Archean greenstone belt magmatism and the continental growth–mantle evolution connection: constraints from Th–U–Nb–LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada
Archean greenstone belt magmatism and the continental growth–mantle evolution connection: constraints from Th–U–Nb–LREE systematics of the 2.7 Ga Wawa subprovince, Superior Province, Canada
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DOI：
10.1016/s0012-821x(99)00283-6
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发表时间：
2000-01
影响因子：
5.3
通讯作者：
A. Polat;R. Kerrich
中科院分区：
文献类型：
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作者：
A. Polat;R. Kerrich
一个广泛的数据库，包括Th-;U-Nb-REE系统，不同的岩浆和沉积岩性的2.7 Ga Wawa绿岩带提供了新的制约机制的地壳生长在南部的上级省，并控制其组成。绿岩带的特点是拼贴的海洋高原，海洋岛弧，海沟浊积岩，这些岩石构造碎片构造组装在一个大的俯冲-增生复合体。并列后，这些不同的岩性集体侵入同运动TTG（英云闪长岩-奥长花岗岩-花岗闪长岩）岩体和超镁铁质长英质岩墙和岩床，俯冲带地球化学特征。洋内玄武岩的特征是近平坦的稀土模式，Nb/U和Nb/Th比值一般大于原始地幔值，与正的Nd值一致。它们与科马提岩共生，这种共生关系被解释为从地幔柱中喷发出来的海洋高原序列。双峰弧火山岩序列、海沟浊积岩和同生TTG岩套的特征是稀土元素分馏，Nb/U和Nb/Th比值小于原始地幔值。混合双曲线之间的海洋高原和岩浆弧序列通过估计的大块大陆地壳的组成，表明在太古代晚期的地壳生长是由构造，沉积和化学混合的海洋高原和弧序列在会聚板块边界。混合计算表明，海洋高原和俯冲带的组分在瓦瓦大陆地壳中分别占6-12%和88- 94%。高Nb/U和Nb/Th比值的高原拉斑玄武岩被解释为弧岩浆作用（低Nb/U和Nb/Th）的补充水库，数亿年前通过俯冲带（高Nb/U，Nb/Th）的海洋岩石圈再循环，并纳入地幔柱，从2.7 Ga高原拉斑玄武岩爆发。高原玄武岩的高Nb/U值与早期从南上级省软流圈地幔中提取大量陆壳原岩（岩浆前体）是一致的。
An extensive database, including Th–;U–Nb–REE systematics, for diverse magmatic and sedimentary lithologies of 2.7 Ga Wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern Superior Province, and controls on its composition. The greenstone belts are characterized by collages of oceanic plateaus, oceanic island arcs, and trench turbidites; these lithotectonic fragments were tectonically assembled in a large subduction–accretion complex. Following juxtaposition, these diverse lithologies were collectively intruded by syn-kinematic TTG (tonalite–trondhjemite–granodiorite) plutons and ultramafic to felsic dykes and sills, with subduction zone geochemical signatures. Intra-oceanic basalts are characterized by near-flat REE patterns, and Nb/U and Nb/Th ratios generally greater than primitive mantle values, consistent with positive ϵNdvalues. They are associated with komatiites, the association being interpreted as an ocean plateau sequence erupted from a mantle plume. Bimodal arc volcanic sequences, trench turbidites, and contemporaneous TTG suites are characterized by fractionated REE, with Nb/U and Nb/Th ratios less than primitive mantle values. Mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust, suggesting that crustal growth in the late Archean was by tectonic, sedimentary, and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries. Mixing calculations suggest that oceanic plateau and subduction zone components in the Wawa continental crust are represented by 6–12% and 88–94%, respectively. High Nb/U and Nb/Th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism (low Nb/U and Nb/Th), hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone (high Nb/U, Nb/Th), and its incorporation into a mantle plume from which 2.7 Ga plateau tholeiites erupted. The variably high Nb/U ratios of the plateau basalts are consistent with early extraction of large quantities of the protoliths (magmatic precursor) of continental crust from the southern Superior Province asthenospheric mantle.