Geology topics
Publications and source records attributed to Esper S. Larsen.
The San Juan Mountains, which are made up chiefly of volcanic rocks, are located in southwestern Colorado and occupy a rudely circular area of over 10,000 square miles, or larger than the state of Massachusetts. They have been studied and mapped geologically by the United States Geological Survey, chiefly by field parties under Whitman Cross and Esper S. Larsen.
The study of the Highwood Mountains was undertaken by a group of men from Harvard University under a grant from the Shaler Memorial Fund of the Department of Geology. The work was under the general direction of Larsen, who, with the assistance of Norman A. Haskell, mapped most of the volcanic rocks. Hurlbut and Griggs worked mostly on the laccoliths, Burgess on the stocks, and Buie on the dikes. The party spent two summers in the field. The laboratory work is far from complete. We cooperated closely in all the work.
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The isotopic compositions and concentrations of lead and uranium have been determined in some separated minerals and the composite of a granite from Monmouth township, Haliburton County, Ontario. The chemical and mass spectrometric methods that were used are described. The age of the zircon from the granite is 1050 million years. Much of the lead, uranium, and thorium exists in chemically unstable and presumably interstitial phases of the granite. A comparison of the observed amounts of uranium, thorium, and lead in the various minerals with those amounts that should have been present, had these three elements existed within the minerals as closed systems, shows a non-balance of these elements in every case. It appears that the granite as a whole has closely approximated a closed system since it was formed with respect to uranium and its decay products, but has been an open system with respect to thorium and its decay products. Interpretations concerning the relationship of these data to lead ores are discussed.
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The late Tertiary volcanism in the San Juan Mountains of southwestern Colorado followed a long interval during which the crust was stable. The volcanic rocks have been divided into four main groups, (1) those of pre‐Potosi (?) age in the eastern area, (2) the Lake Fork quartz latite, San Juan tuff, and Silverton volcanic series, (3) the Potosi volcanic series and the Fischer quartz latite, and (4) the Hinsdale formation separated by long intervals without eruptions. Each of the groups is made up of rocks from basalt to rhyolite with chemical and other peculiarities. During the eruption of a group there was little deformation but there was subsidence after the eruption of two of the groups and doming after the eruption of the last group. The magma moved into the area from the sides and was erupted about as rapidly as it moved.in. For the group named the Potosi volcanic series, the first eruptions had a composition near that of the primary magma ‐ dark quartz latites. Their eruption was followed by a short time without eruptions during which the magma became layered by crystal settling. The next eruptions were rhyolites followed abruptly, probably because of active movement of magna by dark quartz latites. The process was repeated three times. Subsidence caused by withdrawal of magma followed, and after a long time a new and different magma moved into the area and yielded the rocks of the Hinsdale formation. After the Hinsdale, a magma intruded the area with subsequent doming, but without eruptions.
The batholith of Southern and Lower California is exposed continuously from near Riverside, California, southward for a distance of about 350 miles. In central Lower California it is covered in part by younger rocks, but discontinuous bodies extend to the southern end of Lower California, and hence the batholith is probably over 1000 miles long. Its width is about 60 miles. A strip across the northern part of the batholith about 70 miles wide has been studied; the western half was mapped in detail, and the eastern half was covered in rapid reconnaissance. In the area studied the batholith intrudes Triassic sediments and Jurassic(?) volcanic rocks along its western border and Paleozoic sediments along its eastern border. Screens and roof pendants are common within the batholith. The Triassic rocks are mildly metamorphosed in the western part of the area but become progressively more coarsely crystalline toward the east. The Paleozoic rocks are rather coarsely crystalline. The metamorphism in large part preceded the intrusion of the batholith, and only locally was there appreciable contact metamorphism. The batholith and older rocks are overlain by Upper Cretaceous and younger sediments. Small bodies of andesite and basalt are associated with the Tertiary sediments, and small bodies of nepheline basalt of Quaternary age are present in the area. The batholith was intruded in early Upper Cretaceous time. The batholith in the area studied was emplaced by over 20 separate injections. Most of the resulting rock types are found in only one or a few small bodies which are confined to a small area. In the area studied in detail (Pl. 1) five types are present in many large, widely separated bodies, making up about 88 per cent of the area underlain by the batholith. In the eastern half of the batholith three more widespread types are present. In the western half of the body the rocks range fro a gabbro to granite, but in the eastern half several tonalites constitute nearly the whole of the mass. The gabbro is composed of many related rocks. Some have hornblende, some pyroxene; in some the plagioclase is anorthite, in others it is as sodic as andesine-Iabradorite. Some of the tonalites contain abundant inclusions that have been almost completely reworked by the magma and have been softened and stretched into thin discs. These inclusions are well oriented and near the contacts with older rocks they parallel the contacts, but elsewhere they strike about N. 30° W. and dip steeply to the east. One tonalite, whose feldspar is andesine, has scattered crystals with cores of bytownite, and has well-crystallized hornblende with cores of pale uralitic hornblende and remnants of augite. Hornblende and biotite are the predominant mafic minerals of the tonalites and granodiorites. The iron content of the mafic minerals of the gabbros is moderate, and it increases as the rocks become richer in silica. The norms and the modes are shown on a variation diagram (Figs. 11, 12). The chemical analyses of the rocks fall near smooth variation curves (Fig. 4). The general strike of the structures of the area have been about N. 30° W. from Paleozoic to the present time. The Paleozoic and Triassic sediments, the orientation of the inclusions and other structures of the batholith, the elongation of the batholith and the mountain ranges, and the strike of the major faults are in about the same direction. In the batholith and the older sediments the dips are steep to the east. The batholith must have been emplaced by stoping and not by forceful injection. Calculations show that the cooling of a large batholith is chiefly through the roof and not through the walls. Crystallization to a depth of 3 kilometers takes place in about half a million years. The different rocks of the batholith were formed from the intermediate gabbro by crystal differentiation and assimilation in depth. In early Upper Cretaceous time diastrophism folded the older rocks and formed, in depth, a strip of gabbroic magma about 1000 miles long. A small amount of this magma was intruded nearly to the surface. The deep magma differentiated quietly until its upper part attained the composition of a tonalite. Earth movements then occurred at least five times in rapid succession and caused the injection of the different tonalites. Some of these carry abundant inclusions, indicating a widespread shattering of the wall rock shortly before final emplacement. From time to time local movements caused the injections of the different granodiorites. When the deep-seated magma reached the composition of a light-colored granodiorite, widespread diastrophism moved the main granodiorite upward. Further local movement caused the emplacement of the many local granodiorites and granites.
Determination of radioactivity has been made on 43 rocks, carefully selected from the different mapped units of the complex Cretaceous batholith of Southern California; they range from gabbro to granite. The activity of the gabbro averages about 0.3 alphas/mg./hr., that of the tonalites, 0.8, the granodiorite 1.3, and the granites over 2.0. The activity of the average rock of the batholith is about 0.9 alphas/mg./hr. Most of the rocks have been analyzed, and modal compositions are given. The radioactivity and the percentages of K 2 O, SiO 2 and PbO are plotted on a variation diagram. The variation curve for activity nearly parallels that for K 2 O, and the ratio of activity to K 2 O is about 0.5. The radioactivities of some rocks fall far from the variation curve, and degree of activity correlates with K 2 O content. A less-marked correlation between activity SiO 2 and PbO was also noted. The K 2 O, SiO 2 , PbO, and radioactivity concentrate in the residual liquid during crystallization and differentiation. PbO follows K 2 O closely, as it is concentrated in the K 2 O minerals, but radioactivity is very low in the K 2 O minerals and is greatly concentrated in the accessory minerals.
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