?2.6 Ga Tectono-Thermal Event In Western Shandong Province

Early and late Archean tectonic-thermal events were widespread in the western Shandong Province.There are widely distributed plutonic rocks(granitoid rocks)and supracrustal rocks,named as granite-greenstone belt.Based on recent research,Archean basement of the western Shandong has a zonal geographic distribution,which can be divided into A,B and C belts from the northeast to the southwest:a late Neoarchean crustally derived granite belt in the northeast that consists predominantly of 2525-2490 Ma monzogranite and syenogranite(Belt A),a middle to early Neoarchean belt in the center which is mainly composed of 2.75-2.60 Ga TTGs,with some supracrustal rocks and late Neoarchean granitic rocks(Belt B),and a late Neoarchean belt of juvenile rocks in the southwest that is dominated by granodiorite(Belt C).In this thesis,a systematic study was carried out in the Belt B by geological survey,petrology,geochemistry,zircon SHRIMP U-Pb dating and O isotopes.The SHRIMP U-Pb dating results of five gneissic tonalites,six gneissic trodhjemites,one gneissic quartz diorite and one meta-gabbro samples demonstrate that the protolith ages for the magmatic rocks vary from 2.74 Ga to 2.60 Ga and metamorphic and anatectic zircons have ages ranging from 2.63 Ga to 2.59 Ga,with the peak of 2.61 Ga.We show that a strong-2.60 Ga tectono-thermal event occurred in the whole Belt B,as indicated by widely distributed metamorphism,anatexis and magmatism.Based on these evidence:(1)almost continuous magmatic activity occurred from 2.75 to 2.60 Ga,with wide metamorphism and anatexis at?2.6 Ga;(2)?2.6 Ga mafic rocks distributed extensively;(3)there was a "quiet period" between 2.60 Ga and 2.56 Ga,suggesting that 2.60 Ga can be regarded the break between early and late Neoarchean for western Shandong.Although-2.60 Ga metamorphism is only clearly recognized in the WSP until now,we propose that this event also occurred at least in some of other areas of the NCC where the early Neoarchean rocks were well developed.Anatexis was enriched in Taishan area,a typical example for the-2.6 Ga tectonic-thermal event.Therefore,we conducted geochemical studies in Taishan area.All the widespread leucosomes intruded at 2.6 Ga,and are generated by partial melting of gneissic tonalite under water-absent condition,while water-saturated partial melting existed in Xiaolongwan and Caishixi.Nevertheless,partial melting of amphibolites generated potentially small amounts of leucosomes.According to geochemical and microstructural features,the leucosomes were divided into three types,positive Eu anomaly trondhjemitic veins,no Eu anomaly trondhjemitic veins and negative anomaly monzonitic veins,respectively.During the formation process of the three type veins,mineral crystallization differentiation played an important role.Earlier crystallized plagioclases had enriched time and space for forming crystalline framework.When they accumulated,they generated positive Eu anomaly trondhjemitic veins.In the process of ascending accessory melts,plagioclase,K-feldspar and quartz shew a synchronous crystallization,marked by equigranular texture.Due to early stage differentiation of plagioclase,this process resulted in negative Eu anomaly.No Eu anomaly leucosomes,which had no experienced early stage plagioclase differentiation,approximately represent original melts material.Whereas,it still can't exclude the possibility that negtative Eu anomaly melts might be the initial melting products of tonalite.The ?18O(V-SMOW)values of most zircons from analytic samples mainly range between+4.5 ‰ and+6.5 ‰.However,gneissic tonalite in Xiaolongwan has low ?18O(V-SMOW)values of recrystallized zircons and metamorphic rims,significantly lower than the ?18O(V-SMOW)values of magmatic zircons.This suggests that meteoric water must be involved in the metamorphic and anatectic process.The obvious difference in O isotope compositions of zircons from gneissic tonalite and trondhjemitic vein in Xiaolongwan indicate that the trondhjemitic leucosome was not the result of partial melting of the nearby tonalite,although they show a close relationship in field,and the trondhjemitic vein intrusion occurred after anatexis of the tonalite.Mantle plume magmatism played a long-term(2.75?2.6 Ga)effect in western Shandong,however,tectonic setting was different in the nature.At-2.7 Ga,the western Shandong had no or little continental crust,but the partial melting of oceanic plateau root or intra-oceanic subduction might be the key ways for the formation of the-2.7 Ga TTG suites.Mantle plume and overturn caused by upwelling of mantle magma interacted with the continental crust with a certain scale and thickness at 2.6 Ga,leading to adding of mantle materials to the crust,formation of granite by crustal recycling and widespread metamorphism and partial melting.Early Neoarchean metamorphic basement including western Shandong,Anshan-Benxi,eastern Hebei,eastern Shandong et al,consist of eastern block of the North China Craton,which make eatablishing plate tectonics in late Neoarchean possible.2.60 Ga can be regarded the break between early and late Neoarchean for the whole North China Craton.