滇中南中元古代撮科蛇绿混杂岩地质特征及构造背景

doi:10.13745/j.esf.sf.2021.7.16

摘要/Abstract

摘要：

撮科蛇绿混杂岩出露于元江县撮科大寨及周边地区,受到后期构造作用影响发生一定的变质和变形。蛇岩性较为复杂,岩块与基质之间为断层接触。基质以硅质岩、熔结凝灰岩和花岗质砾岩为主,岩块的类型主要有辉石岩、辉长岩、辉绿岩、闪长岩、玄武岩、斜长花岗岩等。其中,撮科蛇绿混杂岩中熔结凝灰岩的SHRIMP锆石U-Pb龄为(1 170±7) Ma(MSWD=0.54,n=12),花岗质砾岩基质具有单一的年龄,为(1 151±5) Ma(MSWD=1.3,n=17);辉长岩岩块的SHRIMP锆石U-Pb年龄为(1 168±8) Ma(MSWD=0.72,n=17),斜长花岗岩岩块的SHRIMP锆石U-Pb年龄为(1 163±9) Ma(MSWD=1.83,n=15),花岗斑岩岩块的SHRIMP锆石U-Pb年龄为(1 186±6) Ma(MSWD=0.92,n=17)。这些年龄指示该蛇绿混杂岩形成于中元古代晚期。撮科蛇绿混杂岩中辉长岩REE标准化曲线整体表现较为平坦的右倾趋势,具有与E-MORB相似的稀土元素特征。在MORB标准化图解中Nb、Ta没有出现明显的负异常,极低的Zr/Nb(8.65)、La/Nb(1.05)、Ba/Nb(10.72)、Th/Nb(0.10)、Th/La(0.09)、Ba/La(10.56)、Zr/Hf(43.4)平均值,且微量元素判别投点主要落入板内玄武岩范围,指示撮科蛇绿混杂岩应属于MORB型蛇绿岩,可能形成于板块伸展的构造环境。熔结凝灰岩锆石的ε_Hf(t)值为-8.7 ~-4.7,平均值为-6.6,所有ε_Hf(t)均为负,Hf同位素模式年龄(t_DMC)为2 534~2 296 Ma,平均为2 408 Ma,指示熔结凝灰岩来自古元古代结晶基底的重熔。撮科蛇绿混杂岩的厘定,对重新认识原大红山群老厂河组的属性,研究扬子陆块西南缘中—新元古代岩浆和构造事件,划分扬子陆块基底大地构造单元具有重要意义。

关键词: 扬子陆块西南缘, 中元古代, 撮科蛇绿混杂岩, 锆石U-Pb年龄, SHRIMP, 地球化学

Abstract:

Outcrops of ophiolitic mélange are found in the Cuoke Dazhai area of Yuanjiang County. The Cuoke ophiolitic mélange has undergone metamorphism and deformation due to late tectonism. The lithology of the ophiolitic mélange is complex and fault contact exists between the block and matrix. The bedrock is mainly composed of siliceous rock, fused tuff and granitic conglomerate. The main types of rock blocks are pyroxenite, gabbro, diabase, basalt, granite, etc. The zircon SHRIMP U-Pb age for tuff in the Cuoke ophiolitic mélange was (1170±7) Ma (MSWD=0.54, n=12); for granite conglomerate matrix (a single age) (1151±5) Ma (MSWD=1.3, n=17), gabbro block (1168±8) Ma (MSWD=0.72, n=17), plagiogranite block (1163±9) Ma (MSWD=1.83, n=15), and granite porphyry block (1186±6) Ma (MSWD=0.92, n=17). These ages indicate the ophiolitic mélange is formed in the Late Mesoproterozoic. The chondrite-normalized REE patterns for gabbro of the Cuoke ophiolitic mélange showed a relatively flat right dipping trend similar to that of E-MORB. In the MORB standardized diagram, no obvious negative NB-TA anomalies were observed. The extremely low average Zr/Nb (8.65), La/Nb (1.05), Ba/Nb (10.72), Th/Nb (0.10), Th/La (0.09), Ba/La (10.56) ratios, extremely low Zr/Hf (43.4) ratio, along with the observation that trace element discrimination points mainly fell inside or near the range of intraplate basalt, suggest the Cuoke ophiolite belongs to MOR-type ophiolite and may be formed in a tectonic environment of plate extension. The ε_Hf(t) values for zircon from fused tuff ranged, all negative, from -8.7 to -4.7, or averaged at -6.6. The Hf isotopic model age ( $t D M C$ ) was 2534-2296 Ma, averaging at 2408 Ma, indicating the tuff is derived from the remelting of the Paleoproterozoic crystalline basement. Characterization of the Cuoke ophiolitic mélange is of great significance for recognizing the attributes of the Laochanghe Formation of the Dahongshan Group and for understanding the Mesozoic and Neoproterozoic magmatic and tectonic events in the southwestern margin of the Yangtze block, as well as for delineating the tectonic units of the basement of the Yangtze block.

Key words: southwestern margin of Yangtze block, mesoproterozoic, Cuoke ophiolitic mélange, zircon U-Pb age, SHRIMP, geochemistry

中图分类号:

P597

张良, 张恒, 龚成强, 丁孝忠, 张传恒, 刘勇, 高林志, 刘燕学. 滇中南中元古代撮科蛇绿混杂岩地质特征及构造背景[J]. 地学前缘, 2022, 29(2): 180-197.

ZHANG Liang, ZHANG Heng, GONG Chengqiang, DING Xiaozhong, ZHANG Chuanheng, LIU Yong, GAO Linzhi, LIU Yanxue. Geological characteristics and tectonic background of the Mesoproterozoic ophiolite mélange in central and southern Yunnan[J]. Earth Science Frontiers, 2022, 29(2): 180-197.

图/表 14

图1 扬子陆块西南缘和研究区地质简图(据文献[7,9]修改) ①攀枝花—楚雄—大红山断裂;②安宁河—绿枝江—希拉河断裂;③皎平—铜厂—杨武断裂;④汤郎—禄表—撮科断裂;⑤红河断裂;⑥化念—石屏—建水断裂;⑦米茂—峨山断裂。

Fig.1 Simplified geological map of the southwestern margin of the Yangtze block and study area. Modified after [7, 9].

图2 撮科混杂岩代表性样品的镜下显微照片 a—辉长岩;b—熔结凝灰岩;c—斜长花岗岩;d—花岗斑岩;e—花岗质砾岩。Pl—斜长石;Qtz—石英;Px—辉石。

Fig.2 Photomicrographs of representative specimen from the Cuoke ophiolitic mélange

图3 撮科蛇绿混杂岩典型岩石野外露头照片 a—撮科蛇绿混杂岩带中的辉长岩;b—熔结凝灰岩;c—斜长花岗岩;d—花岗斑岩;e—花岗质砾岩;f—玄武岩以及斜长花岗岩岩块;g—剖面露头。

Fig.3 Field photos of the Cuoke ophiolitic mélange outcrops

图4 撮科蛇绿混杂岩锆石阴极发光(CL)图像、年龄以及测点位置实线和虚线圆圈分别代表 U-Pb 和 Lu-Hf分析点,所有年龄值使用 207Pb/206Pb 年龄,正负数值为锆石εHf(t)值。

Fig.4 Cathodoluminescence (CL) images, ages and Hf isotope of zircons from sample 20180526-1 from the Cuoke ophiolitic mélange

表1 撮科蛇绿混杂岩SHRIMP锆石U-Th-Pb同位素测定结果

Table 1 Zircon SHRIMP U-Th-Pb dating results for the Cuoke ophiolitic melange

图5 撮科蛇绿混杂岩中辉长岩(20171219-2)、熔结凝灰岩(20180526-1)、花岗质砾岩(20181201-2)、斜长花岗岩(20181201-25)和花岗斑岩(20190411-13)SHRIMP锆石U-Pb谐和图

Fig.5 U-Pb concordia diagrams of gabbro (sample 20171219-2), ignimbrite (20180526-1), granitic conglomerate (20181201-2), plagiogranite (20181201-25) and granite porphyry (20190411-13) samples from the Cuoke ophiolitic mélange

表2 辉长岩全岩地球化学测试结果

Table 2 Results of whole rock geochemical analysis of gabbro

图6 撮科蛇绿混杂岩中辉长岩TAS图解(a)和SiO2-K2O(b)图解(据文献[14,15]修改) 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—玄武质粗面玄武岩;S3—粗面安山岩;T—粗面岩、粗面英安岩;F—副长石岩;U1—碱玄岩;U2—响岩质碱玄岩;U3—碱玄质岩响岩;Ph—响岩。

Fig.6 TAS (a) and K2O vs. SiO2 diagrams (b) for the Cuoke gabbro. Modified after [14-15].

图7 撮科蛇绿混杂岩中辉长岩Nb/Y-Zr/TiO2岩石分类图解(据文献[16]修改)

Fig.7 Zr/TiO2 vs. Nb/Y diagram for gabbroic rocks of the Cuoke ophiolitic mélange. Modified after 16.

图8 撮科蛇绿混杂岩中辉长岩微量元素蛛网图(a)和陨石稀土元素分配图(b)(据文献[17]修改)

Fig.8 Primitive mantle-normalized trace element spider (a) and chondrite-normalized REE patterns (b) of Cuoke ophiolitic mélange. Modified after 17.

表3 撮科蛇绿混杂岩中熔结凝灰岩样品(20180526-1)锆石Hf同位素测试结果

Table 3 Results of Hf isotopic analysis of zircon from ignimbrite sample 20180526-1 from the Cuoke Area

测试点	同位素比值				年龄/Ma	ε_Hf(0)	ε_Hf(t)	t_DM	$t D M C$
测试点	¹⁷⁶Yb/¹⁷⁷Hf	¹⁷⁶Lu/¹⁷⁷Hf	¹⁷⁶Hf/¹⁷⁷Hf	2σ	年龄/Ma	ε_Hf(0)	ε_Hf(t)	t_DM	$t D M C$
20180526-1
1.1	0.065 441	0.002 139	0.281 904	0.000 007	1 177	-30.7	-6.3	1 954	2 403
2.1	0.127 639	0.003 335	0.281 877	0.000 010	1 153	-31.6	-8.7	2 059	2 534
3.1	0.036 120	0.001 287	0.281 896	0.000 008	1 153	-31.0	-6.5	1 921	2 392
4.1	0.039 772	0.001 153	0.281 907	0.000 009	1 179	-30.6	-5.4	1 899	2 346
5.1	0.054 308	0.002 218	0.281 470	0.000 008	1 872	-46.1	-7.2	2 575	2 994
6.1	0.018 690	0.000 610	0.281 860	0.000 008	1 154	-32.3	-7.2	1 937	2 441
7.1	0.040 058	0.001 190	0.281 881	0.000 009	1 180	-31.5	-6.3	1 937	2 405
8.1	0.035 199	0.001 057	0.281 879	0.000 008	1 163	-31.6	-6.7	1 934	2 415
9.1	0.098 364	0.003 063	0.281 930	0.000 009	1 173	-29.8	-6.2	1 966	2 392
10.1	0.030 439	0.000 886	0.281 891	0.000 009	1 164	-31.1	-6.1	1 908	2 377
11.1	0.025 548	0.000 753	0.281 872	0.000 009	1 174	-31.8	-6.4	1 927	2 407
12.1	0.032 932	0.000 995	0.281 927	0.000 009	1 174	-29.9	-4.7	1 863	2 296
13.1	0.045 143	0.001 453	0.281 907	0.000 009	1 174	-30.6	-5.8	1 915	2 365
14.1	0.042 998	0.001 323	0.281 890	0.000 010	1 173	-31.2	-6.3	1 932	2 397
14.1	0.042 998	0.001 323	0.281 890	0.000 010	1 173	-31.2	-6.3	1 932	2 397
15.1	0.044 616	0.001 372	0.281 885	0.000 009	1 153	-31.4	-6.9	1 942	2 422
16.1	0.045 183	0.001 349	0.281 869	0.000 009	1 169	-31.9	-7.1	1 962	2 446
17.1	0.027 815	0.000 841	0.281 874	0.000 008	1 118	-31.8	-7.7	1 930	2 442
18.1	0.038 412	0.001 246	0.281 865	0.000 008	1 159	-32.1	-7.4	1 963	2 457

表3 撮科蛇绿混杂岩中熔结凝灰岩样品(20180526-1)锆石Hf同位素测试结果

Table 3 Results of Hf isotopic analysis of zircon from ignimbrite sample 20180526-1 from the Cuoke Area

测试点	同位素比值				年龄/Ma	ε_Hf(0)	ε_Hf(t)	t_DM	$t D M C$
测试点	¹⁷⁶Yb/¹⁷⁷Hf	¹⁷⁶Lu/¹⁷⁷Hf	¹⁷⁶Hf/¹⁷⁷Hf	2σ	年龄/Ma	ε_Hf(0)	ε_Hf(t)	t_DM	$t D M C$
20180526-1
1.1	0.065 441	0.002 139	0.281 904	0.000 007	1 177	-30.7	-6.3	1 954	2 403
2.1	0.127 639	0.003 335	0.281 877	0.000 010	1 153	-31.6	-8.7	2 059	2 534
3.1	0.036 120	0.001 287	0.281 896	0.000 008	1 153	-31.0	-6.5	1 921	2 392
4.1	0.039 772	0.001 153	0.281 907	0.000 009	1 179	-30.6	-5.4	1 899	2 346
5.1	0.054 308	0.002 218	0.281 470	0.000 008	1 872	-46.1	-7.2	2 575	2 994
6.1	0.018 690	0.000 610	0.281 860	0.000 008	1 154	-32.3	-7.2	1 937	2 441
7.1	0.040 058	0.001 190	0.281 881	0.000 009	1 180	-31.5	-6.3	1 937	2 405
8.1	0.035 199	0.001 057	0.281 879	0.000 008	1 163	-31.6	-6.7	1 934	2 415
9.1	0.098 364	0.003 063	0.281 930	0.000 009	1 173	-29.8	-6.2	1 966	2 392
10.1	0.030 439	0.000 886	0.281 891	0.000 009	1 164	-31.1	-6.1	1 908	2 377
11.1	0.025 548	0.000 753	0.281 872	0.000 009	1 174	-31.8	-6.4	1 927	2 407
12.1	0.032 932	0.000 995	0.281 927	0.000 009	1 174	-29.9	-4.7	1 863	2 296
13.1	0.045 143	0.001 453	0.281 907	0.000 009	1 174	-30.6	-5.8	1 915	2 365
14.1	0.042 998	0.001 323	0.281 890	0.000 010	1 173	-31.2	-6.3	1 932	2 397
14.1	0.042 998	0.001 323	0.281 890	0.000 010	1 173	-31.2	-6.3	1 932	2 397
15.1	0.044 616	0.001 372	0.281 885	0.000 009	1 153	-31.4	-6.9	1 942	2 422
16.1	0.045 183	0.001 349	0.281 869	0.000 009	1 169	-31.9	-7.1	1 962	2 446
17.1	0.027 815	0.000 841	0.281 874	0.000 008	1 118	-31.8	-7.7	1 930	2 442
18.1	0.038 412	0.001 246	0.281 865	0.000 008	1 159	-32.1	-7.4	1 963	2 457

图9 辉长岩La/Yb-Nb/La图解(据文献[19]修改)

Fig.9 La/Yb vs. Nb/La diagram for gabbro. Modified after 19.

图10 撮科蛇绿混杂岩构造环境判别图解(据文献[20,21,22]修改) (a)玄武岩Zr-Zr/Y判别图解;(b)玄武岩Zr/4-2Nb-Y判别图解;(c)玄武岩Th-Hf/3-Ta判别图解;(d)玄武岩Zr-Ti/100-3Y判别图解。WPB—板内玄武岩;MORB—洋中脊玄武岩;IAB—岛弧玄武岩;A1、A2—内碱性玄武岩;A2、C—板内拉斑玄武岩;B—富集型洋中脊玄武岩;D—亏损型洋中脊玄武岩;C、D—火山弧玄武岩;CAB—钙碱性玄武岩;IAT—岛弧拉斑玄武岩;WPAB—板内碱性玄武岩;E-MORB—富集型洋中脊玄武岩;N-MORB—亏损型洋中脊玄武岩;a—钙碱性玄武岩;b—洋中脊玄武岩;c—洋中脊玄武岩、岛弧拉斑玄武岩和钙碱性玄武岩;d—板内玄武岩板。

Fig.10 Tectonic discrimination diagrams for the Cuoke gabbro. Modified after [20-22].

图11 撮科地区熔结凝灰岩锆石εHf(t)-t图解

Fig.11 εHf(t)-t diagram of zircon from ignimbrite in Cuoke area

参考文献 32

[1]	LI Z X, BOGDANOVA S V, COLLINS A S, et al. Assembly, configuration, and break-up history of Rodinia: a synjournal[J]. Precambrian Research, 2008, 160(1/2):179-210. DOI URL
[2]	CAWOOD P A, WANG Y J, XU Y J, et al. Locating South China in Rodinia and Gondwana: a fragment of greater India lithosphere?[J]. Geology, 2013, 41(8):903-906. DOI URL
[3]	CAWOOD P A, ZHAO G C, YAO J L, et al. Reconstructing South China in Phanerozoic and Precambrian supercontinents[J]. Earth-Science Reviews, 2018, 186:173-194. DOI URL
[4]	YIN C Q, LIN S F, DAVIS D W, et al. 2.1-1.85 Ga tectonic events in the Yangtze Block, South China: petrological and geochronological evidence from the Kongling Complex and implications for the reconstruction of supercontinent Columbia[J]. Lithos, 2013, 182/183:200-210. DOI URL
[5]	戴恒贵. 康滇地区昆阳群和会理群地层、构造及找矿靶区研究[J]. 云南地质, 1997, 16(1):1-39.
[6]	吕世琨, 戴恒贵. 康滇地区建立昆阳群(会理群)层序的回顾和重要赋矿层位的发现[J]. 云南地质, 2001, 20(1):1-24.
[7]	曹德斌. 青龙厂幅F48E002001 1/5万地质图说明书[R]. 昆明: 云南省地质矿产勘查开发局, 1995.
[8]	杨红, 刘福来, 杜利林, 等. 扬子地块西南缘大红山群老厂河组变质火山岩的锆石U-Pb定年及其地质意义[J]. 岩石学报, 2012, 28(9):2994-3014.
[9]	任光明, 庞维华, 王立全, 等. 扬子陆块西南缘3.8 Ga碎屑锆石及其地质意义[J]. 地球科学, 2020, 45(8):14.
[10]	尹福光, 孙志明, 任光明, 等. 上扬子陆块西南缘早—中元古代造山运动的地质记录[J]. 地质学报, 2012, 86(12):1917-1932.
[11]	宋彪, 张玉海, 万渝生, 等. 锆石SHRIMP样品靶制作、年龄测定及有关现象讨论[J]. 地质论评, 2002, 48(增刊):26-30.
[12]	LUDWIG K R. Squid 1.02: a user’s manual[M]. Berkeley: Berkeley Geochronology Center, 2002.
[13]	STACEY J S, KRAMERS J D. Approximation of terrestrial lead isotope evolution by a two-stage model[J]. Earth and Planetary Science Letters, 1975, 26(2):207-221. DOI URL
[14]	LE BAS N J, LE MAITRE R W, STRECKEISEN A, et al. A chemical classification of volcanic rocks based on the alkali-silica diagram[J]. Journal of Petrology, 1986, 27(3):745-750. DOI URL
[15]	PECCERILLO A, TAYLOR S R. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey[J]. Contributions to Mineralogy and Petrology, 1976, 58(1):63-81. DOI URL
[16]	WINCHESTER J A, FLOYD P A. Geochemical discrimination of different magma series and their differentiation products using immobile elements[J]. Chemical Geology, 1977, 20:325-343. DOI URL
[17]	SUN S S, MCDONOUGH W F. Chemical and isotopic systematics of oceanic basalts: implication for mantle composition and processes[J]. Geological Society, London, Special Publications, 1989, 42(1):313-345. DOI URL
[18]	MCCULLOCH M T, GAMBLE J A. Geochemical and geodynamical constraints on subduction zone magmatism[J]. Earth and Planetary Science Letters, 1991, 102(3/4):358-374 DOI URL
[19]	WATSON S. Rare earth element inversions and percolation models for Hawaii[J]. Journal of Petrology, 1993, 34:763-783 DOI URL
[20]	PEARCE J A, NORRY M J. Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks[J]. Contributions to Mineralogy and Petrology, 1979. 69(1):33-47 DOI URL
[21]	PEARCE J A, CANN J R. Tectonic setting of basic volcanic rocks determined using trace element analyses[J]. Earth and Planetary Science Letters, 1973, 19(2):290-300. DOI URL
[22]	WOOD D A. The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province[J]. Earth and Planetary Science Letters, 1980, 50(1):11-30. DOI URL
[23]	张恒, 高林志, 张传恒, 等. 扬子板块西南部古元古代岩浆及变质事件:兼论扬子板块对Nuna超大陆事件的响应[J]. 地质通报, 2019, 38(11):1777-1789.
[24]	GREENTREE M R, LI Z X, LI X H, et al. Late Mesoproterozoic to earliest Neoproterozoic basin record of the Sibao orogenesis in western South China and relationship to the assembly of Rodinia[J]. Precambrian Research, 2006, 151(1/2):79-100. DOI URL
[25]	耿元生, 杨崇辉, 杜利林, 等. 天宝山组形成时代和形成环境:锆石SHRIMP U-Pb年龄和地球化学证据[J]. 地质论评, 2007, 53(4):556-563.
[26]	耿元生, 旷红伟, 柳永清, 等. 扬子地块西、北缘中元古代地层的划分与对比[J]. 地质学报, 2017, 91(10):2151-2174.
[27]	尹福光, 孙志明, 白建科. 东川、滇中地区中元古代地层格架[J]. 地层学杂志, 2011, 35(1):49-54.
[28]	ZHU W G, ZHONG H, LI Z X, et al. SIMS zircon U-Pb ages, geochemistry and Nd-Hf isotopes of ca. 1.0 Ga mafic dykes and volcanic rocks in the Huili area, SW China: Origin and tectonic significance[J]. Precambrian Research, 2016, 273:67-89. DOI URL
[29]	张传恒, 高林志, 武振杰, 等. 滇中昆阳群凝灰岩锆石SHRIMP U-Pb年龄:华南格林威尔期造山的证据[J]. 科学通报, 2007, 52(7):818-824.
[30]	李怀坤, 张传林, 姚春彦, 等. 扬子西缘中元古代沉积地层锆石U-Pb年龄及Hf同位素组成[J]. 中国科学: 地球科学, 2013, 43(8):1287-1298.
[31]	刘军平, 曾文涛, 徐云飞, 等. 滇中峨山地区中元古界昆阳群黑山头组火山岩锆石U-Pb年龄及其地质意义[J]. 地质通报, 2018, 37(11):2063-2070.
[32]	刘昊岗, 张恒, 张传恒, 等. 滇中北部昆阳群凝灰岩SHRIMP锆石U-Pb年龄及其地层学意义[J]. 地质通报, 2019, 38(7):1183-1190.