| The Anshan-Benxi greenstone belt(ABGB),locating in the northeastern North China Craton(NCC),preserves Eoarchean to Neoarchean geological signatures and widespread economic BIF deposits,and therefore attracts lots of attention.Based on the geochronological,petrographical,geochemical and tectonic studies on the late Neoarchean supracrustal rocks in the ABGB,this thesis systematically studies the lithological assemblages,petrogenesises,formation ages,tectonic setting and crustal growth of the Late Neoarchean ABGB.The late Neoarchean meta-volcanic rocks of the ABGB are dominantly exposed in the Waitoushan-Gongchangling-Benxi(WGB)areas,and are chiefly composed of plagioclase amphibolites,mica/amphibole plagioclase gneisses and amphibole/mica two-feldspar gneisses,while the Anshan area developes rare metamorphic volcanic rocks and comprises chlorite schists,chlorite quartzite schists and plagioclase amphibolites.Protoliths of these meta-volcanic rocks can be geochemically and petrographically classified as tholeiites(TH),tholeiitic to calc-alkaline basalts(THCB),andesite and dacite-rhyolite.TH basalts exhibit tholeiitic compositions and are characterized by approximately flat REE and trace element patterns with negligible Eu and Nb anomalies,which probably represent hybrid mixtures of N-MROB and arc-like compositions and are equivalent to back-arc basin basalts(BABB).THCB basalts display tholeiitic to calc-alkaline compositions,slightly right-declined REE patterns and obvious negative Nb anomalies as well as nearly consistent(Hf/Sm)PM and Ba/Th ratios,which were produced by partial melting of mantle peridotites previously metasomatized by subducted slab-released melts and minor fluid and were comparable to island arc basalt(IAB).The andesites exhibit distinctly high MgO,Mg#and transition metal elements(e.g.,Cr,Ni and Co),low FeOT/MgO ratios,fractionated REE patterns and evidently negative Nb,Ta and Ti anomalies,which are akin to Phanerozoic Setouchi sanukoid high magnesian andesites(HMAs)and-2.7 Ga Wawa greenstone belt high magnesian andesites and were melts of mantle peridotites previously assimilated by subducted-slab melts/fluids.The dacite-rhyolites possess the highest SiO2 and the lowest MgO,Cr and Ni contents,and were derived from intracrustal recycling.Integrating with geochronology,lithological associations,lithologic spatial distributions and metamorphisms,supracrustal rocks in the AGBG exhibit distinct spatial-temporal zoning distributions as follows:(i)the WGB areas,northeastern ABGB,are primarily composed of～2.57-2.52 Ga arc-related BADR,～2.55-2.52 Ga BABB,multiple thin layers of BIF and relatively lesser～2.54-2.51 Ga meta-sedimentary rocks,with up to high amphibolite facies metamorphism,and(ii)the Anshan area develops abundant～2.56-2.51 Ga meta-sedimentary rocks,massive and thick BIFs,and minor～2.56-2.53 Ga arc BADR and BABBs,with upper greenschist to low amphibolite facies metamorphisms.Such differences suggest that the WGB areas were closer to centers of volcanisms while the Anshan area correlates to a more stable sedimentary environment and was far away from volcanic centers.Therefore,we propose that the Anshan area represents a typical back-arc basin environment while the WGB areas are highly likely to be an arc-back arc transition zone,indicative of a roughly south-dipping subduction polarity.Collectively,geodynamic evolutions for the late Neoarchean ABGB are as follows:firstly,a subduction initiated before～2.57 Ga and produced arc-related magmatisms in the WGB areas and north Liaoning province during～2.57-2.52 Ga,e.g.,arc basalts,HMAs and dacite-rhyolites.Secondly,a back-arc basin opened at least before-2.55 Ga and generated abundant BABBs in the WGB areas,which was probably closed at-2.52 Ga.Subsequently,an arc-continent collision launched and resulted in widespread potassium-rich granites during～2.52-2.49 Ga,accompanied with～2.51-2.48 Ga metamorphisms.The whole ABGB records an episode of significant crustal growth and intra-crustal recycling during the late Neoarchean. |
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