Petrological and Geochronological Study of Amphibolite from Jiaobei Terrane

Chen Shuang; Li Xuping; Duan Wenyong; Sun Guangming; Feng Qingda

Abstract：Jiaobei terrane lies in the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt,eastern block of the North China craton.There is a set of amphibolite closely related to the high-pressure mafic granulite in Jingshan complex of Jiaobei terrane, which is an important component of the Precambrian metamorphic basement.Our petrological and mineralogical study shows that amphibolite records three stages of metamorphism.The peak stage(M1)shows a mineral assemblage Amp1+Pl +Ttn. The temperature calculated by pseudosection modeling and estimated pressure conditions are T=660―715 ℃,P=0.65―0.71 GPa.The mineral assemblage at post-peak stage(M2)is Amp2+Ep+Pl+Chl+Ab,suggesting temperature and pressure con-ditions 537―630 ℃/0.41―0.58 GPa.The mineral assemblage at retrograde stage(M3)is Ab+Prh+Chl+Cal.Prehnite and calcite veins indicate that the amphibolite underwent a prehnite-pumpellyite facies metamorphism with P-T conditions<400 ℃/0.35 GPa.LA-ICP-MS U-Pb geochronology combined with cathodoluminescence(CL)imaging of 21 zircons are all characterized by relatively low Th/U ratios(0.06―0.43),low luminescence and weakly oscillatory zoning.The morphological and internal structure features of zircons suggest that their growths were associated with partial melting,and they yielded 206 Pb/238U weighted mean ages of 1 845±23 Ma(MSWD=0.35).After formation of the mafic rock,it may have experienced granulite facies metamorphism,and then recorded the anataxis event during collisional convergence of the Jiao-Liao-Ji orogenic belt at ~1.85 Ga.The investigated amphibolite recorded amphibolite facies metamorphism,and subsquently two episodes of retrogressive metamorphism during the lift of the belt.