Petrogenesis of the Dullstroom Formation, Bushveld Magmatic Province, South Africa

The Precambrian Dullstroom Formation of South Africa, which is predominantly composed of basaltic andesites interbedded with subordinate sedimentary and felsic volcanic strata, represents the first phase of an extended period of magmatism that was responsible for the Bushveld Magmatic Province, including the extrusive Rooiberg Group and the intrusive Bushveld Complex. New geochemical and isotopic data for the Dullstroom Formation are presented in an effort to elucidate the petrogenetic processes operative during the initiation of this magmatic episode. The volcanic units of the central portion of the Dullstroom Formation have been subdivided into at least three interbedded compositional groups: low Ti mafic to intermediate units, high-Ti mafic to intermediate units, and high Mg felsic units. High Ti and low Ti volcanic units are similar in some compositional characteristics to basalts of the nearby northern and southern provinces, respectively, of the Mesozoic Karoo continental flood basalts. Isotopic and compositional data for low Ti Dullstroom strata are consistent with bulk assimilation into a melt similar in composition to a southern Karoo basalt of 20% upper continental crust accompanied by 20% fractional crystallization of pyroxene and plagioclase. Isotopic and compositional data for high Ti Dullstroom strata are consistent with magma mixing of 30% northern Karoo K-rich basalt and 70% southern Karoo basalt followed by 20% assimilation of upper continental crust and 20% fractional crystallization of pyroxene and plagioclase. Compositions of high Mg felsic volcanic strata are consistent with 25% assimilation of a mixture of silica-rich sedimentary rock and upper continental crust into a melt similar in composition to low Ti volcanic units with 25% fractional crystallization of pyroxene and plagioclase. However, it has been suggested that compositions of these high Mg felsic strata may also be consistent with interaction of a crustal melt. Assimilation, fractional crystallization, and magma mixing that apparently affected these Dullstroom Formation volcanic strata may have occurred in a series of shallow magma chambers. These data are consistent with the suggestion that Dullstroom Formation volcanic rocks are the result of a mantle plume. Mantle plume origin also is suggested by the large volume of intrusive and extrusive strata associated with this magmatic episode. These data do not support the hypothesis that the Bushveld Complex and the Rooiberg Group formed by impacts of a cluster of comets or asteroids. Received: 14 October 1998 / Accepted: 17 May 1999     

Rb–Sr and Sm–Nd isotopic compositions of the Rooiberg Group, South Africa: early Bushveld-related volcanism

The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes).

Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for ⁸⁷Sr/⁸⁶Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for ¹⁴³Nd/¹⁴⁴Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement.     

Petrologic, stratigraphic and tectonic significance of Mesozoic volcanic rocks in the Río Wampú area, Eastern Honduras

A thick sequence of dark-red lava flows and clastic rocks with abundant volcanic debris crops out along the Río Wampú of eastern Honduras. Lithologic characteristics of these rocks, field interpretations of the Río Wampú stratigraphy, and limited K---Ar age determinations on volcanic samples indicate that these redbeds and associated volcanic units are correlative with the Upper Cretaceous Valle de Angeles Group of central Honduras. These strata uncomformably overlie dark clastic units which are tentatively correlated with the Jurassic Agua Fría Formation of the Honduras Group. The lava samples are predominantly microporphyritic andesites and basaltic andesites with abundant fine-grained phenocrysts of plagioclase, clinopyroxene and Fe---Ti oxides. Immobile-element whole-rock and mineral concentrations indicate a high-K tholeiitic composition. Elevated TiO₂ whole-rock and clinopyroxene concentrations found in the basaltic andesites apparently reflect a primary magmatic characteristic. Based on geochemical and modal data, it appears that these rocks were erupted from partial mantle melts which were modified by substantial fractionation of modal phases. The presence of widespread Mesozoic redbeds and associated volcanics in Honduras was previously cited as evidence of regional extensional tectonics. However, structural, geochemical and stratigraphic data collected during this study indicate that the Río Wampú volcanic suite was formed during a compressional (subduction-related) event. Similar high-TiO₂ basaltic andesites and andesites are reported along several active continental margins.     

The origin of low-K rhyolites from the mariana frontal arc

Low-K rhyolites and overlying 2-pyroxene andesites, both of Late Eocene age, comprise the oldest volcanic units exposed on Saipan. The mineralogy and geologic setting of these rocks indicate they were erupted in a volcanic arc setting. The presence of andesite and lack of basalts of similar age suggest that the rhyolites are not part of a typical bimodal (basalt/rhyolite) suite.Major and trace element data indicate the Saipan andesites were not parental to the rhyolites. Out of various models evaluated for derivation of the rhyolites, the most reasonable involves crystal fractionation of a boninite series andesite that was very depleted in LIL elements. This andesite probably evolved from more mafic magmas which in turn were derived from the sub-arc mantle. Isotopic data suggest the mantle source for these magmas may have contained a minor seawater component.     

内蒙古扎鲁特旗地区中侏罗统塔木兰沟组的厘定

扎鲁特旗地区发育的一套中性火山岩,前人曾将其置于二叠纪“大石寨组”、“青凤山组”,缺少古生物资料和测年数据.本次1：25万区域地质调查,在原1：20万划分的“大石寨组”、“青凤山组”的安山岩中采集了同位素测年样品,采用全岩激光⁴⁰Ar/³⁹Ar法测年,对火山岩样品进行了年代学研究,结果表明火山岩年龄为（172.7±2.0）~（165.2±1.3）Ma.鉴于这套火山岩地层岩石组合特征清楚,时代依据充分,在区域上具有一定的延展性和可填图性,故将这套火山岩地层重新厘定为中侏罗统塔木兰沟组.     

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。     

山西吕梁地区吕梁群变基性火山岩原岩形成年龄、地球化学特征及形成环境

华北克拉通古元古代中期构造环境及演化至今尚存有争议,该时期相关岩浆作用的成因研究可为这些问题的解决提供重要证据。本文对华北克拉通中部山西吕梁地区古元古代中期形成的吕梁群近周营组基性火山岩开展综合研究,获得2 180±19 Ma 的形成年龄,岩石地球化学低TiO₂、高Al₂O₃ 和K₂O,属钙碱性系列。微量元素富集Rb、Ba、Th、U、Sr 等大离子亲石元素,亏损Nb、Ta、Ti 和P等高场强元素,富集Pb,具轻稀土略富集、铕弱异常的右倾稀土模式,与典型消减带弧岩浆产物特征一致。同位素组成显示正ε_Nd（t）= 1.0～2.6 和ε_Hf（t）= 1.8～9.2, 指示它们来源于亏损地幔,低的Ti/Y（246～385）和Ce/Y 值（0.99～1.77）进一步揭示,地幔源区受到俯冲流体的交代和沉积物的参与,是楔尖晶石二辉橄榄岩与石榴石二辉橄榄岩过渡带略亏损地幔区发生部分熔融,并经历了橄榄石、单斜辉石等矿物分离结晶作用而形成。结合同期埃达克岩-高镁安山岩-富Nb玄武岩-安山岩组合以及弧花岗岩类的出现提出,2 200 Ma 华北中部带已演化至活动大陆边缘弧的构造环境。     

Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan,NW China: implications for the tectonic evolution and Phanerozoic continental growth

Situated in the southwest of the Central Asian Orogenic Belt (CAOB), the South Tian Shan (STS) Block is a key area for understanding the final accretion of the CAOB. A suite of volcanic rocks interbedded with continental sediments from the Xiaotikanlike Formation lies along the southwestern edge of the Tian Shan orogen. Laser-ablation-inductively coupled plasma-mass spectrometer U–Pb dating provided a crystallization age of 295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early Permian magmatic event. The volcanic rocks show a variable composition, with dominant rhyolites and dacites, subordinate basaltic andesites and few basalts. The felsic rocks are enriched in K and exhibit remarkably negative anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated with their high negative ε _Nd(t) values and old Nd model ages suggest that they are most likely sourced from ancient lower crustal rocks. The mafic rocks are characterized by high Mg#, Cr, Ni contents, negative Nb, Ta anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile elements. The mafic rocks are thus inferred to derive from enriched subcontinental lithospheric mantle. The petrographic and geochemical characteristics of the Xiaotikanlike Formation volcanic rocks indicate that they were generated under a post-collisional regime. Therefore, the final collision between the Tarim Craton and the Kazakhstan–Yili terrane took place before Early Permian, most probably at Late Carboniferous. Differing from other tectonic units of the CAOB, the recycling of ancient lithospheric crust played a significant role in the continental growth of the STS Block.     

松辽盆地南部早白垩世火山岩的地球化学特征及其成因

对松辽盆地南部早白垩世火石岭组和营城组火山岩进行的岩石地球化学研究表明,火石岭组火山岩主要以玄武质粗面安山岩和粗安岩为主,存在少量粗面英安岩,其Si O2=53.8%~68.6wt%、K2O+Na2O=7.70%~9.59%、Mg#=26.72~33.89,以富集轻稀土和大离子亲石元素(LILEs,如Rb、Ba、Th、U等),亏损重稀土和高场强元素(HFSEs,如Nb、Ta、Ti、P等)为特征,这些特征与俯冲带火山岩类似。营城组火山岩以流纹岩为主,存在少量的玄武质粗面安山岩、粗面安山岩和英安岩,基性端元具有低硅(Si O2=49.3%~56.6%)、低碱(K2O+Na2O=4.57%~6.60%)、富镁(Mg#=33.49~59.51)的特征,主要属于钙碱性系列,暗示其原始岩浆主要来源于地幔;酸性端元的Si O2=65.4%~74.9%,K2O+Na2O=3.23%~7.75%,低镁(Mg#=21.76~39.18)。营城组火山岩稀土配分型式呈右倾型,富集轻稀土和大离子亲石元素,亏损重稀土和高场强元素,与俯冲环境火山岩相似。其中,营城组流纹岩地球化学特征类似A型花岗岩,暗示形成于伸展环境。结合区域研究成果,认为松辽盆地南部早白垩世火山岩的形成与古太平洋板块向欧亚大陆之下的俯冲作用有关。火石岭组火山岩形成于活动大陆边缘环境,营城组火山岩的形成与岩石圈拆沉作用引发的伸展背景有关。     

龙木错—双湖古特提斯洋俯冲记录——羌塘中部日湾茶卡早石炭世岛弧火山岩

尽管龙木错—双湖结合带经历早古生代—三叠纪的长期演化过程,但已有资料显示羌塘中部沿该带的岩浆活动主要与碰撞作用有关,仍缺乏与俯冲相关火成岩的报道。报道龙木错—双湖结合带西段日湾茶卡东的一套那底岗日组火山岩中安山岩的年龄、岩石及地球化学特征,利用LA-ICP-MS方法测得安山岩锆石U-Pb年龄为330.6±2.8Ma,表明火山活动的时间为早石炭世。该套岩石因遭受硅质流体交代,SiO2含量变化较大（54.84%~72.84%）,在Nb/Y-Zr/TiO2×0.00001图解中全部落入安山岩区,属于低—中钾钙碱性系列;微量元素蛛网图明显富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具火山岛弧特征。日湾茶卡安山岩是在羌塘中部发现的早石炭世岛弧型火山岩,为龙木错—双湖古特提斯洋盆晚古生代俯冲作用提供直接的年代学和岩石学证据。     

Geochemical constraints on the genesis of the volcanic rocks in the southeast of Isfahan area, Iran

Late Miocene–Pliocene to Quaternary calc-alkaline lava flows and domes are exposed in southeast of Isfahan in the Urumieh Dokhtar magmatic belt in the Central Iran structural zone. These volcanic rocks have compositions ranging from basaltic andesites, andesites to dacites. Geochemical studies show these rocks are a medium to high K calc-alkaline suite and meta-aluminous. Major element variations are typical for calc-alkaline rocks. The volcanic rocks have SiO₂ contents ranging between 53.8% and 65.3%. Harker diagrams clearly show that the dacitic rocks did not form from the basaltic andesites by normal differentiation processes. They show large ion lithophile elements- and light rare earth elements (LREE)-enriched normalized multielement patterns and negative Nb, Ti, Ta, and P. Condrite-normalized REE patterns display a steep decrease from LREE to light rare earth elements without any Eu anomaly. These characteristics are consistent with ratios obtained from subduction-related volcanic rocks and in collision setting. The melting of a heterogeneous source is possible mechanism for their magma genesis, which was enriched in incompatible elements situated at the upper continental lithospheric mantle or lower crust. The geochemical characteristics of these volcanic rocks suggested that these volcanic rocks evolved by contamination of a parental magma derived from metasomatized upper lithospheric mantle and crustal melts.     

Emeishan large igneous province, SW China

In recent years, there have been major advances in our understanding of the Emeishan large igneous province (LIP) of SW China following publication of a number of LIP-focused investigations of the terrain and associated rocks. This paper reviews the current state of knowledge. The volcanic and upper-intrusive portion of the province is relatively small (0.3×10⁶ km³), even when offset fragments, eroded sections and buried portions are included in the volume calculation. The most reliable radiometric age dates (zircon U–Pb SHRIMP from an associated layered intrusive body several kilometers in area) indicate generation at 259 Ma, consistent with the end-Guadalupian (end Middle Permian) stratigraphic age. In addition, several Ar–Ar dating studies have been carried out, mainly on the volcanic rocks, with a number of reported dates 253–251 Ma (Late Permian), but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data (possibly due to a monitor standard miscalibration). The Ar–Ar investigations have also yielded a large number of secondary ages, which are clustered at 175, 142, 98 and 42 Ma, and these are inferred to record sub-regional tectonic events that affected the western Yangtze Block as East Asia was assembled and later deformed by India’s collision–indentation into Asia. Magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1–2 my. The geochemistry of the volcanic rocks and bio-lithostratigraphic studies of the underlying Maokou Formation suggests a mantle plume generated the province. The basalts can be classified into low and high Ti groups with different parental magmas. The low Ti basalts are confined to the western part of the province and are overlain by the high Ti basalts. The low Ti magmas formed at shallow mantle depths in the spinel–garnet field transition zone (60–80 km), whereas the high Ti magmas formed by low degrees of partial melting within the deeper garnet stability field. This observation suggests a deepening of the melting column as the lithosphere thickened due to under-plating and a transition from peak basalt generation to the waning stage. Outstanding issues, which might focus future studies, are also outlined.     

南秦岭中段西乡群火山岩岩石成因

南秦岭中段新元古代中期（730-845Ma）西乡群（自下而上包括孙家河组、大石沟组和白勉峡组）火山岩喷发于大陆板内裂谷环境。它们极有可能与导致Rodinia超大陆裂谷化裂解的地幔柱活动有关。根据岩石地球化学数据．南秦岭中段新元古代中期西乡群裂谷基性熔岩总体上属于低Ti／Y（LT,Ti／Y〈500）岩浆类型。LT熔岩又可进一步划分为LT1和LT2等2个亚类。LT1熔岩以高Nb／La（0．87～0．98）、低Thw／NbN（≈1）、缺乏Nb—Ta和Ti的亏损、具有“大隆起”式微量元素原始地幔标准化分配型式、（^87SrSr^86Sr）（t）=0．703869、εNd（t）=4．83为特征,属于拉斑玄武质岩浆系列;LT2熔岩以低Nb／La（〈0．75）、高ThN／NbN（〉1．4）、Nb—Ta和Ti亏损明显和Sr—Nd同位索比值变化较大为特征。元素和同位素数据表明,西乡群裂谷火山岩的化学变化不是由一个共同的母岩浆结晶分异作用所产生。孙家河组、大石沟组和自勉峡组中TiO2含量大于1．09％的火山岩的母岩浆经受了辉长岩质结晶分离作用。而白勉峡组中TiO2含量小于0．69％的基性熔岩的化学演化则是受控于单斜辉石（cpx）±橄榄石（ol）结晶分离作用。西乡群火山岩系中,基性、中性和酸性熔岩间为分异结晶关系。南秦岭中段新元古代中期西乡群裂谷火山岩系极有可能是源于共同的地幔柱,该地幔柱组分的成分为;εNd（t）≈＋5,^87Sr／^86Sr（t）≈0．704,La／Nb≈0．7。南秦岭中段新元古代中期西乡群裂谷基性熔岩存在空间上的地球化学变化：LT1熔岩的母岩浆,没有受到明显的大陆岩石圈混染,保存了鲜明的地幔柱信号;而大陆地壳或大陆岩石圈混染作用对于LT2熔岩的形成则有着重要贡献。研究揭示,南秦岭中段新元古代中期西乡群裂谷基性熔岩的母岩浆总体上产生于上涌地幔柱上部层位（地幔柱头）3GPa?     

Archaean greenstone belt from the Central African Republic (Equatorial Africa)

The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites.The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.     

西藏措勤晚白垩世英安岩的厘定与地球化学特征

野外地质调查和SHRIMP锆石U/Pb地质年代学研究表明,拉萨地块中带措勤地区发育一套形成于晚白垩世中期的英安岩(88.5±1.2 Ma)。该套火山岩具有岛弧火山岩的地球化学特征,为地壳深部岩浆岩部分熔融形成的Ⅰ型岩浆岩,经历了明显的斜长石分离结晶作用。与同时代的侵入岩相比,该套火山岩为高度演化的岩浆,Rb/Sr比值较高,但Sr/Y比值较低。在拉萨地块内部,广泛发育的90～85 Ma的岩浆作用与新特提斯洋的北向俯冲作用相关。     

Geochemistry of Archean Metavolcanic Rocks from Kadiri Schist Belt, Andhra Pradesh, India

The basic volcanic group exposed in the Kadiri schist belt includes high Mg-basalt, basalt, basaltic andestite and dacite. The basalts are tholeiitic in composition and high Mg-basalts, basaltic andesites and dacites show calc-alkaline affinity. Major and trace element characteristics suggest that the volcanic suite has been derived from an initial tholeiitic magma which has given rise to an early basaltic type and a later calc-alkaline type of rocks. An island arc and active continental margin tectonic setting was inferred for these rocks.     

Geochronology and geochemistry of late Carboniferous volcanic rocks from northern Inner Mongolia,North China: Petrogenesis and tectonic implications

Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.     

Geochemical Characteristics and Tectonic Significance of Early Carboniferous Volcanic Rocks of Kalamaili,Xinjiang

Shanlianglishi Formation strata volcanic rocks are widely distributed in western fault zone of Kalamaili. Studies show that the rocks are basalt, andesite and basaltic andesite, calc-alkaline series with SiO₂ content of samples ranging from 49.43% to 59.08%, TiO₂ (1.18%~2.49%), P₂O₅ (0.21%~0.92%) and MgO (3.81%~6.28%). Samples have relatively high HFSE content (Ti, Zr, Y) and also high La/Nb (2.98~1.96), Zr/Y (9.36~4.01) ratio. The primitive mantle normalized trace element spider shows samples enriched LILE (Rb, Ba, K, U) and Pb, but relatively depleted Nb, Ta and Ti. Rare earth element distribution of REE shows slightly enriched LREE, (La/Yb)_N=1.93~9.59, and that Eu is weakly negative anomaly (δEu=0.87~0.95). Combined with geochemical characteristics of WPB and VAB. Shanlianglishi Formation strata formed in the stretched after collision environmen. After partial melting of the enriched mantle that was previously metasomatized by the subduction fluid and with participation of the subduction sediment, magma experienced a certain degree of fractional crystallization and contamination by earth crust, and then erupted to form the Shanlianglishi Formation strata volcanic rocks. In eraly Late-Carboniferous, extensional movement was the most intense volcanic activity, which reached its peak in this area.     

大兴安岭南段晚中生代双峰式火山作用

大兴安岭南段晚中生代克头鄂博组山岩表现出双峰式特征，主要由玄武质安山岩、英安岩和流纹岩组成。基性火山岩属于代钾拉斑系列，轻微富集LREE，Eu异常不明显（Eu/Eu=0.99-1.04)和HREE无明显分馏的特征（Dy/YbcN=1.030-1.089);富集大离子亲石元素（LILE）而亏损高场强元素（HFSE），尤其是强烈亏损Nb,Ta。英安岩和流纹岩为钙碱性系列，在REE配分模式上为LREE富集型，其中英安岩为Eu弱负异常（Eu/Eu=0.81-1.01)，流纹岩的Eu负异常明显（Eu^*/Eu=0.65-0.76)；在微量元素蛛网图上，英安岩类似于基性火山岩，流纹岩除了具LILE富集和HFSE亏损特征外，还显示出Sr,P,Ti等元素的强烈亏损，可能与岩浆演化过程中斜长石、磷灰石的分离结晶作用相关。晚中生代双峰式火山岩分离结晶的结果。流纹岩表现出较高的La/Sm比值和很高的K／P、K／Ti比值，其成因可能与地壳混染作用或与大陆中、下地壳重熔作用有关。结合区域晚中生代盆岭构造格局特征、大兴安岭南段晚中生代双峰式火山岩形成于造山后阶段，是岩石圈快速伸展体制下导致受早期流体交代的岩石圈地幔发生减压部分熔融作用的产物。     

湖北北部耀岭河群、武当山群、随县群(大狼山群)之我见

川、陕、豫、鄂边境和湖北武当山地区的耀岭河群—武当群，鄂北随州—应山地区的随县群（大狼山群），大别山西南麓孝感和黄冈地区长江北岸的原红安群上部地层实为同一地层。原耀岭河群是一套变基性火山岩，它岩性单一，厚度不大，应废群复组。其下为一套变质（含砾）含晶屑岩屑酸性火山岩，命名为柳林组。再下是一套变质碎屑沉积岩、火山质沉积岩，命名为天星庙组。耀岭河组在鄂东、鄂中缺失；柳林组、天星庙组横贯东西。建议三组统称武当山群。武当山群上覆有含化石的古生界和上震旦统碳酸盐岩地层，其间没有角度不整合界线；之下未见底，时代为早震旦世