滇西劍川始新世富堿巖漿巖鋯石U-Pb年代學(xué)與Sr-Nd-Hf同位素地球化學(xué)及其對巖石成因的制約*

沈陽 鄭遠(yuǎn)川** 張愛萍 王梓軒 王璐 徐培言 吳昌炟

1. 中國地質(zhì)大學(xué)地球科學(xué)與資源學(xué)院，北京 1000832. 云南大學(xué)資源環(huán)境與地球科學(xué)學(xué)院，昆明 6500911.

肇始于新生代的印度-歐亞大陸碰撞造山作用(Yin and Harrison, 2000; Dingetal., 2003; Zhangetal., 2012)，晚碰撞階段(40～26Ma)主要是以大規(guī)模走滑剪切為特征(侯增謙等, 2006a, b)，以青藏高原東緣三江地區(qū)表現(xiàn)最為顯著，發(fā)育了一系列由北北西至南北向深大斷裂組成的大規(guī)模走滑斷裂系統(tǒng)，即著名的金沙江-哀牢山走滑斷裂系統(tǒng)(Houetal., 2003)。沿走滑斷裂廣泛發(fā)育新生代富堿侵入巖和鉀質(zhì)火山巖，構(gòu)成著名的長達(dá)～1000km的金沙江-紅河富堿巖漿巖帶(張玉泉等, 1987; 鄧萬明等, 1998)，同時，在此巖漿巖帶內(nèi)形成有與富堿巖漿巖在成因和時空分布密切相關(guān)的斑巖型銅-鉬-金等多金屬成礦帶(侯增謙等, 2004, 2015)。因此，對帶內(nèi)富堿巖漿巖的地球化學(xué)和巖漿演化的有效限定對于完善區(qū)域富堿巖漿巖成因和金屬礦產(chǎn)的資源評價與找礦具有重要意義。

三江地區(qū)的大量新生代富堿巖漿巖總體可將其劃分為淺成富堿侵入巖和鉀質(zhì)-超鉀質(zhì)火山巖兩類，其成因爭論較多，不同的模式包括：巖石圈減薄軟流圈上涌引起交代富集地幔部分熔融模式(Chungetal., 1998; 張玉泉等, 2000)、殼幔過渡帶部分熔融模式(鄧萬明等, 1998; Wangetal., 2001)、揚(yáng)子板塊西向俯沖模式(Liuetal., 2000; 侯增謙等, 2004)、蘭坪-思茅大陸板片沿斷裂帶東向俯沖模式(Wangetal., 2001)、大規(guī)模走滑誘發(fā)的“島弧型”地幔熔融模式(王建等, 2003)，金云母-石榴石-單斜輝石巖脈低程度部分熔融模式(Jiangetal., 2006)。這些模型雖可解釋地球化學(xué)特征，但是三江富堿巖漿巖分布范圍廣、類型多樣、產(chǎn)狀復(fù)雜，且上述模式都是基于某個地區(qū)或礦區(qū)研究提出。同時，該區(qū)淺成富堿侵入巖與鉀質(zhì)-超鉀質(zhì)的火山巖伴生，而部分模式的提出將兩者混為一體，缺乏對區(qū)域新生代富堿巖漿巖的精細(xì)劃分。目前，對三江新生代富堿巖漿巖巖石性質(zhì)和成因的認(rèn)識依然存在偏差。

滇西劍川位于青藏高原東南緣的三江南段地區(qū)(圖1a)，處于三江造山帶與揚(yáng)子板塊西南緣的結(jié)合部位，記錄了青藏高原隆升過程中大地構(gòu)造演化及深部動力學(xué)的關(guān)鍵信息。目前對區(qū)內(nèi)新生代巖漿巖的研究仍然較為薄弱，特別是研究區(qū)內(nèi)富堿巖漿巖的巖石成因及其與三江富堿巖漿巖帶的時空關(guān)系尚不明確，制約了對該區(qū)巖漿屬性、地質(zhì)演化和成礦地質(zhì)條件的正確認(rèn)識。本文對這些巖漿巖開展了野外調(diào)查和室內(nèi)工作，發(fā)現(xiàn)存在花崗巖和正長巖兩類巖石。通過系統(tǒng)采集花崗巖和正長巖樣品并開展鋯石U-Pb定年、全巖主微量與Sr-Nd同位素以及鋯石Hf同位素研究，以期精確厘定其成巖年齡、巖漿源區(qū)和演化關(guān)系，補(bǔ)充完善三江富堿巖漿巖的巖石成因，探討其動力學(xué)過程和成礦潛力。

1 地質(zhì)背景與巖石特征

滇西劍川位于青藏高原東南緣的三江南段地區(qū)(圖1a)，處于三江褶皺造山帶和揚(yáng)子板塊西南緣的結(jié)合部位。三江地區(qū)南段由于經(jīng)歷了特提斯構(gòu)造活動(Lietal., 2018)、陸內(nèi)造山作用以及新生代構(gòu)造轉(zhuǎn)換的改造，造就了該區(qū)復(fù)雜的地質(zhì)構(gòu)造系統(tǒng)，褶皺和斷裂較為發(fā)育。區(qū)域上各時代地層較為齊全，自前寒武系蒼山群至第四系沉積物均有分布。三江南段揚(yáng)子克拉通西緣一側(cè)的巖漿巖較為發(fā)育，巖石類型復(fù)雜，明顯受到區(qū)域斷裂構(gòu)造的控制(Lietal., 2018)，二疊紀(jì)玄武巖廣為分布，但以新生代火山-侵入巖最為發(fā)育，次為不同時期的酸性、堿性侵入巖以及新生代基性火山巖。新生代巖漿巖主要是一系列沿金沙江-哀牢山斷裂帶兩側(cè)(劍川、鶴慶、祥云、姚安和金平等)分布的富堿侵入巖體(圖1a; 侯增謙等, 2004; Luetal., 2012; 沈陽等, 2018)，此外零星分布有煌斑巖脈和鉀質(zhì)火山巖，如堿玄巖、粗面巖和鉀質(zhì)云煌巖等(Guoetal., 2005; Huangetal., 2010; Luetal., 2015)。富堿侵入巖巖性復(fù)雜，以二長花崗斑巖、花崗斑巖、石英二長斑巖、透輝石正長巖和石英正長斑巖為主(侯增謙等, 2004; Luetal., 2012; 沈陽等, 2018)。富堿侵入巖具有富堿(K2O+Na2O>8.0%)、高鉀(K2O/Na2O>1.0)的特征，屬于高鉀鈣堿性和鉀玄巖系列巖石；通過鋯石U-Pb法測得區(qū)域內(nèi)富堿侵入巖的侵位時間主要集中在37～32Ma(Luetal., 2012)。

樣品采自劍川縣城西北，根據(jù)1:20萬蘭坪幅和維西幅區(qū)調(diào)資料(趙永嘉等, 1974; 王自康等, 1984)，劍川新生代富堿侵入巖主要呈巖株與巖枝狀產(chǎn)出，形態(tài)多不規(guī)則，且侵位的最新層位為新近紀(jì)劍川組沉積地層(圖1b)。區(qū)內(nèi)出露地層包括中-古元古界石鼓片巖，分布于研究區(qū)東北緣；古近系寶相寺組礫巖、雜砂巖，金絲廠組砂巖夾鈣質(zhì)砂巖、粉砂巖，以及新近系雙河組砂巖、泥巖，劍川組凝灰質(zhì)砂礫巖、火山角礫巖等大面積分布于中南部(圖1b)。

橫穿研究區(qū)內(nèi)各富堿巖漿巖體共采集了13件新鮮的巖石樣品(圖1b)。根據(jù)野外和室內(nèi)工作共識別出4種巖性，即石英二長斑巖、花崗斑巖和正長斑巖、粗面巖。石英二長斑巖(15SG02-2，圖2a)為斑狀結(jié)構(gòu)，斑晶主要為斜長石(30%～40%)，鉀長石(40%～30%)、石英(10%～15%)和黑云母(5%～10%)，基質(zhì)為顯晶質(zhì)主要是石英和長石等，副礦物為磷灰石、榍石、鋯石和鐵鈦氧化物等約1%～5%?；◢彴邘r(14YL01-1，圖2c)為斑狀結(jié)構(gòu)，斑晶主要為石英(15%～25%)、斜長石(25%～30%)，鉀長石(35%～30%)和黑云母(5%～10%)，基質(zhì)主要是石英和長石等，副礦物為磷灰石、榍石、鋯石和鐵鈦氧化物等。角閃正長斑巖(16JCs01-1，圖2b)為斑狀結(jié)構(gòu)，斑晶主要為鉀長石(30%～40%)、角閃石(35%～20%)和黑云母(10%～15%)，基質(zhì)主要是石英、正長石和少量的角閃石等，副礦物為磷灰石、鋯石和鐵鈦氧化物等約1%～5%。粗面巖(16JCb03-3，圖2d)為斑狀結(jié)構(gòu)，斑晶主要為鉀長石(30%～40%)和透輝石(30%～35%)，基質(zhì)為隱晶質(zhì)，副礦物以鐵鈦氧化物等為主。

圖1 金沙江-紅河斷裂帶新生代富堿巖漿巖地質(zhì)圖

(a)青藏高原東緣新生代構(gòu)造和富堿巖漿巖分布圖(據(jù)Wangetal., 2001; 侯增謙等, 2004)；(b)滇西劍川研究區(qū)地質(zhì)圖(據(jù)趙永嘉等, 1974[注]趙永嘉, 劉禮國, 王義昭等. 1974. 蘭坪幅(G-47-X Ⅵ)1:20萬區(qū)域地質(zhì)調(diào)查報告; 王自康等, 1984[注]王自康, 張遠(yuǎn)志, 彭興階等. 1984. 維西幅(G-47-X)1:20萬區(qū)域地質(zhì)調(diào)查報告修改)及采樣位置

Fig.1 Geological map of Cenozoic alkali-rich magma along Jinsha River-Red River fault zone

(a) geologic map showing Cenozoic structure and distribution of the alkali-rich magmatic rocks in the eastern Tibetan Plateau (modified after Wangetal., 2001; Houetal., 2004); (b) geological map of the studied area with the location of samples

圖2 滇西劍川富堿巖漿巖正交偏光下顯微鏡照片(a)石英二長斑巖；(b)角閃正長斑巖；(c)花崗斑巖；(d)粗面巖. Q-石英；Pl-斜長石；Kf-鉀長石；Am-角閃石；Bi-黑云母；Di-透輝石Fig.2 Microphotos under cross-polarized lights of the alkali-rich magmatic rocks in the Jianchuan area of western Yunnan(a) quartz monzonite porphyry; (b) amphibole syenite porphyry; (c) granite porphyry; (d) trachyte. Qz-quartz; Pl-plagioclase; Kfs-K-feldspar; Am-amphibole; Bt-biotite; Di-diopside

2 測試方法

巖石樣品主微量元素在武漢上譜分析科技有限責(zé)任公司和北京大學(xué)造山帶與地殼演化教育部重點(diǎn)實(shí)驗(yàn)室完成測試。其中，主量元素用XRF法測定， 微量元素采用等離子質(zhì)

譜ICP-MS(Agilent 7700e)測定，主微量數(shù)據(jù)的質(zhì)量優(yōu)于10%。樣品Sr-Nd同位素分析在中國科學(xué)技術(shù)大學(xué)放射性成因同位素地球化學(xué)實(shí)驗(yàn)室完成，同位素比值的測試采用MAT-262熱電離質(zhì)譜計完成，Rb-Sr同位素比值測定采用Ta金屬帶和Ta發(fā)射劑；Sm-Nd比值測定采用Re金屬帶。標(biāo)準(zhǔn)溶液NBS987的重復(fù)測量結(jié)果為87Sr/86Sr=0.710249±0.000012(2σ, n=38)，標(biāo)準(zhǔn)溶液La Jolla重復(fù)測量結(jié)果為143Nd/144Nd=0.511869±0.000006(2σ, n=25)。Sr和Nd同位素比值測量精度優(yōu)于0.003%。測得同位素比值采用86Sr/88Sr=0.1194和146Nd/144Nd=0.7219進(jìn)行質(zhì)量分餾校正。重復(fù)分析標(biāo)準(zhǔn)溶液NBS 987和La Jolla，分別得到87Sr/86Sr值為0.710249±0.000012和143Nd/144Nd值0.511869±0.000006。全巖Rb-Sr、Sm-Nd分析的全流程本底分別是<200pg、<100pg。詳細(xì)的分析流程可以參見Chenetal. (2000)。

鋯石LA-ICP-MS U-Pb測年在中國地質(zhì)大學(xué)(北京)地學(xué)實(shí)驗(yàn)中心礦物激光微區(qū)分析實(shí)驗(yàn)室(MilmaLam)完成，儀器為NewWave193UC型準(zhǔn)分子激光器和Agilent 7900四級桿型等離子質(zhì)譜儀(ICP-MS)。分析中，年齡計算采用國際91500作為外標(biāo)，元素含量采用美國標(biāo)準(zhǔn)物質(zhì)局人工合成玻璃NIST610作為外標(biāo)，29Si作為內(nèi)標(biāo)元素進(jìn)行校正。數(shù)據(jù)校正采用ICPMSDataCal處理樣品的同位素比值和元素含量數(shù)據(jù)(Liuetal., 2008a, b)，普通鉛校正按Andersen (2002)完成，年齡計算和諧和圖繪制采用ISOPLOT(Ludwig, 2003)完成。

鋯石在完成定年后，進(jìn)行相同位置原位Hf同位素分析，在中國地質(zhì)科學(xué)院地質(zhì)研究所大陸構(gòu)造與動力學(xué)重點(diǎn)實(shí)驗(yàn)室利用Neptune Plus型多接收等離子質(zhì)譜和GeoLasPro 193nm激光剝蝕(LA-MC-ICP-MS)系統(tǒng)完成。測試過程中采用He作為剝蝕物質(zhì)載氣，剝蝕直徑采用44μm，測定時使用鋯石GJ-1作為參考， 實(shí)驗(yàn)過程中的相關(guān)儀器運(yùn)行條件及詳細(xì)的分析流程參見侯可軍等(2007)。分析過程中鋯石標(biāo)準(zhǔn)GJ-1的176Hf/177Hf測試加權(quán)平均值和誤差分別為0.282007±0.000025(2σ)。計算初始176Hf/177Hf時，Lu的衰變常數(shù)采用1.865×10-11/y(Schereretal., 2001)，εHf(t)值的計算時采用球粒隕石Hf同位素值176Lu/177Hf=0.0336，176Hf/177Hf=0.282785(Bouvieretal., 2008)。在Hf的地幔模式年齡計算中，虧損地幔176Hf/177Hf現(xiàn)在值采用0.28325，176Lu/177Hf采用0.0384(Griffinetal., 2000)，地殼模式年齡計算時采用平均地殼176Lu/177Hf=0.015(Griffinetal., 2002)。

表1劍川富堿巖漿巖主量(wt%)、微量元素(×10-6)與Sr-Nd同位素成分

Table 1 Whole-rock major (wt%), trace (×10-6) elements, Sr-Nd isotopic data of the Jianchuan alkali-rich magmatic rocks

樣品號14JC01-214JC01-314JP08-116JCS01-116JCB03-216JCB03-314JC02-114YL01-114LJS01-116JCS20-116JCS20-215SG01-415SG02-2巖性透輝石正長斑巖角閃正長斑巖粗面巖花崗斑巖石英二長斑巖系列鉀玄質(zhì)巖埃達(dá)克巖年齡(Ma)35.735.835.136.1SiO261.4962.1063.5154.8854.0253.9469.1469.9368.0167.9567.9268.9168.65TiO20.610.580.420.780.800.800.360.260.370.310.310.260.26Al2O315.4315.5315.5714.5814.3314.5015.2414.2014.7313.3013.3015.6915.36FeOT4.634.363.456.126.336.322.431.422.462.002.011.621.77MnO0.100.100.090.130.130.130.060.020.050.040.040.030.05MgO1.291.321.443.483.873.810.880.841.061.081.120.810.94CaO3.933.703.435.285.725.502.381.942.433.823.482.002.03Na2O4.104.264.713.752.892.654.513.494.123.123.095.225.08K2O6.816.586.126.707.117.103.874.243.933.713.704.514.70P2O50.310.300.230.750.780.790.190.070.170.150.150.140.13LOI0.340.310.241.811.981.900.373.282.054.153.860.390.53Total99.5599.6199.6098.9498.6598.1499.6999.8599.6799.8699.2199.7699.69Mg#33354351525239514449504749A/CNK0.730.740.750.630.630.660.961.020.950.830.860.920.90Li26.732.825.343.934.635.025.789.529.046.647.219.317.8Be6.556.437.6913.129.6315.023.473.234.093.413.343.012.81Sc11.8913.149.2013.2413.9213.855.303.636.565.415.422.922.78V1181108012313613544.0827.6746.2138.4237.9013.3921.71Cr41.3931.3769.2844.0355.6952.885.888.215.1317.3117.360.556.20Co11.1810.179.1019.3021.3721.234.712.675.314.924.572.722.70Ni19.0016.1027.2020.6324.9024.725.264.045.4410.5610.173.274.63Cu42.4629.048.9783.6686.0189.323.453.2612.697.557.381.951.97Zn72.266.649.010810811138.725.540.840.239.018.436.3Ga20.8720.5120.8821.3920.7322.2919.7920.3821.7518.0017.9620.5119.83Rb256266243230287316120153122126130123121Sr1685163116392080237723841471370111597496512331277Y28.828.324.933.2292.3155.913.910.316.212.011.78.87.9Zr361369380399339344141155154152152143133Nb26.7326.7429.2227.8324.0524.838.838.398.488.508.447.607.50Ba228022172140198426062873174592425511386137916541742Hf7.637.678.309.778.918.913.703.434.404.224.333.854.56Ta1.661.741.861.731.601.600.780.690.680.580.590.840.85Pb48.150.442.4106.7104.4111.445.023.257.238.938.949.944.6Th23.923.726.244.234.236.510.414.712.810.010.39.08.6U6.697.146.318.893.153.222.463.205.685.525.262.252.08La51.349.350.964.1139.8231.828.538.032.025.425.121.519.4Ce99.597.998.8120.5125.3137.955.062.160.147.146.841.137.4Pr12.1011.8711.7713.6223.4649.556.567.397.155.375.254.944.60Nd45.345.343.452.5101.8198.224.726.028.518.017.716.916.0Sm11.7511.289.9510.2221.3838.456.016.346.543.523.533.963.84Eu2.472.352.202.527.189.691.411.191.640.850.871.161.13Gd8.097.817.088.1038.6737.054.053.864.632.612.682.972.78Tb1.061.030.911.125.255.200.510.500.580.380.390.380.35Dy5.575.434.756.0630.5427.852.612.402.962.072.081.791.69Ho1.061.040.901.126.745.420.480.390.560.380.390.310.30Er2.882.812.443.0318.4315.051.280.971.461.061.010.810.75Tm0.440.440.380.451.981.920.190.150.220.160.150.120.11

續(xù)表1

Continued Table 1

樣品號14JC01-214JC01-314JP08-116JCS01-116JCB03-216JCB03-314JC02-114YL01-114LJS01-116JCS20-116JCS20-215SG01-415SG02-2巖性透輝石正長斑巖角閃正長斑巖粗面巖花崗斑巖石英二長斑巖系列鉀玄質(zhì)巖埃達(dá)克巖年齡(Ma)35.735.835.136.1Yb2.812.772.392.939.1012.341.181.041.391.081.040.750.74Lu0.430.430.350.441.351.930.170.160.210.150.150.110.11Eu/Eu?0.770.770.800.850.760.780.870.740.910.860.871.031.05Sr/Y58.6 57.6 65.8 62.6 8.1315.3 106 36.1 68.9 81.2 82.3 140 161 (La/Yb)N13131516111317261617172119Zr/Sm30.732.738.239.115.98.9423.524.523.543.143.236.234.7Nb/Ta16.115.415.716.115.015.511.312.112.514.614.49.098.82(87Sr/86Sr)i0.70700.70700.70710.70730.70590.7056143Nd/144Nd0.51250.51250.51240.51250.51250.5126εNd(0)-3.5-3.3-4.4-2.5-2.6-0.4εNd(36Ma)-3.3-3.0-4.2-2.2-2.2-0.2tDM (Ga)1.281.181.251.161.030.98

通過以上測試，本文獲得了13件樣品的主、微量元素和6件Sr-Nd同位素(表1)、4件樣品的鋯石U-Pb定年(表2)和鋯石Hf同位素(表3)測試結(jié)果。

3 結(jié)果

3.1 鋯石U-Pb年代學(xué)和Hf同位素

本文對劍川富堿巖漿巖體中的2件花崗斑巖(16JCs20-1、14YL01-2)和2件正長斑巖(14JP08-1、16JCs01-1)樣品進(jìn)行了鋯石U-Pb定年(表2、圖3)。這4件樣品中的多數(shù)鋯石為典型的巖漿鋯石，顯示較好的生長環(huán)帶(圖3)，部分呈不完整的棱角狀或渾圓狀。

花崗斑巖16JCs20-1中，測得18顆鋯石的Th/U比值為0.46～1.27(>0.40)，表明測定的鋯石均為巖漿成因(Hoskin and Schaltegger, 2003)，測點(diǎn)的206Pb/238U年齡比較集中為36.9～34.5Ma，加權(quán)平均年齡為36.1±0.2Ma(MSWD=1.1)(圖3a)。樣品14YL01-2中，選擇33顆鋯石開展測試，測點(diǎn)的206Pb/238U年齡比較分散，變化為2366～34.3Ma(圖3c)。其中，8顆新生代巖漿鋯石的Th/U比值為0.44～0.81，測點(diǎn)的206Pb/238U加權(quán)平均年齡為35.1±0.4Ma(MSWD=2.5)(圖3e)；繼承鋯石的206Pb/238U年齡介于2366～65.1Ma之間(圖3c)。透輝石正長斑巖14JP08-1中，16顆鋯石的Th/U比值為0.47～0.95(>0.40)，表明測定的鋯石為巖漿成因(Hoskin and Schaltegger, 2003)，測點(diǎn)的206Pb/238U年齡比較集中為37.5～34.9Ma，加權(quán)平均年齡為35.7±0.1Ma(MSWD=3.2)(圖3b)。角閃石正長斑巖16JCs01-1中，對32顆鋯石進(jìn)行測定，測點(diǎn)的206Pb/238U年齡比較分散，變化為2456～35.1Ma(圖3d)。其中，8顆新生代巖漿鋯石的Th/U比值為0.44～0.81，測點(diǎn)的206Pb/238U加權(quán)平均年齡為35.8±0.4Ma(MSWD=0.8)(圖3f)。繼承鋯石測點(diǎn)的206Pb/238U年齡介于2456～138Ma之間(圖3d)。

圖3 滇西劍川富堿巖漿巖體的鋯石U-Pb年齡諧和圖Fig.3 U-Pb concordia diagrams of the alkali-rich magmatic rocks from Jianchuan, western Yunnan

對已測年齡的4個樣品進(jìn)行鋯石Lu-Hf同位素分析，結(jié)果見表3?；◢彴邘r16JCs20-1中鋯石176Hf/177Hf為0.282787～0.282871，εHf(t)值為+1.3～+4.3，Hf同位素地殼模式年齡(tDMC)為0.84～1.03Ga。樣品14YL01-2中8顆巖漿鋯石176Hf/177Hf為0.282788～0.282878，εHf(t)值為+1.3～+4.5，Hf同位素地殼模式年齡(tDMC)為0.83～1.03Ga。透輝石正長斑巖14JP08-1中16顆鋯石176Hf/177Hf為0.282734～0.282852，εHf(t)值為-0.6～+3.6，Hf同位素地殼模式年齡(tDMC)為0.89～1.15Ga。角閃石正長斑巖16JCs01-1中8顆巖漿鋯石176Hf/177Hf為0.282705～0.282877，εHf(t)值為-1.6～+4.5，Hf同位素地殼模式年齡(tDMC)為0.83～1.22Ga。

3.2 全巖主微量元素

劍川花崗斑巖和石英二長斑巖的SiO2含量變化為67.92%～69.93%(表1)，除個別樣品落入石英二長巖范圍外，多數(shù)樣品落入了花崗巖區(qū)內(nèi)(圖4a)；正長斑巖和粗面巖的SiO2含量變化范圍為53.94%～63.51%(表1)，樣品主要落在正長巖和二長巖區(qū)內(nèi)(圖4a)。劍川富堿巖漿巖樣品均具有較高的K2O含量，花崗斑巖和石英二長斑巖的K2O含量變化范圍為3.70%～4.70%(表1)，在K2O-SiO2圖解中均落入高鉀鈣堿性巖石系列(圖4b)；而正長斑巖和粗面巖的K2O含量變化范圍為6.12%～7.10%(表1)，屬鉀玄質(zhì)巖石系列(圖4b)。

在球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖中(圖5a)，樣品強(qiáng)烈富集輕稀土(LREE)，具有明顯輕重稀土分餾特征。正長斑巖和粗面巖的(La/Yb)N為11～16(表1)，與區(qū)域基性巖具有一致的稀土元素配分模式(圖5a)；而花崗斑巖和石英二長斑巖的(La/Yb)N為16～26(表1)，具有更為顯著的輕重稀土分餾趨勢(圖5a)。同時，樣品都沒有明顯的Eu異常(0.74～1.05)(圖5a)，表明沒有斜長石的分離結(jié)晶。在原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖中(圖5b)，不同巖性的巖石均顯示大離子親石元素和Pb富集、高場強(qiáng)元素虧損等特征，其中Nb、Ta和Ti呈明顯的“TNT”負(fù)異常，暗示具有島弧巖漿的特征。此外，花崗斑巖和石英二長斑巖具有高的Sr(370×10-6～1471×10-6)、Sr/Y、La/Yb比值和低的Yb(0.74×10-6～1.39×10-6)、Y(7.92×10-6～16.2×10-6)含量(表1)，呈現(xiàn)埃達(dá)克質(zhì)巖漿的屬性。

表2劍川地區(qū)富堿巖漿巖鋯石LA-ICPMS U-Pb定年結(jié)果

Table 2 Zircon LA-ICPMS U-Pb data of the Jianchuan alkali-rich magmatic rocks

測點(diǎn)號含量(×10-6)ThUPbTh/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ16JCs20-1花崗斑巖011116150342.40.740.048410.001610.037140.001230.005570.000081204937135.80.5021071955.547.51.120.046290.001590.036350.001270.005710.00008134836136.70.5031862228773.50.810.049380.002490.038340.001870.005680.000091668338236.50.6041030137743.80.750.046340.002380.036360.001860.005750.00011167436236.90.705146.8196.95.60.750.049220.001570.038760.001410.005690.000081585839136.60.506593.3704.428.40.840.043360.001810.033690.001410.005650.00007-1047034136.30.4072062225485.10.910.047810.001940.036890.001360.005640.00007906037136.30.5081844144672.01.270.048790.002510.037840.001950.005650.000091388938236.30.6091092215949.80.510.04770.002140.037290.001590.005680.00011856337236.50.710796.4173032.10.460.04970.001660.038250.001210.00560.0000618155381360.411873.8110739.90.790.046990.001630.036280.001260.005610.000074953361360.512206.9439.4162.40.470.04570.002270.035460.001620.00570.00012-185635236.60.813340.8590.215.70.580.048210.002040.036950.001470.00560.0000711067371360.514141.6196.638.50.720.046140.001570.035630.00120.005610.0000654936136.10.415982.0110754.50.890.049850.001940.037820.001370.005540.000061886338135.60.416366.1477.415.20.770.045920.002220.034150.001760.005360.00008-68134234.50.517917.6125336.60.730.047280.001730.03650.001320.005610.000076359361360.418485.8660.419.50.740.046570.001910.035780.001460.005620.00008276236136.10.514YL01-2花崗斑巖0188.3486.98.30.180.162830.002268.21160.119710.365640.0043424851122551320092002882.6178326.70.490.069050.001121.435380.023910.150710.0018590016904109051003922.714689.80.630.160020.002328.918940.134820.404090.0048724561223301421882204968.6188513.90.510.046720.001940.035570.001440.005520.00006356635135.50.405449.2227413.30.200.051720.001420.336320.009110.047140.0006527337294729740640.4126.314.60.320.152550.002248.078960.124080.383960.0046523751222401420952207558.5105291.40.530.062880.001510.998670.023730.115140.001567042870312703908657.8933.313.40.700.046750.002040.035680.001520.005530.00006367136135.60.40971.0356.548.60.200.078040.002431.408750.041440.130930.001331148638931779381075.3688.377.50.110.160340.002819.178080.136630.415160.0038524593023561422381811222.8361.640.40.620.046920.002640.035440.001960.005480.000064512335235.20.412127.9915.4200.40.140.155290.002089.052850.110060.422710.003752405102343112273171341.6318.644.30.130.139590.002355.139960.072970.267050.002422222301843121526121415612030183.20.770.04920.001940.03740.001440.005510.000061577037135.40.41566.4336.927.70.200.076730.001461.981620.033220.18730.00169111439110911110791660.5235.551.90.260.066170.001041.162660.0170.127410.00116812167838773717151.0136765.70.110.071390.001181.600310.024610.162540.00159691797010971818639.9144463.40.440.046840.001580.042030.001380.006510.00007415242141.80.419124.8270.93.40.460.047480.001380.035870.001010.005480.00005734935.8135.20.320132.1943.911.00.140.061140.001070.90450.016410.107270.00137644196549657821506.6161917.10.310.066070.001291.345390.026830.147660.001958092186612888112275.01732643.30.040.052240.001020.308890.006210.042880.00056296242735271323361.8992.724.20.360.067010.000961.288060.016910.139380.0012483814840884172471.8951.98.20.080.067820.001171.159330.018680.123950.0011586319782975372530.959.52.40.520.051620.002150.072290.002950.010150.000112697471365.10.726388.1481.848.50.810.049850.001960.042110.001620.006120.000071886842239.30.427115.8472.971.10.240.06710.001061.300630.0190.140560.00128841168468848728342.8566.5263.70.610.046750.002010.034460.001450.005340.00006366934134.30.429147.1289.514.60.510.047310.001540.03480.001090.005340.00005657235134.30.330491.21148140.50.430.065450.001291.162720.020320.128850.00117789427831078173118451916272.90.960.066110.001021.200630.017160.131680.00119810168018797732617.8125757.90.490.112230.001395.185660.066750.333820.00252183613185011185712

續(xù)表2

Continued Table 2

測點(diǎn)號含量(×10-6)ThUPbTh/U同位素比值年齡(Ma)207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ33264.1430.2190.70.610.046720.001650.035480.001220.005510.00005355735135.40.314JP08-1透輝石正長斑巖01179.4195.17.10.920.05050.004290.036280.002560.005550.0001221812136335.70.80267.6119.74.40.560.056340.008020.039180.005780.005550.0001746627639636103171.5180.37.30.950.054450.004370.040120.002870.005580.0001139012540335.90.704125.6147.35.60.850.05260.004010.040220.002640.00560.00013312108403360.805101.9175.55.40.580.055530.004360.039150.002390.005570.000124349739235.80.80699.8145.55.70.690.048870.004250.035750.00280.005540.0001514112536335.610775.5133.54.00.570.046130.004950.031980.002730.005550.00014413932335.70.90895.7159.04.10.600.049830.004780.036270.002820.005540.0001418713136335.60.909139.0166.47.10.840.050030.004410.035240.002590.005470.0001319612435335.20.81083.7145.33.70.580.054260.00510.038950.003070.005530.0001438213439335.60.911136.2225.55.90.600.04810.003670.035650.002340.005580.000110411136235.90.712157.0211.78.50.740.050570.003610.037330.002280.00550.0001122110237235.40.71376.3114.45.70.670.056450.005440.038060.002730.005520.0001547011138335.511484.1129.63.90.650.049950.004570.037340.002870.005590.0001519312637335.90.91598.1143.85.40.680.050240.004710.035250.002370.005520.0001320611035235.50.81678.5168.15.60.470.050290.004490.035370.002350.005470.0001220811035235.20.816JCs01-1角閃正長斑巖0176.4677.4102.20.110.067940.001171.22030.024340.129810.00148672481011787802256.2568.4562.70.450.049750.00150.039260.001520.005680.000121835239136.50.80397.5463.483.80.210.066220.001261.178580.02460.128960.001658132379111782904235.1539.525.30.440.050570.002460.039170.001950.005690.00016221653923710574.9309.418.10.240.053480.00160.294510.011220.039720.0008834947262925150688.1564.797.80.160.065680.001041.238790.019360.136540.0009579621818982550753.0102091.60.050.064350.0010.828910.019270.092970.001487532461311573908110.2576.9102.50.190.067190.001041.19680.019610.128810.00116844207999781709264.4136010.00.190.052890.002080.158630.010430.021610.0011332469150913871036.61078128.70.030.066030.0011.15490.015180.126850.00095807327807770511140.6309.6325.50.450.047290.001590.035910.001190.005520.00007645336135.50.4121126139313700.810.046050.001820.035650.001190.005620.000128436136.10.813182.5100745.40.180.059370.001270.341810.018750.040790.0018758154299142581214129.81187177.30.110.066240.0011.205130.01740.131250.00107814178038795615179.0287.9279.20.620.047170.002290.036990.001780.005690.00009587537236.50.616332.2164213.10.200.176240.0019710.504790.194890.430510.0060726181424801723082717239.113389.60.180.117330.001535.915780.088270.364510.0029819161519641320041418503.1158111300.320.134980.001547.286760.087170.390360.0025621641221471121251219149.1495.2360.70.300.122450.001576.729980.088070.397510.0028319921320771221581320147.5662.4410.00.220.143710.003667.41850.150960.374380.0057322724521631820502721131.1265.2224.90.490.049880.001730.037970.00130.005530.000071895638135.60.422514.2145914.70.350.117250.001425.832450.069150.359880.0023119151219511019821123178.6212810200.080.142050.001587.706410.087390.392340.0025522521121971021341224346.3179613.70.190.155950.0019810.00060.13280.463790.0033224121324351224561525346.7150012.20.230.122810.001876.457670.108560.380250.0037119981720401520771726149.4318.1284.90.470.050050.003120.037280.002250.005450.000119710337235.10.727233.0656.4512.90.350.145410.001898.013320.126790.399020.005652293122233142165262881.01318546.90.060.117570.001315.744570.06170.352950.002161920111938919491029323.91104861.40.290.147750.001638.570670.133860.418610.005132320122293142254233081.31594407.20.050.117620.001543.927910.086530.240410.0038819201916191813892031396.9713.2824.50.560.048480.001960.037250.001470.00560.0000812366371360.53267.7903.2446.10.070.148990.001738.540130.136170.413440.00498233413229014223123

圖4 劍川富堿巖漿巖主量元素成分圖(a)硅-堿圖(Middlemost, 1994)；(b)硅-鉀圖(Peccerillo and Taylor, 1976)；(c-f)哈克圖解. 文獻(xiàn)數(shù)據(jù)：Guo et al., 2005; Huang et al., 2010; Lu et al., 2013, 2015; 洪濤等, 2015; 黃永高等, 2018Fig.4 Major elemental diagrams of the Jianchuan alkaline-rich magmatic rocks(a) total alkalis vs. silica (Middlemost, 1994), (b) potassium vs. silica (Peccerillo and Taylor, 1976), (c-f) Harker variation diagrams. Literature data: Guo et al., 2005; Huang et al., 2010, 2018; Lu et al., 2013, 2015; Hong et al., 2015

圖5 劍川富堿巖漿巖球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖(a,標(biāo)準(zhǔn)化值據(jù)Boynton, 1984)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b,標(biāo)準(zhǔn)化值據(jù)Sun and McDonough, 1989)數(shù)據(jù)來源：Guo et al., 2005; Huang et al., 2010; Lu et al., 2015Fig.5 Chondrite-normalized REE patterns (a, normalization values after Boynton, 1984) and primitive mantle-normalized trace-element spidergrams (b, normalization values after Sun and McDonough, 1989) of the alkali-rich magmatic rocks in JianchuanData sources: Guo et al., 2005; Huang et al., 2010; Lu et al., 2015

圖6 劍川富堿巖漿巖Sr-Nd同位素圖解(底圖據(jù)He et al., 2016)Fig.6 Sr-Nd isotopic diagram of the alkali-rich magmatic rocks in Jianchuan (base map after He et al., 2016)

3.3 Sr-Nd同位素地球化學(xué)

劍川富堿巖漿巖體Sr-Nd同位素測試結(jié)果顯示，兩類巖石均具有富集的Sr-Nd同位素成分(圖6)，其中，正長巖的87Sr/86Sr比值為0.7072～0.7073，εNd(t)為-4.2～-3.1，Nd同位素二階段模式年齡為1.18～1.28Ga(表1)；而花崗巖的87Sr/86Sr比值為0.7058～0.7080，εNd(t)為-2.2～-0.2，Nd同位素二階段模式年齡為0.98～1.16Ga(表1)。

4 討論

4.1 鋯石U-Pb年代學(xué)

本文測得劍川花崗巖年齡為36.1～35.1Ma，正長巖年齡為35.8～35.7Ma(圖3)，表明兩類巖石均形成于始新世，與前人研究的滇西金沙江-紅河富堿巖漿巖帶的埃達(dá)克質(zhì)花崗巖(北衙、馬廠箐、巍山、永勝和白蓮村等)和鉀玄質(zhì)正長巖(喇叭山、六合和賓川等)成巖時限一致。統(tǒng)計前人已發(fā)表的年齡數(shù)據(jù)，滇西新生代富堿巖漿巖形成時代集中分布37～32Ma(Luetal., 2012; Dingetal., 2016; Heetal., 2016; 劉金宇等, 2017; 黃永高等, 2018)。

測年結(jié)果還表明，鋯石老核的年齡在138～2456Ma(圖3c, d)之間，包括251～297Ma、753～971Ma、1857～1985Ma和2213～2456Ma等4個年齡組。這些年齡值反映繼承鋯石源于多種不同時代的巖石，與揚(yáng)子板塊西緣多期巖漿或變質(zhì)作用事件一致(洪濤等, 2015)，進(jìn)一步證實(shí)其揚(yáng)子的親緣性。

圖7 劍川富堿巖漿巖的埃達(dá)克巖判別圖(a) Sr/Y-Y圖；(b) La/Yb-Yb圖(Lu et al., 2013)；(c) SiO2-Mg#圖(Hou et al., 2004)；(d) Zr/Sm-Nb/Ta圖(Foley et al., 2002).文獻(xiàn)數(shù)據(jù)：Guo et al., 2005; Huang et al., 2010; Lu et al., 2013, 2015; 洪濤等, 2015; 黃永高等, 2018Fig.7 Adakite discrimination diagrams of the alkali-rich magmatic rocks in Jianchuan(a) Sr/Y vs. Y; (b) La/Yb vs. Yb (Lu et al., 2013); (c) SiO2 vs. Mg# (Hou et al., 2004), and (d) Zr/Sm vs. Nb/Ta (Foley et al., 2002). Data sources: Guo et al., 2005; Huang et al., 2010, 2018; Lu et al., 2013, 2015; Hong et al., 2015

4.2 巖石成因

4.2.1 埃達(dá)克質(zhì)花崗巖成因

劍川的花崗斑巖和石英二長斑巖具有高SiO2、Al2O3、Sr、Sr/Y、La/Yb和低的Y、Yb含量(表1)，呈現(xiàn)埃達(dá)克質(zhì)巖漿的屬性(圖7a)。目前，埃達(dá)克質(zhì)巖漿的成因主要有4種解釋：(1)俯沖板片部分熔融的產(chǎn)物(Defantetal., 1990)；(2)直接來源于交代地幔楔的初始玄武巖漿經(jīng)歷分離結(jié)晶和地殼混染的產(chǎn)物(Castilloetal., 1999; Macphersonetal., 2006)；(3)拆沉下地殼的部分熔融(Kay and Kay, 1993; Xuetal., 2002)；(4)增厚基性下地殼的部分熔融(Atherton and Petford, 1993; Chungetal., 2003; Houetal., 2004)。

源于俯沖板片部分熔融的埃達(dá)克質(zhì)巖漿通常富鈉(K2O/Na2O<0.4)，且Sr-Nd同位素特征與MORB相似(Martinetal., 2005)。但是劍川花崗巖有著高的K2O/Na2O比值(0.86～1.22)和明顯不同于MORB的Sr-Nd同位素特征(圖6)，表明該埃達(dá)克質(zhì)巖漿并不是俯沖板片熔融的產(chǎn)物。然而，交代地幔直接熔融形成的是低硅埃達(dá)克質(zhì)巖漿，通常有低SiO2(<60%)和高M(jìn)gO含量(>4.0%)(Martinetal., 2005)。這些特征與劍川花崗巖的特征明顯不一致(表1)，應(yīng)排除此種巖漿成因。此外，已有地震研究資料表明現(xiàn)今滇西地殼厚度為40～55km(Xuetal., 2007; Lietal., 2008)。趙欣等(2003)利用滇西富堿侵入巖體中基性捕擄體成分估算得古新世地殼厚度大約為55km，說明自古新世以來滇西地殼厚度沒有發(fā)生明顯減薄。因此，拆沉下地殼部分熔融形成劍川埃達(dá)克質(zhì)巖漿的可能性也可以排除。那么劍川埃達(dá)克質(zhì)巖漿最可能的成因是形成于增厚的基性下地殼的部分熔融，這一認(rèn)識也獲得同位素和微量元素模擬的支撐(圖6、圖7a, b)。

劍川花崗巖的Sr-Nd同位素組成和同時期富堿侵入巖中角閃巖捕擄體同位素相似，這種角閃巖捕擄體可能代表了揚(yáng)子西緣新生下地殼的組分(Houetal., 2017; 周曄等, 2017)。在(87Sr/86Sr)i-εNd(t)圖解中(圖6)花崗巖均落在角閃巖與金沙江洋中脊玄武巖(MORB)的演化線上。同時，花崗巖具有高SiO2含量和低Mg#值，在SiO2-Mg#圖中(圖7c; Houetal., 2004)樣品均落在實(shí)驗(yàn)獲得的增厚下地殼熔融成因的區(qū)域。樣品微量元素結(jié)果顯示高Sr/Y和La/Yb，低Y和HREE含量(圖7)，表明熔融過程中石榴石作為主要?dú)埩粝鄬?dǎo)致重稀土明顯分異(Rappetal., 1995)。相對平坦的HREE配分模式說明在埃達(dá)克質(zhì)巖漿形成過程中角閃石相較石榴石占據(jù)主導(dǎo)作用(Moyen, 2009)。且樣品具有低的Nb/Ta比值，在Zr/Sm-Nb/Ta圖中(圖7b; Foleyetal., 2002)樣品均落在角閃巖部分熔融區(qū)域。說明劍川花崗巖很可能源自石榴角閃巖相增厚下地殼的部分熔融。

表3劍川富堿巖漿巖的Hf同位素成分

Table 3 Zircon Hf isotopic data of the Jianchuan alkali-rich magmatic rocks

測點(diǎn)號176Yb/177Hf176Lu/177Hf176Hf/177Hf±2σ176Hf/177Hf (i)εHf(0)εHf(t)tDM (Ma)tDMC (Ma)fLu/Hf16JCs20-1花崗斑巖,t=36.1Ma10.0203950.0008230.282790130.2827890.61.46521025-0.9820.0210610.0008480.282815160.2828151.52.3617968-0.9730.0164760.0006690.282818150.2828171.62.4610962-0.9840.0220260.0008150.282837160.2828372.33.1585917-0.9850.0282710.0011370.282813140.2828121.42.2625974-0.9760.0213210.0008370.282845120.2828442.63.3575901-0.9770.0266780.0009270.282855160.2828552.93.7561877-0.9780.0228300.0009030.282801130.2828001.01.86381000-0.9790.0222230.0008820.282787130.2827870.51.36571031-0.97100.0273910.0010630.282807150.2828061.22.0632986-0.97110.0208800.0008330.282796130.2827950.81.66441012-0.97120.0302020.0011570.282788170.2827870.61.36611030-0.97130.0191220.0007510.282805160.2828041.21.9630991-0.98140.0212950.0007940.282791130.2827910.71.56491022-0.98150.0144090.0005690.282805140.2828041.21.9627991-0.98160.0225390.0009130.282822150.2828221.82.6608952-0.97170.0191940.0007590.282801160.2828001.01.86351000-0.98180.0398780.0013270.282871290.2828703.54.3545842-0.9614YL01-2花崗斑巖,t=35.1Ma40.0243660.0006340.28282200.2828221.82.5604953-0.9880.0235540.0006440.28278800.2827880.61.36511030-0.98110.0288340.0006830.28287800.2828783.84.5525826-0.98140.0123110.0004390.28279400.2827940.81.66391015-0.99190.0545870.0017110.28281800.2828171.62.4626962-0.95280.0207740.0007520.28282400.2828231.82.6603949-0.98290.0328220.0009210.28283600.2828362.33.0588921-0.97330.0082770.0002680.28285000.2828492.73.5559890-0.9914JP08-1透輝石正長斑巖,t=35.7Ma10.0223930.0007370.282819260.2828181.72.4610960-0.9820.0171240.0006180.282775210.2827740.10.96691059-0.9830.0097600.0003820.282770250.282770-0.10.76721070-0.9940.0139290.0005170.282793230.2827930.81.56421017-0.9850.0136700.0005260.282734210.282734-1.3-0.67251151-0.9860.0155450.0005380.282754240.282753-0.60.16981107-0.9870.0099210.0003750.282816220.2828161.52.3608966-0.9980.0213620.0007160.282756230.282756-0.60.26971101-0.9890.0100100.0003860.282743200.282743-1.0-0.27091130-0.99100.0143320.0004720.282763270.282763-0.30.56831085-0.99110.0136940.0004900.282785220.2827850.51.26531036-0.99120.0129170.0005710.282852170.2828512.83.6561885-0.98130.0243960.0011130.282809230.2828081.32.1630983-0.97140.0152050.0006290.282836180.2828362.33.0584920-0.98150.0197710.0007740.282766220.282765-0.20.56851079-0.98160.0184690.0007270.282800190.2827991.01.76371003-0.9816JCs01-1角閃正長斑巖,t=35.8Ma20.0335140.0012790.282877130.2828763.74.5536829-0.9630.0163400.0007030.282863140.2828633.24.0547859-0.98110.0266810.0010220.282854160.2828542.93.7564880-0.97120.0503050.0018280.282705180.282703-2.4-1.67931219-0.94150.0194160.0008400.282841140.2828412.43.2580909-0.97210.0516240.0019080.282871130.2828703.54.3553842-0.94260.0572330.0019370.282838160.2828372.33.1602918-0.94310.0457610.0011670.282775190.2827740.10.86801060-0.96

圖8 劍川富堿巖漿巖鋯石U-Pb年齡-εHf(t)同位素圖解文獻(xiàn)數(shù)據(jù)：Lu et al., 2013; Ding et al., 2016. 揚(yáng)子克拉通西緣中-古生代的巖漿鋯石數(shù)據(jù)來源: Huang et al., 2009; Sun et al., 2009; Xu et al., 2008Fig.8 Zircon U-Pb age vs. εHf(t) diagram of the alkali-rich magmatic rocks in JianchuanData sources: Lu et al., 2013; Ding et al., 2016. Data for Pre-Cenozoic magmatic zircons from western Yangtze Craton are from Huang et al., 2009; Sun et al., 2009; Xu et al., 2008

新元古代是揚(yáng)子板塊西緣新生下地殼的主要形成時期(Sunetal., 2009)?；◢弾r鋯石εHf(t)值為+1.3～+4.5，相應(yīng)的二階段模式年齡為0.83～1.03GMa(表3、圖8)，表明該埃達(dá)克質(zhì)巖漿是新元古代形成的新生鎂鐵質(zhì)下地殼熔融的產(chǎn)物。綜上所述，具有埃達(dá)克質(zhì)屬性的劍川花崗巖是增厚的石榴角閃巖相新生下地殼部分熔融的產(chǎn)物。此外，與典型的埃達(dá)克巖相比，劍川埃達(dá)克質(zhì)花崗巖具有高鉀的特征，可能是區(qū)域鉀質(zhì)-超鉀質(zhì)巖漿注入到新生下地殼熔體中發(fā)生巖漿混合作用的結(jié)果(Guoetal., 2007; Yangetal., 2015)。

4.2.2 正長巖成因

劍川正長斑巖和粗面巖具有較低的SiO2含量(53.94%～63.51%)，較高的K2O含量(6.12%～7.10%)和K2O/Na2O比值(1.30～2.68)，屬鉀玄質(zhì)的正長巖。前人對正長巖類的成因提出多種模式：(1)高壓條件下長英質(zhì)地殼物質(zhì)的熔融(Huang and Wyllie, 1981)；(2)與俯沖相關(guān)的交代富集巖石圈地幔的熔融(Lynchetal., 1993)，或堿性玄武質(zhì)巖漿分離結(jié)晶的產(chǎn)物(Thorpe and Tindle, 1992; Yangetal., 2005)；(3)巖漿混合作用形成，幔源鎂鐵質(zhì)巖漿與其誘發(fā)熔融的長英質(zhì)巖漿混合后結(jié)晶分異的產(chǎn)物(Riishuusetal., 2005; 楊進(jìn)輝等, 2007)，或幔源硅不飽和的堿性巖漿和殼源花崗質(zhì)巖漿混合形成(Dorais, 1990)。目前多數(shù)學(xué)者主張正長巖的形成有幔源組分的貢獻(xiàn)(Harrisetal., 1999)。

劍川正長巖可能是交代富集的巖石圈地幔部分熔融形成的基性巖漿(滇西鉀質(zhì)-超鉀質(zhì)巖)演化的產(chǎn)物，證據(jù)如下：(1)滇西新生代正長巖與基性鉀質(zhì)-超鉀質(zhì)巖在時空分布上具有非常密切的關(guān)系(圖1a)。時間上，前人通過全巖、黑云母和鉀長石等礦物的40Ar-39Ar定年獲得區(qū)域新生代基性巖的形成時代為36.6～32.6Ma(Guoetal., 2005; 李勇, 2012; Huangetal., 2010; Luetal., 2015)，其中劍川江尾塘橄欖粗玄巖(JC-JWT)獲得全巖40Ar-39Ar年齡為32.6Ma(李勇, 2012)，與正長巖鋯石U-Pb年齡一致(36.5～33.4Ma; Luetal., 2012)?？臻g上基性巖總是與正長巖伴生(圖1)，在劍川江尾塘、新華、馬登和甸心村等地出露有始新世橄欖粗玄巖、堿性玄武質(zhì)和煌斑巖等(李勇, 2012; Luetal., 2015)。(2)在哈克圖解中，正長巖和基性巖的K2O、MgO、CaO、Fe2O3T和Al2O3與SiO2都呈現(xiàn)較好的線性關(guān)系(圖4b-f)，反映兩者之間連續(xù)的巖漿演化過程。同時，正長巖和基性巖具有相似的、平行的稀土和微量元素配分模式。在球粒隕石標(biāo)準(zhǔn)化圖中(圖5a)，兩者均強(qiáng)烈富集輕稀土(LREE)，具有明顯的輕重稀土分餾特征，且樣品都沒有明顯的Eu異常。在微量元素原始地幔歸一化圖中(圖5b)，均顯示LILEs和Pb富集、HFSEs虧損等特征。這一系列相似的元素特征說明兩者具有一致的巖漿源區(qū)。(3)正長巖與基性巖具有相似的Sr-Nd同位素組成(圖6)。按照Depaolo and Wasserburg (1976) 計算方法可知t=36Ma時，正長巖εSr(t)為+35.40～+38.04，相應(yīng)εNd(t)在-3.05～-4.16之間，而正的εSr(t)和負(fù)的εNd(t)性質(zhì)為富集地幔所特有的同位素特征。

4.3 地球動力學(xué)意義

前人研究認(rèn)為，印度-亞洲大陸約在新生代初開始碰撞后，青藏高原東緣進(jìn)入隆升和大規(guī)模的陸內(nèi)變形時期，形成長達(dá)1000km的金沙江-紅河走滑拉分?jǐn)嗔褞?。沿該斷裂帶及其兩?cè)較寬的范圍內(nèi)，成群產(chǎn)出鉀質(zhì)-超鉀質(zhì)火山巖和中酸性富堿高鉀的淺成侵入巖，即金沙江-紅河富堿巖漿巖帶(張玉泉等, 1987; 鄧萬明等, 1998)。已有的不同方法測得年代學(xué)數(shù)據(jù)顯示其成巖年齡集中分布在37～32Ma之間(Guoetal., 2005; Huangetal., 2010; Luetal., 2012, 2015; Dingetal., 2016)。劍川花崗巖和正長巖的侵位年齡(36～35Ma)與金沙江-紅河富堿巖漿巖帶巖漿活動峰期年齡高度一致，為始新世晚期。

研究區(qū)構(gòu)造上位于鹽源-金平富堿斑巖銅金成礦帶，劍川富堿巖漿巖成巖峰期年齡與北衙等多金屬礦集區(qū)成礦相關(guān)富堿斑巖的時代相近(Luetal., 2012; Houetal., 2017)。且劍川花崗巖具有與區(qū)域成礦富堿斑巖相似的埃達(dá)克質(zhì)巖漿屬性和同位素特征(侯增謙等, 2004, 2015; Houetal., 2017)。此外，研究區(qū)已有桃花鐵銅礦床和桃花鉛鋅多金屬礦床(陳喜峰等, 2013; 周云鳳和李治平, 2013)的相關(guān)報道；在野外地質(zhì)調(diào)查過程中也發(fā)現(xiàn)石鼓鎮(zhèn)桃花巖體(石英二長斑巖)中局部發(fā)育大量的黃鐵礦化和接觸帶角巖化-硅化-絹英巖化等特點(diǎn)。新一輪的區(qū)域地質(zhì)調(diào)查顯示(黃永高等, 2018)，劍川-石鼓等地發(fā)現(xiàn)明顯的Au、Pb、Zn、Cu、Ag等礦化元素異常，且在與金沙江斷裂帶平行的NW-SE向呈串珠狀展布，具有以斑巖體與圍巖接觸帶為中心的不對稱帶狀分布特點(diǎn)。可以推測，在劍川石鼓地區(qū)的花崗巖可能具有形成斑巖-熱液型多金屬礦床的成礦潛力。

花崗巖和正長巖在滇西形成時代一致，在空間分布上密切伴生，表明兩者受控于相同的構(gòu)造過程。如上述討論，正長巖和基性巖巖漿的形成都需要交代巖石圈地幔的部分熔融過程。同時，埃達(dá)克質(zhì)巖漿起源于增厚基性下地殼的部分熔融。交代大陸巖石圈地幔和增厚下地殼的熔融過程都需要大量的地幔熱(Thompson and Connolly, 1995; Petfordetal., 2000)。然而，按照現(xiàn)今地?zé)崽荻仍谙碌貧ず蛶r石圈地幔深度積累的放射熱無法提供誘發(fā)熔融的熱量(Turneretal., 1993; Houetal., 2004)。劍川花崗巖和正長巖鋯石測年結(jié)果顯示形成于36～35Ma，屬于印亞大陸碰撞的晚碰撞階段(侯增謙等, 2006a, b)，在此時期滇西并無洋殼俯沖作用發(fā)生，所以不存在洋殼脫水熔融。那么，最可能導(dǎo)致巖石圈地幔和增厚下地殼熔融的機(jī)制是巖石圈地幔的減薄，使熱的軟流圈物質(zhì)減壓、上涌(Thompson and Connolly, 1995)，從而提供巨量的熱。巖石圈地幔減薄可以通過兩種方式實(shí)現(xiàn)：巖石圈地幔的整體拆沉(Bird, 1979; Kay and Kay, 1993)或巖石圈下部發(fā)生對流減薄(England and Houseman, 1989; Platt and England, 1994; Houseman and Molnar, 1997)。在滇西，鉀質(zhì)-超鉀質(zhì)巖漿起源于富集巖石圈地幔(Guoetal., 2005; Huangetal., 2010; Luetal., 2015)，暗示巖石圈地幔并未被全部剝離。從區(qū)域而言鉀質(zhì)-超鉀質(zhì)巖漿體積較小，也未有源于軟流圈的巖漿活動報道，所以巖石圈地幔整體拆沉模式在研究區(qū)并不適用。同時，在滇西地區(qū)晚始新世-漸新世時期形成大量伸展背景下的裂谷盆地(圖1a; Chungetal., 1998)，可能是深部巖石圈地幔對流減薄的響應(yīng)(England and Houseman, 1989; Platt and England, 1994)。因此，巖石圈地幔減薄的機(jī)制最有可能是地幔的下部發(fā)生對流減薄，從而導(dǎo)致軟流圈物質(zhì)上涌，使殘留的巖石圈地幔和增厚下地殼發(fā)生部分熔融。

揚(yáng)子板塊西緣之下的巖石圈地幔在1000～800Ma經(jīng)歷了原特提斯洋板塊俯沖(Zhang, 2017)，受俯沖流體的交代作用影響，形成了富集的巖石圈地幔(Zhouetal., 2002a, b)。自新生代印度與歐亞大陸發(fā)生碰撞，使滇西地殼和巖石圈大幅度縮短并加厚(Xuetal., 2007; Lietal., 2008)。在37～32Ma之間，加厚的巖石圈超過最大穩(wěn)定性，巖石圈地幔發(fā)生減薄，熱的軟流圈物質(zhì)上涌。地球物理研究也證實(shí)，新生代滇西存在著軟流圈地幔物質(zhì)上涌(劉建華等, 1989)，支持了巖石圈地幔減薄的觀點(diǎn)。在這一過程中上涌的熱的軟流圈取代了冷的巖石圈地幔的下部，使殘留的巖石圈地幔發(fā)生熔融形成了劍川地區(qū)的基性巖漿(煌斑巖和鉀質(zhì)-超鉀質(zhì)火山巖)，經(jīng)分離結(jié)晶演化形成正長巖。幔源熔體因密度差而在地殼底部發(fā)生底墊，并且幔源的熱的基性巖漿進(jìn)入增厚的下地殼，誘發(fā)其發(fā)生部分熔融形成了具有埃達(dá)克質(zhì)巖漿屬性的花崗巖。

5 結(jié)論

(1)劍川富堿巖漿巖體鋯石U-Pb定年得成巖年齡為35.1～36.1Ma，是滇西金沙江-紅河富堿巖漿巖帶內(nèi)巖漿活動主要時期的產(chǎn)物。

(2)劍川富堿巖漿巖體據(jù)礦物組合和地球化學(xué)特征可劃分為花崗巖和正長巖兩種巖石類型，都具有富堿、高鉀的地球化學(xué)特征?；◢弾r起源于增厚的鎂鐵質(zhì)新生下地殼的部分熔融，正長巖是交代富集的巖石圈地幔熔融產(chǎn)生的基性巖漿演化的產(chǎn)物。

(3)劍川花崗巖和正長巖形成于印度與歐亞大陸碰撞的晚碰撞階段。是由巖石圈地幔發(fā)生對流減薄導(dǎo)致了軟流圈物質(zhì)的上涌，從而使殘留的巖石圈地幔和增厚下地殼發(fā)生熔融，并經(jīng)一定程度的分異演化，最后在淺成環(huán)境侵位的產(chǎn)物。

致謝感謝唐菊興研究員、張開均研究員和另一位匿名評審專家提出寶貴的修改意見。本研究在野外調(diào)查和室內(nèi)實(shí)驗(yàn)分析工作中得到中國地質(zhì)大學(xué)(北京)馬睿、張馳、李鑫、劉思岐和高雷等同學(xué)的熱心幫助，在此表示衷心感謝！