Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities
Volatile-rich Metasomatism in the Cratonic Mantle beneath SW Greenland: Link to Kimberlites and Mid-lithospheric Discontinuities
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DOI：
10.1093/petrology/egy009
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发表时间：
2017-12-01
影响因子：
3.9
通讯作者：
Gerdes, Axel
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Sun, Jing;Gerdes, Axel
自从发现了一些地球上最古老的地壳，并在岩石圈地幔中发现了巨大的钻石潜力以来，格陵兰的克拉通部分一直是科学调查的热点，但对其特征的描述仍然不完整。我们对格陵兰西南部北大西洋克拉通发现的一套新的金伯利岩橄榄岩包体进行了详细的岩石学和现场分析，以揭示地幔交代的时间和性质，及其与低体积熔体(如金伯利岩)形成的联系和地球物理上可检测到的不连续面。识别出两种类型的矿物学和交代样式，赋存于两个深度段。第一种类型为二辉橄榄岩、方辉橄榄岩和硅质岩，部分含金云母，赋存深度约100-170公里。随着TiO2、MnO、Na2O含量的增加和NiO含量的降低，形成了矿物Mg#不断降低的趋势。这些系统学归因于约150 Ma金伯利岩的水合硅酸盐熔体前体在裂谷、减压和岩石圈减薄过程中发生的交代作用。这种交代伴随着石榴石的渐进性分解，在结构上以占据先前粗颗粒的辉石-尖晶石组合为明显，在成分上通过在新形成的单斜辉石中与石榴石相容的元素(Y、Sc、In、重稀土元素)的浓度增加而明显。伴生的硫化物饱和度以铜、镍和钴的贫化为标志。残留的、更多的二氧化硅不饱和和潜在的更多的氧化熔体向上渗流并交代浅层岩石圈地幔，岩石圈地幔由含金云母的结构平衡的橄榄岩(包括辉石)组成，显示出最近辉石破裂的证据。这是第二种类型的岩性，出现在类似于90-110公里深的地方，并被推断为具有高度亏损的原岩。这类岩石在成分上与二辉橄榄岩不同，橄榄石具有较高的钙铝比，但铝和钒含量较低。虽然低铝可能部分反映了较低的平衡温度，但低V可归因于在较低压力下本质上更多的氧化地幔和氧化交代作用的组合。浅层克拉通地幔岩石圈的强烈交代作用与克拉通西北部记录的强烈亏损形成对比，后者在590-550 Ma延伸至210公里深，表明北大西洋克拉通解体时岩浆源的逐渐变浅也记录了类似于40公里岩石圈地幔的损失。富含金云母的岩性集中在一个狭窄的深度区间(类似于90-110公里)，与负的地震速度梯度重叠，这被解释为格陵兰西部之下的中-岩石圈不连续。这被认为是小体积富挥发分岩浆作用的表现，为中生代金伯利岩、超镁铁煌斑岩和碳酸盐岩横跨北大西洋克拉通侵位铺平了道路。
The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterization of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, to unravel the timing and nature of mantle metasomatism, and its link to the formation of low-volume melts (e.g. kimberlites) and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognized. The first type comprises lherzolites, harzburgites and dunites, some phlogopite-bearing, which occur from similar to 100-170 km depth. They form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to c. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally evident by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, heavy rare earth elements) in newly formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidizing melts percolated upwards and metasomatized the shallower lithospheric mantle, which is composed of phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene-breakdown. This is the second type of lithology, which occurs at similar to 90-110 km depth and is inferred to have highly depleted protoliths. This type is compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. Whereas low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidizing mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550 Ma extended to >210 km depth, and suggests loss of, similar to 40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (similar to 90-110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.