Rick Varne

Tasmanian Tertiary basalts, the Balleny plume, and opening of the Tasman Sea (southwest Pacific Ocean)

A seamount chain extending from the Balleny Islands to the East Tasman Plateau records the passage of the Australian and Antarctic plates over the Balleny plume. A poorly known seamount chain trending northeast from the East Tasman Plateau across the Tasman Sea to the western edge off the Lord Howe Rise represents a possible older trace of the plume. Late Cretaceous inception of this plume, and of another beneath Marie Byrd Land on the stationary Antarctic plate, may have been involved in the initiation of spreading at ∼80 Ma in the Tasman Sea and southwest Pacific Ocean. The Balleny plume isotopic and trace element signature, indicative of a high U/Pb mantle source, is recorded in Cenozoic Tasmanian basalts but is not present in the adjacent Victorian mafic lava-field province, located farther from the plume trace.

Ancient subcontinental mantle: A source for K-rich orogenic volcanics

Active island-arc volcanoes west of the arc/continent collision between the Sunda arc and northwest Australia are erupting mafic volcanics that range from calc-alkaline basalts through shoshonitic trachybasalts to leucitites. These mafic arc volcanics display a threefold enrichment in K, Rb, Sr, Ba, La, and Nb contents along the arc from Bali to Flores, increasing toward the collision zone. The enrichment is very well correlated with increasing 87Sr/86Sr and decreasing 143Nd/144Nd values. The geochemical and isotopic data imply that the arc volcanics are being generated from two source materials; one yielding material poor in K-group elements, with relatively low 87Sr/86Sr ( 0.707) values and low 143Nd/144Nd values. The K-rich material appearing in the arc volcanoes is being derived from ancient subcontinental mantle involved in the collision zone. Eastern Sunda K-rich mafic volcanism first appeared after the collision began. Before the collision, ancient northwest Australian mantle erupted K-rich, diamond-bearing ultramafics with high 87Sr/86Sr values and low 143Nd/144Nd values. These form part of a typical ultrapotassic continental volcanic association, yet they have Ba/La and Ba/Nb values characteristic of arc volcanism, showing their parent mantle to be geochemically and isotopically capable of supplying the K-rich material appearing in the eastern Sunda arc volcanoes. Terranes possibly underlain by similar “Gondwanaland” mantles surround the collision zone, which is the site of large-scale mixing between oceanic and continental lithospheres. Other petrogenetic theories seem incapable of accounting for the regional variations in eastern Sunda arc geochemistry. Striking geochemical and isotopic resemblances between eastern Sunda leucitites and K-rich orogenic volcanics elsewhere in the world, such as those of the Roman region, imply similar origins involving K-rich subcontinental mantle material. Where a “K-h” correlation occurs, it might only be a signal that subduction of a lithospheric slab beneath a continental margin is inducing melting in old mantle, deep in the continental lithosphere.

Crustal assimilation versus subducted sediment input in west Sunda arc volcanics: an evaluation

SummaryNew geochemical and Sr, Nd, and Pb isotopic analyses of Quaternary to Cretaceous sediments from the northeastern Indian Ocean are used to estimate the composition of the sedimentary material subducted along the Sunda Trench, and to evaluate the effects of crustal contamination versus subducted sediment input in the Quaternary volcanics of the west Sunda arc. Two sediment endmember components are identified: siliceousclastic (SS) and calcareous-organogenic (CS); the latter can be regarded as SS strongly diluted by organogenic material, mainly CaCO3. Siliceous-clastic sediments are characterised by a component isotopically and geochemically similar to “typical” average upper crust. Consequently, LILE and LREE concentrations, LILE/LILE and LILE/LREE values, and Sr, Nd, and Pb isotopic ratios in West Sunda arc volcanics cannot easily distinguish between assimilation of crustal material by uprising magmas and contamination of their mantle source by bulk sediments. Post-Miocene siliceousclastic sediments sampled in the vicinity of the Sunda arc are largely derived from the arc itself, and therefore should not be used to evaluate the extent of sediment contamination of Indonesian arc volcanics. In addition, geophysical evidence suggests that post-Miocene sediments are largely accreted rather than subducted, and existing10Be isotopic data imply that post-Miocene sediments are not recycled by present-day volcanism. As the least contaminated arc volcanics occur in the eastern section of the west Sunda arc, where the highest sediment fluxes have been calculated, subduction of pre-Miocene northeastern Indian Ocean sediments or incorporation of fluids released from them into the sources of west Sunda are volcanics seem unable to reproduce the range and spatial distribution of Sr, Nd, and Pb isotopic values in the arc volcanics. By contrast, assimilation of crustal material by uprising melts derived from Indian Ocean-type mantle wedge, up to a maximum of approximately 10% for the most contaminated arc volcanics, seems better able to account for the Sr, Nd, and Pb isotope systematics of the arc volcanics, and is consistent with the variations in crustal thickness and composition along the arc, and with the spatial distribution of Sr, Nd, and Ph isotope values in mafic arc volcanics. These conclusions are also supported by the low10Be coupled with high B/Be values, and by the positive B/Be-SiO2 and B/Be-87Sr/86Sr correlations in calc-alkaline Sunda arc volcanics.ZusammenfassungNeue geochemische und Sr-, Nd-, und Pb-Isotopenanalysen von quartären und kretazischen Sedimenten des nordöstlichen Indischen Ozeans werden dazu herangezogen, um die Zusammensetzung des entlang des Sunda-Tiefseegrabens subduzierten Sedimentmaterials und die Effekte von Krustenassimilation versus Subduktionsbedingtem Sedimenteintrag in die quartären Vulkanite des westlichen Sundabogens zu evaluieren. Zwei sedimentäre Endglieder wurden identifiziert: eine silikatischklastische (SS) und eine karbonatisch-organische (CS) Komponente, letztere kann als durch organisches Material, insbesondere durch CaCO3. stark verdünnte SS-Komponenete betrachtet werden. Silikatisch-klastische Sedimente sind durch eine Komponente charakterisiert, die isotopisch und geochemisch ähnlich der „typischen” durchschnittlichen Oberkruste ist. Daher erlauben LILE und LREE Konzentrationen, LILE/LILE und LILE/LREE Werte, sowie Sr-, Nd-, und Pb-Isotopenverhältnisse von Vulkaniten des West-Sundabogens keine einfache Unterscheidung zwischen Assimilation von Krustenmaterial durch aufsteigende Magmen und Kontamination ihrer Mantelquelle durch Sedimente. Post-miozäne silikatisch-klastische Sedimentproben, die in der Nähe des Sunda-Bogens genommen wurden, stammen großteils selbst vom Inselbogen und sollten daher nicht für eine Abschätzung des Ausmasses der Sedimentkontamination im indonesischen Vulkanbogens verwendet werden. Ferner gibt es geophysikalische Evidenz, die eher für Akkretion als Subduktion dieser Sedimente spricht und auch die10Be Isotopendaten zeigen, daß post-miozäne Sedimente nicht vom derzeitigen Vulkanismus rezykliert werden. Da die am geringsten kontaminierten Vulkanite im Ostabschnitt des Sunda-Bogens, von wo der höchste Sediment-Flux berechnet worden ist, zu finden sind, scheint die Subduktion prämiozäner Sedimente des nordöstlichen Indischen Ozeans oder die Beteiligung von aus diesen in die Quelle der Vulkanite Westsundas freigesetzten Fluiden, nicht auszureichen, um den Streubereich und die räumliche Verteilung von Sr-, Nd-, und Pb-Isotopenwerten in den Vulkaniten zu reproduzieren. Im Gegensatz dazu scheint Assimilation von Krustenmaterial - bis zu 10% für die am stärksten kontaminierten Vulkanite — durch aufsteigende Schmelzen, die aus dem „Indischen Ozean-Typ” Mantelkeil stammen, die Sr-, Nd- und Pb-Isotopensystematik der Vulkanite besser zu erklären. Sie ist auch konsistent mit der Variation der Krustendicke und Zusammensetzung entlang des Vulkanbogens und mit der räumlichen Verteilung der Sr-, Nd-, und Pb-Werte in den mafischen Vulkaniten. Diese Schlußfolgerungen werden durch die niedrigen10Be, gekoppelt mit hohen B/Be Werten und durch die positive B/Be-SiO2 und B/Be-87Sr/86Sr Korrelationen in den kalk-alkalischen Sunda-Vulkaniten, untermauert.

Macquarie Island: Its Geology, Structural History, and the Timing and Tectonic Setting of its N-MORB to E-MORB Magmatism

Macquarie Island is an exposure above sea level of the Macquarie Ridge Complex, on the boundary between the Australian and Pacific plates south of New Zealand. Geodynamic reconstructions show that at ca. 12-9.5 Ma, oceanic crust of the Macquarie Island region was created at this plate boundary within a system of short spreading-ridge segments linked by large-offset transform faults. At this time, the spreading rate was slowing (<10 mm/yr half-spreading rate) and magmatism was waning, Probably before 5 Ma, and possibly before the extinct spreading ridge had subsided, the plate boundary became obliquely convergent, and crustal blocks were rotated, tilted, and uplifted along the ridge to form the island. Planation by marine erosion has exposed sections through the oceanic crust. The magmatism that built the oceanic crust produced melts similar in composition to the widespread normal to enriched mid-oceanic-ridge basalt IN-to E-MORB) suite found in many spreading ridges, but the melts ranged beyond E-MORB to primitive, highly enriched, and silica-undersaturated compositions. These compositions form one end member of a continuum from MORB but seem not to have been derived from a MORB-source mantle, despite sharing a Paclfic MORB isotopic signature. The survival of these primitive melts may be due to their origin in a slow-spreading system that must have been closing down as extension along the plate boundary gave way to transpression, putting a stop to the upwelling of asthenosphere and decompression melting. In a more energetic. faster-spreading system, mixing would have been more efficient, the presence of this end member could not easily have been inferred from its isotopic composition, and the igneous rocks would have resembled a typical N-to E-MORB suite. Macquarie Island may therefore provide a type example of magmatism at a very slow spreading ridge and a clue to the origins of E-MORB.

Al-spinels in primitive arc volcanics

SummaryAl-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island arc volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the saine olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localized Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) in congruent breakdown of amphibole in amphibole-rich cumulates in sub-arc, or sub-OIB volcano magma chambers. The latter reaction produces a melt with ∼ 20–22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the saine reaction, are incorporated into the magma during recharge, and subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene.ZusammenfassungAl-reiche Spinelle (100Cr/(Cr + Al) < 5, Al2O3 > 50 Gew.%) sind in alpinen Peridotiten, in terrestrischen und lunaren mafischen und ultramafischen Kumulaten und in manchen metamorphen Gesteinen weit verbreitet, aber sie scheinen in terrestrischen, vulkanischen Gesteinen selten zu sein. Wir beschreiben hier das Vorkommen von Al-reichen Spinell-Inklusionen in Olivinkristallen von Inselbogen-Vulkaniten von 5 neuen Lokalitäten: Bukit Mapas (Sumatra) und Ost-Bali im Sunda-Bogen und die Inseln Epi, Merelava und Ambrym im Vanuatu-Bogen. Relativ Cr-reiche Spinelle kommen häufiger auch als Einschlüsse in denselben Olivin-Kristallen vor, und es scheint, daß Chrom-arme Aluminiumspinelle im Ungleichgewicht mit ihren basaltischen Mutterschmelzen stehen. In den untersuchten Gesteinen kommen Al-reiche Spinelle zusammen mit Silikatgläsern und Aluminium-reichen Klinopyroxenen in Schmelzeinschlüssen in Olivinen vor. Diese Assoziation weist auf einen Ursprung hin, der Kontamination durch lokalisierte Al-reiche “pockets” von Schmelze involviert; dies steht im Gegensatz zu einem Ursprung als Xenokristalle. Wir schlagen zwei Mechanismen vor, die diese Aluminium-reiche Schmelze in basaltischen Magmakammern erzeugen können: (1) lokalisierte Produktion von Aluminium-reicher Schmelze durch vollkommene Auflösung von assimilierten gabbroischen Gesteinen aus der unteren Kruste. In diesem Modell kann die Aluminium-reiche Schmelze Al-reiche Spinelle kristallisieren, die dann anschließend als feste Einschlüsse von Phenokristallen in der basaltischen Mutterschmelze eingefangen werden oder als Schmelzeinschlüsse, in denen Al-reiche Spinelle und Al-reiche Klinopyroxene als Tochterphasen kristallisieren. (2) Inkongruenter Zerfall von Amphibol in Amphibol-reichen Kumulaten in Magmakammern unter Inselbögen oder unter OIB-Vulkanen. Die letztgenannte Reaktion erzeugt eine Schmelze mit ungefähr 20–22% Al2O3, Aluminium-haltigen Klinopyroxen, Al-reichen Spinell und Olivin. Mischung zwischen diesen Produkten des Zerfalls von Amphibol und basaltischer Mutterschmelze kann während der ganzen Evolution eines magmatischen Systems stattfinden, aber besonders während der Zufuhr neuer heißer Magnesium-reicher Basalte. Aluminium-haltige Spinelle und Klinopyroxene, die während des Zerfalls von Amphibol entstanden sind oder vielleicht aus einer Aluminium-haltigen Schmelze in derselben Reaktion produziert wurden, werden während der Neuzufuhr in das Magma inkorporiert und im Anschluß daran, zusammen mit den koexistierenden Cr-Spinellen, von kristallisierendem Olivin und Klinopyroxen eingefangen.

Sumatran granitoids and their relationship to Southeast Asian terranes

Abstract Three subparallel, N-S-trending granitoid provinces occur in Southeast Asia: the Eastern Granitoid Province of peninsular Malaysia; the Central Granitoid Province, extending from northwestern Thailand to the western part of peninsular Malaysia and to the “Tin Islands” (Indonesia); and the Western Granitoid Province, in western Thailand and Burma. It has been asserted that each of these granitoid provinces is restricted to one of the Southeast Asian terranes defined using Gondwanan Palaeozoic stratigraphic relations and faunal distributions. Very little is known about possible continuations of these terranes in Sumatra, and here we use the “granite basement terrane” concept and the geochemical and isotopic compositions of Sumatran granitoids and fragmental rocks to extend the Southeast Asian terrane boundaries into Sumatra and investigate the nature of the Sumatran lithosphere. Granitoids west of the Semangko fault and within the basement of the Quaternary volcanic arc have low initial 87 Sr 86 Sr values ( Granitoids in eastern Sumatra, east of the Semangko fault, including those of the “Tin Islands”, of Bukit Batu close to Palembang, the Hatapang pluton and granodiorites and fragmental volcanics of the Lake Toba area, and possibly Sijunjung pluton in central Sumatra, all share high 87 Sr 86 Sr values and other S-type isotopic and compositional similarities, and seem to be related to the Central Granitoid Province S-type granitoids of the sibumasu terrane (= SIam-Burma-MAlaysia-SUmatra; Metcalfe, 1984). These similarities suggest that the granitoids define a “granite basement terrane” which may form the basement of the sibumasu terrane. They also imply that the magmatism of the Toba caldera was derived from the same crustal source. Granitoids that are geochemically and isotopically similar to those of the Western Province are not yet known in Sumatra, and granitoids similar to those of the Eastern Province are rare. The distribution of Sumatran granitoids suggests that the Semangko fault and the Sunda Strait may mark the southwesternmost and southeasternmost limits of the sibumasu terrane, and that the boundary between the Central and the Eastern Granite Provinces may run through the “Tin Islands”.

Tectonic setting of cambrian rifting, volcanism and ophiolite formation in western tasmania

Abstract In western Tasmania, small Precambrian regions are surrounded by a ramifying system of troughs filled with Cambrian sedimentary and volcanic rocks, and ophiolite complexes. The volcanic associations include a rift-related olivine tholeiite association, dacite-rhyolite and andesite associations, and a low-Ti, high-Mg andesite-tholeiite ophiolite association, and may have formed during a long-lived period of crustal thinning, punctuated by episodes of crustal rupturing, magmatism, and small scale rifting. Such extensional tectonism could occur in an active continental margin associated with strike-slip faulting of regional scale, and the volcanic associations may together constitute an igneous assemblage characteristic of magmatism in a transcurrent tectonic regime within an active continental margin undergoing break-up. The western Tasmanian Cambrian palaeogeography and volcanism preserve a transitional stage between the Late Proterozoic Kanmantoo regime of sedimentary basins with little volcanism developed at the rifting margin of the Proterozoic craton, and the tectonic regime of the Palaeozoic Lachlan Fold Belt where the Cambrian volcanic rocks are dominated by island-arc associations and the rift-related olivine tholeiite association is absent. Eastern Australian lithosphere may have grown by the insertion of newly-formed igneous complexes within the stretched and rifted continental margin, as well as by the accretion of “terrenes” and the addition of packets of subduction complexes which developed off-shore.

Al-rich spinel in primitive arc volcanics

SummaryAl-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island arc volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the saine olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localized Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) in congruent breakdown of amphibole in amphibole-rich cumulates in sub-arc, or sub-OIB volcano magma chambers. The latter reaction produces a melt with ∼ 20–22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the saine reaction, are incorporated into the magma during recharge, and subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene.ZusammenfassungAl-reiche Spinelle (100Cr/(Cr + Al) < 5, Al2O3 > 50 Gew.%) sind in alpinen Peridotiten, in terrestrischen und lunaren mafischen und ultramafischen Kumulaten und in manchen metamorphen Gesteinen weit verbreitet, aber sie scheinen in terrestrischen, vulkanischen Gesteinen selten zu sein. Wir beschreiben hier das Vorkommen von Al-reichen Spinell-Inklusionen in Olivinkristallen von Inselbogen-Vulkaniten von 5 neuen Lokalitäten: Bukit Mapas (Sumatra) und Ost-Bali im Sunda-Bogen und die Inseln Epi, Merelava und Ambrym im Vanuatu-Bogen. Relativ Cr-reiche Spinelle kommen häufiger auch als Einschlüsse in denselben Olivin-Kristallen vor, und es scheint, daß Chrom-arme Aluminiumspinelle im Ungleichgewicht mit ihren basaltischen Mutterschmelzen stehen. In den untersuchten Gesteinen kommen Al-reiche Spinelle zusammen mit Silikatgläsern und Aluminium-reichen Klinopyroxenen in Schmelzeinschlüssen in Olivinen vor. Diese Assoziation weist auf einen Ursprung hin, der Kontamination durch lokalisierte Al-reiche “pockets” von Schmelze involviert; dies steht im Gegensatz zu einem Ursprung als Xenokristalle. Wir schlagen zwei Mechanismen vor, die diese Aluminium-reiche Schmelze in basaltischen Magmakammern erzeugen können: (1) lokalisierte Produktion von Aluminium-reicher Schmelze durch vollkommene Auflösung von assimilierten gabbroischen Gesteinen aus der unteren Kruste. In diesem Modell kann die Aluminium-reiche Schmelze Al-reiche Spinelle kristallisieren, die dann anschließend als feste Einschlüsse von Phenokristallen in der basaltischen Mutterschmelze eingefangen werden oder als Schmelzeinschlüsse, in denen Al-reiche Spinelle und Al-reiche Klinopyroxene als Tochterphasen kristallisieren. (2) Inkongruenter Zerfall von Amphibol in Amphibol-reichen Kumulaten in Magmakammern unter Inselbögen oder unter OIB-Vulkanen. Die letztgenannte Reaktion erzeugt eine Schmelze mit ungefähr 20–22% Al2O3, Aluminium-haltigen Klinopyroxen, Al-reichen Spinell und Olivin. Mischung zwischen diesen Produkten des Zerfalls von Amphibol und basaltischer Mutterschmelze kann während der ganzen Evolution eines magmatischen Systems stattfinden, aber besonders während der Zufuhr neuer heißer Magnesium-reicher Basalte. Aluminium-haltige Spinelle und Klinopyroxene, die während des Zerfalls von Amphibol entstanden sind oder vielleicht aus einer Aluminium-haltigen Schmelze in derselben Reaktion produziert wurden, werden während der Neuzufuhr in das Magma inkorporiert und im Anschluß daran, zusammen mit den koexistierenden Cr-Spinellen, von kristallisierendem Olivin und Klinopyroxen eingefangen.

Al-spinels in primitive arc volcanics@@@Al-Spinelle in primitiven Inselbogen-Vulkaniten

SummaryAl-rich spinels (100Cr/(Cr + Al) < 5, Al2O3 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar mafic and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island arc volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the saine olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localized Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-Al melt in basaltic magma chambers are suggested: (1) localized high-Al melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-Al melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) in congruent breakdown of amphibole in amphibole-rich cumulates in sub-arc, or sub-OIB volcano magma chambers. The latter reaction produces a melt with ∼ 20–22% of Al2O3, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the saine reaction, are incorporated into the magma during recharge, and subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene.ZusammenfassungAl-reiche Spinelle (100Cr/(Cr + Al) < 5, Al2O3 > 50 Gew.%) sind in alpinen Peridotiten, in terrestrischen und lunaren mafischen und ultramafischen Kumulaten und in manchen metamorphen Gesteinen weit verbreitet, aber sie scheinen in terrestrischen, vulkanischen Gesteinen selten zu sein. Wir beschreiben hier das Vorkommen von Al-reichen Spinell-Inklusionen in Olivinkristallen von Inselbogen-Vulkaniten von 5 neuen Lokalitäten: Bukit Mapas (Sumatra) und Ost-Bali im Sunda-Bogen und die Inseln Epi, Merelava und Ambrym im Vanuatu-Bogen. Relativ Cr-reiche Spinelle kommen häufiger auch als Einschlüsse in denselben Olivin-Kristallen vor, und es scheint, daß Chrom-arme Aluminiumspinelle im Ungleichgewicht mit ihren basaltischen Mutterschmelzen stehen. In den untersuchten Gesteinen kommen Al-reiche Spinelle zusammen mit Silikatgläsern und Aluminium-reichen Klinopyroxenen in Schmelzeinschlüssen in Olivinen vor. Diese Assoziation weist auf einen Ursprung hin, der Kontamination durch lokalisierte Al-reiche “pockets” von Schmelze involviert; dies steht im Gegensatz zu einem Ursprung als Xenokristalle. Wir schlagen zwei Mechanismen vor, die diese Aluminium-reiche Schmelze in basaltischen Magmakammern erzeugen können: (1) lokalisierte Produktion von Aluminium-reicher Schmelze durch vollkommene Auflösung von assimilierten gabbroischen Gesteinen aus der unteren Kruste. In diesem Modell kann die Aluminium-reiche Schmelze Al-reiche Spinelle kristallisieren, die dann anschließend als feste Einschlüsse von Phenokristallen in der basaltischen Mutterschmelze eingefangen werden oder als Schmelzeinschlüsse, in denen Al-reiche Spinelle und Al-reiche Klinopyroxene als Tochterphasen kristallisieren. (2) Inkongruenter Zerfall von Amphibol in Amphibol-reichen Kumulaten in Magmakammern unter Inselbögen oder unter OIB-Vulkanen. Die letztgenannte Reaktion erzeugt eine Schmelze mit ungefähr 20–22% Al2O3, Aluminium-haltigen Klinopyroxen, Al-reichen Spinell und Olivin. Mischung zwischen diesen Produkten des Zerfalls von Amphibol und basaltischer Mutterschmelze kann während der ganzen Evolution eines magmatischen Systems stattfinden, aber besonders während der Zufuhr neuer heißer Magnesium-reicher Basalte. Aluminium-haltige Spinelle und Klinopyroxene, die während des Zerfalls von Amphibol entstanden sind oder vielleicht aus einer Aluminium-haltigen Schmelze in derselben Reaktion produziert wurden, werden während der Neuzufuhr in das Magma inkorporiert und im Anschluß daran, zusammen mit den koexistierenden Cr-Spinellen, von kristallisierendem Olivin und Klinopyroxen eingefangen.