Geochemical characteristics of island-arc volcanic rocks in the Nan-Nam Pat-Phetchabun zone,northern Thailand

Late Permian-Early Triassic (P₂-T₁) volcanic rocks distributed on the eastern side of ocean-ridge and oceanic-island basalts in the Nan-Uttaradit zone were analyzed from aspects of petrographic characteristics, rock assemblage, REE, trace elements, geotectonic setting, etc., indicating that those volcanic rocks possess the characteristic features of island-arc volcanic rocks. The volcanic rock assemblage is basalt-basaltic andesite-andesite. The volcanic rocks are sub-alkaline, dominated by calc-alkaline series, with tholeiite series coming next. The chemical composition of the volcanic rocks is characterized by low TiO₂ and K₂O and high Al₂O₃ and Na₂O. Their REE patterns are of the flat, weak LREE-enrichment right-inclined type. The trace elements are characterized by the enrichment of large cation elements such as K, Rb and Ba, common enrichment of U and Th, and depletion of Nb, Ta, Zr and Hf. The petrochemical plot falls within the field of volcanic rocks, in consistency with the plot of island-arc volcanic rocks in the Jinsha River zone of China. This island-arc volcanic zone, together with the ocean-ridge/oceanic island type volcanic rocks in the Nan-Uttaradit zone, constitutes the ocean-ridge volcanic rock-island-arc magmatic rock zones which are distributed in pairs, indicating that the oceanic crust of the Nan-Uttaradit zone once was of eastward subduction. This work is of great significance in exploring the evolution of paleo-Tethys in the Nan-Uttaradit zone.     

泰国北部清迈带洋岛型火山岩特征研究

泰国北部清迈带洋岛型火山岩常被下石炭统及上二叠统浅水碳酸盐岩覆盖,主要岩石类型有夏威夷岩和钾质粗面玄武岩。以碱性系列为主,极个别亚碱性。主要化学成分以高TiO2、高P2O5和中K2O为特征;稀土元素强烈的轻稀土富集右倾斜型;微量元素模式呈K-Ti上隆富集型;多种成分投图落于洋岛玄武岩区及碱性玄武岩区,属于洋岛型火山岩。与中国云南德钦、耿马(昌宁-孟连带)洋岛玄武岩类似,它们是清迈带古特提斯洋壳的极重要组成部分。     

川西甘孜-理塘结合带洋岛型玄武岩地球化学、年代学特征及其地质意义

甘孜-理塘结合带作为三江特提斯造山带与松潘-甘孜造山带的重要组成部分,发育大量的洋岛-海山岩石组合,洋岛玄武岩的存在标志着洋盆已发育成熟的洋壳.木里地区的洋岛-海山岩石组合由"基性火山岩+碳酸盐岩"构成,对基性火山岩的地球化学特征分析表明：基性火山岩SiO₂含量为40.17%~49.19%,TiO₂含量1.77%~4.86%,Al₂O₃含量9.67%~15.39%,MgO含量3.85%~17.75%,全碱含量（K₂O+Na₂O）0.87%~6.33%,属碱性玄武岩系列.稀土总量∑REE值较高,为106.21×10^-6~378.83×10^-6,（La/Yb）_N比值为9.23~39.41,为轻稀土富集型,Eu、Ce无异常,配分模式为右倾型.微量元素表现出大离子亲石元素Rb、Ba、Th、K富集,而亏损高场强元素Nb、Zr.稀土元素及微量元素特征与标准洋岛型玄武岩相近,源区为软流圈地幔石榴石橄榄岩,且部分经历了岩浆结晶分异,形成于洋岛台地环境.玄武岩的锆石U-Pb定年结果为218.96~221.71 Ma,表明洋岛型玄武岩形成于晚三叠世中期.     

Study on the geochemical characteristics of arc-volcanic rocks in the Chiang Rai-Lampang belt of northern Thailand

On the basis of the petrographic characteristics, rock assemblages, petrochemistry, REEs, trace elements and geotectonic settings, the authors described the characteristics of continental marginal arc-volcanic rocks in the Late Permian-Early Triassic (P2-T1) volcanic rocks distributed on the eastern side of the ocean-ridge/oceanic island basalts in the Chiang Mai belt. The volcanic rock assemblage is basaltic andesite-andesite-rhyolite. The volcanic series is dominated by the calc-alkaline series, with the tholeitic series coming next. The chemical composition of the volcanic rocks is characterized by high Al2O3; the REE distribution patterns are of the LREE-enrichment rightward incline type; the large cation elements are highly enriched, and the volcanic rocks are generally enriched in U and Th and depleted in Ti, Cr and P. The petrochemical plot falls within the field of island-arc volcanic rocks, in consistency with the projected points of continental marginal arc-volcanic rocks in the Lancangjiang belt. These continental mar-ginal arc-volcanic rocks, together with ocean-ridge/oceanic island-type volcanic rocks in the Chiang Mai belt, con-stitute the ocean-ridge volcanic rock-arc magmatic rock belts distributed in pairs, indicative of eastward subduction of the oceanic crust in the Chiang Mai belt. This result is of great importance in exploring the evolution of the pa-leo-Tethys in the Chiang Mai belt.     

西准噶尔洋中海山及其俯冲带处地质效应

随着研究的不断深入,在中亚造山带(CAOB)不断有不同时代的洋岛玄武岩(OIB)被识别出来。在中亚造山带西南缘的西准噶尔地区的多条蛇绿混杂岩带中,也存在具有OIB特征的玄武岩。这些玄武岩呈枕状,与超基性岩、辉长岩、块状玄武岩、灰岩及紫红色硅质岩等紧密共生。地球化学研究表明枕状玄武岩均为碱性系列,具有较高的TiO2含量(大多>2.5%)、强烈富集轻稀土元素、无明显Nb、Ta负异常,与典型的OIB极为相似,认为其可能形成于大洋板内与地幔柱有关的海山环境。通过对海山的发展阶段分析认为,西准噶尔地区海山应该发展到爆炸海山阶段,因为其中发育大量的枕状熔岩。海山中火山岩或火山碎屑沉积物富集大离子亲石元素和高场强元素,海山的俯冲将对弧及弧后地区火山岩地球化学产生明显影响,而西准噶尔地区泥盆纪-石炭纪火山岩中恰恰存在海山的信号。因此,海山俯冲模式可能能更好地解释西准噶尔地区火山岩中存在OIB特征火山岩的成因。另外,海山俯冲还存在潜在的资源效应,因此应该寻找和研究古海山及火山岛链俯冲的迹象,将对进一步找金铜等矿提供可靠依据。     

Tectonic Implications of the Carboniferous Volcanic Rocks in the Eastern Junggar — Evidence from Zircon U‐Pb Ages and Geochemistry

Well Drilling shows that the volcanic rocks from the Carboniferous Batamayineishan Formation in the Eastern Junggar basin are mainly composed of volcaniclastic rocks (av. 52%) and volcanic lavas (32%), with a small amount of volcanic pyroclastic lavas (av. 11%). The volcanic lavas are basalt‐basaltic andesite‐andesite‐dacite assemblage. The LA‐ICP‐MS zircon U‐Pb dating of the andesite and the dacite yielded 325～321 Ma and 310 Ma ages, respectively, which is of high agreement with the published age (300 Ma) of basalts from this Formation, it is implied that an important volcanic activity occurred in Junggar basin in the late Carboniferous. The lavas have low TiO₂ and high Na₂O, indicating a calc‐alkaline series. Geochemical data show that they are characterized by LREE‐enriched patterns with slightly negative Eu anomalies. The rocks have high large ion lithophile element (LILE), and low high field strength element (HFSE) concentrations, with strong negative Nb, Ta and Ti anomalies. From basic through intermediate to felsic, the depletions in Sr, Ti and P of the studied volcanic rocks increase gradually. These geochemical characteristics indicate that the volcanic rocks are magmatic evolution products attributed to partial melting of mantle‐derived spinelle lherzolite related to oceanic subduction in an island‐arc setting. In combination with the LA‐ICP‐MS zircon U‐Pb dating, it is inferred that subduction of the Junggar Ocean in eastern Junggar basin lasted to the Late Carboniferous. Consequently, the final closure of the Junggar Ocean occurred most likely after 310 Ma.     

藏北中生代火山岩的岩石化学与地球化学初步研究

在西藏冈底斯山以北、唐古拉山以南地区,分布着侏罗纪到白垩纪的基性和中性火山岩。它们与本区的超基性岩空间上接近,或者呈断层接触。对这些岩石组合的成因有不同的解释。有人认为是大陆碰撞缝合带的蛇绿岩(常承法与郑锡澜,1974),有人认为是岛弧与边缘海的产物。近来,张旗(1983)在丁青发现了比较连续的蛇绿岩组合。要正确地对这些岩石的构造环境进行解释,必须进行详细的岩石学、岩石化学和微量元素地球化学研究。本文对采自拉弄沟、罗布中(图1)两地的玄武岩和采自切里湖的安山岩的岩石化学和微量元素地球化学做了初步研究,这里将初步结果及讨论作一报道。     

New data on the Early Cenozoic volcanism of the Kolyuchinskii-Mechigmen graben,Chukotka Peninsula

The isotopic-geochemical composition of the Late Paleocene-Early Eocene basalts in the central part of the Kolyuchinskii-Mechigmen graben on the east of the Chukotka Peninsula is studied. The distribution of major and trace elements and the elemental ratios point to the formation of basalts under conditions of marginal-continental rifting. The basalts of Mount Otdel’naya are characterized by a combination of depleted, intraplate, and suprasubduction geochemical features, which differentiates them both from the rocks of suprasubduction volcanic belts and tholeiites and alkaline lavas of the continental rifts and oceanic island arcs. The specific peculiarities of basalts of Mount Otdel’naya are extremely high, relative to similar basalts of spreading zones of the region and the ⁸⁷Sr/⁸⁶Sr ratios, which reflect the involvement of carbonate material in the magma generation zone.     

Composition, sources, and mechanism of continental crust growth in the Lake zone of the Central Asian Caledonides: I. Geological and geochronological data

Data on the composition, inner structure, and age of volcanic and siliceous-terrigenous complexes and granitoids occurring in association with them in the Caledonian Lake zone in Central Asia are discussed in the context of major relations and trends in the growth of the Caledonian continental crust in the Central Asian Foldbelt (CAFB). The folded structures of the Lake zone host basalt, basalt-andesite, and andesite complexes of volcanic rocks that were formed in distinct geodynamic environments. The volcanic rocks of the basalt complex are noted for high concentrations of TiO₂ and alkalis, occur in association with fine-grained siliceous siltstone and siliceous-carbonate rocks, are thus close to oceanic-island complexes, and were likely formed in relation to a mantle hotspot activity far away from erosion regions supplying terrigenous material. The rocks of the basalt-andesite and andesite complexes have lower TiO₂ concentrations and moderate concentrations of alkalis and contain rock-forming amphibole. These rocks are accompanied by rudaceous terrigenous sediments, which suggests their origin in island-arc environments, including arcs with a significantly dissected topography. These complexes are accompanied by siliceous-terrigenous sedimentary sequences whose inner structure is close to those of sediments in accretionary wedges. The folded Caledonides of the Lake zone passed through the following evolutionary phases. The island arcs started to develop at 570 Ma, their evolution was associated with the emplacement of layered gabbroids and tonalitetrondhjemite massifs, and continued until the onset of accretion at 515–480 Ma. The accretion was accompanied by the emplacement of large massifs of the tonalite-granodiorite-plagiogranite series. The postaccretionary evolutionary phase at 470–440 Ma of the Caledonides was marked by intrusive subalkaline and alkaline magmatism. The Caledonides are characterized by within-plate magmatic activity throughout their whole evolutionary history, a fact explained by the accretion of Vendian-Cambrian oceanic structures (island arcs, oceanic islands, and back-arc basins) above a mantle hotspot. Indicators of within-plate magmatic activity are subalkaline high-Ti basalts, alkaline-ultrabasic complexes with carbonatites and massifs of subalkaline and alkaline gabbroids, nepheline syenites, alkaline granites, subalkaline granites, and granosyenites. The mantle hotspot likely continued to affect the character of the lithospheric magmatism even after the Caledonian folded terrane was formed.     

新疆阿尔泰山南缘玛音鄂博断裂南侧变质基性岩的发现及其地质意义

位于额尔齐斯-玛音鄂博大断裂带南侧的阿热勒托别变质基性岩产于下石炭统姜巴斯套组一套细碎屑沉积岩中.岩石组成以变质玄武岩为主, 它们具有相对高的TiO₂、MgO和低SiO₂、K₂O的特点; 在稀土元素组成上, 显示轻稀土弱富集的配分模式, 无明显Eu异常; 微量元素组成显示大离子亲石元素富集, 并具有明显的正Th异常和弱的Nb负异常、高的Nb含量(> 2×10-6) 以及HFSE基本上和MORB相当的特点; 其Zr/Nb比值落在MORB范围之内, 而Ti/V比值略比MORB高, 表明其为MORB源的亏损地幔在相对较低熔融程度下熔融的产物.其ε_Nd(t) 值为+7.40~+8.35, 略低于MORB, 但高于洋岛以及大陆板内玄武岩.因此该变质基性岩总体上兼有火山弧和洋中脊玄武岩特征, 因而其可能形成于弧后环境, 代表了弧后盆地扩张早期的产物.      

班公湖-怒江板块缝合带发现侏罗纪洋岛型岩石组合

那东洋岛大地构造位置位于班公湖-怒江板块缝合带西段,洋岛型岩石组合平行于蛇绿混杂岩带南侧展布,夹持于侏罗系木嘎岗日群之中.那东洋岛玄武岩与木嘎岗日群复理石砂板岩呈过渡接触关系.那东洋岛岩石组合主要由3种岩石组成:玄武岩、玄武质砾岩、灰岩或礁灰岩,那东洋岛的岩石组合具有多旋回性.对那东洋岛中的玄武岩进行地球化学分析,玄武岩为OIB型玄武岩.以往研究认为班公湖-怒江洋盆在休罗纪处于大洋演化阶段,那东洋岛的发现对揭示班公湖-怒江洋在侏罗纪的演化历史具有重要意义.     

Petrology,geochemistry and geochronology of the Zhongcang ophiolite,northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ~(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).     

Evolution of an intra‐oceanic island arc during the Late Silurian to Late Devonian,New England Fold Belt

Geochemical studies of volcanic rocks in the Gamilaroi terrane and Calliope Volcanic Assemblage, New England Fold Belt, eastern Australia, indicate that the setting in which these rocks formed changed in both space and time. The Upper Silurian to Middle Devonian basalts of the Gamilaroi terrane show flat to slightly light rare‐earth element (LREE) depleted chondrite normalised patterns, depletion of high field strength elements (HFSE) relative to N‐MORB, low Ti/V and high Ti/Zr ratios, high Ni, Cr and large‐ion lithophile element (LILE) contents, features characteristic of intra‐oceanic island arc basaltic magmas. They are associated with low‐K, less mafic volcanics, showing moderate LREE enrichment, low Nb and Y contents and Rb/Zr ratios. The depletion of HFSE in the basalts indicates that the magmas were derived from a refractory source in a supra‐subduction zone setting. The presence of such a zone implies that the arc was associated with a backarc basin, the location of which was to the west where a wide backarc region existed from the Middle Silurian. This polarity of arc and backarc basin suggests that the subduction zone dipped to the west. In contrast to their older counterparts, Middle to Upper Devonian basalts of the Gamilaroi terrane have MORB‐like chondrite normalised patterns and higher Ti and lower LILE contents. Moreover, they have low Ti/Zr ratios and MORB‐like Ti/V ratios and HFSE contents, features typical of backarc basins. Dolerites of the Gamilaroi terrane also have predominantly backarc basin signatures. These features suggest that both the basalts and dolerites have been emplaced in an extensional environment produced during the rifting of the intra‐oceanic island arc lithosphere. A progressive increase in Ti/V ratios, and TiO₂ and Fe₂O₃ contents at constant MgO, of stratigraphically equivalent basalts, towards the north‐northwest part of the belt, is consistent with either greater extension to the north or melting of a more fertile magma source. By contrast, basalts in the southeast part of the terrane have moderately high Ti/Zr and low Ti/V ratios and in some samples, exhibit depletion of HFSE, compositional features transitional between island arc and backarc basin basalts. The Lower to Middle Devonian mafic rocks in the Calliope Volcanic Assemblage show both LREE enriched and depleted chondrite normalised REE patterns. Further, the majority have high Ti/Zr ratios and low Zr contents as well as relatively high Th contents relative to MORB. These features are common to rocks of Middle Devonian age as well as those of Early Devonian age and are suggestive of eruption in an arc setting. Thus, the data from this study provide new evidence for the evolution of the New England Fold Belt from the Late Silurian to the Late Devonian and reveal a history more complicated than previously reported.     

东天山寨北山铜矿区钠质火山岩年代学、地球化学特征及其成因

寨北山矿区海相火山岩为一套富钠的玄武安山玢岩、安山岩、英安岩、流纹岩组合,属于钙碱性系列岩石,具有低MgO（0.51%~5.93%,平均2.54%）、FeO（0.54%~6.39%,平均2.84%）和钛（TiO₂=0.09%~1.10%,平均0.58%）,富铝（Al₂O₃=12.23%~17.75%,平均15.20%,A/CNK=0.79~1.42,平均1.11）以及富钠（Na₂O/K₂O平均为7.30）、富水的特征。火山岩中斜长石主要为钠长石,少量更长石。轻、重稀土分馏较明显（（LREE/HREE）_N=3.68~9.00）,微量元素显示大离子亲石元素（如Th、U、Rb）、轻稀土的富集和高场强元素（如Nb、Ta、Ti、P）相对亏损的特征。获得矿区雅满苏组钠质玄武安山玢岩SHRIMP锆石U-Pb谐和年龄为（337.6±3.3）Ma,为早石炭世火山活动的产物。火山岩岩石学及地球化学特征表明研究区钠质火山岩可能形成于俯冲带近大陆方向的岛弧构造环境,是早石炭世洋壳俯冲熔融产生的岩浆在海底喷发过程中与海水相互反应后,经低变质相作用产生的。成矿元素在钠长石化过程中可能被淋滤出来进入含矿热液,后期在适当的温压等条件下沉淀形成本区的矿床。     

Geochronology of the Dong Tso Ophiolite and the Tectonic Environment

The wedge shaped Dong Tso ophiolitic block is distributed near the transition point from the western to the middle sub-belt of the Bangong-Nujiang suture zone.The ophiolite is characterized by well-developed cumulate rocks that are mainly composed of cumulate and massive gabbros.In the cumulate gabbros,the adcumulate amphiboles are distributed extensively around the plagioclase and residual pyroxene grains; hence,the rocks are named adcumulate amphibole-gabbro.In this study,the formation age of the ophiolite has been estimated to be 166 ± 4 million years （Ma） by the sensitive high-resolution ion microprobe （SHRIMP） Ⅱ U-Pb isotopic analysis of the zircons from the adcumulate amphibole-gabbro; the 40Ar/39Ar plateau age was estimated to be 148.19 ± 1.53 Ma,which should represent the emplacement time of the ophiolite,by isotopic dating of the pure amphibole mineral from the amphibole-schist.Two different suits of volcanic lavas have been recognized in this work.The purple colored pillow basalts have high TiO2 and P2O5 contents,and are rich in light rare earth elements （LREEs）,large-ion lithospheric elements （LILEs） and high-field-strength elements （HFSEs）,the characteristics that are the typical of the oceanic island basalt （OIB）.On the other hand,other massive basaltic andesites of celadon color are poor in MgO; rich in Fe2O3,LREEs,LILEs,and HFSEs; and especially characterized by negative Nb and Ta anomalies,the properties that establish the andesites as continental arc volcanic rocks.It is concluded that hotspots had developed in the old Dong Tso basin,the oceanic basin that had been developing from middle Jurassic （166 Ma） or even before and emplaced northward in late Jurassic （about 148 Ma）.     

Quaternary high-Nb basalts: existence of young oceanic crust under the Sanandaj–Sirjan Zone,NW Iran

Quaternary basaltic volcanoes are distributed in the northern part of the Sanandaj–Sirjan Zone (N-SSZ). Those in the Ghorveh area of the N-SSZ are characterized by low SiO₂, high alkalis, and LILE + LREE enrichment. They also have high Mg numbers (Mg# = 65–70) and high contents of Cr (>300 ppm), Ni (>177 ppm), and TiO₂ (>1.5 wt.%), suggesting that they crystallized directly from primary magma. The basalts are classified as high-Nb basalts (HNB), with Nb concentrations greater than 20 ppm. Their ⁸⁷Sr/⁸⁶Sr values range from 0.7049 to 0.7053 and their ?⁰_Nd values lie between –0.2 and 1.1. The small negative values of ?⁰_Nd indicate involvement of continental material in the evolution of the source magma in the area. Based on these new chemical and isotopic data and their relationship to the Plio-Quaternary volcanic adakites in northern Ghorveh, we propose that the partial fusion of metasomatized mantle associated with adakitic magma was responsible for generation of the HNB rocks following late Miocene collision of the Arabian and Iranian plates. Rollback of Neotethyan oceanic spreading and mantle plume activity caused a thinning of the northern SSZ lithosphere; furthermore, the S wave tomography model beneath the N-SSZ supports this hypothesized lithospheric thinning. The HNB rocks have close spatial proximity and temporal association with adakites, which were formed by the subduction of young (<25 Ma) oceanic crust. Our discussion clarifies the role of the oceanic slab in the post-collision generation of the HNB basalts in this area. Our data confirm the relationship of the HNB rocks to the subduction zone instead of to the oceanic island basalt (OIB) type magma in extensional zones.     

陕西商州地区丹凤变质火山岩的地球化学特征

陕西商州地区丹凤变质火山岩具有洋内岛弧火山岩地球化学特征,它们是分别来自不同源区的拉斑玄武和钙碱性2个系列共存的一套变质火山岩。其Th/Ta比值高及Ni、Ta、Ti、Y和Yb含量低,表明岩石受到消减带组分的影响。种种证据表明,丹凤变质火山岩是早古生代华北地块南缘消减带之上洋内岛弧环境的产物。     

The petrogenesis of modern and ophiolitic lavas reconsidered: Ti-V and Nb-Th

Tectonic discrimination diagrams are a key tool for understanding ancient volcanic rock origins. In this contribution we compile over 15,000 whole rock compositions to re-evaluate the Ti-V discrimination diagram and compare it to another commonly used tool, the Nb/Yb-Th/Yb diagram. We have reformulated the Ti-V diagram into a log–log plot to distinguish samples more clearly at lower concentrations. The compilation shows that MORB are dominated by Ti/V = 20–43, whereas juvenile arc tholeiites and boninites are characterized by Ti/V < 20 generally, although there is minor overlap at the boundary (Ti/V = 20–22). Plume-related volcanic rocks (ocean island basalts, oceanic plateaux, and continental flood basalts) generally have Ti/V > 43, although there may be significant overlap with MORB-like ratios for ridge-centered OIB and for some oceanic plateaux. About 56% of alkaline OIB have Ti/V > 70. Back-arc basins are dominantly MORB-like. Melt models show that MORB and juvenile arc volcanics most likely formed under different ?O₂ conditions, but are permissive of similar ?O₂ if the arc rocks form by much higher melt fractions. The Nb/Yb vs. Th/Yb plot clearly distinguishes most oceanic basalts (MORB, plateaux, OIB) from subduction-related volcanic rocks (boninite, juvenile arc tholeiite, calc-alkaline) and from flood basalts. We propose here a new two-proxy diagram of Ti/V vs. Th/Nb, which incorporates the advantages of both.     

东昆仑巴颜喀拉浊积盆地二叠纪火山岩

东昆仑巴颜喀拉浊积盆地内沿扎拉依-哥琼尼洼断裂带及约古宗列断裂带的断夹块内出露有一套二叠纪马尔争组火山岩, 火山岩呈构造岩片形式产出, 扎拉依-哥琼尼洼断裂带火山岩岩石组合主要为玄武岩、玄武安山岩, 少量玄武岩具枕状构造, 岩石SiO₂含量均匀, TiO₂的含量较高, ALK的含量较低, 为拉斑系列的玄武岩, 玄武岩稀土总量较高, 稀土配分曲线为轻稀土富集型, 与洋岛型火山岩的稀土配分曲线相一致, 火山岩大离子亲石元素较富集, 高场强元素及重稀土元素较平坦, 稀土、微量元素特征及构造环境判别显示其形成于较富集的洋岛环境, 少数为洋中脊的构造环境.约古宗列断裂带火山岩岩石组合为玄武岩、玄武安山岩, 岩石SiO₂含量较高, TiO₂含量较低, 均 < 1%, 为钙碱性系列火山岩, 玄武安山岩的稀土配分曲线与扎拉依-哥琼尼洼断裂带一致, 而英安岩的轻稀土富集程度高, 与岛弧高钾安山岩的稀土配分曲线相吻合, 构造环境判别显示其形成于岛弧构造环境.根据扎拉依-哥琼尼洼断裂带两侧火山岩成分的差异以及断裂带两侧巴颜喀拉群碎屑物成分的差异, 可以把巴颜喀拉山三叠纪浊积盆地进一步划分为北亚带和南亚带.      

昌宁-孟连缝合带干龙塘-弄巴蛇绿岩地球化学及Sr-Nd-Pb同位素组成研究

本文对三江古特提斯昌宁-孟连带中段弄巴-干龙塘蛇绿混杂岩进行了详细的主量、微量元素及Sr-Nd-Pb同位素地球化学研究。结果表明,弄巴玄武岩包括拉斑系列和碱性系列,弄巴拉斑玄武岩具有高TiO2和低K2O的特征,(La/Yb)N介于1.87～2.38之间,岩石的Sr-Nd-Pb同位素组成和典型MORB十分相似,结合岩石较高的Th/Yb和低的Zr/Nb值,可以认为弄巴拉斑玄武岩具有富集型洋脊玄武岩(E-MORB)的特征,可能起源于富集的地幔源区或是亏损地幔源区和地幔柱发生交代作用的结果。弄巴碱性玄武岩具有较高的TiO2(2.38%)和K2O(2.37%)含量,(La/Yb)N=11.19,富集轻稀土,表现出典型的碱性OIB的特征,可能是大洋板内热点浅部熔融的产物。干龙塘拉斑玄武岩具有高TiO2、Mg#,低K2O和亏损轻稀土等特征,表现出N-MORB的地球化学特征,岩石的Sr-Nd-Pb与MORB相似,表明岩石起源于亏损的地幔源区。