New chronological constraints for Cryogenian to Cambrian rocks in the Three Gorges,Weng'an and Chengjiang areas,South China     

《Gondwana Research》2014,25(3):1027-1044

The Neoproterozoic and Cambrian were two of the most dramatic periods in the history of the Earth, because large multi-cellular animals first appeared then in the so-called “Cambrian Explosion”. To better understand this event, many paleontological and geochemical studies now focus on rocks in South China, because of the fossiliferous succession and good exposure. Since the recognition of the Yangtze Gorges and Chengjiang area as type localities of the Sinian (Ediacaran) and Meishucunian (Early Cambrian) Systems, both sections have been intensively investigated. In order to decipher the relationships between the evolution of life and surface environmental changes, it is necessary to understand their paleontological, geochemical and geo-chronological constraints.This study presents new chronological constraints for the Cryogenian to Cambrian rocks in the Three Gorges, Weng'an and Chengjiang areas, South China. We discovered two tuff layers, one at the base of the Shuijingtuo Fm at Three Gorges and the other at the bottom of the Dahai Member in Chengjiang. In addition, we collected sandstones from Neoproterozoic strata in the Three Gorges, Chengjiang and Weng'an areas for provenance analysis. Zircons, separated from the tuff layers, provide new Nano-SIMS U–Pb ages of 526.4 ± 5.4 Ma at the base of the Shuijingtuo Fm, and 523.9 ± 6.7 Ma at the bottom of the Dahai Member. The tuffaceous beds occur at an unconformity, and nodules are common in the Three Gorges, Meishucun and Taoying sections, indicating that major and relatively wide-scale volcanic and sedimentological events occurred at ca. 525 Ma. Moreover, carbonate carbon isotope chemostratigraphies at Morocco, Siberia, Three Gorges and Meishucun display different characteristics during the Tommotian. One possibility is that the South China Ocean was separated from an outer ocean at that time. Detrital zircons in sandstones have age populations at ca. 2.7, 1.8, 1.6, 1.0 and from 0.9 to 0.75 Ga. indicating that the paleo-hinterland of the Nanhua and Kangdian rift basins was geologically complex. Despite the lack of ca. 1.6 Ga rocks in the Yangtze and Cathaysia Blocks, these data are nevertheless interpreted to indicate derivation of the zircons from basement rocks in the Yangtze craton.  相似文献   

Reproductive strategy of the bradoriid arthropod Kunmingella douvillei from the Lower Cambrian Chengjiang Lagerstätte,South China     

《Gondwana Research》2014,25(3):983-990

The bivalved bradoriid arthropod Kunmingella douvillei (Mansuy, 1912) is the most common species in the Lower Cambrian Chengjiang Lagerstätte. Its soft anatomy has been reported based on well-preserved specimens. However, as with other Cambrian arthropods, its reproductive behavior is poorly documented. Rare specimens of K. douvillei preserved with eggs have been reported. Our new collections confirm that the eggs were directly attached to the last three pairs of biramous appendages and verify the head–trunk boundary suggested by previous work. The brooding behavior of K. douvillei was most probably related to its small body size, as in living crustaceans. The K. douvillei female could carry 50–80 eggs ranging from 150–180 μm in diameter, possibly indicating a K-reproductive strategy; its ontogenetic and reproductive mode might have been an adaptation in response to increasing predatory pressure across the Ediacaran–Cambrian boundary. All of the three main directions of development in the life history of living marine invertebrates, namely planktotrophic larvae, lecithotrophic larvae, and brood care, most likely evolved by Stage 3 of the Cambrian.  相似文献   

Sedentary habits of anthozoa-like animals in the Chengjiang Lagerstätte: Adaptive strategies for Phanerozoic-style soft substrates     

《Gondwana Research》2014,25(3):966-974

Substrates have perpetually played a crucial role in shaping the morphology of animal bodies, particularly the sedentary apparatus of benthic suspension feeders. In-situ preservation of sedentary forms from Cambrian Lagerstätten, especially the Lower Cambrian Chengjiang Lagerstätte, Yunnan Province, southwestern China, provides a unique opportunity for studying ancient interactions between the benthos and their substrates. The sedentary apparatuses of anthozoa-like animals Xianguangia sinica Chen & Erdtmann, 1991, Archotuba conoidalis Hou et al., 1999 and Archisaccophyllia kunmingensis Hou et al., 2005 from the Chengjiang Lagerstätte are investigated in their sedimentary context. X. sinica bears a cylindrical, rigid column and a basal, rounded, cuticularized attachment disc that partially attached into the soft muddy substrate; A. kunmingensis has a prominent, flexible pedal-shaped disc that attached directly to the substrate by expanding its disc surface area; whereas A. conoidalis has a unique, posterior-tapering tube and probably might have secreted mucus to firmly attach to the ‘sclerotized’ substrates. These three sedentary taxa employed different adaptive strategies, i.e., the ‘iceberg’, the ‘snowshoe’ and one newly defined but later widespread in Phanerozoic period, the ‘anchoring-nail’, to cope with the soft substrates. Furthermore, the anchoring-nail mode, sclerite-sticking behavior represented by Archotuba is a Phanerozoic innovative strategy in response to the appearance of pelagic larvae and small shelly fossils, especially to the first widespread biomineralization event at the beginning of the Cambrian, whereas the other two modes have already developed for about 90 Ma from the Ediacaran.  相似文献   

The Paleozoic evolution of Central Tianshan: Geochemical and geochronological evidence     

《Gondwana Research》2014,25(2):797-819

A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~ 450–400 Ma, ~ 370–350 Ma and ca. 340 Ma. The first stage (450–400 Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)_CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350 Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~ 365 Ma) granitoids fit within the A-type field whereas the younger (~ 352 Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340 Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf T_DM^C model age populations: ca. 2400 Ma, ca. 1400 Ma and ca. 1100 Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~ 2400 Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~ 1400 Ma) and the assembly of Rodinia (~ 1100 Ma).  相似文献   

A review of Precambrian palaeomagnetism of Australia: Palaeogeography,supercontinents, glaciations and true polar wander     

《Gondwana Research》2014,25(3):1164-1185

This is the first review of the Australian Precambrian palaeomagnetic database since that undertaken by Idnurm and Giddings (1988) 25 years ago. In this period the data have almost tripled in number from about 60 to more than 170 and while some segments of the pole path are now quite well defined, overall the data are sparse. It is debatable whether the extant rock record amenable to palaeomagnetism is complete enough for full palaeogeographic histories to be reconstructed. The SWEAT connection is apparently ruled out for Rodinia as both the 1200 Ma and 1070 Ma poles from (ancestral) Australia and Laurentia disallow it. However, older palaeopoles do support a SWEAT-like configuration for the pre-Rodinia supercontinent Nuna but the geological reasoning for SWEAT applies to Rodinia so a Nuna SWEAT is less than gratifying. The concept of a “grand-pole” is introduced here, which includes all the “key-pole” features but is predicated on the condition that two or more independent laboratories are in agreement.Precambrian data from Australia include the oldest palaeopole yet defined, the record of one of the oldest geomagnetic polarity reversals, the most definitive evidence for low-latitude Neoproterozoic glaciation, the first study of BIFs and the timing/nature of iron-ore genesis, the most unusual ‘field test’ (impact melt rock and ejecta horizon host rocks), some of the best examples of complete contact tests and the timing of craton assembly. Some old caveats that can no longer be ignored, such as corrections for inclination flattening and the permitting of rotations between contiguous intracontinental cratons to bring conflicting palaeopoles into alignment are required. Care should be exercised when inferring palaeolatitudes from sedimentary derived palaeoinclinations. TPW should only be considered if there is evidence from more than one, and preferably more, independent continents. Future work identified includes a complete magnetostratigraphic study of ~ 300 my Adelaidean succession, better age constraints for the Adelaidean and Officer Basin successions and a better age for the Gawler Craton GB dykes.  相似文献   

Ontogeny and dimorphism of Isoxys auritus (Arthropoda) from the Early Cambrian Chengjiang biota,South China     

《Gondwana Research》2014,25(3):975-982

The morphology of Isoxys auritus Jiang, 1982 is reinterpreted in the light of abundant new specimens from the Early Cambrian Chengjiang biota, South China. I. auritus was a bivalved arthropod, its shield armed with two cardinal spines sub-equal in length. Two morphotypes (shield with and without ornamentation) which are of several original differences were interpreted as sexual dimorphs. 81 specimens examined here, which range between 4.8 mm and 47.0 mm, represent a successive developmental sequence. The earliest stages were characterized by short cardinal spines, large spherical eyes, a pair of elongated antennulae, and seven pairs of post-antennular appendages. The slim antennula is uniramous, consisting of nine articles, each armed with short spines. It differs from that of great appendage and lacks any grasping function. During the ontogeny, the body length increases, accompanied by addition of trunk somites and appearance of primary reticulated ornaments, and both cardinal spines become evident. In the fully grown adult, there are up to 11 pairs of post-antennular appendages, equipped with the stout endopod composed of 6 or 7 podomeres lacking endites, and a terminal claw; the paddle shaped exopod is fringed with long setae along its posterior margin. Secondary reticulation of the shield has developed inside each primary one; the cardinal spines more elongated; and the ratio of length to height of shield increases. However, the size of eye and antennula relative to the body length significantly decreases. Accordingly, intraspecific variation, including sexual dimorphs and developmental change, is evident in I. auritus. Recognizing such differences is important for detecting possible synonymies in the genus. Furthermore, the ontogenetic changes of I. auritus described here, particularly the postembryonic segment addition and possible allometric growth may also improve our understanding of development of Cambrian arthropods in Burgess-shale type preservation, especially those possessing such a large shield.  相似文献   

Cratonic reactivation and orogeny: An example from the northern margin of the North China Craton     

《Gondwana Research》2014,25(3-4):1203-1222

Reactivation of cratonic basement involves a number of processes including extension, compression, and/or lithospheric delamination. The northern margin of the North China Craton (NCC), adjacent to the Inner Mongolian Orogenic Belt, was reactivated in the Late Paleozoic to Early Mesozoic. During this period, the northern margin of the NCC underwent magmatism, N–S compression, regional exhumation, and uplift, including the formation of E–W-trending thick-skinned and thin-skinned south-verging folds and south-verging ductile shear zones. zircon U–Pb SHRIMP ages for mylonite protoliths in shear zones which show ages of 310–290 Ma (mid Carboniferous–Early Permian), constraining the earliest possible age of deformation. Muscovite within carbonate and quartz–feldspar–muscovite mylonites from the Kangbao–Weichang and Fengning–Longhua shear zones defines a stretching lineation and gives ⁴⁰Ar/³⁹Ar ages of 270–250 Ma, 250–230 Ma, 230–210 Ma, and 210–190 Ma. Deformation developed progressively from north to south between the Late Paleozoic and Triassic. Exhumation of lower crustal gneisses, high-pressure granulites, and granites occurred at the cratonic margin during post-ductile shearing (~ 220–210 Ma). An undeformed Early Jurassic (190–180 Ma) conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis. Deformation was induced by convergence between the southern Mongolia and North China cratonic blocks, and the location of this convergent belt controlled later deformation in the Yanshan Tectonic Province. This province formed as older E–W-trending Archean–Proterozoic sequences were reactivated along the northern margin of the NCC. This reactivation has features typical of cratonic basement reactivation: compression, crustal thickening, remelting of the mid to lower crust, and subsequent orogenesis adjacent to the orogenic belt.  相似文献   

Beyond the Cambrian Explosion: From galaxy to genome     

《Gondwana Research》2014,25(3):881-883

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Partitioning of the Cretaceous Pan-Yangtze Basin in the central South China Block by exhumation of the Xuefeng Mountains during a transition from extensional to compressional tectonics?     

《Gondwana Research》2014,26(4):1644-1659

The formation of a series of intermountain basins is likely to indicate a geodynamic transition, especially in the case of such basins within the central South China Block (CSCB). Determining whether or not these numerous intermountain basins represent a division of the Cretaceous Pan-Yangtze Basin by exhumation of Xuefeng Mountains, is key to understanding the late Mesozoic to early Cenozoic tectonics of the South China Block (SCB). Here we present apatite fission track (AFT) data and time–temperature modeling in order to reconstruct the evolution history of the Pan-Yangtze Basin. Fourteen rock samples were taken from a NE–SW-trending mountain–basin system within the CSCB, including, from west to east, the Wuling Mountains (Wuling Shan), the south and north Mayang basins, the Xuefeng Mountains (Xuefeng Shan) and the Hengyang Basin. Cretaceous lacustrine sequences are well preserved in the south and north Mayang and Hengyang basins, and sporadically crop out in the Xuefeng Mountains, whereas Paleogene piedmont proluvial–lacustrine sequences are only found in the south Mayang and Hengyang basins. AFT results indicate that the Wuling and Xuefeng mountains underwent rapid denudation post-84 Ma, whereas the south and north Mayang basins were more slowly uplifted from 67 and 84 Ma, respectively. Following a quiescent period from 32 to 19 Ma, both the mountains and basins have been rapidly denuded since 19 Ma. Both the AFT data and sedimentary facies changes suggest that the Cretaceous deposits that cover the south–north Mayang and Hengyang basins through to the Xuefeng Mountains define the Cretaceous Pan-Yangtze Basin. Integrating our results with tectonic background for the SCB, we propose that rollback subduction of the paleo-Pacific Plate produced the Pan-Yangtze Basin, which was divided into the south–north Mayang and Hengyang basins by the abrupt uplift and exhumation of the Xuefeng Mountains from 84 Ma to present, apart from a period of tectonic inactivity from 32 to 19 Ma. This late Late Cretaceous to Paleogene denudation resulted from movement on the Ziluo strike–slip fault, which formed due to intra-continental compression most likely associated with the Eurasia–Indian plate subduction and collision. Sinistral transpression along the Ailao Shan–Red River Fault at 34–17 Ma probably transformed this compression to the extrusion of the Indochina Block, and produced the quiescent window period from 32 to 19 Ma for the mountain–basin system in the CSCB. Therefore, the initiation of exhumation of the Xuefeng Mountains at 84 Ma indicates a switch in tectonic regime from Cretaceous extension to late Late Cretaceous and Cenozoic compression.  相似文献   

The mafic–ultramafic complex of Aniyapuram,Cauvery Suture Zone,southern India: Petrological and geochemical constraints for Neoarchean suprasubduction zone tectonics     

《Journal of Asian Earth Sciences》2014

Several Precambrian mafic–ultramafic complexes occur along the Cauvery Suture Zone (CSZ) in Southern Granulite Terrain, India. Their origin, magmatic evolution and relationship with the associated high-grade rocks have not been resolved. The Aniyapuram Mafic–Ultramafic Complex (AMUC), the focus of the present study in southern part of the CSZ, is dominantly composed of peridotites, pyroxenites, gabbros, metagabbros/mafic granulites, hornblendites, amphibolites, plagiogranites, felsic granulites and ferruginous cherts. The rock types in the AMUC are structurally emplaced within hornblende gneiss (TTG) basement rocks and are highly deformed. The geochemical signature of the amphibolites indicates tholeiitic affinity for the protolith with magma generation in island arc-setting. N-MORB normalized pattern of the amphibolites show depletion in HFS-elements (P, Zr, Sm, Ti, and Y) and enrichment of LIL-elements (Rb, Ba, Th, Sr) with negative Nb anomalies suggesting involvement of subduction component in the depleted mantle source and formation in a supra-subduction zone tectonic setting. Our new results when correlated with the available age data suggest that the lithological association of AMUC represent the remnants of the Neoarchean oceanic lithosphere.  相似文献