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Columbia River Basalt Group
Foeken, J. Quaternary Geochronology , 4 1 , pp. Full text not currently available from Enlighten. We have used cosmogenic 3 He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano.
3 Geochemistry and dating. Analytical methods. Obvious crustal and mantle xenolith fragments were re- moved from the basalt sample.
Erosion resulting from the Missoula Floods has extensively exposed these lava flows, laying bare many layers of the basalt flows at Wallula Gap , the lower Palouse River , the Columbia River Gorge and throughout the Channeled Scablands. As the molten rock came to the surface, the Earth’s crust gradually sank into the space left by the rising lava.
This subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin or Columbia River Plateau. The northwesterly advancing lava forced the ancient Columbia River into its present course. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes.
In these ancient lake beds are found fossil leaf impressions, petrified wood , fossil insects, and bones of vertebrate animals. Both flows are similar in both composition and age, and have been attributed to a common source, the Yellowstone hotspot.
Influence of olivine grains on K-Ar dating of Late-Cenozoic olivine basalts in eastern China
For more than three decades potassium-argon K-Ar and argon-argon Ar-Ar dating of rocks has been crucial in underpinning the billions of years for Earth history claimed by evolutionists. Dalrymple argues strongly:. Hualalai basalt, Hawaii AD 1. Etna basalt, Sicily BC 0.
Dating crystalline groundmass separates of altered Cretaceous seamount basalts by the Ar/Ar incremental heating technique. A.A.P. Koppers, H.
The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. Like the underlying flood basalts, these shields are composed of alkaline lavas. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement.
Younger shields e. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting.
The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia.
Columbia River Basalt Group
These major tholeiitic dikes agree in radiometric date with the North Mountain Basalt of Nova Scotia and with other tholeiitic flows of the Newark Supergroup that have been paleontologically assigned to the Hettangian. New paleomagnetic results are also reported for the Caraquet and Avalon dikes. This discrepancy is likely due to insufficient averaging out of paleosecular variation and demonstrates that caution is needed in paleomagnetically correlating among the Early Jurassic flows and intrusions of eastern North America.
Basaltic magma that erupted simultaneously over large areas of Earth’s surface—so-called flood basalts—may have released prodigious volumes of SO2, CO2.
Basalt , extrusive igneous volcanic rock that is low in silica content, dark in colour, and comparatively rich in iron and magnesium. Some basalts are quite glassy tachylytes , and many are very fine-grained and compact. It is more usual, however, for them to exhibit porphyritic structure, with larger crystals phenocrysts of olivine , augite , or feldspar in a finely crystalline matrix groundmass. Olivine and augite are the most common porphyritic minerals in basalts; porphyritic plagioclase feldspars are also found.
Basaltic lavas are frequently spongy or pumiceous; the steam cavities become filled with secondary minerals such as calcite , chlorite, and zeolites. Basalts may be broadly classified on a chemical and petrographic basis into two main groups: the tholeiitic and the alkali basalts. Tholeiitic basaltic lavas are characterized by calcic plagioclase with augite, pigeonite or hypersthene, and olivine rarely as the dominant mafic minerals; basalts without olivine are also well represented.
Tholeiitic basalts, which contain 45 to 63 percent silica, are rich in iron and include the tholeiites basalts with calcium-poor pyroxene. The active volcanoes of Mauna Loa and Kilauea in Hawaii erupt tholeiitic lavas.
Barry, T. Lithos , pp. A well-documented stratigraphy indicates at least GRB flow fields or individual eruptions , and on this basis suggests an average inter-eruption hiatus of less than 4, years. Isotopic age-dating cannot resolve time gaps between GRB eruptions, and it is difficult to otherwise form a picture of the durations of eruptions because of non-uniform weathering in the top of flow fields and a general paucity of sediments between GR lavas.
Where sediment has formed on top of the GRB, it varies in thickness from zero to cm of silty to fine-sandy material, with occasional diatomaceous sediment.
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Noble gas geochronology
For basaltic rocks older than about 30 Ma, the dating of plagioclase separates is We also show that for some continental flood basalts, even wt% of sericite.
The mechanism of continental growth and tectonic evolution of the CAOB remain debated. The MORB-like rocks were formed in a fore-arc setting, but the OIB-like mafic rocks were formed by the intra-plate magmatism related to mantle plume activities. The Early Paleozoic intrusions are occurred as small stocks with a dominant composition of diorite, trondhjemite, and granodiorite. Their protoliths are calc-alkaline andesite and tholeiitic and alkaline basalts, formed in an arc related and seamount setting, respectively.
It is clearly show that the West Junggar was under an intra-oceanic subduction regime during the Early Paleozoic, and the initial subduction of the southern PAO might have occurred in the Early Cambrian. Based on our observations, and in combination with previous work, we propose the plume-induced subduction initiation model for the Early-Middle Cambrian tectonic evolutionary of the Junggar Ocean. Our new model not only shed light on subduction initiation dynamics of the southern PAO, but also contribute to tectonic evolution of the CAOB.
New Technique for Dating Volcanic Rocks
However, a causative relationship between volcanism and warming remains speculative, as the timing and tempo of CRBG eruptions is not well known.
axes but with points opposite those of Uncertainties in Age Dating the prevailing The radiogenic argon and helium contents of three basalts erupted all striae.
Intellectual merit. Continental and oceanic basaltic extrusive rocks are the most common volcanic rock types on the earth’s surface and their temporal and spatial evolution are critical for the understanding of plate tectonics, mantle melting processes, paleomagnetism, continental flood basalt provinces, etc. At the same time, basaltic rocks, especially when aphanitic and altered, are often difficult to date.
Magnetite Fe3O4 is found in nearly all types of extrusive rocks and common in basaltic to intermediate volcanic rock types. We anticipate that this approach and results from the proposed case studies should be of significant interest to a large and diverse portion of the geosciences community interested in the continental and oceanic realm. This project will rigorously develop and calibrate the dating of magnetite and explore its geological application to both continental and oceanic basaltic rocks.
Deep-ocean basalts: inert gas content and uncertainties in age dating.
We report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon K—Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of Stepwise thermal and AF demagnetization generally isolated a high temperature component HTC of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component LTC in some samples. Rock magnetic analysis identifies fine-grained pseudo-single domain PSD magnetite and titanomagnetite as primary carriers of the remanence.
Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul
Dating young basalts with a combination of cosmogenic 3He and 21Ne (3Hec and 21Nec) age-pairs is a useful method (Fenton et al., ). Both nuclides are.
A new technique makes it possible to determine reliable ages for some very young volcanic rocks, Jet Propulsion Laboratory geologist told the American Geophysical Union meeting in Philadelphia today. Alan R. Gillespie said he has dated basaltic flow that erupted , years ago. The lava flow, at Sawmill Canyon on the east slope of California’s Sierra Nevada mountains, forced its way through the million-year-old granite of the Sierra. Determining accurate dates for recent geologic events will allow geologists to sort out the complex climatic and faulting history of the largest single mountain range in the continental United States.
Gillespie says his ,year-old lava flow lies beneath moraines from two of the major glacial periods of the Sierra — the recent Tioga and the earlier Tahoe. That, he says, puts an older limit on the the Tahoe glaciation it can be no older than , years , which has been the subject of considerable controversy among geologists. Gillespie’s results confirm that the Tahoe glaciation probably occurred during the last major ice age in North America and Europe — the Wisconsin glaciation. He has also dated — at , years old — another lava flow that lies beneath yet-older glacial moraine in the same Sawmill Canyon.
That ,year date argues for the presence of previously undated glacial period that occurred between the Tahoe and the still earlier Sherwin period. The importance of recognizing and dating individual glacial periods in the Sierra is that they can then be related to the major glaciers that swept much of the United States, putting accurate dates to those events. By understanding the chronology of the ice ages in the past, scientists hope to better understand and predict climatic trends in the future.