Upper Quaternary vegetation dynamics and palaeoclimatology of the La Chonta bog area (Cordillera de Talamanca,Costa Rica)

Two pollen records from La Chonta bog (2310 m altitude) and one pollen record of a soil profile (2430 m altitude) at a short distance from the bog permit the reconstruction of the vegetational history and climatic sequence of probably the last ca. 80 000 yr of the Cordillera de Talamanca, Costa Rica. The three pollen records can be correlated on the basis of three radiocarbon-dated horizons (> 39 900 yr BP, 34 850 yr BP and 1390 yr BP) and palynostratigraphy. Pollen concentration data provide indirect time control between radiocarbon-dated horizons of the La Chonta 2 core. During the Early Glacial successive fluctuations of the upper forest line occurred between ca. 2300 m and 2800 m altitude and the bog is alternately situated close to the subalpine rainforest belt and in the uppermontane rainforest belt. During the Pleniglacial (ca. 50 000-13 000 yr BP) the forest line fell to ca. 2000 m altitude and paramo vegetation surrounded the La Chonta site. During the Late Glacial the forest line shifted rapidly in ca. 270 yr to 2700–2800 m altitude and the lake became surrounded by Quercus-dominated upper montane rainforest during a period of ca. 1655 yr. A distinct climate cooling (by 2–2.5°C) followed during ca. 580 yr and the upper forest line dropped to ca. 2400 m altitude. This temperature rebound is probably equivalent to the Younger Dryas event. In the depression extensive Alnus carr and swamp vegetation developed. Finally, during the Holocene, the forest line shifted to 3300–3500 m elevation and the lower montane rainforest reached close to ca. 2300 m altitude. In the later part of the Holocene subparamo scrub with Hypericum, Puya, Compositae, Escallonia and Ericaceae largely replaced Alnus carr and the regional and local vegetation closely resembles the presentday composition. During the Last Glacial to Holocene transition a shift of the upper forest line of ca. 1400 m is inferred, corresponding to a glacial temperature depression at 2300 m altitude of ca. 8°C.     

Holocene vegetation and fire dynamics in the supra‐mediterranean belt of the Nebrodi Mountains (Sicily,Italy)

High‐resolution pollen, macrofossil and charcoal data, combined with accelerator mass spectrometry ¹⁴C dating and multivariate analysis, were used to reconstruct Holocene vegetation and fire dynamics at Urio Quattrocchi, a small lake in the supra‐mediterranean belt in the Nebrodi Mountains of Sicily (Italy). The data suggest that after 10 000 cal a BP increasing moisture availability supported closed forests with deciduous (Quercus cerris, Fagus sylvatica and Fraxinus spp.) and evergreen (Quercus ilex) species. Species‐rich closed forest persisted until 6850 cal a BP, when Neolithic activities caused a forest decline and affected plant diversity. Secondary forest with abundant Ilex aquifolium recovered between 6650 and 6000 cal a BP, indicating moist conditions. From 5000 cal a BP, agriculture and pastoralism led to the currently fragmented landscape with sparse deciduous forests (Quercus cerris). The study suggests that evergreen broadleaved species were more important at elevations above 1000 m a.s.l. before ca. 5000 cal a BP than subsequently, which might reflect less human impact or warmer‐than‐today climatic conditions between 10 000 and 5000 cal a BP. Despite land use since Neolithic times, deciduous supra‐mediterranean forests were never completely displaced from the Nebrodi Mountains, because of favourable moist conditions that persisted throughout the Holocene. Reconstructed vegetation dynamics document the absence of any pronounced mid‐ or late‐Holocene ‘aridification’ trend at the site, an issue which is controversially debated in Italy and the Mediterranean region. Copyright © 2012 John Wiley & Sons, Ltd.     

Temperature variability and vertical vegetation belt shifts during the last ∼50,000 yr in the Qilian Mountains (NE margin of the Tibetan Plateau, China)

A 13.94-m-long sediment core, collected from a medium-sized lake in the Qilian Mountains (NE Tibetan Plateau, China), was analysed palynologically at 81 horizons. The interpretation of indicator taxa yielded various vertical shifts of the vegetation belts. These palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts. Our main findings are the following: A short period of the late Marine Isotope Stage 3 (around ∼46,000 yr ago) was characterized by interglacial temperature conditions with a tree line above its present-day altitude. During the LGM, the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts, indicating that the climate was colder by ∼4-7°C than today. Markedly higher temperatures were inferred from higher arboreal pollen frequencies between ∼13,000 and ∼7,000 yr ago with a Holocene temperature optimum and a maximal Picea-Betula mixed-forest expansion between ∼9,000 and ∼7000 yr ago, when temperatures exceeded the present-day conditions by at least 1-2°C. Alpine steppes and meadows and sub-alpine shrub vegetation dominated around the lake since the middle Holocene, suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods.     

Millennial-scale shifts in microtidal ecosystems during the Holocene: dynamics and drivers of change from the Po Plain coastal record (NE Italy)

Framed into a robust stratigraphic context, multivariate analyses on the Holocene palaeobiological record (pollen, benthic foraminifers, ostracods) of the Po coastal plain (NE Italy) allowed the investigation of microtidal ecosystems variability and driving parameters along a 35-km-long land–sea transect. Millennial-scale ecosystem shifts are documented by coeval changes in the meiofauna, reflecting variations in organic matter–water depth (shallow-marine environments) and degree of confinement-salinity (back-barrier settings). In-phase shifts of vegetation communities track unsteady water-table levels and river dynamics in freshwater palustrine areas. Five environmental–ecological stages followed one another crossing four tipping points that mark changes in relative sea level (RSL), climate and/or fluvial regime. At the culmination of Mediterranean RSL rise, after the 8200 event, remarkable growth of peatlands took place in the Po estuary, while low accumulation rates typified the shelf. At the transgressive–regressive turnaround (~7000 cal a bp ), the estuary turned into a delta plain with tidally influenced interdistributary embayments. River flow regime oscillations after the Climate Optimum (post-5000 cal a bp ) favoured isolation of the bays and the development of brackish wetlands surrounded by wooded peatlands. The youngest threshold (~800 cal a bp ), which led to the establishment of the modern delta, reflects a major avulsion of the Po River.     

Interaction between protokimberlite melts and mantle lithosphere: Evidence from mantle xenoliths from the Dalnyaya kimberlite pipe,Yakutia (Russia)

The Dalnyaya kimberlite pipe(Yakutia,Russia) contains mantle peridotite xenoliths(mostly Iherzolites and harzburgites) that show both sheared porphyroclastic(deformed) and coarse granular textures,together with ilmenite and clinopyroxene megacrysts.Deformed peridotites contain high-temperature Fe-rich clinopyroxenes,sometimes associated with picroilmenites,which are products of interaction of the lithospheric mantle with protokimberlite related melts.The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe.Coarse granular xenoliths fall on a geotherm of 35 mWm-2 whereas deformed varieties yield a 45 mWm-2)geotherm in the 2-7.5 GPa pressure interval.The chemistry of the constituent minerals including garnet,olivine and clinopyroxene shows trends of increasing Fe~#(=Fe/(Fe+Mg))with decreasing pressure.This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels.Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals,determined by LA-ICP-MS.Orthopyroxenes,some clinopyroxenes and rare garnets are depleted in Ba,Sr,HFSE and MREE and represent relic lithospheric mantle.Re-fertilized garnet and clinopyroxene are more enriched.The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene.Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets.Olivines show decreases in Ni and increases in Al,Ca and Ti from Mg-rich varieties to the more Fe-rich,deformed and refertilized ones.Minerals showing higher Fe~#(0.11-0.15) are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels.In P-f(O_2) diagrams,garnets and Cr-rich clinopyroxenes indicate reduced conditions at the base of the lithosphere at-5 log units below a FMQ buffer.However,Cr-poor clinopyroxenes,together with ilmenite and some Fe-Ca-rich garnets,demonstrate a more oxidized trend in the lower part of lithosphere at-2 to 0 log units relative to FMQ.Clinopyroxenes from xenoliths in most cases show conditions transitional between those determined for garnets and megacrystalline Cr-poor suite.The relatively low diamond grade of Dalnyaya kimberlites is explained by a high degree of interaction with the oxidized protokimberlite melts,which is greater at the base of the lithosphere.     

Factors controlling the heterogeneous distribution of Cr(VI) in soil, plants and groundwater: Evidence from the Assopos basin, Greece

The effective influence of industry or ultramafic rocks by natural processes to soil, plants and groundwater contamination by chromium, which is often a subject of debate, was investigated for the case of the Assopos basin, Greece. The Neogene Assopos basin, is mainly composed by Tertiary and Quaternary sediments of more than 400 m thick and is characterized by brittle type deformation (fault zones, faults). Chromium in soil, ranging from 67 to 204 ppm, is mostly hosted in chromite, Fe-chromite, Cr-bearing goethite and silicates.Special attention was given to the plants, which are a major source of organic matter that serves as the driving force for Cr(VI) reduction. The increase of the Fe, Mn and Ni contents, with the increasing Cr content in the plant-roots, in particular at the external parts of roots and those of bulb-type plants, suggest reduction and immobilization of Cr(VI) and that redox reactions play a significant role to the translocation processes from root to shoot.Groundwater samples from the Assopos aquifer showed a wide spatial variability, ranging from <2 to 180 ppb Cr_total content [almost same to the Cr(VI)-values] despite their spatial association. The presence of Cr(VI)-contaminated groundwater at depths >200 m is attributed to a direct injection of Cr(VI)-rich industrial wastes at depth rather than that Cr(VI) is derived from the Assopos river or by the interaction between water and Cr-bearing rocks. The heterogeneous distribution of Cr in groundwater may be related with the intense neotectonic deformation, as is exemplified by several sharp tectonic contacts between sediment types, while the Cr content in soil is mostly depend on the transported chromite grains.     

中国阿尔泰造山带花岗岩Pb同位素组成特征:幔源成因佐证及陆壳生长意义

为进一步对阿尔泰造山带花岗岩进行物源示踪研究,本文选择几个较典型的同造山和后造山不同类型的花岗岩以及相伴生的基性岩进行长石Pb同位素的测定。结果显示花岗岩206Pb/204Pb范围为17.997~18.921,平均值为18.269;207Pb/204Pb范围为15.460~15.599,平均值为15.528;208Pb/204Pb范围为37.661~38.262,平均值为37.954;其μ值为9.19~9.71,集中于9.30~9.60,与典型的壳源花岗岩明显不同。在源岩判别图解上,主要落在洋岛玄武岩和岛弧玄武岩的范围内,所有点远离上地壳、下地壳和深海沉积物,其源区性质类似于洋岛玄武岩和岛弧玄武岩,与花岗岩同时代的伴生基性岩Pb同位素也具有相似的特征,说明两者可能具有相似的物源特征,即幔源组分。这与报道的Sr、Nd同位素的特征相一致,进一步证明阿尔泰花岗岩具有幔源组分。这种特点与其他造山带(如华南、喜马拉雅)明显不同,显示阿尔泰花岗岩的特殊性。该研究从另一个侧面证明中亚造山带存在一定规模的显生宙陆壳生长。     

The Circle of Hydro-Hegemony between riparian states,development policies and borderlands: Evidence from the Talas waterscape (Kyrgyzstan-Kazakhstan)

Since the 1990s, transboundary water management has come to play a key role both in global environmental politics debates and in the shaping of international development policies, specifically in the Global South. As a consequence, a growing body of literature in the framework of critical hydropolitics has emerged reflecting on the role that power, discourses, and strategies play in shaping transboundary water policies and in influencing riparian relations. The focus on a state-centric perspective, however, often has led to neglect of the role of international development actors in shaping these policies. Through a critical application of the Circle of Hydro-Hegemony (CHH) and ethnographic qualitative field research in borderlands, this contribution aims to analyse how the establishment of a development initiative known as the Chu-Talas Commission, supported by the United Nations Economic Commission for Europe (UNECE) and other donors, has influenced and shaped transboundary water politics in the Talas waterscape, which is shared by Kyrgyzstan and Kazakhstan. The evidence shows that despite the international narration of the Chu-Talas Commission as a success story for water cooperation in Central Asia, Kazakhstan, through the deployment of both material and bargaining power strategies, has been able to shape UNECE development policies in its favour, impose its agenda on Kyrgyzstan, and emerge as the basin hydro-hegemon.     

~(207)Pb/~(206)Pb Zircon Dating of the Huangtuling Hypersthene-Garnet-Biotite Gneiss from the Dabie Mountains, Luotian County, Hubei Province, China: New Evidence for Early Precambrian Evolution

The Huangtuling hypersthene-garnet-biotite gneiss at Luotian County, Hubei Provine, is a typicalgranulite-facies rock of the Dabie Group Complex in the Dabie orogenic belt. Investigations on the morphology andoccurrence of zircons and their internal structures shown in the thin sections lead to the recognition of three types ofzircons, which are in good agreement with the types identified on the basis of morphology, colour and external fea-tures from the related zircon concentrates. The observation of zircons in the rock reveals that part of type 1 zirconsshow signs of a double-layered structure. The interval part existed in the protolith prior to the granulite-facies meta-morphism. Type 2, the prismatic zircons which mainly occur in garnet and hypersthene are metamorphic minerals ofthe granulite-facies metamorphism. Type 3, the round multifaceted zircons in felsic minerals and biotite, are proba-bly attributed to a later geological event related to migmatization. The ~(207)Pb/~(206)Pb zircon dating by direct evaporationon (thermal evaporation ion mass spectrometer) yields ages ranging from 2814 Ma to 1992 Ma. The age discrepancyamong these different zircon types is conspicuous. The yellow-brown(type 1) zircons give ages of 2814±29 Ma to2527±6 Ma, the prismatic euhedral zircons (type 2), 2456±7 Ma to 2254±4 Ma, and the round multifaceted zircons(type 3), 1992±10 Ma. The results are geologically interpreted in consideration of the complicated behaviours of zir-cons during Precambrian geological evolution of the Dabie area. (1) If the protolith of the gneiss is a sedimentaryrock, then type 1 zircons are clastic ones and the ages 2814±29 Ma and 2811±27 Ma may reflect the minimum age ofthe rocks of its source region. also the first geological event in the area. Sedimentation of the protolith occurred be-tween 2814 Ma and 2527 Ma, probably close to 2814 Ma. If the protolith is a volcanic rock, then the formation age ofthe supracrustal rocks of the Dabie Group Complex is around 2814 Ma. The age 2456±7 Ma reflects the time whenthe granulite-facies metamorphism took place. The later migmatization event is dated at aboat 1992±10 Ma, and isprobably the latest early Precambrian event in the area. The present work provides geochronological evidence for the existence of the Dabie Archaean craton, whichhad probably experienced 3 or 4 geological events during its early Precambrian evolution.     

Petrology and mineralogy of granulite-facies mafic xenoliths (Sardinia, Italy): Evidence for KCl metasomatism in the lower crust

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T = 850–900 °C, P = 0.8–1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56–72 mol%) + Fe–Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO₂ = 4.7–6.5 wt.%; F = 0.24–1.19 wt.%; Cl = 0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6–50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks.     

Sand dune migration as a factor of geoheritage loss: Evidence from the Siwa Oasis (Egypt) and implications for geoheritage management

Anthropogenic damage of geoheritage is documented widely, but natural processes can also lead to geoheritage loss. For instance, sand dune migration causes submergence of unique geological and palaeontological sites in desert environments of the Sahara. The Siwa Oasis in the Western Desert of Egypt boasts rich geoheritage, which is represented in many localities. Three of them in the southern part of the oasis are outcrops of highly-fossiliferous limestones. Palaeontological, sedimentary, palaeogeographical, and geomorphological types of geoheritage are recognized there. Sand dune activity on the study area is registered both visually and with remote sensing techniques. Denudation and destruction of naturally-exposed rocks is documented. Evidence of outcrop submergence with sand is found in all cases. The localities are situated in the pathway of rapid (up to ?10 m/yr) dune migration. One locality may disappear within one–two years. Sand dune migration has to be considered as a factor of geoheritage loss in the Siwa Oasis, and the relevant protection of the studied localities is necessary. Geopark creation and improvement of water use in the oasis can also help significantly, as well as the reference to archaeological experience of excavation and protection of heritage sites submerged by sands. More generally, geoheritage conservation should be integrated with a program for sustainable oasis development.     

Triassic Nappe in the Central Part of the Southern Central Asian Orogenic Belt (Ejinaq, NW China): Evidence from Structural Analysis and Geothermochronology

The thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt(CAOB). However, the timing, structural style and kinematic processes of the thrust nappe remain controversial, particularly the detail of the thrust nappe in the Guaizihu region(110 km east of Ejinaq). In this study, we investigate new field mapping, seismic sections, geochronology and low-temperature thermochronometric dating to provide constraints on the history of...     

South Equatorial Current (SEC) driven changes at DSDP Site 237, Central Indian Ocean, during the Plio-Pleistocene: Evidence from Benthic Foraminifera and Stable Isotopes

This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum–Uvigerina proboscidea (Au–Up), Pullenia bulloides–Bulimina striata (Pb–Bs), Globocassidulina tumida–Nuttallides umbonifera (Gt–Nu), Gyroidinoides nitidula–Cibicides wuellerstorfi (Gn–Cw) and Cassidulina carinata–Cassidulina laevigata (Cc–Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au–Up and Pb–Bs biofacies), dominate the early Pliocene interval (5.6–4.5 Ma) of global warmth. An intense Indo-Pacific ‘biogenic bloom’ and strong Oxygen Minimum Zone extended to intermediate depths (1000–2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt–Nu, Gn–Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc–Cl biofacies).     

Oligo-Miocene foraminiferal record (Miogypsinidae, Lepidocyclinidae and Nummulitidae) from the Western Taurides (SW Turkey): Biometry and implications for the regional geology

The marine Oligo-Miocene units of western Taurides, deposited under different tectonic regimes (in Bey Da?lar? platform in foreland and coeval sequences in hinterland), were studied to establish a high-resolution biostratigraphic framework. Biometric study of the full spectrum of larger foraminifera in a regional scale allowed us correlating them with the shallow benthic zonation (SBZ) system introduced by [Cahuzac, B., Poignant, A., 1997. Essai de biozonation de l’Oligo-Miocène dans les bassins européens à l’aide des grands foraminifères néritiques. Bulletin de la Société géologique de France 168, 155–169], and to determine the ages of these sites on zonal precision for the first time. In correlating these assemblages to standard shallow benthic zones, planktonic data were also used whenever possible. Taxa, classified under the genera Nummulites, Miogypsina, Miolepidocyclina, Nephrolepidina, Eulepidina, Heterostegina, Operculina and Cycloclypeus (?) and their assemblages, closely resemble to the fauna described from European basins. These groups characterize the SBZ 22B to 25 zones referring to a time interval from early Chattian to Burdigalian. However, a main gap in late Chattian (SBZ 23) and in early part of the Aquitanian (SBZ 24) is also recorded in the platform succession. In the meantime, rare Eulepidina in the Burdigalian levels suggest a clear Indo-Pacific influence. Based on the discovery of early Chattian (SBZ 22B) deposits (previously mapped under Eocene/Miocene units), the Oligo-Miocene stratigraphy of the Bey Da?lar? platform is also revised. A more precise chronology for regional Miocene transgression is presented based on the miogypsinid evolutionary scale.     

Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La_N/Yb_N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La_N/Yb_N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La_N/Yb_N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd_N/Yb_N up to 20.7 × Ch; La_N/Yb_N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites (εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites (εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.     

藏南白朗地区宗卓组混杂岩的物质来源和形成时代——来自岩石学和碎屑锆石U-Pb定年的证据

本文通过对藏南白朗地区宗卓组混杂岩"基质"进行岩石学、碎屑锆石形态学和U-Pb同位素测年等研究,结果表明:"基质"中碎屑岩岩屑成分以中酸性熔岩为主,石英颗粒在阴极发光下主要呈不发光或弱发光,为低温变质成因的自生石英再沉积;碎屑锆石以岩浆成因锆石为主,少量变质成因锆石,多因经历了一定距离的搬运磨蚀作用而呈浑圆状、椭圆状;碎屑锆石年龄绝大多数分布于422~2867 Ma(43.5%)和71~205 Ma(52.2%),在中元古代(1032~1569 Ma)和早白垩世(102~142 Ma)形成两个年龄主峰。综合研究将研究区宗卓组混杂岩中"基质"的形成时代限定为晚白垩统马斯特里赫特期,雅鲁藏布江缝合带北侧冈底斯弧与南侧印度大陆北缘均对其物源具有一定的贡献。