Discovery of the Dagele Eclogite in East Kunlun,Western China and Its Zircon SHRIMP U-Pb Ages:New Constrains on the Central Kunlun Suture Zone

Objective Eclogites are important indicators of ancient plate boundaries or paleosuture zones.Despite their great geological significance,very few investigations have been carried out in the Kunlun region.The Central East Kunlun fault zone was believed to be an Early Paleozoic suture zone,but there has been no reliable evidence for this,though studies on ophiolite,granite,and basic granulite indicate that the Early Paleozoic orogeny occurred in the East Kunlun.This work focused on the Dagele eclogites

The Age of the Original Silurian Badangshan Formation and its Ductile Deformation in the Northern Margin of North China Craton: New Evidence from Zircon SHRIMP U-Pb Ages

Objective Many previous researches have documented the tectonic evolution of the northern margin of North China Craton.However,whether the age of original Silurian Badangshan Formation in the study area belongs to the Archean,Silurian or Devonian remains controversial,and the ductile deformation time of this formation also lack

SHRIMP Zircon U-Pb Ages of the Wajilitag Igneous Complex: Constraints on the Origin of A-type Granitoids in the Tarim Large Igneous Province, NW China

Objective Large igneous provinces(LIPs)are sites of spatially contiguous,rapidly emplaced magmatic rocks,which represent the physical and chemical transfer of material from the mantle to the crust.Exposed within some continental LIPs are felsic and mafic plutonic and volcanic rocks.Although their volumes are minor compared to the flood basalts,the plutonic rocks of continental LIPs

SHRIMP Zircon U-Pb Age of the Sidingheishan Mafic-Ultramafic Intrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China and its Petrogenesis implication

Objective The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the North Tianshan Mountains.This work used zircon U-Pb age data,bulk rock major and trace elements,Sr-Nd-Pb isotope data to assess mantle source characteristics and crustal assimilation of the parental magma of the Sidingheishan intrusion.We have also discussed the tectonic evolution of the southern margin of the Central Asian Orogenic belt in the Late Paleozoic.

New Zircon SHRIMP U-Pb Ages of the Langjiu Formation Volcanic Rocks in the Shiquanhe Area,Western Lhasa Terrane and their Implications

Objective The potassic and ultrapotassic rocks relating to the India-Eurasia collision and continual plate convergence are widely distributed in the Lhasa terrane.These rocks are very important to understand the deep processes of the India-Eurasia collision and the uplift and evolution of the Tibetan Plateau.Although high-potassic volcanic rocks are also exposed in the western Lhasa terrane,their

中亚-蒙古造山带东段的锡林郭勒杂岩:早华力西期造山作用的产物而非古老陆块?——锆石SHRIMP U-Pb年代学证据

锡林郭勒杂岩是华北板块北缘古生代褶皱带内出露面积最大的变质岩系,以前多被当着前寒武纪的古老地块。本文通过对该杂岩中副片麻岩和正片麻岩的锆石SHRIMP U-Pb年代学研究发现,副片麻岩中的锆石多为岩浆锆石,其^(206)Pb/^(238)U加权平均年龄为406±7Ma,指示它们的原岩主要是由近同期(略早些)的岩浆岩风化后就近沉积的产物,该年龄应代表源区(岛弧型?)花岗岩的形成时间,同时也是副片麻岩原岩沉积的下限年龄。正片麻岩中岩浆锆石的^(206)Pb/^(238)U加权平均年龄为382±2Ma,代表花岗片麻岩原岩的侵位年龄。岩石中锆石的变质增生边的形成年龄为337±6Ma,代表锡林郭勒杂岩发生变质和变形的时间,该变质事件可能与贺根山缝合带内所发生的一次主要的碰撞造山作用有关。这些年龄资料充分说明,锡林郭勒杂岩并非古老地块,而是华力西早期岩浆作用、沉积作用和变质作用事件的产物.整个事件是在较短的时间范围内(～70Ma)完成的,推测该杂岩发育在碰撞造山带的弧前环境。中亚-蒙古造山带东南部(内蒙古的中、东部)碰撞前的构造格局可能不是典型的多岛洋体制,由于缺少古老的陆块,造山过程更多的表现为大洋的大陆化过程,即洋内俯冲形成岛弧,岛弧在被动大陆边缘拼贴聚合转化为新的大陆。

New detrital zircon U-Pb ages from BIF-related metasediments in the Ntem Complex (Congo craton) of southern Cameroon, West Africa

Banded Iron Formations (BIFs) were formed by contemporaneous events of active sediments supply and the venting of a hydrothermal fluid source at the Mid-Ocean-Ridge. BIFs within the Ntem Complex at the northern edge of the Congo Craton are intercalated with metasandstones and siltstones. SHRIMP U-Pb analysis on detrital zircons obtained from these metasediments gave variable ages from over 3000 Ma to 1000 Ma with the maximum age of deposition clustered around 1200 Ma and the peak of deposition at 1800 Ma. This age range suggested that the sub-basin was opened sometime in the Archean and remained active up till the Neoproterozoic. Zircons with Archean ages have a provenance linked to the charnockitic suite and the high-K granites within the Ntem Complex. The Paleoproterozoic ages are attributed to clastic inputs from the neigbouring Nyong Series west of the Ntem Complex. Also the peak of deposition in the Proterozoic could probably be explained by the globally recognized intense crust-forming processes in the Early Proterozoic time. The provenance of the younger Neoproterozoic ages is tied to various lithologies within the northern mobile belts of the Adamawa-Yade massifs and correlates with Neoproterozoic sedimentation ages in the Yaoundé, Lom and Poli series. The Neoproterozoic ages obtained are comparable to those obtained from metasediments of the Amazonian Craton and provide evidence of Pre-Gondwana assemblage of the Congo and the S?o Francisco Cratons.

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.

SHRIMP zircon U-Pb age and significance of Early Paleozoic volcanic rocks in East Kunlun orogenic belt, Qinghai Province, China

Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.

SHRIMP U-Pb zircon age of the youngest exposed pluton in eastern China

Analysis using the Sensitive High Resolution Ion Microprobe (SHRIMP) shows that the Yongsheng pluton, located to the south of Huadian County, Jilin Province, in the North China Craton, has a zircon U-Pb isotopic emplacement age of (31.6±1.3) Ma. It is therefore the youngest exposed pluton so far recognized in eastern China. Although geochemical data indicate intensive crystal fractionation, the Sr-Nd isotopic features suggest that the magmatic source region of the lithospheric mantle was weakly depleted. This implies that the previous enriched lithospheric mantle had been replaced by juvenile asthenospheric mantle before or during the Palaeogene. This recognition has great significance for future work on the Mesozoic-Cenozoic geological evolution and deep geological processes in eastern China.

SHRIMP zircon U-Pb ages from coal beds across the Permian-Triassic boundary, eastern Yunnan, southwestern China

The first SHRIMP zircon U e Pb ages from coal beds close to the end-Permian mass extinction are reported from the C_1 coal seam in the Yantang Mine in Laibin Town, Xuanwei County, eastern Yunnan Province.Zircons were extracted from kaolinite claystone layers, defined as tonsteins(volcanic ash deposits), in the subseam B_1 and B_3 of the coal seam C_1.The U-Pb ages are 252.0 ± 2.3 Ma and 250.3 ± 2.1 Ma for the sub-seam B_1 and B_3, respectively. Within analytical uncertainties, these U-Pb ages include the time period of the onset of the mass extinction at 251.941 ± 0.037 Ma, which was obtained from the marine Meishan section in Zhejiang Province, ~1600 km away from the Yantang Mine. These new ages represent not only the first and closest ages to the PTB mass extinction in terrestrial coal beds, but also ages from the nearest site to the Emeishan volcanoes investigated so far. Therefore these new data provide the most accurate stratigraphic horizon of terrestrial facies of the end-Permian extinction in South China. The Emeishan volcanoes were likely the source of volcanic ash in the coal seams at the Xuanwei County and broader areas in South China. Furthermore, the minerals and geochemistry characteristics of the C_1 coal seam also implied the influences of contemporaneous volcanic activities.

Tectonic evolution of the Western Kunlun orogenic belt in northern Qinghai-Tibet Plateau:Evidence from zircon SHRIMP and LA-ICP-MS U-Pb geochronology

The Western Kunlun Range in northern Qinghai-Tibet Plateau is composed of the North Kunlun Terrane,the South Kunlun Terrane and the Karakorum-Tianshuihai Terrane. Here we report zircon SHRIMP and LA-ICP-MS U-Pb ages of some metamorphic and igneous rocks and field observations in order to pro-vide a better understanding of their Precambrian and Palaeozoic-early Mesozoic tectonic evolution. Based on these data we draw the following conclusions: (1) The paragneisses in the North Kunlun Terrane are likely of late Mesoproterozoic age rather than Palaeoproterozoic age as previously thought,representing tectonothermal episodes at 1.0―0.9 Ga and ～0.8 Ga. (2) The North Kunlun Terrane was an orogenic belt accreted to the southern margin of Tarim during late Mesoproterozoic to early Neopro-terozoic,the two episodes of metamorphisms correspond to the assemblage and breakup of Rodinia respectively. (3) The Bulunkuole Group in western South Kunlun Terrane,which was considered to be the Palaeoproterozoic basement of the South Kunlun Terrane by previous studies,is now subdivided into the late Neoproterzoic to early Palaeozoic paragneisses (khondalite) and the early Mesozoic metamorphic volcano-sedimentary series; the paragneisses were thrust onto the metamorphic vol-cano-sedimentary series from south to north,with two main teconothermal episodes (i.e.,Caledonian,460―400 Ma,and Hercynian-Indosinian,340―200 Ma),and have been documented by zircon U-Pb ages. (4) In the eastern part of the South Kunlun Terrane,a gneissic granodiorite pluton,which intruded the khondalite,was crystallized at ca. 505 Ma and metamorphosed at ca. 240 Ma. In combination with geochronology data of the paragneiss,we suggest that the South Kunlun Terrane was a Caledonian accretionary orogenic belt and overprinted by late Paleozoic to early Mesozoic arc magmatism.

SHRIMP single zircon U-Pb dating of the Kongling high-grade metamorphic terrain: Evidence for >3.2 Ga old continental crust in the Yangtze craton

Single zircons from two trondhjemitic gneisses and two clastic metasedimentary rocks without Eu anomaly of the Kongling high-grade metamorphic terrain are dated by the in situ SHRIMP U-Pb method. The results show that the trondhjemitic magma emplaced at 2947-2903 Ma. Concordant age of as old as 3.3 Ga is present in the detrital zircons from the clastic metasedimentary rocks. Together with the depleted mantle Nd model age (TDM =3.2-3.3 Ga) of the clastic metasedimentary rocks, this documents the presence of Paleoarchean continental crust in the Yangtze craton.

The Early Jurassic magmatism in northern Guangdong Province, southeastern China: Constraints from SHRIMP zircon U-Pb dating of Xialan complex

This paper reports SHRIMP zircon U-Pb ages of 196±2 Ma for granite, and 195±1 Ma for gabbro from the Xialan complex in the Meizhou area, northern Guangdong Province. These results shed new light on the calm stage of magmatic activity in southeastern China during 200-180 Ma, and revealed that the back-arc extension induced by the subduction of the western Pacific plate may have begun at 195 Ma at least. Field observation on the fresh outcrops allows us to recognize some features formed by magma mixing. A part of the gabbro has a fine-grained rim of 20-30 cm at the margin, and thins gradually to-ward the granite; numerous dark fine-grained to microcrystalline dioritic enclaves developed in the granite. These enclaves vary in shape and size, dark minerals concentrated at the margin of enclaves, and the contact between enclaves and host rock is either obvious or obscure, or gradational. In addi-tion, needle-shaped apatites are included in the enclaves. The REE patterns of gabbros, as well as the trace element patterns, are generally consistent with those of granitic rocks. The above characters further suggest that in the Early Jurassic the injection of basic magma had melted deep continental crust and produced acidic magma, and the Xialan complex was produced by the mixing of them.

Structure and deformation around the Gyirong basin, north Himalaya, and onset of the south Tibetan detachment system

Gyirong basin and its adjacent area are located at a special position in the Himalayan orogen, where the south Tibetan detachment system (STDS) and N-S trending rift converged. The north Himalayan orogen here can be divided into five petrologic-tectonic units successively from south to north: 1) the Greater Himalayan crystalline complex (GHC); 2) the STDS shear zone; 3) the Tethyan Himalayan sedimentary sequence (THS); 4) the late Cenozoic sedimentary basins, such as Gyirong and Oma basins; and 5) the Malashan gneiss dome. Structural studies show that this area experienced four stages of deformation: 1) the earlier south-directed thrusting, preserved both in the GHC and THS; 2) top-down-to-north slip along the STDS, normal faults related to this slip formed the early controlling structures of the Cenozoic basins,and the tilted pattern of the blocks between the basins indicated a north-directed slip; 3) east-west extension, the resultant N-S trending normal fault formed the eastern boundary of the basins; and 4) late gravitational collapse. Zircon SHRIMP U-Pb dating on the syn-deformational (leuco-) granite along the STDS indicates that the major activity of the STDS occurred at ca. 26 Ma, but its onset may have begun as early as ca. 36 Ma.

Geological and chronological evidence of Indo-Chinese strike-slip movement in the Altyn Tagh fault zone

A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with syn-shear anatexis. Among them, one is the anatectic long co-lumnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461-547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239-244 Ma for the long-columnar anatectic zircon. This type of zircon is direc-tionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, represent-ing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth horn-blende in the same mylonite