Age determination on rock samples from Svalbard

A linear, N-S-trending belt of elliptical, positive magnetic anomalies occurs in central Nordaustlandet, northeast Svalbard. They extend from the Caledonian and older complexes in the vicinity of Duvefjorden, southwards beneath the western margin of Austfonna and the offshore areas covered by Carboniferous and younger strata, to the vicinity of Edge¯ya. One of the strongest anomalies occurs in inner Duvefjorden where it coincides with a highly magnetic quartz monzonite-granite pluton at Djupkilsodden. U-Pb and Pb-Pb zircon dating of this post-tectonic pluton defines an age of c. 415 Ma, this being based on the Pb-Pb analyses of three specimens (Pb-Pb ages of 414±10 Ma, 411±10 Ma and 408±10 Ma) and a U-Pb discordia with an upper intercept at 417+18/-7 Ma. Neighbouring felsic plutons in central Nordaustlandet, including the Rijpfjorden and Winsnesbreen granites, lack magnetic signatures in their exposed parts, but have a similar Caledonian age. The central Nordaustlandet magnetic anomalies appear to be part of a circa 300 km long linear belt of late Silurian or early Devonian post-tectonic plutonism that characterizes the Caledonian basement of eastern Svalbard. Felsic intrusions of similar age further west in Spitsbergen are likewise both highly magnetic (Hornemantoppen batholith) and largely non-magnetic (Newtontoppen batholiths / Chydeniusbreen granitoid suite). They all appear to have been intruded at the end of the main period of Caledonian terrane assembly of the northwestern Barents Shelf.

Chemistry of volcanic ash from Celebes and Sulu Sea Basins

During Leg 124, off the Philippines, volcanic material was recovered in deep-sea sediments dating from the late Oligocene in the Celebes Sea Basin, and from the early Miocene in the Sulu Sea Basin. Chemical and petrological studies of fallout ash deposits are used to characterize volcanic pulses and to determine their possible origin. All of the glass and mineral compositions belong to medium-K and high-K calc-alkaline arc-related magmatic suites including high-Al basalts, pyroxene-hornblende andesites, dacites, and rhyolites. Late Oligocene and early Miocene products may have originated from the Sunda arc or from the Sabah-Zamboanga old Sulu arc. Late early Miocene Sulu Sea tuffs originated from the Cagayan arc, whereas early late Miocene fallout ashes are attributed to the Sulu arc. A complex magmatic production is distinguished in the Plio-Quaternary with three sequences of basic to acidic lava suites. Early Pliocene strata registered an important activity in both Celebes Sea and Sulu Sea areas, from the newly born Sangihe arc (low-alumina andesite series) and from the Sulu, Zamboanga, and Negros arcs (high-alumina basalt series and high-K andesite series). In the late Pliocene and the early Pleistocene, renewal of activity affects the Sangihe-Cotobato arc as well as the Sulu and Negros arcs (same magmatic distinctions). The last volcanic pulse took place in the late Pleistocene with revival of all the present arc systems.

Mineral and chemical composititon of sediments from ODP Hole 119-740A

ODP Hole 740A is located on the inner part of the East Antarctic continental shelf in Prydz Bay, at the seaward end of a major onshore rift structure known as the Lambert Graben. Drilling at this site led to the recovery of some 65 m of continental sediments (Prydz Bay red beds) that form part of a much thicker (2-3 km) pre-continental breakup sequence, the development of which may be related to the initiation and rifting of the Lambert Graben. Palynological and paleomagnetic studies have not been able to determine the age of the sediments; they may be equivalent to the onshore late Permian Amery Group or younger. The succession consists predominantly of sandstone, siltstone, and claystone arranged in erosively based, pedogenically influenced fining-upward sequences up to 5 m thick. These were deposited by shallow, braided streams draining an extensively vegetated alluvial plain, with sufficient topographic relief to trap fine-grained sediment and inhibit rapid channel shifting. Pedogenic processes were initiated on the alluvial plain, but climatic conditions were generally unsuitable for extensive pedogenic carbonate formation and the development of mature soil profiles. The sediments were probably derived from a rapidly uplifted fault block terrain composed of upper Proterozoic and Archaean gneisses lying to the southeast of the depositional site. Uplift may have taken place along the tectonically active seaward extension of the eastern faulted margin of the Lambert Graben, which passes immediately southeast of Hole 740A. Differences in mineralogical composition between the Amery Group and the Prydz Bay red beds probably reflect differences in rock composition in the source area. The age of the Prydz Bay red beds has still to be resolved.