Abstract:

The zircon U-Pb ages were dated by SHRIMP method for eight basaltic rocks occurred in the eastern area of Zhejiang Province，which were erupted during the Late Mesozoic and named as Lower and Upper rock series (LRS & URS). The weighted average 206Pb/238U age of the basalts from LRS is (120±1) Ma with Th/U ratios ranging from 0.65 to 2.44. The weighted average 206Pb/238U age of the basalts from URS is (103±2) Ma, with Th/U ratios ranging from 0.57 to 3.03. These data suggest that the zircons from both rock series are of magmatic origin and represent the formation times of the basalts of LRS and URS. Thus, it can be concluded that the formation time of the URS and LRS is significantly different, and the age data are consistent with the previous researches. It can also be found that most of the zircons from the LRS basalts have a relatively homogeneous ages with only a few ancient zircon relicts, which indicates that they have little crustal contamination during petrogenesis. Some zircon grains show core-rim structure with a core age of about 132 Ma and a rim age of about 120 Ma, which may indicate that the inherited grains were captured from the magma source, and imply that there exist magmatic rocks formed at about 130 Ma in the mantle or mantle-crustal boundary during the LRS magma activity. The magmatic zircons from URS basalts are very few, many detrital grains were dated with the ages covering almost all of the magmatic events in the South China block. It is suggested that the URS basaltic rocks were contaminated by clastic rocks which contained the detrital grains with different ages.

Key words: zircon SHRIMP U-Pb dating, basalts, Late Mesozoic, eastern Zhejiang Province