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1.
西藏玛依岗日地区侵入脉岩为辉长岩,通过采集辉长岩样品,观察显微照片,并进行主量元素、微量元素和稀土元素含量测试。结果表明：Na2O与K2O含量变化范围不大,全碱(K2O+Na2O)含量为186%～411%,样品K2O/Na2O值的范围为025～066。岩石富集轻稀土、亏损重稀土,铕无正负异常。总体富集Hf、La、Nd、Ti,亏损P、Yb、Y等元素。地球化学特征表明其形成于陆内裂谷环境,岩浆来源于富集地幔,受到硅铝质地壳物质的混染。结合辉长岩围岩为晚石炭世—早二叠世浅变质岩系以及早二叠世晚期之后的地层中不发育岩墙群的事实,而且根据前人对藏北羌塘南部地区基性岩墙群为晚石炭世—早二叠世的年龄约束,推断研究区内南北向辉长岩可能为古特提斯洋拉张初期的产物。  相似文献   

2.
Mafic intrusives emplaced within the mélange zone of the Manipur Ophiolitic Complex are subalkalinetholeiitic affinity with Fe-enrichment. Based on the field occurrences, textures-mineralogy and whole-rock compositions, these mafic intrusives can be identified as type-I (gabbro intrusives) and type-II (basalt-dolerite dykes). The type-I resembling enriched-type mid-ocean ridge basalt (E-MORB) shows moderate LREE enrichment (LaN/SmN = 2.5–2.6), slightly enriched MORB normalized HFSE patterns possibly represent melts derived from enriched MORB sub-oceanic mantle sources by small degree of partial melting. The other type-II has normal-type mid-ocean ridge basalt (N-MORB) geochemical features, as it exhibits nearly flat to depleted LREE (LaN/SmN = 1.0–0.6), flat MORB normalized HFSE patterns with slight LREE/HREE depletion (CeN/YbN = 1.37–0.46). It might have been derived from depleted MORB type sub-oceanic mantle source. The MORB signature displayed by these mafic intrusives indicates that they are dismembered fragments of oceanic crust generated at mid-ocean spreading ridge system and support the hypothesis that the Manipur ophiolites was initially formed in the divergent plate margin.  相似文献   

3.
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   

4.
Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.  相似文献   

5.
Gabbro inclusions from Tindfjallajökull are divided into two types: I. Panidiomorphic gabbros of non-cumulative origin composed of plagioclase + olivine ± clinopyroxene and interstitil vesicular glass. They have formed in equilibrium with the host magma and may either represent a marginal facies or a highly solidified magma body. In the latter case the host magma or part of it could be mobilized interstitial liquid. II. Allotriomorphic-hypidiomorphic tholeiitic olivine gabbro and diorite xenoliths with scarce Ti-pargasite which have undergone less than 10% partial melting in the host magma forming melts of alkali basaltic or Hekla andesite-like compositions dependent on the original mineral assemblage. Such liquids, enriched in K2O and possibly other incompatible elements, may contaminate basaltic magmas rising slowly through a gabbroic lower crust. Large scale production of andesites by partial melting of such rocks is not possible but would need more hydrous or differentiated source rocks.  相似文献   

6.
Mantle xenoliths from Tenerife show evidence of metasomatismand recrystallization overprinting the effects of extensivepartial melting. The evidence includes: recrystallization ofexsolved orthopyroxene porphyroclasts highly depleted in incompatibletrace elements into incompatible-trace-element-enriched, poikiliticorthopyroxene with no visible exsolution lamellae; formationof olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depletedclinopyroxene at the expense of orthopyroxene; the presenceof phlogopite; whole-rock CaO/Al2O3 >> 1 (Ca metasomatism) inrecrystallized rocks; and enrichment in incompatible elementsin recrystallized rocks, relative to rocks showing little evidenceof recrystallization. The ‘higher-than-normal’ degreeof partial melting that preceded the metasomatism probably resultsfrom plume activity during the opening of the Central AtlanticOcean. Sr–Nd isotopic compositions are closely similarto those of Tenerife basalts, indicating resetting from theexpected original mid-ocean ridge basalt composition by themetasomatizing fluids. Metasomatism was caused by silicic carbonatitemelts, and involved open-system processes, such as trappingof elements compatible with newly formed acceptor minerals,leaving residual fluids moving to shallower levels. The compositionsof the metasomatizing fluids changed with time, probably asa result of changing compositions of the melts produced in theCanary Islands plume. Spinel dunites and wehrlites representrocks where all, or most, orthopyroxene has been consumed throughthe metasomatic reactions. KEY WORDS: Canary Islands; Tenerife; mantle xenoliths; geochemistry; Ca metasomatism; open-system processes; lithosphere; ocean islands  相似文献   

7.
A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjected to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxene - Cr-spinel increase at the expense of garnet and orthopyroxene with increasing degrees of metasomatism. The most metasomatised xenolith is a wehrlite. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but enriched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemical composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculations show that the modally metasomatised rocks became enriched in incompatible elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Major elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metasomatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poor environment. The data suggest that the xenoliths represent the mantle wallrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and incompatible element-enriched component of this melt percolated into the wallrock along a strong temperature gradient and caused the observed range of metasomatism.  相似文献   

8.
We present new geochemical data for the upper mantle and crustal sections(whole-rock major and trace element compositions)as well as mineral chemical data,from the Northern Carbibbean ophiolites in the Habana-Matanzas region in Western Cuba.These ophiolites are part of the Northern Cuban Ophiolitic Belt(NCOB),extending for more than 1000 km along the island.The upper mantle peridotites are composed mainly of refractory harzburgite with tectonite textures,and show convex-downward patterns depleted in MREE normalized to chondrite values(Mc Donough and Sun,1995).These geochemical trends are characteristic for depleted mantle wedge peridotites metasomatized by slab-derived,LREE enriched melts.The NCOB also includes abyssal peridotites having lower LREE/HREEratiosanddisplayingrelativelyhomogeneous and flat patterns from MREE to HREE.These peridotites represent fragments accreted into the continental margin from a subducted oceanic lithosphere.Gabbro and dolerite units in the NCOB are systematically depleted in High Field Strength Elements(HFSE:Nb,Ta,Hf,Ti)and REE with respect to N-MORB(1 X N-MORB).Their melt evolution was affected by subduction input.Spatially associated granitic rocks have a volcanic arc geochemical affinity.Some mafic extrusive rocks within the NCOB exhibit boninitic signatures,and may represent the products of subduction initiation magmatism,whereas other extrusive rock occurrences display N-MORB to E-MORB geochemical fingerprints,slightly modified by subduction derived fluids.Using these geochemical data and constraints,we present a tectonomagmatic model for the evolution of the NCOB within the framework of the Caribbean geology.  相似文献   

9.
The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K2O: 0.01 to 0.4 oxide wt%, P2O5: <0.01 to 0.2 oxide wt%, LaN: 7 to 35, YbN: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K2O: 0.32 to 0.9 oxide wt %, P2O5: 0.02 to 0.35 oxide wt%, LaN: 11 to 60, YbN: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998  相似文献   

10.
Laser ablation microprobe data are presented for olivine, orthopyroxeneand clinopyroxene in spinel harzburgite and lherzolite xenolithsfrom La Palma, Hierro, and Lanzarote, and new whole-rock trace-elementdata for xenoliths from Hierro and Lanzarote. The xenolithsshow evidence of strong major, trace element and Sr isotopedepletion (87Sr/86Sr 0·7027 in clinopyroxene in themost refractory harzburgites) overprinted by metasomatism. Thelow Sr isotope ratios are not compatible with the former suggestionof a mantle plume in the area during opening of the AtlanticOcean. Estimates suggest that the composition of the originaloceanic lithospheric mantle beneath the Canary Islands correspondsto the residues after 25–30% fractional melting of primordialmantle material; it is thus significantly more refractory than‘normal’ mid-ocean ridge basalt (MORB) mantle. Thetrace element compositions and Sr isotopic ratios of the mineralsleast affected by metasomatization indicate that the upper mantlebeneath the Canary Islands originally formed as highly refractoryoceanic lithosphere during the opening of the Atlantic Oceanin the area. During the Canarian intraplate event the uppermantle was metasomatized; the metasomatic processes includecryptic metasomatism, resetting of the Sr–Nd isotopicratios to values within the range of Canary Islands basalts,formation of minor amounts of phlogopite, and melt–wall-rockreactions. The upper mantle beneath Tenerife and La Palma isstrongly metasomatized by carbonatitic or carbonaceous meltshighly enriched in light rare earth elements (REE) relativeto heavy REE, and depleted in Zr–Hf and Ti relative toREE. In the lithospheric mantle beneath Hierro and Lanzarote,metasomatism has been relatively weak, and appears to be causedby high-Si melts producing concave-upwards trace element patternsin clinopyroxene with weak negative Zr and Ti anomalies. Ti–Al–Fe-richharzburgites/lherzolites, dunites, wehrlites and clinopyroxenitesformed from mildly alkaline basaltic melts (similar to thosethat dominate the exposed parts of the islands), and appearto be mainly restricted to magma conduits; the alkali basaltmelts have caused only local metasomatism in the mantle wall-rocksof such conduits. The various metasomatic fluids formed as theresults of immiscible separations, melt–wall-rock reactionsand chromatographic fractionation either from a CO2-rich basalticprimary melt, or, alternatively, from a basaltic and a siliceouscarbonatite or carbonaceous silicate melt. KEY WORDS: mantle xenoliths; mantle minerals; trace elements; depletion; carbonatite metasomatism  相似文献   

11.
柴北缘察汗诺地区广泛发育中生代辉长岩、闪长岩、花岗闪长岩和花岗岩。LA-ICP-MS锆石U-Pb测年结果表明, 辉长岩206 Pb/238 U加权平均年龄为246±0.7 Ma, 形成于中三叠世早期。辉长岩SiO2(50.61%~54.41%)和全碱含量(2.36%~3.72%)低, 里特曼指数为0.54~1.12, 属于中钾钙碱性系列岩石; Mg#(73~79)和Cr(342×10-6~753×10-6)、Ni(45.1×10-6~145×10-6)含量高, LREE/HREE为4.74~5.67, 富集U、Th等大离子亲石元素, 亏损Nb、Ta、Ti等高场强元素, 呈现出与典型俯冲作用密切相关的弧岩浆岩特征。锆石Lu-Hf同位素分析结果显示, 辉长岩εHf(t)值为-2.24~+1.37, 研究表明察汗诺辉长岩岩浆源区因受俯冲板片流体交代而富集, 岩石主要起源于富集地幔的部分熔融。空间上, 察汗诺辉长岩与东昆仑布尔汗布达、鄂拉山以及西秦岭同仁-泽库等同时代的钙碱性岩浆岩, 共同构成了古特提斯洋向北俯冲的大陆边缘弧。  相似文献   

12.
The Northern Apennine ophiolites are remnants of the MiddleJurassic–Early Cretaceous lithosphere from the LigurianTethys. New trace element and Nd–Sr isotope investigationswere performed on: (1) the rare gabbros associated with thesubcontinental mantle rocks from the External Liguride ophiolites;(2) the gabbro–peridotite association from the poorlyknown ophiolitic bodies from Cecina valley (Southern Tuscany).Clinopyroxenes from the External Liguride and Cecina valleygabbros have similar trace element compositions, which are consistentwith formation from normal mid-ocean ridge basalt (N-MORB) magmas.Sm–Nd mineral isochron ages are 179 ± 9 Ma foran External Liguride gabbro and 170 ± 13 Ma and 173·5± 4·8 Ma for two different gabbroic bodies fromthe Cecina valley ophiolites. These ages are interpreted todate the igneous crystallization of the gabbros and are slightlyolder than the oldest pelagic sediments of the Ligurian Tethys.Initial  相似文献   

13.
Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U–Pb and Lu–Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ∼1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions (ɛHf=+4.1–+7.8) and 1.7–1.6 Ga depleted mantle model ages, consistent with 1.7–1.6 Ga felsic protoliths being derived from “juvenile” Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions (ɛHf=−0.7 to +13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.  相似文献   

14.
Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary mélanges of the Northern Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature (~850°C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene, Ti-pargasite and spinel. U–Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean concordia U–Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature of the pyroxenites (initial εNd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle. In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush. A thermal event in Late Permian–Middle Triassic caused the partial resetting of zircon U–Pb system.  相似文献   

15.
Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent. Received: 24 September 1996 / Accepted: 12 August 1997  相似文献   

16.
Major and trace element and Sr-Nd-Pb-O-C isotopic compositions are presented for carbonatites from the Cape Verde (Brava, Fogo, Sáo Tiago, Maio and Sáo Vicente) and Canary (Fuerteventura) Islands. Carbonatites show pronounced enrichment in Ba, Th, REE, Sr and Pb in comparison to most silicate volcanic rocks and relative depletion in Ti, Zr, Hf, K and Rb. Calcio (calcitic)-carbonatites have primary (mantle-like) stable isotopic compositions and radiogenic isotopic compositions similar to HIMU-type ocean island basalts. Cape Verde carbonatites, however, have more radiogenic Pb isotope ratios (e.g. 206Pb/204Pb=19.3-20.4) than reported for silicate volcanic rocks from these islands (18.7-19.9; Gerlach et al. 1988; Kokfelt 1998). We interpret calcio-carbonatites to be derived from the melting of recycled carbonated oceanic crust (eclogite) with a recycling age of ~1.6 Ga. Because of the degree of recrystallization, replacement of calcite by secondary dolomite and elevated ‘13C and ‘18O, the major and trace element compositions of the magnesio (dolomitic)-carbonatites are likely to reflect secondary processes. Compared with Cape Verde calcio-carbonatites, the less radiogenic Nd and Pb isotopic ratios and the negative Ɨ/4 of the magnesio-carbonatites (also observed in silicate volcanic rocks from the Canary and Cape Verde Islands) cannot be explained through secondary processes or through the assimilation of Cape Verde crust. These isotopic characteristics require the involvement of a mantle component that has thus far only been found in the Smoky Butte lamproites from Montana, which are believed to be derived from subcontinental lithospheric sources. Continental carbonatites show much greater variation in radiogenic isotopic composition than oceanic carbonatites, requiring a HIMU-like component similar to that observed in the oceanic carbonatites and enriched components. We interpret the enriched components to be Phanerozoic through Proterozoic marine carbonate (e.g. limestone) recycled through shallow, subcontinental-lithospheric-mantle and deep, lower-mantle sources.  相似文献   

17.
Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.  相似文献   

18.
 Lherzolite xenoliths in Miocene to Pleistocene basalts from five sites in the Hamar-Daban range in southern Siberia provide sampling of the mantle close to the axis of the Baikal rift. These anhydrous spinel lherzolites commonly have foliated fabrics and spongy rims around clinopyroxene, and many contain accessory feldspar. The feldspar occurs in reaction zones adjacent to spinel and orthopyroxene (where it appears to have been formed by the reaction: spl+opx+cpx+fluid →fs+ol) and less commonly as thin, irregular veins. The feldspars have variable compositions but are generally alkali-rich; their K2O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from orogenic lherzolites (usually <0.1% K2O). The temperature range for the Hamar-Daban xenolith suite (950–1010° C) is more restricted than for spinel peridotite xenoliths from other occurrences in the Baikal area. The feldspar-bearing lherzolites yield equilibration temperatures similar to or slightly lower than feldspar-free ones. The majority of the Hamar-Daban lherzolites are fertile and clinopyroxene-rich, as for most other occurrences in the Baikal region. Trace element compositions of selected xenoliths and their clinopyroxenes were determined by ICP-MS, INAA and proton microprobe. Feldspar-bearing xenoliths are enriched in alkalies indicating that feldspar formation is associated with addition of material and is not simply due to isochemical phase changes. Most xenoliths and their clinopyroxenes studied are depleted in light REE and have contents of Sr, Zr and Y common for fertile or moderately depleted mantle peridotites. Few are moderately enriched in LREE, Sr, Th and U. Sr-Nd isotope compositions of clinopyroxenes indicate long-term depletion in incompatible elements similar to unmetasomatised xenoliths from other occurrences south and east of Lake Baikal. The formation of feldspar and of spongy aggregates after clinopyroxene, and the enrichment in alkalies appear to be recent phenomena related to infiltration of an alkali-rich, H2O-poor fluid into spinel peridotites. Received: 20 March 1995 / Accepted: 26 June 1995  相似文献   

19.
MORB 是玄武岩中研究得最详细的玄武岩类,可分为N-MORB 和E-MORB 两类。通常认为,N-MORB 和OIB 都是独立的端元,分别来自亏损和富集的地幔源岩,而E-MORB 则是N-MORB 与OIB 混合的结果。本文研究表明,E-MORB 具复杂的成因,洋脊深度、洋脊扩张速率及源区部分熔融程度及压力不是造成E-MORB 富集的主要原因。压力及部分熔融程度对玄武岩成分的影响远小于地幔不均一性的影响。推测E-MORB 可能有两个主要的形成方式：1） 由较深处略富集的地幔发生部分熔融而成;2） 由N-MORB 与OIB 混合形成。玄武岩微量元素频率直方图表明,N-MORB 基本上保持了来自亏损地幔源区的特征;OIB 则多多少少受到外来物质加入或与N-MORB 混合的影响; E-MORB 则是N-MORB 受OIB 影响的产物。OIB 与E-MORB 似乎没有本质上的区别, 仅仅是受影响和混合程度的不同而已。OIB 富集LILE,可能既有继承了来自源区的特征（深部富集地幔、循环的古洋壳、循环的陆壳、大陆岩石圈地幔、LVZ 熔体层或早期交代岩脉等）,也可能有外来物质加入的影响（与N-MORB 发生不同程度的混合作用）。3 类玄武岩的87Sr/86Sr 和143Nd/144Nd 同位素频率分布与早先的结论一致,但206Pb/204Pb、207Pb/204Pb和208Pb/204Pb同位素频率分布显示OIB 具有更加复杂的特征。  相似文献   

20.
Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO2-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ34S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996  相似文献   

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