盧寅花，王力，張國(guó)峰

1)吉林大學(xué)地球科學(xué)學(xué)院，長(zhǎng)春，130061； 2)吉林省第一地質(zhì)調(diào)查所，長(zhǎng)春，130061

內(nèi)容提要：黃綠山地區(qū)位于柴達(dá)木盆地北緣造山帶西段，區(qū)內(nèi)花崗巖廣泛分布，相對(duì)匱乏同位素的研究成果以及巖漿活動(dòng)和變質(zhì)作用的地質(zhì)記錄。本文對(duì)前人在地質(zhì)填圖中認(rèn)為原屬于華力西期的奧長(zhǎng)花崗斑巖侵入體進(jìn)行巖相學(xué)、年代學(xué)、地球化學(xué)和全巖Sr-Nd同位素研究，探討其成因類型和源區(qū)，并在此基礎(chǔ)上討論了其地球動(dòng)力學(xué)意義。LA-ICP-MS鋯石U-Pb定年結(jié)果顯示，黃綠山奧長(zhǎng)花崗斑巖結(jié)晶時(shí)代為466±3 Ma，為加里東期中奧陶世的產(chǎn)物。樣品具有高SiO2和Na2O，低K2O的特點(diǎn)；在SiO2—K2O圖解上，除一個(gè)樣品落入鈣堿性區(qū)域之外，其余樣品全部落入低鉀拉斑系列。樣品具有右傾的稀土分配特點(diǎn)((La/Yb)N介于4.41～10.01)，具有微弱的Eu異常(0.94～1.09)。樣品具有較低的10000Ga/Al(<2.6)以及極低的P2O5(平均為0.06%)，說明黃綠山奧長(zhǎng)花崗斑巖為I型花崗巖；結(jié)合其中元古代的Nd二階段模式年齡(1.03～1.21 Ga)，認(rèn)為黃綠山奧長(zhǎng)花崗斑巖為中元古代下地殼再活化部分熔融的產(chǎn)物。在花崗巖構(gòu)造圖解上，樣品落入VAG區(qū)域，顯示其與洋殼俯沖具有密切的聯(lián)系，結(jié)合其與奧陶系灘間山群的伴生關(guān)系和具有喜馬拉雅型花崗巖的特征，因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖產(chǎn)于與洋殼俯沖相關(guān)的弧后盆地環(huán)境中；弧后盆地的存在暗示洋殼俯沖仍未結(jié)束，地體碰撞尚未開始，因此柴達(dá)木盆地北緣洋盆至少在466 Ma仍未關(guān)閉。

造山巖漿活動(dòng)能記錄一些重要的地球動(dòng)力學(xué)過程所經(jīng)歷的時(shí)間與性質(zhì)，如俯沖、板片斷離和巖石圈拆沉。因此，通過對(duì)這些巖漿巖的時(shí)空分布、巖石構(gòu)造結(jié)構(gòu)組合和變異特征的研究，可以重建出造山帶的古地球動(dòng)力學(xué)演化和巖石圈活化改造的過程。位于青藏高原北緣的阿爾金—祁連—柴北緣早古生代造山系是原特提斯構(gòu)造域最北部的構(gòu)造拼合體，該拼合體被認(rèn)為是原特提斯洋俯沖—增生—閉合以及碰撞作用的產(chǎn)物(Seng?r and Natal’ in, 1996; Pan Guitang et al., 2012)，其中柴達(dá)木盆地北緣(柴北緣)造山帶中發(fā)育典型的超高壓變質(zhì)帶(Zhang Guibin et al., 2008, 2009, 2014；Yu Shengyao et al., 2015a)，在折返的變質(zhì)地體中的泥質(zhì)片麻巖(楊經(jīng)綏等，2001)和榴輝巖(Zhang Jianxin et al.，2010)中均發(fā)現(xiàn)柯石英，表明同碰撞過程中大陸地殼向地幔深處俯沖的造山過程。造山作用的不同階段伴隨著巖漿作用，具有反映各種地球動(dòng)力學(xué)背景特征的地球化學(xué)特征，如俯沖相關(guān)弧巖漿、同碰撞巖漿和碰撞后巖漿。而以往對(duì)柴北緣造山帶巖漿作用的研究主要集中在俯沖相關(guān)的弧巖漿作用和碰撞后巖漿作用，分別跟蹤原特提斯洋巖石圈的長(zhǎng)期俯沖(Wu Cailai et al., 2014，2019；康珍等，2015)和大陸俯沖后深俯沖地殼的剝露(Yu Shengyao et al.，2015a，Wu Cailai et al.，2019)。然而，俯沖到碰撞轉(zhuǎn)換期巖漿作用很少受到關(guān)注，反應(yīng)在柴北緣造山帶在470～450 Ma期間的巖漿活動(dòng)報(bào)道記錄較為稀少(Wu Cailai et al., 2019)，但這些巖石提供了大洋板片俯沖末期到碰撞初期殼幔相互作用過程的關(guān)鍵信息，可以記錄從大洋俯沖到大陸俯沖的構(gòu)造轉(zhuǎn)變。因此，從原特提斯洋板片俯沖到柴達(dá)木陸塊俯沖的時(shí)間和詳細(xì)的地球動(dòng)力學(xué)過程仍然不清楚。此外，由于柴北緣造山帶中發(fā)育典型的超高壓變質(zhì)帶，因此前人研究成果主要聚焦于其中的榴輝巖及其變質(zhì)圍巖上(Chen Danling et al., 2009; Mattinson et al., 2007, 2009)，而對(duì)本區(qū)的巖漿活動(dòng)研究則較為薄弱，與區(qū)域上廣泛分布的花崗質(zhì)侵入體數(shù)量比，現(xiàn)有的年代學(xué)研究稍顯薄弱(吳才來等, 2001a, 2001b)，尤其是放射性同位素研究成果較為匱乏，模糊了對(duì)柴北緣地區(qū)巖漿作用和地殼生長(zhǎng)和改造過程的認(rèn)識(shí)；這些前人研究成果中存在局限性都限制了柴北緣造山帶古地球動(dòng)力學(xué)演化過程的重建。黃綠山奧長(zhǎng)花崗斑巖侵入體位于柴北緣造山帶西部，巖體產(chǎn)于高壓—超高壓地質(zhì)體之外，侵位于未變形的淺變質(zhì)奧陶系地層中，推測(cè)為原地的產(chǎn)物，可以為本區(qū)動(dòng)力學(xué)背景提供良好的制約；并且該巖體的地質(zhì)年代學(xué)和地球化學(xué)研究尚屬空白，因此本文對(duì)柴北緣灘間山地區(qū)的黃綠山奧長(zhǎng)花崗斑巖進(jìn)行巖相學(xué)和地質(zhì)年代學(xué)，以及全巖主微量和全巖Sr-Nd同位素的研究，準(zhǔn)確厘定了其結(jié)晶時(shí)代并探討其源區(qū)屬性，在此基礎(chǔ)上對(duì)還原其地球動(dòng)力學(xué)背景也具有一定的指示意義。

1 區(qū)域地質(zhì)背景及樣品描述

1.1 區(qū)域地質(zhì)背景

柴北緣造山帶位于青藏高原北緣，該帶沿西北—東南延伸約800 km，西部以阿爾金走滑斷層為界，東部以瓦洪山—溫泉斷層為界。北西向的魚卡—烏蘭斷裂將其分為南部和北部?jī)蓚€(gè)地質(zhì)特征明顯不同的構(gòu)造單元(圖1；陸松年等，2004)，其中北部構(gòu)造單元為歐龍布魯克微陸塊，沿烏蘭—德令哈—?dú)W龍布魯克—全吉山—達(dá)肯達(dá)坂山一線地區(qū)呈北西西向分布，由古元古代至中元古代變質(zhì)基底和新元古代至新生代沉積蓋層組成，變質(zhì)基底包括德令哈雜巖、達(dá)肯大坂巖群和萬洞溝群。南部構(gòu)造單元是一個(gè)早古生代俯沖—碰撞造山帶，即柴北緣造山帶(從都蘭北部的沙柳河—野馬灘一帶，向西北延伸至錫鐵山、綠梁山和賽什騰山一帶)由含榴輝巖的花崗質(zhì)片麻巖以及早古生代灘間山群島弧火山巖構(gòu)成(陸松年等，2004；王慧初等，2006)。區(qū)域地質(zhì)填圖表明，帶內(nèi)地層主要為奧陶系灘間山群的海相火山沉積巖。

圖1 柴北緣造山帶區(qū)域構(gòu)造地質(zhì)圖(據(jù)Wang Xiaoxia et al., 2015修改)Fig. 1 Geological map showing the regional structures of the North Qaidam Orogenic Belt (modified after Wang Xiaoxia et al., 2015)

黃綠山地區(qū)位于柴北緣造山帶的西北部(圖1)，大柴旦鎮(zhèn)北西約92.5 km處。區(qū)內(nèi)奧陶系灘間山群依據(jù)巖性組合可分為兩個(gè)巖組，下巖組為灰白、灰綠色變粒巖、綠泥片巖、千枚巖夾白云石大理巖、英安巖以及砂屑生物灰?guī)r等；上巖組為變安山質(zhì)晶屑凝灰?guī)r、變安山巖、變英安巖等，是一套類島弧火山巖建造。黃綠山奧長(zhǎng)花崗斑巖以巖株的形式侵位于古生界奧陶系灘間山群下巖組地層中。本區(qū)出露的巖漿巖主要為花崗巖類和輝長(zhǎng)巖類(圖2)。

圖2 柴達(dá)木盆地北緣黃綠山及鄰區(qū)地質(zhì)簡(jiǎn)圖(據(jù)青海省地質(zhì)礦產(chǎn)勘查局，1991修改)Fig. 2 Geological sketch map of Huanglvshan area and surrounding area, northern margin of the Qaidam Basin (modified from Bureau of Geology and Mineral Resources of Qinghai Province, 1991#)

1.2 樣品描述

本次分析的樣品均集中采自黃綠山巖體，取樣位置為：N38°15′34″、E94°31′37″。巖石樣品呈灰白色，斑狀結(jié)構(gòu)，塊狀構(gòu)造，由基質(zhì)(約65%)和斑晶(約35%)兩部分組成；斑晶主要為斜長(zhǎng)石(0.5～1.5 mm，含量約15%)，石英(0.3～1 mm，含量約15%)，正長(zhǎng)石(0.3～1 mm，含量約5%)?；|(zhì)則主要由長(zhǎng)石，石英和黑云母細(xì)小晶體組成(圖3)。樣品發(fā)育有微弱的絹云母化和高嶺土化。

2 分析方法

2.1 鋯石LA-ICP-MS年代學(xué)

本次研究所用的樣品破碎與鋯石挑選在河北省廊坊區(qū)域地質(zhì)調(diào)查研究所實(shí)驗(yàn)室完成，鋯石制靶、反射光、陰極發(fā)光圖像在中國(guó)科學(xué)院地質(zhì)與地球物理研究所完成。LA-ICP-MS鋯石U-Pb年代學(xué)測(cè)試在吉林大學(xué)東北亞礦產(chǎn)資源評(píng)價(jià)國(guó)土資源部重點(diǎn)實(shí)驗(yàn)室完成。激光剝蝕使用德國(guó)相干公司(Coherent)COMPExPro型ArF準(zhǔn)分子激光器，質(zhì)譜儀為美國(guó)安捷倫公司7500A型四極桿等離子質(zhì)譜。本次測(cè)試激光束斑直徑32 μm，激光能量密度10 J/cm2，剝蝕頻率8 Hz。通過標(biāo)準(zhǔn)鋯石91500(1062 Ma)作為外標(biāo)進(jìn)行同位素比值校正，標(biāo)準(zhǔn)鋯石PLE/GJ-1/Qing Hu為監(jiān)控盲樣。元素含量選用國(guó)際標(biāo)樣NIST610為外標(biāo)，Si為內(nèi)標(biāo)元素進(jìn)行計(jì)算，NIST612和NIST614為監(jiān)控盲樣。使用Glitter軟件進(jìn)行同位素比值及元素含量的計(jì)算。諧和年齡及圖像使用Isoplot/Ex(3.0)給出。普通鉛校正使用Anderson(2002)給出的程序方法計(jì)算。

2.2 巖石地球化學(xué)測(cè)試

樣品的主量元素和微量元素是在吉林大學(xué)測(cè)試實(shí)驗(yàn)中心測(cè)定。采用X—射線熒光光譜儀測(cè)定主量元素，相對(duì)標(biāo)準(zhǔn)偏差為2%～5%。微量元素和稀土元素分析則是采用美國(guó)安捷倫科技有限公司的Agilent 7500A型耦合等離子體質(zhì)譜儀測(cè)試(Z/T0223-2001)，樣品的測(cè)試均經(jīng)過國(guó)際標(biāo)樣BHVO-2、BCR-2和國(guó)家標(biāo)樣GBW07103、GBW07104監(jiān)控，微量元素和稀土元素的分析精度為：元素含量大于10×10-6的絕對(duì)誤差小于5%，元素含量小于10×10-6的絕對(duì)誤差小于10%。

2.3 Sr—Nd同位素測(cè)定

Sr-Nd同位素測(cè)試在核工業(yè)北京地質(zhì)研究院分析測(cè)試研究中心完成，測(cè)試方法為TIMS，測(cè)試儀器型號(hào)為ISOPROBE-T，參照GB/T16272-1999，實(shí)驗(yàn)過程中相對(duì)濕度為50%，溫度為20℃，同位素誤差以2σ計(jì)算。其中Sr同位素分析采用ISOPROBE-T熱電離質(zhì)譜計(jì)，采用單帶M+，可調(diào)多法拉第接收器接收。質(zhì)量分餾用n(86Sr)/n(88Sr)=0.1194校正，標(biāo)準(zhǔn)測(cè)量結(jié)果：NBS987為0.710250±0.000007實(shí)驗(yàn)室流程本底：Rb=0.2 ng/g，Sr=0.2 ng/g。Nd同位素分析采用ISOPROBE-T熱電離質(zhì)譜計(jì)，采用三帶M+，可調(diào)多法拉第接收器接收。質(zhì)量分餾用n(146Nd)/n(144Nd)=0.7219校正，標(biāo)準(zhǔn)測(cè)量結(jié)果：JMC為n(143Nd)/n(144Nd)=0.512109±0.000003，全流程本底Sm-Nd小于50 ng。

3 分析結(jié)果

3.1 鋯石LA-ICP-MS年代學(xué)

HLS-TC2-N2樣品來自黃綠山奧長(zhǎng)花崗斑巖。樣品中挑選的鋯石呈無色透明、金剛光澤，鋯石顆粒為近等軸粒狀，長(zhǎng)寬比介于1:1到1.5:1之間(圖4)。陰極發(fā)光(CL)圖像顯示，大多數(shù)鋯石晶體呈現(xiàn)典型的巖漿鋯石振蕩環(huán)帶(Pidgeon et al., 1998; Corfu et al., 2003)。23個(gè)分析點(diǎn)測(cè)試結(jié)果顯示Th的含量為358×10-6～1457×10-6，U的含量為557×10-6～1268×10-6，Th/U為0.39～1.15(>0.1)，具有巖漿鋯石特征(Weaver, 1991)(表1)。n(207Pb)/n(235U )—n(206Pb)/n(238U)圖解顯示，鋯石測(cè)點(diǎn)均落在諧和線之上(圖5a)，23個(gè)鋯石測(cè)點(diǎn)得出的U-Pb年齡介于461～467 Ma之間，加權(quán)平均年齡為466±3 Ma (MSWD=0.055，n=23)(圖5b)，因此，該年齡代表了奧長(zhǎng)花崗斑巖的結(jié)晶時(shí)代。

圖4 柴北緣黃綠山奧長(zhǎng)花崗斑巖鋯石形態(tài)及陰極發(fā)光CL圖像Fig. 4 Cathodoluminescence (CL) images of zircons from oligoclase granite porphyry in Qaidam

圖5 柴北緣黃綠山奧長(zhǎng)花崗斑巖鋯石U-Pb年齡諧和圖Fig. 5 Zircons U-Pb age concordia diagram of oligoclase granite porphyry in Qaidam

3.2 地球化學(xué)特征3.2.1 主量元素

巖石分析結(jié)果及特征值見表(表2)：樣品具有高SiO2(73.29%～74.37%)和高Na2O(4.81%～5.75%)，低K2O(0.67%～1.73%)，MgO(0.75%～1.27%)，TFeO(1.79%～2.33%)和Al2O3含量(11.59%～13.69%)的特點(diǎn)；全堿含量ALK=5.75%～6.77%，平均值為6.53%；樣品具有變化較大的Mg#(38.6～54.4)和A/CNK(0.84～1.24)。樣品在TAS圖解上落入花崗巖區(qū)域(圖6a)，在An—Ab—Or圖解上落入奧長(zhǎng)花崗巖區(qū)域(圖6b)，在SiO2—K2O圖解上主要落入低鉀拉斑系列(圖6c)，在A/CNK—A/NK圖解上，樣品主要呈現(xiàn)過鋁質(zhì)的特征，其中具有較高燒失量的樣品落入準(zhǔn)鋁質(zhì)，推測(cè)為后期熱液蝕變作用形成(圖6d)。

表2 柴北緣黃綠山奧長(zhǎng)花崗斑巖主量元素(%)、微量元素(×10-6)和稀土元素(×10-6)含量及相關(guān)參數(shù)Table 2 Major elements (%)，trace element (×10-6) and rare earth elements (×10-6) contents of oligoclase granite porphyry in Qaidam

圖6 巖石TAS圖解(a，據(jù)Irvine and Baragar，1971)和巖石An—Ab—Or圖解(b，據(jù)O’Connor，1965)和巖石 K2O—SiO2圖解(c，據(jù)Peccerillo and Taylor，1976)和巖石A/CNK—A/NK圖解(d，據(jù)Maniar and Piccoli，1989)Fig. 6 Plots of TAS (a, after Irvine and Baragar, 1971), plots of An—Ab—Or (b, after O’Connor, 1965), plots of K2O—SiO2 (c, after Peccerillo and Taylor, 1976) and plots of A/CNK—A/NK (d, after Maniar and Piccoli, 1989)

3.2.2 微量元素

黃綠山奧長(zhǎng)花崗斑巖樣品稀土含量較低(ΣREE介于44.01×10-6～67.27×10-6；具有明顯右傾的稀土分配特征(圖7a)，(La/Yb)N介于4.41～10.01，LREE/HREE介于6.08～9.06范圍內(nèi)；樣品Eu異常不明顯，δEu值介于0.94～1.09。

樣品在微量元素蛛網(wǎng)圖上顯示：相較于原始地幔，其具有富集大離子親石元素 (如Rb、Ba、K等)的特點(diǎn)。相對(duì)虧損部分高場(chǎng)強(qiáng)元素Nb、Ta、Ti、P等(圖7b)。

3.3 Sr-Nd同位素

全巖Sr-Nd同位素?cái)?shù)據(jù)分析結(jié)果見表3，并繪制在圖8中。初始n(87Sr)/n(86Sr)和εNd(t)值使用465 Ma的鋯石U-Pb年齡計(jì)算，全巖初始Sr比值[n(87Sr)/n(86Sr)]i=0.70212～0.70461，εNd(t)為-0.1～2.1，利用二階段模式(Liew and Hofmann，1988)計(jì)算出Nd同位素年齡(TDM2)也相對(duì)均一，范圍介于1.03～1.21 Ga。黃綠山奧長(zhǎng)花崗斑巖樣品的Sr-Nd同位素組成均勻，與同時(shí)代的團(tuán)魚山I型花崗巖 (Wu Cailai et al., 2009) 較為相似，明顯高于大柴旦的S型花崗巖(吳才來等，2007)(圖8)。

表3 柴北緣黃綠山奧長(zhǎng)花崗斑巖Sr-Nd同位素組成Table 3 Sr-Nd isotopic compositions of oligoclase granite porphyry in North Qaidam

4 討論

4.1 成因巖石學(xué)

樣品燒失量LOI在1.40到2.65之間，平均值為1.70,最大值不超過3%，暗示了熱液蝕變對(duì)樣品成分影響影響有限，并且樣品在稀土球粒隕石標(biāo)準(zhǔn)化圖解和微量元素蛛網(wǎng)圖上的曲線都近似平行(圖7a，b)，進(jìn)一步說明這些樣品可以用來討論巖石成因。

圖7 柴北緣黃綠山奧長(zhǎng)花崗斑巖稀土元素球粒隕石標(biāo)準(zhǔn)化配分圖(a，標(biāo)準(zhǔn)化值據(jù)Boynton，1984)和 奧長(zhǎng)花崗斑巖微量元素原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖(b，標(biāo)準(zhǔn)化值據(jù)Sun and Mcdonough，1989)Fig. 7 Chondrite-normalized REE patterns (a, normalizing values are from Boynton, 1984) and primitive mantle (PM)-normalized trace element spider diagrams (b, normalizing values are from Sun and McDonough, 1989) for oligoclase granite porphyry in Qaidam

花崗巖類按其源區(qū)和地球化學(xué)特征傳統(tǒng)上分為 I型、S型或A型(Chappell, 1974)。樣品具有低的10000Ga/Al(1.01～1.34，<2.6)，Zr+Nb+Ce+Y(<350×10-6)，鋯飽和溫度(TZr介于722～760 ℃，<800℃)以及銪異常，這些特征與A型花崗巖的特征不相符，因此黃綠山奧長(zhǎng)花崗斑巖不屬于A型花崗巖。黃綠山奧長(zhǎng)花崗斑巖在10000Ga/Al—Zr和10000Ga/Al—CaO/(K2O+Na2O)圖解上落入I和S型花崗巖區(qū)域(圖9a、b)，因此黃綠山奧長(zhǎng)花崗斑巖應(yīng)屬于I或S型花崗巖。磷灰石在S型花崗巖中溶解度極高，因此會(huì)隨著結(jié)晶分異的進(jìn)行，S型花崗巖P2O5會(huì)逐漸升高(Chappell, 1999; Chappell and White, 2001)，這與樣品P2O5含量(平均0.06%)極低的特點(diǎn)并不一致；此外，樣品具有相對(duì)較低的[n(87Sr)/n(86Sr)]i和正的εNd(t)，與柴北緣造山帶老基底巖石來源的大柴旦S型花崗巖相距甚遠(yuǎn)(圖8)，因此其不可能來源于古老的沉積物質(zhì)的部分熔融；因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖不屬于S型花崗巖，而屬于I型花崗巖。

圖9 10000Ga/Al—Zr圖解(a)和10000Ga/Al—CaO/(K2O+Na2O) (b)圖解Fig. 9 Diagrams of 10000Ga/Al—Zr (a) and 10000Ga/Al—CaO/(K2O+Na2O) (b)

黃綠山奧長(zhǎng)花崗斑巖中未發(fā)現(xiàn)由于基性巖漿注入形成的暗色包體，而且樣品SiO2含量高(73.80%～74.37%)且變化幅度小，而巖漿混合作用會(huì)造成不同的樣品具有顯著不同的地球化學(xué)成分，因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖不可能由巖漿混合作用形成。樣品Na2O/K2O均>1.50，表示奧長(zhǎng)花崗斑巖為較高程度的部分熔融(鄧晉福等，2015)，另部分樣品具有較高的Mg#值(>45)則可能是由于高硅熔體的局部不均勻抽取造成的(Barnes et al., 2019)。

樣品Eu異常不明顯，說明斜長(zhǎng)石結(jié)晶分異作用微弱，而斜長(zhǎng)石是低壓下巖漿結(jié)晶分異過程中典型的分異礦物，因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖不可能由幔源巖漿結(jié)晶分異作用形成。與原始地幔相比(Rb/Sr=0.0342，Nb/Ta=17.5，Th/Nb=0.177；Sun and McDonough，1989)，樣品具有高的Rb/Sr(0.12～0.30)和Th/Nb(1.25～1.38)，低的Nb/Ta(9.1～13.0)比值，與大陸地殼成分類似(Th/Nb=0. 44， Nb/Ta=11，Rb/Sr=0.35；Gao Shan et al., 2004)；結(jié)合I型花崗巖一般被認(rèn)為來自下地殼鎂鐵質(zhì)—中性火成巖源區(qū)或與它們成分相當(dāng)?shù)淖冑|(zhì)巖源區(qū)(例如斜長(zhǎng)角閃巖；Gao Peng et al., 2016)；因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖主要來源于大陸下地殼源區(qū)，樣品低的相容元素含量如Cr、Co、Ni也符合下地殼熔體特點(diǎn)。此外，樣品在[n(87Sr)/n(86Sr)]i—εNd(t)圖解上落入洋殼來源的高鍶低釔中酸性巖下方(圖8)，加之樣品具有較低的Sr/Y比值(8.0～14.4)，因此認(rèn)為黃綠山奧長(zhǎng)花崗斑巖并非來源于俯沖洋殼部分熔融所產(chǎn)生的高鍶低釔中酸性熔體；而且樣品與錫鐵山大陸型榴輝巖區(qū)域較為接近，說明巖體源區(qū)與柴北緣造山帶下地殼成分相同，結(jié)合其1.03～1.21 Ga的二階段Nd模式年齡(TDM2)，認(rèn)為黃綠山奧長(zhǎng)花崗斑巖為柴北緣造山帶中元古代下地殼部分熔融的產(chǎn)物。

4.2 構(gòu)造環(huán)境指示意義

區(qū)域上的火成巖證據(jù)表明祁連古生代和柴達(dá)木地塊之間存在一個(gè)早古生代的原特提斯洋的支洋盆(Song Shuguang et al., 2017；Wu Cailai et al., 2009)。錫鐵山地區(qū)出露的形成時(shí)代為542 Ma左右的鎂鐵質(zhì)巖漿巖具有鈣堿性和島弧型的微量元素特征(孫國(guó)超，2020)；在柴北緣造山帶東段，灘間山群內(nèi)的群王尕秀弧型輝長(zhǎng)巖的年齡為522～468 Ma (朱小輝等，2010)，在柴北緣造山帶西部，島弧火山巖、輝長(zhǎng)巖和鈣堿性I型花崗巖等深成巖體形成于514～470 Ma區(qū)間內(nèi)(袁桂邦等，2002；史仁燈等，2004；Wu Cailai et al., 2009)；烏蘭地區(qū)具有島弧特征的火成巖形成于506～494 Ma (Li Xiucai et al., 2018)；魚卡—落鳳坡地區(qū)弧后蛇綠巖雜巖，包括一套橄欖巖、火山巖、變輝長(zhǎng)巖和斜長(zhǎng)花崗巖的組合，年齡為535～493 Ma(朱小輝等，2014)。通過以上前人研究成果，說明柴北緣洋盆至少從542 Ma向相鄰陸塊開始俯沖，至少持續(xù)到470 Ma(圖10)。柴北緣超高壓變質(zhì)帶中的片麻巖和大陸型榴輝巖中含柯石英包體的鋯石的形成時(shí)代介于423～449 Ma (Song Shuguang et al., 2005; Zhang Guibin et al., 2008, 2009, 2014；Yu Shengyao et al., 2015a)。與同期446 Ma同碰撞過鋁質(zhì)花崗巖一起標(biāo)志著大陸俯沖作用的進(jìn)行(Wu Cailai et al., 2002)；即研究區(qū)至少在446 Ma之后即處于同碰撞構(gòu)造體制當(dāng)中。由于柴北緣巖漿活動(dòng)在470～450 Ma時(shí)代范圍內(nèi)具有較為沉寂的特點(diǎn)(Wu Cailai et al. 2019; 圖10)，并且無變質(zhì)作用的記錄；因此研究區(qū)缺少俯沖晚期(洋盆關(guān)閉之前)巖漿巖活動(dòng)的制約，即洋盆俯沖結(jié)束或地體開始碰撞的時(shí)間尚不明確(圖10)。

圖10 柴北緣地區(qū)巖漿巖結(jié)晶時(shí)代統(tǒng)計(jì)直方圖(巖漿巖時(shí)代數(shù)據(jù)引自史仁燈，2004；盧新祥等，2007；孟繁聰?shù)龋?005；吳才來等，2007；Wu Cailai et al.，2002，2004，2009，2014，2019；Zhao Zhixin et al.，2017；Yu Shengyao et al.，2012；Song Shuguang et al.，2014；吳鎖平，2008；趙志新，2018；角閃巖相變質(zhì)時(shí)代據(jù)Chen Xin et al.，2020；榴輝巖變質(zhì)時(shí)代據(jù)Mattinson et al.，2006；Zhang Guibin et al.，2008，2014和Yu Shengyao et al.，2015a)Fig. 10 Statistic histogram of magmatic rock crystallization ages in the Northern Margin of Qaidam (Magmatic age data from Shi Rendeng et al., 2004& ; Wu Cailai et al., 2002, 2004, 2007&, 2009, 2014, 2019；Lu Xinxiang et al. , 2007&；Meng Fancong et al., 2005&；Zhao Zhixin et al., 2017；Yu Shengyao et al., 2012；Song Shuguang et al., 2014；Wu Suoping, 2008&；Zhao Zhixin, 2018&；Amphibolite facies metamorphism age data from Chen Xin et al., 2020；Eclogite metamorphic age data from Mattinson et al., 2006； Zhang Guibin et al., 2008, 2014 and Yu Shengyao et al ., 2015a)

樣品在花崗巖構(gòu)造環(huán)境判別圖解中(圖11a～d)，黃綠山奧長(zhǎng)花崗斑巖樣品都落入火山弧型花崗巖VAG范圍內(nèi)(Pearce et al., 1984)，說明黃綠山花崗巖的形成與洋殼過程俯沖作用密切相關(guān)。此外，黃綠山花崗斑巖屬于低鉀拉斑系列，巖石類型屬于奧長(zhǎng)花崗斑巖(圖6b)；樣品Sr<300×10-6和Yb<2×10-6，屬于低鍶低釔型花崗巖，顯示源區(qū)相對(duì)較淺，巖石低鉀低鈣也指示巖漿源區(qū)較淺(張旗等，2010)。這種巖石類型多與伸展構(gòu)造背景相關(guān)(張旗等，2010)，暗示了黃綠山奧長(zhǎng)花崗斑巖形成于伸展環(huán)境中。而侵入體圍巖為奧陶系灘間山群，其產(chǎn)于島弧或者弧后盆地環(huán)境中(吳冠斌等, 2010)，考慮到黃綠山侵入體侵位年齡(466 Ma)與灘間山群形成時(shí)代接近，因此認(rèn)為黃綠山形成于弧后盆地的伸展構(gòu)造背景中。

圖11 柴北緣黃綠山奧長(zhǎng)花崗斑巖構(gòu)造環(huán)境判別圖解(據(jù)Pearce，1996)Fig. 11 Discrimination diagrams of tectonic environment for oligoclase granite porphyry in North Qaidam(after Pearce, 1996) VAG—火山弧花崗巖；ORG—洋脊花崗巖；WPG—板內(nèi)花崗巖；syn-COLG—同碰撞花崗巖；post-COLG—后碰撞花崗巖 VAG—volcanic arc granites; ORG—ocean ridge granites; WPG—within plate granites; syn-COLG—syn-collision granites; post-COLG— post-collision granites

綜上，弧后盆地的存在暗示了洋殼俯沖仍未結(jié)束，地體碰撞尚未開始。因此黃綠山奧長(zhǎng)花崗斑巖記錄了柴北緣洋盆俯沖晚期的巖漿作用，標(biāo)志著柴北緣洋俯沖作用至少持續(xù)到466 Ma。

5 結(jié)論

(1)獲得柴北緣造山帶黃綠山奧長(zhǎng)花崗斑巖結(jié)晶時(shí)代為465.7±3.4 Ma，屬于中奧陶世的產(chǎn)物。

(2)認(rèn)為柴北緣造山帶黃綠山奧長(zhǎng)花崗斑巖屬于I型花崗巖，為中元古代下地殼物質(zhì)部分熔融的產(chǎn)物。

(3) 柴北緣造山帶黃綠山花崗閃長(zhǎng)巖為弧后盆地的伸展環(huán)境下產(chǎn)物，指示柴北緣洋盆俯沖作用至少持續(xù)到466 Ma。

致謝：感謝河北省廊坊區(qū)域地質(zhì)調(diào)查研究所實(shí)驗(yàn)室、吉林大學(xué)東北亞礦產(chǎn)資源評(píng)價(jià)國(guó)土資源部重點(diǎn)實(shí)驗(yàn)室以及核工業(yè)北京地質(zhì)研究院分析測(cè)試研究中心提供的測(cè)試幫助；感謝審稿專家、編輯對(duì)本文提出的寶貴意見和悉心指導(dǎo)。。

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