Geology of volcanic rocks in the south half of the Ishpeming greenstone belt, Michigan
By: by Theodore J. Bornhorst and Rodney C. Johnson.
Contributors: Johnson, R. C.
Series: Contributions to Precambrian geology of Lake Superior region ;; ch. P; U.S. Geological Survey bulletin ;; 1904-P
Publisher: U.S. G.P.O.; For sale by USGS Map Distribution
Published: 1993
Place of publication: Washington, DC; Denver, CO
Language: eng
Number of pages: 13
Subjects: Volcanic ash, tuff, etc. -- Michigan.; Geology, Stratigraphic -- Archaean.; Geology -- Michigan.; Kitchi Formation.; Mona Formation.
Other titles: Ishpeming greenstone belt, Michigan.; Volcanic rocks, south half Ispheming greenstone belt, Michigan.
Description: The south half of the Ishpeming greenstone belt is domi- nated by a thick succession of subaqueously erupted volcanic rocks. A regional structural interpretation of the belt provides a framework for placing this succession in a stratigraphic order. The south half of the Ishpeming greenstone belt preserves the remnants of overlapping subaqueous volcanic complexes within an arc system. The Kitchi Formation, as redefined herein, comprises the lower half of the volcanic succession and is subdivided into three informal units. The basal basalt flow unit is composed of pillowed, tholeiitic, basalt flows and minor banded iron-formation and dacitic tuff. It is overlain by the second and third units, which are tuff and lahar (volcanic debris flow) units that inter- finger with one another. These units range in composition from calc-alkalic basalt to dacite. The Kitchi Formation represents an evolving tholeiitic, mafic shield to calc-alkalic, mafic to felsic subaqueous volcano. The Mona Formation, which is redefined herein, is in structural contact with the Kitchi Formation, but is likely younger. It is subdivided into three informal units. The base consists of a basalt flow unit, which is composed of pillowed, tholeiitic to magnesium-rich basalt flows and minor amounts of banded iron-formation and tuff. The basalt is overlain by a calc-alkalic dacite to rhyolite lapilli-tuff unit and a laterally continuous upper tuff unit of calc-alkalic basalt to andesite. The Mona Formation represents an evolving tholeiitic, mafic shield to mafic-felsic calc-alkalic, mafic to felsic subaqueous volcano. The Deer Lake Peridotite, an ultramafic sill complex that intrudes the Kitchi Formation, is speculatively interpreted as a subvolcanic ultramafic komatiite base of the Mona Formation.
Notes: Includes bibliographical references (p. 13).
Title on p. [4] of cover: Volcanic rocks, south half Ispeming greenstone belt, Michigan.