Theodore J. Bornhorst

Volcán El Chichón, Mexico: Pre-1982 S-rich eruptive activity

Abstract Mapping and sampling of the interior of El Chichon Volcano was made possible by the exposures of the 1982 explosion crater. The 1-km-diameter, 200-m-deep crater exposes rocks produced during several eruptive episodes: volcanic domes, talus breccias, plinian airfall deposits and pyroclastic-flow deposits of compositionally similar alkalic hornblende-trachyandesites with phenocrysts of anhydrite. All of the rocks, except those leached of anhydrite, have extremely high S contents. El Chichon consists of a volcanic dome complex, airfall debris, and pyroclastic outflow sheets. Prior to 1982, its last major activity was about 650 ± 100 yr B.P.

Quaternary silicic pyroclastic deposits of Atitlán Caldera, Guatemala

Abstract Atitlan caldera has been the site of several silicic eruptions within the last 150,000 years, following a period of basalt/andesite volcanism. The silicic volcanism began with 5–10 km3 of rhyodacites, erupted as plinian fall and pyroclastic flows, about 126,000 yr. B.P. At 85,000 yr. B.P. 270–280 km3 of compositionally distinct rhyolite was erupted in the Los Chocoyos event which produced widely dispersed, plinian fall deposits and widespread, mobile pyroclastic flows. In the latter parts of this eruption rhyodacite and minor dacite were erupted which compositionally resembled the earliest silicic magmas of the Atitlan center. As a result of this major eruption, the modern Atitlan (III) caldera formed. Following this event, rhyodacites were again erupted in smaller (5–13 km3) volumes, partly through the lake, and mafic volcanism resumed, forming three composite volcanoes within the caldera. The bimodal mafic/silicic Atitlan volcanism is similar to that which has occurred elsewhere in the Guatemalan Highlands, but is significantly more voluminous. Mafic lavas are thought to originate in the mantle, but rise, intrude and underplate the lower crust and partly escape to the surface. Eventually, silicic melts form in the crust, possibly partly derived from underplated basaltic material, rise, crystallize and erupt. The renewed mafic volcanism could reflect either regional magmato-tectonic adjustment after the large silicic eruption or the onset of a new cycle.

Trace-element sanidine/glass distribution coefficients for peralkaline silicic rocks and their implications to peralkaline petrogenesis

Abstract Sanidine/glass distribution coefficients for 11 trace elements have been determined on six peralkaline and two subalkaline silicic rocks. Distribution coefficients for Na, Sc, Fe, Cs, La, Ce, Sm, Tb and Lu from this study and the literature show little variation, within analytical uncertainty, for silicic rocks of peralkaline and subalkaline affinity. Distribution coefficients for Eu and Rb show a marked decrease with increasing peralkalinity. This variation may be the result of the decrease in the degree of polymerization from subalkaline to peralkaline silicic melts. Previous studies involving modelling of peralkaline rocks have selected, incorrectly, Eu and Rb sanidine/glass distribution coefficients determined from subalkaline silicic rocks.

Partitioning of Gold in Young Calc-Alkalic Volcanic Rocks from Guatemala

Gold analyses of whole rock, mineral, and groundmass of seven calc-alkalic volcanic rocks (basalt to rhyolite) show that gold is less abundant in the dominant phenocrystic phases than in the whole rock; groundmass concentrations are also lower than whole rock. Crystal-liquid partition coefficients are <1 for plagioclase and hornblende, and about 1 for olivine, pyroxene, magnetite, and biotite. If only the principal phases are considered, the bulk gold partition coefficient is typically <1, yet gold seems to be depleted rather than enriched in many evolved magmas. A significant proportion of gold in these rocks is in neither the groundmass nor the dominant phenocrysts but instead is contained in tiny sulflde blebs that are found in these rocks. During crystallization the continual separation of a sulflde phase that concentrates gold is consistent with compatible gold behavior and petrography. Further data is needed to evaluate this hypothesis.

Greenockite and Associated Uranium-Vanadium, Minerals from the Huron River Uranium Prospect Baraga County, Michigan

4A. E. Seaman Mineral Museum and Department of Physics Michigan Technological University 1400 Townsend Drive Houghton, Michigan 49931 jaszczak@mtu.edu T he Huron River uranium prospect is one of several baseand preciousmetal explorations in the vicinity of the Huron River drainage system and the western flanks of the adjacent Huron Mountains (Kalliokoski and Lynott 1987). Although there are no true mines, historic or modern, in this part of Baraga County, the area is intriguing in that it lies nearly equidistant between two world-class mining districts: the famous Michigan native copper district to the northwest, and the Marquette Range iron (and gold) mining district to the southeast. Classical geological features such as a well-exposed Precambrian unconformity at the Big Eric’s Crossing of the Huron River (also a popular camping site) draw geologists and students to the region each year. Figure 1. Contact between mineralized brecciated quartzite and Michigamme Slate at the Huron River uranium prospect; Mark Elder photo.

Archean Appinites from the Northern Complex, Michigan

The northern complex is an Archean greenstone-granitoid terrane located in Upper Michigan at the southern margin of the Superior Province, interpreted as a product of N-directed subduction followed by continental collision. Amphibole-bearing granitoids in the northern complex have comparable field relationships, textures and compositions to Paleozoic appinites emplaced during the Caledonian orogeny in the British Isles. We suggest that the rocks in the northern complex represent an Archean appinite suite. The northern complex appinites vary from olivine-normative hornblende cumulates to quartz-normative hornblende diorites. Hornblendite is characterized by high Mg-numbers and elevated Cr, Ni, and Sc content whereas diorite is more fractionated with lower Mg-numbers and is comparatively enriched in the large-ion-lithophile elements (e.g., K, Rb, Sr, and Ba). Rare-earth elements show negligible Eu anomalies and a progressive increase of

Inter-laboratory comparison of X-ray fluorescence analyses of eruptive products of El Chichon Volcano, Chiapas, Mexico

[Ce/Yb]_{N}

Application of discriminant function analysis to the felsic rocks of the Bushveld Complex, South Africa

ratios from hornblendites to diorites. Geochemical characteristics are comparable to subduction-related magmas. The northern complex appinites were emplaced during continental collision along the Great Lakes tectonic zone with magmas derived from mantle that was metasomatized prior to collision during N-directed subduction beneath the northern complex. The northern complex appinites document another similarity between Archean and Phanerozoic collisional orogens.

Petrochemistry of the Fish Cove rhyolite, Keweenaw peninsula, Michigan, U.S.A.

Abstract An inter-laboratory comparison has been made of X-ray fluorescence analyses of 10 samples of lava and pumices from El Chichon Volcano, Chiapas, Mexico. Some determinations of major-element constituents agree within analytical uncertainty, whereas others exchibit significant bias. Analyses carried out at the Michigan Technological University (MTU) laboratory are systematically lower in MgO (26–48%), Fe total (5–18%), CaO (4–15%) and higher in K 2 O (0–15%) than analyses made at the U.S. Geological Survey (USGS) Denver laboratory. These differences are ascribed in part to a complex combination of calibration assumptionsand mineralogical and particle-size effects inherent in the use of pressed rock-powder pellets in the analytical procedure of the MTU laboratory. Other, but as yet unknown, differences in sample preparation and/or analytical technique may also be important; effects related to natural sample inhomogeneityare believed to be insignificant. The inter-laboratory differences in the analytical data complicated accurate assessment of whether El Chichon magmas have changed composition during the past 300 000 a. Knowledge of such change is needed for understanding petrogenetic history and for such related studies as evaluation of volcanic hazards.

Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift: COMMENT

Abstract The technique of multivariate analysis was used to investigate the geochemical relationships between the felsic rocks of the Bushveld Complex. The Bushveld granite and Rooiberg felsite form two distinct geochemical groups based on their major element compositions, possibly indicating that they originated from separate and genetically unrelated magmas. A discriminant function based on six major oxides was found to be 90 percent effective in distinguishing between the two groups. These conclusions have important implications for the petrogenesis of the Bushveld Complex.