James B. Gill

The formerly continuous Vitiaz Arc broke into its Vanuatu and Fijian portions during a reversal of subduction polarity in the Miocene. Basaltic volcanism in Fiji that accompanied the breakup ranged from shoshonitic to low‐K and boninitic with increasing distance from the broken edge of the arc that, presumably, marks the broken edge of the slab. The Sr‐Pb‐Nd isotope ratios of the slab‐derived component in the breakup basalts most closely match those of the isotopically most depleted part of the Samoan seamount chain on the Pacific Plate that was adjacent to the site of breakup at 4–8 Ma, and differ from those of subsequent basalts in spreading segments of the surrounding backarc North Fiji and Lau Basins. Subduction of the Samoan Chain along the Vitiaz Trench Lineament may have controlled the limit of polarity reversal and, hence, where the double saloon doors (Martin, 2013) opened. Prior to breakup, Fijian volcanics were more similar isotopically to the Louisville Seamount Chain.

The Havre Trough (HT) backarc basin in the southwest Pacific is in the rifting stage of development. We distinguish five types of basalt there based on their amount and kind of slab component: backarc basalts (BAB) with little or no slab component, modified BAB with slight amounts, reararc (RA) with more, remnants of the preexisting arc (Colville Ridge horsts), and arc front volcanoes within the HT. Previous subarc mantle is quickly removed and replaced by more fertile mantle with less slab component. The ambient mantle is “Pacific” isotopically, and more enriched in Nb/Yb and Nd and Hf isotope ratios north of the Central Kermadec Discontinuity at 32°S than to the south. The contrast may reflect inheritance in the south of mantle that was depleted during spreading that formed the southern South Fiji Basin and a higher degree of melting because of a wetter slab‐derived flux. The slab component also differs along strike, more like a dry melt in the north and a supercritical fluid in the south. The mass fraction of slab component increases southward in the backarc as well as the arc front. RA volcanoes have the most slab component (1%–2%) and form indistinct ridges at high angles to, and <50 km behind, frontal volcanoes. Backarc basalts have less and occur throughout the basin. Slab components are distributed further into the backarc, and more irregularly, during the rifting than spreading stage of backarc basin development. The rifting stage is disorganized geochemically as well as spatially.

We present new Sr, Nd, Hf and Pb isotope data for Early Miocene shoshonitic and high‐K lavas dredged from seamounts in the South Fiji Basin, southwest Pacific Ocean. Our analyses provide a useful reference dataset for this distinctive compositional suite. The shoshonitic lavas formed in an intra‐oceanic back‐arc basin setting and are broadly coeval with nearby arc and back‐arc basin lavas. Further work in the region will be needed before the tectonic implications of the shoshonitic and high‐K lavas are fully understood.

SBAs for the FRCS(Tr&Orth) Examination - December 2020

IODP Site U1437 is located in the Izu rear‐arc region, approximately 330 km west of the Izu‐Bonin trench axis. The oldest four units (Units IV through Unit VII) include volcaniclastic sediment and in situ hyaloclastites. They have ages of about 6–15 Ma, shortly after cessation of Shikoku backarc basin opening. Three magma types are identified by their distinct geochemistry; they are similar types to those found in the modern Izu arc (Rear Arc Seamount Chain [RASC]‐type, Rift‐type, and volcanic front [VF]‐type). RASC‐type has the most enriched Nd and Hf isotope and fluid‐immobile trace element ratios and dominates from 9 to 6 Ma. Rift‐type, dominant from 15 to 9 Ma, is similar to VF‐type in Nd‐Hf isotopes but has the least radiogenic Sr and Pb, and intermediate La/Yb and Nb/Yb, indicating a more fertile mantle source and less hydrous slab component than VF‐type. Less common and randomly distributed VF‐type sediments have the most radiogenic Sr and Pb, and the highest Ba/(Th, LREE [light rare earth element]) ratios, and are interpreted to be distally derived. The genesis of mafic Unit VII samples (~15 Ma) was modeled using the Arc Basalt Simulator. Results are most similar to those for basalts in the modern rift environment indicating the addition of ~1% of a melt‐rich slab component generated at ~125 km, to a Philippine Sea Plate ambient mantle that was more depleted than DMM (depleted MORB mantle). The initial post‐Shikoku basin magmatism in the Izu rear‐arc generated Rift‐type magmas for about 6 million years before the distinctive RASC‐type magmatism began, which then became increasingly enriched.

Felsic magmas from the Izu‐Bonin rear arc have compositions that resemble average continental crust in some respects. In order to understand their origin, we studied 1.1–4.4 Ma tephras in a rear‐arc drill core from International Ocean Discovery Program Expedition 350, Site U1437. They provide a well‐dated record of changing magmatic compositions during the early stages of the most recent episode of Izu‐Bonin arc rifting. Based on our comprehensive recontextualization of published analyses of 2. However, trace element and Hf isotope ratios preclude rear‐arc rhyolite derivation by partial melting of the Oligocene‐Eocene arc basement. Thus, we favor a model where fractional crystallization is more important than crustal melting in producing intra‐oceanic arc rhyolites in this region.

The International Ocean Discovery Program Expedition 350 drilled between two Izu rear‐arc seamount chains at Site U1437 and recovered the first complete succession of rear‐arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear‐arc seamounts came into existence at this site, were recovered in the deepest part of the hole (∼15–16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear‐arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear‐arc seamount chains. It is suggested that following the opening of the Shikoku back‐arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at ~ 9 Ma by the start of rear‐arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back‐arc Basin opening.

The inorganic portion of tuffaceous mud and mudstone in an oceanic island arc can be mostly volcanic in origin. Consequently, a large volume of submarine volcaniclastic material is as extremely fine‐grained as products of subaerial eruptions (75% of the mud is volcanic, and that most of it was derived from proximal rear arc volcanic sources. It faithfully preserves integrated igneous geochemical information about arc evolution in much the same way that terrigenous shales track the evolution of continental crust. In addition, their high sedimentation rate enables high resolution study of climate cycles, including the effects of Pleistocene glaciation on the behavior of the Kuroshio Current in the Shikoku Basin south of Japan.