Zircon U-Pb dating on the Mesozoic volcanic suite from the Qingshan Group stratotype section in eastern Shandong Province and its tectonic significance

A geochronological study of zircon U-Pb on the volcanic rocks from the stratotype section of the Qingshan Group within the Jiaozhou Basin, eastern Shandong Province, is presented. The zircons were analyzed using the method of in situ ablation of a 193 nm excimer laser system coupled with an up to date ICP-MS system. Among the three formations of the Qingshan Group, zircons recovered from the lowest part of the Houkuang Fm. were dated at 106±2 Ma (95% confidence, the same below), whereas those from the lower and upper parts of the Shiqianzhuang Fm. were given ages of 105±4 Ma and 98±1 Ma, respectively. A spatially decreasing trend for the Mesozoic magmatic timing from west to east in the province is observed through comparing the data of this study with those by previous works on the Qingshan volcanic lavas occurring at western Shandong and within the Yishu fault zone. The Qingshan volcanic rocks are constituent of the 'Shoshonite Province' in East China. Exposed at most provinces of central East China along the Tan-Lu fault and the Yangtze fault zones, these volcanic suites are characterized by shoshonite and high-K calcalkalic rocks in lithology and thought to be correlated with the partial melting of continental mantle in genesis. It is also shown that the Qingshan potassic volcanic suite from eastern Shandong basins is distinctly younger than those from other ar-eas of the shoshonite province. By contrary, ages of the Mesozoic to Cenozoic alkaline basalts, sourced by asthenospheric mantle, from both northern Huaiyan basin and northern Dabie belt along the Tan-Lu fault zone and from the Ningwu, Lishui and Luzong basins along the Yangtze fault zone are observably older than those occurring within eastern Shandong. The revealed temporal and spatial patterns in magmatism for the two types of volcanic suites make an important geochronological con-straint on the Mesozoic to Cenozoic dynamic evolution model of the subcontinental lithosphere in East China.     

LA-ICP-MS zircon U-Pb dating from granitoids in southern basement of Songliao basin:Constraints on ages of the basin basement

Seven LA-ICP-MS zircon U-Pb datings from granitoids in the southern basement of the Songliao basin were done in order to constrain the ages of the basin basement. The cathodoluminescence (CL) images of the zircons from seven granitoids indicate that they are euhedral-subhedral ones with striped ab-sorption and obvious oscillatory zoning rims. The dating results show that a weighted mean 206Pb/238U age is 236±3 Ma for quartz diorite (sample No.T6-1) located in the western slope of the basin,that weighted mean 206Pb/238U ages are 319±1 Ma (2126 m) and 361±2 Ma (1994 m) for diorite (sample No.YC1-1) and granite (sample No.YC1-2) located in northern part of southeastern uplift of the basin,respectively,and that weighted mean 206Pb/238U ages are 161±5 Ma,165±2 Ma,165±1 Ma and 161±4 Ma for samples Q2-1,SN121,SN122,and SN72 granitoids located in southern part of southeastern uplift of the basin,respectively. The statistical results of ages suggest that the middle Jurassic granitoids con-stitute the main part of basement granitoids,and that the Hercynian and Indo-Sino magmatisms also occur in the basin basement. It is implied that the Songliao basin should be a rift one formed in the intracontinent or active continental margin settings in the late Mesozoic after the Middle Jurassic orogeny took place.     

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.     

The Pengguan tectonic dome of Longmen Mountains, Sichuan Province: Mesozoic denudation of a Neoproterozoic magmatic arc-basin system

Neoproterozoic igneous and metamorphic complexes occur as tectonic domes in the Longmen Mountains of the western margin of the Yangtze Block, and are important in reconstructing the Rodinian supercontinent and constraining the timing and mechanism of tectonic denudational processes. The Pengguan dome consists of granitic intrusions and metamorphic rocks of the Huangshuihe Group and is tectonically overlain by ductilly deformed Sinian to Paleozoic strata. The plutonic intrusions consist of granites with abundant amphibolite enclaves. New LA-ICP-MS zircon U-Pb dating yielded an emplacement age of 809±3 Ma and a protolith age of 844±6 Ma for the granite. The granitic rocks have geochemical signatures typical of A-type granites, indicating their formation under an extensional environment, by melting of newly formed tonalite-trondhjemite-granodiorite (TTG) rocks. A detachment fault, characterized by variable ductile shear deformation of S-C fabric and ESE-ward kinematics, separates the Pengguan dome from the Sinian-Paleozoic cover. 40Ar/39Ar dating of muscovite from the mylonite in the detachment fault of the dome demonstrates that ductile deformation occurred at ~160 Ma. This study indicates the existence of a Neoproterozoic magmatic arc-basin system, which was denudated by a Jurassic middle crustal ductile channel flow along the Longmenshan thrust belt.     

Late Cretaceous uplift history of the Cretaceous volcanic arc in Southwest Japan: Provenance analysis of the Yuasa–Aridagawa basin based on U–Pb zircon ages

The Ryoke Metamorphic complex has undergone low‐P/T metamorphism and was intruded by granitic magmas around 100 Ma. Subsequently, the belt was uplifted and exposed by the time deposition of the Izumi Group began. The tectonic history of uplift, such as the timing and processes, are poorly known despite being important for understanding the spatiotemporal evolution of the Ryoke Metamorphic Belt. U–Pb zircon ages from sedimentary rocks in the forearc and backarc basins are useful for constraining uplift and magmatism in the provenance. U–Pb dating of detrital zircons from 12 samples (four sandstones and eight granitic clasts) in the Yuasa–Aridagawa basin, a Cretaceous forearc basin in the Chichibu Belt of Southwest Japan, gave mostly ages of 60–110 Ma. Granitic clasts contained in conglomerate suggest that granitic intrusions predate the formation of Coniacian and Maastrichtian conglomerate. Emplacement ages of granitic bodies originated from granitic clasts in Coniacian conglomerate are (110.2 ±1.3) Ma, (106.1 ±1.8) Ma, (101.8+5.8–3.8) Ma, and (95.3 ±1.4) Ma; for granitic clasts in Maastrichtian conglomerate, (89.6 ±1.8) Ma, (87.3+2.4–1.8) Ma, (85.7 ±1.2) Ma, and (82.7 ±1.2) Ma. The results suggest that detrital zircons in the sandstones were mainly derived from volcanic eruptions contemporaneous with depositional age, and plutonic rocks of the Ryoke Metamorphic Belt. Zircon ages of the granitic clast samples also indicate that uplift in the provenance began after Albian and occurred at least during the Coniacian to Maastrichtian. Our results, together with the difference of provenance between backarc and forearc basins suggest that the southern marginal zone of the Ryoke Metamorphic Belt was uplifted and supplied a large amount of clastic materials to the forearc basins during the Late Cretaceous.     

Paleoproterozoic granitic gneisses of the Dinggye and LhagoiKangri areas from the higher and northern Himalaya, Tibet: Geochronology and implications

Granitic gneisses have been widely found in crystalline rocks in the Dinggye area of the Higher Himalaya (HHM) and the LhagoiKangri area of the North Himalaya (NHM), Tibet. In the HHM, the gneisses intruded in the granulite-amphibolite facies metamorphosed sedimentary rocks, known as Nyalam group. In the NHM, the gneisses intruded in the amphibolite facies metamorphosed ones, known as LhagoiKangri group. These granitic gneisses are peraluminous monzonitic granites in terms of their mineral assemblage, and are considered as being derived from metamorphosed sedimentary rocks by anatexis based on the transitional relationship of the gneisses with their migmatitized wall rocks. Zircons are similar in crystal shape and interior structure from both gneisses. Most of them are euhedral or subhedral elongated prism-shaped transparent crystals, with fine oscillatory zoning, showing the magmatic genesis. Some of them are short prism-shaped and with relict core inherited from magma source and oscillatory zoning mantle crystallized from magma. SHRIMP U-Pb dating of zicons shows that both the granitic gneisses in the HHM and NHM are Paleoproterozoic (1811.6±2.9 Ma and 1811.7±7.2 Ma, respectively). These ages are similar to those (1815 to 2120 Ma) from granitic gneiss which is widely distributed in the Lesser Himalaya (LHM). The ages of inherited zircons (>2493.9±7.0 Ma, 2095.8± 8.8 Ma, 1874±29 Ma) exhibit the possible presence of several thermal events in Paleoproterozoic. All of the ages suggest the same India basement beneath the different units in Himalaya area, and do not support the idea that the HHM and NHM are accretionary terranes in Pan-Africa orogenic event. The fact that the basement in HHM is as old as or even younger than LHM is inconsistent with the presently prevalent orogenic models such as either extrusion of low-viscosity mid-crust or orogenic channel.     

Opening time and filling pattern of the Neoproterozoic Kangdian Rift Basin, western Yangtze Continent, South China

The Neoproterozoic Kangdian Rift Basin is a continental rift basin in the western Yangtze Continent. Determining its time of opening and subsequent filling pattern is an important aspect of understanding the formation and evolution of the Yangtze Continent, South China. The Luliang Formation is the early filling in the eastern part of this rift basin, and its deposition age and filling sequence are significant for studies of the regional stratigraphic correlation, opening time, and filling pattern of this basin. Having been correlated to the upper part of the Mesoproterozoic Kunyang Group or to the Neoproterozoic Chengjiang Formation, the age and regional stratigraphic correlation of the Luliang Formation have long been uncertain. This is due to a lack of reliable absolute age constraints. To address this issue, the first zircon SHRIMP U-Pb geochronology has been established for this formation, yielding two high-precision isotopic ages of 818.6±9.2 and 805±14 Ma for the tuff layers at the bottoms of the lower and upper members of the Luliang Formation, respectively. Given the error factor, the bottom age of the lower member of the Luliang Formation can be interpreted as ca. 820 Ma, corresponding to the bottom age of the lower part of the Banxi Group, which is the early filling of the Neoproterozoic Xianggui Rift Basin, a representative basin of the Neoproterozoic rift basins in the Yangtze Continent, South China. The bottom age of the upper member of the Luliang Formation can be interpreted as ca. 800±5 Ma, corresponding to the bottom age of the Chengjiang Formation in the western part of the Neoproterozoic Kangdian Rift Basin and also corresponding to the bottom age of the upper part of the Banxi Group in the Neoproterozoic Xianggui Rift Basin. These ages indicate that the Neoproterozoic Kangdian Rift Basin shares the same opening time and filling sequences as those of the other Neoproterozoic rift basins in South China. Basin analysis shows that the Neoproterozoic Kangdian Rift Basin is a typical half-graben basin, with its main boundary fault in the west and basin center in the east. This basin consisted of mini unidirectional half-graben basins in its juvenile stage and simplified to become a large united half-graben basin in its mature stage, i.e., a supradetachment basin.     

下扬子天目山盆地火山岩锆石LA-ICP-MS定年及地质意义

天目山盆地是下扬子江南隆起带保存较完整的中生代火山盆地,中生代火山岩系岩性自下而上主要为流纹岩-英安岩-安山岩。对盆地内黄尖组下段流纹岩和英安岩分别进行了锆石 LA-ICP MS定年,分别获得了133．6±1．5 Ma（MSWD＝0．73）和135．0±2．1 Ma（MSWD＝0．78）的锆石U-Pb年龄,指示天目山盆地黄尖组火山岩时代为早白垩世。天目山盆地火山活动起始时间和长江中下游地区晚中生代火山活动基本一致,说明江南隆起带和长江中下游地区在早白垩世均处于强烈拉张环境。     

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.     

Geochemistry and chronology of the Jiehekou Group metamorphic basic volcanic rocks in the Lüliang Mountain area,Shanxi, China

Samples were systematically collected from metamorphic basic volcanic rocks in the Jiehekou and Xiyupi areas on both sides of the Lüliang Mountains, Shanxi Province and analyzed for their major elements, trace elements and rare earth elements (REE). The geochemical characteristics of their major, trace and rare-earth elements indicated that the metamorphic basic volcanic rocks in this area were emplaced in the tectonic environment like a modern continental rift. Sm-Nd and Rb-Sr isotope chronological studies demonstrated that the Jiehekou Group metamorphic basic volcanic rocks were formed during the 2600-Ma crust/mantle differentiation event, and were transformed by granulite facies metamorphism during the late Neo-Archaean period (2500 Ma ±), making the Sm-Nd systematics of the rocks reset. During the late Paleoproterozoic period (1800 Ma ±) the Rb-Sr systematics of the rocks were disturbed again in response to the Lüliang movement. Since the extent of disturbance was so weak that the Sm-Nd systematics was not affected, the age of 1600 Ma ± obtained from this area seems to be related to local magmatic activities within the craton. Research results lend no support to the idea that the Lüliang Group was formed during the Archaean. Instead, it should be formed during the Proterozoic.     

Characteristics and dynamic settings of the Central-east Asia multi-energy minerals metallogenetic domain

That more than 82 percent of proved sandstone-type uranium deposits coexist with proved oil-gas or coalfields in the world reflects the fact of coexistence and accumulation of multi-energy minerals including oil, gas, coal and uranium in the same basin. Especially, this phenomenon is most typical in the Central-east Asia energy basins. Across China, Mongolia and some central Asian countries, the giant Central-east Asia metallogenetic domain (CEAMD) stretches more than 6,000 km from Songliao Basin of China in the east to the Caspian Sea in the west. The multi-energy minerals distribution characteristics of the domain include: their spatial distribution is complicated and ordered; the ore-bearing horizon relates closely to the geographical region; the accumulation/mineralization and localization time is the same or close; the occurrence setting and accumulation/mineralization have close correlation; and they have rich provenance for all the minerals. All of these imply that they have close relations between each other under a unified geodynamic background. The exogenetic uranium mineralization process in CEAMD can be divided into five phases using time limits of 100 Ma, (50±2) Ma, 20±(2–4) Ma, 8–5 Ma. The major mineralization periods and their differences in each primary uranium-bearing basin are identical to the oil-gas accumulation and localization periods and phases in the same basin, and are also in response to regional tectonics and controlled in general by the regional geodynamic environment. For industrial application and commercial exploitation, it is suggested that an important period for coexistence, accumulation and localization of oil, gas, coal and uranium and their interaction mainly occur in the late/last and post basin evolution. Through generalized analysis and comparison of accumulation/mineralization environment of the energy basins in CEAMD, the authors propose that the relatively stable regional tectonic background and moderate (weaker) structural deformation probably are necessary for formation, coexistence and preservation of large and medium-scaled sandstone-type uranium ore deposits, oil-gas fields and coalfields, while basins in favor of coexistence and accumulation are those intracratonal, intermediary massif basins and corresponding reformed basins.     

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.     

LA-ICP-MS zircon U-Pb dating and phenocryst EPMA of dikes,Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the al- tered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes. Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr2O3 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GS1 and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously pub- lished data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula. These also imply that the intensive crust-mantle interaction and asthenospheric underplating had oc- curred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).     

Protolith natures and U-Pb sensitive high mass-resolution ion microprobe (SHRIMP) zircon ages of the metabasites in Hainan Island, South China: Implications for geodynamic evolution since the late Precambrian

Abstract Metabasites within the Paleozoic volcanic‐clastic sedimentary sequences in Hainan Island, South China, show large differences not only in the nature of protoliths, but also in zircon U‐Pb sensitive high mass‐resolution ion microprobe (SHRIMP) ages. The protoliths for the Tunchang area metabasites have intraoceanic arc geochemical affinities. In the east‐central island gabbroic to diabasic rocks and pillow lavas are also present, while the Bangxi area metabasites with back‐arc geochemical affinities in the northwest island consist of basaltic, gabbroic and/or picritic rocks. Three types of zircon domains/crystals in the Tunchang area metabasites are defined. Type 1 is comagmatic and yields concordant to approximately concordant 206 Pb/238 U ages ranging from 442.1 ± 13.7 Ma to 514.3 ± 30.2 Ma with a weighted U‐Pb mean age of 445 ± 10 Ma. Type 2 is inherited and yields a weighted 207 Pb/206 Pb mean age of 2488.1 ± 8.3 Ma. Type 3 is magmatic with a 207 Pb/206 Pb age of ca 1450 Ma. Magmatic zircons in the Bangxi area metabasites yield a weighted U‐Pb mean age of 269 ± 4 Ma. We suggest 450 Ma is the minimum age for crystallization of protoliths of the Tunchang area metabasites, because the age range of ca 440–514 Ma probably corresponds to both the time of igneous crystallization and the high‐temperature overprint. The presence of abundant inherited zircons strongly favors derivation of these rocks from a NMORB‐like mantle proximal to continental crust. A protolith age of ca 270 Ma for the Bangxi area metabasites probably records expansion of an epircontinental back‐arc basin and subsequent generation of a small oceanic basin. The presence of ophiolitic rocks with an age of ca 450 Ma, not only in Hainan Island, but also in the Yangtze block, highlights the fact that the South China Caledonian Orogeny was not intracontinental in nature, but characterized by an ocean‐related event.     

Petrological, geochemical and chronological constraints for the tectonic setting of the Dongco ophiolite in Tibet

The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.     

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)     

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.     

Time and tectonic setting of the Xixiang Group: Constraints from zircon U-Pb geochronology and geochemistry

The present paper reports the research development in the timing and tectonic setting of the Xixiang Group, northwestern margin of the Yangtze block. U-Pb concordant ages of (946 ± 18) Ma and (904±18) Ma of zircons were obtained from the upper and lower units of the Xixiang volcanic succession, respectively. Elemental and Nd isotope characteristics show that the volcanic suite has the affinities of arc island magmatism as well as an evolution trend of tectonic setting from front-arc to back-arc. The volcanic rocks suffered from some extensive post-stage tectono-thermal events, resulting in the resetting of their Rb-Sr isotopic system.     

Chronological and geochemical studies of granite and enclave in Baimashan pluton,Hunan, South China

Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low εNd(t) values but high (87Sr/86Sr)i ratios, and old T2DM (ca. 1.9-2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.     

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.