TitaniQ Records of P-T-D Paths from Metapelites during Burial Metamorphism and Orogenesis: Evidence for the Role of Pressure Solution Creep

TitaniQ (Ti-in-quartz) thermobarometry of metapelites recording crenulation cleavage development during prograde metamorphism reveals that quartz grains in different fabric domains may record distinct stages of P-T-D paths and further testifies to the importance of pressure solution creep in the rheology of the upper and middle continental crust. This study integrates microstructural analyses, petrologic modeling, cathodoluminscence (CL) imaging, and secondary ion mass spectrometry (SIMS) of samples with known tectonic histories. Samples were collected along E-W transects across the Rowe-Hawley Belt (RHB) and the Connecticut Valley-Gaspe Trough (CVG) of central Vermont. Rocks of the RHB record both the Taconic and Acadian orogenies; CVG rocks record the Acadian orogeny only. CL and SIMS analyses were conducted on quartz in different microstructural contexts such as fold hinges, microlithons and cleavage domains, inclusion suites defining internal foliations in garnet, and pressure shadows. In each generation of foliation, metapelites of the RHB and CVG display pressure solution seams, flattened quartz grains neighboring micas in cleavage domains, and more equant quartz grains with foam textures in quartz-rich microlithons and hinges of microfolds. Quartz grains in prograde fabric domains lack lattice preferred orientations. CL (415 nm) zoning patterns of quartz are typically complex and associated with sharp boundaries. Patterns include: rounded-to-anhedral dark cores with bright rims (indicating an increase in [Ti] from core to rim); bright cores with dark rims; dark cores with bright mantles, and dark rims; patchy zoning; and striated grains. RHB chlorite-garnet grade metapelites yielded [Ti] from 0.4-157 ppm; a small fraction of analyses yielded [Ti] greater than that predicted for peak metamorphic conditions and are interpreted as detrital signatures. Garnet-staurolite grade CVG metapelites yielded [Ti] of 2.2-9.8 ppm. The degree of geochemical recycling of quartz inferred from the results of CL, SIMS, and microstructural analyses cannot be accounted for by dynamic recrystallization, volume diffusion, or changes in the volume of quartz predicted by petrologic models. In both the RHB and CVG, dark cores with low [Ti] are interpreted as quartz growth during burial metamorphism and cleavage development via pressure solution. Brighter mantles and rims with increasing [Ti] relative to cores can be linked to pressure solution or quartz-producing metamorphic reactions depending on microstructural context. Likewise, dark rims relative to cores or mantles record either rapid tectonic burial or fluid influx during exhumation. Our results demonstrate that TitaniQ analyses from complexly deformed metapelites yield important insight into P-T-D histories, but require an integrated approach to analyzing quartz in different fabric domains, and that pressure solution creep played a significant role throughout the P-T-D path.