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The isovolume locus for a diamond-mineral inclusion pair occurs where the relative volumes of the two minerals respond identically to changes in pressure-temperature (P-T) conditions. Thirty potential inclusion minerals have been assessed with a simple linear model of this system. The remnant pressure on the inclusion (diamond at the Earth's surface) can be estimated by extrapolating the conditions of diamond formation to 0[degrees]C using the slope of the isovolume locus. Remnant pressures can be positive (isovolume locus lower than formation pressure) or negative (locus higher than formation pressure), the latter indicating the inclusion has decompressed completely. When placed in order of increasing isovolume slope, this mineral list defines a pressure preservation index (PPI). Published work confirms that the model is quantitative up to diamond formation pressures of 50 Kb and qualitative beyond. Ten minerals are identified as key PPI indicators--in decreasing order of PPI they are: sanidine, coesite, dolomite, (sphene, garnet, diopside, zircon), magnesiowustite, spinel, [Mg.sub.0.9][Fe.sub.0.1]Si[O.sub.3] perovskite. For most diamond formation conditions, inclusions of the first three minerals will retain high remnant pressures, whereas the last three will decompress completely. The central four minerals in brackets will have a highly variable response because their isovolume loci pass through diamond formation conditions. Preliminary calculations suggest that fluid inclusions ([H.sub.2]O, C[O.sub.2]) fall at or near the top of the list. For inclusion-bearing diamonds at the Earth's surface, the model predicts: (1) remnant pressures; (2) confining pressure on inclusion during delivery; (3) the stability of high-pressure polymorphs; (4) retention vs. resetting of radiometric ages; (5) those minerals suited for determining the conditions of formation based on measured remnant pressure; (6) those minerals capable of stabilizing a microdiamond inclusion; and (7) that supercritical fluids preserve inclusions of microdiamond in most minerals.

The enigmatic early Paleozoic tectonic setting of the Lachlan orogen in Australia has obscured reconstructions of east Gondwana. Recognition of an intraoceanic mafic island arc in this paper establishes the presence of a convergent plate boundary in the Tasmanides from the Early Ordovician to the Early Silurian. This in turn facilitates the updating of supercontinent correlations.

Abstract: Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (±100 °C and ±4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth''s surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin. [Copyright &y& Elsevier]

Raman spectroscopy is used to identify minerals in isolated sealed inclusion chambers in some New South Wales (Australia) diamonds and to determine the remnant internal pressure on each inclusion with values of each: 3.03, 3.31 and 3.54 - 3.62 GPa for coesite, 0.42, 1.36 and 2.3 GPa for diopsidic omphacite, and 0.8 GPa for grossular garnet, each determination for a different uncut stone. The diamond stress due to remnant pressure was found to be proportional to the maximum strain birefringence around each inclusion, as measured by the Berek compensator, with the constant (K = 1000 / 5.8, R2 = 0.93) independent of the inclusion mineral, size, orientation and shape within the limits of error. Thus, simple birefringence measurements of diamond adjacent to inclusions can be used to estimate remnant pressures on inclusions. The calibration shows that one of the stressed diamonds has an anomalously low strain birefringence. [ABSTRACT FROM AUTHOR]

Investigation of coarse (>2 mm) heavy-mineral concentrates from the White Hills Gravel near St Arnaud in western Victoria provides new evidence for the age and provenance of this widespread palaeoplacer formation. A prominent zircon - sapphire - spinel assemblage is characteristic of Cenozoic basaltic-derived gemfields in eastern Australia, while a single diamond shows similar features to others found in alluvial deposits in northeastern Victoria and New South Wales. Dating of two suites of zircons by fission track and U - Pb (SHRIMP) methods gave overlapping ages between 67.4 ± 5.2 and 74.5 ± 6.3 Ma, indicating a maximum age of Late Cretaceous for the formation. Another suite of minerals includes tourmaline (schorl - dravite), andalusite, rutile and anatase, which are probably locally derived from contact metamorphic aureoles in Cambro-Ordovician basement metapelites intruded by Early Devonian granites. U - Pb dating of rutile grains by laser ablation ICPMS gave an age of 393 ± 10 Ma, confirming an Early Devonian age for the regional granites and associated contact metamorphism. Other phases present include pseudorutile, metamorphic corundum of various types, maghemite and hematite, which have more equivocal source rocks. A model to explain the diverse sources of these minerals invokes recycling and mixing of the far-travelled basalt-derived suite with the less mature, locally derived metamorphic suite. Some minerals have probably been recycled from Mesozoic gravels through Early Cenozoic (Paleocene - Eocene) drainage systems during various episodes of weathering, ferruginisation and erosion. Comparison between heavy-mineral assemblages in occurrences of the White Hills Gravel may allow distinction between depositional models advocating either separate drainage networks or coalescing sheets. Such assemblages may also provide evidence for the present-day divide in the Western Uplands being the youngest expression of an old (Late Mesozoic - Early Cenozoic) stable divide separating north- and south-flowing streams or a much younger feature (ca 10 Ma) which disrupted mainly south-flowing drainage. [ABSTRACT FROM AUTHOR]

The Kabadah Formation outcrops in central New South Wales as a thrust package 66 km long, interleaved with Lower Silurian Canowindra Volcanics and situated between the Junee - Narromine and Molong Volcanic Belts of the Ordovician Macquarie Arc. The Kabadah Formation contains Early Silurian corals and Llandovery graptolites. Its provenance is complex, with detrital fragments of mafic - intermediate volcanic rocks, free crystals of pyroxene, chromite and ultramafic clasts, detrital volcanic quartz, garnet, and clasts of welded S-type rhyolitic volcanic rocks; and rare clasts from uplifted fold-belt rocks (granite and metamorphosed and deformed sediments). The variety of these clasts suggests that the Kabadah Formation records the Benambran collision of the Macquarie Arc with Ordovician quartz-rich sedimentary rocks, with detritus also derived from coeval Early Silurian mafic and felsic magmatism. The major source of detritus was from the short-lived emergent Fifield arc that formed from the subduction of an older backarc basin. The Kabadah Formation accumulated in an upward-shallowing Early Silurian marine basin between phases of the Benambran Orogeny. [ABSTRACT FROM AUTHOR]

The first recorded igneous activity of the now-fragmented Macquarie Arc in central New South Wales began in Early Ordovician (Lancefieldian - Bendigonian) times. In the westernmost Junee - Narromine Volcanic Belt, mafic to intermediate lavas of the Nelungaloo Volcanics, intruded by monzonite at 481 Ma, pass up into sandstone, conglomerate and siliceous siltstone of the Lancefieldian to early Bendigonian Yarrimbah Formation. Further east, in the northern Molong Volcanic Belt, the Mitchell Formation consists of volcaniclastic conglomerate, sandstone and siltstone with minor lavas and is succeeded conformably by volcaniclastic granule conglomerate, sandstone and siltstone of the middle - late Bendigonian Hensleigh Siltstone. This earliest phase of volcanism in the Macquarie Arc is dominated by high-K calc-alkaline to shoshonitic evolved basalt, basaltic andesite and andesite, but felsic lavas with SiO2 >63% are absent. Age-corrected εNd values for five lavas range from +6.2 to +7.8, indicating an absence of any old continental crustal component in the petrogenesis of these volcanic rocks. Together, these data suggest formation of the Macquarie Arc as an intra-oceanic arc, probably built on Cambrian oceanic crust. [ABSTRACT FROM AUTHOR]

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee - Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee - Narromine Volcanic Belt have ages that cluster in the 470 - 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley - Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee - Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley - Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian - Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian - Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee - Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the Junee - Narromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group, and their radiogenic isotope signatures show significant crustal involvement. [ABSTRACT FROM AUTHOR]