Volcanogenic belts of the marginal sea lithosphere in the Russian Northeast and their ore potential

According to the concepts of accretionary tectonics, the region of interest was a dynamically evolving active continental margin during Mesozoic/Cenozoic time; this is reflected in the generation of nine volcano-plutonic belts that successively evolved from northwest to southeast. Most of these evolved in parallel with the present-day location of the Kuril-Kamchatka deep-sea trench: the Late Jurassic/Early Cretaceous Uda-Murgali belt (UMVB) the Uyandina-Yasachnaya (UYVB), the Oloi belt (OVB), the Late Cretaceous/Paleogene Okhotsk-Chukchi belt (OChVB), the Late Cretaceous/Paleogene East-Sikhote-Alin’ belt (ESVB), the Eocene/Oligocene Koryak-West-Kamchatka belt (KWKVB), the Oligocene/Quaternary Central Kamchatka belt (CKVB), and the Pliocene/Quaternary East Kamchatka belt (EKVB). The successively younger age of the volcanic belts since the Early Cretaceous is in correspondence with the displacement of the volcanic arc-trench system toward the Pacific Ocean. Apart from the above-mentioned volcanogenic belts, the Omolon craton terrane also contains the pre-accretionary Devonian Kedon marginal volcanogenic belt (KVB). All the volcanogenic belts and the surrounding perivolcanic zones of tectono-magmatic activation (TMA) form the world-largest metallogenic province with a polychronous volcanogenic-plutonogenic metallization of various compositions.     

Zones of activation and volcanism

The zones of tectono-magmatic activation (TMA) that bound and traverse the major volcanogenic belts in the Russian Northeast were previously thought to be satellites of these belts. We even suggested an appropriate term, “perivolcanic zones” [Sidorov et al., 1978]. However, these supra-subduction and their perivolcanic zones are generally radically different from the deep-seated TMA zones of the Chukchi or Omsukchan types in that they are less deep and are short-lived. In contrast to volcanogenic belts, which are characterized by numerous small volcanic features with numerous mineralization occurrences of base and noble metals among regional propilitization fields, it is exactly in the zones of activation of these types that long-lived, volcano-plutonic centers with major mineralization fields are formed.     

Volcanic,subvolcanic and plutonic series of the northeastern asia and the general problem of the volcano-plutonic formations

The petrology of the last few decades considers two main divisions of magmatic associations: the volcanic and the plutonic ones. But new field data obviously show that modern systems of igneous rocks should have three, rather than two, main divisions: volcanic, plutonic and volcano-plutonic. These three types of rock assemblages reflect the overall progress of igneous activity within the diverse natural processes. Only by admitting the presence of accompanying volcanic phenomena from shallow to intermediate plutonic phenomena, and vice versa, one can gain an understanding of magmatic evolution actually existing under certain geological conditions. The principal type of volcano-plutonic formations is associated with the late and postorogenic stages of tectono-magmatic cycles and is endowed with common petrographic and metallogenic features. The well studied volcano-plutonic complexes from the Late Mesozoic « East Asian belt », here described, illustrate this point of view. Some additional examples from certain other belts of folding have proved the regularity of such a trend in the tectono-magmatic evolution.     

东北亚中生代火山岩的地球动力学意义

东北亚中生代火山岩可划分为陆内环状火山岩带、纬向火山岩带和陆缘北北东向火山岩带,本文通过对俄罗斯、蒙古、中国东北、日本和朝鲜半岛在内的东北亚中生代火山岩分布图的编制,以及火山岩地球化学对比研究,认为东北亚中生代火山岩是古亚洲洋构造域向太平洋构造域转换时期,深部地幔地球化学过程以及东亚大陆与古太平洋板块相互作用的产物。在晚古生代至早中生代古亚洲构造域的闭合和欧亚大陆形成过程中,古亚洲域冷板块向地幔深部潜入而引发的热地幔柱的上升,是东北亚中生代大地构造演化和岩浆作用的重要控制因素。     

Geochemistry and geochronology of Late Triassic volcanic rocks in the Chiang Khong region, northern Thailand

The Chiang Khong segment of the Chiang Khong–Lampang–Tak Volcanic Belt is composed of three broadly meridional sub‐belts of mafic to felsic volcanic, volcaniclastic, and associated intrusive rocks. Associated sedimentary rocks are largely non‐marine red beds and conglomerates. Three representative Chiang Khong lavas have Late Triassic (223–220 Ma) laser ablation inductively coupled mass‐spectroscopy U–Pb zircon ages. Felsic‐dominated sequences in the Chiang Khong Western and Central Sub‐belts are high‐K calc–alkaline rocks that range from basaltic to dominant felsic lavas with rare mafic dykes. The Western Sub‐belt lavas have slightly lower high field strength element contents at all fractionation levels than equivalent rocks from the Central Sub‐belt. In contrast, the Eastern Sub‐belt is dominated by mafic lavas and dykes with compositions transitional between E‐mid‐oceanic ridge basalt and back‐arc basin basalts. The Eastern Sub‐belt rocks have higher FeO* and TiO₂ and less light rare earth element enrichment than basalts in the high‐K sequences. Basaltic and doleritic dykes in the Western and Central sub‐belts match the composition of the Eastern Sub‐belt lavas and dykes. A recent geochemical study of the Chiang Khong rocks concluded that they were erupted in a continental margin volcanic arc setting. However, based on the dominance of felsic lavas and the mainly non‐marine associated sediments, we propose an alternative origin, in a post‐collisional extensional setting. A major late Middle to early Late Triassic collisional orogenic event is well documented in northern Thailand and Yunnan. We believe that the paucity of radiometric dates for arc‐like lavas in the Chiang Khong–Lampang–Tak Volcanic Belt that precede this orogenic event, coupled with the geochemistry of the Chiang Khong rocks, and strong compositional analogies with other post‐collisional magmatic suites, are features that are more typical of volcanic belts formed in a rapidly evolving post‐collisional, basin‐and range‐type extensional setting.     

Circular geomorphologic and geological features in the Japanese islands

The available geomorphologic, geological, geophysical, and paleogeographic data were used to identify and reconstruct, to varying degrees of accuracy, dome-block and tectono-magmatic circular uplifts within the Japanese island arc, which contain the main mountain massifs and the centers of orogenic granitoid magmatism on Kyushu, Honshu, Shikoku, and Hokkaido islands. By analogy with the magmatic domes of mountain features in the adjacent continental land (Sikhote-Alin’), they are identified with Cretaceous/Paleogene granitoid focus systems of regional rank. It was found that these elements of the structural setting have preserved their significance up to the present, in spite of the fact that destructive tectonogenetic tendencies have been dominant since the Late Oligocene/Miocene. We also hypothesize that relicts of a major dome-block feature (Nampo) exist within the northern Philippine Sea. The local circular features on these islands reflect the entire diversity of focus formations at volcanic arcs and in the volcano-plutonic belts at active continental margins. Our study showed that the morphotectonic base of the Japanese Islands is not a chaotic collage of terranes, but rather a consolidated system of Cretaceous/Paleogene central-type orogenic uplifts that are evolving in an inherited manner with superimposed Late Cenozoic magmatic, block, and fault features. The Japanese Islands contain an abundance of circular features of varying ranks and ages, indicating the essential control of deep injected dislocations and of the magmatic factor in the structure and evolution of the region.     

Miocene volcanism in eastern Oregon: An example of calc-alkaline volcanism unrelated to subduction

Three upper Miocene calc-alkaline volcanic centers are spaced about 100 km apart in the northeast-trending transition zone between the Columbia Plateau and Basin and Range geologic provinces. These centers are transverse to contemporaneous andesitic vents of the western Cascades Sardine Formation. Some of the Miocene volcanic features in northeast Oregon resemble those of linear volcanic belts that are associated with subduction, but structural patterns do not support the existence of an upper Miocene subduction zone in northeast Oregon. Major east-trending fold axes and northwest-trending faults indicate that north-south compression and east-west extension were the dominant deformational events in this region. It is suggested that the origin of these calc-alkaline rocks is related to their particular tectonic setting and not to subduction.     

South Aegean volcanic arc: Geochemical variations and geotectonic implications

The Aegean volcanic arc is one of the most important geological structure of the Mediterranean area. It is a belt of volcanic centers consisting of products ranging from basaltic, andesitic, dacitic to rhyolitic in composition, all of them displaying a typical calc-alkaline chemical character. The most abundant rock types are represented by andesites and dacites. Minor amounts of basalts and rhyolites occur mainly in the central-eastern sector of the arc. The REE, Rb, Sr, Ba, Th, Ta, Hf, Zr, Ni, Co, V and Cr abundances determined in 27 representative samples from different centers suggest that: 1) the intermediate and acidic terms are products of crystal/liquid fractionation processes starting from basic parent magmas: 2) large variations in incompatible elements occur in the most basic samples that are interpreted as evidence for heterogeneously LIL element-enriched mantle source; 3) plagioclase played a role in the evolution of the volcanic centers of the eastern and central arc different from that played in the volcanoes of the western sector. Along the arc, the differences in the distribution of lithological types, in the volumes of erupted material, in the volcanological characteristics of the different centers as well as in the patterns of trace element distribution in the volcanites are considered to be connected with the prevailing tectonic regime affecting the various sectors of the arc.     

Structural features and petroleum geology of the fold-thrust belt in the southern Tarim basin, China

The west Kunlun fold-thrust belt (WKFTB) and the Altun fold-thrust belt (AFTB) are respectively located in the southern margin of the Tarim basin, NW China. The analyses of typical structures and regional dynamics of the fold-thrust belts reveal their different structural and petroleum features and mechanisms. WKFTB differs from AFTB by abundant fault-related folds and triangles zones, and was formed by northward extrusion of the west Kunlun orogen. AFTB was affected synchronously by northward extrusion of the Altun orogen and the sinistral strike-slipping of the Altun Fault, so it is characterized by the minor scale and the monotonous structural styles. The Aqike anticline and the Aqike fault, of which the strikes are orthogonal to the strike of the fold-thrust belts, are regarded as the adjustive structures between both of the fold-thrust belts. The oil-gas pools of WKFTB develop mainly in the faulted-related anticline traps, but the oil-gas pools of AFTB develop mainly in the low fault-block and anticlines traps related with the paleo-uplifts. There are different exploration countermeasures for both of the fold-thrust belts.     

扬子板块东北缘中元古代的大地构造划分

扬子板块东北缘存在四条主要的中元古代变质带,自南向北依次为江南变质带、沿江变质带、云台一张八岭变质带和连云港一泗阳变质带。它们分别为中元古代的古弧后盆地、火山岛弧、裂谷及弧前盆地,扬子板块东北缘中元古代为活动大陆边缘构造体系。苏（北）胶（南）变质造山带应解体,其中一部分属扬子大陆边缘体系。     

Epithermal mineralization in the Kedon Paleozoic volcano-plutonic belt,Northeast Russia: Geochemical studies of Au–Ag mineralization

This paper is concerned with the geochemical features and conditions of generation for Paleozoic Au–Ag mineralization in the pre-accretionary Kedon (D_2-3) volcano-plutonic belt (KVB), which is situated within the Omolon cratonic terrane (Northeast Russia). We present new data on the compositions and concentrations of trace elements and rare-earth elements (REEs) in the host rocks and volcanogenic ores of epithermal Au–Ag fields. The ores are found to be enriched in a wide range of trace elements. The general features of the ores under study include a low level of REE concentrations, an obvious enrichment in light REEs, and considerable variations in europium anomalies, ranging from low negative to low- and strongly positive. Fluid inclusions were studied to show that the ore-forming solutions were hydrocarbonate potassium in composition. The mineral content of the fluid increases toward later, low-temperature phases of the mineralization. We found a tendency of potassium increasing from the earlier oreless quartz to productive quartz going to great depth, as well as a slow decrease in the concentrations of Na⁺, Ca⁺⁺, and Cl^–. The productive quartz shows direct correlation in Ag–K and a reverse correlation in Ag–Na. The results indicate andesite magmas and meteoric water as the most likely sources of the fluids that have formed epithermal Au–Ag ores in the KVB deposits.     

Geochemistry of the Early Miocene volcanic succession of Northland,New Zealand,and implications for the evolution of subduction in the Southwest Pacific

Latest Oligocene and Early Miocene volcanic rocks occur on the Northland Peninsula, New Zealand, and record the inception of Cenozoic subduction-related volcanism in the North Island that eventually evolved to its present manifestation in the Taupo Volcanic Zone. This NW-striking Northland Arc is continuous with the Reinga Ridge and comprises two parallel belts of volcanic centres ca. 60 km apart. A plethora of tectonic models have been proposed for its origins. We acquired new trace element and Sr–Nd isotope data to better constrain such models. All Northland Arc rocks carry an arc-type trace element signature, however distinct differences exist between rocks of the eastern and western belt. Eastern belt rocks are typically andesites and dacites and have relatively evolved isotope ratios indicating assimilated crustal material, and commonly contain hornblende. Additionally some eastern belt rocks with highly evolved isotope compositions show fractionated REE compositions consistent with residual garnet, and some contain garnetiferous inclusions in addition to schistose crustal fragments. In contrast, western belt rocks are mostly basalts or basaltic andesites with relatively primitive Sr–Nd isotope compositions, do not contain hornblende and show no rare earth element evidence for cryptic amphibole fractionation. Eastern and western belt rocks contain comparable slab-derived fractions of fluid-mobile trace elements and invariably possess an arc signature. Therefore the difference between the belts may be best explained as due to variation in crustal thickness across the Northland Peninsula, where western belt centres erupted onto a thinner crustal section than eastern belt rocks.The consistent arc signature throughout the Northland arc favours an origin in response to an actual, if short-lived subduction event, rather than slab detachment as proposed in some models. No Northland Arc rocks possess a convincing adakite-like composition that might reflect the subduction of very young oceanic lithosphere such as that of the Oligocene South Fiji Basin. Therefore we favour a model in which subduction of old (Cretaceous) lithosphere drove subduction.     

Space–time relationships between volcanic associations of different alkalinities: The Belogolovskii Massif,Sredinnyi Range,Kamchatka. Part II. Geochemistry of volcanic rocks and magma sources

Data are presented relating to volcanic series in the Belogolovskii Massif, Sredinnyi Range, Kamchatka. We discuss new geochronologic data, the distributions of rare elements and platinum elements in the rocks, and list the isotope characteristics of volcanic series with normal and moderate alkalinities. We show that the Late Pliocene to Early Pleistocene rocks that belong to the moderate alkaline series of the Belogolovskii volcanic massif are different from rocks in the normally alkaline series of the Late Miocene to Middle Pliocene volcanogenic basement in having higher concentrations of the HFSE and LILE components. We propose a model for the generation of moderate alkaline magmas involving a heterogeneous depleted and a heterogeneous enriched source of material. According to the isotope data, one of these sources may be the subducted oceanic lithosphere of the Pacific and the Commander-Islands type, while the other source was recycled material of the Indian MORB type.     

DISCUSSION ON THE SEISMOGENIC STRUCTURE OF ZHAN-JIANG BAY AREA FROM THE VIEW OF DEEP FAULT SYSTEM INTERPRETED BASED ON THE GRAVITY DATA

The neotectonics in Zhanjiang Bay area is almost the inferred faults and there are not any active faults seen on the ground surface. So it is difficult for research on the seismogenic structure. This paper analyzes and interpretes the gravity data that can reflect the feature of deep faults and then discusses the seismogenic structure of Zhanjiang Bay area in combination with its geology and earthquake activity. There is a huge NEE-trending high gravity gradient belt lying in the coastal region among Guangdong, Guangxi, and Hainan, and Zhanjiang Bay is located in this gravity gradient belt. We analyzed and interpreted more than eighty images obtained with many different methods one by one, then, got the result that Zhanjiang Bay area is embraced by two giant fault belts trending in the NEE and NW direction respectively, and its interior is crossed over by the NE-trending fault belt. These three fault belts are well shown in the gravity images, especially the NEE-trending fault belt and NW one. The gravity isolines and gradient belts or the thick black stripes of the NEE-and NW-trending fault belts are displayed apparently. Also, these gravity structures are good in continuity, extend vastly and cut deeply. What is more, the NEE-trending fault belt plays a leading and region-controlling part. It shows good continuity, and cuts off the NW-and NE-trending faults frequently and intensively. The NW-trending fault belt also is good in continuity and cuts the NEE-and NE-trending faults relatively frequently and strongly, but it is restricted by the NEE-trending one. Last, the continuity of the NE-trending fault is worse and the strength cutting off NE-and NW-trending faults is significantly weak, just in some segments and in the shallow positions. According to the characteristics above and combined with the analyses of geological structure and earthquake activity, the conclusion can be drawn that the NEE-trending fault is the controlling structure and the main seismogenic structure in Zhanjiang Bay area, and the NW-trending fault is the second one. They conjugate and act together. Therefore, Zhanjiang Bay has the tectonic condition for generating M_S=6.5 earthquakes.     

中国大陆东南缘主要构造带基底结构

利用中国大陆东南缘四条北西向人工地震测深剖面的初至Pg波走时,使用有限差分层析成像方法反演剖面基底速度结构,并通过初始模型选取、射线数分布、走时拟合等手段分析结果的可靠度.研究结果表明,剖面表层速度比华北克拉通的地表速度高,闽东火山断拗带地表速度普遍高于西侧的闽西南坳陷带.相邻构造带的分界线两侧均有明显的速度横向变化,剖面东段的闽东火山断拗带内的东南沿海一带,速度变化较大,地层的褶皱变形要强于西段.基底埋深自西向东呈逐渐变浅的趋势,剖面东段的闽东火山断拗带基底起伏剧烈,基底深度与闽西南坳陷带相比较浅,与闽西北隆起带相比较深.西段的闽西北隆起带基底变化较为平缓,基底深度相对较浅;闽西南坳陷带基底深度明显加深,在龙岩新泉盆地内最深接近4 km.基底形态的起伏变化揭示出东南缘基底是坳陷与隆起并存的构造特征,与研究区近代的地质构造特征相吻合.邵武—河源、政和—大埔和长乐—诏安断裂带都切割基底,其中,政和—大埔断裂4 km深度下方有比较明显的低速异常带,结合本文以及现有的研究结果,进一步确认政和—大埔断裂是不同断块构造单元的分界.     

南北地震带南段远震P波走时层析成像研究

南北地震带南段位于青藏高原东南缘,是青藏高原与扬子克拉通的过渡地带.本文收集了该区域内90个固定台站和356个流动台站的远震波形数据,采用波形互相关方法拾取了88691个P波走时残差数据,应用FMTT(Fast Marching Teleseismic Tomography)层析成像方法获取了南北地震带南段深部的三维P波速度结构.结果显示了研究区深部的结构具有显著的不均匀性:腾冲火山地区深部400km以浅的深度内分布着明显的低速异常;四川盆地西南部下方300km内具有较强的高速异常;在上地幔顶部,沿川滇菱形块体周边的大型断裂带及川滇菱形块体南端分布着显著的低速异常,这些低速异常为青藏高原物质向东南方向挤出提供了必要的通道;保山地块下方存在一东倾的高速异常带,该高速异常带为印度板块岩石圈向东俯冲的体现.     

The Geochemistry of Volcanogenic Mineralization in the Northwestern Segment of the Pacific Ore Belt: Northeast Russia

This paper considers the geochemistry of volcanogenic mineralization in the northeastern segment of the Pacific Ore Belt (Northeast Russia). We give new evidence for the compositions and concentrations of trace and rare-earth elements (REE) in the ores of volcanogenic fields: Au-Ag epithermal (of various types and ages), Cu-Mo-Au porphyritic, Au-Bi related to granitoidal intrusions, Sn-Ag subvolcanic and kies polymetallic enriched in Au and Ag, as well as REEs in alkaline volcanic rocks. Geochemical signatures have been compiled for 17 formation types of volcanogenic fields. It was found that the ore-forming fluids in most fields belonged to an NaCl-H₂O hydrothermal system that was enriched in Cl relative to F; the values of Y/Ho in the ores of nearly all types correspond with the interval of ratios characteristic for present-day hydrothermal fluids in backarc basins; most of the ores that we studied had near-chondrite spectra with configurations similar to those of the REE spectra in volcanic rock sequences of the andesite-diorite series. Comparative analysis of REE spectra in the distribution of trace elements over classes of gold concentration shows synchronous enrichment of ores in similar sets of trace elements. The high Co/Ni ratio in volcanogenic ores probably reflects the superposition of a later magmatic fluid upon an earlier mineralization. Samples from ores of volcanogenic fields, except for Kuroko, show δCe and δEu varying from negative to mildly positive values, thus indicating low-oxidizing conditions during deposition. It was found for Au-Ag epithermal ores that they are enriched in a wide range of trace elements; they have low concentrations of REEs, the light REEs are more abundant than the heavy ones, and the Eu anomalies vary considerably from small negative to low and high positive values. The results provide evidence of an exhalation hydrothermal origin of the Khotoidokh field. It has been shown that the REEs in the ores of the Bol’shoe field are of the type that is most valuable to industry. The results can be used to deal with practical problems: determining the formation type and evaluating the industrial value of a field; detecting accessory components in ores; and discriminating between the types of geochemical anomalies (in rocks or in soil) and stray fluxes as to the potential of a field.     

天山山前主要推覆构造区的地壳缩短

利用平衡地质剖面方法研究天山山前主要褶皱带的地壳缩短,其中3条平衡剖面分别横跨天山南簏的柯坪逆断裂-褶皱带和库车逆断裂-褶皱带,2条剖面横跨天山北簏的玛纳斯活动逆断裂-褶皱带,其余1条剖面横跨吐鲁番中央隆起逆断裂-褶皱带。柯坪活动逆断裂-褶皱带、库车逆断裂-褶皱带、玛纳斯逆断裂-褶皱带和吐鲁番盆地的地壳缩短量分别为40~45km、27~37km、8·5~10·5km和6~7km。天山山前活动逆断裂-褶皱带在EW向上互不重叠,它们的缩短量大致代表了该经度上新生代的最小地壳缩短量,反映出天山地壳缩短由西向东减小的趋势。假定天山山前活动逆断裂-褶皱带开始形成的时间为距今2·5Ma的西域砾岩沉积期,考虑到博阿断裂、塔拉斯-费尔干纳断裂在SN向上的缩短活动分量,上述4个地段的最小缩短速率分别为15·4~17·3mm/a、12·7~16·5mm/a、3·8~4·5mm/a和2·3~2·7mm/a。活动走滑断裂在天山内部特定位置向左偏转,走向由NW转为NWW,在断裂转折的部位走滑活动量转化为天山SN向的缩短变形     

Cambrian greenstone belts in Victoria: Marginal sea-crust slices in the Lachlan Fold Belt of southeastern Australia

The oldest rocks in the mainland southeastern Australian segment of the (Palaeozoic) Lachlan Fold Belt are Cambrian greenstones which outcrop in three narrow linear belts separated by Lower Palaeozoic marine troughs in which many thousands of meters of predominantly greywacke-shale sediments accumulated. The greenstone belt nearest the Australian craton, the Mt. Stavely Greenstone Belt, is composed of calc-alkalic meta-andesites, metadacites and intermediate and acid pyroclastics. The Heathcote Greenstone Belt, of central Victoria, consists of three segments; the northern and southern segments are very similar and their internal stratigraphy, petrology, and geochemistry suggest they represent an incomplete, disrupted ophiolite. However, the central segment of the Heathcote Greenstone Belt, which is more intensely deformed and metamorphosed than the northern and southern segments, is composed of a calc-alkalic volcanic suite dominated by meta-andesites. The Mt. Wellington Greenstone Belt of eastern Victoria shows remarkable overall similarities to the northern segment of the Heathcote Greenstone Belt and evidence, including the presence of fault slices of gabbro and peridotites, suggests that this belt too is a much disrupted ophiolite. We interpret the ophiolites to have been the crust of a marginal sea which developed by rifting of thin continental-type crust at the leading edge of a palaeo-Australian plate in the early Cambrian. Meta-andesites and associated rocks of the Mt. Stavely Greenstone Belt were probably erupted onto this thin continental crust above a Benioff zone, and a rifted-off fragment of this thin continental crust bearing a cover of calc-alkalic volcanic rocks has been preserved as the central segment of the Heathcote Greenstone Belt during later deformation events.