The Neoproterozoic high-Mg dioritic dikes in South China formed by high pressures fractional crystallization of hydrous basaltic melts
The Neoproterozoic high-Mg dioritic dikes in South China formed by high pressures fractional crystallization of hydrous basaltic melts
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DOI：
10.1016/j.precamres.2017.04.009
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发表时间：
2017-04
影响因子：
3.8
通讯作者：
Qi‐Wei Li;J. Zhao
中科院分区：
文献类型：
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作者：
Qi‐Wei Li;J. Zhao
扬子地块西缘新元古代同德中基性侵入体中大量侵入岩脉，为研究Rodinia超大陆裂解过程中高镁闪长岩的成因提供了理想的机会。脉岩的岩石由细粒至中粒闪长岩组成，具有高Mg#（40-69）、低SiO2（50.98- 61.01wt%）和可变Na 2 O含量（1.94- 5.50wt%）。稀土元素配分模式为右倾配分模式，具轻微的负-正Eu异常（Eu/Eu_（？）= 0.90-1.35）。它们的原始地幔标准化微量元素模式以富集Ba和Pb，亏损Nb、Ta和Ti为特征。它们还显示出高且可变的Sr/Y（12-115）和（La/Yb）N比（3-21）。高镁闪长岩具有相对稳定的87 Sr/86 Sr比值（0.704862-0.706320）和正εNd值（+0.6-+2.0）。这些地球化学特征表明同德高镁闪长岩脉不是俯冲洋板部分熔融或富集地幔源区的产物。相反，它们更有可能是由高压条件下含水镁铁质岩浆中石榴石和角闪石矿物组合的分离结晶形成的。同德岩体侵位后，岩脉沿着岩浆热收缩断裂侵入。提出的高镁闪长岩脉成因新模式表明，扬子地块西缘在新元古代早期处于活动大陆弧环境。
Numerous ∼830 Ma dikes intruded into the Neoproterozoic Tongde mafic-intermediate intrusion at the western margin of the Yangtze Block, providing an ideal opportunity for examining petrogenesis of high-Mg diorites during the breakup of the supercontinent Rodinia. Rocks from the dikes are composed of fine to medium grained diorites which have high Mg#(40–69), low SiO2(50.98–61.01 wt%), and variable Na2O contents (1.94–5.50 wt%). They show right inclined REE patterns with slightly negative to positive Eu anomalies (Eu/Eu∗= 0.90–1.35). Their primitive mantle-normalized trace element patterns are characterized by enrichment of Ba and Pb and depletion of Nb, Ta and Ti. They also show high and variable Sr/Y (12–115) and (La/Yb)Nratios (3–21). The high-Mg diorites have relatively constant initial87Sr/86Sr ratios (0.704862–0.706320) and positive εNd values (+0.6–+2.0). These geochemical features reveal that the Tongde high-Mg dioritic dikes were not produced by partial melting of subducted oceanic slab or enriched mantle source. Rather, they were more likely formed by fractional crystallization of garnet and hornblende-bearing mineral assemblages from hydrous mafic magmas under high pressure conditions. The dikes intruded along the fractures resulted from magma thermal contraction after emplacement of the Tongde intrusion. The new model proposed for the petrogenesis of the high-Mg dioritic dikes suggest that the western margin of the Yangtze Block was in an active continental arc setting during the early Neoproterozoic.