Geochemical relationships in the Sudbury igneous complex; origin of the main mass and offset dikes
Geochemical relationships in the Sudbury igneous complex; origin of the main mass and offset dikes
复制标题
萨德伯里火成岩杂岩中的地球化学关系；
DOI：
10.2113/gsecongeo.92.3.289
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发表时间：
1997
期刊：
影响因子：
5.8
通讯作者：
K. P. Farrell
中科院分区：
文献类型：
--
作者：
P. Lightfoot;R. Keays;G. Morrison;A. Bite;K. P. Farrell
岩石学模型与构成1.85 Ga萨德伯里火成杂岩的不同岩石类型有关，受到广泛的新地球化学数据的约束。我们发现，主要质量长英质苏长岩，过渡带石英辉长岩和花岗斑岩具有相似的高度不相容微量元素（例如，La/Sm = 4.5-7，La/Nb = 2.8-4.2，Th/Zr = 0.04-0.05），这些变化与岩浆类型的结晶和分异作用相一致，岩浆主要（>80%）来自上地壳，地幔源区贡献较小。虽然目前还没有确凿的证据表明，岩浆是作为陨石撞击产生的熔体片在原地产生的，我们没有发现这个模型应该被拒绝的主要原因。然而，我们认为，一个小的贡献，幔源苦橄岩浆是需要解释丰富的镍，铜，铂族元素（PGE）在萨德伯里矿床，以及组成的超镁铁质包裹体（MgO = 12-36重量%; Fo（sub 68-87）橄榄石，含450- 3，700 ppm镍，和丰富的富铬尖晶石），镁铁质苏长岩（8- 14wt%MgO）和次层（6- 12wt%MgO）的镁组成。我们认为，萨德伯里杂岩的主体是通过合并多达20%的幔源苦橄质岩浆而形成的，这些岩浆沿着撞击事件产生的地壳裂缝侵位。这些苦橄质岩浆进入熔体片作为一个密集的羽流，大力与它混合，并由于显着的成分转变，混合岩浆形成岩浆硫化物，通过岩浆柱下沉，耗尽熔融镍，铜，铂族元素。由于长英质苏长岩和花岗斑岩的不相容微量元素的比例难以区分，我们认为没有必要通过不同来源的岩浆结晶来推导这些岩石单元。长英质苏长岩和花岗斑岩的成分差异是岩浆就地分异的结果。我们发现，主体具有许多与大多数偏移脉石英闪长岩相似的成分特征（例如，帕金支堤：La/Sm = 6.3，La/Nb = 4.5，Th/Zr = 0.05）和Whistle矿海湾相关浅色长花岗岩（La/Sm = 6.2，La/Nb = 5.0，Th/Zr = 0.02）。这些岩石的成分介于长英质苏长岩和花岗斑岩之间，因此结晶自相同的岩浆类型;可以说，未矿化的石英闪长岩提供了萨德伯里杂岩结晶的原始岩浆的最佳估计。详细地说，偏移脉内和偏移脉之间的成分存在微妙的变化，最大的差异是北岭和南岭偏移之间的差异;北部山脉的偏移脉岩切割太古代花岗岩类和片麻岩，具有高Sr、La/Yb、La/Sm和Gd/Yb以及低TiO 2，而南部山脉的脉岩切割早元古代沉积物、基性火山岩和侵入体，具有低Sr、La/Yb、Gd/Yb，La/Sm，高TiO 2。这些差异可能是由于不同围岩在筑堤过程中的同化作用造成的。Creighton矿的一个强烈矿化的偏置岩脉具有与主体不同的地球化学变化，在Creighton矿的情况下，与当地矿化子层更相似。这些数据表明矿化石英闪长岩和无矿化石英闪长岩具有不同的地球化学成分，这些特征可能具有找矿价值。
Petrological models relating the different rock types constituting the 1.85 Ga Sudbury Igneous Complex are constrained with extensive new geochemical data. We show that the main mass felsic norite, transition zone quartz gabbro, and granophyre have similar ratios of the highly incompatible trace elements (e.g., La/Sm = 4.5-7, La/Nb = 2.8-4.2, Th/Zr = 0.04-0.05) and that these variations are consistent with the crystallization and differentiation of magma types largely (>80%) derived from the upper crust, with a smaller contribution from a mantle source. Although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact, we find no principal reason why this model should be rejected. However, we propose that a small contribution of mantle-derived picritic magma is required to explain the abundant Ni, Cu, and platinum-group elements (PGE) in the Sudbury deposits, as well as the compositions of the ultramafic inclusions (MgO = 12-36 wt %; Fo (sub 68-87) olivines with 450-3,700 ppm Ni, and abundant chrome-rich spinel), and the magnesian composition of the mafic norite (8-14 wt % MgO) and the sublayer (6-12 wt % MgO). We believe that the main mass of the Sudbury Complex achieved its present composition through incorporation of up to 20 percent mantle-derived picritic magma emplaced along crustal fractures produced by the impact event. These picritic magmas entered the melt sheet as a dense plume, vigorously mixing with it, and due to the marked compositional shift, the mixed magma formed magmatic sulfides which sank through the magma column, depleting the melt in Ni, Cu, and PGE. Since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements, we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources. Rather, the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma. We show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites (e.g., the Parkin offset dike: La/Sm = 6.3, La/Nb = 4.5; Th/Zr = 0.05) and of embayment-related leucocratic norites from the Whistle mine (La/Sm = 6.2, La/Nb = 5.0, Th/Zr = 0.02). These rocks have compositions intermediate between the felsic norite and the granophyre, and therefore crystallized from the same magma type; arguably, the unmineralized quartz diorites provide the best possible estimate of the original magma from which the Sudbury Complex crystallized. In detail, there are subtle variations in composition within and between offset dikes, with the largest difference being between the North and South Range offsets; the North Range offset dikes cut Archean granitoids and gneisses and have elevated Sr, La/Yb, La/Sm, and Gd/Yb and low TiO 2 whereas the South Range dikes cut Early Proterozoic sediments, mafic volcanics, and intrusions, and have low Sr, La/Yb, Gd/Yb, La/Sm, and high TiO 2 . These differences may be caused by the assimilation of different country rocks during emplacement of the dike. A strongly mineralized offset dike at the Creighton mine has geochemical variations that are different when compared to the main mass, and in the case of Creighton, are more similar to the local mineralized sublayer. These data suggest that mineralized and barren quartz diorites have different geochemical compositions, and that these traits may be of value in mineral exploration.