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RISS 인기검색어
- 검색키워드
- 저자 : LeMasurier, Wesley E. (검색결과 2 건)
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LeMasurier, Wesley E.,Choi, Sung Hi,Hart, Stanley R.,Mukasa, Sam,Rogers, Nick Elsevier 2016 Lithos Vol.260 No.-
<P><B>Abstract</B></P> <P>Basalt-trachyte volcanoes in the Marie Byrd Land (MBL) Cenozoic province lie along the Amundsen Sea coast on the north flank of the West Antarctic rift. Basalts here are characterized by OIB-like geochemistry, restricted ranges of <SUP>87</SUP>Sr/<SUP>86</SUP>Sr (0.702535–0.703284) and <SUP>143</SUP>Nd/<SUP>144</SUP>Nd (0.512839–0.513008) and a wide range of <SUP>206</SUP>Pb/<SUP>204</SUP>Pb (19.357–20.934).</P> <P>Basalts at three MBL volcanoes display two anomalies compared with the above and with all other basalts in West Antarctica. They include <SUP>143</SUP>Nd/<SUP>144</SUP>Nd (0.512778–0.512789) values at Mt. Takahe and Mt. Siple that are 2σ lower than other West Antarctic basalts, and Ba/Nb, Ba/La, and Ba/Th values at Mt. Murphy and Mt. Takahe that are 3–8 times higher than normal OIB. Isotope and trace element data do not support crustal and lithospheric mantle contamination, or the presence of residual mantle amphibole or phlogopite as explanations of these anomalies.</P> <P>The apparent coincidence of these anomalies with the site of a pre-Cenozoic convergence zone along the Gondwanaland margin suggests a subduction influence. Major episodes of subduction and granitic plutonism took place in MBL during the Devonian, Permian, and Late Cretaceous. Relicts in the source region, of components from these subducted slabs, provide a credible explanation for the uncoupling of Ba from other large ion lithophile elements (LILE), for its erratic distribution, and for the anomalously low <SUP>143</SUP>Nd/<SUP>144</SUP>Nd at Mt. Takahe.</P> <P>The last episode of subduction ended ~85Ma, and was followed by continental break-up, rifting and lithospheric attenuation that produced the West Antarctic rift as we know it today. Thus, the enigmatic geochemical signatures in these three volcanoes seem to have been preserved roughly 61–85m.y. after subduction ended. New calculations of source melting depth and a new determination of lithospheric thickness suggest that the source of the anomalies resides in a fossil mélange diapir that rose from the Cretaceous subducting slab, became attached to the base of the lithosphere at 80–100km depth, and remained there during the subsequent plate motion and source remobilization history of this region.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Three MBL volcanoes display Ba and Nd isotope anomalies unique to West Antarctica. </LI> <LI> Neither lithospheric contamination, nor mantle amphibole explains the anomalies. </LI> <LI> Residual subduction component contamination provides the best explanation. </LI> <LI> Subduction ended ~85Ma followed by formation of the West Antarctic rift. </LI> <LI> The anomaly source is a fossil mantle diapir attached to the base of the lithosphere. </LI> </UL> </P>
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LeMasurier, Wesley,Choi, Sung Hi,Kawachi, Yosuke,Mukasa, Sam,Rogers, Nick Elsevier 2018 Lithos Vol.296 No.-
<P><B>Abstract</B></P> <P>Mt. Takahe is a large, late Quaternary trachyte shield volcano that rises through 2000+m of the West Antarctic ice sheet. It is composed mostly of <I>ne</I>-trachyte, <I>hy</I>-<I>ol</I>-trachyte, and <I>qz</I>-trachyte flows, with subordinate basanite, intermediate rocks, and pantellerites.</P> <P>All rock types can be adequately modeled by fractional crystallization of basanite - the only basaltic rock exposed here. The <I>ne</I>-trachytes can be explained by a single stage of low-pressure fractionation near the base of the upper crust. Models of oversaturated rocks require a period of evolution at a depth of ~35km, below the stability field of plagioclase, where fractionation of kaersutite and associated high pressure minerals will yield silica oversaturated residual magmas. This is then followed by a period of fractionation at a depth of ~3km, where peralkalinity and Fe-enrichment are acquired. Pantellerite compositions span virtually the entire spectrum of peralkalinity, Fe-enrichment, LILE–enrichment, and SiO<SUB>2</SUB> values, and seem to represent a range of residence times in upper crustal vs., upper mantle magma chambers.</P> <P>Mt. Takahe is unusual among Marie Byrd Land volcanoes for its geochemical anomalies. These include the lowest <SUP>143</SUP>Nd/<SUP>144</SUP>Nd ratios in West Antarctica, and unusually high but unpredictable Ba values. These anomalies are believed to originate in a pre-85Ma subduction mélange at the base of the lithosphere, which seems to be the source of Mt. Takahe basaltic rocks.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Extreme pantellerite compositions reflect upper mantle vs. upper crust residence. </LI> <LI> Geochemical anomalies reflect a paleosubduction signature. </LI> <LI> Wide range of oversaturated & undersaturated felsic rocks reflects complex plumbing. </LI> <LI> Multi-storied plumbing created by stationary plate over mantle plume for 20my </LI> </UL> </P>
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