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辽东半岛南辽河群锆石U-Pb年代学及其地质意义
引用本文:李壮,陈斌,刘经纬,张璐,杨川.辽东半岛南辽河群锆石U-Pb年代学及其地质意义[J].岩石学报,2015,31(6):1589-1605.
作者姓名:李壮  陈斌  刘经纬  张璐  杨川
作者单位:北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871;合肥工业大学资源与环境工程学院, 合肥 230009;北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871;北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871;北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871
基金项目:本文受科技部973项目(2012CB416603)资助.
摘    要:本文报道了辽东半岛古元古代胶-辽-吉活动带南辽河群中变质火山岩和沉积岩的锆石U-Pb年代学数据。变质流纹岩的锆石具典型的岩浆振荡环带结构和较高的Th/U比值(0.3),锆石U-Pb年龄为~2.2Ga,该年龄可代表其原岩形成年龄,在误差范围内与古元古代辽吉花岗岩年龄一致,表明辽吉花岗岩并不是辽河群的基底。变质玄武岩的锆石阴极发光强度较弱、弱分带或无分带,同时具较低的Th/U比值(0.1),为典型的变质成因锆石,锆石U-Pb年龄为~1.9Ga,代表其变质时代。变质沉积岩的碎屑锆石年龄主要介于1981~3520Ma之间:峰期年龄为2033Ma和2092Ma的锆石年龄信息暗示辽东半岛至少存在一期2000~2100Ma的岩浆事件,并且该时期的中酸性岩浆岩是南辽河群沉积岩的一个重要物源;峰期年龄为2155Ma、2446Ma、2509Ma、2594Ma、2668Ma、2790Ma、3356Ma、3467Ma和3520Ma的锆石年龄信息,区域上与古元古代辽吉花岗岩、辽河群火山岩及太古宙基底年龄相吻合,暗示它们为南辽河群沉积岩提供了重要物源。沉积岩中最年轻的碎屑锆石U-Pb年龄为~2.0Ga,可代表其沉积时的最大年龄。所以,辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶-辽-吉活动带的形成演化与弧-陆碰撞有关,而不是许多人坚持的裂谷环境。

关 键 词:辽东半岛  南辽河群  年代学  火山-沉积-变质  华北克拉通
收稿时间:9/7/2014 12:00:00 AM
修稿时间:2014/11/11 0:00:00

Zircon U-Pb ages and their implications for the South Liaohe Group in the Liaodong Peninsula, Northeast China
LI Zhuang,CHEN Bin,LIU JingWei,ZHANG Lu and YANG Chuan.Zircon U-Pb ages and their implications for the South Liaohe Group in the Liaodong Peninsula, Northeast China[J].Acta Petrologica Sinica,2015,31(6):1589-1605.
Authors:LI Zhuang  CHEN Bin  LIU JingWei  ZHANG Lu and YANG Chuan
Institution:Key Laborary of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China;School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China;Key Laborary of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China;Key Laborary of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China;Key Laborary of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China
Abstract:Liaodong Peninsula is located in the southeastern part of Liaoning Province, the basement rocks of which consist mainly of the Archean Anshan-Fushun Complex in the north (Liaobei-Jinan Block), Archean Jinzhou Complex in the south (Liaonan Block), and the Paleoproterozoic Liaohe Group and granitoid intrusions (Jiao-Liao-Ji Belt) in between. The Paleoproterozoic Liaohe Group is divided by the Qinglongshan-Zaoerling shear zone into two parts, the South Liaohe Group and North Liaohe Group, with more volcanic rocks in the former and more clastic and carbonate rocks in the latter. The South Liaohe Group consists of three rock units, with a volcanic-rich sequence in the lower part (the Lieryu and Gaojiayu formations), a carbonate-rich sequence in the middle (the Dashiqiao Formation), and a pelitic sequence in the upper part (the Gaixian Formation). We report in the paper the zircon U-Pb age data for two meta-volcanic rocks from the Lieryu Formation and three meta-sedimentary rocks from the Gaixian Formation of the Paleoproterozoic South Liaohe Group in the Liaodong Peninsula. Zircon analyses on the meta-rhyolite (LZ3) yield 207Pb/206Pb ages of ca.2.2Ga, with the range of 2178~2226Ma. These zircons show oscillatory and sector-zoning, and high Th/U ratios (>0.3), typical of magmatic origin; thus the age probably represents the formation age of the protolith of the meta-volcanic rocks, which are regionally comparable to those of the Paleoproterozoic Liaoji granitoids, indicating that the Liaoji granitoids are not the basement of the Liaohe Group sedimentary-volcanic rocks as previously thought. Zircon analyses on the meta-basalt (LZ12) yield 207Pb/206Pb ages of ca.1.9Ga, with the range of 1850~1949Ma. Zircon grains from the meta-basalt show relatively dark CL images, with patchy zoning or irregular structures and low Th/U ratios (<0.1). These features suggest that they should be formed during the process of fluid-present metamorphism, and their U-Pb ages of ca.1.9Ga represent the metamorphic age. The dating result of the detrital zircons from the meta-sedimentary rocks (LC3, LC5 and LB3) shows that the 207Pb/206Pb ages vary from 1981Ma to 3520Ma, with characteristic age probability peaks at ca.2033Ma, 2092Ma, 2155Ma, 2446Ma, 2509Ma, 2594Ma, 2668Ma, 2790Ma, 3356Ma, 3467Ma and 3520Ma. The characteristic age probability peaks of 2033Ma and 2092Ma indicate that more than one magmatic event occurred at 2000~2100Ma in the Liaodong Peninsula seldom recognized before, and that 2000~2100Ma acid rocks are an important contributor to the provenance for the South Liaohe Group. The characteristic age probability peaks of 2155~3520Ma are mainly consistent with the formation of regionally distributed Liaoji granitoids and coeval volcanic rocks in the Jiao-Liao-Ji Belt and the granitoids of the Archean basement, suggesting that these rocks provided the major component of the provenance for the South Liaohe Group. The deposition time of the meta-sedimentary rocks is later than ca.2.0Ga as limited by the youngest detrital zircons. These new zircon ages for the South Liaohe Group reveal that the volcanism-deposition-metamorphism events of the South Liaohe Group occurred at 2.2~1.9Ga, and lasted for at least 300Myr. Integration of our new data with recent studies on lithology, metamorphism, geochemistry and geochronology of Archean-Paleoproterozoic basement rocks is inconsistent with the rifting model, but rather, supports the Paleoproterozoic arc-continent collision model, as follows: (1) The detrital zircon spectra of the South Liaohe Group are similar to that of back-arc basin; (2) There is a lack of rock associations that are typical of the rift, such as OIB, phonolite and carbonatite; (3) The Paleoproterozoic metamorphic volcanic rocks and Liaoji granitoids show geochemical characteristics typical of arc magmas; (4) The South Liaohe and North Liaohe groups have different rock associations and metamorphic histories (P-T-t paths); (5) The Liaobei-Jinan and Liaonan blocks show considerable differences in terms of lithological units, geochronology and metamorphic features. Therefore, we propose that the Paleoproterozoic Jiao-Liao-Ji Belt formed in the arc-continental collision setting, rather than in a continental rifting setting as previously thought by most workers.
Keywords:Liaodong Peninsula  South Liaohe Group  Geochronology  volcanism-sedimentary-metamorphism  North China Craton
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