全球洋岛和洋底高原海山中的中酸性岩——数据挖掘及其意义

Intermediate-acid rocks in seamount of global ocean island and oceanic plateau：Data mining and its significance

洋岛类型的海山和洋底高原类型的海山主要由玄武岩组成， 中酸性岩出露很少， 因此很少引起学术界的关注。 我们认为即便中酸性岩很少， 研究他们也是很有意义的， 因为这些中酸性岩不仅可能与洋岛和洋底高原玄武岩的构造背景有关， 也可能与洋岛和洋底高原在形成中酸性岩时的温度和压力变化有关， 这对正确认识洋岛和洋底高原形成的地球动力学背景是很有意义的。 本次研究收集了 GEOROC 数据库中的数据总量为 44 404 个， 经过清洗留下有效数据 3 908 个， 分为洋岛中酸性岩（OIG）、洋底高原中酸性岩（OPG）和洋底高原中酸性岩中的岛弧部分（OPAG）3 类。 学术界通常认为， 洋岛代表热点， 洋底高原温度相对较低， 属于大火成岩省。 然而， 本文对全球洋岛和洋底高原的中酸性岩浆岩的研究表明， 分布在洋岛和洋底高原的中酸性岩主要是板内环境的富碱性的粗面岩和碱性流纹岩， 洋底高原边缘可能受俯冲带的影响， 部分中酸性岩具有岛弧的地球化学特征。 按照中酸性岩的 Sr-Yb 分类， 可知洋岛中酸性岩大多属于南岭型和广西型， 洋底高原海山中酸性岩主要属于南岭型， 广西型很少， 指示洋底高原中酸性岩形成的深度比洋岛浅， 温度也比洋岛低， 同时表明中酸性岩大多形成的压力较低而温度很高， 指示伴有高热中酸性岩的洋岛和洋底高原是地球上的热点。 本文认为， 洋岛和洋底高原中酸性岩并非学术界普遍认为的是玄武岩和碧玄岩分离结晶形成的， 因为洋岛和洋底高原岩浆岩如果是双峰式分布的话， 则不大可能是分离结晶的， 其中的酸性岩部分可能是玄武岩部分熔融形成的。 同时有些岩浆演化的关系表明， 中酸性岩在哈克图解中与玄武岩受不同因素制约， 说明并非演化的关系。

The ocean island and oceanic plateau types of seamounts are mainly composed of basalts, and intermediate-acid rocks are rarely exposed. So relatively little attention has been paid to the academic community. We think that it is meaningful to study the above mentioned intermediate-acid rocks, which may be not only related to the tectonic setting of Ocean Island and Oceanic Plateau basalts, but also know the changes of temperature and pressure during the formation of intermediate-acid rocks. This also makes sense for a correct understanding of the geodynamic setting of Ocean Island and Oceanic Plateau. The data collected from the GEOROC database are 44 404 and 3 908 are effective. The data are divided into Ocean Island intermediate-acid magmatic rocks（OIG）, Oceanic Plateau intermediate-acid rocks（OPG）and the island arc section of the Oceanic Plateau（OPAG）. Academia is generally believed that ocean island on behalf of hot spots, the oceanic plateau temperature is relatively low, which belongs to large igneous rock province. However, the study of intermediate-acid rocks in the global ocean island and oceanic plateau shows that the intermediate-acidic rocks distributed in the ocean island and oceanic plateau are mainly alkaline-rich trachyte and alkaline rhyolite in the intraplate environment. The oceanic plateau margin may be affected by the subduction zone, and some intermediate-acid rocks have the geochemical characteristics of the island arc. According to the Sr-Yb classification of the intermediate-acid rocks, we can know the intermediate-acid rocks of ocean island are mostly of Nanling-type and Guangxi-type, while the intermediate-acid rocks of the oceanic plateau are mostly of Nanling-type and a few are Guangxi-type. Which indicated that the intermediate-acid rocks of the oceanic plateau formed shallower than the ocean island, and the temperature also lower than the ocean island. It is also indicated that the intermediate-acid rocks were mostly formed with low pressure and high temperatures, indicating that ocean island and oceanic plateau with high-temperature intermediate-acid rocks are hot spots on earth. We believe that ocean island and oceanic plateau intermediate-acid rocks are not all formed by the separation and crystallization of basalt and basanite that is generally accepted by academics, because if ocean island and oceanic plateau magmatic rocks are bimodal distribution, which are unlikely to be separation and crystallization. The acid rocks may be formed by the partial melting of the basalts. Meanwhile, some magmatic evolutionary relationships indicate that the intermediate-acid rocks are constrained by different factors with basalts. It shows that the two are restricted by different factors, but not evolutionary relationship.