Lava Stratigraphy Research Articles

Basaltic Lava Characteristic in Goa Pandan Area, Sukadana, East Lampung: Inferences from Stratigraphy and Petrography Analysis

The presence of extensive basalt formations in East Lampung has its own attractions in the field of geology, particularly the presence of Goa Pandan as a lava cave and tourist attraction in East Lampung. The presence of basalt lava in the southern part of the South Sumatra Basin has drawn attention to the presence of this lava, but detailed research on the characteristics of the lava and its formation process is still very rare. This study aims to determine the eruption period of the basalt lava and magma evolution process based on the correlation between lava stratigraphy and petrography analysis. Field observations show a lava sequence that forms Goa Pandan. Each lava sequence is characterized by autobreccia and vesicular structures on the surface. In addition, the presence of columnar joints, sheeting joints, massive lava, and other additional structures indicate the characteristics of low-viscosity basalt lava. The presence of mineral structures and abundance under the microscope clearly shows the magma formation process when basalt lava flowed on the surface. Resorption-overgrowth of plagioclase and pyroxene minerals indicates an open system when basalt lava flowed on the surface. In addition, the presence of zoning and patching in plagioclase minerals indicates that magma variability is influenced by temperature. The documentation of this lava stratigraphy can serve as a basis for further understanding of magma characteristics and formation processes. There is still much geological work that can be done in the research area to get a detailed picture of the evolution process of magma and the presence of basalt lava in this area.

Forearc lava stratigraphy of the Beila Ophiolite, north-central Tibetan Plateau: Magmatic response to initiation of subduction of the Bangong-Nujiang Meso-Tethys Ocean

It has been suggested that boninites and associated volcanic rocks found in supra-subduction zone (SSZ) ophiolites form in a forearc setting, meaning that these rocks could provide crucial constraints on understanding the magmatic response to subduction initiation. Here, we report newly documented forearc basalts (FAB) and boninites from the Beila Ophiolite of the Bangong–Nujiang Suture Zone, north-central Tibetan Plateau. The lava stratigraphy, from bottom to up, includes FAB, basaltic boninites, and boninites, similar to lavas from the Izu–Bonin–Mariana (IBM) forearc. The FAB has geochemical characteristics similar to those of IBM forearc basalts, with nearly flat REE patterns, and shows enrichment in Rb, Ba, Pb, and Sr and depletion in Nb, Ta, and Zr, suggesting an origin by decompression melting of a depleted mantle source with minor input of subducting plate materials. The basaltic boninites and boninites show similar petrological features, as well as porphyritic textures with phenocrysts of olivine, clinopyroxene, and minor orthopyroxene. The basaltic boninites and boninites display uniform major- and trace-element characteristics that are similar to those of typical boninites, with high MgO (>10.78 wt%), Cr, and Ni contents, extremely low TiO2 (<0.21 wt%), rare earth element (REE), and high-field-strength element (HFSE) contents, and U-shaped REE patterns. In addition, the basaltic boninites and boninites have lower ɛNd(t) values than those of the FAB and display variable degrees of influence by slab melts and fluids. Geochemical modeling further suggests that the boninites were generated later during residual highly depleted mantle fluxing by the subducting plate. Together with contemporary boninitic and arc-related magmas along the Bangong-Nujiang Suture Zone and adjacent terranes (the North Lhasa and South Qiangtang), it suggests that the whole Meso-Tethys Ocean was subducted since the Middle Jurassic. Moreover, the Beila lava stratigraphy with FAB overlain by boninite is similar to that found in the IBM forearc and in many ophiolites (e.g., Troodos, Oman, and Xigaze), and represents a continuous geochemical transition from MORB-like to boninitic affinities. This similarity suggests that initiation of subduction might be commonly involved in the formation of ophiolites.

Exploring the links between volcano flank collapse and the magmatic evolution of an ocean island volcano: Fogo, Cape Verde

Mass-wasting of ocean island volcanoes is a well-documented phenomenon. Massive flank collapses may imply tens to hundreds of km3 and generate mega-tsunamis. However, the causal links between this large-scale, low-frequency instability, and the time–space evolution of magma storage, crystal fractionation/accumulation, lithospheric assimilation, and partial melting remains unclear. This paper aims at tracking time variations and links between lithospheric, crustal and surface processes before and after a major flank collapse (Monte Amarelo collapse ca. 70 ka) of Fogo volcano, Cape Verde Islands, by analysing the chemical composition (major, trace elements, and Sr–Nd–Pb isotopes) and age-controlled stratigraphy (K–Ar and Ar–Ar dating) of lavas along vertical sections (Bordeira caldera walls). The high-resolution sampling allows detecting original variations of composition at different time-scales: (1) a 60 kyrs-long period of increase of magma differentiation before the collapse; (2) a 10 kyrs-long episode of reorganization of magma storage and evacuation of residual magmas (enriched in incompatible elements) after the collapse; and (3) a delayed impact at the lithospheric scale ~ 50 kyrs after the collapse (increasing EM1-like materiel assimilation).

The main pulse of the Siberian Traps expanded in size and composition

Emplacement of large volumes of (sub)volcanic rocks during the main pulse of the Siberian Traps occurred within <1 m.y., coinciding with the end-Permian mass extinction. Volcanics from outside the main Siberian Traps, e.g. Taimyr and West Siberia, have since long been correlated, but existing geochronological data cannot resolve at a precision better than ~5 m.y. whether (sub)volcanic activity in these areas actually occurred during the main pulse or later. We report the first high precision U-Pb zircon geochronology from two alkaline ultramafic-felsic layered intrusive complexes from Taimyr, showing synchronicity between these and the main Siberian Traps (sub)volcanic pulse, and the presence of a second Dinerian-Smithian pulse. This is the first documentation of felsic intrusive magmatism occurring during the main pulse, testifying to the Siberian Trap’s compositional diversity. Furthermore, the intrusions cut basal basalts of the Taimyr lava stratigraphy hence providing a minimum age of these basalts of 251.64 ± 0.11 Ma. Synchronicity of (sub)volcanic activity between Taimyr and the Siberian Traps imply that the total area of the Siberian Traps main pulse should include a ~300 000 km2 area north of Norilsk. The vast aerial extent of the (sub)volcanic activity during the Siberian Traps main pulse may explain the severe environmental consequences.

Geomagnetic Secular Variations at the Permian‐Triassic Boundary and Pulsed Magmatism During Eruption of the Siberian Traps

AbstractThe tempo of Large Igneous Province emplacement is crucial to determining the environmental consequences of magmatism on the Earth. Based on detailed flow‐by‐flow paleomagnetic data from the most representative Permian‐Triassic Siberian Traps lava stratigraphy of the northern Siberian platform, we present new constraints on the rate and duration of the volcanic activity in the Norilsk and Maymecha‐Kotuy regions. Our data indicate that volcanic activity there occurred during a limited number of short volcanic pulses, each consisting of multiple individual eruptions, and that the total duration of discrete eruption pulses did not exceed ~10,000 years (hiatuses are not included). Our study confirms the occurrence of a thick interval in the lower part of the Norilsk lava sections, which contains a record of geomagnetic reversal and excursion. Based on combined evidence from paleomagnetic secular variation and typical timescales for such reversals, we conclude that the ~1‐km‐thick lava stratigraphy, corresponding to ~20,000 km3 of basalt, of the Kharaelakh, Norilsk, and Imangda troughs was formed during a brief, but voluminous, eruptive period of several thousand years or less. Our data further suggest that the ore‐bearing Norilsk‐type intrusions are coeval or nearly coeval with the boundary between the Morongovsky and Mokulaevsky formations. We calculated a new Siberian Permian‐Triassic paleomagnetic pole Norilsk‐Maymecha‐Kotuy (NMK): PLat = 52.9°, PLong = 147.1°, A95 = 4.3°, K = 23.2, and N = 49 lava flows. It is shown that geomagnetic field variations circa 252 Ma were similar to those observed in the latest Cenozoic.

Geochemistry and 40Ar/39Ar geochronology of the Nandurbar‐Dhule mafic dyke swarm: Dyke‐sill‐flow correlations and stratigraphic development across the Deccan flood basalt province

The ENE‐WSW‐trending Nandurbar‐Dhule swarm is the best developed tholeiitic dyke swarm in the Deccan Traps. We obtained 40Ar/39Ar ages of 67.06 ± 0.60, 67.49 ± 0.89, and 63.43 ± 0.48 Ma (2σ internal errors) on three of its largest dykes (36–79 km long), indicating that swarm emplacement spanned ≥2.5 million years under regional crustal extension. Our Sr‐Nd‐Pb isotopic data, combined with previously available geochemical data, identify multiple magma injections in some dykes and also identify probable feeder dykes of some lavas in the lower Western Ghats sequence and in Saurashtra, each ~200 km away. Several dykes are compositionally distinct from hitherto analysed lavas; &gt;50% of the analysed Nandurbar‐Dhule dykes are isotopically like the Mahabaleshwar and Panhala formations of the upper Western Ghats sequence, covering a very narrow isotopic range, but have the distinctive chemical signatures of the high‐TiO2 Kolhapur Unit of the southernmost Western Ghats. These dykes thus possess a unique combination of isotopic and chemical characteristics not hitherto known in Deccan tholeiites, cross‐combining features of different eruptive units in the Wai Subgroup of the Western Ghats. This new, “Nandurbar‐type” chemical‐isotopic flavour is however frequently observed in dykes, sills, and lavas in the Pachmarhi, Shahdol, and Mandla areas 450–600 km to the east, and in Deccan‐age dykes cutting through the Early Cretaceous Rajmahal Traps of eastern India. Varied geochemical evidence indicates that the northern and northeastern Deccan lava stratigraphy (such as the Pavagadh section and the Pachmarhi‐Shahdol‐Jabalpur‐Mandla areas) is largely independent of the Western Ghats lava stratigraphy.

The Pushtashan juvenile suprasubduction zone assemblage of Kurdistan (northeastern Iraq): A Cretaceous (Cenomanian) Neo-Tethys missing link

The Pushtashan suprasubduction zone assemblage of volcanic rocks, gabbros, norites and peridotites occurs in the Zagros suture zone, Kurdistan region, northeastern Iraq. Volcanic rocks are dominant in the assemblage and consist mainly of basalt and basaltic andesite flows with interlayered red shale and limestone horizons. Earlier lavas tend to be MORB-like, whereas later lavas display island arc tholeiite to boninitic geochemical characteristics. Tholeiitic gabbros intrude the norites and display fractionation trends typical of crystallisation under low-pressure conditions, whereas the norites display calc-alkaline traits, suggesting their source included mantle metasomatised by fluids released from subducted oceanic crust. Enrichment of Rb, Ba, Sr, Th and the presence of negative Nb anomalies indicate generation in a suprasubduction zone setting. Trondhjemite and granodiorite intrusions are present in the volcanic rocks, gabbros and norites. SHRIMP U-Pb dating of magmatic zircons from a granodiorite yields a mean 206Pb/238U age of 96.0 ± 2.0 Ma (Cenomanian). The initial ɛHf value for the zircons show a narrow range from +12.8 to +15.6, with a weighted mean of +13.90 ± 0.96. This initial value is within error of model depleted mantle at 96 Ma or slightly below that, in the field of arc rocks with minimal contamination by older continental crust. The compositional bimodality of the Pushtashan suprasubduction sequence suggests seafloor spreading during the initiation of subduction, with a lava stratigraphy from early-erupted MORB transitioning into calc-alkaline lavas and finally by 96 Ma intrusion of granodioritic and trondhjemitic bodies with juvenile crustal isotopic signatures. The results confirm another Cretaceous arc remnant preserved as an allochthon within the Iraqi segment of the Cenozoic Zagros suture zone. Implications for the closure of Neo-Tethys are discussed.

Stratigraphy of amethyst geode-bearing lavas and fault-block structures of the Entre Rios mining district, Paraná volcanic province, southern Brazil

The Entre Rios mining district produces a large volume of amethyst geodes in underground mines and is part of the world class deposits in the Paraná volcanic province of South America. Two producing basalt flows are numbered 4 and 5 in the lava stratigraphy. A total of seven basalt flows and one rhyodacite flow are present in the district. At the base of the stratigraphy, beginning at the Chapecó river bed, two basalt flows are Esmeralda, low-Ti type. The third flow in the sequence is a rhyodacite, Chapecó type, Guarapuava subtype. Above the rhyodacite flow, four basalt flows are Pitanga, high-Ti type including the two mineralized flows; only the topmost basalt in the stratigraphy is a Paranapanema, intermediate-Ti type. Each individual flow is uniquely identified from its geochemical and gamma-spectrometric properties. The study of several sections in the district allowed for the identification of a fault-block structure. Blocks are elongated NW and the block on the west side of the fault was downthrown. This important structural characterization of the mining district will have significant consequences in the search for new amethyst geode deposits and in the understanding of the evolution of the Paraná volcanic province.

Geochemistry of the Palitana flood basalt sequence and the Eastern Saurashtra dykes, Deccan Traps: clues to petrogenesis, dyke–flow relationships, and regional lava stratigraphy

Recent studies of large mafic dyke swarms in the Deccan Traps flood basalt province, India, indicate that some of the correlative lava flows reached several hundred kilometers in length. Here we present field, petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data on the Palitana lava sequence and nearby dykes in the Saurashtra region of the northwestern Deccan Traps. These rocks are moderately evolved, many with low-Ti-Nb characteristics. We infer that most dykes are notably (and systematically) less contaminated by ancient continental crust than the Palitana flows, but four dykes are equally or significantly more contaminated, with some of the most extreme Sr-Nd isotopic compositions seen in the entire Deccan Traps (initial eNd is as low as −18.0). A Bhimashankar-type and a Poladpur-type dyke are present several hundred kilometers from the type section of these magma types in the Western Ghats escarpment. We find no geochemical correlations between the Palitana sequence and three subsurface sequences in NE Saurashtra containing abundant picritic rocks, surface lavas previously studied from Saurashtra, or the Western Ghats sequence. Intriguingly, the Eastern Saurashtra dykes cannot have been feeders to any of these lava sequences. Feeder dykes of these sequences may be located in southwestern or central Saurashtra, or in the Dhule-Nandurbar-Dediapada areas across the Gulf of Cambay, 200–300 km east of Palitana. Our results indicate polycentric flood basalt eruptions not only on the scale of the Deccan Traps province, but also within the Saurashtra region itself.

Geochemical stratigraphy of lavas and fault-block structures in the Ametista do Sul geode mining district, Paraná volcanic province, southern Brazil

The Paraná volcanic province is the world's largest producer of amethyst geodes, mostly from the Ametista do Sul mining district (20 by 30km) in southern Brazil. The description of the lava flows in the Ametista do Sul mining district, and adjacent areas, combined with flow-by-flow geochemistry and scintillometry, defines the stratigraphy of the flows. Nine flows were identified in the Ametista do Sul region and four additional flows were identified in the Frederico Westphalen region, totalizing 13 flows in the stratigraphy of the Ametista do Sul mining district. Six of the flows belong to Pitanga magma type (more than 3wt.% TiO2); which occur at the base of the local stratigraphy and the other seven flows belong to Paranapanema magma type (2–3wt.% TiO2) at the top of the stratigraphy. The amethyst-bearing geodes occur in the uppermost three Pitanga flows, known as Veia Alta (main producer), Veia do Meio and Veia Baixa. These flows were highly altered (more than 60vol.%) to clay minerals by a low temperature alteration process. The massive presence of clay minerals is an important component that controls the rheology of the rock, transforming the basalt into a very low grade smectite and celadonite metabasalt. Two Paranapanema flows, COOGAMAI and Linha Alta, from Ametista do Sul were identified in a lower position, at the base of the section in Frederico Westphalen which were correlated using scintillometry and flow-by-flow geochemistry. In addition, Veia Alta and Veia do Meio flows were also identified in lower positions, compared to Ametista do Sul. Thus, the identification of these flows in different places with different elevations is an evidence of the presence of downthrown blocks in a fault-block structure, lowering the stratigraphy more than 200m to the west in four steps. The tectonic patterns that generate this structure are three NW-strike lineaments. The identification of the COOGAMAI flow at the base of the Frederico Westphalen region, suggests that the Veia Alta geode-hosting flow is below the ground or below the elevation of 250m in this region. These evidences show that the fault-block structure is controlling vertically the deposit of amethyst-bearing geodes and is of major significance in the exploration for new amethyst geode deposits in the region.

Geochemical and scintillometric characterization and correlation of amethyst geode-bearing Paraná lavas from the Quaraí and Los Catalanes districts, Brazil and Uruguay

AbstractGeochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Paraná volcanic province) in Quaraí (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Catalán, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO2. Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti (&lt;2.0 wt% TiO2) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49±3.2, 123±10.3, 62±4.7, 94±4.6, ~45 and ~85. Colada Catalán has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay.

Phase Equilibria Constraints on Relations of Ore‐bearing Intrusions with Flood Basalts in the Panxi Region, Southwestern China

Abstract:There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu‐Ni‐Platinum Group Element (PGE)‐bearing sulfide deposits and large mafic layered intrusions that host giant Ti‐V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore‐bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q‐Pl‐Ol‐Opx‐Cpx system) has been carried out to elucidate whether ore‐bearing Panzhihua, Xinjie and Limahe intrusions are co‐magmatic with the picrites and flood basalts (including high‐Ti, low‐Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica‐undersaturated olivine basalt and silica‐saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low‐Ti basalts are silica‐undersaturated, whereas the Limahe gabbro‐diorites and high‐Ti basalts are silica‐saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high‐Ti basalts cannot be co‐magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low‐Ti basalts. Additionally, the gabbro‐diorites and peridotites of the Limahe intrusion are not co‐magmatic, and the former appears to be derived liquid from high‐Ti basalts.

Deep Drilling into a Mantle Plume Volcano: The Hawaii Scientific Drilling Project

Summary The Hawaii Scientific Drilling Project drilled and cored two holes in Hilo, Hawaii, the deeper reaching a depth of 3508 mbsl, and it retrieved a total of 4600 meters of rock core (525 meters from the Mauna Loa volcano and the remainder from the Mauna Kea volcano). The Mauna Loa core extends the continuous lava stratigraphy of that volcano back to 100 ka and reveals major changes in lava geochemistry over that time period. The Mauna Kea core spans an age range from about 200 ka to perhaps 700 ka, and when combined with surface outcrops, it provides a 700-kyr record of the lava output from a single volcano. During the time covered by the lavas from the core, the volcano drifted some 60–80 km across the melting region of the Hawaiian mantle plume, and therefore the HSDP rock core provides the first systematic cross-sectional sampling of a deep mantle plume. The geochemical characterization of the core, which involved an international team of forty scientists over a period of fifteen years provides information about mantle plume structure and ultimately about the deepest parts of the Earth’s mantle. The study of the lava core (which still continues) has provided unprecedented information about the internal structure of a large oceanic volcano and the time scale over which volca-noes grow. The hole also provides an intriguing glimpse of a complex subsurface hydrological regime that differs greatly from the generalized view of ocean island hydrology. Drilling conditions were favorable in the subaerial parts of the volcanic section, where coring was generally fast and efficient. The submarine part of the lava section, made up primarily of volcanogenic sediments and pillow lavas, proved considerably more difficult to drill. Some of the difficulties and considerable additional expense were due to pressurized aquifers at depth and a few critical mistakes made while setting casing. Even with the more difficult conditions, the project retrieved about 2400 meters of nearly continuous core from the submarine section of Mauna Kea. Overall, the HSDP project was highly successful even though the original target depth was about 20% deeper than the final hole depth. As expected, the project results answer several important questions about oceanic volcanoes, mantle plumes, and ocean island water resources, but they raise many more that might be addressed with further moderate-depth drilling in other Hawaiian volcanoes.

No vertical axis rotations during Neogene transpressional orogeny in the NE Gobi Altai: coinciding Mongolian and Eurasian early Cretaceous apparent polar wander paths

SUMMARY In this paper, we test the role of vertical axis rotations during transpressional mountain building. To this end, we carried out a palaeomagnetic study in the NE Gobi Altai of southern Mongolia, sampling widely exposed lower Cretaceous lavas allowing comparison of rotation histories of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends at the eastern termination of the Bogd strike-slip zone. We provide new 40 Ar/ 39 Ar ages to show that the stratigraphy of mafic lavas and fluvio-lacustrine sediments on the southern flanks of Mt Ih Bogd and Mt Baga Bogd have ages between ∼125 and ∼122 Ma, and a mafic sill that intrudes the sequence has an age of 118.2 ± 0.8 Ma. The lavas are older than previously dated lavas south of Artz Bogd, with ages of 119‐115 Ma. Palaeomagnetic results from the 119‐115 Ma lavas south of Artz Bogd show a significant steeper inclination than both results from 125 to 122 Ma lavas of Baga Bogd and Ih Bogd, as well as from newly sampled and previously published younger lavas and necks of the 107‐92 Ma Tsost Magmatic Field and Shovon and Khurmen Uul basalts. We explain this result by insufficient averaging of secular variation and small errors induced by overcorrection of bedding tilt. We show that individual lavas in the SE Artz Bogd locality represent individual spot readings of the Earth’s magnetic field and integrate all results obtained from lower Cretaceous lavas in the Gobi Altai. We present a pole, or rather, an apparent polar wander path without significant plate motion from the reference positions of Eurasia, from ∼125 to 95 Ma, with n = 126, λ = 80.8 ◦ , φ = 158.4 ◦ , κ = 25.3, A95 = 2.5, palaeolatitude = 48.2 with a scatter Sλ = 16.7 (Sl = 15.3, Su = 17.8) and a regionally consistent direction for the Gobi Altai of D/I = 11.1 ◦ /65.9 ◦ , � D/� I = 3.8 ◦ /1.9 ◦ . This is one of the best-determined palaeopoles/APWP’s for Asia. Formation of the Ih Bogd, Baga Bogd and Artz Bogd restraining bends was thus not associated with vertical axis rotations larger than our error margin of ∼10 ◦ .

The Mount Pavagadh volcanic suite, Deccan Traps: Geochemical stratigraphy and magmatic evolution

The patterns of eruption and dispersal of flood basalt lavas on the surface, or as magmas in dykes and sills within the crust, determine the volcanological and stratigraphic development of flood basalt provinces. This is a geochemical and Sr-isotopic study of lavas of varied compositions that outcrop around Mount Pavagadh (829 m), Deccan Traps, an important outlier north of the main basalt outcrop. Most of the ∼550-m thick exposed section at Pavagadh is made up of subalkalic basalts rich in the incompatible elements (particularly Nb, Ba, and Sr). Picrite and rhyolite–dacite flows also occur, the latter capping the sequence. The relatively high initial 87Sr/ 86Sr ratios (up to 0.7083) and chemical characteristics of the rhyolitic rocks of Pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis. An assimilation–fractional crystallization (AFC) model involving the picrite lava and either a southern Indian or a western Indian granite as the contaminant explains the geochemical and Sr-isotopic variation in the basalts and the rhyolites quite well. A systematic comparison of the basaltic lavas (with binary plots, normalized multielement patterns, and discriminant function analysis) to the well-established lava stratigraphy of the Western Ghats, 400–500 km to the south, precludes any chemical–genetic relationships between the two. Basalts exposed in sections closer to Pavagadh (∼150–200 km), in the Toranmal, Navagam, and Barwani–Mhow areas, have several flows with some similar chemical characteristics. However, the Pavagadh sequence is significantly different from all of these sequences geochemically, petrogenetically, and in magnetic polarity, to be considered independently built. This result is significant in terms of eruptive models for the Deccan Traps, as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast province.

Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii

238 U- 230 Th and U-Pb dating of zircons from leucocratic plutonic xenoliths indicating that lava stratigraphy is an incomplete monitor of magmatic evolution within subsurface res- ervoirs. Our results indicate that diorites from Mauna Kea record postshield evolution over tens of thousands of years when the depth of magma storage increased and highly evolved lavas began erupting. In contrast, diorites and syenogabbros from Hualalai record generation of evolved alkalic magma during the shield stage, and growth of a deep composite pluton ABSTRACT Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ 238 U- 230 Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hualalai volcanoes, Hawaii, reveals that impor- tant episodes of magmatic evolution are not necessarily refl ected in the stratigraphy or com- positions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of postshield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 ± 9 ka and 261 ± 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stra- tigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma.

Chronology of Vesuvius? activity from A.D. 79 to 1631 based on archeomagnetism of lavas and historical sources

The activity of Vesuvius between A.D. 79 and 1631 has been investigated by means of precise archaeomagnetic dating of primary volcanic deposits and taking into account the stratigraphy of lavas and tephra, historical written accounts, archaeological evidence related to the developing urbanisation, and radiocarbon ages. We found that the historical records are highly useful in constraining the timing of the main events, even if the data are often too scarce and imprecise for ascertaining the details of all phases of activity, especially their magnitude and emplacement of all the deposit types. In addition, some eruptions that took place in the 9th and 10th centuries appear to be unnoticed by historians. The archaeomagnetic study involved 26 sites of different lavas and 2 pyroclastic deposits. It shows that within the 15 centuries which elapsed between A.D. 79 and 1631, the effusive activity of Vesuvius clustered in the relatively short period of time between A.D. 787 and 1139 and was followed by a 5-century-long repose period. During this time Vesuvius prepared itself for the violent explosive eruption of 1631. The huge lavas shaping the morphology of the coast occurred largely through parasitic vents located outside the Mount Somma caldera. One of these parasitic vents is located at low elevation, very close to the densely inhabited town of Torre Annunziata. Among the various investigated lavas, a number of which were previously attributed to the 1631 eruption, none is actually younger than the 12th century. Therefore it is definitively concluded that the destructive 1631 event was exclusively explosive.

Zeolite Parageneses in the North Atlantic Igneous Province: Implications for Geotectonics and Groundwater Quality of Basaltic Crust

Zeolites are among the most common products of chemical interaction between groundwaters and the Earth's crust during diagenesis and low-grade metamorphism. The unique crystal structures of zeolites result in large molar volumes, high cation-exchange capacities, and reversible dehydration. These properties influence both the stability and chemistry of zeolites in geologic systems, leading to complex parageneses and compositional relationships that provide sensitive indicators of physicochemical conditions in the crust. Observations of zeolite occurrence in Tertiary basaltic lavas in the North Atlantic region indicate that individual zeolite minerals are distributed in distinct, depth-controlled zones that parallel the paleosurface of the plateau basalts and transgress the lava stratigraphy. The zeolite zones are interpreted to have formed at the end of burial metamorphism of the lavas. Relative timing relations between various mineral parageneses and crustal-scale deformal features indicate that the minerals indicative of the zeolite zones formed within 1 million years after cessation of volcanism. Empirical correlation between the depth distribution of zeolite zones and the temperatures of formation of zeolites in geothermal systems provides estimates of regional thermal gradients and heat flow in flood-basalt provinces. Similarly, the orientations of zeolite zones can be used to distinguish synvolcanic and post-volcanic crustal deformation. Because zeolites that characterize the individual zones display different ion-exchange selectivities for various cations, reactions between groundwaters and zeolites in basaltic aquifers can result in depth-controlled zones where individual elements are concentrated in the crust. This is established for Sr, which is concentrated by at least an order of magnitude in heulandite, resulting in an overall Sr enrichment of lavas in the heulandite-stilbite zeolite zone.

Change of stress field in Beta‐Atla‐Themis Region on Venus, estimated from surface geometry of dike swarms, lava stratigraphy and crater density

We estimate the orientations of crustal stress on Venus from surface geometry of dike swarms associated with volcanoes. The inferred stress field sometimes has various orientations even in the same region. Using lava stratigraphy, volcanoes in the Beta‐Atla‐Themis (BAT) region are classified into three groups: volcanoes before, during and after the formation of corona chains. Dike swarms in each group show a similar stress orientation. Between the first two groups, the compressional stress orientation had changed from a N‐S trend to a trend parallel to the strike of the corona chains. The stress fields during and after the corona chain formation are similar. The crater densities of the two volcano groups are 1.67±0.94 × 10−6 and 0.86±0.36 × 10−6 km−2, respectively, which indicates a probable age difference (&gt;108yr). The stress field and volcanic activity before the corona chain formation in the BAT are probably associated with adjacent rift activity in Beta Regio. Those during the corona chain formation may be attributed to a mantle upwelling centered at Atla Regio.

Hafnium Isotope Stratigraphy of Mauna Loa and Mauna Kea Lavas from The Hawaii Scientific Drilling Project Core

Hafnium Isotope Stratigraphy of Mauna Loa and Mauna Kea Lavas from The Hawaii Scientific Drilling Project Core