S. A. Gibson

Subcontinental mantle plumes, hotspots and pre-existing thinspots

The magmatism occurring when a hot convective mantle plume is sited beneath a lithospheric plate may be more complex if the latter is continental, rather than oceanic, because of the characteristic local physical inhomogeneity of continents. Thus, the surface volcanic expression of the plume (hotspot) may be displaced from immediately above its rising stem, if the continent has been previously locally thinned nearby. The magmatism of the British Tertiary Igneous Province and Parana basin, South America, appears to fit this model, and it may also explain why the Miocene Columbia River basalts of the NW United States overlie an Early Tertiary sedimentary basin.

High-Ti and low-Ti mafic potassic magmas: Key to plume-lithosphere interactions and continental flood-basalt genesis

The role of mantle plumes in the genesis of continental flood-basalts (CFB) remains controversial, primarily due to our limited knowledge of the composition of the subcontinental lithospheric mantle (SCLM). In this study we use the widespread Cretaceous mafic potassic magmatic rocks, emplaced around the margins of the Paranasedimentary basin, to probe large-scale compositional variations of the SCLM beneath southern Brazil and Paraguay. On the basis of Ti contents, together with major-, trace-element and isotopic ratios, these mafic potassic rocks may be subdivided into high-Ti and low-Ti groups. The former have relatively high average TiO2 (4.64), CaO/Al2O3 (1.74) and 143Nd/144Ndi (0.51232), together with low La/Nb (1.1) and 87Sr/86Sri (0.7050). The latter are characterised by much lower average TiO2 (1.77), CaO/Al2O3 (0.72) and 143Nd/144Ndi (0.51182), together with higher La/Nb (2.01) and 87Sr86Sri (0.7068). These high-Ti and low-Ti groups are spatially separate and their distribution correlates with tectonic setting; the low-Ti magmas are associated with cratonic regions, whereas the high-Ti magmas are in Proterozoic mobile belts. The distribution of the subgroups of mafic potassic magmatic rocks correlates closely with the geochemical provinciality of the Early Cretaceous high-Ti and low-Ti Paranaflood-basalts. This is the first reported occurrence of extensive low-Ti mafic potassic magmatism associated both spatially and temporally with the low-Ti region of a major CFB province. Our study further reveals similar relationships between tectonic setting and the geochemical provinciality of mafic potassic magmas and continental flood-basalts across Gondwana. We use the bulk-rock compositions and radiogenic isotopic ratios of both the high-Ti and low-Ti mafic potassic magmatic rocks as end members in models of CFB genesis. Mixing calculations involving Sr and Nd isotopic ratios indicate that the flood-basalts may contain up to 50% of mafic potassic lithosphere-derived melts. Overall, the results of our geochemical modelling agree with geophysical arguments that the convecting asthenosphere is the predominant source of CFB magmas.

Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes

A comparison of high-MgO magmas from both oceanic and continental settings reveals that they exhibit wide variations in their bulk-rock contents of FeO* (9–16 wt% at 15 wt% MgO). The high-FeO* picrites (ferropicrites) range from Archean to recent in age but are relatively rare at the Earth’s surface, typically forming thin isolated flows near the base of thick lava piles in large igneous provinces. They are characterised by high contents of compatible trace elements (e.g. Cr=400–1650 parts per million (ppm) and Ni=250–1050 ppm) and unaltered samples (Parana–Etendeka and Madagascar) have relatively smooth, normalised multi-element patterns that lack significant relative enrichments in strongly incompatible elements (e.g. [Ba/La]n=0.5–1.0) and [La/Nb]n=1.2–1.4). The ferropicrites are distinguished from other picritic rocks (e.g. Deccan, Hawaii, West Greenland) by their relatively low abundances of Al2O3 (∼10 wt%) and heavy rare-earth elements (HREEs, e.g. Lu=<10×chondrite). They have 87Sr/86Sri ratios of ∼0.7048 and ϵNd values of ∼+4 that are comparable to those of ocean-island basalts. Modelling calculations of combined Sr, Nd and Pb isotopic ratios indicate that some of the ferropicrites may have assimilated upper or lower crust but this appears to have had little effect on major element abundances. The high-FeO* contents of world-wide ferropicrites, relative to ‘normal’ picrites, cannot simply be attributed to variations in degrees of partial melting and/or depth of melt segregation of an anhydrous lherzolite mantle source. Quantitative partition modelling suggests that the contributing parental melts of the ferropicrites were derived by adiabatic decompression melting of a mantle source that was similar in composition to experimentally studied Fe-rich peridotite PHN1611. The parental melts of the Parana–Etendeka ferropicrites appear to have been generated by ∼10% partial melting, at high pressures (45–35 kbar) and high mantle potential temperatures (Tp=1550°C). The relatively low volume of world-wide ferropicrites and their association with igneous rocks of ‘normal’ FeO* contents in mantle plume-related igneous provinces suggest that the former may be derived from Fe-rich streaks in mantle plume starting-heads.

Strongly potassic mafic magmas from lithospheric mantle sources during continental extension and heating: evidence from Miocene minettes of northwest Colorado, U.S.A.

Abstract Minette occurs as sparse dykes and sills in the Upper-Miocene Elkhead Mountains igneous province, NW Colorado. At the time of magma emplacement, the region was undergoing crustal extension and probably also heating of the sub-continental lithospheric mantle by the approaching Yellowstone plume. The predominant magmatism of the province comprises lava field remnants and hypabyssal plutons. Elemental variation within the minette suite is explicable in terms of fractional crystallisation which involved phlogopite separation from even the most magnesian (MgO = 10.68%) samples. Wide ranges of incompatible-element abundances and ratios occur in the minettes with MgO > 6.0%. Some of these ratios (e.g. Ti/Zr andLa/Nb) correlate well with 143 Nd/ 144 Nd in this suite. The minettes have a combination of relatively low values of both 87 Sr/ 86 Sr (0.70387–0.70413) and 143 Nd/ 144 Nd (0.51201–0.51227), with 206 Pb/ 204 Pb (17.28–17.47), 207 Pb/ 204 Pb (15.45–15.54) and 208 Pb/ 204 Pb (36.55–37.02). Taken together, these isotopic characteristics fall far outside the range of all oceanic igneous rocks and therefore rule out an exclusively asthenospheric source for the magmas. Genetic models involving crustal contamination of either basaltic (s.l.) or lamproitic liquids do not appear to explain satisfactorily the geochemical features of the minettes. Alternative models invoke either separate subcontinental lithospheric mantle sources for each magma batch or mixing between upwelling basaltic liquids and varying amounts of ultrapotassic lithospheric melts. Both models fit the geochemical data reasonably well but the latter is, in addition, consistent with a recent analysis by D. McKenzie [8] of the physical constraints on strongly potassic magma genesis during continental lithospheric extension and/or heating above a mantle plume. A brief survey of the tectonic settings of minettes and ultrapotassic rocks worldwide shows that a strong case can be made for their association in space and time with heating and/or thinning of sub-continental lithospheric mantle.

Major element heterogeneity in Archean to Recent mantle plume starting-heads

Variations in the bulk-rock compositions of primary high-Mg melts provide important constraints on the thermal and chemical structure of their mantle source regions. Picrites and komatiites of Archean to Recent age exhibit a wide range of FeO and Al2O3 at a given MgO content. Those with FeO* contents (total Fe as FeO)>12.5 wt% have low Al2O3 contents (typically<10 wt%), and also fractionated heavy rare-earth-element ratios ([Gd/Yb]n=1.25–3.75), that are consistent with melt generation in the garnet-stability field. The high-Fe magnesian melts typically have positive ϵNd values, which are similar or slightly lower than those of co-existing picrites and komatiites, and suggest that the convecting mantle was the predominant melt source region. The high-Fe magnesian magmas typically occur at, or close to, the base of igneous successions of Archean (e.g. Onverwacht Group, S. Africa and Superior Province, Canada) and Phanerozoic age (e.g. Siberia, Parana-Etendeka, and the North Atlantic Igneous Province) and appear to represent some of the earliest magmas to be generated during melting of mantle plume starting-heads. Generation of high-Fe magnesian melts cannot be readily explained by high-pressure melting of fertile peridotite in a dynamic melting regime. A comparison of the bulk-rock compositions of Fe-rich picrites with the results of recent experimental studies on basalt–peridotite mixtures suggests that the high-Fe magnesian melts may have been generated by moderate amounts of partial melting of ‘re-fertilised’ peridotite at potential temperatures of ≥1450°C and pressures ≥4.5 GPa. This hybrid Fe-rich peridotite is thought to result from a series of progressive mixing and reaction processes between subducted oceanic crust (eclogite) and convecting mantle. These findings suggest that compositional heterogeneity, involving streaks of recycled oceanic crust in a peridotite host, may have been a characteristic of mantle plume starting-heads since Archean times.

Erratum to “High-Ti and low-Ti mafic potassic magmas: Key to plume—lithosphere interactions and continental flood-basalt genesis” [Earth Planet. Sci. Lett. 136 (1995) 149–165]

Abstract The role of mantle plumes in the genesis of continental flood-basalts (CFB) remains controversial, primarily due to our limited knowledge of the composition of the subcontinental lithospheric mantle (SCLM). In this study we use the widespread Cretaceous mafic potassic magmatic rocks, emplaced around the margins of the Parana sedimentary basin, to probe large-scale compositional variations of the SCLM beneath southern Brazil and Paraguay. On the basis of Ti contents, together with major-, trace-element and isotopic ratios, these mafic potassic rocks may be subdivided into high-Ti and low-Ti groups. The former have relatively high average TiO2 (4.64), CaO Al 2 O 3 (1.74) and 143 Nd 144 Nd i (0.51232), together with low La Nb (1.1) and 87 Sr 86 Sr i (0.7050). The latter are characterised by much lower average TiO2 (1.77), CaO Al 2 O 3 (0.72) and 143 Nd 144 Nd i (0.51182), together with higher La Nb (2.01) and 87 Sr 86 Sr i (0.7068). These high-Ti and low-Ti groups are spatially separate and their distribution correlates with tectonic setting; the low-Ti magmas are associated with cratonic regions, whereas the high-Ti magmas are in Proterozoic mobile belts. The distribution of the subgroups of mafic potassic magmatic rocks correlates closely with the geochemical provinciality of the Early Cretaceous high-Ti and low-Ti Parana flood-basalts. This is the first reported occurrence of extensive low-Ti mafic potassic magmatism associated both spatially and temporally with the low-Ti region of a major CFB province. Our study further reveals similar relationships between tectonic setting and the geochemical provinciality of mafic potassic magmas and continental flood-basalts across Gondwana. We use the bulk-rock compositions and radiogenic isotopic ratios of both the high-Ti and low-Ti mafic potassic magmatic rocks as end members in models of CFB genesis. Mixing calculations involving Sr and Nd isotopic ratios indicate that the flood-basalts may contain up to 50% of mafic potassic lithosphere-derived melts. Overall, the results of our geochemical modelling agree with geophysical arguments that the convecting asthenosphere is the predominant source of CFB magmas.

Triggering of the largest Deccan eruptions by the Chicxulub impact

New constraints on the timing of the Cretaceous-Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental fl ood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1–1.0 J/m 3 throughout the upper ~200 km of Earth’s mantle, suffi cient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava fl ows, which together may account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous-Paleogene time, a huge pulse of mantle plume–derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava fl ows known on Earth. High-precision radioisotopic dating of the main-phase Deccan fl ood basalt formations may be able either to confi rm or reject this hypothesis, which in turn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed signifi cantly to the CretaceousPaleogene extinction.

Magmatic expression of lithospheric thinning across continental rifts

Abstract Studies of magmatism associated with continental rifting have traditionally focused only on volcanism within the downfaulted axial zone and along its immediate flanks. Teleseismic travel-time delay studies during the last decade have confirmed the results of earlier gravity surveys of rifted areas, showing that thinning at the base of the continental lithosphere occurs throughout a zone up to about 10 times wider than the physiographic expression of the rift. It is, therefore, logical to consider rifting-related magmatism on the same scale. Potential sources of mafic magmas in rift zones are the thinned subcontinental lithospheric mantle (SCLM), the convecting mantle beneath the continental plate and mixtures of the two. Detailed elemental and radiogenic isotope geochemical studies show that, during the initial extension of continental rifts, the associated mafic magmatism tends to be: (1) relatively sodic and from predominantly convecting mantle sources at the rift axis; (2) relatively potassic and from predominantly lithospheric mantle sources at the margins of the thinned-plate zone. This underlying geochemical pattern is obscured in many instances by such processes as crustal contamination and magma mixing within open-system reservoirs. The mafic ultrapotassic component that provides a distinctive input to SCLM-source magmas appears to be largely fusible at temperatures well below the dry solidus of SCLM; so that, in some cases, prolonged magmatism at a site causes removal of most or all of the potassic lithosphere-source melt (as mafic ultrapotassic magmas or as a contribution to mixed-source melts) without destruction of that lithosphere segment as a geophysically defined unit. Such a zone of refractory lithosphere permits subsequent, recognisable, convecting mantle source melts to penetrate it and reach the surface. These principles are illustrated by discussion of the Neogene-Quaternary magmatism of the Rio Grande, East African, Rhine and Baikal rifts, in the context of the most recent published models of their geophysical structures to depths > 200 km. Teleseismic and gravity studies identify lithospheric thinning beneath the Rio Grande, East African and Baikal rifts across zones 700–800 km wide. The failure of the southern Rhine graben to show a similar deep seismic structure may be a result of efficient buoyant migration of low-viscosity mafic alkalic melt out of the underlying mantle during the 7 Ma period since magmatism ceased, causing seismically defined asthenosphere to revert to lithosphere. A 700 km geochemical traverse across the Rio Grande rift at ~ 37°N, focusing on Oligocene-Miocene magmatism minimally affected by post-genesis processes, shows a clear symmetrical pattern of relatively sodic volcanics at the rift axis and mafic, ultrapotassic magmatism on its outer flanks. The geochemistry of these contrasting magma types is consistent with the view that they originated predominantly within the convecting mantle and SCLM, respectively. The same geochemical pattern is detectable in the volcanism within the equatorial segment of the East African rift system but it is complicated in two zones: east of the Gregory rift and southwest of Lake Kivu, by the effects of previous Cretaceous-Palaeogene magmatism. Limited, published, appropriate, geochemical data show that regional compositional variation in the volcanics associated with the Baikal rift appear to fit the pattern proposed here as a general model for rifting-related magmatism.

Asthenosphere-derived magmatism in the Rio Grande rift, western USA: implications for continental break-up

Abstract Magmas that are generated at continental rift zones provide an insight into the processes operating during the early stages of continental break-up. Our detailed study of mafic volcanism along the axis of the Rio Grande rift shows that, throughout both phases (30–17 and < 13 Ma) of its evolution, magmas with compositions interpreted as melts from the asthenospheric mantle have reached the surface. This recognition of early phase (26 Ma) magmas with incompatible trace element concentrations and radiogenic isotope ratios resembling those normally associated with ocean-island basalts and small seamounts (OIB) is significant because: (1) magmas dominated by the composition of asthenosphere-derived melts are not usually thought to be characteristic of early-phase continental rifting; (2) Tertiary mafic magmatism of an age greater than late Miocene in Colorado and New Mexico was hitherto regarded as subduction-related. Previous studies have shown that the final erupted composition of asthenosphere-derived melts is determined by the potential temperature of the convecting mantle, the amount and rate of lithosphere extension, fractional crystallization and crustal contamination. However, in the Rio Grande rift and elsewhere, such as the Basin and Range province, Eifel, NW Sardinia and the Cameroon Line, the final composition of these melts is also significantly influenced by earlier magmatic episodes. During the initial stages of asthenosphere melt generation the magma batches that first penetrate may heat a previously undisturbed segment of lithosphere and mix with strongly potassic, low temperature melt fractions. When these segments have been subsequently temporarily purged of such fusible potassic fractions the asthenosphere-derived melts can rise unimpeded through the sub-continental lithosphere.

Oligocene lamproite containing an Al-poor, Ti-rich biotite, Middle Park, Northwest Colorado, USA

Abstract A small 33 ±0.8 Ma lamproite pluton is exposed in the midst of a 23−26 Ma basalt-rhyolite province in Middle Park, NW Colorado. It contains abundant phlogopite phenocrysts in a fine-grained groundmass of analcime pseudomorphs after leucite, biotite, potassic fichterite, apatite, ilmenite and accessory diopside. The phlogopite phenocryst cores contain ~4 wt.% TiO2, 1% Cr2O3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ~2% Al2O3, ~8% TiO2 and F <1.5%. Apart from those elements affected by leucite alteration, both the elemental and isotopic composition of this lamproite are close to those of the Leucite Hills, Wyoming. Its Nd-isotopic model age (TDM = 1.6 Ga) is outside the Leucite Hills range but within that of other Tertiary strongly potassic magmatism in the region underlain by the Wyoming craton. Evidence from both teleseismic tomography and the mantle xenoliths within other western USA mafic ultrapotassic igneous suites shows that the total lithospheric thickness beneath NW Colorado was probably ~150−200 km at 33 Ma, when the Middle Park lamproite was emplaced. This is an important constraint on tectonomagmatic models for the Cenozoic evolution of this northernmost part of the Rio Grande rift system.