A. P. Dickin

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.

Isotopic evidence for the crustal evolution of the Frontenac Arch in the Grenville Province of Ontario, Canada

Abstract Nine late-tectonic granitoid bodies from the Central Metasedimentary Belt (CMB) of the Grenville Province were studied by the Sr, Nd and O isotopic systems. They occur on either side of the Rideau Lake Fault (RLF). Four bodies south of the fault were dated by the U-Pb zircon method. Three of the ages are tightly constrained at 1166 ± 3 Ma. This is in contrast to ages reported for late-tectonic plutons north of the fault between 1090 and 1076 Ma. t Nd DM model ages range from 1211 to 1480 Ma, indicating that the plutons were derived from a mixed source involving older crust. Exposed pelitic and psammitic metasediments in the area have t DM ages averaging 1790 Ma. The observed Sr-Nd-O relationships indicate that marble was involved in the evolution of the granitoids. The granites are chemically similar to the Mid-Proterozoic anorogenic plutons that occur elsewhere in North America. Their crystallization ages are not found elsewhere in the CMB in Ontario, but they are similar to ages found to the east in the Adirondacks, New York. This suggests that the Frontenac-Adirondack area represents a separate Penokean-age block assembled with the rest of the CMB ∼ 1070 Ma ago.

Evolution of magma source regions in the Rio Grande rift, southern New Mexico

Early magmatism of the southern Rio Grande rift was strongly controlled by the thermal history of the lithosphere. The compositions of igneous rocks in southern New Mexico record a general shift in magma source regions through the Cenozoic from lithosphere to asthenosphere and from upper crust to lower crust and, finally, to little crustal involvement. Extension began as early as 36 Ma, coincident with the onset of bimodal volcanism of the middle Tertiary ignimbrite flare-up and production of half grabens after a short episode of post-Laramide subduction-related volcanism. Oligocene mafic magmas have incompatible trace element patterns similar to modern continental-arc basalts (low Nb, Ta, and Ti; high Rb/Nb, Ba/Nb, and La/Nb). The most mafic magma has 87Sr/86Sr and ϵNd values near bulk Earth (0.704771 and +0.2, respectively). The Oligocene mafic to intermediate-composition suites evolved toward slightly higher 87Sr/86Sr, lower ϵNd, and nonradiogenic Pb isotopic compositions (87Sr/86Sr = 0.70440–0.70785; ϵNd = −2.2 to −4.8; 206Pb/204Pb = 17.039–18.084, 207Pb/204Pb = 15.387–15.498; 208Pb/204Pb = 37.094–38.130) and are interpreted as partial melts of slightly hydrated lithosphere that were contaminated in the lower crust. Contemporaneous rhyolitic magmas have more radiogenic Sr and Pb isotopic compositions (87Sr/86Sr = 0.7111; ϵNd = −4.5; 206Pb/204Pb = 18.435; 207Pb/204Pb = 15.538; 208Pb/204Pb = 38.607) and record the involvement of an upper-crustal component. Silicic volcanism ceased abruptly at 28.5 Ma, but mafic to intermediate-composition lithospheric magmatism persisted until 24 Ma. Although extension continued, forming half grabens with sedimentary fill, the period between 24 and 10 Ma was amagmatic. This lack of volcanism and the end of lithosphere-dominated magma genesis reflect the effective scavenging of the hydrated parts of the lithospheric mantle by middle Tertiary magmatism, which produced a refractory, infertile, subcontinental lithosphere. Volcanism resumed at 10 Ma with sporadic eruption of tholeiitic and alkalic basalts having trace element patterns similar to oceanic-island basalts (high Nb, Ta, and Ti; low Ba/Nb and La/Nb), depleted Sr and Nd isotopic compositions (87Sr/86Sr = 0.70297–0.70396; ϵNd = +4.7 to +7.3), and more-radiogenic Pb isotopic compositions (206Pb/204Pb = 18.460–19.698; 207Pb/204Pb = 15.461–15.684; 208Pb/204Pb = 38.091–39.411) than the lithosphere-derived suites. The composition of these late Cenozoic basalts records a dramatic shift in source region to upwelling asthenosphere that melted by adiabatic decompression.

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.

Origin of Archean ferropicrites: geochemical constraints from the Boston Creek Flow, Abitibi greenstone belt, Ontario, Canada

Abstract New major-element, trace-element and Nd isotope data for non-cumulate samples from the Fe-enriched Boston Creek Flow (BCF) have permitted the development of a petrogenetic model for the origin of this geochemically unusual flow. The data reveal high contents of Nb, Ta, LREE, Ti, Ni, Co and Ir, as well as in Fe, and low contents of Al ( Al 2 O 3 TiO 2 = 6 ), HREE, Y, V, Sc, Pd and Pt compared to komatiitic rocks and typical tholeiitic rocks of similar magnesian content. Primitive mantlenormalized incompatible trace-element profiles for the BCF peak in the TaNbLa region [ ( La SM ) N = 2.0, ( La Yb ) N = 5.4, ( Th Ta ) N = 0.7 ] and bottom in the Al-Sc region [ ( La Sc ) N = 11.0, ( La Al ) N = 9.7 ], and reveal slight depletions in Zr and Hf relative to MREE [( Sm Zr ) N = 1.3] and Ti . ϵNd (T = 2720 Ma) values for the flow are +2.5, indicating derivation of the parental melt from a long-term depleted mantle source. These geochemical characteristics are somewhat similar to those of modern oceanic within-plate basalts. These unusual geochemical characteristics and comparisons to Proterozoic ferropicrite flows at Pechenga, Russia, indicate that the BCF is ferropicritic rather than komatiitic in affinity. Alteration-, assimilation and magma mixing-, and crystal fractionation processes cannot explain the BCF. Therefore, the distinctive geochemistry of the BCF is concluded to have been a feature of its mantle source and is attributed to mantle processes. Models involving melting of core material in the mantle source may explain the enrichment of Fe and high Ni in the BCF, but they are contradicted by the relatively low Pt and Pd. Models involving olivine equilibration in the mantle are consistent with the high Ni (and Co), similar composition of assumed BCF parental melt equilibrium olivine (at 25 kbar) and that of modern Fe-rich lherzolite olivine (Fo85), and comparison of the MgO and FeO content of the melt with those of experimentally-derived melts that are saturated in olivine at high pressure. Models involving garnet equilibration in the mantle could explain the low Al, HREE, Y, V and Sc, but are contradicted by the very low Al2O3 content of the assumed parental melt compared with those of experimentally-derived melts of similar FeO and MgO content that are saturated in garnet at high pressure. Alternatively, the unusual geochemistry of the BCF can be explained by a two-source-component mixing model, somewhat like those commonly accepted for oceanic within-plate basalts petrogenesis. The first source component is peridotite depleted by extraction of melt(s) prior to generation of the BCF parental melt, which explains the relatively high compatible-element contents and low Al, HREE, Sc, Pd and Pt contents. The second source component is highly enriched small-degree melt fractions formed at mantle depths sufficient to stabilize garnet (majorite?), which explains the relative enrichments in Fe, Ti and the highly incompatible elements. Mixing of the two source components must have occurred immediately prior to melting to maintain the depleted Nd isotope composition, and may have been facilitated by metasomatic amphibole stabilization at relatively shallow (⩽ 100 km) depths in the mantle.

ISOTOPIC EVIDENCE FOR DISTINCT CRUSTAL SOURCES OF NORTH AND SOUTH RANGE ORES, SUDBURY IGNEOUS COMPLEX

Lead isotope ratios were measured on Fe-Cu-Ni sulphide ores and associated silicate phases from nine mines on the North and South Ranges of the Sudbury Igneous Complex (SIC). These data are compared with the lead isotope signatures of country rocks and shock-melted Sudbury Breccias around the complex. Both ore and feldspar data from the South Range fall along a 1.85 Ga reference line (the age of the igneous complex) and indicate a magmatic origin for lead in the ore. Data from the North Range are also consistent with a magmatic origin, but are displaced to lower 206Pb/204Pb and 207Pb/204Pb ratios. These two suites fall close to the lead isotope composition of country rocks adjacent to the South and North ranges, respectively. Furthermore, there is a close correspondence between lead signatures of the SIC and Sudbury Breccia on each side of the complex. The evidence suggests an origin for the SIC by shock melting of different crustal units on the two sides of the complex. This is consistent with the impact of a meteorite on the southward-thickening apron of Huronian strata which overlay the margin of the Superior craton at 1.85 Ga.

Quaternary volcanism in northwestern Colorado: Implications for the roles of asthenosphere and lithosphere in the genesis of continental basalts

Abstract Quaternary volcanic rocks were erupted at four locations in NW Colorado; Dotsero (4150 y.B.P.), Willow Peak (undated), McCoy (0.64 m.y. B.P.), and Triangle Peak (1.98-1.87 m.y.B.P.). At Triangle Peak, there are at least eleven lava flows, but eruptions at the other locations were monogenetic. Dotsero was the only hydrovolcanic eruption. The volcanic rocks are alkali basalts, containing the phenocryst assemblage: olivine, Fe-Ti oxide, ± clinopyroxene, ± plagioclase. The basalts are chemically similar to OIB, as would be expected from their intraplate crustal setting. Nevertheless, they have La/Ta, K/Ta, Ba/Ta and K/La ratios which are significantly higher than those of oceanic OIB. These differences cannot be explained by contamination during uprise of OIB-like basalt by continental crust of reasonable composition. It is, therefore, logical to assume that the Quaternary magmas contained a component of partial melt of subcontinental lithospheric mantle. This conclusion is in accord with the low 143 Nd/ 144 Nd ratios of the basalts. The geochemistry of the Quaternary basalts can be explained by mixing between three separate mafic magma end-member groups that were erupted in the same area during the Miocene. Group 1 magmas were OIB, representing partial melts of OIB-source asthenosphere. Group 2 magmas were minettes, with low 143 Nd/ 144 Nd ratios, regarded as partial melts of sub-Colorado lithospheric mantle. Group 3 magmas had high La/Ta ratios, and, generally, low LIL/HFS ratios. During the Miocene, the latter group of magmas are interpreted to have been derived by partial melting of asthenosphere that had been modified by subduction of oceanic lithosphere below the North America plate. The presence of a component of Group 3 magma in the Quaternary basalts indicates that the mantle source of this group was trapped for 8 m.y. in an uppermost asthenospheric layer which experienced very sluggish flow. We propose that this layer is equivalent to the thermal boundary layer — situated between the rigid part of the lithospheric mantle (above), and the convecting asthenosphere (below) — originally identified by calculations of the thermal histories of lithospheric plates.

Osmium isotope evidence for a crustal origin of platinum group elements in the Sudbury nickel ore, Ontario, Canada

Abstract Sulphide ores from the International Nickel Companys (INCO) Creighton Mine, Sudbury, were analysed for osmium isotope ratios by ICP-MS. Rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix. Calculated initial 187 Os 188 Os ratios at 1.85 Ga (the emplacement age of the Sudbury complex) cluster around 0.60. The occurrence of less radiogenic compositions is attributed to post-emplacement, open-system behaviour of the Re Os system. The Creighton results strongly overlap the initial ratios of published osmium data from two other Sudbury mines, suggesting a narrow range of isotope ratio in the original ore, within the range of estimated osmium isotope compositions in the country rock at 1.85 Ga, using published and new data. Therefore, the sulphide ores of the Sudbury complex can have an entirely crustal source without a mantle-derived contribution. This conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis, since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex. The extraction of platinum group elements (PGE) from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure.