Siderophile and chalcophile metal variations in Tertiary picrites and basalts from West Greenland with implications for the sulphide saturation history of continental flood basalt magmas
Siderophile and chalcophile metal variations in Tertiary picrites and basalts from West Greenland with implications for the sulphide saturation history of continental flood basalt magmas
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DOI：
10.1007/s00126-006-0112-4
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发表时间：
2007-03
影响因子：
4.8
通讯作者：
R. Keays;P. Lightfoot
中科院分区：
文献类型：
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作者：
R. Keays;P. Lightfoot
拥有 6500 万年历史的大陆洪水玄武岩出现在西格陵兰岛的 Qeqertarssuaq 岛和 Nuussuaq 半岛上，其中包括约 1,000 米的苦味熔岩和离散的 10 至 50 米厚的高度污染的玄武岩。在 Qeqertarssuaq，熔岩被分配到 Vaîgat 和 Maligât 地层，其中前者包括 Naujánguit 岩层，该岩层由含 7–29 wt% MgO、80–1,400 ppm Ni、5.7–9.4 ppb Pt 和 4.2–12.9 ppb Pd 的苦铁矿组成。 Naujánguit 段包含两层受污染的玄武岩，即 Asûk 和 Kûgánguaq，其中 SiO2 含量较高 (52–58 wt%)，MgO 含量较低至中等 (7.5–12.8 wt%)。这些熔岩的普遍特征是铜和镍丰度较低（平均为 40 ppm 镍和 45 ppm 铜）以及极低的铂（0.16–0.63 ppb）和钯（0.13–0.68 ppb）丰度，就 Asûk 而言，它们含有页岩捕虏体以及天然铁和硫铁矿的液滴。努苏瓦克的受污染玄武岩（B0 至 B4 成员）通常也缺乏镍、铜和铂族元素 (PGE)。来自 Qeqertarssuaq 的所有受污染玄武岩和一些来自 Nuussuaq 的玄武岩的地球化学特征（特别是不相容的微量元素如 Th/Nb 的比率）记录了似乎来自熔岩正下方的三角洲页岩的化学贡献。这表明岩浆的污染发生在岩浆通过沉积岩中发育的管道系统迁移期间，因此发生在高地壳水平。镍、铜和铂族元素的消耗以及地壳污染产生的地球化学特征也是诺里尔斯克地区西伯利亚陷阱玄武岩的一个特征。这些玄武岩属于厚度为 0 至 500 米、约 5,000 至 10,000 平方公里的 Nadezhdinsky 地层，该地层位于诺里尔斯克地区的中心。西伯利亚和西格陵兰岛之间的一个主要区别是，Cu 和 Ni 含量最低的 Nadezhdinsky 组样品中 PGE 的消耗比西格陵兰岛污染的玄武岩要严重得多。此外，与 Nadezhdinsky 地层相比，西格陵兰岛受污染和贫金属的火山岩的体积非常显着；当地中心很少包含超过 15 个细流，总厚度 <50 m，更常见的是 10-20 m，因此每个系统喷发部分的体积可能比 Nadezhdinsky 大厦小两个数量级。西格陵兰中心沿着断层带并列，这些断层带似乎与第三纪三角洲的沉降有关，因此沿着南北结构的侵位似乎是熔岩和支线侵入物分布的主要控制因素。这使我们认为格陵兰岛系统很小，硫化物的分离发生在地壳的高位，而在诺里尔斯克，饱和事件发生在深处，随后含硫化物的岩浆就位到地壳的高位。因此，期望西格陵兰溢流玄武岩经历诺里尔斯克系统规模的矿化过程可能是不合理的。
Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ∼1,000 m of picritic lavas and discrete 10- to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7–29 wt% MgO, 80–1,400 ppm Ni, 5.7–9.4 ppb Pt and 4.2–12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52–58 wt%) and low to moderate MgO (7.5–12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16–0.63 ppb) and Pd (0.13–0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril’sk region. These basalts belong to the 0- to 500-m thick, ∼5,000- to 10,000-km3Nadezhdinsky Formation, which is centered in the Noril’sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metal-depleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10–20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N–S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril’sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril’sk system.