Petrology of volcanic rocks from Kaula Island, Hawaii

Abstract

A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a “post-erosional” stage of volcanism.