Late Mesozoic Magmatism And Tectonic Setting Of Bairin Left Banner-Jarud Banner Area In The Southern Great Xing'an Range

The Balinzuo Banner-Jarud Banner area is located in the eastern part of Inner Mongolia and geographically southern part of the Great Xing'an Range.The products of intensive late Mesozoic magmatism in this area constitute part of a NE-trending magmatism belt that extends across northeast China and its adjacent area.Currently,the tectonic evolution of the Great Xing'an Range in the Mesozoic remains a matter of debate.The key disagreement is on the spatial scope and time range of the influence from the Mongol-Okhotsk tectonic system and Paleo-Pacific tectonic system.The relatively large distance between the study area and the Mongol-Okhotsk suture zone or Paleo-Pacific subduction zone makes it an ideal region for the research on the transformation of remote effects of the Mongol-Okhotsk tectonic system and Paleo-Pacific tectonic system.In this study we present petrographical,geochemical,geochronological and zircon Hf isotopic data of late Mesozoic magmatic rocks in the Balinzuo Banner-Jarud Banner area.These data provide new insights into the geochronological framework,time-space distribution,petrogenesis and tectonic setting of late Mesozoic magmatic rocks in this area.The present results,together with the basin evolution,faults and time-space distribution of late Mesozoic magmatic rocks in northeast China,are used to reveal the tectonic and magmatic evolution of the southern part of the Great Xing'an Range in the late Mesozoic.The volcanic rocks in the study area are classified in terms of ages and lithology.Combined with zircon U-Pb ages of intrusive rocks and other previously published geochronological data in the area,late Mesozoic magmatism in the study area is divided into three periods,which are the late Jurassic(a peak at 154Ma),the early Early Cretaceous(two peaks at 140 Ma and 130Ma),and the late Early Cretaceous(a peak at 125Ma).It is noted that there is a brief gap between the late Jurassic and the early Cretaceous.Magmatism significantly intensified in the early Cretaceous.The late Jurassic intrusive rocks are dominated by syenogranites,monzonites,quartz monzonites and granodiorites,but also contain some granite porphyries.Among these rocks,granodiorites are recognized as I-type granites,while quartz monzonites and granite porphyries are recognized as A-type granites.The zircon ?Hf(t)values of intrusive rocks range from 3.1~11.6,with a TDM2 age of 586Ma~1369Ma.Contemporaneous acidic volcanic rocks consist of rhyolites and rhyolitic crystal tuffs.Rhyolitic crystal tuffs(sample 416TW16,164Ma)from Xinmin formation have similar geochemical features with S-type granites.Rhyolites(sample 16TW02,162Ma)are recognized as highly fractionated I-type rhyolites,while other acidic volcanic rocks are A-type rhyolites.The zircon ?Hf(t)values of volcanic rocks range from-0.7~12.1,with a TDM2 age of 549Ma~1714Ma.Based on these pieces of information,it is suggested that the primary magma of these magmatic rocks results from partial melting of newly-formed lower crust.As we investigate the early-middle Jurassic calc-alkaline rocks and subduction-related deposits,their emplacement or ore-forming ages become younger from Erguna massif to the northern part of Xing'an massif and the middle part of the Great Xing'an Range.It suggests that the early Jurassic magmatic rocks were formed in association with an active continental margin created by the southeastward subduction of Mongol-Okhotsk oceanic plate.The occurrences of middle Jurassic S-type muscovite monzogranites and C-type adakitic rocks are associated with the scissors-like closure of the western part of Mongol-Okhotsk ocean(to the west of 120° east longitude).This closure is also responsible for contemporaneous metamorphic events and thrust nappe events in northern Hebei and western Liaoning.The late Jurassic magmatic rocks in the southern part of the Great Xing'an Range all plot in the field of post-collisional granites in the tectonic discrimination diagram.In the early period,magmatic rocks are dominated by I-type granites,highly fractionated I-type rhyolites and rhyolitic crystal tuffs with characteristics of S-type granites.It indicates that the early late Jurassic magmatic rocks were formed in a transition stage from compression to extension environment in which the thickened crust started to collapse.When it comes to the middle and late period of late Jurassic,magmatic rocks consist of A-type granites or rhyolites,suggesting a full-scale extension environment.We conclude that magmatic rocks in the southern part of the Great Xing'an Range were formed in a back-arc extensional environment created by the retreat of subducted slab which occurred after the southward subduction of the eastern part of Mongol-Okhotsk ocean(to the east of 120° east longitude).The early Early Cretaceous intrusive rocks are composed of quartz monzonite porphyries,diorites,quartz diorites,granodiorites,syenogranites,and monzonites.The zircon ?Hf(t)values of Derite and Babaoshan monzonites,which are recognized as I-type granites,range from-1.6~3.1 and-11.7~2.6,with TDM2 ages of 918~1776Ma and 1385~2678Ma,respectively,suggesting that the magma resulted from partial melting of newly formed lower crust.Quartz monzonite porphyries and granodiorites are recognized as adakitic rocks,with relatively low Mg O contents and Mg# values.Their zircon ?Hf(t)values range from 2.3~5.8 and 3.0~8.3,with TDM2 ages of 565~900Ma and 868~1355Ma,respectively,suggesting that the magma was generated by partial melting of lower crust during thickening of the crust.Contemporaneous intermediate-acid volcanic rocks are classified as calc-alkaline series with features of arc-type volcanic rocks.Their ?Hf(t)values(ranging from-11.6~13)show that they are products of partial melting of lithosphere mantle metasomatized by fluids relating to subduction.Generally,they were formed in an active continental margin environment,and some material from ancient crust was introduced during the emplacement of the magma.The existence of ancient captured zircons in early Early Cretaceous magmatic rocks and their negative ?Hf(t)values indicate that some material from ancient crust is involved during evolution of the magma,as evidenced by the existence of material from ancient crust in middle and southern part of the Great Xing'an Range.The Sr-Nd-Pb isotopic data of early Cretaceous magma in the southern part of the Great Xing'an Range imply that the mantle source region is metasomatized by subduction-related fluids.Together with contemporaneous tectonic events in the region,it is concluded that the early Early Cretaceous magmatism occurs in association with the Flat-slab subduction of Mongol-Okhotsk ocean.Alkali-feldspar granites,syenogranites and granite porphyries constitute the late Early Cretaceous intrusive rocks,which are classified as A-type granites and highly fractionated I-type granites.Contemporaneous acidic volcanic rocks are dominated by rhyolites and rhyolitic crystal tuffs which have similar geochemical characteristics with A-type rhyolites.The zircon ?Hf(t)values of intrusive and volcanic rocks range from-4.6~8.6 and-0.1~10.2,with TDM2 ages of 842~2035Ma and 692~1634Ma,suggesting that the magma was caused by partial melting of newly formed lower crust.These features reveal the existence of extensional environment across the region,as corroborated by widespread A-type granites,metamorphic core complex and rift basins in the region.The peak age of early Cretaceous magmatism in the southern part of the Great Xing'an Range differs from that of Songliao Basin and its adjacent region to the east.Moreover,the ages of magmatism in the Great Xing'an Range become younger from south to north,while the ages of magmatism in Songliao basin and its adjacent region to the east get younger from east to the west.Together with geophysical data,it is concluded that the late Early Cretaceous magmatism in the Great Xing'an Range is mostly caused by the subduction of Paleo-Pacific plate toward Eurasia plate and the large-scale upwelling of asthenosphere after Mongol-Okhotsk oceanic plate collapsed.In specific,the late Early Cretaceous magmatism(125Ma)in the study area is more related to the large-scale upwelling of asthenosphere.This study shows how the southward subduction of Mongol-Okhotsk ocean evolve in space and time.During the late Permian-Triassic,the Mongol-Okhotsk oceanic plate subducted southeast,leading to the occurrence of active continental margin in the northern part of Xing'an massif and Erguna massif.As the scissor-like closure of Mongol-Okhotsk ocean occurred in the late Triassic,starting from west to east,the influence of subducted oceanic crust extended southeast,reaching the southern part of the Great Xing'an Range in the late middle Jurassic.During the late middle Jurassic-late Jurassic,the eastern part of Mongol-Okhotsk ocean gradually closed.With the subduction zone migrating northeast,the subduction slab retreated,resulting in the back-arc extensional environment in the Great Xing'an Range and the north margin of North China craton.During the late Jurassic-early Early Cretaceous,the closure of Mongol-Okhotsk ocean accelerated,leading to the southward Flat-slab subduction of oceanic slab.Finally,the subducted slab was underneath the southern part of the Great Xing'an Range.In the late Early Cretaceous,the closure of entire Mongol-Okhotsk ocean finally completed,followed by the collapse of subducted oceanic slab starting from south to north,which is responsible for the extensional environment in the Great Xing'an Range and the north margin of North China craton.