Geological characteristics and tectonic background of the Mesoproterozoic ophiolite mélange in central and southern Yunnan

doi:10.13745/j.esf.sf.2021.7.16

Abstract

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

CLC Number:

P597

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.

Figures/Tables 14

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

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

Fig.3 Field photos of the Cuoke ophiolitic mélange outcrops

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

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

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

Table 2 Results of whole rock geochemical analysis of gabbro

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

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

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

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

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

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

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

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

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