Nd-Sr isotope features of Cretaceous volcanic rocks in northwestern Zhejiang

Abstract：Cretaceous volcanic rocks in Zhejiang Province occur on two different basement tectonic units:the Yangtze block and Cathaysian block. There is great difference in Nd isotopic composition between volcanic rocks of the two different tectonic units. The εNd values of volcanic rocks of the Yangtze block are higher, ranging between –8.9 and –0.9 with a mean of –4.6 (n=33), whereas the εNd values of volcanic rocks of the Cathaysian block range from –1.41 to +1.2 with a mean of –6.7 (n=69). Furthermore, the εNd values of volcanic rocks of the two blocks increase with time and the model ages show a very marked trend of decrease with time. In the Yangtze block the mean εNd and t2DM values of acid volcanic rocks from cycle I formed at 135-124 Ma B．P． to cycle III formed at 110-900 Ma B．P． are -6.2,-2.4 and -2.2 and 1.42, 1.11 and 1.09 Ga respectively. In the Cathaysian block the mean εNd and t2DM values of acid volcanic rocks of the contemporaneous three cycles are -10.1, -7.2 and -5.5 and 1.75, 1.49 and 1.35 Ga respectively. It is inferred that the Cretaceous rhyolitic rocks developed widely in Zhejiang Province probably originated by partial melting of the basement metamorphic rocks. Continuous underplating of basalt resulted in significant increase of introduced mantle-derived materials with time.