2012 Vol. 23, No. 5
We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo area from the Kongling Group in the northern Huangling anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim textures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660-808 degrees C (714 degrees C in average), all...
SHRIMP zircon U-Pb dating of the Neoproterozoic Maoping series (Sandouping (rock suite) granites exposed in the southern part of the Huangling anticline shows that the formation time of Sandouping biotite-hornblende tonalite intrusion, Jinpansi hornblende-biotite tonalite intrusion, and Longtanping monzogranite are 863 +/- 9, 842 +/- 10, and 844 +/- 10 Ma, respectively. Their geochemical features include A/CNK=0.98-1.06, from metaluminous to weakly peraluminous, delta=1.37-1.53, Sm/Nd=0.17-0.24, and Rb-N/Yb-N=1.1-3.62. These ..
The Miaowan ophiolite is a highly dismembered ophiolitic complex cropping out near the northern margin of the Yangtze craton. The rocks of this complex consist of, from bottom to top, harzburgite tectonite locally containing podiform chromite, dunite, layered and isotropic gabbro, a sheeted dike complex (SDC), meta-pillow lavas with chert pods and layers...
In this paper the authors briefly introduce and review the new progress of the newly discovered Proterozoic Miaowan (庙湾) ophiolite and Neoproterozoic Huangling (黄陵) granitoids in the southern Huangling anticline in the Yangtze craton, and the tectonic evolution significance of assemblage and breakup of Rodinia and Columbia supercontinent in South China.
We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo (赵家阳坡) area from the Kongling (崆岭) Group in the northern Huangling (黄陵) anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim textures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660–808 ℃ (714 ℃ in average), all indicating that the BIFs underwent granulite facies metamorphism. The age of zircons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indicating that there was a significant granulite facies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the assembly of the Columbia supercontinent with South China. Moreover, the REE pattern is characterized by depletion in LREE while relatively flat in HREE, LaN/YbN=0.26, with a positive Eu anomaly (Eu/Eu*=1.59), which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.
SHRIMP zircon U-Pb dating of the Neoproterozoic Maoping (茅坪) series (Sandouping (三斗坪) rock suite) granites exposed in the southern part of the Huangling (黄陵) anticline shows that the formation time of Sandouping biotite-hornblende tonalite intrusion, Jinpansi (金盘寺) hornblende-biotite tonalite intrusion, and Longtanping (龙潭坪) monzogranite are 863±9, 842±10, and 844±10 Ma, respectively. Their geochemical features include A/CNK=0.98–1.06, from metaluminous to weakly peraluminous, δ=1.37–1.53, Sm/Nd=0.17–0.24, and RbN/YbN=1.1–3.62. These indicate that the granite rocks are supersaturated SiO2 calc-alkaline granitoids. The characteristic of Sr-Nd isotopic composition is that the values of ɛNd(t) and εSr(t) are −12.4 to −11.0 and 20.2–32.2, respectively. It also suggests that the material source of the granite rocks mainly originated from the crust, and they formed in a volcanic arc tectonic environment. These facts suggest that the occurrence of Neoproterozoic granitoids in the southern part of the Huangling anticline should be related to an arc environment along an active continental margin caused by southward subduction of oceanic crust beneath the northern Yangtze craton, and the formation age is not later than 863 Ma.
The Miaowan (庙湾) ophiolite is a highly dismembered ophiolitic complex cropping out near the northern margin of the Yangtze craton. The rocks of this complex consist of, from bottom to top, harzburgite tectonite locally containing podiform chromite, dunite, layered and isotropic gabbro, a sheeted dike complex (SDC), meta-pillow lavas with chert pods and layers, and tectonically intercalated marble. The SDC is a very important and significant part of the Miaowan ophiolitic sequence, and grades downward into gabbro and ultramafic rocks, and upward into meta-pillow lavas. Some dikes preserve one-way chilled margins, typical of extensional ophiolitic settings, whereas most preserve double chilled margins, in cases where the chilling direction can be determined. The SDC is mainly composed of meta-diabase (dolerite), meta-plagiogranite, and small amounts of meta-gabbro and ultramafic rocks. LA-ICP-MS zircon dating yields an upper intercept age of 1 026±79 Ma for one meta-plagiogranite, 1 043±23 Ma for a second meta-plagiogranite and 1 096±32 Ma for one meta-gabbro at the bottom of the SDC, suggesting formation of the SDC at circa 1 026–1 096 Ma, consistent with the recently determined formation age of the Miaowan ophiolite. Sparse geochemical data on the meta-diabase indicate that the protolith was a sub-alkaline, low-potassium tholeiite similar to mid-ocean ridge basalt (MORB). The chondrite-normalized rare earth element (REE) patterns of the meta-diabase are generally flat ((La/Yb)N=0.56–0.94), with a slight depletion in LREE, but no obvious Eu anomalies. Given that the meta-plagiogranites show evidence of formation in a suprasubduction zone environment, we suggest that the basalts were originally island arc tholeiites, perhaps formed in an extensional forearc setting. The geochemistry of the meta-diabase and plagiogranite from the sheeted dikes, together with regional relationships, all agree with the previous interpretations that the Miaowan ophiolite formed in a suprasubduction zone setting.
The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.
The stratigraphic, structural and metamorphic features of the basal thrust belt of the ca. 1.0 Ga Miaowan (庙湾) ophiolite in the southern Huangling (黄陵) anticline, show that it can be divided into three tectono-lithostratigraphic units from north to south: mélange/wildflysch rock units, flysch rock units, and sedimentary rock units of the autochthonous (in situ) stable continental margin. The three units underwent thrust-related deformation during emplacement of the Miaowan ophiolitic nappe, with kinematic indicators indicating movement from the NNE to SSW, with the metamorphic grade reaching greenschist-amphibolite facies. LA-ICP-MS U-Pb geochronology of zircons from granite pebbles in the basal thrust-related wildflysch yield ages of 859±26, 861±12 and 871±16 Ma; whereas monzonitic granite clasts yield an age of 813±14 Ma. This indicates that the formation age of the basal thrust belt is not older than 813±14 Ma, and is earlier than the earliest formation time of the majority of the Neoproterozoic Huangling granitoid intrusive complex, which did not experience penetrative ductile deformation. These results suggest that the northern margin of the Yangtze craton was involved in collisional tectonics that continued past 813 Ma. This may be related to the amalgamation of the Yangtze craton with the Rodinia supercontinent. Through comparative study of lithology, zircon geochronology, REE patterns between granodiorite and tonalite pebbles in the basal thrust-zone conglomerate, it can be concluded that the pebbles are the most similar to the Huanglingmiao (黄陵庙) rock-mass (unit), implying that they may have come from Huanglingmiao rock-mass. Zircon cores yield xenocrystic ages of 2 074±120 Ma, suggesting that the protolith of the Neoproterozoic Huangling granitoid intrusive complex may have originated from partial melting of older basement rocks, that is to say there may be Paleoproterozoic crystalline basement in the southern Huangling anticline. The ages of xenocrystic zircons in the granite pebbles in the basal-thrust conglomerate/wildflysch show a correlation with the age spectra from Australia, implying that the terrain that collided with the northern margin of the Yangtze craton and emplaced the Miaowan ophiolite at ca. 813 Ma may have been derived from the Australian segment of Rodinia.
Planation surfaces (PS) play a major role in reconstruction of the evolutionary history of landforms in local areas. Thus, objective and consistent mapping of planation surfaces from remotely sensed data (e.g., satellite imagery, digital elevation models (DEMs)) is paramount for interpreting the geomorphological evolution. Due to the lack of dated sedimentary covers and the difficulties of comprehensive field work, the number and age of planation surfaces in the Southwest Hubei (湖北) Province of China are still controversial. In order to map the PS in the study area, four data visualization techniques including ETM+ false color composite, grey-scale DEM, shaded relief model (SRM) and painted relief model (PRM) were examined. It is found that the PRM is the most optimal technique for planation surface mapping. The study area was successfully mapped by visual interpretation of a PRM derived from ASTER GDEM. The mapped PS was divided into five classes in terms of elevation according to previous studies, varying from 1 700–2 000 (PS1), 1 300–1 500 (PS2), 1 000–1 200 (PS3), 800–900 (PS4) to 500–600 (PS5) m. The results were partially compared with the published works. It is revealed that this method of mapping enjoys a higher accuracy and can reduce the time and effort required in the traditional mapping to a large extent. The results also demonstrated that the PRM is an effective tool for geomorphological feature mapping with considerable accuracy. The preliminary results can serve to facilitate locating representative samples for the planation surfaces dating, thus to determine the ages of PS in the study areas.
Based on multi-beam bathymetric data and 2D high-resolution, multi-channel seismic profiles, combing ODP1148 drilling data, the morphology, internal sedimentary architecture, and evolution pattern of 17 deepwater canyons from the Middle Miocene to present are documented in the northern Baiyun (白云) sag (BS), Pearl River Mouth basin (PRMB), and northern South China Sea (SCS). There exist six seismic architectural elements in these canyons, including basal erosive surfaces (BES), thalweg deposits (TD), lateral migration packages (LMP), mass transport deposits (MTD), canyon margin deposits (CMD), and drape deposits (DD). According to the stratigraphical ages and geometrical features of these canyons, their formation and evolution processes are divided into three stages: (1) Middle Miocene scouring-filling, (2) Late Miocene lateral migration, and (3) Pliocene-Quaternary vertical overlay. An auto-cyclic progressive process of eroding and filling by turbidity currents results in the scouring-filling and vertical overlay; bottom currents are responsible for the remarkable asym- metry between the two flanks of canyons; and faults are inherent dynamic forces triggering these can yons. It is inferred that these canyons are caused by the double effects of turbidity and bottom currents under the control of faults as inherent dynamic forces.
Since the concept of petroleum migration-accumulation dynamic system (PMADS) was introduced in the 1990s, its definition, division and even denomination have long been in disagreement. This article aims to further discuss and try to clarify this concept and to explore more reasonable approaches to the division of PMADSs on the basis of our studies on petroleum migration and accumulation in some petroliferous basins of China, especially those from western China. According to the studies, we define PMADS as a relatively independent natural hydrocarbon migrating and accumulating system within a petroleum system. Its extent is supposed to cover the whole migration pathway starting from a hydrocarbon provenance throughout to an accumulation or a group of accumulations of same source. Just like petroleum systems, a petroleum migration-accumulation system is also an important natural unit for petroleum exploration and assessment in petroleum basins. Its position in a basin is right between a petroleum system and a play. A petroleum system has either a number of migration-accumulation systems or one system only. However, the key element of a petroleum system is the source of hydrocarbon; a petroleum system has a distinct hydrocarbon kitchen and is different from its neighboring ones in terms of hydrocarbon kitchens. In contrast, different migration-accumulation systems within the same petroleum system have a shared source kitchen but different characteristics of migration and accumulation to form hydrocarbon accumulations. As far as the division of petroleum migration-accumulation systems (PMASs) is concerned, 8 major approaches are proposed, including those based on pool-forming dynamic mechanisms, location of hydrocarbon accumulation relative to the source kitchen, phase states of migration and accumulation, migration patterns and directions, geneses of accumulations, reservoir pressure systems, depositional systems and facies, and integrated method, of which the integrated approach is underlined with regard to the division of PMASs in a complex petroleum system like those in West-China superimposed basins.
We present a case study of applying MT (magnetotellurics) and CSAMT (controlled source audio-frequency magnetotellurics) for geophysical exploration in Jiangxia (江夏), which is located in new industrial developing suburb, where artificial noises are severe. In order to know deep buried structure, fracture status, and characteristics of underground geothermal development about 2 km, we acquired MT and CSAMT data to image subsurface structure through inversion and joint interpretation. The electrical terms of the 2D MT inversion can be divided into three ranges of resistivity values: (1) a highly resistive (> 350 Ω·m) layer mainly characteristic of limestone, dolomitic limestone, leuttrite, silicarenite, and packsand; (2) an intermediate resistivity (250–350 Ω·m) layer mainly constituted by siliceous shale, siltstone, battie, and ampelitic limestone; and (3) a low resistivity (20–250 Ω·m) layer, from surface to −100 m, which is related to lacustrine alluvium of Quaternary period; the deep low resistivity layer is interpreted to be representative of the geothermal field. The result of the 2D CSAMT inversion reveals two layers of different electrical resistivities: (1) the first resistive layer (20–250 Ω·m), which is related to lacustrine alluvium of Quaternary period and the heat source, and (2) the second resistive layer (250–3 000 Ω·m). The heat source appears to be bounded within the middle of exploration area and shows the N-S trend. Its depth ranges from more than 1.2 to less than 0.7 km, and its resistivity values range from 20 to 250 Ω·m in the northeast part of Jiangxia. Comparing the results of MT and CSAMT method, the positive anomalies are similar and can be assumed to be generated by the same source.
Energy cone is a unique but characteristic slope that describes the extent of deposits left around a volcano by various flowage phenomena, usually regarded as the boundary of pyroclastic and the source region of lahar. The energy cone value is determined as 0.07 by the energy-line model combined with the parameters of plume height and gradient, and the energy cone spread extent is defined by the numerical simulation method LAHARZ to simulate with this value based on the 1: 50 000 digital elevation model of Tianchi (天池) Volcano, and the source region profiles in the north and south slope can prove the correctness of this threshold. This energy cone threshold and extent can be used as the reference of pyroclastic flow and lahar simulation.