Geochemical evolution within the Tonga-Kermadec-Lau Arc-Back-arc systems: The role of varying mantle wedge composition in space and time

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga-Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc - back-arc system. Geochemical variations in Tonga-Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO and F) contents increase eastwards towards the modern arc. These observations suggest thai depletion is due to melt extraction during back-arc extension and vokanism, together with a long 'residence time' of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293-325] 'HO-rich component' composition), into the overlying, but variably depleted mantle wedge

Temporal development of the Atherton Basalt Province, North Queensland.

The Atherton Basalt Province is centred on the Atherton Tableland, ∼km southwest of Cairns in north Queensland. Forty-eight K-Ar age determinations and four U/Th analyses from these basalts provide information on the distribution of the volcanics over time. Volcanism commenced at 7.1 Ma and continued into the Early Holocene, with volumetric peaks in activity occurring 3.5-3 Ma and 2-1Ma. The province shows a change with time from eruptions of voluminous lava flows that built relatively large shield volcanoes, to the production of less voluminous lavas and pyroclastics associated with cinder cones during the last million years. Phreatomagmatic, maar-forming eruptions are also preserved among the most recent eruptions. Previous radiocarbon dating of swamp sediments suggested that volcanic activity may have occurred within the past 10 000 years. No systematic change over time is apparent in the location of the volcanoes of the province. Rather, a source region ∼ 80 km in diameter for the province as a whole has evolved over time. Extensive partial melting in the upper mantle may have led to the ascent and adiabatic melting of lesser volumes of mantle material from deeper levels. Changes in lithospheric stresses in north Queensland, caused by the docking of the Ontong Java Plateau and subsequent development of northward subduction at the San Cristobal Trench, may have allowed the ascent of magma from this mantle region

Tectonic Subdivision of the Prince Charles Mountains: A Review of Geologic and Isotopic Data

The Prince Charles Mountains have been subject to extensive geological and geophysical investigations by former Soviet, Russian and Australian scientists from the early 1970s. In this paper we summarise, and review available geological and isotopic data, and report results of new isotopic studies (Sm-Nd, Pb-Pb, and U-Pb SHRIMP analyses); field geological data obtained during the PCMEGA 2002/2003 are utilised. The structure of the region is described in terms of four tectonic terranes. Those include Archaean Ruker, Palaeoproterozoic Lambert, Mesoproterozoic Fisher, and Meso- to Neoproterozoic Beaver Terranes. Pan-African activities (granite emplacement and probably tectonics) in the Lambert Terrane are reported. We present a summary of the composition of these terranes, discuss their origin and relationships. We also outline the most striking geological features, and problems, and try to draw attention to those rocks and regional geological features which are important in understanding the composition and evolution of the PCM and might suggest targets for further investigations

Variations in the geochemistry of magmatism on the East Pacific Rise at 10?30'N since 800 ka

Samples of volcanic rock, collected from the flanks of the East Pacific Rise at 10°30'N, were used to investigate changes in the geochemistry of magmatism at the ridge axis, over the past 800 ka at this location. We show that there have been large variations in the major element chemistry of the lavas erupted at the spreading axis on this ridge segment over this period. For example, the average MgO content of lavas erupted at the ridge axis increased from about 3.0% at 600 ka, to about 7.0% at 300 ka. Since 300 ka the average MgO content has systematically decreased, and the average MgO content of lavas collected from within the neovolcanic zone at 10°30'N is 6.0%. These temporal changes in major element chemistry are not accompanied by systematic changes in isotope composition or incompatible trace element ratios, and are interpreted to reflect changes in the average rate of supply of melt to the ridge axis during this period. The data support previous arguments that changes in melt supply rate over periods of 100-1000 ka have an important influence on the major element chemistry of the lavas erupted at fast spreading ridges. At 10°30'N, the melt supply rate appears to have been relatively low for much of the past 800 ka. Samples younger than 50 ka, collected from within 3 km of the ridge axis at 10°30'N (inside the neovolcanic zone), have a smaller range in major element chemistry compared to the samples dredged from the ridge flanks. Variations in the chemistry of lavas erupted over periods of less than about 100 ka may be controlled by the geometry of the magma plumbing system beneath the ridge axis