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Shoji Arai - One of the best experts on this subject based on the ideXlab platform.
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chemical homogeneity of high cr chromitites as indicator for widespread invasion of boninitic melt in mantle peridotite of bir tuluha ophiolite northern arabian shield saudi arabia
Ore Geology Reviews, 2017Co-Authors: Abdel Monem Habtoor, Ahmed H Ahmed, Norikatsu Akizawa, Hesham M Harbi, Shoji AraiAbstract:Abstract The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by Dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and Dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and Dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the Dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in Dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated Dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.
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sulfide rich Dunite within a thick moho transition zone of the northern oman ophiolite implications for the origin of cyprus type sulfide deposits
Lithos, 2013Co-Authors: Hironori Negishi, Akihiro Tamura, Shoji Arai, Satoko Ishimaru, Hisayoshi Yurimoto, Shoichi Ito, Norikatsu AkizawaAbstract:Peculiar Dunites, in part wehrlitic, that contain up to 3 vol.% sulfides from a thick (~ 1000 m) Moho transition zone (MTZ) are found along Wadi Thuqbah in the northern Oman ophiolite. We discuss their relevance to the formation of Cyprus-type massive sulfide deposits near the surface. Field observations suggest that the sulfide-rich MTZ Dunites are of late-intrusive origin. The sulfides form composite grains with magnetite and form angular clasts, which are enclosed or cut by magnetite. The sulfide part is composed of homogeneous pyrrhotite and vermicular intergrowth of pyrrhotite and pentlandite. Sulfide inclusions in clinopyroxene comprise pyrrhotite with pentlandite blebs, free of magnetite. Olivines in the sulfide-rich Dunite characteristically show low NiO contents (0.1–0.3 wt.%) relative to a high Fo value (~ 91), and as such they do not lie on a Fo–NiO trend of ordinary sulfide-free MTZ Dunites–wehrlites. This low-Ni olivine was precipitated from a high-Mg magma that had segregated Ni-rich sulfide melts. The pentlandite–pyrrhotite intergrowth was formed by subsolidus exsolution at low temperatures (< 200 °C) from high-temperature mono-sulfide solid solution. Iron released from olivine during serpentinization produced magnetite, which was combined with the sulfides to form the composite grains. In-situ S isotope ratios of the sulfides (δ34S = 0.7–2.8) are slightly higher than mantle values but lie within the range for magmas from oceanic island arcs, such as the Marianas. The δ34S are lower than those for sulfate from seawater and MORB-related sulfides, such as TAG (Trans-Atlantic Geotraverse) deposits. One of the Cyprus type massive sulfide deposits (Aarja) from the crustal section of the same area shows similar S isotope ratios to the sulfides in the Thuqbah sulfide-rich Dunites/wehrlites, indicating their genetic linkage. The Aarja sulfide deposit was formed within the V2 lavas, which are relatively sulfur-rich and of an off-axis origin, as a result of high-temperature seawater circulation. The Thuqbah sulfide-rich Dunite possibly represents an igneous root of the Cyprus-type massive sulfide deposit of Aarja formed in an off-ridge magmatic-hydrothermal system.
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Possibility of titanium transportation within a mantle wedge: formation process of titanoclinohumite in Fujiwara Dunite in Sanbagawa belt, Japan
2012Co-Authors: Satoko Ishimaru, Shoji AraiAbstract:Abstract. Titinoclinohumite-bearing Dunites from Fujiwara, the Sanbagawa metamorphic belt of high-pressure type, Japan, were described to examine the possibility of Ti mobility during metasomatism within the mantle wedge. The Fujiwara Dunite body and surrounding high-pressure Sanbagawa schists possibly form a subduction complex, and the Dunites are a good analogue to the mantle wedge overlying the slab. The Fujiwara Dunites are of deserpentinization origin; the deserpentinized olivine is high in Fo (up to 96) and low in NiO (0.2 to 0.3 wt %), and contains magnetite inclusions. Titanoclinohumites are associated with the deserpentinized olivine, as lamellar intergrowth or veinlets, up to 1 cm in width. Other metamorphic minerals include antigorite, brucite, chlorite, ilmenite, perovskite, Ti-rich ludwigite, and carbonates. The protolith of the Fujiwara Dunite was partially serpentinized cumulative Dunites from intra-plate magma, containing relatively low-Fo (85 to 86) olivines and TiO2-rich (up to 3 wt %) chromian spinels. The metamorphic olivines and titanoclinohumites contain micro-inclusions of methane (CH4) with or without serpentine and brucite. The source of Ti for titanoclinohumite was possibly the Ti-rich chromian spinel, but Ti was mobile through hydrocarbon-rich fluids, which were activated during the metamorphism. The hydrocarbons, of which remnants are carbonates and methane micro-inclusions, were derived from carbonaceous materials or bitumen, possibly incorporated in the precursory serpentinized and brecciated peridotite (= the protolith for the Fujiwara Dunites) before subduction. Ti can be mobile in the mantle wedge if hydrocarbons are available from the subducted slab.
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petrology of Dunite harzburgite with decimeter scale stratification in a drill core from the tari misaka ultramafic complex southwestern japan
Journal of Mineralogical and Petrological Sciences, 2001Co-Authors: Ichiro Matsumoto, Shoji AraiAbstract:A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of Dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe3+ contents, and slightly lower in Cr/(Cr+Al) ratio in Dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to Dunite spinel near (usually within 5 cm) the lithological between Dunite. It was presumed that the Dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt. The petrological and chemical features of this part of the drill is basically similar to those in Dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the Dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed Dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by Dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.
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Petrology of Dunite/harzburgite with decimeter-scale stratification in a drill core from the Tari-Misaka ultramafic complex, southwestern Japan
Journal of Mineralogical and Petrological Sciences, 2001Co-Authors: Ichiro Matsumoto, Shoji AraiAbstract:A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of Dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe3+ contents, and slightly lower in Cr/(Cr+Al) ratio in Dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to Dunite spinel near (usually within 5 cm) the lithological between Dunite. It was presumed that the Dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt. The petrological and chemical features of this part of the drill is basically similar to those in Dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the Dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed Dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by Dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.
Othmar Muntener - One of the best experts on this subject based on the ideXlab platform.
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melt peridotite interaction in the southern lanzo peridotite field textural and geochemical evidence
Lithos, 2007Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
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Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence
Lithos, 2006Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
Paul T. Robinson - One of the best experts on this subject based on the ideXlab platform.
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Petrogenesis of the Kangjinla peridotite in the Luobusa ophiolite, Southern Tibet
Journal of Asian Earth Sciences, 2011Co-Authors: Jingsui Yang, Guolin Guo, Paul T. RobinsonAbstract:Abstract The Kangjinla peridotite in the eastern part of the Luobusa ophiolite consists mainly of fresh harzburgite with less abundant lherzolite and Dunite. The generally depleted nature of the rocks and minerals suggests that they are the residue of partial melting of MOR-like mantle. However, all of the peridotites show some degree of LREE enrichment which is attributed to modification by subduction-related fluids. The principal minerals in the peridotites typically show two stages of mineral growth. Early stage olivine and pyroxene typically form large grains with kink banding and wavy extinction; the pyroxene also shows kinking of twin lamellae. Later stage minerals are normally smaller in size and occur along fractures and cleavage planes within or between the early stage minerals. The late stage minerals show no evidence of deformation. Early stage orthopyroxene and clinopyroxene have higher Al2O3 and Cr2O3 than the later varieties. Fo values of olivine are 90–92 in harzburgite and lherzolite and 92–94 in Dunite. The late-stage olivine has higher Cr2O3 and NiO than the early stage grains. The Cr numbers (Cr# = (Cr × 100)/(Cr + Al)) of chrome spinel in the mantle peridotites are between 30 and 77, being lowest in the lherzolites and highest in the Dunites. On the Cr# vs. Mg# (=(Mg × 100)/(Mg + Fe2+)) diagram the lherzolites and clinopyroxene harzburgites plot in the abyssal peridotite field, whereas the harzburgites and Dunites plot in the island-arc peridotite field. Our preferred explanation for these compositional features is that the mantle peridotite formed in a MOR environment, then was modified altered by later-stage melts and fluids in a suprasubduction zone (SSZ) setting. The Kangjinla peridotites host a number of small podiform chromitites with a wide range of textures, including massive, disseminated, layered, nodular and anti-nodular. The chromitites have uniformly high Cr# (75.6–82.7) and moderately high Mg numbers (56.4–74.1), similar to that elsewhere in the Luobusa ophiolite. Many of the textures suggest precipitation from mafic magma but the chromitites contain a variety of ultrahigh pressure (UHP) minerals that indicate crystallization at depths >120 km.
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REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet
Journal of Petrology, 2004Co-Authors: Meifu Zhou, John Malpas, Paul T. Robinson, Stephen J. EdwardsAbstract:The Luobusa ophiolite, Southern Tibet, lies in the Indus– Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and Dunite. The harzburgite contains abundant pods of chromitite, most of which have Dunite envelopes, and the cpx-bearing harzburgites host numerous Dunite dykes. Dunite also exists as a massive unit similar to those of the mantle–crust transition zones in other ophiolites. All of the Dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing Dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the Dunite. These features are consistent with formation of the Dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone Dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the Dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with Dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone Dunites.
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podiform chromitites in the luobusa ophiolite southern tibet implications for melt rock interaction and chromite segregation in the upper mantle
Journal of Petrology, 1996Co-Authors: Meifu Zhou, John Malpas, Paul T. Robinson, Zijin LiAbstract:The Luobusa ophiolite in the Indus—ladling Zjingbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of Dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100Mg/(Mg + Fe) ] ranging from 89 to 91 and show flat, unfractionated, chondrite-normaliied platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr + Al)] (1866). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have Dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with high cv-numbers (74-82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock reaction removed pyroxene from the peridotites and precipitated olivine, forming Dunite envelopes around the chromitite pods. The melts thus became more boninitic in composition and chromite saturated, leading to precipitation of chromite alone. The interplay of melt—rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition, producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.
Giovanni B. Piccardo - One of the best experts on this subject based on the ideXlab platform.
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melt peridotite interaction in the southern lanzo peridotite field textural and geochemical evidence
Lithos, 2007Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
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Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence
Lithos, 2006Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
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RECENT RESEARCHES ON MELT-ROCK INTERACTION IN THE LANZO SOUTH PERIDOTITE
Ofioliti, 2005Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Gianluca Spagnolo, Eugenio PoggiAbstract:New field, petrographic-structural and petrologic-geochemical investigations have been dedicated to the spinel and plagioclase peridotites cropping out at Mt. Arpone and Mt. Musine, within the Southern Body of the Lanzo Ophiolitic Massif (Western Alps). Field mutual relationships indicate that coarse granular, “reactive” spinel peridotites formed at the expense of pristine lithospheric mantle protoliths. Most of the spinel peridotites were successively transformed in “impregnated”, plagioclase-rich peridotites. Metre-wide bands of granular “replacive” spinel harzburgites and Dunites and decametre- to hectometre-long bodies of granular “replacive” spinel Dunites cut across the plagioclase peridotites, together with some gabbroic dikelets and dikes. It is recognized that all these rock types were formed by melt-related processes: i) the reactive spinel peridotites originated by the diffuse percolation of silica-undersaturated melt increments which dissolved pyroxenes of the pristine lithospheric peridotites and crystallised new olivine; ii) the plagioclase peridotites originated by diffuse percolation of silica-saturated melt increments which crystallised interstitially within the peridotite, causing impregnation and refertilization of previous lithospheric as well as reactive spinel peridotites; iii) the replacive spinel harzburgite and Dunite bands and bodies were formed by the focused percolation of silica-undersaturated MORB-type liquids, which dissolved plagioclase and pyroxenes, along preferential channels within the plagioclase peridotites. Once formed, these channels were utilized by aggregate MORB melts to migrate to shallow crustal levels. This study sheds further light on the evolution of the melt dynamics (from single melt increments to aggregate MORB) and the migration mechanisms (from diffuse to focused porous flow to diking) during the progressive exhumation of this lithospheric mantle section, in response to the lithosphere extension leading to formation of the Jurassic Ligurian Tethys.
Alberto Zanetti - One of the best experts on this subject based on the ideXlab platform.
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Formation of Highly Refractory Dunite by Focused Percolation of Pyroxenite-Derived Melt in the Balmuccia Peridotite Massif (Italy)
Journal of Petrology, 2008Co-Authors: Maurizio Mazzucchelli, Alberto Zanetti, Giorgio Rivalenti, Daniele Brunelli, Elena BoariAbstract:A 50 m thick and 150 m long Dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle.The contacts with the host spinel-facies depleted lherzolite are sharp.The Dunite body is composed of spinel-rich Dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic Dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the Dunite body is fairly refractory (Fo in olivine: 90 7^93 8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the Dunites were generated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive Dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1 5 GPa and focused melt transport occurs at pressures greater than 0 7 GPa. The results show that, depending on the focusing factor assumed, Dunite can form from peridotite at P51 2 GPa and from pyroxenite at P51 1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the Dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich Dunites and the flow structures in the Cr-spinel layers.The proposed mechanism allows Dunite formation to occur well within the spinel stability field, and therefore at greater depth than Dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where Dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slowand ultraslow-spreading ocean ridges.
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melt peridotite interaction in the southern lanzo peridotite field textural and geochemical evidence
Lithos, 2007Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
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Melt/peridotite interaction in the Southern Lanzo peridotite: Field, textural and geochemical evidence
Lithos, 2006Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Othmar MuntenerAbstract:Abstract This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and Dunites crop out at Mt. Arpone and Mt. Musine. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and Dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and Dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm.
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RECENT RESEARCHES ON MELT-ROCK INTERACTION IN THE LANZO SOUTH PERIDOTITE
Ofioliti, 2005Co-Authors: Giovanni B. Piccardo, Alberto Zanetti, Gianluca Spagnolo, Eugenio PoggiAbstract:New field, petrographic-structural and petrologic-geochemical investigations have been dedicated to the spinel and plagioclase peridotites cropping out at Mt. Arpone and Mt. Musine, within the Southern Body of the Lanzo Ophiolitic Massif (Western Alps). Field mutual relationships indicate that coarse granular, “reactive” spinel peridotites formed at the expense of pristine lithospheric mantle protoliths. Most of the spinel peridotites were successively transformed in “impregnated”, plagioclase-rich peridotites. Metre-wide bands of granular “replacive” spinel harzburgites and Dunites and decametre- to hectometre-long bodies of granular “replacive” spinel Dunites cut across the plagioclase peridotites, together with some gabbroic dikelets and dikes. It is recognized that all these rock types were formed by melt-related processes: i) the reactive spinel peridotites originated by the diffuse percolation of silica-undersaturated melt increments which dissolved pyroxenes of the pristine lithospheric peridotites and crystallised new olivine; ii) the plagioclase peridotites originated by diffuse percolation of silica-saturated melt increments which crystallised interstitially within the peridotite, causing impregnation and refertilization of previous lithospheric as well as reactive spinel peridotites; iii) the replacive spinel harzburgite and Dunite bands and bodies were formed by the focused percolation of silica-undersaturated MORB-type liquids, which dissolved plagioclase and pyroxenes, along preferential channels within the plagioclase peridotites. Once formed, these channels were utilized by aggregate MORB melts to migrate to shallow crustal levels. This study sheds further light on the evolution of the melt dynamics (from single melt increments to aggregate MORB) and the migration mechanisms (from diffuse to focused porous flow to diking) during the progressive exhumation of this lithospheric mantle section, in response to the lithosphere extension leading to formation of the Jurassic Ligurian Tethys.
