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Submarine arc-backarc magmatism in the Papua New Guinea (PNG) - Solomons -Vanuatu region

There are two main reasons for resorting to ship-based sampling of the products of arc and backarc magmatism. The first of these is the fact that glass is more commonly preserved in lavas that are quenched in water, and this material is essential for tracking the processes of magmatic evolution. The second is that detailed analysis and quantification of the fluxes of gases in the arc-backarc environment is only really practicable in the submarine realm, where the gases are trapped in the water column. Accordingly, I have been collaborating with colleagues from the Division of Exploration & Mining of CSIRO (North Ryde, Sydney) in the use of the RV Franklin in cruises to the SW Pacific. We have successfully recovered spectacular suites of young, glassy volcanic rocks in the past two years, and discovered newly active backarc spreading centers in the Coriolis Troughs of Vanuatu. In 2002, we have two cruises scheduled for the Bismarck (PNG) and San Cristobal (between the Solomons and Vanuatu) arcs.

Asama volcano in central Honshu, Japan.

Arc magmatism in a region of ridge subduction

It is the fate of all ridges to be subducted, but there are a limited number of examples of this phenomenon occurring at any one time. One of the active examples is in the Solomon Islands, where the Woodlark Ridge is being subducted eastwards beneath the New Georgia Group of volcanoes. A remarkable array of lava types has been erupted in the Pliocene to Recent from these volcanoes, including highly magnesian picrites, andesites, and dacites. We are obtaining comprehensive geochemical analyses of these rocks, determining their ages, and attempting to model the particular processes of magma generation and evolution associated with ridge subduction.

RV Franklin

The deep petrologic structure of island arcs

It is a fact that deep structures of island arcs are rarely exposed (exceptions include the Horoman Complex in Hokkaido, and the Jijal Complex in the Himalayas). Despite the consensus that island arc magmatism is critical in formation of the continental crust, we are ignorant of much of the plutonic processes accompanying the more accessible volcanic products. We can predict that considerable quantities of ultramafic and mafic cumulate complements to the erupted basalt-andesite-dacite arc suites must exist. While there are several explanations for the apparent lack of these complements, there are some fortuitously exposed examples in the South Island of New Zealand where the magmatic processes in the roots of arc volcanoes can be studied. Over the past few years, we have been examining a number of layered, ultramafic-mafic-intermediate plutonic complexes of Permo-Triassic age in the so-called "Median Tectonic Zone" (or Batholith) stretching from Bluff in the south to Nelson in the north of the South Island. Of particular interest has been the discovery of in-situ platinum group minerals in ordinary arc tholeiite magmas, and the recognition of complex cumulate – percolating magma interactions in the plutonic sequences.

The deep petrologic structure of oceanic plateaus

Mantle plumes are ephemeral, and are accompanied on first ascent into the outer regions of the Earth by extensive partial melting – the magmatic products form over-thickened portions of crust known as oceanic plateaus, and the largest current example is the Ontong Java Plateau (OJP) in the SW Pacific. The OJP collided with the Solomons – Vanuatu arc system about 10 million years ago, triggering a reversal in subduction polarity. Following the collision, the margin of the Plateau was locally uplifted in the NE Solomon Islands of Malaita and Santa Isabel. Associated with extensive outcrops of basaltic pillow lavas of the OJP are gabbros and peridotites (also exposed on the island of Choiseul) that seem to be the plutonic equivalents and residual mantle fraction respectively of the erupted magmas. We are studying the genesis of these rocks.

Asama volcano in central Honshu, Japan.

Isolating the variables – the multicomponent sources of arc-backarc magmas

The processes of magma formation in arc-backarc systems are complex involving components from the subducted lithosphere (i.e., metamorphosed sedimentary, igneous and residual mantle lithologies), advecting mantle wedge, and the overriding arc lithosphere. Because there can be considerable ambiguity in the identification of these components and the individual mass fluxes involved, it is important to find natural locations where the component inputs vary. For example, we identified a region of northern Honshu where the active volcanic chain associated with subduction of the Pacific Plate transgresses a major terrane boundary in the arc lithosphere, and showed that a distinct change in the Pb isotopic character of the arc magmas is coincident with this boundary. The importance of this observation is that the budget of Pb in arc magmas is commonly asserted to be controlled by a subducted lithosphere component. But clearly, our models have to become more sophisticated in terms of the mass balances of the different components involved. We have recently extended this type of research effort to a transect along the volcanic front from Hokkaido (Japan) to the southern Kurile Islands (Russia), once again crossing a major lithospheric terrane boundary while the nature of the subducted Pacific Plate is constant.

The geochemical and petrologic changes accompanying high pressure/temperature subduction zone metamorphism

There is clear geochemical and isotopic evidence that some fraction of subducted lithosphere is returned to the Earth's surface, both in arc-backarc magmas and also as exhumed terranes in former plate collision zones. Our current knowledge of the geochemical changes that occur over a range of pressures and temperatures in subducted lithologies is not however, particularly sophisticated. We have been studying the details of these changes in suites of blueschist-eclogite rock types, particularly from Ecuador and New Caledonia. Our aims are to map with laser ablation inductively coupled plasma source mass spectrometry (LA-ICP-MS) the detailed trace element distributions in the various mineral phases comprising these rock types. We are trying to assess what fraction of the highly mobile trace elements such as U, Pb, and Sr (characteristically "over-enriched" in arc magmas) are lost at the various stages of prograde metamorphic evolution of subducting lithosphere.

The petrologic and geochemical evolution of the Rabaul Volcano (PNG)

In 1994, twin volcanic cones either side of the Rabaul caldera (i.e., harbour) erupted in spectacular fashion, causing considerable dislocation to the human population. Eruptions continue from the eastern vent (Tavurvur) while the western vent (Vulcan) is now dormant. This is merely the latest eruption episode of a complex basalt-andesite-dacite volcano. Because of the potential hazard, there has been a long history of monitoring at Rabaul, and more recently, a major AUSAID project to understand the detailed seismic and crustal environment of the volcano. We have undertaken in collaboration with Dr. R. W. Johnson (AGSO-Geoscience Australia) and the Rabaul Volcano Observatory, a detailed petrologic and geochemical study of the products of the latest eruption, and a more extended study of the history of the volcano for the past few thousand years. Given that Rabaul is now one of the best understood active volcanoes in terms of current seismicity, our hope is that significant integration of the petrologic evolution with crustal structures and magma chamber configuration can be achieved.