孔凡梅 劉云 李旭平** 郭敬輝 趙國春
KONG FanMei1,LIU Yun1,LI XuPing1**,GUO JingHui2 and ZHAO GuoChun3
1. 山東省沉積成礦作用實驗室,山東科技大學,青島 266510
2. 中國科學院地質(zhì)與地球物理研究所,北京 100029
3. 香港大學地球科學系,香港
1. Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals,Shandong University of Science and Technology,Qingdao 266510,China
2. Institute of Geology and Geophysics,CAS,Beijing 100029,China
3. Department of Earth Sciences,University of Hong Kong,Pokfulam Road,Hong Kong,China
2014-10-11 收稿,2015-04-03 改回.
華北克拉通東部陸塊在太古代晚期~2.5Ga 發(fā)生過一次重要的俯沖-碰撞拼合事件,將古老的微陸塊焊接到一起,形成了華北克拉通的基本格架(Zhai and Liu,2003;Zhai and Santosh,2011;Zhao and Zhai,2013),此次碰撞引發(fā)區(qū)域性變質(zhì)深熔作用,在俯沖階段形成TTG 片麻巖和石榴黑云斜長片麻巖,與此同時,該正片麻巖被同構(gòu)造花崗巖類巖石侵入(Zhai and Liu,2003;Zhai and Santosh,2011),早期地殼受到強烈的再造(頡頏強等,2013;單厚香等,2013),之后古元古-中元古發(fā)生裂解(Zhai and Liu,2003)。眾多研究成果顯示,在1950 ~1850Ma 期間,華北地塊經(jīng)歷了一次高壓麻粒巖相變質(zhì)事件(Zhao et al.,2001;Zhao and Guo,2012;Zhou et al.,2008;Tam et al.,2011;劉平華等,2011,2013),部分學者認為華北克拉通于此期間發(fā)生陸-陸碰撞、拼合成統(tǒng)一的克拉通變質(zhì)基底(Zhao et al.,2005)。
膠北地塊結(jié)晶基底是華北克拉通東部陸塊的重要組成部分。年代學研究成果顯示,該區(qū)在早前寒武紀經(jīng)歷了多期巖漿-變質(zhì)熱事件,主要表現(xiàn)為三期巖漿事件(~2.9Ga、~2.7Ga和~2.5Ga)和兩期熱變質(zhì)事件(~2.5Ga 和1.8 ~1.9Ga)(Tang et al.,2007;Jahn et al.,2008;Zhou et al.,2008;劉建輝等,2011,2012;徐揚等,2011;董春艷等,2010;Liu et al.,2013a;頡頏強等,2013)。最近的年代學研究表明,膠北地塊變泥質(zhì)巖和基性高壓麻粒巖的變質(zhì)時間為1.85 ~1.95Ga 之間(Zhou et al.,2008;Tam et al.,2011;Zhao and Guo,2012),高壓麻粒巖相峰期變質(zhì)時間為1.85 ~1.9Ga(劉平華等,2011)。
膠北地塊基性高壓麻粒巖賦存于太古代花崗片麻巖和TTG 片麻巖中(劉文軍等,1998;劉平華等,2010;Tam et al.,2012a),其原巖為形成于弧后擴張背景下的拉斑玄武質(zhì)巖石。巖石在膠-遼-吉帶碰撞閉合過程中,先經(jīng)歷了麻粒巖相變質(zhì)作用,隨后又經(jīng)歷了抬升過程中退變質(zhì)和鈣硅酸巖化作用(劉文軍等,1998;劉平華等,2010;李旭平等,2013)。斜長角閃巖與二輝麻粒巖形成于膠北地塊古元古代地層沉積之前的一次地殼伸展作用,后于古元古代晚期(1.95 ~1.80Ga)卷入了碰撞造山作用過程(劉平華等,2013)。
研究資料顯示,膠北地塊鎂鐵質(zhì)巖主要為太古代基性-超基性侵入巖和古元古代基性-超基性侵入巖,例如具2839~2904Ma 的Sm-Nd 模式年齡的福山后變質(zhì)橄欖輝石巖和鋯石U-Pb 年齡為1852 ±9Ma、1868 ±11Ma 的西水夼變輝長巖(王世進等,2009)。研究者于膠北地塊荊山群分布區(qū)發(fā)現(xiàn)超鎂鐵質(zhì)巖(周喜文等,2004;Tam et al.,2011),膠北地塊出露的棲霞TTG 片麻巖中不規(guī)則透鏡狀超鎂鐵巖,變質(zhì)演化P-T-t 軌跡具有典型碰撞造山帶順時針型式,可能是古老陸塊之間碰撞造山的產(chǎn)物(劉平華,2011;劉平華等,2011,2012;Liu et al.,2012,2013b)。本文研究的超鎂鐵巖石,為出露于膠北地塊前寒武紀結(jié)晶基底TTG 片麻巖中的尖晶石橄欖巖、輝石巖和角閃石巖,通過對其進行礦物巖石和主、微量元素的地球化學的分析,探討其形成的熱力學特征及其后期的交代變質(zhì)作用,進而探究該超鎂鐵質(zhì)巖石的原巖性質(zhì)、形成的構(gòu)造背景及其所經(jīng)歷的地質(zhì)作用過程。
膠北地塊位于華北克拉通東部,北臨渤海,位于膠-遼-吉活動帶的南端(Zhao et al.,1999,2000);東南側(cè)大致以五蓮-即墨-牟平斷裂帶為界鄰接蘇魯造山帶;西側(cè)以郯廬斷裂為界鄰接魯西地體;膠北地塊中五蓮-即墨-牟平斷裂帶以北,發(fā)育了中生代膠萊盆地(圖1)。膠北前寒武紀結(jié)晶基底巖石組合及演化歷史與遼東半島非常相似(Zhao et al.,1999,2005;李旭平等,2011)。膠北地塊出露的太古宙陸殼物質(zhì)主要是稱為膠東群的一套形成于晚太古代的變質(zhì)火山沉積巖系,及以粉子山群和荊山群為代表的早元古代變質(zhì)雜巖系列(宋明春和李洪奎,2001;宋明春,2008;李旭平等,2011)。
膠北地塊的核部主要有中新太古代的膠東巖群變質(zhì)火山巖系列、棲霞TTG 質(zhì)片麻巖、花崗質(zhì)片麻巖和少量中新太古代至古元古代鎂鐵質(zhì)巖石及膠東巖群中基性火山巖(宋明春和李洪奎,2001;Jahn et al.,2008)。TTG 片麻巖分布在萊西和棲霞地區(qū),前人研究成果表明至少經(jīng)歷3 期的TTG 巖漿事件,而2.6 ~2.5Ga 期間最為強烈(劉建輝等,2011,2012)。而TTG 片麻巖中分布有眾多大小不等的同期或稍早的透鏡狀超鎂鐵質(zhì)巖侵入體和更早的中太古代斜長角閃巖與高壓麻粒巖。膠北荊山群分布于萊陽-萊西-平度-安丘等地,圍繞膠東群分布,其巖石組合主要為富鋁的片巖-片麻巖、透輝石巖、大理巖、石墨片麻巖、黑云斜長片麻巖和鎂鐵質(zhì)巖石(Tam et al.,2011),經(jīng)歷了高角閃巖相-麻粒巖相變質(zhì)作用。粉子山群則分布在膠北地塊北部的蓬萊和煙臺等地,巖石組合主要為泥砂質(zhì)片巖、片麻巖和大理巖等,其變質(zhì)程度相對較低,經(jīng)歷了高綠片巖相到低角閃巖相變質(zhì)作用(周喜文等,2004;王舫等,2010;Tam et al.,2011)。
圖1 膠北地塊構(gòu)造地質(zhì)簡圖及采樣位置(據(jù)周喜文等,2004;李旭平等,2013 修改)Fig.1 Geological schematic map of the Jiaobei terrane and sample localities (modified after Zhou et al.,2004;Li et al.,2013)
膠北前寒武紀結(jié)晶基底的巖石據(jù)其物質(zhì)組成和形成時代約分為五類:①TTG 質(zhì)-閃長質(zhì)和花崗質(zhì)片麻巖類(劉建輝等,2011;單厚香等,2013;頡頏強等,2013;劉平華等,2014);②新太古代變質(zhì)表殼巖系,在不同地區(qū),巖石類型有所差異,棲霞北部主要巖石類型為斜長角閃巖、黑云斜長片麻巖和黑云變粒巖等(劉平華等,2014);③元古代變質(zhì)表殼巖系,主要為相當于孔茲巖系的含石榴石富鋁片巖-片麻巖、長英質(zhì)片巖-片麻巖、大理巖和鈣鎂硅酸鹽巖(劉平華等,2014),并包括高壓泥質(zhì)麻粒巖(周喜文等,2004;Zhou et al.,2008;王舫等,2010;Tam et al.,2012b,c)和中-新元古代淺變質(zhì)巖系及古元古代變質(zhì)侵入巖(劉平華等,2014);④基性高壓麻粒巖類(白瑾等,1996;劉文軍等,1998;劉平華等,2010;李旭平等,2011,2013;Tam et al.,2012a;Liu et al.,2013a,b;劉平華等,2014);⑤鎂鐵-超鎂鐵質(zhì)巖類(劉平華,2011;劉平華等,2014)。
膠北地塊的超鎂鐵質(zhì)巖石,主要分布在萊西-萊陽、招遠、棲霞一帶;該區(qū)的超鎂鐵質(zhì)巖石多以不規(guī)則透鏡體或者變形巖墻的形式被包裹于花崗質(zhì)或TTG 質(zhì)片麻巖中,并常與高壓基性麻粒巖伴生(劉平華,2011)。前人研究成果表明,膠北棲霞超鎂鐵質(zhì)巖石中鋯石記錄的高壓變質(zhì)時代為1858~1877Ma,此年齡與區(qū)內(nèi)高壓基性麻粒巖記錄的1900 ~1850Ma 峰期高壓變質(zhì)時間相吻合,該超鎂鐵質(zhì)巖退變質(zhì)時代為1820 ~1840Ma(劉平華,2011);元古代荊山群分布區(qū)也存在變質(zhì)基性-超基性巖,已有的研究成果指出其207Pb/206Pb變質(zhì)年齡為1956 ±41Ma 和1884 ±24Ma(Tam et al.,2011)。
本文中超鎂鐵質(zhì)樣品采自萊西福山后村,萊西懷古莊與張各莊之間,招遠大吳家、蘇家莊子等地的太古代基底中(圖2)。研究地區(qū)采集的蛇紋巖、蛇紋巖化橄欖巖、輝石巖和角閃石巖,來自包含在新太古代TTG 片麻巖中的透鏡狀或塊狀巖體(圖2)。招遠地區(qū)的超鎂鐵巖蛇紋巖化較嚴重,萊西地區(qū)所取樣品與劉平華(2011)論述的A 類高鎂超基性巖一致(圖2b),據(jù)山東省地質(zhì)礦產(chǎn)局第四地質(zhì)隊區(qū)調(diào)研究所,為變超基性巖類。取樣點皆落入1∶5 萬地質(zhì)圖上新太古代的基性-超基性變質(zhì)巖區(qū)。本文鑒定觀察和研究分析了26 件樣品,選取其中14 件進行地球化學主、微量元素的分析。
圖2 膠北地塊研究區(qū)前寒武紀基底巖石分布及采樣點位置(據(jù)山東省地質(zhì)礦產(chǎn)局第四地質(zhì)隊區(qū)調(diào)研究所,1995①山東省地質(zhì)礦產(chǎn)局第四地質(zhì)隊區(qū)調(diào)研究所. 1995. 中華人民共和國地質(zhì)圖(1∶50000). 山東省地質(zhì)印刷廠印刷)(a)招遠地區(qū);(b)萊西地區(qū)Fig. 2 Distribution of Precambrian rocks and sample localities in the investigated area of the Jiaobei terrane(a)Zhaoyuan area,(b)Laixi area
礦物的電子探針分析數(shù)據(jù)是在中國海洋大學海底科學與探測技術(shù)教育部重點實驗室完成,儀器型號JXA-8230;分析條件:加速電壓15kV,束流20nA,束斑1μm;數(shù)據(jù)通過ZAF correction produce 校正。對數(shù)據(jù)進行分析處理是采用Ax、Geokit、Minpet 等軟件計算礦物端元組分。主微量元素在中科院地球化學研究所礦床地球化學重點實驗室完成,主量元素采用PW4400 型X 熒光儀測定;微量元素采用ICP-MS 質(zhì)譜分析儀,分析方法如Qi and Grégoire(2000)。根據(jù)對標準樣品GBPC-1de 分析結(jié)果,分析誤差<5%。圖文中礦物縮寫符號據(jù)Whitney and Evans(2010)。
研究區(qū)的超鎂鐵巖樣品均發(fā)生中等-強的蛇紋巖化,其組成礦物主要是橄欖石、斜方輝石、角閃石、蛇紋石等,次要礦物有尖晶石、綠泥石、磁鐵礦及方解石等;副礦物有鈦鐵礦和磷灰石。角閃石巖相對較新鮮。此外26 件研究樣品中均未發(fā)現(xiàn)單斜輝石。根據(jù)礦物組合可以將研究區(qū)巖石分為以下幾種類型:
(1)蛇紋石化尖晶石方輝橄欖巖
巖石發(fā)生中度蛇紋石化,其礦物組合為橄欖石(40% ~50%)、蛇紋石(20% ~30%)、斜方輝石(5% ~15%)、尖晶石(5% ~10%)和角閃石(5% ~10%),還含有少量磁鐵礦、鈦鐵礦和榍石。為表達簡便,后文中將其簡稱為尖晶石橄欖巖或橄欖巖。
其中尖晶石有兩期成因,早期尖晶石(Spl1)被包裹于斜方輝石中,或與早期大顆粒的斜方輝石共生(圖3a,b),呈褐綠色,自形-半自形粒狀,粒徑0.2 ~0.5mm(圖3a,b);晚期尖晶石(Spl2)淡紅褐色,他形晶,通常<0.2mm,與后期形成的角閃石密切共生(3c)。橄欖石也分兩期形成,早期為包裹在大顆粒斜方輝石中的渾圓狀(Ol1)或大顆粒殘斑狀橄欖石,晚期橄欖石顆粒細小(Ol2),分布于大顆粒殘斑狀橄欖石或斜方輝石周圍。早期斜方輝石(Opx1)顆粒較大,約0.5mm~2cm。而晚期斜方輝石(Opx2)交代早期大顆粒斜方輝石(Opx1)的邊緣,或呈不規(guī)則細粒狀產(chǎn)出于早期斜方輝石邊部,并與早期角閃石緊密共生。早期角閃石(Amp2)與晚期橄欖石(Ol2)和晚期斜方輝石(Opx2)共生,晚期角閃石(Amp3)與蛇紋石、磁鐵礦和方解石等礦物共生(圖3f)。
(2)蛇紋石化尖晶石橄欖斜方輝石巖
主要礦物為斜方輝石(40% ~50%)、蛇紋石(25% ~30%)、橄欖石(5% ~10%)、尖晶石(5% ~10%)和角閃石(10% ~20%)。為表達簡便,后文中將其簡稱為尖晶石輝石巖或輝石巖。
其它礦物有綠泥石、磁鐵礦、鈦鐵礦、方解石、磷灰石等。斜方輝石粒徑約0.5 ~2mm,大顆粒斜方輝石中間保留了早期特征(Opx1),邊緣為受交代作用改造的晚期產(chǎn)物。橄欖石多為后期重結(jié)晶形成的細小顆粒。尖晶石為綠色(Spl2),形狀極不規(guī)則,粒徑通常<0.2mm,大多裹在蛇紋石中。角閃石也呈現(xiàn)兩期特征。部分樣品中見方解石細脈。
(3)蛇紋巖
蛇紋石含量大于90%,網(wǎng)格狀結(jié)構(gòu)中殘留了少量的橄欖石,常見蛇紋石完全交代橄欖石卻仍保留了橄欖石顆粒形態(tài)假象,假象顆粒間常呈現(xiàn)三邊平衡結(jié)構(gòu)(圖3e),顯示橄欖石顆粒在蛇紋石化之前經(jīng)歷了變質(zhì)重結(jié)晶作用。少量絹石化斜方輝石,具有斜方輝石假象。還有磁鐵礦、方解石、綠泥石等次生礦物,樣品發(fā)生了程度不同的碳酸鹽化現(xiàn)象。
(4)角閃石巖
巖石由角閃石(90%)、斜方輝石(5%)、尖晶石(1% ~2%)、橄欖石組成,還見有少量斜長石、鈦鐵礦、方解石、伊丁石等礦物,其中尖晶石含量很少,為綠色尖晶石。角閃石也分早晚兩期,分別與斜方輝石-橄欖巖-尖晶石和綠泥石-蛇紋石-磁鐵礦-伊丁石-方解石共生(圖3h)。
采自膠北地塊新太古變質(zhì)基底的尖晶石橄欖巖和尖晶石輝石巖中角閃石全部為綠色鎂角閃石與透閃石,為常見的變質(zhì)角閃石,而非富鈦、富鉻的地幔角閃石(樊祺誠等,1992)。這兩類超鎂鐵質(zhì)巖中含有大量綠色高鋁尖晶石,有學者認為綠色高鋁尖晶石是麻粒巖相的標志性礦物(Evans and Frost,1975;劉平華,2011),則研究樣品中綠色高鋁尖晶石和晚期的橄欖石、斜方輝石代表了麻粒巖相的礦物組合,而鎂普通角閃石的出現(xiàn),又喻示巖石經(jīng)歷了角閃巖相的變質(zhì)作用。依據(jù)巖相學和礦物化學分析認為,采自膠北地塊變質(zhì)基底中的尖晶石橄欖巖的演化可以分為3 個階段,分別為:M1 原巖形成階段,早期殘留的原巖礦物組合為Opx1+
Jiaobei massifOl1+Spl1;M2 麻粒巖-角閃巖相變質(zhì)階段,該階段礦物組合為Opx2+Ol2+Spl2+Amp2;M3 晚期綠片巖相變質(zhì)階段,這階段新生的礦物為Amp3、Srp3、Mag3等,還有綠泥石等礦物。所有的巖石類型中都觀察到有碳酸鹽化現(xiàn)象,常見方解石與蛇紋石共生,部分薄片中局部見方解石細脈切穿蛇紋石,顯示碳酸鹽化與蛇紋石化同期或稍晚的特征。圖3 中礦物代號的下標代表了以上劃分的不同階段。
表1 膠北地塊新太古超鎂鐵巖中橄欖石、斜方輝石和尖晶石的礦物成分(wt%)Table 1 Representative mineral compositions (wt%)of olivine,orthopyroxene and spinel in the ultramafic rocks from Late Archean
分別對研究區(qū)的尖晶石橄欖巖、尖晶石斜方輝巖和角閃石巖等進行礦物成分特征研究,電子探針礦物組分分析結(jié)果列在表1、表2。
斜方輝石 在尖晶石橄欖巖中早期代表地幔巖原巖的斜方輝石,En 為89.45 ~89.96,F(xiàn)s 值為9.55 ~10.01,屬頑火輝石范圍,Mg#為0.90 ~0.91;晚期斜方輝石受到交代作用的改造,En 為88.06 ~88.50,F(xiàn)s 為10.91 ~11.43,也屬于頑火輝石范圍,Mg#~0.89。尖晶石輝石巖中斜方輝石En 為84.52 ~85.31,F(xiàn)s 為14.29 ~15.06,屬古銅輝石范圍,Mg#為0.89 ~0.91。角閃石巖中斜方輝石En 為76.40 ~76.94,F(xiàn)s為21.85 ~23.25,屬于古銅輝石范圍,Mg#為0.79 ~0.80。且尖晶石橄欖巖中斜方輝石Al 的含量低,在Al2O3-TiO2圖解中,顯示其具克拉通橄欖巖中斜方輝石特征(圖4)。
橄欖石 在尖晶石橄欖巖中,早期橄欖石Fo 值范圍為88.42 ~90.40,F(xiàn)eO 含量9.17% ~10.96%,MnO 含量0.13%~0.19%。晚期重結(jié)晶的小顆粒橄欖石Fo 略有降低,為87.01~87.37,F(xiàn)eO 和MnO 的含量升高,分別為12.02% ~12.34%和0.20% ~0.34%。尖晶石輝石巖中橄欖石也表現(xiàn)出與尖晶石橄欖巖相同的變化規(guī)律,早晚兩期的橄欖石呈現(xiàn)Fo 值逐漸降低,而MnO 含量升高的特征(圖5)。
表2 膠北地塊新太古超鎂鐵巖中角閃石、蛇紋石及其他礦物的化學成分(wt%)Table 2 Representative mineral composition (wt%)of amphibole,serpentine and other minerals in the ultramafic rocks from Late Archean Jiaobei massif
圖4 斜方輝石Al2O3-TiO2 圖解灰色區(qū)域和實線圈閉區(qū)域分別代表克拉通橄欖巖和非克拉通橄欖巖(據(jù)Rudnick et al. ,2004 修改)Fig.4 Al2O3-TiO2 diagram of orthopyroxeneGrey area and solid circle represent cratonic peridotite and offcratonic peridotite respectively (after Rudnick et al. ,2004)
圖5 膠北地塊超鎂鐵巖中橄欖石的Fo-MnO 關(guān)系圖解Fig. 5 Fo-MnO diagram of olivines from the ultramafic rocks in Jiaobei terrance
圖6 膠北地塊超鎂鐵巖中尖晶石的XMg-Cr#關(guān)系圖解(a)、Mg-Fe2+-Cr 成分圖解(b,據(jù)Della-Pasqua,1995)和Al2O3-Cr2O3 圖解(c,據(jù)Franz and Wirth,2000)Fig. 6 XMg-Cr# diagram (a),Mg-Fe2+-Cr diagram (b,after Della-Pasqua,1995)and Al2O3-Cr2O3 diagram (c,after Franz and Wirth,2000)of spinels from the ultramafic rocks in Jiaobei terrane
圖7 膠北地塊超鎂鐵巖中角閃石成分特征(a)角閃石的Si-Mg/(Mg +Fe2+)成分分類圖(據(jù)Leake et al. ,1997);(b)Al-Ti 成分圖解(據(jù)靳是琴,1991)Fig. 7 Amphibole chemistry from ultramafic rocks in Jiaobei terrane(a)Si-Mg/(Mg+Fe2+)diagram (after Leake et al. ,1997);(b)Al-Ti diagram (after Jin,1991)
尖晶石 尖晶石橄欖巖中早期的尖晶石Mg#為0.72 ~0.79,Cr#為0.06 ~0.11,Al2O3含量60.25% ~63.03%,Cr2O3為5.60% ~6.77%,F(xiàn)eO 為12.24% ~12.28%,TiO2<0.05%。晚期的尖晶石Mg#降低為0.68 ~0.71,Cr#為0.06~0.12,Al2O3含量也有所降低,而FeO 含量增加。
尖晶石輝石巖中綠色鋁尖晶石Mg#范圍為0.58 ~0.67,Cr#為0.07 ~0.16,Al2O3含量為51.1% ~59.1%,Cr2O36.15% ~14.07%,MgO 14.02% ~16.74%。角閃石巖中也存在少量綠色鋁尖晶石,Mg#為0.49 ~0.51,Cr#0.06 ~0.09。不同階段和巖石類型中的尖晶石,從早期到晚期,從橄欖巖、輝石巖到角閃石巖,Mg#依次降低,F(xiàn)eO 含量依次升高,說明隨著變質(zhì)作用的進行,由于Mg-Fe 擴散作用的影響,Mg 逐漸被Fe 替代(圖6a)。研究的超鎂鐵巖尖晶石,尤其在早期地幔巖階段形成的高Mg、Al 低Cr 尖晶石,其成分區(qū)介于深海橄欖巖的尖晶石和鋁尖晶石成分區(qū)之間(圖6b)。而在Al2O3-Cr2O3關(guān)系上(圖6c),研究區(qū)超鎂鐵巖的尖晶石在成分上主要表現(xiàn)為變質(zhì)交代成因(Evans and Frost,1975;Sar?fak?ogˇlua et al.,2010),其Cr#較低,Mg#高可能主要與其寄主巖石堆晶成因或交代作用有關(guān),幾乎沒有進入地幔分異演化過程(Haggerty,1989;Franz and Wirth,2000),但也可能表明膠北基底的超鎂鐵巖是地幔低部度熔融的產(chǎn)物(Pearce et al.,2000;Choi et al.,2008)。
角閃石 尖晶石橄欖巖和尖晶石輝石巖中所含角閃石明顯不同于地幔環(huán)境形成的富鈦、富鉻的棕色或褐色角閃石(樊祺誠等,1992),這兩種巖石類型中所有角閃石皆為綠色或無色,且TiO2、Cr2O3和Al2O3含量均較低,顯示它們形成于超鎂鐵質(zhì)巖石構(gòu)造折返的變質(zhì)作用過程(劉平華,2011),而非地幔環(huán)境中形成。據(jù)Leake et al.(1997)的角閃石分類,尖晶石橄欖巖和橄欖斜方輝石巖中所含角閃石均屬于鈣質(zhì)角閃石,大部分為鎂角閃石與透閃石(圖7a),都是常見的變質(zhì)角閃石。
角閃石巖中的角閃石,同屬鎂角閃石,也分為兩期,但前者Mg#低,而后者較高,晚期角閃石中較富Mg,因歸于變質(zhì)交代成因的角閃石受到源于超鎂鐵巖蛇紋巖化釋放出富Mg 流體影響的結(jié)果。
角閃石中鈦的含量是受溫度控制的,一般溫度越高,角閃石中鈦的含量也越高。采用Al-Ti-變異圖進行角閃石成因分析(圖7b,據(jù)靳是琴,1991),該圖基于變基性巖石建立的角閃石分類圖件,膠北地塊尖晶石橄欖巖和尖晶石輝石巖中早期的角閃石落在角閃巖相區(qū),而晚期角閃石則分布于綠片巖相區(qū);角閃石巖中早、晚兩期角閃石分別落在了麻粒巖-高角閃巖相和低角閃巖相區(qū),這表明角閃石巖中早期角閃石與巖漿作用有關(guān)。
蛇紋石 交代橄欖石形成網(wǎng)紋狀結(jié)構(gòu),同時析出磁鐵礦。蛇紋石沒有任何受到應力的跡象,表明形成于非擠壓環(huán)境的晚期交代變質(zhì)作用過程。蛇紋石的Mg#多變化于89 ~97 之間,蛇紋巖中蛇紋石的Mg#約95 ~97,而尖晶石橄欖巖和尖晶石輝石巖中Mg#分別為89 ~94 和約92,表明蛇紋巖的成分繼承了原巖的特征。斜長石:在角閃石巖中An 約48.22 ~49.52,為中長石。尖晶石橄欖巖和輝石巖中的綠泥石為鎂綠泥石Mg#約0.91 ~0.93。
圖8 研究區(qū)橄欖巖的主元素變化五角星代表太古代橄欖巖的平均成分(Griffin et al. ,1999);灰色區(qū)域代表世界上典型的克拉通橄欖巖(據(jù)Rudnick et al. ,2004 及其中引用的文獻);十字符號代表原始地幔(McDonough and Sun,1995)Fig.8 Major element compositional variations for the serpentinite peridotites investigatedStar represents average“Archaen”from Griffin et al. (1999);Gray field represents typical cratonic peridotites from the craton of Anzanian,Kaapvaal,Daldyn,Siberia and Slave (after Rudnick et al. ,2004 and references there within);Cross denotes primitive mantle (P. M. )from McDonough and Sun(1995)
表3 膠北地塊新太古代超鎂鐵巖的主量元素(wt%)和微量元素(×10 -6)的分析結(jié)果Table 3 Major (wt%)and trace (×10 -6)elements analyses of the ultramafic rocks from Late Archean Jiaobei massif
巖石中某些元素的地球化學性質(zhì)在變質(zhì)、交代作用過程中可以發(fā)生遷移,尤其是活動元素,如K、Na、Ca 等主元素、輕稀土、大離子親石元素等微量元素。然而有些元素卻相對穩(wěn)定,特別是某些微量元素如Zr、Y、Ti、Nb 等高場強元素,很可能保留了原巖的性質(zhì)(Pearce et al.,2000;Pearce,2008)。對于膠東變質(zhì)基底的鎂鐵-超鎂鐵巖石,盡管漫長復雜的變質(zhì)演化歷史,對一些主、微量元素產(chǎn)生影響,但在多方面的分析和研究中仍可追蹤原巖產(chǎn)出的地質(zhì)信息(Ernst,1989;Rudnick et al.,2004;Deschamps et al.,2013)。即使是蛇紋石化,其相容元素和稀土元素也很少活動,可以幫助鑒別原巖的信息,甚至巖漿作用過程中熔體與巖石的作用關(guān)系(Deschamps et al.,2013)。膠北地區(qū)鎂鐵-超鎂鐵石的14 個樣品的主元素和微量元素的分析結(jié)果列于表3。其中包括9個超基性巖樣品,2 個輝石巖樣品和2 個斜長角閃巖-角閃巖樣品。圖中主元素氧化物成分點是去除水分后,再換算成100%,F(xiàn)e2O3換算成FeOT后的投點。
圖9 Si/Al-(Mg+Fe)/Al (a)和Mg/Ti-Fe/Ti (b)的元素比值圖解Fig.9 The element ratios Si/Al vs. (Mg + Fe)/Al (a)and Mg/Ti vs. Fe/Ti (b)
膠北地塊萊西-招遠地區(qū)超鎂鐵巖的主元素地球化學特征,所研究的三類巖石:蛇紋巖-蛇紋石化尖晶石橄欖巖、尖晶石輝石巖和角閃石巖樣品的SiO2含量分別為37.39% ~43.27%、45.51% ~47.08%、48.69% ~51.13%;MgO 含量高,分別為31.51% ~42.15%、23.13% ~23.62%、15.94%~19.29%;而Al2O3分別0.16% ~2.82%、5.83% ~5.84%、5.22% ~8.64%。10 件蛇紋巖化橄欖巖樣品,其LOI 為7.23% ~13.84%,呈現(xiàn)與蛇紋巖化程度從中等到強是一致的特征。2 件輝石巖和2 件角閃巖樣品中CaO 含量分別為5.07% ~6.07%、11.64% ~13.99%,TiO2和P2O5含量變化相對較小,分別變化于0.32% ~0.34%和0.35% ~0.49%,0.02% ~0.03%和0.03% ~0.04%之間。尖晶石橄欖巖的Mg#在0.86 ~0.93 之間。在MgO 對其他主元素的關(guān)系圖中(圖8),研究區(qū)尖晶石橄欖巖的主要元素Al2O3、FeO、CaO 和SiO2的成分落于世界上典型的克拉通橄欖巖成分區(qū)到原始地幔成分區(qū)之間,表現(xiàn)為克拉通地幔性質(zhì),成分點分布相對離散。根據(jù)Si/Al-(Mg+Fe)/Al 和Mg/Ti-Fe/Ti 主元素的比值圖解(圖9a,b)顯示,尖晶石橄欖巖、尖晶石輝石巖、角閃石巖成分點成線性相關(guān),表明研究區(qū)鎂鐵質(zhì)巖和超鎂鐵巖有著成因上的聯(lián)系(Pearce,1968;Russell and Nicholls,1987,1988)。
研究區(qū)蛇紋巖化尖晶石橄欖巖、尖晶石輝石巖和角閃石巖的主元素、稀土元素和微量元素含量見表3 所示。角閃石巖稀土配分模式(圖10a)顯示基本呈平坦到LREE 略為富集曲線,(La/Sm)N的比值為1.11 ~1.41,(La/Yb)N的比值為0.91 ~1.6,與洋中脊玄武巖類似,與現(xiàn)代大西洋玄武巖的比值非常相近((La/Sm)N為1.09 ~1.70,(La/Yb)N為0.65 ~1.66)(le Roex et al.,1996)。尖晶石橄欖巖和尖晶石輝石巖屬LREE 略富集的右頃型,重稀土配分曲線相對平坦,類似于板內(nèi)拉張環(huán)境玄武巖稀土配分曲線特征。膠北地區(qū)鎂鐵-超鎂鐵巖的稀土總量∑REE 變化于0.46 ×10-6~10.38×10-6,含量變化較大。除了兩個超基性巖有輕微的δEu 負異常(Eu/Eu*= -0.84 ~-0.71),可能是由于結(jié)晶分異引起;其他分析樣品皆為正異常(Eu/Eu*=1.01 ~1.59),可能由早期發(fā)生分離結(jié)晶作用,巖體中斜長石堆晶所引起。樣品表現(xiàn)出LREE 略富集,特別是尖晶石橄欖巖和蛇紋巖表現(xiàn)出較為寬緩的U 型稀土配分樣式,表明受到了流體交代富集作用(Paulick et al.,2006)。
在原始地幔標準化的多元素圖解上(圖10b),LIL(大離子親石元素Sr、K、Rb)基本上沒有異常,說明流體交代作用的改變不很明顯;蛇紋巖化橄欖巖和輝石巖中的Ba 有些正異常,也應該與巖石的碳酸鹽化或角閃石的出現(xiàn)相關(guān),Ba2+可以替代Ca2+進入到方解石晶格中,或替代K+進入交代作用形成的角閃石晶格中。LIL/HFS 比值不高,反映出大離子親石元素和高場強元素含量相當。高場強元素Zr、Hf 沒有明顯的異常出現(xiàn),Ta 的略微正異常,而Nb 卻呈現(xiàn)與Ta 相反的特征,可能和巖石中較多斜方輝石和少量鈦鐵礦的存在有關(guān)(Rollinson,1993),Ti 的正異??赡苁艿街髟睾臀⒘吭胤治龇椒ú煌挠绊?,亦或也可能有含Ti 礦物,如鈦鐵礦的出現(xiàn)有關(guān)。U 和Pb 的正異常反映了變質(zhì)交代作用過程中流體的影響,角閃石的晶格中較之橄欖石和輝石能夠固定更多的U 和Pb。角閃石巖中K 的正異常也源于此因。此外,U作為易溶組分,可隨蛇紋巖化的流體進入到蛇紋石中,使得蛇紋巖化方輝橄欖巖和輝石巖中的U 呈正異常(Deschamps et al.,2013)。
圖10 膠北地塊鎂鐵-超鎂鐵巖球粒隕石標準化稀土元素配分圖(a)和原始地幔標準化微量元素蛛網(wǎng)圖(b)(標準化值據(jù)McDonough and Sun,1995)Fig. 10 Chondrite-normalized REE patterns (a)and primitive mantle-normalized trace element spider diagrams(b ) for mafic-ultramafic rocks in Jiaobei terrane(normalization values after McDonough and Sun,1995)
研究區(qū)樣品主要為膠北地塊出露的前寒武紀變質(zhì)基底中蛇紋巖化尖晶石方輝橄欖巖、尖晶石橄欖斜方輝石巖、角閃石巖。巖石呈透鏡體被包裹在太古代的表殼巖中,自形成后通過漫長的地質(zhì)歷史,至少經(jīng)歷了兩期變質(zhì)和交代作用,早期的麻粒巖-角閃巖相變質(zhì)階段,和晚期經(jīng)歷的綠片巖相變質(zhì)作用,隨后普遍發(fā)生蛇紋巖化和碳酸鹽化。超鎂鐵巖中普遍存在角閃石,巖相學和礦物化學特征顯示其粒狀變晶結(jié)構(gòu)特征主要形成于角閃巖相或綠片巖相的變質(zhì)作用中。膠北地塊蛇紋巖沒有任何受到應力的跡象,說明蛇紋巖化形成于非擠壓環(huán)境的晚期交代變質(zhì)作用過程。
巖相學和礦物化學分析顯示,尖晶石橄欖巖和尖晶石輝石巖記錄了三個階段的礦物組合:早期的Opx1+Ol1+Spl1;中期的Opx2+ Ol2+ Spl2+ Amp2和晚期的Amp3+ Srp3+Mag3+Cal 組合。巖石從形成開始,隨著向地面抬升的過程,先經(jīng)麻粒巖-角閃巖相變質(zhì)作用,繼續(xù)上升的過程中又經(jīng)歷綠片巖相變質(zhì)作用(圖7b)及普遍的蛇紋巖化、碳酸鹽巖化變質(zhì)交代的演化過程,反映了研究區(qū)超鎂鐵巖從形成到近地表的一個抬升過程中的退變演化趨勢。
膠北蛇紋石化尖晶石方輝橄欖巖與Coleman(1977)所提到典型的方輝橄欖巖中SiO2(39.6% ~44.4%)和TiO2(0.01% ~0.15%)變化范圍基本一致。方輝橄欖巖橄欖石Fo 值落入88 ~93 范圍之中,較寬成分變化范圍是堆晶橄欖巖中橄欖石的特征(Duchesne and Charlier,2005;Li et al.,2011),且樣品中原生尖晶石Cr#極低,Cr#與Mg#呈明顯的負相關(guān)性(Dick and Bullen,1984),上述特征都表明膠北尖晶石方輝橄欖巖可能是幔源堆晶系列或極低熔融程度的產(chǎn)物。橄欖巖全巖的高MgO 含量(31.51% ~42%)表現(xiàn)出太古代地幔橄欖巖的性質(zhì)(Herzberg,1993;Francis,2003)。
巖相學和礦物學研究表明,膠北尖晶石橄欖巖原巖尖晶石的具Al,低Cr#(0.06 ~0.12)特征,兩者數(shù)量值介于深海橄欖巖中尖晶石和端元的鋁尖晶石成分之間,表明膠北基底的超鎂鐵巖經(jīng)受了地幔極低度部分熔融。前人的研究成果表明,如膠北基底的超鎂鐵巖這樣Cr#極低的尖晶石是交代變質(zhì)成因(Evans and Frost,1975;Sar?fak?ogˇlua et al.,2010;Haggerty,1989;Franz and Wirth,2000),接近于原始地幔和洋殼形成前的大陸裂谷環(huán)境(Bonatti and Michael,1989),研究樣品中斜方輝石低A2O3和高TiO2特征顯示出克拉通橄欖巖的性質(zhì)(Rudnick et al.,2004)。
橄欖巖的主量化學研究表明,其成分上接近太古代的地幔橄欖巖,是原始地幔低程度部分熔融的產(chǎn)物。研究區(qū)超鎂鐵巖,除了角閃石巖外,研究樣品燒失量較高,顯示巖石樣品中含有較多的H2O 或CO2等揮發(fā)性組分,這與長時間變質(zhì)過程中流體的作用、角閃石的形成,以及蛇紋石化都有關(guān)系。超鎂鐵質(zhì)巖主元素變化的分散性(圖8),以及Si/Al-(Mg +Fe)/Al 和Mg/Ti-Fe/Ti 主元素的比值圖解(圖9),表明原巖與角閃石巖的原巖具有成因聯(lián)系,可能是具有堆晶成因的一套幔源巖漿系列。
蛇紋巖化尖晶石方輝橄欖巖稀土元素配分曲線表現(xiàn)出流體交代作用的影響,輝石巖和角閃石巖的稀土元素配分曲線以及∑REE 含量變化較大,少數(shù)樣品δEu 負異??赡苁怯捎诮Y(jié)晶分異引起;多數(shù)樣品皆為δEu 正異常,表明巖石可能經(jīng)歷了早期發(fā)生分離結(jié)晶作用,巖體中斜長石堆積成晶。所有巖石類型顯示的Zr、Hf、Y 等高場強元素基本無異?,F(xiàn)象等,也顯示研究區(qū)超鎂鐵巖可能形成于裂谷環(huán)境。結(jié)合他人的研究成果(Rudnick et al.,2004;Jahn et al.,2008;劉平華,2011),本文的研究結(jié)果表明前寒武紀巖石在長期復雜的變質(zhì)作用過程中,膠北地區(qū)變質(zhì)基底表現(xiàn)出較明顯的不均一性。
致謝 感謝中國海洋大學教育部海底科學與探測技術(shù)重點實驗室來志慶老師、中國科學院地球化學研究所礦床地球化學重點實驗室漆亮研究員在礦物巖石地球化學分析中給予的幫助;感謝山東省地質(zhì)科學實驗研究院李洪奎研究員對野外工作的指導;衷心感謝審稿專家宋述光教授、劉平華博士和焦淑娟博士提出的寶貴建議。
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