岩石学报  2020, Vol. 36 Issue (10): 3117-3136, doi: 10.18654/1000-0569/2020.10.11   PDF    
滇西龙陵-瑞丽低钠流纹岩年代学、岩石地球化学和锆石Hf同位素组成及其地质意义
韦诚1,2, 戚学祥1, 沈辉3, 吉风宝4, 任玉峰1, 刘旭峰1     
1. 自然资然部深部动力学重点实验室, 中国地质科学院地质研究所, 北京 100037;
2. 北京大学地球与空间科学学院, 北京 100871;
3. 云南省地质矿产勘查院, 昆明 650051;
4. 西藏自然科学博物馆, 拉萨 850000
摘要: 龙陵-瑞丽早白垩世流纹岩带断续延伸约100km,其形成的构造背景是揭示怒江洋演化及腾冲与保山地块关系的重要证据。笔者在野外调研的基础上,对本区流纹岩开展了岩石学、地球化学、锆石U-Pb定年和Hf同位素等方面的研究。结果表明流纹岩具有高硅贫钠、高钾钙碱性和高分异特征;锆石LA-ICP-MS U-Pb年龄分布在129~123Ma区间,代表流纹岩的喷发时代。流纹岩锆石εHf(t)值分布于-12.3~-6.3之间,对应的地壳模式年龄为1591~1826Ma;其微量元素蛛网图、稀土元素配分模式图和Nb/Ta比值、Th/U比值等与大陆中上地壳相似;岩石中无斜长石斑晶、Sr含量极低以及在Al2O3/(MgO+FeOT)]-[CaO/(MgO+FeOT)]和Rb/Ba-Rb/Sr图解上,样品都分布在变泥质岩部分熔融区;表明流纹岩岩浆来源于古老地壳物质部分熔融的产物,其低钠是源区中斜长石含量低或斜长石未参与部分熔融造成的。流纹岩轻稀土元素和大离子亲石元素(Th、U和K)富集,Nb、Ta、Ti、P和Sr强烈负异常;在构造环境判别图解上样品全部落在活动大陆边缘区;在空间上,这些流纹岩呈带状分布,构成俯冲增生杂岩带的一部分,并与同时代侵入岩紧密相邻,成因上类似于区域内同期S型花岗岩。这些特征表明研究区内流纹岩与高黎贡构造带内同时代花岗岩类相似,形成于俯冲的大陆边缘弧环境,即幔源岩浆底侵至下地壳,这一过程提供了足够的热源使壳源物质(可能为富云母、贫斜长石的变沉积岩类)发生部分熔融生成酸性岩浆,它们在经历了高度的结晶分异后喷出地表形成低钠流纹岩,并在弧前或弧间盆地内被浅海相碎屑沉积物覆盖。
关键词: 锆石U-Pb定年    Lu-Hf同位素组成    低钠流纹岩    增生杂岩带    滇西    
Geochronology, geochemistry and zircon Hf isotope of the low Na rhyolite at Longling-Ruili belt, and its geological implications
WEI Cheng1,2, QI XueXiang1, SHEN Hui3, JI FengBao4, REN YuFeng1, LIU XuFeng1     
1. Key Laboratory of Deep-Earth Dynamics, Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
3. Yunnan Institute of Geology & Mineral Resources Exploration, Kunming 650051, China;
4. Tibet Museum of Natural Science, Lhasa 850000, China
Abstract: The Longling-Ruili Early Cretaceous rhyolite belt extended intermittently about 100km,and its tectonic setting can provide key evidence to show the subduction of the Nujiang Ocean and collision between Tengchong and Baoshan blocks. Based on field work,petrological and geochemical studies together with zircon U-Pb dating and Hf isotopic analysis of the rhyolites have been carried out. The results show that the rhyolites are characterized by high-Si and low-Na,high-K calc-alkaline and highly differentiation index (DI). The zircon LA-ICP-MS U-Pb ages range from 129Ma to 123Ma,representing the eruption ages of the rhyolites. The zircon εHf(t) values range from -12.3 to -6.3 with tDMC of 1591~1966Ma. Trace element spider diagram,REE patterns,and Nb/Ta and Th/U values of the rhyolites are similar to those of the middle and upper continent crust. There's no plagioclase phenocrysts in the rock,and Sr content is extremely low,also,all samples fall in the metapolitical partial melting field on the [Al2O3/(MgO+FeOT)]-[CaO/(MgO+FeOT)] and Rb/Ba-Rb/Sr diagrams. These indicate that the rhyolitic magma was derived from partial melting of ancient crustal material. The low Na content was resulted from partial melting of plagioclase-poor rocks or the situation that plagioclase did not melt during the partial melting. With high-K calc-alkaline,enrichment in light REE and large ionic lithophile elements (Th,U and K),and obvious negative anomalies of Nb,Ta,Ti,P and Sr,all samples fall in the active continental margin field on the tectonic discrimination diagram. As a part of subduction and accretionary complexes,the spatially banded rhyolites are closed to the contemporaneous intrusive rocks,their geochemical characteristics are consistent with those of the coeval S-type granites in Gaoligong orogen. All above demonstrates that the rhyolites were formed in an active continental margin related with subduction of Nujiang oceanic crust,i.e.,the rhyolitic magma was formed by the partial melting of crustal material (mica-rich and plagioclase-poor metapelite) caused by ascent mantle-derived magma. After crystallization differentiation,the low-Na rhyolitic magma erupted and covered by neritic facies clastic sediments within the forearc or inter arc basin.
Key words: Zircon U-Pb dating    Lu-Hf isotopic compositions    Low-Na rhyolites    Accretionary complex belt    Western Yunnan    

三江构造带位于青藏高原东南缘,是东特提斯构造域喜马拉雅造山带与东南亚造山带的交接转换部位,是研究东特提斯域洋陆演化的关键地区(图 1a钟大赉, 1998; Yin and Harrison, 2000; Metcalfe, 2002, 2013; Kapp et al., 2005, 2007; Qi et al., 2014, 2015, 2016; Zhang et al., 2017)。腾冲地块和保山地块之间的高黎贡造山带位于三江构造带的西缘,由于印度-亚洲大陆的碰撞,块体逃逸或者侧向挤出叠加了强烈的右行走滑剪切作用,研究区内的岩石都发生了不同程度的变质变形。前人研究成果表明高黎贡造山带内广泛分布的中、新生代侵入岩形成时代集中在130~115Ma和76~50Ma(杨启军等, 2006;丛峰等, 2010, 2011;戚学祥等, 2011; Xu et al., 2012; Cao et al., 2014; Ma et al., 2014; Chen et al., 2015; Qi et al., 2015, 2019; Zhu et al., 2015, 2017; Xie et al., 2016;张诗启等, 2017; Zhang et al., 2018; Lin et al., 2019)。其中,早白垩世侵入岩主要分布于高黎贡构造带内,由规模不等的闪长岩、花岗闪长岩和花岗岩岩体组成,形成于与俯冲有关的大陆边缘弧(杨启军等, 2006;戚学祥等, 2011; Zhu et al., 2015, 2017; Xie et al., 2016; Qi et al., 2019)或形成于保山-腾冲地块碰撞后的加厚地壳环境(Xu et al., 2012)。早白垩世火山岩主要分布于高黎贡构造带东南缘龙陵-瑞丽一带,前人对区内火山岩进行了初步研究,获得龙陵西大荒地流纹岩锆石U-Pb年龄为130Ma(白宪洲等, 2012),潞西轩岗乡团坡糜棱岩化流纹岩锆石U-Pb年龄为121Ma(高永娟等, 2012),但对其空间分布、形成机制和构造背景有待进一步研究。为此,本文在野外专题填图的基础上,通过岩石学、岩石地球化学、同位素年代学和同位素示踪的方法,并结合前人对构造带内同时代侵入岩的研究成果,探讨早白垩世火山岩的时空间分布、岩浆来源及其形成的构造背景,为揭示怒江洋的演化及腾冲与保山地块的关系提供重要证据。

图 1 青藏高原及其东南缘地质概图(a, 据Xu et al., 2015; Qi et al., 2015)、高黎贡造山带地质简图(b,据Qi et al., 2019)和高黎贡造山带东南缘龙陵地质简图(c,据四川省地质调查院区调中心, 2010修编) BNS-班公湖-怒江缝合带;YTS-雅鲁藏布江缝合带;MK-密支那缝合带;LCJF-龙川江断裂;LLRF-泸水-龙陵-瑞丽断裂 Fig. 1 Geological map of the Tibet area and its southeastern stretch (a, after Xu et al., 2015; Qi et al., 2015), Gaoligong Orogen (b, after Qi et al., 2019) and Longling region in the southeastern margin of Gaoligong belt (c) BNS-Banggong-Nujiang Suture; YTS-Yarlung-Tsangpo Suture; MK-Myitkyina Suture; LCJF-Longchuanjiang Fault; LLRF-Lushui-longling-Ruili Fault

① 四川省地质调查院区调中心. 2010. 1/50000龙陵县幅地质图

1 区域地质背景

滇西高黎贡构造带总体呈南北向,北起察隅,经贡山、泸水至龙陵、潞西转为南西向,经瑞丽延至缅甸境内被Sagaing断裂带所截,全长约500km,宽10~20km(图 1a),并与缅甸Mogok构造带相连(Socquet and Pubellier, 2005; Searle et al., 2007; Xu et al., 2012; Eroğlu et al., 2013; Ridd, 2015),构成腾冲地块与保山地块的边界。构造带主体由早古生代高级变质岩、晚古生代浅变质岩和古生代至新生代岩浆岩组成,并在新生代块体旋转挤出过程中叠加右行韧性剪切变形(Tapponnier and Molnar, 1976;钟大赉等, 1991;季建清, 2000;Wang et al., 2006; Song et al., 2010; Xu et al., 2015; Zhang et al., 2017;戚学祥等, 2019),岩石都发生不同程度糜棱岩化,形成花岗质、长英质和角闪质糜棱岩及糜棱岩化片麻岩、片岩和大理岩。构造带东南缘沿龙陵-潞西-瑞丽断裂走向方向分布有超镁铁质岩、硅质岩、深水浊积岩和夹有早白垩世流纹岩层或块体的弧前碎屑沉积岩组成的俯冲增生杂岩带(Xie et al., 2016; Qi et al., 2019)。其中,早白垩世流纹岩夹于浅海相碎屑沉积岩中,与侏罗纪复理石沉积建造一起构成杂岩带的一部分,在空间上两者之间呈断层接触(图 1c)。构造带西侧腾冲地块内出露地层为早古生代高黎贡群(Pz1G)深变质岩系和晚古生代碎屑岩系等(戚学祥等, 2019);构造带东侧主要出露古生界蒲满哨群(PzPm)浅变质沉积岩,局部夹有碳酸盐岩和火山岩(钟大赉等, 1991;钟大赉, 1998)。

2 岩相学特征

龙陵-瑞丽火山岩带主要分布于龙陵-潞西-遮放一带,为一系列规模不等的火山岩层或火山岩透镜体夹于浅海相砂岩、粉砂岩和泥岩这些沉积岩中(图 1b)。其中,在龙陵县城西的火山岩出露规模最大,长约9km,宽约1km,面积约9km2,呈北东向展布;其他火山岩零星分布于芒市-河头一带,出露面积0.5~1.2km2(图 1c)。流纹岩呈灰色-灰白色、斑状结构、块状构造、流纹构造(图 2)。斑晶主要为石英(10%~15%)、钾长石(3%~5%)和白云母(1%~2%),其中,石英多被溶蚀,呈不规则港湾状(图 2f),部分为棱角状的石英晶屑(图 2d),粒度在0.2~0.8mm之间;钾长石为自形-半自形板柱状,0.1~0.6mm,表面具有弱高岭土化(图 2d, f);基质(80%~85%)主要为隐晶质-微晶质(图 2b, d, f);其次含有少量黄铁矿、磁铁矿和锆石等副矿物(1%)。

图 2 高黎贡东南缘流纹岩野外露头和镜下照片 (a、b)龙陵-腾冲剖面;(c、d)龙陵-河头剖面;(e、f)芒市-河头剖面. Kfs-钾长石;Q-石英;Ms-白云母 Fig. 2 Outcrop and photomicrographs of rhyolites from the southeastern margin of Gaoligong Orogen (a, b) from Longling-Tengchong section; (c, d) from Longling-Hetou section; (e, f) from Mangshi-Hetou section. Mineral abbreviations: Kfs-K-feldspar; Q-quartz; Ms-muscovite
3 样品采集及测试方法

样品17QLT-6、16QLH-6和17QLH-8采自龙陵西层状流纹岩,样品16QMH-2采自芒市-河头之间的透镜状流纹岩,采样位置见图 1c

锆石分选在河北省地质调查研究院完成。样品经常规的粉碎和重选,分选出纯度较高的锆石,然后在双目镜下经人工挑选出纯度在99%以上的锆石样品。用环氧树脂将锆石样品和标样固定成圆饼状,用不同型号砂纸和磨料将锆石磨去一半并抛光。然后,在北京离子探针中心对抛光好的锆石进行阴极发光成像观察,查明锆石内部生长层的分布和结构。在此基础上,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用GeoLas 2005 ArF准分子激光剥蚀系统(LA)和Agilent 7500a四级杆质谱(ICP-MS)进行锆石U-Pb同位素定年和锆石成分测试。其中,激光波长193nm,能量密度14J/cm2,频率8Hz,光斑直径为32μm。锆石U-Pb年龄测定采用国际标准锆石91500作为外标校正,以29Si(锆石中SiO2的含量为32.18%)作为内标,测定锆石中U、Th和Pb的含量(Hu et al., 2012)。每测定3~5个点后插入一次标样测定,以便及时校正。采用ICPMSDataCal(V3.7)软件对同位素比值数据进行处理,详细的仪器操作条件和数据处理方法见Liu et al.(2008, 2010)。使用ISOPLOT程序(Ludwig, 2003)进行锆石加权平均年龄计算及谐和图的绘制。

锆石Hf同位素测试分析在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采用LA-MC-ICP-MS(Neptune Plus)完成。激光剥蚀系统为美国Coherent公司生产的GeoLasPro 193nm。实验过程中采用He作为剥蚀物质载气,根据锆石大小,剥蚀直径采用44μm,测定时使用锆石国际标样GJ-1作为参考物质。相关仪器运行条件及详细分析流程见侯可军等(2007)。分析过程中锆石标准GJ-1的176Hf/177Hf测试加权平均值分别为0.282007±0.000025(2σ)。计算初始176Hf/177Hf时,Lu的衰变常数采用1.865×10-11y-1(Scherer et al., 2001),εHf(t)值的计算时采用球粒陨石Hf同位素值176Lu/177Hf=0.0336,176Hf/177Hf=0.282785(Bouvier et al., 2008)。在Hf的地幔模式年龄计算中,亏损地幔176Hf/177Hf现在值采用0.28325,176Lu/177Hf采用0.0384(Griffin et al., 2000),地壳模式年龄计算时采用平均地壳的176Lu/177Hf=0.015(Griffin et al., 2002)。

全岩主量和微量元素化学成分分析在国家地质实验测试中心完成。主量元素采用XRF(X-ray fluorescence)方法进行测定,分析精度优于5%。微量元素采用等离子质谱仪ICP-MS(Inductively Coupled Plasma Mass Spectrometry)方法进行测定,含量大于10×10-6的元素测试精度为5%,而小于10×10-6的元素测试精度为10%。

4 全岩地球化学特征

全岩主量和微量元素数据(表 1)表明研究区内流纹岩SiO2含量较高,除样品16QLH-2为73.69%外,其余都分布在75.76%~81.48%之间,其重要特点是K2O(3.35%~4.54%)和Al2O3(11.87%~16.47%)含量相对稳定,Na2O(0.01%~0.05%)、CaO(0.05%~0.09%)含量非常低,与镜下观察到的岩石中斑晶主要为钾长石和石英,几乎未见斜长石一致。岩石的A/CNK值为2.84~3.78,远高于正常酸性岩类,与Na2O和CaO含量低有关,因此该指数没有实际指示意义,但岩石中富铝矿物白云母的存在,仍反映其过铝质性质。在Nb/Y-Zr/TiO2关系图上,样品落在流纹岩区(图 3a),在Th-Co关系图(图 3b)上,流纹岩样品落在高钾钙碱性系列区,说明研究区内流纹岩属于高硅、低钠钙的高钾钙碱性岩类。在哈克图解上,Al2O3、K2O、FeOT和TiO2含量与SiO2呈负相关(图 4a-e),暗示了可能存在钾长石、镁铁质矿物和铁-钛氧化物的结晶分异。

表 1 滇西龙陵-瑞丽流纹岩的常量元素(wt%)、稀土元素和微量元素(×10-6) Table 1 Chemical compositions of major (wt%), rare earth elements and trace elements (×10-6) for rhyolite from Longling-Ruili belt, western Yunnan

图 3 流纹岩Zr/TiO2-Nb/Y关系图(a,据Winchester and Floyd, 1977)和Th-Co关系图(b,据Hastie et al., 2007) Fig. 3 Zr/TiO2 vs. Nb/Y diagram (a, after Winchester and Floyd, 1977) and Th vs. Co diagram (b, after Hastie et al., 2007) for the rhyolitic rocks

图 4 高黎贡造山带东南缘流纹岩哈克图解 Fig. 4 Harker plots of selected major and trace elements of the rhyolites in the southeastern margin of Gaoligong Orogen

流纹岩的∑REE变化于182.3×10-6~265.4×10-6,LREE/HREE值为5.3~10.0,(La/Yb)N值为6.12~12.62,(La/Sm)N和(Gd/Yb)N分别在4.49~8.69和1.31~1.71之间,Eu/Eu*值为0.49~0.60(图 5b),展示出轻稀土富集、分馏程度较高、重稀土相对亏损、Eu明显负异常的特点。在原始地幔标准化微量元素蛛网图上,流纹岩样品富集大离子亲石元素(Th、U和K)、轻稀土元素,高场强元素(Nb、Ta、P和Ti)和Sr强烈负异常(表 1图 5a),其中的Sr强烈负异常与岩石中Ca或斜长石含量很低有关。

图 5 流纹岩原始地幔标准化微量元素蛛网图(a)和球粒陨石标准化稀土元素模式图(b)(标准化值据Sun and McDonough, 1989) 上、中和下地壳数据引自Rudnick and Gao, 2003;高黎贡早白垩世花岗质岩石数据引自Qi et al., 2019 Fig. 5 Primitive mantle-normalized trace element spider diagrams (a) and chondrite-normalized REE patterns (b) (normalization values after Sun and McDonough, 1989) Upper, middle and lower continental crust values are from Rudnick and Gao, 2003; The data of Cretaceous granitic rocks are from Qi et al., 2019
5 锆石特征及同位素组成 5.1 锆石形态结构和LA-ICP-MS U-Pb定年

为了全面厘定龙陵-瑞丽带中流纹岩的形成时代,笔者选择了芒市-河头剖面(16QMH-2)、龙陵-河头(16QLH-6、17QLH-8)和龙陵-腾冲(17QLT-6)3个剖面中出露的流纹岩样品进行锆石U-Pb定年(图 1c)。3个剖面流纹岩中锆石特征基本一致,均呈自形短柱-长柱状,长约50~150μm,长宽比约1:1~3:1,无色透明-弱淡黄色,锆石晶面整洁光滑。阴极发光图像(图 6)清晰显示出锆石具有典型的岩浆振荡环带,未见新生变质锆石边。根据亮度可分为灰白色和灰黑色二类,这与锆石的Th、U含量有关,但测试所得的206Pb/238U年龄值基本一致(表 2),反映出锆石结晶过程中岩浆内Th、U分布不均一的特点。四组样品锆石的U、Th含量变化较大,分别分布于151×10-6~1213×10-6和153×10-6~1053×10-6之间(表 2),与阴极发光图像相吻合。锆石的Th/U比值均大于0.3,典型的韵律环带亦反映其为岩浆成因(Corfu et al., 2003; Hoskin and Schaltegger, 2003;吴元保和郑永飞, 2004)。

图 6 代表性锆石阴极发光图像 实圈为LA-ICP-MS U-Pb年龄分析点位置,虚线圈为Hf同位素分析点位置 Fig. 6 Representative cathodoluminescence (CL) images of zircon grains from the rhyolites Solid circles indicate the spots of LA-ICP-MS U-Pb dating, dashed circles indicate the spots of Hf isotope analyses

表 2 滇西龙陵-瑞丽流纹岩锆石LA-ICP-MS U-Pb定年数据 Table 2 Zircon LA-ICP-MS U-Pb dating data for rhyolite from Longling-Ruili belt, western Yunnan

锆石LA-ICP-MS定年结果表明,16QMH-2号样品中锆石存在Pb丢失或均一化,致使其部分测点206Pb/238U年龄值远离协和线(图 7a,虚线圈),存在Pb丢失现象;第20号测点206Pb/238U年龄值远大于其他测点值(图 6),可能为混入锆石,因此这些测点未参与年龄加权平均计算。剩余13个测点锆石的206Pb/238U加权平均年龄为129±2.3Ma(MSWD=2.4)(图 7a)。16QLH-6样品中20个测点锆石206Pb/238U年龄分布在119~133Ma之间,除9、13和25号测点误差较大外,其他17颗锆石206Pb/238U加权平均年龄为128±1.8Ma(MSWD=3.9)(图 7b)。17QLH-8样品中20颗锆石206Pb/238U年龄集中分布在120~128Ma之间,206Pb/238U加权平均年龄为126±1.3Ma(MSWD=2.3)(图 7c)。17QLT-6样品中20颗锆石206Pb/238U年龄集中分布在119~128Ma之间,206Pb/238U加权平均年龄为123±1.3Ma(MSWD=2.9)(图 7d)。总体来看,4件样品锆石U-Pb年龄基本一致,集中在123~129Ma之间(图 8a),都在误差范围内。测点都在锆石韵律环带发育区域,其Th/U比值都大于0.3,具有典型的岩浆锆石特征,其LA-ICP-MS锆石U-Pb年龄反映了锆石的结晶年龄,代表流纹岩的喷发时代。

图 7 流纹岩锆石U-Pb年龄谐和图 Fig. 7 U-Pb concordia diagrams for zircons from the rhyolites

图 8 锆石U-Pb年龄、εHf(t)值和Hf同位素二阶段模式年龄柱状图 Fig. 8 Histograms of U-Pb ages (a), εHf(t) values (b) and Hf model ages (c) of zircons from the rhyolites
5.2 锆石Lu-Hf同位素组成

样品16QLH-6、16QMH-2和17QLH-8的Lu-Hf同位素在锆石U-Pb定年的同一颗锆石的相同或其相邻部位测定(图 6),结果见表 3176Hf/177Hf初始比值和εHf(t)值根据同一锆石U-Pb定年数据计算;二阶段模式年龄(tDMC)根据亏损幔源计算(Griffin et al., 2000)。测定结果表明,3个样品共47颗锆石分析点获得的同位素176Hf/177Hf初始比值相对稳定,分别介于0.282391~0.282516、0.282349~0.282496和0.282408~0.282512之间。样品16QLH-6的εHf(t)值为-10.8~-6.3之间,平均为-8.3±0.7(图 8b),显示出-9和-7两个峰值,对应的二阶段模式年龄范围为1591~1869Ma;样品16QMH-2的εHf(t)值为-12.3~-7.0之间,平均为-9.6±0.8(图 8c),峰值为-10,对应的二阶段模式年龄范围为1631~1966Ma;样品17QLH-8的εHf(t)值为-10.2~-6.5之间,平均为-8.3±1.0(图 8d),峰值为-8,对应的二阶段模式年龄为1596~1824Ma。

表 3 锆石LA-ICP-MS Lu-Hf同位素组成 Table 3 Zircon LA-ICP-MS Lu-Hf isotope data
6 讨论 6.1 岩浆成因及演化

研究区流纹岩具有高硅(>73%),高Rb/Sr(29~66)值和高分异指数(DI=85~90),低Zr/Hf(18~26,高分异花岗岩/流纹岩 < 25, Breiter et al., 2014),低Cr(0.97×10-6~2.93×10-6)、Co(0.42×10-6~3.64×10-6)和Ni(0.68×10-6~2.61×10-6)的特点;在哈克图解上,主量元素和微量元素整体随SiO2含量的升高而降低(图 4);且出现过铝质矿物白云母(图 2b),表明岩石经历了高程度的结晶分异,为高分异过铝质流纹岩,与高分异S型花岗岩类似(Miller, 1985; Chappell et al., 2012)。

过铝质长英质火成岩主要有两种可能成因:(1)镁铁质岩浆在下地壳条件下发生部分熔融高度结晶分异可产生高硅富铝岩浆(Li et al., 2007; Ronga et al., 2010; Chappell et al., 2012);(2)变沉积岩,如富粘土的变泥质岩和贫粘土的变质砂岩部分熔融的产物(Patiño Douce and Johnston, 1991; Nabelek and Glascock, 1995; Sylvester, 1998; Clemens, 2003)。

强不相容元素比值是岩浆源区特征的良好指示(Barbarin, 1999; Frost et al., 2001, 2016),研究区流纹岩具有较低的Nb/Ta比值(7.6~11.7),与大陆地壳比值相近(6, Wedepohl et al., 1991; 11~12, Taylor and McLennan, 1985; Green, 1995; Rudnick and Fountain, 1995; 12~13, Barth et al., 2000);其Th(15.5×10-6~34.1×10-6)>10×10-6、U(1.29×10-6~3.85×10-6,平均为2.83×10-6)、Th/U比值(6.9~13.9)和Eu/Eu*比值(0.5~0.6)与中上地壳(Rudnick and Fountain, 1995; Rudnick and Gao, 2003)相当。流纹岩显示出与中上地壳相似的微量元素蛛网图和稀土元素配分曲线(图 5)。在molar[Al2O3/(MgO+FeOT)]-molar[CaO/(MgO+FeOT)]和Rb/Ba-Rb/Sr图解中,所有样品点都落入变泥质岩部分熔融区域(图 9a, b)。以及,流纹岩锆石具有较低的176Hf/177Hf初始比值、εHf(t)值(-12.3~-6.3),Hf二阶段模式年龄集中于1600~1800Ma(图 8),表明流纹岩岩浆来源于中上地壳物质部分熔融。

图 9 流纹岩molar[Al2O3/(MgO+FeOT)]-molar[CaO/(MgO+FeOT)]图解(a, 据Altherr et al., 2000)和Rb/Ba-Rb/Sr图解(b, 据Sylvester, 1998) Fig. 9 Molar Al2O3/(MgO+FeOT) vs. molar CaO/(MgO+FeOT) (a, after Altherr et al., 2000) and Rb/Ba vs. Rb/Sr (b, after Sylvester, 1998) diagrams for the rhyolites

研究成果表明诱发壳源物质部分熔融形成酸性岩浆主要有3种原因:1)地壳增厚加压升温(Harris et al., 1986; Sylvester, 1998)或地壳快速伸展减薄引起的等温降压(Guillot and LeFord, 1995;戚学祥等, 2008);2)高热幔源岩浆上升到地壳中(Xu et al., 2012; Qi et al. 2019);3)地壳中生热元素的作用(Bolhar et al., 2008)。锆石是花岗质岩浆中结晶较早的矿物,岩浆中Zr的分配系数对温度极为敏感,现有的岩石薄片中没有发现金红石矿物,但流纹样品中Zr、Ti和Al2O3含量均随着SiO2含量的升高而降低(图 4i),表明岩体中存在有锆石和金红石作为独立矿物相存在。根据Watson et al. (2006)提出的锆石Ti含量温度计公式计算得到,样品中锆石结晶温度为568~778℃,平均为717℃,说明它们是在相对较高的温度条件下结晶的。尚无证据表明研究区在早白垩世地壳发生明显增厚,亦无淡色花岗岩类岩石出露,因此不支持大陆地壳自身提供足够热源使古老地壳物质发生深熔或重熔作用。同时,高黎贡构造带内无放射性生热元素异常,流纹岩中锆石Th、U含量分别在97×10-6~1053×10-6和127×10-6~1213×10-6,都在正常范围内。流纹岩中Th、U含量分别为15.5×10-6~34.1×10-6和1.29×10-6~3.85×10-6,略高于上地壳平均值(Th=10.5×10-6, U=2.7×10-6),因此由生热元素引起壳源物质部分熔融形成流纹岩岩浆的可能性也可以排除。前人对高黎贡构造带内早白垩世侵入岩进行了广泛的研究,相继发现了同时代岩浆主要来源于幔源组分的中性岩类,如辉长闪长岩体(图 1bεHf(t)=+1.2~+5.4,Qi et al., 2019)、闪长岩体(εHf(t)=+3.6~+6.2,丛峰等, 2011),并认为大部分花岗岩类岩浆来源于混染了不同比例幔源物质的壳源岩浆(εHf(t)=-13.9~+2.9,丛峰等, 2010, 2011; Qi et al., 2019),以及由幔源岩浆上升过程中诱发壳源物质部分融熔形成的S型花岗岩(Xu et al., 2012; Qi et al., 2019)。鉴于此,我们认为本区流纹岩岩浆为壳源物质部分熔融形成的,幔源物质在流纹岩形成过程中提供了热源。

在La-La/Sm关系图(图 10a)上,样品沿部分熔融线和分离结晶线均有分布,说明流纹岩岩浆演化受到了部分熔融和分离结晶作用的双重控制。斜长石的分离结晶导致Eu和Sr的亏损,而钾长石的分离结晶造成Eu和Ba亏损;流纹岩样品中Sr含量极低且趋于稳定,Ba含量则明显降低,反映出岩浆演化过程中主要是钾长石发生分离结晶(图 10b),Eu亏损主要是先期源区贫斜长石和后期钾长石分离结晶造成的。ΣREE与SiO2呈明显负相关(图 4f)以及Ti、P、Nb和Ta负异常暗示了岩石可能经历了含钛矿物(钛铁矿和金红石等)、磷灰石和锆石等高分配系数矿物较高程度的分离结晶(图 10c)。由此可见,龙陵-瑞丽一带早白垩世流纹岩是古老地壳物质部分熔融生成的熔体经历了分离结晶的产物。

图 10 早白垩世流纹岩La-La/Sm关系图(a)、Eu/Eu*-Ba关系图(b,据Eby, 1990)和La-(La/Yb)N关系图(c,据Wu et al., 2003) Pl-斜长石;AF-碱性长石;Zr-锆石;Sph-榍石;Ap-磷灰石;Mon-独居石;Allan-褐帘石 Fig. 10 Diagrams of La vs. La/Sm (a), Eu/Eu* vs. Ba (b, after Eby, 1990) and La vs. (La/Yb)N (c, after Wu et al., 2003) for the Early Cretaceous rhyolites Pl-plagioclase; AF-alkali feldspar; Zr-zircon; Sph-titanite; Ap-apatite; Mon-monazite; Allan-allanite
6.2 低钠流纹岩成因

钾质交代作用、热液蚀变和风化作用会导致长英质火山岩富集K2O、亏损Na2O(Irfan, 1999; Ennis et al., 2000)。低温钾质交代作用是富钠矿物(主要是斜长石)分解后形成冰长石为主的次生矿物,造成岩石富集K2O、Rb、Ba和亏损Na2O、CaO、Sr(Ennis et al., 2000)。高温钾化作用通常由碱性花岗质岩浆在侵位过程中或就位后蚀变而成,蚀变过程中伴随着碱性长石斑晶中出溶钠长石和钾长石交代石英斑晶现象(Taylor et al., 1984)。本文测试样品的Na2O和CaO含量极低,但岩石结构构造和矿物关系保存良好,未见钾化交代矿物等其他蚀变现象(图 2d-f),且主量元素和微量元素显示出较好的线性关系(图 4),表明所研究的流纹岩成分受后期钾化作用和热液蚀变的影响甚微。

流纹岩具有较高的Al2O3和Rb(163×10-6~218×10-6)含量、Eu负异常(0.49~0.60)和Ba含量(273×10-6~644×10-6),极低的Sr、Na2O和CaO含量(表 1)。低钠可能是岩浆源区的固有特征,是沉积岩型原岩源区的反映。实验岩石学研究表明贫Sr过铝质花岗岩/流纹岩岩浆可以由变泥质岩部分熔融生成,而泥质岩的部分熔融首先是白云母和黑云母的部分熔融,当流体压力足够高时,斜长石发生部分熔融(Patiño Douce and Johnston, 1991; Patiño Douce and Harris, 1998; Frost et al., 2016)。如富云母贫斜长石的变泥质岩发生无水脱水熔融时可产生富硅贫钠钙的酸性熔体和富石榴子石、夕线石的残余体(Patiño Douce and Johnston, 1991; Patiño Douce, 1996)。此外,壳源花岗质岩浆的Rb-Sr系统关系可反映部分熔融类型和源岩性质(Knesel and Davidson, 2002; Zeng et al., 2011),在变泥质岩中,Sr和Rb的主要寄主矿物分别是长石和云母,与长石相比,云母具有较高的Rb/Sr比值。研究区流纹岩具有较高的Rb/Sr值(29~66),低Sr含量,且随着Rb含量的增加Sr明显无明显变化,亦佐证了源区主要为云母类矿物的发生部分熔融。在此基础上,熔体发生前文所述钾长石、磷灰石和锆石等的分离结晶作用。

综上所述,低钠流纹岩可能是先期由富云母贫斜长石类变泥质岩部分熔融产生的贫钠、钙熔体,后期经历高度的分离结晶形成。

6.3 构造意义

高黎贡造山带内早白垩世岩浆岩呈带状分布,岩石组合主要为闪长岩、花岗闪长岩和花岗岩,其高钾钙碱性、Nb、Ta负异常、变化较大的εHf(t)值(-12.3~+7.1)和内含同时代暗色包体等特征揭示其形成于与班公湖-怒江洋向拉萨-腾冲地块俯冲有关的活动大陆边缘弧背景,岩浆为洋壳俯冲诱发富集地幔物质部分融熔形成的幔源岩浆与壳源岩浆按不同比例混合的产物(杨启军等, 2006;丛峰等, 2010, 2011;戚学祥等, 2011; Cao et al., 2014; Zhu et al., 2015; 2017; Xie et al., 2016; Zhang et al., 2018; Qi et al., 2019)。研究区内流纹岩构成高黎贡早白垩世岩浆岩带的一部分,夹于下白垩统海相碎屑沉积岩中,构成俯冲增生杂岩带的一部分,并为上白垩统陆相碎屑沉积岩不整合覆盖,其高钾钙碱性属性及大离子亲石元素富集、高场强元素(Nb、Ta、P、Ti)及Sr负异常特征(图 5a)和Hf同位素特征(图 11)与构造带内同时代侵入岩相似,在Th/Ta-Yb和La/Yb-Th/Yb构造环境判别图解上所有样品都落在活动大陆边缘区域(图 12),以及如上节所述,研究区内流纹岩岩浆是由幔源岩浆上升过程中诱发壳源物质部分融熔的产物,且幔源岩浆的形成与怒江洋俯冲有关(Qi et al., 2019),表明其形成构造背景与高黎贡构造带内同时代侵入岩一致,形成于与怒江洋向腾冲地块下俯冲有关的活动大陆边缘弧环境。

图 11 高黎贡造山带东南缘早白垩世流纹岩的锆石εHf(t)与U-Pb年龄图 高黎贡花岗岩类、察隅岩体、班怒带花岗岩类数据来自Zhu et al., 2009a, b, 2016; Xie et al., 2016; Qi et al., 2019 Fig. 11 Plot of εHf(t) vs. U-Pb ages for the Early Cretaceous rhyolites in the southeastern margin of Gaoligong Orogen Data for the granitic rocks of Gaolinggong, Chayu pluton and Banggong-Nujiang belt form Zhu et al., 2009a, b, 2016; Xie et al., 2016; Qi et al., 2019

图 12 高黎贡造山带东南缘早白垩世流纹岩Th/Ta-Yb(a, 据Gorton and Schandl, 2000)和La/Yb-Th/Yb图解(b,据Harris et al., 1986) ACM-活动大陆边缘;MORB-洋中脊玄武岩;WPB-板内玄武岩;WPVZ-板内火山岩;OA-洋内弧 Fig. 12 Th/Ta-Yb (a, after Gorton and Schandl, 2000) and La/Yb-Th/Yb (b, after Harris et al., 1986) diagrams for Early Cretaceous rhyolitic rocks in the southeastern margin of Gaoligong Orogen ACM-Active continental margin; MORB-Mid-ocean basalt; WPB-Within-plate basalt; WPVZ-Within-plate volcanic zone; OA-Oceanic arcs

高黎贡东南缘的龙陵-瑞丽带为一套俯冲增生杂岩带,其岩块主要包含蛇纹岩块、硅质岩块和玄武岩块等,基质主要由含硅质岩、硅质泥岩夹层的复理石沉积建造构成深海-半深海相混杂岩和夹有早白垩世流纹岩层或透镜体的浅海-半深海相碎屑岩组成(刘本培等, 2002; Qi et al., 2019)。前者反映腾冲地块与保山地块间存在洋壳俯冲作用,后者具有明显的弧前或弧间盆地沉积的特点,进一步说明龙陵-瑞丽流纹岩带是高黎贡早白垩世岩浆岩带的一部分,形成于与怒江洋向腾冲地块下俯冲有关的活动大陆边缘弧环境,上白垩统陆相沉积岩不整合覆盖在这套增生杂岩带之上标志着俯冲作用的结束。

鉴于此,我们认为龙陵-瑞丽流纹岩带与同时代侵入岩带一致,形成于怒江洋向腾冲地块下俯冲的大陆边缘弧,即洋壳向下俯冲过程中诱发地幔楔物质部分熔融形成基性岩浆(Kepezhinskas et al., 1996; Iwamori, 1998; Nakamura and Iwamori, 2009)。这些高热的幔源岩浆上升过程中促使壳源物质(富云母贫斜长石的变沉积岩类)部分熔融形成酸性岩浆,经岩浆房结晶分异后沿构造薄弱带喷出到弧前或弧间盆地内,并被碎屑沉积物覆盖(图 13)。

图 13 高黎贡造山带东南缘早白垩世岩浆活动示意图 Fig. 13 Schematic tectonic model of the origin of Early Cretaceous magmatism in the southeastern margin of Gaoligong Orogen
7 结论

(1) 滇西龙陵-瑞丽流纹岩为低钠、高分异的高钾钙碱性流纹岩。锆石LA-ICP-MS U-Pb定年结果表明流纹岩的喷发时代为123~129Ma。

(2) 流纹岩中无斜长石斑晶,具有负的εHf(t)值(-12.3~-6.3)、与大陆中上地壳相似的Nb/Ta和Th/U比值,在岩石成因判别图解中,所有样品点都落入变泥质岩部分熔融区域,表明流纹岩岩浆来源于壳源物质部分熔融的产物,其低钠含量可能是源区中斜长石含量低造成的。

(3) 流纹岩地球化学特征与高黎贡构造带内同时代形成于大陆边缘弧环境的侵入岩一致,并夹于海相碎屑沉积岩中构成俯冲增生杂岩带的一部分,表明其形成于与俯冲有关的大陆边缘弧环境,喷发在弧前或弧间盆地中。

致谢      孟繁聪研究员和蔡明海教授认真审阅了初稿并提出了中肯的修改意见;野外工作得到了云南省地质勘查局王臣新高级工程师的大力帮助;中国地质大学(武汉)地质过程与矿产资源国家重点实验室胡兆初教授协助完成锆石U-Pb定年和Hf同位素测试;在此一并表示衷心感谢!

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