New field observations from the Pontiac subprovince, Superior Province, supported by whole-rock geochemistry provide strong evidence for extensional magmatism and volcanism during deposition of the ca. 2685 Ma Pontiac sediments. Contacts between basaltic and komatiitic volcanic rocks and sedimentary rocks are concordant and weakly deformed, inconsistent with an allochthonous emplacement model proposed for volcanic successions in the Pontiac subprovince. Structural observations and aeromagnetic imagery show that the volcanic rocks are present as thin (<500 m), laterally continuous (>30 km) conformable successions within the turbiditic succession and were emplaced prior to the earliest deformation of the Pontiac subprovince. Sediment-matrix breccias with fluidal and blocky igneous clasts identical to the compositions of the mafic–ultramafic rocks occur along intrusive contacts with Pontiac Group wacke. These textures are interpreted as peperite, representing the interaction of magma or lava with wet, unconsolidated sediment. Peperite was observed along the margins of tholeiitic-komatiitic rocks and calc-alkaline lamprophyre intrusions, indicating that these magmas were broadly synchronous with the deposition of the Pontiac sediments. The volcanic rocks are characterized as tholeiitic basalt and Al-depleted to Al-undepleted komatiite, have high Nb/Th, flat to positive light rare earth element (REE) slopes, and flat heavy REE slopes, consistent with partial melting of a mantle plume. The calc-alkaline lamprophyre intrusions have low Nb/Th and prominent negative light and heavy REE slopes, consistent with partial melting of a previously metasomatised mantle. The presence of autochthonous komatiitic and tholeiitic rocks is consistent with the Pontiac subprovince developing as an extensional basin within the Abitibi subprovince in response to an upwelling mantle plume ca. 2.7 Ga. This model is incompatible with uniformitarian evolutionary models for the Superior craton interpreting the sedimentary subprovinces as accretionary prisms that formed during subduction processes. Non-uniformitarian models invoking stagnant-lid tectonics predict the formation of extensional basins and ascent of depleted mantle-derived magmas associated with upwelling mantle plumes, but the ability to produce source-metasomatism magmas without subduction is highly contested. The evolution of the Pontiac basin may thus reflect a transitional period in the Neoarchean where stagnant-lid and proto-subduction styles of tectonism were operating intermittently.

在全岩地球化学的支持下，来自苏必利尔省庞蒂亚克次省的新实地观测为约 2685 马庞蒂亚克沉积物。玄武质和科马提质火山岩与沉积岩之间的接触一致且变形较弱，与提出的庞蒂亚克次省火山序列的异地侵位模型不一致。构造观测和航磁图像显示，火山岩在浊积层序内以薄 (<500 m)、横向连续 (>30 km) 的顺应层序存在，并且在庞蒂亚克次省最早变形之前就已被侵位。沉积基质角砾岩具有与镁铁质-超镁铁质岩石的成分相同的流体和块状火成碎屑，沿着与庞蒂亚克群瓦克的侵入接触发生。这些纹理被解释为矿质岩，代表岩浆或熔岩与潮湿的松散沉积物的相互作用。沿拉斑-科马提质岩石和钙碱性斑岩侵入体的边缘观察到了钙钛矿，表明这些岩浆与庞蒂亚克沉积物的沉积大致同步。火山岩的特征为拉斑玄武岩和贫铝至未贫铝科马提岩，具有高 Nb/Th、平坦至正的轻稀土元素 (REE) 斜坡和平坦的重稀土斜坡，与地幔柱的部分熔融一致. 钙碱性萤石侵入体具有低 Nb/Th 和显着的负轻重稀土斜率，与先前交代地幔的部分熔融一致。原生科马提岩和拉斑岩的存在与庞蒂亚克亚省在阿比蒂比亚省内发展为一个伸展盆地相一致，以响应上升流地幔柱。2.7 Ga。这个模型与上克拉通的均变演化模型不兼容，该模型将沉积亚省解释为在俯冲过程中形成的增生棱柱。引用停滞盖构造的非均匀模型预测了伸展盆地的形成和与上涌地幔柱相关的耗尽地幔衍生岩浆的上升，但是在没有俯冲作用的情况下产生源交代岩浆的能力受到高度争议。因此，庞蒂亚克盆地的演化可能反映了新太古代的一个过渡时期，在那里停滞盖和原始俯冲类型的构造运动间歇性地运行。