A discussion on the boundary between diagenesis and metamorphism with reference to zeolite facies

Abstract A Triassic formation in the Turpan-Hami and Ordos basin of China gives two typical examples of occurrence of laumontite belonging to diagenetic facies. Sandstones of the Upper Triassic Yanchang Formation in the eastern sector of the Ordos basin are formed at the mature stage of mesodiagenesis at temperatures in the range of 71-120°C, while the temperatures of the oil-bearing beds Chang-6 and Chang-7 where laumontite is present are about 81-88°C. The laumontite-bearing beds of the Middle Triassic in the Hami depression are presently at the supermature stage of organic matter, the corresponding diagenetic temperature being about 140 °C. The term zeolite facies should be rejected, and the features of metamorphic stage should be clearly denned, such as closely packed grains, absence of pores and cements, and characteristic mineral assemblages including sericite, epidote, muscovite, illite (2M₁), chlorite (IIb, β= 97°), dickite, pyrophyllite, graphite, chlorozeolite formed at temperatures of 200°C and Ro values of 2.5%; the stage is also characterized by schistosity and illite crystallinity of IC = 0.42°Δ 2θ. The anchimetamorphic zone is characterized by some characteristic minerals such as paragonite, rectorite, albite, laumontite, illite (1M), chlorite (Ib, P = 90°+ IIb, β= 97°) formed at temperatures of 130-200°C and Ro values of 1.3-2.5% as well as other quantitative parameters of organic matter.     

鄂尔多斯地块早古生代构造格局及演化

本文通过沉积、构造和沉降历史的综合分析，揭示了鄂尔多斯早古生代碳酸盐岩台地演化历史，恢复了不同演化阶段的古构造格局，并探讨了古构造的演化机制。     

柯树北花岗岩体中金－铀矿脉的初步研究

柯树北花岗岩体中的金－铀矿脉主要产出在岩体的西南部。该岩体Ｕ－Ｐｂ同位素年龄１５３Ｍａ，定位在两条深大断裂的复合部位。通过岩石化学及稀土元素特征研究表明，岩体与华南重熔再生岩浆成因的陆壳改造型花岗岩相一致。矿脉为石英－硫化物型，金、铀矿化与微晶石英和细晶、微晶黄铁矿关系密切。岩体的铀、金丰度高于本区任何其它岩石，矿脉有往深部变好的趋势，推测本区具有良好的成矿前景。     

Fine structure of seismotectonics in western Shillong massif,north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.     

黄河矿晶体结构的重新测定及空间群对称的讨论

黄河矿 BaCe(CO₃)₂F(a=0.5072(1)nm,c=3.846(1)nm;V=856.8×10^-3nm³;Z=6)晶体结构的重新测定,采用中心对称空间群 R3m-D_3d⁵,基于337个可观察的单晶 X 射线数据(I>2σ₁)和30个可变参数,R=0.031。原子坐标和均方根振幅(据各向异性位移参数计算,O.013-0.025nm)表明,没有必要降低对称性,即没有必要采用相应的非中心对称空间群或其它次群。黄河矿的结构中,原子形成似层状排列,且垂直于 c 轴;碳酸根通过 Ba 与 Ce 的联结形成三维结构。C(1)O₃,和 C(2)O₃,中的 C 原子偏离三个氧组成的平面,其偏离的距离分别为0.0066(15)nm 和 0.0029(15)nm。     

PRESENT-DAY KINEMATICS OF THE ORDOS BLOCK AND ITS SURROUNDING AREAS FROM GPS OBSERVATIONS

Located among the South China block, Tibetan plateau, Alxa block and Yinshan orogenic belt, the Ordos block is famous for its significant kinematic features with stable tectonics of its interior but frequent large earthquakes surrounding it. After the destruction of the North China Craton, the integrity, rotation movement and kinematic relations with its margins are hotly debated. With the accumulation of active tectonics data, and paleomagnetic and GPS observations, some kinematic models have emerged to describe rotation movement of the Ordos block since the 1970's, including clockwise rotation, anticlockwise rotation, clockwise-anticlockwise-alternate rotation, and sub-block rotation, etc. All of these models are not enough to reflect the whole movement of the Ordos block, because the data used are limited to local areas.
In this study, based on denser geophysical observations, such as GPS and SKS splitting data, we analyzed present-day crustal and mantle deformation characteristics in the Ordos block and its surrounding areas. GPS baselines, strain rates, and strain time series are calculated to describe the intrablock deformation and kinematic relationship between Ordos block and its margins. SKS observations are used to study the kinematic relationship between crust and deeper mantle and their dynamic mechanisms, combined with the absolute plate motion(APM)and kinematic vorticity parameters. Our results show that the Ordos block behaves rigidly and rotates anticlockwise relative to the stable Eurasia plate(Euler pole: (50.942±1.935)°N, (115.692±0.303)°E, (0.195±0.006)°/Ma). The block interior sees a weak deformation of~5 nano/a and a velocity difference of smaller than 2mm/a, which can be totally covered by the uncertainties of GPS data. Therefore, the Ordos block is moving as a whole without clear differential movement under the effective range of resolution of the available GPS datasets. Its western and eastern margins are characterized by two strong right-lateral shearing belts, where 0.2°~0.4°/Ma of rotation is measured by the GPS baseline pairs. However, its northern and southern margins are weakly deformed with left-lateral shearing, where only 0.1°/Ma of rotation is measured. Kinematics in the northeastern Tibetan plateau and western margin of the Ordos block can be described with vertical coherence model with strong coupling between the crust and deeper mantle induced by the strong extrusion of the Tibetan plateau. The consistency between SKS fast wave direction and absolute plate motion suggests the existence of mantle flow along the Qinling orogenic belt, which may extend to the interior of the Ordos block. SKS fast wave directions are consistent with the direction of the asthenosphere flow in Shanxi Rift and Taihang Mountains, indicating that the crustal deformation of these areas is controlled by subduction of the Pacific plate to North China. The week anisotropy on SKS in the interior of Ordos block is from fossil anisotropy in the craton interior. After comparing with the absolute plate motion direction and deformation model, we deem that anisotropy in the interior of Ordos block comes from anisotropy of fossils frozen in the lithosphere. In conclusion, the Ordos block is rotating anticlockwise relative to its margins, which may comes from positive movement of its margins driven by lithospheric extrusion or mantle flow beneath, and its self-rotation is slight. This study can provide useful information for discussion of kinematics between the Ordos block and its surrounding tectonic units.     

前前陆盆地层系砂岩成岩环境特征及其孔隙演化——以鄂尔多斯盆地西缘二叠系为例

鄂尔多斯西缘二叠系为前前陆盆地层系，砂岩成岩作用仍受前陆盆地构造的影响，不同构造分区砂岩储层的成岩环境存在一定的差别。研究发现，西缘逆冲带砂体经历了浅埋藏弱压实和溶蚀作用，储集物性最优；斜坡带砂岩储层不仅残留少量粒间孔隙，而且溶蚀作用形成的次生孔隙对储集空间的改善十分明显；压实压溶作用和胶结充填作用严重损害前渊带砂岩原生孔隙，但后期的凝灰质溶蚀蚀变作用对储层性能有一定的改善，仍不失为天然气的有效储层。     

Study on Shear Wave Velocity Structure and Velocity Ratio Beneath Ordos Block and Its Eastern and Southern Margins

Using pure S wave fitting method, we studied the shear wave velocity structures under the Ordos block and its eastern and southern marginal areas. The results show that the velocity structure beneath Yulin station in the interior of Ordos block is relatively stable, where no apparent change between high and low velocity layers exists and the shear wave velocity increases steadily with the depth. There is a 12km thick layer at the depth of 25km under this station, with an S wave velocity (Vs=3.90km/s) lower than that at the same depth in its eastern and southern areas (Vs≥4.00km/s). The crust under the eastern margin of Ordos block is thicker than that of the Yulin station, and the velocity structures alternate between the high and low velocity layers, with more low velocity layers. It has the same characteristic as having a 10km-thick low velocity layer (Vs=3.80km/s) in the lower crust but buried at a depth of aout 35km. Moreover, we studied the Vp/Vs ratio under each station in combination with the result of P wave velocity inversion. The results show that, the average velocity ratio of the Yulin station at the interior of Ordos block is only 1.68, with a very low ratio (about 1.60) in the upper crust and a stable ratio of about 1.73 in the mid and lower crust, which indicates the media under this station is homogenous and stable, being in a state of rigidity. But at the stations in the eastern and southern margins of the Ordos block, several layers of high velocity ratio (about 1.80) have been found, in which the average velocity ratio under Kelan and Lishi stations at the eastern margin is systemically higher than that of the general elastical body waves (1.732). This reflects that the crust under the marginal areas is more active relatively, and other materials may exist in these layers. Finally, we discussed the relationship among earthquakes, velocity structures beneath stations and faults.     

鄂尔多斯北缘断陷盆地区强震孕育的卫星遥感影像研究

利用多时相、多波段卫星图像,研究1979年五原6.0级地震、1989年大同-阳高6.1级地震和1996年包头西6.4级地震震区的构造活动信息,结合前人的烈度调查资料,探讨这些强震发生的地质构造环境。研究表明:1979年五原6.0级地震发生在NE向海子堰断裂与NW向五原西断裂交会的部位。五原6.0级地震高烈度区等震线长轴呈NE向,与海子堰断裂一致,是五原地震的发震构造。1996年包头西6.4级地震发生在由陡崖和沟槽地貌显示的NEE向乌拉山北缘断裂与NW向新生砂石厂断裂交会的部位。包头西6.4级地震高烈度区等震线长轴呈NE向,与乌拉山北缘断裂接近,该断裂是包头西6.4级地震的发震构造。2次地震高烈度区长轴与低烈度区长轴走向相差近90°,这是因为除发震构造外,烈度区还受一组与之交会的共轭断裂活动的影响。1989年大同-阳高6.1级地震,发生在从六棱山腹地向大同-阳高盆地延伸的NNE向大王村-西要泉断裂上,该断裂是大同-阳高地震的发震构造     

鄂尔多斯盆地东部太原组古岩溶特征

鄂尔多斯盆地下二叠统太原组为碳酸盐与陆源碎屑混合沉积,主要由灰岩、岩屑石英砂岩、泥岩和煤岩组成。太原组沉积时期鄂尔多斯盆地东部为碳酸盐与陆源碎屑混合沉积的陆表海潮坪沉积体系和三角洲沉积体系。北部发育三角洲前缘沉积。南部发育碳酸盐潮下带—潮间带及陆源碎屑潮下带。太原组中普遍发育冲刷充填现象,说明短期的沉积间断发生频繁。太原组中岩溶角砾岩发育,主要为镶嵌状角砾岩,见到少量裂缝角砾岩和紊乱角砾岩。太原组中的岩溶形成于同生期,没有形成岩溶地貌。与现代碳酸盐沉积物相比,太原组灰岩具有较高的Mn和Fe含量,明显低的Sr含量和δ18O值。这是由于埋藏作用和淡水淋滤所引起的,低的Sr含量和δ18O值主要由淡水淋滤所致。与大多数地质学家研究得出的二叠纪生物和非生物成因的方解石的δ13C值相比较,太原组方解石的δ13C值无明显差异。宏观和微观特征均说明太原组存在古岩溶。岩溶作用使得灰岩的储集性能在一定程度上得到了改善。