B. G. J. Upton

The chemistry of zircon: Variations within and between large crystals from syenite and alkali basalt xenoliths

Single grains of zircon can contain zones indicating several generations of crystal growth, each of which should reflect the chemical and physical conditions occurring at the time of its formation. Trace element analyses have been made of large zircon crystals from rocks of alkaline affinities by ion microprobe. The chondrite-normalised rare earth element (REE) concentrations increase rapidly from La to Lu, as would be expected from the decrease in ionic radius and consequent easier substitution into the Zr site within the zircon lattice. Lanthanum, praseodymium, and neodymium are considerably lower than values observed in bulk analyses of zircon. The partition coefficients for the light rare earth elements (LREEs), between zircon and melt or whole rock, must therefore be significantly lower than those calculated using bulk analyses. Cerium is enriched relative to neighbouring REEs due to the presence of Ce4+. Estimates of partition coefficients of Ce3+ and Ce4+ between zircon and melt demonstrate that although the Ce anomalies are large the Ce4+Ce3+ ratio is very small (less than 3 × 10−3). The size of the Ce anomaly is variable and should be capable of providing information on oxygen fugacity changes.

Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting

Abstract A lithospherically weak zone embracing the southern margin of the Greenland Archaean craton and the Ketilidian Julianehab batholith underwent repeated rifting during the interval 1350–1140 Ma, accompanying breakup of Palaeopangaea. The Gardar Province comprises the area affected by the faulting and asssociated alkaline magmatism. While an estimated 2–5 km of Proterozoic cover has been eroded, rift-fill successions have been preserved in early fault-bounded basins. The orientation of dyke swarms changed from WNW–ESE to nearly NE–SW during Gardar times. The principal swarms are inferred to occupy zones of lithospheric thinning and graben development. Central-type intrusive complexes, largely of syenites and nepheline syenites, reached shallow levels. The principal magmatic evolution was from transitional olivine basalt through to phonolites. Accompanying silica-oversaturated magma generation involved greater degrees of crustal assimilation. Anorthositic xenoliths in the Gardar intrusions imply the presence of an extensive anorthositic complex at depth, regarded as an integral part of the North American Proterozoic massif anorthosite association. The most primitive Gardar basalts are themselves relatively evolved, probably as a result of olivine±pyroxene fractionation during crustal underplating. The Gardar basic rocks are troctolitic with elevated Al 2 O 3 /CaO ratios: their incompatible element patterns suggest a significant input from lithospheric sources. The Ca-deficient nature of the Gardar basalts is attributed to an origin involving lithospheric mantle depleted by previous melting events. Trace element and isotopic signatures suggest considerable heterogeneity in the mantle sources which are ascribed to differential metasomatism of clinopyroxene-poor peridotite sources by small-fraction asthenospheric melts. The ultramafic lamprophyre/carbonatite association that recurred throughout the period is inferred to have originated from melting of metasomites deep within the lithospheric mantle. Affinities between the alkaline intrusions over an interval of >100 Ma characterize the Gardar as a coherent magmatic province and support the contention that the magmas are largely of lithospheric origin. The energy required to generate the very large requisite volumes of primitive magmas may have been supplied by successive mantle plumes. The Gardar magmatism pre- and post-dates the ∼1.27 Ga Mackenzie Igneous Events of North America but wholly pre-dates the ∼1.1 Ga Keweenawan magmatism associated with the Midcontinent Rift.

Beyond EM-1: Lavas from Afanasy-Nikitin Rise and the Crozet Archipelago, Indian Ocean

Lavas from Afanasy-Nikitin Rise, possibly the Late Cretaceous product of the Crozet hotspot, cover a wide range of isotopic compositions that includes the lowest ( 206 Pb 204 Pb) t (to 16.77) and ϵ Nd ( t ) (to −8) values yet found among oceanic islands or spreading centers worldwide, as well as high ( 87 Sr/ 86 Sr) t (to 0.7066). In contrast, young basalts from the Crozet Archipelago exhibit a narrow range of variation around ϵ Nd ∼ +4, 87 Sr/ 86 Sr ∼ 0.7040, and 206 Pb/ 204 Pb ∼ 19.0, closely resembling that of shield lavas of the Reunion hotspot. The Afanasy-Nikitin rocks also have much higher Ba/Nb, Ba/Th, and Pb/Ce than modern oceanic island or ridge lavas, as well as high La/Nb. The data do not obviously support the Crozet plume model but, assuming the model to be plate tectonically correct, would indicate that the plume-source composition either changed dramatically or that Afanasy-Nikitin magmatism involved significant amounts of nonplume mantle. The low 206 Pb/ 204 Pb, low ϵ Nd lavas provide the best evidence to date of the sort of material that, by variably contaminating much of the Indian mid-ocean-ridge basalt (MORB) source asthenosphere, may be responsible for the isotopic difference between most Indian MORB and Pacific or North Atlantic MORB. The combined isotopic and trace element results suggest an ultimate origin in the continental crust or mantle lithosphere for this material, although whether it was cycled through the deep mantle or resided at shallow levels in the convecting mantle cannot currently be determined.

Formation and composition of the lower continental crust: Evidence from Scottish xenolith suites

Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo-Carboniferous. The extremely variable Al2O3 (up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite- and mantle-normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high-grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra-site variations that are consistent with crystal-liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNd and 206Pb/ 204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm-Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm-Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian-derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the average age of the crust.

The Rum Layered Suite

Abstract Palaeocene igneous activity in the Rum Central Complex culminated in the formation of the ultrabasic and gabbroic rocks of the Layered Suite. Its three components, the Eastern Layered Series, the Western Layered Series and the Central Series, represent a continuum in time during which replenishments of picritic (MgO 15-20 wt.%) and basaltic magmas ponded in thin sill-like bodies at the Lewisian gneiss - Torridonian sandstone unconformity, each contributing incrementally to a layered cumulate sequence. The magmas were probably guided during ascent by the long-lived Long Loch Fault. Peridotite (olivine - chrome-spinel) cumulates formed from picritic magma. The residual (basaltic) magma mixed with resident residual magma from earlier batches, and with small amounts of siliceous rheomorphic melts from country rocks, forming (isotopically contaminated) allivalitic (= troctolitic), plagioclase - olivine cumulates or, less commonly, gabbroic (plagioclase - olivine - clinopyroxene) cumulates. Residual basaltic magma was probably also intruded as gabbroic sheets and plugs, and extruded as lavas. Widespread slump and shear structures indicate mechanical instability of unconsolidated cumulate mushes, especially in the allivalites. Ultrabasic breccias are common in the Central Series, and are attributed to (i) disruption of earlier cumulates as new batches of magma rose along an elongate, north-south feeder zone and (ii) collapse of cumulates into this zone during episodic magma withdrawal. Equilibrated textures, lack of compositional zoning in olivine and pyroxene, offsets between compositional and modal variation at unit and other lithological boundaries, the occurrence of finger structures and other replacement features, and the compositional modification of ultrabasic rocks adjoining late-stage gabbroic veins, all attest to the pervasive influence of migrating intercumulus liquids during crystallization and consolidation of the cumulates.

Early Tertiary magmatism in NE Greenland

Abstract Between 73° and 74°N in NE Greenland, a Lower Series of quartz tholeiite lavas is overlain by an Upper Series which varies from quartz tholeiitic to basanitic. The Lower Series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the Iceland plume to ascend to shallow levels. Relocation of eruption sites up to 150 km (?) further west, preceeded Upper Series activity. In this, small-degree melts, generated principally from garnet-lherzolite facies sources, were followed by products of more extensive, shallower melting which gave rise successively to the Upper Series lavas, a post-Upper Series basaltic dyke swarm and its associated caldera complex. Palaeomagnetic observations and 40Ar/39Ar dating suggest that the Lower Series (and part of the Upper Series) erupted in C24r at 58–57 Ma. Rejuvenescent activity at c. 32 Ma, marking the termination of volcanism, may relate to decompressive mantle melting associated with late regional uplift.

Geochemical evolution of the Ivigtut granite, South Greenland: a fluorine-rich "A-type" intrusion

Abstract The Ivigtut alkali granite stock lies within the Proterozoic Gardar alkaline igneous province of South Greenland. This small (300 m across) granite body once contained the worlds largest body of cryolite, now mined out. The granite surrounding the cryolite body has been extensively metasomatised by F- and CO 2 -rich fluids, leading to zonal enrichment in HFSE and REE. Variations in the Rb–Sr and Sm–Nd systematics of the granite can be correlated with the degree of metasomatism. Unaltered granites have highly variable initial 87 Sr / 86 Sr , and initial e Nd ratios of around −3, suggesting formation through crustal contamination of a mantle-derived magma. Metasomatised granites show higher initial e Nd ratios, which can be interpreted to suggest that the metasomatic fluid was derived from a mantle source.

Timing of Proterozoic magmatism in the Gardar Province, southern Greenland

The Gardar Province in southern Greenland is believed to have experienced three episodes of rifting, all associated with the extrusion of large volumes of alkalic magma. A nepheline-syenite of the Motzfeldt Center yields a U-Pb apatite age of 1.35 ± 0.01 Ga. An augite-syenite from the Ilimaussaq Complex yields a Sm-Nd pyroxene, alkali feldspar, magnetite age of 1.13 ± 0.05 Ga. On the basis of field relationships, the Eriksfjord Formation lavas have been interpreted as the oldest manifestation of Gardar magmatism. Sm-Nd plagioclase, pyroxene, wholerock ages of 1.17 ± 0.03 Ga and 1.20 ± 0.03 Ga for the middle member of the Eriksfjord Formation, however, indicate that these lavas may be part of the youngest magmatic activity. The available chronological data indicate that the Gardar Province was active for at least 200 m.y., from 1.35 to 1.12 Ga. It is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood-basalt provinces.

Noble gas systematics in basalts and a dunite nodule from Réunion and Grand Comore Islands, Indian Ocean

Noble gas isotope data are reported for olivine crystals in: (a) three basalt samples and one dunite nodule from Reunion; and (b) from one basalt sample from Grande Comore. Although the three Reunion basalts were selected to cover different phases of the volcanism, the data reveal rather similar 3He/4He ratios (1.8·10−5–2.1·10−5; 13–15 RA) but variable 40Ar/36Ar ratios (430–1900). The dunite has a 40Ar/36Ar of ∼ 3000. The 3He/4He ratios from Reunion are distinctly higher than those for MORB whereas the 40Ar/36Ar ratios of the Reunion basalts are lower than MORB values. Such noble gas isotopic signatures are similar to those reported from Hawaiian and Icelandic basalts. The Grande Comore sample, by contrast, showed a 3He/4He value slightly below that for MORB, with a relatively low 40Ar/36Ar of ∼ 380. On the basis of these data we conclude that the Reunion and Grande Comore magmas may have originated from hot-spot mantle sources with distinctly different noble gas isotopic compositions.

Crust-mantle interaction in the evolution of the Ilímaussaq Complex, South Greenland: Nd isotopic studies

Sm-Nd isotopic compositions were determined for the peralkaline Ilimaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial ɛNd(≈ 0), whereas a few samples trend towards ɛNd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilimaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust.