Samuel S. Goldich

A Study in Rock-Weathering

Four districts furnished samples for a study in rock-weathering. Mineralogical data and fourteen new chemical analyses are presented in tables and diagrams. The granite gneiss of Morton, Minnesota, shows a marked loss of soda and lime in early stages of weathering but a more gradual loss of potash and baria. The progress of the decomposition is interpreted from a series of six samples of residual clay. Kaolinite is an end product. New chemical analyses of material from the Medford, Massachusetts, diabase are presented as a check on the older data. Ferrous and ferric iron determinations on samples of diabase and of glacial till suggest that the till is less oxidized than the underlying diabase, indicating preglacial weathering of the diabase. A diabase on the north shore of Lake Superior, Minnesota, is weathered locally to a depth of 40 feet but shows little chemical change except oxidation. An amphibolite from the Black Hills, South Dakota, yielded beidellite or related clay minerals by decomposition of its hornblende. Calcite has been leached, and other minerals attacked. On the basis of the present and earlier studies, a mineral-stability series in weathering is proposed. The arrangement of the common rock-forming minerals in this series is found to coincide with the reaction series. Experimental work on the attack of silicate minerals by water is in accord with the suggested mineral-stability series.

Age of the Morton and Montevideo Gneisses and Related Rocks, Southwestern Minnesota

Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Geochemistry of the Cenozoic Volcanic Rocks of Ross Island and Vicinity, Antarctica

The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series : basanitoid → trachybasalt → phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Trans-antarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% Na₂O, 5.6% K₂O, and 57% SiO₂. The basanitoids contain relatively large amounts of Ba (~400 ppm), Sr (~1,000 ppm), and Rb (~30 ppm). The concentrations of these and other trace elements in the various rock types support the model of small degrees of partial melting of peridotitic mantle for the derivation of the basanitoid magma, and of magmatic differentiation for the subsequent development of the rock series.

Rhyolitic Tuff Flows in Southern Peru

The numerous and widespread sillar deposits in the western Andes of southern Peru are rhyolitic tuff flows which are not welded. Two types may be easily distinguished by color in the field. One, a salmon-colored sillar, is composed largely of minute glass shards and pumice fragments and contains small amounts of oligoclase and biotite and, rarely, quartz. The other, a white sillar, is relatively free of hematite colorant, is hard, and consists largely of secondary axiolitic and spherulitic growths of potash feldspar and (rarely) tridymite with relatively little glass. The white sillar contains quartz, oligoclase, and biotite fragments. Both types of sillar contain foreign material, mostly andesite, probably from the vent walls. Five chemical analyses of sillars from the Arequipa region show remarkable uniformity, regardless of color or degree of recrystalliza-tion, and closely approach the composition of average rhyolite. Sr, Li, Rb, and Cs are present in amounts typical of rhyolitic and granitic rocks. In the upland valleys of Apurimac and Cusco departments single rhyolitic tuff flows have been traced for 20 miles. These sillar deposits are in remnants of mature valleys which are probably of early Pleistocene age. Youthful streams have cut canyons as much as 2,300 feet below the valleys which guided the course of the tuff flows. Similar relationships hold in the Arequipa region, where many deposits of both salmon and white sillar have accumulated in a broad basin between the Yura and Chili rivers. The sillar deposits all appear to be related to dominantly andesitic volcanic centers. Physiographic evidence suggests that eruption of these rhyolitic tuff flows is connected with relatively sudden epeirogenic movement. Andesitic volcanism both preceded and followed the principal early Pleistocene rhyolitic eruptions. The volume of rhyolitic sillar is small compared to that of andesite flows and pyroclastics.

Determination of ferrous iron in silicate rocks

Abstract Present-day rock analyses commonly are deficient because FeO and other constituents (H 2 O and CO 2 ), which are not amenable to instrumental analysis, are not determined. Ferrous iron, however, can be determined with good precision in most silicate rocks by a simple, rapid, and inexpensive method using a fused-silica flask in place of a large Pt crucible for sample dissolution. Comparative data on 18 standard reference samples are given.

STRATIGRAPHY AND PETROLOGY OF THE BUCK HILL QUADRANGLE, TEXAS

Tertiary tuff beds and related sediments with intercalated lava flows, aggregating over 2000 feet, are named the Buck Hill volcanic series for outcrops in the Buck Hill quadrangle, Brewster County, Texas. The volcanic series rests unconformably on the Upper Cretaceous Boquillas formation. The Lower Cretaceous is represented by the Georgetown limestone, the Grayson clay, and the Buda limestone. The lower 900–1000 feet of the Buck Hill volcanic series is named the Pruett formation, and the upper 1000–1400 feet, the Duff tuff. Lava flows up to 325 feet thick, designated the Cottonwood Spring basalt, separate these formations. A limestone pebble conglomerate and arkosic sandstone at the base of the Pruett mark the Cretaceous-Tertiary unconformity. The Pruett tuff commonly is calcareous and grades to beds of fossiliferous freshwater limestone. A massive layer of breccia-conglomerate occurs near the top of the formation, approximately 60 feet below the base of the Cottonwood Spring basalt. Lava flows intercalated with the Pruett sediments, from oldest to youngest, are the Crossen trachyte, the Sheep Canyon basalt, and the Potato Hill andesite. The Duff formation is chiefly silicic tuff with minor breccia and a few thick beds of stream conglomerate. It is overlain by the Mitchell Mesa rhyolite, the youngest rock of the volcanic series in the quadrangle. In the southern part of the quadrangle Straddlebug Mountain and Buck Hill are small intrusives of syenite; similar syenitic intrusives cut the Cretaceous, the Pruett tuff, and the Sheep Canyon basalt in the Elephant Mountain area. The igneous rocks are alkalic and chemically resemble the analyzed rocks from the Terlingua-Solitario region to the south. The main structural features are west and northwest-trending normal faults. Two major fault zones divide the quadrangle into three blocks with a middle down-dropped segment. Differential hardness of the rocks of the Buck Hill volcanic series controls the topography.

Geology and geochemistry of the Rainy Lake Area

Abstract Major stratigraphic units in the Rainy Lake area include graywackes (Coutchiching), mafic and felsic volcanic rocks (Keewatin), and conglomerates and associated finer-grained clastic rocks (Seine), Tonalites (Laurentian) intruded the Coutchiching and Keewatin, and, following a period of folding, contributed clasts to the younger Seine sediments. Granitic plutosn (Algoman) were emplaced during a younger igneous and metamorphic event. Radiometric dating places all of the major rock-forming events between 2700 and 2600 Ma ago. Uplift and stabilization 26000 to 2400 Ma ago resulted in widespread retrograde metamorphism and local cataclasis. Compositional differences between the Coutchiching gneisses in Rice Bay and the lower grade graywackes in the Bears Passage belt reflect different source terranes. Major rock types in the Keewatin are massive and pillow basalts of tholeiitic composition, and felsic rhyodacites with quartz phenocrysts; intermediate rocks are uncommon. Gabbroic sills were emplaced in the Keewatin as the volcanic pile thickened, and anorthosite cumulates reflect differentiation of larger bodies of magma. Sills of leucotonalite and leucogranodiorite are abundant at Rice Bay as slightly discordant bodies in the Coutchiching gneisses and in the overlying Keewatin volcanic rocks. Laurentian intrusions are highly sheared and altered tonalites and granodiorites. Algoman intrusions are less deformed, elliptical-shaped plutons of granodiorite and quartz monzonite. The Ottertail Lake Pluton, the largest body, is zoned from biotite-hornblende granodiorite at the margin to leucocratic quartz monzonite at the center. Angular inclusions of Keewatin rocks are abundant at the margin. These decrease in size and angularity inward where they are last recognized as small mafic clots and schlieren in quartz monzonite. The diversity and general bimodal character of the major igneous rocks units have resulted from different source materials, degrees of melting, and the crustal history of the magmas — fractional crystallization and contamination. Keewatin volcanic rocks evolved through differentiation of tholeiitic magmas, and similarities between the rhyodacites and the Laurentian tonalites suggest that these may be related. Leucocratic sills abundant in Rice Bay are not genetically related to the Keewatin rocks but have some similarities to the Algoman rocks. Marked variations in the Ottertail Lake stock probably resulted from both contamination and differentiation. The origin of these magma is not clear but deviation from the same source as the Keewatin rocks is unlikely. With the development and thickening of the crust, a greater diversity of material became available for magmatic recycling.

Glauconite from the precambrian belt series, Montana

Glauconite from the upper part of the Missoula Group of the Belt Series, Flathead County, Montana, has been dated at 1070 million years by potassium-argon and rubidium-strontium analyses. This is the first glauconite of Precambrian age reported in North America.

Investigations in Radioactivity-Dating of Sediments

Two samples of glauconite from the Upper Cambrian Franconia formation in Goodhue County, Minnesota, give an average K-A age of 440 m. y. Illite from the Siyeh limestone, Belt series, Glacier Park, Montana, is dated at 740 m. y. by K-A and 780 m. y. by Rb-Sr techniques. Pipestone from the Sioux quartzite at Pipestone, Minnesota, gives a K-A age of 1.20 b. y., and the time of deposition of the Sioux formation is placed in the early Middle Keweenawan or Lower Keweenawan. Geologic history of sediments subsequent to deposition is most important in attempting to date the time of deposition. X-ray crystallographic studies coupled with radioactivity-dating should lead to a better understanding of the history of sediments and of the changes that commonly are attributed to diagenes s.

Lunar and Terrestrial Ilmenite Basalt

A basalt hornfels from the Keweenawan Duluth complex in Minnesota contains 7 percent by weight of titanium dioxide and is similar in many respects to the Apollo 11 samples. Hornfels texture, as well as primary textures in lunar rocks, resemble those in Keweenawan rocks.