Anomalously young volcanoes on old hot-spot traces: I. Geology and petrology of Cocos Island

Cocos Island is the summit of a seamount on the aseismic Cocos Ridge, a proposed trace of the Galapagos hot spot. The island lies on a portion of the ridge that is middle Miocene in age, but K/Ar and paleomagnetic dates indicate that Cocos is only about 2 m.y. old. Cocos thus offers a rare opportunity for an on-land study of seamount volcanism superimposed on an early hot-spot volcanism. Cocos Island was built in three major stages that define three lithostratigraphic units: (1) shield-building, (2) explosive volcanism, and (3) post-explosive volcanism stages. All Cocos rocks belong to the typical oceanic island alkali basalt-to-trachyte series and have fairly homogeneous Sr ( 87 Sr/ 86 Sr = 0.70299-0.70308), Nd ( 143 Nd/ 144 Nd = 0.512952-0.513001), and Pb ( 206 Pb/ 204 Pb = 19.214-19.251; 207 Pb/ 204 Pb =15.553-15.596; 206 Pb/ 204 Pb = 38.899-39.036) isotopic ratios. The Cocos rock series was generated by fractional crystallization of olivine, clinopyroxene, plagioclase, ilmenite, and apatite from similar alkali basalt parental magmas. Sr, Nd, and Pb isotopic ratios indicate that Cocos and Galapagos volcanic rocks may have come from a common, although heterogeneous, mantle reservoir, and this implies that the young Cocos volcano is still a part of the Galapagos hot-spot signal.