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Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30-52% in LZa troctolites, decreasing to 4-12% at the top of LZb olivine-gabbros and remaining low (1-13%; 4{middle dot}6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe-Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P2O5 content of the residual magma to [~]1{middle dot}7 wt % at the level of apatite-in, suggesting that the magma contained [~]49 wt % SiO2 and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.

The concept of variable textural maturity at grain junctions in igneous rocks was first introduced by Holness et al. (2005). The papers of Holness et al. (2007a, 2007b) demonstrate how this concept can be applied to cumulates, and present a hypothesis to explain the observation of excursions in textural maturity, either localized, or stepwise, within a cumulate pile. The comments by McBirney et al. refer to difficulties they have in accepting our hypothesis, and we welcome the opportunity to clarify some confusions in Holness et al. (2007a, 2007b). Here we identify the issues to be addressed with separate headings.

The Palaeogene Kangerlussuaq Intrusion (∼50 Ma) of East Greenland displays concentric zonation from quartz-rich nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre; modal layering and igneous lamination are locally developed but there are no internal intrusive contacts. This is an apparent violation of the phase relations in Petrogeny's Residua System. We propose that this intrusion is layered, grading from quartz syenite at the bottom to nepheline syenite at the top. Mineral and whole-rock major and trace element data and Sr–Nd–Hf–Pb isotope data are presented that provide constraints on the petrogenesis of the intrusion. Radiogenic isotope data indicate a continuously decreasing crustal component from the quartz nordmarkites ( 87 Sr/ 86 Sr = 0·7061; ɛ Ndi = 2·3; ɛ Hfi = 5·2; 206 Pb/ 204 Pb meas = 16·98) to the foyaites ( 87 Sr/ 86 Sr = 0·7043–0·7044; ɛ Ndi = 3·8–4·9; ɛ Hfi = 10·7–11·1; 206 Pb/ 204 Pb meas = 17·78–17·88); the foyaites are dominated by a mantle isotopic signature. The average Mg-number of amphibole cores becomes increasingly primitive, varying from 26·4 in the nordmarkites to 57·4 in the pulaskites. Modal layering, feldspar lamination and the presence of huge basaltic xenoliths derived from the chamber roof, now resting on the transient chamber floor, demonstrate bottom-upwards crystallization. The intrusion cannot, therefore, have formed in a system closed to magmatic recharge. The lack of gneissic xenoliths in the nordmarkites suggests that most contamination took place deeper in the crust. In the proposed model, the nordmarkitic magma formed during crustal assimilation in the roof zone of a large, silica-undersaturated alkali basaltic/basanitic, stratified magma chamber, prior to emplacement in the uppermost crust. The more primitive syenites, terminating with foyaite at the top of the intrusion, formed as a consequence of repeated recharge of the Kangerlussuaq Intrusion magma chamber by tapping less contaminated, more primitive phonolitic melt from deeper parts of the underlying chamber during progressive armouring of the plumbing system.

The clinopyroxene^plagioclase^plagioclase dihedral angle, cpp, in gabbroic cumulates records the time-integrated thermal history in the sub-solidus and provides a measure of textural maturity. Variations in cpp through the Layered Series of the Skaergaard intrusion, East Greenland, demonstrate that the onset of crystallization of clinopyroxene (within LZa), Fe^Ti oxides (at the base of LZc) and apatite (at the base of UZb) as liquidus phases in the bulk magma is recorded by a stepwise increase in textural maturity, related to an increase in the contribution of latent heat to the total heat loss to the surroundings and a reduction in the specific cooling rate at the crystallization front of the intrusion. The onset of both liquidus Fe^Ti oxide and apatite crystallization is marked by a transient increase in textural maturity, probably linked to overstepping before nucleation. Textural maturation at pyroxene^plagioclase^plagioclase triple junctions effectively ceases in the uppermost parts of the Layered Series as a result of the entire pluton cooling below the closure temperature for dihedral angle change, which is 10758C. Solidification of the Layered Series of the Skaergaard intrusion occurred via the upwards propagation of a mush zone only a few metres thick.

Textural maturity describes the extent to which a rock has evolved from the initial reaction-controlled texture towards textural equilibrium controlled by the minimization of interfacial energy. Solidification in a magma chamber results in the formation of an impingement texture by the random juxtaposition of planar-sided grains. Orthocumulates, in which the initial melt-filled pores are pseudomorphed by later-crystallizing phases, have an ophitic or intersertal texture immediately after complete solidification, which then evolves towards solid-state equilibrium by rounding of initially planar grain boundaries and an increase in the median dihedral angle subtended at the junctions of two primocrystic grains with the interstitial phase.The bulk of the increase in angle occurs just below the solidus temperature in kilometre-scale mafic plutons. Quantification of textural maturity via measurement of dihedral angle populations in troctolitic and gabbroic cumulates from the Rum Eastern Layered Intrusion and the Skaergaard Intrusion demonstrates that the rocks preserve a record of thermal events related to magma chamber replenishment and the onset of chamber-wide convection.Textural maturity is also a function of the liquidus phase assemblage: for systems in which only olivine and plagioclase are liquidus (i.e. cumulus) phases in the main magma body above the crystal mush, the texture is significantly less mature than that in systems in which clinopyroxene is an additional liquidus phase. The difference in textural maturity reflects differences in the cooling and solidification rate, and demonstrates directly that the liquidus phase assemblage plays a role in determining the thermal history of plutons.

The major element composition of plagioclase, pyroxene, olivine, and magnetite, and whole-rock Sr/Sr data are presented for the uppermost 2.1 km of the layered mafic rocks (upper Main Zone and Upper Zone) at Bierkraal in the western Bushveld Complex. Initial Sr/Sr ratios are near-constant (0.7073 ± 0.0001) for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation. The 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite) layers and changes up-section from gabbronorite (An72 plagioclase; Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayalite ferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overall fractionation trend is, however, interrupted by reversals characterized by higher An% of plagioclase, higher Mg# of pyroxene and olivine, and higher V2O5 of magnetite. In the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite. These reversals in normal fractionation trends define the bases of at least nine major cycles. We have calculated a plausible composition for the magma from which this entire succession formed. Forward fractional crystallization modeling of this composition predicts an initial increase in total iron, near-constant SiO2 and an increasing density of the residual magma before magnetite crystallizes. After magnetite begins to crystallize the residual magma shows a nearconstant total iron, an increase in SiO2 and decrease in density. We explain the observed cyclicity by bottom crystallization. Initially magma stratification developed during crystallization of the basal gabbronorites. Once magnetite began to crystallize, periodic density inversion led to mixing with the overlying magma layer, producing mineralogical breaks between fractionation cycles. The magnetitite and nelsonite layers mainly occur within fractionation cycles, not at their bases. In at least two cases, crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically, and triggered the proposed density inversion, resulting in close, but not perfect, coincidence of mineralogical breaks and packages of magnetitite layers.

© The Author 2015. The Cambridge Drill Core provides a continuous sample of the lower part of the floor cumulates of the Skaergaard Intrusion, including ~150M of stratigraphy from the unexposed Hidden Zone. Bulkrock geochemistry together with olivine mineral compositions and augite-plagioclase-plagioclase dihedral angles from the drill core are interpreted as a record of the early history of the Skaergaard magma chamber. A detailed geochemical and microstructural study of the mode and morphology of augite reveals no unambiguous markers that can be used to pinpoint the first appearance of cumulus augite in the stratigraphy sampled by the drill core. The early history of the Skaergaard magma chamber involved the arrival of multiple small batches of magma, each with a variable load of olivine and plagioclase phenocrysts. The region of the core between -120 and -108.8m records at least three separate, but closely spaced, influxes of new magma, with more magma added at -85 and -65 m. The last influx of magma is recorded by the cumulates just below the lowest exposed horizons of the Layered Series, involving a large volume of magma that inflated the chamber to its final size. The Skaergaard magma chamber therefore formed by the progressive inflation of what was likely to have originated as a sill intruded at the discontinuity between the Precambrian gneisses and the overlying plateau lavas. The 'Skaergaard parental magma' should therefore be viewed as the integrated and mixed composition of the various magma influxes that filled the chamber. ispartof: JOURNAL OF PETROLOGY vol:56 issue:6 pages:1199-1227 status: published

contaminated layered intrusions known. Thousands of recrystallized Strontium and neodymium isotopic data for mafic cumulates, chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins, and adjacent crustal rocks of the Hasvik Layered Intrusion, are thought to represent the remnants of the assimilated material. North Norwegian Caledonides, are reported together with new The xenoliths spalled off the roof during magma emplacement, and, mineralogical and whole-rock analytical data to constrain the extent together with the elevated temperatures (400–600°C) of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks, led to a high degree of assimilation in the magma chamber. Initial Sr/Sr (700 Ma) of 0·7045 and eNd Hasvik magma chamber. (700 Ma) of+3·03 for the chilled margin, which has a tholeiitic composition akin to the chilled rocks of the Skaergaard intrusion, demonstrate that the parental magma was derived from a depleted mantle source. The basal cumulates (0–335 m) show an up-section decrease in Sr/Sr from 0·7045 to 0·7038 and a correlative