Lanfang He

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active volcanoes have been studied using geophysical methods. However, the internal structure of ancient volcano complexes remains poorly understood. We investigated ancient volcano complexes by comparing magnetotelluric (MT) observations from Zhibo (ZB) ancient volcano with active mid-oceanic ridge volcanoes from Iceland and intracontinental volcanoes from north China. The MT responses of magma chambers in these active volcanoes showed similar low-resistivity values ranging from several to tens of Ω·m, indicating a comparable resistivity of the active magma. Assuming that the ancient active volcano chambers had a similar resistivity to that of current active volcanoes, we reconstructed the ancient Carboniferous volcano complex in ZB using the ratio of the lower portion of the MT responses from ZB ancient volcanic edifices and active volcanoes. The results implied the existence of fossil magma chambers at a depth of 5 to 7 km marking the site of a former volcanic center. This finding supports the magmatic origin of the ZB volcanic rock-hosted iron deposits.

Volcanoes serve as the primary pathways for heat and material transfer from Earth’s interior to its surface, providing valuable insights into subsurface processes. Active and potentially active volcanoes have influenced human history and are closely related to current tectonic activity. Consequently, many active volcanoes have been studied using geophysical methods. However, the internal structure of ancient volcano complexes remains poorly understood. We investigated ancient volcano complexes by comparing magnetotelluric (MT) observations from Zhibo (ZB) ancient volcano with active mid-oceanic ridges volcanoes from Iceland and intracontinental volcanoes from north China. The MT responses of magma chambers in these active volcanoes showed similar low-resistivity values ranging from several to tens of Ω·m, indicating a comparable resistivity of the active magma. Assuming that the ancient active volcano chambers had a similar resistivity to that of current active volcanoes, we reconstructed the ancient Carboniferous volcano complex in ZB using the ratio of the lower portion of the MT responses from ZB ancient volcanic edifices and active volcanoes. The results implied the existence of fossil magma chambers at a depth of 5 km marking the site of a former volcanic center. This finding supports the magmatic origin of the ZB volcanic rock-hosted iron deposits.

蛇绿岩记录了地球特别是大洋演化的历史,是地球科学关注的热点。同时,蛇绿岩也是多种关键金属矿床的赋矿岩体,在铬铁矿、铂族矿产、贵金属找矿中地位突出。围绕蛇绿岩构造环境、地球化学、地质年代学、岩石学研究已取得重大进展,在地质固碳和地质氢新能源研究方面也有突破。但蛇绿岩的内部结构,特别是小尺度的内部结构,至今仍是蛇绿岩研究的薄弱环节。本文介绍了蛇绿岩内部结构研究方法与进展,依托矿床电磁学观测,结合地质、钻探结果,讨论了蛇绿岩的内部结构特征,特别是蛇绿岩内部超基性岩体的内部结构特征。矿床电磁学分析结果表明:从岩石物理分析到电磁法探测都发现西藏罗布莎、东巧蛇绿岩内部发育显著的高低阻电阻率异常分带。CSAMT探测结果表明新疆萨尔托海蛇绿岩内部也有类似的电性结构分带特征。依据罗布莎、东巧、萨尔托海、Semail蛇绿岩的地球物理探测和钻探结果以及乌拉尔一些矿区的剥露与深钻结果,我们提出了蛇绿岩电阻率-蚀变带结构模型。基于这一模型,蛇绿岩中的超基性岩主要发育高阻新鲜-弱蚀变方辉橄榄岩和低阻蚀变-强蚀变纯橄岩-蛇纹岩两个岩相结构带,两个岩相之间的过渡带是铬铁矿富集成矿的有利空间。电阻率-蚀变带模型可用于铬铁矿成矿预测以及蛇绿岩内部地质作用研究。

This special issue of Mesozoic is dedicated to the memory of a great specialist in carbonate sedimentology, Dr Juan Li, who passed away on July 19th, 2023. Juan Li sadly left us while logging the Paleocene-Eocene thermal maximum (PETM) interval in the Tibetan Himalaya. This special issue has collected contributions to Mesozoic–Cenozoic geological studies from the Qinghai-Tibet Plateau from colleagues, friends, and fellow scientists to express our remembrance and regret.

白云鄂博是全球最大的稀土矿, 也是我国著名的铁矿区。稀土白云岩是白云鄂博稀土矿主要赋矿岩体, 大部分铁矿也赋存于其中。然而, 稀土白云岩的岩石电磁学特征至今没有清楚的认识, 影响了电磁法勘探资料解释及稀土白云岩发育规律的认识。本文测量了白云鄂博稀土白云岩、铁矿石样品和部分围岩的岩石复电阻率, 为分析岩矿石复电阻率的分布规律和影响因素, 还测量了样品的磁化率数据。利用手持XRF分析仪对样品进行成分粗分析, 部分样品进行了主微量分析和显微镜下分析。结果显示, 矿区岩石电阻率变化范围跨度大, 而且相同岩性岩石电阻率差异较大。矿区围岩与矿石(铁、稀土)之间有较为明显的磁化率差异、轻稀土元素含量(LREE: La、Ce、Pr、Nd)差异显著。岩石组分与岩石电磁属性的交汇结果显示, 岩石磁化率与岩石Fe₂O₃^T含量正相关。镜下分析发现, 磁铁矿的分布与联结程度影响白云鄂博铁矿石电阻率。白云鄂博稀土白云岩具有中高阻、甚至高阻特征, 岩石中大量发育的高阻矿物是影响其电磁学特征的重要影响因素。岩石电磁学分析为白云鄂博电磁法勘探数据解释和隐伏矿预测提供了重要的岩石物理依据。

Electromagnetic signals in geophysics are frequently disturbed by various interference in field data acquisition. Denoising for passive electromagnetic methods such as magnetotelluric (MT) or audio magnetotelluric (AMT) data is significant to improve data quality and finally imaging to the geoelectrical structure. Conventionally, most denoising methods are employed in frequency domain and few of them are applied in time domain. However, a great number of irregular noise in the electromagnetic time series prove difficulty to be removed. We propose a denoising method, using Encoder-Decoder consisted of Long Short-Term Memory cells (ED-LSTM), to reduce the effect of the step noise and the random-impulsive noise. Supervised learning and transductive learning are used for the denoising of the step noise and the random-impulsive noise, respectively. Our results indicate that the step and random-impulsive noise could be successfully removed from the raw time series. The result indicate that ED-LSTM could potentially to be wildly used in the electromagnetic time series denoising and then improve data quality.

The Bayan Obo deposit in Inner Mongolia is a crucial source of rare earth elements-niobium-iron, with significant economic importance for China. The ore-bearing dolomite is believed to have originated from mantle-derived magma, indicating high prospectivity in the deeper sections. However, the deep distribution of the ore-bearing dolomite remains poorly understood. Here, we develop a 3D S-wave velocity (VS) structure down to 2.5 km depth beneath the Bayan Obo using ambient noise tomography techniques, which used 25 days of continuous waveform data recorded by 312 short-period seismometers. Combined with previous geological, geochemical, and geophysical studies, our results reveal the possible 3D spatial distribution of the ore-bearing dolomite beneath the Bayan Obo. The results indicate that the VS structures beneath the main, east, and west pits change from relatively high-velocity (high-V) zones (with VS of approximately 2.8–3.2 km/s) at shallow depths (depth to approximately 1.5–1.8 km) to significant low-velocity (low-V) zones (with VS < 2.6 km/s) at deeper depths (depth below approximately 1.8 km). We suggest that the content of ore-bearing dolomite may decrease significantly at deeper depths (depth below approximately 1.8 km) beneath these pits, which possibly indicates a diminished prospecting potential there. Our results also indicate a significant high-V zone (with VS > 3.4 km/s) at depths of approximately 1–2.5 km between the main and west pits. Its lithology remains unclear and is proposed to be a key scientific issue in the future to improve our understanding of the ore-controlling structure associated with the tectonic events in the Bayan Obo deposit.

Crustal structure and fluid or melt originating in the deep crust and mantle are critical in regional magmatic mineral systems. However, the crustal structure and the processes that entrain and focus fluids from a deep-source region to a metallogenic belt remain relatively undisclosed. We present a magnetotelluric (MT) study of the eastern Qimantagh Metallogenic Belt (QMB) in the northern Tibetan Plateau. Data from 33 MT stations in two sections and 7 dispersed stations are acquired using a surface electromagnetic prospecting (SEP) system in frequency band ranges from 320 Hz to 0.00034 Hz. Data are converted by Bostick conversion and two-dimensional (2D) nonlinear conjugate gradient inversion. Our MT results reveal the geoelectrical crustal structure of the QMB, which consists of a southern low-resistivity domain that reflects the Kumukuri rift, a high-resistivity middle domain that represents the southern QMB in the central Kunlun belt, and a northern low-resistivity domain that covers the northern QMB and southwestern Qaidam block. We present a comprehensive tectonic and geophysical model of QMB based on the MT interpretation and geological analysis. We infer the high-resistivity domain as a reflection of a rigid crust and detached lithospheric mantle, this belt separate the QMB into northern and southern QMB. Most of the mineral deposits are found in the northern low-resistivity domain of QMB. Our study and findings provide an understanding of the tectonic evolution of the northern Tibetan Plateau, the crustal structure that controls the temporal and spatial distribution of magmatic rocks, and the geological signature associated with mineral deposits.