曾令君, 周?棟, 星玉才, 趙太平, 姚軍明, 包志偉*

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盧氏八寶山花崗斑巖地球化學(xué)特征及成因研究

曾令君1,2, 周?棟1,2, 星玉才3, 趙太平1, 姚軍明1, 包志偉1*

(1. 中國(guó)科學(xué)院 廣州地球化學(xué)研究所 礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州?510640; 2. 中國(guó)科學(xué)院大學(xué), 北京?100049; 3. 河南省地礦局 第一地質(zhì)調(diào)查院, 河南 洛陽(yáng)?471023)

河南省盧氏縣八寶山巖體位于華北克拉通南緣、東秦嶺造山帶西段。巖體呈筒狀, 可能為古火山機(jī)構(gòu)巖頸相的超淺成侵入體。巖體的形成時(shí)代為(146±2) Ma(LA-ICPMS鋯石U-Pb年齡)。巖體邊緣相為鉀長(zhǎng)花崗斑巖、中心相為黑云母二長(zhǎng)花崗斑巖。巖體出現(xiàn)以鐵為主的多金屬礦化, 與區(qū)內(nèi)同時(shí)代成礦花崗巖有著顯著的差別。巖石SiO2含量較高(64.8%～73.5%), ACNK值為0.98 ~ 1.56, 屬于準(zhǔn)鋁質(zhì)-過(guò)鋁質(zhì)鉀玄巖系列花崗巖類(lèi)。輕稀土相對(duì)富集(La/Yb)N=2.29～23.8, Eu異常不顯著(Eu=0.61 ~ 1.39), Rb、Ba、U和K等元素強(qiáng)烈富集, Sr、Nb、Ti和P等元素顯著虧損, 屬于高分異的I型花崗巖, 并且鉀長(zhǎng)花崗斑巖具有相對(duì)較高的演化分異程度。巖體具有較低的Nd()初始值(-19.46～-16.4)和較古老的二階段模式年齡(2.49～2.27 Ga), 與鋯石Hf同位素模式年齡一致, 表明該巖體的源區(qū)物質(zhì)以殼源物質(zhì)為主。斑巖體為碰撞后的拉伸環(huán)境下地殼物質(zhì)部分熔融的產(chǎn)物。根據(jù)黑云母地質(zhì)壓力計(jì)估算, 八寶山巖體的的侵位深度為1.89～2.55 km, 與該巖體淺成相的地質(zhì)產(chǎn)狀一致, 推測(cè)其深部可能存在有利于斑巖Cu-Au-Mo-Pb-Zn的多金屬成礦區(qū)段。

高分異I型花崗巖; Nd同位素; 斑巖; 八寶山巖體; 華北克拉通南緣

0 引 言

秦嶺造山帶是中國(guó)大陸中央造山帶的重要組成部分, 總體上是由華北、秦嶺和揚(yáng)子三板塊沿兩個(gè)主縫合帶(商丹和勉略帶)經(jīng)歷早古生代、晚古生代晚期和中生代早期三次板塊俯沖、碰撞造山作用形成的; 之后又經(jīng)歷了中新生代強(qiáng)烈的陸內(nèi)造山作用疊加, 是一個(gè)典型的大陸復(fù)合造山帶[1]。秦嶺造山帶, 特別是華北克拉通南緣中生代花崗質(zhì)巖漿活動(dòng)強(qiáng)烈, 與之有關(guān)的Mo、W、Pb、Zn和Au等多金屬礦產(chǎn)特別發(fā)育, 是我國(guó)最重要的多金屬成礦帶之一[2–3]。華北南緣中生代花崗巖時(shí)空分布、源區(qū)組成、成因演化規(guī)律、成礦專(zhuān)屬性及其與秦嶺造山帶演化和華北克拉通中生代巖石圈減薄過(guò)程的聯(lián)系一直是一個(gè)研究的熱點(diǎn)[4–6]。

河南省盧氏縣八寶山巖體位于華北克拉通南緣、東秦嶺造山帶的西段, 巖體為火山頸相的超淺成斑巖, 目前已發(fā)現(xiàn)的礦化類(lèi)型以鐵礦化為主, 并伴有少量的銅、鉬和鉛鋅礦化。該巖體目前已獲得了精確的同位素年齡(146±2) Ma (LA-ICPMS鋯石U-Pb[7]), 但是該巖體及有關(guān)礦床目前尚缺少深入的地質(zhì)、地球化學(xué)研究。為探討巖體的地質(zhì)、地球化學(xué)特征及成因過(guò)程, 本文在詳細(xì)的野外地質(zhì)工作基礎(chǔ)上, 對(duì)八寶山巖體進(jìn)行了巖石地球化學(xué)及同位素地球化學(xué)研究, 在此基礎(chǔ)上探討了巖體成因。

1?區(qū)域地質(zhì)背景

秦嶺造山帶是中國(guó)大陸中央造山帶的重要組成部分, 它由2個(gè)主縫合帶(商丹和勉略縫合帶)和3個(gè)塊體(華北、秦嶺和揚(yáng)子)組成[1,8]。華北南緣是秦嶺造山帶中最重要的中生代構(gòu)造-巖漿-成礦帶, 其范圍北至三寶斷裂(三門(mén)峽-寶豐), 南部以欒川斷裂為界(圖1)。

盧氏八寶山地區(qū)的大地構(gòu)造屬于華北克拉通南緣、東秦嶺造山帶的西段。華北南緣結(jié)晶基底為新太古界太華群中深變質(zhì)巖系, 蓋層為古元古界熊耳群、中元古界官道口群、新元古界欒川群和陶灣群。區(qū)內(nèi)出露的地層主要是中元古代的碳酸鹽巖和碎屑巖。

八寶山地區(qū)主要發(fā)育了一系列近東西向的逆沖斷層和擠壓破碎帶及復(fù)式褶皺。并且其褶皺形態(tài)自北向南由開(kāi)闊逐漸變?yōu)榫€(xiàn)狀緊密型, 斷裂密度相應(yīng)由小變大。區(qū)內(nèi)成礦巖體均為中生代巖漿作用的產(chǎn)物, 巖性包括閃長(zhǎng)巖、鉀長(zhǎng)花崗斑巖及二長(zhǎng)花崗斑巖。巖體出露面積為0.6 ~ 1.3 km2, 呈北北東向的近平行排列, 自西向東依次為: 蒲鎮(zhèn)溝閃長(zhǎng)巖帶、銀家溝-夜長(zhǎng)坪花崗斑巖帶、后瑤峪-八寶山花崗斑巖帶、郭家河閃長(zhǎng)巖帶。工業(yè)礦床主要見(jiàn)于銀家溝-夜長(zhǎng)坪花崗斑巖帶和后瑤峪-八寶山花崗斑巖帶上, 而中性閃長(zhǎng)巖帶僅發(fā)現(xiàn)零星的礦化。區(qū)內(nèi)礦產(chǎn)較為豐富, 包括業(yè)已探明的曲里小型鐵鋅銅礦床[10]、銀家溝大型多金屬硫鐵礦床[11]、夜長(zhǎng)坪大型鎢鉬礦床[12]、后瑤峪小型鐵鉛鋅礦床[13]及八寶山小型鐵銅礦床。

2?巖體地質(zhì)及巖相學(xué)特征

八寶山巖體位于河南省盧氏縣境內(nèi)。區(qū)內(nèi)出露的地層主要是中元古界官道口群含硅質(zhì)條帶、條紋的白云巖。巖體侵入中元古界官道口群白云巖中, 與圍巖邊界平直, 呈巖株?duì)町a(chǎn)出。巖體地表出露似一頭東尾西的魚(yú)形, 長(zhǎng)約2 km, 面積約1.05 km2。在600 m標(biāo)高以上, 巖體與圍巖接觸帶內(nèi)傾, 呈喇叭形, 600 m標(biāo)高以下接觸帶陡直呈筒狀。鐵礦體主要產(chǎn)于巖體和圍巖的接觸帶上。

圖1?秦嶺造山帶構(gòu)造簡(jiǎn)圖及中生代花崗巖體分布圖(據(jù)文獻(xiàn)[9])

1–第三系; 2–中生代碎屑沉積巖; 3–古生代海相碳酸鹽巖與陸源碎屑巖; 4–古元古界秦嶺群黑云斜長(zhǎng)片麻巖和斜長(zhǎng)角閃巖; 5–中元古界熊耳群火山巖及中上元古代海相碎屑沉積巖; 6–新太古界太華群片麻巖、麻粒巖和混合巖; 7–加里東期花崗巖; 8–白堊紀(jì)火山巖; 9–燕山期花崗巖; 10–燕山期閃長(zhǎng)巖; 11–推斷斷裂; 12–斷裂; 13–城市。

巖體可分為兩個(gè)巖相: 中心相為黑云母二長(zhǎng)花崗斑巖、邊緣相為鉀長(zhǎng)花崗斑巖, 兩巖相一般呈漸變過(guò)渡關(guān)系, 局部見(jiàn)指狀穿插關(guān)系。鐵礦化主要發(fā)生在巖體與圍巖的接觸帶。巖體及圍巖中分布較多不同巖性的巖脈, 如正長(zhǎng)斑巖脈、花崗斑巖脈、閃長(zhǎng)斑巖脈(圖2)。巖體與圍巖接觸帶局部發(fā)育有夕卡巖, 但夕卡巖與鐵礦化無(wú)明顯的空間聯(lián)系。據(jù)野外產(chǎn)狀及定年結(jié)果(LA-ICPMS鋯石U-Pb, (146±2) Ma),兩種不同的巖性應(yīng)為同期巖漿作用的產(chǎn)物[7]。

黑云二長(zhǎng)花崗斑巖呈灰白色, 斑狀結(jié)構(gòu), 塊狀構(gòu)造(圖3a)。斑晶主要是斜長(zhǎng)石、鉀長(zhǎng)石、石英及少量黑云母、角閃石(圖3b)。石英斑晶占15%, 大小0.75 ~ 1.25 mm, 斑晶呈他形, 邊緣有被熔蝕的現(xiàn)象(圖3c); 斜長(zhǎng)石斑晶占30%, 顆粒較大, 約2 ~ 3.5 mm, 自形程度較好, 主要為中長(zhǎng)石, 具有環(huán)帶構(gòu)造、常見(jiàn)聚片雙晶和卡納復(fù)合雙晶, 也有少量更長(zhǎng)石, 具有細(xì)而密的聚片雙晶, 有些斜長(zhǎng)石完全被絹云母置換, 呈斜長(zhǎng)石的假像; 鉀長(zhǎng)石約25%, 大小0.6 ~ 2 mm, 鉀長(zhǎng)石表面較渾濁, 內(nèi)部和邊緣有熔蝕現(xiàn)象, 有些鉀長(zhǎng)石內(nèi)部發(fā)生了碳酸鹽化、絹云母化和泥化(圖3d)。黑云母和角閃石含量較少(約5%), 鏡下可見(jiàn)角閃石的黑云母化及黑云母綠泥石化現(xiàn)象(圖3e)?；|(zhì)和斑晶成分相同, 主要是鉀長(zhǎng)石、斜長(zhǎng)石和石英, 含量約占30%。副礦物以磁鐵礦、榍石和磷灰石為主, 含量很少。

圖2?八寶山巖體地質(zhì)圖

據(jù)河南省盧氏縣地質(zhì)勘查研究所《1﹕2000河南省盧氏縣八寶山鐵銅礦區(qū)地形地質(zhì)及工程布置圖(2011年)》修改。1–第四系; 2–中元古界官道口群; 3–黑云母二長(zhǎng)花崗斑巖; 4–鉀長(zhǎng)花崗斑巖; 5–花崗斑巖; 6–正長(zhǎng)斑巖; 7–閃長(zhǎng)斑巖; 8–斷層。

圖3?八寶山巖體典型巖石的手標(biāo)本和巖相學(xué)特征

(a)黑云母二長(zhǎng)花崗斑巖手標(biāo)本特征; (b)黑云母二長(zhǎng)花崗斑巖鏡下特征; (c)他形石英, 邊緣被熔蝕; (d)鉀長(zhǎng)石碳酸鹽化; (e)黑云母綠泥石化; (f)鉀長(zhǎng)花崗斑巖手標(biāo)本特征; (g)鉀長(zhǎng)花崗斑巖鏡下特征; (h)鉀長(zhǎng)石的卡斯巴雙晶, 發(fā)生泥化。

鉀長(zhǎng)花崗斑巖呈肉紅色, 斑狀結(jié)構(gòu), 塊狀構(gòu)造(圖3f)。斑晶主要是鉀長(zhǎng)石和石英(圖3g), 含量55%。石英斑晶25%, 粒度比黑云母二長(zhǎng)花崗斑巖中的石英斑晶稍大, 約1 ~ 2.5 mm, 斑晶呈他形, 石英斑晶熔蝕現(xiàn)象明顯; 鉀長(zhǎng)石斑晶30%, 粒度較大2 ~ 5 mm, 鉀長(zhǎng)石具有卡斯巴雙晶(圖3h), 有些鉀長(zhǎng)石發(fā)生泥化、絹云母化、碳酸鹽化, 并且多數(shù)鉀長(zhǎng)石和石英斑晶的邊緣有被熔蝕的現(xiàn)象, 這是淺成斑巖特有的現(xiàn)象[14]。斑晶和基質(zhì)成分相同, 由鉀長(zhǎng)石和石英組成, 含量約占45%。副礦物以黃鐵礦、鋯石和金紅石為主, 含量很少。

3?分析方法

本文選取了33件較新鮮的花崗巖樣品進(jìn)行全巖主元素和微量元素、Nd同位素和黑云母電子探針?lè)治觥?/p>

主元素和微量元素分析在中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室完成。主元素采用堿熔玻璃片X射線(xiàn)熒光光譜法(XRF)測(cè)試, 分析儀器為Rigaku ZSX 100e, 分析精度優(yōu)于2%。樣品處理工作主要包括樣品的燒失、稱(chēng)樣(0.7 ~ 1 g的樣品于一次性痰杯中, 加入8倍量的LiBO4)以及在XRF玻璃熔片震蕩電熱熔融爐中將樣品熔成玻璃片待測(cè)。微量元素分析采用酸溶法。用ICP-MS測(cè)試, 所用質(zhì)譜儀為T(mén)hermo Plasma-Quad Excell ICP-MS。在質(zhì)譜分析過(guò)程中用Rh元素作為內(nèi)標(biāo)來(lái)進(jìn)行內(nèi)部校正, 純的元素標(biāo)樣作為外部校正[15]。實(shí)驗(yàn)室標(biāo)樣BHVO-2(玄武巖)、GSR-1(花崗巖)、GSR-2(花崗巖)和GSR-3(花崗巖)作為參考標(biāo)準(zhǔn), 分析精度優(yōu)于5%, 詳細(xì)分析流程見(jiàn)劉穎等[16]。

Nd同位素分析測(cè)試在中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室的Micromass ISOPROBE型多接收電感耦合等離子體質(zhì)譜儀(MC-ICPMS)上進(jìn)行。對(duì)Nd同位素分析所需樣品采用酸淋濾法進(jìn)行預(yù)處理, 以減少后期蝕變作用對(duì)同位素比值的影響。對(duì)Sm和Nd的分離和純化是用石英交換柱用1.7 mL Teflon粉末作為交換介質(zhì)完成的。實(shí)驗(yàn)所用的Nd標(biāo)樣為國(guó)際標(biāo)樣Shin-Etsu JNdi-1[17]。143Nd/144Nd比值用143Nd/144Nd=0.72190進(jìn)行校正。詳細(xì)的實(shí)驗(yàn)流程和分析方法見(jiàn)梁細(xì)榮 等[18]和韋剛健等[19]。

電子探針定量分析在南京大學(xué)內(nèi)生金屬礦床成礦機(jī)制研究國(guó)家重點(diǎn)實(shí)驗(yàn)室完成, 使用的儀器是JEOL JXA-8800M電子探針。工作條件為: 加速電壓15 kv, 加速電流10 nA, 束斑直徑小于1 μm, 所有測(cè)試數(shù)據(jù)都進(jìn)行了ZAF (atomic number effect, absorption effect and fluorescence effect) 修正。使用的標(biāo)樣均為美國(guó)國(guó)家標(biāo)準(zhǔn)局的礦物標(biāo)樣。詳細(xì)的實(shí)驗(yàn)流程和分析方法見(jiàn)Amli.[20]和Roeder[21]。

4?分析結(jié)果

4.1?全巖主元素和微量元素地球化學(xué)

八寶山斑巖體巖石主元素和微量元素分析結(jié)果見(jiàn)表1。

由于巖石普遍、強(qiáng)烈的蝕變作用(絹云母化、高嶺土化和碳酸鹽化等), 樣品的燒失量較高, 因此在進(jìn)行地球化學(xué)投圖時(shí)將樣品去除燒失量后, 重新?lián)Q算為100%。鉀長(zhǎng)花崗斑巖和黑云母二長(zhǎng)花崗斑巖都比較富硅, SiO2變化于64.8% ~ 73.5%之間。巖體堿含量較高, 鉀長(zhǎng)花崗斑巖的Na2O和K2O含量分別為0.24% ~ 1.98%和8.00% ~ 11.7%, K2O+Na2O總量為8.32% ~ 13.0%; 黑云母二長(zhǎng)花崗斑巖的Na2O和K2O含量分別為2.02% ~ 3.88%和3.93% ~ 5.76%, K2O+Na2O總量為8%左右。在TAS巖石分類(lèi)圖解上(圖4)樣品位于堿性系列和亞堿性系列的過(guò)渡區(qū)。其中, LSB-31樣品由于鉀長(zhǎng)石化較強(qiáng)烈, K2O含量較高, 導(dǎo)致在TAS圖解上落入正長(zhǎng)巖范圍。其余的點(diǎn)主要落入花崗巖和石英二長(zhǎng)巖范圍, 與巖石的實(shí)際礦物組成的定名基本一致。由于巖體富鉀, 鉀長(zhǎng)花崗斑巖(K2O/Na2O=4.97 ~ 24.7)和黑云母二長(zhǎng)花崗斑巖(K2O/Na2O=1.01 ~ 2.79)樣品落入鉀玄巖系列(圖5)。

表1?八寶山花崗巖體主元素(%)、微量元素及稀土元素(μg/g)組成分析結(jié)果

(續(xù)表1)

樣號(hào)鉀長(zhǎng)花崗斑巖 LSB-8LSB-10LSB-11LSB-15LSB-19LSB-20LSB-26LSB-31LSB-32LSB-33LSB-39LSB-40LSB-43LSB-44LSB-47LSB-51LSB-67 Hf3.933.844.083.763.823.833.324.393.963.464.7755.354.723.554.394.13 Ti168613261277130210718268161321103112017327469097109261399915 Y18.313.014.21713.310.114.621.312.011.78.0111.813.815.619.314.29.62 Ga19.117.518.117.917.418.415.120.016.616.217.918.718.618.918.117.916.8 1000*Ga/Al2.482.222.072.312.312.372.182.302.142.092.252.372.282.442.42.262.23 La41.27.6723.516.735.54.947.558.7320.227.217.812.324.968.833.949.77.24 Ce74.018.544.138.159.610.517.418.243.247.427.322.445.212868.187.911.9 Pr8.092.75.025.326.011.372.572.315.865.142.622.594.8713.48.19.182.87 Nd26.911.316.420.919.75.5811.19.4920.617.28.249.3116.740.62829.911.7 Sm4.182.262.383.963.091.412.732.243.102.531.451.752.745.454.34.152.45 Eu1.060.620.750.850.670.640.610.700.830.630.650.710.821.050.691.140.63 Gd3.691.842.273.282.71.362.342.512.512.361.291.612.484.63.853.61.93 Tb0.560.340.340.520.410.250.440.520.390.360.190.290.410.60.590.510.32 Dy3.232.152.193.152.381.712.733.652.262.021.311.832.492.953.482.71.85 Ho0.680.490.510.660.510.390.580.820.470.450.30.440.550.630.770.560.39 Er2.161.481.671.951.571.131.642.381.491.391.031.411.531.922.271.681.18 Tm0.330.240.260.320.260.190.270.390.240.230.190.230.240.290.360.280.2 Yb2.411.732.042.231.971.301.722.721.81.61.321.811.772.072.541.961.45 Lu0.360.290.310.340.330.210.260.430.280.250.240.30.290.310.370.310.23 ΣREE16951.710198.413531.051.955.210310863.95710527015719351.3 LREE15543.192.385.912624.441.941.793.810058.149.195.225714318243.7 HREE13.48.589.6212.410.16.579.9813.49.478.695.867.929.7513.414.211.67.55 LREE/HREE11.55.029.596.8712.53.724.193.109.9111.59.916.29.7619.21015.65.79 δEu0.800.900.970.700.701.390.710.900.880.781.411.280.940.620.510.870.86 (La/Yb)N12.23.178.265.3512.92.703.132.298.0312.19.614.8710.123.89.5518.13.59 (La/Sm)N6.362.186.372.727.412.251.782.514.206.947.924.545.878.155.097.721.91 (Gd/Yb)N1.260.880.911.211.130.861.110.761.151.210.80.731.161.831.251.511.1 TZr(℃)797782806799773798756777807796824834839808786825781

(續(xù)表1)

樣號(hào)鉀長(zhǎng)花崗斑巖黑云母二長(zhǎng)花崗斑巖 LSB-68LSB-75LSB-76LSB-78LSB-79LSB-80LSB-87LSB-92LSB-93LSB-94LSB-12LSB-13LSB-46LSB-48LSB-7-1LSB-7-16 SiO268.572.971.470.669.57270.771.469.670.564.867.865.671.363.267.3 TiO20.200.220.250.210.200.200.350.230.220.130.540.390.480.140.450.25 Al2O314.114.614.314.113.514.514.615.214.113.515.914.315.913.715.613.9 Fe2O3T1.951.141.712.743.330.761.430.552.532.094.144.444.652.354.431.13 MnO00000.080.010.880.010.020.040.050.030.070.010.060.03 MgO0.120.120.110.110.160.10.660.100.100.451.631.131.400.641.280.54 CaO0.070.110.080.080.270.190.650.210.200.793.32.083.070.933.231.99 Na2O0.530.560.640.630.580.630.450.610.640.533.432.593.872.763.882.02 K2O9.468.399.589.079.3310.19.9110.310.39.604.075.393.935.654.305.76 P2O50.010.010.030.010.120.10.150.10.100.040.280.200.280.030.280.13 LOI4.61.491.441.842.290.882.610.721.501.791.421.250.332.171.675.61 Total99.699.699.699.599.699.699.499.699.599.599.799.799.899.898.999.6 K2O/Na2O17.614.714.914.315.91621.916.915.918.01.182.071.012.041.102.79 ANK1.261.461.251.301.221.21.271.241.151.191.581.411.491.281.421.46 ACNK1.251.431.231.281.171.161.151.201.111.050.991.020.981.110.931.06 K2O+Na2O9.998.9610.29.709.9110.810.310.91110.17.507.997.818.428.187.78 D.I.89.692.093.091.390.694.792.894.592.893.488.289.389.591.987.286.3 Rb110204240150249261333350269264128124112153124196 Ba908232737382049270237133397410639002629149120492258223328001865 Th21.815.813.825.220.418.417.825.519.720.416.014.516.723.616.118.6 U4.737.944.426.048.114.313.827.356.523.023.082.382.297.13.713.87 Nb23.830.142.420.132.228.527.940.131.237.02318.621.035.923.024.5 Sr13411810821713511099.617710290.5646462643136635150 Zr13912815115113214713116515412819216720691.2180100 Ta1.702.703.572.012.882.542.392.712.153.721.803.151.681.731.802.20 Hf4.253.703.994.093.683.943.614.514.13.714.754.285.073.044.93.3 Ti613126615301942125812122216166312597503051224826468130.401.10 Y15.326.919.114.418.112.619.521.415.216.215.69.921714.716.910.7 Ga1619.520.417.01816.717.522.517.816.020.318.119.315.718.214.8

(續(xù)表1)

樣號(hào)鉀長(zhǎng)花崗斑巖黑云母二長(zhǎng)花崗斑巖 LSB-68LSB-75LSB-76LSB-78LSB-79LSB-80LSB-87LSB-92LSB-93LSB-94LSB-12LSB-13LSB-46LSB-48LSB-7-1LSB-7-16 1000*Ga/Al2.142.512.662.262.512.132.262.782.372.232.402.402.282.162.192.00 La20.613.315.048.625.78.5315.420.86.7817.147.618.962.839.753.218.9 Ce38.830.638.482.151.524.834.848.120.534.485.936.411063.792.835.4 Pr4.674.415.508.836.224.224.816.793.524.459.834.37126.35103.93 Nd16.618.321.429.622.216.919.726.51616.534.316.641.119.432.412.8 Sm3.193.613.844.423.753.483.995.043.73.265.312.685.882.685.302.17 Eu0.610.870.841.060.980.671.101.150.780.761.431.061.690.611.270.56 Gd2.803.293.223.83.382.793.844.212.792.854.402.324.962.643.641.59 Tb0.470.650.540.530.550.440.610.690.490.480.600.330.690.380.530.25 Dy2.764.313.192.753.212.53.743.982.912.883.331.853.572.302.911.52 Ho0.591.030.710.590.710.490.770.850.610.620.660.370.720.550.540.31 Er1.743.092.221.642.091.412.182.381.751.761.871.091.901.601.651.04 Tm0.270.450.350.260.340.230.340.370.280.280.270.150.310.250.270.20 Yb2.023.162.421.822.381.662.572.561.931.931.731.122.101.811.731.35 Lu0.330.490.370.300.40.270.360.400.290.290.270.180.310.260.280.23 ΣREE95.687.798.118612368.594.312362.487.719787.524814220680.2 LREE84.671.28517411158.679.910851.376.618480.123313219473.7 HREE11.016.513.011.7139.8214.415.411.011.113.17.4614.59.8411.56.49 LREE/HREE7.684.316.5114.98.545.975.537.014.626.8814.010.71613.416.811.3 δEu0.610.760.710.770.830.640.840.740.710.740.871.260.930.700.840.88 (La/Yb)N7.313.014.4219.17.753.674.295.832.516.3419.712.021.415.622.110.0 (La/Sm)N4.162.372.527.094.411.582.482.661.183.385.784.546.909.576.481.74 (Gd/Yb)N1.140.861.091.721.171.381.231.361.191.212.091.711.951.205.620.97 TZr(℃)803796799811780791798803788796785783790745769761

注：Fe2O3T為全鐵; ACNK=Al2O3/(CaO+Na2O+K2O), 分子數(shù)比; ANK= Al2O3/( Na2O+K2O), 分子數(shù)比; Mg#=Mg2+/(Mg2++Fe2+)×100;Eu=2EuN/(SmN+GdN); (La/Yb)N為球粒隕石標(biāo)準(zhǔn)化值, 標(biāo)準(zhǔn)化值引自文獻(xiàn)[22]; 鋯石飽和溫度Zr=12900/[2.95+0.85+ln(496000/rmet)],=(Na+K+2Ca)/(Al×Si),rmelt為全巖鋯含量。

圖4?八寶山巖體巖石TAS圖解

圖5?八寶山花崗斑巖K2O-SiO2圖解(據(jù)文獻(xiàn)[23], 樣品符號(hào)同圖4)

盡管兩類(lèi)巖石在主元素的含量上有一定差異, 但其鋁含量均較高(Al2O3=13.5% ~ 16.3%), ACNK (Al2O3/(CaO+Na2O+K2O), 分子數(shù)比)=0.98～1.56, 在A/NK-A/CNK圖解(圖6)中, 為準(zhǔn)鋁質(zhì)-過(guò)鋁質(zhì)花崗巖。因此, 八寶山巖體為準(zhǔn)鋁質(zhì)-過(guò)鋁質(zhì)的鉀玄質(zhì)巖漿巖。

從圖7可以看到, 八寶山花崗斑巖的主元素呈連續(xù)演化的特征, TiO2、Al2O3、Fe2O3T、MgO、CaO和P2O5含量均隨SiO2含量的增加而降低。中心相的黑云母二長(zhǎng)花崗斑巖到邊緣相的鉀長(zhǎng)花崗斑巖SiO2含量依次增加, 除了K2O, 其他氧化物含量逐漸減少, 暗示鉀長(zhǎng)花崗斑巖和黑云母二長(zhǎng)花崗斑巖存在結(jié)晶分異的演化關(guān)系, 且鉀長(zhǎng)花崗斑巖具有相對(duì)較高的演化程度。

圖6 八寶山花崗斑巖A/NK-A/CNK圖解(據(jù)文獻(xiàn)[24], 樣品符號(hào)同圖4)

八寶山花崗巖體的稀土元素總量較低,SREE含量變化范圍在51.3~ 271 μg/g之間。在球粒隕石標(biāo)準(zhǔn)化分布模式圖上呈向右傾斜的平滑曲線(xiàn)(圖8a), LREE/HREE值在3.72 ~ 19.2之間, (La/Yb)N=2.29 ~ 23.8。黑云母二長(zhǎng)花崗斑巖((La/Yb)N=10.0~22.1)比鉀長(zhǎng)花崗斑巖((La/Yb)N=2.29 ~ 12.2, 除了LSB-44, (La/Yb)N=23.8)的輕重稀土分餾明顯。(La/Sm)N值為1.18 ~ 9.57, (Gd/Yb)N值為0.76 ~ 2.09, 表明輕稀土分餾程度較重稀土顯著。巖石Eu異常不顯著(Eu=0.61 ~ 1.39)。其中鉀長(zhǎng)花崗斑巖的Eu負(fù)異常(Eu=0.61 ~ 0.97, 除了LSB-20、LSB-39以及LSB-40)比黑云母二長(zhǎng)花崗斑巖(Eu=0.70 ~ 1.26)明顯。兩個(gè)不同巖性斑巖的稀土元素組成的差異可能與早期獨(dú)居石等富輕稀土礦物及斜長(zhǎng)石的分離結(jié)晶作用有關(guān),晚期巖相相對(duì)虧損輕稀土元素及Eu。

在微量元素原始地幔標(biāo)準(zhǔn)化圖解上(圖8b), 表現(xiàn)為Rb、Ba、U和K等大離子親石元素的強(qiáng)烈富集、高場(chǎng)強(qiáng)元素Zr、Hf和Ta無(wú)明顯異常, Sr、Nb、Ti和P等元素顯著虧損。與黑云母二長(zhǎng)花崗斑巖相比, 鉀長(zhǎng)花崗斑巖具有明顯高的Rb、Ba、Th、U和K等大離子親石元素含量, 更高的Rb/Sr比值和更強(qiáng)烈的Ti和P虧損(圖7和圖8b), 可能與巖漿分異過(guò)程中斜長(zhǎng)石和鐵鈦氧化物等礦物的結(jié)晶分異有關(guān)。

4.2?全巖Nd同位素組成

選取10個(gè)樣品進(jìn)行Nd同位素分析, 結(jié)果見(jiàn)表2。八寶山巖體的143Nd/144Nd初始比值介于0.511560~0.511730之間;Nd()=-19.2～-16.4, 均為負(fù)值。在計(jì)算Nd模式年齡時(shí), 對(duì)于Sm/Nd值在-0.5～-0.3之間的樣品, 用單階段模式計(jì)算DM年齡是合適的, 但對(duì)于和大陸地殼Sm/Nd比相差較大的樣品, 即Sm/Nd>-0.3或<-0.5, 用單階段模式計(jì)算DM年齡就會(huì)產(chǎn)生較大的偏差[27]。在這種情況下, 采用兩階段Nd模式年齡計(jì)算方法能有助于得到較合理的DM年齡值[28–29]。八寶山巖體的Sm/Nd值變化于-0.24～-0.57之間, 大部分樣品的Sm/Nd<-0.5, 與太古宙大陸地殼Sm/Nd平均值(-0.4)[30]相比偏差較大, 為了最大限度減少因地殼演化階段內(nèi)Sm-Nd分餾對(duì)Nd模式年齡計(jì)算值產(chǎn)生的影響, 本文采用兩階段Nd模式年齡計(jì)算方法。DM2年齡介于2.49～2.27 Ga之間, 老的Nd兩階段模式年齡和負(fù)的Nd()值指示八寶山花崗巖體主要來(lái)源于古老的地殼物質(zhì)。

圖7?主元素和微量元素對(duì)SiO2變化圖解(據(jù)文獻(xiàn)[25]和[26], 樣品符號(hào)同圖4)

圖8?八寶山花崗斑巖球粒隕石標(biāo)準(zhǔn)化REE分布模式圖及原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖

球粒隕石和原始地幔標(biāo)準(zhǔn)化值據(jù)文獻(xiàn)[22]。

表2?八寶山花崗巖體全巖Nd同位素分析測(cè)試結(jié)果

4.3?黑云母電子探針?lè)治鼋Y(jié)果

黑云母二長(zhǎng)花崗斑巖中的黑云母具有富鎂貧鐵的特征, 其FeO和MgO的質(zhì)量分?jǐn)?shù)分別為11.5%～13.3%和13.8%～18.6%, Al2O3的質(zhì)量分?jǐn)?shù)為13.3%～14.6%。在Mg2+-(Fe2++Mn2+)-(Fe3++Al3++Ti4+)三角圖中(圖9), 所有黑云母的成分投點(diǎn)均在鎂質(zhì)黑云母區(qū)域內(nèi), 表明本區(qū)黑云母的成分類(lèi)型為鎂質(zhì)黑云母。

5?討?論

5.1 成因類(lèi)型

八寶山花崗斑巖的暗色礦物主要是角閃石和黑云母, 巖體均具有高的SiO2含量(64.8% ~ 73.5%)和全堿含量(K2O+Na2O=8.32% ~ 13.0%), 巖石較高的分異指數(shù)、LREE/HREE及Rb/Sr比值的變化范圍等均表明它們經(jīng)歷了高程度的結(jié)晶分異。

對(duì)于I型、S型花崗巖的劃分, 鋁飽和指數(shù)ACNK=1.1僅適用于未經(jīng)強(qiáng)烈結(jié)晶分異的花崗巖[33], 對(duì)于分異程度較高的八寶山花崗斑巖并不適合。實(shí)驗(yàn)研究表明, P2O5在弱過(guò)鋁質(zhì)和強(qiáng)過(guò)鋁質(zhì)巖漿中隨SiO2增加變化趨勢(shì)不同, 這種性質(zhì)成功地用于區(qū)分I型和S型花崗巖類(lèi)[34–38]。本文的數(shù)據(jù)顯示, 八寶山花崗巖主要為準(zhǔn)鋁質(zhì)到過(guò)鋁質(zhì)花崗巖, 在圖7中P2O5與SiO2含量呈負(fù)相關(guān)關(guān)系, 與I型花崗巖演化趨勢(shì)一致。

A型花崗巖的重要特征是形成溫度高、無(wú)水、鋁質(zhì)、富集HFSE和非造山成因, 尤其是高溫、無(wú)水特點(diǎn), 是A型花崗巖區(qū)別于高分異花崗巖的重要參數(shù)[39]。而八寶山花崗斑巖具有一系列不同于A型花崗巖的特點(diǎn): (1)八寶山巖體的FeOT/MgO比值主要介于2～8, 有別于A型花崗巖顯著富鐵的特征(FeOT/MgO>10[40]); (2)巖體的Zr、Nb、Ce和Y等元素的含量均較低, Zr+Nb+Ce+Y = 192～340 μg/g,小于350 μg/g[40]。在區(qū)分A型花崗巖與分異的I型花崗巖的有關(guān)判別圖解上(圖10), 樣品幾乎全部落入高分異花崗巖區(qū)域, 并且在Ce-SiO2判別圖解中(圖7), 投點(diǎn)均落在I型花崗巖區(qū)域; (3) 雖然八寶山巖體的巖漿溫度很高(鋯石飽和溫度[41]變化于745～839 ℃, 見(jiàn)表1), 但由于其含有角閃石和黑云母等含水礦物, 與A型花崗巖的定義不符。綜上分析, 八寶山巖體應(yīng)屬于高分異I型花崗巖。

圖9 黑云母的Mg2+-(Fe3++Al3++Ti4+)-(Fe2++Mn2+)圖解(底圖據(jù)文獻(xiàn)[31], Fe2+和Fe3+的計(jì)算據(jù)文獻(xiàn)[32])

個(gè)別黑云母二長(zhǎng)花崗斑巖和鉀長(zhǎng)花崗斑巖樣品具有高Sr低Y的微量元素組成, 顯示埃達(dá)克質(zhì)巖類(lèi)似的地球化學(xué)特征[42–43], 反映花崗巖源區(qū)可能存在石榴子石殘留相。事實(shí)上華北南緣不少150～130 Ma的花崗巖具有不同程度的類(lèi)似埃達(dá)克巖的組成特點(diǎn)[44–45], 表明該時(shí)間段內(nèi)華北南緣由揚(yáng)子向華北克拉通之下俯沖所形成的加厚下地殼尚未拆沉。

5.2?物質(zhì)來(lái)源

八寶山巖體具有高的SiO2(64.8% ~ 73.5%), 相對(duì)低的MgO含量(0.10% ~ 1.40%)和Mg#(8.78 ~ 49.3), 顯示出地殼來(lái)源的特征。全巖Nd同位素分析結(jié)果顯示巖體具有較低的Nd()(－19.46～－16.4)和較古老的模式年齡(NdDM2=2.49～2.27 Ga)。

圖10 八寶山花崗斑巖成因類(lèi)型判別圖

A–A型花崗巖; FG–分異M+I+S型花崗巖; OGT–非分異M+I+S型花崗巖。據(jù)文獻(xiàn)[40], 樣品符號(hào)同圖4。

根據(jù)殼源巖石的Nd同位素研究, 華北塊體殼源巖石Nd同位素模式年齡主要分布于3.6～1.8 Ga范圍內(nèi), 并且主要集中于3.6～3.3 Ga、2.8～2.6 Ga和2.2～2.0 Ga區(qū)段內(nèi), 華北陸塊主體形成于太古宙, 并以2.8～2.6 Ga時(shí)期為最強(qiáng)的陸殼增生期, 其次, 古元古代也是較主要的地殼增生期[46]。揚(yáng)子塊體殼源巖石Nd同位素模式年齡主要分布于2.4～1.0 Ga范圍內(nèi), 在3.4～2.5 Ga區(qū)間內(nèi)也有少量分布, 這表明揚(yáng)子塊體陸殼主體形成于1.0 Ga前的元古宙[47–49], 太古宙古陸核較少[47]。即揚(yáng)子地殼的年齡相對(duì)年輕。

由于八寶山巖體產(chǎn)于華北克拉通南緣, 其巖漿源區(qū)可能為: (1) 華北地塊南緣的中-下地殼巖石(即太華群和/或熊耳群)[50]; (2)俯沖于華北克拉通南緣地殼之下的揚(yáng)子陸殼及南秦嶺陸殼[51]。

八寶山花崗斑巖的兩階段Nd模式年齡主要介于2.49～2.27 Ga之間。2.49～2.27 Ga的模式年齡不同于華北克拉通主體形成于太古宙并以2.8～2.6 Ga為強(qiáng)烈的地殼增長(zhǎng)期, 而與揚(yáng)子板塊古元古代和中元古代為主的地殼生長(zhǎng)期相符合。此外, 八寶山巖體的Nd()值也與Jahn.[52]歸納出來(lái)的華北克拉通古老下地殼的Nd(=130 Ma)值(-44～-32)相差很多。因此, 八寶山花崗巖體的巖漿源區(qū)更可能為揚(yáng)子克拉通的地殼物質(zhì)。

綜上所述, 可以推斷八寶山巖體的源區(qū)可能是俯沖的揚(yáng)子克拉通及南秦嶺陸殼與太華群和熊耳群的混合, 但主要來(lái)源于南秦嶺和揚(yáng)子陸塊的部分熔融, 這與Hf同位素分析結(jié)果一致[7]。

5.3?巖石成因

八寶山巖體的鋯石定年結(jié)果表明, 鉀長(zhǎng)花崗斑巖和黑云母二長(zhǎng)花崗斑巖是同一期巖漿作用的產(chǎn)物, 只是經(jīng)歷了不同程度的結(jié)晶分異[7]。從黑云母二長(zhǎng)花崗斑巖到鉀長(zhǎng)花崗斑巖, 巖石的礦物組成中石英和堿性長(zhǎng)石含量逐漸升高, 斜長(zhǎng)石的含量逐漸減少, 鐵鎂礦物含量明顯下降; 隨著SiO2含量的升高, TiO2、Al2O3、Fe2O3T、MgO、CaO和P2O5逐漸降低, 而全堿含量逐漸升高, Nb/Ta和La/Y沒(méi)有什么變化(圖7)。顯示了黑云母二長(zhǎng)花崗斑巖和鉀長(zhǎng)花崗斑巖之間的同源巖漿分異演化的趨勢(shì)。

在稀土元素分布模式和微量元素蛛網(wǎng)圖上(圖8), 兩者具有一定的相似性及演化關(guān)系, 據(jù)此推斷兩者可能是由同一母巖漿分異形成的。這與兩種巖石的鋯石Hf同位素和全巖的Nd同位素分析結(jié)果也相當(dāng)一致[7]。例如, 黑云母二長(zhǎng)花崗斑巖和鉀長(zhǎng)花崗斑巖輕、重稀土分餾和Eu負(fù)異常程度的差別顯示獨(dú)居石和斜長(zhǎng)石等結(jié)晶分異的影響, 這與主元素的分析結(jié)果相一致。八寶山花崗斑巖具有富硅、明顯虧損Nb、Sr、P、Ti和Eu等地球化學(xué)特征, 指示其母巖漿經(jīng)歷了顯著的分離結(jié)晶演化。Nb、Ti元素的虧損指示了富Ti礦物相的分離, 而P的強(qiáng)烈虧損表明發(fā)生了磷灰石的分離結(jié)晶, Sr和Eu的虧損與斜長(zhǎng)石和鉀長(zhǎng)石的分離結(jié)晶作用有關(guān)[34]。與黑云母二長(zhǎng)花崗斑巖相比, 鉀長(zhǎng)花崗斑巖相對(duì)富集Rb、Ba、Th、U和K等大離子親石元素, 顯示了強(qiáng)烈分異的地球化學(xué)特征, 表明兩類(lèi)花崗巖之間存在一定的分異演化關(guān)系。鉀長(zhǎng)花崗斑巖的Rb/Sr和Ba/Sr比值明顯高于黑云母二長(zhǎng)花崗斑巖(圖7), 這也表明鉀長(zhǎng)花崗斑巖具有更高程度的分異-演化。

在Ba-Sr對(duì)數(shù)圖解上(圖11a), 鉀長(zhǎng)花崗斑巖和黑云母二長(zhǎng)花崗斑巖都落在鉀長(zhǎng)石和斜長(zhǎng)石的結(jié)晶演化線(xiàn)上, 這些數(shù)據(jù)的變化趨勢(shì)也進(jìn)一步證明了巖體形成過(guò)程中主要經(jīng)歷了鉀長(zhǎng)石和斜長(zhǎng)石的分異。相對(duì)于主要礦物來(lái)說(shuō), 副礦物的結(jié)晶分異對(duì)稀土元素含量變化影響較大。在(La/Yb)N-La變異圖解上(圖11b), 從巖體樣品點(diǎn)的分布來(lái)看, 八寶山花崗斑巖存在獨(dú)居石和褐簾石的結(jié)晶分異, 它們可能對(duì)稀土元素的含量有一定影響。根據(jù)巖體的巖石地球化學(xué)特征可以看出鉀長(zhǎng)花崗斑巖的演化程度更高。

5.4?構(gòu)造意義

前人的研究指出, 從晚侏羅世到早白堊世, 秦嶺造山帶處于擠壓到伸展的轉(zhuǎn)換階段, 并且大規(guī)模發(fā)育鉀長(zhǎng)花崗巖和富堿中酸性火山巖[1–2,5]。在Pearce.[58–59]提出的花崗巖構(gòu)造背景判別圖上(圖12), 鉀長(zhǎng)花崗斑巖和黑云母二長(zhǎng)花崗斑巖都屬于碰撞后花崗巖范圍。我們對(duì)華北南緣其他大型斑巖礦床巖體地球化學(xué)數(shù)據(jù)做了總結(jié)(如金堆城、南泥湖和東溝等), 發(fā)現(xiàn)八寶山巖體的形成構(gòu)造背景與華北南緣的其他同時(shí)代巖體的形成構(gòu)造背景一致(圖12)。

八寶山巖體巖石地球化學(xué)、鋯石U-Pb定年和Hf、Nd同位素組成研究所獲得的結(jié)論, 也與秦嶺造山帶的形成和演化的研究結(jié)果相吻合。秦嶺造山帶是以新元古代到中生代初期的多板塊、多類(lèi)型碰撞造山為主體, 并遭受中新生代強(qiáng)烈陸內(nèi)造山作用疊加改造的一個(gè)復(fù)合型造山帶[1]。其形成和演化主要經(jīng)歷了3個(gè)不同構(gòu)造演化階段: (1)晚太古代-古元古代造山帶前寒武紀(jì)結(jié)晶基底的形成演化; (2)新元古代-中三疊世, 以現(xiàn)代板塊構(gòu)造體制為基本特征的板塊構(gòu)造演化; (3)中生代以來(lái)的陸內(nèi)造山作用與構(gòu)造演化[1]。原秦嶺造山帶的華北板塊、揚(yáng)子板塊和秦嶺微板塊在三疊紀(jì)中、晚期完成了全面碰撞造山后, 形成的強(qiáng)烈擠壓縮短增厚的巖石圈根, 在中生代新的殼幔動(dòng)力學(xué)系統(tǒng)中, 發(fā)生強(qiáng)烈殼幔物質(zhì)交換、伸展減薄, 造成顯著的巖石圈去根作用[1,8], 伴隨大量巖漿侵位發(fā)育了與陸內(nèi)構(gòu)造-巖漿活動(dòng)有關(guān)的大規(guī)模熱液礦床的成礦作用。

圖11?八寶山花崗斑巖造巖礦物(a)和副礦物(b)分離結(jié)晶判別(圖解底圖據(jù)文獻(xiàn)[53], 樣品符號(hào)同圖4)

PlAn15–斜長(zhǎng)石(An=15); PlAn50–斜長(zhǎng)石(An=52); Ms–白云母; Bt–黑云母; Kfs–鉀長(zhǎng)石; Grt–石榴子石; Amp–角閃石; Allan–褐簾石; Mon–獨(dú)居石; Ap–磷灰石; Zr–鋯石;Sph-榍石。磷灰石的分配系數(shù)據(jù)文獻(xiàn)[54]; 鋯石分配系數(shù)據(jù)文獻(xiàn)[55]; 褐簾石分配系數(shù)據(jù)文獻(xiàn)[56]; 獨(dú)居石分配系數(shù)據(jù)文獻(xiàn)[57]。

圖12?八寶山花崗巖體的構(gòu)造環(huán)境判別圖(底圖據(jù)文獻(xiàn)[58–59], 數(shù)據(jù)來(lái)自文獻(xiàn)[2,60–62], 樣品符號(hào)同圖4)

ORG–大洋中脊花崗巖; WPG–板內(nèi)花崗巖; VAG–火山弧花崗巖; Syn-COLG–同碰撞花崗巖; post-COLG–后碰撞花崗巖。陰影區(qū)是金堆城、湯家坪、南泥湖和東溝巖體。

八寶山巖體的形成年齡正對(duì)應(yīng)于秦嶺造山帶在晚侏羅世-早白堊世的擠壓向伸展的轉(zhuǎn)變階段[63–65]。八寶山巖體的形成機(jī)制可概括為: 在晚侏羅世-早白堊世的擠壓向伸展轉(zhuǎn)換時(shí)期, 至少在八寶山斑巖形成時(shí)華北南緣加厚地殼尚未拆沉, 而這種加厚的下地殼可能主要源于俯沖于華北南緣陸殼之下的揚(yáng)子及南秦嶺地殼, 俯沖陸殼在上涌地幔熱源影響下, 發(fā)生部分熔融形成的酸性巖漿上升侵位, 從而形成八寶山巖體碰撞后花崗巖。

5.5 巖體形成深度與找礦前景

對(duì)黑云母二長(zhǎng)花崗斑巖中的黑云母進(jìn)行了電子探針?lè)治? 分析結(jié)果見(jiàn)表3。利用黑云母中TAl的含量估算八寶山巖體形成的壓力以及深度[66](表3)。黑云母結(jié)晶的壓力為52 ~ 57 MPa, 對(duì)應(yīng)的侵位深度為1.89 ~ 2.55 km, 表明該巖體形成于較淺的環(huán)境, 這與巖相學(xué)顯示的特征相符。而斑巖成礦系統(tǒng)的深度一般在1 ~ 6 km, 最大成礦深度達(dá)9 km。因此巖體被剝蝕程度較低, 可以推斷八寶山斑巖成礦系統(tǒng)未受到構(gòu)造抬升的破壞, 深部成礦和找礦潛力較大。

在八寶山巖體周?chē)植贾恍┭嗌狡诘闹兴嵝孕“邘r體, 均呈北北東向有規(guī)律地平行排列。這些巖體的展布具有明顯的方向性、等距性和分帶性規(guī)律。特別是銀家溝巖體在巖性和成巖成礦年齡與八寶山巖體比較接近(表4)。除了蒲陣溝巖體巖性為中酸性外, 其余巖體的巖性均為酸性, 并且礦體與鉀長(zhǎng)花崗斑巖關(guān)系最為密切。通過(guò)與盧氏地區(qū)不同巖體成礦特征的對(duì)比, 我們推測(cè)這些巖體屬于相似的成礦體系。并且通過(guò)鉆探和野外觀察發(fā)現(xiàn)八寶山主要是在巖體內(nèi)部和圍巖的接觸帶中發(fā)現(xiàn)有鐵銅礦體, 可能在巖體的外圍會(huì)有鉛鋅礦體存在。

表3?黑云母礦物的電子探針?lè)治鼋Y(jié)果

注：(kbar)=3.03×TAl-6.53(±0.33)[66]; 上地殼靜巖壓力梯度=27.5 MPa/km;TAl指在O=22基礎(chǔ)上的黑云母中鋁陽(yáng)離子總數(shù)。

表4?盧氏地區(qū)巖體、礦體概況

八寶山巖體為含黑云母和角閃石的富水巖漿體系, 黑云母組成以較高的Mg/Fe比值、低鋁、貧鈣、K/Na>>30(108 ~ 475)與全球典型的含礦斑巖特征一致[14]; 鋯石Ce4+/Ce3+的比值也很高(286 ~ 786)[7], 這種高氧化性的富水巖漿有利于斑巖銅鉬(金)礦床的形成, 所以其深部或邊緣應(yīng)存在有利的成礦部位, 如較大的巖枝、巖體外緣的突變部位。

6?結(jié)?論

(1)八寶山巖體主要由中心相的黑云母二長(zhǎng)花崗斑巖和邊緣相的鉀長(zhǎng)花崗斑巖組成, 屬于準(zhǔn)鋁質(zhì)-過(guò)鋁質(zhì), 鉀玄巖系列的高分異I型花崗巖類(lèi)。兩者具有同源巖漿演化分異的趨勢(shì), 并且鉀長(zhǎng)花崗斑巖演化分異程度更高。

(2)全巖Nd同位素分析結(jié)果表明, 八寶山巖體的源區(qū)可能主要來(lái)源于南秦嶺和揚(yáng)子陸塊的部分熔融, 但在巖漿形成過(guò)程中可能混入了少量太華群和熊耳群的物質(zhì)。

(3)根據(jù)黑云母地質(zhì)壓力計(jì)估算, 八寶山巖體的的侵位深度為1.89 ~ 2.55 km, 表明該巖體形成侵位較淺, 剝蝕程度較小, 斑巖成礦體系可能得到很好的保存, 在其深部可能存在有利于斑巖Cu-Au-Mo-Pb-Zn的多金屬成礦區(qū)段。

野外工作得到了盧氏縣北方礦業(yè)有限公司的大力支持; 主元素和微量元素分析、Nd同位素分析得到了中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室的支持; 黑云母電子探針?lè)治龅玫搅四暇┐髮W(xué)內(nèi)生金屬礦床成礦機(jī)制研究國(guó)家重點(diǎn)實(shí)驗(yàn)室的支持, 在此一并表示感謝。同時(shí)作者要特別感謝兩位審稿專(zhuān)家和責(zé)任編輯提出的建設(shè)性的修改意見(jiàn)和寶貴的評(píng)論。

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Geochemistry and petrogenesis of the Babaoshan granite porphyry in Lushi county, Henan Province

ZENG Ling-jun1,2, ZHOU Dong1,2, XING Yu-cai3, ZHAO Tai-ping1, YAO Jun-ming1and BAO Zhi-wei1*

1. Key Laboratory for Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou?510640, China; 2. University of Chinese Academy of Sciences, Beijing?100049, China; 3. No.1 Institute of Geological Survey, Geological Bureau of Henan Province, Luoyang?471023, China

The Babaoshan granite porphyries in Lushi county, Henan province, is tectonically located at the southern margin of North China Craton, in the western part of the East Qinling orogen. The granite porphyries occur as circular column which might have been the neck of volcanic edifice. Formed at (146±2) Ma (LA-ICPMS zircon U-Pb), the intrusion is unique for the associated iron predominated ore deposit with accompanying Cu-Mo-Pb-Zn mineralization, which makes it a meaningful target for metallogenetic investigation in the East Qinling area. The Babaoshan granite intrusion is a zoned granitic stock consisting of syenogranite porphyry in the outer zone and biotite monzogranite porphyry in the inner part. The granite porphyries have high SiO2(64.8%－73.5%) contents, with ACNK values varying between 0.98－1.56. It can be classified as metaluminous- peraluminous granite. The rocks are relatively enriched in LILE such as Rb, Ba, U, K and LREE ((La/Yb)N= 2.29－23.8), depleted in Sr, Nb, Ti, and P, with insignificant Eu anomalies. The geochemical characteristics of the Babaoshan granite porphyries suggest a highly fractionated I-type origin, while the syenogranite porphyry is more fractionated. The lowNd() (-19.46－-16.4) and rather old model ages (NdDM2= 2.46 Ga to 2.27 Ga) are consistent with Hf isotopic compositions. It demonstrates that the rocks might have been derived from partial melting of crustal material. It possibly derived from partial melting of the lower crust under extensional tectonic regime following the collision condition. Biotite geobarometer calculations show that the Babaoshan granite porphyries may form at depth of 1.89 km to 2.55 km, which is consistent with the sub-volcanic occurrence. We deduce that there might be an underlying porphyry Cu-Mo-Pb-Zn polymetallic mineralization section at depths.

highly fractionated I-type granite; Nd isotope; porphyry; Babaoshan granite; southern margin of the North China Craton

P581; P597; P595

A

0379-1726(2013)03-0242-20

2012-11-14;

2013-01-14;

2013-02-05

國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃項(xiàng)目(2012CB416602)

曾令君(1989–), 女, 碩士研究生, 礦床學(xué)專(zhuān)業(yè)。E-mail: zenglingjun0213@163.com

BAO Zhi-wei, E-mail: baozw@gig.ac.cn, Tel: +86-20-85290105