低硅碱性岩浆的起源和构造背景

IF 6 2区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY
Qiushi Zhou, Rui Wang
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引用次数: 0

摘要

儘管鹼性火成岩在地球上較為罕見,但由於其礦物組合和地球化學成分的差異,使鹼性火成岩成為火成岩中最多元化的一類。鹼性火成岩的一般特徵是矽不飽和及鉀-鈉(在某些情況下)富集。虽然没有统一的碱性火成岩分类方案,但总碱量-SiO2 或 K2O-SiO2 等图表可用于对碱性火成岩进行分类。硅饱和碱性岩(SiO2>52 wt.%)的成因相对复杂,因为往往涉及多期岩浆活动,因此大多数研究集中于硅不饱和碱性岩浆的成因。在过去几十年中,进行了大量的岩石学实验研究。最初,人们认为痕量元素的富集只能通过地幔橄榄岩的低度部分熔化来实现,但实验熔体无法再现天然碱性火成岩的地球化学成分。随后的研究主要集中在含碳酸盐的蚀变岩上,这些蚀变岩代表了俯冲洋壳的平均成分。虽然实验研究表明,硅不饱和熔体和碱过饱和熔体可从斜长岩中生成,但一些研究认为,天然硅不饱和碱性火成岩是不相容元素多级源富集的结果。地幔的低度部分熔体可在岩石圈地幔中沉积,形成由辉石、闪石和辉石组成的矿脉。包含这些成分的熔融实验表明,生成的熔体能更好地再现自然形成的硅不饱和碱性岩浆。缺硅碱性火成岩出现在不同的构造环境中。板块内和板块分异环境(即主要是大陆裂谷)中的碱性火成岩通常由地幔过渡带(MTZ)的俯冲板块或变质岩石圈地幔等再循环成分造成。在汇聚板块边界,硅不饱和碱性岩浆可通过熔融镁质岩浆或在弧后深度集中分解辉绿岩而获得。我们汇编了碱性火成岩的全球数据,发现大陆碰撞带的硅不饱和碱性火成岩富含钾,与其他构造环境中的碱性火成岩不同。我们认为,俯冲大陆沉积物是一个重要的富钾末端成分,为碰撞带的碱性岩浆提供了大量的钾。此外,俯冲板块中富钾矿物的固结意味着钾只能参与俯冲带的岩浆活动。在板块深度达到300千米之前,云母族矿物、钾长石、蒸发矿物等富钾矿物逐渐被消耗,很少参与深部过程。与此相反,辉石等相对富含Na的矿物可随蚀变洋壳进入地幔深部,由此产生的熔体可通过对流地幔流进入岩石圈。这就在岩石圈地幔中形成了闪石等变质矿物,这些矿物熔化后生成富含Na和低硅的碱性岩浆。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Origin and tectonic setting of low-Si alkaline magma

Despite of the relatively rare occurrence of alkaline igneous rocks on Earth, they are the most diverse group of igneous rocks due to variations in their mineral assemblages and geochemical compositions. Alkaline igneous rocks are generally characterized by Si undersaturation, and K-Na (and in some cases C) enrichment. Although there is no unified alkaline igneous rock classification scheme, diagrams such as total alkalis-SiO2 or K2O-SiO2 can be used to classify alkaline igneous rocks. The origins of silica-saturated alkaline rocks (SiO2>52 wt.%) are relatively complex because multiple episodes of magmatism are often involved, therefore most studies have focused on the origins of silica-undersaturated alkaline magmas. Numerous experimental petrological studies have been conducted in the past few decades. Initially, it was considered that enrichment of trace elements was only achieved by low-degree partial melting of mantle peridotite, but the experimental melts could not reproduce the geochemical composition of natural alkaline igneous rocks. Subsequent studies have focused on carbonate-bearing eclogites that represent the average component of subducted oceanic crust. Although experimental studies indicate that silica-undersaturated and alkali-oversaturated melts can be generated from eclogites, some studies have considered that natural silica-undersaturated alkaline igneous rocks are the result of multi-stage source enrichment by incompatible elements. Low-degree partial melts of the mantle can be emplaced in the lithospheric mantle, forming veins consisting of phlogopite, amphibole, and pyroxene. Melting experiments including these components show that the produced melts better reproduce the naturally occurring silica-undersaturated alkaline magmas. Silica-deficient alkaline igneous rocks occur in various tectonic settings. Those in intraplate and divergent settings (i.e., mainly continental rifts) normally involve contributions from recycled components such as subducted slabs at the mantle transition zone (MTZ) or metasomatized lithospheric mantle. At convergent plate boundaries, silica-undersaturated alkaline magmas can be derived by the melting of mélange or the focused breakdown of phlogopite at back-arc depths. We compiled global data for alkaline igneous rocks and discovered that silica-undersaturated alkaline igneous rocks in continental collisional zones are K-rich and differ from those from other tectonic settings. We suggest that subducted continental sediment is an important K-rich end-member, which contributes a large amount of K to the alkaline magmas in the collision zone. Moreover, the solidus of K-rich minerals in the subducted plate implies that K can only participate in magmatism in the subduction zone. Before the plate reaches a depth of ∼300 km, the mica-group minerals, K-feldspar, evaporitic minerals, and other K-rich minerals are gradually consumed and rarely participate in deeper processes. In contrast, relatively Na-rich minerals such as pyroxene can be transported into the deeper mantle with eclogitic oceanic crust, and the resultant melt can enter the lithosphere owing to convective mantle flow. This forms metasomatic minerals, such as amphibole, in the lithospheric mantle, which melt to generate Na-rich and low-Si alkaline magmas.

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来源期刊
Science China Earth Sciences
Science China Earth Sciences GEOSCIENCES, MULTIDISCIPLINARY-
CiteScore
9.60
自引率
5.30%
发文量
135
审稿时长
3-8 weeks
期刊介绍: Science China Earth Sciences, an academic journal cosponsored by the Chinese Academy of Sciences and the National Natural Science Foundation of China, and published by Science China Press, is committed to publishing high-quality, original results in both basic and applied research.
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