蔣俊毅 蘇尚國 崔曉亮 劉璐璐 孟維一 王菁姣

1. 中國地質(zhì)大學(xué)(北京)地球科學(xué)與資源學(xué)院，北京 1000832. 中國地質(zhì)調(diào)查局地學(xué)文獻中心文獻數(shù)據(jù)室，北京 1000831.

克拉通是指地殼中早已固結(jié)，并在至少自古生代以來的地質(zhì)歷史時期里長期保持穩(wěn)定、很少受到破壞的部分，一般形成于前寒武系(翟明國，2008；Pearson，2009)。成熟的克拉通具有面積大、厚度大、密度低、含水量低和地溫梯度低的巖石圈(Pollack，1986)，它能較好地記錄早期地球演化的進程，并浮于軟流圈之上?？死ɑ静粫l(fā)生巖石圈變形或其他大規(guī)模巖漿作用，由此克拉通上可以保留很多后續(xù)較新的地質(zhì)作用記錄，對克拉通的研究具有不可忽視的重要意義(Wildeetal.，2001，2003)。然而近期研究表明，克拉通可能并不像過去所認為的那樣穩(wěn)定，而是會發(fā)生減薄甚至破壞(翟明國，2008；朱日祥等，2012)，如華北克拉通東部，有關(guān)其減薄的機制仍存在諸多爭議。

華北克拉通減薄的機制目前主要有兩種觀點較為流行：①拆沉作用(Gaoetal.，1993，2002，2004；鄧晉福等，1994，1996，2000，2003，2006，2009，2015a，b；高山和金振民，1997；Dengetal.，1998；吳福元和孫德有，1999; 吳福元等，2003；許文良等，2004，2006，2009；Xuetal.，2006a，b)；②機械-熱侵蝕作用(Fan and Menzies，1992；徐義剛，1998，1999，2004，2006；徐義剛等，2009；Xu，2001；Xuetal.，2004，2008)，以及與之相似的熔體-橄欖巖相互作用(Zhangetal.，2004，2007，2008；張宏福和楊岳衡，2007)和巖石圈地幔水化模型(Niu，2005)。這兩種觀點是對減薄過程的各有側(cè)重(朱日祥，2018)，即前者著重于物理過程，強調(diào)自上而下地由擠壓造山變質(zhì)形成的密度較大的下地殼和巖石圈地幔向下運動進入軟流圈中引發(fā)大規(guī)模巖漿上涌，能讓地殼巖石圈下沉的巖性只有榴輝巖，而榴輝巖一般產(chǎn)于匯聚大陸板塊邊界的超高壓變質(zhì)造山帶中(Gaoetal.，1993)；后者則側(cè)重于化學(xué)過程，著重于從下向上的由軟流圈上涌帶來的大量熱量對巖石圈底部進行烘烤軟化，軟化的巖石圈不斷從上覆的巖石圈地幔剝離，而引發(fā)軟流圈的大規(guī)模上涌的一個可以模擬的條件是加入大量的水(Xiaetal.，2013；夏群科和郝艷濤，2013)，由此強調(diào)了華北克拉通的減薄與貫穿早白堊世的古太平洋向華北克拉通下方俯沖有關(guān)(吳福元和孫德有，1999；Niu，2005；Zhengetal.，2007, 2009)。雖然兩種模型側(cè)重點不同，但都不約而同地強調(diào)了華北克拉通減薄是殼幔物質(zhì)相互作用的結(jié)果。

華北克拉通東部的早白堊世西石門雜巖體是與克拉通減薄密切相關(guān)的一套巖石組合，本文通過對其進行詳細的地質(zhì)調(diào)查和礦物學(xué)、巖石學(xué)、地球化學(xué)、年代學(xué)的綜合研究，揭示了西石門雜巖體的成因，并借此探討白堊紀華北克拉通東部巖石圈減薄機制。

1 地質(zhì)概況

1.1 研究區(qū)地質(zhì)概況

研究區(qū)位于華北克拉通東部，太行山造山帶的中南段(羅照華等，1997，1999)，向西為以太行山造山帶為主的華北克拉通中部地塊，向南為磁縣-大名斷層，南約500km后進入中國中部中央造山帶，北側(cè)為贊皇隆起帶，東側(cè)不出50km即進入廣袤的華北平原凹陷區(qū)(蔡本俊等，1987)。

研究區(qū)內(nèi)廣泛發(fā)育燕山期基性-中性-堿性巖漿巖，總體的出露面積約為350km2，多呈巖株、巖蓋小規(guī)模產(chǎn)出，受NNE-NE向深大斷裂控制分為三個巖漿巖帶呈串珠狀分布。如圖1所示，自東向西的三個巖漿巖帶分別為：東部的堿性洪山雜巖體、中部的中酸性武安雜巖體和西部的基性-中性符山雜巖體。區(qū)內(nèi)巖漿巖的形成時代約為148～120Ma，三個巖漿巖帶內(nèi)均有鐵礦床發(fā)育，體現(xiàn)了本區(qū)巖漿巖與鐵礦成礦的密切關(guān)系(章百明等，1996；陳永健等，2014)。

圖1 華北克拉通燕山期巖漿巖分布簡圖(a)和研究區(qū)區(qū)域地質(zhì)簡圖(b)(據(jù)蘇尚國等，2017)Fig.1 Sketch map showing the distribution of Yanshanian igneous rocks in North China Craton(a)and sketch geological map of study area(b)(after Su et al.，2017)

圖2 河北武安市西石門雜巖體地質(zhì)圖Fig.2 The geological map of Xishimen Complex in Wuan, Hebei Province

位于中部的武安雜巖體規(guī)模較大，其主體自北向南由綦村巖體、西石門雜巖體、坦嶺雜巖體以及固鎮(zhèn)雜巖體4個規(guī)模較小的雜巖體組成，各雜巖體呈巖蓋、巖株狀產(chǎn)出，巖性為一系列中酸性巖(Sunetal.，2014，2015；Zhangetal.，2015；Liuetal.，2016)。二長閃長巖和二長巖構(gòu)成了武安雜巖體的主要巖性，顏色多見淡粉色、淡灰色，中細粒結(jié)構(gòu)和似斑狀結(jié)構(gòu)共存；雜巖體還發(fā)育有閃長巖和正長巖，結(jié)構(gòu)上也有中細粒和似斑狀之分，這兩種巖性皆可見脈狀產(chǎn)出于二長巖之中(Chenetal.，2008；Shenetal.，2013，2015)。武安雜巖體的不同巖性之間常為漸變過渡，部分巖性之間接觸接線截然。雜巖體內(nèi)部含多種包體，其中圍巖捕擄體、片麻質(zhì)包體和MME最為常見，前兩者的具體巖性包括變質(zhì)石英砂巖、奧陶系灰?guī)r等；MME的大小不一，從小于1cm到大于10cm不等，多為閃長質(zhì)包體，少數(shù)為正長質(zhì)包體(Lietal.，2013； Li and Santosh，2014)。

圖3 西石門雜巖體中各侵入體之間的侵入關(guān)系 (a、b)二長閃長巖與二長巖不均勻混合；(c)正長巖侵入二長巖；(d)二長巖中的暗色包體；(e)斑狀角閃二長巖侵入二長巖，流線發(fā)育；(f)正長巖脈侵入斑狀角閃正長巖中Fig.3 The intrusive relationship of igneous rocks in the Xishimen Complex (a，b)inhomogeneous mixing of monzodiorite and monzonite；(c)syenite intrudes into monzonite；(d)MME in monzonite；(e)porphyry amphibolite monzonite intrudes into monzonite and the linearflowage structure；(f)syenite veins intrudes into porphyry amphibolite syenite

1.2 西石門雜巖體巖相學(xué)特征

西石門雜巖體是武安雜巖體中出露面積最大、鐵礦儲量最高的雜巖體，雜巖體呈巖株狀產(chǎn)出，此外，巖株旁還可見個別巖墻和小巖體。雜巖體與同屬中部巖帶的綦村、坦嶺等雜巖體共同呈北北東向產(chǎn)出，出露面積約35km2。雜巖體主體的巖性為二長閃長巖-二長巖-正長巖，并被晚期閃長質(zhì)和長英質(zhì)脈體穿插，各巖性的地表特征及各侵入巖之間關(guān)系見圖2。根據(jù)野外調(diào)查結(jié)果，從巖石結(jié)構(gòu)上，可以清晰地將雜巖體分為具等粒結(jié)構(gòu)的和具不等粒結(jié)構(gòu)的兩組組合，且后者切穿前者(圖3e)。不同的結(jié)構(gòu)和清晰的侵入關(guān)系，指示了西石門雜巖體顯然不是單一巖漿活動所形成的單元。王濤等(2017)將由2個以上年齡基本相同且具有成因聯(lián)系的單元構(gòu)成的復(fù)式巖基稱為巖套(或序列)，故本文將西石門雜巖體中被侵入的、具等粒結(jié)構(gòu)的巖性組合定義為早期巖套，將侵入其他巖性的、具不等粒結(jié)構(gòu)的巖性組合定義為晚期巖套。

早期巖套為具等粒結(jié)構(gòu)的二長閃長巖-二長巖-正長巖；晚期巖套為以斜長石、角閃石為斑晶的具似斑狀結(jié)構(gòu)的斑狀二長巖-斑狀正長巖，此外還出露正長巖脈體。根據(jù)野外地質(zhì)調(diào)查和分析測試結(jié)果，后者的侵位時間確實較前者稍晚，雜巖體中廣泛發(fā)育有巖漿侵入的現(xiàn)象(圖3)。西石門雜巖體中的包體成分復(fù)雜，既包括太古代片麻巖和元古代砂巖等包體，也包括先存暗色巖漿巖和極富鐵質(zhì)的巖石。雜巖體的巖相學(xué)特征見圖4。

1.2.1 早期巖套巖相學(xué)特征

早期巖套的共同特點是具等粒結(jié)構(gòu)，故也可以稱為等粒結(jié)構(gòu)巖套，巖性主要包括二長閃長巖、二長巖和正長巖。

二長閃長巖呈深灰色，細粒結(jié)構(gòu)，塊狀構(gòu)造。角閃石含量約5%～15%，為淺黃色-褐色，他形粒狀，顆粒大小為0.2×0.4mm～0.7×0.8mm，主要成分為韭閃石質(zhì)普通角閃石和韭閃石，部分顆粒周圍見Fe質(zhì)析出；斜長石含量約50%～60%，呈自形-半自形板狀，顆粒大小為0.2×0.4mm～0.6×0.8mm，主要成分為中長石，含有少量拉長石，牌號范圍為An31Ab66Or3～An52Ab46Or2。鉀長石含量約25%～30%，多呈不規(guī)則狀形成于斜長石之間，分布不均勻；鈦鐵氧化物不足1%，粒徑多小于0.1mm，零星分布。

二長巖呈灰色，細粒結(jié)構(gòu)，塊狀構(gòu)造。角閃石含量約5%～10%，為淺黃色-褐色，他形粒狀，顆粒大小為0.3×0.5mm～0.6×0.8mm，主要成分為韭閃石質(zhì)普通角閃石和淺閃石，部分晶體發(fā)育有暗化邊結(jié)構(gòu)；斜長石含量約45%～55%，呈自形-半自形板狀，顆粒大小為0.4×0.5mm～0.8×1mm，主要成分為中長石，還含有少量奧長石和鈣長石，牌號范圍為An21Ab78Or1～An51Ab46Or3。鉀長石含量約35%～45%，分布也不均勻；鈦鐵氧化物不足1%，粒徑多小于0.1mm，零星分布。

正長巖呈淡紅色，中粗粒結(jié)構(gòu)，塊狀構(gòu)造，由鉀長石、斜長石、石英、角閃石組成，含極少量磁鐵礦、磷灰石。鉀長石含量約50%～60%，呈半自形板狀，顆粒大小為0.1×0.2mm～0.6×0.8mm，主要成分為透長石，牌號范圍為An0Ab3Or97～An1Ab27Or72；斜長石含量約25%～35%，呈半柱狀結(jié)構(gòu)，顆粒大小為0.2×0.4mm～0.5×0.7mm，主要成分為鈉長石，并含有少量奧長石和中長石，牌號范圍為An1Ab98Or1～An48Ab51Or1；石英含量約5%，呈他形粒狀，顆粒大小為0.1×0.1mm～0.3×0.4mm；角閃石含量約5%，呈半自形長柱狀，顆粒大小為0.1×0.1mm～0.3×0.5mm，少部分為他形粒狀，正長巖中角閃石的種類范圍較廣，主要為韭閃石、韭閃石質(zhì)普通角閃石和淺閃石質(zhì)普通角閃石，以及少量淺閃石。部分地區(qū)的正長巖強烈高嶺土化。

1.2.2 晚期巖套巖相學(xué)特征

晚期巖套的共同特點是具似斑狀結(jié)構(gòu)，故也可以稱為似斑狀結(jié)構(gòu)巖套，巖性主要包括斑狀二長巖和斑狀正長巖，其中各巖性又因主要斑晶不同具體命名也不同，當以角閃石為主時稱為斑狀角閃二長巖和斑狀角閃正長巖，當以斜長石為主時稱為斑狀斜長二長巖。

斑狀二長巖呈灰白色，似斑狀結(jié)構(gòu)，塊狀構(gòu)造。斑晶為角閃石、斜長石。角閃石呈長柱狀，自形-半自形，顆粒大小為2×3mm～6×10mm，含量約8%～10%，個別見晶體連生，主要成分為韭閃石、含鐵韭閃石、韭閃石質(zhì)普通角閃石、淺閃石質(zhì)普通角閃石和含鐵韭閃石質(zhì)普通角閃石以及少量的淺閃石，邊緣見反應(yīng)邊(圖4g, h)，反應(yīng)邊的主要成分為黑云母、鈉長石、淺閃石和磁鐵礦；斜長石呈板狀、粒狀，自形-半自形，顆粒大小為2×3mm～5×7mm，含量約2%～8%，主要成分為拉長石、中長石、奧長石和少量鈉長石，牌號范圍為An6Ab93Or1～An66Ab34Or0。角閃石與斜長石斑晶可以同時存在，兩者的相對含量導(dǎo)致巖石名稱非常復(fù)雜，此研究將所有含斑晶的二長巖全部歸為斑狀二長巖?；|(zhì)為顯晶質(zhì)，由輝石、角閃石、鉀長石、斜長石、石英和磁鐵礦、鈦鐵物等組成，礦物粒度較細，總體不超過0.8mm。輝石含量約5%～8%，半自形粒狀結(jié)構(gòu)；角閃石含量約15%～25%，成分上全部屬于硅質(zhì)淺閃石；石英約5%～10%，他形粒狀結(jié)構(gòu)；鉀長石含量約25%～30%，半自形-他形，粒狀結(jié)構(gòu)，主要成分為透長石；斜長石含量約35%～40%，自形-半自形，粒狀結(jié)構(gòu)，主要成分為中長石、鈉長石以及少量奧長石、拉長石，牌號范圍為An1Ab98Or1～An52Ab44Or3；磁鐵礦/鈦鐵礦約2%，他形粒狀結(jié)構(gòu)。巖石整體風化程度弱，但遭受不同程度的鈉化改造，蝕變程度一般。

斑狀正長巖呈淡紅色，似斑狀結(jié)構(gòu)，絕大部分斑晶為角閃石，極少出現(xiàn)斜長石斑晶，巖石具塊狀構(gòu)造，基質(zhì)由鉀長石、斜長石、石英、角閃石組成，含極少量磁鐵礦、磷灰石。角閃石斑晶呈自形長柱狀，顆粒大小為0.6×1.5mm～6×10.5mm，含量約10%；極少數(shù)呈斑晶狀的斜長石粒徑可達6×8mm。基質(zhì)中角閃石呈半自形長柱狀，顆粒大小為0.1×0.2mm～0.3×0.5mm，含量約5%。鉀長石含量約45%～55%，呈半自形板狀，顆粒大小為0.1×0.3mm～0.6×1.0mm；斜長石含量約30%～35%，主要成分為中長石，呈半自形板狀，顆粒大小為0.2×0.4mm～0.6×0.8mm；石英含量約5%，呈他形粒狀，粒徑約0.1～0.2mm。

除上述主要巖性的巖石外，研究區(qū)還出露少量正長巖脈切穿上述所有巖性。正長巖脈主體呈淡紅色，細粒結(jié)構(gòu)，塊狀構(gòu)造，由鉀長石、斜長石、石英組成，含少量角閃石，其中部分角閃石呈斑晶狀產(chǎn)出，此外還含極少量磁鐵礦、磷灰石。鉀長石含量約50%～55%，呈半自形板狀，顆粒大小為0.1×0.2mm～0.3×0.6mm；斜長石含量約30%～40%，呈半自形板狀，顆粒大小為0.1×0.2mm～0.2×0.5mm；石英含量約10%，呈他形粒狀，粒徑約0.1～0.2mm；角閃石含量約5%，呈半自形長柱狀，顆粒大小為0.1×0.1mm～0.1×0.2mm，弱綠泥石化、綠簾石化，部分地區(qū)角閃石呈斑晶狀，顆粒大小為1.0×1.5mm～8.0×10.5mm，含量可達1%。巖石整體風化蝕變程度較低。含角閃石斑晶的正長巖脈與不含斑晶的正長巖脈之間為過渡關(guān)系，前者應(yīng)當是正長巖脈在前進中較富流體的頭部冷凝結(jié)晶形成的。

在西石門地區(qū)，上述這些巖性之間侵入關(guān)系時而明顯時而難辨，這可能與它們的侵位年齡非常接近，侵位和冷卻時間過于相近而引起的巖漿混合有關(guān)，在一些地區(qū)也觀察到了巖漿混合的現(xiàn)象(霍延安等，2019)。但總體而言具有等粒結(jié)構(gòu)的巖套略早于具似斑狀結(jié)構(gòu)的巖套，它們最后共同呈一整體被正長質(zhì)巖脈穿插(圖3f)。

2 樣品及實驗分析方法

全巖主微量分析測試在河北廊坊市更新地質(zhì)有限公司完成。除FeO外的主量元素檢測標準為GB/T14506.28-2010，使用檢測儀器為Axiosmax X射線熒光光譜儀，F(xiàn)eO含量為滴定法測得。全巖微量元素分析采用ICP-MS溶液法分析。詳細樣品分析過程以及ICP測試條件見Qietal.(2000)。

全巖Sr-Nd同位素和Pb同位素分析測試在中國地質(zhì)大學(xué)(北京)科學(xué)研究院同位素實驗室完成。首先稱量0.1～0.2g全巖粉末樣品，使用混合酸(HF+HNO3+HClO4)溶樣，待樣品完全溶解后蒸干，加入6mol/L的HCl溶液，將樣品轉(zhuǎn)化為氯化物蒸干。再用0.5mol/L HCl溶解，使用Rb-Sr和Sm-Nd交換柱進行元素分離提純和元素提取。最后用ISOPROBE-T熱電離質(zhì)譜儀完成元素比值測定。Pb同位素詳細的分析測試方法見Tayloretal.(2015)，標準值自Wuetal.(2017)。

鋯石U-Pb分析測試在北京離子探針中心SHRIMP II上完成。詳細分析方法見Williams(1998)。測試時一次流O2-強度為3～5nA，測試束斑直徑為25～30μm。標樣為M257(U=840×10-6)和TEM年齡為417Ma(Blacketal.，2003)，分別用于鋯石U含量和年齡校正。試驗所得數(shù)據(jù)的后期處理使用SQUID(Ludwigetal.，2001)和ISOPLOT完成。

鋯石Lu-Hf同位素分析測試在內(nèi)蒙古自治區(qū)地質(zhì)調(diào)查院分析測試中心完成。測試儀器為多接收電感耦合等離子體質(zhì)譜儀(NeptunePlus)和激光剝蝕系統(tǒng)(GeoLasHD193nm)。實驗使用載氣是He，剝蝕直徑為32μm，標準鋯石為GJ-1和Ple?ovice。鋯石標樣GJ-1的176Hf/177Hf加權(quán)平均值為0.282007±0.000025(2σ)。計算εHf(t)值時，采用的Hf同位素標準值為球粒隕石176Lu/177Hf=0.036，176Hf/177Hf=0.283(Blichert-Toft and Albarède，1997)；計算地幔模式年齡時，采用的標準為虧損地?，F(xiàn)今值176Hf/177Hf=0.253，176Lu/177Hf=0.038；計算地殼模式年齡則采用176Hf/177Hf=0.015(Griffinetal.，2000)。

單礦物化學(xué)成分電子探針分析測試在中國冶金地質(zhì)總局山東測試中心完成。實驗所用的電子探針儀器型號為(JEOL)JXA-8230。測試分析電壓、電流和檢出角分別為15kV、20nA和40°，分析束斑為1～9μm，使用ZAF法校正。主量元素的峰值積分時間和背景積分時間分別為10s和5s；微量元素的峰值積分時間和背景積分時間分別為20s和10s。實驗標準礦物為美國SPI礦物、金屬標準以及中國國家標樣GSB。

3 西石門雜巖體年代學(xué)特征

西石門雜巖體的巖性較為復(fù)雜，選取其中的樣品B6077(二長閃長巖)作為二長閃長巖代表、樣品KS0816-1(二長巖)作為二長巖代表、樣品TW2595(斑狀角閃二長巖)作為斑狀二長巖代表、樣品TW2832(斑狀角閃正長巖)作為斑狀正長巖代表進行鋯石U-Pb年代學(xué)測試。具體的測試結(jié)果見表1，鋯石CL圖像見圖5，所得的U-Pb協(xié)和曲線圖見圖6。

圖5 西石門雜巖體鋯石CL圖像Fig.5 Cathodoluminescence images of zircons in the Xishimen Complex

根據(jù)鏡下觀察和測試結(jié)果，相關(guān)樣品中的鋯石可以分為3類。第一類鋯石發(fā)育較明的顯震蕩環(huán)帶，呈半自形-自形粒狀、短柱狀、長柱狀，長軸長多不超過100μm，Th/U值較高(0.57～4.86)，總體具有源生巖漿鋯石的特征；第二類鋯石Pb丟失嚴重，呈他形粒狀，在CL圖像下呈現(xiàn)出明顯的元素交換通道和交代增生邊，這一類鋯石受后期熱液改造蝕變強烈，改造之前的特征已不明顯；第三類鋯石也發(fā)育了較明顯的震蕩環(huán)帶，但呈現(xiàn)出他形-半自形橢球狀，粒度較第一類鋯石明顯增大，多數(shù)大于50×120μm，Th/U值較低(0.09～0.85)，這一部分鋯石屬于捕獲鋯石或繼承鋯石。

表1西石門雜巖體鋯石SHRIMPU-Pb測試結(jié)果

Table 1 SHRIMP U-Pb isotope data of the Xishimen Complex

測點號含量(×10-6)同位素年齡(Ma)同位素比值UTh206Pb238U1σ207Pb206Pb1σ208Pb232Th1σ238U206Pb%誤差207Pb206Pb%誤差207Pb235U%誤差二長閃長巖(早期巖套)B6077-1.19974217143.12.214089140344.571.50.04883.80.154.1-1.2370857144.02.4265113136344.251.70.05154.90.165.2-2.16141814139.92.228161137345.591.60.05192.70.163.1-2.211115401141.22.19486138245.141.50.04793.60.153.9-3.17672304141.72.2305108136444.991.60.05254.70.165.0-4.1528988141.62.318576125645.021.60.04983.30.153.7-5.1331732140.42.4106123137345.431.70.04815.20.155.5-6.1313661136.12.380129132346.851.70.04765.40.145.7-6.23991021141.02.3138121134345.231.60.04885.10.155.4-7.14171145142.02.458153137444.891.70.04726.40.146.6-8.1249452139.72.5195227141545.641.80.05009.80.1510.0-9.1380778138.22.7-147181135446.142.00.04347.30.137.6-10.1361697141.22.420498137445.171.70.05024.20.154.5-11.1262491138.02.4152115133346.201.80.04914.90.155.2-12.1277493139.22.4316170138445.821.70.05277.50.167.7-13.1296587141.62.331280139345.011.70.05263.50.163.9-14.1277504137.32.3163114133446.451.70.04934.90.155.2-15.1242407137.32.420296134446.461.80.05024.10.154.5-16.1374858140.22.3183171138345.481.70.04977.30.157.5-17.1420877140.52.3224175134345.381.70.05067.60.157.8-17.2238513140.12.6475170142445.511.90.05667.70.177.9-18.1273533136.22.3310153131446.851.70.05266.70.156.9-19.1159177138.52.5248138136546.031.90.05126.00.156.3-20.13411080137.52.488231133346.371.80.04789.70.149.9-21.1255547136.32.4-162170129346.791.70.04316.80.137.1-22.17081322134.32.12104126347.491.60.04614.30.134.6二長巖(早期巖套)KS0816-1-1.1242391136.22.4262153133446.831.80.05156.70.156.9-2.18613841131.92.018474126248.371.50.04983.20.143.5-3.1299640131.62.4-276326127448.501.80.041212.80.1212.9-4.15571932136.72.332163135346.661.70.05282.80.163.3-6.1435712139.02.244570139345.891.60.05583.20.173.6-7.1374757134.72.3210153131347.381.70.05036.60.156.8-8.17192766135.92.136577130246.921.60.05393.40.163.8-9.15731496132.62.2219241127348.121.70.050510.40.1410.5-9.2211246138.92.6380148134545.931.90.05426.60.166.9-10.1355882138.92.4319138140345.901.70.05286.10.166.3-11.1356765136.62.3154163131346.701.70.04917.00.157.2-12.1250534132.42.4362114130448.181.80.05385.10.155.4-13.1309678136.72.6141282136546.661.90.048912.00.1412.2-14.1493961133.92.320252126447.641.70.046410.50.1310.6-15.18603598139.22.2334107136245.811.60.05314.70.165.0-16.15151103137.32.2174109126346.441.60.04964.70.155.0

續(xù)表1

Continued Table 1

測點號含量(×10-6)同位素年齡(Ma)同位素比值UTh206Pb238U1σ207Pb206Pb1σ208Pb232Th1σ238U206Pb%誤差207Pb206Pb%誤差207Pb235U%誤差斑狀二長巖(晚期巖套)TW2832-1.1113482504.921.62502122524362.111.00.16450.710.771.2-2.116594122.91.8228211123851.951.50.05079.20.139.3-3.166132556.525.12490142640612.051.20.16330.810.961.4-4.1197782340.618.72398132329292.281.00.15470.89.331.2-5.1116111959.417.520611420901122.821.00.12730.86.231.3-6.1109322162.819.32248162114402.511.00.14170.97.791.4-7.1109372441.320.92494112478392.171.00.16370.610.391.2-9.1118562560.621.82496112559392.051.00.16390.611.021.2-10.181252403.322.82477122398422.211.10.16200.710.091.4-11.1126312482.221.52504122450412.131.00.16470.710.671.3-12.195362457.524.62469112453412.151.20.16120.710.321.4-14.198392419.921.52427112422362.201.10.15730.79.881.3-15.1149242469.920.42494122271382.141.00.16370.710.541.2-16.1151382527.721.32531102455452.081.00.16730.611.081.2-17.1122552181.019.52245122087392.481.10.14150.77.851.3-18.110570123.92.0111231119751.541.60.04829.80.139.9-19.112122126.03.41031701401750.662.70.04817.20.137.7-20.110662126.02.085245135950.651.60.047710.30.1310.5-21.12871362457.647.12503112442652.152.30.16450.610.532.4-22.192342470.851.02506152388792.142.50.16490.910.622.6-25.195472508.750.22472232374782.102.40.16161.410.602.8-26.189542514.450.42524262539732.102.40.16661.610.962.9-27.165112536.253.824951624031112.072.60.16380.910.892.7-30.1382702431.252.12448242361742.182.60.15931.410.062.9-32.13471432537.947.8248792517602.072.30.16300.510.842.3-36.1142432548.249.52471232314682.062.40.16151.410.792.7-37.1100202435.761.025031421941242.183.00.16460.910.423.1斑狀正長巖(晚期巖套)TW2595-3.1102161126.62.159281121550.411.70.047211.80.1311.9-4.1130117128.42.1154268127749.721.60.049111.40.1411.6-5.1164356129.32.783253131449.372.10.047710.60.1310.9-6.1212411122.22.0244140117452.271.60.05116.10.136.3-7.17573124.32.4221276131751.361.90.050611.90.1412.1-8.1169242126.92.0166342121650.321.60.049414.70.1414.7-10.1160235122.22.065165121452.241.60.04736.90.127.1-11.1349612128.61.6283184128349.641.30.05198.00.148.1-12.1263461129.02.92518401231049.462.30.051236.50.1436.6-13.1323528124.51.7147334122451.281.40.049014.20.1314.3-15.1212330129.11.6107132128349.451.20.04825.60.135.7-16.1183210123.62.0120410122851.651.60.048417.40.1317.5-17.1289774126.01.540178124250.661.20.04687.50.137.6

圖6 西石門雜巖體相關(guān)樣品的鋯石U-Pb協(xié)和圖 (a)二長閃長巖(樣品B6077)；(b)二長巖(樣品KS0816-1)；(c)斑狀正長巖(樣品TW2832)；(d)斑狀二長巖(樣品TW2595)Fig.6 U vs. Pb concordia diagrams of zircons in the Xishimen Complex (a)Monzodiorite (Sample B6077)；(b)monzolite (Sample KS0816-1)；(c)porphyry syenite (Sample TW2832；(d)porphyry monzonite (Sample TW2595)

3.1 早期巖套鋯石SHRIMP U-Pb年代學(xué)

從二長閃長巖獲取了22顆鋯石共26個數(shù)據(jù)點，都屬于巖漿鋯石，所獲得的206Pb/238U平均加權(quán)年齡為139.52±0.92Ma(MSWD=1.10)，代表了二長閃長巖的侵位年齡。從二長巖獲取了17顆鋯石共17個數(shù)據(jù)點，都屬于巖漿鋯石，所獲得的206Pb/238U平均加權(quán)年齡為135.6±1.1Ma(MSWD=1.3)，代表了二長巖的侵位年齡。根據(jù)霍延安等(2019)研究，在與西石門雜巖體緊鄰的固鎮(zhèn)雜巖體中正長巖的206Pb/238U平均加權(quán)年齡為131.7±1.2Ma(MSWD=1.2)，同時還存在大量2300～2700Ma的捕獲和繼承鋯石，前者代表了正長巖的侵位年齡。從二長閃長巖-二長巖-正長巖巖石年齡逐漸變小，這與中酸性巖漿的演化過程是一致的，暗示西石門雜巖體中由不同巖性組成的早期巖套可能具有一定程度的同源性。但在后續(xù)研究中，對來自西石門雜巖體和鄰近的坦嶺、固鎮(zhèn)等雜巖體的多個二長閃長巖和二長巖樣品進行過多次多種手段的測年分析，都未能從中找到年齡較大的捕獲或繼承鋯石，又暗示了正長巖源區(qū)可能與二長閃長巖-二長巖源區(qū)不一致。

3.2 晚期巖套鋯石SHRIMP U-Pb年代學(xué)

從斑狀二長巖獲取了27顆鋯石共27個數(shù)據(jù)點，其中有4顆鋯石的206Pb/238U年齡較小，平均加權(quán)年齡為124.4±2.1Ma(MSWD=0.53)，該年齡與西石門雜巖體其他巖性的年齡在誤差范圍內(nèi)有連續(xù)性，應(yīng)當是斑狀正長巖侵位的真正年齡；其余鋯石年齡較大，都屬于捕獲或繼承鋯石，且同樣集中在約2400～2600Ma的范圍內(nèi)，故同樣使用207Pb/206Pb年齡標記這部分鋯石的年齡。從斑狀正長巖獲取了12顆鋯石共12個數(shù)據(jù)點，都屬于巖漿鋯石，所獲得的206Pb/238U平均加權(quán)年齡為126.1±1.6Ma(MSWD=1.8)，代表了斑狀正長巖的侵位年齡。此外，在鄰區(qū)坦嶺雜巖體的斑狀二長巖-斑狀正長巖中都發(fā)現(xiàn)了大量年齡約2000～2600Ma的繼承和捕獲鋯石(劉璐璐等，2017，2019)，指示晚期巖套各巖性源區(qū)具有相似性，即同樣都有古老的陸殼基底參與。與早期巖套的年齡139.52±0.92Ma～131.7±1.2Ma相比，晚期巖套的年齡126.1±1.6Ma～124.4±2.1Ma總體較小。

4 西石門雜巖體地球化學(xué)特征

4.1 主量元素地球化學(xué)特征

西石門雜巖體各巖性的主量元素測試結(jié)果見表2所示。如圖7中所示，雜巖體巖性總體為一套二長閃長巖-二長巖-正長巖的巖石組合，根據(jù)TAS分區(qū)對野外地質(zhì)調(diào)查中不易區(qū)分的二長閃長巖和二長巖進行區(qū)分，但二長巖與正長巖、斑狀二長巖與斑狀正長巖主要依靠鏡下觀察鑒別命名。

圖7 西石門雜巖體TAS圖解(底圖據(jù)Wilson，1989) (1)橄欖輝長巖；(2)輝長巖；(3)輝長閃長巖；(4)閃長巖；(5)花崗閃長巖；(6)花崗巖；(7)副長石輝長巖；(8)二長輝長巖；(9)二長閃長巖；(10)二長巖；(11/15)正長巖；(12)副長石巖；(13)副長石二長閃長巖；(14)副長石二長正長巖；(16)副長石正長巖Fig.7 TAS diagram of the Xishimen Complex(after Wilson，1989) (1)olivine gabbro；(2)gabbro；(3)gabbro diorite；(4)diorite；(5)granodiorite；(6)granite；(7)parafeldspar gabbro；(8)monzogabbro；(9)monzodiorite；(10)monzolite；(11/15)syenite；(12)parafeldspar diorite；(13)parafeldspar monzonite；(14)parafeldspar syenite；(16)parafeldspar syenite

圖8 西石門雜巖體SiO2-K2O圖解(底圖據(jù)Miller et al.，1999)Fig.8 The SiO2 vs. K2O diagram of the Xishimen Complex(after Miller et al.，1999)

雜巖體中晚期巖套的酸性程度較早期巖套的酸性程度稍高，晚期巖套的SiO2含量范圍為57.00%～67.51%，MgO含量范圍為0.61%～3.37%，(Na2O+K2O)含量范圍為8.02%～11.75%(圖8)，A/CNK范圍為0.72～1.05；早期巖套的SiO2含量范圍為51.72%～68.56%，MgO含量范圍為0.17%～4.88%，(Na2O+K2O)含量范圍為5.62%～11.65%，A/CNK范圍為0.56～1.24。在哈克圖解(圖9)中，除了Na2O、K2O外，兩個巖套都表現(xiàn)出相似的相關(guān)性和較連續(xù)的巖漿分離結(jié)晶演化過程，指示由不同巖性組成的西石門雜巖體具有一定的同源性，同時較低的MgO含量指示成巖巖漿主體來源于地殼物質(zhì)。

值得一提的是，西石門雜巖體中Na2O、K2O含量的異常與區(qū)域內(nèi)普遍遭受的鈉化和鉀化蝕變作用有關(guān)。鈉化、鉀化作用在露頭上表現(xiàn)為巖石顏色發(fā)白，在鏡下表現(xiàn)為斜長石斑晶普遍鈉長石化、絹云母化，角閃石斑晶普遍綠簾石化、黑云母化。鈉化和鉀化蝕變導(dǎo)致整個雜巖體全堿含量升高，但由于未在巖石中發(fā)現(xiàn)堿性礦物共生組合，故不能將雜巖體定義為堿性雜巖體。此外，鉀化蝕變還會導(dǎo)致雜巖體整體鉀質(zhì)含量升高，使雜巖體顯示出了目前如高鉀鈣堿性巖石系列一樣的地球化學(xué)特征，部分樣品甚至出現(xiàn)了鉀玄巖系列的地球化學(xué)特征(圖8)，但鉀化作用的具體程度難以模擬，在研究雜巖體來源和演化時須謹慎。

4.2 微量元素地球化學(xué)特征

西石門雜巖體各巖性的微量元素測試結(jié)果見表2所示，不同巖套不同巖性所得的稀土和微量元素配分圖(圖10)具有相似的特征。在稀土元素配分圖中，整個雜巖體總體表現(xiàn)出輕微右傾性的輕稀土富集的特征，ΣREE為62.76×10-6～212.5×10-6，(La/Yb)N值為3.34～16.56，不僅各巖性之間稀土元素的配分沒有很大差異， 在早期巖套和晚期巖套之間元素配分也沒有很大差異：早期巖套的ΣREE為62.76×10-6～217.5×10-6，(La/Yb)N值為3.34～11.25，晚期巖套的ΣREE為137.5×10-6～214.7×10-6，(La/Yb)N值為5.12～16.56；這種相似性暗示了兩者的同源性。大部分樣品中Eu異常表現(xiàn)得不明顯，為0.69～1.34，僅個別樣品δEu低至0.48，這既可能指示形成雜巖體的巖漿未經(jīng)過斜長石的結(jié)晶分異過程，也可能僅僅指示巖漿上侵時氧逸度較高。雜巖體整體Sr/Y值較低，早期巖套中絕大多數(shù)樣品的Sr/Y值范圍為7.75～38.33，僅有2個樣品的Sr/Y值較高為52.19～65.49；晚期巖套中絕大多數(shù)樣品的Sr/Y值范圍為3.22～48.2，僅有1個樣品Sr/Y值較高為59.74。在微量元素配分圖解中，不同巖性的巖石都明顯虧損Ta、Nb、Ti，輕微虧損Y，輕微富集Ba，顯示了殼源物質(zhì)與巖漿源區(qū)具有一定程度的聯(lián)系。

表2西石門雜巖體巖全巖主量元素(wt%)和微量元素(×10-6)分析測試結(jié)果

Table 2 Contents of major(wt%)and trace(×10-6)elements of the Xishimen Complex

樣品號B2595-1AB2595-1CB2592B2594-6B2596aB6001B6004B6026B6026-1aB6029B6029-1B6280巖性斑狀斜長二長巖斑狀角閃二長巖斑狀角閃正長巖SiO263.6763.3566.8666.2766.8859.8359.9158.6157.0060.1659.5861.49TiO20.460.490.320.350.290.610.670.600.690.590.640.59Al2O316.5916.0916.4716.7016.4617.6617.4417.8817.6218.0417.8017.28Fe2O33.092.902.352.872.243.633.824.654.873.493.632.90FeO1.261.790.520.320.742.052.321.482.620.920.981.25MnO0.1000.1200.0510.0530.0700.0700.0950.0490.0590.0870.0770.095MgO1.381.580.630.610.841.792.371.912.241.261.461.22CaO2.932.941.491.581.243.382.923.853.683.564.113.20Na2O5.064.765.095.005.195.645.665.935.605.725.625.76K2O4.344.865.304.965.023.613.013.383.864.694.494.70P2O50.200.190.120.130.130.280.300.380.450.280.320.27Total99.0999.0699.2098.8499.198.5698.4898.7298.7098.8098.7198.76Li28.3026.7010.7013.908.217.707.374.477.798.6311.209.91Be1.841.772.571.642.031.451.412.062.012.121.712.19Sc13.2013.908.879.609.449.5811.2010.6013.409.1110.4010.10V74.074.146.650.646.288.4103.0110.0133.087.795.290.2Cr28.2037.408.1717.6016.208.9911.909.047.804.515.409.22Co8.7510.504.024.705.3910.508.819.8713.904.434.678.00Ni15.1017.203.807.707.294.975.273.084.212.802.854.76Cu14.0023.2029.7022.307.723.494.572.814.542.257.908.53Zn86.478.153.782.371.731.735.119.025.327.729.523.4Ga19.717.819.718.218.621.421.020.721.321.721.322.3Ge1.111.061.010.870.93///////As1.732.071.381.411.871.050.920.610.681.341.451.23Rb98.6110.0126.0114.0125.060.855.040.758.664.966.069.3Sr600491293418351586620833690491506508Y18.618.614.115.812.316.317.619.821.018.219.019.9Zr253152241171140149149164148181205195Nb7.457.287.626.925.997.717.766.746.707.418.249.23Mo0.590.580.480.560.470.260.160.110.120.170.160.17Ag0.630.140.230.350.09///////Cd0.050.050.030.050.04////0.050.050.05In0.030.030.030.030.02///////Sn0.050.050.050.060.061.150.981.051.332.091.171.29Sb0.190.200.240.320.250.070.10/0.070.061.830.11Cs1.351.411.671.461.010.280.550.190.130.200.200.18Ba7386894908828901096130886982510109441080La33.931.733.830.431.027.527.227.532.127.025.532.0Ce65.062.859.858.854.955.653.362.871.359.158.066.2Pr7.527.446.406.815.736.917.368.068.927.607.578.13Nd27.126.821.925.220.627.629.932.234.129.629.932.3Sm4.514.643.644.463.335.185.675.906.325.485.895.95Eu1.101.080.941.050.951.631.801.851.961.791.951.97Gd3.924.133.103.842.924.124.504.635.354.524.884.96Tb0.530.560.430.530.400.600.650.710.770.660.700.71Dy2.883.032.342.862.133.303.634.014.163.473.793.83Ho0.570.600.460.550.430.650.680.760.840.690.740.76Er1.711.771.441.541.241.721.822.202.171.791.871.99

續(xù)表2

Continued Table 2

樣品號B2595-1AB2595-1CB2592B2594-6B2596aB6001B6004B6026B6026-1aB6029B6029-1B6280巖性斑狀斜長二長巖斑狀角閃二長巖斑狀角閃正長巖Tm0.250.240.210.220.180.250.270.320.330.280.290.31Yb1.711.611.461.441.181.731.822.122.281.841.962.07Lu0.240.220.220.200.180.260.260.310.350.270.290.30Hf5.313.615.634.073.564.274.364.394.424.715.255.22Ta0.470.480.450.470.390.550.550.380.360.450.430.52W0.690.680.490.630.910.380.390.240.170.240.150.29Tl0.660.710.870.800.860.150.120.100.150.180.190.17Pb21.0017.0015.2013.7016.305.135.564.244.386.665.5110.20Bi0.050.060.080.090.07///////Th4.173.796.204.754.893.483.713.633.654.043.784.58U0.860.780.990.950.970.990.760.830.890.850.811.10δEu0.810.760.860.780.931.091.101.091.041.111.121.12Sr/Y32.2626.420.7826.4628.5435.9535.2342.0732.8626.9826.6325.53(La/Yb)N12.0111.9314.0312.7915.929.639.067.868.538.897.889.37樣品號B6298-1B6282B6390B6475B6692PM3-3b1PM3-3b2PM3-6b1B6391B6595B6932-1B6288-2巖性斑狀斜長二長巖斑狀角閃二長巖斑狀角閃正長巖正長巖SiO260.5458.8658.4760.6358.1860.1660.1759.7261.2558.7667.5163.98TiO20.570.490.650.530.670.490.480.450.530.620.280.47Al2O317.4317.8817.4517.9516.8918.1718.3918.5617.5417.0216.5917.36Fe2O33.742.103.203.683.912.463.414.221.532.861.360.72FeO2.132.021.581.082.090.870.870.650.951.640.431.38MnO0.0790.0670.1100.1000.1100.1300.0830.0440.1300.0870.0240.085MgO1.882.062.551.652.622.633.370.721.492.100.621.18CaO4.385.844.822.994.062.231.532.834.705.580.914.39Na2O4.968.306.077.225.427.008.015.087.765.266.156.18K2O3.060.462.582.734.313.091.394.992.114.385.603.37P2O50.280.270.400.270.450.260.280.360.270.430.080.20Total99.0598.3697.8998.8498.7397.4897.9897.6398.2698.7399.5599.32Li5.966.1334.1013.6019.2061.1068.4022.9015.4017.302.954.31Be1.682.542.192.652.652.382.481.982.542.441.392.30Sc9.245.9112.508.2112.3014.5013.0011.6010.5012.603.137.64V89.275.1123.070.5131.044.058.286.151.5131.029.553.3Cr16.907.389.895.2314.408.555.403.9910.409.865.6010.10Co11.004.117.945.469.4915.205.915.894.997.529.484.71Ni5.962.345.522.124.788.024.154.762.815.781.412.72Cu2.733.376.284.414.24114.005.438.985.365.1110.902.37Zn23.813.948.041.033.873.855.020.744.329.87.921.5Ga22.319.819.823.121.321.021.021.721.221.419.821.9Ge/////1.151.221.00////As0.621.360.982.202.272.081.413.091.452.150.760.99Rb55.220.545.554.878.980.939.5105.034.566.189.841.4Sr80564236345561535134250727164282616Y16.720.419.522.518.220.720.218.715.719.925.423.1Zr169169181205168157155146174114384308Nb8.006.766.897.266.895.785.475.705.766.2611.4012.00Mo0.150.160.120.110.190.240.120.210.100.130.330.33Ag/////0.110.070.07////Cd//0.060.07/0.030.030.030.08///

續(xù)表2

Continued Table 2

樣品號B6298-1B6282B6390B6475B6692PM3-3b1PM3-3b2PM3-6b1B6391B6595B6932-1B6288-2巖性斑狀斜長二長巖斑狀角閃二長巖斑狀角閃正長巖正長巖In/0.060.060.080.070.070.050.030.060.06//Sn1.210.991.151.261.100.060.040.041.231.181.281.51Sb0.05/0.230.080.051.570.160.180.070.060.060.05Cs0.255.9110.200.150.170.160.110.750.110.150.100.16Ba169435077467281512305139291097986353984La33.520.230.830.330.231.926.428.235.531.831.235.9Ce60.347.265.166.563.659.854.152.874.368.481.981.4Pr7.395.607.888.436.527.256.576.438.748.779.1910.10Nd28.823.828.631.927.126.224.424.530.235.534.838.9Sm5.274.955.375.715.384.854.684.545.286.506.166.88Eu1.771.641.701.711.721.541.441.431.732.080.931.36Gd4.204.354.905.174.734.214.024.104.195.435.845.27Tb0.610.660.680.780.650.590.590.590.610.810.870.81Dy3.233.893.664.033.513.463.363.313.183.984.924.45Ho0.660.790.750.840.710.690.680.650.620.771.020.88Er1.732.151.992.491.862.051.941.961.682.182.922.34Tm0.280.350.310.390.290.300.280.280.270.320.510.38Yb1.832.392.112.531.871.951.901.891.792.063.392.56Lu0.290.380.320.410.290.300.280.280.270.310.520.38Hf4.524.454.695.144.463.693.483.474.663.669.178.05Ta0.440.370.390.370.350.350.320.360.480.330.720.64W0.190.190.410.170.225.490.340.640.350.190.130.23Tl0.16/0.090.120.200.120.060.330.100.130.120.11Pb5.314.025.155.653.182.691.904.486.194.321.693.91Bi/////0.130.040.040.05/0.22/Th4.462.934.003.424.003.332.983.864.853.347.747.68U1.371.040.881.001.170.890.811.140.961.231.361.14δEu1.161.091.020.971.051.051.021.021.131.080.480.69Sr/Y48.231.4718.6220.2233.7916.9616.9327.1117.2632.263.2226.67(La/Yb)N11.095.128.857.269.799.918.429.0412.029.365.588.50樣品號B6032B6018B6029-1B6299B6925B6071-2B6023-1B6072B6073-1B6077B6294B6296巖性斑狀角閃二長巖正長巖二長巖二長閃長巖SiO260.1162.6662.2957.6568.5664.6552.1561.2259.6851.7252.1364.94TiO20.560.370.480.510.220.400.750.660.710.820.840.19Al2O317.6817.8817.1417.7117.0516.7216.6516.5816.8617.2417.1915.83Fe2O32.802.391.033.111.022.442.593.073.535.125.590.60FeO1.020.881.620.450.081.382.652.883.033.723.150.03MnO0.1100.0500.0780.0270.0230.0960.0710.1400.1600.1200.0980.024MgO1.371.441.580.690.171.053.411.621.913.803.770.21CaO4.992.243.994.580.582.659.024.024.036.557.245.01Na2O6.035.466.015.875.654.807.424.584.594.834.607.92K2O3.595.304.644.666.004.880.943.844.012.332.290.56P2O50.340.200.240.260.0210.180.540.330.350.360.380.062

續(xù)表2

Continued Table 2

樣品號B6032B6018B6029-1B6299B6925B6071-2B6023-1B6072B6073-1B6077B6294B6296巖性斑狀角閃二長巖正長巖二長巖二長閃長巖Li15.0011.006.2414.703.1427.909.0818.0015.0026.8011.0011.70Be1.782.022.161.521.473.082.212.882.531.251.171.91Sc15.304.648.169.241.916.8214.7011.2012.1019.0018.501.70V73.647.862.787.314.759.3123.094.0104.0190.0203.012.2Cr6.575.9110.4020.303.437.769.468.498.5023.4025.902.08Co4.362.384.896.041.156.345.3810.1010.4019.3018.600.98Ni7.811.633.685.121.143.742.433.093.729.3811.300.88Cu7.281.881.979.0913.108.653.574.085.855.664.201.33Zn28.233.116.011.022.761.914.179.185.035.530.61.9Ga20.219.919.521.617.319.213.222.021.920.221.318.0Ge1.07///////////As2.520.790.781.260.770.540.450.500.531.461.670.22Rb49.076.047.679.285.0106.017.8100.093.337.339.16.5Sr37838531522020449943675277845993383Y22.216.216.713.822.715.422.520.820.318.716.910.7Zr16415518415923525713019518510179353Nb5.345.986.787.5710.107.807.129.989.104.604.248.43Mo0.150.160.100.270.430.670.190.450.370.120.110.07Ag0.10///////////Cd0.04///////////In0.04//////0.060.060.060.06/Sn0.051.271.221.091.101.351.711.451.670.800.650.49Sb0.180.07/0.18/0.10/0.070.060.130.12/Cs0.330.250.090.560.311.6211.301.290.830.180.210.23Ba786771741176093588620489795313621317110La24.820.422.726.824.034.732.631.533.220.521.611.4Ce51.245.660.549.573.265.473.867.570.248.043.417.5Pr6.746.057.896.018.927.659.308.258.676.475.742.58Nd26.322.829.524.437.127.836.531.434.127.125.610.4Sm5.094.085.054.496.944.857.016.196.605.615.021.85Eu1.701.151.201.401.151.351.631.751.861.811.850.54Gd4.543.253.943.526.313.995.815.425.824.554.361.73Tb0.650.520.580.500.940.560.840.790.810.680.610.24Dy3.683.003.232.674.922.934.574.104.083.693.411.53Ho0.710.600.620.530.950.590.870.790.800.760.690.37Er1.991.771.801.402.531.632.372.151.991.951.801.23Tm0.300.280.270.230.400.270.340.320.310.300.260.26Yb1.971.971.871.562.351.872.182.182.001.891.712.07Lu0.320.300.280.230.330.290.300.320.300.280.250.36Hf3.784.154.674.096.216.563.765.734.923.142.588.87Ta0.330.400.380.400.790.410.380.580.460.250.220.60W0.370.400.310.540.160.440.230.310.460.240.240.39Tl0.120.220.090.160.290.68/0.600.580.090.10/

續(xù)表2

Continued Table 2

樣品號B6032B6018B6029-1B6299B6925B6071-2B6023-1B6072B6073-1B6077B6294B6296巖性斑狀角閃二長巖正長巖二長巖二長閃長巖Bi0.03///////////Th2.983.383.953.894.345.483.934.463.891.971.995.09U1.030.990.871.100.661.091.360.830.850.400.331.29δEu1.090.970.831.080.530.940.790.930.921.101.220.93Sr/Y17.0323.7718.8615.948.9932.419.3836.1538.3324.5555.217.75(La/Yb)N7.636.287.3610.416.1911.259.068.7610.066.577.663.34樣品號B6007B6028B6392-1B6875aB6920PM3-5b10bPM3-5b14KS0816-1PM3-5b15KS0816-2B6872B6872-2TW2595巖性二長巖二長閃長巖斑狀角閃正長巖SiO261.0156.9661.6356.1357.3955.6457.2965.3454.7461.2757.6458.1466.10TiO20.490.590.570.880.750.780.630.340.610.650.500.490.33Al2O318.6319.0518.1221.0119.6120.0117.416.6617.6216.3814.6014.6016.52Fe2O33.312.673.745.013.706.582.752.292.933.323.212.792.23FeO0.680.720.500.680.820.251.751.450.922.783.222.940.82MnO0.1700.1100.0900.0970.1200.1100.0980.0840.1200.1400.0780.0650.089MgO1.071.260.881.002.110.862.120.841.381.604.884.740.84CaO3.369.252.143.604.273.305.912.1210.414.005.095.692.00Na2O7.536.586.125.515.804.876.084.836.384.685.525.005.16K2O1.980.134.741.200.992.842.845.210.463.860.540.624.83P2O50.260.360.280.550.480.490.400.160.390.320.220.210.13Total98.4997.6998.8195.6896.0395.7297.2899.3195.9798.9995.595.2999.05Li22.9019.105.8434.6054.7028.6027.9015.6023.6022.6011.7013.2024.10Be1.832.122.091.821.791.991.723.251.593.081.021.002.10Sc14.8013.6010.2019.4016.5016.6017.206.2215.409.6313.6014.0011.40V66.576.273.277.569.1109.099.844.269.192.4132.0129.047.7Cr6.265.259.429.237.207.296.556.385.5116.40208.00223.0022.00Co3.667.366.9810.907.127.575.335.534.6511.5022.4019.005.68Ni5.235.704.098.094.197.194.802.284.483.6754.5063.3011.70Cu5.947.435.7123.407.1515.009.0511.106.287.9487.7025.5015.90Zn32.122.248.729.930.032.324.456.223.572.332.630.5110.0Ga21.827.321.822.319.223.821.521.521.422.616.916.617.4Ge1.001.51/1.121.051.191.10/1.00///1.01As2.4411.300.883.292.832.272.680.672.450.760.630.341.31Rb38.01.471.523.118.944.844.4129.06.2101.08.59.0101Sr372219575323320363373436301774548630399Y17.820.715.822.622.222.919.618.322.220.510.59.614.3Zr1481412501691491551432741252101068898Nb5.345.176.546.225.785.964.919.724.8410.303.373.376.93Mo0.160.130.140.330.170.220.250.470.110.590.130.070.61Ag0.100.11/0.150.080.080.12/0.15///0.10Cd0.040.04/0.040.040.060.03/0.03///0.07In0.050.07/0.060.060.060.06/0.060.06//0.03Sn0.060.061.030.050.050.080.041.570.041.500.720.690.06Sb0.190.160.070.750.280.180.200.070.180.070.06/0.35

續(xù)表2

Continued Table 2

樣品號B6007B6028B6392-1B6875aB6920PM3-5b10bPM3-5b14KS0816-1PM3-5b15KS0816-2B6872B6872-2TW2595巖性二長巖二長閃長巖斑狀角閃正長巖Ba65912712077234801110971750168944238305951La21.327.631.336.833.735.128.338.224.837.114.413.634.7Ce42.554.160.571.069.574.454.564.449.470.436.032.264.5Pr5.316.847.367.888.339.456.607.866.298.603.983.516.77Nd20.425.728.030.432.434.026.028.625.732.316.514.924.6Sm3.995.024.865.846.065.785.215.025.265.953.252.954.10Eu1.421.931.701.861.881.841.771.281.681.751.130.990.97Gd3.584.484.245.315.225.114.624.524.655.182.912.463.31Tb0.530.640.560.730.720.710.630.670.650.770.410.380.46Dy2.973.513.034.174.073.963.473.653.674.142.312.042.59Ho0.590.720.610.840.790.760.670.720.730.810.450.410.48Er1.742.051.702.422.312.241.991.972.202.131.151.101.32Tm0.260.290.270.350.320.320.290.340.300.350.190.170.19Yb1.731.921.832.322.092.161.912.161.932.131.191.111.27Lu0.270.280.290.350.310.330.300.340.300.330.190.170.18Hf3.363.256.074.143.573.623.446.723.025.733.102.582.73Ta0.320.300.370.380.320.320.300.560.270.610.180.180.42W0.521.210.180.540.570.320.440.361.020.260.090.073.86Tl0.080.010.320.050.030.070.070.770.010.60//0.82Pb2.333.6610.304.081.748.842.3815.102.1513.102.373.2229.3Bi0.040.06/0.070.100.060.11/0.08///0.11Th2.782.895.043.863.192.952.796.782.795.371.831.924.69U0.880.800.871.571.541.631.261.151.160.820.640.550.91δEu1.151.251.151.031.031.041.110.831.040.971.131.130.81Sr/Y20.910.5836.3914.2914.4115.8519.0323.8313.5637.7652.1965.4927.90(La/Yb)N7.468.7110.379.619.779.858.9810.727.7910.567.337.4316.56樣品號TW2832PM3-5b1aPM3-5b3aPM3-5b4cPM3-5b7bB6033B6068B6073-2B6286B6288B6393B6969B6691巖性斑狀角閃二長巖二長巖SiO260.6257.4660.0758.9856.7859.3960.8359.2757.9855.8856.5358.7455.37TiO20.470.450.620.790.710.710.470.660.650.710.790.600.84Al2O316.0217.6717.0918.5619.7119.4318.4320.2918.4719.0418.1018.1920.48Fe2O33.163.743.633.894.714.042.873.682.614.344.974.665.45FeO2.360.850.750.950.950.380.520.280.501.821.550.981.28MnO0.0920.1800.1600.0770.0880.0790.0660.0910.0760.0700.0860.1500.072MgO3.381.290.901.161.050.780.831.281.201.961.421.371.30CaO5.547.624.613.904.292.963.773.588.605.735.213.674.54Na2O4.195.104.605.534.236.755.476.247.045.894.056.155.65K2O2.603.255.863.313.721.564.970.520.280.734.383.460.60P2O50.230.390.360.420.460.400.280.320.430.440.510.350.55Total98.6797.9998.6797.5696.7096.5098.5096.2397.8496.6197.5998.3296.14Li4.4611.304.4118.4028.9023.9018.1022.0015.7020.5021.2045.0028.10Be1.461.621.371.881.842.221.591.901.882.071.951.702.00Sc14.5014.5014.3014.9015.7016.8013.4016.9018.4016.5015.8014.4018.10V99.783.094.496.9106.074.273.162.370.5124.0131.096.8121.0Cr136.004.344.097.496.987.366.616.376.145.256.718.929.17Co14.706.346.347.056.516.533.149.184.327.546.496.3112.90

續(xù)表2

Continued Table 2

樣品號TW2832PM3-5b1aPM3-5b3aPM3-5b4cPM3-5b7bB6033B6068B6073-2B6286B6288B6393B6969B6691巖性斑狀角閃二長巖二長巖Ni48.804.093.425.835.305.0613.004.828.475.195.518.186.19Cu5.796.836.197.2910.607.2718.609.516.609.3315.8013.9010.70Zn38.424.820.627.429.027.131.625.129.421.025.542.422.1Ga18.524.024.522.522.722.621.620.726.620.422.220.922.6Ge1.111.130.971.211.251.211.021.081.311.981.350.951.51As0.663.952.852.293.122.561.921.662.061.962.726.722.63Rb44.455.7119.047.053.721.169.36.94.410.958.961.09.2Sr902317246383381313452301253391411597422Y15.120.719.625.120.423.815.221.323.020.624.821.427.4Zr95132195179149176155148145154156132167Nb5.654.285.736.545.326.404.725.875.545.485.765.346.65Mo0.150.160.170.280.170.200.130.180.170.180.190.170.32Ag0.180.070.110.080.100.110.090.080.080.070.600.090.08Cd0.040.040.030.050.040.050.040.040.040.030.020.040.03In0.040.070.070.050.050.050.050.040.060.060.060.050.06Sn0.040.040.040.060.050.060.030.050.060.040.050.040.06Sb0.350.270.140.900.250.260.260.200.220.280.200.350.20Cs0.321.711.462.191.661.001.380.881.489.471.700.142.45Ba126066910909631020473108023011163710601000428La27.721.620.334.528.433.916.927.831.932.828.231.739.6Ce52.747.742.168.453.768.433.558.367.761.156.459.075.0Pr6.136.385.228.246.388.384.136.808.417.057.257.229.22Nd22.926.121.833.326.132.617.726.533.727.730.728.737.3Sm4.445.404.336.494.996.203.475.136.085.186.085.487.08Eu1.341.711.461.961.881.891.431.671.971.932.121.652.14Gd3.764.634.035.554.685.353.084.625.504.825.554.886.56Tb0.500.640.580.790.650.760.450.650.770.670.780.660.91Dy2.683.633.314.493.634.152.503.684.283.744.443.745.05Ho0.540.710.680.900.730.810.500.730.810.740.910.741.00Er1.532.122.082.682.172.431.482.092.392.202.622.182.91Tm0.210.290.320.370.310.350.220.300.340.310.370.320.39Yb1.411.932.122.421.992.331.521.872.102.092.382.042.60Lu0.210.290.350.350.300.340.240.270.300.320.340.310.36Hf2.553.214.564.323.774.293.723.463.493.873.863.124.12Ta0.360.300.390.390.320.380.320.420.330.350.330.330.38W7.390.590.940.331.530.670.320.280.500.210.240.467.27Tl0.150.120.300.110.120.060.150.020.010.030.130.110.02Pb7.835.833.448.438.336.938.234.633.705.985.8010.4014.30Bi0.030.110.050.110.140.090.050.380.050.110.090.170.06Th2.732.693.773.673.113.762.923.303.013.113.422.673.69U0.910.741.291.341.091.380.831.671.291.171.360.971.37δEu1.011.051.071.001.201.011.341.051.051.191.120.980.96Sr/Y59.7415.3112.5515.2618.6813.1529.7414.131118.9816.5727.915.4(La/Yb)N11.916.785.808.648.658.826.749.019.219.517.189.429.23

注：“/”表示低于檢測限或未檢測.通過巖石結(jié)構(gòu)來區(qū)分早期和晚期巖套，帶“斑狀”的屬于晚期巖套；相應(yīng)地，不帶“斑狀”的屬于早期巖套

表3西石門雜巖體Sr、Nd同位素數(shù)據(jù)

Table 3 Sr, Nd isotope data of the Xishimen Complex

圖10 西石門雜巖體原始地幔標準化微量元素蛛網(wǎng)圖(標準化值據(jù)McDonough and Sun, 1995)和球粒隕石標準化稀土元素配分圖(標準化值據(jù)Sun and McDonough，1989)Fig.10 Primitive mantle-normalized trace element spider diagrams (normalization values after McDonough and Sun, 1995) and Chondrite-normalized REE patterns (normalization values after Sun and McDonough, 1989) of Xishimen Complex

4.3 同位素地球化學(xué)特征

4.3.1 Sr-Nd同位素地球化學(xué)特征

西石門雜巖體的全巖Sr-Nd同位素數(shù)據(jù)結(jié)果見表3，相關(guān)圖解見圖11。根據(jù)不同巖性分別使用相應(yīng)的代表年齡進行計算，結(jié)果顯示西石門雜巖體總體(87Sr/86Sr)i范圍較低且集中，為0.7063～0.7073，其中晚期巖套的(87Sr/86Sr)i為0.7063～0.7071，早期巖套的(87Sr/86Sr)i為0.7064～0.7073，總體較前者略高。(143Nd/144Nd)i總體范圍較窄為0.5116～0.5118，εNd(t)的范圍為-12.7738～-18.0488，其中晚期巖套的εNd(t)為-18.0488～-12.7738，早期巖套的εNd(t)為-16.9346～-14.9763，變化范圍較前者稍小。

4.3.2 Pb同位素地球化學(xué)特征

西石門雜巖體的全巖Pb同位素測試結(jié)果見表4，相關(guān)圖解見圖12。同樣地，不論是早期巖套還是晚期巖套，Pb同位素比值變化范圍都較窄，其中206Pb/204Pb范圍為17.1204～18.1739，207Pb/204Pb范圍為15.3609～15.4940，208Pb/204Pb范圍為37.4326～38.2946。

4.3.3 鋯石Hf同位素

對已經(jīng)獲得U-Pb年齡的部分鋯石樣品進行了進一步的Hf同位素原位測試，獲得的結(jié)果如表5和圖13所示。對于年齡較小的鋯石樣品，使用206P/238U年齡計算；對于年齡較大的鋯石樣品，使用207Pb/206Pb年齡計算。由于西石門雜巖體的正長巖中未能發(fā)現(xiàn)巖漿鋯石，故采用了鄰區(qū)固鎮(zhèn)雜巖體的正長巖(樣品ZZ0726-2)數(shù)據(jù)作為分析樣本。

二長閃長巖中鋯石的176Hf/177Hf范圍為0.282196～0.2828240，計算獲得εHf(t)=-17.5～4.7，tDM1=1577～730Ma，tDM2=2597～957Ma。二長巖中鋯石的176Hf/177Hf范圍為0.282171～0.282602，計算獲得εHf(t)=-18.4～-3.6，tDM1=1580～1068Ma，tDM2=2672～1557Ma。正長巖中鋯石的176Hf/177Hf范圍為0.282317～0.282429，計算獲得εHf(t)=-13.5～-9.8，tDM1=1289～1398Ma，tDM2=2008～2301Ma。斑狀二長巖中較年輕鋯石的176Hf/177Hf范圍為0.281322～0.281703，計算獲得εHf(t)=-48.6～-35.2，tDM1=2644～2171Ma，tDM2=4866～3890Ma；年齡較大鋯石的176Hf/177Hf范圍為0.281303～0.281370，計算獲得εHf(t)=1.9～5.1，tDM1=2677～2600Ma，tDM2=2888～2708Ma。斑狀正長巖中鋯石176Hf/177Hf范圍為0.281576～0.282161，計算獲得εHf(t)=-36.6～-19.0，tDM1=2324～1585Ma，tDM2=4216～2708Ma。

表4西石門雜巖體的Pb同位素數(shù)據(jù)

Table 4 Pb isotope data of Xishimen Complex

樣品號巖性206Pb/204Pb±2σ207Pb/204Pb±2σ208Pb/204Pb±2σ(206Pb/204Pb)i(207Pb/204Pb)i(208Pb/204Pb)iB6026-1a斑狀角閃二長巖17.7505±415.6503±338.5526±1017.593315.443337.8758B6298-1斑狀斜長二長巖17.5254±115.5910±138.1345±417.382215.400637.5164PM3-3b1斑狀角閃二長巖18.3094±315.6672±238.8650±718.173915.494038.2946PM3-6b1斑狀角閃二長巖17.7244±115.6201±138.4771±417.575315.423837.8352B6073-1二長閃長巖17.4273±315.5976±338.1530±817.280715.401737.5168B6077二長閃長巖17.2605±315.5494±238.0447±617.120415.360937.4326B6288-2正長巖17.6738±215.6121±238.8336±417.530415.422938.2091B6007二長巖18.0481±115.6437±138.6858±417.898415.449838.0494B6028二長巖17.9228±115.6639±138.5943±417.774915.470937.9629B6068二長巖17.5172±215.6071±238.1168±617.365715.405637.4636

表5西石門雜巖體鋯石Hf同位素測試結(jié)果

Table 5 Zircon Hf isotopes of the Xishimen Complex

測點號Age(Ma)176Yb/177Hf2σ176Lu/177Hf2σ176Hf/177Hf2σεHf(0)εHf(t)tDM1 (Ma)tDM2 (Ma)fLu/Hf二長閃長巖(早期巖套)B6077-1.2144.00.1442060.0011840.0054370.0000290.2824070.000029-12.9-10.313522044-0.84-2.2139.90.0925660.0013490.0035360.0000310.2822970.000028-16.8-14.114452341-0.89-3.1141.70.0786330.0061410.0031280.0002000.2821960.000035-20.4-17.515772597-0.91-4.1141.60.0842840.0010150.0036890.0000600.2822270.000023-19.3-16.515552514-0.89-5.1140.40.0816900.0017730.0032840.0000500.2822230.000030-19.4-16.615442530-0.90-6.2141.00.1527390.0022160.0056870.0000540.2824630.000035-10.9-8.412741905-0.83-7.1142.00.2251900.0045380.0085290.0001460.2828400.0000422.44.7730957-0.74-8.1149.70.1931820.0032780.0070400.0001380.2826810.000032-3.2-0.69631350-0.79二長巖(早期巖套)KS0816-1-1.1136.20.1044580.0005880.0039730.0000260.2822700.000034-17.7-15.115032406-0.88-2.1131.90.1181940.0015140.0045090.0000900.2823050.000030-16.5-14.014742317-0.86-3.1131.60.1738900.0016060.0063770.0000280.2824520.000036-11.3-9.013181930-0.81-4.1136.70.0950930.0001270.0034500.0000170.2822680.000029-17.8-15.114852418-0.90-6.1139.00.0603660.0013320.0023710.0000530.2821710.000028-21.3-18.415802672-0.93-7.1134.70.1049170.0018560.0041280.0000500.2823050.000025-16.5-13.914572318-0.88-8.1135.90.1620130.0012200.0062320.0000650.2826020.000030-6.0-3.610681557-0.81正長巖(早期巖套)ZZ0726-2-2.1122.90.1227000.0025000.0047060.0000600.2824290.000033-12.1-9.812892008-0.86-3.1125.30.1083200.0044290.0041060.0001610.2824020.000035-13.1-10.713092081-0.88-10.1128.00.0774020.0010650.0031260.0000310.2823170.000028-16.1-13.513982301-0.91斑狀二長巖(晚期巖套)TW2832-2.1122.90.0249670.0024360.0010080.0000940.2817030.000053-37.8-35.221713890-0.97-7.124940.0124060.0001720.0004840.0000070.2813560.000020-50.15.126102712-0.99-9.124960.0124490.0000830.0004760.0000010.2813380.000017-50.74.526352755-0.99-10.124770.0126840.0004440.0005000.0000160.2813650.000024-49.85.026002708-0.98-12.124690.0099840.0001330.0004050.0000050.2813030.000018-51.92.826772858-0.99-14.124270.0098040.0001050.0003810.0000040.2813050.000021-51.91.926732888-0.99-18.1123.90.0128330.0008520.0005260.0000330.2814660.000045-46.2-43.524654497-0.98-19.1126.00.0072260.0000760.0002980.0000020.2813220.000017-51.3-48.626444866-0.99-20.1126.00.0214010.0016490.0009140.0000700.2815670.000041-42.6-39.923524232-0.97-32.124870.0253390.0005160.0009930.0000190.2813700.000024-49.64.626272744-0.97斑狀正長巖(晚期巖套)TW2595-3.1126.60.0524730.0012470.0019760.0000290.2820990.000030-23.8-21.216672869-0.94-4.1128.40.0197580.0020360.0007780.0000720.2816730.000036-38.9-36.121993966-0.98-5.1129.30.0457310.0009280.0016990.0000170.2820020.000027-27.2-24.617923117-0.95-6.1122.20.0448810.0003680.0016630.0000030.2820990.000024-23.8-21.316532877-0.95-7.1124.30.0162030.0014030.0006550.0000500.2815760.000044-42.3-39.623244216-0.98-8.1126.90.0575210.0017280.0021560.0000610.2821610.000021-21.6-19.015852708-0.94

圖11 西石門雜巖體(87Sr/86Sr)i-(143Nd/144Nd)i圖解(底圖據(jù)Zindler and Hart, 1986) 華北克拉通下地殼數(shù)據(jù)引自Jahn et al.，1999Fig.11 (87Sr/86Sr)i vs.(143Nd/144Nd)i correlation diagram of the Xishimen Complex(after Zindler and Hart, 1986) Data of lower crust of North China Craton from Jahn et al.，1999

圖12 西石門雜巖體208Pb/204Pb-206Pb/204Pb和207Pb/204Pb-206Pb/204Pb圖解(底圖據(jù)Zindler and Hart, 1986) 華北克拉通數(shù)據(jù)引自Ying et al., 2010Fig.12 208Pb/204Pb vs. 206Pb/204Pb and 207Pb/204Pb vs. 206Pb/204Pb correlation diagrams of the Xishimen Complex(after Zindler and Hart, 1986) Data of North China Craton from Ying et al., 2010

圖13 西石門雜巖體鋯石Hf同位素同位素特征Fig.13 Zircon Hf isotope characteristics of the Xishimen Complex

圖14 斑狀角閃二長巖中的斑晶角閃石 (a)樣品PM3-3b1中編號為1的角閃石斑晶，單偏鏡照片；(b)同一顆角閃石斑晶的背散射照片，照片中紅點表示電子探針測點位置，數(shù)字表示測點號. 測試結(jié)果見表6Fig.14 Porphyry amphibole in porphyry amphibole monzonite (a)monopolarized micrograph of No. 1 amphibole phenocryst in Sample PM3-3b1；(b)backscattered micrograph of the same amphibole phenocryst，red dots in the photo indicate the location of the electron probe，and the numbers indicate the series of measurement. The results are shown in Table 6

圖15 西石門雜巖體角閃石結(jié)晶深度頻數(shù)統(tǒng)計圖Fig.15 The histogram of the crystallization depth of amphibole in the Xishimen Complex

5 西石門雜巖體中的角閃石研究

西石門雜巖體各巖性中普遍發(fā)育有角閃石。在早期巖套中，角閃石晶體與其他造巖礦物粒度差異有限，總體表現(xiàn)為平衡結(jié)晶的關(guān)系。在晚期巖套中，角閃石晶體可呈斑晶狀產(chǎn)出，這些斑狀角閃石往往保留有較好的環(huán)帶(圖14)，顯然是早期巖漿房的產(chǎn)物，同時在基質(zhì)中也有大量粒度較小但與周圍造巖礦物呈平衡結(jié)晶關(guān)系的角閃石晶體?？傊?，廣泛發(fā)育的角閃石為直接研究巖體的演化和侵位提供了良好的研究對象(Leakeetal.，1997)。為了反演雜巖體的侵位過程，基于Ridolfietal.(2010)的角閃石單礦物溫壓計對角閃石結(jié)晶的平衡溫度、壓力條件進行估算，該溫壓計對估算火成巖中的角閃石形成條件具有廣泛的適應(yīng)性。根據(jù)西石門雜巖體中角閃石的電子探針分析測試結(jié)果，計算所得的角閃石平衡結(jié)晶環(huán)境如表6、圖15所示。

5.1 早期巖套中的角閃石

早期巖套中的角閃石在Ridolfietal. (2010)分類中屬于鎂質(zhì)普通角閃石和鎂質(zhì)綠鈉閃石。根據(jù)計算，可以歸納出早期巖套中角閃石主要集中形成于兩個物理化學(xué)環(huán)境下：①溫度較高、壓力較大、氧逸度較高、平衡融體含水量也較高，即T=942.22～970.47℃、P=372.05～442.57MPa、lg(fO2)=-10.47～-10.06、H2Omelt=4.84%～6.52%，指示的形成深度為14.05～16.71km；②溫度較低、壓力較低、氧逸度較低、平衡融體含水量也較低，即T=883.66～940.51℃、P=194.95～339.04MPa、lg(fO2)=-11.34～-9.94、H2Omelt=3.33%～5.42%，指示的形成深度為7.36～12.80km。同一巖性中記錄的兩個物理化學(xué)環(huán)境，指示了巖漿上侵過程中經(jīng)歷了多個大規(guī)模結(jié)晶階段，即巖漿曾在多級巖漿房中停留的階段，而后者(環(huán)境②)可能指示了早期巖套的最終定位環(huán)境。

表6西石門雜巖體角閃石元素含量(wt%)及形成環(huán)境計算結(jié)果

Table 6 Chemical composition(wt%)and forming condition of hornblende from the Xishimen elements

測點號SiO2TiO2Al2O3FeOMnOMgOCaONa2OK2OFClTotalT(℃)P(MPa)深度(km)ΔNNOlg(fO2)H2Omelt早期巖套B6073-1-1-143.011.9710.4913.520.2913.1911.132.030.780.15/96.59912.08265.1610.010.86-10.795.32B6073-1-1-241.561.8812.1513.280.2312.8311.502.090.890.10/96.55959.02406.3315.350.65-10.166.13B6073-1-1-343.291.5310.9411.350.1114.8910.992.040.940.25/96.44925.01287.4910.861.48-9.944.76B6073-1-1-541.781.9312.2013.750.1712.6711.362.090.86//96.84953.46405.7115.320.61-10.296.26B6073-1-1-641.911.9212.1913.160.2212.5111.362.140.850.04/96.32951.46412.8615.590.51-10.426.52B6073-1-1-741.641.9612.0713.280.2212.6511.352.200.870.15/96.41954.79400.8815.140.56-10.326.13B6073-1-1-841.622.0311.9413.180.2013.0311.452.170.940.06/96.64958.51384.0214.500.67-10.165.60B6073-1-1-942.671.9611.0613.520.1813.1211.212.020.890.31/96.99925.57304.4111.500.81-10.595.42B6073-1-1-1041.931.9112.2714.330.0712.3910.962.020.910.11/97.01942.22409.8415.480.61-10.476.36B6073-1-1-1142.241.6711.9414.370.0812.8111.542.120.880.02/97.72945.54372.0514.050.71-10.335.96B6068-1-144.322.359.4212.710.1314.1111.142.020.810.16/97.23890.20198.087.481.04-11.014.23B6068-1-243.572.609.8912.300.1314.1811.132.120.890.19/97.05910.49223.858.450.98-10.714.03B6068-1-344.172.249.3212.850.1314.0510.782.050.830.32/96.83883.66194.957.361.12-11.064.09B6288-2-1-242.221.8912.0211.830.0713.5111.602.191.320.380.0196.91960.50390.7914.760.74-10.064.84B6288-2-1-341.492.1612.4713.130.0712.5611.522.301.330.330.0697.28970.47442.5716.710.37-10.255.12B6288-2-1-441.902.1212.3813.280.0212.4211.372.201.220.27/97.12957.70430.3916.250.38-10.455.62B6288-2-1-541.272.1512.4213.330.0912.5411.412.291.260.180.0496.90969.74439.1016.580.42-10.225.20B6288-2-1-641.492.1712.2912.870.0712.4611.612.201.370.470.0196.85967.68431.6416.300.33-10.345.12B6288-2-2-242.342.0210.3514.110.2711.9711.442.071.530.310.1396.62918.64272.8910.310.33-11.203.76B6288-2-2-342.511.9710.2213.990.2812.1811.172.041.640.240.1296.46911.80262.349.910.47-11.183.36B6288-2-2-442.652.0610.4715.020.1611.7911.072.021.750.190.1497.45909.44273.8510.340.35-11.343.33B6288-2-2-542.071.7811.1514.190.3512.6011.362.371.060.560.0397.43937.76318.3112.020.59-10.604.82B6288-2-2-642.261.7211.2813.600.3512.1111.732.351.240.430.0496.97940.51339.0412.800.30-10.845.01B6288-2-2-742.831.8510.5114.220.3812.4911.332.251.230.310.0597.41917.44271.3810.250.55-11.004.28晚期巖套(斑晶)B6026-1a-1-143.381.5211.4411.840.2014.7711.132.160.91//97.41934.54315.1211.901.38-9.875.05B6026-1a-2-141.421.4612.1515.260.2511.4111.232.230.85//96.26942.70419.3215.840.33-10.746.77B6026-1a-2-242.221.5412.2315.540.2311.5411.292.180.90//97.69936.28407.8415.400.34-10.856.72B6026-1a-2-343.101.4911.2912.000.1814.3211.302.161.02//96.95935.12313.6511.851.22-10.024.88B6026-1a-2-444.881.529.6810.330.1315.6411.252.031.03//96.54898.19208.187.861.64-10.273.74B6026-1a-2-542.691.3411.7013.830.1913.1911.172.120.90//97.13931.95349.2413.190.96-10.325.83B6026-1a-3-143.961.2911.2311.720.1314.9211.432.331.05//98.20931.72297.3111.231.36-9.944.52B6026-1a-3-244.141.5710.1513.400.2213.5011.112.210.92//97.22895.76238.339.000.98-10.974.76

續(xù)表6Continued Table 6測點號SiO2TiO2Al2O3FeOMnOMgOCaONa2OK2OFClTotalT(℃)P(MPa)深度(km)ΔNNOlg(fO2)H2OmeltB6026-1a-4-144.831.339.1912.290.1114.0611.261.900.81//95.76872.35191.807.241.26-11.124.77B6026-1a-4-242.951.1411.0712.100.0413.9611.422.080.88//95.65927.10308.7911.661.20-10.175.48B6026-1a-4-342.911.3811.3012.710.0713.7211.462.100.90//96.55932.02321.0112.121.06-10.235.50B6026-1a-4-542.091.4910.3917.930.149.7411.271.811.17//96.02887.99283.0610.69-0.08-12.155.84B6026-1a-4-1040.641.3512.6618.140.119.1010.992.011.10//96.09933.24503.3619.01-0.30-11.507.57B6026-1a-4-1244.641.469.5112.840.1013.7711.231.980.84//96.35878.94206.107.781.13-11.134.83PM3-3b1-1-146.412.036.8913.930.1513.4511.381.860.860.14/97.09819.24108.064.080.94-12.523.23PM3-3b1-1-241.362.0311.7915.720.2311.5611.332.591.150.47/98.24951.64375.7514.190.16-10.794.95PM3-3b1-1-341.712.0612.1713.060.0313.0511.472.221.330.26/97.35964.10404.3115.270.61-10.124.75PM3-3b1-1-442.142.1112.2412.520.1013.7311.562.211.240.46/98.29966.76395.2214.930.80-9.94.77PM3-3b1-1-542.271.0712.1113.430.0913.1911.702.361.200.16/97.56954.89394.0114.880.85-10.045.21PM3-3b1-1-641.632.1112.1312.740.0713.0411.462.211.300.05/96.74964.70405.0015.300.59-10.134.85PM3-3b1-1-741.062.0812.2013.160.0812.7011.352.131.250.31/96.31966.57421.8415.930.56-10.135.13PM3-3b1-1-841.562.0812.3712.780.0913.3111.542.221.290.15/97.39972.43420.5515.880.68-9.914.83PM3-3b1-1-941.892.1112.3912.960.1113.2511.462.291.290.17/97.93968.01416.0215.710.64-10.034.85PM3-3b1-1-1041.821.8812.8114.510.1712.2611.572.191.320.24/98.77965.94459.7017.360.39-10.295.56PM3-3b1-1-1141.132.1212.6214.600.1412.1011.292.281.350.04/97.66968.77453.1717.110.34-10.305.12PM3-3b1-1-1241.972.0712.6413.010.0813.2311.492.241.280.37/98.38969.88437.5216.520.64-9.995.13PM3-3b1-1-1341.982.1212.6013.040.1012.9311.462.251.300.25/98.02967.16439.4216.600.52-10.155.23PM3-3b1-1-1441.422.0612.4313.140.0713.0211.582.231.290.21/97.42973.19430.4016.250.59-9.994.96PM3-3b1-1-1541.992.0712.5613.250.0813.1411.422.241.34//98.07966.87429.0616.200.63-10.054.94PM3-3b1-1-1640.891.9312.6914.520.1111.9811.572.271.290.06/97.30974.53470.8717.780.30-10.255.44PM3-3b1-1-1740.681.9912.8915.180.1011.6611.522.161.400.23/97.81976.11492.7118.610.23-10.295.41PM3-3b1-1-1840.682.0912.8814.760.0911.7911.452.141.320.22/97.43975.96493.3418.630.25-10.265.60PM3-3b1-1-1940.882.0512.7914.310.0911.8411.062.181.300.09/96.58966.45486.5418.380.31-10.355.68PM3-3b1-1-2041.641.8612.9914.740.1012.3611.262.351.350.13/98.77968.19475.5117.960.46-10.185.30PM3-3b1-1-2140.581.9612.7914.780.1311.8311.462.221.370.17/97.30976.99485.7418.340.29-10.225.32PM3-3b1-1-2242.082.1812.4812.910.1113.4411.442.281.300.01/98.22968.57416.8015.740.68-9.974.80PM3-3b1-1-2341.351.7612.6414.840.1212.1311.332.291.260.24/97.96963.83452.1617.080.44-10.285.45PM3-3b1-1-2440.762.0312.8415.150.0911.5211.512.191.28//97.36972.98490.0018.510.16-10.405.76PM3-3b1-1-2541.501.9512.6414.490.0912.1911.352.221.290.19/97.90963.52451.1317.040.40-10.335.45PM3-3b1-1-2641.592.1012.4113.250.1013.3811.472.221.250.20/97.97971.57416.0115.710.74-9.874.81PM3-3b1-1-2741.692.0612.4313.280.0713.3511.502.221.350.31/98.23971.13417.3815.760.72-9.94.63PM3-3b1-1-2840.922.0712.8314.540.0911.6511.262.151.37//96.88968.51491.2718.550.21-10.425.64PM3-3b1-1-2940.422.1012.8915.340.0911.3911.522.211.36//97.31978.41501.1218.930.10-10.375.52PM3-3b1-1-3040.861.9912.8114.920.1011.6811.562.161.380.11/97.56973.53484.7018.310.21-10.355.49PM3-3b1-1-3140.701.9612.8814.700.1611.7011.392.171.250.16/97.06973.41497.0818.770.25-10.305.88PM3-3b1-1-3240.782.0212.8415.120.1111.9111.462.201.300.16/97.89975.11479.5418.110.32-10.225.44PM3-3b1-1-3340.962.0612.7113.760.1012.3911.662.211.340.17/97.37979.54470.5117.770.38-10.095.24PM3-3b1-1-3442.012.0712.3013.220.0912.9011.342.221.260.43/97.84958.88411.5915.540.57-10.255.16PM3-3b1-1-3542.132.0712.5313.620.1012.8311.432.291.260.32/98.55962.52426.3516.100.51-10.245.23PM3-3b1-1-3641.471.8912.4314.400.1212.3011.122.061.510.12/97.42956.91431.2416.290.53-10.314.86PM3-3b1-1-3740.792.0312.6814.370.1012.0611.902.181.380.18/97.67981.92470.5717.770.26-10.175.21PM3-3b1-1-3841.781.9812.6814.360.1213.2011.322.371.350.15/99.30970.53428.1116.170.72-9.914.58PM3-3b1-1-3941.322.0712.5113.400.0712.9911.562.131.320.16/97.53973.81436.3916.480.62-9.954.97

注：“/”表示低于檢測限或未檢測

5.2 晚期巖套中的角閃石

晚期巖套中的斑晶角閃石在Ridoflietal. (2010)分類中屬于鎂質(zhì)普通角閃石、鎂質(zhì)綠鈉閃石和鈣鎂質(zhì)韭閃石。根據(jù)計算，可以歸納出晚期巖套中的斑晶角閃石主要集中形成于3個物理化學(xué)環(huán)境下：③溫度較高、壓力較大、氧逸度較高、平衡融體含水量也較高，即T=933.24～985.60℃、P=375.75～553.87MPa、lg(fO2)=-11.50～-9.87、H2Omelt=4.34%～7.57%，指示的形成深度為14.19～20.92 km，這個環(huán)境條件與早期巖套中角閃石集中形成的環(huán)境①相似；④溫度中等、壓力中等、氧逸度中等、平衡融體含水量也中等，即T=872.35～935.12℃、P=191.80～349.24MPa、lg(fO2)=-12.15～-9.87、H2Omelt=3.74%～5.84%，指示的形成深度為7.24～13.19km，這個環(huán)境條件與早期巖套中角閃石集中形成的環(huán)境②相似；⑤溫度較低、壓力較小、氧逸度較低、平衡融體含水量也較低，即T=819.24～820.65℃、P=108.06～111.00MPa、lg(fO2)=-12.52～-12.35、H2Omelt=3.10%～3.23%，指示的形成深度為4.08～4.19km，這個環(huán)境條件與早期巖套中角閃石集中形成的環(huán)境無法對應(yīng)。晚期巖套斑晶角閃石中記錄的3個角閃石集中形成環(huán)境都可以與早期巖套中角閃石記錄的環(huán)境大致對應(yīng)，前者的高溫、高壓、高氧、富水環(huán)境(環(huán)境③)和中溫、中壓、中氧、含水環(huán)境較(環(huán)境④)后者的范圍略寬。

晚期巖套中的基質(zhì)角閃石在Ridoflietal. (2010)分類中以鎂質(zhì)綠鈉閃石為主。斑狀角閃二長巖中基質(zhì)角閃石的成分范圍較集中，這可能是巖漿冷卻時快速結(jié)晶的結(jié)果。根據(jù)計算，斑狀角閃二長巖中的基質(zhì)角閃石的主要集中形成于一個物理化學(xué)環(huán)境下：⑥溫度較低、壓力較低、氧逸度較低、平衡融體含水量也較低，即T=642.34～700.47℃、P=26.81～40.72MPa、lg(fO2)=-14.97～-13.63、H2Omelt=1.75%～3.62%，指示的形成深度為1.01～1.54km。晚期巖套基質(zhì)角閃石中記錄的低溫、低壓、低氧、低水環(huán)境(環(huán)境⑥)代表了雜巖體整體最后定位的環(huán)境，指示雜巖體最終定位于淺成部位。

所有角閃石測點的深度頻數(shù)統(tǒng)計圖如圖15所示。不同巖性中相同的角閃石集中形成環(huán)境的記錄，指示整個西石門雜巖體經(jīng)歷了復(fù)雜而又相似的巖漿演化定位過程。

6 討論

6.1 巖漿侵位時間

西石門雜巖體的巖性復(fù)雜，根據(jù)TAS圖解對雜巖體的巖性進行了分類(圖7)，其主要為二長閃長巖、二長巖和正長巖。同時根據(jù)野外地質(zhì)調(diào)查的結(jié)果，雜巖體中即使是相同的巖性也存在很大的結(jié)構(gòu)差異，故可再根據(jù)雜巖體的結(jié)構(gòu)，將其細分為具等粒結(jié)構(gòu)的二長閃長巖、二長巖、正長巖及具似斑狀結(jié)構(gòu)的斑狀二長閃長巖和斑狀二長巖，這些巖性之間具有或明或暗的接觸關(guān)系(圖3)，總體而言具有相同結(jié)構(gòu)的巖性之間的侵入接觸關(guān)系比具不同結(jié)構(gòu)的巖性之間的關(guān)系更難以判斷，且具似斑狀結(jié)構(gòu)的巖石較具等粒結(jié)構(gòu)的巖石侵位時間稍晚，即如前所述的晚期巖套確實晚于早期巖套。

精確的鋯石SHRIMP U-Pb測試結(jié)果也與上述野外地質(zhì)現(xiàn)象相符合。雜巖體中二長閃長巖年齡為139.52±0.92Ma，二長巖年齡為135.6±1.1Ma，正長巖年齡為131.7±1.2Ma，它們共同組成了早期巖套，并表現(xiàn)出了隨著巖漿酸性程度增加年齡變小的趨勢。斑狀二長巖年齡為126.1±1.6Ma，斑狀正長巖年齡為124.4±2.1Ma，它們組成了晚期巖套，并同樣地表現(xiàn)出了隨著巖漿酸性程度增加年齡變小的趨勢，晚期巖套中還保留有大量年齡大于2000Ma的捕獲和繼承鋯石。由此得出，西石門雜巖體中，早期巖套的侵位時間為139.52±0.92Ma～131.7±1.2Ma，平均年齡為135.6Ma，晚期巖套的侵位時間為126.1±1.6Ma～124.4±2.1Ma，平均年齡為125.3Ma。西石門雜巖體的整體侵位時間為139.52±0.92Ma～124.4±2.1Ma，這與華北克拉通東部巖漿活動到達頂峰的時間一致(Griffinetal.，1998；朱日祥等，2012；Lietal.，2012；Jinetal.，2015；Niu，2015)。

值得注意的是，盡管早期巖套與晚期巖套具有明顯不同的結(jié)構(gòu)，但它們在主量元素、微量元素或Sr-Nd、Pb和鋯石Hf同位素測試結(jié)果中，均沒有表現(xiàn)出特別明顯的差異，相反卻都表現(xiàn)出了相近的巖漿演化關(guān)系和一定程度的同源性。

6.2 巖漿的源區(qū)和演化

有關(guān)華北克拉通早白堊世殼幔相互作用的引發(fā)機制一直廣受爭議，但不論是拆沉引發(fā)抑或是俯沖板片、地幔對流失穩(wěn)等引發(fā)，在這一時期內(nèi)華北克拉通減薄以及在自山西至山東約1000km、500000km2的范圍內(nèi)留下了大量的火成巖體是不爭的事實，而且這些火成巖體或多或少都經(jīng)歷了殼幔相互作用(趙越，2004；Parfenovetal.，2011；Wangetal.，2011)。由于僅在晚期巖套中發(fā)現(xiàn)了捕獲和繼承鋯石，可以假設(shè)其總體來源與早期巖套不同，顯然是以古老的殼源物質(zhì)為基礎(chǔ)，并且可能代表了新一次深部巖漿(熱)事件或構(gòu)造運動。換句話說，在早期巖套已經(jīng)侵位成為固結(jié)或半固結(jié)狀態(tài)之后，新形成的晚期巖漿才開始侵入，它們兩者可能難以發(fā)生巖漿混合作用。

在(87Sr/86Sr)i-(143Nd/144Nd)i圖解(圖11)中，西石門雜巖體的早期巖套和晚期巖套幾乎重疊在一起，都落在了EMI型地幔與華北下地殼之間，故合理的推論是這兩個巖套的巖漿源區(qū)都是來自EMI型地幔與華北下地殼的混合，即西石門雜巖體兩期巖漿活動的源區(qū)都是來自EMI型地幔與華北下地殼的混合。另一方面，在208Pb/204Pb-206Pb/204Pb和207Pb/204Pb-206Pb/204Pb圖解(圖12)中，西石門雜巖體中絕大部分的樣品卻都落在了華北克拉通范圍內(nèi)，兩種同位素的結(jié)果出現(xiàn)了矛盾。上述殼幔物質(zhì)混合成巖的推論與雜巖體中SiO2(51.20%～68.56%)、MgO(0.17%～4.88%)、Cr(大部分為 2.08×10-6～37.4×10-6)、Ni(大部分為0.88×10-6～37.4×10-6)含量較低，富集LREE、虧損HFSE以及(87Sr/86Sr)i較高(0.7063～0.7073)、(143Nd/144Nd)i較低(0.5116～0.5118)的情況是一致的。

圖16 西石門雜巖體SiO2-微量元素圖解Fig.16 SiO2 vs. minor elements diagrams of the Xishimen Complex

結(jié)合西石門雜巖體中各巖性的侵位形成時機以及捕獲或繼承鋯石的產(chǎn)出情況，可以假設(shè)在約139.52～131.7Ma時，殼源物質(zhì)參與雜巖體的組建的程度有限，雜巖體的主要物質(zhì)來源仍然是富集地幔；直到約126.1～124.4Ma時，殼源物質(zhì)才更直接地大量參與了雜巖體的組建，并在晚期巖套中留下了大量的捕獲或繼承鋯石。但是，這樣的假設(shè)與上文中西石門雜巖體中兩套巖套源區(qū)一致的推論相矛盾，因為殼源物質(zhì)參與程度的不同必然導(dǎo)致各巖性的同位素組成出現(xiàn)差異，特別是在先形成的以二長閃長巖-二長巖-正長巖為主的早期巖套與后形成的以斑狀二長閃長巖-斑狀正長巖為主的晚期巖套之間，然而這種差異并未在同位素組成中體現(xiàn)出來。

故一個更為合理的解釋是，西石門雜巖體中同位素的組成不能指示成巖物質(zhì)來源于典型EMI型地幔與華北克拉通下地殼混合，同時在晚期巖套中才出現(xiàn)的大量捕獲和繼承鋯石也不能指示華北克拉通下地殼物質(zhì)直接大量參與了雜巖體的組裝。雜巖體實際上的巖漿源區(qū)的地球化學(xué)特征應(yīng)當介于典型EMI型地幔與華北克拉通下地殼之間一個較狹窄的區(qū)域，譬如白堊紀富集巖石圈地幔、大別山白堊紀花崗巖、華北克拉通埃達克巖(Chenetal.，2013)等理論端元，這個區(qū)域的形成可能與華北克拉通自中生代以來長期遭受多方向的碰撞擠壓和俯沖有關(guān)(Nieetal.，1990；Renetal.，2002；Oxman，2003；Zhou and Li，2017；鄭永飛等，2018)。在這個狹窄的區(qū)域內(nèi)，殼幔物質(zhì)趨于混合均勻，但在外界物理化學(xué)條件變化(如俯沖板片脫水、巖石圈拆沉后上彈等)時，不同端元仍會發(fā)生部分熔融，先形成了二長閃長質(zhì)-二長質(zhì)巖漿，并因源區(qū)石榴子石的存在在演化時富集LREE、虧損HFSE。這個演化過程在SiO2-微量元素圖解(圖16)中，表現(xiàn)為在二長閃長巖-二長巖中，SiO2與Sc和V都表現(xiàn)出了連續(xù)的負相關(guān)性，指示二長巖應(yīng)當是二長閃長巖巖漿演化的結(jié)果。而盡管正長質(zhì)巖石也表現(xiàn)出了SiO2與Sc和V的負相關(guān)性，但它與二長閃長巖-二長巖的SiO2含量并不連續(xù)，同樣指示正長質(zhì)巖石來源與二長閃長巖-二長巖不同。在上述整個過程中，盡管不同巖漿活動時部分熔融程度不同，但半封閉巖石圈地幔環(huán)境內(nèi)同位素組成范圍已經(jīng)較小，因此就算各端元部分熔融程度差別很大，也不會對所產(chǎn)生巖漿的同位素組成造成顯著的影響。由此可以在相近的地質(zhì)年代中于同一區(qū)域內(nèi)形成僅在晚期巖套中包含大量捕獲和繼承鋯石，而在其他具相近同位素組成的中酸性巖石中沒有捕獲或繼承鋯石的現(xiàn)象。

圖17 西石門雜巖體Y-Sr/Y圖解(底圖據(jù)Defant and Drummond，1990)Fig.17 Y vs. Sr/Y diagram of Xishimen Complex(after Defant and Drummond，1990)

鄧晉福等(2004)指出，地幔橄欖巖的局部熔融只能產(chǎn)生玄武巖，在含水條件下只能產(chǎn)生拉斑玄武巖或高鎂閃長巖，即鈣堿性安山巖巖漿不能直接來源于地幔橄欖巖，并進一步指出，鈣堿性安山巖巖漿的來源與榴輝巖，即加厚陸殼相關(guān)。值得注意的是在所有的西石門雜巖體樣品中，有兩件二長閃長巖樣品B6872和B6872-2顯示出了較明顯的低SiO2(57.64%～58.14%)、富Cr(208×10-6～223×10-6)、富Ni(54.5×10-6～63.3×10-6)、較高Sr/Y比(52.19～65.49)，同時低MgO(4.74%～4.88%)、輕微富集輕稀土元素((La/Yb)N=7.33～7.43)的特征(圖17)。與此同時，不論是在早期巖套還是在晚期巖套中，都未出現(xiàn)明顯的負Eu異常，這標志著一個加厚陸殼的存在(鄧晉福等，1996)。但它們與一般由板片俯沖產(chǎn)生的埃達克巖特征相比，更加富K2O、87Sr/86Sr>0.704，這些特征都與一般的埃達克巖不符，而與“C型埃達克巖”(張旗等，2001)更接近，暗示西石門雜巖體中早期巖套與晚期巖套都可能與區(qū)域內(nèi)太平洋板片的俯沖關(guān)聯(lián)較弱，它們應(yīng)當是陸內(nèi)巖漿作用的產(chǎn)物，是軟流圈地幔上侵到加厚陸殼底部導(dǎo)致巖石圈底部基性巖部分熔融形成的。陸內(nèi)巖漿作用的開始時間早于早期巖套的定位時間139.5Ma，所產(chǎn)生的二長閃長質(zhì)巖漿在中途巖漿房發(fā)生了些許的輝石和角閃石結(jié)晶分異從而形成了二長質(zhì)巖漿，這些二長質(zhì)巖漿斷續(xù)灌入西石門雜巖體中，從而形成了現(xiàn)在在露頭尺度上或明或暗的接觸關(guān)系。直到約126.1Ma時，另一次強烈的地質(zhì)運動才產(chǎn)生了晚期巖套。

La-La/Sm圖解常用于判別巖漿之間的演化關(guān)系(韓吟文和馬振東，2003)，在西石門雜巖體的La-La/Sm圖解(圖18)中，早期巖套和晚期巖套都分別表現(xiàn)出了明顯的與部分熔融相關(guān)的傾斜趨勢，指示西石門雜巖體中不同的巖漿成分與部分熔融程度關(guān)系更緊密，且其中早期巖套整體趨勢較晚期巖套平坦，可能指示早期巖套的分離結(jié)晶程度較晚期巖套稍高。根據(jù)哈克圖解(圖9)，早期巖套中CaO、TiO2、MgO、FeOT和P2O5都隨SiO2降低，這可能指示早期巖套不僅發(fā)生了輝石和角閃石的結(jié)晶分異，還發(fā)生了鈦鐵礦和磷灰石的分異。

圖18 西石門雜巖體La-La/Sm圖解Fig.18 La vs. La/Sm diagram of Xishimen Complex

6.3 大地構(gòu)造意義

西石門雜巖體中一個顯而易見的“矛盾”在于，具有相同成分、相近源區(qū)和相似巖漿演化過程的多期中酸性巖漿在西石門這同一個區(qū)域內(nèi)產(chǎn)出了兩種結(jié)構(gòu)不同的巖套——具等粒結(jié)構(gòu)的早期巖套和具似斑狀結(jié)構(gòu)的晚期巖套。一般而言，控制巖漿巖不同結(jié)構(gòu)的關(guān)鍵因素在于巖漿就位時的“過冷度”，即巖漿侵入就位的過程中巖漿溫度與圍巖溫度的差(Burgisser and Bergantz，2011)。

在西石門地區(qū)的各類巖石中普遍發(fā)育有角閃石，根據(jù)角閃石形成的物理化學(xué)環(huán)境判斷，形成西石門雜巖體的多源巖漿都經(jīng)歷了多個中途侵位結(jié)晶階段。在早期巖套中，角閃石的大量晶出集中在①T=942.22～970.47℃、P=372.05～442.57MPa和②T=883.66～940.51℃、P=194.95～339.04MPa兩個環(huán)境中，根據(jù)計算所得到的深度分別為14.05～16.71km和7.36～12.80km兩處，前者指示了早期巖套巖漿上侵過程中中途經(jīng)歷的深部巖漿房，后者則指示了早期巖套最終的定位深度，即7.36km的中深部。在晚期巖套中，角閃石的大量晶出主要集中在③T=933.24～985.60℃、P=375.75～553.87MPa，④T=872.35～935.12℃、P=191.80～349.24MPa和⑤T=642.34～820.65℃、P=26.81～111.00MPa三個環(huán)境中，根據(jù)計算所得到的深度分別為14.19～20.92km、7.24～13.19km和1.01～4.19km三處，前兩者也應(yīng)當指示了晚期巖套巖漿上侵過程中中途經(jīng)歷的深部巖漿房，最后的深度同樣指示了晚期巖套最終的定位深度，即1.01km的淺部。即早期巖套的最終定位深度不淺于7.36km，晚期巖套的最終定位深度不淺于1.01km。

盡管角閃石沒有直接的證據(jù)記錄兩次中深部和淺部成巖階段的時間，但鋯石SHRIMP U-Pb年齡已經(jīng)明確指示早期巖套的侵位時間為139.5～131.7Ma，平均年齡為約135.6Ma，晚期巖套的侵位時間為126.1～124.4Ma，平均年齡為約125.3Ma。這兩個不同結(jié)構(gòu)、不同侵位時間的巖套在西石門地區(qū)產(chǎn)出在同一個雜巖體中，指示在約10.3Ma的時間范圍內(nèi)，當時的西石門雜巖體所處的巖漿房被整體由7.36km的中深部快速抬升至1.01km的淺部，抬升了6.35km。即在早期巖套定位完成之后，華北克拉通東部發(fā)生了大規(guī)模的抬升作用，使當時的西石門雜巖體由7.36km的中深處上移至1.01km的淺處，與此同時新的二長質(zhì)和正長質(zhì)巖漿灌入并定位形成了晚期巖套，最終雜巖體整體冷卻后被正長巖巖脈切穿。同時，晚期巖套中殘留的大量捕獲和繼承鋯石也是晚期巖漿快速上升的證據(jù)。

圖19 華北克拉通東部早白堊世巖石圈減薄模式圖Fig.19 The lithospheric thinning model in eastern North China Craton during Early Cretaceous

結(jié)合晚期巖套中含有大量殼源物質(zhì)的正長質(zhì)巖漿的出現(xiàn)，引發(fā)這次抬升運動的原因很可能是深部巖石圈拆沉引發(fā)的淺部巖石圈上彈從而引起的減壓熔融作用。華北克拉通在侏羅紀-白堊紀期間巖石圈至少減薄了120km(Menziesetal.，1993；鄧晉福等，2000)，此時也正是華北克拉通巖漿活動的頂峰，上涌的軟流圈地幔會破壞巖石圈底部密度較高的部分(Hirschmannetal.，2009；Greenetal.，2010)，導(dǎo)致其拆沉脫離上部巖石圈后，上部巖石圈由于平均密度減少會在地幔浮力作用下向上回彈，回彈過程中靜巖壓力逐步減小，引發(fā)地殼巖石圈發(fā)生大規(guī)模的減壓熔融作用。此時由于陸內(nèi)巖漿活動減弱，新形成的二長閃長質(zhì)巖漿非常稀少，先存巖漿房中殘余的巖漿整體進一步向二長質(zhì)巖漿演化。上涌的軟流圈地幔還導(dǎo)致晚期巖套的Pb同位素組成整體較早期巖套的Pb同位素組成略微更靠近EMI型地幔，也導(dǎo)致正長巖中(87Sr/86Sr)i較高(0.7067)、εNd(t)較低，同樣更接近于EMI型地幔。伴隨回彈引發(fā)的巖漿快速上升讓晚期巖漿快速侵位至淺部位置，使得西石門雜巖體出現(xiàn)了具似斑狀結(jié)構(gòu)的晚期巖套，并在其中保留了很多尚未來得及溶解的捕獲和繼承鋯石。華北克拉通東部早白堊世巖石圈減薄模式見圖19。

7 結(jié)論

(1)根據(jù)西石門雜巖體形成時代和巖石結(jié)構(gòu)特征將其劃分為兩套巖石組合，先侵位的具等粒結(jié)構(gòu)的早期巖套和后侵位的具似斑狀結(jié)構(gòu)的晚期巖套。早期巖套由二長閃長巖-二長巖-正長巖系列組成，鋯石U-Pb年齡為139.5～131.7Ma(平均135.6 Ma)；后者由斑狀二長巖-斑狀正長巖系列組成，年齡為126.1～124.4Ma(平均125.3Ma)，此外這些巖石中還存在大量年齡約2500Ma的繼承鋯石。

(2)早期巖套巖石成分為SiO2=51.72%～68.56%、MgO=0.17%～4.88%，部分樣品表現(xiàn)出埃達克巖的特征；晚期巖套巖石成分為SiO2=57.00%～67.51%、MgO=0.61%～3.37%。此外，兩套巖石的微量、稀土元素和同位素組成也很相似，但晚期巖套巖石中鋯石具有更負的εHf(t)，顯示其具有更多的殼源物質(zhì)組成。

(3)角閃石溫度計、壓力計指示，早期巖套定位的溫度、壓力條件為T=883.66℃、P=194.95MPa，相當于7.36km深處。晚期巖套定位的溫度、壓力條件為T=642.34℃、P=26.81MPa，相當于1.01km深處。結(jié)合兩者的侵位時間差，說明在約10.3Myr內(nèi)，地殼被快速抬升了6.35km。

(4)早白堊世華北克拉通東部軟流圈上涌，造成中國東部大規(guī)模巖漿活動。在約135.6Ma時陸內(nèi)巖漿作用開始影響研究區(qū)，軟流圈地幔上涌促使地殼物質(zhì)熔融、混合形成了早期巖套，同時大規(guī)模的巖漿活動及地殼褶皺變形導(dǎo)致陸殼加厚。之后巖石圈地幔發(fā)生拆沉作用，地殼快速抬升。在約125.3Ma后，軟流圈地幔繼續(xù)上涌，回彈的陸殼開始發(fā)生大規(guī)模的熔融形成了晚期巖套。據(jù)此認為拆沉作用是中生代華北克拉通巖石圈減薄的主要機制。