內(nèi)蒙古白乃廟銅鉬礦區(qū)侵入巖LA-MC-ICP-MS鋯石U-Pb定年及地質(zhì)意義

馮曉曦,姚書振，王家松，魏佳林,李光耀,王佳營，馮旭彪

(1. 中國地質(zhì)大學(xué)(武漢)資源學(xué)院，湖北武漢 430074； 2. 天津地質(zhì)調(diào)查中心，天津 300170；3. 深圳市冠欣礦業(yè)集團(tuán)，廣東深圳 518048)

內(nèi)蒙古白乃廟銅鉬礦區(qū)侵入巖LA-MC-ICP-MS鋯石U-Pb定年及地質(zhì)意義

馮曉曦1,2,姚書振1，王家松2，魏佳林2,李光耀2,王佳營2，馮旭彪3

(1. 中國地質(zhì)大學(xué)(武漢)資源學(xué)院，湖北武漢 430074； 2. 天津地質(zhì)調(diào)查中心，天津 300170；3. 深圳市冠欣礦業(yè)集團(tuán)，廣東深圳 518048)

白乃廟銅鉬礦是華北地塊北緣最重要的大型銅鉬礦，礦區(qū)中部花崗閃長斑巖與成礦關(guān)系密切。為探討成巖成礦關(guān)系，筆者選取花崗閃長斑巖進(jìn)行了LA-ICP-MS鋯石U-Pb定年和巖石地球化學(xué)特征研究?；◢忛W長斑巖測年分別為432.4±2.2 Ma、440.7±2.1Ma和443±1Ma，巖體侵位時(shí)代為早志留世，與銅礦石中輝鉬礦Re-Os等時(shí)線年齡440.5±4.4Ma(待刊)基本一致。巖體屬高鉀鈣堿性巖石組合系列，稀土曲線右傾，δEu 14.83～-19.54，弱虧損，富集Rb、Ba、Th、U、K，虧損Nb、Ta、Ti，具有島弧花崗巖的地球化學(xué)性質(zhì)。結(jié)合區(qū)域地質(zhì)背景，認(rèn)為白乃廟銅鉬礦成礦與早志留世花崗閃長斑巖巖漿期后熱液活動(dòng)密切相關(guān)，成礦構(gòu)造背景為早古生代古亞洲板塊向華北板塊俯沖形成的白乃廟古火山島弧帶。

LA-ICP-MS鋯石U-Pb定年 花崗閃長巖 成巖成礦 白乃廟銅鉬礦

Feng Xiao-xi，Yao Shu-zhen，Wang Jia-song，Wei Jia-lin，Li Guang-yao，Wang Jia-ying，F(xiàn)eng Xu-biao. LA-MC-ICP-MS zircon U-Pb dating of the pluton in the Bainaimiao Cu-Mo deposit of Inner Mongolia and its geological significance[J]. Geology and Exploration, 2014, 50(2):0369-0381.

1 引言

白乃廟銅(鉬)礦位于內(nèi)蒙古四子王旗白音朝克圖蘇木，礦區(qū)由南、北礦帶，十三個(gè)礦段組成。2010年以后，深圳冠欣礦業(yè)在白乃廟銅(鉬)礦Ⅱ、Ⅲ、Ⅳ礦段深部發(fā)現(xiàn)厚大銅礦體，找礦成果正逐步擴(kuò)大。

白乃廟銅(鉬)礦為華北地塊北緣唯一的大型銅(鉬)礦床，以其獨(dú)特的成礦地質(zhì)條件吸引許多專家學(xué)者從地質(zhì)、地球化學(xué)、構(gòu)造演化、成巖成礦時(shí)代等進(jìn)行了許多研究。一些學(xué)者對(duì)礦區(qū)中部與成礦關(guān)系密切的花崗閃長斑巖進(jìn)行了成巖時(shí)代的研究。王東方(1986)測得花崗閃長斑巖的Rb-Sr模式年齡為386Ma?；◢忛W長斑巖呈巖床產(chǎn)于綠片巖中，認(rèn)為是島弧背景下中淺成巖漿分異產(chǎn)物。胡驍(1988)認(rèn)為白乃廟島弧巖系的深成巖花崗閃長巖、花崗閃長斑巖、石英閃長巖、英云閃長巖等巖石Rb-Sr模式年齡為400±30Ma(386、419、340Ma)，認(rèn)為巖體屬成弧期同熔型花崗巖。唐克東(1992)測得白乃廟含礦花崗閃長斑巖U-Pb年齡為466Ma(鋯石)，花崗閃長斑巖K-Ar年齡為450Ma(角閃石)，白云母花崗巖K-Ar年齡為429.8Ma(白云母)，認(rèn)為白乃廟銅礦是早古生代環(huán)大洋島弧發(fā)育中晚期形成的斑巖型銅礦。聶鳳軍等(1995)測得花崗閃長斑巖巖體Sm-Nd同位素等時(shí)線年齡為440±40Ma(2δ)，εNd(t)=-3.2±1.7(2δ)，認(rèn)為北礦帶的形成過程與加里東期(440Ma)花崗閃長斑巖的侵位和結(jié)晶分異密切相關(guān)。陳衍景(2009)測得侵入白乃廟組的花崗閃長斑巖侵位年齡為430±6Ma(n=13，MSWD=1.1)，白乃廟北礦帶容礦破碎斑巖年齡為445±6Ma(n=14，MSWD=2.4)，輝鉬礦Re-Os等時(shí)線年齡為444±30Ma(MSWD=2.8)，認(rèn)為礦區(qū)存在早古生代斑巖型銅礦化。由上述可見，礦區(qū)花崗閃長斑巖成巖時(shí)代的精確厘定是研究白乃廟銅(鉬)礦床成礦機(jī)制的關(guān)鍵要素，但上述研究表明巖體侵位時(shí)代為386Ma～466Ma，時(shí)代跨度80Ma，不利于對(duì)礦床成礦的認(rèn)識(shí)。

為了進(jìn)一步精確厘定白乃廟銅鉬礦成礦成巖關(guān)系，筆者選擇礦區(qū)花崗閃長斑巖進(jìn)行LA-ICP-MS鋯石U-Pb定年和巖石化學(xué)特征研究(Hoskinetal,2003 ;Donaldetal,2003)，初步探討研究白乃廟銅鉬礦成巖成礦時(shí)代和地質(zhì)構(gòu)造背景。

2 地質(zhì)背景及含礦巖體特征

2.1 地質(zhì)背景

白乃廟銅(鉬)礦床位于華北地臺(tái)北緣中段，南鄰華北地臺(tái)北緣赤峰-白云鄂博深大斷裂，北依溫都爾廟俯沖-增生雜巖帶(圖1)。

礦區(qū)出露的地層有新元古代白云鄂博群，寒武系白乃廟組，中志留統(tǒng)徐尼烏蘇組，二疊系下統(tǒng)三面井組，白堊系下統(tǒng)固陽組等。主要成礦圍巖白乃廟組為一套中淺變質(zhì)的綠片巖，其原巖為一套海底噴發(fā)的基性-中酸性火山熔巖、凝灰?guī)r，夾少量正常沉積的碎屑巖和碳酸鹽巖，為淺?；鹕匠练e建造(圖2)。

礦區(qū)構(gòu)造復(fù)雜，主要以斷裂構(gòu)造為主。東西向構(gòu)造是白乃廟礦區(qū)主要的控巖控礦構(gòu)造，控制了早古生代基性-中酸性火山噴發(fā)及花崗閃長斑巖等中酸性巖漿活動(dòng)。與成礦相關(guān)的花崗閃長斑巖呈巖株?duì)顤|西向展布，侵入于白乃廟組(圖2)。

北礦帶礦體主要產(chǎn)在花崗閃長斑巖中，巖體受東西向構(gòu)造控制，基本是順層侵入于綠片巖中。礦體大部分產(chǎn)于巖體中，巖體控制明顯，少數(shù)礦體受構(gòu)造控制明顯，礦體具有從巖體外延至圍巖中的現(xiàn)象。南礦帶礦體主要產(chǎn)于綠片巖中，礦體產(chǎn)狀與圍巖基本一致。礦體呈似層狀、透鏡體狀、單層或者多層，平行或者斜列式產(chǎn)出。礦體具有膨大、收縮、分枝、尖滅等現(xiàn)象。圍巖蝕變主要有：鉀長石化、黑云母化、硅化、絹云母化、綠泥石化、綠簾石化、碳酸鹽化，前三種蝕變與成礦關(guān)系最為密切。礦石類型有花崗閃長斑巖型和綠片巖型。

圖1 內(nèi)蒙古中部地區(qū)區(qū)域地質(zhì)構(gòu)造圖(李文博等,2007,略改)Fig.1 Regional geological map of the central section of Inner Mongolia (after Li et al. ,2007 with slight modification) 1-南蒙古生代大陸邊緣；2-南蒙古生代弧增生雜巖；3-索倫克爾縫合帶晚古生代增生雜巖；4-華北早古生代大路邊 緣；5-華北前寒武紀(jì)克拉通；6-蛇綠巖；7-高壓變質(zhì)巖1-Paleozoic continental margin of south Mongolia; 2-Paleozoic arc-accreted complex of south Mongolia; 3-late Paleozoic accretionary complex in the the Solonker suture zone; 4-early Paleozoic continental margin of North China; 5-North China Precambrian craton; 6-ophiolite; 7-high-pressure metamorphic rocks

圖2 白乃廟銅鉬礦地質(zhì)構(gòu)造略圖(來自李進(jìn)文2007，略改)Fig.2 Simplified geological map of the Bainaimiao copper-molybdenite deposit (modified from Li et al.,2007) 1-第四系；2-下白堊統(tǒng)固陽組礫巖；3-下二疊統(tǒng)三面井組砂巖；4-中志留統(tǒng)徐尼烏蘇組細(xì)碎屑巖；5-白乃廟組第五巖段；6-白乃廟組第四巖段；7-白乃廟組第三巖段；8-白乃廟組第二巖段；9-白乃廟組第一巖段；10-中元古界白云鄂博群白音寶拉格組；11-志留世花崗閃長斑巖；12-二疊世石英閃長巖；13-二疊世白云母花崗巖;14-花崗斑巖脈;15-石英脈;16-礦化帶、礦體及礦段編號(hào);17-實(shí)測及性質(zhì)不明斷層;18-白乃廟斷裂帶;19-地質(zhì)界線/不整合地質(zhì)界線;20-巖石化學(xué)樣品采集位置及編號(hào); 21-測年樣品采集位置及編號(hào)1-Quaternary; 2-low Cretaceous Guyang Fm. conglomerate; 3-low Permian Sanmianjing Fm. sandstone; 4-middle Silurian Sanmianjing Fm. fine-grained clastic rock; 5-fifth lithological member of Bainaimiao Fm.; 6-fourth lithological member of Bainaimiao Fm.; 7-third lithological member of Bainaimiao Fm.; 8-second member of Bainaimiao Fm.; 9-first member of Bainaimiao Fm. ;10-Mesoproterozoic Bayan Obo Group Baiyin Baolage Fm.; 11-top Silurian granodiorite;12-Permian quartz diorite; 13-Permian muscovite granite; 14-granite porphyry vein;15-quartz vein; 16-mineralization zone, orebody and number of ore section;17-measured and unknown fault; 18-Bainaimiao fracture zone;19-geological boundary / unconformity; 20-position and number of petrochemistry sample; 21-position and number of sam-ple for dating

2.2 巖體特征

花崗閃長斑巖主要出露于礦區(qū)中部、北部，南礦帶綠片巖中也有零星出露。受區(qū)域變質(zhì)作用，蝕變強(qiáng)烈。風(fēng)化面多呈灰黃-褐黃色?；◢忛W長斑巖體呈近東西向侵入白乃廟組，出露面積分別為0.5km2。巖石多為灰白-淺肉紅色，似片麻狀構(gòu)造、班狀構(gòu)造。具花崗結(jié)構(gòu)，花崗碎斑結(jié)構(gòu)，交代殘余。巖石由石英(35%)、堿性長石(20%)、斜長石(30%)和暗色礦物(角閃石、黑云母)的蝕變礦物組成。副礦物見有磁鐵礦(<1%)、磷灰石(<1%)和金紅石(<1%)，零星分布。蝕變礦物有綠泥石、絹云母、白云母、方解石及金屬礦物黃鐵礦、黃銅礦、輝鉬礦等。

石英碎斑呈它形粒狀，具波狀消光。堿性長石呈它形板狀，微斜長石可見格子雙晶，鈉長石可見聚片雙晶。斜長石呈它形-半自形板狀，可見環(huán)帶構(gòu)造，顆粒中心較具較強(qiáng)的絹云母、簾石、碳酸鹽化。碎基含量60%，粒徑小于0.5mm，由石英、堿性長石碎粒和蝕變礦物組成。石英碎粒呈它形粒狀集合體分布，長石呈它半條-粒狀碎粒分布。蝕變礦物綠泥石、簾石、白云母、絹云母、方解石及鐵質(zhì)呈它形粒狀、片狀集合體、略具定向性分布(圖3)。

圖3 礦區(qū)中部D016點(diǎn)花崗閃長斑巖(4×10正交偏光)比例尺Fig.3 Granodiorite of D016 dot from the central deposit(4×10 orthogonal polarization)

3 巖石化學(xué)特征

對(duì)礦區(qū)中部花崗閃長斑巖采集4件樣品進(jìn)行巖石化學(xué)分析，分析結(jié)果列于表1。

表1 花崗閃長斑巖的主量元素氧化物(%)、微量元素和稀土元素(×10-6 )分析結(jié)果

續(xù)表1

Continued Table 1

樣品號(hào)D016D017D016-1D010H2O-0.440.370.380.39CO21.740.0212.240.018Cu51.84200129975Pb5.908.825.948.68Cr8.4810.407.788.42Li15.48.6417.4021.10Rb81.69691.5122Cs4.121.344.483.34W7.641716.0312Sr436642493436Ba936322843472V140168140155Sc12.410.311.115Nb9.6210.69.6311Ta0.580.620.630.66Zr126125125127Hf3.713.753.783.84Be1.441.311.631.57Ga15.516.615.316.8U1.894.661.713.42Th13.89.4916.316.5La35.115.135.446Ce54.636.456.667Pr7.24.277.049.17Nd27.216.826.134.4Sm4.923.784.666.06Eu1.51.291.421.7Gd4.183.794.044.99Tb0.570.640.550.71Dy33.942.853.84Ho0.60.820.570.75Er1.622.41.612.11Tm0.260.390.260.34Yb1.82.821.862.36Lu0.280.430.270.35Y1521.114.619.3F7361470979985

注：天津地質(zhì)礦產(chǎn)研究所測試，2013年。

本文所研究的4件樣品的SiO2：60.09%～65.21% ；A12O3：15.40%～16.47%;MgO:1.58%～2.73%;K2O:2.52%～3.78%;Na2O:3.18%～4.00%；A/CNK為1.42～1.79。采用Geokit2012軟件對(duì)數(shù)據(jù)進(jìn)行處理(路遠(yuǎn)發(fā)，2004)。

在REE元素球粒隕石標(biāo)準(zhǔn)化分布型式圖上顯示平坦型，略右傾特征(圖4)，∑REE值為113.97～199.08, LREE/HREE值為2.13～4.93，Eu弱虧損，δEu值為14.83～19.54，LaN/YbN=3.62～13.17，LaN/SmN=2.51～4.78，GdN/YbN=0.62～0.75，REE富集型，輕稀土分餾程度要高于重稀土，為典型的花崗巖漿巖的特征(李洪普等，2011)。

圖4 白乃廟銅鉬礦侵入巖球粒隕石標(biāo)準(zhǔn)化稀土配分模式圖(C1球粒隕石據(jù)Sun and Mc Donough,1989)Fig.4 Chondrite-normalized REE patterns of granite in the Baibaimiao Cu-Mo deposit(chondrite from Sun and Mc Donough,1989)

在微量元素標(biāo)準(zhǔn)化模式圖上, 元素含量均高于原始地幔巖，相對(duì)富集Rb、Th、K 和La、Sm,相對(duì)虧損Ba、Ta、Nb、Ti、P和Yb(圖5)，Nb/Ta為15.28～17.10(正常花崗巖Nb/Ta約11)，Zr/Hf為33.1～33.9(正常花崗巖Zr/Hf約36～39)，Rb/Sr為0.15～0.28，具島弧的鈣堿性巖系的地球化學(xué)特征(趙振華，1997;梁亭等,2008;熊欣等,2011)。

圖5 白乃廟銅鉬礦侵入巖原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(C1球粒隕石據(jù)Sun and Mc Donough,1989)Fig.5 Primitive-mantle normalized spidergram in the Baibaimiao Cu-Mo deposit(chondrite from Sun and Mc Donough,1989)

4 鋯石樣品測試

4.1 測試方法

鋯石樣品 Tj08采自Ⅻ礦段、Tj09采自Ⅸ礦段、Tj11采自Ⅷ礦段(圖2)。

鋯石樣品靶的制作和鋯石陰極發(fā)光照相在北京離子探針中心完成。LA-MC-ICP-MS鋯石U-Pb定年在天津地質(zhì)礦產(chǎn)研究所實(shí)驗(yàn)室完成。

4.2 測年結(jié)果

3件鋯石樣品在透射光下為無色，顆粒自形程度較好，多呈柱狀，長軸變化于100～150μm，個(gè)別達(dá)300μm，長短軸之比在1∶1～1∶2之間。在陰極發(fā)光圖像中，大多數(shù)鋯石均發(fā)育較好的振蕩環(huán)帶，Th/U一般大于0.5,顯示為巖漿鋯石(圖6、圖7,表2～4)，該年齡代表了巖漿結(jié)晶年齡(余吉遠(yuǎn)等，2010)。測年結(jié)果分述如下：

Tj08樣品測試了33粒鋯石，鋯石U的含量為204×10-6～1228×10-6，平均為546×10-6，Th/U為0.39～ 1.60 ,Th/U一般大于0.5。206Pb/238U年齡范圍在314.4±2.91Ma～442.2±7.22Ma，加權(quán)平均年齡為432.4 ±2.2 Ma，MSWD =1.8, 擬合概率0.008(圖6)。大部分鋯石振蕩環(huán)帶清晰，為巖漿鋯石，測年為418.3±3.71Ma～442.2±7.22Ma,在誤差范圍內(nèi)與諧和年齡一致。此外還有3粒鋯石206Pb/238U年齡為：Tj0801點(diǎn)314.4±2.91Ma、Tj0806點(diǎn)368.3±3.29Ma、Tj0818點(diǎn)372.7±3.58Ma，前兩者具環(huán)帶，但顆粒小，第三粒無環(huán)帶，略具晶形，三者可能為熱液鋯石，與變質(zhì)熱液疊加有關(guān)。

Tj09樣品測試了31粒鋯石，鋯石U的含量為51×10-6～1706×10-6，平均為703×10-6，Th/U大多0.52～ 1.43，Tj0909點(diǎn)為0.2，為基底捕獲鋯石，Tj0919點(diǎn)為0.06，原因不明。206Pb/238U年齡范圍在372.3±2.24Ma～1742.2±12.05Ma，加權(quán)平均年齡為440.7±2.1 Ma，MSWD = 3.1, 擬合概率0(圖6)。大部分鋯石振蕩環(huán)帶清晰，為巖漿鋯石，測年為423.3±2.54Ma～450.5±3.08Ma。Tj0906、Tj0909點(diǎn)、Tj0910點(diǎn)均為捕獲基底鋯石。

Tj11樣品測試了32粒鋯石。鋯石U的含量為454×10-6～4445×10-6，平均為715×10-6，Th/U大多為0.50～ 1.01。206Pb/238U年齡范圍在383.0±2.43Ma～1487.8±10.17Ma，加權(quán)平均年齡為443±1 Ma，MSWD = 0.34, 擬合概率1。大部分鋯石振蕩環(huán)帶清晰，為巖漿鋯石，測年為440.1±2.69Ma～446.3±2.73Ma。Tj1110點(diǎn)與Tj1111點(diǎn)測年分別為1238.3±10.1Ma、1487.8±10.17Ma，Tj1128點(diǎn)測年為1288.3±11.94Ma，均為捕獲基底鋯石。

圖6 白乃廟銅鉬礦花崗閃長巖樣品Tj08、Tj09、Tj11鋯石陰極發(fā)光照片圖像Fig.6 CL images of zircon grains for granodiorite sample Tj08, Tj09, and Tj11 from Bainaimiao Cu-Mo deposit

圖7 白乃廟銅鉬礦區(qū)Tj08、Tj09、Tj11花崗閃長巖鋯石U-Pb諧和曲線圖Fig.7 U-Pb concordia diagram of zircon in granodiorite from Tj08, Tj09 and Tj11 samples of Bianaimiao Cu-Mo deposit

分析號(hào)含量(×10-6)同位素比值年齡(Ma)Tj08PbU206Pb/238U1σ207Pb/235U1σ207Pb/206Pb1σ208Pb/232Th1σ232Th/238U1σ206Pb/238U1σ207Pb/235U1σ1406680.05000.00050.68870.01070.09990.00140.01670.00040.82940.0054314.42.91532.08.252283830.07020.00090.54560.00840.05640.00080.02800.00040.39000.0062437.45.56442.16.823445800.06880.00070.56720.00700.05980.00070.01980.00020.74320.0016428.74.38456.25.654435520.07010.00060.55750.01200.05770.00120.02710.00060.55580.0034436.73.89449.99.665354620.06810.00080.55130.00870.05870.00090.01900.00020.83160.0035424.95.25445.97.066558500.05880.00050.51970.00400.06410.00050.01110.00011.05500.0135368.33.29424.93.277506510.06920.00070.55920.00750.05860.00080.01740.00020.84170.0077431.24.19451.06.068364650.07000.00060.57550.00620.05960.00060.02240.00020.65490.0084436.43.90461.65.009577540.06800.00040.54060.00620.05760.00070.01490.00010.98660.0036424.22.43438.85.0610294010.06920.00110.53020.00730.05550.00070.01570.00040.68300.0162431.66.56432.05.9511415420.07030.00060.57720.00560.05950.00060.02210.00010.54420.0018438.03.88462.64.4812486360.07040.00080.54970.00470.05660.00040.01970.00020.66830.0045438.54.79444.83.8113273680.06780.00060.55200.01780.05910.00180.02180.00050.60340.0042422.84.01446.314.3714445810.06950.00070.54790.00680.05710.00080.02220.00020.61050.0021433.44.16443.65.5115253510.06960.00090.55830.00730.05820.00070.02150.00020.44250.0051433.55.76450.45.8916486000.07040.00100.56250.00590.05800.00050.02110.00020.80030.0019438.56.06453.24.7717425410.06970.00100.56820.00600.05910.00060.02020.00020.79520.0055434.46.43456.94.84189112280.05950.00060.59520.00510.07250.00060.01340.00011.60050.0063372.73.58474.24.0719192440.07030.00070.55390.01610.05710.00130.02020.00070.71000.0063438.24.34447.512.9820455610.07050.00100.55540.00560.05720.00060.02080.00010.82910.0046439.06.48448.54.5021354680.07010.00050.56150.00850.05810.00090.02530.00050.46810.0026436.73.10452.56.8622405030.06920.00060.53990.00830.05660.00090.02300.00020.76560.0042431.44.00438.36.7723425740.06700.00060.56460.00660.06110.00071.100.00040.77100.0147418.33.71454.55.2924162040.07010.00060.56680.02090.05860.00203.420.00070.39280.0017436.93.95456.016.8125536950.06850.00070.55500.00540.05870.00060.960.00020.80040.0077427.24.23448.34.3926516760.07020.00110.56360.00510.05820.00050.790.00020.63150.0052437.46.73453.84.1427465990.07050.00070.56700.00620.05830.00061.020.00020.58820.0029439.24.30456.15.0028344560.06750.00070.55850.00940.06000.00101.700.00020.94450.0043421.04.16450.67.5729344070.07000.00100.55980.00810.05800.00101.710.00010.99090.0247436.36.28451.46.5030385100.06920.00070.55870.00800.05860.00081.410.00020.62610.0044431.14.43450.76.4231415130.07100.00120.54520.00710.05570.00071.200.00020.66830.0040442.27.22441.95.7632496110.06890.00080.54800.00640.05770.00061.090.00020.83760.0085429.65.22443.75.1633283690.07030.00100.55370.00960.05710.00101.770.00040.56060.0038437.96.35447.47.76

注：天津地質(zhì)礦產(chǎn)研究所，2013年4月測試。

表3 白乃廟花崗閃長斑巖LA-ICP-MS鋯石U-Pb測試結(jié)果表Table 3 Zircon LA-ICP-MS U-Pb dating results of of granodiorite from Bainaimiao deposit

注：天津地質(zhì)礦產(chǎn)研究所，2013年4月測試。

表4 白乃廟花崗閃長斑巖LA-ICP-MS鋯石U-Pb測試結(jié)果表Table 4 Zircon LA-ICP-MS U-Pb dating results of of granodiorite from Bainaimiao deposit

注：天津地質(zhì)礦產(chǎn)研究所，2013年4月測試。

圖8 白乃廟銅鉬礦花崗巖類(La/Yb)N-YbN圖解(a,Martin,199)和Sr/Y-Y圖解(b,Difant,1993)Fig.8 Diagrams of(La/Yb)N(a,Martin,1999)-YbN and Sr/Y-Y(b,Defant,1993)for granites in Bainaimiao Cu-Mo deposit

5 討論

5.1 巖體的成巖時(shí)代

一般認(rèn)為，巖漿鋯石的封閉溫度較高(>850℃)，其結(jié)晶年齡代表了巖體的侵入年齡。研究表明巖漿鋯石環(huán)帶清晰，Th/U值高，通常在0.5以上(Hoskinetal,2003; Donaldetal.,2003; Davisetal,2003)?；◢忛W長斑巖Th/U 值絕大部分大于0.5，多具清晰振蕩環(huán)帶，并有少量鋯石具有核幔結(jié)構(gòu),核部應(yīng)為繼承源巖的成分，鋯石晶域的年齡可解釋為該巖石的侵位年齡(李鵬等,2011;叢源等,2012)。

本文得到的花崗閃長斑巖LA-MC-ICP-MS鋯石年齡加權(quán)平均值為435.5±1.9 Ma、439.8±2.5Ma、443.0±1.0Ma。與陳衍景(2009)測得的花崗閃長斑巖侵位年齡為430±6Ma～445±6Ma基本一致，晚于唐克東(1992)測得的含礦花崗閃長斑巖U-Pb年齡466Ma(鋯石)約30Ma，早于胡驍(1988)測得的花崗閃長斑巖Rb-Sr年齡(400Ma)約30Ma。綜合分析認(rèn)為白乃廟礦區(qū)花崗閃長斑巖侵位時(shí)代435.4Ma～443.0Ma，為早志留世。

5.2 巖漿演化與大地構(gòu)造環(huán)境探討

圖9 白乃廟銅礦花崗巖類(Y+Nd)-Rb 圖解(a) 和Nb-Y 圖解(b)(Pearce et al.,1984)Fig.9 Diagrams of (Y+Nd)-Rb and Nb-Y for granite in Bainaimiao Cu-Mo deposit(Pearce et al.,1984)

本次所研究的4 件花崗巖類巖石化學(xué)樣品具有低場強(qiáng)元素Rb、K、Ba、Sr和高場強(qiáng)元素Sc、Y、Th、U元素正異常，Nb、P、Ti 、Ta高場強(qiáng)元素負(fù)異常，為俯沖環(huán)境下巖漿的典型特征(張建新等,2003;惠衛(wèi)東等,2013, 姚志剛等，2010)，Eu虧損，推測為洋殼向陸殼俯沖到達(dá)一定深度時(shí)，洋殼中的角閃巖相會(huì)發(fā)生脫水，形成榴輝巖相，水就會(huì)攜帶角閃巖相中的Rb、Cs、Sr、Ba進(jìn)入地慢楔，具有島弧巖石的一般地球化學(xué)特征。

本文樣品在LaN/YbN-YbN圖解(圖8a)和Sr/Y-Y圖解(圖8b)部分落入埃達(dá)克巖區(qū)與經(jīng)典島弧巖石區(qū)交叉區(qū)，主要落入經(jīng)典島弧巖石區(qū)，說明巖石具有島弧成巖特征；在Rb-(Y+Nb)圖解上(圖9a)和Nb-Y花崗巖構(gòu)造環(huán)境判別圖解上(圖9b)，主要落入火山島弧花崗巖+同碰撞花崗巖區(qū)，說明巖石形成與弧陸碰撞有關(guān)；在K2O-SiO2分類圖上(圖10a)，投點(diǎn)主要落入高鉀鈣堿性系列區(qū)，個(gè)別點(diǎn)落入高鉀鈣堿性系列與鉀玄巖系列、鈣堿性系列過渡區(qū)，反映了巖漿來源環(huán)境變化大;在K2O-Na2O分類圖上(圖10b)，投點(diǎn)主要分布在I型花崗巖區(qū)，部分落入I型與A型花崗巖過渡區(qū)，反映了巖漿來源于與幔源混染的火成巖。綜合分析認(rèn)為巖石形成于與碰撞有關(guān)的火山島弧構(gòu)造環(huán)境。

圖10 白乃廟銅礦花崗巖類SiO2-K2O圖解(a)和K2O-Na2O圖解(b)Fig.10 Diagrams of SiO2-K2O and K2O-Na2O for granite in Bainaimiao Cu-Mo deposit

白乃廟銅(鉬)床礦中部花崗閃長斑巖的結(jié)晶年齡及巖石化學(xué)特征均說明了白乃廟地區(qū)早古生代中酸性巖漿形成演化和銅鉬礦成礦具有密不可分的關(guān)系，這種關(guān)系和早古生代該區(qū)古亞洲洋板塊向華北板塊強(qiáng)烈俯沖相對(duì)應(yīng)，可以認(rèn)為白乃廟銅鉬礦床的成礦主動(dòng)力來自板塊俯沖。

早古生代古亞洲洋板塊沿西拉木倫河-溫都爾廟一線向華北地臺(tái)俯沖(陳從云等，1987)，形成一帶有洋殼的A型俯沖(芮宗瑤等，2007)，形成一條近東西向的大規(guī)模的鐵、金、銅多金屬礦帶(王東方，1986)。寒武至奧陶系白乃廟組中基性海相火山作用在達(dá)茂旗北部-白乃廟-圖林凱一帶形成東西延長達(dá)300km的島弧型基性火山熔巖噴發(fā)(許立權(quán)，2003;高計(jì)元等，2001;劉敦一，2003)，形成了黑礦型的白乃廟銅、鉬礦床原始礦源層(文獻(xiàn)①;李進(jìn)文等，2007)，同時(shí)也形成了在白乃廟地區(qū)特有的”溝、弧、盆”構(gòu)造體系(陳從云等，1987)。巖漿及期后含礦熱液疊加改造原礦源層形成了具島弧和斑巖型銅礦特點(diǎn)的白乃廟銅鉬礦床(辛河斌，2006)。

6 結(jié)論

(1) 白乃廟銅鉬礦床中具有古島弧的地球化學(xué)性質(zhì)的花崗閃長斑巖形成于(432.4±2.2)Ma-(443±1)Ma。早古生代古亞洲洋殼向華北板塊消減、俯沖，早志留世洋殼熔融形成的巖漿侵入白乃廟組中基性島弧火山巖形成花崗閃長斑巖，巖漿及期后熱液形成白乃廟銅鉬礦床。

(2) 花崗閃長斑巖侵位時(shí)代與主要成礦時(shí)代輝鉬礦Re-Os基本一致，且北礦帶礦體主要產(chǎn)于花崗閃長斑巖巖體中，成礦作用與早志留世花崗閃長斑巖侵位密切相關(guān)。

致謝 野外地質(zhì)調(diào)查、采樣和樣品測試工作期間得到了天津地質(zhì)調(diào)查中心司馬獻(xiàn)章院長的大力幫助，在此一并表示謝意。

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LA-MC-ICP-MS Zircon U-Pb Dating of the Pluton in the Bainaimiao Cu-Mo deposit of Inner Mongolia and its Geological Significance

FENG Xiao-xi1,2,YAO Shu-zhen1,WANG Jia-song2,WEI Jia-lin2, LI Guang-yao2,WANG Jia-ying2,FENG Xu-biao3

(1.Institute of Earth Resources, China University of Geoscience, Wuhan, Hubei 430074; 2.Tianjin Center of China Geology Survey , Tianjin 300170; 3.Guanxin Mining Group of Shenzhen City, Shenzhen, Guangzhou 518048)

The Bainaimiao Cu-Mo deposit has become the foremost Cu-Mo deposit in the north margin of the North China plate, and its mineralization is closely related to the pluton. In order to study the relationship between diagenesis and mineralization, this work made zircon LA-MC-ICPMS dating of the granodiorite and study of its geochemistry. The dating yielded ages of 432.4±2.2 Ma, 440.7±2.1Ma and 443±1Ma for this rock.The emplacement age of the intrusion is the lower Silurian, which is accordant with the isochron age of the Re-Os isochron age 440.5±4.4Ma. Moreover, trace elements Rb, Ba, Th, U, and K are enriched , while the elements Cs, Nb, and Ti are depleted. The REE distribution pattern is of the right-dip type, δEu is in 14.83～-19.54, and there is a negative anomaly of Eu, which is similar to the geochemical characteristics of the island-arc granite and high-K calc-alkaline volcanic rock series. We suggest that this Cu(Mo) deposit formed in the early Silurian and was associated with magmatic activity in the early Silurian. Its tectonic setting was the Bainaimiao island arc zone that was produced by the Early Paleozoic underthrusting of the paleo-Asian oceanic plate beneath the northern margin of the North China craton.

LA-MC-ICP-MS zircon U-Pb dating，arc pluton，magmatism and mineralization， Bainaimiao Cu-Mo deposit

2013-06-21；

2014-01-13；[責(zé)任編輯]郝情情。

中國地質(zhì)調(diào)查局項(xiàng)目(1212011085256)資助。

馮曉曦(1972年—)，男，1996年畢業(yè)于成都理工學(xué)院，獲學(xué)士學(xué)位，在讀博士生，高級(jí)工程師，長期從事地質(zhì)調(diào)查和研究工作。E-mail：tjfengxiaoxi@163.com。

姚書振(1947年—)，學(xué)士，礦床學(xué)教授，博士生導(dǎo)師，長期從事礦床學(xué)、礦田構(gòu)造學(xué)和區(qū)域成礦學(xué)的教學(xué)與研究工作。E-mail：szyao@cug.edu.cn。

P618.41

A

0495-5331(2014)02-0369-13