| Our study area,Taili,where belong to the Liaoning Province,located in the eastern part of the north margin of the North China Craton?NCC?.It is an ideal place to study the natural rheology of rocks due to the intensive and extensive deformation.The Taili area is dominated by granitic rocks.According to the detailed U-Pb zircon age,four stages of the magmatic activity occurred in the area,including the late Archean granitic gneiss??2.5 Ga?and?hornblende?biotite plagioclase gneiss,the late Triassic??222Ma?porphyritic granitic gneiss,felsic leptynite and granitic aplite,the late Jurassic??156 Ma?biotite monzogranite and some mafic veins,and the early cretaceous??140Ma?intermediate-mafic dykes.In general,the Taili is characterized by NE-SW sinistral shear with intensive deformation behaviors as indicated by a series of shear deformation symbols,such as S-C fabrics,boundinage structures,rotated feldspar porphyroclasts etc.Three stages of deformation can be subdivided.The first stage is featured by a steep,generally NE striking granitic gneiss and?hornblende?biotite plagioclase gneiss.Rootless fold and melt facies are well-developed pointing to the anatexis magmatism within amphibolite facies conditions,reflecting that the deformation formed at the depth of?25–21 km.The early zircon have high?Hf?t?values ranging from-4.4 to+6.5,with corresponding TDM2=3.0–2.6 Ga,possibly indicate their derivation from the mantle materials associated with extensive dehydration,and subsequently influenced by the young crustal which account for the a"shell source"in geochemical signatures.The second stage of deformation limit to the central part,manifested by the Triassic porphyritic granitic gneiss,felsic leptynite and granitic aplite.Ductile shear and mylonites developed,concomitant with greenschist facie metamorphism?450–550°C?.The porphyritic granitic gneiss display typical sinistral shear,indicating the deformation formed at the depth of?16–12 km.Variable deformation patterns in these granitic rocks with a clear transition from NE to NEE-trending.Contemporaneous felsic veins show uniform deformation behaviors with variable morphology features,which can be basically divided into the NS,NE and NW directions.The Triassic porphyritic granitic gneiss and felsic leptynite show typical adakitic signature as evidenced by the high K2O/Na2O values?>0.5?,Sr/Y and?La/Yb?N,reflecting the thickening lithosphere relate to the Indosinian orogenesis in region.These Triassic adakitic rocks have low?Hf?t?values of-14.1 and-22.7,with corresponding TDM2=2.1–1.8 Ga.Their?Nd?t?range between-12.8 and-13.1,with TDM2of?2.0 Ga,indicating that the Mesozoic granitic rocks are affected by a addition of ancient lower crustal materials.The last stage deformation mainly occurred in the late Jurassic biotite monzogranite,which was dominated by brittle and ductile deformation.Faults,joints and cleavage are well developed,reflecting the deformation formed at the depth of<8km.The biotite monzogranite have low?Hf?t?values of-29.36 and-38.08,indicating the ancient crustal involved into the magmatism.Mafic veins are developed in the region with insignificant deformation,pointing to the deformation occurred at the depth of 0–3 km.Overall,the tectonic deformation was obviously stronger than that in the south.From north to south,the compression direction changed from NW to nearly NS.We obtain the stress difference?51.2 Mpa?of the quartz in biotite monzogranite by using dislocation density method,which is larger than that in the north?35–38Mpa?.All the results in Taili are larger than the contemporaneous deformed rocks in the Yiwulushan area?North of the Taili??9.6–13.9 Mpa?,indicating a deformation trend from strong to the weak from the south to north along the eastern margin of NCC.The Flinn values?K=[ln?X/Y?/ln?Y/Z?]?)of the granitic rocks ranging from 1.26to 3.25,pointing to the general shear.The granitic gneiss and biotite monzogranite have high vorticity>0.71,and 0.21–0.66 of the porphyritic granitic gneiss,reflecting the early is dominated by the simple shear and pure shear for the later.Viscosity ratios??a/?b?between the granitic gneiss and porphyritic granitic gneiss are 3.5–4.67,and felsic veins and?hornblende?biotite plagioclase gneiss are 2.8–6.Combined with detailed field observations,the porphyritic granitic gneiss is stronger than that of early?hornblende?biotite plagioclase gneiss,granitic aplite and garnet-bearing granitic aplite show stronger competency than that of granitic gneiss and?hornblende?biotite plagioclase gneiss.The grain size is the dominant factor controlling the rheology,i.e.,the larger the particle size,the higher the rock viscosity is.The proportion of the mineral content is another dominant factor;the higher the feldspar content,the lower the mica content,the higher the rock viscosity is.From Archean to early Mesozoic,the lithosphere under the eastern NCC uplifted nearly 9 km,associated with the deformation pattern change from the early rootless fold and melt facies to the ductile shear and mylonitic.The exposed Triassic adakitic rocks reflect the growing of lithospheric thickness during this period,relating to the subduction of the Paleo-Asian ocean in the north of NCC.The late Mesozoic tectonic framework in the region is mainly controlled by the Pacific tectonic domain,accompanied by a significant of lithospheric thinning and decratonization,which account for a series of brittle deformation.The pre-Jurassic magma activity was dominated influenced by the collision of the Paleo-Asian ocean and the Yangtze plate.The multi-stage tectonic movement resulted in the complex deformation assemblages in the eastern part of the NCC,which lasted until the late Mesozoic. |
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