Geodynamic conditions of volcanism and magma formation in the Kurile-Kamchatka island-arc system

The conditions of magma formation were reconstructed on the basis of characteristic features of the evolution of the Kurile-Kamchatka island-arc system, structural and chemical zoning patterns of volcanic complexes, and available published data on peridotite and basalt melting and stability of hydrous minerals. It was shown that the volcanic arc of the Sredinnyi Range of Kamchatka occurs now at the final stage of subduction, whereas subduction beneath the volcanic arc of eastern Kamchatka began at the end of the Miocene, after its jump into the present-day position. The volcanism of Southern Kamchatka and the Kuriles has occurred under steady-state subduction conditions since the Miocene and is represented by typical island-arc magmas. The latter are generated in a mantle wedge, where the melting of water-saturated peridotite occurs in a high-temperature zone under the influence of fluid. The formation of the frontal and rear volcanic zones was related to the existence of two levels of water release from various hydrous minerals. During the initial and final stages of subduction, as well as in the zone of Kamchatka—Aleutian junction, partial melting is possible in the upper part of the subducted slab in contact with a hotter mantle material compared with the mantle in a steady-state regime. This is responsible for the coexistence of predominant typical island-arc rocks, rocks with intraplate geochemical signatures, and highly magnesian rocks, including adakites.     

The beginning of volcanic activity within Sredinny metamorphic Massif (Sredinny Range,Kamchatka)

For the first time, the age of the beginning of the volcanic activity within Sredinny metamorphic Massif is determined (7–6 Ma). We suppose that this event was caused by the collision of Kamchatka with the Kronotsk arc that started about 7 Ma from accretion of Shipunsky peninsula. We demonstrate that at least two types of rocks were erupted within Sredinny Range of Kamchatka in late Miocene times: typical islandarc rocks were produced in the central and northern parts of the Range, and hybrid type rocks—in its southernmost part.     

Cretaceous sedimentary–volcanogenic complexes of the Kamchatka Isthmus: Structure,composition and geodynamic formation conditions

New data on the chemical and rare-element composition and age of the rocks referred earlier to the Iruney suite of the Kamchatka Isthmus are received. In the recent structure these rocks compose the structural–strata complexes of the nappe-folded Lesnovsky Rise. Radiolarian analysis data substantiate that the deposits belonging to the Ening series and the middle and upper parts of the Iruney suite were formed in a single sedimentation basin in the Campanian time. The discovery of a new occurrence of Prunobrachidae representatives on the Kamchatka Peninsula allows us to draw wide interregional correlations and reconstruct the sedimentation conditions. The studied volcanites relate to different igneous series and were formed in geodynamic conditions of the marginal sea and the volcanic arc. The igneous rocks of the Ening stratum are similar to the N-MOR and OI basalts that were formed within the marginal sea (Iruney Marginal Sea) basin. The Upper Cretaceous formations of the eastern slope of the Sredinny Range were formed within the volcanic rise with the island-arc type of volcanism. The younger Eocene igneous rocks of the neo-autochthon (granites and granodiorites) and the volcanic rocks of the Kinkil suite mark a new orogenic stage of development of the Kamchatka margin.     

Garnet-Pyroxenite-Derived End-Member Magma Type in Kamchatka: Evidence from Composition of Olivine and Olivine-Hosted Melt Inclusions in Holocene Rocks of Kekuknaisky Volcano

Late Quaternary volcanoes of Sredinny Range (Kamchatka) attract geoscientists’ attention by their unusual geochemical features and geodynamic setting. They produced volcanic rocks that are enriched relative to N-MORB in most of incompatible trace elements (except HREE), including strong enrichment in large-ion lithophile elements, and show a negative Nb–Ta anomaly, which is typical for rocks formed in supra-subduction settings. However, modern subduction of the Pacific Plate does not reach the most part of Sredinny Range, as inferred by mapping of Wadati–Benioff zone or seismic tomography. We constrain the source of parental magmas for Sredinny Range volcanic rocks by combining major and trace element geochemical data for olivine and naturally quenched olivine-hosed melt inclusions for Holocene tephra layers of the Kekuknaisky field. Composition of the most magnesian olivine (Ni > 2000 ppm, Fe/Mn ≈ 75 at Mg# ~ 84–85 mol %) and geochemical characteristics of the most primitive melts (FC3MS = 0.61 ± 0.04 (2s)) are consistent with their derivation from a pyroxenite source, while elevated LREE/HREE ratios in lavas indicate that it contained garnet. This garnet-bearing pyroxenite likely originated from the lower crust or lithospheric mantle. Its melting could have occurred due to delamination and sinking into the hotter mantle.     

Petrological-geochemical features of the Cretaceous and Cenozoic intrusive magmatism of Kamchatka,the melt sources,and the geodynamic settings

A comparative analysis of the geological setting and composition was carried out for the Cretaceous, Eocene, and Miocene-Pliocene granitoids of Kamchatka. New petrochemical, geochemical, and isotope data are reported. The alkaline granitoids and granites of the Sredinny Range in Kamchatka have an enriched isotope composition and elevated contents of Rb, Th, U, and LREE as compared to their analogs in the eastern part of the region. The largest scale Cretaceous crustal magmatism was formed in a setting of intense tectonic motions and metamorphism. The smaller scale Eocene magmatism produced crustal granitoid melts in the Sredinny Range of Kamchatka and mantle initially basaltic melts that evolved to granites in the southeastern Kamchatka and Ganalsky Range. These processes were accompanied by the rejuvenated of the older crust and the local formation of a new crust. The low-volume crustal-mantle Miocene-Pliocene magmatism of variable composition was developed in volcanic belts, forming the upper crustal horizons on the existing crystalline basement.     

Petrology and geochemistry of the Cretaceous granitoid magmatism of Central Kamchatka,exemplified by the Krutogorova and Kol’ intrusive complexes

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ～80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.     

Two stages of granite formation in the Sredinny Range, Kamchatka: Tectonic and geodynamic setting of granitic rocks

The newly formed continental crust in southern Kamchatka was created as a result of the Eocene collision of the Cretaceous-Paleocene Achaivayam-Valagin island arc and the northeastern Asian margin. Widespread migmatization and granite formation accompanied this process in the Sredinny Range of Kamchatka. The tectonic setting and composition of granitic rocks in the Malka Uplift of the Sredinny Range are characterized in detail, and the U-Pb (SHRIMP) zircon ages are discussed. Two main stages of granite formation—Campanian (80–78 Ma ago) and Eocene (52 ± 2 Ma ago) have been established. It may be suggested that granite formation in the Campanian was related to the partial melting of the accretionary wedge due to its under-plating by mafic material or to plunging of the oceanic ridge beneath the accretionary wedge. The Eocene granitic rocks were formed owing to the collision of the Achaivayam-Valagin ensimatic island arc with the Kamchatka margin of Eurasia. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago. As a result, the island-arc complexes were thrust over terrigenous sequences of the continental margin. The thickness of the allochthon was sufficient to plunge the autochthon to a considerable depth. The autochthon and the lower portion of the allochthon underwent high-grade metamorphism followed by partial melting and emplacement of granitic magma 52 ± 2 Ma ago. The anomalously rapid heating of the crust was probably caused by the ascent of asthenospheric magma initiated by slab breakoff, while the Eurasian Plate plunged beneath the Achaivayam-Valagin arc.     

Late Cretaceous-Paleocene magmatic complexes of central Kamchatka: Geological settings and compositional features

A comparative analysis of the Late Cretaceous-Paleocene volcanism was conducted for four areas of Kamchatka: the Pravyi Tolbachik-Levaya Shchapina-Adrianovka interlfuve (the northern part of the Tumrok Range), the area south of the Ipuin River and Mt. Khrebtovaya (the northern Valaginsky Range), the area of Mt. Savul’ch (the upper reaches of the Kitil’gina River, northern Valaginsky Range), and the Kirganik-Levaya Kolpakova interfluve (the Sredinny Range). New petrochemical, geochemical, and isotopic data on the volcanic rocks from these areas are reported. The examination of this material, together with already published data on volcanic and plutonic rocks of similar composition and age, made it possible to establish the following: (1) the considered basaltoids are ascribed to the subalkali basalt-trachyandesite series with transition toward a meymechite-picrite rock association; (2) the alkali content in the rocks of the Valaginsky-Tumrok-Sredinny ranges increases simultaneously with the increase of the Rb content, while the contents of HFSE and radioactive elements decrease and then again increase. Two trends are identified in the Yb_n-Ce_n diagram: a positive trend spanning most of the volcanic and plutonic rocks and a negative trend defined by the data points of the meymechite-picrite association. The first trend reflects the rock evolution during crystallization differentiation, while the second trend was produced by different degrees of melting of initial protolith. The possible geodynamic reconstructions of this volcanism are discussed as well.     

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies (t = 620–650°C, P _s = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100?90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.     

Petrology and Geochemistry of the Volcanic Rocks Dredged from the Geophysicist Seamount in the Kurile Basin: Evidence for the Existence of Thinned Continental Crust

Dredged samples from the Geophysicist seamount volcano in the northeastern part of the Kurile Basin include volcanic and volcanoclastic rocks ranging from basalt to andesite. The rocks have geochemical features typical of high-K island-arc calc-alkaline volcanism. They are enriched in LILE and depleted in Zr, Ti, Nb, Ta and Y. The chondrite-normalized REE patterns are characterized by enrichment of LREE similar to those of island-arc lava from the submarine volcanoes of rear-arc zone of the Kurile Island Arc. The volcanic rocks have a wide range of ⁸⁷Sr/⁸⁶Sr ratios (0.70287-0.70652), varying ¹⁴³Nd/¹⁴⁴Nd and Pb isotopic ratios. Their trace-element compositions and Sr-Nd-Pb isotope signatures may be explained by a small addition of crustal continental component to mantle-derived magmas that suggest the existence of thinned continental basement under the eastern part of the Kurile Basin.     

Cenozoic volcanic rocks of North Kamchatka: In search of subduction zones

Two belts of subaerial volcanic rocks—the Eocene Kinkil belt and the Neogene belt of the Sredinny Range—extend along the Kamchatka Isthmus. It is suggested that their formation is related to subduction of the oceanic lithosphere beneath the continental margin of North Kamchatka. The oceanic lithosphere consumed in the subduction zones could have been formed as a result of active spreading in the Komandorsky Basin. In the simplest case, both spreading and subduction reflect the northwestward motion of the lithosphere of the Komandorsky Plate relative to Kamchatka, the Shirshov Ridge, and the Aleutian Basin combined into one relatively immobile plate conventionally called the North American Plate. The authors perform a simulation of conjugate spreading and subduction. The most important parameter determining the regional geodynamics—the velocity of the Komandorsky Plate moving relative to the North American Plate—is taken as 2.5, 5.0, and 7.5 cm/yr. The calculated ages of the onset and end of volcanic activity in the aforementioned belts are compared with the dates obtained with the isotopic and paleontological methods. For the Eocene Kinkil belt, where volcanism started 44 Ma ago, the model age of the onset of subduction depends on the accepted velocity of the motion of the Komandorsky Plate and varies from 54 Ma at the velocity of 2.5 cm/yr to 47.5 Ma at the velocity of 7.5 cm/yr. It can be assumed that the model of fast subduction in this age interval is most consistent with the geological data. For the Miocene-Pliocene belt of the Sredinny Range, assuming the velocity of the motion of the Komandorsky Plate at 5.0 and 7.5 cm/yr, multiple rifting at the boundary with the Shirshov Ridge should be assumed. Therefore, for the end of the Neogene, a model with low velocity (2.5–5.0 cm/yr, i.e., about 4.0 cm/yr) is preferable.     

THE LITHOSPHERIC EVOLUTION IN THE QIANGTANG BLOCK OF NORTHERN TIBET PLATEAU: EVIDENCE FROM CENOZOIC VOLCANISM

THE LITHOSPHERIC EVOLUTION IN THE QIANGTANG BLOCK OF NORTHERN TIBET PLATEAU: EVIDENCE FROM CENOZOIC VOLCANISM     

Petrogeochemical features of Oligocene-Early Miocene volcaniclastic rocks of the Sea of Japan

Petrographic and geochemical studies showed that the Oligocene-Early Miocene volcaniclastic rocks from the southern part of the Sea of Japan are ascribed to the high-potassium aluminous rocks of the subalkaline volcanic series of active continental margins. A comparative analysis revealed the spatiotemporal relation of Oligocene-Early Miocene subaerial volcanism of the Sea of Japan with Late Cretaceous and Eocene-Early Miocene ignimbrite volcanism of the East Eurasian margin. This allows us to refer the volcaniclastic rocks of the Sea of Japan to a stage of ignimbrite volcanism that occurred during relative quiescence against a general extension in the continental margin setting.     

Lateral structural variability in zone of eocene island-arc-continent collision,Kamchatka

The lateral variability of structural elements in the collision zone of the Cretaceous-Paleocene Achaivayam-Valagin island arc with the northeastern Asian margin is considered. The similarity and difference of Eocene collision structural elements in the north and the south of Kamchatka are shown. In northern Kamchatka, the continent-arc boundary is traced along the Lesnaya-Vatyn Thrust Fault, which completed its evolution about 45 Ma ago. The thin, near-horizontal allochthon of this thrust, composed of island-arc rocks, overlies the deformed but unmetamorphosed terrigeneous sequences of the Asian margin. The general structure of this suture in the Kamchatka Isthmus and southern Koryakia is comparable with the uppermost subduction zone, where a thin lithospheric wedge overlaps intensely deformed sediments detached from the plunging plate. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago with thrusting of island-arc complexes over terrigenous rocks of continental margin. However, the thickness of the allochthon was much greater than in the north. Immediately after this event, both the autochthon and lower part of allochthon were deformed and subsided to a significant depth. This subsidence gave rise to metamorphism of both the autochthon (Kolpakov and Kamchatka groups, Kheivan Formation) and lower allochthon (Andrianovka and Khimka formations). The anomalously fast heating of the crust was most likely related to the ascent of asthenospheric masses due to slab breakoff, when the Eurasian Plate was plunging beneath the Achaivayam-Valagin arc.     

大别山北缘西段双桥中生代火山岩地球化学及锆石U-Pb同位素年代学

首次对大别山北缘西段(河南省境内)中生代火山岩进行了锆石原位U-Pb同位素测年。结果表明,罗山县双桥火山岩集中区石英安山岩锆石LA-ICP-MS U/Pb年龄为133.1±1.5 Ma,与大别东段中生代火山岩主体毛坦厂组的时代一致,属早白垩世,稍早于或与大别山区大规模花岗岩岩基侵位时代相同;与长江中下游地区的宁芜和庐枞盆地火山岩喷发时间同步,可能同属中国东部中生代巨量岩浆活动的重要组成部分,与太平洋板块斜向俯冲引起的大陆岩石圈重力不稳而产生的拆沉导致岩石圈强烈减薄深部动力学过程相联系。大别山西段早白垩世火山岩属高钾钙碱性系列粗安岩-英安岩-流纹岩组合。中酸性火山岩显示轻稀土元素富集、重稀土元素强烈亏损、弱或基本无负Eu异常的稀土元素配分模式以及高Sr低Yb特征,反映岩浆部分熔融源区残余有石榴石,说明当时存在加厚的成熟陆壳。高(87Sr/86Sr)i(0.707 56)、极低的εNd(t)(-20.1)和大离子亲石元素富集、高场强元素Nb、Ta明显亏损,显示岩浆源区的壳源性质。以上特征反映大别山地区该时期岩石圈伸展程度和软流圈上涌规模远不如长江中下游同时期以幔源为主的中基性火山岩发育的宁芜和庐枞地区。     

中国东北地区中生代构造体制的转变:来自火山岩时空分布与岩石组合的制约

东北地区中生代经历了蒙古-鄂霍茨克构造体系向太平洋构造体系的转换,形成了不同期次火山活动。本文归纳总结了露头区与覆盖区中生代火山岩的年代学、空间分布、岩石组合以及地球化学特征,揭示了两个构造域的时空分布范围。该区火山岩锆石U-Pb年龄统计结果表明中生代存在五期火山活动:早-中侏罗世(190~160Ma)、晚侏罗世(160~145Ma)、早白垩世早期(145~120Ma)、早白垩世晚期(120~100Ma)、晚白垩世早期(100~90Ma)。早-中侏罗世火山岩分布较少,火山岩仅分布在大兴安岭西部满洲里地区和东部张广才岭以及南侧辽宁北票-朝阳地区,火山岩属于高钾钙碱性系列,为蒙古-鄂霍茨克海闭合和法拉隆板块双俯冲作用的产物。晚侏罗世东北地区火山活动明显增强,主要分布在大兴安岭地区,张广才岭以及小兴安岭也有少量分布。西部大兴安岭地区以粗面安山岩、粗面岩为主,属于同碰撞造山成因,为蒙古-鄂霍茨克海闭合造山环境产物。东部以中酸性、酸性岩为主,为法拉隆板块背离欧亚大陆,岩石圈伸展引起的壳源物质熔融产物。早白垩世早期火山活动最为强烈,火山岩主要分布在大兴安岭地区。岩性以高钾钙碱性系列的粗面玄武安山岩、粗面安山岩、安山岩、粗面岩为主,为蒙古-鄂霍茨克海闭合造山后伸展环境产物。早白垩世晚期火山岩主要分布在松辽盆地内部。火山岩以中酸性岩为主,属于中钾-高钾钙碱性系列,为伊泽奈崎板块俯冲引起的弧后拉张,软流圈上涌导致年轻地壳熔融的产物。晚白垩世早期火山岩仅分布在小兴安岭及吉林、黑龙江省东部地区。火山岩为一套玄武岩、玄武安山岩、安山岩和英安岩组合,属于中钾钙碱性系列,是伊泽奈崎-库拉板块高角度俯冲的大陆边缘岩浆活动产物。东北地区中生代不同期次火山岩记录了蒙古-鄂霍茨克构造域向太平洋构造域转换过程及其时空影响范围。     

Environmental,palaeogeographic, and tectonic setting of the Mullions Range Volcanics,New South Wales

The Mullions Range Volcanics in the Mullion Creek‐Kerrs Creek area consist of Middle to Late Silurian rhyolite and dacite lavas‐and intraformational clastic rocks, lying on the eastern flank of the Molong Rise. They are the products of a shallow water (maximum depth c.500 m) volcanic pile that separated the broad Narragal Lagoon on the Molong Rise from the Hill End Trough to the E. Facies and thickness changes within the Mullions Range Volcanics suggest a major volcanic centre to the S or SE of the area studied, where the pile may have become emergent. Facies changes in the overlying Bay and Cunningham Formations support the existence of a volcanic pile, which had a topographic influence on subsequent deposition. The Mullions Range Volcanics probably represent the silicic phase of volcanism associated with the initiation of rifting to form the Hill End Trough.     

Review Section

ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.     

云南思茅大平掌矿区火山岩的地球化学特征及构造意义

大平掌矿区细碧岩-石英角斑岩建造为典型的双峰式火山岩组合,缺失SiO_2为52％～61％之间的中性火山岩。火山岩的TiO_2含量低及其它岩石化学特征、稀土元素地球化学特征均表明这套火山岩很可能形成于岛弧环境。不相容元素N-MORB标准化型式以Ba、Nd、Sm相对富集和Nb、Sr、Ti的相对亏损为特征,其中Ba 富集和Nb亏损更是岛弧火山作用的主要特征之一。大平掌矿区细碧岩具非常低的 Nb/Y(<0.15)和Zr/TiO_2(<0.01)比值,石英角斑岩的低Zr/TiO_2和Nb/Y特征以及Zr/Y比值(3.34～4.23)、(La/Yb)_N比值(0.47～2.50)变化范围都可以反映出火山岩形成于岛弧环境。大平掌火山岩的Sr、Nd同位素特征与世界上典型岛弧火山岩的Sr、Nd特征相似,其岩浆来源于亏损地幔,且经受地壳混染和(或)海水热液蚀变的影响。岩石中富含的放射成因铅也与岛弧环境密切相关。因此,大平掌铜多金属矿床的形成很可能受岛弧环境下的双峰式火山作用控制。     

“三江”义敦岛弧带玄武岩喷发序列与裂谷—岛弧转化

岩石-构造组合是恢复古板块构造历史的最有效手段之一,同时是表征古板块边界与板内环境的最重要的地质证据。本文拟从岩石-构造组合角度,通过对义敦岛弧带玄武岩,特别是前岛弧期玄武岩喷发序列、岩石组合、地球化学特征和其形成背景的研究,试图从较深层次上揭示岩浆-构造内在联系,探索义敦古岛弧的形成与发展。