Late Triassic alkaline complex in the Sulu UHP terrane: Implications for post-collisional magmatism and subsequent fractional crystallization

Abstract

Continental subduction and its interaction with overlying mantle wedge are recognized as fundamental solid earth processes, yet the dynamics of this system remains ambiguous. In order to get an insight into crust-mantle interaction triggered by partial melting of subudcted continental crust during its exhumation, we carried out a combined study of the Shidao alkaline complex from the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic (K2O: 3.4-9.3 wt.%) gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field studies suggest that the mafic rocks are earlier than the felsic ones in sequence. LA-ICPMS zircon U-Pb dating on them gives Late Triassic ages of 214 ± 2 to 200 ± 3 Ma from mafic to felsic rocks. These ages are slightly younger than the Late Triassic ages (225-210 Ma) of the felsic melts from partial melting of the Sulu UHP terrane during exhumation. The alkaline rocks have wide ranges of SiO2 (49.7-76.7 wt.%), MgO (8.25-0.03 wt.%), Ni (126.0-0.07 ppm), and Cr (182.0-0.45 ppm) contents. The contents of MgO, total Fe2O3, CaO, TiO2 and P2O5 decrease with increasing SiO2 contents. The contents of Na2O, K2O, and Al2O3 increase from gabbro to amphibole syenite, and decrease from amphibole syenite to granite, respectively. The alkaline rocks have characteristics of an arc-like pattern in trace element distribution, e.g., enrichment of LREE, LILE (Rb and Ba), Th and U, depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic rocks to the felsic rocks, the (La/Yb)N ratios and the contents of the total REE, Sr and Ba decrease but the Rb contents increase. The alkaline rocks with high SiO2 contents also display features of an A2-type granitoids, e.g., high contents of total alkalis, Zr and Nb and high ratios of Fe2O3 T /MgO, Ga/Al, Yb/Ta and Y/Nb, suggesting a post-collisional magmatism during exhumation of the Sulu UHP terrane. The alkaline rocks have homogeneous initial 87 Sr/ 86 Sr ratios (0.7058-0.7093) and negative εNd(t) values (-18.6 to-15.0) for whole-rock. The Sr-Nd isotopic data remain almost unchanged with varying SiO2 and MgO contents, suggesting a fractional crystallization (FC) process from the same parental magma. Our studies suggest a crust-mantle interaction in continental subduction interface as follows: (1) hydrous felsic melts from partial melting of subducted continental crust during its exhumation metasomatized the overlying mantle wedge to form a K-rich and amphibole-bearing mantle; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex under a post-collisional setting; and (3) the alkaline magma experienced subsequent fractionational crystallization mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar.

Petrogenesis of Cretaceous granites in the southern Sulu Orogen: Implications for the flare‐up of post‐collisional magmatism

“…And a scenario for the origin of the monzogranites in the Sulu belt is presented in Figure 12. During 205–185 Ma (Figure 12a), the secondary intracontinental compression between the NCC and the Yangtze Block further thickened the crust and induced the transformation of the thickened lower crust to eclogite (Liu et al, 2008a; Wang et al, 2016; Wang, Liu, Zhou, & Li, 2013; Wu & Yong, 2013; Xu, Zhang, Wang, & Liu, 2016). Moreover, the density of eclogite is higher than that of lithospheric mantle peridotite (Levander, Niu, Lee, & Cheng, 2006).…”

Early Cretaceous uplift of the Jiaobei Terrane: Evidence from the detrital zircon and sediment compositions of sandstones in the Jiaodong Peninsula, China

“…The protolith ages of gneisses and eclogites are the Neoproterozoic (Z. Q. Xu et al, 2009) with UHP metamorphism at 240–220 Ma determined by U–Pb ages of zircon rims overgrown during the metamorphic process (R. Y. Zhang et al, 2009). In the southeastern part of the Sulu Orogen, the Late Triassic alkaline intrusions (214–200 Ma) have been merely identified throughout the Jiaodong Peninsula with their formation at the post‐collision stage (H. J. Xu, Zhang, Wang, & Liu, 2016; Z. F. Zhao et al, 2012).…”

Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

“…The Sulu UHP metamorphic terrane is commonly regarded as the eastern extension of the Dabie UHP metamorphic terrane, which was displaced by the Tanlu Fault (Figure 1a). The Sulu‐Dabie orogenic belt was unconformably overlain by Jurassic clastic sedimentary rocks and Cretaceous volcaniclastic deposits, and intruded by post‐collisional Mesozoic granites (Figure 1b; e.g., Goss, Wilde, Wu, & Yang, 2010; Guo, Chen, Zhang, Siebel, & Zhai, 2005; Xu, Ma, & Ye, 2007; Xu, Zhang, Wang, & Liu, 2016).…”