Magma plumbing systems in large igneous provinces: Inferences from cyclical variations in Palaeogene East Greenland basalts (original) (raw)
Related papers
Generation and Polybaric Differentiation of East Greenland Early Tertiary Flood Basalts
Journal of Petrology, 1997
Major element, trace element, isotope, and petrographic studies of magma differentiation; mantle melting the basal volcanics in the Kangerdlugssuaq Fjord region of the East Greenland margin provide constraints on the generation and subsequent differentiation and contamination histories of magmas during the early stage of continental rifting creating the North Atlantic ocean basin. Most primitive lavas of the succession are INTRODUCTION shown to have accumulated olivine. Corrections for crystal ac-The distinctive geochemical features of continental flood cumulation yield estimates for erupted primitive liquids with MgO basalts (CFBs) are strongly controlled by the composition contents between 9 and 13 wt %. The occurrence of suspended and structure of the continental lithosphere and under-Fo 88 olivine phenocrysts further requires the existence of parental lying mantle. Significant controversy remains concerning magmas with MgO contents up to 17 wt % and FeO contents up the balance of contributions from the lithosphere and to 14 wt %. Quantitative modeling of fractionation indicates the mantle, and the mechanisms by which the various that Kangerdlugssuaq magmas primarily differentiated at moderate source components contribute to basalt petrogenesis.
Geochimica et Cosmochimica Acta, 2023
The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid-to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.
Journal of Petrology, 2000
mantle melting zone, accompanied by an increase in the average Drilling along a 63°N transect off SE Greenland during Ocean degree of melting with time from >4% to >12%. These modest Drilling Program (ODP) Legs 152 and 163 recovered a succession degrees of melting imply mantle temperatures only >100°C hotter of volcanic rocks representing all stages in the break-up of the than normal upper mantle. Upwelling mantle must therefore have volcanic rifted margin. The rocks range from pre-break-up continental been fed dynamically to the melt zone to generate the igneous crust tholeiitic flood basalt, through syn-break-up picrite, to truly oceanic of 18 km thickness deduced from seismic and gravity studies. Nbasalt forming the main part of the seaward-dipping reflector MORB-like magmas dominated the earliest part of the succession sequence (SDRS). All the lava flows recovered from the transect although a few flows of 'Icelandic' basalt were erupted in the prewere erupted in a subaerial environment. 40 Ar-39 Ar dating shows break-up phase. In contrast, the post-break-up magmas had an that the earliest magmas were erupted at >61 Ma and has Icelandic mantle source. This suggests that the developing head of confirmed that the main part of the SDRS was erupted during C24r the ancestral Iceland plume was compositionally zoned, with a core (56-53 Ma) following continental break-up. Magma represented by of Icelandic mantle surrounded by a thick outer zone of hot, depleted the pre-break-up lava flows was stored in crustal reservoirs where upper mantle. it evolved by fractional crystallization and assimilation of continental crust. Trace element and radiogenic isotope data show that the contaminant changed, through time, from lower-crustal granulite to a mixture of granulite and amphibolite, suggesting storage of magma at progressively shallower levels in the crust. The degree of contamination declined rapidly as break-up proceeded, and the youngest rocks sampled in the transect are uncontaminated by continental basement. Variation of, for example, Sc/Zr and Sm/ KEY WORDS: flood basalt; geochemistry; Greenland; Palaeogene; Sr-Nd-Pb isotopes Lu through the succession suggests a shallowing of the top of the * Corresponding
Journal of Petrology, 2003
thick lithospheric cap, with 3 He contributed from volatile-rich fluids We present elemental and isotopic (Sr-Nd-Pb-Hf-Os-He) data from elsewhere in the Icelandic plume. The exact nature of the recycled on primitive alkalic lavas from the Prinsen af Wales Bjerge, component is not yet resolved, although Hf isotope compositions rule East Greenland. Stratigraphical, compositional and 40 Ar-39 Ar data out any significant role for recycled pelagic sediment, and the low indicate that this inland alkalic activity was contemporaneous with 187 Os/ 188 Os limits the participation of recycled basaltic material the upper parts of the main tholeiitic plateau basalts and also postand argues instead for a contribution from the mantle section of the dated them. The alkalic rocks show a marked crustal influence, recycled slab. indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high 3 He/ 4 He isotope ratios (R/R A 12•4-18•5), sub-chondritic 187 Os/ 188 Os i (0•120-0•126), low Nd i (>+4) and Hf i KEY WORDS: alkalic lavas; flood basalts; high 3 He/ 4 He; East Greenland; (>+6) that plot below the 'Nd-Hf mantle array', and trace recycled lithosphere; Iceland plume element characteristics similar to HIMU ocean island basalt (OIB). The uncontaminated magma is inferred to have more radiogenic 206 Pb/ 204 Pb values (>19•2) than the plateau basalts and Icelandic basalts, and thus represents a possible 'enriched' component to
Contributions To Mineralogy and Petrology, 2005
The lavas of the Zig-Zag Dal Formation of eastern North Greenland constitute a Mesoproterozoic tholeiitic flood basalt succession up to 1,350 m thick, extending >10,000 km2, and underlain by a sill complex. U-Pb dating on baddeleyite from one of the sills thought to be contemporaneous with the lava extrusion, gives an age of 1,382±2 Ma. The lavas, subdivided from oldest to youngest into Basal, Aphyric and Porphyritic units, are dominantly basaltic (>6 wt.% MgO), with more evolved lavas occurring within the Aphyric unit. The most magnesian lavas occur in the Basal unit and the basaltic lavas exhibit a generalised upward decrease in Mg number (MgO/(MgO + Fe2O3T)) through the succession. All of the lavas are regarded as products of variable degrees of olivine, augite and plagioclase fractionation and to be residual after generation of cumulates in the deep crust. The basaltic lavas display an up-section fall in the ratio of light to heavy rare-earth elements (LREE/HREE) but an up-section rise in Zr/Nb, Sc, Y and HREE. The older lavas (Basal and Aphyric units) are characterised by low ɛNd and ɛHf in contrast to higher values in the younger (Porphyritic unit) lavas. The Porphyritic Unit basalts are characterised by a notable enrichment in Fe and Ti. The Zig-Zag Dal succession is inferred to reflect an increase in melt fraction in the sub-lithospheric mantle, with melting commencing in garnet-lherzolite facies peridotites and subsequently involving spinel-facies mantle at increasingly shallow depths. Melting is deduced to have occurred beneath an attenuating continental lithosphere in conjunction with ascent of a mantle plume. Lithospheric contamination of primitive melts is inferred to have diminished with time with the Porphyritic unit basalts being products of essentially uncontaminated plume-source magmas. The high iron signature may reflect a relatively iron-rich plume source.
The 200- to 300-m-thick volcano-sedimentary sequence in the Kangerluluk Fjord, associated with penecontemporaneous and late-tectonic dykes, as well as a synvolcanic plutonic suite, represents an integral component of the Palaeoproterozoic Ketilidian Mobile Belt, south Greenland. The ca. 1808-Ma Kangerluluk supracrustal sequence contains four distinct mappable lithofacies: (a) a conglomerate–sandstone lithofacies; (b) a pyroclastic lithofacies; (c) a volcanic lithofacies; and (d) a peperite lithofacies. The volcanic lithofacies, up to 200 m thick, is characterized by shallow-water subaqueous brecciated and pillowed flows. Flows are either (a) feldspar–phyric, or (b) feldspar–pyroxene–phyric, with 0.2- to 3-cm-size plagioclase and 0.2- to 3-cm-size pyroxene that constitute between 20 and 30% (locally up to 50%) of the flows. Mafic dykes intruded wet unconsolidated pyroclastic lithofacies, resulting in the formation of peperites. Geochemically, the volcanic and pyroclastic units represent a distinct tholeiitic magmatic suite enriched in incompatible trace elements including Th, La, Yb, Zr and Nb, and exhibiting (La/Yb)n~10. The plutonic suite and associated dykes display a calc-alkaline trend with enriched LREE and unfractionated flat HREE patterns, Lan ranging between 50 and 100, (La/Yb)n ratios between 8 and 22, and negative Nb and Ti anomalies on the mantle-normalized, incompatible multi-element patterns. The pillowed flows lie in the continental flood basalt field on the Y–Nb–Zr discrimination diagram, and display a Nb anomaly and K2O-enrichment that collectively suggest a crustal component and/or a subduction-modified mantle source. The geology, stratigraphy of the Kangerluluk area and geochemistry can be used to infer a change in magma genesis from arc to rift volcanism. The 1850- to 1800-Ma calc-alkaline Julianehåb batholith represents a magmatic arc that rifted during crustal extension, allowing for the ascent of mantle-derived mafic magma. The geochemistry of the mafic volcanic flows, synvolcanic dykes and pyroclastic deposits favours a crustal component in magma genesis and offers new insights into the tectonic evolution of the Ketilidian Mobile Belt.
1998
Leg 152, located on the southeast Greenland Margin, successfully recovered igneous rocks from three drill sites along a transect at 63°N: 915, 917, and 918. The margin is characterized by a 150-km-wide sequence of seaward-dipping reflectors (SDRS), and thus has an architecture typical of volcanic rifted margins. Site 917 was located close to the inboard, feather-edge of the SDRS, on the continental shelf, and drilling recovered rocks ranging in composition from picrite to dacite. Site 915 was immediately oceanward of Site 917, and Site 918 was located on the continental rise, in the main sequence of the SDRS. Drilling at both Sites 915 and 918 recovered basalt. Ash horizons were cored in the sediment column at Site 918 and Site 919 in the Irminger Basin. The oldest recovered lavas (the Lower and Middle Series from Site 917), erupted approximately 61 m.y. ago, sit on steeply dipping pre-rift metasediments. They are variably contaminated by ancient amphibolite-and granulite-facies crust, consistent with eruption in a continental setting before plate breakup. A few of the Lower Series lavas are high-MgO basalts. The amount of contamination decreased dramatically in the succeeding Upper Series lavas, which also show evidence for a rapid shallowing of the average depth of melting, and contain a high proportion of picrites and high-MgO basalts. These observations are consistent with attenuation of the lithosphere and rapid egress of magmas to the surface during eruption of the Upper Series lavas. Basalt from Site 915 is only slightly contaminated by continental crust, and basalts from Site 918, erupted during C24r time (approximately 56−53 Ma), are uncontaminated. The eruption environment and the composition of the lavas indicate a clear role for the Iceland plume, in that (1) some lava flows from the Lower and Upper Series at Site 917 represent high-MgO magmas and, by inference, had high liquidus and source temperatures; (2) the main sequence of the SDRS were erupted subaerially, thus requiring support from hot, buoyant mantle; and (3) the basalts from Sites 915 and 918 more closely resemble depleted Icelandic tholeiites than normal mid-oceanridge basalts. Leg 152 has shown that igneous activity was under way by 61 Ma on the southeast Greenland Margin, at the same time as in West Greenland and the British Isles, over 2000 km apart in their pre-drift locations.
Mineralium Deposita, 2007
Sixty-five million year old continental flood basalts crop out on Qeqertarssuaq Island and the Nuussuaq Peninsula in West Greenland, and they include ∼1,000 m of picritic lavas and discrete 10-to 50-m-thick members of highly contaminated basalts. On Qeqertarssuaq, the lavas are allocated to the Vaîgat and Maligât Formations of which the former includes the Naujánguit member, which consists of picrites with 7-29 wt% MgO, 80-1,400 ppm Ni, 5.7-9.4 ppb Pt and 4.2-12.9 ppb Pd. The Naujánguit member contains two horizons of contaminated basalts, the Asûk and Kûgánguaq, which have elevated SiO2 (52-58 wt%) and low to moderate MgO (7.5-12.8 wt%). These lavas are broadly characterized by low Cu and Ni abundances (average, 40 ppm Ni and 45 ppm Cu) and very low Pt (0.16-0.63 ppb) and Pd (0.13-0.68 ppb) abundances, and in the case of the Asûk, they contain shale xenoliths and droplets of native iron and troilite. The contaminated basalts from Nuussuaq, the B0 to B4 members, are also usually Ni-, Cu-, and platinum-group elements (PGE)-depleted. The geochemical signatures (especially the ratios of incompatible trace elements such as Th/Nb) of all of the contaminated basalts from Qeqertarssuaq and some of those from Nuussuaq record what appears to be a chemical contribution from deltaic shales that lie immediately below the lavas. This suggests that the contamination of the magmas occurred during the migration of the magmas through plumbing systems developed in sedimentary rocks, and hence, at a high crustal level. Nickel, Cu, and PGE depletion together with geochemical signatures produced by crustal contamination are also a feature of Siberian Trap basalts from the Noril'sk region. These basalts belong to the 0-to 500-m thick, ∼5,000to 10,000-km 3 Nadezhdinsky Formation, which is centered in the Noril'sk Region. A major difference between Siberia and West Greenland is that PGE depletion in the Nadezhdinsky Formation samples with the lowest Cu and Ni contents is much more severe than that of the West Greenland contaminated basalts. Moreover, the volumes of the contaminated and metaldepleted volcanic rocks in West Greenland pale is significant when compared to the Nadezhdinsky Formation; local centers rarely contain more than 15 thin flows with a combined thickness of <50 m and more typically 10-20 m, so the volume of the eruptive portions of each system is probably two orders of magnitude smaller than the Nadezhdinsky edifice. The West Greenland centres are juxtaposed along fault zones that appear to be linked to the subsidence of the Tertiary delta, and so emplacement along N-S structures appears to be a principal control on the distribution of lavas and feeder intrusions. This leads us to suggest that the Greenland system is small and segregation of sulphide took place at high levels in the crust, whereas at Noril'sk, the saturation event took place at depth with subsequent emplacement of sulphide-bearing magmas into high levels of the crust. As a consequence, it may be Miner Deposita : [319][320][321][322][323][324][325][326][327][328][329][330][331][332][333][334][335][336] unreasonable to expect that the West Greenland flood basalts experienced mineralizing processes on the scale of the Noril'sk system.
Mineralogical Magazine, 2003
Basalts from the volcano-sedimentary Eriksfjord Formation (Gardar Province, South Greenland) were erupted at around 1.2 Ga into rift-related graben structures. The basalts have compositions transitional between tholeiite and alkaline basalt with MgO contents <7 wt.% and they display LREE-enrichment relative to a chondritic source. Most of the trace element and REE characteristics are similar to those of basalts derived from OIB-like mantle sources. Initial 87 Sr/ 86 Sr ratios of clinopyroxene separates range from 0.70278 to 0.70383 and initial e Nd values vary from -3.2 to +2.1. The most unradiogenic samples overlap with the field defined by carbonatites of similar age and can be explained by mixing of isotopically depleted and enriched mantle components. Using AFC modelling equations, the Sr-Nd isotope data of the more radiogenic basalts can successfully be modelled by addition of <5% lower crustal granulite-facies gneisses as contaminants. d 18 O v-smow values of separated clinopyroxene range from +5.2 to +6.0% and fall within the range of typical mantle-derived rocks. However, up to 10% mixing with an average lower crustal component are permitted by the data.