Seismological and SAR signature of unrest at Nisyros caldera, Greece

Nisyros island, a Quaternary volcanic center located at the SE of the Aegean Volcanic Arc, has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions, the last one being in 1887. The recent long lasting episode of unrest (1995–1998) in the area is the first instrumentally documented providing information on the behavior of the volcano. Evidence from seismicity and SAR interferometry suggests that the presently active part of the Kos–Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island defining an area much smaller than the whole volcano-tectonic area. Seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from SAR interferometry. These observations help us to discuss the different elements controlling the behavior of the volcanic system such as: the existence, location and timing of magma chamber inflation, the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion, the seismic failure and migration of hypocenters indicating shallow magma transport.     

The Cretaceous Messum igneous complex, S.W. Etendeka, Namibia: reinterpretation in terms of a downsag-cauldron subsidence model

The Messum igneous complex (MIC) lies within the ENE-trending zone of Lower Cretaceous (132 Ma) Damaraland intrusive complexes in Namibia, intruded into Pan-African Damara basement. It is defined by a roughly circular structure 18 km in diameter, the bounding ring fault exposed along the eastern sector. Encircling Messum are the volcanic sequences of the Goboboseb Mountains, comprising a lower basalt series (Tafelkop and Tafelberg types) followed, with intervening basalts, by four voluminous quartz latite (QL) eruptions (Goboboseb and Springbok QL units).Inferred stages of development are: (a) an initial very broad basaltic lava shield, comprising the Tafelberg and Tafelkop basalts, and Messum crater basalts (MCB; possibly ponded in near-vent lava lakes). Embedded within the lower basaltic sequence is a localised rhyolite-dominated eruptive centre (ca. 5 km in diameter), interpreted as a funnel caldera located towards the centre of the MIC. (b) Downsagging, extending northwards from Messum, following the Goboboseb QL eruptions (≥2300 km³). Ponding of overlying basaltic units. (c) Climactic Springbok QL eruption (≥6300 km³) producing further downsag together with the inward radial dip of all volcanic units towards the MIC. Ring fault initiation. (d) Cauldron subsidence emplacement of a granitoid suite, forming the MIC ‘moat’ (area between the ring fault and the core region). (e) Intrusion of gabbroic cone sheets into incompletely solidified granitic melts within the southeastern moat. Resulting hybridisation and magma mingling produced extensive development of heterogeneous granitoid and hybrid dioritic lithologies. (f) Cone sheet intrusions of the eastern gabbros into more highly solidified granitoids of the southeastern moat. (g) Intrusion of thick (1–2 km) western gabbro cone sheets, exhibiting local fine-scale layering, into solidified granitoids, mainly within the western moat. Minor late-stage granitic intrusions. (h) 2–3 Ma quiescent period followed by quartz- and ne-syenite intrusions, and finally basanite dykes, emplaced within the MIC core. Accompanying differential uplift of the core.Uplift/resurgence within the MIC has accompanied intrusion of the moat granitoids and mafic cone sheets, thereby juxtaposing volcanic and intrusive sequences. Phases of both subsidence and uplift have characterised the MIC. The NW Scotland Tertiary central igneous complexes and Messum show evidence of a number of parallel developments, but also important differences. The MIC differs markedly from caldera systems within the western USA and circum-Pacific. Messum is therefore suggested to represent a distinct class of intrusive/extrusive central complex, although probably common in large igneous provinces.     

福建戴云山巨型环状体的成因探讨

通过对戴云山巨型环状体西南端上涌地区4幅1：5万区域地质调查，结合区域地质资料综合研究，从火山产物及火山构造空间分布格局分析，初步认为它是以石牛山地区为中心的大型破火山组合群体，其中包括不同时期，不同级别，不同类型的火山构造，是大型破火山经过多期，多旋回，继承性和迁移喷发，同一时期，旋回火山构造多呈同心环状（卫星式）分布，不同时期，旋回火山构造往中心迁移叠置或继承套叠构成巨型环状火山构造面貌。     

天姆尖破火山口地球物理特征

本系统论述了天姆尖地区地层、岩石的重、磁参数特征和该区重磁异常的分布特征，并利用这些特征对天姆尖地区地质构造、隐伏（半隐伏）岩体、深部构造和火山构造进行了推断和解释，得出了该区具有寻找富大火山岩型铀矿远景的结论。     

浙江省庆元破火山地球物理地球化学特征及地质意义

庆元破火山以中心式仙桃山岩体为中心,高、中、低温成矿元素异常依次由内向外分布。它们各自拥有不同的空间展布,矿物组成,结构构造,围岩蚀变和伴生元素特征。通过地球物理与地球化学有机结合,深入探讨破火山的空间分布特征,可以开拓寻找不同类型的矿产资源及矿产资源远景评价的思路。     

Magma ascent and eruptive processes from textural and compositional features of Monte Nuovo pyroclastic products, Campi Flegrei, Italy

Geochemical and textural studies were carried out on alkaline products of the AD 1538 Monte Nuovo eruption. Due to the integration of the volcanological study with eyewitness reports, the dynamics and timing of each phase of the eruption and the volume of emitted magmas are known in detail. On this basis, unique in Campi Flegrei, the relations between magma chamber mechanisms, eruptive styles, magma ascent dynamics and volatile exsolution processes have been explored. Glass and phenocryst compositions indicate that the erupted magma has a homogeneous phono-trachytic composition. Textures and compositions of phenocrysts indicate that they crystallised at equilibrium with the melt in the magma chamber, likely as a mushy boundary layer along the chamber wall, where the temperature was below the liquidus temperature of the crystal free-chamber core. The estimated crystallisation temperature is 850±40°C. The magma phase relations in Petrogenys Residua System suggest that phenocryst crystallisation occurred at between 100 and 200 MPa, corresponding to depths ranging from 3 to 8 km. The microlite composition and their close genetic relations with vesicles indicate that groundmass crystallisation occurred during the eruption as a consequence of magma degassing and vesiculation induced by decompression during its ascent toward the surface. Crystal size distributions reveal that microlites grew in two stages of undercooling that we define as: (1) magma migration onset upward from the chamber and (2) magma rising through the conduit to the surface, possibly lasting tens of days and few days, respectively. These results provide information on the physical conditions that characterise pre- and syn-eruptive processes, which may be useful in order to define eruptive scenarios and to evaluate short-term precursors. Furthermore, the collected data provide for the first time information on degassing-induced crystallisation during the eruption of a highly evolved alkaline magma.Editorial responsibility: M. Carroll     

雁荡山破火山口构造特征及成岩物质来源

雁荡山破火山口构造以其独特的火山地貌形态，完美的环状断裂和环状圈层分布的岩相，环形展市的航磁异常和地球化学异常，显示出一个地质─地球物理、地球化学、岩相─构造配置较完整的典型破火山口模式，依壳、幔二元混合模拟计算．火山岩和侵入岩源区物质组成为UC56.2DM43．8和UC56．4DM43．6，两者来自相似源区．源区处于地壳下部。     

Geochemistry of caldera-forming and resurgent magmas in the Pliocene Hiwada caldera,northeast Japan: An example of a monotonous intermediate system

The 2.9-Ma Hotokezawa Ignimbrite, which was ejected from the Aizu caldera cluster in the northeast Japan arc, is a typical monotonous intermediate ignimbrite, with 40–50 vol% crystals and an eruptive volume of >140 km³ dense-rock equivalent. This ignimbrite filled Hiwada caldera and was deformed by post-caldera plutonic intrusions that formed a resurgent dome. The Hotokezawa Ignimbrite is a calc-alkaline, medium-K dacite to rhyolite with SiO₂ contents of 67.9–71.3 wt%, and has homogeneous trace-element abundances and Sr–Nd isotopic ratios. These geochemical features suggest that the Hotokezawa magma was formed by partial melting of amphibolitic crustal rocks. This crystal-rich magma did not appear during the post-caldera stage. Therefore, it is plausible that the chamber of eruptible magma was emptied by the caldera-forming eruption. In contrast, post-caldera plutonic rocks exhibit a variety of compositions and have a clear SiO₂ gap corresponding to the caldera-forming magma: the early pluton (tonalite) and later ones (quartz porphyry, granite porphyries, and granite) contain 62.0–66.6 and 71.2–76.5 wt% SiO₂, respectively. The tonalite and the Hotokezawa Ignimbrite form a continuous trend in their major-element variations. The Sr–Nd isotopic ratios of the ignimbrite and tonalite overlap, but those of the porphyries and granite are more enriched. The early tonalite represents the more basic part of the Hiwada caldera system that was held in small pockets separate from the main magma chamber, because its trace-element abundances are varied and distinct from those of the Hotokezawa Ignimbrite. The distinct compositional change from the Hotokezawa Ignimbrite to the late porphyries and granite indicates that the partial melting crust generating felsic magma was renewed by the subsequent intrusion of the mantle melts. The new felsic magma ascended through subsidence-related faults into the shallow caldera system and emplaced as laccoliths forming the resurgent dome.     

Submarine Hydrothermal Mineralization on the Izu–Bonin Arc,South of Japan: An Overview

Abstract

Considerable effort has been expended in studying the Izu–Bonin Arc over the past 15 years. In particular, 43 dives of the Shinkai 2000 have been undertaken there to discover and evaluate the extent of submarine hydrothermal activity and mineralization. Most effort has been focused on Myojin Knoll (23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives).

The Izu–Bonin Arc is divided in two by the Sofugan Tectonic Line. Eight submarine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the arc is quite different from that of the southern sector. Volcanic rocks from the northern sector are more acidic than those from the southern sector.

Evidence for submarine hydrothermal mineralization has been observed at four seamounts along the Izu–Bonin Arc (Myojin Knoll, Myojinsho, Suiyo Seamount, and Kaikata Caldera), and submarine hydrothermal activity is evident at another three seamounts along the arc (Kurose Hole, Mokuyo Seamount, and Doyo Seamount).

The most extensive submarine hydrothermal mineral deposit so far located on the Izu–Bonin Arc is the Sunrise deposit at Myojin Knoll. This deposit, at least 400 m in diameter and 30 m high, is associated with black smoker venting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hydrothermal vents there was 278°C. Some of the sulfide chimneys contained as much as 49 μg/g Au and 3,400 μg/g Ag. The sunrise deposit is one of the largest submarine volcanic massive sulfide deposits so far discovered in midocean ridge, backarc, or arc settings and has an estimated mass of 9 × 10⁶ t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide deposits may be found in other caldera along the Izu–Bonin Arc.

The geological variability along the arc, the high seismicity, the occurrence of active volcanism and submarine hydrothermal venting, and a proven submarine hydrothermal mineral potential coupled with the proximity of the region to Japan suggest that the Izu–Bonin Arc could profitably serve as a natural laboratory for the long-term monitoring of the seafloor.     

New 40Ar/39Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

Since the 1960s, an early explosive activity in the Mont-Dore Massif is associated with a major pyroclastic rhyolitic eruption (5–7 km³) known as the “Grande Nappe” (GN). This event, linked to the formation of a 6-km-diameter cryptic caldera named “Haute Dordogne”, was before our investigation dated by ⁴⁰Ar/³⁹Ar at 3.07 ± 0.04 Ma. Our new single-crystal laser fusion ⁴⁰Ar/³⁹Ar dates obtained on two outcrops of the GN (Rochefort-Montagne and Ludières) questioned several hypotheses made concerning this “landmark” event of the Mont-Dore Massif history. We demonstrate that: (1) the GN rhyolitic eruption has occurred much later than previously estimated (i.e. 2.77 ± 0.02–0.07 Ma full external uncertainties); (2) the correlation made between the Vendeix rhyolitic complexes (intra-caldera position) dated back to 2.74 ± 0.04 Ma and the GN is proposed; (3) xenocryst contamination could be very high (i.e. 70% for the Rochefort-Montagne GN outcrop) and explains the noticeable older age obtained previously; (4) a link between the GN eruption and the formation of a caldera is questionable; the hypothesis of a northward-oriented blast channeled eastward toward the paleo-Allier River is thus proposed.