Mafic Lava Flows Research Articles

Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.

How lava flows: New insights from applications of lidar technologies to lava flow studies

Mafic lava flows are common; for this reason, they have long been a focus of volcanological studies. However, field studies of both older and active flows have been hampered by difficulties in field access; active flows are hot, whereas older flows have rough and jagged surfaces that are difficult to traverse. As a result, morphometric studies of lava flows have generally lagged behind theoretical studies of flow behavior. The advent of laser scanning (LS) (i.e., lidar, light detection and ranging) technologies, both airborne mapping (ALSM) and terrestrial (TLS), is promoting detailed studies of lava flows by generating data suitable for production of high-resolution digital elevation models (DEMs). These data are revolutionizing both the visual and quantitative analysis of lava flows. First and foremost, this technology allows accurate mapping of flow boundaries, particularly in vegetated areas where bare earth imaging dramatically improves mapping capabilities. Detailed imaging of flow surfaces permits mapping and measurement of flow components, such as channels, surface folds, cracks, blocks, and surface roughness. Differencing of preeruptive and posteruptive DEMs allows analysis of flow thickness variations, which can be related to the dynamics of lava emplacement. Multitemporal imaging of active flows provides information not only on the rates and locations of individual flow lobes, but also measurement of pulsed lava transport. Together these new measurement capabilities can be used to test proposed models of channel formation, lava tube formation, rates of flow advance, and flow conditions within lava channels; they also provide new ways to assess the hazard and risk posed by lava flow inundation. Early published studies illustrate the potential of applying lidar to volcanic terrain; it is clear, however, that the availability of high-resolution digital topography is poised to revolutionize the study of mafic lava flows.

Soltan Maidan Complex (SMC) in the eastern Alborz structural zone, northern Iran: magmatic evidence for Paleotethys development

Soltan Maidan Complex is located in the eastern Alborz zone, Iran. Thickness of this complex ranges from 436 to 1,286 m and its age is Upper Ordovician–Silurian. It is the most important and voluminous Lower Paleozoic magmatic activity in Iran. This complex consists of mafic lava flows, agglomerates, and tuffs accompanied with some sedimentary intercalations. Field geology evidence indicate both subaerial and submarine eruptions during accumulation of this complex and also occurrence of a long-time exhumation event in the study area from Upper Ordovician to Middle Devonian time. The volcanic rocks of Soltan Maidan Complex indicate a range composition of basalt to basaltic-andesite and show transitional to alkaline nature of the primary magma along with evolution via fractional crystallization process. These rocks represent fairly high degrees of partial melting (between 14–20 %) from a common garnet peridotitic mantle source in an intra-continental rift setting. Relatively high Nb and Zr contents (i.e., 21.68 and 189.63 ppm, respectively) show the generation of magma from an enriched mantle source. Also, Th/La (with an average of 0.13) and Zr/Nb (with an average of 9.11) ratios of the samples illustrate mixing between an OIB-like magma with an EMI-type mantle source. The primary magma shows lack or minor role of the crustal contamination during its ascent. According to the many evidences, Soltan Maidan magmatism has occurred in the Upper Ordovician–Silurian times in relevant to a cycle of rift-related magmatic events and breakup of the northern margin of Gondwana in the process of Paleotethys development in the north of Iran.

Porphyry Cu–Au mineralization in the Mirkuh Ali Mirza magmatic complex, NW Iran

The Mirkuh Ali Mirza Cu–Au porphyry system in East Azerbaijan Province is located on the western part of the Cenozoic Alborz–Azerbaijan volcanic belt. The belt is also an important Cu–Mo–Au metallogenic province in northwestern Iran. The exposed rocks in the study area consist of a volcaniclastic sequence, subvolcanic rocks and intermediate to mafic lava flows of Neogene age. The volcanic rocks show a typical subduction-related magmatic arc geological and geochemical signature, with low concentration of Nb, Ta, and Ti. Mineralization is hosted by Neogene dacitic tuff and porphyritic dacite situated at the intersections of northeast and northwest faults. Field observations, alteration zonation, geochemical haloes and isotopic data of the Mirkuh Ali Mirza magmatic complex show similarities with typical convergent margin Cu–Au porphyry type deposits. The following features confirm the classic model for Cu–Au porphyry systems: (a) close spatial association with high-K calcalkaline to shoshonitic rock related to post-collision extensional setting (b) low grade Cu (0.57%) (c) stockworks as well as disseminated sulfides (c) zonality of the alteration patterns from intense phyllic at the center to outward weak-phyllic, argillic, and propylitic (d) the presence of a pyritic halo (e) accompanied by sheeted veins and low-sulfidation epithermal gold (f) mineralization spatially associated with intersection of structures, (g) genetically related to diorite porphyry stocks at depth (h) geochemical zonation of (Cu±Au±Ag±Bi)→(Cu+Mo±Bi±Au±Pb±Zn±As)→(Au+Mo±Pb±Zn)→(As+Ag+Sb+Mn+Ba+Pb+Zn+Hg)→Hg from center to outwards (i) The range of sulfur isotopic values is approximately zero (interpreted to have magmatic source) and similar to other subduction-related porphyry Cu deposits.

Contrasting interactions of two magmas with the same water-saturated hosts, Big Bend National Park, Texas

Rhyolitic and mafic dikes cut lower Tertiary sediments, tuffs and mafic lavas of the Eocene–Oligocene Chisos Formation in Big Bend National Park, Texas. Two contrasting magmas injected into the same hosts under the same conditions provide a “controlled” experiment on magma–host interaction. Peralkaline rhyolite dikes are extensive and stand in bold topographic relief. On contact surfaces against mudstones, close-packed spheroids of chilled rhyolite indicate vigorous movement of magma within envelopes of fluid. Against sandstones, the rhyolite dike contacts are more irregular and locally protrude as blunt fingers into the wall rock. Against mafic lava flows, dike contacts are planar, and the rhyolite is less altered. Mafic dikes cut the rhyolite dikes. Some contacts of these mafic dikes are peperitic, indicating intrusion into soft wet sediment and tuff. The same hosts of the older rhyolitic dikes must also have been water-saturated and unconsolidated. Higher viscosity and lower temperature prevented rhyolite magma from forming detached bodies, or engulfing its host, to form peperites.Estimated pressures and contact temperatures approximate or exceed the critical point of water, indicating that the fluid envelopes at rhyolite dike contacts were very unstable, because even small temperature fluctuations would have produced wide variations in fluid density around the critical point. The resulting volume instability triggered rapid pulsations in the fluid envelope, deforming the contacts. Where permeability of the host was low, hot fluid accumulated at contacts, but where permeability was high, fluid could escape before reaching high temperature.

Mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into mantle and crustal processes

Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux.

Origin of the volcanic complexes of La Désirade, Lesser Antilles: Implications for tectonic reconstruction of the Late Jurassic to Cretaceous Pacific-proto Caribbean margin

La Désirade Island is located on the hanging wall of the present-day Lesser Antilles subduction zone and consists of a suite of Mesozoic igneous rocks capped by Neogene limestone. The basement suite contains Kimmeridgian to Tithonian (~ 153–145 Ma) mafic lava flows and pillow basalts overlain by felsic flows and breccias and intruded by a Mid-Berriasian (~ 144 Ma) trondhjemite pluton and intermediate to felsic dykes. The mafic rocks form a ~ 300 m thick sequence which trace element geochemistry reveals to contain, in stratigraphic order: (1) tholeiites with a very weak subduction signature; (2) calc-alkaline and tholeiitic arc rocks containing pelagic and terrigenous sediment slab-related components and (3) arc tholeiites with a minor subduction signature. The mantle wedge source was depleted and did not contain a significant plume-related component. The overlying felsic rocks show similar trace element patterns and incompatible trace element ratios to the mafic units. Factors such as pelagic sedimentary deposition and re-working, low eruption rates and the presence of MORB-like and felsic rocks are best explained by an origin at a back-arc spreading ridge. This back-arc was most likely in the proto-Caribbean (Colombian Marginal) seaway and was related to east-dipping Andean/Cordilleran subduction. Other sites in the Greater Antilles and Central America older than the ~ 135 Ma westward acceleration of North America appear to corroborate a latest Jurassic–Early Cretaceous east-dipping arc system. The preservation of La Désirade in the fore-arc of the present Antilles arc is consistent with Mid-to-Late Cretaceous inception of west-dipping subduction along the former back-arc axis which had previously given rise to La Désirade.

Petrology and K-Ar chronology of the Neogene-Quaternary Middle Atlas basaltic province, Morocco

AbstractThe Middle Atlas basaltic province is the largest and youngest volcanic field in Morocco. A hundred well-preserved strombolian cones and maars emitted numerous mafic lava flows, which cover a surface of ca. 960 km2. The study of 103 samples from these volcanic units allows us to distinguish four lava types, to map their distribution, to provide new K-Ar ages and to discuss their petrogenesis on the basis of their petrologic and geochemical (major and trace element) features. Nephelinites represent only 1.2% of the total surface of volcanic units; they are scattered all over it as small monogenic volcanoes built during the Middle and Late Miocene (16.25-5.87 Ma) and the Plio-Quaternary (3.92-0.67 Ma). The three other types are exclusively Plio-Quaternary (3.77-0.60 Ma). Basanites cover 22.5% of the volcanic field area, and generally overlie the more widespread alkali basaltic flows (68.5% of the plateau surface). Finally, subalkaline basalts form the El Koudiate cone and associated flows (7.8% of the surface of the volcanic units) and their origin is ascribed to contamination of alkali basaltic magmas by the upper continental crust. We show that nephelinites, basanites and alkali basalts derive from temporally increasing but small degrees of partial melting of variably enriched spinel– and garnet-bearing peridotites from the base of the North-African lithospheric mantle, at depths of ca. 70 km. Partial melting was caused by an asthenospheric uprise, which induced dehydration melting of parga-site-bearing enriched peridotites, metasomatised during an earlier Cenozoic plume-related magmatic event.

Back-arc basalts from the Loncopue graben (Province of Neuquen, Argentina)

Young basaltic back-arc volcanoes occur east of the main Andes chain at about 37.5°–39°S in the Loncopue graben, Province of Neuquen, Argentina. These olivine-rich basalts and trachybasalts have up to 8% MgO, with high Ni and Cr contents, but highly variable incompatible element concentrations. Mafic lava flows and cinder cones at the southern end of the graben lack phenocrystic plagioclase. The northern samples have relative Ta–Nb depletions and K, Pb and LREE enrichment. These samples strongly resemble rocks of the nearby arc volcanoes Copahue and Caviahue, including their Fe–Ti enrichment relative to the main Andes arc rocks. The Sr, Nd and Pb isotope ratios show that the source regions of these back-arc basalts are enriched in subducted components that were depleted in the aqueous mobile elements such as Cs, Sr and Ba as a result of prior extractions from the subducted complex below the main arc. Some mafic flows show slightly low 206Pb/ 204Pb and 143Nd/ 144Nd values as well as incompatible trace element ratios similar to southern Patagonia plateau back-arc basalts, suggesting contributions from an EM1 mantle source. Geothermometry and barometry suggest that the basalts crystallized and fractionated small amounts of olivine and spinel at ∼ 35 km depth at temperatures of 1170–1220 °C, at about QFM + 0.5 to QFM + 1 with 1–2% H 2O, and then rose rapidly to the surface. The Loncopue graben back-arc basalts are transitional in composition between the South Patagonia back-arc plateau basalts and the Caviahue and Copahue arc volcanoes to the northwest. The EM1 source endmember is possibly the subcontinental lithospheric mantle. Strong variations in incompatible element enrichment and isotopic compositions between closely spaced cinder cones and lava flows suggest a heterogeneous mantle source for the Loncopue graben volcanics.

New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California

We undertook a 40 Ar/ 39 Ar study of young mafic and silicic lavas at Mammoth Mountain and the Long Valley caldera (east-central California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29- and 41 ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafic lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence (∼190–160 ka), the Mammoth sequence (∼120–58 ka), the northwest caldera sequence (∼41–29 ka), and the Inyo chain sequence (∼9 ka–present). In each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162 ± 2 ka and 99 ± 1 ka for bracketing mafic lava flows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999).

Geologic Setting of Volcanic-Associated Massive Sulfide Deposits in the Kamiskotia Area, Abitibi Subprovince, Canada

The Upper Archean volcanic succession in the Kamiskotia area (Abitibi greenstone belt, Timmins region) hosts a series of past-producing copper-zinc volcanic-associated massive sulfide (VMS) deposits. All of these occur within a restricted, east-facing stratigraphic interval in the upper part of the Kamiskotia Volcanic Complex. New U-Pb ages for this interval, ranging from 2701.1 ± 1.4 to 2698.6 ± 1.3 Ma, and an age of 2703.1 ± 1.2 Ma from the lower part of the Kamiskotia Volcanic Complex, indicate that the complex is likely part of the Blake River assemblage (2701–2697 Ma) rather than the older Tisdale assemblage (2710–2703 Ma). The Kamiskotia Volcanic Complex consists largely of felsic and mafic lava flows, and VMS mineralization appears to have generally developed at or near the sea floor close to inferred synvolcanic faults. New U-Pb ages of 2714.6 ± 1.2 and 2712.3 ± 2.8 Ma from the northeast-facing volcanic succession in the northern part of the study area (Loveland, Macdiarmid, and Thorburn Townships) indicate that it forms part of the Kidd-Munro assemblage (2719–2710 Ma). A west-northwest–trending faulted contact is inferred between this older succession and the Kamiskotia Volcanic Complex rocks to the south. The Kidd-Munro assemblage rocks are coeval with the Kidd Volcanic Complex, which hosts the giant Kidd Creek VMS deposit 30 km to the east of the study area. The lower part of the succession, in south-central Loveland Township, consists of high silica FIIIb rhyolites. These rocks are geochemically similar to ore-associated FIIIb rocks from Kidd Creek and seem likely to represent the most prospective part of this succession. Future exploration in the Kamiskotia Volcanic Complex is probably best focused on the along-strike extension of the VMS-hosting interval and, in particular, on areas close to the intersections of synvolcanic faults. Mafic and felsic volcaniclastic strata which can be replaced by VMS mineralization, and felsic coherent facies flows and/or domes, appear to be important potential targets.

Correction to “Ground‐penetrating radar sounding in mafic lava flows: Assessing attenuation and scattering losses in Mars‐analog volcanic terrains”

Correction to “Ground‐penetrating radar sounding in mafic lava flows: Assessing attenuation and scattering losses in Mars‐analog volcanic terrains”

Ground‐penetrating radar sounding in mafic lava flows: Assessing attenuation and scattering losses in Mars‐analog volcanic terrains

We conducted low‐frequency (16 to 100 MHz) ground‐penetrating radar surveys on the eroded lava flows at Craters of the Moon (Idaho, USA) volcanic field to evaluate the potential of future radar‐sounding investigations on Mars to map shallow subsurface features. Radar‐sounding profiles were obtained from three locations: above a lava tube, across a volcanic rift, and over a scoria cone. Results were combined with laboratory permittivity and magnetic permeability measurements of field‐collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth. Our results demonstrate a constrained performance for low‐frequency sounding radars to characterize mafic, arid volcanic terrains that contain a significant amount of ferro‐oxides (∼14%), mainly in the form of olivine and magnetite. Penetration depths of 35 m were achieved at a frequency of 100 MHz, and depths of 80 m were achieved at 16 MHz, with an effective dynamic range of 60 dB. Results indicate that for frequencies below 100 MHz, the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses. Over our frequency range, the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses, ground porosities, and degree of heterogeneity rather than the sounding frequency. In light of these results, we suggest average attenuation and scattering losses measured in terms of dB/m and discuss the expected penetration depth for the Mars orbital radar‐sounding instruments SHARAD and MARSIS in mafic volcanic terrains.

EVIDENCE OF EARLY PALAEOZOIC CONTINENTAL RIFTING FROM MAFIC METAVOLCANICS OF SOUTHERN PELORITANI MOUNTAINS (NORTH-EASTERN SICILY, ITALY)

Pre-Hercynian magmatic rocks are widespread in the Palaeozoic basement of the Peloritani Mountain Belt. The metabasites of the Mongiuffi-Melia area (SE Peloritani) represent the largest magmatic products of Early Palaeozoic. These rocks occur as mafic lava flows and metavolcanoclastites often preserving relict igneous textures. They were weakly metamorphosed during the Hercynian orogenesis and show sub-greenschist metamorphic assemblages overprinting the original igneous parageneses. The geochemical features of these rocks indicate an alkali basaltic composition and a within-plate to P-MORB affinity. Basaltic protoliths of metabasites were produced by partial melting of variably enriched mantle sources and experienced fractionation and little crustal contamination during their ascent. Overall data are consistent with a geodynamic environment related to an early stage of tectonically dominated continental rifting. These metabasites represent the only evidences of a Cambro-Ordovician extensional event in the Peloritani domain of the former Alboran microplate. Their features are consistent with a possible location of this terrane at the south-western termination of the South Armorican Ocean during the Early Palaeozoic.

U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation

Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units—the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland)—these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ 13 C excursions, lasting at most ∼50– 80 m.y.

Permian volcanism in the Central Western Carpathians (Slovakia): Basin-and-Range type rifting in the southern Laurussian margin

The 1,500- to 2,000-m-thick Permian volcano-sedimentary Malužina Formation of the uppermost nappe of the Central Western Carpathians (a segment of the Alpine-Carpathian orogenic belt) occurs in several fault blocks distributed across Slovakia. This unit is a part of a post-Variscan overstep suite that followed accretion of the Gothic terranes to Laurussia. It consists of three upward-fining megacycles of semi-arid/arid, fluvial-lacustrine clastic redbeds and local dolomites and evaporites. Abundant intercalated volcanic rocks are predominantly mafic lava flows; volcaniclastic rocks and dykes are subordinate. Felsic rocks are represented by rare volcaniclastics and dykes. Compositionally, the mafic rocks are rift-related continental tholeiites with enriched light REE patterns having (La/Yb)n ratios between 2 and 5.5 and with mantle-normalized patterns characterized by negative Nb-Ta anomalies. The rocks were derived from subcontinental lithospheric mantle and were affected by crustal contamination. It is inferred that the volcanism of the Malužina Formation formed in a Basin and Range tectonic setting in which rifting followed collision of the Palaeo-Tethys ridge with the trench bordering southern Laurussia. This model can be applied to other Permian volcanic suites of rift basins in the Eastern Alps and Carpathians over a strike-length of about 1,000 km, which indicates the width of the slab window.

Geologic Setting and Genesis of the Mule Canyon Low-Sulfidation Epithermal Gold-Silver Deposit, North-Central Nevada

The Mule Canyon mine exploited shallow, low-sulfidation, epithermal Au-Ag deposits that lie near the west side of the Northern Nevada rift in northern Lander County, Nevada. Mule Canyon consists of six small deposits that contained premining reserves of about 8.2 Mt at an average grade of 3.81 g Au/tonne. It is an uncommon mafic end member of low-sulfidation Au-Ag deposits associated with tholeiitic bimodal basalt-rhyolite magmatism. The ore is hosted by a basalt-andesite eruptive center that formed between about 16.4 to 15.8 Ma during early mafic eruptions related to regionally extensive bimodal magmatism. Hydrothermal alteration and Au-Ag ores formed at about 15.6 Ma and were tightly controlled by north-northwest- to north-striking high-angle fault and breccia zones developed during rifting, emplacement of mafic dikes, and eruption of mafic lava flows. Hydrothermal alteration assemblages are zoned outward from fluid conduits in the sequence silica-adularia, adularia-smectite, smectite (intermediate argillic), and smectite-carbonate (propylitic). All alteration types contain abundant pyrite and/or marcasite ± arsenopyrite. Field relations indicate that silica-adularia alteration is superimposed on argillic and propylitic alteration. Little or no steam-heated acid-sulfate alteration is present, probably the result of a near-surface water table during hydrothermal alteration and ore deposition. Two distinct ore types are present at Mule Canyon: early replacement and later open-space filling. Replacement ores consist of disseminated and vesicle-filling pyrite, marcasite, and arsenopyrite in argillically altered or weakly silicified rocks. Ore minerals consist of Au-bearing arsenopyrite and arsenian pyrite overgrowths on earlier-formed pyrite and marcasite. Open-space filling ores include narrow stockwork quartz-adularia veins, banded and crustiform opaline and chalcedonic silica-adularia veins, silica-adularia cemented breccias, and sparse carbonate-pyrite and/or marcasite veins. Ore minerals consist mostly of electrum and Ag sulfide and selenide minerals, with minor to major amounts of pyrite, marcasite, and arsenopyrite, and local stibnite. Both types of ores have similar geochemical signatures, characterized by high Au, Ag, As, Sb, and Se contents, locally high Hg, Mo, Tl, and W contents, and low Cu, Pb, and Zn contents. Stable isotope data indicate that ore fluids consisted dominantly of meteoric water that evolved by deep circulation through Paleozoic sedimentary rocks at low water/rock ratios (about 1) and high temperatures (>200°C). Calculated isotopic compositions of ore fluids are δ 18OH2O= –3 to –7 per mil, δ DH2O= –107 to –124 per mil, δ 13CCO2= 0 to –6 per mil, and δ 34SH2S= –3 to +8 per mil. The ore fluids obtained much of their H2S and CO2 and probably scavenged ore metals and trace elements from the Paleozoic sedimentary rocks. Some H2S and CO2 may have been derived from degassing Miocene magmas. Mule Canyon formed at shallow depths, probably about 100 m below the paleosurface. Ore fluids were dilute, nearly neutral in pH, reduced, H2S-rich, and CO2-bearing. Peak temperatures in ore zones reached 230° to 265°C at nearly lithostatic pressures when some crystalline quartz ± adularia precipitated, but most ore formed at temperatures <200°C at near hydrostatic pressures and was accompanied by precipitation of opaline and chalcedonic silica± adularia ± calcite and dolomite. Deposition of gold in As-rich overgrowths on pyrite and/or marcasite in disseminated ores occurred owing to decreasing H2S in the ore fluids resulting from sulfidation reactions. Later electrum and Ag selenide precipitation in open spaces occurred owing to boiling, loss of H2S to the vapor phase, and cooling. Mule Canyon is similar to most other low-sulfidation Au-Ag deposits associated with Miocene tholeiitic bimodal basalt-rhyolite magmatism in the Great Basin, such as Sleeper, Midas, and Buckhorn. Major differences at Mule Canyon are the high Fe sulfide contents of some ores and altered wall rocks, the paucity of silicification, and the presence of only narrow and discontinuous gold-bearing siliceous veins. These differences primarily are due to the mafic composition of the host rocks. Mule Canyon differs from low-sulfidation deposits associated with subduction-related calc-alkaline porphyry intrusions, such as the Comstock lode, in that a much smaller proportion of water, salts, and metals was derived from crystallizing magmas.

Mesozoic volcanism in the Middle East: geochemical, isotopic and petrogenetic evolution of extension-related alkali basalts from central Lebanon

Mesozoic picritic and alkali basalts from central Lebanon represent a significant part of an extension-related Upper Jurassic to Upper Cretaceous discontinuous volcanic belt which occurs throughout the Middle East. Volcanism was associated with an episode of intraplate extension that followed a period of continental break-up, where Mesozoic micro-continental blocks separated from Gondwana as the Neotethys ocean opened in Jurassic times. This volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m, along with some minor pyroclastic flows. These flows are stratigraphically intercalated with thick carbonate platform deposits. The basalts are made up of about 15–20% olivine (Fo78–91), 30–35% clinopyroxene (salite), 40–50% plagioclase (An56–71) and opaque Fe–Ti oxides (∼5%). Geochemically, the rocks exhibit a relatively wide range of SiO2 (40.4 to 50.5 wt%) and MgO (5.1 to 15.5 wt%) contents, are relatively enriched in TiO2 (1.7 to 3.7 wt%) and vary in composition from alkali picrite and basanite to alkali basalt. The Mg numbers range from 0.56 to 0.70, with an average of 0.63. The rocks are enriched in incompatible trace elements such as Zr (86–247 ppm), Nb (16–66 ppm) and Y (19–30 ppm). Such compositions are typical of those of HIMU-OIB and plume-related magmas. The REE patterns are fractionated ((La/Yb)N = 11), LREE enriched, and are generally parallel to subparallel. Elemental ratios such as K/P (1.1–4.7), La/Ta (11–13), La/Nb (0.57–0.70), Nb/Y (0.68–1.55) and Th/Nb (0.20–0.36) suggest that crustal contamination was minor or absent. This may be related to a rapid ascent of the magma, in agreement with the nature (mafic, oceanic-like) and the small thickness (about 12 km) of the Mesozoic crust of the Eastern Mediterranean region. The 143Nd/144Nd isotopic compositions of the lavas range from 0.512826 to 0.512886, and 87Sr/86Sr from 0.702971 to 0.703669, suggesting a HIMU-like mantle source. Trace element compositions indicate a melt segregation depth of 100–110 km, well within the garnet lherzolite stability field. The geochemical characteristics of the rocks are typical of within-plate alkali basalts, and suggest that the magmas were derived from a fertile, possibly plume-related, enriched mantle source. Petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting (F = 1.5%) of a primitive garnet-bearing mantle source (garnet lherzolite).

Neoproterozoic magmatism in Southwestern Algeria (Sebkha el Melah inlier): a northerly extension of the Trans-Saharan orogen

The Neoproterozoic Sebkha el Melah inlier is a part of the Pan-African Trans-Saharan orogenic belt that is exposed in northwestern Africa east of the West African craton. The inlier is composed of a 4–5 km thick sequence of fine-grained marine to fluvial clastic sedimentary rocks intercalated with, and conformably overlain by, mafic lava flows and proximal volcaniclastic deposits, 600–1000 m thick. The lava flows and associated minor intrusives are mainly shoshonites. Their geochemical characteristics are indicative of subduction-related magmas and are characterized by relative depletion of Nb, Ta and Ti with respect to rare-earth elements and Th. Their positive but highly variable ε Nd values (+1–+5) are interpreted to reflect contamination of mantle-derived mafic melts (∼+6) by continental crust. It is suggested that the Sebkha el Melah shoshonitic rocks formed in a backarc or rifted arc setting. Their location, close to the Trans-Saharan suture, is interpreted to be the result of subduction erosion which removed the forearc and possibly also part of the arc. The shallow source (<60 km) for the Sebkha el Melah volcanic rocks is related to flat-slab subduction. Traced along strike to the south, the >620 Ma, Neoproterozoic volcanic suites of the Trans-Saharan belt change to typical continental, Andean margin calc-alkaline rocks in the northwestern Hoggar and an oceanic island arc complex in Mali. This may indicate that the rate of convergence of the West African craton and the Tuareg (Saharan) paleocontinent increased from south to north due to a change in the angle of convergence across the margin (oblique in the south to orthogonal in the north), resulting from the curve of the eastern margin of the West African craton that swings from N–S to NW–SE.

Magma composition and viscosity as controls on peperite texture: an example from Passamaquoddy Bay, southeastern Canada

Silurian mafic lava flows and rhyolite sills in the Passamaquoddy Bay area, southeastern Canada, interacted with wet sediment to produce peperitic breccia. The pahoehoe-type mafic flows flowed over and bulldozed into wet, unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts. Soft-sediment deformation structures in the siltstone adjacent to the flows, irregularly shaped vesicles in the siltstone, and siltstone-filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement. Sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts. Evidence for the rhyolitic magma interacting with wet sediment includes sediment-filled vesicles in rhyolite, quartz-filled vesicles in siltstone and angular, non-vesicular shards of rhyolite in clastic dykes. Peperite formed by this process is exclusively blocky. The rhyolitic magma in the Passamaquoddy Bay sequence and other examples of high-viscosity magma producing only blocky peperite suggest that in contrast to low-viscosity magmas, which produce a combination of blocky and fluidal peperite, only blocky peperite will form associated with highly viscous rhyolite magma. The sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that, in this case, sediment texture did not control peperite type. The change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and/or confining pressure, and the generation and maintenance of a steam film at the magma/wet sediment interface.