Early Yanshanian post-orogenic granitoids in the Nanling region Petrological constraints and geodynamic settings

Early Yanshanian magmatic suites predominate absolutely in the Nanling granite belt. They consist mainly of monzogranite and K-feldspar granite. There occur associations of early Yanshanian A-type granitoids (176 Ma-178 Ma) and bimodal volcanic rocks (158 Ma-179 Ma) in southern Jiangxi and southwestern Fujian in the eastern sector of the granite belt and early Yanshanian basalts (177 Ma-178 Ma) in southern Hunan in the central sector of the belt. Both the acid end-member rhyolite in the bimodal volcanic rock association and A-type granitoids in southern Jiangxi have the geochemical characteristics of intraplate granitic rocks and the basic end-member basalt of the association is intraplate tholeiite, while the basaltic rocks in southern Hunan include not only intraplate tholeiite but also intraplate alkali basalt. Therefore the early Yanshanian magmatic suites in the Nanling region are undoubtedly typical post-orogenic rock associations. Post-orogenic suites mark the end of a post-collision or late orogenic event and the initiation of Pangaea break-up, indicating that a new orogenic Wilson cycle is about to start. Therefore it may be considered that the early Yanshanian geodynamic settings in the Nanling region should be related to post-orogenic continental break-up after the Indosinian orogeny and the break-up did not begin in the Cretaceous.     

Petrogenesis and significance of early Yanshanian syenite-granite complex in eastern Nanling Range

According to the statistics for granitoid distribution map of 1/2000000 Nanling region[1], the granitoids in the Nanling region sum up an area of more than 170000 km2, occupying about one fifth of the entire Nanling region. Granitoid rocks in the Nanlingregion are mainly composed of monzogranites (occu- pying more than 84%), granodiorites (about 11%) and syenogranites (about 3%)[2]. There also exists a small amount of syenites (0.12%) with a sum area of about 94 km2[2]. However, nearly half …     

Strongly peraluminous granites of Mesozoic in Eastern Nanling Range,southern China: Petrogenesis and implications for tectonics

The strongly peraluminous granites (SPGs) of Eastern Nanling Range (ENR) are a characteristic of all bearing highly aluminous minerals, such as muscovite±AI-rich biotite±tourmaline±garnet, and lack of cordierite. In respect of petrography, geochemistry, Nd isotope, and single grain zircon U-Pb dating, the representative granite bodies of them are studied. The research shows that these granites were emplaced in two stages, namely 228-225 Ma BP and J2-3 159-156 Ma BP, belonging to Indosinian and early Yanshanian periods, respectively, and they have low εNd(t) values (-10.6--11.1), high A/CNK, Rb/Sr ratios and tDM values (1887-1817 Ma), and REE's tetrad effect (TE1,3=1.13-1.34). In comparison with related geology, petrology and chronology of granites in adjacent regions, it is suggested that Indosinian SPGs of ENR formed in the circumstance of post-collisional extension 20 Ma after the major collision of Indosinian Movement (258-243 Ma BP) in Indo-China Peninsula, and early Yanshanian SPGs formed in the     

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineralization in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. εNd(t) = -11.92-1.96, (143Nd/144Nd)i = 0.5120-0.5125, TDM = 0.70-1.71 Ga,(87Sr/86Sr)i = 0.7043-0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geochemically similar to adakite, which might have been stemmed from the partial melting of thickened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2-4.0 GPa) and high temperature (850-1150℃) surroundings of the lower crust favor both the fluid and adakite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contributes to the Cu-Au mineralization.     

Multiple-aged granitoids and related tungsten-tin mineralization in the Nanling Range, South China

The Nanling metallogenic belt in South China is characterized by well-developed tungsten-tin mineralization related to multiple-aged granitoids. This belt is one of the 5 key prospecting and exploration areas among the 19 important metallogenic targets in China. Important progress has been made in recent years in understanding the Nanling granitoids and associated mineralization, and this paper introduces the latest major findings as follows: (1) there exists a series of Caledonian, Indosinian, and Yanshanian W-Sn-bearing granites; (2) the Sn-bearing Yanshanian granites in the Nanling Range form an NE-SW trending aluminous A-type granite belt that stretches over 350 km. The granites typically belong to the magnetite series, and dioritic micro-granular enclaves with mingling features are very common; (3) the Early Yanshanian Sn- and W-bearing granites possess different petrological and geochemical features to each other: most Sn-bearing granites are metaluminous to weakly peraluminous biotite (hornblende) granites, with zircon ?_Hf(t) values of ca. ?2 to ?8, whereas most W-bearing granites are peraluminous two-mica granites or muscovite granites with ?_Hf(t) values of ca. ?8 to ?12; (4) based on the petrology and geochemistry of the W-Sn-bearing granites, mineralogical studies have shown that common minerals such as titanite, magnetite, and biotite may be used as indicators for discriminating the mineralizing potential of the Sn-bearing granites. Similarly, W-bearing minerals such as wolframite may indicate the mineralizing potential of the W-bearing granites. Future studies should be focused on examining the internal relationships between the multiple-aged granites in composite bodies, the metallogenic peculiarities of multiple-aged W-Sn-bearing granites, the links between melt evolution and highly evolved ore-bearing felsic dykes, and the connections between granite domes and mineralization.     

The Early Paleozoic Tiefosi syn-collisional granite in the northern Dabie Orogen:Geochronological and geochemical constraints

The Tiefosi granitic pluton is located 5 km northwest of Xinyang City,northern Dabie Orogen,and was emplaced in the Proterozoic Qinling Group. SHRIMP zircon U-Pb dating suggests its crystallization at 436 ± 11 Ma. It is composed of monzogranite and syenogranite containing some amounts of muscovite and few mafic minerals. The rocks are characterized by high and restricted SiO2 content,low FeO,Fe2O3 and MgO contents,high K2O/Na2O ratio,and display high-K calc-alkaline and peraluminous (ACNK>1.1) characteristics. They are generally enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE). They can be divided into three groups in light of rare earth elements (REE) and trace elements. Group I is moderate in ΣREE and characterized by the absence of Eu anom-aly,high (La/Yb)N ratio,and moderate Rb/Sr and Rb/Ba ratios. Group Ⅱ has moderately negative Eu anomaly,low (La/Yb)N ratio and high ΣREE contents,Rb/Sr and Rb/Ba ratios. Group Ⅲ displays positive Eu anomaly,moderate (La/Yb)N ratio,and low ΣREE,Rb/Sr and Rb/Ba ratios. The calculated εNd(440Ma) values of the rocks vary from 8.8 to 9.9 and Nd depleted mantle model ages are about 2.0 Ga,which resemble those of the paragneisses from the Qinling Group. The results indicate that the Tiefosi granite is crust-derived,syn-collisional S-type granite. Generation of Group I was related to low degree melting of the Qinling Group,while Group Ⅱ was formed by fractionational crystallization of plagioclase from Group I magmas,and Group Ⅲ resulted possibly from magma mingling with plagioclase cumulates. The Tiefosi granite was formed within crustal level related to the collision between the North China and South China blocks in the Early Paleozoic time.     

Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to~19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.     

渤海湾盆地区燕山期构造特征与原型盆地

结合近几年来渤海湾盆地区深层地震勘探与解释的成果，重点论述了渤海湾盆地区燕山期构造特征与盆地原型，提出燕山期构造变形样式总体在纵向上可分为三个构造层，分别称为上部、中部和下部；横向上总体可分为三个构造带，西部为向西逆冲的薄皮逆冲带，中部为冲断—走滑带，东部为厚皮褶皱—冲断带，主体由两期挤压方向皆为NW—SE向的褶皱—逆冲变形形成；并将其演化分为三个阶段：燕山早期、燕山中期和燕山晚期。但是，渤海湾盆地区燕山期的构造变形特征和原型盆地有所变化，其空间上的差异是基底构造格局及其空间差异叠合的结果。综合其它研究结果还表明，渤海湾盆地区燕山期构造是在西太平洋大陆边缘弧的挤压构造背景下，陆内壳下拆沉和壳内挤出逃逸构造的综合动力作用下形成的。     

南岭东段钨锡多金属矿床地质特征、成矿模式及找矿方向

南岭东段地区是世界著名的钨多金属矿集区。近年来中国地质调查局在该区实施钨多金属矿调查评价,找到了一批具有中大型远景的与花岗岩有关的新类型钨锡多金属矿床,特别是在复式岩体中的花岗岩型新类型钨钼多金属矿床（简称＂体中体＂式钨钼多金属矿床）和受中、上泥盆统地层控制的层控矽卡岩型钨锡多金属矿床的发现,不仅实现了南岭东段地区找矿新突破,而且对整个南岭乃至我国华南地区钨锡多金属矿找矿具有重要指导作用。本文根据近年来南岭东段地区钨锡多金属矿找矿新成果,通过该区钨锡多金属矿床地质特征的总结,建立了南岭东段地区钨锡多金属矿成矿模式,提出了南岭东段地区今后钨锡多金属矿找矿方向应重点注意与燕山期复式花岗岩有关的＂体中体＂式钨钼多金属矿床和中、上泥盆统控制的层控矽卡岩型钨锡多金属矿床的寻找。     

Petrological, geochemical and chronological constraints for the tectonic setting of the Dongco ophiolite in Tibet

The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.     

Silurian A-type granitoids in the southern margin of the Tongbai-Dabieshan: Evidence from SHRIMP zircon geochronology and geochemistry

The Dabieshan Orogenic belt is well known for the exhumation of early Mesozoic ultrahigh-pressure (UHP) metamorphic rocks and Jurassic–Cretaceous emplacement of voluminous granitoids. However, the tectonic evolution in the orogen during the Paleozoic, especially its magmatic response to tectonism has not received much attention. As indicated by published data, the Dabieshan orogenic belt contains different records of Paleozoic magmatic-tectonic association in different tectonic units. Occ…     

Geological,tomographic, kinematic and geodynamic constraints on the dynamics of sinking slabs

We use geodynamic models with imposed plate velocities to test the forward-modeled history of subduction based on a particular plate motion model against alternative seismic tomography models. We utilize three alternative published reference frames: a hybrid moving hotspot-palaeomagnetic, a hybrid moving hotspot-true polar wander corrected-palaeomagnetic, and a Subduction Reference Frame, a plate model including longitudinal shifts of subduction zones by matching subduction volumes imaged by P-wave tomography, to assess which model best predicts present day mantle structure compared with seismic tomography and volumetrically derived subduction history. Geodynamic modeling suggests paleo-longitudinal corrections applied to the Subduction Reference Frame result in lower mantle slab material beneath North America and East Asia accumulating up to 10–15° westward of that imaged by tomography, whereas the hybrid models develop material offset by 2–9°. However, the Subduction Reference Frame geodynamic model produces slab material beneath the Tethyan Domain coinciding with slab volumes imaged by tomography, whereas the hybrid reference frame models do not, suggesting regional paleo-longitudinal corrections are required to constrain slab locations. We use our models to test inferred slab sinking rates in the mantle focusing on well-constrained regions. We derive a globally averaged slab-sinking rate of 13 ± 3 mm/yr by combining the ages of onset and cessation of subduction from geological data and kinematic reconstructions with images of subducted slabs in the mantle. Our global average slab-sinking rate overlaps with the 15–20 mm/yr rate implied by mantle convection models using a lower mantle viscosity 100 times higher than the upper mantle.     

Role of mantle-derived magma in genesis of early Yanshanian granites in the Nanling Range,South China: <Emphasis Type="Italic">in situ</Emphasis> zircon Hf-O isotopic constraints

Although a number of petrographic observations and isotopic data suggest that magma mixing is common in genesis of many granite plutons, it is still controversial whether the mantle-derived magmas were involved in granites. We carried out in this study a systematic analysis of in situ zircon Hf-O isotopes for three early Yanshanian intrusions dated at ca. 160 Ma from the Nanling Range of Southeast China. The Qinghu monzonite has very homogeneous zircon Hf-O isotopic compositions, εHf(t) =11.6±0.3 and δ18O=5...     

Geochemical characteristics of zircons in the Ashizuri A‐type granitoids: An additional granite topology tool for detrital zircon studies

Zircon is resistant to alteration over a wide range of geological environments, and isotopic ratios within the mineral provide constraints on ages and their parental magmas. Trace element compositions in zircon are also expected to reflect those of their parent magmas, and have a potential as essential indicators for their host rocks. Because most detrital zircons that accumulate at river mouths are derived primarily from granitoids, the classification of zircon within granitoids is potentially meaningful. This study employs the conventional classification scheme of granites (I‐, S‐, M‐, and A‐types). To clarify geochemical characteristics of zircons in A‐type granites, trace element compositions of zircons extracted from the A‐type Ashizuri granitoids were examined. Zircons from the Ashizuri granitoids commonly show enrichments of heavy rare earth elements and positive Ce anomalies, indicating that these zircons were igneous in origin. In addition, zircons in these A‐type granites are characterized by enrichments of Nb, Y, Ta, Th, and U and strong negative Eu anomalies, which exhibit good positive correlations with those in their whole rocks. This fact indicates that these signatures in zircons reflect well those in their parental bodies and are useful in identifying zircons derived from A‐type granite. Based on compilations of available data, zircons from A‐type granites can be clearly discriminated from other‐types of granites within Nb/Sr–Eu anomaly, U/Sr–Eu anomaly, Nb/Sr–U/Sr, and Nb/Sr–Ta/Sr cross‐plots. All indices used in these diagrams were selected based on the geochemical features of both zircon and whole rock of A‐type granites. Application of these discrimination diagrams to detrital zircons will likely provide further insights. For example, some Hadean detrital zircons plot in similar fields to A‐type granites, implying the existence of A‐type magmatism in the Earth's earliest history.     

现代地震预警系统中的时间延迟效应分析

地震预警系统(EEW)中时间延迟制约着地震预警的时效性,缩短地震预警延迟时间能显著提高地震预报效能和应急反应能力。通过分析地震预警系统的延迟因素,分解从地震发生到预警信息发布过程的各个环节,计算出每个环节的时间延迟值,并对原地、异地和混合地震预警模式的地震预警水平、预警盲区半径进行对比,分析三种地震预警模式的时间延迟效应。结果显示,预警系统的时间延迟主要包含数据传输和数据处理两方面,实验室理论最少延迟时间为Δt=3.9s;主流预警系统平均延迟时间为Δt=14.3s;最先进的预警系统延迟只有Δt=8.7s。时间延迟同样影响着浅源地震(0~60km)的预警盲区半径,二者呈正相关性,当Δt=8.7s时,盲区半径最高达52.2km;当Δt=3.9s时,盲区半径最大只有23.4km。分析认为,有效缩短地震预警系统中的延迟时间,不仅能提高地震预警的时效性,同时能降低预警的盲区范围,对整个监测预报、预警和防灾减灾事业也有积极的推动作用。     

Geochemical and geochronological constraints on the origin and emplacement of the Shimo-ondori diorites in Shikoku,Southwest Japan

In this study, the LA-ICP-MS zircon U–Pb dating of the Shimo-ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo-ondori diorites are suggesstive that they could be influential for the estimate of the early-Cretaceous tectonic evolution for the eastern Asian margin. Their whole-rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO₂ and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (ε_Hf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge-mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo-ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo-ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.     

2003年民乐-山丹6.1级地震震情预警检验研究

2003年民乐—山丹6.1级地震前,甘肃省地震局于9月20日提出了祁连山中东段地区6级左右地震短期预报意见,并采取了多种行之有效的短临跟踪措施,得到了中国地震局、甘肃省委、省政府的高度评价和表彰奖励。根据本次成功震例预报实践中积累的不同阶段预报指标体系,利用层次分析法,确定了祁连山地震带震情指标预警模型。依据预警模型及震情预警指标分层结果,将各类定量和定性指标转化为无量纲的以1～4表示的指标值,按成对比较法比较下层各指标两两之间对上层某指标的重要程度,计算权重向量,并对模型的整体层次进行一致性检验。研究结果表明,该地震前祁连山地震带震情预警值为3.1389,依据预警等级判定标准,震情预警级别为红色。     

早期地震预警方法研究现状及展望

地震是破坏性最大的自然灾害之一,它具有突发性、毁灭性、次灾严重、无灾有害等特点,会给人们的生命财产造成巨大的损失。为了达到减轻地震灾害的目的,除了加强工程结构抗震设计外,人们最先想到的就是地震预报。如果能预知地震的发生时间与地点而先将人员撤离地震区、关闭次生灾害源,无疑会将地震灾害降到最低。但地震预报这个世界性的科学难题在短期内很难取得突破性进展。为此,许多国家进而投资发展地震预警系统和地震应急控制系统。本文对国内外地震预警方法研究现状进行了阐述,并对地震预警的发展趋势进行了展望。     

Early Paleozoic ridge subduction in the Chinese Altai: Insight from the abrupt change in zircon Hf isotopic compositions

Zircons were separated from granitoids, gneisses, and sedimentary rocks of the Chinese Altai. Those with igneous characteristics yielded U-Pb ages of 280-2800 Ma, recording a long history of magmatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating that prior to that time the magmas came from both ancient and juvenile sources, whereas younger magmas were derived mainly from juvenile material. This may imply that the lithosphere was signifi- cantly modified...     

中国地震预警延迟时间算法及其对预警盲区的影响研究

通过分析预警系统的预警步骤来探讨影响获得预警时间的因素,并与瑞士地震台网预警延迟各步骤进行对比,得出中国地震预警系统仪器延迟时间为7.6s。提出地震波到台站所需时间的算法,较震中假设在台站中间的传统算法更为准确,在此算法下采用双台法和四台法计算接收地震波所需时间,得到中国地震预警延迟时间。讨论双台法和四台法在不同震源深度和台站密度下对预警盲区大小的影响,在台间距小于20km时,2种方法预警盲区差异不大。通过理论计算得出,在台站到达一定密度时,预警系统仪器延迟时间缩短比台站加密对预警盲区的缩小更有效。