Magmatic Rocks Of Mesozoic Volcanic Basins In Central Erguna Block

This study focuses on magmatic rocks from volcanic basins in central Erguna Block by systematic analyses on the basement intrusive rocks and volcanic caprocks within the basin,and intrusive rocks in adjacent areas,using LA-ICP-MS zircon U-Pb dating,whole-rock geochemistry,zircon Hf isotope,whole-rock Sr-Nd-Pb isotope,as well as mineral geochemistry of biotite,amphibole,and plagioclase techniques.The ages and compositions of the basement intrusive rocks and volcanic caprocks are constrained,the sources of multi-stage magmas,magma evolution,and petrogenesis of magmatic rocks are deciphered.Integrating the Mesozoic magmatism in Erguna Block,Xiang'an Block,and Mongolia-Russian Transbaikal,the geodynamics of different tectonic evolution stages of the Erguna Block are determined.After the in-depth analyses on magmatism,the nature and tectonic evolution of the volcanic basins in central Erguna Block are determined,and the coupling and genetic relationships between magmatism-basin evolution and Mesozoic geodynamics of the Erguna Block are explored,the main findings are as follows:1.Basement intrusive rocks of the basinsThe ages of basement intrusive rocks include Late Triassic-Middle Triassic(256-238 Ma)and Late Triassic-Early Jurassic(234-182 Ma).Late Triassic-Middle Triassic intrusive rocks mainly include bojites,quartz monzodiorites,quartz monzonites,syenogranites,and monzogranites.Bojites and quartz monzodiorites were derived from lithospheric mantel metasomatized by subduction fluids,showing subduction-related geochemical signatures.Bojites were originated by 2%-3% equilibrium melting of 2% rutile-bearing lherzolites;quartz monzodiorites experienced moderate crustal contamination and fractional crystallization.Quartz monzonites,syenogranites,and monzogranites are high-K calcalkaline I-type granites,and were mainly derived from partial melting of Mesoproterozoic continental crust with the involvement ofVold crustal materials,exhibiting geochemical affinities with magmatic arcs.Late Triassic intrusive rocks are dominated by monzogranites and syenogranites,they were derived from partial melting of Mesoproterozoic with the involvement of old crustal materials.Early Jurassic intrusive rocks are high-K calcalkaline I-type monzogranites that were originated from partial melting of K-rich metabasalt of Mesoproterozoic lower continental crust,with the involvement of minor mantel components,indicating the crust was thickened moderately.The intrusive rocks near the basins are located in the Tayuan area,the eastern margin of the Erguna Block,including Late Triassic syenogranites(210 Ma)and Middle Jurassic quartz monzonites(172-167 Ma).Triassic syenogranites have A-type geochemical characteristics,and were derived from partial melting of felsic magmatic rocks in the shallow crust,implying the eastern Erguna Block was situated in an extensional environment at 210 Ma.Middle Jurassic quartz monzonites are characterized by adakitic affinity,and were originated from partial melting of Meso-Neoproterozoic continental lower crust.The P-T conditions for the primary magma are >15 kbar and 750~950 °C,and indicating the shortening and thickening of the continental crust.2.Volcanic caprocks of the basinsVolcanic caprocks of the volcanic basins are dominated by Early Jurassic(193-175 Ma)and Early Cretaceous(140-112 Ma)rocks.Here focuses on the Xiangyangtun Basin which has the most representative volcanic caprocks.Volcanic caprocks of the Xiangyangtun Basin mainly consist of Early Cretaceous rocks(121-114 Ma).These rocks can be further subdivided to the early-stage basalt-trachyte-rhyolite sequences that are characterized by phenocryst-rich trachyte,and late-stage trachyte-vitrophyric trachyte-volcanic glass sequences that are characterized by phenocryst-free trachyte,obsidian,and pitchstone.The early-stage rocks belong to I-type volcanic rocks,the primary magma was derived from partial melting of the Meso-Neoproterozoic continental crust,and probably experienced fractional crystallization during magma evolution with contamination of old crustal country rocks.The late-stage rocks are highly-fractionated I-type volcanics.With increasing of the fractionation,the geochemical signatures of the rocks evolve to A-type rhyolites.The elemental evolutionary trend from I-type to A-type volcanics indicates that the primary magma has undergone intensive fractional crystallization,and forms the highly-fractionated volcanic rocks at the late-stage.3.Nature and tectonic evolution of the volcanic basinsThe eruption manner of magmas in the basins is dominated by fissure eruptions controlled by NE-trending fault systems.The nature of the basins is extensional fault basins.The basins have experienced three stages of evolution: an initial stage for basin faulting(T3-J1),a compressional state(J2),and an intensive extensional faulting stage that controls the formation of the basins(J3-K1),indicating there different tectonic backgrounds corresponding to the three stages.4.Geodynamics of the Mesozoic magmatismThe spatiotemporal distribution and elemental variation of the Late Permian-Middle Triassic and Late Triassic-Early Jurassic magmatic rocks on both sides of the Mongol-Okhotsk suture belt indicate that the oceanic plate was characterized by bidirectional subduction during these time.The geochemical signatures of Late Permian-Middle Jurassic(256-238 Ma)and Late Triassic-Early Jurassic(234-182 Ma)basement intrusive rocks of the volcanic basins and intrusive rocks from adjacent areas suggest an Andean-type continental arc setting in the south side of the ocean basin.The magmatic lull on the south side of the ocean basin(225-215 Ma)is probably associated with the initiation of crustal thickening during 237-225 Ma.The protracted magmatism(210 Ma to Early Jurassic)is intensive in the Erguna and Xing'an blocks.The eastern edge of the Erguna Block and the Xinag'an Block were situated in a back-arc setting at least after 216 Ma,and the setting controlled the initial faulting of the volcanic basins.During the Middle Jurassic the continental crust of the Erguna Block was thickened as triggered by the continuous southward subduction of the Mongol-Okhotsk oceanic plate,and accompanied by the intrusive of high-K calcalkaline I-type granites.The continental crust was intensively shortened and thickened with the formation of thickened lower continental crust-derived adakitic rocks.The volcanic basins were uplifted and eroded with developing compressional structures.The Great Xing'an Range archives the most significant magmatic gap since the Late Permian(177-167 Ma),indicating the western Mongol-Okhotsk oceanic basin was closed at this time.During the Late Jurassic-Early Cretaceous,The Erguna Block was dominated mainly by the post-collisional extension of the Mongol-Okhotsk tectonic regime.The Early Cretaceous highly-fractionated volcanic rocks in the volcanic basins are similar to those highly-fractionated granites developed in typical extensional areas worldwide.The volcanic basins evolve to the intensive extensional faulting stage,the voluminous volcanic rocks are the products of fissures eruptions controlled by NE-trending extensional fault systems.