Granites of the St. Cloud Area

Geological Society of Minnesota  

GSM LECTURE, MONDAY, FEBRUARY 23, 2009

At The University of Minnesota

Granites of the St. Cloud Area

Terry Boerboom, M.Sc,    Senior Scientist, Minnesota Geological Survey 

Date:  Monday, February 23, 2009      Lecture Begins:  7:30 PM

Location:  The University of Minnesota, East Bank, Electrical Engineering/Computer Science Bldg., Room 3-210

ABSTRACT: Granites of the St. Cloud area and east-central Minnesota

The granitic rocks of the St. Cloud area and east-central Minnesota have long been a source of both building materials and geologic wonder.  Quarrying for dimension stone began in the St. Cloud district in the late 1800’s and peaked in the 1930’s, when there were at least 18 different quarry companies operating in the area.  Currently, there are 5 dimension stone quarries and at least 4 crushed-rock aggregate quarries active in east-central Minnesota, all in granitic rocks.

In an area that extends at least 100 miles northeast-southwest from the near Mille Lacs Lake to beyond southern Stearns County, there are at least 15 different granitoid intrusions that form a semi-continuous mass known as the East-Central Minnesota batholith.  These granitic rocks were classically considered to be older than 1,800 Ma, and to have been emplaced during the Penokean orogeny.  However recent exhaustive geochronologic studies have shown that nearly all of the intrusions are between 1,772 and 1,799 Ma in age.  This age coincides with deformation and metamorphism associated with the Yavapai Orogeny.  The Yavapai orogenic belt crosses southern Minnesota into Wisconsin, but also affected crustal rocks to the north, in east-central Minnesota, at the same time the ECMB was emplaced.

This lecture will summarize both the cultural/quarry history of the granites in the St. Cloud area, and look at geologic reinterpretations based on the results of recent geochronological studies.

RESEARCH INTERESTS: 

Precambrian geology and mineral deposits: 

Geologic mapping and related studies of igneous and 

metamorphic rocks in northern, central and southwestern Minnesota,

Igneous petrology, mineral deposits, metallic minerals 

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