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1.
Meta-igneous mafic and ultramafic rocks, which constitute about 60% of the granulitic xenoliths enclosed in the Neogene alkali basalts of the Bournac pipe (French Massif Central) have well preserved magmatic trends of element variations. The meta-igneous suite was probably derived from at least two different parental magmas and it may be a part of a gabbroic complex which resembles mafic bodies associated with anorthosites. The xenoliths are also very similar to many other granulitic xenoliths and to meta-igneous mafic granulitic massifs. This indicates that the gabbroic intrusions may be widespread in the lower crust and the close association of gabbroic rocks with meta-sedimentary granulites suggests a model for the composition of the lower continental crust.  相似文献   

2.
Toshio  Nozaka 《Island Arc》1997,6(4):404-420
Abstract Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P-T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and meta-sediments; (ii) garnet gabbro (Group 111) and corundum anorthosite (Group IV); (iii) spinel pyroxenite (Group 11); and (iv) spinel peridotite and pyroxenite (Group I). Groups I1 and I11 show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P-T conditions of equilibration, Groups I1 and I11 are interpreted as formed from basaltic magma that intruded beneath the crust-mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre-existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine-grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back-arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow-level magmatism at the back-arc side.  相似文献   

3.
Spinel-lherzolite xenoliths have been found in olivine tholeiite near Andover in the Tasmanian Tertiary volcanic province. They show a high-pressure mineralogy of predominant olivine (Mg90), with aluminous enstatite (Mg90) and lesser aluminous diopside and chrome-bearing spinel, and resemble lherzolite xenoliths commonly found in undersaturated lavas. Such xenoliths are unusual in tholeiitic basalts and the occurrence directly attests to a mantle origin for at least some tholeiitic magmas.The lherzolites are accompanied by doleritic and pyroxenitic xenoliths and by olivine, orthopyroxene, clinopyroxene and plagioclase xenocrysts. If near-liquidus phases are represented amongst the xenocrysts, then the magnesian number of the host basalt and its xenocryst assemblage provisionally suggest a magma derived by more than 15–20% partial melting of mantle peridotite, before commencing xenocryst crystallisation at pressures between 8–13 kbar.With this new record, lherzolite-bearing lavas in Tasmania now cover an extremely wide compositional range, extending from highly undersaturated olivine melilitite to olivine tholeiite. They also include a considerable number of fractionated alkaline rocks that are only sparsely reported in the literature as lherzolite hosts. This latter group contains representatives of a previously suggested but unestablished alkaline fractionation series based on olivine nephelinite, viz. calcic olivine nephelinite → sodic olivine nephelinite → potassi-sodic olivine nephelinite → mafic nepheline benmoreite → mafic phonolite.Lherzolite and megacryst-bearing lavas are relatively more abundant in peripheral parts to the main basalt sequences in Tasmania. This suggests that they developed in fringing zones of less intense mantle melting which enhanced stagnation and fractionation of magmas within the mantle before eruption. Calculated crustal thicknesses under these areas suggest that the magmas were generated at pressures exceeding 6–11 kbar, with the Andover tholeiitic magma exceeding 9 kbar.  相似文献   

4.
Abstract   Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo86) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism.  相似文献   

5.
The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (～30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.  相似文献   

6.
Studies on the deep-seated xenoliths from global volcanoes reveal that the present petrological crust-mantle boundary between the lower crust and the upper mantle actually is a transitional layer from mainly mafic granulites to ultramafic spinel lher-zolites[1,2], i.e. a transitional zone distinctive from the seismological Moho[3]. Oceanic lithosphere crust- mantle transitional zone can be established from the study on the exposed ophiolites. However, as for the continental lithosphere, since …  相似文献   

7.
Paleozoic lamprophyres exhibit good exposures in the western part of the Central–East Iranian microcontinent. These rocks crop out as volcanoes, dykes, and plugs. The constituent minerals are amphibole, clinopyroxene, plagioclase, K‐feldspar, olivine, Cr‐spinel, titanite, biotite, and ilmenite. The main textures in volcanic lamprophyres are porphyritic, trachytic, microlithic, and variolitic, whereas in dykes and plugs, intergranular texture is common. These lamprophyres are regionally metamorphosed in some areas. Petrographical and geochemical characteristics of the studied rocks suggest that they are classified as alkaline lamprophyres and camptonites. They are enriched in alkalis (Na2O + K2O), large ion lithophile elements, and light rare earth elements, and the features of trace element concentrations are similar to those of within‐plate basalts. This study suggests that the lamprophyres were derived from different degrees of partial melting of metasomatized amphibole‐bearing spinel lherzolite. Subduction of Paleo‐Tethys oceanic crust from the Early to late Paleozoic resulted in enrichment in fluids in the mantle, and lamprophyric magmatism occurred along the minor and major faults. This limited but typical lamprophyric magmatism in a broad area of Central Iran suggests that, in spite of the long length of the Paleozoic (～250 my), it was a relatively calm era from the viewpoint of magmatism in Central Iran.  相似文献   

8.
Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe2+) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe3+ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe2+ and Fe3+. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe2+‐ and Fe3+‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe2+‐ and Fe3+‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe2+‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe2+) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe2+)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.  相似文献   

9.
Late Cenozoic alkali basalts in the Ganseong area of South Korea contain abundant ultramafic xenoliths and clinopyroxene megacrysts. Anhydrous clinopyroxene‐rich wehrlite–clinopyroxenites make up the majority of the xenolith population and range from wehrlite through olivine clinopyroxenite to clinopyroxenite. This study investigates the petrogenesis of wehrlite–clinopyroxenite xenoliths and clinopyroxene megacrysts on the basis of petrography and mineral and whole‐rock chemistry. Observations such as an absence of carbonate or apatite, high Ti/Eu ratio, and clinopyroxene‐dominated mineralogy lead us to rule out peridotite–melt reactions as the origin of the Ganseong wehrlites– olivine clinopyroxenites. The whole‐rock compositions (e.g. high abundance of CaO at a given MgO content and low abundance of incompatible elements, such as U, K, P, and Ti compared with mafic melts) indicate that the pyroxenites do not represent crystallized magma itself, but are rather cumulates with a small amount of residual liquid. Anhydrous and orthopyroxene‐free mineral assemblages, crystallization sequence of olivine→clinopyroxene→plagioclase, and mineral chemistries (e.g. low Cr# and high TiO2 abundances in spinels and high TiO2 and Na2O abundances in clinopyroxenes at a given Mg#) suggest that relatively anhydrous intraplate alkaline basalt is the most likely candidate for the parent magma. Texture and compositions of the clinopyroxene megacrysts preclude a cognate origin via high‐pressure crystallization of the host magma. The clinopyroxene megacrysts occupy the Fe‐rich end of the compositional trends defined by wehrlite–pyroxenite clinopyroxenes. Progressive decreases in Mg# and an absence of significant compositional gaps between pyroxenite xenoliths and clinopyroxene megacrysts indicate fractionation and differentiation of a similar parental magma. We suggest that the clinopyroxene megacrysts represent fragments of pegmatitic clinopyroxenites crystallized from more advanced fractionation stages of the evolution of a series of magmatic liquids formed Ganseong wehrlite–clinopyroxenites.  相似文献   

10.
Abstract   A 8.65 Ma adakitic intrusive sheet exposed near Monglo village in the Baguio District of Northern Luzon contains a suite of ultramafic and mafic xenoliths including in order of abundance: spinel dunites showing typical mantle-related textures, mineral and bulk rock compositions, and serpentinites derived from them; amphibole-rich gabbros displaying incompatible element patterns similar to those of flat or moderately enriched back-arc basin basalt magmas; and amphibolites derived from metabasalts and/or metagabbros of identical affinity. A single quartz diorite xenolith carrying a similar subduction-related geochemical signature has also been sampled. One amphibolite xenolith provided a whole-rock K–Ar age of 115.6 Ma (Barremian). We attribute the origin of this suite to the sampling by ascending adakitic magmas of a Lower Cretaceous ophiolitic complex located at a depth within the 30–35 km thick Luzon crust. It could represent an equivalent of the Isabela-Aurora and Pugo-Lepanto ophiolitic massifs exposed in Northern Luzon.  相似文献   

11.
SEONG HEE  CHOI  JONG IK  LEE  CHUNG-HWA  PARK  JACQUES  MOUTTE 《Island Arc》2002,11(4):221-235
Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.  相似文献   

12.
Distribution of trace elements in spinel and garnet peridotites   总被引：6，自引：1，他引：5  
The distribution of trace elements in the upper mantle has been discussed on the basis of the trace element abundances in bulk rocks and constituent minerals of two spinel and garnet facies peridotite xenoliths in alkali basalts from eastern China. The data presented are consistent with the suggestion that highly incompatible elements (Rb, Ba, Th, U, Sr, Nb, Ta) mainly reside in intergranular components, and to a lesser extent in fluid inclusions in minerals. The LILE composition in olivine and orthopyroxene can be seriously affected by the presence of fluid inclusions. Consequently the subsolidus partitioning of the LILE cannot be used to infer the olivine-melt and orthopyroxene-melt partition coefficients for these elements. There is a significant difference in (Opx/Cpx)HREE ratios for spinel and garnet peridotites, suggesting a P-T control on equilibrium partition coefficients.  相似文献   

13.
Ghodrat Torabi 《Island Arc》2010,19(2):277-291
The Jandaq lamprophyres occur as eight mostly parallel dykes, which cross‐cut Eocene volcanic and sedimentary rocks of the Pis‐Kuh Formation in dominant north to south direction. These lamprophyres are mainly composed of kaersutite, clinopyroxene, olivine, feldspar, ilmenite, and spinel as primary minerals. The rocks studied here are enriched in alkalis, TiO2, large ion lithophile elements, and light rare‐earth elements (LREE), with SiO2 content between 41.7 and 46.2 wt%, and are classified as camptonite and alkaline lamprophyre according to the mineralogical and chemical characteristics. These rocks exhibit positive Eu anomalies (Eu/Eu* = 1.08–1.39) and are characterized by strong enrichment in LREE relative to heavy REEs, and also by varied Zr/Hf ratios. The geochemical features of the rocks suggest that the lamprophyre magmas were derived from low‐degree melting of an amphibole garnet lherzolite that experienced strong metasomatism by carbonate‐rich fluids in response to dehydration melting from the subducted slab. The Jandaq lamprophyric magmatism has been attributed to the former subduction of the Central–East Iranian microcontinent confining oceanic crust from the Triassic to Eocene, and decompression melting induced by the extensional basin of the Jandaq area in the early Oligocene.  相似文献   

14.
In the Late Cenozoic West Carpatian alkali olivine basalts spinel peridotite xenoliths have been found. Their mineral composition corresponds to those found in this type of xenoliths in alkali basalts throughout the world (Mg-rich olivine, Cr-diopside, clinopyroxene, spinel). For the studied West Carpathian alkali olivine basalts kaersutite amphibole and clinopyroxene magacrysts are characteristic. The presence of the xenoliths under consideration in the boundary zone between Tatra and Pannonian blocks is the consequence of tectonic conditions (thickness of the Crust) in the area mentioned as distinct from the other West Carpathian areas of alkali olivine basalt occurrences.  相似文献   

15.
The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km2, it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km2 of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.  相似文献   

16.
In three representative nodules contained in an alkali-olivine basalt, a succession of cumulate cycles has been noted: $$\begin{gathered} 1) olivine - orthopyroxene + \varepsilon (Cpx + Sp.); \hfill \\ 2) ortho - clinopyroxene + \varepsilon (Ol. + Sp. + Plag); \hfill \\ 3) ortho - clinopyroxene + Plag + \varepsilon (Ol. + Sp.). \hfill \\ \end{gathered} $$ The element distribution in the minerals enables us to say that these mafic and ultramafic nodules formed near the stability line of plagioclase at about 10 kb. These cumulates, which belong to a comagmatic series, from picrites to basalts, were formed in the upper mantle. They are associated with norites — Plag. + Opr. + Cpx. +≡ (Il. + Bi) — belonging to the same series, but crystallized in the deep part of the crust. On the other hand, these norites could be xenoliths taken away from an infragranitic basement of granulite facies.  相似文献   

17.
Jin  Zhang  Hong-fu  Zhang  Ji-feng  Ying  Yan-jie  Tang  Li-feng  Niu 《Island Arc》2008,17(2):231-241
Abstract The occurrence of the Pishikou mafic dike in the Qingdao region, China provides important constraints on the origin of Late Cretaceous (86–78 Ma) mafic magmatism on the eastern North China craton. The Pishikou mafic dike is distributed in the Cretaceous Laoshan granitoid body, Qingdao region and contains peridotitic and granulitic xenoliths, xenocrysts, and megacrysts. Rocks from the Pishikou mafic dike are basanites and have low SiO2 (< 42 wt%) and Al2O3 (12.5 wt%) contents, and high MgO (> 8 wt%), total alkalis (Na2O + K2O > 4.8 wt%, Na2O/K2O > 1), TiO2 (> 2.5 wt%), CaO (> 9 wt%) and P2O5 (> 1 wt%). In trace element abundances, they are highly enriched in large ion lithophile elements (LILEs) and light rare‐earth elements (LREEs) (ΣREE = 339–403 ppm, (La/Yb)N = 39–42) without high field strength element (HFSE) depletion. These rocks have radiogenic Sr and Pb, and less radiogenic Nd isotopic compositions [(87Sr/86Sr)i > 0.7059, εNd ≈ 2.7–3.8 (206Pb/204Pb)i ≈ 18.0 ± 0.1]. The diagnostic elemental ratios, such as Nb/La, Nb/U, and Nb/Th, are compatible with those of mid‐oceanic ridge basalts (MORBs) and oceanic island basalts (OIBs). Therefore, the Pishikou mafic dike has a geochemical feature completely different from those of the Early Cretaceous mafic dikes from the Qingdao region, but similar to those of back‐arc basalts from the Japan Sea. This geochemical feature suggests that the Pishikou mafic dike was derived from an asthenosphere source, but contaminated by materials from the subducted Pacific slab. The discovery of this mafic dike thus provides a petrological evidence for the contribution of subducted Pacific slab to the Late Cretaceous magmatism in the Qingdao region of the eastern North China craton.  相似文献   

18.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

19.
The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/86Sr (around 0.703) and 143Nd/144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/144Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange (87Sr/86Sr and 143Nd/144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.  相似文献   

20.
琼北新近纪碱性橄榄玄武岩中广泛出露大型地幔岩捕虏体 (最大尺度 2 0cm) ,但前人尚未在第四纪玄武岩中发现过此类捕虏体。经过对采自第四纪玄武岩中 14 0余个样品的微观结构观测 ,发现天然氧化作用揭示出样品中部分粗粒橄榄石晶体发育位错构造 ;同时由橄榄石和辉石晶粒组成的某些集合体型斑晶的结构 ,与新近纪玄武岩中大型地幔岩捕虏体的结构相同 ,从而确定第四纪玄武岩中存在橄榄石捕虏晶和微型地幔岩捕虏体 (最大尺度为 4mm)。捕虏晶和微型捕虏体的存在表明第四纪玄武岩岩浆与新近纪一样都来源于上地幔 ;同时 ,地幔岩捕虏体由大型演变为微型反映了岩浆上升速度的逐步降低 ,以及火山活动强度的减弱  相似文献   

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