BLUE CALCITE IN THE GRENVILLE PROVINCE: EVIDENCE OF MELTING

There are indications that marble can melt in a post-collision tectonic environment like that in the Grenville province. Regionally developed temperatures and pressures are estimated to have been at least 750̊C and 7–8 kilobars in the Gatineau Park area, north of Ottawa (Canada). Our attention was focused on occurrences of blue marble along Highway 5, close to Old Chelsea and Wakefield, Quebec. The samples are generally devoid of macroscopic inclusions, but one does contain yellow and dark green inclusions. They were imaged with a Zeiss Sigma HDVP SEM using the novel large-area imaging module Atlas 5. Overview image mosaics with the BSE and CL signals were acquired at a resolution of 150 nm/pixel and 90 nm/pixel. In addition, regions of interest were imaged at 15 nm/pixel. Samples devoid of visible inclusions contain numerous sets of micrometric to nanometric inclusions, locally preferentially aligned. Euhedral barite and subhedral anhydrite crystallites ~200 nm to 540 µm across occur as single inclusions throughout. Polymineralic inclusions range from 50 µm to 700 µm in size and contain enstatite, diopside, orthoclase, albite, titanite, and phlogopite, whereas others, contain pyrrhotite + chalcopyrite + magnetite + galena. The blue calcite sample with visible inclusions contains yellow enstatite decorated with a cathodoluminescent outer boundary that is As-rich, and green apatite that is sulfatic. The enstatite and apatite inclusions are roundish and range from 234 µm to 3.6 mm in size. Some enstatite crystals contain globules of calcite that are accompanied by a small (5 to 25 µm) crystal of barite. We contend that the enstatite and apatite crystallized from a silicocarbonatitic melt of crustal origin, probably rather quickly. In this context, these minerals were constrained to accept As and sulfate in their structure. We consider the particles of silicates and sulfates to have been constituents of the fluxed melt. The recurrent globular clusters of sulfides + magnetite suggest that an oxygenated sulfide melt also nucleated in the carbonate stew. We still have not pinpointed the specific cause of the blue color, but our investigation is ongoing.