岩石学报  2013, Vol. 29 Issue (11): 3901-3913   PDF    
滇西保山地块中-新生代岩浆作用及其构造意义
董美玲1,2, 董国臣1, 莫宣学1, 朱弟成1, 聂飞1, 于峻川1, 王鹏1, 罗微1     
1. 地质过程与矿产资源国家重点实验室, 中国地质大学地球科学与资源学院, 北京 100083;
2. 国土资源实物地质资料中心,燕郊 101149
摘要: 保山地块上的中-新生代岩浆活动以蚌渺岩体和桦桃林岩体为代表,蚌渺岩体岩石类型为石英二长岩、二长花岗岩,锆石U-Pb年龄为83~85Ma,桦桃林岩体为二云母花岗岩,锆石U-Pb年龄为60~66 Ma,表明该地区在晚白垩世和古新世发生了两次不同的岩浆作用。蚌渺岩体和桦桃林岩体都以全碱含量高为特征,K2O/Na2O>1,桦桃林岩体属于高钾钙碱性系列,蚌渺岩体更加倾向于钾玄岩系列。它们的铝饱和指数A/CNK的值均大于1.1,属铝过饱和系列。这两个岩体岩石总体上富集大离子亲石元素和Pb,亏损高场强元素。蚌渺岩体较桦桃林岩体明显富集轻稀土元素[(La/Yb)N=65.2~67.2],而桦桃林岩体较蚌渺岩体显示明显的负Eu异常(δEu=0.17~0.35)。2个岩体中3件样品41个测点的锆石εHf(t)值变化范围较大(-15.8~-1.1),蚌渺岩体和桦桃林岩体的Hf同位素地壳模式年龄分别集中于2.0~1.2Ga和2.0~1.5Ga。全岩锆饱和温度蚌渺高于桦桃林,说明这两个岩体源区岩石的熔融程度不同。这些地球化学特征指示中-新生代花岗岩类为S型花岗岩,主要来源于古元古代-中元古代地壳物质的重熔,并不同程度地混入了幔源物质。保山地块、腾冲地块和拉萨地体在中-新生代存在几乎同期的岩浆活动,但保山地块与拉萨地体由于岩石地球化学特征不同,不存在对比性,而与腾冲地块存在共同点。保山地块与腾冲地块上的中-新生代岩浆作用与新特提斯洋的闭合和印欧大陆的碰撞息息相关。
关键词: 中-新生代花岗岩类     地球化学     锆石Hf同位素     保山地块     滇西    
The Mesozoic-Cenozoic magmatism in Baoshan Block, western Yunnan and its tectonic significance
DONG MeiLing1,2, DONG GuoChen1, MO XuanXue1, ZHU DiCheng1, NIE Fei1, YU JunChuan1, WANG Peng1, LUO Wei1     
1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China;
2. Cores and Samples Center of Land and Resources, Yanjiao 101149, China
Abstract: The Bangmiao and Huataolin plutons, located in Baoshan Block of the Tethyan belt, western Yunnan, are representative of the Mesozoic-Cenozoic magmation. The Bangmiao pluton consists mainly of quartz monzonite and monzogranite with zircon U-Pb ages of 83~85Ma. The Huataolin two-mica granite pluton was dated at 60~66Ma by zircon U-Pb method. These ages indicate that there were two magmatic events during the Late Cretaceous and Paleocene. The two plutons are high-K calc-alkaline and peraluminous, with K2O/Na2O>1 and A/CNK>1.1, but the Bangmiao pluton is apt to shoshonitic. These rocks are enriched in large-ion lithophile elements (LILEs) and lead, and depleted in high field-strength elements (HFSEs). The Bangmiao pluton is obviously enriched in light rare-earth elements (LREEs) [(La/Yb)N=65.2~67.2] in comparison with the Huataolin pluton. However, the Huataolin shows evident negative Eu anomalies (δEu=0.17~0.35). 41 analyses from three samples of the two plutons give a wide range zircon εHf(t) values (-15.8~-1.1) and varying Hf-isotope crustal model ages (2.0~1.2Ga and 2.0~1.5Ga). The Bangmiao pluton has higher zircon saturation temperatures of the whole rock than the Haotaolin pluton, which indicate that the molten degree of anatexis in the magma source is different. Such geochemical signatures indicate that the Mesozoic-Cenozoic granitoids are S-type granite in character and were derived mainly from anatexis of Paleoproterozoic to Mesoproterozoic crustal materials with varying contributions from mantle-derived components. The Mesozoic-Cenozoic granitoids in Baoshan Block are geochemically comparable to that in Tengchong Block, which can be related to the closure of the Neotethys ocean and the collision between the India and Eurasia, however, are incomparable to the nearly coeval rocks reported in Lhasa Terrane.
Key words: Mesozoic-Cenozoic granitoids     Geochemistry     Zircon Hf isotope     Baoshan Block     Western Yunnan    
1 引言

许多地质学者普遍认为,在冈瓦纳大陆和劳亚大陆最后的碰撞之前,至少有两个大洋盆闭合,即古特提斯洋和新特提斯洋,其中部分学者甚至提出中特提斯洋(Metcalfe, 1996)的存在。晚古生代以来,在劳亚和冈瓦纳这两个超大陆之间不断地有微小陆块,如Cimmerian地体(Sengǒr et al., 1988)、Sibumasu地体和拉萨地体等(Wang et al., 2001; Jin, 2002),从冈瓦纳北部裂离出来,增生到位于北部的古亚洲地体,构成了全球规模特提斯造山带。而滇西特提斯构造带是东特提斯造山带的重要组成部分之一,该地区的大地构造格局和演化对全球特提斯研究具有重要的地质意义,因此,长期以来成为众多研究者的关注热点。滇西特提斯构造带是由多个地块和地块相间的构造带组成的,如腾冲地块、保山地块、思茅地块和昌宁-孟连结合带等(钟大赉等, 1998)。经受晚古生代-早中生代古特提斯和中生代-新生代新特提斯造山作用的影响,这些微陆块岩浆活动频繁。拉萨地体和腾冲地块的中-新生代岩浆作用已被广泛研究,但保山地块相对薄弱。作者对出露于保山地块蚌渺-桦桃林地区的花岗岩类开展了野外调研,本文目的是报道蚌渺-桦桃林花岗岩类锆石U-Pb年代学、锆石Hf同位素数据和全岩元素地球化学数据,约束其岩浆源区,并探讨其构造意义。

2 地质背景和样品

滇西保山地块在大地构造上位于藏-滇-泰-马中间板块中段,向南与缅甸掸邦地块相连,东与昌宁-孟连结合带以柯街-南定河断裂为界,西与腾冲地块以泸水-潞西-瑞丽断裂为界,北部在碧江一带由于澜沧江断裂和怒江断裂汇拢而消失(图 1a)。最老地层是震旦系-寒武系公养河群,在晚寒武世至中侏罗世期间,保山地块以滨海台地沉积为主,缺失上石炭统、上二叠统和下三叠统,其他地层间均为假整合关系。下二叠世发育冈瓦纳相杂砾岩,早二叠世至中侏罗世发育多期大陆火山岩。晚古生代的生物群具有亲冈瓦纳特征。表明该地块是处于稳定的构造环境并与冈瓦纳大陆有亲缘关系(钟大赉, 1998Metcalfe, 1996; Ueno, 2000)。保山地块岩浆活动主要以前寒武纪末期、早古生代、中生代晚期和新生代为主(图 1b),前寒武纪末期以西盟老街子花岗岩体为代表,规模较小,以岩株为主,其Rb-Sr等时线年龄为687Ma(李文昌等, 2010),同时在潞西志本山地区发育二云母花岗岩,其Rb-Sr等时线年龄为645Ma(张玉泉等, 1990);早古生代以平河岩体为代表,它是保山地块出露面积最大的花岗岩体,其锆石U-Pb年龄为466~502Ma(Chen et al., 2007; Liu et al., 2009; 董美玲等, 2012);中生代晚期和新生代岩浆呈小岩株零星产出,前者同位素年龄约80~100Ma(Chen et al., 2007; 金世昌和庄凤良, 1988)。

图 1 滇西保山地块构造地质简图(a, 据Shen et al., 2002修改)和保山地块中-新生代花岗岩类的地质简图(b, 据云南省地质局, 1965修改) Fig. 1 Simplified tectonic map of the Baoshan Block in western Yunnan Province (a, modified after Shen et al., 2002) and simplified geological map of the Mesozoic-Cenozoic granitoids in Baoshan Block (b)

①云南省地质局. 1965. 1:20万潞西幅地质图

中-新生代的岩浆活动集中于龙陵-潞西地区,出露面积约180km2,呈岩株产出。这些岩株侵位于下古生界-中寒武统公养河群类复理石和砂页岩建造(陈国达等, 2004),泥盆纪、志留纪的灰岩、粉砂岩、页岩和早古生代平河花岗岩基,并将平河岩基分成南北两部分(图 1b)。中生代晚期岩性为石英二长岩,二长花岗岩,岩石整体灰白色,中粗粒状,似斑状结构(图 2a);斑晶多为钾长石和斜长石,少量角闪石和黑云母(图 2b);斜长石自形-半自形板状,可见聚片双晶和卡纳复合双晶,钾长石具细微格子状双晶,微弱的泥化,角闪石绿色,可见两组呈菱形的解理,对称消光,黑云母褐色,片状,石英呈它形粒状(图 2c);副矿物为锆石、钛铁矿和磁铁矿等。新生代岩性为二云母花岗岩,灰白色,中细粒状,花岗结构(图 2d);主要矿物为钾长石、斜长石、石英,次要矿物为黑云母和白云母(图 2e);斜长石呈半自形板状,双晶纹不清晰,钾长石半自形-他形粒状,含量可达45%以上,具卡式双晶,微弱的碳酸盐化,片状黑云母和白云母含量相当,白云母自形程度高,石英呈他形粒状,个别具波状消光(图 2f);副矿物组合为锆石+磷灰石+榍石+磁铁矿,可见特征富铝矿物-石榴子石(图 2e, f)。

图 2 中-新生代花岗岩类标本及显微镜下特征 Fig. 2 Rock photograph and microphotographs of the Mesozoic-Cenozoic granitoids in Baoshan Block
3 分析方法

主量元素在核工业北京地质研究院采用XRF法在6735仪器上分析完成,分析精度优于5%。微量元素在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)利用Agilent 7500a ICP-MS 分析完成。详细的测试方法和分析流程参见Liu et al. (2008a)的文章。分析结果见表 1

表 1 中-新生代花岗岩类的全岩主量元素(wt%)和微量元素(×10-6)分析结果 Table 1 Whole-rock major (wt%) and trace (×10-6) elemental data of the Mesozoic-Cenozoic granitoids

锆石阴极发光图像在中国地质科学院完成。锆石U-Pb同位素定年在中国地质大学(武汉)地质过程与矿产资源国家重点实验室利用LA-ICP-MS分析完成。测试仪器为Agilent 7500a,激光剥蚀系统为GeoLas 2005。激光剥蚀斑束直径为32μm,激光剥蚀深度为20~40μm。对分析数据的离线处理采用软件ICPMSDataCal完成。详细的仪器操作条件和数据处理方法同Liu et al.(2008b, 2010a, b) 详细描述。分析结果见表 2

表 2 中-新生代花岗岩类的锆石LA-ICP-MS U-Pb分析结果 Table 2 LA-ICP-MS zircon U-Pb analytical data of the Mesozoic-Cenozoic granitoids

锆石Hf 同位素组成分析是基于阴极发光(CL)图像和锆石U-Pb定年测试的基础上进行的。锆石Lu-Hf 同位素原位分析是在天津地质矿产研究所同位素实验室完成的,所用的测试仪器为NEPTUNE多接收器电感耦合等离子质谱仪(MC-ICP-MS),利用193nm FX激光器对锆石进行剥蚀,激光剥蚀的束斑直径为50μm,能量密度为10~11J/cm2,频率为8~10Hz。详尽的分析技术和实验参见耿建珍等(2011) 的论述。样品BM1148和DP1145均在相同的测年样品上进行锆石Hf同位素分析,而样品HTL1141由于锆石颗粒较小且挑选颗粒不多,故在同一地区与其具有近似年龄的样品HTL1146上进行了锆石Hf同位素分析。分析结果见表 3

表 3 中-新生代花岗岩类的锆石Hf同位素数据 Table 3 Hf isotopic data of zircon grains from the Mesozoic-Cenozoic granitoids
4 分析结果 4.1 锆石U-Pb年代学

本文在详细调研的基础上,对岩体不同部位代表性样品进行了锆石LA-ICP-MS U-Pb 测年。样品中锆石颗粒自形程度高,多为长柱状晶体,部分短柱状,长约100~200μm,长宽比为1.2:1~2:1,锆石阴极发光图像可见清晰的生长环带(图 3d),锆石的Th/U比值绝大部分大于0.1,为典型的岩浆成因锆石(Hoskin and Schaltegger, 2003)。在分析过程中,我们选择锆石环带边部打点,在剔除个别异常点后,锆石的U、Th、Pb同位素成分数据及谐和年龄列于表 2

图 3 中-新生代花岗岩类锆石阴极发光图像和U-Pb年龄谐和图 Fig. 3 Cathodoluminescence (CL) images of zircon grains and concordia plots of the Mesozoic-Cenozoic granitoids in Baoshan Block

样品BM1148中18个测点的206Pb/238U加权平均年龄值为83.4±1.1Ma(MSWD=0.99),在U-Pb年龄谐和图上分析点均分布在谐和线上或其附近(图 3a),显示很好的谐和性,表明锆石形成后U-Pb同位素体系是基本封闭的,没有U或Pb同位素的明显丢失或加入,测试结果真实可信。样品HTL1141中10个测点的206Pb/238U加权平均年龄为59.9±3.6Ma(MSWD=13),在U-Pb年龄谐和图上分析点均分布在谐和线上并且沿线分布(图 3b),可能是锆石中U含量过高的影响结果(表 2)。样品DP1145的206Pb/238U加权平均年龄值集中在两组,分别为85.4±4.2Ma(MSWD=1.8)和66.1±1.8Ma(MSWD=1.02)(图 3c),反应岩浆源区中含有晚白垩世的继承组分。本次测定的年龄结果均在允许误差范围内,可以确定蚌渺和桦桃林这两个岩株的形成年龄分别为83~85Ma和60~66Ma,属晚白垩世和古新世。

4.2 全岩地球化学

根据常量元素化学成分计算的标准矿物进行岩石分类,蚌渺岩体岩石类型为石英二长岩和二长花岗岩,属二长岩向花岗岩的过渡类型,桦桃林岩体为花岗岩(图 4),与岩相学特征相吻合。蚌渺岩体,SiO2为64.92%~65.06%,TiO2为0.49%~0.63%,Al2O3为15.08%~16.19%,MnO为0.05%~0.08%,MgO为2.01%~2.49%,CaO为2.85%~3.08%,P2O5为0.26%~0.39%,全碱(K2O+Na2O)为7.86%~8.34%;而桦桃林岩体,SiO2为73.17%~74.59%,TiO2为0.06%~0.12%,Al2O3为14.20%~15.02%,MnO为0.04%~0.06%,MgO为0.15%~0.23%,CaO为0.29%~0.59%,P2O5为0.25%~0.35%,全碱(K2O+Na2O)为6.46%~8.49%(表 1)。蚌渺岩体和桦桃林岩体都以全碱含量高为特征,K2O/Na2O>1,桦桃林岩体岩石系列属于高钾钙碱性系列,蚌渺岩体更加倾向于钾玄岩系列(图 5a),它们的铝饱和指数A/CNK的值均大于1.1,属铝过饱和系列,投点均落在I-S型花岗岩分界的S型一侧(图 5b),标准矿物计算结果中出现刚玉分子,也说明它们可能是S型花岗岩。相对于桦桃林花岗岩,蚌渺岩体的MgO、CaO和TiO2较高,分别表现为样品BM1148-1标准矿物计算后出现透辉石和含量较高的钛铁矿(表 1)。

图 4 中-新生代花岗岩类的QAP图解(据Streckeisen, 1976) Fig. 4 QAP classification diagram for the Mesozoic-Cenozoic granitoids in Baoshan Block(after Streckeisen, 1976)

图 5 中-新生代花岗岩类的K2O-SiO2图解(a, 实线据Peccerillo and Taylor, 1976; 虚线据Middlemost, 1985)和A/CNK-A/NK图解 (b, 据Maniar and Piccoli, 1989) Fig. 5 K2O-SiO2 (a, after Peccerillo and Taylor, 1976; Middlemost, 1985) and A/CNK-A/NK (b, after Maniar and Piccoli, 1989) plots for the Mesozoic-Cenozoic granitoids in Baoshan Block

桦桃林岩体的稀土总量∑REE在30.7×10-6~63.5×10-6之间,明显低于蚌渺岩体稀土总量(294×10-6~371×10-6)(表 1),桦桃林岩体(La/Yb)N值(5.4~8.5)同样低于蚌渺岩体 (La/Yb)N值(65.2~67.2),说明岩石轻/重稀土分馏强烈,而且前者较后者分馏弱;桦桃林岩体δEu值(0.17~0.35)<1,具有明显的负铕异常,岩浆结晶分异作用十分明显,蚌渺岩体δEu值(0.91~1.04)≈1,表明岩浆结晶分异不明显(图 6a)。微量元素原始地幔标准化图解中(图 6b),蚌渺岩体富集Rb、Th等大离子亲石元素(LILEs)和Pb,相对亏损Nb、Ta、P、Zr、Ti等高场强元素(HFSEs)。桦桃林岩体富集Rb、U、Pb和P,相对亏损Nb、Nd、Zr、Ti,Sr、Ba呈明显负异常,表明花岗岩岩浆部分熔融或结晶分异过程中具有斜长石的分离。蚌渺岩体和桦桃林岩体中高场强元素含量分别为,Zr 33.6×10-6~210×10-6;Hf 1.42×10-6~5×10-6;Nb 22×10-6~37.2×10-6,其富集程度基本相当于原始地幔的十倍,与S型花岗岩接近,低于典型A型花岗岩,高于Ⅰ型花岗岩。

图 6 中-新生代花岗岩类的稀土元素球粒陨石标准化配分曲线图(a, 标准化值据Boynton, 1984)及微量元素原始地幔标准化蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 6 Chondrite-normalized REE patterns (a, normalization values after Boynton, 1984) and primitive mantle-normalized trace element patterns (b, normalization values after Sun and McDonough, 1989) for the Mesozoic-Cenozoic granitoids in Baoshan Block
4.3 锆石Hf同位素特征

本文分析的3件样品锆石的176Yb/177Hf和176Lu/177Hf比值范围分别为0.1271~0.0121和0.0038~0.0004(表 3),176Lu/177Hf比值几乎都小于0.002,表明这些锆石在形成以后,仅具有较少的放射成因Hf的积累,因而可以用初始176Hf/177Hf 比值代表锆石形成时的176Hf/177Hf 比值(吴福元等, 2007)。考虑到3件样品的fLu/Hf的平均值在-0.99~-0.89之间,明显小于镁铁质地壳的fLu/Hf(-0.34, Amelin et al., 2000)和硅铝质地壳的fLu/Hf(-0.72, Vervoort et al., 1996),故二阶段模式年龄更能反映其源区物质从亏损地幔被抽取的时间(或其源区物质在地壳的平均存留年龄)。样品BM1148共18颗锆石的εHf(t)值变化-3.9~-1.1之间,对应的地壳模式年龄tDMC为1.4~1.2Ga;样品DP1145和HTL1146共23颗锆石的εHf(t)值显示较大的变化范围,原因可能是这两件样品中存在来自于蚌渺岩体的继承锆石,对应于蚌渺岩体年龄范围的锆石颗粒εHf(t)值集中在-12.3~-2.7,而年龄较小的桦桃林岩体的锆石εHf(t)值变化在-15.3~-5.2,对应的地壳模式年龄tDM C分别为2.0~1.3Ga和2.0~1.5Ga(表 3图 7)。

图 7 中-新生代花岗岩类的εHf(t)值与U-Pb年龄值直方图 Fig. 7 εHf(t) values and U-Pb ages histograms of the Mesozoic-Cenozoic granitoids in Baoshan Block
5 讨论 5.1 岩浆侵位时代

蚌渺岩体侵入的最高层位为泥盆系,桦桃林岩体侵位于蚌渺岩体及寒武系中,它们的侵位时代尚未被精确的确定。早期研究利用Rb-Sr全岩等时线方法和白云母K-Ar法,认为桦桃林岩体侵位于64~66Ma(陈吉琛, 1987),最近,Chen et al. (2007) 采用LA-ICP-MS U-Pb定年方法,获得桦桃林岩体的侵位时代为61Ma和78Ma,年龄差距很大,而蚌渺岩体尚未有定年数据。本文采用LA-ICP-MS定年方法,在蚌渺岩体1件代表性的样品中获得了谐和度很好的年龄83.4±1.1Ma(BM1148), 在桦桃林岩体2件代表性样品中获得了2组年龄,一组是较年轻的59.9±3.6Ma和66.1±1.8Ma,一组是和蚌渺岩体相当的85.4±4.2Ma,可能是来源于蚌渺的继承锆石,这也可能正是Chen et al. (2007) 获得的两个年龄差别很大的原因。上述可靠的锆石U-Pb年龄数据表明,蚌渺岩体侵位于83~85Ma,桦桃林岩体侵位于60~66Ma,分别形成于两次岩浆事件,即晚白垩世和古新世。

5.2 源区性质

随着Hf同位素测试技术的成熟,近年来锆石原位Hf同位素示踪研究越来越受到人们的重视(Vervoort et al., 1996; Amelin et al., 2000; Scherer et al., 2000; Griffin et al., 2002)。Hf同位素也被应用于岩石成因的探讨。研究表明(Peter and Roland, 2003; 吴福元等, 2007),不同地球化学储库(如亏损地幔、球粒陨石和地壳等)其176Hf/177Hf组成以及εHf(t)值明显不同,如具有低的176Hf/177Hf以及εHf(t)值的岩石往往指示其源区为地壳或经过地壳的混染,而具有较高的176Hf/177Hf以及εHf(t)值的岩石直接来自地幔或由幔源物质分异的新生壳源物质。蚌渺岩体和桦桃林岩体具有负的锆石εHf(t)值,在εHf(t)值-U-Pb年龄图解(图 8)中,所有样品点均落在球粒陨石演化线附近及以下的地壳区域,但蚌渺岩体具有相对较高的负值,其平均值为-5.1,而桦桃林岩体平均值为-9.3,明显低于蚌渺岩体,两者具有不明显的演化作用(图 8),表明蚌渺岩浆可能来源于古元古代-中元古代新生地壳的重熔,而桦桃林岩体则是在中生代进一步成熟陆壳的基础上深熔而成。

图 8 中-新生代花岗岩类的εHf(t)值-U-Pb年龄图解 图中阴影部分数据来自Chu et al., 2006Xu et al., 2012 Fig. 8 εHf(t) values vs. U-Pb ages plot of the Mesozoic-Cenozoic granitoids in Baoshan Block

两个岩体的锆石εHf(t)值均具有较大的变化范围,可能暗示其源区发生了不同程度幔源物质的注入(朱弟成等, 2009; Zhu et al., 2009, 2011, 2012)。另外,据研究表明(Sylvester, 1998),过铝质花岗岩中的Al2O3/TiO2比值可以作为源区岩石部分熔融温度的指示剂,当岩石的Al2O3/TiO2比值>100时,源区部分熔融温度<875℃,当岩石的Al2O3/TiO2比值<100时,源区部分熔融温度>875℃,且Al2O3/TiO2比值与形成温度成反消长关系。蚌渺岩体的Al2O3/TiO2比值为24~33,均小于100,桦桃林岩体的Al2O3/TiO2比值为120~245,均大于100。结合全岩锆饱和温度(蚌渺771~785℃,桦桃林678~727℃)(表 1),表明蚌渺岩体岩浆的形成温度相对桦桃林岩体要较高一些,这也为蚌渺岩体熔融程度高于桦桃林岩体提供了依据,也就为两个岩体均具有较大变化范围的εHf(t)值提供了佐证。

5.3 岩浆事件及其构造意义

本文的年代学数据表明,滇西保山蚌渺-桦桃林地区的岩浆活动时限为晚白垩世-古新世。实际上,晚白垩世-古新世岩浆活动在保山西缘的腾冲地块非常普遍(吕伯西等, 1993; 杨启军等, 2009; Xu et al., 2012),以腾梁花岗岩和盈江花岗岩为典型代表,腾梁花岗岩全岩Rb-Sr等时线年龄为78~84Ma(江彪等, 2012),锆石U-Pb年龄为68~76Ma和53Ma(杨启军等, 2009; Xu et al., 2012),盈江花岗岩锆石U-Pb年龄为53~66Ma(Xu et al., 2012)。在南冈底斯带东段朗县-米林一带花岗岩类的侵位年龄为79~84Ma(管琪等, 2010),中西部曲水、日喀则地区也发现了晚白垩世岩浆活动(81~87Ma, Ji et al., 2009),大规模的林子宗火山岩形成于65~40Ma(董国臣等, 2002; 莫宣学等, 2003)。这些研究表明,保山地块与腾冲地块以及拉萨地块在晚白垩世-古新世存在几乎同期的岩浆活动。

冈底斯花岗岩成分复杂,以准铝质Ⅰ型花岗岩为主,含有部分S型花岗岩,Ⅰ型花岗岩的εNd均为正值(Chung et al., 2005),花岗岩中铁镁质包体发育,岩浆混合特征明显(江万等, 1999; Mo et al., 2007; 董国臣等, 2011),表明冈底斯岩浆活动频繁,壳幔物质交换作用强烈,锆石εHf(t)值基本为正(图 8),代表新生地壳的熔融产物。腾梁花岗岩以过铝-强过铝S型花岗岩为主,含有少量Ⅰ型花岗岩,花岗岩中铁镁质微粒包体较少,岩浆混合特征不明显,地幔对花岗岩形成的贡献不大,锆石εHf(t)值均为负(图 8),主要由古老中下地壳物质熔融而成(杨启军等, 2009)。保山蚌渺-桦桃林花岗岩属强过铝质S型花岗岩,未见铁镁质包体,锆石εHf(t)值均为负(图 8),来源于古元古代-中元古代地壳的熔融(本文)。以上研究表明,保山地块与拉萨地块在中-新生代岩浆活动上具有明显的不同,不存在对比性,而与腾冲地块上同期的岩浆活动在地球化学特征上存在共同点。

腾冲地块于燕山期与保山地块碰撞拼贴(莫宣学等, 1993; 钟大赉等, 1998),白垩纪末期新特提斯洋开始闭合,随后印度板块与欧亚大陆开始碰撞,时间为65Ma(莫宣学等, 2003),这些俯冲碰撞事件造成了腾冲与保山地区的现今构造格局,地块上中-新生代的花岗岩类岩浆作用正是在这样大的构造背景下应运而生的。

6 结论

(1) 滇西保山地块蚌渺岩体与桦桃林岩体分别形成于83~85Ma和60~66Ma,表明该地区在晚白垩世和古新世发生了两次不同的岩浆作用。

(2) 蚌渺石英二长岩、二长花岗岩和桦桃林花岗岩属高钾钙碱性系列到钾玄岩系列,具有壳源过铝质S型花岗岩特征。蚌渺岩体可能来源于古元古代-中元古代新生地壳的熔融,桦桃林岩体则是在中生代进一步成熟陆壳的基础上深熔而成。

(3) 保山地块、腾冲地块和拉萨地体在中-新生代存在几乎同期的岩浆活动,但保山地块与拉萨地体由于岩石地球化学特征不同,不存在对比性,与腾冲地块存在共同点。保山地块与腾冲地块上的中-新生代岩浆作用与新特提斯洋的闭合和印欧大陆的碰撞息息相关。

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