東北延邊地區(qū)晚中生代柳洞巖體的成因：鋯石U-Pb年代學(xué)和地球化學(xué)證據(jù)

張 超，徐仲元，劉正宏，李世超，時(shí) 溢，范志偉

吉林大學(xué)地球科學(xué)學(xué)院,長春 130061

東北延邊地區(qū)晚中生代柳洞巖體的成因：鋯石U-Pb年代學(xué)和地球化學(xué)證據(jù)

張 超，徐仲元，劉正宏，李世超，時(shí) 溢，范志偉

吉林大學(xué)地球科學(xué)學(xué)院,長春 130061

柳洞巖體位于延邊地區(qū)和龍市南部，大地構(gòu)造位置上處于古洞河斷裂西南側(cè)，龍崗地塊北緣，其巖性為二長花崗巖。為確定其侵位時(shí)代和大地構(gòu)造環(huán)境，對(duì)柳洞巖體進(jìn)行了系統(tǒng)的巖相學(xué)和年代學(xué)研究。N-6和N-7樣品LA-ICP-MS鋯石U-Pb測(cè)年結(jié)果顯示，其加權(quán)平均年齡分別為(118.63±0.69)Ma和(118.58±0.87)Ma。巖體具有高硅(w(SiO2)=70.30%～76.30%)、富堿(w(Na2O+K2O)=7.82%～8.52%)、富鋁(w(Al2O3)=12.30%～15.50%)和低鈦(w(TiO2)=0.16%～0.29%)的特點(diǎn)，屬于高鉀鈣堿性系列。A/CNK值為1.03～1.11，為弱過鋁質(zhì)巖石。分異指數(shù)(DI)為87.99～94.89，固結(jié)指數(shù)(SI)為2.39～5.31，反映了該巖體經(jīng)歷了高程度分異演化作用。δEu為0.63～0.92，具有中等銪負(fù)異常，LREE/HREE和(La/Yb)N值分別為3.46～8.09和10.59～33.94，顯示輕稀土元素相對(duì)富集、重稀土元素虧損的特點(diǎn)。柳洞巖體明顯富集Cs、Rb、Ba、K等大離子親石元素(LILE)，同時(shí)虧損Nb、Ta、Ti等高場強(qiáng)元素(HFSE)以及P元素。綜合巖相學(xué)、年代學(xué)、巖石地球化學(xué)特征及區(qū)域地質(zhì)資料，柳洞巖體具有高分異的高鉀鈣堿性I型花崗巖的特點(diǎn)，形成于后造山向伸展轉(zhuǎn)換的體制環(huán)境下。

地球化學(xué)；鋯石U-Pb年代學(xué)；后造山花崗巖；晚中生代；延邊地區(qū)

0 前言

顯生宙以來，受古亞洲洋構(gòu)造域和濱太平洋構(gòu)造域的影響，延邊地區(qū)巖漿活動(dòng)強(qiáng)烈[1]。柳洞巖體位于延邊地區(qū)和龍市南部，其東側(cè)為中國與朝鮮界河-圖們江，大地構(gòu)造位置上處于龍崗地塊北緣。在1∶20萬大拉子幅的地質(zhì)報(bào)告*吉林省地質(zhì)局區(qū)域地質(zhì)測(cè)量大隊(duì).大拉子幅(1∶20萬)區(qū)域地質(zhì)測(cè)量報(bào)告.長春：吉林省區(qū)域地質(zhì)礦產(chǎn)調(diào)查所，1964.中，前人從地質(zhì)、巖性等方面將柳洞巖體列入華力西晚期侵入旋回，推測(cè)其侵位時(shí)代為晚二疊世，但同時(shí)認(rèn)為該巖體也可能為燕山侵入旋回。Wu等[2]對(duì)柳洞巖體鄰區(qū)進(jìn)行的花崗巖鋯石U-Pb測(cè)年結(jié)果為(114±1)Ma，侵位時(shí)代為白堊紀(jì)。

以上研究[2-3]顯示，柳洞巖體形成時(shí)代及其構(gòu)造背景需要進(jìn)一步厘定。為確定該巖體的形成時(shí)代及巖石地球化學(xué)特征，對(duì)柳洞巖體進(jìn)行了系統(tǒng)的巖相學(xué)、年代學(xué)和巖石地球化學(xué)研究，對(duì)認(rèn)識(shí)延邊地區(qū)燕山晚期的構(gòu)造演化具有重要意義。

1 地質(zhì)背景

延邊地區(qū)位于中亞造山帶東緣，濱太平洋構(gòu)造域西側(cè)，佳木斯地塊、興凱地塊和龍崗地塊交界處[4-5](圖1a)。北西向的古洞河斷裂從區(qū)內(nèi)通過，斷裂北側(cè)為中亞造山帶東緣，南側(cè)為發(fā)育華北地臺(tái)綠片巖-角閃巖相的變質(zhì)巖基底。柳洞巖體呈北東向展布，出露面積約70 km2，巖體侵入到侏羅紀(jì)花崗巖與太古宙變質(zhì)巖基底中，與早白堊世沉積地層斷層接觸 (圖1b)。

2 樣品描述、測(cè)試方法和分析結(jié)果

2.1 樣品描述

樣品采自研究區(qū)圖們江岸邊及牛心山南側(cè)，均為新鮮出露巖石，樣品巖性主要為肉紅色中細(xì)粒二長花崗巖，中細(xì)?；◢徑Y(jié)構(gòu)，塊狀構(gòu)造。主要組成礦物有：條紋長石(約20%)，粒度1～2 mm，形狀不規(guī)則，具有條紋結(jié)構(gòu)，發(fā)生強(qiáng)烈高嶺土化；微斜長石(約20%)，粒度1～3.5 mm，呈板柱狀，格子狀雙晶清晰可見，輕微蝕變；斜長石(約30%)，粒度1～2 mm，呈半自形-自形板柱狀，部分顆粒中心發(fā)生輕微高嶺土化和絹云母化，聚片雙晶發(fā)育,斜長石牌號(hào)An=26，為更長石；石英(>25%)，粒度1～4 mm，呈他形粒狀分布在其他礦物之間，有少量細(xì)小裂紋；黑云母(<5%)，片狀，大多發(fā)生強(qiáng)烈的綠泥石化，局部完全蝕變?yōu)槿渚G泥石；此外還有少量白云母。出現(xiàn)的副礦物有磁鐵礦、長柱狀磷灰石和鋯石(圖2a)。在硯水平附近巖體中見晶洞構(gòu)造，晶洞內(nèi)發(fā)育較好的石英晶簇(圖2b)。

圖1 和龍地區(qū)柳洞巖體分布圖(圖a據(jù)文獻(xiàn)[6]修編)Fig.1 Distribution of Liudong pluton in Helong area(Fig.a modified from reference[6])

a.細(xì)粒二長花崗巖；b.中細(xì)粒二長花崗巖。Pl.斜長石；Pth.條紋長石；Ms.白云母；Q.石英； Ap.磷灰石；Mc.微斜長石；Chl.綠泥石。圖2 柳洞巖體二長花崗巖顯微照片(a)特征和野外照片(b)Fig.2 Microphotographs features(a)and photographs for the Early Cretaceous monzonitic granite(b)

2.2 測(cè)試方法

鋯石樣品在河北省廊坊市區(qū)域地質(zhì)調(diào)查研究所進(jìn)行挑選。先將所采樣品粉碎至80～100目，通過淘洗和電磁方法對(duì)樣品中的鋯石進(jìn)行挑選，在雙目鏡下挑選沒有明顯裂隙和包裹體的鋯石顆粒進(jìn)行制靶。然后采集樣靶透射光、反射光和陰極發(fā)光的圖片。鋯石的制靶、顯微圖像的采集以及鋯石U-Pb同位素測(cè)年分析在西北大學(xué)大陸動(dòng)力學(xué)國家重點(diǎn)實(shí)驗(yàn)室完成。對(duì)鋯石樣品的分析在Agilent7500型ICP-MS和ComPexl02 ArF準(zhǔn)分子激光器以及GeoLas200M光學(xué)系統(tǒng)聯(lián)機(jī)下進(jìn)行，激光束直徑為30 μm，剝蝕深度為20～40 μm。將國際標(biāo)準(zhǔn)鋯石91500作為鋯石年齡外標(biāo)標(biāo)準(zhǔn)物質(zhì)，元素含量則采用NIST SRM610作為外標(biāo)，29Si作為內(nèi)標(biāo)[7-9]。通過Andersen的方法對(duì)實(shí)驗(yàn)測(cè)得數(shù)據(jù)進(jìn)行同位素比值校正以去除普通鉛的影響，鋯石年齡諧和圖通過Isoplot3.0繪制而成，所得同位素比值年齡的誤差均為1σ。

在國家地質(zhì)測(cè)試中心采用X射線熒光光譜儀對(duì)樣品的主量元素進(jìn)行了分析，通過等離子質(zhì)譜儀(X-series)進(jìn)行微量元素的分析。

2.3 分析結(jié)果

圖3 早白堊世二長花崗巖(N-6)中部分鋯石陰極發(fā)光圖像Fg.3 Cathodoluminescence (CL) images of selected zircons from the Early Cretaceous monzonitic granite(N-6)

2.3.1 鋯石U-Pb測(cè)年

為進(jìn)一步確定柳洞巖體的年齡，對(duì)本次工作所取的N-6和N-7兩個(gè)樣品進(jìn)行LA-ICP-MS 鋯石U-Pb定年分析，分析結(jié)果見表1。

N-6中鋯石大小為50～150 μm，呈半自形-自形，多數(shù)為長柱狀，長寬比3∶1～1∶1，內(nèi)部可見少量裂隙和包裹體，陰極發(fā)光(CL)圖像(圖3)下可見清晰的震蕩生長環(huán)帶，鋯石Th/U值為0.39～1.61，暗示其巖漿成因的特點(diǎn)[10]。對(duì)樣品N-6共進(jìn)行25個(gè)鋯石顆粒的測(cè)定，2個(gè)點(diǎn)由于鋯石內(nèi)部裂隙導(dǎo)致鉛丟失致使所得結(jié)果較年輕，其余23個(gè)點(diǎn)206Pb/238U年齡范圍在(115±1)～(123±1)Ma 。

N-7中鋯石大小為100～150 μm，呈半自形-自形，大多數(shù)為長柱狀，長寬比3∶1～2∶1，內(nèi)部可見少量裂隙和包裹體，陰極發(fā)光(CL)圖像(圖4)下可見清晰的震蕩生長環(huán)帶，鋯石Th/U值為0.46～1.34，暗示其巖漿成因的特點(diǎn)[10]。對(duì)樣品N-7進(jìn)行了25個(gè)鋯石顆粒的測(cè)定，1個(gè)點(diǎn)由于鋯石內(nèi)部裂隙導(dǎo)致鉛丟失致使所得結(jié)果較年輕，其余24個(gè)點(diǎn)206Pb/238U年齡范圍在(114±1)～(123±3)Ma。

圖4 早白堊世二長花崗巖(N-7)中部分鋯石陰極發(fā)光圖像Fg.4 Cathodoluminescence (CL) images of selected zircons from the Early Cretaceous monzonitic granite(N-7)

N-6和N-7樣品中鋯石U-Pb年齡諧和圖顯示大部分分析結(jié)果位于諧和線上及其附近，所得出的加權(quán)平均年齡分別為(118.63±0.69)Ma(n=18，MSWD=2.7)和(118.58±0.87)Ma(n=24，MSWD=3.7)(圖5)。結(jié)合所測(cè)樣品鋯石巖漿成因的特點(diǎn)，該年齡解釋為柳洞巖體的侵位年齡，表明和龍地區(qū)柳洞巖體二長花崗巖的侵位時(shí)代為早白堊世晚期。

2.3.2 巖石地球化學(xué)

柳洞巖體二長花崗巖的主量元素和微量元素分析結(jié)果見表2。

主量元素 巖體中w(SiO2)為70.30%～76.30%，全堿w(Na2O+K2O)為7.82%～8.52%，巖石具有低Ti(w(TiO2)=0.16%～0.29%)、富鋁(w(Al2O3)=12.30%～15.50%)的特點(diǎn)。分異指數(shù)(DI)為87.99～94.89，固結(jié)指數(shù)(SI)為2.39～5.31，反映巖體經(jīng)歷了高程度分異演化作用。A/CNK為1.03～1.11，屬于弱過鋁質(zhì)巖石(圖6a)。在TAS圖上樣品全部落入亞堿性系列(圖略)，在w(SiO2)-w(K2O)圖上樣品全部落入高鉀鈣堿性范圍內(nèi)(圖6b)，與中國東北地區(qū)燕山期侵入巖相似。巖石地球化學(xué)特征與Barbarin[11]劃分的高鉀鈣堿性花崗巖類(KCG)相似。

稀土元素 柳洞巖體稀土元素總量(w(REE))為(55.51～124.30)×10-6，質(zhì)量分?jǐn)?shù)較低。所有樣品具有相似的稀土分配模式。LREE/HREE和(La/Yb)N值分別為3.46～8.09和10.59～33.94，指示輕稀土元素相對(duì)富集且分餾明顯、重稀土元素虧損的特點(diǎn)。δEu為0.63～0.92，具有中等Eu負(fù)異常(圖7a)。Eu的負(fù)異常應(yīng)與斜長石在源區(qū)的分離結(jié)晶有關(guān)。

圖5 柳洞巖體二長花崗巖中鋯石LA-ICP-MS U-Pb年齡諧和圖Fig.5 U-Pb concordia diagrams and the average age diagrams summarizing the LA-ICP-MS zircon data for the Early Cretaceous monzonitic granite from Liudong pluton

圖6 延邊和龍地區(qū)二長花崗巖含鋁指數(shù)(a)和w(SiO2)-w(K2O)圖(b)Fig.6 Aluminous index diagram (a) and w(SiO2) versus w(K2O) diagram (b)for the monzonitic granite in Helong, Yanbian area

球粒隕石標(biāo)準(zhǔn)化值據(jù)文獻(xiàn)[12]；原始地幔標(biāo)準(zhǔn)化值據(jù)文獻(xiàn)[13]。圖7 和龍地區(qū)二長花崗巖球粒隕石標(biāo)準(zhǔn)化稀土元素配分模式圖(a)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b) Fig.7 Chondrite-normalized REE pattern(a) and primitive mantle-normalized trace element pattern (b) for the Early Cretaceous monzonitic granite in Helong area

樣號(hào)w(232Th)/10-6w(238U)/10-6Th/UU-Th-Pb同位素比值207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ年齡/Ma207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σN-6.1*111714510.770.058820.002470.137530.005480.016920.00022561641315108.01.0N-6.276016560.460.049010.002320.124640.005850.018440.000121481091195117.80.8N-6.3188317981.050.049910.001130.127470.003020.018450.00012191421223117.90.8N-6.4155817550.890.047530.001050.124080.002960.018880.0001876381193121.01.0N-6.5100014140.710.050440.001120.130480.003020.018710.00013215411253119.50.8N-6.6*112116990.660.059040.001870.148690.004540.018320.00023569451414117.01.0N-6.75758090.710.050360.001400.126960.003530.018330.00014211501213117.10.9N-6.85635571.010.047720.001570.119940.003950.018280.0001785581154117.01.0N-6.9151516170.940.052580.001420.132410.003190.01840.00014311411263117.50.9N-6.10151418070.840.048380.001010.12260.002600.018370.00013118371172117.40.8N-6.11100011830.850.047240.001450.122870.003730.018940.0001762521183121.01.0N-6.1257715620.370.046650.001000.120690.002680.018750.0001331371162119.80.8N-6.13119914290.840.048390.001130.123260.002810.018530.00012118421183118.40.7N-6.14310123111.340.051480.000950.129540.002100.018360.00013262251242117.30.8N-6.1572914380.510.049550.001640.129370.004130.018970.00020174541244121.01.0N-6.16115216560.700.047630.001060.121630.002650.018500.0001181411172118.20.7N-6.17155416820.920.049400.001090.127230.002800.018660.00010167411223119.20.7N-6.18*146318330.800.075010.003480.126030.005560.012190.00017106996121578.01.0N-6.192374680.510.048590.002630.121790.006440.018300.00025128941176117.02.0N-6.20132918110.730.048650.001060.12720.002780.018900.00013131391223120.70.8N-6.21*49510260.480.047050.00270.117830.006610.018160.00022521261136116.01.0N-6.22*8309990.830.055620.002090.142710.005360.018580.00016437681355119.01.0N-6.2372614620.500.050090.001840.126560.004780.018290.00020199671214117.01.0N-6.24*78513790.570.056920.001790.152280.004980.019330.00019488551444123.01.0N-6.25*70210070.700.057160.002580.142080.006580.018010.00020498821356115.01.0N-7.199513410.740.049350.001150.126230.002910.018600.00015164391213118.80.9N-7.2143810351.390.053740.001540.134400.003810.018200.00015360491283116.30.9N-7.398615410.640.050500.000940.129800.002490.018650.00015218301242119.10.9N-7.4216524290.890.050320.00280.126050.006840.018170.000232101291216116.01.0N-7.598610750.920.049640.001170.127050.003160.018520.00014178431213118.30.9N-7.6141412271.150.048900.001330.128760.003460.019110.00016143471233122.01.0N-7.7118814780.800.053300.001510.136340.004270.018410.00015341561304117.61.0N-7.8139410671.310.05540.004030.139570.007830.019240.00048429811337123.03.0N-7.9104113380.780.052460.001120.137320.002890.019010.00016305331313121.41.0N-7.1098114880.660.050080.00240.128980.006280.018650.00032199821236119.02.0N-7.115086120.830.052160.001810.134650.004550.018800.00018293591284120.01.0N-7.128257941.040.050930.002780.12880.007330.018340.000312381001237117.02.0N-7.137568720.870.053990.003700.136370.009170.018490.000323711211308118.02.0N-7.1450610050.500.051350.002000.126360.005080.017790.00018257741215114.01.0N-7.151755370.330.053290.005190.138640.013120.019120.0003834217813212122.02.0N-7.16331220511.610.051000.000900.130700.002490.018530.00012241321252118.40.8N-7.17133413750.970.047710.001320.121530.003320.018550.0001485501163118.50.9

表1(續(xù))

注：*所標(biāo)樣品未參加年齡計(jì)算。

表2 延邊和龍地區(qū)二長花崗巖主量元素和微量元素分析結(jié)果

表2(續(xù))

注：主量元素質(zhì)量分?jǐn)?shù)單位為%；微量元素質(zhì)量分?jǐn)?shù)單位為10-6。

微量元素 在原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(圖7b)中，柳洞巖體明顯富集Cs、Rb、Ba、K等大離子親石元素(LILE)，同時(shí)虧損Nb、Ta、Ti等高場強(qiáng)元素(HFSE)以及P。Nb/Ta值為11.00～16.27。高場強(qiáng)元素Nb、Ta的虧損，反映了巖漿來源于地殼或者受到地殼物質(zhì)的混染，其中，P、Ti的虧損應(yīng)與磷灰石、鈦鐵礦、榍石及角閃石、黑云母等含P、Ti礦物的分離結(jié)晶有關(guān)[14-16]。Rb/Ta值為11.88～16.27，處于地幔平均值與地殼平均值之間[17]，暗示其源區(qū)可能為殼?；煸碵18]。

3 討論

3.1 柳洞巖體侵位時(shí)代

關(guān)于柳洞巖體的侵位時(shí)代前人曾認(rèn)為是晚二疊世，但也推測(cè)可能為燕山期花崗巖。本次工作所采年齡樣品位于圖們江沿岸，其形成時(shí)代在1∶20萬大拉子幅*吉林省地質(zhì)局區(qū)域地質(zhì)測(cè)量大隊(duì).大拉子幅(1∶20萬)區(qū)域地質(zhì)測(cè)量報(bào)告.長春：吉林省區(qū)域地質(zhì)礦產(chǎn)調(diào)查所，1964.和1∶25萬延吉市幅地質(zhì)報(bào)告*吉林省地質(zhì)調(diào)查院.延吉市幅區(qū)域地質(zhì)調(diào)查報(bào)告.北京：全國地質(zhì)資料館，2007.中定為晚二疊世。Wu等[2]、張艷斌等[3]將該處定為侏羅紀(jì)花崗巖。二長花崗巖的CL圖像及Th/U值(0.39～1.34)顯示具有典型的巖漿成因，其定年結(jié)果應(yīng)代表了花崗巖的形成時(shí)代。LA-ICP-MAS鋯石U-Pb定年結(jié)果顯示，樣品N-6和N-7加權(quán)平均年齡分別為(118.63±0.69)Ma和(118.58±0.87)Ma，并非前人所認(rèn)為的晚二疊世或侏羅紀(jì)花崗巖。柳洞巖體侵位時(shí)代為早白堊世晚期。

3.2 構(gòu)造環(huán)境

柳洞巖體富硅、富堿、富鋁和低鈦，稀土元素質(zhì)量分?jǐn)?shù)較低，輕重稀土元素分異較強(qiáng)，富集Ba、Rb、U、K等大離子親石元素，相對(duì)虧損Nb、Ta、P、Ti等元素， A/CNK為1.03～1.11，屬于弱過鋁質(zhì)巖石，DI值為87.99～94.89，顯示了高分異I型花崗巖的特點(diǎn)。野外所見的晶洞構(gòu)造暗示柳洞巖體侵位深度應(yīng)較淺，且經(jīng)歷了高程度的分異演化[19]。

IAG.島弧花崗巖類；CAG.大陸弧花崗巖類；CCG.大陸碰撞花崗巖；POG.后造山花崗巖類;RRG.與裂谷有關(guān)的花崗巖類；CEUG.與大陸的造陸抬升有關(guān)的花崗巖類。圖8 柳洞巖體早白堊世二長花崗巖的主量元素構(gòu)造環(huán)境判別圖解(底圖據(jù)文獻(xiàn)[21])Fig.8 The discrimination diagrams of the main elements of the Early Cretaceous monzonitic granite(base map modified after reference[21])

柳洞巖體侵位年齡為(118.63±0.69) Ma和(118.58±0.87) Ma，屬于早白堊世晚期，且具有高鉀鈣堿性高分異I型花崗巖的特點(diǎn)。主要造巖礦物有石英、斜長石、條紋長石和少量黑云母及白云母，具有后造山花崗巖的特點(diǎn)。巖石學(xué)和地球化學(xué)特征可以對(duì)應(yīng)Barbarin[11]的高鉀鈣堿性(KCG)類型。Liegeois[20]認(rèn)為高鉀鈣堿性巖石主要形成于同碰撞巖石圈加厚之后的伸展垮塌向非造山板內(nèi)的過渡階段。在Maniar等[21]的w(SiO2)-w(Al2O3)(圖8a)和w(SiO2)-w(FeOt)/(w(FeOt)+w(MgO))(圖8b) 構(gòu)造判別圖上，柳洞巖體樣品落入后造山環(huán)境中。

在Pearce等[22-23]的w(Ta+Yb)-w(Rb)(圖9a)和w(Y+Nb)-w(Rb)(圖9b)的構(gòu)造判別圖解中，大部分樣品落入后碰撞環(huán)境中。該區(qū)存在中侏羅世的C型埃達(dá)克巖(待發(fā)表資料)，暗示了該早白堊世高鉀鈣堿性花崗巖可能形成于地殼加厚之后的伸展垮塌構(gòu)造環(huán)境中。

前人根據(jù)巖漿活動(dòng)的特點(diǎn)，認(rèn)為晚三疊世-早侏羅世期間中國東部就已經(jīng)開始受到古太平洋構(gòu)造域的影響[5-6,24-25]。中國東部早白堊世存在具有活動(dòng)大陸邊緣特點(diǎn)的火山巖和I型花崗巖[26-28]，暗示了中國東部早白堊世巖漿活動(dòng)可能與古太平洋構(gòu)造域的活動(dòng)有關(guān)。而張旗等[29]根據(jù)中侏羅世-早白堊世中國東部廣泛分布的C型埃達(dá)克巖推測(cè)中國東部高原的存在，且該時(shí)期大面積的巖漿活動(dòng)與古太平洋構(gòu)造域的活動(dòng)關(guān)系不大，早白堊世的巖漿活動(dòng)可能與中國東部高原的垮塌有關(guān)。華北東部地區(qū)早白堊世變質(zhì)核雜巖的識(shí)別[30-31]及中國東部地區(qū)廣泛分布的A型花崗巖[32]和雙峰式火山巖[33]表明，早白堊世期間中國東部處于地殼伸展變形環(huán)境中[32-35]，但對(duì)其動(dòng)力學(xué)機(jī)制存在較大的爭議[29,34,36-37]。

Zhang Yanbin等[3]對(duì)鄰區(qū)百里坪巖體巖漿巖進(jìn)行鋯石U-Pb測(cè)年，認(rèn)為早白堊世晚期巖體侵位構(gòu)造環(huán)境應(yīng)為伸展環(huán)境。郭春麗等[38]對(duì)遼東半島飲馬灣山巖體進(jìn)行的鋯石U-Pb測(cè)年顯示為(120±4)～(129±2)Ma，并認(rèn)為該期巖漿活動(dòng)發(fā)育在伸展環(huán)境背景下。曾濤等[39]通過對(duì)東北新開嶺地區(qū)晚中生代花崗巖的研究認(rèn)為，研究區(qū)早白堊世期間處于擠壓之后的伸展環(huán)境。汪洋等[40]對(duì)燕山地區(qū)侏羅紀(jì)-白堊紀(jì)巖漿巖研究認(rèn)為，早白堊世期間研究區(qū)處于后造山崩塌的環(huán)境中。葛文春等[18]對(duì)大興安嶺中生代玄武巖的研究認(rèn)為，晚侏羅世-早白堊世期間的巖漿活動(dòng)受地幔柱活動(dòng)的影響。柳洞巖體侵位時(shí)代與前人所測(cè)的百里坪巖體、飲馬灣山巖體和新開嶺地區(qū)巖體年齡誤差范圍內(nèi)一致，且?guī)r石地球化學(xué)特征上與新開嶺地區(qū)白堊紀(jì)巖體類似，結(jié)合區(qū)域地質(zhì)資料及中國東部大地構(gòu)造環(huán)境[15-16,18,30-31,33-35,38-40]，延邊地區(qū)早白堊世期間，應(yīng)處于伸展變形的環(huán)境中。高鉀鈣堿性花崗巖指示一種構(gòu)造體制的變化[41]，結(jié)合野外柳洞巖體特征和地球化學(xué)特征，筆者認(rèn)為延邊地區(qū)柳洞巖體形成于后造山向地殼伸展變形轉(zhuǎn)換環(huán)境中。

4 結(jié)論

1)延邊地區(qū)柳洞巖體LA-ICP-MS鋯石U-Pb加權(quán)平均年齡為(118.63±0.69)Ma和(118.58±0.87)Ma，巖體侵位時(shí)代為早白堊世晚期，并非前人所認(rèn)為的晚二疊世或侏羅紀(jì)。

2)柳洞巖體富硅、富堿、富鋁和貧鈦，LREE/HREE和(La/Yb)N值分別為3.46～8.09和10.59～33.94，輕重稀土元素分異較強(qiáng)，DI值為87.99～94.89，野外見有晶洞構(gòu)造。野外地質(zhì)特征及巖石地球化學(xué)特征具有高分異I型花崗巖的特點(diǎn)。Nb/Ta值及Nb、Ta負(fù)異常暗示其源區(qū)可能為殼幔混源。

3)柳洞巖體具有后造山花崗巖的特點(diǎn)，結(jié)合其巖石地球化學(xué)特征及中國東部早白堊世構(gòu)造環(huán)境，柳洞巖體侵位應(yīng)與地殼伸展變形有關(guān)，且延邊地區(qū)早白堊世處于后造山向伸展轉(zhuǎn)換體制環(huán)境下。

感謝西北大學(xué)大陸動(dòng)力學(xué)國家重點(diǎn)實(shí)驗(yàn)室在鋯石制靶和CL圖像采集以及LA-ICP-MS 鋯石U-Pb測(cè)年中給予的支持。衷心感謝給予幫助的鄭常青教授。

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Petrogenesis of the Late Mesozoic Liudong Pluton in Yanbian Area NE China：Evidence from Zircon U-Pb Geochronology and Geochemistry

Zhang Chao, Xu Zhongyuan, Liu Zhenghong, Li Shichao, Shi Yi, Fan Zhiwei

CollegeofEarthSciences,JilinUniversity,Changchun130061，China

The Liudong pluton is located in the southern Helong area of Yanbian area, the southwest segment of Gudonghe fault and north margin of Longgang block. It’s composed of monzonitic granite. Systematic researches on petrography and geochronology have been carried out to determine the emplacement age and tectonic environment of the pluton. The LA-ICP-MS zircon U-Pb dating yields the weighted mean age of (118.63±0.69) Ma and (118.58±0.87) Ma for N-6 and N-7 sample, respectively. The rock is weak peraluminous and belongs to the high potassium calc-alkaline series, characterized by high Si (70.30%-76.30%), high alkali (7.82%-8.52%), high aluminum (12.30%-15.50%) and low titanium (0.16%-0.29%), with A/CNK value of 1.03-1.11. Such geochemical characteristics asDI(87.99-94.89)andSI(2.39-5.31) indicate the rock experienced a highly fractionated evolving stage. The rock is of LREE enrichment (LREE/HREE=3.46-8.09) and HREE depletion ((La/Yb)N=10.59-33.94). The Liudong pluton is characterized by the strong enrichment in large ion lithophile elements (LILE), such as Cs, Pb, Ba, K, depletion in high field-strength elements (HESE), such as Nb, Ta, Ti, and P, and middle negative Eu anomalies (δEu = 0.63～0.92). Combining petrography, geochronology, rock geochemical characteristics, and regional geological material, it can be concluded that the Liudong pluton belongs to the highly fractionated high-potassium calc-alkaline I-type granite and emplaced in the transferring stage from a post-orogenic to an extensional tectonic setting.

geochemistry; zircon U-Pb geochronology; post-orogenic granite; Late Mesozoic period; Yanbian area

10.13278/j.cnki.jjuese.201401112.

2013-06-15

國家自然科學(xué)基金項(xiàng)目(40972135，41272223)；中國地質(zhì)調(diào)查局項(xiàng)目(1212011085481)

張超(1986-)，男，博士研究生，主要從事構(gòu)造與巖漿活動(dòng)的研究，E-mail:bosewell@163.com

徐仲元(1963-)，男，教授，主要從事構(gòu)造地質(zhì)學(xué)研究，E-mail:xuzy@jlu.edu.cn。

10.13278/j.cnki.jjuese.201401112

P588.12

A

張超，徐仲元，劉正宏，等.東北延邊地區(qū)晚中生代柳洞巖體的成因：鋯石U-Pb年代學(xué)和地球化學(xué)證據(jù).吉林大學(xué)學(xué)報(bào):地球科學(xué)版,2014,44(1):145-157.

Zhang Chao, Xu Zhongyuan, Liu Zhenghong,et al.Petrogenesis of the Late Mesozoic Liudong Pluton in Yanbian Area NE China：Evidence from Zircon U-Pb Geochronology and Geochemistry.Journal of Jilin University:Earth Science Edition,2014,44(1):145-157.doi:10.13278/j.cnki.jjuese.201401112.