華北克拉通太古宙研究若干進(jìn)展

萬(wàn)渝生,董春艷,頡頏強(qiáng),劉守偈,馬銘株,謝士穩(wěn),任 鵬,孫會(huì)一,劉敦一

中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所北京離子探針中心,北京 100037

華北克拉通太古宙研究若干進(jìn)展

萬(wàn)渝生,董春艷,頡頏強(qiáng),劉守偈,馬銘株,謝士穩(wěn),任 鵬,孫會(huì)一,劉敦一

中國(guó)地質(zhì)科學(xué)院地質(zhì)研究所北京離子探針中心,北京 100037

華北克拉通存在3.8 Ga以上的地質(zhì)演化歷史。本文對(duì)近年來(lái)我們?cè)诎氨?、冀東、魯西、膠東和陰山地區(qū)研究取得的進(jìn)展作了簡(jiǎn)要介紹。在鞍山,深溝寺雜巖獲得～3770 Ma,3600–3660 Ma,～3450 Ma,3310–3330 Ma和 ～3120 Ma鋯石年齡,與白家墳雜巖和東山雜巖的巖漿事件十分類(lèi)似。在冀東,對(duì)曹莊雜巖副變質(zhì)斜長(zhǎng)角閃巖和石榴黑云片麻巖定年,發(fā)現(xiàn)大量3.5–3.8 Ga 碎屑鋯石,并獲得 ～2.5 Ga變質(zhì)鋯石年齡,冀東地區(qū)很可能存在始太古代巖石,可把曹莊雜巖形成時(shí)代限定在2.5 Ga和3.4 Ga之間。在魯西,把太古宙基底從東北到西南依次劃分為A、B、C三個(gè)巖帶: A帶主要為新太古代晚期的殼源花崗巖,B帶主要為新太古代早期的巖石,C帶主要為新太古代晚期的新生巖漿巖。魯西是華北克拉通新太古代早期巖石分布最廣泛的地區(qū),也是華北克拉通確認(rèn)新太古代早期和晚期表殼巖系共存的唯一地區(qū)。在膠東,規(guī)模巨大的2.9 Ga巖漿熱事件被識(shí)別出來(lái),它是強(qiáng)烈地幔添加作用的產(chǎn)物?？赡茏鳛閹r漿板底墊托作用的結(jié)果,2.9 Ga巖石與2.7 Ga巖石一道,在新太古代晚期(～2.5 Ga)遭受強(qiáng)烈變質(zhì)改造。在陰山地塊,除2.5 Ga表殼巖和侵入巖外,還存在2.7 Ga英云閃長(zhǎng)巖和2.6 Ga花崗巖。新獲得的資料表明陰山地塊與華北克拉通東部陸塊具有類(lèi)似的早前寒武紀(jì)地質(zhì)演化歷史。還對(duì)華北克拉通太古宙變質(zhì)基底的鋯石年齡和Hf同位素組成及全巖Nd同位素組成進(jìn)行了統(tǒng)計(jì)研究。結(jié)合前人工作,初步總結(jié)了華北克拉通太古宙地質(zhì)演化特點(diǎn)。認(rèn)為華北克拉通與其它克拉通類(lèi)似,新太古代早期—中太古代晚期是陸殼形成增生的主要時(shí)期,但最強(qiáng)烈廣泛的構(gòu)造熱事件存在于新太古代晚期。在我們新提出的構(gòu)造區(qū)劃中,劃分出了三個(gè)古陸塊(>2.6 Ga),即東部古陸塊、南部古陸塊和中部古陸塊。

華北克拉通; 太古宙; 巖漿作用; 變質(zhì)作用; 鋯石定年; Nd-Hf同位素

華北克拉通是中國(guó)規(guī)模最大的克拉通,也是全球存在>3.8 Ga巖石的少數(shù)地區(qū)之一(Liu et al.,1992; Song et al.,1996; Wan et al.,2005,2012a)。>2.6 Ga巖石在華北克拉通廣泛存在(圖1)。然而,華北克拉通以存在強(qiáng)烈新太古代晚期構(gòu)造巖漿熱事件而聞名(沈其韓等,2005; Wan et al.,2011a; Zhai and Santosh,2011)。本文簡(jiǎn)單介紹了近年來(lái)我們?cè)谝恍┑湫偷貐^(qū)取得的主要研究進(jìn)展和華北克拉通鋯石定年和Hf同位素組成分析及全巖Nd同位素組成分析的統(tǒng)計(jì)研究結(jié)果,并對(duì)一些問(wèn)題作了簡(jiǎn)要討論。

1 典型地區(qū)

1.1 鞍本地區(qū)

鞍本(鞍山—本溪)位于華北克拉通東北部,早前寒武紀(jì)基底主要由新太古代晚期表殼巖系和花崗質(zhì)巖石組成。新的研究進(jìn)一步確定新太古代晚期花崗質(zhì)巖石主要為正長(zhǎng)花崗巖(圖2)(Wan et al.,2015a)。正長(zhǎng)花崗巖輕重稀土分異程度存在較大變化,但通常具有強(qiáng)的負(fù)銪異常,全巖Nd同位素和鋯石Hf同位素組成表明它們形成于更古老陸殼物質(zhì)的再循環(huán)作用。表殼巖(鞍山巖群)以不同但通常較小的規(guī)模存在于花崗質(zhì)巖石中,以發(fā)育條帶狀鐵建造(BIF)而聞名,成為我國(guó)最大型鋼鐵基地。近年來(lái)鞍本地區(qū)早前寒武紀(jì)研究的另一重要進(jìn)展是在鞍山東部發(fā)現(xiàn)大范圍分布的中太古代富鉀花崗巖,它們與鞍山地區(qū)鐵架山花崗巖的形成時(shí)代和組成特征十分類(lèi)似,為鐵架山花崗巖的東延部分(圖2)(董春艷等,未發(fā)表數(shù)據(jù))。

鞍山位于鞍本地區(qū)的西南部,已在多處發(fā)現(xiàn)3.8 Ga巖石,包括石英閃長(zhǎng)巖和奧長(zhǎng)花崗巖兩大類(lèi)。白家墳雜巖是最早發(fā)現(xiàn)3.8 Ga巖石(糜棱巖化奧長(zhǎng)花崗巖)的雜巖(Liu et al.,1992),但之后的進(jìn)一步研究表明該雜巖包括了3.1–3.8 Ga糜棱巖化奧長(zhǎng)花崗質(zhì)巖石和其它類(lèi)型巖石,3.8 Ga巖石僅局部存在。東山雜巖也是由不同類(lèi)型3.1–3.8 Ga巖石組成,3.8 Ga變質(zhì)石英閃長(zhǎng)巖空間上與3.8 Ga條帶狀?yuàn)W長(zhǎng)花崗巖共生,其發(fā)現(xiàn)為鞍山地區(qū)增加了一種新的類(lèi)型3.8 Ga巖石(Wan et al.,2005; Liu et al.,2008)。我們對(duì)新發(fā)現(xiàn)的深溝寺雜巖進(jìn)行了深入解剖。該雜巖位于東山雜巖和白家墳雜巖之間。在～50 m的剖面中,分辨出3.1–3.8 Ga等不同時(shí)代的巖石(圖3)。除奧長(zhǎng)花崗質(zhì)巖石外,還存在二長(zhǎng)花崗巖和變質(zhì)基性巖。它們之間的相互關(guān)系清楚。

圖1 華北克拉通早前寒武紀(jì)地質(zhì)簡(jiǎn)圖(給出了>2.6 Ga巖石和鋯石分布)Fig. 1 Simplified geological map of the North China Craton,showing the locations of rocks and zircons of >2.6 Ga

圖2 華北克拉通鞍山—本溪地區(qū)地質(zhì)簡(jiǎn)圖(Wan et al.,2015a)Fig. 2 Geological map of the Anshan–Benxi area,North China Craton (after Wan et al.,2015a)

圖3 鞍山深溝寺雜巖地質(zhì)剖面(Wan et al.,2012a)Fig. 3 Photographic mosaic showing the section of the Shengousi Complex in the Anshan area (after Wan et al.,2012a)

上述三個(gè)雜巖所記錄的構(gòu)造巖漿熱事件十分類(lèi)似(圖4)。最近,Wang等(2015)在鞍山鍋底山地區(qū)發(fā)現(xiàn)了3.8 Ga片麻狀?yuàn)W長(zhǎng)花崗巖,空間上也與3.3 Ga花崗質(zhì)巖石共生?？梢钥隙?.1–3.8 Ga巖石在鞍山地區(qū)曾十分廣泛地分布。值得注意的是,盡管3.8 Ga巖石目前僅在鞍山被發(fā)現(xiàn),但3.8 Ga鋯石在鞍山外圍歪頭山地區(qū)已被大量發(fā)現(xiàn)。3.8 Ga巖石在整個(gè)鞍本地區(qū)無(wú)疑廣泛分布。經(jīng)過(guò)進(jìn)一步工作,有可能在歪頭山地區(qū)發(fā)現(xiàn)3.8 Ga巖石。

1.2 冀東地區(qū)

華北克拉通僅在鞍山地區(qū)發(fā)現(xiàn)3.8 Ga巖石,但冀東地區(qū)曹莊雜巖發(fā)現(xiàn)了大量3.8 Ga碎屑鋯石。曹莊雜巖由角閃巖相-麻粒巖相變質(zhì)花崗質(zhì)巖石和表殼巖組成(圖5),分辨出的最古老巖石為3.3–3.4 Ga花崗質(zhì)巖石(Nutman et al.,2011; 劉守偈等,未發(fā)表資料)。表殼巖包括黑云石榴片麻巖、黑云變粒巖、斜長(zhǎng)角閃巖、大理巖、鈣硅酸鹽巖、鉻云母石英巖和BIF,其形成時(shí)代不是3.5 Ga(Jahn et al.,1987),而僅能限制在2.5–3.4 Ga之間(Liu et al.,2013a; Nutman et al.,2014)。早期的工作在鉻云母石英巖中發(fā)現(xiàn)大量3.55–3.88 Ga碎屑鋯石(Liu et al.,1992; Wu et al.,2005a; Wilde et al.,2008; Nutman et al.,2011),Liu等(2013a)最近在石榴黑云片麻巖和副變質(zhì)斜長(zhǎng)角閃巖中發(fā)現(xiàn)大量3.4–3.8 Ga碎屑鋯石和2.5 Ga變質(zhì)鋯石。圖6a給出了石榴黑云片麻巖的鋯石定年結(jié)果。結(jié)合鉻云母石英巖定年結(jié)果,碎屑鋯石年齡峰值為～3.83 Ga、～3.67 Ga、3.55 Ga和～3.41 Ga(圖6b)。>3.3 Ga碎屑鋯石年齡分布與鞍山地區(qū)巖石的類(lèi)似。然而,不同類(lèi)型變質(zhì)碎屑沉積巖中都存在大量3.4–3.88 Ga碎屑鋯石,表明其物源區(qū)很近,就在冀東地區(qū)。為了發(fā)現(xiàn)這些古老巖石,有必要在冀東地區(qū)開(kāi)展進(jìn)一步工作。

圖4 鞍山地區(qū)白家墳、東山和深溝寺變質(zhì)雜巖的太古宙巖漿作用年齡記錄對(duì)比(Wan et al.,2012a)Fig. 4 Comparison of the Archean magmatic records of the Baijiafen,Dongshan and Shengousi complexes in the Anshan area (after Wan et al.,2012a)

圖5 冀東黃柏峪地區(qū)地質(zhì)簡(jiǎn)圖(據(jù)陳濤,1988; Liu et al.,2013a; Nutman et al.,2011修改)Fig. 5 Simplified geological map of the Huangbaiyu area,eastern Hebei (modified after CHEN,1988; Liu et al.,2013a; Nutman et al.,2011)

圖6 冀東黃柏峪地區(qū)碎屑鋯石定年結(jié)果(Liu et al.,2013a)Fig. 6 Zircon dating results of detrital zircons from metasedimentary rocks in the Huangbaiyu area,eastern Hebei (after Liu et al.,2013a)

1.3 魯西地區(qū)

根據(jù)大量地質(zhì)和鋯石定年結(jié)果,Wan等(2010,2011a)把魯西地區(qū)太古宙基底劃分為三個(gè)帶。A帶:位于東北部,主要由新太古代晚期混合巖和殼源花崗巖組成; B帶,位于中部,主要由新太古代早期巖石組成; C帶,位于西南部,主要由新太古代晚期幔源巖石組成。萬(wàn)渝生等(2015)最近對(duì)B帶和C帶之間界線的位置進(jìn)行了修改(圖7)。

魯西地區(qū)表殼巖零星分布于花崗質(zhì)巖石中,包括泰山巖群、孟家屯巖組和濟(jì)寧巖群。在早期的研究中,泰山巖群被劃分為雁翎關(guān)巖組、柳杭巖組和山草峪巖組,認(rèn)為它們都形成于新太古代早期(曹?chē)?guó)權(quán),1996)。然而,近年來(lái)的研究表明只有雁翎關(guān)巖組和柳杭巖組的下部形成于新太古代早期,而山草峪巖組和柳杭巖組上部形成于新太古代晚期。孟家屯巖組形成于新太古代早期。原認(rèn)為形成于古元古代晚期的濟(jì)寧巖群形成于新太古代晚期。根據(jù)新的資料,把魯西地區(qū)表殼巖作了重新劃分(Wan et al.,2012b)。新太古代早期表殼巖為雁翎關(guān)—柳杭巖系,包括原雁翎關(guān)巖組、原柳杭巖組下部和孟家屯巖組,主要由變質(zhì)基性巖(斜長(zhǎng)角閃巖)和變質(zhì)超基性巖組成,也有少量變質(zhì)碎屑沉積巖。部分變質(zhì)基性巖還保留很好的枕狀構(gòu)造,部分變質(zhì)超基性巖還保留鬣刺結(jié)構(gòu); 新太古代晚期表殼巖為山草峪—濟(jì)寧巖系,包括原山草峪巖組、原柳杭巖組的上部和原濟(jì)寧巖群,主要由黑云變粒巖、云母片巖、BIF、變質(zhì)礫巖和變質(zhì)中酸性火山沉積巖組成,部分黑云變粒巖還保留很好的沉積構(gòu)造。魯西是迄今為止華北克拉通發(fā)現(xiàn)新太古代早期和晚期表殼巖的唯一地區(qū)。

圖7 魯西地區(qū)地質(zhì)圖(Wan et al.,2010,2011a; 萬(wàn)渝生等,2015)Fig. 7 Geological map of western Shandong (after Wan et al.,2010,2011a; WAN et al.,2015)

圖8 魯西新太古代早期巖漿侵入巖的鋯石年齡變化圖(Wan et al.,2015b)Fig. 8 Zircon age variation diagram for early to middle Neoarchean magmatic rocks in western Shandong (after Wan et al.,2015b)

圖9 魯西新太古代晚期巖漿侵入巖的鋯石年齡變化圖(2010)Fig. 9 Zircon age variation diagram for different types of intrusive rocks in western Shandong(2010)

魯西地區(qū)2.6–2.75 Ga花崗質(zhì)巖石廣泛分布,主要出露于B帶(Wan et al.,2011a,2014)?？砂阳斘鞯貐^(qū)新太古代地質(zhì)演化劃分為新太古代早期(2.6–2.75 Ga)和新太古代晚期(2.5–2.6 Ga)兩個(gè)階段,理由如下: 1)存在從2.6–2.75 Ga連續(xù)的巖漿鋯石年齡記錄(圖8); 2) ～2.6 Ga變質(zhì)作用和深熔作用十分發(fā)育(杜利林等,2003,2005; 任鵬等,未發(fā)表資料); 3)～2.7 Ga表殼巖廣泛分布,但～2.6 Ga表殼巖迄今未識(shí)別出來(lái)(Lu et al.,2008; Wan et al.,2012b); 4)存在2.56–2.60 Ga的構(gòu)造巖漿作用“寂寞期”; 5)存在～2.6 Ga基性-超基性巖漿作用。

圖10 膠東地區(qū)地質(zhì)圖(Wan et al.,2015c)Fig. 10 Geological map of eastern Shandong(after Wan et al.,2015c)

新太古代晚期巖漿作用十分強(qiáng)烈,其年齡峰值為2.52–2.53 Ga。C帶中的新太古代晚期巖漿巖包括輝長(zhǎng)巖、閃長(zhǎng)巖、英云閃長(zhǎng)巖、花崗閃長(zhǎng)巖及二長(zhǎng)花崗巖和正長(zhǎng)花崗巖,可見(jiàn)不同類(lèi)型巖石之間的巖漿混合作用。A帶中的殼源花崗巖包括二長(zhǎng)花崗巖和正長(zhǎng)花崗巖,它們空間上與不同類(lèi)型混合巖(深熔巖石)共生,還包裹時(shí)代更老的TTG等類(lèi)型巖石。殼源花崗巖是深熔巖石進(jìn)一步演化的產(chǎn)物,魯西為研究它們兩者之間相互關(guān)系及殼源花崗巖形成過(guò)程提供了很好的實(shí)例。值得注意的是,年齡大于2.53 Ga的巖石通常顯示強(qiáng)烈的變形,而年齡小于2.52 Ga的巖石通常無(wú)變形或僅弱變形(圖9),表明在2.52–2.53 Ga期間魯西地區(qū)構(gòu)造體制發(fā)生了從擠壓向伸展的轉(zhuǎn)換(Wan et al.,2010)。類(lèi)似情況在華北克拉通其它地區(qū)也存在(Wilde et al.,2005; Yang et al.,2013)。

圖11 膠東棲霞地區(qū)中太古代巖石野外照片(Xie et al.,2014)Fig. 11 Field photographs of Mesoarchean rocks in Qixia,eastern Shandong(after Xie et al.,2014)

圖12 陰山西烏蘭不浪地區(qū)地質(zhì)圖(據(jù)董曉杰等,2012; 馬銘株等,2013修改)Fig. 12 Geological map of the Xi Ulanbulang area,Yinshan(modified after DONG et al.,2012a; Ma et al.,2013)

1.4 膠東地區(qū)

圖10給出了膠東地區(qū)2.5 Ga、2.7 Ga和2.9 Ga巖石樣品的空間分布。2.9 Ga巖石主要分布在馬莊河—河西夼一帶,呈近東西向分布,面積達(dá)幾十平方千米。巖石類(lèi)型包括石英閃長(zhǎng)巖、英云閃長(zhǎng)巖和高硅奧長(zhǎng)花崗巖,以英云閃長(zhǎng)巖為主(Xie et al.,2014)。在棲霞東部的周家溝地區(qū),2.9 Ga英云閃長(zhǎng)巖與2.9 Ga高硅奧長(zhǎng)花崗巖直接接觸(圖11)。兩者都遭受強(qiáng)烈變質(zhì)變形,片麻理方向一致。2.7 Ga花崗質(zhì)巖石與2.9 Ga花崗質(zhì)巖石密切共生,但空間分布關(guān)系還不清楚。2.7 Ga花崗質(zhì)巖石主要分布在東部,面積在100 km2以上,呈北東—南西向延伸。巖石類(lèi)型主要為英云閃長(zhǎng)巖。該區(qū)2.5 Ga花崗質(zhì)巖石也廣泛分布。由于遭受新太古代晚期和古元古代晚期變質(zhì)變形和深熔作用強(qiáng)烈改造,2.5 Ga、2.7 Ga和2.9 Ga英云閃長(zhǎng)巖在野外難以相互區(qū)分。

在膠東,早期研究認(rèn)為存在中太古代表殼巖(唐格莊巖群)和新太古代表殼巖(膠東巖群)。實(shí)際上,中太古代表殼巖比以往認(rèn)為的要少得多。只有棲霞黃崖底地區(qū)可能存在少量的2.9 Ga表殼巖,它們以包體形式存在于2.9 Ga英云閃長(zhǎng)巖中(Jahn et al.,2008)。同樣,以往認(rèn)為的新太古代晚期膠東巖群多為變質(zhì)侵入巖,但膠東地區(qū)確實(shí)存在～2.5 Ga表殼巖,其中包括BIF。

1.5 陰山地塊

陰山地塊近年來(lái)研究取得重要進(jìn)展(范宏瑞等,2010; 董曉杰等,2012; Dong et al.,2012; Jian et al.,2012; 劉利等,2012; 馬銘株等,2013; Ma,2013a,2013b,2014a,2014b; Zhang et al.,2014)。其中最重要的是西烏拉不浪地區(qū)2.7 Ga英云閃長(zhǎng)巖的發(fā)現(xiàn),這是西部陸塊發(fā)現(xiàn)2.7 Ga巖石的唯一地區(qū)。英云閃長(zhǎng)巖呈近南北向分布,面積大于10 km2(圖12)。巖石遭受強(qiáng)烈變質(zhì)變形和深熔作用改造,空間上與新太古代晚期興和巖群共生(董曉杰等,2012; 馬銘株等,2013)。圖13給出了陰山地塊早前寒武紀(jì)變質(zhì)基底鋯石年齡直方圖。結(jié)合前人工作,可把陰山地塊早前寒武紀(jì)變質(zhì)基底特征小結(jié)如下: (1)～2.5 Ga巖漿作用十分強(qiáng)烈,但也有～2.6 Ga和～2.7 Ga的TTG花崗質(zhì)巖石存在,并有更古老鋯石存在。(2)～2.5 Ga表殼巖包括不同類(lèi)型變質(zhì)火山巖和火山沉積巖,并有BIF存在; ～2.5 Ga深成侵入巖包括輝長(zhǎng)巖、閃長(zhǎng)巖、TTG和正長(zhǎng)花崗巖等。該區(qū)還未發(fā)現(xiàn)～2.7 Ga表殼巖。(3)2.7–2.8 Ga期間,存在強(qiáng)烈的地幔添加作用。(4)～2.5 Ga巖石主要為～2.7 Ga古老陸殼物質(zhì)殼內(nèi)再循環(huán)作用產(chǎn)物,地幔添加也起了重要作用。(5)存在古元古代中晚期(1.9–2.2 Ga)巖漿作用。(6)發(fā)育～2.5 Ga變質(zhì)作用。(7)發(fā)育古元古代晚期(1.86–1.94 Ga)變質(zhì)作用。以上許多特征與華北克拉通東部陸塊類(lèi)似。一般情況下,僅通過(guò)基底的對(duì)比來(lái)確定其相互關(guān)系較為困難。但是,考慮到新太古代—古元古代構(gòu)造巖漿熱事件的相似性和2.5 Ga構(gòu)造熱事件在全球范圍內(nèi)的有限分布,我們認(rèn)為,華北克拉通西部和東部陸塊太古宙基底的明顯相似性,可能表明它們形成于類(lèi)似的地質(zhì)作用過(guò)程,2.5 Ga左右華北克拉通已成為一個(gè)統(tǒng)一的整體,并一道經(jīng)歷了古元古代地質(zhì)演化歷史。

圖13 陰山地塊早前寒武紀(jì)變質(zhì)基底鋯石年齡直方圖(Wan et al.,2015c)Fig. 13 Age histogram of zircons from early Precambrian rocks in Yinshan Block(after Wan et al.,2015c)

2 鋯石年齡分布和同位素地球化學(xué)

2.1 鋯石年齡分布

Wan等(2011b)收集了華北克拉通早前寒武紀(jì)鋯石年齡,鋯石年齡分布直方圖指出新太古代晚期和古元古代晚期是華北克拉通2個(gè)最重要的構(gòu)造巖漿熱事件時(shí)期。新的研究支持了這一認(rèn)識(shí)。如果把華北克拉通作為一個(gè)整體看待,存在從3.8 Ga到1.8 Ga連續(xù)的鋯石年齡記錄,最顯著的年齡峰值為～2.52 Ga和～1.85 Ga,年齡峰谷為～3.6 Ga、～3.2 Ga、～2.85 Ga、～2.65 Ga、～2.25 Ga和2.0 Ga(圖14)。雖然全球范圍內(nèi)存在～2.3 Ga鋯石年齡峰谷(Condie et al.,2009),但在華北克拉通南緣及其它地區(qū)近年來(lái)發(fā)現(xiàn)了大量這一時(shí)代的巖石(Diwu et al.,2014)。

2.2 全巖Nd同位素組成

Wu等(2005b)總結(jié)了華北克拉通全巖Nd同位素資料,Nd同位素虧損地幔模式年齡直方圖表明2.6–3.0 Ga為陸殼形成主要時(shí)期。在他們收集的樣品中,包括了許多形成時(shí)代為古元古代或更年輕的巖石。為了更好地反映華北克拉通太古宙陸殼形成增生情況,我們主要使用太古宙巖石樣品,但包括了部分2.45–2.5 Ga巖石樣品。把樣品劃分為四個(gè)部分,即變質(zhì)超基性-中性巖、TTG及相關(guān)巖石(例如黑云變粒巖)、殼源花崗巖和變質(zhì)沉積巖。從圖15a看,地幔添加主要時(shí)期為3.8 Ga、3.3–3.35 Ga、3.1 Ga、2.9 Ga、2.8 Ga和2.5–2.75 Ga,這些時(shí)期也存在殼內(nèi)再循環(huán)作用。鞍山地區(qū)一些3.8 Ga巖石具有負(fù)的εNd(t)值,可能表明殼內(nèi)再循環(huán)作用在始太古代就已存在。隨著時(shí)間演化,殼內(nèi)再循環(huán)作用變的更為重要。一些>3.3 Ga巖石樣品具有異常高的εNd(t)值,它們主要來(lái)自鞍山地區(qū)。主要原因可能是: 1)巖石Sm、Nd含量低,分析誤差較大; 2)巖石普遍遭受強(qiáng)烈改造; 3)一些早期取自白家墳雜巖的巖石樣品形成時(shí)代不是3.8 Ga而更年輕。在兩階段Nd模式年齡直方圖中,數(shù)據(jù)主要集中在2.65–2.85 Ga之間,最大年齡峰值為～2.80 Ga,在3.2–3.8 Ga之間數(shù)據(jù)呈高原平臺(tái)形式分布,年齡低谷為～3.2 Ga(圖15b)。與TTG巖石相比,變質(zhì)超基性-中性巖模式年齡通常更為年輕一些。

圖14 華北克拉通太古宙巖石的鋯石年齡直方圖(Wan et al.,2015c)Fig. 14 Age histogram for zircons from the early Precambrian basement of the North China Craton (after Wan et al.,2015c)

圖15 華北克拉通太古宙巖石Nd同位素組成(Wan et al.,2015c)Fig. 15 Nd isotope composition of Archean rocks of the North China Craton (after Wan et al.,2015c)

圖16 華北克拉通太古宙巖石的鋯石Hf同位素組成(Wan et al.,2015c)Fig. 16 Hf isotope composition of zircons from Archean rocks of the North China Craton (after Wan et al.,2015c)

2.3 鋯石Hf同位素組成

Geng等(2012)和Wang和Liu(2012)收集了華北克拉通早前寒武紀(jì)巖石的鋯石Hf同位素組成資料,指出陸殼形成和增生時(shí)期主要為新太古代早期(2.8–2.7 Ga)。我們的數(shù)據(jù)主要來(lái)自太古宙巖石,部分來(lái)自2.45–2.49 Ga巖石,一些碎屑鋯石數(shù)據(jù)來(lái)自古元古代碎屑沉積巖。計(jì)算參數(shù)見(jiàn)Bouvier等(2008)、Griffin等(2000)和S?derlund等(2004)。在εHf(t)-鋯石年齡圖中,對(duì)于巖漿鋯石和變質(zhì)鋯石僅使用207Pb/206Pb加權(quán)平均年齡和上交點(diǎn)年齡,排除了強(qiáng)烈鉛丟失(不諧和度>40％)數(shù)據(jù)。對(duì)于碎屑鋯石,僅使用1σ<20 Ma和不諧和<15％的單個(gè)數(shù)據(jù)。華北克拉通鋯石Hf同位素組成分布特征與全巖Nd同位素組成分布特征類(lèi)似,在3.55–3.8 Ga、3.45 Ga、3.3–3.35 Ga、2.9 Ga和2.5–2.8 Ga存在地幔添加(圖16a)。殼內(nèi)再循環(huán)作用始于3.8 Ga并延續(xù)到3.25 Ga,在2.90–3.25 Ga期間,殼內(nèi)再循環(huán)作用比地幔添加作用更為重要。與全巖Nd同位素組成相比,鋯石Hf同位素組成似乎更能反映其原有特征,揭示出更為明顯的地幔添加和殼內(nèi)再循環(huán)作用。在Hf同位素虧損地幔模式年齡直方圖中,數(shù)據(jù)主要集中在2.85–2.6 Ga之間,年齡峰值為～2.8 Ga(圖16b)。與全巖Nd同位素模式年齡分布相比,鋯石Hf同位素模式年齡分布呈現(xiàn)出更明顯的高原平臺(tái)現(xiàn)象,從3.2 Ga到4.1 Ga出現(xiàn)更大的年齡變化。這種全巖Nd同位素和鋯石Hf同位素模式年齡分布并不意味著地幔添加的連續(xù)發(fā)生,而在相當(dāng)程度上與新生和再循環(huán)物質(zhì)混合有關(guān)。

3 華北克拉通太古宙基底形成演化

3.1 演化特點(diǎn)

1)老于2.8 Ga巖石僅局部發(fā)現(xiàn),例如鞍本、冀東、膠東、魯山和信陽(yáng),所占面積小于華北克拉通太古宙基底出露面積的5％。然而,沉積巖中的> 2.8 Ga碎屑鋯石和巖漿巖中的>2.8 Ga外來(lái)鋯石廣泛分布。所發(fā)現(xiàn)的>3.4 Ga巖石主要為花崗質(zhì)巖石,這些年齡的超基性-基性巖石也應(yīng)存在,但由于缺乏定年對(duì)象難以確定。2.6–2.75 Ga巖漿巖最近在許多地區(qū)都被鑒別出來(lái),2.5–2.55 Ga巖石也比以往認(rèn)為的有更廣泛的分布。

2)只有少量>2.6 Ga表殼巖被識(shí)別出來(lái),包括鞍山地區(qū)的3.35 Ga陳臺(tái)溝表殼巖和2.9–3.0 Ga鐵架山表殼巖、膠東地區(qū)2.9 Ga黃崖底表殼巖、魯山地區(qū)2.82 Ga下太華表殼巖、魯西地區(qū)2.71–2.75 Ga雁翎關(guān)—柳杭巖系。然而。2.51–2.56 Ga表殼巖在華北克拉通幾乎每一個(gè)新太古代巖石出露區(qū)都存在,雖然其比例較小(通常<10％)。玄武質(zhì)和中-酸性火山-沉積巖是新太古代晚期表殼巖的主要巖石類(lèi)型。

3)侵入巖的巖石類(lèi)型隨時(shí)間而變化。3.1–3.8 Ga和2.7–2.9 Ga侵入巖分別主要為奧長(zhǎng)花崗巖和英云閃長(zhǎng)巖,也存在輝長(zhǎng)質(zhì)和閃長(zhǎng)質(zhì)巖石。最古老大規(guī)模分布的富鉀花崗巖是鞍本地區(qū)的鐵架山巖體?；◢忛W長(zhǎng)巖只是在新太古代晚期才大量出現(xiàn),空間上與奧長(zhǎng)花崗巖和英云閃長(zhǎng)巖共生,新太古代晚期殼源花崗巖(主要為二長(zhǎng)花崗巖和正長(zhǎng)花崗巖)廣泛分布。

4)TTG巖石的稀土組成在3.3–3.45 Ga期間發(fā)生大的變化,由輕重稀土分異不強(qiáng)轉(zhuǎn)變?yōu)檩p重稀土強(qiáng)烈分異。這被認(rèn)為是地球冷卻的結(jié)果(Wan et al.,2005),但Moyen(2011)認(rèn)為T(mén)TG巖石稀土組成變化與地球冷卻過(guò)程無(wú)關(guān)。新太古代晚期殼源花崗巖的稀土模式和微量元素組成存在很大變化,表明隨時(shí)代變化物源區(qū)組成及巖漿形成演化過(guò)程變得更為復(fù)雜。

5)從鋯石年齡直方圖看,如果把華北克拉通作為一個(gè)整體看待,從3.8 Ga到2.5 Ga幾乎存在連續(xù)的地質(zhì)記錄,不過(guò)在～3.5 Ga、～3.2 Ga、2.85 Ga和～2.65 Ga存在年齡低谷。Nd-Hf同位素組成特征表明在幾乎每一個(gè)構(gòu)造巖漿熱事件時(shí)期地幔添加和殼內(nèi)再循環(huán)都起來(lái)重要作用。新太古代是華北克拉通陸殼形成增生的最重要時(shí)期。

6)>2.8 Ga變質(zhì)鋯石年齡記錄十分稀少,～2.6 Ga變質(zhì)鋯石年齡在魯西廣泛獲得,最重要的太古宙變質(zhì)事件發(fā)生在～2.5 Ga?？赡鼙砻鞯貧ぶ皇堑搅酥刑糯蟛虐l(fā)生明顯增厚并在新太古代晚期達(dá)到高峰,這也許暗示構(gòu)造體制發(fā)生了重大改變,板塊構(gòu)造在新太古代晚期已起作用。

7)華北克拉通最重要的太古宙礦產(chǎn)是BIF及一些Cu-Zn硫化物礦。盡管BIF形成時(shí)代從古太古代到古元古代都存在,但最重要的BIF形成時(shí)期為新太古代晚期(2.51–2.55 Ga)(萬(wàn)渝生等,2012; 張連昌等,2012)。相對(duì)穩(wěn)定的構(gòu)造環(huán)境看來(lái)是形成大規(guī)模BIF礦產(chǎn)的重要控制因素。華北克拉通金礦主要形成于中生代,但通常認(rèn)為太古宙表殼巖(綠巖帶)是其重要的物源區(qū)(翟明國(guó),2010)。

3.2 華北克拉通陸殼形成主要時(shí)期

華北克拉通在3.8 Ga以前就有陸殼形成的地質(zhì)記錄,但2.8 Ga以前的地質(zhì)記錄已多被后期構(gòu)造熱事件破壞。根據(jù)鋯石年齡直方圖,新太古代晚期是華北克拉通最為重要的構(gòu)造巖漿熱事件時(shí)期,也有許多證據(jù)表明這是陸殼增生的重要時(shí)期(Liu et al.,2009; Li et al.,2010; Wan et al.,2010; Geng et al.,2012; Ma et al.,2012; Wang and Liu,2012)。然而,全巖Nd同位素組成和鋯石Hf同位素組成表明,與新太古代晚期相比,新太古代早期可能是陸殼增生的更重要時(shí)期。新太古代早期巖石記錄相對(duì)較少與后期陸殼物質(zhì)遭受破壞有關(guān)。例如,在魯西地區(qū),～2.5 Ga殼源花崗巖的Nd-Hf同位素組成表明它們?yōu)椤?.7 Ga新生陸殼物質(zhì)再循環(huán)產(chǎn)物。此外,迄今在華北克拉通8個(gè)地區(qū)已發(fā)現(xiàn)新太古代早期巖石(Wan et al.,2014)。要確定是新太古代早期或是新太古代晚期為華北克拉通陸殼增生的最主要時(shí)期,仍需開(kāi)展進(jìn)一步研究。無(wú)論如何,新太古代無(wú)疑是華北克拉通陸殼形成增生的最重要時(shí)期,這與全球其它主要克拉通一致。新太古代陸殼迅猛增生可能表明全球構(gòu)造體制發(fā)生了重大轉(zhuǎn)換,板塊構(gòu)造已起作用,而這又可能與地球從熱向冷的熱狀態(tài)改變有關(guān)。這與前面從變質(zhì)作用時(shí)代演化研究得出的認(rèn)識(shí)一致。

3.3 華北克拉通太古宙基底構(gòu)造區(qū)劃

華北克拉通早前寒武紀(jì)構(gòu)造區(qū)劃主要是針對(duì)新太古代晚期—古元古代變質(zhì)基底,因?yàn)楦爬蠋r石僅零星分布。Zhai和Santosh(2011)與Zhao和Zhai(2013)總結(jié)了前人有關(guān)華北克拉通構(gòu)造早前寒武紀(jì)變質(zhì)基底不同的構(gòu)造劃分方案。

如果華北克拉通在新太古代晚期板塊構(gòu)造體制已起作用,應(yīng)存在一些更為古老的陸塊。前面提到,魯西地區(qū)～2.6 Ga構(gòu)造巖漿熱事件十分發(fā)育,可把2.6 Ga作為新太古代早期和晚期的年代界線。這也可能適用于整個(gè)華北克拉通,原因包括2.56–2.59 Ga巖石和鋯石年齡記錄十分缺乏。根據(jù)古老巖石和鋯石的空間分布,我們?cè)谌A北克拉通劃分出三個(gè)古老陸塊(>2.6 Ga),即東部古陸塊、南部古陸塊和中部古陸塊(圖17; 中國(guó)地質(zhì)科學(xué)院,2015)。

圖17 華北克拉通古陸塊(>2.6 Ga)分布圖(Wan et al.,2015a)Fig. 17 Distribution of ancient (>2.6 Ga) terranes in the North China Craton (after Wan et al.,2015a)

1)東部古陸塊

該古陸塊分布于華北克拉通東部,包括鞍本、冀東、魯西和膠東等地區(qū),是研究程度最高的古陸塊。最古老的巖石和鋯石都主要分布在該古陸塊中。在鞍本,已發(fā)現(xiàn)2.9–3.8 Ga巖石; 在冀東,已發(fā)現(xiàn)3.0–3.4 Ga巖石和3.3–3.8 Ga碎屑鋯石; 在魯西,已發(fā)現(xiàn)大量2.6–2.75 Ga巖石; 在膠東,已發(fā)現(xiàn)2.7–2.9 Ga巖石和更古老鋯石。新太古代早期以來(lái),這些不同地區(qū)之間相互空間關(guān)系可能未發(fā)生重大改變(不考慮郯廬斷裂等的影響)。冀東地區(qū)3.3–3.8 Ga碎屑鋯石來(lái)自近源,不是來(lái)自鞍本地區(qū),雖然兩地中太古代以前的鋯石年齡分布類(lèi)似。除古老巖石和鋯石外,新太古代晚期殼源花崗巖的同位素組成研究也表明古老陸殼物質(zhì)的廣泛分布。值得注意的是,不同地區(qū)2.5 Ga殼源花崗巖同位素組成特征與其早期演化歷史有關(guān)。例如,鞍本地區(qū)廣泛分布的2.5 Ga正長(zhǎng)花崗巖,其全巖εNd(t)和巖漿鋯石εHf(t)都十分的低,而魯西地區(qū)2.5 Ga殼源花崗巖的全巖εNd(t)和巖漿鋯石εHf(t)大都為正值。這是因?yàn)榘氨揪哂泻荛L(zhǎng)的地質(zhì)演化歷史,而魯西大規(guī)模的地幔添加作用發(fā)生在新太古代早期。在魯西,古陸塊在邊界在B帶和C帶之間(圖7)。冀東地區(qū)古陸塊邊界還不清楚,需進(jìn)一步工作來(lái)確定。

2)南部古陸塊

該古陸塊在華北克拉通南緣呈近東西向分布,包括霍邱、信陽(yáng)、魯山、中條等地,已發(fā)現(xiàn)3.65 Ga、2.8 Ga和2.6–2.7 Ga巖石。2.6–3.6 Ga碎屑鋯石和外來(lái)鋯石在更多地區(qū)存在(Liu et al.,2012c)。在該古陸塊西側(cè)的北秦嶺古生代變質(zhì)火山-沉積巖中發(fā)現(xiàn)大量3.5–4.1 Ga鋯石,被認(rèn)為來(lái)自于華北克拉通南緣。華北克拉通南緣存在4.1 Ga以上的演化歷史。>2.6 Ga巖石和鋯石的廣泛發(fā)現(xiàn)是近年來(lái)華北克拉通南緣早前寒武紀(jì)研究取得的重要進(jìn)展。

3)中部古陸塊

該古陸塊分布于華北克拉通中部,包括贊皇、阜平、恒山、張家口、承德等地,已發(fā)現(xiàn)2.7 Ga的TTG花崗質(zhì)巖石,規(guī)模通常不大,但最近路增龍等(2014)在阜平分辨出面積在150 km2以上的TTG巖石。2.5 Ga殼源花崗巖的廣泛分布也表明古老陸殼物質(zhì)的存在。然而,在所有古陸塊中,該古陸塊所發(fā)現(xiàn)的>2.6 Ga古老陸殼物質(zhì)最少,并存在較多2.5 Ga的TTG花崗質(zhì)巖石。仍需進(jìn)一步工作確定該古陸塊是否存在。

除上述三個(gè)古陸塊外,>2.6 Ga巖石和鋯石在其它地區(qū)也存在。在陰山,已發(fā)現(xiàn)面積較大的2.7 Ga英云閃長(zhǎng)巖和更古老的鋯石(Dong et al.,2012; 董曉杰等,2012; Jian et al.,2012; 馬銘株等,2013)。2.5 Ga殼源巖漿巖石的鋯石Hf同位素組成特征表明其來(lái)自2.7 Ga巖石的再循環(huán)。在陰山地塊也可能存在>2.6 Ga古老陸塊。

4 結(jié)論

1)華北克拉通經(jīng)歷了從2.5 Ga到>3.8 Ga長(zhǎng)時(shí)間的復(fù)雜地質(zhì)演化,但>2.8 Ga巖石僅局部存在。鞍本和冀東是已發(fā)現(xiàn)>3.8 Ga巖石和鋯石的最重要地區(qū)。

2)與其它許多克拉通不同,華北克拉通最重要的構(gòu)造熱事件發(fā)生在新太古代晚期。然而,Nd-Hf同位素研究表明最重要的陸殼形成增生時(shí)間為中太古代晚期—新太古代早期,又與其它許多克拉通類(lèi)似。

3)在華北克拉通劃分出了三個(gè)古陸塊(>2.6 Ga),即東部古陸塊、南部古陸塊和中部古陸塊,華北克拉通迄今所發(fā)現(xiàn)古老巖石和鋯石幾乎都分布在這三個(gè)古陸塊中。

致謝: 作為近年來(lái)工作的初步總結(jié),本文涉及到的內(nèi)容多已發(fā)表或即將發(fā)表。在鋯石年齡和Hf同位素及全巖Nd同位素圖解中,使用了大量文獻(xiàn)資料,由于篇幅所限未在參考文獻(xiàn)中一一列出。在工作和研究過(guò)程中,得到許多老師和同行的支持和幫助。深表謝意。

Acknowledgements:

This study was supported by China Geological Survey (Nos. 1212011120151,12120114021301,1212010811033 and 12120115070301),Ministry of Science and Technology (Nos. 2012CB416600 and 2015FY310100) and National Natural Science Foundation of China (Nos. 1172127 and 41472169).

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Some Progress in the Study of Archean Basement of the North China Craton

WAN Yu-sheng,DONG Chun-yan,XIE Hang-qiang,LIU Shou-jie,MA Ming-zhu,XIE Shi-wen,REN Peng,SUN Hui-yi,LIU Dun-yi
Beijing SHRIMP Center,Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037

The North China Craton has experienced a long geological history which can be traced back to ca.3.8 Ga ago. This paper reports some new progress made in recent years in the study of North China Craton in such areas as Anshan-Benxi,eastern Hebei,western Shandong,eastern Shandong and Yinshan. In Anshan,the Shengousi Complex provides evidence of magmatic events occurring at ～3770 Ma,3660–3600 Ma,～3450 Ma,3330–3310 Ma,and ～3120 Ma. These data are similar to data from the Baijiafen and Dongshan complexes in showing protracted,episodic Archaean crustal evolution. In eastern Hebei,detrital and metamorphic zircons from para-amphibolite and garnet-biotite gneiss of the Caozhuang Complex gave ages of 3.8–3.4 Ga and ～2.5 Ga,respectively. Combined with previous work,the new geochronological results suggest that a significant amount of Eoarchean crustal fragments may occur in eastern Hebei and that the sedimentary protoliths of the paragneisses were deposited between 3.4 and 2.5 Ga. In western Shandong,the Archean basement can be divided into three belts: a late Neoarchean crust-derived granite belt in the northeast that consists predominantly of 2525–2490 Ma monzogranite and syenogranite and banded gneisses (Belt A),a middle to early Neoarchean belt in the center which is mainly composed of 2.75–2.60 Ga TTGs and supracrustal rocks (Belt B),and a late Neoarchean belt of juvenile rocks in the southwest that is dominated by granodiorite,gabbro,quartz diorite and tonalite,with some monzogranite and syenogranite (Belt C). Western Shandong is the only area where both early and late Neoarchean supracrustal rocks were identified. In eastern Shandong,a major magmatic event was identified at 2.9 Ga,which resulted in a major period of juvenile crustal addition from the depleted mantle. Together with 2.7 Ga rocks,the 2.9 Ga rocks underwent strong metamorphism at 2.5 Ga,probably as a result of magmatic underplating. In the Yinshan Block,there are 2.7 Ga tonalites and 2.6 Ga granites and older xenocrystic zircons in 2.5 Ga granitoid rocks. Data obtained in recent years indicate that the Yinshan Block is similar in early Precambrian geology to the Eastern Block. The authors also summarized zircon U-Pb dating and Nd-Hf isotopic features of Archean basement of the North China Craton. Combined with previous work,the authors discussed general features of the Archean crust formation and evolution of the North China Craton. It is concluded that the most important production of continental crust occurred during the late Mesoarchean to early Neoarchean period,but the most intensive and widespread tectono-thermal event took place at the end of the Neoarchean. In this new tectonic model,the authors have defined and outlined three ancient terranes (>2.6 Ga),namely Eastern Ancient Terrane,Southern Ancient Terrane and Central Ancient Terrane.

North China Craton; Archean; magmatism; metamorphism; zircon dating; Nd-Hf isotopes

P534.2; P597.3; P539.6

A

10.3975/cagsb.2015.06.01

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2015-06-28; 改回日期: 2015-07-15。責(zé)任編輯: 閆立娟。

萬(wàn)渝生,男,1958年生。博士,研究員。主要從事前寒武紀(jì)地質(zhì)和SHRIMP U-Pb同位素年代學(xué)研究。通訊地址: 100037,北京市西城區(qū)百萬(wàn)莊大街26號(hào)。E-mail: wanyusheng@bjshrimp.cn。