甘肃省文县刘家坪蓟县系变基性火山岩地球化学特征及构造环境

doi:10.19657/j.geoscience.1000-8527.2018.06.13

摘要/Abstract

摘要：

详细的野外地质调查显示,甘肃省刘家坪蓟县系为一套变基性火山岩,岩石以变玄武岩为主。岩石地球化学特征表明:A/CNK平均值为0.59%,具有铝不饱和特征;Na₂O/K₂O值为47.00~144.00,表现为钠质;在TAS图解中,显示原岩为亚碱性系列;在AFM图解中,样品点均落入拉斑玄武岩系列;稀土元素含量∑REE为58.22×10^-6~95.59×10^-6,平均为67.81×10^-6;δEu=0.83~1.17,δCe=0.88~0.95,稀土配分模式为平坦型;微量元素中,高场强元素Zr、P、Y、Yb相对亏损,Th、Hf、Nb相对富集,低场强元素K亏损明显,Rb、Ba相对富集,且Zr/Y值为2.38~3.91,Ti/Zr值为92.30~147.23,Th/Ta值为1.77~2.21,大于1.60。Sm-Nd同位素测试结果表明,变玄武岩等时线年龄为(1 387±32) Ma。根据地球化学构造环境判别图,结合区域地质背景,认为刘家坪蓟县系变基性火山岩主要形成于与岛弧相关的构造环境,这一认识对研究碧口地块的构造演化具有重要意义。

关键词: 蓟县系, 变基性火山岩, Sm-Nd年龄, 地球化学特征, 构造环境

Abstract:

Detailed field geological survey suggested that the Jixian Group volcanic rocks in Liujiaping are mainly meta-basalts. Geochemical analysis showed that the rocks are aluminum unsaturated (average A/CNK=0.59%) and sodic (Na₂O/K₂O=47.00-144.00). In the TAS diagram, the basalts are sub-alkaline. In the AFM diagram, the samples fall into the tholeiitic basalt field. The total REE contents (∑REE) vary in the range of 58.22×10^-6-95.59×10^-6(average 67.81×10^-6 ). The rocks show anomalies of Eu (δEu=0.83-1.17) and Ce (δCe=0.88-0.95), with flat chondrite-normalized REE distribution pattern. The trace element geochemistry is characterized by distinct depletions of Zr, P, Y and Yb, and relative enrichments of Th, Hf and Nb. There is also strong depletion in K and relative enrichments in Rb and Ba. The values of Zr/Y are 2.38-3.91; Ti/Zr are 92.30-147.23; and Th/Ta are 1.77-2.21, more than 1.60. The meta-basalt yielded a whole-rock Sm-Nd isochron age of (1,387±32) Ma. Based on geochemical tectonic discrimination and regional geological background, it is considered that the Jixian Group meta-basalts in Liujiaping were formed in an island arc setting, which is important to the tectonic evolution study of the Bikou terrane.

Key words: Jixian Group, meta-basalt, Sm-Nd age, geochemical characteristic, tectonic setting

中图分类号:

毛艳丽, 王滔, 李鸿睿, 潘中奎. 甘肃省文县刘家坪蓟县系变基性火山岩地球化学特征及构造环境[J]. 现代地质, 2018, 32(06): 1254-1262.

MAO Yanli, WANG Tao, LI Hongrui, PAN Zhongkui. Geochemical Characteristics and Tectonic Setting of Meta-mafic Volcanic Rocks from Jixian Group in Liujiaping,Wenxian County, Gansu Province[J]. Geoscience, 2018, 32(06): 1254-1262.

图/表 12

图1 甘肃省文县刘家坪地质简图((a)据李佐臣等[21]修编;(b)据《1∶50 000刘家坪幅、三河幅、楚家坝幅区调报告》修编①) 1.第四系;2.清白口系;3.蓟县系白果树岩组一岩段;4.蓟县系白果树岩组二岩段;5.蓟县系阳坝组一岩段;6.蓟县系阳坝组二岩段;7.石英脉;8.断层角砾岩;9.地质界线;10.早期构造面理接触界线;11.实测逆断层;12.早期韧性剪切面理接触界线;13.韧性剪切带;14.背形构造;15.向形构造;16.S-C组构;17.断层编号;18.韧性剪切带编号;19.硅化;20.剖面线;21.同位素年龄测试采样点及编号;22.岩石地球化学测试采样点

Fig.1 Regional geologic map in Liujiaping of Wenxian County, Gansu Province ((a) after LI et al.[21]; (b) modified from the 1∶50,000 regional geologic map of the Liujiaping, Sanhe, and Chujiaba areas)

图2 刘家坪蓟县系变基性火山岩野外和显微照片(正交偏光) (a)变玄武岩;(b)绿泥绿帘石岩;Aug.普通辉石;Pl.斜长石;Q.石英;Ep.绿帘石;Chl.绿泥石

Fig.2 Field photos and thin section micrographs (crossed polar) of the Jixian meta-basalts in Liujiaping

表1 蓟县系变基性火山岩主量元素含量(wB/%)及相关参数

Table 1 Major element contents (%) and parameters of the Jixian meta-basalts

样品编号	SiO₂	TiO₂	Al₂O₃	Fe₂O₃	FeO	MnO	MgO	CaO
PM26-23	50.02	1.60	14.70	2.19	8.58	0.20	7.58	10.59
PM26-35	48.78	0.96	14.12	2.75	8.53	0.17	7.73	12.44
PM26-58	48.53	1.13	13.54	2.19	9.83	0.20	7.91	11.72
PM26-76	46.93	1.05	13.12	2.39	10.28	0.22	8.11	10.42
PM26-82	49.13	1.09	13.48	5.00	7.93	0.18	5.62	12.75
PM26-110	48.16	1.11	14.27	3.48	9.77	0.19	6.42	11.60
样品编号	Na₂O	K₂O	P₂O₅	烧失量	总量	A/CNK	FeO^T	Mg^#
PM26-23	2.22	0.02	0.18	3.04	100.92	0.64	10.55	56.15
PM26-35	1.44	0.01	0.09	2.38	99.39	0.56	11.00	55.60
PM26-58	1.45	0.03	0.09	2.69	99.31	0.57	11.80	54.44
PM26-76	1.57	0.02	0.08	5.57	99.75	0.61	12.43	53.77
PM26-82	0.47	0.01	0.08	5.10	100.82	0.56	12.43	44.63
PM26-110	1.69	0.02	0.10	3.60	100.40	0.60	12.90	47.01

表2 蓟县系变基性火山岩稀土、微量元素含量(wB/10-6)及相关参数

Table 2 REE and trace element contents (10-6) and parameters of the Jixian meta-basalts

样品编号	Sr	Rb	Ba	Th	Ta	Nb	Zr	Hf	Ti	Sc	Cr
PM26-23	86.30	6.00	10.50	8.00	4.00	8.20	108.00	7.50	9 968.00	33.60	254.00
PM26-35	230.00	9.00	22.80	10.10	5.00	14.00	51.00	8.10	6 394.00	35.00	184.00
PM26-58	133.00	16.60	25.10	7.80	4.30	10.10	59.00	2.80	6 498.00	40.40	214.00
PM26-76	125.00	45.90	28.00	9.90	5.30	9.90	48.00	5.60	7 067.00	37.20	131.00
PM26-82	226.00	72.30	8.30	8.40	3.80	8.60	56.10	9.70	7 493.00	31.40	130.00
PM26-110	164.00	77.30	10.50	7.80	4.40	12.00	58.00	8.80	7 329.00	36.60	170.00
样品编号	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er
PM26-23	7.20	17.80	2.90	14.80	4.30	1.20	4.59	0.92	5.91	1.15	3.29
PM26-35	3.90	9.60	1.60	8.20	2.60	0.92	2.83	0.62	4.07	0.82	2.39
PM26-58	4.80	11.40	1.90	9.70	2.90	1.08	3.20	0.68	4.37	0.89	2.55
PM26-76	3.60	9.50	1.60	8.30	2.60	1.04	2.88	0.63	4.12	0.85	2.46
PM26-82	3.40	9.40	1.60	8.20	2.50	0.95	2.75	0.61	3.98	0.81	2.31
PM26-110	4.10	10.70	1.70	9.00	2.80	0.94	2.99	0.66	4.31	0.88	2.56
样品编号	Tm	Yb	Lu	Y	∑REE	∑Ce/∑Y	δEu	δCe	(La/Yb)_N	(Ce/Yb)_N
PM26-23	0.49	3.02	0.42	27.60	95.59	2.44	0.83	0.91	1.39	1.33
PM26-35	0.36	2.36	0.34	19.90	60.51	1.94	1.04	0.90	0.96	0.92
PM26-58	0.37	2.28	0.34	20.90	67.36	2.16	1.09	0.88	1.22	1.13
PM26-76	0.36	2.24	0.32	20.20	60.70	1.92	1.17	0.92	0.93	0.96
PM26-82	0.35	2.15	0.31	18.90	58.22	1.96	1.11	0.94	0.92	0.98
PM26-110	0.38	2.41	0.36	20.70	64.49	2.01	1.00	0.95	0.99	1.00

图3 蓟县系变基性火山岩TAS图解和AFM图解(底图分别据LE BAS等[25]和IRVINE等[26]) (a)中:Pc.苦橄玄武岩;B.玄武岩;O1.玄武安山岩;O2.安山岩;O3.英安岩;R.流纹岩;S1.粗面玄武岩;S2.玄武质粗面安山岩;S3.粗面安山岩;T.粗面岩、粗面英安岩;F.副长石岩;U1.碱玄岩、碧玄岩;U2.响岩质碱玄岩;U3.碱玄质响岩;Ph.响岩;Ir.Irvine 分界线,上方为碱性,下方为亚碱性。(b)中:A=K2O+Na2O;F=FeO+0.9Fe2O3;M=MgO

Fig.3 Plots of TAS and AFM for the Jixian meta-basalts (base map after LE BAS et al.[25] and IRVINE et al.[26])

图4 蓟县系变基性火山岩稀土元素球粒陨石配分模式和微量元素N-MORB标准化蛛网图(底图分别据SUN等[27]和PEARCE等[30])

Fig.4 Chondrite-normalized REE patterns and N-MORB normalized multi-element spider diagram of the Jixian meta-basalts (base map after SUN et al.[27] and PEARCE et al.[30])

图5 白果树岩组变玄武岩Sm-Nd等时线分布图

Fig.5 Sm-Nd isochron diagram of the Baiguoshu Formation meta-basalts

图6 白果树岩组变玄武岩143Nd/144Nd-1/Nd图解

Fig.6 143Nd/144Nd-1/Nd diagram of the Baiguoshu Formation meta-basalts

表3 白果树岩组(Jxb)变玄武岩Sm-Nd含量及同位素组成

Table 3 Sm-Nd contents and isotopic compositions of the Baiguoshu Formation meta-basalts

样号	岩性	w(Sm)/10^-6	w(Nd)/10^-6	¹⁴⁷Sm/¹⁴⁴Nd	¹⁴³Nd/¹⁴⁴Nd±1σ	T_DM±1σ/Ma
1	变玄武岩	8.951	40.170	0.134 8	0.512 317±0.000 004	1 606±8
2	变玄武岩	8.111	37.130	0.132 2	0.512 318±0.000 004	1 554±7
3	变玄武岩	2.343	7.601	0.186 5	0.512 800±0.000 003	1 956±17
4	变玄武岩	2.356	7.460	0.191 1	0.512 826±0.000 004	2 177±27
5	变玄武岩	2.887	9.254	0.188 7	0.512 832±0.000 010	1 933±60
6	变玄武岩	3.207	10.390	0.186 8	0.512 826±0.000 004	1 831±22
7	变玄武岩	2.633	8.461	0.188 3	0.512 815±0.000 005	2 004±30
8	变玄武岩	2.489	7.922	0.190 1	0.512 827±0.000 004	2 079±26

图7 蓟县系变基性火山岩的构造环境的F1-F2和F2-F3关系图(底图据PEARCE [37]) WPB.板内玄武岩;OFB.洋底(洋中脊)玄武岩;SHO.钾玄岩;CAB.钙碱性玄武岩;LKT.低钾(岛弧)拉斑玄武岩

Fig.7 F1-F2 and F2-F3 tectonic discrimination diagrams for the Jixian Group meta-basalts (base map after PEARCE[37])

图8 蓟县系火山岩w(Zr)-w(Zr)/w(Y)图解(底图据PEARCE等[38])

Fig.8 Plot of Zr vs.Zr/Y for the Jixian Group meta-basalts (base map after PEARCE et al.[38])

图9 TiO2-MnO-P2O5图解(底图据ELLEN等[39])

Fig.9 Ternary plot of TiO2-MnO-P2O5 for the Jixian Group meta-basalts (base map after ELLEN et al.[39])

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