DECIPHERING MESOPROTEROZOIC AND PALEOZOIC EVENTS PRESERVED IN THE ISOTOPIC COMPOSITIONS OF ZIRCON AND TITANITE: SEM IMAGING, SHRIMP U-PB GEOCHRONOLOGY, AND EMP ANALYSIS

Complex age zoning in zircon and titanite reveals multiple independent Proterozoic and Paleozoic growth events in the Baltimore Gneiss, eastern Piedmont, MD. Zircons from 3 felsic gneisses contain igneous (volcanic?) cores, dated by SHRIMP microanalysis, at ~1.25 Ga. Zircon overgrowths, identified by cathodoluminescence imaging, formed at about 1.22, 1.16, and 1.02 Ga, presumably in response to Grenvillian events. A foliated biotite granite, intrusive into felsic gneiss, contains inherited zircon cores (1.18-1.31 Ga) and 1075 ± 15 Ma oscillatory-zoned (i.e. magmatic) overgrowths interpreted to date the time of crystallization of the granite.

The foliated biotite granite contains complexly zoned titanite. Backscattered-electron (BSE) imaging reveals that many of the titanite grains contain three growth stages: cores, intermediate mantles, and rims. Electron microprobe traverses across several zoned titanite grains show regular variations in the concentrations of several minor elements including at least 2.5 wt % Al2O3 and smaller amounts of Fe2O3, Y2O3, and Nb2O5. Abrupt changes in concentrations of all four elements confirm that the boundaries between core, mantle and rim, visible in BSE, reflect discontinuities in the chemical environment in which titanite crystallized. SHRIMP ages from these three zones are 374 ± 8, 336 ± 8, and 301 ± 12 Ma, respectively. Extrapolating and interpolating from limited existing thermochronologic data on cooling ages, these titanites formed at temperatures of ~ 640-680º C, ~500º C, and ~325º C. We surmise that the chemically homogeneous titanite cores completely recrystallized during late Devonian metamorphism; they may have been part of an equilibrium assemblage that developed at upper amphibolite facies conditions. Intergrowths of these titanites with fabric-forming biotite suggest that the Carboniferous events causing the crystallization of mantles and rims also led to the partial replacement of biotite. This replacement was probably associated with the latest development or strengthening of associated foliation, now dated here to be Carboniferous. In contrast, Proterozoic zircon was inert during Devonian metamorphism and Carboniferous deformation. Thus, titanite that can be related to fabric-forming events may be useful as a structural geochronometer.