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1.
 Mantle peridotites of the Internal Liguride (IL) units (Northern Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical (ICP-MS and SIMS techniques) and isotopic investigations on very fresh samples have been performed with the major aim to constrain the timing and mechanism of their evolution and to furnish new data for the geodynamic interpretation. The data are also useful to discuss some general geochemical aspects of oceanic-type mantle. The studied samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration temperatures (1150–1250° C). Whole rocks, and constituent clinopyroxenes, show a decoupling between severe depletion in highly incompatible elements [light rare earth elements (LREE), Sr, Zr, Na, Ti] and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong M(middle)REE/H(heavy)REE fractionation. These compositional features indicate low-degree (<10%) fractional melting, which presumably started in the garnet stability field, as the most suitable depletion mechanism. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on carefully handpicked clinopyroxene separates, indicate an extremely depleted signature (87Sr/86Sr=0.702203–0.702285; 143Nd/144Nd=0.513619–0.513775). The Sm/Nd model ages suggest that the IL peridotites melted most likely during Permian times. They could record, therefore, the early upwelling and melting of mid ocean ridge basalt (MORB) type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the Western Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusions and HT-LP retrograde metamorphic re-equilibration, prior to their emplacement on the sea floor. The trace element chemistry of IL peridotites also provides useful information about the composition of oceanic-type mantle. The most important feature concerns the occurrence of Sr and Zr negative anomalies (relative to “adjacent” REE) in both clinopyroxenes and bulk rocks. We suggest that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions. Received: 15 February 1995/Accepted: 4 August 1995  相似文献   
2.
 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998  相似文献   
3.
 Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with “kamafugite” lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary or near-primary mantle silicate melts of “kamafugitic” composition are transitional with Ca-carbonatite liquid and provides direct evidence of immiscibility of carbonatite from “kamafugite” magma. It is inferred that a primary mantle origin of Ca-carbonatites is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface. The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that their genesis depends on similar source and associated tectonic conditions. Received: 17 January 1995 / Accepted: 14 June 1995  相似文献   
4.
Mantle peridotites of the External Liguride (EL) Units (Northern Apennines) mainly consist of fertile spinel-lherzolites partially recrystallized to plagioclase-facies assemblages, and are consequently appropriate to investigate the interphase element partitioning related to the transition from spinel- to plagioclase-facies stability field. Evidence for the development of the plagioclase-facies assemblage is mainly given by: (1) large exsolution lamellae of orthopyroxene and plagioclase within spinel-facies clinopyroxene; (2) plagioclase rims around spinel; (3) granoblastic domains made up of olivine+plagioclase±clino-and orthopyroxene. In situ major and trace [REE (rare-earth elements), Ti, Sc, V, Cr, Sr, Y, Zr and Ba] element mineral analyses have been performed, by electron and ion probe, on selected samples which show the progressive development of the plagioclase-bearing assemblage. The main compositional variations observed during the change from spinel- to plagioclase-facies minerals are as follows: (1) clinopyroxenes decrease in Al, Na, Sr, Eu/Eu* and increase in Y, V, Sc, Cr, Zr and Ti; (2) amphiboles decrease in Eu/Eu*, Sr, Ba and increase in Zr and V; (3) spinels decrease in Al and increase in Cr and Ti. The most striking feature is the decoupling in the behaviour of similarly incompatible elements (D about 0.1) in clinopyroxene, e.g. Sr decrease is mirrored by Zr increase. Massbalance calculations indicate that the trace element interphase redistribution documented in the EL peridotites occurred in a closed system and in response to the metamorphic reaction governing the transition from the spinel- to the plagioclase-facies stability field. The observed element partitioning reveals, moreover, that subsolidus re-equilibration processes in the upper mantle produce HFSE (high-field-strength element)/REE fractionation in minerals, which must be evaluated for a reliable determination of mineral-melt distribution coefficients. The results of this study furnish evidence for subsolidus metamorphic evolution during decompression, without concomitant partial melting processes. This is consistent with the interpretation that the EL peridotites represent subcontinental lithospheric mantle emplaced at the surface in response to lithospheric thinning and tectonic denudation mechanisms related to the Triassic-Jurassic rifting of the Ligure-Piedmontese basin.  相似文献   
5.
Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREEN/LREEN>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREEN/LREEN29; Yb 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx2+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.  相似文献   
6.
Mantle xenoliths hosted by the historic Volcan de San Antonio, La Palma, Canary Islands include veined spinel harzburgites and spinel dunites. Glasses and associated minerals in the vein system of veined xenoliths show a gradual transition in composition from broad veins to narrow veinlets. Broad veins contain alkali basaltic glass with semi-linear trace element patterns enriched in strongly incompatible elements. As the veins become narrower, the SiO2-contents in glass increase (46 → 67 wt% SiO2 in harzburgite, 43 → 58 wt% in dunite) and the trace element patterns change gradually to concave patterns depleted in moderately incompatible elements (e.g. HREE, Zr, Ti) relative to highly incompatible ones. The highest SiO2-contents (ca. 68% SiO2, low Ti-Fe-Mg-Ca-contents) and most extreme concave trace element patterns are exhibited by glass in unveined peridotite xenoliths. Clinopyroxenes shift from LREE-enriched augites in basaltic glass, to REE-depleted Cr-diopside in highly silicic glass. Estimates indicate that the most silicic glasses represent melts in, or near, equilibrium with their host peridotites. The observed trace element changes are compatible with formation of the silicic melts by processes involving infiltration of basaltic melts into mantle peridotite followed by reactions and crystallization. The Fe-Mg interdiffusion profiles in olivine porphyroclasts adjacent to the veins indicate a minimum period of diffusion of 600 years, implying that the reaction processes have taken place in situ in the upper mantle. The CaO-TiO2-La/Nd relationships of mantle rocks may be used to discriminate between metasomatism caused by carbonatitic and silicic melts. Unveined mantle xenoliths from La Palma and Hierro (Canary Islands) show a wide range in La/Nd ratios with relatively constant, low-CaO contents which is compatible with metasomatism of “normal” abyssal peridotite by silicic melts. Peridotite xenoliths from Tenerife show somewhat higher CaO and TiO2 contents than those from the other islands and may have been affected by basaltic or carbonatitic melts. The observed trace element signatures of ultramafic xenoliths from La Palma and other Canary Islands may be accounted for by addition of small amounts (1–7%) of highly silicic melt to unmetasomatized peridotite. Also ultramafic xenoliths from other localities, e.g. eastern Australia, show CaO-TiO2-La/Nd relationships compatible with metasomatism by silicic melts. These results suggest that silicic melts may represent important metasomatic agents. Received: 15 November 1998 / Accepted: 17 May 1999  相似文献   
7.
双河洞中大熊猫-剑齿象动物群化石的发现   总被引：1，自引：0，他引：1  
对贵州省境内的双河洞历次科考中发现的第四纪动物群化石进行初步研究和总结,认为目前对双河洞第四纪动物群研究尚处于起步阶段,化石的发现者都是洞穴科考专家,古生物知识相对欠缺且重视不够是洞穴化石研究进展缓慢的原因。为了加快双河洞第四纪动物群的研究进程,以今年中法联合科考过程中发现的大熊猫-剑齿象动物群化石为样本,采用ICP-MS铀系定年方法对这批大熊猫骨骼化石碳酸盐岩包裹物的年龄进行测试,结果表明该化石群化石距今6.75万年,初步判定此动物群可能生存于中更新世-晚更新世早期。这批哺乳动物化石及其地层信息将为贵州（尤其是黔北）第四纪的研究提供重要资料。   相似文献   
8.
New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at [8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of [8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the [8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE. Received: 17 March 1999 / Accepted: 11 June 1999  相似文献   
9.
Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN0.5) and HREE (GdN/ YbN1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN 0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.  相似文献   
10.
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