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1.
Melt inclusions in clinopyroxenes of olivine foidite bombs from Serra di Constantinopoli pyroclastic flows of the Vulture volcano (Southern Italy) were studied in detail. The rocks contain abundant zoned phenocrysts and xenocrysts of clinopyroxene, scarce grains of olivine, leucite, haüyne, glass with microlites of plagioclase and K-feldspar. The composition of clinopyroxene in xenocrysts (Cpx I), cores (Cpx II), and in rims (Cpx III) of phenocrysts differs in the content of Mg, Fe, Ti, and Al. All clinopyroxenes contain two types of primary inclusion-pure silicate and of silicate-carbonate-salt composition. This fact suggests that the phenomena of silicate-carbonate immiscibility took place prior to crystallization of clinopyroxene. Homogenization of pure silicate inclusions proceeded at 1 225 – 1 190°C. The composition of conserved melts corresponded to that of olivine foidite in Cpx I, to tephrite-phonolite in Cpx II, and phonolite-nepheline trachyte in Cpx III. The amount of water in them was no more than 0.9 wt.%. Silicate-carbonate inclusions decrepitated on heating. Salt globules contained salts of alkali-sulphate, alkali-carbonate, and Ca-carbonate composition somewhat enriched in Ba and Sr. This composition is typical of carbonatite melts when decomposed into immiscible fractions. The formation of sodalite-haüyne rocks from Vulture is related to the presence of carbonate-salt melts in magma chamber. The melts conserved in clinopyroxenes were enriched in incompatible elements, especially in Cpx III. High ratios of La, Nb, and Ta in melts on crystallization of Cpx I and Cpx II suggest the influence of a carbonatite melt as carbonatites have extremely high La/Nb and Nb/Ta and this is confirmed by the appearance of carbonatite melts in magma chamber. Some anomalies in the concentrations and relatives values of Eu and especially Ga seems typical of Italian carbonatite related melts. The mantle source for initial melts was, most likely, rather uniform, undepleted and was characterized by a low degree of melting and probable presence of garnet in restite.  相似文献   

2.

Distinctly different groundmass mineralogy characterise the hypabyssal facies, Mesoproterozoic diamondiferous P3 and P4 intrusions from the Wajrakarur Kimberlite Field, southern India. P3 is an archetypal kimberlite with macrocrysts of olivine and phlogopite set in a groundmass dominated by phlogopite and monticellite with subordinate amounts of serpentine, spinel, perovskite, apatite, calcite and rare baddeleyite. P4 contains mega- and macrocrysts of olivine set in a groundmass dominated by clinopyroxene and phlogopite with subordinate amounts of serpentine, spinel, perovskite, apatite, and occasional gittinsite, and is mineralogically interpreted as an olivine lamproite. Three distinct populations of olivine, phlogopite and clinopyroxene are recognized based on their microtextural and compositional characteristics. The first population includes glimmerite and phlogopite–clinopyroxene nodules, and Mg-rich olivine macrocrysts (Fo 90–93) which are interpreted to be derived from disaggregated mantle xenoliths. The second population comprises macrocrysts of phlogopite and Fe-rich olivine (Fo 81–89) from P3, megacrysts and macrocrysts of Fe-rich olivine (Fo 85–87) from P4 and a rare olivine–clinopyroxene nodule from P4 which are suggested to have a genetic link with the precursor melt of the respective intrusions. The third population represents clearly magmatic minerals such as euhedral phenocrysts of Fe-rich olivine (Fo 85–90) crystallised at mantle depths, and olivine overgrowth rims formed contemporaneously with groundmass minerals at crustal levels. Close spatial association and contemporaneous emplacement of P3 kimberlite and P4 lamproite is explained by a unifying petrogenetic model which involves the interaction of a silica-poor carbonatite melt with differently metasomatised wall rocks in the lithospheric mantle. It is proposed that the metasomatised wall rock for lamproite contained abundant MARID-type and phlogopite-rich metasomatic veins, while that for kimberlite was relatively refractory in nature.

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3.
Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary nephelinite-carbonatite complex of Napak, eastern Uganda, show large isotopic variations that can only be attributed to open-system behaviour. Possible explanations of the data include mixing between nephelinitic melts derived from an isotopically heterogeneous mantle, or interaction between a HIMU melt and mafic granulites. In both models crystal fractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data from clinopyroxene phenocrysts from the olivine-bearing nephelinites, suggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interpretation of crystal fractionation in an open magma system that was undergoing continuous isotopic change. This study emphasises the importance of using combined isotopic data from both whole rock and mineral phases to interpret the evolutionary history of a single eruptive centre.  相似文献   

4.
 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995  相似文献   

5.
The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 1–2 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (1–2 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements *Corresponding author.  相似文献   

6.
ABSTRACT

The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.  相似文献   

7.
The major element composition of plagioclase, pyroxene, olivine,and magnetite, and whole-rock 87Sr/86Sr data are presented forthe uppermost 2·1 km of the layered mafic rocks (upperMain Zone and Upper Zone) at Bierkraal in the western BushveldComplex. Initial 87Sr/86Sr ratios are near-constant (0·7073± 0·0001) for 24 samples and imply crystallizationfrom a homogeneous magma sheet without major magma rechargeor assimilation. The 2125 m thick section investigated in drillcore comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite)layers and changes up-section from gabbronorite (An72 plagioclase;Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayaliteferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overallfractionation trend is, however, interrupted by reversals characterizedby higher An% of plagioclase, higher Mg# of pyroxene and olivine,and higher V2O5 of magnetite. In the upper half of the successionthere is also the intermittent presence of cumulus olivine andapatite. These reversals in normal fractionation trends definethe bases of at least nine major cycles. We have calculateda plausible composition for the magma from which this entiresuccession formed. Forward fractional crystallization modelingof this composition predicts an initial increase in total iron,near-constant SiO2 and an increasing density of the residualmagma before magnetite crystallizes. After magnetite beginsto crystallize the residual magma shows a near-constant totaliron, an increase in SiO2 and decrease in density. We explainthe observed cyclicity by bottom crystallization. Initiallymagma stratification developed during crystallization of thebasal gabbronorites. Once magnetite began to crystallize, periodicdensity inversion led to mixing with the overlying magma layer,producing mineralogical breaks between fractionation cycles.The magnetitite and nelsonite layers mainly occur within fractionationcycles, not at their bases. In at least two cases, crystallizationof thick magnetitite layers may have lowered the density ofthe basal layer of melt dramatically, and triggered the proposeddensity inversion, resulting in close, but not perfect, coincidenceof mineralogical breaks and packages of magnetitite layers. KEY WORDS: layered intrusion; mineral chemistry; isotopes; magma; convection; differentiation  相似文献   

8.
L.I. Panina  L.M. Usoltseva 《Lithos》2008,103(3-4):431-444
To find out the reasons responsible for the diversity of igneous rocks forming the alkaline-ultrabasic carbonatite Krestovskiy massif (the Maimecha–Kotui province, Russia) we have studied melt inclusions in clinopyroxene of trachydolerites, porphyric melanephelinites, and tholeiites. It was established that the homogenization temperatures of inclusions in these rocks are rather close: 1140–1180 °C, 1190–1230 °C, and 1150–1210 °C, respectively. Compositions of melt inclusions in clinopyroxenes from different rocks are significantly different. The chemical composition of clinopyroxene of trachydolerites corresponds to that of trachybasalts and their derivatives. The inclusions are enriched in Sr, Ba, P, and S and their total sum of alkalies (at K ≥ Na) is never less than 5–6 wt.%. Inclusions from the rims of clinopyroxene phenocrysts in porphyric melanephelinites are similar in composition also to inclusions in trachydolerites. But in the cores of clinopyroxene phenocrysts the composition of inclusions corresponds to nephelinite melt. The composition of some melt inclusions in the intermediate and cores zones of clinopyroxene from porphyric melanephelinite has high SiO2 (53–55 wt.%), MgO (8–9 wt.%) and a low (1–2 wt.%) total sum of alkalies (at Na ≥ K) and is depleted in Al2O3 (6–7 wt.%), which is similar to the composition of basaltic komatiites. The composition of inclusions in tholeiites is also basic, highly magnesian, and low-alkaline, Sr and Ba are rare to absent. Compared to the inclusions of basaltic komatiite composition, the inclusions in tholeiites are enriched in Al and depleted in Ca, Ti, and P. The melts trapped in clinopyroxenes from different rocks contain low (0.014–0.018 wt.%) water but they are enriched in F: from 0.37 wt.% in nephelinite melts to 0.1–0.06 wt.% in tholeiite and basaltic komatiite melts. Inclusions in all the rocks under study, host clinopyroxene, and the rocks themselves are significantly enriched in incompatible elements (1–2 orders of magnitude relative to the mantle norm). In tholeiites, the partitioning of these elements is rather uniform, while in trachydolerites and especially in melanephelinites it is contrasting with a drastic depletion in HREE relative to LREE, MREE, and HFSE. A conclusion is made that the Krestovskiy massif was formed by no less than three mantle-derived magmas: melanephelinite, tholeiite and basaltic komatiite. Magmas were generated in different magma sources at different depths with various degrees of enrichment in incompatible elements. These magmas were, most likely, dominated by melanephelinite magma. In intermediate chambers this magma differentiated to form derivative melts of nephelinite, trachydolerite–trachyandesite–trachyte compositions. Komatiite-basalt melts were, most likely, derivatives of primitive meimechite magmas.  相似文献   

9.
We report the results of LA-ICP-MS analyses of rock forming minerals in clinopyroxene-apatite-K feldspar-phlogopite (CAKP) metasomatic xenoliths and primary carbonatite melt inclusions (CMI) hosted in apatite (Ap) and K feldspar (Kfs). The xenoliths are from the Cretaceous lamprophyre dikes of the Transdanubian Central Range, Hungary. The CMI in Ap have phosphorus dolomitic composition as opposed to CMI in Kfs, which display dolomitic alkali-aluminosiliceous character. The melts found in CMI in Ap and in Kfs likely formed by liquid-liquid separation from an originally carbonate- and phosphorous-rich melt. Primitive mantle (PM) normalized trace element distributions of both Ap- and Kfs-hosted CMI (n = 60 and 20, respectively) reveal a strong negative Ti-anomaly, and an extreme enrichment in incompatible elements (U, Th, LILE and LREE) relative to HREE, Sc, V, Ni and Cr. Rarely, apatites contain unique CMI, which show major- and trace-element signature transitional to K feldspar-hosted CMI. This is due to heterogeneous entrapment of an immiscible phosphorous-bearing carbonatite melt and a carbonate-bearing alkali aluminosiliceous melt, which is a further evidence for their co-existence. CMI reveal that U, Th, Pb, Nb, Ta, P, Sr, Y and REE partitioned into the phosphorous-bearing carbonatite melt, whereas Cs, Rb, Na, K, B, Al, Zr and Hf preferred the silicate-bearing liquid.PM normalized REE pattern (high LREE/HREE), elevated Zr and Hf contents and negative Ti anomaly of clinopyroxene (Cpx) indicate that its formation is genetically linked to carbonatite metasomatism attested by CMI. Trace element partitioning between the studied Cpx and CMI is in accordance with experimentally determined trace element distributions between Cpx and carbonatite melt. Cpx, which occur in samples with high modal proportion of apatite represent mantle section, which interacted with a higher amount of “initial” carbonatite melt than Cpx from apatite-poor xenoliths. This is confirmed by higher Cr, Ni, V, Sc, Ti and lower Zr, as well as Hf concentration in Cpx from xenoliths with low modal abundance of Ap. CMI reveal that Ti, V, Ni and Cr were in lower concentration in the “initial” carbonatite melt than in PM. Contrarily, Zr and Hf were more abundant in this melt than in PM. Consequently, a continuously migrating “initial” carbonatite melt, increased Zr and Hf concentration, and decreased Ti, Sc, V, Ni and especially Cr in the clinopyroxenes. Our findings suggest that the studied CAKP rocks were formed by carbonatite melt metasomatism, which occurred in an open system in the upper mantle.  相似文献   

10.
This paper presents a study of melt and fluid inclusions in minerals of an olivine-leucite phonolitic nephelinite bomb from the Monticchio Lake Formation, Vulture. The rock contains 50 vol.% clinopyroxene, 12% leucite, 10% alkali feldspars, 8% hauyne/sodalite, 7.5% nepheline, 4.5% apatite, 3.2% olivine, 2% opaques, 2.6% plagioclase, and < 1% amphibole. We distinguished three generations of clinopyroxene differing in composition and morphology. All the phenocrysts bear primary and secondary melt and fluid inclusions, which recorded successive stages of melt evolution. The most primitive melts were found in the most magnesian olivine and the earliest clinopyroxene phenocrysts. The melts are near primary mantle liquids and are rich in Ca, Mg and incompatible and volatile elements. Thermometric experiments with the melt inclusions suggested that melt crystallization began at temperatures of about 1200 °C. Because of the partial leakage of all primary fluid inclusions, the pressure of crystallization is constrained only to minimum of 3.5 kbar. Combined silicate–carbonate melt inclusions were found in apatite phenocrysts. They are indicative of carbonate–silicate liquid immiscibility, which occurred during magma evolution. Large hydrous secondary melt inclusions were found in olivine and clinopyroxene. The inclusions in the phenocrysts recorded an open-system magma evolution during its rise towards the surface including crystallization, degassing, oxidation, and liquid immiscibility processes.  相似文献   

11.
Summary Spongy textures are observed in anhydrous Group 1 mantle xenoliths (harzburgite, lherzolite and wehrlite) hosted in Tertiary alkali basaltic lavas from the Hessian Depression, Germany. These textures are developed only on clinopyroxene and spinel, and occur as rims or cross-cutting veinlets and patches showing optical continuity with the host grain. They are often associated with pools of amorphous glassy material. There is no preferential development of spongy domains against the xenolith-lava contact suggesting that the host magma did not play any significant role in their formation. Spongy clinopyroxene and spinel occur in all rock types, but, are more pervasive in wehrlite. Chemically, spongy domains of clinopyroxene and spinel are more refractory than unaffected areas, which is consistent with their formation through a partial melting event. The associated glassy material shows chemical characteristics which suggest that the melt pools are genetically related to the development of the spongy textures. The partial melting event was probably triggered by the infiltration of a low-density fluid. The fluid may have evolved from a silicate melt responsible for the metasomatic Fe-enrichment recorded in wehrlite. In this context, the more pervasive development of spongy clinopyroxene in wehrlite may be explained by a higher concentration of the evolved fluid phase at proximity to its silicate melt source. Received March 15, 2000; revised version accepted September 6, 2001  相似文献   

12.
达拉库岸镁铁-超镁铁质岩体位于东昆仑造山带南带之喀拉米兰晚古生代沟弧系,主要由二辉橄榄岩、单辉橄榄岩、橄榄二辉岩、单辉辉石岩和辉长岩组成。辉长岩的锆石U-Pb谐和年龄为244.4±1.5Ma,属于中三叠世。岩石普遍弱富集稀土元素和大离子亲石元素(Rb、Ba、Sr),亏损不相容元素(Nb、Ta)。岩体原生岩浆为高镁拉斑玄武质岩浆(Mg O 11.4%,Fe O10.8%)。元素地球化学和Nd、Sr同位素组成特征表明岩浆源区为富集岩石圈地幔,岩浆运移和侵位过程中遭受不同程度地壳物质的同化混染作用。依据橄榄石组分模拟获得橄榄石结晶过程中母岩浆达到硫饱和,并发生硫化物的熔离作用。从岩体特征、矿石结构、原生岩浆性质、深部硫化物熔离和物探信息等方面综合分析,岩体具有形成岩浆型铜镍硫化物矿床的良好条件。  相似文献   

13.
A technique is described for determining the cooling historyof olivine phenocrysts. The technique is based on the analysisof the diffusive re-equilibration of melt inclusions trappedby olivine phenocrysts during crystallization. The mechanismof re-equilibration involves diffusion of Fe from and Mg intothe initial volume of the inclusion. The technique applies toa single crystal, and thus the cooling history of differentphenocrysts in a single erupted magma can be established. Weshow that melt inclusions in high-Fo olivine phenocrysts frommantle-derived magmas are typically partially re-equilibratedwith their hosts at temperatures below trapping. Our analysisdemonstrates that at a reasonable combination of factors suchas (1) cooling interval before eruption (<350°C), (2)eruption temperatures (>1000°C), and (3) inclusion size(<70 µm in radius), partial re-equilibration of upto 85% occurs within 3–5 months, corresponding to coolingrates faster than 1–2°/day. Short residence timesof high-Fo phenocrysts suggest that if eruption does not happenwithin a few months after a primitive magma begins cooling andcrystallization, olivines that crystallize from it are unlikelyto be erupted as phenocrysts. This can be explained by efficientseparation of olivine crystals from the melt, and their rapidincorporation into the cumulate layer of the chamber. Theseresults also suggest that in most cases erupted high-Fo olivinephenocrysts retain their original composition, and thus compositionsof melt inclusions in erupted high-Fo olivine phenocrysts donot suffer changes that cannot be reversed. Short residencetimes also imply that large unzoned cores of high-Fo phenocrystscannot reflect diffusive re-equilibration of originally zonedphenocrysts. The unzoned cores are a result of fast efficientaccumulation of olivines from the crystallizing magma, i.e.olivines are separated from the magma faster than melt changesits composition. Thus, the main source of high-Fo crystals inthe erupted magmas is the cumulate layers of the magmatic system.In other words, olivine-phyric rocks represent mixtures of anevolved transporting magma (which forms the groundmass of therock) with crystals that were formed during crystallizationof more primitive melt(s). Unlike high-Fo olivine phenocrysts,the evolved magma may reside in the magmatic system for a longtime. This reconciles long magma residence times estimated fromthe compositions of rocks with short residence times of high-Foolivine phenocrysts. KEY WORDS: melt inclusions; olivine; picrites; residence time; diffusion  相似文献   

14.
Xenoliths record two distinct events in the mantle below theQuarternary West Eifel Volcanic Field, Germany. The first, duringthe Hercynian Orogeny, led to widespread formation of secondary,Ti-poor amphibole, clinopyroxene and phlogopite. The signatureof the second event, related to Quaternary volcanism, variesacross the field. At Dreiser Weiher and Meerfelder Maar, thisevent is characterized by amphibole–phlogopite–clinopyroxeneveins, hosted in lherzolite and harzburgite xenoliths broughtto the surface by sodic olivine nephelinite–basanite suitelavas. These veins formed from crystallization of sodic magmathat flowed along fractures in the mantle. At Rockeskyller Kopf,Gees and Baarley, the Quaternary event is characterized by wehrlitexenoliths, many of which have phlogopite–clinopyroxeneveins, that were transported by potassic foid suite lavas. Wehrliteformed by reaction of lherzolite–harzburgite, with a largevolume of potassic magma that flowed along grain boundariesrather than in fractures. During reaction, orthopyroxene wasconsumed and secondary clinopyroxene, olivine and phlogopiteprecipitated. Veins formed in wehrlites only during periodicover-pressure events. The composition of the magmas parentalto the veins is similar to the lavas that carried the xenolithsto surface, indicating that the source of foid and olivine nephelinite–basanitesuite magma is domainal, as was the flow regime and magma flux. KEY WORDS: Eifel; mantle xenoliths; metasomatism; trace elements  相似文献   

15.
The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.  相似文献   

16.
Olivine nephelinite, basanite, and transitional alkaline basaltlavas of the Quaternary Fort Selkirk volcanic complex in thecentral Yukon represent three distinct alkaline magma serieswhich have evolved along diverging paths. They cannot be relatedby low-pressure crystal-liquid fractionation, and systematicisotopic differences make it difficult to derive them by variabledegrees of melting of a common mantle source. Field evidencerequires, however, that these three magma series are intimatelyrelated in time and space, and they share a number of anomalouschemical characteristics including low Ca/Na ratios with respectto the majority of terrestrial equivalents. When the effectsof differential olivine fractionation are ignored, the compositionalspectrum of the Fort Selkirk lavas approximates a binary mixingline between transitional alkaline basalt and olivine nephelinite.A population gap along this mixing line, located between thecompositions of the nephelinite and basanite lavas, coincideswith the compositions of amphibole and/or amphibole–garnet–clinopyroxeneassemblages observed in mantle xenoliths. This compositionalgap may represent a thermal divide separating two minimum-meltcompositions in a mantle source consisting of a lherzolite hostcut by amphibole–garnet–clinopyroxenite veins. Theolivine nephelinite endmember may have been derived by earlymelting in the amphibole–garnet–clinopyroxeniteveins, whereas the transitional alkaline basalt would representmore extensive melting of the host lherzolite.  相似文献   

17.
Alkaline-basic dike from the Yllymakh Massif (Central Aldan) has been studied. Its partially crystallized matrix contains corroded phenocrysts of olivine and hypidiomorphic phenocrysts of clinopyroxene and pseudo-, epileucite. It was found that phenocrysts of clinopyroxene contain abundant primary inclusions, Ti-magnetite and apatite bear only single inclusions, whereas olivine is enriched in secondary inclusions, which are confined to the cleavage of host mineral (along second and third pinacoids) and its cracks. The homogenization temperatures of the primary inclusions in clinopyroxene and secondary inclusions in olivine are approximately equal and lie within 1260–1240°C. The compositions of melt inclusions in olivine and clinopyroxene are also similar and corresponded to the malignite-pseudoleucite phonolite-monzonite pulaskites, which are developed at the Yllymakh Massif. Unheated inclusions in apatite and Ti-magnetite compositionally approach monzonites and nepheline syenites—tinguaites, respectively. It was concluded that the alkaline basaltoid magma was presumably parental magma for the entire rock complex of the Yllymakh Massif. Its crystallization and differentiation presumably provided all observed rock variety from ultrabasics (early derivatives located at depth) and malignites (later derivatives) to leucite phonolites, monzonites, and alkaline pulaskites, which were obtained during subsequent stages of the melt evolution. The parental magma, and especially its derivatives, were enriched in BaO (0.8–0.1 wt %), Cl (0.1–0.3 wt %) and trace elements (primarily, LREE and MREE), which are several times higher than mantle values. At the same time, ion microprobe (SIMS) study showed that derivative melts were dry: contained only 0.01–1.13 wt % H2O. The trend of melts conserved in the minerals and the massif rocks corresponds to the evolution of alkalinebasaltoid magma with increase in Si, Al, alkalis and decrease in Mg, Ca, and Fe, i.e. the Bowen trend. The considered alkaline-basic dike was presumably formed from the derivative of leucite-phonolite melt, which during emplacement captured olivine xenocrysts from previously fractionated ultrabasic rocks. The parental magma was presumably derived by high-degree melting of garnet-spinel-facies depleted mantle at some influence of crustal material.  相似文献   

18.
The Massif du Sud is a large ophiolitic complex that crops out in the southern region of New Caledonia (SW Pacific). It is dominated by harzburgite tectonite that locally shows a transitional gradation to massive dunite up section. Clinopyroxene, orthopyroxene and plagioclase progressively appear in dunite up to the transition to layered wehrlite and orthopyroxene–gabbro. The dunite–wehrlite and wehrlite–gabbro contacts are parallel and the latter defines the paleo-Moho.Highly depleted modal, mineral and bulk rock compositions indicate that harzburgites are residues after high degrees (20–30%) of partial melting mainly in the spinel-stability field. Their relative enrichment in HFSE, LREE and MREE is due to re-equilibration of melting residues with percolating melts. Dunite formed in the Moho transition zone by reaction between residual mantle harzburgite and olivine-saturated melts that led to pyroxene dissolution and olivine precipitation. Rare clinopyroxene and plagioclase crystallized in interstitial melt pores of dunite from primitive, low-TiO2, ultra-depleted liquids with a geochemical signature transitional between those of island arc tholeiites and boninites.Ascending batches of relatively high-SiO2, ultra-depleted melts migrated through the Moho transition zone and generated wehrlite by olivine dissolution and crystallization of clinopyroxene, orthopyroxene and plagioclase in variable amounts. These liquids were more evolved and were produced by higher degrees of melting or from a more depleted source compared with melts that locally crystallized clinopyroxene in dunite. Ultra-depleted magmas, non-cogenetic with those that formed the Moho transition zone, ascended to the lower crust and generated gabbroic cumulates with subduction-related affinity. Thus, the ultramafic and mafic rocks in the Moho transition zone and lower crust of the Massif du Sud ophiolite are not products of fractional crystallization from a single magma-type but are the result of migration and accumulation of different melts in a multi-stage evolution.The record of high partial melting in the mantle section, and migration and accumulation of ultra-depleted subduction-related melts in the Moho transition zone and lower crust support that the Massif du Sud ophiolite is a portion of forearc lithosphere generated in an extensional regime during the early phases of the subduction zone evolution. Our results show the existence of different types of ultra-depleted melt compositions arriving at the Moho transition zone and lower crust of an infant intraoceanic paleo-arc. Ultra-depleted melts may thus be a significant component of the melt budget generated in oceanic spreading forearcs prior to aggregation and mixing of a large range of melt compositions in the crust.  相似文献   

19.
Plagioclase-bearing peridotites are commonly associated with gabbroic rocks sampled around the Moho Transition Zone. Based on mineral chemistry, texture, and spatial relations, the formation of plagioclase-bearing peridotites has been attributed to impregnation of basalt into residual peridotites. We conducted reactive dissolution and crystallization experiments to test this hypothesis by reacting a primitive mid-ocean ridge basalt with a melt-impregnated lherzolite at 1,300 °C and 1 GPa and then cooling to 1,050 °C as pressure decreased to 0.7 GPa. Crystallization during cooling produced lithologic sequences of gabbro–wehrlite or gabbro–wehrlite–peridotite, depending on reaction time. Wehrlitic and peridotitic sections contain significant amounts of plagioclase interstitial to olivine and clinopyroxene and plagioclase compositions are spatially homogeneous. Clinopyroxene in the wehrlite–peridotite section is reprecipitated from the melt and exhibits poikilitic texture with small rounded olivine chadacrysts. Mineral composition in olivine and clinopyroxene varies spatially, both at the scale of the sample and within individual grains. Olivine grains that crystallized close to the melt–peridotite interface are enriched in iron due to their proximity to the basaltic melt reservoir. Consistent with many field studies, we observed gradual spatial variation in olivine and clinopyroxene composition across a lithologically sharp boundary between the gabbro and wehrlite–peridotite. Plagioclase compositions show no obvious dependence on distance from the melt–rock interface and were precipitated from late-stage trapped melts. Compositional trends of olivine, pyroxene, and plagioclase are consistent with previous experimental results and natural observations of the Moho Transition Zone. Different lithological sequences form based primarily on the melt–rock ratio, composition of the melt and host peridotite, and thermochemical conditions, but are expected to grade from gabbro to wehrlite or troctolite to peridotite. Plagioclase-bearing peridotite represents the low melt–rock ratio end member where pyroxene is only partially replaced by olivine and melt, whereas dunite is expected to form where melts overwhelm and consume all other phases. This study confirms that under nominally anhydrous conditions, the gabbro–wehrlite–plagioclase-peridotite sequence can be formed by reaction between basalt and lherzolite and subsequent crystallization at intermediate to low pressures. Melt–rock reaction is a fundamental process in the formation of new crust at the shallowest part of the melting column where pyroxene-undersaturated melts percolate through depleted peridotite.  相似文献   

20.
Despite evidence for its involvement, the importance of amphibolein controlling the compositions of mafic alkaline magmas remainsunder-appreciated. Relatively small variations in large ionlithophile elements (LILE) with respect to other incompatibleelements, such as light rare-earth elements (LREE) or Th, requirethat amphibole was an important residual phase during the productionof Late Tertiary to Recent olivine nephelinite (Ol-NEPH) magmasbeneath the northern Canadian Cordillera. The erupted maficmagma types define a continuous array from Ol-NEPH to hypersthene-normativeolivine basalt (Hy-NORM AOB). The overall compositional arrayhas a sense of curvature which is counter to binary mixing,but can be modeled by two distinct linear melting trends: onefrom Ol-NEPH to basanite (BASAN) compositions, during whichamphibole controlled the composition of the melt, and the ratiosof LILE/LREE change significantly, but the ratios of high fieldstrength elements (HFSE) remain relatively constant; the otherfrom BASAN to Hy-NORM AOB corresponding to the melting of alherzolite assemblage, following the exhaustion of amphibole,across which the ratios of LILE/LREE remain relatively constant,but the ratios of HFSE change significantly. Other intraplate alkaline suites, such as those of the HawaiianIslands, show similar evidence for the involvement of residualamphibole in the genesis of Ol-NEPH to BASAN magmas. The meltingof any amphibole-bearing mantle assemblage is likely to be atwo-step process, regardless of whether the amphibole is segregatedas veins or distributed interstitially. In a water-undersaturatedenvironment, the first stage of melting is controlled by thebreakdown of amphibole, which produces silica-saturated liquidsbelow 12 kbar and silica-undersaturated liquids at greater depths,with little contribution from other mineral phases. In the secondstage, following the exhaustion of amphibole, the major elementcompositions of subsequent melts change rapidly to equilibratewith a lherzolite mineralogy, but the incompatible trace-elementcharacteristics of the former amphibole persist. KEY WORDS: amphibole; mafic alkaline magmas; northern Canadian Cordillera; trace elements *Corresponding author  相似文献   

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