Geochemical Characteristics and Tectonic Significance of Late Triassic Lamprophyre in the Beishan Region, Gansu Province
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摘要:
甘肃北山地区位于中亚造山带南缘,是研究中亚造山带构造演化的关键区域。煌斑岩多为岩石圈地幔在伸展背景下低程度部分熔融形成的碱性岩石,确定其形成时代和岩石成因可为区域构造演化提供新的依据。笔者对北山地区柳园南煌斑岩进行了系统的岩石学、地球化学和锆石U-Pb年代学及Hf同位素研究。柳园南煌斑岩的锆石U-Pb年龄为(228.2±1.1) Ma(晚三叠世)。煌斑岩中富含金云母和角闪石等富挥发性组分的矿物。岩石地球化学分析表明:柳园南煌斑岩属于超钾质煌斑岩,母岩浆为碱性岩浆系列;全岩微量元素具有明显的Nb-Ta和Zr-Hf负异常,锆石εHf(t)值为0.5~4.9,平均值为+2.8,具俯冲洋壳板片流体交代特征。柳园南煌斑岩中橄榄石Mn/Fe与Ca/Fe值对比表明,柳园南煌斑岩源区为富金云母的方辉橄榄岩地幔;微量元素模拟计算表明,地幔源区具有富集的特征。因此,柳园煌斑岩岩浆源区为被俯冲板片流体交代的岩石圈地幔。结合前人对中亚造山带南缘构造演化的研究,北山南部地区在晚三叠世时已进入陆内伸展阶段,减压作用促使被俯冲流体交代的岩石圈地幔发生低程度部分熔融,形成柳园煌斑岩脉。
Abstract:The Beishan region is located at the southern margin of the Central Asian Orogenic Belt (CAOB) and is a key area for studying the tectonic evolution of the CAOB. Lamprophyres are the product of low-degree partial melting of the subcontinental lithospheric mantle in the extensional background. Their formation age and petrogenesis can play a significant role in ascertaining the regional tectonic evolution. In this paper, systematic petrological, geochemical and zircon U-Pb chronology and Hf isotope studies were carried out on the lamprophyre in the Liuyuan area in southern Beishan region. The zircon U-Pb age of the Liuyuannan lamprophyre is (228.2±1.1) Ma (Late Triassic). The lamprophyre is rich in volatile component minerals such as phlogopite and hornblende. The Liuyuannan lamprophyre belongs to ultrapotassic lamprophyre, and its parent magma is an alkaline magma series. The Liuyuannan lamprophyre are characterized by pronounced negative Nb-Ta and Zr-Hf anomalies, positive zircon εHf (t) values ranging from 0.5 to 4.9, with an average value of +2.8. These data indicate that the mantle source was metasomatized by the subducted melts/fluids. The Mn/Fe and Ca/Fe ratios of olivine crystals in the Liuyuannan lamprophyre indicate that the mantle source is phlogopite-rich harzburgite mantle; trace element simulations indicate that the mantle source of Liuyuannan lamprophyre is an enriched peridotite-type mantle. Therefor, the source of the Liuyuan lamprophyre should be the lithospheric mantle metasomatized by subducting slab fluids. Combined with previous studies on the tectonic evolution of the southern of the CAOB, we believe that the Beishan region has entered intracontinental extensional environment in the Late Triassic. Decompression promoted low-degree partial melting of the lithospheric mantle metasomatized by subduction fluids, resulting in the formation of the Liuyuan lamprophyre.
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中亚造山带是全球最大的显生宙增生型造山带,是由大量增生杂岩、岩浆弧、蛇绿岩及大陆碎片等构成的复杂拼贴体(Windley et al., 2007;肖文交等, 2019;王文宝等,2024;吴妍蓉等,2024)。尽管前人对中亚造山带的形成与演化历史进行了大量研究,但关于中亚造山带最终拼贴以及进入陆内阶段的时间,目前仍存在不同的认识。部分研究者根据中亚造山带南缘泥盆纪区域性角度不整合(Xu et al., 2013)、A型花岗岩的形成时代(Shi et al., 2010)、高压变质岩的形成时代(高俊等, 2006;Gao et al., 2011)等,认为中亚造山带最终拼贴时间为泥盆纪或石炭纪;也有研究者根据中亚造山带南缘发育的二叠纪蛇绿混杂岩(Song et al., 2015),三叠纪蓝片岩的发现(初航等, 2013;Zhang et al., 2015),以及大量二叠纪—三叠纪花岗岩研究(Li et al., 2017;Feng et al., 2020;Wu et al., 2021),认为中亚造山带最终拼贴时间为晚二叠世—中三叠世(Xiao et al., 2009, 2018);还有学者认为中亚造山带最终拼贴时间持续到晚三叠世(Mao et al., 2021)。
北山造山带位于中亚造山带南缘中段(图1a),是研究中亚造山带南缘构造演化的关键区域。关于北山地区进入陆内演化阶段的时间,前人多根据不同成因类型的花岗岩、闪长岩、辉长辉绿岩等岩浆作用的地质年代学和地球化学特征进行约束(Li et al., 2012;高峰, 2018;俞胜, 2022),但由于增生造山作用的复杂性以及岩浆源区及演化过程的多样性,使得岩石地球化学解释通常具有多解性。相比之下,煌斑岩是一类富含暗色矿物斑晶和挥发性组分(如H2O和CO2)的浅成岩,具有明显的斑状结构,斑晶可以由橄榄石、辉石、角闪石和金云母等镁铁质矿物组成,通常以岩脉的形式出露(Rock, 1987, 1991)。煌斑岩大多是由岩石圈地幔在伸展背景下发生低程度部分熔融形成的(Vaughan et al., 2003),可以较好地约束中亚造山带在北山地区的构造演化。因此,笔者对北山地区柳园南煌斑岩进行了详细的岩石学,岩石地球化学,锆石U-Pb年代学和Hf同位素地球化学分析,研究其形成时代、成因及构造背景,为北山地区早中生代构造演化提供新的依据。
图 1 中亚造山带构造单元简图(a) (据Jahn et al., 2004修改), 北山造山带构造简图(b) (据Xiao et al., 2010修改)和柳园地区地质简图(c) (据甘肃省地质矿产局,1966修改)Figure 1. (a) Sketch of tectonic units of the Central Asian Orogenic Belt, (b) Beishan Orogenic Belt, and (c) geological map of the Liuyuan area1. 地质概况
1.1 区域地质概况
北山造山带位于中亚造山带南缘,北侧为中天山造山带,南侧为塔里木–华北克拉通之间区域(图1a)。北山造山带由古生代期间位于古亚洲洋不同区域的前寒武纪微陆块、岛弧和蛇绿混杂岩等不同构造单元拼贴而成,各构造单元从北到南依次为:雀儿山岛弧、红石山蛇绿混杂岩带、黑鹰山岛弧、旱山岛弧、星星峡-石板井蛇绿混杂岩带、马鬃山岛弧、红柳河-洗肠井蛇绿混杂岩带、双鹰山岛弧、花牛山岛弧、柳园蛇绿混杂岩带和石板山岛弧(图1b)(Xiao et al., 2010, 2018;肖文交等, 2019)。
北山地区煌斑岩出露于柳园蛇绿混杂岩带南、北两侧的花牛山岛弧和石板山岛弧,区内主要断裂为NWW向和近EW向展布,出露地层主要包括下寒武统双鹰山组灰黑色碳质板岩、浅灰白色大理岩,为一套海相沉积细碎屑岩黑色岩系沉积建造;奥陶系花牛山群片麻岩、混合岩、变质砂岩和千枚岩等变质岩;石炭系下统红柳园组碎屑岩夹灰岩,为一套海相的沉积碎屑岩建造;二叠系金塔组和双堡塘组,主要由玄武岩、英安岩与砂岩、粉砂岩组成,为一套中基性火山岩与沉积碎屑岩互层组合(左国朝等,1990;刘雪亚等,1995)。区内岩浆活动强烈,其中以泥盆纪、二叠纪和三叠纪花岗质岩体最为发育(Zuo et al.,1990,1991;江思宏等,2006;许伟,2019),辉长岩次之,镁铁-超镁铁质岩体大多在辉长岩体中零星分布(高文彬等,2020)。研究区内也发育三叠纪脉岩包括辉绿岩脉、酸性岩脉以及煌斑岩脉(图1c)(过磊等,2018;孙海瑞等,2020)。
1.2 北山地区煌斑岩脉地质特征
与其他造山带不同,中亚造山带中已报道的煌斑岩脉较少,目前已知的仅有俄罗斯Altai-Mongolian地区的Chuya煌斑岩(Ar-Ar年龄 236~234 Ma)(Vasyukova et al.,2011)和北山造山带柳园西煌斑岩(Ar-Ar年龄 为240~220 Ma)(刘畅等,2006)。柳园地区煌斑岩脉近EW向延伸,分别出露于柳园西、东南和西南3处。笔者在柳园西煌斑岩研究基础上,选择柳园东南和西南两处煌斑岩进行研究,这两处煌斑岩侵入于下二叠统金塔组中,接触界线截然。煌斑岩脉外表呈深褐色,宽为1.0~2.0 m,延伸几百米,走向为~240°,倾角近于直立(图2a,图2b)。
柳园南煌斑岩镜下具典型煌斑结构,矿物主要为橄榄石(30%~40%)、单斜辉石(25%~35%)、金云母(25%~35%)和少量角闪石(1%~3%),副矿物(2%~4%)主要为磷灰石、磁铁矿、铬铁矿、钛闪石等(图2e)。其中橄榄石少数具自形结构,大多数呈浑圆状,表面分布有很多不规则裂纹,正高突起,粒径为0.5~3.0 mm,较小橄榄石大多被金云母包裹,部分橄榄石边缘和裂隙中发生蛇纹石化(图2c)。金云母粒径变化较大(1.0~4.0 mm),呈片状或细长棱柱状,具黄褐色–棕褐色多色性,边缘偶见绿色(图2d)。单斜辉石多位于橄榄石和金云母颗粒之间,半自形–他形结构,粒径为~1 mm,部分发生阳起石化和透闪石化(图2c)。与柳园西煌斑岩相比(刘畅等, 2006),柳园南煌斑岩中含有较多的橄榄石、单斜辉石等镁铁质矿物。
2. 样品与分析测试方法
为了保证数据的可靠性,本研究对柳园南煌斑岩样品进行了系统的显微镜下鉴定,挑选新鲜且具代表性的煌斑岩样品进行单矿物和全岩地球化学组分分析。其中锆石样品(HBY-1)质量大于25 kg,采样位置:N41°00′24.00″,E95°44′14.30″。锆石分选工作在河北省区域地质矿产调查研究所实验室采用浮选和电磁选法完成。利用阴极发光及背散射图像对锆石颗粒内部结构进行分析,选择无裂隙、无继承核、具有环带的锆石进行U-Pb同位素和Hf同位素分析。
锆石U-Pb测定在长安大学西部矿产资源与地质工程教育部重点实验室完成。实验室使用美国Photo Machines公司的 Analyte Excite 193 nm 气态准分子激光剥蚀系统,与美国 Agilent公司7700X型四极杆等离子体质谱仪联用。激光束斑直径为 35 μm,频率5 Hz,能量密度5.9 J/cm2 ,具体测试过程参考文献(栾燕等, 2019)。锆石U-Pb定年使用PleŠovice(338 Ma)和Qinghu(159 Ma)为外部监控样,选用91500(
1065 Ma)为校准,本次标样测试值均符合推荐值(Wiedenbeck et al., 1995;Slama et al., 2008)。对分析数据的离线处理采用 ICPMSDataCal 程序, 包括对空白及锆石样品的信号选择、仪器灵敏度漂移校正、锆石元素含量及 U-Th-Pb 同位素比值和年龄计算等(Liu et al., 2008)。锆石谐和年龄图绘制采用Isoplot 3.7(Ludwig, 2003)完成。锆石Lu-Hf同位素测试在西北大学大陆动力学国家重点实验室采用Neptune型多接收等离子质谱仪联合Newwave UP213激光剥蚀系统,分析时激光束斑直径为40 µm,激光剥蚀时间为26 s,重复频率为10 Hz,标准锆石
91500 和TEMORA的176Hf/177Hf测定结果分别为0.282296 ±0.000013 (2σ)和0.282680 ±0.000013 (2σ),位于参考值误差范围内(Woodhead et al., 2004)。176Lu的衰变常数选用1.876×10−11/a(Albarede et al., 2006),球粒陨石176Lu/177Hf和176Hf/177Hf分别选用0.033200 和0.282772 用来计算岩石样品的εHf(t)值(Bouvier et al., 2008)。仪器的运行条件、详细的分析流程、数据矫正方法及锆石标准参考值详见(侯可军等, 2007)。岩石造岩矿物电子探针分析与全岩主量、微量元素测试均在长安大学西部矿产资源与地质工程教育部重点实验室完成。电子探针分析测试仪器为JXA-iHP200F型场发射电子探针,工作电压20 kV,电流2.0×10−8 A,束斑直径1 μm,分析误差为2%,实验数据使用ZAF方法校正。主量元素测定采用XRF荧光光谱法进行分析,测试仪器为日本岛津XRF-1800型波长色散X射线荧光光谱仪。选用国家标准样品GBW07112(辉长岩)为质量监控标样,采用外标法校正,分析误差小于3%。微量和稀土元素分析测试采用安捷伦7700E型电感耦合等离子体质谱(ICP-MS)分析仪,以BCR-2(玄武岩)为标样进行质量监控,采用外标法校正,分析误差小于5%。
3. 分析结果
3.1 锆石年代学
柳园南煌斑岩中锆石呈无色透明、半自形、长-短柱状,长宽比为1∶1~2∶1。阴极发光图像显示,大多数锆石具良好的晶形以及清晰的震荡环带,为典型的岩浆锆石(图3a)(王梓桐等,2022;牛腾等,2023)。在11个测试点中(表1),锆石Th和U含量分别为164×10−6~505×10−6和111×10−6~541×10−6,Th/U值为0.8~1.6,表明其为岩浆成因(Hoskin et al., 2003),锆石年龄代表柳园南煌斑岩的形成年龄。11个分析数据表明锆石谐和年龄为(228.2±1.1) Ma(n=11,MSWD=0.004)(图3b),该年龄与柳园西煌斑岩脉中金云母Ar-Ar年龄(220~240 Ma)(刘畅等,2006)和中亚造山带北部俄罗斯Chuya地区煌斑岩中金云母Ar-Ar年龄(236~234 Ma)(Vasyukova et al., 2011)接近,表明柳园地区煌斑岩脉都形成于晚三叠世。
表 1 柳园南煌斑岩LA-ICP-MS锆石U-Pb同位素分析结果Table 1. Analysis results of LA-ICP-MS zircon U-Pb isotope from Liuyuannan lamprophyre测试点 Pb(10−6) Th(10−6) U(10−6) Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 206Pb/238U 1σ 207Pb/235U 1σ HBY-1 13.6 183 111 1.64 0.0530 0.0031 0.2375 0.0136 0.0367 0.0006 232.4 3.5 239.3 12.1 HBY-2 22.2 256 329 0.78 0.0508 0.0024 0.2517 0.0120 0.0359 0.0005 227.1 3.4 228.0 9.7 HBY-3 14.3 189 116 1.63 0.0534 0.0035 0.2720 0.0159 0.0374 0.0008 236.6 4.7 244.3 12.7 HBY-4 24.0 304 277 1.09 0.0505 0.0038 0.2432 0.0180 0.0350 0.0005 222.0 3.3 221.0 14.7 HBY-5 38.3 471 428 1.10 0.0510 0.0020 0.2535 0.0096 0.0364 0.0007 231.1 4.5 229.4 7.7 HBY-6 20.2 244 224 1.09 0.0504 0.0028 0.2543 0.0142 0.0364 0.0005 230.4 3.2 230.1 11.5 HBY-7 29.7 377 380 0.99 0.0496 0.0030 0.2375 0.0136 0.0349 0.0007 221.3 4.6 216.4 11.2 HBY-8 41.1 505 541 0.93 0.0504 0.0020 0.2514 0.0107 0.0361 0.0006 228.6 3.8 227.7 8.7 HBY-9 21.3 274 217 1.26 0.0508 0.0030 0.2463 0.0115 0.0352 0.0006 223.2 3.9 233.5 9.4 HBY-10 14.7 171 173 0.99 0.0508 0.0031 0.2582 0.0141 0.0376 0.0008 238.4 5.2 233.3 11.4 HBY-11 13.1 164 175 0.94 0.0508 0.0022 0.2517 0.0158 0.0358 0.0005 226.7 3.2 227.9 12.8 3.2 矿物晶体化学
笔者对柳园南煌斑岩中橄榄石、单斜辉石晶体核部进行了主量元素分析,分析结果见表2。橄榄石晶体具有富Mg特征,Fo牌号(100×Mg/(Mg+Fe))从86.5到92.3不等,平均为90.6。橄榄石晶体中Ni含量为
1572 ×10−6~4495 ×10−6,平均值为2917 ×10−6,范围远大于地幔橄榄石Ni含量(2200×10−6 ~3400 ×10−6,Foley et al., 2013);Ca含量为1572×10−6 ~4495 ×10−6, Fo牌号最高的橄榄石具有最低的Ca浓度(图4b);橄榄石Al含量为16×10−6~476×10−6,平均值为227×10−6,同样远高于地幔橄榄石Al含量值(Al<130×10−6, Foley et al., 2013)。橄榄石中次要元素(Ca、Ni、Al等)含量表明柳园南煌斑岩中橄榄石形成于岩浆分离结晶作用,而不是地幔橄榄岩捕虏体。此外,橄榄石中Ni和Ca含量明显不同于洋岛玄武岩和大洋中脊玄武岩中橄榄石含量,而位于煌斑岩区域附近(图4a、图4b)。煌斑岩中单斜辉石En牌号为43.8%~45.5%、Wo牌号为47.8%~49.2%、Fs牌号为6.7%~7.0%,属于透辉石。表 2 柳园南煌斑岩橄榄石及单斜辉石成分表Table 2. Composition of olivine and clinopyroxene from Liuyuannan lamprophyre分析编号 矿物
名称SiO2(%) Al2O3(%) MgO(%) CaO(%) Cr2O3(%) FeO(%) MnO(%) NiO(%) Total(%) Fo牌号 HBY-2d-1.2 橄榄石 39.82 0.02 46.03 0.17 0.02 12.88 0.27 0.21 99.47 86.5 HBY-2d-2.1 橄榄石 40.77 0.04 48.86 0.18 0.02 10.05 0.19 0.29 100.44 89.7 HBY-2d-2.4 橄榄石 41.14 0.04 48.83 0.18 0.11 10.19 0.14 0.33 100.99 89.6 HBY-2d-3.1 橄榄石 41.12 0.05 50.27 0.17 0.05 9.07 0.19 0.35 101.37 90.9 HBY-2d-5.2 橄榄石 40.37 0.03 48.43 0.17 0.16 11.16 0.22 0.26 100.89 88.7 HBY-2d-6.1 橄榄石 41.76 0.00 50.86 0.12 0.10 8.41 0.17 0.53 101.96 91.6 HBY-2d-8.1 橄榄石 41.06 0.07 50.41 0.15 0.06 8.45 0.02 0.47 100.71 91.5 HBY-2d-9.1 橄榄石 40.45 0.02 48.24 0.16 0.08 9.84 0.10 0.20 99.21 89.8 HBY-2d-9.2 橄榄石 41.79 0.04 48.70 0.17 0.05 10.24 0.18 0.30 101.53 89.5 HBY-2d-10.1 橄榄石 40.76 0.06 49.10 0.16 0.12 9.56 0.12 0.33 100.24 90.2 HBY-2d-11.1 橄榄石 42.31 0.09 49.70 0.22 0.03 7.81 0.11 0.42 100.83 92.0 HBY-2d-12.1 橄榄石 41.24 0.03 49.80 0.17 0.14 8.71 0.09 0.43 100.73 91.1 HBY-2d-13.1 橄榄石 41.65 0.03 51.04 0.14 0.20 8.03 0.11 0.36 101.65 92.0 HBY-2d-14.1 橄榄石 41.61 0.05 51.17 0.13 0.12 8.23 0.08 0.46 101.91 91.8 HBY-2d-15.1 橄榄石 41.59 0.07 50.65 0.18 0.04 7.90 0.08 0.43 100.94 92.0 HBY-2d-16.1 橄榄石 41.36 0.01 50.56 0.13 0.00 7.64 0.09 0.57 100.41 92.3 分析编号 矿物
名称SiO2(%) Al2O3(%) MgO(%) CaO(%) Cr2O3(%) FeO(%) MnO(%) TiO2(%) Total(%) Wo牌号 En牌号 Fs牌号 HBY-2d-4.2 单斜
辉石51.16 3.23 15.40 24.09 0.93 4.40 0.10 0.55 100.28 49.20 43.78 7.01 HBY-2d-5.3 单斜
辉石52.37 2.90 15.91 24.27 1.04 4.43 0.13 0.52 101.97 48.67 44.40 6.93 HBY-2d-7.1 单斜
辉石50.82 3.03 15.67 24.03 0.74 4.27 0.14 0.56 99.84 48.87 44.35 6.77 HBY-2d-7.3 单斜
辉石52.60 2.07 15.81 23.15 0.54 4.16 0.17 0.43 99.40 47.83 45.45 6.71 注:Total(%)为主量元素总含量。 3.3 岩石主量、微量元素地球化学
柳园南煌斑岩主量、微量元素分析结果见表3。为了消除蚀变作用对主量元素数据的影响,将全岩主量元素扣除烧失量后重新进行100%计算。柳园南煌斑岩SiO2含量为44.3%~45.2%,全碱(Na2O+K2O)含量为2.3%~3.1%,K2O>Na2O,里特曼指数(Na2O+K2O)2/(SiO2-43)为2.4~5.4,在SiO2–(Na2O+K2O)相关图中柳园南煌斑岩位于碱性岩区域(图5a)。此外,柳园南煌斑岩MgO含量为24.9%~26.8%,FeOT含量为10.5%~11.1%,Mg#值Mg2+/(Mg2++Fe2+)为0.67~0.68,具有富镁的特征。与其相反,柳园西煌斑岩MgO含量为12.2%~14.1%,K2O含量为5.3%~6.1%(刘畅, 2006);Chuya煌斑岩MgO含量为11.3%~16.1%,K2O含量为5.3%~9.4%(Vasyukova et al., 2011)。在煌斑岩分类图中柳园南煌斑岩位于超钾质煌斑岩区域,而柳园西和Chuya煌斑岩位于过钾质-钾镁煌斑岩区域(图5b)。
表 3 柳园南煌斑岩主量元素(%)及微量元素(10−6)分析结果Table 3. Analysis results of major and trace elements of Liuyuannan lamprophyre样品编号 HBY-1a HBY-2b HBY-2c HBY-2d HBY-1c HBY-2a 岩性 煌斑岩 煌斑岩 煌斑岩 煌斑岩 煌斑岩 煌斑岩 SiO2 43.1 43.1 42.4 42.9 TiO2 0.53 0.64 0.52 0.53 Al2O3 5.51 5.89 5.14 5.16 Fe2O3 10.4 10.1 10.6 10.5 MnO 0.15 0.15 0.16 0.16 MgO 24.35 23.94 25.63 25.55 CaO 8.86 9.29 8.53 9.05 Na2O 0.42 0.55 0.57 0.61 K2O 1.78 2.42 1.90 1.91 P2O5 0.23 0.22 0.23 0.21 烧失量 4.00 3.29 3.66 3.17 Total 99.4 99.6 99.4 99.8 Mg#值 0.67 0.67 0.67 0.68 m/f值 2.00 2.03 2.07 2.08 里特曼指数 2.39 5.38 4.99 4.92 Li 22.2 25.2 19.9 13.1 24.9 14.2 Sc 39.9 42.1 10.6 15.1 30.3 29.7 Rb 86.1 108.5 80.8 85.4 91.2 87.1 Sr 810 1170 1210 1099 409 569 Y 12.3 11.8 11.4 11.0 11.3 10.2 Zr 72.4 67.3 65.9 64.0 67.6 60.9 Hf 1.98 1.68 1.53 1.61 1.96 1.80 Nb 4.06 3.94 3.80 3.72 4.08 3.68 Ta 0.27 0.23 0.23 0.21 0.29 0.25 La 34.1 32.6 33.0 32.0 35.6 32.0 Ce 76.6 72.2 73.0 72.4 74.5 68.3 Pr 9.77 9.14 9.30 9.12 9.34 8.82 Nd 39.2 36.5 37.0 37.0 37.2 34.8 Sm 6.25 6.10 5.96 5.88 6.29 6.06 Eu 2.08 2.17 1.93 1.90 1.78 1.69 Gd 5.62 5.30 5.45 5.18 5.42 5.13 Tb 0.55 0.50 0.51 0.50 0.58 0.55 Dy 2.56 2.32 2.28 2.32 2.29 2.15 Ho 0.44 0.41 0.40 0.40 0.45 0.42 Er 1.26 1.08 1.06 1.08 1.15 1.08 Tm 0.14 0.14 0.13 0.13 0.15 0.14 Yb 1.05 0.93 0.86 0.86 0.99 0.90 Lu 0.15 0.13 0.13 0.11 0.14 0.13 Pb 73.7 56.7 96.0 53.5 70.4 54.0 Th 4.74 4.10 3.41 3.35 4.41 3.68 U 1.38 1.26 1.34 1.20 1.39 1.19 Eu异常 1.07 1.17 1.03 1.05 0.93 0.93 LREE 180 170 171 169 176 162 HREE 168 159 160 158 165 152 REE 11.8 10.8 10.8 10.6 11.2 10.5 LREE/HREE值 14.3 14.7 14.8 14.9 14.7 14.5 (La/Yb)N 23.4 25.3 27.4 26.6 25.9 25.5 注:Total为主量元素总含量;LREE为轻稀土元素总含量;HREE为重稀土元素总含量;REE为稀土元素总含量。 图 5 柳园南煌斑岩SiO2-(K2O+Na2O)(a)与K/(K+Na)-K/Al(b)相关图I.钠质煌斑岩;I’.弱钾质煌斑岩;II.钾质煌斑岩;III.超钾质煌斑岩;IV.过钾质煌斑岩;V.钾镁煌斑;岩底图据Irvine等(1971)、路凤香等(1991);柳园西、Chuya煌斑岩数据分别引自刘畅等(2006)和Vasyukova(2011)Figure 5. (a)(Na2O+K2O)- SiO2 and (b) K/(K+Na) - K/Al diagrams of the Liuyuannan lamprophyre柳园南煌斑岩样品REE丰度呈均匀的趋势(ΣREE含量为162×10−6~180×10−6),低的(La/Sm)N值(5.29~6.72,均值为5.74),球粒陨石标准化稀土元素蛛网图呈右倾型,富集轻稀土元素(图6a),没有明显的Eu异常(δEu = 0.93~1.17)。原始地幔标准化微量元素蛛网图中列出不受后期蚀变作用影响的不相容元素配分模式,可见明显的Nb-Ta、Zr-Hf负异常,相似于岛弧玄武岩微量元素配分模式(图6b)。与中亚造山带同时代煌斑岩相比,柳园南煌斑岩微量元素含量低于柳园西和Chuya煌斑岩,但它们具有相似的稀土元素及微量元素配分模式。
图 6 柳园南煌斑岩球粒陨石标准化稀土元素配分模式图(a)和原始地幔标准化微量元素蛛网图(b)球粒陨石标准化数据引自 Boynton(1984);原始地幔标准化数据引自Sun等(1989);洋岛玄武岩、岛弧玄武岩数据引自Li 等(2015);柳园西、Chuya煌斑岩数据分别引自刘畅等(2006)和Vasyukova等(2011)Figure 6. (a) Chondrite-normalized REEs pattern and (b) Primitive Mantle-normalized trace elements Spider diagram of Liuyuannan lamprophyre3.4 锆石Hf同位素特征
柳园南煌斑岩Hf元素分析结果见表4,柳园南煌斑岩中锆石(176Lu/177Hf)的变化范围为
0.0008 ~0.0029 ,绝大部分都小于0.0020 ,表明放射成因176Hf值可忽略不计,测试所得(176Hf/177Hf)的变化范围为0.282660 ~0.282784 ,可代表岩石形成时锆石中的Hf同位素值(吴福元等, 2007)。fLu/Hf值为−0.9~−1,变化范围较小,显示出较为均一的特征。以成岩年龄228 Ma 计算,得出的锆石εHf (t) 值为0.5~4.9,平均值为+2.8。表 4 柳园南煌斑岩中锆石Hf 同位素分析结果Table 4. Zircon Hf isotopic data of Zircon crystals from Liuyuannan lamprophyre点号 样品编号 176Yb/177Hf 2σ 176Lu/177Hf 2σ 176Hf/177Hf 2σ 年龄
(Ma)(176Hf/177Hf)i εHf(0) εHf(t) TDM
(Ma)fLu/Hf 1 HBY-1 0.030890 0.000175 0.000772 0.000003 0.282660 0.000022 228 0.282657 −3.96 0.93 834 −0.98 2 HBY-2 0.037702 0.000322 0.000953 0.000007 0.282707 0.000020 228 0.282702 −2.32 2.54 773 −0.97 3 HBY-3 0.056320 0.000131 0.001362 0.000004 0.282718 0.000019 228 0.282713 −1.90 2.90 764 −0.96 4 HBY-4 0.060112 0.000074 0.001400 0.000001 0.282710 0.000023 228 0.282704 −2.18 2.61 776 −0.96 5 HBY-5 0.056959 0.000328 0.001366 0.000007 0.282784 0.000018 228 0.282778 0.41 5.21 671 −0.96 6 HBY-6 0.050719 0.000570 0.001433 0.000012 0.282711 0.000021 228 0.282704 −2.17 2.62 777 −0.96 7 HBY-7 0.039301 0.000143 0.000993 0.000003 0.282762 0.000020 228 0.282757 −0.37 4.48 696 −0.97 8 HBY-8 0.077620 0.000778 0.001884 0.000012 0.282652 0.000018 228 0.282644 −4.23 0.49 870 −0.94 9 HBY-9 0.049260 0.000207 0.001325 0.000005 0.282687 0.000018 228 0.282682 −3.00 1.81 808 −0.96 10 HBY-10 0.125049 0.000434 0.002978 0.000010 0.282677 0.000022 228 0.282665 −3.35 1.21 860 −0.91 11 HBY-11 0.051215 0.000306 0.001212 0.000006 0.282774 0.000023 228 0.282768 0.06 4.88 683 −0.96 12 HBY-12 0.044043 0.000164 0.001235 0.000006 0.282696 0.000020 228 0.282691 −2.69 2.13 794 −0.96 13 HBY-13 0.028124 0.000061 0.000732 0.000002 0.282748 0.000018 228 0.282745 −0.85 4.05 710 −0.98 4. 讨论
4.1 地壳混染作用
在讨论柳园地区煌斑岩成因之前,需首先考虑地壳混染作用的影响(Zhang et al., 2009;Kou et al., 2018)。原始地幔标准化的微量元素蛛网图中,柳园南煌斑岩具有明显的Nb-Ta、Zr-Hf负异常,该特征类似于新生代全球岛弧玄武岩特征(图6b)。此外,大陆地壳物质同样具有明显的Nb-Ta负异常(Rudnick et al., 2003),因此同化混染地壳物质也可形成这种特征。假设地壳混染作用是导致柳园南煌斑岩Nb-Ta负异常的主要原因,通过亏损地幔与大陆地壳Nb/Yb与Th/Yb值进行模拟计算。设定上地壳Nb/Yb和Th/Yb值分别为6.0和5.25(Rudnick et al., 2003),亏损地幔Nb/Yb和Th/Yb值为1.0和0.6(Sun et al., 1989)。根据质量守恒原理,两端元混合作用模拟计算表明形成柳园南煌斑岩Nb-Ta负异常需要>70%的上地壳物质混染(图7a),幔源岩浆侵位过程中很难发生如此高程度的地壳混染作用(Bowen, 1928)。此外,地壳物质仅具有Nb-Ta负异常特征,因此地壳混染作用很难解释柳园南煌斑岩中明显的Nb-Ta、Zr-Hf负异常(图6b),该特征更可能是俯冲流体交代作用导致的结果。
图 7 柳园南煌斑岩Nb/Yb-Th/Yb(a)与Th/Nb-εHf(t)(b)相关图图a底图据 Pearce(2008);上地壳和亏损地幔各元素组分引自Rudnick等(2003)和Sun等(1989);图b中俯冲板块衍生流体组分和大洋循环沉积物组分引自Veroot等(1999)、Kessel等(2005)和Chauvel等(2008)Figure 7. (a)Nb/Yb-Th/Yb and (b)Th/Nb-εHf(t) covariance diagrams of Liuyuannan lamprophyre大洋板片在深俯冲过程中随着温度和压力的升高而发生变质脱水反应,板片流体组分由俯冲洋壳以及携带沉积物中释放的含水流体和/或熔体组分决定,通常具有富集大离子亲石元素和亏损高场强元素特征(McCulloch et al., 1991)。笔者选择对岩浆源区与演化过程更敏感的Hf同位素组分,以及代表俯冲板片流体组分的大离子亲石元素(Th)与高场强元素(Nb)比值进行模拟计算。设定亏损地幔组分为:εHf(t) = 12、Hf = 0.6×10−6、Th= 0.1×10−6、Nb= 3.1×10−6(Zindler et al., 1986);俯冲板块衍生流体组分平均值为:εHf(t) = 14.5、Hf = 12×10−6、Th = 2.6×10−6、Nb = 1.8×10−6(Kessel et al., 2005;Chauvel et al., 2008);大洋循环沉积物组分平均值为:εHf(t) = −6.9、Hf = 4.1×10−6、 Th=12×10−6、Nb=7.6×10−6(Veroot et al., 1999;Chauvel et al., 2008)。由于柳园南煌斑岩(HBY-1)中锆石具有非常低的Lu/Hf 值,因此文中用锆石εHf(t)值(平均值+2.8)近似代表母岩浆Hf同位素组分(Kinny et al., 2003;Mole et al., 2018),用HBY-1全岩微量元素Th=4.4×10−6、Nb=4.1×10−6代表煌斑岩母岩浆组分。模拟计算表明:由15%循环大洋沉积物和85%大洋上部地壳流体组成板片流体组分交代亏损地幔可以很好的解释柳园南煌斑岩的微量元素以及锆石Hf同位素特征(图7b)。
此外,柳园南煌斑岩样品全岩地球化学组分具较高的MgO含量,Mg#(0.67~0.68)、Ni(740×10−6~863×10−6)、Cr(
1069 ×10−6~2259 ×10−6),接近地幔熔体组分(Mg# 0.7~0.8、Ni>400×10−6~500×10−6、Cr>500×10−6,Frey et al., 1978)。煌斑岩中橄榄石Fo值最高达~92,接近地幔橄榄石Fo值(>90,Mao et al., 2022)。进一步表明,柳园煌斑岩原始岩浆演化过程中地壳物质的混染作用影响较弱,煌斑岩中明显的Nb-Na、Zr-Hf负异常,较低的εHf(t) 值特征为柳园南煌斑岩岩浆源区曾受到俯冲板片流体交代作用的结果( Elliott et al., 1997;Duggen et al., 2005;Wang et al., 2020)。4.2 岩浆源区
确定俯冲流体交代作用后,应进一步确定其地幔源区类型(即二辉橄榄岩地幔或辉石岩地幔)(Straub et al., 2011)。柳园南煌斑岩具有富集大离子亲石元素,亏损高场强元素特征,此外,轻、重稀土元素之间具有明显的分馏(图6)。所有这些特征都与苏鲁造山带和胶东半岛煌斑岩特征一致(Guo et al., 2004;Ma et al., 2014;Hou et al., 2022);这些煌斑岩被认为是岩石圈地幔低程度部分熔融的产物。实验研究表明,与含金云母橄榄岩熔体处于平衡的橄榄石将具有较高的Ca/Fe、低的Mn/Fe值(Ammannati et al., 2016)。图8a显示,柳园南煌斑岩中较早结晶的橄榄石(Fo牌号>91.5)都位于富金云母+方辉橄榄岩熔融区域(Förster et al., 2018)。方辉橄榄岩通常形成于次大陆岩石圈地幔逐渐演化过程中,是地幔二辉橄榄岩部分熔融形成拉斑玄武质岩浆之后的残余地幔物质(Brown et al., 1993)。
文中用稀土元素(REE)丰度和比值模型,进一步讨论柳园南煌斑岩地幔源区部分熔融程度。选择不受地幔矿物(尤其是石榴子石和尖晶石)组分影响的高不相容元素La和低不相容元素Sm,通过非批次熔融方程进行模拟计算。两种不同组分被用于代表可能的地幔趋势:亏损的MORB地幔(DMM)来代表对流的软流圈地幔(McKenzie et al., 1991),原始地幔(PM)用来代表原始地幔组分(Sun et al., 1989)。柳园南煌斑岩最佳拟合区域为具有富集特征的尖晶石二辉橄榄岩发生~5%部分熔融形成的岩浆(图8b)。
随着地壳岩石的不断形成,岩石圈地幔将逐渐亏损不相容元素。岩浆源区的富集特征可以通过俯冲流体的交代作用、深部岩浆的底侵作用、裂谷中含水熔体的停滞和结晶、新生流体的逸出作用等过程实现。结合煌斑岩地球化学特征,笔者认为俯冲流体交代作用使得岩石圈地幔中不相容元素再次富集,可以很好地解释柳园南煌斑岩岩浆源区中富集不相容元素的特征,也很好地解释了煌斑岩源区中富含金云母等富水矿物特征。综合上述证据可知柳园南煌斑岩原始岩浆为被俯冲流体交代的岩石圈地幔发生~5%部分熔融形成的富含挥发性组分的岩浆。
4.3 柳园煌斑岩构造背景
北山地区煌斑岩脉分布于柳园西、东南和西南3处,煌斑岩走向均近于EW向。煌斑岩中角闪石Ar-Ar年龄(刘畅等, 2006)和锆石U-Pb年龄表明柳园地区煌斑岩均形成于晚三叠世。柳园地区煌斑岩中都含有大量富挥发性组分的金云母,具有较高的(Na2O+K2O)含量,属于碱性岩石系列。柳园南煌斑岩中含有较多的橄榄石和单斜辉石晶体,因此具有较高的MgO含量和较低的K2O含量,位于超钾质煌斑岩区域;而柳园西煌斑岩位于过钾质–钾镁煌斑岩区域(图5)。由于微量元素在橄榄石与硅酸盐岩浆之间具有较低的分配系数(Schnetzler et al., 1970),柳园南煌斑岩具有较低的稀土、微量元素浓度,但在球粒陨石标准化稀土元素蛛网图中,柳园地区煌斑岩都具有相似的右倾斜率;在原始地幔标准化微量元素蛛网图中,柳园地区煌斑岩都具有明显的Nb-Ta、Zr-Hf负异常(图6)。因此,虽然柳园地区煌斑岩脉具有不同的矿物学含量和地球化学数据,但相似的野外地质特征、地质年代学以及地球化学特征表明柳园地区煌斑岩应形成于同一构造–岩浆事件。
柳园地区煌斑岩出露于柳园断裂南北两侧的花牛山岛弧和石板山岛弧地区。已有地质资料表明:沿柳园–帐房山断裂两侧断续分布辉铜山–帐房山蛇绿混杂岩带形成时间为446~362 Ma,这些蛇绿岩残片可代表花牛山岛弧和石板山岛弧之间洋壳俯冲时间的上限(余吉远等, 2012)。中亚造山带南缘南天山–柳园–索伦克尔缝合带中分布的高温高压变质岩带形成时间为465~315 Ma(Vasyukova et al., 2011),这些高温高压变质岩形成于洋壳俯冲之后的折返过程,代表了古亚洲洋俯冲结束的时间(杨高学, 2023),因此,北山造山带在晚石炭世(约315 Ma)之后进入弧–陆碰撞及后碰撞伸展阶段。北山地区广泛分布着早二叠世镁铁–超镁铁质岩石(Xue et al., 2018)、辉绿岩脉(高文彬等, 2021;Xu et al., 2021)以及高钾和富碱的花岗质岩体(张文等, 2010;Zhang et al., 2012;Liu et al., 2019),表明该时期北山地区处于碰撞后伸展背景。此外,也有学者对东天山-北山地区的晚三叠世花岗岩进行了研究,柳园地区晚三叠世花岗岩体属于高分异的I型花岗岩和A型花岗岩(Li et al., 2012;Mao et al., 2021),东天山地区中三叠世尾亚复式岩体中也发育中三叠世具 A 型特征的正长花岗岩(238 Ma)与碱性辉长岩 (242 Ma;Zhang et al., 2005;Feng et al., 2021)。研究表明:大陆裂谷内形成的 A 型花岗岩常与碱性辉长岩共生,而造山后伸展环境下的 A 型花岗岩则与钙碱性花岗岩或钙碱性辉长岩紧密共生(邓晋福等, 1999)。综上所述,笔者认为北山南部柳园地区在晚三叠世造山过程应已结束,进入陆内伸展阶段(Zhang et al., 2008;Zhou et al., 2008;王国强等, 2021)。
Vaughan等(2003)指出全球范围内不同构造背景下由交代岩石圈地幔熔融形成的富K质岩浆作用都与拉张伸展背景存在关联。具体而言,被交代的含水的、富挥发性组分的富集岩石圈地幔受到伸展作用的影响,将会发生低程度部分熔融形成富K质岩浆(McKenzie, 1989;Foley et al., 1992),该岩浆将沿着构造薄弱带上升到浅部地壳(图9)。在许多造山带演化的晚期转变为板内伸展的阶段,通常形成一系列富K质岩浆作用,如苏鲁造山带早白垩世煌斑岩(Hou et al., 2022)、秦岭造山带晚三叠世煌斑岩(孙万龙等, 2022)和欧洲Iberian地区Variscan造山带中晚古生代煌斑岩(Scarrow et al., 2011)等。柳园地区煌斑岩形成于晚三叠世,地球化学特征表明其源自于富集流体交代的岩石圈地幔,岩墙侵入可作为地壳伸展的标志(刘畅等, 2006)。将其与柳园地区构造演化相结合表明,古生代期间古亚洲洋在北山地区的俯冲-增生作用使得俯冲板片脱水形成的高度富集大离子亲石元素而亏损高场强元素的流体,这些富集流体上升交代使得岩石圈地幔发生富集作用。晚三叠世时,柳园地区进入板内伸展作用(图9),减压熔融使得被早期俯冲流体交代的岩石地幔发生低程度部分熔融,形成煌斑岩母岩浆,并沿着断裂侵入,形成煌斑岩岩脉。此外,柳园地区煌斑岩与中亚造山带北部俄罗斯Chuya地区煌斑岩具有相似的成岩时代与岩石地球化学特征表明中亚造山带在晚三叠世时处于伸展背景。
5. 结论
(1) 柳园南煌斑岩锆石U-Pb年龄为(228.2±1.1) Ma,该年龄与柳园西煌斑岩脉中金云母Ar-Ar年龄(220~240 Ma)和中亚造山带北部俄罗斯Chuya地区煌斑岩中金云母Ar-Ar年龄(236~234 Ma)接近,表明柳园地区煌斑岩脉都形成于晚三叠世。
(2) 煌斑岩中富含金云母、角闪石等富挥发性组分矿物,属于碱性岩石系列;岩石地球化学特征表明其源区具有俯冲流体交代特征;煌斑岩中橄榄石Mn/Fe与Ca/Fe值表明柳园南煌斑岩源区为富金云母的方辉橄榄岩地幔;结合煌斑岩大地构造位置表明其岩浆源区为被俯冲流体交代的岩石圈地幔。
(3) 依据前人对北山地区构造演化的研究,北山南部地区在晚三叠世时已进入陆内伸展阶段,减压作用促使被俯冲流体交代的岩石圈地幔发生低程度部分熔融,形成柳园地区煌斑岩。
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图 1 中亚造山带构造单元简图(a) (据Jahn et al., 2004修改), 北山造山带构造简图(b) (据Xiao et al., 2010修改)和柳园地区地质简图(c) (据甘肃省地质矿产局,1966修改)
图c中年龄数据引自刘畅(2006)、李舢等(2011)、Li等(2017)、孙海瑞等(2020)
Figure 1. (a) Sketch of tectonic units of the Central Asian Orogenic Belt, (b) Beishan Orogenic Belt, and (c) geological map of the Liuyuan area
图 4 柳园南煌斑岩的橄榄石晶体Fo-Ni(a)和Fo-Ca(b)相关图
煌斑岩、Hawaii和MORB数据引自Sobolev等(2007)、Foley等(2013)和Prelević等(2013)
Figure 4. (a)Fo-Ni and (b)Fo-Ca diagrams of olivine crystals from Liuyuannan lamprophyre
图 5 柳园南煌斑岩SiO2-(K2O+Na2O)(a)与K/(K+Na)-K/Al(b)相关图
I.钠质煌斑岩;I’.弱钾质煌斑岩;II.钾质煌斑岩;III.超钾质煌斑岩;IV.过钾质煌斑岩;V.钾镁煌斑;岩底图据Irvine等(1971)、路凤香等(1991);柳园西、Chuya煌斑岩数据分别引自刘畅等(2006)和Vasyukova(2011)
Figure 5. (a)(Na2O+K2O)- SiO2 and (b) K/(K+Na) - K/Al diagrams of the Liuyuannan lamprophyre
图 6 柳园南煌斑岩球粒陨石标准化稀土元素配分模式图(a)和原始地幔标准化微量元素蛛网图(b)
球粒陨石标准化数据引自 Boynton(1984);原始地幔标准化数据引自Sun等(1989);洋岛玄武岩、岛弧玄武岩数据引自Li 等(2015);柳园西、Chuya煌斑岩数据分别引自刘畅等(2006)和Vasyukova等(2011)
Figure 6. (a) Chondrite-normalized REEs pattern and (b) Primitive Mantle-normalized trace elements Spider diagram of Liuyuannan lamprophyre
图 7 柳园南煌斑岩Nb/Yb-Th/Yb(a)与Th/Nb-εHf(t)(b)相关图
图a底图据 Pearce(2008);上地壳和亏损地幔各元素组分引自Rudnick等(2003)和Sun等(1989);图b中俯冲板块衍生流体组分和大洋循环沉积物组分引自Veroot等(1999)、Kessel等(2005)和Chauvel等(2008)
Figure 7. (a)Nb/Yb-Th/Yb and (b)Th/Nb-εHf(t) covariance diagrams of Liuyuannan lamprophyre
图 8 柳园南煌斑岩中橄榄石100×(Ca/Fe)-100×(Mn/Fe)相关图(a)与全岩La/Sm-La相关图(b)
图a各区域数据引自Förster等(2018);图b微量元素元素分配系数引自McKenzie等(1991)、McKenzie等(1995);DMM、PM和N-MORB引自Sun等(1989)
Figure 8. (a) 100×(Ca/Fe)-100×(Mn/Fe) Covariance diagram of the olivine crystals from Liuyuannan lamprophyre and (b) whole-rock La/Sm-La Covariance Diagram
表 1 柳园南煌斑岩LA-ICP-MS锆石U-Pb同位素分析结果
Table 1 Analysis results of LA-ICP-MS zircon U-Pb isotope from Liuyuannan lamprophyre
测试点 Pb(10−6) Th(10−6) U(10−6) Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 206Pb/238U 1σ 207Pb/235U 1σ HBY-1 13.6 183 111 1.64 0.0530 0.0031 0.2375 0.0136 0.0367 0.0006 232.4 3.5 239.3 12.1 HBY-2 22.2 256 329 0.78 0.0508 0.0024 0.2517 0.0120 0.0359 0.0005 227.1 3.4 228.0 9.7 HBY-3 14.3 189 116 1.63 0.0534 0.0035 0.2720 0.0159 0.0374 0.0008 236.6 4.7 244.3 12.7 HBY-4 24.0 304 277 1.09 0.0505 0.0038 0.2432 0.0180 0.0350 0.0005 222.0 3.3 221.0 14.7 HBY-5 38.3 471 428 1.10 0.0510 0.0020 0.2535 0.0096 0.0364 0.0007 231.1 4.5 229.4 7.7 HBY-6 20.2 244 224 1.09 0.0504 0.0028 0.2543 0.0142 0.0364 0.0005 230.4 3.2 230.1 11.5 HBY-7 29.7 377 380 0.99 0.0496 0.0030 0.2375 0.0136 0.0349 0.0007 221.3 4.6 216.4 11.2 HBY-8 41.1 505 541 0.93 0.0504 0.0020 0.2514 0.0107 0.0361 0.0006 228.6 3.8 227.7 8.7 HBY-9 21.3 274 217 1.26 0.0508 0.0030 0.2463 0.0115 0.0352 0.0006 223.2 3.9 233.5 9.4 HBY-10 14.7 171 173 0.99 0.0508 0.0031 0.2582 0.0141 0.0376 0.0008 238.4 5.2 233.3 11.4 HBY-11 13.1 164 175 0.94 0.0508 0.0022 0.2517 0.0158 0.0358 0.0005 226.7 3.2 227.9 12.8 表 2 柳园南煌斑岩橄榄石及单斜辉石成分表
Table 2 Composition of olivine and clinopyroxene from Liuyuannan lamprophyre
分析编号 矿物
名称SiO2(%) Al2O3(%) MgO(%) CaO(%) Cr2O3(%) FeO(%) MnO(%) NiO(%) Total(%) Fo牌号 HBY-2d-1.2 橄榄石 39.82 0.02 46.03 0.17 0.02 12.88 0.27 0.21 99.47 86.5 HBY-2d-2.1 橄榄石 40.77 0.04 48.86 0.18 0.02 10.05 0.19 0.29 100.44 89.7 HBY-2d-2.4 橄榄石 41.14 0.04 48.83 0.18 0.11 10.19 0.14 0.33 100.99 89.6 HBY-2d-3.1 橄榄石 41.12 0.05 50.27 0.17 0.05 9.07 0.19 0.35 101.37 90.9 HBY-2d-5.2 橄榄石 40.37 0.03 48.43 0.17 0.16 11.16 0.22 0.26 100.89 88.7 HBY-2d-6.1 橄榄石 41.76 0.00 50.86 0.12 0.10 8.41 0.17 0.53 101.96 91.6 HBY-2d-8.1 橄榄石 41.06 0.07 50.41 0.15 0.06 8.45 0.02 0.47 100.71 91.5 HBY-2d-9.1 橄榄石 40.45 0.02 48.24 0.16 0.08 9.84 0.10 0.20 99.21 89.8 HBY-2d-9.2 橄榄石 41.79 0.04 48.70 0.17 0.05 10.24 0.18 0.30 101.53 89.5 HBY-2d-10.1 橄榄石 40.76 0.06 49.10 0.16 0.12 9.56 0.12 0.33 100.24 90.2 HBY-2d-11.1 橄榄石 42.31 0.09 49.70 0.22 0.03 7.81 0.11 0.42 100.83 92.0 HBY-2d-12.1 橄榄石 41.24 0.03 49.80 0.17 0.14 8.71 0.09 0.43 100.73 91.1 HBY-2d-13.1 橄榄石 41.65 0.03 51.04 0.14 0.20 8.03 0.11 0.36 101.65 92.0 HBY-2d-14.1 橄榄石 41.61 0.05 51.17 0.13 0.12 8.23 0.08 0.46 101.91 91.8 HBY-2d-15.1 橄榄石 41.59 0.07 50.65 0.18 0.04 7.90 0.08 0.43 100.94 92.0 HBY-2d-16.1 橄榄石 41.36 0.01 50.56 0.13 0.00 7.64 0.09 0.57 100.41 92.3 分析编号 矿物
名称SiO2(%) Al2O3(%) MgO(%) CaO(%) Cr2O3(%) FeO(%) MnO(%) TiO2(%) Total(%) Wo牌号 En牌号 Fs牌号 HBY-2d-4.2 单斜
辉石51.16 3.23 15.40 24.09 0.93 4.40 0.10 0.55 100.28 49.20 43.78 7.01 HBY-2d-5.3 单斜
辉石52.37 2.90 15.91 24.27 1.04 4.43 0.13 0.52 101.97 48.67 44.40 6.93 HBY-2d-7.1 单斜
辉石50.82 3.03 15.67 24.03 0.74 4.27 0.14 0.56 99.84 48.87 44.35 6.77 HBY-2d-7.3 单斜
辉石52.60 2.07 15.81 23.15 0.54 4.16 0.17 0.43 99.40 47.83 45.45 6.71 注:Total(%)为主量元素总含量。 表 3 柳园南煌斑岩主量元素(%)及微量元素(10−6)分析结果
Table 3 Analysis results of major and trace elements of Liuyuannan lamprophyre
样品编号 HBY-1a HBY-2b HBY-2c HBY-2d HBY-1c HBY-2a 岩性 煌斑岩 煌斑岩 煌斑岩 煌斑岩 煌斑岩 煌斑岩 SiO2 43.1 43.1 42.4 42.9 TiO2 0.53 0.64 0.52 0.53 Al2O3 5.51 5.89 5.14 5.16 Fe2O3 10.4 10.1 10.6 10.5 MnO 0.15 0.15 0.16 0.16 MgO 24.35 23.94 25.63 25.55 CaO 8.86 9.29 8.53 9.05 Na2O 0.42 0.55 0.57 0.61 K2O 1.78 2.42 1.90 1.91 P2O5 0.23 0.22 0.23 0.21 烧失量 4.00 3.29 3.66 3.17 Total 99.4 99.6 99.4 99.8 Mg#值 0.67 0.67 0.67 0.68 m/f值 2.00 2.03 2.07 2.08 里特曼指数 2.39 5.38 4.99 4.92 Li 22.2 25.2 19.9 13.1 24.9 14.2 Sc 39.9 42.1 10.6 15.1 30.3 29.7 Rb 86.1 108.5 80.8 85.4 91.2 87.1 Sr 810 1170 1210 1099 409 569 Y 12.3 11.8 11.4 11.0 11.3 10.2 Zr 72.4 67.3 65.9 64.0 67.6 60.9 Hf 1.98 1.68 1.53 1.61 1.96 1.80 Nb 4.06 3.94 3.80 3.72 4.08 3.68 Ta 0.27 0.23 0.23 0.21 0.29 0.25 La 34.1 32.6 33.0 32.0 35.6 32.0 Ce 76.6 72.2 73.0 72.4 74.5 68.3 Pr 9.77 9.14 9.30 9.12 9.34 8.82 Nd 39.2 36.5 37.0 37.0 37.2 34.8 Sm 6.25 6.10 5.96 5.88 6.29 6.06 Eu 2.08 2.17 1.93 1.90 1.78 1.69 Gd 5.62 5.30 5.45 5.18 5.42 5.13 Tb 0.55 0.50 0.51 0.50 0.58 0.55 Dy 2.56 2.32 2.28 2.32 2.29 2.15 Ho 0.44 0.41 0.40 0.40 0.45 0.42 Er 1.26 1.08 1.06 1.08 1.15 1.08 Tm 0.14 0.14 0.13 0.13 0.15 0.14 Yb 1.05 0.93 0.86 0.86 0.99 0.90 Lu 0.15 0.13 0.13 0.11 0.14 0.13 Pb 73.7 56.7 96.0 53.5 70.4 54.0 Th 4.74 4.10 3.41 3.35 4.41 3.68 U 1.38 1.26 1.34 1.20 1.39 1.19 Eu异常 1.07 1.17 1.03 1.05 0.93 0.93 LREE 180 170 171 169 176 162 HREE 168 159 160 158 165 152 REE 11.8 10.8 10.8 10.6 11.2 10.5 LREE/HREE值 14.3 14.7 14.8 14.9 14.7 14.5 (La/Yb)N 23.4 25.3 27.4 26.6 25.9 25.5 注:Total为主量元素总含量;LREE为轻稀土元素总含量;HREE为重稀土元素总含量;REE为稀土元素总含量。 表 4 柳园南煌斑岩中锆石Hf 同位素分析结果
Table 4 Zircon Hf isotopic data of Zircon crystals from Liuyuannan lamprophyre
点号 样品编号 176Yb/177Hf 2σ 176Lu/177Hf 2σ 176Hf/177Hf 2σ 年龄
(Ma)(176Hf/177Hf)i εHf(0) εHf(t) TDM
(Ma)fLu/Hf 1 HBY-1 0.030890 0.000175 0.000772 0.000003 0.282660 0.000022 228 0.282657 −3.96 0.93 834 −0.98 2 HBY-2 0.037702 0.000322 0.000953 0.000007 0.282707 0.000020 228 0.282702 −2.32 2.54 773 −0.97 3 HBY-3 0.056320 0.000131 0.001362 0.000004 0.282718 0.000019 228 0.282713 −1.90 2.90 764 −0.96 4 HBY-4 0.060112 0.000074 0.001400 0.000001 0.282710 0.000023 228 0.282704 −2.18 2.61 776 −0.96 5 HBY-5 0.056959 0.000328 0.001366 0.000007 0.282784 0.000018 228 0.282778 0.41 5.21 671 −0.96 6 HBY-6 0.050719 0.000570 0.001433 0.000012 0.282711 0.000021 228 0.282704 −2.17 2.62 777 −0.96 7 HBY-7 0.039301 0.000143 0.000993 0.000003 0.282762 0.000020 228 0.282757 −0.37 4.48 696 −0.97 8 HBY-8 0.077620 0.000778 0.001884 0.000012 0.282652 0.000018 228 0.282644 −4.23 0.49 870 −0.94 9 HBY-9 0.049260 0.000207 0.001325 0.000005 0.282687 0.000018 228 0.282682 −3.00 1.81 808 −0.96 10 HBY-10 0.125049 0.000434 0.002978 0.000010 0.282677 0.000022 228 0.282665 −3.35 1.21 860 −0.91 11 HBY-11 0.051215 0.000306 0.001212 0.000006 0.282774 0.000023 228 0.282768 0.06 4.88 683 −0.96 12 HBY-12 0.044043 0.000164 0.001235 0.000006 0.282696 0.000020 228 0.282691 −2.69 2.13 794 −0.96 13 HBY-13 0.028124 0.000061 0.000732 0.000002 0.282748 0.000018 228 0.282745 −0.85 4.05 710 −0.98 -
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