林 偉,薛生升,贠孟超
(山西省地球物理化學(xué)勘查院,山西運城 044000)
中條山地區(qū)郭家溝基性-超基性巖體的巖石成因及地質(zhì)意義
林 偉,薛生升,贠孟超
(山西省地球物理化學(xué)勘查院,山西運城 044000)
文章通過對中條山地區(qū)郭家溝基性-超基性巖體的巖石學(xué)、礦物學(xué)和巖石地球化學(xué)方面的分析研究,討論了該巖體的形成時代、巖石成因、及其地質(zhì)意義。郭家溝巖體侵入時代為元古代,可能與絳縣群形成時代較為相近(2200Ma左右)。郭家溝巖體具有巖相分帶現(xiàn)象,邊緣相為輝長巖,向內(nèi)漸變過渡為含長輝石巖、變余輝石巖(角閃石巖)等,在巖體的中心發(fā)育著磁鐵透閃石巖;巖石中MgO與其他主量元素線性相關(guān),各類巖石地球化學(xué)特征較為相似,顯示出巖石受分離結(jié)晶作用的影響,具有巖漿同源演化的趨勢;巖石總體顯示出低硅、 低鎂、 高鐵、 高鈦、高鈣的特征,相對富集LREE和LILE,而虧損HREE和Nb、Ta、Zr、Y等高場強元素,輕重稀土分餾明顯,具有較弱的Eu異常, V含量較高,具有阿拉斯加型侵入巖體和與火山弧相關(guān)的巖漿特征。從巖石組合及地球化學(xué)特征結(jié)合區(qū)域構(gòu)造演化,認為巖體形成于大陸邊緣弧的構(gòu)造環(huán)境,從側(cè)面也說明絳縣群的形成環(huán)境應(yīng)該是大陸邊緣俯沖后的伸展環(huán)境,而非陸內(nèi)裂谷。
基性-超基性巖體 巖石成因 大陸邊緣弧 郭家溝 中條山
Lin Wei,Xue Sheng-sheng,Yun Meng-chao. Petrogenesis and geological significance of the Guojiagou basic-ultrabasic intrusion in Zhongtiao Mountains [J]. Geology and Exploration, 2016, 52(6):1116-1128.
中條山地區(qū)位于秦嶺-大別造山帶北緣,華北克拉通中部造山帶南緣位置。其前寒武紀變質(zhì)雜巖是華北克拉通中部造山帶的一個重要組成部分(圖1a),主要由新太古代到古元古代的涑水雜巖及其上覆的絳縣群、中條群、擔(dān)山石群變質(zhì)層狀巖系和未變質(zhì)的西陽河群沉積-火山巖系組成(圖1b)。在涑水雜巖中出露一系列的基性-超基性巖體,分布于中條山中北段西麓(夏縣大廟-絳縣大盆岔,圖1b)。這些巖體雖然被發(fā)現(xiàn)較早,但研究程度很低,還未見公開發(fā)表的文獻資料,僅1:20萬運城-三門峽幅區(qū)域地質(zhì)調(diào)查中曾作過較詳細的調(diào)查,并對其中幾個巖體進行了普查找礦工作。本文通過野外實際工作,選擇郭家溝附近巖體為研究對象,對中條山基性-超基性巖的成因和地質(zhì)意義進行了探討,同時也為中條山前寒武紀變質(zhì)雜巖的研究提供一些有用的線索。
區(qū)內(nèi)出露地層主要是前寒武紀變質(zhì)巖(圖1b),從老至新分別為太古界-古元古界的涑水群,古元古界絳縣群、中條群和擔(dān)山石群,中元古界西陽河群。其中涑水雜巖主要由不同時期的TTG質(zhì)片麻巖(西姚石英閃長質(zhì)片麻巖、寨子英云閃長質(zhì)片麻巖等)、非TTG質(zhì)花崗巖類(橫嶺關(guān)、解州、煙莊巖體等)、少量的變質(zhì)表殼巖系組成(張瑞英,2015)。絳縣群分為上(橫嶺關(guān)亞群)下(銅礦峪亞群)兩個亞群,橫嶺關(guān)亞群底部為薄層的變質(zhì)石英砂礫巖,上部由一套變質(zhì)泥質(zhì)-半泥質(zhì)巖石組成;銅礦峪亞群從下至上由石英巖、絹英片巖、絹英片巖、變凝灰?guī)r、變超鉀質(zhì)流紋巖、凝灰?guī)r、變超鉀質(zhì)基性火山巖和鈉質(zhì)火山-沉積巖系組成(劉玄,2015;薛克勤,2006)。中條群與絳縣群和涑水群呈不整合接觸,其底部為變質(zhì)石英砂(礫)巖,向上依次為砂質(zhì)板巖、含藻白云石大理巖、黑色片巖、雜色含藻大理巖以及絹云母片巖(劉玄,2015;薛克勤,2006;中條山銅礦地質(zhì),1978)。擔(dān)山石群與下伏的絳縣群和中條群以及上覆西陽河群都呈角度不整合,主要由一套變質(zhì)砂礫巖組成。西陽河群主要由安山質(zhì)火山巖組成(劉玄,2015;薛克勤,2006)。汝陽群主要是由砂質(zhì)泥質(zhì)碎屑巖和碳酸鹽巖組成的一套沉積巖 (孫大中和胡維興,1993)。
中條山基性-超基性巖指分布于山西省南部中條山脈中北段西麓。全區(qū)共發(fā)現(xiàn)超基性巖體百余個,其中多數(shù)侵入于太古界涑水群雜巖。區(qū)內(nèi)巖體主要分布于夏縣大廟-澗底河以北,長約50km,寬2~8km的范圍內(nèi)。據(jù)它們在平面上的分布特點,大致可將其劃分為大盆岔-郭家溝-店上-馬峰溝巖帶,七峪-馬家廟-澗底河巖帶,南川口-牛家峪巖帶和范家窯巖帶。原1:20萬三門峽幅報告認為超基性巖帶展布到東北部郭家溝附近即漸滅,本次通過野外查證、對比研究認為該超基性巖帶繼續(xù)延伸到絳縣大盆岔附近,之后被第四系覆蓋。這些“巖帶”大致沿走向北北東10°~30°方向呈雁行狀排列,其中以前兩個巖帶為最主要,包含巖體多,出露條件好;而后兩個巖“帶”僅共發(fā)現(xiàn)六個巖體,且多為黃土掩蓋,這些被覆蓋的巖體可通過綜合地質(zhì)、地球化學(xué)、地球物理等方法對其進行識別(段毅君,2014),工作程度較低。巖體規(guī)模一般寬10余米至數(shù)十米,長數(shù)十米至百余米,部分較大者可寬100~300m,長數(shù)百米至1500m。產(chǎn)狀以大致順層長透鏡狀居多,亦有部分成等軸狀,或局部斜交圍巖呈侵入接觸關(guān)系,往深部常楔形尖滅或分叉。大中條地區(qū)斷裂以 NE 向和 NW 向為主(馮旭亮,2015)。巖體的侵入受北東-南西向構(gòu)造以及圍巖層理控制,巖體長軸絕大多數(shù)呈北東-北北東方向。
圖1 中條山地區(qū)大地構(gòu)造位置簡圖(a,據(jù)Zhai et al.,2011改繪);中條山地區(qū)區(qū)域地質(zhì)簡圖(b,據(jù)孫大中等,1991年改繪)Fig. 1 Regional tectonic sketch map of the Zhongtiao Mountain region(a,adapted after Zhai et al.,2011);geological sketch map of the Zhongtiao Mountain region (b,adapted after Sun et al.,1991) 1-第四系; 2-寒武-奧陶系; 3-西陽河群; 4-擔(dān)山石群; 5-中條群;6-銅礦峪亞群; 7-橫嶺關(guān)亞群; 8-基性-超基性巖體; 9-煙莊巖體; 10-橫嶺關(guān)巖體; 11-冷口表殼巖; 12-TTG巖套; 13-地質(zhì)界線; 14-斷裂; 15-絳縣群; 16-涑水雜巖1-Quaternary; 2-Cambrian-Ordovician; 3-Xiyanghe Group;4-Danshishan Group; 5-Zhongtiao Group; 6-Tongkuangyu Subgroup;7-Henglingguan Subgroup;8-basic-ultrabasic intrusion;9-Yanzhuang intrusion;10-Henglingguan intrusio;11-Lengkou supracrustal rock;12-TTG srite;13-geological boundary; 14-fault; 15-Jiangxian group; 16-Sushui complex
中條山基性超基性巖巖石類型較簡單,并具有巖相分帶特征,大部分中小巖體均屬輝石巖類,次為角閃石巖類,中心部分可達橄欖輝石巖類。根據(jù)7個揭露的巖體特征總結(jié)為中心相一般為橄欖巖、斜輝橄欖巖,多發(fā)生蛇紋石化和透閃石化;中間相為透閃蛇紋巖、蛇紋透閃巖或透閃巖;邊緣相一般為致密的角閃石巖、輝石巖,向外過渡為輝長巖;最外緣與圍巖接觸帶過渡為斜長角閃巖。
郭家溝巖體位于中條山基性-超基性巖帶的東北位置,與店上巖體相距3km左右,屬于中條山超基性巖帶的東北端。巖帶呈北東-南西向展布,長度在10km,寬度在幾十米到幾百米,連續(xù)出露寬度最寬在500m,巖體整體呈脈狀分布,部分呈橢球狀、紡錘狀分布,長軸方向與巖脈整體方向一致。走向10°~30°之間,傾向東南。巖體侵位受北東-南西向構(gòu)造和圍巖片麻理控制,多出現(xiàn)分支復(fù)合現(xiàn)象。根據(jù)野外產(chǎn)出特征,巖體侵位于涑水雜巖(圖2a、圖2c),野外多觀察到巖體順片麻巖層理侵位,也發(fā)現(xiàn)有以小巖枝透鏡狀產(chǎn)出切穿片麻理形式侵位片麻巖中,或橫切橫嶺關(guān)變二長花崗巖弱片麻理(圖2b),部分地段被晉寧期輝綠巖切穿。目前認為除晉寧期輝綠巖外,在該區(qū)基性-超基性巖侵位時代應(yīng)當(dāng)是最晚。巖石類型主要以變余輝石巖(角閃石巖)為主,次為(蝕變)輝長巖、透閃石巖(原巖有輝石巖,二輝巖,含橄欖二輝巖)、透閃石陽起石巖等。在地表開挖處可見到巖體具有巖相分帶現(xiàn)象:邊緣相為(蝕變)輝長巖,向內(nèi)則漸變過渡為含長輝石巖、變余輝石(角閃石巖)等,在巖體的中心發(fā)育著磁鐵透閃石巖(原巖為二輝巖、含橄欖石二輝巖)等(圖2c)。
圖2 郭家溝基性-超基性巖體野外及顯微照片F(xiàn)ig. 2 Photos showing outcrop and micrographs of the Guojiagou basic-ultrabasic rocks a-基性-超基性巖體侵位于斜長片麻巖;b-基性-超基性巖體侵位于橫嶺關(guān)花崗巖;c-郭家溝巖體巖相分帶示意圖;d-滑石中包裹的粒狀橄欖石;e-透閃巖中保留的原輝石假象;f-輝石巖礦物組成;g-輝長巖中的輝長結(jié)構(gòu);h-(蝕變)輝長巖中礦物組成;Ol-橄欖石;Tc-滑石;Mt-磁鐵礦;Tr-透閃石;Hb-普通角閃石;Pl-斜長石;Q-石英a-basic-ultrabasic rocks emplaced in plagiogneiss; b-basic-ultrabasic rocks emplaced in the granite of Henglingguan;c-facies zonation schematic of Guojiagou rocks;d-granular olivine is wrapped in talc;e-pyroxene is reserved in tremolite;f-composition of pyroxenite;g-gabbro structure in gabbro;h-the composition of(alteration) gabbro;Ol-olivine;Tc-talc;Tr-tremolite;Mt-magnetite;Hb-hornblende;Pl-plagioclase;Q-quartz
磁鐵透閃石巖呈淺灰白色、灰色,中粗粒狀變晶結(jié)構(gòu),塊狀構(gòu)造,巖石主要由透閃石(70%~75%)、滑石(10%~15%)、磁鐵礦(15%±)組成,局部滑石中包裹有粒狀橄欖石(圖2d)。滑石由斜方輝石蝕變而來,部分透閃石集合體保留原單斜輝石假象(圖2e)。
變輝石巖(角閃石巖),呈灰黑色帶綠色調(diào),變余自形粒狀結(jié)構(gòu)、中粗粒變晶結(jié)構(gòu),塊狀構(gòu)造,巖石主要由角閃石(85%~90%)、變余斜長石(3%~7%)、磁鐵礦(3%~5%)組成,角閃石具有不同種類,有普通角閃石、透閃石、陽起石等,局部地方保留有原短柱狀輝石假象,斜長石多發(fā)生絹云母化(圖2f)。
(蝕變)輝長巖,呈深灰綠色,變余中粗粒狀輝長結(jié)構(gòu)(圖2g),塊狀構(gòu)造,巖石主要由角閃石(40%~58%)、變余斜長石(40%~55%)、黑云母(5%±)、磁鐵礦(1%±)組成,角閃石為普通角閃石,多個角閃石聚集在一起,顯示出原輝石晶體假象,斜長石多已絹云母化,呈集合體產(chǎn)出,集合體呈半自形-自形長石晶形,部分地段輝長巖中可見到石英,顆粒較小,呈水滴狀分布在角閃石中(圖2h),可能是輝石向角閃石蝕變過程中析出的。
3.1 分析方法
主量和微量元素分析均在在長安大學(xué)西部礦產(chǎn)資源與地質(zhì)工程教育部重點實驗室完成。主量元素分析在X射線熒光光譜儀完成 (XRF法);微量元素采用Thermo-X7電感耦合等離子體質(zhì)譜儀(ICP-MS)進行測定。執(zhí)行標(biāo)準(zhǔn)參照DZ/0223-2001。儀器工作參數(shù):Power:1200w,Plasmagas:13L/min ,nebulizer gas:0.64L/min,Auxiliary gas:0.80L/min。
3.2 主量元素地球化學(xué)
郭家溝巖體中透閃石巖、變輝石巖(角閃石巖)主量元素總體顯示出低硅、 低鎂、 高鐵、 高鈦、高鈣的特征(表1)。透閃石巖SiO2含量為37.23%~38.46%,平均37.85%;MgO含量為18.66%~21.09%,平均19.88%;TFe2O3含量為27.49%~29.15% ,平均28.32%;TiO2含量為0.72%~0.80%,平均0.76%;CaO含量為4.55%~7.30%,平均5.93%。變輝石巖SiO2含量為42.50%~45.95%,平均44.23%;MgO含量為 6.34%~11.05%,平均8.70%;TFe2O3含量為16.98%~19.51% ,平均18.25%;TiO2含量為0.73%~1.03%,平均0.88%;CaO含量為9.03%~11.33%,平均10.18%(表1)。而輝長巖中主量元素含量變化較大,SiO2含量為47.70%~52.76%;MgO含量為 4.52%~13.58%;TFe2O3含量為9.27%~17.58% ;TiO2含量為0.39%~2.45%,;CaO含量為7.22%~12.38%(表1)。
表1 郭家溝巖體主量和微量元素分析數(shù)據(jù)
續(xù)表1
Continued Table 1
樣品編號G-10G-23G12G-15G22G-04G-06G-08G-17G-25巖石名稱透閃石巖透閃石巖變輝石巖變輝石巖變輝石巖輝長巖輝長巖輝長巖輝長巖輝長巖Sm1.480.932.122.912.817.713.103.314.323.51Eu0.310.100.560.791.012.300.750.961.120.81Gd1.480.832.353.092.827.682.993.154.453.53Tb0.200.100.310.420.391.170.430.450.540.46Dy1.110.541.892.402.136.362.422.322.942.63Ho0.210.090.360.430.381.290.510.450.510.49Er0.590.271.061.240.993.311.311.131.421.37Tm0.080.040.160.170.130.450.190.150.200.20Yb0.430.210.870.960.752.761.230.861.091.16Lu0.070.040.140.170.110.430.200.140.180.20ΣREE23.5914.1443.9347.1435.44173.8475.8767.0198.2671.08LREE19.4212.0236.7938.2627.74150.3966.5958.3686.9361.04HREE4.172.127.148.887.7023.459.288.6511.3310.04LR/HR4.665.675.154.313.606.417.186.757.676.08(La/Yb)N4.516.484.564.253.367.326.7310.0010.136.72(La/Sm)N1.201.341.721.290.822.412.452.392.352.04(Gd/Yb)N2.783.202.192.603.042.251.972.963.302.46δEu0.640.350.770.801.090.910.750.910.780.70Cs0.300.060.070.200.101.230.110.050.190.11Ba120.4789.1387.85161.01159.50666.80136.7048.41145.80164.16Rb0.700.011.5210.024.1672.0015.395.6712.4823.83Th0.862.251.501.910.984.974.091.942.092.67U0.780.570.550.401.221.111.980.670.901.34Nb2.481.331.952.751.1313.755.321.003.214.45Ta0.160.120.100.250.080.930.460.070.190.38Sr17.5263.3610.96114.00146.39383.90141.3091.92203.30221.25Zr30.0216.2331.5831.3822.56186.0048.0035.9891.4234.46Hf0.900.591.031.290.925.121.871.122.521.26Y4.602.168.409.557.7827.3610.459.3611.2110.80V497.02183.83635.67680.801017.58332.80226.70143.70704.76288.04Li0.531.283.896.864.9330.463.823.938.797.82Be0.240.081.121.000.391.281.471.020.571.23Sc12.9712.7612.9642.2457.8328.4257.1058.8949.5738.99Cr671.08505.13703.58318.50320.02126.10361.40400.0048.341892.43Co157.5394.33100.4052.2853.4146.7148.8346.5354.9444.18Ni165.48246.97174.7972.34110.1261.0877.96119.6046.04101.90Cu14.0174.22147.0620.77731.9095.8948.541030.0022.6647.69Zn114.01190.08132.60109.78110.76133.00128.4046.86109.0054.71Ga8.317.8815.9817.4414.8624.4413.6910.3116.369.42Cd0.100.260.090.070.210.190.050.030.200.12In0.030.020.070.100.110.090.070.060.080.05Pb2.9611.322.062.8440.8213.403.073.346.076.41Bi0.100.040.260.170.490.070.180.190.120.28
注:主量元素單位為%;微量元素單位為10-6; LOI為燒失量。測試單位:長安大學(xué)西部礦產(chǎn)資源與地質(zhì)工程教育部重點實驗室;測試時間:2014年6月。
3.3 稀土和微量元素地球化學(xué)
郭家溝透閃石巖類稀土總量ΣREE在14.14×10-6~23.59×10-6之間,LR/HR為4.66~5.67,(La/Yb)N在4.51~6.48之間,可見輕稀土相對富集,重稀土相對虧損,在球粒隕石標(biāo)準(zhǔn)化圖解中呈右傾,輕重稀土分餾相對明顯(圖3);(La/Sm)N為1.20~1.34,(Gd/Yb)N為 2.78~3.20,說明輕稀土元素內(nèi)部分異作用較弱,而重稀土元素內(nèi)部具有一定的分異作用;兩個樣品δEu為 0.35、0.64,為銪的負異常。微量元素地球化學(xué)特征表現(xiàn)為Ba、Th、U、La、Ce等元素相對富集,而Rb、Nb、Ta顯著虧損,Zr和Y則表現(xiàn)為弱的負異常(圖3)。
圖3 郭家溝巖體球粒隕石標(biāo)準(zhǔn)化稀土元素配分曲線圖和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖Fig.3 Chondrites standardized REE and normalized trace element distributions pattern for the Guojiagou rocksa-球粒隕石標(biāo)準(zhǔn)化稀土元素配分曲線圖;b-原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖;第一組-透閃石巖;第二組-變輝石巖;第三組-輝長巖a-chondrites standardized REE;b-normalized trace element distribution pattern; first group-tremolite;second group-metamorphic pyroxenol-ite;third group-gabbro
輝石巖類稀土總量ΣREE在35.44×10-6~47.14×10-6之間,LR/HR為3.60~5.15,(La/Yb)N在3.36~4.56之間,可見輕稀土相對富集,重稀土相對虧損,在球粒隕石標(biāo)準(zhǔn)化圖解中呈右傾,輕重稀土分餾相對明顯(圖3);(La/Sm)N為0.82~1.72,(Gd/Yb)N為 2.19~3.04,說明輕稀土元素內(nèi)部分異作用較弱,而重稀土元素內(nèi)部具有一定的分異作用;3個樣品δEu為 0.77~1.09,未顯示明顯的銪負異常。微量元素地球化學(xué)特征表現(xiàn)為Ba、Th、U、La、Ce、Sr等元素相對富集,Nb、Ta顯著虧損,Rb、Zr和Y則表現(xiàn)為較弱虧損(圖3)。
輝長巖類稀土總量ΣREE在67.01×10-6~173.84×10-6之間,LR/HR為6.08~7.67,(La/Yb)N在6.72~10.13之間,可見輕稀土相對富集,重稀土相對虧損,在球粒隕石標(biāo)準(zhǔn)化圖解中呈右傾,輕重稀土分餾較明顯(圖3);(La/Sm)N為1.60~2.45,(Gd/Yb)N為 1.97~3.30,說明輕、重稀土內(nèi)部具有一定的分異作用;δEu為 0.70~0.91,為弱的銪的負異常。微量元素圖解上表現(xiàn)為Cs、Ba、Rb、Th、U等元素相對富集,Nb、Ta顯著虧損, Sr、Zr和Y則表現(xiàn)為弱虧損(圖3)。
4.1 郭家溝巖體的時代
對中條山地區(qū)基性-超基性巖的侵入時代主要有以下兩種看法:(1)晚太古代侵入體(穆書漢,1983);(2)古元古代侵入體(呂梁期)(山西省地質(zhì)局,1:20萬運城、三門峽幅地質(zhì)圖說明書,1972)。
中條山地區(qū)基性-超基性巖體主要分布在夏縣-絳縣一帶,位于涑水雜巖的東北部,總體呈北東東向雁行展布,侵位于TTG質(zhì)片麻巖、非TTG質(zhì)花崗巖類(橫嶺關(guān)、煙莊巖體等)、冷口變質(zhì)火山巖系等太古代涑水雜巖中,可見該超基性雜巖體的形成時代應(yīng)該是夏縣-絳縣一帶涑水雜巖中最晚的。
近年來,針對中條山中部-東北部地區(qū)的涑水雜巖發(fā)表了大量高精度的年齡數(shù)據(jù)(圖4、表2),這些年齡數(shù)據(jù)顯示煙莊巖體形成最晚,年齡數(shù)據(jù)為(2351±37)Ma,而該基性-超基性雜巖體形成時代又晚于煙莊巖體。郭家溝巖體順片麻巖層理侵位,也發(fā)現(xiàn)有以小巖枝透鏡狀產(chǎn)出切穿片麻理形式侵位片麻巖中,或橫切橫嶺關(guān)變二長花崗巖弱片麻理,橫嶺關(guān)變二長花崗巖年齡數(shù)據(jù)為(2435.9±12)Ma,可見郭家溝巖體形成時代也應(yīng)該古元古代。
圖4 涑水雜巖中各巖石單元的形成年齡分布圖(據(jù)白謹?shù)龋?997;張瑞英,2015)Fig. 4 The distribution for the age of each rock unit in the Sushui complex (after Bai et al.,1997;Zhang,2015)1-第四系; 2-寒武系-奧陶系;3-西陽河群;4-擔(dān)山石群;5-中條群;6-銅礦峪亞群; 7-橫嶺關(guān)亞群;8-基性-超基性巖體;9-煙莊巖體;10-橫嶺關(guān)巖體;11-冷口表殼巖;12-TTG巖套;13-地質(zhì)界線;14-斷裂;15-絳縣群;16-涑水雜巖1-Quaternary; 2-Cambrian-Ordovician; 3-Xiyanghe Group;4-Danshishan Group;5-Zhongtiao Group;6-Tongkuangyu Subgroup; 7-Henglingguan Subgroup;8-basic-ultrabasic intrusion;9-Yanzhuang intrusion;10-Henglingguan intrusion;11-Lengkou supracrustal rock;12-TTG srite;13-geological boundary; 14-fault; 15-Jiangxian group; 16-Sushui complex
序號巖石類型測試方法同位素年齡資料來源1TTG片麻巖SHRIMPU-Pb法(2560±6)Ma田偉等,2005;郭麗爽等,20082TTG片麻巖LA-ICP-MS法(2561±15)Ma張瑞英等,20133TTG片麻巖LA-ICP-MS法(2553±21)Ma張瑞英等,20134TTG片麻巖LA-ICP-MS法(2543±21)Ma張瑞英等,20135冷口變質(zhì)火山巖LA-ICP-MS法(2562±22)Ma張瑞英等,20136橫嶺關(guān)花崗巖2435.9MaYuetal,20067煙莊花崗巖LA-ICP-MS法(2351±37)Ma張瑞英等,2012
4.2 郭家溝巖體的成因
郭家溝巖體在橫向上具有分帶現(xiàn)象:邊緣相為輝長巖,向內(nèi)則過渡為含長輝石巖、輝石角閃石巖、輝石巖等,在巖體的中心發(fā)育著含磁鐵礦透閃石巖(原巖為二輝巖、含橄欖石二輝巖)等;稀土元素球粒隕石配分曲線和微量元素原始地幔配分曲線較為相似,稀土總量并由透閃石巖至輝長巖稀土總量有規(guī)律的升高,說明巖體為同源巖漿演化的產(chǎn)物;在 FeOT- MgO- Na2O + K2O 圖上,郭家溝巖體的大部分樣品落在弧堆晶巖區(qū)(圖5a) 。在 Al2O3-MgO- CaO 圖上, 郭家溝巖體大部分樣品也落在鎂鐵、超鎂鐵堆晶巖或二者的過渡范圍內(nèi)(圖5b)。在MgO與其它主量元素哈克圖解中可發(fā)現(xiàn)(圖6):與SiO2、Al2O3、Na2O、K2O、CaO呈較好的負相關(guān)性,與P2O5、TiO2呈弱的負相關(guān),與TFe2O3、MnO呈正相關(guān)。顯示出各類巖石受分離結(jié)晶作用的影響,也體現(xiàn)出巖漿具有同源巖漿的演化趨勢。
圖5 郭家溝巖體FeOT - MgO- Na2O+K2O圖(a,據(jù)Irvine and Baragar,1971)和 Al 2O3 - MgO- CaO 圖( b,據(jù)Coleman,1977)Fig. 5 Diagrams of FeOT - MgO- Na2O + K2O (a,after Irvine and Baragar,1971) and Al2O3 - MgO- CaO ( b,after Coleman,1977)第一組-透閃石巖;第二組-變輝石巖;第三組-輝長巖first group-tremolite;second group-metamorphic pyroxenolite;third group-gabbro
郭家溝巖體主量元素特別是輝石巖以貧硅、貧鎂、富鐵、富鈣、富鈦等特征;稀土元素配分曲線總體呈右傾,輕稀土相對富集,重稀土相對虧損,輕重稀土分餾相對明顯,絕大多數(shù)樣品銪異常不明顯;在原始地幔標(biāo)準(zhǔn)化的微量元素蛛網(wǎng)圖中(圖3),相對富集LREE和LILE,顯著虧損Nb、Ta等HFSE,具有弱的Zr、Y負異常。另外,郭家溝巖體特別是輝石巖中V含量較高(表1)。
總之,郭家溝巖體的巖相及元素地球化學(xué)特征與阿拉斯加型雜巖體特征(張魁武等,1988;Himmelberg, Loney,1995;張旗,2014)相符,應(yīng)該屬于阿拉斯加型侵入體。
4.3 郭家溝巖體的大地構(gòu)造背景及地質(zhì)意義
中條山從構(gòu)造位置上地處華北板塊南緣,位于華北板塊中部造山帶的南緣(Zhao,1998,1999,2000,2005)。研究其前寒武紀的地質(zhì)構(gòu)造背景及演化對探索整個華北板塊前寒武紀的構(gòu)造演化具有重要的地質(zhì)意義。
涑水雜巖主要由變質(zhì)表殼巖、大量TTG質(zhì)巖石及花崗巖體組成,其主體形成時間為2800~2350Ma。這個時期陸核也逐漸形成,大規(guī)模的TTG巖漿侵入,顯示了重要的地殼生長事件和陸核邊緣火山弧的環(huán)境信息(白謹,1997;郭麗爽,2008;張瑞英,2013),橫嶺關(guān)巖體和煙莊巖體的侵入,巖石特征指示了巖石形成于活動大陸邊緣和同碰撞構(gòu)造環(huán)境(孫大中、胡維興,1993;郭麗爽,2008;張瑞英,2013)。前已及述,中條山超基性雜巖體的形成時代應(yīng)該是夏縣-絳縣一帶涑水雜巖中最晚的,其形成的大地構(gòu)造背景也應(yīng)該為活動大陸邊緣。
郭家溝巖體為鈣堿性巖石組合,其輕稀土(LREE)表現(xiàn)為富集,稀土配分曲線右傾,并富集LILE,虧損HFSE(Nb、Ta等),這些特征顯示出島弧的環(huán)境信息。再結(jié)合區(qū)域上的大地構(gòu)造背景,從而說明郭家溝巖體是侵入在大陸邊緣弧環(huán)境的巖漿分離結(jié)晶的產(chǎn)物。
圖6 郭家溝巖體MgO-其它主量元素協(xié)變圖Fig. 6 Plots of MgO vs. other oxides of the Guojiagou intrusion showing major- element variations with fractional crystallization第一組-透閃石巖;第二組-變輝石巖;第三組-輝長巖first group-tremolite;second group-metamorphic pyroxenolite;third group-gabbro
2550Ma~2350Ma,中條山地區(qū)存在大量的TTG巖體及花崗巖體侵入事件,顯示了重要的地殼生長事件和陸核邊緣火山弧的俯沖環(huán)境信息,2350Ma-2200Ma之間則存在明顯的巖漿活動中斷期(張晗2012,張瑞英,2015),一些學(xué)者用大量的年齡數(shù)據(jù)表明絳縣群應(yīng)形成于古元古代,其下部橫嶺關(guān)亞群變質(zhì)沉積巖最大沉積年齡為約2200Ma(李秋根等,2008;劉玄等,2015),而上部銅礦峪亞群火山-凝灰?guī)r極有可能形成于2160~2190Ma之間(孫海田和葛朝華,1990 ,孫大中等,1991;孫大中和胡維興,1993;陳文明和李樹屏,1998;趙鳳清,2006;張晗2012),侵位于銅礦峪巖群之中的次火山巖形成時代可能為2120~2160Ma之間(許慶林,2010;張晗,2012;李寧波等,2013)??梢?200Ma以后,中條山又開始了新一期的構(gòu)造巖漿活動。
在晚太古代時期,中條山、五臺和恒山地區(qū)具有相一致的動力學(xué)背景,它們表現(xiàn)明顯的親緣性(田偉等,2005)。2.1 ~2.3Ga,華北克拉通中部造山帶呂梁-恒山-五臺-阜平地區(qū)存在一期重要的與雁門關(guān)基性-超基性巖體近同時的巖漿侵入、火山噴發(fā)和沉積的構(gòu)造-巖漿地質(zhì)事件(Wangetal.,2010;Zhaoetal.,2004,2006;2007;萬加亮和王志洪,2012),且處于大陸邊緣弧的構(gòu)造環(huán)境(Wangetal.,2010),這是該造山帶自~2.5Ga 晚太古代強烈島弧巖漿活動后又一期重要的地殼增生事件(萬加亮和王志洪,2012)。將中條山地區(qū)郭家溝巖體與五臺地區(qū)雁門關(guān)巖體特征(萬加亮和王志洪,2012)進行對比,其巖石地球化學(xué)特征、巖石成因、構(gòu)造環(huán)境都具有很大的相似性。縱觀整個華北克拉通中部造山帶,在晚太古代-古元古代時期,中條山、五臺和恒山地區(qū)具有較為相似的的動力學(xué)背景,進而說明中條山基性-超基性巖體的侵入可能與絳縣群形成時代較為相近。
實際上,人們對絳縣群形成的大地構(gòu)造環(huán)境一直以來存在爭議。有些學(xué)者認為中條山地區(qū)在早元古代的大地構(gòu)造環(huán)境為裂谷環(huán)境(《中條山銅礦地質(zhì)》 編寫組,1978; 胡維興,1987;真允慶,1992;薛克勤,2006;劉玄,2015),而另一些研究者根據(jù)研究絳縣群中橫嶺關(guān)亞群的變質(zhì)沉積巖和銅礦峪亞群的雙峰式火山巖的巖石地球化學(xué)特征,認為絳縣群在早元古代的大地構(gòu)造環(huán)境為活動大陸邊緣構(gòu)造系統(tǒng)中的弧后盆地的伸展環(huán)境(李秋根等,2008;Lietal.,2009; Liuetal.,2012a; 張晗,2012),也可能經(jīng)歷了由俯沖造山到造山后伸展的地質(zhì)演化過程(楊崇輝等,2015)。若中條山基性-超基性巖的侵入與絳縣群的形成為同一期構(gòu)造巖漿活動事件,而基性-超基性巖體顯示出活動大陸邊緣的環(huán)境信息,進而從側(cè)面也說明絳縣群的形成環(huán)境應(yīng)該是大陸邊緣的俯沖后的伸展環(huán)境,而非陸內(nèi)裂谷。
(1)郭家溝巖體具有巖相分帶現(xiàn)象,邊緣相為輝長巖,向內(nèi)漸變過渡為含長輝石巖、變余輝石巖(角閃石巖)等,在巖體的中心發(fā)育著磁鐵透閃石巖;巖石中MgO與其他主量元素線性相關(guān),各類巖石地球化學(xué)特征較為相似,顯示出巖石受分離結(jié)晶作用的影響,具有巖漿同源演化的趨勢;
(2)郭家溝巖石總體顯示出低硅、 低鎂、 高鐵、 高鈦、高鈣的特征,相對富集LREE以及LILE,而虧損HREE和Nb、Ta、Zr、Y等高場強元素,輕重稀土分餾明顯,具有較弱的Eu異常, V含量較高,具有阿拉斯加型侵入巖體和與火山弧相關(guān)的巖漿特征。
(3)郭家溝巖體侵入時代為元古代,可能與絳縣群形成時代較為相近(2200Ma左右),形成于大陸邊緣弧的構(gòu)造環(huán)境,從側(cè)面也說明絳縣群的形成環(huán)境應(yīng)該是大陸邊緣俯沖后的伸展環(huán)境,而非陸內(nèi)裂谷。
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Petrogenesis and Geological Significance of the Guojiagou Basic-Ultrabasic Intrusion in Zhongtiao Mountains
LIN Wei,XUE Sheng-sheng,YUN Meng-chao
(ShanxiProvinceGeophysical,GeochemicalExplorationInstitute,Yuncheng,Shanxi044000)
According to the detailed study of petrology, mineralogy, geochemistry on the Guojiagou basic-ultrabasic intrusion in Zhongtiao Mountains, this work discussed the formation age, petrogenesis and geological significance of this intrusion. The age of this intrusion is the Proterozoic, which is similar with the Jiangxian group (about 2200Ma). The Guojiagou intrusion displays lithofacies zonation, with marginal facies of gabbro, gradual transition to inward with plagioclase-containing pyroxenite and palimpsest pyroxenolite (amphibolite), and central magnet tremolitite. The MgO content and other major elements in rocks have a linear correlation, and the geochemical characteristics of all kinds of rocks are similar, suggesting their influence by fractional crystallization, and a magmatic homologous evolution trend. These rocks generally show low silicon, low magnesium, high iron, high titanium, high calcium, LREE and LILE enrichment, loss of HREE, HFSE of Nb, Ta, Zr, Y, relatively obvious REE fractionation, weak Eu anomaly and high V content. They own characteristics of the Alaska intrusive rocks and volcano arc related magmatic features. Combined the rock assemblages and geochemical characteristics with the regional tectonic evolution, it is inferred that the pluton was formed in a continental marginal arc tectonic environment, and that the formation environment of the Jiangxian group should be a continental margin extensional environment after subduction, rather than an intracontinental rift.
basic-ultrabasic intrusion, petrogenesis, continental arc, Guojiagou, Zhongtiao Mountains
2016-07-29;[修改日期]2016-10-19;[責(zé)任編輯]郝情情。
林 偉(1982年- ),男,地質(zhì)工程師,2009年畢業(yè)于長安大學(xué)礦物學(xué)、巖石學(xué)、礦床學(xué)專業(yè),獲碩士學(xué)位,現(xiàn)從事固體礦產(chǎn)資源勘查工作。E-mail:179438543@qq.com。
P595
A
0495-5331(2016)06-1116-13