Neogene Lavas Research Articles

Geology of Las Minas: an example of an exhumed geothermal system (Eastern Trans-Mexican Volcanic Belt)

ABSTRACT The Las Minas area corresponds to an exhumed geothermal system considered a proxy for the deep part of the nearby Los Humeros active geothermal system. The stratigraphic succession is made up of: Palaeozoic-Miocene granitoids, a thick Jurassic- Cretaceous carbonate succession, Neogene lava flows and volcano-sedimentary deposits. Linked to a Miocene magma intrusion, marble and skarn rock-volumes developed by contact metamorphism and geothermal fluid flow. Faults are arranged in SW- and NNW-striking systems. These controlled the morphological evolution and favored Neogene-Quaternary dyke emplacement. Faulting gave rise to a tectonic depression where lacustrine sediments and pyroclastics deposited. Skarn rocks are mainly located at fault intersections and along pre-existing discontinuities, suggesting the role of bedding and/or fractures in channeling deep fluids. Results give inputs for exploration at depth of Los Humeros geothermal system.

Extension of the Middle Miocene Kleifakot geomagnetic instability event in Ísafjörður, Northwest Iceland

Detailed records of past geomagnetic polarity transitions and excursions are seldom found in extrusive volcanics. One type of such events seems to involve very irregular variations of the magnetic field direction. Several examples of that type have been discovered in paleomagnetic surveys on the Neogene lava pile of Iceland. Lava sequences spanning the most notable event are accessible in the two tributary fjords Mjóifjörður and Ísafjörður south of Ísafjarðardjúp, Northwest Iceland. Paleofield direction results from about 80 sampling sites in these sequences have been described in previous publications. The present paper adds 20 sampled sites in Ísafjörður, extending the area where parts of this „Kleifakot instability event“ of the geomagnetic field are recorded, to 5–6 kilometers along the fjord. Intermediate paleofield directions in the collection often agree closely with each other in correlated strata, even more than 4 kilometers apart. Events like this can provide valuable correlation tools for future stratigraphic mapping and various studies on volcanological features in the relatively uncharted region around Ísafjarðardjúp and beyond. Globally, recognition of the existence of such events will aid in the interpretation of results in other kinds of paleomagnetic studies. The role of geomagnetic paleo-intensity determinations is discussed briefly in this context.

Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area.Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions.Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

WITHDRAWN: Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

Abstract This paper describes the Neogene lava–sediment mingling from the Abu Treifiya Basin, Cairo–Suez district, Egypt. The lava–sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators. The identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the Abu Treifiya Basin. Characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops. The peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area. Lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive/intrusive basaltic rocks forming peperitic breccias to lava–sediment contacts at a large to small scales, respectively. Peperites encompass five main facies types ascribed to: (i) carbonate sediments-hosted fluidal and blocky peperites, (ii) lava flow-hosted blocky peperites, (iii) volcaniclastics-hosted fluidal and blocky peperites, (iv) sandstone/siltstone rocks-hosted blocky peperites, and (iv) debris-flows-hosted blocky peperites. Soft sediment deformation structures, vesiculated sediments, sediments filled-vesicles, and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments. All the peperites in this study could be described as blocky or fluidal, but mixtures of different clast shapes occur regardless of the host sediment. The presence of fluidal and blocky juvenile clasts elucidates different eruptive styles, reflecting a ductile and brittle fragmentation. The gradual variation from fluidal to blocky peperite texture, producing the vertical grading is affected by influencing factors, e.g., the viscosity, magma temperature, confining pressure, sediment fluidization, and vapor film at the magma–sediment interactions. Peperites in the study area record deposition within a shallow marine and fluvio-lacustrine environment accumulated in a rift-related basin developed during pre- to syn-rift phase, respectively. The facies transitions (peperites) in this area resulted from the explosive and sediment depositional processes, which were mingled separately by volcanism under contrast geological conditions. The development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the Abu Treifiya Basin. Hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows.

Discussion on ‘Displacement of the Great Divide in north Queensland associated with Neogene lava flows’ by P. W Whitehead and P. N. Nelson

One of the longest and most controversial debates in Australian geomorphology has centred around the issue of drainage evolution. Since E. C. Andrews began to extol the virtues of the Davisian geographic cycle in Australia in 1903, decades of research generated conclusions that in the words of Young (1978) were ‘often as much the product of theory as of fact.’ There was a lack of objective testing of hypotheses, which led to the persistence of error. Despite major challenges to the geographic cycle and its value in understanding the evolution of landscapes in eastern Australia throughout the latter part of the 20th century some aspects of its influence continue to the present day. Whitehead & Nelson's (2014) recent interpretation of drainage evolution in the Barron River catchment is a case in point. Their uncritical examination of field evidence has led them to conclude that Pleistocene basalt flows from a volcano 30 km upstream led to the blockage of the Barron River causing it to adopt a new eastwards course towards the Coral Sea. This, according to the authors, also resulted in the westward migration of the continental drainage divide in this region. As will be demonstrated here though, there is abundant field evidence to show this could not have happened and the present course of the Barron River developed well before the Pleistocene.

Reply to discussion by J. Nott (2014) on ‘Displacement of the Great Divide in north Queensland associated with Neogene lava flows’

Click to increase image sizeClick to decrease image sizeKEYWORDS: Barron RiverMitchell Riverbasaltdrainage evolutioneastern highlandscontinental divide

Displacement of the Great Divide in north Queensland associated with Neogene lava flows

In north Queensland, Australia, the ‘Great Divide’ forms the border between catchments draining into the Gulf of Carpentaria, including the Mitchell River, and those draining into the Coral Sea, including the Barron River. Until recently, it was commonly proposed that what is now the upper Barron River previously drained into the Mitchell River. However, little evidence was presented, and the assertion has been disputed. Our examination of borehole data, combined with accurate surveying of bedrock in the present Barron River channel, provides definitive evidence that paleochannels of the Mitchell River previously drained what is now the upper Barron River subcatchment. Lava that flowed down these channels at ca 1.79 Ma is evident in some of the boreholes and is exposed in the Barron River channel. The lava flows blocked the river channel, diverting the headwaters of the paleo-Mitchell River east into the Barron River, resulting in the western migration of the Great Divide. The consequent reduction in stream energy available to the truncated headwaters of Mitchell River has led to channel infill and aggradation of more than 40 m since the diversion of the Barron River. Subsurface paleochannels may be directing groundwater across the present drainage divide from the upper Barron River catchment into the Mitchell River catchment.

In situ U–Pb zircon geochronology of Neogene garnet-bearing lavas from Slovakia (Carpatho-Pannonian region, Central Europe)

Zircons armored in garnet from four Neogene lavas of Slovakia (three andesites and one dacite) were dated in situ, using excimer laser ablation ICP-MS. For the andesites, the 66 analytical points yield a lower intercept age of 13.3±0.1Ma (MSWD=1.2), which is older than the age of 12.4±0.2Ma recorded for the dacite. U–Pb ages obtained in this study are significantly younger than the Lower Badenian ages (K–Ar and fission-track dating coupled with biostratigraphic correlations) classically assumed for the garnet-bearing lavas of the Western Carpathians, with a minimum discrepancy of 3Ma. Accessorily, a composite zircon with an inherited core reveals that some Pan-African crustal component is present at depth.Garnet-bearing andesites and dacites potentially bear critical information about melt generation in the lower crust and possible recycling of pre-existing crust into igneous rocks. Practically, the problem is to assess the nature of the melt from which the garnet actually grew. For this purpose, zircon grains included into garnet may help to clarify the beginning of the magma history. It seems likely that zircon was not grown from the bulk dacitic or andesitic melt but rather from a specific rhyolitic batch. Later mixing and consecutive change in magma chemistry explain that free zircons in the melt were almost totally dissolved, whereas zircons shielded into garnets were preserved. Finally, owing to their Miocene age, the zircon grains armored into garnet preclude any restitic origin for their host and definitely demonstrate the magmatic origin of garnet.

A paleomagnetic and rock-magnetic study of a neogene lava flow sequence in La Gomera (Canary Islands, Spain)

We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (TC = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (TC = 120 to 200°C) and an intermediate Curie-temperature phase (TC = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α95 = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A95 = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter SB = 5.9 with an upper confidence limit Sup = 8.0 and a lower confidence limit Slow = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.

Late Cenozoic volcanic activity in western Georgia: Evidence from new isotope geochronological data

The products of volcanic activity from the Kutaisi area and Guria (western Georgia) were studied in terms of isotope geochronology to determine the age of rocks and to confirm their attribution to Cenozoic formations. The results obtained show that the erupted rocks in the Kutaisi area were formed during the three pulses of Mesozoic volcanic activity: the Bajocian, Kimmeridgian-Tithonian, and Turonian-Santonian. It was shown that no displays of Late Cenozoic volcanism occurred in this region of the western Georgia. Because of this, its inclusion into the Central Georgian neovolcanic province, earlier supposed, seems to be improper. By the data of isotope geochronology, Guria is the only region of western Georgia where volcanic activity occurred in post-Paleogene period. Two pulses of young volcanism were revealed: of about 15.5 and 9-7.5 My. The former was related to the introduction of syenite intrusion, and the latter, to subaqueous exudation of subalka- line Neogene lavas. All the outcrops of Neogene rocks we found and dated in Guria fit within the well-pro- nounced sublatitudinal linear band which probably represents the occurrence in the Middle Miocene of a local zone of extension appearing under conditions of total compression during the collision of the Eurasian and Ara- bian lithospheric plates.

Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: geochronological constraints and new field evidence

AbstractRecent field work and review of radiometric data obtained from Neogene lavas and plutonic rocks exposed in the Eastern Central Patagonian Cordillera (46–48ºS), which overlie subducted segments of the South Chile Ridge, suggest important Late Miocene to Pleistocene morphological changes in relation to base level variations and/or tectonic events. We present new field observations from a region south of Lago General Carrera‐Buenos Aires, between the main Cordillera and the Meseta del Lago Buenos Aires, demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5–4 Ma and after 3 Ma. We also show that the 12 Ma basaltic flows of the Meseta del Lago Buenos Aires (∼2000 m a.s.l.) have been subjected to deep incision, with younger lavas dated at 1.2 Ma partially filling the valleys. These incisions are thought to reflect progressive eastward tilting of the entire meseta. Our new observations, together with additional features from Central Patagonia, strongly suggest that tectonic events led to a regional widespread morphological change after 5 Ma. The coincidence in time and space between the subduction of segments of the South Chile Ridge at 6 and 3 Ma causing opening of a slab window, and strong base level variations in the studied area, suggests a cause‐and‐effect relationship. In Central Patagonia, compressional tectonics ended well before extensional events reported here. Causes of uplift and further extension are probably completely disconnected. The uplift is purely tectonic in origin and occurred prior to the subduction of the South Chile Ridge. Extension should be a consequence of this subduction.

Cenozoic magmatism in the western Ross Embayment: Role of mantle plume versus plate dynamics in the development of the West Antarctic Rift System

Geochemical and chronological data for Cenozoic plutons and dikes from northern Victoria Land (Antarctica), were used to propose a tectonic‐magmatic model for this portion of the West Antarctic Rift System (WARS). The seven major plutons are compositionally bimodal, with gabbroic and syenitic portions. Among the 180 studied dikes, most are 1 m thick and have alkali basalt‐basanite‐tephrite compositions, along with minor intermediate rocks. Trachytic‐rhyolitic dikes (up to 50 m thick) are by far less common. The 40Ar‐39Ar data for dikes indicate middle Eocene to early Oligocene ages, the oldest found to date for igneous activity throughout the WARS. The geochronological‐structural framework provides evidence for coeval emplacement of dikes on two main NW‐SE and N‐S striking trends, whereas plutonic‐subvolcanic activity occurred in adjacent crustal sectors at different times. Mafic dikes display major and trace element distribution typical of basalts from oceanic islands, such as high ratios of Nb and Ta to large ion lithophile and heavy rare earth elements, coupled with prominent negative K and Pb anomalies in the primitive mantle‐normalized multielement diagrams. Initial isotopic compositions are within the ranges of 0.70299–0.70372 for 87Sr/86Sr and 4.2–6.3 for εNd. These features are shared by younger Neogene lavas. Geochemical modeling for both mafic dikes and lavas indicates an enriched magma source characterized by a residual potassic hydrous phase, thus pointing out a mantle source nearly uniform throughout 50 m.y. Comparable geochemical signature are reported for the magma source of the other Cenozoic volcanic provinces of the WARS and the whole Antarctic Plate. The new data for the early igneous rocks of the rift put in evidence a chronologic‐structural link between magmatic evolution, regional tectonics and plate dynamics which suggests a model for WARS magmatism that is alternative to current plume hypotheses. We propose that magma genesis and emplacement are related to reactivation of preexisting translithospheric faults, which promoted local decompression melting of an enriched mantle that was previously veined during the decompression episode associated with the amagmatic late Cretaceous extensional rift phase.

On low-latitude virtual geomagnetic poles in Icelandic basalt lava sequences

In two paleomagnetic studies from the Neogene lava sequences of Iceland published in 1977, of the order of 10% of the reliably determined remanence directions corresponded to virtual pole positions below 40° latitude. This was a larger proportion than found in most other volcanic locations. To explain it, Harrison [Harrison, C.G.A., 1980. Secular variation and excursions of the Earth's magnetic field. J. Geophys. Res. 85, 3511–3522] suggested that some 10% of the observed virtual geomagnetic poles (VGP) from Iceland might be distributed at random over the globe. These “random poles” can be envisaged as dating from periods when the geomagnetic field is weak and dominated by variable non-dipole terms. Much additional data on remanence directions and intensities has been collected since then, and the present study analyses results from over 3500 lava flows in Iceland. The relative frequency and strength of virtual dipoles in this collection vary with the pole latitude in a similar way as in the collection studied by Harrison. Normalizing of remanence intensity data by the use of anhysteretic remanence (ARM) may improve the resolution of this type of analysis.

Cretaceous and Neogene lavas of Sabah - origin and tectonic significance

A combination of field and geochemical investigations of the Cretaceous and Neogene lavas of Sa bah (CNLS) allow some control on the relative timing of events in its tectono-magmatic history. The low-K tholeiitic lava (Telupid basalt of Cretaceous age) or so-called 'boninitic suite', interpreted as a product of partial melting of hydrated oceanic lithosphere in the mantle wedge, suggesting that the first magma formed in response to intra-oceanic subduction. As subduction proceeded, the magma composition changed to the calc-alkaline suite (high-K calc-alkaline lavas of Neogene Tungku and Tanjung Batu andesites), probably because of the hydrated asthenosphere of the mantle wedge confirming an oceanic, supra-subduction zone origin for this volcanic arc assemblage. An evolutionary sequence can be envisaged for the CNLS which begins with the establishment of an oceanic island arc where supra-subduction zone extension led to the genesis of tholeiitic lava and/or boninitic (Telupid basalt) and followed by formation of the volcanic arc (Tungku andesite). Next, followed by Tungku arc-splitting, as extension continued, a marginal basin (Sulu Sea) developed. Later, incomplete closing of the Sulu Sea caused the southwards subduction beneath the older arc and formation of the Tanjung Batu andesite.

New palaeomagnetic results from Icelandic Neogene lavas

SUMMARY In this paper, some statistical properties of new collections of palaeomagnetic direction and intensity data are presented and compared with results from older collections. The new data come mostly from three composite surveys in the basalt lava pile of Iceland, totalling about 1000 lavas of ages 7-14 Myr. The palaeomagnetism of all these lavas converges on a reproducible long-term pattern in several important features of the Neogene field behaviour. Previous conclusions which are confirmed in this paper include: (1) the distribution of intermediate virtual geomagnetic poles (VGPs) is not confined to preferred longitude intervals; (2) normal- and reverse-polarity states of the geomagnetic field are essentially equivalent as regards the intensity of the field, mean field directions, and secular variation; (3) the virtual geomagnetic dipole moment fdls off with decreasing VGP latitude, by a factor of 3.5-4 between polar and equatorial latitudes: (4) a significant reduction in the between-flow angular dispersion of VGPs may have occurrcd during the last 14 Myr; and (5) palaeomagnetic directions from Icelandic basalt lavas are stable and reliable.

The geomorphology of andesite rock forms in the Vtáčnik mountain range, Central Slovakia

The paper treats of the morphogenetic characteristic of andesite rocks in the Vtáčnik mountain range, Central Slovakia. Rock outcrops are the relics of Neogene lava flows and buttes, which achieved their present forms as a result of frost weathering and block slides down the sides.