Ray W. Kent

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.

The formation and fate of large oceanic igneous provinces

Abstract Large igneous provinces are conspicuous features of late Phanerozoic geology, and include continental flood basal@ rifted continental margin volcanic sequences and oceanic plateaus. The latter are formed in an environment which typically recycles back into the mantle on a time scale of < 200 m.y., but because comparisons have recently been made between oceanic plateaus and Precambrian greenstone belt sequences, new questions arise about their formation, their fate and their preservation. Here we review some critical aspects of three oceanic plateaus, Ontong Java, Kerguelen and the Caribbean/Colombian abducted plateau, and comment on their make-up and the factors governing their preservation, with particular relevance to ancient terranes. Many large igneous provinces can be linked to mantle plumes. Where plumes ascend beneath spreading ridges, their energy is transformed into a large melt volume, producing over-thickened plateau crust. Where the spreading rate is low in relation to magma supply, the plateau may become subaerial (e.g. Iceland), but with fast spreading the plateau remains submarine. Thicker lithosphere may result in plume incubation before magma extrusion, and there are many intermediate situations where plumes could readily break through thin lithosphere (oceanic or continental). Because magma supply exceeds extension rate, plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of Phanerozoic ophiolites. Precambrian greenstones could represent imbricated oceanic plateaus, or plumes penetrating thin continental lithosphere. The initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust. When they collide with active margins they choke the subduction zone, causing subduction “flip” or “backstep” and the development of extensive talc-alkaline arc volcanism on top of the plateau sequences. However, after X= 100 m.y. they are potentially negatively buoyant, so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite, they may be able to spontaneously subduct.

Lithospheric uplift in eastern Gondwana: Evidence for a long-lived mantle plume system?

Recent work has demonstrated that modern drainage systems on rifted continental margins may be the product of ancient lithospheric doming in areas of flood-basalt magmatism. This uplift is thought to reflect the existence of widespread mafic underplating and excess melt generation at rift zones. Here I show that uplift due to the initiation of a thermal anomaly (plume head) in the asthenosphere may be evidenced by the formation of discrete rift structures and persistent paleodrainage (sediment dispersal) patterns for up to 150 m.y. prior to the commencement of igneous activity and final continental breakup. Volcanism thus occurs as a comparatively late stage geochemical response to the long-term residence of hot plume material beneath continental mantle lithosphere.

Crustal make-up of the northern Andes: evidence based on deep crustal xenolith suites, Mercaderes, SW Colombia

Samples of the deep crust and upper mantle in the Northern Andes occur as abundant xenoliths in the Granatifera Tuff, a late Cenozoic vent in the Mercaderes area of SW Colombia. The lower crustal assemblage includes granulites, hornblendites, pyribolites, pyroxenites and gneisses; mafic rocks predominate, but felsic material is also common. P-T conditions for the pyribolite assemblages (i.e. Hbl+Fs/Scp+Grt+Cpx+Qtz±Bt), which are the best constrained, are 720-850 � C and 10-14 kbar, consistent with a deep-to-lower crustal origin. A notable feature of this xenolith suite is that it is dominated by hornblende. However, mineral reactions within the suite show that there is a transition from amphibolite to granulite facies, and there is a probable restite-melt relationship represented within the suite. However, the latter appears to be dominated by hornblende and garnet. The mafic rocks mostly lack the high Cr and Ni that would be expected of cumulates. Neither do they possess the positive Sr and Eu anomalies that would be consistent with resite or cumulate models for the lower crust. They bear greatest similarity to oceanic basalts (s.l.). The Rb contents of the xenoliths, whether mafic or silicic, are very low, and the more silicic members of the suite tend to have small positive Sr and Eu anomalies, which are transitional to adakitic compositions. The Sr isotopic compositions of the xenoliths lie between 0.704 and 0.705; however, the Nd isotopic compositions are much more variable, indicating considerable long-term heterogeneity. Few of the xenoliths can be compositionally recognised as metasedimentary; however, a sedimentary component is evident in the Pb isotopic compositions. Within these constraints, our favoured model is a deep crust formed by basaltic components (subduction-accretion?), and minor sediment, which is subject to an increase in thermal gradient to produce the granulites, any melting being dominated by hornblende-out reactions involving garnet. However, there is no evidence of any pervasive crustal melting, leading to the conclusion that the voluminous Andean magmatism arises from the mantle wedge. Crown Copyright D 2002 Published by Elsevier Science B.V.

Mineraloy and 40 Ar/ 39 Ar geochronology of orangeites (Group II kimberlites) from the Damodar Valley, eastern India

Abstract A suite of ultramafic-mafic alkaline igneous rocks in the Damodar Valley, eastern India, contains carbonate, phosphate and titanate minerals that are not characteristic or common in minettes or lamproites, but are typical of orangeites (Group II kimberlite) from southem Africa. Phlogopite grains from the Damodar alkaline rocks yield mean 40Ar/39Ar arges of 116.6±0.8 Ma, 113.5±0.5 Ma and 109.1 ±0.7 Ma (1 σ errors) using laser dating techniques. These ages are similar to the Rb-Sr ages of African orangeites, which lie mostly in the range 121 to 114 Ma. Prior to this study, only one possible occurrence of orangeite (the ~820 m.y.-old Aries pipe, Western Australia) was known outside the Kaapvaal craton and its environs. If the Damodar alkaline rocks are bona fide orangeites, it is likely that they were generated at depths of >150 km, within the stability field of diamond.

Magnesian basalts from the Hebrides, Scotland: chemical composition and relationship to the Iceland plume

Abstract The Hebridean Tertiary igneous province lay 700–900 km south of a presumed plume centre, and >500 km landward of a rifted continental margin prior to opening of the northeast Atlantic. The Province includes a small number of magnesian basalts containing olivine phenocrysts with core compositions equal to, or in excess of, Fo89. Olivine-liquid equilibrium calculations suggest that these phenocrysts formed from liquids with MgO contents of 14–15 wt%, and in rare instances, 18–20 wt%. The liquidus temperatures of these magmas imply mantle potential temperatures of 1350–1460°C at distances up to 900 km from the axis of the Iceland plume. Their emplacement at c. 60 Ma implies either a long-established hotspot, or active channeling of plume material along the base of the plate towards a region undergoing considerable lower crustal extension.

Emplacement of Hebridean Tertiary flood basalts: evidence from an inflated pahoehoe lava flow on Mull, Scotland

The lower 200 m of the Tertiary lava pile on Mull, western Scotland, consists mainly of small volume (0.01–1 km3), high-magnesian basaltic lava flows. Eruption of these flows probably occurred from fissures or point-source vents, producing pahoehoe-textured sheets averaging about 5 m in thickness. Certain lavas cropping out in northwest Mull greatly exceed this average thickness and may represent inflated pahoehoe flows akin to those described at Kilauea, Hawai’i. Structural features of a 16–30 m thick lava at Port Haunn, northwest Mull, that conform to this proposal include sub-horizontal sheets of amygdales up to 14 cm thick separated by amygdale-poor zones, pipe amygdales at the base of the flow and a low-relief flow top. Alternating olivine-rich and olivine-poor bands in the Port Haunn lava are suggestive of lava pulses during a continuous eruption, also consistent with inflation. Elsewhere in western Mull, there is evidence for filled lava tubes and pahoehoe toes, both of which are characteristic of inflated sheet flows formed on gentle ground slopes (<4° and perhaps <0.5°). Fluid dynamical considerations suggest that each sheet flow in the lower part of the Mull lava sequence probably was active for a short period, permitting the build up of small lava fields over several months or years. This is commensurate with the small volume and limited areal extent of the Tertiary Mull lavas when compared to basalts in igneous provinces such as the Columbia River and Deccan.

Coal—magma interaction: an integrated model for the emplacement of cylindrical intrusions

Olivine-bearing lamproite magmas intruded into Permian coal seams in northeast India occur as root-like cylinder stockworks, extending for up to several kilometres up-dip along the bedding planes of their sedimentary host. Clusters of eight or more conduits are typical, linked by thin tubular cross-branches. Cylindrical geometry may arise by injection of hot, low-viscosity fluid through a slot, with the development of multiple tube-like instabilities at the interface between the moving fluid and a higher-viscosity host. This behaviour appears more complex than the models of Chouke, van Meurs & van der Pod, and Saffman & Taylor, which predict the development of a single dominant tube in porous or layered flow. Cylinder emplacement may be an essentially passive process, in which the sediment column is reduced by expulsion of heated pore fluids at the head of the moving intrusion, creating a space into which the melt can propagate. Generation of a superheated vapour envelope by non-nucleated film boiling of these fluids around the hot lamproite magma (the Leidenfrost effect) allows melt flow to be maintained in a lengthening tube thermally insulated from the surrounding coal, in a manner analogous to submarine lava tubes. Cooling of the magma through the Nukiyama temperature (the temperature at which maximum evaporation of the heated fluid occurs) may give rise to violent surface boiling and the formation of large vapour bubbles at the magma–coal interface. Implosion of these bubbles could then result in the formation of shock breccias, comparable to hyaloclastites produced by bubble or surface film collapse in the vicinity of pillow lava tubes. The operation of such a process around lamproite magma is suggested by the presence of complex breccias composed of highly fragmented coal, sandstone, and lamproite, at the termini of certain cylinders. Surface and subsurface exposures of the cylinders reveal the presence of a carbonate–chlorite–clay halo surrounding each intrusion, resulting from the alteration of mafic mineral phases by fugitive volatiles released from the protective vapour jacket. The coal seams proximal to intrusion clusters are relatively undeformed, with no evidence of assimilation by the invading melts. The coals have experienced extensive carbonization, probably as a result of slow conductive heating from the cooling lamproite bodies, or fluids derived therefrom. Field observations indicate that these thermal effects are not merely confined to the coal–melt interface, but occur for some considerable distance away from the intrusions, producing large areas of naturally coked coal.

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.

Discussion on application of palynological data to the chronology of the Palaeogene lava fields of the British Province

Andrew C. Kerr & Ray W. Kent write: In a recent paper, Bell & Jolley (1997) used palynological data to propose that the Mull Tertiary lava succession and the upper Antrim lavas are significantly younger (by 2–4 Ma) than the Skye Main Lava Series. In support of this proposal, they correlated the StaVa magma sub-type (found at the base of the Mull lava pile) with the two Preshal More-type flows on Skye, and suggested that these two magma types are compositionally similar. We contend that neither of these suggestions is correct, and discuss data that are at variance with the conclusions reached by Bell & Jolley. Eruption of the Mull lava succession was believed by Bell & Jolley to have started at c. 55 Ma, about 4 Ma after the commencement of igneous activity on Skye. The suggestion of a 55 Ma age for Mull basalts ignores four 40Ar/39Ar incremental heating ages (weighted mean=60.0±0.5 Ma, 1σ error) obtained by Mussett (1986) on lavas from the lower part of the Mull Plateau Group. Bell & Jolley believed these ages to be unreliable, yet they satisfy all of the acceptance criteria of Lanphere & Dalrymple (1978), i.e., they can be considered as reliable estimates of the true crystallization ages of these basalts. Furthermore, Mussett’s (1986) 40Ar/39Ar data for the Mull lavas are consistent with Rb–Sr and 40Ar/39Ar ages obtained for Mull granites (58.1±1.6 Ma to 56.5±1.0 Ma, Walsh et al. 1979; Mussett 1986). All of these radiometric ages