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1.
This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits.

An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone.  相似文献   


2.
The Late Cretaceous Goiás alkaline province (GAP) is composed of alkaline plutonic bodies (north), diatremes with subordinate flows, dykes, and plugs (central), and dominant lava flows and pyroclastics (south). In this work, we describe field and petrographic aspects of coherent kamafugites from Santo Antônio da Barra in the southern GAP and Águas Emendadas in the central GAP. Intensive variables inferred from simulations using whole-rock chemical data constrain the behavior of these kamafugitic magmas from their origin in the mantle to their final emplacement as upper-crust magma chambers, diatreme structures, and lava flows. In most cases, the evidence indicates that differentiation in both deep- and shallow-seated magma chambers intervened in their evolution. The discordance between the Precambrian basement and the Phanerozoic sedimentary rocks is the most likely site where the shallow chambers were established, whereas the deeper chambers were probably located in the upper crust. CO2 seems to be the most important volatile phase. An interplay of various possible evolution paths involving crystal fractionation, magma mixing, and liquid immiscibility may explain the range of observed features in GAP kamafugites.  相似文献   

3.
Permian to Cretaceous mélange of the McHugh Complex on the Kenai Peninsula, south-central Alaska includes blocks and belts of graywacke, argillite, limestone, chert, basalt, gabbro, and ultramafic rocks, intruded by a variety of igneous rocks. An oceanic plate stratigraphy is repeated hundreds of times across the map area, but most structures at the outcrop scale extend lithological layering. Strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation, forming broken formation and mélange. Deformation was noncoaxial, and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing, generally thinly sedimented oceanic plate. Soft-sediment deformation processes do not appear to have played a major role in the formation of the mélange. A model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to σ1 are contracted in both the brittle and ductile regimes, layers at 30–45° to σ1 are extended in the brittle regime and contracted in the ductile regime, and layers at angles greater than 45° to σ1 are extended in both the brittle and ductile regimes. Imbrication in thrust duplexes occurs at deeper levels within the wedge. Many structures within mélange of the McHugh Complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge. A displacement field for the McHugh Complex on the lower Kenai Peninsula includes three belts: an inboard belt of Late Triassic rocks records west-to-east-directed slip of hanging walls, a central belt of predominantly Early Jurassic rocks records north–south directed displacements, and Early Cretaceous rocks in an outboard belt preserve southwest–northeast directed slip vectors. Although precise ages of accretion are unknown, slip directions are compatible with inferred plate motions during the general time frame of accretion of the McHugh Complex. The slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates, which became more oblique with time. They are not considered indicative of strain partitioning into belts of orogen-parallel and orogen-perpendicular displacements, because the kinematic data are derived from the earliest preserved structures, whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion-related fabrics.  相似文献   

4.
We present the results of a U–Pb perovskite age study of kamafugites from Mata da Corda (MC) and Santo Antônio da Barra (SAB), Minas-Goiás alkaline province, Brazil. Perovskite crystals were separated from MC mafurites, ugandites, and cognate pyroxenites, as well as from SAB melilite mafurite. The range of ages of Brazilian kamafugitic samples is 15 Ma. The 206Pb/238U perovskite ages generally cluster into three age groupings: 88–90, 80–81, and 75–76 Ma. The two younger periods of kamafugitic magmatism occur in the MC area, whereas the older samples are from the SAB area. These new age results provide the first robust evidence of a progressive eastward younging of mafic alkaline magmatism, most likely related to a mantle plume hotspot track.  相似文献   

5.
The Raspas Metamorphic Complex of southwestern Ecuador is regarded as the southernmost remnant of oceanic and continental terranes accreted in the latest Jurassic–Early Cretaceous. It consists of variably metamorphosed rock types. (1) Mafic and ultramafic rocks metamorphosed under high-pressure (HP) conditions (eclogite facies) show oceanic plateau affinities with flat REE chondrite-normalized patterns, Nd150 Ma ranging from +4.6 to 9.8 and initial Pb isotopic ratios intermediate between MORB and OIB. (2) Sedimentary rocks metamorphosed under eclogitic conditions exhibit LREE enriched patterns, strong negative Eu anomalies, Rb, Nb, U, Th, Pb enrichments, low Nd150 Ma values (from −6.4 to −9.5), and high initial 87Sr/86Sr and 206,207,208Pb/204Pb isotopic ratios suggesting they were originally sediments derived from the erosion of an old continental crust. (3) Epidote-bearing amphibolites show N-MORB affinities with LREE depleted patterns, LILE, Zr, Hf and Th depletion, high Nd150 Ma (>+10) and low initial Pb isotopic ratios.The present-day well defined internal structure of the Raspas Metamorphic Complex seems to be inconsistent with the formerly proposed interpretation of a “tectonic mélange”. The association of oceanic plateau rocks and continent-derived sediments both metamorphosed in HP conditions suggests that the thin edge of the oceanic plateau first entered the subduction zone and dragged sediments downward of the accretionary wedge along the Wadatti–Benioff zone. Subsequently, when its thickest part arrived into the subduction zone, the oceanic plateau jammed the subduction processes, due to its high buoyancy.In Ecuador and Colombia, the latest Jurassic–Early Cretaceous suture involves HP oceanic plateau rocks and N-MORB rocks metamorphosed under lower grades, suggesting a composite or polyphase nature for the latest Jurassic–Early Cretaceous accretionary event.  相似文献   

6.
The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the São Luis-West Africa craton and the São Francisco-Congo-Kasai cratons. New Sm/Nd and U/Pb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma.The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandão massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic São Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province.Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (TDM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandão massif to the east yields both Transamazonian and Archean U/Pb zircon and Nd (TDM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaço Group.Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeú-Paraíba (SPP), Riacho Pontal (SRP), and Piancó-Alto Brígida (SPAB) fold belts in the central domain formed ˜ 1.0 ± 0.1 Ga, based on U/Pb ages of zircons. Nd model ages (TDM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments.Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the São Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandão massifs and the older crustal blocks of the central domain were originally part of the São Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.  相似文献   

7.
The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.  相似文献   

8.
A low-angle thrust fault places high-PT granulites (hangingwall) of the Internal Zone of the Neoproterozoic Brasília Belt (Tocantins Province, central Brazil) in contact with a lower-grade footwall (External Zone) comprised of nappes of distal passive margin- and back-arc basin-related supracrustals. The footwall units were emplaced at  750 Ma onto proximal sedimentary rocks (Paranoá Group) of the São Francisco paleo-continent passive margin. The high-PT belt is comprised of 645–630 Ma granulite-facies paragneiss and orthogneiss, and mafic–ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at  750 Ma. These complexes occur within lower-grade metasedimentary rocks in the hangingwall of the Maranhão River Thrust, which forms the Internal Zone–External Zone boundary fault to the north of the Pirineus Zone of High Strain. Detailed lithostructural studies carried out in Maranhão River Thrust hangingwall and footwall metasedimentary rocks between the Niquelândia and Barro Alto complexes, and also to the east of these, indicate the same lithotypes and Sm–Nd isotopic signatures, and the same D1–D2 progressive deformation and greenschist-facies metamorphism. Additionally, footwall metasedimentary rocks exclusively display a post-D2 deformation indicating that the Maranhão River Thrust propagated through upper crustal rocks of the Paranoá Group relatively late during the tectonic evolution of the belt. Fault propagation was a consequence of intraplate underthrusting during granulite exhumation. The results allow for a better tectonic understanding of the Brasília Belt and the Tocantins Province, as well as explaining the presence of the Pirineus Zone of High Strain.  相似文献   

9.
Axial ratio—fluctuation measurements made on markers (oncolites, oolites, quartz grains, and accretionary lapilli) on bedding-perpendicular sections of unstrained sedimentary rocks have been homogeneously strained mathematically by set amounts, using published transformation equations, to provide a simulation of natural tectonic deformation. The resultant data have been used as test input to the strain analysis computer programs of Dunnet and Siddans (1971) and Matthews et al. (1974), which were devised to assess tectonic strain in assemblages of strained elliptical markers, with the use of limited specific assumptions concerning the nature of the pre-tectonic fabrics of the markers. The tests have shown that even slight deviations in the initial fabrics from the assumptions involved in the use of the strain analysis methods may lead to significant errors in the strain determinations. Errors are particularly large when fabrics which were imbricate in the unstrained state are used with the mistaken assumption that they were bedding-symmetric.  相似文献   

10.
Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0 × 0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations.The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites.The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P2O5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions.  相似文献   

11.
Keilite (Fe>0.5,Mg<0.5)S, the iron-dominant cubic analog of niningerite, (Mg>0.5,Fe<0.5)S, occurs in enstatite chondrites [Shimizu, M., Yoshida, H., Mandarino, J.A., 2002. The new mineral species keilite, (Fe,Mg)S, the iron-dominant analog of niningerite. Can. Mineral. 40, 1687–1692]. I find that keilite occurs only in enstatite chondrite impact-melt rocks and impact-melt breccias. Based on the phase relations in the system MgS–MnS–CaS–FeS [Skinner, B.J., Luce, F.D., 1971. Solid solutions of the type (Ca,Mg,Mn,Fe)S and their use as geothermometers for the enstatite chondrites. Am. Mineral. 56, 1269–1296], I conclude that keilite formed from niningerite or alabandite (Mn>0.5,Fe<0.5)S by reaction with troilite (FeS) at elevated temperatures of well above 500 °C (the lowest equilibration temperature of keilite), but it is likely that the maximum temperatures during melting experienced by keilite-bearing impact-melt rocks and impact-melt breccias were considerably higher, perhaps >1500 °C, as indicted by the occurrence of euhedral enstatite that formed from a melt [McCoy, T.J., Dickinson, T.L., Lofgren, G.E., 1999. Partial melting of the Indarch (EH4) meteorite: a textural, chemical, and phase relations view of melting and melt migration. Meteorit. Planet. Sci. 34, 735–746]. Based on the classifications of the keilite-bearing meteorites as impact-melt rocks and impact-melt breccias and my own textural observations, I conclude that this elevated temperature was reached as a result of impact and not internal heating and melting, followed by fast cooling, thus, quenching in keilite. Enstatite chondrite impact-melt rocks and impact-melt breccias that do not contain keilite may have been more deeply buried after impact and, hence, cooled slowly and were annealed so that FeS exsolved from keilite, concomitant with the formation of niningerite, alabandite or various (Mn,Mg,Fe) mixed sulfides.  相似文献   

12.
The impact of gigantic meteorites momentarily creates extremely high pressures and temperatures, leading to the formation of distinctive objects — impact breccias and impactites — at the points where they fall. The Zhamanshin crater is one such place. This annular structure has a diameter of about 10 km, contoured by a low rim made up of a body of shattered Paleozoic rocks thrown out from the ring's center, from depths exceeding 100 - 200 m. Among these breccias have been found pieces and lenticular bodies of remelted rocks — impactites, called zhamanshinites. These thermally altered rocks from a series ranging from baked clays to cinder-like, partly recrystallized substances, glass, and black drops and spattered droplets (up to 1 - 5 cm) of isotropic glass that undoubtedly hardened as they fell through the air. The refused neogenic substances formed by selective evaporation are characterized by a considerable deficit of K and Na, excess Al2O3 and SiO2, a lack of H2O, and a sharp predominance of Fe+2 over Fe+3. Their characteristic shape and distinctive structure and composition suggests that these remelted droplets are tektites, which were called irgizites when first found in the USSR. Their occurrence in the meteorite crater confirms the idea of a genetic unity between tektites and impactites. Selective evaporation of the more volatile elements is characteristic of the surfaces of small planets lacking an atmosphere, but is not characteristic of the earth's surface. The processes and substances occurring at the impact points of gigantic meteorites are therefore unique on the earth, but highly typical of the surfaces of the Moon, Mars, Mercury, and other relatively small celestial bodies.  相似文献   

13.
In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.  相似文献   

14.
A Nappe system south to southwest of the São Francisco Craton represents the southern extension of the Brasília belt and describes an inverted metamorphic pile of greenschist facies toward amphibolite facies. The Aiuruoca-Andrelândia nappe is one of the nappes of this system. The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambu and Aiuruoca (MG), consists of a structural-metamorphic domain transported toward the E-NE. There is a metamorphic transition, from the kyanite zone to kyanite and sillimanite coexistence, until the sillimanite zone. Metapelitic rocks preserve high-pressure parageneses (Rt–Ky–Grt–Ms–Bt–Pl–Qtz) and contain retrograde eclogitic rocks. Sil–Pl–Qtz coronitic intergrowths around garnets are common decompressive textures. Kyanite schists register the Pmax of 11 kbar at 660 °C and define a decompressive path until 6–7.5 kbar at 650 °C. These P–T conditions represent the equilibrium in S2 schistosity (amphibolite facies) and the beginning of the cooling path in the Ky–Sil transition. The decompressive path suggests an extrusional process, immediately after burying at about 60 km. Exhumation controlled by convergent events, related to the São Francisco Plate subduction and tectonic erosion, took these units, isothermally, to higher levels (20–33 km). Later, the metamorphic path shifted toward near-isobaric cooling.  相似文献   

15.
The mineralogy and chemistry of some analcime-rich igneous rocks — analcimites from the potassic Highwood province, Montana, and an analcime tinguaite from the Dunedin Volcanic Complex, New Zealand — have been investigated to obtain some indication of the KNa substitution in natural analcimes. The analcimes have potash-poor compositions, the K2O revealed by the rock analyses being represented in the mode mainly by Or-rich feldspar (hyalophane or sanidine). These data and the evidence afforded by other analcime compositions confirm earlier conclusions based on experimental studies (Peters, Luth and Tuttle, 1966), namely that analcime solid solutions may contain up to approximately 2% K2O. Some aspects of analcime-rich igneous assemblages are also discussed.  相似文献   

16.
Summary The paper reviews the published work, and presents new data, on the four occurrences of carbonatite that have been recognised in Italy since 1990. All are Recent in age. Three are extrusive and comprise tuffs and breccias while the fourth is high level and consists of tuffisites. They form diatremes with tuff rings, three of them being intimately associated with melilititic tuffs and lavas. Two of the occurrences include carbonatitessensu stricto i.e. calcite is > 50% by volume, while the other two are `carbonatitic', primary igneous calcite generally being 20-40%, but thin ash layers are true carbonatites. The tuffs and breccias are mixtures of carbonatitic and melilitic lapilli and bombs set in calcite-rich ash. All occurrences contain mantle debris in the form of small xenoliths, xenocrysts and cores to concentric lapilli, of olivine, pyroxene and phlogopite characterised by high Mg#, Ni and Cr. In one occurrence concentric lapilli are built of sharply bounded layers of melilitite, carbonate-bearing melilitite and calcite-phyric carbonatite around wehrlitic cores. New whole rock analyses for all four localities are given and electron microprobe analyses of calcite indicate the presence of significant Sr, Ba and REE, which is typical of carbonatitic calcite. The melilitites and carbonatites are taken to be consanguineous and to have separated immiscibly during rapid transport to the surface, earlier solidification of the melilitite producing final carbonatitic liquids. The presence of the carbonatites and melilitites in central Italy is taken as evidence that this igneous province is unlikely to be subduction related; instead the spatial distribution of these rocks in a zone east of the Roman Igneous Province is considered to reflect thicker underlying lithosphere.
Die Karbonatite Italiens: Geologische Position, Petrologie und regionale Bedeutung
Zusammenfassung Diese Arbeit gibt einen Überblick über bereits veröffentlichte Ergebnisse und bringt neue Daten zu den vier Karbonatitvorkommen, die in Italien seit 1990 nachgewiesen wurden. Alle sind rezenten Alters. Drei davon sind extrusiv und umfassen Tuffe und Brekzien, während das vierte subvulkanisch ist und aus Tuffisiten besteht. Sie bilden Diatreme mit Tuff-Ringen, wobei drei von ihneneng mit melilithischen Tuffen und Laven assoziiert sind. Zwei der Vorkommen umfassen Karbonatite sensu strictu, i.e. Calcit > 50% Vol.%, während die anderen zwei karbonatitisch sind und nur 20–40% primären magmatischeu Calcit enthalten; dünne Aschenlagen bestehen aber aus echten Karbonatiten. Die Tuffe und Brekzien stellen Mischungen von karbonatitischen und melilithischen Lapilli und Bomben in einer Calcit-reichen Asche dar. Alle Vorkommen führen Reste von Mantelgesteinen in der Form kleiner Xenolithe, Xeno-Kristalle und als Kerne von kozentrischen Lapilli; hier liegt Olivin, Pyroxen und Phlogopit mit hohem Mg#, Ni, und Cr vor. In einem Vorkommen werden konzentrische Lapilli aus scharf begrenzten Lagen von Melilitit, Karbonat-führendem Melilitit und Calcit-phyrischem Karbonatit um Wehrlit-Kerne aufgebaut. Neue Gesamtgesteinsanalysen von allen vier Lokalitäten, sowie Mikrosondenanalysen von Calciten, lassen die Anwesenheit von beachtlichen Gehalten an Sr, Ba, und REE erkennen, was typisch für karbonatitische Calcite ist. Die Melilithite und Karbonatite sind gleichen Ursprungs und haben sich während raschen Transportes an die Oberfläche durch Magmenentmischung gebildet. Die frühere Verfestigung des Mililithites führte dann zur Bildung von karbonatitischen Schmelzen. Die Anwesenheit von Karbonatiten und Melilithiten in Zentralitalien wird als Hinweis darauf gesehen, daß diese magmatische Provinz nicht Subduktions-bezogen ist, die räumliche Verteilung dieser Gesteine in einer Zone östlich der römischen magmatischen Provinz wird vielmehr als Hinweis auf eine mächtigere Lithosphäre gesehen.
  相似文献   

17.
Proterozoic metamorphosed sequences are identified in the Transversal Zone (TZ) domain of the Borborema geological province, Northeast Brazil. This TZ domain is located between the well-known E–W Patos and Pernambuco continental shear zones. In its eastern part, in the Taquaritinga region, a large mass of augen gneisses with a conspicuous horizontal to subhorizontal tectonic foliation forms one of the most important rock types in the region that displays U–Pb zircon ages ca. 1.52 Ga. Paleoproterozoic orthogneisses dated by U–Pb on zircon at ca. 1.97 Ga and older paragneisses and banded gneisses represent basement rocks, which were cross-cut by these Mesoproterozoic augen gneisses, and have been in turn intruded by plutonic rocks in upper Neoproterozoic (U–Pb and Rb–Sr, ca. 0.6 Ga) times.Chemical analyses of major, minor, and trace elements (including REE) for the basement orthogneisses indicate calcalkaline affinities and a signature very similar to volcanic arc granites, representing crustal accretion during the Paleoproterozoic Transamazonian/Eburnean orogenesis in the region. In turn, the chemical data for augen gneisses indicate that they are relatively homogeneous and evolved metaluminous metaplutonic rocks with characteristics very similar to A-type granites generated and emplaced in an extensional anorogenic setting. Relatively high 87Sr/86Sr initial ratio and negative Nd(t) are signatures of crustal components in these rocks.Based on geochemical, geochronological, and structural data, the Taquaritinga region is composed of Paleoproterozoic (>1.97 Ga) rocks intruded by Mesoproterozoic (ca. 1.5 Ga) anorogenic granites and Neoproterozoic granites (ca. 0.6 Ga). These data also suggest that the tectonometamorphic structures displayed by Meso and Neoproterozoic suites were developed by the Brasiliano/Pan-African orogeny and that the record of Transamazonian/Eburnean orogeny is restricted to basement rocks. This means that there is no evidence for a compressional event in Mesoproterozoic times (Cariris Velhos=Grenville) as suggested for the central and western part of the TZ. It is important to remark that the Taquaritinga augen gneisses are, up to now, the only unit that represents magmatic pulses associated with extensional episodes with this age (ca. 1.5 Ga) in the TZ and in the whole Borborema Province.

Resumen

A zona Transversal da Província Borborema está geologicamente situada entre os lineamentos Pernambuco e Patos. Na sua parte oriental, na região de Taquaritinga (PE), dominam augen gnaisses com uma penetrativa e proeminente foliação tectônica subhorizontal, com idade de cristalização de 1.52 Ga obtida em zircão. Estas rochas são intrusivas em ortognaisses e paragnaisses do embasamento com idade mínima em torno de 1.97 Ga, e são intrudidas por rochas plutônicas associadas ao Ciclo Brasiliano (ca. 0.6 Ga).Análises de elementos maiores, menores e traços (incluindo terras raras) nos augen gnaisses mostram caráter metaluminoso para esta suíte ígnea e fortes semelhanças com os granitos tipo-A gerados em ambiente anorogênico. Razões iniciais 87Sr/86Sr relativamente altas e Nd(t) negativos são assinaturas indicadoras de forte contribuição crustal nestes augen gnaisses.A conjunção dos dados geoquímicos, geocronológicos e de campo indicam que na região de Taquaritinga onde são identificadas rochas de idades paleo (>1.95 Ga), meso (ca. 1.5 Ga) e neoproterozóicas (ca. 0.6 Ga), o ciclo orogênico Transamazônico é restrito às rochas do embasamento, e que todas as estruturas dúcteis identificadas nas rochas meso e neoproterozóicas foram desenvolvidas durante o Ciclo orogênico Brasiliano. Não foram encontradas evidências que atestem a existência do Ciclo Cariris Velhos nesta parte da Província Borborema. Contudo, está bem marcado que nesta Zona Transversal e na Província Borborema, os augen gnaisses de Taquaritinga são, até o momento, os únicos marcadores de pulsos magmáticos associados com eventos extensionais crustais datados do Mesoproterozóico inferior.  相似文献   

18.
The paper discusses geological, mineralogical, petrographic, and geochemical data on the Ureg Nuur volcanoplutonic association of high-Mg volcanic and subvolcanic rocks located among Vendian–Cambrian accretionary structures in the Mongolian Altay. These rocks have a high potassium alkalinity (K2O/Na2O up to 1.2), are enriched in LILE and Sr, and have negative Zr–Hf and Nb anomalies in multielement spectra; this confirms the suprasubduction type of the source of melts. The geological setting and established age (512.4 ± 6.1 Ma, 39Ar–40Ar dating of biotite phenocrysts) evidence picritic magmatism at the accretionary stage of the development of the Altay fragment of the Paleoasian ocean. This indicates a large igneous province related to a mantle plume.  相似文献   

19.
La série sédimentaire du Crétacé moyen et supérieur étudiée dans l'Ouest du Bassin de Paris et sur la bordure du Massif armoricain comporte sept pulsations transgressives qui peuvent être reconnues en fonction de la disposition géomètrique des dépôts et de l'enchaînement vertical des faciès.Les épisodes transgressifs atteignent leur phase paroxysmale, en alternance avec des stades de régression, successivement:
1. (1) à la fin de l'Aptien supérieur —(régression début Albien)
2. (2) au milieu de l'Albien supérieur —(régression fin Albien-début Cénomanien)
3. (3) au milieu du Cénomanien inférieur —(régression fin Cénomanien inférieur)
4. (4) à la fin du Cénomanien moyen —(régression fin Cénomanien)
5. (5) au milieu du Turonien inférieur —(régression du Coniacien)
6. (6) au Santonien puis Campanien —(régression fin Campanien)
7. (7) au Maestrichtien —(régression fin Maastrichtien)
Ces dépôts crétacés présentent une succession de séquences séparées par des discontinuités et des surfaces d'érosion plus ou moins marquées. Les caractères et la répartition de ces séquences sont analysés en domaine de sédimentation terrigène littorale et dans le bassin envahi par la craie. Il est alors tenté d'interpréter les variations bathymétriques d'une part en fonction d'évènements tectoniques locaux, d'autre part en relation avec des variations eustatiques plus générales.Middle and Upper Cretaceous deposits studied in the western Paris Basin and on the Armorican Massif margin show seven transgressive pulses that can be detected in the geometrical arrangement of the sedimentary bodies and the vertical succession of facies.The transgressive episodes, each delimited by a regressive phase, reached their peak during:
1. (1) late Late Aptian —(Early Albian regression)
2. (2) mid Late Albian —(Late Albian-Early Cenomanian regression)
3. (3) mid Early Cenomanian —(late Early Cenomanian regression)
4. (4) late Middle Cenomanian —(Late Cenomanian regression)
5. (5) mid Early Turonian —(Coniacian regression)
6. (6) Santonian-Campanian —(Late Campanian regression)
7. (7) Maastrichtian —(Late Maastrichtian regression)
The Cretaceous sediments occur as a series of sequences, separated by more or less important discontinuities and erosion surfaces. The features and distribution of these sequences have been studied in the nearshore region of terrigenous sedimentation and in the Chalk basin. An attempt is made to relate the bathymetric changes to either tectonic movements or more widespread eustatic oscillations.  相似文献   

20.
The Ulundi Formation of the lowermost Fig Tree Group in the type area in the Barberton Mountain Land consists of a sequence of quartz—sericite schists, banded cherts and tuffaceous rocks up to 40 m thick. It overlies talc—carbonate schists assigned to the upper part of the Onverwacht Group. In the cherts, which in parts represent the sulfide facies of a banded-iron formation, silicified spheroids have been observed which appear to be diagenetically altered, locally zeolitized accretionary lapilli. Portions of the cherts formed through silicification of fine-grained felsic tuffs and contain abundant stylolite seams along which 4% of the rock thickness has been removed by intrastratal solution during deformation.A supposedly organogenic spheroidal structure, Ramsaysphera, has been previously described from the upper part of the Ulundi Formation. The spheroids, with diameters of up to 8 mm, occur in a distinct bed of up to at least 1.6 m thickness and can be traced in outcrop over an area which originally covered 17 × 30 km. The bed was deposited in fairly still water of several 100 m depth. The spheroids accumulated with only little finer-grained detrital material, the interstices between them being filled by cherty silica and minor amounts of dolomite. Up to 65% of the spheroids consist of tectonically elongated, originally round fragments of a micro-felsitic groundmass with remnants of phenocrysts. The majority of the remaining spheroids consists of concentrically banded bodies of siliceous material with minor amounts of chlorite and sericite, as well as small dolomite crystals. These spheroids represent highly altered granules of volcanic glass deposited from volcanic ash clouds through a body of water. The prototype of Ramsay-sphera also belongs to this group of spheroids, and there is little microscopic or macroscopic evidence for its explanation as an organic structure.  相似文献   

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