岩石学报  2019, Vol. 35 Issue (9): 2911-2925, doi: 10.18654/1000-0569/2019.09.19   PDF    
滇西允沟岩组碎屑锆石年龄谱对相关地块亲缘性的约束
赵林涛1,2, 李三忠1,2, 吕勇3, 林宇3, 刘永江1,2, 牟墩玲1,2, 廖家飞3, 潘明3     
1. 海底科学与探测技术教育部重点实验室, 中国海洋大学海洋地球科学学院, 青岛 266100;
2. 青岛海洋科学与技术国家实验室海洋地质功能实验室, 青岛 266237;
3. 中国地质科学院岩溶地质研究所, 桂林 541004
摘要: 滇西允沟岩组为一套低绿片岩相的泥质浅变质岩,岩性以云母石英片岩、云母石英千枚岩、云母片岩、细晶灰岩为主,局部夹硅质岩,一直以来被认为是前寒武纪变质基底的重要组成部分。其原岩以碎屑岩为主,含有部分灰岩、白云岩,是一套形成于特提斯洋被动大陆边缘的半深海-深海沉积物,因此,是研究特提斯洋构造演化的重要窗口。但是,对允沟岩组的形成时代至今仍存在较大争议,为此本文选取允沟岩组中的片岩进行了碎屑锆石U-Pb精确定年。分析结果表明,允沟岩组原岩沉积时代为新元古代晚期-寒武纪期间(551~491Ma),主要由新太古代、新元古代及少量古元古代碎屑物质组成。其中,大量太古宙晚期碎屑锆石表明其源区有太古宙基底的存在,而1749Ma这组碎屑锆石可能与哥伦比亚超大陆聚散有关,956Ma和848Ma这两组锆石记录的事件可能是对罗迪尼亚超大陆三阶段裂离事件的前两期的响应。此外,还含有少量泛非运动信息,但泛非运动对其源区的影响极其有限。结合前人研究成果和本文碎屑锆石年龄谱,显示允沟岩组形成于原特提斯洋的被动陆缘,与印度板块、南羌塘地块具有显著的亲缘性。
关键词: 允沟岩组    锆石U-Pb定年    昌宁-孟连缝合带    滇西    原特提斯洋    
Detrital zircon age spectra of the Yungou Formation and its constrain to the related block affinity, western Yunnan
ZHAO LinTao1,2, LI SanZhong1,2, LÜ Yong3, LIN Yu3, LIU YongJiang1,2, MU DunLing1,2, LIAO JiaFei3, PAN Ming3     
1. MOE Key Laboratory of Submarine Geosciences and Prospecting Techniques, College of Marine Geosciences, Ocean University of China, Qingdao 266100, China;
2. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China;
3. Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China
Abstract: The Yungou Formation in western Yunnan Province, a suit of metapelites of lower greenschist facies, consists dominantly of mica quartz schist, mica quartz phyllite, mica schist and fine crystalline limestone, with some siliceous rocks. It has always been considered as an important part of the Precambrian metamorphic basement. The protoliths are mainly clastic rocks with some limestone and dolomite, belonging to a set of semi-deep sea sediments formed in the passive continental margin of the Tethys Ocean. So the Yungou Formation witnessed the origin, development and extinction of the Tethys Ocean, and played a great role in understanding the tectonic evolution of the Tethys Ocean. However, its forming age is still under controversial. In this paper, the zircon U-Pb dating of schists from the Yungou Formation, southwestern Yunnan Province, has been carried out. The analysis results show that the depositional age of the protolith of the Yungou Formation is in Late Neoproterozoic-Cambrian (551~491Ma), and mainly composed of Neoarchean, Neoproterozoic and some Paleoproterozoic detrital rocks. The discovery of massive Neoarchean detrital zircons indicates the existence of the Archean basement in the source area. The 1749Ma zircons may be related to the Columbia Supercontinent from assembly to breakup. The zircons of 956Ma and 848Ma may be a response to the former two stages of the three-stage-rifting evolution of the Rodinia Supercontinent. In addition, the dating result also revealed the existence of pan-African tectono-thermal event, but its impact on the source region of the Yungou Formation is extremely limited. Combined with previous study, the U-Pb age spectra of detrital zircons shows that the Yungou Formation developed on the passive continental margin of the Proto-Tethys Ocean and when it deposited this block has the good affinity of the Indian and South Qiangtang blocks.
Key words: Yungou Formation    Zircon U-Pb dating    Changning-Menglian Suture    Western Yunnan    The Proto-Tethys Ocean    

滇西昌宁-孟连缝合带是西南三江地区一条主要的构造带(刘本培等,1993钟大赉,1998Feng,2002赖绍聪等,2010Fan et al., 2015Wang et al., 2018),作为亲冈瓦纳地块(拉萨地块、腾冲地块、保山地块)与兰坪-思茅地块的分界线。昌宁-孟连缝合带组成复杂,岩片类型较多,代表古特提斯洋的俯冲消亡记录,但是,该带的研究始终存在俯冲极性之争(张旗等,1996钟大赉,1998Ueno and Hisata, 2001; Wang et al., 2018),古特提斯洋是向东俯冲还是向西俯冲涉及到该带哪侧代表被动陆缘或主动陆缘,为此该带的深入研究进而可以确定古特提斯洋云南段的构造演化;另一方面,昌宁-孟连缝合带作为西南三江成矿带中的一条重要构造带(李永森和陈炳蔚,1991王义昭等,2000邓军等, 2010, 2012张鹏飞等,2017邹光富等,2017),对该构造带的研究,无疑对解决三江成矿带的成因和构造特点、确定其矿产资源前景,具有重要的战略意义。

一直以来,昌宁-孟连缝合带都是国际研究热点,前人研究取得了丰硕成果,但以往研究主要侧重于昌宁-孟连缝合带物质组成、运动学特征、构造演化等方面(翟明国等,1990赵大贤,1990从柏林等,1993刘本培等,1993周德进等,1995王义昭和丁俊,1996张旗等,1996钟大赉,1998李峰和段嘉瑞,1999沈上越等,2002段向东,2013; Yang et al., 2014Fan et al., 2015; Wang et al., 2018Zheng et al., 2019王慧宁等,2019)。该带及邻区变质岩广泛出露,其中在缝合带西部地区发育一套浅变质岩,称为允沟岩组,为勐统-大雪山变质岩带组成部分。虽然前人已经对允沟岩组做过一些研究,但是对其形成时代依然存在较多的争议。1965年云南省地质局第一区域地质测量大队变质岩组将昌宁县勐统街一带以泥沙质细碎屑沉积岩为主的浅变质岩命名为勐统群,并将其置于崇山群之上、澜沧群之下,时代划为前寒武纪(云南省地质矿产局,1996)。云南省地质局第一区域地质测量大队(1965)在1:100万下关幅地质图说明书中,认为勐统群可能属奥陶纪至志留纪。云南省地质矿产局区域地质调查大队(1980、1981)将勐统群自下而上划分为立果组、储家山组、昌宁组、回头山组(云南省地质矿产局,1996)。1:20万沧源幅区域地质报告(云南省地质矿产局区域地质调查大队, 1986)中将这套浅变质岩划分为王雅岩组与允沟岩组,1:25万凤庆幅区域地质报告(云南省地质调查院, 2008)中将勐统群上部的昌宁组+回头山组对比为1:20万沧源幅所建立的允沟岩组(Pt3y),同时根据昌宁组中发现的微古植物化石:Margominuscula sp.、Tremayosphaeridium sp.、Protoleiosphaeridum densumRetinarites irregularis,这些多数是国内外前寒武纪地层中常见的分子,综合分析将其时代地层对比为新元古界。段向东(2013)依据Rb-Sr同位素测年样的寒武系芒告岩组,将其地质时代对比为新元古代。可见,对允沟岩组的形成时限、变质时代、构造归属性研究依然存在巨大争论,这严重制约了该区特提斯洋大地构造演化的研究。

① 云南省地质局第一区域地质测量大队. 1965. 1:100万下关幅地质图说明书

② 云南省地质矿产局区域地质调查大队. 1980. 1:20万保山幅区域地质调查报告

③ 云南省地质矿产局区域地质调查大队. 1981. 1:20万凤庆幅区域地质调查报告

④ 云南省地质矿产局区域地质调查大队. 1986. 1:20万沧源幅区域地质调查报告

⑤ 云南省地质调查院. 2008. 1:25万凤庆幅区域地质调查报告

尽管允沟岩组内部构造发育,但后期构造对其改造相对较小,其中保留了大量前寒武纪地质信息,是研究三江地区构造演化的重要窗口,有助于更好地认识滇西原特提斯洋的形成与演化。为此,本文选取了昌宁地区允沟岩组中的千枚状片岩、石英片岩,在对锆石年龄谱进行详细分析的基础上,通过对比周边构造单元特征,为允沟岩组的形成时代、大地构造归属性提供更多约束。

1 区域地质背景

研究区的大地构造位置处滇西三江地区的昌宁-孟连缝合带上,东为扬子地块,西为印度板块,南为印支地块,北为羌塘地块和康滇地块(图 1a)。昌宁-孟连缝合带主要是沿澜沧江两侧分布,受澜沧江断裂控制,以西为保山-耿马地块,以东为思茅-临沧地块(钟大赉,1998Feng, 2002赖绍聪等,2010王保弟等,2013)。带内出露泥盆系温泉组、泥盆系-石炭系南段组、石炭系-二叠系平掌组和鱼塘寨组、二叠系大明山组、新近系芒棒组,皆呈断块产出,带内未见蛇绿混杂岩出露。其中温泉组、南段组均表现为被动大陆边缘的斜坡相浊流沉积。平掌组玄武岩与上覆的鱼塘寨组碳酸盐岩组成了典型的洋岛玄武岩-海山碳酸盐岩构成的双层结构。鱼塘寨组与大明山组表现为基本连续稳定的碳酸盐台地沉积(段向东,2013)。澜沧江断裂影响制约着该区域的火山-岩浆活动、变质变形作用以及成矿作用(王国芝等,2001胡斌,2002)。澜沧江断裂长期活动形成了澜沧江构造带,构造变形十分复杂,发育板块边界聚合挤压形成的大规模构造混杂堆积,同斜与倒转褶皱和逆冲推覆带或滑脱带强烈发育,伴生劈理、片理、线理等小型构造,并且发育有多条深层韧性剪切带和高温韧性剪切带(段建中和谭筱虹,2000钟康惠等,2004张波等,2008)。前寒武纪变质杂岩体呈长条带状展布,自维西至景洪,及两侧的腾冲-瑞丽和哀牢山均有分布。前人对昌宁-孟连缝合带内岩石学、同位素年代学、岩石地球化学、构造地质学等方面的研究成果,对正确恢复不同演化阶段的地层层序,分析其沉积环境、岩浆起源及演化,探讨古特提斯洋的构造演化以及古特提斯重建,提供了重要信息(从柏林等,1993刘本培等,1993张旗等,1996钟大赉,1998赖绍聪等,2010Ito et al., 2016; Wang et al., 2018; Zheng et al., 2019)。

图 1 三江地区构造单元(a)和昌宁地区变质岩分布(b) Fig. 1 Tectonic units of the Sanjiang area (a) and distribution of metamorphic rocks in the Changning area (b)

允沟岩组位于昌宁-孟连缝合带西侧,总体呈NW-SE向展布,长约20km,宽约2~10km(图 1b)。允沟岩组为一套泥质浅变质岩,昌宁地区岩性以灰白色云英片岩为主,变质程度为低绿片岩相。根据岩石组合特征将其从下至上可划分为三个岩段(图 2)。第一岩段厚约1167m,主要为紫红色、灰黄色、灰绿色二云片岩、石英二云片岩、二云石英片岩互层,局部夹片理化硅质岩;第二岩段厚约1225m,灰白色灰黄色石英片岩、绢云母片岩、绢云母石英片岩组成;第三岩段厚约416m,主要为灰色中厚层白云岩,绿灰色板岩,灰色绢云石英千枚岩,深灰色细晶灰岩。原岩以碎屑岩为主,含有部分灰岩、白云岩,局部夹硅质岩。

图 2 允沟岩组地层柱状图 Fig. 2 The stratigraphic column of Yungou Formation

允沟岩组向南延伸至勐统、耿马地区,其中,勐统地区允沟岩组的详细研究表明,其原岩建造为类复理石夹基性-酸性火山-沉积建造,物源成分以长英质岩石为主,并含有火成岩的物源及风化或再沉积的古老沉积物,其形成的构造环境属于被动大陆边缘(许威,2014);而耿马地区允沟岩组主要为绢云石英千枚状片岩、绢云石英千枚岩、浅变质粉晶灰岩、细晶灰岩,恢复其原岩为硅质岩、含泥质灰岩夹细碎屑岩、条带状灰岩(云南省地质调查院,2008)。

昌宁地区的允沟岩组东侧与西盟群、温泉组呈断层接触,西侧被古近系、新近系地层不整合覆盖。允沟岩组内部发育两组脆性断裂构造,其中一组断裂呈北北西向延伸,与澜沧江断裂带平行,多数为澜沧江断裂带的次级走滑断裂,在澜沧江两侧的部分韧性剪切带中还见有大量的S-C组构、旋转眼球体、牵引构造等(张波等,2008),显示了明显的左行走滑剪切特征,它们是印度-欧亚板块碰撞过程中,印支地块向南逃逸的产物(段建中和谭筱虹,2000)。另外一组断裂受南汀河断裂控制,呈北北东向延伸,并将北北西向的走滑断裂错断。此外,还发育多条韧性剪切带。多期构造叠加使得原生层序遭受强烈破坏,已经难以恢复其原生层序。允沟岩组内部见晚三叠世岩浆岩侵入,其锆石U-Pb年龄为216.2±1.7Ma(未发表),为古特提斯洋俯冲碰撞后期的产物。

2 采样位置及分析方法

本次研究共采集3件典型样品,岩性分别为黑云石英片岩(D1206-2、D5729-2)、白云母石英千枚状片岩(PM023-16),采样位置见图 1。样品D1206-2采自昌宁县田园镇麦地顶附近,D5729-2采自昌宁县温泉乡西侧,样品PM023-16采自于昌宁县城西北茶叶箐附近的实测剖面PM023。

样品D1206-2和D5729-2在镜下鉴定结果均为黑云石英片岩(图 3a)。岩石呈深灰绿色,鳞片变晶结构,片状构造,主要由粒径≤0.15mm的粒状变晶石英、斜长石和粒径≤0.25mm的显微鳞片-片状变晶黑云母、白云母及副矿物等组成。石英、斜长石已重结晶成他形粒状,镶嵌分布,其中斜长石多已高岭石化、绿泥石化,仅保留晶形轮廓,局部见残余。显微鳞片状-片状黑云母、白云母呈断续-连续杂乱-半定向排列构成片状构造。黑云母多呈显微鳞片变晶状,石英呈等粒显微-细粒变晶状,二者粒度较细,可能与岩石受热接触变质作用有关。呈粒状的不透明矿物星散分布。

图 3 允沟岩组变质岩正交偏光镜下特征 Bt-黑云母; Pl-斜长石; Qtz-石英; Ms-白云母 Fig. 3 Characteristics of phenocrysts under crossed nicols of metamorphic rocks in Yungou Formation Bt-biotite; Pl-plagioclase; Qtz-quartz; Ms-muscovite

样品PM023-16的镜下鉴定结果为浅红褐色绢-白云母石英千枚状片岩(图 3b),岩石呈浅红褐色,粒状鳞片变晶结构,千枚状构造,主要由粒径0.01~0.2mm的粒状变晶石英和粒径≤1.0mm的显微鳞片-片状变晶绢-白云母、隐晶质铁泥质及少量副矿物等组成。泥质多已重结晶成显微鳞片-片状变晶水云母-白云母,绢云母-白云母连续定向排列构成千枚状-片状构造。石英重结晶呈他形粒状,彼此镶嵌,不均匀分布。隐晶质、尘粒状铁泥质主要浸染于云母中呈条痕状聚集产出,平行片理方向,少数呈粒状星散分布。由于岩石中矿物已完全变质重结晶,推断原岩可能为砂质泥岩类、泥质胶结的石英杂砂岩类。

本文采用了激光剥蚀等离子体质谱法(LA-ICP-MS)进行锆石U-Pb年龄的测定。锆石分选流程如下:首先,将样品初碎至150目,用重选和磁选的方法分选出锆石颗粒;然后,在双目镜下挑选出晶形、颗粒大小最具代表性的锆石颗粒,并与锆石标样一起置于双面胶上,灌上环氧树脂制靶,固化后磨至一半并抛光,使锆石内部暴露;最后,抛光后将待测锆石做透射光、反射光显微照相和阴极发光照相,以检查锆石的外部和内部结构。随后,结合背散射图像(BSE)和阴极发光图像(CL),挑选出较自形、裂纹较少、包裹体少的锆石,选择环带清晰、较均匀、干净微区部位作为测试靶点,进行U-Pb同位素年龄测定。

测试工作在中国地质科学院国家地质实验测试中心完成。测试使用LA-ICP-MS,激光束斑直径24~32μm,频率为5Hz。样品经剥蚀后,以氦气作为载气,再和氩气混合后进入ICP-MS进行分析。每个测试流程包含了10个未知样品点分析,并在首尾分别插入2个GJ-1标样和1个Plesovice标样,以确保样品分析结果的精确度和准确度。U-Pb年龄数据处理采用ICP-MS DataCal4.3程序,数据进行了普通铅校正,年龄计算和图谱制作运用Isoplot3.0处理(Ludwig, 2003)。单点分析的同位素比值及年龄的误差为1σ,U-Pb平均年龄的误差为1σ

3 锆石特征和U-Pb定年结果 3.1 黑云石英片岩(D1206-2)

CL图像(图 4a)揭示锆石颗粒粒径分布于50~150μm之间,主要集中于80~150μm。锆石颗粒普遍具晶棱圆化,呈半自形-他形,少量港湾状外形,多数锆石颗粒呈现无分带、云雾状分带、弱分带、弱面状分带结构特征,可能是受后期热液作用影响所致。其中,部分锆石(如图 4a的6、8、19、21号点等)环带发育,为典型岩浆锆石特征,部分锆石见有变质增生边及核部的继承锆石,几乎所有锆石都具有一定的磨圆度,边缘还发育一定宽度的白色蚀变边,这可能是后期热液作用叠加于变质重结晶作用之上的产物。

图 4 允沟岩组变质岩锆石阴极发光图像 Fig. 4 Cathodoluminescence (CL) images of zircons from the metamorphic rocks in Yungou Formation

对该样品进行了30个锆石颗粒的U-Pb同位素分析,测定结果显示(表 1),其中3个点明显不和谐,16个点协和度较差(72%~89%)。这表明这些碎屑锆石都发生过不同程度的Pb丢失,它们的真实年龄应该老于207Pb/206Pb所显示的年龄。仅有11个点协和度大于90%,落在协和曲线上或者附近(图 5a)。这些锆石的年龄在541~2570Ma之间,Th/U比值为0.10~1.44,多数大于0.3,这表明其主体为岩浆锆石。碎屑锆石年龄谱(图 5b)显示,碎屑锆石年龄明显分为两组,第一组年龄为2339~2570Ma,峰值年龄为2513Ma;第二组锆石年龄为658~832Ma,样品最年轻碎屑锆石年龄为658Ma,这表明原岩沉积年龄应该小于658Ma。

表 1 允沟岩组变质岩碎屑锆石U-Pb定年结果 Table 1 U-Pb dating results of detrital zircons from metamorphic rocks of the Yungou Formation

图 5 允沟岩组碎屑锆石U-Pb年龄谐和图与碎屑锆石年龄谱图(由各样品中谐和度>90%的锆石组成) Fig. 5 U-Pb concordia plot and age spectra of detrital zircons from Yungou Formation (date used are those with concordance >90%)
3.2 黑云石英片岩(D5729-2)

CL图像(图 4b)揭示锆石颗粒粒径分布于50~100μm之间,主要集中于60~90μm。锆石颗粒普遍具晶棱圆化,呈半自形-他形,少量港湾状外形,多数锆石颗粒呈现云雾状分带、弱分带结构特征,可能是受后期热液作用影响所致。部分锆石表面可见有岩浆振荡环带,锆石颗粒边缘发育一定宽度的白色蚀变边,这表明发生过后期热液作用叠加于变质重结晶作用之上。

对D5729-2进行了30个锆石的U-Pb同位素分析,测定结果显示(表 1),11、12、24三点谐和度较低,其余点26个点谐和度在99~109之间(图 5c),U-Pb年龄范围在551~2536Ma之间,Th/U比值为0.14~1.13,多数大于0.3。碎屑锆石年龄谱(5d)显示,年龄分布范围较广,可分为2490~2538Ma、1766~2139Ma、1038~1468Ma、551~992Ma四组,其年龄峰值分别为2515Ma、1779Ma、1125Ma、961Ma。其中最年轻锆石年龄为551Ma,指示其原岩沉积时代应该晚于551Ma。

3.3 白云母石英千枚状片岩(PM023-16)

CL图像(图 4c)揭示锆石颗粒粒径分布于70~150μm之间,主要集中于100~150μm。锆石颗粒普遍具晶棱圆化,呈半自形-他形,少量港湾状外形,锆石颗粒呈现弱分带结构特征,可能是受后期热液作用影响所致;多数岩浆锆石内部见有震荡环带,少量颗粒边缘出现一定宽度的白色蚀变边,也可能变质重结晶作用后受到了后期热液作用叠加。

对PM023-16白云母石英千枚状片岩中的锆石进行30个测试点的U-Pb同位素分析,测定结果显示(表 1),8、20号点协和度较低,其余26个点谐和度在83~106之间,U-Pb年龄范围在857~2740Ma之间(图 5e),Th/U比值为0.13~1.24,多数大于0.3。碎屑锆石年龄谱(图 5f)显示,碎屑锆石年龄变化范围较大,可以分为2477~2740Ma、1658~1859Ma、856~1371Ma三组,主要年龄峰值分别为2505Ma、1737Ma、953Ma。最年轻碎屑锆石年龄为857Ma,这表明原岩沉积应晚于857Ma。

4 讨论 4.1 源区特征:太古宙锆石年龄记录与源区地质事件

昌宁孟连缝合带通常认为是古特提斯洋的主洋盆之一,在其周缘发育有较多的古老变质岩系,包括大勐龙群、澜沧群、崇山群、西盟群等。前人对此已经做了大量岩石学、年代学研究(彭兴阶和罗万林,1982吴世泽等,1984刘增池,1985翟明国等,1990赵大贤,1990朱炳泉等,2001王丹丹等,2014),认为大勐龙群形成时代为1750~1900Ma,澜沧群形成时代为700~900Ma,崇山群形成于1000~1900Ma之间(翟明国和从柏林,1993)。王丹丹等(2014)在西盟群中得到524~1802Ma的碎屑锆石年龄。邢晓婉和张玉芝(2016)在西盟群帕可组变质岩中得到了多颗2755~2284Ma的碎屑锆石。本文在允沟岩组3件样品中也获得了23个新太古代-古元古代早期的锆石数据,年龄在2339~2740Ma之间,峰值年龄为2517±18Ma(MSWD=0.56),锆石在CL图像中显示为残留岩浆振荡环带的老核或呈灰色板状。结合其Th/U比值多数大于0.2的特征,这表明这组锆石主要为岩浆锆石,代表新太古代晚期的一期岩浆活动,说明此时古老基底已经开始形成。因而,推测在这套变质岩的源区存在太古宙古老基底。

样品D5729-2和PM023-16碎屑锆石年龄组成(图 5d, f)极为相似,均含有2500Ma、1750Ma、950Ma左右的3个年龄峰值。这表明二者可能来源于同一源区或相近地区。样品D1206-2碎屑锆石年龄谱(图 5b)中,仅含有2513Ma的年龄峰值,缺乏1750Ma、950Ma左右的年龄峰值,表现出与样品D5729-2和PM023-16的明显差异,这很可能是由于样品D1206-2原岩形成之后,遭受后期地质作用改造或晶体自身遭受放射性损伤等原因引起部分放射性成因铅丢失,因而,导致2483~541Ma之间的16个年龄数据谐和度偏低,故其实际年龄应该大于207Pb/206Pb所显示的年龄。

在允沟岩组3件样品78个有效分析点中,选取谐和度>90%的59个数据点做碎屑锆石年龄谱(图 6a),以含有较多的新太古代、新元古代碎屑锆石为典型特征,明显形成了2个强峰与2个弱峰,峰值年龄分别为2517Ma、956Ma与1749Ma、848Ma。其中,2517Ma这组碎屑锆石来自古老地壳物质再循环,进一步表明在其源区可能有太古宙古老基底的存在。

图 6 允沟岩组与其他大陆的碎屑锆石年龄谱对比(据Li et al., 2018修改) 数据来源:扬子地块(Chen et al., 2013Liu et al., 2008Wang et al., 2012, 2013a, bYin et al., 2013Zhang et al., 2006);拉萨地块(Zhu et al., 2011a, bLeier et al., 2007Guo et al., 2016);华夏地块(Yu et al., 2007, 2008, 2010Yao et al., 2011, 2015Xu et al., 2007Wu et al., 2010Wan et al., 2007, 2010Li et al., 2002, 2012, 2014Yan et al., 2011a, bZhou et al., 2015Wang et al., 2010);西澳大利亚板块(Cawood and Nemchin, 2000Ksienzyk et al., 2012Clark et al., 2000Veevers et al., 2005Downes et al., 2016);北澳大利亚板块(Camacho et al., 2015Rösel et al., 2014);印度板块(Turner et al., 2014Zhuang et al., 2015Malone et al., 2008Hughes et al., 2015);滇缅泰马地块(Cai et al., 2017Usuki et al., 2013);北羌地块(Gehrels et al., 2011彭虎等,2014);南羌塘地块(Dong et al., 2011Zhu et al., 2011b) Fig. 6 Comparison of U-Pb age spectra of detrital zircons between the Yungou Formation and the other continents (after Li et al., 2018) Data sources:Yangtze Block (Chen et al., 2013; Liu et al., 2008; Wang et al., 2012, 2013a, b; Yin et al., 2013; Zhang et al., 2006); Lhasa Block (Zhu et al., 2011a, b; Leier et al., 2007; Guo et al., 2016); Cathaysia Block (Yu et al., 2007, 2008, 2010; Yao et al., 2011, 2015; Xu et al., 2007; Wu et al., 2010; Wan et al., 2007, 2010; Li et al., 2002, 2012, 2014; Yan et al., 2011a, b; Zhou et al., 2015; Wang et al., 2010); Western Australia Plate (Cawood and Nemchin, 2000; Ksienzyk et al., 2012; Clark et al., 2000; Veevers et al., 2005; Downes et al., 2016); northern Australia Plate (Camacho et al., 2015; Rösel et al., 2014); India Plate (Turner et al., 2014; Zhuang et al., 2015; Malone et al., 2008; Hughes et al., 2015); Sibumasu Block (Cai et al., 2017; Usuki et al., 2013); North Qiangtang Block (Gehrels et al., 2011; Peng et al., 2014); South Qiangtang Block (Dong et al., 2011; Zhu et al., 2011b)

1749Ma的年龄峰虽然较弱,对应于古元古代末期,但与全球1800Ma哥伦比亚超大陆最终聚合时间相近(李三忠等,2016潘桂棠等,2016),因此,这组锆石可能表明源区曾经历了哥伦比亚超大陆聚合事件。956Ma、848Ma对应时代为新元古代,这两组锆石可能与罗迪尼亚超大陆的三个阶段裂离(~950Ma、850~825Ma、780~650Ma)的前两个事件对应(李三忠等,2016),这也表明源区曾经历了罗迪尼亚超大陆裂解事件。

虽然只有一颗锆石的年龄为551Ma,但最可能是冈瓦纳大陆内部广泛发育的泛非运动(500~600Ma)的记录。但是,周边地区发现了大量泛非运动的信息,邢晓婉等(2015)在西蒙群帕可组碎屑锆石中发现了569Ma的年龄峰值。王舫等(2017)在澜沧群浅变质岩中获得了大量530~570Ma的碎屑锆石年龄。杨学俊等(2012)在公养河群基性火山岩中得到了499Ma的锆石年龄。宋述光等(2007)在怒江早古生代片麻状花岗岩中发现了462~519Ma的锆石年龄记录。这些都表明,允沟岩组形成于冈瓦纳大陆的某个被动陆缘。

4.2 原岩沉积年龄:原特提斯洋存在证据

研究区允沟岩组作为滇西一个重要的变质地体,其原岩形成时代、变质时代一直备受争议。由于缺乏系统的精确定年,导致众多学者对该变质岩形成时代认识不一致。允沟岩组原岩成分以长英质岩石为主,并含有火成岩的物源及风化或再沉积的古老沉积物,其形成的构造环境为被动大陆边缘(许威,2014)。本次研究的允沟岩组3件样品(D1206-2、D5729-2、PM023-16)的原岩均为碎屑岩类。样品中最年轻的碎屑锆石协和年龄分别为658Ma、551Ma、857Ma,因而限定其沉积时代下限应该晚于这些年龄。

此外,在允沟岩组下部(即原昌宁组)产微古植物:Margominuscula sp.、Tremayosphaeridium sp.、Protoleiosphaeri-dumdensumRetinaritesirregularis,且多数为前寒武纪地层中的常见分子(云南省地质调查院,2008)。允沟岩组与下伏王雅岩组为断层接触,在1:5万色树坝幅、孟定街幅区域地质调查中(云南省地质矿产局区域地质调查大队, 2000),王雅岩组之上的寒武系芒告岩组变质酸性火山岩中获得Rb-Sr同位素年龄值491±18Ma(段向东,2013),而允沟岩组中并未见有这期岩浆活动的年龄信息,也未见西盟群帕可组461Ma的花岗片麻岩年龄记录(邢晓婉和张玉芝,2016),且滇西地区广泛发育的500~470Ma的岩浆活动信息(Chen et al., 2007Liu et al., 2009杨学俊等,2012)在允沟岩组中也并未发现。据此,可将允沟岩组沉积年龄上限限定在491Ma之前。

① 云南省地质矿产局区域地质调查大队. 2000. 1:5万色树坝幅(国内部分)、孟定街幅区域地质调查联测报告

综上所述,允沟岩组主体应该形成于寒武纪期间,可能有少部分形成于新元古代末期,原岩形成时代应该在新元古代末期-寒武纪期间(551~491Ma),属于原特提斯洋沉积记录,而非通常认为的古特提斯洋记录。允沟岩组主要由新太古代物质跟新元古代物质组成,其中少量碎屑锆石记录了古元古代末期与泛非运动的地质信息。相较于邻区广泛发育的泛非运动年龄记录,推测其源区可能远离泛非运动强烈地区。此外,由于此套变质岩变质程度较低,锆石变质增生环带较窄,未能得出变质年龄。

4.3 大地构造关联:地块亲缘性判断

沉积体系中的碎屑锆石年谱是重建大地构造背景、建立古大陆构造演化和判断不同大陆间亲缘关系的最有效方法之一(Li et al., 2018)。图 6显示了允沟岩组与拉萨地块、滇缅泰马地块、印度板块、扬子地块、华夏地块、西澳大利亚板块、北澳大利亚板块、北羌塘地块、南羌塘地块碎屑锆石年谱。

将允沟岩组与不同板块碎屑锆石年谱相比较(图 6a-j),拉萨地块的主要峰值为1168Ma、559Ma、347Ma,与允沟岩组主要峰值年龄明显不同。滇缅泰马地块以富含古元古代-中元古代、晚古生代碎屑锆石,缺乏新元古代碎屑锆石为特征,与允沟岩组明显不同。扬子地块主要峰值年龄为818Ma,同时缺乏泛非运动年龄信息,可以排除其为物源区的可能。华夏地块峰值年龄为952Ma,这与允沟岩组中956Ma这组锆石峰值相吻合,但华夏地块中古元古代锆石年龄峰值与允沟岩组差距较大,因而排除其作为允沟岩组物源区的可能。西澳大利亚板块与北澳大利亚板块年龄峰值分别为1166Ma、1884Ma,这也与允沟岩组不符。北羌塘地块以发育大量晚古生代碎屑锆石为特征,与允沟岩组不相符。

但是,南羌塘地块早元古代-新元古代的4个年龄峰值均与允沟岩组吻合较好,这表明南羌塘地块可能是物源区之一。同时,印度板块在新太古代-新元古代出现多个年龄峰值,也与允沟岩组吻合较好,因此推测允沟岩组的碎屑锆石可能来源于印度板块与南羌塘地块,这些地块具有亲缘性。

5 结论

(1) 根据本文3件样品锆石U-Pb同位素结果及前人研究成果表明,允沟岩组中含有大量新太古代锆石年龄信息(2517Ma),表明其源区有太古宙古老基底的存在,此外源区还记录了哥伦比亚超大陆聚合、罗迪尼亚超大陆裂解的重大地质事件。

(2) 据允沟岩组3个样品的最小碎屑锆石年龄分别为658Ma、551Ma、857Ma,及允沟岩组下部地层中发现的前寒武纪微古植物化石,以及其中含少量的泛非运动的信息、缺乏区域上较为发育的500Ma~470Ma的年龄信息,推测其原岩形成于新元古代末期-寒武纪期间(551~491Ma),其主体形成于寒武纪期间,少量可能形成于新元古代末期。这表明其形成与原特提斯洋形成有关。

(3) 通过与印度板块、南羌塘地块碎屑锆石年谱对比,发现允沟岩组含有大量太古宙年龄信息以及哥伦比亚超大陆汇聚时期、罗迪尼亚超大陆三阶段裂解信息及少量泛非运动信息,与印度板块、南羌塘地块形成了较好的对应关系,因而推断允沟岩组沉积时所在地块与印度板块、南羌塘地块具有较好的亲缘性。

致谢      本文在成文过程中得到了多位老师和同学的帮助,特别是中国地质科学院岩溶地质研究所洞穴室区调组各位老师、同事对本人在参与云南1:5万西邑等四幅区域地质调查中给予的巨大帮助,在此表示感谢。同时,也感谢四位匿名审稿人的对本文的认真审阅并提出宝贵的修改意见。

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