浙東南晚中生代花崗巖的鋯石U-Pb年代學(xué)、地球化學(xué)及其地質(zhì)意義

高萬里，王宗秀，王對興，李春麟

1.中國地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所/中國地質(zhì)科學(xué)院頁巖油氣評價重點(diǎn)實(shí)驗(yàn)室，北京 1000812.中國地質(zhì)大學(xué)(北京)地球科學(xué)與資源學(xué)院，北京 1000833.石家莊經(jīng)濟(jì)學(xué)院資源學(xué)院，石家莊 050031

浙東南晚中生代花崗巖的鋯石U-Pb年代學(xué)、地球化學(xué)及其地質(zhì)意義

高萬里1，王宗秀1，王對興2，3，李春麟1

1.中國地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所/中國地質(zhì)科學(xué)院頁巖油氣評價重點(diǎn)實(shí)驗(yàn)室，北京 1000812.中國地質(zhì)大學(xué)(北京)地球科學(xué)與資源學(xué)院，北京 1000833.石家莊經(jīng)濟(jì)學(xué)院資源學(xué)院，石家莊 050031

浙東南地區(qū)(江紹斷裂帶東南)是位于華南東北部瀕太平洋的沿海地區(qū)，是理解古太平洋板塊俯沖作用的重要地區(qū)。本次研究選取巖坦、梁弄、新鋪3個典型的巖體進(jìn)行巖相學(xué)、鋯石年代學(xué)和地球化學(xué)研究，并結(jié)合前人對該地區(qū)花崗巖體的研究結(jié)果，探討古太平洋板塊俯沖與巖漿活動之間的關(guān)系。LA-ICP-MS鋯石U-Pb定年結(jié)果顯示：新鋪花崗巖的年齡為(145.8±1.4) Ma，表明浙東南地區(qū)晚侏羅世仍存在巖漿活動的記錄；梁弄花崗巖和巖坦花崗巖的形成時代分別為(106.2±1.4)和(94.7±1.4) Ma，代表早白堊世晚期典型的巖漿活動。地球化學(xué)特征上，3個巖體均富SiO2、Al2O3，具有高的A/CNK，屬高鉀鈣堿性花崗巖；稀土元素球粒隕石標(biāo)準(zhǔn)化分布型式圖中具顯著的負(fù)Eu異常，稀土元素總量偏低；微量元素原始地幔標(biāo)準(zhǔn)化分布型式圖中富集Rb、Cs、U、Th、Pb，虧損Ba、Sr、Nb、Ti，為典型的殼源型花崗巖。結(jié)合已有的資料，本次研究表明,新鋪花崗巖形成在由侏羅紀(jì)擠壓向白堊紀(jì)伸展轉(zhuǎn)變的構(gòu)造背景下，梁弄花崗巖和巖坦花崗巖形成在巖石圈減薄的伸展構(gòu)造背景下，它們形成均受到了古太平洋板塊俯沖作用的影響。

浙東南；晚中生代；花崗巖；LA-ICP-MS；地球化學(xué)；古太平洋板塊

0 引言

中國東南部晚中生代以來發(fā)生了大規(guī)模的火山活動和巖漿侵入，構(gòu)成舉世矚目的中生代火山-侵入雜巖帶，是瀕太平洋地區(qū)一個宏偉的構(gòu)造-巖漿帶的組成部分[1]。前人曾對該區(qū)大規(guī)模巖漿活動的地質(zhì)背景進(jìn)行了深入研究，并分別提出了幾種解釋這一巖漿活動的動力學(xué)模型,主要分為兩類：板內(nèi)活動作用，諸如陸陸碰撞模型[2]，大陸拉張裂解模型[3-7]，地幔柱模型[8-10]；古太平洋板塊俯沖模型，包括平板俯沖模型[11-12]，俯沖角度變化模型[13-16]。隨著越來越多的地質(zhì)數(shù)據(jù)的累積，太平洋板塊俯沖作用逐漸被人們所接受。然而前人所建立的俯沖模型中均認(rèn)為古太平洋板塊的漂移方向是北西向；孫衛(wèi)東[17-18]研究則表明，太平洋板塊在地質(zhì)歷史上曾發(fā)生過多次的轉(zhuǎn)向。其中，在125～140 Ma，其漂移方向顯示為南西向。而且，太平洋洋殼年齡展布(130～170 Ma)也顯現(xiàn)出自北向南逐漸變老的特征[19]，顯示太平洋當(dāng)時漂移方向應(yīng)該總體向南。因此，古太平洋板塊的俯沖作用與華南晚中生代巖漿活動的關(guān)系仍需進(jìn)一步研究。

浙東南地區(qū)(江紹斷裂帶東南)是位于華南東北部瀕太平洋的沿海地區(qū)，是理解古太平洋板塊俯沖作用的重要地區(qū)。浙東南地區(qū)晚中生代巖漿活動的規(guī)律可以為理解古太平洋板塊俯沖作用提供關(guān)鍵線索。筆者以浙東南地區(qū)晚中生代花崗巖為研究對象，在對區(qū)內(nèi)花崗巖已有年齡全面甄別、清理的基礎(chǔ)上，選取區(qū)內(nèi)梁弄、巖坦和新鋪3個巖體進(jìn)行巖相學(xué)、鋯石U-Pb年代學(xué)及巖石地球化學(xué)研究(取樣位置見圖1)。并結(jié)合區(qū)內(nèi)已有的花崗巖數(shù)據(jù)，探討巖漿活動的構(gòu)造背景及與古太平洋板塊俯沖之間的關(guān)系。

1 區(qū)域地質(zhì)背景及樣品特征

中國東南部出露了較大規(guī)模的前震旦紀(jì)變質(zhì)巖層，保留有較多的古構(gòu)造形跡[20]。其中前震旦紀(jì)的變質(zhì)基底以面狀分布的片巖、片麻巖為主，構(gòu)成中國東南部最古老的陸殼基底之一，稱為華夏古陸[21]。中生代特別是晚中生代以來，該區(qū)發(fā)生了大規(guī)模的巖漿侵入與火山活動，形成了巨厚的火山沉積地層[6, 13]。除在江紹斷裂帶附近出露少量的元古宙基底之外，區(qū)內(nèi)大部分為晚中生代火山沉積巖所覆蓋，巖性以流紋巖、凝灰?guī)r為主，有少量的玄武巖、安山巖等中基性巖；晚中生代侵入巖主要以花崗巖類為主，中基性巖主要以包體的形式出現(xiàn)。同時區(qū)內(nèi)零星分布一些晚白堊世-新生代的盆地。

巖坦巖體位于永嘉縣巖坦鎮(zhèn)，巖體出露面積約56 km2。巖體主要侵位于上侏羅統(tǒng)諸暨組與黃山組中。前人的區(qū)域地質(zhì)調(diào)查中，認(rèn)為其屬于燕山晚期第三階段巖漿侵入的產(chǎn)物。巖性主要為鉀長花崗巖與正長花崗巖。樣品DY11-79采自巖坦鎮(zhèn)，坐標(biāo)為N28°27′05″，E120°43′22″。巖性為細(xì)中粒正長花崗巖，細(xì)中粒花崗結(jié)構(gòu)，局部文象結(jié)構(gòu)，塊狀構(gòu)造。巖石由斜長石、鉀長石、石英、黑云母組成。斜長石呈半自形板狀，雜亂分布，粒度一般為0.2～2.0 mm，少部分為2.0～5.0 mm；核部多具絹云母化、硅化、不均勻高嶺土化等，少見環(huán)帶構(gòu)造。鉀長石呈他形粒狀，雜亂分布，粒度一般為2.0～5.0 mm，少部分為0.2～2.0 mm；具高嶺土化等，晶內(nèi)嵌布少量斜長石等小包體，部分與石英呈文象狀交生。石英呈他形粒狀，填隙狀分布，粒度一般為0.2～2.0 mm，少部分為2.0～3.0 mm；黑云母呈葉片狀、星散狀分布，粒度一般為0.15～1.5 mm，少部分綠泥石化(圖2a、b)。

梁弄巖體位于余姚縣梁弄鎮(zhèn)，受北北東向斷裂的控制，為一個由燕山晚期和喜馬拉雅期花崗巖構(gòu)成的復(fù)合巖體，面積88.19 km2。巖體侵入于下白堊統(tǒng)和上侏羅統(tǒng)中，又被上新統(tǒng)嵊縣群玄武巖所覆蓋。樣品DY11-137采自梁弄鎮(zhèn)，采樣坐標(biāo)為N29°45′54″,E120°59′36″，為燕山晚期侵入巖。巖性為灰白色石英閃長巖，塊狀構(gòu)造，中粒等粒結(jié)構(gòu)，礦物成分主要為斜長石(55%～65%)、角閃石(15%～25%)、鉀長石(10%)、石英(5%)(圖2c、d)。

新鋪巖體位于遂昌縣北部雙溪口鎮(zhèn)新鋪村，呈北東向展布。該處為燕山早晚期侵入巖構(gòu)成的復(fù)合巖體，巖體北部侵入到元古宙八都群中，南部為蘇村晶洞鉀長花崗巖侵入。樣品DY11-158采自新鋪巖體的北部，采樣坐標(biāo)為N28°47′17″，E119°19′24″。巖性為斑狀鉀長花崗巖，巖石主要由斑晶和基質(zhì)組成，斑晶主要為鉀長石，粒度大小為0.5 mm左右，體積分?jǐn)?shù)約15%；基質(zhì)主要為長英質(zhì)礦物(圖2e、f)。

2 LA-ICP-MS鋯石U-Pb年代學(xué)

鋯石按常規(guī)方法分選，最后在雙目鏡下挑純；將分選鋯石用雙面膠粘在載玻片上，罩上PVC環(huán)，然后將環(huán)氧樹脂和固化劑進(jìn)行充分混合后注入PVC環(huán)中，待樹脂充分固化后，將樣品靶從載玻片上剝離，并對其進(jìn)行打磨和拋光；然后對靶上的樣品進(jìn)行反射光和透射光照相，以及陰極發(fā)光(CL)照相(圖3)。3個花崗巖樣品的CL圖像分析均在中國地質(zhì)科學(xué)院國土資源部成礦作用與資源評價重點(diǎn)實(shí)驗(yàn)室完成。鋯石的Th、U、Pb同位素分析在國土資源部成礦作用與資源評價重點(diǎn)實(shí)驗(yàn)室采用激光燒蝕電感耦合等離子質(zhì)譜儀(LA-MC-ICP-MS)完成。采用的儀器、相關(guān)參數(shù)及測試流程參見文獻(xiàn)[22]。分析結(jié)果應(yīng)用ICPMSDatacal程序[23]和Ludwig的Isoplot程序進(jìn)行數(shù)據(jù)處理[24]，并采用208Pb校正法進(jìn)行普通鉛校正[25]。實(shí)驗(yàn)分析結(jié)果見表1，測試數(shù)據(jù)的誤差均為1σ。

進(jìn)行定年的鋯石以透明為主，局部半透明，裂紋不發(fā)育，無核或核部小。從鋯石CL圖像(圖3)上可以看出，3個巖體鋯石多為長柱狀晶體，長寬比為1∶1至3∶1，顆粒大小為80～215 μm，鋯石的Th/U為0.71～2.99。CL圖像顯示較弱，部分鋯石發(fā)育鋯石核，呈現(xiàn)典型的核幔結(jié)構(gòu)。所有樣品均發(fā)育典型的振蕩環(huán)帶結(jié)構(gòu)，顯示鋯石均為典型的巖漿鋯石。測點(diǎn)都選擇在韻律環(huán)帶結(jié)構(gòu)清晰的部位，盡可能避開核部。本次研究中，主要根據(jù)206Pb/238U來確定巖體的形成時間。所有鋯石U-Pb年代學(xué)測定結(jié)果列于表1，圖3為年齡諧和圖。

巖坦正長花崗巖(DY11-79)鋯石Th/U為1.74～2.99，13顆鋯石的206Pb/238U表面年齡比較一致，為92.43～101.08 Ma。在協(xié)和曲線上，具有較好的群落性，13個測點(diǎn)的206Pb/238U的加權(quán)平均年齡為(94.7±1.4)Ma，MSWD=2.9，代表巖體的結(jié)晶年齡。

梁弄石英閃長巖(DY11-137)的15顆鋯石進(jìn)行了同位素分析，除去1個測點(diǎn)可能由于Pb丟失的緣故偏離協(xié)和曲線外，其余14個測點(diǎn)均落在協(xié)和曲線上，具有較好的群落性。206Pb/238U表面年齡比較一致，為100.00～108.57 Ma，14個測點(diǎn)的206Pb/238U的加權(quán)平均年齡為(106.2±1.4)Ma，MSWD=1.4，代表巖體的結(jié)晶年齡。

新鋪鉀長花崗巖(DY11-158)的16顆鋯石進(jìn)行了同位素分析，除去1個測點(diǎn)明顯偏離協(xié)和曲線外，其余15個測點(diǎn)均具有較好的群落性。206Pb/238U表面年齡比較一致，為140.51～150.10 Ma，15個測點(diǎn)的206Pb/238U加權(quán)平均年齡為(145.8±1.4)Ma，MSWD=0.84，代表巖體的結(jié)晶年齡。

圖1 浙東南花崗巖分布圖及取樣點(diǎn)位置圖Fig.1 Map of granite distribution and sample location in southeastern Zhejiang

a、b.巖坦正長花崗巖；c、d.梁弄石英閃長巖；e、f.新鋪斑狀鉀長花崗巖。圖2 樣品的顯微特征照片F(xiàn)ig.2 Photomicrographs of the samples

圖3 代表性鋯石陰極發(fā)光圖像與鋯石U-Pb年齡協(xié)和圖Fig.3 Representative zircon CL images with analyzed spots and zircon U-Pb concordia diagram

點(diǎn)位wB/10-6ThUTh/U同位素比值207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ年齡/Ma207Pb/206Pb206Pb/238UDY11-7979.1146.5277.011.900.04900.00250.09880.00510.01460.0003150.09±115.7393.59±1.7979.251.6629.721.740.05640.00520.11720.01340.01500.0005477.82±202.696.02±3..2179.3220.84119.481.850.04830.00400.10200.00930.01440.0001122.31±175.992.43±0.7179.497.5942.922.270.04710.00420.09900.00900.01530.000453.80±196.2797.95±2.5079.5223.65114.171.960.05330.00260.11190.00540.01530.0003338.95±111.197.77±1.9879.6629.93249.592.520.05140.00100.10540.00280.01490.0002261.18±46.2995.17±1.2079.7389.70194.932.000.04760.00140.09650.00300.01470.000179.72±68.5194.29±0.7679.8140.6249.062.870.05230.00300.10490.00610.01460.0003298.21±134.2493.55±1.7279.9149.7565.042.300.04860.00250.10580.00640.01580.0004127.87±72.22101.08±2.8079.10382.18212.361.800.05280.00180.10790.00270.01490.0002320.43±79.6295.23±1.4379.11299.51100.172.990.04870.00150.10270.00320.01530.0002200.08±74.9998.16±1.4879.12118.8966.311.790.04820.00340.09860.00830.01480.0005109.35±224.0494.64±3.4779.1368.6933.352.060.04650.00340.10040.00730.01570.000333.43±157.39100.51±2.00DY11-137137.159.9143.401.380.05130.00460.11070.01000.01570.0004253.77±211.09100.22±2.29137.246.5631.041.500.05580.00620.12870.01790.01670.0010442.64±251.82106.54±6.50137.368.8559.691.150.05330.00470.11860.00990.01640.0007342.65±199.98104.64±4.50137.4102.4083.331.230.04770.00120.10430.00280.01600.000283.43±61.11102.10±1.55137.568.5453.921.270.05200.00250.12060.00580.01700.0004287.10±107.39108.47±2.49137.6106.4981.691.300.05020.00150.11650.00350.01690.0003211.19±100.91108.25±1.69137.759.9360.171.000.05430.00340.11500.00880.01560.0008388.94±140.73100.00±5.28137.861.8353.571.150.05120.00280.11670.00590.01660.0003250.07±125.91106.32±1.87137.9154.69110.431.400.05130.00120.11790.00320.01670.0003253.77±53.69106.79±1.65137.10184.63162.511.140.04880.00090.11170.00240.01670.0003138.98±42.59106.50±1.66137.11320.18149.362.140.05210.00090.12110.00200.01700.0002287.10±34.36108.57±1.22137.1257.3648.861.170.04830.00180.11140.00430.01670.0002122.31±85.18106.86±1.50137.1373.4449.051.500.05950.00330.12880.00800.01590.0006583.36±122.21101.57±3.93137.14116.3167.641.720.05130.00170.11660.00410.01650.0002253.77±75.91105.64±1.44137.1592.0285.681.070.05020.00180.11560.00580.01660.0004211.19±81.47106.35±2.69DY11-158158.1152.69172.180.890.05130.00060.15830.00300.02240.0004253.77±27.28143.00±2.22158.237.6750.180.750.05150.00130.16690.00550.02360.0005261.18±54.62150.10±3.43158.3139.9373.361.910.04820.00110.15250.00400.02300.0004122.31±51.85146.50±2.74158.439.2338.641.020.05090.00200.16240.00700.02320.0006238.96±88.88147.87±3.58158.5255.61286.990.890.04890.00270.15310.02050.02250.0018142.68±162.02143.56±11.39

表1(續(xù))

3 巖石地球化學(xué)

將全巖地球化學(xué)樣品破碎成<1 cm的巖屑，從中挑選出200 g新鮮無蝕變、無晚期巖脈的巖屑，用1 mol/L的HCl溶液浸泡，去除表面污染，然后用去離子水反復(fù)洗凈，烘干，用剛玉板小型鄂式破碎機(jī)細(xì)碎，最后用瑪瑙球磨機(jī)磨至約200目。樣品的主要元素及痕量元素(稀土元素除外)在中國地質(zhì)科學(xué)院地球物理與地球化學(xué)研究所用X熒光光譜法測定，稀土元素用電感耦合等離子體發(fā)射光譜(ICP-AES)測定，輕稀土元素(La、Ce、Pr、Nd、Sm)相對標(biāo)準(zhǔn)誤差RSD<5，重稀土元素(Eu-Lu)相對標(biāo)準(zhǔn)偏差<10%。

3.1 主量元素

主量元素、微量元素如表2所示。3個巖體的花崗巖具有高SiO2、Al2O3、K2O，低MgO、CaO、P2O5特征：w(SiO2)為73.28%～75.79%，w(Na2O+K2O)為8.39%～9.46%；w(K2O)>w(Na2O)，K2O/Na2O為1.13～1.57，在圖4a上落入高鉀鈣堿性系列范圍；A/CNK=1.05～1.15，鋁飽和指數(shù)高，在圖4b上基本都落入偏鋁質(zhì)-過鋁質(zhì)區(qū)域，屬于準(zhǔn)鋁質(zhì)或弱過鋁質(zhì)高鉀鈣堿性花崗巖。

3.2 微量元素特征

由稀土元素球粒隕石標(biāo)準(zhǔn)化分布型式圖(圖5a)可知，3個巖體的稀土元素總量(∑REE)為(183.43～247.24)×10-6，富集輕稀土，LREE/HREE為1.17～6.64，(La/Yb)N=2.09～18.95，其中輕稀土分餾較重稀土更為顯著；(La/Sm)N和(Gd/Yb)N值分別為2.31～7.57和0.65～1.51，銪負(fù)異常明顯，其中尤以新鋪花崗巖(δEu=0.07)顯著，指示成巖過程中經(jīng)歷了明顯的斜長石分離結(jié)晶。微量元素原始地幔標(biāo)準(zhǔn)化分布型式圖(圖5b)上，3個巖體基本上富集Rb、Cs、Th、U、Pb，明顯虧損Nb、Ba、Sr、Ti，屬于低Ba、Sr花崗巖。Rb、Ba、Sr、Ti質(zhì)量分?jǐn)?shù)的變化主要受造巖礦物控制，Rb升高和Sr、Ba的降低是鉀長石和斜長石、黑云母分離結(jié)晶造成的，Ti的負(fù)異常表明它們經(jīng)歷了鈦鐵礦的分離結(jié)晶作用。

4 討論

4.1 巖石成因

新鋪、梁弄和巖坦巖體的主量元素特征顯示，它們均為富硅(73.28%～75.79%)、富鋁(12.81%～13.64%)、富堿(w(Na2O+K2O)=8.39%～9.46%)，鉀大于鈉(K2O/NaO=1.13～1.57)。稀土元素均呈現(xiàn)“海鷗”式的分配模式，巖坦巖體與梁弄巖體La/Yb=10.08～18.95，比新鋪巖體(La/Yb=2.09)明顯偏高，顯示出更明顯的輕重稀土元素分餾。銪負(fù)異常明顯，巖坦巖體及梁弄巖體δEu為0.52～0.54，而新鋪花崗巖δEu=0.07更為顯著，指示其源區(qū)有穩(wěn)定的斜長石存在。微量元素特征為，富集大離子親石元素(Cs、Rb、K)，虧損高場強(qiáng)元素(Nb、Ti、P)，表明這3個巖體均為高分異I型花崗巖或殼源型花崗巖。

表2 巖坦、梁弄及新鋪花崗巖的主量元素及微量元素成分

注：ALK=w(Na2O)+w(K2O)；A/CNK=w(Al2O3)/w(CaO+Na2O+K2O)；δEu=(Eu/0.735)/(0.5(Sm/0.195+Gd/0.259))；DI為分異指數(shù)。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。

1-7源自文獻(xiàn)[26-32]，8源自本文實(shí)測；w(K2O)-w(SiO2)底圖據(jù)文獻(xiàn)[33]，A/CNK-A/NK底圖據(jù)文獻(xiàn)[34]。a.w(K2O)-w(SiO2)圖解； b.A/CNK-A/NK圖解。圖4 浙東南晚中生代花崗質(zhì)巖石分類圖解Fig.4 Classification diagram for Late Mesozoic granities from southeastern Zhejiang

球粒隕石標(biāo)準(zhǔn)值源自文獻(xiàn)[35]，原始地幔標(biāo)準(zhǔn)值源自文獻(xiàn)[36]；數(shù)據(jù)來源同圖4。圖5 浙東南花崗質(zhì)巖石稀土元素球粒隕石標(biāo)準(zhǔn)化配分圖(a)及原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b)Fig.5 Chondrite-normalized REE patterns (a)and primitive mantle normalized element spider diagram(b) for granitic rocks from southeastern Zhejiang

a底圖據(jù)文獻(xiàn)[37];b底圖據(jù)文獻(xiàn)[38]。圖6 巖坦、新鋪和梁弄巖體的Rb/Ba-Rb/Sr圖解(a)及A/MF-C/MF圖解(b)Fig.6 Rb/Ba-Rb/Sr (a) and A/MF-C/MF (b) diagrams of the Yantan, Xinpu and Liangnong granites

在不相容元素比值上，梁弄巖體Rb/Nb值為17.79，高于巖坦巖體(7.23)和新鋪巖體(6.35)，均大于全殼平均值(5.36)，表明它們主要為殼源組分熔融形成的。巖坦巖體的Nb/Ta值為16.03，高于新鋪巖體(8.86)和梁弄巖體(9.15)，指示其成巖過程中可能有更多的地幔物質(zhì)參與。關(guān)于源區(qū)物質(zhì)成分特征，在Rb/Sr-Rb/Ba圖解上(圖6a)，新鋪巖體投影于右上方的富黏土源巖區(qū)域內(nèi)，而梁弄巖體及巖坦巖體投影于砂質(zhì)源巖區(qū)域內(nèi)；在A/MF-C/MF圖解上(圖6b)，它們則落入了變質(zhì)雜砂巖的部分熔融區(qū)域內(nèi)，而新鋪巖體則落入?yún)^(qū)域外，這表明新鋪巖體的成巖過程中有更多地殼物質(zhì)的參與。

4.2 地質(zhì)意義

鋯石LA-ICP-MS U-Pb年齡分析表明，新鋪鉀長花崗巖的形成時代為(145.8±1.4)Ma，這與廣山巖體花崗斑巖((147.2±1.7)Ma)，柵溪巖體黑云母花崗巖((150±2.6)Ma)[39]以及在浙西北發(fā)現(xiàn)的淳安開嶺角巖體(151 Ma)和里陳家?guī)r體(148 Ma)[40]等形成時代一致，均屬于晚侏羅世晚期巖漿活動的產(chǎn)物。新鋪花崗巖(145.8±1.4)Ma的年齡表明在浙江東南部燕山期巖漿演化過程中，沒有缺失侏羅紀(jì)侵入活動的記錄。此后，位于浙東南地區(qū)的蘇村鉀長花崗巖侵位于133 Ma[41]，洪公鋁質(zhì)A型花崗巖侵位于124 Ma[32]，均形成在早白堊世伸展構(gòu)造背景下。而新鋪花崗巖地球化學(xué)特征與產(chǎn)于由擠壓向拉張轉(zhuǎn)變過程中形成的富鉀鈣堿性花崗巖(KCG)特征有相似之處[42]；前人大量的地質(zhì)研究表明，華南乃至整個中國東南部燕山期區(qū)域構(gòu)造經(jīng)歷了從侏羅紀(jì)擠壓到白堊紀(jì)拉張的構(gòu)造環(huán)境，伴隨有一系列花崗巖巖漿活動[13, 43-45]。邢光福等[46]通過區(qū)域地質(zhì)構(gòu)造及地層對比研究認(rèn)為，華南中生代構(gòu)造體制轉(zhuǎn)折結(jié)束于晚侏羅世( 149.8～142.3 Ma),這表明新鋪花崗巖可能形成于由擠壓向伸展轉(zhuǎn)變的過程中。

syn-COLG.同碰撞花崗巖；WPG.板內(nèi)花崗巖；VAG.火山弧花崗巖；ORG.洋脊花崗巖。底圖據(jù)文獻(xiàn)[53]；數(shù)據(jù)來源同圖4。圖7 浙東南晚中生代花崗巖構(gòu)造環(huán)境判別圖Fig.7 Tectonic discrimination diagrams of Late Mesozoic granites from southeastern Zhejiang

梁弄石英閃長巖與巖坦正長花崗巖LA-ICP-MS鋯石U-Pb年代分別為(106.2±1.4)Ma和(94.7±1.4)Ma，與黃巖望海崗巖體[27]、曹門堿性花崗巖體[28]、石平川花崗巖體[30]等形成時代一致，為早白堊世晚期巖漿活動的產(chǎn)物。該時期花崗巖常與A型花崗巖共生，形成特征的I-A型花崗巖體[47]，均形成在伸展構(gòu)造背景下[5]。除了I-A型花崗巖組合之外(如舟山普陀山，桃花島[47]，瑤坑堿性花崗巖[31])，該區(qū)還發(fā)育雙峰式的火山巖(玄武巖-流紋巖)及雙峰式侵入巖(輝長巖-花崗巖)，它們共同構(gòu)成了陸緣伸展型構(gòu)造火成巖組合[48]。Li[5]的研究表明，該時期形成的花崗巖與巖石圈的幕式減薄有關(guān)，地球化學(xué)特征顯示出弧巖漿巖的特征，暗示巖石圈的減薄與板塊俯沖有關(guān)。

東南沿海地區(qū)晚中生代普遍出現(xiàn)A型花崗巖或堿性巖，一系列NNE、NE向的斷裂、盆地的展布，淺成鎂鐵質(zhì)巖脈的侵入均表明晚中生代中國東南部整體上處在伸展拉張的構(gòu)造環(huán)境下[5-7, 49-51]。但是關(guān)于伸展性質(zhì)仍然存在爭論，目前爭論的焦點(diǎn)在于該時期的伸展屬于板內(nèi)伸展還是弧后伸展。Li[5]曾指出華南白堊紀(jì)鈣堿性巖漿活動與巖石圈伸展引發(fā)的減壓熔融作用有關(guān)，與俯沖作用無關(guān)；Li等[11]用平板俯沖模型來解釋華南1 300 km巖漿活動，提出190～90 Ma的巖漿活動與俯沖板片的塌陷和拆沉作用有關(guān)[12]。與此相反，一些學(xué)者基于晚中生代巖漿巖帶呈北東走向展布，且?guī)r漿巖年齡有從內(nèi)陸至沿海逐漸變年輕的趨向這一基本事實(shí)[13, 52]，提出用古太平洋板塊俯沖來解釋華南晚中生代的巖漿活動，認(rèn)為中國東南部晚中生代處在活動大陸邊緣的構(gòu)造環(huán)境下。與典型的活動大陸邊緣花崗巖主要落入火山弧花崗巖(VAG)區(qū)域不同，浙東南花崗巖在構(gòu)造環(huán)境圖解(圖7)上大多跨越火山弧(VAG)及板內(nèi)花崗巖(WPG)區(qū)域，這一特征與大陸弧后盆地伸展環(huán)境下形成的花崗巖相似[54]。Gilder等[4]在研究中國裂谷分布時曾指出，中國東部中生代的伸展環(huán)境是伴隨古太平洋板塊向華南大陸之下俯沖消減作用發(fā)生的，這一時期的伸展環(huán)境主要是由于弧后擴(kuò)張作用所導(dǎo)致的。毛建仁等[55]認(rèn)為燕山晚期整個中國東部主要受到庫拉板塊向歐亞大陸北北西向俯沖的太平洋構(gòu)造體系控制，處于活動大陸邊緣及其弧后擴(kuò)張盆地的構(gòu)造環(huán)境下；隨著時代的更新，大陸巖石圈不斷伸展-減薄，擴(kuò)張作用不斷增強(qiáng)，由早期庫拉板塊向歐亞大陸俯沖碰撞-伸展走滑演化為晚期的伸展-裂解。舒良樹等[56]通過對比北美西部與中國東南部盆嶺構(gòu)造的異同，認(rèn)為中國東南部與北美西部中、新生代時期均受太平洋構(gòu)造體制制約，早白堊前后俯沖角度的變化導(dǎo)致弧后進(jìn)一步伸展塌陷，巖石圈減薄。劉國興等[57]對華南東南沿海地區(qū)的巖石圈電性結(jié)構(gòu)研究也發(fā)現(xiàn)，東部地殼由于太平洋板塊的俯沖作用發(fā)生過劇烈減薄。因此，中國東南部晚中生代花崗巖形成在古太平洋板塊俯沖導(dǎo)致的弧后拉張背景、巖石圈減薄的背景下。

5 結(jié)論

1)新鋪、梁弄和巖坦巖體的主量元素特征顯示，它們均為弱過鋁質(zhì)-準(zhǔn)鋁質(zhì)的高鉀鈣堿性花崗巖，分異演化程度高(DI=94.58%～97.84%)；稀土元素均呈現(xiàn)“海鷗”式的分配模式，銪負(fù)異常明顯，富集大離子親石元素(Cs、Rb、K)，虧損高場強(qiáng)元素(Nb、Ti、P)，表明這3個巖體均為殼源型花崗巖。

2)LA-ICP-MS鋯石U-Pb測年結(jié)果表明：新鋪花崗巖年齡為(145.8±1.4)Ma，表明浙東南存在晚侏羅晚期的巖漿活動，其形成于侏羅紀(jì)擠壓向白堊紀(jì)伸展轉(zhuǎn)變的構(gòu)造背景下；梁弄和巖坦花崗巖的結(jié)晶年齡分別為(106.2±1.4)Ma和(94.7±1.4)Ma，代表浙東南早白堊世晚期典型的巖漿活動，二者均形成在伸展構(gòu)造背景下。整體上，新鋪、巖坦和梁弄巖體均形成在古太平洋板塊俯沖影響的巖石圈拉張減薄構(gòu)造背景下。

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Zircon U-Pb Geochronology, Geochemistry of Late-Mesozoic Granite in Southeastern (SE) Zhejiang Province and Its Tectonic Implication

Gao Wanli1, Wang Zongxiu1, Wang Duixing2,3, Li Chunlin1

1.Institute of Geomechanics,Key Lab of Shale Oil and Gas Geological Survey/ Chinese Academy of Geological Science, Beijing 100081,China2.School of Earth Science and Resources, China University of Geosciences,Beijing 100083,China3.School of Resources, Shijiazhuang University of Economics,Shijiazhuang 050031,China

The southeastern (SE) Zhejiang (southeast side of the Jiangshan-shaoxing fault) situated in coastal area of the Pacific Ocean, is an important area to understand the subduction of the Paleo-Pacific plate. Yantan, Xinpu and Liangnong plutons in SE Zhejiang are chosen for petrography, zircon geochronology and geochemistry study. Combined with previous research results of the granite in this area,the authors discuss the relationship between the subduction of the Paleo-Pacific plate and magmatism. LA-ICP-MS zircon U-Pb dating results show that the Xinpu granite has an crystallization age of (145.8±1.4) Ma, and emplaced in the first stage of Early Cretaceous in SE Zhejiang Province, and the Liangnong granodiorite and Yantan syenogranite, with emplacement ages of (106.2 ± 1.4) and (94.7 ±1.4) Ma, respectively, were resulted from the Late Cretaceous magmatism. Late Mesozoic granites are charactered by such geochemical characteristics as enrichment of SiO2, Al2O3, and high ratio of A/CNK, which indicates the granite belongs to the high-K calc-alkaline granite. In the primitive mantle-normalized distribution patterns, these granitic rocks are enriched in Rb, Cs，U,Th, Pb, and depleted in Ba,Sr, Nb, Ti. Their REE patterns are highly fractionated, strongly negative Eu anomalies and low total REE, which is corcandant with those of the crust-derived granite. The authors also conclude that the Xinpu granite (ca.145.8 ± 1.4 Ma) formed in the tectonic setting shifting from compression in the Late Jurassic to extension in Cretaceous, and Liangnong granite (ca. 106.2 ± 1.4 Ma) and Yantan granite (ca. 94.7 ± 1.4 Ma) are formed in the extensional dynamic setting which are all influenced by subduction of the Paleo-Pacific plate.

southeastern Zhejiang; Late-Mesozoic; granite; LA-ICP-MS; geochemistry; Paleo-Pacific plate

10.13278/j.cnki.jjuese.201403112.

2013-09-25

中國地質(zhì)調(diào)查局地質(zhì)大調(diào)查項(xiàng)目 (1212011121068)

高萬里(1985-)，男，博士研究生，主要從事巖石大地構(gòu)造方面研究，E-mail:gwanli851202@163.com

王宗秀(1959-)，男，研究員，主要從事構(gòu)造地質(zhì)方面研究，E-mail:wzxmail@vip.sina.com。

10.13278/j.cnki.jjuese.201403112

P588.12

A

高萬里，王宗秀，王對興，等.浙東南晚中生代花崗巖的鋯石U-Pb年代學(xué)、地球化學(xué)及其地質(zhì)意義.吉林大學(xué)學(xué)報(bào):地球科學(xué)版,2014,44(3):861-875.

Gao Wanli, Wang Zongxiu, Wang Duixing，et al. Zircon U-Pb Geochronology, Geochemistry of Late-Mesozoic Granite in Southeastern(SE) Zhejiang Province and Its Tectonic Implication.Journal of Jilin University:Earth Science Edition,2014,44(3):861-875.doi:10.13278/j.cnki.jjuese.201403112.