Geochronology and Petrogenesis of the Archean Silicic Volcanoplutonic Series of the Verkhovtsevo Greenstone Structure,Ukraine

Geological and isotope-geochemical studies of acid volcanics in the Verkhovtsevo greenstone belt and surrounding tonalite-trondhjemite plutons within the central Dnieper gneiss-green- stone terrain were conducted in the search for genetic relationships and increased understanding of the petrogenesis of acid melts. The acid volcanic and plutonic rocks are similar in mineral composition and form a unified calc-alkaline-like trend from dacite/tonalite to rhyolite/ trondhjemite. Dacites and tonalites have the same rare-earth element (REE) patterns with moderately fractionated light and heavy REE as well as small negative Eu anomalies. Rhyolite and trondhjemites have less-fractionated REE patterns with larger negative Eu anomalies. Whole-rock data for the acid volcanic and plutonic rocks yielded a single isochron of 3117 ± 204 Ma, εNd = +1.14 ± 0.80.

The data suggest a temporal and genetic relationship between the acid volcanics of the greenstone sequences and the surrounding plutonic rocks; both appear to belong to a single suite. The positive eNd value tends to suggest that a source of their melts can be traced to mafic materials rather than to older sialic crust. Petrochemical data and REE-model calculations suggest that dacite/tonalite liquids might have formed during partial melting of a mafic source, such as Archaean tholeiite TH-1 in equilibrium with hornblende-pyroxene-plagioclase restite. Subsequent differentiation of these melts in equilibrium with titanoilmenite-pyroxene-plagioclase cumulate may have given rise to the trondhjemites and rhyolites. Such a mineralogy of the restite and cumulate phases suggests that felsic melts containing little water in the Verkhovtsevo greenstone belt were generated at depths up to 30 km, probably in the greenstone belt's mafic basement.     

Young bimodal volcanism at Medicine Lake volcanic center,northern California

The last 10,000 years of activity at the Medicine Lake volcanic center in northern California is characterized by bimodal mafic and siliceous volcanism. Interflow element variations are complex and exhibit a discontinuity for most elements between 57 and 62 per cent SiO₂. No simple linear or curvilinear element trends exist between the mafic (Modoc) and siliceous (glass) volcanics.The geochemical variation patterns exhibited by volcanic rocks from the Medicine Lake volcanic center preclude any simple model for magma origin involving either varying degrees of melting or of fractional crystallization. A model is tentatively invoked for the andesites and basalts involving ? 35 per cent melting of eclogite (of altered rise tholeiite composition) in a descending slab followed by varying amounts of fractional crystallization and perhaps magma mixing. Up to 20 per cent of shallow fractional crystallization of plagioclase and minor Ti-magnetite seems to be required by the Sr, Eu anomaly, and TiO₂ distributions.Compositional variation and high δO¹⁸ values in most dacite glass flows are best interpreted in terms of a crustal origin involving up to 50 per cent partial melting of average continental crust. Rhyolite glasses may have formed by small degrees of melting (20–30 per cent) of this crust followed by 5–10 per cent of shallow fractional crystallization (removing dominantly plagioclase) or by 40–50 per cent fractional crystallization of a dacite parent (~63 per cent SiO₂) produced in the crust. The shallow fractional crystallization is necessary to explain the low Sr contents and large negative Eu anomalies in the rhyolites. Dacites from the Composite Flow are tentatively interpreted to have formed by shallow mixing of a hybrid magma (composed of varying amounts of andesite and dacite) with rhyolite prior to and during eruption.     

Neoproterozoic Volcanic Rocks in the Southern Quruqtagh of Northwest China:Geochemistry,Zircon Geochronology and Tectonic Implications

The Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts,alkaline andesites,and calc-alkaline rhyolites.The rhyolites are characterized by variably fractionated enrichment in light rare earth elements（LREE） and flat in heavy rare earth elements（HREE）,and strongly negative Eu anomalies.Compared to the rhyolites,the andesites also exhibit enrichment in LREE and flat HREE（chondrite-normalized values of La/Yb,and La/Sm are 13.30-41.09,3.18-6.89 respectively）.Their rare earth element patterns display minor negative Eu anomalies.Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb,Zr,Ti,and Hf on spider diagrams.Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 743±7 Ma,741±2 Ma,and 727±4 Ma.These ages are interpreted to represent the timing of volcanic eruptions. According to geochemistry and rock type,these volcanic rocks formed within a continental island-arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.     

闽中地区马面山群东岩组变质岩形成的古构造环境研究

闽中地区马面山群东岩组地层主要为绿片岩为主的一套古火山沉积建造。其主要岩性类型包括各种成分的绿片岩、大理岩、石英片岩及变粒岩类。绿片岩显示海底火山喷发特征,变粒岩原岩为中酸性岩类。东岩组变质岩岩石化学研究表明,绿片岩的原岩应为玄武岩类。变粒岩类主要属于英安岩及流纹岩。这些特征反映东岩组具双峰式火山岩特征,形成于大陆内部张性环境。绿片岩稀土元素特征也显示和大陆拉张环境中的火山岩类稀土特征非常相似,属大陆拉斑玄武岩;微量元素分布显示出该组变质岩原岩类似于大洋岛和大陆裂谷的板内碱性玄武岩。因此闽中地区中元古代可能处于板内古裂谷环境。     

Geochemistry of the Yandal belt metavolcanic rocks,Eastern Goldfields Province,Western Australia

Archaean felsic metavolcanic rocks occur throughout the Yandal belt in the north of the Eastern Goldfields of Western Australia where they are most abundant in the higher parts of the stratigraphy. With the exception of the Spring Well Sequence at the southern end of the belt, these rocks are typically dacites showing geochemical affinities with Archaean high‐Al trondhjemite‐tonalite‐dacite (TTD) suites. They have high Sr, Al₂O₃, and (La/Yb)_N; low Y, Nb, Zr and heavy rare‐earth elements (HREE); and lack a significant Eu anomaly. In contrast, broadly coeval mafic volcanic rocks have flat REE patterns and trace‐element compositions more typical of modern backarc basin basalts. The Spring Well Sequence is readily distinguished lithologically and geochemically from the remainder of the Yandal belt. Spring Well basaltic andesites are geochemically similar to modern calc‐alkaline arc magmas, i.e. negative Nb–Ta anomalies and enrichment of both large‐ion lithophile elements (LILE) and light rare‐earth elements (LREE). Andesites and rhyolites, both abundant in the Spring Well Sequence, have elevated LILE relative to high field strength elements, and moderate to strong negative Nb, Ta, Sr and Ti anomalies. Rhyolites have low Sr/Y and relatively flat REE patterns ((La/Yb)_N = 4.2–5.0). The chemistry and lithostratigraphic associations of the Yandal belt, with the exception of the Spring Well area, suggest a similarity with the Kalgoorlie Terrane, which is supported by published geochronological data. In contrast, the abundance of rhyolite, distinctive calc‐alkaline chemistry and ca2690 Ma age of the Spring Well Sequence suggests a possible association with ca2692 Ma bimodal calc‐alkaline arc‐rift sequence at Teutonic Bore and similar rocks at Melita and Jeedamya, 150 km south of Spring Well. The abundance of TTD dacite and tholeiitic basalt throughout the Yandal belt suggests magma generation from both decompression partial melting of mantle peridotite to produce backarc tholeiitic magma, and partial melting of subducted oceanic lithosphere to produce high‐Al dacite‐tonalite magma. Based on field relationships of the lithological associations, spatial geochemical patterns and published geochronological data, a shallow, west‐dipping subduction model is postulated for the Yandal belt. In this model, widespread tholeiitic basalt and TTD dacite volcanic sequences are thought to have formed in a backarc basin west of a predominantly submerged continental margin volcanic arc. The dominance of dacite in the upper stratigraphy of the Yandal belt could indicate the development of a secondary volcanic ridge or arc in this basin. The Spring Well Sequence is interpreted to occupy the northern preserved portion of the primary arc, remnants of which now extend south through Teutonic Bore to the Melita and Jeedamya volcanic centres. South of Spring Well, volcanic sequences become distinctly bimodal with basalt and high silica rhyolite suggesting an increasing influence of arc extension toward the south.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

内蒙古朱日和地区早古生代岩浆岩年代学、地球化学特征及其构造意义

中亚造山带早古生代的构造演化一直存在不同的认识,特别是其俯冲转为碰撞的时限.内蒙古朱日和南部的额尔登陶勒盖地区出露的早古生代岩浆岩,对于限定白乃庙岛弧的延伸方向、岛弧演化和拼贴时间具有重要意义.对额尔登地区出露的火山岩和侵入岩进行了精确的锆石SHRIMP和LA-ICP-MS U-Pb定年、锆石Lu-Hf同位素和地球化学分析以确定其时代和成因.研究获得4个岩浆岩年龄,1件英安岩年龄为434.0±4.4 Ma,2件花岗岩年龄分别为432.8±2.6 Ma和428.1±1.8 Ma,此3件样品形成于早中志留世;另有1件流纹岩年龄为411.8±1.0 Ma,形成于早泥盆世.早中志留世侵入岩为低钾、过铝质钙碱性中酸性岩石,富集Rb、Th,弱富集Zr、Hf,明显亏损Nb、Ta、P、Ti.在稀土配分曲线上,侵入岩均不存在Eu的负异常.其中2件花岗岩样品的锆石基本为正ε_Hf（t）值（-1.19~11.51,3.32~10.28）,T_DM2为684~1 493 Ma及759~1 202 Ma,主要来自新生地壳;早中志留世火山岩具有中-低钾、偏铝质-过铝质、钙碱性酸性岩石特征,富集Rb、Th,弱富集Zr、Hf,明显亏损Nb、Ta、P、Ti.在稀土配分曲线上,多数火山岩不存在Eu负异常.英安岩中的锆石具有正ε_Hf（t）值（3.70~7.94）,T_DM2为912~1 183 Ma,主要来自新生地壳;而早泥盆世流纹岩（411 Ma）存在明显的Eu负异常,其锆石具有负ε_Hf（t）值（-14.95~-7.07）,T_DM2为1.8~2.3 Ga,应为古老地壳再循环的产物,与早中志留世岩浆岩源区明显不同,可能代表构造环境的转变.综合区域地质和前人研究资料表明,早志留世英安岩和花岗岩形成于俯冲岛弧环境,白乃庙岛弧向东可延伸至本区,而早泥盆世流纹岩可能形成于后碰撞环境.      

Dokhan volcanics of Gabal Monqul area,North Eastern Desert,Egypt: geochemistry and petrogenesis

The Dokhan volcanics are represented by a thick stratified lava flows succession of basalt, andesite, imperial porphyry, dacite, rhyodacite, rhyolite, ignimbrites, and tuffs. These lavas are interbanded with their pyroclastics in some places including banded ash flow tuffs, lithic tuffs, crystal lapilli tuffs, and agglomerates. They are typical calc–alkaline and developed within volcanic arc environment. All rocks show moderate enrichment of most large ion lithophile elements relative to high field strength elements (HFSE). The incompatible trace elements increase from basalt through andesite to rhyolite. The felsic volcanics are characterized by moderate total rare earth elements (REE) contents (162 to 392 ppm), less fractionated patterns {(Ce/Yb)_N = (1.24 to 10.93)}, and large negative Eu anomaly {(Eu/Eu*) = (0.15 to 0.92)}. The mafic volcanics have the lowest REE contents (61 to 192 ppm) and are relatively steep {(Ce/Yb)_N = (3.2 to 8.5)}, with no negative Eu anomalies {(Eu/Eu*) = (0.88 to 1)}. The rhyolite displays larger negative Eu anomaly (Eu/Eu* = 0.28) than those of other varieties, indicating that the plagioclase was an early major fractionating phase. The mineralogical and chemical variations within volcanics are consistent with their evolution by fractional crystallization of plagioclase and clinopyroxene.     

Early Cretaceous continental arc-related volcanic rocks in the Duobuzha area,northern Tibet: implications for evolution history of the Bangong–Nujiang Ocean

New whole-rock major and trace elements data, zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb ages, and zircon Hf isotope compositions were analysed for Early Cretaceous volcanic rocks, also called Meiriqieco Formation (MF) in the Duobuzha area of the Southern Qiangtang–Baoshan Block (SQBB), northern Tibet. Our aim is to clarify their petrogenesis and tectonic setting, and constrain the evolution process on the northern margin of Bangong–Nujiang suture zone (BNSZ) during Early Cretaceous time. The MF volcanic rocks are mainly composed of andesites with subordinate basalts and rhyolites with high-K calc-alkaline affinity. Zircon LA-ICP-MS U–Pb dating for two andesite and one rhyolite samples give uniform ages within error of ca.113, 114, and 118 Ma, respectively, indicating they were erupted on the Early Cretaceous. The MF andesites have variable zircon ε_Hf(t) values (+0.5 to +10.5), which is different from those of MF rhyolites (+7.9 to +10.7). All the MF rocks are enriched in large ion lithophile elements, and depleted in high field strength elements, yielding the affinity of arc rocks. The MF basalts were most likely derived from the mantle wedge that was metasomatized by fluids released from subducting slab with the involvement of subducted sediments. The MF rhyolites were generated by partial melting of the juvenile mafic lower crust. The MF andesites are interpreted to have formed by mixing of the magmas that parental of the MF basalts and the MF rhyolites. In addition, a couple of distinctly magmatic sources are identified in the SQBB, and this may be related to mantle components injected into the continental crust. Combined with published geological data in the BNSZ and SQBB, we consider that the MF volcanic rocks are formed in a continental arc setting, suggesting that BNO were subducting during the Early Cretaceous time in the Duobuzha area.     

内蒙古西乌旗晚石炭世–早二叠世伸展事件——来自大石寨组火山岩的证据

内蒙古西乌旗地区发育一套中酸性火山岩,空间展布特征显示其为大石寨组火山岩的西延部分。为查明该火山岩的形成时代及构造属性,对其进行了岩石学、年代学和岩石地球化学研究。研究结果表明该火山岩主要由安山岩及流纹岩组成。锆石LA-MC-ICP-MS U-Pb定年结果显示其喷发时代为275~311 Ma,属晚石炭世–早二叠世。岩石地球化学特征表明中性岩富钙贫镁,富集LREE及K、Rb、Ba,亏损Nb、Ta,具有弱Eu异常,安山质岩浆可能是由基性岩浆分离结晶形成的,但在形成过程中受到了陆壳物质混染。酸性岩贫钙镁,富硅碱,具有显著的Eu负异常,亏损Nb、Ta、Sr、P、Ti,反映岩石成因与中下地壳的熔融及其后期的分离作用有关。在构造判别图解中,中性岩具有板内玄武岩特征,而酸性岩具有A2型花岗岩特征。结合区域已发表资料推断,西乌旗大石寨组火山岩形成于造山后伸展环境,暗示古亚洲洋至少在早二叠世之前已经闭合。     

Origin of felsic volcanism in the Izu arc intra-arc rift

An intra-arc rift (IAR) is developed behind the volcanic front in the Izu arc, Japan. Bimodal volcanism, represented by basalt and rhyolite lavas and hydrothermal activity, is active in the IAR. The constituent minerals in the rhyolite lavas are mainly plagioclase and quartz, whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite. Both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range. The petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts. The hydrous mineral-free narrow mineral compositions and low-Al₂O₃ affinities of the IAR rhyolites are produced from basaltic middle crust under anhydrous low-temperature melting conditions. The IAR basalt lavas display prominent across-arc variation, with depleted elemental compositions in the volcanic front side and enriched compositions in the rear-arc side. The across-arc variation reflects gradual change in the slab-derived components, as demonstrated by decreasing Ba/Zr and Th/Zr values to the rear-arc side. Rhyolite lavas exhibit different across-arc variations in either the fluid-mobile elements or the immobile elements, such as Nb/Zr, La/Yb, and chondrite-normalized rare earth element patterns, reflecting that the felsic magmas had different source. The preexisting arc crust formed during an earlier stage of arc evolution, most probably during the Oligocene prior to spreading of the Shikoku back-arc basin. The lack of systematic across-arc variation in the IAR rhyolites and their dry/shallow crustal melting origin combines to suggest re-melting of preexisting Oligocene middle crust by heat from the young basaltic magmatism.     

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.     

滇东南建水火山岩LA-ICP-MS锆石U-Pb测年、地球化学特征及地质意义

滇东南建水地区位于师宗-弥勒构造带的南段,区内出露一套玄武岩–安山岩–英安岩–流纹岩组合。研究这套火山岩的形成时间和形成环境对认识滇东南构造格局有着重要意义。本文首次对建水火山岩进行LA-ICP-MS锆石U-Pb测年,获得两个样品的年龄分别为261.9±2.2 Ma(MSWD=0.80)、264.8±1.7 Ma(MSWD=1.12),属中二叠世晚期,代表建水火山岩的形成时间。建水玄武岩与安山岩具有低TiO_2含量(0.50%~0.88%)、高Mg~#(52.0~64.5)、弱富集LREE((La/Yb)_N=1.42~3.44)、富集LILE(Rb,Th,U,Pb)、轻微亏损HFSE(Nb,Ta,Ti)的特点,具典型岛弧玄武岩的特征;英安岩和流纹岩高ΣREE含量(139.5×10~(–6)~313.6×10~(–6))、富集LREE((La/Yb)N=4.16~9.78)和LILE(Rb,Ba,Th,U)、亏损HFSE(Nb,Ta)、强亏损Sr、Ti、Y等元素的特点与典型的岛弧流纹岩相似,但高钾(K_2O含量平均7.73%)、钙碱性(δ=0.93~2.94)、强过铝质(A/CNK=1.13~2.10)的特点使之区别于一般岛弧酸性火山岩,而具有一些上地壳部分熔融形成的S型花岗岩的特点。综合建水火山岩岩石组合、地球化学和区域地质背景,认为建水火山岩形成于活动大陆边缘的弧后盆地伸展环境,是地幔部分熔融形成的玄武质岩浆结晶分异与上地壳混染作用的共同结果,与滇桂交界处岛弧火山岩、两广交界处岛弧玄武岩等同为哀牢山洋向北俯冲的产物。     

北山南带音凹峡地区酸性火山岩年代学、地球化学研究:二叠纪裂谷岩浆作用的新证据

北山造山带位于中亚造山带南缘中段地区,是中亚造山带、塔里木克拉通和华北克拉通的构造结合部。年代学资料表明北山南带晚古生代存在一期重要的岩浆活动,所形成的岩石类型包括了镁铁-超镁铁质杂岩、花岗岩类以及酸性火山岩。音凹峡地区位于北山南带,该地区广泛分布着厚度巨大的二叠系。这套地层主要是由酸性火山岩及火山碎屑岩组成,流纹岩的锆石LA-ICP-MS U-Pb年龄为273±1 Ma,为早二叠世。针对音凹峡地区酸性火山岩的地球化学研究发现,其具有高SiO_2、K_2O+Na_2O、Al_2O_3含量、低Fe_2O_3~T、Mg O、P_2O_5含量;富集轻稀土元素,重稀土元素无分异,具有明显的Eu负异常;微量元素方面,酸性火山岩富集Cs、Rb、Th、U、Zr和Hf等元素,而相对亏损Ba、Nb、Ta、Sr、P、Ti等元素。酸性火山岩的锆石ε_(Hf)(t)值为-6.0~3.9,具有较老的Hf同位素模式年龄,t_(DM2)=1046~1669 Ma。音凹峡酸性火山岩的地球化学证据表明这套火山岩系可能由中-新元古代壳源岩石部分熔融形成,并与幔源岩浆进行了不同程度的混合,之后发生了分离结晶作用。综合音凹峡地区同时代镁铁质岩石、花岗岩类的研究成果,以及该地区二叠系沉积建造及火山岩特征分析可知,北山南带在早二叠世应处于大陆裂谷的构造背景。     

拉萨地体北部永珠地区早白垩世岩浆岩地球化学、锆石U-Pb年代学、Hf同位素组成及其地质意义

拉萨地体北部出露大面积早白垩世岩浆岩,对它们的成因和形成机制的研究,有助于揭示拉萨地块白垩纪时期的岩浆作用过程及动力学背景.通过岩石学、地球化学和同位素地质学方法对拉萨地体北带永珠地区早白垩世中-酸性岩浆岩进行了研究.结果显示黑云母二长花岗岩、流纹岩和安山岩的锆石LA-ICP-MS U-Pb年龄分别为118±1.0 Ma、121±0.8 Ma和115±0.8 Ma,代表了其侵入和喷出时代.黑云母二长花岗岩、花岗斑岩和流纹岩为高钾钙碱性过铝质-强过铝质岩浆岩（A/CNK=1.01~1.35）,亏损高场强元素Nb、P、Ti和大离子亲石元素Ba、Sr,富集大离子亲石元素Rb、K和放射性元素U、Th;稀土配分图显示LREE富集,HREE近平坦,Eu明显负异常,为形成于大陆边缘的岛弧岩浆岩特征.黑云母二长花岗岩和流纹岩的锆石Hf初始比值ε_Hf（t）分别为-1.21~3.01和-0.68~5.35,对应的两阶段模式年龄分别为0.99~1.26 Ga和0.84~1.22 Ga,为壳幔混源岩浆.安山岩为高钾钙碱性,亏损Nb、Ta、P、Ti、U和Sr,富集Rb、K和Th,稀土配分图显示LREE富集,HREE近平坦,Eu轻微负异常,为形成于大陆边缘弧的岩浆岩.结合前人研究成果,分析认为永珠地区早白垩世岩浆岩形成于班公湖-怒江特提斯洋壳南向俯冲作用下的大陆边缘弧环境,由俯冲的班公湖-怒江中特提斯洋板片在深部脱水熔融,进而诱发上覆地幔楔部分熔融形成基性岩浆上涌,导致下地壳物质发生部分熔融形成酸性岩浆,它们在上升过程中按不同比例混合,形成中性和酸性岩浆侵入到地下或喷出地表,形成侵入岩和火山岩.      

西秦岭合作—美武地区侏罗纪郎木寺组火山岩地球化学特征及构造环境浅析

西秦岭合作—美武地区郎木寺组火山岩主要以中性的安山质火山岩为主,其次为英安岩及英安质火山碎屑岩和酸性的流纹质火山岩,为一套高K、低Ti、准铝质的钙碱性系列火山岩,微量元素总体富集Rb、亏损Th、La、Nd,Nb,稀土总量不高,含量不稳定,变化范围较大,轻重稀土分馏较明显,Eu存在微弱的负异常。通过对岩石学、岩石地球化学分析,表明其形成于活动大陆边缘弧环境,岩浆来源于地幔楔或俯冲板块上升过程中产生的俯冲岩浆带,为合作—美武地区火山岩岩石成因及构造环境研究提供依据。     

Origin,age and petrogenesis of Neoproterozoic composite dikes from the Arabian‐Nubian Shield,SW Jordan

The evolution of a Pan‐African (c. 900–550 Ma) suite of composite dikes, with latite margins and rhyolite interiors, from southwest Jordan is discussed. The dikes cut the Neoproterozoic calc‐alkaline granitoids and high‐grade metamorphic rocks (c. 800–600 Ma) of the northern Arabian‐Nubian Shield in Jordan and have been dated by the Rb‐Sr isochron method at 566±7 Ma. The symmetrically distributed latite margins constitute less than one‐quarter of the whole dike thickness. The rhyolite intruded a median fracture within the latite, while the latter was still hot but completely solidified. The dikes are alkaline and bimodal in composition with a gap in SiO₂ between 61 and 74 wt%. Both end members display similar chondrite‐normalized rare earth element patterns. The rhyolites display the compositional signature of A‐type granites. The (La/Lu)_N values are 6.02 and 4.91 for latites and rhyolites, respectively, and the rhyolites show a pronounced negative Eu anomaly, in contrast to the slight negative Eu anomaly of the latites. The chemical variability (e.g. Zr/Y, Zr/Nb, K/Rb) within and between latites and rhyolites does not support a fractional crystallization relationship between the felsic and mafic members of the dikes. We interpret the magma genesis of the composite dikes as the result of intrusion of mantle‐derived mafic magma into the lower crust in an extensional tectonic regime. The mafic magma underwent extensive fractional crystallization, which supplied the necessary heat for melting of the lower crust. The products of the initial stages of partial melting (5–10%) mixed with the fractionating mafic magma and gave rise to the latite melts. Further partial melting of the lower crust (up to 30%) produced a felsic melt, which upon 50% fractional crystallization (hornblende 15%, biotite 5%, feldspars 60%, and quartz 20%) gave rise to the rhyolitic magma. Copyright © 2004 John Wiley & Sons, Ltd.     

Basalt-rhyolite volcanism by MORB-continental crust interaction: Nd,Sr-isotopic and geochemical evidence from southern San Joaquin Basin,California

The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km³ of the Tecuya volcanic rocks. The basalts have _Nd(t) values of +2 to +6 and (⁸⁷Sr/⁸⁶Sr)_i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)_N =2.4–5.5; La=28–150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr–Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. _Nd(t) values in rhyolites vary from +1.5 to +3.7 while (⁸⁷Sr/⁸⁶Sr)_i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)_N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu^*=0.75) and depletion of Ti and P. Mixing relations in (⁸⁷/⁸⁶Sr)_i – _Nd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin.     

华北板块南缘熊耳群火山岩岩石类型及火山岩系列

基于大量的岩相学和岩石化学资料，作者认为，熊耳群火山岩的岩石类型主要为橄榄玄粗岩，安粗岩，粗面英安岩和流纹岩，其次为高钾玄武安山岩、橄榄粗安岩，高钾安山和高钾英安岩；火山岩系列主要为橄榄玄粗岩系，其次为高钾钙碱性岩系；熊耳群火山岩可能是大陆边缘弧弧后火山岩或碰撞后弧火山岩。同时，作者在文中列出了４４个代表性火山岩样品的岩石化学资料和９１３个火山岩，次火山－侵入岩样品的平均化学成分资料。     

准噶尔盆地西北缘晚石炭系-早二叠系火山岩岩石学和地球化学研究

晚石炭纪-早二叠纪,在准噶尔盆地西北缘,发育区域性的火山岩建造,岩性以酸性英安岩和流纹岩为主,同时发育少量中性玄武安山岩和安山岩,多属于中-低钾的钙碱性系列。样品全碱(Na₂O+K₂O)含量为0.99％ ～8.12％,K₂O的含量较低,为3.01％ ～0.05％,稀土元素总量(∑ REE)较低。稀土元素轻重分异,轻稀土元素(LREE)相对于重稀土元素(HREE)富集:(La/Yb)_N=0.76～5.18,且部分样品显示轻微的Eu负异常和Ce正异常:δEu=0.533～1.148,δCe=0.979～1.224。微量元素中大离子亲石元素(LILE)富集,高场强元素Nb、Ta亏损。分析认为石炭系末期-二叠系早期,研究区所处构造环境为洋内弧附近的前弧盆地,发育的火山岩是母源岩浆经历了一定程度分异作用后的产物,以岛弧环境为主,兼具有部分板内大陆环境特征。