Abstract Three submarine Diamante cross-chain volcanoes in the southern Mariana arc mark a magma-healed zone of along-arc (north–south) extension that allows either mafic mantle-derived basalts or felsic magmas from the middle of thickened arc crust to erupt. The largest volcano is East Diamante, with a well-developed (5×10 km) caldera that formed via violent felsic submarine eruptions beginning nearly 0.5 Ma. One or more of these eruptions also formed a giant submarine dune field extending 30 km to the NW of the volcano. Felsic igneous activity continues at least as recently as c. 20 000 years ago, with emplacement of resurgent dacite domes, some hot enough to power the only black smoker hydrothermal system known in the Mariana arc. In contrast, felsic eruptions do not occur on the two volcanoes to the west, implying that the mid-crustal felsic zone does not underlie the thinner crust of the Mariana Trough back-arc basin. Diamante cross-chain lavas define a medium K suite; mafic lava phenocryst assemblages show arc-like associations of anorthite-rich plagioclase with Fe-rich olivine. Magmatic temperatures for a basaltic andesite and three dacites are c. 1100 °C and c. 800 °C, respectively, typical for cool, wet, subduction-related felsic magmas. Felsic magmas formed under low-P crustal conditions. The Diamante cross-chain is the southernmost of at least seven and perhaps eight Mariana arc volcanoes in a c. 115 km long arc segment characterized by felsic eruptions. This is the ‘Anatahan Felsic Province’, which may have formed above a mid-crustal tonalite body that formed by fractionation or was re-melted when heated by c. 1200 °C mafic, mantle-derived magmas. Across- and along-arc variations suggest that felsic eruptions and dome emplacement occurred when midcrustal tonalite was remobilized by intrusions of mafic magma, while north–south extension facilitated the development of conduits to the surface. Supplementary material: Detailed Hyperdolphin ROV dive tracks, Cook 7 dredge locations, 40 Ar/ 39 Ar analytical data, analytical methods, major and selected trace element analyses of whole rock samples, and compositional data for minerals are available at http://www.geolsoc.org.uk/SUP18611

Abstract The Tambien Group of northern Ethiopia (Tigre), with probable correlatives in Eritrea, is a 2–3-km-thick siliciclastic–carbonate succession that was deposited in an intra-oceanic arc platform setting within the southern Arabian–Nubian Shield (ANS) area (southern extension of the Nakfa Terrane) of the Mozambique Ocean. Its deposition occurred prior to ocean closure between converging fragments of East and West Gondwana and concomitant structural emergence of the East African Orogen (EAO). The Tambien Group is well exposed and best studied in the Mai Kenetal and Negash synclinoria, where litho- and chemostratigraphy (including δ 13 C carb , 87 Sr/ 86 Sr) provide the basis for a composite reference section. Two glaciogenic intervals have been suggested from exposures within the Didikama and Matheos Formation in the Negash Synclinorium. No reliable palaeomagnetic data exist to constrain the palaeolatitude of Tambien Group deposition and the southern ANS, but palaeogeographic reconstructions and evaporite pseudomorphs in lower carbonate units (Didikama Formation) imply low to intermediate latitudes (<45°). Integration of available geochronological information (regional magmatism and detrital zircon) suggests c. 775–660 Ma as a plausible window constraining deposition of the prospective glacial intervals. The Tambien Group appears to preserve a coherent chemostratigraphic framework that can be effectively subdivided according to shifts in δ 13 C carb polarity [polarity intervals A (+), B (–), C (+), D (–)]. Slates underlying and interstratified with polarity interval A carbonate preserve evidence of extreme chemical weathering that lessened prior to deposition of polarity interval B carbonate. Tambien Group carbonate units have sedimentological characteristics consistent with both shallow and deeper marine depositional settings. The lower prospective glacial interval lacks diagnostic sedimentological evidence of synglacial deposition, but is overlain by negative δ 13 C carbonate (polarity interval B) with sedimentological characteristics consistent with well-documented cap-carbonate successions. The upper prospective glacial interval in the Negash Synclinorium (Matheos Diamictite) best exhibits characteristics consistent with glaciogenic deposition (matrix-supported polymictic clasts, possible dropstones, possible bullet-nosed and striated clasts). In contrast to pericratonic rift margin settings that are common for Cryogenian glaciogenic deposits, palaeogeographic reconstructions for the 775–660 Ma timeframe place northern Ethiopia within an intra-oceanic setting that was likely far removed from cratonic hinterlands. More work on Tambien Group sedimentology, geochronology and palaeogeography is required to better evaluate the extent and timing of glacial conditions associated with the prospective glaciogenic intervals. Supplementary material Supplementary Table 21.1 of Tambien Group geochronological age constraints is available at http://www.geolsoc.org.uk/SUP18462 .

Abstract Evidence of Early- to Mid-Cryogenian ( c. 780 Ma and c. 740 Ma) glacial activity is summarized for the northern Arabian–Nubian Shield (ANS), including structural framework, stratigraphy, lithological descriptions and relationships with younger and older units, banded iron formation chemostratigraphy, other characteristics, geochronological constraints, and discussion. The ANS is a broad tract of juvenile continental crust, formed from accreted arc-backarc basin terranes developed around the margins of the Mozambique Ocean. As a result, these successions formed in marine environments at some distance from continental margins. Deposits include banded iron formation (BIF) and possibly glacial diamictite scattered over broad regions of the Central Eastern Desert of Egypt, NW Arabia and possible correlative units in NE Sudan. The older ( c. 780 Ma) examples (Meritri group, NE Sudan; basal Mahd group, Arabia) occur in the central ANS, on the southern flank of an important lithospheric boundary, an ophiolite-decorated suture zone. Mahd group diamictite is thin (1–5 m thick) and rests above the earliest (Cryogenian) ANS unconformity. The Meritri group interval near Port Sudan is much thicker and part of a deformed passive margin. Both Mahd and Meritri group deposits need further study before they are accepted as glaciogenic; confirmation of this interpretation would indicate that Neoproterozoic glacial activity began at least as early as 780 Ma ago. The younger ( c. 740 Ma) glacial deposits include diamictite and BIF: the Atud diamictite and BIFs of the Central Eastern Desert of Egypt and the correlative Nuwaybah diamictite and BIF of NW Arabia. Northern ANS-BIF is a well-layered chemical sediment of interlaminated hematite-magnetite and jasper. A glacial origin for the Atud-Nuwaybah diamictites is inferred because large clasts and matrix zircons have ages (Palaeoproterozoic and Neoarchean) and compositions (especially quartzite, arkose, and microdiamictite) that require transport from outside the ANS Cryogenian basin. Northern ANS-BIF may also reveal glacial influence, having been deposited in response to reoxygenation of a suboxic ocean. The 740 Ma diamictite and/or BIF may correlate with Tambien Group diamictites in Ethiopia ( Miller et al. 2011 ). Northern ANS diamictite and BIF were deposited in an oceanic basin of unknown size, as indicated by association with abundant ophiolites; they are strongly deformed, obscuring many primary features. There is no strong evidence for or against Ediacaran glaciation in the ANS, largely because the region was uplifted at this time. The c. 600 Ma ANS peneplain may have been partly cut by Ediacaran glaciation. Some of the post-accretionary basins of Arabia could preserve glaciogenic deposits of Ediacaran age, but assessing this possibility requires further investigation.