Subaqueous volcanism in the Paleo-Pacific Ocean based on Jurassic basaltic tuff and pillow basalt in the Raohe Complex, NE China
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Pillow lava
Drilling on Leg 29 recovered basalts at five sites.Three of the basalts were from pillow flows, one from a massive lava flow, and one from an intrusion.No major differences between the rocks from the five sites are found in major oxide, normative mineralogical, and minor element composition.However, there are minor differences of unknown significance between the sites close to New Zealand and the sites close to Australia.Chemically, the Leg 29 basalts are intermediate in composition between oceanic tholeiites and oceanic alkalic basalts.
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THE plateau-lavas of Antrim cover an area of approximately 1,550 square miles—the largest area of Tertiary lavas in the British Isles. Although from the earliest daysof geological science these lavas have attracted many observers, very little has been done in the way of petrological study and, curiously enough, the dominant olivine-basalt lava has never been analysed.
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Andesites
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Mio-Pliocene Chilcotin Group basalt flows form a 50 000 km 2 lava plateau in south-central British Columbia. Two periods of eruptive activity at 2–3 and 6–10 Ma are indicated by a compilation of available age data, including 10 new K–Ar age determinations, and basalts from these two periods are chemically indistinguishable. The Chilcotin Group consists of thin, crudely columnar-jointed pahoehoe flows, some thick, tiered flows, pillow lava and pillow breccia, and rare silicic tephra layers. The presence of many vesicle sheets and cylinders and collapsed pahoehoe toes suggests that the basalts were volatile-rich. Known vents (gabbro and basalt plugs) for the basalt flows form a northwest trend along the axis of the lava plateau. The plateau appears to have formed from the overlap of many low-profile shield volcanoes and is similar in morphology to other plains basalts such as the Snake River Plain and parts of Iceland. Glaciation has stripped off an unknown volume of the flat-lying basalt flows.
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A basaltic tuff formation (Upper Basaltic Tuff of the Janggi Group) occurs in close association with basalt (Yeonil Basalt) at the Tertiary Janggi basin. The purpose of this paper is to describe the occurrence of the basaltic tuff and associated basalt and to determine their mode of formation. The basaltic rocks of the study area show four distinct lithofacies, all of which are originated from the interaction of basaltic magma with external water. The four lithofacies include (l) sideromelane shard hyaloclastite, (2) pillow breccia, (3) entablature-jointed basalt, and (4) in-situ breccia. The sideromelane shard hyaloclastite constitutes most of the Upper Basaltic Tuff and has a gradual contact with the pillow breccia. The pillow breccia consists of a poorly sorted mixture of isolated and broken pillows, and small basalt globules and fragments engulfed in a volcanic matrix of sideromelane shard hyaloclastite. The entablature-jointed basalt occurs as a small body within the hyaloclastite. It is characterized by irregularly-curved joints known as entablature. The in-situ breccia occurs as a marginal facies of entablature-jointed basalt, and its width varies from 10 to 30m. The result of this study indicates that the basaltic tuff and associated basalts of the study area were produced by the volcanic activity of same period and the basaltic tuff was formed by subaqueous eruption of basaltic lava followed by nonexplosive quench fragmentation.
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The retreat of Lillooet Glacier (LG) has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite. The lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow-dominated tindar. Chemically, the LG volcanic rocks are basalt to basaltic andesite, and, as a group, they represent the highest-silica, Quaternary mafic products in the Garibaldi volcanic belt (GVB). Like other northern GVB (alkaline) basalts, they lack the Nb–Ta depletion signature typically associated with subduction-related products. Geochemical and petrologic analysis indicates that the LG basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage: olivine + plagioclase. Thermodynamic modeling establishes shallow, crustal, pre-eruptive storage conditions at <2 kbar (1 kbar = 100 MPa; or 7.5 km) and an H 2 O content of 0.5–1 wt.%. We estimate that the LG basalts were erupted at the peak of, or during the waning stages of, Fraser glaciation (17–13 ka). The eruption produced an englacial lake that was >150 m deep and that appears to have been sustained throughout the entire eruption (i.e., no discernible passage zone). Using hydrostatic constraints, we calculate a minimum overlying paleo-ice thickness of >645 m and a paleo-ice surface elevation of >1895 m above sea level.
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These first measurements of specific surface area (SSA) of bulk samples of subsurface marine basalts were undertaken to determine the potential area available for microbial colonization. SSA ranged from 0.3 to 52 m 2 /g of basalt with the lowest value coming from pillow basalt and the highest value from breccia. The average for massive and pillow basalts combined was 2.3 m 2 /g. The total specific surface area of the extrusive volcanic rocks of the ocean crust is estimated to be 10 24 m 2 . This surface area could provide attachment for up to 10 34 cells if cell density is the same as that of experimentally colonized basalt surfaces. Independent measures and calculations of biomass in basalts suggest that cell densities on surfaces are only 10 −4 times those in laboratory experiments and, therefore, the surface area of basalt does not limit microbial biomass in the igneous ocean crust.
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