Eight samples of basaltic-to-rhyolitic calc-alkalic lavas erupted from the Rabaul caldera on the island of New Britain have Sr87/Sr86 ratios ranging from 0.7035 to 0.7040 and averaging 0.7038. The mean strontium-isotope ratio is in the middle of an extremely limited range of values for other calc-alkalic island-arc lavas of the western and southwestern circum-Pacific region. The constancy of Sr87/Sr86 in the Rabaul lavas is compatible with derivation of the andesitic and more silicic members of the series by fractional crystallization of a basaltic magma. However, the strontium-isotope data do not preclude different degrees of partial melting of an isotopically uniform source in the mantle as a possible mechanism for producing the compositional variety of lavas at Rabaul.

* Present address: Department of Geology, University of Auckland, Private Bag, Auckland, New Zealand.

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