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The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn-Pb-Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO.sub.2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatures of 880-940 °C, and Fe-Ti oxide equilibrium indicates oxygen fugacity of ca. FMQ + 1.5. Melt inclusions have high-SiO.sub.2 rhyolite compositions (> 75 wt%, on an anhydrous basis), and the plot of normative mineral compositions in the granitic triplot indicates low pressure of magma stalling and crystallisation (< 1 kbar) at cotectic compositions. Melt inclusion metal contents plotted vs incompatible element Y suggest contrasting behaviour of different metals during fractionation and degassing. Zinc was mostly retained in the melt during crystallisation, whereas other metals, such as Pb, Cu, Sn and Mo, were released to an exsolving fluid phase. The latter may have thus been transferred to the hydrothermal system from a degassing magma. Shallow storage of relatively hot magma would have induced vigorous hydrothermal circulation on the seafloor, a precondition for ore deposition., Author(s): Andrea Agangi [sup.1], Pearlyn C. Manalo [sup.1], Ryohei Takahashi [sup.1], Apivut Veeravinantanakul [sup.1], Marlina A. Elburg [sup.2] Author Affiliations: (1) grid.251924.9, 0000 0001 0725 8504, Graduate School of International [...]

Although large volumes of magma and fluid move through the crust, the sites where these volumes once resided are difficult to recognize. We show that some (foliation) boudinage and shear band structures, which are classically interpreted as resulting from externally imposed extension or shearing, are better explained by the ductile collapse of magma-or fluid-filled voids. Experiments show that the shape of the collapse structures depends on the original shape of the void and the direction of confluence of its wails when its contents are drained. This can lead to both symmetrical boudin-like and asymmetrical shear band-like structures. The recognition of the true nature of these structures necessitates a reevaluation of strain measurements reported in the literature and the tectonic models based on them. Identifying the 'missing volumes' clarifies the way magma makes granite plutons many cubic kilometers in volume in the upper crust.

Byline: Ronald J. Bakker (1), Marlina A. Elburg (2) Abstract: A set of Palaeozoic diopside--titanite veins are present in Mesoproterozoic metagranites and metasediments that constitute the basement (Mt Painter Inlier) of the Adelaide Fold Belt (South Australia). These massive veins (up to 1 m) of pegmatitic nature contain large crystals of diopside, LREE--Y-enriched titanite (up to 40 cm in length) and minor amounts of quartz. They can be used to trace the system's development from a high-temperature magmatic stage through to a massive hydrothermal event. The pegmatitic origin of these veins is evident from a complex fluid-melt inclusion assemblage, consisting of a highly saline inhomogeneous fluid and relicts of melt. Immiscibility of melt and heterogeneous highly saline fluids (exceeding 61 eq. mass% NaCl) is preserved in primary inclusions in diopside and secondary inclusions in titanite, indicating relatively shallow conditions of formation (510 +- 20degC and 130 +- 10 MPa). Graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites. The system evolved into hydrothermal conditions, which can be deduced from a later population of only fluid inclusions (homogeneous and less saline, [approximately equal to] 40 eq. mass% NaCl), trapped around 350 +- 20degC and 80 +- 10 MPa. During quartz crystallization, the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals, which were trapped at 200 +- 20degC and 50 +- 10 MPa. Brecciation and a palaeo-geothermal system overprinted the pegmatitic veins with an epithermal hematite--quartz assemblage and lesser amounts of bladed calcite and fluorite, in an intermittently boiling hydrothermal system of fairly pure [H.sub.2]O at 100--140degC and 1--5 MPa. Remobilization of LREEs and Y from titanite and/or the granitic host rock is evidenced by precipitation of apatite, allanite and wakefieldite in an intermediate stage. Occasional incorporation of radioactive elements or minerals, presumably U-rich, in the fluorite is responsible for radiolysis of [H.sub.2]O to H.sub.2. Author Affiliation: (1) Department of Applied Geosciences and Geophysics, Mineralogy and Petrology, University of Leoben, Peter-Tunner-Str. 5, 8700, Leoben, Austria (2) Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000, Ghent, Belgium Article History: Registration Date: 27/06/2006 Received Date: 15/10/2005 Accepted Date: 14/06/2006 Online Date: 01/08/2006 Article note: Communicated by J. Touret

A set of Palaeozoic diopside-titanite veins are present in Mesoproterozoic metagranites and metasediments that constitute the basement (Mt Painter Inlier) of the Adelaide Fold Belt (South Australia). These massive veins (up to 1 m) of pegmatitic nature contain large crystals of diopside, LREE-Y-enriched titanite (up to 40 cm in length) and minor amounts of quartz. They can be used to trace the system's development from a high-temperature magmatic stage through to a massive hydrothermal event. The pegmatitic origin of these veins is evident from a complex fluid-melt inclusion assemblage, consisting of a highly saline inhomogeneous fluid and relicts of melt. Immiscibility of melt and heterogeneous highly saline fluids (exceeding 61 eq. mass% NaCl) is preserved in primary inclusions in diopside and secondary inclusions in titanite, indicating relatively shallow conditions of formation (510 [+ or -] 20[degrees]C and 130 [+ or -] 10 MPa). Graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites. The system evolved into hydrothermal conditions, which can be deduced from a later population of only fluid inclusions (homogeneous and less saline, [approximately equal to] 40 eq. mass% NaCl), trapped around 350 [+ or -] 20[degrees]C and 80 [+ or -] 10 MPa. During quartz crystallization, the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals, which were trapped at 200 [+ or -] 20[degrees]C and 50 [+ or -] 10 MPa. Brecciation and a palaeo-geothermal system overprinted the pegmatitic veins with an epithermal hematite-quartz assemblage and lesser amounts of bladed calcite and fluorite, in an intermittently boiling hydrothermal system of fairly pure [H.sub.2]O at 100-140[degrees]C and 1-5 MPa. Remobilization of LREEs and Y from titanite and/or the granitic host rock is evidenced by precipitation of apatite, allanite and wakefieldite in an intermediate stage. Occasional incorporation of radioactive elements or minerals, presumably U-rich, in the fluorite is responsible for radiolysis of [H.sub.2]O to [H.sub.2].