Primary Magmas at the Volcanic Front of the NE Japan Arc : Coeval Eruption of Crustal Low-K Tholeiitic and Mantle-derived Medium-K Calc-Alkaline Basalts at Azuma Volcano
Primary Magmas at the Volcanic Front of the NE Japan Arc : Coeval Eruption of Crustal Low-K Tholeiitic and Mantle-derived Medium-K Calc-Alkaline Basalts at Azuma Volcano
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日本东北弧火山前缘的原生岩浆：东火山地壳低钾拉斑岩和地幔衍生的中钾钙碱性玄武岩同时喷发
DOI：
10.1093/petrology/egs065
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发表时间：
2012
影响因子：
3.9
通讯作者：
Yoshiyuki Tatsumi
中科院分区：
文献类型：
--
作者：
Toshiro Takahashi;Yuka Hirahara;Takashi Miyazaki;Ryoko Senda;Qing Chang;Jun-Ichi Kimura;Yoshiyuki Tatsumi
长期以来，人们一直认为日本东北弧的低钾拉斑玄武岩（TH）和中钾钙碱性（CA）熔岩是由幔源玄武岩岩浆分离结晶而成，后者是由同一原始TH玄武岩经分离而成的镁铁质和长英质岩浆混合而成。根据对枣火山斜长石斑晶的Sr同位素微区分析，提出了另一种观点：（1）低K TH玄武安山岩是由下地壳角闪岩熔融形成的;（2）中K CA玄武岩-安山岩是由幔源玄武岩和地壳TH熔融混合形成的。为了进一步研究“原生”低K TH和中K CA玄武岩的成因，我们调查了东火山的玄武岩和安山岩。东是东北日本弧火山前缘的第四纪喷发中心，喷发出两种类型的玄武岩：（1）放射成因Sr（87 Sr/86 Sr = 0·7058-0·7062）低K TH玄武岩，无岩浆混合同化作用;（2）非放射成因Sr（87 Sr/86 Sr = 0·7039-0·7041）中钾CA玄武岩，具有岩浆混合的证据。伴生的中间熔岩体积庞大，都是（3）中等放射性Sr（87 Sr/86 Sr = 0·7044-0·7055）中钾安山岩，它们都具有CA亲合性，有严格岩浆混合的证据，但没有地壳同化作用。低K TH玄武岩的同位素组成与东下地壳花岗岩类的同位素组成相似，其组成表明它可能是由高度下地壳角闪岩熔体形成的。中钾CA玄武岩的基块为玄武质，含富镁橄榄石（Fo 89）和钙质斜长石斑晶（An 90），Sr（87 Sr/86 Sr = 0·7037-0·7038）为非放射成因，表明其起源于原始地幔熔体。玄武质基底的主量元素和微量元素分析表明，原始岩浆成分接近高钾。我们的结论是，幔源玄武岩在东是一个高到中等K岩浆，后来与低K TH玄武岩熔体从角闪岩下地壳混合，形成中等K CA玄武岩和安山岩的结果。这支持了低K TH玄武岩的下地壳起源的观点，同时需要重新评估的起源的跨弧变化的钾含量的幔源原生弧玄武岩，高至中K CA玄武岩的地球化学相当类似的高K弧后玄武岩在东北日本弧。
It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derived from a common primary TH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the ‘primary’ low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (87Sr/86Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (87Sr/86Sr = 0·7039–0·7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (87Sr/86Sr = 0·7044–0·7055) medium-K andesites, all of which have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneath Azuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-K CA basalt has a basaltic groundmass with Mg-rich olivine (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (87Sr/86Sr = 0·7037–0·7038), suggesting that it originated from a primary mantle melt. Major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-K TH basalts and simultaneously requires a reappraisal of the origin of the across-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc.