松潘甘孜地块四姑娘山花岗岩锆石U-Pb年代学证据及与汶川地震的关系
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摘要
松潘甘孜地体东部的花岗岩主要形成于印支晚期至燕山初期,其中四姑娘山花岗岩体的锆石普遍具有岩浆锆石的特征。通过四姑娘山花岗岩体的黑云母花岗闪长岩中的23粒锆石的锆石激光探针U-Pb定年,确定其岩浆结晶年龄为燕山早期(191±1)Ma,此年龄对解决龙门山断裂带形成的初始时间有重要意义。虽然在松潘甘孜地体多数岩体长轴走向和矿物定向均呈北西向,但过去区调中划分的北西向的金川—理县构造岩浆带不能代表这种楔入作用的产物,而应重新划分为北东向的道孚—金川—小金—黑水构造岩浆带,四姑娘山岩体是这个北东向构造岩浆带中的典型。这些花岗岩主要是扬子地块沿龙门山构造带向松潘甘孜地体内楔入导致松潘甘孜地体中下地壳低速层发生部分熔融的结果。地球物理资料显示,四姑娘山地区是有"山根"的,这些"山根"主要由巨大的花岗岩基组成,它们不是引起汶川地震的原因,但是减弱了汶川地震的地震波向西北方向的青藏高原传递,并对汶川地震时的龙门山断裂带向西南段的扩张起了一定的阻挡作用,即降低了汶川地震的大量余震在龙门山断裂带西南段发生的概率。
The granitoid intrusions in the eastern part of the Songpan-Garzê terrane were mainly formed in Late Indo-Chinese-Early Yenshanian period,and the Four-Girl Mountain Granite(FGMG)is one of these intrusions.Zircons from FGMG commonly have characteristics of magma zircons.LA-ICP-MS zircon U-Pb dating of 23 zircon grains from biotite granodiorite of the FGMG yielded a weighted 206Pb/238U mean age of(191±1)Ma(Early Yenshanian),which is considered to be the magmtic crystallization age of FGMG.This age has great significance in understanding the initial formation time of the Longmenshan fault belt.The NW-trending Jingchuan-Lixian tectono-magmatic belt defined during the previous regional geological survey can't represent the product of the collision between he Yangtze massif and the Songpan-Garzê terrane,although the major axes and mineral directions of most single granitic masses assume parallel NW-trending arrangement in the Songpan-Garzê terrane.Therefore,the NE-trending Douhu-Jingchuan-Xiaojing-Heishui tectono-magmatic belt should be reasonably established.The Four-Girl Mountain Granite(FGMG)pluton is a representative granitic mass and represents the product of partial fusing in the low velocity layer existent in the middle-lower crust of the Songpan-Garzê terrane when the Yangtze massif was wedged into the Songpan-Garzê terrane along the Longmenshan structural belt.Deep geophysical exploration shows that the Four-Girl Mountain area has "roots of mountain",which are made up of many great granitic plutons.These roots of mountain did not cause the Wenchuan Earthquake,but might have weaken the damage influence during the northwestward seismic wave propagation towards the Tibetan Plateau,and probably prevented the southwestward expansion of the Longmenshan fault belt when the Wenchuan Earthequake occurred.In other words,the probability of aftershocks that occurred along the southwest section of the Longmenshan fault belt was reduced by the roots of mountain.
The granitoid intrusions in the eastern part of the Songpan-Garzê terrane were mainly formed in Late Indo-Chinese-Early Yenshanian period,and the Four-Girl Mountain Granite(FGMG)is one of these intrusions.Zircons from FGMG commonly have characteristics of magma zircons.LA-ICP-MS zircon U-Pb dating of 23 zircon grains from biotite granodiorite of the FGMG yielded a weighted 206Pb/238U mean age of(191±1)Ma(Early Yenshanian),which is considered to be the magmtic crystallization age of FGMG.This age has great significance in understanding the initial formation time of the Longmenshan fault belt.The NW-trending Jingchuan-Lixian tectono-magmatic belt defined during the previous regional geological survey can't represent the product of the collision between he Yangtze massif and the Songpan-Garzê terrane,although the major axes and mineral directions of most single granitic masses assume parallel NW-trending arrangement in the Songpan-Garzê terrane.Therefore,the NE-trending Douhu-Jingchuan-Xiaojing-Heishui tectono-magmatic belt should be reasonably established.The Four-Girl Mountain Granite(FGMG)pluton is a representative granitic mass and represents the product of partial fusing in the low velocity layer existent in the middle-lower crust of the Songpan-Garzê terrane when the Yangtze massif was wedged into the Songpan-Garzê terrane along the Longmenshan structural belt.Deep geophysical exploration shows that the Four-Girl Mountain area has "roots of mountain",which are made up of many great granitic plutons.These roots of mountain did not cause the Wenchuan Earthquake,but might have weaken the damage influence during the northwestward seismic wave propagation towards the Tibetan Plateau,and probably prevented the southwestward expansion of the Longmenshan fault belt when the Wenchuan Earthequake occurred.In other words,the probability of aftershocks that occurred along the southwest section of the Longmenshan fault belt was reduced by the roots of mountain.
引文
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四川省地质调查院.1∶25万炉霍县幅区域地质调查报告,2008.
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四川地矿局化探队.1∶5万上孟、下孟幅区域地质调查报告,1987.
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[2]Hu Jianmin,Meng Qinten,Shi Yuruo,et al.SHRIMP U-Pbdating of zircons from granitoid bodies in the Songpan-Ganziterrane and its implications[J].Acta Petrol Sinica,2005,21(3):867-880(in Chinese with English abstract).
[3]袁海华,张志兰,张平.龙门山老君沟花岗岩体的隆升及冷却史[J].成都地质学院学报,1991,18(1):17-22.Yuan Haihua,Zhang Zhilan,Zhang Ping.The uplifting and coolinghistories of Laojungou granite in the western margin of the centralsection mountain[J].Journal of Chengdu College of Geology,1991,18(1):17-22(in Chinese with English abstract).
[4]王全伟,王康明,戴宗明.川西花岗岩及其成矿系列[M].北京:地质出版社,2008,156-237.Wang Quanwei,Wang Kangming,Dai Zongming,et al.Granitesand its Metallogenic Series in Western Sichuan,China[M].Beijing:Geological Publishing House,2008:156-237(in Chinese).
[5]赵永久,袁超,周美夫.川西老君沟和孟通沟花岗岩的地球化学特征、成因机制及对松潘-甘孜地体基底性质的制约[J].岩石学报,2007,23(5):995-1006.Zhao Yongjiu,Yuan Chao,Zhou Meifu,et al.Geochemistry andpetrogenesis of Laojungou and Mengtonggou granites in westernSichuan,China:constraints on the nature of Songpan-Ganzibasement[J].Acta Petrologica Sinica,2007,23(5):995-1006(inChinese with English abstract).
[6]赵永久,袁超,周美夫.松潘甘孜造山带早侏罗世的后造山伸展:来自川西牛心沟和四姑娘山岩体的地球化学制约[J].地球化学,2007,36(2):139-152.Zhao Yongjiu,Yuan Chao,Zhou Meifu,et al.Post-orogenicextension of Songpan-Garze orogen in Early Jurassic:Constraintsfrom Niuxingou monzodiorite and Siguniangshan A-type granite ofwestern Sichuan,China[J].Geochimica,2007,36(2):139-152.
[7]Honglin Yuan,Shan Gao,Xiaoming Liu,et al.Accurate U-Pb Ageand Trace Element Determinations of Zircon by Laser Ablation-Inductively Coupled Plasma Mass Spectrometry[J].Geoanalytical andGeostandard Research,2004,28(3):353-370.
[8]Andersen T,Griffin W L,Pearson N J.Crustal Evolution in the SWPart of the Baltic Shield:The Hf Isotope Evidence[J].Journal ofPetrology,2002,43(9):1725-1747.
[9]Ludwig K.Isoplot/Ex 2.49.A Geochronological Toolkit forMicrosoft Excel[CP/DK].Berkeley Geochronology Center,Berkeley,CA,USA,2001.Special Publication No.1a.
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[11]Chappell B W,White A J R.I-and S-type granites in the Lachlanfold belt,Transactions of the Roval of Edinburgh[J].EarthSciences,1992,83:1-26.
[12]Chappel1,B.W.Alluminium saturation in I-and S-type granitesand the characterization of fractionated hapogranites[J].Lithos,1999,46:535-551.
[13]Wu Fuyuan,Bor-ming Jahn,Simon Wilde,et al.Phanerozoiccrustal growth:U-Pb and Sr-Nd isotopic evidence from thegranites in northeastern China[J].Tectonophysics,2000,328:89-113.
[14]许志琴,侯立玮,王宗秀.中国松潘-甘孜造山带的造山过程[M].北京:地质出版社,1992,7-17.Xu Zhiqin,Hou Liwei,Wang Zongxiu,et al.Orogenic Processesof the Songpan-Garze Orogenic Belt of China[M].Beijing:Geological Publishing House,1992:7-17(in Chinese).
[15]莫宣学.青藏高原地质研究的回顾与展望[J].中国地质,2010,37(4):841-853.Mo Xuanxue.A review and prospect of geological researches onthe Qinghai-Tibet Platea[J].Geology in China,2010,37(4):841-853(in Chinese).
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[17]Tarney J,Jones C E.Trace element geochemistry of orogenicigneous rocks and crustal growth models[J].J.Geol.Soc.London,1994,151(5):855-868.
[18]王谦身,滕吉文,张永谦,等.龙门山断裂系及邻区地壳重力均衡效应与汶川地震[J].地球物理学进展,2008,23(6):1664-1670.Wang Qianshen,Teng Jiwen,Zhang Yongqian,et al.The effect ofcrustal gravity isostasy and Wenchuan earthquake in Longmenshanfaults and adjacent area[J].Progress in Geophysics,2008,23(6):1664-1670(in Chinese with English abstract).[19]侯遵泽,杨文采.中国重力异常的小波变换和多尺度分析.[J]地球物理学报,1997,40:84-95Hou Ziunze,Yang Wencai.Wavelet transform and multi-scaleanalysis on gravity anomalies of China[J].Acta Geophysica Sinica,1997,40(1):84-95(in Chinese with English abstract).[20]楼海,王椿镛,吕智勇,等.2008年汶川Ms8.0级地震的深部构造环境—远震P波接收函数和布格重力异常的联合解释[J].中国科学(D辑),2008,38(10):1207-1220.Lou Hai,Wang Chunyong,Lv Zhiyong,et al.Deep tectonicenvironment of the 2008 Wenchuan Ms8.0 Earthquake:Thecombined interpretation of the P receiver function of teleseismicand Bouguer gravity anomalies[J].Science in China(series D),2008,38(10):1207-1220(in Chinese).[21]吴建平,黄媛,张天中,等.汶川Ms 8.0级地震余震分布及周边区域P波三维速度结构研究[J].地球物理学报,2009,52(2):320-328.Wu Jianping,Huang Yuan,Zhang Tianzhong,et al.Aftershockdistribution of the Ms8.0 Wenchuan earthquake and threedimensional P-wave velocity structure in and around source region[J].Chinese Journal of Geophysics,2009,52(2):320-328(inChinese).[22]杨华.青藏高原地球物理特征分析[J].物探与化探,1986(5):321-329.Yanghua.Analysis of the Comprehensive GeophysicalCharacteristics of the Qinghai-Tibet Plateau[J].Geophysical andGeochemical Exploration,1986,10(5):321-329(in Chinese withEnglish abstract).[23]丁燕云,李占奎.武都-文县-理县一线地球物理场界线的发现及意义[J].中国地质,2009,36(6):1251-1256.Ding Yanyun,Li Zhankui.The discovery of the geophysical fieldboundary along Wudu-Wenxian—Lixian line and its significance[J].Geology in China,2009,36(6):1251-1256(in Chinese withEnglish abstract).[24]蔡学林,王绪本,朱介寿,等.汶川8.0级特大地震震源断裂特征及其动力学分析[J].中国地质,2010,37(4):952-966.Cai Xuelin,Wang Xubenz,Zhu Jieshou,et al.Characteristics andgeodynamic analysis of the focal fault for the Great 80MsWenchuan Earthquake[J].Geology in China,2010,37(4):952-966(in Chinese with English abstract).[25]徐纪人,赵志新.汶川8.0级大地震震源机制与构造运动特征[J].中国地质,2010,37(4):967-977.Xu Jiren,Zhao Zhixin.Causative mechanism and related tectonicmovement of the 2008 Wenchuan M8 Earthquake[J].Geology inChina,2010,37(4):967-977(in Chinese with English abstract).
四川省地矿局区域地质调查队.1∶20万小金幅区调报告(地质),1984
四川省地质调查院.1∶25万马尔康县幅区域地质调查报告(地质),2009
四川省地矿局区域地质调查队.1∶20万新龙幅区调报告(地质),1984.
四川省地矿局区域地质调查队.1∶20万小金幅区调报告(地质),1984.
四川省地质调查院.1∶25万马尔康县幅区域地质调查报告(地质),2009.
青海省地质调查院.1∶25万久治县幅区域地质调查报告,2006.
四川省地质调查院.1∶25万阿坝县幅区域地质调查报告,2006.
四川省地质调查院.1∶5万勒青贡等5幅区域地质调查报告,2009.
四川省地矿局区域地质调查队.1∶20万阿坝幅区调报告(地质),1984.
四川省地质调查院.1∶25万炉霍县幅区域地质调查报告,2008.
四川省地矿局区域地质调查队.1∶20万马尔康幅区调报告(地质),1979.
四川地矿局化探队.1∶5万上孟、下孟幅区域地质调查报告,1987.
四川省地矿局区域地质调查队.1∶20万小金幅区调报告(地质),1984.
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