Early Cretaceous Magmatism And Its Constraints On Rare And Rare Earth Metal Mineralization In Kunduleng-Xizhelimu Area,Southern Great Xing’an Range

The Baerzhe deposit（also known as the 801 deposit）,located in the Kunduleng-Xizhelimu area in the southern Great Xing’an Range,is a super-large Zr-Nb-Be-U-Th-REE rare and rare earth metal deposit discovered in China in the 1975.The ore-bearing rock is an Early Cretaceous peralkaline granite（alkaline granite）;in 2014,alkaline rhyolites（referred as riebeckite rhyolite porphyry in this paper）of rare and rare earth metal mineralization in the Early Cretaceous were found in the Aliwula area.Extensive scholarly attention has been devoted to understanding the petrogenesis and mineralization processes of these peralkaline felsic rocks,yielding numerous research findings.However,there is still a great controversy about the origin and metallogenic process of peralkaline felsic magma.Moreover,there is a lack of research on the genetic relationship between the contemporary volcanic-intrusive rocks with different lithology and geochemical characteristics,and the Early Cretaceous magmatism process and petrogenesis model in the study area have not been established.Based on the new theory of magmatic system,through field geological investigation,petrography,isotope chronology,whole-rock and mineral geochemistry,and zircon Hf-O isotope analysis,this paper carried out detailed research on different types of contemporary igneous rocks in the study area,and tried to establish the magmatic system by combining multiple types of magmatic rocks.At the same time,this paper also further established a magma chamber model of peralkaline felsic subvolcanic rocks-intrusive rocks,and limited the enrichment mechanism of rare metal and rare earth elements and the physical and chemical conditions of magma-hydrothermal evolution process through mineral geochemical analysis,fluid inclusion and numerical simulation calculation.The research led to the following conclusions:（1）The rock association and its formation age were identified.The Early Cretaceous volcanic-intrusive rocks in the study area are composed of intermediate rocks,peraluminous and peralkaline felsic rocks,which were formed in a short period between 137-120 Ma.The previously identified 802 alkaline granite and Aliwula alkaline rhyolite have been reclassified in this paper as peralkaline felsic subvolcanic rocks,comprising riebeckite rhyolite porphyry,riebeckite granite porphyry,and riebeckite fine-grained granite.Simultaneously,peralkaline felsic sub-volcanic rocks have been newly discovered in the Baerzhe deposit area.The intermediate rocks were formed in two periods of-137Ma and 126Ma-121Ma;the formation age of peraluminous felsic rocks is between132 and 121Ma;the peralkaline felsic sub-volcanic rocks and 801 arfvedsonite granite were formed between 127-120 Ma.（2）The petrogenesis of different type of rocks was determined,and the magma chamber model of peralkaline felsic subvolcanic rocks and granites was established.The peraluminous intermediate rocks showing low SiO2 content,low Rb/Sr and（La/Yb）N ratios,withεHf（t）between+4.9 and+10.7 and characteristic of LILEs enrichment and Nb,Ta and other elements depletion,were formed by partial melting of fluid metasomatized lithospheric mantle.Peraluminous and peralkaline felsic rocks are characterized by high SiO2 and Na2O+K2O content and low Ca O and Mg O content,with high differentiation index（DI）and iron index（Fe-index）.The high Rb/Sr ratios（0.77-3.35）and positiveεHf（t）values（+6.0-+12.5）suggest that the peraluminous felsic rocks with highly fractionated I-type granites characteristic,are the product of partial melting of K-rich mafic lower crustal materials through magma differentiation.The peralkaline felsic rocks show the characteristics of A-type granite,and theε_Hf（t）values are between+3.1 and+10.6.The trace element characteristics of zircons show that the peralkaline felsic magma and the peraluminous magma are different batches of melts extracted from the same magma chamber.The two-end member mixing model and the oxygen isotope composition of zircons further reveal that the peralkaline felsic magma was formed by the mixing of the peraluminous felsic magma deriving from the K-rich basic lower crust and the deep-source alkali-rich fluid.Whole-rock geochemistry and mineral characteristics reveal that the peralkaline felsic subvolcanic rocks（fine-grained riebeckite granite-granite porphyry-rhyolite porphyry）and the 801 arfvedsonite granite are formed a common magma source.The unmineralized arfvedsonite granite represent the residual crystal-rich mushes,whereas the peralkaline felsic subvolcanic rocks and the mineralized arfvedsonite granite are formed by the extracted interstitial melt from crystal mushes and the mixing of the unerupted residual melt with the residual crystal accumulation,respectively.（3）The Early Cretaceous magmatic system in the study area was constructed.The temporal and spatial distribution,petrography,and mineral composition characteristics of the rock association in the study area,combined with the petrogenesis of different rock series and the crystallization depth indicated by zircon ¹⁷⁶Lu/¹⁷⁷Hf isotope,collectively suggest that there is a transcrustal magmatic system in the study area during the Early Cretaceous.Under the extensional tectonic settings,the upwelling of the asthenosphere promotes the partial melting of the enriched lithosphere.This process gives rise to mantle-derived magma partially erupted or invaded the crust to form andesite-diorite porphyrite and the other part underplating.while promoting the melting of the lower crust material,it also inputs heat and fluid for the felsic magma chamber in the middle-upper crust,which reactivate the felsic magma chamber.The interstitial melt extracted from the felsic crystal mushes directly erupted or invaded the crust to form peraluminous volcanic rocks and peraluminous granite porphyry;another portion of the extracted melt mixed with deep source,giving rise to peralkaline felsic magma.This peralkaline felsic magma emplaced or erupted in the shallow crust,thus forming a series of arfvedsonite granite-riebeckite rhyolite porphyry-granite porphyry-fine grained granite.（4）The mineralization process of peralkaline felsic rocks is clarified.The rare and rare earth metal mineralization process of 801 arfvedsonite granite can be delineated into three distinct stages:source enrichment,magmatic differentiation enrichment,and hydrothermal metasomatism enrichment.The alkali-rich fluid from the deep source brings ore-forming elements,which makes the peralkaline felsic magma have higher Zr,Nb and REE contents.With the progress of magma fractional crystallization,incompatible ore-forming elements are gradually enriched in residual magma.The enrichment of ore-forming elements in peralkaline felsic subvolcanic rocks is mainly achieved by magma fractional crystallization.For the 801 arfvedsonite granite with higher mineralization degree,fluid-melt interaction in the late stage of fractional crystallization is the main reason for its mineralization.The ore-forming elements such as Zr and Nb can be highly enriched in the hydrosilicate magmatic hydrothermal fluid.With the decrease of temperature and the crystallization of albite,Zr precipitates in the form of zircon-quartz aggregates.The transformation from reduced melt to oxidized fluid in the magmatic-hydrothermal stage is the reason for the precipitation of columbite.Magmatic hydrothermal fluids activate and re-enrich ore-forming elements such as REE through metasomatism.Finally,the mixing of atmospheric water triggers the crystallization and precipitation of REE minerals.