Geochronological And Geochemical Constraints On The Petrogenesis Of Permian Dolerite Dyke Swarms In The Beishan Orogenic Belt, Nw China

Extensive Early Permian mafic-ultramafic intrusions, doleritic dykes, and basalts crop out within the Beishan area, southern Central Asian Orogenic Belt (CAOB). We present new geochronological and geochemical data for Gubaoquan dolerite dyke swarms in the Beishan orogenic belt. Zircon U-Pb Dating of the Gubaoquan dykes indicates that they were emplaced during the Early Permian (280.7 +/- 4 Ma), that was coeval with Yinaoxia and Podong mafic dykes in Beishan area. The dykes are characterized by low Mg# (47-84) in the clinopyroxene crystals, and the content of whole-rock Fe2O3 (t), MgO, and alkali (Na2O + K2O) range from 12.5-17.4, 4.06-5.51, and 2.8-4.4 wt.%, respectively. The samples from the Gubaoquan dykes have high and variable Ba/La (5.93-14.2) and Ba/Nb (15.0-37.3) ratios but low Th/Yb (0.17-0.24) ratios. The rocks show slightly enrichments in LREE, HFSE, Th, and Hf, and depletion in Nb and Ta. The epsilon Nd (t = 280 Ma) values and initial Sr-87/Sr-86 ratios of the Gubaoquan dykes show variations ranging from 6.4 to 6.8 and 0.706240 to 0.707546, respectively. These data suggest that the parental magmas for the Gubaoquan dykes were probably derived from partially decompressed melting of upwelling depleted asthenosphere mantle that was metasomatized previously by subducted fluids.