Devonian ignimbrites of central Victoria: explosive magmas from multiple sources, and deep crustal structure in the Selwyn Block

IF 1.2 4区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
J. Clemens
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引用次数: 0

Abstract

Abstract The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. KEY POINTS Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas.
维多利亚中部泥盆纪熔结凝灰岩:来自多个来源的爆炸性岩浆和Selwyn地块的深部地壳结构
摘要在维多利亚州中部形成每个泥盆纪S型火山杂岩的岩浆是通过地壳源区不同岩石类型组合的部分熔融而产生的。更多的硅化流纹岩是独立于流纹岩-英安质岩浆形成的,而不是不同的。在每个复合体中,每个熔结凝灰岩单元的化学和同位素特征也是不均匀的。因此,在澳大利亚古生代的这一地区,岩浆异质性至少在三个层面上发生——单个岩石单元、复合体和整个地区。这意味着,作为熔结凝灰岩岩浆的逻辑来源的Selwyn地块可能在三维上具有高度的不均匀性。各种杂岩年龄的统计显著差异表明,深地壳部分熔融的热源本身在一定程度上是局部的,可能反映了地幔衍生的镁铁质岩浆的大量异步侵入,很可能是相对小批量的。因此,在这里的地壳深处不应该有厚的辉长岩底层或板内层。较薄的热镁铁质岩浆体可能侵位在不同的地方和不同的水平,引发了局部高级变质条件。已公布的各种火山单元的辐射测量日期表明,地壳的岩浆加热发生在至少17 Myr的时期,尽管最大强度发生在约375至约371之间 马。因此,这里的深地壳模型应该包括许多可能很薄的变质沉积单元,这些单元具有高度可变的年龄和原始粘土含量,以及垂直和水平分散并偶尔侵入的晚泥盆纪镁铁质岩石的岩床。硬壳衍生的I型岩浆的主要变火成岩来源通常位于较高的结构水平,在那里它们没有经历足以引起形成I型火山岩浆所需的高温熔融反应的变质温度。关键点维多利亚州中部的每个晚泥盆纪S型火山岩浆都是通过不同的地壳源岩组合的熔融形成的,很可能是在塞尔温地块内。流纹岩独立于流纹岩-英安质岩浆形成,而不是区别。深部地壳部分熔融的镁铁质岩浆热源以大量、相对小批量、水平和垂直分布的片状侵位。I型岩浆的主要变质火成岩和浅地壳来源没有经历形成I型火山岩浆所需的更高温度。
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来源期刊
Australian Journal of Earth Sciences
Australian Journal of Earth Sciences 地学-地球科学综合
CiteScore
2.80
自引率
8.30%
发文量
45
审稿时长
6-12 weeks
期刊介绍: Australian Journal of Earth Sciences publishes peer-reviewed research papers as well as significant review articles of general interest to geoscientists. The Journal covers the whole field of earth science including basin studies, regional geophysical studies and metallogeny. There is usually a thematic issue each year featuring a selection of papers on a particular area of earth science. Shorter papers are encouraged and are given priority in publication. Critical discussion of recently published papers is also encouraged.
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