Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough

Doleritic sill complexes, which are an important component of volcanic continental margins, can be imaged using 3D seismic reflection data. This allows unprecedented access to the complete 3D geometry of the bodies and an opportunity to test classic sill emplacement models. The doleritic sills associated with basaltic volcanism in the North Rockall Trough occur in two forms. Radially symmetrical sill complexes consist of a saucer-like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined, commonly ragged, outer rim. Bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano. With an elongate, concave upwards, trough-like geometry bilaterally symmetrical sills climb away from the magma source from which they originate. Both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge, producing hybrid sill complexes. Radial sill complexes consist of a series of radiating primary flow units. With dimensions up to 3 km, each primary flow unit rises from the inner saucer and is fed by primary magma tube. Primary flow units contain secondary flow units with dimensions up to 2 km, each being fed by a secondary magma tube branching from the primary magma tube. Secondary flow units in turn are composed of 100-m scale tertiary flow units. A similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes, with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex. This branching flow pattern, as well as the interaction between flow units of varying orders, provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger. The data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer, grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening. The branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex, suggesting that the general features of the laccolith emplacement model also apply.Editorial responsibility: J. Stix     

Doppler radar sounding of volcanic eruption dynamics at Mount Etna

Besides their common use in atmospheric studies, Doppler radars are promising tools for the active remote sensing of volcanic eruptions but were little applied to this field. We present the observations made with a mid-power UHF Doppler radar (Voldorad) during a 7-h Strombolian eruption at the SE crater of Mount Etna on 11–12 October 1998. Main characteristics of radar echoes are retrieved from analysis of Doppler spectra recorded in the two range gates on either side of the jet axis. From the geometry of the sounding, the contribution of uprising and falling ejecta to each Doppler spectrum can be discriminated. The temporal evolution of total power backscattered by uprising targets is quite similar to the temporal evolution of the volcanic tremor and closely reproduces the overall evolution of the eruption before, during and after its paroxysm. Moreover, during the sharp decrease of eruptive activity following the paroxysm, detailed analysis of video (from camera recording), radar and seismic measurements reveals that radar and video signals start to decrease simultaneously, approximately 2.5 min after the tremor decline. This delay is interpreted as the ascent time through a magma conduit of large gas slugs from a shallow source roughly estimated at about 500 m beneath the SE crater. Detailed analysis of eruptive processes has been also made with Voldorad operating in a high sampling rate mode. Signature of individual outburst is clearly identified on the half part of Doppler spectra corresponding to rising ejecta: temporal variations of the backscattered power exhibit quasi periodic undulations, whereas the maximum velocity measured on each spectrum displays a sharp peak at the onset of each outburst followed by a slow decay with time. Periodicity of power variations (between 3.8 and 5.5 s) is in agreement with the occurrence of explosions visually observed at the SE vent. Maximum vertical velocities of over 160 m s^–1 were measured during the paraoxysmal stage and the renewed activity. Finally, by using a simplified model simulating the radar echoes characteristics, we show that when Voldorad is operating in high sampling rate mode, the power and maximum velocity variations are directly related to the difference in size and velocity of particles crossing the antenna beam.Editorial responsibility: A. Woods     

Nonlinear Time Series Analysis of Volcanic Tremor Events Recorded at Sangay Volcano,Ecuador

The assumption that volcanic tremor may be generated by deterministic nonlinear source processes is now supported by a number of studies at different volcanoes worldwide that clearly demonstrate the low-dimensional nature of the phenomenon. We applied methods based on the theory of nonlinear dynamics to volcanic tremor events recorded at Sangay volcano, Ecuador in order to obtain more information regarding the physics of their source mechanism. The data were acquired during 21–26 April 1998 and were recorded using a sampling interval of 125 samples s^–1 by two broadband seismometers installed near the active vent of the volcano. In a previous study Johnson and Lees (2000) classified the signals into three groups: (1) short duration (<1 min) impulses generated by degassing explosions at the vent; (2) extended degassing chugging events with a duration 2–5 min containing well-defined integer overtones (1–5 Hz) and variable higher frequency content; (3) extended degassing events that contain significant energy above 5 Hz. We selected 12 events from groups 2 and 3 for our analysis that had a duration of at least 90 s and high signal-to-noise ratios. The phase space, which describes the evolution of the behavior of a nonlinear system, was reconstructed using the delay embedding theorem suggested by Takens. The delay time used for the reconstruction was chosen after examining the first zero crossing of the autocorrelation function and the first minimum of the Average Mutual Information (AMI) of the data. In most cases it was found that both methods yielded a delay time of 14–18 samples (0.112–0.144 s) for group 2 and 5 samples (0.04 s) for group 3 events. The sufficient embedding dimension was estimated using the false nearest neighbors method which had a value of 4 for events in group 2 and was in the range 5–7 for events in group 3. Based on these embedding parameters it was possible to calculate the correlation dimension of the resulting attractor, as well as the average divergence rate of nearby orbits given by the largest Lyapunov exponent. Events in group 2 exhibited lower values of both the correlation dimension (1.8–2.6) and largest Lyapunov exponent (0.013–0.022) in comparison with the events in group 3 where the values of these quantities were in the range 2.4–3.5 and 0.029–0.043, respectively. Theoretically, a nonlinear oscillation described by the equation ++g(x)=fcost can generate deterministic signals with characteristics similar to those observed in groups 2 and 3 as the values of the parameters ,,f, are drifting, causing instability of orbits in the phase space.     

2021年3—5月全球火山活动简报

正1)吉林省地震局,长春1301172)中国地震局火山研究所,长春130117     

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks,Japan, a possible high-pressure dehydration product of antigorite serpentinite

Spinifex-like textured metaperidotites from the Higo Metamorphic Rocks (HMR), west-central Kyushu, Japan, may be formed by high-pressure dehydration of antigorite, and may indicate deep subduction of serpentinite reaching a pressure–temperature condition of 1.6 GPa and 740–750 °C. Three rock types have been identified based on mineral assemblage and rock texture: Type I (L) consisting of medium-grained (1–5 cm long) olivine + enstatite + chromite ±tremolite with secondary talc and anthophyllite that occurs in low-grade metamorphic rocks of the biotite zone, Type I (H) of coarse-grained (up to 10 cm long) olivine + enstatite (with clinoenstatite lamella) + chromite ±tremolite with secondary talc that occurs in high-grade metamorphic rocks of the garnet-cordierite zone, and Type II composed of Al-spinel + chlorite + olivine + apatite + ilmenite with minor sodic gedrite in the garnet-cordierite zone together with Type I (H). Olivines in all rock types are mostly serpentinized during exhumation. The chromite-olivine thermometer gives 560–690 °C for Type I (L) rocks, and the spinel-olivine thermometer gives 610–740 °C for Type II rocks. The peak metamorphic pressure will be higher than 1.6 GPa based on the location of the experimentally determined invariant point (P = 1.6 GPa and T = 670 °C) of antigorite + forsterite + enstatite + talc + H₂O. This estimate is consistent with the occurrence of chlorite in Type II rocks, which is stable up to 890 °C at 2.0 GPa. The spinifex-like textured metaperidotites occur as small bodies in the low P/T type gneisses, implying tectonic juxtaposition of them probably during exhumation of the HMR. Recent findings of medium pressure (0.9–1.2 GPa) granulites and gneisses from the HMR may indicate that the HMR has a deep root into the wedge mantle from which the spinifex-like textured metaperidotites have derived.     

西藏罗布真矿区林子宗群火山岩锆石U-Pb年龄、地球化学特征及其地质意义

罗布真铅锌多金属矿床位于冈底斯火山-岩浆活动带的中南部,矿区内广泛发育林子宗群火山岩,岩性为英安岩和流纹质晶屑凝灰岩。LA-ICP-MS锆石U-Pb定年结果显示,罗布真矿区内林子宗群英安岩的成岩年龄为(49.14±0.86) Ma,应属于帕那组火山岩地层,并非前人所认为的典中组。岩石具有高K、低Ti的特征,属于高钾钙碱系列;相对富集Rb、Th、U、K、Zr、Hf等元素,亏损Nb、Ta、P、Ti、Sr等元素,且轻重稀土分馏明显,具有明显的负铕(δEu=0.50~0.65)异常。在印度-亚洲大陆汇聚的过程中,新特提斯洋向欧亚大陆俯冲,约50 Ma发生了大规模的火山、岩浆作用,同时岩浆-热液成矿作用也进入高峰期,林子宗群火山岩的形成时代与该时代一致,形成于陆陆碰撞形成的板内环境。罗布真矿区林子宗群火山岩年龄的厘定,揭示了冈底斯带林子宗群帕那组火山岩具有赋矿潜力,对认识该地区林子宗群火山岩与热液脉型铅锌多金属矿化的关系具有指导意义。     

春风油田石炭系火山岩油气层综合判识研究

火山岩油气藏是一种特殊类型油气藏,受岩性、岩相、储集空间类型等因素影响较大。常规测井仅能对储层进行评价,录井仅能对岩性及油气显示进行评价。利用测录井资料无法快速判识油气层现状,制约了现场完井方式及试油方案的及时决策,并影响钻井周期、成本及对区块的潜力评价。通过岩心描述、镜下薄片储集空间及含油性观察,结合测井解释和试油结果,总结了春风油田石炭系火山岩油藏岩石类型及含油性特征;火山岩储集空间类型、影响因素、识别方法及物性有效性;孔隙、裂缝等储集空间有效性电性识别方法;建立了春风油田石炭系油气层综合判识准则。本次研究可快速对该区石炭系完钻探井进行油气层综合判识,指导现场完井方式决策,提高勘探效益。     

冈底斯中段厅宫地区林子宗火山岩锆石U-Pb年龄和地球化学特征

林子宗火山岩事件是印度-亚洲大陆主碰撞过程的火山作用响应,对冈底斯带的构造演化具有重要意义。在冈底斯中段厅宫地区通过系统的LA-ICP-MS锆石U-Pb同位素测年,获得林子宗群典中组底部、年波组上部和帕那组顶部的火山岩锆石U-Pb年龄分别为(66.4±0.35)Ma、(53.6±1.5)Ma和(45±0.6)Ma。对林子宗火山岩的岩石地球化学分析表明,该火山岩具弧火山岩特征,早期表现出陆缘弧火山岩的特点,至中期年波旋回钾玄岩出现,标志着陆内岩浆作用的开始,到晚期帕那旋回则显示出陆壳重熔和地壳加厚的特征,记录了新特提斯洋消减俯冲到陆陆碰撞的地质信息。     

Paleoproterozoic volcanic rocks in the southern margin of the North China Craton,central China:Implications for the Columbia supercontinent

The volcanic rocks of the Xiong'er Group are situated in the southern margin of the North China Craton(NCC).Research on the Xiong er Group is important to understand the tectonic evolution of the NCC and the Columbia supercontinent during the Paleoproterozoic.In this study,to constrain the age of the Xiong'er volcanic rocks and identify its tectonic environment,we report zircon LA-ICP-MS data with Hf isotope,whole-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopes of the volcanic rocks of the Xiong'er Group.The Xiong'er volcanic rocks mainly consist of basaltic andesite,andesite.dacite and rhyolite,with minor basalt.Our new sets of data combined with those from previous studies indicate that Xiong'er volcanism should have lasted from 1827 Ma to 1746 Ma as the major phase of the volcanism.These volcanics have extremely low MgO.Cr and Ni contents,are enriched in LREEs and LILEs but depleted in HFSEs(Nb,Ta,and Ti),similar to arc-related volcanic rocks.They are characterized by negative zircon ε_(Hf)_(t) values of-17.4 to 8.8,whole-rock initial ~(87)Sr/~(86)Sr values of 0.7023 to 0.7177 andε_(Nd)(t) values of-10.9 to 6.4.and Pb isotopes(~(206)Pb/~(204)Pb =14.366-16.431,~(207)Pb/~(204)Pb =15.106-15.371,~(208)Pb/~(204)Pb= 32.455-37.422).The available elemental and Sr-Nd-Pb-Hf isotope data suggest that the Xiong'er volcanic rocks were sourced from a mantle contaminated by continental crust.The volcanic rocks of the Xiong'er Group might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by oceanic subduction in the Archean.Thus,we suggest that the subduction-modified lithospheric mantle occurred in an extensional setting during the breakup of the Columbia supercontinent in the Late Paleoproterozoic,rather than in an arc setting.     

华北地台本溪组,太原组火山事件沉积特征及时空分布规律

作者对华北地台24个煤田及煤矿区进行了详细的研究,在本溪组及太原组中共发现6期12次火山事件层。这些火山事件沉积遍及全区,主要为凝灰岩、沉凝灰岩及凝灰质沉积岩,它们在时空分布上有一定规律。火山碎屑可能来自华北地台以北及以南若干不同的火山源,而岩浆可能来自下地壳。