Mesoproterozoic Crystal Plastic Deformation of the Music Pass Pluton, Sangre de Cristo Range, Colorado
Category: Research Poster
Author(s): Hari Brogan, John Singleton, Dylan Frawley
Presenter(s): Hari Brogan
Mentors(s): John Singleton, Dylan Frawley
In southern Colorado, the Sangre de Cristo Range contains the Mesoproterozoic Music Pass pluton (quartz monzonite ca. 1.43 Ga). A pervasive subvertical ENE-WSW-striking foliation averages 116/81 SSW and suggests folding about a steeply-plunging axis, with an average mineral lineation of 258/64. Microstructural and electron backscatter diffraction analyses of quartz and hornblende indicate grain boundary migration dynamic recrystallization and dislocation creep at high temperature (>500°C) matching amphibolite grade deformation. Minimal rotation of macroscopic foliation across localized shear zones suggests coaxial deformation, but weakly asymmetric fabrics may indicate north/northeast-side-up shear. To evaluate 3D strain geometry, cut and scanned mutually perpendicular faces (XZ, YZ and XY planes) of orientated samples were used for both edge fabric ellipse and shape matrix eigenvector analyses. Lode’s strain symmetry values (v) range from 0.04–0.35, indicating plane strain to slightly oblate strain. Natural strain (ε) values range from 0.38–0.71. U-Pb geochronology of an apatite separate yielded a date of 1338±15 Ma, which records the timing of cooling below ~500–450°C. Deformation in the Music Pass pluton likely involved a period of high-temperature (>500°C), coaxial dominated strain under ~NE-SW directed contraction shortly after pluton emplacement (between ~1.43-1.34 Ga), followed by a late-stage subtle folding of foliation about a steeply west-plunging axis.