MIDDLE TO LATE JURASSIC VOLCANICS OF THE TIAOJISHAN FORMATION, NE CHINA: EVIDENCE OF MELTING OF HYDRATED MANTLE-WEDGE DURING PALEOPACIFIC SUBDUCTION

In northeastern China, the Middle to Late Jurassic (ca. 165-152 Ma) Tiaojishan Formation consists of a northeast-southwest trending belt of predominantly mafic lavas with occasional interbedded coarse-grained clastics. Many workers (as noted by Gao, et al., 2004) have documented and interpreted the origin of Mg-rich to adakitic volcanics within the Tiaojishan Formation and other temporally equivalent magmatic suites in eastern China, to partial melting of: 1) a foundering lower crust beneath the eastern block of the North China craton from Late Triassic to Late Jurassic, 2) overthickened ecologitic Archean crust, 3) subducted oceanic crust from the Monglo-Okhotsk and/or the Paleo-Pacific plates, or 4) hydrous melting of mantle wedge material. In this study, whole-rock major and trace element geochemical analyses of lavas and tuffs from the Chengde basin region northeast of Beijing are used to suggest that volcanics of the Tiaojishan Formation in this region are representative of a typical convergent margin arc. The high-K (1.66-5.74 wt% K₂O) calc-alkaline volcanic suite ranges in composition from basaltic trachyandesite to rhyolite (53-78 wt% SiO₂). Mafic lavas of this suite (54-64 wt% SiO₂) have Mg#’s (molar 100*Mg/(Mg+Fe⁺²)) ranging from 48-52. Although not adakitic in composition (Sr/Y ratios; 48-58), the mafic suite is similar in composition, for example, to modern enriched, hi-Mg andesites (as defined by Kelemen, et. al., 2003) of central and western Aleutian arc magmas. Trace element ratios (e.g. Th/La and Ce/Pb) and high Pb_N (N = chondrite normalized) are consistent with mantle wedge partial melting induced by dehydrating slab sediment. REE ratios (La/Yb (19-44), and, LIL and HFSE enrichment trends on chondrite and MORB normalized plots, also mimic modern Aleutian arc magma compositions. High HREE concentrations Yb_N, and Lu_N (~ 6-13 ppm) are not consistent with partial melting of garnet-rich Archean crust or eclogitic slab material. The Middle to Late Jurassic age of volcanism in the Tiaojishan Formation, and its position in the eastern block of the North China craton, is most consistent with a Paleo-Pacific arc system origin.