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U-Pb (SHRIMP) and 40Ar-39Ar geochronological constrainsts on the evolution of the xingxingxia shear zone,NW China, A Triassic segment of the Altyn Tagh fault system [查看] YuWang、GuihuaSun、JinyiLi

The development of structures and their age along the segment of the Altyn Tagh fault system, and the eastward extension of the Tianshan orogenic belt, remain speculative.Recent investigations on the structural framework, granitic intrusions, and metamorphic rocks in the eastern Tianshan and adjacent areas show that the NE-striking Xingxingxia sinistral ductile shear zone,NW China, is subparallel to the Altyn Tagh fault zone and is superposed on the eastern Tianshan orogenic belt. U-Pb zircon sensitive high-resolution ion microprobe (SHRIMP) dating, and muscovite, biotite,and K-feldspar 40Ar/39Ar thermochronology indicate that sinistral shear along the Xingxingxia shear zone initiated at ~240–235 Ma,broadly at the same time as initial formation of the Altyn Tagh fault zone, but later than initiation of dextral strike-slip motion along~EW-trending eastern Tianshan orogenic belt at ~270–245 Ma. Formation of the Xingxingxia ductile shear zone was associated with Gondwanaland convergence along the southern margin of the Eurasian continent during the Late Permian–Early Triassic.

The Early Jurassic tectono-magmatic events in southern Jiangxi and northern Guangdong provinces, SE China Constraints from the SHRIMP zircon U-Pb dating [查看] X.Q.Yu、G.G.Wu、XixiZhao、J.F.Gao、Y.J.Di、Y.Zheng、Y.P.Dai、C.L.Li、J.T.Qiu

The Jurassic to Cretaceous magmatism in the South China Block (SCB) has important geological significance for its tectonic evolution, particularly, the tectonic regime change. We report here new SHRIMP zircon U–Pb ages, geochemical and Sr–Nd isotopic results from the Early Jurassic magmatic rocks in eastern Nanling region of southern Jiangxi and northern Guangdong provinces, SE China. The SHRIMP zircon U–Pb analyses yield consistent Early Jurassic ages of 193 ± 2 Ma for granodiorite and 196 ± 1 Ma for gabbrodiabase from the Hanhu complex of southern Jiangxi Province. Field observation indicates that the gabbro-diabase coexists with syenite in granodiorite, and that the mafic components infiltrate into the syenite.Combined with the SHRIMP zircon U–Pb ages of 196 ± 2 Ma for granites and 195 ± 1 Ma for gabbros of the Xialan complex of northern Guangdong province, these contemporary ages represent an earliest tectono-magmatic events during the Yanshanian orogeny. These results reveal that there are tectonic extension and the post-collisional and intraplate magmatism events at or before 196 Ma, in contrast to the wide believed quiescent stage (205–180 Ma) of magmatic activity in SE China. The confirmation of these tectono-magmatic events in the eastern Nanling area thus sheds new light on the quiescent stage of magmatic activity in SE China during 205–180 Ma and provides new constraints to understand the Early Jurassic tectonic evolution of SE China.

The age of the base of the Paleoproterozoic Hutuo Group in the Wutai Mountains area, North China Craton SHRIMP zircon U-Pb dating of basaltic andesite [查看] DULiLin、YANGChongHui、GUOJingHui、WANGWei、RENLiuDong、WANYuSheng、GENGYuanSheng

Basalt and basaltic andesite metamorphosed at greenschist facies occurs with conglomerate layers at the bottom of the Paleoproterozoic Hutuo Group in the Wutai Mountains area, North China Craton. Detailed geological surveying confirms that these volcanic rocks are conformable within the neighboring sedimentary rocks. The SHRIMP results on basaltic andesite are divided into two groups. In one group the 207Pb/206Pb ages are from 2433 to 2558 Ma, which is consistent with the basement crustal age in Fuping and Wutai areas. In the other group, 13 grains yielded a weighted mean 207Pb/206Pb age of 2140±14 Ma. The latter is interpreted as the eruption age of the basaltic andesite, and gives the age of the base of the Hutuo Group. This result further suggests that the Hutuo Group formed in the middle Paleoproterozoic, not the early Palaeoproterozoic or late Archaean, as thought before,and is related to a 2.2−2.1 Ga rifting event in the Central North China Craton.

Tectonic setting and significance of 2.3-2.1 Ga magmatic events in the Trans-North China Orogen New constraints from the Yanmenguan mafic-ultramafic intrusion in the Hengshan-Wutai-Fuping area [查看] ZhihongWang、SimonA.Wilde、JialiangWan

Adetailed study of the∼2.2 GaYanmenguan mafic–ultramafic intrusion in the Hengshan area is presented with the aim of clarifying the tectonic history of the Trans-North China Orogen in the North China Craton during the period 2.3–2.1 Ga. SHRIMP U–Pb dating of magmatic zircons from a gabbro indicates emplacement at ∼2193 Ma. The whole rock and mineral geochemistry of this intrusion are akin to Alaskan-type complexes, and are typical of subduction-related magmas. The parental magma likely originated from the depleted mantle wedge above a subduction zone, and experienced fractional crystallization of olivine and clinopyroxene. The continental arc, represented by the mafic–ultramafic intrusions and contemporary granitic and volcanic rocks, together with the Lüliang back-arc basin constitute a marginal arc–back-arc basin system that evolved between 2.3 and 2.1 Ga. Based on geochronological datasets and available data on magmatism, metamorphism and structural deformation, a comprehensive new tectonic model for the evolution of the TNCO during the Neoarchean to Paleoproterozoic is constructed: (1) ∼2.5 Ga intra-oceanic arc–back-arc basin system and its accretion at the end of the Neoarchean; (2) a long hiatus in magmatism and tectonism between ∼2.5 and 2.3 Ga; (3) ∼2.3–2.1 Ga marginal arc–back-arc basin magmatism developed in the accreted Hengshan–Wutai–Fuping arc terranes and the Lüliang terrane;(4) ∼2.1 Ga collision, resulting in closure of the ∼2.3–2.1 Ga Lüliang back-arc basin, accretion of the Lüliang–Hengshan–Wutai–Fuping arc terranes, and their attachment to the Western Block of the North China Craton; (5) the intrusion of ∼1970–1910Ma post-collisional mafic dykes in an extensional event;(6) final collision between the Western and Eastern Blocks between∼1.9 and 1.8 Ga, resulting in extensive magmatism, tectonism and metamorphism; and lastly (7) post-1.8 Ga crustal extension.

Subducting sediment-derived arc granitoids evidence from the Datong pluton and its quenched enclaves in the western Kunlun orogen, northwest China [查看] Shi-YongLiao、Yao-HuiJiangShao-YongJiang、Wan-ZhiYang、QingZhou、Guo-DongJin、PengZhao

Abstract This paper presents detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of the Datong pluton and its quenched enclaves from the western Kunlun orogen, northwest China, in an attempt to achieve a better understanding on the origin of diverse arc magmas. The Datong host granitoids are intermediate to acid in composition (SiO2=57.5~73.1 wt.%), and exhibit high-K calc-alkaline to shoshonitic affinities.The quenched enclaves are silica-rich ultrapotassic rocks. Detailed SHRIMP zircon U–Pb dating indicates that the Datong pluton was emplaced in Ordovician time (473.4–447.7 Ma), which places the Datong pluton in an active continental margin setting, rather than a syn-collision setting of Early Silurian age. The Datong host granitoids were derived by partial melting of subducted sediments, with the subsequent melt interacting with the overlying mantle wedge during its ascent. Partial melting of the veined mantle wedge hybridized by sediment-derived melts generated the silicarich ultrapotassic magma, which was injected into the Datong granitoid magma chamber and quenched, resulting in enclaves hosted by granitoids. This contribution provides evidence that arc magmas can be derived directly by partial melting of subducted sediments, which is helpful to further understand the origin of diverse arc magmas.

SHRIMP U-Pb zircon chronology of ultrahigh-temperature spinel–orthopyroxene–garnet granulite from South Altay orogenic belt,northwestern China [查看] ZILONGLI、YINQILI、HANLINCHEN、M.SANTOSH、WENJIAOXIAO、HUIHUIWANG

Abstract Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U–Pb ages are reported from an ultrahigh-temperature (UHT) spinel–orthopyroxene–garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China.This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan–Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al2O3 contents up to 9.3 wt%. The peak temperatures of metamorphism were >950°C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P–T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 ±8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490–500 Ma. Since the granulite was metamorphosed at temperatures >900°C, exceeding the closure temperature of U–Pb system in zircon, a possible interpretation is that the 499±8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a ~500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo-Asian Ocean and associated accretion-collision tectonics of the Siberian and Kazakhstan–Junggar Plates followed by rapid exhumation leading to decompression.

SHRIMP U-Pb dating for a K-bentonite bed in the Tieling Formation,North China [查看] SUWenBo、LIHuaiKun、HUFFWD、ETTENSOHNFR、ZHANGShiHong、ZHOUHongYing、WANYuSheng

A SHRIMP U-Pb zircon age of 1437±21 Ma was obtained for a recently discovered K-bentonite bed in the Tieling Formation,situated northeast of Beijing at the boundary between Liaoning and Hebei provinces, on the northern margin of the North China Craton (NCC). The SHRIMP U-Pb age places Tieling Formation near the end of the Calymmian Period of the early Mesoproterozoic Era. In addition, a SHRIMP U-Pb zircon date of 1372±18 Ma was acquired for K-bentonite beds in the overlying,dark-shale-dominated Xiamaling Formation from the same location northeast of Beijing. This date assigns a similar Mid-Mesoproterozoic (Ectasian Period) age for the Xiamaling Formation, as have previously determined dates from other sections northwest of Beijing. These dates indicate that the Tieling and Xiamaling formations, as well as the related succession in the eastern part of the Yanshan Mountains, represented by the well-known Meso- to Neoproterozoic standard section in Jixian, can be correlated well with sections northwest of Beijing in the western part of the Yanshan Mountains. In other words, the boundary between the Calymmian and Ectasian periods in the northern parts of the NCC is marked by the unconformity between the Tieling and Xiamaling formations in the northern NCC. This boundary was previously ascribed to the “Qinyu Orogeny” and thought to be of Grenville age. In this regard, the conventional “Qinyu Orogeny” should now be regarded as a short-lived regional uplift during Early Mesoproterozoic time, rather than a result from the Grenvillian assembly of the NCC to the Rodinia Supercontinent (~1.0 Ga).

SHRIMP U-Pb dating and Hf isotopic analyses of Middle Ordovician meta-cumulate gabbro in central Qiangtang, [查看] ZHAIQingGuo、WANGJun、LICai、SULi

Metabasites consisting of metamorphic ultra-mafic rocks, cumulate gabbro, gabbro (diabase), basalt, and plagiogranite are exposed at the Taoxinghu area in central Qiangtang, northern Tibetan Plateau. Zircon SHRIMP U-Pb dating for the cumulate gabbro yields a weighted mean age of 467±4 Ma, which is the oldest and most reliable magmatic age in this area. Zircon 176Hf/177Hf ratios range from 0.282615 to 0.282657, with εHf(t) values of 5.02±0.28, indicating that the cumulate gabbro was mainly derived from the depleted mantle. In addition, geochemical data of metabasites suggest that they have similar characteristics to those in the mid-ocean ridge basalts (MORB). The Taoxinghu metabasites may represent the fragment of Early Paleozoic ophiolite in the “Central Uplift” of the Qiangtang, northern Tibetan Plateau.

Progress and controversy in the study of HP-UHP metamorphic terranes in the West and Middle Central China Orogen [查看] LiuLiang、YangJiaxi、ChenDanling、WangChao、ZhangChengli、YangWenqiang、CaoYuting

ABSTRACT: During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world.The in situ zircon U-Pb dating using LA-ICPMS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475–509, 420–457, and 485–514 Ma,respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20–80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these regions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present,the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle depth of stishovite stability field (>9 GPa) is occasional or universal; (2) the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field (>300 km); (3) the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in metamorphism of the HP-UHP rocks in the West and Middle Central China orogen, but also in continental deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction.

Occurrence of Neoproterozoic low-grade metagranite in the western Beihuaiyang zone, the Dabie orogen [查看] LIUYiCan、LIULiXiang、GUXiaoFeng、LIShuGuang、LIUJia1、SONGBiao

Based on detailed field investigations and petrographic observations, we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone, the Dabie orogen. This study reports the results of zircon U-Pb dating and preliminary petrographic observations on two metagranite samples. The studied rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their U-Pb ages are 726 ± 6 and 758 ± 12 Ma, respectively, similar to those for the Luzhenguan complex in the eastern segment of the Beihuaiyang zone. In combination with previously determined 635 ± 5 Ma low-grade metagabbro, this study suggests the occurrence of at least two types of Neoproterozoic intrusive rocks in the Beihuaiyang zone, the northern margin of the South China Block (SCB): 726−758 Ma metagranite and 635 Ma metagabbro. These rocks occur within the metamorphosed Ordovician volcanic zone (originally named the Dingyuan Formation) and are in tectonic contact to each other, but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid- and late-Neoproterozoic mafic and felsic magmatism in the Suizhou and Zaoyang area, Hubei Province, but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks, it appears that these Neoproterozoic low-grade rocks are exotic and they may have been detached and scraped from subducting SCB crust in the early Triassic during the initial continental subduction, and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.

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