Conditions of basalt formation in the Dzhida zone of the Paleoasian Ocean

Petrological and geochemical studies performed with invoking data on the compositions of clinopyroxenes have clarified the conditions of formation of Vendian-Cambrian basaltic complexes in the Dzhida zone of the Paleoasian Ocean (northern Mongolia and southwestern Transbaikalia). The research was based on a comparative analysis with reference igneous basaltic associations. Of special importance are our microprobe data on trace and rare-earth elements in clinopyroxenes from igneous rocks of different present-day geodynamic settings, namely, N-MORB (Mid-Atlantic Ridge, Central Atlantic), OIB (Bouvet Island, South Atlantic), WPB (within-plate tholeiitic plateau basalts of the Siberian Platform), and boninites of ensimatic arcs (Izu-Bonin island arc, Pacific). The studies have shown that the paleo-oceanic structures in the district of the Urgol guyot formed during geodynamic processes under the impact of mantle plumes on oceanic spreading crust, which resulted in oceanic basaltic plateaus and within-plate oceanic islands. All these structures were later superposed by typical island-arc structure-lithologic associations. Formation of basalt complexes in the Dzhidot guyot district proceeded with a stronger effect of enriched plume melts of within-plate oceanic islands as compared with the Urgol guyot. This is evidenced from petrochemical and geochemical data showing the development of OIB-type magmatic systems on the oceanic basement. Data on clinopyroxenes confirm the participation of mantle plume in this process, which led to the evolution of magmas from typical oceanic basalts (MORB) to plateau basalts and OIB.     

Basaltoids and carbonatite tuffs of Ambinsky volcano (Southwestern Primorye): Geology and genesis

Geological-petrological data were first obtained on the Early Miocene basaltoids and spinel-fassaite carbonatite tuffs of the Ambinsky volcanic structure in southwestern Primorye. The geological study of Ambinsky volcano allowed the reconstruction of stratigraphic sections across lava and pyroclastic basaltic rocks and stratified carbonatite tuffs. The chemical compositions of rocks and mineral phenocrysts from basalts and carbonatite tuffs are reported. The basaltoids are classed with undifferentiated moderately alkaline within-plate basalts. Evidence of carbonate-silicate immiscibility was found in the basaltoids and carbonatite tuffs. It was suggested that the formation of the carbonatite melt associated with simultaneous basification and abundant crystallization of spinel, fassaite, as well as oversaturation of the silicate system in Ca was caused by limestone assimilation, subsequent transformation of the melt, and liquid immiscibility. Thermal decomposition of carbonates with dissolution of released CaO in magma and accumulation of CO₂ in a closed magmatic chamber gave rise to the autoclave gas effect and, correspondingly, heavy explosive eruptions atypical of such volcanic rocks. The genesis of carbonatite tuffs of Ambinsky volcano can serve as a model example of exsolution of carbonate melt in the moderately alkaline nonagpaitic basaltic system.     

Stratigraphical framework of basaltic lavas in Torres Syncline main valley,southern Parana-Etendeka Volcanic Province

The Paraná-Etendeka Volcanic Province records the volcanism of the Early Cretaceous that precedes the fragmentation of the South-Gondwana supercontinent. Traditionally, investigations of these rocks prioritized the acquisition of geochemical and isotopic data, considering the volcanic stack as a monotonous succession of tabular flows. Torres Syncline is a tectonic structure located in southern Brazil and where the Parana-Etendeka basalts are well preserved. This work provides a detailed analysis of lithofacies and facies architecture, integrated to petrographic and geochemical data. We identified seven distinct lithofacies grouped into four facies associations related to different flow morphologies. The basaltic lava flows in the area can be divided into two contrasting units: Unit I - pahoehoe flow fields; and Unit II - simple rubbly flows. The first unit is build up by innumerous pahoehoe lava flows that cover the sandstones of Botucatu Formation. These flows occur as sheet pahoehoe, compound pahoehoe, and ponded lavas morphologies. Compound lavas are olivine-phyric basalts with intergranular pyroxenes. In ponded lavas and cores of sheet flows coarse plagioclase-phyric basalts are common. The first pahoehoe lavas are more primitive with higher contents of MgO. The emplacement of compound pahoehoe flows is related to low volume eruptions, while sheet lavas were emplaced during sustained eruptions. In contrast, Unit II is formed by thick simple rubbly lavas, characterized by a massive core and a brecciated/rubbly top. Petrographically these flows are characterized by plagioclase-phyric to aphyric basalts with high density of plagioclase crystals in the matrix. Chemically they are more differentiated lavas, and the emplacement is related to sustained high effusion rate eruptions. Both units are low TiO₂ and have geochemical characteristics of Gramado magma type. The Torres Syncline main valley has a similar evolution when compared to other Large Igneous Provinces, with compound flows at the base and simple flows in the upper portions. The detailed field work allied with petrography and geochemical data are extremely important to identify heterogeneities inside the volcanic pile and allows the construction of a detailed lithostratigraphical framework.     

Late Cenozoic Volcanism of the Vitim Depression and Its Correlation with the Vitim Lava Plateau (Western Transbaikalia) according to K–Ar Geochronology

New data on the geology, age, and composition of the Late Cenozoic volcanic rocks of the Vitim Depression, a satellite of the Vitim Lava Plateau (VLP), have been obtained. Two-stage volcanic activity has been revealed. In the Middle Miocene (14–13 Ma), a basalt and trachybasalt (hawaiite) lava covers were formed. In the Pleistocene (1.6–0.7 Ma), basanite erupted from scoria cones. Both stages of volcanism were coeval to similar pulses of volcanism within the VLP. However, unlike the latter, there were no Pliocene and Late Miocene eruptions within the Vitim Depression. Additionally, the Vitim Depression basalts have depleted geochemical characteristics comparative to those of the VLP. These features of the Vitim Depression volcanics imply their autonomous formation, whereas the synchroneity of the initial and final stages of volcanism in the depression and in the VLP reflects the existence of a common regulating mechanism of igneous activity.     

Erratum to: Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Geochemistry and geodynamic setting of Late Cretaceous-Miocene basalts in the southern Korean Peninsula

Newly obtained data highlight strong geological and geochemical differences between Late Cretaceous-Paleogene and Eocene-Middle Miocene volcanic rocks in the southern Korean Peninsula. The rocks are spatially separated and differ in the proportions of acid and basic varieties. The Late Cretaceous-Paleogene basalts are similar to suprasubduction rocks in having high Al₂O₃, LILE, and Th contents, and low TiO₂ and HFSE contents. The Miocene basalts have a composition intermediate between those of subduction and within-plate rocks. Compared to subduction rocks, they are lower in radiogenic Sr, K, LILE (Cs, Rb, Ba), and Th and higher in MgO, Ni, Ti, and HREE. A drastic change in U, Ba, Rb, Ce, Th, and ⁸⁷Sr/⁸⁶Sr in the basic volcanic rocks of the southern Korean Peninsula at the Late Cretaceous-Paleogene boundary suggests a decreasing sedimentary contribution to the magma. The latter testifies to a change in the direction of the motion of the oceanic and continental plates, increasing compressional forces and, finally, the cessation of subduction. The synthesis of the original authors and published data on Cenozoic volcanism of the southern Korean Peninsula and the eastern Sikhote Alin showed that the tectonic evolution of the eastern Eurasian margin occurred in four stages: Late Cretaceous-Paleogene, Eocene-Oligocene, Early, and Middle-Miocene.     

Cenozoic Volcanism in South China Sea and Its Vicinity and South China Sea Spreading

The rock series, rock types and Sr-Nd isotopic dating of the Cenozoic volcanic rocks in the South China Sea are similar to those in its vicinity. On the basis of the spreading age of the South China Sea, the Cenozoic volcanic rocks are divided into three stages: the pre-spreading stage, the spreading stage and the post-spreading stage. The deep process characteristics of the asthenosphere and lithosphere may be inferred from the study on primary basaltic magma. The top layers of the asthenosphere both in the spreading stage and in the pre-spreading stage are closer to the earth surface than that in the post-spreading stage. From the pre-spreading stage to the spreading stage, the top layer of the asthenosphere decreased in depth, while the amount of interstitial partial melts increased. The evolution of the primary basaltic magma shows a progressive evolution sequence of the rifting volcanism and a faster lithospheric spreading velocity. From the spreading stage to the post-spreading stage, the top layer of the asthenosphere gradually increased in depth, but the amount of interstitial partial melts decreased. The evolution of primary basaltic magma shows a retrogressive evolution sequence of the rifting volcanism and a gradual decrease in the lithospheric spreading velocity. The depth recognized by the study on the Cenozoic volcanism demonstrates the deep environment for the formation and evolution of the South China Sea.     

中部拉萨地体南侧吉瓦地区早白垩世则弄群火山岩的发现及意义

西藏尼玛县吉瓦地区措勤-多瓦后陆拗陷带内分布的火山岩LA-ICP-MS锆石U-Pb年龄为120.3~126.5Ma左右,重新厘定为早白垩世则弄群,否定了前人归属为上新世乌郁群(N2wy)及始新世帕那组(E2p)的认识。岩石以酸性火山岩为主,中基性火山岩为辅,酸性岩类主要为火山碎屑岩类和熔岩类,典型岩石类型为流纹质熔结凝灰岩和流纹岩等,中基性岩主要为玄武安山岩,安山玄武岩等,杏仁构造普遍发育。研究区大量的流纹质熔结凝灰岩的出现反映了吉瓦地区的火山岩主要为陆相火山喷发形成。地球化学特征显示轻稀土富集,负Eu异常明显,富集K、Rb、Th、U等大离子亲石元素,相对亏损Nb、Ta、P、Ti等高场强元素。酸性火山岩具有A型花岗质岩浆岩特征,基性岩具有板内玄武岩亲缘性,这一特征可能与班公湖-怒江洋壳岩石圈南向俯冲过程中发生的板片断离有关。中部拉萨地体南侧早白垩世火山岩的发现,使班公湖-怒江洋壳南向俯冲在晚侏罗世-早白垩世的岩浆活动在原来的基础上向南延伸70~80km,火山岩地层时代的重新归位对研究冈底斯带早白垩世地球动力学背景及建立地质年代学格架提供了新的约束资料,具有重要的科学意义。吉瓦地区早白垩世则弄群火山岩可能受到了班公湖-怒江特提斯洋壳向南、雅鲁藏布江洋壳向北的双向俯冲制约。     

Geochemistry of the late phanerozoic mafic dykes from the Moyar shear zone, South India, and its implications on the spatial extent of Deccan Large Igneous Province

The late Phanerozoic dykes of the Moyar shear zone mark a prominent intrusive structure in the Precambrian crystalline rocks of northern Kerala. The dykes, having variable strike length and width, show a predominant NW-SE trend and basaltic composition with SiO₂ ranging from 48.59 % to 49.53 % and normative quartz/olivine. The chondrite normalized REE patterns are fractionated, parallel to sub-parallel, and are generally uniform but with negative Eu-anomalies. Chemical characteristics are typical of MORB or within-plate basalts and suggest derivation of melt from a fertile or plume-related mantle source with a considerable correlation to Deccan basalts. This is consistent with the regional geological setting including the volcanism, associated with a Proterozoic crustal scale shear zone, occurring long before the onset of seafloor spreading in the Indian Ocean. The possibility of redefining the southern limit of the Deccan Large Igneous Province is examined using the characteristic features of the dykes.     

大兴安岭哈拉哈河-绰尔河第四纪火山分期: K-Ar年代学与火山地质特征

大兴安岭中部哈拉哈河-绰尔河第四纪火山区分布有34座火山,这些火山总体呈北东向带状分布,火山岩分布面积约400 km2,岩性主要为碱性橄榄玄武岩.根据火山地质特征,结合火山岩K-Ar测年结果,哈拉哈河-绰尔河第四纪火山可进一步划分为早、中、晚更新世和全新世4期.早更新世火山岩,由于被后期火山岩覆盖,主要分布于火山区周边和出露在河谷中.中更新世火山活动最强,不论火山数量(27座)还是熔岩流规模都超过该区第四纪火山的一半以上.晚更新世时期火山活动趋弱,火山活动范围缩小,只局限于小范围区域.全新世火山活动又进入新的高峰期,强爆破式喷发和规模宏大的熔岩流,以及保存完好的熔岩流地貌是全新世火山之特点.     

“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。     

Composition and formation settings of the siliceous-volcanogenic complexes of the Nizhneussuriisk segment,Kiselevka-Manoma terrane,West Sikhote Alin

This paper reports new data on the mineralogical-petrographical composition, genetic types of sedimentary rocks, and geochemical features of the volcanic rocks of the Snarsky area of the Nizhneussuriisk segment of the Kiselevka-Manoma accretionary complex, which is developed on the right bank of the lower reaches of the Ussuri River. The Middle Jurassic-Aptian sedimentary rocks of the area are represented by pelagic radiolarian cherts, semipelagic siliceous-clayey deposits, subordinate shallow limestones, volcanomictic conglomerates, and turbidite sandstones. The basalts of the area are represented by alkaline basalts resembling within-plate ocean-island basalts and N-MORB-type tholeiitic basalts. Rock associations of two physicogeographical and geodynamic settings are distinguished: the pelagic setting with within-plate alkaline volcanic rocks and the hemipelagic one with volcanic edifices of spreading basalts.     

云南马关新生代钾玄质玄武岩的岩石学与地球化学特征及构造环境

云南马关地区广泛出露了一套富含大量幔源包体和捕掳晶的新生代玄武岩.对该地区的新生代玄武岩进行了岩石学和地球化学方面的研究, 为该地区自新生代印度-欧亚大陆碰撞以来发生的岩浆活动及壳幔物质交换等问题提供重要的信息和岩浆岩约束.研究表明: 该区玄武岩具有富碱且富钾的特征, 其中全碱含量w(K₂O+Na₂O)变化在2.94%~8.23%之间、K₂O/Na₂O在0.44~6.72之间, 21个样品平均的K₂O/Na₂O比值为1.26.采用火山岩类型系列与划分方法, 确定本区多数岩石属于钾玄岩类的玄武岩或碧玄岩.岩石具有富集LREE和LILE的特征, 经球粒陨石/原始地幔标准化后的稀土元素配分形式和微量元素蜘蛛图均具有与OIB相似的特征.岩石中含有大量深源岩石包体, 斑晶含量少且结晶程度低, 相容元素Ni含量较高等特点, 符合原生岩浆的基本特征; 唯有Mg#(0.49~0.72)偏低, 可能与源区本身的性质或者源区发生的壳幔混合作用等因素有关.根据钾质岩石构造环境判别标准, 显示岩石形成于板内环境, 其成因与印度-欧亚大陆的碰撞诱发的软流圈物质向高原东南方向侧向挤出有关.      

Geodynamics and sources of preaccretionary magmatism in western Mongolia

The paper reports original isotopic and geochemical data on Early Precambrian lavas in the Ozernaya Zone in Mongolia. According to their normalized trace-element patterns, the rocks are classified into the following groups: (1) rocks similar to N-MORB; (2) rocks similar to E-MORB; (3) basalts enriched in trace elements, with HFSE minima; and (4) basalts depleted in trace elements, with HFSE minima. All of the lava types could be produced in an island arc—backarc basin system. The magmatic rocks of group (1) were likely formed in a spreading backarc basin, and those of group (2) were likely generated within the influ- ence zone of a hotspot or were derived from heterogeneous upper mantle domains. The lavas of group (3) seem to be fragments of an ensimatic, relatively primitive island arc. The basalts and basaltic andesites of group (4) were likely produced by mixing melts of groups (1) and (3). The fact that lavas of groups (1) and (4) sometimes intercalate within a single stratigraphic section suggests that the extension and subduction zones were closely spaced and operated simultaneously. The magmas of groups (1), (2), and (3) were derived from different mantle sources, which possessed different ratios of trace elements and were different in isotopic composition.     

A Lower Palaeozoic shallow water sequence in the eastern European Variscides (SW Poland): provenance and depositional history

The Gackowa Formation of inferred Cambrian —Ordovician age is part of the metamorphosed pre-Variscan basement of the southern Kaczawa Mountains of the Sudetes region, south-west Poland. Previously variously interpreted as lavas, tuffs and sandstones, it is shown to consist of about 200 m of originally well-sorted siliclastic sedimentary rocks within a sequence of predominantly mafic volcanic rocks. Four facies have been distinguished based on relict sedimentary structures and textures and these suggest that deposition took place on a storm-dominated shelf, mostly above the wave base. The petrography, detrital zircon morphologies and geochemistry suggest affinity with, and derivation in large part from, acid volcanic rocks at a comparable stratigraphic position in a separate tectonostratigraphic unit to the south. The Gackowa Formation and its acid volcanic source rocks have a clear continental geochemical signature, in contrast with the mantle-derived basic/bimodal volcanic rocks below and above; it is suggested that the parent acid magma originated by crustal melting during the ascent of these mantle-derived magmas. A continental crust setting is inferred for the deposition of the Gackowa Formation sandstones, thus suggesting a continental setting for the associated within-plate basalts. The entire volcanic/sedimentary succession resembles elements of the Lower Palaeozoic of Germany, and all may have formed in an initial rift setting during Late Precambrian to Ordovician times.     

First evidence for the Middle Triassic volcanism in South Primorye

A detailed study of a relatively well-exposed fragment of the Barabash Formation in the southern part of the Voznesenka terrane is carried out to specify the geodynamic settings of the Permian volcanogenic and volcanogenic-sedimentary complexes in South Primorye. It is established that the basaltic flows juxtaposed in the studied sequence originated from sharply different sources. The geochemical characteristics indicate that the basalts from the sequence base were presumably derived by melting of oceanic lithospheric mantle or asthenosphere, while the source of the overlying basalts was lithospheric mantle reworked by a subduction process. The basalts are subsequently overlain by tuffaceous–terrigenous and terrigenous rocks and limestones with remains of Capitanian (Middle Permian) fauna. Accessory zircons extracted from the tuffaceous–terrigenous rocks yield an U–Pb concordant age of 233.3 ± 3.3 Ma (Middle Triassic Ladinian Stage) for the youngest zircon population. The obtained data lead us to conclude that the Barabash Formation is a tectonostratigraphic rather than stratigraphic unit and may be a fragment of the Triassic accretionary wedge. The obtained data cast doubt on the accepted assignment of this unit to the Voznesenka terrane. It is more logical to include it in the Laoelin–Grodekov terrane, which represents a fragment of the Late Paleozoic active continental margin. This suggests that the boundary between these blocks should be specified and the timing of the final stage of amalgamation of the Laoelin–Grodekov terrane with the terranes of the Bureya–Khanka orogenic belt should be revised.     

Provenance of Lower Cretaceous Wölong Volcaniclastics in the Tibetan Tethyan Himalaya: Implications for the final breakup of Eastern Gondwana

Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wölong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wölong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ～ 140 Ma to ～ 119 Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust.The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (～ 102 Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent.     

扬子地块北缘西乡群孙家河组火山岩形成时代及元素地球化学研究

西乡群孙家河组为一套低绿片岩相浅变质火山-沉积岩系,主要由基性-中基性-酸性火山岩和凝灰岩、沉凝灰岩、泥岩、硅质岩组成,火山岩岩石类型包括玄武岩、安山岩、英安岩和流纹岩.LA-ICPMS锆石U-Pb定年揭示流纹岩形成时代为832.9±4.9Ma,辉石玄武岩的形成时代为845.0±17Ma,两者在误差范围内一致,属新元古代同期岩浆作用产物.元素地球化学研究表明,孙家河组玄武岩属拉斑玄武岩系列,具有受地壳混染的板内玄武岩的地球化学特点.玄武岩-安山岩-英安岩主量元素成分投点呈规律性变化、REE球粒陨石标准化及微量元素原始地幔标准化分配型式具有一致性并相互重叠,不相容元素Th和相容元素Cr相关模拟图中沿分离结晶线分布,证明玄武岩-安山岩-英安岩为同一基性岩浆分离结晶的产物.REE和微量元素分配型式以及微量元素比值对的显著差异,暗示流纹岩与玄武岩-安山岩-英安岩来源于不同源区.Sr-Nd同位素研究表明,玄武岩-安山岩-英安岩样品的ε_(Nd)(t)值均大于0以及在ε_(Nd)(t)-(~(87)Sr/~(86)Sr)_t图解中位于OIB成分区,表明其源区应为与洋岛玄武岩类似的地幔源区;流纹岩样品具有可与基性熔岩相比拟的ε_(Nd)(t)值,暗示流纹岩最有可能是初生玄武质地壳部分熔融而成.本文所研究的原划孙家河组火山岩系列的形成时代、构造环境的确定以及扬子陆块乃至世界上同一时间内普遍发育大陆裂谷岩浆岩组合的地质事实,说明原划孙家河组以及西乡群中的确存在新元古代的组成部分,它们应是新元古代大陆裂谷的产物,它和扬子地块820M8后造山裂解环境花岗岩均是新元古代晚期Rodinia超大陆裂解作用的岩浆响应.     

Geochronology and genesis of subalkaline basaltic lava rivers at the Dzhavakheti highland,lesser Caucasus: K-Ar and Sr-Nd isotopic data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (⁸⁷Sr/⁸⁶Sr = 0.7039–0.7042; ∈_Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈_Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The ⁸⁷Sr/⁸⁶Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.