胡 娟,劉曉春,曲 瑋,崔建軍

中國地質(zhì)科學院地質(zhì)力學研究所,北京 100081

桐柏造山帶古元古代變質(zhì)基性巖的鋯石U-Pb年齡及其地質(zhì)意義

胡 娟,劉曉春*,曲 瑋,崔建軍

中國地質(zhì)科學院地質(zhì)力學研究所,北京 100081

桐柏造山帶位于秦嶺和大別造山帶之間,其北側(cè)對應于北秦嶺中高級變質(zhì)雜巖,南側(cè)對應于大別山高壓/超高壓變質(zhì)雜巖。鋯石U-Pb定年在一個榴輝巖中獲得其原巖侵位年齡和鋯石重結(jié)晶或生長年齡分別為(1961±23) Ma和(1949±51) Ma,在一個石榴角閃巖(退變榴輝巖？)中獲得其變質(zhì)年齡為(1936±26) Ma,表明這兩種基性巖石在二疊-三疊紀高壓變質(zhì)之前經(jīng)歷了古元古代變質(zhì)作用的影響。由桐柏—大別—蘇魯造山帶可利用的年代學資料確定,揚子陸塊北緣發(fā)育的古元古代構(gòu)造熱事件可分為兩幕,第一幕發(fā)生在約1.97～1.93 Ga,可能與Columbia超大陸的聚合有關(guān); 第二幕發(fā)生在約1.85～1.82 Ga,或者代表Columbia超大陸匯聚的繼續(xù),或者是陸塊邊緣增生過程的結(jié)果。

古元古代; 兩幕變質(zhì); 揚子陸塊; 桐柏造山帶

揚子陸塊是中國大陸最重要的古老陸塊之一,了解其早期形成和演化歷史對充分認識中國大陸地殼的組成、構(gòu)造格局的演變乃至全球構(gòu)造事件的對比均具有重要意義。近年來,在揚子陸塊不同地區(qū)的巖石中發(fā)現(xiàn)了一系列2.0～1.8 Ga構(gòu)造熱事件的年代學記錄(凌文黎,1996; Maruyama et al.,1998; 周漢文等,1998; Qiu et al.,2000; 高山等,2001; Ling et al.,2001; Ayers et al.,2002; 吳元保等,2003; 夏群科等,2003; Chen et al.,2003a,b; Yang et al.,2003; 鄭永飛,2003; Bryant et al.,2004; Li et al.,2004; 沈其韓等,2005; Hacker et al.,2006; Liu et al.,2006;Zhang et al.,2006a,b,c; 趙風清等,2006; Zheng et al.,2006; Wu et al.,2006,2008,2009; Liu et al.,2008a; Sun et al.,2008; 熊慶等,2008; 彭敏等,2009;張麗娟等,2011),其中基巖部分主要來自于揚子陸塊北緣的黃陵背斜(崆嶺群)及其附近區(qū)域,以及大別—蘇魯造山帶中的黃土嶺麻粒巖,而碎屑鋯石的年代學信息則遍布揚子陸塊。這有可能說明,至少揚子陸塊的北緣已卷入到古元古代強烈造山作用之中。而且,與華北陸塊一樣,古元古代造山作用在揚子陸塊早期基底形成過程中可能起到了重要作用。

桐柏造山帶西連秦嶺造山帶,東接大別—蘇魯造山帶(圖1),其北側(cè)發(fā)育與北秦嶺相似的古生代中高級變質(zhì)單元,南側(cè)則發(fā)育二疊紀末-三疊紀高壓變質(zhì)單元,屬于大別—蘇魯高壓/超高壓變質(zhì)帶的西延部分(Liu et al.,2008b,2010,2011)。同位素年代學研究表明,高壓變質(zhì)單元中的變質(zhì)沉積巖均含有古元古代巖漿巖或變質(zhì)鋯石的年齡峰值,但榴輝巖的原巖及其伴生的變質(zhì)酸性火成巖多形成于新元古代(Liu et al.,2008b,2010)。進一步詳細的鋯石U-Pb定年在一個榴輝巖和一個石榴角閃巖(退變榴輝巖？)樣品中獲得了古元古代構(gòu)造熱事件的年齡記錄,這也是在揚子陸塊基巖中獲得的不可多得的古元古代年代學信息。本文報道了兩種變質(zhì)基性巖(即鳳凰咀榴輝巖和武勝關(guān)石榴角閃巖)的鋯石 U-Pb同位素年代學結(jié)果,并結(jié)合桐柏造山帶以及整個揚子陸塊中的同期年代學記錄來探討揚子陸塊北緣古元古代構(gòu)造熱事件的意義,以期加深對揚子陸塊早前寒武紀陸殼增生歷史與構(gòu)造演化過程的理解。

1 區(qū)域地質(zhì)背景

桐柏造山帶中的高壓變質(zhì)單元以桐柏雜巖為核心,向北為北部榴輝巖帶,向南為南部榴輝巖帶(圖2),這些構(gòu)造單元基本上可以與西大別高壓/超高壓變質(zhì)地體相對比(Liu et al.,2004a,b)。北部榴輝巖帶主要出露于鴻儀河—固廟—桐柏一帶,向東延伸到譚家河北部,在大地構(gòu)造部位上相當于西大別地區(qū)的滸灣剪切帶,但缺少石炭紀榴輝巖。主要組成巖石為花崗質(zhì)糜棱巖、白云鈉長片麻巖、白云石英片巖和大理巖,榴輝巖及其退變產(chǎn)物石榴角閃巖以團塊、透鏡體或不規(guī)則條帶產(chǎn)于白云鈉長片麻巖和大理巖中。南部榴輝巖帶主要出露于七尖峰花崗巖體以西的黑龍鎮(zhèn)—雙河鎮(zhèn)—大阜山一帶,向東延伸到蔡家河,相當于西大別地區(qū)的紅安巖群。主要由一套變質(zhì)火山巖-沉積巖系組成,包括白云鈉長片麻巖、白云石英片巖和大理巖,榴輝巖也以團塊、透鏡體或不規(guī)則條帶產(chǎn)于白云鈉長片麻巖和大理巖中。此外,該帶還產(chǎn)出有大量的變質(zhì)輝長巖和石榴角閃巖塊體,以大阜山變質(zhì)基性-超基性巖體為代表。北、南兩條榴輝巖帶的峰期變質(zhì)條件分別為530～610°C、1.7～2.0 GPa和 460～560°C、1.3～1.9 GPa(劉曉春等,2005; Liu et al.,2008b)。SHRIMP 鋯石U-Pb以及礦物-全巖Rb-Sr和Lu-Hf定年揭示榴輝巖的原巖形成于新元古代,而峰期變質(zhì)時代為晚二疊世(約 255 Ma)(Liu et al.,2008b; Cheng et al.,2011),明顯早于大別—蘇魯?shù)貐^(qū)的高壓/超高壓變質(zhì)巖(約240～220 Ma)。此外,在變質(zhì)沉積巖的碎屑鋯石中獲得2.49 Ga、1.93 Ga和1.85～1.82 Ga三個年齡峰值,反映其物源來自于古老的揚子地塊。

桐柏雜巖的主體是已強烈糜棱巖化的粗?；◢徺|(zhì)片麻巖,其內(nèi)含有規(guī)模不等的細粒閃長質(zhì)-花崗質(zhì)片麻巖、斜長角閃巖、副片麻巖、大理巖和鈣硅酸鹽巖等捕擄體(包體),這些捕擄體與周圍的糜棱巖化花崗質(zhì)片麻巖一起發(fā)生了塑性變形。Liu等(2010)在這些變質(zhì)巖捕擄體中發(fā)現(xiàn)了退變榴輝巖,礦物反應結(jié)構(gòu)和石榴石成分環(huán)帶指示其經(jīng)歷了綠簾角閃巖相、榴輝巖相和角閃巖相三個演化階段,由相關(guān)系推測榴輝巖相變質(zhì)作用的 P-T范圍位于＜700°C、＞1.2 GPa區(qū)域內(nèi)。對退變榴輝巖、石榴角閃巖和閃長質(zhì)-花崗質(zhì)片麻巖等變質(zhì)火成巖進行的鋯石 U-Pb定年結(jié)果表明(Liu et al.,2010),這些變質(zhì)巖的原巖年齡介于 933 Ma和 742 Ma之間,而變質(zhì)時代為232～220 Ma。變質(zhì)沉積巖的碎屑鋯石則產(chǎn)生2960 Ma和1966 Ma兩組年齡數(shù)據(jù)。占絕對優(yōu)勢的花崗質(zhì)片麻巖是在早白堊世(約 140 Ma)侵位的,而非前人所認為的新元古代(Kr?ner et al.,1993)。

2 樣品及分析方法

2.1 樣品描述

鳳凰咀榴輝巖(樣品 SHZ17-1)呈團塊狀產(chǎn)于南部榴輝巖帶大理巖中,出露寬度約50 m (圖3a)。榴輝巖呈淡綠色,主要由石榴石(35%)、綠輝石(25%)、角閃石(20%)、多硅白云母(7%)、綠簾石(3%)、石英(10%)和少量金紅石+榍石組成(圖 3b)。斑狀變晶結(jié)構(gòu),變形明顯。石榴石變斑晶多呈渾圓狀,粒徑0.5～1.0 mm,含有大量的模糊狀包裹體,經(jīng)電子探針檢測主要是綠簾石,其次是石英,也有角閃石,一般具有凈邊結(jié)構(gòu)。綠輝石多呈細粒集合體狀,但個別顆粒稍大。角閃石也為細小集合體,與綠輝石平衡共生,背散射圖像分析顯示環(huán)帶狀結(jié)構(gòu)。多硅白云母葉片稍大,定向性不如其他柱狀礦物。綠簾石均為長柱狀,背散射圖像分析也顯示環(huán)帶狀結(jié)構(gòu)。金紅石顆粒較大者保存較好,較小者僅保留在榍石的核部。

武勝關(guān)石榴角閃巖(樣品 WSG01-4)呈 3 m ×10 m大小的透鏡體夾于桐柏雜巖的黑云斜長片麻巖中(圖 3c)。主要組成礦物是石榴石(30%)、角閃石(45%)、斜長石(20%)、石英(5%)和少量綠簾石、鈦鐵礦,未見單斜輝石和金紅石(圖3d)。石榴石均為它形,粒徑 0.5～1.0 mm,具有明顯的白眼圈構(gòu)造,由斜長石和少量角閃石構(gòu)成。石榴石內(nèi)含有分散狀包裹體,主要是石英、榍石和少量綠簾石。角閃石呈細粒集合體分布,可含有蠕蟲狀、它形的斜長石包體。斜長石呈斑塊狀,或與角閃石共存,或圍繞石榴石生長。石英顆粒稍大,可能與石榴石屬于同一期次。綠簾石有兩種產(chǎn)狀,其一是生長在鈦鐵礦邊緣,其二是呈充填脈狀,與少量綠泥石共存,代表晚期綠片巖相迭加。以上礦物結(jié)構(gòu)表明,武勝關(guān)石榴角閃巖可能是榴輝巖的退變產(chǎn)物。

2.2 分析方法

鋯石的分選和挑純由河北省區(qū)域地質(zhì)礦產(chǎn)調(diào)查研究所實驗室完成。將待測鋯石顆粒制成環(huán)氧樹脂樣品靶,拋光至其中心部位。先進行光學和陰極發(fā)光(CL)顯微結(jié)構(gòu)觀察和照相,并由電子探針分析檢測礦物包裹體的成分,在此基礎(chǔ)上選擇合適的鋯石顆粒和(或)區(qū)域進行U-Pb年齡測定。鳳凰咀榴輝巖樣品 SHZ17-1的鋯石 U-Pb定年由中國地質(zhì)科學院地質(zhì)研究所北京離子探針中心SHRIMP Ⅱ完成。分析流程和原理參考 Compston等(1992)的有關(guān)描述,測試孔徑約為25～30 μm。用澳大利亞國家地質(zhì)調(diào)查局標準鋯石 TEM (417 Ma)進行元素間分餾的校正,應用澳大利亞國立大學地球科學學院標準鋯石SL13 (572 Ma,U含量238×10-6)標定所測鋯石的U、Th和Pb含量。利用中國科學院地質(zhì)與地球物理研究所CAMECA IMS-1280二次離子質(zhì)譜儀(SIMS)對武勝關(guān)石榴角閃巖樣品 WSG01-4的鋯石進行了U-Pb定年。鋯石 SIMS年齡測定的方法和原理與Williams (1998)的描述類似。仍使用標準鋯石SL13來標定待測樣品的 U、Th和 Pb的含量,每測定 3個樣品點之后測定一個標準鋯石 TEM (417 Ma)點,用來進行不同元素間同位素分餾校正。數(shù)據(jù)處理采用SQUID1.03和ISOPLOT程序,利用實測的204Pb含量來進行普通Pb校正。單個測點同位素年齡的誤差為1σ,加權(quán)平均年齡具95%的置信度。

3 地質(zhì)年代結(jié)果

3.1 鳳凰咀榴輝巖(樣品SHZ17-1)

榴輝巖樣品SHZ17-1中的鋯石多為不規(guī)則狀、渾圓狀或短柱狀,晶體大小在200～400 μm之間。陰極發(fā)光圖像顯示多數(shù)鋯石顆粒具有核-邊結(jié)構(gòu)(圖4a,b),核部呈條帶狀構(gòu)造,與輝長巖中的典型鋯石特征相似(Corfu et al.,2003); 邊部呈均勻的灰白色,似交代重結(jié)晶或變質(zhì)增生成因(Xia et al.,2009)。大部顆粒發(fā)育不規(guī)則的裂隙,其內(nèi)被強發(fā)光物質(zhì)充填,表明它們可能經(jīng)歷了流體改造。

在16顆鋯石上分析了18個點,其中13個點位于核部,5個點位于邊部(表1)?？傮w來看,鋯石核的U 和 Th 含量較高,分別為 99×10-6～564×10-6和86×10-6～757×10-6,Th/U 比值也較高,為 0.75～1.34,進一步說明它們?yōu)閹r漿成因; 鋯石邊部的 U和 Th含 量 相 對 較 低 , 分 別 為 46×10-6～69×10-6和29×10-6～41×10-6,Th/U 比值為 0.56～0.77,與典型的變質(zhì)成因鋯石(Th/U＜0.1)略有區(qū)別。兩個鋯石部位的U-Pb年齡未見明顯的差別,但邊部誤差偏大(圖5)。13個鋯石核部分析點給出的不一致線上交點年齡為(1961±23) Ma (MSWD = 1.5),可能代表了原始輝長巖的侵位時間; 不一致線的下交點年齡為(533±880) Ma,誤差大,因此不具有實際地質(zhì)意義。5個鋯石邊部分析點中,7.2的年齡數(shù)據(jù)明顯年輕,排除此點,其它4個分析點的207Pb/206Pb加權(quán)平均年齡為(1949±51) Ma (MSWD = 0.042),可能代表鋯石重結(jié)晶或變質(zhì)增生的時間。

3.2 武勝關(guān)石榴角閃巖(樣品WSG01-4)

石榴角閃巖樣品 WSG01-4中的鋯石均為卵形或渾圓狀,粒徑一般為20～100 μm。陰極發(fā)光圖像揭示大多數(shù)鋯石顯示均勻的灰色,少數(shù)發(fā)育不明顯的面狀/扇狀環(huán)帶(圖 4c,d),顯然屬于變質(zhì)成因(Corfu et al.,2003)。有些鋯石顆粒發(fā)育一個非常狹窄的,不連續(xù)的強發(fā)光邊,可能反映了晚期生長。礦物包裹體比較發(fā)育,主要成分是角閃石、石英、磷灰石和榍石,但未見石榴石,說明鋯石與巖石基質(zhì)中礦物組合的形成時間可能并不一致。

在23顆鋯石上分析了23個點(表2)。總體來看,除個別分析點(如5.1)外,鋯石中的Th、U含量普遍較低(分別為 3.7×10-6～13.4×10-6和 0.1×10-6～2.9×10-6),Th/U 比值為 0.01～0.30,且絕大多數(shù)＜0.10,進一步證明了它們的變質(zhì)成因(Williams et al.,1987; Gebauer et al.,1997; Rubatto et al.,2000)。所有23個分析點均位于不一致線上或其附近,證明其經(jīng)歷了不同程度放射性成因 Pb的丟失。23個分析點產(chǎn)生的上交點年齡為(1936±26) Ma(圖 6),下交點年齡為(116±210) Ma (MSWD=1.3),其中上交點年齡與207Pb/206Pb加權(quán)平均年齡(1932±26) Ma (MSWD=1.3),在誤差范圍內(nèi)一致,結(jié)合鋯石的內(nèi)部結(jié)構(gòu)和包裹體類型將其解釋為角閃巖相變質(zhì)作用的發(fā)生時代; 下交點年齡誤差范圍較大,因此不具有地質(zhì)意義。

4 討論與結(jié)論

本文在榴輝巖中獲得其原巖侵位年齡和鋯石重結(jié)晶或變質(zhì)增生年齡分別為(1961±23) Ma和(1949±51) Ma,在石榴角閃巖中獲得其變質(zhì)年齡為(1936±26) Ma,表明這兩種基性巖石經(jīng)歷了古元古代變質(zhì)作用的影響。在兩個樣品中均未獲得約255 Ma的榴輝巖相變質(zhì)年齡,可能緣于榴輝巖形成溫度較低而使鋯石未能重新生長,或生長邊太窄而無法測定,這種現(xiàn)象在桐柏—大別地區(qū)的中低溫榴輝巖中普遍存在(Liu et al.,2004a,2008b,2010)。在桐柏造山帶的變質(zhì)沉積巖中已報道了4組碎屑鋯石年齡,分別為(1966±22) Ma (樣品 WHD04-2,變質(zhì)年齡)、(1930±7) Ma (樣品 DFS06-1,巖漿年齡)、(1846±5) Ma (樣品 SHZ03-5,變質(zhì)年齡)和(1820±6) Ma (樣品 TB02-1,變質(zhì)年齡)(Liu et al.,2008b,2010),這些碎屑鋯石年齡數(shù)據(jù)與本文的基巖定年結(jié)果相吻合?？磥?在桐柏造山帶中記錄的古元古代構(gòu)造熱事件似可分為兩幕,第一幕發(fā)生在約1.97～1.93 Ga,第二幕發(fā)生在約 1.85～1.82 Ga,二者間隔約100 Ma。

實際上,在桐柏造山帶中獲得的古元古代兩幕年代學圖譜在大別—蘇魯造山帶不同類型的三疊紀超高壓變質(zhì)巖中也已識別出來。這些超高壓巖石包括榴輝巖、硬玉石英巖、大理巖和花崗質(zhì)片麻巖,時代主要集中在1.99～1.92 Ga和1.86～1.80 Ga (周漢文等,1998; Maruyama et al.,1998; Ayers et al.,2002;Chen et al.,2003a; 吳元保等,2003; 夏群科等,2003;Yang et al.,2003; Li et al.,2004; Hacker et al.,2006;Liu et al.,2006; Zheng et al.,2006; Wu et al.,2006,2008; Sun et al.,2008)。甚至在中生代花崗巖中也有類似的繼承鋯石的年代學記錄(Bryant et al.,2004)。在揚子陸塊內(nèi)部,約2.0～1.93 Ga的古元古代變質(zhì)年齡信息主要來自于揚子北緣的崆嶺群太古宙變質(zhì)基底及其周圍的新元古代沉積蓋層(Qiu et al.,2000;高山等,2001; Ling et al.,2001; 趙風清等,2006;Zhang et al.,2006a,b,c; Liu et al.,2008a; Wu et al.,2009),尚未見第二幕變質(zhì)作用的年齡報道,但這一時期有基性或花崗質(zhì)巖漿的局部侵入(熊慶等,2008;彭敏等,2009; 張麗娟等,2011)。

現(xiàn)有同位素年代學資料表明,約2.0～1.93 Ga的巖漿活動和變質(zhì)作用在揚子陸塊北部廣泛發(fā)育,應代表一次重要的碰撞造山事件。一般認為,這一碰撞造山事件可能與元古代Columbia超大陸的聚合有關(guān)(Zhang et al.,2006a,b; Zheng et al.,2007; Wu et al.,2008,2009),并導致了揚子陸塊早期基底的形成。出露于黃陵背斜的圈椅埫A型花崗巖和基性巖脈以及華山觀環(huán)斑花崗巖的侵入時代集中在約1.85 Ga (熊慶等,2008; 彭敏等,2009; 張麗娟等,2011),表明這一時期揚子陸塊應處于伸展構(gòu)造體制下,可能與Columbia超大陸的裂解有關(guān)(彭敏等,2009; 張麗娟等,2011)。然而,在桐柏—大別—蘇魯造山帶中還發(fā)育一期 1.86～1.80 Ga的構(gòu)造熱事件,有可能說明在揚子陸塊北緣存在另一期造山作用。這期造山作用或者與華北陸塊中發(fā)育的同期古元古代碰撞造山帶的性質(zhì)相似(Zhao et al.,2003),代表Columbia超大陸匯聚的繼續(xù),或者是陸塊邊緣增生過程的結(jié)果。

致謝: 鋯石測年得到中國地質(zhì)科學院地質(zhì)研究所北京離子探針中心張維博士和中國科學院地質(zhì)與地球物理研究所李獻華研究員、李秋立博士、唐國強和劉宇工程師的協(xié)助,張建新研究員對論文提出了寶貴的修改意見,在此致謝！

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Zircon U-Pb Ages of Paleoproterozoic Metabasites from the Tongbai Orogen and Their Geological Significance

HU Juan,LIU Xiao-chun,QU Wei,CUI Jian-jun
Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing100081

The Tongbai orogen is situated at the junction between the Qinling and Dabie orogenic belts.Its northern part corresponds to the North Qinling medium- to high-grade metamorphic complexes,whereas the southern part belongs to the westward continuation of the Dabie HP/UHP metamorphic complexes.Zircon U-Pb dating reveals a protolith age of (1961±23) Ma and a zircon recrystallization or growth age of (1949±51) Ma for an eclogite,and a metamorphic age of (1936±26) Ma for a garnet amphibolite (retrograded eclogite?).These data suggest that these rocks might have suffered Paleoproterozoic metamorphism prior to Permian-Triassic HP metamorphism.Based on the available geochronological data from the Tongbai-Dabie-Sulu orogen,the authors have inferred the existence of two episodes of Paleoproterozoic tectonothermal events in northern Yangtze block.The first episode occurred at 1.97-1.93 Ga,probably related to the assembly of the Columbia supercontinent.The second episode took place at 1.85-1.82 Ga,resulting either from the protracted assembly of the Columbia supercontinent or from the accretionary orogenesis along the northern margin of the Yangtze block.

Paleoproterozoic; two metamorphic episodes; Yangtze block; Tongbai orogen

P597.3; P588.348 文獻標志碼: A doi: 10.3975/cagsb.2012.03.04

本文由國家973項目(編號: 2009CB825006)和中國地質(zhì)調(diào)查局國土資源大調(diào)查項目(編號: 1212010711812)聯(lián)合資助。

2012-03-03; 改回日期: 2012-04-09。責任編輯: 張改俠。

胡娟,女,1981年生。博士研究生。主要從事變質(zhì)巖石學研究。E-mail: hujuan1314@163.com。

*通訊作者: 劉曉春,男,1962年生。研究員。主要從事巖石學研究。E-mail: liuxchqw@yahoo.com.cn。