Early Proterozoic diabases in the Republic Mine, Northern Michigan have undergone pre-metamorphic hydrothermal alterations, followed by regional and retrograde metamorphism. High calcic plagioclase, pyroxene and olivine were completely decomposed in all the diabases during pre-metamorphic hydrothermal alteration which is characterized by increasing acidity and oxygen fugacity, the distinct loss of silicon, calcium and sodium, and gain of water, other major elements and most trace elements. Chemical composition of the diabases near the contact between the metadiabases and banded iron formation in the keel of the syncline was then changed from mafic to peraluminous during later hydrothermal alteration which may be related to fluid-driven regional metamorphism. This later hydrothermal alteration is characterized by distinct loss of total rock mass, water and iron group elements and gain of most felsic chemical components. Regional metamorphism is characterized by low-pressure amphibolite facies with peak pressure ~ 2.25 kb and temperature ~ 609 °C and with constant low oxygen fugacity (logfo2 = -21.2 ~ -20.0). The mineral assemblage Am+Pl+Q±Bt±Spn±Carb of mafic metadiabases was formed near the culmination of the regional metamorphism. The mineral assemblage Grt+Ms+Bt±And±Kfs of peraluminous metadiabases, however, was formed during retrograde phase of regional metamorphism which is characterized by decreasing temperature recorded by zoned garnets and their coexisting micas. The layer faults along the contact between the metadiabases and banded iron formation and axial faults cutting the keel of the syncline were well developed during F2 deformation of the Penokean Orogeny. They served as channels for fluid flow and controlled the amount of hydrothermal fluids and heat carried by them so as to determine the intensity and the distribution of pre-metamorphic hydrothermal alteration and retrograde metamorphism.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Xinping Yin in partial fulfillment of the requirements for the degree of Master of Science, September 1994.
Metamorphism and Hydrothermal Alteration of Early Proterozoic Diabases in the Republic Mine, Northern Michigan.
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