Paper No. 57-8
Presentation Time: 10:45 AM
AN UNUSUAL DISJUNCTIVE CLEAVAGE IN GRENVILLIAN CARBONATES FOLDED AT THE UPPER GREENSCHIST FACIES (SOUTHERN ONTARIO, CANADA)
We present an unusual occurrence of axial planar cleavage in metamorphosed carbonates. At the outcrop- and hand-sample scale, the axial planar cleavage appears to be a spaced disjunctive cleavage, of the sort commonly associated with pressure solution during sub-greenschist facies deformation. Yet at the thin-section scale, axial planar cleavage is defined by oriented mica and elongate calcite grains. These rocks are in the Elzevir Terrane of the Composite Arc Belt, also known as the Central Metasedimentary Belt, and they are likely deformed during peak regional metamorphism of the Ottawan phase of the Grenville Orogeny. The mineral assemblage is calcite and mica (phlogopite), with lesser dolomite, quartz, muscovite (possibly detrital), chlorite, and plagioclase. Accessory minerals include graphite, apatite, rutile, hematite, and sulfides. The mineralogy of the rock requires that it is on the high-grade side of dolomite + Kspar -> phlogopite + calcite reaction. In addition to this assemblage, two other lines of evidence constrain peak conditions of deformation and metamorphism to the upper greenschist facies (about 450-500 degrees Celsius): 1) two reliable thermometers (the calcite-graphite isotopic thermometer and calcite-dolomite solvus thermometer), and 2) mineral assemblages, including the co-existence of albite and oligoclase in these metamorphosed carbonates and associated mafic rocks. Variations in grain size and mode define layering, which is likely transposed bedding (the protolith is limestone/dolomite with varying proportions of pelitic material). Folds are open to tight, sub-similar to sub-parallel, with cm- to dm-scale wavelengths. Cleavage is strictly axial planar; it rarely fans about fold axial planes. The uniform compositions of calcite, dolomite, and phlogopite, in addition to relatively straight grain boundaries, are evidence for neo-crystallization of mica and recrystallization of calcite (rather than pressure solution) as the mechanism for axial planar cleavage formation. This apparent contradiction between a low-grade fabric and higher-grade textures and metamorphic processes is the focus of ongoing research. No current ideas about spaced cleavage formation appear to explain the spaced cleavage common in this belt of marble; if it is not pressure solution, what is it?