Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic Craton
Hafnium isotope constraints on the origin of Mesoarchaean andesites in southern West Greenland, North Atlantic Craton
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DOI：
10.1144/sp449.2
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发表时间：
2016-09
期刊：
影响因子：
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通讯作者：
K. Szilas;J. Tusch;J. Hoffmann;A. Garde;C. Münker
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文献类型：
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作者：
K. Szilas;J. Tusch;J. Hoffmann;A. Garde;C. Münker
摘要西格陵兰南部的许多表壳岩带都有浅色斜长角闪岩，它们通常保存着火山岩结构，被解释为变质安山岩。这些岩石与拉斑玄武岩组合物的中斜斜长角闪岩有关，显示枕状熔岩结构，因此支持在海洋环境中喷发。在这里，我们提出了大块岩石Lu-Hf同位素数据的变质安山岩约3071马Qussuk表壳带。令人惊讶的是，我们发现的证据参与源近变质安山岩的Hf同位素组成，而变质玄武岩显示更多的亏损组合物，约+4。微量元素模拟表明，结合地壳同化（AFC）的分离结晶是不能够生产的地球化学成分的变质安山岩从玄武岩熔体。相反，这些变安山岩指向大程度（c。50%）的岩浆混合，涉及镁铁质和长英质端元。这可能表示：（1）岩浆房作用;（2）地幔楔上有地幔成分的叠加;（3）原始镁铁质地壳的大程度熔融。考虑到西格陵兰南部的西太平洋地壳中存在大量独立的水平构造和变质证据，这些钙碱性变安山岩和拉斑玄武岩很可能是由中太古代俯冲带火山作用产生的。
Abstract Numerous supracrustal belts in southern West Greenland host leucoamphibolites, which commonly preserve volcaniclastic textures, and are interpreted as meta-andesites. Such rocks are associated with mesocratic amphibolites of tholeiitic basaltic compositions, which display pillow-lava structures and, thus, support eruption in an oceanic environment. Here we present bulk-rock Lu–Hf isotope data for meta-andesites from the approximately 3071 Ma Qussuk supracrustal belt. Surprisingly, we find evidence for the involvement of a source with near-chondritic Hf-isotope composition in the meta-andesites, whereas the metabasalts display more depleted compositions, with around +4. Trace element modelling indicates that fractional crystallization in combination with crustal assimilation (AFC) is not capable of producing the geochemical compositions of the meta-andesitic rocks from a basaltic melt. Instead, these meta-andesites point to large degrees (c. 50%) of magma mixing, involving mafic and felsic end members. This may either represent: (1) a magma chamber process; (2) mantle-wedge overprinting by a silicic component; or (3) large degrees of melting of primitive mafic crust. Given that there is abundant independent structural and metamorphic evidence for horizontal tectonics in the Archaean crust of southern West Greenland, it is likely that these calc-alkaline meta-andesites and tholeiitic metabasalts were produced by Mesoarchaean subduction zone volcanism.