Cumulates from primitive rift-related East Greenland Paleogene magmas: Petrological and isotopic evidence from the ultramafic complexes at Kælvegletscher and near Kærven
Cumulates from primitive rift-related East Greenland Paleogene magmas: Petrological and isotopic evidence from the ultramafic complexes at Kælvegletscher and near Kærven
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DOI：
10.1016/j.lithos.2006.03.036
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发表时间：
2006-11
期刊：
影响因子：
3.5
通讯作者：
P. Holm;Niels-Ole Prægel
中科院分区：
文献类型：
--
作者：
P. Holm;Niels-Ole Prægel
在东格陵兰岛中部Kangerlussuaq峡湾的Kærven-KælVegletscher地区，出现了许多古近纪超镁铁质侵入岩。我们介绍了其中五个地区的地质、地球化学和同位素数据：Kælgreletscher、Kartografvig、North Kærven、East Kærven和‘Mordor’。侵入体侵位于太古代的地下室，KælVegletscher侵入体的北部被Kangerlussuaq侵入体截断，Kærven侵入体被Kærven Gabbro截断。超镁铁质岩石范围从覆盖在辉石岩和二辉橄榄岩上的纯橄榄岩到橄榄单斜辉石岩，并显示橄榄石和橄榄石-铬铁矿堆积结构，范围从橄榄石-铬铁矿-单斜辉石-单斜辉石和偶尔的单斜辉石-橄榄石堆积体。纯橄榄石矿物由早期形成的橄榄石(FO86-)±铬尖晶石(Cr#69-95)和偶尔的间隙单斜辉石(MG#-97)组成，而辉石和橄榄单斜辉石保留了较多的富铁橄榄石(FO77-85)，并在铬铁矿中保留了较少的镁单斜辉石(MG#77)和较低的铬#(Cr#66)。超镁铁质岩石被解释为几乎由原始地幔岩浆形成的堆积体，在KælVegletscher杂岩中，这些岩浆反复侵入到以前侵入的脆性岩石中。单斜辉石地球化学表明，岩浆为拉斑玄武岩，其二氧化钛含量较低，为1-2wt.%。堆积体中捕获液体的相对比例是使用微量元素模拟的，这表明接触岩石中的捕获液体可能是侵入体内部岩石的2-3倍。局部地壳中较高的Ba/Nb、Rb/Nb和Ba/La比值低于原始地幔，表明大多数岩石的地壳混染作用很小。HREE模拟表明，熔融提取的温度约为3 Gpa。同位素组成范围：Ma)=−3.7–+3.8,206Pb/204Pb=15.44–17.83,207Pb/204Pb=14.87–15.39为55 Ma，ε为−14-+34,208Pb/204Pb为36.41-38.15，为ε-Nd55。除了少数受到强烈地壳污染的样品外，有两种成分可以解释大部分的同位素变化。一种是岩石圈地幔成分，Kærven端元，具有非放射性成因的铅、锶和放射性成因的Nd，但由于岩石中低的锶/铅比值和极低的不相容元素丰度，不能排除地壳混染的可能性。另一个端元与冰岛玄武岩中最非放射性成因的铅相似，在组成上与北大西洋端元[Ellam，R.M.，Stuart，F.M.，2000]相似。北大西洋玄武岩的次岩石圈来源：共同端元的证据和意义。J.汽油。41,919-932]。没有迹象表明重要的IREM(冰岛)放射性端元[Ellam，R.M.，Stuart，F.M.，2000]。北大西洋玄武岩的次岩石圈来源：共同端元的证据和意义。J.汽油。41,919-932]在Kangerlussuaq以北和以东的熔岩中也可以看到。研究表明，55 Ma前大陆裂解时冰岛地幔热柱的两个组分在Kangerlussuaq地区之下是分离的并相邻的。建议在格陵兰岛中东部的古近纪火成岩中，分别以NaEM和IREM为主，追踪这些羽流成分的范围。
A number of Paleogene ultramafic intrusions occur in the Kærven–Kælvegletscher area of the Kangerlussuaq Fjord, central East Greenland. We present geological, geochemical and isotopic data from five of these: Kælvegletscher, Kartografvig, North Kærven, East Kærven and ‘Mordor’. The intrusions are emplaced in the Archaean basement and the northern part of the Kælvegletscher intrusion is truncated by the Kangerlussuaq intrusion and the North Kærven by the Kærven Gabbro. The ultramafic rocks range from dunite over wehrlite and lherzolite to olivine clinopyroxenite and display olivine and olivine–chromite cumulate textures ranging into olivine–chromite–clinopyroxene and occasional clinopyroxene–olivine cumulates. Dunite mineralogy consists of early formed olivines (Fo86–89)±chrome–spinels (Cr#69–95) with occasional interstitial clinopyroxene (Mg#89–97), while wehrlites and olivine clinopyroxenites retain more Fe-rich olivine (Fo77–85) and range to less magnesian clinopyroxene (Mg#77) and lower Cr# in chromite (Cr#66). The ultramafic rocks are interpreted as cumulates formed by almost primary mantle-derived magmas, which in the Kælvegletscher complex were recurrently intruded into brittle rocks of previous intrusions. Clinopyroxene geochemistry indicates that the magmas were tholeiitic and had relatively low TiO2=1–2 wt.%. Relative proportions of trapped liquids in the cumulates are modelled using trace elements, which suggest that contact rocks may contain 2–3 times as much trapped liquid as rocks from the interior part of intrusions. Lower than primordial mantle values of Ba/Nb, Rb/Nb and Ba/La ratios, which are high in the local crust, indicate little crustal contamination for most rocks. HREE modelling suggests melt extraction at around 3 GPa. Isotopic compositions range: εSr(55 Ma)=−14–+34, εNd(55 Ma)=−3.7–+3.8,206Pb/204Pb=15.44–17.83,207Pb/204Pb=14.87–15.39 and208Pb/204Pb=36.41–38.15. Except for a few samples that are strongly crustally contaminated, two components explain most of the isotopic variation. One is suggested to be a lithospheric mantle component, the Kærven end-member, with unradiogenic Pb, Sr and radiogenic Nd, but crustal contamination cannot be ruled out because of low Sr/Pb ratios and very low abundances of incompatible elements in the rocks. The other end-member approximates the most unradiogenic Pb in Icelandic basalts and is similar in composition to the NAEM [North Atlantic End-Member [Ellam, R.M., Stuart, F.M., 2000. The Sub-lithospheric Source of North Atlantic Basalts: evidence for, and Significance of, a Common End-member. J. Petrol. 41, 919–932]. There is no indication of the important IREM (Icelandic Radiogenic End-Member [Ellam, R.M., Stuart, F.M., 2000. The Sub-lithospheric Source of North Atlantic Basalts: evidence for, and Significance of, a Common End-member. J. Petrol. 41, 919–932] otherwise seen in lavas to the north and east of Kangerlussuaq. It is indicated that two components of the Icelandic mantle plume at continental break-up 55 Ma ago were isolated and adjacent under the Kangerlussuaq area. Paleogene igneous rocks in Central East Greenland dominated by NAEM and IREM, respectively, are suggested to trace the extent of these plume components.