香港世界地质公园粮船湾组碎斑熔岩柱状节理构造特征

在香港世界地质公园西贡火山岩园区 ,白垩纪粮船湾组流纹质碎斑熔岩普遍发育完整的柱状节理 ,构成世界上面积最大的流纹质火山岩石柱群.文章在详细野外观察与测量的基础上 ,分析了粮船湾组石柱群原生柱状节理构造特征 ,初步探讨了其成因.结果表明 ,该区火山岩石柱截面形态以六边形为主 ,少量五边形及不规则状 ,直径以1~3m为主 ;完整的单个石柱高一般20~30m ;石柱整体向NW和SW陡倾.但是 ,粮船湾组火山岩非全部发育柱状节理 ,按石柱发育程度可分为极发育、一般发育(被破坏)和不发育三种类型.通过统计大量原生柱面的产状 ,发现各个方向的柱面均有发育 ,其无序性符合冷凝收缩的成因模式 ,认为后期次生节理构造及海蚀作用对原生柱状节理有一定程度的破坏.     

香港国家地质公园粮船湾组火山岩岩石学研究

在香港国家地质公园西贡火山岩园区,核心地质景观是白垩纪粮船湾组(Kkh)火山岩优美的六方形石柱(柱状节理),它们的岩石类型长期存在熔岩和火山碎屑岩之争。笔者等经野外调查和薄片岩石学研究,确认粮船湾组火山岩实属一种特殊的熔岩——流纹质碎斑熔岩,以普遍的柱状节理、斑晶具有碎斑结构和珠边结构、基质发育霏细结构和流动构造为特征;它们不仅代表了香港地区中生代最晚期火山喷发的产物,而且构成了西贡破火山机构的中央侵出相岩穹。推断粮船湾组火山岩石柱是地球上已知面积最大的流纹质碎斑熔岩石柱群(～150 km2),目前所见的火山岩石柱仅是长期剥蚀后的残余部分。     

香港世界地质公园碎斑熔岩柱状节理构造特征

在香港世界地质公园西贡火山岩园区,白垩纪粮船湾组流纹质碎斑熔普遍发育完整的柱状节理,构成世界上面积最大的石柱群。在详细野外观察与测量基础上,了粮船湾组石柱群柱状节理构造特征,探讨了成因。石柱截面形态以六边形为主,少量五边形及不规则状,直径以13m为主;完整的单个石柱高一般2030m;石柱整体向NW和SW陡倾。粮船湾组火山岩非全部发育柱状节理,按发育程度可分为极发育、一般发育和不发育三种类型通过统计大量原生柱面产状,各个方向的柱面均有发育,其无序性符合冷凝收缩成因模式后期次生构造及海蚀作用对原生柱状节理有一定程度的。     

大兴安岭北段斯木科流纹质碎斑熔岩特征及成因

在大兴安岭北段斯木科地区发育一种具有独特结构特征的火山岩石类型——流纹质碎斑熔岩,其具有熔岩和熔结凝灰岩的双重结构.岩石富Si、碱,Na2O/K2O=0.61~0.82,轻稀土富集,Eu亏损强烈,富集Rb、Th,亏损Ba、K,与同期流纹岩的地球化学特征相同,其区别主要在岩相学上.二者均形成于造山后伸展环境,属于由于岩浆向上运移、定位过程的不同而形成的"同质异相体".     

松辽盆地东南隆起区营城组柱状节理流纹岩特征和成因

松辽盆地东南隆起区下白垩统营城组出露柱状节理流纹岩。柱状节理属于原生构造节理,岩相为火山通道相次火山岩亚相-侵出相,柱体排列方式为近水平式、倾斜式、近直立式,截面为不规则的四方柱、五方柱、六方柱,柱体规模属于中小型,流纹岩中常发育隐爆角砾结构。通过对柱状节理流纹岩成分及结构构造的研究,表明本区的流纹岩柱体直径与其对应的显微组构斑晶大小成正比例相关。柱状节理的成因是熔岩冷却收缩形成的,由于受到种种内外因素制约,形成的柱状节理形态各异。     

初洞流纹质碎斑岩──一种底辟构造岩

初洞岩体是一个罕见的复杂地质体。它以其独特的岩石外貌、多期次的矿物世代、罕见的组构特征及复杂的变形机制而不同其它岩体。在研究其岩石结构构造基础上，对反映其成因信息的组构特征进行了理论解释，并将其命名为流改质碎斑岩──一种底辟构造岩。进一步研究后提出：流纹斑岩、碎斑熔岩及流纹质碎斑岩组成“三位一体”的次火山侵入杂岩体，并依此建立起特殊构造环境下岩石分阶段逐渐自碎的一种新底辟构造机制。     

The Geological Significance of a Tuffite Interlayer in the Cretaceous Port Island Formation in Northeastern Hong Kong

The Port Island Formation(PIF), a typical Cretaceous red bed in Hong Kong, is dominated by non-fossiliferous, reddish clastic rocks, making it difficult to determine the sedimentary age of PIF precisely. Previous studies assigned the PIF to Late Cretaceous provisionally only on the basis of its stratigraphic sequence and lithology. This study identified a tuffite interlayer in the PIF and a zircon UPb age of 128.2±2.7 Ma by LA-ICP-MS method was obtained. It's the first time to date the depositional age of the PIF with a reliable chronological constraint. With the support of stratigraphic evidence, we concluded that the geological age of PIF should be Early Cretaceous rather than Late Cretaceous. Based on the volcanic history of Hong Kong and Southeast China and the distribution of the PIF in Mirs Bay, it is believed that there was no volcanic activity in Hong Kong in ca. 128 Ma. The tuffite interlayer discovered in PIF was formed by the deposition of volcanic ash, which might originate from remote region outside Hong Kong, in an aquatic environment on Port Island. The identification of the tuffite interlayer, as the response to a volcanic event, has great significance not only to the studies of establishment and regional correlation of the strata system and the geological evolution in Hong Kong,but also to the study of volcanic activities in Southeast China.     

香港九龙复活破火山的鉴别及其地质意义

香港主城区座落在香港岛一九龙地区的九龙花岗岩体上,该花岗岩体平面形态呈直径约11km的圆形,周边火山岩群山环绕,构成特殊的圆形盆地地貌,以往曾有人认为是陨石坑。笔者根据前人地质资料分析并经实地考察,认为该地貌形态实为大致以尖沙咀为中心的一个大型晚中生代复活破火山机构,九龙花岗岩体为破火口塌陷后侵位的中央侵入体,周围火山地层产状围斜内倾,发育环形断裂（主要为高角度正断层）和放射状断裂,具典型复活破火山特征,可称之为“九龙复活破火山”。     

Analysis of rainstorm-induced slide-debris flows on natural terrain of Lantau Island, Hong Kong

Lantau Island, the largest outlying island of the territory of Hong Kong, experienced a severe rainstorm on 4–5 November 1993, which induced >800 slope failures on natural terrain there. Detailed field investigations were carried out to study the failure modes, in relation with various influencing factors. It was found that the occurrence of slide-debris flows has a close relationship with bedrock geology, slope gradient, vegetation cover and micro landform. The failure modes of slide-debris flows may be classified into translational slides and rotational slides, and the former are predominant. Analysis of the hydrological response of colluvial slopes during the rainstorm indicated that the majority of the failures were caused by the development of a perched water table in the thin surface layer of colluvium of volcanic origin due to infiltration during the heavy rain. Undisturbed soil samples from south Lantau have been subjected to anisotropically consolidated undrained compression tests at comparatively low stress levels. Constant deviatoric stress path tests (CQD) simulating the stress path in the field at in situ stress levels have been performed to investigate soil behavior. The CQD test results indicate that the material of slopes at undisturbed state is brought to dilation because of the increase in pore water pressure caused by infiltration of rain water. For a translational slide, the displacement, resulting from dilation, may destroy cohesion along the failure surface and locally within the interior of the slide. The surplus water during the intense rainstorm was able to equilibrate the reduction in pore pressure caused by dilation, and the dilation and displacement may be further increased. The strain-softening after significant strains triggered debris flow mobilization. However, for a rotational slide, the increase in pore water pressure caused by surplus water infiltration during the intense rainstorm could not equilibrate the reduction in pore pressure caused by dilation, much or even all of the sliding block could not mobilize into a debris flow.     

Late Jurassic to Early Cretaceous volcanism of Hong Kong: Insights from the Ping Chau Formation

The vast expanse of Mesozoic igneous rocks in Hong Kong contain important geological records of late Mesozoic magmatic events and tectonic processes from the coastal region of Southeast China. Of these,the Ping Chau Formation in the northwestern New Territories is the youngest known stratum. We perform a detailed study of the volcanic rocks of the Ping Chau Formation utilizing zircon U-Pb dating,with major and trace elements geochemistry. LA-ICP-MS zircon U-Pb data reveal Early Cretaceous age from two volcanic rock samples, with zircon crystallization from magmas at 140.3 ± 0.8 Ma and 139.3 ± 0.9 Ma,respectively. These rocks have high contents of total alkalis(Na_2O + K_2O = 5.58-9.45 wt.%), high-field-strength elements and light rare earth elements, conspicuous negative Eu anomalies, and depletions in Nb, Ta, Ti, Sr, Ba and P. Using this data, in combination with previous studies on the late Mesozoic volcanic belt in Southeast China, we propose that the volcanic rocks of the Ping Chau Formation probably originated from deep melting of the crust in a back-arc extensional setting induced by the subduction of the paleo-Pacific Plate. This formation represents the final stages of Early Cretaceous volcanic activity in Hong Kong, as associated with large-scale lithospheric extension, thinning and magmatism. Our results provide new information that can be used in evaluating the significance of Early Cretaceous volcanism and tectonics in Southeast China.     

Regional bias of landslide data in generating susceptibility maps using logistic regression: Case of Hong Kong Island

On the basis of 1,834 landslide data for Hong Kong Island (HKI), landslide susceptibility maps were generated using logistic regression and GIS. Regional bias of the landslide inventory is examined by dividing the whole HKI into a southern and a northern region, separated by an east-west trending water divide. It was found that the susceptibility map of southern HKI generated by using the southern data differs significantly from that generated by using northern data, and similar conclusion can be drawn for the northern HKI. Therefore, a susceptibility map of HKI was established based on regional data analysis, and it was found to reflect closely the spatial distributions of historical landslides. Elevation appears to be the most dominant factor in controlling landslide occurrence, and this probably reflects that human developments are concentrated at certain elevations on the island. Classification plot, goodness of fit, and occurrence ratio were used to examine the reliability of the proposed susceptibility map. The size of landslide susceptible zones varies depending on the data sets used, thus this demonstrates that the historical landslide data may be biased and affected by human activities and geological settings on a regional basis. Therefore, indiscriminate use of regional-biased data should be avoided.     

Geochemistry and zircon U-Pb geochronology of the rhyolitic tuff on Port Island,Hong Kong: Implications for early Cretaceous tectonic setting

Early Cretaceous rhyolitic tuffs, widely distributed on Port Island, provide insights into the volcanism and tectonic setting of Hong Kong. In this paper we present petrological, geochronological and geochemical data of the rhyolitic tuff to constrain the diagenesis age and petrogenesis of the rocks, tectonic setting and early Cretaceous volcanism of Hong Kong. The first geochronological data show that the zircons in the volcanic rocks have U-Pb age of 141.1–139.5 Ma, which reveals that the rhyolitic tuff on Port Island was formed in the early Cretaceous (K₁). Geochemically, these acid rocks, which are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), belong to the high K calc-alkaline to shoshonite series with strongly-peraluminous characteristic. The geochemical analyses suggest that the volcanic rocks were derived from deep melting in the continental crust caused by basaltic magma underplating. Based on the geochemical analysis and previous studies, we concluded that the rhyolitic tuffs on Port Island were formed in a back-arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.     

香港西贡滨珊瑚REE的地球化学特征及其与海平面变化的关系

用ICP-MS技术测定了位于珠江口西岸香港西贡滨珊瑚在1991～2002年(分辨率达到0.5a)的稀土元素(REE)含量。实验结果显示香港滨珊瑚REE具有Ce负异常、重稀土富集这种典型的海水相分布模式。香港滨珊瑚中REE含量处于目前已见报道中的高值范围,与巴布亚新几内亚珊瑚的REE含量相当。通过与不同地区的比较,香港珊瑚高REE含量很可能就是毗邻的珠江水体高REE的直接反映,而且同南海北缘半封闭边缘海的性质有关。香港珊瑚REE的来源主要为珠江口及其相邻的陆缘沉积物。此外,1991～2002年香港珊瑚REE含量年际下降趋势十分明显,Ce负异常和重稀土富集的程度也呈加剧的趋势,分布模式向海水相转化。REE含量与海平面的年际变化之间有显著的负相关关系(r=-0.7～-0.9),而且重稀土与海平面的相关系数优于轻稀土。上述结果揭示了热带滨珊瑚对近期全球变暖、极冰融化和海水膨胀所引起的海平面快速上升的响应。