C. Kent Brooks

Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting

Abstract A new collection of mantle xenoliths in Tertiary dykes from the Wiedemann Fjord area in Southeast Greenland shows that this part of the central Greenland craton is underlain by highly depleted peridotites. The samples are mostly spinel harzburgites with highly forsteritic olivines (Fo 87–94 , average Fo 92.7 ). This, together with unusually high modal olivine contents (70–>95%), places the Wiedemann harzburgites in a unique compositional field. Relative to depleted Kaapvaal harzburgites with comparable Fo in olivine, the Wiedemann samples have considerably lower bulk SiO 2 (average 42.6 wt% versus 44–49 wt%). Spinel compositions are similar to those in other sub-cratonic harzburgites. Pyroxene equilibrium temperatures average 850°C, which is above an Archaean cratonic geotherm at an inferred pressure of 1–2 GPa, but low enough so that it is unlikely that the xenoliths represent residual peridotites created during Tertiary magmatism. Among mantle samples, the Wiedemann harzburgites are, in terms of their bulk composition, most similar to harzburgites from the ophiolites of Papua New Guinea (PNG) and New Caledonia (NC). One hypothesis is that the Wiedemann harzburgites, along with PNG and NC harzburgites, formed via dissolution of pyroxene from previously depleted peridotites, possibly beneath a volcanic arc. If so, higher spinel Cr/Al in Wiedemann samples may reflect a deeper origin compared to PNG and NC peridotites. Alternatively, using proposed primitive mantle compositions as a protolith, the Wiedemann harzburgites can be modeled as the residue after extraction of some 40% melt. The composition of this calculated hypothetical melt in terms of CaO, Al 2 O 3 , FeO, MgO and SiO 2 is similar to published experimental data on high degree melts of peridotite at 2–3 GPa. Munro-type komatiites lie close to these calculated and experimental melts but are slightly displaced towards low degree experimental melts at higher pressure (e.g., 6 GPa). We conclude that the Wiedemann harzburgites formed as a residue after about 40% melting, and that they may represent shallow, refractory residues after polybaric melting initiated at pressures ≥7 GPa and continuing to relatively low pressures (2–3 GPa or less). Extraction and aggregation of polybaric melts would produce liquids similar to Munro-type komatiites.

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.

Importance of iron-rich tholeiitic magmas at divergent plate margins: A reappraisal

When a tholeiitic liquid differentiates, it may give rise to either iron-poor, silica-rich (rhyolitic) differentiates or to iron-rich, silica-poor differentiates, as found in the Skaergaard intrusion. Iron-rich differentiated liquids are rare among erupted rocks, but are found in small quantities from several localities at divergent plate margins. Among the Tertiary basalts and intrusions in East Greenland, normal erupted basalts may be mixtures of primitive liquid and differentiated iron-rich liquid, which exists at depth but normally does not reach the surface because of its high density. The evolved liquids of the Skaergaard intrusion were of this kind. Data from mid-oceanic ridges confirm this view. We believe that iron-rich differentiated liquids, despite their scarcity on the surface, are much more voluminous at depth, as picrites, at the other side of the density minimum attained during liquid evolution, are thought to be. The trend toward iron enrichment develops when the tholeiitic magma differentiates in a closed system at a relatively low oxidation state, whereas the trend toward silica enrichment and iron depletion occurs when the magma has interacted with the oxidized and hydrated surroundings in the crust.

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland‐Iceland ridge were emplaced 5‐9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd=Dy(N) in the range 2 to 5.6 and Yb=Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd=Dy(N) than earlier breakup related flood basalts. The moderately low Yb=Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd=Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest 3 He= 4 He ratios (>10 R=Ra) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin.

Structural and magmatic segmentation of the Tertiary East Greenland Volcanic Rifted Margin

Abstract The Tertiary East Greenland Volcanic Rifted Margin is characterized by massive magmatic construction that produced a distinctive crustal architecture including: (1) a thick pile of flood basalts continuing offshore as seismically imaged ‘seaward-dipping reflector sequences’; (2) an extensive margin-parallel mafic dyke swarm; and (3) shallow crustal gabbroic plutons and deeper crustal ‘underplated’ material. These igneous units developed in the framework of an asymmetrical, crustal-scale fold, or ‘flexure’, that accommodated major subsidence along the continent-ocean transition. Extensive exposures along the margin reveal that the flexure and associated igneous structures define rift segments separated by various types of structural discontinuities. First-order segments occur between major triple-rift junctions as at Kangerlussuaq. At an intermediate scale, second-order accommodation zones bound margin segments c. 100 km in length with long-lived structural and/or magmatic expressions. Third-order discontinuities, spaced at tens of kilometres, correspond to smaller accommodation zones at abrupt along-strike changes fault or magmatic structures. Outcrop-scale transfer and transform faults occur at still smaller scales. Some of the larger accommodation zones appear to be related to pre-existing Precambrian structures and may have helped localize relatively late, post-flexure alkalic intrusions. The style of segmentation provides a link between similar segmentation patterns in continental rifts and mid-ocean ridge spreading centres that persist long after continental separation.

An ocean-ridge type magma chamber at a passive volcanic, continental margin; the Kap Edvard Holm layered gabbro complex, East Greenland

The gabbros of the Tertiary Kap Edvard Holm Layered Serieshave a stratigraphic thickness of more than 5000 m. Earlier work has shown that the range in cumulus mineral compositions is restricted (plagioclase An 81 —An 51 ; olivine Fo 85 —Fo 66 ; pyroxenes Ca 43 Mg 46 Fe 11 to Ca 43 Mg 37 Fe 20 ). Field evidence of magma injections is common, which together with the restricted range in mineral chemistry suggests that the magma chamber was frequently replenished by a less fractionated magma. A detailed study of a 600 m section (900–1500 m) in the Lower Layered Series reveals a period of crystallization when the magma chamber behaved as a closed system (900–1300 m). The rocks formed during this periodare well-laminated olivine–gabbros (900–110 m), which evolved to well-laminated oxide-gabbros (1100–1300 m). Compositional trends in the cumulusminerals are towards more evolved compositions (plagioclase An 64 —An 58 , pyroxene Mg# from 80 to 76) with stratigraphic height. From 1300 m to 1500 m, granular olivine-gabbros dominate, with moreprimitive mineral compositions (plagioclase An 67 —An 76 , pyroxene Mg# from 78 to 82). The transition olivine–gabbro to oxide-gabbro at 1100m is a consequence of fractional crystallization, and it is shown how changes in activities of FeO and Fe 2 0 3 in the magma are reflected in the total iron content of plagioclases.The transition from oxide-gabbro to olivine-gabbro at 1300 m results from replenishment by less evolved basaltic magma. On the basis of calcic pyroxene chemistry and the mineral crystallization sequence it is concluded that the Kap Edvard Holm Layered Series crystallized from a tholeiitic magma similar to MORB. Melanogabbroic units occur throughout the intrusion as discordant to subconcordant sill-like bodies 0.2–2.0 m thick. The melanogabbroic units consist of Cr-rich augite-olivine-plagioclase heteradcumulates and contain deformed mica crystals of pre-emplacement origin. These units crystallized from a wet, MgO-rich magma which was injected into the layered host gabbros after the formation of the cumulus pile, but before the magma was completely solidified. The Kap Edvard Holm Layered Series has several parallels with the plutonic part of ophiolite sequences. These include: cumulus mineral assemblage, compositions of the minerals and the restricted range in compositions with stratigraphic height; field evidence of repeated replenishment of basaltic magma; dyke swarms overlying the roof zone of the magma chamber; and the existence of a late intrusive suite of wet, MgO-rich magma. These parallels suggest that the processes involved in the formation of the Kap Edvard Holm Layered Series were similar to those involved in the formation of the crustalpart of many ophiolites and beneath present-day spreading ridges. The Kap Edvard Holm Layered Series is therefore believed to represent a shallow-level magma chamber which acted as a reservoir for basaltic flows at the continental margin during the opening of the North Atlantic Ocean.

Petrochemistry of the Volcanic Rocks of the North Atlantic Ridge System

The North Atlantic has been a key area since Vine [1] demonstrated the regularity of the magnetic anomalies along the Reykjanes Ridge while Iceland and the transverse ridge extending from the Faeroes to East Greenland represent the trace of a hot spot generated throughout the development of this part of the North Atlantic. It is our intention here to review the chemistry of oceanic tholeiites such as are recovered from abyssal parts of the mid-ocean rift system, contrast them with those of hot spot areas such as Iceland, and try to trace the chemical development of these lavas throughout the 60 m.y. spreading history. In addition the alkaline lavas of the area will be briefly described.

A tertiary lamprophyre dike with high pressure xenoliths and megacrysts from Wiedemanns Fjord, East Greenland

Lamprophyric dikes, mainly of camptonite/monchiquite affinities occur in the Wiedemanns Fjord area. One example contains a complex assemblage of olivine-orthopyroxene-chrome spinel nodules, megacrysts of kaersutite, diopside, strongly reverse-zoned green salite and various spinel phases. Microprobe analyses are presented for all these phases and for the lilac-coloured titansalites and strongly-coloured kaersutites of the groundmass. It is concluded that these minerals record evolution under various P, T and oxidation regimes during the formation of a lamprophyric parent magma. The nodules provide evidence for deep fractures in this area supposedly associated with early rifting in the North Atlantic.

Methane-bearing, aqueous, saline solutions in the Skaergaard intrusion, east Greenland

Solutions of H2O−NaCl−CH4 occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 μm diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH4, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C−O−H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.

Tertiary faulting and pseudotachylytes in the East Greenland volcanic rifted margin: Seismogenic faulting during magmatic construction

Volcanic rifted margins have voluminous magmatic constructions at continent-ocean transitions including thick sequences of flood basalts, dike swarms, and mafic plutons developed in crustal-scale flexures. In the Tertiary East Greenland volcanic rifted margin, upper crustal dilation and flexure were accommodated by extensive faulting, locally pervasive cataclastic deformation, and pseudotachylyte generation. The widespread distribution and large volumes of pseudotachylyte suggest that regional seismogenic faulting was vigorous during rapid crustal subsidence, dike intrusion, and the start of sea-floor spreading.