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L. C. Calk

Publications and source records attributed to L. C. Calk.

12 recordsLinked to original sources
USGS2012

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km 3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H 2 O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H 2 O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.

British Columbia↗
USGS1998

A king-sized theropod coprolite

Fossil faeces (coprolites) provide unique trophic perspectives on ancient ecosystems. Yet, although thousands of coprolites have been discovered, specimens that can be unequivocally attributed to carnivorous dinosaurs are almost unknown. A few fossil faeces have been ascribed to herbivorous dinosaurs, but it is more difficult to identify coprolites produced by theropods because other carnivorous taxa coexisted with dinosaurs and most faeces are taxonomically ambiguous. Thus sizeable (up to 20 cm long and 10 cm wide) phosphatic coprolites from Belgium and India that have been attributed to dinosaurs might have been produced by contemporaneous crocodylians or fish. But there is no ambiguity about the theropod origin of the Cretaceous coprolite we report here. This specimen is more than twice as large as any previously reported carnivore coprolite, and its great size and temporal and geographic context indicate that it was produced by a tyrannosaur, most likely Tyrannosaurus rex. The specimen contains a high proportion (30-50%) of bone fragments, an it rare tangible evidence of theropod diet and digestive processes.

Nature↗
USGS1991

Degassing and differentiation in subglacial volcanoes, Iceland

Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60°C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation.

Journal of Volcanology and Geothermal Research↗
USGS1991

Degassing and differentiation in subglacial volcanoes, Iceland

Within the neovolcanic zones of Iceland many volcanoes grew upward through icecaps that have subsequently melted. These steep-walled and flat-topped basaltic subglacial volcanoes, called tuyas, are composed of a lower sequence of subaqueously erupted, pillowed lavas overlain by breccias and hyaloclastites produced by phreatomagmatic explosions in shallow water, capped by a subaerially erupted lava plateau. Glass and whole-rock analyses of samples collected from six tuyas indicate systematic variations in major elements showing that the individual volcanoes are monogenetic, and that commonly the tholeiitic magmas differentiated and became more evolved through the course of the eruption that built the tuya. At Herdubreid, the most extensively studies tuya, the upward change in composition indicates that more than 50 wt.% of the first erupted lavas need crystallize over a range of 60??C to produce the last erupted lavas. The S content of glass commonly decreases upward in the tuyas from an average of about 0.08 wt.% at the base to < 0.02 wt.% in the subaerially erupted lava at the top, and is a measure of the depth of water (or ice) above the eruptive vent. The extensive subsurface crystallization that generates the more evolved, lower-temperature melts during the growth of the tuyas, apparently results from cooling and degassing of magma contained in shallow magma chambers and feeders beneath the volcanoes. Cooling may result from percolation of meltwater down cracks, vaporization, and cycling in a hydrothermal circulation. Degassing occurs when progressively lower pressure eruption (as the volcanic vent grows above the ice/water surface) lowers the volatile vapour pressure of subsurface melt, thus elevating the temperature of the liquidus and hastening liquid-crystal differentiation. ?? 1991.

Journal of Volcanology and Geothermal Research↗
USGS1986

Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada

An assemblage of pyroxenite, peridotite, and mafic granulite xenoliths contained in the toe of a 10 Ma trachybasalt flow remnant overlying Late Cretaceous granitoids indicates the presence of a mafic-ultramafic complex beneath the Sierra Nevada batholith. Olivine-free pyroxenites that include orthopyroxenites, websterites, and clinopyroxenites are dominant. Primary igneous textures are displayed by some pyroxenites, but commonly are masked by recrystallization. Fe-rich harzburgites and lherzolites are rare. A few of the ultramafic xenoliths contain ovoid opaque patches that are apparently pseudomorphs after garnet and have pyralspite garnet compositions. A pressure corresponding to a lower crustal depth of approximately 40 km has been determined from two of these xenoliths using a garnet-orthopyroxene geobarometer. Abundant mafic granulites can be subdivided into those containing 12 per cent or less A 12 O 3 and chemically gradational with pyroxenites and others containing more than 15 per cent A 12 O 3 and showing considerable scatter on oxide variation diagrams. The high-alumina granulite xenoliths have relatively low 87 Rb/ 86 Sr but high 87 Sr/ 86 Sr, whereas low-alumina and ultramafic xenoliths have a wide range of 87 Rb/ 86 Sr, but lower 87 Sr/ 86 Sr; the isotopic data indicate an age for the complex roughly the same as that of overlying granitoid plutons. However, the granitoids have initial 87 Sr/ 86 Sr ratios intermediate between the high-alumina and ultramafic xenoliths, suggesting that they may have resulted from mixing of basaltic magma, represented by the ultramafics, and crustal materials, with subsequent crystal fractionation. The trachybasalt may represent a partial melt of the ultramafic rocks. Rocks analogous to the Chinese Peak xenoliths are exposed in the Giles complex of central Australia, a series of several deformed layered mafic and ultramafic intrusions, emplaced in a granulite facies terrain. Contemporaneous development of mafic-ultramafic complexes and the Sierra Nevada batholith may explain the present day thick (˜ 50 km) crust in this region

Journal of Petrology↗
USGS1978

Fusion of granodiorite by basalt, central Sierra Nevada

A trachybasalt plug, 100 m in diameter, has partially fused inclusions and wallrock of porphyritic granodiorite near Tuolumne Meadows, Yosemite National Park, Calif. Granodiorite surrounding the plug was altered within distances of about 3.5 m. Within this distance, (1) trace amounts of glass occur along fractures and grain boundaries, (2) biotite has been altered from dark olive to dark reddish brown and contains bands of fine reddish iron oxide grains, and (3) optic axial angles of potassium feldspar decrease toward the contact. As much as 20 volume percent of glass occurs in the granodiorite in a reentrant and in inclusions within the plug. Detailed study of two partially fused samples shows that, relative to unfused granodiorite, SiO 2 is clearly depleted in both samples, whereas K 2 O is depleted in one but not the other. Total iron, Al 2 O 3 , MgO, Na 2 O, and H 2 O show apparent increases in both samples; other constituents show no significant changes. Chemical analyses suggest that much original material was lost from the partially fused rocks, probably by mass migration of melt rather than by chemical diffusion. The composition of glasses now present in the partially fused rocks was dominated by melting of quartz and feldspars, whereas the composition of the early-formed "lost" melt was strongly influenced by subsolidus reaction and subsequent melting of biotite.

California↗
USGS1971

Kaersutite - A product of reaction between pargasite and basanite at Dish Hill, California

Paragasitic amphibole, occurring interstitially and as veins in peridotite inclusions in basanite, has reacted with the host basanite to form kaersutitic amphibole. The amphibole compositions vary with respect to distance from the edge of the xenolith; iron, titanium, and potassium contents are higher and magnesium, silicon, sodium, and chromium contents are lower closer to the basanite. Pargasite was exposed to the basanite when peridotite blocks broke open along amphibole veins during transport to the surface. Small amphibole fragments isolated in the basanite show the most reaction; compositional gradients in interstitial and vein amphibole are steep into peridotite inclusions where the amphibole was shielded from reaction. The compositions of amphiboles so modified have no direct bearing on high pressure fractionation trends if the amphibole is cognate, or on the bulk composition of the upper mantle if it is accidental.

California↗