SHRIMP zircon U-Pb dating of the intrusives in the Tongling metallogenic cluster and its dynamic setting

This paper selected five typical Mesozoic intrusives from the Tongling metallogenic cluster (Xiaotongguanshan, Fenghuangshan, Xinqiao, Dongguashan, and Shatanjiao plutons), and made a systemic SHRIMP zircon U-Pb dating for the five plutons, which produced an age range of 151.8±2.6- 142.8±1.8 Ma. This work put an accurate constraint on the formation age of the intrusives in the Tongling metallogenic cluster. These age data indicate that magmatic activity reached a peak during Late Jurassic. The intrusive sequence of magma is generally from quartz monzonite (porphyry) through monzonite to granodiorite to quartz monzodiorite to pyroxene monzodiorite to gabbro-diabase. The intrusives of different lithology differed in crystallization age, probably implying the intrusives in the Tongling area underwent an evolutional process of magma, which was closely related to geodynamical setting in the depths of the area. A dynamic model was presented for the origin of the igneous rocks in the study area as follows. The assembly between the Yangtze craton and the North China craton fini- shed at the end of T3, and then the stage of another compressional orogeny began in the Tongling area, i.e., Pacific dynamic system. Along with the subduction of the Izanagi plate underneath the Eurasian plate at J2-J3, NW-trending compression toward the East China continent was produced, and compres- sional deformation also took place, forming NE-trending fold and resulting in thickening of the crust in the Tongling area. High-density eclogite-facies rocks were produced in the low part of the crust, re- sulting in the delamination of mantle lithosphere and lower crust, and upwelling of materials in as- thenosphere. Decompression melting produced basaltic magma, and the materials in lower crust were heated by the underplating of the basaltic magma. Thus, melting of lower crust yielded granitic magma, which intruded along deep and large faults through various geological processes (J3-K1). The SHRIMP U-Pb zircon age of 151.8±2.6-142.8±1.8 Ma for intrusives in the Tongling area suggests that the de- lamination of lithosphere mantle and lower crust at least began at middle-late stage of Late Jurassic, resulting in sharp thinning of lithosphere and intense extension of middle-upper crust. Thus, a lot of decollements were produced between cover and cover, basement and cover, and middle and lower crust. This was structural layering or detachment of lithosphere in the Tongling area. Three concordant ages for old inherited cores of magmatic origin (747-823 Ma) indicated that there were obvious mag- matism in the Tongling area during Neoproterozoic, and a little more of the Neoproterozoic igneous source rocks participated in the formation of Mesozoic intrusives.     

Constraining the stepwise migration of the eastern Tibetan Plateau margin by apatite fission track thermochronology

Granites sampled from Garzê-Litang thrust, Longmen Shan thrust, Garzê and Litang strike-slip faults in the eastern Tibetan Plateau have been analyzed with apatite fission track thermochronological method in this study. The measured fission track apparent ages, combined with the simulated annealing mod- eling of the thermal history, have been used to reconstruct the thermal evolutionary histories of the samples and interpret the active history of the thrusts and faults in these areas. Thermal history mod- eling shows that earlier tectonic cooling occurred in the Garzê-Litang thrust in Miocene (~20―16 Ma) whereas the later cooling occurred mainly in the Longmen Shan thrust since ~5 Ma. Our study sug- gests that the margin of eastern Tibetan Plateau was extended by stages: through strike-slip faults deformations and related thrusts, the upper crust formed the Garzê-Litang margin in the Miocene epoch and then moved to the Longmen Shan margin since ~5 Ma. During this process, the deformations of different phases in the eastern Tibetan Plateau were absorbed by the thrusts within them and conse- quently the tectonic events of long-distance slip and extrusion up to hundreds of kilometers have not been found.     

Geochronology of the volcanic rocks in the Lu-Zong basin and its significance

The Lu-Zong (Lujiang-Zongyang) basin is one of the most important volcanic basins in the middle and lower reaches of the Yangtze River area, China. It comprises four shoshonitic volcanic units, which are, in an ascending order, the Longmenyuan, Zhuanqiao, Shuangmiao and Fushan Groups. The LA-ICP MS U-Pb zircon ages of the four units are: 134.8±1.8 Ma for the Longmenyuan Group, 134.1±1.6 Ma for the Zhuanqiao Group, 130.5±0.8 Ma for the Shuangmiao Group, and 127.1±1.2 Ma for the Fushan Group. The results indicate that all volcanic rocks in the Lu-Zong basin were formed in the Early Cretaceous from about 135 Ma to 127 Ma, lasting 8-10 Ma. There were no Jurassic volcanic activities in all the volcanic basins including the Lu-Zong basin in the middle and lower reaches of the Yangtze River area. This work has provided new chronological results for the further study and understanding of the tec- tonic, magmatic and metallogenic processes of eastern China in the Mesozoic.     

Re-Os isotope dating of molybdenites in the Huang-shaping Pb-Zn-W-Mo polymetallic deposit, Hunan Province, South China and its geological significance

The large-scale Huangshaping Pb-Zn-W-Mo polymetallic deposit is located in the central Nanling min- eralization zone, South China. Six molybdenite samples from the Huangshaping deposit were selected for Re-Os isotope measurement in order to define the mineralization age of the deposit. It yields a Re-Os isochron age of 154.8±1.9 Ma (2σ ), which is in accordance with the Re-Os model ages of 150.9― 156.9 Ma. This age is about 7 Ma younger than their host granite porphyry, which was dated as 161.6±1.1 Ma by zircon U-Pb method using LA-ICPMS. All these ages demonstrate that the Huang- shaping granite and related Pb-Zn-W-Mo deposit occurred in the middle Yanshanian period, when many other granitoid and related ore deposits emplaced and formed, e.g. the Qitianling granite and Furong tin deposit, the Qianlishan granite and giant Shizhuyuan W-Sn-Mo-Bi deposit and Jinchuantang Sn-Bi deposit in the nearby area. They constitute the main part of the magmatic-metallogenic belt of southern Hunan, and represent the large-scale metallogeny in middle Yanshanian in the area. The lower rhenium content in molybdenite of Huangshaping deposit suggests that the ore-forming material was mainly of crust origin.     

A paleoisthmus linking southern South America with the Antarctic Peninsula during Late Cretaceous and Early Tertiary

Paleontological and stratigraphical data indicate that a link between terminal southern South America and the Antarctic Peninsula probably was an irregularly narrow continuous land from Late Cretaceous (Campanian) through the Eocene period. In paleogeographical feature it is more like modern Isthmus of Panama. A paleoisthmus called the isthmus of Scotia is hypothesized as a type of connection between them. It lasted about 35 Ma and extended about 700–900 km long and 100–200 krn wide. This isthmus played an important role in biotic dispersal and migra-tion among South America, Antarctica, Australia and New Zealand blocks during this interval. Project supported by the Science and Technology Con~rnission, the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 49572076).     

U-Pb zircon geochronology and Hf isotope study of metamorphosed basic-ultrabasic rocks from metamorphic basement in southwestern Zhejiang: The response of the Cathaysia Block to Indosinian orogenic event

A combined study using LA-ICP-MS U-Pb dating, Hf isotopes, trace elements and the Ti-in-zircon geo-thermometer was carried out on zircons from the metamorphosed basic-ultrabasic rocks in the meta-morphic basement of the Cathaysia Block, southwestern Zhejiang Province. The formation and meta-morphic ages of the rocks from the metamorphic basement of the Cathaysia Block were determined based on zircon U-Pb geochronology. The age for the magmatic crystalline zircons from the protolith is about 1.85 Ga. The εHf(t) values of the older zircons were from ?7 to ?3, with two-stage model Hf ages (TDM2LC) of about 2.9 to 3.4 Ga, indicating that the source material was derived from anatexis and recy-cling of the Archean crust. The newly formed metamorphic zircons yielded U-Pb ages of 260―230 Ma. The metamorphic temperature calculated using the Ti-in-zircon geothermometer ranged from 610 to 720℃, consistent with the results from petrographic observations, indicating that the Cathaysia Block experienced an amphibolite facies metamorphism during the Indosinian. Results from this study pro-vided an important timeframe for the tectonic evolution in South China and the Southeast Asia during the Late Permian and Early Triassic times.     

Deformation features of garnet-bearing granites from Huwan,western Dabie Mountains

Foliated garnet-bearing granite, usually associated with high pressure and ultrahigh -pressure (UHP) metamophic rocks, is a particular rock-type extensively exposed in the Mesozoic Dabie-Sulu orogenic belt of China. This study focuses on deformation features and SHRIMP zircon dating of foliated garnet granite in a high-pressure metamorphic unit from Huwan, western Dabie Mountains in order to resolve discrepancies in current versions of its petrogenesis and structural evolution. SHRIMP dating reveals a zircon age of 762 ± 15 Ma (MSWD=1.7) for Huwan granites, representing the Middle to Late Neoproterozoic age of intrusion and crystallization. Field and microstructural studies show that the Huwan granite body underwent multiple-stage deformation. The deformation was manifested by an early stage of rootless folding and imposition of relict foliation (S₁); an Indosinian main stage marked by imposition of north-dipping penetrative gneissosity (S₂) and development of ductile shear zones under NNE-SSW directed compression; and a final Indosinian stage of southward thrusting of the Huwan high-pressure unit. Shallow level extension prevailed after the Late Triassic, giving rise to south-dipping thrust faults and north-dipping normal faults. Supported by the National Natural Science Foundation of China (Grant Nos. 40802046 and 40334037) and the Project of Science & Technology Research and Development from Sinopec (Grant No. P02009)     

Evolutionary events of Cainozoic Ostracoda of western Philippine Sea and paleoceanographical implications

The Oligocene ostracod fauna of IISDP Site 292 of the western Philippine Sea was marked by high density, diversity and origination, probably reflecting a favorable period for deep-sea Ostraroda; while the sharp decrease in dtmsity and diversity as well as the repeatedly occurring vxtinctions in thc. Neogene may indicate the deterioration in bottom-water environmcnts at that tlme. Five rriajor faunal changes have been recognized in terms of density, diversity and evolutionary artivity (origination and extinction), occurring in the earliest Oligocene (PI8), mid-Oligocene (P21), earliest Miocene (N4), early Middle Miocene (N8) and late Pliocene (N21). respectively. These changes in ostracod faunas are suggested to represent short-term events related to global paleoceanographical changes and to record the Cainozoic cooling history of the West Pacific Ocean and Philippine Sea deep-water. Project supported by the National Natural Science Foundation of China (Grant No. 49676287).     

塔里木盆地东北缘晚新生代钻孔岩心磁性地层年代学及其构造意义

塔里木盆地的高分辨率沉积记录对于理解青藏高原隆升、亚洲内陆干旱化乃至全球气候变化至关重要.建立可靠的地层年代标尺对于研究塔里木盆地晚新生代沉积环境演化、构造运动及古气候变化具有重要意义.本文对塔里木盆地东北缘库尔勒地区的两个全取心钻孔ZK3（深500 m）、ZK5（深300 m）进行详细的磁性地层学研究,结果表明,ZK3孔中更新统底界为54.8 m,下更新统底界为167.0 m,上新统底界为432.0 m,钻孔底部年龄约为6.2 Ma,属上中新统上部;ZK5孔中更新统底界为64.7 m,下更新统底界为241.5 m,钻孔底部年龄约为3.2 Ma,属上上新统.基于上述磁性地层年代标尺,通过沉积速率分析发现ZK3孔在3.0—3.6 Ma之间沉积速率明显增大,反映了塔里木盆地北部天山在此期间的快速隆升.通过东西部多个盆地地质剖面沉积速率的对比分析发现,这期构造活动在区域上具有准同期活动特征,在时代上与晚中新世以来青藏高原快速隆升的时代一致,可能与青藏高原的隆升扩展效应有关.

     

Cyclostratigraphy of the Induan (Early Triassic) in West Pingdingshan Section, Chaohu, Anhui Province

The Induan sequence in the West Pingdingshan Section, Chaohu, Anhui Province, displays a series of superimposed mudstone/limestone cycles. The lithological character of the cycles, combined with power spectral and wavelet analysis of magnetic susceptibility readings, reveals 12 short eccentricity and 56 precession Milankovitch cycles - obliquity cycles are not apparent. The uniformity of cycle thicknesses indicates a stable depositional setting making this section ideal to perform various geo-logical studies. Accordingly, the Induan Stage is estimated to have lasted 1.1 Ma, and the depositional rate for this part of the section is about 3.7 cm/ka. This places the Induan-Olenekian boundary in the West Pingdingshan Section at about 251.5 Ma based on an age of 252.6 Ma for the Permian-Triassic boundary.     

New index of ferromanganese crusts reflecting oceanic environmental oxidation

Ferromanganese crusts (hereinafter crusts) form in aerobic environment and the environmental oxida-tion degree is recorded by the redox sensitive element Co in the crusts. The ages of the layers from the surface to bottom of the crusts are determined, and main element contents at high resolution along the depth sections of three crusts from the Pacific Ocean are analyzed by an electron microprobe. Thus the variations of Co/(Fe Mn) and Co/(Ni Cu) with age/depth of the crust layers are obtained. By comparing the ratios of Co/(Fe Mn) and Co/(Ni Cu) with the δ 18O curves of the Pacific benthic foraminifera, we find that these two ratios can reflect the variation of the environmental oxidation state under which the crust layers deposit. The evolution of the oxidation degree reflected by the two indexes resembles the evo-lution of temperature since the Oligocene reflected by the δ 18O curves of the Pacific benthic foraminif-era. This suggests that the crust-forming environment after the Oligocene is controlled mainly by the oxygen-rich bottom water originated from the Antarctic bottom water (AABW). However it is not the case prior to the Oligocene. Furthermore it suggests that the environmental oxidation degree controls the formation of the crusts and the Co contents in the crusts. This explains why the Co contents in the crusts increase with time up to now.     

Tectono-thermal evolution of middle-south section of the Da Hinggan Mountains

The tectonic settings of the different stages of the magmatic activity in the middle-south section of the Da Hinggan Mts. are analyzed through measuring the isotopic ages of the Mesozoic volcano-plutonic rocks from this area, and thus the tectono-magmatic evolution series are consequently determined as the initial mantle upwelling marked by the Late Triassic invasion of basic-ultrabasic rocks containing mantle-source enclaves, middle-upper crust extension marked by intrusion of the Early-Middle Jurassic diobase dike swarms, dramatic ruption of the Late Jurassic trachitic volcanic rocks, the Early Cretaceous nonorogenic alkalic-subalkalic granite invasion and the formation of the basic dike swarms and basalts. It is thus inferred that the uprise of the Da Hinggan Mts. in the Mesozoic is closely reiated to the upwelling of the deep magma in the mantle upwarping settings. Project supported by the National Natural Science Foundation of China (Grant No. 249472143).     

Preliminary analysis on the relationships between Tibetan Plateau NDVI change and its surface heat source and precipitation of China

The tectonic-sedimentary cycles and environmental evolution in Southwest Monsoon area since 2.78 Ma were reconstructed, based on the analyses of lithology, grain size, carbonate content, loss-on-ignition (LOI) and pollen in a 737.72 m sediment core from the Heqing Basin, China. The results indicated that the lake basin was formed in about 2.78 Ma. Continuous lacustrine sediment was preserved since 2.65 Ma. Three stages of tectonic-sedimentary cycle were revealed, which were 2.78–1.55, 1.55–0.99 and 0.99–0 Ma. The environmental evolution in Heqing area was influenced by uplift of the Qinghai-Tibetan Plateau, the resulting variation of Southwest Monsoon and the earth orbital-scale periodicity. Supported by the Key Projects of the National Natural Science Foundation of China (Grant Nos. 40331003 and 40625007)     

Constraints on the age of the Tuchengzi Formation by LA-ICP-MS dating in northern Hebei-western Liaoning,China

Accurately determining the age of the Tuchengzi Formation has direct influence on confirming the boundary between the Jurassic and the Cretaceous systems in northern Hebei-western Liaoning, and on related geological problems in China. However, the Tuchengzi Formation mainly consists of sedimentary rocks, with a poor fossil record and especially lack of index fossils. The Tuchengzi Formation is also lack of the type of volcanic rocks that can provide an isotopic age. Therefore, the age of the Tuchengzi Formation has been uncertain. Based on our systematic dating of the tuff interbedded in the Tuchengzi Formation of Chengde and Jinlingsi-Yangshan basins in northern Hebei-western Liaoning, combined with the dating results of previous researchers, here we suggest that the age range of the Tuchengzi Formation in northern Hebei-western Liaoning is from 147 Ma to 136 Ma. It implied that the Tuchengzi Formation was mainly formed in the Early Cretaceous. Supported by National Natural Science Foundation of China (Grant No. 90714010)     

N2-Ar-He systematics and source of ore-forming fluid in Changkeng Au-Ag deposit, central Guangdong, China*

Changkeng Au-Ag deposit is a newly-discovered new type precious metal deposit. N₂-Ar-He systematics studies and³He/⁴He and δD- δ¹⁸O composition analyses show that the ore-forming fluid of the deposit is composed mainly of formation water (sedimentary brine) but not of meteoric water, which was thought to be source of the ore-forming fluid by most previous researchers. The content of mantle-derived magmatic water in the ore-forming fluid is quite low, usually lower than 10%. According to the source of the ore-forming fluid, the Changkeng Au-Ag deposit should belong to sedimentary brine transformed deposits. From the Late Jurassic to the Early Cretaceous Period, with deposition and accumulation of thick sediments in Sanzhou Basin, the formation water in the sedimentary layers was expelled from the basin because of overburden pressure and increasing temperature. The expelled fluid moved laterally along sedimentary layers to the margin of the basin, and finally moved upward along a gently-dipping interlayer fault. Because of a decline in pressure and temperature, ore minerals were deposited in the fault. Project supported by the National Natural Science Foundation of China (Grant No. 495020291, the Natural Science Foundation of Zhongshan University, the Research Foundation of National Key Laboratory of Metallogenesis in Nanjing University (Grant No. 039704) and the Lingnan Foundation.     

Chronology and geochemistry of the volcanic rocks in Woruo Mountain region,northern Qiangtang depression: Implications to the Late Triassic volcanic-sedimentary events

A suite of sedimentary-volcaniclastic rocks intercalated with the volcanic rocks unconformably overlies the Triassic Xiaochaka Formation in the Woruo Mountain region, Qiangtang Basin, northern Tibet. The vitric tuff from the base of these strata gives a SHRIMP zircon U-Pb age of 216 ± 4.5 Ma, which represents the age of the Late Triassic volcanic-sedimentary events in the Woruo Mountain region, and is consistent with that of the formation of the volcanic rocks from the Nadi Kangri Formation in the Nadigangri-Shishui River zone. There is a striking similarity in geochemical signatures of the volcanic rocks from the Woruo Mountain region and its adjacent Nadigangri-Shishui River zone, indicating that all the volcanic rocks from the Qiangtang region might have the same magmatic source and similar tectonic setting during the Late Triassic. The proper recognition of the Late Triassic large-scale volcanic eruption and volcanic-sedimentary events has important implications for the interpretation of the Late Triassic biotic extinction, climatic changes and regressive events in the eastern Tethyan domain, as well as the understanding of the initiation and nature, and sedimentary features of the Qiangtang Basin during the Late Triassic-Jurassic.     

New SHRIMP U-Pb age from the Wuqiangxi Formation of Banxi Group: Implications for rifting and stratigraphic erosion associated with the early Cryogenian (Sturtian) glaciation in South China

In South China, the Wuqiangxi Formation of the Banxi Group and its equivalents underlie the early Cryogenian (Sturtian) glacial deposits but their thickness varies from <200 m to >2000 m. In the Guzhang section of western Hunan, the Wuqiangxi Formation is only 152 m thick, and an ash bed 58 m below the glacial diamictite yielded a SHRIMP U-Pb age of 809.3±8.4 Ma. In contrast, 90 km south of the Guzhang section towards the basin in Zhijiang area where the Wuqiangxi Formation is ~2200 m thick, an age of 725±10 Ma has been reported from the top of this unit, 300 m below the glacial diamictite. These ages provide new evidence for the regional stratigraphic correlation across the Nanhua basin, and suggest unusually large (>2 km) stratigraphic erosion potentially associated with the Sturtian glaciation in South China. The magnitude of erosion may imply significant uplifting and tectonotopography at the onset of the Sturtian glaciation.     

Faulting on the Anninghe fault zone, Southwest China in Late Quaternary and its movement model

The Anninghe fault is one of the significant earthquake-generating fault zones in the Southwest China. Local his-torical record shows that a M≥7 strong earthquake occurred in the year of 1536. On the basis of the detailed air-photographic interpretation and field investigation, we have acquired the following knowledge: 1 The average sinistral strike-slip rate since the Late Pleistocene is about 3~7 mm/a; 2 There is important reverse faulting along the fault zone besides the main left-lateral strike-slip motion, and the shortening rate across the Anninghe fault zone due to the reverse faulting is about 1.7~4.0 mm/a. If the Xianshuihe fault zone is simply partitioned into the Anninghe and Daliangshan faults, we can also get a slip rate of 3~7 mm/a along the Daliangshan fault zone, which is the same as that on the Anninghe fault zone. Moreover, on the basis of our field investigation and the latest knowledge concerning the active tectonics of Tibetan crust, we create a dynamic model for the Anninghe fault zone.     

Metamorphic characteristics and geotectonic implications of the high-pressure granulites from Namjagbarwa, eastern Tibet

A large area of high-pressure garnet-kyanite granulite is exhumed in the Namjagbarwa area, which provides a window for observing the deep crust rocks and structures of the Tibetan Plateau. Three mineral assemblages can have been distinguished in the garnet-kyanite HP granulites by petrography, i.e. M₁. Mus+Bi+P1+Q, M₂. Gt+Ky +perphite/antiperphite+Rt+Q, M₃. Gt+Sill+Cord+Sp+Ilm ± Opx. Metamorphic conditions of the peak granulite assemblages (M₂) formatted by thickening of crusts, with available isotopic ages of 45–69 Ma, are at 1.4—1.8 Gpa and 750—850°. Their retrograde assemblages overprinted by decompressure during the uplift, with available isotopic ages of 18—23 Ma, were formed at 0.60—0.70 Gpa, 621—726°. The thermobarometric evaluation, petrogenetic grid and corresponding isotopic ages indicate a clockwise isothermal decompression metamorphic path. The HP granulite metamorphic history indicates that the collision of the Indian Plate with the Eurasian Plate had begun at 70 Ma, far earlier than the widely accepted 45 Ma. Project supported by the National Natural Science Foundation of China (Grant No. 49732100), the National Key Project for Basic Research, and the Chinese Academy of Sciences Project for Tibetan Research Project (GrantNos. KZ951-A1-204, KZ95T-06).     

Carbon isotopic fractionation in the process of Fischer-Tropsch reaction in primitive solar nebula

Carbon isotopic fractionation of low-weight molecules of the products of Fischer-Tropsch reaction under the conditions of Earth's accumulation region in primitive solar nebula has been experimentally studied. Among the reaction products, carbon dioxide has the heaviest isotopic composition; carbon isotopic composition of methane has a large variation; different from biogenic natural gases, carbon isotopic compositions among ethane, propane and butane yield a reverse distribution pattern. It suggests that primordial hydrocarbons that were captured in the interior of the Earth during its accretion possess a reverse carbon isotopic distribution pattern. Project supported by the National Natural Science Foundation of China (Grant No. 49233060).