Origin of the Pegmatites of Maine

Edson S. Bastin
1910 The Journal of geology  
During a portion of the summers of 1906 and 1907 I had the opportunity to study in some detail the pegmatite deposits of Maine, and at intervals during succeeding years have been able to pay brief visits to commercially important pegmatite deposits in Connecticut, New York, Pennsylvania, and Maryland. Without entering into the details of the field occurrences, which will be described in a forthcoming bulletin of the U.S. Geological Survey, I wish to summarize here the more important scientific
more » ... esults of these studies. Particular acknowledgment is due to Dr. Whitman Cross of the U.S. Geological Survey, for valuable criticism and advice in this work. The pegmatite deposits in the state of Maine all belong to the type commonly known as granite pegmatite, its dominant minerals being the same as those which are most abundant in ordinary granites. Pegmatites are most abundantly developed in the western and southwestern part of the state, and are invariably associated with granites. Excellent exposures in the feldspar and gem quarries, on glaciated ledges, and especially along the seashore in the Boothbay Harbor region, afford unusual opportunities for detailed field studies. General geologic relations.-The pegmatite masses vary in size from extremely small stringers intimately injected between the foliae of schists and thus forming injection gneisses, to batholitic masses ' Published with the permission of the Director of the U.S. Geological Survey. Vol. XVIII, No. 4 297 This content downloaded from on August 14, 2016 08:13:22 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). EDSON S. BASTIN a mile long and one-half mile wide, showing coarse pegmatitic textures throughout. Most commonly the pegmatite masses are roughly lens-shaped and lie parallel to the foliation of the inclosing schistose rocks, their attitude being dikelike or sill-like, according as the schists lie at steep or gentle inclinations. The rocks associated with the pegmatites are granite gneiss, granites of various textures, and schists of sedimentary origin. The field relations show that the pegmatites are invariably intrusive into the sedimentary schists, frequently cutting sharply across the schist foliae though usually intruded parallel to them. Characteristic contact metamorphic minerals are sometimes developed. Into the granite gneisses the pegmatites are also in some instances distinctly intrusive, but in other cases their relations indicate that the two rocks are nearly contemporaneous and probably related in origin. Fig. i . Microscopic examination of the fine-grained granite and the pegmatite in this occurrence shows that the mineral species in the two rocks are identical, the sole differences being in the texture and the proportions of the constituents. In other instances small irregular segregationlike masses with pegmatitic texture are wholly inclosed by normal granite. Although in certain instances distinct dikes of pegmatite cut the granites and in other instances dikes of granite cut the pegmatites, there is no evidence that the two rocks are of widely different ages or that there was more than one general period of granite and pegmatite intrusion. The granites are known to be of late Silurian or early Devonian age, and it is probable that the pegmatites are to be similarly correlated. With the exception of certain diabases, the granites and pegmatites are the youngest known igneous rocks of the state. The relations between the pegmatites and the granites indicate beyond reasonable question that the two rock types are genetically related. Evidence of this is found (i) in the fact that the predominant minerals in both rocks are the same; (2) in the occurrence of granite in all districts where pegmatites are found, and (3) in numerous observed instances of transition from granite to pegmatite. One of the most instructive instances illustrating such transition is exposed on the shore of Boothbay Harbor, and is illustrated in Mineral composition and texture.-As already stated, the dominant 298 This content downloaded from on August 14, 2016 08:13:22 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). ORIGIN OF THE PEGMATITES OF MAINE pegmatite minerals are those characteristic also of the normal granites; namely, quartz, orthoclase and microcline, albite or oligoclase, muscovite, biotite, and black tourmaline. Accessory constituents nearly always present are garnet, magnetite, and green opaque beryl. Accessory minerals which are present only in certain pegmatites number !o 5 feet FIG. I.-Granite grading into pegmatite and both intrusive in schists. Shore of Southport Island near south entrance of Townsend Gut. over fifty species; the most important are lepidolite, amblygonite, spodumene, blue, green, and pink tourmaline, transparent blue, green, or golden beryl, colorless to amber-colored topaz, and rose and amethystine quartz. A number of pegmatites have been successfully worked for certain of these gem minerals. In possibly ninety-nine one-hundredths of the pegmatite masses in the state no unusual minerals are present, the constituents being the same as in the normal granites and the proportions also somewhat 299
doi:10.1086/621741 fatcat:tshk43k6tvadvkn3nwzn4u7pcu