M. Storey

Consequences of plume-lithosphere interactions

Abstract Splitting or thinning of lithosphere above a mantle plume can result in voluminous melt generation, leading to the formation of large igneous provinces, or LIPs. Examples of LIPs include continental flood basalt provinces and oceanic plateaus. Basaltic samples from the Ontong Java Plateau, Nauru Basin and Manihiki Plateau, which are among the largest of the LIPs, have isotopic compositions within the range of ocean island basalts. The majority of continental basalts, however, record a trace element and isotopic contribution from the lithosphere through which they have erupted. We are thus unable to reconcile the available compositional data with models which derive the isotopic and large-ion lithophile element-enriched character of continental flood basalts solely from sub-lithospheric mantle plume sources. A combination of mantle sources is indicated, with the thermal energy being supplied by voluminous melts from a plume, and the lithospheric components in continental flood basalts being inherited by contamination of plume-derived melts by low melting point hydrous and carbonated fractions in the lithosphere. Successive injection of plume-derived melts serves to heat the lithosphere, reducing its viscosity and making it susceptible to rupture if allowed by regional plate forces. Furthermore, the lithosphere, including the mechanical boundary layer, may be thinned by thermal stripping from below, allowing the plume mantle to ascend and decompress further. Such a system has the potential for positive feedback leading to rapid melt generation. While we do not exclude recent models of LIP formation which require the sudden impact of a new mantle plume, we favour a model whereby the thermal anomaly builds gradually, incubating beneath a steady-state lithospheric cap.

An oceanic flood basalt province within the Caribbean plate

Abstract The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar–39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ∼90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau (i.e., large volume, rapidly erupted, regionally extensive volcanism). The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galapagos hotspot at its inception. The trace-element and isotopic compositions of the ∼90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curacao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, and/or tectonic emplacement of the plateau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa Rica (63 Ma). The latter occurrence conceivably formed over the Galapagos hotspot and subsequently accreted to the western edge of the plateau during subduction of the Farallon plate.

40Ar/39Ar geochronology of the West Greenland Tertiary volcanic province

Abstract 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 40Ar/39Ar dating study shows that volcanism commenced in West Greenland between 60.9 and 61.3 Ma and that ∼80% of the Paleocene lava pile was erupted in 1 million years or less (weighted mean age of 60.5±0.4 Ma). Minimum estimates of magma production rates (1.3×10−4 km3 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 1 m/year) lateral spreading of the Iceland plume head at the base of the Greenland lithosphere at ∼62 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 ∼55 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.

Are oceanic plateaus sites of komatiite formation

During Cretaceous and Tertiary time a series of oceanic terranes were accreted onto the Pacific continental margin of Colombia. The island of Gorgona is thought to represent part of the most recent, early Eocene, terrane-forming event. Gorgona is remarkable for the occurrence of komatiites of middle Cretaceous age, having MgO contents up to 24%. The geochemistry of spatially and temporally associated tholeiites suggests that Gorgona is an obducted fragment of the oceanic Caribbean Plateau, postulated by Duncan and Hargraves (1984) to have formed at 100 to 75 Ma over the Galapagos hotspot. Further examples of high-MgO oceanic lavas that may represent fragments of the Caribbean Plateau occur in allochthonous terranes on the island of Curacao in the Netherlands Antilles and in the Romeral zone ophiolites in the southwestern Colombian Andes. These and other examples suggest that the formation of high-MgO liquids may be a feature of oceanic-plateau settings. The association of Phanerozoic komatiites with oceanic plateaus, coupled with thermal considerations, provides a plausible analogue for the origin of some komatiite-tholeiite sequences in Archean greenstone belts.

40Ar39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track

Abstract The East Greeland 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 by40Ar 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 Glestcher 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 Aputiteˆq, Kap Edvard Holm, 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 (lavas, 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 ∼3.9-5.0 cm/yr between ∼60 and ∼49 Ma, consistent with estimates from seismic studies of submerged flood basalts.

LARGE IGNEOUS PROVINCES - SITES OF PLUME IMPACT OR PLUME INCUBATION

Large igneous provinces, representing prodigious volumes of basalt erupted through continental and oceanic crust, are believed to be associated with high-temperature mantle plumes incident at the base of the lithosphere. Recent plume initiation models for continental flood-basalt volcanism suggest that material in the plume will intersect the solidus shortly after arriving, or impacting, beneath the lithosphere, so that melting is near-synchronous with plume impact. Beneath continents, however, melting of a plume head is inhibited by the presence of a thick (>125 km) mechanical boundary layer, which must be thinned and removed by conductive heating and melt injection before significant basalt production can occur. This necessitates a period of plume incubation, characterized by lithosphere extension and doming, by the establishment of long- lived paleodrainage patterns, and, laterally, by the intrusion of alkalic magmas. Field evidence from rive major igneous provinces indicates that plume incubation is a more appropriate model than simple plume impact for continental flood-basalt volcanism.

RELATION BETWEEN ALKALIC VOLCANISM AND SLAB-WINDOW FORMATION

Ridge crest-trench interactions along continental destructive plate margins may result in the development of slab-free windows beneath the continental margin. Slab windows were generated at various locations along the Pacific margin of the Americas and the Antarctic Peninsula during the past 70 m.y. Slab-window formation is temporally and spatially associated with mafic, alkalic volcanism. Lavas erupted above the loci of slab windows are geochemically indistinguishable from some ocean-island, plume-related basalts. However, generation of slab-window basalts from deep-seated mantle plumes requires the fortuitous initiation of plume activity following cessation of subduction. Asthenospheric upwelling and associated decompressional melting following slab-window formation are probably promoted by removal of subducted oceanic lithosphere from beneath the continental margin following the cessation of subduction. Major lithospheric extension is not a prerequisite for alkalic volcanism in this case. The close association of subduction-related volcanism and within-plate alkalic volcanism within the geologic record may also be explained by this mechanism.

Long-lived postbreakup magmatism along the East Greenland margin: Evidence for shallow-mantle metasomatism by the Iceland plume

4 0 Ar/ 3 9 Ar dating has identified a succession of middle Miocene (14-13 Ma) basaltic lavas in East Greenland that overlie Eocene flood basalts that were erupted during continental breakup ca. 56-55 Ma. The long postbreakup magmatic history (∼40 m.y.) of the East Greenland margin precludes a simple relationship between this later igneous activity and the track of the Iceland hotspot. Chemical and isotopic data suggest that the postbreakup magmas were produced from mantle that had been metasomatized by light rare earth element-enriched, H 2 O- and CO 2 -bearing melts originating from the Iceland plume. Episodic melting of recently metasomatized shallow mantle beneath Greenland and the North Atlantic can explain both the composition and the long-lived nature of postbreakup magmatism along the East Greenland margin, as well as lavas on Jan Mayen Island that have enriched, Icelandic-type isotopic signatures.

Origin of hybrid lavas from Agua de Pau volcano, Sao Miguel, Azores

Summary The island of Sao Miguel in the Azores exhibits bimodal volcanism: eruptives from Agua de Pau volcano and the ‘Waist’ consist almost entirely of basalt or trachyte with rare intermediate products. Although some intermediate lavas are fractionates from basalt, the majority are mixed-magma hybrids anomalously enriched in europium, barium and potassium. These features clearly indicate that they have also accumulated alkali feldspar. The source of the alkali feldspar contaminant must be the least evolved trachyte in the Agua de Pau system, because barium, europium and strontium are rapidly depleted in liquids and crystals with differentiation to more evolved trachytes. Many less evolved trachytes are also accumulative in alkali feldspar, suggesting a genetic association with the hybrid lavas. Geochemical modelling indicates that the hybrid lavas are the result of contamination of basalt by accumulative trachyte containing up to 75% alkali feldspar. Field and petrological evidence points to the episodic injection and ponding of basalt beneath trachytic magma at Agua de Pau volcano. One trachytic lava, with alkali feldspar-contaminated basaltic inclusions, represents a sample of this interface. Density calculations indicate that highly accumulative trachyte can have a neutral or negative buoyancy with respect to basalt and could descend through the interface. Resorption of alkali feldspar will lower the bulk density of the contaminated basalt, with the effect of favouring even greater contamination by overlying accumulative trachyte. The most attractive mechanism for achieving the accumulative trachyte compositions is the slumping of side-wall cumulates and the formation of magmatic density currents which flowed along the compositional interface between trachyte and basalt. Flowage differentiation and crystal settling would result in local ‘parcels’ of denser highly accumulative trachytic magma. The process is illustrated with results from a simple laboratory experiment.

Petrology of Early Cretaceous flood basalts and dykes along the rifted volcanic margin of eastern India

An approximately 220-m thick sequence of Early Cretaceous flood basalts (the Rajmahal Basalt Group) crop out over some 4300 km2 in Bihar, eastern India, forming the leading edge of a seaward-dipping reflector sequence emplaced during the break-up of India and Australia/East-Antarctica. Geochemical data support a division of the basalts and associated dykes into high-Ca and low-Ca magma types. High-Ca tholeiites have CaO contents >10.0 wt%, mg# 50.3–59.6 and K2O 0.11–0.55 wt%. LaNb ranges from 1.29 to 3.62. Rocks of the low-Ca magma type have ≤ 10.5 wt% CaO, mg# 52.1–70.7 and K2O 0.26–1.1 wt%. LaNb is between 1.6 and 3.29. These element abundances and ratios are similar to those of Cretaceous tholeiites from the central Kerguelen Plateau (ODP Site 120–749). Plate reconstructions indicate that the plateau lay adjacent to the Indian continental margin during Early Cretaceous times. It is shown that certain of the Rajmahal basalts (low-Ca magma type) have been contaminated by Indian upper crust, whilst others (high-Ca lavas) retain the near-flat mantle-normalized trace element patterns of oceanic plateau tholeiites.