Lakehead University Geology - Journal Yearbook (Thunder Bay, Ontario Canada)

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Dr. Henri Loubat Geological Engineer, PhD., Geneva Assistant Professor, Lakehead University I became particularly attracted by petrology in 19ol when I found the opportunity of studying a set of metamorphic rocks from California. These rocks belonged to the glaucophane-schist and eclogite facies of metamorphism. At that time, I was fascinated by two problems exhibited by these classic Californian types: the process of diaphtoresis , and the phenomenon of converging facies. The glaucophane-schist may be due to a particular regional metamorphism acting on greywackes and ophiolites; but frequently it results from a retrograde process of modification from a high degree of metamorphism (amphibolite or eclogite) toward lower degrees of alteration. Both these glaucophane-bearing types are to be found, closely associated, in California, and they exemplify very well the retroinorphosis ( diaphthoresis) and facies convergence . It is easy to realize the interest of the study of those facies, if we know that eclogite themselves are not always considered as metamorphic rocks.... Soon after, receiving a grant from the University of Geneva I studied briefly greenstones, sediments and serpentines from Cuba. The degree of alteration of my samples was somewhat discouraging for a sucessful petrographical and geochemical investigation, but this was a first contact with igneous submarine rocks. During the next four years my main petrographic activity was devoted to surveying the Versoyen region - this area of the Alps is geologically very attractive. We should know that the Alps are subdivized longitudinally by a major tectonic thrust plane, the trace of which is called the Pennic front thrust . This line runs parallel to the chain, subdivizing it into two equal bands: the internal Alps (toward Italy) and the external Alps (toward France and Switzerland). The former is very rich in ophiolites, strongly metamorphic and intensely disturbed by the tectonic. The latter is not metamorphic, lacking ophiolites, and gently folded. The Versoyen, which is located exactly on the border between France and Italy, is the last ophiolitic area we could find when leaving the internal Alps going toward France, ’with this marginal location, this area offers the weakest degree of metamorphism we could have in the Alps for submarine volcanic rocks. A study based on field, microscopic and chemical investigations leads to the following conclusions: ' We have there a well-preserved submarine volcanic complex composed of the piling of various related units; at the bottom, along with slabs of aplitic granites and gneiss (fragments of the Alpine basement?), thin layers of serpentinites . Above, thick lenses of diff erenciated gabbros; still above, a set of sills, inter-bedded with black schists. At the top, an enormous amount of pillow-lavas. This rather logical sequence so well known by anglo-saxon geologists, was never recognized before anywhere in the Alps, although it is quite probable that it is actuall ' omnipresent, but was always disturbed by complex diastrophism. Petrology 17

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chlorite, illite and an interbedding of chlorite and illite. The difference in colour is probably- caused by the higher oxidation state of the dark yellowish brown silts. Varved sediments, probably of Pleistocene age, underlie the Recent sediments in the area of study. North of Coppermine Point the varved sediments are greenish gray in colour whereas to the south of Coppermine Point the varves have been oxidized to pale brown in colour. The mineralogical composition of the varved sediments consists of orthoclase, microcline, quartz, chlorite, illite, an interbedding of chlorite and illite, dolomite and calcite. It was noted that there is a decrease in the percentage of calcite in the varved sediments northward of St. Mary ' s River. JOHN S. MOTHERSILL LIMNOGEOLOGICAL STUDIES OF THE EASTERN PART OF THE LAKE SUPERIOR BASIN. 16

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Moreover there was an interesting point about the metamorphism of the volcanic system: the intensity of metamorphism was steadily decreasing from the bottom to the top (very weakly metamorphic pillows) . I suggested an eventual direct relationsnip between the depth of the crustal segment and a kind of late-magmatic auto alteration (deuteric alteration). These are the reasons why I remain personally interested in the results of dredging operations, close to any mid-oceanic ridge. If there were some metamorphic rocks already there, they may well be attributed to some process of alteration of that kind. As soon as you are occupied by such questions, generally many other interesting problems arise. Studying recently, in collaboration with the Bedford Canadian Oceanic Institute, a set of dredged specimens, I momentarily shift toward another topic of interest: the origin of the dredged serpentinized ultrabic fragments. Among many interesting and mysterious facts, we found evidence of an intense pre-serpentinization cataclasis, probably due to a creeping process at the base of the crust, and presumably related to the oceanic floor spreading. Many metamorphic specimens from N. lat. on the Mid Atlantic Ridge are still waiting for an investigation; we may say already that they uniformly belong to the green schist facies only. It is fascinating to meet, near Thunder Bay, in the heart of the Canadian Shield, greenstone belts showing perfectly preserved relics of submarine basic lavas, exactly similar to the one formed very recently. It is extremely promising to compare their detailed petrological properties, with those recent equivalent rocks, because we may suppose some kind of difference between now and 3 billion years ago in the superficial environment and in the relations between continents, oceanic crust and mantle. In this comparison, as a petrologist, I foresee the possibility of detecting an evolution of the earth ' s crust, evolution about which we do not even have a serious hypothetical model yet. The geology school at Lakehead has therefore, a wonderful field of investigations for a long and sucessful future o BONGARD LESLIE GO. LTD. 204 ARTHUR ST. THUNDER BAY (P), ONT. STOCKBROKERS BOND DEALERS MUTUAL FUNDS D. F. BARON MGR. W. N. AUBRY S. MALINOSKI J. M. SCOTT TELEPHONE 344-6618