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華北克拉通懷安陸塊新太古代低鋁和高鋁TTG 片麻巖的地球化學(xué)特征與成因*

2015-03-15 11:24張華鋒王浩錚豆敬兆張少穎
巖石學(xué)報(bào) 2015年6期
關(guān)鍵詞:巖系片麻巖熔融

張華鋒 王浩錚 豆敬兆 張少穎

ZHANG HuaFeng1,WANG HaoZheng2,DOU JingZhao1 and ZHANG ShaoYing1

1. 中國(guó)地質(zhì)大學(xué)地球科學(xué)與資源學(xué)院,北京 100083

2. 中國(guó)科學(xué)院地質(zhì)與地球物理研究所,北京 100029

1. School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China

2. Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China

2014-6-31 收稿,2015-03-01 改回.

太古宙高級(jí)變質(zhì)地體中灰色片麻巖以英云閃長(zhǎng)巖-奧長(zhǎng)花崗巖-花崗閃長(zhǎng)巖為主,簡(jiǎn)稱(chēng)TTG(Jahn et al.,1981,1984),是早前寒武紀(jì)下地殼的重要組成,其成因研究對(duì)認(rèn)識(shí)早期大陸生長(zhǎng)和演化具有重要意義(Arth and Baker,1976;Barker and Arth,1976;Barker et al.,1976;Martin,1986,1987,1994,1999;Smithies,2000;Whalen et al.,2002,2004;Condie,2005;Foley et al.,2002,2003;Rapp et al.,2003;Halla et al.,2009;Moyen,2011;Martin et al.,2014)。前人依據(jù)巖石地球化學(xué)特點(diǎn)將TTG 巖系分為高鋁和低鋁系列(Barker and Arth,1976;Barker et al.,1976;Barker,1979),SiO2=70%,Al2O3<15%的為低鋁TTG,其重稀土含量高并具有負(fù)的Eu 異常,而高鋁系列Al2O3>15%,重稀土含量相對(duì)低,具正或無(wú)Eu 異常。Halla et al. (2009)則根據(jù)北歐地區(qū)大量新太古代TTG 巖系的巖石地球化學(xué)特征重新定義高鋁和低鋁TTG 概念,劃分出低鋁高重稀土系列(Eu 負(fù)異常)和高鋁低重稀土系列(Eu 無(wú)或正異常),并在SiO2-Al2O3含量關(guān)系圖中大致劃出二者的界線(xiàn)。他們認(rèn)為高鋁和低鋁系列分別來(lái)自洋殼高壓和低壓熔融。高鋁系列的巖石地球化學(xué)特點(diǎn)類(lèi)似顯生宙埃達(dá)克巖(Martin,1999),又被稱(chēng)為低鎂埃達(dá)克巖(Rapp et al. ,1999),由洋殼部分熔融而成(Martin,1986)、洋底高原(Condie,2005)或俯沖的洋底高原(Martin et al. ,2014)以及基性下地殼熔融而成(Smithies,2000;Whalen et al. ,2002,2004;Zhang et al. ,2013a;Qian and Hermann,2013)。低鋁系列則認(rèn)為由偏基性巖漿分離結(jié)晶或斜長(zhǎng)角閃巖低壓熔融產(chǎn)物(Arth and Barker,1976;Barker and Arth,1976;Arth et al. ,1978;Drummond and Defant,1990;Drummond et al. ,1996)。迄今人們對(duì)于TTG 的成因及其機(jī)制仍存在諸多不同認(rèn)識(shí)(Martin et al. ,2014)。

華北克拉通中北部懷安陸塊出露大量早前寒武紀(jì)變質(zhì)巖石,主要由經(jīng)歷麻粒巖相變質(zhì)作用的灰色片麻巖組成,其中的酸性巖石類(lèi)似太古宙TTG 巖系(錢(qián)祥麟等,1985;劉宇光,1989;趙宗溥,1993;Zhai,1996;伍家善等,1998),巖石化學(xué)顯示主要為英云閃長(zhǎng)巖及少量花崗閃長(zhǎng)巖和奧長(zhǎng)花崗巖(Zhai,1996;劉富等,2009;Liu et al. ,2012)。鋯石SHRIMP 或LA-ICP-MS U-Pb 年齡表明主體形成于2.55 ~2.45Ga 期間(Zhao et al. ,2008;Liu et al. ,2012;Zhang et al. ,2012a)。其中識(shí)別出少量低鋁巖系,巖性為奧長(zhǎng)花崗質(zhì)片麻巖(Zhang et al. ,2012a),但是它們一直缺乏詳細(xì)地巖石成因分析及其與高鋁巖系的對(duì)比工作。為此,在前人的研究成果基礎(chǔ)上,本文以華北克拉通懷安陸塊內(nèi)低鋁和高鋁TTG 片麻巖為例,探討其巖漿演化與成因等問(wèn)題。

1 地質(zhì)背景

懷安陸塊位于恒山雜巖以北,宣化盆地以西,與西部孔茲巖帶以大同斷裂相隔,北部則以赤城-尚義斷裂為界與紅旗營(yíng)子群分隔(圖1)。其內(nèi)出露的變質(zhì)巖類(lèi)被稱(chēng)為懷安雜巖,主要由經(jīng)歷了麻粒巖相變質(zhì)作用的灰色片麻巖類(lèi)和殘塊狀分布的變質(zhì)表殼巖類(lèi)組成(孔茲巖系),其研究歷史悠久,時(shí)代歸屬和命名幾經(jīng)更迭。在20 世紀(jì)50 年代前被廣泛稱(chēng)作“桑干群”,60 年代至90 年代初被稱(chēng)為“集寧群”。90 年代初被稱(chēng)為“馬市口群”(麻粒巖-片麻巖)和“下白窯群”(孔茲巖系)(趙宗溥,1993)或“葛胡窯灰色片麻巖系”和“豐鎮(zhèn)群”(陳亞平等,1990)。80 年代,出露于張家口-宣化一帶的地質(zhì)體被歸入早中太古代“遷西群”和晚太古代“遵化群”(張春華等,1990)。劉宇光(1989)提出該區(qū)主要為下地殼成因的TTG 巖套組成,經(jīng)歷了3.5 ~3.0Ga、2.8 ~2.7Ga 和2.4 ~2.6Ga 三次巖漿-熱事件,并將其命名為“懷安群”,將原“上集寧群”統(tǒng)稱(chēng)為“集寧群”。

圖1 華北克拉通(a,據(jù)Zhao et al.,1999)及懷安陸塊(b,據(jù)Zhai,1996 修改)地質(zhì)簡(jiǎn)圖圖中編號(hào)為樣品位置及其編號(hào)Fig.1 Geological skeleton map of the North China Craton (a,after Zhao et al.,1999)and the Huai’an terrane (b,modified after Zhai,1996)

崔文元(1982)認(rèn)為上集寧群的變質(zhì)表殼巖相當(dāng)于印度出露的孔茲巖系(khondalites),原巖為一套古島弧或活動(dòng)大陸邊緣沉積,而下集寧群麻粒巖系原巖為中性?shī)A基性熔巖。錢(qián)祥麟等(1985)則認(rèn)為上集寧群屬于晚太古代陸內(nèi)沉積,而下集寧群為遭受麻粒巖相變質(zhì)的一套TTG 深成巖體,形成于中晚太古代,二者為不整合接觸關(guān)系。吳昌華和韓光(1989)亦認(rèn)為二者之間存在不整合,但“上集寧群”為古元古代沉積蓋層而不是太古宙沉積,其中的基性麻粒巖為更晚期侵入兩套巖系中的巖漿巖。沈其韓等(1989)認(rèn)為“下集寧群”以火山作用為主,“上集寧群”以沉積作用為主,彼此之間可能不存在不整合,而是連續(xù)的沉積。

總之,本區(qū)高級(jí)變質(zhì)地質(zhì)體可大致分為灰色片麻巖和富鋁的變質(zhì)表殼巖兩部分。灰色片麻巖中酸性巖石的地球化學(xué)特征類(lèi)似TTG,時(shí)代分布在2.55 ~2.45Ga 之間(Zhao et al.,2008;Liu et al.,2012;Zhang et al.,2012a;劉樹(shù)文等,2011;Su et al.,2014),Nd-Hf 同位素顯示為新太古代陸殼增生的產(chǎn)物(劉敦一等,1997;Wu et al.,2005;Liu et al.,2012;Zhang et al.,2012a;Geng et al.,2012;Su et al.,2014)。TTG 片麻巖中分布有大量席狀、透鏡狀和巖株?duì)罨愿邏郝榱r(王仁民等,1991;翟明國(guó)等,1992),變質(zhì)軌跡顯示為減壓降溫軌跡,變質(zhì)時(shí)代分布在1.93 ~1.80Ga(Guo et al.,2005;張華鋒等,2006;Wang et al.,2010;王洛娟等,2011;羅志波等,2012),與區(qū)域上分布的基性高壓麻粒巖年齡相似,例如,恒山地區(qū)(Zhang et al.,2006a)和承德北部(毛德寶等,1999)。

對(duì)于富鋁片麻巖,其變質(zhì)作用研究顯示曾經(jīng)歷過(guò)較復(fù)雜的變質(zhì)歷史,總體呈順時(shí)針變質(zhì)P-T 軌跡特征(金巍等,1991;Liu et al.,1993;賀高品等,1991;盧良兆等,1996;劉福來(lái)和沈其韓,1999;Liu et al.,2014;Cai et al.,2014)。其原巖沉積環(huán)境則有穩(wěn)定克拉通盆地沉積(Condie et al.,1992)和大陸邊緣兩種認(rèn)識(shí)(盧良兆等,1996);鋯石同位素年齡顯示孔茲巖沉積和變質(zhì)時(shí)代為古元古代(劉喜山等,1992;吳昌華和韓光,1989;Wan et al.,2009;Zhao et al.,2010;Li et al.,2010;Zhang et al.,2012b)。

2 樣品及分析方法

圖2 華北克拉通懷安陸塊低鋁奧長(zhǎng)花崗質(zhì)片麻巖野外地質(zhì)特征Fig.2 Field map of the Low-Al trondhjemite from the Huai’an terrane,NCC

低鋁奧長(zhǎng)花崗質(zhì)片麻巖的樣品取自山西天鎮(zhèn)縣米薪關(guān)鎮(zhèn)下陰山村東(圖1),巖體周?chē)鸀榈谒南蹈采w,未能觀察到與圍巖高鋁TTG 片麻巖的直接接觸關(guān)系,推測(cè)邊界因后期構(gòu)造運(yùn)動(dòng)發(fā)生了片麻理平行化作用,確切的地質(zhì)關(guān)系無(wú)法準(zhǔn)確判斷。巖體外貌肉紅色,應(yīng)與巖石表面風(fēng)化作用有關(guān)。所取樣品鏡下觀察局部存在弱絹云母化,但無(wú)強(qiáng)烈絹云母或鉀長(zhǎng)石化現(xiàn)象,其全巖化學(xué)成分富鈉也說(shuō)明不存在后期的強(qiáng)烈鉀化作用。巖體內(nèi)部可見(jiàn)大小不一的透鏡狀基性高壓麻粒巖(圖2a),其變質(zhì)鋯石U-Pb 年齡分布在2.0 ~1.8Ga 之間(張華鋒等,2006)。巖石呈片麻狀構(gòu)造,中細(xì)粒變晶結(jié)構(gòu),礦物組成主要為奧長(zhǎng)石(55%),石英(40%),堿性長(zhǎng)石(5%)以及少量石榴石(1%)、黑云母,副礦物為鋯石、獨(dú)居石(圖3)。鋯石SHRIMP U-Pb 定年結(jié)果顯示為2.53Ga 左右,εHf(t)=+6 ~+10,tDMC變化在2467 ~2790Ma 之間,平均2605Ma。Lu-Hf 同位素組成表明該巖體物源為新生地殼物質(zhì)(Zhang et al.,2012a)。

高鋁片麻巖樣品分別采自懷安蔓菁溝與興和黃土窯石墨礦區(qū)(圖1),樣品弱片麻狀構(gòu)造、條帶狀、塊狀構(gòu)造,中-粗粒變晶結(jié)構(gòu)。礦物組合變化較大,暗色礦物從偏基性巖石的含單斜輝石、紫蘇輝石、黑云母到酸性巖石含少量黑云母的巨大變化特點(diǎn)。偏基性巖石以二輝石為主,似紫蘇花崗巖礦物組合(圖3)。紫蘇輝石他形分布在單斜輝石和斜長(zhǎng)石之間,應(yīng)為后期麻粒巖相變質(zhì)成因。本文所取英云閃長(zhǎng)巖樣品多呈淡黃色、青灰色,與肉紅色鉀質(zhì)花崗巖極易區(qū)分。礦物主要由斜長(zhǎng)石(45% ~55%)、石英(25% ~35%)、堿性長(zhǎng)石(5% ~10%)組成,暗色礦物為二輝石(0 ~5%)、黑云母少量(1%)。巖體內(nèi)部同樣發(fā)育基性麻粒巖透鏡體或巖席。高鋁巖系的鋯石U-Pb 年齡分布在2.55 ~2.45Ga 之間(Zhao et al.,2008;Liu et al.,2012;Zhang et al.,2012a;Su et al.,2014),Nd-Hf 同位素顯示為新生地殼產(chǎn)物(Wu et al.,2005;Liu et al.,2012)。

巖石主、微量元素分析在西北大學(xué)大陸動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室完成,分別采用XRF 和ICP-MS 分析測(cè)試。大多數(shù)主量元素測(cè)試的精確度和準(zhǔn)確度優(yōu)于5%,測(cè)試國(guó)際標(biāo)樣BHVO-1,AGV-1 和GSP-1 等絕大多數(shù)微量元素的分析精確度和準(zhǔn)確度優(yōu)于10%,結(jié)果見(jiàn)表1。

3 分析結(jié)果

3.1 主量元素

數(shù)據(jù)顯示低鋁奧長(zhǎng)花崗質(zhì)片麻巖的SiO2含量變化在76.5% ~79.5%之間(表1),明顯高于本文及前人(Liu et al.,2012)獲得的高鋁TTG 巖系(66.6% ~77%);所有樣品的Na2O/K2O 比值變化在2.4 ~6.2,顯示出富鈉特征;Na2O含量分別為3.69% ~4.55%(低鋁巖系)和3% ~5.91%(高鋁巖系),K2O 分別為0.91% ~1.15%(低鋁巖系)和0.99%~1.77%(高鋁巖系)。低鋁奧長(zhǎng)花崗質(zhì)片麻巖的Al2O3(11.01% ~12.61%)、CaO(1.27% ~1.59%)、Na2O、K2O 均低于高鋁巖系(見(jiàn)表1),并顯示出相對(duì)低鎂,高鐵的特征,其Mg 指數(shù)(Mg#=100 ×Mg2+/(Mg2++Fe2+),F(xiàn)eO =0.9FeOT)變化在18 ~53,明顯低于高鋁巖系的鎂指數(shù)(Mg#=35 ~54);在CIPW 標(biāo)準(zhǔn)礦物An-Ab-Or 分類(lèi)圖中(圖4,O’Connor,1965),所有數(shù)據(jù)分別落入英云閃長(zhǎng)巖和奧長(zhǎng)花崗巖區(qū),而前人少量數(shù)據(jù)落入花崗閃長(zhǎng)巖區(qū),巖石顯示出富鈉、鈣和貧鉀的特征。懷安陸塊中高鋁TTG 巖系的SiO2含量與Al2O3、CaO、P2O5、FeO、MgO 等元素含量呈較好的線(xiàn)性正相關(guān)關(guān)系(圖5),而Na2O 保持不變。低鋁奧長(zhǎng)花崗質(zhì)片麻巖隨著SiO2含量的升高,Al2O3、MgO 含量呈逐漸降低的線(xiàn)性關(guān)系,F(xiàn)eO 則表現(xiàn)出先增多,后降低的趨勢(shì)(圖5c),而CaO、P2O5含量保持基本不變(圖5b,f),暗示后期麻粒巖相變質(zhì)作用對(duì)其改造較弱,可以用于成因分析解釋。

圖3 顯微礦物結(jié)構(gòu)特征(a-d)為低鋁奧長(zhǎng)花崗質(zhì)片麻巖;(e-h)為英云閃長(zhǎng)質(zhì)片麻巖(高鋁巖系). (a)單偏光下石榴石+鈦鐵礦+黑云母特征;(b)為圖(a)在正交鏡下石榴石+黑云母+石英+奧長(zhǎng)石組合特征;(c)正交鏡下較大的鈦鐵礦顆粒與黑云母、石榴石共生,顆粒間為鋯石;(d)長(zhǎng)石顆粒間的他形石英顆粒中包裹豆莢狀獨(dú)居石;(e)單偏光下偏基性的英云閃長(zhǎng)質(zhì)片麻巖(高鋁巖系)(12HTY34)暗色礦物組合為單斜輝石+紫蘇輝石+鈦鐵礦;(f)為(e)在正交鏡下特征,主要礦物為斜長(zhǎng)石+石英+二輝石+鈦鐵礦;(g)同上,為偏基性英云閃長(zhǎng)質(zhì)片麻巖(高鋁巖系)樣品中暗色礦物組合特征為二輝石,淺色礦物以石英、斜長(zhǎng)石為主;(h)為酸性英云閃長(zhǎng)質(zhì)片麻巖(高鋁巖系)(12HTY06)礦物組合特征,主要為斜長(zhǎng)石+石英,少量鉀長(zhǎng)石;暗色礦物很少. 礦物縮寫(xiě)(Whitney and Evans,2010):Pl-斜長(zhǎng)石;Kfs-鉀長(zhǎng)石;Qz-石英;Gt-石榴石;Cpx-單斜輝石;Opx-紫蘇輝石;IIm-鈦鐵礦;Zrn-鋯石;Bt-黑云母;Mnz-獨(dú)居石Fig.3 Microscopic photos of the Late Archean Low-Al trondhjemite (a-d)and High-Al TTGs (e-h)from the Huai'an terrane

表1 懷安陸塊高鋁和低鋁TTG 片麻巖的主量(wt%)和微量元素(×10 -6)組成特征Table 1 Major (wt%)and trace (×10 -6)elements analytical results for the Low-Al and High-Al TTG gneisses in the Huai’an terrane,NCC

圖4 標(biāo)準(zhǔn)礦物Ab-An-Or 分類(lèi)圖解(據(jù)O’Connor,1965;Barker,1979)Fig. 4 Rock classification based on normative feldspar variation (after O’Connor,1965)and the fields drawn after Barker (1979)

3.2 微量元素

低鋁奧長(zhǎng)花崗質(zhì)片麻巖與高鋁TTG 巖系的稀土特征明顯不同。從稀土/球粒隕石配分模式圖中(圖6a),明顯可以看出低鋁奧長(zhǎng)花崗質(zhì)片麻巖的重稀土含量相對(duì)較高,在球粒隕石的10 倍左右,并呈現(xiàn)出從Gd 向Lu 逐漸升高的趨勢(shì),從而導(dǎo)致稀土配分模式為海鷗型,具有較低的(La/Yb)N比值(4 ~6),相對(duì)低的(Gd/Yb)N(0.35 ~1)和Eu/Eu*[=EuN/(SmN×GdN)1/2]負(fù)異常為特征。高鋁TTG 巖系的稀土則顯示出輕、重稀土分餾強(qiáng)烈的特征(圖6a),其(La/Yb)N比值變化在20 ~70,具正異常和無(wú)異常兩種情況,樣品DJG01 和12HTY06 不僅具有強(qiáng)烈的正銪異常,同時(shí)還表現(xiàn)出稀土總量相對(duì)低以及輕、重稀土分餾強(qiáng)烈的特征。

從微量元素蛛網(wǎng)圖中可以看出(圖6b),所有樣品均表現(xiàn)出相對(duì)虧損Rb、Th、Nb、P、Ti 等元素的特征,但明顯存在兩點(diǎn)不同:①低鋁奧長(zhǎng)花崗質(zhì)片麻巖相對(duì)虧損Sr、Ba,且Sr 的豐度也明顯低于高鋁TTG 巖系(表1);②低鋁奧長(zhǎng)花崗質(zhì)片麻巖相對(duì)富集高場(chǎng)強(qiáng)元素(HFSE),如Zr 及重稀土元素,并具有高Y 含量的特征(>23.6 ×10-6),而高鋁TTG 巖系的Y含量低,變化范圍較大(0.64 ×10-6~13.7 ×10-6)。

對(duì)平定小金川土司僧格桑與大金川土司索諾木聯(lián)合發(fā)動(dòng)的叛亂一事,史書(shū)使用了平鋪直敘的敘述方式,而管世銘的詩(shī)歌《薄伐》采用比興的手法,以“螳臂當(dāng)車(chē)”“坐井觀天”等成語(yǔ)比擬反叛者的行為,對(duì)他們的自大狂妄加以嘲諷?!皩蚁瓤姑?,恃險(xiǎn)殊披猖”指出大小金川屢次違抗朝廷,發(fā)動(dòng)戰(zhàn)爭(zhēng),譴責(zé)了大小金川土司的猖狂之態(tài)。“釋此茍不誅,何以勸柔良”則申明若是放任大小金川的叛逆行為,其他土司就會(huì)紛紛效仿,朝廷將難以應(yīng)對(duì),從而點(diǎn)出了朝廷鎮(zhèn)壓叛亂的必要性和緊迫性。

4 討論

4.1 巖漿演化

兩類(lèi)巖石的稀土總量差異明顯(圖5g),但是均表現(xiàn)出隨SiO2含量升高而降低的趨勢(shì),特別是低鋁奧長(zhǎng)花崗質(zhì)片麻巖的稀土總量降低相對(duì)更快,與SiO2含量之間的斜率更大,說(shuō)明二者巖漿經(jīng)歷過(guò)不同程度的演化。

Lu/Hf 比值可以明顯地區(qū)別本文中的兩類(lèi)巖石(圖7a,b,e),低鋁奧長(zhǎng)花崗質(zhì)片麻巖的Lu/Hf =0.1 ~0.16,高鋁TTG 巖系的Lu/Hf=0.01 ~0.07。前者的Lu/Hf 隨著Mg#的降低而減小,呈正相關(guān)關(guān)系,后者的Lu/Hf 比值與Mg#無(wú)明顯相關(guān)性(圖7a)。前者的重稀土總量(ΣHREE)隨著Lu/Hf的減小略有降低,而后者則呈現(xiàn)快速降低趨勢(shì)(圖7b),暗示二者源區(qū)殘留相和演化過(guò)程中的分離礦物相不同。

為了進(jìn)一步分析本文兩類(lèi)巖石地球化學(xué)成因與關(guān)系,我們依據(jù)前人提供的巖漿鋯飽和溫度公式(Watson and Harrison,1983;Miller et al.,2003),對(duì)兩類(lèi)巖石進(jìn)行計(jì)算。結(jié)果顯示,低鋁奧長(zhǎng)花崗質(zhì)片麻巖的溫度變化在767 ~846℃之間,平均799℃,而高鋁TTG 巖系的溫度明顯較低,除樣品DPG04 因鋯含量很低獲得不合理的溫度外(608℃),其它變化在675 ~789℃,平均734℃。目前,所有鋯石定年數(shù)據(jù)均未發(fā)現(xiàn)繼承鋯石,所以,我們認(rèn)為鋯飽和溫度應(yīng)代表巖漿溫度和源區(qū)最小熔融溫度(Miller et al.,2003)。兩類(lèi)巖石的稀土總量(ΣREE)以及重稀土總量(ΣHREE)隨鋯飽和溫度(TZr)的降低而減少,具正相關(guān)關(guān)系(圖7c,d),可能與巖漿冷卻過(guò)程中某些礦物的分離結(jié)晶作用有關(guān),從而引起兩類(lèi)巖石的稀土總量以及重稀土均發(fā)生不同程度地降低。另外,隨著Eu/Eu*正異常的增大,高鋁TTG 巖系的Lu/Hf 比值降低,(La/Yb)N則呈增大趨勢(shì)(圖7e,f),而低鋁奧長(zhǎng)花崗質(zhì)片麻巖的Lu/Hf 減小時(shí),Eu/Eu*異常和(La/Yb)N基本保持不變。

綜上所述,懷安陸塊內(nèi)的高鋁TTG 巖系的巖漿分離結(jié)晶礦物應(yīng)具有如下特征:1)稀土總分配系數(shù)高,且重稀土的高于輕稀土的,如此能夠引起稀土總量降低而(La/Yb)N逐漸增大;2)對(duì)Lu/Hf 比值不產(chǎn)生顯著影響,即Lu、Hf 分配系數(shù)相近,導(dǎo)致殘余巖漿Lu/Hf 比值變化不大情況下,其它元素或比值發(fā)生明顯地或強(qiáng)烈地變化;3)具Eu/Eu*正異常,某種礦物堆晶作用導(dǎo)致巖石呈正銪異常增大趨勢(shì);4)分離結(jié)晶能夠引起巖漿中Al、Ca、Mg、Fe 的明顯降低(圖5)。

上述特征說(shuō)明本區(qū)高鋁TTG 巖系的巖漿曾經(jīng)歷過(guò)以角閃石為主,單斜輝石為輔,少量磷灰石的分離結(jié)晶作用,巖漿晚期可能發(fā)生過(guò)斜長(zhǎng)石的堆晶作用。這可以合理地解釋巖漿中稀土總量隨著溫度的降低而減少,輕、重稀土分餾顯著增大并伴有正銪異常逐漸增大的趨勢(shì)(圖7)。由于角閃石的Lu、Hf 分配系數(shù)相近,其分離結(jié)晶作用不能引起殘余巖漿中Lu/Hf 比值的明顯變化,而單斜輝石雖然兩元素的分配系數(shù)均小于1,但對(duì)于Lu 的分配系數(shù)更高,接近Hf 的2 倍(Rollinson,1993)。因此,少量單斜輝石的分離結(jié)晶可以導(dǎo)致殘余巖漿中Lu/Hf 比值降低趨勢(shì)。

圖5 SiO2與其它元素或比值的哈克圖解Fig.5 Harker diagrams showing the covariations between SiO2 and some major and trace elements and element ratios

對(duì)于低鋁奧長(zhǎng)花崗質(zhì)片麻巖而言,副礦物獨(dú)居石、磷釔礦的分離結(jié)晶作用可能是引起其微量元素變化的主要原因。這些副礦物具有較高的Lu/Hf 比值,分離結(jié)晶能夠引起殘余巖漿Lu/Hf 比值和稀土總量逐漸降低的趨勢(shì)。顯微鏡下可見(jiàn)獨(dú)居石分布在較大的石英晶體中(圖3)。該巖體礦物以?shī)W長(zhǎng)石+ 石英為主(圖3),奧長(zhǎng)石分離結(jié)晶不僅會(huì)引起Al2O3的降低,還會(huì)引起Na2O 和CaO 的明顯變化,然而這些特征我們并沒(méi)有觀察到(圖5)。所以,該巖體的主要礦物相未曾發(fā)生過(guò)顯著的分離結(jié)晶或堆晶作用。

圖6 新太古代懷安陸塊中低鋁奧長(zhǎng)花崗質(zhì)片麻巖和高鋁TTG 片麻巖的球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖(a)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b)(標(biāo)準(zhǔn)化值據(jù)Sun and McDonough,1989)Fig.6 Chondrite-normalized REE distribution patterns (a)and primitive mantle-normalized trace elements spider diagram (b)for Archean Low-Al and High-Al TTGs in the Huai’an terrane (normalization values after Sun and McDonough,1989)

4.2 低鋁奧長(zhǎng)花崗質(zhì)片麻巖的成因與源區(qū)

本文低鋁、高硅、富重稀土特征的奧長(zhǎng)花崗巖在早前寒武紀(jì)地殼中不多見(jiàn)(Rollinson,1988;Zhao and Cooper,1993),其地球化學(xué)成因可能有以下幾種情況:1)副礦物分離結(jié)晶或堆晶作用(如,石榴石、鋯石、褐簾石、磷釔礦等);2)低硅英云閃長(zhǎng)巖分離結(jié)晶;3)低壓下的小程度部分熔融。

第一種成因最早由Rollinson (1988)提出。他對(duì)蘇格蘭新太古代經(jīng)歷麻粒巖-角閃巖相變質(zhì)的酸性花崗巖-奧長(zhǎng)花崗質(zhì)片麻巖巖脈(SiO2=72% ~76%)做過(guò)詳細(xì)的地球化學(xué)成因研究(Rollinson,1988)。該類(lèi)巖石銪正異常與高鋁TTG巖系一致,但重稀土明顯強(qiáng)烈富集。根據(jù)巖石高硅和低Ce/Y 比值及高Y 含量的變化關(guān)系等,前人提出在高溫富硅的酸性巖漿中,富重稀土的副礦物,如石榴石和鋯石等很容易發(fā)生堆晶作用,從而產(chǎn)生重稀土富集的現(xiàn)象。

低鋁奧長(zhǎng)花崗質(zhì)片麻巖的Zr 含量增加,Mg 指數(shù)以及Lu/Hf 比值降低(圖7a,c),暗示其巖漿早期沒(méi)有大量鋯石分離或早期巖漿中的Zr 含量未處于過(guò)飽和狀態(tài)。鋯飽和溫度計(jì)算結(jié)果顯示,其巖漿溫度分布在770 ~850℃之間,而本區(qū)高鋁TTG 巖系的計(jì)算結(jié)果顯示巖漿溫度相對(duì)較低(680 ~790℃,圖7d)。因此,低鋁奧長(zhǎng)花崗質(zhì)片麻巖不可能由溫度更低的高鋁TTG 巖系分離結(jié)晶而成。

Zhang et al. (2012a)曾根據(jù)熔融實(shí)驗(yàn)結(jié)果和巖體地球化學(xué)特征做過(guò)分析,提出該巖體來(lái)自新生基性地殼物質(zhì)在低壓條件下,斜長(zhǎng)石和輝石、角閃石為殘留相的低部分熔融產(chǎn)物。本文的巖石地球化學(xué)數(shù)據(jù)顯示,低鋁奧長(zhǎng)花崗質(zhì)片麻巖的Lu/Hf 比值明顯高于本區(qū)廣泛分布的高鋁TTG 巖系。二者的鋯石原位Hf 同位素組成也明顯具有一致的特征(圖8)。前者的176Lu/177Hf 明顯高于后者,并具有更高的176Hf/177Hf 比值。鋯石微量元素特征是其結(jié)晶環(huán)境中元素特征的反映(Zhang et al.,2012a)。二者作為同時(shí)代產(chǎn)物,均來(lái)自新生基性地殼熔融的產(chǎn)物,也應(yīng)具有相似的Lu/Hf 比值。根據(jù)礦物分配系數(shù)(Rollinson,1993),基性地殼熔融過(guò)程中,能夠?qū)е氯垠w中Lu 和Hf 明顯解耦的礦物只有石榴石。在相同的熔融條件下,源區(qū)無(wú)石榴石殘留,地殼會(huì)產(chǎn)生相對(duì)高Lu/Hf比值的熔體。另外,源區(qū)如果大量殘留角閃石,會(huì)導(dǎo)致巖漿具有較低的中稀土含量,因?yàn)榻情W石的中稀土分配系數(shù)大于1(Rollinson,1993)。

綜上所述,我們認(rèn)為本文低鋁奧長(zhǎng)花崗質(zhì)片麻巖的原始巖漿應(yīng)該來(lái)自基性巖石低壓下的低部分熔融(第三種成因)。這種富鈉巖石不可能由太古宙中酸性巖石(閃長(zhǎng)質(zhì)片麻巖和高鋁TTG 巖系)低壓下的低部分熔融而成,其重熔產(chǎn)物通常都是相對(duì)富鉀的花崗巖。此類(lèi)巖石的成因均歸結(jié)為基性巖漿分異或基性地殼低壓熔融的產(chǎn)物(文獻(xiàn)見(jiàn)前述)。所以,本區(qū)低鋁奧長(zhǎng)花崗質(zhì)片麻巖很可能是角閃巖相-輝石麻粒巖相條件下(<10 ~8kbar),由新生基性地殼在低壓條件下發(fā)生部分熔融的結(jié)果,其源區(qū)殘留相礦物以輝石+斜長(zhǎng)石為特征,含少量或沒(méi)有角閃石的殘留為特征(圖9)。

圖7 華北克拉通懷安陸塊新太古低鋁和高鋁TTG 巖系的微量元素比值協(xié)變圖解圖例同圖5Fig.7 Plots of trace element ratios relationships for Archean Low-Al and High-Al TTGs in the Huai’an terrane,NCC

4.3 高鋁TTG 片麻巖成因與源區(qū)

太古宙TTG 的成因涉及到早期地殼形成和演化的機(jī)制和過(guò)程,但對(duì)于TTG 成因尚存不同觀點(diǎn)。例如,Martin(1999)認(rèn)為T(mén)TG 與顯生宙Adakites 類(lèi)似,應(yīng)具有類(lèi)似的成因,而Smithies (2000)和Condie (2005)則認(rèn)為二者M(jìn)g#值方面差異較大,成因機(jī)制不同。根據(jù)地球化學(xué)模擬和實(shí)驗(yàn)巖石學(xué)結(jié)果,認(rèn)為其源巖可能為:(1)榴輝巖(Arth and Hanson,1972;Jahn et al.,1981;Rapp et al.,2003),(2)石榴石角閃巖或石榴石麻粒巖(Martin,1987;Foley et al.,2002,2003;Qian and Hermann,2013)。

圖8 華北克拉通懷安陸塊新太古代低鋁奧長(zhǎng)花崗質(zhì)片麻巖和高鋁TTG 片麻巖的鋯石Lu-Hf 同位素組成特征對(duì)比Fig. 8 Zircon in situ Lu-Hf isotopic compositions of Archean Low-Al and High-Al TTGs in the Huai’an terrane,NCC

總體上,對(duì)于TTG 的成因起源存在三種認(rèn)識(shí):①俯沖洋殼的熔融(Martin,1986;Defant and Drummond,1990;Drummond and Defant,1990;Martin,1987,1994,1999);②底侵增厚的基性下地殼(Atherton and Petford,1993;Petford and Atherton,1996;Smithes,2000;Qian and Hermann,2013;Zhang et al.,2013a)或海底高原(Condie,2005;Martin et al.,2014);③地幔巖部分熔融并經(jīng)結(jié)晶分異而成(Arth and Barker,1976;Barker and Arth,1976;Kamber et al.,2002;Kleinhanns et al.,2003)。而低鋁TTG 系列則有基性巖石分離結(jié)晶(Arth and Barker,1976;Barker and Arth,1976)或基性地殼在低壓條件下部分熔融(Smithes,2000;Whalen et al.,2004)以及低壓下洋殼部分熔融而成(Halla et al.,2009)。

本文樣品中的Cr、Ni 含量低于太古宙TTG 平均值(Drummond et al.,1996)。如按照Kleinhanns et al. (2003)的模式,由富水地幔楔部分熔融形成的TTG 巖漿具有較高的Cr、Ni 含量,該巖漿再經(jīng)結(jié)晶分異作用形成低Cr、Ni 含量的花崗質(zhì)巖漿為太古宙鉀質(zhì)花崗閃長(zhǎng)巖-花崗巖-二長(zhǎng)花崗巖(GGM;De wit,1998)。因此本文中低Cr、Ni 的花崗質(zhì)巖石不可能由幔源巖漿直接衍生而來(lái)。前人的巖石化學(xué)數(shù)據(jù)中有少數(shù)樣品(3 個(gè)樣品)的SiO2含量在63% ~66.5%,Cr 含量高于100 ×10-6(Liu et al.,2012),是否符合幔源巖漿分離結(jié)晶的產(chǎn)物呢?這些樣品的Mg#值在55 ~61 之間,而且區(qū)域上與這些酸性巖漿伴生著大量同時(shí)代的輝長(zhǎng)-閃長(zhǎng)質(zhì)片麻巖,它們的SiO2變化在51% ~64%(Zhai,1996;Liu et al.,2012),部分屬于紫蘇花崗巖(耿元生和劉敦一,1997)。在野外,彼此漸變過(guò)渡,無(wú)法明確識(shí)別地質(zhì)關(guān)系。所以,我們認(rèn)為前人部分?jǐn)?shù)據(jù)也可能反映的是閃長(zhǎng)質(zhì)片麻巖和高鋁TTG巖系局部混合或混染特征。前人解釋為俯沖板片熔體與地幔楔反應(yīng)的結(jié)果(Liu et al.,2012),對(duì)此尚需更多工作甄別。根據(jù)前人的研究成果,懷安陸塊中的高鋁TTG 巖系應(yīng)來(lái)自俯沖板片或基性下地殼的熔融(Martin,1986,1987,1994,1999;Smithes,2000;Condie,2005;Qian and Hermann,2013;Zhang et al.,2013a)。相對(duì)于本區(qū)的低鋁奧長(zhǎng)花崗質(zhì)片麻巖,高鋁TTG 巖系具有高Al、Ca 含量和Sr、Ba 的正異常特征,同時(shí)顯示出很低的Lu/Hf 比值(0.01 ~0.07),這表明源區(qū)殘留相礦物應(yīng)以石榴石和角閃石為特征。本文高鋁TTG 巖系(本文和前人數(shù)據(jù))中相對(duì)低硅(66.63%)、高鈣(5.12%)、鎂(1.96%)、重稀土總量(11 ×10-6)和Lu/Hf 比值(0.07)的樣品(12HTY34),可能更接近原始巖漿,卻具有弱的銪負(fù)異常(圖7e)。如前所述,高鋁TTG 巖系在巖漿演化過(guò)程中,以角閃石和單斜輝石的分離結(jié)晶為特征,巖石地球化學(xué)數(shù)據(jù)并沒(méi)有顯示出斜長(zhǎng)石在早期發(fā)生過(guò)明顯地分離作用,所以弱的負(fù)銪異常應(yīng)反映巖漿本身特點(diǎn)。如果原始巖漿為部分熔融產(chǎn)物而不是由基性巖石分離而來(lái),那么,其地球化學(xué)特征則暗示源巖非榴輝巖(Martin,1987;Foley et al.,2002,2003;Qian and Hermann,2013)。

圖9 太古代地溫梯度與玄武巖溫壓穩(wěn)定相圖(據(jù)Xiong et al.,2006;推測(cè)的太古代地溫梯度線(xiàn)據(jù)Martin,1986)Fig.9 Archean geothermal gradient and P-T diagram showing liquidus and solidus of hydrous tholeiite as solidus of dry tholeiite(after Xiong et al.,2006;Martin,1986)

綜上所述,懷安陸塊中新太古代高鋁TTG 巖系應(yīng)由石榴石角閃巖熔融為特征,殘留相有石榴石、輝石、角閃石和少量斜長(zhǎng)石為特征,非榴輝巖相熔融產(chǎn)物。另外,本文TTG 巖系的稀土含量和(La/Yb)N比值隨著巖漿溫度增高分別增多和降低的特征,說(shuō)明溫度對(duì)花崗質(zhì)巖石稀土元素含量起著一定的控制作用,并非只是壓力(Halla et al.,2009;Moyen,2011)。這與本區(qū)古元古鉀質(zhì)花崗質(zhì)片麻巖的稀土變化趨勢(shì)一致(Zhang et al.,2011)。

4.4 高鋁和低鋁TTG 片麻巖的形成機(jī)制

4.4.1 地幔柱機(jī)制

華北克拉通的2.9 ~2.7Ga 和2.6 ~2.5Ga 兩期增生事件的關(guān)系和范圍尚存不同意見(jiàn)(Zhai et al.,2005;翟明國(guó),2013;Yang et al.,2009;第五春榮等,2012;Wang and Liu,2012;劉敦一等,1997;Wu et al.,2005;Liu et al.,2012;Diwu et al.,2014),其巖漿作用演化特征和形成機(jī)制尚不清楚。

新太古代是否有板塊機(jī)制曾有過(guò)不同認(rèn)識(shí)(Davies,1992;王仁民等,1997),可能新太古代已經(jīng)開(kāi)始有類(lèi)似現(xiàn)代板塊性質(zhì)的俯沖作用(Zhai and Windley,1990),但其規(guī)??赡芎苄?。前人對(duì)太古宙TTG 巖系的研究,發(fā)現(xiàn)Mg#有隨時(shí)間逐漸增大的趨勢(shì),認(rèn)為可能與新太古代末板塊俯沖和地幔楔的形成有關(guān)(Martin and Moyen,2002)。洋殼可以產(chǎn)生平坦俯沖,相對(duì)冷的板片只要足夠時(shí)間加熱(50Myr)即可熔融形成埃達(dá)克巖(Gutscher et al.,2000),也有學(xué)者認(rèn)為太古宙洋殼為平坦俯沖,但是TTG 熔體是底侵引起的基性下地殼熔融而成,與埃達(dá)克巖地球化學(xué)性質(zhì)不同(Smithies,2000)。無(wú)論如何,懷安陸塊南部晚太古代五臺(tái)雜巖中的島弧、洋中脊型玄武巖和Adakite 的形成,表明新太古代末可能存在過(guò)俯沖作用(Wang et al.,2004)。本區(qū)大量TTG 片麻巖主要形成于晚太古代2550 ~2450Ma 之間(趙宗溥,1993;劉敦一等,1997;伍家善等,1998;Zhao et al.,2008;Liu et al.,2012;Su et al.,2014),時(shí)代上與恒山-五臺(tái)雜巖一致(Kr?ner et al.,2005;Wilde et al.,2002,2004;Wilde and Zhao,2005;Zhang et al.,2006b)。因此,懷安陸塊中的高鋁TTG 巖系可能來(lái)自新太古代俯沖板片的熔融(Liu et al.,2012)。但是,我們需要考慮一個(gè)重要現(xiàn)象,即全球乃至整個(gè)華北克拉通均存在2.5Ga 左右的增生事件并形成大量高鋁TTG 質(zhì)巖石(Condie,1998,2000;Zhai et al.,2000;Zhai and Liu,2003;Wu et al.,2005;Wang and Liu,2012;Jian et al.,2012;Geng et al.,2006,2012;Yang et al.,2008,2009;Zhao et al.,2005;第五春榮等,2012)。因此,地幔柱作為一種可能的增生機(jī)制不能排除(Zhao et al.,1999;Geng et al.,2006;Yang et al.,2008)或地幔柱與俯沖板片共同作用的結(jié)果(Condie,1998)。需要指出,早前寒武紀(jì)地質(zhì)演化的復(fù)雜性是各種演化機(jī)制和模型產(chǎn)生的重要原因之一,譬如,冀東地區(qū)形成在2.55 ~2.5Ga 的TTG 質(zhì)片麻巖也有認(rèn)為是島弧環(huán)境俯沖的產(chǎn)物而非地幔柱機(jī)制(Nutman et al.,2011)。因此,地幔柱模式仍需要諸多證據(jù)。

4.4.2 俯沖模式

本文低鋁高硅的奧長(zhǎng)花崗質(zhì)片麻巖的原始巖漿不可能由洋殼低壓熔融。其形成深度即便按照推測(cè)的太古宙地溫梯度也無(wú)法熔融(圖9)。如果俯沖板片能夠在較低壓力下發(fā)生部分熔融,那么也會(huì)產(chǎn)生大量廣泛分布的同性質(zhì)的低鋁高硅TTG 巖系。然而目前為止,華北克拉通中部高級(jí)變質(zhì)地體中僅懷安報(bào)道有此類(lèi)巖石的存在。而且,本文的低鋁奧長(zhǎng)花崗質(zhì)片麻巖的巖石化學(xué)性質(zhì)與Halla et al. (2009)所指的低鋁TTG 巖系的巖石地球化學(xué)性質(zhì)有很大不同。前人所指的低鋁TTG 巖系,其SiO2含量相對(duì)較低且變化范圍大,輕、重稀土分餾相對(duì)更強(qiáng)烈。而本文的低鋁奧長(zhǎng)花崗質(zhì)片麻巖的巖漿溫度明顯高于本區(qū)同時(shí)代的高鋁TTG 巖系。如果前者是低壓洋殼熔融,后者則必然是洋殼高壓條件下熔融的產(chǎn)物。俯沖產(chǎn)生的巖漿要穿過(guò)地幔楔,高壓深部條件下形成的熔體與地幔作用后的溫度應(yīng)該更高,而低壓條件下形成的熔體與淺部地幔楔作用,巖漿的溫度應(yīng)該相對(duì)更低(詳見(jiàn)Xiong et al.,2006),這與本文數(shù)據(jù)結(jié)果相反。因此,本文低鋁奧長(zhǎng)花崗質(zhì)片麻巖不太可能由洋殼低壓熔融而成。

假如~2.53Ga 的低鋁奧長(zhǎng)花崗質(zhì)片麻巖來(lái)自俯沖洋殼的熔融,其熔融壓力低的特點(diǎn)可以用板片斷裂模式加以解釋(Davies and von Blanckenburg,1995;Schoonmaker et al.,2005)。俯沖板片在陸核對(duì)接初期,板片的拖拽力和陸核碰撞阻力共同導(dǎo)致洋殼發(fā)生撕裂形成不斷發(fā)育的斷裂帶并導(dǎo)致大洋巖石圈減壓熔融而形成輝長(zhǎng)質(zhì)巖漿的底侵作用,這進(jìn)一步引發(fā)局部洋殼的低壓低部分熔融現(xiàn)象,從而形成低鋁奧長(zhǎng)花崗質(zhì)巖漿的侵入。隨著俯沖板片斷裂帶的發(fā)育可以最終導(dǎo)致俯沖板片的斷離。板片斷裂模式可以解釋新太古代多種高溫巖漿先后出現(xiàn)的特征(Zhang et al.,2013b)。板片斷裂發(fā)育過(guò)程中會(huì)引起軟流圈物質(zhì)上涌規(guī)模的逐漸增大,引起巖石圈地幔以及洋殼不同程度的大范圍熔融,形成拉斑玄武質(zhì)巖漿底侵和洋殼熔融成因的酸性巖漿。底侵造成基性下地殼重熔形成TTG 巖系和高溫紫蘇花崗巖類(lèi),而先期侵入下地殼的TTG 則可能發(fā)生重熔并形成2.5 ~2.45Ga 的鉀質(zhì)花崗巖類(lèi)的侵入活動(dòng)(趙宗溥,1993;伍家善等,1998;Zhai and Liu,2003;Zhang et al.,2011;劉樹(shù)文等,2011;Zhou et al.,2011)。這種模式下形成的TTG 巖系之物源應(yīng)具有多元性,例如,不同深度上的洋殼和新生大陸地殼,其形成時(shí)間上,應(yīng)晚于2.53Ga 的低鋁奧長(zhǎng)花崗質(zhì)巖漿,結(jié)束的標(biāo)志為大量鉀質(zhì)花崗巖巖漿的形成。

該模式對(duì)陸-陸(弧)對(duì)接產(chǎn)生板片斷裂比較合適。五臺(tái)-恒山地區(qū)大量新太古代表殼巖(前莊旺表殼巖)被2.55 ~2.45Ga 的恒山片麻巖侵入(田永清,1991),五臺(tái)雜巖的基性和中酸性火山巖被時(shí)間非常相近的2.55 ~2.45Ga 的新太古代花崗巖類(lèi)侵入(田永清,1991),符合板片斷裂模式下大量巖漿侵入大陸弧前盆地沉積巖和火山巖中的特點(diǎn)(Schoonmaker et al.,2005)?;◢従G巖帶成因可以采用這種模式解釋?zhuān)侨蚩死ǖ幕◢従G巖帶是圍繞高級(jí)變質(zhì)地體(灰色片麻巖)分布(Windley,1995),所以俯沖板片圍繞高級(jí)變質(zhì)地體呈卵形分布,且均在新太古末發(fā)生斷裂這一模式來(lái)解釋不盡合理。因此,筆者認(rèn)為本區(qū)低鋁奧長(zhǎng)花崗質(zhì)巖漿更可能來(lái)自基性下地殼的熔融,而非俯沖板片。

最近,Martin et al.(2014)依據(jù)前人提出的大陸幕式生長(zhǎng)現(xiàn)象并結(jié)合玄武巖地球化學(xué)特征,提出高鋁TTG 巖系的源區(qū)不是俯沖板片的MORB 玄武巖,而是俯沖的洋底高原玄武巖,并認(rèn)為這可能是太古宙TTG 巖系與顯生宙Adakite 巖系均形成在俯沖環(huán)境而地球化學(xué)特征不同的重要原因之一。然而,作者沒(méi)有解釋同時(shí)期伴生的低鋁TTG 巖系的成因。

本區(qū)同時(shí)代還形成大量輝長(zhǎng)質(zhì)-閃長(zhǎng)質(zhì)巖漿的侵入(Zhai,1996;Liu et al.,2012),結(jié)合本區(qū)同時(shí)代低鋁和高鋁TTG 巖系的源區(qū)礦物特點(diǎn),筆者認(rèn)為前者更可能是底侵巖漿,它們導(dǎo)致下地殼在不同深度上熔融形成廣泛分布的高鋁TTG 巖漿和少量低壓熔融的低鋁TTG 巖系。該熱事件的結(jié)束標(biāo)志是大量2.5 ~2.45Ga 鉀質(zhì)花崗巖巖漿的侵入。對(duì)于2.5 ~2.4Ga 鉀質(zhì)花崗巖的成因意義仍有不同看法(Zhai and Liu,2003;Zhao et al.,2006;劉樹(shù)文等,2011;Zhou et al.,2011;Zhang et al.,2011;Liu et al.,2012),其分布不只局限在懷安陸塊,而是整個(gè)華北克拉通(趙宗溥,1993),它們主要來(lái)自下地殼TTG 的重熔,說(shuō)明當(dāng)時(shí)大陸地殼已經(jīng)廣泛存在并具有相當(dāng)厚度,大規(guī)模陸殼生長(zhǎng)已經(jīng)完成,暗示新太古末巖漿底侵熱事件的結(jié)束。同時(shí),我們也不能忽略TTG 和鉀質(zhì)花崗巖的形成均屬于地殼重熔作用,反映深部地殼存在過(guò)高級(jí)變質(zhì)作用(Martin,1987;程裕淇等,2004;Kemp et al.,2007),其廣泛形成也標(biāo)志著深部地殼存在過(guò)區(qū)域麻粒巖相變質(zhì)作用。在此,筆者認(rèn)為地幔柱(或地幔上涌)或局部與俯沖板片共同作用的模式(Condie,1998)能夠解釋?xiě)寻?,乃至整個(gè)華北克拉通太古宙廣泛分布的TTG 巖漿和變質(zhì)作用。

5 結(jié)論

(1)懷安片麻巖中低鋁奧長(zhǎng)花崗質(zhì)片麻巖來(lái)自新增生基性地殼在角閃巖-麻粒巖相條件下低部分程度熔融的熔體,巖漿經(jīng)歷副礦物獨(dú)居石的分離結(jié)晶作用;同期形成的高鋁TTG 巖系由新生基性地殼在石榴石角閃巖相條件下熔融而成,巖漿經(jīng)歷了角閃石和輝石為主的分離結(jié)晶作用和斜長(zhǎng)石的堆晶作用。

(2)TTG 巖系的微量元素含量特征不只與壓力有關(guān),同時(shí)也受結(jié)晶溫度影響。

(3)新太古代懷安片麻巖可能是地幔柱或地幔柱與俯沖板片共同作用下的結(jié)果。

致謝 衷心感謝西北大學(xué)大陸動(dòng)力學(xué)實(shí)驗(yàn)室主微量元素分析實(shí)驗(yàn)室全體員工的辛勤測(cè)試工作;對(duì)中國(guó)科學(xué)院地質(zhì)與地球物理研究所翟明國(guó)研究員在野外工作上的大力支持及對(duì)初稿的鼓勵(lì)表示衷心感謝;對(duì)共同參加野外考察的成都理工大學(xué)地球科學(xué)與資源學(xué)院倪志耀教授、中國(guó)科學(xué)院地質(zhì)與地球物理研究所彭澎研究員、李鐵勝博士、王偉博士、趙磊博士等表示真誠(chéng)的謝意。中國(guó)科學(xué)院廣州地球化學(xué)研究所趙太平研究員、黃小龍研究員,中國(guó)科學(xué)院地質(zhì)與地球物理研究所錢(qián)青副研究員、周艷艷博士,西北大學(xué)第五春榮博士對(duì)本文提出了諸多批評(píng)和修改建議,糾正了文中許多謬誤并對(duì)部分文字進(jìn)行了修正,大大地提高了本文質(zhì)量,作者對(duì)他們辛勤勞動(dòng)和無(wú)私奉獻(xiàn)深表謝意。

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