内蒙古色日崩地区白音高老组流纹岩岩石地球化学特征及地质意义

内蒙古西乌旗色日崩地区早白垩世白音高老组流纹岩为高硅（SiO₂=78.32%~86.80%）、富碱（K₂O+Na₂O=3.84%~7.63%）、高铝（Al₂O₃=8.59%~13.70%）,低MgO、CaO、TiO₂、MnO,稀土元素与微量元素地球化学特征显示其类似于A型花岗岩特征;低Ti/Y、Nb/Y与Ti/Zr比值,高Rb/Nb与Rb/Y比值以及Sr、Eu强烈亏损,表明白音高老组流纹岩为上地壳物质部分熔融成因,形成于早白垩世非造山板内伸展环境.     

Pattern and origin of spherulites in a rhyolite dike swarm,northeastern Cheongsong, Korea

Several NNW-trending Cretaceous rhyolite dikes in the northeastern Cheongsong area of southern Korea contain spherulitic fabrics and are locally quarried as an ornamental stone. The dikes, part of the Cheongsong dike swarm, contain a variety of spherulites which are characterized by radial array of acicular intergrowths from fibrous quartz and alkali feldspars. They are classified as some simple, multiple and compound spherulite types from flower-like patterns, and the types are different each other in spatial distribution within the dikes. The various radial fabrics suggest that they crystallized under the different cooling mechanisms and processes in rapidly cooling dikes. Spheroids lacking a flower-like pattern may be precipitated after compositional segregation in a supercooled magma. Simple acicular spherulites could be crystallized by devitrification of true glass which cooled rapidly below the transformation temperature, whereas layered multiple spherulites could result from a crystallization of supercooled magma at relatively rapid cooling rates above the transformation temperature. Acicular multiple spherulites could be produced from a combination of the above three mechanisms operating at slower cooling rates.     

内蒙古扎木廷火山岩年代学、地球化学与中生代构造体制转换

大兴安岭火山带晚中生代火山岩的起源、演化及形成的构造背景一直存在争议.文章对出露于扎木廷地区白音高老组火山岩进行LA-ICP-MS锆石U-Pb定年,结果显示流纹岩及粗面岩分别形成于(131.6±1.2)Ma和(126.9±1.5)Ma,与白音高老组第二段火山沉积夹层中动植物化石鉴定结果一致,表明形成于早白垩世早期.流纹...     

Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption,northern Sumatra

The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre-eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO₂ and lower incompatible element concentrations in the low-SiO₂ PIV/PV, than the high-SiO₂ PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite-MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera-wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet-stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co-ignimbrite clouds (PI- and PIII-dominant), where associated ignimbrites isolated earlier (PIV- and PV-rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre-eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.     

西藏谢通门县切琼地区典中组流纹岩锆石U-Pb年龄、Hf同位素及地球化学特征

西藏中拉萨地块切琼地区广泛发育的林子宗群火山岩被认为是印度大陆与亚洲大陆碰撞火山作用的响应,对于揭示大陆碰撞的时限和过程具有重要的意义。本文对区内流纹岩进行了LA-ICP-MS锆石U-Pb年龄、Hf同位素及全岩元素地球化学研究,结果显示:研究区流纹岩年龄为67.1±0.1 Ma,符合林子宗群典中组火山岩年龄范围。该流纹岩具有富Si、高K、低Ti、过铝质特征;富集轻稀土元素,亏损重稀土元素,(Gd/Yb)_N=0.57~1.16,普遍出现负Eu异常(δEu=0.05~0.08),相对富集大离子亲石元素(Pb、Rb、Th、U),亏损高场强元素(P、Ti),并且明显亏损Ba和Sr,属于高钾钙碱性过铝质系列,具有陆源弧火山岩的特点;锆石¹⁷⁶Hf/¹⁷⁷Hf值在0.282673~0.282804之间,_εHf(t)在?2.1~+2.3之间,平均为?0.6,t_2DM变化于875~1117 Ma之间。综合分析表明,切琼地区林子宗群典中组流纹岩形成于俯冲构造环境,源于古老地壳物质再循环,且混染了少量幔源物质。对比区域上相关矿床,结合切琼地区已发现矿点,表明切琼地区典中组火山岩具有一定寻找浅成低温热液型、矽卡岩型矿床的潜力。     

内蒙古中部苏尼特左旗大石寨组流纹岩SHRIMP锆石U-Pb年龄、地球化学特征及其构造意义

内蒙古中部苏尼特左旗地区大石寨组酸性火山岩SHRIMP锆石U-Pb测年结果显示,2件流纹岩样品的206Pb/238U年龄加权平均值分别为272.1±3.1Ma和271.4±1.7Ma,说明大石寨组流纹岩喷发于早二叠世。锆石Hf同位素分析结果表明,εHf(t)值为+10.8~+13.1;Hf-Nd同位素耦合关系计算显示,其与同期形成的大石寨组玄武岩Nd同位素组成特点非常相似。全岩地球化学分析表明,该流纹岩高Si O_2,贫Ca O、Fe_2O_3*和Mg O,大部分样品属于过铝质、钙碱性流纹岩;富集轻稀土元素,轻、重稀土元素分馏中等,具明显的负Eu异常,富集Cs、Rb、Th、U和Zr,亏损Ba、Sr、Nb、Ta、P和Ti,指示其是富水的钙碱性基性地幔岩浆分离结晶作用的产物。结合区域地质资料,认为苏尼特左旗流纹岩形成于大陆岛弧背景。     

沽源火山盆地燕山期次流纹斑岩地质地球化学特征

燕山期次流纹斑岩主要分布在沽源火山盆地盖层的上侏罗统张家口组火山岩中,活动时间以燕山晚期为主.次流纹斑岩体在空间分布上受断裂构造和燕山晚期火山构造控制,与铀钼矿化关系密切,矿化主要产在次流纹斑岩体接触带或其顶部的隐爆角砾岩中.次流纹斑岩与围岩呈侵入接触关系,呈岩株、岩墙、岩脉等形态产出.岩石地球化学研究表明:次流纹斑岩...     

大兴安岭旁开门金银矿床赋矿围岩的锆石U——Pb 年龄及其地质意义

大兴安岭旁开门金银矿床赋存在上侏罗统甘河组基性岩和下白垩统富林组中酸性火山岩中,其赋矿围岩蚀变流纹岩中单颗粒锆石U-Pb 年龄测定表明,21 个成分点中有1 组不谐和年龄( 141 ± 2 Ma、129 ± 2 Ma) 和3 组谐和年龄,后者的加权平均值分别为116. 2 ± 3. 4 Ma、112. 5 ± 1. 1 Ma 和 108. 2 ± 4. 2 Ma。结合锆石晶形和CL 图像特征,进一步确定流纹岩的成岩时代为早白垩世晚期,岩浆房冷却到岩浆热液期经历约8 Ma,成矿作用与富林组火山作用有关,发生在108 Ma 之后,其成矿与中国东北部陆缘晚中生代浅成热液金矿大规模成矿时代相一致。     

大兴安岭南部钠闪石流纹岩的岩石成因:年代学和地球化学证据

本文报道了在大兴安岭南部白音高老组新发现的钠闪石流纹岩的锆石U-Pb定年结果和岩石地球化学资料,以便揭示富碱钠闪石流纹岩的成因与区域构造演化.钠闪石流纹岩中锆石为自形-半自形的长柱状、短柱状,具有明显岩浆成因的震荡生长环带,结合其Th/U比值(1.2～1.9),暗示其为岩浆成因.锆石LA-ICP-MS U-Pb定年结果表明,其206Pb/238U年龄介于134 ～ 149Ma之间,其加权平均年龄为141Ma,表明钠闪石流纹岩形成于早白垩世早期.锆石εHf(t)=+9.07～+12.08,二阶段Hf模式年龄介于415 ～616Ma之间.岩石地球化学资料表明,钠闪石流纹岩以富硅、碱、铁质和贫钙、镁质为特征,在地球化学上,具有典型的A型流纹岩特征.该类岩石不仅具有高的稀土元素总量(REE=307×10-6～ 1395×10-6)、显著的Ce正异常(Ce/Ce* =6.52 ～ 18.6)和显著的Eu负异常(Eu/Eu*=0.007 ～0.009),而且表现出高场强元素(如Zr、Hf、Nb、Ta)的显著富集、大离子亲石元素(如Ba、Sr)的强烈亏损和很高的Ga/A1比值(104×Ga/Al =4.88～6.41).上述资料表明,钠闪石流纹岩的原始岩浆应是古俯冲蚀变洋壳部分熔融的产物,并形成于蒙古-鄂霍茨克缝合带闭合后的岩石圈伸展构造环境.     

新疆博格达东缘色皮口地区柳树沟组流纹岩地球化学、LA-MC-ICP-MS锆石U-Pb年代学及地质意义

新疆色皮口地区位于博格达造山带东段北部,区域内的上石炭统柳树沟组流纹岩分布较为广泛。色皮口地区流纹岩SiO2介于70.81%～77.62%,碱总量6.87%～9.02%,K2O/Na2O为0.74～1.04,K2O/TiO2和K2O/P2O5比值分别为10.38～18.83、72.45～242.14,A/CNK指数介于1.43～1.55,具有高硅、高铝质的特点。不相容元素K、Rb、Th、Ba富集,高场强元素Nb、Ta、Zr、Hf富集,ΣREE141.76×10-6～228.2×10-6,(La/Yb)N4.11～7.42,δEu0.27～0.50,流纹岩稀土总量较低,轻重稀土分异显著,铕负异常明显。流纹岩原始地幔标准化蛛网图显示Sr、Nb、Ta、P和Ti负异常,而Sr、P、Ti谷深,显示较强的岩浆分异作用。流纹岩的锆石LA-MC-ICP-MSU-Pb定年结果为314.0±1.1Ma(n=17,MSWD=1.6,Th/U比值在0.39～0.80之间),这表明流纹岩形成时代为晚石炭世。结合区域上前人的Sr-Nd、O和Hf同位素成果,推测区域分布的流纹岩可能来源于两个不同的源区:亏损地幔的幔源基性火成岩经过部分熔融或幔源玄武岩浆结晶分异和地壳物质部分熔融,可能代表了裂谷闭合时期构造应力场由拉张到挤压频繁转换作用的产物。