Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO₂) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific.Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-backarc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

中文翻译：

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现代海洋背景下镁铁质火山岩的地球化学特征及其对太古代镁铁质岩浆作用的影响

绿岩带以镁铁质火山岩为主，其地球化学特征表明一系列可能的地球动力学影响。人们提出了许多与现代构造环境的类比。对现代海洋的越来越多的勘探和对海底火山岩的全面采样现在提供了独特的机会来表征不同的熔体来源和岩石成因，可以更接近地与绿岩带进行比较。在这项研究中，我们对来自各种环境的2,850 多个独特的海底基性火山岩 (<60 wt % SiO ₂ ) 样品进行了高质量的地球化学分析，包括洋中脊、洋脊-热点交汇处、洋内弧和弧后扩张中心以及海洋岛屿。汇编的数据显示了绿岩带中发现的玄武岩全部成分的显着地球化学变化。这种多样性被解释为是由于地壳厚度变化、干熔融与湿熔融条件、地幔混合和污染造成的。特别是，不同的融化条件与地幔异质性、复杂的地幔流态和短暂的构造域有关，例如与扩散扩散、重叠扩散中心和三重连接相关的构造域。这些在西太平洋的微板镶嵌中得到了充分记录。通过主成分分析和编译数据的无监督层次聚类相结合，揭示了现代海洋环境中镁铁质火山岩成分的系统差异。大多数弧后系统的镁铁质火山岩具有强烈的贫乏地幔特征和众所周知的俯冲相关化学特征，例如大离子亲石元素（LILE）富集与强负Nb-Ta异常和低重稀土元素（HREE）相结合。这与太古代绿岩带中的镁铁质火山岩形成鲜明对比，后者没有显示出或至少显示出较弱的俯冲相关化学物质、较少的地幔耗尽、较少的湿熔融以及可变的地壳污染。这些差异被解释为地幔温度较低、地壳较薄以及当今环境中与俯冲相关的过程的结果。然而，在许多现代弧后盆地中，包括劳盆地、东斯科舍洋脊、布兰斯菲尔德海峡和马努斯盆地等，都出现了与太古代绿岩带地球化学特征相同的基性岩，这些盆地具有地幔源丰富、热流高的特点。 ，以及小规模微板马赛克典型的复杂扩散机制。这些类型的环境被认为有利于现代和古代绿岩带中的火山成因块状硫化物（VMS）矿床，因此基性火山岩的特定地球化学特征对于贱金属勘探的区域选择可能很重要。