Petrogenesis And Tectonic Implication Of Granitic-Mafic Rocks Along Dawu-Luhuo Area Within Songpan-Ganzi Terrane

Songpan-Ganzi(SPGZ)terrane is located on the northeastern margin of the Qinghai-Tibet Plateau,and is enclosed in a huge inverted triangle between the terranes of West Qinling,Qaidam,Kunlun and North Qiangtang terranes and the cratons of North China and South China.The SPGZ terrane is called“Geological Bermuda”,covered by the extremely thick Triassic flysch deposits,which makes it difficult to discuss its basement property,subduction-collision process and magmatism history.The key issues such as the basement property,tectonic evolution and petrogenetic model of the Mesozoic magmatic rocks within SPGZ are still controversial.In this paper,the Late Triassic magmatic rocks in the southern SPGZ are selected as the research object.The granitic-mafic rocks distributed along Dawu-Luhuo area are studied systematically in the field,petrology,geochronology,mineral and whole rock geochemistry.By using these data,this study discussed their composition diversity and tectonic background,revealed the basement property of SPGZ,established the geochronological framework and genetic relationship between granitoids and mafic rocks,and proposed a new Mesozoic tectonic evolution model for the northeastern Paleotethys which the SPGZ is located.The main research results are as follows:(1)Zircon U-Pb dating results show that the Dawu granodiorite was formed at 223.5?217.4Ma,and the Luhuo monzonite crystallization age is 219.4 Ma.Granitic samples are I-type granitoids,depleted in Ba,Nb,Ta,Ti elements and displaying the characteristics of active continental margin and volcanic arc granite.Luhuo monzonites have high Ba and Sr contents,relatively high and uniform?_Hf(t)value(–3.69?–1.65),indicated that they are formed by partial melting of the enriched juvenile crustal sources.Dawu granodiorites have dark inclusions and dispersed?_Hf(t)value(–13.51?0.41)which indicate that they were formed in the mixing of mantle and ancient crust derived melt.Based on the regional information,this study believes that the Ganzi-Litang ocean not only subducted to the south,but also had the history of north subduction.The Late Triassic Dawu-Luhuo granitoids were formed by the subduction of Ganzi-Litang ocean under the Songpan-Ganzi terrane.(2)According to the differences of geochemical component,the mafic rocks exposed along Dawu-Luhuo area can be divided into four groups with various characteristic,which can restrict the subduction feature and the process of Ganzi-Litang ocean.The weighted mean age of group one is 223.0±3.8 Ma,which has the characteristics of N-MORB,a few samples with arc features fall into the continental back-arc tholeiite area.Some samples have ultra high?_Nd(t)values(8.73-9.43),which are considered to be derived from the remelting of depleted mantle.The group three mafic rocks have OIB characteristics,enriched in high field strength elements such as Nb,Ta,Zr,Hf,obtained the weighted mean age of 218.4±7.6 Ma,Their?_Hf(t)value(2.29?4.48)and(La/Nb)_MORB(0.55?0.86)are similar to those of Emeishan mantle plume high Ti basalts,but their low Ti O₂ content(1.06%?1.56%)and the positive correlation between Ti O₂and Fe O^T/Mg O indicate that they have tholeiite characteristics,similar to the active continental margin basalt.Their source may be the previous enrichment component remains in the mantle.The composition of the group two mafic rocks straddles between the group one and group three mafic rocks,most of them fall into the E-MORB area,showing the characteristics of magmatic mixing.The group four samples have IAT characteristics,enriched in Ba,Sr,Pb,Th,U elements and depleted in Nb,Ta,Ti elements,obtained the weighted mean age of 218.9±3.5 Ma.Their high?_Hf(t)value(4.61?5.39)show they are formed in the continental back-arc with the dual characteristics of MORB and IAT,derived from a metamorphic mantle wedge mixed with depleted mantle.The mafic rocks in this area formed at the same time but have various geochemical components,which may be controlled by oceanic ridge subduction.This study believe that the SPGZ experienced the ridge subduction of the Ganzi-Litang ocean during Late Triassic.The depleted asthenosphere upwelling through“Slab window”led to the hybrid and melting of different components in the mantle wedge,forming these four groups of compemporaneous,but different geochemically characterized mafic rocks.In addition,the underplating of a large amount of basaltic magma caused the partial melting of the various depths of crust,forming abundant granitic intrusions within SPGZ terrane,and developed the Dawu-Luhuo back-arc rift(which may be the precursor of the Xianshuihe fault).(3)The Granitic and mafic rocks not only provide important evidence for the discussion of tectonic evolution,but also carry a lot of basement information.The zircons in the granodiorite obtained the two-stage Hf model age of 2.2?1.2 Ga,indicating that there may be an unexposed Proterozoic basement.A large number of ancient zircons have been found in several late Triassic mafic rocks(n>20),with the age consistent with the age of the ctustal growth and tectonic thermal events in the Yangtze Craton.Abundant Paleoproterozoic zircons are found in 19LD3-15basalt and yield mean age of 2475±14 Ma and upper intercept age of 2484±18 Ma,which is consistent with the age of the ancient zircon core in Menggu granite within error(2459±41Ma),may represent the same Paleoproterozoic basement.The captured zircons from Qiuluo basalt(21YL19-1)reveaved Archean basement information firstly,the main age of 2.8?2.7 Ga of zircons,yield mean age of 2779±18 Ma and upper intercept age of 2811±24 Ma.These Archean to Proterozoic zircons can be compared with the Yudongzi complex in the western Yangtze craton.The Lu-Hf isotopic evidence of captured zircons revealed that SPGZ terrane experienced the evolution history including the crust growth at 2.8?2.7 Ga(?_Hf(t)=0.30?3.64),reworking of ancient crustal and crust growth at 2.5?2.3 Ga(?_Hf(t)=-2.3?8.0),significant reworking of ancient crustal at 2.0?1.8 Ga(?_Hf(t)=-13.0?3.8)and the formation of Neoproterozoic crust at 1.0?0.7 Ga(?_Hf(t)=-11.6?12.7).Combined with multidisciplinary research results,this study proposed that the SPGZ terrane has the continental crustal basement and evolution history similar to that of the Yudongzi complex which is likely to be the westward extension of the Yangtze Craton.The Triassic SPGZ basin is a large back-arc deposit basin developed on the Archean-proterozoic ancient basement,affected by the northward subduction of the Ganzi-Litang ocean.The Ganzi-Litang ocean occurred ridge subduction at the Late Triassic which triggered a large-scale magmatic event in this region.