zone, located south of Glorieta Baldy peak, Santa Fe, New Mexico. The shear zone juxtaposes the Paleoproterozoic Shaggy Peak granite in the footwall with supracrustal amphibolites and felsic schists of the Thompson Peak metamorphic succession in the hanging wall. The Shaggy Peak granite yields a preliminary 207Pb/206Pb weighted mean crystallization age of 1687 ± 3 Ma (MSWD = 0.67).

Near the contact between the granite and amphibolite, sheared granite displays an intense gneissic foliation with grain size reduction relative to the undeformed granite. In lower strain domains, muscovite occurs as large 3-7 mm blocky grains with ragged edges and kink bands at high angles to the dominant foliation. In higher strain domains, muscovite and biotite occur as 1mm or smaller grains aligned with the dominant foliation. Thin section observations reveal C-S and C’ foliation with top-to-south, reverse shear sense. The S-plane is defined by aligned quartz grains and the C-plane is defined by aligned muscovite grains. The C’ foliation, characterized as shear bands, cut through the S and C planes. New, recrystallized quartz grains are smaller with sutured grain boundaries. Amphibolite near the contact preserves an intense foliation and finer grain size but hornblende and plagioclase are recrystallised and preserve no obvious shear sense. Felsic schist adjacent to amphibolite also exhibits intense foliation. Thin section observations reveal C-S and C’ foliation with top-to-south, reverse shear sense. The C-plane is defined by alignments of muscovite grains and the S-plane is defined by aligned quartz grains. C’ is defined by kinked and sheared muscovite.

Preliminary muscovite and biotite Rb/Sr analyses from in-situ LA-QQQ-ICP-MS yield isochron dates between about 1429 and 1361 Ma for muscovite and a maximum age of 1331 Ma for biotite. These dates are similar to U–Th-Pb monazite-xenotime chemical ages previously reported for the Thompson Peak area. We interpret N-S to NNW-SSE contractional deformation associated with the Shaggy Peak shear zone to be associated with the Mesoproterozoic ca. 1.43-1.35 Picuris Orogeny.