Origin of two contrasting latest Permian-Triassic volcanic rock suites in the northern North China Craton: implications for early Mesozoic lithosphere thinning

摘要

New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian-Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian-Early Triassic at ca. 255-245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO_2 and K_2O, moderate initial ~(87)Sr/~(86)Sr, low negative εNd(t) and εHf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust-mantle boundary in syncollisional to post-collisional/ post-orogenic tectonic settings. The late rock suite, erupted during the Middle-Late Triassic at ca. 238-228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high A1_2O_3, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial ~(87)Sr/~(86)Sr; high negative εNd(t) and εHf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle-Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.