Columnar Joints Research Articles

Columnar jointing in Paleoproterozoic carbon-bearing rocks (Onega Basin, NW Russia): implications for carbon migration in sedimentary rocks

ABSTRACT Columnar jointing occurs commonly in igneous rocks and rarely in sedimentary rocks. We report the results of combined field and petrological studies, Raman spectroscopy and column geometry analysis of columnar jointing in Paleoproterozoic carbon-bearing rocks, Eastern Fennoscandian Shield, NW Russia. The columnar rocks occur near the contact with the saucer-shaped dolerite sill emplaced in the unconsolidated organic rich sediments. Columnar jointing aureole (1 to 7 m) and column architecture strongly depends on the contact morphology. Columnar jointing is caused by the contraction of organic-rich sediments due to generation and further release of gas and oil rich fluid. The results of integrated studies indicate that fluidization driving the jointing is associated with not only the dehydration, but also with the mobilization of gas and oil rich fluid due to the thermal maturation of organic matter. Columnar jointing is associated with intensive fluid circulation and devolatilization evidenced by the numerous veins, globules, and vugs filled with migrated carbonaceous matter and secondary minerals in the columnar rocks as well as high vesicularity of dolerites near the contact. The redistribution of the mobile elements (K, Rb, LREE) along with the significant enrichment of S near the contact also indicates the intensive fluid circulation. The study contributes to understanding of mechanism of columnar jointing in sedimentary rocks and formation the fracture networks in organic-rich sediments.

Coastal boulder production controlled by columnar joints of ignimbrite and extreme waves: insights from the high-energy coast of Pantelleria Island (Sicily Channel, Mediterranean Sea)

Coastal boulder production controlled by columnar joints of ignimbrite and extreme waves: insights from the high-energy coast of Pantelleria Island (Sicily Channel, Mediterranean Sea)

The Hidden Paradises: Geology and Geotourism Potentials in Lambu District, West Nusa Tenggara

Lambu District of Bima Regency, West Nusa Tenggara Province has three extraordinary geological features that can potentially be developed into geotourism objects: Pulau Kelapa, Tanjung Meriam, and Pantai Pink. These three objects can be accessed by water transportation. Pulau Kelapa is a geomorphological feature in the form of crescent-shaped hills. The land use as savanna with views of small island cluster resulted from volcanic activities, and can be observed from the top of the hills. Tanjung Meriam forms a headland morphology protruding towards the beach, composed of andesitic-basaltic igneous rocks in the form of columnar joints that extended spectacularly along the coastline. Tanjung Meriam is said to be the only largest columnar joint site in Indonesia that is still pristine and does not undergo any shape changes by human activities. The pink color of the sand at Pantai Pink is caused by the pigment of decomposed foraminifers that formed sand grains stretching along the beach area. In addition, the clean and clear sea water complements the beauty and aesthetics of the beach. Geologically, the area is made of by the Layered Limestone Formation (Tml) and Old Volcano Products (QTvs). Geomorphosite assessment for geotourism purpose, according to Kubalikova (2013), showed that the feasibility value of Pulau Kelapa, Tanjung Meriam and Pantai Pink is 57.91%, 51.66% and 51,66% respectively. Tourism development in this area is still limited to individual visits; yet to be managed further by the local government while at the same time the potential is enormous. Therefore, planning is needed to develop geotourism potential by considering scientific and educational (geology, biology and local culture) and tourism values for sustainable development

The Influence of Rock Slope Scales of Joint Network and Slope Height on the Stability and Failure Mechanisms

This paper demonstrates the influence of slope geometry and scale of the joint networks on the stability and failure mechanisms in discontinued rock masses. These analyses are based on the finite element method (FEM) to calculate the factor of safety (FOS) and to simulate the explicit discontinuity deformations for different scales of rock masses. The study was carried out through examples of regular jointed models having different slope heights and irregular jointed dry rock slopes. Joints usually decrease the strength of the slopes and suggestively increase slope stability problems. The result shows that the slope stability decreases with increased block size and shape and that the irregular columnar joints are more unfavorable to the slope stability than the regular joints. Furthermore, with dropping the factor of safety and randomly distributed joints the rock failure mechanism appears to shift from structurally controlled for the 50 m slopes towards a step-like failure path for the 100 m and 200 m slopes.

Investigation on the failure mechanism of the collapse of the columnar jointed basalt in underground cavern

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress.

A zone of columnar joints beneath the roof of a granitic pluton: The Okueyama granite, southwestern Japan

AbstractIgneous rocks are fractured during cooling from magma to form cooling joints, which are typically columnar joints in volcanic rocks, while orthogonal joints are considered typical for plutonic rocks. We performed a 3D study of joint systems in a granitic batholith of the Okueyama granite in western Japan, which has its roof and its internal structures from the roof to 1000 m downward exposed. We used an unmanned aerial vehicle (UAV) to observe the joints in outcrops from various angles. Based on our study, we propose a schematic model for joint systems in a granitic pluton. A granitic pluton has zones of rock columns below the roof and next to the wall. The rock column zone below the roof is as thick as 300 m, and its higher portions form steep cliffs, probably because of increased resistance to weathering. The axes of the rock columns are nearly vertical below the roof and gently plunge next to the walls, with high intersection angles with the wall. The distribution of columnar joints near only the roof and walls suggests that the granite cooled more rapidly near the roof and walls than in the core of the pluton. When the granite was jointed by parallel joints during cooling, the rock slabs between the parallel joints near the roof and the walls are subdivided into columns with polygonal cross‐sections. This suggests that the granite was fractured by parallel joints at a temperature immediately below the solidus, after which the rock slabs were subdivided into rock columns during further cooling.

Lava–water–sediment interaction: Dolomite U[sbnd]Pb dating marks the Cenozoic Ocean–continent transformation in the southwestern Tianshan Mountains

The regression of the Neotethys Ocean from the southwestern Tianshan Mountains remains debatable because of notable sedimentary discontinuities and lack of fossil evidence. This study aimed to explain this problem by applying comprehensive carbonate UPb dating to newly recognised pyroclastic rock layers in the Tuoyun Basin, southwestern Tianshan Mountains. These rocks, referred to as peperite in the early literature, are not like typical pyroclastic rocks; rather, they are formed by the mixing of magma with wet sediment. Field observations have revealed that the pyroclastic rock layers are overlying Cenozoic pillow basalts and then covered by the later columnar jointing basalts, recording a transformation from marine to terrestrial environment. The dolomite-dominated mineralogy and major element analysis indicate that the pyroclastic rock is dolomitic in composition. The sedimentary origin is supported by the δ18OSMOW values (23.7‰ to 36‰) and δ13C values (−6.48‰ to −7.08‰) of the bulk-rock pyroclastic rock and the δ34S values (−20.79‰ to 23.86‰) of native sulfur. However, the relatively depleted initial ratios of 87Sr/86Sr (0.7048–0.7065) and εNd(t) values (−0.52 to +3.10) suggest the possibility of isotopic exchange with the conjugate basalts. For carbonate from Tuoyun pyroclastic rock, UPb isotope dating using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) produced well-defined Lutetian ages of 46.1 ± 0.5–41.7 ± 0.6 Ma. This indicates that the Tuoyun region was in a marine environment during the Lutetian epoch and then transformed into a continental environment after the marine regression. In this study, we successfully applied the UPb technique by LA–MC–ICP–MS to constrain the timing of ocean–continent transformation through carbonate UPb dating. These findings explain the regression of the Neotethys Ocean from the southwestern Tianshan Mountains and offer novel insights into the geological evolution history of this region.

Investigation on effect evaluation of seepage control system of large underground powerhouse of the Baihetan hydropower station

The Baihetan Hydropower Station, situated in the lower reaches of Jinsha River in China, ranks as the second largest hydropower project. Effective seepage control in the underground powerhouse area is paramount due to the complex geological conditions on the left bank compared to the right bank. To assess the efficiency of the seepage control system, a combination of field measurements and numerical simulations was employed. In the session of field measurements, the characteristics of seepage field such as the groundwater level, hydraulic head, and leakage quantity in the impoundment process were analyzed. It became evident that the seepage patterns within the underground powerhouse were heavily influenced by geological structures, particularly the staggered zones C3 and C2, along with the columnar joint basalt. Notably, the leakage quantity of these areas rate considerably increased during impoundment, peaking at 1049.7 L/ min at the water level of 816 m in 2021. A finite element model with the seepage control system was developed based on the stable seepage theory, simulating the seepage field under two different water levels (816 m and 825 m).The numerical simulation results indicated that the grouting curtain and drainage hole arrays had a substantial lowering the groundwater levels. Comparing with the measurements, the inaccuracy of calculated total leakage quantity was 0.7 %, which was within the pumping capacity. The study provided a method of combining field measurement and numerical simulation to evaluate the effect of a complicated seepage control system for a large hydropower project. This method comprehensively revealed the seepage field of the underground powerhouse during the impoundment. It proved to have guiding significance for the operational stability of the hydropower station.

Petrology and Geochemistry of Microgabbro in Watugajah Area, Gedangsari, Southern Mountains

An intrusive igneous rock with a columnar joint structure having a height of approximately 55 m cropped out in the Watugajah area, Gedangsari, Gunung Kidul Regency, Yogyakarta Special Province. This region geologically is in the western part of the Southern Mountains, part of the Baturagung Subzone, which belongs to the Kebo Formation. This study aims to characterize the igneous rock in the Watugajah area of the Southern Mountains. It suggests the petrogenetic information of this igneous rock and gives a new consideration of the geological history in the Kebo Formation.Geological field mapping was managed in the research area to identify the correlation with other rock units. Rock samples were collected to characterize their composition by petrographical and geochemical analyses. The results suggest the igneous rocks intruded sedimentary rocks of the Kebo Formation, which consists of intercalations coarse sandstone and tuff, fine sandstone and siltstone, and tuff and fine sandstone units. The igneous rocks are classified as microgabbro and basalt in the form of the sill and dyke intrusions. The microgabbro and basalt consist of plagioclase, clinopyroxene, and magnetite, formed by the cooling of magma in the shallow depths. Based on geochemical studies, the igneous rocks of the Watugajah area were formed by subduction zone tectonic setting with the tholeiitic affinity. Keywords: Gedangsari, microgabbro, Southern Mountains, subduction, Watugajah.

Igneous Rock Associations 29. The Nenana Magnetitite Lava Flow, Alaska Range, Alaska

Magnetitite deposits like El Laco (Chile) are rare and have controversial origins. An unusual magnetitite lava flow overlying a rhyolite unit occurs in the north-central Alaska Range and originally covered ~ 750 km2 of the Miocene Nenana basin. Dating of the rhyolite and relationships between the magnetitite and sedimentary rocks indicate that both are of Late Miocene age. The magnetitite flow is mainly magnetite with some post-eruptive alteration to hematite. Both the rhyolite flow and the magnetitite flow are vesicular, but the magnetitite flow also has small, millimetre-scale columnar jointing. The vesicular zones in the magnetitite flow grade into massive rock on the scale of a thin section, suggesting a degassing lava origin. Samples of the magnetitite flow contain between 12 and 26 wt.% SiO2 and between 45 and 75 wt.% FeO. Rare earth elements (REE) and trace elements from the magnetitite and rhyolite have similar patterns but with lesser abundance in the magnetitite. Both the rhyolite and the magnetitite have light-REE-enriched REE profiles with negative Eu anomalies. Electron microscopic analysis shows that most of the silica and trace element content of the magnetitite flow comes from very finely disseminated silicate minerals and glass in the magnetite. This suggests that the magnetitite was derived from a magma that had undergone unmixing into a silica-rich phase and an iron-rich phase prior to its eruption. Fractures and vesicles within the magnetitite flow contain minor rhyolitic glass and minerals suggesting that the rhyolite magma invaded columnar joints in the solidified magnetitite flow, and is a subvolcanic sill-like body at the studied locality. The magnetitite flow erupted prior to the emplacement of the rhyolite, which may be extrusive on a regional scale. The features of the Nenana magnetitite, and its geological relationships, are consistent with genetic models that invoke unmixing of magma into immiscible Fe-rich and Si-rich liquids during ascent.

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.

A Case Study on Tunnel Excavation Stability of Columnar Jointed Rock Masses with Different Dip Angles in the Baihetan Diversion Tunnel

Columnar jointed rock mass (CJRM) formed by intact rock divided by special symmetrical columnar joints is a special type of rock with poor mechanical properties, strong anisotropy, and weak self-supporting ability, severely affecting the excavation safety and stability of underground tunnels. In this study, taking the Baihetan hydropower station as the engineering background, CJRM geological numerical models with different dip angles that combined well with the natural CJRM were generated based on the geological statistical parameters of the engineering site and were verified to have high rationality and accuracy. Tunnel excavation and overloading tests were carried out on these numerical models, and the results showed that the stress and displacement distributions after excavation exhibited strong anisotropic characteristics under different dip angles, and the positions where engineering safety problems are most likely to occur are the side walls, which are prone to stress-structure-controlled failure mode. The self-supporting ability at different dip angles after excavation from weak to strong are 45°, 60°, 75°, 90°, 30°, 0°, and 15°. The safety factors assessed by overloading for CJRM with dip angles of 0–90° degrees were 2.5, 2.6, 2.6, 1.8, 2.1, and 2.2, respectively, providing a valuable reference for the construction safety and support measures of CJRM excavation.

Decay of ignimbrite fairy chimneys of Arizona's Basin and Range Province, USA

AbstractFairy chimneys of ignimbrite are well studied in Cappadocia in central Anatolia, Turkey. The ignimbrite landforms of Arizona's Basin and Range, USA, in contrast, remain relatively unexplored in geomorphic scholarship, despite decades of geological research on the characteristics of the ignimbrites themselves. This research focuses on the rock‐decay processes that modify Arizona's Basin and Range fair chimneys. Dissolution of glass and groundmass, measured along joints using digital image processing of back‐scattered electron microscope imagery, reveal a 3× higher porosity than inside the rock interior. This method also measured a 3‐4× increase in dissolution near the base of the chimney compared with the rock interior, leading to notch development that promotes chimney mass wasting and subsequent ballistic impacts. Epilithic organisms studied with electron microscopy both enhance rock decay and promote case hardening. These observations are consistent with prior understanding of how fairy chimneys evolve over time: columnar joints exposed on cliff faces experience enough decay to allow erosion of joint material, this erosion separates the chimney from the rock face and notch development near the chimney base promotes mass wasting. The rock‐decay processes studied here likely operate on very different timescales. Mass wasting events leading to ballistic impacts occur at observable timescales. Varnish microlamination dating, using ultrathin sections of rock varnish formed on fairy chimneys, indicates that the case hardening of the top of fairy chimneys are much more stable than the sides; the tops last experienced surface detachment during the early Holocene, as opposed to the sides that experienced flaking throughout the late Holocene. The timescale of basal notch development is not known, with the exception that the very surface of notches erode at rates too fast to accumulate rock varnish. The timescales of epilithic organisms enhancing rock decay and dissolution of columnar joints are not known.

Dyke or pipe? Contributions of magnetic fabrics to the reconstruction of the geometry of an eroded subvolcanic body (Cadí basin, Pyrenees)

The reconstruction of the geometry of igneous bodies in inverted basins is often a difficult task, because most of their volume can be either below or above the topographic surface. Indirect methods, such as the determination of their internal fabric can give clues to constrain geometrical reconstructions. The Sant Salvador intrusion, located in the Late Carboniferous-Permian Cadí basin (Central-Eastern Pyrenees), is a good example for testing the application of magnetic fabrics to shallow igneous bodies because of (i) its characteristics in terms of crystal composition/orientation, showing a homogeneous petrofabric controlled by plagioclase crystals 0.4 mm in size, (ii) its good, though partial, preservation, (iii) its position in the stratigraphical succession, between volcaniclastic sediments and unconformably overlying red beds, and (iv) the many unknown factors related to the intrusion, including the geometry of the body and its depth and the amount of erosion after intrusion. Furthermore, the Sant Salvador intrusion provides opportunity for checking the correspondence between the petrofabric and the magnetic fabrics when the magnetic carriers of susceptibility (with an average value of 5217·10−6 SI) are mainly ferromagnetic. In this case the main magnetic carriers are magnetite and also hematite, the latter resulting from oxidation processes at the paleosurface. The results of Anisotropy of Magnetic Susceptibility show a bimodality in the K1 disposition after bedding restitution: vertical in the deeper areas of the intrusion and subhorizontal, E-W, in the uppermost stratigraphic positions with an intermediate necking zone in which transition from vertical to horizontal flow is interpreted to occur. Bands deflecting the mineral foliation at the micro scale have also a tectonic or thermal origin but do not seem to interfere with the magnetic ellipsoid axes. Columnar jointing is interpreted to be parallel to the long axis of the intrusion (and therefore to magma flow, and perpendicular to the paleosurface. All these features indicate that the Sant Salvador intrusion can be interpreted as a subvertical pipe feeding a sill that was eroded before the sedimentation of the overlying red beds.

Emplacement dynamics of the plumbing system and lava pile of the Paraná Magmatic Province in Morro da Igreja, Santa Catarina, Brazil

The mechanisms of magma ascent, transport and emplacement of the volcanic pile from LIPs are key issues regarding the understanding of the complex construction of volcanic systems and magma transport through the crust. The integrated approach of morphology of the volcanic and subvolcanic bodies, whole rock geochemistry and ASM data provides a robust tool to unravel the flow dynamics of the volcanic bodies and the sequence of the magmatic events. Such approach allows building up a model of construction of the lava pile considering the role played by the plumbing system. This paper investigates the emplacement mechanism of sill and lava flows of the Serra Geral Group (SGG), in the southern part of Mesozoic Paraná-Etendeka Magmatic Province (PEMP) in Brazil. Geologic and geochemical analysis were integrated with anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic remanent magnetization (AARM) and rock magnetism experiments on 14 sampling sites in the Morro da Igreja region, top of the lava pile in Santa Catarina. The area is composed of low-Ti basaltic (with predominance of rubbly pahoehoe) and silicic lava flows, both intruded by a tabular sill that reaches a few hundred meters wide. The mafic lava flows are composed of Gramado, Urubici and Esmeralda magma-type basalts and andesite basalts. The silicic lava flows are massive or foliated, classified as Palmas type dacites. The sill has regular columnar joints and is classified as Esmeralda type. Magnetic mineralogy data suggests magnetite or Ti-poor magnetite as the main magnetic mineral for all the igneous rocks, and AARM results show anomalous fabric for the lava flows and normal or intermediate fabric for the sill. Besides the anomalous fabric sites, AMS data provide reliable directional data to infer flow direction for the sill, with initial propagation towards NE, followed by a preferential SE direction of magma flow. For the silicic rocks, the presence of vertical and inclined magnetic foliation suggest a lava dome geometry. The dynamics of the magmatic flow of the sill and lava flows associated with the compositional characteristics of the rocks allows stablishing the stratigraphy of the magmatic events in the area as well as the proposition of an emplacement model for the sequence.

The geometry and dimensions of columnar jointing structures in volcanic rocks of Colombia

Las estructuras de disyunción columnar en rocas volcánicas corresponden al patrón de diaclasas que resulta de la contracción y deformación de los materiales durante el enfriamiento post emplazamiento. Estas pueden desarrollarse tanto en lavas como en depósitos de corrientes de densidad piroclástica (CDP) soldadas. En este estudio se analizaron visualmente 394 polígonos en 14 afloramientos de rocas volcánicas y depósitos volcaniclásticos soldados con disyunción columnar en Colombia, y computacionalmente, 349 polígonos definidos por procesamiento de imágenes. Estas medidas, junto con las variaciones relativas de tamaños de las columnas, fueron utilizadas para estimar la homogeneidad, el orden y la madurez de los patrones de disyunción. La relación entre tamaños de estrías y longitud de lado de columnas se usó para aproximar el mecanismo de disipación calórica dominante durante el emplazamiento de lavas y CDP. La inspección visual de afloramientos reveló que en lavas domina la geometría hexagonal, mientras que en depósitos de CDP dominan los pentágonos y tetrágonos. El análisis computacional indica que, en general, la disyunción columnar en Colombia tiende a organizarse en geometría hexagonal óptima. Específicamente, el afloramiento denominado Cristales, ubicado en el flanco suroriental del Nevado del Ruiz, en la cordillera Central, presenta el mayor grado de madurez; el Domo Victoria, un volcán monogenético de la cordillera Central, tiene el mayor grado de homogeneidad; y el sitio Los Ataúdes, ubicado en el flanco occidental de la cordillera Occidental, muestra el mayor grado de ordenamiento. Las estrías usualmente están entre 10 y 25% del lado de columnas. Los valores del número de Péclet entre 0,28 y 0,41 indican que en el emplazamiento de las lavas dominó la transferencia de calor conductiva, aunque no se puede descartar la disipación de calor convectiva asistida por diaclasas.

Salt Polygons and Porous Media Convection

From fairy circles to patterned ground and columnar joints, natural patterns spontaneously appear in many complex geophysical settings. Here, we investigate the origins of polygonally patterned crusts of salt playa and salt pans. These beautifully regular features, approximately a meter in diameter, are found worldwide and are fundamentally important to the transport of salt and dust in arid regions. We show that they are consistent with the surface expression of buoyancy-driven convection in the porous soil beneath a salt crust. By combining quantitative results from direct field observations, analog experiments, and numerical simulations, we further determine the conditions under which salt polygons should form, as well as how their characteristic size emerges.6 MoreReceived 1 June 2022Revised 5 December 2022Accepted 25 January 2023DOI:https://doi.org/10.1103/PhysRevX.13.011025Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasGeophysical fluid dynamicsGeophysicsPattern formationTechniquesChaos & nonlinear dynamicsFluid DynamicsNonlinear DynamicsInterdisciplinary Physics

The obscuring effect of magma recharge on the connection of volcanic-plutonic rocks

Abstract The current debate on volcanic-plutonic connection is centered on whether efficient liquid-crystal segregation dominates the evolution of a mushy reservoir to produce evolved, crystal-poor rhyolite and cumulate leftover. However, magma recharge may remarkably influence the evolution of a mushy reservoir and obscure the evidence of liquid-crystal segregation. This complexity poses a challenge to exploring the connection of volcanic-plutonic rocks. This study investigates the Qinzhou Bay granitic complex (~250–248 Ma) from South China, which contains crystal-poor (<19 vol%) peraluminous rhyolites and subsequent crystal-rich (28–54 vol%) porphyries. Although the rhyolite and porphyry units have a close spatio-temporal link, they do not share a fractionation trend and similar whole-rock Sr-Nd-O isotopic compositions; thus, a direct connection is not evidenced. We further present textural analyses, mineral and melt inclusion compositions, thermobarometry (the combination of Ti-in-zircon thermometer and Ti-in-quartz thermobarometer), and thermodynamic modeling to examine the alternative interpretations, i.e., the two units may have intrinsically independent origins or the connection of the two units has been obscured. For the rhyolite unit, thermobarometric results reveal a polybaric storage system consisting of middle (>600 ± 80 MPa) and upper (~150 ± 40 and ~60 ± 20 MPa) crustal reservoirs. Variations in quartz Fe content and chlorine-rich, metaluminous melt inclusions suggest that magma hybridization with less-evolved metaluminous magmas occurred at both crustal levels. In particular, the elevated Fe contents in the quartz population that crystallized at the shallowest level (~60 ± 20 MPa) suggest that recharge magmas were directly injected into the shallowest reservoir. Deviation of the whole-rock composition from the liquid evolution trend recorded in melt inclusions suggests a combined effect of magma mixing and crystal-melt segregation processes in upper crustal reservoirs. Thermodynamic modeling and mass balance calculations suggest that the whole-rock composition of the rhyolite could be reproduced by mixing between regionally exposed dacites and segregated melts at crystallinities of 50–60% (using parental magma represented by the least-evolved melt inclusion). For the porphyry unit, thermobarometric results reveal magma storage at middle (more than 450 ± 40 to 550 ± 40 MPa) and upper (110 ± 20 to 140 ± 20 MPa) crustal levels. The small-scale oscillatory zonation of plagioclase, the pervasive resorption of quartz and alkali feldspar, and the presence of peraluminous microgranular enclaves in the porphyries suggest a recharge event of metasediment-sourced magmas, triggering reactivation and convection of the reservoir. Autoclastic and overgrowth textures of quartz, plagioclase, and alkali feldspar phenocrysts and development of columnar jointing suggest that the reactivated porphyritic magmas ascended and emplaced at ultrashallow levels (~30 ± 10 MPa). Because of the similar storage pressures, the porphyries may represent remobilized cumulates of rhyolitic magmas, whereas the texture and geochemistry of the cumulate-liquid pair were modified, a key factor rendering a cryptic connection between the rhyolite and porphyry. Alternatively, the plumbing systems feeding the rhyolite and porphyry units are horizontally independent or vertically discrete, but this circumstance is inconsistent with the same evolution trend of quartz Fe and Al contents of the rhyolite and porphyry. Our study highlights that whole-rock composition may record blended information of complex processes, and caution should be taken when whole-rock composition is used to extract information of a single process. Multi-method constraints are required to evaluate the influence of recharge processes on the modification of liquid-cumulate records, and big data analysis on the basis of geochemistry should be conducted with caution to avoid biased understanding.

A Petrological Study of Columnar Joints with Geoheritage Value in the Cenozoic Sedimentary Basin, Korea

A comparative petrological study was conducted on the columnar joints of Guryongpo, Pohang, Yangnam, Gyeongju, and Gangdong-hwaam, Ulsan in the Cenozoic Miocene sedimentary basin. Horizontal columnar joints, in these columnar joints, are characteristically developed, and the cross-sectional shape of each columnar joint is predominantly quadrilateral and pentagonal in Guryongpo and Yangnam columnar joints, and pentagonal to hexagonal in Gangdong-hwaam columnar joint. As a result of XRF analysis, it was confirmed that Guryongpo columnar joint is andesite, Yangnam columnar joint is basalt, and Gangdong-hwaam columnar joint is basaltic andesite, all of which are sub-alkaline. They are easily distinguished because Guryongpo columnar joint contains small-sized plagioclase (1∼2 mm), while rather large plagioclase phenocrysts (several mm∼1 cm) are uniformly distributed both in Yangnam and Gangdong-hwaam columnar joints. The value of whole rock magnetic susceptibility (WRMS) are also well classified according to the rock composition. In addition, in the tectonic discriminant diagram, these columnar joints correspond to the orogeny zone and island-arc tholeiite regions of the orogeny, but the AFM diagram and WRMS analysis show that each columnar joint was formed from the different magmas.

Hydrothermal alteration and corresponding reservoir significance of the Permian Emeishan basaltic lavas, west Sichuan, China

Reservoir spaces, such as vesicles, ‘secondary’ amygdales, dissolution caverns and geodes, are widely developed in the Emeishan basaltic lavas in the Zhoudaping section, Leshan, west Sichuan, China. The dissolution characteristics, cementation sequences, hydrothermal activity stages, as well as fluid types, and their effects on the reservoir capacity were investigated for each stage. Macroscopically, the dissolution features present as irregular dissolution zones, which are characterised by a light red colour. Microscopically, in the dissolved zone, the cementation-filling minerals are associated with complex fill sequences, such as quartz/laumontite/chlorite–chlorite/saponite–epidote/celadonite–cryptocrystalline chlorite–laumontite/calcite/quartz. The U–Pb geochronology shows that the age of chlorite fill in amygdales is 235.3 ± 19.6 Ma; the coarse-crystalline quartz inside dissolution caverns/geodes, 124.47 ± 5.63 to 123.84 ± 5.63 Ma; and the siliceous mineral-filled amygdales, 118.34 ± 3.70 to 114.08 ± 3.76 Ma, which correspond to the early Late Triassic and the mid–late Early Cretaceous, respectively. Combined with geochemical characteristics of post-dissolution fill, the amygdales are affected by two stages of hydrothermal activity: chlorite filling of the amygdales corresponds to post-magma hydrothermal fluids during the early Late Triassic, and the siliceous mineral-fill in amygdales corresponds to deep-sourced hydrothermal fluids during the mid–late Early Cretaceous. The geodes/dissolution caverns result from a single stage of hydrothermal activity related to the mid–late Early Cretaceous deep-source low-temperature hydrothermal fluid. The Late Triassic post-magma hydrothermal fluids are generally destructive to pores, and tectonic-related dissolution of deep-sourced hydrothermal fluids has a positive effect on the formation of reservoir spaces, greatly enhancing fluid storage and flow capacities of the volcanic lavas. We recommend the multi-stage hydrothermal dissolution during Late Triassic–Early Cretaceous and faults, fractures and columnar joints be the focus of hydrocarbon exploration. KEY POINTS The reservoir spaces developed in the Zhoudaping section, such as amygdales, dissolution caverns and geodes, were controlled by different stages and types of hydrothermal alteration. Amygdales are the product of two hydrothermal events, which correspond to post-magma hydrothermal fluids during the early Late Triassic and the deep-sourced hydrothermal fluids of the mid–late Early Cretaceous. Geodes/dissolution caverns are affected by deep-source low-temperature hydrothermal fluids in the mid–late Early Cretaceous. The multi-stage hydrothermal dissolution during the Late Triassic–Early Cretaceous and faults, fractures and columnar joints should be the focus of hydrocarbon exploration.