新疆東天山二紅洼巖體巖漿演化特征與成礦潛力分析

段星星，杜輝，劉拓，李文明，夏明哲

(1.中國地質(zhì)地質(zhì)調(diào)查局西安地質(zhì)調(diào)查中心，陜西 西安　710054； 2.長安大學(xué)地球科學(xué)與資源學(xué)院，陜西 西安　710054；3.西部礦產(chǎn)資源與地質(zhì)工程教育部重點(diǎn)實(shí)驗(yàn)室，陜西 西安　710054)

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新疆東天山二紅洼巖體巖漿演化特征與成礦潛力分析

段星星1，杜輝1，劉拓1，李文明1，夏明哲2,3

(1.中國地質(zhì)地質(zhì)調(diào)查局西安地質(zhì)調(diào)查中心，陜西 西安710054； 2.長安大學(xué)地球科學(xué)與資源學(xué)院，陜西 西安710054；3.西部礦產(chǎn)資源與地質(zhì)工程教育部重點(diǎn)實(shí)驗(yàn)室，陜西 西安710054)

二紅洼巖體位于康古爾-黃山韌性剪切帶東段，主要由含長二輝橄欖巖、橄欖輝長巖、輝長蘇長巖和淡色輝長巖組成。巖石相對(duì)富集LREE，虧損高場強(qiáng)元素(Nb、Ta、Ti)。巖體原生巖漿為普通拉斑玄武巖(MgO=7.3%)。通過對(duì)元素地球化學(xué)和Nd、Sr、Pb同位素體系研究證明，巖漿源區(qū)以軟流圈物質(zhì)為主，混入了少量富集巖石圈地幔組分，巖漿遭受了約5%上地殼物質(zhì)的混染。與同巖帶典型含礦巖體對(duì)比研究表明，巖體在巖石組合、原生巖漿性質(zhì)、同化混染程度等方面存在顯著差異，這些因素可能制約了成礦潛力。

成礦潛力；巖漿演化；二紅洼巖體；東天山

東天山北部的鎂鐵-超鎂鐵質(zhì)巖帶主要沿康古爾深大斷裂產(chǎn)出，從西到東依次有土墩、二紅洼、香山、黃山南、黃山東、葫蘆、圖拉爾根等典型含礦巖體(圖1a)。目前，在該帶發(fā)現(xiàn)的銅鎳資源儲(chǔ)量大于1×106t,僅次于金川礦床，已成為中國第二個(gè)世界級(jí)的巖漿銅鎳硫化物礦床成礦帶。近年來，對(duì)黃山、黃山東、香山、圖拉爾根等巖體巖石成因及成礦作用方面的研究取得了顯著進(jìn)展(秦克章等，2002；毛景文等，2002；韓寶福等，2002；ZHOU et al.，2004；PIRAJNO et al.，2004；夏明哲等，2008)。然而，位于該帶西段二紅洼巖體研究相對(duì)程度相對(duì)較低，王潤民等對(duì)巖體初步評(píng)價(jià)認(rèn)為二紅洼巖體屬于鐵質(zhì)系列基性-超基性巖體，有一定的成礦潛力(王潤民，1987)。仲勇研究表明巖漿起源于上地幔，原生巖漿屬于拉斑玄武巖，對(duì)形成銅鎳硫化物礦床是有利的(仲勇，1991,1993)。孫濤等依據(jù)橄欖石組分特征論證在巖漿結(jié)晶過程中母巖漿達(dá)到了硫飽和，并發(fā)生有硫化物的熔離作用(孫濤等，2014)。通常巖體類型、構(gòu)造環(huán)境、巖漿源區(qū)性質(zhì)、原生巖漿、巖漿分異程度、質(zhì)量平衡、巖漿含水量和同化混染等特征是篩選有利于賦含巖漿硫化物礦床巖體的關(guān)鍵因素。因此，筆者在巖石學(xué)、礦物學(xué)和地球化學(xué)研究基礎(chǔ)上，論證巖漿源區(qū)性質(zhì)和巖漿演化過程，并與同一巖帶典型含礦巖體特征進(jìn)行對(duì)比研究，深入探討巖體成礦前景。

1　地質(zhì)概況

二紅洼巖體位于新疆省哈密市東南約115km處，是沿康古爾深大斷裂分布的鎂鐵-超鎂鐵巖群中出露面積較大的巖體之一。二紅洼巖體由南、北兩部分組成，中間被第三系葡萄溝組覆蓋(圖1b)。巖體平面形態(tài)成橢圓狀，出露面積約8km2。其中，巖體南部侵位于中元古界的黑云母片巖，而北部基本被第三系和第四系覆蓋，接觸關(guān)系不清。

圖1　二紅洼巖體地質(zhì)略圖Fig.1　Simplified geological map of Erhongwa

2　巖相學(xué)及造巖礦物晶體化學(xué)

2.1巖相學(xué)特征

二紅洼巖體屬于復(fù)式巖體，分為2個(gè)侵入期次：第一侵入期次以輝長蘇長巖和淡色輝長巖為主，巖性穩(wěn)定，分布于巖體南北邊緣；第二侵入期次為由北向南依次為含長二輝橄欖巖、橄欖輝長巖，基性程度依次降低。鉆孔顯示含長二輝橄欖巖和橄欖輝長巖構(gòu)成韻律旋回重復(fù)出現(xiàn)，在750m處二輝橄欖巖相中賦存稀疏浸染狀硫化物。第二侵入期次形成的含長二輝橄欖巖和橄欖輝長巖與第一侵入期次巖石界限截然，巖石結(jié)構(gòu)和礦物組合突變。綜合分析各巖相的侵位順序?yàn)榈x長巖—輝長蘇長巖—橄欖輝長巖—含長二輝橄欖巖—橄欖巖。

巖體巖石新鮮，蝕變作用較弱。常見的結(jié)構(gòu)有輝長結(jié)構(gòu)、自形-半自形粒狀結(jié)構(gòu)、包含結(jié)構(gòu)等典型的基性-超基性巖結(jié)構(gòu)(圖2)，以塊狀構(gòu)造為主。各巖相結(jié)晶作用末期均有褐色普通角閃石晶出，反映了揮發(fā)分在巖漿晚期階段富集。

圖2　二紅洼橄欖輝長巖中的包橄結(jié)構(gòu)和含長結(jié)構(gòu)圖Fig.2　Microphotographs of ol-gabbro of the Erhongwa intrusion

2.2礦物晶體化學(xué)特征

巖體主要造巖礦物有橄欖石、輝石、斜長石和少量普通褐色角閃石，礦物晶體化學(xué)組成見表1。其中，橄欖石Fo值為72～79，均屬貴橄欖石。黃山東、黃山、香山和葫蘆巖體的橄欖石Fo值為80～89，明顯高于二紅洼巖體(圖3a)。

輝石以單斜輝石為主，見有少量的斜方輝石。單斜輝石主要為透輝石、頑火輝石。而黃山東、黃山、香山和葫蘆巖體中斜方輝石數(shù)量較多(圖3b)，二輝橄欖巖和蘇長巖為主要的含礦巖相。此外，巖體中斜長石An為71.4～84.1，Ab為13.1～28.7，Or為0.4～1.1，均為培長石。

圖3　(a)橄欖石Fo特征及(b)輝石的Wo-En-Fs圖解(MORIMOTO et al.,1988)Fig.3　(a) Fo of olivine (b) Wo-En-Fs diagram( After MORIMOTO et al.,1988)

巖性樣號(hào)礦物SiO2TiO2Al2O3Cr2O3FeOMnOMgONiOCaONa2OK2OTotal組成淡色輝長巖ehw6-1-2Ol38.730.010.020.0024.100.3537.40.050.000.010.01100.67Fo74Fa26淡色輝長巖ehw6-1-4Ol38.050.020.080.0025.550.4035.840.130.080.030.03100.21Fo72Fa28淡色輝長巖ehw6-1-5Ol38.090.000.050.0023.380.4037.810.090.000.010.0199.83Fo74Fa26淡色輝長巖ehw6-1-6Ol38.320.000.000.0323.560.3737.60.040.030.010.0199.97Fo74Fa26含長二輝橄欖巖ehw2-3-2Ol36.620.000.050.0618.940.3338.830.070.060.010.0194.97Fo79Fa21含長二輝橄欖巖ehw2-3-5Ol39.150.000.020.0020.910.1640.070.050.000.000.00100.35Fo78Fa22含長二輝橄欖巖ehw2-3-6Ol39.030.010.020.0020.050.0940.110.050.030.010.0199.41Fo78Fa22含長二輝橄欖巖ehw3-3-2Ol39.010.000.000.0321.870.2539.650.060.050.010.01100.95Fo77Fa23橄欖輝長巖ehw4-1-1Ol39.110.000.010.0521.240.2639.220.040.000.010.0199.96Fo77Fa23橄欖輝長巖ehw4-1-3Ol38.890.000.010.0521.240.4839.840.060.030.000.00100.62Fo77Fa23橄欖輝長巖ehw4-1-4Ol38.810.000.020.0021.400.2540.080.060.050.000.00100.67Fo77Fa23橄欖輝長巖ehw4-1-5Ol39.050.010.000.0020.430.3440.330.020.000.000.00100.19Fo78Fa22含長二輝橄欖巖ehw2-3-3cpx51.230.403.130.066.360.2115.010.0022.240.360.0299.09En44Fs10Wo46含長二輝橄欖巖ehw2-3-4cpx50.660.384.210.085.100.2015.820.0022.750.480.0099.74En45Fs8Wo46含長二輝橄欖巖ehw3-3-1cpx51.620.393.100.086.820.2316.20.0020.530.340.0199.33En47Fs11Wo42含長二輝橄欖巖ehw3-3-2cpx50.50.332.070.125.590.1310.310.0024.730.440.0299.25En33Fs10Wo47橄欖輝長巖ehw4-1-8opx49.640.251.7816.9111.290.2318.570.001.060.320.27100.43En72Fs25Wo3淡色輝長巖ehw6-1-3pl52.110.0233.120.000.260.050.000.0011.642.530.0799.81An72Ab28淡色輝長巖ehw6-1-3pl47.350.0233.460.000.260.050.000.0016.422.130.0799.77An81Ab19淡色輝長巖ehw6-2pl49.330.0231.470.080.350.000.250.0015.582.090.1399.31An80Ab20淡色輝長巖ehw7-1-1pl48.180.0532.420.020.360.000.030.0015.732.030.1598.97An81Ab19淡色輝長巖epw7-1-2pl48.460.0631.590.110.390.000.030.0016.22.30.1999.33An79Ab21橄欖輝長巖ehw4-1-1pl48.920.0031.770.030.280.000.010.0016.012.070.0699.15An81Ab19橄欖輝長巖ehw4-1-4pl49.060.0131.260.030.250.040.070.0016.651.750.0099.11An84Ab16橄欖輝長巖ehw4-1-5pl48.920.0232.590.000.240.010.030.0015.782.130.0699.77An80Ab20橄欖輝長巖ehw4-1-7pl49.220.0031.290.000.290.000.010.0015.902.190.0898.98An80Ab20含長二輝橄欖巖ehw2-3-3pl48.050.0432.390.020.250.030.030.0016.731.410.1199.05An86Ab14含長二輝橄欖巖ehw2-3-4pl49.80.0231.060.040.260.030.000.0015.572.250.0999.12An79Ab21

3　巖石地球化學(xué)

3.1主量元素地球化學(xué)

巖體中各巖相主量元素、微量元素分析數(shù)據(jù)見表2。其中SiO2為46.54%～50.29%，屬基性巖。與巖相學(xué)特征變化相對(duì)應(yīng)，Al2O3、FeO+Fe2O3、MgO、CaO、Na2O含量變化大。MgO為6.54%～16.02%，平均為9.75%。各類巖石的m/f值為2.38～3.43，為鐵質(zhì)超基性巖。在SiO2-Na2O+K2O圖上(圖4a)，它們都投影于亞堿性區(qū)。在圖4b上，樣品均落入拉斑玄武巖系列區(qū)。

3.2微量元素地球化學(xué)

巖石稀土元素豐度較低，∑REE=10.54×10-6～17.46×10-6，平均為14.32×10-6，為球粒隕石的3～8倍。(La/Sm)N=0.64～1.46，(La/Yb)N=0.9～2.08，(Gd/Yb)N=1.23～1.5，表明輕重稀土元素之間和輕、重兩組稀土元素內(nèi)部的元素之間分餾程度較弱。δEu=1.34～2.09，顯示不同程度的正Eu異常。若不考慮Eu異常，稀土元素配分曲線大體表現(xiàn)為平坦型特征(圖5a、表3)。

表2　巖體主量元素分析數(shù)據(jù)表(%)

圖4　巖石化學(xué)分類圖解Fig.4　Petrogeochemical classification diagram

巖體的原始地幔標(biāo)準(zhǔn)化的蛛網(wǎng)圖(圖5b)型式基本一致，巖石普遍富集大離子親石元素(Cs、Rb、Ba、Sr)，顯著虧損Nb，適度虧損Zr、Hf、Ti，部分虧損Ta。此外，大部分樣品的不相容元素、適度不相容元素與原始地幔相應(yīng)元素的比值均不超過10，說明這些巖石相對(duì)虧損大多數(shù)不相容元素和適度不相容元素。

圖5　(a)球粒隕石標(biāo)準(zhǔn)化的稀土元素配分曲線圖;(b)原始地幔標(biāo)準(zhǔn)化的多元素蛛網(wǎng)圖(標(biāo)準(zhǔn)值據(jù)SUN et al., 1989)Fig.5　(a)Chondrite-normalized REE patterns；(b)PM-normalized trace elements spider diagram

北 巖 體樣品EHW1/1EHW3/3EHW2/1EHW2/3EHW1/2EHW1/3EHW3/1EHW3/2巖性含長二輝橄欖巖橄欖輝長巖南 巖 體EHW4/1EHW5/1EHW5/2EHW7/1EHW6/1EHW6/2橄欖輝長巖輝長蘇長巖淡色輝長巖Li2.632.061.281.192.53.232.542.64.911.452.243.971.861.93Sc45.7543.5814.8511.8238.8529.3449.7251.9341.4519.0630.6615.2519.221.24V149.8122.175.2481.8310282.18149.1143.8109.383.2978.18128.448.155.99Cr1300338.4558.2668.6835.2406.5409.5425.9905.61002984.8112.8307.3383.4Mn838.8813.5624.8679.51065838898.5907.1749.1732.5548.9964.2475.7450.4Co54.5259.7458.9857.9380.0266.8860.7964.2154.9552.0949.8152.7752.8344.68Ni117.768.33105.397.04165122.863.7366.4193.46109.7105.122.1493.0680.3Cu54.3959.1941.0542.8155.1747.5949.5946.6454.5467.1843.3532.9934.2837.35Zn46.6338.6836.4339.0555.0350.3545.2541.2538.142.2534.1251.5431.229.8Rb1.411.411.762.242.262.851.370.962.162.012.253.151.861.7Sr414.9424.2478.6444.8337.6441.4390.5380.2436.7447.6484.9452564.6557.8Y7.336.965.265.066.4668.657.76.585.895.925.353.53.78Zr16.0713.4714.7215.6718.2821.9417.2512.617.1516.8516.4115.5510.189.97Hf0.520.450.410.430.550.60.560.430.530.490.50.460.290.28Nb0.260.20.270.280.340.530.280.160.30.290.240.240.160.14Ta0.020.020.240.230.020.010.020.020.020.240.210.240.250.23Cs0.210.090.150.130.170.30.10.070.120.150.120.430.090.1Ba25.0825.3426.2728.6824.833.2824.1219.7326.5128.2831.6637.7141.6631.76Pb0.540.580.650.610.660.920.630.450.660.690.780.90.70.81Th0.290.290.270.30.410.510.250.170.350.460.340.420.320.27U0.050.070.050.070.080.10.050.070.070.080.080.260.070.09La1.281.271.351.311.541.871.421.081.451.561.551.71.381.39

續(xù)表3

北 巖 體樣品EHW1/1EHW3/3EHW2/1EHW2/3EHW1/2EHW1/3EHW3/1EHW3/2巖性含長二輝橄欖巖橄欖輝長巖南 巖 體EHW4/1EHW5/1EHW5/2EHW7/1EHW6/1EHW6/2橄欖輝長巖輝長蘇長巖淡色輝長巖Ce3.413.193.443.363.934.653.963.053.753.83.73.922.963.08Pr0.520.50.480.470.540.650.610.480.540.530.520.530.410.43Sm1.10.960.790.81.0111.151.060.980.90.840.830.620.6Nd3.243.062.732.743.283.563.763.123.192.992.882.922.192.34Eu0.560.530.470.460.490.520.580.530.520.50.480.480.470.47Gd1.31.230.951.031.221.21.481.311.281.091.081.040.70.79Tb0.230.230.150.170.210.190.270.240.20.180.180.170.110.12Dy1.531.511.121.41.261.731.581.441.141.171.150.680.79Ho0.290.280.190.20.260.240.330.290.250.230.220.230.130.15Er0.90.880.580.660.790.741.040.930.840.670.670.730.410.47Tm0.110.110.070.080.10.090.130.120.10.090.080.090.050.06Yb0.820.780.540.60.720.660.880.80.690.650.590.680.390.45Lu0.110.110.080.080.10.090.120.110.10.080.090.090.050.05∑REE15.414.6212.8213.0715.616.7417.4614.7115.3214.3914.0614.5610.5411.18

3.3Nd、Sr、Pb同位素地球化學(xué)

巖體的Nd、Sr同位素組成相對(duì)均一，εNd(t) =+6.3～+7.7，εSr(t) =-9～-11.5(t=274Ma)，顯示虧損型地幔特征。在εNd(t) -(87Sr/86Sr)i相關(guān)圖上，二紅洼巖體均投影在了第二象限，位于OIB范圍內(nèi)，而黃山東、黃山、葫蘆和香山巖體Nd、Sr同位素組成變化較大，具有EMII的演化趨勢(圖6a)。

巖體的(206Pb/204Pb)i=17.814～17.927，(207Pb/204Pb)i=15.445～15.494，(208Pb/204Pb)i=37.296～37.395，在初始Pb同位素相關(guān)圖中，數(shù)據(jù)點(diǎn)都落在地球等時(shí)線右側(cè)，表明富含放射成因的Pb同位素(圖6b、圖6c)。在初始Pb同位素以及Pb與Nd、Sr同位素相關(guān)圖上，數(shù)據(jù)點(diǎn)位于MORB范圍內(nèi)和其附近，充分證明巖漿源區(qū)具有與MORB相似的Nd、Sr、Pb同位素組成(圖6d、表4)。

4　討論

4.1同化混染

世界級(jí)銅鎳硫化物礦床研究表明，同化混染作用(如巖漿演化過程中陸殼、圍巖中Si、Fe、S組分的加入)是形成巖漿硫化物礦床的關(guān)鍵因素之一。二紅洼巖體內(nèi)捕虜體或殘留頂蓋不發(fā)育，表明巖體形成過程中同化混染作用弱。一般來說，經(jīng)過地殼物質(zhì)混染的巖漿其Nd、Sr同位素都會(huì)發(fā)生變化。然而，由于同化混染過程中的混染物往往是具有低固相線溫度的花崗巖、沉積巖及其變質(zhì)等價(jià)物，這些巖石都具有較高的Sr豐度和87Sr/86Sr值，所以，同化混染作用對(duì)Sr同位素組成的影響往往比對(duì)Nd同位素組成的影響大。二紅洼巖體Nd、Sr同位素相對(duì)均一，εNd(t)=+6.3～+7.7，(87Sr/86Sr)i=0.703 4～0.703 5，對(duì)同化混染作用敏感的207Pb/204Pb值也沒有明顯升高，表明巖漿演化過程中同化混染作用較弱。采用MORB地幔端元和上地殼的Nd、Sr同位素特征定量模擬獲得巖漿演化過程中受到小于5%地殼物質(zhì)混染，與其他典型含礦巖體相比混染程度最低(圖7)。

表4　二紅洼巖體Nd、Sr、Pb同位素分析數(shù)據(jù)表

注： Pb初始同位素計(jì)算參數(shù)λ1=1.551 25×10-10，λ1=9.848 5×10-10，λ1=0.494 75×10-10，t=274Ma。

圖6　(a)εNd(t)-(87Sr/86Sr)i相關(guān)圖(據(jù)ZINDER et al.,1986) ;(b)207Pb/204Pb-206Pb/204Pb相關(guān)圖；(c)208Pb/204Pb-206Pb/204Pb相關(guān)圖(據(jù)ALLEGRE et al.,1988;ZINDER et al.,1986) ;(d)εNd(t)-(208Pb/204Pb)i相關(guān)圖(據(jù)ALLEGRE et al.,1988;ZINDER et al.,1986) (巖體Nd、Sr、Pb同位素?cái)?shù)據(jù)來自夏明哲博士論文)Fig.6　(a)εNd(t) versus initial 87Sr/86Sr of the Erhongwa intrusion (After ZINDER et al.,1986) ；(b) 207Pb/204Pb and 206Pb/204Pb of the Erhongwa;(c) 208Pb/204Pb versus 206Pb/204Pb of the Erhongwa (After ALLEGRE et al.,1988;ZINDER et al.,1986)；(d)(208Pb/204Pb)i versus εNd(t) of the Erhongwa(After ALLEGRE et al.,1988;ZINDER et al.,1986)

4.2原生巖漿

通常與鎂鐵-超鎂鐵質(zhì)巖漿有關(guān)的銅鎳硫化物礦床的原生巖漿是成礦作用的關(guān)鍵。與銅鎳硫化物礦床相關(guān)的原生巖漿系列主要可分為兩類：一類是科馬提巖巖漿；一類是苦橄-拉斑玄武巖巖漿。在二紅洼巖體中單斜輝石的SiO2-Al2O3圖解中，所有的單斜輝石位于亞堿性巖區(qū)，表明其原生巖漿屬于亞堿性系列。Mg#(Mg#=Mg/(Mg+Fe))也是鑒別原生巖漿的重要標(biāo)志之一。GREEN認(rèn)為，與地幔橄欖巖平衡的原生巖漿的Mg#=0.63～0.73(GREEN，1975)；FREY認(rèn)為，Mg#=0.68～0.73(FREY，1978)；HESS認(rèn)為Mg#>0.68(HESS，1992)。如果以Mg#=0.65～0.73代表了原生巖漿和近于原生巖漿的Mg#范圍，二紅洼巖體中3件樣品的Mg#=0.71～0.73，基本接近原生巖漿范圍，其MgO含量平均值為7.45%。利用橄欖石-熔體平衡原理也可以估算進(jìn)入巖漿房中原生巖漿的MgO含量。Mg-Fe在橄欖石-熔體之間的分配系數(shù)為一相對(duì)穩(wěn)定的值，即KdOl-Melt=(FeO/MgO)Ol/(FeO/MgO)melt=0.3～0.33(ROEDER,1970)。然而，由于早期結(jié)晶的橄欖石與殘余晶間液體之間要發(fā)生再平衡作用，使得早期結(jié)晶橄欖石比其在原生巖漿中結(jié)晶時(shí)的橄欖石中的Mg含量低，因此，F(xiàn)o值最高的橄欖石組分可能更接近于液相線橄欖石的組成。以巖體中橄欖石最高的Fo=79估算出與其處于平衡狀態(tài)巖漿MgO含量為7.1%。兩種方法約束原生巖漿MgO含量基本一致，明顯低于該巖帶典型含礦巖體原生巖漿MgO含量(11.9%～9.6%)。由此可見，二紅洼巖體原生巖漿應(yīng)為普通的拉斑玄武巖。

[虧損地幔值DM(N-MORB)：εNd(300Ma)=9；(87Sr/86Sr)i=0.702 2；上地殼值UC：εNd(300Ma)=-9；(87Sr/86Sr)i=0.715]圖7　二紅洼巖體同化混染模擬圖Fig.7　Contamination modeling diagram of Erhongwa

4.3巖漿源區(qū)

二紅洼巖體的(87Sr/86Sr)i值為0.703 4～0.703 5，

εNd(t)值為+6.3～+7.7，且Pb同位素組成與MORB型虧損地幔Pb同位素一致。此外，巖體中除一件樣品外，其余樣品Zr/Nb值為42～59，這在MORB的Zr/Nb值(10～60)范圍內(nèi)(DAVIDSON，1996)，Sm/Nd值為0.27～0.30，平均值為0.32，基本上位于MORB的范圍(ANDERSON，1994)。這些證據(jù)表明巖漿源區(qū)主體應(yīng)屬于軟流圈地幔。而且在主量元素中，TiO2、K2O、Na2O的相容性最低，巖體各種巖石的TiO2平均含量僅為0.31%，K2O的平均含量為0.1%，Na2O為1.67%，說明不相容的主量元素也是虧損的。然而，巖體在沒有受到明顯的陸殼組分的混染的情況下，其(87Sr/86Sr)i明顯高于正常洋中脊玄武巖的(87Sr/86Sr)i=0.702 29～0.703 11；(143Nd/144Nd)i低于正常洋中脊玄武巖(143Nd/144Nd)i0.513 0～0.513 3(SUNDERS，1988)。巖體普遍含有一定量的普通褐色角閃石，具有Nb、Ti的虧損，部分也有Ta虧損，暗示其源區(qū)可能受到早期俯沖板片/俯沖沉積物所釋放流體的交代作用。在Nb/Zr-Th/Zr和Th/Yb-Nb/Yb相關(guān)圖上顯示巖漿源區(qū)存在被俯沖改造的富集巖石圈地幔(圖8)。由此可見，巖體源區(qū)主體應(yīng)該是軟流圈地幔，可能同時(shí)也有很少量俯沖流體改造的富集巖石圈地幔的組分加入。

圖8　(a)Th/Yb-Nb/Yb圖解;(b)Nb/Zr-Th/Zr 圖解Fig.8　(a)Th/Yb vs Nb/Yb diagram ;(b)Nb/Zr vs Th/Zr diagram

5　成礦潛力

黃山、黃山南、黃山東、香山、二紅洼等巖體位于同一構(gòu)造帶，沿康古爾深大斷裂分布，巖體形成時(shí)限約270～285Ma，應(yīng)為后碰撞時(shí)期的產(chǎn)物。Nd、Sr同位素地球化學(xué)研究表明它們的巖漿源區(qū)性質(zhì)基本一致，以虧損地幔組分為主，含有一定數(shù)量流體交代改造的富集型巖石圈地幔。除此之外，二紅洼巖體在巖石組合、原生巖漿性質(zhì)、同化混染程度等方面與該帶典型含礦巖體存在顯著差異。

世界上重要的巖漿硫化物礦床都賦存在高度分異的巖體中，而分異弱的巖體往往不成礦或貧礦。二紅洼巖體地表出露面積大，巖漿分異差，巖體以輝長巖相為主，超基性巖石數(shù)量有限，僅見少量含長二輝橄欖巖。此外，賦存巖漿硫化物礦床的巖體原生巖漿均為高鎂拉斑玄武巖、苦橄巖和科馬提巖，巖漿演化過程中普遍受到一定程度的同化混染作用，而二紅洼巖體原生巖漿為普通拉斑玄武巖，并且?guī)r漿巖漿過程中受到較弱同化混染作用。現(xiàn)有鉆探成果顯示深部二輝橄欖巖相中賦存星點(diǎn)狀硫化物，未見稀疏侵染狀或稠密侵染狀礦石。依據(jù)巖體分布特征、巖石組合、地球化學(xué)等綜合判斷，二紅洼巖體與該帶典型含礦巖體比較，關(guān)鍵成礦指標(biāo)存在明顯差異，這可能不同程度制約了成礦潛力，巖體淺部找礦難度大。

6　結(jié)論

(1) 二紅洼巖體主要由含長二輝橄欖巖、橄欖輝長巖、輝長蘇長巖和淡色輝長巖組成。巖石化學(xué)組成屬拉斑玄武巖系列。巖石相對(duì)富集LREE，普遍富集普遍富集大離子親石元素(Cs、Rb、Ba、Sr)，顯著虧損Nb，適度虧損Zr、Hf、Ti，部分樣品虧損Ta。

(2) 巖體Nd、Sr、Pb同位素組成屬虧損型地幔，原生巖漿為普通拉斑玄武巖，巖漿演化過程中發(fā)生了較弱的同化混染作用程度，遭受約5%上地殼物質(zhì)混染。

(3) 與同一巖帶的典型含礦巖體對(duì)比研究發(fā)現(xiàn)，二紅洼巖體在巖石組合、原生巖漿性質(zhì)、同化混染程度等方面存在顯著差異，綜合研究表明巖體淺部找礦難度大。

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Magmatic Evolution and Mineralization Potential of the Erhongwa Intrusion in East Tianshan Mountains, Xinjiang

DUAN Xingxing1, DU Hui1, LIU Tuo1, LI Wenming1, XIA Mingzhe2,3

(1.Xi’an Center of Geological Survey,China Geological Survey,Xi’an 710054, Shaanxi,China;2.College of Earth Science and Resources, Chang’an University,Xi’an 710054, Shaanxi,China;3.Key Laboratory of Western China’s Mineral Resources and Geological Engineering, Ministry of Education, Xi’an 710054, Shaanxi,China)

Erhongwa intrusion lies in the eastern part of Kongguer-Huangshan fault belt. It consists of pl-bearing iherzolite, olivine gabbro,gabbronorite and leucogabbro. Most of these rocks are slightly enriched in LREE and depleted in HFSE(Nb, Ta and Ti).The primary magma of this intrusion may be ordinary tholeiite (MgO=7.3%). Element geochemistry and Nb, Sr, Pb isotope characters suggest that this magma source is composed of asthenosphere components, mixed with a small amount of lithospheric mantle components, and this magma was subjected to becontaminated about 5% by contiental crust. Compared with others intrusion in this mafic-ultrumafic belt, the Erhongwa intrusion has some significant differences in rock association, primary magma nature and assimilation,which may restrict the mineralization potential in this area.

mineralization potential;magmatic evolution;Erhongwa intrusion;East Tianshan Mountains

2015-12-02；

2016-05-16

國家自然基金項(xiàng)目(41102045)和中央高校基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金(2013G1271100)資助

段星星(1983-)，男，湖南郴州人，工程師，2012年畢業(yè)于長安大學(xué)巖石學(xué)、礦物學(xué)、礦床學(xué)專業(yè)，現(xiàn)從事礦床勘查及勘查地球化學(xué)工作。E-mail: 86000536@qq.com

P588.12

A

1009-6248(2016)03-0050-11