Porphyritic Basalt Research Articles

Characteristics and Controlling Factors of Particle Crushing in Volcaniclastic Sediments under Compaction: The Quaternary Pyroclastic Deposits of Hongtu Hill, Changbaishan Area, Northeastern China

Recently, many scholars have conducted experimental mechanical compaction studies on sandstones, carbonates, and mudstones to visually study the mechanical compaction process and reservoir evolution of sedimentary rocks. However, experimental mechanical compaction studies on the evolution of the compaction process of pyroclastic rocks have been ignored. Volcaniclastic rock reservoirs are widely distributed across the world and strongly influenced by the crushing of pyroclastic particles. In this study, we analyzed the characteristics and controlling factors of the crushing of pyroclastic particles during compaction diagenesis from a microscopic perspective through experimental mechanical compaction. These results can provide quantitative compaction background parameters for the quantitative study of pyroclastic rock reservoir evolution. We took pyroclastic samples from Hongtu Hill in the Changbaishan area as an example, and experimental mechanical compaction experiments were conducted. Furthermore, image surface porosity and particle analysis statistical methods were used, and the variations in the effective porosity and image surface porosity under different axial stresses were studied. The results showed that, after compaction, the effective porosity did not exhibit a decreasing trend with increasing axial stress but rather a normal distribution trend that initially increased and then decreased. In the compaction experiment, the pyroclastic particle crushing process was segmented with increasing axial stress, and there was an obvious compaction band in the initial stage of the compaction, called the particle rearrangement stage (10–30 MPa). Furthermore, there were relatively non-successive compaction localization areas in the later stage of compaction, called the particle crushing stage (50–70 MPa), which was represented by vitreous basalt particles surrounded by porphyritic basalt particles. During experimental mechanical compaction, the smaller the compactness, the smaller the solidity, and the larger the slenderness of the particles, the more likely the particles were to break during compaction. Particles containing intragranular pores and vitreous basalt particles were easily crushed.

Cheakamus basalt lavas, British Columbia: a Pleistocene record of rapid, continuous eruption within a mountainous drainage system

The Cheakamus basalts are a voluminous (1.65 km3) set of Late Pleistocene, valley-filling lavas erupted from a vent situated near present-day Conflict Lake, in the alpine Callaghan Valley near Whistler, British Columbia. Geochemical and petrographic properties suggest these olivine–plagioclase porphyritic basalt lavas derive from a single batch of magma affected by minor sorting of phenocrysts and xenocrystic plagioclase. Thirty-four sites sampled for paleomagnetic directions record a mean pole direction of 345.2°/73.0° (α95 = 1.3°) and show no statistical variation nor drift with stratigraphic position. These data suggest that the Cheakamus basalt lavas were emplaced in a single paleomagnetic moment—a period of time significantly less than 2000 years. 40Ar/39Ar geochronometry on three lava samples returns a weighted mean age estimate of 15.95 ± 7.9 ka (2σ) and field evidence, including well-glaciated lava flow surfaces overlain by till, indicate the eruption coincided with the early stages of the Fraser glaciation (∼20–18 ka). The lavas preserve features indicative of a landscape hosting diverse and dynamic paleoenvironments. Subaerial eruption of basalt lava filled an ice-free Callaghan Creek drainage system before inundating and damming of the paleo-Cheakamus River creating an upstream rising body of water. Periodic overtopping of the lava dam resulted in syn-eruptive intermittent flooding and overtopping of lavas expressed by discontinuous lenses of interflow sediment. Rare instances of enigmatic cooling columns may also indicate localized ice contact with glaciers that partially filled the Cheakamus Valley. The displacement of the modern Cheakamus River and the long-term damming and formation of Callaghan and Conflict lakes remain direct indicators of the control and impact these basaltic eruptions have had on the geomorphology of the present-day Callaghan and Cheakamus valleys.

Fouimba and Goma Mounts Greenstone Belts Litho-Structural Analysis Related to Côte D’Ivoire Birimian Geodynamic Setting and Implying in West-Africa Craton Gold Deposits

Mount Fouimba and Mount Goma (Seguela) greenstone belts petro-structural studies combine remote sensing, geophysics, petrography and structural analysis. In view of establishing mapping details of paleoproterozoic geological formations, geological setting rocks observed are essentially magmatic formations, such as two-mica granite, granodiorites, and porphyritic basalts; and a few metamorphics which are metatonalite, amphibolites and amphibo-lopyroxenites. Remote sensing, such as Landsat 8 OLI satellite imagery and geophysical data, has been combined to show regional NNE-SW shear zone. Tectonic structures and microstructures have enabled to identify two main deformation phases: D1 phase corresponding to compression, and D2 is a transpression phase. Mechanisms responsible for deformations are respectively flattening and transpression. Geological formations derived from mantle origin but contain crustal components, and their tectonic setting occurred during subduction.

Slope stability analysis of deep-seated landslides using limit equilibrium and finite element methods in Debre Sina area, Ethiopia

Slope failure is a recurring natural hazard in the western margin of the Main Ethiopian Rift and especially around the Debre Sina area. To minimize the damage caused by failure events, a detailed investigation of landslide-prone areas identified using numerical modelling plays a crucial role. The main aim of this study is to assess the stability of slopes and to evaluate and compare safety factors calculated by the different available numerical methods. Stability analyses of slopes prone to different types of failures were performed with different techniques. The stability was assessed for slopes of complex geometry composed of aphanitic basalt, porphyritic basalt, tuff, and colluvium (poorly sorted clayey sand to silty sand) using the limit equilibrium method and the shear strength reduction method based on finite elements. Furthermore, numerical analysis was done under static and pseudo-static loading using the horizontal seismic coefficient to model their stability during a seismic event. Satellite images were used to select failure-prone slopes based on slope properties and identified past landslides, as well as to derive structural and geological information for the numerical models. The slope stability analysis indicates that the studied slopes are unstable, and any small-scale disturbance will further reduce the factor of safety and cause failure. The slope stability of landslide prone hills in the study area strongly depends on the saturation conditions and the seismic load.

Geology of a Neogene caldera cluster in the Borgarfjörður eystri–Loðmundarfjörður area, Eastern Iceland

The Borgarfjörður eystri–Loðmundarfjörður area in Eastern Iceland hosts several volcanic centres in a relatively small area. Large volumes of silicic, as well as an unusual amount of intermediate rocks, occur in this area, alongside other volcanic and sub-volcanic features that formed in the Miocene between 14 and 12:2 Ma. We compiled the first comprehensive geological map of the area and summarise the geology based on more than 40 years of fieldwork. We identify regionally extensive marker horizons that comprise intermediate (icelandite) and mafic (olivine basalt and porphyritic basalt) lavas. These marker horizons, along with new paleomagnetic data and some previous radiogenic dating, allow us to bracket a phase of magmatic activity that is one of the oldest in Eastern Iceland. We describe the prominent features of the volcanic centres, including remnants of collapse calderas in Njarðvík, Dyrfjöll, Breðuvík and other ignimbrite-producing vent structures in Kækjuskörð and Herfell. Notably, the area also contains extremely well exposed examples of volcanic vents, cone sheets, and unique caldera-lake sediments. We conclude with open questions to inspire future research on this understudied area.

Petrogenesis of Early Permian basalts in the Turpan-Hami basin, NW China: Implications for the spatial limits of the Tarim mantle plume

Controversies remain as to the area of the Tarim Large Igneous Province (TLIP), in particular, the relationship between extensive Permian magmatism in the Central Asian Orogenic Belt (CAOB) and the activity of Tarim plume. Here we report the geochronological, mineralogical and geochemical characteristics of the Permian basalts from the Turpan-Hami (Tu-Ha) basin, located in southwestern CAOB. LA-ICP-MS zircon U-Pb dating yielded an age of 298.4 ± 2.6 Ma, temporally close to that of the Tarim plume. The basalts could be classified into lower porphyritic basalt with phenocrysts of plagioclase, olivine, minor clinopyroxene, and upper aphyric basalts. Geochemically, the former is characterized by relatively higher Na2O + K2O (3.97–5.13 wt%), low (La/Yb)N (∼2) and depleted Sr-Nd values [εNd(t)= +8.2 to +8.7; (87Sr/86Sr)i = 0.7032–0.7034], whereas the latter have lower Na2O + K2O (4.19–4.43 wt%) content, higher (La/Yb)N (∼4–6) and more enriched Sr-Nd values [εNd(t)= +4.6 to +6.2; (87Sr/86Sr)i = 0.7033–0.7048]. The compositional variation of plagioclase phenocrysts and pressure estimation (∼7kbar) indicate that Tu-Ha magmas were stored and evolved in a lower crustal magma chamber which experienced at least one-time magma replenishment. The estimated mantle potential temperature (1270–1480 °C) in Tu-Ha basin is similar to MOR, with no obvious thermal anomaly. Furthermore, higher Na/Ti ratio of quartz-normative Tu-Ha basalts suggest a shallower melting depth for Tu-Ha basin than central area of TLIP, which against a typical plume model. These characteristics alongside regional geologic records collectively suggest that the formation of Permian Tu-Ha basalts is more closely linked post-orogenic extension, implying that the extensive Permian magmatism in the CAOB is unrelated to the Tarim plume.

Spectral characteristics, petrography and opaque mineralogy of the Oligo-Miocene basalts at Wadi Abu Qada- Wadi Wata area, west-central Sinai, Egypt

The Oligo-Miocene basaltic exposures at Wadi Abu Qada – Wadi Wata area, west-central Sinai Peninsula occur as intra-continental fissure-fed eruptions in the form of km-scale sill attaining thickness of about 30 m and dyke of about 70 m thick, intruding/extruding the Early Cretaceous to Early Eocene sedimentary sequences. Stratigraphy of the study area was dealt with in the context of the geologic setting of these Oligo-Miocene basalts, which are genetically related to the Gulf of Suez rifting in the late Oligocene. These basaltic exposures were discriminated from their surrounding sedimentary rocks using ASTER, Landsat-8 and Sentinel-2 datasets. Spectral characteristics of these basalts were determined by remotely-sensed satellite data as well as by using a handled ASD mineral identifier. Advanced image processing techniques were applied and many images processing algorithms were used in image enhancement including Principal Component Analysis (PCA), Band ratios, Decorrelation Stretch (DS), Mixture Tuned Match Filter (MTMF), and ASTER mineral's indices. All these techniques in conjunction with petrographical investigations were applied in lithological discrimination of the studied Oligo-Miocene basalts and new geological mapping were accomplished and verified by field data. The considered Oligo-Miocene basaltic rocks are mainly olivine dolerite with minor olivine porphyritic basalts. The main mineralogical components are plagioclase, augite and olivine with subordinate Fe-Ti oxides (ferriilmente and titanomagnetite).

The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism

Abstract The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts of the Columbia River magmatic province. Field mapping along with analytical data of tuff samples identify two calderas, the Castle Rock and Ironside Mountain calderas, as the venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both calderas lie within the area of the proposed general storage sites of main-phase Columbia River Basalt magmas. The Castle Rock caldera formed during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The northwestern boundary of the caldera is roughly defined by the juxtaposition of over 300 m of densely welded rheomorphic intra-caldera tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following caldera collapse, fluvial and lacustrine volcaniclastic sediments were deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite and icelandite (Fe-rich andesite), which correlate stratigraphically to upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and lavas are interbedded with sediments and Dinner Creek Tuff unit 4. The Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner Creek Tuff unit 2, which lies ~15 km north of the Castle Rock caldera. The caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera, rheomorphic, and partially welded tuff are bound by Weathersby Formation shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic andesite and icelandite dikes and sills that are also stratigraphically correlative to upper Grande Ronde Basalt lavas intruded into the tuff, mostly along the margins of the caldera, which altered much of the tuff. Mafic lavas within the study area that closely pre- and post-date Dinner Creek Tuff units were correlated with regional units of the Columbia River Basalt Group. Porphyritic and aphyric mafic lava flows underlying Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are westward extensions of fractionated tholeiitic magmas as seen in late-stage Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally, porphyritic basalt lava flows that overlie the Hunter Creek Basalt and volcaniclastic sediments at the Castle Rock caldera are correlative with the 13.5 Ma Tim's Peak Basalt. At Castle Rock, pre-caldera Columbia River Basalt Group lavas appear to lap onto a mid-Miocene topographic high that stretches northward and westward for tens of kilometers based on stratigraphic data, and it may be related to regional uplift at initial impingement of the mantle upwelling to produce the Columbia River Basalt Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal volcanism of the Columbia River magmatic province. Eruption of rhyolites is closely pre- and post-dated by the eruption of local and regional tholeiitic lavas belonging to the Columbia River Basalt Group. The local eruption of evolved tholeiitic lavas likely concealed calderas, but these lavas also illustrate the close proximity of mafic and rhyolitic magmas at depth at these rhyolite centers. Consequently, the stratigraphy of both the Castle Rock and Ironside Mountain calderas somewhat differs from that of rhyolite calderas dominated by silicic and calc-alkaline intermediate pre- and post-caldera volcanism.

Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from Stromboli volcano, have been investigated over the last two decades. Among these, a heterogeneous (composite) glassy sample (B1) containing intimately mingled porcellanite and buchite lithotypes was selected to be studied through new FESEM–EDX and QEMSCAN™ mineral mapping investigations, coupled with the already available bulk rock composition data. This xenolith was chosen because of the unique and intriguing presence of abundant Cu–Fe sulphide globules within the buchite glass in contrast with the well-known general absence of sulphides in Stromboli basalts or their subvolcanic counterparts (dolerites) due to the oxygen fugacity of NNO + 0.5–NNO + 1 (or slightly lower) during magma crystallization. The investigated sample was ejected during the Stromboli paroxysm of 5 April 2003 when low porphyritic (LP) and high porphyritic (HP) basalts were erupted together. Both types of magmas are present as coatings of the porcellanite–buchite sample and were responsible for the last syn-eruptive xenoliths’s rim made of a thin crystalline-glassy selvage. The complex petrogenetic history of the B1 pyrometamorphic xenolith is tentatively explained in the framework of the shallow subvolcanic processes and vent system dynamics occurred shortly before (January–March 2003) the 5 April 2003 paroxysm. A multistep petrogenesis is proposed to account for the unique occurrence of sulphide globules in this composite pyrometamorphic xenolith. The initial stage is the pyrometamorphism of an already hydrothermally leached extrusive/subvolcanic vent system wall rock within the shallow volcano edifice. Successively, fragments of this wall rock were subject to further heating by continuous gas flux and interaction with Stromboli HP basalt at temperatures above 1000 °C to partially melt the xenolith. This is an open system process involving continuous exchange of volatile components between the gas flux and the evolving silicate melt. It is suggested that the reaction of plagioclase and ferromagnesian phenocrysts with SO2 and HCl from the volcanic gas during diffusion into the melt led to the formation of molecular CaCl in the melt, which then was released to the general gas flux. Sulphide formation is the consequence of the reaction of HCl dissolved into the melt from the gas phase, resulting in the release of H2 into the melt and lowering of fO2 driving reduction of the dissolved SO2.

Petrogenesis of basaltic and associated rhyolitic rocks of the Alaje Formation from the Aiba area in Tigray, Northern Ethiopia

An integrated field, petrological, as well as major and trace element geochemical study of a suite of basaltic and associated rhyolitic rocks of the Alaje Formation from the continental flood basalts of the Aiba area of northern Ethiopia has been conducted. The study constrained the geochemical and petrogenetic association of the basaltic and rhyolitic rocks. The results show that the Alaje Formation is constituted by three layers of transitional to tholeiitic basaltic suites intercalated with three layers of alkaline, pantelleritic trachytes/trachy dacites and comenditic rhyolites. The dominant phenocrysts in the porphyritic basalts are plagioclase, with some clinopyroxene, Fe–Ti oxide and olivine. The basalts: (i) have high-Ti concentrations, (ii) show uniform LREE enriched and slightly HREE depleted pattern, and (iii) have low Y/Nb and Zr/Nb ratios, all implying their co-genesis and evolution from homogeneous source parental magma, and their derivation by shallow level fractional crystallization of magma sourced from an enriched component of the mantle. The associated rhyolitic rocks: (i) contain some plagioclase phenocrysts, (ii) plot along the differentiation lines of the associated basalts, (iii) show strong negative anomalies in P, Ti, and Sr, (iv) show general LREE enrichment and HREE depletion, and (v) have very low La/Nb, Rb/Nb, Ta/Nb and Th/Nb ratios. All these suggest their derivation by low-pressure fractionation of mantle derived basaltic magma, with little or no crustal contamination. The rhyolitic rocks of the Alaje Formation from the Aiba area have nearly the same geochemical signatures as other well-constrained rhyolitic rocks of similar stratigraphic context and age in other parts of the Northwestern Ethiopian plateau.

Tectonic conditioning revealed by seismic refraction facilitates deep-seated landslides in the western escarpment of the Main Ethiopian Rift

Landslide is a geo-hazard phenomenon that has been taking lives and causing severe property damages all over the world mostly in mountainous areas. The Main Ethiopian Rift has a unique tectonic setting with complex geological and geomorphological features, coupled with continuously deteriorating environmental conditions, which made its escarpments vulnerable for landslides. The study area is located near the Debre Sina town, within the Yizaba Wein locality, which has been severely affected by frequent landslide problems. This work was carried out using a multidisciplinary approach based on geological, geomorphological, kinematic analysis and geophysical survey. Seismic refraction investigations were carried out along the Yizaba Wein landslide main scarp to determine the depth to the bedrock and to the failure plane, to assess the stability of the slope, to locate possible structural features and to identify the extent of recent landslide activity, and to study the subsurface situation. The seismic measurements were made along three nearly orthogonal survey lines in the recently affected area. A high-resolution 2D P-wave survey was conducted using a 24-channel seismic unit. The seismic refraction results revealed four layers of geomaterials with distinct physical characteristics that contained a subsurface landslide anomaly within the layers. The layers were interpreted according to the major lithological units, from top to bottom: (i) clay, loosely cemented colluvial sediments and highly weathered agglomeratic basalt; (ii) highly to moderately fractured porphyritic basalt, ignimbrite-volcanic ash and rhyolite/trachyte; (iii) moderately to slightly fractured ignimbrite, rhyolite/trachyte and basalt and (iv) very strong, massive, fresh rock/bedrock. Faults and weak zones have also been identified in the bedrock based on the abundance of fractures and subsidiary faults resulting from damage of rocks and change of lithology due to variable fault rock formation strongly influencing the wavefield distribution which usually causes a local decrease of the velocity value. The main findings show that the landslide in the Yizaba Wein locality was caused by its complex geological-structural setting and downslope movement of the underlying pyroclastic sediment facilitated by heavy rainfall. Considering the similar geological and tectonic settings, similar mechanisms can be assumed for other landslides along the rift margins and associated highlands of Ethiopia.

Petrography of aggregates in Luzon, Philippines: Identification of components and deleterious materials

Petrography is one of a series of standard tests used to assess an aggregate’s components, mechanical qualities, durability, chemical stability, and alkali reactivity. In this study, aggregate materials were collected from rock exposures and/or alluvial deposits from four areas near Metro Manila, Philippines: Bulacan, Rizal, Pampanga, and Zambales. Transmitted light microscopy was conducted to identify rock types and characterise physical and chemical properties that may present potential problems when used as aggregate materials. The results show that the aggregates vary in terms of rock types and alteration type. Samples from Bulacan are mostly porphyritic basalt and fine to coarse-grained sandstone with veinlets of silica and carbonate. The presence of cavities and microfractures caused mainly by vesicles from the volcanic rocks was also observed. Rizal aggregates are composed predominantly of chloritized basalts and andesites with minor clastic rocks and tuffs. The aggregates from Zambales are products of erosion of the Zambales Ophiolite, mixed with the lahar deposits from the Pinatubo eruption. On the other hand, Pampanga aggregates are mostly lahar deposits, containing pumice, a poor choice for aggregate composition due to its low hardness, brittleness and vesiculated texture. Aside from the lithological classification, potentially alkali-reactive constituents were also observed in selected samples from the four sampling areas.

Evaluating weathering of palaeosols in Cameroon (Central Africa) as a tool for paleoenvironmental reconstruction

Palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas (1400 mm mean annual rainfall, 22 °C mean annual temperature) of the Adamaoua region in north-central Cameroon by means of macro- and micromorphology, geochemical analyses and bulk mineralogy. The studied sequence at Mangoli is about 11 m deep and is divided into three sections, in terms of weathering products: (1) Precambrian granite, located in the base; (2) vesicular porphyritic basalt, in the middle section; (3) aphanitic basalt in the uppermost layer. The products of weathering from the Precambrian granite consist of the following horizons, from bottom to top: a saprolite with granite structure, a yellowish to whitish fine saprolite, and Btg and Bt horizons. The saprolite horizon is dominated by quartz, feldspars (plagioclases and microcline), and muscovite. Primary minerals, except quartz, show weathering, while clay fills the pores and fractures and kaolinite dominates the clay mineralogy. The granite weathering products are slightly acidic (pH ranging from 5.9 to 6.3) and display low values of Fe extracted with dithionite-soluble Fe (Fed) and high Chemical Index of Alteration (CIA) and Mineral Index of Alteration (MIA) values. The middle section of the sequence consists of different areas of the basalt’s spheroidal weathering zone, from the more advanced to the less weathered phase. The clay mineralogy is dominated by kaolinite. The zones with the most alteration are the fractures, where thick, reddish clay coatings are observable. In these areas, the minerals are well crystallized. The kaolinite crystals share areas with Fe oxides. The upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons. The lowest is a C horizon, overlaid by Bt horizons. The C horizon shows a high degree of weathering in some areas, but it is still possible to identify the primary minerals. Their products of weathering are slightly acidic (pH = 6) and have high Fed, CIA and MIA values, which are indicative of intense weathering. The results suggest warm and humid climates during the formation of all of the palaeosols, similar to modern environmental conditions. We conclude that Palaeosol 1, formed from the granite, represents a soil formed during the Mid-Miocene Climatic Optimum, when the temperatures are assumed to have been 3–6 °C warmer than today. Features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau. Meanwhile, the former top of the plateau is located on the slopes and lower altitudes; the burial of soils by volcanic materials at Mangoli increased the altitudes of the former plateau and resulted in landscape inversion (slopes). These palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide.

Hydrothermal Alteration Associated with Vein-Type Sulphide Mineralization at Lappadata Prospect, South Sulawesi, Indonesia: A Preliminary Study

In Lappadata area, Bone Regency, South Sulawesi Province, Indonesia, indication of vein-type sulphide mineralization is occurred. This paper describes a recent study of the prospect which focused on hydrothermal alteration associated with the mineralization. In this study, alteration and mineralization samples have been collected selectively and representatively from the field and then analyzed in laboratory using petrography, ore microscopy, and X-ray diffraction methods. The study resulted that host rocks of the mineralization are porphyritic basalt and limestone members of the Early Eocene to Late Oligocene Salo Kalupang Formation. Hydrothermal alteration mineral assemblages include quartz, chlorite, muscovite, sericite, calcite, actinolite, epidote, illite, kaolinite, and pyrite, which generally zoned from distal-propylitic mainly characterized by chlorite, calcite and epidote to proximal-argillic characterized by sericite and illite. The mineralization occurred in quartz vein and disseminated with identified hypogene ores of sphalerite, chalcopyrite, galena, and pyrite as well as supergene ores of covellite, chalcocite, and malachite. Study of the hydrothermal alteration stability temperatures suggested that the mineralization was formed in temperature range of about 200 to 300°C, from a near-neutral pH hydrothermal fluid. The whole characteristics of hydrothermal alteration, ore and gangue minerals, mineralization types, as well as range of formation temperature and composition of the responsible hydrothermal fluid indicated that the mineralization in Lappadata prospect is an epithermal type.

The Zealandia Volcanic Complex: Further evidence of a lower crustal “hot zone” beneath the Mariana Intra‐oceanic Arc, Western Pacific

AbstractThis paper addresses formation of felsic magmas in an intra‐oceanic magmatic arc. New bathymetric, petrologic, geochemical, and isotopic data for Zealandia Bank and two related volcanoes in the south‐central Mariana arc is presented and interpreted. These three volcanoes are remnants of an older andesitic volcano that evolved for some time and became dormant long enough for a carbonate platform to grow on its summit before reawakening as a rhyodacitic volcano. Zealandia lavas are transitional between low‐ and medium‐K and tholeiitic and calc‐alkaline suites. They define a bimodal suite with a gap of 56–58 wt% SiO2; this suggests that mafic and felsic magmas have different origins. The magmatic system is powered by mantle‐derived basalts having low Zr/Y and flat rare earth element patterns. Two‐pyroxene thermometry yields equilibration temperatures of 1000–1100 °C for andesites and 900–1000 °C for dacites. Porphyritic basalts and andesites show textures expected for fractionating magmas but mostly fine‐grained felsic lavas do not. All lavas show trace element signatures expected for mantle and crustal sources that were strongly melt‐depleted and enriched by subduction‐related fluids and sediment melts. Sr and Nd isotopic compositions fall in the normal range of Mariana arc lavas. Felsic lavas show petrographic evidence of mixing with mafic magma. Zealandia Bank felsic magmatism supports the idea that a large mid‐ to lower‐crustal felsic magma body exists beneath the south‐central Mariana arc, indicating that MASH (mixing, assimilation, storage, and homogenization) zones can form beneath intra‐oceanic as well as continental arcs.

Distribution and gas-bearing properties of Permian igneous rocks in Sichuan Basin, SW China

Abstract Based on the analysis of outcrop, seismic, logging and drilling data, combined with exploration practice, the characteristics, distribution, reservoir performance and gas-bearing properties of Permian igneous rocks in Sichuan Basin are studied. The study shows that central volcanic eruptive facies are developed in Sichuan Basin, and their lithological assemblages and distribution characteristics show obvious differences. The igneous rocks are mainly distributed in three regions: the southwestern part of the basin has dominantly large- scale overflow facies basalts; the central and western part of the basin, Jianyang-Santai area, develop intrusive rocks, volcanic lavas (basalts) and pyroclastic rocks; and the eastern part of Sichuan, Dazhou-Liangping area, only develop diabase and basalts. Five aspects of understandings are achieved: (1) The Upper Permian igneous rocks can be divided into intrusive rocks and extrusive rocks, with the extrusive rocks as the main body. The chemical compositions of the extrusive rocks are characterized by both alkaline basalt and tholeiitic basalt, and belong to the subalkaline type of transitional basalt magma eruption. (2) There are obvious rhythmic structures vertically among overflow facies basalt, and the single rhythmic layer consists of, from bottom up, pyroclastic rocks (undeveloped), gray and dark gray porphyritic basalts (unstable), dark gray and purple microcrystalline-cryptocrystalline basalts, dark greyish green porous and amygdaloid basalts; the central volcanic eruption shows the rhythm and the vertical sequence of volcanic clastic rocks (agglomerates and breccias), volcanic lava, tuffaceous lava from bottom to top. (3) The pore types of basalt and pyroclastic rocks are diverse, mainly dissolution pore and de-vitrification micropore, but their physical properties are different. Basalt is characterized by ultra-low pore permeability, small reservoir thickness, and reservoirs are distributed in the upper and middle parts of the cycle, with poor lateral comparability. Volcanic clastic rocks are medium to high porous reservoirs (Well YT1: porosity: 8.66%−16.48%, average 13.76%) with large thickness and good reservoir quality. (4) Natural gas in basalts in southwestern basin mainly comes from Middle Permian, and natural gas in volcanic clastic rocks in central and western basin comes from Cambrian Qiongzhusi Formation. (5) Analysis of igneous reservoir-forming conditions in different areas shows that there are relatively insufficient gas sources and great differences in preservation conditions in southwestern basin. Reservoirs are poorly developed and gas-bearing is complex. The Jianyang-Santai area in the central and western part of Sichuan Basin has abundant hydrocarbon sources, developed reservoir, favorable preservation conditions and favorable gas geological conditions, and it is a favorable area for gas exploration.

Geology, Petrology and Geochemistry of Volcanic Rocks Around Arba Minch, Southern Ethiopia

The study area is subpart of the southern main Ethiopian Rift and geographically bounded between UTM (Universal Transverse Mercator) coordinates of 662000m-669000m latitude and 336000m-341000m longitude. The main aim of this study is to investigate the geological, petrographic and geochemical properties of volcanic rocks around Arba Minch, southern Ethiopia. Both field and laboratory analytical techniques are employed to characterize minerals and the rock types. Eight thin section samples are prepared to study petrographic properties of minerals and five best representative samples are analyzed for whole rock chemistry using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). Petrographically, samples from the porphyritic basalt show phenocrysts of olivine and plagioclase within the fine-grained groundmass. There are rounded to subrounded holes or vesicles that appear white under plane polarized light observations and dark under cross polarized light observations which further confirms vesicular nature of the basalt. Thin section sample from rhyolite outcrop shows phenocrysts of K-feldspar and quartz within tiny fine-grained groundmass of feldspar minerals. Aphanitic basalt, Porphyritic basalt, vesicular basalt, rhyolite and dolerite are the rock units found in the study area. The geochemical results of the analyzed samples indicate silica ranges of (48.5-60.1)%. Based geochemical classifications made, rock units are classified into rhyolite, basalt, hawaiite and mugherite rock types. Trace element analytical results show all samples lie on the within plate tectonic setting. The rare earth and trace element ratios and spider diagrams confirmed that most samples are derived from magmas of asthenospheric source with little crustal contamination.

Geochemical characteristics of basalts from Andaman subduction zone: Implications on magma genesis at intraoceanic back‐arc spreading centres

This study reports new petrological and geochemical data for the ocean floor volcanic rocks dredged from the back‐arc basin of the Andaman Sea and elucidates their geochemical characteristics and petrogenetic aspects to provide insights into the melt generation processes, oceanic crust emplacement, and tectonomagmatic evolution of the Andaman back‐arc basin. The studied samples are porphyritic basalts with olivine, plagioclase, and clinopyroxene representing the phenocrysts and the groundmass composition marked by plagioclase microlites, granular clinopyroxenes, and glass. Geochemical characteristics of these basalts reflect sub‐alkaline, tholeiitic composition of precursor magma that experienced clinopyroxene‐Ti magnetite fractionation. The overall geochemical trend for these basalts reflects LILE–HFSE–REE depleted MORB‐type chemistry which is modified by input from subduction‐derived components. Pronounced enrichment in selected LREE (Nd) and incompatible fluid‐mobile trace elements like Ba with positive primitive mantle‐normalized La and Sr anomalies in the majority of these basalts are attributed to transport and influx of slab‐dehydrated fluids and sediments into the depleted back‐arc mantle asthenosphere from the dehydrating subducted oceanic lithosphere in the volcanic arc front during initial stage of back‐arc rifting in the Andaman Sea. The studied basalts were derived by 10%–30% partial melting of a heterogeneous spinel lherzolitic mantle at a shallow depth. The chemical heterogeneity of the spinel lherozolite source suggests an enrichment‐depletion history of back‐arc mantle correlatable with the transitional tectonic evolution of the back‐arc basin from an initial rifting to matured spreading stage. The depleted MORB‐type mantle (DMM) beneath a juvenile back arc experienced infiltration of fluids and subduction‐derived components (SDC) from proximal subducting oceanic slab that gradually diminished with increasing distance from the arc and progressive back‐arc spreading. The episodes of melt generation and magma genesis invoke a gradual geodynamic transition from incipient back‐arc rifting to matured back‐arc spreading stages in the tectonomagmatic evolution of the Andaman back‐arc basin system. Oceanic crust generated during the incipient rifting stage sampled arc‐type melts enriched and hydrated by subduction inputs and fluid flux, while oceanic crust formed during matured back‐arc extension stage inherited trace element depleted MORB‐type mantle signatures devoid of subduction influence.

Characteristics and Productivity of Volcanic Rock Aquifers in Kola Diba Well Field, North-central Ethiopia

Characterization the different volcanic rock aquifers and evaluation of their respective productivity was done in Kola Diba well field (221 km2), north-central Ethiopia. Collections and analyses of pertinent secondary data were followed by detailed field investigation. Geological and structural investigations was conducted in the field paying particular attention to the different features of the overlying sediments and underlying volcanic rocks units that made them to be a water bearing formations. The structural data were analyzed using Stereonet v7.3 software. Hydrogeological logs, Semi‐log scale (specialized plot) and Log‐Log scale (diagnostic plot) were prepared to determine the different aquifer types and also to characterize the nature of each aquifer. Inventory of groundwater structures (boreholes and hand dug wells) were also conducted as part of hydrogeological investigation. Pumping test data were collected from ten deep wells and were analyzed using Theis and Neuman methods using Aquifer Test v3.5 software. Stratigraphically, the geology of the area from bottom to top is aphanitic basalt, porphyritic basalt and alluvial sediments. The rocks are affected by different degree of weathering and fractures. The geological formations of the area are categorized hydrogeologically into intergranular and fractured medium and fracture medium. The major aquifers are confined, unconfined and leaky. Hydraulic conductivity values of these different aquifers ranges from 0.0048 to 1.46 m/day with mean of 0.444 m/day, transmissivity ranges from 0.5 to 474 m2/day and specific capacity ranges from 0.026 to 0.88 l/sec/m with mean values of 86.878 m2/day and 0.325 88 l/sec/m, respectively. On the basis of their corresponding transmissivity and specific capacity value, the different aquifers of the studied area were categorized into four aquifer potentiality groups: moderate to low, low to weak, weak and very weak aquifers potentiality. Any future development of groundwater should be focused on the moderate to low potentiality aquifers.

Mineralogy and Fluid Inclusion Microthermometry of Epithermal Gold-Base Metal Mineralization at Anggai, Obi Island, Indonesia

Epithermal gold-base metal mineralization at Anggai village, Obi island, Indonesia has been identified through an exploration program by Broken Hill Proprietary Company during 1995 to 1996. This paper describes a preliminary study on the prospect which focused on its ore and gangue mineralogy, hydrothermal alteration, fluid inclusion microthermometry as well as geochemistry, to elucidate the formation conditions and ore grades. Fresh and altered rocks and mineralization samples were collected from field work to be studied using petrography and ore microscopy, X-ray diffraction analysis, atomic absorption spectrometry, and fluid inclusion microthermometry. The study resulted that host rocks of the mineralization are fine-grained porphyritic basalt and andesite of the Oligocene to Early Miocene Bacan Volcanics. Hydrothermal alteration mineral assemblages include quartz, chlorite, epidote, albite, less biotite, sericite, clay, and pyrite. Mineralization styles are crustiform quartz vein and dissemination, where sulphides clustered and disseminated in closed-space both in quartz gangue and in strongly altered host rocks. Under the microscope, main hypogene ore minerals identified include galena, sphalerite, chalcopyrite, and pyrite which hosts fine-grained gold. Fluid inclusion microthermometric study on the vein quartz resulted in homogenization temperature of 220 to 230oC and salinity of 2.0 to 2.5 wt.% NaCl equivalent. Chemical analysis of nine selected samples resulted in highest grades of 98 g/t Au, 275 g/t Ag, 0.61% Cu, 48% Pb, and 5.35% Zn.