2. 中国科学院青藏高原研究所大陆碰撞与高原隆升重点实验室,北京 100085;
3. 中国地质大学地球科学与资源学院,北京 100083
2. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibet Plateau Research, Chinese Academy of Sciences, Beijing, 100085, China;
3. School of Earth Science and Resources, China University of Geosciences, China University of Geosciences, Beijing, 100083, China
斑岩型矿床提供了世界上约75%金属Cu、50%金属Mo、20%金属Au以及W、Sn、Pb、Zn等金属(Silltoe et al., 2010),其经济意义重大,因此也一直得到科学界的关注,并成为经久不衰的研究热点。近年来,我国地质学者在青藏高原及周边的斑岩型矿床成矿理论研究及矿产勘查方面取得了一系列创新成果及找矿突破,指导并发现了冈底斯Cu-Mo矿带、玉龙Cu-Mo矿带以及滇西Cu-Au-Mo矿带(芮宗瑶等,2003;侯增谦等,2003;郑有业等,2004;姜耀辉等,2008;邓军等, 2010, 2011, 2012;Deng et al., 2010;杨立强等,2010),提出这些矿床形成于陆陆碰撞造山环境,而有别于环太平洋地区形成于岛弧及陆源弧环境的斑岩型矿床。冈底斯成矿带位于拉萨地块南缘,沿着雅鲁藏布缝合带北侧近东西向展布,东起工布江达,西至昂仁,长约700km,包括我国最大的驱龙Cu矿床(>10Mt)以及甲玛、雄村、冲江、朱诺等一批超大型及大中型矿床(图 1)。研究表明,该成矿带斑岩型矿床按成矿环境可分为俯冲型及碰撞型两种类型,分别对应与新特提斯洋俯冲及印度-欧亚陆陆碰撞造山环境。冈底斯俯冲型斑岩矿床规模较小,目前仅发现雄村超大型Cu-Au矿床,成矿时代为173~165Ma (唐菊兴等,2010),而碰撞型斑岩矿床规模较大,成矿集中在16~13Ma,形成了我国最大的驱龙Cu-Mo矿床(>10Mt)以及甲玛、达布、冲江、朱诺等一批超大型及大中型斑岩型Cu-Mo矿床(芮宗瑶等,2003;Hou et al., 2004)。由于中新世强烈的斑岩成矿作用,冈底斯斑岩成矿带通常也特指中新世斑岩型矿床的空间分带。然而,近年的研究表明,冈底斯还发育始新世及渐新世斑岩成矿作用,如吉如Cu-Mo (49.2±1.7Ma;郑有业未刊数据)和沙让Mo矿床(51±1.0Ma;唐菊兴等,2009),以及明则斑岩型Mo矿床(30.26±0.69Ma;闫学义等,2010)、冲木达矽卡岩型Cu-Au矿床(40.5±5.6Ma;李光明等,2006)、努日矽卡岩型W-Cu-Mo矿床(23.62±0.97Ma;闫学义等,2010)。这些渐新世矿床位于冈底斯中新世斑岩成矿带的南部,集中分布在雅鲁藏布缝合带北侧的泽当-桑日一带,部分学者提出了克鲁-冲木达成矿带(闫学义等,2010),以示区别冈底斯中新世成矿带。
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图 1 青藏高原大地构造分区图(a, 据Zhu et al., 2011)和冈底斯东南缘泽当区域地质与矿床分布图(b, 据Harrison et al. 2000修改) Fig. 1 Tectonic framework of the Tibetan Plateau (a, after Zhu et al., 2011) and regional geological map showing the localities of major deposits in the Zedong area, southeastern Gangdese (b, modified after Harrison et al. 2000) |
冈底斯中新世斑岩成矿带研究程度较高,与斑岩成矿相关的岩石主要为二长花岗斑岩、花岗闪长斑岩、石英二长斑岩、花岗斑岩等,成岩时代集中在18~12Ma,岩石具有高钾钙碱性-钾玄岩特征,且显示埃达克质岩石特征。但克鲁-冲木达成矿带研究程度相对较低,为什么该带发育斑岩型Mo矿化(如明则矿床),而很少出现类似冈底斯的斑岩型Cu-Mo矿化,该带与斑岩型Mo矿化有关的岩石类型及成因是否不同于冈底斯中新世斑岩Cu-Mo矿床。为此本文选择该成矿带内的程巴-明则斑岩型与夕卡岩型矿床的含矿岩石开展了岩石学、元素地球化学、锆石SHRIMP U-Pb定年与Hf同位素、以及辉钼矿Re-Os年代学研究。本研究将有助于深入分析克鲁-冲木达斑岩成矿系统的发育特征,并揭示冈底斯渐新世与中新世斑岩成矿作用的联系与区别。
2 区域地质背景与矿床地质特征冈底斯造山带位于印度河-雅鲁藏布江缝合带及班公湖-怒江缝合带之间,是一条近东西向延伸的构造-岩浆岩带,长约2500km,宽约150~300km,从腹地向东、西构造结两侧逐渐变窄,以其内部发育的洛巴堆-米拉山断裂和狮泉河-纳木错蛇绿混杂岩带为界,由南向北通常被划分为南冈底斯、中冈底斯及北冈底斯(潘桂棠等,2006)。研究表明,中冈底斯可能为一个古老的微陆块,具有前寒武纪(古元古代-太古代)结晶基底,上覆沉积盖层主要为石炭-二叠纪浅变质碎屑岩及早侏罗世-晚白垩世酸性火山岩和火山碎屑岩组成,侵入岩主要以早白垩世和晚三叠-早侏罗世为主;南冈底斯和北冈底斯则是由特提斯洋俯冲分别在中冈底斯的南、北两侧增生和拼贴而成(Zhu et al., 2011)。但是,在南冈底斯东部的林芝岩群中发现的寒武纪花岗岩则表明局部地区可能存在类似与中部拉萨地块出露的前寒武纪结晶基底(Dong et al., 2010)。
研究区主要位于南冈底斯南缘的泽当地区,Aitchison et al.(2000)根据该地区发育的白垩纪拉斑玄武岩质弧玄武岩和玄武安山岩,认为其为新特提斯大洋俯冲形成的洋内岛弧,伴随着特提斯洋的俯冲消亡及印度-欧亚陆陆碰撞而拼贴于拉萨地体南缘。该区出露的地层主要为三叠系姐德秀组碎屑岩-碳酸盐岩建造、白垩系麻木下组钙碱性岛弧火山岩及碳酸盐岩-碎屑岩建造以及白垩系比马组岛弧安山岩-英安岩-沉积岩系,并被新近纪罗布莎岩群不整合覆盖。区域构造线主要为北部的北倾冈底斯逆冲断裂带和南部的南倾仁布-泽当逆冲断裂带,冈底斯逆冲断裂带以北大面积出露古生代-中生代地层及侵入其中的白垩纪-第三纪冈底斯岩基,仁布-泽当逆冲断裂带以南大面积出露晚三叠世砂岩和千枚岩,两条逆冲断裂带之间分布着晚白垩世和第三纪正长岩、第三纪砾岩以及基性-超基性岩(图 1)。
克鲁-冲木达成矿带位于冈底斯中新世斑岩成矿带南部,距雅鲁藏布江缝合带最近处仅6km。明则、程巴、冲木达是该成矿带三个较典型的矿床,其呈近东西向分布在西藏山南地区泽当镇东部。明则、程巴曾分别称为明则矿段和程巴矿段,二者又合并称为明则矿床(韩逢杰,2006),并且一直被后续研究者统称为明则斑岩型Mo矿床(闫学义等,2010;侯增谦等,2012),实际上矿区斑岩型Mo矿化发育在程巴地区,而明则地区发育的是夕卡岩型Cu矿化。并且近年来,中国冶金地质勘察工程总局第二地质勘查院在山南地区开展了大量的勘查工作,扩大了明则和程巴的资源量,同时考虑到二者矿床类型不同,故本文分别称为明则夕卡岩型Cu矿床和程巴斑岩型Mo矿床。程巴矿床出露的地层主要为三叠系姐德秀组砂质板岩,花岗闪长岩和二长花岗岩呈岩基产出(矿区称之为陈坝岩体),含矿二长花岗斑岩呈岩株侵位其中。该矿床以斑岩型Mo矿化为主,伴生Cu矿化,矿石以石英细脉-网脉状和浸染状矿化为特征,矿石矿物主要为辉钼矿、黄铜矿、黄铁矿。明则矿床位于程巴矿床西边,矿区出露的地层主要为白垩系比马组变质粉砂岩及三叠系姐德秀组砂质板岩,侵入体为始新世娘古处岩体,岩石类型为细粒黑云母花岗闪长岩,局部被晚期二长岩-石英二长闪长岩侵入。该矿区发育矽卡岩型Cu-Mo矿化,矿体受控于北倾逆冲断裂,在断裂带两侧依次发育石榴子石矽卡岩→绿帘石矽卡岩→黑云母角岩→浅变质粉砂岩蚀变分带,成矿过程可分为干矽卡岩阶段、湿矽卡岩阶段、氧化物阶段、石英-硫化物阶段和石英-碳酸盐阶段,类似经典的岩浆接触交代型(夕卡岩型)矿床,但夕卡岩矿化带两侧未见岩浆岩与围岩的侵入接触现象。矿化主要分布在绿帘石矽卡岩带,矿石矿物主要为黄铜矿、孔雀石及蓝铜矿。冲木达矿床位于程巴矿床东边,发育夕卡岩型Cu-Au矿化(李光明等,2006),矿化主要产在比马组层纹状灰岩、微晶灰岩与似斑状花岗闪长岩-二长花岗岩中,矿石矿物主要有黄铜矿、辉铜矿、黝铜矿和斑铜矿,另有少量的磁铁矿、黄铁矿、辉钼矿等。
3 样品采集与分析方法本文主要采集了明则矿区与夕卡岩矿化关系密切的二长岩开展了成岩年代学研究,以探讨夕卡岩成矿时代,同时开展了程巴矿区矿石中辉钼矿Re-Os定年及含矿花岗闪长岩的锆石SHRIMP U-Pb定年,据此分析了明则和程巴斑岩型矿化与夕卡岩型矿化之间的成因联系。并且,本文还采集了明则二长岩和程巴花岗闪长岩(图 2)开展了岩石地球化学及Hf同位素分析。明则二长岩呈半自形微细粒等粒结构,主要矿物为斜长石(35%~45%)、钾长石(25%~30%)、角闪石(15%~20%)、黑云母(5%~10%)、石英(2%~5%);程巴花岗闪长岩呈中粗粒花岗结构-似斑状结构,主要矿物为斜长石(40%~50%)、钾长石(15%~25%)、石英(20%~25%)、角闪石(5%)、黑云母(5%~10%)。
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图 2 野外及显微照片 (a)-明则二长岩; (b)-程巴花岗闪长岩;(c)-程巴花岗闪长岩显示花岗结构;(d)-明则二长岩中角闪石中包裹的磁铁矿,辉石显示角闪石溶蚀边结构;(e)-明则二长岩中钾长石晶体包含角闪石、斜长石、榍石,而呈现“筛状”结构; (f)-明则二长岩中早期的斜长石颗粒被晚期的斜长石穿插,并发育斜长石的溶蚀边结构.Q-石英; Kfs-钾长石; Pl-斜长石; Hbl-角闪石; Bt-黑云母; Spn-榍石; Px-辉石; Mag-磁铁矿 Fig. 2 Field photographs and microphotographs (a)-Mingze monzonlite, (b)-Chengba granodiorite, (c)-granitic texture for the Chengba granodiorite, (d)-magnetite emplaced in the hornblende which was rimmed by pyroxene for the Mingze monzonlite, (e)-screening texture charactered by K-feldspar with inclusions of plagioclase, hornblende and sphene for the Mingze monzonlite, (f)-late fine grained plagioclase intruded into early subhedral plagioclase and the resorption texture of plagioclase shown in the Mingze monzonlite. Q-quartz; Kfs-K-feldspar; Pl-plagioclase; Hbl-hornblende; Bt-biotite; Spn-sphene; Px-pyroxene; Mag-magnetite |
锆石分选由河北省区域地质矿产调查研究所实验室完成,样品破碎过筛后通过重-磁分选后,最后在双目镜下挑出锆石。分选出来的锆石多为无色透明、长柱状自形-半自形晶体,少量发生了碎裂。程巴样品中锆石的平均晶体长约150~300μm,长宽比约1.5:1~3:1;明则样品中锆石的平均晶体长约80~150μm,长宽比约1.5:1~2:1。锆石制靶和阴极发光(CL)图像分析在中国科学院地质与地球物理研究所电子探针实验室完成,锆石光学显微照相在中国地质大学(北京)完成。U、Th及Pb同位素组成分析在澳大利亚Curtin大学实验室完成,所用的锆石标样为TEMORA (年龄为417Ma),同位素分析所用的仪器为SHRIMP Ⅱ,测试程序及流程参见Williams (1998),数据处理采用Isoplot软件(Ludwig, 1999),普通Pb由实测的204Pb校正,所有测点误差均为1σ,所采用的206Pb/238U加权平均年龄具95%的置信度。锆石Hf同位素在中国地质大学地质过程与矿产资源国家重点实验室(GPMR)完成,测试仪器Neptune Plus型MC-ICP-MS,采用Geolas 2005激光剥蚀系统进行剥蚀,激光束斑大小为44μm,分析的位置同锆石SHRIMP U-Pb定年的测点位置一致,所用的锆石标样为91500,具体的测试程序及流程参见Yuan et al. (2008)。辉钼矿Re-Os同位素分析在国家地质实验测试中心完成,样品从程巴矿区钻孔中不同深度的4件含辉钼矿脉的矿石中挑选,测试程序及流程参见Du et al. (2004)。明则和程巴样品的主量、微量与稀土元素在河北省区域地质矿产调查研究所实验室测定,主量元素用XRF测定,微量与稀土元素用ICP-MS法测定,数据的质量优于10%。
4 分析结果 4.1 锆石U-Pb年龄锆石U-Pb年龄的分析结果见表 1和图 3。程巴花岗闪长岩中锆石的14个分析样点的U和Th含量分别为303×10-6~2754×10-6和148×10-6~799×10-6,Th/U比值为0.26~0.92,类似于岩浆锆石的比值(Hoskin and Black, 2000)。这14个样点具有相似的206Pb/238U年龄,介于27.6~29.9Ma,基本落入协和线上,其平均年龄为28.4±0.4Ma (MSWD=0.71)。明则二长岩中锆石的12个分析样点的U和Th含量分别为512×10-6~2305×10-6和551×10-6~2325×10-6,Th/U比值为0.34~1.87,12个样点具有相似的206Pb/238U年龄,介于28.7~31.8Ma,基本落入协和线上,平均年龄为30.4±0.6Ma (MSWD=1.8)。
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表 1 冈底斯程巴花岗闪长岩及明则二长岩锆石SHRIMP U-Pb年代学数据 Table 1 Zircon SHRIMP U-Pb data of the Chengba granodiorite and Mingze monzonlite, Gangdese |
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图 3 程巴花岗闪长岩(CB-3)及明则二长岩(MZ-14)中锆石U-Pb年龄谐和图 Fig. 3 U-Pb concordia diagrams for zircons from Chengba granodiorite (CB-3) and Mingze monzonlite (MZ-14) |
程巴矿区4件辉钼矿Re-Os同位素分析结果见表 2和图 4。辉钼矿Re含量较高,为86×10-6~713×10-6,所获得的Re-Os等时线年龄为30.2±0.9Ma (MSWD=2.4)。
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表 2 程巴斑岩型Mo矿床矿石中辉钼矿Re-Os同位素分析结果 Table 2 Molybdenite of Re-Os isotopic data of Chengba porphyry Mo deposit |
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图 4 程巴矿床辉钼矿Re-Os等时线年龄图 Fig. 4 Re-Os isochron age diagram for molybdenite samples from the Chengba porphyry Mo deposit |
程巴花岗闪长岩与明则二长岩样品的元素地球化学数据见表 3。程巴花岗闪长岩具有较高的SiO2含量(65%~67%)、较低的MgO含量(1.67%~2.13%)及Mg#值(49.5~51.1);K2O含量为3.2%~4.1%,属于高钾钙碱性岩系;A/CNK为0.87~0.92,属于准铝质岩系,与I型花岗岩类似。而明则二长岩具有相对较低的SiO2 (55%~57%)、较高的MgO含量(3.5%~6.9%)和Mg#值(57.6~67.2);K2O含量为2.7%~5.0%,总体属于钾玄岩系;A/CNK为0.65~0.79,属于准铝质岩系(图 5)。
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表 3 程巴花岗闪长岩和明则二长岩主量(wt%)和微量(×10-6)元素分析结果 Table 3 Whole rock geochemical data of the Chengba granodiorite and Mingze monzonlite (Major elements: wt%; Trace elements: ×10-6) |
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图 5 程巴花岗闪长岩和明则二长岩主量及微量元素图解 (a)-硅碱图(Middlmost, 1994);(b)-QAP图(Bowden et al., 1984);(c)-K2O vs.SiO2图(Peccerillo and Taylor, 1976);(d)-MgO vs. SiO2图;(e)-Mg# vs. SiO2图;(f)-La/Yb vs. La图.数据来源:38Ma卧龙寄主岩及包体(Guan et al., 2012);30Ma冲木达寄主岩及包体(姜子琦等,2011);板片熔融(Zhu et al., 2009);下地壳熔融(Hou et al., 2004; Guo et al., 2007; Gao et al., 2010);AFC-同化混染与分离结晶(Stern and Kilian, 1996) Fig. 5 Discrimination diagrams for the Mingze monzonlite and Chengba granodiorite (a)-SiO2 contents against K2O+Na2O (Middlmost, 1994); (b)-Q-A-P (Bowden et al., 1984); (c)-SiO2 contents against K2O (Peccerillo, 1976); (d)-SiO2 contents against MgO; (e)-SiO2 contents against Mg#; (f)-La/Yb ratios versus La contents. Data sources: 38Ma Wolong host granitoids and mafic enclaves (Guan et al., 2012); 30Ma Chongmuda host granitoids and mafic enclaves (Jiang et al., 2011); Slab melting (Zhu et al., 2009); Lower-crustal melting (Hou et al., 2004; Guo et al., 2007; Gao et al., 2010); Mantle melt and crustal Assimilation-fractional crystallization (AFC) (Stern and Kilian, 1996) |
尽管上述2类岩石类型不同,但他们具有相似的稀土元素和微量元素特征(图 6),均显示轻稀土富集、弱的Eu负异常(δEu=0.71~0.86)、富集大离子亲石元素(LILE,例如Rb、Th、U、K)和亏损高场强元素(HSFE, 例如Nb、Ta、Ti)等特征。但相比较而言,明则二长岩的REE含量明显高于程巴花岗闪长岩,而轻重稀土元素分异程度[(La/Yb)N=17.9~29.5]又低于后者[(La/Yb)N=29.2~39.6]。并且,二长岩相容元素含量较高,Cr为34×10-6~379×10-6(平均为170×10-6),Ni为48×10-6~116×10-6(平均为74×10-6);而花岗闪长岩的相容元素含量较低,Cr为20×10-6~39×10-6(平均为34×10-6),Ni为16×10-6~25×10-6(平均为19×10-6)。此外,二长岩具有相对较高的Y (21.3×10-6~27.1×10-6)和较低的Sr/Y比值(24~49),而花岗闪长岩具有较低的Y (11.1×10-6~13.9×10-6)和较高的Sr/Y比值(54~68),二者分别落入岛弧型岩浆岩和埃达克质岩范围内(图 7)。
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图 6 明则二长岩与程巴花岗闪长岩稀土元素配分图(a)和微量元素蛛网图(b)(标准化值据Sun and McDonough, 1989) Fig. 6 Chondrite-normalized rare earth element patterns (a) and primitive-mantle-normalized trace element patterns (b) for the Mingze monzonlite and Chengba granodiorite (normalization values after Sun and McDonough, 1989) |
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图 7 明则二长岩与程巴花岗闪长岩Sr/Y vs. Y图(a)和(La/Yb)N vs. YbN图(b) 图例同图 5 Fig. 7 Discrimination diagrams of Sr/Y ratios vs. Y contents (a) and (La/Yb)N ratios vs. YbN contents (b) for the Mingze monzonlite and Chengba granodiorite Symbols are same as those in Fig. 5 |
Hf同位素分析结果见表 4。明则二长岩样品中锆石的176Yb/177Hf和176Lu/177Hf值分别为0.0196~0.0869和0.0005~0.0022,εHf(t)值为+2.8~+6.8,模式年龄为446~593Ma,地壳模式年龄为673~928Ma。程巴花岗闪长岩样品中锆石的176Yb/177Hf和176Lu/177Hf值分别为0.0126~0.0331和0.0004~0.0010,εHf(t)值为+4.2~+6.1,模式年龄为456~531Ma,地壳模式年龄为714~840Ma。
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表 4 程巴花岗闪长岩及明则二长岩锆石Hf同位素数据表 Table 4 Zircon Hf isotopic data of the Chengba granodiorite and Mingze monzonlite |
明则二长岩与矿区夕卡岩型Cu矿化关系密切,二长岩的锆石SHRIMP U-Pb年龄为30.4±0.6Ma (MSWD=1.8),暗示明则夕卡岩Cu矿化时代约30Ma。辉钼矿Re-Os同位素年代学数据表明程巴斑岩型Mo矿化时代也是约30Ma,与明则夕卡岩Cu矿床几乎同时期形成。
锆石SHRIMP U-Pb年代学研究表明,程巴矿区含矿花岗闪长岩的侵位时代为28.4±0.4Ma (MSWD=0.71),该年龄与冲木达二长花岗岩的锆石U-Pb年龄基本一致(27.7±1.1Ma;莫济海等,2008)。此外,前人在该区新生代花岗岩体中还获得了一组约30Ma的年龄数据,例如Harrison等(2000)获得程巴花岗闪长岩1个样品的锆石U-Pb年龄为30.4±0.4Ma (MSWD=1.1),Chung等(2009)获得程巴花岗闪长岩2个样品的锆石U-Pb年龄分别为30.3±0.6Ma (MSWD=1.4)和31.0±0.5Ma (MSWD=5.2),姜子琦等(2011)获得冲木达石英二长岩的锆石U-Pb年龄为30.2±0.7(MSWD=3.7)。这些数据表明程巴和冲木达岩体可能属于同一个岩体,其侵位时代为28~31Ma。
5.2 岩石成因 5.2.1 明则二长岩明则二长岩中显示辉石的角闪石溶蚀边结构以及钾长石巨晶中包含角闪石、斜长石而呈现的“筛状”结构(图 2),这些均是岩浆发生混合作用的重要显微岩相学证据(王玉往等,2012)。二长岩具有较低的SiO2含量,较高的MgO与相容元素(Cr、Ni、Co、V)含量及Mg#值(图 8),其最初的成分可能为玄武质,但由于与酸性岩浆的混合作用或镁铁质矿物的分离结晶作用而导致其地化特征偏离原生的玄武质岩浆而呈现闪长质特征。区域大面积出露的程巴与冲木达花岗闪长岩-二长花岗岩的成岩时代与二长岩基本一致,表明该区可能发生了基性岩浆与酸性岩浆的混合作用,程巴与冲木达花岗岩体中大量出现的闪长质包体可能也是这个作用的产物。
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图 8 明则二长岩与程巴花岗闪长岩Cr vs. Ni图(a)和Ni vs. Mg#图(b) 数据来源:板片熔融和下地壳熔融范围据Guan et al. (2012).图例同图 5 Fig. 8 Discrimination diagrams of Ni contents vs. Cr contents (a) and Ni contents vs. Mg# values (b) for the Mingze monzonlite and Chengba granodiorite Data sources: Slab melting and Lower-crustal melting (Guan et al., 2012). Symbols are same as those in Fig. 5 |
二长岩的Nb/U和Ce/Pb比值分别为1.9~3.2和10.1~15.1,类似于陆壳(Nb/U=6.2,Ce/Pb=3.9;Rudnick and Fountain, 1995),而明显不同于洋中脊玄武岩(MORB)和洋岛玄武岩(OIB)(Nb/U=47,Ce/Pb=27;Hofmann et al., 1986),表明二长岩的母岩浆不太可能是由软流圈地幔部分熔融形成。二长岩地化特征与冈底斯东段卧龙花岗岩中的38Ma的镁铁质包体具有相似的地球化学特征,后者被认为是富集岩石圈地幔部分熔融的结果(Guan et al., 2012)。此外,冲木达花岗闪长岩中也发现了31Ma的闪长质包体(姜子琦等,2011),其地球化学特征与明则二长岩类似。基于上述数据,明则二长岩很可能是岩石圈地幔部分熔融形成的熔体高度演化的产物。
明则二长岩显示较高的K2O含量,可能与源区存在富钾矿物相(金云母、富钾角闪石和钾长石)。二长岩的Eu负异常不明显,表明钾长石不太可能是富钾矿物相。此外,与金云母平衡的熔体具有高的Rb/Sr (>0.1)和低的Ba/Rb (<20)比值,而与角闪石平衡的熔体具有低的Rb/Sr (<0.06)和高的Ba/Rb比值(>20)(Furman and Graham, 1999),明则二长岩的Rb/Sr为0.16~0.21,Ba/Rb比值为7.5~11.5,表明源区富钾矿物相主要为金云母。因此,冈底斯东段南缘岩石圈地幔可能存在金云母,这与藏东玉龙斑岩铜矿带深部含金云母的岩石圈地幔(Jiang et al., 2006)以及辽东地区侏罗纪含角闪石、金云母的岩石圈地幔(Jiang et al., 2005)等具有相似性。
5.2.2 程巴花岗闪长岩程巴渐新世花岗闪长岩和冈底斯中新世含矿斑岩在地化特征方面具有很大的相似性,后者在一系列文献中被称为埃达克质岩。目前,部分国内外学者提出不能将斑岩矿区发育的这套钙碱性-高钾钙碱性花岗岩类全部论证为埃达克岩,而埃达克岩应特指那些与俯冲的年青大洋板片熔融有关的岩石(Richards and Kerrich, 2007; Richards, 2009; 毛景文等,2010)。并且,对于冈底斯中新世这套含矿斑岩的成因认识存在多种观点,包括增厚的下地壳(Chung et al., 2003; Hou et al., 2004; Guo et al., 2007; Guan et al., 2012),俯冲或残留的新特提斯洋板片(Qu et al., 2004),以及俯冲板片熔体交代的上地幔(Gao et al., 2007, 2010)的部分熔融。本文基于以下几点认为程巴渐新世花岗闪长岩与冈底斯中新世含矿斑岩可能具有相似的成因,主要为增厚的下地壳部分熔融的产物。
(1)克鲁-冲木达成矿带渐新世花岗闪长岩-二长花岗岩大面积出露,而以明则二长岩为代表的中基性出露面积很小,因此,这些中酸性花岗岩体不太可能是明则母岩浆结晶分异的产物。La-La/Yb相关图(图 5f)也不支持结晶分异作用,而显示部分熔融特征。
(2)程巴花岗闪长岩具有较高的K2O含量,较低的MgO含量及Mg#值,以及低含量的相容元素(Cr、Ni、Co、V),明显不同于俯冲大洋板片脱水所释放流体交代上覆楔形地幔,并诱发软流圈地幔部分熔融而形成的钙碱性岩浆(如拉萨地块南缘早白垩世俯冲型岩浆岩;Zhu et al., 2009),而类似于冈底斯中新世含矿斑岩(图 5)。考虑到新特提斯大洋板片在~50Ma已发生断离(Wen et al., 2008; Lee et al., 2009),而程巴花岗闪长岩的侵位时代约为30Ma,因此该花岗闪长岩体的形成环境是大陆碰撞而不是洋陆俯冲环境。
(3)程巴花岗闪长岩的岩浆源区也不太可能来自富集岩石圈地幔(Gao et al., 2007, 2010)。要形成花岗闪长岩这种高硅的长英质熔体,需要富集岩石圈地幔发生比二长岩熔体更加低程度的部分熔融(Jiang et al., 2006),这种过程通常会导致大量不相容元素进入长英质熔体。然而,二长岩相对花岗闪长岩具有较高的不相容元素特征并不支持花岗闪长岩的岩浆源区和二长岩的源区相同(图 6)。
(4)程巴花岗闪长岩的岩浆锆石具有正的εHf(t)值(+4.0~+6.1),显示亏损地幔的特征。研究证明,藏南新生的增厚下地壳具有亏损地幔特征,其形成与镁铁质岩浆底侵作用有关(Mo et al., 2008; Chung et al., 2009)。
5.3 壳幔相互作用与明则斑岩型Mo矿化的指示意义上述研究表明,明则二长岩的形成与富集岩石圈地幔部分熔融有关,其钾玄岩特征可能归因于源区含有金云母。研究表明,大洋板片流体(或熔体)与地幔橄榄岩反应可形成金云母辉石岩,其部分熔融可以形成钾质岩浆(Wyllie and Sekine, 1982; Turner et al., 1996; Rogers et al., 1998; Jiang et al., 2006)。因此,明则二长岩富集岩石圈地幔源区可能遭受新特提斯洋板片流体(或熔体)改造。洋壳板片本身富水,在脱水过程能把大量的水、硫、卤素、金属,以及亲流体的大离子亲石元素(LILE)输入到岩石圈地幔,从而使其部分熔融形成的岩浆富水、富硫,并具有较高的氧逸度,在这种高氧逸度情况下,硫主要以硫酸盐的形式溶解于岩浆之中,导致通常优先向硫化物分配的Cu、Au等开始作为不相容元素向硅酸盐熔浆中富集,从而使岩浆具有萃取和容载Cu、Au等金属元素的能力(Oyarzun et al., 2001; Mungall, 2002)。本文的元素分析结果(表 3)也支持这一认识,明则二长岩样品中Cu含量较高,为252×10-6~5662×10-6(平均值为2781×10-6),而程巴花岗闪长岩样品中Cu含量仅为7.4×10-6~63.7×10-6,表明来自受俯冲板片流体交代的富集地幔的幔源岩浆含矿能力较高。
明则二长岩与程巴-冲木达花岗闪长岩体中发育的闪长质包体在岩石地球化学及成因方面具有相似性(图 5、图 7、图 8),很可能属于富集岩石圈地幔部分熔融形成的同一套岩浆系统。这套岩浆不仅能够形成二长岩,而且能与花岗闪长岩的原生岩浆发生岩浆混合作用,从而部分还能以包体的形式发育在花岗闪长岩中。这与野外观察到的程巴和冲木达花岗闪长岩体中发育大量闪长质包体的现象一致。并且,冈底斯中新世含矿斑岩中也发育大量的镁铁质包体,包体中也证明含有大量岩浆成因的金属硫化物-氧化物(黄铜矿、斑铜矿、磁铁矿)产出(杨志明等,2008)。这种岩浆混合作用实质上代表了岩石圈地幔与下地壳之间的一种壳幔作用方式(Sun et al., 2010),来自富集地幔的幔源岩浆不仅自身含有丰富的Cu、S和H2O,而且在与长英质岩浆混合过程中向后者提供了部分Cu、S和H2O。这种壳幔相互作用对克鲁-冲木达矿带成矿带内斑岩-夕卡岩矿化具有重要的控制作用。
既然程巴渐新世花岗闪长岩与冈底斯中新世含矿斑岩具有相似的地球化学特征及Hf同位素特征,且两个区域均发生了强烈的壳幔相互作用,但为什么明则发育斑岩型Mo矿化,而其北部的冈底斯发育斑岩型Cu-Mo矿化。一种合理的解释是,程巴地区在渐新世发育的矿化实际上是斑岩型Cu-Mo矿化,由于斑岩型Cu-Mo矿床一般在垂向上具有上Cu下Mo的分带特征,成矿后由于该区发生了强烈的隆升与剥蚀,导致上部的Cu矿体被剥蚀而现今仅保留Mo矿体,这与笔者通过磷灰石裂变径迹分析获得的程巴地区剥蚀量大于北部冈底斯成矿带的剥蚀量(笔者未刊数据)具有较好的一致型。因此,克鲁-冲木达矿带内斑岩型Cu矿资源潜力较弱。
6 结论(1)明则夕卡岩型Cu矿化与二长岩关系密切,成矿时代与二长岩结晶时代应大致相同,约为30Ma。该年龄也与程巴斑岩型Mo矿化时代一致。
(2)明则二长岩是富集岩石圈地幔部分熔融的产物,程巴花岗闪长岩是增厚下地壳部分熔融的产物。
(3)明则二长岩与程巴-冲木达花岗闪长岩中发育的暗色微粒包体可能具有相同的岩浆源区及成因,并暗示区域发生了幔源镁铁质岩浆与壳源长英质岩浆的混合作用,这种壳幔作用方式导致了克鲁-冲木达矿带斑岩型Cu-Mo矿化,但由于该区隆升剥蚀强烈,上部的斑岩型Cu矿体基本被剥蚀而没有保存下来,因此现今表现为斑岩型Mo矿床。
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2013, Vol. 29





