辽南金州拆离带糜棱状花岗岩脉体变形特征及锆石SHRIMP U-Pb年龄——韧性拆离时限的新证据

Deformation characteristics and zircon SHRIMP U-Pb age of the mylonitized granite vein in Jinzhou detachment fault of southern Liaoning:new evidence for constraint on the ductile detachment time

辽南金州拆离断层带中发育糜棱状花岗岩脉。野外观察与显微构造分析显示该脉体为同构造变形脉体,与围岩太古宙片麻岩一起经历了伸展韧性剪切变形。长石Fry法的应变测量结果显示样品的付林参数K=0.83,罗德参数υ=0.09,应变强度Es=0.71,表明该岩石应变以平面应变为主,且有限应变较强;运动学涡度为Wk=0.89,表明剪切作用类型以简单剪切为主。对该花岗岩脉进行锆石SHRIMP U-Pb年代学测定,10颗岩浆锆石的206Pb/238U年龄加权平均值为129±2 Ma(MSWD=1.6),代表岩脉侵位年龄。结合该区研究的最新成果,表明金州拆离断层至少在129 Ma已经开始韧性变形,辽南地区构造体制在此时已经从缩短转折到伸展。

Extensive crustal extension existent in northeast Asian continent since Late Mesozoic has resulted in the formation of one of the world＇s largest extensional provinces. Metamorphic core complexes （MCCs） from Lake Baikal to the southern margin of North China constitute a typical extensional phenomenon in this province. The duration and mechanism of such large-scale crustal extension, especially the mechanism of the extension as to whether the stretching took place simultaneously in this region or it was gradually expanded, deserve further research. Obviously the precise chronological limit of the beginning of each metamorphic core complex is an effective means to solve this problem. Syn-tectonic mylonitized granite （vein） is an ideal symbol body for studying the kinematics, dynamics and chronology of the metamorphic core complexes, and is also an important object for determining the structural features and deformation time. Southern Liaoning was a rather intense and typical lithospheric thinning and destruction area in North China Craton during Late Mesozoic, with the formation of the Liaonan metamorphic core complex serving as one of the marks. The authors tentatively studied the deformation features and zircon SHRIMP U-Pb geochronology of the mylonitized granite vein exposed in the Jinzhou detachment fault of the Liaonan MCC, with the purpose of providing new evidence for constraint on the ductile detachment time. The vein intruded into Archean mylonitic biotite plagioclase gneiss along the mylonitic foliation of the wall rock and underwent a strong extensional ductile shear deformation together with the wall rocks. Field observations and micro-structural analysis show that the vein is syn-kinematic. Fry＇s finite strain measurements of feldspars show that the sample＇s Flinn K values is 0.83, Lodυvalues is 0.09, and strain intensity （Es） is 0.71, indicating that the mylonite is dominated by plane strain. The kinematic vorticity number （Wk） is 0.89, implying that the deformation is dominated by simple shear. SHRIMP U-Pb dating of the magmatic zircon from the mylonitized granite vein yields an age of 129±2 Ma （n=10, MSWD=1.6）, suggesting that the vein was emplaced in Early Cretaceous. Recent researches have shown that the Liaonan MCC resulted from regional and lithosphere thinning in the North China Craton in Late Mesozoic. The formation of the Jinzhou detachment fault and the uplift on its eastern side are the major indicator of the turning point of the tectonic regime from contraction to extension. In combination with the results obtained by the authors, it is suggested that the Jinzhou detachment fault started its ductile shearing at least at 129±2 Ma and the tectonic regime changed from contraction to extension at this time.