Origin of the Transgressive granophyres from the Layered Series of the Skaergaard intrusion, East Greenland

Journal of Volcanology and Geothermal Research, 52 ( 1992 ) 185-207

185

Elsevier Science Publishers B.V., Amsterdam

Origin of the Transgressive granophyres from the Layered Series of the Skaergaard intrusion, East Greenland Marc Hirschmann Department of Geological Sciences, AJ-20, University of Washington, Seattle, WA 98195, USA (Received March 9,1991; revised version accepted January 12, 1992 )

ABSTRACT Hirschmann, M., 1992. Origin of the Transgressive granophyres from the Layered Series of the Skaergaard intrusion, East Greenland. In: D.J. Geist and C.M. White (Editors), Special Issue in Honour of Alexander R. McBirney. J. Volcanol. Geotherm. Res., 52: 185-207. The Transgressive granophyres are a swarm of silicic dikes and sills in the Layered Series of the Skaergaard intrusion that range in composition from ferrodiorites to granites. Although originally thought to be derived by differentiation of Skaergaard magma, accumulated evidence indicates that the origin of these granophyres is more complex and involves contributions from melted country rocks. In the present study it is shown that field relations are inconsistent with any genetic relationship between the Transgressive granophyres from the Layered Series and differentiated Skaergaard magma. Although new Sr isotope analyses confirm that these granophyres are at least partly derived by fusion of Archean country rock, field relations preclude derivation of this crustal component from partially melted gneissic xenoliths from the Skaergaard magma chamber or from melting along the contact between the gneisses and the Skaergaard pluton. Major- and trace-element compositions, together with Sr isotope ratios indicate that the Transgressive granophyres from the Layered Series represent a cogenetic suite of magmas related to each other primarily by a closed system process such as crystal fractionation, and that the parental magma that gave rise to these granophyres was formed beneath the Skaergaard intrusion by contamination of differentiated post-Skaergaard mafic magmas with partially fused Archean crust.

Introduction

Since the pioneering work of Wager and his coworkers, research on the Skaergaard intrusion has deeply influenced petrologic thinking about such topics as the liquid line of descent, differentiation, and magma chamber dynamics (e.g., Wager and Deer, 1939; Wager and Brown, 1967; McBirney, 1975; McBirney and Correspondence to: M. Hirschmann, Department of Geological Sciences, AJ-20, University of Washington, Seattle, WA 98195, USA.

Noyes, 1979; Hunter and Sparks, 1987). Although the Skaergaard is one of the most extensively studied intrusions in the world, there are still fundamental controversies about its igneous evolution. In particular, there is debate about the path of the liquid line of descent and the relationship, if any, between differentiation of the Skaergaard magma and evolution of silicic liquids (Hunter and Sparks, 1987, 1990; McBirney and Naslund, 1990; Morse, 1990; Brooks and Nielsen, 1990). An issue related to the postulated evolution of silicic liquids from the Skaergaard intrusion that has also generated considerable disagreement

0377-0273/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

18 6

M. HIRSCHMANN

is the origin of silicic granophyres* that are temporally and spatially associated with the Skaergaard complex. Wager and Brown ( 1967 ) distinguished five separate classes of granophyres in the Skaergaard intrusion: ( 1 ) Transgressive granophyres, (2) the Tinden acid granophyre, ( 3 ) the Sydtoppen transitional granophyre, (4) melanogranophyres (called "hedenbergite granophyres" in Wager and Deer, 1939), ( 5 ) clot-cored granophyres of the Marginal Border Group. The Transgressive granophyres comprise a swarm of small dikes and sills, primarily in the Layered Series, and were given their name by Wager and Deer (1939) because they transgress the igneous layering of the Skaergaard gabbros. In addition to these relatively small bodies, there are two thick silicic sills in the Upper Border Group, the Sydtoppen and Tin-

W

E

Fig. 1. Schematic cross section of Skaergaard intrusion. Shaded area in Layered Series (LS) is region where Transgressive granophyres are abundant. Shaded area in Upper Border Group (UBG) represents approximate location of Tinden and Sydtoppen Sills. *Silicic rocks associated with layered intrusions have been traditionally termed "granophyres" in the field and in the literature, but many, particularly the Transgressive granophyres in the Layered Series of the Skaergaard intrusion, lack granophyric texture. In order to avoid confusion and to remain consistent with previous literature, the term "granophyre" is used in this report, but in the present context it is used as a rock name, not as a textural description.

den granophyres (Fig. 1 ). The Tinden sill directly overlies the Sydtoppen sill, which in turn overlies the Basistoppen sill, a differentiated mafic sheet that intruded the Upper Border Group after the Skaergaard intrusion solidified (Naslund, 1989). The combined thickness of these granophyres ranges from 30 to 80 m, and the larger body, the Tinden, extends for 1.5 km (Naslund, 1989). The most obvious difference between the Transgressive granophyres and the Tinden and Sydtoppen sills is that of size, and the petrogenesis of these bodies has often been considered together (e.g., Wager and Deer, 1939; Leeman and Dasch, 1978 ). The petrogenesis of the melanogranophyres and the clot-cored granophyres have been considered elsewhere (McBirney and Nakamura, 1974; Kays et al., 1981 ) and will not be discussed further here. The origin of the Transgressive, Sydtoppen, and Transgressive granophyres have been the subject of considerable research and speculation. Wager and Deer (1939) concluded that the Transgressive granophyres, as well as the Tinden and Sydtoppen sills, were products of extreme differentiation of the Skaergaard magma, but S7Sr/86Sr analyses of the Tinden sill and several Transgressive granophyres (Hamilton, 1963) demonstrated that these granophyres have substantially more radiogenic strontium than do the Skaergaard gabbros. In light of these data, Hamilton (1963 ) and Wager and Brown (1967) reinterpreted the Tinden and Sydtoppen sills as differentiates of Skaergaard magma that had been contaminated by melted gneissic xenoliths, but Wager and Brown (1967) persisted in interpreting the Transgressive granophyres as late differentiates that were filter-pressed from the Layered Series, perhaps as a result of crustal flexuring. Radiogenic isotope analyses by Leeman and Dasch (1978) showed that Tinden, Sydtoppen, and Transgressive granophyres have a crustal component, but that the isotopic ratios

THE TRANSGRESSIVE GRANOPHYRES (LAYERED SERIES - SKAERGAARD INTRUSION, EAST GREENLAND)

of the Tinden and Sydtoppen sills are distinct from each other and from the Transgressive granophyres. Although not ruling out the possibility that some part of the granophyres could be derived from Skaergaard differentiates, Leeman and Dasch emphasized the role of crustal fusion in derivation of the granophyres. Oxygen isotope studies have also indicated that the Transgressive granophyres from the Layered Series are at least partly derived from melting Archean country rocks (Taylor and Forrester, 1979). The most detailed physical model for the origin of the Transgressive granophyres is that of Norton et al. (1984), who proposed that blocks of gneiss entrained during inflation of the magma chamber remained as discrete entities during the solidification of the gabbroic liquid, and that the still molten gneissic blocks were remobilized and emplaced as the Transgressive granophyres after the gabbros crystallized. Although the Transgressive granophyres have been studied as tangential components of numerous previous investigations, there has been to date no systematic appraisal of their petrogenesis. In the present study, the Transgressive granophyres from the Layered Series are investigated for the purpose of understanding their origin and to determine what, if any, relationship they may have to the Skaergaard gabbros that they intrude.

Geologic setting The Skaergaard intrusion is an Eocene differentiated tholeiitic layered body of gabbro approximately 10 km by 7 km in plan view, located in the Kangerdlugssuaq region of East Greenland (Fig. 2 ). The intrusion has been divided into three primary units: the Layered Series, which accumulated on the floor of the magma chamber, the Marginal Border Group, which crystallized on the walls of the chamber, and the Upper Border Group, which solidified beneath its roof (Wager and Deer, 1939 ). The Layered Series is further subdivided on the ba-

18 7

sis of mineralogical and textural variations into Lower, Middle and Upper Zones (Wager and Deer, 1939). The upper parts of the Skaergaard pluton intrude early Tertiary basalts and the lower portions intrude Archean gneiss. The gneiss and the basalts are separated by a thin sequence of Cretaceous arkosic sediment. The gneisses of the region, which yield a whole-rock Pb isochron of 2.98 Ga that has been interpreted as the age of metamorphism (Leeman et al., 1976), are primarily metagranites, with subordinate amounts of metasediments and mafic and ultramafic amphibolites (Kays et al., 1989). Numerous small dikes, sills, and pods intruded the Skaergaard intrusion soon after it solidified. The most abundant of these are mafic dikes that occupy perhaps 10% of the outcrop area of the southern part of the intrusion. These dikes range widely in composition, dimension, and texture. (For a more complete description, see Nielsen, 1978; and Brooks and Nielsen, 1978). Cross cutting relationships demonstrate that these mafic dikes are younger than the silicic granophyres. (Bird et al., 1986)

Field relations Transgressive granophyres intrude the Layered Series and the Upper Border Group of the Skaergaard intrusion. They are present in all major divisions of the Layered Series except LZa, but dikes and sills of Transgressive granophyre are most abundant in three areas near Uttentals Sound: in the Upper Zone south of Forbindelses glacier, in the Middle Zone north of Forbindelses glacier, and in the Middle and Lower Zones on Kraemers Island (Fig. 2 ). This region is roughly coincident with a gravity high that marks the feeder of the Skaergaard magma chamber (Fig. 1) (Blank and Gettings, 1973 ). The granophyres in this region form a complex swarm of dikes and sills that locally achieve densities of 3 intrusions per meter. The total volume of Transgressive granophyres in the

188

M. HIRSCHMANN

t

Transgressive granophyres

-7 Skaergaard gabbros

BII

Basistoppen sill Gneiss Water

Glaciers

Fig. 2. Map of western portion of Skaergaard intrusion showing location of Transgressive granophyres in the Layered Series of the Skaergaard intrusion. Modified and augmented after Bird et al. (1986). For a more general map of the Skaergaard intrusion, see Wager and Brown ( 1967 ).

Layered Series probably constitutes between 0.001 and 0.01 km 3. For comparison, the volume of the Tinden sill is probably greater than 0.1 km 3 (Naslund, 1989). The largest individual Transgressive granophyres in the Layered Series are sills in UZa that extend for more than a kilometer and are locally up to 6 meters wide. Other bodies range from this size down to crack-filling veins less than 1 m m thick and extending only a few tens of cm, but typical dikes and sills are 30-150 cm thick and 10-200 m long. The granophyres have sharp, well defined contacts with the enclosing gabbros and fill complex and irregular fractures, many of which have multiple parallel and subparallel branches (Fig. 3). Dikes change orientations, in some instances as much as 120 ° over their strike length. Even when straight, fracture orientations undulate and dike thicknesses vary irregularly. Some frac-

tures show a lateral (strike-slip) sense of displacement. The Transgressive granophyres are spatially related to several generations of high-temperature mineralized fractures. Cross-cutting relations between these veins and the granophyre dikes and sills allow reconstruction of the thermal and temporal relationship between emplacement of the granophyres and the cooling of the Skaergaard pluton (Norton et al., 1984; Bird et al., 1986). The Transgressive granophyres cross-cut pyroxene-oxide veins; are cut by veins that contain both Al-poor and Al-rich amphiboles; and are either later than or contemporaneous with veins that contain only one type of amphibole (Bird et al., 1986 ). Phase equilibria constraints and geothermometry are consistent with temperatures of formation for these assemblages of 950°C, < 500°C, and 500-750°C, respectively (Bird

18 9

THE TRANSGRESSIVE GRANOPHYRES (LAYERED SERIES - SKAERGAARD INTRUSION, EAST GREENLAND)

Fig. 3. Typical outcrops of Transgressivegranophyresfrom UZa (A) and from MZ (B). Note hammers for scale. et al., 1986). These relations constrain the temperature of the enclosing Skaergaard gabbros to be between 500 and 750°C at the time the granophyres intruded. In areas where Transgressive granophyres are abundant, cross-cutting relations indicate at least four intrusive events. Contacts of crosscutting dikes and sills are razor sharp and show evidence of brittle fracture (Fig. 4). Enough time must have elapsed between each intrusive event for the previous intrusion to have cooled below, or at least near, its solidus. For compositions qualitatively similar to the Transgressive granophyres, melting experiments at 0.2 GPa give a solidus temperatures between 675 and 700°C (Naney, 1983; Whitney, 1988 ). Combined with cross-cutting relations with mineralized fractures, these observations constrain the temperature of the enclosing gabbros at the time of intrusion of granophyres to have been 600 _+ 100 ° C. Wager and Deer (1939) failed to observe fine-grained margins in the granophyres at their

contacts with the enclosing gabbros and therefore inferred that the granophyres intruded the Skaergaard while the main intrusion was brittle, but still hot. Detailed examination in the field, however, reveals that some granophyres do in fact have thin fine-grained selvages along their margins, and there are also rare thin ( < 20 cm) granophyres that are chilled throughout

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Fig. 4. Sketch map of felsic granophyre sill cutting mafic granophyre dike and Skaergaardgabbro from nearly vertical cliff in UZa, south of Forbindelsesglacier.

190

M. H1RSCHMANN

Fig. 5. Chilled granophyre from MZ consists of phenocrysts of plagioclase (pl), alkali feldspar (aj~p), quartz (q) and biotite (bt) in matrix of quartz and feldspar. Biotite phenocryst is recrystallized to fine-grained aggregate and matrix is partially silicified and altered. Field of view is 4 mm.

(Fig. 5 ). This demonstrates that the temperature of the intruding granophyric liquids was greater than that of the gabbros into which the granophyres were emplaced. A preponderance of the Transgressive granophyre dikes in the Layered Series dip inwards towards the presumed feeder of the Skaergaard intrusion and roughly parallel with the walls of the main intrusion, though in the Upper Zone the largest bodies intruded the gabbros as sills (Bird et al., 1986). In combination with their close spatial association with hydrothermal veins (Bird et al., 1986), this suggests the granophyres intruded along fractures formed as a consequence of cooling and contraction of the Skaergaard gabbros. The inward dips of these dikes also suggest that the granophyres were derived from a source beneath the Skaergaard pluton.

Petrography The most marie granophyres are fine-grained ferrodiorites or quartz diorites which contain 60-70% plagioclase and 15-30% marie con-

stituents (chiefly green and brown amphibole). The most felsic granophyres are medium-grained leucocratic granites containing 20-35% plagioclase; 30-40% perthitic alkali feldspar, 25-35% quartz, and 1 to 2% brown biotite. All gradations between these end members are present. Additional primary phases in the marie granophyres include clinopyroxene, biotite, rare orthopyroxene, rare olivine and abundant accessory apatite and zircon. In addition to biotite, some felsic granophyres contain subordinate brown amphibole and common accessory zircon and apatite. Actual granophyric texture is not common in the Transgressive granophyres. Most intrusions lack granophyrie intergrowths altogether whereas others have sparse interstitial granophyre and only a few have more than 5%. The granophyres are equigranular, fine- to medium-grained hypidiomorphic granular rocks with average grain sizes from 0.5 m m to 3 m m (Fig. 12). Small marie enclaves are ubiquitous in the Transgressive granophyres. Although they are more common in the maflc granophyres, they

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

are present in all but the most felsic dikes and sills. They comprise up to 2% of a given intrusion and range from 0.5 cm to l0 cm in diameter. The margins of the enclaves are irregular, cuspate, and, in some instances, diffuse. In thin section it is difficult to define a host/enclave boundary, as the only differences between the enclaves and hosts are an increase in mafic constituents and a slight decrease in grain size in the former. Petrographic evidence alone suggests that the enclaves are not xenoliths. The form of their margins, consistency of texture, and mineral assemblages all suggest that these inclusions were liquids at the time that they were incorporated into the granophyres. Their finer grain size suggests that they were cooled during their incorporation in the host, and hence are probably not restitic. They may instead represent commingled mafic liquids. There are no identifiable inclusions in the Transgressive granophyres from the Layered Series that could represent xenoliths of country rock. The degree of alteration of the Transgressive granophyres ranges from incipient to extensive. Above UZb, nearly all granophyres are strongly altered, but granophyres from the rest of the Layered Series are less affected. These observations are consistent with the oxygen isotope data of Taylor and Forrester (1979), which indicates that granophyres above UZb interacted significantly with meteoric waters and that those from UZb and below did not. The most common alteration features are feldspars clouded with clay minerals and partly chloritized or otherwise degraded mafic grains.

Analytical methods Forty to 80 grams of each sample were chipped and crushed to < 200 mesh powder with an alumina jaw crusher and shatterbox. Major elements, except TiO2 and P205, were

191

determined by atomic absorption at the University of Oregon. TiO2 and P205 were determined by colorimetry and ferric/ferrous ratios were established by titration. Ba, Co, Cr, Cu, Li, Ni, Rb, Sr, and Zn were determined by atomic absorption at the University of Oregon and Ga, Nb, V, Y, Zr were analyzed by XRFS at Washington State University. Ht', Ta, Th, U, and REE abundances were determined by INAA. Approximately 400-mg samples were irradiated for 2 hours at 1 MW in the TRIGA reactor at Oregon State University. Gamma spectra were acquired with an 8192 channel ADCAM ORTEC multichannel analyzer. Short-count observations were acquired between 3 and 6 days after irradiation. Intermediate-count observations were acquired between 8 and 14 days after irradiation. Short and intermediate counts lasted 4000 and 8000 s, respectively. Data were reduced with ADCAM ORTEC software. All INAA work was done at the Oregon State University Radiation Center. For Sr isotopic analyses, approximately 400 mg of powder were weighed and dissolved by heating in teflon beakers with concentrated, vapor-distilled HF and HNO3. After drying, the residue was taken up in solution with dilute HC1 and divided into three separate aliquots for strontium isotope ratio, strontium isotope dilution, and rubidium isotope dilution determinations. Separate Rb and Sr spike solutions were added to the respective isotope dilution aliquots. Sr and Rb were separated by cation exchange using DOWEX 80-X (Bio-Rad Corp. ) resin. Purified separates were loaded on tantalum filaments. Isotopic ratios were determined with a VG 354 automated multicollector mass spectrometer and elemental abundances were determined using the isotope dilution aliquots run on a VG Micromass 30 120 sector mass spectrometer, both at the University of Alberta. All analyses are reported relative to a 87Sr/86Sr value of 0.71028 for NBS standard SRM 987.

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THE TRANSGRESSIVE GRANOPHYRES (LAYERED SERIES - SKAERGAARD INTRUSION, EAST GREENLAND)

times richer in K20 and "incompatible" trace elements (U, Hf, REE). The more silicic granophyres are broadly of granitic composition. They are more potassic than rhyolites from tholeiitic oceanic settings, such as Iceland (e.g., MacDonald et al., 1987 ), but compositionally similar to some of the felsic granophyres from continental tholeiitic provinces such as W. Scotland (e.g., Centers 2 and 3 of Mull; Walsh et al., 1979). The compositions of Transgressive granophyres define a trend on an AFM projection

Major- and trace-element compositions The Transgressive granophyres are metaluminous to mildly peraluminous (normative corundum < 1%) silicic rocks that range from 62 to 75 wt.% SiO2, but the majority of Transgressive granophyres contain greater than 66 wt.% SiO2 (Table 1, Fig. 6). The more mafic granophyres and enclaves contain between 58.6 and 66.8% SiO2 and 9.3 to 4.4% FeO ° and have compositional similarities to icelandites (e.g., MacDonald et al., 1987), though they are 2-4

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194

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Fig. 7. AFM projection showing composition of Transgressive granophyres (dots). The trend of granophyre compositions resembles that of the tholeiitic Thingmuli lavas (Carmichael, 1964 ), but is distinct from proposed Skaergaard liquid lines of descent (LLOD) (Hunter and Sparks, 1987; McBirney and Naslund, 1990).

that is similar to the lavas of Thingmuli (Carmichael, 1964) and that is markedly less enriched in iron than the Skaergaard liquid line of descent as proposed by either Hunter and Sparks (1987) or McBirney and Naslund (1990) (Fig. 7). Although the Transgressive granophyres apparently comprise a suite of differentiated tholeiitic liquids, they do not provide evidence to support either of these postulated liquid lines of descent. Major-element concentrations of the Transgressive granophyres show strong correlations with SiO2 content (Fig. 6). Felsic Kangerdlugssuaq gneisses span a similar range of compositions, but element concentrations of these gneisses do not show similar correlations with each other or with the Transgressive granophyres (Fig. 6). In the Transgressive granophyres, concentrations of elements that are strongly included in mafic minerals and feldspar (e.g., Co, Cr, Ni, Sc, Cu, Sr, Eu) decrease with increasing SiO2 (Fig. 8 ). Some elements that are generally excluded from major rockforming minerals (e.g., Th) show statistically significant positive correlations with SiO2, but others (Rb, Ba U, Hf, LREE) show only very general trends or vary non-systematically. In the case of elements highly soluble in H20 (e.g.,

Rb, Ba), these variations may be attributable to hydrothermal alteration. The lack of strong correlations between some trace elements in the granophyres is probably caused by bulk distribution coefficients near 1, as would be the case if accessory minerals such as apatite and zircon were involved in the differentiation process. Mafic enclaves plot on extensions of compositional trends for most major and trace elements (Figs. 6 and 8), a relation consistent with their representing cognate inclusions of cogenetic liquids. This relation is inconsistent with a restitic origin for the enclaves unless the compositional variation for all the granophyres can be attributed to restite unmixing (Chappel et al., 1987 ), as there is no reason to expect different processes to produce identical trends.

Sr isotopes Eleven Transgressive granophyres from the Layered Series and two mafic enclaves were analyzed for Sr isotopes. Rb/Sr ratios of analyzed samples vary from 0.17 to 2.9 for the granophyres and from 0.10 to 0.30 for the enclaves. Granophyres and enclaves plot on an

19 5

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

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isochron that gives an age of 55.2_+ 0.9 m.y. (2tr) (Fig. 9). The mean square of weighted deviates (MSWD) for these data is 7.5, a value that is substantially greater than that required for statistical validity at the 95% confidence level (Brooks et al., 1972), so in the absence of other evidence, the data give an errorchron, rather than an isochron. Independent evi-

dence, however, corroborate this age. Brooks and Gleadow (1977) obtained a fission-track age of 54.6 _+3.4 m.y. for the Skaergaard intrusion. The exact time interval between emplacement of the Skaergaard gabbros and the Transgressive granophyres is not known, but the field evidence cited above, combined with the thermal model of Norton et al. (1984) sug-

196

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Fig. 9. Rb-Sr isochron for Transgressive granophyres. The isochron was regressed by the method o f Brooks, et al, (1972).

gests it was not substantially more than 0.3 m.y. Stratigraphic evidence for the age of the Skaergaard intrusion and radiometric ages for closely associated intrusions elsewhere in Kangerdlugssuaq region are also consistent with an age for the Skaergaard gabbros and granophyres near 55 m.y. (Wager and Brown, 1967). Initial S7Sr/86Sr ratios ( = 87Sr/a6Sr55) range from 0.70836___2 to 0.70913+3 (Table 2). These ratios are larger than those of Skaergaard gabbros (87Sr/86Sr55=0.7042-0.7046, Stewart and DePaolo, 1990) and those of the post-Skaergaard marie dike swarms (87Sr/ 86Sr55=0.703-0.706, G. Goles, pers. commun., 1987). Mafic enclaves have S7Sr/a6Sr55

ratios that are similar, but not identical, to their hosts (Table 2). These differences may reflect differences inherited from slightly different parent liquids, in which case the enclaves must represent inclusions of chilled mafic granophyre liquid, but may also reflect different degrees of reaction with altering fluids of initially isotopically homogeneous material. The small differences in 87Sr/86Sr55 among the granophyres could reflect second order heterogeneities in their source materials or could have been caused by post-emplacement redistribution of Sr. S7Sr/S6Sr55 in the granophyres correlate qualitatively with their degree of alteration, as observed in thin section. More altered samples have smaller 87Sr/86Sr55 than do their more pristine counterparts (Fig. 10), probably owing to interaction with meteoric fluids that equilibrated isotopically with Skaergaard gabbros and overlying Tertiary basalts (STSr/S6Sr55=0.704-0.705) during hydrothermal circulation (Norton and Taylor, 1979). Partially altered granophyres would consequently be expected to have smaller 8 7 5 r / 865r55 than do their fresh counterparts. Additional evidence for post-emplacement changes of strontium isotopes in the Trans-

TABLE 2 Sr isotope data for the Transgressive Granophyres Sample

Location

Rb (ppm)

Sr (ppm)

875r/86Sro

87Sr/S6Sr55

SG l l a SG l l c SG 16 SG 34 SG77 SG 83 SG84 SG 114 SG115 SG 120a SG 120b SG 61 SG60 SG 57 SG 59

UZahost UZaenclave UZa MZ MZ MZ MZ UZa UZa UZahost UZaenclave UZa dike interior UZa dike contact UZa gabbro contact UZa gabbro interior

70.3 _+0.1 55.3 +_0.1 52.9 +_0.2 74.3 -+0.1 89 -+6" 110.0 _+0.1 90.3 +_0.1 94.0 +0.1 94.8 _+0.1 56.6 _+0.1 32.7 +0.1 80.3 -+0.1 80 _+8" 1.70_+ 0.01 1.59 + 0.01

261 _+2 243 _+2 264.1 _+0.1 184.2-+0.7 170 _+12" 131.0_+0.1 80 +_6" 79.3-+0.1 40.0+0.1 296 _+2 277 +1 211.1 +_0.1 183 _+16" 266.5 _+4 258 _+3

0.70908_+2 0.70929_+2 0.70881 _+2 0.70925-+2 0.71006_+3 0.71085_+2 0.71087_+5 0.71139_+2 0.71362+2 0.70917_+2 0.70889+2 0.70997 -+ 3 0.70933_+2 0.70446 _+2 0.70446 +_2

0.70849+_2 0.70879+_2 0.70837_+2 0.70836+_2 0.70893_+14 0.70900+_2 0.70838+17 0.70876_+2 0.70843_+4 0.70875_+2 0.70863+_2 0.70913 _+3 0.70837_+11 0.70445 _+2 0.70445 _+2

* Rb and Sr analyses by atomic absorbtion. All other Rb and Sr analyses by isotope dilution.

197

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

gressive granophyres comes from examination of the margin and core of a single 70-cm-thick granophyric dike from UZa. Petrographic examination of the contact between this granophyre and its wallrock reveals the presence of microfractures filled with hydrous minerals and quartz parallel to and within a few millimeters of the interface. The feldspars in the granophyre are cloudier within 1 or 2 cm of the contact than those further away; mafic minerals in the gabbros are converted to amphiboles at the contact and have reaction rims of oxides within a few millimeters of the contact. 87Sr/86Sr55 at the center of the dike (SG-61, Table 2 ) is 0.70913 + 3 and at the margin it is 0.70837___ 11 (SG-60, Table 2). Diffusive exchange between the granophyre dike and the enclosing gabbros appears not to have been an important process, as 87Sr/a6Sr55 of the gabbro at the dike/gabbro contact (SG-57, 0.70445 _+2, Table 2 ) is indistinguishable from 875r/86Sr55, 220 cm from the contact (SG-59). Although the compositions in the interior and margin of the dike are otherwise nearly identical (Table 1 ), the difference in their 878r/86Sr55 ratio spans the whole range observed for the Transgressive granophyres. This suggests that observed variation in 878r/86Sr55 ratios of the

Transgressive granophyres was caused by postemplacement processes and that the sources of the Transgressive granophyres had similar or identical 878r/S6Sr ratios.

The Tinden and Sydtoppen sills One sample of the Tinden sill from its southernmost outcrop south of Brodregletscher was analyzed for Sr isotopes. This result, along with a compilation of all previously published strontium isotope data from the Tinden and Sydtoppen sills, is listed in Table 3. Although the precision of some of the data is poor, several conclusions may be drawn: (1) S7Sr/S6Sr55 ratios of the Tinden and Sydtoppen sills are greater and less than, respectively, those of the Transgressive granophyres from the Layered Series. These sills are, therefore, not directly related to the Transgressive granophyres. (2) 87Sr/S6Srss ratios of the Sydtoppen sill are similar to that determined for the Basistoppen Sill (Stewart and DePaolo, 1990), which it directly overlies. This observation supports the conclusion of Naslund (1989) that this sill (referred to by Naslund (1989) as

TABLE3 Sr isotope data for the Tinden and Sydtoppen Sills Sample

Rb

Sr

87Sr/86Sro

87Sr/S6Sr55

Source

86.5+0.9" 90 +2%* 67 +5% 125 + 5% 96 +10% 50 +10%

92+2" 92+2% ° 62+5% 65 + 5% 92+10% 130+10%

0.72889+4 0.71591+3 0.71371+10 0.72516+ 10 0.732 +3 0.710 +3

0.72595+8 0.71369+12 0.7113+2 0.7200+ 3 0.730+3 0.709+3

1 2 3 3 4 4

120+5% 88+5% 306 + 5%

0.70797+ 10 0.70821+10 0.70667 + 4

0.7068+2 0.7067+2 0.7063 + 3

3 3 3

Tinden Sill SG 97 SK48 SK48 DK 20 EG3058 EG5206

Sydtoppen Sill SK 88A'" SK 88B** EG-4489

66 61 47

+5% +5% + 5%

Sources: ( 1 ) this study; (2) Stewart and Depaolo (1990); (3) Leeman and Dasch ( 1978); (4) Hamilton (1963). * Rb and Sr analyses by isotope dilution. All other samples analyzed by XRF. ** SK 88A and SK 88B are separately crushed splits of the same sample.

198

M. HIRSCHMANN

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

GRN-1 ), is derived in large part by differentiation of the Basistoppen magma. (3) The Tinden sill is isotopically heterogeneous and 875r/SrSr55 does not correlate with composition, as indexed by Rb/Sr (Table 3 ). These data are difficult to reconcile with Hunter and Spark's (1987, 1990) assertion that this sill is a mixture of differentiated Skaergaard liquid with partially fused gneiss, as the compositions do not form a mixing trend. They are more easily reconciled with the hypothesis of Naslund (1989) that the Tinden sill (referred to by Naslund (1989) as GRN2) is derived primarily from a raft of partially melted gneiss that was emplaced in temporal association with the intrusion of the Basistoppen Sill. If this is the case, then mixing of the constituents of the sill was insufficient to homogenize isotopic features inherited from the country rock from which they were derived. Discussion

Derivation of granophyresfrom within the Skaergaard intrusion The elevated 878r/a6Sr55 ratios of the Transgressive granophyres, relative to the Skaergaard gabbros corroborates inferences from previous investigations (e.g., Hamilton, 1963; Leeman and Dasch, 1978) that these silicic bodies cannot be pristine differentiates of the Skaergaard magma. The isotopic data do not, however, specifically exclude derivation of the granophyres by contamination of late-stage Skaergaard magmas with crustal melts (Wager and Brown, 1967; Hunter and Sparks, 1987), although the consistency of 875r/a6Sr55 ratios among the Transgressive granophyres from the Layered Series would seem to preclude mixing

199

of isotopically disparate magmas at the level that these bodies were emplaced. More concrete evidence bearing on the possible derivation of the Transgressive granophyres from the Layered Series from Skaergaard differentiates, contaminated or otherwise, comes from field relations. The granophyres cannot have been derived locally from the enclosing gabbros if the granophyres were hotter than the enclosing rocks at the time they intruded. Field relations described above demonstrate that the Transgressive granophyres intruded the upper part of the Layered Series when the local temperature of the Layered Series gabbros was 600 + 100 ° C. Comparison of granophyre compositions to melting experiments of comparable natural compositions (Helz, 1976; Beard and Lofgren, 1991 ) indicates that the low-pressure, watersaturated (0.2 GPa) liquidus o f the more mafic Transgressive granophyres is > 900 °C and the liquidi of all but the most felsic granophyres >800°C. This demonstrates that the granophyres could not have been derived from locally-generated Skaergaard differentiates. This conclusion is consistent with inferred multiple episodes of granophyre intrusion and solidification and with the observation of local chilled margins described above. Because the Layered Series cooled upwards from the floor, the regions of the intrusion beneath the site that any particular granophyre was emplaced were always cooler than the gabbros into which that granophyre intruded (Norton and Taylor, 1979). No part of the Transgressive granophyres could have been derived from liquids extracted from these relatively cold underlying gabbros. It is probable that these regions were completely solid at the time the granophyres intruded, as the solidus

Fig. 10. 875r/86Sr55decreaseswith increasingalteration. (A) Feldspars and mafic minerals are largelyunaltered; 875r/ 86Sr55= 0.70913 _+3. (B) Alkalifeldspars are cloudedwith phyllosilicates;plagioclaseand mafic mineralsremain largely unaltered; 87Sr/86Sr55=0.70876_ 2. (C) Feldspars are strongly altered to phyllosilicates;mafic minerals are altered to chlorite; 87Sr/86Srs~= 0.70836 +_2. amph= ampibole; chl=chlorite; other abbreviations as in Fig. 5. Fields of view are 4 mm long.

200

of the gabbros was almost certainly greater than 600_+ 100°C (McBirney, 1975), but this line of reasoning is independent of the actual gabbro solidus. The region of the Skaergaard intrusion that is stratigraphically higher than the locus of granophyre intrusions could potentially have been hot enough to be the sources of felsic liquids at the time of granophyre emplacement, but intrusion of the Transgressive granophyres downwards from a stratigraphically higher level is physically implausible. Granophyric liquids are less dense than Fe-rich cumulates and there is no reasonable mechanism that can be invoked that might overcome buoyancy forces and cause granophyric liquids to intrude downwards into denser rock. These arguments apply equally to pristine Skaergaard differentiates or to hypothesized contaminated Skaergaard differentiates (e.g., Wager and Brown, 1967). Contaminated Skaergaard differentiates at or below the level at which the Transgressive granophyres were emplaced would have been cooler than the granophyre's liquidus and there is no plausible mechanism to force intrusion of such contaminated differentiates downward from above. The field relations used above may also be applied to constrain the possible location of sources of fused country rock that are constituents of the granophyric magmas. Partially melted gneisses could not have remained molten within the Layered Series below the locus of granophyre intrusion at the time the latter were emplaced, as these regions would have been cooler than 600_+ 100 ° C. According to the thermal model of Norton and Taylor ( 1979 ), the temperatures along the walls and near the base of the intrusion were about 400 °C at the time temperatures in the interior of the Layered Series were 600 °C - too cool to preserve any partial melts produced at the contact earlier in the cooling history of the intrusion (Kays et al., 1981 ). Sources higher in the intrusion, such as magmas derived from melted gneiss blocks ponded near the roof of the intrusion,

M. H1RSCHMANN

are again ruled out because of buoyancy relations. The Transgressive granophyres could not, therefore, have been derived from remobilization of melted gneissic xenoliths enclosed in the Skaergaard gabbros as suggested by Norton et al. (1984), nor from wall rock melted along the gabbro-gneiss contact. Because the Transgressive granophyres cannot have been derived from differentiated Skaergaard magma, from melts of country rock generated along the Skaergaard/gneiss contact, nor from xenoliths within the intrusion, they must be derived from a compositionally and thermally independent magmatic system originating outside the Skaergaard pluton.

Evidencefor closedsystem cogenesis of Transgressive granophyres White et al. (1989) presented evidence that granophyres in the nearby Vandfaldsdalen Macrodike were generated by mixing of mafic liquids with fused gneisses. This does not appear to have been the case for the Transgressive granophyres. Strontium isotope ratios in the Transgressive granophyres vary over an extremely narrow range, relative to those of the country rock (Fig. 11 ) and do not correlate with SiO2 content (Fig. 12) or any other compositional index (Sr, Rb/Sr, Mg#, etc.). Significant variations in the degree of differentia1.0

i. Go ~(/) co

~ granophyres T amph|bo/l~es ~ felslcgnelsses| 0.9 metasedlments j

0 A

#o 0.8 0.7 O.O

A

0,5

1.O

1.5

2.0

2.5

3.0

Rb/Sr

Fig. 11. 87Sr/S6Sr55ratios of Transgressive granophyres and countryrocks. Data on countryrocksfrom Pankhurst et al. (1976); Kays et al. (1989); Stewart and DePaolo (1990) and Blichert-Toft et al. (1992). Gneisses with 87Sr/a6Sr55greaterthan 1.0 have been excludedfrom plot for clarity.

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

0.7090

co

0.7086

co

•

•

•

0.7082

. . . . . . . . . . . . . . . . . . 58 64 70

76

Wt. % SiO 2

Fig. 12. STSr/a6Sr55 versus SiO2 in the Transgressive granophyres. The lack of correlation indicates that isotopic variation among granophyres is not caused by mixing or contamination processes. Symbols are the same as in Fig. 6.

tion of the granophyres, as indexed by their Rb/Sr ratio, are accompanied by negligible, non-systematic, variations in S7Sr/S6Srss (Fig. 11). Compositional variation among the Transgressive granophyres is therefore not consistent with magma mixing (in the conventional sense) or AFC processes. The similarity of 87Sr/86Srs5 ratios over the broad range of major- and trace-element compositions observed in the Transgressive granophyres from the Layered Series is remarkable, given the tremendous variation in S7Sr/ 86Srs5 found in the country rocks of the Kangerdlugssuaq region (Fig. 11 ). The virtual lack of variation in S7Sr/a6Sr55 in these granophyres suggests that they represent a cogenetic suite of magmas that are related to each other by predominantly closed system processes. This inference is supported by the observation that major and trace elements show consistent trends on variation diagrams (Figs. 6 and 8 ). Recently, it has been shown experimentally that Sr isotope ratios homogenize diffusively on a much smaller time scale than other relevant chemical gradients (Baker, 1989; Lesher, 1990). This suggests the possibility that magmas related by mixing could potentially be more homogeneous with respect t o 878r/a6Sr than would otherwise be expected, if the length scale of diffusive exchange is relevant to the mixing process (Lesher, 1990). Blichert-Toft et al. ( 1992 ) have presented evidence that this

201

phenomenon may explain some of the chemical variation of the crustally-contaminated rocks associated with the East Greenland macrodikes. Blichert-Toft et al. (1992) further suggest that diffusively-controlled magma mixing may also be responsible for formation of the Transgressive granophyres. Given that the evidence for closed-system cogenesis of the Transgressive granophyres rests in large part on the Sr isotope data, this process could conceivably have been important in the genesis of these granophyres. On the other hand, this hypothesis requires the distance between mixing components to have been sufficiently small to allow complete homogenization of the Sr isotope ratios during diffusive exchange. In the hybrid rocks described by Blichert-Toft et al. ( 1992 ) from the Miki Fjord macrodike, 87Sr/86Sr55 ranges from 0.704 to 0.756 over a distance of less than 5 m. The entire range of magmas represented by the Transgressive granophyres would have had to have coexisted over a length scale of perhaps just a few tens of centimeters in order to diffusively equilibrate with respect to 87Sr/86Sr, assuming temperatures and time scales were not orders of magnitude different than those relevant to the macrodike example. Even assuming that it is possible that such compositionally diverse liquids resided in a boundary layer long enough to equilibrate isotopically, it would seem unlikely that such a boundary layer could be sampled to produce the Transgressive granophyres without bringing with it evidence of the isotopically unequilibrated reservoirs that would have to have resided in close proximity. The isotope data, as well as major- and traceelement compositions, are consistent with derivation of the range of Transgressive granophyre compositions by closed system processes such as partial melting of a homogeneous source or fractionation. Restite unmixing appear to be excluded by non-monotonic variation of such elements as Sr and Ba (Fig. 8) (Chappel et al., 1987). Therefore, any proposed source for the Transgressive Grano-

202

phyres must account for the compositions of the most primitive granophyres. Possibilities include partial melts of country rock and hybridized differentiates of mafic magmas. The viability of each of these possibilities will now be considered.

Derivation of granophyres by partialfusion of country rock Initial strontium isotope ratios of the granophyres lie at the low end of the observed range of 87Sr/86Srss for the Kangerdlugssauq gneisses (Fig. 11 ), which suggests the possibility that the the Transgressive granophyres could have been derived by variable degrees of partial fusion of basement rocks. Basement lithologies represented in the region that could conceivably partially melt to form felsic liquids are metasediments, granitic and tonalitic gneisses, and mafic amphibolites (Kays et al., 1989), but the metasedimentary rocks all have exceptionally high radiogenic Sr c o n t e n t s (87Sr/86Sr55 > 0.75, Fig. 11 ) and are, therefore, not potential source rocks for the Transgressive granophyres. Most of the granitic and tonalitic gneisses also have very radiogenic Sr (Fig. 11 ), and all are too rich in silica ( > 65 wt.% SiO2 ) and too poor in iron ( < 4.0 wt.% FeO*) to have generated partial melts similar in composition to the mafic granophyres (60 wt.% SiO2, 9% wt. FeO °). Because the Transgressive granophyres are inferred to be cogenetic, they could not have been derived by partial melting of metagranitic rocks if this process fails to explain the existence of the mafic granophyres. This leaves the mafic amphibolites, some of which have approximately the right 87Sr/a6Sr55 ratios (Fig 11 ), and which could potentially form partial melts with the range of major- element compositions (e.g., Helz, 1976; Beard and Lofgren, 1991) observed in the Transgressive granophyres. Unfortunately, the mafic amphibolites are far too poor in incompatible elements (LREE, Zr, Rb, Hf, Th, etc. ) to form partial melts with trace-element abundances similar to those of

M. HIRSCHMANN

the Transgressive granophyres. Concentrations of these elements in the amphibolites are 1/3 to 1/10 those observed in the Transgressive granophyres (Kays et a1.,1981; 1989). Partial melts of Kangerdlugssuaq amphibolites could therefore have LREE and other incompatible element abundances similar to the Transgressive granophyres only at small melt fractions ( < 30%, Figs. 13 and 14), but partial melts of amphibolites with melt fractions less 1000"

A ~ ~ 0 0 . ~ 0 l-

~

30 % melt

o

~o o

10,,

V

amphibolite I

I

I

I

I

I

B .r-

p. 0 J¢

U 0

1

I

I

I

I

I

o .r"

100 0 J¢

o

8 10 1

Le

Ce

Nd SmEu

Tb

YbLu

BMafic Granophyres Fig. 13. Chondrite-normalized rare earth element abundances ofmafic granophyres compared with: (A) average amphibolite from Kangerdlugssuaq gneisses (Kays et al., 1981, 1989) and composition resulting from 30% melting of average amphibolite; (B) average composition of lower crust (Taylor and McClennan, 1985) and composition resulting from 30% melting of lower crust; and (C) compositions ofFG- 1 mafic dikes (Brooks and Nielsen, 1978 ). Partial melt compositions are calculated for the limiting case where bulk REE Kd's equal 0; actual melt compositions should, therefore, be less than shown. Rock analyses normalized to chondrite abundances given by Taylor and McClennan ( 1985 ).

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D ) 80

OO

so

o

o

IMG

Q.

4o

o

o

20

no AA

0

•

FG-1 '

'

'

100

•

'

'

'

200

Zr

,

i

i

,

300

,

i

,

i

,

400

(ppm)

Fig. 14. Hypothetical behavior of trace elements during batch melting of mafic amphibolites for limiting case where bulk distribution coefficients are zero. MG (black dot) is average mafic granophyre; AA (diamond) is average marie amphibolite from Kandgerlugssuaq region (Kays et al., 1989; Blichert-Toft, 1992); LC (shaded box) is average lower crust composition from Taylor and McClennan (1985). Lines emanating from AA and LC are paths of partial melt compositions with small diamonds and shaded boxes giving specific compositions at 50, 40 and 30% melting of amphibolite and lower crust. Open squares are compositions of FG-1 dike swarm (Brooks and Nielsen, 1978 ) and open circles are compositions of granitic gneisses (Kays et al., 1981, 1989).

than 30% are invariably more silica-rich ( > 70 wt.% SiO2; Helz, 1976; Beard and Lofgren, 1991 ) than all but the most felsic granophyres. The Transgressive Granophyres could not therefore have been derived by varying degrees of partial melting of the mafic amphibolites. Generation of the range of Transgressive granophyre compositions from a mixture of partial melts of diverse basement lithologies is improbable. Because many excluded element concentrations (e.g., LREE, Zr, Hf, Th ) in the granitic gneisses are 1/3 to 1/2 of those found in the Transgressive granophyres (Kays et al., 1981, 1989 ), partial melts of granitic gneisses could match trace-element compositions of the granophyres if the melt fraction were relatively small, but would be more silicic than the marie granophyres (ie. Whitney, 1988 ). A mixture of such melts with partially melted amphibolite could have major- and trace-element concentrations similar to the mafic granophyres if the mixture was dominated by a component from a mostly melted amphibolite and if the degree of melting of the metagranite was very small.

203

This is an unlikely combination of materials, as the temperatures required to generate a high proportion of melt in a mafic amphibolite (1000°C, Helz, 1976; Beard and Lofgren, 1991 ) would generate large melt fractions in granitic rocks (Whitney, 1988 ). Leeman and Dasch ( 1978 ) suggested on the basis of Pb isotope ratios that the silicic granophyres of the Skaergaard intrusion were generated by partial melting of granulites, though granulites are not exposed in the Kangerdlugssuaq region. Evidence for a granulitic source for the Skaergaard granophyres rests largely on analysis of a single sample, DS- 13, a silicic rock from LZc which has Pb isotope ratios that are distinctly different from all other analyzed rocks from the Kangerdlugssuaq region (Leeman and Dasch, 1978). The S7Sr/S6Sr55 ratio for this rock (0.7287; Leeman and Dasch, 1978 ) is significantly different from all other analyzed Transgressive granophyres (for which 87Sr/S6Sr55=0.7085 ___7, n = 15; this study and Leeman and Dasch, 1978 ). It is likely that DS13 is unrelated to the Transgressive granophyres from the Layered Series. It could instead be related to younger peralkaline rhyolite dikes that intrude the lower part of the Layered Series (Brooks and Nielsen, 1982 ), or to some of the granophyres from the Upper Border Group that have higher 87Sr/86Sr55 ratios. Although isotopic evidence for lower crustal component in the Transgressive Granophyres from the Layered Series is weak, a brief examination of the possibility that these granophyres formed from partial melting of lower crustal rocks is worthwhile. Average lower crustal compositions, however, (e.g., Taylor and McClennan, 1985 ) are only slightly richer in LREE than the amphibolites outcropping in the Kangerdlugssuaq region (Figs. 13 and 14). Partial fusion of typical lower crustal granulites could yield liquids with LREE abundances similar to those found in the Transgressive granophyres at melt fractions less than 30% (Fig. 13 ), but such liquids would presum-

204

ably be more silica-rich than the mafic granophyres. Partial melts of average lower crustal rocks would also be much less enriched in incompatible trace elements than are the mafic granophyres (Fig. 14 ). The possibility of lower crustal rocks with unusual geochemical characteristics cannot be ruled out, but there is no compelling observational evidence for derivation of the Transgressive granophyres by partial melting of granulitic lower crust.

Derivation of granophyresfrom hybrid differentiates The mafic dikes that intruded the Skaergaard gabbros soon after solidification of the Skaergaard and the granophyres span a wide range of compositions and provide evidence for storage and differentiation of magma beneath the Skaergaard intrusion (Nielsen, 1978; Brooks and Nielsen, 1978 ). 87Sr/86Sr ratios indicate that some of the mafic dikes have interacted with felsic crust, though none have Sr as radiogenic as the Transgressive granophyres (Brooks and Nielsen, 1978 ). It is possible that magmas similar to those represented by the mafic dikes could have reacted with gneissic basement rocks to form hybrid magmas with the geochemical characteristics of the mafic granophyres. A key geochemical feature of the Transgressive granophyres is the marked enrichment in typically incompatible trace elements (LREE, HFSE, etc.) at intermediate silica contents ( < 6 5 wt.%). The most likely mechanism for generating enriched abundances of these elements accompanied by only moderate silica enrichment (in metaluminous liquids) is extended fractionation along a tholeiitic liquid line of descent such as that seen in the postSkaergaard mafic dikes. For example, the FGl dike swarm that cut the Skaergaard intrusion soon after emplacement of the Transgressive granophyres display enrichments in LREE and other incompatible trace elements similar to those observed in the Transgressive grano-

M. HIRSCHMANN

phyres (Figs. 13 and 14), without silica enrichment (Brooks and Nielsen, 1978). Liquids similar to these are the most reasonable precursors to the mafic granophyres. The granophyres have slightly greater concentrations of some incompatible elements (e.g., Rb, Fig. 14) than the differentiated FG-1 dikes, but differences in these abundances may be accounted for by contamination with minor amounts of partially fused basement. Such contamination is, of course, also required to explain the elevated 87Sr/86Sr55 ratios of the granophyres. The wide range of compositions of mafic magmas and of crustal contaminants that could have potentially participated in the genesis of the Transgressive granophyres prevents identification of specific end-member components, but there are enough compositional clues to place broad constraints on potential endmembers of the hybrid parent of the granophyres. The high Fe/Mg ratio (Fig. 7), low compatible element concentrations (e.g., Ni, Co, Table 1, Fig. 8), and high incompatible trace-element concentrations (Figs. 8 and 14) of the mafic granophyres indicates that the mafic magmatic component was akin to the more evolved dikes of the FG-1 swarm, rather than a relatively undifferentiated basalt similar to more primitive FG- 1 dikes or the THOL1 basalts of Nielsen (1978). The small 87Sr/ 86Sr55 ratios of the granophyres, relative to the majority of the Archean gneisses suggests that the contribution of fused gneiss was probably volumetrically subordinate to differentiated mantle-derived magma, although this could be partly accounted for by the presumably greater concentration of Sr in the mantle-derived component, relative to the crustal-derived component. As discussed above, a heat source other than the cooling Skaergaard pluton was required to maintain granophyric magmas at their liquidus temperature at the time of the emplacement of the Transgressive granophyres. The heat required to partially fuse country rock in regions immediately below the Skaergaard

T H E T R A N S G R E S S I V E G R A N O P H Y R E S ( L A Y E R E D SERIES - S K A E R G A A R D I N T R U S I O N , EAST G R E E N L A N D )

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pluton can only have been provided by intrusion of mafic or intermediate magmas. Melting of the crustaUy-derived component of the granophyres indicated by the isotope data was almost certainly brought about by direct contact with such magmas, a process that would undoubtedly be accompanied by some degree of hybridization. It is, therefore, not surprising that the geochemical characteristics of the Transgressive granophyres are inconsistent with simple partial melting of the country rocks and instead are consistent with derivation from a hybridized differentiated melt. Participation of differentiated mafic or intermediate magmas is required to provide both thermal and material input in order to explain the genesis of the Transgressive granophyres.

sition of the gabbros may have temporarily hindered transport of mafic magmas past the base of the intrusion. The intrusion also may have caused local distortion of the regional stress regime in a manner that discouraged dike propagation through the area. Prolonged storage of mafic magmas beneath the Skaergaard intrusion would presumably facilitate differentiation and hybridization. (3) The close association of the Transgressive granophyres with mineralized fractures (Norton et al., 1984) suggests that the granphyres were intruded along pathways created during cooling and fracturing of the Skaergaard pluton. These pathways did not exist in the unfractured gneissic country rocks.

Relation of the Transgressive granophyres to the Skaergaard intrusion

Conclusions

The observation that the Transgressive granophyres of the Layered Series are restricted to the central part of the Skaergaard intrusion, lacking related counterparts in either other divisions of the intrusion or in the country rocks, is not likely to be simply coincidental. Though the granophyres are not directly descended from the Skaergaard magma and were not derived from country rocks that melted owing to the heat of the crystallizing Skaergaard intrusion, there must be an indirect relationship between the Skaergaard pluton and the genesis of the Transgressive granophyres. The larger intrusion may have contributed to generation and emplacement of the granophyres in several ways: ( 1 ) Conductive heating through the floor of the intrusion raised the temperature of the rocks directly below the intrusion, relative to surrounding rocks, thus making reaction between later mafic magmas and country rocks more likely. (2) Non-brittle behavior of the Skaergaard pluton, owing first to presence of melt and later to temperatures above the brittle-ductile tran-

The Transgressive granophyres represent a cogenetic suite of magmas that are related to each other primarily by closed system processes, most probably fractionation. Sr isotope ratios adjusted back to 55 m.y.B.p, cluster around 0.709, indicating participation of fused basement, but the range of 87Sr/86Srs5 is small compared to that ofgneissic country rocks. All of the variation in 87Sr/86Srs5 may be attributed to the effects of hydrothermal alteration. The Transgressive granophyres from the Layered Series are not descended from contaminated differentiates of Skaergaard magma and field relations preclude derivation of the granophyres from partially fused gneissic xenoliths or from partially melted wallrock. Geochemical relations, in particular incompatible trace-element abundances, also preclude derivation of the granophyres solely from fusion of gneissic country rocks. The accumulated evidence is, however, consistent with derivation of the Transgressive granophyres from the Layered Series from hybrid melts formed from reaction of differentiated mafic magmas with felsic country rocks.

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Acknowledgments I wish to thank A.R. McBirney for suggesting that the Transgressive granophyres would make an interesting topic of study and D.J. Geist for inviting me to contribute to this volume in honor of Mac's retirement. Thanks to H.R. Naslund and H. Anderson for providing logistical support in East Greenland and to R.St.J. Lambert for access to the isotope facilities at the University of Alberta. Thanks also to G. Bergantz, I.S. McCallum, and H.R. Naslund for helpful discussions; to A.R. McBirney, A.D. Johnston, G.G. Goles, M.A. Kays and D.L Whitney for critical reviews of earlier versions of this manuscript; to D. Bostok, D. Krstic, J. MacKinnon, and C. McBirney for technical assistance; and to C.E. Lesher, and an anonymous reviewer for helpful reviews of the present version. This work was supported by NSF grant EAR-8503428 to A.R. McBirney and by a Grant-in-Aid from Sigma Xi and a GSA research grant to the author.

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