Petrochemistry of hornblende gabbro from Tong' an of Fujian Province shows typical characteristics of low-MgO and high-alumina basalts. The amphibole in it is calcium-rich tschermakitic hornblende and the plagioclases in it are anorthites (An =91 - 95 ). In basic arc magmas, the high content of H2O content of the low-MgO and high-alumina basaltic magmas leads to crystallization of high-calcium plagioclase. The 40Ar-39Ar age of the tschermakitic hornblende is 129. 620?. 152 Ma. It suggests that hornblende gabbro from Tong'an is the product of magmatic activities of "early extensional stage" during the early Cretaceous in the southeastern coast of China.