陳雙雙, 蘇玉平, 鄭建平, 魏 穎

(中國(guó)地質(zhì)大學(xué)(武漢) 地球科學(xué)學(xué)院, 地質(zhì)過程與礦產(chǎn)資源國(guó)家重點(diǎn)實(shí)驗(yàn)室, 湖北 武漢 430074)

0 引 言

中亞造山帶經(jīng)歷了碰撞造山、后碰撞和板內(nèi)演化三個(gè)階段, 是全球顯生宙陸殼增生與板內(nèi)演化最顯著的地區(qū)之一[1]。新疆北部是中亞造山帶的重要組成部分, 具有完整且明顯的陸殼增生演化過程[2]。特別是古生代經(jīng)歷了復(fù)雜的地質(zhì)演化, 中生代以后開始進(jìn)入相對(duì)穩(wěn)定的板內(nèi)演化階段[3]。該地區(qū)廣泛發(fā)育古生代蛇綠巖套[4–6], 如 447～531 Ma的唐巴勒蛇綠巖[4]、(472±8.4) Ma的洪古勒楞蛇綠巖[5]及391 Ma的達(dá)拉布特蛇綠巖[6]; 至晚古生代準(zhǔn)噶爾地體及周邊廣泛發(fā)育后碰撞花崗巖[7–9], 表明古準(zhǔn)噶爾洋的閉合演化過程。與古生代碰撞-后碰撞巖漿活動(dòng)相比, 在新疆北部地區(qū)很少有關(guān)于中新生代巖漿活動(dòng)的報(bào)道, 僅在較老資料中有過百口泉、白堿灘等地區(qū)中生代玄武巖的記載[10], 但由于當(dāng)時(shí) K-Ar同位素年代學(xué)測(cè)試精度的限制, 沒有得到普遍認(rèn)同[3]。近年來, 不斷發(fā)現(xiàn)的中新生代巖漿活動(dòng)[3,11–15], 提供了揭示新疆北部地區(qū)中新生代巖石圈深部過程的重要窗口[16–17]。在詳細(xì)的野外地質(zhì)和室內(nèi)巖相學(xué)基礎(chǔ)上, 本次工作對(duì)克拉瑪依玄武巖開展了全巖主元素、微量元素和Sr-Nd同位素以及鋯石U-Pb年代學(xué)、Hf同位素的全面分析, 擬對(duì)巖漿活動(dòng)的成因機(jī)制及構(gòu)造背景進(jìn)行探討, 以期為新疆北部顯生宙地幔演化提供資料。

1 地質(zhì)背景

西準(zhǔn)噶爾位于西伯利亞板塊、哈薩克斯坦板塊和塔里木板塊交匯區(qū)域, 其與相鄰三大板塊的關(guān)系一直受到地質(zhì)學(xué)界的關(guān)注。泥盆紀(jì)末至石炭紀(jì)初西準(zhǔn)噶爾地區(qū)廣泛發(fā)育的蛇綠巖套和島弧火山巖, 表明古準(zhǔn)噶爾洋的俯沖消減過程[4–6]; 晚古生代準(zhǔn)噶爾地體及周邊廣泛發(fā)育的后碰撞花崗巖, 表明古洋盆的閉合及區(qū)域進(jìn)入后碰撞期階段[7–9]。王京彬等[18]將西準(zhǔn)噶爾地區(qū)“后碰撞”的時(shí)間厘定為石炭紀(jì)至二疊紀(jì), 而韓寶福等[9]將準(zhǔn)噶爾后碰撞深成巖漿的活動(dòng)時(shí)間限定于早石炭中晚期至早二疊世末期。晚二疊世之后西準(zhǔn)噶爾地區(qū)進(jìn)入了穩(wěn)定的板內(nèi)演化階段[19]。所以西準(zhǔn)噶爾地區(qū)經(jīng)歷了主碰撞-后碰撞-板內(nèi)環(huán)境這樣一套完整的構(gòu)造-巖漿活動(dòng)的演化歷史。與古生代碰撞-后碰撞的巖漿活動(dòng)相比, 具有確切年齡依據(jù)的中新生代幔源巖漿活動(dòng)較少。近幾年來, 在新疆北部地區(qū)報(bào)道的中新生代巖漿活動(dòng)記錄主要有克拉瑪依玄武巖(192 Ma)[3]、托云玄武巖類(100～40.36 Ma)[11–13]、哈拉喬拉橄欖玄武巖(20～10 Ma)[14–15](圖1a), 這些巖漿活動(dòng)主要沿著斷裂帶或薄弱帶分布。

本次研究的玄武巖(圖1b)位于克拉瑪依市西南10 km 處的侏羅統(tǒng)八道灣組底部(45.5348°N, 84.7576°E),與下伏下石炭統(tǒng)太勒古拉組地層呈角度不整合接觸關(guān)系, K-Ar年齡在170～190 Ma之間[10]。研究區(qū)域內(nèi)發(fā)育了一系列 NE-NNE向斷層, 其周圍地層中還發(fā)育了大量的 NE-NNE向和 NWW-NW 向基性巖墻群[21]。研究區(qū)域內(nèi)主要出露晚古生代至中新生代地層, 中生代地層出露較全。下伏地層是下石炭統(tǒng)太勒古拉組, 其地層產(chǎn)狀陡傾, 存在各種斷裂和褶皺構(gòu)造。侏羅紀(jì)地層覆蓋于太勒古拉組地層之上, 兩者呈角度不整合接觸關(guān)系[3]。

2 巖石學(xué)特征

本次研究的玄武巖具有明顯的柱狀節(jié)理(圖2a),樣品新鮮, 呈致密塊狀構(gòu)造, 顯微鏡下顯示斑狀結(jié)構(gòu)。斑晶主要由富Ca斜長(zhǎng)石、普通輝石和橄欖石組成, 包括深灰色橄欖玄武巖和深黑色含菱鐵礦橄欖玄武巖。

圖1 新疆北部地區(qū)中新生代火山巖分布圖(a) (據(jù)文獻(xiàn)[20])和克拉瑪依區(qū)域地質(zhì)簡(jiǎn)圖及采樣點(diǎn)(b) (據(jù)文獻(xiàn)[21])Fig.1 Distribution of Mesozoic-Cenozoic volcanic rocks in northern Xinjiang (a) (modified from reference [20]); The simplified geological map of the Karamay region and sampling locations (b) (modified from reference [21])1–第四紀(jì)沉積物; 2–下白堊統(tǒng)吐谷魯組沉積巖; 3–中-上侏羅統(tǒng)沉積巖; 4–下侏羅統(tǒng)八道灣組火山沉積巖; 5–上二疊統(tǒng)小泉溝組沉積巖; 6–下石炭統(tǒng)太勒古拉組凝灰?guī)r-中基性火山巖; 7–花崗巖-花崗閃長(zhǎng)巖; 8–角閃花崗巖-黑云母花崗巖; 9–中基性巖脈; 10–采樣點(diǎn)。1–quaternary sediment; 2–lower Cretaceous Tugulu formation sediment; 3–Middle-Upper Jurassic sediment; 4–Lower Jurassic Badaowan formation sediment; 5–Lower Permian Xiaoquangou formation sediment; 6–Lower Carboniferous Tailegula formation tufa and intermediate-basic volcanic rock;7–granite and granodiorite; 8–hornblende-granite and biotite-granite; 9–intermediate-basic dyke; 10–sampling site.

深灰色橄欖玄武巖(圖2b), 斑晶主要由富Ca斜長(zhǎng)石、普通輝石和橄欖石組成, 還含有少量磁鐵礦,樣品較新鮮, 沒有明顯的蝕變。富 Ca斜長(zhǎng)石(45%～50%)呈自形-半自形板狀, 粒徑 0.2～0.6 mm;普通輝石(20%～25%)呈板狀、粒狀, 粒徑 0.05～0.2 mm; 橄欖石(5%)呈不規(guī)則粒狀, 粒徑很小?；|(zhì)具有間粒間隱結(jié)構(gòu), 主要由斜長(zhǎng)石、玻璃和少量磁鐵礦組成。

深黑色含菱鐵礦橄欖玄武巖(圖 2c), 斑晶由富Ca斜長(zhǎng)石、普通輝石、橄欖石和碳酸鹽礦物菱鐵礦組成。菱鐵礦呈顆粒狀, 粒徑較大, 約為0.5 mm, 薄片中無色, 閃突起, 具有明顯的十字消光[21](圖2d)?；|(zhì)也具有間粒間隱結(jié)構(gòu)。

圖2 克拉瑪依玄武巖野外及鏡下照片F(xiàn)ig.2 Photomicrographs and field photos of Karamay basalts(a)克拉瑪依玄武巖明顯的柱狀節(jié)理; (b)深灰色橄欖玄武巖的正交偏光顯微照片; (c)深黑色含菱鐵礦橄欖玄武巖的正交偏光顯微照片; (d)鏡下可見特征碳酸鹽礦物菱鐵礦。Ol–橄欖石; Cpx–單斜輝石; P1–斜長(zhǎng)石; G–玻璃; Mag–磁鐵礦; Sid–菱鐵礦。(a) The columnar jointing structure of Karamay basalts; (b) Microphotographs of cross-polarized light of the dark grey olivine basalt; (c) Microphotographs of cross-polarized light of the dark-coloured siderite-bearing olivine basalt; (d) Siderite (carbonate minerals) can be seen. O1–olivine,Cpx–clinopyroxene, Pl–plagioclase, G–glass, Mag–magnetite, Sid–siderite.

3 實(shí)驗(yàn)分析方法

全巖主元素和微量元素分析均在中國(guó)地質(zhì)大學(xué)(武漢)地質(zhì)過程與礦產(chǎn)資源國(guó)家重點(diǎn)實(shí)驗(yàn)室完成, 主元素采用 XRF法分析, 測(cè)試精度優(yōu)于 1%。利用標(biāo)樣 GBW07113評(píng)估實(shí)驗(yàn)室 XRF測(cè)試數(shù)據(jù)的準(zhǔn)確度(表1)。微量元素分析采用兩酸(HNO3+HF)酸溶方法對(duì)樣品粉末進(jìn)行溶解, 采用 ICP-MS(Agilent 7500a)測(cè)定元素含量。分別用AGV-2、BHVO-2、BCR-2、RGM-1和GSR-1這5個(gè)標(biāo)樣評(píng)估實(shí)驗(yàn)室ICP-MS測(cè)試數(shù)據(jù)的準(zhǔn)確度(表2), 大多數(shù)元素的相對(duì)誤差小于5%, 部分元素相對(duì)誤差小于10%。Sr-Nd同位素(表3)組成在中國(guó)地質(zhì)大學(xué)(武漢)的MAT 261固體質(zhì)譜儀上分析, Nd和 Sr同位素比值分別采用146Nd/144Nd=0.7219和86Sr/88Sr=0.1194進(jìn)行校正。

鋯石的所有測(cè)試分析均在澳大利亞 Macquarie大學(xué)GEMOC中心完成。由巖石樣品經(jīng)過破碎、重液分離和磁選初步分選出鋯石, 再在雙目鏡下分選出晶型完好、顆粒大于50 μm的鋯石, 并將代表性鋯石制成環(huán)氧樹脂樣品靶, 然后在顯微鏡下觀察并進(jìn)行CL照相。鋯石U-Pb同位素(表4)分析在Agilent 7500電感耦合等離子質(zhì)譜與Merchantek/NWR 213 nm激光熔蝕探針聯(lián)機(jī)上進(jìn)行。U-Pb同位素?cái)?shù)據(jù)由Glitter軟件處理,206Pb/238U加權(quán)平均年齡及諧和年齡圖由Isoplot程序計(jì)算繪制。鋯石Hf同位素(表5)在 Nu Plasma多接受器的電感耦合等離子質(zhì)譜與Merchantek/NWR 213nm激光熔蝕探針的聯(lián)機(jī)上進(jìn)行。選用Blichert-Toft et al.[26]建議的球粒隕石值計(jì)算 εHf(t), 用 Griffin et al.[27]建議的虧損地幔值和大陸平均地殼的176Lu/177Hf=0.015[28]來計(jì)算虧損地幔模式年齡(TDM)和平均大陸地殼模式年齡(Tcrust)。

表1 克拉瑪依玄武巖主元素分析結(jié)果 (%)Table 1 Major element analytical results (%) of Karamay basalts

表3 新疆北部晚古生代和中新生代火山巖的Sr-Nd同位素特征Table 3 Sr-Nd isotopes of Late Paleozoic and Mesozoic-Cenozoic volcanic rocks in northern Xinjiang

表4 克拉瑪依玄武巖鋯石LA-ICPMS U-Pb分析數(shù)據(jù)Table 4 Zircon LA-ICPMS U-Pb analytical data of Karamay basalts

表5 克拉瑪依玄武巖鋯石Hf同位素成分Table 5 Hf isotopic compositions of zircon in Karamay basalts

4 分析結(jié)果

4.1 主元素和微量元素特征

克拉瑪依玄武巖 SiO249.6%～51.2%, FeOT10.4%～12.7%, MgO 3.58%～6.81%, CaO 6.74%～7.98%, K2O+Na2O 5.27%～5.60%, Mg#值在 0.26～0.55之間。與正常的大洋中脊玄武巖相比, 堿含量明顯偏高, 尤其是鉀含量。利用Zr/TiO2-Nb/Y圖解[29]對(duì)克拉瑪依玄武巖進(jìn)行分類(圖3), 顯示它們屬于堿性玄武巖系列, 里特曼指數(shù)σ=3.87～4.68(表1)。

這些玄武巖的稀土元素總量較高(113～118 μg/g), 具有富集輕稀土元素特征, 稀土分布模式曲線均向右傾斜(圖 4a), 且(La/Yb)N比值較高(4.99～10.1), 表現(xiàn)較強(qiáng)的輕重稀土元素分異特征。它們沒有Eu的負(fù)異常(δEu=1.02～1.06)。在微量元素蛛網(wǎng)圖(圖 4b)中, 相對(duì)富集大離子親石元素 LILE(如Rb、Ba和 K 等), Nb-Ta、Zr-Hf表現(xiàn)為相對(duì)富集, Ba、Sr顯示明顯正異常, Th、P為負(fù)異常。所有微量元素含量都高于原始地幔值, 且多數(shù)元素高出10倍以上,并且稀土分布模式曲線和蛛網(wǎng)圖與典型的OIB特征都極為相似, 但顯示更為虧損的特點(diǎn)(圖4)。

圖3 克拉瑪依玄武巖Zr/TiO2-Nb/Y圖解(據(jù)文獻(xiàn)[30])Fig.3 Plots of Zr/TiO2-Nb/Y for Karamay basalts (after reference [30])

圖4 克拉瑪依玄武巖稀土元素球粒隕石標(biāo)準(zhǔn)化分布模式(a)和微量元素原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖(b)Fig. 4 Chondrite normalized REE patterns of Karamay basalts (a);Primitive mantle normalized trace element patterns of Karamay basalts (b)球粒隕石、原始地幔、E-MORB、N-MORB及OIB資料據(jù)文獻(xiàn)[31]。The data of Chondrite, Primitive mantle, E-MORB, N-MORB and OIB are from reference [31].

4.2 全巖Sr-Nd同位素特征

克拉瑪依玄武巖的初始 Sr同位素(87Sr/86Sr)i變化范圍很小(0.7048～0.7049), εNd(t)值具有相對(duì)較低的正值(+2.95～+3.02), Nd同位素二階段模式年齡TDM2為 725～731 Ma(表 3)。相比于晚古生代基性火山巖(較高的正εNd(t)值、較低的(87Sr/86Sr)i值, 表3),這些玄武巖表現(xiàn)出虧損程度相對(duì)較低的特征, 且落在DM與EMⅠ地幔演化范圍內(nèi)(圖5)。

4.3 鋯石U-Pb年代學(xué)和Hf同位素特征

在近3 kg的樣品中經(jīng)人工重砂分選發(fā)現(xiàn)10顆鋯石。所有鋯石沒有完整的形態(tài), 但多發(fā)育典型的巖漿結(jié)晶結(jié)構(gòu)特點(diǎn) (圖6)。除KL5a偏離諧和曲線外,其他顆粒都落在諧和曲線上, 其中 1個(gè)鋯石分析點(diǎn)(KL4b)得到了相對(duì)較老的206Pb/238U年齡(424.1 Ma),其余 8個(gè)鋯石分析點(diǎn)集中分布, 給出了(357.3±5.1)Ma (MSWD=1.6)的諧和年齡(圖 6)。這些鋯石的 U和 Th 的含量分別為 86～245 μg/g 和 56～257 μg/g (表4), Th/U比值介于0.55～0.76之間, 也具巖漿鋯石特征。

圖5 晚古生代和中新生代火山巖的εNd(t)-(87Sr/86Sr)i (a)和εNd(t)-t關(guān)系圖(b)Fig.5 Plots of εNd(t) vs. initial 87Sr/86Sr (a) and εNd(t) vs. age (b) for late Paleozoic and Mesozoic-Cenozoic volcanic rocks圖5a改自文獻(xiàn)[32], MORB數(shù)據(jù)來自文獻(xiàn)[33], OIB數(shù)據(jù)來自文獻(xiàn)[34],EMⅠ和EMⅡ數(shù)據(jù)來自文獻(xiàn)[35]。在圖5a和圖5b中, 晚古生代基性火山巖包括西準(zhǔn)噶爾玄武安山巖[22]、富鈮玄武巖[20]、三塘湖盆地玄武巖[24]和柳園玄武巖[25]; 中新生代基性火山巖包括克拉瑪依玄武巖[3]、托云玄武巖[11–13]和哈拉喬拉玄武巖[14–15]。Fig.5 a is modified from Reference [32], the data of MORB are from reference [33], the data of OIB are from Reference [34], the data of enriched mantle Ⅰ (EMⅠ) and Ⅱ (EMⅡ) are from reference [35];In Fig.5a and Fig.5b, Late Paleozoic basic volcanic rocks include Western Junggar basaltic andesite[22], Nb-enriched basalt[20], Santanghu basalt[24], Liuyuan basalts[25]; Mesozoic-Cenozoic basic volcanic rocks include Karamay basalt[3], Tuoyun basalt[11–13], Halaqiaola basalt[14–15].

圖6 克拉瑪依玄武巖中鋯石LA-ICPMS U-Pb諧和曲線圖Fig.6 LA-ICPMS U-Pb concordia diagram of zircons from Karamay basalts

顆粒 KL4b(424 Ma)的初始 Hf同位素比值(176Hf/177Hf)i=0.282720, εHf(t)值為+7.48; 其余 8 顆鋯石(357 Ma)的(176Hf/177Hf)i值為 0.282730～0.282960、εHf(t)值為+6.23～+10.22(表 5)。它們的模式年齡 TDM和 Tcrust分別在 581～741 Ma和 712～968 Ma之間。

5 討 論

5.1 克拉瑪依玄武巖成巖年齡及石炭系地層關(guān)系

本研究所得克拉瑪依玄武巖鋯石的主體 U-Pb諧和年齡為(357.3±5.1) Ma, 該年齡不能代表克拉瑪依玄武巖的噴發(fā)時(shí)代, 而是代表了圍巖太勒古拉組地層時(shí)代。支持該結(jié)論的主要證據(jù)包括有: 徐新等[3]對(duì)這套克拉瑪依玄武巖進(jìn)行40Ar/39Ar定年, 確定其形成年齡為(192.7±1.3) Ma; 薛云興等[21]系統(tǒng)研究了這套橄欖玄武巖中菱鐵礦成因, 明確指出其應(yīng)形成于侏羅紀(jì)造山期后的板內(nèi)巖漿活動(dòng); 郭麗爽等[36]曾對(duì)太勒古拉組玄武巖進(jìn)行鋯石U-Pb定年, 測(cè)得結(jié)果為(357.5±5.4) Ma, 與我們所得鋯石 U-Pb年齡(357.3±5.1) Ma相當(dāng)一致; 特別是從野外產(chǎn)狀上看,克拉瑪依玄武巖呈角度不整合覆蓋在太勒古拉組火山-沉積巖之上[21], 且發(fā)育明顯的柱狀節(jié)理(圖 2a);并且侏羅紀(jì)玄武巖與周邊古生代玄武巖具有明顯不同的地球化學(xué)特征。此外, 鋯石具有較高的正 εHf(t)值 (+6.23～+14.49)和 較 高 的 (176Hf/177Hf)i值(0.282725～0.282977), 其中1個(gè)鋯石分析點(diǎn)KL4a的εHf(t)值高達(dá)+14.49, 這都暗示了其具有較虧損地幔源區(qū)特征, 而下文“巖石成因”中分析克拉瑪依玄武巖具有與 OIB型源區(qū)相似的幔源源區(qū)特征, 這就更加暗示該巖漿鋯石不是來自于克拉瑪依玄武巖,而更可能是捕獲于太勒古拉組地層的繼承鋯石。

西準(zhǔn)噶爾地區(qū)下石炭統(tǒng)地層的上下關(guān)系一直存在著爭(zhēng)議。一些學(xué)者[36]認(rèn)為石炭系從下到上分別為太勒古拉組、包古圖組和希貝庫拉斯組; 也有學(xué)者[37]認(rèn)為從下到上為希貝庫拉斯組, 包古圖組和太勒古拉組。廖卓庭等[38]根據(jù)生物化石將這三個(gè)組的上下關(guān)系定為: 包古圖組、希貝庫拉斯組和太勒古拉組。我們?cè)诳死斠赖貐^(qū)獲得的太勒古拉組地層時(shí)代為(357.3±5.1) Ma, 該年齡大于(345.6±6.2) Ma 的包古圖組安山巖[39]、328～342 Ma的包古圖組蝕變凝灰?guī)r[37]和(335.6±7.8) Ma的包古圖組侵入巖體[40], 并且野外產(chǎn)狀表明包古圖組呈角度不整合伏于希貝庫拉斯組之下[36,41], 根據(jù)希貝庫拉斯組與夏爾甫東巖體的侵入接觸關(guān)系, 推斷希貝庫拉斯組形成年齡應(yīng)早于293 Ma[42]。據(jù)此推測(cè)西準(zhǔn)噶爾下石炭統(tǒng)地層從下到上依次為太勒古拉組、包古圖組和希貝庫拉斯組, 這樣可能更合適。

5.2 巖石成因

克拉瑪依侏羅紀(jì)玄武巖 SiO2含量為49.56%～51.22%, 不可能是由下地殼巖石部分熔融形成, 應(yīng)來源于地幔源區(qū)。需要討論的是克拉瑪依玄武巖的幔源原始巖漿的分異作用、地殼混染程度、地幔源區(qū)性質(zhì)以及是否發(fā)生交代富集作用。

5.2.1 地殼混染與結(jié)晶分異作用

這些玄武巖的堿含量明顯偏高(尤其是鉀含量),屬于堿性玄武巖系列(σ=3.87～4.68)。該玄武巖Nb、Ta含量較高, 沒有顯示虧損的特征, Zr、Hf也沒有明顯的正異常, Nb/Ta比值為16.7～18.1, Zr/Hf比值約為 40.0, 分別與原始地幔(Nb/Ta=17.5, Zr/Hf=36.3)相近而遠(yuǎn)高于大陸地殼值(Nb/Ta=12.1, Zr/Hf =11.0);較低的 Th/Ce比值(0.06)和較低的 Th/La比值(0.13～0.14), 與幔源巖漿的 Th/Ce比值(0.02～0.05)和Th/La比值(約0.12)相當(dāng)而明顯低于地殼的Th/Ce比值(約 0.15)和 Th/La 比值(約 0.30)[31,43]。此外, Zr/Yb比值(124～139)、Th/La 比值(0.13～0.14)、Ta/Yb 比值(1.58～1.83)、Nb/Yb 比值(28.0～30.8)和 La/Yb 比值(12.3～14.1)也都具有與原始地幔比值相似且遠(yuǎn)偏于地殼比值的特征[31,44]。在Nb-Ta異常圖中(圖7c、圖7d), 克拉瑪依玄武巖遠(yuǎn)離UCC區(qū)域, 也暗示了其受到的地殼混染作用很小。盡管鋯石是捕獲于早期巖漿活動(dòng)(太勒古拉組地層)的繼承鋯石, 但是, 不論從巖相學(xué)角度還是從地球化學(xué)角度, 都可以明顯看出, 克拉瑪依玄武巖受地殼物質(zhì)影響程度相對(duì)有限,特別是沒有古老地殼物質(zhì)的混染; 至于受古生代太勒古拉組火山巖混染確實(shí)無法完全排除, 因此我們只能采用逼近的方法, 利用其地球化學(xué)數(shù)據(jù)討論源區(qū)特征。

圖7 晚古生代和中新生代火山巖的Th/Yb-Nb/Yb (a)、Th/Y-Sm/Th (b)、Ta*-Nb* (c)和Nb/Ta-Nb (d)關(guān)系圖Fig.7 Plots of Th/Yb-Nb/Yb (a), Th/Y-Sm/Th (b), Ta*-Nb* (c) and Nb/Ta-Nb (d)圖7a改自文獻(xiàn)[20, 45], HS表示較深的幔源, LS表示較淺的幔源。圖7b改自文獻(xiàn)[46], PM、OIB和N-MORB數(shù)據(jù)來自文獻(xiàn)[31]。圖7c改自文獻(xiàn)[47], OIB、E-MORB、N-MORB數(shù)據(jù)來自文獻(xiàn)[48], PM數(shù)據(jù)來自文獻(xiàn)[31], UCC數(shù)據(jù)來自文獻(xiàn)[49]。圖7d改自文獻(xiàn)[50], 球粒隕石、OIB、PM、DM數(shù)據(jù)來自文獻(xiàn)[51, 52], UCC數(shù)據(jù)來自文獻(xiàn)[50]?；曰鹕綆r數(shù)據(jù)來源與圖5相同。Fig.7 a is modified from reference [20, 45]; HS represents deep mantle source, while LS is superficial mantle source. Fig.7b is modified from reference[46]; the data of PM, OIB and N-MORB are from reference [31]. Fig.7c is modified from reference [47]; the data of OIB, E-MORB and N-MORB are from reference [48]; the data of PM are from reference [31]; the data of UCC are from reference [49], Fig.7d is modified from reference [50]; the data of chondrite, OIB, PM, and DM are from reference [51, 52]; the data of UCC are from reference [50]. The data source of basic rocks are identical with those of Fig.5.

La和La/Sm相關(guān)圖呈一條水平直線, 暗示了玄武巖巖漿上升過程中結(jié)晶分異作用具有一定影響。Eu 沒有明顯的負(fù)異常(δEu=1.02～1.06), Ba、Sr表現(xiàn)出較弱的正異常以及Nb、Ta相對(duì)富集(圖4), 且TiO2與 FeOT、K2O和 Na2O沒有明顯相關(guān)關(guān)系, 表明斜長(zhǎng)石和鈦鐵氧化物、角閃石的結(jié)晶分異不明顯。而MgO與FeOT、Cr與Ni呈明顯正相關(guān)關(guān)系, 且相對(duì)較低的 Ni、Cr含量(表 2)以及橄欖石和單斜輝石的斑晶(圖2), 都暗示著橄欖石和單斜輝石的結(jié)晶分異在巖漿演化過程起一定作用。

5.2.2 源區(qū)特征

克拉瑪依侏羅紀(jì)玄武巖的稀土分布模式曲線和蛛網(wǎng)圖(圖4)都具有明顯右傾的特點(diǎn), 富集輕稀土元素和大離子親石元素(如Rb、Ba和K等), 與OIB的分布曲線模式很相似, 且其元素含量相比于 OIB都較為虧損, 暗示其具有與 OIB型源區(qū)相似但較為虧損的源區(qū)特征。它們的Nb/U比值(56.4～58.0)與OIB(Nb/U=52.0±15.0)相當(dāng)[53]; La/Nb 比值(0.43～0.46)遠(yuǎn)低于地殼(La/Nb=1.50～2.20)和原始地幔(La/Nb=0.98～1.00), 與 OIB(La/Nb=0.68)接近且略低于OIB的La/Nb比值[53]; Ce/Pb、Nb/Pb比值也有同樣特點(diǎn)。在εNd(t)-(87Sr/86Sr)i圖解(圖5)中, 相比于晚古生代基性火山巖, 侏羅紀(jì)玄武巖源區(qū)虧損程度相對(duì)較低, 且落在OIB的幔源源區(qū)范圍內(nèi)(圖7), 指示著這些玄武巖源區(qū)具有與OIB型源區(qū)相似但較為虧損的源區(qū)特征。

侏羅紀(jì)玄武巖強(qiáng)烈富集輕稀土元素和強(qiáng)不相容元素以及表現(xiàn)Ba、Sr的正異常, 暗示了巖石圈地?？赡芙?jīng)歷過交代富集作用。斑晶菱鐵礦的存在和偏高的Zr/Hf比值(40.0～41.6), 也通常被認(rèn)為是碳酸鹽熔體交代地幔過程的結(jié)果[54–58]。綜上所述, 我們推測(cè)克拉瑪依侏羅紀(jì)玄武巖源區(qū)可能經(jīng)歷過交代富集作用, 使其巖漿源區(qū)虧損程度相對(duì)較低且具有與OIB型源區(qū)相似的幔源源區(qū)的特征。

稀土元素的豐度和比值可以用來推測(cè)源區(qū)深度及部分熔融的程度[59–60]?？死斠蕾_紀(jì)玄武巖具有相對(duì)較高的 Sm/Yb比值(4.17～4.71)和較高的La/Yb比值(12.3～14.1)指示其源區(qū)深度可能在80 km以下深處的石榴石二輝橄欖巖地幔[46,61], 并且其部分熔融程度較低, 處于5%～10%之間(圖8)。

圖8 晚古生代和中新生代火山巖的Sm/Yb-Sm (a)和Sm/Yb-La/Yb (b)關(guān)系圖Fig.8 Plots of Sm/Yb-Sm (a) and Sm/Yb-La/Yb (b) for late Paleozoic and Mesozoic-Cenozoic volcanic rocksDM端元數(shù)據(jù)來源文獻(xiàn)[62], La 0.206 μg/g, Sm 0.299 μg/g, Yb 0.347 μg/g, La/Yb=0.594, Sm/Yb=0.862; PM 端元數(shù)據(jù)來源文獻(xiàn)[31], La 0.687 μg/g, Sm 0.444 μg/g, Yb 0.493 μg/g, La/Yb=1.394, Sm/Yb=0.901;虛線和實(shí)線分別代表DM熔融曲線和PM熔融曲線, 尖晶石二輝橄欖巖(Ol0.530+Opx0.270+Cpx0.170+Sp0.030和 Ol0.060+Opx0.280+Cpx0.670+Sp0.110)和石榴石二輝橄欖巖(Ol0.600+Opx0.200+Cpx0.100+Gt0.100和 Ol0.030+Opx0.160+Cpx0.880+Gt0.090)熔融曲線來自文獻(xiàn)[59, 60], 曲線上的數(shù)字代表熔融程度; 基性火山巖數(shù)據(jù)來源與圖5相同。The data of Depleted Mantle (DM) are from reference [62], La=0.206 μg/g, Sm=0.299 μg/g, Yb=0.347 μg/g, La/Yb=0.594, Sm/Yb=0.862;Primitive Mantle (PM) are from reference [31], La=0.687 μg/g,Sm=0.444 μg/g, Yb=0.493 μg/g, La/Yb=1.394, Sm/Yb=0.901; Dashed and solid curves are the melting trends from DM and PM, Melting curves for spinel lherzolite(Ol0.530+Opx0.270+Cpx0.170+Sp0.030 and Ol0.060+ Opx0.280+Cpx0.670+Sp0.110) and garnet lherzolite (Ol0.600+Opx0.200+Cpx0.100+Gt0.100 and Ol0.030+Opx0.160+Cpx0.880+Gt0.090) are from reference [59, 60],Numbers along melting curves represent the degree of partial melting.The data source of basic rocks are identical with those of Fig.5.

5.3 構(gòu)造演化過程

克拉瑪依侏羅紀(jì)玄武巖具有相對(duì)高的Ti/Y比值和低的 Hf/Ta比值, 類似于板內(nèi)玄武巖[63]。用Hf-Th-Ta (圖 9a)、Nb-Zr-Y 三角圖 (圖 9b)判別, 結(jié)果也是一致的。實(shí)際上, 從區(qū)域大地構(gòu)造演化和對(duì)比分析來看, 新疆北部在中新生代時(shí)早已經(jīng)進(jìn)入了板內(nèi)環(huán)境[2,3,9,18,19]。

新疆北部晚古生代基性火山巖具有較低的(87Sr/86Sr)i值(低至 0.7034)和較高的正 εNd(t)值(高達(dá)+8.84), 具有虧損地幔源區(qū)特征, 主要位于 DM 區(qū)域;而中新生代玄武巖具有相對(duì)較高的(87Sr/86Sr)i值(高達(dá) 0.7056)和相對(duì)較低的正 εNd(t)值(低至+0.29), 位于 DM 和 EMⅠ混合的地幔演化區(qū)域, 虧損程度相對(duì)較低(圖5)。同樣, 圖7a和圖7b也表現(xiàn)出晚古生代與中新生代火山巖巖漿源區(qū)的虧損富集的差別。此外, 從源區(qū)深度來看, 晚古生代巖漿源區(qū)主要處于較淺的尖晶石穩(wěn)定深度且源巖熔融程度較高, 而中新生代的巖漿源區(qū)則主要位于較深的石榴子石穩(wěn)定深度范圍, 熔融程度較低(圖 8); 并且, 晚古生代基性火山巖受到較大程度的地殼物質(zhì)混入作用, 可能與俯沖環(huán)境有關(guān)[61,66], 而中新生代玄武巖則受到很小程度的地殼混染(圖7c, 圖7d, 圖9)。

由晚古生代到中新生代, 地幔源區(qū)由較淺的虧損地幔逐步向較深的虧損程度相對(duì)較低的幔源演化、熔融程度以及地殼物質(zhì)混入程度逐漸變低。引起這種變化的原因可能是俯沖的洋殼物質(zhì)以及拆層的巖石圈在地幔一定深度發(fā)生變質(zhì)重熔后, 與原虧損地幔和巖石圈地幔不斷交代富集的結(jié)果[2,67–70]。此外, 俯沖板片和拆層巖石圈在地幔中重熔可形成富集Nb-Ta的熔體[71–72], 而高Nb-Ta的熔體交代地幔是導(dǎo)致克拉瑪依玄武巖富集Nb-Ta的主要原因[73–74]。

根據(jù)上述分析, 新疆北部地區(qū)晚古生代到中新生代的殼-幔作用過程, 可能包括: (1)古準(zhǔn)噶爾洋發(fā)生俯沖消減, 俯沖板片在一定的溫壓條件下釋放出流體和熔體, 導(dǎo)致上覆相對(duì)虧損的地幔楔部分熔融(圖 10a), 形成了 331～375 Ma的較虧損的且受地殼物質(zhì)混染的基性火山巖[22–23]。(2)早期的俯沖板片持續(xù)下沉最終在300 Ma左右發(fā)生板片斷離[20]。由于古準(zhǔn)噶爾洋的閉合及板塊的碰撞增生, 使巖石圈不斷增厚而最終發(fā)生拆層去根作用[75–77], 導(dǎo)致軟流圈地幔急劇上升[20,70–80](圖10b)。虧損的軟流圈地幔上涌底侵并發(fā)生部分熔融, 在侵入地殼過程中受到地殼物質(zhì)或早期俯沖帶物質(zhì)的混染[2,9,81], 形成了260～290 Ma的較虧損的且受到較大程度地殼物質(zhì)混染的玄武巖[24–25]。(3)拆層巖石圈和俯沖下沉的洋殼物質(zhì)下降到地幔一定深度, 開始發(fā)生變質(zhì)重熔作用, 形成富集堿質(zhì)、Nb-Ta、大離子親石元素(LILE)和輕稀土元素(LREE)的熔/流體。這些富集熔/流體在上升過程中, 與原虧損地幔和巖石圈地幔不斷地交代平衡, 逐步改造為較富堿、富含不相容元素的虧損程度相對(duì)較低的地幔。繼而沿著斷裂帶或薄弱帶快速上涌噴出地表, 形成了中新生代虧損程度較低的、源區(qū)深度較深的且僅受到較小程度地殼混染的玄武巖(圖10c)。

圖9 晚古生代和中新生代火山巖的Hf-Th-Ta (a) (據(jù)文獻(xiàn)[64])和Nb-Zr-Y (b) (據(jù)文獻(xiàn)[65])構(gòu)造判別圖Fig.9 Tectonic discrimination diagrams of Hf-Th-Ta (a) (after reference [64]) and Nb-Zr-Y (b) (after reference [65]) for late Paleozoic and Mesozoic-Cenozoic volcanic rocks圖9a中: T–島弧拉斑玄武巖; CAB–鈣堿性玄武巖; N-MORB–洋中脊玄武巖; E-MORB+WPT–富集的洋中脊玄武巖和板內(nèi)拉斑玄武巖; WPA–板內(nèi)堿性玄武巖。圖 9b中: WPA–板內(nèi)堿性玄武巖; WPA+WPT–板內(nèi)堿性玄武巖和板內(nèi)拉斑玄武巖; E-MORB–富集的洋中脊玄武巖;N-MORB+CAB–洋中脊玄武巖和鈣堿性玄武巖; WPT+CAB–板內(nèi)拉斑玄武巖和鈣堿性玄武巖?；曰鹕綆r數(shù)據(jù)來源與圖5相同。In Fig.9a, IAT–Island-Arc Tholeiites, CAB–Calc-Alkaline Basalts, N-MORB–N-type Mid-Ocean Ridge Basalts, E-MORB+WPT–E-type Mid-Ocean Ridge Basalts+Within-Plate Tholeiites, WPA–within-plate alkaline basalt; In Fig.9b, WPA–within-plate alkaline basalt, WPA+WPT–within-plate alkaline basalt+Within-Plate Tholeiites, E-MORB–E-type Mid-Ocean Ridge Basalts, N-MORB+CAB–N-type Mid-Ocean Ridge Basalts+Calc-Alkaline Basalts, WPT+CAB–Within-Plate Tholeiites+Calc-Alkaline Basalts; The data source of basic rocks are identical with those of Fig.5.

6 主要認(rèn)識(shí)

(1)克拉瑪依侏羅紀(jì)玄武巖中獲得鋯石的 U-Pb諧和年齡(357.3±5.1) Ma代表了圍巖太勒古拉組地層時(shí)代, Hf同位素表明太勒古拉組火山巖可能來自虧損地幔。

(2)侏羅紀(jì)玄武巖源區(qū)具有與OIB型源區(qū)相似但較為虧損的源區(qū)特征。該玄武巖可能來源于＞80 km處的石榴子石二輝橄欖巖穩(wěn)定存在的地幔源區(qū)的低程度部分熔融?？死斠佬鋷r僅受到很小程度的地殼混染, 橄欖石和單斜輝石的結(jié)晶分異具較明顯的控制作用。

(3)與晚古生代島弧及后碰撞基性火山巖不同,侏羅紀(jì)玄武巖形成于相對(duì)穩(wěn)定的板內(nèi)構(gòu)造環(huán)境, 其地幔源區(qū)虧損程度不及晚古生代火山巖源區(qū), 可能與俯沖下沉洋殼和拆層巖石圈在一定深度發(fā)生變質(zhì)重熔形成富集熔體并不斷與地幔發(fā)生交代富集作用有關(guān)。

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