扬子地块东南缘新元古代下江群地层白云母40Ar-39Ar年龄及其地质意义❋

来志庆， 韩宗珠， 李三忠， 部雪娇， 刘 博

(中国海洋大学海洋地球科学学院 海底科学与探测技术教育部重点实验室，山东 青岛 266100)

扬子地块东南缘新元古代下江群地层白云母40Ar-39Ar年龄及其地质意义❋

来志庆， 韩宗珠❋❋， 李三忠， 部雪娇， 刘 博

(中国海洋大学海洋地球科学学院 海底科学与探测技术教育部重点实验室，山东 青岛 266100)

古太平洋板块在燕山期的俯冲后撤致使华南板块的大地构造背景由造山增厚转换为伸展减薄，但目前对于俯冲后撤的起始时间仍有争议，且针对扬子地块东南缘甚少有年龄数据来制约此次中生代构造属性的转换。本文对扬子地块东南缘下江群地层的变质矿物白云母开展40Ar-39Ar同位素测年研究，并分别获得(103±2)Ma和(102±2)Ma的坪年龄。结合已有研究数据，上述年龄表明扬子地块东南缘在早白垩世晚期(～100Ma)已完全转变为太平洋构造域，并由陆内造山转换为伸展扩张构造阶段。此时古太平洋板块的俯冲后撤已启动和逐渐东移，并最终导致华夏地块东部晚白垩世(～86Ma)大量花岗岩和双峰式火山岩的形成。

扬子地块；下江群；太平洋板块；俯冲后撤

扬子地块作为华南板块的主要组成构造单元，位于特提斯与古太平洋两大构造域的接触带。受太平洋板块俯冲与后撤的影响，扬子地块乃至整个东亚的大地构造在中生代发生了重要转折[1-2]。燕山期陆内构造作用导致扬子地块乃至整个华南板块普遍发生强烈褶皱和冲断变形[3-6]，形成一系列NE—SW走向的断陷盆地并伴随着巨量岩浆侵入和火山活动[7-8]。有学者强调华南褶皱构造主要形成于印支期[9]。华南晚中生代断陷盆地的形成与古太平洋俯冲无关，其白垩纪岩浆活动是岩石圈伸展减薄造成减压熔融的结果[8]。但也有研究者认为，古太平洋板块的西向俯冲后撤导致NE—SW向白垩纪伸展盆地群的形成，太平洋板块的远距离平板俯冲造成华南板块在1200km范围内广泛分布火成岩和构造带[10]，而板块俯冲逐渐后退变陡导致华南地区中晚侏罗世以来岩浆活动由内陆向海岸线方向的迁移[11-12]。古太平洋板块自二叠纪中期向华南板块之下俯冲[13]，在欧亚大陆东部形成造山带[14]。其大陆边缘在中生代(250～90Ma)属于安第斯型[15-16]，同时造成华南板块大量造山期火成岩的形成，其侏罗纪岩浆活动的峰期为170～140Ma[17-18]；由于古太平洋板块的俯冲后撤，其在晚白垩世(约90Ma)后转变为西太平洋型，同时造成华夏地块形成大量晚白垩世A型花岗岩和双峰式火山岩，峰期主要为107～86Ma[6,19-22]。目前古太平洋板块俯冲后撤的起始时间仍有争议，也有学者认为俯冲后撤始于早白垩世甚至晚侏罗纪(早于120Ma)[23-24]。因此，华南中生代大地构造演化历史是极其复杂和存在争议的，也一直是地学界研究的热点。但前人研究主要通过火成岩地球化学和年代学证据制约古太平洋板块俯冲后撤和构造属性的转换，且主要集中于华夏地块，关于扬子地块的大地构造转换事件的研究甚少。扬子地块东南缘存在大量的华南典型变质沉积岩地层-下江群及其相应地层，下江群变质变形明显，岩石内变质矿物白云母变形及定向排列显著，且含量较多。本文以下江群变质地层为研究对象，利用白云母矿物开展40Ar-39Ar定年研究，旨在制约扬子地块东南缘燕山期构造属性转换事件，并进一步讨论太平洋板块俯冲后撤对华南板块的影响。

1 区域地质概况及样品岩石学特征

1.1 区域地质概况

扬子地块西邻印度板块，北邻华北板块，东南与华夏地块相邻[25]。其与华夏地块之间为雪峰山陆内复合构造系统，是两大陆块长期相互运动和作用过程中形成的陆内变形带(见图1a)。扬子地块存在太古代-古元古代结晶基底(崆岭群等)，基底周围为中元古代-新元古代褶皱带，褶皱带与震旦系盖层不整合接触[26-27]。

(a:华南板块构造简图[34]，b：研究区域地质图及采样位置，c：上白垩统与新元古界地层不整合接触关系。a:Tectonic map of Yangtze Block[34];b: Sampling sites and location of the study area in Yangtze Block;c: Unconformity between upper Cretaceous and Neoproterozoic stratas.)

本文研究区位于扬子地块东南缘，其基底地层为新元古代绿片岩相火山沉积地层(970～825Ma)，与上覆新元古代地层(815～750Ma)以及震旦纪盖层(<750Ma)不整合接触，并有过铝质S型花岗岩(825～815Ma)侵入[26-29]。早新元古代变质火山沉积地层(970～825Ma)以梵净山群(贵州)、四堡群(广西)、冷家溪群(湖南)、双桥山群(江西)、上溪群(安徽)和双溪坞群(浙江)为代表[27,30-31]。晚新元古代变沉积地层(815～760Ma)主要有下江群(贵州)、丹洲群(广西)、板溪群(湖南)、登山群(安徽)和历口群(浙江)等[26]。侵入梵净山群及相应地层的新元古代花岗侵入体(825～815Ma)主要有三防、本洞、摩天岭和元宝山等岩体[32-33]。

梵净山群及相应地层主要分布于黔东南及桂北九万大山、元宝山一带，与上覆下江群呈平行不整合至角度不整合接触关系，主要是由灰色、灰绿色变质细砂岩、 变质粉砂岩及泥质粉砂岩组成的海相碎屑岩系。

下江群及相应地层是一套沉积序列和沉积构造背景特殊的地层单元，主要为一套陆源碎屑夹大量晶屑凝灰岩和沉凝灰岩夹层沉积地层，包括变质砂岩、板岩、千枚岩、片岩、大理岩等[35-36]。其沉积厚度大, 沉积速率高, 成熟度低。作为扬子地块基底的下江群及对应地层经历绿片岩相变质作用，并发育大量的直立褶皱和开阔褶皱。

区域内还出露南华-震旦系、寒武系、奥陶系、泥盆系、石炭系、二叠系、侏罗系、白垩系等地层(见图1b)。南华-震旦系与丹洲群为整合接触关系，主要岩性为一套轻变质的含砾砂泥岩、砂岩、泥岩等。石炭系为碳酸盐台和碎屑岩沉积。白垩系为一套红层沉积，主要岩性有砾岩，杂砂岩和粉砂岩等，与下伏地层为不整合接触关系(见图1c)。

1.2 样品岩石学特征

本次研究在基于野外详细地质调查的基础上，采集下江群变沉积岩地层样品两件并进行40Ar-39Ar测年。测试样品岩石学特征主要有：

样品GX2007-140为黑色千枚岩，变晶结构，千枚状构造，主要矿物有白云母(50%)+石英(25%)+磁铁矿(25%)。白云母含量高，但矿物颗粒较小，变形定向排列明显；石英破碎并定向排列，沿片理分布，最大者200μm，多数小于100μm。斜长石少见，且碳酸盐化和云母化明显(见图2a)。

(a:样品GX2007-140；b：样品GX2007-152。a:Sample GX2007-140;b:Sample GX2007-152.)

样品GX2007-152为青灰色千枚岩，变晶结构，千枚状构造，变质程度低于样品GX2007-140。主要矿物有石英(50%)+白云母(30%)。镜下石英呈条带状，最大者小于100μm；白云母颗粒较小，定向排列明显(见图2b)。

2 40Ar-39Ar年代学实验方法及结果

样品经清洗、室温风干后逐步破碎至0.30～0.45mm，在双目镜下分别挑选出无蚀变白云母单矿物，纯度在99%以上。样品用铝箔包裹，镉箔屏蔽，与参考标准样品(黑云母ZBH- 2506，年龄为132.5Ma)一起放于核反应堆照射。每个样品分多阶段，50W CO2激光器输出功率从4.0%开始加热，最高加热至18.0%。所有的数据在回归到时间零点值后再进行质量歧视校正、本底校正和干扰元素同位素校正。Ca、K产生的干扰同位素校正因子为：(36Ar/37Ar)(Ca)= 0.00026726，(39Ar/37Ar)(Ca)= 0.0008984，(40Ar/39Ar)K=0.00597，38Ar/36Ar (a)=0.1869，38Ar/36Ar (a)=5.543×1010a-1，J=0.008196。样品测定由中国科学院同位素年代学和地球化学重点实验室完成，测试方法参照文献[37]，同位素数据处理和年龄计算采用ArArCALC软件[38]，测试结果如下：

样品GX2007-140获得坪年龄为(103±2)Ma， 主坪的39Ar释放量达到77%，等时线年龄为(107±1)Ma(MSWD=1.43)。样品GX2007-152获得坪年龄为(102±2)Ma，主坪的39Ar析出占总量的99%。样品的等时线年龄为(99±5)Ma(MSWD=1.43)，拟合很好(见图3)。

图3 下江群变沉积岩白云母40Ar/39Ar同位素坪年龄和等时线年龄

3 讨论

矿物坪年龄是矿物冷却至其相应封闭温度时记录的封闭温度年龄，因此，矿物坪年龄实际上是构造热事件的冷却年龄[39]。本研究样品白云母均一的“主坪”揭示其受后期改造并不强烈，说明白云母形成过程中保持了较理想的同位素封闭体系，受外界因素干扰较少，样品40Ar-39Ar宽阔坪年龄的39Ar释放率为70%～99%，年龄数据可靠，与相似的等时线年龄数据对照能够代表变形的冷却年龄。

目前，对于扬子地块和华夏地块的碰撞形成江南-雪峰造山带的时间仍有争议。有学者认为碰撞始于古生代[40-41]，也有学者认为两地块在新元古代就已发生碰撞并拼合成一体[31-33]。但华南板块在中生代之前完成拼合是已得到共识的，且最新研究成果已将碰撞时间界定至825～815 Ma(见图4a)[42]。

晚三叠世，伴随着古特提斯洋的消减闭合，华南板块北缘与华北板块发生碰撞并拼合到一起(见图4b)，此次构造事件使得扬子地块边缘受到的强烈的碰撞与挤压作用。华北板块和华南板块的碰撞是中生代最重要的一次地质事件，两板块约在早中侏罗世结束碰撞并造成东亚的最终形成[43]。

燕山期(侏罗-白垩纪)，华南板块主要处于太平洋板块俯冲导致的大地构造转换阶段。古太平洋板块NW向俯冲于华南板块之下[44]，有效弹性厚度与热流值以及地震学的综合分析说明古太平洋板块已俯冲至扬子克拉通(四川盆地)下方[49]。华南板块广泛发育向内陆不断迁移的造山带和山前前陆盆地,多层逆冲推覆系统也说明扬子地块内部的造山事件是自侏罗纪-白垩纪的一次由东南向西北方向渐进的变形[32, 50]。磷灰石裂变径迹年龄揭示，自湘鄂西向川东华蓥山构造变形发展的时代从 165～95Ma具有递进变新的趋势[51]。中侏罗世岩浆岩展布方位和太平洋俯冲带边界平行[52]。另外，雪峰造山带加里东-印支-燕山运动自南东向北西不断穿时拓展说明其力来源于南东方向[54]。所以扬子地块在中侏罗世后已完全受古太平洋板块活动的影响。

(YB：扬子地块；CB：华夏地块；IN：印度地块；IC：印度支那地块；AN：南极洲地块；AU：澳大利亚地块；LA：劳伦古大陆；NCB：华北地块；SCB：华南地块；Q：羌塘地块；S：滇缅马地块；SA：南美陆块；AF：非洲陆块; MA:马达加斯加陆块; TB：塔里木盆地；QB：柴达木盆地；QT：羌塘地体；LT：拉萨地体；SGT：松潘-甘孜地体；SB：四川盆地；QDOB：秦岭大别造山带. YB: Yangtze block; CB: Cathaysia Block; IN: Indian Block; IC: Indochina Block; An: Antarctica Block; Au: Australia Block; LA: laurentia; NCB: North China block; SCB: South China Block; Q = Qiangtang Block; S =Sibumasu Block; SA: South American plate; AF:African plate; MA:Madagascar landmass; TB: Tarim basin; QB: Qaidam basin; QT: Qiangtang terrane; LT= Lhasa terrane; SGT:Songpan-Ganzi terrane; SB: Sichuan basin;QDOB: Qinling-Dabie orogeny.)

由于古太平洋板块NW向俯冲，燕山期构造热事件分布于自海岸线向内陆延伸的1300km范围内，而大范围的变形带分布于近2000km的范围内，远至四川盆地[54]。华夏地块和扬子地块形成大量的造山期火成岩，其侏罗纪岩浆作用的高峰期为170～140Ma[17-18]。后由于古太平洋板块的俯冲后撤致使华夏地块与扬子地块形成大量伸展扩张性质的火成岩，其主要岩浆峰期为107～86Ma[19,21-22,56-57]。但是，目前对于华南板块在140～100Ma之间的构造属性仍存在造山压缩和伸展扩张的异议[21-24]。部分学者认为140～100Ma之间出现的A型花岗岩具有造山期后的特征，并认为华南板块在100Ma之前已经改变为伸展扩张构造环境[21]。也有学者认为晚白垩世(约98Ma)I型花岗岩出现说明在华夏地块在100Ma之前为俯冲造山环境[22]。因此，不妨认为华南板块区域构造环境由挤压到伸展并非一蹴而就，两者之间可能存在一个过渡期。140～100Ma，华南板块可能处于陆壳造山增厚与伸展减薄的过渡期，古太平洋板块NW向持续俯冲过程中，由于俯冲距离和板片密度不断加大，最终导致俯冲板片的断离和拆沉，由此引起软流圈上涌并导致岩石圈地幔和地壳部分熔融而形成A型与I型花岗岩以及双峰式火山岩。

野外观察研究区晚白垩地层不整合于晚元古代地层之上(见图1c)。扬子地块西部的四川盆地晚侏罗-早白垩沉积岩存在大量的NE向向斜和背斜，盆地内晚白垩地层与早白垩地层不整合接触，晚白垩与新第三纪河流湖泊相沉积并没有发生褶皱变形。中国东部的扩张和裂谷形成大量的NE—SW向裂谷盆地，在湘鄂西地区出露少量扩张期的碎屑岩说明扬子地块在晚白垩世经历一次伸展扩张事件[55]。晚白垩世伸展构造性质火成岩在空间上呈NE—SW向展布，且年龄较老的多在内陆发育(如广西锡田花岗岩)[56-57]，年龄较年轻的岩浆岩(如台湾90～86Ma的A型花岗岩)则主要集中于东南沿海和台湾地区，说明晚白垩世岩浆活动存在由内陆向沿海逐渐迁移的规律[6,8-9]。另外，扬子地块磷灰石裂变径迹年龄也存在自西至东由约90Ma逐渐变小的规律[58]。因此，作者认为本文所得40Ar-39Ar年龄(103±2)Ma和(102±2)Ma代表了古太平洋板块俯冲后撤的起始时间，此时扬子地块已由陆内造山完全转换为伸展构造环境，并形成大量的北东向盆山构造。古太平洋俯冲板片的后撤以及后续俯冲角度的增大使得俯冲作用对于扬子地块乃至整个华南板块的影响逐渐东移，并在华夏地块东部和下扬子地区形成大量的晚白垩世花岗岩和双峰式火成岩。

此外，印度-欧亚板块的陆陆碰撞始于60Ma。晚始新世(约40Ma)藏南最高海相层可能是印度-亚洲大陆碰撞完成的标志[59-60]。最新研究认为印度板块和欧亚板块的碰撞始于55Ma，板块边缘的弧岩浆作用主要集中于80～40Ma，并自南向北迁移，并在70～43Ma再次回迁至西藏南部[61]。 因此，印度板块和欧亚板块的陆陆碰撞构造事件在下江群地层中并未有地质记录。

4 结论

(1)下江群变沉积岩样品的白云母40Ar/39Ar定年结果分别为(103±2)Ma和(102±2)Ma，指示扬子地块东南缘在早白垩世晚期经历一次构造热事件。

(2)早白垩世晚期(～100Ma)，古太平洋板块的俯冲后撤使得华南板块由陆内造山转变为伸展构造环境，且由于后续俯角度的增大导致华夏地块东部巨量晚白垩世(～86Ma)花岗岩和双峰式火山岩的形成。

[1] 张岳桥, 董树文, 李建华, 等. 华南中生代大地构造研究新进展[J]. 地球学报, 2009, 33(3): 257-279. Zhang Y Q, Dong S W, Li J H, et al. The new progress in the study of Mesozoic tectonics of South China [J]. ActaGeoscienticaSinica, 2009, 33(3): 257-279.

[2] 周雪瑶, 于津海, 王丽娟, 等. 粤西云开地区基底变质岩的组成和形成 [J]. 岩石学报, 2015, 031( 03) : 855-882. Zhou X Y, Yu J H, Wang L J, et al. Compositions and formation of the basement metamorphicrocks in Yunkaiterrane, western Guangdong Province, South China [J]. Acta Petrologica Sinica, 2015, 31(3) : 855-882.

[3] 杨绍祥, 余沛然. 浦市辰溪浅层叠瓦式推覆构造特征及地质找矿意义 [J]. 湖南地质, 1995, 14(1): 31-34. Yang S X, Yu P R. The characteristics of shallower imbricate Nappe structure along Pushi-Chenxi and its significance in searching mineral resources [J]. Hunan Geology, 1995, 14(1): 31-34.

[4] Li J W, Zhou M F, Li X F, et al. The Hunan-Jiangxi strike-slip extension of the Tan-Lu fault [J]. Journal of Geodynamics, 2001, 32: 333-354.

[5] Li S Z, Kusky T M, Zhao G C, et al. Two-stage Triassic exhumation of HP-UHP terranes in the DabieOrogenfo china: constrain from structural geology [J]. Tectonophyysics, 2010, 490: 267-293.

[6] Li Z X, Li X H, Chung S L, et al. Magmatic switch-on and switch-off along the South China continental margin since the Permian: Transition from an Andean-type to a Western Pacific-type plate boundary [J]. Tectonophysics, 2012, 532-535: 271-290.

[7] Shu L S, Zhou X M, Deng P, et al. Mesozoic tectonic evolution of the Southeast China Block: New insights from basin analysis [J]. Journal of Asian Earth Science, 2009, 34: 376-391.

[8] Li J H, Zhang Y Q, Dong S W, et al. Cretaceous tectonic evolution of South China: A preliminary synthesis [J]. Earth-Science Reviews, 2014, 134: 98-136.

[9 ] Wang Y J, Zhang F F, Fan W M, et al. Tectonic setting of the South China Block in the early Paleozoic: Resolving intracontinental and ocean closure models from detrital zircon U-Pb geochronology [J]. Tectonics, 2010, 29: TC6020.

[10] Li Z X, Li X H. Formation of the 1300-km-wide intracontinental orogeny and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model [J]. Geology, 2007, 35: 179-182. [11] Zhou X M, Li W X. Origin of Late Mesozoic igneous rocks in southeastern China: Implications for lithosphere subduction and underplating of mafic magmas [J]. Tectonophysics, 2000, 326: 269-287.

[12] Zhou X M, Sun T, Shen W Z, et al. Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: a response to tectonic evolution [J]. Episodes, 2006, 29: 26-33.

[13] Meng L F, Li Z X, Chen H L, et al. Geochronological and geochemical results from Mesozoic basalts in southern south China Block support the flat-slab subduction model [J]. Lithos, 2012, 132-133: 127-140.

[14] Dong S W, Zhang Y Q, Zhang F Q,et al. Late Jurassic-Early Cretaceous continental convergence and intracontinentalorogenesis in East Asia: A synthesis of the Yanshan Revolution [J]. Journal of Asian Earth Sciences, 2015: doi: http: //dx. doi. org/10. 1016/j. jseaes. 2015. 08. 011.

[15] Maruyama S. Pacific-type orogeny revisited: Miyashiro-type orogeny proposed [J]. Island Arc, 1997, 6: 91-120.

[16] Li J H, Zhang Y Q, Dong S W, Shi,W, et al. Structural and geochronological constraints on the Mesozoic tectonic evolution of the North Dabashan zone, South Qinling, central China [J]. Journal of Asian Earth Sciences, 2013,64: 99-114.

[17] Wu F Y, Yang J H, Wilde S A, et al. Geochronology, petrogenesis and tectonic implications of Jurassic granites in the Liaodong Peninsula, NE China [J]. Chemical Geology, 2005,221: 127-156. [18] Li X H, Li Z X, Li W X, et al. U-Pb zircon, geochemical and Sr-Nd-Hf isotopic constraints on age and origin of Jurassic I- and A-type granites from central Guangdong, SE China: A major igneous event in response to foundering of a subducted flat-slab? [J]. Lithos,2007,96: 186-204.

[19] 邱检生, 肖娥, 胡建, 等. 福建北东沿海高分异型花岗岩的成因: 锆石年代学、地球化学和同位素制约 [J]. 岩石学报,2008, 24: 2468-2484. Qiu J S, Xiao E, Hu J, et al. Petrogenesis of highly fractionated I-type granites in the coastal area of northeastern Fujian Province: Constraints from zircon U-Pb geochronology, geochemistry and Nd-Hfistopes [J]. Acta Petrologica Sinica, 2008, 24: 2468-2484.

[20] Wong J, Sun M, Xing G F,et al. Geochemical and zircon U-Pb and Hf isotopic study of the Baijuhuajianmetaluminous A-type granite: Extension at 125-100 Ma and its tectonic significance for South China [J]. Lithos, 2009, 112: 289-305.

[21] Wang F Y,Liu S A, Li S G, et al. Zircon U-Pb ages, Hf-O isotopes and trace elements of Mesozoic high Sr/Y porphyries from Ningzhen, eastern China: Constraints on their petrogenesis, tectonic implications and Cu mineralization [J]. Lithos, 2014,200-201: 299-316.

[22] Zhao J L, Qiu J S, Liu L, et al. The Late Cretaceous I- and A-type granite association of southeast China: Implications for the origin and evolution of post-collisional extensional magmatism [J]. Lithos,2015, doi: 10. 1016/j. lithos. 2015. 10. 018.

[23] Yan J, Liu J M, Li Q Z, et al. In situ zircon Hf-O isotopic analyses of late Mesozoic magmatic rocks in the Lower Yangtze River Belt, central eastern China: Implications for petrogenesis and geodynamic evolution [J]. Lithos, 2015, 227: 57-76.

[24] Su Y P, Zheng J P, Griffin W L, et al. Petrogenesis and geochronology of Cretaceous adakitic, I- and A-type granitoids in the NE Yangtze block: Constraints on the eastern subsurface boundary between the North and South China blocks [J]. Lithos, 2013, 175-176: 333-350.

[25] Li S Z,Santosh M, Zhao G C,et al．Intracontinental deformation in a frontier of super- convergence: A perspective on the tectonic milieu of the South ChinaBlock [J]. Journal of Asian Earth Sciences, 2012, 49: 311-327．

[26] Wang H Z, Mo X X. An outline of the tectonic evolution of China [J]. Episodes, 1995, 18: 6-16.

[27] Zhao G C, Cawood P A. Precambrian geology of China [J]. Precambrian Research, 2012, 222-223: 13-54.

[28] Wang J, Li Z X. History of Neoproterozoic rift basins in South China: implications for Rodinia break-up [J]. Precambrian Research, 2003, 122: 141-158.

[29] Yao J L, Shu L S, Santosh M. Neoproterozoic arc-related mafic- ultramafic rocks and syn-collision granite from the western segment of the JiangnanOrogen, South China: constraints on the Neoproterozoic assembly of the Yangtze and Cathaysia Blocks [J]. Precambrian Research, 2014, 243: 39-62.

[30] 舒良树, 卢华复, 贾东, 等. 华南武夷山早古生代构造事件的40Ar/39Ar同位素年龄研究 [J]. 南京大学学报(自然科学), 1999, 35(6): 668-674. Shu L S, Lu H X, Jia D, et al. Study of the40Ar/39Ar isotopic age for the early Paleozoictectonothermal event in the Wuyishan region, South China [J]. Journal of Nanjing University (Natural Sciences), 1999, 35(6): 668-674.

[31] Wang X L, Zhou J C, Qiu J S, et al. Geochemistry of the Meso- to Neoproterozoic basic-acid rocks from Hunan Province, South China: Implications for the evolution of the western Jiangnanorogeny [J]. Precambrian Research, 2004, 135: 79-103.

[32] Li X H. U-Pb zircon ages of granites from the southern margin of the Yangtze Block: timing of NeoproterozoicJinning Orogeny in SE China and implications for Rodinia assembly [J]. Precambrian Research, 1999, 97: 43-57.

[33] Wang X L, Zhou J C, Qiu J S, et al. LA-ICP-MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks fromNorthern Guangxi, South China: implications for tectonic evolution [J]. Precambrian Research, 2006, 145: 111-130.

[34] Du Q D, Wang Z J, WangJ, et al. Geochronology and paleoenvironment of the pre-Sturtian glacial strata: Evidence from the Liantuo Formation in the Nanhua rift basin of the Yangtze Block,South China [J]. Precambrian Research, 2013,233: 118-131.

[35] 张传恒,刘耀明, 史晓颖, 等. 下江群沉积地质特征及其对华南新元古代构造演化的约束 [J]. 地球学报, 2009,30(4): 495-504. Zhang C H, Liu Y M, Shi X Y, et al. Sedimentological feature of the Xiajiang Group and their constraints on the Neoproterozoic tectonic evolution of South China [J]. Acta Geoscientica Sinica, 2009,30(4): 495-504.

[36] 汪正江, 王剑, 段太忠, 等. 扬子克拉通内新元古代中期酸性火山岩的年代学及其地质意义 [J]. 中国科学: 地球科学, 2010, 40(11): 1543 -1551. Wang Z J, Wang J, Duan T Z, et al. Geochronology of middle Neoproterozoic volcanic deposits in Yangtze Craton interior of South China and itsimplications to tectonic settings [J]. Science China: Earth Science, 2010, 40(11): 1543 -1551.

[37] Qiu H N, Jiang Y D. Sphalerite40Ar/39Ar progressive crushing and step-wise heating techniques [J]. Earth and Planetary Science Letters, 2007, 256: 224-232.

[38] Koppers A A P．ArArCALC: Software for 40Ar/39Ar age calculations [J]. Computer Geoscience, 2002, 28(5): 748-765.

[39] McDougall I, Hrrison T M. Geochronology and thermochronology by the 40Ar/39Ar method [M]. Oxford: Oxford University Press, 1988: 269.

[40] Chen X, Rong J Y. From biostratigraphy to tectonics with Ordovician and Silurian of South China as an example [J]. Geoscience, 1999, 13: 385-389.

[41] Gu X X, Liu J M, Zheng M H, et al. Provenance and tectonic setting of the Proterozoic turbidites in Hunan, South China: geochemical evidence [J]. Journal of Sedimentary Research, 2002, 72: 393-407.

[42] Zhao G C. JiangnanOrogen in South China: Developing from divergent double subduction [J]. Gondwana Research, 2015, 27: 1173-1180.

[43] Yang Z, Besse J. New Mesozoic apparent polar wander path for South China: Tectonic consequences [J]. Journal of Geophysical Research Solid Earth, 2001, 106(B5): 8493-8520.

[44] SunWD, DingX, HuYH, et al. The golden transformation of the Cretaceousplate subduction in the west Pacific [J]. Earth and Planetary Science Letters, 2007, 262 (3-4): 533-542.

[45] Wang Qi F, Deng J, Li C S, et al. The boundary between the Simao and Yangtze blocks and their locations in Gondwana and Rodinia: Constraints from detrital and inherited zircons [J]. Gondwana Research, 2014, 26: 438-448.

[46] Reid A, Wilson C J L, Shun L, etal. Mesozoic plutons of the Yidun Arc, SW China: U/PbgeochronologyandHf isotopic signature [J]. Ore Geology Reviews, 2007, 31: 88-106.

[47] Shi H C, Shi X B, Glasmacher U A, et al. The evolution of eastern Sichuan basin, Yangtze block since Cretaceous: Constraints from low temperature thermochronology [J]. Journal of Asian Earth Sciences (2015), doi: http: //dx. doi. org/10. 1016/j. jseaes. 2015. 11. 008.

[48] Garzantia E, Hu X M. Latest Cretaceous Himalayan tectonics: Obduction, collision or Deccan-related uplift? [J]. Gondwana Research, 2015, 28(1): 165-178.

[49] Deng B, Li Z W,Liu S G, et al. Structural geometry and kinematic processes at the intracontinentalDaloushan mountain chain: Implications for tectonic transfer in the Yangtze Block [J]. ComptesRendus Geoscience, 2015,http: //dx. doi. org/10. 1016/j. crte. 2015. 06. 009.

[50] Wang B, Zhang G W, Yang Z Y, et al. New Mesozoic paleomagnetic results from the northeastern Sichuan basin and their implication [J]. Tectonophysics, 2013, 608: 418-427.

[51] 梅廉夫, 刘昭茜, 汤济广, 等. 湘鄂西-川东中生代陆内递进扩展变形: 来自裂变径迹和平衡剖面的证据 [J]. 地球科学,2010, 35(2): 161-174. Mei L F, Liu S X, Tang J G, et al. Mesozoic intra-Continental progressive deformation in westernHunan-Hubei-Eastern Sichuan Provinces of China: Evidencefrom Apatite fission track and balanced cross-section [J]. Earth Science-Journal of China University of Geosciences, 2010, 35(2): 161-174.

[52] 金宠, 李三忠, 王岳军, 等. 雪峰山陆内复合构造系统印支-燕山期构造穿时递进特征 [J]. 石油与天然气地质, 2009, 30(5): 598-607. Jin C, Li S Z, Wang Y J, et al. DiachronousandprogressivedeformationduringtheIndosinian-Yanshanianmovements oftheXuefengMountainintracontinentalcompositetectonicsystem [J]. Oil and Gas Geology, 2009, 30(5): 598-607.

[53] 刘博. 雪峰陆内复合构造系统: 深部构造特征及其动力学演化[D]. 青岛: 中国海洋大学, 2009. Liu B. Features and Evolution of Deep Structures in the XuefengIntracontinental Tectonic System [D]. Qingdao: Ocean University of China, 2009.

[54] Wang Y J, Fan W M, Zhang G W, et al. Phanerozoic tectonics of the South China Block: Key observations and controversies [J]. Gondwana Research, 2013, 23: 1273-1305.

[55] Shen C B, Mei L F, Xu S H. . Fission track dating of Mesozoic sandstones and its tectonic significance in the Eastern Sichuan Basin, China [J]. Radiation Measurements, 2009,44: 945-949.

[56] 蔡明海, 何龙清, 刘国庆, 等. 广西大厂锡矿田侵入岩结石年龄及其意义 [J]. 地质论评, 2006, 52(3): 409-414. Cai M H, He L Q, Liu G Q, et al. SHRIMP zircon U-Pbdating of the intrusive rocks in the DachangTin-polymetallicore field, Guangxi and their geological significance [J]. Geological Review, 2006, 52(3): 409-414.

[57] 陈富文, 李华芹, 杨玉萍. 广西龙头山斑岩型金矿成岩成矿锆石年代学研究 [J]. 地质学报, 2008, 82(7): 921-926. Chen F W, Li H Q, Yang Y P. Zircon SHRIMP U-Pbchronology of diageneticmineralization of theLongtoushanporphyry gold orefield,Gui County , Guangxi [J]. ActaGeologicaSinica, 2008, 82(7): 921-926.

[58] Tang S L, Yan D P, Qiu L, et al. Partitioning of the cretaceous Pan-Yangtze Basin in the central South China Block by exhumation of the Xuefeng Mountains during a transition from extensional to compressional tectonics? [J]. Gondwana Research, 2014, 25: 1644-1659.

[59] 李国彪. 西藏南部古近纪微体古生物及盆地演化特征[D] . 北京: 中国地质大学, 2004: 1-171. Li G B. Paleogene micropaleontology and basin evolution in southern Tibet [D]. Beijing: China University of Geosciences, 2014: 1-171.

[60] 莫宣学, 潘桂棠. 从特提斯到青藏高原形成: 构造-岩浆事件的约束 [J]. 地学前缘, 2006, 13(6): 43-51. Mo XX, Pan G T. From the Tethys to the formation of the Qinghai-Tibet Plateau: constrained by tectono-magmaticevents [J]. Earth Science Frontiers, 2006, 13(6): 43-51.

[61] Zhu D C, Wang Q, Zhao Z D, et al. Magmatic record of India-Asia collision [J]. Scientific reports, 2015, 5: 14289, doi: 10. 1038/srep14289.

责任编辑 徐 环

40Ar-39Ar Dating for Muscovite in Neoproterozoic Meta-sedimentary Rocks of Xiajiang Group in Southeastern Yangtze Block and Its Geological Significance

LAI Zhi-Qing， HAN Zong-Zhu， LI San-Zhong， BU Xue-Jiao， LIU Bo

(The Key Lab of Submarine Geosciences and Prospecting Techniques，Ministry of Education； College of Marine Geosciences，Ocean University of China，Qingdao 266100，China)

During Yanshanian, tectonic of southern Yangtze Block converted due to rollback of Pacific plate. nevertheless，there was no accurate geochronology to constrain the conversion and onset of rollback. Therefore, muscovite40Ar-39Ar ages were measured, and samples GX2007-140 and GX2007-152 collected from Xiajiang Group yielded plateau ages (103±2)Ma and (102±2)Ma, respectively.40Ar-39Ar thermochronology indicates that rollback of Pacific plate had also began and migrated eastward since early Cretaceous(～100 Ma), and tectonics had change from intracontinental orogeny into extension in southeastern margin of Yangtze Block. At this time, Huge amounts of cretaceous granites and bimodal volcanic rocks (～86 Ma) in eastern margin of Eurasian Plate were eventually formed.

Yangtze block；Xiajiang group；Pacific Ocean plate；rollback

中石化总公司重大科技攻关项目(G0800-06-ZS-281)；国家自然科学基金项目(41376053)资助 Supported by the Key Proggram of Petro China(G0800-06-ZS-281); The National Natruanal Science Foundation of China (41376053)

2015-11-10；

2015-12-14

来志庆(1983-)，男，博士生，主要从事岩石地球化学和海洋地质学研究。E-mail:zqlai@ouc.edu.cn

❋❋ 通讯作者： E-mail：hanzongzhu@ouc.edu.cn

P597

A

1672-5174(2016)05-094-07

10.16441/j.cnki.hdxb.20150391

来志庆， 韩宗珠， 李三忠, 等. 扬子地块东南缘新元古代下江群地层白云母40Ar-39年龄及其地质意义[J]. 中国海洋大学学报(自然科学版), 2017, 47(5): 94-100.

LAI Zhi-Qing， HAN Zong-Zhu， LI San-Zhong， et al.40Ar-39Ar Dating for muscovite in neoproterozoic meta-sedimentary rocks of Xiajiang group in Southeastern Yangtze block and its geological significance [J]. Periodical of Ocean University of China, 2017, 47(5): 94-100.