Petrogenesis of Granites from Dangchuan Area in West Qinling Orogenic Belt and Its Tectonic Implication
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摘要: 对西秦岭造山带党川地区的党川花岗岩和石门花岗岩进行了LA-ICP-MS锆石U-Pb定年、元素地球化学和Sr-Nd-Pb同位素组成的研究.结果表明, 党川花岗岩和石门花岗岩的岩浆结晶年龄分别为438±3Ma和220±2Ma.在岩石地球化学特征上, 党川花岗岩类似于C-型埃达克质岩石, 岩浆产生于增厚地壳物质的部分熔融, 而石门花岗岩类似于普通的地壳深熔型花岗岩.党川花岗岩的ISr=0.70660~0.70929, εNd (t) =-2.24~-4.48;石门花岗岩的ISr=0.70581~0.70804, εNd (t) =-3.73~-4.72.Sr-Nd同位素组成进一步指示它们的岩浆派生于地壳物质.然而, 在Pb同位素组成上, 党川花岗岩和石门花岗岩存在着明显的差异.党川花岗岩以相对富放射成因Pb同位素组成为特征, 初始Pb同位素比值为: 206Pb/204Pb=18.288~18.484, 207Pb/204Pb=15.677~15.693, 208Pb/204Pb=38.182~38.283;而石门花岗岩以相对低的放射成因Pb同位素组成为特征, 初始Pb同位素比值为: 206Pb/204Pb=17.989~18.189, 207Pb/204Pb=15.560~15.567, 208Pb/204Pb=37.982~38.000.这表明党川花岗岩和石门花岗岩的岩浆来自于不同地壳物质的部分熔融.区域分析表明, 西秦岭党川地区中古生代和早中生代的岩浆事件、岩石成因机制及岩浆源区均可与东秦岭地区北秦岭构造单元相对比, 由此说明西秦岭党川地区是东秦岭地区北秦岭构造单元的西延, 并且东秦岭地区早中生代南秦岭块体向北秦岭块体的大陆俯冲作用向西一直延至到西秦岭地区.
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关键词:
- 西秦岭造山带 /
- 花岗岩 /
- U-Pb锆石定年 /
- Sr-Nd-Pb同位素 /
- 岩石成因
Abstract: This paper carries out a study of U-Pb zircon dating using LA-ICP-MS method, geochemical and Sr-Nd-Pb isotopic compositions of the Dangchuan and Shimen granites in Dangchuan area of the West Qinling orogenic belt.The results show that the Dangchuan and Shimen granites have magma crystallization ages of 438±3Ma and 220±2Ma, respectively.Geochemical signatures indicate that the Dangchuan granite is similar to C-type adakitic rock that resulted from partial melting of thickened crustal material and the Shimen granite is similar to common anatexis-type granite.The Dangchuan granite has initial Sr isotopic ratios (ISr) of 0.70660-0.70929 and εNd (t) values of-2.24 to-4.48, and the Shimen granite has ISr of 0.70581-0.70804 and εNd (t) values of-3.73 to-4.72.The Sr-Nd isotopic composition of the Dangchuan and Shimen granites provides an additional eveidence that both the granites were derived from crustal source.However, Pb isotopic composition of the Dangchuan granite is distinct from that of the Shimen granite.The Dangchuan granite is characterized by more radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=18.288-18.484, 207Pb/204Pb=15.677-15.693 and 208Pb/204Pb=38.182-38.283, whereas the Shimen granite is characterized by relatively less radiogenic Pb isotopic composition with initial Pb isotopic ratios of 206Pb/204Pb=17.989-18.189, 207Pb/204Pb=15.560-15.567 and 208Pb/204Pb=37.982-38.000.It is strongly suggested that the magmas of the Dangchuan and Shimen granites were derived from different crustal sources.The Early Paleozoic and Early Mesozoic tectono-magma events, their petrogenesis and magma sources of Dangchuan area in the West Qinling orogenic belt can be comparable with those of the North Qinling tectonic unit in the East Qinling orogenic belt, indicating that the North Qinling tectonic unit can be extended to Dangchuan area in the West Qinling orogenic belt.In the East Qinling orogenic belt, the South Qinling continental crust was subducted beneath the North Qinling crust due to Early Mesozoic continental collision between the Yangtze and North China blocks.This continental subduction also occurred in the West Qinling orogenic belt.-
Key words:
- West Qinling orogenic belt /
- granite /
- U-Pb zircon dating /
- Sr-Nd-Pb isotopes /
- petrogenesi
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秦岭造山带以宝成铁路划分为东、西两段, 东段称之为东秦岭造山带, 西段称之为西秦岭造山带.在东秦岭造山带, 商丹断裂是秦岭地区发生早古生代洋-陆相互作用和其后发生大陆碰撞作用的主边界, 具有地缝合线意义(张国伟等, 2000).以商丹断裂为界, 东秦岭造山带可划分为北侧的北秦岭构造单元和南侧的南秦岭构造单元, 这两个构造单元具有不同的地质演化, 其中北秦岭构造单元具有活动大陆边缘性质, 以发育早古生代和早中生代岩浆侵入作用为特征, 而南秦岭构造单元主要发育早中生代岩浆侵入作用, 缺少早古生代岩浆侵入作用(Zhang et al., 1996, 1997a; 卢欣祥等, 2000; 张本仁等, 2002).现有研究表明北秦岭早中生代花岗岩类的岩浆源区主要来自于南秦岭地壳物质, 从而证明了在大陆碰撞阶段, 南秦岭块体俯冲叠置于北秦岭块体之下(Zhang et al., 1996, 1997a; 张本仁等, 2002).东秦岭造山带北秦岭构造单元的构造-岩浆事件及花岗岩类的岩浆产生机制是否向西扩展到西秦岭造山带, 已成为当今秦岭造山带地质研究中的基础问题之一.这对全面讨论秦岭造山带不同区段的构造演化及它们的构造衔接关系具有重要科学意义.位于西秦岭造山带的党川地区成为研究上述问题的关键地区, 该区花岗岩类广泛出露.然而, 该区花岗岩类的年代学和地球化学的研究程度较为薄弱. 李永军等(2005)在对党川地区花岗岩类进行区域地质调查的基础上, 根据全岩Rb-Sr同位素等时年龄和TIMS锆石U-Pb年龄讨论了党川地区花岗岩类的形成时代, 主要获得了2组年龄: 分别为174~225 Ma和391~430 Ma, 但由于测定方法的限制, 这些年龄结果带有一定程度的不确定性.本文选择党川地区2个代表性岩体, 开展了LA-ICP-MS锆石U-Pb定年、地球化学和Sr-Nd-Pb同位素地球化学的研究, 据此讨论它们的岩石成因及其构造意义.
1. 地质背景简述
研究区在地理位置上位于甘肃省天水市的党川地区; 在构造位置上, 位于宝鸡-成都铁路线以西的西秦岭造山带(图 1).该区北侧以宝鸡-天水断裂为界与祁连造山带的东端相邻, 南侧-西南侧为北西西向的唐藏-天水断裂, 该断裂被认为是东秦岭地区商丹断裂的西延, 在天水市以西尖灭, 并与祁连山南缘断裂相接(张国伟等, 2000; 冯益民等, 2002).
图 1 西秦岭党川地区地质简图(Zhang et al., 2006a)DC.党川花岗岩; SM.石门花岗岩Fig. 1. Simplified geological map of Dangchuan area in West Qinling研究区出露的前寒武纪基底岩系为古元古代秦岭岩群, 该岩群受到高角闪石相的变质作用, 主要由含石榴石黑云斜长片麻岩组成, 含少量斜长角闪岩、大理岩和变粒岩等(甘肃省地质矿产局, 1989); 显生宙地层主要为早古生代李子园群(丹凤群) 和草滩沟群(二郎坪群) 及中生代(白垩纪) 陆相砂砾岩.李子园群和草滩沟群均为火山-沉积岩系(丁仨平等, 2004; 裴先治等, 2006), 它们分布在秦岭岩群的南北两侧.党川地区暴露的地层系统类似于宝鸡-成都铁路线以东的东秦岭造山带北秦岭构造单元(裴先治等, 2006).
党川地区花岗岩类广泛分布(图 1), 它们主要侵位于秦岭岩群和草滩沟群中.党川地区花岗岩先前称之为党川复式岩体(甘肃省地质矿产局, 1989).根据近期的地质调查, 党川地区花岗岩类可解体为7个岩体单元, 并可能存在早古生代和早中生代两期岩浆作用(李永军等, 2005).本文选择党川地区解体出的党川岩体和石门岩体进行了重点研究.党川岩体在空间上呈不规则状展布, 出露面积约200 km2; 主要岩石类型为中细粒黑云母二长花岗岩, 岩石呈灰白色, 块状构造; 主要矿物组成为石英(25%~28%)、斜长石(35%~45%)、钾长石(20%~30%) 和黑云母(5%~7%).石门岩体分布于党川岩体的西侧, 形态呈长椭圆状, 出露面积约50 km2; 主要岩石类型为中粗粒黑云母二长花岗岩, 岩石为肉红色, 块状构造; 主要矿物组成为: 石英(28%~32%)、斜长石(30%~35%)、钾长石(35%~40%) 和黑云母(3%~5%).
2. 分析方法
样品无污染粉碎至200目以下.主量元素在湖北地质实验研究所用XRF方法测定, 分析准确度优于5%;微量元素在中国地质大学(武汉) 用ICP-MS方法测定, 分析准确度优于10%, 其中稀土元素分析准确度优于5%.
全岩Sr-Nd-Pb同位素测定在西北大学大陆动力学国家重点实验室用Nu Plasma仪器测定.Sr和Nd同位素的分馏校正分别采用86Sr/88Sr=0.119 4和146Nd/144Nd=0.721 9, 在分析期间, NBS987给出87Sr/86Sr=0.710 244±14 (2σ), J & M标准给出143Nd/144Nd=0.511 825±6 (2σ), BCR-2标准给出143Nd/144Nd=0.512 644±6 (2σ), 全程Sr空白 < 4 ng, Nd空白 < 1 ng.Pb同位素的分馏校正采用205Tl/203Tl=2.387 5.在样品测量期间, NBS981标准给出206Pb/204Pb=16.937±1 (2σ), 207Pb/204Pb=15.491±1, 208Pb/204Pb=36.696±1;BCR-2标准给出206Pb/204Pb=18.742±1 (2σ), 207Pb/204Pb=15.620±1, 208Pb/204Pb=38.705±1.测量的Pb空白为0.1~0.3 ng.详细的Sr-Nd-Pb同位素分析方法见Zhang et al. (2006a).
用于锆石U-Pb年代学测定的样品, 在廊坊地质服务有限公司利用标准技术对锆石进行了分选.锆石制靶后, 进行了锆石阴极发光照像, 以观察锆石的内部结构.锆石U-Pb年龄在中国地质大学地质过程与矿产资源国家重点实验室利用LA-ICP-MS方法测定, 激光束斑直径为32μm.实验中采用He作为剥蚀物质的载气, 锆石91500作为外标, NIST610作为内标, 分析方法及仪器参数类似于Yuan et al. (2004).锆石测定点的同位素比值、U-Pb表面年龄和U-Th-Pb含量计算采用GLITTER程序.采用Andersen (2002)方法对普通Pb进行校正, 并采用ISOPLOT程序(Ludwig, 2001) 进行锆石加权平均年龄计算及谐和图的绘制.
3. 结果
3.1 U-Pb锆石年龄
样品0588 (北纬34°18′43.6″, 东经106°08′03.6″) 采集于党川岩体中细粒黑云母二长花岗岩.该样品中的锆石大部分为无色透明, 自形程度较好.在阴极发光图像上, 大多具有较好的岩浆型锆石的振荡环带(图 2a), 但这些岩浆型锆石内部含有较多的继承锆石, 它们呈浑圆状, 一般分布在锆石的核部(图 2a), 在CL图像上呈现灰白色到白色.样品0593 (北纬34°23′37.8″, 东经106°06′51.2″) 采集于石门岩体中粗粒黑云母二长花岗岩.该样品的锆石自形程度较好, 它们呈长柱或短柱状, 大部分锆石具有岩浆型锆石的振荡环带(图 2b), 继承型锆石较为少见.
样品0588和0593锆石U-Pb LA-ICP-MS分析数据列于表 1.
表 1 党川花岗岩和石门花岗岩锆石LA-ICP-MS U-Pb同位素分析数据Table Supplementary Table U-Pb zircon LA-ICP MS chronological data of Dangchuan granite and Shimen granite样品0588测定了23颗锆石23个点, 其中岩浆型锆石15点, 继承型锆石8点.岩浆型锆石的Th/U比值变化于0.08~0.73之间, 继承型锆石的Th/U比值变化于0.16~1.23之间.在U-Pb谐和图上(图 3), 岩浆型锆石的测定点主要位于谐和线上, 它们的206Pb/238U年龄变化于436~442 Ma之间, 206Pb/238U年龄的加权平均年龄为438±3 Ma (MSWD=0.59), 该年龄解释为党川花岗岩的岩浆结晶年龄.继承型锆石的207Pb/206Pb年龄变化于845~2 465 Ma之间, 反映岩浆源区中含有元古代的地壳组分.
样品0593测定了11颗锆石11个点, 其中岩浆型锆石10点, 继承型锆石1点.岩浆型锆石的Th/U比值变化于0.21~0.73之间, 继承型锆石的Th/U比值为0.47.在U-Pb谐和图上(图 3), 岩浆型锆石的测定点位于谐和线上或稍偏谐和线的右侧, 它们的206Pb/238U年龄变化于212~231 Ma之间, 206Pb/238U年龄的加权平均值为220±2 Ma (MSWD=0.66), 该年龄解释为石门花岗岩的岩浆结晶年龄.1个继承型锆石的206Pb/238U年龄为417 Ma.
3.2 主量元素和微量元素
党川花岗岩和石门花岗岩的主量元素和微量元素数据见表 2.
表 2 党川花岗岩和石门花岗岩主量元素(%) 和微量元素(10-6) 数据Table Supplementary Table Major element (%) and trace element (10-6) data of Dangchuan granite and Shimen granite由表 2可见, 党川花岗岩SiO2=72.31%~74.08%.它们的K、Na含量显示有较大范围的变化: K2O=3.19%~5.32%, Na2O=2.77%~4.08%, K2O/Na2O=0.78~1.92.党川花岗岩Al2O3=13.90%~15.26%, 铝指数A/CNK=1.06~1.12, 表明岩石为过铝质.石门花岗岩SiO2=74.45%~78.83%, Al2O3=11.03%~13.76%, K2O=4.60%~5.23%, Na2O=2.70%~3.95%, K2O/Na2O=1.16~1.79.它们的A/CNK=1.04~1.08, 表明石门花岗岩也属于过铝质岩石.党川花岗岩与石门花岗岩比较, 前者以相对低SiO2含量和相对高Al2O3含量为特征, 而后者以相对高SiO2含量和相对低Al2O3含量为特征.
在微量元素组成上(表 2), 党川花岗岩和石门花岗岩均以高Rb含量和低Cr、Co、Ni含量为特征, 其中党川花岗岩Rb含量变化于113×10-6~181×10-6, 石门花岗岩Rb含量变化于125×10-6~255×10-6.党川花岗岩与石门花岗岩相比, 党川花岗岩明显富Sr, 贫Nb、Ta、Y和重稀土元素, 而石门花岗岩相对贫Sr, 富Nb、Ta、Y和重稀土元素.其中, 党川花岗岩Sr=254×10-6~337×10-6, Y=5.9×10-6~11×10-6; 石门花岗岩Sr=114×10-6~158×10-6, Y=8.5×10-6~14.3×10-6, 由此导致党川花岗岩具有高的Sr/Y比值(25~49), 而石门花岗岩具有较低的Sr/Y比值(10~19).在稀土元素组成上, 党川花岗岩和石门花岗岩均为轻稀土富集型的稀土元素组成模式(图 4), 但两者存在明显的差异, 党川花岗岩的轻、重稀土元素的分异程度高于石门花岗岩.党川花岗岩(La/Yb) N=20.8~66.8, 负Eu异常不明显, Eu/Eu*=0.79~0.99;石门花岗岩(La/Yb) N=8.9~11.4, 存在中等程度的负Eu异常, Eu/Eu*=0.47~0.67, 反映岩浆源存在残留的斜长石或在岩浆结晶过程中斜长石的分离作用.
图 4 稀土元素组成模式(球粒陨石标准化值据Taylor and McLennan, 1985)Fig. 4. Chondrite normalized REE patterns3.3 Sr-Nd同位素
党川花岗岩和石门花岗岩的Sr-Nd同位素数据见表 3.
表 3 党川花岗岩和石门花岗岩Sr-Nd同位素组成Table Supplementary Table Sr and Nd isotopic compositions for Dangchuan granite and Shimen granite以党川花岗岩岩浆结晶年龄t=438 Ma计算, 获得党川花岗岩ISr值变化于0.706 60~0.709 29之间, εNd (t) 值变化于-2.24~-4.48之间.以石门花岗岩岩浆结晶年龄t=220 Ma计算, 获得石门花岗岩ISr值变化于0.705 81~0.708 04之间, εNd (t) 值变化于-3.73~-4.72之间.党川花岗岩和石门花岗岩的Nd同位素亏损地幔模式年龄(TDM) 分别为1.13~1.58 Ga和1.15~1.32 Ga.上述结果表明党川花岗岩和石门花岗岩应来自于地壳物质的部分熔融, 这与党川花岗岩和石门花岗岩含有古老继承型锆石的观察结果是一致的.在εNd (t) -ISr图(图 5) 上, 它们的ISr和εNd (t) 相关关系不明显, 反映岩浆中无明显幔源物质的加入.
3.4 Pb同位素
党川和石门花岗岩全岩Pb同位素组成见表 4. 由表 4可见, 党川和石门花岗岩以高放射成因的铅同位素组成为特征.党川花岗岩现今的铅同位素比值为: 206Pb/204Pb=18.481~18.820, 207Pb/204Pb=15.695~15.718, 208Pb/204Pb=38.715~39.823;石门花岗岩现今的铅同位素比值为: 206Pb/204Pb=18.402~18.559, 207Pb/204Pb=15.580~15.585, 208Pb/204Pb=38.401~38.442.根据全岩U、Th、Pb含量和铅同位素比值, 以t=438 Ma计算的党川花岗岩的初始铅同位素比值为: (206Pb/204Pb)t=18.288~18.484, (207Pb/204Pb)t=15.677~15.693, (208Pb/204Pb)t=38.182~38.283;以t=220 Ma计算的石门花岗岩的初始铅同位素比值为: (206Pb/204Pb)t=17.989~18.189, (207Pb/204Pb)t=15.560~15.567, (208Pb/204Pb)t=37.982~38.000.以上花岗岩初始铅同位素比值表明, 党川花岗岩相对富放射成因铅同位素组成, 而石门花岗岩相对贫放射成因铅同位素组成, 指示它们的岩浆源区存在着一定的差异.党川花岗岩和石门花岗岩的全岩初始铅同位素比值分别与东秦岭地区早古生代和早中生代花岗岩类全岩初始铅同位素比值相类似(Zhang et al., 1996, 1997a; 张本仁等, 2002).
表 4 党川花岗岩和石门花岗岩全岩Pb同位素组成Table Supplementary Table Whole-rock Pb isotopic compositions for Dangchuan granite and Shimen granite4. 讨论
4.1 岩石成因
西秦岭党川地区早古生代党川花岗岩(438 Ma) 和早中生代石门花岗岩(220 Ma) 在主量元素和微量元素组成上存在着明显的差异, 反映它们可能有着不同的成因或岩浆源区组成.
早古生代党川花岗岩具有强分异的稀土元素组成模式, 无明显的Eu异常, 亏损重稀土元素(如Yb=0.38×10-6~0.72×10-6) 和Y (5.86×10-6~11×10-6), 具有较高的Sr/Y和(La/Yb) N比值.如图 6, 党川花岗岩样品主要落入埃达克岩范围.这表明党川花岗岩具有埃达克岩的地球化学属性, 但党川花岗岩具有高的K2O含量及高的K2O/Na2O比值, 并且具有演化的Sr和Nd同位素组成, 明显区别于岛弧背景中由俯冲洋壳板片部分熔融形成的岛弧型埃达克岩(Defant and Drummond, 1990), 而与增厚下地壳部分熔融形成的C型埃达克质岩(Chung et al., 2003; Hou et al., 2004; Wang et al., 2005; Zhang et al., 2006c) 相类似.党川花岗岩的地球化学特征指示岩浆源区残留相中含有石榴石, 无或少量斜长石, 岩浆起源于增厚下地壳的部分熔融作用.党川花岗岩的这一成因机制与东秦岭北秦岭构造单元中产出的早古生代灰池子花岗岩(埃达克质) 的成因机制相一致(李伍平等, 2001), 它们反映在西秦岭和东秦岭地区存在着统一的一次早古生代的地壳加厚作用.这一地壳加厚作用应归因于秦岭地区在志留纪时, 华北陆块与扬子陆块发生对接与碰撞作用(Gao et al., 1995; 张本仁等, 2002).
图 6 (a) (La/Yb) N-YbN图(Matin, 1999); (b) Sr/Y-Y图(Defant and Drummond, 1990)Fig. 6. (a) (La/Yb) N vs. YbN diagram; (b) Sr/Y vs. Y diagram早中生代石门花岗岩高SiO2、低Al2O3, K2O/Na2O=1.16~1.79, A/CNK=1.04~1.08, 富集Rb, 低Sr/Y比值(10.4~18.5) 和(La/Yb) N比值(8.9~11.4), 在稀土元素组成模式上存在中等程度的负Eu异常, 并且Isr=0.705 81~0.708 04, εNd (t) =-3.73~-4.72.这些元素和Sr-Nd同位素地球化学特征清晰地表明石门花岗岩是正常厚度地壳物质深熔作用的产物.早中生代是我国主要大陆块体发生强烈碰撞的时代, 从此奠定了我国大陆现今的构造格局, 并结束了我国大陆块体具有洋盆分隔的历史(张国伟等, 2004).在大别地区, 早中生代发生大陆深俯冲与高压/超高压变质作用(Xu et al., 1992; Ames et al., 1996; Hacker et al., 1998; Li et al., 2000; Ayers et al., 2002); 在东秦岭地区(包括南秦岭和北秦岭构造单元), 早中生代广泛发育花岗岩类岩浆的侵入作用(Zhang et al., 1996, 1997a; 卢欣祥等, 2000; Sun et al., 2002; 张本仁等, 2002; 张成立等, 2005), 这些花岗岩类的形成归因于华北陆块与扬子陆块的碰撞作用所诱发地壳物质部分熔融的产物(张本仁等, 2002).位于西秦岭的石门花岗岩在成因机制上应类似于东秦岭地区这些早中生代花岗岩类.
尽管早古生代党川花岗岩和早中生代石门花岗岩的元素地球化学和Sr-Nd同位素组成指示它们均为地壳物质部分熔融的产物, 但它们在Pb同位素上组成上存在着明显的差异(图 7).党川花岗岩相对富放射成因Pb同位素组成, 而石门花岗岩相对贫放射成因Pb同位素组成.这不符合相同岩浆源区形成不同时代花岗岩所呈现的Pb同位素比值随时间的变化规律.由此表明, 党川花岗岩和石门花岗岩具有不同地壳物质的源区(见下讨论).
图 7 (a) 206Pb/204Pb-207Pb/204Pb图; (b)206Pb/204Pb-208Pb/204Pb图北秦岭早古生代花岗岩的Pb同位素组成范围据Zhang et al. (1997a); 北秦岭和南秦岭早中生代花岗岩类的Pb同位素组成范围据Zhang et al. (1997a, 2007) 和张宏飞等(1997b)Fig. 7. (a)206Pb/204Pb vs.207Pb/204Pb diagram; (b)206Pb/204Pb vs.208Pb/204Pb diagram4.2 构造意义
在东秦岭造山带, 北秦岭构造单元以发育早古生代和早中生代两期花岗岩类为特征, 而南秦岭构造单元主要发育早中生代花岗岩类, 缺少早古生代花岗岩类(卢欣祥等, 2000; 张本仁等, 2002).西秦岭党川地区发育早古生代和早中生代两期花岗岩, 它们的岩石成因机制与北秦岭构造单元相应时期花岗岩类的成因机制相一致.这表明西秦岭党川地区是东秦岭造山带北秦岭构造单元的西延.同时, 党川地区的花岗岩类的形成时代和岩石形成机制也可与宝鸡-天水断裂以北的祁连造山带花岗岩类形成时代和成因机制相对比(张宏飞等, 2006a; Zhang et al., 2006b), 由此建议东秦岭造山带的北秦岭构造单元向西一直延至到祁连造山带, 宝鸡-天水断裂无构造单元的分隔意义.这与祁连造山带东部地区武山和清水基性火山岩系分别对应于东秦岭丹凤群和二朗坪群基性火山岩系的区域分析是一致的(裴先治等, 2003, 2004, 2005, 2006).
东秦岭造山带在早中生代华北陆块与扬子陆块碰撞阶段, 通过花岗岩类的源区示踪, 已证明北秦岭早中生代花岗岩类的岩浆源区并不是来北秦岭陆壳物质, 而是来自于南秦岭陆壳物质, 从而指示南秦岭块体向北秦岭块体的大陆俯冲作用(Zhang et al., 1996, 1997a; 张本仁等, 2002).东秦岭地区陆壳俯冲作用是否向西一直延至到西秦岭地区受到人们的关注.在这方面, 花岗岩的Pb同位素组成特征提供了重要证据.
前面指出, 西秦岭党川地区早古生代党川花岗岩和早中生代石门花岗岩具有不同的初始Pb同位素组成, 反映它们的岩浆来自于不同的地壳物质.图 7指示党川花岗岩的初始Pb同位素组成与北秦岭早古生代花岗岩类的初始Pb同位素组成相似, 而石门花岗岩的初始Pb同位素组成与北秦岭和南秦岭早中生代花岗岩的初始Pb同位素组成相似.因此, 党川地区不同时代花岗岩Pb同位素的组成变化类似于北秦岭相应时代花岗岩Pb同位素的组成变化.根据北秦岭不同时代花岗岩类Pb同位素组成对岩浆源区的分析(Zhang et al., 1996, 1997a; 张本仁等, 2002), 并考虑到西秦岭党川地区与北秦岭地区相应时代花岗岩类在Pb同位素组成上的可比性, 西秦岭党川花岗岩的岩浆应来自于北秦岭地壳物质的部分熔融, 而石门花岗岩的岩浆应来自于俯冲南秦岭地壳物质的深熔作用, 从而说明在西秦岭党川地区, 同样存在大陆壳的俯冲与叠置, 并且东秦岭地区早中生代南秦岭块体向北秦岭块体的大陆俯冲作用向西一直延至到西秦岭地区.
5. 结论
西秦岭党川地区党川花岗岩形成于早古生代(438±3 Ma), 石门花岗岩形成于早中生代(220±2 Ma).党川花岗岩具有C-型埃达克质岩石的地球化学特征, 岩浆来自于增厚下地壳的部分熔融, 下地壳增厚作用起因于早古生代华北与扬子陆块的初始碰撞作用.石门花岗岩形成于地壳的深熔作用, 与早中生代时华北与扬子陆块的强烈碰撞和大陆俯冲作用相联系.西秦岭党川地区花岗质岩浆事件和岩石成因机制与东秦岭地区北秦岭构造单元相对比, 表明西秦岭党川地区是东秦岭地区北秦岭构造单元的西延部分.西秦岭党川和石门花岗岩的Pb同位素组成指示它们的岩浆产生于不同地壳物质的部分熔融, 其中党川花岗岩的岩浆来自于北秦岭块体地壳物质的部分熔融, 而石门花岗岩的岩浆应来自于俯冲南秦岭地壳物质的深熔作用, 由此表明东秦岭地区早中生代南秦岭块体向北秦岭块体的大陆俯冲作用向西一直延至到西秦岭地区.
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图 1 西秦岭党川地区地质简图(Zhang et al., 2006a)
DC.党川花岗岩; SM.石门花岗岩
Fig. 1. Simplified geological map of Dangchuan area in West Qinling
图 4 稀土元素组成模式(球粒陨石标准化值据Taylor and McLennan, 1985)
Fig. 4. Chondrite normalized REE patterns
图 6 (a) (La/Yb) N-YbN图(Matin, 1999); (b) Sr/Y-Y图(Defant and Drummond, 1990)
Fig. 6. (a) (La/Yb) N vs. YbN diagram; (b) Sr/Y vs. Y diagram
图 7 (a) 206Pb/204Pb-207Pb/204Pb图; (b)206Pb/204Pb-208Pb/204Pb图
北秦岭早古生代花岗岩的Pb同位素组成范围据Zhang et al. (1997a); 北秦岭和南秦岭早中生代花岗岩类的Pb同位素组成范围据Zhang et al. (1997a, 2007) 和张宏飞等(1997b)
Fig. 7. (a)206Pb/204Pb vs.207Pb/204Pb diagram; (b)206Pb/204Pb vs.208Pb/204Pb diagram
表 1 党川花岗岩和石门花岗岩锆石LA-ICP-MS U-Pb同位素分析数据
Table 1. U-Pb zircon LA-ICP MS chronological data of Dangchuan granite and Shimen granite
表 2 党川花岗岩和石门花岗岩主量元素(%) 和微量元素(10-6) 数据
Table 2. Major element (%) and trace element (10-6) data of Dangchuan granite and Shimen granite
表 3 党川花岗岩和石门花岗岩Sr-Nd同位素组成
Table 3. Sr and Nd isotopic compositions for Dangchuan granite and Shimen granite
表 4 党川花岗岩和石门花岗岩全岩Pb同位素组成
Table 4. Whole-rock Pb isotopic compositions for Dangchuan granite and Shimen granite
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[1] Ames, L., Zhou, G. Z., Xiong, B. C., 1996. Geochronology and isotopic character of ultrahigh-pressure metamorphism withimplications for collision of the Sino-Korean and Yangtze cratons, central China. Tectonics, 15: 472-489. doi: 10.1029/95TC02552 [2] Andersen, T., 2002. Correction of common lead in U-Pb analyses that do not report 204Pb. Chem. Geol. , 192: 59-79. doi: 10.1016/S0009-2541(02)00195-X [3] Ayers, J. C., Dunkle, S., Gao, S., et al., 2002. Constraints on timing of peak and retrograde metamorphismin the Dabie Shan ultrahigh-pressure metamorphic belt, east-central China, using U-Th-Pb dating of zircon and monazite. Chem. Geol. , 186: 315-331. doi: 10.1016/S0009-2541(02)00008-6 [4] Bureau of Geology and Mineral Resources of Gansu Province, 1989. Regional geology of Gansu Province. Geological Publishing House, Beijing, 1-752 (in Chinese). [5] Chung, S. L., Liu, D. Y., Ji, J. Q., et al., 2003. Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet. Geology, 31: 1021-1024. [6] Defant, M. J., Drummond, M. S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature, 347: 662-665. doi: 10.1038/347662a0 [7] Ding, S. P., Pei, X. Z., Li, Y., et al., 2004. Analysis of the disintegration and tectonic setting of the "Liziyuan Group" in the Tianshui area, western Qinling. Geological Bulletin of China, 23 (12): 1209-1214 (in Chinesewith English abstract). [8] Feng, Y. M., Cao, X. D., Zhang, E. P., et al., 2002. Structure, process and dynamics of West Qinling orogenic belt. Xi'an Map Publisher, Xi'an, 263 (in Chinese). [9] Gao, S., Zhang, B. R., Gu, X. M., et al., 1995. Silurian-Devonian provenance changes of South Qinling basins: Implications for accretion of the Yangtze (South China) to the North China cratons. Tectonophysics, 250: 183-197. doi: 10.1016/0040-1951(95)00051-5 [10] Hacker, B. R., Ratschbacher, L., Webb, L., et al., 1998. U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie orogen, China. Earth Planet. Sci. Lett., 161: 215-230. doi: 10.1016/S0012-821X(98)00152-6 [11] Hou, Z. Q., Gao, Y. F., Qu, X. M., et al., 2004. Origin of adakitic intrusives generated during mid-Miocene eastwest extension in southern Tibet. Earth Planet. Sci. Lett. , 220: 139-155. doi: 10.1016/S0012-821X(04)00007-X [12] Li, S. G., Jagoutz, E., Chen, Y. Z., et al., 2000. Sm-Nd and Rb-Sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China. Geochim. Cosmochim. Acta, 64: 1077-1093. doi: 10.1016/S0016-7037(99)00319-1 [13] Li, W. P., Wang, T., Wang, X. X., 2001. Source of Huichizi granitoid complex pluton in northern Qinling, central China: Constrained in element and isotopic geochemistry. Earth Science—Journal of China University of Geosciences, 26 (3): 269-278 (in Chinese with English abstract). [14] Li, Y. J., Li, S. C., Yang, J. Q., et al., 2005. The "disintergration" of granite bodies in the Dangchuan area of Western Qinling: Evidence from the isotopic dating ages. Bulletin of Mineralogy, Petrology and Geochemistry, 24 (2): 114-120 (in Chinese with English abstract). [15] Lu, X. X., Xiao, Q. H., Dong, Y., et al., 2000. Granitoid tectonic map of Qinling belt. Geological Publishing House, Beijing (in Chinese with English abstract). [16] Ludwig, K. R., 2001. Users manual for Isoplot/Ex (rev. 2.49): A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center, Special PublicationNo. la: 55. [17] Martin, H., 1999. Adakitic magmas: Modern analogues of Archaean granitoids. Lithos, 46: 411-429. doi: 10.1016/S0024-4937(98)00076-0 [18] Pei, X. Z., Ding, S. P., Hu, B., et al., 2004. Definition of the Guanzizhen ophiolite in Tianshui area, western Qinling, and its geological significance. Geological Bulletin of China, 23 (12): 1202-1208 (in Chinese with English abstract). [19] Pei, X. Z., Guo, J. F., Li, Y., et al., 2003. Discovery of the Carboniferous in the Yanghe (Wushan) -Gucheng (Lixian) area and its geological significance. Geological Bulletin of China, 22 (7): 494-499 (in Chinese with English abstract). [20] Pei, X. Z., Li, Y., Lu, S. N., et al., 2005. Zircons U-Pb ages of the Guanzizhen intermediate-basic igneous complex in Tianshui area, West Qinling, and their geological significance. Geological Bulletin of China, 24 (1): 23-29 (inChinese with English abstract). [21] Pei, X. Z., Liu, H. B., Ding, S. P., et al., 2006. Geochemical characteristics and tectonic significance of the meta-volcanic rocks in the Liziyuan Group from Tianshui area, western Qinling orogen. Geotectonica et Metallogenia, 30 (2): 193-205 (in Chinese with English abstract). [22] Sun, W. D., Li, S. G., Chen, Y. D., et al., 2002. Timing of synorogenic granitoids in the South Qinling, Central China: Constraints onthe evolution of the Qinling-Dabie orogenic belt. J. Geology, 110: 457-468. doi: 10.1086/340632 [23] Taylor, S. R., McLennan, S. M., 1985. The continental crust: Its composition and evolution. Blackwell Scientific Publication, Oxford, 1-132. [24] Wang, Q., McDermott, F., Xu, J. F., et al., 2005. Cenozoic K-rich adakitic volcanic rocks in the Hohxil area, northern Tibet: Lower-crustal melting in an intracontinental setting. Geology, 33: 465-468. [25] Xu, S. T., Okay, A. I., Ji, S. Y., et al., 1992. Diamond from the Dabie Shan metamorphic rocks and its implication for tectonic setting. Science, 256: 80-82. doi: 10.1126/science.256.5053.80 [26] Yuan, H. L., Gao, S., Liu, X. M., et al., 2004. Accurate U-Pb age and trace element determinations of zircon by laser ablation-inductively coupled plasmamass spectrometry. Geostandards and Geoanalytical Research, 28: 353-370. doi: 10.1111/j.1751-908X.2004.tb00755.x [27] Zhang, B. R., Gao, S., Zhang, H. F., et al., 2002. Geochemistry of Qinling orogenic belt. Science Press, Beijing, 1-187 (in Chinese). [28] Zhang, C. L., Zhang, G. W., Yan, Y. X., et al., 2005. Origin and dynamic significance of Guangtoushan granitic plutons to the north of Mianlue zone in southern Qinling. Acta Petrologica Sinica, 21 (3): 711-720 (in Chinese with English abstract). [29] Zhang, G. W., Guo, A. L., Yao, A. P., 2004. Western QilingSongpan continental tectonic node in China's continental tectonics. Earth Science Frontiers, 11 (3): 23-32 (in Chinese with English abstract). [30] Zhang, G. W., Zhang, B. R., Yuan, X. C., et al., 2000. Qinling orogenic belt and continental dynamics. Science Press, Beijing, 855 (in Chinese with English abstract). [31] Zhang, H. F., Gao, S., Zhang, B. R., et al., 1997a. Pb isotopes of granitoids suggest Devonian accretion of Yangtze (South China) craton to North China craton. Geology, 25: 1015-1018. [32] Zhang, H. F., Ouyang, J. P., Ling, W. L., et al. 1997b. Pb, Sr, and Ndisotopie composition of Ningshan granitoids, South Qinling and their deep geological information. Acta Petrol. Mineral. , 16 (1): 22-32 (in Chinese with English abstract). [33] Zhang, H. F., Jin, L. L., Zhang, L., et al., 2006a. Pb and Nd isotopic compositions of basement and granitoid in the Qilianshan: Constraints on tectonic affinity. Earth Science—Journal of China University of Geosciences, 31 (1): 57-65 (in Chinese with English abstract). [34] Zhang, H. F., Zhang, B. R., Harris, N., et al., 2006b. U-Pb zircon SHRI MPages, geochemical and Sr-Nd-Pb isotopic compositions of intrusive rocks from the LongshanTianshui area in the southeast corner of the Qilian orogenic belt, China: Constraints on petrogenesis and tectonic affinity. J. Asian Earth Sci. , 27: 751-764. doi: 10.1016/j.jseaes.2005.07.008 [35] Zhang, H. F., Zhang, L., Harris, N., et al., 2006c. U-Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Garze fold belt, eastern Tibetan plateau: Constraints on petrogenesis and tectonic evolution of the basement. Contrib. Mineral. Petrol. , 152: 75-88. doi: 10.1007/s00410-006-0095-2 [36] Zhang, H. F., Jin, L. L., Zhang, L., et al., 2007. Geochemical and Pb-Sr-Nd isotopic compositions of granitoids from western Qinling belt: Constraints on basement nature and tectonic affinity. Science in China (Series D), 50: 184-196. [37] Zhang, H. F., Zhang, B. R., Zhao, Z. D., et al., 1996. Continental crust subduction and collision along Shangdantectonic belt of East Qinling, China: Evidence from Pb, Nd and Sr isotopes of granitoids. Science in China (Series D), 39: 273-282. [38] 丁仨平, 裴先治, 李勇, 等, 2004. 西秦岭天水地区"李子园群"的解体及其构造环境浅析. 地质通报, 23 (12): 1209-1214. doi: 10.3969/j.issn.1671-2552.2004.12.007 [39] 冯益民, 曹宣铎, 张二朋, 等, 2002. 西秦岭造山带结构造山过程及动力学. 西安: 西安地图出版社, 263. [40] 甘肃省地质矿产局, 1989. 甘肃省区域地质志. 北京: 地质出版社, 1-752. [41] 李伍平, 王涛, 王晓霞, 2001. 北秦岭灰池子花岗质复式岩体的源岩讨论: 元素-同位素地球化学制约. 地球科学——中国地质大学学报, 26 (3): 269-278. https://www.cnki.com.cn/Article/CJFDTOTAL-DQKX200103008.htm [42] 李永军, 李锁成, 杨俊泉, 等, 2005. 西秦岭党川地区花岗岩体的"解体"及同位素年龄证据. 矿物岩石地球化学通报, 24 (2): 114-120. https://www.cnki.com.cn/Article/CJFDTOTAL-KYDH200502004.htm [43] 卢欣祥, 肖庆辉, 董有, 等, 2000. 秦岭造山带花岗岩类大地构造图. 北京: 地质出版社. [44] 裴先治, 丁仨平, 胡波, 等, 2004. 西秦岭天水地区关子镇蛇绿岩的厘定及其地质意义. 地质通报, 23 (12): 1202-1208. doi: 10.3969/j.issn.1671-2552.2004.12.006 [45] 裴先治, 郭俊锋, 李勇, 等, 2003. 西秦岭武山杨河-礼县固城地区石炭纪地层的发现及其意义. 地质通报, 22 (7): 494-499. doi: 10.3969/j.issn.1671-2552.2003.07.006 [46] 裴先治, 李勇, 陆松年, 等, 2005. 西秦岭天水地区关子镇中基性岩浆杂岩体锆石U-Pb年龄及其地质意义. 地质通报, 24 (1): 23-29. [47] 裴先治, 刘会彬, 丁仨平, 等, 2006. 西秦岭天水地区李子园群变质火山岩的地球化学特征及其地质意义. 大地构造与成矿学, 30 (2): 193-205. [48] 张本仁, 高山, 张宏飞, 等, 2002. 秦岭造山带地球化学. 北京: 科学出版社, 1-187. [49] 张成立, 张国伟, 晏云翔, 等, 2005. 南秦岭勉略带北光头山花岗岩体群的成因及其构造意义. 岩石学报, 21 (3): 711-720. https://www.cnki.com.cn/Article/CJFDTOTAL-YSXB200503013.htm [50] 张国伟, 张本仁, 袁学诚, 等, 2000. 秦岭造山带与大陆动力学. 北京: 科学出版社, 1-855. [51] 张国伟, 郭安林, 姚安平, 2004. 中国大陆构造中的西秦岭-松潘大陆构造结. 地学前缘, 11 (3): 23-32. https://www.cnki.com.cn/Article/CJFDTOTAL-DXQY200403004.htm [52] 张宏飞, 靳兰兰, 张利, 等, 2006a. 基底岩系和花岗岩类Pb-Nd同位素组成限制祁连山带的构造属性. 地球科学——中国地质大学学报, 31 (1): 57-65. https://www.cnki.com.cn/Article/CJFDTOTAL-DQKX200601008.htm [53] 张宏飞, 欧阳建平, 凌文黎, 等, 1997b. 南秦岭宁陕地区花岗岩类Pb、Sr、Nd同位素组成及其深部地质信息. 岩石矿物学杂志, 16 (1): 22-32. -