RESOLVING THE TIMING OF MINERAL GROWTH AND FABRIC DEVELOPMENT WITH TITANITE U-PB GEOCHRONOLOGY

In polymetamorphic terranes, it is problematic to link ages from geochronometers to metamorphic fabrics and, therefore, to a specific deformation event(s). It is necessary then to analyze a mineral which may preserve multiple age domains. Titanite has been shown to retain multiple age and elemental domains in single crystals through high-grade metamorphism. In this study, titanite U-Pb geochronology is used to examine whether ages are thermally reset along a sample transect towards a mylonitic shear zone in NW Argentina. A secondary goal of this work is to understand the conditions under which titanite resists resetting.

A combination of petrographic observations and electron microprobe analyses reveal the textures and compositional domains in titanite and other minerals, including garnet and hornblende. Titanite are elongate, wrapped by the mylonitic fabric, and have patchy elemental and BSE zoning. Garnet has distinct cores with prograde zoning and thin rims, which appear to be in equilibrium with the fabric defining minerals. Hornblende preserves two growth domains: inclusion rich cores and thin overgrowth rims in equilibrium with the fabric defining minerals. In-situ U-Pb and trace element data was collected in titanite from four samples using SHRIMP-RG and from one sample using LASS-ICPMS and SHRIMP-RG. All samples preserve lower-intercept ages between ~900Ma and 1.0Ga. We observed no correlation between age and BSE or elemental domains; these domains correlate with Al and Nb variations. Zr-in-titanite temperatures indicate upper amphibolite facies conditions, ~660ºC-710ºC.

Given these results, we conclude that titanite U-Pb ages and temperatures reflect original Grenville metamorphism. ⁴⁰Ar/³⁹Ar hornblende cooling ages of 515 Ma suggested titanite may be reset at the shear zone but overprinting P-T of 560ºC and 0.8 GPa, fluid infiltration, and high-strain did not cause significant lead loss. Overprinting conditions and cooling ages suggest that rims of garnet and hornblende likely correlate to Paleozoic metamorphism, while textural evidence and titanite ages suggest garnet and hornblende cores grew during the Proterozoic. Titanite records older metamorphic events within this polymetamorphic terrane, even within shear zones.