DISCOVERY OF FERROPICRITES AND HIGH-MAGNESIAN ANDESITES FROM THE ERDENETSOGT FORMATION, CENTRAL MONGOLIA
DISCOVERY OF FERROPICRITES AND HIGH-MAGNESIAN ANDESITES FROM THE ERDENETSOGT FORMATION, CENTRAL MONGOLIA
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中蒙古额尔德涅佐格特地层中发现铁锰矿和高镁质安山岩
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
A. Tamura
中科院分区：
文献类型：
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作者：
Ganbat Erdenesaikhan;A. Ishiwatari;D. Orolmaa;S. Arai;A. Tamura
对汉盖地区Tsetserleg吸积地体Erdenetsogt组绿岩进行了新的地球化学和岩石学研究，重点介绍了新发现的苦榴质和安山岩。这些岩石主要出现在Erdenetsogt组的下部，以块状熔岩、岩基和岩脉的形式与五颜六色的层状带状燧石和粉砂岩密切相关。研究的绿岩原岩包括(1)与海洋高原玄武岩有亲缘关系的羽状拉斑绿岩，(2)弧状钙碱性安山岩。羽状岩的化学特征是轻稀土元素（LREE）富集程度与拉斑岩相似，并存在高FeO* (bb0 ~ 14 wt%)和MgO （12 ~ 22 wt%）的硅铁铁矿，这是大火成岩省（包括海洋高原）的特征。因此，它们的拉斑岩组成和高铁和高钛含量需要源幔橄榄岩的熔融，并加入一些富铁和富钛的玄武岩物质。该安山岩具有玻璃状结构，MgO含量高（高达7 wt%）， LREE富集，且Nb富集，类似于高镁安山岩（HMAs）的sanukite型。我们推测汉盖拉斑绿岩可能代表了上志留统—下泥盆统汉盖—亨特古陆—海洋深水区发育的海洋高原的上半部分。汉盖HMAs可能是由于石炭世海洋高原与中蒙古地块活动大陆边缘碰撞导致俯冲带向海洋后退后，年轻大洋板块俯冲而产生的。
New geochemical and petrological results are presented for greenstones from the Erdenetsogt Formation hosted by the Tsetserleg accretionary terrane in the Hangay region, with particular emphasis on newly found picritic and andesitic rocks. These rocks occur mostly in the lower portion of the Erdenetsogt Formation as massive lavas, sills, and dykes closely associated with varicolored bedded ribbon cherts and siltstones. The protoliths of the studied greenstones comprise (1) plume-derived tholeiitic greenstones with oceanic plateau basalt affinity, (2) arc-derived, calc-alkaline andesites. The plume-derived rocks are characterized by chemical signatures such as slight LREE enrichment similar to that of tholeiitic OIB and the existence of ferropicrite with high FeO* (>14 wt%) and MgO (12–22 wt%), which is characteristic of large igneous provinces (LIPs), including oceanic plateaus. Therefore, their tholeiitic composition and high-Fe and -Ti contents require melting of the source mantle peridotite with addition of some recycled Fe- and Ti-rich basaltic material. The andesites are characterized by glassy texture, high MgO content (up to 7 wt%), and significant LREE enrichment with depletion in Nb and resemble sanukite type of high-magnesian andesite (HMAs). We infer that the Hangay tholeiitic greenstones probably represent an accreted upper section of an oceanic plateau that developed in the deep-water region of the Hangay-Henteypaleo-ocean in the Upper Silurian-Lower Devonian. The Hangay HMAs may have been produced by subduction of young oceanic plate after an oceanward back-stepping of the subduction zone that was a result of the collision during the Carboniferous of the oceanic plateau and the active continental margin of the Central Mongolian Massif.