俯冲板片中微量元素的活动性:西天山榴辉岩相运输脉的制约

西天山的基性高压变质岩显示互相连接的榴辉岩相脉体网络来源于蓝片岩的进变质脱水作用,通过这些网脉可以洞察在俯冲带高压条件下远程流体流动中的流体-岩石相互作用和元素负载。岩相学证据表明外来流体的渗透作用产生运输脉,而流体持续的沟道式流动导致主岩蓝片岩的淋滤。脉和蓝片岩蚀变带中锂(Li)的含量几乎是蓝片岩主岩中锂的两倍,它支持流体是外部来源的设想。这些流体触发了蓝片岩主岩的榴辉岩化作用而形成蓝片岩蚀变带。由于成脉流体中的微量元素含量低,所以导致与流体发生化学反应的主岩中所有微量元素的强烈淋滤。在此过程中有53%~81%的微量元素被活化,与大离子亲石元素和轻稀土元素的损失相符合,它们的损失量几乎是重稀土元素和高场强元素损失量的两倍。     

Equilibration History of the Basel Alpine-Type Peridotite, Red Mountain, New Zealand

The Red Mountain alpine peridotie forms the basal, dominantlyharzburgitic tectonite portion of an ophiolite suite in SouthIsland, New Zealand. Olivine and pyroxene Mg/Fe compositionsare constant for individual lithologies, but generally increasethrough the series harzburgite, orthopyroxenite, harzburgiticdunite, dunite. An olivine-clinopyroxene dominated transitionalperidotite along the western margin of the mass has more Fe-richsilicates than in the harzburgitic suite. Fe-Mg silicate-spinelrelationships and the distribution of Al between coexistingpyroxene and spinel indicate nearly complete post-layering equilibration.A partial re-equilibration is suggested by narrow compositionalrims on pyroxenes and spinel. Relative to the mineral cores,the rims show enhanced partitioning of Al into spinel relativeto pyroxene. The Fe-Mg relationships between silicates and spinel,and the compositional variations from cores to rims of pyroxenesand spinels indicate that the rims formed at lower temperaturesthan the mineral cores. This conclusion is supported by theapplication of several geothermometers, which give average temperaturesof equilibration and partial re-equilibration of 1000–1070?C and 920–1030 ?C, respectively. Pyroxene overgrowthson olivine probably represent pre-equilibration cooling phenomena.Equilibration pressures cannot be estimated with precision becauseRed Mountain pyroxenes have Al contents that vary as a functionof whole-rock Al₂O₃, and other compositional variables, as wellas of T and P of equilibration. The lack of plagioclase in theharzburgite tectonites, and the wide range of (Al/Cr)_spinelindicate equilibration at fairly high pressures, probably atdepths within the 25–80 km range. The transitional peridotiteprobably formed by re-equilibration of residual crystals withbasaltic melt at shallower (<25 km) depths, and is evidencesupporting the conclusion that the ultramafic and mafic partsof the ophiolite suite at Red Mountain represent complementaryparts of the same melting event.     

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–YarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant 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 themantle–crust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such melt–rock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: melt–rock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet     

Holocene evolution of the Anichab Pan on the south-west coast of Namibia

Coastal sediment-filled depressions (pans) are one of the few areas that contain Quaternary records of sea-level and palaeoenvironmental change along the western margin of southern Africa. Anichab is a 128 km² salt-encrusted pan on the hyper-arid southern coast of Namibia with an emergent, well-preserved and in-place mid-Holocene mollusc assemblage. The molluscs are typical of subtidal sands on the sheltered side of offshore islands but include several warm-water species no longer found living along this coast. The Holocene evolution of the pan was largely influenced by changes in sea level and supply of sand along the coast. Calibrated radiocarbon ages of mollusc shells indicate a maximum Holocene sea level of ca 2 m above mean sea level (msl) from 7·0 to 6·3 ka and a return to near present-day sea level by 5·3 ka. The pan surface is 2 m below msl and has been emergent since 4·9 ka from the build up of sandy beaches and coastal dunes. A thin (1–4 cm) halite crust occurs over much of the pan surface but a layer of halite-cemented sand up to 40 cm thick is restricted to the central pan. Gypsum occurs near the subsurface brine interface and is limited by calcium to the edges of the pan. Nodules of calcite-cemented sand are forming in brackish, relatively high alkalinity subsurface waters in the south-east corner of the pan and nodules of aragonite-cemented sand are forming in brines 1 m below the central pan surface. Although modern dolomite has been reported from coastal lagoons of Brazil and Australia, carbonate cements are a minor feature of Anichab Pan and dolomite was restricted to a single reworked nodule most likely of Late Pleistocene age. Therefore, Anichab Pan does not appear to be a modern analogue to extensive, mixed-water dolomite cements found in Upper Pleistocene sediment-filled depressions on the Namibian shelf.     

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees

The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association