| The Neoproterozoic era,bridging the Proterozoic and the Phanerozoic,witnessed a series of significant events,including the extreme climate,i.e.,"Snowball Earth"events and the transition from supercontinent Rodinia to Gondwanaland.However,many critical scientific questions remain regarding both the supercontinent evolution and "Snowball Earth" events.1)The lack of reliable isotopic(e.g.,Sr)and geochemical signatures from the cap dolostones(transgressive system tracts)of post-glaciation,precludes detailed understanding of the paleooceanography or paleoenvironment change during the transition from glacial to interglacial states;2)How to reconcile the discordance over the Australia-Laurentia connection near the center of Rodinia,Meanwhile,as one important craton within the supercontinent,Tarim Block's paleoposition during this interval has not been well constrained.In this dissertation,we conducted a systematic field,stable isotopic(C-O?Sr and Mg),and paleomagnetic study of the well exposed Neoproterozoic successions that contain un-deformed glacial deposits in the Aksu-Wushi area of northwestern margin of Tarim Block.According to the detailed field investigations,including cross-section measuring in key sections,a new Cryogenian diamictite and atop cap dolostones,i.e.,diamictite-cap carbonate(DCC)couplet is discovered for the first time.The newly discovered cap carbonate fills the gap that was postulated to exist between the underlying diamictites and the overlying strata in previoius studies.Analyses of the regional stratigraphy,sedimentological characteristics,paleontological markers,as well as the available radiometric ages allow to establish a complete Neoproterozoic stratigraphic framework.Within this framework,the DCC couplet is correlated to the Marinoan glaciation of ca.640 Ma and the subsequent deglaciation.In order to characterize the paleooceanography or paleoenvironment after Marinoan glaciation,besides the measurements of stable C isotopes,we analyzed the Sr and Mg isotopes of the cap dolostones of the DCC couplet using a newly developed step-leaching technique.This technique can effectively extract primary geochemical signatures from the cap dolostones by eliminating contaminations from diagenitic or later alterations.To better understand the correlations,additional Sr-isotope data were extracted from the coeval cap dolostones(i.e.,the Zhamoketi Formation)from the Quruqtagh area(NE Tarim)as well.The cap dolostones of the DCC show negative C-isotope(?13C)values in a range of ca.-2.2?-2.5‰,typical of Marinoan cap carbonates.The 87Sr/86Sr and ?26Mg values can be categorized into two groups:samples from the lower dolostone have average values of 0.7079 ± 0.0007(1?)for 87Sr/86Sr ratios and-1.46 ± 0.01‰(1?)for?26Mg,respectively;and samples from the upper subunit have average values of 0.70687 ± 0.0003(la)for 87Sr/86Sr and-1.60 ± 0.06‰(la)for ?26Mg.Based on these isotope features,the lower and upper cap-dolostone subunits of the DCC may correspond to the glacial meltwater plume and overturn ocean deposition stages,respectively.Also,the near overlaps of 87Sr/86Sr ratios between the Zhamoketi cap dolostones and the lower cap dolostones of the DCC may suggest that the two cap dolostones were formed in a same water plume.The slight discrepancies in 87Sr/86Sr and ?26Mg of Marinoan cap dolostones from Tarim and Australia/Mongolia,appear to suggest different searwater environments where cap dolostones were precipitated.We carried out detailed paleomagnetic and rock paleomagnetic investigations of the Neoproterozoic successions that contain Cryogenian diamictite sequence of Qiaoenbrak Formation(Fm)and early Ediacaran Sugetbrak Fm at six sections in the study area.The stable characteristic remanent magnetizations(ChRMs),obtained from the diamictite portion within Qiaoenbrak Fm,-correspond to a paleopole at ?=6.3°S,? = 17.50E,A95= 9.1° with a paleolatitude of 16.3 ± 5.6°;while the ChRMs from the ' cap carbonate' and red sandetones of early Ediacaran Sugetbrak Fm(lower unit)yield a younger paleopole at ?=21.1°S,?=87.40E,A95=7.0° with a paleolatitude of 27.4 ± 5.60.Based on both stability tests and rock magnetic experiments,the two paleopoles are likely of primary origin.Combined stratigraphy with recent zircon U-Pb chronologic data,their ages are approximately assigned to?730 Ma and?630Ma,respectively.The two poles document a large-scale rotation of Tarim at near-constant paleolatitudes during the Cryogenian time,and the rotation is coeval with Rodinia breakup.Together with newly reported geological constraints,Tarim Block possibly occur either together with South China in the between Australia-East Antarctica and Laurentia,or by itself alone in that role,forming an extended or alternative "missing-link" configuration for Rodinia.At the same time,indications of subduction-related magmatism in Tarim's Neoproterozoic record suggest that Rodinia breakup was dynamically linked to subduction retreat along its northern margin.Such a model is akin to early stages of Jurassic fragmentation within southern Gondwana,and implies more complicated dynamics of supercontinent breakup than superplume impingement alone.Additionally,the mid-to low-paleolatitudes,of Neoproterozoic glacial deposits or cap carbonates atop the diamictites determined from paleomagnetic data,provide new supporting evidence for the Neoproterozoic "Snowball Earth" hypothesis. |
PDF Full Text Request