A. D. Saunders

THERMAL AND CHEMICAL STRUCTURE OF THE ICELAND PLUME

Basaltic lavas, forming thick offshore seaward-dipping reflector sequences (SDRS) and onshore igneous provinces around the North Atlantic margins, represent melting of anomalously hot mantle in the head of the ancestral Iceland plume. Some of these lavas are chemically and isotopically indistinguishable from recent Icelandic basalt, but others more closely resemble basalt erupted at normal segments of mid-ocean ridges (N-MORB). In this paper we show that Icelandic basalt and N-MORB define parallel tight arrays on a plot oflog(Nb/Y) against log(Zr/Y), with N-MORB relatively deficient in Nb. Deficiency or excess of Nb, relative to the lower bound of the Iceland array, may be expressed as ΔNb=1.74+log⁡(Nb/Y)−1.92log⁡(Zr/Y)such that Icelandic basalt has ΔNb > 0 and N-MORB has ΔNb < 0. ΔNb is a fundamental source characteristic and is insensitive to the effects of variable degrees of mantle melting, source depletion through melt extraction, crustal contamination of the magmas, or subsequent alteration. We use new and published Nb, Zr and Y data to identify the mantle sources for Palaeocene and Eocene basaltic lavas erupted around the Atlantic margins in order to deduce the thermal and compositional structure of the head of the ancestral Iceland plume. The results show that the head of the plume was zoned, with an axial zone of Icelandic mantle surrounded by a thick outer shell of anomalously hot but compositionally normal N-MORB-source mantle. The zoning is very similar in scale and character to that seen today along the Reykjanes Ridge and is difficult to reconcile with the initiation of rifting and SDRS formation through the impact of a large plume head originating solely from the lower mantle. The thick outer shell of hot, depleted upper mantle, which formed more than half the volume of the plume head, suggests that at least part of the plume originated in the thermal boundary layer at the base of the upper mantle.

Geochemical characteristics of basaltic volcanism within back-arc basins

Summary Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB and basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.

Subduction of pelagic sediments: implications for the origin of Ce-anomalous basalts from the Mariana Islands

Attempts to assess the significance of subducted sediments in the genesis of island arc magmas have been limited by the lack of trace element data on pelagic oozes. Accordingly, we have analysed a series of pelagic clays and nannofossil oozes from the Nazca Plate for REE and other trace elements. A calculated average––Pacific Authigenic Weighted Mean Sediment (PAWMS)––exhibits light REE-enrichment (Lan/Ybn∼4.5), high contents of Ba and Sr, but low abundances of Rb, Nb and Ta. Most significant, however, is the occurrence of large, negative Ce anomalies (Ce/Ce*~0.2). We have attempted to model the contribution of PAWMS-type material to the source of the magmas of the Mariana Island Arc, an intra-oceanic arc far removed from the effect of continent-derived detritus. Only small amounts of pelagic sediment, between 0.3 and 0.5% of the source, are required to develop the small negative Ce anomalies, high Ba/La ratios, and high LIL/HFS element ratios characteristic of these and other arc lavas. However, a small fluid contribution from the dehydrating subducted oceanic crust is required to produce the high Rb/Ba observed in several Mariana suites. The ternary mixing between sediment, mantle host and dehydrating oceanic crust also produces very low Nb and Ta abundances in the arc lava source. However, the very high abundance of Sr (>1000ppm) and the estimated high 87Sr/86Sr ratio (~0.7150) of PAWMS, results in a model 87Sr/86Sr ratio of 0.7070. This is far higher than the measured ratios in the Mariana arc lavas (0.7033–0.7040) and may suggest that the subducted sediment has a lower Sr content (<200 ppm) or a lower 87Sr/86Sr ratio, or that the carbonate-rich component is not involved in source contamination. Volumetrically it would appear that much of the sediment approaching the Mariana trench (~45 km3 Ma−1 per km of arc) may be recycled into the deeper mantle. This will have the effect of introducing high Ba, Sr, Th, 87Sr/86Sr, 208Pb/204Pb and 207Pb/204Pb material into the mantle. However, incorporation of such material cannot alone satisfactorily account for the trace element and isotope chemistry of ocean islands; oceanic sediments have LIL/Ta, Nb ratios far too high to produce the trace element characteristics of most intraplate magmas.

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.

Fluid Influence on the Trace Element Compositions of Subduction Zone Magmas

Subduction zones represent major sites of chemical fractionation within the Earth. Element pairs which behave coherently during normal mantle melting may become strongly decoupled from one another during the slab dehydration processes and during hydrous melting conditions in the slab and in the mantle wedge. This results in the large ion lithophile elements (e.g. K, Rb, Th, U, Ba) and the light rare earth elements being transferred from the slab to the mantle wedge, and being concentrated within the mantle wedge by hydrous fluids, stabilized in hydrous phases such as hornblende and phlogopite, from where they are eventually extracted as magmas and contribute to growth of the continental crust. High-field strength elements (e.g. Nb, Ta, Ti, P, Zr) are insoluble in hydrous fluids and relatively insoluble in hydrous melts, and remain in the subducted slab and the adjacent parts of the mantle which are dragged down and contribute to the source for ocean island basalts. The required element fractionations result from interaction between specific mineral phases (hornblende, phlogopite, rutile, sphene, etc.) and hydrous fluids. In present day subduction magmatism the mantle wedge contributes dominantly to the chemical budget, and there is a requirement for significant convection to maintain the element flux. In the Precambrian, melting of subducted ocean crust may have been easier, providing an enhanced slab contribution to continental growth.

The Iceland plume in space and time: a Sr-Nd-Pb-Hf study of the North Atlantic rifted margin

New Sr–Nd–Pb–Hf data require the existence of at least four mantle components in the genesis of basalts from the the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf–Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (∼95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ∼55 Ma ago. Thus, Hf–Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume [Thirlwall, J. Geol. Soc. London 152 (1995) 991–996; Hards et al., J. Geol. Soc. London 152 (1995) 1003–1009; Fitton et al., Earth Planet. Sci. Lett. 153 (1997) 197–208]. They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of ‘enriched’ streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive ΔNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower eNd and eHf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its eNd and eHf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative ΔNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower eHf for a given eNd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be ‘polluted’ not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting [Klein and Langmuir, J. Geophys. Res. 92 (1987) 8089–8115] along the MAR in the North Atlantic.

Geochemistry of Cenezoic volcanic rocks, Baja California, Mexico: Implications for the petrogenesis of post-subduction magmas

Abstract Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO2, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, Lan/Ybn ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. 87Sr/86Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have 87Sr/86Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction. It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH2O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component. The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.

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.

Geological–tectonic framework of Solomon Islands, SW Pacific: crustal accretion and growth within an intra-oceanic setting

The Solomon Islands are a complex collage of crustal units or terrains (herein termed the ‘Solomon block’) which have formed and accreted within an intra-oceanic environment since Cretaceous times. Predominantly Cretaceous basaltic basement sequences are divided into: (1) a plume-related Ontong Java Plateau terrain (OJPT) which includes Malaita, Ulawa, and northern Santa Isabel; (2) a ‘normal’ ocean ridge related South Solomon MORB terrain (SSMT) which includes Choiseul and Guadalcanal; and (3) a hybrid ‘Makira terrain’ which has both MORB and plume=plateau affinities. The OJPT formed as an integral part of the massive Ontong Java Plateau (OJP), at c. 122 Ma and 90 Ma, respectively, was subsequently affected by Eocene‐Oligocene alkaline and alnoitic magmatism, and was unaffected by subsequent arc development. The SSMT initially formed within a ‘normal’ ocean ridge environment which produced a MORB-like basaltic basement through which two stages of arc crustal growth subsequently developed from the Eocene onwards. The Makira terrain records the intermingling of basalts with plume=plateau and MORB affinities from c. 90 Ma to c. 30 Ma, and a contribution from Late Miocene‐present-day arc growth. Two distinct stages of arc growth occurred within the Solomon block from the Eocene to the Early Miocene (stage 1) and from the Late Miocene to the present day (stage 2). Stage 1 arc growth created the basement of the central part of the Solomon block (the Central Solomon terrain, CST), which includes the Shortland, Florida and south Isabel islands. Stage 2 arc growth led to crustal growth in the west and south (the New Georgia terrain or NGT) which includes Savo, and the New Georgia and Russell islands. Both stages of arc growth also added new material to pre-existing crustal units within other terrains. The Solomon block terrane collage records the collision between the Alaska sized OJP and the Solomon arc. Initial contact possibly first occurred some 25‐20 Ma but it is only since around 4 Ma that the OJP has more forcefully collided with the Solomon arc, and has been actively accreting since that time, continuing to the present day. We present a number of tectonic models in an attempt to understand the mechanism of plateau accretion. One model depicts the OJP as splitting in two with the upper 4‐10 km forming an imbricate stack verging to the northeast, over which the Solomon arc is overthrust, whilst deeper portions of the OJP (beneath a critical detachment surface) are subducted. The subduction of young (<5 Ma), hot, oceanic lithosphere belonging to the Woodlark basin at the SSTS has resulted in a sequence of tectonic phenomena including: the production of unusual magma compositions (e.g. Na‐Ti-rich basalts, and an abundance of picrites); an anomalously small

A new plate tectonic model of the Caribbean; implications from a geochemical reconnaissance of Cuban Mesozoic volcanic rocks

Accreted terranes, comprising a wide variety of Jurassic and Cretaceous igneous and sedimentary rocks, are an important and conspicuous feature of Cuban geology. Although the Mesozoic igneous rocks are generally poorly exposed and badly altered, we have collected and geochemically analyzed 25 samples that place new constraints on plate tectonic models of the Caribbean region. From our recognizance sampling, six main lava types have been identified within the Mesozoic igneous rocks of Cuba: rift basalts, oceanic tholeiites, backarc basin lavas, boninites, island arc tholeiites (IAT), and calc-alkaline lavas. We suggest that the rift-related basalts may have formed during the development of the proto-Caribbean, as the Yucatan block rifted away from northern South America in Jurassic–Early Cretaceous time. The Early Cretaceous oceanic tholeiites have flat rare earth element patterns, and are compositionally similar to Pacific mantle plume–derived oceanic plateaus of similar age. The Early Cretaceous arc-related rocks are either backarc basalts, boninites, or relatively trace element–depleted IAT lavas. A limited amount of geochemical and field evidence hints that two parallel arc systems existed in the western proto-Caribbean area in Early Cretaceous time. This leads us to speculate that in the proto-Caribbean at this time there was a western arc with a northeast-dipping subduction zone erupting IAT lavas (with Farallon plate being consumed), and a more eastern boninitic arc with a southwest-dipping subduction zone (with proto-Caribbean plate being consumed). This latter arc was relatively short lived and after being aborted was mostly eroded away. The Cretaceous primitive (IAT) arc survived and, later in Cretaceous time, as this arc system moved into the widening gap between North and South Americas, calc-alkaline lavas began to be erupted. The evidence suggests that the change from IAT to calc-alkaline lavas was gradual and not abrupt. These new data, although limited, provide geochemical constraints on the tectonic development of the northern part of the Caribbean plate. In consequence, we present a new plate tectonic model for this area of the Caribbean.