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    SHRIMP U-Pb zircon geochronology of high-grade rocks and charnockites from the eastern Amery Ice Shelf and southwestern Prydz Bay, East Antarctica Constraints on Late Mesoproterozoic to Cambrian tectonot [查看] XiaochunLiu、YueZhao、BiaoSong、JianLiu、JianjunCui
    The eastern Amery Ice Shelf (EAIS) and southwestern Prydz Bay are situated near the junction between the Late Neoproterozoic/Cambrian high-grade complex of the Prydz Belt and the Early Neoproterozoic Rayner Complex. The area contains an important geological section for understanding the tectonic evolution of East Antarctica. SHRIMP U–Pb analyses on zircons of felsic orthogneisses and mafic granulites from the area indicate that their protoliths were emplaced during four episodes of ca. 1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma and ca. 1060–1020 Ma. Subsequently, these rocks experienced two episodes of high-grade metamorphism at N970 Ma and ca. 930–900 Ma, and furthermore, most of them (except for some from the Munro Kerr Mountains and Reinbolt Hills) were subjected to high-grade metamorphic recrystallization at ca.535 Ma. Two suites of charnockite, i.e. the Reinbolt and Jennings charnockites, intrude the Late Mesoproterozoic/Early Neoproterozoic and Late Neoproterozoic/Cambrian high-grade complexes at N955 Ma and 500 Ma, respectively. These, together with associated granites of similar ages, reflect late- to post-orogenic magmatism occurring during the two major orogenic events. The similarity in age patterns suggests that the EAIS–Prydz Bay region may have suffered from the same high-grade tectonothermal evolution with the Rayner Complex and the Eastern Ghats of India. Three segments might constitute a previously unified Late Mesoproterozoic/Early Neoproterozoic orogen that resulted from the long-term magmatic accretion from ca. 1380 to 1020 Ma and eventual collision before ca. 900 Ma between India and the western portion of East Antarctica. The Prydz Belt may have developed on the eastern margin of the Indo-Antarctica continental block, and the Late Neoproterozoic/Cambrian suture assembling Indo-Antarctica and Australo-Antarctica continental blocks should be located southeastwards of the EAIS–Prydz Bay region.
    Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China Evidence from zircon U–Pb age, trace element and mineral inclusion [查看] Yi-CanLiu、Xiao-FengGu、Shu-GuangLi、Zhen-HuiHou、BiaoSong
    The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complex zone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South China Block and display a multiple metamorphic evolution. However, the exact timing of the UHP metamorphism in the NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. In order to constrain the ages of peak UHP metamorphism and subsequent retrogression during continental subduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combined petrological, trace element and U–Pb isotopic study. In combination with petrological data, the present zircon SHRIMP and LA-ICPMS U–Pb dating provides precise constraints on the timing of multistage metamorphic events on the eclogites in the region. The U–Pb isotope in zircon from the eclogites records the times of multiple discrete events in the history of the rock such as Neoproterozoic, 238±2 Ma, 222±4–227±2 Ma,210±4–215±2 Ma, 199±2 Ma and 176±2–188±2 Ma. Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. By U–Pb age, trace element and mineral inclusion of zircon, at least two episodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226±3 and 214±3 Ma, respectively. The younger ages of 199±2 and 176–188 Ma most likely record the granulitefacies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of 238±2 Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus,Neoproterozoic mafic lower continental crust was subducted to depths greater than 120 km (corresponding to the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226±3 Ma. Then these UHP metamorphic rocks were exhumed to about 60 km depth at 214±3 Ma and experienced highpressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustal levels and overprinted by granulite- and amphibolite-facies metamorphism, respectively.
    Age, geochemistry, and tectonic implications of a late Paleozoic stitching pluton in the North Tian Shan suture zone, western China [查看] Bao-FuHan、Zhao-JieGuo、Zhi-ChengZhang、LeiZhang、Jia-FuChen、BiaoSong
    The Central Asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on Earth due to closure of the paleo–Asian Ocean in the Phanerozoic. Among major suture zones in the North Xinjiang region of western China, the North Tian Shan suture zone, because of collision between the Yili terrane in the south and the Junggar terrane in the north, contains the youngest ophiolitic rocks and may represent the terminal stage of development of the Central Asian orogenic belt in western China, but the timing of the suture zone remains poorly constrained. A sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age of 316 ± 3 Ma (i.e., the beginning of the late Carboniferous) from the undeformed Sikeshu pluton, which crosscuts the suture zone, places a crucial upperage bound for the time of collision between the Yili and Junggar terranes. This event occurred later than, or nearly concurrent with, other accretion-collision events in the North Xinjiang region, implying that final terrane amalgamation was completed in the late Carboniferous. The Sikeshu pluton shares geochemical characteristics of the widespread late Carboniferous to Permian postcollisional A-type and I-type granitoids with depleted-mantle–like Sr-Nd isotopic signatures in the North Xinjiang region. They all occurred during a protracted (ca. 320–270 Ma) episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the Central Asian orogenic belt. The same postcollisional magmatism also generated Cu-Ni-sulfi de–bearing, mafi c-ultramafi c magmatic complexes complexes,adakites, and porphyry-type coppermolybdenum–bearing magmatic rocks in the North Xinjiang region.
    SHRIMP zircon U-Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China [查看] BeiXua、ShuhaiXiao、HaiboZou、YanChen、Zheng-XiangLi、BiaoSong、DunyiLiu、ChuanmingZhoug、XunlaiYuang
    The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U–Pb method.Volcanic bed V1 near the base of the Bayisi diamictite yields a 740±7Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725±10Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615±6Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725±10Ma and 615±6 Ma, and the Hankalchough diamictite between 615±6Ma and ∼542Ma (i.e., the Neoproterozoic–Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635Ma Nantuo and 582Ma Gaskiers glaciations, respectively.However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ∼750Ma and ∼650 Ma.
    Zircon SHRIMP U-Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet [查看] Da-RenWen、DunyiLiu、Sun-LinChung、Mei-FeiChu、JianqingJi、QiZhang、BiaoSong、Tung-YiLee、Meng-WangYeh、Ching-HuaLo
    The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma),respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma.We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.
    Zircon SHRIMP U-Pb and in-situ Lu–Hf isotope analyses of a tuff from Western Beijing Evidence for missing Late Paleozoic arc volcano eruptions at the northern margin of the North China block [查看] Shuan-HongZhang、YueZhao、BiaoSong、Yue-HengYang
    Zircon SHRIMP U–Pb and in-situ Lu–Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U–Pb zircon dating of the tuff yielded a 206Pb/238U weighted mean age of 296±4 Ma (95% confidence, MSWD=3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline,I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu–Hf analysis results of most zircons from the tuff yielded initial 176Hf/177Hf ratios from 0.282142 to 0.282284 and εHf(t) values from −15.9 to −10.7. These Lu–Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some arc volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic.
    The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton Magmatism related to a Paleoproterozoic orogen [查看] Shuan-HongZhang、Shu-WenLiu、YueZhaoa、Jin-HuiYang、BiaoSong、Xiao-MingLiu
    The AMGRS is a suite of anorthosite-mangerite-alkali granitoid-rapakivi granite in the northern North China Craton (NCC).New zircon SHRIMP and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating of the Damiao anorthosite, Changsaoying K-feldspar granite, Lanying anorthosite and quartz syenite and the Gubeikou K-feldspar granite yielded emplacement ages of 1726±9 Ma, 1753±23 Ma, 1739±43 Ma, 1712±15 Ma and 1692±19 Ma, respectively. Combined with previous geochronological data from the Damiao norite and mangerite and Shachang rapakivi granite, it indicates that the whole AMGRS was intruded between 1750 Ma and 1680 Ma. The Damiao anorthosites and norites exhibit a high Mg#, and this decreases from mafic (up to 63) to felsic (<10) end members. The alkali granitoids of the AMGRS are characterized by high contents of alkali (K2O+Na2O), HSFE, TFeO/MgO, molecular (K2O+Na2O)/Al2O3 and Ga/Al values, that are typical features of A-type granites. All of the diverse components of the AMGRS have similar Sr–Nd–Hf isotopic compositions with low whole-rock initial 87Sr/86Sr ratios of 0.703–0.704, εNd(T) values of −5.0 to −6.3 (with TDM model ages of 2.34 to 2.58 Ga), and zircon εHf(T) values ranging from −4.1 to −7.5 (with Hf isotopic model TDM and TCDM ages ranging from 2.32 Ga to 2.43 Ga and 2.70 Ga to 2.88 Ga,respectively).This suggests that these rocks are the fractional crystallization products of one parental magma. Zircon Hf and wholerock Sr–Nd–Pb isotopic systematics combined with high La/Yb (>10) and intermediate Th/Ta ratios, suggest that the parental magma of the AMGRS could have been derived from EM-I lithosphere mantle enriched by recycled Archean continent crustal materials, accompanied by some assimilation of lower crustal components. The mafic to felsic compositions of the AMGRS are different from typical bimodal rocks formed in an anorogenic intra-continent rift setting, but resemble those emplaced during postcollisional/post-orogenic extensional tectonic setting. This is compatible with their age of <1.85 Ga, indicating they postdate the continent–continent collision between the Western and Eastern blocks of the NCC at ∼1.85 Ga.
    SHRIMP U-Pb zircon geochronology and geochemistr y of metavolcanic and metasedimentary rocks in Northwestern Fujian, Cathaysia block, ChinaTectonic implicatio ns and the need to redefine lithostratigraphic units [查看] YushengWan、DunyiLiu、MeihuiXu、JianminZhuang、BiaoSong、YuruoShi、LilinDu
    Northwestern Fujian Province is one of the most important Pre-Palaeozoic areas in the Cathaysia Block of South China. Metavolcano sedimentary and metasedimentary rocks of different types, ages and metamorphic grades (granulite to upper greenschist facies) are present, and previously were divided into several Formations and Groups. Tectonic contacts occur between some units, whereas(deformed) unconformities have been reported between others. New SHRIMP U–Pb zircon ages presented here indicate that the original lithostratigraphy and the old “Group” and “Formation” terminology should be abandoned. Thus the “Tianjingping Formation” was not formed in the Archaean or Palaeoproterozoic, as previously considered, but must be younger than its youngest detrital zircons (1790 Ma) but older than regional metamorphism (460 Ma). Besides magmatic zircon ages of 807 Ma obtained from metavolcano-sedimentary rocks of the “Nanshan Formation”and 751–728 Ma for the “Mamianshan Group”, many inherited and detrital zircons with ages ranging from 1.0 to 0.8 Ga were also found in them. These ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break-up of the Rodinia supercontinent. The new zircon results poorly constrain the age of the “Mayuan Group” as Neoproterozoic to early Palaeozoic (728–458 Ma),and not Palaeoproterozoic as previously thought. Many older inherited and detrital zircons with ages of 3.6, 2.8, 2.7, 2.6–2.5, 2.0–1.8 and 1.6 Ga were found in this study. A 3.6 Ga detrital grain is the oldest one so far identified in northwestern Fujian Province as well as throughout the Cathaysia Block. Nd isotope tDM values of eight volcano-sedimentary and clastic sedimentary rock samples centre on 2.73–1.68 Ga, being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation.These rocks underwent high grade metamorphism in the early Palaeozoic (458–425 Ma) during an important tectono-thermal event in the Cathaysia Block.
    Metamorphism in the Central Zone of the North China Craton SHRIMP U-Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province [查看] YushengWan、SimonA.Wilde、DunyiLiu、ChangxiuYang、BiaoSong、XiaoyanYin
    This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua F Group gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84±0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua F Group, gives a magmatic zircon age of 2.14±0.02 Ga and a metamorphic zircon age of ¨1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua FGroup_ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.
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