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Parental Magma of the Skaergaard Intrusion: Constraints from Melt Inclusions in Primitive Troctolite Blocks and FG-1 Dykes

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Parental Magma of the Skaergaard Intrusion: Constraints from Melt Inclusions in Primitive Troctolite Blocks and FG-1 Dykes Contrib Mineral Petrol (2010) 159:61–79 DOI 10.1007/s00410-009-0416-3 ORIGINAL PAPER Parental magma of the Skaergaard intrusion: constraints from melt inclusions in primitive troctolite blocks and FG-1 dykes Jakob K. Jakobsen Æ Christian Tegner Æ C. Kent Brooks Æ Adam J. R. Kent Æ Charles E. Lesher Æ Troels F. D. Nielsen Æ Michael Wiedenbeck Received: 17 December 2007 / Accepted: 8 June 2009 / Published online: 14 July 2009 Ó The Author(s) 2009. This article is published with open access at Springerlink.com Abstract Troctolite blocks with compositions akin to the distinct La/SmN and Dy/YbN ratios that link them to the Hidden Zone are exposed in a tholeiitic dyke cutting across lowermost volcanic succession (Milne Land Formation) of the Skaergaard intrusion, East Greenland. Plagioclase in the regional East Greenland flood basalt province. New these blocks contains finely crystallised melt inclusions major- and trace element compositions for the FG-1 dyke that we have homogenised to constrain the parental magma swarm, previously taken to represent Skaergaard magmas, to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7– overlap with the entire range of the regional flood basalt 14.1 wt.% FeOT. These compositions are lower in FeOT succession and do not form a coherent suite of Skaergaard and higher in SiO2 than previous estimates and have like melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism. Communicated by J. Hoefs. Keywords Skaergaard intrusion Á Melt inclusions Á J. K. Jakobsen Á C. Tegner Bulk composition Á Parental magma Á East Greenland Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark J. K. Jakobsen (&) Introduction Nordic Volcanological Centre, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland e-mail: [email protected] The Eocene Skaergaard intrusion in East Greenland has been considered a type example of extreme fractional C. K. Brooks crystallisation of small basaltic magma chambers in the Geological Museum, Øster Voldgade 5-7, upper crust for almost 70 years (Irvine et al. 1998; 1350 Copenhagen K, Denmark McBirney 1995; Wager and Brown 1968; Wager and Deer A. J. R. Kent 1939). A central problem for geochemical and petrological Department of Geosciences, Oregon State University, modelling of the Skaergaard intrusion is identification of 104 Wilkinson Hall, Corvallis, OR 97331, USA the parental magma composition. This bears directly on C. E. Lesher important issues such as the liquid line of descent, the Department of Geology, University of California, Davis, origin of the Pd–Au mineralisation hosted in the intrusion, One Shields Avenue, Davis, CA 95616, USA volume estimates of exposed and unexposed parts of the intrusion, and the links between the intrusion and the T. F. D. Nielsen Geological Survey of Denmark and Greenland, Øster Voldgade contemporaneous flood basalt succession. Indeed, identifi- 10, 1350 Copenhagen K, Denmark cation of parental magma compositions is a general prob- lem for most plutonic rocks. M. Wiedenbeck Previous estimates of the parent magma have been Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum, Telegrafenberg C161, based on chilled samples from the margins of the intru- 14473 Potsdam, Germany sion (Hoover 1989a; Wager 1960), related dykes (Brooks 123 62 Contrib Mineral Petrol (2010) 159:61–79 and Nielsen 1978, 1990) and mass-balance calculations Brooks 2004). The emplacement of the intrusion is inti- (Ariskin 2002; Nielsen 2004). These methods all point to mately linked to an episode of crustal extension, rifted a ferro-tholeiitic composition. However, even small dif- margin formation and outpourings of a 6–10-km thick ferences of less than a few wt.% SiO2 or FeO, for flood basalt succession during the opening of the North- example, may result in a huge variation in modelled east Atlantic (Andreasen et al. 2004; Irvine et al. 1998; magma compositions with prolonged fractional crystalli- Tegner et al. 1998a). The box-shaped intrusion was em- sation (e.g. Bowen or Fenner trends). Melt inclusions placed at the unconformity between the Archean basement provide alternative and direct information about the and the overlying flood basalts and was accommodated by magma from which layered mafic intrusions crystallised steep boundary faults (Irvine 1991; Nielsen 2004). Pres- (Li et al. 2005; Spandler et al. 2000, 2005). For sure estimates demonstrate that the intrusion was em- Skaergaard, Hanghøj et al. (1995) showed that melt placed into a thin volcanic cover, at a depth equivalent to inclusions (now fully crystallised) represent an iron-rich a pressure of 0.7 ± 0.5 kbar at the roof contact, but ferrobasalt magma from which the Middle Zone crystal- subsided during crystallisation, due to emplacement of lised. Furthermore, melt inclusions in apatite and olivine flood basalts over the intrusion, to a depth equivalent to of the Upper Zone demonstrate that the evolved magma 3.0 ± 1.3 kbar at the roof contact (Larsen and Tegner exsolved into iron- and silica-rich melts by silicate-sili- 2006). cate liquid immiscibility (Jakobsen et al. 2005). Traditionally, the intrusion is divided into: (a) the Here we examine melt inclusions in plagioclase of Layered Series (LS), which is the most prominent unit primitive plagioclase-olivine (troctolite) cumulate rocks and crystallised upwards from the bottom of the magma to evaluate the parental magma composition. An obstacle chamber; (b) the Upper Border Series (UBS) that solidi- to this approach is that the most primitive cumulate fiedfromthetopanddownwards;and(c)theMarginal rocks of the Skaergaard intrusion remain unexposed in Border Series (MBS) that crystallised inwards from the the Hidden Series (McBirney 1996; Wager and Brown walls (Naslund 1984; Wager and Brown 1968;Wager 1968). We therefore examine metre-sized blocks of and Deer 1939). Each of these units is subdivided into primitive troctolite exposed in a *15 m wide basaltic zones and subzones based on their cumulus mineralogy dyke cutting the intrusion (Irvine et al. 1998). We first (see McBirney 1996 and references therein). The intru- describe the petrography, mineral chemistry, and whole sion has undergone extreme fractionation which is rock composition of the troctolitic blocks and show that clearly reflected in the composition of the latest cumu- it is plausible that they originate from the unexposed part lates with six cumulus phases and pure iron end-mem- of the Skaergaard intrusion, as first speculated by Brooks bers for pyroxene (hedenbergite) and olivine (fayalite) and Nielsen (1990). We then describe the petrography and sodium-rich plagioclase.TheUBSandtheLSmeet and major and trace element composition of melt at the Sandwich Horizon, which is generally believed to inclusions trapped in cumulus plagioclase of these represent the final liquid (McBirney 1996; Wager and troctolite blocks. We also re-examine the major and trace Brown 1968). element composition of the related FG-1 dykes (Brooks The lowest part of the LS is unexposed and is tradi- and Nielsen 1978, 1990). The compositions of homo- tionally referred to as the Hidden Zone (HZ), its relative genised melt inclusions (47.4–49.0 wt.% SiO2;13.4– size is unknown but various estimates for HZ range from T 14.9 wt.% Al2O3; 10.7–14.1 wt.% FeO ) are believed to 15 to 70% of the entire intrusion (Blank and Gettings 1973; represent a plausible magma composition. We discuss Maaløe 1976; Norton et al. 1984; Wager 1960). The low- the merits of this composition relative to the FG-1 dykes ermost part of LS denoted Lower Zone a (LZa) and the and other suggested parental Skaergaard magmas and drilled part of HZ have essentially the same mineralogy their petrogenetic links to the overlying flood basalt (Maaløe 1976) and consist of cumulus plagioclase and succession. olivine with poikilitic augite. Inverted pigeonite and interstitial oxides occur in small amounts. The MBS per- tinent for this contribution forms the eastern and western Geological setting side of the intrusion (Fig. 1a) and ranges from 70 to 600 m in width (Hoover 1989b). Wager and Brown (1968) divi- The *55 Ma Skaergaard layered mafic intrusion mea- ded the MBS into an outer ‘tranquil division’ and an inner sures approximately 11 9 7 km in outcrop (Fig. 1a) and ‘banded division’. The outer edge of the tranquil division is is located at the mouth of the Kangerlussuaq fjord, East in places highly variable in compositions and appears to Greenland (Brooks and Gleadow 1977; Gleadow and represent pulses of magma (Holness et al. 2007; Hoover Brooks 1979; Hirschmann et al. 1997; Hamilton and 1989b). 123 Contrib Mineral Petrol (2010) 159:61–79 63 31°45' 31°40' 31°35' 1 N 0 50 100 m 68° 68° 13' 13' ? gneiss LZ ? KT- 39 locality MZ 3 MBS 2 Campsite Dyke Kræmer UZ Island Campsite MZ dyke Forbindelses Glacier 1 MBS Uttental Sound Home 68° Bay UZ 68° 10' Locality 10' Troctolite with troctolites Basistoppen blocks Sheet Campsite UBS North House foundation EG4507 Uttental Sou Skaergaard Bay locality basalt 0 1 2 km a nd b Seawater 31°45' 31°40' 31°35' Plagioclase ultraphyric dyke Fig. 1 Location and map of the Campsite Dyke. a Map of the area (inset on Fig. 1a) showing the complex array of dykes, their Skaergaard intrusion with the Campsite Dyke marked with a grey cutting relations and location of the studied troctolite blocks. Dashed line. Sample locations for chilled margins discussed in the text are line/lighter shading indicates that the dykes are not well exposed. shown; EG4507 (Wager 1960) and KT-39 (Hoover 1989a). FG-1 Square boxes with numbers refer to the inferred relative ages of the dykes are 10 km off the map to the west. MBS Marginal Border dykes (Irvine et al.
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