广东河台金矿区印支期花岗岩与混合岩成因联系及大地构造意义*
2020-04-01焦骞骞贺昌坤董有浦许德如陈根文陈诚师爽高亦文
焦骞骞 贺昌坤 董有浦 许德如 陈根文 陈诚 师爽 高亦文
1. 昆明理工大学国土资源工程学院,昆明 6500932.东华理工大学核资源与环境国家重点实验室,南昌 3300133.中国科学院广州地球化学研究所,中国科学院矿物学与成矿学重点实验室,广州 5106404. 广东省地质局第五地质大队,肇庆 5266001.
两广交界(粤西-桂东南)的云开大山地区位于扬子板块与华夏板块之间的过渡地带。由于其所处的特殊大地构造位置,因此不同学者曾先后使用云开隆起(广西壮族自治区地质矿产局, 1985; 汪劲草等, 1994)、云开地体(康云骥, 2001; Wanetal., 2010)、云开褶皱带(郭福祥, 1994)、云开地块(Wangetal., 2007a)、云开构造带(Wangetal., 2007b)、云开造山带(Liang and Li, 2005; Lietal., 2010; Fengetal., 2014)等术语概括其大地构造属性。扬子板块与华夏板块自新元古代拼合形成华南板块后,又至少先后经历了加里东期、印支期和燕山期三期构造-热事件(Wangetal., 2011)。受多期构造-热事件的影响,云开地区加里东期、印支期和燕山期花岗岩均有出露,并且还发育大面积混合岩和多条韧性剪切带。因此,云开地区是国内研究花岗岩与混合岩岩石成因关系的一个良好场所而受到关注(张伯有和俞鸿年, 1992; 王联魁等, 2003)。
花岗岩与混合岩之间可能的成因联系是成因岩石学研究的重要课题(Johannesetal., 2003; Sepahietal., 2013; Sugaetal., 2016),而岩浆的形成、分离结晶、上升、侵位,是地壳物质及能量从深部到浅部的重要交换过程,对研究大陆地壳演化有重要意义,因此前人对此进行过大量研究(England and Thompson, 1986; Simpsonetal., 2000; Solar and Brown, 2001; Annenetal., 2006; Brown, 2007; Qiuetal., 2014)。混合岩多在深熔作用下产于变质程度高的地层中(Sawyer, 1998, 2001),由于原岩在不同温-压条件下发生熔融和分离结晶的程度有所不同,因此形成的混合岩的岩石结构构造、矿物组成及化学成分也有所差异,总的可以分为两大类,即变熔混合岩和全熔混合岩(Brownetal., 1995; Milordetal., 2001; Solar and Brown, 2001; Sugaetal., 2016)。变熔混合岩熔融程度较低,整体还处于固态,仍保留了部分熔融前的原岩结构,长英质脉体(熔融体)与暗色基体(残留体)间呈厘米级间隔(Sawyer, 1994, 2001; Brown, 1994; Brownetal., 1995)。而全熔混合岩中,原岩熔融程度和熔体分离程度都较高,结构几乎完全破坏,岩石结构均一化,矿物颗粒增大,残留的原岩固体部分已失去凝聚力,与熔融体一起形成高粘度的“晶粥”,或者成为能迁移的塑性体,其流变学特征已类似于岩浆(Sawyer, 1998; Milordetal., 2001; Whiteetal., 2005; Sugaetal., 2016)。Milordetal. (2001)根据岩石中暗色矿物(主要是黑云母)含量,将全熔混合岩分为三类:深色、中色和浅色全熔混合岩,黑云母含量分别为>30%、10%~30%和<10%。重熔花岗岩一般是浅色的,在成因上可能与其周围的混合岩有关,是混合岩中熔融部分与残余部分分离后在合适的位置聚集结晶而成(Milordetal., 2001; Solar and Brown, 2001),另外可能还有一些外部岩浆及含水流体的加入,从而增加了熔体的量(Johannesetal., 2003)。然而,在云开地区,混合岩周边却分布着一些暗色的S型花岗岩岩体,例如在河台金矿区西北部出露的印支期云楼岗岩体,它在成因上与河台金矿区中大量的混合岩是什么关系呢?张伯有和俞鸿年(1992)认为云开地区混合岩是由糜棱岩改造而来,而花岗岩则是混合岩化的最终产物,但是缺少相应的年代学及地球化学证据。王联魁等(2003)按形成方式将花岗岩分为三类,即混合岩建造、深熔花岗岩建造和岩浆花岗岩建造,并对云开地区花岗岩的岩石学、地球化学特征及形成条件进行了较为详细的研究,但是并未深入讨论它们之间的成因关系。因此,本文将以云开地区河台金矿区内云开群混合岩及云楼岗岩体为研究对象,通过岩石学、矿物学、地球化学、地质年代学的研究,探讨暗色的S型花岗岩与混合岩之间的成因关系,从而进一步丰富岩浆岩的成因岩石学理论,并为云开地区大地构造演化提供新的证据。
图1 华南地区大地构造及印支期花岗岩分布图(a, 据Qiu et al., 2014修改)和云开地区地质简图(b, 据丘元禧和梁新权, 2006; 彭松柏等, 2006修改)
1 构造背景
华南板块由扬子板块和华夏板块构成,其界线为江山-绍兴缝合带(图1a),但由于多期强烈构造事件的叠加,该缝合带向南西延伸逐渐模糊不清,导致两板块的南西边界有较大争议(图1a, Yanetal., 2006; Wan, 2012)。云开地区位于华夏板块的西北部边界,区域内出露的云开群和高州杂岩过去通常被作为华夏地块的前寒武变质基底,其上覆未变质-弱变质的奥陶系-白垩系(图1b),然而,最新研究表明,这两套地层实为同一套地层,统称为云开群,两者沉积时代相同,为晚新元古代-早古生代,并非古元古代-早新元古代(叶真华等, 2000; 邝永光等, 2001; Wangetal., 2007a; Wanetal., 2010; Chenetal., 2012; 周雪瑶等, 2015; 焦骞骞等, 2017)。云开群变质程度由绿片岩相到麻粒岩相,岩性为片岩、千枚岩、板岩,局部为片麻岩、角闪岩、麻粒岩、混合岩。锆石U-Pb定年表明,云开群经历了两期变质作用,一期集中在约440Ma,与加里东运动有关;在印支期约240Ma时,云开群局部又发生了再活化,形成变质程度更高的岩石,例如,混合岩、硅线石-石榴子石-堇青石片麻岩(Wangetal., 2007a, 2012; Wanetal., 2010)。
云开地区有大量花岗质侵入体,时代从加里东期到燕山期均有分布。信宜花岗岩锆石U-Pb年龄为460~430Ma,其形成与加里东期造山活动有关(图1b, 彭松柏等, 2006; Wangetal., 2007a; Wanetal., 2010)。云开地区出露的印支期S型花岗岩年龄集中在260~245Ma,少量为230Ma,并且主要沿断裂分布(图1b, Chenetal., 2011)。例如,在广宁-罗定动力变质带北东段,特别是河台金矿区附近(邱小平, 2004);在防城-灵山断裂带附近大容山-十万大山也有大量S型花岗岩(祁昌实等, 2007; Jiaoetal., 2015)。在燕山期侵入形成大量未变形的I型花岗岩沿着云开地区西南边缘分布(蔡明海等, 2002; 邱小平, 2004; Wangetal., 2007a; Linetal., 2008)。区域上构造线以NE-NNE方向为主,可见几条近于平行的剪切带系统,例如防城-灵山断裂F1,罗定-广宁断裂F2,吴川-四会断裂F3等(图1b)。这些区域性大断裂(韧性剪切带)主要是在印支期造山作用下经过约248~220Ma和220~200Ma两期构造活动形成的(Wangetal., 2007a; 丁汝鑫等, 2015; Jiaoetal., 2017),并且控制着区域内矿产的分布和产出。例如,沿着罗定-广宁断裂有新洲金矿区、河台金矿区、罗定金矿区的分布,其中河台金矿区是本文的主要研究对象。
2 矿区地质及岩石学特征
河台矿区及外围出露的地层主要有云开群、奥陶系、志留系(图2)。云开群分布在矿区北部,为一套整体无序的变质岩组成,岩性以变粒岩、片麻岩、混合岩为主,局部遭受强烈韧性剪切作用形成糜棱岩系列岩石,河台金矿的矿体就产于这些糜棱岩带(ML9、ML11、ML12、ML13、ML18)中。奥陶系与志留系分布在矿区南部,以薄层浅变质砂岩、粉砂岩、及薄层板岩为主,通过F1断裂与云开群地层接触。矿区西部出露印支期云楼岗岩体;矿区东北部为燕山期的伍村巨斑状黑云母花岗岩,单颗粒锆石U-Pb年龄153.6±2.1Ma(翟伟等, 2005)。
图2 河台矿区地质图(据陈骏和王鹤年, 1993修改)
2.1 云楼岗岩体岩石学特征
云楼岗岩体出露面积大于100km2,岩性主要为中粗粒黑云母二长花岗岩、黑云母斜长花岗岩,具花岗结构、块状构造(图3a, b),局部受剪切形成糜棱岩化花岗岩。岩石主要由长石(55%)、石英(20%)、黑云母(20%)和白云母(5%)组成(图3c, d)。其中黑云母呈自形-半自形,与长石和石英近于同时形成,在其边部分布有较小的白云母。前人利用全岩Rb-Sr法、单颗粒锆石全熔法等获得年龄为242~209Ma(崔遥, 1989; 叶伯丹, 1989)。由于这些方法精度不高或者缺少锆石的形态结构特征,因此可靠性难以评价。本次研究将利用LA-ICP-MS锆石U-Pb法对其进行精确测年。
图3 岩石样品野外及镜下照片
2.2 云开群混合岩岩石学特征
河台矿区内分布着大面积的云开群混合岩,这些混合岩大多属于全熔混合岩,混合岩化之前的岩石结构已完全破坏,岩石中矿物颗粒均一化、粗粒化。从颜色及矿物成分看,属于中色-浅色的全熔混合岩,主要由长石(55%)、石英(25%)、黑云母(10%),白云母(5%)和绢云母集合体(5%)组成(图3e, f)。由于距离剪切带较近,受其影响,长石边缘有明显细粒化现象,绢云母分布于长石裂隙中,表明其晚于长石,可能与后期热液活动有关。
3 样品采集及实验方法
9个花岗岩样品采自河台矿区云楼岗岩体东部,并且靠近云开群混合岩的位置(图2)。7个混合岩样品采自高村金矿-140m中段和云西金矿+10m中段(图2)。野外所采集的花岗岩样品表面风化较强(图3a, b),首先对其进行实验前处理,将表面风化部分去除掉,以免影响实验结果。
3.1 LA-ICP-MS锆石U-Pb定年
锆石分选在河北省诚信服务有限公司完成,采用常规方法将样品粉碎至80目以上,并采用电磁选方法进行分选。在双目镜下挑选出晶形和透明度较好,无裂纹,粒径足够大的锆石颗粒作为测试对象。锆石制靶和阴极发光(CL)图像在重庆宇劲科技有限公司完成。锆石年龄测试在中国科学院广州地球化学研究所矿物学与成矿学中国科学院重点实验室完成,使用仪器为LA-ICP-MS,仪器型号为Resolution M50 Agilent 7500a,厂家Resonetics Agilent,光斑为29μm。采用标准锆石Plesovice(206Pb/238U加权平均年龄为337.13±0.37Ma(Slmaetal., 2008))和Temora(206Pb/238U加权平均年龄为416.6±1.0Ma(Blacketal., 2003))作为外标,元素含量采用 NIST SRM610作为外标,29Si作为内标元素(锆石中SiO2含量为32.8%)(袁洪林等, 2003),分析方法参考Yuanetal. (2004)方法;普通铅校正采用Andersen (2002)推荐的方法;锆石的同位素比值及微量稀土元素含量计算采用ICPMSDATECAL程序(Liuetal., 2008, 2010),年龄计算及谐和图的绘制采用Isoplot 2006(Ludwig, 2003)。
3.2 全岩地球化学和Sr-Nd同位素测试
将新鲜的花岗岩和混合岩样品磨碎到200目,以备主微量和Sr-Nd同位素测试。测试工作在中国科学院广州地球化学研究所同位素地球化学国家重点实验室完成。
主量元素利用Rigaku 100e XRF进行测试,分析结果的误差(相对标准偏差值)小于3%(H2O+除外)。样品粉末加入Li2B4O7(1:8),在V8C自动熔融机(Analymate,中国)熔融机上加热至1150~1200℃,制成均一的玻璃片,然后进行X射线荧光(XRF)分析。
微量元素分析在Agilent 7700X型电感耦合等离子体质谱仪(ICP-MS)上进行分析,稀土元素和Y分析误差小于4%,其它微量元素在3%~7%之间。将约40mg样品粉末放入Teflon杯中并加入HF+HNO3,100℃加热7天溶解。之后烘干并加入3%的HNO3,进行测试。利用Rh作为内标进行校正(Liuetal., 1996)。
分离纯化后的Sr和Nd溶液在Micromass Isoprobe型多接收器等离子质谱仪(MC ICP-MS)上进行87Sr/86Sr和142Nd/144Nd比值测定,详细的测试方法见Lietal. (2006)。测试过程中的质量分馏效应分别采用86Sr/88Sr=0.1194和146Nd/144Nd=0.7219进行校正。Shin Etsu JNdi-1标准的143Nd/144Nd和87Sr/86Sr测定值为分别0.512115(2σ)和0.710260(2σ)。
表1云楼岗花岗闪长岩和云开群混合岩锆石LA-ICP-MSU-Pb年龄
Table 1 Zircon LA-ICP-MS U-Pb ages of the Yunlougang granodiorite and the Yunkai Group migmatite
测点号ThU(×10-6)Th/U同位素比值同位素年龄(Ma)协和度207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ(%)样品5H031云楼岗花岗闪长岩-0819.04880.040.05140.000.28720.020.04020.0038710927414258594-1255.06920.080.05010.000.26940.010.03930.002119724210248397-1343.45460.080.05060.000.27780.010.04020.0076.011522912249491-1444.84310.100.05080.000.27860.010.04010.001769924410249397-154.552940.020.04970.000.27160.010.03990.0030911125611250397-1610.81930.060.05190.000.28400.020.04000.0028016025413250498-171273990.320.04990.000.27350.010.03990.00235932509251499-1848.21220.390.05330.000.29760.010.04060.0021114424213252496-1959.51160.520.05440.000.30920.020.04080.001879824510252397-2038.192.60.410.05050.000.27350.010.04000.0018911924411252496-2251.61930.270.05260.000.28660.010.03960.0028315825413253399-2466.66050.110.05090.000.27980.010.03980.0021712924611253497-2513.23600.040.04960.000.27130.010.03940.00217942519253499-2686.22450.350.05180.000.28450.020.03960.002809425710253398-2721.65730.040.05190.000.28830.010.04010.002329625010254498-2826.81110.240.04840.000.26990.010.04030.0025710725612254499-2923.54680.050.04990.000.27990.010.04030.002339424910254397-3053.65020.110.05050.000.28060.010.04000.001178624311255595-3138.41010.380.05020.000.26950.020.03990.00191852518255398-3263.22010.310.04750.000.25260.010.03940.0034310426512257496样品HT112云开群混合岩-103687950.460.05240.000.27870.010.03870.00302622506245498-121948380.230.05120.000.25850.010.03660.00256472335232399-131072000.540.05250.000.32540.010.04530.003068128610285899-144252961.440.05550.000.26710.010.03500.00432632406222492-161911930.990.04990.000.26000.010.03830.001911122358242596-182002580.780.05270.000.31320.020.04330.0031764277122731098-225.308110.010.04970.000.26780.010.03910.00189542415247397-232052690.760.05220.000.26130.010.03640.00295632366231397-2510217580.060.04960.000.28540.010.04170.00176372556263596-271012680.380.05160.000.27770.010.03900.00333612497247599-29991210.820.05270.000.28260.010.03900.003228525310247697-31803890.460.05230.000.33620.010.04670.002986829410294899-3637.213010.030.05640.000.38210.010.04910.00478383298309793-373693491.060.05360.000.28250.010.03830.00367522536242495-387334551.610.05270.000.27440.010.03780.00317522466239497-853.03790.140.05710.000.43760.010.05520.004945236911347793-95536000.920.05130.000.27670.010.03920.00254462485248499
图4 云楼岗花岗闪长岩(a)和云开群混合岩(b)LA-ICP-MS锆石U-Pb年龄协和图、代表性的锆石颗粒特征及年龄
图5 岩石地球化学分类TAS图解(a, 底图据Miyashiro, 1978; Middlemost, 1994)和A/NK-A/CNK图解(b, 底图据Chappell and White, 1992; Chappell, 1999)
图6 哈克图解(g,底图据Roberts and Clemens, 1993)
表2云楼岗岩体和云开群混合及变质沉积岩主量(wt%)和微量(×10-6)元素含量
Table 2 Major (wt%) and trace (×10-6) element contents of the Yunlougang pluton, the Yunkai Group migmatite and metasedimentary
样品号5H1715H1725H1735H1745H1755H1765H0315H0325H033HT112HT51HT52HT54HT57HT60HT80G0103-1G0104-1G0105-1岩性云楼岗岩体云开群混合岩云开群变质沉积岩SiO268.168.6867.6267.6367.8868.1165.9465.9466.0673.5072.2572.4673.8671.1671.9570.5762.9859.3575.28Al2O314.9414.7715.115.114.714.9616.0515.9416.0413.3314.4714.4611.8113.8914.5314.7820.0719.6710.16Fe2OT34.674.234.684.654.154.685.165.215.161.871.822.173.862.832.174.144.636.555.41MnO0.110.080.10.10.080.110.120.120.120.080.070.070.110.120.090.080.030.090.18TiO20.580.530.580.580.530.570.640.640.640.210.190.280.420.320.170.490.740.741.06MgO1.761.651.81.811.631.771.941.931.950.580.460.511.540.900.351.560.862.811.34CaO2.662.192.52.52.152.682.932.952.921.100.340.800.901.350.830.760.060.572.5K2O2.853.873.283.293.832.882.742.752.774.626.414.853.664.536.264.765.24.550.96Na2O2.862.562.752.832.562.93.043.023.062.402.263.030.872.651.840.770.462.121.92P2O50.210.210.210.210.190.210.260.260.260.330.150.180.150.250.360.120.090.10.19LOI1.110.920.821.121.610.980.830.880.882.020.990.922.411.870.852.033.942.990.66Total99.8599.799.4599.8299.3299.8699.6599.6399.85100.0599.4199.7399.6099.8799.41100.0799.499.899.8A/CNK1.181.191.191.181.201.171.211.201.201.221.281.241.681.181.281.893.082.081.16σ1.31.611.481.521.641.331.461.451.471.622.572.100.671.832.271.111.602.720.26Cs16.0915.8115.0816.4615.6615.4118.4821.420.546.7586.6066.1117.9528.6333.80113.98Sc8.638.578.829.348.218.747.878.979.015.2484.795.2888.6547.054.74211.028.612.78.2Ba265.3537.5355.3375.2528.4265.9137.8158161.1554233.5261.5348.6420.3186.6365.32444861167Ti311129913099322729433152280531993197116210601609232617268863035V65.6362.4264.4468.6261.4768.2559.5670.7371.1621.0111.5216.0849.1933.815.37365.27Mn794.7616697.2741.6595.6805.7735843.1867.5550.6569.3514.7820.9853.4685609.3Co7.566.717.468.156.817.427.17.848.172.9781.9882.6024.1843.6651.7166.543Cr34.5339.0840.1842.0936.2235.4635.3839.0743.1525.1717.0115.5155.9920.2419.5593.5187113104Ni14.3913.7616.0617.6911.2715.7713.9817.0417.588.1464.0415.02315.697.3652.74428.03Cu11.6511.729.9811.039.7413.5421.822.9423.310.7913.876.32120.6869.5911.5939.9Zn72.7367.7473.7868.758.7773.0772.8474.5379.7135.2267.78162.856.0844.4735.1948.82Pb18.9924.2220.0617.2121.3317.7810.8412.8412.5918.3551.7360.643.91611.735.892.738Ga20.119.5319.4318.8418.4219.9717.9419.9320.1712.5816.7418.5617.6319.1918.0719.31Ge1.671.841.71.731.731.572.011.081.471.6991.4861.9271.9841.8761.6032.364Rb183.9177.1157.9169.7177.5163.1152.5171.8171173.1274.1225.6185227.2267.225621624380Sr112.8117.5106.5116.5115.6115.984.8103.8104.955.5352.0356.8227.9984.1753.0527.7331107165Hf1.631.481.121.465.590.411.511.532.143.5842.1744.0994.8023.7781.8082.688Zr61.7560.4361.662.4566.0757.3962.1768.6381.4412465.13128.8159.4123.550.0589.02190118579
续表2
Continued Table 2
样品号5H1715H1725H1735H1745H1755H1765H0315H0325H033HT112HT51HT52HT54HT57HT60HT80G0103-1G0104-1G0105-1岩性云楼岗岩体云开群混合岩云开群变质沉积岩Nb16.1615.2215.9616.615.0816.3915.1217.4119.426.38710.8111.8411.5715.519.10613.6191825Ta1.821.241.571.441.491.511.21.584.521.4291.2731.131.2551.3830.5621.687282633Th14.6613.8714.2715.5714.6514.9212.1214.3914.026.0716.98719.9914.513.015.12615.63191946U3.583.373.743.673.383.723.744.034.489.4385.737.6594.7738.8819.0388.0843.14.75.7La31.0227.2525.3331.0227.1431.6723.1226.5129.9812.2913.3430.8827.2827.627.60335.2563.2752.5282.74Ce60.4253.4549.9461.353.1960.8646.2952.5559.6924.1826.5561.7352.5450.5516.2367.3792.4490.27146.2Pr7.466.716.197.616.687.525.886.537.462.9993.2317.5486.3435.9222.0798.1210.878.9813.86Nd25.9823.6621.7226.6923.3426.720.8423.1726.0111.511.8627.6423.220.678.20729.6845.137.556.5Sm5.024.544.125.074.515.14.064.55.12.5292.4725.1594.2643.8872.3925.5358.857.4410.5Eu0.380.380.320.350.380.40.30.330.450.7270.5280.6330.720.6690.4260.9941.411.261.42Gd5.084.564.155.064.615.164.024.585.312.5052.154.0073.7813.3432.7114.9557.556.9110Tb0.620.560.490.610.550.630.490.570.650.4530.3470.5160.5620.4640.6210.7611.230.931.38Dy2.842.552.312.712.572.942.312.723.022.7742.0092.563.1982.283.974.4636.145.77.27Ho0.420.380.350.410.380.440.340.420.470.5680.4040.4720.6580.3840.7380.9111.231.161.43Er1.111.040.971.121.071.180.971.161.31.5031.0991.2691.7620.9041.9232.4833.673.334.38Tm0.10.090.080.090.090.10.080.10.120.2240.1770.1910.2630.1310.2860.3750.480.470.6Yb0.930.820.780.890.820.980.780.921.061.4491.171.2371.6980.8891.782.4112.942.763.83Lu0.090.070.070.080.070.090.070.090.110.2260.1790.1920.2730.1420.2550.3760.450.490.69Y12.6211.3810.2412.0811.5112.2210.6411.4113.2414.0710.6311.8815.989.60520.8523.17282633∑REE141.5126.1116.8143125.4143.8109.5124.1140.763.965.5144.0126.5117.949.2163.7245.6219.7340.8LREE130.3116107.6132115.2132.3100.5113.6128.754.258.0133.6114.3109.336.9146.9221.9198.0311.2HREE11.210.19.21110.211.59.110.6129.77.510.412.28.512.316.723.721.829.6LREE/HREE11.611.511.71211.311.511.110.810.75.67.712.89.412.83.08.89.49.110.5(La/Yb)N23.9323.8423.2925.0023.7423.1821.2620.6720.296.088.1817.9111.5222.293.0610.4915.4413.6515.50(La/Sm)N3.993.873.973.953.884.013.683.803.793.143.483.864.134.592.054.114.624.565.09δEu0.230.260.240.210.250.240.230.220.260.880.700.430.550.570.510.580.530.540.42δCe110.970.970.980.980.970.970.970.980.990.990.980.971.000.980.861.021.06Fe2O3/MgO2.652.562.602.572.552.642.662.702.653.203.994.242.513.146.142.655.382.334.0410000Ga/Al2.542.492.432.352.362.522.112.362.371.782.182.422.822.612.352.46Rb/Sr1.541.511.631.411.481.461.661.631.803.125.273.976.612.705.049.236.972.270.48Nb/Ta8.8812.2710.1711.5310.1210.8512.6011.024.304.478.4910.489.2211.2116.208.060.680.690.76
注:云开群变质沉积岩数据引自Wanetal. (2010). A/CNK=Al/(Ca+Na+K),δEu=Eu/Eu*=EuN/(SmN×GdN)1/2,δCe=Ce/Ce*=CeN/(LaN×PrN)1/2
图7 全岩原始地幔标准化微量元素蛛网图(a,标准化值据Sun and McDonough, 1989)和球粒陨石标准化稀土元素配分图(b,标准化值据Boynton, 1984)
图8 云楼岗岩体和云开群混合岩U-Pb年龄-εNd(t)图解(a, 底图据周新民, 2007)和 (87Sr/86Sr)i-εNd(t)图解(b, 底图据Wang et al., 2011)
3.3 黑云母化学成分分析
对薄片进行详细观察后,选择5个具有代表性的样品薄片进行电子探针(EPMA)分析。电子探针分析在中国科学院广州地球化学研究所矿物学与成矿学中国科学院重点实验室电子探针(EMPA)实验室完成,所用仪器为日本电子JOEL公司生产的JXA-823V型电子探针,实验中加速电压为15kV,束流为2.0×10-8,束斑大小为5μm,矿物定量分析误差不超过3%。黑云母结构式以22个O离子数为标准进行计算,Fe2+和Fe3+利用林文蔚和彭丽君(1994)提供的方法进行估算。
4 实验结果
4.1 地质年代学
云楼岗岩体(样品5H031)中的锆石晶形良好,多为自形晶,长柱状,长100~200μm,长宽比1:3~1:4。CL图像上,锆石颗粒颜色较暗,不透明,具有典型的核-幔结构(图4a)。锆石继承核形态多样,而锆石幔部具有明显的震荡环带,Th/U比为0.02~0.52,具典型的深熔岩浆锆石的特点。锆石幔部20个点的206Pb/238U年龄为258~248Ma(表1、图4a),加权平均年龄为253±1.6Ma(MSWD=0.14)。
云开群混合岩(样品HT112)中的锆石形态多样,自形-他形,大多为不规则状,锆石颗粒相对较小,长80~150μm,长宽比1:2~1:3。在CL图像上锆石发光性也有差别,多数发暗光,少数晶形较好的发亮光,且具有震荡环带(图4b)。选择17颗形态不同的锆石中进行U-Pb定年,获得的206Pb/238U年龄值较分散,分布在347~222Ma(表1、图4b),但是其中有10个年龄集中在248~231Ma,加权平均年龄为240.3±5.1Ma(MSWD=3.8)。另外有6颗锆石年龄较老且分散,在347~263Ma,可能是由于原岩在熔融过程中,锆石没有完全被改造而继承有原岩的年龄信息,因而偏大。
4.2 全岩主微量
表2为云楼岗岩体的10个样品和云开群混合岩的6个样品的主微量元素分析测试结果。
4.3 全岩Sr-Nd同位素
表3为5个云楼岗岩体样品和7个云开群混合岩样品的Sr-Nd同位素测试结果。
表3云楼岗岩体和云开群混合及变质沉积岩全岩Sr-Nd同位素
Table 3 Sr-Nd isotopic composition of the Yunlougang pluton, the Yunkai Group migmatite and metasedimentary
样品号Sm(×10-6)Nd(×10-6)147Sm144Nd143Nd144Nd2σ(Err.)εNd(0)εNd(t)tDM2(Ma)Rb(×10-6)Sr(×10-6)87Rb86Sr87Sr86Sr2σ (Err.)87Sr86Sr()i云楼岗岩体5H1715.0225.980.11680.5120020.000006-12.41-9.841821183.9112.84.730.73096160.0000070.713945H1724.5423.660.11600.5120820.000005-10.86-8.251692177.1117.54.370.73247240.0000110.716745H1745.0726.690.11480.5120000.000004-12.44-9.801818169.7116.54.220.7323120.0000090.717115H1754.5123.340.11690.5120040.000006-12.37-9.801818177.5115.64.450.73241250.0000080.716395H1765.1026.700.11550.5120000.000004-12.44-9.821819163.1115.94.080.73241250.0000080.71773云开群混合岩HT1122.5311.500.13290.5118290.000006-15.78-13.842133173.155.59.070.76565350.0000080.73469HT512.4711.860.12600.5118450.000008-15.47-13.312091274.152.015.350.78292850.0000110.73051HT525.1627.640.11280.5118210.000006-15.93-13.372096225.656.811.560.76951570.0000070.73006HT544.2623.200.11110.5117670.000005-17.00-14.392178185.028.019.230.76387770.0000110.69823HT573.8920.670.11370.5120210.000010-12.04-9.511783227.284.27.840.74114940.0000110.71440HT602.398.210.17620.5119470.000009-13.49-12.872055267.253.114.670.77290060.0000110.72283HT805.5429.680.11270.5117140.000009-18.02-15.462264256.027.726.930.78960090.0000130.69768云开群变质沉积岩G0103-18.0943.130.11340.5118880.000008-14.60-10.001910G0104-17.1837.080.11710.5117720.000001-16.90-12.402170G0105-111.0859.250.11310.5117100.000012-18.10-13.402170
注:云开群变质沉积岩引自Wanetal. (2010)
云楼岗岩体的87Sr/86Sr比值测试结果为0.7309616~0.7324724。按照253Ma年龄计算岩体的初始Sr-Nd同位素组成,计算结果为初始比值(87Sr/86Sr)i为0.71394~0.71773,εNd(t)为-9.84~-8.25(图8a),Nd模式年龄为1821~1692Ma。可见,云楼岗岩体的Sr-Nd同位素组成与华南东部印支期S型花岗岩及东澳大利亚拉克兰褶皱带(LFB)的Sr-Nd同位素组成是基本一致的(图8b, Healyetal., 2004; Wangetal., 2011)。
云开群混合岩的87Sr/86Sr比值测试结果为0.7411494~0.7896009。基于240Ma年龄计算岩体的初始Sr-Nd同位素,初始比值(87Sr/86Sr)i为0.69768~0.73469,εNd(t)为-15.46~-9.51(图8a),Nd模式年龄为2264~1783Ma。
4.4 黑云母成分
云楼岗岩体和云开群混合岩中的黑云母特征相似,都为棕红色叶片状,长0.5~3mm。表4为电子探针测试的黑云母化学成分和计算得到的离子含量。黑云母AlⅣ和Fe#(Fe#=Fe(T)/(Fe(T)+Mg2+))含量可以用来判断岩石的过铝质和氧化还原状态(Shabanietal., 2003)。
云楼岗岩体中黑云母中Fe#为0.48~0.52、Mg#为0.48~0.53、AlⅣ为2.612~2.758apfu(每个结构单元中的原子数)、Ti4+为0.189~0.332apfu。在Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn)图上(图9a)显示为Mg质黑云母,而在10×TiO2-(FeOT+MnO)-MgO三角图解上(图9b)显示为受到改造再平衡的黑云母。利用黑云母中Ti(Henryetal., 2005)和AlT含量(Uchidaetal., 2007)可以作为地质温度计,计算得到的黑云母形成温度为589~679℃,平均651℃;压力为245~278MPa,平均266MPa;相当于9.27~10.51km,平均10.08km。
云开群混合岩中的黑云母与云楼岗岩体中的一样,也是Mg质的,并且是受到改造再平衡的黑云母(图9a, b),其中Fe#、Mg#值分别为0.44~0.56和0.44~0.56,AlⅣ和Ti4+含量分别为2.663~2.880apfu和0.243~0.488apfu。计算得到形成温度为623~733℃,平均664℃;压力为211~323MPa,平均266MPa;相当于深度7.98~12.21km,平均10.06km。
图9 黑云母Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn)分类图解(a, 底图据Foster, 1960)、10×TiO2-(FeOT+MnO)-MgO图解(b, 底图据Nachit et al., 2005)和Ti-Mg/(Fe+Mg)温压图解(c, 底图据Henry et al., 2005)
5 讨论
5.1 云楼岗岩体与云开群混合岩的成因联系
表4云楼岗岩体和云开群混合黑云母化学组成(wt%)
Table 4 Geochemical compositon of the biotites in the Yunlougang pluton and the Yunkai Group migmatite (wt%)
测点号SiO2TiO2Al2O3FeOMnOMgONa2OK2OH2OTotalSi4+AlⅣAlⅥTi4+Fe3+Fe2+Mn2+Mg2+(wt%)(apuf)Fe#Mg#T(℃)P(MPa)H(km)云楼岗岩体5H031-136.672.8717.6818.040.359.960.089.092.0796.815.3092.6910.3240.3130.1762.0080.0422.1490.50.5672.5260.59.855H031-236.872.6217.6018.150.3510.080.069.242.0797.045.332.670.3260.2850.1812.0130.0432.1730.50.5657.3254.89.635H031-336.183.0417.7718.240.339.980.109.512.0797.215.2422.7580.2750.3320.1692.0420.0412.1550.510.49679.8266.010.055H031-436.602.7317.6717.910.3510.020.049.392.0796.795.3082.6920.3260.2980.1741.9980.0432.1670.50.5664.6261.59.885H031-536.562.2217.9018.390.2910.140.089.342.0796.995.2992.7010.3550.2420.1722.0570.0352.1910.50.5629.0273.010.325H031-636.282.8417.8718.710.349.740.089.292.0797.215.2592.7410.3090.3090.1732.0950.0412.1060.520.48666.5271.210.255H031-735.952.8417.5917.960.319.700.089.022.0495.505.2852.7150.3310.3140.1712.0360.0382.1250.510.49671.0269.910.205H031-836.452.6917.8918.480.339.910.069.502.0797.385.2712.7290.3180.2930.1722.0630.042.1370.510.49659.8270.210.215H031-936.352.8217.7018.380.409.520.099.132.0696.465.2992.7010.3380.3090.1762.0650.052.0690.520.48666.5267.810.125H031-1036.202.7417.9618.830.379.700.089.032.0796.985.2562.7440.3270.2990.1712.1150.0462.10.520.48661.1277.510.495H031-1136.762.5917.3218.260.3510.020.089.212.0696.655.3432.6570.3080.2830.1902.0290.0432.1710.510.49654.0245.49.275H031-1236.512.8317.6617.690.339.940.099.272.0696.385.312.690.3340.310.1721.9790.0412.1550.50.5671.0263.39.955H031-1336.332.8417.5217.710.329.890.099.262.0595.985.3092.6910.3240.3120.1741.9900.0392.1540.50.5672.0260.59.855H031-1436.492.8717.8118.030.349.970.099.272.0796.945.2842.7160.3220.3120.1702.0140.0412.1530.50.5672.0267.510.115H171-137.111.7317.7017.090.3210.600.049.572.0796.245.3882.6120.4140.1890.1721.9030.042.2950.470.53589.2263.99.975H171-236.421.9417.6717.220.3610.540.139.392.0595.715.3272.6730.3710.2130.1671.9390.0442.2970.480.52610.2269.310.185H171-336.272.6117.9418.240.3410.220.079.852.0797.605.2422.7580.2950.2830.1662.0380.0412.2010.50.5656.2272.110.285H171-436.662.6218.0417.920.2910.280.069.752.0897.705.2742.7260.330.2830.1641.9910.0352.2040.490.51658.4273.010.325H171-537.212.0917.9217.220.2910.470.069.632.0896.965.3632.6370.4040.2260.1681.9070.0352.2490.480.52621.4268.410.145H171-635.402.4717.0316.460.299.700.069.521.9992.935.3382.6620.3630.280.1681.9080.0372.180.490.51656.6263.69.965H171-736.282.3117.8317.650.3410.350.099.552.0696.475.2822.7180.340.2530.1641.9860.0422.2470.490.51639.3273.610.345H171-836.691.8917.9517.400.2710.470.109.402.0696.225.3332.6670.4060.2070.1641.9510.0332.2690.480.52604.6278.110.51平均36.472.5517.7317.910.3310.050.089.372.0696.555.3022.6980.3380.2790.1712.0060.0402.1790.500.50651.5266.910.09云开群混合岩HT104-135.802.8018.4518.670.189.130.149.502.0696.735.2212.7790.390.3070.1552.1220.0221.9840.530.47663.5307.211.61HT104-235.712.5218.1118.510.189.610.089.452.0596.225.2332.7670.3590.2770.1602.1080.0232.10.520.48648.3294.211.12HT104-335.202.2618.5418.760.179.540.119.532.0496.155.1762.8240.3860.250.1492.1580.0222.0910.520.48630.4319.612.08
续表4
Continued Table 4
测点号SiO2TiO2Al2O3FeOMnOMgONa2OK2OH2OTotalSi4+AlⅣAlⅥTi4+Fe3+Fe2+Mn2+Mg2+(wt%)(apuf)Fe#Mg#T(℃)P(MPa)H(km)HT104-435.002.1717.9418.230.169.180.109.322.0094.095.2442.7560.4090.2440.1572.1270.022.050.530.47623.8306.011.56HT104-535.652.2718.7018.400.199.380.159.532.0596.315.2162.7840.4380.250.1472.1050.0242.0460.520.48630.4323.312.22HT104-634.894.4217.6518.390.169.430.139.052.0496.155.122.880.170.4880.1612.0960.022.0630.520.48733.3271.210.25HT104-735.572.5118.3018.860.199.500.059.382.0596.405.2092.7910.3650.2760.1582.1510.0232.0730.530.47645.7303.311.46HT104-835.382.5617.2718.890.159.690.079.322.0295.345.252.750.2680.2850.1832.1610.0182.1430.520.48653.2261.59.88HT104-935.262.1918.4718.450.199.440.159.362.0395.535.2062.7940.4180.2430.1492.1290.0232.0770.520.48625.2320.212.10HT13-136.313.0416.4819.410.269.340.079.642.0496.585.3352.6650.1870.3360.2272.1580.0322.0450.540.46676.2211.27.98HT13-236.212.8317.0219.360.289.250.069.642.0496.685.3112.6890.2510.3120.2052.1700.0342.0230.540.46664.3237.88.99HT13-336.262.9617.1318.930.209.300.069.712.0596.595.3112.6890.2650.3260.1972.1220.0242.0310.530.47673.2242.19.15HT13-436.292.9516.5919.240.229.380.069.722.0496.505.3342.6660.2060.3260.2212.1440.0272.0560.530.47673.2217.28.21HT13-536.272.9717.2018.900.279.380.089.682.0596.785.3012.6990.2610.3260.1942.1170.0332.0430.530.47673.2243.99.22HT13-635.832.7117.8320.370.278.960.058.972.0597.045.2382.7620.3080.2980.1852.3050.0331.9520.560.44653.3277.210.48HT13-736.572.7417.6219.560.279.010.079.742.0797.645.3082.6920.3190.2990.1912.1830.0331.9490.550.45655.6259.39.80HT13-835.112.8516.2618.750.208.750.059.221.9793.165.3372.6630.2470.3260.2142.1700.0261.9840.550.45669.7228.78.64HT13-935.822.6517.0118.710.269.280.079.492.0295.315.3152.6850.2880.2950.1942.1280.0332.0520.530.47657.0247.89.37HT13-1035.753.1417.0619.750.269.320.089.232.0496.635.2512.7490.2020.3470.2012.2240.0332.040.540.46681.2241.29.11HT13-1136.203.0117.0619.220.199.450.109.532.0596.815.2932.7070.2310.3310.2012.1490.0242.0610.530.47675.6237.28.97HT13-1236.142.9517.0219.620.289.110.049.652.0596.865.2992.7010.2370.3260.2062.1990.0351.9910.550.45669.7237.28.97HT5-136.602.6918.0116.450.1611.670.0710.062.1097.815.2332.7670.2660.2890.1501.8170.0192.4870.440.56673.7266.010.05HT5-236.562.7517.5716.610.1511.800.119.692.0997.325.252.750.2210.2970.1621.8330.0182.5260.440.56678.0247.29.34HT5-336.662.7017.8916.630.1511.890.099.992.1098.105.2282.7720.2330.290.1551.8290.0192.5270.440.56674.3257.59.73HT5-436.492.7718.3916.370.1311.820.109.902.1198.075.1952.8050.2780.2960.1401.8090.0162.5090.440.56677.5281.110.63HT5-536.742.7918.4116.600.1711.400.0910.062.1198.365.2222.7780.3030.2990.1441.8290.022.4150.450.55676.6280.510.60HT5-636.312.7317.8516.270.1811.210.109.822.0796.545.2542.7460.2950.2970.1511.8180.0222.4180.450.55675.5268.410.14平均35.952.7717.6218.440.209.820.089.562.0596.515.2552.7450.2890.3050.1762.0800.0252.1380.510.49664.1266.210.06
注:结构式以22个氧原子进行计算. Fe#=FeT/(FeT+Mg2+);Mg#=Mg2+/(FeT+Mg2+);温度T={[ln(Ti)-a-c(XMg)3]/b}0.333(Henryetal., 2005);压力P=[3.03×TAl-6.53(±0.33)]×100 (Uchidaetal., 2007);深度H=P/ρg, ρ=2700kg/m3, g=9.8m/s2
然而,云楼岗岩体与云开群混合岩、变质沉积岩之间地球化学特征在许多方面也有区别。首先,云楼岗岩体的SiO2含量比云开群混合岩要低。云开群混合岩的稀土配分曲线比较平,与云开群变质沉积岩形态类似,但是比较低(图7b)。云楼岗岩体的稀土配分曲线与之有较大差别,尽管轻稀土元素LREE与云开群变质沉积岩特征类似,但是重稀土元素则较曲折,不似云开群变质沉积岩那般平直。另外,云楼岗岩体的 (La/Yb)N为22.80,大于云开群变质沉积岩((La/Yb)N=14.86)及混合岩((La/Yb)N=11.36),因此其稀土配分曲线整体斜率也更大。全岩Rb/Sr比可以作为指标用来判别熔体成因(Harris and Inger, 1992; Harrisetal., 1995; Solar and Brown, 2001)。云楼岗岩体的Rb/Sr比值相对较低,为1.4~1.8,可能有外部岩浆的混入,使得云开群变质沉积岩发生重熔,并且由于混合岩浆熔融程度高,SiO2较低,从而导致Rb/Sr分馏程度较低。而云开群混合岩的Rb/Sr比值高,为2.7~9.2,分馏程度高,说明云开群变质沉积岩中的云母发生了脱水熔融。当然,仅是云母脱水熔融不足以提供足够的熔体形成混合岩,推测可能还有其它外部流体的渗入导致水流熔融(Wardetal., 2008; Sawyeretal., 2011)。大陆地壳中产生水流熔融的许多地方都靠近主要剪切带,这些剪切带为含水流体渗入到大陆地壳中提供了通道(Sawyer, 2010)。云开地区存在着一期左旋的剪切作用,时间在约240Ma(Jiaoetal., 2017),与本文所得到的混合岩年龄一致,可以作为含水流体的通道使云开群变质沉积岩发生水流熔融。因此,总体来看,云开群混合岩的地球化学特征与云开群变质沉积岩更加类似,表明混合岩仅来自于变质沉积岩的直接熔融;而云楼岗岩体则有可能来自于混合岩浆,并且由于外部岩浆的加入,使得其SiO2含量降低。另外,从Sr-Nd同位素特征来看,云楼岗岩体具有比云开群混合岩和变质沉积岩低的(87Sr/86Sr)i比值,高的εNd(t);前者的Nd模式年龄为1821~1692Ma(平均1794Ma),而后两者近乎一致为2178~1783Ma(平均2086Ma)和2170~1910Ma(平均2083Ma),前者明显小于后两者。当幔源的含水玄武岩侵入下地壳中时,能够形成一系列岩床,同时在地壳深部产生热区域,在这些热区域附近,由玄武岩不完全结晶残留下来的硅酸盐熔体可能与部分熔融的地壳岩石发生混合,从而形成混合硅酸盐熔体(Annenetal., 2006)。这种混合而成的硅酸盐熔体可能具有闪长岩和花岗闪长岩的成分(Sugaetal., 2016)。因此,在形成云楼岗花岗闪长岩体时,可能有来自地幔的年轻熔体成分加入到局部重熔的下地壳的熔体中从而形成混合岩浆。Jiaoetal. (2015)对云开地区的大容山S型花岗岩杂岩体的成因进行了详细研究,研究表明其形成于约249Ma,同样也是混合岩浆结晶形成的,有少量地幔物质的加入,与云楼岗岩体具有类似的成因,可见其成因在区域上有一定的普遍性。而云开群混合岩与变质沉积岩有近乎一致的Sr-Nd同位素特征及Nd模式年龄,因此,云开群混合岩是变质沉积岩部分熔融并在原地结晶形成的,没有明显的外部熔体(地幔物质)的混入。
从地质年代学上看,前人曾在云开群局部变质程度较高的片麻岩中获得242~236Ma的印支期变质重置年龄(Wangetal., 2007a; Wanetal., 2010),与本文获得的云开群混合岩的U-Pb年龄240.3±5.1Ma近乎一致,可见,云开地区在印支期发生过一次广泛的构造热事件。然而,云楼岗岩体的年龄是253±1.6Ma,在分析误差范围内比混合岩中变质锆石的年龄早了10Myr,因此,云楼岗岩体并非附近云开群混合岩化的产物,与河台矿区的混合岩也不是同源的。
5.2 大地构造意义
印支期造山运动奠定了中国东部的构造格局,对于区域构造发展、岩浆活动及变质作用有重要的影响(Huangetal., 1987; Ren, 1991, 1996; Wangetal., 2007a)。在华南板块印支期过铝质花岗岩以大规模的岩盖及广泛分布的小型侵入体形式产出,分布面积约21000km2,约占整个华南花岗岩的12.3%(图1a; 湖南省地质矿产局, 1988; 孙涛, 2006; Maoetal., 2011)。其中包括60%强过铝质(A/CNK>1.1)S-型花岗岩露头,30%的弱过铝质(A/CNK=1.0~1.1)和10%的钙碱性I型花岗岩(邓希光等, 2004; Sunetal., 2005; Qiuetal., 2014)。然而,由于出露的印支期花岗岩年龄范围较广,在约260~210Ma都有产出(Chuetal., 2012a, b; Wangetal., 2007a, 2013; Qiuetal., 2014),使得印支期构造岩浆事件的时空分布格局及终结时间存在争议(Wangetal., 2013)。华南板块位于两大碰撞造山带之间,北部与华北板块之间为大别苏鲁超高压变质带,西南与印支板块之间为宋马带(图1a)。由于位于这些造山带之间,华南板块在印支期整体处于压缩环境,并且广泛发育印支期花岗岩。华南板块西南部大量的花岗岩形成在258~242Ma,可能主要与古特提斯洋的闭合导致印支板块北部与华南板块的陆陆碰撞有关(Nametal., 1998; Carteretal., 2001; Wangetal., 2010);而华南板块与华北板块碰撞的时间相对较晚,因此华南板块北部花岗岩多为240~225Ma(Lietal., 1993; Zheng, 2008; Zhaoetal., 2013)。
在河台地区,印支期岩浆岩、混合岩和韧性剪切带并存,说明它们与造山活动有密切关系(Solar and Brown, 2001; Johannesetal., 2003; Sepahietal., 2013)。但三者的形成时间又有所不同,可能代表造山作用不同演化阶段的产物。云楼岗岩体的侵入(约253Ma)和云开群混合岩的形成(约240Ma)可能都与古特提斯洋的闭合以及华南板块与印支板块之间的陆陆碰撞有关,但是岩浆作用的时间却比混合岩化的时间早了10Myr。云楼岗岩体形成在253Ma的同碰撞压缩条件下,地壳加厚导致下地壳岩石局部发生深熔,并且有少量年轻的地幔物质加入。随后,在240Ma随着造山带的坍塌剥蚀,导致云开地区地壳降压熔融从而形成混合岩(Wangetal., 2012)。而此时,在空间上岩浆岩则出现在广东中部,甚至更东部的湖南江西一带(图1a)。可见,碰撞对华南板块的影响随时间逐渐向东迁移。前人研究表明,河台地区剪切带的形成时间有两期,早期为左旋,发生在约240Ma,晚期为右旋,发生在约204Ma(Jiaoetal., 2017)。混合岩化的发生时间与早期左旋运动的时间大致相当。这些剪切带和混合岩代表了碰撞晚期的逆冲走滑作用和变质沉积岩的降压熔融(Wangetal., 2007b, 2012)。综上所述,云开地区印支期的花岗岩可能是华南板块和印支板块碰撞高峰期的作用产物,而云开群混合岩和韧性剪切带则可能是终了期的产物。
6 结论
(1)云楼岗花岗闪长岩体与邻近的云开群混合岩不是同源的,不是混合岩化的最终产物。岩体可能是地壳深熔并有少量年轻的地幔物质混入形成的混合岩浆结晶而成。而云开群混合岩则可能仅是云开群变质沉积岩原地熔融形成的。
(2)云楼岗岩体的形成时间是约253Ma,云开群局部混合岩化的时间是约240Ma,可能分别代表的是华南板块和印支板块碰撞高峰期和终了期的产物。