滇西九頂山斑狀花崗巖LA-ICP-MS鋯石U-Pb定年及地球化學(xué)特征

刀 艷, 李 峰, 王 蓉, 吳 靜, 范柱國, 陸世才

(昆明理工大學(xué) 國土資源工程學(xué)院，云南昆明 650093)

滇西九頂山斑狀花崗巖LA-ICP-MS鋯石U-Pb定年及地球化學(xué)特征

刀 艷, 李 峰, 王 蓉, 吳 靜, 范柱國, 陸世才

(昆明理工大學(xué) 國土資源工程學(xué)院，云南昆明 650093)

九頂山斑狀花崗巖產(chǎn)于揚(yáng)子板塊西緣與金沙江-哀牢山深大斷裂構(gòu)造帶東側(cè)交會部位，是礦區(qū)內(nèi)復(fù)式小雜巖體群中出露面積最大的巖體。巖體斑晶含量約占30%，主要由正長石、斜長石、石英和少量的黑云母、角閃石等組成；基質(zhì)為中細(xì)粒的石英、斜長石和正長石等組成，副礦物主要有磷灰石和鋯石等。斑狀花崗巖中鋯石U-Pb年齡為34.7±0.6 Ma，為喜馬拉雅早中期(始新世)的產(chǎn)物。斑狀花崗巖具有高鉀富堿高鋁的特征，屬堿性-堿鈣性，高鉀鈣堿性系列-鉀玄巖系列，大部分落入A型花崗巖區(qū)域。斑狀花崗巖輕稀土富集、重稀土虧損(La/Yb)N=28.90～42.28，負(fù)Eu異常(δEu=0.69～0.93)不明顯，富集大離子親石元素Rb、Ba、U、Th和虧損高場強(qiáng)元素Nb、Ta、Zr、Hf。通過斑狀花崗巖地球化學(xué)特征、圖解判別等的綜合研究，認(rèn)為九頂山斑狀花崗巖是殼?；旌铣梢?，形成于青藏高原新生代碰撞造山-造山期后拉張的構(gòu)造環(huán)境。

斑狀花崗巖 鋯石U-Pb年齡 地球化學(xué) 九頂山 云南

Dao Yan, Li Feng, Wang Rong, Wu Jing, Fan Zhu-guo, Lu Shi-cai. Zircon U-Pb geochronology and geochemistry of porphyritic granite in the Jiudingshan area, western Yunnan[J]. Geology and Exploration, 2014, 50(3):0533-0542.

0 引言

九頂山礦區(qū)地處青藏高原東南緣三江成礦帶的金沙江-哀牢山大成礦帶，在構(gòu)造上處于古近紀(jì)以來印度-歐亞大陸碰撞引起的地殼縮短和變形的過渡帶(Deweyetal., 1988)，具有長期而復(fù)雜的地質(zhì)演化歷史。長期以來，前人在該地區(qū)進(jìn)行了大量的地質(zhì)特征、巖石學(xué)、地球化學(xué)、同位素、成巖成礦構(gòu)造、成巖成礦動力學(xué)等研究(葛良勝等，2002；彭建堂等，2005；王治華等，2009，2011；周曉峰，2010；郭曉東等，2010；和文言等，2011)，而對單一巖體斑狀花崗巖的研究尚不全面。近年來研究發(fā)現(xiàn)，巖漿侵入時代為喜馬拉雅期，最早獲得斑狀花崗巖K-Ar年齡值為46.5～64.8Ma(西南冶金310隊(duì)，1981，轉(zhuǎn)自郭曉東，2009)，郭曉東等(2011)和楚亞婷等(2011)通過U-Pb法分別獲得鋯石年齡為33.78±0.21Ma和36.17 ± 0.36Ma，而對斑狀花崗巖的地球化學(xué)，同位素等研究相對缺乏。因此本文采用LA-ICP-MS鋯石U-Pb測年、主微量元素的確定，以期探討斑狀花崗巖的形成年齡、成因及其構(gòu)造環(huán)境。

1 地質(zhì)背景及巖石學(xué)特征

1.1 地質(zhì)背景

九頂山斑巖型銅鉬金多金屬礦床大地構(gòu)造位置處于揚(yáng)子板塊西緣與金沙江-哀牢山深大斷裂構(gòu)造帶東側(cè)交會部位。而金沙江-哀牢山成礦帶是西南三江地區(qū)重要的多金屬成礦帶之一，也是世界上著名的構(gòu)造巖漿帶之一，構(gòu)造運(yùn)動頻繁，巖漿活動強(qiáng)烈，成礦作用特殊，形成了豐富多樣的銅、鉬、金多金屬礦產(chǎn)資源。

礦區(qū)內(nèi)出露古生界下奧陶統(tǒng)向陽組(O1x)長石石英砂巖、粉砂巖、碳泥質(zhì)細(xì)砂巖夾條帶狀灰?guī)r、泥質(zhì)白云巖透鏡體以及下泥盆統(tǒng)康廊組(D1k)灰?guī)r地層和第四系(Q)地層。下奧陶統(tǒng)向陽組上亞段構(gòu)成九頂山富堿侵入體圍巖，是主要的賦礦地層(葛良勝等，2002)，本次研究的斑狀花崗巖就侵位于這套巖系中。

礦區(qū)構(gòu)造復(fù)雜，具有多期多階段活動特點(diǎn)，主要有區(qū)域性NW向金沙江-哀牢山深大斷裂、NE向斷裂及近EW向的隱伏斷裂構(gòu)造。礦區(qū)褶皺也較發(fā)育，主要包括區(qū)域性向陽復(fù)背斜的南端、金廠箐-人頭箐背斜、寶興廠(銅廠)-亂硐山向斜和雙馬槽向斜，它們對巖體及礦體的就位也起到明顯的控制作用。

1.2 斑狀花崗巖巖體特征

斑狀花崗巖廣泛分布與礦區(qū)北接觸帶、寶興廠、冷風(fēng)箐一帶，呈巖株?duì)町a(chǎn)出；總體呈灰白色，似斑狀結(jié)構(gòu)，塊狀構(gòu)造(圖2a)；斑晶占30%，主要由正長石(12%)、斜長石(10%)、石英(8%)和少量的黑云母、角閃石(3%)組成，其中正長石粒徑多在0.7～3.2mm，斜長石粒徑為0.7～3.2mm，石英為0.5～1.0mm；基質(zhì)為顯晶質(zhì)，由中細(xì)粒的石英、斜長石和正長石等組成。副礦物有鋯石、榍石、磷灰石和磷釔石等。顯微鏡下，基質(zhì)為顯微花斑結(jié)構(gòu)、顯微文象結(jié)構(gòu)(圖2b)，斑晶斜長石呈環(huán)帶結(jié)構(gòu)，正長石交代斜長石呈云霧狀，有時在斜長石的邊緣可見蠕蟲狀的石英，石英具有熔蝕現(xiàn)象，角閃石可見兩組解理的橫切面。

圖1 研究區(qū)地質(zhì)簡圖(據(jù)西南冶金地質(zhì)勘探公司310地質(zhì)隊(duì)，1981①修改)Fig.1 Regional geological map of research area in Jiudingshan, western Yunnan① 1-第四系; 2-下泥盆統(tǒng)康廊組灰?guī)r; 3-下奧陶統(tǒng)向陽組四段二亞段紫灰泥質(zhì)粉砂巖夾泥灰?guī)r透鏡體; 4-堿長花崗斑巖(γπ61-4); 5-花崗斑巖(γπ61-3);6-斑狀花崗巖(πγ61-2);7-正長斑巖(ξπ61-1);8-早期煌斑巖;9-晚期煌斑巖; 10-輝長巖; 11-巖體界線; 12-斷層(F1為響水?dāng)嗔?；F3為亂硐山斷裂)1-Quaternary; 2-limestone of lower Devonian Kanglang Group; 3-gray argillaceous siltstone with marl lens at the bottom of the second sub-member of the four member in lower Ordovician Xiangyang Group; 4-alkali feldspar granite porphyry; 5-granite porphyry; 6-porphyries granite; 7-syenite porphyry; 8-early lamprophyre; 9-late lamprophyre; 10-gabbro; 11-rock mass boundary; 12-fault (F1-Xiangshui fault; F3-Luandongshan fault)

圖2 九頂山斑狀花崗巖(a)及其鏡下特征(b)Fig.2 Porphyritic granite from Jiudingshan, west Yunnan (a) rock sample, (b) microscopic photograph

SpotωB/10-6Pb232Th238UTh/U同位素比值同位素年齡(Ma)207Pb/206Pb207Pb/235U206Pb/238U208Pb/232Th207Pb/206Pb207Pb/235U206Pb/238U208Pb/232Th比值1σ比值1σ比值1σ比值1σ年齡1σ年齡1σ年齡1σ年齡1σ11JDS037_018.21158710591.500.05520.00310.03950.00220.005260.00010.001710.000142012139.32.133.80.634.51.511JDS037_028.1766113310.500.05330.00280.03770.00190.005250.00010.001680.000134312037.51.933.80.533.91.411JDS037_038.4791512690.720.04850.00260.03630.00200.005380.00010.001730.000112411936.21.934.60.535.01.211JDS037_046.194659710.480.05250.00280.03980.00200.005520.00010.001710.000130912539.61.935.50.634.41.611JDS037_055.854118980.460.05460.00280.04130.00210.005590.00010.001930.000139411741.12.136.00.639.01.811JDS037_069.62174812461.400.05470.00290.04000.00200.005360.00010.001620.000039812339.82.034.40.432.70.811JDS037_075.746268630.730.05610.00330.04100.00220.005360.00010.001720.000145713140.82.234.50.634.71.311JDS037_088.28118911571.030.05550.00300.04020.00190.005370.00010.001610.000143212040.11.934.50.532.51.011JDS037_104.954037520.540.05460.00330.04130.00250.005630.00010.001800.000139413741.12.436.20.736.31.8

2 LA-ICP-MS 鋯石U-Pb定年

本次測年的樣品采自九頂山礦區(qū)2640中段探礦坑口SZ11-6，樣品(JDS037)地理位置：N25°31.426′，E100°25.825′。

鋯石挑選在河北廊坊地質(zhì)調(diào)查研究院完成，鋯石陰極發(fā)光(CL)顯微照相和鋯石U-Pb測年分析在中國地質(zhì)大學(xué)(武漢)地質(zhì)過程與礦產(chǎn)資源國家重點(diǎn)實(shí)驗(yàn)室完成的。詳細(xì)的分析方法及儀器參數(shù)可參考Chipley等(2007)。普通鉛校正采用Andersen(2002)的方法進(jìn)行，樣品的U-Pb年齡諧和圖繪制和年齡權(quán)重平均計算均采用Isoplot/Ex-ver3(Ludwig，2003)完成，分析結(jié)果列于表1。

從斑狀花崗巖(JDS037)中挑選出的鋯石晶體呈無色，形態(tài)多為長柱狀，鋯石大小約為83～138μm，長寬比近于2∶1，晶面和錐體形態(tài)完好。鋯石陰極發(fā)光圖像顯示該組鋯石發(fā)育明顯的震蕩環(huán)帶(圖3)，為巖漿成因鋯石的典型特征(Belousovaetal., 2002)，且232Th/238U比值均大于0.4，為0.46～1.50，具有巖漿鋯石的特點(diǎn)(Hoskin and Schaltegger, 2003; 吳元寶等，2004)。

九頂山斑狀花崗巖JDS037測試的10顆鋯石U-Pb同位素分析，第9顆鋯石樣品偏離值過大， 舍棄不用，絕大多數(shù)數(shù)據(jù)在諧和線上，其206Pb/238U年齡變化于33.8±0.5Ma～36.2±0.7 Ma，所有測點(diǎn)均投影在諧和線附近，諧和度在95%以上，其加權(quán)平均年齡為34.7±0.6 Ma(n=9，MSWD = 2.3，probability = 0.017)(圖4)，這個年齡代表了研究區(qū)斑狀花崗巖的結(jié)晶年齡。

表2 九頂山斑狀花崗巖主量元素化學(xué)分析測試數(shù)據(jù)%Table 2 Analysis of major elements(ωB/%) of porphyritic granite from Jiudingshan intrusion, Yunnan

注：*表示樣品數(shù)；①-本文；②-何明勤等,2004；④-Whalenetal., 1987。

圖3 九頂山斑狀花崗巖鋯石陰極發(fā)光圖像和測點(diǎn)年齡(Ma)Fig.3 Cathodoluminescence images and dating spots of the zircon of porphyritic granite in Jiudingshan (Ma)

圖4 九頂山斑狀花崗巖LA-ICP-MS 鋯石U-Pb年齡諧和圖解Fig.4 LA-ICP-MS zircon U-Pb Concordia diagrams of porphyritic granite from Jiudingshan

3 巖石地球化學(xué)特征

3.1 主量元素特征

本文對6組斑狀花崗巖樣品進(jìn)行分析，其中3組為本次分析樣品，另外3組來源于何明勤等(2004)。數(shù)據(jù)分析表明(表2)，斑狀花崗斑巖類ω(SiO2)含量在68.96%～77.28%之間，ω(K2O)為4.59%～6.60%，ω(Na2O)為1.15%～3.80%，ω(Al2O3)為10.96%～14.46%，ω(MgO)為0.07%～1.06%，ω(TFeO)為1.01%～3.85%，ω(K2O/Na2O)為1.21～5.74，ω(K2O+Na2O)為7.75%～8.55%。

從化學(xué)分析結(jié)果來看，斑狀花崗巖具有高硅(68.96%～77.28%)、富堿ω(K2O+Na2O=7.75%～8.55%)、高鋁(A/CNK=0.93～1.12)的特點(diǎn)。在Al2O3-FeOt-MgO三元圖解(圖5)中，投點(diǎn)位于過鋁質(zhì)花崗巖區(qū)域，在SiO2-K2O圖解上表現(xiàn)為鉀玄巖-高鉀鈣堿性系列(圖6)花崗巖。

圖5 九頂山斑狀花崗巖AFM三元圖解(據(jù)Irvine and Baragar, 1971)Fig.5 AFM triangular diagrams porphyritic granite from Jiudingshan

樣號LaCePrNdSmEuGdTbDyHoErTmYbLuY∑REELREE/HREEδEuδCe(La/Yb)N(La/Sm)N文獻(xiàn)來源3*48.4987.69.7933.475.251.373.440.432.130.300.900.130.810.1410.52204.779.890.930.940.555.81②4*60.30107.0013.940.307.261.534.470.833.020.571.490.271.410.2115.10257.668.410.760.8428.905.23③JDS-041-163.29117.3515.1841.058.131.767.250.783.480.671.800.241.560.2318.25341.018.960.690.9136.186.45①JDS-17466.72113.2210.9634.925.041.204.410.472.110.431.180.161.070.1512.33254.3710.40.760.942.288.33①

①注：①-本文；②-何明勤等(2004)；③-葛良勝等(2002)。

圖6 九頂山斑狀花崗巖SiO2-K2O圖解(底圖據(jù)Peccerillo et al., 1976)Fig.6 SiO2 versus K2O diagram of porphyritic granite from Jiudingshan (base diagram from after Peccerillo et al., 1976)

3.2 稀土元素和微量元素

稀土元素分析的樣品有4組(表3)，其中2組為本次實(shí)驗(yàn)采樣，數(shù)據(jù)分析由貴陽地球化學(xué)研究所完成，另外2組數(shù)據(jù)來自葛良勝等(2002)的4個樣品均值以及何明勤等(2004)的3個樣品均值。稀土總量為∑REE=204.77×10-6～341.01×10-6(均值為264.45×10-6)，∑LREE/∑HREE = 8.41～10.40，表現(xiàn)為LREE富集，球粒隕石標(biāo)準(zhǔn)化圖解為平滑的右傾曲線(圖7)，負(fù)銪異常δEu(0.69～0.93)不明顯。樣品的(La/Yb)N=28.90～42.28，具有明顯的輕重稀土元素分餾；(La/Sm)N=5.23～8.33，輕稀土分異明顯。

圖7 九頂山斑狀花崗巖稀土元素球粒隕石標(biāo)準(zhǔn)化配分圖(球粒隕石標(biāo)準(zhǔn)據(jù)Sun and McDonough,1989)Fig.7 Chondrite-normalized REE patterns of porphyritic granite from Jiudingshan (chondrite from Sun and McDonough, 1989)

九頂山斑狀花崗巖微量元素共3組數(shù)據(jù)(表4)，其中本次數(shù)據(jù)2組，另一組來源于何明勤等(2004)的3組數(shù)據(jù)的平均值。大離子親石元素(LILE)Rb(177.15×10-6～318.02×10-6)、Ba(485.17×10-6～1355.13×10-6)、U(6.66×10-6～9.32×10-6)、Th(21.31×10-6～38.06×10-6)富集，而高場強(qiáng)元素(HFSE)Nb(10.40×10-6～16.20×10-6)、Ta(0.86×10-6～0.99×10-6)、Zr(70.45×10-6～181.23×10-6)、Hf(2.42×10-6～5.02×10-6)相對虧損，Nb和Ta元素具有明顯負(fù)異常(圖8)。

圖8 九頂山斑狀花崗巖微量元素蛛網(wǎng)圖(原始地幔標(biāo)準(zhǔn)據(jù) Sun and McDonough,1989)Fig. 8 Trace element spider diagram of porphyritic granite from Jiudingshan (primitive mantle data from Sun and McDonough,1989)

樣號RbBaThUNbTaSrZrHfTiVCrCoNiCuZn文獻(xiàn)來源JDS-041-1177.15485.1738.069.3216.200.99816.43181.235.0244.7420.715.7613.5638.3132.60①JDS-174179.831214.5730.096.9813.570.86697.28132.733.7833.2927.714.6314.536.7139.84①3*318.021355.1321.316.6610.400.91500.2670.452.420.3234.5134.935.3320.63536.6781.79②

注：①-本文；②-何明勤等(2004)。

4 討論

4.1 斑狀花崗巖年代學(xué)意義

關(guān)于九頂山巖體，尤其是與成礦有關(guān)的斑狀花崗巖的成巖時代，前人開展了大量的研究工作，Liang(2007)通過全巖SHRIMP 年齡獲得與Cu-Mo有關(guān)的花崗巖為35.0Ma，駱耀南等(1998)應(yīng)用全巖Rb-Sr法獲得與Cu-Mo有關(guān)的花崗巖年齡為36Ma，和文言等(2011)鋯石U-Pb測年獲得37.93Ma，楚亞婷等(2011)應(yīng)用鋯石LA-ICP-MS U-Pb 年齡得出36.17Ma以及郭曉東等(2011)也是通過LA-ICP-MS U-Pb 年齡獲得33.78Ma的年齡值，時代為古近紀(jì)始新世(E2)。本文測試的斑狀花崗巖LA-ICP-MS鋯石U-Pb年齡為(34.7±0.6)Ma，進(jìn)一步證實(shí)九頂山斑狀花崗巖體與前人獲得正長斑巖、煌斑巖、花崗斑巖和堿長花崗斑巖的年齡一致。研究區(qū)巖漿侵位為4個階段，而斑狀花崗巖和煌斑巖組合屬于第Ⅱ階段巖漿侵位階段，在年齡上4個階段的巖體均為喜馬拉雅早-中期巖漿活動的產(chǎn)物，同時與青藏高原新生代碰撞造山中的晚碰撞階段(40～26 Ma)(侯增謙等，2006)相吻合。郭曉東等(2011)以及和文言等(2011)提出斑狀花崗巖與銅鉬礦化有密切關(guān)系，本次野外觀察也觀察到斑狀花崗巖體內(nèi)的少量礦化現(xiàn)象，斑狀花崗巖成巖年齡為33.78～37.93Ma，但是目前尚未有斑狀花崗巖銅鉬礦化年齡的研究，因此斑狀花崗巖是否與主成礦階段相關(guān)還未可知。根據(jù)前人的研究，斑巖型銅鉬礦為礦區(qū)主要的成礦階段，已有數(shù)據(jù)顯示礦化年齡為33.9±1.1～35.8±1.6(王登紅等，2004；曾普勝等，2006；邢俊兵等；2009；楚亞婷等，2011)。從斑狀花崗巖成巖年齡以及主成礦階段的年齡分析，斑狀花崗巖的侵入與主要的銅鉬礦化沒有直接的聯(lián)系，該巖體是否與研究區(qū)次成礦階段有關(guān)系，有待進(jìn)一步確認(rèn)。

4.2 巖石成因類型及源區(qū)特征

九頂山斑狀花崗巖組成SiO2含量在68.96%～77.28%之間，平均72.15%；堿質(zhì)含量K2O+Na2O 為7.75%～8.55%，平均8.22%，與世界范圍內(nèi)的A型花崗巖(K2O+Na2O=8.72%；Whalenetal., 1987)相當(dāng)。在Na2O-K2O圖解(圖9)中5個點(diǎn)落在A型區(qū)域，有1個點(diǎn)落在S型花崗巖范圍內(nèi)。稀土組成中總稀土含量較高(∑REE =264.45×10-6)和相對富集輕稀土，顯示了A型花崗巖的組成典型特點(diǎn)。

圖9 九頂山斑狀花崗巖Na2O-K2O判別圖解(底圖據(jù)Collins et al., 1982)Fig. 9 Na2O versus K2O diagram of genetic types of porphyritic granite from Jiudingshan (after Collins et al., 1982)

侯增謙等(2003)認(rèn)為，弧造山環(huán)境含礦斑巖主要為鈣堿性和高鉀鈣堿性系列，而碰撞造山環(huán)境含礦斑巖則主要為高鉀鈣堿性系列和鉀玄巖系列，成巖物質(zhì)可能來源于上地幔。在高鉀巖石中Rb、Sr、Ba 的含量趨于高值，而Nb、Ta、Ti 相對虧損，不相容元素這種高含量特點(diǎn)表明九頂山富堿斑巖體具有殼源物質(zhì)的參與，而Sr、Nd、Pb 同位素組成變化范圍較小，表明其物質(zhì)來源于富集地幔源區(qū)(何明勤等，2004)。張玉泉等(1997)通過對哀牢山-金沙江富堿侵入巖帶巖石的微量、稀土元素的地球化學(xué)特征進(jìn)行系統(tǒng)研究認(rèn)為其物質(zhì)來源于地幔源區(qū)。胡祥昭等 (1995) 認(rèn)為包括馬廠箐巖體在內(nèi)的滇西北的富堿花崗斑巖的物質(zhì)來源于地殼深部或上地幔。鄧萬明等(1998)認(rèn)為滇西北的富堿斑巖物質(zhì)來自“殼-?；旌蠈印钡牟糠秩廴凇．叓I(xiàn)武等(2005)通過對姚安和馬廠箐的富堿斑巖的研究認(rèn)為馬廠箐巖體物質(zhì)來源于富集地幔。趙欣等(2004)認(rèn)為滇西北的富堿斑巖物質(zhì)來源于富集地幔。由此可以看出九頂山礦區(qū)富堿斑巖的物質(zhì)來源于殼-?；旌蠋菦]有爭議的。

圖10 九頂山斑狀花崗巖構(gòu)造環(huán)境判別圖(底圖據(jù)Pearce et al.,1984)Fig. 10 Trace discrimination diagrams for the tectonic setting of porphyritic granites (base diagram from after Pearce et al.,1984)

4.3 構(gòu)造環(huán)境判別

前人對包括九頂山復(fù)式雜巖體在內(nèi)的滇西北的富堿花崗斑巖的產(chǎn)出構(gòu)造環(huán)境的認(rèn)識有爭議。曾普勝等(2002) 認(rèn)為形成于剪切走滑擠壓為主、局部引張的構(gòu)造環(huán)境；趙欣等(2004)認(rèn)為形成于后碰撞弧環(huán)境；畢獻(xiàn)武等(2005)認(rèn)為形成于大陸弧環(huán)境；而鄧萬明等(1998)和葛良勝等(2002)認(rèn)為形成于碰撞后板內(nèi)構(gòu)造環(huán)境。富堿侵入巖物質(zhì)來源較深，一般源于上地幔，形成于拉張環(huán)境中(涂光熾，1987)。在花崗巖構(gòu)造環(huán)境判別的研究中，Pearceetal(1984)最早系統(tǒng)地討論了花崗巖與其形成的構(gòu)造環(huán)境問題。Pearceetal(1984)將花崗巖分為4種基本類型：洋脊花崗巖(ORG)、火山弧花崗巖(VAG)、板內(nèi)花崗巖(WPG)和同碰撞花崗巖(S-COLG)。Pearce等還認(rèn)為，Rb、Y(Yb)和Nb(Ta)是花崗巖最有效的判別指標(biāo)。將九頂山礦區(qū)斑巖體的相應(yīng)微量元素含量分別投到Pearce的(Yb+Ta)-Rb、(Y+Nb)-Rb、Ta-Yb、Nb-Y等圖解中(圖10)，樣品投在同碰撞花崗巖-火山弧花崗巖。九頂山的6個樣品投影點(diǎn)在R1-R2圖解(圖11)中落在碰撞造山-造山期后范圍內(nèi)。

圖11 九頂山斑狀花崗巖 R1-R2圖解(底圖據(jù)Bachelor and Bowden, 1985)Fig.11 Diagrams showing R1 vs.R2 of porphyritic granites from Jiudingshan (after Batchelor and Bowden,1985)

5 結(jié)論

(1) 九頂山礦區(qū)斑狀花崗巖LA-ICP-MS鋯石U-Pb年齡為34.7±0.6Ma，為屬于喜馬拉雅運(yùn)動早期始新世(E2)巖漿活動，處于滇西新生代富堿巖漿活動高峰期(45～30Ma)范圍內(nèi)，是青藏高原碰撞造山帶的晚碰撞造山作用(40～26 Ma)的產(chǎn)物。

(2) 九頂山斑狀花崗巖具有高鉀富堿高鋁的特征，為高鉀鈣堿性系列-鉀玄巖系列，并表現(xiàn)出A型花崗巖的特征。

(3) 根據(jù)斑狀花崗巖的巖石化學(xué)、稀土元素和微量元素地球化學(xué)特征等綜合因素，判斷九頂山斑狀花崗巖是殼幔源混合成因的斑狀花崗巖，形成于青藏高原新生代碰撞造山-造山期后拉張的構(gòu)造環(huán)境。

致謝 中國地質(zhì)大學(xué)地質(zhì)過程與礦產(chǎn)資源國家重點(diǎn)實(shí)驗(yàn)室宗克清博士和葉曉峰同學(xué)在鋯石 U-Pb 同位素測試分析中提供了幫助，評審專家及編輯對本文提出寶貴的修改意見，在此一并表示衷心的感謝！

[注釋]

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Zircon U-Pb Geochronology and Geochemistry of Porphyritic Granite in the Jiudingshan area, West Yunnan

DAO Yan, LI Feng, WANG Rong, WU Jing, FAN Zhu-guo, LU Shi-cai

(Department of Land Resources Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093)

The Jiudingshan porphyritic granite, the largest exposed rock of the Jiudingshan complex, is located at the junction of the west Yangtze plate and the east Ailaoshan-Jinshajiang fault. Megacrysts (10%) of porphyric granite contain orthoclase, plagioclase, quartz and biotite, and matrix comprises quartz, plagioclase and orthoclase. The accessory mineral assemblages consist of apatite, zircon, and so on. LA-ICP-MS zircon U-Pb dating of 10 zircon grains from Jiudingshan porphyritic granite gives a concordant age of 34.7±0.6 Ma, the Eocene. The porphyric granite is petrochemicaly characterized by high potassium, rich alkali and high alumina, belonging to high K calc alkaline series and shoshonite series, which can be classified as A-type granite. The intrusion shows LREE enrichment, HREE depletion, weak negative Eu anomaly (δEu=0.69～0.93) and a smoothly right-declined REE distribution pattern, enrichment of LILE (Rb, Ba, U, Th) and depletion of HFSE (Nb, Ta, Zr, Hf) with negative Nb and Ta. Geochemical characteristics in discrimination diagrams of porphyritic granite in Jiudingshan indicate that this rock body was mainly derived from a crust source mixed with mantle materials and formed at the Cenozoic syn-collision and post-orogenic stage of the Tibetan plateau in an extensional tectonic setting.

porphyritic granite, zircon U-Pb dating, geochemistry, Jiudingshan, Yunnan

2013-11-19；

2014-02-21；[責(zé)任編輯]郝情情。

國家危機(jī)礦山接替資源勘查項(xiàng)目(編號：20089943)和云南省自然科學(xué)基金項(xiàng)目(編號：2011FZ035)資助。

刀 艷(1988年-)，女，2011年畢業(yè)于昆明理工大學(xué)，獲學(xué)士學(xué)位，碩士研究生在讀，主要從事礦床學(xué)研究。E-mail：daoyan_1120@126.com。

P595,P597+.3

A

0495-5331(2014)03-0533-10