Pyroclastic Research Articles

Discovery of shale oil in alkaline lacustrine basins: The Late Paleozoic Fengcheng Formation, Mahu Sag, Junggar Basin, China

A new type of shale oil in alkaline lacustrine sediments has been discovered in the Late Paleozoic Fengcheng Formation, Mahu Sag, Junggar Basin, China. The fine-grained sedimentary rocks deposited in this alkaline lacustrine environment can be divided into four types and eight sub-types: mudstone (with no alkali minerals), including massive dolomitic mudstone, and massive and laminated calcareous mudstone; dolomite, including massive argillaceous dolomite (with alkali minerals), and massive and laminated argillaceous dolomite (with no alkali minerals); evaporites; and pyroclastic rocks. The massive argillaceous dolomite (with alkali minerals) and pyroclastic rocks have the highest shale oil potential, with average oil saturation index (OSI) values of 344.67 and 124.65 mg HC/g TOC, respectively. Shale oil exploration in the representative well MY1 indicates that the Fengcheng Formation is thick and contains abundant natural fractures, brittle minerals, and mobile oil. The entire Fengcheng Formation is oil-bearing and contains three concentrated stratigraphic intervals of shale oil (i.e., sweet spots). Well MY1 indicates that, compared with source rocks developed in marine and sulfate-type saline basins, the fine-grained sedimentary rocks deposited in alkaline lacustrine environments can also have high shale oil potential. The co-existence and regular distribution of conventional and unconventional reservoirs in the Fengcheng Formation indicate that it is an ideal exploration target for multiple resource types.

Preliminary study of the origin of mafic mineral megacrysts in volcanic rocks in the southwestern part of Kulon Progo Mountains, Indonesia

The Bagelen district in the southwestern part of Kulon Progo Mountains has a unique aspect in the form of the occurrence of mafic mineral megacrysts in volcanic rocks. This research is conducted to determine the type, the distribution, and the origin of those megacrysts, which are abundant in pyroclastic rocks. Detailed geological mapping on an area of 4 x 4 km with a scale of 1: 25,000 and petrographic analysis were done to explain the geological aspects that led to the presence of mafic mineral megacrysts in the study area. Petrographic analysis shows that the types of mafic mineral megacryst are hornblende, clinopyroxene, and plagioclase. Their features under the microscope observation are oscillatory zoning, half oscillatory zoning, sieve, and breakdown texture. These minerals are found as single crystal fragments and also as clinopyroxene and hornblende cumulate fragments in pyroclastic breccia. Based on mineralogical composition and texture, these mafic mineral megacrysts were formed by fractional crystallization process in the form of crystal settling mechanism. These minerals were initially accumulated in the bottom of magma chamber, then carried upward by rapid magma rising in association with explosive eruption event of Mount Ijo. These minerals were then transported on the surface through the mechanism of pyroclastic flow and were deposited in the valley to form an alluvial fan morphology in the southwestern part of Kulon Progo Dome.

Pyroclastic Deposits Identification using Near-Surface Seismic Refraction Tomography in Rawa Dano Volcanic Complex, Banten, Indonesia

Rawa Dano is a caldera lake which resulted from Dano Purba Volcano’s massive eruption, and it produced a huge amount of pyroclastic deposits that typically formed complex volcaniclastic series. Due to the lack of information regarding the subsurface properties of Rawa Dano area, therefore in this study, a low-energy seismic refraction survey was carried out to identify the distribution of pyroclastic deposits resulted from intensive volcanic eruptions. The data were acquired from two lines in two different sites. Variations of longitudinal velocity in the seismic vertical cross-section suggest that there are more than one type of deposits existed in the area. The results show two main refractors which are related to the deposition of different facies. The seismic velocity shown in the upper part of the seismic tomography model indicates that the pyroclastic deposit has a great thickness. This finding suggests that the eruptions happened massively. By combining the results from both sites, it could be inferred that the preceding one is even bigger in magnitude. The result is in agreement with the earlier surface geological study, which explains a similar conclusion. This research demonstrates the capability of seismic refraction tomography to map the distribution and condition of volcanic deposits around Rawa Dano Volcanic Complex.

Re-interpretation of distribution of Lautan Pasir caldera-forming eruption products, Bromo-Tengger Caldera Complex, East Java

We present the result of morphostratigraphic analysis and geological field observation to identify the distribution of scoria flow deposit related to the caldera-forming eruption of Lautan Pasir caldera as the youngest caldera in Bromo-Tengger caldera complex. Morphostratigraphic analysis derived from digital elevation model and volcanic stratigraphy shows that products of Lautan Pasir caldera-forming eruption(s) mainly distributed to the north filled up the Sapikerep valley, and further to the north formed Sukapura distal fan. While to the south, it filled the valley between Ijo and Old Semeru. Field observation in the deeply dissected part of Sapikerep valley found two massive pyroclastic flow deposits separated by lava flow. Above the lava flow is scoria rich pyroclastic flow deposit typical of Lautan Pasir caldera-forming eruptions. Below the lava flow is scoria poor pyroclastic deposit of older products, most likely, of Ngadisari caldera. The distribution of Ngadisari caldera-forming eruption products is very limited considering the following massive volcanism of Lautan Pasir caldera. This outcrop might provide the key information to further reconstruct the volcanism stage of Bromo-Tengger caldera complex

The Sector Development at Tanjung Sakti, Mount Dempo, Bengkulu by Using Integration of Remote Sensing Data and TOPEX Gravity Satellite

Mount Dempo is the highest volcano in South Sumatra, which lies between the Bukit Barisan mountains and Gumai. The mountain located in Dempo Makmur Village, Sub-district of Pagar Alam, Lahat Regency, South Sumatra is located at an altitude of 3173 meters above sea level with coordinates of 4.03 ° S 103.13 °E. Mount Dempo’s morphology is formed by pyroclastic deposits consisting of Tuff and Sand rocks. Mount Dempo’s vegetation is dominated by Cassia sp. and Camellia sinensis for upper vegetation, while Strobilanthes hamiltoniana and Strophanthus membranifolium dominate the undergrowth. The purpose of this study is to identify geological structures to predict geothermal prospect areas by integrating remote sensing data and TOPEX Gravity Satellite Data. The remote sensing data used in this study is Landsat 8. This data is used to analyze Land Surface Temperature (LST) from a single thermal infrared band, surface emissivity based on Normalization Difference Vegetation Index (NDVI) from the study area and determine structure delineation. Gravity Satellite Data is used to map gravity anomalies in the volcanic complex of Mount Dempo. Gravity data processing produces a high anomaly zone in the northern part of the study area and is predicted as a prospect area because it is assumed to be related to the plutonic body. High density contrast indicates that there is an error in that area. In line with the error, there are several hot springs because the error serves as a pathway for geothermal fluid to rise to the surface. The study believes that with all the facts stated above, the spots which are located in Tanjung Sakti, Mount Dempo district are very prospective to be developed as a geotourism complex, in which could also increase the welfare of the local citizens.

Geologic and geomorphologic study of the Terra Murata and Centane-Panoramica sites (Island of Procida, Naples Bay, Southern Tyrrhenian sea) aimed at solving some applied geological and geotechnical problems

A geologic and geomorphologic study aimed at solving some geological and geotechnical problems, regarding the massive seepage of meteoric waters in the coastal cliffs of the Island of Procida (Naples Bay, southern Italy), composed of both tuffs and loose pyroclastic deposits, has been carried out in the geosites of Terra Murata (Middle Ages village and coastal cliff towards the Corricella Bay) and Centane-Panoramica (coastal cliff facing on the Tyrrhenian sea). A detailed geologic and geomorphologic survey has allowed to suggest solutions to the applied geological and geotechnical problems related to the occurrence of massive seepages of waters at the physical interface between pyroclastic rocks and loose pyroclastic deposits, characterized by different density, permeability and porosity and also controlled by a dense network of fractures, involving the pyroclastic deposits cropping out in the selected areas. Field sampling and geotechnical laboratory analyses have been carried out to calculate the values of main geotechnical parameters of the yellow tuffs cropping out at the Terra Murata promontory. At the same time, a detailed monitoring of the seepages of waters has been carried out through a detailed geological survey of the tuff outcrops of the promontory. The obtained results have suggested a strong control from both the geomorphologic instability of the coastal cliff and tectonic setting. At the Centane-Panoramica geosite the geological survey, coupled with geotechnical analyses and standard penetrometric tests, has accordingly evidenced that the geomorphologic instability was mainly concentrated in the sectors of the tuff coastal cliffs facing seawards towards the Tyrrhenian sea.

Accumulation and Distribution of Natural Gas Reservoir in Volcanic Active Area: A Case Study of the Cretaceous Yingcheng Formation in the Dehui Fault Depression, Songliao Basin, NE China

Tight gas sandstone and volcanic gas reservoirs have received global attention in the energy arena for further exploration and exploitation attempts. Considering the Yingcheng Formation of Dehui fault depression in the Songliao Basin as an example, this study focused on the accumulation and distribution of natural gas reservoirs in volcanic area in a fault depression basin. Volcanic activities occurred in the Yingcheng Formation, which is distributed centrally in the northwest of the study area. During the sedimentation of the Yingcheng Formation, fan-delta, lacustrine, and nearshore subaqueous fan facies were deposited. The source rocks of the Yingcheng Formation have high abundance of organic matter mainly in type III at high-overmature stages, indicating favorable conditions for gas production. The porosity of volcanic reservoir is 3.0%-14.8%, the permeability is 0.0004 mD-2.52 mD, and the pore types are mainly secondary dissolved pores and fractures. Besides, the porosity of the tight sandstone reservoir is 0.5%-11.2%, and the permeability is 0.0008 mD-3.17 mD. The pore types are mainly interparticle pores, with a small proportion of intraparticle pores and microfractures. The intrusion of late volcanic magma provided sufficient heat for the thermal maturity progression of organic matter in Yingcheng Formation and promoted the generation of natural gas in large quantities. Volcanic rocks formed at the early and middle stages of volcanic activities occupied the sedimentary space and hindered the development of sedimentary sand bodies to a certain extent. However, volcanic rocks can become the seal to promote the formation of tight sandstone gas traps. Comparing tight sandstone reservoirs with volcanic ones, the latter are less affected by compaction; thus, their petrophysical properties do not vary much with depth, showing more homogeneous characteristics. The pyroclastic rocks influenced by volcanic activity and the secondary pores formed by dissolution in the later stages also provide reservoir space for gas accumulation. Ultimately, the tight sandstone and volcanic rocks in the study area form a complex gas reservoir system, which can become a reference for exploration and exploitation of natural gas in other petroliferous fault depressions that are affected by volcanisms.

Unraveling the Holocene Eruptive History of Flores Island (Azores) Through the Analysis of Lacustrine Sedimentary Records

Lacustrine sequences from active volcanic settings usually hold a rich and continuous record of tephra layers, providing a critical source of information to reconstruct a most complete eruptive history of a region. Lake sedimentary records on volcanic islands are particularly useful as the typical small size of these islands and their steep subaerial and submarine slopes lead to a lower preservation of potential erodible pyroclastic deposits. Here we explore the lacustrine sedimentary record of Lagoa da Lomba, a crater lake in the central upland area of Flores Island (Azores), to gain insight into the recent eruptive history of this island. The strategic location of Lagoa da Lomba, half distance between the two clusters of recent volcanic activity of the island, together with its long-lasting record, back to 23.52 cal kyr BP, makes this lake a privileged site to investigate the Holocene volcanic history of Flores. Based on a detailed stratigraphic characterization of sediments from a lake transect of three cores, supported by glass shard geochemistry and radiocarbon dating, we recognized four Holocene eruptive events taking place between 6.28 and 2.36 cal kyr BP, demonstrating that the Holocene volcanic activity at Flores Island may have lasted longer than previously reported. Glass shard geochemistry from the different tephra layers suggests three populations, basaltic to trachybasaltic in composition, where the last eruption is the least evolved endmember. Two of the four eruptive events correlate with subaerially-exposed pyroclastic sequences, in terms of stratigraphy and geochemistry. The most recent event recorded at Lagoa da Lomba was constrained to 3.66 – 2.36 cal kyr BP and linked to an eruption sourced from Lagoa Comprida Volcanic System. The second most recent eruptive event was sourced from Lagoa Funda Volcanic System and dated at 3.66 cal kyr BP. Our observations show that Flores experienced vigorous volcanic activity during the Late Holocene. Therefore, contrary to what is assumed, the possibility of future eruptions should be properly considered, and the volcanic hazard here should not be underestimated. Moreover, we highlight the importance of tephrostratigraphy in recent lake sediments to reconstruct past volcanic activity, especially at small volcanic islands, such as Flores, where exposure is poor due to erosion within the limited subaerial area and the dense vegetation.

Magma chamber decompression during explosive caldera-forming eruption of Aira caldera

Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.

Variations in tephra stratigraphy created by small‐scale surface features in sub‐polar landscapes

We explore the effect small‐scale surface features have on influencing the morphology and grain‐size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub‐polar landscapes. Icelandic thúfur, small cryogenic earth mounds, are used to assess how and why the morphology and GSD of tephra layers vary over such formations. Through measurement of tephra layer thickness and GSD, Hekla 1947 and Grímsvötn 2011 tephra layers are analysed. Results indicate that such microtopographic features do indeed alter the form of tephra deposits and therefore the tephra layer that is preserved in the stratigraphy. Tephra thickness is significantly greater in hollows than on the thúfur crests. There is greater variation in tephra thickness measurements from thúfur in comparison to control measurements from a surface where thúfur are absent. Thúfur crests contain larger grain sizes than hollows, for both H1947 and G2011 tephras; however this was only statistically significant for the G2011 tephra. Such morphological patterns are thought to arise from an interplay of tephra characteristics, altered topography from the thúfur formations and earth surface processes operating at the sites. This study provides insight into the potential of tephra layer morphology and internal structures as indicators of Quaternary landforms and processes. Additionally, it provides important context for the appropriate sampling of tephra layers to infer volcanological processes, as the characteristics of preserved layers do not necessarily reflect those of the original fall‐out.

Rapid changes from arid to humid conditions during the onset of the Paraná–Etendeka Igneous Province: can volcanic gas emissions from continental flood basalts affect the precipitation regime?

Abstract Despite the intriguing correlation between continental flood basalt provinces and environmental crises, little is known about how the local/regional sedimentary systems and environment respond to flood basalt volcanism. Active sedimentary systems, and their interaction with volcanism, provide an important rock record to understand palaeoenvironments in volcanic settings. The Paraná–Etendeka Igneous Province is a well-known example of a continental flood basalt emplaced on a dry desert environment, but evidence has also shown the existence of humid conditions during the volcanic episode. This work describes and interprets non-volcanic sedimentary and volcaniclastic rocks interbedded with Paraná–Etendeka Igneous Province lavas in southernmost Brazil to better understand the palaeoenvironmental process and changes during the onset of volcanism. Non-volcanic sedimentary rocks record the existence of ephemeral sheet-like flows and ponds/lakes while volcaniclastic rocks document hydromagmatic activity, supporting a change to more humid conditions. Stratigraphic constraints indicate that this change started with the onset of volcanism and affected the whole province. We suggest that SO 2 degassing from the Paraná–Etendeka province may have caused a net global surface cooling, resulting in precipitation redistribution and a local increase in rainfall. This hypothesis may help explain the cooling and increased humidity observed elsewhere to be closely related to the Paraná–Etendeka emplacement.

Weathering on volcanic edifices under semiarid climates: insights from a regional assessment of the composition of Fogo Island regoliths (Cape Verde)

Abstract In this study, the geochemistry and mineralogy of regoliths formed on Fogo Island (Cape Verde), a polygenic strato-volcano built during the Quaternary, are used to assess the geomorphological factors that control the early stages of basalt weathering. Fogo Island soils are mainly derived from relatively homogenous silica-undersaturated basaltic rocks. However, a discernible exotic component is recognized in areas most exposed to prevailing winds via ratios of non-mobile elements that are hosted in different amounts by basaltic rocks (e.g. Th, Sc and Ti). The weathering extent is evidenced by a relative depletion in mobile elements (e.g. Na, Ca, Mg) and an enrichment in non-mobile elements (e.g. Ti, Fe, Sc, Al), the decomposition of the most labile minerals (olivines) and the formation of secondary components (phyllosilicates and some Fe-oxides, such as hematite-goethite), along with the enrichment in quartz supplied from non-volcanic areas. Weathering depends on bedrock age and type (pyroclastic deposits v. lava-flows). In particular, soils covering older volcanic units tend to be more affected by chemical alteration than those overlying younger units. In addition, more intense weathering is observed in locations characterized by a combination of moderate elevation, slopes with low gradient and relatively high rainfall. The present investigation shows that even in low humidity environments recently formed basalts are affected by weathering, with the extent of chemical decomposition being mainly determined by the age of surface exposure and local orographic/climatic features.

Implications of reworking processes on tephra distribution during volcanic eruptions: The case of Parícutin (1943–1952, western Mexico)

AbstractParícutin in western Mexico is considered one of the most studied monogenetic volcanoes worldwide. The volcano began its formation in the middle of a cornfield in February 1943 and lasted until March 1952. Descriptions of the eruption documented the remobilization of unconsolidated primary ashfall by rainfall and wind in minor proportions. Despite these descriptions, the resulting reworked deposits have not yet been described in stratigraphic sequence. We present the first stratigraphic study of interfingered primary pyroclastic and reworked deposits of Parícutin's succession. We identify 13 units, of which seven correspond to reworked deposits dispersed around the volcano. These deposits display sedimentary structures produced by tephra remobilization due to lahars and stream flows. In addition, some layers display dunes and ripples generated by dust storms. By using GIS tools, we integrated the existing data with our new composite stratigraphic column, the distribution map of the syn‐eruptive reworked deposits and a new isopach map. This new evaluation reveals that more than 70% of the total thicknesses around the volcano correspond to syn‐eruptive reworked deposits (R1–R4 units). Therefore, previous studies had overestimated the distribution of primary tephra from the Parícutin explosive phases. The lowest and flattest areas with wide rill networks, which are located 4–6 km north of the volcano, are composed of up to 90% reworked deposits. In contrast, proximal locations with gentler slopes located at medium altitudes better preserve pyroclastic deposits. This study brings new light to understanding the sedimentary processes that occur during volcanic eruptions and highlights the importance of recognizing pyroclastic and reworked deposits during monogenetic eruptions.

Geology and conceptual model of the Domuyo geothermal area, northern Patagonia, Argentina

The western slope of Cerro Domuyo in northern Patagonia is characterized by thermal springs with boiling waters, Quaternary silicic domes, and pyroclastic deposits that suggest the existence of a geothermal reservoir. According to geochemical studies, the reservoir may have a temperature of 220°C and one of the largest advective heat fluxes reported for a continental volcanic center. In this paper, we propose a more refined conceptual model for the Domuyo geothermal area, based on a geological survey supported by UPb, UTh, and ArAr geochronology and by magnetotelluric and gravity surveys. Our study indicates that the Domuyo Volcanic Complex (DVC) is a Middle Pleistocene dome complex overlying middle Miocene to Pliocene volcanic sequences, which in turn cover: 1) the Jurassic-Early Cretaceous Neuquén marine sedimentary succession, 2) silicic ignimbrites dated at ~186.7Ma, and 3) the Paleozoic metamorphic basement intruded by ~288Ma granite bodies. The volcanic cycle in the DVC is distinctly bimodal, characterized by the emplacement of massive silicic domes and less voluminous olivine basalts on its southern slope. A major collapse of the central dome at ~600ka produced a voluminous (19.4km3 and 133km2) block-and-ash flow, and associated pyroclastic flows, that filled a valley to the southwest at distances up to ~30km from Cerro Domuyo summit. This was followed by a period of intense effusive activity that formed the Cerro Guitarra, Cerro Las Pampas, Cerro Domo, and Cerro Covunco silicic domes. The last two domes are the youngest and largest edifices, dated at 0.50Ma (ArAr age) and 0.25Ma (UTh age). Pre-Cenozoic successions were affected by N-S reverse and thrust faults that were later displaced by an ENE-WSW-trending transtensional belt. The basement rocks at the northern termination of the Cordillera del Viento anticlinorium were also displaced towards the east-northeast by this belt, which is observed NNW of Cerro Domuyo. The DVC was emplaced within this zone of crustal weakness. The integration of geologic observations with magnetotelluric and gravity data, allowed us to develop an updated conceptual model of the geothermal system. The geothermal reservoir is inferred at a depth of less than 2km within pre-Pliocene fractured rocks, bounded by ~WSW-ENE trending faults and sealed by the pyroclastic deposits and rhyolitic lavas of the DVC. The location of most thermal springs is not directly controlled by faults. Instead, flows emerge at the contact between the fractured and faulted basement and the caprock.

Chrono-stratigraphy of the youngest (last 1500 years) rhyolitic eruptions of Lipari (Aeolian Islands, Southern Italy) and implications for distal tephra correlations

The youngest (last 1500 years) volcanic eruptions of Lipari, within the Aeolian Archipelago, produced the prominent pumice cone of Monte Pilato and the obsidian lava flows of Rocche Rosse and Forgia Vecchia, concentrated in the north-eastern sector of the island as well as highly dispersed white-coloured, fine-grained tephra layers of rhyolitic composition in terrestrial and marine settings on the regional scale. Here we describe in detail the stratigraphy of pyroclastic successions and lava flows erupted by different vents - Monte Pilato, Forgia Vecchia, Lami, and Rocche Rosse - combining field observations, sedimentological characteristics of the tephra deposits, and major and trace element compositions of the volcanic glass. All the pyroclastic materials consist of aphyric pumice lapilli and ash with a largely homogeneous rhyolitic composition. The Monte Pilato and Forgia Vecchia deposits primarily consist of highly vesicular pumice fragments and subordinate obsidian clasts, whilst Rocche Rosse and Lami are characterized by moderately vesicular juvenile fragments with a more significant fraction of obsidian. The Lami tephra also contains peculiar pumice clasts with a fibrous texture and breadcrust bombs.Stratigraphic relationships, and paleomagnetic and 14C ages of the lava and pyroclastic deposits are combined with the archaeological information and historical reports, enabling us to provide an accurate chrono-stratigraphic framework for the youngest eruptions of Lipari. Following the 8th century CE eruption of Monte Pilato, which produced a pumice cone and a obsidian lava flow, activity resumed in the second half of 13th century CE with the explosive eruption of Forgia Vecchia that culminated in the emission of a bilobate obsidian lava flow. This eruption was shortly followed by the explosive eruptions of Lami and Rocche Rosse, the latter concluded with the emission of the widely renowned obsidian lava flow.By integrating stratigraphy and geochemistry of tephra deposits with a new chronological scheme, our work facilitates the refinement of proximal-to-distal correlation of Lipari's rhyolitic tephra in continental marine environments of the central Mediterranean area in the last 1500 years. A fine-grained, rhyolitic ash found on Stromboli (~40 km NE from Lipari) has an origin from the Monte Pilato and thus, constrains tephra dispersion towards the NE. Very similar ash beds dispersed southwards and interlayered within the near-source deposits of La Fossa, Vulcano island (~10 km from Lipari) exhibit features that are consistent with the younger activities of the Rocche Rosse eruption. A possible link between previously identified rhyolitic ash layers identified in marine cores of the Ionian Sea and the Forgia Vecchia eruption are postulated, although the age and textural characteristics of these distal tephra are not univocal in indicating a correlation to either Monte Pilato or Forgia Vecchia.

Permian-triassic magmatic and thermal events in the Dunhuang orogenic belt: implications for subduction records of the Paleo-Asian Ocean

ABSTRACT The final subduction and suture of the Paleo-Asia Ocean (PAO) are controversial, and the key to this question is to locate the latest subduction/final suture of the accretionary type orogenic belt. Previous studies mainly focused on the Tianshan, Beishan, Alxa, and Solonker Orogens for this issue. As a Phanerozoic orogenic belt located further south of the Beishan-Alxa Tectonic Belts, the Dunhuang Orogenic Belt (DOB) is becoming a research hotspot for revealing the evolutionary history of the southernmost Central Asia Orogenic Belt (CAOB). The youngest subduction-related arc magmatism is pivotal for constraining the lower limit of subduction of a lost ocean. This study presents integrated investigations of field observation, whole-rock geochemistry, and geochronology, targeting diabase dykes, intermediate-acid magmatic rocks, and garnet-bearing metabasite samples in the middle part of the DOB. LA-ICP-MS U-Pb dating of zircon yielded episodic magmatism of ca. 274–273 Ma for Xiaohongshan granite and Dongbatu syenogranite, ca. 250 Ma for Keziletage syenite, ca. 243–239 Ma for diabase and granitic dykes in the Kalatashitage area and Mogutai syenogranite, and maximum deposition age of ca. 262 Ma for pyroclastic rock in the Kalatashitage area. Most of those Permian-Triassic magmatic rocks show continental arc-related geochemical features during the subduction stage. The negative εHf(t) values and ancient TDM2 model ages of zircon indicate they were mainly derived from the partial melting of ancient crustal materials. The hornblende 40Ar/39Ar ages of ca. 258–250 Ma of the garnet-bearing metabasite samples also reflect a high heat flow setting during this period possibly triggered by arc activities. These Permian-Triassic magmatic and thermal records in the DOB are similar to contemporaneous records in surrounding South Tianshan-Alxa-Beishan orogens. We suggest that the final subduction of the PAO in the DOB possibly at least lasted to the Middle Triassic.

PALEOZOIC XENOLITHS IN EOCENE PLUTONS: THE EVIDENCE FOR THE DESTRUCTION OF PRE-JURASSIC CRYSTALLINE BASEMENT BENEATH ADJARA–TRIALETI BELT, LESSER CAUCASUS

In Georgia the Paleogene Adjara–Trialeti riftogenic belt (length 350 km, width 50–2 km) is dominantly composed of trachytic and trachytic–andesitic pyroclastic deposits, though plutonic rocks also play an important role in the structure. In this article, we report new data on the (LA-ICP-MS) U–Pb zircon geochronology and petrochemistry of the plutons, their xenoliths and restite from this belt. The results indicate that the magmatism in the basin began in the Early Eocene (~50 Ma) associated with the formation of pyroclastic rocks. The mafic intrusions (~46–44 Ma) led to the assimilation and contamination of sialic crust and formation of monzo-syenite melts emplaced at ~43–42 Ma. The Eocene monzo-syenite plutons contain xenoliths of Paleozoic granites (312±7 to 474±5 Ma) and tholeiitic basalts that contain inherited zircon grains ranging in age from Neo-Proterozoic (747±33 Ma, 632±29 Ma) to Cambrian (515±9 Ma). Paleozoic granite xenoliths show complete mineralogical and age similarity to the Adjara–Trialeti belt adjacent pre-Jurassic massif granites. Inherited zircon grains most likely are captured by magmas during ascent that cuts through the Gondwana-derived old continental crust. Obtained results and regional geological analysis demonstrate that the riftogenic basin of the Adjara–Trialeti belt developed on the pre-Jurassic crystalline basement, from Late Cretaceous to Eocene into a back-arc extensional regime to post-collision geodynamic setting.

High pressure experimental investigation of clinopyroxene dissolution in a K-basaltic melt

Dissolution of clinopyroxene (cpx) in a K-basaltic melt from the Campi Flegrei Volcanic District (Italy) has been investigated through dissolution and dissolution-crystallization experiments at pressure of 0.8 GPa, superliquidus temperature of 1350 °C, and dwell times between 0.5 and 1 h. The obtained dissolution rates range from 7.9·10−6 cm s−1 to 6.1·10−6 cm s−1 as a function of dwell time. In the dissolution-crystallization experiment (1300 °C; 0.8 GPa; 2 h), the formation of overgrowth rims accompanied by new cpx crystals suggests that the injection of recharge magmas in basaltic reservoir may lead to inverse or oscillatory zonation. The interaction between cpx and K-basaltic melt at ~1570 °C was studied by in situ radiography using synchrotron radiation combined with the Paris-Edinburgh press. This resulted in cpx resorption to occur depending on the temperature conditions with respect to the liquidus temperature of the cpx (TcpxL). The calculated cpx dissolution rates are ~5·10−3 cm s−1 at T ≤ TcpxL and ~ 3·10−2 cm s−1 at T ≥ TcpxL. The role of crystal dissolution in the estimation of magma residence times has been also tested for a natural magmatic system by interpolating the dissolution rates (~10−5–10−6 cm s−1) with the textural data of cpx phenocrysts from the Agnano-Monte Spina pyroclastic deposit at Campi Flegrei caldera (Campanian region, Italy). Results from calculations indicate that the time required for partial or complete resorption of phenocrysts varies from ~0.5 to ~40 h, and that the effect of crystal dissolution may be relevant to estimate magma residence times whether significant dissolution occurs during magma mixing processes.

Trace elements mineral chemistry of sulfides from the Woxi Au‐Sb‐W deposit, southern China

AbstractThe Woxi Au‐Sb‐W deposit is one of the largest polymetallic ore deposits in the Xuefengshan Range, southern China, hosted in low‐grade metamorphosed Neoproterozoic volcaniclastic rocks. The orebodies of the deposit are predominantly composed of banded quartz veins, which are strictly controlled by bedding and faults. Petrographic observations and geochemical results are reported on the occurrence of Au and properties of the ore‐forming processes for different stages in the deposit. The veins extend vertically up to 2 km without obvious vertical metal zoning. The ore‐forming process can be subdivided into four mineralization stages: Pre‐ore stage; Early stage (scheelite‐quartz stage); Middle stage (pyrite‐stibnite‐quartz stage); and Late stage (stibnite‐quartz sage). Four types of pyrite (Py0, Py1, Py2, and Py3) were identified in the ores and host‐rock: Py0 occurs as euhedral grains with voids in the core, ranging in size from 50 to 100 μm and formed mainly in the Pre‐ore stage and Early stage; Py1 occurs as subhedral grains. Small grains (around 10 μm) of Py1 form irregularly shaped clusters of variable size ranging from tens to hundreds of μm and mainly formed in the Middle stage; Euhedral‐subhedral fine‐grained Py2 formed in the Late stage; Minor subhedral fine‐grained Py3 was deposited in the Late‐stage. Stibnite is widely distributed in the Middle and Late stage ore veins. No systemic difference was recognized in mineralogical features among stibnite formed in different stages. In addition to native gold, the lattice bound Au+1 widely exists in Py1 and Py2 in the deposit, and widespread Py1 is considered as the main Au‐bearing mineral with the highest Au contents. Most elements (such as Co, Ni, Cu, As, Sb, Ba, and Pb) are considered to occur as solid solution within the crystal lattice and/or invisible nanoparticles in sulfides minerals. The Co/Ni ratio of most pyrite is lower than 1, suggesting that the metals in the ore‐forming fluid are sourced from sedimentary rocks. The coupled behavior between Au and As; Au and Sb suggests that the substitution of As and Sb in pyrite can enhance the incorporation of Au. Variation of trace elements in pyrites of different stages suggests some information on the mineralization processes: Large ion lithophile elements (such as Ba and Pb) are enriched in Py0 indicating that water‐rock reaction occurred in the Early stage; Fine‐grained Py1 with a heterogeneous distribution of elements suggests fast crystallization of pyrite in the Middle stage.

Mineralogical and physicochemical constraints on the hydrothermalism of volcanic rocks in the Paraná Igneous Province, northwestern Paraná state, Brazil

Hydrothermally affected basaltic flows and volcaniclastic rocks in northwestern Paraná (Brazil) display textural features associated with secondary minerals, reflecting the temperature intensity and evolution of the hydrothermal process. Determination of the secondary mineralogy and the geochemical processes associated with their precipitation are addressed in this paper. The lithotypes that comprise the bulk of the Paraná Igneous Province have millimetric to centimetric vesicles filled by secondary minerals. Field data, petrographic descriptions, mineral characterization and mineral chemistry were used to analyze these secondary mineral phases. Matrix material in volcaniclastic rocks (e.g. shards, glass pisolites) precedes the precipitation of the secondary minerals. Coalescence and necking features are common in vesicles located on top of basaltic flows. Clay minerals identified as vesicle and/or fracture infillings include celadonite, illite/smectite (I/S) interestratifications, as well as montmorillonite and nontronite, with morphologies typical of authigenic volcanic processes such as cauliflower-like and honeycomb. The occurrence of different zeolites species is primarily controlled by CaO and Na2O activity in the hydrothermal fluid. Heulandite is the most pervasive zeolite, followed by stilbite and analcime. Calcite occurs in the center of vesicles, as the last crystallizing phase, or isolated in monomineralic vesicles. Vesicle formation occurred by magma degassing processes at temperatures of up to 1150 °C. However, infilling of these voids occurred later with the percolation of hydrothermal fluids external to the system. Although random from a geographical perspective, gradual decrease in temperature and fluid evolution were responsible for the systematic change in the mineral phases within individual vesicles. The hydrothermal fluid was marked by a Ca–Na signature, high cation/hydrogen ratio and low to intermediate temperatures, ranging between 50 and 200 °C. The controls for precipitation were mainly temperature and the chemical composition of the fluid, with high alkalinity and low H+ activity.