Petrology of picritic basalts from Kamloops, British Columbia; primary liquids from a Triassic-Jurassic arc
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ABSTRACT The mid-Miocene Strawberry volcanic field of northeastern Oregon is an example of intracontinental flood volcanism that produced lavas of both tholeiitic and calcalkaline compositions derived by open-system processes. Until now, these dominantly calc-alkaline lavas have not been considered to have a petrogenetic origin similar to that of the flood basalts of the Pacific Northwest because of their calc-alkaline composition. These lavas are situated in between and co-erupted with the dominant volcanic field of the Columbia River Basalt Group (CRBG). Due to the timing, location, and diversity of these erupted units, the Strawberry Volcanics may hold valuable information about the role of crustal modification during large magmatic events such as hotspot volcanism. The earliest eruptions of the Strawberry Volcanics began at 16.2 Ma and appear continuous to 15.3 Ma, characterized by low-silica rhyolite. High-silica, A-type rhyolite eruptions followed at 15.3 Ma. The silicic eruptions continued until 14.6 Ma, with an estimated total volume up to ~100 km3. The first eruptions of the intermediate lava flows occurred at 15.6 Ma and continued with both tholeiitic and calc-alkaline, and transitional, lavas until 12.5 Ma. Volume estimates of the intermediate lavas are ~1100 km3. The mafic lavas are sparse (~2% of total volume) and are distributed throughout the upper sequences, and they appear to be near last to arrive at the surface. Herein, we show that the Strawberry Volcanics are not only related in time and space to the Columbia River Basalt, but they also share some chemical traits, specifically to the Steens Basalt. Evidence of this similarity includes: overlapping normalized incompatible trace-element patterns, selected trace-element ratios, and radiogenic isotopes. Furthermore, we compared the Strawberry rhyolites to the other mid-Miocene rhyolites of eastern Oregon associated with the inception of the Yellowstone–Snake River Plain hotspot and found overlapping eruption ages, trace and rare earth element compositions, and “A-type” rhyolite characteristics. This research concludes that the Strawberry Volcanics were part of the regional basalt to rhyolite magmatism of the Yellowstone–Snake River Plain hotspot.
Flood basalt
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Petrogenesis
Geochronology
Isotope Geochemistry
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A clinopyroxene–basalt geothermobarometry perspective of Columbia Plateau (NW‐USA) Miocene magmatism
Abstract The origin of NW‐USA Columbia River Basalt Group Miocene magmatism and its relation to tectonism has been widely debated and is still open to study. We investigated the pre‐eruptive evolution of the magmas, to constrain pressures and temperatures of the ascending magmas, and plumbing conditions. We determined major element concentrations of 17–6 Ma tholeiites, and applied clinopyroxene – liquid geothermobarometry to calculate pre‐eruptive pressures and temperatures. These ranged from 0 to 0.66 GPa and 1120 to 1222 °C, respectively, defining two age‐related parallel trends in a P–T diagram. This indicates a consistent crustal evolution of the magmas, and records at least two distinct initial temperatures. Using clinopyroxene interdiffusion coefficients we estimated magma ascent speeds ≥ 0.6 km yr −1 . Possible geological explanations for the calculated parameters are: lower‐crust magma chamber processes; magmatism and tectonism feed‐back consistent with an extensional environment.
Geothermobarometry
Magma chamber
Orogeny
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Distribution of and geological relationships amongst Middle Jurassic, Middle and Upper Jurassic, Mid- to Late Cretaceous, and Tertiary volcanic, sedimentary, and plutonic units are better known from bedrock mapping in the Hallett Lake (NTS 93F/15) area in 1997. Undivided Hazelton Group rocks include maroon-grey heterolithic and monolithic laharic boulder breccia, volcanic grit, and mudstone that are localized in northeast-trending fault blocks. Middle Jurassic Naglico Formation (?) clinopyroxene-phyric volcaniclastic rocks, laharic breccia and siliceous siltstone occur within one of these fault blocks. Eocene Ootsa Lake Group dacitic and rhyolitic crystal tuff, pyroclastic rocks, breccia, flow-layered lavas, vitrophyre, glassy domes, and rare, amygdaloidal, plagioclase-porphyritic basalt overlie various Mesozoic basement units. Eocene Endako Group clinopyroxene-plagioclase porphyry basaltic flow and flow breccia rocks conformably overlie the Ootsa Lake Group and with it, are gently tilted. Small- and large-scale, northeast-trending, down-to-the-southeast, synvolcanic faults apparently localized the distribution of Ootsa Lake Group and Endako Group.
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During the period 1983 to 1986, the large number of papers, books, and monographs published on the petrology and geochemistry of igneous rocks reflects the vitality and continued growth of this central earth science discipline. While an attempt has been made to prepare a reasonably complete bibliography, the accompanying text, of necessity, reflects a subjective selection of topics. If some papers have escaped my net I apologize and assure the authors that the omissions are inadvertent. A list of books and monographs is provided in the first section of the bibliography. Papers dealing with isotopic and/or trace elements studies on igneous rocks are included in the bibliography if these papers also contain a significant petrologic component. The review by R. Carlson in this volume contains a complete reference list on this topic.
Igneous petrology
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TRACE (psycholinguistics)
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The upper Paleozoic Fennell Formation in south-central British Columbia consists of basalts and associated marine sediments metamorphosed at low greenschist facies conditions. Although the microphenocrysts of plagioclase and Fe–Ti oxides are almost completely altered, those of augite and amphibole have survived this metamorphism. In the vicinity of the Chu Chua mineral deposit, relict augite microphenocrysts, which constitute a major proportion of the microphenocryst assemblage, are enriched in Al and Ti and are similar in composition to those from alkalic and transitional basalts. Relict amphiboles are also enriched in Ti (4.5–5.9% TiO 2 ) and are classified as kaersutites. The occurrence of kaersutite and the chemistry of relict augites indicate that in this area the Fennell Formation basalts were originally alkalic and transitional in composition. On conventional Ti–(Zr/P 2 O 5 ) and (Nb/Y)–(Zr/P 2 O 5 ) immobile-element discrimination diagrams, both the kaersutite-bearing and kaersutite-free rocks plot in the tholeiitic basalt field. Accordingly, it is suggested that these diagrams may not provide clear evidence for the magmatic composition of altered volcanic rocks.Based on the lead isotopic compositions, petrographic features, and alkalic character of the Fennell Formation basalts, it is interpreted that these basalts were formed in a tectonic setting similar to that of present-day oceanic islands or seamounts.
Greenschist
Amphibole
Incompatible element
Alkali basalt
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