ALBERT

Description: The late Miocene and younger mafic back-arc lavas in the southern Puna of the central Andean plateau have been attributed to the aftermath of crustal and mantle lithospheric delamination or foundering. In this paper, we analyze in more detail the nature of the back-arc mafic suite magmas, including the conditions of magma generation in the mantle and of magma evolution during ascent and ponding in the crust, using extensive compositional data for phenocryst minerals and olivine-hosted melt inclusions in combination with published and new whole-rock chemical and isotopic data. We estimate that the primary melts last equilibrated with an enriched mantle source at temperatures near 1375°C and pressures near 2 GPa, which is near the base of the seismically determined ~60 km thick crust. A mantle source geochemically enriched by continental material introduced through delamination and subducted erosion processes is required to explain the coincidence of the high 87 Sr/ 86 Sr ratios (〉0·705) and high Sr concentrations (〉700 ppm) of the most primitive lavas (e.g. 9–10 wt % MgO, olivine Fo 88 ). The crystallization conditions inferred from mineral–melt equilibria indicate that olivine ( T = 1320–1220°C) was followed by clinopyroxene ( T = 1230–1140°C). Clinopyroxene–melt equilibration pressures of 0·7 to near 1 GPa in the most mafic samples indicate that the magmas crystallized at mid-crustal depths of 20–35 km, within a region of inferred partial melt accumulation based on the presence of low seismic velocity zones. Olivine-hosted melt inclusions indicate relatively dry melts (maximum 0·5 wt % H 2 O) with unusual high-Al basaltic compositions, which are attributed to the high-pressure suppression of plagioclase crystallization. A first stage of crustal contamination before mid-crustal accumulation and crystallization of the mafic magmas is suggested by high O-isotope ratios in olivine phenocrysts and negative Eu anomalies in clinopyroxene from the plagioclase-free mafic lavas. Mixing models based on trace elements and radiogenic isotopes suggest assimilation of silicic melt in the lower crust, similar to contemporaneous glassy dacites with steep REE patterns and negative Eu anomalies. A second stage of crustal assimilation at shallower depths is indicated by the mismatch of incompatible elements in clinopyroxene relative to bulk-rock compositions, by strong positive correlations of radiogenic isotopes with wt % SiO 2 , and by petrographic observation of partly resorbed and reacted quartz xenocrysts. Mixing calculations require the erupted magmas to have assimilated in total some 15–25% crust.

Description: Koegel Fontein, about 350 km north of Cape Town, is the only known early Cretaceous anorogenic igneous complex along the volcanic rifted margin of South Africa. The oldest rocks of the complex are minor granite and syenite intrusions at 144 Ma, which were followed by tholeiitic and alkaline basalt dykes, then by microsyenite and quartz porphyry dykes. The youngest and largest igneous unit is the 135 Ma Rietpoort Granite, with an exposed diameter of about 20 km. The country rocks are Mesoproterozoic gneisses of the Namaqua–Natal Province, which in many places were deformed and retrogressed by Pan-African tectonism. Whole-rock 18 O values from the Rietpoort Granite and smaller plutonic units (syenite, granite) are in the range 6–9 (outliers 4 and 17). Quartz 18 O values from all units are in a narrow range and indicate magma 18 O values between 6 and 8. In contrast to the syenites and granites, most mafic and silicic dyke units have 18 O 〈6, as low as –4·1. Quartz porphyry dykes that are compositionally similar to the Rietpoort Granite have a bimodal distribution of 18 O values in both whole-rock and quartz phenocrysts. The magma 18 O values estimated from the phenocryst data define a ‘normal group’ identical to the Rietpoort Granite (6–8) and a ‘low- 18 O group’ (0 to –2). The microsyenite and mafic dykes also yield negative 18 O values, but the strong hydrothermal alteration of these rocks and lack of fresh phenocrysts make a primary origin of the low 18 O values unlikely and untestable. Whole-rock D values of igneous units and basement rocks average –99, which corresponds to a palaeo-meteoric water with 18 O as low as –9. This is much lower than the expected value for meteoric water at the time of emplacement, given the low latitude (30–40°S). Quartz veins cutting the mafic dykes have 18 O values as low as –2, which attest to hydrothermal fluids having low 18 O values. Country rocks in the study area have a large range of 18 O (–3 to 10), with the majority below the mantle value of 6. The low 18 O values of the country rocks, although prevalent in the roof pendant of the Rietpoort Granite, do not appear to have originated from a meteoric–hydrothermal system established by the intrusions. We suggest instead that the Koegel Fontein complex was emplaced into a structurally controlled zone in the Namaqualand basement whose 18 O values had been lowered by interaction with meteoric fluid during reactivation along Pan-African shear zones. Initial emplacement of the magmas caused thermal dehydration of the country rocks and expulsion of low- 18 O fluids. This was followed by local partial melting of the altered crust with formation of low- 18 O crustal magmas. The O isotope data provide new constraints on the crustal vs mantle source of the Koegel Fontein magmas. The Rietpoort Granite and ‘normal 18 O’ quartz-porphyry dykes crystallized from magmas with 18 O values of 7–8, Nd of –5 to –7, and initial 87 Sr/ 86 Sr of 0·716–0·732, which fit a model for 30–50% meta-igneous crust similar to the local Namaqua gneisses, with a minor component of low- 18 O crust. The ‘low- 18 O’ quartz porphyry magma had an identical Nd isotope composition, but lower initial 87 Sr/ 86 Sr (0·709–0·725) and 18 O (0 to –2), which we attribute to melting or assimilation of hydrothermally altered basement rocks with Rb and 18 O depletion.

Description: The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order, from bottom to top, pyroxenite–norite–anorthosite. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the UG2 chromitite and the Merensky Reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral–melt evolution. The anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates. Here, chromite compositions change abruptly from high and constant Mg/(Mg + Fe 2+ ) and Fe 2+ /Fe 3+ ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers. Furthermore, the composition of disseminated chromites varies depending on the host silicate assemblage; for example, in Ti, V and Zn contents. Importantly, the abrupt change in chromite composition across the chromitite–silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid. Chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re-equilibration with intercumulus liquid. Among those features is the decoupling of alkalis from other incompatible lithophile elements. In comparison with cumulus plagioclase, intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible Li, K and Rb. Strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers. To explain these features, we propose a new model of post-cumulus recrystallization, which intensifies the modal layering in the crystal–liquid mush, producing the observed sequence of nearly monomineralic layers of chromitite, pyroxenite and anorthosite that define the cyclic units. The crucial element of this model is the establishment of redox potential gradients at contacts between chromite-rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt. Because basaltic melts are ionic electrolytes with Na + as the main charge carrier, the redox potential gradient induces electrochemical migration of Na + and other alkali ions. Selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units. Sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions, which may eventually result in the formation of sharply divided monomineralic layers. The observed variations in ferric/ferrous iron ratios in chromite from the cyclic units and Fe distribution in plagioclase imply a redox gradient of the order of 0·9 log-units f O 2 , equivalent to a potential gradient of 60 mV. Preliminary estimates suggest that the resulting electrochemical flux of Na + ions is sufficient to mobilize about one-third of the total Na content of a 1 m thick mush layer within 10 years. The proposed electrochemical effect of post-cumulus crystallization is enhanced by the presence of cumulus chromite but, in principle, it can operate in any type of cumulates in which ferrous and ferric iron species are distributed unequally between crystalline and liquid phases.