A prolonged granitoid formation in Saglek Block,Labrador:Zonal growth and crustal reworking of continental crust in the Eoarchean  

作　　者：Tsuyoshi Komiya Shinji Yamamoto Shogo Aoki Keiko Koshida Masanori Shimojo Yusuke Sawaki Kazumasa Aoki Shuhei Sakata Takaomi D.Yokoyama Kenshi Maki Akira Ishikawa Takafumi Hirata Kenneth D.Collerson 

机构地区：[1]Department of Earth Science and Astronomy,Graduate School of Arts and Sciences,The University of Tokyo,Tokyo 153-8902,Japan [2]Department of Earth and Planetary Sciences,Tokyo Institute of Technology,2-12-1 Ookayama,Meguro,Tokyo 152-8551,Japan [3]Laboratory for Planetary Sciences,Kyoto University,Kitashirakawa Oiwakecho,Kyoto 606-8502,Japan [4]School of Earth Sciences,The University of Queensland,Brisbane,Queensland,Australia [5]KDC Consulting,33 Cramond St,Witston,Qld 4051,Australia

出　　处：《Geoscience Frontiers》2017年第2期355-385,共31页地学前缘（英文版）

基　　金：supported by JSPS grants(Nos.23253007 and 26220713)from the Ministry of Education,Culture,Sports,Science and Technology of Japan;supported by from the Ministry of Education,Culture,Sports,Science,and Technology of Japan and by the Mitsubishi Foundation

摘　　要：The Archean continental crusts account for ca.20% of the present volume,but the thermal history of the Earths＇ mantle suggests much more continental crusts were formed in the early Archean.Because the Archean continental crust underwent severe metamorphism,it is important to avoid influence by the later thermal events.We carried out a comprehensive geochronological work of Cathodoluminescence（CL） observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages.The zircons were classified into three domains of core,mantle and rims,and the cores were further classified into three groups of inherited,altered and zoned cores based on the zonation on the CL images.We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St.John＇s Harbour South（SJHS） area,and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence.The first and second generations comprise mafic rocks and lack magmatic zircons.We conducted CL imaging and U-Pb dating of zircons from the third,sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 L 25,3892 ± 33 and 3897 ± 33 Ma for each,supporting the presence of the over 3.9 Ca Iqaluk Gneiss.The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world.Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to 〉3.9 Ga,and tentatively classified into six groups based on their ages.The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area,and the orthogneisses become young as it is away.The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldeThe Archean continental crusts account for ca.20% of the present volume,but the thermal history of the Earths＇ mantle suggests much more continental crusts were formed in the early Archean.Because the Archean continental crust underwent severe metamorphism,it is important to avoid influence by the later thermal events.We carried out a comprehensive geochronological work of Cathodoluminescence（CL） observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages.The zircons were classified into three domains of core,mantle and rims,and the cores were further classified into three groups of inherited,altered and zoned cores based on the zonation on the CL images.We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St.John＇s Harbour South（SJHS） area,and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence.The first and second generations comprise mafic rocks and lack magmatic zircons.We conducted CL imaging and U-Pb dating of zircons from the third,sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 L 25,3892 ± 33 and 3897 ± 33 Ma for each,supporting the presence of the over 3.9 Ca Iqaluk Gneiss.The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world.Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to 〉3.9 Ga,and tentatively classified into six groups based on their ages.The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area,and the orthogneisses become young as it is away.The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the olde

关 键 词：Eoarchean Geochronology Uivak Gneiss Iqaluk Gneiss Continental recycling The oldest rocks 

分 类 号：P588.121[天文地球—岩石学]