东天山卡拉塔格地区镁铁质岩体年代学、岩石地球化学研究*

2019-11-13 01:30周国超王玉往石煜解洪晶李德东郭博然
岩石学报 2019年10期
关键词:东天山辉长岩塔格

周国超 王玉往 石煜 解洪晶 李德东 郭博然

1. 中国地质大学地球科学与资源学院,北京 1000832. 北京矿产地质研究院,有色金属矿山深部资源勘查工程技术研究中心,北京 1000121.

新疆东天山地区发育有大量的二叠纪镁铁-超镁铁质岩体,形成了多个岩浆型铜镍硫化物矿床及钒钛磁铁矿矿床(秦克章, 2000; 毛景文等, 2002; Maoetal., 2008; Tangetal., 2012; Suetal., 2011, 2012; 王玉往等, 2009, 2010, 2013; 石煜等, 2017a, b; Shietal., 2018),这些岩体和矿床沿区域断裂分布在多个构造单元内,是我国重要的正岩浆矿床集中产区(王京彬和徐新, 2006; 刘德权等, 2005; Qinetal., 2003)。前人研究在岩浆源区、岩浆演化、成矿作用及构造背景等方面取得了许多重要的成果,认为其成岩成矿时代集中在268~300Ma(韩宝福等, 2004; 三金柱等, 2010; 李锦轶等, 2006; 唐冬梅等, 2009; Qinetal., 2011; Sunetal., 2013a, b),母岩浆主要来自亏损地幔(邓宇峰等, 2011; Tangetal., 2012, 2013; 赵云等, 2016),在演化过程中发生了不同程度的岩石圈物质混染,混染作用导致了岩浆中硫达到饱和而成矿(钱壮志等, 2009; 孙涛等, 2010; Hanetal., 2013; Maoetal., 2016)。对于该期岩浆活动的构造背景认识分歧较大,主要有:地幔柱成因(Zhouetal., 2004; Pirajnoetal., 2008);俯冲作用所致(毛启贵等, 2006; Hanetal., 2010, 2013);后碰撞阶段幔源岩浆底垫作用所导致的地壳伸展作用的结果(韩宝福等, 2004; 王京彬和徐新, 2006; Wangetal., 2008);后碰撞伸展作用和地幔柱叠加(Qinetal., 2011; Suetal., 2011)。

另一方面,关于区内铜镍硫化物矿床的成因,一些学者认为,东天山地区的铜镍硫化物矿床形成于岩浆通道成矿系统(Maoetal., 2014a; Zhaoetal., 2016),该系统上部的同时期玄武岩和辉绿岩脉等可能由于大规模的剪切走滑作用而在随后的地质历史中被剥蚀掉,只保留了下部赋矿的岩浆通道,形成造山带背景下特殊的岩浆通道成矿系统(Qinetal., 2011; Maoetal., 2008),有别于保留了厚层玄武岩的Noril’sk矿床(Naldrett, 1992)。前人研究在大南湖岛弧带和吐哈盆地发现了~280Ma的辉长岩和玄武岩,且地球化学特征与东天山地区镁铁-超镁铁质岩相似(周鼎武等, 2006; 唐冬梅等, 2017; Maoetal., 2014b),暗示这可能是该区域岩浆通道成矿系统未被剥蚀的上部层位。卡拉塔格地区月牙湾铜镍硫化物矿床的发现(毛启贵等, 2018; 孙燕等, 2018)进一步说明东天山二叠纪铜镍硫化物成矿作用在大南湖岛弧带北缘也有响应。笔者及北京矿产地质研究院项目组在卡拉塔格地区发现了一系列镁铁质岩体,成岩时代为早二叠世,主要岩石类型为辉长岩、橄榄辉长岩、橄长岩、淡色辉长岩和辉绿岩,是研究东天山二叠纪岩浆演化与成矿作用新的切入点。本文对卡拉塔格地区镁铁质岩体进行岩石学、年代学、岩石地球化学及同位素等方面的研究,以探讨其构造背景、岩浆演化及其与东天山镁铁-超镁铁岩体和吐哈盆地玄武岩的关系。

1 区域地质背景

1.1 东天山区域地质特征

中亚造山带位于西伯利亚克拉通和塔里木-华北克拉通之间(图1a),是显生宙以来最大的增生造山带之一(Jahnetal., 2000; Gaoetal., 2009; Xiaoetal., 2013)。东天山地区位于中亚造山带南缘,在长期的演化过程中经历了极其复杂的裂解与拼合,构造环境多样(秦克章等, 2012; Maoetal., 2008; Xiaoetal., 2004, 2009),自北向南依次划分为博格达-哈尔里克构造带、吐哈盆地、觉罗塔格构造带和中天山地块(秦克章等, 2012; 王玉往等, 2006),其中觉罗塔格构造带由北向南划分为大南湖-头苏泉岛弧、康古尔剪切带和雅满苏岛弧(Xiaoetal., 2004, 图1b)。区域内出露的地层较全,以康古尔塔格深大断裂为界,以北出露大柳沟组、大南湖组、头苏泉组和企鹅山群,为一套钙碱性岛弧火山岩和内碎屑岩建造;以南出露梧桐窝子组、干墩组和雅满苏组,主要为滨浅海相火山-沉积岩系建造组合(刘德权等, 1992; 姬金生等, 1994; Zhangetal., 2017)。区域断裂构造主要呈EW向展布,沿康古尔塔格断裂和阿奇克库都克-沙泉子断裂及其次级断裂发育跨构造单元分布的众多时代一致的小型镁铁-超镁铁质岩体,并形成多个铜镍矿床集中区,如黄山-镜儿泉成矿区(三金柱等, 2010; 夏明哲等, 2008; 王玉往等, 2009; 秦克章等, 2012; Sunetal., 2013a)、白鑫滩-路北成矿区(赵冰冰等, 2018; Fengetal., 2018)和白石泉-天宇成矿区(唐冬梅等, 2009; Tangetal., 2011; 毛启贵等, 2006)。

图1 东天山及临区构造位置图(a, 据Jahn et al., 2000)和构造单元划分(b, 据Xiao et al., 2004)及镁铁-超镁铁质岩体分布图(据Mao et al., 2016修改) ①沁城断裂;②康古尔塔格断裂;③苦水断裂;④阿齐克库都克-沙泉子断裂;⑤星星峡断裂;⑥卡瓦布拉克断裂Fig.1 The map of tectonic position (a, after Jahn et al., 2000) and tectonic units (b, after Xiao et al., 2004) and distribution of mafi-ultramafic intrusions (modified after Mao et al., 2016) in eastern Tianshan ① Qincheng fault; ② Kanggurtag fault; ③ Kushui fault; ④ Aqqikkuduk-Shaquanzi fault; ⑤ Xingxingxia fault; ⑥ Kawabulak fault

图2 卡拉塔格地区地质构造及镁铁质岩体分布图(底图据毛启贵等, 2010; Deng et al., 2016修改)Fig.2 Geological map and locations of mafic intrusions in Kalatag area (modified after Mao et al., 2010; Deng et al., 2016)

1.2 卡拉塔格地质特征

卡拉塔格地区位于大南湖-头苏泉古生代岛弧带北缘(图1b),又被称为东天山晚古生代构造“天窗”(秦克章等, 2001; 李文铅等, 2006),整体为一个隆起带。卡拉塔格隆起带核部发育奥陶系-志留系火山碎屑岩,边部由内向外发育连续的泥盆系-二叠系的火山-沉积岩地层(秦克章等, 2001; 毛启贵等, 2010)。区内主要构造为NW向断裂,与区域构造线展布方向一致,早期为张性,晚期转变为压性,是控制卡拉塔格地区火山作用、岩浆侵入和成矿作用的主干断裂。卡拉塔格地区侵入岩十分发育,加里东期侵入岩以卡拉塔格岩基为代表,产出花岗闪长岩、二长花岗岩等;海西期侵入体以闪长岩、石英斑岩、二长斑岩岩枝或岩株产出(毛启贵等, 2010, 2018; Dengetal., 2016)。此外,在卡拉塔格地区发育多个镁铁质岩体 (图2),主要岩性为辉长岩、橄榄辉长岩、淡色辉长岩和辉绿岩等,部分镁铁质岩体发育铜镍硫化物矿化。

表1卡拉塔格地区镁铁质岩体特征

Table 1 Charactersitics of mafic intrusions in Kalatag area

岩体K1K2K3K4K5K6月牙湾洪湖湾形态椭圆形脉状椭圆形椭圆形近圆形椭圆形月牙状圆形走向北西北西北北西南北北西南北北西南北长(m)100~300200100~400500~100020050~1002000300宽(m)50~1004050~200200~50010030~60100~500300面积(km2)0.020.0060.060.40.020.0050.80.1围岩玄武岩安山岩安山岩安山岩安山岩安山岩玄武岩玄武岩岩性辉长岩、辉绿岩辉绿岩橄榄辉长岩、橄榄辉绿岩、淡色辉长岩淡色辉长岩、辉长岩、橄榄辉长岩橄长岩、伟晶橄长岩橄榄辉长岩、伟晶橄榄辉长岩橄榄辉长岩、橄长岩、淡色辉长岩淡色辉长岩、辉长岩、辉绿岩特征中心粗,边缘细,发育钛铁矿化辉绿岩脉中心粗,边缘细,围岩发育烘烤边橄榄辉长岩侵位晚于辉长岩,围岩发育烘烤边岩性单一,见伟晶状囊体岩性单一,见伟晶状囊体岩体北倾、底部橄长岩中发育铜镍矿化中心粗,边部细,发育钛铁矿化

2 岩体地质

2.1 岩体特征

卡拉塔格地区镁铁质岩体分布广泛,由东向西分别为K2、K1、K3、K4、K5、K6、月牙湾和洪湖湾等8个岩体,其中洪湖湾、K4岩体见铜镍矿化,月牙湾岩体赋存有小型铜镍硫化物矿床(孙燕等, 2018)。笔者对上述岩体进行了详细调查(图3、表1)。这些镁铁质岩体分布在卡拉塔格断裂和卡北断裂之间,受北西向断裂构造控制,侵位于大柳沟组、大南湖组和脐山组地层之中。岩体形态多为北西、北北西走向的椭圆形,岩体规模较小,长100~2000m,宽50~500m,出露面积<1km2,单个岩体岩性较为单一,以辉长岩、橄榄辉长岩、辉绿岩和淡色辉长岩为主。岩体的围岩主要为中基性火山岩,其中K2、K3、K4、K5和K6等岩体围岩为安山岩(图4a),K1、月牙湾和洪湖湾岩体围岩为玄武岩(图4b),。K1、K3和洪湖湾岩体显示中心粗边部细的单期岩浆冷凝结晶的特征;K2岩体为一条辉绿岩脉;K4岩体至少发育两期岩浆作用,橄榄辉长岩相侵位晚于辉长岩相;K5和K6岩体岩性单一,发育伟晶岩囊体;在K1、K3、K4和洪湖湾岩体辉长岩相中发育<10%的钛铁矿,偶见橄榄辉长岩中发育微量黄铁矿和黄铜矿(<1%)。月牙湾岩体形态特殊,呈向北西拖尾的月牙状,岩体北倾,由北向南依次发育淡色辉长岩、橄榄辉长岩和橄长岩,在橄长岩与橄榄辉长岩接触部位发育铜镍矿化。

图3 卡拉塔格地区镁铁质岩体地质简图Fig.3 Sketch geological maps of mafic intrusions in Kalatag area

图4 K4岩体(a)和月牙湾岩体(b)与围岩接触关系Fig.4 The contacting relation of wall rock and intrusion in K4 (a) and Yueyawan (b)

2.2 岩石特征

卡拉塔格地区镁铁质岩体的岩石类型有辉长岩、橄榄辉长岩、橄长岩、淡色辉长岩和辉绿岩。各岩类分布情况见图3,辉长岩为K1、K4和洪湖湾岩体的主体相;橄榄辉长岩在K3和K6岩体主体、K4岩体边部及月牙湾岩体中部发育;橄长岩主要发育在K5岩体和月牙湾岩体南部;淡色辉长岩以局部囊体形式在K3、K4和洪湖湾岩体出露,在月牙湾岩体北部出露面积较大;辉绿岩主要发育在K1、K3和洪湖湾岩体边缘。各岩石类型具体特征如下:

辉长岩:浅灰绿色,普遍发生弱蚀变,粒状结构、变余辉长结构,块状构造。主要由斜长石和单斜辉石组成(图5a),斜长石为中-拉长石,粒径0.5~2mm,含量40%~45%,发生高岭土化、钠黝帘石化蚀变;单斜辉石为普通辉石和透辉石,粒径0.5~2mm,含量20%~30%,部分发生阳起石化、透闪石化、绿泥石化等蚀变;一般含褐色普通角闪石,粒径0.5~1mm,含量约10%,不均匀分布于斜长石和单斜辉石颗粒之间;见钛铁矿,含量5%~10%,发育在蚀变的辉石颗粒中。

橄榄辉长岩:灰绿色,岩石较新鲜,包橄结构、嵌晶结构、粒状结构等(图5b),条带状构造、块状构造。斜长石为拉长石,自形长板状,粒径1~3mm,含量40%~60%;单斜辉石为普通辉石,半自形-他形粒状,粒径1~3mm,含量20%~30%,充填在长石间隙;橄榄石半自形-他形粒状,粒径0.5~1mm,含量10%~25%,发育在斜长石和单斜辉石晶粒之间或被单斜辉石包裹。发育少量(<5%)黑云母和钛铁矿等副矿物和微量磁黄铁矿、黄铁矿、黄铜矿等硫化物。矿物颗粒粗大者呈伟晶橄榄辉长岩,斜长石颗粒达10mm,交织结构(图5c),单斜辉石和橄榄石发育在斜长石晶粒间隙。

橄长岩:灰黑色,较新鲜,中-细粒似斑状结构(图5d),块状构造。斑晶和基质均由斜长石和橄榄石组成,斑晶斜长石为中-拉长石,自形板状,含量60%~70%,斑晶粒度2~4mm,基质粒度<0.5mm;橄榄石他形粒状,含量25%~30%,斑晶颗粒1~2mm,基质颗粒<0.2mm。含5%~10%单斜辉石,他形粒状,粒径0.5~1mm,充填在斜长石颗粒的间隙或包裹橄榄石,也见微量钛铁矿和黄铜矿微小颗粒发育。橄长岩矿物颗粒粗大者可形成伟晶状橄长岩,交织结构,斜长石颗粒可达5~10mm,橄榄石2~4mm,局部皂石化、绿泥石化蚀变。

淡色辉长岩:灰白色,蚀变中粗粒结构,交织结构,块状构造。主要由斜长石、单斜辉石、角闪石组成。斜长石为中-拉长石,部分更长石,粒径1~2mm,含量60%~70%,发生高岭土化、绢云母化、钠黝帘石化等蚀变,斜长石残余晶形完整,呈净边结构(图5e)。单斜辉石多发生透闪石和绿泥石化蚀变,粒径1~2mm,含量10%~15%。含10%~20%的褐色普通角闪石,微量黑云母、钛铁矿、磁铁矿等,偶见石英。

辉绿岩:灰绿色,中细粒辉绿结构、交织结构(图5f)、包橄结构,块状构造。主要由中基性斜长石、单斜辉石、橄榄石和普通角闪石等矿物组成。斜长石为针柱状、交织状,有时具有定向性,长轴<2mm,含量50%~60%;单斜辉石呈他形微粒状,粒径<1mm,含量10%~20%,充填在斜长石间隙,绿帘石化蚀变;橄榄石他形微粒状,粒径<1mm,含量0~10%,大多充填在斜长石和辉石矿物颗粒之间,少量被辉石和角闪石包裹;角闪石(约10%)发育在其它矿物间隙,多阳起石化和绿泥石化蚀变;含少量(5%~10%)钛铁矿和微量(<1%)黄铜矿。

3 样品采集及分析方法

3.1 样品采集

本次研究挑选了新鲜、有代表性的样品进行锆石U-Pb定年、全岩主微量和Sr-Nd-Pb同位素分析测试。岩石样品均采自岩体地表露头,用于锆石U-Pb定年的样品分别来自洪湖湾辉长岩(H56-9)和K4辉长岩(K43-7),采样坐标分别为42°45′10.59″N、91°18′60.01″E和42°39′59.48″N、91°43′52.40″E,采样位置见图3。

3.2 分析方法

全岩主微量分析在核工业北京地质研究院分析测试研究中心完成。主量元素测试方法为X射线荧光光谱法,采用AxiosmAX X射线荧光光谱仪,分析精度优于1%。微量元素分析采用酸溶法制备样品并在ELEMENT XR电感耦合等离子质谱仪上测试,分析精度优于3%。详细的分析方法见Gaoetal. (2002)。

Sr-Nd-Pb同位素分析在中国科学院地质与地球物理研究所完成。铅同位素测试采用美国Thermofisher公司Triton Plus型热电离质谱仪。将纯化好的样品用微量盐酸溶解,并加1微升硅胶和磷酸混合发射剂于铼灯丝表面,质谱测量温度为1250±50℃。国际标样NIST981被用于监控质谱仪状态,铅同位素质量分馏校正系数为每质量单位1.2‰。Sr-Nd同位素测试采用Thermofisher Triton Plus多接收热电离质谱仪,全流程本底Sr和Nd分别小于250pg和100pg,用88Sr/86Sr=8.375209和146Nd/144Nd=0.7219对Sr和Nd同位素比值进行校正,用国际标样NBS-987和JNdi-1对仪器稳定性进行监测,详细的分析方法见Lietal. (2012, 2015)。

4 分析结果

4.1 锆石U-Pb年龄

洪湖湾岩体辉长岩中的锆石均呈透明的长柱-短柱状,自形-半自形晶体;锆石长100~200μm,长宽比1.5~3;锆石阴极发光图像较暗,岩浆结晶环带一般不明显,部分可见环带状结构。锆石U、Th、Pb含量分别为90×10-6~376×10-6、100×10-6~491×10-6和28×10-6~145×10-6;,Th/U为1.1~1.96(表2),表明所测样品均为岩浆锆石(Rubatto, 2002)。LA-ICP-MS测试结果显示23个有效数据点206Pb/238U加权平均年龄为281.8±1.2Ma (MSWD=1.05),所有数据点均分布在谐和曲线上或附近,谐和年龄为282.2±0.6Ma (MSWD=1.05)(图6a)。

表2洪湖湾岩体和K4岩体辉长岩LA-ICP-MS锆石U-Pb测试结果

Table 2 LA-ICP-MS U-Pb age of zircons from gabbros of Honghuwan and K4 intrusions

测点号含量(×10-6)UThPbTh/U同位素比值年龄(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σHh56-9(洪湖湾辉长岩)-01223310851.390.0534800.0008490.3281940.0055720.0444920.000379350.130.6288.24.3280.62.3-022714841251.790.0523860.0007060.3202960.0045360.0443480.000389301.929.6282.13.5279.72.4-032243761041.680.0532570.0010490.3268660.0057060.0447540.000559338.944.4287.24.4282.23.5-042183681031.690.0529270.0006510.3273910.0052830.0447670.000379324.127.8287.64.0282.32.3-05155224601.440.0531830.0008950.3244390.0057860.0442710.000432344.538.9285.34.4279.22.7-0690100281.100.0530240.0011000.3217420.0070460.0440180.000427331.546.3283.25.4277.72.6-07160207581.300.0520360.0008210.3173760.0052600.0442450.000362287.137.0279.94.1279.12.2-083404811341.420.0526120.0006170.3280010.0045160.0451850.000367322.327.8288.03.5284.92.3-102243701011.650.0542700.0009730.3357490.0063410.0448570.000354383.438.9293.94.8282.92.2-11172197581.150.0525950.0008270.3189430.0072950.0446490.000474322.335.2281.15.6281.62.9-12187352961.880.0511040.0007480.3165570.0053410.0449580.000483255.633.3279.34.1283.53.0-13137197551.440.0518170.0007100.3290880.0055680.0460620.000502276.026.9288.94.3290.33.1-14133182511.360.0524170.0009130.3278390.0059440.0454110.000411301.938.9287.94.5286.32.5-151933791011.960.0529020.0007370.3271460.0049720.0448760.000415324.133.3287.43.8283.02.6-16143191551.330.0537540.0018030.3255590.0063860.0445090.000513361.275.9286.24.9280.73.2-172904901341.690.0527100.0006760.3253720.0044560.0447880.000364316.729.6286.03.4282.42.2-183124361251.400.0517220.0005990.3207940.0037150.0450560.000407272.321.3282.52.9284.12.5-19184259731.410.0527990.0012610.3208220.0052420.0444660.000538320.453.7282.54.0280.53.3-203024651331.540.0532850.0007720.3279720.0048830.0447740.000527342.731.5288.03.7282.43.2-21249325921.310.0534410.0015080.3242080.0047530.0445490.000502346.463.0285.13.6281.03.1-223764911451.310.0510970.0008720.3157390.0077810.0447000.000601255.638.9278.66.0281.93.7-23159218611.380.0528410.0007480.3265530.0048820.0448530.000414320.426.9286.93.7282.82.6-25190233681.230.0520800.0008590.3171370.0055190.0441830.000418287.138.9279.74.3278.72.6

续表2

Continued Table 2

测点号含量(×10-6)UThPbTh/U同位素比值年龄(Ma)207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σK43-7(K4岩体辉长岩)-018069200.860.0504910.0011580.3070840.0074930.0441660.000479216.784.2271.95.8278.63.0-02127137391.080.0526060.0007960.3258840.0055550.0450040.000487322.335.2286.44.3283.83.0-03160226611.410.0526100.0009190.3164790.0059650.0436880.000436322.338.9279.24.6275.72.7-047360180.820.0529210.0011590.3202090.0072760.0440610.000560324.150.0282.15.6278.03.5-05135182491.340.0524950.0006920.3167920.0042860.0438670.000426305.634.3279.43.3276.82.6-06132118340.890.0542030.0009920.3296970.0062000.0441730.000376388.940.7289.34.7278.72.3-07144191511.330.0525650.0006380.3180880.0041700.0439490.000387309.330.6280.43.2277.32.4-08108110301.020.0521780.0009830.3159150.0070860.0438370.000430300.147.2278.85.5276.62.7-09116105300.910.0526120.0010050.3183700.0063560.0439990.000470322.310.2280.64.9277.62.9-10120109310.910.0528030.0010600.3179800.0064480.0437720.000379320.446.3280.35.0276.22.3-117868200.870.0510520.0010810.3095810.0068430.0440510.000433242.752.8273.95.3277.92.7-12130121340.930.0543680.0011680.3288460.0076110.0439110.000422387.145.4288.75.8277.02.6-13160193521.210.0528230.0008090.3230500.0055680.0443810.000437320.439.8284.24.3279.92.7-142204911312.230.0531080.0006990.3287220.0056230.0448960.000533344.529.6288.64.3283.13.3-15123175471.420.0521480.0008710.3158570.0059010.0439020.000373300.137.0278.74.6277.02.3-16108125341.160.0529790.0009780.3211050.0062330.0440130.000426327.840.7282.84.8277.72.6-17125122340.970.0521430.0008750.3158450.0051290.0440230.000362300.137.0278.74.0277.72.2-18119109310.920.0518140.0009370.3144740.0056100.0441180.000409276.040.7277.64.3278.32.5-19147196541.330.0528030.0008850.3269790.0064560.0448560.000435320.441.7287.34.9282.92.7-20145106320.730.0519360.0009170.3226840.0064370.0450700.000511283.440.7284.04.9284.23.2-219985250.860.0517940.0009100.3144520.0064880.0439760.000468276.040.7277.65.0277.42.9-22105105300.990.0533810.0010570.3217400.0063980.0437700.000415346.446.3283.24.9276.22.6-2310899280.920.0527810.0008620.3157900.0055220.0434140.000431320.441.7278.74.3274.02.7-24139168451.210.0509740.0010160.3075170.0063120.0437740.000388239.044.4272.34.9276.22.4-259899281.010.0526910.0010620.3220410.0073890.0443030.000511316.713.9283.55.7279.43.2

表3卡拉塔格地区镁铁质岩石主量元素(wt%)和微量元素(×10-6)分析结果

Table 3 Major (wt%) and trace (×10-6) elements composition of mafic intrusions in Kalatag district

样品号K3-1K4-8K4-12K6-1K6-2K4-16H-4K1-5K1-8K5-1K5-3K5-4H-2K3-2K4-2K3-8H-9K2-3岩性橄榄辉长岩辉长岩橄长岩淡色辉长岩辉绿岩SiO247.9547.8747.7546.1148.0147.2351.3645.4745.0849.1947.0347.9957.4264.4353.2548.5949.6549.35TiO21.261.731.531.141.551.662.700.200.490.810.640.641.831.000.672.011.731.49Al2O318.5917.2917.7217.0519.4816.9714.8621.7622.6722.2919.0020.7614.9912.2321.7116.4516.8216.07Fe2O3T10.7711.2411.0012.5612.2410.4111.457.659.687.2310.148.778.285.764.2312.2810.329.37FeO8.247.538.239.908.015.076.305.524.705.728.397.474.913.862.728.335.805.37MnO0.150.160.160.170.140.150.170.140.130.100.140.120.170.120.060.180.200.14MgO7.917.247.7510.945.635.364.156.415.165.0810.408.512.852.051.016.925.297.23CaO8.888.788.858.027.829.526.9813.3511.919.288.259.084.707.768.408.456.909.40Na2O3.593.593.493.154.084.314.641.731.954.173.163.455.564.385.553.794.372.76K2O0.260.280.250.190.250.281.080.340.500.340.250.181.390.071.430.451.351.20P2O50.210.150.220.180.180.160.430.030.060.170.110.120.720.240.310.370.350.24LOI0.371.631.220.460.623.932.142.902.351.290.830.352.071.913.370.463.032.75Total99.9499.9699.9499.9699.9999.9899.9599.9899.9899.9599.9599.9699.9999.95100.0099.95100.00100.00Mg#65.5365.5665.0968.6358.1967.6756.6069.6968.4963.7571.0569.2953.4751.2642.3762.1964.3672.72Li6.209.609.146.285.957.4412.703.523.896.726.615.0713.002.455.185.8322.804.64Be0.830.660.690.650.550.561.360.120.220.600.410.402.080.931.131.221.050.75Sc20.8027.1024.6017.2013.8038.4032.5042.3031.409.5010.509.7222.0020.608.6830.2024.9044.20V16021417814946726026518741792847913410665228219283Cr15517713910618157741413349463244100105279Co52464771533627343234635213129483432Ni1101031001794732102831531381483126914369Cu5182623641554674106292423244124685939Zn1061201101061081171585571657465149925514615291Ga18.1018.2017.5015.7018.6018.5019.7015.6017.7018.1014.9015.8021.7014.1022.0019.8018.1017.10Rb2.262.042.201.291.673.1011.804.835.142.541.871.3214.500.5421.104.6522.5015.90Sr47239734845560941474439345256548553347959601422802305Y22.1023.3029.3017.5015.3024.0033.904.405.8315.0010.3010.7054.8039.3031.7035.8026.6028.60Mo0.350.550.420.270.310.370.700.090.200.290.200.221.140.470.450.620.540.81Cd0.100.070.100.090.060.110.160.050.060.050.060.050.240.140.070.140.160.11In0.060.070.060.050.050.070.090.040.040.040.030.030.100.100.040.080.070.08Sb0.060.050.040.050.060.120.100.080.090.050.030.040.070.230.050.070.320.19Cs0.090.220.180.380.140.140.120.130.170.270.150.180.110.030.260.170.460.09

续表3

Continued Table 3

样品号K3-1K4-8K4-12K6-1K6-2K4-16H-4K1-5K1-8K5-1K5-3K5-4H-2K3-2K4-2K3-8H-9K2-3岩性橄榄辉长岩辉长岩橄长岩淡色辉长岩辉绿岩Ba88857359101812868010471535435628322149273327La6.364.635.955.275.655.6416.401.632.964.953.503.5528.1011.8010.4011.4011.8013.90Ce17.0012.8016.6013.9014.1014.7039.903.205.9012.908.939.2066.0028.9025.9029.9029.5029.30Pr2.752.192.822.262.172.425.840.500.862.081.421.499.904.423.964.664.443.94Nd13.6011.7014.6011.6010.7012.5027.602.454.1010.306.997.3546.5021.3019.5022.9021.7017.90Sm3.533.444.192.972.593.576.710.720.952.541.731.8210.805.534.995.935.274.36Eu1.491.681.561.351.401.412.170.300.411.361.011.053.301.381.952.081.821.47Gd3.183.103.772.512.213.135.540.600.852.151.511.649.044.864.315.124.253.99Tb0.690.700.850.550.480.721.130.120.170.470.320.341.791.080.921.110.900.86Dy3.853.954.783.032.614.076.070.780.992.511.791.859.386.105.195.974.734.81Ho0.820.901.080.660.580.901.250.170.230.560.390.401.951.431.161.331.001.08Er2.152.252.821.711.472.333.120.460.571.411.001.045.043.762.983.402.512.78Tm0.380.430.520.310.270.420.530.080.110.250.190.190.880.710.530.620.440.52Yb2.412.613.141.921.722.633.340.530.751.621.101.185.414.553.263.852.693.24Lu0.350.380.460.280.240.380.470.080.110.230.170.160.770.650.440.540.370.45Pb1.941.401.521.491.501.833.340.580.611.381.390.997.643.923.503.313.613.24Bi0.010.000.000.00<0.0020.000.000.000.010.000.010.000.020.060.000.010.010.02Th0.220.150.210.140.230.280.810.200.320.120.100.091.351.100.590.470.581.42U0.090.100.110.070.100.140.300.110.200.080.050.060.600.670.230.200.220.44Nb2.751.992.392.312.452.086.770.290.621.961.311.3210.803.802.024.955.3018.60Ta0.190.160.160.150.160.150.440.020.030.140.090.090.680.250.160.350.331.18Zr11710012292839415881379515531815692185104117Hf2.672.563.042.071.902.443.950.270.431.811.211.247.284.192.294.382.522.89∑REE58.5650.7663.1448.3146.1954.82120.111.6218.9543.3330.0431.26198.996.4785.5098.8191.4288.60LREE44.7336.4445.7237.3536.6140.2498.628.8015.1934.1323.5824.46164.673.3366.7076.8774.5370.87HREE13.8314.3217.4210.969.5814.5821.462.823.769.206.466.8034.2623.1418.8021.9416.8917.73LREE/HREE3.232.542.623.413.822.764.603.124.043.713.653.604.803.173.553.504.414.00(La/Yb)N1.891.271.361.972.361.543.522.192.842.192.282.163.731.862.292.123.153.08δEu1.331.541.181.471.741.261.061.361.361.731.871.820.990.801.251.131.141.06

注:Mg#=100×Mg2+/(Mg2++Fe2+)

表4卡拉塔格地区镁铁质岩石Sr、Nd、Pb同位素分析数据

Table 4 Sr, Nd, Pb isotopic data of mafic intrusion in Kalatag district

样品号K3-1K4-12K6-2K4-16H-4K1-5K51-1H5-4H-2K3-2K4-2K3-8H-9岩性橄榄辉长岩辉长岩橄长岩淡色辉长岩辉绿岩Rb (×10-6)2.262.201.673.1011.804.832.541.3214.500.5421.104.6522.50Sr (×10-6)47234860941474439356553347959.360142280287Rb/86Sr0.01390.01830.00790.02170.04590.03560.01300.00720.08770.02650.10170.03190.081287Sr/86Sr0.7034160.7032000.7035110.7034500.7037960.7046330.7032700.7033520.7042330.7047170.7038100.7035270.7041482σ0.0000120.0000140.0000120.0000130.0000110.0000150.0000130.0000100.0000140.0000110.0000120.0000110.000011(87Sr/86Sr)i0.7033600.7031300.7034800.7033600.7036100.7044900.7032200.7033200.7038800.7046100.7034100.7034000.703820Nd (×10-6)13.6014.6010.7012.5027.602.4510.307.3546.5021.3019.5022.9021.70Sm (×10-6)3.534.192.593.576.710.722.541.8210.805.534.995.935.27147Sm/144Nd0.15690.17350.14630.17270.14700.17670.14910.14970.14040.15700.15470.15660.1468143Nd/144Nd0.5129880.5130380.5129610.5130120.5128650.5129470.5129720.5129680.5128700.5129480.5129720.5129510.5128822σ0.0000200.0000100.0000090.0000090.0000080.0000090.0000090.0000100.0000080.0000080.0000100.0000080.000009(143Nd/144Nd)i0.5127000.5127200.5126930.5126960.5125960.5126230.5126990.5126940.5126130.5126600.5126890.5126640.512613εNd(t)8.258.648.118.176.226.748.238.136.557.478.037.546.55U (×10-6)0.090.110.100.140.300.110.080.060.600.670.230.200.22Th (×10-6)0.220.210.230.280.810.200.120.091.351.100.590.470.58Pb (×10-6)1.941.521.501.833.340.581.380.997.643.923.503.313.61206Pb/204Pb17.979417.880017.994617.978918.275418.301017.947917.939218.193318.583318.001518.030118.12452σ0.00290.00180.00220.00180.00240.00290.00270.00230.00150.00170.00310.00160.0022207Pb/204Pb15.482015.453215.484215.468615.521215.563315.479415.477815.497615.501815.460815.487815.49382σ0.00250.00150.00200.00170.00220.00260.00250.00250.00120.00120.00250.00150.0017208Pb/204Pb37.695837.544237.715537.633337.974338.108937.669937.664237.847537.900537.631837.742237.77692σ0.00570.00380.00490.00380.00490.00650.00640.00570.00300.00300.00680.00380.0042(206Pb/204Pb)i17.845017.680017.810017.767018.025017.779017.780017.786017.976018.103017.821017.867017.9590(207Pb/204Pb)i15.475015.443015.475015.458015.508015.536015.471015.470015.486015.477015.451015.479015.4850(208Pb/204Pb)i37.592037.423037.579037.497037.755037.801037.593037.587037.688037.645037.482037.616037.6320

注: Rb、Sr、Sm、Nd同位素计算参数: λ(Sr)=1.42×10-11a-1, λ(Nd)=0.654×10-11a-1, (87Sr/86Sr)CHUR=0.7045, (143Nd/144Nd)CHUR=0.512638; Pb初始同位素计算参数: λ1=1.55125×10-11a-1, λ2=9.8485×10-11a-1, λ3=0.49475×10-11a-1,t=280Ma

图5 卡拉塔格地区镁铁质岩石显微照片 (a)辉长岩(单斜辉石阳起石、绿泥石化蚀变,正交偏光);(b)橄榄辉长岩(斜长石嵌晶结构,正交偏光);(c)伟晶橄榄辉长岩(伟晶交织状斜长石,正交偏光);(d)橄长岩(似斑状结构,正交偏光);(e)淡色辉长岩(斜长石净边结构,单偏光);(f)辉绿岩(斜长石交织结构,正交偏光). Cpx-单斜辉石;Pl-斜长石;Ol-橄榄石;Tre-透闪石;Act-阳起石;Chl-绿泥石;Di-透辉石;Ilm-钛铁矿Fig.5 Mircophotographs of mafic intrusions in Kalatag area (a) gabbro (actinolitization and chlorophyllization alterated clinopyroxene, crossed polar); (b) olivine gabbro (poikiloblastic texture of plagioclase, crossed polar); (c) olivine gabbro pegmatite (pegmatitic interlaced texture of plagioclase, crossed polar); (d) troctolite (porphyritic texture, crossed polar); (e) leucogabbro (metasomatic edulcoration border texture of plagioclase, plane polar); (f) diabase (interlaced texture of plagioclase, crossed polar). Cpx-clinopyroxene; Pl-plagioclase; Ol-olivine; Tre-tremolite; Act-actinolite; Chl-chlorite; Di-diopside; Ilm-ilmenite

图6 洪湖湾岩体辉长岩(a)和K4岩体辉长岩(b)锆石U-Pb谐和年龄图Fig.6 U-Pb concrodia diagrams for zirons from gabbros in Hoghuwan (a) and K4 (b) intrusions

K4岩体辉长岩中的锆石呈透明的长柱状或短柱状,少量不规则状,呈自形-半自形晶体;锆石长50~200μm,长宽比1~3;锆石阴极发光图像较暗,岩浆结晶环带一般不明显,部分可见环带状结构。锆石U、Th、Pb含量分别为73×10-6~220×10-6、60×10-6~491×10-6和18×10-6~131×10-6,Th/U为0.73~2.23(表2),表明所测样品均为岩浆锆石(Rubatto, 2002)。LA-ICP-MS测试结果显示25个有效数据点206Pb/238U加权平均年龄为278.2±1.2Ma (MSWD=0.95),所有数据点均分布在谐和曲线上或附近,谐和年龄为278.3±0.6Ma (MSWD=0.9)(图6b)。

4.2 主量元素

卡拉塔格地区8个镁铁质岩体主要岩石类型的主、微量分析数据见表3。其中淡色辉长岩SiO2含量介于53.25%~64.43%之间,属中性岩类;其它岩石类型SiO2含量介于45.08%~51.36%之间,属于基性岩类;各岩石类型Mg#介于42.4~72.7之间。与东天山二叠纪镁铁-超镁铁质岩(黄山岩体SiO2=36.9%~53.9%,Mg#介于60.7~84.2,Dengetal., 2015;葫芦岩体SiO2=29.67%~56.14%,Mg#介于42.2~87.4,Hanetal., 2013)相比,卡拉塔格地区岩体基性程度普遍较低(或酸度较高),岩浆演化程度较高。主量元素图解显示卡拉塔格地区镁铁质岩体与东天山镁铁质岩地球化学特征相似,且向同时期吐哈玄武岩过渡(图7)。在SiO2-(Na2O+K2O)图解上,只有1个淡色辉长岩样品落在亚碱性区域,其余样品均落在碱性和亚碱性分界线上(图8)。

图7 卡拉塔格地区镁铁质岩体主要氧化物与SiO2相关性图解 东天山镁铁质岩石数据来源:①据Deng et al. (2014)和赵冰冰等(2018);②据赵冰冰等(2018)和毛启贵等(2006);③据Chai et al. (2008);④据Han et al. (2013)和夏明哲等(2008);⑤据唐冬梅等(2017);图9数据来源同此图Fig.7 Diagrams of oxides versus SiO2 of mafic intrusions in Kalatag area Data of mafic rocks in eastern Tianshan are from follows: ① after Deng et al. (2014) and Zhao et al. (2018); ② after Zhao et al. (2018) and Mao et al. (2006); ③ after Chai et al. (2008); ④ after Han et al. (2013) and Xia et al. (2008); ⑤ after Tang et al. (2017) ; data sources inFig.9 are same as thig figure

图8 卡拉塔格镁铁质岩体SiO2-(Na2O+K2O) 图解(底图据Irvine and Baragaer, 1971)Fig.8 Diagram of SiO2 vs. (Na2O+K2O) (base map after Irvine and Baragaer, 1971) for mafic intrusions in Kalatag area

4.3 微量元素

卡拉塔格地区各岩石类型稀土元素总体特征相似,显示轻稀土轻微富集的缓右倾型稀土配分曲线(图9a),∑REE变化范围为11.62×10-6~198.9×10-6,LREE为8.80×10-6~164.6×10-6,HREE为2.82×10-6~34.26×10-6,轻重稀土元素分馏不明显,∑LREE/∑HREE=2.54~4.80,(La/Yb)N为1.27~3.73,无铕异常或较弱的铕正异常(δEu=0.80~1.87)。稀土配分型式与东天山镁铁岩相似并向同时期玄武岩过渡,并显示与OIB相似的特征(图9c)。

样品微量元素普遍高于原始地幔值,总体表现为Rb、Ba、Sr、K正异常,Th、Nb、Ta负异常(图9b)。区域内同时期镁铁质岩石显示Sr正异常,玄武岩显示Sr负异常,卡拉塔格地区镁铁质岩石主体为Sr正异常,也有一些样品显示Sr负异常(图9d),说明卡拉塔格镁铁质岩石具有过渡性质。

4.4 Sr、Nd、Pb同位素

Nd、Sr、Pb同位素分析数据见表4。卡拉塔格地区镁铁质岩体(87Sr/86Sr)i=0.70313~0.70461,εNd(t)=+6.22~+8.64,(206Pb/204Pb)i=17.68~18.103,(207Pb/204Pb)i=15.443~15.536,(208Pb/204Pb)i=37.423~37.801,具有高Nd、低Sr、低Pb,且变化范围较窄的特征。Sr-Nd同位素组成与二叠纪塔里木地区玄武岩和镁铁-超镁铁质岩明显不同,样品点投影在洋岛玄武岩(OIB)、岛弧玄武岩(GVAB)和活动大陆边缘(ACM)区域内(图10a),(206Pb/204Pb)i-(87Sr/86Sr)i图解中样品点投影在MORB和OIB范围内(图10b)。Pb同位素组成位于地球等时线右侧并落在MORB范围内(图10c, d)。总体来看,卡拉塔格地区镁铁质岩同位素特征与黄山、黄山东等东天山镁铁-超镁铁质岩及吐哈盆地玄武岩相似,显示亏损型地幔源区特征。

5 讨论

5.1 时代及构造背景

本文厘定卡拉塔格地区洪湖湾岩体和K4岩体辉长岩年龄分别为282.2±0.6Ma和278.3±0.6Ma,本文讨论的该区其它岩体年龄为275~285Ma(毛启贵等, 2018)。孙燕等(2018)报道了月牙湾岩体橄榄辉长岩成岩年龄为281.3±2Ma,Maoetal. (2014b)测定沙尔湖岩体辉长岩成岩年龄为286.5±2.1Ma,显示成岩时代均为早二叠世,与东天山地区广泛发育的铜镍矿化镁铁-超镁铁质岩石形成时代一致(韩宝 福等, 2004; 三金柱等, 2010; 李锦轶等, 2006; 唐冬梅等, 2009; Qinetal., 2011; Sunetal., 2013b)。

图9 卡拉塔格地区镁铁质岩体及区域镁铁质岩石球粒陨石标准化稀土元素分布型式图(a、c)和原始地幔标准化微量元素蛛网图(b、d) (MORB、OIB及标准化值引自Sun and McDonough, 1989)Fig.9 Chondrite-normalized REE patterns (a, c) and PM-normalized trace elements spider diagram (b, d) of mafic rocks in Kalatag and adjacent region (data for MORB, OIB and normalizing values after Sun and McDonough, 1989)

图10 卡拉塔格地区镁铁质岩体Sr-Nd-Pb同位素图解 数据来源:吐哈盆地玄武岩引自夏林圻等(2006);黄山东引自Sun et al. (2013a);黄山引自夏明哲(2009);亏损地幔(DM)引自Zindler and Hart (1986);活动大陆边缘(ACM)引自Hawkesworth (1982);全球岛弧玄武岩(GVAB)引自http://www. petdb. Org;塔里木镁铁-超镁铁质岩引自Zhou et al. (2009);塔里木玄武岩引自Yuan et al. (2012)Fig.10 Plots of Sr-Nd-Pb isotopes for the mafic intrusion in Kalatag area Data sources: the Tuha basalts from Xia et al. (2006); the Huagnshandong intrusion from Sun et al. (2013); the Huagnshan intrusion from Xia (2009); a depleted mantle-derived melt (DM) from Zindler and Hart (1986); active continental margin (ACM) from Hawkesworth (1982); global volcanic arc basalts (GVAB) from a public database (http://www.petdb.org); Permian Tarim mafic-ultramafic rocks from Zhou et al. (2009); Permian Tarim basalts from Yuan et al. (2012)

图11 卡拉塔格地区镁铁质岩体εNd(t)对(87Sr/86Sr)i (a)、(Th/Nb)N图解(b)和SiO2对εNd(t) (c)、(87Sr/86Sr)i图解(d) 亏损地幔(DMM)和原始地幔标准化数据引自Sun and McDonough (1989);上地壳(UC)和下地壳(LC)数据引自Rudnick and Gao (2003);早二叠世A型花岗岩数据引自汪传胜等(2009)Fig.11 Diagrams of εNd(t) vs. (87Sr/86Sr)i (a), εNd(t) vs. (Th/Nb)N (b), SiO2 vs. εNd(t) (c) and SiO2 vs. (87Sr/86Sr)i (d) for mafic intrusions in Kalatag area Data for the depleted mantle-derived melt (DMM) and normalizing values from Sun and McDonough (1989). Data for the upper crust (UC) and Lower crust from Rudnick and Gao (2003). Data for the coeval A-type granite in the region from Wang et al. (2009)

图12 卡拉塔格地区镁铁质岩体La/Nb-La/Ba图解(a,据Fitton et al., 1991)和 (206Pb/204Pb)i-(143Nd/144Nd)i 图解(b,据Deckart et al., 2005)Fig.12 Diagrams of La/Nb vs. La/Ba (a, after Fitton et al., 1991) and (206Pb/204Pb)i vs. (143Nd/144Nd)i (b, after Deckart et al., 2005) for mafic intrusions in Kalatag area

图13 卡拉塔格地区镁铁质岩体Th/Yb-Ba/La (a)和εNd(t)-(Nd/Hf)N (b)图解 亏损地幔(DMM)和原始地幔标准化数据引自Sun and McDonough (1989);蚀变洋壳流体(AOCF)数据引自Hauff et al. (2003)和Kelley et al. (2003);全球俯冲沉积物(GLOSS)数据引自Chauvel et al. (2009)和Plank and Langmuir (1998);俯冲沉积物流体(SSF)数据引自Chauvel et al. (2009)和Johnson and Plank (2000);板片流体(SF)相应值为90% AOCF和10% SSF混合Fig.13 Plots of Th/Yb vs. Ba/La (a) and εNd(t) vs. (Nd/Hf)N (b) for the mafic intrusion in Kalatag area Data for the depleted mantle-derived melt (DMM) and normalizing values from Sun and McDonough (1989). Data for the altered oceanic crust fluid (AOCF) from Hauff et al. (2003) and Kelley et al. (2003). Data for the global subducting sediment (GLOSS) from Chauvel et al. (2009) and Plank and Langmuir (1998). Data for the subducted sediment fluid (SSF) from Chauvel et al. (2009), Johnson and Plank (2000). Data for the slab fluid (SF) are the mixture of 90% AOCF and 10% SSF

图14 东天山地区二叠纪幔源岩浆侵位模式图Fig.14 A brief model for Permian mantle-derived magma emplacement in eastern Tianshan

关于新疆东部二叠纪幔源岩浆活动构造背景的争论主要在于其是否与塔里木地幔柱有关(夏林圻等, 2006; Qinetal., 2011; Maoetal., 2008; Pirajnoetal., 2008),或者为碰撞后伸展环境(Zhouetal., 2004; 王玉往等, 2009; Wangetal., 2008; 邓宇峰等, 2012)。Qinetal. (2011)认为地幔柱叠加作用为东天山和北山镁铁-超镁铁质岩的侵位及形成铜镍矿床提供了的热源。Xieetal. (2014)研究认为俯冲板片撕裂导致的软流圈上涌也可以形成与岩浆硫化物成矿有关的玄武质岩浆,并不需要地幔柱提供热源;而且地幔柱能量的影响范围可能有限(宋谢炎等, 2018)。前文研究表明卡拉塔格地区镁铁质岩石主量元素(图7)、稀土元素和微量元素特征(图9)与二叠纪塔里木玄武岩明显不同,且Sr、Nd、Pb同位素具有显著区别(图10a, b),说明卡拉塔格地区镁铁质岩的形成可能与塔里木地幔柱活动无关。石煜(2018)总结了东天山地区εNd(t)随时间变化规律,认为随着时间变化亏损软流圈物质稳定上涌,其过程中并未出现地幔柱物质的剧烈加入过程,因此可能也不存在地幔柱叠加的过程。

汪传胜等(2009)、陈希节和舒良树(2010)、陈希节等(2016)研究认为哈尔里克地区早二叠世(297~285Ma)的基性岩墙群、钾长花岗岩和碱性花岗岩及双峰式火山岩组合是碰撞后拉张阶段的典型标志;康古尔-黄山构造带铜镍硫化物矿区的A型花岗岩可能侵位于造山晚期的伸展环境(孙赫等, 2010);吐哈盆地早二叠世玄武岩微量元素特征显示与区域性伸展作用密切相关(周鼎武等, 2006; 唐冬梅等, 2017);白建科等(2018)研究认为吐哈盆地南缘的企鹅山群玄武岩(314±3.5Ma)形成于伸展环境下,且捕获有中天山地块的碎屑锆石。以上研究从多个方面说明北天山洋已于晚石炭世早期之前闭合消失,东天山地区早二叠世为后碰撞伸展环境(韩宝福等, 2004; 王京彬和徐新, 2006; 顾连兴等, 2006)。

综上所述,本文认为在早二叠世时期,北天山洋已经闭合,增生造山主碰撞过程已经结束,包括卡拉塔格在内的东天山地区镁铁质-超镁铁质岩体形成于早二叠世后碰撞伸展环境,而与塔里木地幔柱活动无关,因此,可能不存在地幔柱叠加俯冲碰撞阶段或者叠加后碰撞伸展阶段的作用过程。

5.2 岩浆源区

一般来说,岩浆源区、同化混染和结晶分异是影响岩石地球化学特征的重要因素,在讨论岩浆源区性质时,首先要排除岩浆上升过程中同化混染和结晶分异作用的影响(李全忠等, 2008)。分别以亏损地幔(DMM)、上地壳(UC)和下地壳(LC)为端元进行的模拟表明母岩浆经历了<5%的上地壳混染(图11a),加入同时期A型花岗岩端元的模拟结果表明上地壳混染程度约5%(图11b)。对同化混染作用很敏感的Sr同位素和207Pb/204Pb比值没有明显升高(表4),在锆石的CL图像上没有发现核幔结构,也没有发现古老的年龄(图5),卡拉塔格地区各岩石的全岩εNd(t)值和(87Sr/86Sr)i值与SiO2含量之间没有相关性(图11c, d)。以上证据说明卡拉塔格镁铁质岩体的初始岩浆上升过程中没有发生明显的同化混染作用。结晶分异作用可以影响岩石中元素的含量,但同位素组成和不相容微量元素比值不因结晶分异而变化(Dengetal., 2015; Tangetal., 2012)。因此,卡拉塔格地区岩石同位素组成和不相容微量元素比值可以指示其岩浆源区。

卡拉塔格地区镁铁质岩体各类岩石具有与MORB和OIB相似的Sr-Nd-Pb同位素特征(图10),微量元素Zr/Nb=6.29~51.04,Sm/Nd=0.23~0.29,分别与MORB对应比值一致(Anderson, 1994),表明其岩浆源区为亏损地幔(Saundersetal., 1992)。Sr-Nd同位素组成具有与弧火山岩相似的特征(图10a),相对较低的Th/U(平均2.08)和较高的La/Nb(平均2.84)、Ba/Nb(61.87)比值与俯冲成因的岩浆岩(如阿拉斯加型岩体)相似(邓宇峰等, 2011),表明岩浆源区遭受了俯冲事件的改造。岩石微量元素显示Nb、Ta的强亏损和Ti弱亏损(图9),表明岩浆源区混合了富集地幔组分。La/Nb-La/Ba图解显示卡拉塔格地区镁铁质岩体母岩浆主要来自俯冲交代的岩石圈地幔并有软流圈地幔的加入(图12a)。利用亏损型地幔端元(N-MORB)和富集型岩石圈地幔(EMⅡ, EMⅠ, Deckartetal., 2005)两端元模拟Sr、Nd同位素组成,表明卡拉塔格地区镁铁质岩石母岩浆主要来自亏损地幔源区,并有大约10%的EMⅡ型富集地幔组分加入(图12b)。前人研究表明,EMⅡ型地幔端元可能是由古板块俯冲带入的陆源沉积物通过壳-幔交代作用形成(Hart, 1988),或者是有俯冲洋壳析出流体交代地幔楔形成(李曙光, 1994)。

卡拉塔格地区镁铁质岩石(Th/Yb)N和(Th/Nb)N分别为0.33~2.54和0.49~5.75,远高于MORB的相应值(平均值分别为0.17和0.43, Sun and McDonough, 1989),可能是地幔源区俯冲板片流体的交代作用影响所致(Songetal., 2011; Sunetal., 2013b),Th/Yb-Ba/La图解也表明岩浆源区存在流体交代改造作用(图13a)。以亏损地幔(DMM)、全球俯冲沉积物(GLOSS)、蚀变洋壳流体(AOCF)和板片流体(SF)进行模拟投图,模拟方法见Tangetal. (2013),结果表明,母岩浆在地幔源区遭受了5%~15%的板片流体交代(图13b),结果与遭受10%左右的富集地幔组分交代的模拟一致(图12b),表明前文所述的富集地幔组分可能是通过板片流体交代作用加入到亏损地幔之中。综上所述,卡拉塔格地区镁铁质岩体源区主要为亏损地幔,并有遭受了5%~15%俯冲流体交代而形成的富集地幔的加入。

5.3 侵位模式

前文讨论表明,卡拉塔格地区镁铁质岩石形成于早二叠世后碰撞伸展环境之下,岩浆源区为亏损地幔,并受到俯冲板片流体的交代,与东天山地区镁铁-超镁铁质岩体和吐哈玄武岩的构造环境和岩浆源区相似(夏明哲等, 2008; Tangetal., 2011, 2012, 2013; 唐冬梅等, 2017),说明它们形成于同一构造岩浆系统。在地球化学特征方面,卡拉塔格地区镁铁质岩体的主量元素和微量元素具有吐哈盆地玄武岩和东天山镁铁-超镁铁质岩体中镁铁质岩相过渡的特征(图6、图8),Sr、Nd、Pb同位素特征一致(图9)。前人对东天山地区含铜镍矿的镁铁-超镁铁质岩体母岩浆成分进行了研究,结果表明黄山南和天宇镁铁-超镁铁质岩石的母岩浆MgO含量分别为12.4%(Maoetal., 2016)和12.61%(Chaietal., 2008),而卡拉塔格地区月牙湾岩体母岩浆MgO含量为6.5%(本人未发表数据),吐哈盆地二叠纪玄武岩母岩浆MgO含量可能更低(唐冬梅等, 2017)。因此,卡拉塔格地区镁铁质岩体与吐哈玄武岩和东天山镁铁-超镁铁质岩体具有相同的岩浆源区,原始岩浆在上升过程中发生分异形成了各自的母岩浆,并经历不同的地质过程侵位或喷出地表(图14)。在早二叠世,构造岩浆活动进入碰撞后伸展阶段,被俯冲板片流体交代改造过的岩石圈地幔被上涌的软流圈物质加热而部分熔融,形成东天山二叠纪构造-岩浆-成矿系统的岩浆源区,原始岩浆在上升到地壳中的阶段性岩浆房中后发生了分异:吐哈盆地玄武岩可能是整个构造-岩浆-成矿系统的前导性喷发岩流,其母岩浆来自阶段性岩浆房分异的上部位置;东天山镁铁-超镁铁质岩体是岩浆成矿的终端岩浆房,母岩浆来自阶段性岩浆房的下部位置,MgO含量较高,母岩浆侵位速度较慢,遭受了较高程度的地壳物质混染而使硫化物发生熔离成矿;卡拉塔格地区镁铁质岩体具有过渡性质,可能是前导性侵入岩体,母岩浆侵位速度较快,地壳混染程度较低(月牙湾岩体含矿,地壳混染程度较高)。因此,由岩体侵位模式和现今保存特征可以推测,从吐哈盆地到康古尔剪切带,剥蚀厚度可能逐渐加大,在剥蚀较浅的区域存在形成铜镍硫化物矿床的潜力。近年来在大南湖岛弧带北缘发现了月牙湾铜镍矿床,其南缘发现了白鑫滩和路北铜镍矿床(图1),其成矿特征与东天山黄山、香山等典型铜镍矿相似。综上所述,卡拉塔格镁铁质岩体、吐哈玄武岩和东天山镁铁-超镁铁质岩体形成于同一构造岩浆系统,在剥蚀较浅的大南湖岛弧带具有形成铜镍硫化物矿床的潜力。

6 结论

(1)东天山卡拉塔格地区出露多个镁铁质岩体,面积出露面积<1km2。主要岩石类型有辉长岩、橄榄辉长岩、橄长岩、淡色辉长岩和辉绿岩,2个辉长岩LA-ICP-MS锆石U-Pb年龄为分别为282.2±0.6Ma和278.3±0.6Ma。

(2)卡拉塔格地区镁铁质岩石SiO2含量介于45.08%~64.43%之间,Mg#介于42.4~72.7之间;轻稀土轻微富集,轻重稀土元素分馏不明显,无铕异常或较弱的铕正异常;富集Rb、Ba、Sr、K,亏损Th、Nb、Ta;Nd、Sr、Pb同位素组成显示岩浆源区为俯冲交代的岩石圈地幔,并被软流圈物质混染。母岩浆在源区经历了俯冲板片流体的交代(5%~15%),上地壳混染较弱(<5%)。

(3)卡拉塔格地区镁铁质岩体的形成可能与塔里木地幔柱作用无关,而是形成于后碰撞伸展的构造环境,其与东天山二叠纪铜镍矿化镁铁-超镁铁质岩体和吐哈盆地玄武岩形成于同一构造岩浆系统,在剥蚀较浅的大南湖岛弧带具有形成铜镍硫化物矿床的潜力。

致谢本文野外工作得到哈密红石矿业公司张锐总经理的支持与协助;同位素分析得到了中国科学院地质与地球物理研究所李潮峰和李友连老师的指导;全岩主、微量分析得到了核工业北京地质研究院刘牧老师的帮助;审稿人对本文提出了建设性的修改意见;作者在此致以诚挚的谢意。

在肖序常院士90华诞之际,谨以此文表示衷心的祝福!

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