Geochemical systematics of 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Fe-rich basalt rhyolite associations: Backarc rift oceanic crust?
Geochemical systematics of 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Fe-rich basalt rhyolite associations: Backarc rift oceanic crust?
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DOI：
10.1016/j.lithos.2007.07.009
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发表时间：
2008-02
期刊：
影响因子：
3.5
通讯作者：
R. Kerrich;A. Polat;Q. Xie
中科院分区：
文献类型：
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作者：
R. Kerrich;A. Polat;Q. Xie
我们提出了来自以前相邻的阿比提比省和瓦瓦省的新太古代Kinojevis和Manitouwadge-Winston湖火山岩的新的地球化学数据。2702-2701 Ma Kinojevis群（KG）为阿比提比绿岩带，以富铁-富镁玄武岩流（Mg-number= 30-56）为主，具有拉斑分选倾向，磁条带明显。在球粒陨石的4 ~ 15倍，稀土元素模式接近平坦。归一化后的La/Sm和Gd/Yb比值分别为0.70 ~ 1.17和0.96 ~ 1.34，后者表明石榴石在残留物中有不同的参与。玄武岩不与太古宙上地壳的平均成分混合在一起，因此很可能是在海洋环境中喷发的。拉斑岩型高硅流纹岩也具有近平坦的REE模式，具有明显的负Eu异常，与玄武岩熔体的极端分馏产物一致。玄武岩的铌含量和Zr/Nb比值（1.7 ~ 6.4 ppm; 10 ~ 24）大多介于太古宙原始海洋弧拉玄武岩（PAT）和海洋高原拉玄武岩（OPB: 2 ~ 9 ppm; 12 ~ 20）之间，与科马岩伴生，可能代表了地幔柱喷发的海洋高原。同样，KG玄武岩的Nb/Th比值（4-8，原始地幔为8）介于太古宙PAT（2-6）和OPB（8 - 20）之间。在成分上，这些玄武岩与近代弧后玄武岩（BAB）相当，处于洋中脊玄武岩（MORB）和洋中脊玄武岩（PAT）之间，它们被解释为弧后裂谷中的洋壳，来自枯竭的上地幔软流圈的轻度含水熔融。Kinojevis玄武岩主要位于log Nb/Y和Zr/Y坐标上的morb -海洋岛玄武岩（OIB）阵列上，而结构上覆的Blake River玄武岩则穿过该阵列，从枯竭的羽流软流层到埃克岩“末端成员”。因此，它们的软流层来源和地球动力学背景可能有所不同。Wawa省Ma Manitouwadge和Winston绿岩带2722-2680的拉斑玄武岩和fiii型流纹岩在成分上与Kinojevis相似。玄武岩和流纹岩在空间上和暂时上与火山块状硫化物矿床（VMS）相关，表明KG具有类似矿床的潜力。KG被解释为洋内弧后地壳。同样，Wawa火山和成矿组合被解释为沉积在弧后环境中，位于Schreiber-Hemlo俯冲-增生杂岩的西北偏北。
We present new geochemical data for Neoarchean Kinojevis and Manitouwadge–Winston Lake volcanic rocks from the formerly contiguous Abitibi and Wawa subprovinces. The 2702–2701 Ma Kinojevis Group (KG), Abitibi greenstone belt, is dominated by Fe- to Mg-rich basalt flows (Mg-number=30–56) having a tholeiitic fractionation trend, and prominent magnetic striping. REE patterns are near-flat at 4 to 15 times chondrite. Normalized La/Sm and Gd/Yb ratios span 0.70 to 1.17 and 0.96 to 1.34 respectively, the latter indicative of a variable involvement of garnet in the residue. Basalts do not plot on mixing lines with average Archean upper crustal composition, and accordingly were likely erupted in an oceanic setting. Tholeiitic high-Si rhyolites also feature near-flat REE patterns, with pronounced negative Eu anomalies, consistent with extreme fractionation products of basaltic melts. Niobium contents and Zr/Nb ratios (1.7–6.4 ppm; 10–24) of basalts are mostly intermediate between respective values for Archean primitive oceanic arc tholeiites (PAT), and ocean plateau tholeiitic basalts (OPB: 2–9 ppm; 12–20) associated with komatiites, likely representing ocean plateaus erupted from mantle plumes. Similarly, Nb/Th (4–8, versus 8 for primitive mantle) ratios in KG basalts are intermediate between ranges for Archean PAT (2–6) and OPB (8–20). Compositionally, these basalts are commensurate with recent backarc basalts (BAB) that are intermediate between mid-ocean ridge basalts (MORB) and PAT: they are interpreted as oceanic crust in a backarc rift, from mildly hydrous melting of depleted upper mantle asthenosphere. Kinojevis basalts plot mostly on the MORB–ocean island basalt (OIB) array in log Nb/Y versus Zr/Y coordinates, whereas structurally overlying Blake River basalts cross that array from depleted plume asthenosphere to adakite “end-members”. Consequently, their asthenosphere sources and geodynamic settings may have differed. Tholeiitic basalts and FIII-type rhyolites in the 2722–2680 Ma Manitouwadge and Winston greenstone belts, Wawa subprovince, are compositionally akin to their Kinojevis counterparts. Basalts and rhyolites in both belts are spatially and temporarily associated with volcanogenic massive sulphide deposits (VMS), indicative of the potential of the KG for similar deposits. The KG is interpreted as intra-oceanic backarc crust. Similarly, the Wawa volcanic and metallogenic associations are interpreted to have been deposited in a backarc setting, to the north-northwest of the Schreiber-Hemlo subduction–accretion complex.