Matachewan Porphyry, donated to the Peter Russell Rock Garden by the Central Canadian Federation of Mineralogical Societies in 2007

Monday, December 24, 2007

Bill Plavac and Peter Russell

Extraction and delivery of Matachewan Porphyry to University of Waterloo by the CCFMS project team

On November 2, 2007, four members of the CCFMS and two local Matachewan contractors set out to extract several large boulders of Matachewan Porphyry from a site in the Bannockburn Township near Matachewan in the Kirkland Lake area of Ontario. 
 
The Matachewan Porphyry is made of pink to cream rectangular feldspar phenocrysts from .6 cm across to  2.5 cm ( 1/4 to 1 inch) floating in  a dark grey to black country rock.  
Matachewan Porphyry
The Team arrived with backhoe, float, blasting equipment, diamond saw, Pionjar drill and small tools ready for any issue that might make extraction difficult. The backhoe operator quickly went to work and removed trees and rocks that were in the way of successful extraction of the specimens. 
 
After several hours of work to clear a way for removal of the specimens, the first large piece was successfully removed from the outcrop. The .90 m by .90 m by 1.2m (4 ft by 4 ft by 3 ft) piece weighing several tonnes was destined for the University of Waterloo. Several hours later four more large pieces of Matachewan Porphyry were successfully removed from the outcrop and loaded onto a flatbed truck with the backhoe. They were transported to the town of Matachewan ready for loading on November 5, 2007 on a transport truck headed for Waterloo and Peterborough. 
 
On November 9, 2007, the University of Waterloo specimen was unloaded and planted in the university’s Rock Garden.   
 

Members of the Team were:  

CCFMS: Bill Plavac, Doug Dingeldein, Bob Beckett, John Calder
 
ON SITE: JP Boucher, Gerard Bernatchez
 

Matachewan Porphyritic diabase

PlumeFormed during early Proterozoic volcanism 2.45-2.47 Ga, this rock was injected into the Superior Province crust as part of the Matachewan dyke swarm. This dyke swarm is one of the largest plume-generated radiating dyke swarms on Earth and covers over 300,000 km2. It consists of three smaller swarms, which radiate from the Sudbury area.  
 
To begin the process of dyke formation the Canadian Shield were subjected to stresses as a mantle plume of molten magma 1000 km in diameter pushed up against the brittle crust causing radiating cracks to appear. Mantle plumes are formed as hot rock from the mantle moved upwards, partly melting when they reach shallow depths in the Earth’s crust.  The plume pushed up against the crust centered north of Barrie, Ontario, forming cracks trending northwest. These radiating Matachewan Dykescracks filled with diabase and other volcanic rocks from the reservoirs below.  

Fractional crystallization 

The magma in the reservoirs interacted with rocks forming the magma chamber to produce rocks of different chemistry. As the magma cooled some minerals started to crystallize. In this rock large feldspars grew. This magma then erupted and formed the porphyritic rocks such as our diabase. As the crystals formed and were removed from the magma, the chemistry of the liquid changed and smaller crystals grew. This process is called fractional crystallization.  
 
The dykes on the Matachewan area are approximately 20m wide and are about 700 to 800 kilometres long. Dykes forming today on Iceland with a width of one metre move along the fracture at about one kilometre and hour. The Matachewan dykes may have taken a month to flow along the entire length of the swarm. 
 

As the lava flows into the fractures forming the dykes the lava cools against the surrounding rocks forming chilled margins.

As the lava flows into the fractures forming the dykes the lava cools against the surrounding rocks forming chilled margins. This example is from the Isle of Skye, Scotland.  

References 

Fractional Crystallization 
http://www.earth.northwestern.edu/pe ople/seth/demos/FRAC/frac.html 
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