岩石学报  2022, Vol. 38 Issue (1): 253-266, doi: 10.18654/1000-0569/2022.01.16   PDF    
腾冲地块晚白垩世-古近纪富锡花岗岩成因: 岩浆源区及分异演化条件
崔晓琳1, 张琦玮2, 吴华英3, 宋志杰1, 张慧娟1, 戴荔果1     
1. 东华理工大学地球科学学院, 南昌 330013;
2. 中国地质大学地质过程与矿产资源国家重点实验室, 北京 10008;
3. 中国冶金地质总局矿产资源研究院, 北京 100025
摘要: 腾冲地块锡成矿作用主要与晚白垩世-古近纪岩浆活动相关, 但仅矿区花岗岩体具有较高的锡含量(平均25×10-6), 区域上同时代的非成矿花岗岩体并未发生锡的富集。本文通过搜集分析现有研究数据, 总结了富锡成矿岩体和区域非成矿岩体在岩浆源区、演化条件和结晶分异程度的异同: 区域非成矿岩体锆石εHf(t)值(-15.1~+3.39)表明, 自东北向西南幔源物质加入有升高的趋势, 但区域玄武岩中低含量的Sn(1.61×10-6)表明幔源物质混入不利于岩浆中锡的富集。晚白垩世部分非成矿岩体与成矿岩体具有相同εHf(t)(-9.7)值, 表明其皆源于古老地壳物质的部分熔融, 但岩体均表现为准铝质-弱过铝质特征, 且锆石Hf同位素(tDM2=1724Ma)和全岩Nd同位素(tDM2=1836Ma)二阶段模式年龄基本一致, 因此其源区可能并非富锡的高黎贡山群变质沉积岩, 而可能是其中未经风化的变质花岗岩。根据腾冲地块地层厚度(28km)和莫霍面深度(47~35km)推断岩浆源区至少位于地下30km(8.4kbar), 由于仅靠地温梯度(25℃/km)无法达到初始熔融温度(>1066℃), 源区部分熔融过程很可能受地幔热的影响。根据Fe2O3/FeO比值, 非成矿岩体(0.59)与成矿岩体(0.48)均具有较低的氧逸度, 属钛铁矿系列, 但成矿岩体的结晶分异程度明显高于非成矿岩体, 且成矿岩体富含挥发分, 高含量的挥发分降低了岩浆固结温度(650~550℃), 延长了结晶分异时间, 促进了锡在晚期岩浆中的富集。因此腾冲地块富锡花岗岩主要是普通岩浆在低氧逸度环境下发生高度结晶分异的结果。
关键词: 腾冲地块    锡矿床    富锡花岗岩    岩浆源区    演化条件    结晶分异    
Formation of the Late Cretaceous-Paleogene Sn-rich granites in the Tengchong block: The magma origin and differentiation conditions
CUI XiaoLin1, ZHANG QiWei2, WU HuaYing3, SONG ZhiJie1, ZHANG HuiJuan1, DAI LiGuo1     
1. College of Earth Sciences, East China University of Technology, Nanchang 330013, China;
2. State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 10008;
3. Institute of Mineral Resources Research, China Metallurgical Geology Bureau, Beijing 100025, China
Abstract: Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25×10-6 on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon εHf(t) values (-15.1~+3.39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1.61×10-6) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same εHf(t) values of -9.7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (tDM2) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma), the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8.4kbar) based on the regional strata thickness (28km) and the Moho depth (47~35km). The initial melting temperature (1066℃) is unable to be reached according to geothermal gradient (25℃/km), therefore, the partial melting process is likely influenced by mantle heat. According to Fe2O3/FeO values, both the unmineralized granitoids (0.59) and the tin mineralized granitoids (0.48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650~550℃), prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.
Key words: Tengchong block    Tin deposit    Sn-rich granitoids    Magma origin    Evolution condition    Fractionation    

锡(Sn)是一种关键金属元素,也是我国的传统优势矿产资源(Mao et al., 2018; 翟明国等, 2019),原生锡矿床,如锡石-硫化物型、石英脉型、云英岩型和矽卡岩型,通常和同生花岗岩有着密切的成因联系(陈郑辉等, 2015; 夏庆霖等, 2018)。与锡矿化相关的同生花岗岩普遍发生锡的富集,其锡含量可达10n×10-6甚至100n×10-6,富锡花岗岩也通常被认为是成矿元素Sn的直接来源(Lehmann, 1990; 毛景文等, 1991, 2009; Audt'at et al., 2000)。目前,关于富锡花岗岩的生成途径,主要有两种解释:(1)花岗岩起源于富Sn源区(Schuiling, 1967; 毛景文等, 1991; Romer and Kroner, 2015, 2016);(2)普通花岗岩经历了高度结晶分异过程(Groves and McCarthy, 1978; Ishihara et al., 1979; Štemprok, 1990)。此外,受控于Sn元素本身的地球化学性质(隋清霖等,2020),源区发生部分熔融的温度、压力以及分异演化过程中熔体氧逸度和挥发分也是影响锡富集程度的关键因素(Ishihara, 1977; 蒋少涌等,2006; Mao et al., 2013; Wolf et al., 2018)。

腾冲地块锡矿产资源丰富,锡成矿作用与晚白垩世-古近纪岩浆活动密切相关(Hou et al., 2007; 邓军等, 2011, 2013, 2016),目前已知锡矿床(点)达70多处,其中大型矿床6处,中型矿床9处,是东南亚巨型锡矿带的北延部分(Hall, 2012; Zaw et al., 2014; Gardiner et al., 2015)(图 1a)。腾冲地块与锡矿化相关的花岗岩中锡含量平均为25×10-6,最高达100×10-6以上(毛景文等, 1991; 曹华文等, 2013),与上地壳(Sn的平均丰度为2.1×10-6)和普通花岗岩(Sn的平均丰度为3×10-6)(Lehmann, 1990; Rudnick and Gao, 2003)相比,锡发生了明显的富集,然而区域上同时代形成的非成矿花岗岩却并未发生锡的富集(毛景文等,1991)。锡在花岗岩体中的初步富集是同生热液锡矿床形成的必备前提,前人对腾冲地块富锡花岗岩成因的研究较少,且均强调富锡地层对富锡花岗岩形成的重要意义,例如张士鲁(1986)基于成矿岩体多侵位于富锡的石炭系勐洪群,认为初始岩浆并不富锡,侵位后岩浆富锡是由于汲取了地层中的Sn;毛景文(1988)认为成矿岩体与勐洪群之间并无明显的接触交代关系,因此否定了勐洪群是矿源层这一说法,提出富锡岩浆起源于前寒武纪高黎贡群的部分熔融,高黎贡山群中锡元素的含量是地壳丰度值的2~4倍,富锡花岗岩可能是高黎贡山群经重熔和分异作用的结果。近年来,关于腾冲地块晚白垩世-古近纪花岗岩体的研究主要集中在岩石成因类型、岩浆源区和区域构造背景的探讨(江彪等, 2012; 马楠等, 2013; 张玙等, 2013; Chen et al., 2014, 2015; Cao et al., 2016, 2017; Wu et al., 2019),有关锡的物质来源及在岩浆中的富集机制的研究则十分匮乏。本文在前人研究的基础上,对腾冲地块晚白垩世-古近纪富锡成矿花岗岩体和同期非成矿岩体的元素地球化学及Nd-Hf同位素数据进行了系统整理和分析,总结了富锡花岗岩体和非成矿岩体在岩浆源区、结晶分异程度和岩浆演化条件方面(温压条件、氧逸度和挥发分等)的差异,结合锡的地球化学性质,初步探讨了腾冲地块富锡花岗岩成因。

图 1 东南亚地区特提斯构造域地质简图(a,据Lehmann and Mahawat, 1989; Li et al., 2016)及腾冲地块晚白垩世-古近纪岩浆岩和典型锡多金属矿床分布图(b,据Xie et al., 2016) Fig. 1 Simplified tectonic map of the Tethyan system in Southeast Asia (a, modified after Lehmann and Mahawat, 1989; Li et al., 2016) and Late Cretaceous-Paleogene granitoids and typical Sn-polymetallic ore deposits of the Tengchong block (b, modified after Xie et al., 2016)
1 区域地质特征

腾冲地块位于西南三江特提斯造山带西南端、东喜马拉雅构造结的南侧翼,东以怒江-瑞丽断裂带与保山地块为邻,西以密支那缝合带与西缅地块相接(Searle et al., 2007; Xu et al., 2012; Deng et al., 2014a, b, 2017)(图 1a)。区域上存在古元古界变质基底——高黎贡山群,是由黑云母质斜长变粒岩、斜长片麻岩、云母片岩、云母石英片岩以及各类岩石共同组成的杂岩体(李再会等, 2012Eroǧlu et al., 2013),集中分布于怒江-瑞丽断裂以及古永-腾冲断裂以西。志留系-泥盆系砂岩、上石炭统-下二叠统和白垩系沉积岩零星分布,第四纪火山岩少量发育。自中生代以来,随着中特提斯洋壳俯冲汇聚、陆块增生造山和新特提斯洋消减、印度-欧亚碰撞造山等事件的影响,腾冲地块发育强烈的岩浆活动,中新生代岩浆岩出露面积达50%(Deng et al., 2014b; 邓军等,2020; 王庆飞等,2020Wang et al., 2021),其中早白垩世花岗岩(128~108Ma)主要分布在槟榔江-大盈江断裂以东,晚白垩世-古近纪花岗岩侵入体(76~55Ma)主要分布在中部,古近纪(56~51Ma)花岗岩和辉长岩侵入体分布在古永-腾冲断裂以西。锡多金属矿化主要和中部的晚白垩世-古近纪岩浆活动相关,晚白垩世(ca.80~70Ma)主要发育云英岩型小龙河大型Sn矿床和新岐中型Sn-稀有金属矿床,以及百花脑大型稀有金属矿床(马楠等, 2013; Chen et al., 2014; Deng et al., 2014b; Cao et al., 2016);古近纪(ca.55~45Ma)主要发育云英岩型和石英-硫化物型Sn矿床,如来利山大型Sn矿床(Hou et al., 2007; Chen et al., 2014; Deng et al., 2014b金灿海等, 2013)(图 1b)。

2 典型锡矿床 2.1 晚白垩世小龙河Sn矿床

小龙河锡矿床锡的总储量为6.56万t,目前开采的锡矿石总量达2.62万t,品位在0.18%~0.42%之间。自西向东分为小龙河矿段、万旦山矿段、黄家山矿段和大松坡矿段四部分,矿区地层出露极少,石炭系空树河群变质沉积岩呈残盖状分布。矿区构造线和控矿断裂构造均呈近NS走向,矿体主要赋存在NNW向的次级断裂中(Cao et al., 2016; Cui et al., 2019)(图 2a)。前人采用多种测年方法对小龙河锡矿床成岩成矿年龄进行了精确限定,矿区黑云母二长花岗岩的定年结果(76.01~70.3Ma)和热液矿物云母(71.9~70.6Ma)、锡石(75.5~71.6Ma)的定年结果高度吻合(马楠等, 2013; Chen et al., 2014, 2015; Cao et al., 2016; Wu et al., 2019)(表 1),证明黑云母二长花岗岩即为小龙河锡矿床的成矿岩体。矿区黑云母二长花岗岩在区域上属于古永花岗岩基的一部分,以复式岩体形式产出,其中似斑状黑云母二长花岗岩分布在矿区西北部,中粗粒黑云母二长花岗岩位于矿区中部,出露面积最大,两者无明显接触界限,细粒黑云母二长花岗岩呈脉状侵入中粗粒黑云母二长花岗岩中;此外中粗粒黑云母二长花岗岩中常发育似伟晶结构的包体和细晶岩脉。矿区岩体普遍发生钠长石化、云英岩化、硅化和绿泥石化蚀变。矿体主要以脉状形式充填在雁行排列的密集断裂带内,少量呈层状、似层状产于岩体与围岩的接触带。矿石矿物主要为锡石,少量黑钨矿、黄铁矿和铌钽铁矿;脉石矿物主要为石英、黄玉、萤石、绢云母、金红石等。锡石呈自形、半自形粒状结构分布在浸染状、脉状和块状矿石中(Chen et al., 2015; Cao et al., 2016; Wu et al., 2019)。

图 2 晚白垩世小龙河锡矿床(a)和古近纪来利山锡矿床(b)地质简图(据Wu et al., 2019) Fig. 2 Schematic geologic maps of the Late Cretaceous Xiaolonghe Sn deposit (a) and the Paleogene Lailishan Sn deposit (b) (modified after Wu et al., 2019)

表 1 腾冲地块晚白垩世-古近纪锡矿床与成矿岩体形成年龄 Table 1 Ages of the Late Cretaceous-Paleogene Sn deposits and the related mineralized granites in Tengchong block
2.2 古近纪来利山Sn矿床

来利山Sn矿床锡的总储量为5.8万t,矿石平均品位为0.63%。自北向南由淘金处、三个硐、老熊窝三个主矿段和东侧的丝光坪矿段组成,出露地层同样为石炭系空树河群浅变质砂岩、板岩等。区域构造线方向以NE为主,次为NWW,矿体主要赋存在NE向的张扭性断裂中。矿区黑云母二长花岗岩沿NE向断裂侵位于石炭系地层中,同样呈复式岩体产出,由中粗粒黑云母二长花岗岩和似斑状黑云母二长花岗岩组成,岩体之间呈相变接触关系(图 2b) (金灿海等, 2013; Chen et al., 2014; Cao et al., 2017; Wu et al., 2019)。根据前人对来利山锡矿床成岩成矿时代所做的研究工作(表 1),矿区黑云母二长花岗岩的定年结果(51.31~50.2Ma)和热液矿物云母(50.4~ 48.4Ma)、锡石(47.4Ma)的定年结果基本一致,证明黑云母二长花岗岩即为来利山锡矿床的成矿岩体(Chen et al., 2014; 林进展等, 2015; Cao et al., 2017; 孙转荣等, 2017; Wu et al., 2019)。矿体主要发育于NE向角砾岩化破碎带及岩体与地层接触带中,呈透镜状、囊状、透镜状产出。近矿围岩蚀变包括云英岩化、黄铁矿化、硅化、绿泥石化、绿帘石化、高岭土化等。矿石类型主要为云英岩型和锡石-硫化物型(即角砾岩型),其中云英岩型发育于云英岩化花岗岩中,矿物组合为石英-萤石-白云母-锡石;锡石-硫化物型发育于角砾岩化破碎带内,角砾成分为长英质脉和云英岩,硫化物胶结,矿物组合主要为黄铁矿-黄玉-锡石,此外还包括磁黄铁矿、少量黄铜矿、磁铁矿、方铅矿、闪锌矿、辉铋矿等(金灿海等, 2013; Chen et al., 2014; Cao et al., 2017; Wu et al., 2019)。

3 晚白垩世-古近纪成矿岩体特征

腾冲地块与锡矿化相关的岩体主要集中在小龙河锡矿区和来利山锡矿区。晚白垩世小龙河成矿岩体和古近纪来利山成矿岩体均为黑云母二长花岗岩,经对比分析两者在矿物组成、主微量元素组成和Hf、Nd同位素组成方面高度相似(张玙等,2013Chen et al., 2015林进展等,2015Cao et al., 2016, 2017孙转荣等, 2017Wu et al., 2019)。在矿物组成上,主要矿物均为石英(20%~35%)、钾长石(30%~40%)、斜长石(20%~30%)和黑云母(5%~10%),副矿物包括锆石、磷灰石、金红石、钛铁矿等,在QAP图解上(图 3a),主要位于二长花岗岩区域内。元素地球化学研究显示,小龙河和来利山锡矿床的成矿岩体均具有高硅富碱特征,SiO2平均含量为73.72 %,碱质含量(Na2O+K2O)平均8.51 %;A/CNK平均值为普遍小于1.1,平均值为1.05,表现为弱过铝质特征(图 3b);K2O平均含量达5.17%,在SiO2-K2O图解上,均属于高钾钙碱性-钾玄岩过渡系列(图 3c);CaO(平均1.06%)、FeOT(平均1.74%)、MgO(平均0.27%)、TiO2(平均0.17%)及P2O5(平均0.05%)含量均较低,Mg#(=MgO/(MgO+FeOT))也较低,平均值为15.7;成矿岩体Fe2O3/FeO比值较低(平均0.48),均属于钛铁矿系列,在Fe2O3/FeO-SiO2图解上,落入Sn矿床成矿范围内(图 3d)。

图 3 腾冲地块晚白垩世-古近纪成矿岩体与非成矿岩体分类图 (a)QAP分类图(据Streckeisen, 1976);(b)A/CNK-A/NK图(据Maniar and Piccoli, 1989);(c)SiO2-K2O图(据Peccerillo and Taylor, 1976; Middlemost, 1985);(d)SiO2-Fe2O3/FeO图(据Wu et al., 2019).数据来源:张玙等, 2013高永娟等, 2014; Ma et al., 2014; Chen et al., 2015; 林进展等, 2015; Qi et al., 2015; 巫嘉德等, 2015; 周新平等, 2015; Cao et al., 2016, 2017; 孙转荣等, 2017; Wu et al., 2019; 图 4图 5数据来源同此图 Fig. 3 Classification diagrams of the Late Cretaceous-Paleogene mineralized and unmineralized granitoids in Tengchong block (a) QAP classification diagram (Streckeisen, 1976); (b) SiO2 vs. K2O plot (Peccerillo and Taylor, 1976; Middlemost, 1985); (c) A/CNK vs. A/NK plot (Maniar and Piccoli, 1989); (d) SiO2 vs. Fe2O3/FeO plot (Wu et al., 2019). Data sources: Zhang et al., 2013; Gao et al., 2014; Ma et al., 2014; Chen et al., 2015; Lin et al., 2015; Qi et al., 2015; Wu et al., 2015, 2019; Zhou et al., 2015; Cao et al., 2016, 2017; Sun et al., 2017; also in Fig. 4 and Fig. 5

微量元素数据显示,小龙河和来利山锡矿床的成矿岩体均具有高分异花岗岩特征,例如两者均强烈富集大离子亲石元素(LILE)Rb、Th、U、K和高场强元素Nb、Ta,相对亏损Ba、Sr、P、Eu和Ti(图 4a, c),稀土元素配分模式显示明显的四分组效应(图 4b, d)。Zr/Hf比值范围较大13.9~71.3,平均值为33.5,Rb/Sr比值(平均36.76)高,同时岩浆分异指数(DI)(平均91.10)高,在Rb-Ba-Sr高分异花岗岩判别图解中(图 5),大部分样品集中在高分异花岗岩区域内;根据全岩Zr含量,由锆饱和温度计得出(Watson and Harrison, 1983),成矿岩体岩浆初始结晶温度范围为737~1066℃, 平均849℃。

图 4 腾冲地块晚白垩世-古近纪成矿岩体与非成矿岩体原始地幔标准化微量元素蛛网图(a、c, 标准化值据Sun and McDonough, 1989)和球粒陨石标准化稀土元素配分曲线图(b、d, 标准化值据Boynton, 1984) Fig. 4 Primitive mantle-normalized trace element patterns (a, c, normalization values after Sun and McDonough, 1989) and chondrite-normalized REE patterns (b, d, normalization values after Boynton, 1984) of the Late Cretaceous-Paleogene mineralized and unmineralized granitoids in Tengchong block

图 5 腾冲地块晚白垩世-古近纪成矿岩体与非成矿岩体Rb-Ba-Sr分异程度判别图 Fig. 5 Discrimination diagram for the differentiation degree of the Late Cretaceous-Paleogene mineralized and unmineralized granitoids in Tengchong block

小龙河和来利山锡矿区成矿岩体中锆石Hf同位素、全岩Nd同位素的测试结果基本一致。锆石Hf同位素组成显示,εHf(t)数据比较集中(-11.5~-7.5),平均值为-9.7,对应二阶段模式年龄tDM2(Ma)=1847~1562Ma, 平均值为1724Ma(图 6a)(Chen et al., 2015Cao et al., 2016);εNd (t)集中在-12.4~-11.4之间(Chen et al., 2015),平均值为-11.9(图 6b),对应的二段模式年龄tDM2(Ma)=1884~1789Ma, 平均值为1836Ma, 与锆石Hf同位素计算结果基本一致。

图 6 腾冲地块晚白垩世-古近纪成矿岩体与非成矿岩体锆石εHf(t)-年龄图(a)及全岩εNd(t)-(87Sr/86Sr)i图(b)(据徐容等, 2018) 数据来源:图a据Xu et al., 2012; 高永娟等, 2014; Chen et al., 2015; 林进展等, 2015; Qi et al., 2015; Cao et al., 2016, 2017; 图b据Chen et al., 2015 Fig. 6 Plots of zircon εHf(t) vs. ages (a) and whole-rock εNd(t) vs. (87Sr/86Sr)i (b) for the Late Cretaceous-Paleogene mineralized and unmineralized granitoids in Tengchong block (after Xu et al., 2018) Data sources: Xu et al., 2012; Gao et al., 2014, 2016, 2017; Chen et al., 2015; Lin et al., 2015; Qi et al., 2015 in Fig. 6a; Chen et al., 2015 in Fig. 6b
4 晚白垩世-古近纪非成矿岩体特征

晚白垩世古永岩基由北向南主体由三岔河岩体(65Ma)、古永岩体(76~72Ma)、侯桥岩体(69~65Ma)、磁洲坝岩体(68~65Ma)、老坪岩体(76.6Ma)、新岐岩体(76~73Ma)、绮罗岩体(73.2Ma)和象陀山岩体(65.0Ma)组成(Xu et al., 2012; Chen et al., 2015; Qi et al., 2015; 周新平等, 2015)。岩体出露面积在16km2(绮罗岩体)~160km2(磁州坝岩体),均以黑云母二长花岗岩为主,除侯桥岩体呈粗粒结构,其它岩体均呈粗粒似斑状结构。与同时期的小龙河锡矿床成矿岩体相比,两者在矿物组成上基本相同,但在元素组成上有明显差异。主量元素数据显示,非成矿岩体同样表现为高硅富碱的二长花岗岩特征(图 3a),属于高钾钙碱性-钾玄岩系列过渡系列(图 3c),但其SiO2含量(平均72.8%)和碱质含量(Na2O+K2O平均7.89%)均比同时期成矿岩体低。除绮罗岩体表现出强过铝质的特征外(A/CNK=1.14~1.17),其他岩体均表现为准铝质-弱过铝质特征(A/CNK平均值为1.0)(图 3b)。根据Fe2O3/FeO比值, 侯桥(0.50)、磁州坝(0.75)、绮罗(0.51)、象驼山岩体(0.18)氧逸度较低,属钛铁矿系列,三岔河岩体(1.16)和老坪岩体(1.35)氧逸度较高,属磁铁矿系列(Qi et al., 2015; 周新平等, 2015)(图 3d)。微量元素数据显示,晚白垩世非成矿岩体的结晶分异程度比同期小龙河锡矿床成矿岩体低,具体表现在,大离子亲石元素(LILE)Rb、K、Th、U、Zr、Hf和高场强元素Nb、Ta的富集程度较低(图 4a, c),稀土总量较低,整体表现为轻稀土富集的右倾模式(ΣLREE/ΣHREE平均为10.3),Eu负异常较弱(δEu平均0.42)(图 4b, d)。Zr/Hf比值范围较集中25.2~43.1,平均值为30.9,和未经锆石结晶分异的普通花岗岩类似,Rb/Sr比值(0.67~11.18,平均值为3.19)较小,同时岩浆分异指数(DI平均值为86.3)较低,在Rb-Ba-Sr高分异花岗岩判别图解中(图 5),大部分样品集中在普通花岗岩区域内,少量为高分异花岗岩。根据全岩Zr含量,由锆饱和温度计得出(Watson and Harrison, 1983),晚白垩世非成矿岩体岩浆初始结晶温度范围为674~986℃,平均813℃。

古近纪非成矿岩体主体分部主要由西盈江地区那邦-铜壁关岩体(56~52Ma)组成(Xu et al., 2012; Ma et al., 2014)。近几年又有许多小型古近纪侵入岩体,在勐连、小塘岩体中发现(高永娟等, 2014巫嘉德等,2015)。西盈江地区岩体则由花岗闪长岩、片麻状花岗岩和二长岩组成,主微量元素成分变化较大,SiO2含量62.71%~78.68%,K2O含量1.62%~7.40%,整体表现为高钾钙碱性,弱过铝质花岗岩。微量元素特征与晚白垩世非成矿岩体类似(图 4a, c),岩浆分异指数(DI平均值为84.2)低,稀土总量低且元素分布具有“海鸥型”的特征(ΣLREE/ΣHREE平均为12.6),Eu负异常较弱(δEu平均0.57)(图 4b, d),表明古近纪非成矿岩体结晶分异程度较低(图 5)。根据全岩Zr含量,由锆饱和温度计得出(Watson and Harrison, 1983),古近纪非成矿岩体岩浆初始结晶温度范围为674~865℃,平均779℃。

晚白垩世-古近纪非成矿岩体εHf(t) 值分布范围较大(-15.1~+3.39),其中晚白垩世古永岩体(平均值-9.7),侯桥岩体(平均值-9.6)、磁州坝岩体(平均值-9.7)、绮罗岩体(平均值-9.7)的εHf(t)值与成矿岩体类似,晚白垩世老坪岩体(平均值-7.5)、三岔河(平均值-7.4, Qi et al., 2015)以及古近纪腾梁地区岩体(平均值-6.9,高永娟等,2014),铜壁关岩体(平均值-7.6, Xu et al., 2012),εHf(t)值均比成矿岩体略高,西部那帮岩体(~53Ma)出现正的εHf(t)值(平均1.52)(图 6a)。从空间分布上看,从东北到西南,εHf(t) 表现出逐渐升高的趋势,反映岩浆源区地幔物质加入量有逐渐增多的趋势(Xu et al., 2012)。

5 讨论 5.1 岩浆源区

根据成矿岩体锆石Hf同位素和全岩Nd同位素数据,晚白垩世-古近纪成矿岩体的εNd(t) 平均值为-11.9,对应二段模式年龄(tDM2)为1836Ma(Chen et al., 2015),锆石εHf(t)平均值为-9.7,对应二阶段模式年龄为1724Ma(Chen et al., 2015林进展等, 2015Cao et al., 2016);表明矿区成矿岩体源于古老地壳物质的重熔。因此,成矿元素锡很可能也来自古老地壳物质。毛景文(1988)认为,腾冲地块古元古代高黎贡山群为成矿岩体的岩浆源区,不仅因为成矿岩体Nd同位素和锆石Hf同位素的二阶段模式年龄计算结果与之相符,更因为高黎贡群地层中的锡含量较高,其中锡、铷、锂的含量比地壳克拉克值高出2~4倍,局部片岩、片麻岩Sn含量可分别高达24.2×10-6和32.7×10-6(毛景文, 1988; 毛景文等, 1991)。

这一结论看似合理,但却与成矿岩体的岩石地球化学特征不符。成矿岩体具有高分异花岗岩特征,但其铝饱和指数却偏低,A/CNK平均值平均值仅为1.05,基本呈现准铝质-弱过铝质特征(图 3b),而高黎贡山群是一套中深变质沉积岩系,恢复后原岩主要为泥质砂岩与粉砂质泥岩、钙质砂岩夹灰岩和少量中基性火山岩、火山碎屑岩(李静等, 2008)。当高黎贡山群发生部分熔融时很可能产生过铝质岩浆,而在过铝质岩浆的分异演化过程中,铝不饱和矿物(如角闪石、黑云母、长石、石英等主要造岩矿物)发生结晶分离会导致残余岩浆中的铝含量会越来越高,而白云母、堇青石、刚玉等过铝质矿物只有在过铝质岩浆中才能发生结晶分离,且这些过铝质矿物的结晶分离不会改变岩浆的过铝质性质。因此,在没有外界物质混入的条件下,无论过铝质岩浆是否发生高分异演化,都将形成过铝质花岗岩。而成矿岩体和大部分非成矿岩体均显示准铝制或弱过铝质特征,因此其源区可能并非高黎贡山群变质沉积岩。此外,成矿岩体的锆石Hf同位素(1724Ma)和全岩Nd同位素(1836Ma)计算所得的二阶段模式年龄基本一致,这表明源区岩石自形成之后始终保持相对封闭状态,即在发生部分熔融前,源区岩石未遭受过强烈的化学风化作用,在部分熔融过程中又无幔源物质加入或沉积岩混染。而高黎贡山群变质沉积岩作为强烈风化作用后的产物,风化过程中大量的矿物分解和物质流失会导致原岩Sm-Nd及Lu-Hf同位素体系的破坏,因此由高黎贡山变质沉积岩部分熔融形成的花岗岩其Hf同位素、Nd同位素二阶段模式年龄结果无法耦合。由此也可推断腾冲地块锡矿床成矿岩体的岩浆源区并非高黎贡山群变质沉积岩。

李静, 张虎, 曾庆荣, 胡绍斌, 彭程, 赵云江, 余赛赢, 邓仁宏, 赵志芳, 包家凤. 2008. 腾冲县幅和潞西市幅1/25万区域地质调查报告. 昆明: 云南省地质调查院

高黎贡山群厚逾15000m, 且尚未见底, 在局部地区逐渐过渡到片麻状花岗岩(毛景文, 1988),其中斜长角闪岩和花岗片麻岩Nd模式年龄分别为1094~840Ma和2717~2218Ma(钟大赉,1998)。在区域上,高黎贡山群向西南延入缅甸Mogok变质岩系,近年来,越来越多的高精度年代学研究表明高黎贡山岩群和缅甸境内的Mogok岩群实际上是由498Ma、136Ma、76Ma、53Ma、20Ma的花岗岩、糜棱岩、花岗片麻岩组成的杂岩体(Barley et al., 2003; Searle et al., 2007; 李再会等, 2012; Eroǧlu et al., 2013)。结合成矿岩体的准铝质-弱过铝质特征和Hf、Nd同位素计算的二阶段模式年龄,我们认为腾冲地块成矿岩体更可能起源于高黎贡山群中变质花岗岩体的部分熔融。

根据区域非成矿岩体εHf(t)值,晚白垩世古永岩体(平均-9.7),侯桥岩体(平均-9.6)、磁州坝岩体(平均-9.7)、绮罗岩体(平均-9.7)与成矿岩体(平均-9.7)类似,但仅绮罗岩体表现出强过铝质特征(A/CNK=1.14~1.17),其他岩体均表现为准铝质-弱过铝质特征,因此推断绮罗岩体的岩浆源区可能为泥质沉积岩,也可能是岩浆侵位过程中受到了泥质沉积岩的同化混染,而非成矿岩体(古永、侯桥、磁州坝岩体)与成矿岩体具有相同的岩浆源区。晚白垩世其他非成矿岩体如老坪岩体(平均-7.5)、三岔河(平均-7.4)以及西部的古近纪腾梁地区岩体(平均-6.9),铜壁关岩体(平均-7.6)的εHf(t)值均比成矿岩体高,至腾冲地块西端那帮岩体(~53Ma)出现正的εHf(t)值(平均+1.52)。空间上自东向西、年龄上从老到新,展现出εHf(t)值逐渐增大的趋势,可能表明源区发生部分熔融过程中幔源物质加入量有逐渐升高的趋势(Xu et al., 2012)。腾冲地块地层中玄武岩的锡含量仅为1.61×10-6(毛景文等,1991),低于上地壳Sn的平均丰度(2.1×10-6),所以幔源物质的加入不利于富锡花岗岩的形成。

5.2 结晶分异程度

相比于区域非成矿花岗岩体,腾冲地块富锡成矿岩体显示明显高分异特征,例如矿区岩体均呈复式岩体产出,古近纪来利山岩体由中粗粒黑云母二长花岗岩和似斑状黑云母二长花岗岩组成,晚白垩世小龙河矿区成矿岩体包括似斑状、中粗粒和细粒黑云母二长花岗岩,此外中粗粒黑云母二长花岗岩中常发育似伟晶结构的包体和细晶岩脉,矿物中含有锂白云母、电气石、萤石等富含挥发分的矿物,这些均是岩浆发生高度结晶分异的显著岩相学标志(吴福元等,2017);在元素组成上,成矿岩体通常高硅、富碱,贫镁铁,富集大离子亲石元素和高场强元素(K、Rb、Th、U、Zr、Hf),亏损Sr、Ba、P、Ti、Eu(图 4a, c),高Rb/Sr比值(图 5)、高岩浆分异指数、且稀土元素呈现明显的四分组效应(图 4b, d)。据研究,稀土元素的四分组效应是富含挥发分(F、Cl、B等)的残余岩浆在结晶分异晚期出溶流体和熔体发生相互作用的结果(吴福元等,2017)。

岩浆分异演化作用是锡在岩浆中发生富集的重要途径(Lehmann, 1990; Blevin and Chappell, 1992; 陈骏等, 2000; Gomes and Neiva, 2002; Chen et al., 2013),在岩浆分异演化过程中,矿物的结晶分离直接影响残余熔体中Sn的含量,例如,锡在石英、钾长石和斜长石中的分配系数很小,这些矿物的结晶分离将导致残余熔体中锡的富集(陈骏等, 2000; 隋清霖等,2020);锡在角闪石、黑云母和Fe-Ti副矿物中的分配系数很高,这些矿物结晶分离会使残余熔体中锡含量下降(Lehmann, 1990; 陈骏等, 2000; 隋清霖等,2020)。通过岩浆的高分异演化过程,岩浆中的锡含量可以富集到平均地壳的10倍以上(陈骏等, 2000)。因此,在无富锡源区的背景下,腾冲地块富锡花岗岩的形成更可能是普通岩浆发生高分异作用的结果,但受控于锡本身的地球化学性质,岩浆结晶分异过程中温压条件、氧逸度和挥发分对岩浆中锡的富集程度有重要影响(Ishihara, 1977; Ishihara et al., 1979; 蒋少涌等, 2006; Mao et al., 2013; Wolf et al., 2018)。鉴于此,尽管区域非成矿岩体如老坪岩体(Qi et al., 2015)和古近纪腾梁地区花岗岩(高永娟等, 2014)也具有高分异花岗岩的特征,但老坪岩体(Fe2O3/FeO=1.35)氧逸度较高,属磁铁矿系列,而古近纪腾梁地区花岗岩在源区部分熔融时有幔源物质混入,这些均不利于锡在晚期岩浆中发生富集。

5.3 岩浆演化条件 5.3.1 温压条件

温压条件控制着源区含锡矿物的分解,Sn在岩体中多以类质同象的方式存在于含Fe、Ti、Nb、Ta的矿物中,如角闪石、黑云母,磁铁矿、金红石、榍石、钛铁矿、铌钽铁矿等(Linnen and Cuney, 2005; Farges et al., 2006; 王汝成等, 2008)。只有含锡矿物发生分解,Sn才能顺利进入熔体中。据研究,腾冲地区莫霍面的深度在47~35km之间,且从东北到西南有变浅的趋势。在腾冲地区, 除去地表未出露的寒武系、奥陶系、志留系以外, 从白垩系到前寒武系, 累计厚度约26km, 高黎贡群厚15000m,且尚未见底。由于我们推测成矿岩体的岩浆源于高黎贡山群中的变质花岗岩体,因此,岩浆起源深度可能位于地下30km。若按照岩石密度为2.8g/cm3计算,30km处的净岩压力为8.2kbar,此时黑云母的最低分解温度为860℃(图 7)。由全岩Zr饱和温度计(Watson and Harrison, 1983)得出的成矿岩体的初始固结温度最高达1066℃,若按照地壳平均地热梯度为25℃/km计算,在地温梯度影响下30km的地下仅能达到750℃,此温度无法使黑云母的分解,更离1066℃的初始固结温度相去甚远,因此源区部分熔融过程很可能受到地幔热的影响。

图 7 腾冲地块晚白垩世-古近纪成矿岩体源区部分熔融相图(据wolf et al., 2018) 绿线代表黑云母分解线;橙线代表固相线,阴影区有熔体出现;黄星代表腾冲地块锡成矿岩体源区黑云母分解的最小温压条件 Fig. 7 Equilibrium phase diagram for partial melting of the Late Cretaceous-Paleogene mineralized granitoids in Tengchong block (after wolf et al., 2018) The green line denotes biotite-out line. The orange line represents the solidus and shaded fields contain melt. The yellow star stands for the least pressure and corresponding temperature of biotite decomposition in the magma origin of the Sn mineralized granitoids in Tengchong block

腾冲地块晚白垩世-古近纪的岩浆活动形成于新特提斯洋俯冲的构造背景下。新特提斯洋的演化始于中侏罗世,至晚白垩世时期,新特提斯洋由低角度俯冲转变成高角度俯冲,这为软流圈上涌提供了通道,同时导致弧后地壳持续加厚和隆升,当隆升至顶点时引发加厚地壳的伸展垮塌,在弧后拉张的环境下,地壳发生减压熔融形成晚白垩世岩浆活动(Xu et al., 2012; Chen et al., 2015; Cao et al., 2016; Wu et al., 2019);到古近纪时期,俯冲板片回撤和断离导致软流圈上涌加速了地壳物质熔融,并形成了古近纪花岗岩(Chen et al., 2014; Cao et al., 2017; Wu et al., 2019)。在上述岩浆形成过程中,均可能受地幔热的影响。在岩浆分异演化过程中挥发分大量聚集,促进了岩浆结晶分异作用形成高分异花岗岩。

5.3.2 氧逸度

氧逸度对源区含锡矿物的分解和岩浆分离结晶过程中锡的富集程度均有重要影响。实验证明,黑云母、榍石、磁铁矿等含锡矿物稳定性随氧逸度升高而增加,因此高氧逸度不利于Sn从含锡矿物中迁出,从而会导致熔体中亏损Sn (Wolf et al., 2018; 隋清霖等, 2020)。此外,当氧逸度较高时,岩浆中的锡主要以Sn4+形式存在(Linnen et al., 1995)。由于Sn4+(0.061nm)的离子半径与Ti4+(0.061nm)、Nb5+(0.064nm)、Ta5+(0.064nm)、Fe3+(0.065nm)十分相近,在结晶分异过程中Sn4+容易以类质同象的方式进入早期结晶的含铁、钛、铌、钽的矿物(如角闪石、黑云母、磁铁矿、钛铁矿、榍石、金红石等)中(Linnen and Cuney, 2005; Farges et al., 2006; 王汝成等, 2008),因此不易在晚期熔体中发生富集; 而当氧逸度较低时,则Sn在岩浆中主要以Sn2+形式存在,由于Sn2+离子半径偏大(0.093nm)很难进入矿物晶格,因此易于在晚期熔体中发生富集(Blevin and Chappel, 1992; Farges et al., 2006; Wang et al., 2017)。

腾冲地块小龙河和来利山成矿花岗岩中磁铁矿较少出现,常见钛铁矿,且Fe2O3/FeO比值(平均值0.48)较低,属于钛铁矿系列,在Fe2O3/FeO-SiO2图解上,均落入典型Sn矿床区内(图 3d),表明岩浆氧逸度较低,有利于源区含锡矿物的分解和迁出以及岩浆结晶分异过程中锡的富集。区域非成矿岩体中,老坪(Fe2O3/FeO=1.35)和三岔河岩体(Fe2O3/FeO=1.16)均属磁铁矿系列(图 3d),其较高的岩浆氧逸度可能是锡在岩浆中未发生富集的重要原因。

5.3.3 挥发分

腾冲地块小龙河和来利山矿区的成矿岩体中常发育锂铁云母、萤石、黄玉、锂电气石、绿柱石等矿物,表明岩浆分异过程中存在大量的挥发分(Li、B、F等)。挥发分元素是岩浆发生高分异演化的促进剂。实验研究表明,熔体中加入F可导致熔体粘度呈指数降低,且熔体降低粘度的效应随着SiO2含量的增加而增强,岩浆粘度降低可有效促进矿物的结晶分离(Thomas et al., 2000; 张德会等, 2004);当体系F含量达到6%时,钠长花岗岩的固相线温度会下降至475℃(Manning and Henderson, 1984),固相线温度下降延长了岩浆寿命,为岩浆的结晶分异作用提供了充分的时间(Webster et al., 1997)。毛景文(1988)根据包裹体测定、二长石温度计和Ab-Or-Q实验相图初步确定腾冲地区富锡成矿岩体的最低固结温度范围为650~550℃, 侵位深度为4~1km,而岩浆源区至少在30km的下地壳,部分熔融温度至少在1066℃。在挥发分元素的影响下,岩浆在经历了如此长距离的侵位和漫长的演化过程后才逐渐冷却固结,在此过程中石英、钾长石和斜长石等造岩矿物的大量结晶分离会使锡在残余岩浆中的浓度升高,因此挥发分通过延长岩浆结晶分异过程,间接促进了锡在熔体中的富集。

6 结论

(1) 腾冲地块晚白垩世-古近纪富锡成矿岩体可能源于高黎贡山群中未遭受风化作用的变质花岗岩体的部分熔融。

(2) 成矿岩体源区至少位于地下30km处,部分熔融过程中很可能受地幔热源的影响;幔源物质锡含量低,非成矿岩体中幔源物质的混入不利于锡的富集。

(3) 成矿岩体属钛铁矿系列,富含挥发分,且结晶分异程度高,成矿岩体中锡的富集主要是低氧逸度条件下岩浆发生高分异演化的结果。

致谢      本文野外地质资料得到云南省腾冲县金山地矿科技服务有限责任公司肖常先工程师、陈国相工程师、黄体庄工程师、严大炳书记、舒家良经理以及其他工作人员的帮助,在此感谢各位的支持;感谢邓军教授和王庆飞教授对本文的指导;感谢两位匿名评审人对本文详细而全面的审阅,他们的意见使本文的质量得到很大提升。

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