Volcanic Necks Research Articles

Magnetic logs from the Lopra-1/1A and Vestmanna-1 wells, Faroe Islands

Susceptibility measurements from cores (representing basalt, lapilli-tuffs and tuffs) and magnetic logs from the Lopra-1/1A well are presented. The basalts fall into high- and low-susceptibility groups with no overlap. The high-susceptibility basalts (seven cores) have susceptibilities between 4 and 88 ×10–3 SI and consist of basalt with < 1% vesicles from thick massive units. The low-susceptibility basalts are intergranular, intersertal or hypocrystalline and contain no or very little (< 1%) visible magnetite, are generally more altered than the high-susceptibility basalts and have susceptibilities in the range from 0.6 to 1.4 × 10–3 SI (seven cores). The susceptibility of ten volcaniclastites of lapilli-tuff or tuff varies from 0.4 to 3.8 × 10–3 SI. The cores from the Lopra-1/1A well reveal a bimodal distribution of magnetic susceptibility. Low susceptibilities ranging from 0.4 to 4 are characteristic of altered basalts poor in magnetite, lapilli-tuffs and tuffs. Thus single measurements of susceptibility are of little use in discriminating between these three types of rock. Susceptibility logs from the Lopra-1/1A well show that the variation below 3315 m distinguishes clearly between volcaniclastics (hyaloclastites) with low and fairly constant susceptibility and basalt beds of between 5 and 10 m thickness (with high susceptibility). The volcaniclastics comprise some 60–70% of the sequence between 3315 and 3515 m with the maximum continuous sediment layer being 80 m thick. A 1½ m core of solid basalt at 2381 m and sidewall cores of basalt from the Lopra1/1A well have a mean susceptibility of 22.1 ± 3.5 × 10–3 SI (standard deviation (σ) = 23.6, number of samples (N) = 46), while samples of hyaloclastite (lapilli-tuff and tuff) have a mean susceptibility of 0.85 × 10–3 SI (σ = 0.39, N = 17). The mean values of the rock magnetic parameters for 303 basalt plugs from the Vestmanna-1 well are: Qave = 13.3 ± 0.6 (σ = 11), Save = 11.8 ± 0.6 × 10–3 SI (σ = 11) and Jave = 4.64 ± 0.25 A/m (σ = 4.4). The reversely polarised, lowermost (hidden) part of the c. 4½ km thick lower basalt formation correlates with Chron C26r. The upper (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n and the more than 2.3 km thick middle and upper basalt formations correlate with Chron C24n.3r.

Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits, New Brunswick, Canada

The Mount Pleasant Sn–W–Mo–Bi–In base metal deposits that are spatially and temporarily related to multiple, Late Devonian granitic intrusions are located in southern New Brunswick, Canada. Regional rocks include a sequence of Ordovician–Silurian detrital rocks, with minor interbedded volcanic rocks, which were deformed in the Devonian Acadian orogeny. The Mount Pleasant subvolcanic–eruptive complex underlies part of a 17 × 12 km caldera which constitutes a sequence of Upper Devonian volcanic and sedimentary rocks, intruded by subvolcanic–plutonic rocks (e.g. the Mount Pleasant intrusions). Brittle deformation affected the rocks within parts of the caldera.At the Mount Pleasant, along the southwestern margin of this caldera, two mineralised zones, termed the Fire Tower Zone and the North Zone, occur within volcanic necks about 1 km apart. In both zones, multiple, successive Late Devonian granitic intrusions were emplaced into interbedded rhyolitic tuffs/sedimentary breccias overlain by rhyodacitic lavas. The volcanic necks are defined by magmatic–hydrothermal breccias. In the Fire Tower Zone, fine grained granite, granite porphyry and porphyritic granite were emplaced successively. The first is associated with porphyry type W–Mo–Bi–In mineralisation. Three successive generations of granitic intrusions (known as granites I, II and III) also occur in the North Zone. Here, similar but less expressive mineralisation occurs in granite I whereas the others are associated with several significant tin deposits.In both zones, the granitic rocks are pale, porphyritic, generally fine grained, biotitic, contain micrographic intergrowths of K-feldspar and quartz in K-feldspar phenocrysts, and are modally of alkaline-feldspar type. The granitic intrusions may be classified as crustal A-type, with intraplate, high heat production and peraluminous characteristics that are typical of a continental rift environment. Petrographic and geochemical evidence suggests that they formed under conditions of fluid saturation and undercooling. Incompatible and immobile trace element and elemental ratio data indicate that they are highly evolved tin bearing granites with a higher tin potential in the North Zone than in the Fire Tower Zone. There is no solid evidence for petrochemical equivalence between individual granites from these two zones.The endogranitic tin zone is an irregular, almost tabular, 400 m long body enclosed in granite II. The form and petrographic, mineralogical and chemical evidence as well as metal zoning show that the endogranitic tin zone is hosted in a distinct phase of granite, termed the endogranitic tin zone granite. Based on chemical data from thirty North Zone granite samples, but including only one sample from the endogranitic tin zone granite, incompatible (and immobile) trace element evidence indicates that granite III is more evolved and has a higher tin potential than granite II, and only suggests that the highest potential for tin may be found in a probable late differentiate of granite III, the endogranitic tin zone granite, which would have intruded the highest portions of granite III and extended upward into granite II. The exploration significance of this is that important targets for tin mineralisation are deep portions of granite II close to granite III, where possible differentiates of the latter might have intruded the former.

New crayfishes (Decapoda: Parastacidae: Euastacus) from northeastern New South Wales, Australia

Routine astacological surveys in northeastern New South Wales have revealed four new species of crayfish. Three species are allied to the "setosus complex", a group of small and poorly spinose Euastacus previously recorded only from Queensland: E. girurmulayn n.sp. from the Nightcap Range, E. guruhgi n.sp. from the Tweed volcanic plug and E. jagabar n.sp. from the Border Ranges. These three species are differentiated chiefly on features of the sternal keel, spination and antennal squame. Euastacus dalagarbe n.sp., recorded from the Border Ranges, has affinities with a growing group of crayfish displaying morphological traits intermediary between the setosus complex and more characteristically spinose Euastacus. It differs markedly in spination of the chelae, and in the nature of the lateral processes of the pereiopods. All of these taxa occur in association with the much larger and more spinose E. sulcatus. An unusual crayfish specimen of uncertain status is also discussed.

Geology, geochronology and tectonic setting of late Cenozoic volcanism along the southwestern Gulf of Mexico: The Eastern Alkaline Province revisited

A NNW-trending belt of alkaline mafic volcanic fields parallels the Gulf of Mexico from the U.S. border southward to Veracruz state, in eastern Mexico. Previous studies grouped this volcanism into the so-called “Eastern Alkaline Province” (EAP) and suggested that it resulted from Gulf-parallel extensional faulting migrating from north to south from Oligocene to Present. On the basis of new geologic studies, forty-nine unspiked K–Ar and two 40Ar– 39Ar ages, we propose a new geodynamic model for the volcanism along the southwestern Gulf of Mexico. We studied in detail four of the six recognized fields of mafic alkaline volcanism in Veracruz state: 1) The lavas flows of Tlanchinol area (7.3–5.7 Ma), 2) the Alamo monogenetic field and Sierra de Tantima (7.6–6.6 Ma), 3) the Poza Rica and Metlatoyuca lava flows (1.6–1.3 Ma) and 4) the Chiconquiaco–Palma Sola area (6.9–3.2 Ma). Other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast: the Middle Miocene lavas at Anegada High, offshore port of Veracruz, and the Middle to Late Miocene volcanism at the Los Tuxtlas. The existence of major Neogene extensional faults parallel to the Gulf of Mexico (i.e., ∼N–S to NNW–SSE) proposed in previous works was not confirmed by our geological studies. Elongation of volcanic necks, vent alignment, and faults mapped by subsurface data trend dominantly NE to ENE and NW to NNW. These directions are parallel to transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico. Ascent of mafic magmas was likely facilitated and controlled by the existence of these pre-existing basement structures. Coupled with previous studies, our data demonstrate the occurrence of three magmatic episodes in Veracruz: 1) A Middle Miocene (∼15–11 Ma) episode in southern Veracruz (Palma Sola, Anegada, and Los Tuxtlas); 2) A Late Miocene to Early Pliocene (∼7.5–3 Ma) pulse of mafic alkaline volcanism throughout the study region; and 3) A Late Pliocene to Quaternary transitional to calc–alkaline volcanism in southern Veracruz (Palma Sola, Los Tuxtlas). Whereas the first and third episodes may be considered part of the subduction-related Trans-Mexican Volcanic Belt, the second pulse of mafic alkaline volcanism has a more complex origin. The absence of significant extensional faulting precludes a rift origin. We favor a model in which a transient thermal anomaly and melting of the mantle was triggered by the tearing and detachment of part of the subducted slab.

Subducted carbonates, metasomatism of mantle wedges, and possible connections to diamond formation: An example from California

We investigated calcite globules and veins in two spinel-garnet peridotite xenoliths from the sub- Sierra Nevada mantle. The studied xenoliths were entrained in a Miocene (11 Ma) volcanic plug. These carbonates are associated spatially with silicate glass inclusions, suggesting that they are primary inclusions-inclusions that formed at high temperature in the mantle and not at or close to the Earth's surface. The host peridotites represent samples of the lithospheric mantle wedge beneath the Mesozoic California magmatic arc, as indicated by radiogenic isotopic ratios measured on clinopyroxene separates [87Sr/86Sr(11 Ma) = 0.7058-0.7061, εNd (11 Ma) = -1.9 to -0.7]. Mineral chemistry of the peridotite major phases is typical of a mantle section that was depleted of melt. The δ18O values of olivine and orthopyroxene from the two samples are also typical of mantle rocks (δ18O = 6-6.5‰). In contrast, calcite veins have δ18O of 18-20‰ and δ13C of -14‰, arguing for a subducted sedimentary origin for these carbonates. Presumably, the carbonates were expelled from the downgoing slab and fluxed into the overlying mantle wedge as CO2- or CO2-H2O-rich fluids or melts. The trace-element patterns of two analyzed calcite veins are typical of the arc signatures (e.g., depletions in high-fieldstrength elements) seen in calc-alkaline magmatic rocks worldwide. However, the cores of peridotite clinopyroxenes do not show that pattern, suggesting that the arc-like trace element signature was introduced via the recycled carbonate agent. A connection between mantle wedge carbonation and diamond formation in a subduction environment is proposed based on these observations.

The endogranitic tin zone, Mount Pleasant, New Brunswick, Canada and its metallogenesis

The Mount Pleasant Sn–W–Mo deposits occur within the margin of an eroded caldera underlain by a Devono–Mississippian intrusive-subvolcanic-eruptive complex of granitic, porphyritic and felsic pyroclastic rocks. The separate North and Fire Tower mineralised zones are centred in volcanic necks defined by hydrothermal breccias within this complex. The oldest of three successive granites in both Zones hosts W–Mo mineralisation, whereas the two younger granites in the North Zone contain various Sn deposits. One of these, the endogranitic tin zone, is an irregular but generally tabular body hosted within a distinctive, highly evolved phase of the third, and youngest granite.The endogranitic tin zone was formed by successive stages of sericitisation, chloritisation/topazification-silicification, brecciation, dense chloritisation-topazification and late replacement. It contains quartz, chlorite, topaz, fluorite, cassiterite, arsenopyrite, base metal sulphides, pyrite, stannite, wolframite, native bismuth, molybdenite, kaolinite and carbonates. The ore minerals occur about equally in veinlets and disseminations. Cassiterite was deposited in all stages, but more abundantly during brecciation and particularly during dense chloritisation-topazification where it accompanies a distinctive, avocado-green, Fe-rich chlorite. The host, endogranitic tin zone granite, has undergone significant depletions and enrichments in major and minor elements.Fluid inclusions indicate that the chloritised/topazified-silicified rock formed from early, low salinity dominantly meteoric fluids at ∼200–225°C. Later, saline, magmatic-hydrothermal fluids, probably differentiated in the upper part of the endogranitic tin zone granite, mineralised the brecciated and densely chloritised-topazified rocks at similar temperatures. Latest low salinity meteoric fluids formed the late replacement bodies at 175–185°C. Sn was deposited under acidic conditions at oxygen fugacity of 10−39–10−34 atm from dominant SnCl20 and SnF+ complexes due to fluid oxidation and neutralisation, and cooling.Endogranitic tin zone cassiterite has a trace element signature that is intermediate between that of cassiterite from deep-seated greisens and pegmatites, and shallower cassiterite–sulphide and Bolivian porphyry–tin ores. The late replacement cassiterite, however, is high in Nb, Ta, Ti and Fe, and thus has a pegmatitic signature. This, together with their size and distribution within the endogranitic tin zone and elsewhere in the complex, suggests that the late replacement bodies are aborted pegmatites formed when the magmatic-hydrothermal fluids became largely diluted by the late meteoric fluids. Wood tin and colloform cassiterite contain abundant Fe, resembling wood tin in the high-level topaz rhyolites and Russian subvolcanic tin deposits. Sphalerite contains abundant In, as do other high-level, stanniferous polymetallic ores. The endogranitic tin zone at Mount Pleasant appears to be unique, differing from both greisen– and porphyry–tin deposits and no close analogues are known elsewhere.

Mesozoic mafic alkaline magmatism of southern Scandinavia

More than 100 volcanic necks in central Scania (southern Sweden) are the product of Jurassic continental rift-related mafic alkaline magmatism at the southwest margin of the Baltic Shield. They are mainly basanites, with rarer melanephelinites. Both rock groups display overlapping primitive Mg-numbers, Cr and Ni contents, steep chondrite-normalized rare earth element patterns (LaN /YbN = 17–27) and an overall enrichment in incompatible elements. However, the melanephelinites are more alkaline and have stronger high field strength element enrichment than the basanites. The existence of distinct primary magmas is also indicated by heterogeneity in highly incompatible element ratios (e.g. Zr/Nb, La/Nb). Trace element modelling indicates that the magmas were generated by comparably low degrees of melting of a heterogeneous mantle source. Such a source can best be explained by a metasomatic overprint of the mantle lithosphere by percolating evolved melts. The former existence of such alkaline trace element-enriched melts can be demonstrated by inversion of the trace element content of green-core clinopyroxenes and anorthoclase which occur as xenocrysts in the melanephelinites and are interpreted as being derived from crystallization of evolved mantle melts. Jurassic magmatic activity in Scania was coeval with the generation of nephelinites in the nearby Egersund Basin (Norwegian North Sea). Both Scanian and North Sea alkaline magmas share similar trace element characteristics. Mantle enrichment processes at the southwest margin of the Baltic Shield and the North Sea Basin generated trace element signatures similar to those of ocean island basalts (e.g. low Zr/Nb and La/Nb) but there are no indications of plume activity during the Mesozoic in this area. On the contrary, the short duration of rifting, absence of extensive lithospheric thinning, and low magma volumes argue against a Mesozoic mantle plume. It seems likely that the metasomatic imprint resulted from the earlier Permo-Carboniferous rifting episode which affected the entire study area and clearly was accompanied by plume activity (Ernst and Buchan in American Geophysical Union, pp 297–337, 1997). Renewed rifting in Jurassic times triggered decompression melting in the volatile-enriched lithospheric mantle and the alkaline melts generated inherited the earlier “stored” plume signature.

Cretaceous arc volcanism of Byers Peninsula, Livingston Island, Antarctica : new petrological, geochemical and isotope data

Abstract On Byers Peninsula, the largest summer ice-free area in the South Shetland Islands, a thick Upper Jurassic-Lower Cretaceous sedimentary sequence and penecontemporaneous igneous activity records the progressive emersion of a volcanic arc. The oldest rocks are represented by voluminous basaltic flows intercalated within late Jurassic shallow marine sediments. Following the emersion, Lower Cretaceous strata were deposited. Welded and nonwelded dacitic to rhyolitic ignimbrites are exposed locally at the base of the continental succession. However, the majority of the outcrops consists of Lower Cretaceous basaltic to basaltic andesite plugs and dykes, representing the roots of a volcanic arc, which form most of the topographic highs of the peninsula. Byers Peninsula lavas are porphyritic with mineralogical associations typical of orogenic series. Major, trace and rare-earth element data show that all the rocks are relatively poor in K2O, enriched in large ion lithophile elements and that they have a clear subduction imprint (Nb-Ta anomaly). Furthermore, a tholeiitic tendency is shown by the REE patterns of some basaltic lavas. Sr- Nd- and Pb isotopic data support an origin from the sub-arc asthenosphere. Byers lavas have isotope signatures similar to that of the South Sandwich intra-oceanic arc. The enriched nature of most of the samples, revealed by Pb isotope ratios and Th/Yb enrichment, indicates that the MORB-like mantle source has been modified by fluids and sediments derived from the subducting slab. No clear temporal chemical change is observed during the evolution of this near-trench volcanic arc.

Interior layered deposits of Valles Marineris, Mars: analogous subice volcanism related to Baikal Rifting, Southern Siberia

The relatively unknown volcanism of the Azas Plateau in the Tuva Republic is linked with the Baikal Rift system and it presents many excellently preserved tuyas that are volcanic edifices formed by subice and underwater eruptions. These tuyas were formed in the Quaternary, when eruptions characterized by trachybasalt and basanite compositions coincided with extensive glaciation in southern Siberia. Their stratigraphy is typically composed of very thick, weak and often layered hyaloclastite sequences, and near horizontal lava cap layers produced by subaerial eruptions. The interior layered deposits (ILDs) in Valles Marineris of Mars, in general, overlie unconformably canyon floors, canyon walls, and in some cases, chaotic terrain with onlap geometry. Many of these ILD mounds have either flat-topped or domal profiles. Thin layers of a few to tens of meters in thickness are extensively observed with relatively limited post-depositional deformations, stacking up to form hundreds to possibly thousands of meters-thick sequences. One example shows possible cross-beds, but bedforms in the layers are rare within the studied MOC images. These characteristics are indicative of low-energy depositional environments such as lacustrine basins or air-falls of dust or volcanic ash rather than sediments deposited in strong currents. Volcanic sheet flows and distal facies of turbidites, however, could be also similar to some thin layers of the ILDs. The ILD mounds are also associated with possible volcanic landforms such as feeding dikes, volcanic necks and vents. Cap units, if layered, are in general near horizontal whereas lower strata could have an angular unconformity relationship with the cap layers, which can be interpreted as lava-fed deltas. These observations support the hypothesis of subice and/or underwater volcanism for the formation of the ILDs. However, not all ILDs have ideal tuya morphology and stratigraphy, implying more complex history of the ILDs in Valles Marineris. The complex interactions between rift formation, associated volcanism, ice coverage due to climate change and floods are observed in the Baikal Rift system of southern Siberia and these could be direct analogues for the Valles Marineris region of Mars.

Carbonate platform growth influenced by contemporaneous basaltic intrusion (Albian of Larrano, Spain)

Abstract Carbonate platform growth in active tectonic settings may be strongly influenced by the structural evolution of the basin, including volcanic activity. In this paper, the sedimentary–tectonic evolution of the Duranguesado carbonate platform (Larrano) in northern Spain is described, and an evolutionary depositional model is presented. The Albian Duranguesado carbonate platform deposits are dominated by rudist and coral limestones with small intervening argillaceous limestone‐filled troughs (10–30 m deep). The platform succession is divided into two parts. The lower platform deposits were intruded by volcanics and tilted before the deposition of the upper platform succession. A volcanic vent plug filling an upward‐flaring pipe occurs in the lower carbonate platform succession. The timing of intrusion is well constrained to the early Albian. Tectonic extension and active deep‐seated strike‐slip faults induced magma ascent at a fault intersection. Sedimentological analysis of these areas indicates that, before volcanic intrusion, they acted as weakened zones of extension, and were slightly more subsident basinal areas. These sites contain anomalous accumulations of siliceous sponge spicule deposits, linked to the release of hydrothermal fluids on the sea floor. After the phase of intrusion, tilting and erosion of the platform and its associated volcanic products occurred. As the intrusion cooled, the platform downwarped, leading to the formation of an overlying perched capping basin bordered by carbonate platforms. Over the volcanic plug, carbonate mounds grew on local palaeohighs. The presence of strike‐slip and magma emplacement in the Duranguesado platform is related to movements between the Iberian and European lithospheric plates, which were accommodated by rotation and lateral movement of the crust along wrench faults.

Fohn lamproite and a possible Late Eocene — pre‐Miocene diatreme field, Northern Bonaparte Basin, Timor Sea

Seismic profiles in the Northern Bonaparte Basin, Timor Sea, northern Australia, disclose an east‐northeast‐trending 120 × 25 km swathe of over 40 circular to subcircular features excavated in the pre‐Miocene erosional surface and buried by Lower Miocene sediments. The larger structures, typified by the Fohn Structure, include central structural highs overlying narrow vertical corridors of upward‐bulging seismic horizons (bulge‐forms). Associated with these structural highs are troughs that overlie broader vertical zones of slower seismic velocity (crater‐form features). Smaller circular features (Dc <2.0 km) include both crater‐form and bulge‐form structures. The first type shows zones of seismic velocity crater‐form structures under craters excavated below the base‐Miocene horizon. The second type exhibits vertical seismic bulge‐form features directly beneath palaeotopographic highs developed at the same horizon. Identification in drill cuttings of fragments of olivine‐leucite lamproite at Fohn 1, at depths of 690–880 m, suggests that this structure is a lamproite diatreme consisting of a massive volcanic plug ringed by pyroclastics, analogous to the classic champagne‐glass structure of alkaline volcanic diatremes. The seismic morphometry of several of the larger structures of the field is analogous to Fohn, suggesting that they too may be diatremes. The smaller bulge‐form features may represent deeply eroded diatreme feeder necks. Crater‐like structures underlain by seismic velocity crater‐forms are interpreted as relatively little‐eroded maars with low‐velocity infill, probably consisting of volcanic and country rock breccia as at Fohn 1, as well as unconsolidated clastics of the transgressive Lower Miocene Oliver Sandstone Member. Lamproite fragments at Fohn 1 are dominated by apatite‐rich assemblages consisting of saponite‐altered olivine, analcime and nontronite‐altered zoned leucite, high‐Ti K‐bearing diopside, Ti‐rich richterite, Ti‐rich phlogopite, Mg‐rich ilmenite, Cr‐bearing priderite, Ba and Sr‐rich fluorapatite and rare‐earth element‐rich apatite. Textural relations suggest early crystallisation of olivine, leucite and K‐bearing diopside followed by crystallisation of alkali amphibole, priderite, ilmenite and apatite in alkali‐Tindash;Fe–rich groundmass. Fohn 1 lamproites have very high whole‐rock P levels, high Na,Sr and Y and low K,Ba,Zr and Nb levels compared to Early Miocene West Kimberley lamproites. The mineral paragenesis suggests crystallisation of olivine‐leucite assemblages at temperatures >1100°C and pressures <0.1 GPa, followed by crystallisation of amphibole and phlogopite under water‐saturated conditions at temperatures below ∼1020°C.

Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40–60 mGal (10 −5m/s 2) and 800–1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5±2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE–SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with well differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5×10 −2 SI units with an average Koenigsberger ratio ( Q n) of almost one and average direction of remanent magnetization of D=147.4° and I=+56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m 3, respectively and susceptibility of 3.14×10 −2 SI units with a Q n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D=337° and 340° and I=−38° and −50° which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.

Chemical and isotopic composition of fumarolic gases and the SO 2 flux from Volcán de Colima, México, between the 1994 and 1998 eruptions

Volcán de Colima is the most active volcano in Mexico, including a significant effusive eruption in 1991 and a weak explosive event in 1994. Geochemical monitoring of the volcano started in January 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater (Z3, ∼800°C, and Z2, ∼400°C). Chemical compositions of volcanic gases from the Z2 fumarole showed a peak in S/Cl in the middle of 1997, 14–15 months before the start of the 1998 lava eruption. High-temperature volcanic vapors from Z3 changed progressively in water isotopic composition and showed a weak trend of increasing HCl/HF and a peak in S/Cl at the same time. Starting from the summer of 1997, the volcanic water became gradually enriched in deuterium, which probably indicated an increasing contribution from deep, less degassed magma. A negative correlation between δD and Cl content for the Z3 fumarolic gases is explained by shallow degassing of the Colima hot lava plug. This hypothesis is consistent with published data on Cl contents and solubilities in magmas and supported by the observed positive trends of S/Cl and HCl/HF with δD for the Z3 fumarole. Until September 1998, the SO 2 flux from the volcano was at a low level (<100 tons/day), and often below the detection limit of the instrument (∼30 tons/day). A sharp increase in the SO 2 flux was recorded 1 month before the eruption started. It jumped from 400±50 tons/day to 1600 tons/day 2 days before new lava emerged on 20 November 1998. The SO 2 flux correlated well with seismicity.

Les pitons volcaniques d'Éthiopie sont les conduits d'alimentation des trachytes des volcans boucliers

Abstract Volcanic plugs in northern Ethiopia have silica-saturated, alkali-rich trachytic compositions. They are enriched in incompatible trace elements and show negative Sr, Ba, Eu and Ti anomalies. Their compositions are distinct from those of felsic rocks within trap sequences, which are rhyolitic. The plugs are not the feeders to the trap volcanism but instead may be related to overlying shield volcanoes.

The Cheslatta Lake suite: Miocene mafic, alkaline magmatism in central British Columbia

New mapping, mineralogical, and geochemical studies help characterize late Tertiary primitive, alkaline, sodic basanite, alkali olivine basalt, transitional basalt, and diabase in the Nechako River, Whitesail Lake, and McLeod Lake map areas of central British Columbia and distinguish the Miocene Cheslatta Lake suite. The suite encompasses scattered erosional remnants of topographically distinct, columnar-jointed, olivine-phyric basalt and diabase volcanic necks, dykes, and associated lava flows north of the Anahim volcanic belt and west of the Pinchi Fault. Volcanic centres at Alasla Mountain and at Cutoff Creek, near Cheslatta Lake, are proposed as type areas. Olivine, plagioclase, and pyroxene phenocrysts, megacrysts, and (or) xenocrysts; common ultramafic xenoliths; and rare but significant plutonic and metamorphic xenoliths are characteristic. Basanite, transitional basalt, and alkali olivine basalt groundmass contain plagioclase, clinopyroxene, Fe-Ti oxides, feldspathoid, olivine, and apatite. The Cheslatta Lake suite is characterized by its alkaline character, olivine-rich (&gt;10 wt.%) normative mineralogy, and silica-undersaturated nature (&gt;1 wt.% normative nepheline; hypersthene-normative rocks are uncommon). Mg numbers vary between 72–42. Some samples encompass near-primitive mantle melt compositions. Cheslatta Lake suite rocks in the Nechako River area are distinguished from the underlying Eocene Endako and stratigraphically higher Neogene Chilcotin groups basaltic andesite lavas within the study area, and from the Chilcotin Group basalt in the type area south of the Anahim volcanic belt, by form, preserved thickness, phenocryst–xenocryst mineralogy, amygdule abundance, included xenoliths, isotopic age, and major and incompatible, high field strength, and rare-earth trace element contents.

Lapilli tuff as evidence of Early Jurassic Strombolian-type volcanism in Scania, southern Sweden

Central Scania, southernmost Sweden, is penetrated by more than one hundred basaltic volcanic necks of Jurassic age. Pyroclastics, related to the volcanic necks, have been investigated to reveal the volcanic environment, and the eruptive character required to produce the pyroclastics. Based on textural and petrographical evidence, a Strombolian-type eruptive character is suggested. The sediment, a lapilli tuff, is clay altered due to early diagenetic processes, but traces of palagonite reveal a former basaltic composition of the lapilli tuff. The volcanoes were part of a monogenetic volcanic field and deposition of the pyroclastic sediment took place on land, probably close to oceanic water.

Multiple Centers of Mineralization in the Indio Muerto District, El Salvador, Chile

The porphyries that formed copper deposits in the Indio Muerto district include both 44 Ma rhyolitic subvolcanic intrusions and 42 to 41 Ma granodioritic stocks and dikes. All are aligned for 4 km along a north-northeast trend, which is the apparent hinge line of a 58 Ma trap-door caldera. Three separate centers of mineralization were associated with the earlier rhyolites, Cerro Pelado, Old Camp, and probably M Gulch-Copper Hill. Four separate centers were associated with the later granodiorites, M Gulch-Copper Hill, O Nose, Turquoise Gulch, and Sector Granito. Patterns of lithology, structure, mineralization, and alteration in the various centers, which have been documented by systematic mapping partially supported by studies of mineralogy and geochemistry, are presented and compared to the previously described Turquoise Gulch center. Cerro Pelado is a 750-m diameter rhyolitic volcanic neck, intruding host andesite at the northeast end of the district. Very weak Cu but much stronger Mo mineralization, with analogies to Climax-type deposits, is associated with this intrusion. Surficial sericitic alteration with weak pyrite-(chalcopyrite-bornite)-specularite-gold mineralization shows decreasing sericite, pyrite, Cu, and Au contents downward. This strong vertical zonation grades downward to alkali feldspar-biotite alteration with increasing molybdenite and magnetite content below an abrupt top of quartz veining. The Old Camp is a 250-m diameter, roughly cylindrical body of disseminated chalcopyrite-bornite and quartz veining with K feldspar-biotite alteration, centered within a rhyolitic quartz porphyry dike. This dike was emplaced and mineralized within 1 m.y. of Cerro Pelado. Mineralization zones outward through chalcopyrite-pyrite to pyrite with sericite-chlorite and then chlorite-epidote-albite alteration. A supergene sulfide enrichment blanket has been largely oxidized. M Gulch-Copper Hill was first mineralized in and around quartz porphyry dikes with quartz veining, disseminated chalcopyrite-bornite, and K feldspar-biotite alteration. Roughly 1 m.y. later, intrusion of a series of feldspar porphyries and igneous breccia displaced the center of the mineralization at M Gulch-Copper Hill. Late-stage hydrothermal activity reworked earlier mineralization and superimposed pyrite-chalcopyrite assemblages with sericite. Aside from some rich D veins containing pyrite-bornite-digenite, economic grade in the M Gulch-Copper Hill pit is mostly due to supergene enrichment. Ore at O Nose was largely supergene chalcocite-covellite on chalcopyrite-pyrite, centered on early QG feldspar porphyry and brecciated and biotized andesite. Subsequent intrusion of intramineral L feldspar porphyry and late hydrothermal activity apparently reworked earlier mineralization. At the contact, this produced a fringing chalcopyrite-pyrite zone in andesite, which is sharply zoned upward to pyrite-bornite-chalcopyrite with higher grade. This is apparently the same L porphyry that intruded the center of the main Turquoise Gulch orebody and controlled transitional and late patterns of alteration and mineralization described previously. The supergene enrichment blanket which makes ore in Sector Granito, to the southwest of Turquoise Gulch, is connected to the main orebody but primary mineralization is spatially separate and related to a different set of intrusions. Although less well known than other centers, a chalcopyrite-bornite core is apparently related to unusually intense, texture-destructive K feldspar alteration in early equigranular X porphyry and later G feldspar porphyry. This core mineralization grades outward to pyrite-sericite and abruptly upward to pyrite-bornite-chalcopyrite with sericite-andalusite-pyrophyllite-diaspore. Extending the trend to the southwest is the barren Granite Gulch feldspar porphyry, emplaced between the two mineralizing periods. 40 Ar/ 39 Ar dating demonstrates that two episodes of mineralization associated with the early rhyolitic and later granodioritic intrusions each lasted approximately 1 to 1.4 m.y., from 44.5 to 43.5 and 42.3 to 40.9 Ma, respectively. The barren Granite Gulch porphyry and perhaps the early and equigranular X and O porphyries were intruded between 43 and 42 Ma. Magmatic activity, therefore, occurred episodically over at least 3 m.y. The individual centers are best explained as expressions of different cupolas on a magma chamber, which evolved from granitic to granodiorite to quartz diorite with progressive injection of new magma. Despite our best efforts to date 16 carefully selected samples, it was not possible to define the sequence and duration of the partially overlapping centers within each of the larger mineralization periods. The different centers exhibit a wide diversity of characteristics, which mirror variations on the general theme of porphyry Cu-Mo development. Changing hydrothermal input from an evolving and growing magma chamber and major remobilization of early mineralization by both intrusion and hydrothermal activity are major factors in this diversity.

Tertiary alkaline volcanics in the Nunatak Region, Northeast Greenland: new observations and comparison with Siberian maymechites

Highly alkaline ultrabasic lavas which occur in the remote Nunatak Region of Northeast Greenland between latitudes 73°45′N and 74°25′N are approximately contemporaneous with the better known, voluminous Tertiary tholeiitic plateau basalts of the coastal region (68°N–71°N, 74°N–76°N). Recent reconnaissance mapping extended the known outcrop of the alkaline ultrabasic lavas southwards and northwards, at the same time revealing several new volcanic plugs, assumed to be feeders to the lavas. Rounded dunitic xenoliths were recovered from one of these plugs. The xenoliths have olivine with forsterite contents too low, and chrome–spinel too titaniferous, to be of mantle origin. Rather, these features are consistent with crystallisation from the picritic host magmas, and the xenoliths are thus regarded as cognate. The alkaline igneous rocks are all ultramafic with high MgO contents (up to 17 wt.%), high TiO 2 and FeO (up to 8.5 wt.%, and 18.1 wt.%, respectively) and very low silica and aluminium (generally 37–44 wt.% SiO 2 and 5–10 wt.% Al 2O 3). The lavas have phenocrysts of olivine, clinopyroxene and iron–titanium oxides. Some have discernable perovskite in the groundmass. Common mica in the groundmass testifies to the hydrous nature of the parental magma. Chemically, the Nunatak Region volcanics are rich in incompatible elements as well as in Ni and Cr, and range from melilitites to basanites. Compared to melilitites from most other settings, those from the Nunatak Region have lower Al 2O 3, and higher FeO and TiO 2 for a given MgO. There is, however, close chemical affinity with the strongly undersaturated, primitive lavas, so-called maymechites (formerly meimechites [Arndt, N., Chauvel, C., Czamanske, G., Fedorenko, V., 1998. Two mantle sources, two plumbing systems: tholeiitic and alkaline magmatism of the Maymecha River basin, Siberian flood volcanic province. Contrib. Mineral. Petrol. 133, 297–313]), described from the Maymecha region in the Siberian Traps. As recently proposed for the origin of the maymechites, it is suggested that volcanic rocks of the Nunatak Region formed from low degrees of mantle melting (<5%) at very high pressures (>5GPa) when a hydrated mantle transgressed its wet solidus during ascent beneath a thick cratonic lithosphere. Trace-element profiles of the nunatak lavas suggest recycled oceanic crust as an important component in the mantle source. The occurrences of both the Nunatak Region volcanic rocks and the Maymecha volcanics at the rims of ascending mantle plumes during continental rifting suggest that some mantle plumes may have a hydrous component.

瀬戸内海西部，芸予諸島の後期中新世火山岩類（芸予火山岩類）及び前‐中期中新世火山岩類（瀬戸内火山岩類）について

Miocene volcanic rocks were discovered in the Geiyo Islands, western Setouchi region of southwest Japan. These occur as dikes or volcanic necks with marginal pyroclastic rocks. The volcanic rocks are composed of high-alkali tholeiitic olivine basalt, olivine andesite and bronzite andesite, in which the olivine andesite from Oge-shima chemically corresponds to high magnesian andesite. Whole rock K-Ar age determinations were carried out on olivine basalts, olivine andesites and bronzite andesites. An olivine andesite from Osakishimo-shima and two bronzite andesites from Okamura-shima yielded late Miocene ages of 8.2±0.7 Ma, 7.8±0.4 Ma and 8.0±0.4 Ma, respectively. It follows that subalkaline volcanism distinctively occurred at late Miocene age in the western Setouchi region. Accordingly, we give a name of “Geiyo volcanic rocks” to these volcanic rocks, which include late Miocene high-alkali tholeiitic olivine basalt reported from the adjacent area. On the other hand, olivine basalts from Okubi-shima and Ko-oge-shima give middle Miocene ages of 12.8±0.6 Ma and 15.4±0.8 Ma, respectively. In addition, an olivine andesite from Oge-shima has been dated as 17.4±0.9 Ma, indicating early Miocene age. On the basis of the radiometric ages, petrography and whole rock chemistry, these volcanic rocks are considered to be a member of the Setouchi volcanic rocks.

The Kinetics of the Smectite to Illite Transformation in Cretaceous Bentonites, Cerro Negro, New Mexico

AbstractThe thermal effects, as well as the survivability and origins of microorganisms in Cretaceous rocks, are evaluated from the timing and extent of the smectite to illite transformation in Cretaceous bentonites collected from cores outside the thermal aureole of the Pliocene Cerro Negro volcanic neck. Overall, randomly ordered mixed-layered illite-smectite (I-S) is the predominant clay mineral in these bentonites, and the K-Ar ages of I-S range from 36 to 48 Ma (21 analyses, two additional analyses were outside this range). Increased temperature from burial is thought to be the primary factor forming I-S in these bentonites. Kinetic model calculations of the smectite to illite transformation are also consistent with I-S formed by burial without any appreciable thermal effects due to the emplacement of Cerro Negro. In a core angled toward Cerro Negro, the percentages of illite layers in I-S from the bentonite closest to Cerro Negro are slightly higher (32-37%) than in most other bentonites in this study. The K-Ar ages of the closest I-S are slightly younger as a group (38-43 Ma; Average = 41 Ma; N = 4) than those of I-S further from Cerro Negro in the same core (41-48 Ma; Average = 44 Ma; N = 6). A small amount of illite in this I-S may have formed by heat from the emplacement of Cerro Negro, but most illite formed from burial. Vitrinite reflectance, however, appears to record the effects of heating from Cerro Negro better than I-S. Tentatively, the temperature of this heat pulse, based on vitrinite data alone, ranged from 100 to 125°C and this is most evident in the CNAR core. The upper temperature, 125°C, approximates the sterilization temperatures for most microorganisms, and these temperatures probably reduced a significant portion of the microbial population. Thermophiles may have survived the increased temperatures from the combined effects of burial and the intrusion of Cerro Negro.