柏道遠(yuǎn), 鐘?響, 賈朋遠(yuǎn), 熊?雄, 黃文義

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南嶺西段加里東期越城嶺巖體鋯石SHRIMP U-Pb年齡、地質(zhì)地球化學(xué)特征及其形成構(gòu)造背景

柏道遠(yuǎn)*, 鐘?響, 賈朋遠(yuǎn), 熊?雄, 黃文義

(湖南省地質(zhì)調(diào)查院, 湖南 長沙?410016)

越城嶺巖體位于南嶺西段, 主體為南部的加里東期花崗巖, 北部為印支期花崗巖。加里東期花崗巖自早至晚依次為中細(xì)粒斑狀黑云母二長花崗巖、細(xì)中粒斑狀黑(二)云母二長花崗巖、細(xì)粒斑狀黑(二)云母二長花崗巖和細(xì)粒黑(二)云母二長花崗巖。巖體東部和西部花崗巖分別具塊狀構(gòu)造和片麻狀構(gòu)造。巖體西緣尚疊加了燕山期左行走滑-伸展型韌性剪切帶。對(duì)中細(xì)粒斑狀黑云母二長花崗巖和(糜棱巖化)細(xì)中粒斑狀黑云母二長花崗巖各進(jìn)行了1個(gè)樣品的鋯石SHRIMP U-Pb年齡測試, 分別得到(436.6±4.8) Ma、(430.5±4.3) Ma的年齡值, 反映花崗巖形成于早志留世晚期。巖石高硅、富鋁、高鉀、中堿, SiO2含量68.35%～78.10%, 平均73.29%; Al2O3含量11.95%～15.55%, 平均14.18%; K2O含量4.12%～5.62%, 平均4.95%; 全堿(Na2O+K2O)含量為6.18%～8.30%, 平均7.58%; K2O/Na2O值在1.36～2.82之間, 平均1.94。ASI值1.04～1.66, 平均1.23?？傮w屬高鉀鈣堿性系列過鋁質(zhì)花崗巖類。大多數(shù)樣品Ba、Nb、Sr、P、Ti表現(xiàn)為較強(qiáng)烈虧損, Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等則相對(duì)富集; ∑REE含量為50.43～328.81 μg/g, 平均173.39 μg/g;Eu值0.21～0.68, 平均為0.40; (La/Yb)N值為0.54～14.04, 平均7.93;Sr值為0.71912和0.72415,Sr()值為208和279,Nd()值為–11.76～–7.80,2DM為1.80～2.12 Ga。A/MF-C/MF圖解顯示源巖為泥質(zhì)巖和碎屑巖。上述地球化學(xué)特征表明花崗巖為S型花崗巖, 是陸殼碎屑巖石部分熔融的產(chǎn)物?；◢弾r氧化物構(gòu)造環(huán)境判別圖解指示巖體形成于后碰撞構(gòu)造環(huán)境。基于巖石成因、構(gòu)造環(huán)境判別以及區(qū)域構(gòu)造演化過程, 推斷加里東期越城嶺花崗巖的具體形成機(jī)制為：奧陶紀(jì)末—志留紀(jì)初的北流運(yùn)動(dòng)導(dǎo)致地殼增厚、升溫, 早志留世中晚期在擠壓減弱、應(yīng)力松弛的后碰撞-減壓構(gòu)造環(huán)境下, 中、上地殼酸性巖石發(fā)生部分熔融并向上侵位。

鋯石SHRIMP U-Pb定年; S型花崗巖; 北流運(yùn)動(dòng); 后碰撞構(gòu)造環(huán)境; 越城嶺; 湘西南

0?引?言

早古生代晚期的構(gòu)造事件在華南形成了強(qiáng)烈的褶皺和斷裂變形[1–5], 并引發(fā)了強(qiáng)烈的花崗質(zhì)巖漿活動(dòng)[1,2,6–8]。地質(zhì)工作者對(duì)加里東期構(gòu)造-巖漿事件進(jìn)行過大量研究并取得了豐富成果, 認(rèn)識(shí)到早古生代晚期的構(gòu)造事件自早至晚可分為寒武紀(jì)末—奧陶紀(jì)初的郁南運(yùn)動(dòng)、中奧陶世末的都勻運(yùn)動(dòng)、奧陶紀(jì)末—志留紀(jì)初的北流運(yùn)動(dòng)(崇余運(yùn)動(dòng))、志留紀(jì)后期的廣西運(yùn)動(dòng)等多期次[5,8–12]; 在該構(gòu)造事件中, 古陸逐漸向北西擴(kuò)展[13–15]、造山帶具“由南向北”擠壓趨勢[12]; 相關(guān)的花崗質(zhì)巖漿活動(dòng)時(shí)代主要為440～390 Ma[2]或460～410 Ma[7]等。盡管如此, 華南早古生代晚期的構(gòu)造事件仍有若干問題需要進(jìn)一步研究, 如不同期次構(gòu)造運(yùn)動(dòng)的影響范圍及其背景差異; 再如關(guān)于加里東期花崗巖的構(gòu)造成因, 或認(rèn)為與板內(nèi)造山環(huán)境下沿?cái)嗔寻l(fā)生中上地殼部分熔融有關(guān)[6], 或認(rèn)為與板內(nèi)碰撞造山之后的后碰撞或后造山伸展-減薄有關(guān)[2,7,16,17], 或認(rèn)為陸-弧-陸碰撞所致[18]。

湖南省加里東期花崗巖發(fā)育, 近些年來已獲得九嶷山雪花頂巖體[19]、彭公廟巖體[20]等部分巖體的鋯石SHRIMP 和LA-ICPMS U-Pb年齡, 但高精度年齡數(shù)據(jù)總體偏少。位于湘西南的越城嶺巖體和苗兒山巖體為以加里東期花崗巖為主、中生代花崗巖為次的大型復(fù)式巖體, 目前已獲得印支期花崗巖(228.7±4.1) Ma和(216.8±7.5) Ma的鋯石LA-ICPMS U-Pb年齡[21], 尚缺乏加里東期精確同位素定年。

湘桂交界的越城嶺-苗兒山一帶為加里東運(yùn)動(dòng)強(qiáng)烈影響地區(qū), 其西側(cè)為NNE向城步-新化大斷裂。該斷裂西側(cè)發(fā)育早志留世沉積而東側(cè)無志留系出露[22]。東側(cè)志留系缺失成因(沉積缺失還是后期剝蝕所致?)的確定直接關(guān)系到對(duì)湘中南-雪峰山地區(qū)早古生代構(gòu)造事件的期次及其影響范圍的正確認(rèn)識(shí), 但目前對(duì)此問題缺乏深入研究。

針對(duì)前述地質(zhì)問題, 本研究對(duì)加里東期越城嶺巖體進(jìn)行了鋯石SHRIMP U-Pb測年, 獲得(436.6± 4.8) Ma和(430.5±4.3) Ma的巖漿事件年齡; 并對(duì)巖體地質(zhì)地球化學(xué)特征進(jìn)行了詳細(xì)研究, 結(jié)果表明花崗巖為加厚地殼在后碰撞環(huán)境下重熔形成的S型花崗巖, 從而確定城步-新化斷裂以東發(fā)生了北流運(yùn)動(dòng)并導(dǎo)致志留紀(jì)期間遭受剝蝕而無沉積。這一成果為湘桂地區(qū)加里東期花崗巖年代補(bǔ)充了資料, 為區(qū)域早古生代晚期構(gòu)造事件期次及影響范圍提供了重要約束。

1?地質(zhì)概況

越城嶺巖體跨湘桂交界, 總體呈NNE向狹長帶狀展布, 總面積約1365 km2。巖體西面為苗兒山大型復(fù)式巖體, 兩者以新寧白堊紀(jì)盆地相隔(圖1)。越城嶺巖體北段主要為印支期花崗巖; 南段大部為加里東期花崗巖, 并有早燕山期小巖體侵入。重力異常特征顯示巖體向東傾斜延伸[23]。在巖體東側(cè), 加里東期花崗巖與南華系—奧陶系呈侵入接觸, 外接觸帶具角巖化、大理巖化變質(zhì); 與泥盆系跳馬澗組呈沉積接觸(圖1)。

圖1?越城嶺巖體地理位置(a)及地質(zhì)略圖(b)

1–花崗巖; 2–片麻狀花崗巖; 3–地質(zhì)界線; 4–角度不整合界線; 5–斷裂; 6–韌性剪切帶; 7–鋯石SHRIMP U-Pb年齡樣品位置及編號(hào); K–白堊系; D—C–泥盆系—石炭系; Nh—O–南華系—奧陶系;K–晚燕山期花崗巖;J–早燕山期花崗巖;T–印支期花崗巖;S–加里東期花崗巖;Sa–加里東期越城嶺巖體中細(xì)粒斑狀黑云母二長花崗巖;Sb–加里東期越城嶺巖體細(xì)中粒斑狀黑(二)云母二長花崗巖;Sc–細(xì)粒斑狀黑(二)云母二長花崗巖;Sd–細(xì)粒黑(二)云母二長花崗巖

1–granite; 2–gneissic granite; 3–geological boundary; 4–uncomformity; 5–fault; 6–ductile shear zone; 7–location and serial number of zircon SHRIMP U-Pb dating sample; K–Cretaceous; D—C–Devonian—Carboniferous; Nh—O–Nanhuan—Ordovician;K–late Yanshanian granite;J–Early Yanshanian granite;J–Early Yanshanian granite;T–Indosinian granite;S–Caledonian granite;Sa–medium-fine-grained porphyritic biotite monzogranite of Caledonian Yuechengling pluton;Sb–fine-medium-grained porphyritic biotite-dimicaceous monzo-granite of Caledonian Yuechengling pluton;Sc–fine-grained porphyritic biotite-dimicaceous monzogranite of Caledonian Yuechengling pluton;Sd–fine-grained biotite-dimicaceous monzogranite of Caledonian Yuechengling pluton of Caledonian Yuechengling pluton

據(jù)巖性特征和本次工作查明的相互之間的侵位關(guān)系, 加里東期越城嶺花崗巖主要可分為4個(gè)侵入期次, 自早至晚依次為中細(xì)粒斑狀黑云母二長花崗巖(Sa)、細(xì)中粒斑狀黑(二)云母二長花崗巖(Sb)、細(xì)粒斑狀黑(二)云母二長花崗巖(Sc)和細(xì)粒黑(二)云母二長花崗巖(Sd)(圖1), 其中以細(xì)中粒斑狀黑(二)云母二長花崗巖分布最廣(主體花崗巖), 其他分布局限。此外, 局部尚見細(xì)粒斑狀正長(堿長)花崗巖脈(S)。花崗巖具塊狀構(gòu)造, 但西部發(fā)育片麻狀構(gòu)造或疊加韌性剪切變形, 變形強(qiáng)烈者可成為花崗質(zhì)糜棱巖。

越城嶺巖體西緣發(fā)育韌性剪切帶和片麻狀花崗巖, 北段和南段分別切割、疊加于印支期花崗巖和加里東期花崗巖之上。在南段, 韌性剪切帶東側(cè)與加里東期片麻狀花崗巖、塊狀花崗巖呈過渡關(guān)系, 西側(cè)以脆性正斷裂與白堊系分界。

前人對(duì)巖體西緣韌性剪切帶特征和成因已進(jìn)行過探討[24–27], 筆者對(duì)南段李家壯-咸水口一帶再次進(jìn)行了地質(zhì)剖面觀測。剖面西部發(fā)育寬近3 km的較連續(xù)韌性剪切帶, 帶內(nèi)巖石主要為眼球狀花崗質(zhì)糜棱巖。剖面東部約10 km范圍內(nèi)主要出露(弱)片麻狀-糜棱巖化細(xì)中粒斑狀黑云母二長花崗巖, 變形強(qiáng)度及面理發(fā)育程度總體自西向東變?nèi)? 內(nèi)部尚發(fā)育多條寬2～20 m不等的韌性剪切帶。片麻理和糜棱面理(C面理)的產(chǎn)狀較穩(wěn)定, 傾向多在290°～310°之間, 傾角一般20°～45°。EW向剖面上S-C組構(gòu)指示正向滑動(dòng)(圖2); 剪切面理(C面理)上的拉伸線理向南側(cè)伏, 側(cè)伏角60°～75°, 指示正向滑動(dòng)的同時(shí)兼具左行走滑。糜棱巖XZ面上礦物強(qiáng)烈拉長、軸比大, 而YZ面上軸比小, 表明以簡單剪切為主(圖3)。

圖2?糜棱巖中S-C 組構(gòu)指示正向滑動(dòng)

從上述巖石、變形特征以及北段印支期花崗巖卷入韌性剪切帶來看, 加里東期主體花崗巖原生構(gòu)造為塊狀構(gòu)造(東部)或(弱)片麻狀構(gòu)造(西部), 具簡單剪切機(jī)制的韌性剪切變形應(yīng)為巖體上部在燕山期向西伸展滑動(dòng)所形成, 而非巖體侵位期構(gòu)造擠壓產(chǎn)物。結(jié)合巖體向東傾斜延伸特征[23]分析, 巖體西部片麻狀構(gòu)造應(yīng)與巖漿由下而上、由東向西侵位時(shí)造成的擠壓有關(guān)。

2?鋯石SHRIMP U-Pb定年

2.1?測年樣品及分析方法

對(duì)中細(xì)粒斑狀黑云母二長花崗巖(Sb)和(糜棱巖化)細(xì)中粒斑狀黑云母二長花崗巖(Sd)分別采集樣品SH01、SH12進(jìn)行鋯石SHRIMP U-Pb測年。

圖3?同一塊糜棱巖標(biāo)本XZ面(a)與YZ面(b)變形差異

將花崗巖樣品粗碎后進(jìn)行人工分選淘洗, 再于雙目鏡下挑選出晶形較好且透明度較高的鋯石, 送實(shí)驗(yàn)室分析。將樣品鋯石與標(biāo)準(zhǔn)鋯石TEM(年齡為417 Ma)在玻璃板上用樹脂固定、拋光, 然后進(jìn)行反射光照相以及陰極發(fā)光掃描電鏡圖像分析, 以確定沒有裂紋及包體不發(fā)育的分析熔樣點(diǎn)。鋯石的陰極發(fā)光照相在中國地質(zhì)科學(xué)院礦床地質(zhì)研究所電子探針研究室完成。鋯石SHRIMP U-Pb分析在北京離子探針中心SHRIMPⅡ上完成, 分析原理和流程見Compston.[28]、Williams.[29]和簡平等[30]資料。應(yīng)用標(biāo)準(zhǔn)鋯石TEM(417 Ma)進(jìn)行元素間的分餾校正。一次離子流O–2強(qiáng)度為5～8 nA。一次離子流束斑直徑為25～30 μm。樣品點(diǎn)清洗時(shí)間為120～180 s。數(shù)據(jù)處理采用Ludwig博士編寫的Squid1.02及Isoplot程序[31–32]。普通鉛根據(jù)實(shí)測204Pb進(jìn)行校正。

2.2?測試結(jié)果

樣品SH01和SH12分別分析了9顆、12顆粒鋯石U-Pb同位素組成。所分析鋯石為透明的自形晶體, 陰極發(fā)光圖像均顯示出巖漿結(jié)晶成分環(huán)帶(圖4), 且Th/U比值高, 為0.12～1.06(大于0.1), 表明鋯石為巖漿成因。

圖4?加里東期越城嶺巖體花崗巖鋯石陰極發(fā)光圖像

樣品SH01的9個(gè)測點(diǎn)U-Pb年齡分析結(jié)果見表1和圖5。除測點(diǎn)9.1年齡偏大外, 其余8個(gè)測點(diǎn)呈水平線狀集中分布于諧和線左側(cè), 其206Pb/238U加權(quán)平均年齡為(436.6 ± 4.8) Ma(2σ), MSWD=0.25, 為花崗巖成巖年齡。順便指出, 8個(gè)測點(diǎn)偏離諧和線的線狀分布特征與207Pb 難以測準(zhǔn)有關(guān), 并表明所測鋯石顆粒在形成后U-Pb 同位素體系封閉, 不影響206Pb/238U比值[33]。

圖5?樣品SH01鋯石SHRIMP U-Pb年齡諧和圖

樣品SH12 的12個(gè)測點(diǎn)U-Pb年齡分析結(jié)果見表1和圖6。4.1、8.1、11.1和12.1等4個(gè)測點(diǎn)屬離群值; 其余8個(gè)測點(diǎn)呈水平線狀分布于諧和線的兩側(cè),206Pb/238U年齡值相近, 變化于425.8～440.8 Ma之間, 給出加權(quán)平均年齡為(430.5±4.3) Ma (2σ), MSWD= 1.9, 為花崗巖樣品成巖年齡。

上述(436.6 ± 4.8) Ma和(430.5 ± 4.3) Ma的 U-Pb年齡在誤差范圍內(nèi)一致或接近, 表明中細(xì)粒斑狀黑云母二長花崗巖和(糜棱巖化)細(xì)中粒斑狀黑云母二長花崗巖均形成于早志留世晚期, 為同一巖漿事件產(chǎn)物, 前者略早于后者。

圖6?樣品SH01鋯石SHRIMP U-Pb年齡諧和圖

表1?加里東期越城嶺巖體鋯石SHRIMP U-Pb同位素分析結(jié)果

注：206Pbc為全部206Pb中屬于普通鉛206Pb所占的百分比。Pb*為放射性成因鉛的含量, 普通鉛根據(jù)實(shí)測204Pb進(jìn)行校正

3?巖石地球化學(xué)特征

3.1 主元素地球化學(xué)特征

加里東期越城嶺巖體樣品的主元素分析結(jié)果見表2。在所分析的13個(gè)樣品中, 樣品HX14-1采自細(xì)粒斑狀堿長花崗巖脈, 其主元素組成顯著區(qū)別于其他樣品, SiO2含量低, 僅為65.17%(SiO2及以下主元素含量值均系無水化處理結(jié)果, 故與表2略有差別); K2O和全堿(ALK)含量極高, 分別為9.09%、13.50%, 屬典型堿性花崗巖。

其他12個(gè)樣品主元素特征如下。SiO2含量變化較大且總體含量較高, 為68.35%～78.10%, 平均為73.29%; 自早至晚含量總體變高。隨著SiO2含量的增加, Al2O3、FeOT、TiO2、MgO、CaO和P2O5等均呈規(guī)律減少, Na2O和K2O無明顯變化(圖7), 反映可能為同來源和同時(shí)代產(chǎn)物。Al2O3含量較高, 為11.95%～15.55%, 平均為14.18%。K2O含量較高,為4.12%～5.62%, 平均為4.95%; 全堿(ALK)含量中等, Na2O+K2O為6.18%～8.30%, 平均為7.58%。K2O大于Na2O, K2O/Na2O值在1.36～2.82之間, 平均為1.94。FeOT含量變化較大, 為1.15%～4.25%, 平均2.59%。TiO2、MgO、CaO和P2O5含量平均分別為0.26%、0.71%、1.17%和0.09%。

根據(jù)Frost.[34]提出的修改的堿鈣指數(shù)(Na2O+K2O–CaO)、鋁飽和指數(shù)ASI等地球化學(xué)變量及ANK, 加里東期越城嶺巖體花崗巖總體屬鈣堿性-堿鈣性及過鋁質(zhì)花崗巖(圖8a、圖8b)。ASI均大于1.0 (1.04～1.66), 平均1.23, 屬弱過鋁-強(qiáng)過鋁質(zhì)。在K2O-SiO2圖中, 總體屬高鉀鈣堿性系列(圖8c)。

在(K2O+Na2O)-SiO2圖解中, 加里東期越城嶺花崗巖主要落于花崗巖區(qū)(圖9), 樣品HX14-1落入正長巖區(qū)。

3.2?微量和稀土元素地球化學(xué)特征

加里東期越城嶺巖體花崗巖微量和稀土元素分析結(jié)果分別見表3和表4, 不相容元素對(duì)原始地幔標(biāo)準(zhǔn)化分布曲線和稀土元素球粒隕石標(biāo)準(zhǔn)化分布曲線分別見圖10和圖11。

表2?加里東期越城嶺巖體的主元素組成(%)

注: 樣品由湖北武漢綜合巖礦測試中心測定, 除CO2采用非水滴定法、H2O+采用高溫加熱-濃硫酸吸收-重量法外, 其他氧化物均采用X射線熒光光譜法(XRF)分析; 樣品采用無水四硼酸鋰作為熔劑。ASI=Al/(Ca-1.67P+Na+K)(分子比); ANK=Al/(Na+K)(分子比); A/MF=Al2O3/ (MgO+FeO)(分子比)

表3?加里東期越城嶺巖體微量元素分析結(jié)果(μg/g)及有關(guān)參數(shù)

注: 樣品由湖北武漢綜合巖礦測試中心測定分析, 分別采用X射線熒光光譜法(XRF)、電感耦合等離子體質(zhì)譜法(ICP-MS)、原子熒光光譜法(AFS)、發(fā)射光譜法(ES)等方法測定。除W、Mo采用堿溶法測定外, 其他微量元素均采用酸溶法測定

圖7?加里東期越城嶺巖體主要氧化物Harker圖解

在不相容元素對(duì)原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖上(圖10), 與相鄰元素相比, 大多數(shù)樣品的Ba、Nb、Sr、P、Ti表現(xiàn)為強(qiáng)烈虧損, 而Rb、(Th+U+K)、(La+Ce)、Nd、(Zr+Hf+Sm)、(Y+Yb+Lu)等則相對(duì)富集, 顯示出一般殼源花崗巖特征。Nb明顯相對(duì)Ta虧損, 也暗示花崗巖具有殼源花崗巖特征[39]。Rb、Ba、Sr、Ti含量的變化主要與造巖礦物有關(guān), Rb升高和Sr、Ba降低一般由鉀長石、斜長石和黑云母分離結(jié)晶所造成, Ti負(fù)異常反映出鈦鐵礦的分離結(jié)晶作用。P虧損暗示存在磷灰石的分離結(jié)晶。值得指出的是, 樣品HX06-1與其他樣品相反, 未顯示出P的虧損, 而是顯示(Zr+Hf)虧損, 可能與鋯石的分離結(jié)晶有關(guān)。

稀土元素含量變化較大(表4), ∑REE為50.43～328.81 μg/g, 平均為173.39 μg/g?！艭e/∑Y為0.40～4.59, 平均為2.95; (La/Yb)N值為0.54～14.04, 平均7.93, 顯示輕稀土明顯富集。但其中樣品HX407-1的∑Ce/∑Y和(La/Yb)N值分別僅為0.40、0.54, 明顯為重稀土相對(duì)富集; 樣品HX06-1和HX06-2的∑Ce/∑Y值分別為1.45和1.44, 顯示輕稀土相對(duì)重稀土略有富集。Eu具較明顯虧損,Eu值0.21～0.68, 平均為0.40, 暗示經(jīng)歷了斜長石的分離結(jié)晶作用。稀土分布曲線大多明顯向右傾斜(圖11), 反映多數(shù)樣品輕稀土相對(duì)富集且輕、重稀土元素均具明顯分餾; 樣品HX407-1總體左傾, 反映重稀土富集; 樣品HX06-1和HX06-2總體呈近水平海鷗狀, 反映輕、重稀土分餾不明顯。樣品HX407-1重稀土的富集可能與流體分異和氟化物的形成有關(guān)[40–41]; 而其輕稀土的減少可能為早期巖漿中褐簾石的分離結(jié)晶作用所造成[42–43]。

圖8?加里東期越城嶺巖體地球化學(xué)分類圖解

(a)和(b)據(jù)Frost.[34]; (c)據(jù)Peccerillo.[35]

(a) and (b) after reference Frost.[34]; (c) after reference Peccerillo.[35]

圖9?加里東期越城嶺巖體(K2O+Na2O)-SiO2圖解(據(jù)Middlemost[36])

綜上, 除個(gè)別樣品外加里東期越城嶺巖體花崗巖微量元素蛛網(wǎng)圖及稀土元素分布曲線形態(tài)總體一致, 暗示其為同時(shí)代、同來源的產(chǎn)物。

3.3?鍶、釹同位素地球化學(xué)特征

為了提供花崗巖成因與物質(zhì)來源信息, 對(duì)加里東期越城嶺巖體進(jìn)行了4個(gè)樣品的鍶、釹同位素分析, 具體由武漢地質(zhì)礦產(chǎn)研究所同位素室完成。

Rb-Sr同位素測定方法??采用陽離子樹脂(Dowex50×8)交換法分離和純化Rb、Sr, 用熱電離質(zhì)譜儀MAT261分析銣、鍶同位素組成, 用同位素稀釋法計(jì)算試樣中的Rb、Sr含量及鍶同位素比值, 用GBW04411、NBS607和NBS987標(biāo)準(zhǔn)物質(zhì)分別對(duì)分析流程和儀器進(jìn)行監(jiān)控; 全流程Rb、Sr空白分別為0.4×10–9g和0.8×10–9g。

Sm-Nd同位素分析方法??取加入145Nd+149Sm混合稀釋劑和不加稀釋劑的樣品各1份, 以氫氟酸和高氯酸溶解后用Dowex50×8陽離子交換樹脂進(jìn)行分離和純化。加了稀釋劑的樣品用于Sm、Nd含量質(zhì)譜分析; 未加稀釋劑的解吸液上P507有機(jī)萃取樹脂柱分離和純化Nd以用于釹同位素比值分析。Sm、Nd含量和釹同位素比值質(zhì)譜分析采用熱電離質(zhì)譜儀Triton完成, Sm、Nd含量采用同位素稀釋法公式計(jì)算得到。用GBW04419和ZkbzNd(JMC)標(biāo)準(zhǔn)物質(zhì)對(duì)分析流程和儀器進(jìn)行監(jiān)控。全流程N(yùn)d、Sm空白分別為1×10–10g和0.7×10–10g。

圖10?加里東期越城嶺巖體微量元素原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖

原始地幔值據(jù)Sun.[37]

Primitive mantle data are quoted from Sun.[37]

圖11?加里東期越城嶺巖體稀土元素球粒隕石標(biāo)準(zhǔn)化分布模式

球粒隕石值據(jù)Taylor.[38]

Chondrite data are quoted from Taylor.[38]

同位素測試數(shù)據(jù)及有關(guān)參數(shù)值列于表5。根據(jù)巖體的鋯石SHRIMP U-Pb年齡, 在計(jì)算有關(guān)參數(shù)時(shí)樣品年齡取值433 Ma。由于小于BABI(玄武質(zhì)無球粒隕石最初初始值)0.69897±0.00003的值不能用于討論巖石成因[45], 因而不考慮樣品HX06-2和HX17的初始Sr值(0.64721)。剔除樣品HX06-2和HX17的鍶同位素值, 則加里東期越城嶺巖體花崗巖Sr值為0.71912和0.72415,Sr()值為208和279,Nd()值為–11.76～–7.80,2DM(兩階段Nd模式年齡)為1.80～2.12 Ga。

4?討?論

4.1?巖漿成因

加里東期越城嶺巖體東部與前泥盆紀(jì)地層呈清楚的侵入接觸關(guān)系, 外接觸帶見熱接觸變質(zhì), 鏡下表現(xiàn)出典型的巖漿結(jié)晶結(jié)構(gòu), 表明花崗巖為巖漿成因。

巖體的巖石地球化學(xué)及鍶、釹同位素地球化學(xué)特征表明其為陸殼重熔型花崗巖, 且源巖為中、上地殼酸性巖石而非下地殼基性巖; 巖漿過程受分離結(jié)晶作用控制。具體分析如下。

(1) 巖石大部分屬ASI值大于1.1的強(qiáng)過鋁(SP)花崗巖(表2, 圖8), 而前人研究表明強(qiáng)過鋁花崗巖多為地殼物質(zhì)熔融產(chǎn)物, 屬S型殼源花崗巖類[46]。

(2) 微量元素蛛網(wǎng)圖(圖10)上巖石顯示出一般殼源花崗巖特征(見前述)。

表4?加里東期越城嶺巖體稀土元素分析結(jié)果(μg/g)及有關(guān)參數(shù)

注: 樣品由湖北武漢綜合巖礦測試中心測定分析, 采用電感耦合等離子體質(zhì)譜法(ICP-MS)分析。樣品處理采用酸溶法

表5?加里東期越城嶺巖體鍶、釹同位素組成及有關(guān)參數(shù)計(jì)算

注:Nd()、Sr()、Sr和2DM的計(jì)算據(jù)陳江峰等[44]; 計(jì)算參數(shù)為:=433 Ma; (143Nd/144Nd)CHUR=0.512638, (147Sm/144Nd)CHUR=0.1967, (87Sr/86Sr)UR= 0.7045, (147Sm/144Nd)CC=0.118, (147Sm/144Nd)DM=0.2137, (143Nd/144Nd)DM=0.513151,Sm=6.54×10–12/a,Rb= 1.42×10–11/a。參數(shù)中下角字母代表的含義: UR代表鍶同位素均一儲(chǔ)庫; CHUR代表球粒隕石均一儲(chǔ)庫; DM代表虧損地幔; CC代表大陸地殼

(3) 湘桂內(nèi)陸帶花崗巖的釹模式年齡(DM)背景值為1.8～2.4 Ga[47–48], 基底時(shí)代主要為1.7～2.7 Ga間[49]。湘東南地區(qū)具地幔物質(zhì)加入的早燕山期花崗巖2DM多在1.22～1.76 Ga之間[49]。由此可見, 加里東期越城嶺巖體的兩階段Nd模式年齡值(1.80～2.12 Ga)與區(qū)域基底地殼相當(dāng), 并明顯高于具地幔物質(zhì)加入的湘東南早燕山期殼源花崗巖。鑒此, 推斷巖體源于基底地殼的重熔。此外, 巖體Sr值(0.71912和0.72415)與高于大陸地殼0.719的Sr平均值[50], Sr、Nd同位素組成(Sr()=208和279,Nd()= –11.76～–7.80)與澳大利亞東南部Lachlan褶皺帶S型花崗巖(Sr()= 77～204,Nd()= –6.1和–9.8)[51]相近, 樣點(diǎn)在Nd()-Sr() 圖解(圖12)中落入華南S型花崗巖區(qū), 也說明巖體源于地殼重熔。

圖12?加里東期越城嶺巖體εNd(t)-εSr(t)圖解

I–華南I型花崗巖; S–華南S型花崗巖, 據(jù)劉昌實(shí)等[52]

I–I-type granite of South China; S–S-type granite of South China, after Liu.[52]

(4) Allégre.[53]指出花崗巖Sr()值大于0, 反映同位素與中、上地殼的親緣性, 而非麻粒巖相下地殼。因此, 加里東期越城嶺巖體Sr()值為208～279, 暗示源巖主要為中、上地殼酸性巖石, 而不是下地殼基性巖。這一認(rèn)識(shí)得到了A/MF-C/MF圖解的支持, 圖中樣點(diǎn)主要落入變質(zhì)泥質(zhì)巖和變質(zhì)雜砂巖部分熔融區(qū)(圖13)。

(5) 前述微量元素和稀土元素特征顯示出明顯的分離結(jié)晶作用, La/Sm-La圖解中也顯示出良好的分離結(jié)晶趨勢(圖14), 因此推斷產(chǎn)生巖性分異的巖漿過程主要受分離結(jié)晶作用控制。

圖13?加里東期越城嶺五團(tuán)巖體A/MF-C/MF圖解(據(jù)Alther et al. [54])

圖14?加里東期越城嶺巖體La/Sm-La圖解

4.2?加里東期越城嶺巖體形成構(gòu)造環(huán)境及機(jī)制

在Maniar.[55]提出的多組主元素構(gòu)造環(huán)境判別圖解中, 加里東期越城嶺巖體的分析樣品主要落入IAG+CAG+CCG區(qū), 且有較多樣品位于與POG重疊區(qū)之外(圖15), 因此應(yīng)屬于IAG+CAG+CCG組類型。巖石大部分屬ASI值大于1.1的強(qiáng)過鋁(SP)花崗巖, 進(jìn)而可進(jìn)一步判斷為大陸碰撞花崗巖類(CCG)(包括同碰撞與后碰撞花崗巖)[56]。

已有研究表明, 贛湘桂一帶早中生代構(gòu)造事件自早至晚可分為寒武紀(jì)末—奧陶紀(jì)初的郁南運(yùn)動(dòng)、奧陶紀(jì)末—志留紀(jì)初的北流運(yùn)動(dòng)(崇余運(yùn)動(dòng))以及志留紀(jì)后期的廣西運(yùn)動(dòng)等3個(gè)期次[5,8–12,57]; 前述U-Pb年齡表明加里東期越城嶺花崗巖形成于早志留世晚期。因此, 推斷花崗巖形成主要與稍早的北流運(yùn)動(dòng)有關(guān)。結(jié)合前述巖漿成因, 推斷加里東期越城嶺花崗巖形成機(jī)制為：北流運(yùn)動(dòng)(陸內(nèi)造山運(yùn)動(dòng))導(dǎo)致中、上地殼疊置、增厚和升溫, 擠壓峰期之后的早志留世中晚期在擠壓減弱、應(yīng)力松弛的后碰撞構(gòu)造環(huán)境下, 中、上地殼酸性巖石減壓熔融并向上侵位, 從而形成越城嶺花崗巖體。

4.3?加里東期越城嶺花崗巖對(duì)區(qū)域構(gòu)造背景的制約

越城嶺巖體前述構(gòu)造-巖漿事件可得到區(qū)域資料的佐證。鄰區(qū)彭公廟巖體(435.3±2.7) Ma和(436.2±3.1) Ma[20]、大寧巖體(419±6.4) Ma[58]、寧岡巖體(433.8±2.2) Ma[59]、海洋山巖體(431±7) Ma[60]、苗兒山巖體(428.5±3.8) Ma(筆者擬另撰文)等鋯石LA-ICPMS或SHRIMP U-Pb年齡, 在誤差范圍內(nèi)與加里東期越城嶺花崗巖年齡一致, 應(yīng)均因北流運(yùn)動(dòng)(崇余運(yùn)動(dòng))強(qiáng)烈擠壓和陸殼增厚所致。

湘南地區(qū)南華紀(jì)—早古生代沉積中缺少火山巖發(fā)育, 沉積物組成自西向東呈漸變特征, 說明引發(fā)加里東期越城嶺花崗質(zhì)巖漿活動(dòng)的構(gòu)造運(yùn)動(dòng)屬板內(nèi)造山運(yùn)動(dòng)。此為近年來已有研究得出的華南早古生代構(gòu)造運(yùn)動(dòng)屬板內(nèi)造山運(yùn)動(dòng)的認(rèn)識(shí)[2,61–64]提供了佐證。

區(qū)域上, 城步-新化大斷裂以西發(fā)育早志留世兩江河組和珠溪江組前陸盆地復(fù)理石沉積, 斷裂以東至湘東南卻缺失志留系[22], 而缺失屬沉積缺失還是后期剝蝕所致, 尚無可信的證據(jù)確認(rèn)。顯然, 城步-新化斷裂東側(cè)越城嶺巖體加里東期花崗巖的發(fā)育及其反映的北流運(yùn)動(dòng)變形和增厚事件, 充分說明斷裂以東在奧陶紀(jì)末已褶皺造山, 因此志留系的缺失屬沉積缺失而非后期剝蝕造成。

總之, 奧陶紀(jì)末—早志留世的北流運(yùn)動(dòng)造成了城步-新化斷裂以東造山抬升成陸并遭受剝蝕, 斷裂以西(現(xiàn)雪峰造山帶)因東側(cè)逆沖塊體的重力荷載而為前陸盆地的構(gòu)造古地理格局。值得指出的是, 位于城步-新化斷裂以西的北鄰白馬山加里東期花崗巖的鋯石SHRIMP U-Pb年齡為(411.0±4.5) Ma(湖南省地質(zhì)調(diào)查院, 1? 25萬邵陽幅區(qū)域地質(zhì)調(diào)查報(bào)告, 2013), 顯著晚于越城嶺加里東期巖體, 印證了早志留世構(gòu)造格局的認(rèn)識(shí)。從時(shí)代看, 白馬山加里東期花崗巖應(yīng)與影響范圍更廣的志留紀(jì)后期廣西運(yùn)動(dòng)有關(guān)。

圖15?加里東期越城嶺巖體構(gòu)造環(huán)境氧化物判別圖

底圖據(jù)Maniar.[55]。IAG–島弧花崗巖; RRG–與裂谷有關(guān)的花崗巖; CAG–大陸弧花崗巖類; CEUG–大陸造陸抬升花崗巖類; CCG–大陸碰撞花崗巖類; POG–后造山花崗巖類

After Maniar.[55]. IAG–Island-arc granite; RRG–Granite related to rift; CAG–Continent-arc granite; CEUG–Continent emerge-uplift granite; CCG–Continent-collision granite; POG–Post-orogenic granite

5?結(jié)?論

(1) (436.6±4.8) Ma、(430.5±4.3) Ma的鋯石SHRIMP U-Pb年齡值, 反映加里東期越城嶺花崗巖形成于早志留世晚期。

(2) 巖體總體為高鉀鈣堿性系列過鋁質(zhì)花崗巖類, 屬S型花崗巖, 源巖主要為中、上地殼酸性巖石。

(3) 花崗巖在北流運(yùn)動(dòng)強(qiáng)擠壓之后應(yīng)力相對(duì)松弛、壓力降低的板內(nèi)后碰撞構(gòu)造環(huán)境下, 因地殼增厚而升溫的中、上地殼巖石減壓熔融并向上侵位而形成。

審稿人對(duì)論文進(jìn)行了認(rèn)真審閱并提出了建設(shè)性修改意見, 在此表示衷心感謝。

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The zircon SHRIMP U-Pb dating, geochemical characteristics and tectonic setting of Caledonian Yuechengling pluton in the western segment of the Nanling Mountains

BAI Dao-yuan*, ZHONG Xiang, JIA Peng-yuan, XIONG Xiong and HUANG Wen-yi

(Hunan Institute of Geology Survey, Changsha?410016, China)

Yuechengling pluton in the western segment of the Nanling Mountains is mainly composed of Caledonian and Indosinian granites in the southern and northern parts, respectively. Caledonian granites change with time from medium-fine-grained porphyritic biotite monzogranite, fine-medium-grained porphyritic two-mica monzogranite, fine-grained porphyritic two-mica monzogranite and fine-grained two-mica monzogranite. The Caledonian granites in the eastern part are characterized by massive and gneissic structures. Late Mesozoic extensional-type ductile shear zones occur in the western margin of this pluton. U-Pb zircon dating for a porphyritic biotite monzogranite sample and a mylonitic porphyritic biotite monzogranite sample yield weighted average ages of (436.6±4.8) Ma and (430.5±4.3) Ma, respectively, which indicate that the Yuechengling pluton was formed in late Early Silurian. The rocks are silicon-high (SiO2= 68.35%―78.10%, 73.29% on average), aluminium-rich (Al2O3= 11.95%―15.55%, 14.18% on average), potassium-high (K2O = 4.12%―5.62%, 4.95% on average) and alkali-moderate (Na2O+K2O = 6.18%―8.30%, 7.58% on average) with K2O/Na2O value of 1.36―2.82 (1.94 on average) and ASI value of 1.04―1.66 (1.23 on average), thus belong to high-K Calc-alkaline series perluminous granitoids. Most samples are depleted in Ba, Nb, Sr, P and Ti, and enriched in Rb, (Th+U+K), (La+Ce), Nd, (Zr+Hf+Sm) and (Y+Yb+Lu). The studied sampels have ∑REE values of 50.43―328.81 μg/g (173.39 μg/g on average),Eu values of 0.21―0.68 (0.40 on average), (La/Yb)Nvalues of 0.54―14.04 (7.93 on average),Srvalues of 0.71912 and 0.72415,Sr() values of 207 and 279,Nd() values of –11.76―–7.80 and2DMages of 1.80―2.12 Ga. The A/MF-C/MF diagram indicates that the Caledonian Yuechengling granites were form by partial melting of mudstones and clastic rocks. All above geochemical characteristics indicate that the granites are S-type granitoid. Multiple oxide-diagrams for discrimination of structural environment show that the granites were formed in post-collisional tectonic setting. According to petrogenesis, discrimination of structural environment and regional tectonic evolution, the formation mechanism of Caledonian Yuechengling granites is inferred as follows:the thickening of crust in Beiliu Movement (in the late Ordovician to the early Silurian) raised the crust temperature,granites formed through parting melting in post-collisional tectonic setting in middle-late Early Silurian.

zircon SHRIMP U-Pb dating; S-type granitoid; Beiliu Movement; post-collisional tectonic setting; Yurchengling; southwestern Hunan Province

P597

A

0379-1726(2015)01-0027-16

2013-08-03;

2013-12-08;

2014-01-23

中國地質(zhì)調(diào)查局“湖南1︰25萬武岡市和永州市幅區(qū)調(diào)修測”項(xiàng)目(1212011120793);“中國地質(zhì)構(gòu)造區(qū)劃綜合研究與區(qū)域地質(zhì)志修編”項(xiàng)目(1212010811032)

柏道遠(yuǎn)(1967–), 男, 研究員級(jí)高級(jí)工程師, 長期從事區(qū)域地質(zhì)調(diào)查與基礎(chǔ)地質(zhì)研究。

BAI Dao-yuan, E-mail: daoyuanbai@sina.com; Tel: +86-731-89807272