喜马拉雅造山带的变质作用与部分熔融

喜马拉雅造山带的核心由高级变质岩系和淡色花岗岩构成,是研究碰撞造山作用和板块构造的天然实验室。本文评述了喜马拉雅造山带变质作用和部分熔融研究取得的新进展和存在的争议,主要内容包括:(1)造山带核部具有"三明治"结构,高级变质和部分熔融的高喜马拉雅系列(GHS)夹持在较低级变质的特提斯喜马拉雅系列(THS)和低喜马拉雅系列(LHS)之中,GHS的变质作用程度具有向上和向下部构造层位降低的特征。高喜马拉雅系列主要由高压麻粒岩相到榴辉岩相的变质岩组成,具有1.2~1.6GPa和700~800℃峰期变质条件,顺时针型变质作用P-T轨迹,其进变质以增温增压为特征,退变质早期为近等温或增温降压过程,晚期为降温降压和近等压降温过程;(2)在造山带西段,紧邻缝合带产出的超高压变质岩具有4.4~4.8GPa和560~760℃的峰期变质条件和顺时针型P-T轨迹,并在退变质中期出现加热过程;(3)尽管造山带的高压和超高压变质岩形成在中、高温条件下,但岩石中的石榴石都保存有明显的主量和微量元素生长成分环带特征;(4)造山带变质核下部发育反转的中、高压型变质序列;(5)在造山带核部,变泥质和长英质麻粒岩的强烈部分熔融主要是增压、增温进变质过程中的白云母和黑云母脱水熔融,和近等温或增温降压过程中的黑云母脱水熔融,可以形成花岗质和英云闪长质熔体。加厚下地壳的高变质温度足以使各种成分岩石(包括基性岩)发生深熔,而不需要外来热源;(6)造山带变质核经历了长期的变质演化过程,其进变质始于~47Ma,峰期变质发生在~25Ma,退变质持续到~15Ma。这些岩石也记录了持续的(超过20Myr)高温变质和部分熔融过程。在造山带西段的超高压变质岩具有~46Ma的峰期变质年龄和~40Ma的退变质年龄,所以经历了一个快速俯冲与折返过程;(7)印度大陆西缘与岛弧的碰撞(造山带西段)和印度大陆东缘与大陆弧的碰撞时间一致,为~50Ma;(8)在造山带西段,印度大陆的深和陡俯冲形成了超高压变质岩;而在造山带中段,印度大陆的平缓俯冲形成了中高压变质岩;(9)构造变质不连续在变质核中广泛存在。多重有序逆冲和无序逆冲导致的岩片叠置控制着造山带的地壳结构;(10)现有的构造模型,包括楔形挤出、隧道流、临界楔和构造楔模型,都不能全面合理地解释造山带变质核部的折返机制。     

华北地区第三、 四活动期平静幕后期中强地震活动与未来强震

本文通过分析、 研究华北地区第三、 四活动期各平静幕后期的中强地震活动特征, 认为: ① 华北地区第三、 四活动期9个活跃幕中有7个在首发强震前3年内发生中强地震, 其中第四活动期1～4活跃幕首发强震前2年内都发生了中强地震, 未来第5活跃幕首发强震前, 以发生中强地震作为序幕的可能性较大; ② 华北地区平静幕后期的中强地震活动, 对其后活跃幕首发强震在时间和地点上有一定指示作用。 由于华北地区可能即将进入新的强震活跃幕, 该研究对判断华北地区未来强震趋势具有实用意义。     

运用防屈曲支撑的混凝土框架结构的抗震性能研究

简要介绍地震安全社区的实现途径以及防屈曲支撑的构成和性能参数。运用ABAQUS软件对有无安装防屈曲支撑的钢筋混凝土框架模型进行动力时程分析,总结分析防屈曲支撑对钢筋混凝土框架结构的减震效果。结果表明,防屈曲支撑能大大降低钢筋混凝土框架结构在罕遇地震作用下的层间位移角,提高建筑物抗震性能,使地震安全社区中抗震设防烈度为Ⅶ度的建筑物能够抵御Ⅷ度罕遇地震。     

辽宁青城子铅锌金银矿矿集区围岩原岩恢复及其构造背景

辽宁青城子矿集区产出在辽东裂谷凹陷带,是中国北方重要的铅锌金银多金属矿集区.矿体与含矿岩系呈整合产出,受一定层位和岩性控制,其中大石桥组是铅锌的矿源层,具有高金丰度和富As的盖县组是金银的矿源层,变质作用或者韧性剪切作用使得金得到富集而成矿.铅锌矿床围岩主要为大石桥亚群杨树沟岩组大理岩、变粒岩和浅粒岩;金银矿床围岩主要为上部盖县组片岩,少量大石桥组变粒岩.围岩的岩石地球化学特征及原岩恢复表明,青城子铅锌矿围岩中的大理岩原岩为灰岩、白云岩,变粒岩、浅粒岩原岩主要为泥质砂岩,个别为火山岩;金银矿围岩中的片岩类原岩为泥岩、砂质泥岩及粉砂岩,变粒岩原岩为泥质砂岩.青城子铅锌金银围岩中的大理岩和片岩构造环境应属稳定大陆边缘,变粒岩、浅粒岩构造环境应属活动大陆边缘或岛弧.结合区域构造演化历史认为,裂谷活动所造成的地壳拉张、变形及其相关的构造-岩浆活动,为铅锌矿床的形成提供有利环境;不同级别的裂谷盆地内的沉积作用以及海底火山喷气作用、沿同生断裂发生热水沉积作用,为矿源层的形成提供成矿物质和成矿流体;吕粱期变质作用对区内铅锌矿的叠加、富集起重要的控制作用.     

南海东北部构造及块体运动指向的地震相响应(英文)

南海的构造与演化与资源环境等关系密切是本文研究重点。本文针对南海的东北部构造及其块体构造方向,利用所采集的区域地震剖面,通过解析地震相与构造及其演化的关系,提出以下观点:(1)构造分区特点明晰,可划分为五个不同构造单元,构造单元之间既有联系又相互独立;(2)南海沉积盆地无论表现为拉张-弱挤压-强挤压的何种构造格局,其区域构造应力场是统一的;(3)首次发现反射地震剖面上显示出两个浅俯冲点。每个块体构造层呈手风琴风箱式折曲并向东聚敛,体现沉积盆地从发育、成长、结束、消亡不同阶段在南海的表现,其块体俯冲方向以及块体包络区域性倾伏方向均与区域应力场方向一致。     

爆破地震效应主控因素分析及减震措施探讨

爆破地震动机制复杂,影响因素众多,为找出影响爆破地震效应的主控因素,引入灰色系统中灰色关联理论。通过实例分析,得到了影响爆破地震效应的主次关系,指出钻孔超深、孔网参数是影响爆破地震效应的主控因素,通过优化这些爆破参数是降低爆破地震效应最有效的方法。灰色关联分析方法计算简单、结果可靠、使用简便,在爆破工程中有一定的应用价值。     

内蒙古中部地区晚古生代地层接触变质作用研究初探

内蒙古中部晚古生代地层由于中生代岩浆的侵入,部分发生接触变质,变质程度多处于沸石相或葡萄石绿纤石相。通过对典型剖面的泥质岩石中的粘土矿物、伊利石结晶度以及镜质体反射率的分析,得出以下结论：（1）粘土矿物以伊利石和绿泥石为主,只在极少数样品中有伊蒙混层和蒙脱石,多数缺乏伊绿混层;（2）伊利石结晶度为022~040;（3）镜质体反射率Ro值为15~40。其结果表明：在岩浆侵入地区有小规模的接触变质,为极低级变质带,并未发生大规模的低绿片岩相的区域变质,而且这种变质只影响到晚二叠世以前沉积的地层和油气,分布范围有限。     

Tausonite and aluminum-fluorine titanite from the metamorphosed metalliferous sediments of the Triassic chert formation of the Sikhote Alin

Rare strontium mineral tausonite and a peculiar Al- and F-rich titanite variety were found in the metamorphosed metalliferous sediments of the Triassic chert formation of the Sikhote Alin, which are distinguished by the abundance of native elements, intermetallic compounds, and metal solid solutions, as well as the presence of diverse Au, Ag, and PGE minerals. Tausonite was documented in the manganese (metamorphosed siliceous-rhodochrosite) rocks of the Ol’ga mining district and in the “brown cherts” (siliceous rocks with manganese garnet and spessartine) of the Dal’nerechensk district, Primorye. It forms rather numerous grains 2–10 μm across usually occurring as inclusions in quartz or rhodonite. According to the electron microprobe analysis, in addition to Sr, Ti, and O, the mineral contains only Fe³⁺ (up to 0.20 a.f.u.). Aluminum-fluorine titanite was found in the “brown cherts” of the Dal’nerechensk district of Primorye (upper reaches of the Gornaya River). Its crystals are up to 200 × 200 μm in size. The recalculation of the microprobe analyses to crystal chemical formulas indicated that up to half of the Ti sites in the structure of this mineral may be occupied by Al. The decrease of the total positive charge owing to the Al³⁺ substitution for tetravalent Ti⁴⁺ is compensated for by a decrease in the total negative charge owing to F^? substitution for O^2? via the scheme Al³⁺F^? → Ti⁴⁺O^2?. The occurrence of considerable amounts of F substituting for oxygen in the titanite structure and, as a consequence (owing to the crystal chemical features of the mineral), the high Al content were related to the reduced character of the metamorphism of the metalliferous deposits.     

Pressure–temperature conditions of ongoing regional metamorphism beneath the Japanese Islands

We evaluate the pressure–temperature (P–T) conditions of ongoing regional metamorphism at the top of the oceanic crust of the subducted Pacific and Philippine Sea plates through a combination of phase diagrams and hypocenter distribution and based on the dehydration-induced earthquake hypothesis. The brute-force method was employed to find the best match thermal structure to link the hypocenter distribution and dehydration. The estimated thermal structure varies far from the values obtained from numerical simulation. Our estimates are consistent with the qualitative physical prediction for the variation of temperature in different subduction zones and provide quantitative constraints for the models.In northeastern Japan, the P–T path for the top of the oceanic crust turns to the high-T side at a depth of around 90 km. The depth corresponds to the location of the volcanic front and an active convection of the wedge mantle below this depth is suggested. Our computations also reveal the effect of an exceptional scenario beneath the Kanto region. The temperature in the Kanto region, where the cold lid of the Philippine Sea plate prevents heating by the return-flow of mantle wedge above, is much lower than that of northeastern Japan. The subduction of younger Philippine Sea plate leads to a higher-temperature in the oceanic crust. In the central Shikoku region, the thermal structure exhibits high-T/P nature. Heating by shear deformation can explain the high-T/P path in the depth range from 20 to 35 km. The Kyushu area shows moderate type T/P path reaching up to eclogite facies conditions. In the Kii and central Shikoku region, the thermal structure exhibits high-T/P nature. However, the absolute values for the areas seem to have problem in physical context. Our results has risen the significance of sediment subduction in the southwest Japan and requirement for further improvements in this technique including the aspect of variation of the bulk composition of the subducted material.     

Multi-stage zeolite facies mineralization in the Hvalfjördur area,Iceland

The Hvalfjördur area, 30 km north of Iceland’s capital Reykjavik, belongs to the sequence of Late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjördur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled “zeolite zones” occur in the Hvalfjördur area.