Nature and tectonic environment of Late Jurassic volcanic-sedimentary basins in northwestern Hebei Province

Petrological and geochemical studies of Late Jurassic volcanic rocks in the Tiaojishan and Houcheng Formations in northwestern Hebei Province reveal that Late Jurassic volcanic rocks are composed mainly of a shoshonite series derived from the enriched mantle, and some high-potassium acidic rocks of crustal affinity. Overlying the Tiaojishan Formation, the Houcheng Formation consists primarily of fluvio-lacustrine rock associations and is believed to have deposited in rifted basins in an extensional setting. It is marked by coarse-grained sediments of alluvial fans and braided rivers in the lower part and fluvio-lacustrine fine-grained sediments intercalated with volcanic rocks in the upper part, thus generally exhibiting an upward-fining sequence. The stratigraphic and sedimentary evolution from the Tiaojishan to Houcheng Formations reflects the transition from a down-faulted basin to a downwarped basin. Regional compression occurring after the deposition of the Houcheng Formation led to the reversion of the extensional basin of this stage.