青海省天峻县阳康地区花岗岩岩体锆石U-Pb年代学及地球化学特征研究
Geochronology and geochemistry of the granodiorite intrusion in Yangkang area of Qinghai Province and its geological significance
责任编辑: 蒋实
收稿日期: 2020-04-19 修回日期: 2020-12-18
基金资助: |
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Received: 2020-04-19 Revised: 2020-12-18
作者简介 About authors
方永坤(1986-),男,硕士,工程师,矿床地球化学方向。Email:
对南祁连阳康地区花岗闪长岩岩体进行了锆石U-Pb年代学及岩石地球化学研究,以便对其岩石成因和中—南祁连消减拼合作用的开始时间给予制约。花岗闪长岩中LA-ICP-MS锆石U-Pb定年结果表明,该岩体形成于晚奥陶世(444±1.2 Ma, MSWD=1.14),属于高钾钙碱性系列,A/CNK值介于1.007~1.104,属弱过铝质,为I型花岗岩,并且明显富集大离子亲石元素(如K、Ba、Rb)、轻稀土元素(LREE)及Th,相对亏损高场强元素(如Ta、Nb)。上述结果表明,该类岩体的原始岩浆应起源于地壳的部分熔融。综合区域同时代火成岩的研究成果,认为花岗闪长岩岩体形成于活动大陆边缘的构造背景。
关键词:
The formation age and petrogenesis of the Yangkang granites in the Qilian block remain controversial. In this study, LA-ICP-MS zircon U-Pb dating and whole-rock geochemical analysis were conducted to constrain the petrogenetic and tectonic processes during the formation of the granites. The results show that U-Pb age of granodiorite in Yangkang pluton is 444±1.2Ma, (MSWD=1.14), indicating that it was formed in the early Late Ordovician and that the rocks belong to weak peraluminous-strong peraluminous I-type granite.The rocks are enriched with large ion lithophile elements (K,Ba, Rb) and Light Rare Earth Elements (LREEs) and Th, but depleted in the high field strength elements (e.g., Ta-Nb). The above results show that Yangkang granites were derived from partial melting of continental crust. Combined with the regional data, the authors suggest that the Yangkang granites were probably formed in an active continental margin during Late Ordovician.
Keywords:
本文引用格式
方永坤, 曹成刚, 董峻麟, 李领贵.
FANG Yong-Kun, Cao Cheng-Gang, DONG Jun-Lin, LI Ling-Gui.
0 引言
祁连造山带具有极为复杂的大地构造演化历史,是近年来热点研究区之一。该造山带又分为北祁连、中祁连及南祁连3个次级造山带,3者拥有共同的前寒武纪结晶基底[1],其构造演化普遍认为是随着中祁连及北祁连发生大陆裂解,形成北祁连洋盆后,中南祁连地块发生裂解分离,逐渐发育为南祁连洋盆。加里东期末期北祁连洋盆及南祁连洋盆均发生俯冲消减,陆块完成拼合[2]。其中,中祁连与南祁连俯冲拼合时限与机制存在一定争议,其原因主要集中在对该构造带内断续出露的寒武—奥陶系基性—中性火山岩、超基性岩及相关沉积组合形成构造环境认识的差异:如大陆裂谷[3,4,5]、地幔柱[6]、外来块体[7]及沟弧体系[8]等,从而限制了对祁连造山带构造演化的认识。近年来,通过对祁连造山带岩浆岩[9,10]、变质岩、河流碎屑沉积物[11,12,13,14]以及区域构造等的研究,前人对沿碰撞带分布的微陆块在汇聚碰撞过程中的构造过程以及与地壳相关的构造—岩浆历史取得了瞩目的成果。由于南祁连早古生代岩浆活动研究数据较为分散,晚古生代盖层大面积出露等因素导致一些基本问题:构造属性,岩浆—构造历史以及相关的地壳形成,以及南祁连构造带的演化仍在争论中。因此笔者选择阳康地区花岗闪长岩岩体进行LA-ICP-MS锆石U-Pb年代学研究和岩石地球化学分析,探讨花岗闪长岩成因类型和构造环境,为进一步制约南祁连的构造演化提供依据。
1 地质背景
祁连造山带主要分为北祁连、中祁连和南祁连3个构造带(图1)。北祁连构造带发育完整的沟—弧—盆体系岩石组合,包括增生楔、洋岛/海山、蛇绿岩、HP/LT变质带、岛弧和弧前/弧后盆地等地质单元[15,16,17]。中祁连构造带由前寒武纪花岗片麻岩、斜长角闪岩和变沉积岩[4,18]以及叠加于其上的早古生代俯冲和碰撞相关的钙碱性侵入岩共同构成[20,21,22],在湟源县城附近主要发育湟源群(包括刘家台组、东岔沟组、磨石沟组和青石坡组)和花石山群[23]。南祁连构造带岩石组合类型时空差异较大,其中西段是由下古生界杂砂岩、板岩夹硅质岩及灰岩透镜体组成,东段是由前寒武纪黑云母斜长片麻岩、黑云母钾长片麻岩以及少量石榴子石斜长角闪岩和石英岩共同组成(即化隆岩群),同时发育大量与早古生代俯冲、碰撞作用密切相关的花岗岩和镁铁质—超镁铁质岩侵入体[24,25,26]。同时,南祁连构造带北缘断续发育一套由早古生代枕状玄武岩、辉长辉绿岩、超基性岩等构成的蛇绿岩带,自西向东断续出露于盐池湾大道尔吉、天峻县木里、刚察县纳任哇尔玛、湟中—化隆县拉脊山和兰州等地区。拉脊山蛇绿岩是该蛇绿岩带中规模最大的蛇绿岩,同时也是南祁连构造带的重要组成部分,南北两侧分别与中祁连湟源群和南祁连化隆岩群断层接触,主要由变质橄榄岩、超镁铁—镁铁质堆晶岩、辉长岩、辉绿岩、枕状玄武岩和深海硅质岩共同构成[8,27]。
图1
研究区位于南祁连中段,区内地层自元古宇—新生界均有不同程度分布,其中新元古界阳康片岩是测区最老的地层,其次为奥陶系盐池湾组,二叠系巴音河群主要有勒门沟组、草地沟组、哈吉尔组、忠什公组,下—中三叠统郡子河群、上三叠统默勒群出露范围较少,新近系咸水河组、第四系砂砾石多出露于地势较低的主河道两侧。区内岩浆侵入活动微弱,主要呈岩株状分布,主要有辉长岩、石英闪长岩和花岗闪长岩,还有少量的各类脉岩。岩浆侵入时代为主要为新元古代和奥陶纪晚期。断裂构造较发育,按走向大致可划分为NW、NE、SN和EW向断裂,从各组断裂的相互切割关系上看,NW向断裂是测区最早的断裂组,NE向断裂是测区最新的断裂组。区内褶皱构造发育不均,元古宇、奥陶系地层内较为发育,其次为二叠系地层,多为宽缓背向斜,主要以近EW向和NW向的宽缓褶皱为主。
花岗闪长岩岩体位于研究区西南,沿构造呈NW-SE向展布,长5 km,宽约2~3 km,内部发育有较多的变质砂岩包体。该岩体侵入新元古界阳康片岩、奥陶系盐池湾组中,被二叠系地层系不整合覆盖(采样位置见图1)。花岗闪长岩新鲜面呈灰白色,中细粒结构,块状构造,主要由角闪石、斜长石、微斜长石、黑云母及少量的石英组成,矿物大小不等。石英呈不规则状、他形粒状充填,约占20%;微斜长石约占7%~8%,呈不规则状、粒状,粒度为0.2~2.0 mm,具条纹结构和格子双晶,局部见有交代包裹斜长石、黑云母现象;斜长石呈自形板状,粒度0.25~1.55 mm,多具聚片双晶和环带构造,较强帘石化,局部见有强烈的绢云母化,轻微碳酸盐化,约占60%;黑云母呈片状,约占5%~6%,局部见有强烈的绿泥石化,并伴有铁质析出。
2 分析方法
野外挑选新鲜无蚀变的样品,样品编号为Pm04-4。将样品粉碎至200目,利用标准重矿物分离技术分选出锆石。经过双目镜下仔细挑选,将不同特征的锆石粘在双面胶上,并用无色透明的环氧树脂固定;待其固化之后,抛光露出锆石表面,对锆石进行反射光透射光和阴极发光图像采集(CL),在上述工作的基础上进行锆石LA-ICP-MS锆石U-Pb定年。
同位素测年样品在天津地质矿产研究所测试完成。本次测试采用激光单点剥蚀,数据采集采用调峰方式,锆石年龄采用国际标准锆石91500作为外标,标准玻璃NIST SRM610作为内标,校正锆石微量元素。分析方法见文献[31];分析结果采用Glitter 4.0进行处理计算。
野外挑选5件新鲜无蚀变样品(PM07-3、PM07-8、PM17-7、PM17-8、QC8112)进行全岩主微量元素分析,测试工作由自然资源部长春矿产资源监督检测中心承担完成,主量元素利用熔片X-射线荧光光谱法(XRF)测定,并采用等离子光谱和化学法测定进行相互检测。微量元素和稀土元素采用电感耦合等离子质谱法(ICP-MS)测定。主量元素分析精度和准确度优于5%,微量稀土元素分析精度和准确度优于10%。分析方法见文献[32]。
3 分析结果
3.1 LA-ICP-MS锆石测年
图2
图2
阳康地区花岗闪长岩锆石阴极发光图像
Fig.2
An image of granodiorite travertine in Yangkang area
表1 阳康地区花岗闪长岩同位素测年结果
Table 1
样品号 | 含量/10-6 | 同位素比值 | 年龄/Ma | ||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Pb | U | 206Pb/238U | 1σ | 207Pb/235U | 1σ | 207Pb/206U | 1σ | 208Pb/232U | 1σ | 232Pb/238U | 1σ | 206Pb/238U | 1σ | 207Pb/235U | 1σ | 207Pb/206U | 1σ | ||
1 | 63 | 638 | 0.0707 | 0.0004 | 0.5532 | 0.0097 | 0.0568 | 0.0010 | 0.0226 | 0.0004 | 1.6932 | 0.0068 | 440 | 3 | 447 | 8 | 482 | 38 | |
2 | 48 | 628 | 0.0707 | 0.0004 | 0.5410 | 0.0090 | 0.0555 | 0.0009 | 0.0235 | 0.0007 | 0.6166 | 0.0015 | 440 | 3 | 439 | 7 | 432 | 36 | |
3 | 78 | 920 | 0.0710 | 0.0004 | 0.5520 | 0.0077 | 0.0564 | 0.0008 | 0.0240 | 0.0010 | 0.9622 | 0.0009 | 442 | 2 | 446 | 6 | 468 | 31 | |
5 | 23 | 235 | 0.0721 | 0.0006 | 0.5671 | 0.0315 | 0.0571 | 0.0032 | 0.0277 | 0.0018 | 1.3363 | 0.0021 | 449 | 4 | 456 | 25 | 495 | 122 | |
7 | 21 | 247 | 0.0713 | 0.0005 | 0.5598 | 0.0333 | 0.0569 | 0.0034 | 0.0330 | 0.0028 | 0.6607 | 0.0042 | 444 | 3 | 451 | 27 | 489 | 132 | |
8 | 44 | 515 | 0.0706 | 0.0005 | 0.5488 | 0.0134 | 0.0564 | 0.0013 | 0.0355 | 0.0033 | 0.6807 | 0.0014 | 440 | 3 | 444 | 11 | 467 | 51 | |
9 | 27 | 315 | 0.0719 | 0.0004 | 0.5660 | 0.0186 | 0.0571 | 0.0019 | 0.0310 | 0.0029 | 0.7146 | 0.0032 | 448 | 3 | 455 | 15 | 494 | 72 | |
10 | 18 | 201 | 0.0720 | 0.0005 | 0.5630 | 0.0280 | 0.0567 | 0.0028 | 0.0290 | 0.0026 | 0.8715 | 0.0019 | 448 | 3 | 453 | 23 | 482 | 109 | |
11 | 14 | 167 | 0.0718 | 0.0005 | 0.5527 | 0.0353 | 0.0559 | 0.0035 | 0.0300 | 0.0025 | 0.6928 | 0.0006 | 447 | 3 | 447 | 29 | 447 | 140 | |
13 | 20 | 232 | 0.0719 | 0.0005 | 0.5566 | 0.0259 | 0.0561 | 0.0026 | 0.0234 | 0.0015 | 1.0709 | 0.0039 | 448 | 3 | 449 | 21 | 458 | 102 | |
15 | 21 | 258 | 0.0713 | 0.0004 | 0.5546 | 0.0192 | 0.0564 | 0.0020 | 0.0219 | 0.0011 | 0.8837 | 0.0045 | 444 | 3 | 448 | 16 | 470 | 77 | |
16 | 14 | 182 | 0.0716 | 0.0005 | 0.5564 | 0.0243 | 0.0564 | 0.0024 | 0.0201 | 0.0010 | 0.6374 | 0.0030 | 446 | 3 | 449 | 20 | 468 | 95 | |
17 | 37 | 534 | 0.0712 | 0.0004 | 0.5471 | 0.0108 | 0.0557 | 0.0011 | 0.0182 | 0.0009 | 0.3169 | 0.0017 | 443 | 3 | 443 | 9 | 442 | 43 | |
20 | 21 | 318 | 0.0718 | 0.0004 | 0.5666 | 0.0210 | 0.0572 | 0.0021 | 0.0201 | 0.0008 | 0.0628 | 0.0003 | 447 | 3 | 456 | 17 | 499 | 82 | |
23 | 15 | 184 | 0.0708 | 0.0004 | 0.5558 | 0.0207 | 0.0569 | 0.0021 | 0.0201 | 0.0008 | 0.8942 | 0.0039 | 441 | 3 | 449 | 17 | 489 | 81 | |
24 | 32 | 436 | 0.0717 | 0.0004 | 0.5524 | 0.0121 | 0.0559 | 0.0012 | 0.0180 | 0.0007 | 0.4893 | 0.0004 | 447 | 2 | 447 | 10 | 447 | 48 | |
26 | 17 | 211 | 0.0714 | 0.0004 | 0.5634 | 0.0191 | 0.0572 | 0.0019 | 0.0192 | 0.0009 | 0.8418 | 0.0031 | 445 | 3 | 454 | 15 | 499 | 73 | |
30 | 18 | 244 | 0.0712 | 0.0004 | 0.5579 | 0.0207 | 0.0568 | 0.0021 | 0.0205 | 0.0011 | 0.4750 | 0.0007 | 443 | 3 | 450 | 17 | 485 | 81 |
图3
图3
阳康地区花岗闪长岩的U-Pb年龄谐和图(a)和加权平均年龄(b)
Fig.3
Zircon U-Pb concordia diagram (a) and weighted average ages diagram (b) from the granodiorite
3.2 地球化学特征
3.2.1 主量元素
阳康地区花岗闪长岩分析结果及特征值见表2,其中SiO2含量为64.72%~69.05%,在TAS岩石分类图解中(图4),样品均落于亚碱性系列范围、花岗闪长岩区,与定名吻合。全碱含量介于6.23%~7.51%之间,Na2O和K2O分别介于2.37%~3.75%和3.51%~3.91%之间;w(K2O)/w(Na2O)值为0.94~1.63,除Pm17-7号样品外,均显示富钾特征。在w(SiO2)-w(K2O)图解中,样品全部落入高钾钙碱性系列区域(图5a),Al2O3含量较高且变化范围小,介于13.08%~16.87%间,铝饱和指数A/CNK值为1.09~1.15,碱总量(NK值)较高,DI值反映岩浆结晶分异程较低;里特曼指数σ值为1.5~2.6,属钙碱性系列。在ANK-A/CNK图解中,投影点均落于弱过铝质区域(图5b)。因此,阳康地区花岗闪长岩属弱过铝质、高钾钙碱性系列。
表2 花岗闪长岩主量元素、稀土元素和微量元素含量及有关参数
Table 2
指标 | PM07-3 | PM07-8 | PM017-7 | PM017-8 | QC8112 |
---|---|---|---|---|---|
SiO2 | 68.14 | 69.05 | 64.72 | 67.81 | 68.83 |
LOS | 0.69 | 0.9 | 2.02 | 1.82 | 1.33 |
FeO | 3.88 | 3.42 | 2.83 | 3.68 | 3.95 |
Fe2O3 | 0.54 | 0.61 | 0.71 | 1.06 | 0.85 |
TiO2 | 0.38 | 0.29 | 0.37 | 0.53 | 0.55 |
Al2O3 | 14.9 | 14.06 | 16.87 | 13.53 | 13.08 |
CaO | 2.52 | 1.91 | 2.92 | 2.29 | 2.58 |
MgO | 0.44 | 0.37 | 1.19 | 1.89 | 1.73 |
K2O | 3.91 | 3.9 | 3.51 | 3.81 | 3.86 |
Na2O | 3.6 | 3.38 | 3.75 | 2.47 | 2.37 |
MnO | 0.1 | 0.08 | 0.06 | 0.08 | 0.08 |
P2O5 | 0.07 | 0.07 | 0.1 | 0.15 | 0.15 |
Total | 99.17 | 99.15 | 99.05 | 99.12 | 99.36 |
ALK | 7.51 | 7.28 | 7.26 | 6.28 | 6.23 |
Na2O/K2O | 0.92 | 0.87 | 1.07 | 0.65 | 0.61 |
A/CNK | 1.007 | 1.059 | 1.104 | 1.094 | 1.024 |
Mg# | 61.75 | 54.58 | 76.85 | 77.93 | 80.13 |
La | 35.5 | 103 | 27.7 | 41.2 | 43.6 |
Ce | 84.6 | 187 | 50.8 | 81.2 | 86.5 |
Pr | 11.5 | 20.9 | 5.71 | 9.12 | 9.71 |
Nd | 47.1 | 72.7 | 20.5 | 32.8 | 34.1 |
Sm | 9.56 | 11.70 | 5.28 | 5.78 | 5.88 |
Eu | 1.95 | 2.29 | 0.96 | 0.94 | 0.82 |
Gd | 7.81 | 8.52 | 4.23 | 4.73 | 4.64 |
Tb | 1.37 | 1.42 | 0.47 | 0.8 | 0.79 |
Dy | 7.54 | 6.99 | 2.49 | 4.29 | 4.04 |
Ho | 1.49 | 1.34 | 0.5 | 0.83 | 0.79 |
Er | 4.13 | 3.68 | 1.39 | 2.29 | 2.19 |
Tm | 0.69 | 0.6 | 0.24 | 0.38 | 0.38 |
Yb | 4.24 | 3.85 | 1.51 | 2.42 | 2.42 |
Lu | 0.65 | 0.61 | 0.24 | 0.39 | 0.36 |
Y | 38.6 | 33.8 | 13.8 | 22.2 | 21.5 |
ΣREE | 256.70 | 458.40 | 133.06 | 209.37 | 217.72 |
LREE | 190.21 | 397.59 | 109.6 | 171.04 | 180.61 |
HREE | 27.92 | 27.01 | 9.66 | 16.13 | 15.61 |
w(LREE)/w(HREE) | 6.81 | 14.72 | 11.35 | 10.6 | 11.57 |
[w(La)/w(Yb)]N | 5.64 | 26.75 | 18.34 | 17.02 | 18.02 |
δEu | 0.67 | 0.67 | 0.98 | 0.53 | 0.46 |
δCe | 1.00 | 0.92 | 0.92 | 0.97 | 0.97 |
Li | 41.8 | 16.1 | 37.8 | 58.1 | 57.2 |
Ga | 21.6 | 19.5 | 20.7 | 17.6 | 17.6 |
Rb | 102 | 125 | 138 | 152 | 152 |
Sr | 357 | 222 | 354 | 218 | 225 |
Nb | 11.6 | 14.3 | 9.23 | 12.9 | 13.2 |
Ba | 1089 | 1277 | 815 | 813 | 890 |
Ta | 0.77 | 1 | 0.82 | 1.19 | 1.13 |
Th | 5.1 | 19.7 | 9.45 | 19.5 | 18.7 |
Zr | 270 | 496.3 | 157 | 194 | 184 |
Sn | 2.6 | 2.3 | 1.8 | 2.4 | 2.9 |
Se | 0.03 | 0 | 0.02 | 0.06 | 0.03 |
w(Nb)/w(Ta) | 15.06 | 14.30 | 11.26 | 10.84 | 11.68 |
w(Rb)/w(Sr) | 0.29 | 0.56 | 0.39 | 0.70 | 0.68 |
w(La)/w(Nb) | 3.06 | 7.2 | 3.0 | 3.19 | 3.3 |
注:主量元素单位为10-2,微量、稀土元素单位为10-6;ALK=K2O+Na2O,A/CNK=Al2O3/(CaO+Na2O+K2O),分子比;Na2O/K2O为含量比;Mg#=100×(MgO/40.31)/(MgO/40.31+Fe2
图4
图5
图5
w(SiO2)-w(K2O)图解(a)和ANK-A/CNK图解(b)
Fig.5
Diagram of w(SiO2)-w(K2O)(a) and ANK-A/CNK(b)
3.2.2 稀土元素
岩石样品稀土总量ΣREE=133.06×10-6~458.40×10-6,平均260.67×10-6,其中轻稀土总量109.6×10-6~397.59×10-6,重稀土总量9.66×10-6~27.92×10-6,w(LREE)/w(HREE)为6.81~14.72,LREE相对富集,HREE相对亏损。[w(La)/w(Yb)]N为5.64~26.75,δEu值为0.46~0.98,具有负异常,揭示了斜长石经历了分离结晶作用。在球粒陨石标准化稀土配分模式图上,配分曲线显示出明显右倾(图6a),轻重稀土元素分馏明显。ΣREE高于上地壳值168.37[33],δCe介于0.92~1.0之间,平均0.96,接近原始地幔δCe值(δCe=1.00)[8]。
图6
图6
球粒陨石标准化稀土元素配分模式图解(a)和原始地幔标准化微量元素蛛网图(b)
Fig.6
Chondrite-normalized REE patterns (a) and primitive mantle-normalized spider diagram of trace elements (b)for the Yangkang granites
3.2.3 微量元素
花岗闪长岩原始地幔标准化微量元素蛛网图显示(图6b),微量元素的配分模式近似一致。微量元素含量与同中国花岗闪长岩元素丰度[34]相比,仅是Sr、Cr低于平均值,其他元素均高于平均值。其中w(K)/w(Rb)值为195.12~232.12,w(Rb)/w(Sr)值为0.76~0.90,均属刘英俊等[35]划分的同熔型花岗岩。相对于原始地幔,微量元素表现出了富集Ba、Rb等大离子亲石元素和活泼的不相容元素Th的特征,同时富集极不相容元素Zr,相对亏损Nb、Ta等高场强元素,且具有明显的Sr、Ti、P的负异常,类似中、上地壳微量元素蛛网图分布模式[36],并与正常弧花岗质岩石的蛛网图特征基本一致,反映岩石成因与岩浆弧环境相关。
4 讨论
4.1 岩体形成时代
笔者对花岗闪长岩岩体进行了LA-ICP-MS锆石U-Pb年代学分析,结果表明锆石均为岩浆成因,其边部年龄代表了岩体的结晶时代, 该年龄代表了锆石w(206Pb)/w(238U)年龄介于440~448 Ma,加权平均年龄为444±1.2 Ma,MSWD=1.14,说明了花岗闪长岩岩体的形成于晚奥陶世。该年龄与中祁连石包城花岗岩(435±4 Ma)[37]及南祁连北段刚查大寺组花岗闪长岩年龄(435±4 Ma)相近,与中祁连湟源新店黑云母二长花岗岩(446±1 Ma)[38]、南祁裕龙沟黑云角闪辉石岩(442±2 Ma)[39]及车路沟岩体(446±3 Ma)[40]年龄相一致,反映了中南祁连地区在晚奥陶世至早志留世存在一期岩浆事件[41]。
4.2 岩石成因类型及源区性质
花岗岩的成因类型,目前大多数地质学者最常用的分类方案是将其划分为Ⅰ型、S型、A型及M型[42]。暗色矿物(角闪石、堇青石、碱性暗色矿物)是区分Ⅰ型、S型及A型的矿物学标志,阳康地区花岗闪长岩具有角闪石及黑云母等暗色矿物,矿物学角度显示其Ⅰ型花岗岩特征。研究表明[43],铝饱和指数(A/CNK)是区分Ⅰ型、S型花岗岩重要的地球化学参数,Ⅰ型A/CNK值<1.1,S型A/CNK值>1.1,阳康地区花岗岩岩体A/CNK值为1.007~1.104,除Pm17-7外,其余4个样品的A/CNK值均小于1.1,样品全部落入弱过铝质范围,显示Ⅰ型花岗岩成因类型特征。此外,磷灰石的含量作为一重要指标,用于区分Ⅰ型及S型花岗岩[44,45,46,47],阳康地区花岗岩P2O5含量0.07~0.15,明显低于S型花岗岩所具P2O5>0.2的特征[48],w(FeO*)/w(MgO)比值(FeO*代表氧化铁及氧化亚铁总量,下同)为2.45~9.92,不具A型花岗岩w(FeO*)/w(MgO)>10[49]的富铁特征。综上,阳康地区花岗岩为Ⅰ型花岗岩。
阳康地区花岗岩具有高硅、高碱、贫镁、贫铁的特征,富集LREE、LILE元素(K、Ba、Rb)和活泼的不相容元素Th,亏损HFSE元素(Ta、Nb、Ti),显示壳源的地球化学属性。其中w(Nb)/w(Ta)值为 10.84~15.06,平均12.60,接近上地壳值(w(Nb)/w(Ta)=10);w(Rb)/w(Sr)为0.29~0.70,平均 0.53,接近大陆地壳值(w(Rb)/w(Sr)=0.32)。样品中Zr元素的富集,也暗示源区的壳源性质。在w(La)/w(Sm)-w(La)图解中(图7),w(La)/w(Sm)比值与La值呈正相关关系,说明岩浆侵位过程中,以部分熔融作用为主,并伴随一定程度的分离结晶,但结晶分异作用相对较弱。样品中Sr、Eu的亏损,说明岩浆源区有残留的斜长石,P、Ti的亏损显示源区有残留的磷灰石、钛铁矿等物质。
图7
综上所述,本次花岗闪长岩地球化学特征显示为壳源物质部分熔融的产物。
4.3 构造环境
花岗岩类型不仅可以指示动力学环境,而且也能作为动力学演变过程的示踪物,指示动力学的演变过程[50]。花岗闪长岩作为高钾钙碱性岩类,它的出现指示了一种构造体系的变化,其岩石地球化学特征很好地映证了这一点。阳康地区花岗闪长岩以富Si、Al、K,贫Mg、Fe、Ti,富集LILE和LREE,亏损HFSE和HREE为特征,显示岛弧和大陆边缘花岗岩特点。在构造环境判别的w(Nb)-w(Y)图中(图8a),样品落入火山弧和同碰撞花岗岩范围;w(Rb)-w(Y)+w(Nb)图解中图中(图8b),样品落入火山弧花岗岩区域,显示火山弧和同碰撞的环境。综合岩石为高钾钙碱性岩石的特点,且并未出现低钾系列,以上分析,表明阳康地区花岗岩总体显示活动大陆边缘的构造背景[51]。同期的岩浆事件在南祁连构造带广泛发育,如柴达木盆地北缘都兰地区旺尕秀岛弧型辉长岩(468~522 Ma)[52]、440~470 Ma洋内弧火山岩[53]以及445~514 Ma滩间山群弧火山—沉积序列。结合柴达木北西段的敖包山—吕梁山地区的钙碱性I型花岗岩(445~496 Ma)[54]以及党河南山地区奥陶纪的大道尔吉俯冲带型蛇绿岩[55]等岩浆记录都反映了该时期南祁连洋壳的北向俯冲,暗示该时期区域上应为活动大陆边缘构造环境。
图8
图8
w(Nb)-w(Y)图解(a)和w(Rb)-[w(Y)+w(Nb)]图解(b)
Fig.8
Diagram of w(Nb)-w(Y)(a) and w(Rb)-[w(Y)+w(Nb)](b)
5 结论
1) 阳康地区花岗闪长岩中锆石LA-ICP-MS锆石U-Pb年龄介于440~448 Ma,加权平均年龄为444±1.2 Ma(MSWD=1.14),该年龄为代表了岩体的结晶年龄,属奥陶纪晚期。
2) 阳康地区花岗闪长岩以高硅、高碱、贫镁、贫铁的特征,富集LREE元素、LILE元素(K、Ba、Rb)和活泼的不相容元素Th,亏损HREE和HFSE元素(Ta、Nb、Ti),具Sr、Eu的负异常,亏损P、Ti,具有Ⅰ型花岗岩的地球化学属性,其为Ⅰ型花岗岩。
3) 阳康地区花岗闪长岩是地壳部分熔融的产物,形成于活动的大陆边缘构造环境。
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