岩石学报  2019, Vol. 35 Issue (7): 2189-2205, doi: 10.18654/1000-0569/2019.07.15   PDF    
西藏汤白矿区下白垩统比马组砂岩地球化学特征:对冈底斯南缘构造演化的启示
杨宗耀1, 胡古月2, 肖洪天3, 王鹰1, 赵晓彦1     
1. 西南交通大学地球科学与环境工程学院, 成都 611756;
2. 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室, 北京 100037;
3. 成都理工大学地球科学学院, 成都 610059
摘要: 沿雅鲁藏布江北岸广泛分布的比马组为一套上侏罗统至下白垩统的火山-沉积岩组合。本次工作通过对采集于汤白矿区的比马组砂岩进行岩相学和岩石地球化学研究,探讨了其物源特征、岩相古地理及大地构造演化背景。研究区的汤白比马组砂岩以火山岩夹层形式产出,碎屑分选差,磨圆度低,杂基含量高,结构成熟度低,以富含长石和岩屑为特征,为活动岩浆弧构造环境下的沉积物。除碳酸盐胶结物导致的CaO含量变化外,所有样品的元素地球化学特征基本与上地壳平均含量大体一致,且Fe2O3T、MgO、TiO2、P2O5及MnO与大陆岛弧成因砂岩含量相同。样品元素地球化学特征中的低CIA值(46%~68%)和较高的ICV值(0.75~1.14)表明砂岩遭受风化作用弱,成熟度低,粘土矿物(如高岭石和蒙脱石等)含量低,多属于构造活动区首旋回沉积物范畴。在稀土元素地球化学特征方面,汤白比马组砂岩显示右倾的球粒陨石标准化配分图,ΣREE值介于46.16×10-6~90.03×10-6之间,具有与后太古宙平均页岩类似的地球化学特征。δEu值介于1.06~1.36之间,呈明显正异常,为斜长石碎屑所致。δCe介于0.92~0.94之间,呈弱负异常,表明其沉积过程中受海水作用较弱。在微量元素球粒陨石标准化蛛网图中,汤白比马组砂岩显示为亏损Th、Nb、P、Ce、Ti等高场强元素,显示岛弧火山岩地球化学特征。因此,汤白比马组砂岩形成于新特提斯洋向拉萨地体俯冲的活动大陆边缘背景。
关键词: 冈底斯    汤白    砂岩    物源分析    构造背景    活动大陆边缘    
Geochemical characteristics of the Early Cretaceous sandstones from the Tangbai deposit, Tibet: Implications for the tectonic evolution of the southern margin of the Gangdese
YANG ZongYao1, Hu GuYue2, XIAO HongTian3, WANG Ying1, ZHAO XiaoYan1     
1. Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, China;
2. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;
3. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China
Abstract: The Bima Formation, a suite of Late Jurassic to Early Cretaceous volcanic-sedimentary rocks, is widely distributed along the northern Yarlung Zangbo River. In order to analyze the provenance, paleogeography and tectonic evolution of the southern Gangdese, an integrated petrological, petrographic and geochemical study has been carried out on the sandstone samples of the Bima Formation around the Tangbai deposit. These sandstones sandwiched by the volcanic rocks are of poor sorting, roundness and texture maturity, but with high contents of matrix. They are rich in feldspar and lithic fragment, and considered to have formed within an active magmatic arc setting. According to the geochemistry analysis, the contents of major elements of the samples are stable except for CaO, which was influenced by the unhomogeneous carbonatization. In general, the components of these samples are similar to that of the upper crust, while the contents of Fe2O3T, MgO, TiO2, P2O5 and MnO have continental island arc affinity. Low Chemical Index of Alteration (46%~68%) and high Index of Compositional Variability (0.75~1.14) indicate that the weathering was weak. The low compositional maturity and poor in clay minerals such as kaolinite and montmorillonite imply that these sandstones were formed by the first cycle sediments from an active tectonic area. The ΣREEs of the sandstones are from 46.16×10-6 to 90.03×10-6. Chondrite standard curves of REEs have rightward trends which are similar to the Archean average shale. There are positive Eu anomalies in the chondrite-normalized rare element plot where the δEu ranging from 1.06 to 1.36 because of the abundant presence of feldspar fragments. The effect of the sea water cause weak negative Ce anomalies (δCe=0.92~0.94). It can be seen from the spider diagram that the Tangbai sandstones are depleted in HFSE elements such as Th, Nb, P, Ce and Ti, which are typical of island arc volcanic rocks. As a conclusion, the sandstones of the Bima Formation in the Tangbai deposit are formed in an active continental margin which was induced by the subduction of the Yarlung Zangbo Neo-Tethys under the Lhasa terrane.
Key words: Gangdese    Tangbai deposit    Sandstone    Provenance    Tectonic setting    Active continental margin    

地球表面的演化是一个伴随海陆变迁、碰撞造山等地质作用过程的复杂行为,火山-侵入岩是来自地球内部的记录,而沉积岩则是地表地质作用所留下来的产物,地表无处不在的风化、剥蚀、搬运和沉积使地球往事均以记忆残片的形式保存在沉积岩中,物源分析就是要研究这些保留下来的痕迹。自沉积岩的成分是其物源区构造背景的记录这一概念(Blatt, 1967; Dickinson, 1970)被提出以来,物源分析方法在19世纪70年代开始萌芽,最初是对沉积岩中副矿物研究来推测母岩性质,研究程度及可靠性较低。至20世纪80年代,鉴于岩石地球化学及砂岩碎屑组分统计等方法的飞速发展,使得物源分析方法得到了进一步完善,业已成为一门各学科结合的综合研究领域。目前,物源分析方法主要有地球化学法(Bhatia, 1983, 1985Bhatia and Crook, 1986Roser and Korsch, 1986, 1988; McLennan and Taylor, 1991; McLennan et al., 1993)、碎屑骨架法(Dickinson and Suczek, 1979; Valloni and Maynard, 1981; Ingersoll et al., 1984)、同位素法(McCulloch and Wasserburg, 1978; Taylor et al., 1983; Gray and Zeitler, 1997)、重矿物法(Morton, 1987; Morton and Hallsworth, 1999)和沉积岩相研究法(Pettijohn et al., 1972)。沉积岩成岩影响因素较多,各种研究方法都有其应用条件和局限性,但其中以地球化学方法和同位素年代学应用最广。物源分析时应注意将多种方法相结合,综合判别,同时还应该考虑构造抬升、剥蚀作用和化学风化等构造和沉积作用对物源区判定的影响,才能得出合理的结论。本文运用地球化学方法对西藏拉萨地体南缘汤白矿区的比马组(K1b)火山-沉积岩中的砂岩开展物源研究,解剖其物源成分,探讨砂岩形成的大地构造背景,结合早-中侏罗世雄村组(J1-2x)砂岩研究成果(杨宗耀等,2017),进一步约束拉萨地体南缘新特提斯洋俯冲阶段的构造演化过程。

1 地质背景

研究区位于滇藏地层大区之冈底斯-腾冲地层区南部与喜马拉雅地层区北部的交接位置,广泛发育中、新生界地层,局部出露前震旦系和古生界地层(图 1)。由中生界区域地层柱状图(图 2)可见,研究区中生界地层可看出由老至新包括中-下侏罗统雄村组(J1-2x)、上侏罗统麻木下组(J3m),上侏罗-下白垩统林布宗组(J3K1l)、比马组(K1b)、下白垩统楚木龙组(K1c)、塔克那组(K1t)、上白垩统设兴组(K2s),其他地层还包括有上白垩统-古新统旦师庭组(K2-Ed)及古新统典中组(E1d)、始新统年波组(E2n)、帕那组(E2p)、秋乌组(E2q)、渐新统日贡拉组(E3r)、渐新统-中新统大竹卡组(E3-N1d)、中新统芒乡组(N1m)、上新统嘎扎村组(N2g)、宗当村组(N2z)。因此,研究区地层整体上为一套受特提斯洋演化及印度-亚洲陆陆碰撞背景下形成的火山岩和火山沉积岩组合,局部经历低级变质作用,具有海相向陆相过渡的地层学特征。

图 1 汤白矿区地质简图(据唐菊兴等,2005修改) 1-第四系;2-下白垩统比马组火山-沉积岩;3-早侏罗世角闪石英闪长斑岩;4-晚白垩世黑云母花岗岩;5-中新世黑云母二长花岗岩;6-始新世辉绿岩脉;7-始新世煌斑岩脉;8-始新世花岗斑岩脉;9-逆断层;10-平移断层;11-推测断层;12-探矿工程及其编号;13-矿体;14-剖面位置 Fig. 1 Geological map of the Tangbai district 1-Quaternary; 2-Lower Cretaceous Bima Formation volcanic-sedimentary rocks; 3-Early Jurassic hornblende quartz diorite porphyry; 4-Late Cretaceous biotite granite; 5-Miocene biotite monzonitic granite; 6-Eocene diabase dyke; 7-Eocene lamprophyre dyke; 8-Eocene granite porphyry dyke; 9-reverse fault; 10-strike-slip fault; 11-infered fault; 12-exploration trench and its number; 13-orebody; 14-stratigraphic profile

① 唐菊兴,李志军,董树义等. 2005.西藏日喀则市汤白铜矿地质勘查报告.成都:成都理工大学

图 2 冈底斯-腾冲区域中生界地层柱状图 Fig. 2 Mesozoic stratigraphic column of Gangdese-Tengchong area

根据区域调查资料(胡敬仁等,2014),比马组火山-沉积岩沿雅鲁藏布江北岸的汤白、塔马、雄村、野马一带呈断续状分布,在雅鲁藏布江南岸那波一带亦有零星出露。该组火山-沉积岩主体为中性火山岩,夹中酸性火山岩及黑灰色板岩和结晶灰岩、泥灰岩、砂岩、粉砂岩,与下伏地层麻木下组(J3m)地层呈整合接触关系。比马组火山-沉积岩以灰白色大理岩为划分标志,厚度变化较大,但延伸稳定。在拉萨地体东部的桑日、加查、乃东地区,比马组厚度较大且沉积岩夹层较多;西部尼木、南木林、谢通门等地,由于花岗岩侵吞,多呈残体形式出露,其厚度较小,沉积夹层相对较少。该组产双壳类、有孔虫、六射珊瑚、腹足等海相化石,显示其浅海相沉积环境。拉萨地体东部地区生物灰岩记录有风暴岩沉积特征。同时,火山岩韵律性特征表明其沉积形成于陆缘斜坡带的浅水相火山-沉积作用环境。目前关于比马组的时代观点差异较大,最早有早侏罗世比马组火山岩年龄出现的报道(Kang et al., 2014冉孟兰等,2017)。但区域调查资料(胡敬仁等,2014)中在谢通门一带所发现的古生物组合为早白垩世阿尔布期(Alb)-赛诺曼期(Cenoman),其次如闫国强等(2014)报道的比马组安山岩锆石U-Pb谐和年龄为92.04±0.71Ma。而最新报道的南木林地区比马组年龄为古新世46.08±0.47Ma(张运昌等,2019),但认为其实际为古新世年波组火山岩。

本次样品采集于汤白矿区南缘,雅鲁藏布缝合带北侧,雄村矿区以东约15km处(图 1a)的下白垩统比马组(K1b)火山-沉积地层。地层中的火山岩相主要包括玄武安山岩、安山岩、英安岩和条带状变质凝灰岩等;沉积岩相为灰白色中层砂岩夹薄层状粉砂岩和页岩。该套火山-沉积岩普遍受中-低级变质作用,可能与主碰撞期挤压作用相关。研究区岩浆活动强烈,出露中、新生代岩浆岩,主要包括早侏罗世角闪石英闪长斑岩(J1)、晚白垩世黑云母花岗岩(K2)、中新世黑云母二长花岗岩(N1)以及始新世辉绿岩脉、煌斑岩脉和花岗斑岩脉等。研究区主要发育断裂构造,呈近东西向、近南北向和北西向。由于研究区同时为汤白矿区,目前由地表探矿工程控制3条主矿体(1号、2号和3号),主要赋存于早侏罗世角闪石英闪长斑岩中(图 1b)。

2 样品测试方法

本次研究在野外露头上共采集12件样品。样品经粗碎、中碎、细碎三阶段并研磨至200目以下,其中,粗碎、中碎、细碎损耗率分别小于3%、5 %、7%, 缩分误差小于3%,加工过程均在无污染环境中完成。样品的主量元素、微量元素和稀土元素测试工作在西南冶金地质测试中心完成,工作环境温度为19℃,湿度为54%。主量元素测试分析采用X射线荧光光谱法(XRF),仪器为荷兰帕纳科Axios X荧光射线光谱仪,检测依据GB/T14506.28-2010和DZG20-02,分析误差小于5%。微量元素和稀土元素的测定采用等离子发射光谱法、质谱法和X荧光法,仪器为iCAP6300全谱仪、Axios X荧光仪和NexIon 300x ICP-MS,检测标准为DZG20-02和DZG20-06。首先称取40mg研磨样品和3个国家标准(GRS1、GRS2、GRS3)样品置于溶样弹中并用酸溶法制成溶液, 然后在ICP-MS上进行测定。NexIon 300x ICP-MS仪器的检测精度为:含量大于10×10-6的元素分析误差小于5%;含量小于10×10-6的元素分析误差小于10%。

3 沉积学及岩相特征

由采样剖面(图 3)可见,本次研究采集的汤白矿区比马组砂岩以火山岩夹层形式产出。整体上,地层由凝灰质、安山质火山岩厚层夹浅海相碳酸盐岩或碎屑沉积岩构成,且碳酸盐岩在砂岩下部,产状较陡,显示轻微变质作用。碳酸盐岩初始沉积环境为高盐度环境下,阳光充足的的浅海相或湖相环境(Wilson, 1975)。因此,剖面中碳酸盐岩和砂岩以薄层状产出的沉积相表明其初始沉积环境可能为浅海相和陆相交替出现的边缘海或为平均海平面附近的陆坡环境。同时,比马组(K1b)下伏的上侏罗统麻木下组(J3m)地层则主要沉积一套碳酸盐岩,反映了稳定的海相沉积环境。在晚侏罗世至早白垩世期间,雄村岛弧有向北侧拉萨地体不断靠近的趋势。

图 3 汤白矿区比马组剖面图及采样位置 1-英安岩;2-安山岩;3-灰岩;4-晶屑凝灰岩;5-大理岩;6-砂质板岩;7-砂岩;8-凝灰岩;9-火山角砾岩;10-碳质板岩夹砂岩;11-岩层产状;12-采样地点及编号 Fig. 3 Stratigraphic profile of the Bima Formation in Tangbai district, showing the sample positions 1-dacite; 2-andesite; 3-limestone; 4-crystal tuff; 5-marble; 6-sandy slate; 7-sandstone; 8-tuff; 9-volcanic breccia; 10-carbonaceous slate and sandstone interbedding; 11-rock stratum occurrence; 12-sample position and its number

汤白比马组砂岩镜下可见大量长石碎屑,双晶明显(图 4f-l);部分样品含有极高的CaCO3胶结物(图 4a-d),出现CaCO3胶结物含量差异较大的原因是成岩时期环境的变化,可能受制于不同时期喷发的火山岩、火山灰性质或沉积相。整体上,碎屑含量约70%,其中石英50%和长石20%;岩屑含量约30%呈棱角状和次棱角状。石英碎屑多为单晶颗粒,极少见多晶石英碎屑,无次生加大现象,不太可能是再旋回石英碎屑。另外,石英单矿物具有港湾状熔蚀边缘,表面干净光洁等喷出岩石英的典型岩相学特征。填隙物约占整体砂岩的30%,其中:杂基约占填隙物的70%,多为细粒长英质矿物、粘土矿物、少量铁质重矿物;胶结物约占填隙物的30%,为方解石等钙质胶结物、石英及云母等硅酸盐胶结物。其次碎屑分选差,磨圆度低,水动力条件较弱且搬运距离近。汤白比马组砂岩杂基含量高,结构成熟度低,以富含长石和岩屑为特征,且比雄村组砂岩更富长石,这是活动岩浆弧成因砂岩典型特征(Dickinson et al., 1983;杜利林等,2013),物源推测主要为火山岩。

图 4 汤白矿区比马组砂岩显微镜下特征 Qm-单晶石英;Qp-多晶石英;Lv-火山岩岩屑;Cal-方解石;Pl-斜长石;Ser-绢云母 Fig. 4 Microphotographs of the Bima formation sandstones in Tangbai district Qm-single crystal quartz; Qp-polycrystalline quartz; Lv-volcanic lithic; Cal-calcite; Pl-plagioclase; Ser-sericite
4 岩石地球化学特征 4.1 主量元素特征

从汤白比马组砂岩主量元素分析结果(表 1)可见:砂岩SiO2含量在59.51%~69.08%之间,平均65.2%;Al2O3含量在13.44%~18.58%之间变化,平均16.85%;Fe2O3T含量为3.75%~5.27%,平均4.41%;MgO含量为1.19%~2.12%,平均1.56%;Na2O含量为1.23%~3.38%;平均2.18%;K2O含量0.99%~2.48%,平均1.71%;CaO含量2.37%~7.36%,平均4.18%;TiO2含量0.33%~0.49%,平均0.41%;MnO含量0.10%~0.20%,平均0.13%;P2O5含量0.08%~0.13%,平均0.10%。因此,除胶结物(图 4a-d)引发的CaO含量变化外,总体样品主量元素含量比较稳定,与上地壳值(Taylor and McClennan, 1985)一致。其次,汤白比马组砂岩的Fe2O3T、MgO、TiO2、P2O5及MnO与大陆岛弧成因砂岩含量一致(Bhatia,1983),这些元素受物源区的铁镁质矿物、磷灰石等重矿物控制,在碎屑搬运过程中受影响较小,对砂岩的成因判别具有重要意义,相反SiO2、Al2O3、Na2O、K2O和CaO含量则在砂岩成岩及后期风化过程中更容易产生较大变化。在汤白矿区比马组砂岩主量元素Harker图解(图 5)中,SiO2与Al2O3呈明显负相关,这与砂岩碎屑搬运成岩过程密切相关,石英质矿物的增加将造成铝硅酸盐等粘土矿物的减少;MgO、CaO、Na2O和MnO与Al2O3含量无明显的关系,表明这些元素的含量可能不受晚期风化作用控制;Fe2O3T、K2O、TiO2和P2O5与Al2O3有微弱正相关关系,可能受到晚期风化作用影响。

表 1 汤白矿区比马组砂岩全岩元素地球化学组成(主量元素:wt%;稀土和微量元素:×10-6) Table 1 Whole-rock geochemical compositions of Bima Formation sandstones from Tangbai district (major elements: wt%; trace elements: ×10-6)

图 5 汤白矿区比马组砂岩主量元素Haker图解 Fig. 5 Haker diagrams of major elements for the Bima Formation sandstones in Tangbai district

在砂岩log(SiO2/Al2O3)-log(Na2O/K2O)岩石地球化学分类图解中(图 6a),汤白比马组砂岩位于杂砂岩区域,并有向岩屑砂岩转换的趋势。Crook(1974)将杂砂岩按石英含量分为贫石英杂砂岩(石英 < 15%;平均SiO2约58%;K2O/Na2O < < 1)、中等含量石英杂砂岩(石英15%~65%;平均SiO2约68%~74%;K2O/Na2O < 1)和富石英杂砂岩(石英>65%;平均SiO2约89%;K2O/Na2O>1)三大类。汤白比马组砂岩石英含量约35%,SiO2平均65.2%,在Na2O-K2O图解中(图 6b),属于中等-富石英杂砂岩范畴。

图 6 汤白矿区比马砂岩岩石地球化学分类图解(a, 据Pettijohn et al., 1972; b, 据Asiedu et al., 2000) Fig. 6 Geochemistry classification diagrams of the Bima Formation sandstones in Tangbai district (a, after Pettijohn et al., 1972; b, after Asiedu et al., 2000)

汤白比马组砂岩SiO2/Al2O3比值在3.20~5.10之间,变化小,成熟度低。与雄村组砂岩相比(杨宗耀等,2017),富SiO2而贫Al2O3,表明硅质矿物含量较高,而云母及铝硅酸盐粘土矿物含量较低,遭受风化作用弱。K2O/Al2O3比值为0.06~0.14,表明砂岩粘土矿物主要为高岭石和蒙脱石,中碱性长石含量较少(Cox et al., 1995),相比于雄村组砂岩较高的K2O/Na2O比值(0.81~7.21),汤白比马组砂岩为0.41~1.83,反映了一个相对稳定的后期风化过程。Girty et al.(1996)认为Al2O3/TiO2比值一定程度上可反映砂岩物源区特征,Al2O3/TiO2比值为34.92~49.82,其物源区为安山质岩浆弧。

Wedepohl(1969)研究指出上地壳由大约21%石英、41%斜长石和21%钾长石组成,所以长石类矿物在上地壳风化过程中扮演着十分重要的角色,其含量丰富,并与风化作用具有密切关系。上地壳的风化是一个长石退化伴随粘土矿物形成的过程,钾、纳、钙从长石矿物中流失导致硅铝质矿物在风化产物中堆积。Nesbitt and Young(1982)提出CIA用以衡量岩石风化程度:

cc=方解石;dol=白云石;ap=磷灰石

其中氧化物是以克分子量形式参与计算,公式中CaO*仅表示岩石中由硅酸盐形成的CaO,而由碳酸盐(方解石、白云石)和磷酸盐产生的CaO则不计算在内。Fedo et al.(1995)给出了CaO*的详细计算步骤,但在实验过程中由碳酸盐产生的CO2无法测量,所以在计算过程中仅排除磷酸盐产生的CaO。汤白比马组砂岩CIA值为46%~68%之间,表明其遭受风化程度很低。一般情况下,砂岩遭受风化产生粘土矿物,其CIA值多在70%左右,不太可能出现低于50%的情况,平均上地壳CIA值约47%,CIA值为45%~55%代表未遭受风化(McLennan, 1993),而汤白比马组砂岩出现了部分样品CIA低于50%,在A-CN-K图解中位于斜长石-钾长石线之下,且全部小于70%,这是受砂岩中大量CaCO3胶结物所致(图 4a-d),其大大提高了CaO*值从而使CIA大低于实际值。A-CN-K图解中(图 7)CIA投影值与计算结果(表 1)一致,风化趋势线相对于A-CN边略向右偏移,表明晚期风化作用可能带入了少量钾,其反向延长位于安山岩附近,并有部分靠近辉绿岩,表明其物源区可能为安山质岩石,部分可能混入少量基性岩成分。

图 7 汤白比马组砂岩主量元素A-CN-K图解(据McLennan et al., 1993; Fedo et al., 1995) 1-辉绿岩;2-安山岩;3-花岗闪长岩;4-花岗岩;5-A型花岗岩;6-紫苏花岗岩;7-钾长花岗岩;8-微斜长石 Fig. 7 A-CN-K diagram of the sandstones in Tangbai district (after Mclennan et al., 1993; Fedo et al., 1995) 1-diabase; 2-andesite; 3-granodiorite; 4-granite; 5-A-type granite; 6-charnockite; 7-moyite; 8- microcline

砂岩样品中的Al2O3含量常用于反映粘土矿物和硅质矿物的比例,而组份变化指数ICV(Fe2O3T+K2O+Na2O+CaO+MgO+MnO+TiO2)/Al2O3)则能进一步衡量样品中氧化铝与其他主量元素的关系及其成分成熟度(Cox et al., 1995)。一般情况下,含粘土矿物较少而铁镁质矿物和非粘土硅酸盐矿物较多(如长石)的砂岩其ICV值常大于1,而砂岩中高岭石、蒙脱石ICV小于0.3,斜长石族矿物约0.5~0.7。汤白比马组砂岩ICV为0.75~1.14,较高的ICV值表明砂岩成分成熟度较低(Cox et al., 1995),高岭石、蒙脱石等粘土矿物含量低,同时镜下可见大量长石类非粘土硅酸盐矿物,这种成分成熟度低的砂岩多形成于构造活动区的第一次旋回沉积物(Van de Kamp et al., 1976)。

4.2 稀土元素特征

在岩浆或流体演化过程中,Ni、Co等相容元素多进入矿物相或残留相,而不相容元素(如大部分大离子亲石性元素)则更倾向于进入熔体相,因此稀土、微量元素以有别于主量元素的方式记录着不同地质活动。

汤白比马组砂岩稀土元素特征(表 1)显示:稀土总量(ΣREE)为46.16×10-6~90.03×10-6之间,平均值为67.54×10-6。其中轻稀土含量(ΣLREE)为39.62×10-6~74.75×10-6,平均值为58.70×10-6;重稀土含量(ΣHREE)为6.25×10-6~15.28×10-6,平均值为8.84×10-6;轻稀土元素和重稀土元素比值(LREE/HREE)为4.89~8.11,平均6.85;(La/Yb)N比值为4.66~6.20,平均比值为7.00。整体上,汤白比马组砂岩稀土总量与该地区侏罗纪火山-侵入岩稀土总量相似,比雄村组砂岩(杨宗耀等,2017)低约15.00×10-6,暗示其物源区岩石相较于雄村组偏基性。其次,富含石英质矿物的沉积岩会表现出ΣREE下降的稀土地球化学行为(Haskin et al., 1966; Nance and Taylor, 1976),这可能是导致汤白比马组砂岩除Eu外各稀土元素及总量比平均上地壳(Taylor and McClennan, 1985)低约0.5倍,但其稀土配分图(图 8)仍与上地壳一致的原因。

图 8 汤白矿区比马组砂岩球粒陨石标准化(a)和澳大利亚后太古宙页岩标准化(b)稀土元素配分图、不同构造环境下砂岩澳大利亚后太古宙页岩标准化稀土元素配分图(c, 据Bhatia, 1985)及粒陨石标准化微量元素球蛛网图(d) Fig. 8 Chondrite-normalized (a) and PAAS-normalized (b) rare earth elements patterns of the sandstones in Tangbai district, PAAS-normalized rare earth element discriminatory plots for sandstones from various tectonic settings (c, after Bhatia, 1985) and chondrite-normalized trace element spidergram of Bima Formation sandstones in Tangbai district (d)

稀土元素球粒陨石标准化配分模式(图 8a)表现为轻稀土富集的右倾模式,与后太古宙平均页岩相似。δEu值为1.06~1.36,平均值1.21,Eu呈明显的正异常,这是由于砂岩有中大量斜长石碎屑(图 4f-l)。δCe为0.92~0.94之间,平均0.93,具弱δCe负异常,但相较于雄村组砂岩略高,表明其受海水作用较弱。在稀土元素澳大利亚后太古宙(PAAS)标准化模式图(图 8c)中,汤白比马组砂岩与大陆岛弧具有一致的配分模式曲线,且与大洋岛弧配分模式有明显区别,表现为轻稀土元素相对富集。

4.3 微量元素特征

球粒陨石标准化微量元素蛛网图中(图 8d),汤白比马组砂岩亏损Th、Nb、P、Ce、Ti等高场强元素,具有岛弧火山岩特征。Cr/Ni比值为1.71~3.15,说明物源区镁铁质或超镁铁质岩石含量极低(Bauluz et al., 2000)。在地壳尺度上,微量元素中Zr多赋存于锆石中,且大部分重稀土元素及微量元素受锆石影响,而锆石中Zr/Hf比值介于30~40(Murali et al., 1983),汤白砂岩Zr/Hf值为32.8~39.11,说明其Zr、Hf元素含量受碎屑锆石控制。对照上地壳Nb/Ta比值约为12的数据结果(Barth et al., 2000),汤白比马组砂岩Nb/Ta比值为11.3~12.49,平均11.87的地球化学特征表明砂岩中Nb、Ta元素可能主要来源于上地壳岩石单元。

Sr和Ba等碱土金属元素的化学性质相近,但在陆相、海-陆交替相及海相等不同环境中的富集程度具有微弱的差异(邓平,1993刘刚和周东升,2007)。根据蓝先洪等(1987)对中国珠三角地区沉积物中Sr和Ba元素的含量特征研究结果,陆相为Sr小于60×10-6、Ba小于300×10-6,而海相为Sr大于160×10-6、Ba大于400×10-6,海陆两相之间的差异明显。汤白比马组砂岩Sr含量为238.7×10-6~475.1×10-6,平均Sr约368.0×10-6,表现为海相沉积特征;Ba含量141.0×10-6~479.4×10-6,平均Ba约311.4×10-6,且其中有4个样品Ba含量小于300×10-6,6个样品Ba含量介于300×10-6和400×10-6之间,2个样品Ba含量大于400×10-6,因此更倾向于具有海陆交替相特征。其次,Sr/Ba比值能反映沉积环境的古盐度特征,一般认为盐度高的海相环境沉积物Sr/Ba比值一般大于1,而淡水沉积物Sr/Ba比值多小于1(蓝先洪等,1987)。汤白比马组砂岩Sr/Ba比值为0.50~2.69,同样反映汤白比马组砂岩沉积于海陆交替相,结合结合沉积环境和沉积相分析,比马组砂岩应该是形成于海陆交替相。

沉积物中与有机质密切相关的V和Ni元素含量是判断沉积环境的重要指标,其含量取决于沉积物中生物导致的氧化还原反应(Lewan and Maynard, 1982; Lewan, 1984)。Tribovillard et al.(2006)的研究表明U、V和Mo在氧化环境中相比于还原环境具有更高的溶解度,因此还原环境中的沉积物更富集U、V和Mo元素,并同与生物有机质分解有关的Ni、Cu、Zn、Cd元素统称为氧化还原反应敏感微量元素,可作为恢复古环境的指针。Hatch and Leventhal(1992)基于以上原理将页岩全岩V、Ni关系与DOP(Degree of Pyritization)进行对比研究,结果表明V/(V+Ni)比值与DOP具有明显正相关关系,而DOP值又是环境中O2和H2S含量的指示,并以此来判定氧化、还原环境(Raiswell and Berner, 1985; Leventhal and Taylor, 1990)。当V/(V+Ni)>0.60时反映水体为还原环境,V/(V+Ni) < 0.60则为氧化环境(Zhou and Jiang, 2009; Lan et al., 2017),随着V/(V+Ni)值得增大,其贫氧程度逐渐增加,当V/(V+Ni)>0.84时,为极度厌氧环境并伴随H2S出现(Hatch and Leventhal, 1992)。汤白比马组砂岩V/(V+Ni)值为0.91~0.95,表明其沉积环境为极度厌氧的还原环境。

5 讨论 5.1 源区母岩成分

本次研究的汤白矿区比马组砂岩是作为薄层状火山岩的夹层产出,具有分选性差、磨圆度低等磨拉石建造特征。同时,其沉积物的来源多为同时期喷发的火山岩,同时掺杂有原地早期岩石的部分剥蚀成分。砂岩的化学组成严格受物源区控制,受后期交代作用影响不大,可代表原始沉积物的化学成分,能有效的示踪物源区岩石组合特征。

Roser and Korsch (1988)基于全岩地球化学来研究砂岩原岩成分,以主量元素TiO2、Al2O3、Fe2O3T、MgO、CaO、Na2O、K2O和TiO2/Al2O3、Fe2O3T/Al2O3、MgO/Al2O3、Na2O/Al2O3、K2O/Al2O3比值分别建立F1-F2、F3-F4沉积岩原岩成分判别函数,有效地区分了以基性火成物源、中性火成物源、酸性火成物源及再旋回石英质物源为代表的4种沉积地层类型。汤白比马组砂岩样品在F1-F2和F3-F4图解(图 9)中均位于中性火成物源区,并表现有少量基性岩成分的混入,这与A-CN-K图解(图 7)中所反应的原岩成分一致。在微量元素Co/Th-La/Sc和La/Th-Hf(图 10)判别图解中,样品位于安山质中性物源和长英质酸性物源过渡区,为古老沉积物源成分加入的结果(Floyd and Leveridge, 1987Gu et al., 2002)。

图 9 汤白比马组砂岩主量元素物源组成判别图解(据Roser and Korsch, 1988) F1=-1.773TiO2+0.607Al2O3+0.76Fe2O3T-1.5MgO+0.616CaO+0.509Na2O-1.224K2O-9.09;
F2=0.445TiO2+0.07Al2O3-0.25Fe2O3T-1.142MgO+0.438CaO+1.475Na2O+1.426K2O-6.861;
F3=30.638TiO2/Al2O3+12.541Fe2O3T/Al2O3+7.329MgO/Al2O3+12.031Na2O/Al2O3+35.4K2O/Al2O3-6.382;
F4=56.5TiO2/Al2O3+10.879Fe2O3T/Al2O3+30.875MgO/Al2O3+5.404Na2O/Al2O3+11.112K2O/Al2O3-3.89
Fig. 9 Major elements composition discriminatory plots for the provenance of the sandstones in Tangbai district (after Roser and Korsch, 1988)

图 10 汤白比马组砂岩微量元素物源组成判别图解(a, 据Gu et al., 2002; b, 据Floyd and Leveridge, 1987) Fig. 10 Trace elements composition discriminatory plots for the provenance of the sandstones in Tangbai district (a, after Gu et al., 2002; b, after Floyd and Leveridge, 1987)
5.2 构造背景探讨

在CaO-Na2O-K2O(图 11)和(SiO2/20)-(K2O+Na2O)-(TiO2+FeO+MgO)主量元素构造背景三角判别图解中,汤白比马组砂岩大部分落入大陆岛弧环境,部分位于大洋岛弧与大陆岛弧相交位置,成分上与安山岩和太古宙杂砂岩相当。在Al2O3/SiO2-(Fe2O3T+MgO)/(SiO2+K2O+Na2O) (图 12a)和(SiO2/Al2O3)-(K2O/Na2O)(图 13)判别图解中,砂岩样品投于演化岛弧背景,这种演化岛弧背景为陆壳中-酸性火山岩浆弧(Kumon and Kiminami, 1994),按Bhatia(1983)的构造背景分类来区分,演化的岛弧与大陆岛弧相似(图 12b)。

图 11 汤白矿区比马组砂岩主量元素构造判别图解(a, 据Toulkeridis et al., 1999; b, 据Kroonenberg, 1994) OIA-大洋岛弧;CIA大陆岛弧;ACM-活动大陆边缘;PM-被动大陆边缘;BAS-玄武岩;AND-安山岩;GRA-花岗岩;AG-杂砂岩 Fig. 11 Tectonic setting discrimination diagrams based on the major elements for Bima Formation sandstones in Tangbai district (a, after Toulkeridis et al., 1999; b, after Kroonenberg, 1994) OIA-ocean island arc; CIA-continental island arc; ACM-active continental margin; PM-passive margin; BAS-basalt; AND-andesite; AG-Archaean greywacke

图 12 汤白比马组砂岩Al2O3/SiO2-(Fe2O3T+MgO)/(SiO2+K2O+Na2O)构造判别图解(据Kumon and Kiminami, 1994; b, 数据来源于Bhatia, 1983) IIA-不成熟岛弧;EIA-演化的岛弧;MMA-成熟的岩浆弧 Fig. 12 Al2O3/SiO2 vs. (Fe2O3T+MgO)/(SiO2+K2O+Na2O) tectonic setting discrimination diagram (after Kumon and Kiminami, 1994; b, data from Bhatia, 1983) IIA-immature island arc; EIA-evolved island arc; MMA-mature magmatic arc

图 13 汤白比马组砂岩SiO2/Al2O3-K2O/Na2O构造判别图解(据Roser and Korsch, 1986) A1-岛弧构造背景;A2-演化的岛弧背景;ACM-活动大陆边缘;PM-被动大陆边缘 Fig. 13 SiO2/Al2O3 vs. K2O/Na2O tectonic setting discrimination diagram (after Roser and Korsch, 1986) A1-arc setting; A2-evolved arc setting; ACM-active continental margin; PM-passive margin

不稳定微量元素(La、Th、Y、Zr、Ti、Co、Ni等)在后期风化沉积成岩过程中不容易发生改变,利用这些元素相互之间的关系建立图解来反应砂岩沉积时期的物源环境及构造环境是非常有代表性的Bhatia and Crook (1986)。在La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr /10图解(图 14)中,汤白比马组砂岩样品均落入大陆岛弧区域,且与La/SC-Ti/Zr图解(图 15)反映结果一致,样品集中表明微量元素判别具有更高的可信度。其次,在与雄村组砂岩样品对比研究中,可见从比马组砂岩到雄村组砂岩具有从大洋岛弧到大陆岛弧演化的趋势。

图 14 汤白比马组砂岩La-Th-Sc (a)、Th-Sc-Zr/10 (b)、Th-Co-Zr/10 (c)构造判别图解(据Bhatia and Crook, 1986) OIA-大洋岛弧;CIA大陆岛弧;ACM-活动大陆边缘;PM-被动大陆边缘 Fig. 14 Tectonic setting discrimination diagrams of La-Th-Sc (a), Th-Sc-Zr/10 (b), Th-Co-Zr/10 (c) for the sandstones in Tangbai district (after Bhatia and Crook, 1986) OIA-ocean island arc; CIA-continental island arc; ACM-active continental margin; PM-passive margin

图 15 汤白比马组砂岩La/Sc-Ti/Zr构造环境判别图解(据Bhatia and Crook, 1986) OIA-大洋岛弧;CIA大陆岛弧;ACM-活动大陆边缘;PM-被动大陆边缘 Fig. 15 Tectonic setting discrimination diagram of La/Sc vs. Ti/Zr for the sandstones in Tangbai district (after Bhatia and Crook, 1986) OIA-ocean island arc; CIA-continental island arc; ACM-active continental margin; PM-passive margin
5.3 构造演化探讨

新特提斯洋的构造演化模式一直是印度-亚洲大陆汇聚、碰撞过程中的一个焦点,早期关于新特提斯洋的研究表明其打开的时间最早可追溯至石炭纪至早二叠纪期间(Dewey et al., 1988; Pogue et al., 1992; Garzanti, 1999),于晚侏罗世之后开始俯冲(Honegger et al., 1982; Van der Voo et al., 1999),虽然近年来关于碰撞的大量研究发表,但在碰撞时间上仍然存在较多争议,主要集中在古新世-始新世约70~34Ma(Patriat and Achache, 1984; Rowley, 1996; Ding et al., 2005; Leech et al., 2005; Aitchison et al., 2007a; Cai et al., 2011; Hu et al., 2016),之后发生印度-亚洲大陆碰撞并开始之后全球最大规模的造山运动(Yin and Harrison, 2000)。

关于雄村岛弧的年代学(Tang et al., 2015)研究表明,相对于早白垩世泽当和Kohistan-Dras洋内弧(Aitchison et al., 2000, 2007b; McDermid et al., 2002; Bignold et al., 2006; Garrido et al., 2006),雄村地区洋内俯冲的时间可提前至早侏罗世。尽管在泽当地区仍存在洋内弧(Aitchison et al., 2000; Kapp and DeCelles, 2019)和陆缘弧(Zhang et al., 2014; Ding et al., 2016; Hu et al., 2016; Wang et al., 2016, 2017)的争议,但雄村超大斑岩型铜(金)矿床在矿床地质特征、含矿斑岩地球化学、成矿元素组合等方面都与世界上典型洋内俯冲成因斑岩型矿床相似(Tang et al., 2015)。其次,杨宗耀等(2017)对雄村矿区侏罗系砂岩的研究表明其物源成分并无拉萨地体古老成分的加入,更进一步证实了雄村洋内岛弧的存在。

Van der Voo et al. (1999)认为在晚侏罗世时期新特提斯洋仍处于扩张阶段,在早白垩世开始洋内俯冲和安第斯型俯冲于拉萨地体之下,随后洋内俯冲与印度大陆之间的洋壳率先俯冲消减,致使早先形成的洋内岛弧拼贴于印度大陆北缘,并进一步向拉萨地体汇聚。然而最新研究认为拉萨地体南缘沿雅鲁藏布缝合带展布的叶巴组火山是雅鲁藏布新特提斯洋开始俯冲于拉萨地体之下的证据(Zhu et al., 2008; Wei et al., 2017; Liu et al., 2018),表明雅鲁藏布新特提斯洋不晚于早侏罗世便开始俯冲于拉萨地体之下(图 16),其出现表明新特提斯洋洋壳已经开始北向俯冲。目前关于叶巴组火山岩的年代学资料表明其最早形成于190Ma(Zhu et al., 2008),略晚于洋内俯冲开始的时间195Ma(Tang et al., 2015),但我们认为新特提斯洋壳发生洋内俯冲的时间应晚于其俯冲于拉萨地体之下,所以还有更老的叶巴组岩浆活动没有发现。总的来说,新特提斯洋的北向俯冲应不晚于早侏罗世。

图 16 中生代新特提斯洋造演化模式 Fig. 16 Tectonic evolution of the Neo-Tethys in Mesozoic

杨宗耀等(2017)认为在早-中侏罗世时期雄村-汤白地区为大洋岛弧环境,但从本文中比马组砂岩所表现的陆缘弧性质的地球化学特来看,白垩世时期雄村-汤白火山弧可能已经完全拼贴到拉萨地体南缘(图 16),而并非印度大陆北缘的沉积物。以桑日群为代表的的岩浆活动并没有停止,否则这一时期应为被动大陆边缘环境,这也与Pan et al. (2012)所提出的演化模式一致。

6 结论

汤白矿区比马组砂岩产于火山岩夹层中,杂基含量高,结构成熟度低,以富含长石碎屑为特征,形成于构造活动区的第一次旋回沉积物,为活动岩浆弧成因杂砂岩。岩石地球化学及岩石学特征表明其形成于浅海相或海陆交替相环境,主要为中性陆缘弧火成物源,具有典型活动大陆边缘成因特征。

通过对雄村-汤白地区的雄村组砂岩和比马组砂岩的研究,认为雅鲁藏布新特提斯洋俯冲于拉萨地体之下始于早侏罗世,随后开始洋内俯冲,形成雄村-汤白等洋内岛弧,其在早白垩世拼贴于拉萨地体南缘,为活动大陆边缘背景。

致谢      感谢中国科学院青藏高原研究所许强副研究员给本文提出的宝贵意见。感谢编辑部俞良军副主编和审稿专家对本文严格把关并提供了建设性的意见和建议。

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