岩石学报  2019, Vol. 35 Issue (6): 1757-1772, doi: 10.18654/1000-0569/2019.06.08   PDF    
滇西高黎贡构造带东缘中侏罗世玄武岩岩石地球化学特征及其构造意义
刘旭峰1,2, 任玉峰1, 韦诚1,3, 戚学祥1     
1. 中国地质科学院地质研究所, 北京 100037;
2. 中国地质大学, 北京 10008;
3. 北京大学地球与空间科学学院, 北京 100871
摘要:高黎贡构造带东南缘的混杂岩带形成的构造背景复杂。产于混杂岩带以东的中侏罗世玄武岩地球化学特征可能为该岩石形成的构造背景提供依据。滇西中侏罗世玄武岩分布于高黎贡构造带东缘的凤平镇-畹町(南带)和怒江(北带)一带。其中怒江一带玄武岩具有块状构造,气孔杏仁发育,间粒结构,发生了绿片岩相变质。岩石属于亚碱性拉斑系列,具有中等的MgO(5.09%~6.20%)、较高的钛(TiO2 1.64%~2.14%)、铁(FeOT 9.19%~12.80%),且Na2O>K2O。岩石Mg#值(Mg#=Mg2+/(Mg2++Fe2+))为0.51~0.59,反映了源区岩浆经历了明显的辉石和橄榄石分离结晶,同时存在低氧逸度下分离结晶。岩石富含稀土元素(78.52×10-6~99.41×10-6),具明显的轻、重稀土分馏((La/Yb)N=4.06~5.54),无明显Eu异常(δEu=0.91~1.00),斜长石分离结晶不明显。同时,富集大离子亲石元素(Rb、Ba、Th、U)和高场强元素(Nb、Zr、Ti),具有弱的Nb、Zr、Ti负异常。较高的Zr/Y(3.58~5.88)、Ta/Yb(0.29~0.51)、Th/Yb(1.15~1.45)和(La/Nb)N(1.03~1.41)比值,说明怒江一带玄武岩具有裂谷特征。相对高Nb含量(8.83×10-6~15.60×10-6)和富集稀土元素不同于岛弧,周边及下部缺少洋壳和Nb负异常有别于OIB。岩浆源自富集的石榴石地幔橄榄岩部分熔融,在上涌过程中受到了地壳污染,显示Nb负异常。有限的岩浆作用规模和较短的喷发时间反映了高黎贡构造带东缘怒江一带具有被动拉张的裂谷性质,可能与古特提斯洋闭合引起的拉张作用有关。
关键词: 高黎贡构造带     玄武岩     中侏罗世     Nb负异常     滇西    
Geochemistry and implications of the Mid-Jurassic basalts on eastern margin of the Gaoligong tectonic zone, western Yunnan
LIU XuFeng1,2, REN YuFeng1, WEI Cheng1,3, QI XueXiang1     
1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. China University of Geosciences, Beijing 10008;
3. School of Earth and Space Sciences, Peking University, Beijing 100871, China
Abstract: The tectonic settings of the mélange on southeast of the Gaoligong tectonic zone in west Yunnan are very complicated. The geochemistry of the Mid-Jurassic basalts at east part of the mélange may provide tectonic environmental evidence for the formation of the rocks. The basalts distribute in Fengpingzhen-Wanding (south basalt belt) and Nujiang (north basalt belt) areas, where the rocks from the Nujiang area are dark grey, massive, vesicular and amygdaloidal in structure, and intergranular in texture. They all underwent greenschists facies metamorphism, with mineral assemblage of altered chlorite, epidote, albite, zeolite, titanite, potassium-magnesium ferro-aluminosilicate and residual Ti-bearing augite. The rocks are classified as sub-alkaline tholeiitic series, with middle MgO (5.09%~6.20%), rich in Ti (TiO2 1.64%~2.14%), Fe (FeOT 9.19%~12.80%), and Na2O>K2O. The magma of the basalts with Mg# values (Mg#=Mg2+/(Mg2++Fe2+)) of 0.51~0.59, experienced pyroxene and olivine fractional crystallization. Meanwhile, iron-enriched (relative low silicate) fractional crystallization tendency at low oxygen fugacity may exist for the magma. The basalts are rich in REE (∑REE 78.52×10-6~99.41×10-6) and fractional between LREEs and HREEs ((La/Yb)N=4.06~5.54), without obvious Eu anomaly (δEu=0.91~1.00). Meanwhile, they possess relatively high LILE (Rb, Ba, Th and U) and HFSE (Nb, Zr and Ti), with week Nb, Zr and Ti negative anomaly. High Zr/Y (3.58~5.88), Ta/Yb (0.29~0.51) and Th/Yb (1.15~1.45) ratios as well as (La/Nb)N ratios (1.03~1.41) reflect their affinity with continent. Relative high Nb concentrations (8.83×10-6~15.60×10-6) and ∑REE distinguish them from those of island arc; lack of ocean crust around them together with Nb negative anomalies show their difference from seamount/OIB. The week Nb negative anomalies for magma are resulted from crustal contamination during upwelling. High TiO2, FeOT, (La/Yb)N ratios as well as low εNd suggest the basaltic magma sourced from enriched garnet peridotite mantle, with relative lower degree partial melting for the Hongqiqiao and Linggangzhai basalts and higher degree partial melting for the Nujiangxian basalts. Upon the limited scale, spatial distribution and short activation time of the Mid-Jurassic basaltic magmatism, the Nujiang area on eastern Gaoligong tectonic zone may have been extensional and depressed, producing rift magma, which may arise from closure of the Paleo-Tethys.
Key words: Gaoligong tectonic zone     Basalts     Mid-Jurassic     Nb negative anomaly     Western Yunnan    

青藏高原中部呈东西向展布的班公湖-怒江新特提斯缝合带形成于侏罗纪至早白垩世(251~150Ma)(Shi, 2007; Shi et al., 2008a, 2012; Wang et al., 2008; 强巴扎西等, 2009; 黄启帅等, 2012; Liu et al., 2014, 2017; 周涛等, 2014)(图 1),经东构造结向南是否与高黎贡构造带相连一直存在着争议(谭敏和詹五弟, 1990; 刘本培等, 1993, 2002; 陈福坤等, 2006; 杨启军等, 2006; Shi et al., 2008b; Zhu et al., 2009, 2016, 2017; 杨启军和徐义刚, 2011; 谢韬等, 2010; 戚学祥等, 2011; Cong et al., 2011; Xu et al., 2012; 李再会等, 2012; Zhao et al., 2014; Cao et al., 2014; Qi et al., 2015; Xie et al., 2016; 尹福光等, 2006, 2012; 张克信等, 2016)。尽管构造带中早白垩世钙碱性岩浆活动及其与藏北班公湖-怒江带北带内岩浆具有一致性的大陆边缘弧地球化学特征以及年代学为腾冲和保山地块间存在洋陆俯冲提供了证据(杨启军等, 2006; Shi et al., 2008b; Zhu et al., 2009, 2016; 杨启军和徐义刚, 2011; 谢韬等, 2010; 戚学祥等, 2011; Cong et al., 2011; Xu et al., 2012; 李再会等, 2012; Zhao et al., 2014; Cao et al., 2014; Qi et al., 2015, 2019; Xie et al., 2016; 尹福光等, 2012; 张克信等, 2016),但缝合带存在的关键性标志——高黎贡构造带东南缘的混杂岩带是否为蛇绿混杂岩带仍存在较大争议。其中,部分学者根据混杂岩带中的蛇纹石化橄榄岩块岩石特征及硅质岩和复理石沉积认为其是蛇绿混杂岩带(刘本培等, 1993, 2002; 李兴振等, 1999; Xie et al., 2016; 王奕萱等, 2018; Qi et al., 2019)或构造混杂岩带(尹福光等, 2012),但部分学者根据其东部中侏罗统碳酸盐地层中含富集型玄武岩及缺乏洋壳证据,认为腾冲与保山地块之间为大陆裂谷关系(从柏林等, 1993; 张旗等, 1996; 钟大赉, 1998)。鉴于此,本文在野外调研的基础上,通过岩石学、岩石地球化学及中侏罗世沉积地层学方面的研究,重新认识这套玄武岩形成的构造背景,这将有助于理解腾冲-保山地块的构造演化,为青藏高原东缘造山带演化提供重要证据。

图 1 研究区大地构造简图(据Qi et al., 2019) Fig. 1 Sketch map of the tectonic structure around the study area, western Yunnan (after Qi et al., 2019) BNS: Bangong-Nujiang Suture; IBS: Indo-Burma Suture; MBT: Main Boundary Thrust; MK: Myitkyina Suture; YTS: Yarlung-Tsangpo Suture
1 地质背景

高黎贡构造带东缘中侏罗世玄武岩分布于腾冲和保山地块之间,西以泸水-龙陵-瑞丽断裂带为界,东以怒江断裂带与保山地块相隔,中部的郭家寨-三家村断裂带与怒江断裂带近于平行(图 2a)。高黎贡构造带位于青藏高原东构造结东南缘,北起察隅,呈南北向经贡山、泸水至龙陵转为南西向经瑞丽至缅甸为Sagaing断裂带所截,全长约500km,宽10~20km(图 2)。构造带东以泸水-龙陵-瑞丽断裂带,西以龙川江断裂带为界,由高绿片岩相-角闪岩相深变质变形带组成,是早古生代原特提斯洋俯冲增生变质的产物(Qi et al., 2019),并在新生代(35~17Ma)块体旋转挤出过程中叠加大规模右行韧性走滑剪切变形形成的一条大型右行韧性剪切带(Tapponnier and Molnar, 1976; Tapponnier et al., 1982; 钟大赉等, 1991; Wang and Burchfiel, 1997; 钟大赉, 1998; Wang et al., 2006, 2008; 季建清, 2000; 杨启军等, 2006; Lin et al., 2009; Zhang et al., 2012; Xu et al., 2012, 2015; Eroǧlu et al., 2013; Huang et al., 2015)。怒江断裂带为泸水-龙陵-瑞丽断裂带的东部分支,北起潞江坝的坝湾,向南东沿怒江河谷或河谷东侧呈向西弧形展布的一条断裂带,总体倾向W-SW,倾角约30°~60°,以脆性、脆-韧性变形为主。断裂带西侧以早古生代、晚古生代浅变质岩及中生代海相沉积明显不同于东侧保山地块内的沉积系列。郭家寨-三家村断裂带由糜棱岩化片岩和糜棱岩组成,北部向北东倾,中部转向北北东,倾角50°左右。高黎贡东缘侏罗纪玄武岩北带夹于怒江断裂带和郭家寨-三家村断裂带之间,沿怒江沿岸分布,南带玄武岩分布于龙陵-瑞丽混杂岩带与郭家寨-三家村断裂带之间(图 2a)。

图 2 滇西地质构造简图(a, 据Qi et al., 2019)及怒江一带玄武岩采样位置(b, 据云南省地质调查院, 2008) Fig. 2 Geologic sketch map (a, after Qi et al., 2019) and sampling sites (b) in western Yunnan

① 云南省地质调查院.2008. 1:250000腾冲县幅、潞西幅区域地质调查报告

区内出露的地层为古生代奥陶纪-二叠纪滨海-浅海相沉积和中生代中侏罗统浅海陆棚相沉积岩夹玄武岩、早白垩世含流纹岩(陆缘弧)弧前沉积岩及晚白垩世陆相沉积岩。此外,在龙陵-瑞丽一带发育蛇绿混杂岩带(Qi et al., 2019)。其中,中侏罗世沉积岩与早古生代地层之间为断层接触(图 2b),早白垩世含流纹岩弧前沉积岩构成混杂岩带的一部分,晚白垩世陆相沉积岩不整合覆盖在早期地层之上。

滇西高黎贡东缘中侏罗世沉积岩是三江构造带内唯一与拉萨-羌塘地块之间班公湖-怒江缝合带内相对应的中生代海相沉积地层,由勐戛组和柳湾组构成(图 3)。前者为中-厚层状泥晶灰岩、含白云质颗粒泥晶灰岩、颗粒灰岩夹钙质泥岩,中部夹厚200~300m玄武岩,与下伏的二叠纪地层呈断层接触,在施甸太平镇与中下三叠统微晶灰岩呈平行不整合关系;后者自下而上依次为深灰色、灰黑色薄-中厚层介壳灰岩、鲕状灰岩、微晶灰岩段;黄、紫红色页岩及介壳灰岩、玄武岩段和灰色、灰黑色薄-中厚层介壳灰岩、鲕状灰岩、微晶灰岩段,上部整合有中侏罗统龙海组泥页岩和粉砂岩。南带玄武岩在潞西凤平镇-畹町一带出露较完整,但风化强烈,呈红色、黄色,块状,气孔构造及杏仁构造清晰。北带玄武岩较南带新鲜。本文样品采自北带的岗岭寨-红旗桥一带(图 2b),野外露头照片见图 4a-c。所采样品分别命名为红旗桥、岭岗寨和怒江西岸玄武岩。该地区植被覆盖严重,岩石风化强烈,玄武岩呈灰黄色及红褐色。其中,岭岗寨玄武岩与二叠纪灰岩呈断层接触,红旗桥玄武岩和怒江西岸玄武岩夹在侏罗纪灰岩中,但与灰岩之间呈断层接触。张旗等(1996)曾报道了滇西玄武岩εNd值(+2.4~-1.4),但未交代样品采样位置及相应的地球化学数据。

图 3 滇西怒江一带施甸太平镇中侏罗统地层剖面(据四川省地调院, 2012) Fig. 3 Mid-Jurassic stratigraphic section in Taipingzhen, Shidian in the Nujiang area, western Yunnan

② 四川省地调院. 2012. 1:50000清河街幅、镇安街幅、龙陵县幅、龙新幅区域地质调查报告

图 4 玄武岩野外露头和显微结构照片 (a)怒江西岸玄武岩露头,风化面灰黄色,断裂面上部岩石破碎,下部呈块状;(b)岭岗寨玄武岩露头,风化面褐红色,节理发育;(c)红旗桥玄武岩露头,风化面褐红色,填充绿纤石杏仁;(d)填充方解石杏仁的玄武岩透射光照片;(e)较新鲜的红旗桥玄武岩正交偏光照片.斜长石斑晶(中右)和基质钠长石化,部分绢云母化(中下淡绿色),辉石斑晶(中左)为含Ti普通辉石,基质中的辉石含Ti低;(f)较新鲜的怒江西岸玄武岩透射光照片.棕黄色大颗粒为绿泥石,之间残留有含Ti普通辉石.有的大颗粒辉石蚀变为榍石.长条状斜长石格架间为细粒普通辉石,凸起高,Ti含量不等.蓝绿色矿物为富K的铁镁质铝硅酸盐集合体,低凸起,一级灰白干涉色;(g)蚀变玄武岩透射光照片.可见到钠长石、绿泥石、绿帘石、不规则赤铁矿及燧石杏仁(下部);(h)蚀变玄武岩透射光照片.位于中部的短柱状沸石显示一组较完全解理(其他薄片中可见到二组完全解理),低凸起,一级灰干涉色.不透明的赤铁矿细粒集合体或局部集中或沿暗红色-黄色绿泥石边缘分布. Aug-普通辉石;Ab-钠长石;Chl-绿泥石;Cal-方解石;KAS-含K铁镁铝硅酸盐;Zeo-沸石 Fig. 4 Macro- and micrographs of the Jurassic basalts from western Yunnan
2 分析方法

选择相对新鲜、脉体和杏仁含量低的样品,破碎,在双目镜下剔除脉体和杏仁,细碎制成粉末用于全岩化学分析。测试分析在国家地质测试中心完成。主量元素采用X荧光光谱仪(XRF-PW4400)测试,分析精度为2%~8%;微量元素分析采用等离子质谱仪(ICPMS-PE300D)测试,含量大于10×10-6的元素精度为5%,小于10×10-6的元素分析精度为10%。矿物电子探针分析在中国地质科学院地质研究所自然资源部深地动力学重点实验室完成,仪器型号为日本电子公司JXA-8100电子探针,配有牛津IncaEnergy能谱仪。电子探针电子束流为20nA,加速电压15.0kV,电子束斑直径为5μm,当矿物颗粒细小时做相应调整。

3 岩相特征

玄武岩以块状构造为主,局部见枕状构造,气孔和杏仁发育(图 4c, d),气孔+杏仁约占熔岩的30%~50%,大小0.1~20mm不等。岩石大多强烈蚀变,显微镜下显示粗玄结构或间粒结构,即长条状斜长石之间填充细小的辉石和磁铁矿颗粒,不含橄榄石。对于较新鲜的红旗桥玄武岩(图 4c),可见到少量斜长石和辉石斑晶,均呈长柱状,含量不超过5%,长约0.9mm。一般,斜长石长度0.1~0.5mm,未见双晶,有的具中空结构,含量约占60%~65%,辉石和磁铁矿含量约占35%~40%,粒度0.1~0.05mm。对于较新鲜的怒江西岸玄武岩,仅见到不超过5%的少量斜长石斑晶,未见到辉石斑晶(或已蚀变为绿泥石),可见到新鲜的细粒辉石。一般斜长石和辉石的含量及粒度类似于红旗桥玄武岩,不同的是在该样品中还发现淡蓝绿色蚀变矿物(图 4f),有时见榍石和钛铁矿。

矿物化学成分(见表 1)显示辉石的类型属于含Ti普通辉石(图 5),TiO2含量为0.6%~1.4%,多发生了绿泥石化和绿帘石化。蓝色矿物为富含钾的铁镁铝硅酸盐,呈细粒集合体,应为铁镁质矿物蚀变产物。该矿物K2O高达10%且含水,其矿物类型有待进一步确定。斜长石已全部转变为钠长石和少量沸石(图 4h),有的表面绢云母化明显(图 4e)。强烈蚀变岩石中仅保留有粗玄结构,可见到长条状钠长石和细小的粒状绿帘石、绿泥石,暗色铁质以细小的赤铁矿分布于钠长石格架之间(图 4g)。沸石的Ca、Al、Si原子比接近1:2:4,含水12%~14%,可能为浊沸石。玄武岩中填充的杏仁体包括绿纤石(图 4c)、燧石(图 4g)、方解石(图 4d)。其矿物组合表明岩石发生了绿片岩相变质,具有典型的细碧岩化海底蚀变特征。

表 1 滇西侏罗纪玄武岩矿物探针分析结果(wt%) Table 1 Microprobe analytical results of the minerals for the Jurassic basalts, western Yunnan (wt%)

图 5 辉石分类图表(据Morimoto, 1988) 1-透辉石;2-普通辉石;3-易剥辉石;4-顽火辉石;5-斜铁辉石 Fig. 5 Classification of pyroxenes (after Morimoto, 1988)
4 岩石化学特征

12个玄武岩全岩化学成分见表 2。其SiO2含量(47.60%~50.35%)变化范围小,Al2O3含量(15.29%~16.24%)稳定,MgO含量中等(5.09%~6.20%),但TiO2和FeOT含量变化范围相对较大。根据Ti/Y比值分成二组,一组为红旗桥、岭岗寨玄武岩,其Ti/Y比值>490(492~574),TiO2含量为2.01%~2.10%,FeOT含量为11.14%~12.8%,Mg#值为0.51~0.54,Cr含量318×10-6~390×10-6,Ni含量175×10-6~206×10-6。另一组为怒江西岸玄武岩,其Ti/Y比值< 490(434~455),TiO2含量为1.64%~1.68%,FeOT含量为9.19%~10.15%,Mg#值为0.56~0.59,Cr含量249×10-6~266×10-6,Ni含量62.4×10-6~64.2×10-6。显然,红旗桥、岭岗寨玄武岩富钛铁,高Cr和Ni,低Mg#值,而怒江西岸玄武岩正好相反。

表 2 滇西侏罗纪玄武岩全岩主量(wt%)和微量元素化学分析结果(×10-6) Table 2 Major element (wt%) and trace element (×10-6) analyses of the Jurassic basalts, western Yunnan

在哈克图解上(图 6),MgO与Ni之间具有正相关性(红旗桥、岭岗寨玄武岩),表明岩浆存在橄榄石的分离结晶(Rollinson, 1993; Du et al., 2015)。FeOT、Cr与MgO均呈负相关性,反映了FeOT、Cr之间具有较强的相关性,表明岩浆存在辉石分离结晶作用。另外,MgO与Fe2O3、SiO2与Fe2O3均呈负相关关系,表明岩浆(相对低硅富铁)可能存在“Fenner”分离结晶趋势,即在低氧逸度下岩浆铁增加直到晶出磁铁矿和钛铁矿(Brooks et al., 1991; Toplis and Carroll, 1995; Peng et al., 2008; Du et al., 2015)。

图 6 滇西侏罗纪玄武岩MgO与SiO2、Al2O3、TiO2、FeOT、Fe2O3、CaO、Na2O、Ni、Cr及SiO2与Fe2O3之间变异图 氧化物含量采用扣除烧失量经100%归一化的结果 Fig. 6 MgO against SiO2, Al2O3, TiO2, FeOT, Fe2O3, CaO, Na2O, Ni, Cr variation diagrams and SiO2 against Fe2O3 variation diagram of the Jurassic basalts, western Yunnan

由于岩石发生了绿片岩相变质,采用相对稳定的主元素SiO2、TiO2、FeO、MgO和不活动微量元素Zr、Nb、Y进行分类,结果见图 7。怒江西岸玄武岩落在亚碱性玄武质区域(图 7a),红旗桥和岭岗寨玄武岩落在亚碱性和碱性玄武质系列边界附近。进一步采用FeOT/MgO-SiO2和TiO2-FeOT/MgO图表分类,3个采样点岩石均落入了亚碱性的拉斑玄武岩系列区域(图 7b, c)。同时,所有样品FeOT含量较高(9.19%~12.8%),在(TiO2+FeO+Fe2O3)-Al2O3-MgO图表(图 8)上落入了高铁的拉斑玄武岩系列区域内。其富铁的特征类似于华北克拉通早元古代裂谷玄武岩(Du et al., 2015)以及特提斯喜马拉雅造山带三叠纪裂谷玄武岩(朱弟成等, 2006)。

图 7 滇西侏罗纪玄武岩Nb/Y-Zr/TiO2 (a, 据Winchester and Floyd, 1977)、FeOT/MgO-SiO2 (b)和FeOT/MgO-TiO2 (c)(据Miyashiro, 1975)分类图解 Fig. 7 Nb/Y vs. Zr/TiO2 (a, after Winchester and Floyd, 1977), FeOT/MgO vs. SiO2 (b) and FeOT/MgO vs. TiO2 (c) (after Miyashiro, 1975) discrimination diagrams for the Jurassic basalts, western Yunnan

图 8 滇西侏罗纪玄武岩(Fe2O3+FeO+TiO2)-Al2O3-MgO分类图(据Jensen, 1976) Fig. 8 Plot of (TiO2+FeO+Fe2O3)-Al2O3-MgO (after Jensen, 1976) for the Jurassic basalts, western Yunnan

所有样品均富含稀土元素,∑REE为78.52×10-6~99.41×10-6(表 2)。在球粒陨石标准化稀土元素配分曲线图上(图 9a),与N-MORB相比,富集轻稀土、亏损重稀土元素,具明显的轻、重稀土分馏,(La/Yb)N比值为4.06~5.54。δEu数值为0.91~1.00,无明显Eu异常,表明斜长石分离结晶作用不明显。在原始地幔标准化微量元素蛛网图上(图 9b),与N-MORB相比,富集大离子亲石元素(Rb、Ba、Th、U)和高场强元素(Nb、Ti、Zr)。同时,具有Nb、Ti、Zr、Y弱负异常。怒江西岸玄武岩明显比红旗桥和岭岗寨玄武岩亏损高场强元素。另外,前者微量元素Nb、Ta、La、Ce、Zr、Hf、Pb明显低于后者,显示了相对亏损的特征。

图 9 滇西侏罗纪玄武岩球粒陨石标准化稀土元素配分图(a)及原始地幔标准化微量元素蛛网图(b)(标准化值据Sun and McDonough, 1989) Fig. 9 Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagrams (b) of the Jurassic basalts, western Yunnan (normalization values after Sun and McDonough, 1989)
5 讨论 5.1 岩浆源区及演化

本区玄武岩与MORB相比,总稀土含量高(78.52×10-6~99.41×10-6)(图 9a),而且富含大离子亲石元素(Rb、Ba、Th、U)和高场强元素(Nb、Ti、Zr)(图 9b),具有中等到较高的轻重稀土分馏((La/Yb)N=4.06~5.54),表明源区可能是富集的石榴石地幔(Sun and Nesbitt, 1978; Kato et al., 1988; Du et al., 2015)。参考张旗等(1996)发表的滇西玄武岩εNd值(+2.4~-1.4),源区主要为富集地幔和少量亏损地幔的混合。红旗桥和岭岗寨玄武岩(FeOT 11.14%~12.80%)比怒江西岸玄武岩(FeOT 9.19%~10.15%)含铁高,说明前者岩浆来源的压力比后者高(朱弟成等, 2006),因此源区地幔熔融程度低,形成的岩浆更富集不相容元素。本区玄武岩Mg#值为0.51~0.59,为演化岩浆结晶产物(原始岩浆Mg#值>0.68, Frey et al., 1978)。岩石MgO与Ni的正相关关系及FeO与Cr之间较强相关性说明岩浆发生了橄榄石和辉石的分离结晶。同时,MgO与Fe2O3、SiO2与Fe2O3的负相关关系,表明岩浆可能存在低氧逸度条件下的富铁作用。玄武岩浆在上涌过程中由于分离结晶和混染可能改变其微量元素的组成。本区玄武岩斜长石的分离结晶作用不明显(δEu 0.91~1.00),对岩浆的不相容元素影响不大。辉石和橄榄石的结晶对岩浆不相容元素影响较小,而磁铁矿和钛铁矿(主要含Nb、Ti矿物)的结晶可能对岩浆高场强元素含量影响较大,但由于他们在岩浆中的含量较少,对岩浆微量元素特征影响不大(Safonova and Santosh, 2014)。本区所有玄武岩样品La/Nb比值(0.99~1.36)基本上在1.0以上,说明岩浆受到了地壳污染(袁超等, 2005; 夏林圻等, 2006, 2008; 朱弟成等, 2006)。

5.2 构造环境

为了对这些玄武岩形成的构造环境进行判别,选择相对不活动元素Ti、Zr、Nb、Ta、Th、Y和Yb的相关图解,结果见图 10。在Ti/100-Zr-3Y图解上(图 10a),玄武岩落入板内、洋中脊和岛弧拉斑区域;在2Nb-Zr/4-Y图解上(图 10b),落入岛弧和富集的洋中脊区域;在Zr/Y-Zr图解上落入板内区域(图 10c);在Th/Yb-Ta/Yb图解上落入活动大陆边缘区域(图 10d),显示了复杂的构造环境。

图 10 滇西侏罗纪玄武岩构造判别图 (a) Ti/100-Zr-3Y图(Pearce and Cann, 1973);(b) 2Nb-Zr/4-Y图(Meschede, 1986);(c) Zr/Y-Zr图(Pearce, 1983); (d) Th/Yb-Ta/Yb图(Pearce, 1983) Fig. 10 Tectonic discrimination diagrams for the Jurassic basalts, western Yunnan

本区玄武岩上部沉积有浅海相碳酸盐盖层,类似于海山。其主元素TiO2含量1.64%~2.10%,Al2O3 15.29%~16.24%,MgO 5.15%~6.20%,Al2O3/TiO2比值7.63~9.55,也类似于海山主元素特征(TiO2 1.5%~4.2%,Al2O3 9%~19%,MgO 3%~9%,Al2O3/TiO2 < 8.5, Safonova and Santosh, 2014),但微量元素有较大差别。洋岛/海山具有较高的Nb含量(13×10-6~130×10-6,平均31×10-6),(La/Nb)N比值小于1,在微量元素蛛网图上显示Nb正异常(Safonova and Santosh, 2014)。中国南海盆地海山玄武岩也具有Nb、Ta正异常特征(Yan et al., 2019)。而本研究区所有样品Nb含量相对于海山不高(8.83×10-6~15.60×10-6,平均12.48×10-6),且(La/Nb)N比值大于1(1.03~1.41),具有Nb负异常(图 7b)。因此,本区岩石为洋岛/海山玄武岩的可能性不大。另外,虽然洋岛玄武岩也会出现Nb负异常,如OIB型和MORB型岩浆混合(钱青等, 2001; Frey et al., 2015; Safonova and Santosh, 2014; Safonova et al., 2016; Khogenkumar et al., 2016),或是洋内弧污染(张旗等, 1995; Greene et al., 2009; Kamenetsky et al., 1997; Mullen and Weis, 2015; Kiminami et al., 2017),或是陆壳污染(Weis et al., 2001)以及地幔柱本身的特征(即包含了富集组分,也包含有如橄榄岩引起的亏损组分)(White, 2010),但它们与岛弧、洋盆或蛇绿岩相伴。本区玄武岩下部及周围尚未见到亏损的MORB、岛弧火山岩、辉长岩以及与洋相关的放射虫硅质岩等沉积,从根本上排除了洋岛/海山的可能性。此外,施甸太平镇玄武岩中夹有粉砂岩(四川省地调院, 2012),排除了洋内环境的可能性。

本区玄武岩样品Zr/Y比值(3.58~5.88)大于3(图 8c),Ta/Yb比值(0.29~0.51)大于0.1(图 8d),Zr/Nb比值(6.54~9.37)低于25(表 2),不同于典型的岛弧玄武岩(Condie, 1989; McCulloch and Gamble, 1991),显示大陆亲缘性(袁超等, 2005)。当裂谷玄武岩受到大陆岩石圈的混染时,显示较高的La/Nb比值,或显示岛弧特有的Nb、Ta负异常(肖龙等, 2003; Ernst et al., 2005; 袁超等, 2005; 夏林圻等, 2006, 2008; 李献华等, 2008; 余星等, 2009; 李洪颜等, 2013; 陈根文等, 2015; 姚华舟等, 2018)。当裂谷玄武岩受到强烈地壳污染时,出现Nb负异常,其Nb/La比值常常小于0.7(肖龙等, 2003; 袁超等, 2005; 夏林圻等, 2008; 李洪颜等, 2013; 陈根文等, 2015),甚至低至0.2(夏林圻等, 2008)。另外,裂谷玄武岩受到地壳污染会出现Pb正异常(张传林等, 2004; 李洪颜等, 2013; 陈根文等, 2015)。根据Xia (2014)研究,裂谷玄武岩与岛弧玄武岩最大区别还在于前者比后者富集程度要高得多,Nb浓度一般在7×10-6~42×10-6,而且岩石的εNd值较低。本区玄武岩Nb浓度为8.83×10-6~15.60×10-6,Nb/La比值为0.74~1.01(表 2),其微量元素浓度比消减带岛弧玄武岩(Nb < 4×10-6)要高的多(Tatsumi and Eggins, 1995; 袁超等, 2005; 夏林圻等, 2008; Xia, 2014),且εNd值较低(+2.4~-1.4, 张旗等, 1996),因此属于遭受陆壳污染的裂谷岩石。

本区玄武岩厚度仅200~300m,规模有限,不可能是溢流玄武岩(袁超等, 2005)。除了该中侏罗世玄武岩浆作用外,本区及周边再未出现新的基性岩浆活动,且活动时间仅限于中侏罗世早期。因此,该岩浆作用不可能起源于地幔柱(Wilson, 1997; 姚华舟等, 2018; Yan et al., 2019)。在时间上,该岩浆活动晚于东部昌宁-孟连古特提斯洋闭合时代(晚三叠),可能具有被动拉张的性质(张旗等, 1996)。怒江西岸玄武岩比红旗桥和岭岗寨玄武岩Nb/La比值低(前者为0.74~0.75,后者0.94~1.01),Nb、Ta和Ti负异常明显,且微量元素含量低,FeOT低,但Mg#值高,反映了源区亏损成分是增加的。三处玄武岩产出位置近,位于同一地层中,它们喷发时间可能有差异。红旗桥和岭岗寨玄武岩含铁高,来源较深,可能喷发较早,源区地幔熔融程度低,岩浆相对富集,而怒江西岸玄武岩喷发晚,源区地幔熔融程度高,岩浆相对亏损,具有向洋中脊岩浆发展的趋势。

5.3 构造演化

区域上,怒江断裂带以西的潞西(郭家寨-潞西)被划分独立的块体,其西侧为腾冲地块,之间为高黎贡构造混杂岩带,东侧为保山地块(云南省地质调查院, 2008;四川省地调院, 2012)。因其具有早古生代深水沉积和1-O2花岗岩侵入、缺少早二叠世河流-滨海-冰筏浅海-浅海碳酸盐沉积而不同于腾冲和保山地块。另外,潞西因不发育早二叠世溢流玄武岩而不同于保山地块。潞西缺失晚三叠-早侏罗世地层。保山地块三叠纪为局限台地-浅海-陆棚相沉积,缺失早侏罗世沉积(云南省地质调查院, 2008;四川省地调院, 2012)。因此,潞西和保山早侏罗世均处于抬升剥蚀状态。中侏罗世早期潞西发生沉降凹陷,接受滨海-浅海相沉积,之后继续沉降,沿怒江河谷有玄武岩喷发,其上部又沉积了中侏罗世浅海相碳酸盐(J2柳湾组),还有间歇性玄武岩喷发。中侏罗世晚期海退,出现河湖相沉积。该中侏罗世玄武岩发育在怒江断裂和郭家寨断裂带之间(图 2a),岩石化学具有大陆裂谷性质,反映了潞西与保山地块之间的拉张,可能与东部的古特提斯洋碰撞闭合(晚三叠世)之后引起的拉张有关(张旗等, 1996)。

南带玄武岩(凤平-畹町)与北带玄武岩(怒江一带)是否具有相同的岩石化学性质有待进一步研究。另外,潞西一带中侏罗世粗碎屑地层中含有铬铁矿,在三台山的硅质凝灰岩中也发现铬铁矿碎屑,表明在中侏罗世沉积前潞西一带可能就有超基性岩存在,它们与北带玄武岩所在的沉积地层有所区别。总之,北带玄武岩岩石化学特征和地层关系反映了潞西与保山地块之间的裂谷化,还不能据此推断腾冲与保山地块之间也具有同样的关系。

6 结论

(1) 滇西高黎贡构造带东缘怒江一带侏罗纪玄武岩具有块状构造,气孔杏仁发育,粗玄结构,发生了绿片岩相变质。原岩残留的辉石为钛普通辉石,大多蚀变为绿泥石、绿帘石,还有少量蓝色富钾铁镁铝硅酸盐、榍石。原岩中的斜长石全部转变为钠长石、沸石。

(2) 玄武岩为亚碱性拉斑玄武岩系列,经历了明显的橄榄石和辉石以及“Fenner”分离结晶作用。岩石富含稀土元素,具明显的轻、重稀土分馏,斜长石分离结晶作用不明显。玄武岩富集大离子元素和高场强元素,又有弱的Nb、Zr、Ti负异常以及高的Zr/Y和Ta/Yb比值,反映了大陆裂谷环境,其Nb负异常是陆壳混染的结果。

(3) 岩浆起源于富集的石榴石地幔,红旗桥和岭岗寨玄武岩源区地幔熔融程度低,岩浆相对富集,而怒江西岸玄武岩源区地幔熔融程度高,岩浆相对亏损,具有向洋中脊岩浆发展的趋势。

致谢      本文在成稿过程中,史仁灯研究员提出了很多关键问题,对本文的改进起到了至关重要的作用,在此非常感谢史仁灯研究员、熊发挥副研究员、刘建峰研究员和杜利林研究员提出的建议和指正!电子探针分析在自然资源部深地动力学重点实验室完成,感谢毛小红博士的帮助!

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