The geochemistry and crustal origin of the Archaean acid intrusive rocks of the Agnew Dome, Lawlers, western Australia

Acid intrusive rocks of the Agnew Dome intrude a supracrustal sequence, part of the Lawlers-Mt. White greenstone belt. The acid intrusives form a complex batholith with multiple intrusions. Three distinct groups of granitoids have been recognised, both in terms of style of intrusion and on petrological grounds. The oldest intrusion was that of the Lawlers tonalite (granitoid group I) which includes a variety of rock-types from quartz diorite through tonalite to granodiorite. These are characterised by high Na/K, high concentrations of LREE, Ba and Sr, very high La/Yb, low HREE and low Y. Intrusion of the Lawlers tonalite pre-dates the first deformation of the intruded greenstones.A second granitoid suite (group II) occupies the dominant area of the batholith and comprises a diverse group ranging from granodiorite through adamellite to trondhjemite. These have lower Na/K and La/Yb ratios and a higher SiO2 content than the tonalites. Their intrusion pre-dates the main folding of the greenstones (D2).The third granitoid group (group III) comprises highly leucocratitic, true granites of near minimum-melt composition, K- and Rb-rich, but with very low concentrations of P, Zr, Ti, Sr and Ba. They have flat REE patterns with Eu-anomalies. These are clearly the latest intrusive phase and form a network of intersecting dykes, sheets and small plugs.On geochemical grounds the three granitoid suites of Lawlers are not related. The tonalites are likely to have been derived from the fusion of mafic source rocks, while some features of the leucogranites suggest source material must include a K-rich granite component. These features suggest that at least, in this part of the Yilgarn Block, the crust which pre-dates both the 2.65 Ga Lawlers tonalite and the Mt. White-Lawlers greenstone belt must have been of mixed character with mafic and sialic rocks very like the presently exposed Yilgarn Archaean suite

Tectonic Processes Along the Chile Convergent Margin

The Chile subduction zone, spanning more than 3500 km, provides a unique setting for studying, along a single plate boundary, the factors that govern tectonic processes at convergent margins. At large scale, the Chile trench is segmented by the subduction of the Chile Rise, an active spreading center, and by the Juan Fernández hot spot ridge. In addition, the extreme climatic change from the Atacama Desert in the north to the glacially influenced southern latitudes produces a dramatic variability in the volume of sediment supplied to the trench. The distribution of sediment along the trench is further influenced by the high relief gradients of the segmented oceanic lithosphere. We interpret new and reprocessed multichannel seismic reflection profiles, and multibeam bathymetric data, to study the variability in tectonic processes along the entire convergent margin. In central and south Chile, where the trench contains thick turbidite infill, accretionary prisms, some 50–60 km wide, have developed. These prisms, however, are ephemeral and can be rapidly removed by high-relief, morphological features on the incoming oceanic plate. Where topographic barriers inhibit the transport of turbidites along the trench, sediment infill abruptly decreases to less than 1 km thick and is confined to a narrow zone at the trench axis. There, all sediment is subducted; the margin is extending by normal faulting and collapsing due to basal tectonic erosion. The transition from accretion to tectonic erosion occurs over short distances (a few tens of km) along the trench. In the turbidite-starved northern Chile trench, ~1 km of slope debris reaches the trench and is subsequently subducted. There, tectonic erosion is causing pronounced steepening of the margin, associated pervasive extension across the slope and into the emerged coastal area, and consequent collapse of the overriding plate. The volume of subducting material varies little along much of the margin. However, the composition of the material varies from slope debris of upper-plate fragments and material removed from the upper plate by basal erosion, to turbidites derived from the Andes