2002 Vol. 13, No. 3
Permian radiolarian biostratigraphy was reexamined in the bedded chert section in Qinzhou area of southeast Guangxi Zhuang Autonomous Region, South China. On the basis of the analyses of characteristic radiolarian composition, six radiolarian zones are recognized, namely, Pseudoalbaillella longtanensis Zone, Pseudoalbaillella globosa Zone, Follicucullus monacanthus Zone, Follicucullus scholasticus Zone, Follicucullus charveti Zone and Neoalbaillella ornithoformis Zone, in ascending order. Correlation of these radiolarian zones with those in Japan is presented. The Neoalbaillella ornithoformis Zone was firstly discovered in Dachongling Section, which may indicate that pelagic chert of the late Permian existed there, according to the co~occurring conodonts
The rock series, rock types and Sr~Nd isotopic dating of the Cenozoic volcanic rocks in the South China Sea are similar to those in its vicinity. On the basis of the spreading age of the South China Sea, the Cenozoic volcanic rocks are divided into three stages: the pre~spreading stage, the spreading stage and the post~spreading stage. The deep process characteristics of the asthenosphere and lithosphere may be inferred from the study on primary basaltic magma. The top layers of the asthenosphere both in the spreading stage and in the pre~spreading stage are closer to the earth surface than that in the post~spreading stage. From the pre~spreading stage to the spreading stage, the top layer of the asthenosphere decreased in depth, while the amount of interstitial partial melts increased. The evolution of the primary basaltic magma shows a progressive evolution sequence of the rifting volcanism and a faster lithospheric spreading velocity. From the spreading stage to the post~spreading stage, the top layer of the asthenosphere gradually increased in depth, but the amount of interstitial partial melts decreased. The evolution of primary basaltic magma shows a retrogressive evolution sequence of the rifting volcanism and a gradual decrease in the lithospheric spreading velocity. The depth recognized by the study on the Cenozoic volcanism demonstrates the deep environment for the formation and evolution of the South China Sea.
The Xiaoban gold deposit is a large~size deposit recently found in middle area of Fujian Province. It belongs to magmatic hydrothermal type occurred in Mayuan Group metamorphic rocks of Middle Proterozoic and is controlled by low angle fault (detachment) structures. The contents of Au in Mayuan Group metamorphic rocks, Caledonian~Indosinian deformed granite and early Yanshanian granite are higher with Au enrichment coefficient of 2.06~5.68, 5.11 and 6.67 than those in other geological bodies. And the higher enrichment coefficients (>2) of Ag, S, Sn and Te are similar to those of gold ore. Meanwhile, the distribution of Au in Mayuan Group metamorphic rocks and early Yanshanian granite with a low D~value (0.58 and 0.67) is favorable to gold mineralization. REE characteristics of gold ore, ratios of (LREE/HREE), (La/Sm)n, (Yb/Lu)n, (La/Tb)n and (Sm/Nd)n are similar to Mayuan Group metamorphic rocks, only non or little normal Eu abnormal of ore is dissimilar to metamorphic rocks. The δ (34S) of the gold ore, with a high homogenization, is (-4.7×10-3)~(-2.7×10-3). The study of inclusion indicates 180~249 ℃ of mineralization temperature, 3.69 % ~ 11.81 % of salinities and 0.869~0.991 g/cm3 of densities of mineralization fluid. Based on hydrogen and oxygen isotope (δ(18O)=11.0×10-3~11.7×10-3, δ(D)=(-48×10-3)~(-62×10-3) ) and initial w (87Sr)/w (86Sr) =0.715, combining to the analysis of geological history, regional metamorphism and magamtic activity, the authors confirm that the source for the ore fluid was mainly from magmatic, partly from metamorphic water, and with a little influence of meteoric water. Isotopic dating made on Rb~Sr isochron age of 182 Ma, by using alteration minerals of gold~ores from the deposit, indicates that the mineralization occurs in early Yanshanian epoch. This is close to the age of 187 Ma of the Anchun magmatite with a similar alteration and gold mineralization to the Xiaoban gold deposit. The age of early Yanshanian epoch of the Xiaoban gold is indentical with the characteristics of southern China gold metallogenic belt and the geotectonic evolution of the transition from paleo~Asian system and paleo~Tethyan system to paleoPacific active continental margin in eastern Asia.
This paper reports our primary effort in the quantitative reconstruction of paleoclimate based on the thrine in phytoecology of the affinity parent plants in the stratigraphic pollen records. The Eocene pollen data come from our former study on the Mingjia borehole 1 in the Jianghan basin. The fluctuating trend in the parameter curve of climate shows that the climate in the Middle Eocene in the Jianghan basin was more or less comparable with that of the present 22° ~ 26° N, characteristic of a humid, semi~humid central southern subtropical climate. The annual temperature at that time dropped by 1 ℃ ~ 4 ℃ in the Late Eocene, approximately equal to that of the present 23° ~ 28° N of northern~central subtropical climate. However, the climate composite at that time, characterized by higher temperature, small annual range and big fluctuation in precipitation, was quite different from the present one. The average temperature in January in the Middle Eocene, higher than that of today, ranged between 5 ℃ and 9 ℃, indicating that no effect of winter monsoon occurred in the Middle Eocene, though such an effect may have occurred occasionally in the Late Eocene. Major temperature decline is recognized at the depth of 2 100 m in the borehole, as was indicated by the decline in average January temperature, the increase in annual range, and the increase in the deciduous broad leaved types of trees in the spore~pollen assemblage. The sharp fluctuation in the annual precipitation, usually raging from 300 to 1 700 mm, was favorable for the migration and accumulation of salty deposit. When the precipitation was lower than 1 000 mm, ephemera shrub increased at the same depth as that of the salty deposit. It is, therefore, deduced that the formation of the salty deposit was attributed mainly to the dry and hot environment in the high mountains and deep basins. The small annual precipitation and the intense fluctuation are favorable for the sustainable accumulation of the salts, which is different from the present salty accumulation in the Northwest China
Geological environment in Tangshan City is under investigation with reference to the Tangshan Urban Earth Science, geo~hazards maps. The expected loss for urban land utilization is calculated by employing relevant economic mathematic models. Quantitative analysis and comprehensive evaluation are then exercised for the capability of land utilization and a series of charts for the analysis of land use capability are worked out to provide the basis for the formulation of controlling measures for urban planning and to ensure the utmost conformity between landuse and geological environment in urban planning.
An academic geophysical research as a regional gravity survey was made during 1994 in the Western White Nile to infer the shallow crustal structures in the area. The result of the survey was compiled as a Bouguer anomaly map with a contour interval of 2 ×10-5 m/s2. It is found that the negative residual anomalies are related to the Upper Cretaceous sediments (Nubian Sandstone Formation) filling all depressions in the Basement complex surface while the positive residual anomalies are attributed to the relatively shallow or outcropping Basement rocks and the steep gravity gradients are resulting from the sharp contacts between the sedimentary infill and the Basement rocks. To define the geological structures in the area, 9 profiles were studied. For each of the profiles, measured and computed Bouguer gravity anomalies, crustal density model, subsurface geology evaluation were performed. A G~model computer program was applied in the gravity modeling, which is based on the line~integral method of gravity computation. A geological/structural map was proposed showing inferred sedimentary basins, faulting troughs and uplifted Basement block and tectonic trends. The basins are believed to be fault~controlled which developed by extensional tectonics (pull~apart mechanism). As for the mechanism and cause of faulting, the area is considered as a part of the Central Sudan rift system which had been subjected to several tectonic events since Early Cambrian to Tertiary times which resulted in the formation of several fracture systems associated with block subsidence, rifting and basin formation.