The petrogenesi and deep dynamic implications of Middel-Late Neoproterozoic intermediate-mafic rocks on southwestern margin of Yangtze Block

doi:10.13745/j.esf.sf.2025.8.99

Abstract

Abstract:

The early-middle Neoproterozoic (ca.937-820 Ma) intermediate-mafic rocks in the western and southwestern margins of the Yangtze Block are generally considered to have formed in the metasomatized mantle source of subduction zone, but the middle-late Neoproterozoic (ca.820-710 Ma) intermediate-mafic rocks had remained controversial. Based on this, this paper presents systematic geochronology and geochemistry studies for the newly identified Taiyangzhai metamafic rocks and Liujiazhai amphibole gabbro on the southwest margin of the Yangtze Block to investigate their petrogenesis, mantle metasomatism and deep dynamic setting. Zircon U-Pb results show that the Taiyangzhai metamafic rocks and Liujiazhai amphibole gabbro were formed at (713 ± 6.3) Ma, and (712 ± 3.6) Ma, respectively. The Taiyangzhai metamafic dike has varying MgO (3.3%-7.2%), total Fe₂O₃ (8.3%-15.5%), CaO (11.3%-12.7%), high TiO₂ (1.2%-2.6%) and low K₂O (0.26%-0.33%), belonging to the tholeiite series. These rocks are slightly enriched in LREEs and depleted in HREEs, enriched in LILEs (Ba, Pb, Sr) and Ti, but depleted in HFSEs (Nb, Ta, Zr, Hf). They also have varied zircon ε_Hf(t) (-1.9~+1.4) and positive whole-rock ε_Nd(t) (+1.5~+3.9), showing decoupling characteristics. Combined with the high TiO₂, Nb contents, Nb/Y (0.40-0.77) and Nb/Zr (0.13-0.18) ratios, the Taiyangzhai metamafic rocks originated from the mantle source modified by slab melts. Liujiazhai amphibole gabbro has high MgO (5.4%-7.2%), total Fe₂O₃ (14.5%-16.2%) and TiO₂ (2.8%-3.4%), low Mg^# values (40.0-49.6) and varying K₂O (0.5%-2.3%). These amphibole gabbros are enriched in LREEs, depleted in HREEs, enriched in LILEs (Rb, Pb, Sr) and Ti, but depleted in HFSEs (Nb, Ta, Zr, Hf). These samples have varied zircon ε_Hf(t) (-0.7~+5.9) and negative whole rock ε_Nd(t) (-1.8~-1.3). Combined with high K₂O, Nb ((3.4-6.2)×10^-6) and Th ((0.4-1.2)×10^-6) contents, as well as high Nb/Y (0.23-0.33) and Nb/Zr (0.08-0.11) ratios, suggest that they were derived from a mantle source modified by slab melt and sediment melt. These intermediate-mafic rocks formed in an extended back-arc setting. We propose that the western and southwestern margins of the Yangtze Block underwent a tectonic transition from subduction to back-arc extension from the early-middle to middle-late Neoproterozoic.

Key words: southwest margin of the Yangtze Block, Middle-Late Neoproterozoic, intermediate-mafic rocks, mantle source, deep geodynamic setting

CLC Number:

P581

JIANG Xiuwei, LAI Shaocong, ZHU Yu, QIN Jiangfeng, ZHU Renzhi, LIU Min, YANG Hang, YANG Zhen, XUE Wenbin. The petrogenesi and deep dynamic implications of Middel-Late Neoproterozoic intermediate-mafic rocks on southwestern margin of Yangtze Block[J]. Earth Science Frontiers, 2025, 32(6): 303-322.

Figures/Tables 15

Fig.1 Simple geological map of western and southwestern margins of Yangtze Block. Modified after [4-5].

Fig.2 Simple geological map of Ailaoshan complex blet on southwestern margins of Yangtze Block. Modified after [11].

Fig.3 Microscopic observation of different intermediate-mafic intrusions

Table 1 Zircon U-Pb of Taiyangzhai and Liujiazhai Nb-enriched rocks in southwestern margin of Yangtze Block

Fig.4 Diagram of zircon U-Pb data

Fig.5 Diagrams of zircon U-Pb vs εHf(t). Modified after [4-5,21].

Fig.6 Diagrams of (a) SiO2 vs total alkaline (Na2O+K2O); (b) SiO2 vs K2O; (c)Nb vs Nb/U;(d) SiO2 vs TiO2. Modified after [4-5,22-24].

Fig.7 Diagrams of (a, c) whole rocks chondrite normalized REE patterns and (b, d) primitive mantle normalized incompatible element variation. Modified after [25].

Fig.8 Diagrams of whole rocks (87Sr/86S)i vs εNd(t). Modified after [4].

Fig.9 Diagrams of crustal contamination of intermediate-mafic intrusions

Fig.10 Diagrams of mantle metasomatism of ultramafic-mafic and intermediate intrusions. Modified after [4,29-30]. (a)—(Ta/La)N vs(Hf/Sm)N([29]);(b)—Nb/Y vs Ba/La;(c)—Th vs U/Th;(d)—Th/Zr vs Nb/Zr([30]);(e)—εNd(t) vs εHf(t)([4]);(f)—全岩(87Sr/86Sr)i vs εNd(t)。

Fig.11 Tectonic diagrams of of Nb-Ti and arc ultramafic-mafic and intermediate intrusions. Modified after [36-38]. (a)—U-Pb vs V/Sc([36]);(b)—Ti vs V([37]);(c)—Y/15-La/10-Nb/8;(d)—Hf/3-Th-Ta图解([38])。

Fig.12 Metasomatized diagrams of Nb-Ti enrich and arc mafic and intermediate intrusions

Fig.13 Diagrams of time-space relationship between different mantle metasomatism and arc magmatism from margins of Yangtze Block. Modified after [6].

Fig.14 Diagrams of Neoproterozoic structural transition from margins of Yangtze Block. Adapted from [45].

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