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Crustal Contamination of Mafic Magmas: Evidence from a Petrological, Geochemical and Sr–Nd–Os–O Isotopic Study of the Proterozoic Isortoq Dike Swarm, South Greenland

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Crustal Contamination of Mafic Magmas: Evidence from a Petrological, Geochemical and Sr–Nd–Os–O Isotopic Study of the Proterozoic Isortoq Dike Swarm, South Greenland Lithos 74 (2004) 199–232 www.elsevier.com/locate/lithos Crustal contamination of mafic magmas: evidence from a petrological, geochemical and Sr–Nd–Os–O isotopic study of the Proterozoic Isortoq dike swarm, South Greenland Ralf Halamaa, Michael Marksa, Gerhard Bru¨gmannb, Wolfgang Siebela, Thomas Wenzela, Gregor Markla,* a Institut fu¨r Geowissenschaften, Eberhard-Karls-Universita¨tTu¨bingen, Wilhelmstrasse 56, D-72074 Tu¨bingen, Germany b Max-Planck-Institut fu¨r Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany Received 10 June 2003; accepted 10 March 2004 Available online 27 April 2004 Abstract The mid-Proterozoic Isortoq dike swarm in the Gardar Province, South Greenland, comprises a variety of alkaline rocks ranging from gabbroic to syenitic in composition. Major magmatic mineral phases are olivine, clinopyroxene, Fe–Ti oxides, amphibole, plagioclase and alkali feldspar. Quartz occurs in some samples as a late magmatic phase. Liquidus temperatures of olivine-bearing samples range between 1120 and 1145 jC and solidus temperatures are 850–930 jC. Calculated silica activities are highly variable between 0.53 and unity. Oxygen fugacities vary from À 3 to + 1 log units relative to the fayalite–magnetite–quartz buffer. The rocks have MgO contents < 6 wt.% with Mg# between 53 and 17. Primitive mantle-normalized trace element patterns show a relative enrichment of LIL elements with Ba peaks and Nb troughs. Clinopyroxenes show a general enrichment in REE relative to chondritic values with variable slightly positive to prominent negative Eu anomalies. Two of the dikes were dated with Sm–Nd three-point isochrons at 1190 F 44 and 1187 F 87 Ma, respectively. Initial 87Sr/86Sr ratios of mafic mineral separates range from 187 188 0.70289 to 0.70432 and initial eNd values vary from + 0.3 to À 10.7. Whole-rock initial Os/ Os ratios are highly variable 18 including very radiogenic values of up to 7.967. d Ov-smow values of separated clinopyroxene and amphibole range from + 5.2x to + 6.2xand fall within the range of typical mantle-derived rocks, although mixing with a lower crustal component is permitted by the data. Using energy-constrained assimilation-fractional crystallization (EC-AFC) modeling equations, the Sr–Nd isotope data of the more radiogenic samples can successfully be modeled by addition of up to 10% lower crustal granulite-facies Archean gneisses as contaminants. The Os isotopic data also suggest the involvement of old radiogenic crust. In accordance with seismic data, we conclude that a wedge of Archean crust extends from West Greenland further to the south below the present erosion level. D 2004 Elsevier B.V. All rights reserved. Keywords: Sr–Nd–Os–O isotopes; QUILF; EC-AFC modeling; Mafic dike swarm; Gardar province; South Greenland 1. Introduction * Corresponding author. Tel.: +49-7071-2972930. Mafic dike swarms of Proterozoic age are wide- E-mail address: [email protected] (G. Markl). spread in many Precambrian cratons and their intru- 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.03.004 200 R. Halama et al. / Lithos 74 (2004) 199–232 sion indicates a considerable extension of the conti- rocks ranging from gabbroic to syenitic in composi- nental crust (Tarney and Weaver, 1987). Studies of tion, it is well suited to study the influence of crustal rift-related mafic dike swarms are essential for under- assimilation processes on the composition of intra- standing generation of such extensive mafic magma- continental mafic magmas. For that purpose, we tism, and they may be used to identify mantle plumes combine petrological with Sr–Nd–O isotopic and (Ernst and Buchan, 1997, 2001). There is general whole-rock geochemical data. Additionally, we apply agreement that mafic mantle-derived magmas experi- the Re–Os method which is known to have a great ence some degree of crustal contamination during potential as tracer of crustal contamination (e.g., ascent and/or residence in crustal magma chambers Chesley and Ruiz, 1998). The olivine–gabbroic dikes (Mohr, 1987). The crust is both a density filter and a may provide information on the nature of the mantle source of incompatible elements (Lightfoot et al., sources for the dikes and Gardar magmatism in 1991) and it may act as a site of extensive and general. extensive partial crystallization of primitive melts (O’Hara and Herzberg, 2002). However, there is much debate whether the trace element and isotopic charac- 2. Geological setting teristics of intracontinental basalts are mantle-derived or due to crustal contamination. Crustal contamination The mid-Proterozoic Gardar Province in South has been shown to be important in the petrogenesis of Greenland represents a rift-related igneous province flood basalts (Devey and Cox, 1987; Peng et al., with magmatic activity lasting from about 1.35 to 1.14 1994; Chesley and Ruiz, 1998; Baker et al., 2000). Ga (Emeleus and Upton, 1976; Upton and Emeleus, On the other hand, a mantle source enriched in 1987; Upton et al., 2003) (Fig. 1a). The country rocks incompatible trace elements not influenced by a major mainly consist of I-type calc-alkaline granitoids crustal input was postulated for some Proterozoic dike (Julianeha˚b batholith) of Early Proterozoic age (van swarms (Tarney and Weaver, 1987; Condie et al., Bremen et al., 1974; Allaart, 1976; Patchett and 1987; Boily and Ludden, 1991) and other flood Bridgwater, 1984; Kalsbeek and Taylor, 1985). Re- basalts (Molzahn et al., 1996). If a mantle-derived cent work (Garde et al., 2002) indicates that the geochemical signature can be demonstrated, it Julianeha˚b batholith was emplaced mainly between remains difficult to locate the mantle source, i.e., 1.85 and 1.80 Ga. In the northwestern part of the whether the magmas are predominantly derived from province, Archean rocks of the Border and Foreland the subcontinental lithospheric mantle (SCLM) (e.g., Zones of the craton comprise the basement (Allaart, Gallagher and Hawkesworth, 1992; Turner et al., 1976). Apart from 12 major alkaline igneous com- 1996), from an upwelling mantle plume (e.g., LeChe- plexes and a succession of supracrustal lavas and minant and Heaman, 1989; Walker et al., 1997; sediments (Eriksfjord Formation), a large number of Puchtel et al., 1999) or from a mixture of both (e.g., dike rocks with variable chemical composition Ellam et al., 1992; Gibson et al., 1995; Thompson et intruded the basement. Two major dike swarms were al., 1998). emplaced into the Julianeha˚b batholith along WSW– Here we present new petrological, geochemical ENE to SW–NE trends in the Tugtutoˆq–Ilı´maussaq and isotopic data of the Isortoq dike swarm in the and the Nunarssuit–Isortoq zones during the late mid-Proterozoic igneous Gardar Province in South Gardar period ( f 1.20–1.14 Ga). The latter of these Greenland. The Gardar Province represents a failed is the topic of the present paper. rift and comprises numerous mafic dikes of different In the Isortoq area, several generations of gabbro- generations. The Isortoq dike swarm is one major ic to intermediate dikes with variable widths from a Gardar mafic dike swarm which has not been studied few centimeters to several hundred meters occur in great detail before, although the feldspathic inclu- (Bridgwater and Coe, 1970, Fig. 1b). The earliest sions in some of the dikes were the topic of several generation of Gardar dikes belongs to a regional dike investigations (Bridgwater, 1967; Bridgwater and swarm of early-Gardar olivine dolerites (‘‘Brown Harry, 1968; Halama et al., 2002). Because the swarm Dikes’’ or ‘‘BD0 dikes’’) trending WNW–ESE comprises a variety of petrographically diverse dike across the Gardar province. They are dated at R. Halama et al. / Lithos 74 (2004) 199–232 201 Fig. 1. (a) Map of the Gardar Province (after Upton and Emeleus, 1987). (b) Map of the Isortoq region (after Bridgwater and Coe, 1970) with sample localities. 202 R. Halama et al. / Lithos 74 (2004) 199–232 1282 F 5Ma(Upton et al., 2003), predating the diameter of 5 Am was used to avoid Na migration. main late-Gardar dike swarm of the Isortoq area. The bulk compositions of oxy-exsolved titanomag- The late-Gardar dikes of the Nunarssuit–Isortoq netite grains were reconstructed by combining image region were emplaced along WSW–ENE to SW– processing (NIH Image software) of backscattered NE trends. The most spectacular feature of the dike electron images of the exsolved grains with point swarm is the abundance of anorthosite xenoliths and analyses of exsolved ilmenite and broad beam anal- feldspar megacrysts in some of the gabbroic dikes yses of exsolved magnetite (Marks and Markl, (Bridgwater and Harry, 1968; Halama et al., 2002) 2001). that are informally named ‘‘Big Feldspar Dikes’’ Rare earth element (REE) contents in clinopyrox- (BFDs). The feldspathic material varies from single enes were measured by in situ laser ablation induc- feldspar crystals ( < 1 cm to 1 m in size) to anortho- tively coupled plasma-mass spectrometry (LA-ICP- sitic bodies up to 30 m long. It is considered as MS) at the EU Large-Scale Geochemical Facility evidence for an anorthosite body underlying South (University of Bristol) using a VG Elemental Plasma- Greenland (Bridgwater, 1967). BFD 1 can be traced Quad 3 + S-Option ICP-MS equipped with a 266 nm for 30 km and is one of the dikes with the greatest Nd-YAG laser (VG MicroProbe II). Details of the volume of feldspathic material in South Greenland method are described by Halama et al. (2002). The (Bridgwater and Harry, 1968). BFD 2 is a composite precision of trace element concentrations, based on dike that consists of a 1–2-m-wide marginal micro- repeated analyses of standards, is approximately F 5% syenite enclosing a feldspathic alkaline gabbro for element concentrations >10 ppm and F 10% for (Bridgwater and Harry, 1968).
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