馬方彬,郗國(guó)慶,王開(kāi)虎,方??担子?,劉文建
(1.中化地質(zhì)礦山總局地質(zhì)研究院,河北 涿州 072754;2.中化地質(zhì)礦山總局新疆地質(zhì)調(diào)查院,新疆 烏魯木齊 830000;3.中國(guó)煤炭地質(zhì)總局特種技術(shù)勘探中心,北京 100073;4.上海申豐地質(zhì)新技術(shù)應(yīng)用研究所有限公司,上海 201702;5.西安地質(zhì)礦產(chǎn)勘查開(kāi)發(fā)院,陜西 西安 710100)
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東天山紅包山-堿泉
—帶鎂鐵-超鎂鐵質(zhì)巖體的地球化學(xué)特征與巖石成因
馬方彬1,郗國(guó)慶1,王開(kāi)虎2,方福康3,白玉4,劉文建5
(1.中化地質(zhì)礦山總局地質(zhì)研究院,河北 涿州 072754;2.中化地質(zhì)礦山總局新疆地質(zhì)調(diào)查院,新疆 烏魯木齊830000;3.中國(guó)煤炭地質(zhì)總局特種技術(shù)勘探中心,北京 100073;4.上海申豐地質(zhì)新技術(shù)應(yīng)用研究所有限公司,上海 201702;5.西安地質(zhì)礦產(chǎn)勘查開(kāi)發(fā)院,陜西 西安 710100)
摘要:紅包山—堿泉一帶鎂鐵-超鎂鐵質(zhì)巖體位于東天山卡瓦布拉克地區(qū),巖體呈脈狀或透鏡狀產(chǎn)出,出露面積約8km2。巖石類型主要為純橄欖巖、橄欖巖、單輝橄欖巖等。地球化學(xué)數(shù)據(jù)表明,該區(qū)巖體具高M(jìn)g#的特征,屬鎂質(zhì)超基性巖。巖石稀土元素為平緩右傾型配分模式,稀土元素總量(0.96×10-6~10.02×10-6)較低,輕重稀土元素組間分餾較強(qiáng)((La/Yb)N=2.14~4.08),δEu變化于0.63~1.08,可能與斜長(zhǎng)石的結(jié)晶有關(guān)。微量元素蛛網(wǎng)圖表明,巖石具富集大離子親石元素Rb、Sr和高場(chǎng)強(qiáng)元素U、Pb及LREE,而虧損高場(chǎng)強(qiáng)元素Nb(Ta)、Hf、Ti的特征。主微量元素等地球化學(xué)相關(guān)圖解說(shuō)明,紅包山—堿泉一帶巖體原生巖漿為幔源的鈣堿性玄武質(zhì)巖漿,成巖作用以巖漿結(jié)晶分異為主導(dǎo),并受到殼源物質(zhì)的同化混染作用。
關(guān)鍵詞:鎂鐵-超鎂鐵質(zhì);地球化學(xué);結(jié)晶分異;同化混染;東天山
卡瓦布拉克鎂鐵-超鎂鐵質(zhì)巖帶是中化地質(zhì)礦山總局地質(zhì)研究院在進(jìn)行地質(zhì)調(diào)查工作時(shí)發(fā)現(xiàn)的一個(gè)含鈷鎳的巖體帶,是天山地區(qū)鎂鐵-超鎂鐵質(zhì)巖體的重要組成部分。該巖帶位于卡瓦布拉克—喀拉塔格一帶,因其晚古生代巖漿巖廣泛發(fā)育而備受地質(zhì)工作者關(guān)注,目前該地區(qū)已積累了大量的地質(zhì)資料,但開(kāi)展的科學(xué)研究工作極少,尤其是對(duì)鎂鐵-超鎂鐵質(zhì)雜巖體的研究,僅限于其地質(zhì)特征、物化探異常等方面,而有關(guān)鎂鐵-超鎂鐵質(zhì)雜巖體巖石地球化學(xué)方面的研究甚少,對(duì)原生巖漿、巖漿演化、同化混染及構(gòu)造環(huán)境等巖體成因問(wèn)題的論述尚屬空白。筆者選取位于紅包山西部、堿泉南部的紅包山—堿泉一帶鎂鐵-超鎂鐵質(zhì)巖體(圖1b)為研究對(duì)象,通過(guò)分析研究該巖體的地質(zhì)特征、巖石地球化學(xué)特征,以求對(duì)上述問(wèn)題進(jìn)行初步探索,并為該區(qū)后續(xù)的地質(zhì)工作提供參考。
Q.第四系;Kgn.花崗質(zhì)片麻巖;Sh.表殼巖;γδ.花崗閃長(zhǎng)巖;Tgn.灰色片麻巖;∑1.鎂質(zhì)-超鎂鐵雜巖體;Vgn.輝長(zhǎng)質(zhì)片麻巖;ηγ.二長(zhǎng)花崗巖;ζγ.鉀長(zhǎng)花崗巖;1.研究區(qū);2.主要斷裂;Ⅱ1.哈薩克斯坦-準(zhǔn)噶爾板塊;Ⅲ1.塔里木北緣陸緣活動(dòng)帶;Ⅲ11.覺(jué)羅塔格晚古生代溝弧帶;Ⅲ12.巴侖臺(tái)-穹塔格-星星峽地塊;Ⅲ2.塔里木板塊;圖1 (a)東天山東部構(gòu)造簡(jiǎn)圖和(b)研究區(qū)地質(zhì)略圖(據(jù)新疆地勘局第一地質(zhì)大隊(duì),2003)Fig.1 (a)Tectonic sketch map of the eastern East Tianshan and (b)geological sketch map of the study area (No.1 Regional Geological Survey Team, Bureau of Geology and Mineral Resources,2003)
1地質(zhì)背景及巖體特征
卡瓦布拉克鎂鐵-超鎂鐵質(zhì)巖帶位于新疆哈密市西南約220km處,大地構(gòu)造位于卡瓦布拉克斷裂以北與阿奇克庫(kù)都克斷裂以南所夾持的狹長(zhǎng)區(qū)域,屬塔里木北緣陸緣活動(dòng)帶,巴侖臺(tái)-穹塔格-星星峽地塊內(nèi)(蔡雄飛等,2013),巖體呈近東西向帶狀展布,斷續(xù)延伸達(dá)28km,寬50~100m,向西尖滅于黑山以南(圖1a)。已發(fā)現(xiàn)巖體共計(jì)100多個(gè),主要位于東經(jīng)91°38′~92°46′,北緯為41°24′~41°32′,主要呈脈狀、串珠狀或透鏡狀產(chǎn)出,出露面積大小不等,總面積約8km2(新疆地勘局第一地質(zhì)大隊(duì),2003)。研究區(qū)具克拉通結(jié)晶基底,為古老變質(zhì)巖系,區(qū)域上長(zhǎng)期隆起剝蝕,蓋層幾乎剝蝕殆盡(郭華春等,2002),出露地層僅為太古宇的一套深成變侵入巖、表殼巖及第四系沉積物,其中深成變侵入巖巖性有鉀長(zhǎng)花崗質(zhì)片麻巖(Kgn)、輝長(zhǎng)質(zhì)片麻巖(Vgn)及灰色片麻巖(Tgn/TTG)等(圖1b)。區(qū)內(nèi)構(gòu)造較為簡(jiǎn)單,主要受南部呈北西—南東向的卡瓦布拉克斷裂及其次級(jí)斷裂影響。區(qū)內(nèi)侵入巖分布廣泛,主要為花崗閃長(zhǎng)巖、鉀長(zhǎng)花崗巖、二長(zhǎng)花崗巖等中酸性巖體及鎂鐵-超鎂鐵質(zhì)巖體(圖1b)。研究區(qū)鎂鐵-超鎂鐵質(zhì)巖體多呈脈狀、透鏡狀侵入于太古宇鉀長(zhǎng)花崗質(zhì)片麻巖、輝長(zhǎng)質(zhì)片麻巖等深成變侵入巖中(圖1b),與圍巖界線清楚,局部可見(jiàn)圍巖捕虜體。巖體受卡瓦布拉克斷裂構(gòu)造控制明顯,宏觀上多呈斷續(xù)帶狀分布于斷裂兩側(cè),且總體展布方向同構(gòu)造線方向基本一致。巖體巖石類型為橄欖巖、輝橄巖、輝石巖、輝長(zhǎng)巖等基性-超基性巖,各巖石主要特征見(jiàn)表1。
表1 研究區(qū)鎂鐵-超鎂鐵質(zhì)巖體巖石特征表
2樣品及測(cè)試方法
筆者選取研究區(qū)內(nèi)ZK2-1鉆孔中的2件橄欖巖(圖2A、圖2B)、2件輝石橄欖巖(圖2C)及1件輝石巖(圖2D)開(kāi)展全巖主微量元素分析,測(cè)試結(jié)果見(jiàn)表2。全巖的主微量、稀土元素測(cè)試在中化地質(zhì)總局中心實(shí)驗(yàn)室完成,其中主量元素分析采用XRF法,稀土及微量元素使用等離子質(zhì)譜儀(ICP-MS)測(cè)定。
3巖石地球化學(xué)
3.1主量元素地球化學(xué)
本區(qū)鎂鐵-超鎂鐵質(zhì)巖體的全巖組分(表2)整體以低SiO2,高M(jìn)gO和高M(jìn)g#為主要特征,并隨鉆孔深度的變化呈現(xiàn)連續(xù)性的變化:SiO2含量為36.75%~37.07%,MgO含量為32.36%~41.26%,Al2O3(0.22%~1.23%)和TFeO(7.09%~8.30%)含量較為穩(wěn)定,CaO含量為0.08%~4.09%,全堿(K2O+Na2O)(0.09%~0.69%)、TiO2(0.01%~0.08%)和MnO(0.07%~0.14%)等含量較少。樣品Mg#值均較高,變化于0.883~0.918,m/f值為7.47~11.10,屬鎂質(zhì)超基性巖(吳利仁,1963)。由氧化物-MgO相關(guān)性圖解(圖3),TFeO與MgO的含量變化關(guān)系不太明顯,MnO與MgO的含量變化整體呈明顯的正相關(guān)關(guān)系,SiO2、TiO2、Al2O3、CaO等隨MgO含量的變化呈明顯的負(fù)相關(guān)關(guān)系,以上特征基本同巖漿分離結(jié)晶中礦物的晶出順序相對(duì)應(yīng),說(shuō)明巖漿演化過(guò)程中化學(xué)成分的變化整體受分離結(jié)晶作用的主導(dǎo)。
圖2 研究區(qū)典型巖石照片F(xiàn)ig. 2 Photos of typical rocks of the study area
在SiO2-FeO/MgO圖(圖4)上,雜巖體位于鈣堿性玄武巖系列區(qū)及鈣堿性與拉斑系列的過(guò)渡帶區(qū),表明該區(qū)巖漿總體具有由鈣堿性系列向拉斑系列演化的趨勢(shì),即早期巖漿應(yīng)以鈣堿性玄武質(zhì)巖漿為主。前人(孫赫等,2007;秦克章等,2012)研究認(rèn)為東天山地區(qū)鎂鐵-超鎂鐵質(zhì)巖體巖漿具拉斑玄武巖向鈣堿性玄武巖過(guò)渡的趨勢(shì),且鈣堿性巖漿源更有利于該區(qū)巖漿鎳銅硫化物礦床的形成。但本區(qū)鎂鐵質(zhì)-超鎂鐵質(zhì)巖體中FeO的含量較低(表2),這代表高氧逸度環(huán)境,不利于銅鎳硫化物的富集,對(duì)銅鎳硫化物成礦有一定的抑制作用,與東天山地區(qū)天宇、圖拉爾根等典型的銅鎳礦化體中較高的FeO含量形成鮮明對(duì)比。同時(shí),本區(qū)巖體m/f值為7.47~11.10,與前人(吳利仁,1963)研究的與硫化銅鎳礦及鉑礦有關(guān)的巖石m/f值(2~6.5)有所不同。故本區(qū)鎂鐵-超鎂鐵質(zhì)巖體的硫化物銅鎳礦成礦潛力較小。另外,據(jù)電子探針?lè)治黾扮R下觀察*中化地質(zhì)礦山總局地質(zhì)研究院.新疆覺(jué)羅塔格-拜城一帶鐵磷礦資源遠(yuǎn)景調(diào)查報(bào)告, 2015.,該區(qū)局部蛇紋石化橄欖巖中出現(xiàn)鎳礦化,且賦存于含鎳磁鐵礦-鐵鎳磁鐵礦中,與傳統(tǒng)的銅鎳硫化物礦化有所不同,為內(nèi)生氧化鎳型礦化,其具體礦化過(guò)程及成礦模式有待進(jìn)一步深入研究。
3.2稀土元素地球化學(xué)
該雜巖體樣品REE分配模式基本相同,均為輕稀土相對(duì)富集的平緩右傾型(圖5);稀土元素總量較低(∑REE為0.96×10-6~10.02×10-6)(表2),且∑REE的變化由橄欖巖、輝橄巖到輝石巖逐漸增高。其中,(La/Yb)N變化于2.14~4.08,即輕重稀土元素組間分餾較強(qiáng);(La/Sm)N變化于2.36~3.67,(Gd/Yb)N變化于0.49~0.93,即輕稀土元素內(nèi)部分餾程度略強(qiáng),重稀土元素內(nèi)部分餾程度較弱;橄欖巖中δEu=0.63~0.74,具有明顯的負(fù)銪異常,而輝橄巖和輝石巖中有弱的正Eu異?;驘o(wú)異常(δEu=0.89~1.08),可能是由于隨巖漿結(jié)晶分異作用的進(jìn)行,一些斜長(zhǎng)石發(fā)生結(jié)晶,從巖漿中析出所致;所有樣品基本無(wú)鈰異常(δCe為0.86~0.92)。
圖3 巖石主量元素氧化物(%)與MgO(%)相關(guān)圖Fig.3 Diagrams of oxides (%) versus MgO (%) of rocks
圖4 巖石化學(xué)分類圖(據(jù)姜常義等,2012)Fig.4 Geochemical classification diagram of rocks (From JIANG et al., 2012)
3.3微量元素地球化學(xué)
由微量元素蛛網(wǎng)圖(圖5)上可知,該雜巖體樣品原始地幔配分曲線基本相同或相似,微量元素總體富集程度較低,具富集大離子親石元素Rb、Sr和高場(chǎng)強(qiáng)元素U、Pb及LREE,而虧損高場(chǎng)強(qiáng)元素Nb(Ta)、Hf、Ti的特征。Sr在橄欖巖中略微虧損,而在輝橄巖、輝石巖中富集程度依次變強(qiáng),可能是由于元素Sr常以類質(zhì)同像出現(xiàn)在斜長(zhǎng)石中(陶琰等,2007),而隨著巖漿的演化,巖漿中結(jié)晶出的斜長(zhǎng)石數(shù)量不斷增多而致。高場(chǎng)強(qiáng)元素Nb(Ta)的虧損是大陸拉斑玄武巖的普遍特征,同時(shí)從橄欖巖到輝橄巖、輝石巖,Nb(Ta)負(fù)異常逐漸顯著,這些可能都是巖漿在上升過(guò)程中發(fā)生一定程度的陸殼混染作用的結(jié)果(ARNDT et al.,1992)。
圖5 (a)研究區(qū)巖體球粒隕石標(biāo)準(zhǔn)化稀土元素配分曲線圖和(b)原始地幔標(biāo)準(zhǔn)化配分曲線圖(標(biāo)準(zhǔn)化值據(jù)SUN et al.,1989)Fig.5 (a)Chondrite-normalized REE patterns and (b) PM-normalized trace elements spider diagram ofthe study area intrusions(Normalizing values after SUN et al., 1989)
巖性輝石巖輝橄巖輝橄巖橄欖巖橄欖巖樣號(hào)ZK2-1-B1ZK2-1-B4ZK2-1-H8ZK2-1-H10ZK2-1-H88SiO239.3238.5736.1537.0736.75TiO20.080.020.010.010.01Al2O31.230.410.250.270.22Fe2O36.856.576.736.465.51FeO1.451.601.110.881.58MnO0.140.070.100.090.11MgO32.3637.4938.2340.6641.26CaO4.092.171.790.220.08Na2O0.140.090.080.080.09K2O0.550.060.010.010.01P2O50.080.070.000.020.01LOI13.0212.7714.0712.2712.58Total98.7799.8398.5298.0398.19Mg#0.8830.8990.9050.9150.918m/f7.478.849.4110.7211.10Co86.10101.0089.30102.00102.00Ni1922.002348.002104.002236.002268.00Cu4.645.144.664.671.38Rb10.101.661.092.050.87
續(xù)表2
巖性輝石巖輝橄巖輝橄巖橄欖巖橄欖巖樣號(hào)ZK2-1-B1ZK2-1-B4ZK2-1-H8ZK2-1-H10ZK2-1-H88Sr76.0036.4026.105.293.95Zr20.0010.008.0016.009.00Nb0.090.110.180.210.23Mo0.450.370.730.530.26Cd0.030.020.030.020.01Cs0.460.230.100.290.13Ba72.1028.706.585.335.14Hf0.080.120.220.170.20Ta0.090.070.040.060.04Pb2.102.412.071.971.32Bi0.020.020.030.020.02Th1.720.280.260.690.42U0.420.110.050.030.03La1.990.480.170.300.26Ce3.610.890.310.610.54Pr0.490.130.040.090.08Nd1.880.490.180.370.35Sm0.350.110.050.070.07Eu0.110.040.010.020.01Gd0.330.100.030.070.06Tb0.060.010.010.010.01Dy0.390.120.040.100.09Ho0.090.020.010.020.01Er0.270.070.030.050.04Tm0.040.010.010.010.01Yb0.350.090.060.070.06Lu0.050.010.020.010.01Y2.320.650.290.460.41ΣREE10.022.570.961.771.61LREE8.432.140.761.461.31HREE1.590.430.190.320.30LREE/HREE5.324.963.954.564.41LaN/SmN3.672.822.392.692.36GdN/YbN0.780.910.490.840.93LaN/YbN4.083.962.143.313.39δEu0.971.080.890.740.63δCe0.870.860.860.920.92
注:Mg#=Mg2+/(Mg2++TFe2+),m/f=(Mg2++Ni2+)/(Fe3++Fe2++Mn2+),表中涉及到主量元素的計(jì)算及文中主量元素投圖時(shí)均按扣除燒失量后的百分含量計(jì);主量元素含量為%,微量元素含量為10-6。
4討論
4.1同化混染作用
巖石地球化學(xué)研究表明,在巖漿結(jié)晶分異作用過(guò)程中,不同元素在不同礦物之間的相容性存在差異,故巖漿在演化過(guò)程中元素豐度會(huì)隨著結(jié)晶分異作用的進(jìn)行而發(fā)生變化,但總分配系數(shù)相同或相近的元素比值則不受結(jié)晶分異作用影響。故總分配系數(shù)相同或很相近且對(duì)同化混染作用敏感的元素比值(Zr/Nb-Th-Nb、Ce/Yb-La/Yb和Nb/Ce-Nb/U等)間的相關(guān)變化,可以準(zhǔn)確地驗(yàn)證同化混染作用的存在與否及混染程度(CAMPBELL et al.,1993;BAKER et al.,1997;MECDONALD et al.,2001;姜常義等,2006,2007;夏昭德等,2009;TANG et al.,2012;馮宏業(yè)等,2014)。在圖6a、圖6b、圖6c中可以看出,Zr/Nb-Th/Nb、Ce/Yb-La/Yb和Nb/Ce-Nb/U表現(xiàn)出明顯的正相關(guān)性,顯示存在同化混染作用。在巖漿結(jié)晶分異和部分熔融過(guò)程中,Ce與Pb具相似的總分配系數(shù)(HOFMANN,1988;SUN et al.,1989)而使得Ce/Pb值保持不變,且地殼Ce/Pb值小于15,典型地幔Ce/Pb值為25±5(HOFMANN,1988;FURMAN et al.,2004);本區(qū)巖體中巖石樣品的Ce/Pb值(0.15~1.72)介于地殼范圍,說(shuō)明存在地殼的混染作用。同時(shí),在La/Ba-La/Nb圖解(圖6d)中也表明該區(qū)巖漿演化過(guò)程中存在地殼物質(zhì)的混染作用。另外,據(jù)Lassiter and Depaolo(1997)研究,元素La/Sm值大于4.5時(shí),指示存在地殼物質(zhì)的混染。本區(qū)一件輝石巖樣品La/Sm值為5.69,一件輝橄巖樣品La/Sm值為4.36,大于4.5或在4.5附近,暗示存在陸殼物質(zhì)的混染作用。綜上所述,該區(qū)巖漿在上升演化過(guò)程中存在了一定程度的陸殼物質(zhì)的混染作用。
圖6 同化混染地球化學(xué)判別圖解(d據(jù)夏林圻等,2007)Fig.6 Geochemical discriminant diagram of contamination(d After XIA et al. ,2007)
4.2結(jié)晶分異作用
區(qū)內(nèi)ZK2-1鉆孔中,橄欖巖、輝橄巖及輝石巖樣品LREE/HREE(3.95~5.32)值及δEu值(0.63~1.08)相近,稀土元素球粒隕石標(biāo)準(zhǔn)化配分曲線和微量元素原始地幔標(biāo)準(zhǔn)化配分曲線形態(tài)基本相同或相似,表明各巖石為同源巖漿演化的產(chǎn)物。隨著鉆孔深度從深到淺,由橄欖巖、輝橄巖到輝石巖,巖石基性程度逐漸降低,主量元素含量上連續(xù)變化,Al2O3、CaO和K2O含量逐漸升高,而Mg#和MgO則逐漸降低(圖3、表2);稀土元素方面,隨Mg#增大,巖石∑REE逐漸減小,(La/Sm)N逐漸減小(圖7a);微量元素方面,隨Mg#增大,巖石中不相容元素Sr、Y逐漸減小,而相容元素Ni總體逐漸增大(圖7b、圖7c、圖7d)。以上特征表明巖漿演化過(guò)程中受分離結(jié)晶作用的控制(李立興等,2012)。
Mg#值是研究(超)鎂鐵質(zhì)巖漿演化的重要參數(shù)。不同學(xué)者對(duì)地幔橄欖巖平衡的原生巖漿中Mg#值范圍劃定不同:FREY F A et al(1978)認(rèn)為Mg#值范圍為0.68~0.73,GREEN D H(1975)將Mg#值劃為0.63~0.73,而HESS P C(1992)則認(rèn)為Mg#值大于0.68。如果將原生巖漿和近于原生巖漿的Mg#范圍取為0.65~0.73,區(qū)內(nèi)巖石樣品Mg#值為0.88~0.92,說(shuō)明該區(qū)橄欖巖、輝橄巖和輝石巖主要由巖漿早期結(jié)晶的礦物相聚集而成(姜常義等,2006)。
圖7 研究區(qū)巖體微量元素與Mg#相關(guān)圖Fig.7 Diagrams of trace elements versus Mg# of the study area intrusions
區(qū)內(nèi)巖石中普遍存在的包含結(jié)構(gòu)和包橄結(jié)構(gòu),是巖漿結(jié)晶過(guò)程中存在橄欖石和輝石分離結(jié)晶作用的直接地質(zhì)證據(jù)。同時(shí),在地球化學(xué)方面也證實(shí)了橄欖石和輝石分離結(jié)晶的存在。在圖3中,巖石樣品SiO2與MgO(TFeO)具明顯的負(fù)相關(guān)性,顯示有橄欖石和輝石的分離結(jié)晶。據(jù)GREEN TH(1994)的研究,在巖漿結(jié)晶過(guò)程中,元素Co、Ni常以類質(zhì)同象的形式置換橄欖石中的Mg,故Co、Ni元素的豐度受控于巖漿中橄欖石的分離/堆晶作用。樣品中Ni、Co含量普遍較高以及Ni、Co與MgO顯示出明顯的正相關(guān)性反映了橄欖石的分離結(jié)晶/堆晶作用(圖8a、圖8b)。由圖8c及表2中,CaO、Al2O3與SiO2呈較好的正相關(guān)性反映了斜長(zhǎng)石或單斜輝石的分離結(jié)晶作用(夏昭德等,2009)。GEIST D et al(1998)研究表明,巖漿中CaO的含量和CaO/Al2O3比值會(huì)隨著單斜輝石的晶出而降低。在圖8d中,CaO與CaO/Al2O3表現(xiàn)出較好的相關(guān)性,也說(shuō)明存在單斜輝石的分離結(jié)晶。另外,橄欖巖中具負(fù)Eu異常,輝橄巖和輝石巖中有弱的正銪異常(表2),暗示巖漿結(jié)晶過(guò)程中存在少量斜長(zhǎng)石的結(jié)晶/堆晶作用。
綜上所述,研究區(qū)鎂鐵-超鎂鐵質(zhì)巖體在巖漿演化過(guò)程中主要受橄欖石和單斜輝石的分離結(jié)晶作用控制,受斜長(zhǎng)石的堆晶作用控制不明顯。
圖8 分離結(jié)晶判別圖Fig. 8 Discriminant diagram of fractional crystallization
4.3可能的成巖環(huán)境
研究區(qū)鎂鐵-超鎂鐵質(zhì)巖體侵入于中天山島弧帶南緣,測(cè)試樣品主量元素TiO2(0.01%~0.08%)的含量較低,相對(duì)富集大離子親石元素Rb、Sr和高場(chǎng)強(qiáng)元素U及LREE,相對(duì)虧損高場(chǎng)強(qiáng)元素Nb(Ta)、Ti的特征,趨向于島弧巖漿特征(HOLE et al.,1984;THOMPSON et al.,1984)。然而,巖石樣品以高M(jìn)g、Mg#和Cr的特征,顯示為幔源巖漿特征(COX et al.,1980;毛啟貴等,2006);樣品的Ta/Hf(0.182~1.125)、Th/Hf(1.182~21.5)及Th/Ta(4~19.11)值均具有E-MORB特征,顯示其巖漿源于富集型地幔;Th/Ta>1.6,Ta/Hf>0.2,反映巖漿形成環(huán)境不同與板塊匯聚邊緣或大洋板內(nèi),而更接近大陸板內(nèi)特征。筆者采用Hf/3-Th-Ta及Th/Hf-Ta/Hf判別圖(圖9a、圖9b、圖10)研究區(qū)內(nèi)巖體產(chǎn)出的構(gòu)造環(huán)境,在圖9a中樣品投在島弧區(qū)(鈣堿性玄武巖區(qū))及板內(nèi)玄武巖區(qū),圖9b中投在大陸拉張帶(或初始裂谷)玄武巖區(qū),結(jié)合研究區(qū)鎂鐵-超鎂鐵質(zhì)巖體出露于卡瓦布拉克-星星峽離散地體內(nèi)部的地質(zhì)背景,推測(cè)紅包山—堿泉一帶鎂鐵-超鎂鐵質(zhì)巖體可能形成于大陸板內(nèi)初始裂谷或地幔柱熱點(diǎn)區(qū)。
5結(jié)論
(1)紅包山—堿泉一帶鎂鐵-超鎂鐵質(zhì)巖體為巖漿在以分離結(jié)晶作用為主導(dǎo),殼源物質(zhì)混染綜合作用下的產(chǎn)物;其母巖漿可能為具幔源的拉斑玄武質(zhì)巖漿。
A.N型MORB區(qū);B.火山弧玄武巖區(qū);C.堿性板內(nèi)玄武巖區(qū);D.鈣堿性玄武巖區(qū)(D1區(qū))和島弧拉斑玄武巖區(qū)(D2區(qū))圖9 Hf/3-Th-Ta判別圖解(底圖據(jù)PEARCE,1982)Fig. 9 Discriminant diagram of Hf/3-Th-Ta (After PEARCE, 1982)
Ⅰ.板塊發(fā)散邊緣N-MORB區(qū);Ⅱ.板塊匯聚邊緣(Ⅱ1.大洋島弧玄武巖區(qū);Ⅱ2.陸緣島弧及陸緣火山弧玄武巖區(qū));Ⅲ.大洋板內(nèi)洋島、海山玄武巖區(qū)及T-MORB、E-MORB區(qū);Ⅳ.大陸板內(nèi)(Ⅳ1.陸內(nèi)裂谷及陸緣裂谷拉斑玄武巖區(qū);Ⅳ2.陸內(nèi)裂谷堿性玄武巖區(qū);Ⅳ3.大陸拉張帶(或初始裂谷)玄武巖區(qū));Ⅴ.地幔熱柱玄武巖區(qū)圖10 Th/Hf-Ta/Hf判別圖解(底圖據(jù)PEARCE,1982)Fig.10 Discriminant diagram of Th/Hf-Ta/Hf (After PEARCE, 1982)
(2)該巖漿在演化過(guò)程中主要受橄欖石和單斜輝石分離結(jié)晶作用的控制,受斜長(zhǎng)石結(jié)晶作用的控制不明顯。
(3)本區(qū)鎂鐵-超鎂鐵質(zhì)巖體的硫化物銅鎳礦成礦潛力較小,賦存于含鎳磁鐵礦-鐵鎳磁鐵礦中的鎳礦化,與傳統(tǒng)的銅鎳硫化物礦化有所不同,為內(nèi)生氧化鎳型礦化,其具體礦化過(guò)程及成礦模式有待進(jìn)一步深入研究。
致謝:感謝中化地質(zhì)礦山總局地質(zhì)研究院夏學(xué)惠(教高)總工、商朋強(qiáng)高工在論文撰寫過(guò)程中給予的指導(dǎo)和幫助。審稿專家提出了許多寶貴的修改意見(jiàn),進(jìn)一步完善了本文,在此深表感謝!野外工作和資料收集得到覺(jué)羅塔格項(xiàng)目組成員的大力支持和幫助,筆者謹(jǐn)向他們致以衷心感謝!
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收稿日期:2015-04-23;修回日期: 2015-12-26
基金項(xiàng)目:中國(guó)地質(zhì)調(diào)查局“新疆覺(jué)羅塔格-拜城一帶鐵磷礦資源遠(yuǎn)景調(diào)查”(1212011220827)
作者簡(jiǎn)介:馬方彬(1987-),男,河北石家莊人,助理工程師,碩士學(xué)位,礦產(chǎn)普查與勘探專業(yè)。E-mail: mfangbin@126.com
中圖分類號(hào):P618.46 ; P618.31
文獻(xiàn)標(biāo)志碼:A
文章編號(hào):1009-6248(2016)02-0176-13
Geochemical Characteristics and Petrogenesis of Hongbaoshan-Jianquan Mafic-UltramaficIntrusions in East Tianshan, Xinjiang
MA Fangbin1,XI Guoqing1, WANG Kaihu2, FANG Fukang3,BAI Yu4,LIU Wenjian5
(1. Geological Research Institute of China Chemical Geology and Mine Bureau, Zhuozhou 072754, Hebei,China;2.Xinjiang Institute of Geological Exploration, General Bureau of Geology and Mine, Wulumuqi 830000,Xinjiang, China;3.China Coal Geology Engineering Corporation, Beijing 100073, China; 4.Shanghai Shenfeng Institute of Novel Geological Techniques Co., Ltd. Shanghai 201702, China;5.Xi'an Institute of Geological and Mineral Exploration, Xi'an 710100, Shaanxi, China)
Abstract:Located in the Kawabulake area of east Tianshan, the Hongbaoshan-Jianquan manfic-ultramafic intrusions exhibit as vein and lenticular, with outcropped area of ~8km2. It mainly consists of dunite,peridotite and wehrlite. The geochemical data shows that these intrusions belong to magnesian ultramafic rocks, which are characterized by high Mg# and gently LREE-rich pattern with low ∑REE values (0.96×10-6~10.02×10-6) and enrichment of LREE ((La/Yb)N=2.14~4.08). Additionally, they are enriched LILE (Rb and Sr) and HFSE (U and Pb), and depleted HFSE (Nb(Ta), Hf and Ti). The δEu values vary from 0.63 to 1.08, which may be related with the crystallization of plagioclase.The geochemical diagrams of major and trace elements correlation indicate that the primary magma of Hongbaoshan-Jianquan intrusions is cal-alkaline basaltic magma that formed in mantle, their diagenesis are mainly controlled by the magmatic crystallization, and contaminated by the crust during upward of magma.
Keywords:manfic-ultramafic; geochemistry; crystallization differentiation; contamination; East Tianshan