Basalt Flows Research Articles

Lunar Pit Morphology: Implications for Exploration

AbstractLunar pits are small (∼10–300 m wide) collapse features with vertical walls and sometimes overhangs. We have identified almost 300 pits, mostly in ponds of cooled impact melt inside large craters younger than ∼1 billion years. Several of these pits may provide access to lava tubes or other caves, and those in the maria expose the layering record of the top 20–100 m of basaltic lava flows. We investigated the 21 known pits outside of impact melt ponds to determine possible origins, ages, and present‐day access to the lunar subsurface. We used Lunar Reconnaissance Orbiter Narrow Angle Camera images (<2 m per pixel) to produce detailed 3D reconstructions of six pit interiors. We evaluated the general morphology and geologic context of all twenty‐one. While four pits have contexts suggestive of lava tubes, the majority are ambiguous, although all occur in or near the maria. We also propose that pit formation is an ongoing process, as the degree of degradation of a pit is unrelated to the age of the host terrain. Much of the original volume of most pits is now filled with debris, but some exhibit significant overhangs and may have present‐day cave access. Viewing pit walls and floors, even from the rims, requires navigating steep slopes of loose material. Since the floors are also covered with rough debris piles, we recommend simple flying vehicles for initial reconnaissance.

Low‐temperature thermal history of the McArthur Basin: Influence of the Cambrian Kalkarindji Large Igneous Province on hydrocarbon maturation

AbstractThe McArthur Basin of the North Australian Craton is one of the very few places on Earth where extensive hydrocarbons are preserved that were generated from Mesoproterozoic source rocks, prior to the development of extensive multicellular life. It is, however, unclear precisely when hydrocarbons from these source rocks matured, and if this occurred as a singular event or multiple phases. In this study, we present new apatite fission track data from a combination of outcrop and sub‐surface samples from the McArthur Basin to investigate the post‐depositional thermal history of the basin, and to explore the timing of hydrocarbon maturation. Apatite fission track data and thermal modelling suggest that the McArthur Basin experienced a basin‐wide reheating event contemporaneous with the eruption of the Cambrian Kalkarindji Large Igneous Province in the North and West Australian cratons, during which thick (>500 m) basaltic flows blanketed the basin surface. Reheating at ca. 510 Ma coinciding with Kalkarindji volcanism is consistent with a proposed timing of elevated hydrocarbon maturation, particularly in the Beetaloo Sub‐basin, and provides a mechanism for petroleum generation throughout the basin. Subsequent regional cooling was slow and gradual, most likely facilitated by gentle erosion (ca. 0.01–0.006 km/Ma) of overlying Georgina Basin sediments in the Devonian–Carboniferous with little structural reactivation. This model provides a framework in which hydrocarbons, sourced from Mesoproterozoic carbon‐rich rocks, may have experienced thermal maturation much later in the Cambrian. Preservation of these hydrocarbons was aided by a lack of widespread structural exhumation following this event.

Tournaisian volcanism associated with transtensional basin development in western Newfoundland during the amalgamation of Pangea

During the Carboniferous, the assembly of the supercontinent Pangea periodically reactivated strike-slip faults and shear zones that transect the northern Appalachian Orogen. These strike-slip deformation zones represent crustal-scale zones of structural weakness that have long-lived and complex kinematic histories. Their periodic reactivation controlled the evolution of the transtensional sedimentary basins of the region. The continuous interplay between the lithospheric plates, their subcontinental lithospheric mantle, and surficial processes (i.e. oceanic/riverine, biological, atmospheric, gravitational, erosional) controlled the development of the Deer Lake Basin in western Newfoundland. The Saltwater Cove Formation of the Deer Lake Basin is a late Tournaisian lacustrian-deltaic succession interrupted by minor eruption of basalt flows and pyroclastic deposits. The basalts have E-MORB (enriched-mid ocean ridge basalt) to OIB (ocean island basalt) chemical affinities with LREE enriched profiles, lack negative Nb and Ti anomalies and εNd(t=350 Ma) values between +2.8 to +6.4. Pyroclastic deposits include mafic lapilli tuff and intermediate tuff, and have compositions similar to CAB (continental arc basalt), having steep LREE relative to HREE patterns, negative Nb and Ti anomalies, and εNd(t=350 Ma) ranging from −6.1 to −1.0. Geochemical variation within the mafic rocks is explained by an E-MORB source variably influenced by crustal contamination processes. Lithofacies analysis coupled with geochemical and isotopic data support their eruption in a transtensional, intra-orogenic basin setting that formed during the assembly of Pangea. Their moderately primitive chemistry supports the interpretation that faulting, associated with transtensional basin development, facilitated asthenosphere upwelling via edge-driven convection.

Bimodal Holocene flank eruptions at Lanín composite volcano, Southern Volcanic Zone of the Andes: Characterization of their deposits and controls on the vents array

The Pleistocene to Holocene Lanín composite volcano, in the Southern Volcanic Zone of the Andes, has summit and flank eruptions of Holocene age. We present here a characterization of flank vents and their deposits produced during the most recent eruptions of the Lanín volcano. Volcanic facies analysis indicates four unreported Holocene eruptive units sourced from flank vents. The associated deposits have basaltic, basaltic andesitic and trachytic compositions, and were formed mainly by low-volume effusive and mainly Strombolian-like explosive eruptions. Pyroclastic deposits were identified at the NE, E and SW volcano flanks, and correspond to concentrated and diluted pyroclastic density current deposits, in addition to non-welded to welded fall deposits. Basaltic lava flows of pahoehoe morphology were produced in a purely effusive episode. An analysis of the distribution of Lanín's flank vents (dyke's orientation and vent alignments) along fault-kinematic analysis in basement rocks were carried out with the aim of assessing the basement anisotropies that may have controlled the location and orientation of dykes, eruptive fissures and vent alignments. Contrary to expectations, the Liquiñe-Ofqui Fault System does not constitute a first-order control on the orientation of dykes and vents alignments in Lanín. The vents arrangement may be partly influenced by pre-Andean crustal scale fault zones, such as the Mocha-Villarica Fault Zone, and local basement structures. The predominance of flank vents over summit eruptions strongly suggests that the load of the volcanic edifice is a controlling factor.

Chapter 7 Scottish mineral Geological Conservation Review sites – Metamorphic minerals

Much of Scotland north of the Highland Boundary Fault has been affected by regional and, to a lesser extent, thermal metamorphism and minor metasomatism which, through variable temperatures and pressures, has produced a range of metamorphic minerals. This paper, the seventh of eight on Scottish Mineral Geological Conservation Review Sites, describes ten sites of the highest conservation value within the Lewisian, Moine and Dalradian terranes of the Scottish Highlands, together with thermally altered inclusions in Paleogene volcanic rocks. The minerals featured include the constituents of pelitic and semipelitic schist, such as kyanite and staurolite, unusually large garnet with orthopyroxene and clinopyroxene in ultramafic rocks, and calc silicate minerals, such as tremolite, actinolite, diopside and epidote, together with phlogopitic mica. One site is noted as having stratabound pyrite, chalcopyrite and sphalerite along with large concentrations of chromium mica. The presence of kyanite and tourmaline on the Ross of Mull is one of the few significant mineral sites within rocks of the Neoproterozoic-age Moine Supergroup.Six of the sites encompass extremely rare minerals, generated by the metamorphism of precursors with unusual rock compositions. Foremost amongst these are the barium minerals celsian, cymrite and hyalophane, and the first recorded occurrence in Great Britain of armenite. Although a range of amphibole species occur in both meta-igneous and metasedimentary rocks of the Scottish Highlands, the iron-magnesium clinoamphiboles of the grunerite-cummingtonite series are extremely rare. Indeed the site described in this paper is one of only two occurrences of cummingtonite within the Precambrian-age Dalradian Supergroup. In some cases, the regional mineral assemblages, including the cummingtonite-bearing rocks, have been overprinted by subsequent high temperature/low pressure thermal events associated with the emplacement of igneous intrusions. Pre-eminent amongst the thermal sites must be the occurrence of mullite and sapphire in aluminous shale xenoliths entrained within Paleogene-age basaltic lava flows on the Isle of Mull.

New Insights Into Subsurface Stratigraphy Northwest of Ascraeus Mons, Mars, Using the SHARAD and MARSIS Radar Sounders

AbstractThe Tharsis Montes volcanoes on Mars are the source of laterally extensive lava flows and other volcanic deposits generating a complex stratigraphy throughout the Tharsis Volcanic Province. We use SHAllow RADar (SHARAD) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations in a region northwest of Ascraeus Mons to determine the composition, density, thickness, and spatial distribution of these emplaced volcanic materials. We identified subsurface reflectors along 43 SHARAD and five MARSIS observations. Reflectors in the volcanic plains are interpreted to be sequences of basaltic lava flows with interspersed pyroclastic material, dust, or regolith during a hiatus in activity. Others correspond to the base of three major flow fields. Several plain reflectors were detected by both MARSIS and SHARAD. Other notable reflectors were identified near Ascraeus' flank where lava buried glacially derived sediment. We derived thickness and other material properties for flows using their distinct topographic boundary visible in the radar images. Permittivity ranged from 7.0 to 11.2 corresponding to lava flow densities of 3.20–3.52 g/cm3. Flow thicknesses ranged from 19.8 to 60.2 m. Loss tangents were low for the flow fields ranging from 0.024 to 0.043. Loss tangents in the plains ranged from 0.010 to 0.076. Higher loss tangents correspond to lossier regions that may have higher concentrations of radar absorbing minerals like hematite. Surface roughness controls where reflectors are detected. SHARAD detects the base of three out of the four flow fields in this region with muted surface roughness from dust mantling and erosion.

Thermal regime of the northwestern sector of the Norte Basin (Uruguay): geothermal gradient and heat flow

The infilling of Norte Basin comprises thick Paleozoic and Mesozoic sedimentary units and Mesozoic basaltic flows mainly related to the evolution of Western Gondwana. In its northwestern sector, the Guaraní Aquifer System is located at increasing depths toward the west under thick volcanic rock coverage (up to 1,000 m), constituting a low-temperature geothermal system, which is currently used only for recreational and touristic purposes. To understand the thermal regime of the northwestern sector of the Norte Basin, the surface and subsurface temperatures were estimated from a diverse database that includes all the deep wells located in the study area. Geothermal gradients ranged between 14.4 and 29.6 °C/km, showing the highest values in the western region of the Salto Department. The heat flow maintained a mean value of 60 mW/m2, which is characteristic of resemblant continental tectonic settings. Additionally, the heat loss for every well used in this study was estimated while accounting for an outflow from the bottom hole; it was largely negligible because it remained less than the sensible heat. Therefore, the outflow water temperature can be considered as being similar to the reservoir temperature measured using adiabatic measurement equipment. The western region of the Salto Department, south of the Arapey fault, represents the area with the highest potential for future geothermal projects.

Average VNIR reflectance: A rapid, sample-free method to estimate glass content and crystallinity of fresh basaltic lava

The microcrystalline texture in basaltic lava, scoria, and spatter can vary widely from pure glass to holocrystalline due to complex cooling histories after eruption. How quickly a molten rock cools is a function of the environmental surroundings, including water, ice, sustained heat source, and atmospheric conditions. Thus, petrologic texture serves as an indicator of cooling history captured in the rock record. As basalt is a common component of terrestrial bodies across the solar system, relating the abundance of crystalline components to spectral character would allow for a more thorough understanding of the cooling history and emplacement conditions on planetary surfaces. Visible/near-infrared (VNIR) reflectance spectroscopy has been used to examine the absorptions associated with volcanic glass, however, the non-linearity of absorption features in this spectral region requires complex spectral unmixing modeling to achieve modal percentages of minerals. Here we present evidence that average reflectance from 500 to 1000 nm (referred to as R500–1000) of solid surface samples is indicative of the crystal texture and degree of glassiness of basaltic rocks. Several factors, such as sample surface roughness, lichen cover, coatings, weathering, and chemical composition can affect the R500–1000 of a sample. However, our data indicate that these factors can be sufficiently controlled during sample selection to attribute relative glassiness values to basaltic surfaces. This quick and straightforward method requires no sample preparation or modeling and is demonstrated with training data from sixteen rocks from five basaltic flow fields with differing mineralogy, surface qualities, and geochemistry across Idaho and Oregon, USA. We further test our relationship with two published datasets of synthetic and natural basalts, as well as a subset of our own data collected with our methods to examine the sensitivities of the correlation. This method has the potential to broadly identify glassier basaltic lavas across planetary surfaces. This could be applied toward understanding lava eruption temperatures, cooling rates, magma petrogenesis, paleoclimate reconstruction, and astrobiology due to the involvement of water in quenching of lava.

Ediacaran magmatism and rifting along the northern margin of the Tarim craton: Implications for the late Neoproterozoic Rodinia configuration and breakup

Abstract The Tarim craton in modern Central Asia was an important component of the supercontinent Rodinia in the Neoproterozoic, although its paleogeography in Rodinia during that era is still controversial. Here, we present new stratigraphic, geochemical, and geochronological data from the Neoproterozoic sedimentary and volcanic rock successions along the northwestern margin of the Tarim craton and discuss the significance of these data and our interpretations for its tectonic evolution and paleogeographic position within Rodinia. The Lower Ediacaran sedimentary sequence (Sugetbrak Formation) in northwest Tarim includes terrestrial and shallow-marine clastic rocks intercalated with two discrete basaltic lava flows near the top. The Upper Ediacaran sedimentary sequence conformably overlying the volcanic and clastic rocks consists mainly of stromatolitic dolomite (Chigebrak Formation), representing a transgressive shallow-marine environment. Previous U-Pb zircon dating of the basaltic lava flows has constrained the timing of their eruption in the early Ediacaran (615 Ma). Detrital zircon U-Pb dating of a feldspar-quartz-sandstone unit situated between the two lava flows revealed an oldest age of 2517 ± 18 Ma and a youngest age of 612 ± 6 Ma, with a majority of zircon grains (n = 42) dated at 891–754 Ma (Tonian). A quartz-sandstone unit above the upper lava flow revealed an oldest age of 2724 ± 15 Ma and a youngest age of 607 ± 8 Ma, with a missing age group of 891–800 Ma. These data and observations indicate: (1) a major switch in the depositional setting from a terrestrial (synrifting) to shallow-marine environment following the eruption of the upper lava unit; and (2) an abrupt disappearance of the source rocks of the 891–800 Ma zircons and sediments from the provenance of the post-615 Ma (postrifting) sedimentary sequence. The basaltic rocks have low SiO2 and MgO but high total Fe2O3 and TiO2 contents (2.34–3.19 wt%), analogous to high-Ti basalts and continental flood basalts. Their Ti/V ratios (65–88), low Th/Nb ratios (~0.1), and high TiO2/Yb ratios (~1.1) are similar to those of ocean-island basalt (OIB). Combined with their Sm/Yb and La/Sm ratios and Sr-Nd-Pb-Hf isotope values, we infer that magmas of the Sugetbrak basalts were likely derived from partial melting of an enriched mantle source (EM I) in a transitional spinel-garnet lherzolite field. This petrogenetic evolution was a result of mantle plume–influenced rift magmatism during the Ediacaran breakup of Rodinia. The Central Tianshan terrane, which was attached to the northern Tarim craton until the Ediacaran Period, was the provenance of 891–800 Ma (Tonian) zircons in the synrift sedimentary succession. As the Central Tianshan terrane broke away from the Tarim craton after ca. 615 Ma, Tonianaged zircons were no longer available to the depocenter of the postrift sedimentary sequence. The transition from rifting to drifting between the Tarim craton and the Central Tianshan terrane marked the final breakup of Rodinia, a global event that was possibly driven by mantle plume activities coeval with the development of the Central Iapetus magmatic province in Laurentia during the Ediacaran.

Cenozoic drainage reversal on the southern margin of the Colorado Plateau, east-central Arizona, USA

Laramide northeast-flowing streams from the ancestral Mogollon highland beveled gently northeast-dipping Late Proterozoic to Cretaceous strata across the southern Colorado Plateau Transition Zone. Late Eocene renewed uplift rejuvenated northeast-flowing streams incising paleocanyons. Apache paleocanyon was incised into the Mogollon highland, including the north-trending Laramide Apache uplift bounded by major reverse faults/monoclines. The Mogollon Rim sequence aggraded in Apache paleocanyon and on a broad alluvial plain to the east. Middle Cenozoic tectonic subsidence of the Transition Zone, aridification, and volcanism combined to aggrade Apache paleocanyon with sedimentary and volcanic rocks from 37.6 to 18.63 Ma. Emplacement of the Mogollon-Datil caldera complex and Chuska erg on the southeastern Colorado Plateau forced streamflow to northwest-dispersal of fluvio-eolian sediment from 34 to 26 Ma. Following erosion by northwest-flowing streams on the southern Colorado Plateau from ~26 to 16 Ma, lake sediments of the lower Bidahochi Fm were deposited. Southwestward reactivation of Laramide faults was underway by ~25 Ma coeval with extensive 25 to 20 Ma Natanes Plateau basalt flows and extreme crustal thinning southwest of the Transition Zone. As northeastward streamflow gradually diminished, a Superstition field ash flow tuff ended northeastward flow at 18.63 Ma and was followed by a period of sluggish southwest stream flow and ponding until after ~14.84 Ma. Southwestward structural subsidence and possible spillover from ancestral Lake Hopi on the Colorado Plateau southern margin caused incision of the southwest-directed Dagger Canyon paleovalley after 14.84, which followed the path of the earlier Apache paleocanyon and possibly to the Sespe delta on the California coast before opening of Gulf of California. Structural collapse of the Tonto Basin to the west induced deposition of the Dagger Canyon Conglomerate in the Dagger Canyon paleovalley before the modern Salt River incised all previous deposits and became integrated with the Gila River during the Plio-Pleistocene.

Delineation of Drainage Pattern and Water Projects - A Geographical Overview

In the context of present study the term Delineation means bordering of any kind of area by using scale and reveals in maps. In the context of present study the term Delineation is refer to mapping the drainage and check dams in the study area. The region is drained by the Arunavati River a northern tributary of Tapi, flowing towards the south. River Aner in the east marked an eastern boundary, which flows towards south while in the west Nandurbar district and in the east Jalgaon district. The Northern boundary is marked by Madhya Pradesh. The basaltic lava flows in the hilly tract are not favorable for groundwater development but give rise to perennial and seasonal springs. The contact of the basaltic lava flows with alluvial mountain front deposit is demarcated by a major fault. The alluvial deposits are further subdivided into (a) Talus and Scree deposits bordering in the Satpura foothills and (b) Alluvium consisting of sand clays gravel. Secondly, Evapotranspiration is an important parameter to decide the quality of water is available for plant growth. Evapotranspiration in this tehsil is higher during the plant growth period, thus reducing the availability of water for plant growth. Drought Prone Taluka the semi-arid tracts of the State which receives rainfall between 750 to 1000 mm are drought-prone areas. The Shirpur taluka comes in the drought-prone area of the State. The annual normal rainfall is 617 mm received over period of 36 days during south west monsoon. A surface water resource in the Taluka is unevenly distributed. This resource is also not assured as it depends on rainfall. Irrigation projects in the Taluka cover the only small area in southeastern part of the Taluka. Because of this, the use of groundwater for irrigation, drinking water, and an industrial purpose has increased many folds. In entire taluka intensive development of groundwater has led to critical situations resulting in manifestations of problems like declining ground water levels, shortage of water supply, etc. This situation warrants for taking up programmers to augment groundwater resources on strong lines in the whole taluka. In this paper, groundwater resources in different formations of the entire taluka, industrialization, problems caused due to excessive utilization of groundwater, method store adopted in conserving the scarce resources and recharging the aquifers have been discussed.

The Secondary Minerals from the Pillow Basalt of Salsette-Mumbai, Deccan Volcanic Province, India

Secondary minerals occur within the tholeiitic basalts of Salsette Island in the greater Mumbai region, as well as in other localities in the Deccan Volcanic Province (DVP). However, the secondary minerals of Salsette Island show remarkable differences with respect to their mineral speciation and precipitation sequence, which are both due to their unique geological environment. The greater Mumbai region is built up by the Salsette subgroup, which represents the youngest sequence of the DVP. It formed subsequently to the main phase of DVP activity in Danian time (62.5 to 61.5 Ma), in the course of the India–Laxmi Ridge–Seychelles breakup. The main part of the Salsette subgroup consists of tholeiitic basaltic flows with pillows, pillow breccia, and hyaloclastite, which formed in contact with brackish and fresh water in a lagoonal environment. In some places, intertrappeans are represented by fossiliferous shallow water sediments. On the top, trachytic and rhyolitic subaqueous volcaniclastics occur, and some dioritic bodies have intruded nearby. Due to differing fluid rock interactions, several distinctly different secondary minerals developed in the void spaces of the hyaloclastite breccia of the interpillow matrix and in the pillow cavities. The highly permeable hyaloclastite breccia formed an open system, where pronounced precipitation occurred in the early phase and at higher temperatures. In contrast, the pillow cavities were a temporally closed system and contained, for example, more low-temperature zeolites. The genesis of the secondary minerals can be summarized as follows: During initial cooling of the volcanic rocks at about 62 Ma, the first mineralization sequence developed with chlorite, laumontite I, quartz, and calcite I. Ongoing magmatic activity caused reheating and the main phase of precipitation at prehnite–pumpellyite facies conditions. During generally decreasing temperatures, in the range of 270–180 °C, babingtonite, laumontite II, prehnite, julgoldite, yugawaralite, calcite II, ilvaite, pumpellyite, and gryolite developed. The fluid contained SiO2 + Al2O3 + FeO + MgO + CaO, and minor MnO and Na2O, and was predominately mineralized by the decomposition of basaltic glass. Further temperature decreases caused zeolite facies conditions and precipitation of okenite I, scolecite, heulandite, stilbite, and finally chabazite I, in the temperature range of 180 °C to less than 100 °C. As FeO, MgO, and MnO were then absent, an interaction of the fluid with plagioclase is indicated. According to Rb-Sr and K-Ar ages on apophyllite-K, a third phase of precipitation with apophyllite-K, okenite II, and chabazite II occurred in the late Eocene to early Oligocene (30–40 Ma). The new hydrothermal fluid additionally contained K2O, and temperatures of 50–100 °C can be expected.

Mineralogy and bulk geochemistry of a fumarole at Hverir, Iceland: Analog for acid-sulfate leaching on Mars

AbstractIceland’s Námafjall geothermal area exhibits a range of alteration environments. Geochemical and mineralogical analyses of fumaroles and hot springs interacting with Holocene basaltic lavas at Hverir, and with Pleistocene hyaloclastites atop nearby Námaskar∂, reveal different patterns of alteration depending on the water/rock ratio, degree of oxidation, and substrate composition and age. The focus of this study is a transect of a Hverir fumarole that has formed a bullseye pattern of alteration of a Holocene basaltic lava flow. Surface samples and samples collected from shallow pits were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) to constrain changes in mineral assemblage and major elemental composition with both distance and depth. Elemental sulfur is concentrated near the vent, with leached deposits with amorphous silica and anatase nearby and kaolinite, hematite, and jarosite/alunite-group sulfate minerals farther out, with smectites and less altered material at the margins, though smaller-scale mineralogical diversity complicates this pattern.Silica phases include amorphous silica (most samples), cristobalite (some samples in the leached part of the apron), and quartz (minor constituent of a few samples). The silica was concentrated through residual enrichment caused by leaching and is accompanied by a significant enrichment in TiO2 (in anatase). The presence of abundant cristobalite in a surface fumarole-altered Holocene basaltic lava flow most likely reflects cristobalite formed during the devitrification of volcanic glass or precipitation from fumarolic vapors, rather than high-temperature processes. Minor, localized quartz likely reflects diagenetic maturation of earlier-formed amorphous silica, under surface hydrothermal conditions. Natroalunite, natrojarosite, and jarosite are all present and even exhibit compositional zonation within individual crystals, showing that under surface hydrothermal conditions, these minerals can form a significant solid solution.The high iron content of the substrate basalt and the prevalence of Fe-sulfates and Fe-oxide spherules among the alteration products makes this geothermal area an especially useful analog for potential martian hydrothermal environments. The residual enrichment of silica in the leached deposits of the Hverir fumarole apron could serve as an acid-sulfate leaching model in which amorphous silica forms without appreciable sulfur-bearing phases in many samples, a possible analog for silica-rich soils in the Columbia Hills on Mars. The coexistence of hematite spherules and jarosite-group minerals serves as an intriguing analog for a volcanic/hydrothermal model for hematite and jarosite occurrences at Meridiani Planum.

New mammals from the Naskal intertrappean site and the age of India’s earliest eutherians

The first Cretaceous mammals described from India were recovered from the Naskal locality, on the southeastern edge of the Deccan Traps Volcanic Province (DTVP), where it is preserved between two basalt flows. Because the DTVP eruptions spanned the Cretaceous-Paleogene boundary (KPB), it is often unknown whether trap-associated fossil sites are latest Cretaceous (Maastrichtian) or early Paleocene in age. The Naskal locality accounts for nearly half of published mammal records from DTVP-associated sediments as well as a host of other vertebrate microfossils. Its age takes on singular importance in the context of mammalian evolution in India and the effects of the end-Cretaceous mass extinction and subsequent evolutionary radiation of placentals. Here we describe two new mammal species, Indoclemensia naskalensis gen. et sp. nov. and I. magnus sp. nov., from Naskal and present evidence from 40 Ar/ 39 Ar geochronology, magnetostratigraphy, and chemostratigraphy of the over- and underlying basalt flows to refine the age of the Naskal locality and nearby Rangapur locality. In conjunction with palynostratigraphy and vertebrate biostratigraphy, these sites can be confidently restricted to a <100 kyr interval spanning the KPB. The most probable 40 Ar/ 39 Ar age is latest Cretaceous (66.136–66.056 Ma), but an earliest Paleogene age cannot be ruled out. We explore the implications of this age assignment for Deccan chemostratigraphy and Deccan volcanism, Cretaceous-Paleogene (K/Pg) mass extinction, Indian mammalian faunal evolution, and the timing of the origin of placental mammals. • New 40 Ar/ 39 Ar geochronology, magnetostratigraphy, chemostratigraphy, and palynofloras from the Naskal and Rangapur sites. • Age of Naskal and Rangapur are likely Maastrichtian, constrained to within 100 kyr of the Cretaceous-Paleogene boundary. • Two new eutherian mammal species from the Naskal site of India are described. • These sites are ‘transitional’ distinctive in biotic composition from older and younger infra - and intertrappean localities.

Flow Stratigraphy from Boreholes in the Lower Cretaceous Rajmahal Trap, Bengal Basin: Megacrystic Basalt Flows and Its Use as Possible Marker Horizon

Abstract Flow stratigraphy has been carried out in the Rajmahal Traps from exploratory boreholes drilled for coal in the western sector of Bengal Basin. Different volcanic flows have been demarcated according to their broad megascopic characters. These flows are primarily basaltic in nature. It mainly contains plagioclase, augite along with magnetite and Ti-magnetite and shows porphyritic texture. Inter-trappean sediments are largely argillaceous in nature and restricted towards the lower part of this volcanic sequence. However, a prominent megacrystic plagioclase bearing basaltic flow zone occurs persistently in the middle part. It demarcates two separate modes of eruption in the entire volcanic sequence. Episodic volcanism during initial phase results intermittent deposition of inter-trappean sediments below this zone whereas eruption becomes fairly continuous during later stage which inhibits sedimentation above the megacrystic flow. As this megacrystic zone is very prominent and well persistent, it can be treated as marker horizon.

Relationships between Payenia volcanism and deep-seated structural controls: Insights from geophysical methods at Nevado volcanic field, Mendoza, Argentina

The Nevado Volcanic Field (NVF), in southern Mendoza foothills, is part of the Payenia Volcanic Province, a large Quaternary retro-arc volcanic area composed principally of basaltic lava flows and monogenetic volcanoes. Monogenetic volcanism location, distribution and evolution are extremely sensitive to previous rheological discontinuities or faults affecting their basement. With the aim of exploring the subsurface configuration and contributing to a better understanding of this volcanism, we analysed and interpreted aeromagnetic and gravity datasets in addition to structural and volcanic alignments information. Our results show that location of volcanism would have been mainly controlled by NW-SE trending structures. These structures would extend at least to middle crust levels. On the other hand, a secondary control might be related to crustal domains with different densities, which could have affected magma propagation to the surface, hence, controlling the spatio-temporal distribution of volcanoes. The complex interplay between these factors would be responsible for the distribution of volcanic centres. Finally, we discuss the implications of these controls concerning the tectonic stress regime contemporaneous with volcanic activity.

Volcanism of the Late Silurian Eastport Formation of the Coastal Volcanic Belt, Passamaquoddy Bay, New Brunswick

This field trip is an excursion through the exquisite, nearly pristine exposures of a Silurian, felsic-dominated bimodal volcanic and sedimentary sequence exposed in the Passamaquoddy Bay area of southwestern, New Brunswick (Eastport Formation). These rocks form the northwest extension of the Coastal Volcanic Belt that extends from southwestern New Brunswick to the southern coast of Maine. The sequence is significant because it is part of a large bimodal igneous province with evidence for supervolcano-scale eruptions that began to form during the close of the Salinic Orogeny (about 424 Ma), and continued into the Acadian Orogeny (421–400 Ma). The geochemical characteristic of the rocks can be explained by extension related volcanism but the specific drivers of the extension are uncertain. The Passamaquoddy Bay sequence is 4 km thick and comprises four cycles of basaltic-rhyolitic volcanism. Basaltic volcanism typically precedes rhyolitic volcanism in Cycles 1–3. Cycle 4 represents the waning stages of volcanism and is dominated by peritidal sediments and basaltic volcanics. A spectrum of eruptive and emplacement mechanisms is represented ranging from the Hawaiian and Strombolian-type volcanism of the basaltic flows and pyroclastic scoria deposits, to highly explosive sub-Plinian to Plinian rhyolitic pyroclastic eruptions forming pyroclastic density currents (PDC) and high grade rheomorphic ignimbrites. During this field trip we will examine key exposures illustrating this spectrum of eruptive and emplacement processes, and their diagnostic characteristics, along with evidence for the interaction between mafic and felsic magmas and a variety of peperitic breccias formed as a result of emplacement of flows on wet peritidal sediments. The constraints the depositional setting and voluminous bimodal volcanism places on tectonic models will also be considered.

Central vs. lateral growth and evolution of the < 100 ka Peinado composite volcano, southern Central Volcanic Zone of the Andes

Composite volcanoes often display shifts in activity between central and lateral vents that are related to the local stress field and the underlying magmatic plumbing system. The Quaternary Peinado composite volcano in the southern Central Volcanic Zone (CVZ) of the Andes exhibits a striking symmetrical configuration of lateral vents around a main central cone and vent. In this contribution, we carry out detailed mapping, morphometric analysis and unspiked K-Ar dating, together with petrographic and whole-rock major oxides, trace element and Sr-Nd-Pb isotopic analyses, to constrain the growth history of the volcano and the interplay between central and lateral activity. Ten new unspiked K-Ar ages ranging between 36.8 ± 3.8 and 72.6 ± 8.0 ka highlight that Peinado is a young potentially active volcano. A main central cone-building stage ≥70–60 ka of mostly andesitic composition formed the bulk of the edifice. During this stage, vents around the cone base erupted basaltic andesite lava flows that are among the most mafic compositions of CVZ composite volcanoes, possibly related to the location of Peinado on an alignment of mafic monogenetic centers. At ~50–40 ka, the locus of activity shifted to mid-flank vents emplaced around the central vent, with eruption of andesite > basaltic andesite lava flows of similar composition to the central cone. Minor coeval-to-younger activity from the central vent was progressively more felsic, with eruption of dacitic lavas and block and ash flows, and possibly minor fallout. The shift of the bulk of activity from the central to the lateral mid-flank vents was likely related to the effects of the edifice load and magma compositions, with more mafic and dense magmas erupting preferentially through the flank vents and more differentiated magmas from the central vent. Although activity has been dominantly mafic and effusive, an evolution towards more felsic products from the central vent suggests that future activity may be somewhat explosive. The particular configuration of the lateral vents and structures around the central cone, together with other features, point towards a possible future collapse of the edifice, either a sector collapse or a caldera collapse.

The nature of the sub-continental lithospheric mantle beneath Thailand: evidence from xenocrysts in Cenozoic basalts

ABSTRACT Shortly after the cessation of the Cenozoic seafloor spreading in the South China Sea, a period of basaltic magmatism occurred in the Indochina Peninsula. The various types of xenoliths/xenocrysts present in these igneous rocks can provide information about the lithospheric mantle under the Indochina Peninsula. In this study, we present new ages and in situ major and trace element compositions for the xenoliths and olivine xenocrysts from the Wichian Buri and Mae Tha basalt flows in Thailand. The new ages obtained in this study for the Wichian Buri and Mae Tha basalt flows are 9.24 and 2.35 Ma, respectively. Under Thailand, the lithology of the sub-continental mantle lithosphere (SCLM) could be spinel-lherzolite, with a mantle temperature range of 936°C–1096°C. The mantle source of the Cenozoic basalts from Thailand and its adjacent regions could be affect by recycled oceanic crust and carbonates. The origin of spongy textures in clinopyroxene should be resulted from partial melting induced by decompression. SCLM under Thailand were transformed by peridotite-melt interaction, and basaltic melt trapped the mantle xenocryst and xenolith quickly ascended to the surface.

Activation of Ad Damm shear zone, western Saudi Arabian margin, and its relation to the Red Sea rift system

Abstract The Ad Damm shear zone (ADSZ) is a major mylonitic right-lateral structure that bounds the Jeddah terrane to the north from the Asir terrane to the south. High-resolution field mapping coupled with petrological and geochemical analyses indicate that Jeddah terrane is characterized by heterogeneous magmatism with extensive meta-basalt intruded by silicic plutons of varying size. South of the ADSZ, Asir terrane is characterized by larger scale granitic batholiths. A younger generation of Eocene–Miocene basaltic dikes cut the mylonitic shear zone at a high angle. Petrographic analyses of the ADSZ mylonitic rocks indicate dynamic recrystallization and grain-size reduction, suggesting high-temperature recrystallization. Field observations also found a lack of low-temperature fault zone rocks (e.g. gouge) except for isolated brittle slickensides. Spider diagrams of Jeddah, Asir terranes, and ADSZ rocks are characterized by an arc-related signature, which related to the amalgamation of Jeddah and Asir terranes and defined ADSZ as Neoproterozoic structure. In contrast, Eocene–Miocene basaltic dikes and southern basaltic flow are represented by a rift-related signature, which associated with the development of the Red Sea rift system. Offshore, south of the ADSZ, the Red Sea rift exhibits well-developed linear magnetic anomalies and large topographic escarpment perpendicular to the rift margin, but they are not present north of it. In addition, recent seismicity recorded along the ADSZ and differences in the crustal thickness and characteristics of Asir and Jeddah terranes, collectively, make ADSZ acted as an active crustal boundary and still influence the ongoing tectonic evolution of the Red Sea rift.