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长江中下游贵池李湾铜多金属矿区岩浆岩年代学及Hf 同位素地球化学研究*

2015-07-21段留安古黄玲杨晓勇严志忠孙卫东

岩石学报 2015年7期
关键词:闪长岩锆石花岗岩

段留安 古黄玲 杨晓勇** 严志忠 孙卫东

1.中国科学技术大学地球和空间科学学院,合肥 230026

2.武警黄金第七支队,烟台 264004

3.安徽省地勘局324 地质队,池州 247100

4.中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广州 510640

5.中国科学院青藏高原地球科学卓越创新中心,北京 100101

1 引言

长江中下游成矿带是我国重要的铜金多金属成矿带,一直是地质学家关注的热点地区之一(常印佛等,1991;翟裕生等,1992;Pan and Dong,1999;Mao et al.,2006;毛景文等,2009),该成矿带自西向东依次可分为鄂东、九瑞、安庆-贵池、庐枞、铜陵、宁芜和宁镇等7 个矿集区(Yang and Lee,2011;Deng et al.,2011),李湾铜多金属矿床位于安庆-贵池矿集区之贵池地区(图1a)。已有的研究显示长江中下游铜金成矿作用主要集中在140 ±5Ma,与燕山期岩浆活动密切相关(Sun et al.,2003;Mao et al.,2006;Wang et al.,2006,2007;Xie et al.,2009,2012;Li et al.,2009,2010;宋国学等,2010;谢建成等,2012;周涛发等,2008,2012;段留安等,2012,2014)。

图1 池州地区区域地质图(据宋国学等,2010 修改)Fig.1 Simplified geological map of the Chizhou area (modified after Song et al.,2010)

图2 李湾矿区地质简图Fig.2 Geological Map of the Liwan area

近年来安庆-贵池矿集区找矿工作取得了较大进展,如池州市抛刀岭金矿已经是大型规模,并有望达到特大型规模(段留安等,2014)。此外,池州市黄山岭铅锌矿深部及周边地区、马头铜钼矿、高家榜铜钼矿,东至赵家岭金矿等找矿工作也取得了较大进展(图1b)。然而安庆-贵池矿集区基础地质研究相对薄弱。目前人们将研究重点集中在140 ±5Ma 左右的岩浆活动及成矿作用上(宋国学等,2010;张智宇等,2011;刘圆圆等,2012;段留安等,2012,2014;杨贵才等,2014),而忽视了125 ±5Ma 左右的岩浆活动及成矿作用。究其原因,主要源于普遍认为贵池地区花园巩A 型花岗岩不成矿或成矿差,以及与125 ±5Ma 时间段目前没有发现大型金属矿产相关。目前为止,对池州地区125 ±5Ma 左右成矿的矿床实例还没有报道,由于李湾矿床为接触交代矽卡岩型矿床,其成矿应接近或晚于成岩年龄,本文通过对该区与成矿相关的岩浆岩主微量元素、年代学、同位素等研究,对该地区成岩成矿进行再认识和总结,进而弥补贵池地区125 ±5Ma时间段成矿的空缺,对该区域地质探矿工作提供理论支持。

2 地质背景和矿床地质特征

2.1 区域地质背景

安庆-贵池矿集区位于扬子板块的北东缘、大别造山带与江南地块之间的下扬子台褶带中,属于贵池-繁昌断褶束中段,是大别造山带和江南叠覆造山带相互作用的地区,其地质构造位置较特殊(图1a)。该区构造演化大体经历了前寒武纪基底形成、震旦纪-早二叠覆盖沉积和中三叠以来的碰撞造山三个阶段,随后进入太平洋构造域,发生了地壳伸展和大规模岩浆活动。

区域地层发育以碳酸盐建造为主、碎屑岩建造为辅的一套沉积盖层,隶属下扬子地层分区贵池地层小区。古生代-早三叠世地层是本区W、Mo、Pb、Zn、Cu、Au、Ag 等金属矿床主要的容矿层位(唐永成等,1998),本区地层对矿床的控制作用主要表现在矿化类型上的差异,如志留系砂页岩地层易形成斑岩型、热液型矿化及蚀变岩型矿化(段留安等,2013),奥陶系、石炭系碳酸盐岩地层易形成矽卡岩型和热液型矿化(董胜,2006)。该区燕山期岩浆岩发育,以中深-浅成侵入岩为主,大规模的岩体有青阳-九华岩体(750km2,139~142Ma)、谭山岩体(140km2,129~133Ma)、花园巩岩体(220km2,125Ma)(Wu et al.,2012),均呈大型岩基产出的复式岩体,分布于区内的中东部(图1b),岩石类型主要为花岗闪长岩、二长花岗岩和钾长花岗岩。小规模的有马石、抛刀岭、乌石、牛脊背等岩体,多呈小岩株产出,出露面积一般不足2km2(图1b),多属深熔高钾钙碱性系列,与铜多金属矿密切相关。花园巩岩体为A 型花岗岩类岩体,一般认为是沿江地区中生代岩浆活动的最后阶段产物,岩石偏碱性,成矿作用较差(董胜,2006)。

图3 李湾矿区各类岩矿石照片(a)钾长花岗岩;(b)闪长岩;(c)安山玄武玢岩;(d)含多金属矽卡化闪长岩;(e)含辉钼矿闪长岩;(f)含黄铁矿条带状矿石;(g、h)多金属矿化矽卡岩;(i)钾长花岗岩中的闪长岩捕掳体Fig.3 Field photographs for Liwan area(a)moyite;(b)diorite;(c)basaltic-andesitic porphyrite;(d)multi-metal contained skarn diorite;(e)molybdenite bearing diorite;(f)pyrite bearing band ore;(g,h)polymetallic ore bearing skarn;(i)diorite xenoliths in the moyite

区域地质构造处于贵池背向斜带之自来山背斜西段核部,巴山岩体西南外围。自来山背斜总体呈北东-南西延伸,核部地层为奥陶系仑山组下段白云岩,北西翼因岩体侵入破坏支离破碎,南东翼出露地层为奥陶系下统仑山组上段灰岩至志留系高家边组砂页岩,地层倾向南东,倾角44°~65°。沿自来山背斜核部常出露有中酸性花岗斑岩、花岗闪长斑岩等侵入岩,在岩体与围岩的接触带蚀变-交代作用强烈,局部产出铜多金属矿体。

2.2 矿区地质

李湾矿区出露地层为奥陶纪和志留纪地层(图2)。矿区内除褶皱构造外,断裂构造也十分发育,主要有F1、F2和F3三条断裂。F1断裂位于矿区西北侧边缘,为左旋平移断层,呈北北东向展布,为区域性梅村-墩上大断层的一部分;F2断裂位于本区南侧边缘,为北东向展布的逆断层,断层北盘为仑山组上、下段灰岩、白云岩,南盘为红花园、大湾组等地层,断层走向长约4.5km,两端均出勘查区外,断层倾向南东,倾角约50°;F3断裂仅在探矿坑道中见到,推测规模较小,倾向南东120°,倾角约70°,为成岩前正断层,成矿岩体西延受阻,矿化就此中断,断层带较窄,发育有角砾岩。

矿区岩浆岩发育,岩浆岩侵位受自来山背斜构造和北北东左旋平行断层的控制,形成浅成侵入岩体和次火山岩体。其中次火山岩,发育于矿区的北部,岩性为粗面质熔结角砾凝灰岩,覆盖于花岗斑岩和白云岩之上;侵入岩岩性主要为钾长花岗岩(图3a,i)、石英斑岩、花岗斑岩、闪长岩(图3b)及玄武安山玢岩脉(图3c)等。钾长花岗岩在矿区出露较广,局部见有闪长岩捕虏体(图3i),其与地层接触带未见明显金属矿化;安山玄武玢岩仅在+72m 水平坑道中发现,呈近南北向向西突出的弧状展布,倾向西或西北,倾角70°,宽2~3m,构成了主矿体东部边界;闪长岩地表未见出露,仅在坑道和钻孔中见到,从野外地质特征看,与本区铜多金属矿密切相关。闪长岩与围岩的接触带表现出强烈的蚀变和铜-钼-锌等多金属矿化(图3d-f),同时闪长岩内部有时也见有黄铜矿、斑铜矿、闪锌矿、黄铁矿、辉钼矿等金属矿化(图3d,e)。

李湾矿区多年来民企一直在采矿中,从钻孔、坑道及采掘出来的岩矿石看该矿床属于小而富的矽卡岩型铜多金属矿,其矿石类型为含铜黄铁矿、含铜硫矽卡岩、含铅锌矽卡岩等,矿石的主要有益组分为Cu、S,个别矿体含Mo、Pb、Zn、Fe。目前,经稀疏钻探及坑探控制矿体10 个,但多没有完全控制。如Ⅱ号矿体,由ZK803 等3 个钻孔控制,走向NEE,地表延伸56m,倾向SE,倾角60°,斜深200m,斜深大于延伸(斜深尚未完全控制)。矿体最大厚度16.94m,最小厚度1.08m,平均厚度6.96m,铜平均品位0.62%,硫平均品位17.34%,铜硫矿体赋存标高为-106~76m;Ⅵ号矿体,由ZK801、ZK803 控制,总体呈透镜状,向上有分枝,倾向SE,倾角45°,斜深106m,最大厚度22.02m,最小厚度4.16m,平均厚度14.35m,厚度变化系数为64.06%,属较稳定矿体。铜平均品位0.70%,硫平均品位13.90%,铜硫矿体赋存标高-114~-176m。

从区域成矿条件分析,李湾一带铜多金属具有较好的成矿前景,已知的地质特征及坑道采出的闪长岩中发育的铜、钼矿化等信息也显示,浅部的闪长岩与地层的接触带形成高硫矽卡岩型铜多金属矿,而在深部可能具有斑岩型铜钼矿的找矿潜力。

3 样品采集、加工及测试结果

为了揭示李湾矿区岩浆岩的地球化学特征及与铜多金属成矿作用的关系,本文对矿区的钾长花岗岩、闪长岩及玄武安山玢岩进行了主量元素、微量元素、单颗粒锆石U-Pb 定年和锆石原位Hf 同位素分析。

全岩的主量元素和微量元素分析在广州澳实矿物实验室完成。其中常量元素采用ME-XRF06 法,由X 荧光光谱仪测定,分析流程见刘颖等(1996);稀土元素采用ME-MS81法,由等离子体质谱测定;微量元素采用ME-MS61 法,由等离子体质谱测定,具体分析流程见Qi et al.(2000)。

锆石单矿物分选由河北省地勘局廊坊实验室完成,将8~10kg 重的原岩样品粉碎,经常规重选和电磁选后在双目镜下挑选锆石。双目镜下将分选好的锆石根据颜色、自形程度、形态等特征初步分类,挑选出具有代表性的锆石用环氧树脂制靶、打磨和抛光。样品测定之前用体积百分比为3%的HNO3清洗样品表面,以除去样品表面的污染。然后进行锆石显微镜照样(反射光和透射光)和阴极发光(CL)照相,锆石的透反射和阴极发光照相在中国科学技术大学壳-幔物质与环境重点实验室完成。

锆石的激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)原位U-Pb 定年和微量元素分析在中国科学院广州地球化学研究所同位素地球化学国家重点实验室完成。仪器组成及实验参数见Li et al.(2012)。数据处理采用ICPMSDataCal软件(Liu et al.,2008,2010a),年龄计算采用ISOPLOT(3.00版)软件(Ludwig,2003)进行。详细分析方法见Yuan et al.(2004)和Liu et al.(2010a)。

图4 李湾矿区侵入岩地球化学判别图解(a)SiO2-Na2O+ K2O 判别图解(据Le Base et al.,1986);(b)岩石系列SiO2-K2O 图解(实线据Peccerillo and Taylor,1976;虚线据Middlemost,1985)Fig.4 Geochemical discrimination plot of the Liwan intrusions(a)SiO2 vs.Na2O+K2O plot (after Le Base et al.,1986);(b)K2O vs.SiO2 diagram (solid lines after Peccerillo and Taylor,1976;dashed lined after Middlemost,1985)

表1 李湾矿区侵入岩全岩主量组成(wt%)Table 1 Rock major element data for the Liwan intrusions (wt%)

表2 李湾矿区侵入岩微量元素组成(×10 -6)Table 2 Trace elements data for the Liwan intrusions (×10 -6)

图5 李湾矿区侵入岩Harker 图解Fig.5 Harker major element variation diagrams of Liwan intrusions

锆石的微区原位Lu-Hf 同位素分析在西北大学大陆动力学国家重点实验室完成。所用质谱为Nu Plasma 型多接收电感耦合等离子体质谱(MC-ICP-MS),激光剥蚀系统为193nm ArF 准分子激光器的GeoLas 2005。激光斑束直径为44μm,激光脉冲频率为8Hz。具体分析方法和仪器参数详见Yuan et al.(2008)。用176Lu/175Lu=0.02655 (De Biévre and Taylor,1993)和176Yb/172Yb =0.58545 (Chu et al.,2001)作为校正因子来进行同质异位干扰校正,计算样品的176Lu/177Hf和176Hf/177Hf。以标准锆石MON-1、GJ-1、91500 作为外标,其推荐的标准值依次为0.282739 ± 0.000057,0.282015 ±0.000056,0.282307 ±0.000055。在进行εHf(t)计算时,采用176Lu 衰变常数= 1.867 × 10-11year-1(Soderlund et al.,2004),球粒陨石现今的176Hf/177Hf =0.282772 和176Lu/177Hf = 0.0332(Blichert and Albarede,1997)。在进行模式年龄计算时,采用现今的亏损地幔176Hf/177Hf = 0.28325 和176Lu/177Hf = 0.0384(Griffin et al.,2000),现今平均大陆壳的176Lu/177Hf =0.015(Griffin et al.,2002)。

4 分析结果

4.1 元素地球化学特征

主量元素分析结果见表1,微量元素分析结果见表2。

闪长岩具较低的SiO2(45.73%~52.5%,平均为50.61%)和Fe2O3T(6.83%~10.57%,平均为7.92%)含量;全碱含量(K2O + Na2O)为4.58%~6.7%,平均为5.22%;Mg#值为36.3~54.86,平均为45.38,CaO 含量为4.01%~8.27%,平均为5.95%。在TAS 图中,落入闪长岩范围(图4a),岩石具有富铝、镁特征,属于高钾钙碱性系列(图4b)。

图6 李湾矿区岩浆岩稀土元素配分图(a)及微量元素蛛网图(b)(球粒陨石标准值据Sun and McDonough,1989;原始地幔及其他地质储库标准值据McDonough and Sun,1995)Fig.6 Chondrite-normalized REE patterns (a)and primitive mantle-normalized trace elements patterns (b)of Liwan intrusions(chondrite values are from Sun and McDonough,1989;primitive mantle and other geological storage standard values are from McDonough and Sun,1995)

钾长花岗岩具有较高的SiO2(73.29%~74.61%,平均为74.09%)、Al2O3(12.61%~13.07%,平均为12.83%)和Fe2O3T(1.24%~1.38%,平均为1.32%)含量;全碱(K2O +Na2O)含量为3.62%~6.32%,平均为4.65%;TiO2含量为0.18%~0.21%,平均为0.19%;MgO 含量为0.49%~0.57%,平均为0.53%,Mg#值为41.29~47.66,平均为44.39,CaO 含量为1.12%~3.24%,平均为2.08%。在TAS图中,落入花岗岩范围(图4a),属于髙钾钙碱性系列。

玄武安山玢岩具有较低的SiO2含量(42%~43.88%,平均为42.95%),较高的Al2O3含量(22.51%~25.14%,平均为23.66%)。Fe2O3T含量为1.78%~3.4%,平均为2.44%;K2O + Na2O 含量为0.74%~2.44%,平均为1.55%;TiO2含量为0.79%~0.87%,平均为0.84%;MgO含量为2.94%~4.25%,平均为3.86%;Mg#值为69.44~82.17,平均为75.80;CaO 含量为18.19%~22.35%,平均为19.64%。

李湾矿区侵入岩A/CNK 比值为1.03~2.18,显示强过铝质的特征。Harker 图解中Al2O3、Fe2O3T、CaO、P2O5、TiO2与SiO2存在负相关性(图5),表明在岩石形成过程中发生斜长石,辉石,角闪石,钛铁矿的分离结晶作用。

钾长花岗岩ΣREE 为269.4 ×10-6~302.7 ×10-6,平均值为283.8 ×10-6。其LREE/HREE 值为7.54~7.91,平均值为7.69。(La/Yb)N的值为6.48~6.86,平均值为6.65;闪长岩ΣREE 为198.7 × 10-6~308.1 × 10-6,平均值为269.7 ×10-6。其LREE/HREE 值为7.06~10.26,平均值为9.27。(La/Yb)N的值为9.30~13.40,平均值为11.77;玄武安山玢岩ΣREE 为285.1 ×10-6~621.4 ×10-6,平均值为420.7 ×10-6,变化范围较大。其LREE/HREE 值为15.95~25.04,平均值为21.07。(La/Yb)N的值为30.64~74.64,平均值为50.30。玄武安山玢岩稀土总量高,轻重稀土分异明显,轻稀土显著偏高,可能是受到了俯冲流体交代作用。三者整体上具有富集轻稀土、亏损重稀土,轻重稀土分异明显的特点。钾长花岗岩表现出明显的Eu 负异常(图6a),表明源区具有斜长石的分离结晶。稀土配分模式右倾型不同于一般的从地幔分离出来的岩石稀土配分模式(如MORB)(图6a),说明李湾钾长花岗岩岩浆源区并不是从原始地幔或者亏损软流圈地幔分离出来的,而可能是从岩石圈地幔分离的产物。

钾长花岗岩具有典型的A 型花岗岩特征:高总碱(K2O+Na2O)和K2O 含量(3.59%~6.23%),低MgO、TiO2和P2O5含量,富集大离子亲石元素(Rb、Th、U)和高场强元素(Zr、Hf、Nb 和Y),亏损Ba、P、Ti(图6b)。在A 型花岗岩判别图解中(图7),李湾钾长花岗岩落入A 型花岗岩区域,这与区域花园巩A 型花岗岩一致。

4.2 同位素年代学

锆石定年分析结果见表3。

李湾矿区钾长花岗岩锆石多为无色透明-浅黄色自形晶体,多呈长柱状,长宽比多介于1∶1~3∶1。CL 照片显示多数锆石震荡环带发育,Th/U 比值均大于0.4。这些特征表明其为岩浆成因锆石(Hoskin,2000;Sun et al.,2002;吴元保和郑永飞,2004)。27 颗锆石的206Pb/238U 表面年龄为117 ±2Ma~130 ±2Ma,加权平均年龄为122.6 ±1.3Ma (图8a);玄武安山玢岩大部分都是继承锆石,206Pb/238U 表面年龄为151.6±5Ma~2156.4 ±43Ma (图8b);闪长岩锆石多为无色透明-浅黄色自形晶体,多为长柱状,长宽比多介于1∶1~3∶1。CL 照片显示多数锆石震荡环带发育,为典型岩浆成岩锆石(Hoskin,2000;Sun et al.,2002;吴元保和郑永飞,2004),其206Pb/238U 年龄变化于116.8 ±2Ma~147.1 ±2Ma 之间,加权平均年龄为123.4 ±2.4Ma (图8c),说明闪长岩侵位比钾长花岗岩稍早一点,这从野外见钾长花岗岩中含有闪长岩的包裹体等地质现象也能看出(图3i)。玄武安山玢岩继承锆石2156Ma 的年龄值暗示了扬子板块东北缘长江中下游地区存在古元古代基底,古元古代基底参与了长江中下游中生代花岗质岩的形成,并产生了大量的斑岩型和矽卡岩型矿床(涂荫玖等,2001;Gao et al.,2001;Zheng et al.,2006;Zhu et al.,2014)。钾长花岗岩和闪长岩不同的结晶年龄说明李湾地区经历了不同的岩浆演化历史,由于闪长岩与本区矽卡岩紧密相关且本身也含铜钼矿(化),所以其年龄更接近铜多金属成矿的年龄,且与长江中下游地区A 型花岗岩的形成时代相一致(125 ±2Ma;范裕等,2008;Wong et al.,2009;Li et al.,2011)。

表3 李湾矿区岩浆岩锆石LA-ICP-MS U-Pb 定年分析结果Table 3 LA-ICP-MS zircon U-Pb data of the Liwan intrusions

图7 李湾矿区岩体A 型花岗岩判别图解(据Whalen et al.,1987)Fig.7 A-type granites discrimination diagram of Liwan intrusions (after Whalen et al.,1987)

4.3 锆石微量元素与Hf 同位素

所有锆石微量元素测试数据见表4,Hf 同位素分析结果见表5。

钾长花岗岩和闪长岩锆石稀土元素配分模式均显示典型的轻稀土亏损,重稀土富集,Ce 正异常、Eu 负异常的特征(图9a,b)。闪长岩锆石稀土总量(REE)范围为598 ×10-6~2122 ×10-6,均值1587 ×10-6;钾长花岗岩锆石稀土总量(REE)范围为746~2222 ×10-6,均值1343 ×10-6,说明两者的锆石稀土总量相似。闪长岩和钾长花岗岩锆石部分显示较高的轻稀土富集特征,可能是在锆石分析时激光点位置打在了部分富轻稀土矿物(磷灰石)或包裹体上(Wang et al.,2014)。通过晶格应力模型计算获得锆石的Ce4+/Ce3+显示了闪长岩锆石具有较高的Ce4+/Ce3+,平均值分别为497;而钾长花岗岩锆石Ce4+/Ce3+偏低,平均值为246;钾长花岗岩锆石Ce4+/Ce3+变化极大(10~3868),可能显示了锆石结晶时氧逸度变化大导致。闪长岩和钾长花岗岩锆石Eu/Eu*均值为0.21;锆石Ti 温度计计算显示,两者的形成温度大体一致,闪长岩及钾长花岗岩的形成温度均值分别为:708℃及721℃。

钾长花岗岩岩浆锆石均具有均一的Hf 同位素组成,锆石176Hf/177Hf 初始比值为0.28254~0.28267,相应的εHf(t)为-6.7~-2.1,均值为-4.7,二阶段模式年龄为1.12~1.36Ga,平均为1.26Ga(图9c)。李献华等(1991)统计前人数据,发现华南地壳幕式增长,主要有3 期:2.5Ga 或更早,1.8Ga,1.2~1.4Ga。钾长花岗岩的源区对应晚期华南地壳的幕式增长,其锆石的二阶段Hf 模式年龄与皖南地区出露的中元古时代基底的年龄值相近,εHf(t)说明李湾矿区侵入岩具有壳幔混合的岩浆岩源区的性质(图10),钾长花岗岩的初始岩浆可能属于皖南基底的部分熔融和地幔物质的混溶。

表4 李湾矿区岩体锆石微量元素组成(×10 -6)Table 4 Zircon trace element data for the Liwan intrusions (×10 -6)

续表4Continued Table 4

表5 李湾矿区岩体锆石Hf 同位素组成Table 5 Zircon Hf isotope data for the Liwan intrusions

图8 李湾矿区岩体锆石年龄图解Fig.8 Zircon concordia diagrams of zircon from Liwan intrusions Ellipse dimensions are 1σ

5 讨论

5.1 岩石成因

图9 李湾矿区岩体锆石稀土配分图(a、b,标准化值据Sun and McDonough,1989)及钾长花岗岩二阶模式年龄图解(c)Fig.9 Chondrite-normalized REE patterns of zircons from Liwan intrusions (a,b,normalization values from Sun and McDonough,1989)and two second model age plot of Kfeldspar granites (c)

研究表明皖南地区A 型花岗岩初始(87Sr/86Sr)i约为0.707,εNd(t)为-4.2~-7.0,Nd 模式年龄为1.2~1.5Ga,这些岩体可能来源于中元古代的物源区的部分熔融(张舒等,2009)。李湾矿区A 型花岗岩样品与皖南地区其它A 型花岗岩相比具有相似的地球化学特征,同时其锆石εHf(t)为-6.7~- 2.1,均值为- 4.7,二阶段模式年龄平均为1.26Ga,这些特征表明,其成岩可能与皖南A 型花岗岩类似,具有地幔物质与皖南基底古老变质岩混合的源区特征。

图10 岩体U-Pb 年龄与Hf 图解Fig.10 U-Pb age vs.Hf diagram of Liwan intrusions

在锆石微量元素变异图解中(图11),总体上,钾长花岗岩及闪长岩中锆石微量元素具有一定的相关关系,暗示成因上联系。闪长岩锆石Ce4+/Ce3+比值和Eu/Eu*分别为7.28~814(均值327)和0.08~0.75(均值0.35);钾长花岗岩则为10.3~3868(均值247)和0.12~0.28(均值0.21)。利用锆石原位微量元素特征区分出大陆地壳和大洋地壳锆石。在Yb-U 图解中(图12),所有锆石样品的Yb 和U 含量比较集中,具有正相关关系,大都落在镁质锆石区域内,并主要集中在镁质、陆壳和洋壳锆石相重叠区域,暗示本区侵入岩的源区存在岩浆混合的可能性。本区侵入岩中锆石微量元素较明显的相关关系反映其岩浆过程主要为共存结晶作用,并在锆石结晶演化过程中得以记录(Gagnevin et al.,2010)。值得注意的是,随着Yb 和U 含量的增加,两个样品成分点均向洋壳锆石区域偏移,暗示洋壳成分在岩浆源区占有重要的支配。

研究表明随着Ti 含量减少或锆石温度降低,大洋地壳锆石中Hf 和大多微量元素含量(如,U、Th、Y、P 和HREE)明显富集,李湾样品具有这个特征。从锆石Ce4+/Ce3+和Eu异常特征看,显示较高的氧逸度特征,有利于铜多金属成矿(Sun et al.,2004,2011;Ling et al.,2009)。研究表明,俯冲带具有比板内更高的氧逸度特征(Sun et al.,2004,2010,2011;Ling et al.,2009)。该区岩浆岩锆石氧逸度特征进一步证明该地区可能先是经历了太平洋俯冲,而后在板块后撤过程中保留了部分俯冲的信息。其闪长岩最有可能来源于残留的俯冲洋壳部分熔融,并在其上升过程中与富集地幔发生相互作用。同时氧逸度还可以作为一个经验性的指标来区分成矿岩体与不成矿岩体,该区侵入岩具有较高的氧逸度,可能暗示了其具有较大的成矿潜力。

图11 锆石微量元素地球化学变异图解Fig.11 Geochemical variation diagrams of zircon from the Liwan intrusive rocks

图12 岩浆岩锆石Yb-U (a)及U-Y (b)图解(a)中陆壳、镁质和洋壳锆石区域引自数据Grimes et al.(2009);(b)中(1)、(2)和(3)曲线代表了混合模式,来源于Langmuir et al.(1978),一个富U-Y 锆石代表长英质端元,三个贫U-Y 锆石代表镁质端元Fig.12 Yb-U (a)and Th-Y (b)diagrams of zircon(a)continental,mafic and ocean zircon field are after Grimes et al.(2009);(b)(1),(2)and (3)mixing models following Langmuir et al.(1978)using one U-Y-rich zircon to represent the felsic end-member and three U-Y-poor zircons to represent mafic end-members

图13 长江中下游A 型花岗岩Nd-Sr 图解数据来源:MORB 和海相沉积物(Hofmann,2003),扬子板块下地壳(Chen and Jahn,1998;Xing et al.,1994),长江中下游地区A 型花岗岩(Yang et al.,2012;Jiang et al.,2005,2011;张舒等,2009;薛怀民等,2009;Chen et al.,2001)Fig.13 Nd-Sr isotopic components diagram for the A-type granites in the LYRBData source:MORB and marine sediments (Hofmann,2003);the lower crust of the Yangtze blocks (Chen and Jahn,1998;Xing et al.,1994);LYRB A-type granites (Yang et al.,2012;Jiang et al.,2005,2011;Zhang et al.,2009;Xue et al.,2009;Chen et al.,2001)

长江中下游A 型花岗岩的初始Sr-Nd 同位素组成,明显不同于扬子下地壳组成,表明板块熔体遭受富集地幔成分的混染作用(图13,Ling et al.,2009,2011)。其趋向于EMⅡ端元,表明岩浆源区有俯冲沉积物加入(Liu et al.,2010b)(图13)。然而,沉积物通常Cu 含量低,因此不可能是李湾铜矿床的主要来源。鄂东、九瑞、安庆-贵池以及铜陵地区,与铜矿床相关的含矿岩体侵位时间早于135Ma,εHf(t)随着侵位时间变晚εHf(t)减小(Ding et al.,2006;Li et al.,2008;刘园园等,2009;Wu et al.,2012;Yang and Zhang,2012)。这种变化趋势不能通过Hf 的储存演化来解释,而更可能是壳幔混染的部分熔融形成的。早期,深部地壳物质拆沉并交代地幔物质,随着时间推移,更多的浅部地壳物质加入,从而造成了越老的斑岩具有更高的εHf(t)值。相对九瑞、鄂东、铜陵地区,李湾矿区岩体具有高的εHf(t)和老的年龄值,说明在岩石形成过程中壳幔混染,使得李湾铜矿床更多的富集Cu。

综上所述,这些明显的地球化学特征表明李湾和长江中下游地区A 型花岗岩起源于有限俯冲沉积物贡献的俯冲洋壳部分熔融,其上升过程中与富集地幔发生相互作用,同时混染了古元古代基底物质。因此,李湾地区钾长花岗岩是陆壳部分熔融与地幔混溶的结果,而闪长岩可能是板块后撤过程中洋壳与地幔物质的混合。

5.2 构造及成矿意义

在构造环境判别图解中(图14),闪长岩岩体都落在火山弧花岗岩范围内,钾长花岗岩落在板内花岗岩范围,玄武安山玢岩落在火山弧花岗岩范围内,结合区域地质背景,我们认为闪长岩成因是残留的洋壳部分熔融与地幔混合的产物,而钾长花岗岩为陆壳部分熔融与地幔的混合的产物,两者同位于板块后撤过程中的拉张背景中,是地幔物质与不同地质体的混溶结果。闪长岩(Al2O3+ Fe2O3+MgO +TiO2)含量为28.15%~31.36%,Al2O3/(Fe2O3+ MgO + TiO2)比值变化在1.05~1.54,暗示熔体处在一个相对低压的环境。因此,总体上李湾矿区的侵入岩形成于早白垩世高温-低压的板块俯冲后撤拉张的环境。富水的闪长岩与铜多金属成矿直接相关,而稍后的钾长花岗岩侵位则为本地区成矿提供了后期热动力,使成矿元素更加富集。

图14 李湾矿区岩体构造判别图解(据Pearce et al.,1984)WPG-板内花岗岩;ORG-洋中脊花岗岩;VAG-岛弧花岗岩;syn-COLG-同碰撞花岗岩Fig.14 Diagrams of tectonic environment for Liwan intrusive rocks by trace elements (after Pearce et al.,1984)

Mao et al.(2006)认为长江中下游矿集区铜、金矿床的形成可能与古太平洋板块或依泽纳吉板块向欧亚大陆俯冲有关;Ling et al.(2009,2011)研究认为长江中下游成矿带中金属矿床的分布与太平洋和依泽纳吉板块之间的洋脊俯冲有关。李湾矿区侵入岩锆石微量元素显示具有高的Ce 正异常特征,暗示岩浆形成于氧化环境。俯冲板片残留的洋壳部分熔融产生具有高氧逸度的熔体。地幔楔熔融时,Cu、Au等趋向集中在硫化物熔体中,只有当地幔熔融源区呈现高氧化态时,S 元素才能更好地进入硅酸盐熔体,此时,Cu、Au 等成矿元素才能富集于硅酸盐熔体中。因此,本地区经历了140 ±5Ma 的板块俯冲,在随后约20Myr 前后太平洋板块后撤过程中,先期俯冲的残留洋壳部分熔融为铜多金属成矿提供了物质来源。

6 结论

李湾矿区侵入岩锆石U-Pb 年代学研究表明其闪长岩形成于123.4 ±2.4Ma,钾长花岗岩形成于122.6 ±1.3Ma,作为矽卡岩型铜多金属矿床,其成矿应接近或略晚于闪长岩的成岩年龄,因此其成岩成矿与长江中下游地区125Ma 左右的A型花岗岩岩浆活动及成矿时间大体一致。

李湾矽卡岩型铜多金属矿床与闪长岩密切相关,闪长岩全程参与了成矿,而随后的钾长花岗岩侵位为本区成矿提供了后续热动力(热源)。闪长岩可能是板块后撤过程中残留洋壳与地幔的混溶,而钾长花岗岩则是陆壳部分熔融与地幔混溶的结果,两者统一形成于太平洋板块在125Ma 左右俯冲后撤过程中造成的拉张背景环境中。

李湾矿区岩体继承锆石2156Ma 代表了基底岩石年龄,不仅证明了扬子板块东北缘存在古元古代基底物质,同时暗示了有古元古代基底的物质参与了本区深部物质的循环,最终形成壳幔混合型岩体,在与奥陶系石灰岩接触部位产生了矽卡岩型铜多金属矿床。

致谢 本文在成文过程中得到了邓江洪、汪方跃等博士的帮助;两位匿名审稿人给予了很好的指导和建议;在此一并致以衷心的感谢。

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