Geology topics

K.A. Foland

Publications and source records attributed to K.A. Foland.

5 recordsLinked to original sources

USGS2007

Jurassic silicic volcanism in the Transantarctic Mountains: Was it related to plate margin processes or to Ferrar magmatism?

Silicic volcanism in the Transantarctic Mountains, represented by rhyolitic tuff that mainly precedes emplacement of the Ferrar Large Igneous Province, is important in interpretation of the tectonic evolution of the Antarctic sector of Gondwana. Sr and Nd isotope data indicate that the tuffs are not directly related to Ferrar magmatism nor to melting of the underlying Ross orogen crust yet zircon gives a U-Pb age of 182.7±1.8 Ma, similar to the U/Pb age for the Ferrar. Distribution of the silicic tuffs along 1400 km of the Transantarctic Mountains suggests, alternatively, a relationship to the Gondwana plate margin. Although West Antarctica comprises Mesoproterozoic crustal terrains, few analyzed rocks are compatible isotopically with the Lower Jurassic tuffs. The source of the tuffs must lie in unexposed Early Jurassic magmatic centers in West Antarctica or an unexposed crustal terrain beneath the Transantarctic Mountains.

Open-File Report↗

USGS2006

2480 Ma mafic magmatism in the northern Black Hills, South Dakota: A new link connecting the Wyoming and Superior cratons

The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ???2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the northern Black Hills, a 1 km thick, layered sill (the Blue Draw metagabbro) that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift-basin sedimentation. Ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207Pb/206Pb upper-intercept age of 2480 ?? 6 Ma (all age errors ??2??), comparable to two-point 207Pb/206Pb errorchron ages obtained by Pb stepwise leaching of the same titanites. Nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 ?? 16 to 2284 ?? 20 Ma (i.e., differentially reset along U-Pb concordia), and hornblende from an associated metadiorite yields a partially reset date with oldest apparent-age increments ranging between 2076 ?? 16 and 2010 ?? 8 Ma. We interpret these data as indicating that an episode of gabbroic magmatism occurred at 2480 Ma, in response to earlier rifting of the eastern edge of the Wyoming craton. Layered mafic intrusions of similar thickness and identical age occur along a rifted belt in the southern Superior craton (Sudbury region, Ontario). Moreover, these mafic intrusions are spatially aligned using previous supercontinent restorations of the Wyoming and Superior cratons (Kenorland-Superia configurations). This new "piercing point" augments one previously inferred by spatial-temporal correlation of the Paleoproterozoic Huronian (southern Ontario) and Snowy Pass (southeastern Wyoming) supergroups. We propose that layered mafic intrusions extending from Nemo, South Dakota, to Sudbury, Ontario, delineate an axial rift zone along which Wyoming began to separate from Superior during initial fragmentation of the Neoarchean supercontinent at ???2480 Ma. ?? 2006 NRC Canada.

Canadian Journal of Earth Sciences↗

USGS1985

Petrogenesis of the magmatic complex at Mount Ascutney, Vermont, USA - I. Assimilation of crust by mafic magmas based on Sr and O isotopic and major element relationships

The Ascutney Mountain igneous complex in eastern Vermont, USA, is composed of three principal units with compositions ranging from gabbro to granite. Sr and O isotopic and major element relationships for mafic rocks, granites, and nearby gneissic and schistose country rock have been investigated in order to describe the petrogenesis of the mafic suite which ranges from gabbro to diorite. The entire complex appears to have been formed within a short interval 122.2??1.2 m.y. ago. The granites with ??18O near +7.8??? had an initial 87Sr/86Sr of 0.70395(??6) which is indistinguishable from the initial ratio of the most primitive gabbro. Initial 87Sr/86Sr ratios and ??18O values for the mafic rocks range from 0.7039 to 0.7057 and +6.1 to +8.6???, respectively. The isotopic ratios are highly correlated with major element trends and reflect considerable crustal contamination of a mantle-derived basaltic parent magma. The likely contaminant was Precambrian gneiss similar to exposed bedrock into which the basic rocks were emplaced. A new approach to modelling of assimilation during the formation of a cogenetic igneous rock suite is illustrated. Chemical and isotopic modelling indicate that the mafic rocks were produced by simultaneous assimilation and fractional crystallization. The relative amounts of fractionation and assimilation varied considerably. The mafic suite was not produced by a single batch of magma undergoing progressive contamination; rather, the various rocks probably were derived from separate batches of magma each of which followed a separate course of evolution. The late stage granite was apparently derived from basaltic magma by fractionation with little or no crustal assimilation. The early intrusive phases are much more highly contaminated than the final one. The observed relationships have important implications for the formation of comagmatic complexes and for isotopic modelling of crustal contamination. ?? 1985 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1984

Botanical and geological significance of potassium-argon dates from the Juan Fernández Islands

Potassium-argon dating of five basalts from the three main islands of the Juan Fernández (or Robinson Crusoe) Islands of Chile in the southeastern Pacific gives ages of 1.01 ± 0.12 and 2.44 ± 0.14 million years for Masafuera, 3.79 ± 0.20 and 4.23 ± 0.16 for Masatierra, and 5.8 ± 2.1 for Santa Clara. These ages are much younger than that of the underlying oceanic plate and are consistent with the origin of the island-seamount chain from a mantle hot spot beneath the eastward moving Nazca plate. The young age for the archipelago suggests that speciation within endemic genera has occurred within the past 4 to 5 million years. Endemic genera of apparently more ancient origins, such as Lactoris and Thyrsopteris, have apparently dispersed to the islands and survive refugially.

Science↗

USGS1977

Application of Sr and O isotope relations to the petrogenesis of the alkaline rocks of the Red Hill complex, New Hampshire, USA

The Red Hill ring complex in central New Hampshire is composed of apparently cogenetic syenites, nepheline-sodalite syenite, and granite. The ages and petrogenetic relations among five of the six recognized units have been investigated by rubidiumstrontium and oxygen isotope analysis of whole rocks and separated minerals. Whole-rock samples from three syenite units are consistent with a single Rb-Sr isochron which gives an age of 198??3 m.y. and an initial (87Sr/86Sr)o ratio of 0.70330??0.00016 (??2 sigma; ??=1.42?? 10-11y-1). However, Sr isotope data for two other units, nepheline syenite and granite, are not consistent with this isochron but rather indicate higher initial ratios which range from 0.7033 to about 0.707. Whole-rock O isotope analyses give ??18O values which range from+6.2 to+9.3??? Sr and O isotope analyses on mineral separates indicate that observed whole-rock variations in (87Sr/86Sr)o are primary and are not due to any secondary process. The fact that the isotope systematics correlate with rock type, suggests that crustal interaction is likely to have played a significant role in the development of this over-and undersaturated association. Such process(es), while still not fully delineated, could be of fundamental importance to the genesis of associations of critically undersaturated and oversaturated intrusives. The data support the idea that interaction between magmas and crustal materials strongly influenced the compositional relations of similar complexes elsewhere including those of the White Mountain magma series. ?? 1977 Springer-Verlag.

Contributions to Mineralogy and Petrology↗