Browsing by Subject "gabbro"

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    Bulletin No. 2. Preliminary Description of the Peridotytes, Gabbros, Diabases and Andesytes of Minnesota
    (Minnesota Geological Survey, 1887) Wadsworth, M.E.
    It is proposed in the following pages to give a preliminary description of certain types of Minnesota rocks, to serve as a framework for a more complete discussion of the same in the final report. The field relations and to some extent the microscopic characters of the rocks described in the following pages have been given by the state geologist in the annual reports for 1879, 1880, and 1881, and in volume I of the final report. In some cases it will be found that the field relations have led the state geologist to different conclusions concerning the nature of the rocks in question from those adopted by the writer, who has based his views upon the correlation of the microscopic characters of rocks and their field relations, as studied by him elsewhere.
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    Bulletin No. 44. Geology of the Duluth Gabbro Complex near Duluth, Minnesota
    (Minnesota Geological Survey, 1964) Taylor, Richard B.
    Multiple intrusions at Duluth, Minnesota, form a rock series that is here called the Duluth Gabbro Complex. Each of the principal rock types transgresses one or more of the older units. The oldest rock, a coarsegrained anorthositic gabbro that was intruded into the Keweenawan flows, makes up the upper part of the complex. It was intruded by basaltic magma of a second period of magmatic activity which formed rocks that commonly are banded, and hence called the layered series. The gabbroic rocks of the layered series as well as the older anorthositic gabbro are cut by intrusive bodies of ferrogranodiorite and granophyre and by late dikes of basalt and aplite. The lower two-thirds of the complex, the layered series. is composed chiefly of troctolite, olivine gabbro, feldspathic gabbro, and syenogabbro. About 15,000 feet of layered rocks is exposed, locally with rhythmic banding, fluxion structure, and gravity stratification demonstrating bottom accumulation by crystallization under conditions of active magma circulation. A series of samples collected from bottom to top of the layered series shows only limited development of cryptic layering. This lack of cryptic layering may be explained as the result of periodic renewal of magma in the crystallizing chamber or by multiple small intrusions. Cross-cutting relations between the different types of gabbro in the layered series show that the mass originated by multiple intrusion. The lack of chilling effects indicates that the successive intrusions were not greatly separated in time. Near the top of the series a transition exists from gabbro to syenogabbro, and a similar transition may exist from syenogabbro to ferrogranodiorite. The rock series is similar in many respects to that of the Skaergaard Intrusion, but there are important differences that can be explained by dissimilar tectonic history. The poor development of cryptic layering and the absence of ferro gabbro at Duluth, in contrast to their remarkable development in the Skaergaard Intrusion, can be explained by differing tectonic stability. The Skaergaard magma apparently crystallized in a chamber under stable conditions, whereas the Duluth magma seems to have crystallized in an environment of tectonic instability manifested by multiple intrusions of magma. The various rocks of the Duluth Gabbro Complex can be explained by crystallization-differentiation of basaltic magma, although the origin of some, such as the anorthositic gabbro and intrusive peridotite, is puzzling. The rocks of the layered series probably were derived from a basaltic magma approaching the composition of analyzed late basalt dikes. Compared with the analyses of "marginal olivine gabbro" from the Skaergaard Intrusion, the Duluth parent magma seems to have been notably richer in K2O, TiO2, MnO, and P2O5. The Duluth Gabbro Complex is an immense sill-like mass that extends for 150 miles northeast from Duluth, and relations in other parts may differ from those at Duluth.
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    Bulletin No. 8. I. The Anorthosytes of the Minnesota Coast of Lake Superior; II. The Laccolitic Sills of the North-west coast of Lake Superior
    (Minnesota Geological Survey, 1893) Lawson, Andrew
    Among the problems which were named in instructions given to Dr. Lawson when he entered upon the season's work for the Minnesota. Geological and Natural History Survey along the northern coast of lake Superior, one was the following. To determine if possible the date and stratigraphic relations of the gabbro invasion. How well he has answered that question his report on the "Anorthosytes of the Minnesota coast of lake Superior" bears ample testimony. In order, however, that the important bearing which his results have on the geology of the northeastern part of the state, may be made more apparent, it is deemed best to preface this publication with a short exposition of the history of the investigation, and a few paragraphs on the extent of the gabbro rocks and on the significance of the term gabbro.
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    Petrogenesis of the Niğde Mafic Complex, Turkey: implications for the tectonic and geomorphic evolution of Central Anatolia
    (2016-06) Ray, Molly
    The Central Anatolian Crystalline Complex (CACC) is a high-grade metamorphic terrain that developed through a protracted history of collision, ophiolite obduction and arc magmatism during the closure of the Neotethys Ocean. The Central Anatolian Ophiolite (CAO) was emplaced onto the CACC during Cretaceous Neotethyan closure but is now highly dismembered. The Niğde Mafic Complex (NMC) is an association of metamorphosed and variably deformed gabbro, diabase, plagiogranite and ultramafic rocks in the southernmost metasedimentary massif of the CACC - the Niğde Massif. The Niğde Mafic Complex is described in detail for the first time, including its structural, petrologic and geochemical features. Mafic, ultramafic and felsic rocks from two adjacent regions, the Ecemiş Mafic Complex (EMC - east of the Niğde Massif - in the Ecemiş Fault corridor) and the Northern Region (isolated mafic-ultramafic outcrops north of the Niğde Massif) are also examined and compared. In addition, gabbro cobbles from basins adjacent to the Niğde Massif are investigated and their likely provenance is suggested. Whole-rock XRF and ICP-MS data indicate a common, suprasubduction zone, ophiolite origin for rocks of the NMC, EMC and Northern Region, and confirm their correlation to the Central Anatolian Ophiolite. Despite variable mineral assemblages, isotropic gabbro from all three regions has similar geochemical features, including depleted LREE (LaN/YbN=0.40-0.89), high Mg numbers (61-87) and low TiO2 (0.13-0.45 wt. %) that indicate a highly depleted mantle source, consistent with a forearc (or pre-arc) origin. Mineral compositions were determined through microprobe analysis. Gabbro mineral assemblages include magnesian orthopyroxene (En64-79) and calcic plagioclase (An74-95), which are typical of suprasubduction zone, water-saturated conditions for magma genesis. Post-kinematic muscovite, garnet and tourmaline-bearing granitoids intruded the NMC and are likely related to the crustally derived Üçkapılı Granite, limiting the age of ophiolite obduction and deformation to before ~85 Ma. Results of hornblende-plagioclase geothermometry for the NMC indicate mid-upper amphibolite facies deformation and metamorphism at temperatures ranging from ~560 to 785°C, similar to conditions reported for the adjacent metasedimentary rocks of the Niğde Massif. Variations in mineral compositions and assemblages among the NMC, EMC and Northern Region reveal a metamorphic gradient whereby the NMC experienced mid-upper amphibolite-facies conditions associated with ductile deformation, and the EMC and Northern Region display evidence for low-grade metamorphism without significant ductile deformation. Overall these findings reveal that the Central Anatolian Ophiolite was locally deformed and metamorphosed in the mid-crust along with tectonically underlying metasedimentary units. The Ulukıșla and Ecemiş Basins, bordering the Niğde Massif locally contain abundant gabbro cobbles in Tertiary conglomerates. Gabbro cobbles from the Oligocene Çukurbağ Formation in the Ecemiş Basin have a mixture of island-arc tholeiite and boninitic geochemical affinities, whereas all cobbles from Messinian conglomerates of the m3pl unit have boninitic geochemistry, exemplified by Ti/V ratios of less than 10. Based on the presence of metamorphic hornblende and the similarity of geochemical features, gabbro cobbles of the m3pl unit may have been sourced in the Niğde Mafic Complex or in other parts of the CAO. This has implications for our understanding of the landscape evolution of the Niğde Massif region including the former extent of the Niğde Mafic Complex.