Relation between mean stress,thermodynamic, and lithostatic pressure

Pressure is one of the most important parameters to be quantified in geological problems. However, in metamorphic systems the pressure is usually calculated with two different approaches. One pressure calculation is based on petrological phase equilibria and this pressure is often termed thermodynamic pressure. The other calculation is based on continuum mechanics, which provides a mean stress that is commonly used to estimate the thermodynamic pressure. Both thermodynamic pressure calculations can be justified by the accuracy and applicability of the results. Here, we consider systems with low‐differential stress (<1 kbar) and no irreversible volumetric deformation, and refer to them as conventional systems. We investigate the relationship between mean stress and thermodynamic pressure. We discuss the meaning of thermodynamic pressure and its calculation for irreversible processes such as viscous deformation and heat conduction, which exhibit entropy production. Moreover, it is demonstrated that the mean stress for incompressible viscous deformation is essentially equal to the mean stress for the corresponding viscous deformation with elastic compressibility, if the characteristic time of deformation is five times longer than the Maxwell viscoelastic relaxation time that is equal to the ratio of shear viscosity to bulk modulus. For typical lithospheric rocks, this Maxwell time is smaller than c. 10,000 years. Therefore, numerical simulations of long‐term (>10 kyr) geodynamic processes, employing incompressible deformation, provide mean stress values that are close to the mean‐stress value associated with elastic compressibility. Finally, we show that for conventional systems the mean stress is essentially equal to the thermodynamic pressure. However, mean stress and, hence, thermodynamic pressure can be significantly different from the lithostatic pressure.     

南极乔治王岛菲尔德斯半岛湖泊堆积物的物相组成特征及其环境意义

本文根据南极乔治王岛菲尔德斯半岛燕鸥湖、西湖和基太克湖三个堆积剖面物相组成与丰度的资料，讨论其物相组成特征、物质来源及其环境意义。研究结果表明，湖泊堆积物主要来自该岛广泛分布的火山岩。在南极地区干冷环境条件下，源区母岩所经受的风化作用以物理风化为主，化学风化微弱。高岭石、方解石丰度与年龄关系曲线在湖积层与冰碛层之间的陡削变化表明，相应的环境变化是突变式的，可能与这两种堆积物搬运方式的不同和冰碛物的保护作用有关。     

Geodynamic evolution of Korea: A view

Abstract Evidence for South Korean Palaeozoic geodynamic evolution is restricted to the Ogcheon Belt, which is a complex polycyclic domain forming the boundary between the Precambrian Gyeonggi Block to the northwest and the Ryeongnam Block to the southeast. Two independent sub-zones can be distinguished: the Taebaeksan Zone to the northeast and the Ogcheon Zone sensu stricto. The Taebaeksan Zone and Ryeongnam Block display characteristic features of the North China palaeocontinent. This domain remained relatively stable during the Palaeozoic. In contrast, the Ogcheon Belt s. s. is a highly mobile zone that belongs to the South China palaeocontinent and corresponds to a rift that opened during the Early Palaeozoic. In lowermost Devonian times, the rift basin was closed and the Ogcheon Belt was structured in a pile of nappes. From the lack of suture in the Ogcheon Belt it can be inferred that the Gyeonggi Block belongs to the South China palaeocontinent. Thus, the boundary between the North China and South China blocks should be located to the north of Gyeonggi Block, that is, in the Palaeozoic Imjingang Belt. From the Middle Carboniferous, sedimentation started again on a weakly subsiding paralic platform located in the hinterland of the Late Palaeozoic orogen of southwest Japan. In the Late Carboniferous, increasing subsidence recorded extensional tectonics related to the opening of the Yakuno Oceanic Basin (southwest Japan). In the Middle Permian, the end of marine influences in the platform and emplacement of terrestrial coal measures, may be correlated with the closure of the oceanic area and subsequent ophiolite obduction. In Late Permian to Early Triassic times, the Honshu Block (the eastern palaeomargin of the Yakuno Basin) collided with Sino-Korea. Post-collisional intracontinental tectonics reached the Ogcheon Belt in the Middle Triassic (Songnim tectonism). Ductile dextral shear zones associated with synkinematic granitoids were emplaced in the southwest of the belt. In the Upper Triassic, the late stages of the intracontinental transcurrent tectonics generated narrow intramontane troughs (Daedong Supergroup). The Daedong basins were deformed during two tectonic events, in the Middle (?) and Late Jurassic. The Upper Jurassic to Lower Cretaceous basins (Gyeongsang Supergroup), that are controlled by left-lateral faults, may have resulted from the same tectonic event.     

大兴安岭扎兰屯地区晚古生代格根敖包组埃达克岩的发现及地质意义

大兴安岭中段扎兰屯地区晚古生代埃达克岩主要岩石类型为安山岩、粗面安山岩、英安岩和粗面英安岩,取得一个安山岩样品的LA-ICP-MS锆石U-Pb年龄为(316.9±2.4)Ma,代表火山岩喷发的年龄.岩石具有较高Si(Si O2=54.97%~63.80%),富碱并相对略富Na(Na2O/K2O1),富Al(Al_2O_3=14.97%~17.69%),高Sr(715.98×10~(-6)~2100×10~(-6)),低Y(12×10~(-6)~18.41×10~(-6))和Yb(1.02×10~(-6)~1.91×10~(-6))的特点.在原始地幔标准化蛛网图中,富集LREE,亏损HREE,Eu呈微弱正异常(δEu=0.97~1.30).同时Mg值介于0.35~0.57,平均0.46.总体特征属于高钾钙碱性埃达克岩(为C型埃达克岩的一种),来源于增厚的玄武质下地壳的部分熔融.扎兰屯地区晚古生代高钾钙碱性埃达克岩的发现,为兴安地块与松嫩地块的拼贴作用提供了新的线索,对正确认识区域地壳演化有着重要的构造意义,为本区寻找与埃达克岩有关的矿产提供了线索.     

滇西象达地区蚌渺岩体地球化学特征及其构造意义

蚌渺花岗岩体出露于保山微地块与腾冲微地块之间的泸水―潞西海槽内;该海槽是古生代地史时期保山微地块西侧的一个半深水沉积盆地,随着中三叠世时期古特提斯主洋盆的关闭及弧-陆碰撞作用,该海槽内的岩层普遍发生褶皱、断裂及不同程度的变质作用;在三台山一带还发育有蛇绿混杂岩。蚌渺岩体的主量元素显示了堇青石过铝花岗岩(CPG)的特点,稀土及微量元素资料表明其属典型的高Sr低Yb花岗岩类,具有类似于埃达克岩的地球化学特征。蚌渺岩体形成于中侏罗世,是三叠纪碰撞造山作用结束之后陆内应力松弛阶段的岩浆活动,属后碰撞花岗岩。可能涉及俯冲板片或造山带山根在榴辉岩相温-压条件下的部分熔融作用,是造山运动加厚的地壳向正常地壳恢复的重要机制;也是滇西地区晚中生代大规模中酸性岩浆活动及相关成矿作用的前奏。     

青海南部沱沱河冻土水合物潜在区火山岩地球化学特征及地质意义

沱沱河盆地是冻土天然气水合物潜在分布区之一,其内发育下—中二叠统开心岭群九十道班组、上二叠统乌丽群那益雄组、上三叠统结扎群巴贡组、中—渐新统雅西措组4套烃源岩以及不同类型的火山岩。研究表明,火山岩岩石类型主要为玄武岩、玄武安山岩、安山岩、玄武质粗面安山岩和粗面安山岩。火山岩主量元素低TiO_2,Al_2O_3含量较高,K_2O含量较低,K_2ONa_2O;火山岩的稀土元素配分模式为轻稀土富集型;微量元素配分模式呈锯齿状,Ta、Nb、P_2O_5、TiO_2、Y、Yb以及铁族元素Sc、Cr、Ni亏损,综合判断青海南部沱沱河地区火山岩形成于岛弧环境。结合火山岩的地球化学特征,推断火山活动可以加快烃源岩的热成熟,进而促使有机质裂解,产生热解气,为水合物的生成提供气源条件。同时,火山岩的气孔构造发育,连通性较好,有可能成为水合物的储层。火山热液具有的高压使周围的烃源岩产生裂隙,这些裂隙不仅是水合物气源的运移通道,还可以为水合物提供形成场所和储存空间。     

台湾岛以东海域海底地形特征及其构造控制

基于2000年5~6月在台湾岛以东海域调查获得的多波束全覆盖测深等地质和地球物理资料,对该海域海底地形特征进行了研究,探讨了构造对海底地形的控制作用及其构造地质意义.研究表明,琉球岛弧岛坡区和琉球海沟表现为典型的西太平洋沟-弧-盆体系控制下的构造地形;台湾岛东部岛坡等深线近南北向平行密集排列,地形坡度大,弧陆碰撞造就了该区独特的地形特征;花东盆地海底峡谷发育,其形成主要受基底起伏和走滑断裂的控制;加瓜海脊东西两侧水深和地形特征明显不同,但其基底可能属于花东盆地,加瓜海脊的东侧对应了两个不同性质板块的边界;西菲律宾海盆表现为北西向线状脊-槽相间排列,并遭受北东向转换断层的切割,根据海底地形、转换断层和磁异常条带的方向推测,研究区海底形成于距今60~45Ma的西菲律宾海盆北东-南西向扩张期.     

甘肃北山地区早白垩世松柏类植物Podozamites(苏铁杉属)及其古气候意义*

Podozamites(苏铁杉属)为松柏类已灭绝的形态属,是北半球中生代植物群中的重要分子。经鉴定,甘肃北山地区早白垩世早期地层中产出的Podozamites包括Podozamites sp.(苏铁杉未定种)、Podozamites aff. distans(间离苏铁杉相似种)和Podozamites bullus(美丽苏铁杉)3个种。基于Podozamites大化石记录和古地理分布特征,结合中生代古气候分区,对该属的古气候意义进行综合分析。结果表明: 晚三叠世—中侏罗世,Podozamites数量较为丰富,主要集中分布于北半球温暖湿润气候区; 晚侏罗世—早白垩世,随着干热气候带的扩张,Podozamites丰度下降,分布区域也随温热潮湿气候区变迁而发生迁移。推测Podozamites在中生代的古地理分布主要受到温暖潮湿气候带变迁的影响,其更加适宜生存于温暖湿润的气候条件下,可作为温暖湿润气候的指示分子。     

都龙锡锌矿床绿泥石特征及其成矿意义

都龙锡锌超大型矿床是中国第三大锡石硫化物矿床,其中的绿泥石化相当普遍,并与矿化关系密切。本文在岩矿鉴定基础上,利用电子探针对绿泥石进行了微区化学成分研究。研究结果表明,该矿床绿泥石为富铁种属的假鳞绿泥石、鲕绿泥石、蠕绿泥石(铁绿泥石)及铁镁绿泥石,指示形成于还原环境;绿泥石为泥质岩或铁镁质岩受热液交代蚀变的产物,绿泥石结构的离子置换主要体现为Fe对Mg的置换,反映其形成与含铁建造有关;绿泥石的形成温度为231～304℃,平均为269℃,属于中-低温范围。绿泥石的形成机制主要表现为溶蚀-结晶、溶蚀-迁移-沉淀结晶2种。绿泥石可能与锡成矿同期形成,其与矿石矿物的生成关系表明,燕山期岩浆活动对都龙矿床的叠加改造作用显著。     

亚洲地球动力系统的演进与东亚矿产资源效应

本文系统阐述了亚洲中部及邻区地球动力系统的演变进程及其所制约的区域地质和成矿特征,并提出了以下新认识和新观点:①在西伯利亚陆块与中朝—塔里木陆块之间的原"古亚洲洋"区域内是全球地史演化中具有双向侧向陆缘增生与垂向增生同时发展的"双向增生"独特地区。②"古中国陆块群"是位于劳亚古陆和冈瓦纳古陆之间独立存在的古陆。由于它的特殊位置而构成了古亚洲洋与特提斯洋的"分水岭"。③地球动力学的"内动力"归根到底来自地球自身的两个方面:一是地球自转和公转形成的离心力(拉张力)与挤压力,它是地球动力学的基础,二是地球内部永不衰败的"高热能库"。当今地震、火山等等,都是地下过饱和的高热能向地表释放的一瞬间转变为强动能的地质事件。④由地球动力系统演变打造的具有不同特征的断裂系统是控矿储矿的良好空间,因此"断裂系统找矿法"是简便有效的找矿方法之一。据此提出了4个理论指导找矿的试点和验证区。