焦騫騫 賀昌坤 董有浦 許德如 陳根文 陳誠 師爽 高亦文

1. 昆明理工大學(xué)國土資源工程學(xué)院，昆明 6500932.東華理工大學(xué)核資源與環(huán)境國家重點實驗室，南昌 3300133.中國科學(xué)院廣州地球化學(xué)研究所，中國科學(xué)院礦物學(xué)與成礦學(xué)重點實驗室，廣州 5106404. 廣東省地質(zhì)局第五地質(zhì)大隊，肇慶 5266001.

兩廣交界(粵西-桂東南)的云開大山地區(qū)位于揚(yáng)子板塊與華夏板塊之間的過渡地帶。由于其所處的特殊大地構(gòu)造位置，因此不同學(xué)者曾先后使用云開隆起(廣西壯族自治區(qū)地質(zhì)礦產(chǎn)局, 1985; 汪勁草等, 1994)、云開地體(康云驥, 2001; Wanetal., 2010)、云開褶皺帶(郭福祥, 1994)、云開地塊(Wangetal., 2007a)、云開構(gòu)造帶(Wangetal., 2007b)、云開造山帶(Liang and Li, 2005; Lietal., 2010; Fengetal., 2014)等術(shù)語概括其大地構(gòu)造屬性。揚(yáng)子板塊與華夏板塊自新元古代拼合形成華南板塊后，又至少先后經(jīng)歷了加里東期、印支期和燕山期三期構(gòu)造-熱事件(Wangetal., 2011)。受多期構(gòu)造-熱事件的影響，云開地區(qū)加里東期、印支期和燕山期花崗巖均有出露，并且還發(fā)育大面積混合巖和多條韌性剪切帶。因此，云開地區(qū)是國內(nèi)研究花崗巖與混合巖巖石成因關(guān)系的一個良好場所而受到關(guān)注(張伯有和俞鴻年, 1992; 王聯(lián)魁等, 2003)。

花崗巖與混合巖之間可能的成因聯(lián)系是成因巖石學(xué)研究的重要課題(Johannesetal., 2003; Sepahietal., 2013; Sugaetal., 2016)，而巖漿的形成、分離結(jié)晶、上升、侵位，是地殼物質(zhì)及能量從深部到淺部的重要交換過程，對研究大陸地殼演化有重要意義，因此前人對此進(jìn)行過大量研究(England and Thompson, 1986; Simpsonetal., 2000; Solar and Brown, 2001; Annenetal., 2006; Brown, 2007; Qiuetal., 2014)。混合巖多在深熔作用下產(chǎn)于變質(zhì)程度高的地層中(Sawyer, 1998, 2001)，由于原巖在不同溫-壓條件下發(fā)生熔融和分離結(jié)晶的程度有所不同，因此形成的混合巖的巖石結(jié)構(gòu)構(gòu)造、礦物組成及化學(xué)成分也有所差異，總的可以分為兩大類，即變?nèi)刍旌蠋r和全熔混合巖(Brownetal., 1995; Milordetal., 2001; Solar and Brown, 2001; Sugaetal., 2016)。變?nèi)刍旌蠋r熔融程度較低，整體還處于固態(tài)，仍保留了部分熔融前的原巖結(jié)構(gòu)，長英質(zhì)脈體(熔融體)與暗色基體(殘留體)間呈厘米級間隔(Sawyer, 1994, 2001; Brown, 1994; Brownetal., 1995)。而全熔混合巖中，原巖熔融程度和熔體分離程度都較高，結(jié)構(gòu)幾乎完全破壞，巖石結(jié)構(gòu)均一化，礦物顆粒增大，殘留的原巖固體部分已失去凝聚力，與熔融體一起形成高粘度的“晶粥”，或者成為能遷移的塑性體，其流變學(xué)特征已類似于巖漿(Sawyer, 1998; Milordetal., 2001; Whiteetal., 2005; Sugaetal., 2016)。Milordetal. (2001)根據(jù)巖石中暗色礦物(主要是黑云母)含量，將全熔混合巖分為三類：深色、中色和淺色全熔混合巖，黑云母含量分別為>30%、10%～30%和<10%。重熔花崗巖一般是淺色的，在成因上可能與其周圍的混合巖有關(guān)，是混合巖中熔融部分與殘余部分分離后在合適的位置聚集結(jié)晶而成(Milordetal., 2001; Solar and Brown, 2001)，另外可能還有一些外部巖漿及含水流體的加入，從而增加了熔體的量(Johannesetal., 2003)。然而，在云開地區(qū)，混合巖周邊卻分布著一些暗色的S型花崗巖巖體，例如在河臺金礦區(qū)西北部出露的印支期云樓崗巖體，它在成因上與河臺金礦區(qū)中大量的混合巖是什么關(guān)系呢？張伯有和俞鴻年(1992)認(rèn)為云開地區(qū)混合巖是由糜棱巖改造而來，而花崗巖則是混合巖化的最終產(chǎn)物，但是缺少相應(yīng)的年代學(xué)及地球化學(xué)證據(jù)。王聯(lián)魁等(2003)按形成方式將花崗巖分為三類，即混合巖建造、深熔花崗巖建造和巖漿花崗巖建造，并對云開地區(qū)花崗巖的巖石學(xué)、地球化學(xué)特征及形成條件進(jìn)行了較為詳細(xì)的研究，但是并未深入討論它們之間的成因關(guān)系。因此，本文將以云開地區(qū)河臺金礦區(qū)內(nèi)云開群混合巖及云樓崗巖體為研究對象，通過巖石學(xué)、礦物學(xué)、地球化學(xué)、地質(zhì)年代學(xué)的研究，探討暗色的S型花崗巖與混合巖之間的成因關(guān)系，從而進(jìn)一步豐富巖漿巖的成因巖石學(xué)理論，并為云開地區(qū)大地構(gòu)造演化提供新的證據(jù)。

圖1 華南地區(qū)大地構(gòu)造及印支期花崗巖分布圖(a, 據(jù)Qiu et al., 2014修改)和云開地區(qū)地質(zhì)簡圖(b, 據(jù)丘元禧和梁新權(quán), 2006; 彭松柏等, 2006修改)

1 構(gòu)造背景

華南板塊由揚(yáng)子板塊和華夏板塊構(gòu)成，其界線為江山-紹興縫合帶(圖1a)，但由于多期強(qiáng)烈構(gòu)造事件的疊加，該縫合帶向南西延伸逐漸模糊不清，導(dǎo)致兩板塊的南西邊界有較大爭議(圖1a, Yanetal., 2006; Wan, 2012)。云開地區(qū)位于華夏板塊的西北部邊界，區(qū)域內(nèi)出露的云開群和高州雜巖過去通常被作為華夏地塊的前寒武變質(zhì)基底，其上覆未變質(zhì)-弱變質(zhì)的奧陶系-白堊系(圖1b)，然而，最新研究表明，這兩套地層實為同一套地層，統(tǒng)稱為云開群，兩者沉積時代相同，為晚新元古代-早古生代，并非古元古代-早新元古代(葉真華等, 2000; 鄺永光等, 2001; Wangetal., 2007a; Wanetal., 2010; Chenetal., 2012; 周雪瑤等, 2015; 焦騫騫等, 2017)。云開群變質(zhì)程度由綠片巖相到麻粒巖相，巖性為片巖、千枚巖、板巖，局部為片麻巖、角閃巖、麻粒巖、混合巖。鋯石U-Pb定年表明，云開群經(jīng)歷了兩期變質(zhì)作用，一期集中在約440Ma，與加里東運(yùn)動有關(guān)；在印支期約240Ma時，云開群局部又發(fā)生了再活化，形成變質(zhì)程度更高的巖石，例如，混合巖、硅線石-石榴子石-堇青石片麻巖(Wangetal., 2007a, 2012; Wanetal., 2010)。

云開地區(qū)有大量花崗質(zhì)侵入體，時代從加里東期到燕山期均有分布。信宜花崗巖鋯石U-Pb年齡為460～430Ma，其形成與加里東期造山活動有關(guān)(圖1b, 彭松柏等, 2006; Wangetal., 2007a; Wanetal., 2010)。云開地區(qū)出露的印支期S型花崗巖年齡集中在260～245Ma，少量為230Ma，并且主要沿斷裂分布(圖1b, Chenetal., 2011)。例如，在廣寧-羅定動力變質(zhì)帶北東段，特別是河臺金礦區(qū)附近(邱小平, 2004)；在防城-靈山斷裂帶附近大容山-十萬大山也有大量S型花崗巖(祁昌實等, 2007; Jiaoetal., 2015)。在燕山期侵入形成大量未變形的I型花崗巖沿著云開地區(qū)西南邊緣分布(蔡明海等, 2002; 邱小平, 2004; Wangetal., 2007a; Linetal., 2008)。區(qū)域上構(gòu)造線以NE-NNE方向為主，可見幾條近于平行的剪切帶系統(tǒng)，例如防城-靈山斷裂F1，羅定-廣寧斷裂F2，吳川-四會斷裂F3等(圖1b)。這些區(qū)域性大斷裂(韌性剪切帶)主要是在印支期造山作用下經(jīng)過約248～220Ma和220～200Ma兩期構(gòu)造活動形成的(Wangetal., 2007a; 丁汝鑫等, 2015; Jiaoetal., 2017)，并且控制著區(qū)域內(nèi)礦產(chǎn)的分布和產(chǎn)出。例如，沿著羅定-廣寧斷裂有新洲金礦區(qū)、河臺金礦區(qū)、羅定金礦區(qū)的分布，其中河臺金礦區(qū)是本文的主要研究對象。

2 礦區(qū)地質(zhì)及巖石學(xué)特征

河臺礦區(qū)及外圍出露的地層主要有云開群、奧陶系、志留系(圖2)。云開群分布在礦區(qū)北部，為一套整體無序的變質(zhì)巖組成，巖性以變粒巖、片麻巖、混合巖為主，局部遭受強(qiáng)烈韌性剪切作用形成糜棱巖系列巖石，河臺金礦的礦體就產(chǎn)于這些糜棱巖帶(ML9、ML11、ML12、ML13、ML18)中。奧陶系與志留系分布在礦區(qū)南部，以薄層淺變質(zhì)砂巖、粉砂巖、及薄層板巖為主，通過F1斷裂與云開群地層接觸。礦區(qū)西部出露印支期云樓崗巖體；礦區(qū)東北部為燕山期的伍村巨斑狀黑云母花崗巖，單顆粒鋯石U-Pb年齡153.6±2.1Ma(翟偉等, 2005)。

圖2 河臺礦區(qū)地質(zhì)圖(據(jù)陳駿和王鶴年, 1993修改)

2.1 云樓崗巖體巖石學(xué)特征

云樓崗巖體出露面積大于100km2，巖性主要為中粗粒黑云母二長花崗巖、黑云母斜長花崗巖，具花崗結(jié)構(gòu)、塊狀構(gòu)造(圖3a, b)，局部受剪切形成糜棱巖化花崗巖。巖石主要由長石(55%)、石英(20%)、黑云母(20%)和白云母(5%)組成(圖3c, d)。其中黑云母呈自形-半自形，與長石和石英近于同時形成，在其邊部分布有較小的白云母。前人利用全巖Rb-Sr法、單顆粒鋯石全熔法等獲得年齡為242～209Ma(崔遙, 1989; 葉伯丹, 1989)。由于這些方法精度不高或者缺少鋯石的形態(tài)結(jié)構(gòu)特征，因此可靠性難以評價。本次研究將利用LA-ICP-MS鋯石U-Pb法對其進(jìn)行精確測年。

圖3 巖石樣品野外及鏡下照片

2.2 云開群混合巖巖石學(xué)特征

河臺礦區(qū)內(nèi)分布著大面積的云開群混合巖，這些混合巖大多屬于全熔混合巖，混合巖化之前的巖石結(jié)構(gòu)已完全破壞，巖石中礦物顆粒均一化、粗粒化。從顏色及礦物成分看，屬于中色-淺色的全熔混合巖，主要由長石(55%)、石英(25%)、黑云母(10%)，白云母(5%)和絹云母集合體(5%)組成(圖3e, f)。由于距離剪切帶較近，受其影響，長石邊緣有明顯細(xì)?；F(xiàn)象，絹云母分布于長石裂隙中，表明其晚于長石，可能與后期熱液活動有關(guān)。

3 樣品采集及實驗方法

9個花崗巖樣品采自河臺礦區(qū)云樓崗巖體東部，并且靠近云開群混合巖的位置(圖2)。7個混合巖樣品采自高村金礦-140m中段和云西金礦+10m中段(圖2)。野外所采集的花崗巖樣品表面風(fēng)化較強(qiáng)(圖3a, b)，首先對其進(jìn)行實驗前處理，將表面風(fēng)化部分去除掉，以免影響實驗結(jié)果。

3.1 LA-ICP-MS鋯石U-Pb定年

鋯石分選在河北省誠信服務(wù)有限公司完成，采用常規(guī)方法將樣品粉碎至80目以上，并采用電磁選方法進(jìn)行分選。在雙目鏡下挑選出晶形和透明度較好，無裂紋，粒徑足夠大的鋯石顆粒作為測試對象。鋯石制靶和陰極發(fā)光(CL)圖像在重慶宇勁科技有限公司完成。鋯石年齡測試在中國科學(xué)院廣州地球化學(xué)研究所礦物學(xué)與成礦學(xué)中國科學(xué)院重點實驗室完成，使用儀器為LA-ICP-MS，儀器型號為Resolution M50 Agilent 7500a，廠家Resonetics Agilent，光斑為29μm。采用標(biāo)準(zhǔn)鋯石Plesovice(206Pb/238U加權(quán)平均年齡為337.13±0.37Ma(Slmaetal., 2008))和Temora(206Pb/238U加權(quán)平均年齡為416.6±1.0Ma(Blacketal., 2003))作為外標(biāo)，元素含量采用 NIST SRM610作為外標(biāo)，29Si作為內(nèi)標(biāo)元素(鋯石中SiO2含量為32.8%)(袁洪林等, 2003)，分析方法參考Yuanetal. (2004)方法；普通鉛校正采用Andersen (2002)推薦的方法；鋯石的同位素比值及微量稀土元素含量計算采用ICPMSDATECAL程序(Liuetal., 2008, 2010)，年齡計算及諧和圖的繪制采用Isoplot 2006(Ludwig, 2003)。

3.2 全巖地球化學(xué)和Sr-Nd同位素測試

將新鮮的花崗巖和混合巖樣品磨碎到200目，以備主微量和Sr-Nd同位素測試。測試工作在中國科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國家重點實驗室完成。

主量元素利用Rigaku 100e XRF進(jìn)行測試，分析結(jié)果的誤差(相對標(biāo)準(zhǔn)偏差值)小于3%(H2O+除外)。樣品粉末加入Li2B4O7(1:8)，在V8C自動熔融機(jī)(Analymate，中國)熔融機(jī)上加熱至1150～1200℃，制成均一的玻璃片，然后進(jìn)行X射線熒光(XRF)分析。

微量元素分析在Agilent 7700X型電感耦合等離子體質(zhì)譜儀(ICP-MS)上進(jìn)行分析，稀土元素和Y分析誤差小于4%，其它微量元素在3%～7%之間。將約40mg樣品粉末放入Teflon杯中并加入HF+HNO3，100℃加熱7天溶解。之后烘干并加入3%的HNO3，進(jìn)行測試。利用Rh作為內(nèi)標(biāo)進(jìn)行校正(Liuetal., 1996)。

分離純化后的Sr和Nd溶液在Micromass Isoprobe型多接收器等離子質(zhì)譜儀(MC ICP-MS)上進(jìn)行87Sr/86Sr和142Nd/144Nd比值測定，詳細(xì)的測試方法見Lietal. (2006)。測試過程中的質(zhì)量分餾效應(yīng)分別采用86Sr/88Sr=0.1194和146Nd/144Nd=0.7219進(jìn)行校正。Shin Etsu JNdi-1標(biāo)準(zhǔn)的143Nd/144Nd和87Sr/86Sr測定值為分別0.512115(2σ)和0.710260(2σ)。

表1云樓崗花崗閃長巖和云開群混合巖鋯石LA-ICP-MSU-Pb年齡

Table 1 Zircon LA-ICP-MS U-Pb ages of the Yunlougang granodiorite and the Yunkai Group migmatite

測點號ThU(×10-6)Th/U同位素比值同位素年齡(Ma)協(xié)和度207Pb206Pb1σ207Pb235U1σ206Pb238U1σ207Pb206Pb1σ207Pb235U1σ206Pb238U1σ(%)樣品5H031云樓崗花崗閃長巖-0819.04880.040.05140.000.28720.020.04020.0038710927414258594-1255.06920.080.05010.000.26940.010.03930.002119724210248397-1343.45460.080.05060.000.27780.010.04020.0076.011522912249491-1444.84310.100.05080.000.27860.010.04010.001769924410249397-154.552940.020.04970.000.27160.010.03990.0030911125611250397-1610.81930.060.05190.000.28400.020.04000.0028016025413250498-171273990.320.04990.000.27350.010.03990.00235932509251499-1848.21220.390.05330.000.29760.010.04060.0021114424213252496-1959.51160.520.05440.000.30920.020.04080.001879824510252397-2038.192.60.410.05050.000.27350.010.04000.0018911924411252496-2251.61930.270.05260.000.28660.010.03960.0028315825413253399-2466.66050.110.05090.000.27980.010.03980.0021712924611253497-2513.23600.040.04960.000.27130.010.03940.00217942519253499-2686.22450.350.05180.000.28450.020.03960.002809425710253398-2721.65730.040.05190.000.28830.010.04010.002329625010254498-2826.81110.240.04840.000.26990.010.04030.0025710725612254499-2923.54680.050.04990.000.27990.010.04030.002339424910254397-3053.65020.110.05050.000.28060.010.04000.001178624311255595-3138.41010.380.05020.000.26950.020.03990.00191852518255398-3263.22010.310.04750.000.25260.010.03940.0034310426512257496樣品HT112云開群混合巖-103687950.460.05240.000.27870.010.03870.00302622506245498-121948380.230.05120.000.25850.010.03660.00256472335232399-131072000.540.05250.000.32540.010.04530.003068128610285899-144252961.440.05550.000.26710.010.03500.00432632406222492-161911930.990.04990.000.26000.010.03830.001911122358242596-182002580.780.05270.000.31320.020.04330.0031764277122731098-225.308110.010.04970.000.26780.010.03910.00189542415247397-232052690.760.05220.000.26130.010.03640.00295632366231397-2510217580.060.04960.000.28540.010.04170.00176372556263596-271012680.380.05160.000.27770.010.03900.00333612497247599-29991210.820.05270.000.28260.010.03900.003228525310247697-31803890.460.05230.000.33620.010.04670.002986829410294899-3637.213010.030.05640.000.38210.010.04910.00478383298309793-373693491.060.05360.000.28250.010.03830.00367522536242495-387334551.610.05270.000.27440.010.03780.00317522466239497-853.03790.140.05710.000.43760.010.05520.004945236911347793-95536000.920.05130.000.27670.010.03920.00254462485248499

圖4 云樓崗花崗閃長巖(a)和云開群混合巖(b)LA-ICP-MS鋯石U-Pb年齡協(xié)和圖、代表性的鋯石顆粒特征及年齡

圖5 巖石地球化學(xué)分類TAS圖解(a, 底圖據(jù)Miyashiro, 1978; Middlemost, 1994)和A/NK-A/CNK圖解(b, 底圖據(jù)Chappell and White, 1992; Chappell, 1999)

圖6 哈克圖解(g,底圖據(jù)Roberts and Clemens, 1993)

表2云樓崗巖體和云開群混合及變質(zhì)沉積巖主量(wt%)和微量(×10-6)元素含量

Table 2 Major (wt%) and trace (×10-6) element contents of the Yunlougang pluton, the Yunkai Group migmatite and metasedimentary

樣品號5H1715H1725H1735H1745H1755H1765H0315H0325H033HT112HT51HT52HT54HT57HT60HT80G0103-1G0104-1G0105-1巖性云樓崗巖體云開群混合巖云開群變質(zhì)沉積巖SiO268.168.6867.6267.6367.8868.1165.9465.9466.0673.5072.2572.4673.8671.1671.9570.5762.9859.3575.28Al2O314.9414.7715.115.114.714.9616.0515.9416.0413.3314.4714.4611.8113.8914.5314.7820.0719.6710.16Fe2OT34.674.234.684.654.154.685.165.215.161.871.822.173.862.832.174.144.636.555.41MnO0.110.080.10.10.080.110.120.120.120.080.070.070.110.120.090.080.030.090.18TiO20.580.530.580.580.530.570.640.640.640.210.190.280.420.320.170.490.740.741.06MgO1.761.651.81.811.631.771.941.931.950.580.460.511.540.900.351.560.862.811.34CaO2.662.192.52.52.152.682.932.952.921.100.340.800.901.350.830.760.060.572.5K2O2.853.873.283.293.832.882.742.752.774.626.414.853.664.536.264.765.24.550.96Na2O2.862.562.752.832.562.93.043.023.062.402.263.030.872.651.840.770.462.121.92P2O50.210.210.210.210.190.210.260.260.260.330.150.180.150.250.360.120.090.10.19LOI1.110.920.821.121.610.980.830.880.882.020.990.922.411.870.852.033.942.990.66Total99.8599.799.4599.8299.3299.8699.6599.6399.85100.0599.4199.7399.6099.8799.41100.0799.499.899.8A/CNK1.181.191.191.181.201.171.211.201.201.221.281.241.681.181.281.893.082.081.16σ1.31.611.481.521.641.331.461.451.471.622.572.100.671.832.271.111.602.720.26Cs16.0915.8115.0816.4615.6615.4118.4821.420.546.7586.6066.1117.9528.6333.80113.98Sc8.638.578.829.348.218.747.878.979.015.2484.795.2888.6547.054.74211.028.612.78.2Ba265.3537.5355.3375.2528.4265.9137.8158161.1554233.5261.5348.6420.3186.6365.32444861167Ti311129913099322729433152280531993197116210601609232617268863035V65.6362.4264.4468.6261.4768.2559.5670.7371.1621.0111.5216.0849.1933.815.37365.27Mn794.7616697.2741.6595.6805.7735843.1867.5550.6569.3514.7820.9853.4685609.3Co7.566.717.468.156.817.427.17.848.172.9781.9882.6024.1843.6651.7166.543Cr34.5339.0840.1842.0936.2235.4635.3839.0743.1525.1717.0115.5155.9920.2419.5593.5187113104Ni14.3913.7616.0617.6911.2715.7713.9817.0417.588.1464.0415.02315.697.3652.74428.03Cu11.6511.729.9811.039.7413.5421.822.9423.310.7913.876.32120.6869.5911.5939.9Zn72.7367.7473.7868.758.7773.0772.8474.5379.7135.2267.78162.856.0844.4735.1948.82Pb18.9924.2220.0617.2121.3317.7810.8412.8412.5918.3551.7360.643.91611.735.892.738Ga20.119.5319.4318.8418.4219.9717.9419.9320.1712.5816.7418.5617.6319.1918.0719.31Ge1.671.841.71.731.731.572.011.081.471.6991.4861.9271.9841.8761.6032.364Rb183.9177.1157.9169.7177.5163.1152.5171.8171173.1274.1225.6185227.2267.225621624380Sr112.8117.5106.5116.5115.6115.984.8103.8104.955.5352.0356.8227.9984.1753.0527.7331107165Hf1.631.481.121.465.590.411.511.532.143.5842.1744.0994.8023.7781.8082.688Zr61.7560.4361.662.4566.0757.3962.1768.6381.4412465.13128.8159.4123.550.0589.02190118579

續(xù)表2

Continued Table 2

樣品號5H1715H1725H1735H1745H1755H1765H0315H0325H033HT112HT51HT52HT54HT57HT60HT80G0103-1G0104-1G0105-1巖性云樓崗巖體云開群混合巖云開群變質(zhì)沉積巖Nb16.1615.2215.9616.615.0816.3915.1217.4119.426.38710.8111.8411.5715.519.10613.6191825Ta1.821.241.571.441.491.511.21.584.521.4291.2731.131.2551.3830.5621.687282633Th14.6613.8714.2715.5714.6514.9212.1214.3914.026.0716.98719.9914.513.015.12615.63191946U3.583.373.743.673.383.723.744.034.489.4385.737.6594.7738.8819.0388.0843.14.75.7La31.0227.2525.3331.0227.1431.6723.1226.5129.9812.2913.3430.8827.2827.627.60335.2563.2752.5282.74Ce60.4253.4549.9461.353.1960.8646.2952.5559.6924.1826.5561.7352.5450.5516.2367.3792.4490.27146.2Pr7.466.716.197.616.687.525.886.537.462.9993.2317.5486.3435.9222.0798.1210.878.9813.86Nd25.9823.6621.7226.6923.3426.720.8423.1726.0111.511.8627.6423.220.678.20729.6845.137.556.5Sm5.024.544.125.074.515.14.064.55.12.5292.4725.1594.2643.8872.3925.5358.857.4410.5Eu0.380.380.320.350.380.40.30.330.450.7270.5280.6330.720.6690.4260.9941.411.261.42Gd5.084.564.155.064.615.164.024.585.312.5052.154.0073.7813.3432.7114.9557.556.9110Tb0.620.560.490.610.550.630.490.570.650.4530.3470.5160.5620.4640.6210.7611.230.931.38Dy2.842.552.312.712.572.942.312.723.022.7742.0092.563.1982.283.974.4636.145.77.27Ho0.420.380.350.410.380.440.340.420.470.5680.4040.4720.6580.3840.7380.9111.231.161.43Er1.111.040.971.121.071.180.971.161.31.5031.0991.2691.7620.9041.9232.4833.673.334.38Tm0.10.090.080.090.090.10.080.10.120.2240.1770.1910.2630.1310.2860.3750.480.470.6Yb0.930.820.780.890.820.980.780.921.061.4491.171.2371.6980.8891.782.4112.942.763.83Lu0.090.070.070.080.070.090.070.090.110.2260.1790.1920.2730.1420.2550.3760.450.490.69Y12.6211.3810.2412.0811.5112.2210.6411.4113.2414.0710.6311.8815.989.60520.8523.17282633∑REE141.5126.1116.8143125.4143.8109.5124.1140.763.965.5144.0126.5117.949.2163.7245.6219.7340.8LREE130.3116107.6132115.2132.3100.5113.6128.754.258.0133.6114.3109.336.9146.9221.9198.0311.2HREE11.210.19.21110.211.59.110.6129.77.510.412.28.512.316.723.721.829.6LREE/HREE11.611.511.71211.311.511.110.810.75.67.712.89.412.83.08.89.49.110.5(La/Yb)N23.9323.8423.2925.0023.7423.1821.2620.6720.296.088.1817.9111.5222.293.0610.4915.4413.6515.50(La/Sm)N3.993.873.973.953.884.013.683.803.793.143.483.864.134.592.054.114.624.565.09δEu0.230.260.240.210.250.240.230.220.260.880.700.430.550.570.510.580.530.540.42δCe110.970.970.980.980.970.970.970.980.990.990.980.971.000.980.861.021.06Fe2O3/MgO2.652.562.602.572.552.642.662.702.653.203.994.242.513.146.142.655.382.334.0410000Ga/Al2.542.492.432.352.362.522.112.362.371.782.182.422.822.612.352.46Rb/Sr1.541.511.631.411.481.461.661.631.803.125.273.976.612.705.049.236.972.270.48Nb/Ta8.8812.2710.1711.5310.1210.8512.6011.024.304.478.4910.489.2211.2116.208.060.680.690.76

注：云開群變質(zhì)沉積巖數(shù)據(jù)引自Wanetal. (2010). A/CNK=Al/(Ca+Na+K),δEu=Eu/Eu*=EuN/(SmN×GdN)1/2,δCe=Ce/Ce*=CeN/(LaN×PrN)1/2

圖7 全巖原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(a,標(biāo)準(zhǔn)化值據(jù)Sun and McDonough, 1989)和球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖(b,標(biāo)準(zhǔn)化值據(jù)Boynton, 1984)

圖8 云樓崗巖體和云開群混合巖U-Pb年齡-εNd(t)圖解(a, 底圖據(jù)周新民, 2007)和 (87Sr/86Sr)i-εNd(t)圖解(b, 底圖據(jù)Wang et al., 2011)

3.3 黑云母化學(xué)成分分析

對薄片進(jìn)行詳細(xì)觀察后，選擇5個具有代表性的樣品薄片進(jìn)行電子探針(EPMA)分析。電子探針分析在中國科學(xué)院廣州地球化學(xué)研究所礦物學(xué)與成礦學(xué)中國科學(xué)院重點實驗室電子探針(EMPA)實驗室完成，所用儀器為日本電子JOEL公司生產(chǎn)的JXA-823V型電子探針，實驗中加速電壓為15kV，束流為2.0×10-8，束斑大小為5μm，礦物定量分析誤差不超過3%。黑云母結(jié)構(gòu)式以22個O離子數(shù)為標(biāo)準(zhǔn)進(jìn)行計算，F(xiàn)e2+和Fe3+利用林文蔚和彭麗君(1994)提供的方法進(jìn)行估算。

4 實驗結(jié)果

4.1 地質(zhì)年代學(xué)

云樓崗巖體(樣品5H031)中的鋯石晶形良好，多為自形晶，長柱狀，長100～200μm，長寬比1:3～1:4。CL圖像上，鋯石顆粒顏色較暗，不透明，具有典型的核-幔結(jié)構(gòu)(圖4a)。鋯石繼承核形態(tài)多樣，而鋯石幔部具有明顯的震蕩環(huán)帶，Th/U比為0.02～0.52，具典型的深熔巖漿鋯石的特點。鋯石幔部20個點的206Pb/238U年齡為258～248Ma(表1、圖4a)，加權(quán)平均年齡為253±1.6Ma(MSWD=0.14)。

云開群混合巖(樣品HT112)中的鋯石形態(tài)多樣，自形-他形，大多為不規(guī)則狀，鋯石顆粒相對較小，長80～150μm，長寬比1:2～1:3。在CL圖像上鋯石發(fā)光性也有差別，多數(shù)發(fā)暗光，少數(shù)晶形較好的發(fā)亮光，且具有震蕩環(huán)帶(圖4b)。選擇17顆形態(tài)不同的鋯石中進(jìn)行U-Pb定年，獲得的206Pb/238U年齡值較分散，分布在347～222Ma(表1、圖4b)，但是其中有10個年齡集中在248～231Ma，加權(quán)平均年齡為240.3±5.1Ma(MSWD=3.8)。另外有6顆鋯石年齡較老且分散，在347～263Ma，可能是由于原巖在熔融過程中，鋯石沒有完全被改造而繼承有原巖的年齡信息，因而偏大。

4.2 全巖主微量

表2為云樓崗巖體的10個樣品和云開群混合巖的6個樣品的主微量元素分析測試結(jié)果。

4.3 全巖Sr-Nd同位素

表3為5個云樓崗巖體樣品和7個云開群混合巖樣品的Sr-Nd同位素測試結(jié)果。

表3云樓崗巖體和云開群混合及變質(zhì)沉積巖全巖Sr-Nd同位素

Table 3 Sr-Nd isotopic composition of the Yunlougang pluton, the Yunkai Group migmatite and metasedimentary

樣品號Sm(×10-6)Nd(×10-6)147Sm144Nd143Nd144Nd2σ(Err.)εNd(0)εNd(t)tDM2(Ma)Rb(×10-6)Sr(×10-6)87Rb86Sr87Sr86Sr2σ (Err.)87Sr86Sr()i云樓崗巖體5H1715.0225.980.11680.5120020.000006-12.41-9.841821183.9112.84.730.73096160.0000070.713945H1724.5423.660.11600.5120820.000005-10.86-8.251692177.1117.54.370.73247240.0000110.716745H1745.0726.690.11480.5120000.000004-12.44-9.801818169.7116.54.220.7323120.0000090.717115H1754.5123.340.11690.5120040.000006-12.37-9.801818177.5115.64.450.73241250.0000080.716395H1765.1026.700.11550.5120000.000004-12.44-9.821819163.1115.94.080.73241250.0000080.71773云開群混合巖HT1122.5311.500.13290.5118290.000006-15.78-13.842133173.155.59.070.76565350.0000080.73469HT512.4711.860.12600.5118450.000008-15.47-13.312091274.152.015.350.78292850.0000110.73051HT525.1627.640.11280.5118210.000006-15.93-13.372096225.656.811.560.76951570.0000070.73006HT544.2623.200.11110.5117670.000005-17.00-14.392178185.028.019.230.76387770.0000110.69823HT573.8920.670.11370.5120210.000010-12.04-9.511783227.284.27.840.74114940.0000110.71440HT602.398.210.17620.5119470.000009-13.49-12.872055267.253.114.670.77290060.0000110.72283HT805.5429.680.11270.5117140.000009-18.02-15.462264256.027.726.930.78960090.0000130.69768云開群變質(zhì)沉積巖G0103-18.0943.130.11340.5118880.000008-14.60-10.001910G0104-17.1837.080.11710.5117720.000001-16.90-12.402170G0105-111.0859.250.11310.5117100.000012-18.10-13.402170

注：云開群變質(zhì)沉積巖引自Wanetal. (2010)

云樓崗巖體的87Sr/86Sr比值測試結(jié)果為0.7309616～0.7324724。按照253Ma年齡計算巖體的初始Sr-Nd同位素組成，計算結(jié)果為初始比值(87Sr/86Sr)i為0.71394～0.71773，εNd(t)為-9.84～-8.25(圖8a)，Nd模式年齡為1821～1692Ma?？梢?，云樓崗巖體的Sr-Nd同位素組成與華南東部印支期S型花崗巖及東澳大利亞拉克蘭褶皺帶(LFB)的Sr-Nd同位素組成是基本一致的(圖8b, Healyetal., 2004; Wangetal., 2011)。

云開群混合巖的87Sr/86Sr比值測試結(jié)果為0.7411494～0.7896009。基于240Ma年齡計算巖體的初始Sr-Nd同位素，初始比值(87Sr/86Sr)i為0.69768～0.73469，εNd(t)為-15.46～-9.51(圖8a)，Nd模式年齡為2264～1783Ma。

4.4 黑云母成分

云樓崗巖體和云開群混合巖中的黑云母特征相似，都為棕紅色葉片狀，長0.5～3mm。表4為電子探針測試的黑云母化學(xué)成分和計算得到的離子含量。黑云母AlⅣ和Fe#(Fe#=Fe(T)/(Fe(T)+Mg2+))含量可以用來判斷巖石的過鋁質(zhì)和氧化還原狀態(tài)(Shabanietal., 2003)。

云樓崗巖體中黑云母中Fe#為0.48～0.52、Mg#為0.48～0.53、AlⅣ為2.612～2.758apfu(每個結(jié)構(gòu)單元中的原子數(shù))、Ti4+為0.189～0.332apfu。在Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn)圖上(圖9a)顯示為Mg質(zhì)黑云母，而在10×TiO2-(FeOT+MnO)-MgO三角圖解上(圖9b)顯示為受到改造再平衡的黑云母。利用黑云母中Ti(Henryetal., 2005)和AlT含量(Uchidaetal., 2007)可以作為地質(zhì)溫度計，計算得到的黑云母形成溫度為589～679℃，平均651℃；壓力為245～278MPa，平均266MPa；相當(dāng)于9.27～10.51km，平均10.08km。

云開群混合巖中的黑云母與云樓崗巖體中的一樣，也是Mg質(zhì)的，并且是受到改造再平衡的黑云母(圖9a, b)，其中Fe#、Mg#值分別為0.44～0.56和0.44～0.56，AlⅣ和Ti4+含量分別為2.663～2.880apfu和0.243～0.488apfu。計算得到形成溫度為623～733℃，平均664℃；壓力為211～323MPa，平均266MPa；相當(dāng)于深度7.98～12.21km，平均10.06km。

圖9 黑云母Mg-(Fe3++AlⅥ+Ti)-(Fe2++Mn)分類圖解(a, 底圖據(jù)Foster, 1960)、10×TiO2-(FeOT+MnO)-MgO圖解(b, 底圖據(jù)Nachit et al., 2005)和Ti-Mg/(Fe+Mg)溫壓圖解(c, 底圖據(jù)Henry et al., 2005)

5 討論

5.1 云樓崗巖體與云開群混合巖的成因聯(lián)系

表4云樓崗巖體和云開群混合黑云母化學(xué)組成(wt%)

Table 4 Geochemical compositon of the biotites in the Yunlougang pluton and the Yunkai Group migmatite (wt%)

測點號SiO2TiO2Al2O3FeOMnOMgONa2OK2OH2OTotalSi4+AlⅣAlⅥTi4+Fe3+Fe2+Mn2+Mg2+(wt%)(apuf)Fe#Mg#T(℃)P(MPa)H(km)云樓崗巖體5H031-136.672.8717.6818.040.359.960.089.092.0796.815.3092.6910.3240.3130.1762.0080.0422.1490.50.5672.5260.59.855H031-236.872.6217.6018.150.3510.080.069.242.0797.045.332.670.3260.2850.1812.0130.0432.1730.50.5657.3254.89.635H031-336.183.0417.7718.240.339.980.109.512.0797.215.2422.7580.2750.3320.1692.0420.0412.1550.510.49679.8266.010.055H031-436.602.7317.6717.910.3510.020.049.392.0796.795.3082.6920.3260.2980.1741.9980.0432.1670.50.5664.6261.59.885H031-536.562.2217.9018.390.2910.140.089.342.0796.995.2992.7010.3550.2420.1722.0570.0352.1910.50.5629.0273.010.325H031-636.282.8417.8718.710.349.740.089.292.0797.215.2592.7410.3090.3090.1732.0950.0412.1060.520.48666.5271.210.255H031-735.952.8417.5917.960.319.700.089.022.0495.505.2852.7150.3310.3140.1712.0360.0382.1250.510.49671.0269.910.205H031-836.452.6917.8918.480.339.910.069.502.0797.385.2712.7290.3180.2930.1722.0630.042.1370.510.49659.8270.210.215H031-936.352.8217.7018.380.409.520.099.132.0696.465.2992.7010.3380.3090.1762.0650.052.0690.520.48666.5267.810.125H031-1036.202.7417.9618.830.379.700.089.032.0796.985.2562.7440.3270.2990.1712.1150.0462.10.520.48661.1277.510.495H031-1136.762.5917.3218.260.3510.020.089.212.0696.655.3432.6570.3080.2830.1902.0290.0432.1710.510.49654.0245.49.275H031-1236.512.8317.6617.690.339.940.099.272.0696.385.312.690.3340.310.1721.9790.0412.1550.50.5671.0263.39.955H031-1336.332.8417.5217.710.329.890.099.262.0595.985.3092.6910.3240.3120.1741.9900.0392.1540.50.5672.0260.59.855H031-1436.492.8717.8118.030.349.970.099.272.0796.945.2842.7160.3220.3120.1702.0140.0412.1530.50.5672.0267.510.115H171-137.111.7317.7017.090.3210.600.049.572.0796.245.3882.6120.4140.1890.1721.9030.042.2950.470.53589.2263.99.975H171-236.421.9417.6717.220.3610.540.139.392.0595.715.3272.6730.3710.2130.1671.9390.0442.2970.480.52610.2269.310.185H171-336.272.6117.9418.240.3410.220.079.852.0797.605.2422.7580.2950.2830.1662.0380.0412.2010.50.5656.2272.110.285H171-436.662.6218.0417.920.2910.280.069.752.0897.705.2742.7260.330.2830.1641.9910.0352.2040.490.51658.4273.010.325H171-537.212.0917.9217.220.2910.470.069.632.0896.965.3632.6370.4040.2260.1681.9070.0352.2490.480.52621.4268.410.145H171-635.402.4717.0316.460.299.700.069.521.9992.935.3382.6620.3630.280.1681.9080.0372.180.490.51656.6263.69.965H171-736.282.3117.8317.650.3410.350.099.552.0696.475.2822.7180.340.2530.1641.9860.0422.2470.490.51639.3273.610.345H171-836.691.8917.9517.400.2710.470.109.402.0696.225.3332.6670.4060.2070.1641.9510.0332.2690.480.52604.6278.110.51平均36.472.5517.7317.910.3310.050.089.372.0696.555.3022.6980.3380.2790.1712.0060.0402.1790.500.50651.5266.910.09云開群混合巖HT104-135.802.8018.4518.670.189.130.149.502.0696.735.2212.7790.390.3070.1552.1220.0221.9840.530.47663.5307.211.61HT104-235.712.5218.1118.510.189.610.089.452.0596.225.2332.7670.3590.2770.1602.1080.0232.10.520.48648.3294.211.12HT104-335.202.2618.5418.760.179.540.119.532.0496.155.1762.8240.3860.250.1492.1580.0222.0910.520.48630.4319.612.08

續(xù)表4

Continued Table 4

測點號SiO2TiO2Al2O3FeOMnOMgONa2OK2OH2OTotalSi4+AlⅣAlⅥTi4+Fe3+Fe2+Mn2+Mg2+(wt%)(apuf)Fe#Mg#T(℃)P(MPa)H(km)HT104-435.002.1717.9418.230.169.180.109.322.0094.095.2442.7560.4090.2440.1572.1270.022.050.530.47623.8306.011.56HT104-535.652.2718.7018.400.199.380.159.532.0596.315.2162.7840.4380.250.1472.1050.0242.0460.520.48630.4323.312.22HT104-634.894.4217.6518.390.169.430.139.052.0496.155.122.880.170.4880.1612.0960.022.0630.520.48733.3271.210.25HT104-735.572.5118.3018.860.199.500.059.382.0596.405.2092.7910.3650.2760.1582.1510.0232.0730.530.47645.7303.311.46HT104-835.382.5617.2718.890.159.690.079.322.0295.345.252.750.2680.2850.1832.1610.0182.1430.520.48653.2261.59.88HT104-935.262.1918.4718.450.199.440.159.362.0395.535.2062.7940.4180.2430.1492.1290.0232.0770.520.48625.2320.212.10HT13-136.313.0416.4819.410.269.340.079.642.0496.585.3352.6650.1870.3360.2272.1580.0322.0450.540.46676.2211.27.98HT13-236.212.8317.0219.360.289.250.069.642.0496.685.3112.6890.2510.3120.2052.1700.0342.0230.540.46664.3237.88.99HT13-336.262.9617.1318.930.209.300.069.712.0596.595.3112.6890.2650.3260.1972.1220.0242.0310.530.47673.2242.19.15HT13-436.292.9516.5919.240.229.380.069.722.0496.505.3342.6660.2060.3260.2212.1440.0272.0560.530.47673.2217.28.21HT13-536.272.9717.2018.900.279.380.089.682.0596.785.3012.6990.2610.3260.1942.1170.0332.0430.530.47673.2243.99.22HT13-635.832.7117.8320.370.278.960.058.972.0597.045.2382.7620.3080.2980.1852.3050.0331.9520.560.44653.3277.210.48HT13-736.572.7417.6219.560.279.010.079.742.0797.645.3082.6920.3190.2990.1912.1830.0331.9490.550.45655.6259.39.80HT13-835.112.8516.2618.750.208.750.059.221.9793.165.3372.6630.2470.3260.2142.1700.0261.9840.550.45669.7228.78.64HT13-935.822.6517.0118.710.269.280.079.492.0295.315.3152.6850.2880.2950.1942.1280.0332.0520.530.47657.0247.89.37HT13-1035.753.1417.0619.750.269.320.089.232.0496.635.2512.7490.2020.3470.2012.2240.0332.040.540.46681.2241.29.11HT13-1136.203.0117.0619.220.199.450.109.532.0596.815.2932.7070.2310.3310.2012.1490.0242.0610.530.47675.6237.28.97HT13-1236.142.9517.0219.620.289.110.049.652.0596.865.2992.7010.2370.3260.2062.1990.0351.9910.550.45669.7237.28.97HT5-136.602.6918.0116.450.1611.670.0710.062.1097.815.2332.7670.2660.2890.1501.8170.0192.4870.440.56673.7266.010.05HT5-236.562.7517.5716.610.1511.800.119.692.0997.325.252.750.2210.2970.1621.8330.0182.5260.440.56678.0247.29.34HT5-336.662.7017.8916.630.1511.890.099.992.1098.105.2282.7720.2330.290.1551.8290.0192.5270.440.56674.3257.59.73HT5-436.492.7718.3916.370.1311.820.109.902.1198.075.1952.8050.2780.2960.1401.8090.0162.5090.440.56677.5281.110.63HT5-536.742.7918.4116.600.1711.400.0910.062.1198.365.2222.7780.3030.2990.1441.8290.022.4150.450.55676.6280.510.60HT5-636.312.7317.8516.270.1811.210.109.822.0796.545.2542.7460.2950.2970.1511.8180.0222.4180.450.55675.5268.410.14平均35.952.7717.6218.440.209.820.089.562.0596.515.2552.7450.2890.3050.1762.0800.0252.1380.510.49664.1266.210.06

注：結(jié)構(gòu)式以22個氧原子進(jìn)行計算. Fe#=FeT/(FeT+Mg2+)；Mg#=Mg2+/(FeT+Mg2+)；溫度T={[ln(Ti)-a-c(XMg)3]/b}0.333(Henryetal., 2005)；壓力P=[3.03×TAl-6.53(±0.33)]×100 (Uchidaetal., 2007)；深度H=P/ρg, ρ=2700kg/m3, g=9.8m/s2

然而，云樓崗巖體與云開群混合巖、變質(zhì)沉積巖之間地球化學(xué)特征在許多方面也有區(qū)別。首先，云樓崗巖體的SiO2含量比云開群混合巖要低。云開群混合巖的稀土配分曲線比較平，與云開群變質(zhì)沉積巖形態(tài)類似，但是比較低(圖7b)。云樓崗巖體的稀土配分曲線與之有較大差別，盡管輕稀土元素LREE與云開群變質(zhì)沉積巖特征類似，但是重稀土元素則較曲折，不似云開群變質(zhì)沉積巖那般平直。另外，云樓崗巖體的 (La/Yb)N為22.80，大于云開群變質(zhì)沉積巖((La/Yb)N=14.86)及混合巖((La/Yb)N=11.36)，因此其稀土配分曲線整體斜率也更大。全巖Rb/Sr比可以作為指標(biāo)用來判別熔體成因(Harris and Inger, 1992; Harrisetal., 1995; Solar and Brown, 2001)。云樓崗巖體的Rb/Sr比值相對較低，為1.4～1.8，可能有外部巖漿的混入，使得云開群變質(zhì)沉積巖發(fā)生重熔，并且由于混合巖漿熔融程度高，SiO2較低，從而導(dǎo)致Rb/Sr分餾程度較低。而云開群混合巖的Rb/Sr比值高，為2.7～9.2，分餾程度高，說明云開群變質(zhì)沉積巖中的云母發(fā)生了脫水熔融。當(dāng)然，僅是云母脫水熔融不足以提供足夠的熔體形成混合巖，推測可能還有其它外部流體的滲入導(dǎo)致水流熔融(Wardetal., 2008; Sawyeretal., 2011)。大陸地殼中產(chǎn)生水流熔融的許多地方都靠近主要剪切帶，這些剪切帶為含水流體滲入到大陸地殼中提供了通道(Sawyer, 2010)。云開地區(qū)存在著一期左旋的剪切作用，時間在約240Ma(Jiaoetal., 2017)，與本文所得到的混合巖年齡一致，可以作為含水流體的通道使云開群變質(zhì)沉積巖發(fā)生水流熔融。因此，總體來看，云開群混合巖的地球化學(xué)特征與云開群變質(zhì)沉積巖更加類似，表明混合巖僅來自于變質(zhì)沉積巖的直接熔融；而云樓崗巖體則有可能來自于混合巖漿，并且由于外部巖漿的加入，使得其SiO2含量降低。另外，從Sr-Nd同位素特征來看，云樓崗巖體具有比云開群混合巖和變質(zhì)沉積巖低的(87Sr/86Sr)i比值，高的εNd(t)；前者的Nd模式年齡為1821～1692Ma(平均1794Ma)，而后兩者近乎一致為2178～1783Ma(平均2086Ma)和2170～1910Ma(平均2083Ma)，前者明顯小于后兩者。當(dāng)幔源的含水玄武巖侵入下地殼中時，能夠形成一系列巖床，同時在地殼深部產(chǎn)生熱區(qū)域，在這些熱區(qū)域附近，由玄武巖不完全結(jié)晶殘留下來的硅酸鹽熔體可能與部分熔融的地殼巖石發(fā)生混合，從而形成混合硅酸鹽熔體(Annenetal., 2006)。這種混合而成的硅酸鹽熔體可能具有閃長巖和花崗閃長巖的成分(Sugaetal., 2016)。因此，在形成云樓崗花崗閃長巖體時，可能有來自地幔的年輕熔體成分加入到局部重熔的下地殼的熔體中從而形成混合巖漿。Jiaoetal. (2015)對云開地區(qū)的大容山S型花崗巖雜巖體的成因進(jìn)行了詳細(xì)研究，研究表明其形成于約249Ma，同樣也是混合巖漿結(jié)晶形成的，有少量地幔物質(zhì)的加入，與云樓崗巖體具有類似的成因，可見其成因在區(qū)域上有一定的普遍性。而云開群混合巖與變質(zhì)沉積巖有近乎一致的Sr-Nd同位素特征及Nd模式年齡，因此，云開群混合巖是變質(zhì)沉積巖部分熔融并在原地結(jié)晶形成的，沒有明顯的外部熔體(地幔物質(zhì))的混入。

從地質(zhì)年代學(xué)上看，前人曾在云開群局部變質(zhì)程度較高的片麻巖中獲得242～236Ma的印支期變質(zhì)重置年齡(Wangetal., 2007a; Wanetal., 2010)，與本文獲得的云開群混合巖的U-Pb年齡240.3±5.1Ma近乎一致，可見，云開地區(qū)在印支期發(fā)生過一次廣泛的構(gòu)造熱事件。然而，云樓崗巖體的年齡是253±1.6Ma，在分析誤差范圍內(nèi)比混合巖中變質(zhì)鋯石的年齡早了10Myr，因此，云樓崗巖體并非附近云開群混合巖化的產(chǎn)物，與河臺礦區(qū)的混合巖也不是同源的。

5.2 大地構(gòu)造意義

印支期造山運(yùn)動奠定了中國東部的構(gòu)造格局，對于區(qū)域構(gòu)造發(fā)展、巖漿活動及變質(zhì)作用有重要的影響(Huangetal., 1987; Ren, 1991, 1996; Wangetal., 2007a)。在華南板塊印支期過鋁質(zhì)花崗巖以大規(guī)模的巖蓋及廣泛分布的小型侵入體形式產(chǎn)出，分布面積約21000km2，約占整個華南花崗巖的12.3%(圖1a; 湖南省地質(zhì)礦產(chǎn)局, 1988; 孫濤, 2006; Maoetal., 2011)。其中包括60%強(qiáng)過鋁質(zhì)(A/CNK>1.1)S-型花崗巖露頭，30%的弱過鋁質(zhì)(A/CNK=1.0～1.1)和10%的鈣堿性I型花崗巖(鄧希光等, 2004; Sunetal., 2005; Qiuetal., 2014)。然而，由于出露的印支期花崗巖年齡范圍較廣，在約260～210Ma都有產(chǎn)出(Chuetal., 2012a, b; Wangetal., 2007a, 2013; Qiuetal., 2014)，使得印支期構(gòu)造巖漿事件的時空分布格局及終結(jié)時間存在爭議(Wangetal., 2013)。華南板塊位于兩大碰撞造山帶之間，北部與華北板塊之間為大別蘇魯超高壓變質(zhì)帶，西南與印支板塊之間為宋馬帶(圖1a)。由于位于這些造山帶之間，華南板塊在印支期整體處于壓縮環(huán)境，并且廣泛發(fā)育印支期花崗巖。華南板塊西南部大量的花崗巖形成在258～242Ma，可能主要與古特提斯洋的閉合導(dǎo)致印支板塊北部與華南板塊的陸陸碰撞有關(guān)(Nametal., 1998; Carteretal., 2001; Wangetal., 2010)；而華南板塊與華北板塊碰撞的時間相對較晚，因此華南板塊北部花崗巖多為240～225Ma(Lietal., 1993; Zheng, 2008; Zhaoetal., 2013)。

在河臺地區(qū)，印支期巖漿巖、混合巖和韌性剪切帶并存，說明它們與造山活動有密切關(guān)系(Solar and Brown, 2001; Johannesetal., 2003; Sepahietal., 2013)。但三者的形成時間又有所不同，可能代表造山作用不同演化階段的產(chǎn)物。云樓崗巖體的侵入(約253Ma)和云開群混合巖的形成(約240Ma)可能都與古特提斯洋的閉合以及華南板塊與印支板塊之間的陸陸碰撞有關(guān)，但是巖漿作用的時間卻比混合巖化的時間早了10Myr。云樓崗巖體形成在253Ma的同碰撞壓縮條件下，地殼加厚導(dǎo)致下地殼巖石局部發(fā)生深熔，并且有少量年輕的地幔物質(zhì)加入。隨后，在240Ma隨著造山帶的坍塌剝蝕，導(dǎo)致云開地區(qū)地殼降壓熔融從而形成混合巖(Wangetal., 2012)。而此時，在空間上巖漿巖則出現(xiàn)在廣東中部，甚至更東部的湖南江西一帶(圖1a)?？梢?，碰撞對華南板塊的影響隨時間逐漸向東遷移。前人研究表明，河臺地區(qū)剪切帶的形成時間有兩期，早期為左旋，發(fā)生在約240Ma，晚期為右旋，發(fā)生在約204Ma(Jiaoetal., 2017)?；旌蠋r化的發(fā)生時間與早期左旋運(yùn)動的時間大致相當(dāng)。這些剪切帶和混合巖代表了碰撞晚期的逆沖走滑作用和變質(zhì)沉積巖的降壓熔融(Wangetal., 2007b, 2012)。綜上所述，云開地區(qū)印支期的花崗巖可能是華南板塊和印支板塊碰撞高峰期的作用產(chǎn)物，而云開群混合巖和韌性剪切帶則可能是終了期的產(chǎn)物。

6 結(jié)論

(1)云樓崗花崗閃長巖體與鄰近的云開群混合巖不是同源的，不是混合巖化的最終產(chǎn)物。巖體可能是地殼深熔并有少量年輕的地幔物質(zhì)混入形成的混合巖漿結(jié)晶而成。而云開群混合巖則可能僅是云開群變質(zhì)沉積巖原地熔融形成的。

(2)云樓崗巖體的形成時間是約253Ma，云開群局部混合巖化的時間是約240Ma，可能分別代表的是華南板塊和印支板塊碰撞高峰期和終了期的產(chǎn)物。