Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii
Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii
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夏威夷欧胡岛 Trans-Koolau 隧道玄武岩熔岩流和岩脉的地质和地球化学
DOI：
10.1007/s004450050239
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发表时间：
1999
影响因子：
3.5
通讯作者：
R. Wilmoth
中科院分区：
文献类型：
--
作者：
M. C. Jackson;F. Frey;M. Garcia;R. Wilmoth
摘要在长1.6公里的跨库劳(T-K)隧道中，采集了一段200米长的库劳玄武岩样品。该段包括126个AA和Pahoehoe熔岩流，5个岩脉和10个薄火山灰单元。这段火山和熔岩流的物理特征表明，它来自附近的库劳地盾西北裂谷带。据推测，该剖面的顶部位于库劳屏蔽层侵蚀前表面之下500-600米处。因此，与以前研究的库劳熔岩相比，这一部分提供了更深层次的、可能更古老的屏蔽层样本。来自库劳火山的地盾熔岩定义了夏威夷地盾的地球化学终端成员。大多数隧道熔岩具有库劳熔岩独特的常量和微量元素丰度特征(如相对较高的SiO_2含量和Zr/Nb丰度比)。此外，与基拉韦厄火山和莫纳罗亚火山最近喷发的屏蔽熔岩相比，大多数库劳熔岩在给定的氧化镁含量下具有较低的Sc、Y和Yb丰度；这一结果与残余石榴石在形成库劳屏蔽熔岩的部分熔融过程中所起的更重要作用是一致的。残留石榴石特征最强的库劳熔岩具有较高的87Sr/86Sr、187Os/188Os、18O/16O和较低的143Nd/144Nd。这些同位素特征以前被解释为反映了被重结晶为石榴石辉石岩的再循环洋壳的来源成分。该组分还具有较高的La/Nb和相对较低的206Pb204Pb值，其地球化学特征可归因于再循环地壳中的古代远洋沉积物。尽管大多数库劳熔岩定义了夏威夷盾状熔岩的地球化学端元，但据推测，存在相当大的盾内地球化学变异性，以反映源区特征。最古老的T-K隧道熔岩流就是一个例子。库劳熔岩具有最低的~(87)Sr/~(86)Sr、最低的Zr/Nb和最低的La/Nb，最高的~(143)Nd/~(144)Nd比值。在大多数方面，它与基拉韦厄火山的熔岩相似。因此，确定夏威夷地盾端元的库劳地盾的地球化学特征反映了循环洋壳的重要作用，但在库劳地盾的生长过程中，这种结壳在源区中的比例是不同的。
Abstract A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T–K) tunnel. The section includes 126 aa and pahoehoe lava flows, five dikes and ten thin ash units. This volcanic section and the physical characteristics of the lava flows indicate derivation from the nearby northwest rift zone of the Koolau shield. The top of the section is inferred to be 500–600 m below the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides a deeper, presumably older, sampling of the shield. Shield lavas from Koolau Volcano define a geochemical end-member for Hawaiian shields. Most of the tunnel lavas have the distinctive major and trace element abundance features (e.g. relatively high SiO2 content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y and Yb at a given MgO content; this result is consistent with a more important role for residual garnet during the partial melting processes that created Koolau shield lavas. Koolau lavas with the strongest residual garnet signature have relatively high 87Sr/86Sr, 187Os/188Os, 18O/16O, and low 143Nd/144Nd. These isotopic characteristics have been previously interpreted to reflect a source component of recycled oceanic crust that was recrystallized to garnet pyroxenite. This component also has high La/Nb and relatively low 206Pb/204Pb, geochemical characteristics which are attributed to ancient pelagic sediment in the recycled crust. Although most Koolau lavas define a geochemical endmember for Hawaiian shield lavas, there is considerable intrashield geochemical variability that is inferred to reflect source characteristics. The oldest T–K tunnel lava flow is an example. It has the lowest 87Sr/86Sr, Zr/Nb and La/Nb, and the highest 143Nd/144Nd ratio found in Koolau lavas. In most respects it is similar to lavas from Kilauea Volcano. Therefore, the geochemical characteristics of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust, but the proportion of this crust in the source varied during growth of the Koolau shield.