Kinematic and Textural Analysis of Titanite: Establishing a Microstructural Framework for a High-Temperature Deformation Geochronometer

We present new structural data from an integrated petrochronological and microstructural study of the Rosy Finch shear zone (RFSZ), Cretaceous Sierra Nevada batholith, USA. The dextral RFSZ is interpreted to have occurred during emplacement of the Mono Creek pluton. Our study established a high spatial resolution deformation geometry and microkinematic framework of the shear zone for future titanite U-Pb geochronology and established how titanite and quartz manifest as fabric- forming minerals in the shear zone.

We collected sixteen samples along a transect orthogonal to the RFSZ to capture the entire strain gradient within the Mono Creek pluton. Microstructural assessments of quartz and titanite were conducted using optical microscopy, scanning electron microscopy , and electron backscatter diffraction (EBSD) data on polished XZ thin sections. Our observations of the active recrystallization mechanisms, crystallographic preferred orientation (CPO) patterns, and crystallograph i c vorticity axis (CVA) of titanite and quartz during fabric development were used to interpret shear zone kinematics and deformation geometry. High resolution EBSD data was then utilized to compare quartz and titanite grain patterns to test the fidelity of titanite as a recorder of fabric development during syn- emplacement deformation (from magmatic to solid-state deformation).

Misorientation maps of titanite reveal a range of misorientation angle magnitudes that vary within the fabric and intragrain scale. Despite grain scale variations in the degree of misorientation, all titanite subgrains yield a strong misorientation direction that matches the CVA pattern for both quartz and titanite. Vorticity axes, determined using CVA analysis, are consistently vertical across the RFSZ for both quartz and titanite, indicative of wrench deformation. The matching quartz and titanite misorientation directions and CVA distributions indicate that titanite was deforming and crystallizing during progressive fabric development across the shear zone and supports the use of titanite as a high-temperature deformation geochronometer .