A Giret

Petrology and geochemistry of the Kerguelen Archipelago basalts (South Indian Ocean): evolution of the mantle sources from ridge to intraplate position

Kerguelen basic lavas belong to three magmatic series: transitional, mildly alkaline and highly alkaline, showing from the first to the latter, a general increase in alkali and incompatible element contents as well as some of the incompatible trace element ratios [(Ce/Yb)n,Nb/Zr,Nb/Y,Zr/Y,Ti/Zr].Ba/Nb,La/Nb and all ratios involving Th are in the range of those observed for Dupal type OIBs although subtle differences exist mainly reflecting a slight Th enrichment in the Kerguelen basalts relative to other OIBs. New Sr, Nd and Pb isotope data strengthen the previously identified Dupal signature of Kerguelen rocks. While the transitional series, older than 26 Ma, shows slightly depleted Sr and Nd isotopic signatures (87Sr/86Sr 0), the alkaline series, between 26 and about 8 Ma, show a slightly enriched signature which is strengthened in the highly alkaline series (87Sr/86Sr up to 0.70056), younger than 12 Ma. Pb isotope ratios do not show any significant difference between these three series. These isotopic data coupled with trace element geochemistry indicate a mixing process between a depleted MORB component and an enriched OIB-type component, the Dupal plume. The Pb isotope characteristics indicate the presence of an old recycled component (continentally derived sediment or crust itself) incorporated into the deep plume source-region which may be responsible for this enrichment. The Kerguelen Islands show a geodynamic evolution from an early ridge centered stage, above or close to the South East Indian Ridge (SEIR) 45 Ma ago, to the present day intraplate setting. The geochemical and isotopic variations reflect this geodynamic evolution and correspond to variable degrees of mixing between the two extreme components (SEIR, MORB and the Kerguelen plume) whose relative proportions evolve with the archipelago position, i.e. with the distance from the ridge at the time of eruption.

40Ar/39Ar geochronology of flood basalts from the Kerguelen Archipelago, southern Indian Ocean: Implications for Cenozoic eruption rates of the Kerguelen plume

The 6500 km2 Kerguelen Archipelago formed on the northern Kerguelen Plateau (NKP) (4×105 km2) which is a shallow submarine plateau belonging to the Kerguelen large igneous province in the southern Indian Ocean. Flood basalts make up 85% of the archipelago and are interpreted as the most recent volcanism (<40 Ma) from the Kerguelen hotspot which has erupted basalt for the last 115 million years. Based on 40Ar/39Ar incremental heating of acid-leached groundmass separates, we report isochron ages ranging from 29.26±0.87 Ma to 24.53±0.29 Ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago. The oldest dated basalt from the archipelago (∼29 Ma) is much younger than the ∼40 Ma age of conjunction between the hotspot and the Southeast Indian Ridge. Basalt eruption seems to have ceased shortly after ∼24 Ma although small volume, highly evolved lavas and plutons continued to form in the archipelago. The basalt age data suggest an average lava accumulation rate of ∼1.6±0.9 km/my during the Oligocene. The archipelago’s volumetric eruption rate (0.009 km3/yr) is lower than estimates made for the Cretaceous Kerguelen Plateau (1.7 km3/yr) and the Ninetyeast Ridge hotspot track (0.18 km3/yr), suggesting that the late Cenozoic extrusive activity of the Kerguelen plume is waning. Cenozoic volcanism attributed to the Kerguelen plume occurs over a diffuse area with Quaternary eruptions at Heard and McDonald Islands and within the Kerguelen Archipelago. The decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the Cretaceous Kerguelen Plateau overriding and insulating the plume. However, if the undated NKP, which underlies the archipelago, formed during the Cenozoic, then, the crustal production rate of the plume from 40 Ma to the present (∼0.25 km3/yr) would be similar to the crustal production rate (0.23 km3/my) previously estimated for the formation of the Ninetyeast Ridge (∼82–38 Ma).

Coeval potassic and sodic calc-alkaline series in the post-collisional Hercynian Tanncherfi intrusive complex, northeastern Morocco: geochemical, isotopic and geochronological evidence

Abstract The post-collisional late Hercynian Tanncherfi intrusive complex (TIC) is part of a widespread intrusive episode in the Moroccan Meseta. The complex contains a wide range of rock types, from monzogabbros to monzogranites. Two distinct magmatic series are recognized: (1) a potassic (shoshonitic) series consisting of monzogabbros, quartz monzonites and monzogranites; and (2) a sodic (granodioritic) series represented by quartz monzodiorites and granodiorites. All the Tanncherfi plutonic rocks display similar spider-diagram profiles, with LILE and LREE enrichment and Nb, Ta, Ti depletion, which are typical of subduction-related magmas. Combined major, trace element compositions and Sr, Nd isotopic results indicate that the two series have been derived from a LILE- and LREE-enriched continental lithospheric mantle source, under different partial melting and/or depth conditions. Intrusion of the Tanncherfi rocks was not temporally related to subduction and the enrichment of their source is likely to be linked to preceding subduction events. The two series evolved by fractional crystallization, of clinopyroxene, plagioclase, hornblende, biotite, K-feldspar and accessories (Fe–Ti oxide minerals, titanite, apatite and zircon) for the potassic series while the sodic series combined fractional crystallization with assimilation of felsic magmas with lower Sr isotopic ratio than the more mafic term of the series, the quartz monzodiorite. The intrusion of the potassic magmas (344±6 Ma) marks a major change in the tectonic regime of eastern Meseta. These magmas intruded during post-thickening uplift and extension, both probably favored by convective thinning of the lithosphere. This model provides a reasonable mechanism for the genesis of other Hercynian intrusive complexes in Morocco.

Temporal geochemical trends in Kerguelen Archipelago basalts: evidence for decreasing magma supply from the Kerguelen Plume

Abstract The Kerguelen Archipelago, a ∼39 Ma to recent volcanic–plutonic complex, is interpreted to be a manifestation of the Kerguelen Plume. Most, ∼85%, of the surface area is covered by flood basalts ranging in age from ∼29 to 25 Ma. The youngest (∼25 Ma) studied flood basalts are in the Southeast (SE) Province of the archipelago. A composite 460 m section of this southeast flood basalt dominantly consists of evolved (3 to 6% MgO) alkalic basalt and trachybasalt with a few interbedded highly evolved lavas (trachyandesites), a 40–70 m conglomerate which contains lignite beds, and a trachytic breccia/tuff unit. All of the lavas in this composite section have Sr and Nd isotopic ratios that are typical of the Kerguelen Plume; e.g., >80% of the 115 analyzed archipelago lavas with >2.3% MgO have ( 87 Sr / 86 Sr ) i =0.70515±12 and ( 143 Nd / 144 Nd ) i =0.51259±5. These ranges include the southeast flood basalts. Pb isotopes, however, are more variable; these 25 Ma lavas have high 206 Pb / 204 Pb at ∼18.4 to 18.6, relative to other archipelago lavas. The temporal trend of the archipelago flood basalt from older, ∼29 Ma, transitional basalts to younger, ∼25 Ma, alkalic basalt with an increasing proportion of highly evolved lavas and intra-bedded sediments in the relatively young southeast section indicates: (a) a temporal decrease in extent of melting and (b) a decreasing supply of magma from the plume to the crust. These temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge-centered plume at ∼43 Ma to its intraplate setting. Supporting evidence for this interpretation is: (a) the absence of a MORB geochemical signature in these 25 Ma lavas; and (b) the relatively low abundances of heavy rare-earth elements in these southeast lavas which reflect partial melting within the garnet stability field.

Geochemical and Hf–Pb–Sr–Nd isotopic constraints on the origin of the Amsterdam–St. Paul (Indian Ocean) hotspot basalts

The Amsterdam–St. Paul (ASP) Plateau is a recent (≤5 Ma) volcanic rise constructed along the Southeast Indian Ridge (SEIR) by the combined effects of a relatively small mantle plume and a mid-oceanic ridge. The Amsterdam and St. Paul islands are located 100 km away from each other and formed during the last 0.4 Myr; they are the only subaerial features of the ASP Plateau and the two islands are structurally separated by the presence of a SW–NE transform fault. New geochemical analyses and Hf–Pb–Sr–Nd isotopic compositions of 20 basaltic rocks from Amsterdam and St. Paul Islands constrain the nature and origin of the sources involved in the genesis of the ASP hotspot basalts. Aphyric basalts from St. Paul are mildly alkalic, incompatible element-enriched and highly fractionated; they are distinct from the tholeiitic basalts from Amsterdam, from the recently discovered Boomerang active seamount on the ASP Plateau, and from the Kerguelen Archipelago basalts on the Antarctic Plate. The St. Paul and Amsterdam basalts have very limited isotopic variations with distinct 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 176Hf/177Hf isotopic compositions (19.08±0.07, 15.61±0.02, 39.45±0.12, 0.28313±0.00003 for Amsterdam, and 18.70±0.08, 15.56±0.01, 38.87±0.05, 0.28306±0.00002 for St. Paul, respectively) that are not compatible with any direct contribution of the enriched Kerguelen plume end-member. Pb–Nd–Sr isotopic compositions of the St. Paul basalts appear consistent with simple binary mixtures between heterogeneous ambient upper mantle and a highly radiogenic Pb plume component (the ASP plume end-member), particularly expressed in the isotopic compositions of the Amsterdam basalts. However, the Amsterdam basalts have distinctly higher ϵHf than the St. Paul basalts (+13 and +10, respectively) for a given ϵNd (+4) and are inconsistent with such a simple binary mixing scenario. Isotopic systematics in the Amsterdam and St. Paul basalts indicate that the Amsterdam and St. Paul volcanoes were formed by sampling isotopically distinct zones of the ASP plume at a lateral distance of 100 km. Less than 1% variation in the proportion of recycled altered oceanic crust relative to pelagic sediment, combined with minor variations in the proportion of recycled material within the Amsterdam and St. Paul plume sources themselves relative to a peridotitic mantle source, could account for the isotopic differences between the compositions of basalts from these two islands. The particularly high 206Pb/204Pb component recorded in the Amsterdam and St. Paul basalts is also locally recorded at different times and locations within other Indian Ocean basalts (e.g. Ninetyeast Ridge basalts, 38 Ma) and such a component has also contaminated the source of SEIR basalts to varying degrees. The particularly high 206Pb/204Pb component is therefore not exclusive to the Amsterdam–St. Paul plume but is heterogeneously distributed within the Indian Ocean upper mantle. This may reflect the role of the Indian mantle plumes in dispersing recycled material within the Indian upper mantle.

Kerguelen basic and ultrabasic xenoliths: Evidence for long-lived Kerguelen hotspot activity

Abstract The xenoliths from the Southeast Province of the Kerguelen Archipelago derived from the lower crust or the upper mantle, can contribute to define the characteristics of the mantle sources below Kerguelen and improve the constraints on the formation of the Kerguelen Islands and plateau. Our petrographic, geochemical and isotopic (Sr, Nd and Pb) study focuses on peridotites (Type Iα: harzburgite/clinopyroxene-poor Iherzolite and Type Iβ: dunite), 2-pyroxenes-spinel bearing ultrabasic and basic xenoliths [Type IIa: clinopyroxene-rich Iherzolite, wehrlite, (± olivine ± plagioclase) websterite, (± garnet ± sapphirine) metagabbro and anorthosite] and ilmenite metagabbros (Type IIc). The large ranges of isotopic ratios for the xenoliths reflect different degrees of interaction between a depleted MORB-type component, quite abundant in the Type II xenoliths, and the Kerguelen plume, distinctly predominant in the Type I xenoliths. Type I peridotites are residues of a previous partial melting event of the Kerguelen plume; residues that subsequently interacted with a percolating alkaline melt. 2-pyroxenes-spinel bearing ultrabasic and basic xenoliths (Type IIa) and ilmenite metagabbroic xenoliths (Type IIc) are deep cumulates crystallized from tholeiitic magmas. The isotopic results for the xenoliths strengthen the hypothesis of an oceanic origin for the Kerguelen Islands and refute the existence of pieces of old continental crust beneath the Islands and the northern part of the Kerguelen Plateau. They also confirm the importance of plume-spreading ridge interactions throughout the history of the Kerguelen plume. The isotopic and geochemical characteristics of the Type IIa and IIc xenoliths are consistent with the hypothesis of an Iceland-type setting for the northern part of the Kerguelen Plateau. The results for the Type I xenoliths on the other hand suggest a similarity between the Hawaii-type midplate volcanic structure and that of Kerguelen Islands. The isotopic data suggest that the Kerguelen xenoliths were formed recently (≤ 45 Ma), and thus support the hypothesis of the formation of the Plateau by the arrival of the plume at the base of the lithosphere (~ 115 Ma ago). The Plateau would have grown through several pulses of plume activity (~ 115, ~ 80, ~ 40 Ma), while the geotectonic environment changed with time (from a ridge-centered position to the present intraplate position). The occurrence of deep Type IIa and IIc xenoliths can explain the crustal thickening and provides evidence for the growth of oceanic plateaus by vertical accretion.

Incompatible trace element and isotopic (D/H) characteristics of amphibole- and phlogopite-bearing ultramafic to mafic xenoliths from Kerguelen Islands (TAAF, South Indian Ocean)

Alkali basalts from the Kerguelen Islands have entrained numerous phlogopite- and amphibole-bearing ultramafic to mafic xenoliths. These are subdivided into mantle harzburgites, dunites and associated composite xenoliths that represent mantle wall-rock (Type-I) and high pressure (10–15 kbar) segregates (Type II). A lamprophyric dyke containing phlogopite megacrysts has been also studied. Chemical compositions of amphiboles and phlogopites from both xenolith types are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites and alkali basalts and in peridotites and pyroxenites from orogenic lherzolite massifs. Interstitial amphibole and phlogopite in harzburgites and dunites probably formed during diffuse percolation of highly alkaline basic silicate melt within the upper mantle (porous flow). Evidence from composite xenoliths suggest that similar mantle melts migrated through a network of dykes generated by hydraulic fracturing in the Kerguelen upper mantle. The lamprophyre is the surface expression of this highly alkaline magmatic activity. The δD values of −92 to −61‰ SMOW for mica and amphibole of Type I and Type II xenoliths and of the phlogopite megacrysts are within the accepted mantle range. Calculated δD-H 2 O values in equilibrium with amphiboles and micas have a bimodal distribution (- 65 ± 5‰ and −83 ± 5‰) indicating that the percolating fluids were isotopically heterogeneous. The ubiquity of the highly alkaline magmatic activity is probably related to the late intraplate activity of the Kerguelen mantle plume.

Flood basalts from Mt. Capitole in the central Kerguelen Archipelago: Insights into the growth of the archipelago and source components contributing to plume‐related volcanism

The Kerguelen Archipelago, constructed on the submarine Northern Kerguelen Plateau, is attributed to Cenozoic volcanism arising from the Kerguelen hot spot. Geochemical studies of 325 to 1000 m thick lava sections of the ∼30 to 25 Ma flood basalt forming the bulk of the archipelago show a temporal change from older tholeiitic basalt to younger slightly alkalic basalt. This compositional transition is expressed in a 630 m lava section at Mt. Capitole where the lava sequence is lowermost tholeiitic basalt overlain by slightly alkalic basalt overlain by plagioclase-rich cumulates that are mixtures of plagioclase-phyric basalt and more evolved magmas. During growth of the archipelago, magma supply from the hot spot was variable and at times sufficiently low to enable extensive crystal fractionation; e.g., at Mt. Capitole and nearby Mt. Tourmente only 10 of 120 lava flows have >6 wt% MgO. On the basis of this study and previous isotopic data for the ∼34 Ma submarine lavas erupted on the Northern Kerguelen Plateau, other flood basalt sections in the Kerguelen Archipelago, and younger lavas erupted in the archipelago and at Heard Island, there is significant Sr, Nd, Hf, and Pb isotopic heterogeneity that can be explained by two stages of mixing. The first mixing event, best shown by the submarine lavas, is between components that are related to Indian Ocean mid-ocean ridge basalt (MORB) and the Kerguelen hot spot. From ∼34 Ma to 0.7060) and low 143Nd/144Nd (<0.5125) and 176Hf/177Hf (<0.2827) and nonradiogenic Pb isotope ratios (<17.9 for 206Pb/204Pb). We infer that this component was lower continental crust.

Reconnaissance Stable Isotope (C, O, H, S) Study of Paleoproterozoic Gold Deposits of the São Luis Craton and Country Rocks, Northern Brazil: Implications for Gold Metallogeny

Caxias, Areal, and Pedra de Fogo are structurally controlled syn- to late tectonic and post-metamorphic gold deposits of Paleoproterozoic age located in the São Luis craton, northern Brazil. Previous fluid inclusion studies indicated reduced, low-salinity CO2-H2O (±CH4-N2) fluids trapped at 260°-300°C (Caxias, Areal) to 330°-400°C (Pedra de Fogo), and at 2 kbar as responsible for the mineralization. Oxygen isotope ratios of quartz (+10.4 to +16.2‰), chlorite (+7.6‰), and sericite (+5.1‰), and hydrogen isotope ratios of chlorite (-46‰), sericite (-58‰), and fluid inclusion water (-32 to -70‰) indicate fluid Δ18O values of +2.6 to +5.6 per mil (Caxias), +0.6 to +3.5 per mil (Areal), and +10.3 to +12.1 per mil (Pedra de Fogo) and fluid ΔD composition of -24 to -53 per mil (Caxias), -49 to -62 per mil (Areal), and -70 per mil (Pedra de Fogo) at the assumed temperatures. These estimated fluid compositions are consistent with metamorphic sources. Calcite and fluid inclusion CO2 Δ13C values between -3.1 and -10.9 per mil are not diagnostic of a particular origin, but a more negative value (-20.2‰) found at Caxias indicates a greater organic component, at least locally. Sulfide Δ34S values of -2.8‰ to -11.0‰ reflect possible achievement of more oxidized conditions. Hydrogen and oxygen isotope analyses of the country rocks suggest minor isotope disequilibrium and resetting of oxygen isotope geothermometers. This might indicate subsolidus post-crystallization isotopic exchange, linked with metamorphism and/or hydrothermal alteration.