Temperatures of Granulite-facies Metamorphism: Constraints from Experimental Phase Equilibria and Thermobarometry Corrected for Retrograde Exchange

This study assesses temperatures of formation of common granulitesby combining experimental constraints on the P–T stabilityof granulite-facies mineral associations with a garnet–orthopyroxene(Grt–Opx) thermobarometry scheme based on Al-solubilityin Opx, corrected for late Fe–Mg exchange. We appliedthis scheme to 414 granulites of mafic, intermediate and aluminousbulk compositions. Our findings suggest that granulites aremuch hotter than traditionally assumed and that the P–Tconditions of the amphibolite–granulite transition portrayedin current petrology textbooks are significant underestimatesby over 100°C. For aluminous and intermediate granulites,mean corrected temperatures based on our method are 890 ±17 and 841 ± 11°C, respectively (uncertainties reportedas 95% confidence limits on the mean), consistent with minimumtemperatures for orthopyroxene production by fluid-absent partialmelting in these bulk compositions. In contrast, mean temperaturesbased on Grt–Opx Fe–Mg exchange equilibria, usingthe same thermodynamic data, are 732 ± 22 and 723 ±11°C, respectively, well below the minimum temperaturesfor Opx stability. For mafic granulites, the mean correctedtemperature using our method is 816 ± 12°C, similarto the mean temperature of 793 ± 13°C from Fe–Mgexchange. Reasons for the differences between the mafic granulitesand aluminous–intermediate granulites are unclear butmay be due to the lower Al concentrations in Opx in the maficrocks and possible deficiencies in the thermodynamic modellingof these low concentrations. We discuss a number of well-knowngranulite terrains in the context of our findings, includingthe Adirondacks, the Acadian granulites of New England, theincipient charnockites of southern India and Sri Lanka, andthe Kerala Khondalite Belt. Our findings carry implicationsfor thermotectonic models of granulite formation. A computerprogram to perform our thermobarometry calculations, RCLC, isavailable from the Journal of Petrology website at http://www.petrology.oupjournals.orgor from the authors at http://www.geo.ucalgary.ca/~pattison/drm_pattison-rclc.htm. KEY WORDS: granulite-facies metamorphism; thermobarometry; garnet; orthopyroxene     

Mafic Pegmatites Intruding Oceanic Plateau Gabbros and Ultramafic Cumulates from Bolivar, Colombia: Evidence for a 'Wet' Mantle Plume?

The fault-bounded Bolívar Ultramafic Complex (BUC) onthe eastern fringes of the Western Cordillera of Colombia wastectonically accreted onto the western coast of South Americain the late Cretaceous–early Tertiary, along with pillowbasalts of the Caribbean–Colombian Oceanic Plateau (CCOP).The complex consists of a lower sequence of ultramafic cumulates,successively overlain by layered and isotropic gabbroic rocks.The gabbros grade into, and are intruded by, mafic pegmatitesthat consist of large magnesiohornblende and plagioclase crystals.These pegmatites yield a weighted mean ⁴⁰Ar–³⁹Ar step-heatingage of 90·5 ± 0·9 Ma and thus coincidewith the timing of peak CCOP volcanism. The chemistry of theBUC is not consistent with a subduction-related origin. However,the similarity in Sr–Nd–Pb–Hf isotopes betweenthe CCOP and the BUC, in conjunction with their indistinguishableages, suggests that the BUC is an integral part of the plume-derivedCCOP. The parental magmas of the Bolívar complex wereprobably hydrous picrites that underwent 20–30% crystallization.The residual magmas from this fractionation contained     

Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California

Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs.     

The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion havea range of textures and compositions, and among these are materialsrepresenting quenched Skaergaard magma. Pristine chilled marginalgabbro (CMG), however, is not ubiquitous at the intrusive contact,because many of the "contact" rocks have been hydrothermallyor metasomatically altered, contaminated with gneiss or olivinexenocrysts, while others contain accumulated minerals. Materialrepresenting quenched magma appears to be restricted to contactrocks that are texturally and mineralogically similar to diabase,and free of accumulated minerals. Where it exists, the CMG isfound within one to three meters of the exposed intrusive contactexcept at the roof of the intrusion where its thickness is greater.CMG was distinguished from the diverse group of contact rocksby petrographic and geochemical screening of over 80 specimens.Samples of CMG from the eastern and western margins and fromthe roof of the intrusion have relatively uniform compositionsimilar to that of ferrobasalt, and are noticeably richer iniron (mg-number=0?51-0?54), TiO₂ K₂O, and P₂O₅ than other unmodifiedcontact rocks. CMG's also have trace element compositions distinctfrom most other rocks in the outer Marginal Border Series (MBS).They have incompatible element contents up to 3–6 timesgreater than in LZa-type cumulates, negligible Eu anomalies,and Ni and Cr contents and Ni/Cr ratios that are among the lowestof rocks in the outer MBS. The results of melting experiments corroborate selection ofthis material as CMG. The composition of glasses obtained frompartial melting experiments of LZa-type cumulates are essentiallyidentical to those of the CMG. The 1-atm. liquidus phase relationsfor one of the CMG samples (KT-39) is largely consistent withthe sequence and composition of cumulus minerals observed withdistance inward through the MBS and upward through the LayeredSeries. Solidification of magma at the outer margin of the intrusionis interpreted to have involved locally efficient quench crystallizationfollowed by initial primocryst growth in an undercooled transitionzone a short distance inward that finally extended into regionsof near equilibrium crystallization. The similarity in composition between samples of chilled marginalgabbro from the exposed roof and sides of the intrusion, andthose of reconstituted trapped liquid from early cumulates inthe outer MBS suggests that a single magma, similar in compositionto ferrobasalt, was parental to the Skaergaard intrusion. Thisinterpretation corroborates geophysical evidence of a significantlysmaller mass for the intrusion than that estimated by Wager,and provides a basis for revision of models of its chemicalevolution. Samples chosen by Wager as chilled marginal gabbrobelong spatially, texturally, and compositionally to the groupof LZa-type cumulates in the MBS, and should no longer be regardedas chilled marginal gabbro.     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.     

华北强震断层面解和震源深度特征

本文利用远震P波及SH波和区域地震P_(nl)波波形反演方法,测定了华北9次强震的断层面解、震源深度及地震矩.结合其他作者测定的7次地震的结果,分析了华北16次强震断层面解及华北地震震源深度特征.结果表明,华北多数强震为走滑型地震,也有个别正断层型及逆断层型地震;16次地震多数发生在地壳中部10-25km深度范围内.     

Symplectitic augite: evidence for discontinuous precipitation as an exsolution mechanism in Ca-rich clinopyroxene

Abstract Part of the augite in the Artfjället gabbro consists of symplectitic intergrowths between augite and blebs or lamellae of orthopyroxene. Mineral compositions are consistent with formation of these symplectites by exsolution of orthopyroxene from magmatic augite at a temperature of ca . 900–1000°C. The microstructures indicate that the exsolution mechanism is discontinuous precipitation, whereby the boundary of an augite grain sweeps through a neighbouring augite, leaving the symplectite in its wake. The formation of this symplectitic augite is catalysed by the presence of an intergranular water-rich fluid phase, which promotes grain boundary mobility.     

Interpretation of grain-size distributions from measured sediment data, Platte River, Nebraska

Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material curve coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution.     

Deep marine arc apron deposits and syndepositional magmatism in the Alisitos group at Punta Cono, Baja California, Mexico

Rocks exposed at Punta Cono include very fine-grained to coarse-grained tuffs, lapilli tuffs, and tuff breccias deposited in a deep marine environment. Syndepositional basaltic intrusive activity was common. In one locality a hyaloclastite-peperite complex formed. Slumped sections with fluidal basalt ‘clasts’, derived from intrusions that entered the sediment pile from below, are present elsewhere. Abundant soft-sediment folds in fine-grained laminated subaqueous fall-out tuff suggest steep gradients; these are cut by shallow channels filled with coarse-grained tuff, lapilli tuff, and rare tuff breccia. The combination of marine fossils, extreme textural immaturity, abundant slump features, and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano. Recognition of magma-wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments. Products of magma-water-sediment interactions at Punta Maria include: (1) jigsaw-puzzle hyaloclastite, formed by non-explosive hydroclastic fragmentation of magma upon contact with water and water-bearing sediment; (2) peperites, produced by mixing of magma with sediment; and (3) an unusual tuff breccia unit, the result of non-explosive mixing of ‘wisps’of lava with sediment during remobilization of an unconsolidated section. Low-explosivity magma-water-sediment interactions are favoured by relatively high hydrostatic pressures in sub-wave base settings.