ALBERT

Notes: Abstract Granitoid rocks of the compositionally zoned Late Cretaceous Toulumne Intrusive Suite in the central Sierra Nevada, California, have initial87Sr/86Sr values (Sri) and143Nd/144Nd values (Ndi) that vary from 0.7057 to 0.7067 and from 0.51239 to 0.51211 respectively. The observed variation of both Sri and Ndi and of chemical composition in rocks of the suite cannot be due to crystal fractionation of magma solely under closed system conditons. The largest variation in chemistry, Ndi, and Sri is present in the outer-most equigranular units of the Tuolumne Intrusive Suite. Sri varies positively with SiO2, Na2O, K2O, and Rb concentrations, and negatively with Ndi, Al2O3, Fe2O3, MgO, FeO, CaO, MnO, P2O5, TiO2, and Sr concentrations. This covariation of Sri, Ndi and chemistry can be modeled by a process of simple mixing of basaltic and granitic magmas having weight percent SiO2 of 48.0 and 73.3 respectively. Isotopic characteristic of the mafic magma are Sri=0.7047, Ndi=0.51269 andδ 18O=6.0, and of the felsic magma are Sri=0.7068, Ndi=0.51212 andδ 18O=8.9. The rocks sampled contain from 50 to 80% of the felsic component. An aplite in the outer equigranular unit of the Tuolumne Intrusive Suite apparently was derived by fractional crystallization of plagioclase and hornblende from magma with granudiorite composition that was a product of mixing of the magmas described above. Siliceous magmas derived from the lower crust, having a maximum of 15 percent mantle-derived mafic component, are represented by the inner prophyritic units of the Tuolumne Intrusive Suite.

Notes: Zusammenfassung Bamus und Ulawun überragen mit mehr als 400 m alle anderen Hauptvulkane im 1000 km langen Bismarck-Vulkanbogen. Beide Kegel liegen sich unmittelbar gegenüber und sind teilweise miteinander verbunden. Die enge Nachbarschaft, die Ähnlichkeit in der Morphologie und die gleiche Entfernung (70–160 km) von der New Britain Benioff-Zone können Anzeichen dafür sein, daß Bamus und Ulawun in Beziehung miteinanderstehen oder letztendlich die gleiche eruptive Geschichte haben. Aber es gibt wichtige Unterschiede zwischen beiden Vulkanen. Man nimmt an, daß Bamus zum letzten Mal zwischen 1878 und 1894 ausgebrochen ist, im Gegensatz dazu brach Ulawun 17 mal im späten neunzehnten Jahrhundert aus. Darüber hinaus sind die Gesteine von Bamus deutlich unterschieden von denen Ulawuns. Mafische Gesteine findet man im älteren Teil von Bamus (einschließlich eines Boninit-ähnlichen Gesteins oder magnesiumreiche Andesite und Lavaausflüsse). Die jüngeren Gesteine von Bamus sind magnesiumarme Andesite (die meisten der jüngsten Produkte haben die höchsten SiO2-Gehalte aller analysierten Gesteinsproben). Im Gegensatz dazu scheint Ulawun eine einheitliche Anhäufung von Basalten und einigen Andesiten zu sein, diese haben durchgehend geringere Gehalte an CaO und Zr verglichen mit den Bamus-Gesteinen. Es gilt als unwahrscheinlich, daß Ulawun-Basalte ursprünglich Magma repräsentieren, von dem die Bamus-Andesite durch einfache Kristallfraktionierung stammen, bei der Beurteilung der Ergebnisse von least squares Mischungsberechnungen und den relativ hohen Zr-Gehalten der Bamus-Andesite. Die magmatische Geschichte von Bamus und Ulawun scheint beziehungslos, aber da beide Vulkane die höchsten im Bismarck-Vulkanbogen sind, mögen beide für einen weiträumigen gravitativen Kollaps oder für ein calderaartiges Einsinken empfänglich sein. Nord- und ostverlaufende Lineamente und Steilhänge an beiden Vulkanen können tektonisch kontrollierte Merkmale früherer gravitativer Zusammenbrüche sein. Besonders Ulawun könnte in einem kritischen Stadium sein wegen seiner Höhe und seiner steilen Hänge, und aufgrund jener Ausbrüche, die 1978 von einer neuen ostverlaufenden (möglicherweise tektonisch kontrolliert) Kluft tief an der östlichen Flanke des Vulkans stattfanden.

Notes: Abstract New petrologic and geochemical data are presented for a suite of rocks from the Papuan Ultramafic Belt (PUB), Papua New Guinea. Tectonite harzburgites at the base of the ophiolite have extremely refractory, uniform mineralogy, and are exceptionally depleted in lithophile elements. These features are consistent with the proposed origin of these rocks as ‘depleted’ upper mantle, residual after extraction of a basaltic melt. The tectonite peridotites are overlain by a thick sequence of layered ultramafic and mafic cumulates containing olivine, orthopyroxene, clinopyroxene and plagioclase as the major cumulus phases. Early cumulates are characterized by magnesian olivine Mg90, orthopyroxene Mg90 and calcic plagioclase An86, and exhibit cryptic variation towards more iron-rich and sodic compositions. Abundances of ‘incompatible’ elements in the cumulates are extremely low which, together with the nature of the cumulus phases, points to a magnesian olivine-poor tholeiite or magnesian quartz tholeiite parent magma(s) strongly depleted in ‘incompatible’ elements. Highly fractionated iron-rich products of this parent magma type are represented by the LREE-depleted lavas in the overlying basalt sequence which, although resembling the most depleted mid-ocean ridge basalts (MORB) in terms of their low abundances of ‘incompatible’ elements, have higher abundances of transition metals and lower abundances of Ti, HREE and other high valence cations compared to common MORB of similar Mg/(Mg+Fe) ratio. Eocene tonalites intruding the PUB are genetically unrelated to the ophiolites, and appear to be related to the Ti-poor high-Mg andesites of Cape Vogel and similar andesites and dacites at the northern end of the PUB. These rocks are considered to represent the early stages of island-arc magmatism associated with a northeastward-dipping subduction zone in the early Eocene immediately prior to emplacement of the PUB.

Notes: Abstract Pleistocene to Recent stratovolcanoes in the Highlands of Papua New Guinea are made up of calc-alkaline to shoshonitic lava, tuff, agglomerate, ash, and lahar deposits. The volcanic rocks are characterized by high and variable Al, high K and total alkalis, and low Fe, Mg, and Ca. There is a continuous variation between high-K calc-alkaline, low-Si high-K calc-alkaline, and shoshonitic rocks. The elements V, Rb, Sr, Zn, Nb, and Ba are high relative to general andesitic abundances, particularly in the shoshonites. The Highlands volcanic rocks originated either in the base of thick sialic crust which is undergoing stabilization after major orogeny and uplift, or more probably, in eclogite sinking through the underlying mantle. Variation in content of K and other “incompatible” elements was either inherited from the source rocks in the base of the crust, or was produced by zone refining in a thick upper mantle zone containing interstitial fluid rich in these elements. Further variation, mainly in Fe/Mg, Si, and total alkalis, was caused by minor low-pressure crystal fractionation involving olivine, clinopyroxene, and, to a lesser extent, calcic plagioclase.

Notes: Abstract The layered cumulus rocks of the Marum ophiolite complex in northern Papua New Guinea range from highly magnesian dunite, wehrlite, and lherzolite through pyroxenite to norite-gabbro with minor anorthosite and ferronorite-gabbro near the top of the sequence. Most of the cumulates, particularly the gabbroic rocks, are characterised by recrystallised adcumulus textures and all intercumulus melt (mesostasis) has been expelled. Trends in the cumulate sequence from Mg-rich to more Fe-, Ca- and Al-rich compositions are consistent with the formation of the layered sequence by magmatic accumulation from mafic tholeiitic magmas with varying degrees of differentiation. The cumulates are characterised by extremely low levels of ‘incompatible’ elements (K, Ba, Rb, P, Zr, Nb, Hf, Y and REE) at all levels of differentiation. REE patterns are strongly depleted in LREE; HREE abundances range from ≦0.3 chondrites in peridotite to 3 x chondrites in the norite-gabbros. The Marum cumulates resemble low-Ti peridotites and gabbros found in other orthopyroxene-bearing ophiolite sequences. The parent magmas of the Marum cumulates are inferred to have been strongly depleted in ‘incompatible’ trace elements (∼ 2,000 ppm Ti, ∼20 ppm Zr, 6–9 x chondrites HREE with LaN/SmN∼0.5). These abundances are lower than found in typical MORB and back-arc basin basalts or their picritic parents. The dissimilarity of trace element abundances of the inferred Marum parent magmas with MORB-type high-alumina olivine tholeiites supports the conclusion drawn previously from the petrology of the cumulates that the parent magmas to the Marum ophiolite were not of MORB composition but resembled the strongly depleted, Ni-rich magnesian olivine-poor tholeiites and quartz tholeiites of the Upper Pillow Lavas of the Troodos ophiolite. The Marum parent magmas are believed to have been formed by shallow melting of refractory peridotite, and are chemically and genetically distinct from the LREE-enriched high-Ti lavas (Tumu River basalts) which occur in faulted contact. The geochemical data do not permit unequivocal assignment of a tectonic environment for the formation of either the Tumu River basalts or the plutonic suite; their juxtaposition results from thrust emplacement.

Notes: Abstract Bodies of pyroxene-bearing rhyodacite in New England, New South Wales, previously considered to be intrusions, are reinterpreted as ignimbritic flows. Small aggregates of pyroxene and plagioclase are inferred to be either compositionally modified crystalline residuals from partial melting or crystal cumulates. An earlier hypothesis involving mixing of solid biotite-diorite with a rhyolitic liquid is questionable, and the origin of the rhyodacite can be explained in terms of recent experimental work on crystal-liquid equilibria under inferred crustal conditions.

Notes: Abstract In the Lachlan Fold Belt of southeastern Australia, Upper Devonian A-type granite suites were emplaced after the Lower Devonian I-type granites of the Bega Batholith. Individual plutons of two A-type suites are homogeneous and the granites are characterized by late interstitial annite. Chemically they are distinguished from I-type granites with similar SiO2 contents of the Bega Batholith, by higher abundances of large highly charged cations such as Nb, Ga, Y, and the REE and lower Al, Mg and Ca: high Ga/Al is diagnostic. These A-type suites are metaluminous, but peralkaline and peraluminous A-type granites also occur in Australia and elsewhere. Partial melting of felsic granulite is the preferred genetic model. This source rock is the residue remaining in the lower crust after production of a previous granite. High temperature, vapour-absent melting of the granulitic source generates a low viscosity, relatively anhydrous melt containing F and possibly Cl. The framework structure of this melt is considerably distorted by the presence of these dissolved halides allowing the large highly charged cations to form stable high co-ordination structures. The high concentration of Zr and probably other elements such as the REE in peralkaline or near peralkaline A-type melts is a result of the counter ion effect where excess alkali cations stabilize structures in the melt such as alkali-zircono-silicates. The melt structure determines the trace element composition of the granite. Separation of a fluid phase from an A-type magma results in destabilization of co-ordination complexes and in the formation of rare-metal deposits commonly associated with fluorite. At this stage the role of Cl in metal transport is considered more important than F.

Notes: Abstract This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (≈1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al2O3 and CaO 〈 1%, Na2O≤0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ≠ 91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr2O3 (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ≠ 92–94 ) have comparatively high Cr2O3 (0.2%–0.45%) and Na2O (up to 0.18%) but very low Al2O3 contents (0.5%–0.7%). Diopsides (Mg ≠ 92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr2O3) and have low Al2O3 (0.7%–2.2%) and Na2O (0.5%–1.6%) contents. Many have high K2O contents, typically 0.1%–0.4% but up to 1.3% K2O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe2+)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr2O3, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr2O3) titanate resembling armalcolite but containing significant K2O (1%–2.5%), CaO (0.6%–2.2%), ZrO2 (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly 13C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (≥2 Ga) enrichment in incompatible elements.

Notes: Abstract Analyses of major and rare earth elements are presented for co-existing garnet, clinopyroxene, and amphibole from a Kakanui “eclogite”. New and previously published analyses of garnets suggest a gradual increase of Fe and decrease of Mg from xenocrysts through garnet pyroxene eclogitic rocks to amphibole-rich eclogitic rocks. Clinopyroxenes show a parallel increase in Fe/Mg ratio and an increase in Jadeite component and decrease in Tschermak's component. These data indicate crystallization of garnet and clinopyroxene from an alkali-rich undersaturated magma and are consistent with the concept of eclogite fractionation, but rare earth data allow severe constraints to be placed on this process. The eclogites are considered to be deep-seated crystallization products of nephelinite, but eclogite fractionation is small and cannot account for the association of alkali basalt, basanite and nephelinite.

Notes: Abstract Major and trace element data and mineral chemical data indicate that the range in rock types making up the Dunedin volcano has developed by crystal fractionation processes acting upon mantle derived basaltic magmas at various levels in the crust and upper mantle. A diversity among parental materials and the operation of the fractionation process at varying levels in the crust and mantle under varying conditions of pH2O have resulted in a diverse series of overlapping fractionation trends. ‘End member’ series are: basalt-hawaiite-mugearite-benmoreite; basanite-nepheline hawaiitenepheline mugearite-nepheline benmoreite; moderately potassic variants on these series. The phonolitic rocks of the volcano are low pressure differentiates derived by fractional crystallization, involving feldspar, as end member products in all the series outlined above. Quartz normative trachytes of the volcano appear to be differentiates from a distinct saturated or oversaturated magma series of different strontium isotopic and trace element characteristics from the undersaturated magma series.