Intersertal Textures Research Articles

Geodynamic framework of the magmatic activity of Cretaceous Léré basin (Southwestern Chad, Central Africa)

Numerous 5 to 20 m-wide dolerite dykes crosscut vertically to sub-vertically basement granitoids around and within the Cretaceous Léré basin, SW Chad (Central Africa). They extend along strike on several hundred meters length. Léré dolerites exhibit various textures, from classical doleritic texture to sub-ophitic via ophitic and intersertal textures. They are composed of euhedral and subhedral crystals of andesine and labradorite, anorthoclase, augite, olivine, ilmenite and Ti-magnetite. Calcite and amphibole are present. Estimated crystallization temperatures are between 1050 and 1105 ± 50 °C. Occurrence of dyke swarms in Léré basin is controlled by Pan-African strike-slip faults created during an extensional tectonic phase during Paleozoic to Early Mesozoic. These lavas, analyzed with ICP-MS and ICP-AES, are basalt, trachybasalt and basaltic trachyandesite with some continental tholeiite affinity. Dolerites of Léré basin have evolved by limited fractional crystallization differentiation, accompanying by minor assimilation process. Three groups of rocks are identified (according to Y/Nb ratios), linked to different mantle sources which have intermediate compositions between E-MORB and OIB.

Age and implication of Late Ediacaran dykes in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for the rifting of the Rheic Ocean

ABSTRACT The İstanbul-Zonguldak Tectonic Unit is a part of Avalonia-Carolina and represents Far East Avalonia. It includes Ordovician to Carboniferous deposits which unconformably overlie Late Neoproterozoic metamorphic rocks. Timing of its detachment from West Gondwana-land, thus rifting of the Rheic Ocean, is poorly known. Here, I report on dacite, diabase and andesite dykes of Late Ediacaran igneous crystallization ages from the basement rocks and rift-deposits in the İstanbul-Zonguldak Tectonic Unit (NW Turkey). They are folded about a consistent axis and have porphyritic to intersertal textures. U-Pb dating on igneous zircons from folded dykes yielded Late Ediacaran ages of ca. 556.2 ± 2.2 Ma (2σ) and 552.1 ± 2.8 to 549.2 ± 2.3 Ma (2σ). Late Ediacaran dykes show calc-alkaline and alkaline affinities, and contain with-in plate components. Rift-related Late Ediacaran magmatism shows that the Yellice arc changed from an arc to a back-arc basin and the rift-deposits formed during the late Ediacaran due to the arc rifting. In conjunction with the data from literature, I suggest that the rift-related magmatism is related to the rifting event during the late Ediacaran, leading to the detachment of this continental block from the West Gondwana-land, thus to the opening of the Rheic Ocean, and Early Cambrian deformation event correspond to the adding of the Kraishte terrane with Far East Avalonia. The docking of the Kraishte terrane led to the deformation of Late Ediacaran dykes. Overall, these data indicate that the depositional time of sedimentary rocks in the continental fragment should have started during the late Ediacaran instead of Ordovician.

Intracontinental basaltic magmatism in the sedimentary succession of the Tucuruí Group: An important record of the final evolution of the Araguaia Belt

The region of Tucuruí, northeastern Pará, is located in the transition between the Amazon Craton and the Araguaia Belt. In this region, there is an extensive graben filled by a volcano-sedimentary succession, 35 km wide that extends over 100 km in the NNW-SSE direction and which is represented by the Tucuruí Group. This group consists of a succession of basaltic flows and diabase sills interbedded with sub-arkose and siltstones. Together with the metasedimentary rocks of the Tocantins Group, these rocks represent the latest records of the Araguaia Orogen in the extreme North of Brazil. The volcanic deposits consist of basalt flows and volcanic breccias associated with diabase sills. The flows are predominantly composed of massive and locally amygdaloidal basalts and basaltic breccias. The basalts are aphanitic, fine-grained or glassy, holo-to hypocrystalline, and are made up of labradorite, augite, Fe–Ti minerals oxides, sulfides, and apatite. The massive basalts have intergranular or intersertal texture, whereas the amygdaloidal basalts are characterized by fibro-radial zeolites, epidote, and chlorite. The basaltic breccias are composed of elongated, hypovitreous basalt fragments encompassed by a volcanoclastic matrix with siltstone films and anastomosed siliceous-zeolitic materials. The basalts are subalkaline with tholeiitic Ocean Island Basalts affinity, intraplate, and are interpreted as products of fissural volcanism in a subaerial environment. Deformation is predominantly ruptile, and fluid remobilization occurs along faults and fractures, generating veins of quartz, carbonate, chlorite, and epidote by hydrothermal processes at low temperatures (anki-metamorphism). The sedimentary succession of the Tucuruí Group represents a paleo-segment formed in a shallow marine environment influenced by storm waves. The evolution of the succession is related to the installation and fill of a rift basin during the final stage of evolution of the Araguaia Belt at the Neoproterozoic/Paleozoic transition. This event is related to the extensional tectonics that marked the transition from a ductile to ruptile regime in the late stage of evolution of the Araguaia Orogen, and it represents a rare record of the end of the Brasiliano Cycle on the Brazilian Platform.

AS ROCHAS VULCÂNICAS DA ILHA SÃO JORGE, AÇORES (PORTUGAL): PETROGRAFIA E GEOQUÍMICA

A Ilha São Jorge faz parte do Grupo Central do Arquipélago dos Açores (Portugal), situado próximo à junção tríplice, entre as placas litosféricas Norte-Americana, Euro-Asiática e Africana, na região do Atlântico Norte. A vulcanoestratigrafia da ilha é composta por três complexos vulcânicos denominados Topo, Rosais e Manadas. Segundo a bibliografia, a idade da Ilha São Jorge é 1,3 ± 0,0035 Ma (40Ar/39Ar). Com base na petrografia, as rochas apresentam granulometria média a fina, texturas porfirítica, microporfirítica, glomeroporfirítica, intergranular, pilotaxítica, traquítica e intersertal. De acordo com dados geoquímicos, as rochas forma classificadas como basanitos, basaltos alcalinos e hawaítos, compostos por olivina, augita, plagioclásio e minerais opacos, que ocorrem como fenocristais ou inseridos na matriz. Algumas amostras de basalto alcalino e hawaíto apresentam kaersutita, interpretada como xenocristal. Texturas de reabsorção, embainhamento, zonação e bordas de reação são feições comuns em fenocristais e indicam que o magma sofreu descompressão rápida, com variação brusca de temperatura e pressão. A interpretação geoquímica permitiu concluir que as lavas apresentam afinidade alcalina sódica e foram geradas por baixas taxas de fusão parcial, a partir de um magma mantélico enriquecido. Os dados geoquímicos são compatíveis com os de magmas gerados em ambiente geotectônico do tipo intraplaca oceânico (OIB).

STUDYING THE GENESIS OF IGNEOUS ROCKS IN ZARIN-KAMAR REGION (SHAHROOD, NORTHEASTERN IRAN) BY RARE EARTH ELEMENTS

Zarin-Kamar region is located north east of Shahrood (36o37’-36o42’N, 55007’-55012’E). Plutonic rocks in this area belongs to syenite group and their texture is intergranular hypidiomorphic. Volcanic rocks in the area have porphyritic, amygdale intersertal texture. which quartz is also seen among their cavities and porosities. Total concentration of REEs (ΣREE) in the study igneous rocks varies between 450 and 683 ppm. Diagram of Eu/Eu* versus Sr and Eu/Eu* versus Ba show negative anomalies of Eu. This phenomenon as well as Ba and Sr trends show that plagioclase removal has happened during the magma evolution. These rocks have rock has originated from an enriched mantle source. The rate of Dy/Yb in the igneous rocks of the region varies between 1.32 and 2.62. it shows that it stemmed from a garnet lherzolitic source. The rate of (Tb /Yb )N was between 0.97 to 2.25 showing a garnet source. Also other related figures showed that the samples belonged to OIB (Oceanic Island Basalt). The rate of La/Ta was between 6.6 to 14.01. It also showed that they had a source from asthenosphere. The rate of La/Nb was 0.5 to 0.91. It also shows a less crustal contamination among these samples.

Geochemical Features and Petrogenesis of Gökçeada Volcanism, Çanakkale, NW Turkey

Gökçeada Island, which is situated west of Biga Peninsula, has widespread magmatism with variable ages. Lower-Middle Eocene Dağiçitepe volcanics are the oldest volcanic unit in the island and consist of lavas, tuff-tuffites. They are influenced by alteration and almost all minerals, except quartz, are transformed into other minerals. Lower Oligocene Gökçeada andesitic lava/ domes exhibiting hypocrystalline porphyric texture, are the products of NE-SW trending domes/ cryptodomes. The phenocrysts assemblages consist of plagioclase, hornblende, clinopyroxene ± biotite and quartz. Middle Miocene Eşelek volcanics, which occur as lavas and pyroclastic rocks, exhibit hypocrystalline porphyric and intersertal textures. They are composed of plagioclase, hornblende and clinopyroxene crystals. Rhyolitic Dağiçitepe volcanics and andesitic Gökçeada lava/domes have calc-alkaline, andesitic Eşelek volcanics have tholeiitic character. They have geochemical features similar to subduction-related magmas. Lower-Middle Eocene Dağiçitepe volcanics are the products of syn-collisional magmas that have undergone processes of crustal contamination due to thickened crust. Whereas, Lower Oligocene Gökçeada andesitic lava/domes are the products of post-collisional magmas and were derived from metasomatized lithospheric mantle. Middle Miocene Eşelek volcanics were also derived from lithospheric mantle but, the mantle source generating Eşelek volcanics were relatively depleted over time. Geochemical data demonstrate the decreasing role of subduction signature and crustal contamination during the genesis and evolutionof Gökçeada volcanics from Lower-Middle Eocene to Middle Miocene.

Cerromojonite, CuPbBiSe3, from El Dragόn (Bolivia): A New Member of the Bournonite Group

Cerromojonite, ideally CuPbBiSe3, represents a new selenide from the El Dragόn mine, Department of Potosí, Bolivia. It either occurs as minute grains (up to 30 µm in size) in interstices of hansblockite/quijarroite intergrowths, forming an angular network-like intersertal texture, or as elongated, thin-tabular crystals (up to 200 μm long and 40 μm wide) within lath-shaped or acicular mineral aggregates (interpreted as pseudomorphs) up to 2 mm in length and 200 μm in width. It is non-fluorescent, black, and opaque, with a metallic luster and black streak. It is brittle, with an irregular fracture, and no obvious cleavage and parting. In plane-polarized incident light, cerromojonite is grey to cream-white, and weakly pleochroic, showing no internal reflections. Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey. Lamellar twinning on {110} is common. The reflectance values in air for the COM standard wavelengths (R1 and R2) are: 48.8 and 50.3 (470 nm), 48.2 and 51.8 (546 nm), 47.8 and 52.0 (589 nm), and 47.2 and 52.0 (650 nm). Electron-microprobe analyses yielded a mean composition of: Cu 7.91, Ag 2.35, Hg 7.42, Pb 16.39, Fe 0.04, Ni 0.02, Bi 32.61, Se 33.37, total 100.14 wt %. The empirical formula (based on 6 atoms pfu) is (Cu0.89Hg0.11)Σ = 1.00(Pb0.56Ag0.16Hg0.15 Bi0.11Fe0.01)Σ = 0.99Bi1.00Se3.01. The ideal formula is CuPbBiSe3. Cerromojonite is orthorhombic (space group Pn21m), with a = 8.202(1) Å, b = 8.741(1) Å, c = 8.029(1) Å, V = 575.7(1) Å3, Z = 4. Calculated density is 7.035 g·cm−3. The five strongest measured X-ray powder diffraction lines (d in Å (I/I0) (hkl)) are: 3.86 (25) (120), 2.783 (100) (122), 2.727 (55) (212), 2.608 (40) (310), and 1.999 (25) (004). Cerromojonite is a new member of the bournonite group, representing the Se-analogue of součekite, CuPbBi(S,Se)3. It is deposited from strongly oxidizing low-T hydrothermal fluids at a fSe2/fS2 ratio >1, both as primary and secondary phase. The new species has been approved by the IMA-CNMNC (2018-040) and is named for Cerro Mojon, the highest mountain peak closest to the El Dragón mine.

Quijarroite, Cu6HgPb2Bi4Se12, a New Selenide from the El Dragόn Mine, Bolivia

Quijarroite, ideally Cu6HgPb2Bi4Se12, is a new selenide species from the El Dragόn mine, Department of Potosí, Bolivia. It most frequently occurs as lath-shaped thin plates (up to 150 µm in length and 20 µm in width) intimately (subparallel) intergrown with hansblockite, forming an angular network-like intersertal texture. Quijarroite is occasionally also present as sub- to anhedral grains up to 200 µm in length and 50 µm in width. It is non-fluorescent, black and opaque with a metallic luster and black streak. It is brittle, with an irregular fracture and no obvious cleavage and parting. In plane-polarized incident light, quijarroite is weakly pleochroic from cream to very slightly more brownish-cream, displaying no internal reflections. Between crossed polars, quijarroite is moderately anisotropic with pale orange-brown to blue rotation tints. Lamellar twinning on {110} is common; parquet twinning occurs rarely. The reflectance values in the air for the COM (Commission on Ore Mineralogy) standard wavelengths (R1 and R2) are: 46.7, 46.8 (470 nm), 47.4, 48.2 (546 nm), 47.1, 48.5 (589 nm), and 46.6, 48.7 (650 nm). Electron-microprobe analyses yielded a mean composition of Cu 13.34, Ag 1.02, Hg 7.67, Pb 16.87, Co 0.03, Ni 0.15, Bi 27.65, Se 33.52, total 100.24 wt %. The mean empirical formula, normalized to 25 apfu (atoms per formula unit), is (Cu5.84Ag0.26)Σ = 6.10(Hg1.06Ni0.07Co0.01)Σ = 1.14Pb2.27Bi3.68Se11.81 (n = 24). The simplified formula is Cu6HgPb2Bi4Se12. Quijarroite is orthorhombic, space group Pmn21, with a = 9.2413(8), b = 9.0206(7), c = 9.6219(8) Å, V = 802.1(1) Å3, Z = 1. The calculated density is 5.771 g·cm−3. The five strongest X-ray powder-diffraction lines (d in Å (I/I0) (hkl)) are: 5.36 (55) (111), 3.785 (60) (211), 3.291 (90) (022), 3.125 (100) (212), and 2.312 (50) (400). The crystal structure of quijarroite can be considered a galena derivative and could be derived from that of bournonite. It is a primary mineral, deposited from an oxidizing low-T hydrothermal fluid at a f S e 2 / f S 2 ratio greater than unity. The new species has been approved by the IMA-CNMNC (2016-052) and is named for the Quijarro Province in Bolivia, in which the El Dragón mine is located.

Geochemistry of the Desur Lavas, Deccan Traps: Case Study from the Vicinity of Belgaum, Karnataka and their Petrogenetic Inferences

Abstract: Geochemical characteristics of Desur-type basalt flows in the southern and southwestern part of Belgaum in Karnataka, India have been investigated to understand their petrogenesis. The basalts are compact, hard, massive, and show characteristic microporphyritic textures with abundant well-twinned and un-twinned plagioclase phenocrysts and minor clinopyroxene set in a fine-grained groundmass consisting of plagioclase, clinopyroxene, glass and Fe-Ti oxides. Thin sections show sub-ophitic, intergranular and intersertal textures. The basalts are Fe-rich tholeiites (13.4-13.8 wt %), characterized by high TiO2 (3.64 to 3.94 wt %); moderate MgO contents (4.79 to 5.41 wt %), low K2O contents (<0.58 wt %) and low Mg# (42.4-45.9). They are enriched in large ion lithophile elements, moderately enriched in the light rare earths (chondrite-normalized LaN/YbN 3.37-4.24), and exhibit nearly flat heavy rare-earth patterns that lack significant Eu anomalies (Eu/Eu* 0.86-1.10). Primitive-mantle-normalized element patterns for these rocks show characteristic troughs at K and Sr, absence of a Nb anomaly, and a low Zr/Nb ratio (<15), which suggest insignificant contamination by many types of continental crust, whereas, enrichments in the large ion lithophiles, La, P and Th could suggest enriched source characteristics. Based on the geochemical characteristics of the basalts, it is inferred that the Desur basalts representing the youngest flows of the Deccan Basalt Group are derived by partial melting of a peridotite source, and subsequent fractionation gave rise to the compositions of the basalts that are found in the Belgaum region.

Copper–zinc albite porphyry in the Hersai porphyry copper deposit, East Junggar, China: A transition between late magmatic and hydrothermal porphyry copper deposit

A 418Ma albite porphyry dike emplaced in a 429Ma granite pluton in the Hersai porphyry copper deposit (Nom, East Junggar, China) consists of euhedral and pale-orange luminescing albite phenocrysts and groundmass, zircons, blue-violet luminescing apatites and chalcopyrite–sphalerite-rich miarolitic cavities. This dike is characterized by two-domain textures presented by albites and disseminated intergranular miarolitic cavities, and by geochemical features of continental island arc and adakitic intrusions.The miarolitic cavities consist of various minerals that crystallized in the following sequence: epidote, rutile and sphalerite–chalcopyrite solid solution, titanite and K-feldspar; chalcopyrite with pyrite inclusions; and chlorite, zoisite and calcite. The miarolitic cavities exhibit magmatic to hydrothermal internal textures, such as aplitic textures presented by intergrowths of albite, epidote, rutile and sphalerite–chalcopyrite solid solution, zoning and intersertal texture, poikilitic textures, and layered chlorites. The sphalerite contains a high concentration of exsolved chalcopyrite (up to 10 vol.%). The rutiles intergrown with epidotes contain 137 to 528ppm Zr corresponding to Zr-in-rutile temperatures of 507 to 625°C, and the rutiles intergrown with sphalerite–chalcopyrite solid solutions have 381 to 420ppm Zr and Zr-in-rutile temperatures of 585 to 605°C. Mineralization temperatures of sphalerite–chalcopyrite solid solutions are possible approximately 600°C. The formation of the chalcopyrite–sphalerite-rich miarolitic cavities is related to interstitial residual sulfide-rich fluids. The copper–zinc albite porphyry may represent a copper–zinc (Cu–Zn) subtype of porphyry copper deposit that formed as a transition between late magmatic and classical hydrothermal porphyry copper deposits.

Geochemistry and mineralogy of a feldspathic lunar meteorite (regolith breccia), Northwest Africa 2200

The lunar meteorite Northwest Africa (NWA) 2200 is a regolith breccia with a ferroan feldspathic bulk composition (Al2O3 = 30.1 wt.%; Mg# = molar 100 × Mg/(Mg + Fe) = 59.2) and low Th content (0.42 μg/g). Lithologically, NWA 2200 is a diverse mixture of lithic and glassy clasts, mineral fragments, and impact glass spherules, all embedded in a dark glassy matrix. NWA 2200 contains some feldspathic brecciated rock components (ferroan anorthositic granulitic breccia, poikiloblastic granulitic breccia, and glassy melt breccia with an intersertal texture). The bulk compositions of these brecciated components indicate they are derived from ferroan troctolitic or noritic anorthosite lithologies (bulk Al2O3 = 26–30 wt.%; bulk FeO/MgO > 1.0). The bulk composition of NWA 2200 is more ferroan and feldspathic than the Apollo feldspathic regolith samples and feldspathic lunar regolith meteorites, and is also more depleted in incompatible elements (e.g., rare earth elements) than Apollo 16 feldspathic regolith samples. We conclude that NWA 2200 originated from a location different to the Apollo landing sites, and may have been sourced from the ferroan KREEP-poor highlands, “KREEP” materials are enriched in such elements as potassium (K), rare earth elements (REE), phosphorus (P).

Magmatic controls on eruption dynamics of the 1950 yr B.P. eruption of San Antonio Volcano, Tacaná Volcanic Complex, Mexico–Guatemala

San Antonio Volcano, in the Tacaná Volcanic Complex, erupted ~1950yr. B.P., with a Pelean type eruption that produced andesitic pyroclastic surges and block-and-ash flows destroying part of the volcano summit and producing a horse-shoe shaped crater open to the SW. Between 1950 and 800yrB.P. the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome, all from a single magma batch. All products consist of phenocrysts and microphenocrysts of zoned plagioclase, amphibole, pyroxene, magnetite±ilmenite, set in partially crystallized groundmass of glass and microlites of the same mineral phases, except for the lack of amphibole. Included in the andesitic blocks of the block-and-ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins. The enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase, hornblende, pyroxene, and olivine, and minor proportions of phenocrysts of plagioclase, hornblende, and pyroxene. A compositional range obtained of blocks and enclaves resulted from mixing between andesite (866°C±22) and basaltic andesite (enclaves, 932°C±22), which may have triggered the explosive Pelean eruption. Vestiges of that mixing are preserved as complex compositional zones in plagioclase and clinopyroxene-rich reaction rims in amphibole in the andesite. Whole-rock chemistry, geothermometry, experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma (≤900°C) and ended with effusive dacitic magma (~825°C), all of which were stored at ~200MPa water pressure. A complex open-system evolution that involved crustal end-members best explains the generation of effusive dacite from the hybrid andesite. Amphibole in the dacite is rimmed by reaction products of plagioclase, orthopyroxene, and Fe–Ti oxides produced by decompression during ascent. Amphibole in the andesite, however, lacks such rims. Because the andesite was at 866±22°C and the dacite was at ~825°C, the reaction rims indicate that the andesitic magma ascended at 0.023ms−1 during the explosive phase of the eruption, whereas the dacitic magma rose more slowly at ~0.002–0.004ms−1.

Mineral Chemistry of Volcanic Rocks of South Shetland Archipelago, Antarctica

This study presents the results of mineral chemistry data of Meso-Cenozoic volcanic rocks of Livingston, Robert and Ardley islands (South Shetland Arc, Antarctica). These rocks include basalts and basaltic andesites that exhibit pilotaxitic, intergranular, and intersertal textures. Glomeroporphyritic clusters consist of phenocrysts of plagioclase, augite, and Ti-magnetite or only plagioclase. The rocks are composed of plagioclase, augite, olivine, Ti-magnetite, and ilmenite phenocrysts. Geochemistry studies indicate that these rocks are rich in Al2O3, Rb, Ba, Sr, and show enrichment in LREE relative to HREE. The Ni, Cr, Co, and MgO contents are lower than primary compositions, and demonstrate that these magmas are evolved. Mineral chemistry indicates that the volcanic rocks belong to the calc-alkaline series typical of island arcs. Plagioclase crystals are mostly bytownite and labradorite in Livingston and Robert islands. Nevertheless, some Robert Island phenocrysts are of anorthite composition. The plagioclase of volcanic rocks of Ardley Island is only labradorite. Augite is the main mafic phase (Wo23-48, En40-58, Fs9-21) in all studied islands. Olivine shows a chrysolite composition. Oxide phases are represented by Ti-magnetite and ilmenite. The evaluation of whole-rock geochemistry and mineral chemistry supports the hypothesis of fractional crystallization for the evolution of the rocks of Livingston, Robert, and Ardley islands. Geothermometry for olivine-clinopyroxene pairs suggests a crystallization temperature of 962°C ± 10°C (1 bar) and of 965°C ± 10°C (500 bars).

Late Precambrian Balkan-Carpathian ophiolite — a slice of the Pan-African ocean crust?: geochemical and tectonic insights from the Tcherni Vrah and Deli Jovan massifs, Bulgaria and Serbia

The Balkan-Carpathian ophiolite (BCO), which outcrops in Bulgaria, Serbia and Romania, is a Late Precambrian (563Ma) mafic/ultramafic complex unique in that it has not been strongly deformed or metamorphosed, as have most other basement sequences in Alpine Europe. Samples collected for study from the Tcherni Vrah and Deli Jovan segments of BCO include cumulate dunites, troctolites, wehrlites and plagioclase wehrlites; olivine and amphibole-bearing gabbros; anorthosites; diabases and microgabbros; and basalts representing massive flows, dikes, and pillow lavas, as well as hyaloclastites and umbers (preserved sedimentary cover). Relict Ol, Cpx and Hbl in cumulate peridotites indicate original orthocumulate textures. Plagioclase in troctolites and anorthosites range from An60 to An70. Cumulate gabbro textures range from ophitic to poikilitic, with an inferred crystallization order of Ol-(Plag+Cpx)-Hbl. The extrusive rocks exhibit poikilitic, ophitic and intersertal textures, with Cpx and/or Plag (Oligoclase-Andesine) phenocrysts. The major opaques are Ti-Magnetite and Ilmenite. The metamorphic paragenesis in the mafic samples is Chl-Trem-Ep, whereas the ultramafic rocks show variable degrees of serpentinization, with lizardite and antigorite as dominant phases. Our samples are compositionally and geochemically similar to modern oceanic crust. Major element, trace element and rare earth element (REE) signatures in BCO basalts are comparable to those of MORB. In terms of basalt and dike composition, the BCO is a ‘high-Ti’ or ‘oceanic’ ophiolite, based on the classification scheme of Serri [Earth Planet. Sci. Lett. 52 (1981) 203]. Our petrologic and geochemical results, combined with the tectonic position of the BCO massifs (overlain by and in contact with Late Cambrian island arc and back-arc sequences), suggest that the BCO may have formed in a mid-ocean ridge setting. If the BCO records the existence of a Precambrian ocean basin, then there may be a relationship between the BCO and the Pan-African ophiolites from the Arabian–Nubian Shield. We suggest that the BCO is the missing link between the Pan-African and the Avalonian–Cadomian peripheral orogens of Murphy and Nance [Geology 19 (1995) 469].

Química mineral das rochas vulcânicas da península fildes (Ilha Rei George), Antártica

Studies realized in the Fildes Peninsula (southwest King George Island, Antartic) permited the identification of volcanic rocks succession from basic to intermediate composition. Dacites and rhyolites are rare. The age of these rocks vary from Paleocene to Eocene. Geochemistry studies indicate these rocks are rich in A1 2 O 3 , Rb, Ba, Sr and they present enrichment in LREE relative to HREE. It is typical of high-Al volcanic association with calc-alkaline affinity. It is evinced for Alkali Index x A1 2 O 3 diagram. The Ni, Cr, Co, MgO contents of basic rocks are lower than primary compositions and it discloses that these magmas are evolved. 87 Sr/ 86 Sr initial ratios are 0.7033 to 0.7037 and l43 Nd/ 144 Nd are 0.512959 to 0.512967. The values of eNd (t = 55 Ma) range between + 6.52 and + 6.72. The Isotopic and Geochemistry data suggest that the rocks were generated from partial melting of lithospheric mantle in an island arc environment. Fildes Peninsula volcanic rocks present porphyritic, glomeroporphyritic, intergranular and intersertal textures. Mineral chemistry indicates typical compositon of low-K and calc-alkaline series of island arcs. Plagioclase compositions are mostly anorthite and bytownite in the basic rocks and mostly bytownite in the intermediate rocks. Plagioclase matrix is labradorite. Augife is the main mafic phase (Wo 24-37 ,En 44-50 ,Fs 19-26 ) in the basic rocks. The pyroxene composition (Wo 38 , En 45 , Fs 17 ) in the basaltic andesites is similar to basalts. Pigeonite (Wo |(115 , En 5 , 67 , Fs~ 93 ) is unusual phase and it is only presents in the matrix of basalts. Bronzite (Wo 3-4 En 70 ,Fs 26-27 ) occurs as microphenocryst in two samples'bf basaltic andesites. Olivine is common phase in the basic rocks and it shwos crisblite composition. The opaque phase is represented for magnetite. It contains ulvospinel from 16 to 25%. Geothermobarometry of clinopyroxene-orthopyroxene from basaltic andesite (AF-21) was obtained by Ca-QUILF Program. It yielded a crystallization and equilibration temperature between 1065°C to 1070°C for pressures from 1 bar to 1500 bar. The evaluation of Mineral Chemistry, Lithochemistry and Isotopic compostions are according to hypothesis of fractional crystallization for evolution of Fildes Peninsula volcanic rocks. It was not found evidences for interference of other petrogenetic processes.

Le magmatisme basique de la boutonniere d'Igherm (Anti-Atlas occidental, Maroc); un jalon des distensions neoproterozoiques sur la bordure nord du craton ouest-africain

Abstract In the Igherm inlier (western Anti-Atlas, Morocco) doleritic dyke swarms with various directions and gabbroic intrusive bodies were emplaced during Neoproterozoic times, cutting across either Eburnean micaschists and granites or Panafrican limestones and quartzites. All these rocks were deformed by the main Panafrican schistosity and covered by molassic and volcanic Upper Neoproterozoic series. The primary mineralogical assemblages (plagioclase, augite, olivine...) of the mafic rocks are nearly completely replaced by secondary assemblages (albite, actinolite, chlorite, epidote, calcite, quartz, leucoxene, magnetite, hematite...). However, three main groups have been recognized by the means of relative chronology and petrography. The group 1 is earlier, as shown by the intrusive character of the dykes of the other two groups into its gabbroic bodies. Using incompatible trace elements and rare earth elements it appears that this magmatism is truly heterogeneous and that the three groups have different magmatic affinities. The group 1 corresponds to tholeiitic dolerites and gabbros characterized by intersertal and ophitic textures and by high contents in Fe 2 O 3 (12.16 to 16.64%), TiO 2 (1.46 to 2.5%), Zr (90 to 174 ppm), Nb (7 to 13 ppm), Y (21.68 to 38.74 ppm) and V (264 to 419 ppm). The REE contents are low (Sigma REE = 49 to 137 ppm) and the REE patterns are flat [1.99<(La/Yb) N <4.56] showing a relative slight enrichment in LREE and no anomaly in Eu (0.89>Eu/Eu (super *) <1.11). These features as the TiO 2 vs FeO (super *) /MgO and V vs Ti/1000 diagrams are characteristic of anorogenic intraplate magmas. The group 2 corresponds to calc-alkaline dolerites and gabbros showing fine-grained intersertal textures and high contents of Al 2 O 3 (14.10 to 20.64%) and low contents of Fe 2 O 3 (8.35 to 12.91%), TiO 2 (0.68 to 1.41%), Zr (66 to 106 ppm), Nb (5 to 7 ppm), Y (16.41 to 20.75 ppm) and V (144 to 264 ppm). The REE contents vary from 67 to 155 ppm and the REE patterns are fractionated (2.78<(La/Yb) N <6.62) with a strong enrichment in LREE. The slight positive Eu anomaly (0.91<Eu/Eu (super *) <1.37) is related to the wealth of plagioclases frequently observed in these rocks. The TiO 2 contents of these rocks and their low FeO (super *) /MgO ratios give them a calc-alkaline affinity similar to that of calc-alkaline orogenic basalts related to an oceanic subduction. The group 3 corresponds to alkaline dolerites characterized by fine-grained intersertal textures with high contents of TiO 2 (3.85 to 3.97%), P 2 O 5 (0.66 to 0.77%), Nb (33 to 39 ppm), Zr (262 to 287 ppm), Y (39.6 to 47.7 ppm) and REE (Sigma REE = 205 to 218 ppm). The REE patterns are fractionated (7.77<La/Yb) N <6.65) without no Eu anomaly (0.99<Eu/Eu (super *) <1.02). The Ti/V and Y/Nb ratios (65.26 to 74.95 and 1.19 to 1.22, respectively) are those of alkaline rocks found in intraplate environments. The detailed petrographical, geochemical and field studies of the Igherm inlier show that the mafic magmatism is more complex than previously described. The mafic tholeiitic and alkaline magmatism occurring in the Igherm inlier is also present in the other inliers of the Moroccan Anti-Atlas during Neoproterozoic times. On the other hand the calc-alkaline Neoproterozoic mafic magmatism is very rare elsewhere in the Anti-Atlas except in the Siroua Massif and locally in the Bas Draa and Tagragra d'Akka inliers (western Anti-Atlas). The geodynamical environment of this mafic magmatism is linked to a strong extensional tectonic regime occurring at the northern border of the West African craton during Neoproterozoic times. This regime is related to the oceanic opening described in Central Anti-Atlas and to the emplacement of the ophiolites of Bou Azzer and Siroua or occurs immediately after the oceanic opening. The chemical heterogeneities observed in the three defined groups can be related to heterogeneities of mantellic sources and/or various partial melting ratios of the sub-continental mantle. We can assume that this major fissural magmatic event, not precisely dated, is equivalent to that observed in the other Neoproterozoic provinces in Hoggar, Cameroon, north America and Brazil.

Implications for cratering mechanics from a study of the Footwall Breccia of the Sudbury impact structure, Canada

Abstract— The Footwall Breccia layer in the North Range of the Sudbury impact structure is up to 150 m thick. It has been analyzed for several aspects: shock metamorphism of clasts, matrix texture, mineralogy, and geochemistry with respect to major and trace element compositions. The matrix of this heterolithic breccia contains mineral and lithic fragments, which have suffered shock pressures exceeding 10 GPa, along with clasts of breccia dikes originating from the crater basement. The matrix in a zone near the upper contact of the breccia layer is dominated by a dioritic composition with intersertal textures, whereas beneath this zone the matrix is characterized by poikilitic to granular textures and a tonalitic to granitic composition. Major and trace element analyses of adjacent slices of a thin‐slab profile from the breccia show that the matrix is chemically inhomogeneous within a range of 3 mm. The breccia layer has been thermally annealed by the overlying Sudbury Igneous Complex, which is interpreted as a coherent impact melt sheet. The Rb‐Sr isochron age of 1.825 ± 0.021 Ga for the matrix is a cooling age after partial melting of fine grained clastic material by the melt system. Two‐pyroxene thermometry calculations give temperatures in excess of 1000 °C for this thermal overprinting. Clasts were affected by recrystallization, melting, and reactions with the surrounding matrix at that time. The crystallization of the molten matrix resulted in the observed variety of igneous textures. Results of clast population statistics for the Footwall Breccia along with both geochemical considerations and the Sr‐Nd isotopic signature of the matrix indicate that the breccia constituents exclusively derived from the Levack gneiss complex, which forms the local country rock to the breccia layer in the Levack area. K‐feldspar‐rich domains, which tend to replace parts of matrix and felsic gneiss fragments have been formed due to metasomatic activities during the Penokean orogeny, ∼ 1.7 Ga ago.The available observations suggest that the Sudbury structure represents the remnant of a multi‐ring basin with an apparent diameter between 180 and 200 km and a diameter of the transient cavity of about 100 km. For a crater of the size of the Sudbury basin a maximum depth of excavation of ∼21 km and a depth of shock‐melted target rocks of ∼27 km are obtained. In the Sudbury crater, the Footwall Breccia layer represents a part of the uplifted crater floor directly underlying the thick coherent impact melt sheet.

Petrographic evidence for nonlinear cooling rates and a volcanic origin for Apollo 15 KREEP basalts

Most Apollo 15 KREEP basalts have textures indicative of linear cooling rates. Some samples have textures indicative of two‐stage cooling: 15434,8 contains orthopyroxene phenocrysts up to 2 mm long in a much finer grained, intersertal groundmass, and probably has accumulated orthopyroxene; 15404,5 has glomerocrysts of orthopyroxene and plagioclase, also in a much finer grained, intersertal groundmass; clasts in glassy breccia 15358 have abundant (20% to 40%), clear, undevitrified orange‐yellow intersertal glass (glassy intersertal textures) indicative of late quenching. Phenocrysts, by comparison with experiments on dynamic crystallization, do not grow from multiply saturated magmas (as are the Apollo 15 KREEP basalts) at linear cooling rates. Therefore the phenocryst‐bearing samples provide evidence for their initially slow cooling and later rapid cooling. Two‐stage cooling is common in volcanic, dynamic environments but cannot be expected in the static environment of crystallization of impact melt sheets, even basin‐scale ones. Therefore these Apollo 15 KREEP basalts, and by extrapolation all the other clast‐free, homogeneous, meteoritic‐siderophile‐absent Apollo 15 KREEP should be reexamined with the constraint that the Apollo 15 KREEP basalts were produced by igneous processes at 3.9 Ga, but managed to retain the trace element characteristics established at about 4.4 Ga. The common acceptance of total remelting by impact at 3.9 Ga, such as the Imbrium basin event, in the interpretation of the isotopic and chemical data is not permissible.

Gabbroic xenoliths and host ferrobasalt from the southern Juan de Fuca Ridge

Rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12‐km section of the southern Juan de Fuca Ridge. Xenoliths comprise ≪ 1% of the dredge contents and range in size from 1 cm3 (glomerocryst) to 240 cm3. The xenoliths have ophitic to intersertal texture with 5–50% interstitial glass of ferrobasaltic composition more evolved than the host lava. On the basis of texture and mineralogy, the xenoliths have been subdivided into three types: type I, plagioclase + olivine + glass; type II, plagioclase + augite + glass ± olivine; and type III, plagioclase + augite + olivine + glass ± pigeonite (partially inverted) + Fe‐Ti oxides. Mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type I is the least evolved and type III is the most evolved. Application of a graphical pyroxene geothermometer to augite in xenolith types II and III yields crystallization temperatures of 1100°–1200°C and to host‐lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100°–1150°C. Coexisting titanomagnetite‐ilmenite pairs in type III xenoliths yield temperature estimates of 1000°–1070°C and log f02 = −9.7 to −10.8. We infer that the xenoliths represent the partially crystalline “mush” boundary zone of a magma chamber based on the abundance of interstitial glass, zonation of mineral grains in the most crystalline samples, and coherence of chemical trends between interstitial glass, glass inclusions, and mineral phases. The evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber. The erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts.

A composite tholeiite dyke at imachar, Isle of Arran: its petrogenesis and associated pyrometamorphism

This NW.-SE. dyke comprises two texturally distinct tholeiitic members. The outer (earlier) bears porphyritic plagioclases with essential olivine, plagioclase, and augite in a chloritized mesostasis; the inner carries porphyritic plagioclases in an olivine-free matrix showing ophitic and intersertal textures and disseminated varioles. The outer component had little metamorphic effect on its country-rock, but where the inner transgresses the outer to come into contact with the schistose-grits the latter are transformed to buchite. The outer component, after initial chilling, crystallized as a virtually closed system; the inner, on the contrary, shows no signs of chilling and bears evidence of crystallization as an open system. The variolitic structures in the inner component result from liquid immiscibility between droplets of acidic melt and basic host magma, the former originating as buchitic liquid caught up by the dyke magma at its contacts with mobilized country rock. Both components afford textural evidence of the immiscibility of a sulphide melt in the tholeiitic magma. The petrography and thermal behaviour of the inner dyke-member support its recognition as a probable former feeder to fissure eruption.