北祁连川刺沟A型花岗岩的年代学及其意义

引用本文

王建, 谢亘, 施光海, 王妍. 2018. 北祁连川刺沟A型花岗岩的年代学及其意义. 岩石学报, 34(6): 1657-1668

Wang J, Xie G, Shi GH and Wang Y. 2018. Geochronology of the Chuancigou A-type granite in the North Qilian belt and its significances. Acta Petrologica Sinica, 34(6): 1657-1668(in Chinese with English abstract)

北祁连川刺沟A型花岗岩的年代学及其意义

王建¹ , 谢亘^1,2 , 施光海¹ , 王妍¹

1. 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;
2. 武警黄金部队第十一支队, 拉萨 850000

2017-05-05 收稿; 2018-02-01 改回.

基金项目: 本文受地质过程与矿产资源国家重点实验室和武装警察部队黄金指挥部研究项目（201404020604）联合资助

第一作者简介: 王建, 男, 1990年生, 博士生, 矿物学、岩石学、矿床学专业, E-mail:wangjian330921@163.com

通讯作者: 施光海, 男, 1968年生, 教授, 主要从事矿物学、岩石学、矿床学和宝玉石矿床学方面的研究, E-mail:shigh@cugb.edu.cn

摘要：关于北祁连南蛇绿岩带蛇绿岩，前人多认为是形成于早古生代洋中脊的大洋岩石圈残片。本次对川刺沟A型花岗岩和其周围超基性岩的研究，有助于较全面认识该区时代序列与构造格架。川刺沟A型花岗岩的组成矿物为碱性长石（~45%）、石英（~30%）、斜长石（~15%）以及少量的霓辉石（~5%）等，发育条纹和显微文象等结构。岩石具有高SiO₂（72.58%~73.96%）和高Na₂O+K₂O（10.12%~10.39%），A/NK < 1，10000Ga/Al值为2.59~3.49。微量元素型式显示该花岗岩富集Rb、Ba、Th、K，强烈亏损Sr、P、Ti、Nb、Ta，稀土元素型式呈明显负铕异常（δEu=0.56~0.60）的海鸥形。岩相学和地球化学特点均符合A型花岗岩特征，进一步细分可将其归属于A₂型，形成于后造山环境。其周围的超基性岩稀土型式呈"U"型，富集Th、Zr、Hf，推测其经历过俯冲熔/流体交代。SHRIMP锆石U-Pb定年给出一组该A型花岗岩的谐和年龄501±5Ma，可以代表其侵位年龄。该年龄早于北祁连造山带陆-陆碰撞时间，晚于大陆裂谷发生时间。由于A₂型花岗岩后造山环境有两亚类：陆-陆碰撞后造山和活动大陆边缘拉张环境，本次研究结果更加倾向后者，即，该A型花岗岩为在中-晚寒武世俯冲作用影响下，活动大陆边缘弧前发生拉张，软流圈部分熔融产生幔源岩浆底侵形成。进一步推测该地区超基性岩可能与俯冲有关，为非经典定义的蛇绿岩。

关键词: 北祁连早古生代 A型花岗岩 SHRIMP锆石测年活动大陆边缘蛇绿岩

Geochronology of the Chuancigou A-type granite in the North Qilian belt and its significances

WANG Jian¹, XIE Gen^1,2, SHI GuangHai¹, WANG Yan¹

1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;
2. No. 11 Gold Geological Party of the Chinese Armed Police Force, Lhasa 850000, China

Abstract: Some geologists believe that South Ophiolite Belt in North Qilian is a fragment of oceanic lithosphere formed in ocean ridge in Early Paleozoic. Characteristics of an A-type granite and a peridotite in the belt will help to understand the time sequence and structural framework of this area. This A-type granite is mainly composed of alkali feldspar (~45%), quartz (~30%), plagioclase (~15%), and aegirine augite (~5%) and other accessory minerals as well, with perthitic and micrographic textures. The granite is high in SiO₂ (72.58%~73.96%), Na₂O+K₂O (10.12%~10.39%) and Ga/Al (10000Ga/Al=2.59~3.49). Its A/NK ratios are below 1.0, which means the granite is peralkaline. Normalized rare earth element and trace element patterns show that this pluton has seagull-shaped REE patterns with obvious negative Eu anomaly (δEu=0.56~0.60), enriched in Rb, Ba, Th, K and depleted in Sr, P, Ti, Nb, Ta. Petrological and geochemical characteristics suggest that this granite is an A-type one, which is subdivided into A₂-type, i.e., formed in post-orogenic environment. U-shaped REE patterns and the enrichment of Th, Zr, Hf of the Chuancigou peridotite illustrate a source of fossil mantle wedge modified by melts or fluids released from subduction slab. SHRIMP U-Pb analysis on zircon of the granite yields a weighted mean age of 501±5Ma, interpreted as the emplacement age, which is earlier than the event of continent-continent collision and later than continent rifting of the North Qilian orogenic belt. As there are two sub-types for A₂ granite:post-collision and active continental margin, for the Chuancigou A₂ granite, the forming environment should be active continental margin. In the Middle-Late Cambrian fore-arc of active continental margin of North Qilian, it is inferred that asthenosphere melted due to the extension caused by subduction and the magma underplated to form the Chuancigou A-type granite, and the formation of the ultramafic rock in the studied area is estimated to be related to subduction zone and it is not a classically defined ophiolite.

Key words: North Qilian Early Paleozoic A-type granite SHRIMP zircon dating active continental margin ophiolite

A型花岗岩最初定义为碱性(alkaline)、无水(anhydrous)和非造山(anorogenic)的花岗岩(Loiselle and Wones, 1979)。随着时间推移，A型花岗岩的定义与理解已不仅限于以上的3A特点(Bonin, 2007)，如有过铝质的A型花岗岩，偶尔出现并非无水的A型花岗岩，以及形成于后造山环境而不是非造山环境的A型花岗岩等。从化学成分上，A型花岗岩可为过碱质、准铝质和弱过铝质，具有较高的Ga/Al比值和富集HFSE(Zr、Nb、Ce、Y)，这些微量元素特点是判别A型花岗岩的重要地球化学标志(Whalen et al., 1987)。过碱质A型花岗岩中通常会出现如霓石-霓辉石、钠闪石-钠铁闪石、铁橄榄石等含碱性暗色矿物的矿物学标志(吴福元等, 2007)。另外，有别于铝质A型花岗岩，过碱质A型花岗岩被认为由幔源岩浆(可混染部分陆壳)分异形成(Bonin, 1990, 2007; Jahn et al., 2009; Shellnutt et al., 2009; Dostal et al., 2014)。由于除了非造山环境外，还可形成于后造山环境，据构造环境进而将A型花岗岩划分出非造山型和后造山型。相对于非造山型的产生于大陆裂谷和与地幔柱有关的板内非造山环境，后造山型的则产生于陆-陆碰撞后或与俯冲作用有关的环境(Brown et al., 1984; Sylvester, 1989; Eby, 1990, 1992; Hong et al., 1996; Whalen et al., 1996; Förster et al., 1997; Barbarin, 1999; Shi et al., 2004; 张家菁等, 2009)。这种分类相当于Eby划分的A₁和A₂亚类，以及Hong划分的AA和PA亚类(Eby, 1990, 1992; Hong et al., 1996)。然而，已报导的属于俯冲拉张环境下的A型花岗岩实例有限，主要有扬子板块西缘新元古代的大相岭A型花岗岩(Zhao et al., 2008)、华夏板块侏罗纪金鸡岭A型花岗岩和西山A型花岗岩(Jiang et al., 2009)以及土耳其东Pontides地区白垩纪A型花岗岩(Karsli et al., 2012)。

北祁连造山带由蛇绿岩、高压低温变质岩和弧岩浆岩组成(冯益民和何世平, 1995; 夏林圻等, 1996; 宋述光, 1997; 张旗等, 1997; 张招崇等, 1998; 钱青等, 2001; 史仁灯等, 2004; Tseng et al., 2007; 相振群等, 2007; Xia and Song, 2010; 武鹏等, 2012; 夏小洪等, 2012; Xia et al., 2012; Song et al., 2013; Chen et al., 2014)。在垂直于北西-南东的区域构造线方向上，自北往南将其划分为北蛇绿岩带、弧岩浆岩带和南蛇绿岩带，高级蓝片岩相俯冲杂岩带以岩片形式呈北西-南东向出露于弧岩浆岩带中(夏林圻等, 1996; Song et al., 2013)。在此岩带划分中，北蛇绿岩带为弧后盆地的SSZ型蛇绿岩(冯益民和何世平, 1995; 夏林圻等, 1996; 张旗等, 1997; 钱青等, 2001; Xia and Song, 2010)。前人认为古祁连洋板片早古生代向北俯冲(夏林圻等, 1996; Tseng et al., 2007; 夏小洪等, 2012; Song et al., 2013)，南蛇绿岩带形成于洋中脊环境，多被认为是该大洋岩石圈残片，主要依据是其中玄武岩或辉绿岩具有MORB特点(夏林圻等, 1996; 左国朝和吴汉泉, 1997; 史仁灯等, 2004; Tseng et al., 2007; 夏小洪等, 2012)。本次研究中以在北祁连南蛇绿岩带川刺沟地区发现的A型花岗岩以及其周围的超基性岩为研究对象，分析了相应的岩相学、地球化学及花岗岩锆石SHRIMP U-Pb年龄，以期能够更加清晰地认识北祁连南带形成的大地构造环境及演化。

1 地质背景及岩相学

川刺沟A型花岗岩位于祁连县野牛沟乡西15km处的川刺沟，研究区位置上位于北祁连造山带南蛇绿岩带的中段(图 1)。北祁连造山带位于青藏高原北缘，南侧为祁连地块，北侧为阿拉善地块(Song et al., 2013)，沿北西-南东向延伸约1000km，被阿尔金断裂在北西侧截断。研究区出露地层主要为下奥陶统阴沟群，由中基性火山岩、火山角砾岩、凝灰岩、砂岩、砾岩和板岩组成，地层受构造活动强烈改造。该区断裂构造发育，以北西-南东向断裂为主，规模较大、延伸远，为区域性托莱山北坡大断裂的组成部分。

图 1 北祁连造山带区域地质简图(据Song et al., 2013修改) Fig. 1 Simplified regional geological map of the North Qilian orogenic belt (modified after Song et al., 2013)

研究区内出露基性-超基性岩体，主要由蛇纹岩和辉长岩组成，多沿断裂分布，沿北西-南东构造线方向展布，向西一直延伸到红土沟。区内中酸性侵入岩出露较少，仅发现规模较小的花岗岩和正长岩被超基性岩包围。

川刺沟A型花岗岩出露于超基性岩分布区中，北西向延伸，长约1000m，露头不连续，较大露头约为10m×6m(图 2)。岩石新鲜面呈灰白色，块状构造。具条纹结构和显微文象结构(图 3a, b)。主要由碱性长石(~45%)、石英(~30%)、斜长石(~15%)和霓辉石(~5%)，以及副矿物锆石和榍石等组成(图 3c, d)。其中，碱性长石主要呈他形粒状，少量宽板状，大小一般0.2~2.0mm，个别达3.0mm，多由细脉状、网状的条纹长石组成。石英呈他形粒状，大小一般0.2~2.0mm。斜长石呈半自形板状，大小一般0.2~1.0mm，多具细而密的聚片双晶。根据⊥(010)晶带的最大消光角法测得N_p'∧(010)=14，斜长石牌号An=30，属于更长石。霓辉石多为半自形和他形，镜下呈浅绿色(图 3c)，其Na₂O含量为4.64%~6.57%，Fe₂O₃含量为10.28%~16.73%(表 1)。综合以上矿物组成及结构，以及霓辉石的出现，显示该花岗岩过碱，符合A型花岗岩特征(Eby, 1990; 吴福元等, 2007)。

图 2 川刺沟地区地质简图(据武警黄金第六支队, 2012修改^①) Fig. 2 Simplified geological map of Chuancigou

① 武警黄金第六支队. 2012.红土沟-川刺沟地区1:10000综合地质图

图 3 川刺沟A型花岗岩和超基性岩显微照片 (a)条纹结构；(b)显微文象结构；(c)花岗岩矿物组成(单偏光)；(d)花岗岩矿物组成(正交偏光)；(e)网脉状蛇纹石；(f)港湾状绢石. Afs-碱性长石；Qz-石英；Pl-斜长石；Aeg-霓辉石；Srp-蛇纹石；Bst-绢石；Spl-铬尖晶石；Mag-磁铁矿，缩写据Whitney and Evans, 2010 Fig. 3 Photomicrographs of the Chuancigou A-type granite and ultrabasic rock (a) perthite texture; (b) micrographic texture; (c) mineral assemblages of the Chuancigou A-type granite (plane polarizer); (d) mineral assemblages of the Chuancigou A-type granite (crossed polarizer); (e) mesh serpentine; (f) embayed bastite. abbreviation: Afs-alkali feldspar; Qz-quartz; Pl-plagioclase; Aeg-agerine augite; Srp-serpentine; Bst-bastite; Spl-spinel; Mag-magnetite, Abbr. after Whitney and Evans, 2010

表 1 川刺沟A型花岗岩中霓辉石电子探针数据(wt%) Table 1 Electron microprobe analysis of aegirine augite of the Chuancigou A-type granite (wt%)

测点号	HCT-25	HCT-26	HCT-27
SiO₂	51.10	52.76	52.12
TiO₂	0.08	0.11	0.04
Al₂O₃	0.76	0.64	0.63
Fe₂O₃	16.73	10.28	14.94
Cr₂O₃	0.02	0.00	0.01
MnO	0.07	0.08	0.03
MgO	5.36	7.51	5.33
FeO	5.50	7.53	7.00
NiO	0.01	0.00	0.02
CaO	12.13	16.01	12.62
Na₂O	6.57	4.64	6.38
K₂O	0.04	0.06	0.04
P₂O₅	0.04	0.02	0.02
Total	98.4	99.6	99.2
O		6
Si	1.982	2.007	2.006
Ti	0.002	0.003	0.001
Al	0.035	0.029	0.029
Fe³⁺	0.488	0.294	0.433
Cr	0.001	0.000	0.000
Mn	0.002	0.003	0.001
Mg	0.310	0.426	0.306
Fe²⁺	0.178	0.240	0.225
Ni	0.000	0.000	0.001
Ca	0.504	0.653	0.520
Na	0.494	0.342	0.476
K	0.002	0.003	0.002
P	0.001	0.001	0.001
Sum	4.000	4.000	4.000
注：依据电价平衡原理计算得到Fe₂O₃(利用中国科学院地质与地球物理研究所电子探针实验室JXA-8100电子探针进行成分分析，加速电压15kV，束流20nA，束斑直径5μm)

表 1 川刺沟A型花岗岩中霓辉石电子探针数据(wt%) Table 1 Electron microprobe analysis of aegirine augite of the Chuancigou A-type granite (wt%)

花岗岩周围为呈墨绿色超基性岩，蛇纹石化显著，组成矿物为蛇纹石(~90%)，铬尖晶石(~6%)和磁铁矿(~4%)等。蛇纹石有两类，一类(~50%)呈网脉状(图 3e)，具有橄榄石假象，颗粒边界清晰，局部具镶嵌等粒结构；另一类(~40%)为绢石，仍保留斜方辉石形态，多呈港湾状(图 3f)。铬尖晶石(~6%)颗粒内部暗红色，多呈不规则状和港湾状。磁铁矿(~4%)主要沿颗粒间隙分布，少量呈星点状。

2 地球化学

岩石地球化学成分分析在河北省区域地质矿产调查研究所完成。测试前将岩石样品研磨至200目，主量元素和Zr采用Axios^max X射线荧光光谱仪(XRF)测定。其中Fe²⁺采用滴定法测试(检测方法代号为GB/T 14506.14—2010)，H₂O⁺采用重量法测试(检测方法代号为GB/T 14506.2—2010)。微量元素采用X Serise 2等离子体质谱仪(ICP-MS)进行分析(检测方法代号为DZG20-1)。其中，主量元素分析精度和准确度优于5%，微量元素的分析精度和准确度优于10%。

岩石化学成分分析结果显示(表 2)川刺沟花岗岩具高硅(72.58%~73.96%)、富碱(Na₂O+K₂O=10.12%~10.39%)和高FeO^T/(FeO^T+MgO)的特点。岩石A/NK为0.97~0.99，岩石碱度率(AR)为6.2~6.6。花岗岩稀土配分型式(图 4a)呈海鸥形，轻稀土明显富集，轻重稀土分馏明显，(La/Yb)_N为10.4~16.8，负Eu异常(δEu=0.56~0.60)。微量元素标准化型式显示样品相对富集Rb、Ba、Th、K，强烈亏损Sr、P、Ti、Nb、Ta(图 4b)。10000Ga/Al对K₂O+Na₂O、FeO^T/MgO、Zr、Nb图解显示，除HC-1的10000Ga/Al值为2.59在靠近边界处I & S区外，其它点均落入A型花岗岩区域(图 5)。上述特征符合A型花岗岩标准(Loiselle and Wones, 1979; Eby, 1990; Frost et al., 2001)。

图 4 川刺沟A型花岗岩(HC)和超基性岩(HCP)球粒陨石标准化稀土元素配分图(a, 标准化值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(b, 标准化值据Sun and McDonough, 1989) Fig. 4 Chondrite-normalized REE diagram (a, normalization values after Boynton, 1984) and primitive mantle-normalized trace element diagram for the Chuancigou A-type granite (HC) and ultrabasic rock (HCP) (b, normalization values after Sun and McDonough, 1989)

图 5 川刺沟A型花岗岩10000Ga/Al对K₂O+Na₂O、FeO^T/MgO、Zr和Nb图解(底图据Whalen et al., 1987) Fig. 5 10000Ga/Al vs. K₂O+Na₂O, FeO^T/MgO, Zr and Nb diagrams of the Chuancigou A-type granite (after Whalen et al., 1987)

表 2 川刺沟A型花岗岩和超基性岩化学成分分析(主量元素:wt%；稀土和微量元素:×10^-6) Table 2 Chemical analysis of the Chuancigou A-type granite and ultrabasic rock (major elements: wt%; trace elements: ×10^-6)

样品号	HC-1	HC-2	HC-3	HC-4	HC-5	HCP-1	HCP-2
岩性	A型花岗岩					超基性岩
SiO₂	72.58	73.10	73.56	73.03	73.96	39.80	40.70
Al₂O₃	13.14	13.26	13.15	13.39	12.83	1.44	1.56
TiO₂	0.19	0.18	0.17	0.18	0.15	0.028	0.021
Fe₂O₃	1.35	1.34	1.26	1.27	1.23	5.40	4.60
FeO	0.36	0.14	0.12	0.12	0.22	2.66	2.95
CaO	0.98	0.93	0.91	0.97	0.91	0.20	0.47
MgO	0.33	0.16	0.19	0.18	0.18	36.54	35.54
K₂O	6.34	6.45	6.07	6.43	6.54	0.016	0.010
Na₂O	4.02	3.92	4.09	3.95	3.58	0.21	0.20
MnO	0.008	0.007	0.007	0.008	0.009	0.094	0.104
P₂O₅	0.024	0.019	0.018	0.017	0.016	0.008	0.008
H₂O⁺	0.34	0.16	0.13	0.15	0.17	12.72	12.91
LOI	0.55	0.38	0.33	0.32	0.27	13.19	13.47
Total	99.86	99.88	99.89	99.89	99.90	99.59	99.64
K₂O+Na₂O	10.36	10.37	10.16	10.39	10.12
A/NK	0.97	0.99	0.99	0.99	0.99
FeO^T/MgO	4.83	8.46	6.50	6.88	7.32
FeO^T/(FeO^T+MgO)	0.83	0.89	0.87	0.87	0.88
AR	6.53	6.44	6.22	6.22	6.59
La	55.20	58.94	51.82	47.77	46.91	0.20	0.16
Ce	91.99	93.24	85.85	79.57	75.70	0.37	0.32
Pr	8.35	7.83	7.44	7.12	6.53	0.042	0.040
Nd	23.04	20.70	20.15	19.38	17.39	0.16	0.15
Sm	2.92	2.49	2.47	2.47	2.15	0.051	0.037
Eu	0.56	0.50	0.48	0.49	0.42	0.027	0.022
Gd	3.20	2.64	2.54	2.47	2.26	0.070	0.051
Tb	0.46	0.39	0.38	0.38	0.32	0.014	0.008
Dy	2.80	2.16	2.21	2.22	1.92	0.099	0.088
Ho	0.64	0.49	0.51	0.50	0.43	0.021	0.021
Er	2.30	1.71	1.73	1.78	1.45	0.070	0.078
Tm	0.46	0.36	0.36	0.36	0.30	0.014	0.014
Yb	3.57	2.37	2.35	2.38	2.08	0.083	0.094
Lu	0.51	0.46	0.47	0.44	0.42	0.016	0.013
(La/Yb)_N	10.4	16.8	14.9	13.6	15.2	1.58	1.15
(La/Sm)_N	11.9	14.9	13.2	12.2	13.7	2.41	2.70
δEu	0.56	0.59	0.58	0.60	0.58	1.38	1.55
Rb	89.78	105.9	132.1	114.8	97.32	0.74	0.84
Ba	480.0	851.3	702.7	632.9	635.5	29.00	34.63
Th	5.08	7.32	6.71	5.73	5.91	0.35	0.18
U	0.67	1.10	0.98	0.91	0.85	0.037	0.035
Nb	12.85	17.56	17.18	18.44	13.89	0.15	0.11
Ta	0.60	0.60	0.64	1.34	0.49	0.032	0.013
Pb	2.99	3.50	2.52	2.72	2.73	1.08	1.30
Sr	36.60	41.97	45.85	33.43	34.82	6.23	15.07
Zr	256.1	236.0	220.3	220.0	214.8	23.68	22.89
Hf	7.59	7.48	7.11	7.23	6.62	0.67	0.49
Y	19.62	17.60	18.53	18.87	16.02	0.65	0.58
Ga	18.00	24.50	23.27	23.34	21.46	1.18	1.68
10000Ga/Al	2.59	3.49	3.34	3.29	3.16

超基性岩具有较高的烧失量(13.19%~13.47%)，归一化后岩石具高MgO(41.25%~42.29%)、和较低的TiO₂(0.02%~0.03%)、Al₂O₃(1.67%~1.81%)和CaO(0.23%~0.55%)。FeO^T为(8.70%~8.22%)。CIPW标准化后的主要矿物组成为橄榄石(43%~47%)和斜方辉石(43%~46%)。根据IUGS(Streckeisen, 1976)可将其原岩恢复为方辉橄榄岩。

川刺沟超基性岩稀土元素标准化型式呈“U”型，LREE相对富集，(La/Yb)_N为1.15~1.58，(La/Sm)_N为2.41~2.70，δEu为1.38~1.55。微量元素型式显示，岩石富集大离子亲石元素Ba、Pb、Sr和高场强元素Th、Zr、Hf。Th、Zr、Hf和LREE的相对富集，暗示该岩石可能受到俯冲地壳熔/流体的交代和改造(Niu, 2004; Paulick et al., 2006; Zheng, 2012)。

3 SHRIMP锆石U-Pb定年

SHRIMP锆石定年样品为新鲜的A型花岗岩(样品号：HC-1，采样点位置：38°30′28″N、99°21′06″E)。锆石挑选在河北省区域地质矿产调查研究所采用破碎、淘洗和重液分离，然后进行电磁分离，最后在双目显微镜下手工挑选完成。锆石制靶后，在北京离子探针中心获得其阴极发光图像。锆石U-Pb同位素定年也在该中心用SHRIMP-Ⅱ离子探针仪分析。使用标样SL13(572Ma)标定所测样品的锆石U、Th、Pb含量，再用标样TEM(417Ma, Black et al., 2004)进行元素间的分馏校正，根据实测²⁰⁴Pb校正普通Pb，样品分析原理及流程详见文献Williams, 1998。测试过程中仪器质量分辨率约为5000，一次离子流束斑大小为25μm，每分析3~4个样品点进行一次TEM标样测定。实验数据处理以及谐和图的绘制采用SQUID和ISOPLOT软件(Ludwig, 2000, 2001)。测试结果列于表 3，其中单个数据误差为1σ，加权平均年龄计算时采用²⁰⁶Pb/²³⁸U年龄值，误差为2σ。

表 3 川刺沟A型花岗岩SHRIMP锆石U-Pb分析结果 Table 3 SHRIMP U-Pb analysis on zircons of the Chuancigou A-type granite

测点号	U	Th	Th/U	²⁰⁶Pb_c	²⁰⁶Pb^*	²³⁸U/²⁰⁶Pb	±%	²⁰⁷Pb/²⁰⁶Pb	±%	年龄(Ma)
测点号	(×10^-6)		Th/U	(%)	(×10^-6)	²³⁸U/²⁰⁶Pb	±%	²⁰⁷Pb/²⁰⁶Pb	±%	²⁰⁶Pb/²³⁸U	²⁰⁷Pb/²⁰⁶Pb	²⁰⁸Pb/²³²Th
01	504	528	1.05	0.20	34.7	12.47	2.0	0.05763	1.5	496.4±9.4	452±48	469±11
02	405	587	1.45	0.37	28.1	12.39	1.6	0.05828	1.6	498.6±7.6	422±61	505.6±9.7
03	747	663	0.89	0.34	52.0	12.34	1.6	0.05857	1.3	500.5±7.5	445±51	433.6±9.2
04	393	493	1.26	0.47	27.0	12.51	1.7	0.05962	1.6	493.5±8.1	444±76	438±11
05	1272	2150	1.69	2.19	71.8	15.21	1.5	0.07639	0.86	401.8±6.0	558±73	320.4±7.0
06	639	319	0.50	0.22	44.7	12.29	1.5	0.05826	1.3	503.1±7.5	470±33	478.8±9.7
08	293	104	0.35	0.22	22.1	11.42	1.7	0.0588	1.9	539.9±8.6	493±46	547±15
09	187	118	0.63	0.18	13.2	12.16	1.7	0.0574	2.4	508.4±8.3	450±59	499±13
10	544	837	1.54	0.12	39.1	11.95	1.6	0.05793	1.3	517.4±7.9	489±39	496.1±8.8
11	151	95	0.63	0.22	11.4	11.43	2.1	0.0577	2.6	540±11	449±83	502±19
12	168	125	0.74	0.17	11.7	12.35	1.7	0.0609	2.4	501.1±8.3	587±72	431±13
13	948	1201	1.27	0.12	64.7	12.59	1.5	0.05699	1.1	492.0±7.2	452±25	470.8±9.8
14	326	193	0.59	0.00	23.3	12.01	1.6	0.0562	1.8	515.8±7.9	462±40	465±10
15	1114	1179	1.06	0.05	75.7	12.64	1.6	0.05693	0.99	490.6±7.7	474±23	473.2±8.3
16	456	158	0.35	—	33.4	11.72	1.6	0.05738	1.5	527.8±7.9	509±33	561±13
17	128	109	0.85	0.93	8.81	12.52	2.3	0.0667	2.7	491±11	573±130	548±22
18	151	127	0.84	—	11.2	11.57	1.8	0.0545	2.8	536.3±9.2	505±59	535±14
注：表中数据分析误差为1σ，Pb_c代表普通铅，Pb^*代表放射性成因铅；普通铅用实测²⁰⁴Pb校正

表 3 川刺沟A型花岗岩SHRIMP锆石U-Pb分析结果 Table 3 SHRIMP U-Pb analysis on zircons of the Chuancigou A-type granite

所分析锆石具有规则的韵律生长环带(图 6)，其Th/U介于0.35~1.69，为典型岩浆成因锆石。除点05、08、11、16、18外(或因远离主体年龄，或因U含量过高)，对其它所测17个点的数据进行分析计算，得到一组谐和加权平均年龄值501±5Ma。该值可看作该花岗岩的侵位年龄(图 7)。

图 6 川刺沟A型花岗岩锆石阴极发光图片及测点位号和年龄值 Fig. 6 CL image of zircons of the Chuancigou A-type granite, their analysis spot No. and U-Pb age

图 7 川刺沟A型花岗岩SHRIMP锆石U-Pb谐和图及加权平均年龄(未进行加权平均年龄计算的用灰线表示) Fig. 7 SHRIMP zircon concordia diagram and the weighted mean age of the Chuancigou A-type granite (the gray ones are excluded when calculating the weighted mean age)

4 讨论

川刺沟花岗岩属于A₂型。在Eby (1992)的Rb/Nb-Y/Nb图解中(图 8a)，多数样品点分布在A₂区内(图 8a)；在Nb-Y-Ce图解(图 8b)中样品点分布在A₁和A₂界限附近(图 8b)。Eby判别图解使用的数据非孤立的划分为两个区域，其数据是一个连续的系列：从A₂区域的后碰撞环境，到A₁-A₂边界处的后造山环境，再到A₁区域的非造山环境(Bonin, 2007)。在Hong et al. (1996)的判别图解(图 8c)中，样品点投在PA区域中。由于微量元素标准化型式中Ba未显示负异常，暗示该花岗岩为后造山碱性花岗岩(Pearce et al., 1984; Bonin, 1990)，与板内环境的A₁型有区别。陆-陆碰撞后或与俯冲作用有关的环境均可产生A₂型花岗岩。然前人研究中，形成于陆-陆碰撞后造山环境的A₂型花岗岩实例较多，与俯冲环境有关的却鲜有讨论。虽然如此，最近还是发现了与俯冲作用有关的A₂型花岗岩(Zhao et al., 2008; Jiang et al., 2009; Karsli et al., 2012)，这说明活动大陆边缘可能是A₂型花岗岩中常见的产出环境。

图 8 川刺沟A型花岗岩Rb/Nb-Y/Nb、Nb-Y-Ce和10000Ga/Al-R₁图解(底图据Eby, 1992；Hong et al., 1996) Fig. 8 Rb/Nb vs. Y/Nb, Nb-Y-Ce and R₁ vs. 10000Ga/Al diagrams of the Chuancigou A-type granite (after Eby, 1992; Hong et al., 1996)

川刺沟A型花岗岩形成于活动大陆边缘环境。Song et al. (2013)结合前人研究，认为北祁连海底扩张开始于~710Ma之前，于~440Ma出现陆-陆碰撞。Xu et al. (2015)认为熬油沟朱龙关群玄武岩形成于大陆裂谷环境，600~580Ma为北祁连裂谷作用时间。这些研究都认为北祁连大陆裂谷化发生在新元古代，陆-陆碰撞开始于~440Ma。川刺沟A型花岗岩形成于501±5Ma，由于该时间晚于北祁连裂谷作用时间，早于北祁连陆-陆碰撞之后的造山时间，故其形成与北祁连裂谷化作用和陆-陆碰撞后造山作用无直接的关联性。另外，由于川刺沟海相火山-沉积岩在奥陶系广泛发育，该A型花岗岩的陆-陆碰撞后造山环境可能性同样地被排除，和韩宝福等(2010)提出的后碰撞型“钉合岩体”不同。因此该A₂型花岗岩形成于活动大陆边缘环境而非陆-陆碰撞后造山环境。前人在北祁连该时期发现的俯冲作用证据进一步支持该结论。大岔大坂拉斑玄武岩-玻安岩体被认为是在517Ma古祁连洋开始发生俯冲时，地幔楔部分熔融所形成，且岩浆作用一直持续到487Ma(Xia et al., 2012)。在幔源岩浆底侵的影响下，陆壳发生熔融形成中酸性岩体，如规模较大的柴达诺花岗岩基，该岩基年龄(516Ma)与大岔大坂拉斑玄武岩的形成年龄一致(Chen et al., 2014)。其他相关研究中，认为北祁连弧岩浆作用一直持续到460Ma(Tseng et al., 2009; Wu et al., 2011; Song et al., 2013)。因此川刺沟A型花岗岩为活动大陆边缘拉张环境下形成。形成于大陆边缘深水盆地的与蛇绿岩相伴生的硅质岩可视为支持这一观点的沉积学证据(Du et al., 2007; 陆静云等, 2014)。形成于495±14Ma的川刺沟地区辉石玄武岩(夏林圻等, 1996)的幔源母岩浆可能为形成A型花岗岩提供了热源或物源。

川刺沟超基性岩亏损程度较高，“U”型稀土型式和Th、Zr、Hf明显的相对富集，说明该岩体可能受到俯冲板片熔/流体交代。值得注意的是，川刺沟超基性岩与玉石沟超基性岩可能具有相似的形成过程，两者稀土型式相似，都呈“U”型且同属南蛇绿岩带。但川刺沟超基性岩稀土含量较高，为相对较低程度的部分熔融所致。从矿物组成、全岩微量元素含量和包裹体特点可以看出玉石沟超基性岩可能为弧前位置地幔楔部分熔融后的残余，受到俯冲板片熔体的交代(Song et al., 2009)。

川刺沟A₂型花岗岩的确定，对于理解北祁连造山带，特别是北祁连南蛇绿岩带的构造演化具有重要意义。北祁连北蛇绿岩带为弧后盆地的SSZ型蛇绿岩(冯益民和何世平, 1995; 夏林圻等, 1996; 张旗等, 1997; 钱青等, 2001; Xia and and Song, 2010)。依据俯冲杂岩带、弧岩浆岩带和北蛇绿岩带共同组成沟-弧-盆体系，前人推测古祁连洋在早古生代发生向北俯冲，南蛇绿岩带中具MORB特点的玄武岩或辉绿岩可视为洋中脊环境中形成(夏林圻等, 1996; Tseng et al., 2007; 夏小洪等, 2012)。然而，具有MORB特点的玄武岩并不只形成于大洋中脊环境，在弧后盆地和弧前盆地环境中也可产生(Crawford et al., 1981; Reagan et al., 2010; Dilek and Furnes, 2011; Xia et al., 2012)。在北祁连南蛇绿岩带中发现的A型花岗岩和超基性岩体不是形成于大洋中脊，而与俯冲有关。南蛇绿岩带中的超基性岩可能并非经典定义的、代表大洋岩石圈残片的蛇绿岩。

5 结论

(1) 川刺沟花岗岩具有A型花岗岩特点，属于A₂亚型，形成于活动大陆边缘拉张环境。超基性岩为蛇纹石化方辉橄榄岩，可能形成于俯冲环境，并非经典定义的、代表大洋岩石圈残片的蛇绿岩。

(2) 川刺沟A型花岗岩的侵位年龄为501±5Ma。推测该A型花岗岩是在中-晚寒武世俯冲作用影响下，北祁连活动大陆边缘弧前拉张，导致软流圈上涌发生部分熔融，进而产生幔源岩浆发生底侵形成的。

致谢野外地质调查中得到了牛建、杨东秀和武警黄金第六支队官兵的帮助和支持；测试过程中得到了北京离子探针中心刘敦一，河北省区域地质矿产调查研究所李世平和中国科学院地质与地球物理研究所毛骞和马玉光等的帮助和支持；论文修改中得益于审稿人和编辑的建设性的修改意见；在此谨致衷心谢意！

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