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Massive submarine basalt flows were sampled at five sites on the Tristan‐Gough‐Walvis hotspot track in the South Atlantic by International Oceanic Discovery Program Expeditions 391/397T, where the plume was interacting with a mid‐ocean ridge, a setting similar to that the of modern Iceland. High resolution XRF core scans document significant internal chemical variations with depth in these flows. Some of this reflects basal olivine accumulation. However, some examples have “scallop‐shaped” patterns that are interpreted to represent influxes of new magma during flow lobe inflation with successive lava injections focused toward the base of the flow unit. Olivine concentration in the deeper parts of the flow is interpreted to reflect top‐down tapping of a vertically zoned magma chamber, with the upper part of the chamber erupting first, and successive eruptive pulses tapping progressively deeper levels of the stratified chamber. The occurrence of massive submarine lava flows requires high eruptive fluxes relative to pillow lava formation. Propagation of these massive flows is favored by (a) high sea water confining pressures, which inhibit vesiculation and keep effective viscosity low and dissolved volatile content high, and (b) chill zones and thick viscoelastic crusts of quenched lava on the flow tops, which effectively insulate the flow interior from ambient temperatures. The formation of a thin film of super‐heated steam on the upper flow surface may similarly enhance the insulation. Evidence suggests that similar massive flows on the seafloor may extend many kilometers from their vents. 

Abstract New whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the “Leka sector” of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere. 

The Tristan-Gough plume system forms age-progressive volcanism on the African plate over ~130 Ma, extending to the active islands of Gough and Tristan-Inaccessible. Walvis Ridge forms massive ridges and plateaus that split into three narrower ridges of the Guyot Province. International Ocean Discovery Program (IODP) Expedition 391 Site U1577 sampled the extreme eastern flank of Valdivia Bank, an oceanic plateau within the Walvis Ridge. Here we report major and trace element data as well as Sr-Nd-Hf-Pb isotopic compositions of IODP 391 Site U1577. Three massive basalt flow subunits were drilled, separated only by thin chilled margins. The lack of any sediment or significant alteration at the contacts, and their consistent paleomagnetic inclination, all suggest that these flows were erupted in relatively quick succession. Accordingly, geochemical variations are minimal. Samples from Site U1577 form tight clusters that overlap in major and trace elements with previous dredge and Deep Sea Drilling Project (DSDP) drill site samples from the Walvis Ridge. All are less enriched in incompatible trace elements, i.e., Ti, K, P, Sr and Zr, relative to samples from Tristan and Gough islands and the Guyot province, consistent with Walvis Ridge samples formed by higher degrees of partial melting. In contrast to Walvis Ridge dredge samples, Site U1577 samples are shifted slightly towards higher 176Hf/177Hf and lower 208Pb/204Pb isotopic compositions, while overlapping in 207Pb/204Pb vs. 206Pb/204Pb as well as Sr-Nd isotopic compositions. Such elevated 176Hf/177Hf combined with lower 208Pb/204Pb isotopic compositions have otherwise only been reported from the Eastern Rio Grande Rise formed in near-/on-ridge position. Magnetic lineations imply formation of Valdivia Bank by seafloor spreading as well. Site U1577 samples provide geochemical support for this hypothesis whereas dredge samples lack signatures of plume-ridge interaction. Also, with Site U1577 on the eastern flank, it is farthest from the mid-Atlantic Ridge at the time of formation compared to the location of near-by dredge samples. With major and trace element data integrated on the same samples as isotopic compositions, we will address the contrasting possibilities of an integral depleted plume component versus evidence for plume-ridge interaction. 

The Dadeville Complex of Alabama and Georgia (southeastern United States) represents the largest suite of exposed mafic-ultramafic rocks in the southern Appalachians. Due to poor preservation, chemical alteration, and tectonic reworking, a specific tectonic origin for the Dadeville Complex has been difficult to deduce. We obtained new whole-rock and mineral geochemistry coupled with zircon U-Pb geochronology to investigate the magmatic and metamorphic processes recorded by the Dadeville Complex, as well as the timing of these processes. Our data reveal an up-stratigraphic evolution in the geochemistry of the volcanic rocks, from forearc basalts to boninites. Our new U-Pb zircon crystallization data—obtained from three amphibolite samples—place the timing of forearc/protoarc volcanism no later than ca. 467 Ma. New thermobarometry suggests that the Dadeville Complex rocks subsequently experienced deep, high-grade metamorphism, at pressure-temperature conditions of ~7 kbar and ~760 °C. The data presented here support a model for formation of the Dadeville Complex in the forearc region of a subduction zone during subduction initiation and protoarc development, followed by deep burial/underthrusting of the complex during orogenesis. 

The Walvis Ridge system consists of a series of seamounts, ridges, and plateaus formed during the opening of the southern Atlantic Ocean since ~135 Ma. International Ocean Discovery Program Expeditions (IODP) 391 and 397T drilled six sites along the length of the hotspot track to understand the magmatic processes associated with evolving plume-ridge systems. The oldest drilled segment of the ridge system – Frio Ridge – extends from the Etendeka flood basalts in Namibia westward into the Atlantic Ocean. Site U1575 is on the Frio Ridge and is the closest site to the African continent. The site drilled 118.9 m of igneous basement with 70.7 m (59.5%) of recovery. The recovered core consisted of alternating sequences of submarine pillow lavas and sheet flows, some of which were massive (up to 21 m thick). Preliminary major and trace element data demonstrate the basaltic lavas are fractionated (MgO = 4.8-6.4 wt. %) with modest TiO2 contents (1.5-2.7 wt. %). The upper 52 m of igneous section (214-267 mbsf) are geochemically consistent throughout the various eruptive styles. However, an abrupt compositional shift to lavas with lower incompatible element abundances (TiO2, Zr, Sr, Nb, La, etc.) from 274-311 mbsf demonstrates a clear shift in magmatic source contributions. Below this, the lavas return to compositions similar to the upper portion of the hole. Shipboard natural gamma radiation (NGR) and magnetic susceptibility (MS) measurements correlate with mineralogical and compositional changes. Specifically, decreases in NGR correlate well with decreases in K2O, Sr, Y, and Zr. MS is positively correlated with zones containing olivine. Trace element discrimination plots demonstrate a dual character: Ti-V relationships are strongly MORB-like while Th/Nb suggests the lavas have both MORB and plume characteristics, consistent with the formation of the Frio Ridge through plume-ridge interaction. Elevated Zr/Nb and Y/Nb values are also consistent with a hybrid source. The composition of this core contrasts sharply with cores recovered from the younger Guyot Province to the southwest. Sites U1578 and U1585 have episodes of higher TiO2 contents (>3.5 wt. %) with trace element signatures (e.g. low Zr/Nb & Y/Nb) indicative of a pronounced plume component, consistent with an intraplate setting for the formation of the Guyot Province. 

Suprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultra- mafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics—including highly depleted bulk-rock trace element patterns and high Cr# of spinel—characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the “layered ultramafics” that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean–derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike.