Cosmogenic ^10Be dating of Guxiang and Baiyu Glaciations

Guxiang and Baiyu Glaciations are two previously recognized local glaciations of the Tibetan Plateau. They have been widely used as the reference standard for classifying Late Quaternary glaciations on the Tibetan Plateau and its surrounding mountains. However, the numerical chronologies of both glaciations have been lacking. In this study, cosmogenic They dating was undertaken to define the timing of these two glaciations. The surface boulders deposited by the glaciers of the Guxiang and Baiyu Glaciations have exposure ages of 112.9±16.7-136.5±15.8 ka BP and 11.1±1.9-18.5±2.2 ka BP, respectively. It is likely that the Guxiang and Baiyu Glaciations correspond to marine isotope stages 6 and 2, respectively.     

Late Cenozoic history of deep water circulation in the western North Pacific: Evidence from Nd isotopes of ferromanganese crusts

Despite its importance in regulating the global climate, the past deep ocean circulation in the western North Pacific has still been poorly understood. Nd isotopes of ferromanganese crusts have been proven to be a good proxy to monitor paleoceanic circulation changes. In this study, late Cenozoic Nd isotopic records are recovered from two ferromanganese crusts located near Mariana arc but at different water depths (MKD13: 1530 m, MDD53: 2700 m), and their implications for paleocirculation change in this area are explored. From the early to late Miocene, Nd isotopic compositions of MDD53 remained stable, and they were also characterized by the least radiogenic signatures ( Nd4.0 to5.0) compared to crusts of similar water depths in the Miocene North Pacific. Afterward, an abrupt increase in its Nd value occurred in the Pliocene. In contrast, Nd isotopes of MKD13 became more radio- genic with time in the Miocene and were almost invariable thereafter. The continual increase in Nd of shallower crust MKD13 is interpreted as reflecting progressive closure of Indonesian Seaway in the Miocene, while the deep western boundary current sourced from the Southern Pacific may have dominated Nd isotopes of deeper crust MDD53 during the same time interval. The lack of Nd isotopic variation of MKD13 in the Pliocene indicates that there were no changes in Nd sources in shallower waters and the final restriction between the Indian and Pacific may have only occurred since then. Therefore the observed large shift to more radiogenic Nd isotopes of MDD53 in the Pliocene should not be resulted from changes in vertical input from shallower to deeper water. Instead, a decreased ventilation of deep southern component current along the studied water depth range (~2700 m) may have continued in the Pliocene.     

Single grain Rb-Sr dating of euhedral and cataclastic pyrite from the Qiyugou gold deposit in western Henan, central China

The Rb-Sr isotopic dating of pyrite mineral from ore deposits can directly provide mineralization age. However, many geological factors may affect the Rb-Sr isotopic system,which baffles application of this method. Employing ultra-low procedural blank Rb-Sr method,we have dated pyrites separated from the No. 4 breccia pipe of the Qiyugou gold deposit,western Henan Province. Single grains of euhedral pyrite crystal with few microcracks yield an isochron age of 126 ± 11 Ma, which represents time of the main mineralization stage of the deposit. Pyrite grains of cataclastic type show nevertheless scattered Rb-Sr isotopic composition and no reasonable isochron can be defined. Crystal morphology and mineral inclusion studies reveal that Rb and Sr of pyrite mineral probably are preserved mainly in biotite, K-feldspar, and sericite mineral inclusions. The dating results likely suggest that cataclastic pyrite is not suitable for the Rb-Sr dating due to modification of the Rb-Sr isotopic system by later hydrothermal activity of fluid.     

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt——Evidence for existence of Caledonian orogenic root

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak meta-morphic age of eclogites, and reflect the existence of orogenic root related to Caledonian sub-duction and continent-continent collision.     

Lower boundary age of the Nanhua System and the Gucheng glacial stage: Evidence from SHRIMPⅡ dating

After the disintegration of the pro-Sinian System, the choice of the stratotype section of the newly established Nanhua System and determination of its lower boundary age have been one of the most important problems in Neoproterozoic stratigraphy[1—3]. In the Yangtze Gorge area in Yichang of Hubei Province where the standard section of the pro-Sinian System is located, the newly established Nanhua System consists only of the Nantuo formation (tillite) and the Liantuo Formation with tuf-…     

Palaeoproterozoic Khondalite Belt in the western North China Craton： New evidence from SHRIMP datin8 and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ui-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (TDM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TDM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite se- ries in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.     

Geochemistry and zircon U-Pb ages of granulite xenolith from Tuoyun basalts, Xinjiang： Implications for the petrogenesis and the lower crustal nature beneath the southwestern Tianshan

The continental lithosphere growth mainly includes the horizontal accretion at the plate boundaries and vertical accretion within the plate[1]. Mafic magmatic materials, as the products of crust-mantle interaction[2,3], became more and more important in studying the formation and evolu- tion of the lower crust. The previous geologic researcheson Tianshan, extending nearly 2500 km from east to west, and the neighbor area were mainly focused on the Paleozoic collision structure[4 ― 6], Mesozoi…     

SHRIMP ages of detrital zircons from the Changcheng System in the Ming Tombs area, Beijing： Constraints on the protolith nature and maximum depositional age of the Mesoproterozoic cover of the North China Craton

The Mesoproterozoic Changcheng System is widely distributed in the North China Craton. Determining its time of deposition and sources is important to understand the Precambrian crustal evolution of the North China Craton. This paper suggests age distribution patterns for detrital zircons from clastic sediments of the Changcheng System in the Ming Tombs area, Beijing. Samples of feldspar-bearing sandstone (CHc-2) and pure sandstone (CHc-9) were collected from the Changzhougou Formation, which constitutes the basal part of the Changcheng System. Detrital zircons show an age range from 2.35 to 2.60 Ga. However, sample CHc-9 in the upper Changzhougou Formation also contains some zircons with ages of 1.9-1.8 Ga and 2.3-2.1 Ga. The age patterns lead to the following conclusions: (1) Most of the detrital material came from a source area composed predominantly of -2.5 Ga continental crust of the North China Craton; (2) 1.9--1.8 Ga reflects the age record of Palaeoproterozoic continent-continent collisional event in the North China Craton; and (3) the oldest age for deposition of the Changcheng System is 1.8 Ga.     

Formation ages and source regions of the Palaeoproterozoic Gaofan,Hutuo and Dongjiao groups in the Wutai and Dongjiao areas of the North China Craton from SHRIMP U-Pb dating of detrital zircons: Resolution of debates over their stratigraphic relationships

This paper reports detrital zircon age distributions of meta-sedimentary rocks of the Gaofan, Hutuo and Dongjiao groups in the Wutai and Dongjiao areas of the North China Craton. Detrital zircons of a quartzite from the Gaofan Group are mainly ~2.5 Ga in age, with some ~2.7 Ga and older. A quartzite pebble from the basal conglomerate of the Hutuo Group is similar in detrital zircon age distribution to the quartzite of the Gaofan Group. For a meta-feldspar-quartz sandstone from the Dongjiao Group, the age of...