緬甸密支那早白堊世鉀玄質(zhì)巖石成因及其大地構(gòu)造意義

張靖祎, 彭頭平, 范蔚茗, 董曉涵, 劉兵兵, 高劍峰, 梁細(xì)榮, 陳林麗

緬甸密支那早白堊世鉀玄質(zhì)巖石成因及其大地構(gòu)造意義

張靖祎1, 2, 彭頭平1, 3*, 范蔚茗3, 4, 董曉涵1, 2, 劉兵兵1, 2, 高劍峰5, 梁細(xì)榮1, 陳林麗1

(1.中國科學(xué)院 廣州地球化學(xué)研究所, 同位素地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2.中國科學(xué)院大學(xué), 北京 100049; 3.中國科學(xué)院 青藏高原地球科學(xué)卓越創(chuàng)新中心, 北京 100101; 4.中國科學(xué)院 青藏高原研究所, 北京 100101; 5.中國科學(xué)院 地球化學(xué)研究所, 礦床地球化學(xué)國家重點(diǎn)實(shí)驗(yàn)室, 貴州 貴陽 550081)

緬甸地處喜馬拉雅造山帶東段, 緊臨喜馬拉雅東構(gòu)造結(jié)。作為喜馬拉雅造山過程深部動力學(xué)在淺部的響應(yīng), 該區(qū)發(fā)育了大量晚中生代?新生代巖漿巖, 這些巖漿巖是揭示新特提斯演化過程及緬甸不同地塊與青藏高原內(nèi)部各塊體之間大地構(gòu)造屬性的重要窗口。但是, 目前該區(qū)域研究程度非常低, 特別是東緬密支那地區(qū)幾乎是空白。本文首次報道緬甸密支那地區(qū)的一套二長巖, 鋯石U-Pb年齡表明其形成時代為早白堊世(~130 Ma)。巖石地球化學(xué)特征顯示, 這套巖石具有高的SiO2(58.92%~63.73%)、全堿(Na2O+K2O=7.94%~10.14%)、K2O含量(3.88%~6.14%)、K2O/Na2O值(>1), 以及富集LILE、LREE和虧損Nb-Ta-Ti特征, 類似典型的鉀玄質(zhì)巖石。結(jié)合其Sr-Nd同位素組成以及低Sr/Y、La/Yb值和高Y、Yb含量, 暗示密支那鉀玄質(zhì)二長巖來源于受俯沖流體/溶體交代的含金云母巖石圈地幔。綜合區(qū)域上研究資料, 認(rèn)為密支那鉀玄質(zhì)巖漿形成于洋內(nèi)俯沖的島弧環(huán)境, 是俯沖板塊回撤或者斷離后軟流圈上涌導(dǎo)致富集巖石圈地幔部分熔融的產(chǎn)物。因此, 大地構(gòu)造上, 密支那洋并不是青藏高原班公湖?怒江洋的南延, 而是新特提斯洋的重要組成部分。

早白堊世; 鉀玄質(zhì)巖石; 交代富集巖石圈地幔; 洋內(nèi)弧; 密支那

0 引 言

鉀玄質(zhì)巖石(shoshonitic rocks)是一類具有高堿(K2O+Na2O>5%)、高K2O/Na2O值(>0.5)、低TiO2(<1.3%)、Al2O3含量高且變化范圍大(9%~20%)、強(qiáng)烈富集大離子親石元素(LILE)和輕稀土元素(LREE)的富堿高鉀巖漿巖, 包括火山巖(橄欖粒玄巖、鉀玄巖、粗面粒玄巖和橄玄安粗巖等)以及對應(yīng)的淺成和深成侵入巖(Morrison, 1980)。其初始的定義為鉀玄系列(shoshonite)火山巖, 是Iddings (1895)在研究美國黃石公園Shoshone河地區(qū)中基性火山巖時發(fā)現(xiàn)了一套由粗玄巖(absarokite; SiO2=45%~52%)、鉀玄巖(shoshonite; SiO2=52%~57%)和安粗巖(banakite; SiO2=57%~63%)組成的巖石而命名的。后來隨著研究的不斷深入, 一些學(xué)者將酸性火成巖中具有鉀玄系列巖石特征的相關(guān)巖石也歸屬于鉀玄質(zhì)系列(Duchesne et al., 1998; 馬昌前等, 1999; Jiang et al., 2002; 王建等, 2003; Bitencourt and Nardi, 2004; Lu et al., 2013), 包括了高硅(SiO2>63%)部分的火山巖和侵入巖, 如鉀質(zhì)?超鉀質(zhì)英安巖和流紋巖、石英二長巖、石英正長巖、正長巖、輝石正長巖、角閃正長巖等, 使得鉀玄巖成為包含了火山巖和侵入巖在內(nèi)的獨(dú)立完整巖漿巖系列(Duchesne et al., 1998; 李獻(xiàn)華等, 1999, 2000, 2001; 馬昌前等, 1999; Jiang et al., 2002; 王建等, 2003; Bitencourt and Nardi, 2004; 賀振宇等, 2008; 章邦桐等, 2011; Lu et al., 2013)。鉀玄巖可以形成于各種構(gòu)造背景, 如大陸弧、后碰撞弧、初始洋弧、晚期的洋弧以及板內(nèi)環(huán)境(Duchesne et al., 1998; Li, 2000; 李獻(xiàn)華等, 2000, 2001; Jiang et al., 2002; Bitencourt and Nardi, 2004; Williams et al., 2004; Chung et al., 2005; Wang et al., 2006; Gao et al., 2007; Zhao et al., 2009; Chen et al., 2010, 2012; Guo et al., 2013; Liu et al., 2014; Li et al., 2018, 2019; 2020)。但現(xiàn)有研究顯示, 它們大部分形成于俯沖?碰撞有關(guān)的構(gòu)造背景, 主要起源于與俯沖作用有關(guān)的富鉀和LILE的交代地幔(Foley and Peccerillo, 1992; Turner et al., 1996; Lu et al., 2013)。然而, 關(guān)于源區(qū)鉀的富集機(jī)制是與俯沖洋殼沉積物有關(guān)(Turner et al., 1996; Miller et al., 1999; Williams et al., 2004), 還是俯沖的大陸地殼物質(zhì)輸入導(dǎo)致的(Ding et al., 2003; Gao et al., 2007; Zhao et al., 2009), 目前還存在爭議。

緬甸地處喜馬拉雅造山帶的東段, 緊臨喜馬拉雅東構(gòu)造結(jié)(圖1)。作為喜馬拉雅造山過程深部動力學(xué)在淺部的響應(yīng), 該區(qū)發(fā)育了大量晚中生代?新生代巖漿巖(Mitchell, 1993; Acharyya, 2007; Mitchell et al., 2007; Searle et al., 2007; Mitchell et al., 2012)。這些巖漿巖作為特提斯演化的產(chǎn)物, 記錄了特提斯東段不同洋盆的演化信息, 是研究青藏高原特提斯構(gòu)造域大陸/地塊裂解和聚合過程等深部動力學(xué)的重要窗口, 也是揭示中或新特提斯演化過程及緬甸不同地塊與青藏高原內(nèi)部各塊體之間大地構(gòu)造屬性的物質(zhì)記錄。但是, 由于緬甸境內(nèi), 特別是密支那地區(qū), 長期以來因政治因素很難開展相關(guān)的地質(zhì)調(diào)查和研究, 因此, 整個緬甸的基礎(chǔ)地質(zhì)研究顯得非常薄弱。近5年在參與基金重大項(xiàng)目“喜馬拉雅山構(gòu)造結(jié)碰撞變形過程”之二級重點(diǎn)課題“東西構(gòu)造結(jié)大陸巖石圈俯沖的巖漿響應(yīng)”的研究中, 通過與緬甸地質(zhì)學(xué)會和密支那大學(xué)地質(zhì)系同仁開展國際合作, 針對緬甸境內(nèi)中?新生代巖漿巖開展了系統(tǒng)的調(diào)查和相關(guān)的研究, 并首次報道密支那新識別的早白堊世鉀玄質(zhì)二長巖, 給出高精度的鋯石U-Pb年代學(xué)結(jié)果、全巖元素和Sr-Nd同位素數(shù)據(jù), 并探討了該區(qū)鉀玄質(zhì)二長巖的成因及其相關(guān)的巖石大地構(gòu)造意義, 為區(qū)域特提斯演化的動力學(xué)過程提供了重要的約束。

1 區(qū)域地質(zhì)概況

緬甸位于東南亞地區(qū), 處于阿爾卑斯?喜馬拉雅造山帶與印度尼西亞弧體系的交界位置。阿爾卑斯?喜馬拉雅造山帶在喜馬拉雅東構(gòu)造結(jié)位置發(fā)生彎曲, 轉(zhuǎn)為南北走向, 并且向南一直延伸, 經(jīng)過印?緬山脈, 繼續(xù)向南至安達(dá)曼群島和尼科巴群島、蘇門答臘, 直至印度尼西亞的巽他和班達(dá)弧。大地構(gòu)造上, 從西向東可以劃分為四個大的構(gòu)造單元: 印?緬山脈、西緬地塊、密支那?抹谷變質(zhì)帶和屬于滇緬泰馬地塊一部分的撣邦高原(Searle et al., 2007, 2017)。其中印?緬山脈地區(qū)發(fā)育有白堊紀(jì)蛇綠巖, 典型的古靈廟蛇綠巖被認(rèn)為是青藏高原雅魯藏布江縫合帶在東南亞地區(qū)的南延(Liu et al., 2016a, 2016b), 同時該地區(qū)還發(fā)育有三疊紀(jì)的濁積巖、片巖、白堊紀(jì)的混雜堆積巖, 新生代的復(fù)理石和磨拉石沉積(Searle et al., 2007, 2017)。西緬地塊主要被巨厚的新生代沉積所覆蓋, 盆地內(nèi)呈南北向線狀分布有大量的火山口湖和破火山錐, 主要為第四紀(jì)火山活動產(chǎn)物(Searle et al., 2007)。新生代沉積地層之下, 為含菊石化石的白堊紀(jì)頁巖、三疊紀(jì)濁積巖, 上述這些地層共同受中生代?新生代火山巖漿作用的影響(Mitchell, 1993; Mitchell et al., 2007)。密支那?抹谷變質(zhì)帶以長1200 km的大型右旋實(shí)皆走滑斷裂與撣邦高原相隔, 區(qū)內(nèi)主要發(fā)育原認(rèn)為是早白堊世的超基性巖和相關(guān)的高級變質(zhì)巖(Mitchell, 1993; Mitchell et al., 2007)。變質(zhì)帶包括北部的帕敢玉礦帶、杰沙?甘高片巖帶、太公?密支那片巖帶以及南部的抹谷高級變質(zhì)帶(Mitchell, 2018)。帕敢玉礦帶主要由高壓變質(zhì)巖石組成, 包括不同類型的硬玉、角閃巖和蛇紋石化橄欖巖(Shi et al., 2008; Franz et al., 2014), 目前大量玉礦年代學(xué)研究顯示其原巖為晚侏羅世, 經(jīng)歷過多期次的交代變質(zhì)作用(Shi et al., 2003, 2008; 丘志力等, 2008; Qi et al., 2013; Yui et al., 2013; 祁敏等, 2014; 蔡詩詩和張恩, 2018)。杰沙?甘高片巖帶主要由淺變質(zhì)的巖石組成, 如石英巖、藍(lán)晶石片巖、石英云母石榴石片巖(Mitchell et al., 2007)。太公?密支那片巖帶由低變質(zhì)的云母片巖、綠泥石片巖、滑石片巖組成, 局部被濁積巖覆蓋(Searle et al., 2007)。在密支那地區(qū)公文山脈(Kumon Range)還發(fā)現(xiàn)有榴輝巖相的高壓變質(zhì)巖(Enami et al., 2012)。密支那地區(qū)與超基性巖相伴生的中基性巖?酸性巖蛇綠巖的年代學(xué)資料顯示, 其形成時代為侏羅紀(jì)(Yang et al., 2012b; Liu et al., 2016a, 2016b)。南部抹谷變質(zhì)帶主要以大理巖和相關(guān)的泥質(zhì)麻粒巖為主, 大理巖中產(chǎn)紅寶石, 少量大的超基性巖體被認(rèn)為是密支那超基性巖的南延, 零星分布在區(qū)內(nèi)(Mitchell et al., 2007)。撣邦高原發(fā)育與周邊板塊有明顯差異的古生代地層序列(發(fā)生了輕微變質(zhì)的濁積巖序列), 古生代地層之上分別為石炭系?下二疊統(tǒng)冰川陸源碎屑混雜巖和中?上二疊統(tǒng)石灰?guī)r, 各地層之間均呈角度不整合(Gardiner et al., 2016; Searle et al., 2017)。中?新生代的巖漿巖, 特別是花崗質(zhì)巖石大面積分布在中央盆地的北部文多?班茂地區(qū)、密支那和撣邦高原及其以南的沿海地區(qū)(Searle et al., 2007)。

IBR. 印?緬山脈; WBB. 西緬地塊; MMB. 密支那?抹谷變質(zhì)帶; BS-STB. 保山?撣邦高原地塊; IYSZ. 印度斯?雅魯藏布江縫合帶; SGF. 實(shí)皆斷裂; GDS. 岡底斯主俯沖增生造山帶; NBS. 東構(gòu)造結(jié); STD. 藏南拆離系; MCT. 主中央沖斷裂; MBT. 主邊沖斷裂; SSF. 撣邦陡崖斷裂系; JL-GLGF. 嘉黎?高黎貢斷裂; LCF. 瀾滄江斷裂; TPF. 三塔斷裂; MPF. 眉萍斷裂; JMU. 帕敢翡翠礦帶; ARFB. 阿薩姆?若開前陸盆地; CVA. 緬甸中央火山弧; CBB. 察隅?波密變質(zhì)帶; TMSZ. 太公?密支那變質(zhì)帶; CMG. Chuangmagyi濁積巖; SLP. 西隆高原。

2 樣品采集和巖石(相)學(xué)特征

本次研究的4塊新鮮二長質(zhì)巖石樣品采自緬甸密支那Wayan 地區(qū)蛇綠巖帶內(nèi)(25°21′57.43″N; 97°30′10.79″E)(圖2a), 對所有樣品進(jìn)行巖相學(xué)研究及全巖主、微量元素含量測定, 并對其中1塊代表性樣品(16MF-58A)進(jìn)行了鋯石U-Pb年代學(xué)研究, 2塊樣品開展了全巖Sr-Nd同位素分析。

礦物組成顯示, 3個樣品為石英二長巖, 另一個樣品為石英二長閃長巖(圖3)。樣品為中粒結(jié)構(gòu), 塊狀構(gòu)造。除去表面風(fēng)化部分后, 樣品內(nèi)部未見到形變(圖2c), 礦物排布無明顯定向。石英二長巖樣品主要由鉀長石(37%~38%)、斜長石(45%~48%)、石英(8%~10%)、斜方輝石(3%~4%)、單斜輝石(~1%)等組成, 同時含有少量鋯石、鈦鐵礦、磁鐵礦等副礦物。石英二長閃長巖樣品的主要礦物包括鉀長石(~25%)、斜長石(~55%)、石英(~7%)、斜方輝石(~6%)、單斜輝石(~4%), 同時含有少量鋯石、鈦鐵礦、磁鐵礦等副礦物。

3 分析方法

3.1 鋯石U-Pb年代學(xué)

鋯石分選采用傳統(tǒng)的重液相和磁選方法, 然后在雙目鏡下挑選無包體、透明干凈、自形程度好的顆粒, 將代表性的鋯石顆粒制成樹脂靶。將制好的靶面拋光至少1/3鋯石截面處, 并進(jìn)行反射光和透射光照相, 然后開展鋯石陰極發(fā)光圖像(CL圖像)拍攝, CL照相使用儀器為中國科學(xué)院廣州地球化學(xué)研究所同位素國家重點(diǎn)實(shí)驗(yàn)室EPMA-JXA-8100掃描電子顯微鏡匹配Gatan Mono CL3型陰極發(fā)光系統(tǒng)。挑選出具有較好韻律環(huán)帶的鋯石顆粒進(jìn)行U-Pb同位素測年, 測年工作在中國科學(xué)院青藏高原研究所大陸碰撞與高原隆升重點(diǎn)實(shí)驗(yàn)室利用激光剝蝕法(LA-ICP-MS)完成。激光剝蝕系統(tǒng)為193 nm, GeoLas Plus, ICP-MS為Agilent 7500a。U-Pb同位素定年中采用鋯石標(biāo)準(zhǔn)91500(TIMS測定206Pb/238U=1062.4± 0.4 Ma(Wiedenbeck et al., 2004))作外標(biāo)進(jìn)行同位素分餾校正, 每8點(diǎn)個樣品點(diǎn)分析一次91500標(biāo)樣。普通Pb校正參照Andersen (2002), 詳細(xì)分析流程參考Liu et al. (2017)。數(shù)據(jù)處理采用軟件ICPMSDataCal (Liu et al., 2010b, 2010c), 鋯石U-Pb年齡計算和諧和圖制作均采用軟件ISOPLOT 3.0(Ludwig, 2003)完成。具體的分析結(jié)果見表1。

3.2 全巖主量、微量元素和Sr-Nd同位素

全巖主、微量元素和Sr-Nd同位素組成分析測試在中國科學(xué)院廣州地球化學(xué)研究所同位素國家重點(diǎn)實(shí)驗(yàn)室完成。其中主量元素采用XRF法測試; 微量元素利用Perkin-Elmer ELAN 6000 ICP-MS測試, 標(biāo)樣為BCR-1, 大部分元素的分析精度優(yōu)于3%, 具體的分析流程參閱劉穎等(1996); Sr-Nd同位素組成在VG354質(zhì)譜儀上完成, NBS987標(biāo)樣87Sr/86Sr= 0.71026512, La Jolla143Nd/144Nd=0.51186210,86Sr/87Sr= 0.1194,146Nd/144Nd=0.7219。87Rb/86Sr、147Sm/144Nd比值用87Sr/86Sr=0.1194和146Nd/144Nd=0.7219進(jìn)行分餾質(zhì)量校正, 分析流程參考Liang et al. (2000)。

圖2 密支那二長質(zhì)巖石采樣點(diǎn)地質(zhì)圖(a;修改自Liu et al., 2016a)及野外照片(b、c)

Q. 石英; A. 堿性長石; P. 斜長石。

4 分析結(jié)果

4.1 鋯石U-Pb年齡

緬甸密支那二長質(zhì)巖石中大部分鋯石呈黃色至棕色調(diào)的自形晶體, 長柱狀或短柱狀, 顆粒長度介于100~250 μm之間, 長寬比介于1∶1至3∶1之間(圖4)。根據(jù)陰極發(fā)光(CL)圖像上內(nèi)部結(jié)構(gòu)特征, 鋯石可分為兩類: 一類為內(nèi)部結(jié)構(gòu)簡單且沒有內(nèi)核, 其CL圖像呈現(xiàn)巖漿鋯石常見的振蕩環(huán)帶結(jié)構(gòu)(Hoskin and Schaltegger, 2003); 另一類結(jié)構(gòu)略微復(fù)雜, 其中心部位常常含不規(guī)則的、具有熔蝕結(jié)構(gòu)的內(nèi)核, CL圖像中內(nèi)核呈現(xiàn)與外部明亮的環(huán)狀外邊不同的圖案。

表1 密支那二長質(zhì)巖石(16MF-58A)鋯石LA-ICP-MS U-Pb定年數(shù)據(jù)表

圖4 密支那二長質(zhì)巖石鋯石U-Pb年齡諧和圖(a)及代表性鋯石陰極發(fā)光圖片(b)

本次測試共獲得30個有效測試點(diǎn), 所有測點(diǎn)均位于鋯石邊部。鋯石的Th含量為87.9~834 μg/g, U為151~960 μg/g, Th/U值變化于0.4~1.0之間, 且絕大部分鋯石具巖漿成因高Th/U值特征(表1)。所有分析點(diǎn)給出了206Pb/238U年齡范圍為124~147 Ma, 在諧和曲線圖上數(shù)據(jù)點(diǎn)成兩組分布(圖4a), 其中20個年輕的點(diǎn)給出了128.9±1.0 Ma的206Pb/238U加權(quán)平均年齡(MSWD=1.4), 代表了二長質(zhì)巖石的形成時代; 其他10個略老的分析點(diǎn)獲得了141.3±1.9 Ma的206Pb/238U加權(quán)平均年齡(MSWD=1.3), 可能是巖漿作用過程捕獲的鋯石, 與鋯石CL圖像呈現(xiàn)兩種特征類型相對應(yīng)(圖4)。

4.2 元素地球化學(xué)特征

密支那二長質(zhì)巖石全巖主、微量元素測試結(jié)果見表2。4件樣品SiO2含量為58.92%~63.73%, 具有低Mg#值(33~42)、高全堿含量(Na2O+K2O=7.94%~ 10.14%), 以及明顯高K2O(3.88%~6.14%)、Al2O3(16.88%~17.46%)和K2O/Na2O值(0.96~1.55)、低TiO2(0.53%~0.61%)和全鐵(Fe2O3T=3.96%~6.16%)特征(表2)。在TAS圖解中顯示為堿性的石英二長質(zhì)巖石成分(圖5), 在K2O-SiO2和K2O-Na2O圖解中, 都落在鉀玄質(zhì)區(qū)域(圖5)。

樣品REE總量為78.0~91.4 μg/g。在球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖解中, 樣品具有輕稀土元素明顯富集, 輕重稀土元素中等分餾((La/Yb)N=4.1~5.7), 弱的Eu負(fù)異常(Eu/Eu*=0.78~0.88)(圖6a)特征。在原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖中, 樣品富集大離子親石元素(Rb、U等), 虧損Nb、Ta、Ti等高場強(qiáng)元素(圖6b)。與密支那蛇綠巖(Yang et al., 2012b)相比, 這些鉀玄質(zhì)巖石顯示出明顯的稀土和微量元素富集特征(圖6b)。

表2 密支那二長質(zhì)巖石全巖主量(%)、微量元素(μg/g)分析結(jié)果

續(xù)表2:

數(shù)據(jù)來源: 密支那蛇綠巖組合數(shù)據(jù)來自Yang et al., 2012b。

4.3 Sr-Nd同位素

兩個代表性樣品Sr-Nd同位素測試結(jié)果見表3, 結(jié)果顯示, 樣品具有低87Sr/86Sr(0.704388~0.705084)和高143Nd/144Nd值(0.512933~0.512949), 對應(yīng)(87Sr/86Si)i值為0.703613、0.703661 ,Nd()值為+6.9~+7.2 (=130 Ma), 二階段模式年齡為0.34~0.36 Ga。與密支那蛇綠巖相比(Yang et al., 2012b), 其Sr-Nd同位素組成具有更加虧損的特征(圖7)。

5 討 論

5.1 巖石分類

樣品地球化學(xué)特征顯示, 緬甸密支那地區(qū)二長質(zhì)巖石在主量元素上, 具有高的全堿(Na2O+K2O= 7.94%~10.14%)、K2O(3.88%~6.08%)、Al2O3(16.88%~17.46%)和K2O/Na2O值(0.96~1.55), 以及低的TiO2(0.53%~0.61%)和全鐵(Fe2O3T=3.96%~6.16%)的特征, 在K2O-Na2O圖解中落在堿性區(qū)域, 與典型的鉀玄質(zhì)巖石特征非常一致(Morrison, 1980; 賈小輝等, 2017)。微量元素上, 它們明顯富集LILE、LREE, 虧損Nb-Ta-Ti高場強(qiáng)元素, 與青藏高原新生代高硅鉀玄質(zhì)巖石相似(Wang et al., 2012; Lu et al., 2013; Huang et al., 2015)。因此, 緬甸密支那地區(qū)二長質(zhì)巖石應(yīng)該歸類為鉀玄質(zhì)巖石。

數(shù)據(jù)來源: 密支那蛇綠巖組合數(shù)據(jù)來自Yang et al. (2012b); 球粒隕石、原始地幔、OIB、E-MORB、N-MORB數(shù)據(jù)來自Sun and Mcdonough (1989)。

表3 密支那二長質(zhì)巖石Sr-Nd同位素組成

數(shù)據(jù)來源: 密支那蛇綠巖組合數(shù)據(jù)來自Yang et al. (2012b); 西緬地塊白堊紀(jì)?第四紀(jì)弧巖漿巖數(shù)據(jù)來自Mitchell et al. (2012); 岡底斯基性巖漿數(shù)據(jù)來自Ji et al. (2016); 大洋沉積物數(shù)據(jù)來自Chauvel et al. (2008)。

5.2 巖石成因

已有研究表明, 低硅(SiO2<63%)鉀玄質(zhì)巖石主要起源于巖石圈地幔, 其富鉀的特征可能是: ①石榴橄欖巖地幔部分熔融或者其分離結(jié)晶的產(chǎn)物(O’Hara and Yoder, 1967); ②幔源巖漿地殼混染或與殼源巖漿混合的結(jié)果(Benito et al., 1999; Di Battistini et al., 2001; Hebert et al., 2014); ③幔源巖漿縱向分帶富集而成(Harris and Middlemost, 1970); ④來自含金云母的交代巖石圈地幔的部分熔融(Nelson, 1992; Turner et al., 1996; Schiano et al., 2004)。但是, 對于高硅(SiO2>63%)鉀玄質(zhì)巖石的起源和成因, 雖然現(xiàn)有大部分研究顯示為地幔起源(Huang et al., 2010; Wang et al., 2012; Lu et al., 2013), 但是也有部分學(xué)者認(rèn)為可能是殼源的(Conceicao and Green, 2004; Yang et al., 2012a)。如Wyllie (1977)基于相平衡實(shí)驗(yàn)研究和巖相學(xué)對比后指出, 加厚的陸殼部分熔融形成的巖漿為粗面巖(正長巖), 與高硅鉀玄質(zhì)巖石類似, 而不是花崗巖; 鄧晉福等(1996)依據(jù)巖石相平衡理論, 認(rèn)同加厚的陸殼(深度大于50~60 km)巖石部分熔融可以形成鉀玄質(zhì)(正長巖)巖漿; 一些高壓實(shí)驗(yàn)(壓力為 1.3~2.0 GPa)也證實(shí), 長英質(zhì)巖石的高壓部分熔融可以獲得高鉀的硅酸巖漿(Huang and Wyllie, 1981; Patino Douce and Johnston, 1991; Skjerlie and Johnston, 1993, 1996)。這些地殼起源的高硅鉀玄質(zhì)巖石都無一例外的要求其源區(qū)熔融的深度較大, 必須是加厚的地殼, 深度至少達(dá)50~ 60 km(Huang and Wyllie, 1981; Johannes and Holtz, 1990; 鄧晉福等, 1996)。

緬甸密支那高硅(SiO2>63%)鉀玄質(zhì)巖石具有明顯低Sr/Y值(20.2~20.3)和La/Yb值(5.69~6.99)、高Y(18.4~18.8 μg/g)和Yb(2.09~2.16 μg/g)含量, 但不同于加厚下地殼起源的巖漿, 由于其源區(qū)具石榴石的分離結(jié)晶導(dǎo)致熔體具有埃達(dá)克巖特征(Xu et al., 2002; Wang et al., 2006)。而且樣品負(fù)Eu異常特征也說明巖漿源區(qū)存在一定量的斜長石分離結(jié)晶, 與加厚下地殼起源(無Eu負(fù)異常; Xu et al., 2002; Wang et al., 2006)不一致。因此, 密支那高硅鉀玄質(zhì)巖石不可能起源于加厚下地殼(Wyllie, 1977; 鄧晉福等, 1996), 而應(yīng)該是源于地幔?？紤]到密支那鉀玄質(zhì)巖石中低硅(SiO2<63%)樣品具有低Sr/Y值(28.72~35.1)和La/Yb(6.97~7.97)值、高HREE(Y=15.9~18.7 μg/g, Yb=1.57~1.97 μg/g)含量, 反應(yīng)其巖漿的起源不是石榴石相巖石圈地幔, 而是相對淺部的尖晶石相地幔源區(qū)。此外, 基于虧損的Sr-Nd同位素組成(圖7)以及富集LILE和LREE的特征, 密支那鉀玄質(zhì)巖石也不可能是幔源巖漿地殼混染或者殼?；旌系慕Y(jié)果。結(jié)合密支那鉀玄質(zhì)巖石特別是低硅(<63%)樣品高Nb/Ta值(16.4~16.7; Sun and Mcdonough, 1989)、富集LILE、LREE和虧損Nb-Ta-Ti的特征, 說明巖漿源于受俯沖流體交代為主的巖石圈地幔, 而不是OIB和MORB型地幔源區(qū)(Hawkesworth, 1993; Turner et al., 1996)。由于密支那鉀玄質(zhì)巖石具有高的Rb/Sr值(0.14~0.36)、缺乏中稀土元素(MREE)的異常的特征, 其源區(qū)應(yīng)該是含金云母的交代的巖石圈地幔(Nelson, 1992; Turner et al., 1996; Schiano et al., 2004; Coban et al., 2019), 而不是含角閃石的富集地幔(Ge et al., 2002)。因?yàn)? 金云母更富集K、Sr、Ba 和Rb, 但具有略為低的REE、HFSE 和Th 含量(Green and Fallon, 1998)?？紤]到大洋沉積物中富集Th和LREE, 且Th在流體中不易遷移, Rb、Ba、U和Pb是流體容易遷移的元素(Hochstaedter et al., 2001; Kessel et al., 2005), 而密支那鉀玄質(zhì)巖石顯示明顯富集的LILE、LREE、Rb、Ba、U和Pb元素, 以及相對較高的Ba/La(32.9~44.9)值和較低的Th/Nb值(1.03~1.33)特征, 反應(yīng)其地幔源區(qū)除了受流體作用之外, 還受到少量的沉積物溶體的交代作用(Hawkesworth, 1993; Pearce and Peate, 1995; Kessel et al., 2005)。近年來藏南超鉀質(zhì)巖石含金云母地幔包體的發(fā)現(xiàn), 以及元素和同位素研究和模擬, 均證實(shí)了含金云母的地幔橄欖巖在淺部尖晶石相條件下部分熔融可以形成鉀質(zhì)和超鉀質(zhì)巖漿(Liu et al., 2011)。因此, 密支那早白堊世鉀玄質(zhì)巖石源于受俯沖流體/溶體交代的含金云母的巖石圈地幔部分熔融, 巖漿演化過程中存在橄欖石、輝石、長石和磁鈦鐵礦的分離結(jié)晶。

5.3 構(gòu)造背景

已有的密支那蛇綠巖中基性?中酸性巖石鋯石U-Pb年代學(xué)研究結(jié)果顯示, 該區(qū)蛇綠巖不是前人認(rèn)為的屬于早白堊世(130~120 Ma; Mitchell, 1993; Shi et al., 2008), 而應(yīng)該形成于侏羅紀(jì)(180~165 Ma; Yang et al., 2012b; Liu et al., 2016a, 2016b), 時代上與班公湖?怒江帶蛇綠巖(180~165 Ma; Yang et al., 2012b; Liu et al., 2016a, 2016b)相對應(yīng)。對蛇綠巖中超基性巖進(jìn)一步研究發(fā)現(xiàn), 密支那蛇綠巖屬于俯沖交代(SSZ)型蛇綠巖, 而緬甸西部的印緬山脈的蛇綠巖主要以洋中脊(MOR)型為主, 有少量的水化熔融的印跡(Liu et al., 2016b)。與雅魯藏布主要以SSZ型為主(Liu et al., 2010a; Niu et al., 2017)、且形成于靠近亞洲大陸邊緣海盆的蛇綠巖(吳福元等, 2014)相比, 印?緬山脈蛇綠巖可能并不是簡單的雅魯藏布蛇綠巖的南延。同樣地, 基于密支那蛇綠巖以及其他地區(qū)的研究新進(jìn)展(課題組未發(fā)表數(shù)據(jù)), 緬甸東部密支那蛇綠巖可能也不是青藏高原內(nèi)部班?怒帶蛇綠巖的南延。因此, 緬甸境內(nèi)蛇綠巖所代表的洋盆演化要比青藏高原內(nèi)部的蛇綠巖更復(fù)雜。

盡管如此, 歸納分析現(xiàn)有的資料依然可以為密支那洋的構(gòu)造演化提供重要約束。如, 在印多吉湖(Indawgyi Lake)蛇綠巖中獲得輝長巖鋯石U-Pb年齡約為184 Ma(課題組未發(fā)表的數(shù)據(jù)), 且其超基性巖地球化學(xué)與密支那橄欖巖一致(魏超等, 2018), 表明密支那蛇綠巖向西南延伸與印多吉湖蛇綠巖相連。而印多吉湖北部緊鄰帕敢翡翠礦區(qū), 且翡翠玉主要是以脈狀產(chǎn)出于橄欖巖中(施光海等, 2001; Shi et al., 2003, 2008, 2014), 屬于大洋俯沖過程交代變質(zhì)形成的玉石(Shi et al., 2003, 2008), 來自翡翠玉中的鋯石U-Pb年齡結(jié)果顯示很寬的變化范圍, 其中一組老的年齡(175~160 Ma)與密支那蛇綠巖時代一致(圖8; Shi et al., 2008; Yui et al., 2013; 祁敏等, 2014)。因此, 帕敢礦區(qū)蛇綠巖所代表的洋盆在構(gòu)造屬性上也是密支那洋盆的一部分。綜合緬甸帕敢礦區(qū)硬玉巖U-Pb年代學(xué)可見, 其年齡主要分為三組: 175~130 Ma、~120 Ma和90~70 Ma(圖8)。由于大部分鋯石年齡反應(yīng)的是洋殼板片俯沖交代變質(zhì)的時間(Shi et al., 2003, 2008;丘志力等, 2008; Qi et al., 2013; Yui et al., 2013; 祁敏等, 2014; 蔡詩詩和張恩, 2018), 因此密支那洋至少到70 Ma還存在洋殼板片的俯沖作用。此外, 該區(qū)大洋俯沖交代變質(zhì)事件還得到了礦區(qū)變質(zhì)礦物藍(lán)閃石和角閃石的Ar-Ar熱年代學(xué)的證實(shí)(~150 Ma、135 Ma、124 Ma和97 Ma), 反應(yīng)了多期次俯沖有關(guān)的高壓變質(zhì)作用(Qi et al., 2013; Shi et al., 2014)。特別是早白堊世(135~124 Ma)硬玉巖的形成時代(Qi et al., 2013), 暗示該區(qū)存在早白堊世的深俯沖作用。

數(shù)據(jù)來源: 硬玉中鋯石年齡數(shù)據(jù)來自Shi et al. (2008, 2003)、Qi et al. (2013)、Yui et al. (2013)。

本次研究在密支那蛇綠巖帶中新發(fā)現(xiàn)的早白堊世鉀玄質(zhì)巖石, 其地球化學(xué)特征上顯示弧巖漿作用的印跡(圖9), 而且它們低Ce/Yb值特征與斐濟(jì)和波多黎各洋弧鉀玄質(zhì)巖石非常一致(圖9d), 而明顯不同于大陸弧或者弧后的鉀玄質(zhì)巖石以及我國西藏和阿爾卑斯的大陸型鉀玄質(zhì)巖石(Gill et al., 2004), 說明該區(qū)早白堊世時期的巖漿?交代變質(zhì)作用主要發(fā)生在洋弧的構(gòu)造背景。

底圖來源: a 據(jù)Defant and Drummond (1993); b據(jù)Pearce and Peate (1995); c據(jù)Wood (1980); d據(jù)Gill et al. (2004)。

5.4 動力學(xué)機(jī)制

正如前面討論, 密支那鉀玄質(zhì)巖石起源于含金云母巖石圈地幔的部分熔融, 而現(xiàn)有的研究表明, 含金云母的地幔橄欖巖在正常地?zé)崽荻认率呛茈y部分熔融的, 因?yàn)槠溆嬎愠鰜淼牡V物平衡溫度達(dá)到1250 ℃(Liu et al., 2011)。很顯然, 含金云母的地幔橄欖巖如果要發(fā)生部分熔融, 必須有更高的地幔潛能熱貢獻(xiàn)。我們認(rèn)為研究區(qū)具有高的地幔潛熱可能與緬甸西部印緬山脈早白堊世MOR型蛇綠巖形成和密支那東部騰沖地塊早白堊世大量花崗質(zhì)巖漿作用有關(guān), 即在早白堊世密支那洋洋脊擴(kuò)張過程中, 地幔對流必然更加強(qiáng)烈, 從而影響板塊周緣的深部地幔動力學(xué); 而130~115 Ma騰沖地塊東部的大規(guī)模花崗質(zhì)巖漿作用(圖1), 同樣也與密支那洋殼的東向俯沖動力學(xué)發(fā)生變化有關(guān)(Zhang et al., 2018; Dong et al., 2019)。洋脊的擴(kuò)張導(dǎo)致兩側(cè)的洋殼板塊運(yùn)動速度增加, 從而引發(fā)洋殼巖石圈下的軟流圈對流系統(tǒng)的擾動, 與此同時, 俯沖的密支那洋殼板片運(yùn)動也會加速, 板片運(yùn)動學(xué)體制的變化可能會引發(fā)俯沖的板片發(fā)生回撤或者斷離, 導(dǎo)致軟流圈的上涌及減壓部分熔融, 產(chǎn)生的巖漿底侵到弧下富集巖石圈地幔, 引發(fā)巖石圈的部分熔融, 從而形成了早白堊世密支那地區(qū)的鉀玄質(zhì)巖石。如此的機(jī)制, 也會導(dǎo)致俯沖隧道中的部分物質(zhì)因?yàn)樯嫌康能浟魅Φ耐茢D作用, 終止了高壓變質(zhì), 并因?yàn)檐浟魅Φ臒嶙饔靡约傲黧w的作用發(fā)生一次重要熱接觸交代變質(zhì)作用, 如帕敢地區(qū)變質(zhì)巖的130~124 Ma高壓變質(zhì)交代作用(Qi et al., 2013; Shi et al., 2014)。

5.5 大地構(gòu)造意義

綜合研究表明, 密支那蛇綠巖雖然在形成時間上與青藏高原內(nèi)部的班公湖?怒江帶蛇綠巖相同, 但是, 其洋盆的演化存在明顯的差別。帕敢地區(qū)的洋殼俯沖交代變質(zhì)成因的翡翠礦的形成時代最晚, 鋯石U-Pb年齡為~70 Ma(祁敏等, 2014), 表明密支那洋盆的演化至少直到70 Ma還未關(guān)閉。已有的研究顯示, 班公湖?怒江洋盆的演化最晚到100 Ma已經(jīng)完全關(guān)閉了(Zhu et al., 2009)。此外, 從察隅?騰沖地塊早白堊世巖漿作用方式上, 特別巖漿爆發(fā)的時間明顯要比青藏高原內(nèi)部班?怒帶巖漿作用早(Zhang et al., 2018; Dong et al., 2019); 更重要的是, 騰沖地塊晚中生代?早新生代的巖漿作用方式體現(xiàn)出從東往西變年輕的趨勢(Xie et al., 2016), 如此的巖漿作用方式與西部緬甸境內(nèi)的密支那洋殼俯沖的后撤非常吻合, 說明騰沖地塊晚中生代?早新生代的巖漿巖的產(chǎn)生是密支那洋演化過程洋殼向東持續(xù)俯沖作用的結(jié)果。因此, 大地構(gòu)造演化上, 密支那洋盆可能并不是青藏高原班公湖?怒江中特提斯洋的南延, 而是雅魯藏布新特提斯洋的一部分, 可能代表的是西緬地塊和騰沖地塊之間洋盆, 從印緬山脈MOR型蛇綠巖的形成時代看, 新特提斯洋的主大洋盆可能位于密支那?雅魯藏布構(gòu)造線(Yang et al., 2012b)。相反, 印緬山脈所代表的洋盆僅僅是新特提斯大洋盆中的一部分。

6 結(jié) 論

(1) 首次在緬甸密支那地區(qū)識別出一套早白堊世二長質(zhì)巖石, 形成時代約為130 Ma, 其中一組捕獲鋯石的年齡約140 Ma, 暗示區(qū)域內(nèi)存在多期次巖漿作用。

(2) 密支那二長質(zhì)巖石具有高堿(Na2O+K2O> 5%)、K2O(>3.0%)、Al2O3(>15%)和TiO2(<1.0%)的特征, 屬于鉀玄質(zhì)巖石; 元素和Sr-Nd同位素組成表明它們起源于受俯沖流體交代富集的巖石圈地幔的部分熔融, 并經(jīng)過基性礦物和磁鈦鐵礦的分離結(jié)晶而成。

(3) 密支那早白堊世鉀玄質(zhì)巖石形成于洋內(nèi)俯沖的環(huán)境, 是俯沖洋殼板片的斷離或者回撤, 引發(fā)軟流圈的上涌, 熱的軟流圈進(jìn)一步引發(fā)含金云母巖石圈地幔的部分熔融的產(chǎn)物。

(4) 密支那洋并非班公湖?怒江中特提斯洋的南延, 而是新特提斯洋的重要組成部分。

致謝:野外考察得到緬甸地質(zhì)學(xué)會Kyaing Sein 教授, 密支那大學(xué)Hnin Min Soe、Me Me Aung 和Aung Naing Thu 三位老師, 以及中國科學(xué)院青藏高原研究所的丁林院士、蔡福龍副研究員和中國科學(xué)院地質(zhì)與地球物理研究所的吳福元院士、陳意研究員的幫助, 鋯石U-Pb定年測試得到中國科學(xué)院青藏高原研究所岳雅慧的幫助, 一并表示感謝！此外, 非常感謝兩位審稿人的建設(shè)性意見！

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Petrogenesis of the Early Cretaceous Shoshonitic Rocks in Myitkina of Myanmar and its Tectonic Implications

ZHANG Jingyi1, 2, PENG Touping1, 3*, FAN Weiming3, 4, DONG Xiaohan1, 2, LIU Bingbing1, 2, GAO Jianfeng5, LIANG Xirong1and CHEN Linli1

(1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China; 3. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China; 4. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China; 5. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, Guizhou, China)

Myanmar is located in the east of the Himalaya orogenic zone and proximal to the Eastern Syntaxis. Voluminous late Mesozoic-Cenozoic magmatic rocks developed in Myanmar, which are the products of the Himalaya orogeny. The origin and tectonic regime of these igneous rocks are critical for the understanding of the geodynamic evolution of the Neo-Tethys and the tectonic correlation of the different terranes in between Myanmar and Tibetan Plateau. However, little attention has been paid to these igneous rocks, particularly for those in Myitkyina of Myanmar. In this study, a suite of shoshonitic rocks were first recognized, which are characterized by high SiO2(58.92%–63.73%), total alkali (Na2O+K2O=7.94%–10.14%), K2O (3.88%–6.14%), and K2O/Na2O ratios (>1) together with the strong enrichment of LILE and LREE and depletion of Nb, Ta and Ti. LA-ICP-MS zircon U-Pb dating results show that they were emplaced at ~130 Ma. The above signatures coupled with their depleted Sr-Nd isotopic compositions, low Sr/Y and La/Yb ratios, and high Y and Yb contents indicate that the Myitkyina shoshonitic rocks were derived from partial melting of the enriched, phlogopite-bearing lithospheric mantle metasomatized by the subduction-related fluids/melts. Combined with other geological data, we propose that the Myitkyna early Cretaceous shoshonitic rocks were generated in an oceanic arc in response to the slab rollback or breakoff. It is thus noted that the Myitkyina Tethyan Ocean was not closed before ~130 Ma, and the southern continuation of the Bangonghu-Nujiang Meso-Tethys was an important component of the Neo-Tethys.

Early Cretaceous; shoshonitic rocks; enriched lithospheric mantle; oceanic arc; Myitkyina

2020-04-26;

2020-05-06;

2020-12-10

國家自然科學(xué)基金重點(diǎn)項(xiàng)目(92055207)、第二次青藏科考項(xiàng)目(2019QZKK0702)、國家自然科學(xué)基金重大項(xiàng)目(41490613)和國家自然科學(xué)基金面上項(xiàng)目(41672058)聯(lián)合資助。

張靖祎(1992–), 女, 博士研究生, 巖石大地構(gòu)造學(xué)專業(yè)。Email: zhangjingyi@gig.ac.cn

彭頭平(1974–), 男, 研究員, 從事巖石大地構(gòu)造研究。Email: tppeng@gig.ac.cn

P581

A

1001-1552(2021)03-0553-017

10.16539/j.ddgzyckx.2020.06.017