吳才來(lái), 雷 敏, 吳 迪, 李天嘯

1)中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所, 北京 100037; 2)中國(guó)地質(zhì)大學(xué)(北京), 北京 100083

柴北緣烏蘭地區(qū)花崗巖鋯石SHRIMP定年及其成因

吳才來(lái)1), 雷 敏1), 吳 迪2), 李天嘯2)

1)中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所, 北京 100037; 2)中國(guó)地質(zhì)大學(xué)(北京), 北京 100083

柴北緣烏蘭地區(qū)花崗巖鋯石SHRIMP U-Pb定年結(jié)果表明, 哈德森溝巖體的年齡為(413±3) Ma, 許給溝巖體的年齡為(254±3) Ma, 椅落山巖體的年齡為(251±1) Ma, 察汗諾巖體角閃閃長(zhǎng)巖和花崗巖的年齡分別為(249±1) Ma和(248±2) Ma, 察汗河巖體年齡為(240±2) Ma, 曬勒克郭來(lái)巖體的花崗閃長(zhǎng)巖和花崗巖年齡分別為(250±1) Ma和(244±3) Ma。從年齡上看, 這些花崗巖明顯地分為兩期: 早期屬早泥盆世(年齡為413 Ma), 形成的巖石組合為: 石英二長(zhǎng)巖+堿長(zhǎng)花崗巖; 晚期屬晚二疊世—早三疊世(年齡為254～240 Ma),又可進(jìn)一步細(xì)分為254～251 Ma、250～248 Ma、244～240 Ma三次侵位, 對(duì)應(yīng)的巖石組合為: 閃長(zhǎng)巖+花崗閃長(zhǎng)巖+花崗巖。巖石地球化學(xué)研究表明, 早期花崗巖類(lèi)不僅富集大離子親石元素, 而且還富集部分高場(chǎng)強(qiáng)元素(Zr、Y、Nb等), 屬A型花崗巖; 晚期花崗巖類(lèi)富集大離子親石元素, 虧損高場(chǎng)強(qiáng)元素, 屬I(mǎi)型花崗巖。早期花崗巖的87Sr/86Sr比值(0.710 8)和Nd模式年齡(T2DM=2.10 Ga)均高于晚期花崗巖(0.707 6～0.710 7, T2DM=1.41～1.58 Ga), 但晚期花崗巖的εNd(t)值(?11.6)低于早期花崗巖(?4.8 ～ ?6.8), 表明早期A型花崗巖可能起源于古元古代的大陸地殼, 而晚期I型花崗巖起源于中元古代地殼。結(jié)合區(qū)域地質(zhì)構(gòu)造特征, 我們認(rèn)為,早期A型花崗巖的形成與祁連巖石圈拆沉導(dǎo)致歐龍布魯克陸塊北緣減薄、拉伸有關(guān), 也標(biāo)志著宗霧隆裂陷的開(kāi)始; 而晚期I型花崗巖類(lèi)的形成與宗霧隆洋殼向南俯沖于歐龍布魯克陸塊之下有關(guān)。

花崗巖; 鋯石SHRIMP定年; 歐龍布魯克陸塊; 宗霧隆構(gòu)造帶; 烏蘭

柴北緣自1998年發(fā)現(xiàn)榴輝巖以來(lái), 一直是地學(xué)界研究的熱點(diǎn)地區(qū)之一(Yang et al., 1998; 楊經(jīng)綏等, 2000), 許多學(xué)者對(duì)區(qū)內(nèi)的超高壓變質(zhì)作用開(kāi)展了深入的研究, 取得了許多重要成果(宋述光和楊經(jīng)綏, 2001; Song et al., 2003a, b, 2005, 2006, 2009, 2011, 2014a, b; Yang et al., 2005; Zhang et al., 2005, 2008, 2009, 2010, 2014; Mattinson et al., 2006; 宋述光等, 2007, 2009, 2013; Chen et al., 2009; Yu et al., 2012, 2014)。在該區(qū)的榴輝巖及其圍巖片麻巖中發(fā)現(xiàn)了許多超高壓變質(zhì)礦物如柯石英、金剛石和超高壓微結(jié)構(gòu)(Liu et al., 1996, 2002; 楊經(jīng)綏等, 2001; 張建新等, 2002; Song et al., 2005), 因此, 該區(qū)成為中國(guó)境內(nèi)繼蘇魯—大別之后的又一條超高壓變質(zhì)帶(楊經(jīng)綏等, 2000; 陳丹玲等, 2005)。柴北緣地區(qū)呈北西向窄長(zhǎng)帶狀從阿爾金山向東沿伸到鄂拉山, 約800 km(圖1),其南北分別被柴達(dá)木北緣斷裂和中祁連南緣斷裂(宗務(wù)隆—青海南山斷裂)所切, 北西端被阿爾金左行走滑斷裂所切, 東南端被哇洪山斷裂所切(陸松年等, 2002, 2004; 林慈鑾等, 2006)。柴北緣地區(qū)以NW向的魚(yú)卡—烏蘭斷裂為界分為南北兩個(gè)構(gòu)造單元,南部構(gòu)造單元為超高壓變質(zhì)帶, 北部構(gòu)造單元是歐龍布魯克陸塊(青海省地質(zhì)礦產(chǎn)局, 1991)。近些年,對(duì)南部構(gòu)造單元上的花崗巖做了較多的研究工作,并取得了許多成果, 特別是花崗巖的年代學(xué)研究方面取得了長(zhǎng)足進(jìn)展(吳才來(lái)等, 2001, 2004, 2007, 2008, 2010, 2014; 袁桂邦等, 2002; Xiao et al., 2004;孟繁聰?shù)? 2005; 盧欣祥等, 2007; Yu et al., 2012; Song et al., 2014a)。然而, 柴北緣北部構(gòu)造單元上的花崗巖定年及研究工作仍不多見(jiàn)。尤其是和都蘭超高壓地體相鄰的烏蘭地區(qū), 出露眾多的花崗巖侵入體, 這些花崗巖體的時(shí)代及成因以及與都蘭地區(qū)乃至與柴北緣南部構(gòu)造單元上花崗巖有何成因聯(lián)系、所反映的構(gòu)造信息等問(wèn)題仍不清楚。因此, 我們完成了烏蘭地區(qū)花崗巖類(lèi)的詳細(xì)地球化學(xué)和年代學(xué)研究, 試圖對(duì)上述問(wèn)題作一探討。

1 地質(zhì)背景及巖體地質(zhì)特征

柴北緣前寒武紀(jì)基底由一套中-高級(jí)片麻巖、角閃巖和片巖組成, 其上被早古生代奧陶紀(jì)火山巖和砂巖、灰?guī)r覆蓋, 這是一套大陸活動(dòng)邊緣的海相沉積。晚古生代地層由晚泥盆世陸相碎屑巖、火山巖和石炭紀(jì)淺海相沉積巖組成。侏羅紀(jì)和白堊紀(jì)含煤沉積地層覆蓋在晚古生代地層之上(青海省地質(zhì)礦產(chǎn)局, 1991)。柴北緣北部構(gòu)造單元沿烏蘭—德令哈—?dú)W龍布魯克—全吉山—達(dá)肯大坂山一線分布, 由德令哈雜巖、達(dá)肯大坂群和新元古代巖群組成結(jié)晶基底, 其上被寒武—奧陶紀(jì)穩(wěn)定的海相沉積巖所覆蓋(陸松年等, 2002, 2004; 辛后田等, 2002; 王惠初等, 2006); 南部構(gòu)造單元沿都蘭北部的沙柳河—野馬灘—錫鐵山—綠梁山—魚(yú)卡—賽什騰山一線分布,是一條早古生代俯沖-碰撞雜巖帶, 由含榴輝巖的花崗質(zhì)片麻巖和早古生代灘間山群島弧火山巖組成(陸松年等, 2002, 2004; 辛后田等, 2002; 王惠初等, 2006), 其上被泥盆紀(jì)磨拉石沉積巖不整合覆蓋。柴北緣北構(gòu)造單元?dú)W龍布魯克陸塊與中南祁連陸塊之間是土爾根大坂—宗霧隆—青海南山構(gòu)造帶(簡(jiǎn)稱宗霧隆構(gòu)造帶)(彭淵等, 2016)(圖1)。宗務(wù)隆構(gòu)造裂谷帶由石炭紀(jì)宗務(wù)隆群和早—中三疊世郡子河群組成, 兩者呈斷層接觸。宗務(wù)隆群包括土爾根大坂組和果可山組, 是一套半深海相復(fù)理石沉積和中酸性-基性火山巖系。郡子河群屬于淺海相碎屑巖和碳酸鹽巖沉積建造(青海省地質(zhì)礦產(chǎn)局, 1991)。宗務(wù)隆構(gòu)造帶的沉積建造向東可延至青海橡皮山一線, 被認(rèn)為屬于華北陸塊南緣斜坡相沉積, 并與秦嶺—昆侖海盆相鄰(郭安林等, 2007, 2009)。

本文研究的花崗巖體主要分布在柴北緣北部構(gòu)造單元?dú)W龍布魯克微陸塊東部烏蘭地區(qū)(圖1), 與宗霧隆構(gòu)造帶相鄰。從東到西分別為許給溝巖體、椅落山巖體、察汗諾巖體、察汗河巖體、哈德森溝巖體和曬勒克郭來(lái)巖體(圖2)。這些巖體長(zhǎng)軸方向?yàn)镹W向, 部分巖體由若干個(gè)NW向延伸的巖體復(fù)合而成, 形成不規(guī)則狀近EW向的巖基, 如椅落山巖體和曬勒克郭來(lái)巖體。巖體的巖石類(lèi)型主要為角閃閃長(zhǎng)巖、石英閃長(zhǎng)巖、石英二長(zhǎng)巖、花崗閃長(zhǎng)巖和花崗巖。巖體的圍巖主要為晚古生代(C-T2)的灰色片巖、片麻巖、灰白色大理巖、變粒巖及綠色中基性火山巖, 其次有少量的早古生代(∈-O)凝灰?guī)r、安山巖夾片巖、薄層大理巖、白云巖、石英巖、陽(yáng)起透輝石巖等。

圖2 烏蘭地區(qū)花崗巖分布示意地質(zhì)圖(據(jù)1/200 000烏蘭幅和天峻幅地質(zhì)圖修編)Fig. 2 Geological sketch map showing distribution of granites in Wulan area (modified after 1/200 000 Geological Map of Wulan Sheet and Tianjun Sheet)

各巖體主要地質(zhì)特征見(jiàn)表1, 主要巖石類(lèi)型的巖石學(xué)特征見(jiàn)表2。

表1 各巖體地質(zhì)特征Table 1 Geological characteristics of rock mass

表2 巖石學(xué)特征Table 2 Petrologic characteristics of rock mass

圖3 鋯石CL圖像Fig. 3 Cathodoluminescence (CL) images of zircons of the granites from Wulan area

2 鋯石SHRIMP定年

8個(gè)樣品鋯石SHRIMP U-Pb定年數(shù)據(jù)列于表3, 定年方法見(jiàn)吳才來(lái)等(2014)。

樣品CL05-82取自椅落山巖體花崗閃長(zhǎng)巖。該樣品的鋯石為柱狀, 長(zhǎng)寬比為1.5∶1～2∶1。CL圖像顯示, 大多數(shù)鋯石具有明顯的振蕩環(huán)帶, 反映了巖漿結(jié)晶鋯石的特點(diǎn)(Pidgeon et al., 1998; Corfu et al., 2003; Hoskin and Schaltegger, 2003)。少量的鋯石具有老的繼承性核, 如鋯石6、7號(hào)測(cè)點(diǎn)(圖3)。測(cè)定的9顆鋯石14個(gè)測(cè)點(diǎn)得出, U的含量變化于172×10-6到1 190×10-6之間, Th的含量變化于74×10-6到820×10-6之間, Th/U比值除老的繼承性鋯石核外,其余的均大于0.3(0.33～0.71)(表3)。該樣品除老的繼承性鋯石核(6、7號(hào)測(cè)點(diǎn))的年齡為(430.2±3.4) Ma、(405.9±3.2) Ma外, 其余具有環(huán)帶結(jié)構(gòu)的鋯石U-Pb年齡變化于(244.4±1.7) Ma到(267.6±2.9) Ma之間,計(jì)算平均年齡為(251.3±1.5) Ma(圖4), 代表鋯石的結(jié)晶年齡(圖4)。

續(xù)表3

圖4 柴北緣東段花崗巖鋯石207Pb/206Pb-238U/206Pb諧和圖及平均年齡Fig. 4207Pb/206Pb-238U/206Pb concordia diagram and average age of the granites from Wulan area

樣品CL05-84取自察汗諾巖體的角閃閃長(zhǎng)巖,樣品中的鋯石大小不一, 但都呈柱狀, 長(zhǎng)寬比為2:1～3:1。CL圖像顯示, 鋯石內(nèi)部具有寬條帶狀、扇狀結(jié)構(gòu), 部分鋯石外圍具有環(huán)帶結(jié)構(gòu)(圖3)。該樣品12顆鋯石測(cè)得的U、Th含量分別為176×10-6～520×10-6和160×10-6～960×10-6, Th/U比值變化于0.87到1.92之間(表3)。12個(gè)分析點(diǎn)得出U-Pb年齡變化于(244.8±2.9) Ma到(251.7±2.2) Ma之間, 得出平均年齡為(249.1±1.3) Ma, 與207Pb/206Pb-238U/206Pb諧和圖產(chǎn)生的交點(diǎn)年齡為(248.1±2.4) Ma, 在誤差范圍內(nèi)與平均年齡一致(圖4)。

樣品CL05-85取自察汗諾巖體的花崗巖, 鋯石呈柱狀, 長(zhǎng)寬比為1.5∶1～2∶1。CL圖像顏色較深, 鋯石具有環(huán)帶結(jié)構(gòu)和板狀結(jié)構(gòu), 部分含有大小不同的礦物包裹體(圖3)。鋯石的U、Th含量較高, 變化較大, 分別為309×10-6～1 907×10-6和169×10-6～3 257×10-6, Th/U比值通常大于0.5, 變化于0.52～1.76之間(表3)。12顆鋯石分析得出U-Pb年齡變化于(236.4±3.3)～(425.9±3.1) Ma, 其中12號(hào)鋯石測(cè)點(diǎn)為老的繼承性核(年齡為(425.9±3.1) Ma), 其余鋯石測(cè)點(diǎn)得出平均年齡為(247.9±2.1) Ma, 解釋為鋯石的結(jié)晶年齡(圖4)。

圖5 鋯石CL圖像Fig. 5 Cathodoluminescence (CL) images of zircons of the granites from Wulan area

圖6 柴北緣東段烏蘭花崗巖鋯石207Pb/206Pb-238U/206Pb諧和圖及平均年齡Fig. 6207Pb/206Pb-238U/206Pb concordia diagram and average age of the granites from Wulan area

表4 柴北緣東段烏蘭花崗巖類(lèi)化學(xué)成分Table 4 Chemical composition of the granites from Wulan area, eastern segment of North Qaidam

續(xù)表4

圖7 SiO2-(Na2O+K2O)圖Fig. 7 Diagram of SiO2-(Na2O+K2O) for the granites from Wulan area

樣品CL05-88取自曬勒克郭來(lái)巖體花崗閃長(zhǎng)巖,樣品中的鋯石呈柱狀, 長(zhǎng)寬比為1∶1～2∶1。CL圖像顯示出明顯的振蕩環(huán)帶(圖3)。鋯石的U、Th含量分別為231×10-6～554×10-6、120×10-6～438×10-6, Th/U比值大于0.5(變化于0.52～0.95之間)。14顆鋯石測(cè)年產(chǎn)生206Pb/238U年齡變化于(245.0±2.1) Ma到(254.4±1.9) Ma之間, 平均年齡為(249.9±1.4) Ma(圖4)。207Pb/206Pb-238U/206Pb諧和圖得出交點(diǎn)年齡為(250.2±1.3) Ma, 與平均年齡在誤差范圍內(nèi)一致(圖4)。

樣品CL05-90取自哈德森溝花崗巖, 鋯石為柱狀, 長(zhǎng)寬比為1∶1到2∶1之間。CL圖像顏色較深, 少量的鋯石可見(jiàn)同心環(huán)帶, 多數(shù)鋯石含有不規(guī)則的黑色團(tuán)塊, 可能反映了后期流體沿裂隙的改造(圖 5)(Cherniak and Watson, 2000)。16顆鋯石分析結(jié)果表明, U、Th的含量分別為546×10-6～4 060×10-6、195×10-6～2 329×10-6, Th/U比值變化于0.32到1.89之間,分析得出平均年齡為(412.6±3.2) Ma(圖6),207Pb/206Pb-238U/206Pb諧和圖得出交點(diǎn)年齡為(412.4±3.5) Ma(圖6), 兩者非常一致, 代表巖體結(jié)晶年齡。

樣品CL05-92取自察汗河花崗閃長(zhǎng)巖, 鋯石呈自形的短柱狀, 長(zhǎng)寬比為1∶1～1.5∶1。CL圖像顯示出較好的振蕩環(huán)帶(圖5)。U、Th含量分別為131×10-6～644×10-6、108×10-6～530×10-6, Th/U比值大于0.5(變化于0.53到0.91之間)(表3)。除11號(hào)鋯石為老的捕獲鋯石(年齡為(2 474.4±26.8) Ma)外,其余12顆鋯石測(cè)年得出較為一致的結(jié)果, U-Pb年齡為(234.9±1.9)～(247.0±1.7) Ma, 平均為(240.3±2.3) Ma (圖6),207Pb/206Pb-238U/206Pb諧和圖年齡為(242.5±3.2) Ma(圖6), 在誤差范圍內(nèi)與平均年齡一致, 代表巖體結(jié)晶年齡。

樣品CL05-93取自曬勒克郭來(lái)巖體的花崗巖,樣品中鋯石為柱狀, 鋯石長(zhǎng)寬比為1.5∶1～2∶1。盡管鋯石的CL圖像顏色較深, 但部分鋯石仍具有明顯的振蕩環(huán)帶, 部分鋯石核部結(jié)構(gòu)很復(fù)雜, 顯示出受到流體的改造(圖5)。測(cè)定結(jié)果表明, 鋯石的U、Th含量變化較大, 分別為184×10-6～5 733×10-6和102×10-6～16 707×10-6, Th/U比值變化于0.49和3.01之間(表3)。17顆鋯石測(cè)年得出206Pb/238U年齡變化于(209.6±1.1) Ma到(249.8±0.7) Ma之間, 除去受流體改造的鋯石, 得出平均年齡為(243.9±2.9) Ma(圖6),207Pb/206Pb-238U/206Pb諧和圖得出交點(diǎn)年齡為(236.2±7.8) Ma(圖6), 與平均年齡在誤差范圍內(nèi)基本一致, 可解釋為巖體結(jié)晶的年齡(圖6)。

圖8 SiO2-Fe＊和MALI圖(據(jù)Frost et al., 2001; Frost and Frost, 2008; 圖例同圖7 )Fig. 8 Diagrams of SiO2-Fe＊ and SiO2-MALI for the granites from Wulan area (after Frost et al., 2001; Frost and Frost, 2008; symbols as for Fig. 7) Fe*=FeOT/(FeOT+MgO); MALI=Na2O+K2O-CaO

樣品CL9956取自許給溝花崗巖, 該樣品的鋯石呈長(zhǎng)柱狀, 長(zhǎng)寬比在2∶1到3∶1之間。CL圖像表明, 大多數(shù)鋯石具有明顯的振蕩環(huán)帶, 少數(shù)鋯石具有板狀結(jié)構(gòu)(圖5), 這也是典型的巖漿鋯石(Pidgeon et al., 1998)。測(cè)試分析表明, 鋯石的U、Th含量分別為613×10-6～1 030×10-6、305×10-6～811×10-6, Th/U比值為0.44～0.94(表3)。該樣品9顆鋯石測(cè)年得出的年齡變化于(244.0±6.5)～(270.0±12.0) Ma, 計(jì)算平均年齡為(254.2±3.5) Ma, 代表鋯石的結(jié)晶年齡(圖6)。

圖9 A/CNK-A/NK圖(據(jù)Maniar and Piccoli, 1989; 圖例同圖7 )Fig. 9 Diagram of A/CNK-A/NK for the granites from Wulan area(after Maniar and Piccoli, 1989; symbols as for Fig. 7)

圖10 哈克圖解(圖10 g據(jù)Peccerillo and Taylor, 1976; 圖例同圖7 )Fig. 10 Harker diagrams for the granites from Wulan (Fig. 10g after Peccerillo and Taylor, 1976; symbols as for Fig. 7)

根據(jù)上面鋯石SHRIMP U-Pb定年結(jié)果, 烏蘭地區(qū)花崗巖類(lèi)除哈德森溝巖體為(412.6±3.2) Ma(屬早泥盆世)外, 其余各巖體的年齡變化于254～240 Ma之間, 屬晚二疊—中三疊世。早期巖石組合: 石英二長(zhǎng)巖+花崗巖; 晚期花崗巖進(jìn)一步可劃分出三個(gè)侵入次序, 即(1)晚二疊(254～251 Ma), 主要為椅落山巖體和許給溝巖體; (2)早三疊(250～248 Ma), 主要為察汗諾巖體和曬勒克郭來(lái)西巖體; (3)中三疊(244～240 Ma), 主要為察汗河巖體和曬勒克郭來(lái)東巖體。巖石組合分別為: (1)石英閃長(zhǎng)巖+花崗閃長(zhǎng)巖+花崗巖; (2)角閃閃長(zhǎng)巖+花崗閃長(zhǎng)巖+花崗巖; (3)花崗閃長(zhǎng)巖+花崗巖。

3 巖石地球化學(xué)

16個(gè)樣品分析了主量和微量元素, 定年樣品分析了Sr、Nd同位素, 數(shù)據(jù)分別列于表4、5。全巖及同位素分析方法見(jiàn)吳才來(lái)等(2014)。

3.1 主量和微量元素

早期花崗巖類(lèi)石英二長(zhǎng)巖、花崗巖具有60.72～73.37 wt％的SiO2, 13.23～18.07 wt％ Al2O3, 2.08～3.07 wt％ TFeO, 0.37～0.43 wt％ MgO, 0.52～2.2 wt％ CaO, 3.67～6.58 wt％ Na2O, 5.11～5.97 wt％ K2O; 晚期花崗巖類(lèi)除樣品CL05-82-2(角閃閃長(zhǎng)巖)外, 其余的具有64.97～77.69 (wt％)的SiO2, 12.30～16.40 wt％ Al2O3, 0.82～4.61 wt％ TFeO, 0.15～2.24 wt％ MgO, 0.56～5.04 wt％ CaO, 2.98～3.65 wt％ Na2O, 1.62～4.79 wt％ K2O。在硅堿圖上,早期花崗巖類(lèi)樣品分別落入正長(zhǎng)巖區(qū)和花崗巖區(qū),晚期的石英閃長(zhǎng)巖和花崗閃長(zhǎng)巖主要投入花崗閃長(zhǎng)巖區(qū), 二長(zhǎng)花崗巖和正長(zhǎng)花崗巖主要投入花崗巖區(qū),而樣品CL05-82-2落入輝長(zhǎng)巖區(qū)(圖7)(Irvine and Baragar, 1971; Middlemost, 1994)。由圖7可見(jiàn), 烏蘭地區(qū)侵入巖類(lèi)主要為亞堿性系列巖石, 早期花崗巖類(lèi)的全堿含量明顯高于晚期巖石(圖7), 且主要為堿鈣性-堿性的Fe質(zhì)類(lèi)型(圖8)。晚期巖石主要為鈣性-鈣堿性的Mg質(zhì)類(lèi)型, 少量的為Fe質(zhì)類(lèi)型(圖8)(Frost et al., 2001; Frost and Frost, 2008), 且隨著SiO2含量的增加, 巖石由準(zhǔn)鋁質(zhì)到弱過(guò)鋁質(zhì)(圖9)(Maniar and Piccoli, 1989)(表4), 其中, 晚期第一、二次侵位的花崗巖類(lèi)為準(zhǔn)鋁質(zhì), 第三次侵位的巖石為弱過(guò)鋁質(zhì)(圖9)。同時(shí), 早、晚兩期花崗巖類(lèi)隨硅的增加, K2O由中鉀(鈣堿性)到高鉀(高鉀鈣堿性)地增加, TiO2、Al2O3、TFeO (FeO+Fe2O3)、MgO、CaO和P2O5表現(xiàn)出規(guī)律性地減少, 但Na2O變化規(guī)律不同(圖10)。早期的Na2O變小, 晚期第一次花崗巖類(lèi)幾乎不變, 其余各次巖石的Na2O增加。

圖11 花崗巖類(lèi)微量元素蛛網(wǎng)圖(原始地幔值據(jù)McDonough and Sun, 1985; 樣品號(hào)同表1)Fig. 11 Primitive mantle-normalized trace-element spider diagrams(normalized values after McDonough et al., 1985; sample numbers as for Table 1)

早期的花崗巖類(lèi)不僅富集大離子親石元素(LILE)如K、Rb、La和Th, 而且也富集高場(chǎng)強(qiáng)元素(HFS)如Sc、Y、Zr、Hf和Nb(表4), 晚期花崗巖類(lèi)則富集大離子親石元素, 相對(duì)虧損高場(chǎng)強(qiáng)元素(表 4)。在微量元素蛛網(wǎng)圖上, 早期花崗巖類(lèi)具有非常明顯的Ba、Nb、Sr、P和Ti負(fù)異常(圖11); 晚期花崗巖類(lèi)具有相似的微量元素原始地幔標(biāo)準(zhǔn)化模型,即Nb、P、Ti具有負(fù)異常(圖11), 而Sr具有弱的正異常到弱的負(fù)異常, 與早期花崗巖具有明顯的Sr負(fù)異常不同(圖11)(McDonough and Sun, 1985)。

圖13 花崗巖類(lèi)稀土元素球粒隕石標(biāo)準(zhǔn)化圖(稀土球粒隕石值據(jù)Boynton, 1984; 樣品號(hào)同表3)Fig. 13 Chondrite-normalized REE patterns for the granites (normalized values after Boynton, 1984; sample numbers as for Table 3)

3.2 稀土元素

花崗巖類(lèi)稀土總量變化于73.35×10-6到740.89×10-6之間(表4), 早期花崗巖類(lèi)具有最高的稀土總量(480.57×10-6～740.89×10-6), 且隨著SiO2含量的增加, 稀土總量降低(表4); 晚期各次花崗巖類(lèi)稀土總量變化范圍分別為: 73.35×10-6～130.94×10-6、87.6×10-6～214.59×10-6、82.01×10-6～203.54×10-6(表4), 第一、二次花崗巖類(lèi)稀土總量隨SiO2增加而升高, 但第三次花崗巖類(lèi)降低(圖12a, 表4)。兩期花崗巖類(lèi)的稀土總量與Zr的含量成正相關(guān), 表明鋯石可能是稀土元素的主要載體礦物(圖12b, 表4)。所有樣品均富集輕稀土元素, LREE/HREE比值變化于5.25～25.27之間(表4)。相對(duì)而言, 晚期各次花崗巖類(lèi)各樣品的輕重稀土比值變化較大, 分別為5.25～18.02、6.34～25.27、6.14～14.55, 早期花崗巖類(lèi)為10.00～12.03。球粒隕石標(biāo)準(zhǔn)化模型表明, 早期花崗巖類(lèi)具有明顯的負(fù)Eu異常(δEu=0.11～0.15), 晚期花崗巖類(lèi)大多數(shù)不具有Eu負(fù)異常和少數(shù)具有弱的負(fù)Eu異常, δEu分別為0.63～1.15、0.35～0.95、0.30～0.80。各期次花崗巖類(lèi)樣品具有相似的稀土配分曲線, 且基本平行(Boynton, 1984)(圖13)。相比較而言, 各期次巖石樣品的輕稀土元素分異明顯, 重稀土元素分異不明顯,即早期巖石的(La/Sm)N為4.68～5.61, (Gd/Yb)N為1.76～2.62; 晚期各次巖石的(La/Sm)N為2.48～7.21、3.50～9.06、3.75～8.17, (Gd/Yb)N為1.19～2.08、1.19～2.65、0.98～2.01。

3.3 Sr、Nd同位素

選擇部分定年樣品做全巖Sr、Nd同位素分析,結(jié)果見(jiàn)表5。

圖14 柴北緣烏蘭花崗巖類(lèi)(87Sr/86Sr)i-εNd(t)圖解Fig. 14 Isotopic (87Sr/86Sr)i-εNd(t) diagram of the granites from Wulan area

表5 柴北緣東段烏蘭花崗巖類(lèi)Sr-Nd同位素分析Table 5 Sr-Nd isotopic analyses of the granites from Wulan area in eastern section of Northern Qaidam

圖15 花崗巖成因類(lèi)型判別圖解(據(jù)Whalen et al., 1987; 圖例同圖7 )Fig. 15 Discrimination of granite genetic-types (after Whalen et al., 1987; symbols as for Fig. 7)A-A型花崗巖; I, S & M-I型、S型和M型花崗巖; FG-分異的I型花崗巖; OGT-世界I型、S型和M型花崗巖A-A-type granite; I, S & M-I-type, S type and M type granite; FG-fractionated I-type granite; OGT-world I-type, S-type and M-type granite

由表5可見(jiàn), 烏蘭早期花崗巖類(lèi)比晚期花崗巖類(lèi)具有較高的(87Sr/86Sr)i、較大的T2DM和較低的εNd(t)值, 早期花崗巖的(87Sr/86Sr)i、T2DM、εNd(t)值分別為0.710 99、2.10 Ga、–11.6, 晚期花崗巖的分別為: 0.707 567～0.710 691、1.41～1.58 Ga、–4.8 ～ –6.8(表5)。圖14中, 早期花崗巖落入澳大利亞拉克蘭(Lachlan)I型和S型花崗巖區(qū)域之下(Keay et al., 1997; Serhat and Goncuoglu, 2007), 但晚期花崗巖類(lèi)樣品落入I型花崗巖區(qū)(圖14)。

4 討論

4.1 花崗巖成因類(lèi)型

研究表明, 烏蘭早期花崗巖類(lèi)(413 Ma)巖石組合為石英二長(zhǎng)巖+花崗巖, 這些巖石不僅富集大離子親石元素, 而且還富集部分高場(chǎng)強(qiáng)元素(Zr、Y、Nb等), 稀土元素配分曲線以明顯的負(fù)Eu異常為特征(圖13a), 具有A型花崗巖的地球化學(xué)特征(圖15a, b, c, d)。按張旗等(2010)的劃分方案, 這組花崗巖類(lèi)似于華南A型花崗巖, 以低Sr高Yb為特征(圖16)。同時(shí), 本期花崗巖類(lèi)具有較高的10 000×Ga/Al (＞2.6, 2.7～3.6, 平均為3.15)、Zr(277×10-6～380×10-6)和Zr+Nb+Ce+Y(＞350×10-6, 586×10-6～815×10-6, 平均為700.5×10-6)值和較低的MgO(0.37～0.43 wt％)、Ba(151×10-6～250×10-6)和Sr(59.8×10-6～63.0×10-6)的含量, 還具有明顯的Eu負(fù)異常(0.11～0.15)(Whalen et al., 1987)和明顯的Ba、Sr、P、Eu、Ti虧損(表4,圖11, 13), 這些都是A型花崗巖的特征。巖石中沒(méi)有堿性暗色礦物和巖石的A/CNK=0.85～1.06, 表明其屬準(zhǔn)鋁質(zhì)-鋁弱飽和的A型花崗巖(King et al., 1997)。負(fù)的Ti、P、Eu異?？赡芘c含Ti礦物相(如鈦鐵礦和金紅石)、磷灰石、斜長(zhǎng)石和/或鉀長(zhǎng)石的分異有關(guān)。鉀長(zhǎng)石的分餾還可能產(chǎn)生Eu、Ba的同時(shí)負(fù)異常(Wu et al., 2003)。實(shí)驗(yàn)巖石學(xué)和鋯石飽和溫度證明(Clemens et al., 1986), A型花崗巖不可能由I型花崗巖分異產(chǎn)生, 因?yàn)锳型花崗巖需要非常高的溫度(Wu et al., 2003)。從花崗巖的Sr、Yb含量來(lái)看, 烏蘭早期A型花崗巖類(lèi)似于華南南嶺A型花崗巖(圖16)。

烏蘭晚期花崗巖類(lèi)巖石組合為閃長(zhǎng)巖+花崗閃長(zhǎng)巖+二長(zhǎng)花崗巖, 其中, 第一次侵入的花崗巖類(lèi)ASI變化于0.97～1.04之間, 平均為1.0, 第二次的為0.81～1.00, 平均為0.95, 第三次的為1.01～1.08, 平均為10.5, CIPW標(biāo)準(zhǔn)礦物計(jì)算結(jié)果, 前兩次的花崗巖幾乎不出現(xiàn)剛玉, 但第三次花崗巖出現(xiàn)0.6％～1.21％的剛玉?？梢?jiàn), 第一、二次花崗巖屬準(zhǔn)鋁質(zhì), 而第三次花崗巖屬鋁弱過(guò)飽和型。三次巖石隨SiO2含量的增加, P2O5含量明顯地呈線性減少,反映了I型花崗質(zhì)巖漿的演化特點(diǎn)。三次巖石的元素地球化學(xué)以富集大離子親石元素, 虧損高場(chǎng)強(qiáng)元素為特征(圖11), 稀土元素以富集輕稀土、且輕稀土分異明顯重稀土分異不明顯、不具有或具有弱的負(fù)Eu異常為特征(圖13), 表現(xiàn)出島弧I型花崗巖類(lèi)地球化學(xué)屬性。另外, 三次花崗巖類(lèi)的微量元素原始地幔標(biāo)準(zhǔn)化曲線(圖11)和稀土元素球粒隕石標(biāo)準(zhǔn)化曲線(圖13)相同或相似, 表明它們具有相同或相似的物質(zhì)來(lái)源和巖漿演化過(guò)程。在圖16中, 三次花崗巖樣品的Sr、Yb投點(diǎn)位于張旗等(2010)劃分的埃達(dá)克型、閩浙型、喜馬拉雅型和華南型花崗巖的過(guò)渡區(qū)域, 表現(xiàn)出由埃達(dá)克型到閩浙型/喜馬拉雅型向華南型過(guò)渡的I型花崗巖特征(圖16)。此外, 晚期花崗巖類(lèi)Sr、Nd同位素特征與澳大利亞拉克蘭褶皺帶I型花崗巖相似(圖14), 也表明其具有I型花崗巖的地球化學(xué)屬性。

圖16 花崗巖Yb-Sr圖解(據(jù)張旗等, 2010; 圖例同圖7 )Fig. 16 Yb-Sr diagram of granites (after ZHANG et al., 2010; symbols as for Fig. 7)

4.2 花崗質(zhì)巖漿起源

實(shí)驗(yàn)巖石學(xué)證明, 在非常寬的溫度、壓力條件下, 多種源巖的部分熔融均可以產(chǎn)生花崗質(zhì)熔體(Rapp et al., 1991; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Patino Douce and Johnston, 1996, 1998; Winther, 1996; Skjerlie and Patino Douce, 2002), 熔體成分的變化取決于初始熔融物質(zhì)的成分、熔融的溫度和壓力、初始物質(zhì)的含水量(Jogvan et al., 2006),如泥質(zhì)的沉積巖部分熔融可以產(chǎn)生強(qiáng)烈富鋁和富鉀的熔體, 硬砂巖的部分熔融可以產(chǎn)生中等到強(qiáng)烈富鋁的花崗閃長(zhǎng)巖/花崗巖熔體, 玄武質(zhì)巖石的部分熔融可以產(chǎn)生云英質(zhì)-奧長(zhǎng)-花崗閃長(zhǎng)質(zhì)熔體(Rapp et al., 1991; Sen and Dunn, 1994; Wolf and Wyllie, 1994; Rapp and Watson, 1995; Winther, 1996)?？梢?jiàn),只要源巖含水或存在含水相的礦物, 部分熔融就可以產(chǎn)生花崗質(zhì)熔體(Patino Douce and Johnston, 1996, 1998)。研究表明, 柴北緣北部構(gòu)造單元?dú)W龍布魯克地塊出露的基底變質(zhì)表殼巖可以劃分為2個(gè)類(lèi)型:第I類(lèi)變質(zhì)表殼巖的T2DM=2.57～2.83 Ga, εNd(t)=–1.18～2.08, 應(yīng)屬有幔源物質(zhì)混入的變質(zhì)陸源沉積巖; 第II類(lèi)變質(zhì)表殼巖的T2DM=1.61～2.17 Ga, εNd(t)為高的正值(7.23～15.12) (陳能松等, 2007a, b)。烏蘭早期花崗巖類(lèi)的(87Sr/86Sr)St為0.710 80, (143Nd/144Nd)st為0.511 514, εNd(t)為–11.6, T2DM2.10 Ga, 晚期各次侵位的花崗巖類(lèi)Sr、Nd同位素比值相似, 即(87Sr/86Sr)St變化于0.707 57～0.710 69,(143Nd/144Nd)st為0.511 965～0.551 207 4, εNd(t)為–4.8～ –0.68, T2DM為1.41～1.58 Ga, 可見(jiàn), 它們與歐龍布魯克基底兩類(lèi)表殼巖的Sr、Nd同位素特征不同, 表明它們不可能來(lái)自暴露地表的前寒武紀(jì)變質(zhì)巖系的部分熔融。通常認(rèn)為A型花崗巖在地殼伸展期間,伴隨著地幔源巖漿為地殼深熔作用提供熱源, 殼源物質(zhì)部分熔融形成的(Clemens et al., 1986; Ostendorf et al., 2014)。因此, King(1997)認(rèn)為準(zhǔn)鋁質(zhì)A型花崗巖是殼內(nèi)部分熔融形成的。根據(jù)(87Sr/86Sr)St、(143Nd/144Nd)St同位素值和T2DM年齡, 結(jié)合它們的巖石地球化學(xué)特征, 我們認(rèn)為, 烏蘭早期的花崗巖類(lèi)可能起源于古元古代的陸殼物質(zhì), 而晚期的花崗巖類(lèi)起源于中元古代的陸殼物質(zhì), 并可能混合了幔源成分。

4.3 花崗巖形成的構(gòu)造環(huán)境

宗霧隆構(gòu)造帶北邊以青海南山斷裂為界與中南祁連地塊相隔, 南邊以宗霧隆南緣斷裂為界與柴北緣歐龍布魯克地塊相鄰, 向西延至阿爾金斷裂,向東分離西秦嶺與南祁連造山帶(圖1)。宗霧隆構(gòu)造帶與鄂拉山構(gòu)造帶地質(zhì)特征及演化過(guò)程十分相似,可能是由于西秦嶺沿共和坳拉谷強(qiáng)烈斜向碰撞柴達(dá)木—?dú)W龍布魯克地塊, 造成了印支期宗務(wù)隆構(gòu)造帶東段造山隆升及強(qiáng)烈的巖漿活動(dòng)(彭淵等, 2016)。柴達(dá)木東緣花崗巖漿-火山活動(dòng)帶稱為鄂拉山構(gòu)造帶,該構(gòu)造帶上苦?！愂蔡辽呔G構(gòu)造混雜巖帶中玄武巖的40Ar/39Ar年齡為(368.6±1.4) Ma(張智勇等, 2004), 說(shuō)明洋盆從晚泥盆世—早石炭世開(kāi)始打開(kāi),到中石炭世—早二疊世形成了有限的洋盆, 烏蘭地區(qū)歐龍布魯克陸塊北部邊緣泥盆紀(jì)A型花崗巖的出現(xiàn), 是對(duì)這一構(gòu)造事件的響應(yīng)。受西秦嶺向西擠出以及華南地塊向北俯沖碰撞的共同影響, 晚二疊世—中三疊世洋盆向西斜向俯沖, 洋盆閉合收縮(孫延貴, 2004), 形成了鄂拉山構(gòu)造帶上年齡為220～200 Ma(Rb-Sr、K-Ar、U-Pb)的巖體(孫延貴等, 2001; 孫延貴, 2004; 李玉曄, 2008; 李永祥等, 2011)。鄂拉山構(gòu)造巖漿帶巖漿巖主體屬于高鉀鈣堿性花崗閃長(zhǎng)巖, 形成于板塊碰撞及碰撞后階段, 是西秦嶺地塊沿共和坳拉谷向柴達(dá)木地塊下斜向強(qiáng)烈俯沖碰撞的產(chǎn)物(張森琦等, 2000; 孫延貴等, 2001;孫延貴, 2004; 李玉曄, 2008)。對(duì)比宗務(wù)隆構(gòu)造帶與鄂拉山構(gòu)造帶內(nèi)侵入巖特征, 兩者具有相同的巖漿巖類(lèi)型, 相似的構(gòu)造成因, 均為印支期構(gòu)造巖漿活動(dòng)的產(chǎn)物。

除本文報(bào)道的兩期花崗巖外, 前人也報(bào)道過(guò)宗霧隆構(gòu)造帶上天峻南山花崗巖、青海湖南花崗巖、二郎洞二長(zhǎng)花崗巖的鋯石U-Pb年齡均為印支期,加上天峻南山果可山組超鎂鐵質(zhì)-鎂鐵質(zhì)蛇綠巖地體(Rb-Sr年齡(318±3) Ma)的發(fā)現(xiàn)(王毅智等, 2001)以及天峻南山等島弧型高鉀鈣堿性I型花崗巖的產(chǎn)出(郭安林等, 2009), 認(rèn)為宗霧隆構(gòu)造帶是一條具有完整構(gòu)造旋回的印支期造山帶(王毅智等, 2001; 郭安林等, 2009)。

圖17 花崗巖類(lèi)La/Yb-Th/Yb構(gòu)造環(huán)境判別圖解(據(jù)Condie, 1989; 圖例同圖7 )Fig. 17 Geochemical compositions of two episodes of the granites from Wulan area plotted in the tectonic setting discrimination diagrams (after Condie, 1989; symbols as for Fig. 7)

Gorton和Schandl(2000)收集了世界上26個(gè)不同地方的花崗巖和中酸性火山巖的地球化學(xué)資料,利用不相容元素Ta、Th和Yb的豐度和比值, 有效地區(qū)分出大洋島弧、活動(dòng)大陸邊緣和板內(nèi)火山巖帶三種不同的構(gòu)造環(huán)境。其中板內(nèi)火山巖帶的資料來(lái)自冰島、埃塞俄比亞和新墨西哥的瓦勒斯火山, 大陸活動(dòng)邊緣的有希臘、智利、阿根廷、日本、墨西哥、阿拉斯加和湯加—克馬德克及伊豆小笠原弧(Tonga–Kermadec and Izu–Bonin arcs), 大洋島弧的有呂宋島(Luzon arc)。三種構(gòu)造環(huán)境中火成巖的Th逐步富集主要?dú)w因于弧的成分增加, Th/Ta比值1～6是板內(nèi)火山巖帶, 6～20是活動(dòng)大陸邊緣, ＞20～90的是大洋島弧(Gorton and Schandl, 2000)。烏蘭地區(qū)早期花崗巖類(lèi)的Th/Ta比值為27.1～29.4, 平均為28.25,晚期的變化較大, 為7.5～51.4, 平均15.17, 可見(jiàn),本區(qū)早期A型花崗巖類(lèi)Th/Ta比值高于活動(dòng)大陸邊緣火成巖, 而和大洋島弧區(qū)火成巖的相似, 這可能與花崗巖產(chǎn)出的位置和源巖有關(guān)。從圖2可以看出,該A型花崗巖產(chǎn)在歐龍布魯克陸塊的北部邊緣, 哇洪山左行走滑斷裂穿過(guò)巖體, 可能是該斷裂的走滑拉分作用, 導(dǎo)致混入了洋殼成分的古元古代陸殼發(fā)生部分熔融, 形成了類(lèi)似大洋島弧火成巖Th/Ta比值的A型花崗巖。晚期花崗巖類(lèi)除個(gè)別樣品外, 所有樣品的Th/Ta比值落入活動(dòng)大陸邊緣區(qū)的范圍內(nèi)。從Th/Yb-La/Yb圖解(Condie, 1989)來(lái)看, 本區(qū)兩期花崗巖類(lèi)樣品投點(diǎn)主要落在大陸邊緣弧的范圍內(nèi)(圖17), 也說(shuō)明兩期花崗質(zhì)巖漿活動(dòng)發(fā)生在活動(dòng)大陸邊緣, 在Muller和Groves(1994)的圖解上, 本區(qū)早期花崗巖類(lèi)落入板內(nèi)區(qū)(圖18a), 晚期的花崗巖類(lèi)落入與弧相關(guān)的區(qū)域內(nèi); 在圖18b上, 兩期次花崗巖類(lèi)的樣品投點(diǎn)均落入與弧相關(guān)的大陸和碰撞后區(qū)域(圖18b); 在Gorton和Schandl(2000)圖解中,兩期花崗巖投點(diǎn)均落入大洋弧和活動(dòng)大陸邊緣區(qū)域(圖18c), 反映了兩期巖漿作用的構(gòu)造環(huán)境與活動(dòng)大陸邊緣相關(guān)。這與兩期花崗巖體分布在歐龍布魯克微陸塊北部邊緣的地質(zhì)事實(shí)相吻合。

圖18 Y-Zr (a)、Zr/Al2O3-TiO2/Al2O3 (b)和Th/Yb-Ta/Yb (c)構(gòu)造判別圖解(據(jù)Muller and Groves, 1994; Gorton and Schandl, 2000; 圖例同圖7 )Fig. 18 (a) Y-Zr, (b) Zr/Al2O3-TiO2/Al2O3and (c) Th/Yb-Ta/Yb geotectonic discrimination diagrams (after Muller and Groves, 1994; Gorton and Schandl, 2000; symbols as for Fig. 7)

綜上所述, 烏蘭地區(qū)泥盆紀(jì)A型花崗巖的出現(xiàn),標(biāo)志著歐龍布魯克北緣宗霧隆裂谷作用的開(kāi)始, 到晚石炭世(Rb-Sr年齡(318±3) Ma)開(kāi)始出現(xiàn)宗霧隆洋盆(王毅智等, 2001), 晚二疊世—中三疊世洋殼向南俯沖, 形成一系列中酸性火山巖和青海湖南山及天峻南山花崗巖為代表的島弧地體, 晚三疊世洋殼閉合進(jìn)入陸內(nèi)碰撞造山期(郭安林等, 2009; 彭淵等, 2016)。

5 結(jié)論

(1)柴北緣東段烏蘭地區(qū)早期的哈德森溝花崗巖鋯石SHRIMP U-Pb年齡為(413±3) Ma, 烏蘭晚期的許給溝巖體的年齡為(254±3) Ma、椅落山巖體為(251±1) Ma、察汗諾巖體為(249±1) Ma、(248±2) Ma,曬勒克郭來(lái)巖體為(250±1) Ma、(244±3) Ma, 察汗河巖體為(240±2) Ma。

(2)烏蘭地區(qū)早期花崗巖類(lèi)(413 Ma)巖石不僅富集大離子親石元素, 而且還富集部分高場(chǎng)強(qiáng)元素(Zr、Y、Nb等), 稀土元素配分曲線以明顯的負(fù)Eu異常為特征, 同時(shí), 巖石具有較高的10 000×Ga/Al比值和較低的MgO、Ba和Sr的含量, 屬A型花崗巖。晚期花崗巖類(lèi)以富集大離子親石元素, 虧損高場(chǎng)強(qiáng)元素為特征, 稀土元素以富集輕稀土、且輕稀土分異明顯重稀土分異不明顯、不具有或具有弱的負(fù)Eu異常為特征, 屬島弧I型花崗巖。

(3)同位素研究表明, 烏蘭早期A型花崗巖類(lèi)起源于古元古代陸殼物質(zhì)的部分熔融, 與祁連巖石圈拆沉導(dǎo)致歐龍布魯克陸塊北緣減薄、拉伸有關(guān), 它的產(chǎn)出標(biāo)志著歐龍布魯克北緣裂陷的開(kāi)始; 而晚期具有大陸活動(dòng)邊緣I型花崗巖類(lèi)起源于中元古代陸殼物質(zhì)的部分熔融, 并可能有幔源物質(zhì)的加入, 其成因與宗霧隆洋殼俯沖于歐龍布魯克陸塊之下有關(guān)。

Acknowledgements:

This study was supported by China Geological Survey (Nos. 121201102000150005-06, 12120115027001 and 12120114079901), National Natural Science Foundation of China (Nos. 41472063, 40921001, 40472034 and 40672049), and the Science and Technology Project (No. Sino Probe 05-05).

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Zircon SHRIMP Dating and Genesis of Granites in Wulan Area of Northern Qaidam

WU Cai-lai1), LEI Min1), WU Di2), LI Tian-xiao2)
1) Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037; 2) China University of Geosciences (Beijing), Beijing 100083

Zircon SHRIMP U-Pb dating of granites in Wulan area of northern Qaidam indicates that Hadesengou rock mass was formed at (413±3) Ma, Xugeigou rock mass at (254±3) Ma, Yiluoshan rock mass at (251±1) Ma, hornblende diorite and granite of Chahanruo rock mass at (249±1) Ma and (248±2) Ma, Chahanhe rock mass at (240±2) Ma, granodiorite and granite of Shailekeguo rock mass at (250±1) Ma and (244±3) Ma respectively. These granites have two formation periods: 1) the early period belongs to Early Devonian (413 Ma) with the rock association being adamellite+alkali-feldspar granite; 2) the late period belongs to Late Permian–Early Triassic (254～240 Ma) which can be further divided into three emplacements (254～251 Ma, 250～248 Ma and 244～240 Ma) with the rock association being diorite+granodiorite+granite. Geochemical study indicates that the early granitoids are not only enriched in large ion lithophile elements but also enriched in some high field-strength elements (Zr, Y, Nb etc.), thus belonging to A-type granite; the late granitoids are enriched in large ion lithophile elements and depleted in high field-strength elements, thus belonging to I-type granite.87Sr/86Sr ratio (0.710 8) and Nd model age (T2DM=2.10 Ga) of the early granite are both higher than those of the late granite(0.707 6～0.710 7, T2DM=1.41～1.58 Ga). Nevertheless, εNd(t) of the late granite (?11.6) is lower than that of the early granite (?4.8 ～ ?6.8). These data show that the early A-type granite might have originated from Paleoproterozoic continental crust, whereas the late I-type granite originated from Mesoproterozoic crust. Combined with regional geological structural characteristics, the authors consider that the formation of early A-type granite was related to the thinning and stretching of north Oulongbuluke block caused by Qilian lithosphere delamination which also marked the beginning of Zongwulong rift, while the formation of late I-type granite was related to the southward subduction of Zongwulong oceanic crust beneath Oulongbuluke block.

granite; zircon SHRIMP dating; Oulongbuluke block; Zongwulong tectonic zone; Wulan

P588.121; P597.1

A

10.3975/cagsb.2016.04.11

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2016-05-06; 改回日期: 2016-06-26。責(zé)任編輯: 閆立娟。

吳才來(lái), 男, 1960年生。博士, 研究員, 博士生導(dǎo)師。主要從事火成巖巖石學(xué)及其成礦研究。E-mail: wucailai@126.com。