Ca. 2.7 Ga ferropicritic magmatism: A record of Fe-rich heterogeneities during Neoarchean global mantle melting
Ca. 2.7 Ga ferropicritic magmatism: A record of Fe-rich heterogeneities during Neoarchean global mantle melting
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DOI：
10.1016/j.gca.2015.09.023
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发表时间：
2016-07
影响因子：
5
通讯作者：
D. Milidragovic;D. Francis
中科院分区：
文献类型：
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作者：
D. Milidragovic;D. Francis
虽然在地质记录中⩾为13%的陆源苦味岩浆很少见，但在新太古代陆壳加速生长的时期，它们相对常见，约为2.7Ga。最近的证据表明，苦味铁质底侵作用在约2.74-2.70亿年的昂加瓦克拉通改造中发挥了重要作用，这为对比全球新太古代岩浆记录中约2.7亿年的镓铁苦铁矿的产状提供了动力。除了Ungava克拉通的富铁深成岩体外，体积较小的苦铁矿流、火山碎屑沉积和侵入岩构成了苏必利尔省南部和西部Abitibi、Wawa、Wabigoon和Vermil域的新太古代绿岩带地层的一部分。在西丘吉尔、斯拉夫、伊尔格恩、卡阿普瓦尔和卡雷利亚等五个太古代克拉通上也发现了新太古代铁苦铁矿，表明全球普遍存在约2.7亿年的富Ga-Fe岩浆活动。新太古代铁苦铁矿在其微量元素地球化学方面形成了两个不同的群。碱性铁苦岩具有分馏的REE剖面，没有系统的HFSE异常，与现代洋岛玄武岩(OIB)岩浆的微量元素特征大体相似。与原生地幔岩浆相比，约2.7kGa碱性铁皮岩母岩具有较高的Nb/yPm(>；2)、低的Al_2O_3/TiO_2(<；8)和Sc/Fe(⩽_3×10~(−))比值，且富镍。碱性铁皮榴石的高Ni含量与低的Sc/Fe比值与不含橄榄石的石榴石-辉石岩来源一致。第二类铁苦皮石族具有明显的非碱性原生微量元素特征，范围从平坦到LREE亏损，类似于太古代拉斑玄武岩和科马提岩。与碱性铁皮橄榄岩相比，亚碱性铁皮榴石母岩具有平坦的HREE，较低的Nb/yPm(<；2)，较高的Al_2O_3/TiO_2(8~25)和Sc/Fe(⩾4×10~(−)4)比值，相对于热解橄榄岩熔体，亏损Ni，表明它们来自不含石榴石的橄榄岩来源。Nd3+同位素证据表明，碱性铁苦味石源区在岩浆形成前不久(⩽3.0Ga)发生了交代富集，但亚碱性铁苦铁矿没有显示出前驱交代作用的证据。富铁橄榄岩可能伴随着富铁橄榄岩向次生石榴石-辉石岩的转化，熔融实验和硅酸盐液体密度对压力和温度的依赖关系表明，铁苦铁矿不可能是正常地幔在高压和高温下熔融而成的。亚碱性苦铁陨石与钠铁陨石(SNC)和钠铁榴石闪长岩(Hed)分异陨石的地球化学相似性表明，富铁地幔可能来源于富铁球粒陨石的侵入。在至少6个克拉通上产出约2.7亿Ga富铁岩石，通常与普遍存在的科马提岩和镁拉斑玄武岩同时代，这与整个新太古代地幔中存在不均匀的富铁“李子”相一致。2.7Ga以后地质记录中铁苦铁矿的稀少表明，这些富铁李子大多是在全球新太古代地幔熔融期间熔融出来的。
Although terrestrial picritic magmas with FeOTOT⩾13 wt.% are rare in the geological record, they were relatively common ca. 2.7 Ga during the Neoarchean episode of enhanced global growth of continental crust. Recent evidence that ferropicritic underplating played an important role in the ca. 2.74–2.70 Ga reworking of the Ungava craton provides the impetus for a comparison of ca. 2.7 Ga ferropicrite occurrences in the global Neoarchean magmatic record. In addition to the Fe-rich plutons of the Ungava craton, volumetrically minor ferropicritic flows, pyroclastic deposits, and intrusive rocks form parts of the Neoarchean greenstone belt stratigraphy of the Abitibi, Wawa, Wabigoon and Vermillion domains of the southern and western Superior Province. Neoarchean ferropicritic rocks also occur on five other Archean cratons: West Churchill, Slave, Yilgarn, Kaapvaal, and Karelia; suggesting that ca. 2.7 Ga Fe-rich magmatism was globally widespread.Neoarchean ferropicrites form two distinct groups in terms of their trace element geochemistry. Alkaline ferropicrites have fractionated REE profiles and show no systematic HFSE anomalies, broadly resembling the trace element character of modern-day ocean island basalt (OIB) magmas. Magmas parental to ca. 2.7 Ga alkaline ferropicrites also had high Nb/YPM(>2), low Al2O3/TiO2(<8) and Sc/Fe (⩽3 × 10−4) ratios, and were enriched in Ni relative to primary pyrolite mantle-derived melts. The high Ni contents of the alkaline ferropicrites coupled with the low Sc/Fe ratios are consistent with derivation from olivine-free garnet-pyroxenite sources. The second ferropicrite group is characterized by decisively non-alkaline primary trace element profiles that range from flat to LREE-depleted, resembling Archean tholeiitic basalts and komatiites. In contrast to the alkaline ferropicrites, the magmas parental to the subalkaline ferropicrites had flat HREE, lower Nb/YPM(<2), higher Al2O3/TiO2(8–25) and Sc/Fe (⩾4 × 10−4) ratios, and were depleted in Ni relative to melts of pyrolitic peridotite; suggesting they were derived from garnet-free peridotite sources. Neodymium isotopic evidence indicates that the source of alkaline ferropicrites was metasomatically enriched shortly before magma generation (⩽3.0 Ga), but the subalkaline ferropicrites do not show evidence of precursor metasomatism. The metasomatic enrichment of the alkaline ferropicrite sources may have been accompanied by conversion of Fe-rich peridotite to secondary garnet-pyroxenite.Melting experiments on “pyrolitic” compositions and consideration of the dependence of the density of silicate liquids on pressure and temperature, suggest that ferropicrites cannot originate by melting of normal terrestrial mantle (Mg-number = 0.88–0.92) at high pressures and temperatures. The geochemical similarity between the subalkaline ferropicrites and the shergottite–nakhlite–chassigny (SNC) and howardite–eucrite–diogenite (HED) differentiated meteorites suggests, however, that the Fe-rich mantle may originate from the infall of Fe-rich chondritic meteorites. The occurrence of ca. 2.7 Ga Fe-rich rocks on at least six cratons that are commonly coeval with the more ubiquitous komatiites and Mg-tholeiites is consistent with the existence of heterogeneous Fe-rich “plums” throughout the Neoarchean mantle. The paucity of ferropicrites in the post-2.7 Ga geological record suggests that majority of these Fe-rich plums have been melted out during the global Neoarchean melting of the mantle.