華夏地塊龍泉地區(qū)發(fā)現(xiàn)亞洲最古老的鋯石

邢光福,楊祝良,陳志洪,姜 楊,洪文濤,靳國(guó)棟,余明剛,趙希林,段 政

中國(guó)地質(zhì)調(diào)查局南京地質(zhì)調(diào)查中心,江蘇南京 210016

華夏地塊的前寒武紀(jì)基底的物質(zhì)組成與構(gòu)造演化史極為復(fù)雜,是當(dāng)前華南區(qū)域地質(zhì)調(diào)查與研究工作中的重大地質(zhì)問(wèn)題。最近,筆者在華夏地塊武夷地區(qū)變質(zhì)巖系研究中,對(duì)浙江省西南部龍泉巖群的一個(gè)云母石英片巖樣品(編號(hào):113ZJ-14-4)進(jìn)行了碎屑鋯石 SHRIMP U-Pb定年,測(cè)得年齡譜延續(xù)范圍極大(4100～410 Ma,自冥古宙延續(xù)至早泥盆世初),其中兩顆碎屑鋯石(編號(hào)#8、#123)獲得大于4050 Ma的年齡(巖漿成因核部207Pb/206Pb表面年齡變化范圍約 4200～4104 Ma,諧和年齡 4127～4100 Ma),這是華夏地塊北部首次發(fā)現(xiàn)冥古宙鋯石。兩顆冥古宙碎屑鋯石中,#123鋯石的巖漿成因核部諧和年齡為(4127±4) Ma,可能是亞洲迄今發(fā)現(xiàn)的最古老鋯石;#8鋯石發(fā)育核幔邊結(jié)構(gòu),其巖漿成因核部年齡為(4100±7) Ma,變質(zhì)幔年齡為～4070 Ma,可能是地球上最早的變質(zhì)事件記錄。此項(xiàng)成果于2014年6月3日全文在線發(fā)表于Nature旗下的《Scientific Reports》(DOI:10.1038/srep05143),并有幸入選“中國(guó)地質(zhì)調(diào)查局、中國(guó)地質(zhì)科學(xué)院2014年度地質(zhì)科技十大進(jìn)展”(中國(guó)地質(zhì)科學(xué)院,2015)。

華夏地塊是中國(guó)東部大陸重要的組成部分之一,主要由早古生代變質(zhì)巖以及中生代花崗巖類、火山-沉積巖組成,并出露少量前寒武紀(jì)巖石(包括新元古代火山-沉積巖以及極少量的古元古代片麻巖)。研究區(qū)地處華夏地塊北部的浙西南龍泉地區(qū)(圖1)。區(qū)域上出露的前寒武紀(jì)變質(zhì)巖系大致可以劃分為上、下兩套巖石地層單元:下部為古元古代的八都群,主要巖性為黑云斜長(zhǎng)片麻巖、變粒巖、黑云片巖和斜長(zhǎng)角閃巖,經(jīng)歷了角閃巖相的中高溫區(qū)域變質(zhì)作用和較強(qiáng)烈的混合巖化作用(胡雄健等,1993;甘曉春等,1995);上部為新元古代的龍泉群,主要巖性為變粒巖、云母片巖、綠簾斜長(zhǎng)角閃巖、含鐵石英巖和大理巖等,經(jīng)歷了高綠片巖相到低角閃巖相的區(qū)域變質(zhì)作用(胡雄健等,1993;甘曉春等,1995;陳正宏等,2008)。本文所報(bào)道的冥古宙鋯石來(lái)自于龍泉巖群的云母石英片巖。

1 冥古宙地殼物質(zhì)研究現(xiàn)狀

地球早期的形成和演化一直是國(guó)際地學(xué)界關(guān)注的熱點(diǎn),冥古宙(Hadean Eon,約4510～3850 Ma;陸松年等,2005)的古老巖石與礦物(鋯石)則是重要的研究對(duì)象和信息載體。目前全球范圍內(nèi)冥古宙巖石(≥3.8 Ga)僅在格棱蘭、東南極、北美和中國(guó)華北等少數(shù)地區(qū)發(fā)現(xiàn)(Black et a1.,1986;Kinny et a1.,1986;Liu et a1.,1992;Song et a1.,1996;Wan et a1.,2005),其中加拿大 Wopmay造山帶 Acasta的 4016 Ma片麻巖是地球上已知最古老的巖石(Iizuka et al.,2006),但O’Neil等(2008)報(bào)道加拿大Nuvvuangittuq綠巖帶142Nd/144Nd-147Sm/142Nd等時(shí)線年齡為 4.28 Ga。我國(guó)發(fā)現(xiàn)的最古老巖石在華北陸塊鞍山、冀東等地(≥3.8 Ga),它們的鋯石Hf模式年齡約4.0 Ga(Liu et al.,1992;Wan et al.,2005;萬(wàn)渝生等,2009)。揚(yáng)子陸塊最古老巖石是鄂北地區(qū)的崆嶺雜巖,為 3.2～2.9 Ga的TTG質(zhì)片麻巖和混合巖等(Gao et al.,1999;Qiu et al.,2000;Jiao et al.,2009)。華夏地塊發(fā)現(xiàn)的最古老巖石時(shí)代是浙西南的古元古代變質(zhì)花崗巖類(<2.0 Ga,甘曉春等,1995;Liu et al.,2009;Yu et al.,2009,2012),至今尚未發(fā)現(xiàn)冥古宙巖石。但是,地球上能保存至今的冥古宙巖石極為有限,且通常遭受了后期復(fù)雜的地質(zhì)過(guò)程,直接從中獲取原巖的詳細(xì)信息相對(duì)比較困難;相比之下,地球上的古老鋯石則數(shù)量相對(duì)多、地域相對(duì)廣。因此,古老鋯石就成為了解地球形成初期500 Ma內(nèi)地殼演化過(guò)程的主要研究對(duì)象,Valley等(2014)甚至提出:地球早期一定發(fā)生過(guò)地殼的化學(xué)均一化、以及存在巖漿海等,但并不能直接被記錄下來(lái);地球最早階段的演化中唯一的物質(zhì)證據(jù)就是鋯石。

圖1 研究區(qū)大地構(gòu)造位置(a)、龍泉地區(qū)地質(zhì)簡(jiǎn)圖(b)(據(jù)Xing et al.,2014修改)Fig.1 Geotectonic location (a),simplified geological map of Longquan area (b) (modified after Xing et al.,2014)

迄今為止,全球發(fā)現(xiàn)的>4050 Ma冥古宙鋯石主要分布于岡瓦納大陸上,且絕大多數(shù)見(jiàn)于西澳大利亞的Yilgarn克拉通直徑約300 km的狹小區(qū)域內(nèi),尤其以 Jack Hill最為著名(Compston et al.,1986;Maas et al.,1992;Nelson et al.,2000;Peck et al.,2001;Griffin et a1.,2004;Kemp et al.,2006;Harrison et al.,2007,2008),最老的鋯石SHRIMP U-Pb年齡達(dá)(4404±8) Ma,其原位 O 同位素分析表明當(dāng)時(shí)已存在花崗質(zhì)巖漿活動(dòng)和早期的海洋(Wilde et al.,2001;Harrison et al.,2008),其次為(4348±3) Ma(Cavosie et al.,2004)等。最近,Valley 等(2014)采用當(dāng)前最先進(jìn)的原子探針層析技術(shù)(atom-probe tomography),測(cè)得 Jack Hill一顆碎屑鋯石年齡為4374 Ma,被認(rèn)為這是目前地球上最可靠的、最古老的鋯石年齡,表明地殼在地球誕生 1億年后、即在太陽(yáng)系誕生 1.6億年后即已形成。在 Jack Hill的4.25 Ga碎屑鋯石中還發(fā)現(xiàn)了金剛石包裹體,表明當(dāng)時(shí)陸殼可能已有相當(dāng)厚度(Menneken et al.,2007)。Svetlana等(2010)對(duì)西澳 Pilbara克拉通Dresser組弱變質(zhì)玄武巖和層狀硅質(zhì)巖的147Sm-143Nd同位素分析表明,在地球形成之后100～200 Ma內(nèi),地幔即開(kāi)始發(fā)生初始分異,形成富集的或虧損的儲(chǔ)庫(kù);大體與此同時(shí),在4.3 Ga之前即通過(guò)對(duì)流地幔分異開(kāi)始了地殼的分異作用。此外,在世界上其它地區(qū)也發(fā)現(xiàn)了≥3.8 Ga的碎屑鋯石,例如德國(guó)巴伐利亞?wèn)|北(Gebauer et al.,1989)、美國(guó)懷俄明州北部(Mueller et al.,1998)、巴西圣弗朗西斯科克拉通(Hartmann et al.,2006)、尼泊爾西部高喜馬拉雅地區(qū)(Gehrels et al.,2006)及中亞造山帶內(nèi)哈薩克斯坦北部(Kr?ner et al.,2008)等。

我國(guó)最早在華北鞍山地區(qū)發(fā)現(xiàn)冥古宙碎屑鋯石(SHRIMP U-Pb年齡≥3.8 Ga,此外還有4.0 Ga的Hf同位素虧損地幔模式年齡(Liu et al.,1992;Wan et al.,2005;萬(wàn)渝生等,2009)。近年來(lái),我國(guó)多處發(fā)現(xiàn)了冥古宙鋯石。在西藏地塊,普蘭縣石英片巖中首次發(fā)現(xiàn)一顆SHRIMP U-Pb法年齡為(4102.5±4.3) Ma的碎屑鋯石,在當(dāng)時(shí)是亞洲地區(qū)已知最古老的鋯石,并發(fā)育～3.6 Ga變質(zhì)深熔鋯石增生邊(溫春齊等,2006;Duo et al.,2007)。在華北陸塊,南緣的信陽(yáng)魯山地區(qū)存在 Hf同位素虧損地幔模式年齡達(dá) 3.9～4.0 Ga的碎屑鋯石,冀東鉻云母石英巖中則發(fā)現(xiàn)了～3.85 Ga碎屑鋯石(萬(wàn)渝生等,2009)、以及Hf同位素虧損地幔模式年齡達(dá)3.96 Ga的碎屑鋯石(Wu et al.,2005);北秦嶺奧陶紀(jì)火山巖中發(fā)現(xiàn)一顆冥古宙捕虜鋯石,其LA-ICP-MS U-P年齡為(4079±5) Ma,可能是勞亞大陸發(fā)現(xiàn)的最古老碎屑鋯石(王洪亮等,2007),此后第五春榮等(2010)利用該碎屑鋯石進(jìn)行了SHRIMP U-P法驗(yàn)證,測(cè)得原位年齡為(4080±9) Ma,并原地再取樣分選出 3000多顆鋯石,利用LA-ICP-MS U-Pb法對(duì)所有鋯石逐一測(cè)年分析,新獲得兩顆冥古宙鋯石(年齡分別為(4007±29) Ma、(3908±45) Ma,均屬捕獲變質(zhì)成因鋯石);河西走廊晚泥盆世中寧組砂巖中發(fā)現(xiàn)二顆碎屑鋯石LA-ICP-MS U-Pb年齡分別為(3891±17) Ma和(4022±16) Ma,它們的 Hf同位素特征與 Jack Hills>4.0 Ga鋯石的(Harrsion et al.,2008)非常相近,均來(lái)自大陸地殼區(qū)的花崗質(zhì)巖漿(Yuan et al.,2012);Cui等(2013)報(bào)道了華北陸塊最古老的鋯石捕虜晶LA-ICP-MS U-Pb年齡為4.17 Ga;劉建輝等(2014)則推斷華北陸塊可能還存在～4.1 Ga的地殼增生作用。在揚(yáng)子陸塊,Zhang等(2006)測(cè)得湖北宜昌南華紀(jì)蓮沱組砂巖中一顆碎屑鋯石的SHRIMP U-Pb年齡為3.8 Ga,Hf模式年齡3.96～4.00 Ga。在西伯利亞陸塊南緣的中亞造山帶,Wang等(2013)在東準(zhǔn)噶爾阿爾曼泰蛇綠混雜帶的沉積巖塊中也發(fā)現(xiàn)一顆LA-ICP-MS U-Pb法年齡為4.04 Ga的碎屑鋯石,其εHf(t)值為–5.2,Hf模式年齡達(dá) 4474 Ma。

華夏地塊多處發(fā)現(xiàn)了冥古宙(或Hf模式年齡達(dá)冥古宙)的碎屑鋯石,Yu等(2007)測(cè)得粵北南雄地區(qū)潭溪片麻巖中一顆 LA-ICP-MS U-Pb年齡為3755 Ma的碎屑鋯石有4.07 Ga的Hf模式年齡;Yao等(2011)在贛南奧陶紀(jì)砂巖中發(fā)現(xiàn)一顆 LA-ICP-MS U-Pb年齡為3.96 Ga的碎屑鋯石,另有兩顆碎屑鋯石的 Hf 模式年齡分別為 4068 Ma(U-Pb年齡2435 Ma)、4202 Ma(U-Pb年齡 2649 Ma);Yu等(2012)測(cè)得浙西南八都巖群一顆U-Pb年齡為3.6 Ga的碎屑鋯石有3.99 Ga的Hf模式年齡。Xu等(2012)在廣西西大明山早寒武世砂巖中測(cè)得一顆碎屑鋯石的LA-ICP-MS U-Pb年齡為(4107±29) Ma;Li等(2014)報(bào)道了華夏地塊一顆碎屑鋯石的 Hf同位素模式年齡為～4.19 Ga。

以上說(shuō)明,我國(guó)的華夏地塊、揚(yáng)子陸塊、華北陸塊等不同塊體都可能存在冥古宙地殼殘片,而且可能有類似的早期地質(zhì)演化歷史;同時(shí)也意味著全球可能存在多個(gè)>4.0Ga的物源區(qū)(Zhang et al.,2006;Zheng et al.,2007;萬(wàn)渝生等,2009)。此外,華夏地塊的武夷地區(qū)、云開(kāi)地區(qū)和南嶺地區(qū)均有冥古宙年代信息,說(shuō)明曾經(jīng)在大范圍內(nèi)分布有冥古宙物質(zhì),甚至有找到冥古宙地質(zhì)體殘留的可能性。

圖2 冥古宙鋯石CL圖像(據(jù)Xing et al.,2014修改)Fig.2 CL image of Hadean zircon (modified after Xing et al.,2014)

2 華夏地塊冥古宙鋯石發(fā)現(xiàn)的意義

如前所述,Xing等(2014)在華夏地塊武夷地區(qū)發(fā)現(xiàn)兩顆冥古宙碎屑鋯石(>4100 Ma),其中一顆(#123)為亞洲最古老的鋯石(4127 Ma);另一顆(#8)具有～4100 Ma的巖漿成因核,其核部被一約4070～4060 Ma的變質(zhì)幔部包裹(記錄了全球最早的變質(zhì)年齡),在最外部還有兩個(gè)介于 3800～3600 Ma的增生邊(圖2,3)。此外,我們也注意到這兩顆冥古宙鋯石的O同位素、微量元素、Ti含量及結(jié)晶溫度等,與已知的其它地區(qū)冥古宙鋯石有明顯差異。例如,#8鋯石碎屑鋯石核部的巖漿鋯石有異常高的鈦含量(51.5×10-6～54.0×10-6),利用鋯石的鈦溫度計(jì)求得其結(jié)晶溫度為 910oC,明顯高于地球上已知其它冥古宙鋯石的形成溫度(例如:西澳Jack Hills冥古宙碎屑鋯石的平均溫度約700oC)(Xing et al.,2014)。此外,#8碎屑鋯石核部的巖漿鋯石還有極高的氧同位素組成(δ18O值平均7.2‰),與西澳 Jack Hills冥古宙巖漿成因鋯石中的最高值相似,且高于幔源巖漿鋯石值,說(shuō)明其巖漿源區(qū)有蝕變地殼物質(zhì)加入,或者其基巖在4150 Ma之前與地表水經(jīng)歷了同位素交換。另外,在球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖上,兩顆冥古宙碎屑鋯石的巖漿成因鋯石區(qū)均表現(xiàn)為輕稀土虧損、正Ce異常和負(fù)Eu異常;它們還具有具有很高的高(Sm/La)N和 U/Yb比值,與地球上其它殼源鋯石類似(Xing et al.,2014)。因此,#8號(hào)顆粒核部可能結(jié)晶于淺部地殼沉積物部分熔融所形成的花崗巖類中,這與 Jack Hills中那些冥古宙鋯石相似,同時(shí)期高溫對(duì)應(yīng)的高 Ti暗示其源區(qū)內(nèi)的無(wú)水(干的)熔融過(guò)程,這一特殊的成巖機(jī)制目前還尚未見(jiàn)有報(bào)道。

圖3 冥古宙鋯石SHRIMP U-Pb諧和圖(引自Xing et al.,2014)Fig.3 SHRIMP U-Pb concodia diagram of Hadean zircon (after Xing et al.,2014)

另外,冥古宙鋯石顆粒在中國(guó)境內(nèi)不同時(shí)期的沉積巖中零星分布,表明了冥古宙鋯石可以在多階段的地殼重循環(huán)過(guò)程中得以保留。本次研究共對(duì)龍泉巖群云母石英片巖的214顆鋯石進(jìn)行了235個(gè)點(diǎn)的U-Pb定年(未發(fā)表數(shù)據(jù)),發(fā)現(xiàn)了兩顆>40億年的冥古宙鋯石,表明北武夷地區(qū)的變質(zhì)基底雖然巖石形成時(shí)代普遍<20億年,但冥古宙鋯石含量(接近1%)遠(yuǎn)高于澳大利亞等地區(qū),暗示可能有更多的冥古宙鋯石,甚至可能還殘留太古宙-冥古宙巖石。因此有必要對(duì)華夏地塊龍泉地區(qū)的碎屑鋯石進(jìn)行進(jìn)一步的 U-Pb定年工作,從而確定本次研究的這些具特殊地球化學(xué)性質(zhì)的冥古宙鋯石顆粒是否具有代表性,同時(shí)幫助我們限定地球地殼早期的演化過(guò)程。

3 結(jié)論

本次研究在華夏地塊龍泉地區(qū)的硅質(zhì)巖中首次發(fā)現(xiàn)兩顆～4100 Ma的碎屑鋯石,其中一顆為亞洲最古老的鋯石(4127 Ma),另一顆記錄了全球最早的變質(zhì)事件(～4070 Ma);它們極高的結(jié)晶溫度和異常高的氧逸度,證明冥古宙時(shí)地殼性質(zhì)和構(gòu)造環(huán)境存在多樣性,為認(rèn)識(shí)地球早期大陸演化過(guò)程提供了的新證據(jù)。

致謝:北京SHRIMP中心的萬(wàn)渝生研究員和南京大學(xué)的王孝磊教授在本文成文過(guò)程給予了極大的幫助,在此表示感謝。

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