| This thesis studies the formation timing of the purportedly Neoproterozoic Jiageda Formation and the geochronology and geochemistry of the Neoproterozoic-Early Paleozoic intrusive rocks in the Erguna Massif of NE China,with the aim of constraining the Neoproterozoic-Early Paleozoic tectonic evolution and tectonic nature of the Erguna Massif by using igneous rock associations and their spatial-temporal extent as well as regional tectonic events.Main achievements are as follows:1.Rock associations,formation timing and tectonic nature of the purportedly Neoproterozoic Jiageda FormationOn the basis of detailed field geological analysis for the type section of the Jiageda Formation(JF)in Jiageda village of the Erguna Massif,together with the dating results of detrital and magmatic zircons from metasedimentary and volcanic rocks from the JF,it can be concluded that(1)metasedimentary rocks from the base of the type section of the JF are deposited between 790–738 Ma(i.e.,Neoproterozoic),whereas volcanic rocks from the upper section formed during the Late Mesozoic(158–126 Ma);and(2)quartz schist from the Jiageda Formation in the Badaguan area deposited between 178–155 Ma(i.e.,the late Mesozoic).The new data of the present study indicate that the originally defined Jiageda Formation within the Erguna Massif does not represent a standard stratigraphic sequence,but consists of the metasedimentary and volcanic units with diverse ages ranging from the Neoproterozoic to the Mesozoic,probably representing a tectonic V m élange.2.Neoproterozoic-Early Paleozoic magmatic events in the Erguna MassifZircons from the Neoproterozoic-Early Paleozoic igneous rocks in the Erguna Massif,NE China are euhedral-subhedral in shape and display fine-scale oscillatory growth zoning and striped absorption as well as their high Th/U ratios,implying their magmatic origin.Zircon and titanite LA-ICP-MS U-Pb dating results indicate that(1)the Neoproterozoic magmatisms in the Erguna Massif can be subdivided into seven stages: ~929 Ma,~887 Ma,~847 Ma,~819 Ma,~792 Ma,~764 Ma and ~738 Ma;and(2)the Early Paleozoic magmatic events can be subdivided into four stages,i.e.,Middle-Late Cambrian(504–500 Ma with a peak at ca.500 Ma),Early Ordovician(485–475 Ma with a peak at ca.480 Ma),Middle-Late Ordovician(465–454 Ma with a peak at ca.460 Ma),and Early Silurian(439–434 Ma with a peak at ca.440 Ma).3.Spatial-temporal distribution and rock associations of the Neoproterozoic magmatisms in the Erguna Massif,revealing their relationship with the assembly and breakup of the Rodinia supercontinentBased on the results of this study and previous studies,929–887 Ma intrusive rocks in the Erguna Massif are composed of alkali-feldspar granites,monzogranites and granodiorites,similar geochemically to I-type granites within an active continental margin setting,whereas the 847–738 Ma intrusive rocks consist mainly of granitoids as well as subordinate mafic rocks,including alkali-feldspar granites,syenogranites,monzogranites,granodiorites,gabbro–diorites and gabbros.Chemically,they display an evolutionary trend from high-K calc-alkaline to alkaline,as well as A-type granites and a bimodal igneous rock association.Combined with the global magmatic-tectonic-thermal events in the Neoproterozoic,it is proposed that the intrusive rocks with the ages of 927 Ma and 887 Ma in the Erguna Massif formed during the assembly stage of the Rodinia supercontinent,whereas 847–738 Ma intrusive rocks formed under an extensional environment related to the breakup of the Rodinia supercontinent.In addition,zircon ?Hf(t)values(–5.8~+11.7)and whole-rock ?Nd(t)values(–1.81~–0.27)imply that primary magmas of the 847–738 Ma granitoids could be mainly derived from partial melting of a juvenile lower crust,with a contribution of ancient crustal material in their petrogenesis.Initial 87Sr/86 Sr and ?Nd(t)VI values of mafic intrusive rocks range from 0.7047 to 0.7055 and –1.36 to –0.13,respectively,indicating that their primary magma could be derived from partial melting of a depleted lithospheric mantle.In summary,the Neoproterozoic magmatisms within the Erguna Massif recorded the key information about the assembly and breakup of the Rodinia supercontinent as a significant geological event in the world.4.Spatial-temporal distribution,rock associations and tectonic setting of the Early Paleozoic magmatisms in the Erguna Massif,revealing the tectonic nature of the Erguna Massif during the Early PaleozoicA suite of high-K calc-alkalic and aluminous acid-intermediate intrusive rocks are representative as the ~500 Ma magmatic events in the Erguna Massif,and are composed mainly of syenogranites and monzogranites(including A-type granites),as well as minor monzonites and monzodiorites.The ~480 Ma intrusive rocks are dominated by I-type and A-type granites,and comprise subordinate intermediate–mafic rocks such as syenogranites,monzogranites,granodiorites,monzonites,diorites,gabbro–diorites,and hornblende gabbros,belonging chemically to subalkaline series.The ~460 Ma intrusive rocks are composed of alkalic quartz monzonites,gabbro–diorites,and hornblende gabbros as well as minor calc-alkaline granitoids,displaying a bimodal igneous rock association;The ~440 Ma magmatic event consists of alkalic gabbros in the Alongshan area and granitoids in Shibazhan region,constituting a typical bimodal igneous rock association.On the whole,the Early Paleozoic magmatisms within the Erguna Massif display an evolutionary trend from high-K calc-alkaline to alkaline series,and have the coeval bimodal igneous associationse.Combined with regional tectonic analysis,the Early Paleozoic intrusive rocks within the Erguna Massif formed in a post-collisional extension environment related to the amalgamation of the Erguna and Xing'an massifs.In addition,based on the whole-rock major and trace element data,together with zircon Hf isotope and whole-rock Sr-Nd isotope data,it is suggested that the Early Paleozoic intermediate-acidic intrusive rocks could be derived from partial melting of juvenile crustal material,with a contribution of ancient crustal material,whereas the primary magma of mafic rocks was originated by partial melting of a depleted lithospheric mantle modified by subduction-related fluids.In summary,the Erguna Massif experienced a post-collisional and intraplate extensional environment during Early Paleozoic,obviously different from an active continental margin or island-arc setting within the Xing'an Massif during the Early Paleozoic.5.Crustal nature and tectonic affinity of the Erguna MassifBased on the discovery of the Neoproterozoic geological bodies in present study,geochronological data of detrital zircons from Precambrian strata,and the Hf and Nd isotopic model ages of granitoids within the Erguna Massif,together with the results of previous studies,it can be concluded that the Erguna Massif in fact represents an ancient microcontinent that contains at least Neoarchaean to Paleoproterozoic crystalline basement.The Erguna Massif,together with the adjacent massifs(e.g.Central Mongolia and Tuva massifs)near the southern margin of the Siberian Craton,has an affinity to the Tarim Craton at least in terms of these Neoproterozoic magmatic events and the distributions of Precambrian zircon U-Pb ages,obviously different from the North China and Siberian cratons. |
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