Search

This contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break-up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ! 50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1-4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvian- ite + garnet + diopside. The assemblages are successfully modelled with T-X(CO2) fluid pseudosections that suggest achievement of CO2-rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near-peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high-grade portions of aureoles is consistent with eventual levelling-off of temperatures between 350 and 400 ! C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion-wall rock contacts is demonstrated by narrow zones of anomalously low d 13 C and d 18 O values of carbonate minerals. Although the up to 5& decrease of both d 13 C and d 18 O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in d 13 C is attributed to fluid- mediated exchange with organic low- 13 C carbon that is dispersed through the unmetamorphosed rocks and the decrease in d 18 O is attributed to fluid-mediated isotopic exchange with the gabbroic intrusive sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high-temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T-X(CO2) fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation reactions to influx of aqueous fluids during cooling.

Objectives:Assess utility of combined electrophysiological and laser-Doppler Vibrometry (LDV) technique for intraoperative monitoring (IM) of hearing threshold (HT) improvement during ossiculoplasty.Methods:Study Design: A prospective, feasibility study. Setting: Academic, tertiary referral center. Ten patients underwent two-stage canal wall-up tympanoplasty due to chronic otitis media with cholestaetoma. During the second look surgery performed 6 to 9 months later, ossiculoplasty was monitored intraoperatively by LDV to assess prosthesis movability and by electrocochleography from the round window (RW-ECochG), and by auditory brainstem responses (ABR) to evaluate HT improvement. The ear was stimulated acoustically by tone-bursts: 0.5kHz, 1.0kHz, 2.0kHz, and 4.0kHz. Intraoperative HT, defined as the last intensity for which RW-ECochG and ABR wave V was present, were tested for various configurations of prosthesis placement. Simultaneously for the same stimulation, LDV measured prosthesis moveability. In a...