Pyroclastic deposits within the East Greenland Tertiary flood basalts (original) (raw)
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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.
Proceedings of the Ocean Drilling Program. Scientific results, 1998
The lavas drilled during Ocean Drilling Program Leg 152 on the southeast Greenland Margin cover almost the whole compositional variation found in the North Atlantic volcanic province, ranging from picrites to dacites. Evidence for high-MgO melts, indicating a hotter than normal (plume) mantle is now found in several widespread areas, regardless of the distance to the assumed plume center. In most areas of the North Atlantic, several parallel fractionation trends are present, indicating several independent magma types and magma generation events. Fe-and Ti-enriched fractionated three-phase cotectic basalts are characteristic of the North Atlantic province but are not present at the southeast Greenland Margin except for one young sill. The Tertiary basalts are richer in FeOT and poorer in Na 2 O than normal mid-ocean-ridge basalts (MORB), indicating melting of hot plume material under a continental/lithospheric lid. The lavas from the southeast Greenland Margin, the Hatton Bank Margin, and the British Isles apparently formed from less hot mantle than the rest. Parts of this southern mantle were more depleted in incompatible elements than the mantle in other areas. The lavas from Hatton Bank and Site 918 have unusually high Sc contents, unparalleled in modern MORBs. All seaward-dipping reflector sequences drilled up to now (Vøring Margin, Hatton Margin, southeast Greenland Margin) include crustally contaminated rocks in the oldest parts of the sequences. Thick Tertiary dacite successions formed only over the peripheral parts of the mantle plume head. The crustal contaminant at the southeast Greenland Margin is similar to that in the British Isles and Kangerlussuaq and is in strong contrast to the crustal component in the Hatton Bank lavas. This is in accordance with the situation of these areas in different Precambrian structural and age provinces in the North Atlantic region.
1998
The lavas drilled during Ocean Drilling Program Leg 152 on the southeast Greenland Margin cover almost the whole compositional variation found in the North Atlantic volcanic province, ranging from picrites to dacites. Evidence for high-MgO melts, indicating a hotter than normal (plume) mantle is now found in several widespread areas, regardless of the distance to the assumed plume center. In most areas of the North Atlantic, several parallel fractionation trends are present, indicating several independent magma types and magma generation events. Fe-and Ti-enriched fractionated three-phase cotectic basalts are characteristic of the North Atlantic province but are not present at the southeast Greenland Margin except for one young sill. The Tertiary basalts are richer in FeOT and poorer in Na 2 O than normal mid-ocean-ridge basalts (MORB), indicating melting of hot plume material under a continental/lithospheric lid. The lavas from the southeast Greenland Margin, the Hatton Bank Margin, and the British Isles apparently formed from less hot mantle than the rest. Parts of this southern mantle were more depleted in incompatible elements than the mantle in other areas. The lavas from Hatton Bank and Site 918 have unusually high Sc contents, unparalleled in modern MORBs. All seaward-dipping reflector sequences drilled up to now (Vøring Margin, Hatton Margin, southeast Greenland Margin) include crustally contaminated rocks in the oldest parts of the sequences. Thick Tertiary dacite successions formed only over the peripheral parts of the mantle plume head. The crustal contaminant at the southeast Greenland Margin is similar to that in the British Isles and Kangerlussuaq and is in strong contrast to the crustal component in the Hatton Bank lavas. This is in accordance with the situation of these areas in different Precambrian structural and age provinces in the North Atlantic region.
40Ar/39Ar geochronology of the West Greenland Tertiary volcanic province
Earth and Planetary Science Letters, 1998
Paleocene volcanic rocks in West Greenland and Baffin Island were among the first products of the Iceland mantle plume, forming part of a larger igneous province that is now submerged beneath the northern Labrador Sea. A 40 Ar= 39 Ar dating study shows that volcanism commenced in West Greenland between 60.9 and 61.3 Ma and that ¾80% of the Paleocene lava pile was erupted in 1 million years or less (weighted mean age of 60:5 š 0:4 Ma). Minimum estimates of magma production rates (1:3 ð 10 4 km 3 year 1 km 1) are similar to the present Iceland rift, except for the uppermost part of the Paleocene volcanic succession where the rate decreases to < 0:7 ð 10 4 km 3 year 1 km 1 (rift). The timing of onset of volcanism in West Greenland coincides with the opening of the northern Labrador Sea and is also strikingly similar to the age of the oldest Tertiary volcanic rocks from offshore SE Greenland and the British-Irish province. This is interpreted as manifesting the impact and rapid (>1 m=year) lateral spreading of the Iceland plume head at the base of the Greenland lithosphere at ¾62 Ma. We suggest that the arrival, or at least a major increase in the flux, of the Iceland mantle plume beneath Greenland was a contributing factor in the initiation of seafloor spreading in the northern Labrador Sea. Our study has also revealed a previously unrecognised Early Eocene volcanic episode in West Greenland. This magmatism may be related to movement on the transform Ungava Fault System which transferred drifting from the Labrador Sea to Baffin Bay. A regional change in plate kinematics at ¾55 Ma, associated with the opening of the North Atlantic, would have caused net extension along parts of this fault. This would have resulted in decompression and partial melting of the underlying asthenosphere. The source of the melts for the Eocene magmatism may have been remnants of still anomalously hot Iceland plume mantle which were left stranded beneath the West Greenland lithosphere in the Early Paleocene.
Earth and Planetary Science Letters, 1998
The East Greenland Tertiary Igneous Province includes the largest exposed continental flood basalt sequence within the North Atlantic borderlands. More than ten layered gabbro complexes, including the-55 Ma Skaergaard intrusion, and a large dolerite sill complex are the plutonic equivalents of flood basalts; both lavas and intrusions have been regarded as synchronous with continental breakup at 57-54 Ma. We report ten new ages of the mafic intrusions, determined by '"Ar-39Ar incremental heating experiments, demonstrating that the mafic intrusions formed in two distinct time windows. Only Intrusion II of the Imilik Gabbro Complex, the Skaergaard intrusion, and the Sorgenfri Gletscher Sill Complex formed at 57-55 Ma coeval with the eruption of regional flood basalts and continental breakup. Other layered gabbro intrusions at Imilik (Intrusion III), Kruuse Fjord, Igtutarajik, Nordre Aputiteq, Kap Edvard Helm, and Lilloise are distinctly younger and formed between 50 and 47 Ma. Plate-kinematic models indicate the axis of the ancestral Iceland mantle plume was located under Central Greenland at-60 Ma and subsequently crossed the East Greenland rifted continental margin. We propose that tholeiitic magmatism along the East Greenland rifted margin largely occurred in three distinct pulses at 62-59 Ma (lavas and dykes), 57-54 Ma (lava& dykes, sills, and some gabbros) and 50-47 Ma (gabbros, dykes and rare lavas), related to discrete mantle melting episodes triggered by plume impact, continental breakup, and passage of the plume axis, respectively. This model implies northwestward continental drift of Greenland relative to the plume axis by-3.8-5.0 cm/yr between-60 and-49 Ma, consistent with estimates from seismic studies of submerged flood basalts.
Extended correlation of the Paleogene Faroe Islands and East Greenland plateau basalts
Lithos, 2009
New analytical data are presented for 13 enriched high-Ti tholeiitic basalts from the top of the Faroese lava pile that was formed by the time of break-up of the North Atlantic ∼ 56-55 Ma ago and are located on the eastern continental margin of the Atlantic Ocean. The samples fall in three groups according to their Nb/Zr/Y chemistry: High-Ti1, High-Ti2 and High-Ti3. The grouping is related to their stratigraphic position so that the High-Ti1 group is found lowest in the sampled profiles and the High-Ti3 group at the top with the High-Ti2 group in between. The High-Ti1 and High-Ti2 groups have 206 Pb/ 204 Pb = 17.59-18.30 while the High-Ti3 group has 206 Pb/ 204 Pb = 18.88-19.12. The three Faroese lava groups can be correlated with the East Greenland synbreakup basalt formations using their geochemistry, lava morphology and phenocryst contents. The High-Ti1 group correlates with the Milne Land Formation and the High-Ti2 and High-Ti3 groups correlate with the Geikie Plateau Formation and Rømer Fjord Formation, respectively. The discovery of equivalents to the East Greenland Geikie Plateau Formation and Rømer Fjord Formation means that volcanism has proceeded longer than previously thought on the Faroe Islands but with a lot lower intensity than on the other side of the rift. This demonstrates that large parts of the km-thick volcanic successions of the North Atlantic LIP developed in parallel on the juxtaposed rifted continental margins during break-up, although the centre of eruptions moved away from the Faroe Islands during the last stages of volcanism. Pb-isotope ratios indicate that the High-Ti1, High-Ti2, Milne Land Formation and Geikie Plateau Formation have the same mantle source while the High-Ti3 and Rømer Fjord Formation also contain material from another mantle component. High-Ti1 and Milne Land Formation lavas have a higher content of most incompatible elements compared to High-Ti2 and Geikie Plateau Formation. This is unexpected because the Zr/Nb ratio indicates that the High-Ti2 and Geikie Plateau Formation lavas are formed by lower degrees of melting. An explanation may be that High-Ti1 and Milne Land Formation magmas fractionated significant amounts of clinopyroxene at the bottom of the crust before rising to higher crustal levels, whereas High-Ti2 and Geikie Plateau Formation magmas initially fractionated olivine at lower pressures.
Proceedings of the Ocean Drilling Program, 1998
Ocean Drilling Program Leg 152 drilled Hole 917A first through Quaternary and Eocene sediments, then through about 779 m of Paleocene basalt and interlayered dacite that comprise the dipping seismic reflectors along the southeast Greenland margin, and thence into about 53.7 m of steeply dipping and metamorphosed sandstone and siltstone. Ten centimeters of quartz sandstone, probably of fluvial origin, was recovered between the basalt and metamorphosed sedimentary rocks. The sedimentary rocks, believed to be in part volcaniclastic, are mostly well-bedded turbidites and laminites and, in places, are intensely burrowed. Minerals are characteristic of the greenschist facies and consist of albite, chlorite, quartz, graphite, white mica, and epidote with lesser amounts of biotite, sphene/leucoxene, and pyrite. Normalized SiO 2 contents range from about 50 to 60.6 wt% and average about 54.4 wt%. Na 2 O is highly variable with albite-rich specimens having as much as 7.6 wt% and albitepoor specimens as little as 1.5 wt%. K 2 O contents range from a low of 0.04 wt% to a high of 2.32 wt%; variation is related to white mica content. Organic carbon contents range from about 0.3 wt% to 3.28 wt%, with an average of about 1.0 wt%. Mineralogy and chemistry suggest a volcanic (basaltic) source. Temperature of metamorphism was greater than 350°C, and possibly as high as 450°C, as shown by the crystallinity of graphite, chlorite geothermometry, and the presence of metamorphic biotite. The characteristics of similar onshore rocks from East Greenland, within the Late Cretaceous (Maastrichtian)-early Paleocene Ryberg Formation, may suggest a correlation with the metamorphosed sedimentary rocks recovered near the base of Hole 917A. We presume that the sediments were deposited in a basin during early rifting of the continents that led to the formation of the North Atlantic Ocean basin, possibly during the Late Cretaceous-earliest Paleocene interval. The original sediments were tilted, raised above sea level and eroded, and then buried beneath a thick section of basalt and dacite. Metamorphism at high temperatures and low pressure (2-4 kg?) probably took place prior to burial by the volcanic cover. The entire sequence was faulted and tilted during subsequent opening and spreading apart of the North Atlantic Ocean basin.
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
Chemical Geology, 2013
Keywords: Archaean Greenland Supracrustal belt Storø Premetamorphic alteration Seafloor weathering Metamorphosed Meso-to Neoarchaean supracrustal rocks in the central part of the island of Storø (Nuuk region, southern West Greenland), show fieldand geochemical evidence of premetamorphic chemical alteration. This alteration changed basaltic precursors into aluminous lithologies, and following amphibolite grade metamorphism and penetrative ductile deformation, these garnet-biotite schists now resemble adjacent metapelitic schists of sedimentary origin. Mass balance calculations (isocon method), suggests that most major elements (Si, Fe, Mg, Ca, Na and P) were leached during alteration. The calculated overall net mass changes are between −18% and −45%, consistent with breakdown of olivine, pyroxenes, plagioclase and apatite in the basaltic precursor rocks. Major and trace elements such as, K, Cs, Rb, Ba, Pb, Zn, La and Ce were added during this alteration process, whereas high field strength elements (Ti, Al, Zr, Hf and Nb) remained essentially immobile and were thus residually enriched. Interestingly, Th which is generally assumed to be immobile in fluids, was also added during this process. These chemical changes reflect interaction between a basaltic protolith and hydrous fluids that established a new equilibrium and thus a different mineral assemblage. It is proposed that the premetamorphic alteration at Storø was due to low-temperature interaction between seawater and oceanic crust, and thus essentially represents in situ submarine seafloor weathering. This interpretation is consistent with the mass balances reported from well-documented examples in younger settings. New U-Pb zircon geochronology from the arc-related mafic sequences at Storø shows that they comprise at least two distinct age groups: an older anorthosite complex dated at 3051.3 ± 2.6 Ma and a younger supracrustal sequence with age brackets between 2840 and 2710 Ma. The allochthonous nature of these two mafic igneous to sedimentary stacks is consistent with accretionary processes in island arc complexes and a compressional Archaean tectonic setting.