H. R. Naslund

Mechanisms of Formation of Igneous Layering

Abstract Layering is a common, almost ubiquitous, feature of gabbroic and syenitic intrusions. Individual layers, or layered sequences, however, vary greatly in such features as thickness and length, the nature of layer boundaries, internal vertical and lateral variations within layers, and the relationships to other nearby layers. Their modal proportions, grain-sizes, mineral compositions, whole-rock compositions, and textures present in layers and their surrounding host rock, are also quite varied. Given the wide range of these characteristics, it is unlikely that any single layer-forming mechanism can explain all or even most of the known occurrences of igneous layering. A wide variety of layer-forming mechanisms has been proposed. Some operate during the initial filling of a magma chamber, as a result of the settling of crystals carried in suspension, flow segregation during magma transport, magma chamber recharge, or magma mixing. Other proposed mechanisms operate in response to continuous, intermittent, or double-diffusive convection. Layering may also form as the result of mechanical processes, such as gravity settling, crystal sorting by magma currents, magmatic deformation, compaction, seismic shocks, or tectonic deformation. Variations of intensive parameters and kinetic factors, such as fluctuations of rates of nucleation and growth of crystals, oxygen fugacity, pressure, and rates of separation of immiscible liquids, may also be responsible for certain types of layering. During late-stage crystallization and cooling, layering may form in response to porous flow of interstitial liquids, metasomatism, constitutional zone refining, solidification contraction, Ostwald ripening, or contact metamorphism. The simple concept of a magma chamber undergoing differentiation as a result of early-formed crystals settling out of the magma and accumulating in layers on the floor of the chamber, has been discarded by most petrologists in favor of models involving in situ crystallization, in which magma chambers are thought to have the general form of a central mass of nearly crystal-free magma, that gradually loses heat and crystallizes inwards from its margins. The transition from crystal-free magma in the central part of the chamber to completely solidified rock in the outer parts is thought to occur through a marginal zone of crystal-liquid mush. As magmas crystallize and differentiate, components included in early-crystallizing minerals are depleted, while those excluded from these phases are enriched. It is unclear, however, how the latter are effectively transferred through the crystal mush zone, so that crystallization at margins results in differentiation of the body as a whole. It is also not clear what non-steady-state or non-equilibrium processes are responsible for the formation of layering during the crystallization process. Because these two problems are interrelated, an understanding of the formation of igneous layering should eventually lead to a better understanding of the processes responsible for igneous differentiation. The time scales and length scales involved in the formation of igneous layering preclude direct experimentation on silicate melts at magmatic temperatures, and as a result, the origin of these features must be largely deduced from field observations and theoretical considerations. The challenge for the igneous petrologist is to determine which features of igneous layering are diagnostic of a particular mechanism, which reflect subsequent compositional or textural modifications, and which can best discriminate between the plethora of possible mechanisms that have been proposed.

The differentiation of the Skaergaard Intrusion: A discussion of Hunter and Sparks (Contrib Mineral Petrol 95:451?461)

ConclusionsWe find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.

The differentiation trend of the Skaergaard intrusion and the timing of magnetite crystallization: iron enrichment revisited

Abstract Initial studies of the Skaergaard intrusion [L.R. Wager, J. Petrol. 1 (1960) 364–398] and much of the subsequent work [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247; C. Tegner, Contrib. Mineral. Petrol. 128 (1997) 45–51; A.R. McBirney, Contrib. Mineral. Petrol. 132 (1998) 103–105] concluded that the Skaergaard magma followed an iron-enrichment trend with little or no silica enrichment until the final stages of crystallization. Several recent reports [R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 95 (1987) 451–461; R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 104 (1990) 248–254], however, have suggested that the Skaergaard magma began to follow a silica-enrichment trend in Lower Zone c (LZc) of the Layered Series where magnetite first became an abundant mineral. Magnetite in LZc, however, generally occurs in aggregates of magnetite–ulvospinel and ilmenite–hematite that have undergone extensive subsolidus reequilibration and exsolution [E.A. Vincent, Neues Jahrb. Mineral. Abh. 94 (1960) 993–1016; E.A. Vincent, Geochim. Cosmochim. Acta 6 (1954) 1–26; A.F. Buddington, D.H. Lindsley, J. Petrol. 5 (1964) 310C357; H.R. Naslund, J. Petrol. 25 (1984) 185–212; A.R. McBirney, J. Petrol. 30 (1989) 363–397; Y.D. Jang, Petrological, Geochemical, and Mineralogical Variations in the Skaergaard Intrusion, East Greenland (Ph.D. Dissertation), State University of New York, Binghamton, NY, 1999, 219 pp.]. As a result, it is not clear if magnetite in these samples was an equilibrium, liquidus mineral fractionated from the main magma reservoir, or if magnetite crystallized as a later, interstitial mineral and did not directly affect the differentiation trend of the main Skaergaard magma. The timing of the initial crystallization of abundant magnetite and ilmenite is a key factor in understanding the trend of Skaergaard differentiation. Because V is a strongly included element in oxides, and is not strongly included in silicate minerals, the V content of an evolving magma is generally controlled by the fractionation of oxide minerals, in particular magnetite. The initial crystallization of magnetite should, therefore, be accompanied by a sudden decrease in the V content of the evolving magma, and in all of the coexisting mafic phases in equilibrium with that magma as well. The V content in Skaergaard pyroxene does not decrease significantly until the upper part of the Middle Zone (MZ), suggesting that the onset of extensive magnetite fractionation is much later than has previously been thought, and that the magnetite in LZc and the lower part of the MZ might not have been a liquidus phase at that level. The observed V trend in Skaergaard pyroxene can be modeled almost perfectly using published partition coefficients for the coexisting minerals in the Skaergaard intrusion, assuming that no magnetite fractionation occurred until the upper part of the MZ. Independently calculated trends for f O 2 in the Skaergaard magma [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247] change in the upper part of the MZ to more reducing conditions. The onset of magnetite fractionation would remove Fe 2 O 3 from the magma and could initiate such a change. The timing of magnetite fractionation will have a strong effect on whether magma evolves towards iron enrichment or silica enrichment.

Disequilibrium partial melting and rheomorphic layer formation in the contact aureole of the Basistoppen sill, East Greenland

The tholeiitic Basistoppen sill was intruded into the upper part of the Skaergaard complex shortly after the Skaergaard magma had solidified. Heat from the cooling Basistoppen magma caused disequilibrium partial melting in the adjacent Skaergaard ferrogabbros. Olivine, ferrobustamite, and magnetite were selectively melted and removed from the rock as an iron-rich melagabbro magma. Plagioclase acted as a refractory phase during partial melting and was left behind as an anorthositic gabbro restite. Modal and grain-size layering formed rheomorphically in the previously solidified host rocks as a result of partial melting and recrystallization. The rheomorphic layers are distinct from those found elsewhere in the intrusion.The extreme degree of contact metamorphism adjacent to the Basistoppen sill is a consequence of the intrusion of the sill into host rocks that were already near their melting temperature. It is suggested that the slow reaction rates between plagioclase and magma inhibited the dissolution of plagioclase relative to olivine, pyroxene, and opaque oxides and resulted in disequilibrium partial melting. The presence of anorthositic gabbro blocks within the Middle Zone of the Skaergaard intrusion indicates that disequilibrium partial melting may also occur during the assimilation of gabbroic xenoliths by magmas.

Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey

Transects through the lower part of the Palisades sill were made at Fort Lee and Alpine, New Jersey in order to characterize the petrologic signature of previously proposed “reversals” in the normal, tholeiitic differentiation trend. Petrographic and geochemical data include: (1) modal and grain size analyses, (2) bulk rock major and trace element concentrations by DCP-AES, and (3) augite, orthopyroxene, magnetite, and olivine compositions by electron microprobe analysis. Anomalous horizons, defined by increased bulk rock Mg#, Cr, Ni, and Co concentrations and abrupt modal and grain-size changes, occur at 10 m (the well known olivine zone), 27 m, 45 m, and 95m above the basal contact. Thermal models coupled with estimates of the emplacement rate and total magma volume indicate that the olivine zone (OZ) is an early-stage feature, related to the emplacement of initial magma into the Palisades chamber. Stokes Law calculations indicate that the settling velocity of average-sized olivine crystals in a high-titanium, quartz-normative (HTQ) magma is too slow for significant gravity settling to have occurred prior to the solidification of the basal 20 m of the sill. It is suggested that the OZ resulted from the emplacement of a heterogeneous initial magma from a compositionally stratified, sub-Palisades storage chamber located within the upper crust; however, heterogeneity may have been derived directly from the mantle or during rapid ascent. Geochemical models indicate that the OZ contains accumulated olivine that is not in cotectic (or constant) proportions with the other cumulus phases, suggesting a mechanical sorting process. Magma chamber recharge is proposed to have occurred at the 27 m and 45 m levels, when a slightly more-primitive HTQ magma was injected into the Palisades sill chamber. Zones of elevated Mg# and Cr, 6 to 10 m thick, at these two horizons may indicate the thickness of the hybrid magma formed by the mixing of these two compositions. Geochemical models indicate that the rocks at these levels have accumulated excess orthopyroxene relative to samples from the rest of the sill. Normal faulting in the Fort Lee area at the 95 m level has caused repetition of the stratigraphic section, and hence, the sharp reversal observed at this level.

Supersaturation and crystal growth in the roof-zone of the Skaergaard magma chamber

The coarse-grained Upper Border Series rocks of the Skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite, skeletal and hopper crystals of apatite, and less abundant sector-zoned augite crystals and hopper zircon crystals. In addition, the melanogranophyres which occur as pods and lenses in the lower part of the Upper Border Series and the upper part of the Layered Series are characterized by very coarse-grained dendritic ferrohedenbergite crystals. Skeletal, hopper, and sectorzoned crystals are not present in the Layered Series gabbros. The development of these unusual crystal morphologies in the Upper Border Series requires that the roof-zone magma was intermittently supersaturated and indicates that the Skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient. It is suggested that supersaturation developed in the roof-zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization, and as a result of sudden volatile loss during the later stages of crystallization when the Upper Border Series rocks became rigid enough to fracture.