解開瑞, 巫建華, *, 祝洪濤, 吳仁貴, 劉 帥

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大興安嶺南端芝瑞盆地流紋巖年代學(xué)、地球化學(xué)及巖石成因

解開瑞1, 巫建華1, 2*, 祝洪濤3, 吳仁貴1, 劉 帥2

(1. 東華理工大學(xué) 地球科學(xué)學(xué)院, 江西 南昌?330013; 2. 東華理工大學(xué) 核資源與環(huán)境國家重點(diǎn)實(shí)驗(yàn)室培育基地, 江西 南昌?330013; 3. 核工業(yè)243地質(zhì)大隊(duì), 內(nèi)蒙古 赤峰?024006)

芝瑞火山盆地位于大興安嶺南端內(nèi)蒙古克什克騰旗境內(nèi), 處于西拉木倫河-長春縫合帶以南、赤峰-開源斷裂帶以北的遼源地塊, 盆地中的長英質(zhì)火山巖系不整合于下二疊統(tǒng)大石寨組之上、新近系漢諾壩組玄武巖之下, 并見有多處花崗斑巖侵入其中。SHRIMP鋯石U-Pb年齡表明, 盆地火山巖系中流紋巖鋯石的206Pb/238U年齡為(156.9±1.7) Ma, 屬于晚侏羅世早期巖漿活動(dòng)的產(chǎn)物。巖石地球化學(xué)資料表明, 流紋巖具富SiO2、K2O和高FeOT/MgO值, 低Al2O3、CaO和MgO的特征, 屬于高鉀鈣堿性系列巖石, ∑REE含量較高, 輕重稀土分餾明顯, 具右傾的稀土分布模式, 負(fù)Eu異常明顯, 具有Ga、Zr、Nb、Y含量高和Ba、Sr含量低的微量元素特征, 相對富集Rb、Th、U、Pb、Zr、Hf, 相對虧損Ba、Sr、P、Ti及Nb、Ta等元素, 具有A型流紋巖和低Sr-Ba流紋巖的微量元素特征; 在Nb-Y-3Ga和Rb/Nb-Y/Nb圖解上顯示拉張構(gòu)造環(huán)境的A2型花崗巖的特征; 具有較高的Sr初始比值((87Sr/86Sr)i=0.706510~0.709821), 較低的Nd初始比值(Nd() = ?6.32 ~ ?4.44)和相對較年輕的Nd模式年齡(1304~1457 Ma)及較低的Pb同位素組成((206Pb/204Pb)t=17.15~17.80、(207Pb/204Pb)t= 15.41~15.48、(208Pb/204Pb)t= 37.38~37.63), 在Sr-Nd同位素示蹤圖解和Pb同位素構(gòu)造演化圖解上流紋巖同時(shí)具有下地殼和富集地幔印記。元素、同位素地球化學(xué)示蹤指示芝瑞流紋巖可能由起源于富集地幔的中元古代年輕下地殼的部分熔融形成, 且在巖漿上升過程中經(jīng)歷了結(jié)晶分異作用。在(Yb+Ta)-Rb和(Y+Nb)-Rb圖解上, 芝瑞流紋巖均顯示板內(nèi)拉張構(gòu)造環(huán)境, 結(jié)合區(qū)域上分布的同時(shí)代火山巖及A型花崗巖類可以限定流紋巖形成于伸展構(gòu)造背景, 并可能與北部的蒙古-鄂霍茨克縫合帶的演化有關(guān)。

流紋巖; 巖石成因論; 地球化學(xué); Sr-Nd-Pb同位素; 晚侏羅世早期; 芝瑞

0 引 言

近NNE向展布的大興安嶺-燕山中生代火山-侵入巖帶位于華北古板塊與西伯利亞古板塊碰撞形成的興蒙造山帶東段, 該帶面積廣闊、巖石類型多樣、地球化學(xué)特征復(fù)雜且與金屬成礦關(guān)系密切, 備受國內(nèi)外地質(zhì)工作者所矚目。年代學(xué)研究揭示, 大興安嶺-燕山中生代火山-侵入巖帶的巖漿活動(dòng)可分為七個(gè)不同的期次[1?3], 巖石類型研究表明, 該帶的長英質(zhì)火山-侵入巖包括高Ba-Sr型、低Ba-Sr型、A型、I型、埃達(dá)克型等類型[1, 4?8]。該帶的長英質(zhì)火山-侵入巖的巖石成因至少有以下四種: (1)由不同性質(zhì)的下地殼物質(zhì)[8?14]或是淺部地殼物質(zhì)[1]及古俯沖蝕變洋殼[15]部分熔融而成; (2)中基性巖漿的結(jié)晶分異成因[1, 12]; (3)巖漿由不同時(shí)期的地殼物質(zhì)與不同性質(zhì)的地幔物質(zhì)混合[5, 16]或是新老地殼物質(zhì)混合成因[4]; (4)巖漿混合再結(jié)晶分異成因, 如巖石圈地幔部分熔融形成的玄武質(zhì)巖漿和下地殼部分熔融形成的酸性巖漿混合產(chǎn)物的分離結(jié)晶[17]。Sr-Nd-Pb同位素地球化學(xué)研究表明, 該帶的長英質(zhì)火山-侵入巖的同位素組成存在空間上差異性: (1)塔河-喜桂圖斷裂帶以北的額爾古納地塊, 長英質(zhì)火山-侵入巖具有相對較高的(87Sr/86Sr)i、負(fù)低且變化大的Nd()、較大的DM2、較高且變化較小鉛同位素組成的特征[6,18?21]; (2)塔河-喜桂圖斷裂帶與賀根山-黑河斷裂帶之間的興安地塊, 長英質(zhì)火山-侵入巖具有相對較低的(87Sr/86Sr)i、較正高且變化大的Nd()值、較小DM2、較高且變化區(qū)間較大的鉛同位素組成的特征[6,18?22]; (3)賀根山-黑河斷裂帶與西拉木倫河-長春縫合帶之間的松遼地塊, 長英質(zhì)火山-侵入巖具有較高的(87Sr/86Sr)i、高的Nd()、小的DM2、較高的鉛同位素組成[4,6,17,21?25]; (4)赤峰-開源斷裂帶以南的燕山板內(nèi)造山帶, 長英質(zhì)火山-侵入巖具有較高且變化區(qū)間大的(87Sr/86Sr)i、負(fù)低的Nd()、較大的DM2、低的(206Pb/204Pb)i、(207Pb/204Pb)i與(208Pb/204Pb)i特 征[5,16,18,20,24,26]。然而, 西拉木倫河-長春縫合帶與華北克拉通北緣赤峰-開源斷裂帶之間的遼源地塊, 長英質(zhì)火山-侵入巖的年代學(xué)格架, 巖石組合組合類型與地球化學(xué)特征、物質(zhì)來源與構(gòu)造背景, 鈾成礦背景的分析研究相對薄弱[26?27]。本次工作選擇芝瑞盆地流紋巖為研究對象, 擬通過年代學(xué)、地球化學(xué)和Sr-Nd-Pb同位素的系統(tǒng)研究, 對流紋巖的成因進(jìn)行分析, 為揭示華北克拉通北緣中生代的構(gòu)造屬性提供依據(jù), 進(jìn)而加深對大興安嶺-燕山中生代火山-侵入巖帶成因規(guī)律的認(rèn)識。

1 區(qū)域地質(zhì)背景

自古亞洲洋閉合之后, 東北地區(qū)經(jīng)歷了環(huán)太平洋構(gòu)造體系和蒙古-鄂霍茨克構(gòu)造體系的疊加與改造作用[7,28?34], 中國東北部中生代火山巖的時(shí)空分布與巖石組合的綜合研究揭示了兩大構(gòu)造體系演化的多階段性特點(diǎn)[3,29,30], 以中侏羅世早期和侏羅紀(jì)末期兩次陸殼加厚過程和中侏羅世晚期-晚侏羅世早期和早、晚白堊世的區(qū)域性伸展事件與鄂霍茨克洋關(guān)閉和太平洋板塊的俯沖及俯沖間歇期匹配[29,33,35]。綜合俯沖距離(大于1500 km) 問題以及中生代不同時(shí)期的火山巖時(shí)空分布特征, 大興安嶺-燕山中生代火山-侵入巖帶中生代大規(guī)?；鹕交顒?dòng)屬古太平洋板塊遠(yuǎn)程俯沖作用產(chǎn)物的認(rèn)識[1,30]受到質(zhì)疑, 特別是對晚侏羅世時(shí)期環(huán)太平洋構(gòu)造體系的影響范圍存在較大爭議[3,29,33?35]。部分學(xué)者認(rèn)為環(huán)太平洋構(gòu)造體系中生代對東北亞大陸影響的空間范圍主要在松遼盆地及以東地區(qū)[7,29,33?35], 蒙古-鄂霍茨克構(gòu)造體系的影響主要存在于松遼盆地以西包括大興安嶺及額爾古納地區(qū)[14,29,33,35], 并可能擴(kuò)展至華北北緣及燕山板內(nèi)地區(qū)[33,34,36,37], 這種影響得到松遼盆地以西地區(qū)區(qū)域發(fā)育的的中侏羅世晚期-晚侏羅世早期火山巖研究的支持[1,2,13,29,34,38?42], 但是, 除去燕山造山帶地區(qū), 分布于膠東、遼東半島, 延邊-遼北、朝鮮及華北克拉通南緣地區(qū)的中-晚侏羅世火山-侵入巖及垂直于華北克拉通邊界的非惟一收縮變形指示著當(dāng)時(shí)可能存在多向構(gòu)造體系作用[30, 31, 43]。晚侏羅世晚期的擠壓及早白堊世西迄蒙古-鄂霍茨克縫合帶、東抵太平洋之濱以雙峰式火山巖組合、變質(zhì)核雜巖、A型花崗巖等代表的巨型地殼伸展省, 可能與環(huán)太平洋構(gòu)造體系和蒙古-鄂霍茨克構(gòu)造體系的雙重影響相關(guān)聯(lián)[7,30,32?34,36,37]。

地處赤峰市克什克騰旗芝瑞鄉(xiāng)的芝瑞盆地, 位于西拉木倫河-長春縫合帶以南、赤峰-開源斷裂帶以北的遼源地塊上, 屬大興安嶺與燕山火山-侵入巖帶的結(jié)合部位, 是沽源-紅山子鈾成礦帶北端鈾礦勘查區(qū)。盆地內(nèi)構(gòu)造以斷裂發(fā)育為主, 斷裂構(gòu)造主要有EW、NE、NW、SN向四組, 其中以NE向斷裂構(gòu)造最為發(fā)育, 主要見于廣興源-大興永地區(qū), 芝瑞地區(qū)因玄武巖覆蓋構(gòu)造形跡不甚清晰。其中盆地北部受大興永-南窩鋪斷裂帶控制,中帶受百岔河斷裂帶控制,南部受筆連溝斷裂控制, 整體呈NE向展布, 每條帶長幾至十幾km, 寬約1 km。盆地NW為廣興源復(fù)式巖體, 巖體主體由二長巖、二長花崗巖、花崗巖和花崗閃長巖組成, 巖體早期的二長巖和侵位于二長巖的花崗閃長巖的SHRIMP鋯石U-Pb年齡為(263.8±2.1) Ma和(263.3±2.5) Ma[44]。盆地內(nèi)以晚中生代火山巖系為主, 不整合于前中生代地層之上, 被新生代地層不整合覆蓋。盆地NW及SW處靠近盆地邊緣, 剝蝕程度較大、出露有下二疊統(tǒng)大石寨組中-酸性火山巖系或燕山早期花崗巖及多期侵入巖脈, 大石寨組下段為灰-灰褐色英安質(zhì)晶屑凝灰?guī)r、紫紅-褐色熔結(jié)凝灰?guī)r, 上段為紫色、灰褐色安山巖, 厚約150~300 m; 在盆地西部邊緣廣興源-大興永一帶和芝瑞地區(qū)的溝谷中為中生代長英質(zhì)火山巖系出露地區(qū)(圖1b), 火山巖系總體走向呈NE25°左右, 可分兩部分, 下部為灰色酸性凝灰?guī)r及灰紫色、灰黃色凝灰質(zhì)粉砂巖夾泥灰?guī)r透鏡體及薄層砂巖、礫巖為主, 厚40~200 m, 上部為淺灰-灰紫色流紋巖、流紋質(zhì)熔結(jié)凝灰?guī)r, 厚200~500 m, 見有花崗巖、花崗斑巖等侵入體及次火山巖侵入到火山巖系內(nèi)部。芝瑞地區(qū)火山巖系頂部被漢諾壩組和第四系覆蓋, 上新統(tǒng)漢諾壩組(N2h)為黑綠色玄武巖夾薄層砂巖、礫巖、泥煤、柴煤, 第四系(Q)主要為風(fēng)成黃土及砂、礫石等。

本文研究流紋巖樣品取自核工業(yè)243大隊(duì)在芝瑞鎮(zhèn)呂家溝門一帶實(shí)施的多個(gè)鉆孔, 樣品新鮮。流紋巖呈灰紫色, 斑狀結(jié)構(gòu), 流紋構(gòu)造明顯, 斑晶以長石為主, 基質(zhì)為隱晶質(zhì)。顯微鏡下觀察(圖2), 巖石具有斑狀結(jié)構(gòu), 斑晶為石英及堿性長石, 堿性長石具有卡式雙晶, 表面不干凈, 可見部分黏土礦化現(xiàn)象, 斑晶發(fā)育熔蝕反應(yīng)邊, 少量暗色礦物黑云母已發(fā)生綠泥石化?；|(zhì)具有包含霏細(xì)結(jié)構(gòu), 由石英作主晶包含堿性長石霏細(xì)狀質(zhì)點(diǎn)構(gòu)成, 巖石中發(fā)育少量硅質(zhì)條帶定向分布, 少量鐵質(zhì)質(zhì)點(diǎn)沿石英主晶接觸部位分布, 副礦物包括鋯石、磷灰石和磁鐵礦等。

2 分析方法

定年樣品編號為ZR208(42°48′34″N,117°47′19″E), 首先把樣品破碎, 經(jīng)浮選和電磁選等方法后, 淘洗、挑純挑出鋯石晶體。之后將挑選好的待測鋯石與標(biāo)準(zhǔn)鋯石TEM(年齡為417 Ma)一起粘貼, 制成環(huán)氧樹脂樣品靶。干燥后, 打磨、拋光使鋯石中心部分暴露, 然后進(jìn)行反射光、透射光和陰極發(fā)光顯微照相及SHRIMP鋯石U-Th-Pb分析。鋯石挑選由河北省廊坊市誠信地質(zhì)服務(wù)有限公司完成, 鋯石SHRIMP U-Th-Pb分析在北京離子探針中心SHRIMP-Ⅱ上完成, 具體分析工作流程及處理流程見宋彪等[45]資料, 芝瑞盆地流紋巖的鋯石U-Th-Pb分析結(jié)果列于表1。

圖1?芝瑞盆地地理位置(a)及地質(zhì)略圖(b)

1?第四系; 2?新近系漢諾壩組; 3?上侏羅統(tǒng)新民組; 4?下二疊統(tǒng)大石寨組; 5?晚二疊世花崗閃長巖; 6?晚二疊世石英二長閃長巖或石英二長巖; 7?中生代花崗巖; 8?花崗斑巖; 9?花崗巖脈; 10?整合地質(zhì)界限; 11?角度不整合地質(zhì)界線; 12?斷層; 13?居民點(diǎn)

1?Quaternary; 2?Neogene Hannuoba Formation; 3?Upper Jurassic Xinmin Formation; 4?Lower Permian Dashizhai Formation; 5?Late Permian granodiorite; 6?Late Permian quartz-diorite and quartz monzonite; 7?Mesozoic granite; 8?granitic porphyry; 9?granite vein; 10?geological boundary; 11?uncomformity; 12?fault; 13?settlement

圖2?芝瑞盆地流紋巖顯微鏡下照片

(a) 流紋構(gòu)造(-); (b)、(c) 斑狀結(jié)構(gòu), 斑晶為鉀長石和石英, 邊部可見溶蝕反應(yīng)邊, 基質(zhì)霏細(xì)結(jié)構(gòu)(+); Afs?堿性長石, Qtz?石英

(a) rhyotaxitic structure; (b),(c) porphyritic texture, porphyritic texture contains alkali feldspar and quartz, and the matrix has felsitic texture; Afs?alkali feldspar; Qtz?quartz

表1 芝瑞盆地流紋巖SHRIMP鋯石U-Th-Pb同位素分析結(jié)果

注:206Pbc和206Pb＊分別表示普通鉛和放射性成因鉛; 普通鉛根據(jù)實(shí)測204Pb進(jìn)行校正; 誤差為1σ

樣品全巖地球化學(xué)分析測試工作在湖北省武漢綜合巖礦測試中心完成。除FeO含量采用硫酸-氫氟酸溶礦, 重鉻酸鉀滴定法測得外, 其余主元素采用X射線熒光熔片法(XRF) 測定, 測試儀器為X熒光光譜儀(Magix-pro2440), 樣品采用無水四硼酸鋰作為溶劑, 分析精度優(yōu)于2%; 微量元素Sr、Ba用電感耦合等離子體發(fā)射光譜法(ICP-OES)測定, 測試儀器為等離子體發(fā)射光譜儀(ICAP6300), 其余微量元素和稀土元素均采用電感耦合等離子體質(zhì)譜法(ICP-MS)測定、測試儀器為X7型電感耦合等離子體質(zhì)譜儀(Thermoele-mental X7)。對USGS國際標(biāo)準(zhǔn)樣品(BHVO-2)的測定結(jié)果表明, 樣品測定值和推薦值的相對誤差小于10%, 且大多數(shù)微量元素的分析誤差在5%以內(nèi), 各分析方法的實(shí)驗(yàn)流程及其誤差、精度等詳見文獻(xiàn)[46]。

Sr-Nd-Pb同位素比值測試工作在中國科學(xué)院地質(zhì)與地球物理研究所重點(diǎn)實(shí)驗(yàn)室Finnigan MAT 262固體表面熱電離質(zhì)譜計(jì)上完成, Sr、Nd同位素分析采用傳統(tǒng)的陽離子交換樹脂法分別分離富集Sr和Nd元素, 采用146Nd/144Nd=0.7219和86Sr/88Sr=0.1194標(biāo)準(zhǔn)化校正測得Nd和Sr同位素比值。Nd和Sr同位素國際標(biāo)樣AMS和NBS987的測試值分別為143Nd/144Nd= 0.512139±18 (2σ,=28)和87Sr/86Sr = 0.710255±16 (2σ,=33)。鉛同位素采用陰離子交換樹脂來分離富集, 鉛國際標(biāo)樣NBS981的測定結(jié)果為207Pb/206Pb=0.9139±4 (2σ,=65)。

3 分析結(jié)果

3.1 鋯石U-Pb定年

芝瑞盆地流紋巖中的鋯石呈自形短柱狀或雙錐狀, 晶形較完整, 陰極發(fā)光圖像(CL)顯示鋯石具有條帶結(jié)構(gòu)(圖2a), 具有典型巖漿結(jié)晶鋯石的內(nèi)部結(jié)構(gòu)特征, 鋯石顆粒晶形較完整, 顆粒長度約90~140 μm,長寬比約1.3~1.6, 進(jìn)行測試時(shí)所有分析點(diǎn)均處于巖漿環(huán)帶部位(圖3a), 分析結(jié)果顯示普通Pb含量為0.52%~4.92%, U含量為51~161 μg/g, Th含量為25~ 92 μg/g, Th/U值變化于0.44~0.77, 平均值為0.58 (＞0.4), 具有典型的巖漿鋯石成分特征[47]。13個(gè)分析點(diǎn)的206Pb/238U年齡數(shù)據(jù)中(表1), 除分析點(diǎn)1.1與3.1偏離數(shù)據(jù)組之外, 其余11個(gè)測點(diǎn)年齡為155~161 Ma, 且數(shù)據(jù)投影點(diǎn)在U-Pb諧和曲線圖上均位于諧和線附近(圖3b), 剔除點(diǎn)1.1和3.1之后剩余的11個(gè)數(shù)據(jù)給出的206Pb/238U年齡加權(quán)平均值為(156.9±1.7) Ma (MSWD=0.37), 該年齡代表了流紋巖的形成時(shí)代。

3.2 主元素和微量元素

3.2.1 主元素

芝瑞盆地流紋巖主元素和微量元素分析結(jié)果見表2。芝瑞盆地流紋巖具有富硅(SiO273.9%~ 75.8%)、富鉀(K2O 4.55%~6.44%), 低MgO、CaO、P2O5的特征, 屬于鉀質(zhì)巖石(K2O/Na2O 1.07~2.24, Na2O-270%)、富鉀(K2O 4%~6%或更高)、低鋁(12%~13%)、貧鈣和鎂的特點(diǎn)[52]。里特曼指數(shù)介于2.08~2.77之間, 平均為2.59, 屬于堿性-鈣堿性系列巖石(圖5a)。

圖4 芝瑞盆地流紋巖TAS圖解(a)及A/NK-A/CNK圖解(b)

TAS圖解底圖據(jù)[48], A/NK-A/CNK底圖據(jù)[49]

TAS from [48]; A/NK-A/CNK from [49]

3.2.2?微量元素

芝瑞流紋巖稀土元素含量較高, 富集輕稀土元素, 重稀土分餾不明顯, Eu強(qiáng)烈虧損。SREE= 465~573 μg/g,SLREE=414~520 μg/g,SHREE= 47~ 54 μg/g, (La/Yb)N=8.62~10.9, (La/Sm)N= 4.12~4.34, (Gd/Yb)N=1.35~1.55, Eu/Eu*= 0.08~0.09(均值0.08)。在球粒隕石標(biāo)準(zhǔn)化稀土元素(REE)分布圖解上 (圖6a[53]), 流紋巖樣品呈“右傾”的“V”字形, 具有與A型花崗巖及A型流紋巖[42,52,54]相似的稀土分布模式, 輕重稀土分餾明顯, 輕稀土具有一定程度的分餾, 重稀土呈比較平緩的分布模式, Ce不具備正異常而負(fù)Eu異常顯著。

流紋巖高場強(qiáng)元素Nb、Ta、Zr、Hf、Ce、Y和大離子親石元素Rb、Th、U的含量較高, Rb 142~ 240 μg/g (平均值196 μg/g), Th 19.2~26.3 μg/g (平均值23.3 μg/g), Pb 10.7~24.6 μg/g(平均值17.9 μg/g), Zr 700~811 μg/g (平均值為764 μg/g), Hf 18.8~23.2 μg/g(平均值21.2 μg/g); Nb 34.3~50.5 μg/g (平均值44.4 μg/g), Ta 2.24~3.47 μg/g (平均3.05 μg/g); 采用原始地幔成分[53]為標(biāo)準(zhǔn), 對芝瑞盆地流紋巖樣品的微量元素含量進(jìn)行標(biāo)準(zhǔn)化作圖。微量元素蛛網(wǎng)圖解(圖6b)上可以看出, 流紋巖富集Rb、Th、U、Pb、Zr、Hf等元素, 而Ba、Sr、P、Ti和Nb、Ta都表現(xiàn)出明顯的負(fù)異常, Ba含量為59.6~104 μg/g (平均值81.4 μg/g), Sr含量為17.8~27.2 μg/g (平均值22.4 μg/g); Ga含量較高, 為23.9~27.5 μg/g (均值為25.2 μg/g), 10000Ga/Al值介于3.73~4.11 (均值為3.87, 大于2.6), Zr+Nb+ Ce+Y含量為987~1156 μg/g (平均1084 μg/g, 大于350 μg/g), 具有典型A型花崗巖微量元素特征[55?57], 與東北地區(qū)A型花崗巖[54]、碾子溝A型花崗巖[55]以及大興安嶺地區(qū)A型流紋巖[13, 42, 51]相似。在微量元素Harker圖解上(圖略), 芝瑞流紋巖Rb、Ba、Zr、Hf、Nb、Th、Yb隨SiO2含量的升高表現(xiàn)出降低的趨勢, 而Sr、U、Pb隨SiO2含量的升高而變化不大。

圖5?芝瑞盆地流紋巖巖石系列(Na2O+K2O-CaO)-SiO2圖解(a)和(FeOT/MgO)-SiO2圖解(b)

(Na2O+K2O-CaO)-SiO2圖解底圖據(jù)[50], 海莫賽格流紋巖數(shù)據(jù)引自文獻(xiàn)[42], 紅山子流紋巖數(shù)據(jù)引自文獻(xiàn)[51]

(Na2O+K2O-CaO)-SiO2data are quoted from [50], the data for the Haimosaige and Hongshanzi rhyolites are quoted from [41] and [51], respectively

圖6?芝瑞盆地流紋巖稀土元素球粒隕石標(biāo)準(zhǔn)化圖解(a)與微量元素原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖(b)

球粒隕石及原始地幔標(biāo)準(zhǔn)化值據(jù)文獻(xiàn)[53], 海莫賽格流紋巖、紅山子流紋巖及A型花崗巖數(shù)據(jù)分別引自文獻(xiàn)[42,51,54]

Chondrite and primitive mantle data are quoted from reference [53], the data for rhyolites from Haimosaige and Hongshanzi and A-type granite are quoted from references[42, 51, and 54], respectively

表2 大興安嶺芝瑞盆地流紋巖主元素(%)、微量元素(μg/g)分析結(jié)果及有關(guān)參數(shù)

3.3 Sr-Nd-Pb同位素特征

芝瑞盆地流紋巖的Sr-Nd-Pb同位素分析結(jié)果列于表3。3個(gè)流紋巖樣品的(87Sr/86Sr)i變化于0.706510~ 0.709821之間,Sr()值為28.53~75.53(平均51.20), (143Nd/144Nd)i=0.512113 ~ 0.512210,Nd()=–6.32 ~ –4.44, ?Sm/Nd= ?0.42 ~ ?0.40, 在?0.6 ~ ?0.2之間, 給出的有明確地質(zhì)意義的DM2值變化于1304~1457 Ma之間; 鉛同位素組成較低, (206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t分別為17.15~17.80、15.41~15.48和37.38~37.63。

表3?大興安嶺芝瑞盆地流紋巖Sr-Nd-Pb同位素分析結(jié)果及有關(guān)參數(shù)

注: Sr、Nd、Pb同位素比值年齡校正時(shí)采用U-Pb年齡156 Ma。計(jì)算Nd()、Sr()和分餾因子?Sm/Nd的過程中, (87Sr/86Sr)UR=0.7045, (87Rb/86Sr)UR= 0.0827;(143Nd/144Nd)UR=0.512638, (147Sm/144Nd)CHUR=0.1967;虧損地幔兩階段模式年齡(DM2)的計(jì)算公式為:DM2=(1/?ln{1+[(143Nd/144Nd)樣品– (143Nd/144Nd)DM–[(147Sm/144Nd)樣品–(147Sm/144Nd)CC](e–1)]/[(147Sm/144Nd)cc–(147Sm/144Nd)DM], 式中, 下角標(biāo)“樣品”代表樣品測試值, “CC”代表地殼。(147Sm/144Nd)CC=0.118, (143Nd/144Nd)DM=0.51315, (147Sm/144Nd)DM=0.2137;Sr=[(87Sr/86Sr)樣品/(87Sr/86Sr)UR–0.7]×104。(206Pb/204Pb)i= (206Pb/204Pb)s–(U/Pb)s×(Pb/U)×(238U/204Pb)s(e238t–1); (207Pb/204Pb)i= (207Pb/204Pb)s–[(U/Pb)s×(Pb/U)×(238U/204Pb)s/137.88](e235t–1); (208Pb/204Pb)I= (208Pb/204Pb)s–(Th/Pb)s×(Pb/Th)×(232Th/204Pb)s(e232t–1)。衰變常數(shù)采用(147Sm)=6.54×10?12/a,(87Rb)=1.42×10?11/a,238U、232Th、204Pb的相對豐度(%)分別為238U=99.2739%、232Th=100%、204Pb的相對豐度可以計(jì)算得出,235U=238U/137.88= 0.7200%; U、Th、Pb的原子量分別為U=238.03、Th=232.08、Pb=206.42;238=1.55125×10?10/a、235=9.8485×10?10/a、232=0.49475×10?10/a

4?討?論

4.1?形成時(shí)代和巖石地層單位歸屬

最近研究結(jié)果顯示, 大興安嶺-燕遼火山巖帶中侏羅世晚期-晚侏羅世早期火山巖地層以發(fā)育淺色沉積巖和火山巖為特征, 呈不整合或平行不整合伏于土城子組紫紅色粗碎屑巖之下[2, 27], 其中賀根山-黑河斷裂帶以北的大興安嶺北部地區(qū)及赤峰-開源斷裂帶以南的燕山板內(nèi)造山帶分別以塔木蘭溝組和髫髻山組(藍(lán)旗組)鎂鐵質(zhì)火山巖組合為特征, 火山巖的年齡為163~150 Ma[2?3]; 兩者之間的松遼地塊及遼源地塊為新民組高鉀鈣堿性流紋巖-堿性流紋巖組合為主的一套酸性火山熔巖, 火山巖的年齡為165~150 Ma[2, 27]。

芝瑞盆地的火山巖系長期被視為流紋巖-粗面巖組合, 并可與燕山板內(nèi)造山帶的張家口組或大興安嶺地區(qū)興安嶺群對比[58]。新的研究表明, 張家口組或興安嶺群整合于土城子組紫紅色碎屑巖系之上, 時(shí)代屬早白堊世[2,3,27,59]。芝瑞盆地火山巖系底部以火山巖夾灰、灰黃色沉積巖, 上部為淺灰-灰紫色流紋巖、凝灰?guī)r為特征, 剖面結(jié)構(gòu)與紅旗組、新民組構(gòu)成的剖面結(jié)構(gòu)可以對比, 不同于土城子組及其上火山巖組合特征[25, 58], 流紋巖SHRIMP鋯石U-Pb年齡為(156.9±1.7) Ma, 地質(zhì)時(shí)代屬于晚侏羅世早期?？梢? 芝瑞盆地火山巖的巖石組合和地質(zhì)時(shí)代與大興安嶺南部上侏羅統(tǒng)新民組[2,27,42]一致, 而與冀北早白堊世早期張家口組或大興安嶺地區(qū)興安嶺群的巖石組合和地質(zhì)時(shí)代[2,3,59]不同, 應(yīng)將芝瑞地區(qū)火山巖系歸入新民組。

4.2?巖石成因

4.2.1?物質(zhì)來源

(1) 主元素、微量元素制約 芝瑞盆地流紋巖具有弱過鋁質(zhì)特征, 流紋巖高SiO2、FeOT/MgO、Rb和Nb且低MgO和CaO(Al2O3)含量同時(shí)具有Eu負(fù)異常的特征顯示其由地殼淺部鈣堿性花崗質(zhì)脫水熔融而成[60]。Rb/Sr、Ti/Y和Ti/Zr值分別為7.56~11.1 (平均8.75)、19.7~23.7(平均21.0)和1.85~ 2.15(平均1.94), 位于殼源巖漿(Rb/Sr>0.5, Ti/Y<100, Ti/Zr<20)范圍[61?63]內(nèi), 是陸殼熔融的產(chǎn)物; 同時(shí), 流紋巖虧損Ba、Nb、Ta而富集Pb、Zr的微量元素特征指示巖漿源于地殼。芝瑞流紋巖具有低Sr、高Yb(7.16~8.90 μg/g)含量的特征, 屬于非常低Sr高Yb型即南嶺型花崗巖(相當(dāng)于A型花崗巖), 殘留相為斜方輝石+高鈣斜長石, 壓力小于0.8 GPa, 形成的地殼厚度小于30 km[64]; 本區(qū)流紋巖的HREE之間的分餾并不明顯, 它們呈比較平緩且略下凹的分布模式, 表明部分熔融后的殘留物中含有角閃石而不含石榴子石[65]。根據(jù)實(shí)驗(yàn)巖石學(xué)成果, 角閃石和斜長石作為部分熔融殘留相且又不發(fā)生反應(yīng)形成石榴子石的溫度條件介于850~1100 ℃, 壓力小于1.0 GPa, 表明巖漿起源小于35 km[65?66]。上述特征表明芝瑞流紋巖的巖漿可能起源于下地殼淺部。

(2) Sr、Nd、Pb同位素制約 芝瑞流紋巖較高的(87Sr/86Sr)i值和負(fù)Nd()值(?6.32 ~ ?4.44)指示巖漿來源與新生地殼或富集地幔有關(guān)[67],Nd()值與富集地幔的Nd()值(?13 ~ ?8.0[68?70])及華北克拉通北緣幔源堿性玄武巖的Nd()值(?5[26])和燕遼地區(qū)由富集地幔物質(zhì)參與形成的中生代侵入巖的Nd()值[60,71?74]接近(圖7a[75?76]); 在(143Nd/144Nd)i-(87Sr/86Sr)i圖解(圖7b[77?78])上, 樣品投影點(diǎn)靠近EMⅠ富集地幔區(qū)域, 在鉛同位素模式圖解 (圖8[55,77,78])中, 樣品投影點(diǎn)落于北回歸線之上的下地殼區(qū)域, 并靠近于EMⅠ, 指示來源與下地殼密切相關(guān), 且有富集地幔組分參與形成。但流紋巖Nd()值高于華北克拉通古老下地殼(?44 ~ ?32)[76], 低于興安地塊及松遼塊體較高的正Nd()值[4,6], 與漢諾壩二輝麻粒巖包體的Nd()值(?18 ~ ?8)[75]接近; 同時(shí), 流紋巖二階段Nd模式年齡(1304~1457 Ma)高于興安地塊及松遼地塊的DM2值(520~1180 Ma[6,18]), (206Pb/204Pb)t整體小于興安地塊及松遼地塊(17.96~18.58[19,21,24]), 指示流紋巖巖漿源區(qū)物質(zhì)與華北克拉通古老下地殼及興蒙造山帶顯生宙新生地殼的關(guān)系較遠(yuǎn), 可能為中元古代年輕下地殼物質(zhì)。研究已經(jīng)證實(shí), 漢諾壩二輝麻粒巖包體是幔源基性巖漿底侵到下地殼底部構(gòu)成的年輕下地殼的一部分[75,79], 新生代時(shí)被漢諾壩玄武巖漿以包體形式帶到了地表[9], 這不僅說明年輕的基性麻粒巖下地殼確實(shí)存在, 而且為解釋芝瑞流紋巖同時(shí)帶有富集地幔和下地殼物質(zhì)印記提供了依據(jù), 即源于EMⅠ富集地幔的年輕下地殼經(jīng)部分熔融形成的巖漿上升到地表形成了芝瑞流紋巖。

對燕山陸內(nèi)造山帶中元古代與富集地幔有關(guān)的富堿侵入巖類的研究顯示[80?81], 這些巖體的 (87Sr/86Sr)i多數(shù)小于0.7053,Nd()為?7.5 ~ ?3.4,206Pb/204Pb< 15.70, 芝瑞流紋巖具有比這些堿性巖類稍高的Sr和Pb同位素組成, 那么這種變化是如何形成的呢？據(jù)邵濟(jì)安等[82]研究, 華北克拉通北緣地殼自中生代開始受到幔源巖漿的改造作用, 這些經(jīng)受改造的地殼物質(zhì)以晚三疊世赤峰地區(qū)堆晶巖包體和河南營子鎂鐵質(zhì)麻粒巖為代表, 堆晶巖包體(87Sr/86Sr)i=0.7056~0.7065;Nd()= ?6.8~ ?3.4,DM=1586 ~ 1317 Ma, 麻粒巖Nd()= ?10 ~ ?9,DM約為1800 Ma, (206Pb/204Pb)t、(207Pb/204Pb)t和(208Pb/204Pb)t分別為17.29~17.49、15.49~ 15.53、37.44~ 38.00[83?84]。芝瑞流紋巖同位素組成與這些經(jīng)受改造的地殼相近, 其比中元古代堿性巖類稍高的Sr、Pb同位素組成可能是這種改造作用引起的。

另外, 芝瑞流紋巖的同位素組成與西拉木倫晚侏羅世碾子溝花崗巖[55]、華北克拉通北緣河坎子鐵質(zhì)正長巖[71]、霧靈山A型侵入雜巖[60]、千層背花崗巖[60]、大少冷花崗巖[73]、對面溝石英二長巖[72]、礬山正長巖[74]、甲山正長巖[16]、東猴頂鉀長花崗斑巖[5]等中生代侵入體雖有差異, 但整體相對一致。如何解釋這些異同點(diǎn)呢？具體研究顯示, 源于富集地幔物質(zhì)與地殼物質(zhì)混合之后演化而成對面溝、霧靈山、河坎子、房山等侵入體[9, 60,71,72], 東猴頂和甲山堿性侵入體由源于虧損地幔物質(zhì)與下地殼物質(zhì)混合演化形成[5,16]。筆者認(rèn)為, 兩階段模式能解釋這些火成巖的Sr-Nd-Pb同位素特征, 即元古宙不同時(shí)期的幔源巖漿與少量古老下地殼物質(zhì)混合形成年輕下地殼并且在中生代時(shí)期受到幔源巖漿改造, 這種經(jīng)受改造的地殼物質(zhì)經(jīng)部分熔融形成了上述火山巖和侵入巖。

圖7?芝瑞盆地流紋巖εNd(t)-(87Sr/86Sr)i和(143Nd/144Nd)i-(87Sr/86Sr)i圖解

漢諾壩麻粒巖的范圍據(jù)[75], 華北克拉通下地殼、上地殼的范圍據(jù)文獻(xiàn)[76], DMM、EMⅠ、EMⅡ、HIMU和原始地幔為Hart[77]和Zindler.[78]定義的地幔端元, 燕山中生代侵入巖據(jù)文獻(xiàn)[59, 71,73], 碾子溝容礦花崗巖據(jù)文獻(xiàn)[55], 東猴頂鉀長花崗斑巖據(jù)文獻(xiàn)[5]

圖8?芝瑞盆地流紋巖207Pb/204Pb-206Pb/204Pb(a)和208Pb/204Pb- 206Pb/204Pb(b)圖解

圖8a底圖據(jù)文獻(xiàn)[55]。DMM代表虧損地幔端元, EM(I、II)代表富集地幔端元[78]; NHRL代表北半球參考線[77]。對面溝, 碾子溝、東猴頂、甲山侵入巖數(shù)據(jù)據(jù)文獻(xiàn)[5,16,55,72]

4.2.2?巖漿過程

在Harker圖解(圖略)上, 流紋巖全巖主元素、微量元素與SiO2的線性相關(guān)性較好, 隨著SiO2的增高, Al2O3、MnO、FeOT、MgO、CaO、TiO2表現(xiàn)出明顯的負(fù)相關(guān)性, 指示鐵鎂礦物和斜長石的結(jié)晶分異或是部分熔融時(shí)這些礦物作為殘留相留在源區(qū)。貧鈉以及低鋁含量表明巖漿發(fā)生過斜長石的分離結(jié)晶作用或是部分熔融過程中在源區(qū)殘留了斜長石; 對巖漿分異作用敏感的某些微量元素(Cr、La、Yb、Nb、Th等)與SiO2之間線性相關(guān)性較好, 指示巖漿形成過程中分異作用占據(jù)重要地位[85]。較低的Sr和Ba含量、極強(qiáng)的Eu負(fù)異常及很高 Rb/Sr值和DI值, 說明母巖漿在上升過程中經(jīng)歷過大量的長石分離結(jié)晶作用[86], 在La/Sm-La關(guān)系圖(圖略)上樣品變化趨勢線與分離結(jié)晶趨勢相一致, 也指示流紋巖演化過程中發(fā)生了分離結(jié)晶作用。

4.2.3?構(gòu)造環(huán)境

地球化學(xué)特征顯示芝瑞流紋巖屬于鉀質(zhì)巖石, 具備A型流紋巖特征, 鋯石飽和溫度計(jì)[87]研究顯示芝瑞流紋巖的鋯石飽和溫度為936~954 ℃(平均944 ℃), 指示具有A型花崗巖一致的高溫特征。在Rb-(Y+Nb)和Rb-(Yb+Ta)判別圖(圖9a,圖9b)上均落入WPG板內(nèi)花崗巖區(qū), 并且顯示為Post-COLG[88]; 其Y/Nb值介于1.3~1.93之間, 平均為1.55(>1.2), 符合A2型花崗巖的化學(xué)分類, 在構(gòu)造環(huán)境判別圖解Nb-Y-3Ga和Rb/Nb-Y/Nb(圖9c、圖9d)上, 樣品落入A2造山后環(huán)境區(qū)域[89], 說明流紋巖形成于板內(nèi)伸展構(gòu)造背景[56,88,89], 與軟流圈上涌和巖石圈伸展-減薄作用有關(guān)[37]。一般認(rèn)為, 拉張背景下幔源巖漿底侵所帶來的外部熱量是地殼能夠發(fā)生高溫部分熔融的關(guān)鍵[56, 66], 大興安嶺白堊紀(jì)A型花崗巖具有高的鋯石飽和溫度, 其高熱的產(chǎn)生被認(rèn)為可能與伸展構(gòu)造體制下巖石圈減薄導(dǎo)致的地幔巖漿底侵有關(guān)[90], 進(jìn)一步說明晚侏羅世芝瑞地區(qū)處于伸展減薄的環(huán)境之下, 可能存在幔源巖漿的底侵作用。

圖9?芝瑞流紋巖構(gòu)造環(huán)境判別圖解

(a)、(b)底圖據(jù)文獻(xiàn)[88]; VAG代表火山弧花崗巖; WPG代表板內(nèi)花崗巖; ORG代表洋中脊花崗巖; Syn-COLG代表同碰撞花崗巖; Post-COLG代表后碰撞花崗巖。(c)、(d)底圖據(jù)文獻(xiàn)[89], A1代表非造山環(huán)境, A2代表造山后環(huán)境

(a), (b) after reference [88]; VAG ?volcanic arc granite; WPG ?within-plate granite; ORG – ocean ridge granite; Syn-COLG? syn-collision granite; Post-COLG? post-collisional granite. (c), (d) after reference [89], A1? nonorogenic setting, A2? post-orogenic setting

在侏羅紀(jì)的中末期, 包括研究區(qū)在內(nèi)的大興安嶺-燕山地區(qū)處于“東亞多向匯聚”的構(gòu)造體制 中[36?37], 伴隨北部西伯利亞與華北-蒙古聯(lián)合陸塊碰撞形成蒙古-鄂霍茨克構(gòu)造帶作用下[36,37,91], 蒙古-鄂霍茨克洋關(guān)閉使得北西側(cè)的結(jié)晶基底推覆于中侏羅世含煤沉積巖之上[28], 同時(shí)形成孫吳地區(qū)的中侏羅世白云母花崗巖和埃達(dá)克質(zhì)巖石[91], 并使其南部的燕山構(gòu)造帶在中侏羅世發(fā)生南北向擠壓運(yùn)動(dòng), 形成冀北-遼西地區(qū)廣泛發(fā)育的自北向南的逆沖構(gòu)造及區(qū)域性地層不整合[33,34,36,37]。中侏羅世末期-晚侏羅世早期, 松遼盆地以西地區(qū)進(jìn)入后造山伸展活動(dòng), 加厚陸殼坍塌或拆沉形成火山活動(dòng)[33?34], 伴隨有相應(yīng)的侵入巖發(fā)育[31?33], 并且火山活動(dòng)具有自大興安嶺北部向南部冀北遼西地區(qū)變年輕的趨 勢[3, 34], 巖石組合上具有北部以塔木蘭溝組、中南部以新民組和南部冀北遼西地區(qū)以髫髻山組火山巖為代表的中基性-酸性-中基性火山巖為主的南北分帶特征[2,3,13,27,29,38?42,51], 火山巖和侵入巖主要屬于堿性-鈣堿性系列巖石, 具有A型花崗巖特征[7,13,33,38?40,42], 是蒙古-鄂霍茨克洋閉合造山后巖石圈伸展環(huán)境下的產(chǎn)物[29,33,38,40]。對東北亞地區(qū)一些典型變質(zhì)核雜巖核部侵入巖的研究表明, 由于受到鄂霍茨克不對稱造山影響, 這些地區(qū)中下地殼層次的伸展可能在中晚侏羅世就已經(jīng)啟動(dòng)[32]。另外, 內(nèi)蒙古達(dá)來廟鉀長花崗巖[35]、海莫賽格地區(qū)流紋巖[42]、西拉木倫碾子溝二長花崗巖[55]、半砬山鉬礦流紋斑巖[92]、紅山子盆地流紋巖[51]及芝瑞流紋巖這些陸續(xù)厘定的中-晚侏羅世A型花崗巖/流紋巖不僅為遼源地塊在中-晚侏羅世時(shí)已經(jīng)進(jìn)入板內(nèi)拉張環(huán)境提供了巖石學(xué)證據(jù), 而且說明芝瑞流紋巖是晚侏羅世時(shí)期華北克拉通北緣與蒙古-鄂霍茨克縫合帶演化有關(guān)的伸展事 件[29, 32?35]的重要組成部分, 是區(qū)域性伸展環(huán)境的產(chǎn)物。綜上所述, 芝瑞盆地晚侏羅世流紋巖可能形成于與蒙古-鄂霍茨克縫合帶的演化有關(guān)的伸展構(gòu)造環(huán)境, 可能受到華北克拉通北緣巖石圈伸展減薄之下幔源巖漿的底侵作用影響。

5?結(jié)?論

(1) 大興安嶺南部芝瑞盆地火山巖系中流紋巖SHRIMP鋯石U-Pb年齡為(156.9±1.7) Ma, 形成于晚侏羅世早期, 火山巖系以高鉀鈣堿性流紋巖為主, 依照巖石組合特征并結(jié)合已獲得的晚侏羅世早期流紋巖年齡, 應(yīng)將其巖石地層單位歸屬于新民組。

(2) Nd、Sr、Pb同位素特征、常量、微量和稀土元素特征指示芝瑞盆地流紋巖由兩階段模式形成, 中元古代源于EMⅠ富集地幔的巖漿混染少量下地殼物質(zhì)形成年輕下地殼并在中生代期間又受到地幔物質(zhì)的改造影響, 直至晚侏羅世早期板內(nèi)拉張構(gòu)造環(huán)境下發(fā)生部分熔融形成的巖漿在上升過程中經(jīng)歷結(jié)晶分異后噴出地表形成此套火山巖, 是華北克拉通北緣巖石圈伸展減薄的產(chǎn)物。

(3) 芝瑞流紋巖是伸展構(gòu)造環(huán)境的產(chǎn)物, 與大興安嶺北部和華北克拉通北緣晚侏羅世A型花崗巖/流紋巖指示的構(gòu)造環(huán)境一致, 不僅為遼源地塊在晚侏羅世時(shí)已經(jīng)進(jìn)入板內(nèi)拉張環(huán)境提供了巖石學(xué)證據(jù), 而且說明遼源地塊晚侏羅世的伸展事件也可能與蒙古-鄂霍茨克縫合帶的演化有關(guān)。

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Petrogenesis of early Late Jurassic rhyolites from the Zhirui Basin in southern Daxing’an Range: Their chronologic and geochemical constrains

XIE Kai-rui1, WU Jian-hua1,2*, ZHU Hong-tao3, WU Ren-gui1and LIU Shuai2

1. College of Earth Science, East China Institute of Technology, Nanchang?330013, China; 2. State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang?330013, China; 3. No. 243 Geological Party, China National Nuclear Corporation, Chifeng?024006, China

The Zhirui volcanic basin is located in Hexigten Banner, Inner Mongolia, at the southern tip of the Daxing'an Range. Its tectonic position lies in the Liaoyuan Block between the Xar Moron - Changchun suture zone and the Chifeng-Kaiyuan suture zone. The felsic volcanic series in the Zhirui Basin occurs unconformablely above the underlying Dashizhai Formation, and below the overlying basalt of the Hannuoba Formation, and has been intruded by granitic porphyry. The SHRIMP zircon206*Pb/238U age estimates for the rhyolite yield weighted an average age of (156.9±1.7) Ma, indicating an early Late Jurassic age. The whole-rock geochemistry of the rhyolites from the felsic volcanic series shows an A-type magmatic affinity, with a typical enrichment in SiO2, K2O, Ga, Zr, Nb, Y and a high ratio of FeOT/MgO but low contents of Al2O3, CaO and MgO, belonging to K-high calc-alkaline series rocks. Furthermore, they also have significantly higher contents of trace elements such as Ga, Zr, Nb, Y but low levels of Ba and Sr. Relative enrichment in Rb, Th, U, Pb, Zr, Hf, but depletion in Ba, Sr, P, Ti and Nb, Ta, and fractionated REE patterns show strong negative Eu anomalies. In the Nb-Y-3Ga and Rb/Nb-Y/Nb diagrams, all of the six samples show the characteristics of A2-type granites that demonstrate an extensionally tectonic setting. The rhyolites share such features as relatively high (87Sr/86Sr)ivaleus (0.706510~0.709821), lowNd() values (Nd()= ?6.32~ ?4.44) and young Nd-model ages (1304~1457 Ma). They exhibit low radiogenic Pb isotopic compositions, with (206Pb/204Pb)t=17.15~17.80, (207Pb/204Pb)t=15.41~15.48 and (208Pb/204Pb)t=37.38~37.63. The projection points are locateed between the lower crust and the depleted mantle as shown in the Sr-Ndisotope-tracer diagram and located in the lower crust area close to the enriched mantle as shown in the Pb isotope-tracer diagram.These elemental and isotopic characters argue for parental magmas from partial melting of mid-Mesoproterozoic crust materials which were derived from the enriched mantle and their subsequent fractional crystallization. In the (Yb+Ta)-Rb and (Y+Nb)-Rb diagrams, all the rhyolites indicate that they belong to an extensionally tectonic setting within the plate. As viewed from regional correlations with the coeval volcanic rocks, together with the Late Jurassic A-type intrusions widely scattered in NE China, the Zhirui rhyolites are testified to be asssociated with an extensionally tectonic setting within the plate. It is the product of a regional extensional environent associated with the evolution of northern Mongol-Khotsk suture in the Jurassic period, which may be accompanied by mantle-derived magma underplating.

rhyolite; petrogenesis; elements; Sr-Nd-Pb isotopes; early Late Jurassic; Zhirui

P597; P581

A

0379-1726(2016)03-0249-19

2015-10-16;

2015-12-29;

2016-01-28

國家自然科學(xué)基金(41372071); 中國核工業(yè)集團(tuán)公司項(xiàng)目(中核地計(jì)[2008]74號)

解開瑞(1990–), 男, 碩士研究生, 礦產(chǎn)普查與勘探專業(yè)。E-mail: krxie0818@163.com

WU Jian-hua, E-mail: jhwu@ecit.cn; Tel: +86-791-83897549