Charles J. Vitaliano

Evolved lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho

Holocene lavas from Craters of the Moon (COM) National Monument are representative of “differentiated” lavas which occur around the margins of the Snake River Plains (SRP) and they range serially in composition from alkali- and phosphorous-rich ferrobasalts to ferrolatites. Petrographic study indicates that these lavas evolved primarily by cotectic crystallization of olivine, plagioclase, magnetite and apatite in the mafic members of the suite (ferrobasalts), and by cotectic crystallization of plagioclase, magnetite, clinopyroxene and minor olivine in the salic members. Quantitative phase relations in the COM lavas, calculated by means of a leastsquares mixing program, indicate that the observed range in composition among these lavas corresponds to at least 70 percent crystallization of a magma similar to the most mafic COM lavas. Anhydrous one-atmosphere experimental crystallization studies fail to reproduce exactly the inferred phase relations; the discrepancy is attributed to the presence of water in the naturally crystallized magmas. The origin of COM parental magma cannot be unequivocably resolved. Available evidence suggests that COM lavas do not represent melts derived directly from the mantle: (1) high Sr87/Sr86 ratios (0.708 to 0.712), (2) relatively high Fe/(Fe+Mg) and excluded-element content in even the most mafic COM lavas, (3) occurrence of megacrysts of inferred high-pressure origin in the Lava Creek flow. Megacrysts occur in the Lava Creek flow as clusters of labradorite, aluminous clinopyroxene, and olivine. Analogy with the experiments of Thompson [1] and least squares mixing calculations indicate that intermediate (ca. 8 to 10 Kbar) pressure fractionation of such megacrysts from olivine tholeiite magma may yield derivative COM-type liquids.

Petrology of McKinney Basalt, Snake River Plain, Idaho

The McKinney Basalt, a composite pahoehoe and pillow lava unit in the Snake River Plain, has been studied by petrographic, chemical, strontium-isotopic, and experimental methods in an attempt to define its origin and evolutionary history. Differences in the compositions of olivine and plagioclase phenocryst cores and in the whole-rock composition of different samples are interpreted as evidence that McKinney Basalt erupted as sequential surges of lava that represent different degrees of fractional crystallization of an olivine tholeiite parental magma. Olivine and plagioclase geothermometry and experimental studies indicate that McKinney pillow lava erupted at a temperature of about 1190° to 1200°C. Compositions of iron-titanium oxides and plagioclase/glass europium partition coefficients indicate that as McKinney lava cooled, oxygen fugacity was buffered near the quartz-fayalite-magnetite buffer. Experimental studies and chemical compositions of McKinney samples suggest that the parental magma underwent significant crystallization only at relatively low pressures ( Strontium isotopic data suggest that McKinney Basalt, like all other Snake River olivine tholeiites, was derived from a mantle region that is more radiogenic than typical suboceanic mantle.

TIOGA BENTONITE (MIDDLE DEVONIAN) OF INDIANA

The petrography of shale partings in carbonate rocks from eleven cores in the Illinois (Jeffersonville Formation) and Michigan Basins (Detroit River Formation) of Indiana indicates the presence of a K-bentonite which is interpreted as the Tioga Bentonite, an important stratigraphic marker in the middle Devonian rocks throughout the central and eastern United States. The clay mineral composition of the Tioga Bentonite of Indiana is interstratified illite and smectite, usually with admixed kaolinite. This composition stands in striking contrast to the simple illite suite without kaolinite in the normal terrigenous shale partings in the Devonian rocks of Indiana. Euhedral sanidine, high temperature albite, zircon, apatite, and the angularity of quartz grains found associated only with the interstratified clay mineral suite support the volcanic origin of these clay partings.RésuméL’étude pétrographique des filons de schiste dans les roches carbonatées provenant de onze carottages dans les bassins de l’Illinois (Formation de Jeffersonville) et du Michigan (Formation de Detroit River) dans l’état d’Indiana, indique la présence d’une bentonite K, qu’on interprète comme la bentonite de Tioga; cette argile est un marqueur stratigraphique important dans les roches du Dévonien moyen dans tout le centre et l’est des Etats-Unis. La composition en minéraux argileux de la Bentonite de Tioga de l’Indiana est celle d’un interstratifié illite et smectite, comprenant généralement en mélange de la kaolinite. Cette composition présente un contraste frappant avec celle de la série simple de l’illite, qui ne comporte pas de kaolinite dans les filons de schiste détritique des roches dévoniennes de l’Indiana. La présence de sanidine holoédrique, d’ablite haute température, de zircon, d’apatite et le caractère angulaire des grains de quartz trouvés seulement en association avec la série du minéral argileux interstratifié, constituent des arguments pour l’origine volcanique de ces filons argileux.KurzreferatDie Petrographie der Schiefertonschieferungen in Karbonatgestein von elf Punkten der Illinois (Jeffersonville Formation) und Michigan Becken (Detroit River Formation) von Indiana weist darauf hin, daß ein K-Bentonit vorhanden ist. Dieses wird als Tioga Bentonit betrachtet und bildet eine wichtige stratigraphische Markierung in dem mitteldevonischen Gestein, das in den mittleren und östlichen Teilen der Vereinigten Staaten weit verbreitet ist. Was die Tongesteinzusammensetzung des Tioga Bentonits von Indiana anbelangt, finden wir Einlagerungen von Illit und Smektit, in der Regel mit Beimischungen von Kaolinit. Diese Zusammensetzung unterscheidet sich auffällig von dem einfachen Illit ohne Kaolinit in den normalen terrigenen Schiefertonschieferungen der devonischen Gesteine von Indiana. Idiomorphes Sanidin, gegen hohe Temperaturen beständiges Albit, Zirkon, Apatit und die winkelige Beschaffenheit der Quarzkörner, die sich nur bei dem Tongestein mit Einlagerungen finden, erhärten die Auffassung, daß diese Tonschieferungen von vulkanischem Ursprung sind.РезюмеПетрография прослоек одиннадцати колонок глинистого сланца в карбонатной горной породе в Иллинойс (формация Джефферсонвилл) и в бассейне Мичиган, Индиана (формация реки Детройт), указывают на присутствие К-бетонита, расшифровываемого как тиога-бетонит, являющимся важным стратиграфическим напластованием в горных породах центральных и восточных Штатов Америки. Строение глинистого минерала тиога-бетонита Индианы — это переслаивающийся иллит и смектит, обычно с примесью каолинита. Это строение резко отличается от простого иллита без каолинита в нормальных терригенных прослойках глинистого сланца в девонских горных породах Индианы. Санидин, высокотемпературный альбит, цикрон, апатит и скосы зерен кварца, находимые в ассоциации только с переслаивающимися глинистыми минералами подтверждают вулканическое происхождение этих глинистых прослоек.

Cenozoic Volcanic Rocks in the Southern Shoshone Mountains and Paradise Range, Nevada

The Cenozoic volcanic rocks in the southern part of the Shoshone Mountains and most of the Paradise Range consist predominantly of lava flows and pyroclastic rocks ranging in composition from rhyolitic to basaltic, and minor volcaniclastic sedimentary rocks. In the Shoshone Mountains, the stratigraphic column is more than 3,000 m thick, of which all but 150 m consist of lava flows and pyroclastic rocks, the remainder being volcaniclastic sediments. New formation names are given to two units in the lower part of the section that are well exposed near Mission Spring and Third Canyon, respectively. In the Paradise Range, lava flows and pyroclastic rocks of rhyolitic to andesitic composition 625 m thick constitute the entire Cenozoic section. In the lower part of the section, only the Third Canyon Formation of the Shoshone Mountains section is recognized in the Paradise Range; it is identified on the basis of stratigraphic position and characteristic propylitic alteration. A prominent ignimbrite unit higher in the section in both ranges is obviously the correlative of the Toiyabe Quartz Latite originally described in the Round Mountain quadrangle. Published K/Ar dates give an absolute age range of 19.3 ± 0.6 to 21.5 ± 0.6 for this formation in the area of the lone and Paradise Peak quadrangles. Dikes and pluglike intrusive rocks range in composition from rhyolite to trachyandesite. The relative position assigned to the various intrusive rocks, purely on the basis of superposition, is in good chemical and mineralogical agreement with the chemistry and mineralogy of the intruded and overlying rocks in each case, suggesting that most dikes and plugs are feeders. Chemical analyses and C.I.P.W. norms for 14 lava flows and pyroclastic and associated intrusive rocks show that all are oversaturated with respect to quartz, all are quartz-hypersthene normative, and all belong to the calc-alkaline suite. Some of the rocks have co-menditic and potassic rhyolite tendencies.

Capping volcanic rocks: West central Nevada

Cenozoic capping volcanic rocks in the Nevada portion of the Basin and Range Province of the western United States belong to the high alumina calcalkaline igneous series. Varying proportions of plagioclase (An=85 to 45 percent), pyroxene (augite, pigeonite, and hypersthene), olivine, magnetite, biotite, and oxyhornblende indicate a modal range from olivine basalt to andesite.Major element analyses made on randomly collected samples, as well as on samples from systematically measured stratigraphic sections in localities of minimum erosion show ranges in Al2O3 (from 17.5 to 22.5 percent); SiO2 (from 44.0 to 54.0 percent); MgO (from 3.47 to 8.20 percent) and CaO (from 7.19 to 11.90 percent). Na2O/K2O is always greater than 1.0. Ba++ and Sr++ abundances for some of the rocks are in agreement with those suggested, by workers in the field, for average basalt and andesite derived by melting of mantle or lower crust, but for many of the samples the values found are considerably higher.Although the presence of biotite in the earlier flows and oxyhornblende in the later ones along with the presence of much magnetite in all the rocks examined suggests that in part these rocks were derived by crystallization of a melt under conditions of high partial pressure of oxygen, the available trace element data indicates that contamination of the magma with crustal material was also a factor in their developmental history. The case for a parent magma, subsequently fractionally crystallized and contaminated in part, is strengthened by the occurrence of crystal cumulates and highly altered xenoliths in some of the flows.

Contact metamorphism at Rye Patch, Nevada

In the Rye Patch area in the west-central part of the Humboldt Range, Nevada, Triassic (?) limestones interbedded with silty layers have been intruded by igneous rocks, the principal type being a quartz monzonite. The quartz monzonite is surrounded on the north, east, and south by an aureole of contact metamorphism. On the western side faulting has cut the intrusion so that any metamorphic rocks which may have been formed are not seen at the surface. A long narrow contact zone east of the aureole extends northward a considerable distance. Metamorphic changes range from recrystallization of limestone to complete replacement by contact silicates. The contact-metamorphic minerals include garnet, diopside, epidote, clinozoisite, idocrase, tremolite, recrystallized calcite, quartz, and minor scheelite. The sediments are deformed by the intrusion into a small northerly trending arch. Aplite dikes and quartz veins appear related to the arching and also to the formation of scheelite.