Geochronology and Geochemistry of the Ore-Bearing Porphyries in the Baogutu Area (Western Junggar): Petrogenesis and Their Implications for Tectonics and Cu-Au Mineralization
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摘要: 西准噶尔包古图地区的一些小型斑岩体或脉岩与铜金矿化的关系非常密切, 引起了人们的广泛关注.报道了一些斑岩的LA-ICP-MS锆石U-Pb定年结果和岩石地球化学成分资料.Ⅱ、Ⅴ岩体的石英闪长斑岩中岩浆结晶锆石的定年结果分别为314.9±1.7Ma和309.9±1.9Ma, Ⅲ岩体的闪长玢岩中岩浆结晶锆石的定年结果为313.9±2.6Ma, 表明包古图地区的岩体形成于晚石炭世.包古图地区小岩体的岩石富Na、高Sr, 贫Y和Yb, 无明显的Eu异常, 具有埃达克岩特征, 同时有些岩石富集MgO (3.93%~4.78%), 具有高的Mg# (68~74), 类似高镁安山岩.结合区域地质和岩浆岩的资料, 认为: (1) 包古图地区的小岩体形成于晚石炭世的岛弧环境, 并可能与石炭纪的洋脊俯冲有关, 其中埃达克质岩为俯冲洋脊两侧的板片熔融而形成, 而高镁闪长岩类为俯冲板片熔体与地幔橄榄岩相互作用的产物; (2) 包古图地区的铜金矿床也很可能与洋脊俯冲有关, 高氧逸度的板片熔体上升并与地幔橄榄岩相互作用, 这使得地幔中的金属硫化物不稳定, 释放出金属成矿物质, 从而使得Cu、Au等在熔体中不断富集而形成矿床.Abstract: The small porphyry plutons or dikes in the Baogutu area, western Junggar, have attracted wide attentions owing to the close association between them and Cu-Au mineralization.This paper presents new LA-ICP-MS zircon U-Pb age and geochemical data of ore-bearing porphyries in the Baogutu area.The quartz diorite porphyry bodies Ⅱ and Ⅴ and diorite porphyry body Ⅲ have crystallization ages of 314.9±1.7Ma, 309.9±1.9Ma, and 313.9±2.6Ma, respectively, suggesting they were generated in Late Carboniferous.They are characterized by high Na2O/K2O and high Sr values but low Y and Yb contents, and negligible Eu anomalies, similar to adakites.In addition, some samples have high MgO (3.93%-4.78%) and Mg# (47-74) values, similar to high-Mg andesite.Taking into account the data of regional geology and magmatic rocks, we suggest that (1) The Baogutu intrusive rocks were possibly formed in an island-arc setting linking to ocean ridge suduction in Late Carboniferous, the adakitic magmas have likely formed by partial melting of the leading edge of the subducted ridge, and high-Mg diorites possibly originated from the interaction between adakitic melts and mantle peridotite; (2) The Baogutu Cu-Au mineralization might occur above a slab window during ocean ridge suduction, and the interaction between high oxygen fugacity slab melt and mantle peridotite caused the decomposition of metal sulfides and the Cu and Au mineralization.
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Key words:
- zircon U-Pb dating /
- adakite /
- high-Mg diorites /
- ridge subduction /
- slab windows /
- West Junggar /
- Central Asian orogenic belt
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0. 引言
西准噶尔地区位于中亚造山带中南部的阿尔泰造山带和天山造山带之间(图 1a), 西准噶尔褶皱带是在古生代由塔里木、哈萨克斯坦和西伯利亚等板块经聚合-俯冲-增生作用形成的(Coleman, 1989; Feng et al., 1989).西准噶尔地区出露大量晚古生代的中酸性侵入岩, 根据产状可以将这些中酸性侵入岩分成两类: 一类是以巨大花岗岩基形式出露的花岗岩, 如庙儿沟、克拉玛依、阿克巴斯套和红山等岩体(Chen and Jahn, 2004; Chen and Arakawa, 2005; 韩宝福等, 2006; 苏玉平等, 2006); 另一类是以小岩株或岩脉形式出露的石英闪长斑岩、闪长玢岩和花岗闪长斑岩等, 主要分布在包古图地区(金成伟等, 1993; 沈远超等, 1993).基于对区内岩浆岩的研究, 许多研究者对西准噶尔地区晚古生代的构造环境提出了一些不同的认识, 包括洋内俯冲的岛弧(张连昌等, 2006)、岛弧增生(王方正等, 2002)、弧后盆地(金成伟等, 1993; 沈远超和金成伟, 1993)、小洋盆(朱宝清和冯益民, 1994) 和碰撞后(Chen and Arakawa, 2005; 韩宝福等, 2006; 苏玉平等, 2006) 等环境.同时, 由于缺乏高精度同位素年代学资料, 包括西准噶尔地区在内的中亚造山带的大陆地壳的增生时限和方式也一直争论不休(Sengör et al., 1993; Jahn et al., 2000; Yakubchuk, 2002; Xiao et al., 2004a, 2004b; Yakubchuk, 2004; Zhou et al., 2004; 高俊等, 2006; Liu and Pan, 2006; 王强等, 2006b; 肖文交等, 2006; Windley et al., 2007; Wang et al., 2007c; Zheng et al., 2007; Qian et al., 2007).
1.花岗岩类; 2.蛇绿岩; 3.铜金矿点; 4.克拉玛依地体(石炭纪); 5.库鲁木地地体(中-早石炭世); 6.拉巴地体(奥陶纪); 7.艾比湖-可可萨依地体(晚志留-早奥陶世); 8.玛依拉地体(早-中志留世); 9.托里地体(中-晚泥盆世); 10.二叠世沉积物/火山沉积物; 11.断层; 12.中生代沉积物; ①唐巴勒; ②玛依拉; ③达拉布特; ④克拉玛依Fig. 1. Geographic map showing Central Asia (modified from Jahn et al., 2000) (a) and regional geological sketch map showing the West Junggar area (modified from Buckman and Aitchison, 2004) (b)西准噶尔地区也是中亚造山带内一个重要的铜金矿化区, 其中包古图铜金矿区是该地区近几年新发现的一个很有前景的成矿区.该矿区位于准噶尔盆地西缘的达拉布特断裂以南、托里铜金成矿带的东段.包古图地区分布约有二十多个石英闪长斑岩、闪长玢岩、花岗闪长斑岩小岩株或脉岩, 有些小岩株周围出露大量中基性岩脉.最近, 有些学者认为这些与成矿有关的侵入岩可能是由俯冲洋壳熔融形成的埃达克岩(张连昌等, 2006), 也有一些学者对矿床进行了Re-Os同位素定年(宋会侠等, 2007).但是对这些斑岩的成矿年代学的研究, 除了前人对一些岩石进行了一些K-Ar和Rb-Sr年代学测试外(金成伟等, 1993; 沈远超和金成伟, 1993), 目前其形成时代一直缺乏精确的年龄制约.笔者近期对该地区的代表性成矿斑岩岩体进行了系统的主、微量元素分析, 并对那些具有典型埃达克岩特征的岩石进行了锆石LA-ICP-MS U-Pb同位素定年, 发现这些岩石主要形成于晚石炭世.本文将重点报道这一成果, 并讨论其动力学与成矿意义.
1. 区域地质与岩体地质
西准噶尔地区的主要构造特征为NE-SW向断裂带非常发育(图 1b), 由北往南依次为托里、哈图、达拉布特断裂和乌尔木逆冲断层, 它们控制了蛇绿岩和花岗岩的分布.西准噶尔地区的一个特点就是蛇绿岩形态复杂, 均已变形, 由于后期的构造运动多沿着走滑断裂分布, 时代不一致, 跨度大, 从晚寒武到石炭纪均有出现.其中唐巴勒蛇绿岩是该地区最老的蛇绿岩(Coleman, 1989; Feng et al., 1989; Kwon et al., 1989; 肖序常和汤耀庆, 1991; Zhang et al., 1993), 形成时代为晚寒武-奥陶纪: 辉长岩中斜长石和榍石的Pb-Pb年龄为523.2±7.2 Ma (Kwon et al., 1989), 辉长岩的矿物-全岩的Sm-Nd内部等时线年龄为489±53 Ma, 玄武岩的全岩Sm-Nd等时线年龄为447±56 Ma (张弛和黄萱, 1992), 辉长岩锆石SHRIMP U-Pb年龄为531±15 Ma (Jian and Liu, 2005).玛依拉蛇绿岩中发现有放射虫化石, 其时代为志留纪(辉长岩锆石SHRIMP U-Pb年龄为415 Ma以前(Jian and Liu, 2005)).达拉布特蛇绿岩中辉长岩的Sm-Nd年龄为395±12 Ma, εNd (t) =8.9 (张弛和黄萱, 1992), 但最近的SHRIMP年代学研究表明这一蛇绿岩形成于346~347 Ma, 为早石炭世1①.克拉玛依蛇绿岩中蚀变辉长岩的锆石SHRIMP定年结果集中在414.4±8.6 Ma和332±14 Ma (徐新等, 2006). 何国琦等(2007)通过对微体古生物的研究认为克拉玛依蛇绿岩形成于早古生代. 黄汲清等(1990)认为在早石炭世晚期至中石炭世初期准噶尔南缘盆地北天山地区出现了新的洋盆, 称之为石炭纪亚洲洋或北天山洋. 肖序常等(1992)认为早石炭晚期该地区也出现了洋盆并称之为北天山洋. 王福同(2006)认为西准噶尔地区在晚石炭世晚期仍为浅海-次深海环境.所以, 西准噶尔地区石炭纪仍然可能有洋盆出现.西准噶尔地区出露大量晚古生代花岗岩, 其显著特点是均具有正的εNd (t), 这些花岗岩的形成的时间在274~340 Ma (Chen and Jahn, 2004; Chen and Arakawa, 2005; 高山林等, 2006; 韩宝福等, 2006; 苏玉平等, 2006).
包古图矿区（图 2）主要的构造线以南北向为主：无论是地层、褶皱和断裂都以南北向为主，矿区位于南北向的希贝库拉斯复式向斜的东翼，断裂除了南北向的主要断裂外还有北东向的次级小断裂. 矿区出露的地层主要为下石炭统的太勒古拉组、包古图组和希贝库拉斯组的一套巨厚的半深海相-大陆坡相火山-碎屑沉积建造.其中太勒古拉组为杂色凝灰岩、凝灰质粉砂岩和灰绿色凝灰岩互层.包古图组为灰-灰黑色薄层凝灰质粉砂岩和灰绿色凝灰岩互层.希贝库拉斯组为灰色厚层凝灰质砂岩、含砾砂岩和层凝灰岩.区内大约有20个规模较小，大小数平方公里的石英闪长斑岩、闪长玢岩和花岗闪长斑岩类的小岩株或脉岩侵入下石炭世地层中，其中以Ⅱ号和Ⅴ号岩株矿化最好，Ⅰ号、Ⅲ号、Ⅳ号也有矿化.本次研究为Ⅱ号、Ⅴ号和Ⅲ号岩体.
图 2 包古图矿区地质简图(据成勇和张锐, 2006修改)1.希贝库拉斯组; 2.包古图上亚组; 3.包古图下亚组; 4.太古勒组; 5.岩体; 6.断层; 7.金矿点; 8铜矿点; Ⅰ-Ⅴ为岩体编号Fig. 2. Regional geological sketch map showing the Baogutu depositⅡ号岩体位于矿区的中部（图 2），呈菱形，侵位于希贝库拉组，主要岩相由石英闪长斑岩和花岗闪长斑岩组成，其中石英闪长岩主要矿物有长石、角闪石和石英，次要矿物有黑云母和辉石，副矿物有磁铁矿、钛铁矿和榍石等.花岗闪长斑岩主要矿物有长石、角闪石和石英，次要矿物有黑云母，副矿物有磁铁矿和榍石等.Ⅱ号岩体的矿化类型主要为斑岩铜矿.Ⅴ号岩体位于矿区的东部（图 2），出露面积为0.84km2，侵位于包古图组上亚组和希贝库拉组的下亚组地层，主要岩相由石英闪长斑岩和花岗闪长岩组成，矿体的北半部以石英闪长斑岩为主，南半部以花岗闪长斑岩为主，二者关系不清楚，化学成分也差别不大.岩体的主要矿物有斜长石、角闪石、石英和钾长石，其中斜长石为自型，聚片双晶发育，石英含量较少，一般在5%，次要矿物有黑云母和辉石，副矿物有磷灰石和榍石等.Ⅴ号岩体的矿化类型也主要为斑岩铜矿.Ⅲ号矿床也称阔个沙也金矿，位于包古图矿区的中南部（图 2），岩体呈椭圆状，侵位于包古图上亚组，岩相主要位石英闪长岩和闪长玢岩，主要矿物有长石、角闪石和石英，次要矿物有黑云母和辉石，副矿物有磷灰石和榍石等.岩体周围中基性脉岩极为发育，大多呈NE30°~60°平行排列，脉宽0.2~3.0m，长几十米到几公里不等，倾向NW，倾角在70°左右.这些脉岩的岩相由辉绿岩、闪长玢岩、石英闪长斑岩和二长花岗斑岩组成.Ⅲ号岩体的矿化类型也主要为斑岩铜矿.
2. 分析方法
主、微量元素的分析测试均在中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室完成.主量元素分析是用Rigaku RIX 2000型荧光光谱仪(XRF) 分析, 其详细步骤见Li et al. (2005).样品的含量由36种涵盖硅酸盐样品范围的参考标准物质双变量拟合的工作曲线确定, 基体校正根据经验的Traill-Lachance程序进行, 分析精度优于1%~5%.微量元素的分析则采用Perkin-Elmer Sciex ELAN 6000型电感耦合等离子体质谱仪(ICP-MS), 具体的流程见Li (1997).使用USGS标准W-2和G-2及国内标准GSR-1、GSR-2和GSR-3来校正所测样品的元素含量, 分析精度一般为2%~5%.分析数据列于表 1.
表 1 包古图成矿斑(玢) 岩主量(%)、微量(10-6) 元素组成Table Supplementary Table Major element (%) and trace element (10-6) compositions of the Baogutu porphyries (porphyrites)
为精选锆石样品, 先将新鲜的岩石样品粉碎至120目以下, 用常规的人工淘洗和电磁选方法富集锆石, 再在双目镜下用手工方法逐个精选锆石颗粒, 未用任何化学试剂.本次锆石定年样品和主元素和微量元素分析的样品相对应.锆石阴极发光图像研究在中国科学院广州地球化学研究所JXA-8100电子探针仪上完成.锆石LA-ICP-MS U-Pb年龄测定在中国地质大学(武汉) 地质过程与矿产资源国家重点实验室完成.ICP-MS为Agilent公司生产的四极杆ICP-MS Agilent 7500a, 激光剥蚀系统为德国Lamda Physik公司的GeoLas 2005深紫外(DUV) 193 nm ArF准分子(excimer) 激光剥蚀系统.实验中采用He作为剥蚀物质的载气, 激光斑束直径为32 μm.参考物质为美国国家标准技术协会研制的人工合成硅酸盐玻璃NIST SRM610, 锆石U-Pb年龄的测定采用国际标准锆石91500作为外标进行校正, 每隔5个分析点测一次标准, 保证标准和样品的仪器条件一致.在样品分析前后以及每隔20个测点各测一次NIST SRM610, 以Si做内标, 测定锆石中的U、Th、Pb的含量.详细的分析流程及有关参数见(Yuan et al., 2004).元素的比率和元素的含量用GLITTER (4.0版) 来处理, 年龄的计算和谐和图用ISOPLOT (3.00版) (Ludwig, 2003) 来完成.分析数据列于表 2.
表 2 包古围成矿(玢)岩锆石LA-ICPMS定年结果Table Supplementary Table Zircon LA-ICPMS dating of porphyries (porphyrite) in the Baogutu
3. 岩体的锆石LA-ICPMS U-Pb定年
3.1 样品06XJ-145 (与Ⅱ矿体共生的石英闪长斑岩)
绝大多数锆石都是透明自型晶, 棱柱状, 长约50~150 μm, 长宽比在1∶5~3∶1.锆石阴极发光图像显示普通的同心韵律环带(图 3).27个分析点显示锆石具有变化的U (64×10-6~522×10-6)、Th (31×10-6~405×10-6) 含量和U/ Th (0.76~2.65) 比值, 为典型的岩浆锆石.在分析的27个点中的22个点在谐和图上形成单一和集中的聚集束, 206Pb/238U年龄在308±4 (2σ) ~319±4 (2σ) 之间, 加权年龄为314.9±1.7 Ma (图 3a).这个年龄被解释为该样品的结晶年龄.其中有5个点明显偏老, 在370 Ma左右, 可能为捕获晶.
图 3 包古图斑岩LA-ICPMS锆石U-Pb谐和图和锆石CL图像Fig. 3. LA-ICPMS zircon U-Pb concordia diagrams and zircon Cathodoluminescence (CL) images3.2 样品06XJ-147 (与Ⅴ矿体共生的石英闪长斑岩)
绝大多数锆石都是透明自型晶, 长约30~100 μm, 长宽比在1∶1~3∶1.阴极发光图像显示绝大多数锆石具有同心韵律环带, 少量锆石具有扇形分带或者弱的分带(图 3).22个分析点显示锆石具有相对一致的U (56×10-6~126×10-6)、Th (36×10-6~103×10-6) 含量和U/ Th (1.15~1.55) 比值, 为典型的岩浆锆石.在分析的23个点中, 所有点在谐和图上形成单一和集中的聚集束, 206Pb/238U年龄在304±5 (2σ) ~315±4 (2σ) 之间, 加权年龄平均为309.9±1.9 Ma (图 3b), 代表了岩浆的结晶年龄.
3.3 样品06XJ-153 (与Ⅲ矿体共生的闪长玢岩)
该样品的锆石的晶体形状复杂, 长约30~100 μm, 长宽比在1∶1~3∶1, 有些呈棱柱状, 有些呈圆状.棱镜状锆石显示同心韵律环带, 而有些板状锆石无分带, 有些圆形锆石呈弱的或者扇形分带(图 3).22个分析点显示锆石具有变化的U (34×10-6~1 712×10-6)、Th (22×10-6~825×10-6) 含量和U/ Th (0.68~3.26) 比值, 但绝大部分在1.0~2.0之间.在分析的22个点中的19个点在协和图上形成单一和集中的聚集束, 206Pb/238U年龄在305±6 (2σ) ~322±5 (2σ) 之间, 加权年龄平均为313.9±2.6 Ma (图 3c), 这个年龄被解释为该样品的结晶年龄.其中有2个点(6, 13) 明显偏老, 分别为364 Ma和344 Ma, 为捕获晶.分析点5具有不谐和的年龄, 其206Pb/238U年龄为761 Ma, 可能是继承锆石.
4. 成矿斑岩地球化学特征
代表性的主元素和微量元素分析结果见表 1.根据表 1并结合前人已发表的资料, 包古图地区斑岩和中基性脉岩具有一些相似的元素地球化学特征: 岩性变化较大, 从闪长岩到花岗岩, 但以花岗闪长岩和闪长岩为主(图 4a), 脉岩从辉长闪长岩到花岗闪长岩, 以闪长岩为主, 二者在TAS图(Middlemost, 1994) 上除了个别点外均落入亚碱性区, 在AFM图(图 4b) 中均落入钙碱性区.岩体和脉岩的岩石都贫碱、富钠、贫钾, 岩体K2O+Na2O为2.40%~8.33%, 平均为5.80%, Na2O/K2O为0.79~9.29, 平均为2.86, 脉岩K2O+Na2O为4.10%~7.90%, 平均为5.40%, Na2O/K2O为0.86~6.61, 平均为2.27.它们均含有高的Al2O3 (16.08%~17.75%), 变化的MgO (0.46%~4.78%) 含量和Mg# (47~75) (表 1).包古图地区小型斑岩体的一些岩石(如Ⅱ和Ⅴ岩体) 具有高的MgO (3.93%~4.78%) 含量、Mg# (68~74), 以及高的Cr (57.5×10-6~151.9×10-6)、Ni (44.4×10-6~62.0×10-6) 等相容元素含量(表 1), 同时具有高的Sr (561×10-6~716×10-6) 含量和相对低的Y (14.8×10-6~15.4×10-6) 含量.包古图地区斑岩具有相似的稀土和微量元素组成: 稀土元素分布模式为轻稀土富集型, 无明显Eu异常(δEu=0.88~1.0), 但重稀土元素分异不明显(图 4a); 富集大离子亲石元素(LILE), 如Ba、K和Sr具有明显的正异常, 亏损高场强元素(HFSE) 和重稀土元素, 显示Nb、P的负异常, Ti也有弱的负异常(图 4b).以上这些特点显示它们与俯冲带岩浆地球化学特征类似(Wilson, 1989).
图 4 TAS (a) 和AFM (b) 图Fig. 4. (a) TAS and (b) AFM digrams5. 讨论
5.1 形成时代与构造背景
包古图地区的小岩体或脉岩一直缺乏高精度同位素年代学的制约, 一般认为其形成于海西中期, 如沈远超和金成伟(1993)给出的Ⅴ号岩体的全岩-角闪石Rb-Sr为322±30 Ma, 角闪石K-Ar年龄为322±1.4 Ma, Ⅰ号岩体的锆石U-Pb的下交点年龄是305 Ma.这些年代学资料误差较大或精度较低.另外, Rb-Sr、K-Ar定年方法有其本身固有的缺点: (1) Rb-Sr同位素体系易受到后期热扰动的影响而重新产生均一化, 尤其在矿区, 热液蚀变作用强烈, 因此Rb-Sr等时线很难实现精确的同位素定年; (2) 后期地质作用的影响容易使K-Ar封闭体系破坏所引起的放射成因氩的丢失, 所以, K-Ar法测年主要适用于年轻的地质热历史简单的地质体测年.由于包古图地区的小岩体或脉岩普遍受到矿化的影响, 因此, Rb-Sr和K-Ar同位素年龄可能较难精确地限定这些岩浆岩的形成时代.
本文新的年代学结果表明, 包古图石英闪长斑(玢) 岩的锆石均具有韵律环带结构, 岩体的年龄在310~315 Ma, 应该反映了岩浆岩的结晶年龄.所以, 包古图地区斑(玢) 岩形成于晚石炭世.目前虽然对西准噶尔地区蛇绿岩的形成时代一般认为是早古生代, 但最近一些锆石SHRIMP U-Pb年代学的研究表明该地区的蛇绿岩时代有晚古生代石炭纪年龄的信息, 如达拉布特蛇绿岩的SHRIMP年龄为346~347 Ma, 为早石炭世, 达尔布特蛇绿岩中侵入玄武岩中的闪长岩脉有一组325±6 Ma的年龄, 而不整合于蛇绿岩之上地层中的辉石安山岩也有一组336±5 Ma的年龄, 克拉玛依蛇绿岩中蚀变辉长岩中也有一组332±14 Ma的年龄(徐新等, 2006).因此, 西准噶尔地区蛇绿岩的锆石U-Pb年龄中普遍存在早石炭世(347~325 Ma) 的数据, 这与前人认为该地区石炭纪存有洋盆(石炭亚洲洋或北天山洋) 相一致(黄汲清等, 1990; 肖序常等, 1992; 王福同, 2006).笔者所测定的包古图地区斑(玢) 岩的LA-ICP-MS锆石U-Pb年龄(310~315 Ma) 略微晚于该地区存在的早石炭世(347~325 Ma) 蛇绿岩.因此, 笔者认为包古图地区斑(玢) 岩的形成很可能与洋盆在晚石炭世的消减有关, 应该形成在岛弧环境中.
5.2 岩石成因
包古图地区的斑(玢) 岩具有埃达克岩的特征(Kay, 1978; Defant and Drummond, 1990; Martin et al., 2005) : SiO2=56.68%~70.65%, Al2O3=16.08%~17.75%, 高Sr (346×10-6~769×10-6), 低Y (9.51×10-6~15.6×10-6) 和Yb (1.08×10-6~1.55×10-6), 无明显Eu异常(δEu=0.88~1.0) (图 5a), 高的Sr/Y (33~48), 在Sr/Y-Y图中(Defant and Drummond, 1993) 均落入埃达克岩区(图 6).这些岩石样品中, 有些样品具有高MgO、Mg# (图 7) 和高的相容元素Cr和Ni (表 1), 类似典型的埃达克型高镁安山岩(Kay, 1978; Yogodzinski et al., 1995).因此, 包古图地区的斑(玢) 岩具有埃达克岩-高镁闪长岩组合的特点.
Fig. 5. (a) The chondrite-normalized rare earth element (REE) patterns and (b) primitive mantle-normalized multi-element plots
图 6 包古图地区斑(玢) 岩Sr/Y-Y图(Defant and Drummond, 1993)数据来源于表 1, 克拉玛依岩体数据来自于(Chen and Arakawa, 2005)Fig. 6. Sr/Y-Y diagram
图 7 SiO2-MgO (a) 和SiO2-Mg# (b) 图Fig. 7. SiO2-MgO (a) and SiO2-Mg# (b) diagrams目前, 对埃达克质岩浆的形成提出了不同的机制, 主要包括: (1) 俯冲洋壳熔融(Kay, 1978; Defant and Drummond, 1990), (2) 增厚下地壳熔融(Atherton and Petford, 1993; Petford and Atherton, 1996; 张旗等, 2001; Chung et al., 2003; Hou et al., 2004; Condie, 2005; Wang et al., 2005; Zhang et al., 2006b), (3) 拆沉下地壳熔融(Kay and Kay, 1993; Xu et al., 2002; Gao et al., 2004; Wang et al., 2006a, 2007a; Hou et al., 2007; Huang et al., 2008; Liu et al., 2008), (4) 玄武质岩浆的结晶分异(Castillo et al., 1999; Macpherson et al., 2006). 张连昌等(2006)根据包古图斑岩具有MORB的Sr-Nd同位素特征和高的镁指数, 认为这些岩石是洋内俯冲的洋壳熔融形成的, 岩浆在上升过程中受到了地幔橄榄岩的不均匀混染.包古图地区的埃达克岩-埃达克型高镁闪长岩组合具有洋壳俯冲熔融形成的埃达克岩的特征: 具有MORB的Sr-Nd同位素特征, 并且富钠贫钾(表 1), 与洋壳组分类似, 而增厚下地壳熔融形成的埃达克岩往往富钾贫钠(Wang et al., 2005, 2007a; Xiao and Clemens, 2007), 并且下地壳熔融一般具有地壳的同位素特点.岩体具有高的MgO和Mg#, 高于变质玄武岩和榴辉岩的实验熔体(1~4 GPa), 和受橄榄岩混染的熔体相似(Rapp et al., 1999) (图 7).所以包古图地区斑(玢) 岩的形成可能与早石炭世洋盆在晚石炭世的俯冲消减有关, 其中埃达克质岩石为俯冲洋壳的熔融形成, 而高镁闪长岩为俯冲板片熔融形成的岩浆和地幔橄榄岩相互作用的结果.
除了小的斑岩体和脉岩以外, 西准噶尔地区还出露大量的花岗岩基(图 1), 如红山、克拉玛依、阿克巴斯套、庙尔沟等岩体.这些岩体有的具有I型花岗岩特点, 如克拉玛依岩体(Chen and Arakawa, 2005; 高山林等, 2006); 有些类似铝质A型花岗岩, 如红山岩体(苏玉平等, 2006), 这些岩石均具有正的εNd (t) (6~9) 和年轻的Nd同位素模式年龄(约500 Ma).随着近年来SHRIMP和LA-ICP-MS锆石定年技术的应用, 在该区积累了相当数量的高质量岩浆岩的年代学数据资料.图 8是西准噶尔地区花岗岩类锆石U-Pb高精度年龄的汇总, 显示岩浆活动集中在274~340 Ma之间, 有两个峰值, 一个是315 Ma的次级峰值, 另外一个是305 Ma主峰值.而包古图地区的斑(玢) 岩就是属于次级峰期岩浆活动的产物.次级峰期形成的岩浆岩以花岗闪长岩和闪长岩类为主, 在这些岩石的样品中有一部分样品类似埃达克岩和高镁闪长岩类(图 6), 而主峰期形成的岩浆岩以碱长花岗岩为主, 部分岩体的岩石类似A型花岗岩.如果包古图地区埃达克质斑(玢) 岩-高镁闪长岩组合的形成与早石炭世洋盆在晚石炭世的俯冲消减有关, 那么紧随其后的I型和铝质A型花岗岩类的形成很可能与板片窗效应有关: 因软流圈的上涌导致地幔减压熔融, 产生的玄武质岩浆发生大规模底侵, 在来自软流圈或底侵玄武质岩浆热的作用下, 年轻的下地壳发生部分熔融形成花岗岩.
图 8 西准噶尔地区花岗岩体的锆石U-Pb年龄直方图数据除本次研究外还包括Xian et al. (2003)、刘志强等(2005)、高山林等(2006)、韩宝福等(2006)、徐新等(2006)、袁峰等(2006)和Zhou et al. (2006, 2007)Fig. 8. Age histograms for the grantoids in the West Junggar5.3 动力学与成矿意义
5.3.1 动力学背景
西准噶尔地区蛇绿岩时代不一致, 跨度大, 从晚寒武到石炭纪都有出现(Coleman, 1989; Feng et al., 1989; Kwon et al., 1989; 肖序常和汤耀庆, 1991; Zhang et al., 1993; Jian and Liu, 2005; 徐新等, 2006), 甚至同一蛇绿岩也有多组年龄信息, 这些年龄主要集中在530~395 Ma和347~332 Ma.所以, 该地区存在的古洋盆可能有多期, 闭合时间可能也是多期.目前对西准噶尔地区晚古生代构造演化有不同的认识: 很多人将准噶尔地区广泛发育的正εNd (t) 和低(87Sr/86Sr) i的花岗岩做为后碰撞地壳垂向生长的标志, 大量的同位素定年表明, 西准噶尔地区岩体的形成时间大约在274~340 Ma (Chen and Jahn, 2004; Chen and Arakawa, 2005; 高山林等, 2006; 韩宝福等, 2006; 苏玉平等, 2006). 韩宝福等(2006)认为西准噶尔后碰撞深成岩浆活动的时限在340~275 Ma之间, 李华芹等(2000)认为西准噶尔地区后造山花岗岩类的侵位时代发生在中石炭世之后, 所以西准噶尔地区在晚石炭世早已进入后碰撞阶段, 准噶尔洋应该在晚石炭世之前早已关闭.但肖文交等(2006)认为新疆北部在晚石炭世-二叠纪可能仍存在活动陆缘.因此, 古亚洲洋构造域南部复杂增生造山作用最后延至晚石炭世晚期-二叠纪.高山林等(2006)也认为晚石炭世准噶尔地区仍存在洋盆体制.准噶尔盆地南缘的北天山巴音沟蛇绿混杂岩形成于早石炭世(344.0±3.4 Ma) (徐学义等, 2006).最近的研究表明, 在天山北部地区也存在洋壳石炭纪的俯冲和熔融作用, 并且形成了典型的岛弧岩浆岩组合埃达克岩-高镁安山岩-富Nb玄武岩组合(王强等, 2006; Wang et al., 2007c).
笔者认为恰当的地球动力学模型至少要能够能够解释: (1) 岩石组合, 埃达克岩-高镁安山(或闪长质) 岩类和I型花岗岩类-A型花岗岩的成因, 即由钙碱性岩浆到碱性岩浆, 埃达克岩-高镁安山岩组合往往形成于俯冲的岛弧环境(Defant and Drummond, 1990; Martin et al., 2005; Wang et al., 2007c); 而A型花岗岩往往产于伸展背景(Whalen et al., 1987; Eby, 1990, 1992; Wu et al., 2002), 在由洋脊俯冲作用下形成的板片窗上出露有A型花岗岩, 如阿拉斯加的McKinley Sequence岩体(Hung et al., 2007), 南美智利的黑云母正长花岗岩San Lorenzo岩体(Suarez and De La Cruz, 2001), 也被认为是由洋脊俯冲作用所形成的(Cande and Leslie, 1986); (2) 岩石形成的时间, 岩石组合形成在一个非常短的时间间隔, 峰值相隔只有10 Ma (图 8); (3) 成矿问题, 次级峰期的岩石往往和铜金矿有关, 而主峰期的岩石不成矿.另外, 古地理研究显示石炭纪早期西准噶尔地区为深海-次深海环境, 太古勒拉组、包古图组和希贝库拉组均为海相火山碎屑沉积(宋春晖等, 1996;王福同, 2006), 石炭纪晚期为次深海-浅海环境.这与智利三联点地区(Chile Triple Junction) 的上新世火山-沉积组合相似, 而该区在过去的14 Ma以来一直有洋脊俯冲作用(Guivel et al., 1999).
笔者认为洋脊俯冲模式可以解释西准噶尔地区晚古生代构造演化.包古图地区埃达克岩-高镁闪长岩组合与日本、厄瓜多尔、墨西哥巴哈半岛、阿拉斯加、内蒙古等地出露的埃达克岩或高镁安山岩相类似, 而这些具有特殊地球化学特征的岩石往往和洋脊俯冲形成的板片窗作用相关(Sisson et al., 2003; Kamei, 2004; Bellon et al., 2006; Cole et al., 2006; Manya et al., 2007; Pallares et al., 2007; Jian et al., 2008).许多学者认为中亚造山带内的花岗岩具有正εNd (t) 和低(87Sr/86Sr) i的特点, 具有年轻同位素的特征, 认为这些花岗岩是在碰撞后伸展背景下来自软流圈地幔的热玄武质底侵岩浆加热而使年轻的下地壳发生部分熔融形成的(Chen and Jahn, 2004; Jahn et al., 2004; Chen and Arakawa, 2005).为了解释下地壳熔融所需的热源, Kovalenko et al. (2004)认为需要有地幔柱或热点的作用, 但Windley et al. (2007)认为中亚造山带在古生代没有地幔柱和热点的证据.如果将西准噶尔地区包古图斑(玢) 岩附近的一些碱长花岗岩体作为后碰撞的标志(Chen and Arakawa, 2005; 韩宝福等, 2006; 苏玉平等, 2006), 那么如何解释比之仅早10 Ma左右的埃达克岩-高镁闪长岩组合的成因则需要更深入地探讨.但是, 洋脊俯冲可以解释在如此短的时间内形成这两类岩石, 当洋脊俯冲时, 洋脊两侧的板片容易熔融而形成埃达克岩类, 板片熔融形成的岩浆上升和地幔橄榄岩相互作用形成高镁闪长岩类, 几乎于此同时洋脊俯冲所造成的板片窗使得软流圈地幔上涌, 使年轻的下地壳熔融形成一些具有后碰撞特点的花岗岩类, 这和现在欧亚大陆的东侧发生的洋脊俯冲相似(Kinoshita, 1997, 1999, 2002; Maruyama et al., 1997; Marwama, 1997; Kim et al., 2005).另外西准噶尔地区出露大量的中基性脉岩(李辛子等, 2004) 也与在洋脊俯冲地质现象比较类似(Sisson et al., 2003).
几乎所有的俯冲带最终都以和扩张脊相互作用而结束(Sisson et al., 2003), 所以在大洋闭合的时候洋脊俯冲事件可能比今天所知道的多.最近, 刘希军等(2007)提出了东准噶尔克拉麦里蛇绿岩是由准噶尔洋的洋脊俯冲作用形成的, Jian et al. (2008)和Liu et al. (2008)等认为内蒙古地区有洋脊-海沟相互作用.因此, 在中亚造山带的聚合历史和大陆地壳的增生过程中洋脊俯冲可能是一个重要的机制.
5.3.2 铜金成矿意义
自从埃达克岩被识别以来, 越来越多的证据表明埃达克质岩石与斑岩型或热液型铜金矿床在时空上往往密切相关, 具有明显的偏向性和亲和性(Thieblemont et al., 1997; Sajona and Maury, 1998; Defant and Kepezhinskas, 2001; Oyarzun et al., 2001; Bissig et al., 2003; Reich et al., 2003; 王强等, 2003; Wang et al., 2006a, 2007b; Zhang et al., 2006b), 尽管目前对铜金矿化的机制还有不一致的认识(Rabbia et al., 2002; Richards, 2002; 张旗等, 2004; Richards and Kerrich, 2007), 但有研究认为埃达克岩可以作为斑岩型或热液型铜金矿床的找矿标志(Defant et al., 2002).Cu、Au是亲硫元素, 主要赋存于地幔和铁镁质岩石, 所以玄武质洋壳熔融形成的埃达克岩具有良好的成矿物质来源, 同时埃达克岩浆具有高的氧逸度(Mungall, 2002). Mungall (2002)认为板片熔体可以携带大量的Fe2O3, 当携带大量Fe2O3的板片熔体进入到富金属硫化物的地幔楔时, 将会导致地幔楔橄榄岩fO2的增高, 地幔中的金属硫化物将被氧化, Cu、Au等将顺利进入到板片熔体或岛弧岩浆中, 另外俯冲板片富含挥发分H2O和Cl, 挥发分Cl在高温高压下是Cu、Au等金属元素的强烈配合剂, 它们将与Cu、Au形成稳定的配合物随岩浆一起迁移(熊小林等, 2005), 有利于成矿.
包古图矿床辉钼矿的Re-Os同位素年龄为310 Ma (宋会侠等, 2007), 与斑岩的形成时代(315~310 Ma) 相一致, 所以包古图斑岩与铜金矿化有密切的关系.笔者认为, 包古图铜金矿床很可能在由洋脊俯冲作用形成的板片窗上, 如阿拉斯加地区的金矿(Haeussler et al., 1995, 2003).当洋脊俯冲时, 洋脊两侧的板片容易熔融而形成埃达克质岩浆, 埃达克质岩浆上升和地幔橄榄岩相互作用, 其中高氧逸度的岩浆使得地幔中的硫化物不稳定, 以氧化状态的形式溶解于硅酸盐中, 从而使得Cu、Au等金属元素在熔体中不断富集而形成矿床.而洋脊俯冲时由于板片窗作用使得软流圈上涌使下地壳熔融形成的岩浆氧逸度不高且缺乏成矿物质来源, 所以不利于成矿.因此, 西准噶尔地区闪长岩类的小岩株或脉岩与铜金成矿关系密切, 具有重要的铜金勘探意义.
6. 结论
(1) 包古图岩体的形成时代在310~315 Ma, 形成于晚石炭世岛弧环境. (2) 包古图斑岩为埃达克岩-高镁安山岩(闪长岩) 组合, 很可能是由洋脊俯冲作用形成的, 其中埃达克质岩为俯冲洋脊两侧的板片熔融而形成, 而高镁闪长岩为俯冲板片熔体与地幔橄榄岩相互作用的产物. (3) 包古图地区的铜金矿床也很可能与洋脊俯冲有关, 其中由洋脊俯冲作用形成的闪长类斑(玢) 岩小岩株有利于形成Cu、Au矿床.
致谢: 感谢郑建平教授的热情约稿和审稿专家的宝贵建议.室内主量、微量元素分析得到了刘颖和胡光黔老师的帮助, 锆石年代学分析得到了刘勇胜教授和宗克清的帮助, 在此一并表示感谢! -
图 1 中亚造山带地质简图(Jahn et al., 2000) (a) 和西准噶尔地区地质简图(据Buckman and Aitchison, 2004,图 2修改) (b)
1.花岗岩类; 2.蛇绿岩; 3.铜金矿点; 4.克拉玛依地体(石炭纪); 5.库鲁木地地体(中-早石炭世); 6.拉巴地体(奥陶纪); 7.艾比湖-可可萨依地体(晚志留-早奥陶世); 8.玛依拉地体(早-中志留世); 9.托里地体(中-晚泥盆世); 10.二叠世沉积物/火山沉积物; 11.断层; 12.中生代沉积物; ①唐巴勒; ②玛依拉; ③达拉布特; ④克拉玛依
Fig. 1. Geographic map showing Central Asia (modified from Jahn et al., 2000) (a) and regional geological sketch map showing the West Junggar area (modified from Buckman and Aitchison, 2004) (b)
图 2 包古图矿区地质简图(据成勇和张锐, 2006修改)
1.希贝库拉斯组; 2.包古图上亚组; 3.包古图下亚组; 4.太古勒组; 5.岩体; 6.断层; 7.金矿点; 8铜矿点; Ⅰ-Ⅴ为岩体编号
Fig. 2. Regional geological sketch map showing the Baogutu deposit
图 3 包古图斑岩LA-ICPMS锆石U-Pb谐和图和锆石CL图像
Fig. 3. LA-ICPMS zircon U-Pb concordia diagrams and zircon Cathodoluminescence (CL) images
图 4 TAS (a) 和AFM (b) 图
a图据Middlemost (1994); b图据Irvine and Baragar (1971); 数据来源于表 1、张锐等(2006)、沈远超和金成伟(1993)
Fig. 4. (a) TAS and (b) AFM digrams
图 5 包古图地区斑(玢) 岩稀土元素分布模式(a) 和原始地幔标准化微量元素蜘蛛网(b) (标准化数据据Sun and McDonough, 1989; 阴影部分的数据来源于张连昌(2006))
Fig. 5. (a) The chondrite-normalized rare earth element (REE) patterns and (b) primitive mantle-normalized multi-element plots
图 6 包古图地区斑(玢) 岩Sr/Y-Y图(Defant and Drummond, 1993)
数据来源于表 1, 克拉玛依岩体数据来自于(Chen and Arakawa, 2005)
Fig. 6. Sr/Y-Y diagram
图 7 SiO2-MgO (a) 和SiO2-Mg# (b) 图
数据来源于表 1、张锐等(2006)、沈远超和金成伟(1993), 底图转引自(王强等, 2006b)
Fig. 7. SiO2-MgO (a) and SiO2-Mg# (b) diagrams
图 8 西准噶尔地区花岗岩体的锆石U-Pb年龄直方图
数据除本次研究外还包括Xian et al. (2003)、刘志强等(2005)、高山林等(2006)、韩宝福等(2006)、徐新等(2006)、袁峰等(2006)和Zhou et al. (2006, 2007)
Fig. 8. Age histograms for the grantoids in the West Junggar
表 1 包古图成矿斑(玢) 岩主量(%)、微量(10-6) 元素组成
Table 1. Major element (%) and trace element (10-6) compositions of the Baogutu porphyries (porphyrites)
表 2 包古围成矿(玢)岩锆石LA-ICPMS定年结果
Table 2. Zircon LA-ICPMS dating of porphyries (porphyrite) in the Baogutu
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