Late Triassic alkaline complex in the Sulu UHP terrane: Implications for post-collisional magmatism and subsequent fractional crystallization |
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| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;1. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. CAS Key Laboratory of Earth''s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia;1. Department of Geosciences, University of Tübingen, Germany;2. School of Earth, Atmosphere and Environment, Monash University, Australia;3. Institute of Geological Sciences, University of Bern, Switzerland;4. Department of Geology and Geochemistry, VU University Amsterdam, The Netherlands |
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| 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 Fe2O3T/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 87Sr/86Sr 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. |
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