Late Triassic alkaline complex in Sulu UHP terrane: Implications for post-collisional magmatism along the continental subduction zone

Abstract:

In order to insight into crust-mantle interaction triggered by partial melting of the subudcted continental crust during its exhumation, we carried out a combined study on Shidao alkaline complex in the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field researches 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 a bit younger than Late Triassic ages for partial melting of the Sulu UHP terrane during exhumation, indicating syn-exhumation magmatism during continental collision. The alkaline rocks have wide ranges of SiO₂ (49.7 – 76.7 wt.%), MgO (8.25 – 0.03 wt.%),total Fe₂O₃ (9.23 – 0.47 wt.%), CaO (8.39 – 0.39 wt.%), Ni (126.0 – 0.07 ppm), and Cr (182.0 – 0.45 ppm) contents. Other major oxides are regularly changed with SiO₂. The alkaline rocks have characteristics of arc-like patterns in the trace element distribution, e.g., enrichment of LREE and LILE (Rb, Ba, Th and U), depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic to felsic rocks, (La/Yb)_N ratios and contents of the total REE, Sr and Ba are decreased but Rb contents are increased. The alkaline rocks also display features of A2-type granitoids, suggesting a post-collisional magmatism. They have high initial ⁸⁷Sr/⁸⁶Sr ratios (0.70575 and 0.70927) and negative ε_Nd(t) values (-18.6 to -15.0) for whole-rock. The homogeneous initial ⁸⁷Sr/⁸⁶Sr ratios and ε_Nd(t) values of the alkaline rocks are almost unchanged with SiO₂ and MgO contents, suggesting a fractional crystallization (FC) process from a same parental magma. Our studies suggest a series of crust-mantle interaction processes along the continental subduction interface as follows: (1) melts from partial melting of the subducted continental crust during its exhumation metasomatized the overlying mantle wedge; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex; and (3) the alkaline magma was successively experienced crystal fractionation mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar.