An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province
An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province
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DOI：
10.1007/pl00007672
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发表时间：
2000-06
影响因子：
3.5
通讯作者：
P. Hollings;R. Kerrich
中科院分区：
文献类型：
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作者：
P. Hollings;R. Kerrich
太古宙上级省的内奇次省是一系列绿岩带，沿北加勒比地体原大陆南缘沿着向东西延伸600 km。安大略西北部Birch-Uchi绿岩带的2.7 Ga邦联构造地层组合主要由镁铁质、中性和长英质成分的火山岩套组成。拉斑玄武岩的成分范围从Mg# 59-26不断演化到更高的稀土元素含量（La=2-19 ppm; Th/LaPm = 1），具有小的负Nb异常。原始拉斑玄武岩与现代洋内弧玄武岩相似，而演化的成员延伸到更高浓度的Ti，Zr，V，Sc和Y，和较低的Ti/Zr，但更高的Ti/Sc和Ti/V比值的弧后玄武岩的特征。钙碱性玄武岩到英安岩的特征是更分馏的稀土元素（La/Ybn=1-8），高Th/Nbpm比值和更深的负Nb异常;它们与现代大洋弧玄武岩和一些可能符合高镁安山岩。这两套地层被解释为一对弧-弧背层序。第三组富Nb玄武岩（NEB; Nb=9-18 ppm）延伸至极高的TiO 2、Ta、P2 O 5、Sc和V含量，具有强烈分馏的REE和Nb/Ta和Zr/Hf比值大于原始地幔值，而Zr/Sm比值较低。最丰富的流纹岩套具有极其丰富但平坦的微量元素模式，被解释为强烈分馏的拉斑玄武岩液体。第二类与新生代埃达克岩和太古代高铝、高La/Ybnonalites的成分相似，Yb ≤ 0.4ppm，Y ≤ 6ppm，Sc ≤ 8 ppm，La/Ybn为19-30，Zr/Sm为50-59。它们被解释为海洋岩石圈玄武质地壳在一个热的浅俯冲带的熔体。埃达克岩与新生代岛弧中的NEB有关，那里有年轻和/或热海洋岩石圈的浅俯冲，通常有斜俯冲。板状熔融埃达克岩喷发，或交代弧下地幔橄榄岩产生HFSE富集源，随后在诱导地幔对流过程中熔融。太古代埃达克岩-NEB组合在拉斑玄武岩-钙碱性弧及其弧后发育过程中爆发。它们共存于Birch-Uchi绿岩带的邦联组合中，意味着与新生代弧相似的会聚边缘过程。
The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/Lapm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Ybn=1–8), high Th/Nbpmratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO2, Ta, P2O5, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Ybntonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Ybnof 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.