克拉玛依侏罗纪玄武岩地球化学特征及其壳幔作用过程研究

陈双双, 苏玉平, 郑建平, 魏 颖

(中国地质大学(武汉) 地球科学学院, 地质过程与矿产资源国家重点实验室, 湖北 武汉 430074)

0 引 言

中亚造山带经历了碰撞造山、后碰撞和板内演化三个阶段, 是全球显生宙陆壳增生与板内演化最显著的地区之一[1]。新疆北部是中亚造山带的重要组成部分, 具有完整且明显的陆壳增生演化过程[2]。特别是古生代经历了复杂的地质演化, 中生代以后开始进入相对稳定的板内演化阶段[3]。该地区广泛发育古生代蛇绿岩套[4–6], 如 447～531 Ma的唐巴勒蛇绿岩[4]、(472±8.4) Ma的洪古勒楞蛇绿岩[5]及391 Ma的达拉布特蛇绿岩[6]; 至晚古生代准噶尔地体及周边广泛发育后碰撞花岗岩[7–9], 表明古准噶尔洋的闭合演化过程。与古生代碰撞-后碰撞岩浆活动相比, 在新疆北部地区很少有关于中新生代岩浆活动的报道, 仅在较老资料中有过百口泉、白碱滩等地区中生代玄武岩的记载[10], 但由于当时 K-Ar同位素年代学测试精度的限制, 没有得到普遍认同[3]。近年来, 不断发现的中新生代岩浆活动[3,11–15], 提供了揭示新疆北部地区中新生代岩石圈深部过程的重要窗口[16–17]。在详细的野外地质和室内岩相学基础上, 本次工作对克拉玛依玄武岩开展了全岩主元素、微量元素和Sr-Nd同位素以及锆石U-Pb年代学、Hf同位素的全面分析, 拟对岩浆活动的成因机制及构造背景进行探讨, 以期为新疆北部显生宙地幔演化提供资料。

1 地质背景

西准噶尔位于西伯利亚板块、哈萨克斯坦板块和塔里木板块交汇区域, 其与相邻三大板块的关系一直受到地质学界的关注。泥盆纪末至石炭纪初西准噶尔地区广泛发育的蛇绿岩套和岛弧火山岩, 表明古准噶尔洋的俯冲消减过程[4–6]; 晚古生代准噶尔地体及周边广泛发育的后碰撞花岗岩, 表明古洋盆的闭合及区域进入后碰撞期阶段[7–9]。王京彬等[18]将西准噶尔地区“后碰撞”的时间厘定为石炭纪至二叠纪, 而韩宝福等[9]将准噶尔后碰撞深成岩浆的活动时间限定于早石炭中晚期至早二叠世末期。晚二叠世之后西准噶尔地区进入了稳定的板内演化阶段[19]。所以西准噶尔地区经历了主碰撞-后碰撞-板内环境这样一套完整的构造-岩浆活动的演化历史。与古生代碰撞-后碰撞的岩浆活动相比, 具有确切年龄依据的中新生代幔源岩浆活动较少。近几年来, 在新疆北部地区报道的中新生代岩浆活动记录主要有克拉玛依玄武岩(192 Ma)[3]、托云玄武岩类(100～40.36 Ma)[11–13]、哈拉乔拉橄榄玄武岩(20～10 Ma)[14–15](图1a), 这些岩浆活动主要沿着断裂带或薄弱带分布。

本次研究的玄武岩(图1b)位于克拉玛依市西南10 km 处的侏罗统八道湾组底部(45.5348°N, 84.7576°E),与下伏下石炭统太勒古拉组地层呈角度不整合接触关系, K-Ar年龄在170～190 Ma之间[10]。研究区域内发育了一系列 NE-NNE向断层, 其周围地层中还发育了大量的 NE-NNE向和 NWW-NW 向基性岩墙群[21]。研究区域内主要出露晚古生代至中新生代地层, 中生代地层出露较全。下伏地层是下石炭统太勒古拉组, 其地层产状陡倾, 存在各种断裂和褶皱构造。侏罗纪地层覆盖于太勒古拉组地层之上, 两者呈角度不整合接触关系[3]。

2 岩石学特征

本次研究的玄武岩具有明显的柱状节理(图2a),样品新鲜, 呈致密块状构造, 显微镜下显示斑状结构。斑晶主要由富Ca斜长石、普通辉石和橄榄石组成, 包括深灰色橄榄玄武岩和深黑色含菱铁矿橄榄玄武岩。

图1 新疆北部地区中新生代火山岩分布图(a) (据文献[20])和克拉玛依区域地质简图及采样点(b) (据文献[21])Fig.1 Distribution of Mesozoic-Cenozoic volcanic rocks in northern Xinjiang (a) (modified from reference [20]); The simplified geological map of the Karamay region and sampling locations (b) (modified from reference [21])1–第四纪沉积物; 2–下白垩统吐谷鲁组沉积岩; 3–中-上侏罗统沉积岩; 4–下侏罗统八道湾组火山沉积岩; 5–上二叠统小泉沟组沉积岩; 6–下石炭统太勒古拉组凝灰岩-中基性火山岩; 7–花岗岩-花岗闪长岩; 8–角闪花岗岩-黑云母花岗岩; 9–中基性岩脉; 10–采样点。1–quaternary sediment; 2–lower Cretaceous Tugulu formation sediment; 3–Middle-Upper Jurassic sediment; 4–Lower Jurassic Badaowan formation sediment; 5–Lower Permian Xiaoquangou formation sediment; 6–Lower Carboniferous Tailegula formation tufa and intermediate-basic volcanic rock;7–granite and granodiorite; 8–hornblende-granite and biotite-granite; 9–intermediate-basic dyke; 10–sampling site.

深灰色橄榄玄武岩(图2b), 斑晶主要由富Ca斜长石、普通辉石和橄榄石组成, 还含有少量磁铁矿,样品较新鲜, 没有明显的蚀变。富 Ca斜长石(45%～50%)呈自形-半自形板状, 粒径 0.2～0.6 mm;普通辉石(20%～25%)呈板状、粒状, 粒径 0.05～0.2 mm; 橄榄石(5%)呈不规则粒状, 粒径很小。基质具有间粒间隐结构, 主要由斜长石、玻璃和少量磁铁矿组成。

深黑色含菱铁矿橄榄玄武岩(图 2c), 斑晶由富Ca斜长石、普通辉石、橄榄石和碳酸盐矿物菱铁矿组成。菱铁矿呈颗粒状, 粒径较大, 约为0.5 mm, 薄片中无色, 闪突起, 具有明显的十字消光[21](图2d)。基质也具有间粒间隐结构。

图2 克拉玛依玄武岩野外及镜下照片Fig.2 Photomicrographs and field photos of Karamay basalts(a)克拉玛依玄武岩明显的柱状节理; (b)深灰色橄榄玄武岩的正交偏光显微照片; (c)深黑色含菱铁矿橄榄玄武岩的正交偏光显微照片; (d)镜下可见特征碳酸盐矿物菱铁矿。Ol–橄榄石; Cpx–单斜辉石; P1–斜长石; G–玻璃; Mag–磁铁矿; Sid–菱铁矿。(a) The columnar jointing structure of Karamay basalts; (b) Microphotographs of cross-polarized light of the dark grey olivine basalt; (c) Microphotographs of cross-polarized light of the dark-coloured siderite-bearing olivine basalt; (d) Siderite (carbonate minerals) can be seen. O1–olivine,Cpx–clinopyroxene, Pl–plagioclase, G–glass, Mag–magnetite, Sid–siderite.

3 实验分析方法

全岩主元素和微量元素分析均在中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成, 主元素采用 XRF法分析, 测试精度优于 1%。利用标样 GBW07113评估实验室 XRF测试数据的准确度(表1)。微量元素分析采用两酸(HNO3+HF)酸溶方法对样品粉末进行溶解, 采用 ICP-MS(Agilent 7500a)测定元素含量。分别用AGV-2、BHVO-2、BCR-2、RGM-1和GSR-1这5个标样评估实验室ICP-MS测试数据的准确度(表2), 大多数元素的相对误差小于5%, 部分元素相对误差小于10%。Sr-Nd同位素(表3)组成在中国地质大学(武汉)的MAT 261固体质谱仪上分析, Nd和 Sr同位素比值分别采用146Nd/144Nd=0.7219和86Sr/88Sr=0.1194进行校正。

锆石的所有测试分析均在澳大利亚 Macquarie大学GEMOC中心完成。由岩石样品经过破碎、重液分离和磁选初步分选出锆石, 再在双目镜下分选出晶型完好、颗粒大于50 μm的锆石, 并将代表性锆石制成环氧树脂样品靶, 然后在显微镜下观察并进行CL照相。锆石U-Pb同位素(表4)分析在Agilent 7500电感耦合等离子质谱与Merchantek/NWR 213 nm激光熔蚀探针联机上进行。U-Pb同位素数据由Glitter软件处理,206Pb/238U加权平均年龄及谐和年龄图由Isoplot程序计算绘制。锆石Hf同位素(表5)在 Nu Plasma多接受器的电感耦合等离子质谱与Merchantek/NWR 213nm激光熔蚀探针的联机上进行。选用Blichert-Toft et al.[26]建议的球粒陨石值计算 εHf(t), 用 Griffin et al.[27]建议的亏损地幔值和大陆平均地壳的176Lu/177Hf=0.015[28]来计算亏损地幔模式年龄(TDM)和平均大陆地壳模式年龄(Tcrust)。

表1 克拉玛依玄武岩主元素分析结果 (%)Table 1 Major element analytical results (%) of Karamay basalts

表3 新疆北部晚古生代和中新生代火山岩的Sr-Nd同位素特征Table 3 Sr-Nd isotopes of Late Paleozoic and Mesozoic-Cenozoic volcanic rocks in northern Xinjiang

表4 克拉玛依玄武岩锆石LA-ICPMS U-Pb分析数据Table 4 Zircon LA-ICPMS U-Pb analytical data of Karamay basalts

表5 克拉玛依玄武岩锆石Hf同位素成分Table 5 Hf isotopic compositions of zircon in Karamay basalts

4 分析结果

4.1 主元素和微量元素特征

克拉玛依玄武岩 SiO249.6%～51.2%, FeOT10.4%～12.7%, MgO 3.58%～6.81%, CaO 6.74%～7.98%, K2O+Na2O 5.27%～5.60%, Mg#值在 0.26～0.55之间。与正常的大洋中脊玄武岩相比, 碱含量明显偏高, 尤其是钾含量。利用Zr/TiO2-Nb/Y图解[29]对克拉玛依玄武岩进行分类(图3), 显示它们属于碱性玄武岩系列, 里特曼指数σ=3.87～4.68(表1)。

这些玄武岩的稀土元素总量较高(113～118 μg/g), 具有富集轻稀土元素特征, 稀土分布模式曲线均向右倾斜(图 4a), 且(La/Yb)N比值较高(4.99～10.1), 表现较强的轻重稀土元素分异特征。它们没有Eu的负异常(δEu=1.02～1.06)。在微量元素蛛网图(图 4b)中, 相对富集大离子亲石元素 LILE(如Rb、Ba和 K 等), Nb-Ta、Zr-Hf表现为相对富集, Ba、Sr显示明显正异常, Th、P为负异常。所有微量元素含量都高于原始地幔值, 且多数元素高出10倍以上,并且稀土分布模式曲线和蛛网图与典型的OIB特征都极为相似, 但显示更为亏损的特点(图4)。

图3 克拉玛依玄武岩Zr/TiO2-Nb/Y图解(据文献[30])Fig.3 Plots of Zr/TiO2-Nb/Y for Karamay basalts (after reference [30])

图4 克拉玛依玄武岩稀土元素球粒陨石标准化分布模式(a)和微量元素原始地幔标准化蛛网图(b)Fig. 4 Chondrite normalized REE patterns of Karamay basalts (a);Primitive mantle normalized trace element patterns of Karamay basalts (b)球粒陨石、原始地幔、E-MORB、N-MORB及OIB资料据文献[31]。The data of Chondrite, Primitive mantle, E-MORB, N-MORB and OIB are from reference [31].

4.2 全岩Sr-Nd同位素特征

克拉玛依玄武岩的初始 Sr同位素(87Sr/86Sr)i变化范围很小(0.7048～0.7049), εNd(t)值具有相对较低的正值(+2.95～+3.02), Nd同位素二阶段模式年龄TDM2为 725～731 Ma(表 3)。相比于晚古生代基性火山岩(较高的正εNd(t)值、较低的(87Sr/86Sr)i值, 表3),这些玄武岩表现出亏损程度相对较低的特征, 且落在DM与EMⅠ地幔演化范围内(图5)。

4.3 锆石U-Pb年代学和Hf同位素特征

在近3 kg的样品中经人工重砂分选发现10颗锆石。所有锆石没有完整的形态, 但多发育典型的岩浆结晶结构特点 (图6)。除KL5a偏离谐和曲线外,其他颗粒都落在谐和曲线上, 其中 1个锆石分析点(KL4b)得到了相对较老的206Pb/238U年龄(424.1 Ma),其余 8个锆石分析点集中分布, 给出了(357.3±5.1)Ma (MSWD=1.6)的谐和年龄(图 6)。这些锆石的 U和 Th 的含量分别为 86～245 μg/g 和 56～257 μg/g (表4), Th/U比值介于0.55～0.76之间, 也具岩浆锆石特征。

图5 晚古生代和中新生代火山岩的εNd(t)-(87Sr/86Sr)i (a)和εNd(t)-t关系图(b)Fig.5 Plots of εNd(t) vs. initial 87Sr/86Sr (a) and εNd(t) vs. age (b) for late Paleozoic and Mesozoic-Cenozoic volcanic rocks图5a改自文献[32], MORB数据来自文献[33], OIB数据来自文献[34],EMⅠ和EMⅡ数据来自文献[35]。在图5a和图5b中, 晚古生代基性火山岩包括西准噶尔玄武安山岩[22]、富铌玄武岩[20]、三塘湖盆地玄武岩[24]和柳园玄武岩[25]; 中新生代基性火山岩包括克拉玛依玄武岩[3]、托云玄武岩[11–13]和哈拉乔拉玄武岩[14–15]。Fig.5 a is modified from Reference [32], the data of MORB are from reference [33], the data of OIB are from Reference [34], the data of enriched mantle Ⅰ (EMⅠ) and Ⅱ (EMⅡ) are from reference [35];In Fig.5a and Fig.5b, Late Paleozoic basic volcanic rocks include Western Junggar basaltic andesite[22], Nb-enriched basalt[20], Santanghu basalt[24], Liuyuan basalts[25]; Mesozoic-Cenozoic basic volcanic rocks include Karamay basalt[3], Tuoyun basalt[11–13], Halaqiaola basalt[14–15].

图6 克拉玛依玄武岩中锆石LA-ICPMS U-Pb谐和曲线图Fig.6 LA-ICPMS U-Pb concordia diagram of zircons from Karamay basalts

颗粒 KL4b(424 Ma)的初始 Hf同位素比值(176Hf/177Hf)i=0.282720, εHf(t)值为+7.48; 其余 8 颗锆石(357 Ma)的(176Hf/177Hf)i值为 0.282730～0.282960、εHf(t)值为+6.23～+10.22(表 5)。它们的模式年龄 TDM和 Tcrust分别在 581～741 Ma和 712～968 Ma之间。

5 讨 论

5.1 克拉玛依玄武岩成岩年龄及石炭系地层关系

本研究所得克拉玛依玄武岩锆石的主体 U-Pb谐和年龄为(357.3±5.1) Ma, 该年龄不能代表克拉玛依玄武岩的喷发时代, 而是代表了围岩太勒古拉组地层时代。支持该结论的主要证据包括有: 徐新等[3]对这套克拉玛依玄武岩进行40Ar/39Ar定年, 确定其形成年龄为(192.7±1.3) Ma; 薛云兴等[21]系统研究了这套橄榄玄武岩中菱铁矿成因, 明确指出其应形成于侏罗纪造山期后的板内岩浆活动; 郭丽爽等[36]曾对太勒古拉组玄武岩进行锆石U-Pb定年, 测得结果为(357.5±5.4) Ma, 与我们所得锆石 U-Pb年龄(357.3±5.1) Ma相当一致; 特别是从野外产状上看,克拉玛依玄武岩呈角度不整合覆盖在太勒古拉组火山-沉积岩之上[21], 且发育明显的柱状节理(图 2a);并且侏罗纪玄武岩与周边古生代玄武岩具有明显不同的地球化学特征。此外, 锆石具有较高的正 εHf(t)值 (+6.23～+14.49)和 较 高 的 (176Hf/177Hf)i值(0.282725～0.282977), 其中1个锆石分析点KL4a的εHf(t)值高达+14.49, 这都暗示了其具有较亏损地幔源区特征, 而下文“岩石成因”中分析克拉玛依玄武岩具有与 OIB型源区相似的幔源源区特征, 这就更加暗示该岩浆锆石不是来自于克拉玛依玄武岩,而更可能是捕获于太勒古拉组地层的继承锆石。

西准噶尔地区下石炭统地层的上下关系一直存在着争议。一些学者[36]认为石炭系从下到上分别为太勒古拉组、包古图组和希贝库拉斯组; 也有学者[37]认为从下到上为希贝库拉斯组, 包古图组和太勒古拉组。廖卓庭等[38]根据生物化石将这三个组的上下关系定为: 包古图组、希贝库拉斯组和太勒古拉组。我们在克拉玛依地区获得的太勒古拉组地层时代为(357.3±5.1) Ma, 该年龄大于(345.6±6.2) Ma 的包古图组安山岩[39]、328～342 Ma的包古图组蚀变凝灰岩[37]和(335.6±7.8) Ma的包古图组侵入岩体[40], 并且野外产状表明包古图组呈角度不整合伏于希贝库拉斯组之下[36,41], 根据希贝库拉斯组与夏尔甫东岩体的侵入接触关系, 推断希贝库拉斯组形成年龄应早于293 Ma[42]。据此推测西准噶尔下石炭统地层从下到上依次为太勒古拉组、包古图组和希贝库拉斯组, 这样可能更合适。

5.2 岩石成因

克拉玛依侏罗纪玄武岩 SiO2含量为49.56%～51.22%, 不可能是由下地壳岩石部分熔融形成, 应来源于地幔源区。需要讨论的是克拉玛依玄武岩的幔源原始岩浆的分异作用、地壳混染程度、地幔源区性质以及是否发生交代富集作用。

5.2.1 地壳混染与结晶分异作用

这些玄武岩的碱含量明显偏高(尤其是钾含量),属于碱性玄武岩系列(σ=3.87～4.68)。该玄武岩Nb、Ta含量较高, 没有显示亏损的特征, Zr、Hf也没有明显的正异常, Nb/Ta比值为16.7～18.1, Zr/Hf比值约为 40.0, 分别与原始地幔(Nb/Ta=17.5, Zr/Hf=36.3)相近而远高于大陆地壳值(Nb/Ta=12.1, Zr/Hf =11.0);较低的 Th/Ce比值(0.06)和较低的 Th/La比值(0.13～0.14), 与幔源岩浆的 Th/Ce比值(0.02～0.05)和Th/La比值(约0.12)相当而明显低于地壳的Th/Ce比值(约 0.15)和 Th/La 比值(约 0.30)[31,43]。此外, Zr/Yb比值(124～139)、Th/La 比值(0.13～0.14)、Ta/Yb 比值(1.58～1.83)、Nb/Yb 比值(28.0～30.8)和 La/Yb 比值(12.3～14.1)也都具有与原始地幔比值相似且远偏于地壳比值的特征[31,44]。在Nb-Ta异常图中(图7c、图7d), 克拉玛依玄武岩远离UCC区域, 也暗示了其受到的地壳混染作用很小。尽管锆石是捕获于早期岩浆活动(太勒古拉组地层)的继承锆石, 但是, 不论从岩相学角度还是从地球化学角度, 都可以明显看出, 克拉玛依玄武岩受地壳物质影响程度相对有限,特别是没有古老地壳物质的混染; 至于受古生代太勒古拉组火山岩混染确实无法完全排除, 因此我们只能采用逼近的方法, 利用其地球化学数据讨论源区特征。

图7 晚古生代和中新生代火山岩的Th/Yb-Nb/Yb (a)、Th/Y-Sm/Th (b)、Ta*-Nb* (c)和Nb/Ta-Nb (d)关系图Fig.7 Plots of Th/Yb-Nb/Yb (a), Th/Y-Sm/Th (b), Ta*-Nb* (c) and Nb/Ta-Nb (d)图7a改自文献[20, 45], HS表示较深的幔源, LS表示较浅的幔源。图7b改自文献[46], PM、OIB和N-MORB数据来自文献[31]。图7c改自文献[47], OIB、E-MORB、N-MORB数据来自文献[48], PM数据来自文献[31], UCC数据来自文献[49]。图7d改自文献[50], 球粒陨石、OIB、PM、DM数据来自文献[51, 52], UCC数据来自文献[50]。基性火山岩数据来源与图5相同。Fig.7 a is modified from reference [20, 45]; HS represents deep mantle source, while LS is superficial mantle source. Fig.7b is modified from reference[46]; the data of PM, OIB and N-MORB are from reference [31]. Fig.7c is modified from reference [47]; the data of OIB, E-MORB and N-MORB are from reference [48]; the data of PM are from reference [31]; the data of UCC are from reference [49], Fig.7d is modified from reference [50]; the data of chondrite, OIB, PM, and DM are from reference [51, 52]; the data of UCC are from reference [50]. The data source of basic rocks are identical with those of Fig.5.

La和La/Sm相关图呈一条水平直线, 暗示了玄武岩岩浆上升过程中结晶分异作用具有一定影响。Eu 没有明显的负异常(δEu=1.02～1.06), Ba、Sr表现出较弱的正异常以及Nb、Ta相对富集(图4), 且TiO2与 FeOT、K2O和 Na2O没有明显相关关系, 表明斜长石和钛铁氧化物、角闪石的结晶分异不明显。而MgO与FeOT、Cr与Ni呈明显正相关关系, 且相对较低的 Ni、Cr含量(表 2)以及橄榄石和单斜辉石的斑晶(图2), 都暗示着橄榄石和单斜辉石的结晶分异在岩浆演化过程起一定作用。

5.2.2 源区特征

克拉玛依侏罗纪玄武岩的稀土分布模式曲线和蛛网图(图4)都具有明显右倾的特点, 富集轻稀土元素和大离子亲石元素(如Rb、Ba和K等), 与OIB的分布曲线模式很相似, 且其元素含量相比于 OIB都较为亏损, 暗示其具有与 OIB型源区相似但较为亏损的源区特征。它们的Nb/U比值(56.4～58.0)与OIB(Nb/U=52.0±15.0)相当[53]; La/Nb 比值(0.43～0.46)远低于地壳(La/Nb=1.50～2.20)和原始地幔(La/Nb=0.98～1.00), 与 OIB(La/Nb=0.68)接近且略低于OIB的La/Nb比值[53]; Ce/Pb、Nb/Pb比值也有同样特点。在εNd(t)-(87Sr/86Sr)i图解(图5)中, 相比于晚古生代基性火山岩, 侏罗纪玄武岩源区亏损程度相对较低, 且落在OIB的幔源源区范围内(图7), 指示着这些玄武岩源区具有与OIB型源区相似但较为亏损的源区特征。

侏罗纪玄武岩强烈富集轻稀土元素和强不相容元素以及表现Ba、Sr的正异常, 暗示了岩石圈地幔可能经历过交代富集作用。斑晶菱铁矿的存在和偏高的Zr/Hf比值(40.0～41.6), 也通常被认为是碳酸盐熔体交代地幔过程的结果[54–58]。综上所述, 我们推测克拉玛依侏罗纪玄武岩源区可能经历过交代富集作用, 使其岩浆源区亏损程度相对较低且具有与OIB型源区相似的幔源源区的特征。

稀土元素的丰度和比值可以用来推测源区深度及部分熔融的程度[59–60]。克拉玛依侏罗纪玄武岩具有相对较高的 Sm/Yb比值(4.17～4.71)和较高的La/Yb比值(12.3～14.1)指示其源区深度可能在80 km以下深处的石榴石二辉橄榄岩地幔[46,61], 并且其部分熔融程度较低, 处于5%～10%之间(图8)。

图8 晚古生代和中新生代火山岩的Sm/Yb-Sm (a)和Sm/Yb-La/Yb (b)关系图Fig.8 Plots of Sm/Yb-Sm (a) and Sm/Yb-La/Yb (b) for late Paleozoic and Mesozoic-Cenozoic volcanic rocksDM端元数据来源文献[62], La 0.206 μg/g, Sm 0.299 μg/g, Yb 0.347 μg/g, La/Yb=0.594, Sm/Yb=0.862; PM 端元数据来源文献[31], La 0.687 μg/g, Sm 0.444 μg/g, Yb 0.493 μg/g, La/Yb=1.394, Sm/Yb=0.901;虚线和实线分别代表DM熔融曲线和PM熔融曲线, 尖晶石二辉橄榄岩(Ol0.530+Opx0.270+Cpx0.170+Sp0.030和 Ol0.060+Opx0.280+Cpx0.670+Sp0.110)和石榴石二辉橄榄岩(Ol0.600+Opx0.200+Cpx0.100+Gt0.100和 Ol0.030+Opx0.160+Cpx0.880+Gt0.090)熔融曲线来自文献[59, 60], 曲线上的数字代表熔融程度; 基性火山岩数据来源与图5相同。The data of Depleted Mantle (DM) are from reference [62], La=0.206 μg/g, Sm=0.299 μg/g, Yb=0.347 μg/g, La/Yb=0.594, Sm/Yb=0.862;Primitive Mantle (PM) are from reference [31], La=0.687 μg/g,Sm=0.444 μg/g, Yb=0.493 μg/g, La/Yb=1.394, Sm/Yb=0.901; Dashed and solid curves are the melting trends from DM and PM, Melting curves for spinel lherzolite(Ol0.530+Opx0.270+Cpx0.170+Sp0.030 and Ol0.060+ Opx0.280+Cpx0.670+Sp0.110) and garnet lherzolite (Ol0.600+Opx0.200+Cpx0.100+Gt0.100 and Ol0.030+Opx0.160+Cpx0.880+Gt0.090) are from reference [59, 60],Numbers along melting curves represent the degree of partial melting.The data source of basic rocks are identical with those of Fig.5.

5.3 构造演化过程

克拉玛依侏罗纪玄武岩具有相对高的Ti/Y比值和低的 Hf/Ta比值, 类似于板内玄武岩[63]。用Hf-Th-Ta (图 9a)、Nb-Zr-Y 三角图 (图 9b)判别, 结果也是一致的。实际上, 从区域大地构造演化和对比分析来看, 新疆北部在中新生代时早已经进入了板内环境[2,3,9,18,19]。

新疆北部晚古生代基性火山岩具有较低的(87Sr/86Sr)i值(低至 0.7034)和较高的正 εNd(t)值(高达+8.84), 具有亏损地幔源区特征, 主要位于 DM 区域;而中新生代玄武岩具有相对较高的(87Sr/86Sr)i值(高达 0.7056)和相对较低的正 εNd(t)值(低至+0.29), 位于 DM 和 EMⅠ混合的地幔演化区域, 亏损程度相对较低(图5)。同样, 图7a和图7b也表现出晚古生代与中新生代火山岩岩浆源区的亏损富集的差别。此外, 从源区深度来看, 晚古生代岩浆源区主要处于较浅的尖晶石稳定深度且源岩熔融程度较高, 而中新生代的岩浆源区则主要位于较深的石榴子石稳定深度范围, 熔融程度较低(图 8); 并且, 晚古生代基性火山岩受到较大程度的地壳物质混入作用, 可能与俯冲环境有关[61,66], 而中新生代玄武岩则受到很小程度的地壳混染(图7c, 图7d, 图9)。

由晚古生代到中新生代, 地幔源区由较浅的亏损地幔逐步向较深的亏损程度相对较低的幔源演化、熔融程度以及地壳物质混入程度逐渐变低。引起这种变化的原因可能是俯冲的洋壳物质以及拆层的岩石圈在地幔一定深度发生变质重熔后, 与原亏损地幔和岩石圈地幔不断交代富集的结果[2,67–70]。此外, 俯冲板片和拆层岩石圈在地幔中重熔可形成富集Nb-Ta的熔体[71–72], 而高Nb-Ta的熔体交代地幔是导致克拉玛依玄武岩富集Nb-Ta的主要原因[73–74]。

根据上述分析, 新疆北部地区晚古生代到中新生代的壳-幔作用过程, 可能包括: (1)古准噶尔洋发生俯冲消减, 俯冲板片在一定的温压条件下释放出流体和熔体, 导致上覆相对亏损的地幔楔部分熔融(图 10a), 形成了 331～375 Ma的较亏损的且受地壳物质混染的基性火山岩[22–23]。(2)早期的俯冲板片持续下沉最终在300 Ma左右发生板片断离[20]。由于古准噶尔洋的闭合及板块的碰撞增生, 使岩石圈不断增厚而最终发生拆层去根作用[75–77], 导致软流圈地幔急剧上升[20,70–80](图10b)。亏损的软流圈地幔上涌底侵并发生部分熔融, 在侵入地壳过程中受到地壳物质或早期俯冲带物质的混染[2,9,81], 形成了260～290 Ma的较亏损的且受到较大程度地壳物质混染的玄武岩[24–25]。(3)拆层岩石圈和俯冲下沉的洋壳物质下降到地幔一定深度, 开始发生变质重熔作用, 形成富集碱质、Nb-Ta、大离子亲石元素(LILE)和轻稀土元素(LREE)的熔/流体。这些富集熔/流体在上升过程中, 与原亏损地幔和岩石圈地幔不断地交代平衡, 逐步改造为较富碱、富含不相容元素的亏损程度相对较低的地幔。继而沿着断裂带或薄弱带快速上涌喷出地表, 形成了中新生代亏损程度较低的、源区深度较深的且仅受到较小程度地壳混染的玄武岩(图10c)。

图9 晚古生代和中新生代火山岩的Hf-Th-Ta (a) (据文献[64])和Nb-Zr-Y (b) (据文献[65])构造判别图Fig.9 Tectonic discrimination diagrams of Hf-Th-Ta (a) (after reference [64]) and Nb-Zr-Y (b) (after reference [65]) for late Paleozoic and Mesozoic-Cenozoic volcanic rocks图9a中: T–岛弧拉斑玄武岩; CAB–钙碱性玄武岩; N-MORB–洋中脊玄武岩; E-MORB+WPT–富集的洋中脊玄武岩和板内拉斑玄武岩; WPA–板内碱性玄武岩。图 9b中: WPA–板内碱性玄武岩; WPA+WPT–板内碱性玄武岩和板内拉斑玄武岩; E-MORB–富集的洋中脊玄武岩;N-MORB+CAB–洋中脊玄武岩和钙碱性玄武岩; WPT+CAB–板内拉斑玄武岩和钙碱性玄武岩。基性火山岩数据来源与图5相同。In Fig.9a, IAT–Island-Arc Tholeiites, CAB–Calc-Alkaline Basalts, N-MORB–N-type Mid-Ocean Ridge Basalts, E-MORB+WPT–E-type Mid-Ocean Ridge Basalts+Within-Plate Tholeiites, WPA–within-plate alkaline basalt; In Fig.9b, WPA–within-plate alkaline basalt, WPA+WPT–within-plate alkaline basalt+Within-Plate Tholeiites, E-MORB–E-type Mid-Ocean Ridge Basalts, N-MORB+CAB–N-type Mid-Ocean Ridge Basalts+Calc-Alkaline Basalts, WPT+CAB–Within-Plate Tholeiites+Calc-Alkaline Basalts; The data source of basic rocks are identical with those of Fig.5.

6 主要认识

(1)克拉玛依侏罗纪玄武岩中获得锆石的 U-Pb谐和年龄(357.3±5.1) Ma代表了围岩太勒古拉组地层时代, Hf同位素表明太勒古拉组火山岩可能来自亏损地幔。

(2)侏罗纪玄武岩源区具有与OIB型源区相似但较为亏损的源区特征。该玄武岩可能来源于＞80 km处的石榴子石二辉橄榄岩稳定存在的地幔源区的低程度部分熔融。克拉玛依玄武岩仅受到很小程度的地壳混染, 橄榄石和单斜辉石的结晶分异具较明显的控制作用。

(3)与晚古生代岛弧及后碰撞基性火山岩不同,侏罗纪玄武岩形成于相对稳定的板内构造环境, 其地幔源区亏损程度不及晚古生代火山岩源区, 可能与俯冲下沉洋壳和拆层岩石圈在一定深度发生变质重熔形成富集熔体并不断与地幔发生交代富集作用有关。

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