西藏尕尔穷-嘎拉勒铜金矿集区火山岩年代学及地球化学

尕尔穷-嘎拉勒铜金矿集区是班-怒结合带南部的重要矿集区.矿集区内发育大面积火山岩, 利用锆石U-Pb年代学方法, 首次精确测定了研究区内不同地层中火山岩年龄, 并结合其岩石地球化学特征探讨了其形成构造背景.研究表明, 区内朗久组火山角砾岩成岩年龄为141.7.0±0.47 Ma(MSWD=0.43), 多爱组流纹岩年龄为136.80±0.48 Ma(MSWD=0.79), 形成时代为早白垩世初期, 属于班公湖-怒江洋南向俯冲的岩浆作用响应; 原定多爱组火山角砾岩(GE火山角砾岩)年龄为85.20±0.53 Ma(MSWD=3.40), 形成时代为晚白垩世, 不属于早白垩世多爱组产物, 属于洋盆消亡后羌塘陆块与冈底斯陆块汇聚阶段的火山作用产物, 晚于尕尔穷铜金矿的成矿年龄(86.87±0.50 Ma).岩石地球化学特征表明, 区内火山岩均具有相对富集Rb、Th、U等大离子亲石元素(LILE), 而亏损Ta、Nb、Yb、Ti等高场强元素(HFSE)的特征, 显示出弧火山岩特性.结合区域已有火山岩研究资料表明, 在班公湖-怒江洋南向俯冲过程中, 从早白垩世初至中晚期均有比较连续的火山作用, 持续时间约为30 Ma(140~110 Ma); 在班公湖-怒江洋盆消亡后的羌塘陆块与冈底斯陆块汇聚晚阶段, 又伴随有晚白垩世火山作用的发生, 该期火山作用与区内成矿岩体年龄相当, 可能为同一岩浆系统的产物. 关键字: 火山岩; 班公湖-怒江特提斯洋; 俯冲; 碰撞; 尕尔穷-嘎拉勒矿集区; 地球化学.      

滇西南腾冲地块新构造运动阶段初步划分

滇西南腾冲地块中部的梁河盆地地层出露较好,阶梯状地貌发育,并有多期火山活动,是开展新构造研究的良好地点。综合地表调查与年代学研究结果发现,该盆地主要发育新近纪南林组与芒棒组和第四纪地层,各时代地层的沉积及变形特征都明显不同,其中最早的盆地沉积南林组地层的固结及变形程度高;芒棒组的固结及变形程度则稍差;第四纪地层没有固结或弱固结,且基本无变形。该区在地貌上共发育7级阶(台)地,各级地貌面主要形成于早更新世以来。该区的火山活动至少包括3.5~5.0Ma、2.3~2.9Ma、1.0~1.5Ma及0.5~0.7Ma四期。综合梁河盆地中地层、地貌、火山等方面的新构造运动标志,将腾冲地块的新构造运动初步划分为3个阶段:15Ma之前,盆地开始断陷,南林组沉积及掀斜变形;25~2Ma,盆地进一步扩张,芒棒组沉积及掀斜变形,并伴随火山活动;32Ma以来,盆地萎缩抬升,形成多级台地及阶地,并伴随多期火山活动。     

Geochemistry, U-Pb Geochronology and Sr-Nd-Hf Isotopes of the Early Cretaceous Volcanic Rocks in the Northern Da Hinggan Mountains

Whole-rock geochemical, zircon U-Pb geochronological and Sr-Nd-Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2(73.19–77.68 wt%) and Na2O+K2O(6.53–8.98 wt%) contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206Pb/238 U ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate(87Sr/86Sr)i values of 0.704912 to 0.705896, slightly negative εNd(t) values of –1.4 to –0.1, and positive εHf(t) values of 3.7 to 8. Their zircon Hf and whole-rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol-Okhotsk Ocean.     

西藏1∶5万班戈县西南地区四幅区调成果与展望

西藏1∶5万班戈县西南地区四幅区域地质矿产调查为青藏专项地质调查项目。该项目取得的主要成果为:1发现了区内寒武系火山岩呈角度不整合覆盖于念青唐古拉群之上,其内部划分为3个岩性段;新建了上寒武统他多雄组。2对下奥陶统扎扛组进行了解体;新建了芙蓉统龙郊组。3通过详细的地质填图,在永珠蛇绿混杂岩带中新发现一套沉积混杂岩,认为该混杂岩区域延伸稳定;新建了晚侏罗世索尔岩组。4对蛇绿岩填图单元进行了解体,由超镁铁质岩、辉长岩、席状岩墙群、枕状玄武岩岩片、放射虫硅质岩组成,新建了晚侏罗世永珠蛇绿岩群。5确定了念青唐古拉群,新识别出的变质花岗岩形成时代为中新元古代,为研究古印度地盾北缘罗迪尼亚大陆形成期和裂解期的构造热事件提供了新资料。     

392BC—2017AD柴达木盆地东北部降水变化及其与太阳活动和强火山喷发的联系

基于采自柴达木盆地内部牦牛山的祁连圆柏树木样芯,新建采样区域过去2710 a的树轮宽度年表,并计算了器测时期（1957—2017 AD）气象数据与树轮宽度年表的相关系数。结果表明：树轮宽度年表与上一年7月至当年6月降水量变化的相关性最高（r=0.753,n=60,p <0.01）,基于此重建了该区392 BC—2017 AD上一年7月至当年6月的降水量变化。器测时期方差解释量高达64%。重建结果显示,在年代际尺度上存在9个湿润期（106—75 BC、6—39 AD、179—229 AD、581—646 AD、823—914 AD、1026—1095 AD、1378—1414 AD、1567—1609 AD和1985—2017 AD）和7个干旱期（328 BC—297 BC、86—151 AD、694—747 AD、1168—1199AD、1444—1525 AD、1680—1725 AD和1792—1860 AD）,1792—1860 AD是过去2400 a最干旱时期。功率谱分析结果表明,过去2400 a柴达木盆地东北部地区降水量存在2～8 a、40 a、80 a和225 a准周...     

腾冲火山区的地震层析成像及其构造意义

利用滇西南临时台网和固定台站的地震数据反演了腾冲及邻近地区的P波速度结构,着重分析了腾冲火山区和龙陵7级地震震源区的地壳结构特点.研究结果表明,腾冲火山区下方10～20 km深度范围存在明显的低速区,其横向尺度大约在20～30 km之间;推测这一低速区代表了仍处于活动状态的壳内岩浆源,热流通道有可能通过腾冲断裂延伸至地...     

中国火山/火山岩景观地质公园展望

本文概略介绍了中国大陆4个火山（岩）为主题的世界地质公园和16个国家地质公园,以及中国香港和中国台湾的两个地质公园。文章还介绍了这些地质公园火山岩喷发时代、火山类型、大地构造环境和火山（岩）景观特征与分类。它们的喷发时代主要为更新世-全新世、中新世、始新世和白垩纪。四个时期的火山地质公园,在不同年代火山中均具代表性与典型性,为中国火山/火山岩研究提供了真实的材料。作者还针对火山地质公园建设现状提出加强科学研究与实施解说系统工程两条建议。     

江西盛源盆地橄榄玄粗岩系列火山岩长石矿物特征及成因信息

江西盛源盆地橄榄玄粗岩系列火山岩中的长石矿物特征为：斜长石斑晶具有反环带结构和交代净边结构,基质中存在大量的斜长石微晶,且在电子探针下研究发现基质中斜长石微晶具有钾长石环边的矿物学特点,为此类火山岩归属于橄榄玄粗岩系列火山岩提供了矿物学证据。通过对长石矿物组合特征进行研究,探讨该地区橄榄玄粗岩系列火山岩的成因以及成岩时的物理化学条件等方面的信息。     

Volcanic sands of Iceland ‐ Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun

The origin, formation and evolution of volcanic sands are less well known than the formation of the much more common quartz‐rich sand sheets. Combining active volcanism and a cold climate, Iceland is covered for about 21% of its surface by sandy areas. The sands were analyzed in detail at two sites and results reveal their diverse origins. The first site is Dyngjusandur, located north of Vatnajökull, and the second site is the Lambahraun area, located south of Langjökull. At both sites, the sand origin is determined from field observations (wind directions from ventifacts), chemical and mineralogical analyses of rocks and sands. At Dyngjusandur, the sand is dominated by glass grains, a situation typical of sand plains in Iceland. Hyaloclastite ridges presently buried beneath Vatnajökull are the dominant source of the sand, and only large size plagioclase crystals (0.5 cm) in sands seem to be derived from the lava flows. Hyaloclastite ridges were crushed by glaciers and mechanically eroded sediments were washed out by melt‐water onto flood plains. The sand chemical composition is spatially homogeneous and similar to the average composition of neighboring sub‐aerial lava flows, reflecting efficient mixing of distinct sources below the glacier. The presence of sand north of Dyngjujökull can be taken as a way to explore the average chemical composition of non‐exposed volcanic material beneath the glacier. In the case of Lambahraun, prevailing winds indicate several potential sources of sand at the north of the sand sheet. Comparison of chemical and mineralogical analyses of sands and rock samples helped to refine the exact origin. In contrast with the first site, the sand is dominated by crystals and is chemically consistent with a mixture of material derived from the lava flows of Eldborgir and Skersli shield volcanoes. Analysis of the contact between the lava flows and the glacier reveals that basaltic sand grains formed as the result of recent advances of the glacier abrading the rocks. The direct interaction of glacial and fluvio‐glacial activity with basaltic plains appears to be necessary to produce a large amount of sands in a relatively short period of time (<4000 years). This site appears to be an excellent natural laboratory for further studies concerning the sand evolution and physical sorting processes in basaltic material, which have important implications for understanding aeolian processes on Mars. Copyright © 2011 John Wiley & Sons, Ltd.     

Importance of volcanic glass alteration to sediment stabilization: offshore Japan

A minor amount (ca 1 wt%) of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the Nankai Trough subduction zone throughout the Shikoku Basin. The distribution and nature of the cement were examined via secondary and backscattered electron imaging. The amorphous silica occurs as altered material in contact with volcanic glass, coating grains (including grain contacts) and filling pores. Based on chemical and petrographic evidence, the cement is probably sourced from volcanic glass; this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica. Amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay‐dominated sediment throughout the Shikoku Basin. Measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the Shikoku Basin (at sites separated by >500 km). The cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration (i.e. Central America, Cascadia and the Gulf of Alaska).