Age of the Marwar Supergroup,NW India:A note on the U–Pb geochronology of Jodhpur Group felsic volcanics

The Marwar Supergroup(NW Peninsular India)is thought to be of Ediacaran-Cambrian age,based on previous paleontological and geochronological studies.However,direct constraints on the onset of sedimentation within the Marwar basin are still scarce.In this study,we report U–Pb zircon,LA-ICP-MS,and SIMS ages from the Chhoti Khatu felsic volcanic rocks,interlayered with the Jodhpur Group sandstones(Lower Marwar Supergroup).The cathodoluminescence images of the zircons indicate complex morphologies,and core-rim textures coupled with the wide range of ages indicate that they are likely inherited or in the case of thin poorly indurated ash-beds,detrital in origin.The age spectra of 68 zircon analyses from our sampling display a dominant 800–900 Ma age peak corresponding to the age of basement"Erinpura granite"rocks in the region.The youngest inherited zircon from a felsic ash layer yielded a U–Pb age of651 Ma±18 Ma that,together with previous studies and paleontological evidence,indicates a postCryogenian age for the initiation of Marwar sedimentation following a~125 Ma hiatus between the end of Malani magmatism and Marwar deposition.

Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision

This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa.Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision.Magnetite is identified as the main magnetic carrier in our study.The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations.A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks.The authors interpret this AMS fabric to have formed during intrusion rather than deformation.Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities.A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation.The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N±5.8°N for the southern margin of Asia near Lhasa.The paleolatitude corresponds to an in-between position of the Lhasa terrane during about 130‒60 Ma.Furthermore,the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18°±9°for the sampling location since about 83 Ma.In the light of tectonic setting of the dykes,the strike of the southern margin of Asia near Lhasa is restored to trend approximately about 310°,which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.

Strange attractors, spiritual interlopers and lonely wanderers:The search for pre-Pangean supercontinents

The observation is made that there are very strong similarities between the supercontinents Columbia, Rodinia and Pangea. If plate tectonics was operating over the past 2.5 billion years of Earth history, and dominated by extroversion and introversion of ocean basins, it would be unusual for three superconti-nents to resemble one another so closely. The term＇strange attractor＇ is applied to landmasses that form a coherent geometry in all three supercontinents. Baltica, Laurentia and Siberia form a group of＇strange attractors＇ as do the elements of East Gondwana （India, Australia, Antarctica, Madagascar）. The elements of ＂West Gondwana＂ are positioned as a slightly looser amalgam of cratonic blocks in all three super-continents and are referred to as ＇spiritual interlopers＇. Relatively few landmasses （the South China, North China, Kalahari and perhaps Tarim cratons） are positioned in distinct locations within each of the three supercontinents and these are referred to as＇lonely wanderers＇. 〈br〉 There may be several explanations for why these supercontinents show such remarkable similarities. One possibility is that modern-style plate tectonics did not begin until the late Neoproterozoic and horizontal motions were restricted and a vertical style of ＇lid tectonics＇ dominated. If motions were limited for most of the Proterozoic, it would explain the remarkable similarities seen in the Columbia and Rodinia supercontinents, but would still require the strange attractors to rift, drift and return to approximately the same geometry within Pangea. 〈br〉 A second possibility is that our views of older supercontinents are shaped by well-known connections documented for the most recent supercontinent, Pangea. It is intriguing that three of the four ＇lonely wanderers＇ （Tarim, North China, South China） did not unite until just before, or slightly after the breakup of Pangea. The fourth＇lonely wanderer＇, the Kalahari （and core Kaapvaal） craton has a somewhat unique Archean-age geology compared to its nearest neighbors in Gondwana, but very similar to that in western Australia.

The Origin and Tectonic Significance of the Volcanic Rocks of the Yeba Formation in the Gangdese Magmatic Belt, South Tibet

Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174–170 Ma. Positive and variable zircon ε_（Hf）（t） values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements（V and Cr） and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements（Rb, Zr and Hf）. According to the discrimination diagrams of P_2O_5-SiO_2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to Ⅰ-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.

The drift history of the Dharwar Craton and India from 2.37 Ga to 1.01 Ga with refinements for an initial Rodinia configuration

Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement.To reconstruct the Proterozoic paleogeographic history of Peninsular India,we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad.Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga,2.22 Ga,2.08 Ga,1.89-1.86 Ga,1.79 Ga,and a previously undated dual polarity magnetization.We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89-1.86 Ga swarm(s),and a new inverse baked contact test for the 2.08 Ga swarm.Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole(43.1°N,184.5°E;A95=4.3°)to a reliability score of R=7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole(VGP)at 33.0°N,347.5°E(a95=16.9°,k=221,N=2).The new VGP for the Dharwar Craton provides support for the union of the Dharwar,Singhbhum,and Bastar Cratons in the Southern India Block by at least 1.79 Ga.Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6°N,233.1°E(A95=9.2°,N=18;R=5).Two dykes from this group yielded 1.05-1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole.A comparison of the previously published poles with our new 1.05-1.01 Ga pole shows India shifting from equatorial to higher(southerly)latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.

Magnetic field hyperactivity during the early Neoproterozoic: A paleomagnetic and cyclostratigraphic study of the Katav Formation, southern Urals, Russia

We present a detailed magnetostratigraphic and cyclostratigraphic profile through the Riphean(Tonian)Katav Formation in the southern Urals.The study confirms the primary nature of the magnetization in these rocks.The cyclostratigraphic study identified several orbital periods including the 405 ka long eccentricity.This allows us to quantify the reversal frequency in the Katav and our estimates range of 7–12 reversals per million years.Based on our study,we identify an interval of magnetic field reversal hyperactivity in the Neoproterozoic interval.Age estimates for the Katav are contentious and range somewhere between 800 Ma and 900 Ma based on carbonate Pb-Pb ages and stable isotope correlations.The paleomagnetic poles obtained in this study of the Katav(and overlying Inzer)Formation do not fit anywhere on the Baltica apparent polar wander path between 1100 Ma and 900 Ma.Furthermore,they lie 90away from the 900 Ma segment of the path.We tentatively estimate their age to be closer to 800 Ma and perhaps confirm a previously hypothesized pulse of rapid true polar wander between 825 Ma and 790 Ma.