Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

Abstract

An integrated study of zircon U-Pb ages, Hf-O isotopes, whole-rock major-trace elements and Sr-Nd-Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle-Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle-Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1-3.7 wt.% and low Mg# values of 1.6-51.2, high K 2 O contents of 2.0-9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr-Nd-Hf-Pb isotope compositions, with initial 87 Sr/ 86 Sr ratios of 0.7050 to 0.7082, negative ε Nd (t) values of-8.1 to-1.8 and negative ε Hf (t) values of-6.7 to 0.4. Zircon U-Pb dating yields consistent ages of 128±2 to 133±2 Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high  18 O values of 5.7 to 7.8 ‰ and variable ε Hf (t) values of-5.8 to 0.7. The all andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle-Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical compoisiton of andesitesis primarilythat of mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesitic rocks, but they are not the governing factors for their trace element and radiogenic isotope compositions.

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

“…In the diagram that plots (La/Sm) N against La (Figure a), the samples fall along the path of partial melting/mixing but away from that of fractional crystallization, ruling out the possibility that fractional crystallization occurs in the production of the MOT trachyandesite. Moreover, the lack of correlations found between the trace element and SiO 2 (Figure b) cannot be due to fractional crystallization but may be attributed to the mixing of two different magmas (Chen, Zheng, & Zhao, ). In Figure c,d, two types of melt compositions can be identified: one is characterized by high LILE and LREE content levels, positive Zr and Hf anomalies, and negative Sr anomalies, whereas the other is characterized by lower LILE and LREE content levels and by slightly positive Sr anomalies.…”

Structural characteristics and geochronology of thrust faulting in the Jiulianshan anticlinorium, theLowerYangtze region,East China

“…Magmatism in southern Anhui Province can be divided into two separate phases at 150–136 and 136–120 Ma (Wu et al, ). The Late Mesozoic intrusive rocks in the Tongling area formed predominantly between 147 and 135 Ma (Wu et al, ; Xu et al, ), and those in the Ningwu, Luzong, and Fanchang basins were mostly emplaced after 131 Ma, with the exception of a few that were emplaced earlier than 135 Ma (Chen et al, ; Wang, Wang, Yu, Yang, Huang, & Zhang, ). Taken together, Late Mesozoic magmatic activity across the South China Block can be considered to have occurred during four major episodes: 147–136, 136–118, 117–108, and 107–86 Ma (Li et al, ).…”

“…Compilation of concordant zircon U–Pb ages obtained for late Mesozoic intrusions in the Lower Yangtze region (data from Wu et al, ; Xu et al, ; Su et al, ; Wu et al, ; Chen, Zhao, & Zheng, , Chen et al, ; Li et al, ; Wang, Wang, Wang, Yu, Yang, Huang, & Zhang, and references therein; Xie, Fang, Xia, Li, & Sun, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…During the Late Mesozoic, basin‐and‐range tectonics and associated magmatism were prevalent in south‐east China as a result of palaeo‐Pacific Plate subduction (Li et al, ; Wang & Shu, ), which is representative of an extensional tectonic setting. Abundant magmatic activity took place in the Lower Yangtze region between 150 and 120 Ma (Wu, Ji, Sun, Yang, & Li, ; Wu et al, ; Xu et al, ; Chen, Zhao, & Zheng, ; Chen, Zhen, & Zhao, ; Li et al, ; Mao, Li, & Ye, ; Wang, Wang, Yu, Yang, Huang, & Zhang, ). During the same interval, widespread metallogenesis occurred (Hua et al, ; Zhou et al, ; Li, Li, Wang, Li, Liu, Tang, Gao, & Wu, ; Chang et al, ; Yang & Zhang, ) and can be broadly divided into two stages: (a) copper and gold mineralization, which is closely related to earlier (~145–136 Ma) high‐K calc‐alkaline series intrusions, and (b) iron and sulphur mineralization associated with later (~135–123 Ma) shoshonitic series volcanic rocks.…”

“…Therefore, small-scale thrust fault motion happened since the earlier part of the Early Cretaceous. Some faults in the fold-and-thrust Wu et al, 2012;Xu et al, 2012;Su et al, 2013;Wu et al, 2013;Chen, Zhao, & Zheng, 2014, Chen et al, 2016Li et al, 2014;Wang, Wang, Wang, Yu, Yang, Huang, & Zhang, 2014 and references therein; Xie, Fang, Xia, Li, & Sun, 2017) [Colour figure can be viewed at wileyonlinelibrary.com] So, multiple thrust fault motions may have happened in the Maoshan-Jiulianshan nappe. In the Maoshan nappe, fold axial planes in the hanging wall dip predominantly to the ESE, and borehole data show that the Lower Cretaceous Gecun Formation and volcanic rocks occur in the footwall (Zhang, 1986;Zhu, 1988).…”