Petrogenesis of a Phonolite-Trachyte Succession at Mount Sidley, Marie Byrd Land, Antarctica
Petrogenesis of a Phonolite-Trachyte Succession at Mount Sidley, Marie Byrd Land, Antarctica
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DOI：
10.1093/petroj/38.9.1225
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发表时间：
1997-09
影响因子：
3.9
通讯作者：
K. Panter;P. Kyle;J. Smellie
中科院分区：
文献类型：
--
作者：
K. Panter;P. Kyle;J. Smellie
利用主微量元素Sr、Nd、O和Pb同位素资料，研究了上新世晚期（5.7 ~ 4.2 Ma） Mt Sidley火山1.5 Ma的演化过程。一个巨大的（5公里× 5公里）破火山口暴露出熔岩和熔岩，在西德利山深处，并允许其岩浆演化被阐明。主要分为两个碱性岩系：(a)强硅质欠饱和玄武岩-空泡岩系；(b)硅饱和至过饱和碱性玄武岩至粗纤维岩系列。岩石组成分别为77Sr/86Sri（0.7028 ~ 0.7032）、143Nd/144Ndi（0.51285 ~ 0.51290）和δ18O（5.0 ~ 6.0‰），含206Pb/204Pb(>19.5)，为含强烈地幔柱成分的软流圈源。部分熔融模式要求熔融≤2%才能产生原生玄武岩，熔融≤5%才能产生来自同一地幔源的碱性玄武岩。通过从玄武岩中分馏出透辉石、橄榄石、斜长石、含钛磁铁矿、霞石和/或磷灰石，得到35%的辉长岩、25%的辉长岩和20%的phonolite作为残余液体，模拟了phonolite系列的分异。类似的碱基玄武岩矿物组合的分馏结晶模拟了粗面岩系列的成分。然而，许多粗叶组织87Sr/86Sri（0.7033 ~ 0.7042）变化，143Nd/144Ndi（0.51280 ~ 0.51283）低，δ18O（6.5 ~ 8.4‰）高，硅质过饱和，表明其受地壳污染。粗叶细胞的进化经历了两步同化-分步结晶过程（AFC）。第一步是中地壳内钙碱性花岗岩类对碱性玄武岩的污染，高同化结晶速率（高r AFC）产生以Ta和Nb相对于K和Rb的消耗为特征的粗溶岩浆。第二步是通过上地壳内的低r AFC进一步分馏这些岩浆，以产生另一组显示极端不相容元素富集的粗叶组织（例如Zr> 1000p .p.m. /和Th> 100p .p.m.）。
The 1.5 Ma evolution of the Late Pliocene (5.7 to 4.2 Ma) Mt Sidley volcano, Marie Byrd Land, is examined using major and trace elements, Sr, Nd, O and Pb isotopic data. A large (5 km × 5 km) breached caldera exposes lavas and tephras, deep within Mt Sidley, and allows its magmatic evolution to be elucidated. Two alkaline rock series are distinguished: (a) a strongly silica-under-saturated basanite to phonolite series; (b) a more silica-saturated to -oversaturated alkali basalt to trachyte series. Rock compositions in both series fall within a narrow range of 77Sr/86Sri (0.7028–0.7032), 143Nd/144Ndi (0.51285–0.51290) and δ18O (5.0–6.0‰), and with 206Pb/204Pb (>19.5), suggest an asthenospheric source containing a strong mantle plume component. Partial melting models require ≤2% melting to produce primary basanite and ≤5% melting to produce alkali basalt from the same mantle source. The differentiation of the phonolitic series is modeled by fractionation of diopside, olivine, plagioclase, titaniferous magnetite, nepheline and/or apatite from basanite to derive 35% mugearite, 25% benmoreite and 20% phonolite as residual liquids. Fractional crystallization of a similar mineral assemblage from alkali basalt is modeled for compositions in the trachyte series. However, many trachytes have variable 87Sr/86Sri (0.7033–0.7042), low 143Nd/144Ndi (0.51280–0.51283), high δ18O (6.5–8.4‰) and are silica oversaturated, suggesting they are contaminated by crust. The trachytes evolved by a two-step assimilation–fractional crystallization process (AFC). The first step involved contamination of alkali basalt by calc-alkaline granitoids within the middle crust where high assimilation to crystallization rates (high-r AFC) produced trachytic magmas characterized by depletions in Ta and Nb relative to K and Rb. The second step involved further fractionation of these magmas by low-r AFC within the upper crust to produce another suite of trachytes showing extreme incompatible element enrichment (e.g. Zr>1000 p.p.m/ and Th>100 p.p.m.).