Geochronology, geochemistry, and Sr-Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U-Pb ages of 281 & PLUSMN; 4 Ma. Characterized by low SiO2 (44.88-47.73 wt.%) and high Mg-# values (62.2-68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)(n) = 0.72-1.43) but enriched in large ion lithophile elements (LILEs). They show high positive & epsilon;(Nd)(t) values (6.6-8.5) with a range of (Sr-87/Sr-86)(i) (0.703477-0.705789) ratios. In contrast, the type 2 dikes have higher SiO2 content (47.47-51.13 wt.%) and lower Mg-# (40.9-63.0). They are enriched in LREEs ((La/Yb)(n) = 1.79-5.02) and LILEs, and have lower & epsilon;(Nd)(t) values (3.8-7.4) with a variable range of (Sr-87/Sr-86)(i) (0.702873-0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86-460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr-Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04-0.24), low Ba/Rb (0.9-3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr-Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.