Potassium-argon/argon-40-argon-39 geochronology of Cenozoic alkali basalts from the South China Sea

Based on the isotopic chronologic results of Cenozoic alkali basalts from the South China Sea, the characteristics of volcanic activi- ty of the South China Sea after spreading were studied. The potassium - argon ages of eight alkali basalt samples from the South China Sea, and the argon - argon ages of two samples among them are reported. Apparent ages of the whole rock are 3.80 to 7. 91 Ma with an average value of 5.43 Ma （potassium- argon, whole rock）, and there is little difference among samples at the same location, e. g. , 4. 76 - 5.78 Ma for location S（M-12. The argon - argon ages for the two samples are 6.06 and 4. 71 Ma, which lie within the age scope of potassium - argon method. The dating results indicate that rock-forming age is from late Miocene to Pliocene, which is consistent with erupting event for alkali basalts from adjacent regions of the South China Sea. Volcanic activities occur after the cessation of spreading of the South China Sea, which are controlled by lithospheric fault and the spreading center formed during the spreading period of the South China Sea. These dating results, combined with geochemical characteristics of these basalts, the published chronological data for the South China Sea and its adjacent regions, and the updated geophysical data near Hainan Island, suggest that after the cessation of spreading of the South China Sea, there occur widely distributing magmatic activities which primarily is alkali basalt, and the volcanic activity continues to Quaternary. The activity may be relative to Hainan mantle plume originated from core/mantle boundary.

Equilibrium between Clinopyroxene and Host Rocks:Implication for the Magmatic Source and Evolution of Alkali Basalts of the Taohekou Formation in the Northern Daba Mountains,China

The Taohekou Formation is a volcanic-sedimentary terrane formed in the early Silurian in the northern Daba Mountains, China. The volcanic rocks, with dominant alkali basalts and minor mantle xenoliths, are enriched in clinopyroxene phenocrysts. Geochemical analysis shows that the composition of clinopyroxenes from different lithofacies has a close affinity. There is a liner correlation present in composition of clinopyroxenes （including phenocryst, microcrystal and xenocryst） from coarse porphyritic basalts, pillow or fine porphyritic basalts to amygdaloidal basalts. All the cllnopyroxenes, except the clinopyroxenes in mantle xenoliths, show a similar pattern of trace elements and REE, which indicates that they are likely products of successive fractional crystallization from cognate magma. Clinopyroxenes in mantle xenoliths, however, are mantle xenocrysts. The crystallization pressure of clinopyroxenes gradually decreases from mantle xenolith, deep-seated xenocryst, coarse porphritic basalts, pillow or fine porphritic basalts, to amygdaloidal basalts, which are 1.92-4.41 GPa, 1.18-2.36 GPa, 1.13-2.05 GPa, 0.44-0.62 GPa and 0.14-0.28 GPa respectively. Calculation results suggest that the primary magma originated from a mantle region deeper than 68 km and stagnates in intervals of 37-68 kin, 15-20 km and 5-9 km during its ascent. The alkali basalts are characterized by increasing concentrations of Si and alkaline with the magmatic evolution. Meanwhile, they are markedly enriched in LREE, and the patterns of trace elements and REE are similar to those of oceanic island basalts.

Melt Pockets in Garnet Megacrysts from Cenozoic Alkali Basalts of the Shavaryn Tsaram Vicinity,Mongolia

Garnet megacryst with a multiphase inclusion from intraplate alkali basalts of the Shavaryn Tsaram(Tariat,Mongolia)was the object of the study.This unusual aggregate consists of porous glass,Ti-rich biotite,orthopyroxene,spinel,clinopyroxene,olivine,and ilmenite.Win TWQ 2.32 thermodynamic simulation of this system revealed a few intervals of equilibrium.Pressure and temperature adjustment reflected in the paragenetic minerals of the melt pocket.The capture of already crystallised garnet megacryst was at P=0.8-1 GPa and T=1120-1160℃.Mineral crystallisation inside the melt pocket,accompanied by external inputs,occurred at P=0.75-0.95 GPa;T=790-1120℃.Symplectite assemblage formed in the garnet megacryst due to decomposition at(P=0.55-0.7 GPa;T=850-930℃).The study of the oxygen isotope content in primary garnet and biotite of the melt pocket showed that the δ^(18)O_(VSMOW) values are the same and correspond to that of typical mantle xenoliths.However,the chemical and microcomponent composition of the melt pocket minerals reveals a material that differs from basalts and peridotites.Thus,it has been revealed that the multiphase inclusion in the garnet megacryst formed not only on account of the garnet’s substance,but also due to the entrapped material of the Earth’s interior.

Helium and Argon Isotopic Composition of Cenozoic Alkali Basalts and Mantle-Derived Xenoliths from Kuandian, Liaoning Province, China

The noble gas isotopic composition and content data of 2 alkali basalts, 3 lherzolite xenoliths and one clinopyroxene megacryst from the Kuandian region have confirmed the occurrence of a fractionation of noble gases during magmatism. Light noble gases such as He and Ne are high in mobility and appear to be incompatible as compared with heavy ones (such as Kr and Xe). Therefore, light noble gases are abundant in volcanics, especially in the volcanics with bubbles; lherzolite xenoliths have relatively high heavy noble gases. The clinopyroxene megacryst has the lowest abundance of noble gases, probably due to its high P-T origin. Noble gas isotopic composition of the clinopyroxene megacryst reveals that the mantle source beneath the Kuandian area has an MORB-like reservoir with 3He/4He ratio of ～10 Ra (Ra: atmospheric 3He/4He ratio) and 40Ar/36Ar ratio of 345.6. The lherzolite xenoliths possess moderate 3He/4He ratios of 2.59-4.53 Ra, reflecting the loss of primary helium during rock deformation or metasomatism caused by enriched mantle fluids during the up-lifting. The alkali volcanics have very low 3He/4He ratios (0.47-0.61 Ra), indicating a contribution of radiogenic 4He, probably having resulted from crust contamination. Most of the samples have excess 21Ne and 22Ne as compared with atmospheric neon, but Kr and Xe isotopic compositions are indistinguishable from atmospheric values within uncertainties with only individual samples having excess 129Xe, 134Xe and 136Xe.

Fluid Composition and Carbon & Oxygen Isotope Geochemistry of Cenozoic Alkali Basalts in Eastern China

The fluid compositions of Cenozoic alkali basalts in eastern China have been determined by the pyrolysis MS method, meanwhile the carbon and oxygen isotopic compositions of CO 2 released from these samples at different heating temperatures have been analyzed by the vacuum step heating method. The data show the volatile heterogeneity in upper mantle sources and different evolution trends of alkali basaltic magmas in eastern China, and these alkali basaltic magmas may be generated in the oxidizing milieu, as compared with mantle derived xenoliths in these alkali basalts, and exotic volatile components were mixed into these magmas in the process of their formation and development.

Contribution to the Petrography, Geochemistry, and Petrogenesis of Zarqa-Ma’in Pleistocene Alkali Olivine Basalt Flow of Central Jordan

The Zarqa-Ma’in basalt (MB) occurs near a plateau basalt (wadi fills) covering about 15 km2 of Makawir, Ataruz, and Hammat um Hasana cone areas in central Jordan. The tectonic evolution occurred through intraplate volcanism and erupted through fissure systems along the Dead Sea, transforming the fault during Miocene to Pleistocene period. Three stages of eruption of MB have been recorded during Pleistocene from 6 to 0.6 Ma. The petrographic analyses data show that the MB rocks are composed of plagioclase, olivine, pyroxene, and magnetite, including secondary minerals calcite, iddingsite, serpentine, and zeolite. Furthermore, the MB rocks have narrow ranges of major and trace element concentrations, and are of under saturated silica type and belong to sodic alkaline magma series. The geochemical characteristics of MB indicate that MB was derived from a slightly fractionated magma as reflected by its high MgO (6.3 - 11.7 ppm) concentration with Mg number from 0.41 to 0.61, low silica content (40.83 - 47.55 wt%), and high Cr and Ni concentrations (115 - 475 and 105 - 553 ppm, respectively). This basalt exhibited low degree of partial melting (10%) for garnet peridotite mantle source. The model mineral fractionation showed that the MB could be fractionated to clinopyroxene, orthopyroxene, olivine, and plagioclase.

Preliminary Study of Alkali Activation of Basalt: Effect of NaOH Concentration on Geopolymerization of Basalt

The main objective of this study is to examine the possibility of using fresh basalt powder in the preparation of geopolymer pastes. Four NaOH concentrations of 2.5, 5, 7.5 and 10 M were used to alkali activation of basalt. In addition, effect of curing temperature at ambient, 45°C and 65°C were studied. The geopolymer pastes were investigated using FTIR, XRD and SEM-EDS techniques as well as compressive strength up to 90 days. The results were shown the compressive strength of prepared geopolymer increased with concentration of alkali activator up to 90 days. On the other hand, the compressive strength of prepared geopolymer pastes were improved with increased curing temperature. The results showed that there was a change in the chemical and mineral structure, due to the reaction of the sodium hydroxide with the different minerals of the basalt. In addition, the Na/Al and Si/Al ratios were completely different from that of the raw basalt. The geopolymerization reactions occurred at the surface basalt and the unreacted basalt particles actually play a supporting role in the geopolymer properties.

Petrogenesis and Geochemistry of Neogene Basalts in Eastern Anhui

The basalt terrain of the Neogene Huangguoshan and. Guiwu Formations of eastern Anhui on the east side of the Tancheng-Lujiang fault belt is one of a few Cenozoic basalt terrains in eastern China for which detailed geochemical study has not been conducted. This paper reports the abundances of major elements and more than 20 trace elements (including REE) of 22 samples and the Nd, Sr and Pb isotopic compositions of 11 samples from the eastern Anhui basalt terrain, thus more or less systematically revealing the geochemical characteristics of this continental basalt suite. The paper discusses the origin of the basalt suite and the character and process of its mantle source. The basalt suite was derived from a heterogeneous continental lithospheric mantle with end members characteristic of the EMI-type oceanic basalt mantle, which was affected by mantle metasomatism (or enrichment of trace elements) and was characterized by a multi-stage evolution under open conditions.

Petrography, Geochemistry and Petrogenesis of the Basalt Flow at Al Azraq Al Shamali Area, East Jordan

Al Azraq Al Shamali (AZS) basaltic rocks were investigated aiming to understand their mineralogy, petrography and geochemistry features, and to achieve that a total of sixteen representative rock samples were selected for both geochemical and petrographic analysis from several sites in the study area. Petrographic characteristics were analyzed by optical microscopy after preparation thin sections for representative rock samples, which show that all basalt samples have minerals comprising: olivine, plagioclase (labradorite), clinopyroxene (augite), opaque’s and some secondary minerals such as Iddingsite, however, the proportions of each mineral vary between samples. Normative mineralogy by using CIPW Norm showed that AZS basalt samples dominated by olivine, Diopside, and nepheline, and AZS basalt can be normatively classified as alkali olivine basalt. Some textures that may be evident on microscopic examination such as porphyritic, glomeroporphyritic, vesicular, intergranular, and ophitic to sub-ophitic texture. X-Ray Fluorescence was used for whole rock major elements analysis (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O and P2O5 in wt%) and trace element (V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Ce, Nd and Ba in ppm). Geochemical analysis reveal that the basalt is alkaline and includes into Sodic series. AZS basalt are produced under-saturated within intraplate continental environment. The normalized trace element diagrams suggest that the AZS product of the asthenosphere part of the mantle at >100 km depth. Furthermore, the data of studied samples suggest that these rocks evolved from a melt formed by low degrees of partial melting. Also the geochemical variation trends of (AZS) basaltic samples supposing that the composition of these basalt have been influenced by fractional crystallization, without clear evidences for crustal contamination.

Characterization of Mineralogy, Petrography, Geochemistry and Petrogensis of Basaltic Outcrops in Jurf Ed Darawish Area, Central Jordan

This research was conducted to investigate the mineralogy, petrography, geochemistry and petrogensis of the basaltic flows in Jurf Ed Darawish (JDB) area of central Jordan. Sexton representative basalt rock samples were selected from the studied JDB outcrops. Modally, JDB consists of plagioclase, olivine, pyroxene (diopside), opaque’s, calcite and iddingsite minerals. Petrographically, basalt is holocrystalline, hypidiomorphic fine to medium grained and exhibited aphanitic to porphyritic texture. The common textures of the JDB rock samples were aphanitic, porphyritic, trachytic, glomeroporphyritic, sub ophitic, vesicular, and amygdaloidal. Geochemically, all of the inspected samples of JDB are located within Trachy basalt and plate alkaline basalt. The tectonic setting of JDB was plotted within the calcalkaline basalt and continental basaltic field. The rare-earth elements showed enrichment of the Ba and K, depletion of Ce relative to K, and enrichment of Nb and Pb with depletion of Y and positive Nb, Zr and Ti anomalies. Negative anomalies of Ba, Sr, Ti and P may be attributed to the fractionation of feldspar for Ba and Sr depletion apatite for P depletion. The positive Nb peak conforms to the tertiary as well as to recent continental alkali basalt provinces and acts as an indicator to the JDB product for the lithosphere from upwelling of the asthenosphere mantle.

Mineralogy and the Texture of the Basalt in Hail Region, Saudi Arabia

The study is carried out to list mineralogy and texture of the basalt in Hail region. The basalt flows belong to the Arabian Harrat volcanism. Cenozoic volcanism has produced 13, vast, basaltic fields in western Saudi Arabia, covering a total area of about 100,000 sq. km. The volcanism starts in the late Cretaceous time and continues periodically into the Holocene. The basaltic field presently investigated is known as Harrat Al-Hamad (or Harrat As-Shamah) which lies in northwestern Saudi Arabia, and continues northwards into Jordan. Mineralogical analysis of 20 rock samples indicates that the rocks are products of continental intra-plate magmatism;mostly as coarse-grained silica—under saturated olivine-rich alkali basalt. Mineralogical study shows that rocks are olivine, plagioclase, pyroxene, Nepheline and Opaque minerals. They correspond to alkali olivine basalts. Moreover, the main well-defined texture is intergranular, vesicular and amygdaloidal texture.

复合绝缘材料用连续玄武岩纤维老化特性及机理

探究连续玄武岩纤维在各类介质环境下的稳定性对于拓展其在复合绝缘材料上的应用至关重要。为此,文中比对了10组不同产地的连续玄武岩纤维与无碱玻璃纤维在酸性环境、碱性环境、富盐环境以及高温富盐环境下的老化特征差异,分析了连续玄武岩纤维的老化机理,通过灰色关联分析明确了构成玄武岩纤维耐候性的关键成分。结果表明:超过50%的玄武岩纤维样本拥有优于无碱玻璃纤维的耐盐碱环境的能力;碱性环境下,Si、Al、B元素与OH−的反应性造成两类纤维损伤,其中玄纤由于内部高含量的Ca、Mg,更易形成氢氧化物附着于纤维表面,降低了损伤程度;富盐环境下纤维表层的Ca、Na、K等元素扩散至溶液,导致了纤维表层结构疏松,且在介质渗透溶胀的作用下形成缺陷,致使两类纤维强度同步下降,但在老化周期内玄武岩纤维仍呈现出更高的力学特征。综上,玄武岩纤维更适于应用在盐碱重污秽地区的中低压复合绝缘产品或高力学需求的电工材料。

玄武岩纤维对碱矿渣砂浆力学和收缩性能的影响

为加速推进碱激发矿渣(Alkali-Activated Slag,AAS)砂浆的发展和应用,降低其硬化过程中产生的过大收缩,研究了6mm玄武岩纤维对AAS砂浆扩展度、抗折和抗压强度、自收缩、干缩的影响,同时进行了微观扫描电镜测试。结果表明:玄武岩纤维可有效地抑制AAS砂浆干缩并提高其抗折和抗压强度,当玄武岩纤维体积掺量为0.75%时,28d龄期干缩相对降低30%,抗压强度相对提高18.2%。研究还发现,玄武岩纤维会降低AAS砂浆的初始工作度,且还会增大其自收缩。提供了玄武岩纤维对AAS砂浆收缩性能影响的具体规律,为工程实践提供了理论依据。

碱激发混杂玄武岩纤维粉煤灰混凝土耐久性研究

碱激发剂可有效改善粉煤灰混凝土的致密性,基于8%掺量碱激发剂,研究了不同玄武岩纤维(BF)长度(9、12、25 mm)单掺和混掺对粉煤灰混凝土的抗压强度、耐久性和微观结构的影响,建立了BP神经网络对混凝土耐久性进行预测,并结合X射线衍射(XRD)和电子显微镜(SEM)技术对掺碱激发剂后的微观结构进行了表征分析。试验结果表明,当玄武岩纤维单掺时,混凝土的抗压强度和耐久性改善不明显,在300次冻融循环后,3种纤维长度按1∶1∶1混掺时,耐久性最佳;根据SEM图像显示,BF在裂缝处起桥梁作用,表明1∶1∶1混掺能更好地约束裂缝的发展,显著改善混凝土的耐久性;建立的BP神经网络损伤预测模型计算结果的回归系数R 2都大于0.99,表明该模型具有较高的精准性,为提高混凝土耐久性的配合比优化设计提供了参考和指导。

柯坪玄武岩的岩石学、地球化学、Nd、Sr、Pb同位素组成与岩石成因

柯坪玄武岩位于塔里木板块西缘,产于下二叠统库普库兹曼组和开派兹雷克组中。岩石化学组成以富集TFeO、TiO2、P2O5和SiO2不饱和为特征。大部分样品属碱性玄武岩系列,个别属拉斑玄武岩系列。它们的稀土元素和微量元素显示了板内拉张环境玄武岩的地球化学特征。εNd(t)=-1.73--3.69,εSr(t)=+27.56-+56.87,206Pb／204Pb=17.87-18.02,207Pb／204Pb=15.45-15.53,208Pb／204Pb=38.22-38.49。Nd、Sr、Pb同位素组成证明柯坪玄武岩源自于前寒武纪的富集型大陆岩石圈地幔。而且,这种情况在塔里木盆地及周缘地区十分发育的镁铁质岩浆岩中具有广泛的代表性。据此,可以将新疆南部和北部划分为两个明显不同的同位素地球化学省。南部省以富集型地幔的同位素组成为特征,而北部省以亏损型地幔的同位素组成为特征。分属于这两个省的古生代晚期阶段岩浆岩有可能不是同一个地质过程的产物。

青藏高原羌塘中部蓝片岩的地球化学特征及其构造意义

青藏高原羌塘中部的冈玛日 -桃形错地区蓝片岩被认为是板块构造边界的产物 ,通过对其主量元素、微量元素和稀土元素的地球化学特征的综合研究 ,其原岩属于洋岛型碱性玄武岩。再结合该地区的地质研究 ,表明在该地区存在一个古特提斯洋 。

湘南宁远地区碱性玄武岩形成时代的新证据:锆石LA-ICP-MS U-Pb定年

采自宁远保安圩中心铺和李宅湘的碱性玄武岩中锆石的阴极发光图像显示出自形-半自形的晶形和均匀条带状吸收的特征。大部分锆石具有明显的板状环带,这是中基性岩中岩浆结晶锆石的典型结构。保安圩中心铺锆石的Th/U比值介于0.37～0.95之间,保安圩李宅湘锆石的Th/U比值介于0.43～2.53之间,表明它们都具有岩浆成因的特征。对保安圩中心铺玄武岩中20粒锆石和保安圩李宅湘玄武岩中13粒锆石进行了LA-ICP-MSU-Pb年龄测定,结果显示,保安圩中心铺34个测定点的206Pb/238U年龄主要集中在204～221Ma之间,加权平均值为212.3Ma±1.7Ma,保安圩李宅湘23个测定点的206Pb/238U年龄主要集中在196～226Ma之间,其加权平均值为205.5Ma±3.0Ma,表明保安圩玄武岩的结晶年龄为晚三叠世。

中国东部新生代碱性玄武岩及幔源捕虏体中的流体组成

中国东部新生代碱性玄武岩及其幔源三辉橄榄岩捕虏体中的流体组分存在差异,幔源二辉橄榄岩形成于以H_2、CO等还原性气体为主的流体环境中,其流体组分以含H_2高、挥发性气体总量低为特征;碱性玄武岩中流体组分以CO_2、SO_2和挥发性气体总量高为特征,在其形成演化过程中有SO_2等氧化性气体的流体加入。碱性玄武岩及二辉橄榄岩捕虏体中的流体组成表明中国东部上地幔源区存在不均一性,碱性玄武岩浆在形成演化上存在地区差异性。中国东部大陆的上地幔中可能存在P_(H_2)较高的区域。

南秦岭地区存在两种不同构造属性的耀岭河群

广泛分布于南秦岭构造带的耀岭河群变质火山岩对南秦岭构造带的属性，乃至整个秦岭造山带的演化起着十分重要的作用。通过对湖北两郧、陕西商南、镇安和安康凤凰山等地区耀岭河群变质岩石组合、原岩组合、岩石化学和稀土、微量元素等方面的研究表明，在不同地区耀岭河群具有不同的岩石组合、岩石化学和地球化学特征，安康-武当地区中部的耀岭河群为板内拉斑玄武岩-偏碱性玄武岩组合，形成于板内拉张-裂谷环境，而商南耀岭河、镇安和安康凤凰山等地区的耀岭河群则为钙碱性玄武岩-岛弧拉斑玄武岩组合，形成于会聚板块边缘-岛弧构造背景，进一步证实南秦岭地区在6．5～8亿年期间存在两种不同的构造体制。

中国东部新生代碱性玄武岩中的流体组成及其碳、氧同位素地球化学特征

利用热分解质谱法测定了中国东部新生代碱性玄武岩中流体挥发分的组成，并对不同温度段释放出的CO2气体测定了C、O同位素值。流体组成和CO2的C、O同位素值表明中国东部上地幔源区的不均一性，与其中所含幔源岩捕虏体相比，碱性玄武岩浆发育在相对氧化的环境中，并有外来流体组分的加入。