Stratification within the granophyre of the South Range of the Sudbury Igneous Complex

English Abstract:

The Sudbury Igneous Complex (SIC) was formed by a meteorite impact, which generated a large quantity of impact melt in a single pulse. Fractionation and crystallization of the impact melt sheet led to formation of three units of the Main Mass of the SIC. On the South Range of the SIC the Main Mass comprises norite cumulates capped by granophyric residues. These two units are separated by a zone called the Transition Zone quartz gabbro which represents a change from plagioclase+orthopyroxene to alkali feldspar+quartz+clinopyroxene+biotite+Fe oxide+apatite crystallization followed by a rapid increase in the amount of trapped liquid.

Mapping and sample collection during the summer of 2013 led to the identification of a lithological reversal expressed as a layer of gabbro exposed near the middle of the granophyre unit. Its mineralogical make up corresponds to that of transition quartz gabbro zone but it occurs more than 1 km above the top of the TZ quartz gabbro. We have mapped this layer over a continuous strike length greater than 5 km between Whitewater Lake and Moore Lake. The contact is a sharp transition from pink granophyre to coarse-grained quartz gabbro with stellate textured clinopyroxene and a sharp increase in the mafic mineral abundance. Geochemically it is characterised by a drop in silica content to 51.72% and an increase in MgO to 2.56% in comparison to the 72.28% silica and 0.55% MgO content of Lower granophyre layer. Subsequent samples were collected (XRF results pending) in order to substantiate field observations. Modeling using MELTS software will be performed for the purpose of evaluation of possible sources of the upper gabbro layer. Existence of the reversal in the lithology of the Main Mass implies a greater complexity in process of evolution of the SIC than has been previously assumed, possibly indicating that the magmatic system evolved as a stably stratified body throughout its existence.