Marginal Border Series Research Articles

A whole-rock data set for the Skaergaard intrusion, East Greenland

We report a compilation of new and published whole-rock major and trace element analyses for 646 samples of the Skaergaard intrusion, East Greenland. The samples were collected in 14 stratigraphic profiles either from accessible and well-exposed surface areas or from drill core, and they cover most regions of the intrusion. This includes the Layered Series, the Upper Border Series, the Marginal Border Series and the Sandwich Horizon. The geochemical data were obtained by a combination of X-ray fluorescence and inductively coupled plasma mass spectrometry. This data set can, for example, be used to constrain processes of igneous differentiation and ore formation.

Lead isotope geochemistry of plagioclase in the Skaergaard intrusion by LA-ICP-MS: Assessing the effects of crustal contamination and link with East Greenland flood basalts

The Skaergaard intrusion of East Greenland is one of the best studied layered intrusions on Earth and preserves an exceptional rock record produced during low-pressure, closed-system crystallization of Fe-rich tholeiitic magma. The lead isotope compositions of plagioclase feldspar in cumulates and related igneous rocks of the Skaergaard intrusion were determined directly from single spot analyses in thin section by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) (n = 757). Plagioclase primocrysts, typically with <2 ppm Pb, and feldspar in interstitial pockets from the Layered Series and from most of the Marginal Border Series have the same Pb isotope compositions within analytical uncertainty (208Pb/206Pb = 2.20 ± 0.06; 207Pb/206Pb = 0.889 ± 0.028). There are no significant Pb isotope differences between cores and rims of individual plagioclase primocrysts or between different types of interstitial pockets (e.g., interstitial granophyre, symplectites). These Pb isotope compositions are less radiogenic than those from most of the contemporaneous East Greenland flood basalts indicative of crustal contamination at deeper crustal levels prior to emplacement. Plagioclase from three samples of the Marginal Border Series adjacent to host gneisses (<5 m from the contact) have higher 208Pb/206Pb (2.30 ± 0.06, 2.35 ± 0.02, 2.51 ± 0.21) compared to the rest of the intrusion. These results confirm that crystallization of the Skaergaard intrusion occurred under essentially closed-system conditions with localized minor incorporation of “amphibolitic” gneiss. Hydrothermal alteration occurred at near-solidus to subsolidus conditions by circulating meteoric fluid that had equilibrated with the overlying flood basalts. This alteration resulted in albitized interstitial granophyre and rare spot analyses from plagioclase primocrysts characterized by Pb isotope compositions comparable to those of the flood basalts. Granophyres of different origins (e.g., transgressive granophyre, granophyric segregation) have variable Pb isotope compositions (208Pb/206Pb = 2.09–2.51; 207Pb/206Pb = 0.844–1.013) that are distinct from the Skaergaard cumulates. Based on Pb isotope systematics, combined with published SrNd isotopic compositions and 40Ar/39Ar geochronology from East Greenland and Faroe Island basalts, the Skaergaard intrusion is linked to eruption of the earliest of the Plateau Basalts in East Greenland (Milne Land Formation) during continental breakup and opening of the North Atlantic Ocean. This microanalytical study highlights the ability to conduct rapid Pb isotopic measurements in low-Pb concentration materials (e.g., <2 ppm Pb in plagioclase) at high spatial resolution by LA-ICP-MS with precision sufficient to constrain a range of magmatic processes in layered intrusions and other feldspar-bearing rocks preserved in the geological record.

Meso to Neoproterozoic layered mafic-ultramafic rocks from the Virorco back-arc intrusion, Argentina

The Virorco layered mafic-ultramafic intrusion is part of a belt that extends over 100 km from NE to SW in the Eastern Sierras Pampeanas of San Luis, Argentina. The rocks of this belt carry a Fe-Cu-Ni sulphide mineralization in veins and as disseminated and massive ore. Platinum group minerals are associated with the sulphides.The Virorco intrusion exhibits modal, textural and cryptic layering. New results allow the characterization of six layered units (Modal Layered Unit, Pyroxenitic Macro-Layered Unit, Gabbroic Unit, Banded Unit, Hornblende Norite Unit and Gabbronorite Unit) present in three sectors of the intrusion (Eastern, Central and Western). The units from the Western Sector (Banded Unit, Hornblende Norite Unit and Gabbronorite Unit) and the Modal Layered Unit from the Eastern Sector belong to the Marginal Border Series of the intrusion. Meanwhile, the Central sector units (Pyroxenitic Macro-Layered Unit and Gabbroic Unit) are from the Layered Series. The presence of crescumulate texture (Modal Layered Unit) and colloform banding (Banded Unit) are evidences of “in situ” crystallization due to supercooling of a MgO-rich hydrated mafic magma, where cooling proceeded from the walls towards the interior of the magma chamber.In previous studies the mafic-ultramafic rocks have been considered to be Cambrian to Ordovician. Here we present a Sm-Nd whole rock isochron which shows that the formation age of these intrusions is 1002 ± 150 Ma and that the protolith age of the Pringles Metamorphic Complex metasedimentary rocks is 1289 ± 97 Ma. Our study also indicates that the San Luis mafic-ultramafic layered intrusives most probably formed in a back-arc tectonic setting, from an enriched sub-continental mantle, influenced by a subducting slab and/or crust injection into the Pampia Terrane prior to its collision with the Rio de la Plata Craton.

Platinum-group mineralization at the margin of the Skaergaard intrusion, East Greenland

Two occurrences of platinum-group elements (PGEs) along the northern margin of the Skaergaard intrusion include a sulfide-bearing gabbro with slightly less than 1 ppm PGE + Au and a clinopyroxene-actinolite-plagioclase-biotite-ilmenite schist with 16 vol% sulfide and 1.8 ppm PGE + Au. Both have assemblages of pyrrhotite, pentlandite, and chalcopyrite typical for orthomagmatic sulfides. Matching platinum-group mineral assemblages with sperrylite (PtAs2), kotulskite (Pd(Bi,Te)1–2), froodite (PdBi2), michenerite (PdBiTe), and electrum (Au,Ag) suggest a common origin. Petrological and geochemical similarities suggest that the occurrences are related to the Skaergaard intrusion. The Marginal Border Series locally displays Ni depletion consistent with sulfide fractionation, and the PGE fractionation trends of the occurrences are systematically enriched by 10–50 times over the chilled margin. The PGE can be explained by sulfide-silicate immiscibility in the Skaergaard magma with R factors of 110–220. Nickel depletion in olivine suggests that the process occurred within the host cumulate, and the low R factors require little sulfide mobility. The sulfide assemblages are different to the chalcopyrite-bornite-digenite assemblage found in the Skaergaard Layered Series and Platinova Reef. These differences can be explained by the early formation of sulfide melt, while magmatic differentiation or sulfur loss caused the unusual sulfide assemblage within the Layered Series. The PGEs indicate that the sulfides formed from the Skaergaard magma. The sulfides and PGEs could not have formed from the nearby Watkins Fjord wehrlite intrusion, which is nearly barren in sulfide. We suggest that silicate-sulfide immiscibility led to PGE concentration where the Skaergaard magma became contaminated with material from the Archean basement.

Contact Metamorphism of Precambrian Gneiss by the Skaergaard Intrusion

The Tertiary Skaergaard intrusion, East Greenland, intruded at the shallow crustal unconformity between Precambrian amphibolite-facies gneisses and overlying Tertiary Plateau Basalts. Maximum contact metamorphic temperatures in quartzo-feldspathic gneisses were determined in two sample traverses across the aureole on the western contact of the intrusion using a combination of microstructural observations (both optical and cathodoluminescence) and the titanium-in-quartz (TitaniQ) thermometer. The onset of recrystallization of the quartz in the gneisses occurred between 390 and 340 m from the contact whereas H2O-fluxed melting occurred in gneisses closer than 130 m from the contact (where T > ∼ 675°C). The maximum temperature recorded by quartz at the contact is ∼865 ± 70°C. Melt fractions reach 50–60 vol. % in some samples although the melt is heterogeneously distributed on all scales. Minor bands of amphibolite-facies mafic gneiss are extensively reacted to an anhydrous pyroxene-bearing hornfels close to the contact, whereas those further than ∼130 m are overprinted by a greenschist-facies assemblage. Discrepancies between the expected temperature for the amphibolite- to greenschist-facies reaction and temperatures obtained from adjacent quartzo-feldspathic gneisses are consistent with the formation of the anhydrous pyroxene hornfels directly from the mafic gneiss, with the lower-grade greenschist-facies assemblage forming on the retrograde path after the establishment of limited hydrothermal activity. It is unlikely that devolatilization reactions in the gneiss produced sufficient H2O to account for the pegmatitic features formed in the Marginal Border Series in the intrusion. A simple one-dimensional thermal model, neglecting any advection of heat by hydrothermal circulation, was fitted to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared with those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are consistent with a heterogeneous release of latent heat of crystallization.

Crystallization of Interstitial Liquid and Latent Heat Buffering in Solidifying Gabbros: Skaergaard Intrusion, Greenland

New compositional profiles across plagioclase grains from the Layered Series (LS), Marginal Border Series (MBS) and Upper Border Series (UBS) of the Skaergaard intrusion are used to understand the mechanisms of cumulate rock solidification and the fate of the interstitial liquid. The data show that plagioclase crystals display three types of compositional profile over the whole intrusion, as follows. (1) Grains with normal zoning, which dominate the MBS and UBS. These are interpreted as having formed at the top of a crystal mush and then buried in the cumulate pile. Crystallization of the interstitial melt resulted in liquid differentiation and produced normally zoned rims on plagioclase cores. (2) Unzoned crystals, which dominate the upper part of the LS, also crystallized at the top of the mush and were then buried in mush with a low interstitial liquid fraction or one experiencing convective movements that kept the liquid to a constant composition. (3) Crystals with a mantle of decreasing An content followed by a rim of constant composition. Grains showing this complex zoning mostly occur in the lower parts of the LS. Depending on the stratigraphic position within the intrusion, the composition of the rim can be An56, An51 or An40. In the main magma body, these compositions (An contents) correspond to those of plagioclase primocrysts (e.g. cores) at the appearance of cumulus clinopyroxene (An56), Fe–Ti oxides (An51) and apatite (An40). Compositional buffering of plagioclase rims is interpreted as being a consequence of enhanced release of latent heat of crystallization at the appearance of new interstitial phases in the crystal mush. When a new phase saturates, the latent heat contribution to the global enthalpy budget of the system becomes sufficiently high to keep the interstitial melt at its liquidus temperature for a period of time that could exceed thousands of years. Under these conditions, equilibrium, adcumulus growth together with diffusion and possibly advection of chemical components result in the formation of plagioclase rims of constant composition (An content). Efficient thermal buffering of the mush liquid depends on the porosity (i.e. fraction of liquid within the mush) and the degree of compositional homogeneity of the mush. In a heterogeneous and highly porous mush, saturation of the new phase occurs in the coldest part of the mush and the enhanced latent heat release at the saturation of this phase is quickly dissipated to the whole volume of liquid, including the warmest part that is not yet saturated in a new phase. As a consequence, no thermal buffering occurs and interstitial crystallization produces grains with normal zoning. The distribution of the various types of plagioclase grains throughout the Skaergaard intrusion can therefore be used to infer the spatial variability in the physical properties of the crystal mush, such as the residual porosity, both at an intrusion-wide scale and at a millimetre- to centimetre-scale.

Major element variation of the Skaergaard pyroxene and its petrologic implications

AbstractPyroxenes from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed using electron microprobe and mineral separation techniques to examine geochemical variations. In general, pyroxenes from all three series show similar trends in major elements vs % crystallization: SiO2, MgO, Al2O3, and TiO2 progressively decrease, FeO and MnO progressively increase, and CaO, Fe2O3, and P2O5 do not change systematically with differentiation. Pyroxenes in the LS and MBS follow a trend similar to that reported by Wager and Brown. The estimated crystallization temperatures closely follow the general trends of published temperature estimates. Major element variation in Skaergaard pyroxenes shows smooth variations with increasing differentiation, indicating that there was no volumetrically significant injection of new magma into the chamber after the initial emplacement. These results strongly support the idea that the Skaergaard intrusion represents in situ crystallization under a closed system magma chamber.

Lateral Reactive Infiltration in a Vertical Gabbroic Crystal Mush, Skaergaard Intrusion, East Greenland

The Marginal Border Series of the Skaergaard intrusion (East Greenland) crystallized in situ on the vertical walls of the magma chamber. It is subdivided into an outer Unbanded Division and an inner Banded Division. The Banded Division contains abundant centimetre- to decimetre-thick bands dominated by fine-grained mafic minerals, with a morphology evolving from almost planar to deeply scalloped and fingered with increasing distance from the intrusion margin. The morphology of these bands is reminiscent of the reaction fronts described in sedimentary basins infiltrated by reactive fluids. We propose that the banding in the Skaergaard Marginal Border Series is produced by chemical disequilibrium resulting from the suction of primitive liquid from the main magma body into the crystal mush, driven by shrinkage of the mush during solidification. Liquid porous flow results in partial dissolution of evolved pre-existing mafic minerals in the mush. This changes the mush liquid composition to one capable of crystallizing mafic rocks with a very minor plagioclase component. Abrupt solidification of this liquid, resulting in the formation of the colloform bands, is explained by supersaturation of some mafic mineral components (e.g. olivine, clinopyroxene, Fe–Ti oxides) in the infiltrating melt. We suggest that the morphological evolution of the colloform bands is a consequence of increasing crystal mush thickness with progressive differentiation.

Crystallization sequence of the Upper Border Series of the Skaergaard Intrusion: revised subdivision and implications for chamber-scale magma homogeneity

Although it is one of the best-studied layered mafic intrusions in the world, the crystallization sequence of the Skaergaard Intrusion, east Greenland, remains in debate. In particular, it has been argued that the crystallization sequence in the Upper Border Series, which crystallized downwards from the roof of the magma chamber, differs from that in the Layered Series formed at the floor. The proposed deviation would require chemical stratification of the magma, and a reexamination of the crystallization sequence therefore has important implications for understanding the dynamics of the system. Here, we examine a new sample set from the Upper Border Series, combining field observations, petrography and anorthite contents of plagioclase with bulk rock Ti, V, P, Cu and Mn concentrations. We demonstrate that the first phases on the liquidus were plagioclase and olivine followed by augite, then ilmenite and magnetite (simultaneously), sulfides, apatite and finally ferrobustamite (now inverted to hedenbergite). This crystallization sequence represents extreme differentiation along the tholeiitic trend, and it mirrors those at the floor (Layered Series) and walls (Marginal Border Series). We therefore propose a revised subdivision of the Upper Border Series into equivalents of the subzones in the Layered Series denoted by apostrophes (LZa′, LZb′, etc.). Moreover, the first appearance of each of the cumulus phases occurs at similar plagioclase core anorthite contents. The mirror images of the crystallization sequences and the anorthite contents of plagioclase cores in the three series imply that the Skaergaard magma chamber solidified by in situ crystallization along the floor, walls and roof from one, largely homogenous, convecting magma body.

Silicate Liquid Immiscibility within the Crystal Mush: Late-stage Magmatic Microstructures in the Skaergaard Intrusion, East Greenland

Late-stage microstructures developed during the last stages of solidification of the Skaergaard intrusion comprise a wide array of reactive and non-reactive intergrowths. Reactive microstructures ascribed to open-system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts, fish-hook pyroxenes and mafic symplectites. They form by the addition of Fe and Ca and removal of alkalis and silica. Other microstructures include those formed by (internally generated) redox reactions between olivine, Fe–Ti oxides and pyroxene. Non-reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite-rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids. The open-system reactive microstructures occur predominantly in the cumulates on the chamber floor, appearing in LZb and disappearing (to be replaced by a granophyre–ilmenite-rich intergrowth association) in the Upper Zone. They are not common in the Marginal Border Series. Their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a Si-rich immiscible component from the interstitial liquid. The loss of the Si-rich component causes the remaining Fe-rich liquid to react with the primocrysts. Bulk-rock major element data are consistent with little or no preferential loss of the Si-rich liquid from the mush, but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma. The first appearance of the paired conjugate non-reactive intergrowths in MZ* (Marginal Border Series) points to the early onset of immiscibility in the bulk liquid.

Trace element variation in olivine in the Skaergaard intrusion: petrologic implications

Olivine from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion was analyzed to examine trace element variations. In general, olivine from all three series shows similar trends in trace elements with differentiation. Ba, Cs, Rb, and Sr, acted as excluded trace elements and the transition elements, Co, Cr, Cu, Ni, Sc, Zn, and V, are included in Skaergaard olivine. Owing to the high DNi in olivine, Ni decreases with differentiation in both olivine and the calculated Skaergaard magma. The low concentration of Cr in olivine is attributable to early depletion of Cr by pyroxene crystallization. Decreases in the abundances of V and Co are related to the onset of magnetite crystallization and the separation of immiscible Co-rich sulphide. In general, calculated partition coefficients (Di) for Skaergaard olivine show similar trends in the LS, UBS, and MBS. Calculated Di’s for Sc, Cr, and Ga in olivine, progressively increase, and Di’s for Zr, Nb, Hf, Cs, Ta, Th, and U progressively decrease, with differentiation. The systematic variation in trace element contents of Skaergaard olivine during differentiation would appear to preclude the possibility of any significant injection of new magma into the chamber during differentiation. The Ni content in Skaergaard olivine systematically decreases with differentiation as would be expected if the intrusion remained a closed system. In fact, no abrupt increases are observed in the abundances of any of the transition metal in olivine during differentiation.

Melt-rich segregations in the Skaergaard Marginal Border Series: Tearing of a vertical silicate mush

The Marginal Border Series (MBS) of the Skaergaard Intrusion crystallised on the steeply dipping sidewalls of the magma chamber. Melt-rich segregations, previously described as the ‘wavy pyroxene rock’ are a key feature of the otherwise homogeneous outermost parts of the MBS. The lens-shaped segregations consistently strike parallel to the chamber wall and have a moderate dip towards it. The shape, size, grain size and mineralogy of the segregations evolve systematically away from the chamber wall. The segregations become bigger, more widely spaced, chemically more evolved and more irregular in shape with increasing distance from the margin. The segregations were previously interpreted as a nucleation effect parallel to the thermal gradient. However, they dip toward the margin, not parallel to it. We offer a new interpretation: that the segregations represent tearing of poorly consolidated crystal mush during localised sagging of the vertical solidification front. The tears form during a process analogous to ‘hot tearing’ of metal alloys, although the driving force for tearing is probably gravitational collapse of the vertical solidification front, or perhaps contemporaneous faulting during chamber filling. Small, regular tears formed in the thinner, more rapidly cooled outer parts of the MBS, while the larger, irregular tears formed in the coarser grained, inner parts. Movement of interstitial liquid fed the cracks after tearing, and the liquid crystallised as relatively evolved, coarse-grained segregations. The mineral chemistry and texture of the segregations suggest that the tearing occurs relatively early in the crystallisation history.

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context of its regional setting, combined with new data from exploration drilling, has resulted in a revised structural model for the intrusion. It is modelled as an irregular box, c. 11km from north to south, up to 8km from east to west, and 34---4km from the lower to the upper contact. The walls of the intrusion are inferred to follow pre-existing and penecontemporaneous steep faults, and the floor and roof seem largely controlled by bedding planes in the host sediments and lavas, similar to regional sills. The suggested shape and volume are in agreement with published gravimetric modelling. Crystallization along all margins of the intrusion concentrated the evolving melt in the upper, central part of the intrusion, best visualized as an ‘onion-skin’ structure inside the box. The total volume is estimated to c. 280 23km 3 , of which 137% are referred to the Upper Border Series (UBS), 164% to the Marginal Border Series (MBS) and 699% to the Layered Series (LS). In the LS, the Lower Zone (LZ) is estimated to constitute 668%, the Middle Zone (MZ) 135% and the Upper Zone (UZ) 197%. The new volume relationships provide a mass balance estimate of the major and trace element bulk composition of the intrusion. The parental magma to the Skaergaard intrusion is similar to high-Ti East Greenland tholeiitic plateau basalts with Mg number c. 0.45. The intrusion represents the solidification of contemporary plateau basalt magma trapped and crystallized under closed-system conditions in a crustal reservoir at the developing East Greenland continental margin.

Major and trace element composition of Skaergaard plagioclase; geochemical evidence for changes in magma dynamics during the final stage of crystallization of the Skaergaard intrusion

Plagioclase separates from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed to examine major and trace element variations. In general, plagioclase from the LS, UBS, and MBS show similar trends in major elements vs. crystallization: SiO2, Na2O, and K2O progressively increase, and CaO and MgO progressively decrease with fractionation. No abrupt changes in the trends of major components of Skaergaard plagioclase during the differentiation of the intrusion are observed. Trace elements in plagioclase reflect changes in the Skaergaard magma and changes in plagioclase distribution coefficients with differentiation. Sr, Ga, and probably Ba are included elements in Skaergaard plagioclase, but were excluded from the other cumulus phases, and as a result systematically increased in the magma and plagioclase during differentiation. Be, Cs, Hf, Rb, Ta, U, and Zr, and the transition metals Co, Cr, Cu, Ni, Sc, V, and Zn were excluded elements in Skaergaard plagioclase, and remained low in plagioclase during differentiation. Changes in the abundances of these elements in plagioclase during differentiation reflect changes in their abundance in the magma. With the exception of the lower zone, which is enriched in the light rare earth elements, rare earth elements in LS plagioclase, in general, increase with differentiation of the Skaergaard intrusion, but decrease dramatically at the UZa/UZb boundary where abundant apatite first appears. Rare earth elements in UBS plagioclase followed a similar trend to LS plagioclase, except during the initial and final stages of differentiation. UBS plagioclase is much more enriched in rare earth elements during the final 20% of crystallization, except for Eu, which is similar in plagioclase from the two series. The observed trends suggest that the floor and roof sequences became isolated from each other and that the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20% of crystallization. As the Skaergaard magma ceased convection, or convected as isolated cells, during the final stages of differentiation, volatile elements may have accumulated in the UBS magma, resulting in an increase in ƒO2, and a decrease in Eu/Sm in UBS plagioclase. The observed trends of rare earth elements in plagioclase from the LS and UBS fit well with theoretical calculations that assume closed-system crystallization, and would be difficult to reconcile with any model requiring significant discharge of magma from the chamber during the final 20% of crystallization. The enrichment of light rare earth elements in plagioclase, suggests that the lower part of the intrusion re-equilibrated with a late, light rare earth element-rich fluid or melt. The recharge model proposed by earlier workers to explain anomalous Sr and Nd isotopes appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples.

Phase equilibria modeling in igneous petrology: use of COMAGMAT model for simulating fractionation of ferro-basaltic magmas and the genesis of high-alumina basalt

A new version of COMAGMAT-3.5 model designed for computer simulations of equilibrium and fractional crystallization of basaltic magmas at low to high pressures is presented. The most important modifications of COMAGMAT include an ability to calculate more accurately the crystallization of magnetite and ilmenite, allowing the user to study numerically the effect of oxygen fugacity on basalt magma fractionation trends. Methodological principles of the use of COMAGMAT were discussed based on its thermodynamical and empirical basis, including specific details of the model calibration. Using COMAGMAT-3.5 a set of phase equilibria calculations (called Geochemical Thermometry) has been conducted for six cumulative rocks from the Marginal Border Series of the Skaergaard intrusion. As a result, initial magma temperature (1165±10°C) and trapped melt composition proposed to be parental magma to the Skaergaard intrusion were determined. Computer simulations of perfect fractionation of this composition as well as another proposed parent produced petrochemical trends opposite to those followed from natural observations. This is interpreted as evidence for an initial Skaergaard magma containing a large amount of olivine and plagioclase crystals (about 40–45%), so that the proposed and calculated parents are related through the melt trapped in the crystal–liquid mixture. This promotes the conclusion that the Skaergaard magma fractionation process was intermediate between equilibrium and fractional crystallization. In this case the classic Wager's trend should be considered an exception rather than a rule for the differentiation of ferro-basaltic magmas. A polybaric version of COMAGMAT has been applied for the genetic interpretation of a volcanic suite from the Klyuchevskoi volcano, Kamchatka, Russia. To identify petrological processes responsible for the observed suite ranging from high-magnesia to high-alumina basalts, we used the model to simulate the Klyuchevskoi suite assuming isobaric crystallization of a parental HMB magma at a variety of pressures and a separate set of simulations assuming fractionation during continuous magma ascent from a depth of 60 km. These results indicate that the Klyuchevskoi trend can be produced by ∼40% fractionation of Ol–Aug–Sp±Opx assemblages during ascent of the parental HMB magma over the pressure range 19–7 kbar with the rate of decompression being 0.33 kbar/% crystallized (at 1350–1110°C), with ∼2 wt.% of H 2O in the initial melt and ∼3 wt.% of H 2O in the resultant high-Al basalt.

INCLUDED BLOCKS (AND BLOCKS WITHIN BLOCKS) IN THE SKAERGAARD INTRUSION : GEOLOGIC RELATIONS AND THE ORIGINS OF RHYTHMIC MODALLY GRADED LAYERS

The early Eocene Skaergaard intrusion of Greenland includes enormous numbers of rocks of both exotic and cognate origins. The lower parts of the Marginal Border Series contain abundant fragments of feldspathic peridotite that are possibly autoliths, intermixed with occasional xenoliths of Precambrian gneiss and metasomatized Cretaceous–Paleocene sediments derived from adjoining country rocks. The Upper Border Series includes one exceptionally large block of gneiss (several hundred meters across), and numerous smaller fragments, these originating from the intrusion9s footwalls, plus a few pieces of peridotite. The Layered Series contains countless autoliths of troctolite, gabbroic anorthosite, and oxide (magnetite-ilmenite) gabbro, broken from parts of the Upper Border Series that have otherwise been lost to erosion; at the upper midlevel of its western half, it contains a few xenoliths of basalt, derived probably from the now-eroded (Eocene) roof of the intrusion. A distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite, olivine gabbro, and gabbroic anorthosite that may represent parts of the Layered Series still hidden at depth. The Layered Series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across, and they typically are coarser grained than their host cumulates, being in this respect more like Upper Border Series rocks. The autoliths are spread stratigraphically through the lower 70% of the exposed 2500 m thickness of the Layered Series and are generally concentrated in three broad stratigraphic zones. Their physical relationships to their host rocks—particularly the way they indent older layers beneath and are covered by younger layers above—provide abundant evidence that there was generally a sharp, well-defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the Layered Series was forming. The distribution of the autoliths between and through the well-known, rhythmic, thin, modally graded layers shows that these layers were spread by magmatic currents; and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents. Many of the larger autoliths are crudely layered internally, and in places it is evident that their stratification existed before they broke loose; therefore, it must have formed in the Upper Border Series. One particularly large block of oxide gabbro exhibits extraordinarily well-developed modal and textural layering and includes small troctolitic autoliths of an earlier generation, and it provides evidence that currents also spread crystalline materials across the top of the magma body. Many of the very small autoliths in the Layered Series are highly anorthositic in composition, apparently because they were leached of mafic minerals, and some of the larger blocks show local patchy internal replacement by anorthosite. Most large blocks show little sign of postaccumulation modification, and some have thin, fine-grained augite-rich rims or rinds, demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates, they still were effectively armored against extensive chemical change. Also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion; these early dikes transgress small anorthositic replacement pipes in the block, showing that the replacement process also occurred in the upper border environment. Two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal-liquid suspension currents. One, involving density surge currents, has been advocated previously; the other is a new concept based on boundary flow separation and reattachment vortex cells. The two mechanisms are used in complementary ways to illustrate the formation of (1) some of the principal Skaergaard structures involving blocks and layers; (2) modally graded layers in the Layered Series that rhythmically alternate with uniform layers; and (3) modally sorted layers in the Upper Border Series featuring “underside draping” beneath small included blocks. Explanations are provided for (1) why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid, and (2) how the liquid part of a current was fractionated away from the crystalline materials. Modal and grain-size data from Skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents; a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion.

The Skaergaard Layered Series. Part III. Non-dynamic Layering

Layering in the Skaergaard Intrusion has been divided into two INTRODUCTION general types, one produced by magmatic flow and another by Recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe Skaergaard Intrusion (McBirney & Nicolas, 1997) tallization, and, in the case of the Layered Series, by compactionhave distinguished two broad types of layering, one related processes. Modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which, in the Layered and Upper another by processes that operate in situ such as varied Border Series, are diffuse and normally lack strong foliation and nucleation and growth of crystals, recrystallization, or by lineation. In the Marginal Border Series, the layers are thinner and compaction-related mechanisms. We refer to layering sharper; possibly because the rate of accumulation was slower. formed by these latter processes as ‘non-dynamic’ to Oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers, but it was less important than solution and processes. Although most layering combines elements of reprecipitation during slow cooling and Ostwald ripening. Evidence more than one process, the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor. and setting. Layering related to compaction becomes sharper with increasing The distinguishing features of dynamic magmatic layheight in the Layered Series until it suddenly disappears above the ering have been described in Part II of this series (McBirtrough horizon near the base of Upper Zone b. Mechanical sorting ney & Nicolas, 1997). They are best seen near the steep during compaction may have produced crude layering, but if it did margins of the Layered Series in what Wager & Brown the evidence has long since been destroyed by the superimposed effects (1968) referred to as the ‘cross-bedded zone’ where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re-equilibrated with them. Numerical the rocks have a marked foliation and lineation. simulations illustrate how small differences of surface energy caused Layering not associated with magmatic flow takes a by variations of grain size, textural dependence of solubility, and variety of forms, but we can distinguish two general pressure solution can cause segregation of minerals into layers during end-members, each of which has a distinct origin and solution and reprecipitation. characteristics. The first results from varied rates of

Metasomatic replacement in the Skærgaard Intrusion, East Greenland: Preliminary observations

Several unusual types of replacement structures in the Layered Series and Marginal Border Series of the Skaergaard Intrusion indicate that metasomatic changes have altered the modal proportions, and in places, the mineral composition of the gabbro. The geometrical forms and compositional patterns of these features indicate that they result from reactive fluids moving through the rocks at high temperatures shortly after the magma had reached an advanced stage of crystallization. The principal effect of metasomatism has been to produce strongly contrasting mafic and felsic rocks.

The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion have a range of textures and compositions, and among these are materials representing quenched Skaergaard magma. Pristine chilled marginal gabbro (CMG), however, is not ubiquitous at the intrusive contact, because many of the “contact” rocks have been hydrothermally or metasomatically altered, contaminated with gneiss or olivine xenocrysts, while others contain accumulated minerals. Material representing quenched magma appears to be restricted to contact rocks that are texturally and mineralogically similar to diabase, and free of accumulated minerals. Where it exists, the CMG is found within one to three meters of the exposed intrusive contact except at the roof of the intrusion where its thickness is greater. CMG was distinguished from the diverse group of contact rocks by petrographic and geochemical screening of over 80 specimens. Samples of CMG from the eastern and western margins and from the roof of the intrusion have relatively uniform composition similar to that of ferrobasalt, and are noticeably richer in iron (mg-number=0·51-0·54), TiO2 K2O, and P2O5 than other unmodified contact rocks. CMG's also have trace element compositions distinct from most other rocks in the outer Marginal Border Series (MBS). They have incompatible element contents up to 3–6 times greater than in LZa-type cumulates, negligible Eu anomalies, and Ni and Cr contents and Ni/Cr ratios that are among the lowest of rocks in the outer MBS. The results of melting experiments corroborate selection of this material as CMG. The composition of glasses obtained from partial melting experiments of LZa-type cumulates are essentially identical to those of the CMG. The 1-atm. liquidus phase relations for one of the CMG samples (KT-39) is largely consistent with the sequence and composition of cumulus minerals observed with distance inward through the MBS and upward through the Layered Series. Solidification of magma at the outer margin of the intrusion is interpreted to have involved locally efficient quench crystallization followed by initial primocryst growth in an undercooled transition zone a short distance inward that finally extended into regions of near equilibrium crystallization. The similarity in composition between samples of chilled marginal gabbro from the exposed roof and sides of the intrusion, and those of reconstituted trapped liquid from early cumulates in the outer MBS suggests that a single magma, similar in composition to ferrobasalt, was parental to the Skaergaard intrusion. This interpretation corroborates geophysical evidence of a significantly smaller mass for the intrusion than that estimated by Wager, and provides a basis for revision of models of its chemical evolution. Samples chosen by Wager as chilled marginal gabbro belong spatially, texturally, and compositionally to the group of LZa-type cumulates in the MBS, and should no longer be regarded as chilled marginal gabbro.

Petrology of the Marginal Border Series of the Skaergaard Intrusion

Petrology of the Marginal Border Series of the Skaergaard Intrusion