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2009 Vol. 20, No. 4

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Micro-structures of Foliated Eclogites in Dabie-Sulu UHP Belt and Implications for the Earliest Stages of Exhumation of UHP Metamorphic Rocks: An Example from Taohang, Southeastern Shandong, China
Shutian Suo, Zengqiu Zhong, Hanwen Zhou, Zhendong You, Li Zhang
2009, 20(4): 649-658. doi: 10.1007/s12583-009-0057-5

The sub-vertical meso- and micro-structures and fabrics developed in coesite-bearing foliated eclogites in the Taohang (桃行) area, southeastern Shandong (山东), China. The diagnostic structures and fabrics, including penetrative foliation or mylonitic foliation containing mineral and stretching lineations, as well as sheath-like folds, appear to be the development of anastomosing UHP eclogite-facies shear belt arrays hosting massive eclogites. Textural relationships and mineral assemblages indicate that the deformation of foliated eclogites developed closely after the formation of the massive eclogite, prior to the development of the granulite/amphibolite-facies symplectites and coronas, occurring over a very wide pressure range of (31−8)×102 MPa. It presents the structural records of the tectonometamorphic processes as being responsible for the earliest stages of exhumation of the UHP metamorphic rocks. Extensive regional field observations show that the meso- and micro-structures and fabrics recognized in the foliated eclogites at Taohang are remarkably similar or consistent in the whole Dabie (大别)-Sulu (苏鲁) UHP metamorphic belt. This article, thus, supports the idea that earliest stages of exhumation of the UHP metamorphic rocks, from mantle depths to the Moho or the mantle-crust boundary layering, may be attributed mainly to a sub-vertical extrusion and ductile flow along the subduction channel, belonging to a syn-collision exhumation at about 235 to 220 Ma.

U-Pb Zircon Age, Geochemical and Sr-Nd-Hf Isotopic Compositions of Neoproterozoic Granitoids in Northwestern Margin of Yangtze Block (South China): Implications for Neoproterozoic Tectonic Evolution
Rong Liu, Benren Zhang, Hongfei Zhang, Honglin Yuan
2009, 20(4): 659-680. doi: 10.1007/s12583-009-0058-4

The widespread Neoproterozoic magmatism along the Yangtze block carries critical information for understanding the Neoproterozoic evolution of the Yangtze block. In the northwestern margin of the Yangtze block, the Hannan (汉南) intrusive complex includes the Wudumen (五堵门), Erliba (二里坝) and Zushidian (祖师殿) granitoids. Using LA-ICP-MS U-Pb zircon dating method, the Wudumen and Erliba granitoids yielded magma crystallization ages of 785±4 and 778±3 Ma, respectively. Samples from these three granitoids show variable SiO2 contents ranging from 58.8% to 72.6%. They are characterized by enrichment of Al2O3 (14.97%–17.87%), Na2O (3.80%–5.33%) and Sr (504 ppm–741 ppm), and depletion of Y (< 19 ppm) and HREE (e.g., Yb < 1.6 ppm), resulting in high Sr/Y (29–161) and (La/Yb)N (7.3–27.8) ratios. The geochemical features of the granitoids are comparable with those of adakite. The granitoids have zircon ɛHf(t) values of +3.65 to +10.05, whole-rock ɛNd(t) values of −0.09 to +2.98 and whole-rock initial 87Sr/86Sr ratios of 0.703 4–0.703 9, indicating that their magma was derived from a juvenile crustal source. Together with geochemical and Hf-Sr-Nd isotopic compositions, it is suggested that the granitoids formed in island-arc setting and originated from partial melting of a subducted oceanic slab. The results support a model that the Yangtze block was surrounded by ocean and arc magmatism in its northern and northwestern margins in Neoproterozoic.

U-Pb Zircon Ages, Geochemical and Sr-Nd-Hf Isotopic Compositions of Granitoids in Western Songpan-Garze Fold Belt: Petrogenesis and Implication for Tectonic Evolution
Hongming Cai, Hongfei Zhang, Wangchun Xu
2009, 20(4): 681-698. doi: 10.1007/s12583-009-0054-8

Granitoids are widespread in the Songpan-Garze (松潘-甘孜) fold belt, western China. These granitoids provide insight into regional tectono-magmatic events, basement nature and tectonic evolution. However, previous studies mainly focused on the eastern Songpan-Garze fold belt. In this article, five granitoid intrusions from the western Songpan-Garze fold belt have been studied. These intrusions are composed of quartz-diorite and granodiorite. Using LA-ICP-MS zircon dating method, the obtained magma crystallization ages are 219±2 Ma for the quartz-diorite and 216±5 Ma for the granodiorite. The ages, combined with regional geological analyses, show that they formed in a post-collisional tectonic setting. The quartz-diorite and granodiorite display co-linear variation in their chemical compositions. REE compositions for both the quartz-diorite and granodiorite show strongly fractionated patterns with (La/Yb)N=5.02–18.34 and Eu/Eu*=0.44–0.89. The quartz-diorites have initial 87Sr/86Sr ratios (ISr) of 0.709 29–0.711 97 and ɛNd(t) values of −8.6 to −6.1 and the granodiorites have ISr values of 0.705 49–0.709 97 and ɛNd(t) values of −8.3 to −4.3. Zircon Hf isotopic data show ɛHf(t) values of −3.8 to +1.6 for the quartz-diorites and −1.2 to +3.0 for the granodiorites. Geochemical and Sr-Nd-Hf isotopic compositions indicate that the quartz-diorites and granodiorites have similar petrogenesis. We suggest that the magmas for the quartz-diorites and granodiorites were derived from partial melting of lower crustal mafic source, resulting from amphibole dehydration melting reaction. The probing of the magma source reveals that the western Songpan-Garze fold belt contains an unexposed continental basement, which is similar to the eastern Songpan-Garze fold belt. Geodynamically, it is proposed that a lithospheric delamination model can account for the magma generation for the quartz-diorites and granodiorites in the western Songpan-Garze fold belt.

Correlations between Shortening Rate, Uplift Rate, and Inversion Rate in Central Inversion Zone of Xihu Depression, East China Sea Basin
Fengli Yang, Haixiao Yu, Qinglin Zhang, Qianyu Li
2009, 20(4): 699-708. doi: 10.1007/s12583-009-0056-6

Late Miocene shortening rate, uplift rate, and inversion rate in the central inversion zone of the Xihu (西湖) depression, East China Sea basin, were independently determined from a large number of 2D reflection seismic data and methods. Shortening rate was estimated from the 2DMOVE balanced cross-section technology, uplift rate was calculated after absolute erosions were determined from seismic data during the uplifting time, and inversion rate was measured using the improved calculation method by Song (1997). The cross correlations among shortening rate, uplift rate, and inversion rate show a good positive relationship, with some differences existing in local areas. This article analyzes the cross correlation between these structural rates and discusses the dynamics of mechanisms for basin inversion and their influence on hydrocarbon accumulation.

Geological Conditions Favourable for High-Wax Oil Enrichment in Damintun Depression, Bohai Bay Basin
Fangbing Zhu
2009, 20(4): 709-719. doi: 10.1007/s12583-009-0066-4

The Damintun (大民屯) depression, a small (about 800 km2 in area) subunit in the Bohai (渤海) Bay basin, hosts nearly 2×108 t of high-wax oils with wax contents up to 60%. The high-wax oils have high consolidation temperatures and viscosities. The high-wax oils were generated from the fourth member of the Shahejie Formation (Es4), which is also important source rocks for oils in other subunits of the Bohai Bay basin. Yet high-wax oils have not been found in significant volumes elsewhere in the Bohai Bay basin. Geological conditions favourable for high-wax oil enrichment were studied. This study shows that the unusual concentrations of high-wax oils in the depression seem to result from at least three different factors: (1) the presence of organic-matter rich source rocks which were prone to generate wax-rich hydrocarbons; (2) the formation of early overpressures which increased the expulsion efficiency of waxy hydrocarbons; and (3) reductions in subsidence rate and basal heat flows, which minimized the thermal cracking of high molecular-weight (waxy) hydrocarbons, and therefore prevented the high-wax oils from being transformed into less waxy equivalents.

Preferential Petroleum Migration Pathways in Eastern Pearl River Mouth Basin, Offshore South China Sea
Jianning Yu, Huayao Zou, Zaisheng Gong
2009, 20(4): 720-730. doi: 10.1007/s12583-009-0062-8

Secondary petroleum migration in the eastern Pearl River Mouth basin was modeled using the three-dimensional PATHWAYSTM model, which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces. The modeling results have accurately predicted the petroleum occurrences. Most commercial petroleum accumulations are along the predicted preferential petroleum migration pathways (PPMP), and most large fields (petroleum reserves greater than 1×108 t) have more than one preferential petroleum migration pathways to convey petroleum to the traps. The lateral migration distance for oil in the LH11-1 field, the largest oilfield so far discovered in the Pearl River Mouth basin, was more than 80 km. The case study suggests that in lacustrine fault basins, petroleum can migrate over a long distance to form large oilfields without driving force from groundwater flow. The focusing of petroleum originating from a large area of the generative kitchens into restricted channels seems to be essential not only for long-range petroleum migration in hydrostatic conditions, but also for the formation of large oil or gas fields. The strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds with relatively large dipping angles are determined primarily by the morphology of the sealing surfaces at regional scales.

Depositional Characteristics of Lake-Floor Fan of Cretaceous Lower Yaojia Formation in Western Part of Central Depression Region, Songliao Basin
Renchen Xin, Guifan Li, Zhiqiang Feng, Jiangping Liang, Changsong Lin
2009, 20(4): 731-745. doi: 10.1007/s12583-009-0065-5

Based on the integrated subsurface data, including those of over 600 m drilled cores, more than 30 drilled wells and 600 km2 three-dimensional (3D) seismic-reflection data of the study area, the characteristics of the lake-floor fan of lower Yaojia (姚家) Formation have been clarified. An evident lacustrine slope break and a steep slope belt developed in the west of Songliao (松辽) basin during depositional period of Qingshankou (青山口)-Yaojia formations (K2). The slope gradient was about 15 m/km. During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. The wedge-shaped sediment body, which is interpreted as the lowstand system tract of SQy1 (LSTy1), developed in the area below the slope-break. The LSTy1 is pinched out in the west of the study area. As to the thickness of LSTy1, it is thicker in the east with 50 m in its thickness than in the west. The LSTy1, rich in sandstone, can be divided into lower part LSTy1L and upper part LSTy1u based on two onlap seismic reflection phases, and core and logging data clearly. The various sediments' gravity flow deposits developed and the complex of lake-floor fan formed in the LSTy1 under the slope-break in the western part of the central depression region. The lake-floor fan consists of various sediments' gravity flow deposits, including: (1) turbidity deposits with characteristics of Bouma sequences; (2) sand-bearing muddy debrite dominated by mud and mixed by sand; (3) mud-bearing sandy debrites characterized by dominated sand and mixed by mud; (4) sandy debris laminar flow deposits with massive or inclined bedding, and (5) sandy slump deposits developed as deformational sedimentary structure. During the lower lake-level period (LSTy1L), the western clinoform region was erosion or sediment pass-by area; the terrigenous clastic was directly transported to deep-water area, converted to channelized sandy debris flow, and combined with slump derived gravity flow deposited in the lower part of the slope and the deep depression region, which then formed lake-floor fans. During higher lake-level period (LSTy1u), terrigenous clastic deposited in the upper part of the slope, and formed deltaic depositional system; while in the lower part of the slope, the deep depression area was filled by lake-floor fans which were mainly slump deposits. The sandstone bodies of the lake-floor fan are the favorable targets of subtle oil-bearing traps in the western part of the central depression region.

Cenozoic Subsidence Features of Beitang Sag and Relationship with Tectonic Evolution
Tingting Zhang, Hua Wang, Yong Yue, Chuanyan Huang, Liwei Zhang
2009, 20(4): 746-754. doi: 10.1007/s12583-009-0061-9

Based on the application of the EBM basin modeling software and 2-D seismic profiles, the Paleogene and Neogene subsidence histories of the Beitang (北塘) sag are simulated with the backstripping technique, and the relationship between subsidence character and tectonic revolution is discussed. Moreover, the result of the basin modeling reveals that the subsidence history of the Beitang sag has the characteristics of several geological periods, and these succeeding periods have shown certain inheritance and difference characteristics. At the early (Es3) and middle (Es2−Es1) rifting periods, the subsidence reaction of the Beitang sag was mainly in the charge of tectonic activity, while at the late (Ng−Nm+Q) rifting period—post rifting period and post rifting subsidence-acceleration period—the subsidence type is mainly that of thermal subsidence or regional depression effect; from the beginning of the subsidence history to the end, the reason for the basin subsidence has changed from tectonic activity to non-tectonic activity.

Paleozoic Structural Deformation of Bachu Uplift, Tarim Basin of Northwest China: Implications for Plate Drifting
Guangyu He, Zhiliang He, Hongan Zhang, Lu Lin, Qianglu Chen, Yixiong Qian, Yi Gu, Zhixin Zhu, Guoqiang Zhu
2009, 20(4): 755-762. doi: 10.1007/s12583-009-0059-3

Based on high-resolution 2D seismic profiles, the Paleozoic structural deformation characteristics of Bachu (巴楚) uplift of northwestern Tarim basin, NW China, are exhibited in this article. The deformation happened during three main geological periods: the end of Middle-Late Ordovician (O2–3), the end of Early-Middle Devonian (D1–2), and the end of Late Permian (P2). In the Bachu uplift, there developed a series of NW-trending thrust faults and imbricate structures due to the effect of the NW-SE compression stress towards the end of Middle-Late Ordovician (O2–3) (middle Caledonian movement), and there developed some NNE-trending thrust faults and fault blocks under the control of the NEE-SWW compression stress at the end of Early-Middle Devonian (D1–2) (early Hercynian movement). However, at the end of Late Permian (P2) (late Hercynian movement), some NE-trending thrust faults and associated folds developed as a result of the NE-SW compression stress. The first-stage (O2–3) deformation is obviously more violent than those of the latter two stages (D1–2 and P2), which implies that the Tarim plate drifted quickly to the north at around the same time basin.

Distribution and Ecological Risk Evaluation of Organochlorine Pesticides in Sediments from Xinghua Bay, China
Yinghui Wang, Shihua Qi, Xinli Xing, Xiangyi Gong, Junhua Yang, Meihui Xu
2009, 20(4): 763-770. doi: 10.1007/s12583-009-0060-x

The concentrations of organochlorine pesticides (OCPs) in sediments from Xinghua (兴化) Bay, Fujian (福建) Province, China, were determined. The OCP concentrations ranged from 2.48 to 313.88 ng/g, with a more serious contamination in the upper sediment. But the OCP concentration in the upper layer was less than that in the lower sediment at the exit of the bay due to the influence of the Coriolis force. At the joint of the internal and external bays, OCPs accumulated abundantly because of freshwater and seawater mingling. The isomer percentage combined with isomer ratios indicated that DDTs (dichlorodiphengltrichloroethanes) were mainly derived from long-term weather soils, while lindane was introduced recently to the bay. The rivers flowing into the bay contributed more to DDT and HCH (hexachlorocyclohexane) contamination, especially at low tide. Compared with the corresponding quality guidelines from Ingersoll, the sediments from Xinghua Bay were likely to pose biological impairment and should be taken into account.