Abstract:

his paper reports the petrological and geochemical characteristics of Permian alkaline and tholeiitic basalts in the Shuanghu Area, central Qiangtang Block. The alkaline basalts are enriched in LILE and LREE, compared with MORB; they have similar La/Nb ratios and higher Ti/V ratios; in the trace element tectonic setting discrimination diagrams, all the alkaline samples are plotted in the field of OIB. The tholeiitic samples have flat REE pattern that is similar to MORB; they have higher Ti/V ratios than those of the MORB; in the trace element tectonic setting discrimination diagrams, all the tholeiitic samples are plotted in the overlapping field between OIB and MORB; these features suggest that the tholeiitic basalt was derived from a relatively enriched mantle source. When coupled with the regional tectonic setting, it is considered that the Permian alkalinetholeiitic basalts association in the Shuanghu Area was formed in the rift valley setting, and their occurrence may indicate a process from rift valley to juvenile oceanic crust along the LongmucuoShuanghu tectonic zone.

Key words: Qiangtang Block; LongmucuoShuanghu Suture; geochemistry; rift valley