Some Pegmatites Near Gwinn, Michigan

Western Michigan University ScholarWorks at WMU Master's Theses Graduate College 4-1975 Some Pegmatites Near Gwinn, Michigan Michael J. Moss Follow this and additional works at: https://scholarworks.wmich.edu/masters_theses Part of the Geology Commons Recommended Citation Moss, Michael J., "Some Pegmatites Near Gwinn, Michigan" (1975). Master's Theses. 2451. https://scholarworks.wmich.edu/masters_theses/2451 This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. SOME PEGMATITES NEAR GWINN, MICHIGAN by Michael J. Moss A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the Degree of Master of Science Western Michigan University Kalamazoo, Michigan April 1975 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS I wish to thank the members of my thesis committee, Frofessors John D. Grace, W. David Kuenzi, and Phillip Mariotti for their assistance and helpful criticism of this thesis. I would also like to thank Professor George H. Springer of the University of Dayton, who first intro duced me to the geology of this region in upper Michigan. Michael J. Moss ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INFORMATION TO USERS This material was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted. The following explanation of techniques providedis to help you understand markings or patterns which may appear on this reproduction. 1. 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TABLE OF CONTENTS CHAPTER PAGE I INTRODUCTION ........................... 1 Purpose ............................. 1 Location and Regional Geology . 2 Previous W o r k .........................4 II DESCRIPTION OF THE STUDY AREA .... 8 III PE T R O G R A P H Y .............................. 13 Petrographic Descriptions ......... 13 Country R o c k .........................21 P e g m a t i t e s ........................... 23 Contact Zones ...................... 25 IV GEOCHEMISTRY......................... 30 M e t h o d ............................... 30 R e s u l t s ............................... 33 K/Rb Ratios ........................... 40 V ORIGIN OF THE PEGMATITES ...............46 The Problem .........................46 Evidence for a Metasomatic Origin . 46 Evidence for an Igneous Origin . 50 A g e ....................................54 VI CONCLUSIONS .............................. 57 REFERENCES ............. 58 APPENDIX ............................... 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I. INTRODUCTION Purpose The purpose of this study was to determine the origin of a group of pegmatites in the vicinity of Gwinn, Michi gan. The pegmatite bodies occur in many diverse forms including veins, isolated pods, migmatites, and irregular dikes. Thus, there is the question of whether these bodies represent one or more than one, geological event. Compositionally, they are simple pegmatites, containing feldspar, quartz, and very little mica which makes this area suited for this study. This simplicity lends itself to geochemical interpretation essentially free of the effects of varying mineralogy within the pegmatites. The pegmatites are contained in meta-igneous precambrian shield rocks. As a side issue to the "granite" problem, there has long been a controversy as to the origin of pegmatites in metamorphic terrains. Some authors (Tuttle and Bowen, 1958; Jahns and Burnham, 1969) propose that these rocks are formed by crystallization from a water saturated magma, while others (Barth, 1962; Mehnert, 1968) stress the importance of metasomatism in their formation. Sometimes different authors use the same piece of field evidence to support different petro- genetic theories, for example, do migmatites show that 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. magma has invaded the country rock, or do they show that the quartzo-feldspathic material has differentiated out of the rock? This study represents an attempt to answer these and other questions, with regards to a specific group of pegmatites in northern Michigan, using standard petrographic and geochemical means in conjunction with K/Rb trace element data. Location and Regional Geology The study area is located in the vicinity of Gwinn, in Marquette County, in the upper peninsula of Michigan. It is about 20 miles south of the city of Marquette (Fig. 1). All the outcrops studied are in sections 21 and 22, of T45N, R25W. The regional geology is shown in Figure 1. The pegmatites under investigation are contained in lower Precambrian, meta-igneous "granitic" rocks of batholi- thic proportions (Gair and Thaden, 1968). They comprise part of a geological region of igneous and high-grade metamorphic rocks known as the southern complex. The southern complex is separated from the northern complex by the Marquette synclinorium, west of the city of Marquette. This synclinorium, as well as other troughs of the region, is made up of metasedimentary rocks of middle Precambrian, Animikian age. It is in these rocks that the economic mainstay of the region is found, namely Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PALEOZOIC UPPER PRECAMBRIAN QUETTE p€m MIDDLE PRECAMBRIAN LOWER PRECAMBRIAN % 0 10 20 MILES _______________1 it .. .1 Fig. 1. Geologic scetch map of central upper Michigan (after Gair and Thadden, 1968). ui the iron formations. One of the main structural features of the Gwinn area is the Gwinn synclinorium. These rocks are correlated with the middle and upper Animikian of the Marquette area (Allen, 1914). The Gwinn district was also once an iron ore producing area. The first ore was shipped in 1872, and the last ore went out in 1948 (Reed, 1957). The town of Gwinn itself was built by the Cleveland-Cliffs Iron Co. as a model mining town with substantial buildings. The upper Precambrian in the region is represented by basic dikes of the Keweenawan age which cut all earlier formations. Patches of Paleozic limestones are also found in this vicinity. Pleistocene glacial outwash covers most of the bedrock. Previous Work Very little literature has been published about the Gwinn district. Early reports were only concerned with the iron ore deposits. These were summarized by Van Hise and Leith (1911) in their monograph on the geology of the Lake Superior region. Again they were concerned mainly with the Animikian (formerly "Huronian") metasedimentary rocks. They mention the metaigneous rocks as Archean in age (lower Precambrian). These rocks were noted as being "...similiar to the basal granites of the neighboring iron ranges." They also published an outcrop map. The most recently published report on the Gwinn Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. area was by R.C. Allen (1914, 1915) of the Michigan State Geological Survey. He correlated the metasedimentary rocks as middle and late Animikian. He also briefly

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