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首页> 外文期刊>Contributions to Mineralogy and Petrology >Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs
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Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

机译：在哥斯达黎加西北部产生化学上独特的硅质岩浆的岩浆作用过程和海洋弧中幼年大陆壳的演化

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Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO2, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.
机译：哥斯达黎加西北部建立在一个海洋高原上，该高原已发展出上陆壳的化学和地球物理特征。将海洋高原转变为大陆壳的一个主要因素是硅质岩浆的产生，演化和沉积。在哥斯达黎加，加勒比大火成岩省（CLIP）构成了科科斯板块俯冲作用中的压倒性板块，这一过程至少持续了25多年。哥斯达黎加的火成岩年龄大于8 Ma，具有典型的化学成分，这些化学成分是大洋洲玄武岩和洋弧内部的典型特征。相反，年轻的火成岩沉积物含有丰富的硅质岩，其中富含SiO 2 ，碱和轻稀土元素，并且在地球化学上与平均上陆壳相似。地球物理证据（高Vp地震速度）也表明相对较厚（〜40 km），并且在CLIP中增加了演化的火成岩。西北哥斯达黎加的硅质矿床有两个主要的组成组：高钛和低钛组，两者之间没有重叠。这些基团的主要元素和微量元素特征与这些岩浆来自结晶麝香中提取的液体一致，这些糊状酒是通过结晶或通过其固相附近p的部分熔融产生的。相对而言，高Ti硅质液体是从具有低氧逸度的干热晶体糊状物中提取的，其中斜长石和辉石是主要的结晶相，而角闪石的含量也较少。相比之下，低Ti硅质液体是从具有高氧逸度的凉爽，湿润的晶体糊状物中提取的，其中斜长石和闪石是结晶的主要相。减少热干的岩浆在较旧的序列中占主导地位，但是最年轻的序列仅包含冷湿氧化组的岩浆。来自其他海洋弧线的硅质火山岩沉积物（例如伊豆邦宁，玛丽亚纳斯）的化学特征与大陆壳明显不同，而哥斯达黎加西北部硅质火山岩的化学特征几乎与上部大陆壳相同。哥斯达黎加西北部从镁铁质洋弧和海洋内岩浆向长陆大陆壳型岩浆的长英质过渡是由其他几个弧形环境中可能不存在的几个重要因素共同决定的：（1）厚层的热成熟加勒比高原，（2）区域或局部地壳扩展，以及（3）建立上​​地壳储层。

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