An Unusual Example of Coherent Kimberlite From the Muskox Kimberlite (Nunavut, Canada): a Re-evaluation of the Criteria for Recognising Coherent Kimberlite
Abstract
Although there is no published treatise on the criteria for recognising coherent kimberlite (CK), a review of the kimberlite literature will reveal that there is some consensus on its characteristics. For example, in most publications in the last kimberlite conference (8IKC volume 1) in which CK is discussed, the groundmass is described as 'uniform and well-crystallised' and spinel crystals are almost always present in abundance. Macroscopic features and contact relationships, however, are less commonly reported and do not seem to be important for the distinction of CK from fragmental rocks. Here we report contact relationships, facies distributions, textural features and mineralogy for one coherent and two fragmental facies from the Muskox kimberlite (Nunavut, Canada) to initiate debate on the criteria for distinguishing coherent from fragmental kimberlite, which we also refer to as volcaniclastic kimberlite (VK). Particular focus is placed on the coherent facies, which has unusual groundmass characteristics that have not been previously described in the kimberlite literature. Emplaced within granodiorite country rocks, the Muskox kimberlite is a steeply-tapering, single-vent, kimberlite body, measuring 200 x 220 m, that is infilled with VK and minor amounts of CK. VK is divided into two main facies, a light-coloured, country rock-rich facies (VK1) and a dark-coloured, olivine-rich facies (VK2). VK2 is mostly enclosed by VK1, forming an off-centre nested architecture. The contact between the facies is roughly vertical and gradational over distances up to 10 meters, however, in most drillholes that intersect the contact, the two VK facies alternate several times, thus forming an apparent inter-digitating relationship. Both facies are massive, poorly sorted and range from matrix- to clast-supported and contain olivine crystals, country rock lithic clasts of limestone and granodiorite and rare juvenile pellets. The matrix of VK1 consists mainly of serpentine, calcite, phlogopite, diopside and opaque oxides, whereas the matrix of VK2 consists mostly of serpentine, calcite, phlogopite, opaque oxides and monticellite. Although the matrix of VK2 is similar to the matrix of CK described in the literature (e.g., well crystallised with abundant spinels), the facies is considered fragmental because of the irregular distribution of framework components and because it is in gradational contact with VK1. CK occurs as a late-stage sill-like body found in five drillholes at ~200 m depth below the current surface. Sill thickness ranges from ~0.1 to >13 m thick and all contacts with VK infill are sharp. Most CK is crystal-rich (15-25 percent) and contains evenly distributed, commonly aligned, six-sided crystals (<2 mm) that have been completely replaced by serpentine and carbonate, and are interpreted to represent altered olivine phenocrysts. Groundmass minerals include serpentine, phlogopite, opaque minerals and a mineral (2- 3 microns) that appears to be similar in composition to monticellite. Unusual textures include radiating clusters of minerals that are reminiscent of spherulites or snowflake textures, and arcuate fractures, reminiscent of perlitic fractures. Muskox CK illustrates the importance of combining macroscopic and microscopic features for the recognition of CK and, if the textures are indeed products of devitrification, then this may be the first evidence that kimberlite magmas can quench to glass.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V31F..05H
- Keywords:
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- 8400 VOLCANOLOGY;
- 8414 Eruption mechanisms and flow emplacement;
- 8486 Field relationships (1090;
- 3690)