李再會, 唐發(fā)偉, 林仕良, 叢 峰, 謝 韜, 鄒光富

成都地質(zhì)礦產(chǎn)研究所,成都 610081

滇西含綠柱石偉晶巖鋯石U-Pb年代學(xué)及其地質(zhì)意義

李再會, 唐發(fā)偉, 林仕良, 叢 峰, 謝 韜, 鄒光富

成都地質(zhì)礦產(chǎn)研究所,成都 610081

對騰沖-梁河地區(qū)含綠柱石偉晶巖進行了鋯石LA-ICP-MS U-Pb定年和地球化學(xué)分析。結(jié)果表明：含綠柱石偉晶巖的形成時代為(48.1±0.8)Ma(MSWD=4.0)，鋯石形態(tài)特征和微量元素特征顯示，偉晶巖鋯石受到熱液的改造。含綠柱石偉晶巖與55～52 Ma的白云母花崗巖在主量元素、微量元素及稀土元素方面表現(xiàn)出極其相似的特征，為鈣堿性系列，過鋁質(zhì)花崗巖，表現(xiàn)出強烈的Eu虧損，δEu為0.074～0.083，相對富集HREE，(La/Yb)N=1.61～1.92，總體表現(xiàn)出典型的“M”型稀土元素四分組效應(yīng)。含綠柱石偉晶巖是白云母花崗巖漿高度演化的結(jié)果，偉晶巖結(jié)晶溫度為581 ℃，代表了印度-歐亞板塊碰撞導(dǎo)致地殼加厚的構(gòu)造背景。

含綠柱石偉晶巖；地球化學(xué)；鋯石U-Pb年代；稀土四分組效應(yīng)；滇西

0 引言

西南三江地區(qū)作為青藏高原的東延部分，不僅和喜馬拉雅造山帶一樣經(jīng)歷了新特提斯洋俯沖、印度板塊和歐亞板塊俯沖碰撞與隆升等一系列大規(guī)模的構(gòu)造運動，而且以其獨特的構(gòu)造部位，被認為是一個吸收新生代印度-歐亞大陸碰撞變形的調(diào)節(jié)帶[1]。作為一個包括元古宙至第四紀巖漿巖的火成巖省，騰沖花崗巖帶包含豐富的巖石類型，其中含稀有元素偉晶巖是重要的巖石類型之一[2-3]。偉晶巖作為一種獨立的礦床類型，不但在礦床學(xué)上占有不可忽視的地位，而且在示蹤大地構(gòu)造演化的過程中具有重要意義[4]。因此，通過對騰沖-梁河地區(qū)含綠柱石偉晶巖鋯石LA-ICP-MS定年、地球化學(xué)研究，探討偉晶巖的形成時代和成因，旨在為騰沖-梁河地塊新生代構(gòu)造背景研究提供一些新的證據(jù)。

1 地質(zhì)背景及巖石學(xué)特征

西南三江作為典型的復(fù)合造山地區(qū)，完好地記錄了超級大陸裂解→增生→碰撞的完整演化歷史和大陸動力學(xué)過程[5]。其位于云南西南部的騰沖-保山地區(qū)，包括騰沖地塊、保山地塊和其間的高黎貢山變質(zhì)帶，屬于緬泰馬微陸塊的北部[6]。在三疊紀期間，騰沖-梁河地塊(簡稱騰-梁地塊)處于東部古特提斯主洋盆，即昌寧-孟連古特提斯洋封閉時的前陸部位。在新特提斯洋擴張時期，其間又形成屬于班公湖-怒江洋盆的東延分支海槽。該海槽在早侏羅世閉合，并導(dǎo)致騰沖地塊和保山地塊的碰撞，其間形成高黎貢碰撞構(gòu)造帶[6]。以滬水-龍陵-瑞麗大斷裂為界，騰沖-保山地塊出露于地表的基底巖石類型有明顯的差別。東南部以公養(yǎng)河群為代表，時代可能是早古生代，其上為上寒武統(tǒng)-中生界碎屑巖、碳酸鹽巖和玄武巖構(gòu)成的沉積蓋層[7]；西北部以高黎貢山群為代表，混合巖化顯著，時代可能是新元古代[8]，上部主要為弱變形的石炭系-三疊系碳酸鹽巖與碎屑巖沉積，古近系-第四系陸相火山巖、河湖相碎屑沉積等構(gòu)成的沉積蓋層。該地區(qū)出露大量的中生代-新生代花崗巖類和混合巖化花崗巖。新生代火山作用強烈(圖1)。研究區(qū)位于騰-梁地塊內(nèi)。

高黎貢山地區(qū)偉晶巖分布廣泛[2-3]，寶石級的礦物(綠柱石、碧璽、鋰電氣石、黃玉等)主要賦存在偉晶巖中[9]。 騰沖-梁河地區(qū)含綠柱石電氣石白云母偉晶巖呈脈狀侵入白云母花崗巖中(圖2a)。白云母花崗巖的年齡為52～56 Ma[10-11]。白云母花崗巖為中粒花崗結(jié)構(gòu)，礦物組成(體積分數(shù))石英為25%～30%，鉀長石為35%～40%，斜長石為30%，黑云母為3%，白云母為1%，副礦物為磷灰石、鈦鐵礦、電氣石、石榴子石和鋯石等。電氣石白云母偉晶巖脈寬十幾厘米至1 m，長數(shù)百米。電氣石白云母偉晶巖與白云母花崗巖圍巖界線呈突變。電氣石白云母偉晶巖主要礦物為石英、長石、白云母和少量電氣石。偉晶巖中見綠柱石(圖2b)，其晶體長3～4 cm。

圖1 西南三江大地構(gòu)造位置圖(a)及騰沖-梁河地區(qū)地質(zhì)圖(b)Fig. 1 Tectonic sketch map(a) and geological sketch map of Tengchong-Lianghe area(b),weastern Yunnan

圖2 電氣石白云母偉晶巖脈露頭及偉晶巖中綠柱石晶體Fig. 2 Photographs showing field outcrop of pegmatite and beryl crystal in pegmatite from the Tengchong-Lianghe area

2 分析方法

主量元素化學(xué)分析在國土資源部西南礦產(chǎn)資源監(jiān)督檢測中心用X熒光光譜儀射線(儀器型號為AXIOS)完成，分析精度優(yōu)于5%；微量元素在中國科學(xué)院地球化學(xué)研究所礦床地球化學(xué)國家重點實驗室用ICP-MS(儀器型號為ELEMENT)完成，分析精度優(yōu)于5%。

樣品的鋯石分離是在河北省區(qū)域地質(zhì)調(diào)查研究所完成。鋯石按常規(guī)方法分選， 最后在雙目鏡下挑純鋯。將鋯石樣品和標樣一起放在玻璃板上用環(huán)氧樹脂固定，拋光到暴露出鋯石的中心面，用反光和透光照相，然后鍍金，拍攝陰極發(fā)光(CL)圖像。陰極發(fā)光照相在西北大學(xué)大陸動力學(xué)國家重點實驗室的掃描電鏡+Gatan陰極發(fā)光MonoCL3上完成。LA-ICP-MS鋯石U-Pb測定在中國地質(zhì)大學(xué)(武漢)地質(zhì)過程與礦產(chǎn)資源國家重點實驗室完成。使用的 ICP-MS 為 Elan 6100 DRC，激光剝蝕系統(tǒng)為德國 Lamda Physik公司的GeoLas 200 M深紫外(DUV) 193 nm ArF準分子激光剝蝕系統(tǒng)。激光束斑直徑為 32 μm,實驗中采用He作為剝蝕物質(zhì)的載氣。U-Th-Pb同位素組成分析以標準鋯石91500作為外標，NIST610作為內(nèi)標，外標校正方法為每隔4～5個樣品分析點測一次標準樣品，保證標準和樣品的儀器條件一致。詳細的儀器操作條件和數(shù)據(jù)處理方法見文獻[12-14]。對分析數(shù)據(jù)的離線處理采用軟件ICPMSSDataCal完成[12-13]。鋯石諧和圖用ISOPLOT程序[15]獲得。用實測204Pb進行普通鉛校正[16]。因樣品年輕，故采用206Pb/238U年齡。單個點的同位素比值和年齡誤差均為1σ，206Pb/238U年齡的加權(quán)平均值誤差為95%的置信度。

3 鋯石U-Pb年齡結(jié)果

含電氣石白云母偉晶巖脈中鋯石呈無色透明，為自形柱狀-長柱狀，顆粒長徑為100～200 μm，長寬比為1∶1～2∶1，顯示巖漿成因的特征。陰極發(fā)光圖像(CL)顯示，幾乎不顯示韻律環(huán)帶，鋯石內(nèi)部呈多孔狀、斑雜狀，陰極射線發(fā)光弱，不均勻(圖3)，顯示熱液蝕變鋯石的特點[17]。鋯石稀土元素配分模式總體表現(xiàn)為左傾式(富集重稀土)的巖漿型特征[17](圖4)，δCe為弱的正異常到弱的負異常(0.61～2.70)，具明顯的Eu負異常(除3顆鋯石具δEu弱正異常)(表1，圖4)。鋯石的U和Th質(zhì)量分數(shù)變化范圍較大，分別為(2 128～26 220)×10-6和(44.1～2 644.0)×10-6，其Th/U值為0.01～0.10(表2)。11個分析點都沿著諧和線或附近分布(圖5a)，可得到一致性U-Pb諧和年齡。11顆鋯石給出的206Pb/238U年齡范圍為46.7～53.1 Ma，加權(quán)平均年齡為(48.1±0.8)Ma(MSWD=4.0)(圖5b)，代表含綠柱石電氣石偉晶巖鋯石的結(jié)晶年齡。

表1 綠柱石電氣石偉晶巖鋯石LA-ICP-MS原位微量元素分析結(jié)果

注： “-”表示低于檢測限。

表2 騰沖-梁河地區(qū)綠柱石電氣石偉晶巖鋯石LA-ICP-MS U-Pb測年結(jié)果

圖3 偉晶巖(D5061)鋯石的CL圖像、分析點位Fig. 3 CL images of zircons with the analysed spots of pegmatite (D5061)from Tengchong-Lianghe area

圖4 偉晶巖鋯石的稀土元素球粒隕石標準化配分模式[18]Fig. 4 Chondrite-normalized REE patterns for the zircons of pegmatite from Tengchong-Lianghe area[18]

4 地球化學(xué)

4.1 主量元素

偉晶巖的w(SiO2)為77.21%～77.37%，w(Al2O3)為12.73%～12.80%(表3)，全堿(w(K2O+Na2O))為8.01%～8.34%，w(TiO2)為0.06%～0.07%，w(P2O5)為0.005%，w(CaO)為0.41%～0.42%，在花崗巖TAS圖(圖6a)中，落入亞堿性系列花崗巖區(qū)。A/CNK為1.11～1.15，為過鋁質(zhì)花崗巖[20](圖6b)。53～52 Ma的白云母花崗巖與偉晶巖化學(xué)成分極為相似，w(SiO2)為75.71%～76.96%，w(Al2O3)為12.50%～12.98%，全堿含量(w(K2O+Na2O))為8.71%～8.92%，w(TiO2)為0.04%～0.07%，w(P2O5)為0.008%～0.010%，w(CaO)為0.07%～0.40%，A/CNK為1.02～1.13，為準鋁質(zhì)-過鋁質(zhì)花崗巖。相反，58～61 Ma的鉀長花崗巖與偉晶巖化學(xué)組成上有較大的差異，w(SiO2)為64.99%～72.05%，w(Al2O3)為14.17%～16.42%，全堿(w(K2O+Na2O))為5.87%～8.64%，w(TiO2)為0.24%～0.59%，w(P2O5)為0.052%～0.200%，w(CaO)為0.27%～4.06%。從主量元素組成方面，顯示偉晶巖與白云母花崗巖有密切的關(guān)系。

4.2 微量元素

偉晶巖w(∑REE)為(84.5～95.0)×10-6，表現(xiàn)出強烈的Eu虧損，δEu為0.074～0.083，相對富集HREE，(La/Yb)N=1.61～1.92[21]，總體表現(xiàn)出典型的“M”型稀土元素四分組效應(yīng)(圖7a)。白云母花崗巖的w(∑REE)為(93.9～175.9)×10-6，稀土配分模式與偉晶巖很相似，表現(xiàn)出強烈的Eu虧損，δEu為0.036～0.090，相對富集HREE，(La/Yb)N=1.01～1.55，表現(xiàn)出“M”型稀土元素四分組效應(yīng)。偉晶巖經(jīng)歷了明顯的分離結(jié)晶作用，REE總量逐漸降低；而鉀長花崗巖則表現(xiàn)出右傾的LREE富集型模式，(La/Yb)N=6.20～22.50，δEu為0.45～0.48。微量元素原始地幔標準圖(圖7b)中，偉晶巖與白云母花崗巖表現(xiàn)出相似的特征，虧損Ba、Sr、P、Ti，富集Rb、Th、K、Nb、Ta。鉀長花崗巖則表現(xiàn)出虧損Nb、Ta。

圖5 含綠柱石偉晶巖鋯石的LA-ICP-MS U-Pb年齡諧和圖Fig .5 Zircon LA-ICP-MS U-Pb concordia diagram of pegmatite from Tengchong-Lianghe area

a圖:Pc.苦橄玄武巖；B.玄武巖；O1.玄武安山巖；O2.安山巖；O3.英安巖；R.流紋巖；S1.粗面玄武巖；S2.玄武質(zhì)粗面安山巖；S3.粗面安山巖；T.粗面巖、粗面英安巖；F.副長石巖；U1.堿玄巖、碧玄巖；U2.響巖質(zhì)堿玄巖；U3.堿玄質(zhì)響巖；Ph.響巖；Ir.Irvine 分界線，其上方為堿性，下方為亞堿性。b圖:第1組.偉晶巖；第2組.鉀長花崗巖；第3組.白云母花崗巖。圖6 騰沖-梁河偉晶巖及花崗巖TAS圖[19](a)及A/CNK-A/CN圖解(b)Fig. 6 Total alkali vs.SiO2 variation diagram[19](a)and A/CNK-A/CN diagram(b) for the pegmatites and granites

鉀長花崗巖D4160D4310D4179D4142花崗偉晶巖D4289-1D4289-2D4289-3白云母花崗巖D4289LLS8LLS10SiO268．9668．6664．9972．0577．3377．3777．2176．9676．0375．71TiO20．240．450．590．250．070．060．060．070．040．06Al2O316．4215．0315．8814．1712．7512．8012．7312．5012．9412．98

表3(續(xù))

注：樣品LLS8、LLS10數(shù)據(jù)引自文獻[11]；主量元素質(zhì)量分數(shù)單位為%，微量元素質(zhì)量分數(shù)單位為10-6。

圖7 騰沖-梁河地區(qū)偉晶巖稀土元素[18](a)和微量元素(b)配分模式圖[22]Fig. 7 Chondrite normalized REE patterns[18](a) and primitive normalized spider diagrams[22](b) for pegmatite from Tengchong-Lianghe area

5 討論

5.1 偉晶巖中鋯石成因

鋯石可在上地幔高溫高壓條件到近地表熱液條件的廣泛范圍內(nèi)形成，具有高度穩(wěn)定性[17]。已有的研究表明：巖漿鋯石Th/U值較高，大于0.1(一般為0.5～1.5)；變質(zhì)鋯石Th/U值低，小于0.2[23-26]。但近年來，發(fā)現(xiàn)在熱液條件下鋯石可發(fā)生蝕變作用，甚至可從熱液中結(jié)晶形成熱液鋯石(hydrothermal zircon)[17, 27-30]。 熱液改造的和熱液鋯石可用于確定流體加入事件及水/巖相互作用的特點，因此，其微量元素，特別是稀土元素組成特點成為探討成巖或成礦地球化學(xué)過程的重要示蹤[17]。本文花崗偉晶巖鋯石具有典型的巖漿鋯石形態(tài)(完整的自形柱狀)，具有明顯的δEu正異常，鋯石稀土元素球粒隕石標準化型式為左傾式(富集重稀土) 的巖漿型鋯石。鋯石卻具有極低的Th/U值(為0.01～0.10)，和多孔狀構(gòu)造，鋯石稀土元素組成Ce正異常降低或消失，顯示出熱液鋯石的特點。(Sm/La)N-La及Ce/Ce*-(Sm/La)N圖解[29](圖8)上，偉晶巖鋯石分布于巖漿鋯石與熱液鋯石的過渡區(qū)，這些特征顯示了熱液改造鋯石的特點。

5.2 偉晶巖的成因

偉晶巖通常被認為形成于晚期巖漿和早期熱液過程的過渡階段[31]。H2O在偉晶巖的形成過程中起著關(guān)鍵的作用，偉晶巖漿中H2O的質(zhì)量分數(shù)往往超過10%[32]。也有人認為過冷卻在偉晶巖形成中起很重要的作用[33-34]。偉晶巖結(jié)晶過程中常常伴隨著熱液流體的階段性演化[35]，不同階段形成的流體成分不盡相同。通過對偉晶巖中包裹體的研究，綠柱石是在早期熱液階段中結(jié)晶沉淀，溫度條件是520～620 ℃，壓力是3.5～5.0 kPa[35-37]。根據(jù)巖漿巖中鋯石的Ti質(zhì)量分數(shù)，通過公式：T(℃)zircon=(5 080±30)/[(6.01±0.03)-lg(w(Ti))]- 273，可以計算巖漿結(jié)晶溫度[38]。對本文偉晶巖鋯石溫度計算(表1)，偉晶巖平均結(jié)晶溫度為581 ℃，為早期熱液階段[35-37]，表明本文偉晶巖中綠柱石是在熱液階段的早期沉淀的，而且溫度相當(dāng)高。研究表明，在H2O飽和的酸性熔體中熔體不混溶過程對含Be礦物的形成具有重要作用[39]，在演化的巖漿和熔體不混溶階段形成的富含H2O的偉晶巖巖漿中，Be的濃集程度可達到1 000倍以上。

圖8 含綠柱石偉晶巖中鋯石(Sm/La)N-w(La)(a)及Ce/Ce*-(Sm/La)N(b)圖解(底圖據(jù)文獻[29])Fig. 8 Discrimination plots for magmatic and hydrothermal zircon(base map modified after reference[29])

騰沖-梁河地區(qū)新生代巖漿作用廣泛，并具有多期的特點，根據(jù)已有的年代學(xué)資料，包括66～58 Ma的鉀長花崗巖[10, 40-41]、55～52 Ma的白云母花崗巖[10-11]、41～43 Ma的二長花崗巖[11, 42]和24 Ma的花崗巖[43]。顯然，含綠柱石偉晶巖與41～43 Ma和24 Ma的巖漿作用無關(guān)。前文已經(jīng)論述，含綠柱石偉晶巖與52～55 Ma的白云母花崗巖在主量元素、微量元素、稀土元素組成和配分模式極為相似，而與 58～61 Ma的鉀長花崗巖有較大的差別，說明偉晶巖是55～52 Ma的白云母花崗巖漿高度演化的產(chǎn)物。白云母花崗巖的εNd(t)為-7.9～-3.7[44]，εHf(t)為-12.35～-4.5[10]，說明其來源于地殼的部分熔融。

5.3 偉晶巖形成的大地構(gòu)造背景

Cerny等[45-46]將富含(Li、Rb、Cs 、Be、Ga、Sn)<(Nb、Ta、 B、 P、 F )等元素的偉晶巖稱為LCT型偉晶巖。這種LCT型偉晶巖通常與同造山、造山晚期過鋁質(zhì)S型花崗巖密切相關(guān)[45, 47]；與 Li-Be-Na-Cs 礦床相關(guān)的偉晶巖，以過鋁質(zhì)、富含B、 Be、Li、 P 、堿質(zhì)(Na、K)，貧Fe、 Mg、 Ca以及礦物( 如磷灰石)顯示強烈的REE“四分組效應(yīng)”為特征[48]。這意味著它很可能是由沉積的泥質(zhì)巖深熔形成。而泥質(zhì)巖，尤其是與蒸發(fā)巖有關(guān)的黏土巖類，是最富集電氣石的，可提供大量的B[49-50]以及Na、 K、Li、 Cl、 F、P、Fe、 Mn等組分[51-52]。過鋁質(zhì)花崗巖主要位于2個大陸巖石圈匯聚使地殼加厚的部位[53]，即大陸碰撞地殼加厚區(qū)。LCT型偉晶巖是典型的過鋁質(zhì)巖漿體系分異演化的產(chǎn)物，因此，可以合理地推斷騰沖-梁河地區(qū)含稀有金屬偉晶巖的形成與大陸碰撞構(gòu)造背景有密切關(guān)系。騰沖-梁河地區(qū)偉晶巖的形成時間為48.1 Ma，與印度-歐亞板塊碰撞事件相吻合。約65 Ma，印度與歐亞板塊開始碰撞[54-55]，引起地殼加厚。加厚地殼部分熔融形成55～52 Ma的S型白云母花崗巖。隨著巖漿的不斷分離結(jié)晶，殘余巖漿中水和揮發(fā)分含量不斷增加，巖漿黏度大大降低，巖漿流動性增大[32]，偉晶巖漿沿著裂隙向上流動，形成了偉晶巖脈。偉晶巖的形成時代也間接證明了印度與歐亞板塊在48 Ma以前就發(fā)生了碰撞。

6 結(jié)論

1)鋯石LA-ICP-MS U-Pb同位素定年結(jié)果表明，騰沖-梁河地區(qū)含綠柱石偉晶巖的形成年齡為(48.1±0.8)Ma(MSWD=4.0)，鋯石形態(tài)特征及元素特征顯示，這些鋯石受到熱液改造。

2)含綠柱石偉晶巖與白云母花崗巖在主量元素、微量元素及稀土元素方面表現(xiàn)出極其相似的特征，為鈣堿性系列，過鋁質(zhì)花崗巖，表現(xiàn)出強烈的Eu虧損，δEu為0.074～0.083，相對富集HREE，(La/Yb)N=1.61～1.92，總體表現(xiàn)出典型的“M”型稀土元素四分組效應(yīng)。

3)騰沖-梁河地區(qū)含綠柱石偉晶巖是白云母花崗巖漿高度演化的殘余巖漿產(chǎn)物，偉晶巖結(jié)晶溫度為581 ℃。偉晶巖代表了印度-歐亞板塊碰撞導(dǎo)致地殼加厚的構(gòu)造背景。

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Zircon LA-ICPMS U-Pb Geochronology of the Beryl-Bearing Pegmatite and Its Geological Significance,Western Yunnan,Southwest China

Li Zaihui，Tang Fawei，Lin Shiliang，Cong Feng，Xie Tao，Zou Guangfu

ChengduInstituteofGeologyandMineralResources,Chengdu610081，China

Zircon LA-ICP-MS U-Pb dating and geochemical analyses were carried out for the beryl-bearing pegmatite of Tengchong-Lianghe area, western Yunnan. Dating results revealed that the beryl-bearing pegmatite were formed at (48.1±0.8) Ma (MSWD=4.0). Zircon morphology and trace element feature indicate that the zircons suffered from hydrothermal alteration. The beryl-bearing pegmatite shows similar characteristics with respect to major elements, trace elements and rare elements with muscovite granite which formed during 55-52 Ma. They are sub-alkaline series, peraluminous granite with strongly Eu depletion, relatively enriched HREE with (La/Yb)N=1.61-1.92 and showing typical M-type of REE tetrad effect. The genesis of beryl-bearing pegmatite is related to the evolution of muscovite granitoids and the crystallization temperature of beryl-bearing pegmatite is 581 ℃. It indicates the overthickened crust tectonic setting caused by India-Asia continental collision.

beryl-bearing pegmatite；geochemistry；zircon U-Pb dating；REE tetrad effect；western Yunnan

10.13278/j.cnki.jjuese.201402113.

2013-07-28

中國地質(zhì)調(diào)查局地質(zhì)大調(diào)查項目(1212010784007)

李再會(1967-)，男，高級工程師，主要從事巖石學(xué)與區(qū)域地質(zhì)研究，E-mail:lizaihui00@163.com

唐發(fā)偉(1979-)，男，工程師，主要從事地球化學(xué)研究，E-mail:37205958@qq.com。

10.13278/j.cnki.jjuese.201402113

P597

A

李再會, 唐發(fā)偉, 林仕良,等.滇西含綠柱石偉晶巖鋯石U-Pb年代學(xué)及其地質(zhì)意義.吉林大學(xué)學(xué)報:地球科學(xué)版,2014,44(2):554-565.

Li Zaihui，Tang Fawei，Lin Shiliang,et al.Zircon LA-ICPMS U-Pb Geochronology of the Beryl-Bearing Pegmatite and Its Geological Significance,Western Yunnan,Southwest China.Journal of Jilin University:Earth Science Edition,2014,44(2):554-565.doi:10.13278/j.cnki.jjuese.201402113.