Petrogenesis of the Tanzawa plutonic complex, central Japan: Exposed felsic middle crust of the Izu–Bonin – Mariana arc

The Miocene Tanzawa plutonic complex, consisting mainly of tonalite intrusions, is exposed at the northern end of the Izu–Bonin – Mariana (IBM) arc system as a consequence of collision with the Honshu Arc. The Tanzawa plutonic rocks belong to the calc-alkaline series and exhibit a wide range of chemical variation, from 43 to 75 wt% SiO₂. They are characterized by relatively high Ba/Rb and Ce/Nb ratios, and low abundances of K₂O, LIL elements, and rare earth elements (REE). Their petrographic and geochemical features indicate derivation from an intermediate parental magma through crystal fractionation and accumulation processes, involving hornblende, plagioclase, and magnetite. The Tanzawa plutonic complex is interpreted to be the exposed middle crust of the IBM arc, which was uplifted during the collision. The mass balance calculations, combining data from melting experiments of hydrous basaltic compositions at lower-to-middle crustal levels, suggest that parental magma and ultramafic restite were generated by dehydration partial melting (∼ 45% melting) of amphibolite chemically similar to low-K tholeiitic basalt. Partial melting of hydrated mafic lower crust might play an important role in felsic middle-crust formation in the IBM arc.     

Osumilite-bearing Granulites in the Eastern Grenville Province, Eastern Labrador, Canada: Mineral Parageneses and Metamorphic Conditions

Osumilite-bearing metasedimentary gneiss occurs in the contactaureole of the Sand Hill Big Pond gabbronorite complex of Labradorianage in eastern Grenville Province, eastern Labrador, Canada.The osumilite (Os) occurs in association with cordierite (Cd),orthopyroxene (Opx), sillimanite (Sil), sapphirine (Sa), spinel(Sp), K-feldspar (Kf), plagioclase (PI), phlogopite (Ph), hematite(Hm),magnetite (Mt), corundum (Co), and quartz (Q) in zones adjacentto the gabbronorite intrusion. The osumilite-in isogard is delineatedat a distance of 1-3km from the intrusive contact. The osumiliteis characterized by enhanced Mg/(Mg + Fe) ratios (0.89-0.92)and relatively high K₂O contents (4. 13-4.78 wt. % ) The compositionalvariation of the osumilite is best understood by the substitutions(Mg+Fe+Mn)+St=2A1 and (K+Na)+(Mg +Fe+Mn)=Al+vacancy. Symplectiticintergrowth of Opx-Cd-Kf-Q, which partly of completely replacedosumilite, is ubiquitously present. The stable osumilite-bearingassemblages (all with Hm, Mt, and P1) deduced from petrographicfeatures and from the phase relations in the KMAS system areOs-Sa-Cd, Os-Cd-Sil-Q, Os-Cd-Kf-Q, Os-Cd-Opx-Q, and Os-Opx-Kf-Q.The saphirine-bearing assemblages are restricted to silica-deficient(quartz-absent) zones of the gnesis, which include Sa-Os-Cd,Sa-Sil-Cd, Sa-Opx-Sp, Sa-Sp-Cd, and Sa-Sp-Co. Orthopyroxenecoronas mantling phologite reflect the breakdown reaction Ph+ Q= Opx+Kf+vapor under extremely low water activity in thevapour. Petrogenetic grids in MAS, KMAS, and KMAS-H₂O-CO₂appropriateto the mineral assemblages in the metasedimentary gneisses suggestthat the changes of the mineral assemblages in the area studiedreflect dehydration reactions Ph+Sil+Q=Cd+Kf+V and Ph+Q=Opx+Kf+V, and osumilite and sapphirine-forming reactions Opx+Cd+Kf+Q=Os,Cd+Kf+Q=Os+Sil+, Opx+Sil+Kf=Os+Sa, and Opx+Sil =Sa+ Cd. Relativelyhigh Mg/(Mg+Fe²⁺) (0. 64-0.88) in the whole-rock compositionand high oxygen fugacity (near hematite-magnetite buffer), togetherwith exceptionally high temperature ( 1000?C) and extremelylow water activity (0.2) at peak meta-morphic conditions mightstabilize the osumilite- and sapphirine-bearing assemblagesat middle or lower crustal levels. Relatively low water activityis probably caused by the relatively dry precursor, which hadbeen metamorphosed at upper amphibolite facies metamorphic conditionsbefore gabbronorite emplacement.     

A corundum–quartz assemblage from the Eastern Ghats Granulite Belt,India: evidence for high P–T metamorphism?

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.     

Reaction microstructures in corundum‐ and kyanite‐bearing mafic mylonites from the Takahama metamorphic rocks,western Kyushu,Southwest Japan

High‐grade mylonites occur in the Takahama metamorphic rocks, a member of the high‐pressure low‐temperature type Nagasaki Metamorphic Rocks, western Kyushu, Japan. Mafic layers within the mylonites retain reaction microstructures consisting of margarite aggregates armoring both corundum and kyanite. The following retrograde reaction well accounts for the microstructures in the CaO–Al₂O₃–SiO₂–H₂O system: 3Al₂O₃ + 2Al₂SiO₅ + 2Ca₂Al₃Si₃O₁₂(OH) + 3H₂O = 2Ca₂Al₈Si₄O₂₀(OH)₄ (corundum + kyanite + clinozoisite + fluid = margarite). Mass balance analyses and chemical potential modeling reveal that the chemical potential gradients present between kyanite and corundum have likely driven the transport of the CaO and SiO₂ components. The mylonitization is considered to take place chronologically after peak metamorphism and before the above reaction, based on the following features: approximately constant thickness of the margarite aggregates, random orientation of margarite, and local modification of garnet composition at a boudin neck that formed during mylonitization. The estimated peak temperature of 640°C and the pressure–temperature conditions of the above reaction indicate that the mylonitization took place at temperature between 530 and 640°C at pressures higher than 1.2 GPa, approximately equivalent to the depth of the lower crust of island arcs.     

Alteration of the groundwater thermal regime caused by advection

ABSTRACT

Groundwater temperature at an arbitrary depth and at an arbitrary point is determined not only by heat transported by conduction but also by advection caused either by infiltration of rain, snowmelt or irrigated water, or by seepage from surface water bodies. Therefore, characteristic changes of groundwater temperature are observed in recharging and discharging areas within a groundwater flow system. The changes may be one-, two-, or three-dimensional, depending on individual situations. Since heat is a conservative quantity in the subsurface environment, groundwater temperature can be used as a tracer to reveal the regional structure of a groundwater flow system. A case study showing the importance of groundwater temperature in a regional groundwater survey is presented taking Nagaoka plain, Japan, as an example. The groundwater temperatures were measured in observation wells with diameters of 65 to 250 mm and depths of 20 m or more. Marked seasonal changes in temperature depth profiles showing advective effects in the horizontal direction from the Shinano River, and in the vertical direction from upper and lower aquifers, were observed. The temperature depth profiles were classified into six types. The distribution of these types does not contradict the regional structure of the groundwater flow system revealed by the potential distribution. As groundwater temperature is an easily measureable element in a hydrological survey, the method described in the present paper is appropriate for a field study in an uninstrumented groundwater basin.     

Melting experiments on hydrous low-K tholeiite: Implications for the genesis of tonalitic crust in the Izu–Bonin – Mariana arc

A series of water-deficient partial melting experiments on a low-K tholeiite were carried out under lower crustal P–T–H₂O conditions (900–1200 °C, 0.7–1.5 GPa, 2 and 5 wt% H₂O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classified as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu–Bonin – Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust (∼ 50% melting at 1050 °C and below 1.2 GPa). Estimated densities of pyroxenitic restites (∼ 3.9 g/cm³) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm³). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900 °C).     

CHIME monazite dating as a tool to detect polymetamorphism in high‐temperature metamorphic terrane: Example from the Aoyama area,Ryoke metamorphic belt,Southwest Japan

Chemical Th–U–total Pb isochron method (CHIME) monazite dating was carried out for pelitic–psammitic migmatites and the Ao granite (one of the Younger Ryoke granites) from the Aoyama area, Ryoke metamorphic belt, Southwest Japan. The Ao granite gives an unequivocal age of 79.8 ± 3.9 Ma. The monazite grains in migmatites yield an age of 96.5 ± 1.9 Ma with rims and patchy domains of 83.5 ± 2.4 Ma. The 83.5 ± 2.4‐Ma overprinting on migmatites over the garnet–cordierite zone suggests a wide and combined effect of thermal input and fluid activity on the monazite grains caused by the contact metamorphism by the Younger Ryoke granites including the Ao granite. This contact metamorphism has not been detected from the major metamorphic mineral assemblage previously, possibly because the migmatites already possessed the high‐temperature mineral assemblage before the granite intrusions and were immune from contact metamorphism in terms of major metamorphic minerals. However, monazite records contact metamorphism clearly. Therefore, the field mapping of the CHIME monazite age is a powerful tool for recognition of polymetamorphism in high‐temperature metamorphic terrains where later thermal effects can not be easily detected by the growth of new major metamorphic minerals.