怀安麻粒岩锆石U-Pb年代学及Hf同位素:华北北缘下地壳增生再造过程研究

引用本文

魏颖, 郑建平, 苏玉平, 马强. 2013. 怀安麻粒岩锆石U-Pb年代学及Hf同位素:华北北缘下地壳增生再造过程研究. 岩石学报, 29(7): 2281-2294 复制到剪切板

WEI Y, ZHENG JP, SU YP and MA Q. 2013. Zircon U-Pb ages and Hf isotopes of granulites from the Huai’an Complex: Implications for the accretion and reworking of the lower crust beneath the north margin of the North China Craton. Acta Petrologica Sinica, 29(7): 2281-2294(in Chinese with English abstract) 复制到剪切板

怀安麻粒岩锆石U-Pb年代学及Hf同位素:华北北缘下地壳增生再造过程研究

魏颖^1,2, 郑建平^2,3, 苏玉平³, 马强^1,2

1. 中国地质大学研究生院，武汉 430074;
2. 地质过程与矿产资源国家重点实验室，武汉 430074;
3. 中国地质大学地球科学学院，武汉 430074

2013-02-20 收稿; 2013-05-04 改回.

基金项目: 本文受973项目(2012CB416604)和国家自然科学基金项目(41130315、91014007、91214204)联合资助

第一作者简介: 魏颖，女，1990年生，博士生，岩石地球化学专业，E-mail: wyandwz@126.com

通讯作者: 郑建平，男，1964年生，教授，博士生导师，岩石学专业，E-mail: jpzheng@cug.edu.cn

摘要: 怀安杂岩体位于华北克拉通中北部，出露太古代-元古代的角闪岩相-麻粒岩相高级变质岩，记录了华北克拉通复杂的地壳形成和演化史。本文对其中的镁铁质麻粒岩和长英质麻粒岩进行了岩石学及锆石U-Pb年代学和Hf同位素组成研究，并在此基础上对其形成过程及所反映的华北北缘古老下地壳的增生再造过程进行讨论。镁铁质麻粒岩主要矿物组合为Cpx+Pl+Qz±Opx±Hb±Grt，粒状变晶结构，局部发育石榴子石的“白眼圈”现象，表明经历了高压麻粒岩相及后期近等温减压退变质作用过程。选择其中矿物组成为Cpx、Pl和少量Qz的单斜辉石麻粒岩进行锆石定年获得2461±20Ma的原岩形成年龄。这些岩浆锆石的ε_Hf(t)为+2.1~+9.7，最高值接近当时的亏损地幔值，其Hf亏损地幔模式年龄(t_DM)集中在2.6~2.7Ga之间，最小为2412Ma和2461Ma，接近原岩形成年龄。长英质麻粒岩主要组成矿物为Pl+Kfs+Cpx+Qz+Opx，粒状变晶结构。锆石测年获得2458±46Ma的原岩形成年龄。这些锆石ε_Hf(t)范围为+2.9~+5.3，锆石Hf两阶段模式年龄(t_crust)集中在2.8~2.7Ga之间。怀安麻粒岩的这些特征显示怀安地区在~2.5Ga发生亏损地幔的部分熔融，为单斜辉石麻粒岩提供了原岩物质，同时可能引起了先存(≥2.7Ga)地壳的重熔，即存在地壳再造，进一步为长英质麻粒岩提供了原岩物质。单斜辉石麻粒岩和长英质麻粒岩原岩都形成于~2.5Ga。

关键词: 麻粒岩锆石U-Pb 锆石Hf同位素下地壳演化怀安华北

Zircon U-Pb ages and Hf isotopes of granulites from the Huai’an Complex: Implications for the accretion and reworking of the lower crust beneath the north margin of the North China Craton

WEI Ying^1,2, ZHENG JianPing^2,3, SU YuPing³, MA Qiang^1,2

1. Graduate School，China University of Geosciences，Wuhan 430074，China;
2. State Key Laboratory of Geological Processes and Mineral Resources，Wuhan 430074，China;
3. Faculty of Earth Sciences，China University of Geosciences，Wuhan 430074，China

Abstract: The Huai’an Complex, an Archean to Proterozoic amphibolite to granulite facies high-grade terrane locates at the north-center of the North China Craton, recording complicated crustal evolution. This paper is about the generation and implication for reworking of the lower crust according to the researching on the petrography, zircon U-Pb geochronology and Hf isotopic characteristics for the mafic and felsic granulites from the Huai’an Complex. The mafic granulites are mosaic granular blastic structure with mineral assemblage of Cpx+Pl+Qz±Opx±Hb±Grt, local kelyphites of orthopyroxene+plagioclase around garnet suggest granulite facies and subsequent near-isothermal decompression metamorphism. The selective Cpx-granulite composites of Cpx+Pl+Qz. The concordant magmatic zircons from it yield the formation time of 2461±20Ma. The zircon ε_Hf(t) values range from +2.1~+9.7, the highest ε_Hf(t) value is near the contemporaneous depleted mantle and the depleted mantle Hf model age is in the range of 2.6Ga to 2.7Ga, two younger ages are 2412Ma and 2461Ma, similar to the formation time. The felsic granulite is mainly composed of Pl+Kfs+Cpx+Qz+Opx, showing granular blastic structure. The zircons analysis intimates the felsic granulite has formed at 2458Ma. The ε_Hf(t) of all analyses zircons changes from +2.9 to +5.3, two stage Hf model ages distribute between 2.8~2.7Ga. These characteristics demonstrate the partial melting of the depleted mantle in Huai’an at ~2.5Ga, which provided protolith material for the Cpx-granulite, moreover, setted off the remelting of the previous (≥2.7Ga) crust, means the crustal reworking accompanied by the accretion.

Key words: Granulite U-Pb geochronology Zircon Hf isotope Lower crustal evolution Huai’an North China Craton

1 引言

华北克拉通是世界上最古老的克拉通之一,其组分可以追溯到3.8Ga以前(Liu et al., 1992,2008; Song et al., 1996; Zheng et al., 2004a,2012; Wan et al., 2005,2012; 吴福元等,2008),并经历了复杂的演化过程(刘敦一等,1997; Wu et al., 2005; Kusky et al., 2007; Jiang et al., 2010; Zhai et al., 2010; Kusky,2011; Geng et al., 2012; Zhang et al., 2012a; Zhao and Zhai, 2013)。岩浆岩的全岩Nd同位素和锆石Hf同位素研究表明,2.8~2.6Ga可能是华北大陆地壳生长的重要时期(Zheng et al., 2004b,2009,2012; Wu et al., 2005; Geng et al., 2012; Zhang et al., 2013)。华北中部广泛分布的岩石,其时代主要集中在新太古代晚期-古元古代末,新太古代早期的岩石仅在少数地区以包体的形式存在(Guan et al., 2002; Kröner et al., 2005),或者在古元古代岩石中残留少量新太古代早期的继承锆石(刘敦一等,1997; Wilde et al., 1997,2002,2003)。这些新太古代-古元古代的岩石及其中的锆石Hf同位素组成暗示华北克拉通在~2.5Ga也经历了强烈的增生和再造事件(Wan et al., 2005,2012; 刘富等,2009; Diwu et al., 2011; Liu et al., 2011,2012; Zhang et al., 2012b)。翟明国(2011)和Kusky(2011)指出2.5Ga左右华北克拉通有广泛的麻粒岩相和角闪岩相变质作用；郭敬辉等(2002)、Guo et al.(2005)和Zhao et al.(2010)通过华北中北部麻粒岩锆石U-Pb年代学研究发现~1.85Ga的变质作用可能更为显著。经历了3.0~2.5Ga复杂的地壳形成与演化,2.45~1.9Ga华北进入构造相对宁静的时期(Du et al., 2013)。随着研究的深入,2.3~2.0Ga的岩浆事件越来越多的被报道(第五春荣等,2007; 郭素淑和李曙光,2009; 黄道袤等,2012),其地质意义日渐突显。麻粒岩是大陆下地壳的主要组成岩石,对华北中北部麻粒岩地质体的研究不仅可以反演华北太古代-古元古代的下地壳增生和再造历史,而且可能约束华北中部的变质时间。

位于华北中北部的怀安杂岩体,出露晚太古代至古元古代的角闪岩相-麻粒岩相高级变质岩,包括TTG片麻岩、(高压)镁铁质麻粒岩、片麻状花岗岩、紫苏花岗岩、S型花岗岩等(Zhao et al., 2006),翟明国和刘文军(2001)还根据蔓菁沟-瓦窑口-马市口-下白窑-尚义等地代表性岩石的研究建立了下地壳剖面结构。怀安杂岩体可视为探索华北大陆下地壳形成与演化的重要窗口之一,本文选择镁铁质麻粒岩及其中的条带状分布的长英质麻粒岩,展开岩石学、锆石U-Pb年代学及原位Hf同位素研究,以探讨华北北部深部地壳的早期演化过程。

2 地质背景

华北克拉通的构造划分存在一定的争议性,Zhao et al.(2001,2005)根据华北克拉通基底岩石特征将其划分为东部陆块、西部陆块和中部造山带三个主要构造单元。Zhai and Santosh(2011)根据古老花岗岩-绿岩带将华北克拉通划分为胶辽、迁怀、集宁、鄂尔多斯、阿拉善、许昌和徐淮7个不同的陆块。按照Zhai and Santosh(2011)的划分,怀安杂岩体位于集宁陆块中核与鄂尔多斯陆块交汇部位(图 1),也相当于中部带(Zhao et al., 2005)的北缘。

图 1 华北克拉通构造划分简图(据Zhai and Santosh, 2011) Fig. 1 Distribution of tectonic subdivision of the North China Craton(after Zhai and Santosh, 2011)

怀安杂岩体以南是恒山、五台、阜平等地块,北部以尚义-赤城大断裂为界与角闪岩相的红旗营子群变质表壳岩组合相接。怀安杂岩体西边出露典型的孔兹岩系,其岩石组合为石墨-含矽线石-石榴子石片麻岩、石榴子石石英岩、长英质副片麻岩、钙质硅酸盐和大理岩(Yin et al., 2009; Zhao et al., 2010)。孔兹岩系原岩形成时间为2.3~1.9Ga,2.0~1.9Ga发生变质作用。伴随孔兹岩系出露零星的紫苏花岗岩和S型花岗岩,形成于~1930Ma(Peng et al., 2012)。

怀安杂岩体的内部构造略呈穹窿状,穹窿的中心大致在该地体的中心瓦窑堡-新平堡一带(刘富等,2009),它主要由TTG片麻岩系列、(高压)镁铁质麻粒岩、片麻状花岗岩、紫苏花岗岩和S型花岗岩组成,局部含有孔兹岩带的变表壳岩成分(Zhao et al., 2006,2010)。本文研究样品取自怀安杂岩体北部的尚义地上村(N40°43′33.13″,E114°18′7.86″)和兴和马市口国道边(N40°43′57.32″,E114°08′19.52″,图 2),毗邻高压麻粒岩出露点。尚义地上的镁铁质麻粒岩露头较小,岩石组合多变,周围产出大量的肉红色钾质花岗岩。马市口的基性麻粒岩露头长约15m,宽3~4m,其中夹有长英质的麻粒岩条带。

图 2 怀安杂岩体地质简图(据Peng et al., 2012)及采样点 Fig. 2 Simplified geological map of Huai’an complexes in the North China Craton(after Peng et al., 2012),showing sample locations

3 岩相学特征 3.1 尚义地上地区麻粒岩

尚义地上地区麻粒岩呈块状构造,粒状变晶结构,主要组成矿物有单斜辉石(10%~55%)、斜长石(30%~40%)、石英(5%~10%)、斜方辉石(0~15%)和角闪石(0~50%),副矿物为石榴子石、磷灰石和锆石。根据辉石和角闪石含量,进一步可以分为二辉石麻粒岩(如样品DS-2、DS-3、DS-4)、单斜辉石麻粒岩(DS-5)和二辉角闪石麻粒岩(DS-9)。单斜辉石麻粒岩矿物组成为单斜辉石、斜长石和少量石英,不发育斜方辉石(图 3)。二辉石麻粒岩中石榴子石发育有斜长石(部分蚀变为绢云母)和斜方辉石组成的后成合晶(图 3),另有斜方辉石包裹单斜辉石(图 3c),指示二辉石麻粒岩可能经历了近等温降压的退变过程(Guo et al., 2002; 翟明国,2009)。二辉角闪石麻粒岩中角闪石环绕单斜辉石和斜方辉石生长,并将早期的石榴石和辉石隔开(图 3),反映了二辉石麻粒岩的进一步降温退变质(O’Brien and RöTzler,2003; 翟明国,2009)。

图 3 怀安麻粒岩镜下特征 (DS-2)二辉石麻粒岩：“白眼圈”结构；(DS-3,DS-4)二辉石麻粒岩：斜方辉石包裹单斜辉石；(DS-5)单斜辉石麻粒岩：单斜辉石+斜长石+石英；(DS-9)二辉角闪石麻粒岩：角闪石交代斜方辉石；(XH-1)长英质麻粒岩：单斜辉石+斜方辉石+条纹长石+斜长石+石英.Grt-石榴子石;Opx-斜方辉石;Cpx-单斜辉石;Pl-斜长石;Am-角闪石;Qz-石英;Kfs-钾长石 Fig. 3 Selected photographs showing Huai’an granulites

3.2 马市口地区麻粒岩

马市口地区基性麻粒岩呈块状构造,其中的长英质麻粒岩条带发育弱片麻理。基性麻粒岩为粒状变晶结构,主要矿物组成有单斜辉石(5%~60%)、斜方辉石(5%~25%)、斜长石(10%~40%)和角闪石(0~35%),次要矿物有石英(5%)、条纹长石(0~10%)和黑云母(0~10%),副矿物为磷灰石和锆石。根据暗色矿物含量可分为二辉石麻粒岩(如样品XH-2、XH-9)和二辉角闪斜长石麻粒岩(XH-5)。二辉石麻粒岩中含少量黑云母,不含碱性长石；二辉角闪斜长石麻粒岩中不含黑云母,含约10%条纹长石,单斜辉石和斜方辉石含量约为5%。长英质麻粒岩条带(XH-1)(图 3)为粒状变晶结构,矿物组成为斜长石(55%)、条纹长石(20%)、单斜辉石(10%)、石英(10%)和斜方辉石(5%),副矿物为磷灰石和锆石。

4 分析方法

选择尚义地上单斜辉石麻粒岩(DS-5)和马市口长英质麻粒岩条带(XH-1)进行锆石U-Pb年龄、微量元素分析及原位Hf同位素分析。这些分析都在中国地质大学(武汉)地质过程与矿产资源国家重点实验室完成。U-Pb定年利用LA-ICP-MS完成,激光剥蚀系统为GeoLas 2005,ICP-MS为Agilent 7500a。激光剥蚀过程中采用氦气作载气、氩气为补偿气以调节灵敏度。每个时间分辨分析数据包括大约20~30s的空白信号和50s的样品信号。对分析数据的离线处理采用软件ICPMSDataCal完成(Liu et al., 2008,2010a)。详细的仪器操作条件和数据处理方法同(Liu et al., 2008,2010a,b)。 U-Pb同位素定年中采用锆石标准91500作外标进行同位素分馏校正,每分析5个样品点,分析2次91500。锆石标准91500的U-Th-Pb同位素比值推荐值据Wiedenbeck et al.(1995)。锆石样品的U-Pb年龄谐和图绘制和年龄权重平均计算均采用Isoplot/Ex_ver3完成(Ludwig,2003)。锆石原位Hf同位素分析利用Neptune MC-ICP-MS和193nm激光取样系统完成。分析时激光束直径为64μm,激光剥蚀时间约为50s。仪器的运行条件及详细的分析过程参见文献Hu et al.(2012)。数据的离线处理采用ICPMSDataCal完成(Liu et al., 2010b)。

5 分析结果 5.1 锆石U-Pb年龄

DS-5和XH-1样品的锆石稀土元素组成见表 1,U-Pb年龄结果见表 2。

表 1 怀安麻粒岩锆石稀土元素组成(×10^-6) Table 1 LA-ICP-MS zircon rare earth elements results for the Huai’an granulites(×10^-6)

测点号	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	(Gd/Lu)_N	δEu
DS-5单斜辉石麻粒岩
DS-5-01	0.14	5.01	0.16	1.28	0.69	1.81	3.29	1.18	18.7	8.45	50.7	12.1	155	25.2	0.02	3.04
DS-5-02		2.40	0.01	0.12	0.17	0.15	1.26	0.49	8.49	3.78	23.2	5.87	80.5	14.4	0.01	0.72
DS-5-03		2.04	0.03	0.19	0.13	0.13	1.62	0.82	11.9	5.06	30.4	7.72	101	18.1	0.01	0.52
DS-5-04	0.22	4.97	0.18	0.89	0.88	0.78	3.61	1.46	20.7	9.18	55.4	13.3	172	28.4	0.02	1.15
DS-5-05	0.95	4.38	0.24	1.75	0.43	0.31	1.83	0.73	11.3	5.01	30.4	7.78	104	18.5	0.01	0.91
DS-5-06	0.56	7.95	0.53	3.88	1.19	1.79	3.35	1.04	14.6	5.65	32.6	7.60	101	18.4	0.02	2.57
DS-5-07		1.68	0.01	0.10	0.20	0.09	1.42	0.48	8.48	3.66	21.7	5.79	79.7	14.5	0.01	0.39
DS-5-08		1.88		0.19	0.20	0.11	2.28	0.86	14.3	6.55	38.4	9.37	130	22.1	0.01	0.31
DS-5-09		0.56	0.01	0.06	0.17	0.16	0.95	0.27	2.96	0.77	2.97	0.46	4.99	0.60	0.20	0.97
DS-5-10	1.72	9.15	0.76	3.03	0.62	0.56	2.10	0.75	11.7	4.86	28.5	7.28	94.5	17.6	0.01	1.36
DS-5-11		2.58	0.02	0.12	0.30	0.17	2.60	0.89	15.0	6.24	38.6	9.72	128	22.4	0.01	0.40
DS-5-12	0.06	3.29	0.10	0.75	0.59	0.54	2.10	0.74	10.8	4.29	25.9	6.40	86.8	15.5	0.02	1.33
DS-5-13	0.13	3.36	0.11	1.05	0.53	0.52	1.86	0.71	12.6	5.14	28.7	7.18	95.7	17.2	0.01	1.42
DS-5-14	0.31	3.56	0.12	0.61	0.44	0.21	2.20	0.87	14.4	6.35	38.4	9.50	132	22.7	0.01	0.54
DS-5-15		0.17		0.06	0.14	0.23	1.29	0.30	3.75	0.88	2.92	0.46	4.09	0.55	0.29	1.09
DS-5-16	0.15	3.79	0.15	1.09	0.60	1.15	2.63	0.90	12.2	5.18	32.8	8.47	122	25.1	0.01	2.38
DS-5-17	0.03	4.31	0.02	0.22	0.54	0.26	3.10	1.22	19.6	8.14	49.0	11.5	144	23.2	0.02	0.48
DS-5-18	0.01	3.33		0.09	0.40	0.23	2.38	0.68	12.5	5.55	32.0	7.82	97.3	16.4	0.02	0.57
DS-5-19	0.05	2.60	0.08	0.62	0.66	0.31	2.29	0.89	14.9	6.33	37.5	8.87	123	21.7	0.01	0.70
DS-5-20		2.62			0.23	0.21	1.43	0.68	13.0	4.95	32.1	7.43	94.1	16.5	0.01	0.85
DS-5-21		2.25		0.14	0.17	0.16	2.19	0.72	14.2	6.07	35.1	8.29	111	19.0	0.02	0.75
DS-5-22	0.18	5.08	0.24	1.64	0.65	1.62	2.59	0.78	11.2	4.73	27.9	6.81	94.5	16.5	0.02	0.73
DS-5-23	0.22	5.58	0.27	2.09	0.87	2.70	3.28	0.94	12.9	4.76	26.5	5.73	74.0	12.9	0.03	4.30
DS-5-24	1.77	18.5	1.62	11.0	4.53	5.27	8.58	2.02	24.0	8.07	43.6	10.0	132	23.5	0.05	2.55
DS-5-25	2.23	20.9	2.05	13.1	4.33	6.02	7.22	1.56	18.3	5.49	29.5	6.34	81.5	13.9	0.06	3.27
DS-5-26	0.66	5.89	0.39	2.75	1.25	1.48	3.94	1.24	18.5	7.58	43.8	10.6	148	24.9	0.02	1.87
DS-5-27		0.10			0.07	0.07	0.51	0.26	4.75	1.40	5.75	0.97	8.46	1.20	0.05	0.83
DS-5-28	0.59	7.79	0.59	4.26	1.94	1.91	3.74	1.03	13.7	4.97	30.7	8.24	121	21.8	0.02	2.14
DS-5-29	0.83	5.70	0.41	1.60	0.42	0.35	2.97	0.91	16.2	6.74	39.1	9.16	130	21.9	0.02	0.70
DS-5-30	0.06	2.51	0.06	0.41	0.24	0.40	1.76	0.73	10.6	4.32	27.0	6.77	91.5	15.4	0.01	1.37
XH-1长英质麻粒岩
XH-1-01	0.18	48.0	0.48	3.60	3.80	1.34	12.1	3.30	40.0	16.1	91.0	24.0	287	64.0	0.02	0.55
XH-1-03	2.26	36.0	0.76	4.50	2.80	0.89	9.80	3.10	39.0	15.6	82.0	21.3	266	58.0	0.02	0.46
XH-1-04	0.44	35.0	0.41	4.30	4.20	1.67	16.8	5.10	62.0	24.6	129	34.0	393	81.0	0.03	0.53
XH-1-05	0.23	27.0	0.11	1.27	1.71	0.85	6.40	2.33	33.0	13.1	72.0	19.2	228	52.0	0.02	0.69
XH-1-06	0.35	22.6	0.10	0.78	1.27	0.43	6.00	1.65	24.0	11.7	68.0	19.0	238	58.0	0.01	0.40
XH-1-07	2.27	65.0	2.02	16.0	10.8	5.00	22.5	6.50	70.0	26.6	135	35.0	403	82.0	0.03	0.97
XH-1-08	0.29	35.0	0.44	5.60	5.40	1.73	15.8	5.10	58.0	20.8	105	25.4	285	56.0	0.03	0.53
XH-1-09	1.26	27.0	0.59	3.70	2.90	0.80	8.40	2.60	33.0	14.0	73.0	19.5	236	53.0	0.02	0.47
XH-1-10	0.02	19.2		0.52	0.97	0.46	5.80	2.04	26.8	12.2	68.0	19.3	240	55.0	0.01	0.46
XH-1-11	0.14	28.0	0.12	1.71	1.74	0.65	6.50	2.48	31.0	12.6	68.0	17.6	220	49.0	0.02	0.52
XH-1-12	0.32	48.0	0.69	5.40	4.50	1.75	16.1	5.90	81.0	35.0	201	54.0	661	144	0.01	0.56
XH-1-13	1.78	58.0	1.26	9.60	6.60	2.80	16.5	4.40	53.0	20.7	108	28.0	332	71.0	0.03	0.78
XH-1-14	3.10	42.0	1.39	8.80	5.00	1.97	15.5	5.20	63.0	25.0	131	33.0	379	79.0	0.02	0.63
XH-1-15		38.0	0.06	1.50	2.80	0.30	16.5	5.30	65.0	25.0	124	30.0	310	59.0	0.03	0.10
XH-1-16	3.90	31.0	0.85	4.80	2.27	1.35	7.40	2.25	26.5	11.1	67.0	18.6	239	53.0	0.02	0.92
XH-1-17	0.15	35.0	0.44	5.70	5.70	2.29	20.7	6.10	72.0	27.0	136	35.0	423	85.0	0.03	0.57
XH-1-18	0.14	16.2	0.22	1.93	1.33	0.66	4.10	1.23	15.9	6.70	37.0	10.2	125	27.4	0.02	0.80
XH-1-19	1.21	26.3	0.80	5.40	2.70	0.60	7.70	2.26	28.0	11.9	66.0	19.2	246	54.0	0.02	0.37
XH-1-20	0.12	28.0	0.12	1.93	4.10	1.30	14.6	4.80	55.0	21.4	116	30.0	351	77.0	0.02	0.46
XH-1-21	1.32	106	2.47	22.7	20.4	6.50	59.0	17.5	190	640	286	65.0	654	108	0.07	0.53
XH-1-22	0.07	28.0	0.32	3.00	3.00	1.17	7.80	3.00	35.0	14.9	82.0	23.1	291	67.0	0.01	0.70
XH-1-23	0.08	15.8		0.60	0.70	0.33	4.40	1.59	21.5	9.90	60.0	17.2	227	57.0	0.01	0.44
XH-1-24	1.44	22.6	0.80	5.40	2.60	0.86	7.60	2.41	25.8	9.90	53.0	14.3	170	36.0	0.03	0.55
XH-1-25	0.55	32.0	0.54	5.20	4.90	1.68	16.6	5.30	62.0	24.8	129	35.0	408	91.0	0.02	0.51
XH-1-26	0.14	22.2	0.37	2.90	2.44	0.80	7.10	2.54	31.0	12.3	70.0	20.1	248	57.0	0.02	0.55
XH-1-27	0.37	28.0	0.51	4.80	5.80	2.04	18.6	5.90	66.0	24.4	129	33.0	382	84.0	0.03	0.55
XH-1-28	0.01	31.0	0.03	1.02	2.02	0.22	10.3	3.20	37.0	12.9	60.0	14.1	150	27.5	0.05	0.12
XH-1-29	0.05	25.6	0.08	1.21	1.80	0.88	8.00	2.60	34.0	15.1	91.0	25.3	323	73.0	0.01	0.60
XH-1-30	0.40	16.9	0.61	5.40	3.50	1.25	9.70	2.60	27.4	9.30	49.0	13.1	166	35.0	0.03	0.61
XH-1-31	0.01	25.3	0.08	1.31	1.47	0.57	5.50	2.34	32.0	12.8	73.0	20.1	253	55.0	0.01	0.54

表 1 怀安麻粒岩锆石稀土元素组成(×10^-6) Table 1 LA-ICP-MS zircon rare earth elements results for the Huai’an granulites(×10^-6)

表 2 怀安麻粒岩锆石U-Pb年龄结果 Table 2 LA-ICP-MS zircon U-Pb results for the Huai’an granulites

测点号	含量(×10^-6)		比值 Th/U	同位素比值						年龄(Ma)
测点号	Th	U	比值 Th/U		1σ		1σ		1σ		1σ		1σ		1σ
DS-5单斜辉石麻粒岩
DS-5-01	260	804	0.32	0.1629	0.0040	10.8906	0.2755	0.4807	0.0050	2487	42	2514	24	2530	22
DS-5-02	188	518	0.36	0.1569	0.0041	10.5113	0.2784	0.4821	0.0054	2433	44	2481	25	2536	24
DS-5-03	436	917	0.48	0.1636	0.0043	10.4814	0.2731	0.4610	0.0051	2494	44	2478	24	2444	22
DS-5-04	166	411	0.40	0.1665	0.0049	11.0250	0.3187	0.4768	0.0056	2524	50	2525	27	2513	24
DS-5-05	408	941	0.43	0.1585	0.0051	9.7489	0.3136	0.4412	0.0051	2440	55	2411	30	2356	23
DS-5-06	429	1040	0.41	0.1668	0.0049	10.3910	0.2985	0.4471	0.0047	2526	50	2470	27	2382	21
DS-5-07	223	548	0.41	0.1620	0.0044	10.8672	0.2804	0.4825	0.0054	2477	45	2512	24	2538	23
DS-5-08	302	667	0.45	0.1635	0.0041	10.9565	0.2750	0.4803	0.0051	2492	42	2519	23	2529	22
DS-5-09	4	14	0.31	0.1789	0.0183	7.7370	0.5444	0.3710	0.0161	2643	171	2201	63	2034	75
DS-5-10	622	1253	0.50	0.1701	0.0048	11.3640	0.3212	0.4779	0.0056	2559	47	2553	26	2518	24
DS-5-11	205	477	0.43	0.1662	0.0048	11.4918	0.3324	0.4946	0.0062	2520	49	2564	27	2590	27
DS-5-12	283	624	0.45	0.1616	0.0042	11.0900	0.2769	0.4907	0.0060	2473	44	2531	23	2574	26
DS-5-13	424	971	0.44	0.1620	0.0039	10.8815	0.2590	0.4779	0.0048	2477	41	2513	22	2518	21
DS-5-14	501	990	0.51	0.1605	0.0040	10.8103	0.2568	0.4791	0.0056	2461	42	2507	22	2523	24
DS-5-15	1	30	0.03	0.1250	0.0095	6.1301	0.4043	0.3801	0.0139	2029	135	1995	58	2077	65
DS-5-16	458	760	0.60	0.1548	0.0042	10.5434	0.2828	0.4824	0.0063	2400	51	2484	25	2538	28
DS-5-17	388	945	0.41	0.1541	0.0038	10.3105	0.2431	0.4748	0.0053	2392	42	2463	22	2505	23
DS-5-18	257	595	0.43	0.1586	0.0040	10.8837	0.2691	0.4881	0.0057	2440	43	2513	23	2563	25
DS-5-19	268	616	0.44	0.1623	0.0043	11.0185	0.2901	0.4832	0.0057	2480	44	2525	25	2541	25
DS-5-20	180	475	0.38	0.1546	0.0048	10.2905	0.3147	0.4754	0.0065	2398	53	2461	28	2507	29
DS-5-21	127	395	0.32	0.1558	0.0050	9.7693	0.3173	0.4484	0.0064	2410	55	2413	30	2388	28
DS-5-22	254	770	0.33	0.1565	0.0039	10.0870	0.2553	0.4611	0.0051	2418	42	2443	23	2444	22
DS-5-23	121	317	0.38	0.1691	0.0074	9.6402	0.3338	0.4161	0.0065	2550	73	2401	32	2243	30
DS-5-24	659	1402	0.47	0.1535	0.0033	9.6077	0.2081	0.4489	0.0046	2387	36	2398	20	2390	20
DS-5-25	565	1261	0.45	0.1581	0.0036	10.4564	0.2364	0.4755	0.0052	2435	39	2476	21	2508	23
DS-5-26	477	1012	0.47	0.1551	0.0039	10.3558	0.2550	0.4801	0.0058	2403	47	2467	23	2528	25
DS-5-27	1	24	0.04	0.1435	0.0142	6.5480	0.4989	0.3548	0.0125	2270	172	2052	67	1957	59
DS-5-28	446	1030	0.43	0.1632	0.0055	11.0998	0.3545	0.4875	0.0049	2500	57	2532	30	2560	21
DS-5-29	425	950	0.45	0.1564	0.0064	10.6887	0.4166	0.4899	0.0059	2417	70	2496	36	2570	26
DS-5-30	440	967	0.45	0.1554	0.0077	10.3366	0.4827	0.4757	0.0056	2406	84	2465	43	2509	24
XH-1长英质麻粒岩
XH-1-01	1351	1428	0.95	0.1366	0.0025	6.6607	0.1291	0.3516	0.0034	2185	31	2067	17	1942	16
XH-1-03	610	867	0.70	0.1601	0.0031	9.7775	0.1962	0.4396	0.0042	2457	33	2414	18	2349	19
XH-1-04	776	899	0.86	0.1574	0.0033	9.0002	0.1874	0.4116	0.0040	2427	35	2338	19	2222	18
XH-1-05	593	846	0.70	0.1593	0.0037	9.9002	0.2332	0.4465	0.0050	2450	39	2426	22	2380	22
XH-1-06	338	559	0.60	0.1489	0.0037	8.4279	0.2086	0.4093	0.0069	2344	43	2278	22	2212	31
XH-1-07	1831	1626	1.13	0.1383	0.0029	5.6411	0.1299	0.2921	0.0034	2206	36	1922	20	1652	17
XH-1-08	505	532	0.95	0.1572	0.0033	9.7785	0.1977	0.4469	0.0041	2426	35	2414	19	2381	18
XH-1-09	401	667	0.60	0.1570	0.0034	9.9183	0.2275	0.4531	0.0052	2424	37	2427	21	2409	23
XH-1-10	264	524	0.50	0.1436	0.0036	8.4832	0.2075	0.4239	0.0043	2271	43	2284	22	2278	19
XH-1-11	584	818	0.71	0.1548	0.0037	9.7223	0.2326	0.4511	0.0047	2400	41	2409	22	2400	21
XH-1-12	754	1219	0.62	0.1471	0.0031	8.5973	0.1846	0.4203	0.0042	2312	36	2296	20	2262	19
XH-1-13	1524	1628	0.94	0.1436	0.0030	7.0569	0.1519	0.3538	0.0034	2272	35	2119	19	1953	16
XH-1-14	537	885	0.61	0.1563	0.0034	8.6109	0.1944	0.3978	0.0038	2417	32	2298	21	2159	18
XH-1-15	729	498	1.46	0.1622	0.0038	10.0583	0.2438	0.4483	0.0046	2480	39	2440	22	2388	21
XH-1-16	327	540	0.61	0.1312	0.0035	6.9488	0.1917	0.3841	0.0047	2114	47	2105	24	2096	22
XH-1-17	680	843	0.81	0.1539	0.0034	8.8482	0.1958	0.4157	0.0042	2390	37	2322	20	2241	19
XH-1-18	220	432	0.51	0.1333	0.0033	6.8825	0.1601	0.3739	0.0035	2143	43	2096	21	2048	16
XH-1-19	443	829	0.53	0.1599	0.0033	9.8834	0.2065	0.4456	0.0042	2455	40	2424	19	2376	19
XH-1-20	631	817	0.77	0.1584	0.0038	9.3392	0.2288	0.4246	0.0050	2439	41	2372	22	2282	22
XH-1-21	1946	1557	1.25	0.1371	0.0033	5.8651	0.1592	0.3074	0.0047	2191	41	1956	24	1728	23
XH-1-22	458	751	0.61	0.1419	0.0031	7.5882	0.1759	0.3854	0.0049	2251	38	2183	21	2102	23
XH-1-23	324	1086	0.30	0.1605	0.0032	8.2234	0.2005	0.3688	0.0060	2461	34	2256	22	2024	28
XH-1-24	240	864	0.28	0.1499	0.0031	8.8768	0.2256	0.4262	0.0069	2346	35	2325	23	2288	31
XH-1-25	668	907	0.74	0.1526	0.0036	9.4888	0.2409	0.4484	0.0057	2376	35	2386	23	2388	25
XH-1-26	309	588	0.53	0.1350	0.0035	7.6731	0.2001	0.4104	0.0047	2165	44	2193	23	2217	22
XH-1-27	575	761	0.75	0.1474	0.0033	8.8167	0.2130	0.4309	0.0051	2316	39	2319	22	2310	23
XH-1-28	353	111	3.19	0.1533	0.0049	9.2661	0.2873	0.4386	0.0059	2382	54	2365	28	2344	26
XH-1-29	385	709	0.54	0.1603	0.0033	10.0485	0.2136	0.4513	0.0045	2459	35	2439	20	2401	20
XH-1-30	161	346	0.46	0.1555	0.0037	9.2078	0.2142	0.4280	0.0047	2407	41	2359	21	2297	21
XH-1-31	371	637	0.58	0.1512	0.0033	9.0566	0.2033	0.4311	0.0046	2361	37	2344	21	2311	21

表 2 怀安麻粒岩锆石U-Pb年龄结果 Table 2 LA-ICP-MS zircon U-Pb results for the Huai’an granulites

尚义地上单斜辉石麻粒岩(DS-5)中锆石为椭圆形-圆形,大小为50~100μm,根据其内部结构(图 4)可分为两种类型：一类具有核边结构,核部显示弱振荡环带,阴极发光为灰黑色,边部小于10μm,阴极发光呈白色；另一类成分较均一,无内部结构,阴极发光呈白色。第一类锆石具有强烈的正Ce异常,重稀土元素富集(图 5),(Gd/Lu)_N在0.01~0.06之间,Th、U含量分别为121×10^-6~659×10^-6和317×10^-6~1402×10^-6,Th/U比值为0.32~0.60,是岩浆锆石。第二类锆石具有强的正Ce异常,重稀土平坦且轻重稀土分异较弱,(Gd/Lu)_N分别为0.20、0.05、0.29,Th含量分别为4×10^-6、1×10^-6和1×10^-6,U含量分别为14×10^-6、30×10^-6和24×10^-6,Th/U比为0.31、0.03和0.04,为变质锆石。对27颗岩浆锆石和3颗变质锆石核部进行LA-ICP-MS定年分析。3颗变质锆石²⁰⁷Pb/²⁰⁶Pb年龄分别为2643±171Ma、2029±135Ma和2270±172Ma；27颗岩浆锆石²⁰⁷Pb/²⁰⁶Pb年龄分布在2387~2559Ma之间,在U-Pb谐和图(图 6)中获得2463±53Ma(MSWD=1.9)的上交点年龄,其中20颗谐和锆石的²⁰⁷Pb/²⁰⁶Pb加权平均年龄为2461±20Ma(MSWD=0.97),另外三颗谐和锆石的²⁰⁷Pb/²⁰⁶Pb年龄分别为2410±55Ma、2418±42Ma和2387±36Ma。

图 4 尚义地上麻粒岩(DS-5)和马市口麻粒岩(XH-1)锆石CL图像虚线圆圈为锆石U-Pb分析的激光剥蚀点位,实线圆圈为Hf分析位置,锆石年龄值为²⁰⁷Pb/²⁰⁶Pb年龄,括弧内数据为Th/U比值 Fig. 4 Representative CL images of zircons from Dishang(DS-5)and Mashikou granulites(XH-1)

图 5 尚义地上麻粒岩(DS-5)和马市口麻粒岩(XH-1)锆石稀土配分图 Fig. 5 Chondrite-normalized REE patterns of zircons from Dishang(DS-5)and Mashikou granulites(XH-1)

图 6 尚义地上麻粒岩(DS-5)和马市口麻粒岩(XH-1)锆石U-Pb年龄谐和图 Fig. 6 Concordia diagrams of zircon U-Pb dating for Dishang(DS-5)and Mashikou granulites(XH-1)

马市口长英质麻粒岩(XH-1)中锆石为椭圆形或棱镜状,大小为150~200μm,具有核边结构(图 4),核部发育规则的振荡环带,阴极发光为灰黑色,边部小于10μm,阴极发光为白色。它们的稀土元素模式均一(图 5),具有强烈的正Ce异常以及不同程度的负Eu异常(δEu=0.10~0.97),富集重稀土元素,(Gd/Lu)_N在0.01~0.07之间。Th、U含量分别为161×10^-6~1946×10^-6和346×10^-6~1628×10^-6,Th/U比在0.28~3.19之间,都显示了岩浆锆石的特征。对30颗锆石核部进行LA-ICP-MS定年分析,4个谐和点的²⁰⁷Pb/²⁰⁶Pb年龄分别为2114±47Ma、2165±44Ma、2271±43Ma和2316±39Ma,其余26颗锆石在U-Pb谐和图(图 6)中给出2458±46Ma(MSWD=4.1)的上交点年龄。

5.2 锆石Hf同位素组成

DS-5和XH-1样品的锆石Hf同位素组成见表 3。

表 3 怀安麻粒岩锆石Hf同位素结果 Table 3 In situ zircon Hf isotope composition for the Huai’an granulites

测点号	¹⁷⁶Hf/¹⁷⁷Hf	1σ	¹⁷⁶Lu/¹⁷⁷Hf	1σ	¹⁷⁶Yb/¹⁷⁷Hf	1σ	Age(Ma)	(¹⁷⁶Hf/¹⁷⁷Hf)_i	ε_Hf(t)	t_DM(Ma)	t_crust(Ma)
DS-5单斜辉石麻粒岩
DS-5-01	0.281328	0.000009	0.000351	0.000002	0.010473	0.000035	2463	0.281312	3.6	2642	2767
DS-5-02	0.281320	0.000011	0.000384	0.000016	0.010151	0.000391	2463	0.281302	3.2	2656	2791
DS-5-03	0.281491	0.000014	0.000171	0.000011	0.004529	0.000277	2463	0.281483	9.7	2412	2376
DS-5-04	0.281335	0.000007	0.000533	0.000002	0.015348	0.000125	2463	0.281310	3.5	2645	2771
DS-5-05	0.281336	0.000011	0.000328	0.000004	0.009170	0.000136	2463	0.281321	3.9	2630	2747
DS-5-06	0.281360	0.000009	0.000450	0.000009	0.012740	0.000187	2463	0.281339	4.5	2606	2706
DS-5-07	0.281352	0.000010	0.000323	0.000003	0.008562	0.000129	2463	0.281337	4.5	2609	2711
DS-5-08	0.281306	0.000011	0.000327	0.000004	0.009211	0.000146	2463	0.281291	2.8	2670	2815
DS-5-10	0.281302	0.000008	0.000247	0.000003	0.006831	0.000077	2463	0.281290	2.8	2670	2816
DS-5-11	0.281312	0.000009	0.000363	0.000004	0.010032	0.000150	2463	0.281295	3.0	2665	2807
DS-5-12	0.281331	0.000009	0.000394	0.000004	0.011199	0.000175	2463	0.281312	3.6	2642	2767
DS-5-13	0.281319	0.000009	0.000480	0.000005	0.013433	0.000227	2463	0.281296	3.0	2664	2803
DS-5-14	0.281335	0.000008	0.000288	0.000006	0.007917	0.000135	2463	0.281321	3.9	2628	2745
DS-5-15	0.281616	0.000010	0.000009	0.000000	0.000341	0.000013	2029	0.281616	4.4	2235	2380
DS-5-16	0.281322	0.000009	0.000398	0.000005	0.009467	0.000236	2463	0.281303	3.3	2654	2787
DS-5-17	0.281345	0.000010	0.000461	0.000003	0.013930	0.000151	2463	0.281323	4.0	2627	2741
DS-5-18	0.281306	0.000010	0.000360	0.000002	0.010533	0.000107	2463	0.281289	2.8	2672	2818
DS-5-19	0.281301	0.000009	0.000463	0.000002	0.012789	0.000106	2463	0.281279	2.4	2686	2842
DS-5-20	0.281341	0.000008	0.000409	0.000001	0.011914	0.000049	2463	0.281322	3.9	2629	2744
DS-5-21	0.281369	0.000010	0.000398	0.000013	0.010949	0.000209	2463	0.281350	4.9	2591	2680
DS-5-22	0.281324	0.000010	0.000395	0.000005	0.011089	0.000124	2463	0.281305	3.3	2651	2782
DS-5-24	0.281284	0.000006	0.000251	0.000005	0.006777	0.000162	2463	0.281272	2.2	2695	2858
DS-5-25	0.281319	0.000011	0.000451	0.000002	0.012581	0.000064	2463	0.281298	3.1	2661	2798
DS-5-26	0.281457	0.000011	0.000212	0.000007	0.005852	0.000157	2463	0.281447	8.4	2461	2459
DS-5-27	0.281647	0.000007	0.000021	0.000000	0.000787	0.000008	2270	0.281646	11.0	2194	2141
DS-5-28	0.281289	0.000008	0.000398	0.000008	0.009455	0.000322	2463	0.281270	2.1	2697	2861
DS-5-29	0.281321	0.000011	0.000415	0.000022	0.011897	0.000710	2463	0.281301	3.2	2656	2791
DS-5-30	0.281296	0.000007	0.000369	0.000001	0.010389	0.000082	2463	0.281279	2.4	2686	2842
XH-1长英质麻粒岩
XH-1-03	0.281356	0.000009	0.000881	0.000015	0.021699	0.000412	2458	0.281315	3.6	2641	2765
XH-1-04	0.281409	0.000007	0.001118	0.000029	0.030089	0.000964	2458	0.281357	5.0	2584	2668
XH-1-05	0.281350	0.000008	0.000808	0.000009	0.019594	0.000265	2458	0.281312	3.5	2644	2771
XH-1-06	0.281325	0.000008	0.000608	0.000007	0.014308	0.000161	2458	0.281296	2.9	2664	2806
XH-1-07	0.281351	0.000009	0.000734	0.000033	0.021345	0.000953	2458	0.281317	3.6	2637	2759
XH-1-08	0.281386	0.000009	0.001019	0.000010	0.030882	0.000485	2458	0.281338	4.4	2609	2711
XH-1-09	0.281401	0.000007	0.000944	0.000038	0.022900	0.000811	2458	0.281357	5.1	2583	2668
XH-1-10	0.281395	0.000009	0.001041	0.000006	0.025357	0.000122	2271	0.281350	0.5	2599	2816
XH-1-11	0.281402	0.000009	0.001381	0.000022	0.036032	0.000386	2458	0.281337	4.4	2612	2713
XH-1-13	0.281381	0.000008	0.001395	0.000012	0.036829	0.000380	2458	0.281316	3.6	2641	2761
XH-1-14	0.281367	0.000008	0.001039	0.000016	0.029801	0.000450	2458	0.281318	3.7	2636	2755
XH-1-15	0.281370	0.000007	0.000679	0.000011	0.020522	0.000341	2458	0.281338	4.4	2608	2711
XH-1-16	0.281342	0.000008	0.000736	0.000018	0.019726	0.000537	2114	0.281312	-4.4	2650	3008
XH-1-17	0.281364	0.000010	0.001242	0.000040	0.034221	0.001239	2458	0.281306	3.2	2655	2785
XH-1-18	0.281410	0.000013	0.001016	0.000064	0.033953	0.002118	2458	0.281362	5.3	2576	2656
XH-1-19	0.281352	0.000006	0.000920	0.000010	0.021434	0.000131	2458	0.281309	3.4	2649	2777
XH-1-20	0.281378	0.000007	0.001031	0.000005	0.027147	0.000296	2458	0.281330	4.1	2621	2731
XH-1-21	0.281333	0.000008	0.000443	0.000006	0.011972	0.000227	2458	0.281312	3.5	2642	2770
XH-1-23	0.281365	0.000007	0.001012	0.000015	0.025928	0.000314	2458	0.281318	3.7	2638	2758
XH-1-24	0.281376	0.000008	0.001127	0.000017	0.029860	0.000654	2458	0.281323	3.9	2630	2744
XH-1-25	0.281393	0.000008	0.001126	0.000015	0.029899	0.000447	2458	0.281340	4.5	2607	2707
XH-1-26	0.281351	0.000009	0.000845	0.000006	0.021585	0.000132	2165	0.281316	-3.1	2644	2964
XH-1-27	0.281361	0.000006	0.000867	0.000010	0.022421	0.000416	2316	0.281323	0.6	2633	2845
XH-1-28	0.281346	0.000006	0.000348	0.000011	0.010888	0.000395	2458	0.281330	4.1	2618	2730
XH-1-29	0.281380	0.000008	0.001160	0.000046	0.029842	0.001131	2458	0.281326	3.9	2627	2739
XH-1-30	0.281374	0.000007	0.000632	0.000004	0.019776	0.000231	2458	0.281344	4.6	2599	2697
XH-1-31	0.281360	0.000007	0.000794	0.000011	0.022216	0.000375	2458	0.281323	3.8	2629	2746

表 3 怀安麻粒岩锆石Hf同位素结果 Table 3 In situ zircon Hf isotope composition for the Huai’an granulites

单斜辉石麻粒岩(DS-5)在U-Pb测试的基础上对28颗锆石进行原位Hf同位素分析,锆石¹⁷⁶Hf/¹⁷⁷Hf比值为0.281284~0.281647,ε_Hf(t)值分布范围为+2.1~+11.0,Hf亏损地幔模式年龄集中在2.6~2.7Ga,两个较年轻的t_DM为2.4Ga和2.5Ga。²⁰⁷Pb/²⁰⁶Pb年龄为2029Ma和2270Ma的2颗变质锆石的ε_Hf(t)分别为+4.4和+11.0,t_DM均为2.2Ga。

长英质麻粒岩(XH-1)在U-Pb测试的基础上对27颗锆石进行原位Hf同位素分析,锆石Hf同位素分布比较均一,¹⁷⁶Hf/¹⁷⁷Hf比值在0.281325~0.281410之间,锆石ε_Hf(t)值分布于-4.4~+5.3,t_DM分布范围为2.6~2.7Ga,t_crust变化范围为2.7~3.0Ga。²⁰⁷Pb/²⁰⁶Pb年龄为2114±47Ma、2165±44Ma、2271±43Ma和2316±39Ma的锆石ε_Hf(t)值分别为-4.4、-3.1、+0.5和+0.6,这四颗锆石的t_crust变化范围为2.8~3.0Ga,其余23颗锆石的ε_Hf(t)值为+2.9~+5.3,t_crust分布于2.7~2.8Ga之间。

6 讨论 6.1 怀安地区2.7~2.8Ga的地壳物质

大陆地壳来源于原始地幔的分异作用,经历了复杂演化历史,并具有幕式生长的特点(Condie,1998; Condie et al., 2009)。全球大陆地壳的主要生长时期为3.0~2.5Ga,该时期的增生陆壳占现在大陆的36%(Condie,1998),广泛分布的2.7~2.8Ga的构造热事件对全球陆壳生长的影响尤为重要(Zheng et al., 2009,2012; Geng et al., 2012)。华北火成岩具有3.0~2.6Ga(峰期为2.8Ga)的Nd模式年龄值(Wu et al., 2005),岩浆锆石的Hf模式年龄也显示了~2.7Ga的峰期(Zhang et al., 2012a),这些表明2.6~2.8Ga可能是华北大陆地壳生长的重要时期(Zheng et al., 2004a,2009,2012; Wu et al., 2005; Geng et al., 2012; Zhang et al., 2013)。在华北克拉通中北部已有的锆石U-Pb年代学数据也记录了~2.7Ga的热事件：Zheng et al.(2004c)和Jiang et al.(2010)报道了汉诺坝辉石岩捕掳体中2.7Ga的锆石和条带状麻粒岩捕掳体中2.8Ga的锆石；刘敦一等(1997)在怀安2500Ma的英云闪长质片麻岩中发现2761~2613Ma的锆石捕掳晶；Kröner et al.(2005)、Wilde et al.(2005)和陈斌等(2006)在吕梁-五台地区晚太古代花岗岩中发现2.7Ga的古老残留锆石；孙敏和关鸿(2001)在阜平群角闪片麻岩中获得2708±8Ma的古老地壳信息；恒山和阜平地区零星存在2.7Ga的片麻岩(Kröner et al., 2005)；Liu et al.(2009)报道了太华2829Ma和2832Ma的英云闪长质片麻岩。

本文所研究的长英质麻粒岩(XH-1)的锆石Hf同位素组成与上述有相似的特征。长英质麻粒岩中~2.5Ga的锆石Hf两阶段模式年龄为2.7~2.8Ga,~2.1Ga锆石的Hf t_crust为3.0Ga,显示怀安地区存在≥2.7Ga的古老地壳物质,可能追溯到3.0Ga。因为数据有限,这一认识尚需进一步研究。其锆石ε_Hf(t)值均较低,分布在-4.4~+5.9之间,长英质麻粒岩原岩主体可能来自~2.7Ga的新生地壳。

6.2 怀安地区2.5Ga的地壳增生与再造

近年来,刘富等(2009)和Liu et al.(2012)通过怀安TTG片麻岩和闪长质片麻岩地体的锆石U-Pb年龄、Hf同位素以及全岩Sm-Nd同位素研究认为怀安地区在2.5Ga也发生了重要的地壳增生事件,其TTG片麻岩的ε_Nd(t)值和大部分岩浆锆石的ε_Hf(t)值分别与同期的亏损地幔ε_Nd(t)值和ε_Hf(t)值相近,TTG片麻岩和闪长质片麻岩较年轻的Hf亏损地幔模式年龄(2.44~2.73Ga和2.49~2.75Ga)与锆石U-Pb年龄一致。

2.5Ga的热事件在华北中部的分布十分广泛：怀安TTG片麻岩和闪长质片麻岩原岩形成于2500~2550Ma(Zhao et al., 2006,2008; 刘富等,2009; Wang et al., 2010; Liu et al., 2012; Zhang et al., 2012b),并发现2437~2493Ma的花岗岩(Zhang et al., 2011)及2477Ma的紫苏花岗岩(Santosh et al., 2013)；承德地区正片麻岩原岩主要形成于2540~2470Ma(耿元生等,2010)；恒山条带状片麻岩、花岗闪长质片麻岩及与深熔有关的花岗质片麻岩都形成于2530~2500Ma期间,一些闪长质片麻岩在2470Ma左右形成(Kröner et al., 2005; Wang,2009; 耿元生等,2010; Diwu et al., 2011)；五台地块的灰色花岗岩类主要形成于2570~2510Ma(Liu et al., 2004; Kröner et al., 2005; Wilde et al., 2005; 耿元生等,2010)；阜平的TTG系列和花岗质片麻岩主要形成于2540~2500Ma,部分花岗闪长质岩浆活动可以持续到2470Ma(Guan et al., 2002; Zhao et al., 2002; Liu et al., 2004; 耿元生等,2010)；中条山奥长花岗质片麻岩和二长花岗质片麻岩形成时间为2560~2470Ma(郭丽爽等,2008; 耿元生等,2010; 张瑞英等,2012)；登封地区的岩浆活动主要发生在2530~2500Ma之间,一些奥长花岗质岩形成于2560Ma左右,而闪长岩则形成于2490Ma(耿元生等,2010; Diwu et al., 2011)。

本文所研究麻粒岩记录的2458Ma和2461Ma的岩浆事件与上述事件可能反映了同一期次的地质作用。单斜辉石麻粒岩(DS-5)中岩浆锆石ε_Hf(t)为+2.1~+9.7,分布在亏损地幔演化线和球粒陨石之间,少数高ε_Hf(t)值接近当时的亏损地幔值,其锆石U-Pb年龄与亏损地幔模式年龄相近,显示单斜辉石麻粒岩原岩可能来自亏损地幔的部分熔融,反映怀安地区~2.5Ga重要的地壳增生事件。长英质麻粒岩(XH-1)的锆石ε_Hf(t)值较低,多数在球粒陨石线附近,表明源区物质为古老(≥2.7Ga)的先存地壳,暗示了~2.5Ga怀安地区同时发生了大陆地壳再造。可能亏损地幔部分熔融的物质底侵,为陆壳熔融提供热量,并在一定程度对其进行混染。

对比怀安杂岩体中TTG片麻岩及其以南五台-阜平杂岩体、太华片麻岩地体、中条片麻岩地体、登封杂岩体、嵩山花岗岩等中酸性岩中岩浆锆石的Hf同位素组成(陈斌等,2006; Xia et al., 2006; 第五春荣等, 2007,2010; 郭丽爽等,2008; Wang et al., 2010; 刘富,2010; Diwu et al., 2011; Jiang et al., 2011; Zhang et al., 2011; Zhou et al., 2011; Geng et al., 2012; 李创举等,2012),发现：从ε_Hf(t)与U-Pb年龄关系图解(图 7)中可知怀安地区以~2.5Ga的新生地壳为主,同时存在先存地壳重熔。

图 7 华北中部带主要杂岩体锆石ε_Hf(t)和年龄关系图其他数据来源：怀安杂岩体(刘富,2010; Jiang et al., 2011; Zhang et al., 2011; 李创举等,2012); 嵩山花岗岩(Zhou et al., 2011); 登封杂岩(第五春荣等,2007; Diwu et al., 2011); 中条片麻岩(郭丽爽等,2008); 太华片麻岩(第五春荣等,2010; Huang et al., 2010);五台-阜平杂岩体(Xia et al., 2006; 陈斌等,2006; Geng et al., 2012) Fig. 7 Plot of zircon ε_Hf(t)versus age for important complexes in the Trans-North China Craton

6.3 怀安地区2.3~2.0Ga的构造热事件

本文所研究的长英质麻粒岩(XH-1)中有4颗谐和的岩浆锆石²⁰⁷Pb/²⁰⁶Pb年龄分别为2114±47Ma、2165±44Ma、2271±43Ma和2316±39Ma,这期热事件在华北中部、孔兹岩带和南缘都有记录,如：中部恒山-五台-阜平等广泛分布2.2~2.0Ga的花岗岩和变火山岩(Du et al., 2013)；孔兹岩带变表壳岩系中碎屑锆石普遍记录了2.3~2.0Ga的年龄(董春艳等,2007; Yin et al., 2009)；南缘蚌埠地区出露变形花岗岩岩浆结晶年龄为2054Ma(郭素淑和李曙光,2009),豫西也存在2.3Ga的岩浆作用(第五春荣等,2007; 黄道袤等,2012)及2.2~2.0Ga的变表壳岩(第五春荣等,2010)。本文认为2.3~2.1Ga的热事件的研究在怀安地区应进一步受到重视。

单斜辉石麻粒岩(DS-5)的3颗变质锆石分别获得2643±171Ma、2270±172Ma、2029±135Ma,谐和度>92%,在U-Pb谐和图中分别落在不一致线上与谐和线附近,可能是锆石在~1.85Ga麻粒岩相变质(郭敬辉等2001; Guo et al., 2005)发生了Pb丢失造成的。

7 主要认识

(1) 单斜辉石麻粒岩原岩形成于~2.5Ga,其锆石高ε_Hf(t)值在亏损地幔线附近,Hf单阶段模式年龄接近形成年龄,指示怀安地区在2.5Ga左右发生了亏损地幔的部分熔融。

(2) 长英质麻粒岩原岩形成于~2.5Ga,其锆石ε_Hf(t)值较低,Hf两阶段模式年龄在2.7~2.8Ga之间,指示怀安地区可能存在≥2.7Ga的古老地壳物质。怀安地区在~2.5Ga发生亏损地幔的部分熔融,为单斜辉石麻粒岩提供原岩物质的同时,可能引起了先存地壳的重熔,即同时存在地壳再造,进一步为长英质麻粒岩提供了原岩物质。

(3) 怀安地区的最古老地壳物质可能可以追溯到3.0Ga,同时可能存在2.3~2.1Ga的热事件。由于数据有限,这些认识尚需进一步研究和积累。

致谢感谢中国地质大学(武汉)陈明和林向洋在实验分析方面给予的帮助。感谢审稿人刘福来研究员、董春艳博士和刘富博士所提出的宝贵意见。

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岩石学报 2013, Vol. 29 Issue (7): 2281-2294	PDF