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1.
Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust. Received: 26 September 1995 / Accepted: 5 June 1996  相似文献   

2.
HP rocks (eclogites and granulites) occur in the upper unit of the western Iberian allochthonous complexes in Spain and Portugal. This HP metamorphism was considered as Early Variscan or Ordovician in Spain and Precambrian in Portugal. We have dated eclogites retrogressed into granulites in the Bragança massif in NE Portugal by U–Pb on rutile and zircon. The upper intercept of the discordia gives 390±4 Ma that we consider as the age of the HP/HT metamorphism.  相似文献   

3.
Seismic investigations to determine the crustal structure in the southwestern part of the Iberian Peninsula have been initiated in 1970. First experiments were carried out during July 1970, when a series of ten shots was fired off Cabo de Sines (Portugal) in shallow water and recorded up to distances of 185 km along a SE-profile towards Huelva (Spain). The profile was reversed in December 1970, when a series of twelve shots was fired off the south coast near Fuzeta (east of Faro) and recorded up to distances of about 260 km along a NW-profile towards Cabo da Roca west of Lisboa. A considerable increase in the seismic efficiency of the explosions could be achieved by generating standing waves in the water.

The structure deduced exhibits some peculiar features. Below the Palaeozoic sediments a fairly high velocity of 6.4 km/sec is found for the dome-shaped basement in that area. The lower crust, which is separated from the upper crust by a distinct velocity inversion (with a minimum velocity of about 5.3–5.6 km/sec), is characterized by a velocity of 7.1 km/sec. From the geological evidence and the sequence of seismic velocities it must be concluded that the upper crustal block in the southwestern part of the Iberian Peninsula has been uplifted by about 2–5 km since Permo-Triassic time, thus emphasizing the significance of vertical movement in tectonic activity.

The top of the upper mantle (8.15 km/sec) was detected at a depth of 30 km close to the Atlantic coast in the west, while near the Algarve coast in the south the depth to the M-discontinuity is about 34–35 km. This result in conjunction with studies of earthquake focal mechanisms confirms the suggestion that the Iberian block is being underthrust under the African plate.  相似文献   


4.
The world's oldest diamond deposits occur in 2.67 Ga dikes and heterolithic breccias emplaced into greenstone belts of the Wawa and Abitibi Subprovinces, southern Superior Province, Canada. Thousands of white to yellow microdiamonds and macrodiamonds to 5 mm in width have been recovered by non-contaminating fusion techniques. The host rocks exhibit petrographic and compositional features that are characteristic of post-Archean minettes and spessartites of the calc-alkaline or shoshonitic lamprophyre clan. Based on chemical trends and petrographic evidence, host rocks that contain more than 16 wt.% MgO represent lamprophyre magmas that entrained cumulate olivine, probably at the base of the crust. Breccia bodies that are tens of metres wide at the two localities are somewhat atypical of late Archean lamprophyre occurrences in the Superior Province and owe their size to optimum conditions for magma ascent that were required to preserve the diamonds. Abundant altered ultramafic xenoliths occur in the host rocks. The majority of xenoliths studied (10 of 14) display uniform major element compositions similar to websterite cumulate suites derived from crystal fractionation processes at the base of post-Archean volcanic arcs. The xenoliths display highly variable trace element abundances that are characteristic of cryptic metasomatism associated with the flux of an oxidised fluid above a subduction zone.

The tectonic setting of the deposits and the nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 to 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of low-temperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of diamonds in most Phanerozoic subduction scenarios, however, may be too deep to be accessed by the lamprophyric magmas. In contrast, shallow subduction, as invoked for the distinctive occurrence of adakitic (slab-melt) type rocks in the southern Superior Province, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas.

The multiple requirements imposed on Archean tectonic models by occurrences of diamonds in hydrous shoshonitic rock types (spessartite and minette lamprophyres), along with distinctively metasomatised xenoliths, strongly favour plate tectonic subduction models of orogeny. Evidence of slightly earlier mantle plumes, such as 2.7 Ga komatiites, only strengthens the need for a subduction-driven low-temperature thermal anomaly in the Archean mantle prior to lamprophyric magmatism.  相似文献   


5.
The occurrence of Lower Paleozoic mafic magmatic rocks in the Central Iberian Zone (CIZ) of the Variscan Orogen is rare. Amphibolites and metagabbros embedded in the metasediments of the Douro-Beiras Supergroup outcrop at Farminhão, Viseu (central-north Portugal). The protoliths of these two rock types are tholeiites presenting different isotopic signatures (εNd480 = +4.63 to +4.93 and +5.74 to +7.67) and incompatible element ratios (normalized La/Lu up to 4.5 and down to 0.7), respectively which suggests they are not cogenetic. The closely related meta-ultramafic rocks are considered as cumulates generated from the magmas that originated the metagabbros. The elemental and isotopic features of the metagabbros are similar to those reported for amphibolitic occurrences in Tenzuela (near Segovia, Spain) which allows the proposition of a similar age for the mafic rocks of Farminhão. Despite the depleted characteristics of the studied rocks, they are interpreted as having been formed during a continental rifting process characterized by variable degrees of stretching, some 100 Ma after the end of the deposition of the Douro-Beiras Supergroup. This rifting event marks the onset of the Variscan Cycle (s.l.) in the Central Iberian Zone. The occurrence of these metabasic rocks near the confluence between the Porto-Viseu Metamorphic Belt and the Juzbado-Penalva do Castelo Shear Zone suggests that these first-order structures may have worked as weakness zones constraining the ascent of magmas during the Ordovician. The Lower Ordovician metabasic rocks here studied are chemically similar to the abundant lower to medium Cambrian magmatic rocks of the Ossa Morena Zone, also in Iberia, further reinforcing the diachronous character of the opening of the Rheic Ocean that later propagated to the eastern sectors of the European Variscan Belt.  相似文献   

6.
峨嵋山大岩浆省由大量的溢流玄武岩及其伴生的镁铁和超镁铁侵入岩组成。攀西地区的一些层状辉长岩体形成于260Ma,与早期报道的峨嵋山大岩浆省的年代相同。这些岩体中含有巨大的钒钛磁铁矿床,矿体呈似层状及透镜状产在层状辉长岩体的下部层位,不同于典型的层状岩体(如布什维尔德岩体)的磁铁矿床。地球化学资料表明,攀西地区含磁铁矿的岩体是从高度演化的基性岩浆中结晶而成,因为富硅的岩浆分离使得母岩浆高度富集铁、钛和钒。相对围岩来说,磁铁矿石形成较晚,是从氧化物矿浆中结晶的产物。矿石中有丰富的含水矿物相,流体的参与对氧化物矿浆的形成有重要的作用。  相似文献   

7.
K. Vijaya Kumar  K. Rathna 《Lithos》2008,104(1-4):306-326
Mesoproterozoic rift-zone magmatism in the Prakasam Alkaline Province of Eastern Ghats Belt, India is represented by three geochemically distinct primary mafic magmas and their plutonic differentiates. The three mafic magmas correspond to the alkali basaltic dykes, gabbroic dykes and lamprophyric dykes. The dyke activity is synchronous with the host plutons and belongs to the 1350–1250 Ma period Mesoproterozoic magmatism. Geochemical signatures suggest that the alkali basaltic dykes have a source in the thermal boundary layer, which has a history of prior melt extraction followed by enrichment. Both the gabbroic and lamprophyric dykes are derived from lithospheric sources and their geochemical variation can be explained by “vein-plus-wall-rock melting model”. Vein/wall-rock ratio is low for the sources of gabbroic dykes, whereas it is high for the lamprophyric dykes. Geochemistry of the gabbro dykes further indicates preservation of previous arc-signals by the lithosphere beneath the Prakasam Alkaline Province during the Mesoproterozoic. Geochemical signatures of lamproite, which could be a cratonic expression of the rift-triggered magmatism in the Prakasam Province, suggest a general increase in the metasomatic imprint with increasing lithosphere thickness from cratonic margin towards interior. It is found that geochemistry of continental rift-zone magmatism of the Prakasam rift is remarkably similar to that of the Gardar rift of South Greenland. It appears that the geodynamic conditions under which melting occurred in the Prakasam Alkaline Province are similar to that of a propagating rift with variable contributions from the convective mantle and subcontinental lithosphere mantle to the rift-zone magmas. The present study illustrates how fertility and chemical heterogeneity of the lithosphere play significant roles in the creation of enormous geochemical diversity characteristic of continental rift-zone magmatism.  相似文献   

8.
Eslamy peninsula, 360 km2 in area, is located in the eastern coast of Urmieh lake in the northwest of Iran. This peninsula is a complex stratovolcano with a collapsed center, which is elevated due to later intrusions of sub-volcanic masses with trachytic to microsyenitic composition. The composite cone consists of a sequence of leucite tephrite, tephrite, leucite basanite, basanite and related pyroclastic rocks. Magmatic activities in the Eslamy peninsula begin with potassic alkaline to ultrapotassic and basic, silica-undersaturated shoshonitic rocks and they are followed by intrusions of lamprophyric dykes and end with acidic magmatism including trachytic, microsyenitic, syenitic and phonolitic domes. The original magma of the Eslamy peninsula rocks has a potassic alkaline nature (Roman type) rich in LREE and LILE and depleted of HREE. These characteristics suggest that the origin of magma can be from deep mantle with a garnet lherzolite composition, a low partial melting rate which has been contaminated by crustal materials in its way up. Fractional crystallization of olivine, diopsidic clinopyroxene and leucite played an important role in the evolution of magmas. Scrutinizing the geodynamic environment of Eslamy peninsula rocks in discrimination diagrams indicates that these rocks must have been formed in a post-collision magmatic arc setting.  相似文献   

9.
The Spanish-Portuguese Pyrite Belt covers a large area in the SW part of the Iberian Peninsula from Seville to the westcoast of Portugal. Total reserves of aprox. 1.000 million tons of massive sulphide ores have an average content of 46% S, 42% Fe, and 2–4% Cu+Pb+Zn. The stratiform sulphide deposits and accompanying manganese mineralizations are of synsedimentary-exhalative origin. They occur in a Lower Carboniferous, geosynclinal, volcanic-sedimentary rock sequence, strongly folded during the Hercynian Orogeny. A brief outline of the regional geology of this ore province is given, and the geology of three mining districts is described: Lousal (Portugal), La Zarza and Tharsis (Huelva Province, Spain). A close relationship between sulphide and manganese ores with the submarine, acid alkaline volcanism is emphasized. Solfataric activity is responsible for the formation of sulphides in the final stages of volcanic extrusions. The ore concentration in big deposits (ore-lenses with up to 100 million tons of massive sulphides) has been due to inflows of sulphide muds and/or detrital sulphides into newly formed depressions of a contineously changing seafloor topography due to volcano tectonic movements.  相似文献   

10.
http://www.sciencedirect.com/science/article/pii/S1674987113000595   总被引：11，自引：0，他引：11  
Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province(ELIP),SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-(P) oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.  相似文献   

11.
Precise U-Pb ages have been obtained from zircon and baddeleyite in plutonic rocks from the Siljan-Skrim area in the Vestfold Graben Segment in the south-western part of the Permian Oslo Rift. In addition, common lead data from feldspars in the same rocks are presented. The measured common Pb compositions of the feldspars are used to correct for initial common Pb in the zircons. The common Pb data indicate that U has been present in the feldspars since the time of crystallisation, probably in a late magmatic volatile phase in fluid inclusions.

The Siljan-Skrim area comprises two intrusive complexes. The northern Skrim-Mykle complex consists of two large, composite intrusive bodies, the Skrim larvikite and the Mykle ekerite. The southern Siljan-Hvarnes complex consists of a series of monzonitic to syenitic intrusions, which are nested in a systematic way, indicating decreasing emplacement age from east to west.

U-Pb zircon ages range from 281.2 ± 0.6 Ma (2σ) to 279.8 ± 0.7 Ma for three samples from the northern part of the Siljan-Skrim area and from 278.5 ± 0.8 Ma to 277.3 ± 0.8 Ma for four samples from the southern part of the area.

The geochronological data suggest that large amounts of magma were emplaced within very short time spans, i.e. less than 2 Ma for each of the two complexes. The high precision of the U-Pb age results allows discrimination between intrusive activity in the northern and southern parts of the Siljan-Skrim area and suggests that emplacement of magmas was episodic.  相似文献   


12.
Syenites are important or predominant components of several plutonic complexes, emplaced between 456 and 415 Ma along the NW margin of the Caledonian orogenic belt, adjacent to the Lewisian foreland, in W and NW Scotland. Although there are, in detail, chemical differences between the syenites from each centre, they form a well-defined compositional group overall. Ratios amongst their trace elements (especially very high values of La/Nb) are quite different from those trachytes and syenites formed by fractional crystallisation of ocean-island basalts and their continental equivalents, emplaced in regions of anorogenic crustal tension. Instead, the Scottish Caledonian syenites closely resemble chemically the fractional-crystallisation residua of potassic subduction-related magmas, such as the shoshonitic series. A comendite minor intrusion from a swarm associated with the Loch Borralan and Loch Ailsh syenitic complexes is remarkably similar in composition to Recent obsidian from the shoshonitic volcano of Lipari, in the Aeolian Arc. Published Sr- and Pb-isotopic ratios preclude a significant component of either upper (Proterozoic Moine schists) or lower crust (granulite-facies Archaean Lewisian or Proterozoic Grenvillian gneisses) in all these syenites, except in local syenitic facies of the Glenelg-Ratagain complex. Fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas. The phases involved in this process may have included plagioclase, alkali feldspar, pyroxene, amphibole, biotite, garnet, Fe-Ti oxide, sphene, allanite, apatite, zircon and zirconolite, and therefore all the ratios amongst even the so-called incompatible elements may have changed during the evolution of the leucocratic magmas. Nevertheless, a detailed study of the Glen Dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas. These appear to be picritic shoshonite (MgO> 15%, Ni>400ppm, La/Yb20, La/Nb>5) for the Glen Dessarry, Loch Borralan and Loch Ailsh syenites, and picritic ultrapotassic magmas (MgO>15%, Ni>400 ppm, La/ Yb60, La/Nb>7) — such as would crystallise to minettes — for the Loch Loyal and Glenelg-Ratagain syenites. Mafic shoshonites were erupted amongst the 410 Ma Lome lavas of this region and also occur as widespread approximately-contemporaneous volcanic feeder plugs. Minettes of similar age are also common as dykes in NW Scotland. Numerous large tonalite-granite complexes, with minor diorites and gabbros, were emplaced in W and NW Scotland between 435 and 400 Ma. These are generally acknowledged to be mixtures of magmas from crustal and mantle sources. Their more mafic members show compositional features, such as very high Ba and Sr, which group them with the rare syenites and the Lorne lavas of the same region into a distinctive geochemical province, within which shoshonitic and allied magmas were the mantle-derived component. Two models are presented to account for the generation of strongly-potassic, subduction-related magmas several hundred km behind a NW-directed subduction zone, or during the period immediately following continental collision.  相似文献   

13.
Anorthosite—adamellite complexes are the chief manifestations of Elsonian magmatic activity of Paleohelikian age (about ?1500 to ?1400 Ma) in Labrador, Canada. Magmatism of similar age and anorogenic character, though with fewer occurrences of massif anorthosite, is present in a belt across the mid-continent and southwestern United States. Anorthosite—quartz mangerite complexes in the Grenville Province lie along the trend of this belt and, although few ages older than the profound Grenvillian regional metamorphism about ?1100 Ma have been determined on them, circumstantial evidence suggests that these also are dominantly of Paleohelikian age.The Labrador complexes are intruded into high-grade metamorphic terrane, older by at least 200 to 300 Ma than the Elsonian magmatism. Typical association of anorthosite massifs with high-grade metamorphic terranes, in Labrador and elsewhere, is probably due to their intrusion into older, stabilized, cratonic crust. The anorthosite—adamellite (and anorthosite—quartz mangerite) complexes are products of bimodal magmatism, and an anorogenic cratonic setting is considered to be of fundamental importance to development of the suites. Olivine tholeiite magmas fractionate to produce high-A1 tholeiitic magmas at or near the base of the cratonic crust, and these magmas are the parents from which anorthosite massifs develop by plagioclase fractionation at higher levels within the crust. Adamellite (quartz mangerite) magmas develop mainly by partial fusion of deep crustal rocks, caused by heat of crystallization from the fractionating olivine tholeiite magmas in the staging region, at or near the base of the crust, and are intruded upward into the crustal complexes; rapakivi textures and chemistries are characteristic products of these magmas. Ferrodiorites, widely associated with anorthosite massifs, probably form as late-stage fractionation products of basic magmas in the subcrustal staging region and are intruded into the massifs in their final stages of development (before intrusion of adamellite or quartz mangerite magmas).The Neohelikian record, dominated by terrestrial sedimentation, basaltic extrusive and intrusive activity, and alkalic magmatism, began soon after ?1400 Ma in the mid-continent United States, central Labrador and southern Greenland. The lithological assemblages have been interpreted by several authors as similar to those of intracontinental rift zones. The following sequence of events: intrusion of Paleohelikian anorthosite—adamellite complexes (granitic intrusion and/or rhyolitic extrusion only, in some places), strong uplift and erosion, crustal attenuation causing basin formation, Neohelikian terrestrial sedimentation, rifting or incipient rifting, renewed basaltic magmatism, and alkalic magmatism, is believed to record a continuing evolving process of mantle—crust interactions over a broad belt across North America.  相似文献   

14.
云南镇沅金矿煌斑岩的蚀变流体特征黄智龙，王联魁，朱成明’（中国科学院地球化学研究所，贵阳５５０００２）（中国科学院广州地球化学研究所，广州５１０６４０）关键词云南镇沅，煌斑岩，蚀交流体，元素迁移煌斑岩为易受蚀变的一种岩石，国外报道的许多能代表原生岩浆...  相似文献   

15.
Mafic microgranular enclaves, composed of diopside and rare magnesium biotite phenocrysts in a groundmass of diopside, biotite, apatite, Fe-Ti-oxides, and alkali feldspar, are associated with Neoproterozoic Piquiri potassic syenite in southern Brazil. Co-genetic mica and clinopyroxene cumulates present inclusions of pyrope-rich garnet in diopside phenocrysts. Textural evidence, as well as the chemical and mineralogical composition, suggest that enclaves crystallized from a lamprophyric magma and co-mingled with the host syenitic magma. The contrasting temperature between both magmas and the consequent chilling was important for the preservation of some early-crystallized minerals in the mafic magma. Diopside groundmass grains contain micro-inclusions of K-rich augite and phlogopite, and some clinopyroxene phenocrysts and elongate groundmass crystals have potassium-rich cores. The pyrope-rich garnet have high #mg number (67–68), with appreciable amounts of Na2O and K2O comparable to pyrope synthesized at 5 GPa. The extremely high K2O contents of K-rich augite micro-inclusions suggest non-equilibrium with the parental magma, whereas the other K-rich clinopyroxenes are similar to K-clinopyroxenes produced at 5–6 GPa. K-clinopyroxene and garnet in mafic microgranular enclaves suggest that lamprophyric magma started its crystallization at upper mantle conditions, and chilled clinopyroxenes with measurable amounts of K2O are taken as evidence that co-mingling began still at mantle pressures.  相似文献   

16.
The first U–Pb geochronological results on the magmatic alignment of the Los Pedroches batholith are presented. The batholith is composed of a main granodioritic unit, several granite plutons and an important acid to basic dyke complex, all of them intrusive after the main Variscan regional deformation phase, D1, along the boundary between the Ossa-Morena and Central Iberian zones (SW Iberian Massif). Zircons from samples on both extremes of the granodiorite massif record nearly simultaneous magmatic crystallization at ca. 308 Ma, while the emplacement of granite plutons was diachronic between 314 and 304 Ma. The U–Pb results combined with new field and textural observations allow to better constrain the age of Variscan deformations D2 and D3 across the region, while the age of D1 remains imprecise. Transcurrent D2 shearing-tightening of D1 folds occurred around 314 Ma (lower Westphalian) in relation to the emplacement of the first granitic magmas. D3 faults and shear bands bearing a strong extensional component developed at ca. 308 Ma (upper Westphalian), associated to the intrusion of the main granodiorite pluton (granodiorite) of the batholith. Together with available geochemical and geophysical information, these results point to the Variscan reactivation of lithospheric fractures at the origin and subsequent emplacement of hybrid magmas within this sector of the Massif.  相似文献   

17.
The Cretaceous M?gantic intrusive complex of southern Qu?beccontains early noritic gabbrodiorites which represent cumulatesfrom crustally contaminated hawaiite to syenite magmas. Wholerock and mineral chemistry, as well as textural evidence, indicatethat post-cumulus recrystallization and reaction were important,and most of the amphibole and biotite are thought to have formedin this way. A younger plutonic quartz-syenite ringdyke maynot be cogenetic with the gabbro-diorites sice it lacks orthopyroxene.It may, however, be cogenetic with basaltic to riebeckite granitedykes. Fractionation of olivine, plagioclase, aluminous clinopyroxene,and minor Ti-magnetite from critically undersaturated alkalibasaltic magmas generated hawaiitic magmas. The developmentof quartzbearing mugearitic and syenitic residua from the hawaiitescan best be modelled by fractionation of amphibole, plagioclase,olivine, oxides, and apatite. Attempts to model fractionationusing observed phenocrysts (including clinopyroxene) were unsuccessful.Amphibole fractionation is interpreted to have taken place througha reaction with still-porous, higher-temperature cumulates onthe walls of the magma chamber. The plutonic syenites probablyrepresent alkali feldspar cumulates from the residual syeniticmelts. Magnesian calc-alkaline lamprophyres exhibit olivineto phlogopite reaction textures, are enriched in Cr, Ni, K,Rb, Nb, Y, Zr, and Si relative to the basaltic dykes, yet havesimilar incompatible element ratios. Their relation to the basaltsis problematical. The late biotite-granite core to the complexis identical to typical White Mountain granites and may haveformed as an anatectic cap on rising, fractionating, mantlederivedmagmas.  相似文献   

18.
Both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes in the Panxi region (SW China) are spatially and temporally associated with syenitic plutons, which are part of the ~ 260 Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from 1 to 5 m and length from 50 to 300 m. The dikes are mainly composed of K-feldspar, albite, aegirine and arfvedsonite, however, mineral modes are different in the Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes. The major Nb–Ta-bearing mineral in the dikes is pyrochlore, which is closely associated with albite and occurs in places with intensive albitization. Rocks of the Nb–Ta-mineralized syenitic dikes contain more albite and less K-feldspar than the Nb–Ta-poor dikes, and have compositions more evolved than the Nb–Ta-poor dikes, indicating that the Nb–Ta-mineralized syenitic dikes formed from a highly evolved magma. We analyzed the B concentrations and B isotopic compositions of the samples of both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes and associated syenitic pluton using a single column purification method and ICP-AES and MC-ICP-MS techniques. The samples of the Nb–Ta-mineralized syenitic dikes have whole-rock B concentrations ranging from 11.4 to 23.9 ppm and δ11B values from − 17.95 to − 14.54‰, whereas the samples of the Nb–Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5 ppm and δ11B values from − 13.45 to − 10.02‰. The high B concentration of the Nb–Ta-mineralized dikes relative to the Nb–Ta-poor dikes is consistent with that B is incompatible and tends to be rich in more evolved magma. The relatively low δ11B values of the Nb–Ta-mineralized dikes indicate that the B isotopes may have fractionated between fluids and rocks in a transitional, magmatic–hydrothermal stage. We propose that the highly evolved magmas in a transitional, magmatic–hydrothermal stage may have Nb– and Ta–fluorine complexes dissolved in the hydrothermal fluids in the presence of Na+. Albite crystallization due to intensive albitization in this stage resulted in the decrease of Na+ in the fluids, decomposing the Nb– and Ta–fluorine complexes. The released Nb and Ta from the complexes were then dissolved in the fluids and finally entered the lattice of pyrochlore crystals in the stage of albitization.  相似文献   

19.
The relatively complete and well preserved shell of a turtle, from the middle Cenomanian of Nazaré (Portugal), is studied here. It is recognized as a member of the crown group Pleurodira and, more specifically, of Bothremydidae. Pleurodira are one of the two lineages of modern turtles, their origin being in Gondwana. Pleurodira are very abundant in the uppermost Cretaceous record of Europe. However, this new finding is one of the few occurrences in the lower Upper Cretaceous of Laurasia. A single member of Bothremydidae had so far been identified in Portugal: the late Campanian-early Maastrichtian Rosasia soutoi. It was the only Cretaceous turtle identified in that country both at specific and at generic levels, being exclusive of Portugal. The taxon from Nazaré is identified as Algorachelus peregrinus, this form is also present in the contemporaneous beds in Spain, and is the oldest member of Bothremydidae in Laurasia. Algorachelus peregrinus is confirmed here to be a coastal form, which facilitated its spread. The two oldest known bothremydids from Laurasia, the European A. peregrinus and the North American Paiutemys tibert, are compared for the first time. They are recognized as closely related taxa. This study provides new data allowing a more precisely characterization of the oldest so far known dispersal event of Pleurodira in Laurasia, which was performed by an African lineage of Bothremydidae that reached the east coast of the Atlantic Ocean at least in the middle Cenomanian, and the west region of that Ocean at least in the late Cenomanian.  相似文献   

20.
Mesozoic—Cainozoic volcanism was concentrated on the youngest eastern Australian craton. Basaltic activity (with some felsic fractionation) has predominated over Mesozoic interludes of calcalkaline volcanism (rhyolites, dacites, trachytes andesites) and more isolated shoshonitic activity (now represented by appinitic, syenitic, granitic and lamprophyric complexes).Epeirogenic movements and associated sea-floor spreading and orogenic episodes at the continental margins, initiated and controlled much of the volcanism. Basin edges, faults, lineaments and their intersections were important in locating sites of volcanism; some fundamental structural lines have focussed volcanism over 300–600 km.The eastern Mesozoic basaltic volcanism shows a late Jurassic N-S trend from undersaturated to saturated compositions, with increasing intensity of melting towards a major Tasmanian-Antarctic thermo-tectonic event. A late Jurassic-late Cretaceous E-W trend may extend from possible ‘kimberlites’ through shoshonitic to calcalkaline activity with increasing proximity to orogenic movements along the New Zealand ‘Geosyncline’.Cainozoic basaltic volcanism reflects the NNE drift of Australia under Atlantic-Indian-Southern Ocean sea-floor spreading, with a debatable role for subduction along the Tasman Sea margin. The ultimate mechanisms of volcanism are not clearly understood. Drift of cratonic structural weaknesses over thermal anomalies in the mantle, with generation of magmas from a geochemically zoned Lower Velocity Zone under influence of uplifts, lithospheric thickness and periodic release of thermal energy, seems to partly explain observed patterns of E. Australian volcanism.  相似文献   

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