Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling P–T paths.An important new result of this study concerns the differencein shape of the P–T paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked P–T path withan interval of burial cooling (dP/dT –0·006 kbar/°C).The difference in the shape of the P–T paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of P–T paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated P–T paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; P–T path     

Petrology of Submarine Lavas from Kilauea's Puna Ridge, Hawaii

We have studied 30 quenched tholeiitic lava flows recoveredby 20 dredge hauls and one submersible dive along Puna Ridge,the submarine part of the East Rift Zone of Kilauea Volcano,Hawaii Glass grains from numerous additional flows were recoveredin turbidite sands cored in the Hawaiian Trough. These quenchedlavas document variable primary magma compositions; olivineand multiphase crystallization and fractionation; degassing;wall-rock stoping and assimilation; mixing in the crustal reservoirand the rift zone; entrainment of olivine xenocrysts from ahot, ductile, olivine cumulate body; and disruption of gabbrowallrocks in the rift zone. Glass grains in turbidite sands contain up to 15•0wt% MgO,in contrast to < 7•0wt% MgO for the sampled glass rindson lavas. The most forsteritic olivine phenocryst (F0₉₀₇) isin equilibrium with primary Kilauea liquid containing an average16•5 wt% MgO, but ranging from 13•4 to 18•4%.Lavas and glass grains have more restricted P₂O₅/K₂O and TiO₂/K₂Othan glass inclusions in olivine, because more diverse liquidstrapped as glass inclusions are mixed and homogenized beforeeruption. Variable trace element compositions in glass grainsand whole rocks indicate that the primary liquids form by partialmelting of mantle sources retaining clinopyroxene and garnet. Orthopyroxene xenocrysts formed at moderate pressures provideevidence for a sub-crustal staging zone. Chromite and olivinecrystallize in the crustal magma reservoir as the liquid coolsfrom an average 1346C to 1170C. Low viscosities of the primaryliquids (04 Pas) facilitate olivine settling, and the crystallizedolivine forms an olivine cumulate body at the base of the reservoir.Olivine is deformed as the hot ductile dunite body flows downand away from the summit. This flow drives instability of theHilina landslide on Kilauea. Dikes intrude the dunite, and magmaflowing through the dikes disaggregates and entrains olivinexenocrysts in Puna Ridge magmas. Primary liquids pond at or near the base of Kilauea's crustalreservoir because they are denser than more fractionated liquidsthat occupy the upper parts of the reservoir. The sulfur andwater contents of glass rinds indicate that fractionated liquidsnear the top of the reservoir degas at low pressure, a processthat increases their density and causes them to sink to levelswhere they mix with resident undegassed, near-primary liquid.The fractionated liquids near the top of the magma reservoiracquire excess Cl, owing to assimilation of hydrothermally alteredroofrocks. Magma flowing into the rift zone encounters and mixes with low-temperature,multiphase-fractionated melt. The mixed magmas typically containrare orthopyroxene, plagioclase as sodic as andesine, olivineas fayalitic as F0₇₅ and Fe-rich augite derived from the fractionatedmagma. Magma flowing through dikes also dislodged fragmentsof gabbroic wallrocks that occur as xenoliths. The interrelations in the Kilauean submarine lavas between hostglass and glass inclusion compositions, volatile contents andmineral chemistry reveal an extraordinarily complex sequenceof petrogenetic processes and events that are difficult or impossibleto determine in subaerial Kilauea lavas because of crystallization,reequilibration and degassing during or after their eruption. KEY WORDS: submarine lavas; petrology; Kilauea; Hawaii; magma mixing ^*Corresponding authorPresent address: Rosentiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA     

秘鲁南部阿雷基帕段海岸岩基带找矿方向探讨

通过详细的岩相学和锆石U-Pb年龄的研究,秘鲁海岸岩基带阿雷基帕段的填图工作得到了补充,该岩基带岩浆活动可分为7个阶段和4个旋回,各阶段都显示出不同的岩浆容量,其地貌主要表现为岩基和岩脉。旋回Ⅰ和旋回Ⅱ代表侏罗纪岩浆活动阶段（201~145Ma）,岩浆分异为辉长岩到花岗岩;旋回Ⅲ和旋回Ⅳ代表白垩纪岩浆,岩浆分异为英云闪长岩-二长花岗岩、闪长岩与花岗闪长岩及英云闪长岩-花岗闪长岩。对角闪石和黑云母矿物晶体的数量关系研究表明,旋回Ⅰ的侵入岩有大量辉石和角闪石结晶,并伴随金-铁成矿作用;旋回Ⅱ有大量的群集角闪石晶体和呈定向排列或群集的六边形黑云母,且伴随铜-金-铁成矿作用;旋回Ⅲ侵入岩内发育角闪石和黑云母,伴随着金-铜和铁成矿作用;旋回Ⅳ发育大量角闪石和六边形黑云母,岩石更偏长英质,并伴随铜-锌-金-银-铁和铜-金-钼成矿作用。     

The Belle Plaine III Meteorite

Abstract The Belle Plaine III meteorite is one of three meteorites, all of about the same size, found within a 2 mile strip southeast of Belle Plaine, Kansas. The meteorite weighs 23.9 kg, and consists of about 28% metallic minerals and 72% silicates. Nickel-iron, magnetite, hypersthene and olivene are the major constituents. The meteorite is an olivene-hypersthene chondrite.     

Withrow — a New Iron Meteorite from the State of Washington

Abstract. About 1950 an iron meteorite weighing 19 1/4 pounds was found a mile west of Withrow in Douglas County, Washington. The discovery site happens to be a little less than 5 miles SSE from that of the Waterville meteorite. However, the two irons are definitely not from a single fall. Withrow is a medium octahedrite showing secondary recrystallization of the type ascribed to cosmic heating. It was apparently picked up not more than a few years after it fell. Weathering is minimal, and effects of passage through the atmosphere are well preserved.     

Troilite Push-Out Pit on the Surface of a Freshly-Fallen Iron Meteorite

Abstract. The surface of the Withrow meteorite shows an unusual pit about two inches in diameter. This is explained as the result of pushing out of a plug of metal by expansion of an underlying troilite nodule.     

Melting Behavior of Simplified Lherzolite in the System CaO-MgO-Al2O3-SiO2-Na2O from 7 to 35 kbar

Phase equilibrium data have been collected for isobaricallyunivariant melting of simplified Iherzolite compositions inthe system CaO-MgO-Al₂O₃ SiO₂-Na₂O over a pressure range of7–35 kbar. These data permit the melting behavior of awide variety of model lherzolite compositions to be determinedquantitatively by algebraic methods. Two P-T univariant meltingreactions, corresponding to plagioclase to spinel lherzoliteand spinel to garnet lherzolite, are identified as peritectic-typetransitions and have positive Clapeyron slopes. The univariantcurves move to higher pressures and temperatures with increasingNa₂O in the liquid. The effect of the univariant curves on meltingis to produce low-temperature regions and isobarically invariantmelting intervals along lherzolite solidi. In the plagioclaselherzolite stability field, melting of four-phase model lherzoliteis pseudo-invariant, occurring over small temperature intervals(5C) and producing liquids that are quartz tholeiites at <8kbar and olivine tholeiites at >8 kbar. Calculated equilibriumconstants for plagioclase-liquid equilibria show both temperatureand pressure dependence. Plagioclase with anorthite content(AN) >90 mol%, as observed in some oceanic basalts, can crystallizefrom liquids with <1% Na₂O. Melting of spinel lherzoliteis not pseudo-invariant but occurs over large temperature intervals(15–60 C), producing a wide range in liquid compositions,from alkali basalts and alkali picrites at low to moderate degreesof melting (<1–10%) to olivine tholeiites and picritesat higher degrees of melting (>10%). On the basis of limiteddata in the garnet Iherzolite field, melts from garnet lherzoliteare more silica rich for a given degree of melting than meltsfrom spinel lherzolite, and liquid compositions trend towardenstatite with increase in pressure. Source fertility (especiallyNa₂O content) has a strong control on the temperature of meltingand liquid composition. Less fertile sources produce smalleramounts of liquids richer in normative silica. For certain bulkcompositions (high SiO₂ and low Al₂O₃), spinel is not a stablephase along the lherzolite solidus.