Geochemical characteristics of Late Permian-Early Triassic continental volcanic rocks in the Zadoi area,northern Qiangtang,Qinghai-Tibet Plateau

Late Permian-Early Triassic volcanic rocks were for the first time defined in the Telongsai area,Zadoi County,in the eastern part of the northern Qiangtang basin.The volcanic rocks formed in a continental environment and four explosive- effusive volcanic rhythms make up a complete volcanic cycle.These volcanic rocks consist predominantly of strongly altered amygdaloidal olivine basalt,lithic and crystal tuff and amygdaloidal andesite.They are characterized by low TiO_2 contents (1.33-1.76%,equivalent to that of the low-Ti Emeishan basalt),low MgO contents (2.23-7.52%,with a mean of 5.02%), enrichment of LREE (4.16-13.44,with a mean of 6.5),no pronounced Eu anomaly (δEu=0.85-1.02,with a mean of 0.96) and weak negative Nb and Ta anomalies.These geochenfical characteristics are similar to those of continental tholeiite. Discrimination diagrams of various trace elements indicate that these volcanic rocks formed in a continental extensional zone or intracontinental rift zone.The above-mentioned characteristics,combined with the regional research results,suggest that the whole Qiangtang area was very likely to lie in an extensional environment during the Late Paleozoic.