Volcanoes and rocks of St Andrew Strait, Papua New Guinea

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Abstract
When Tuluman volcano, in St Andrew Strait, northern Bismarck Sea, erupted between 1953 and 1957, it produced acid rocks similar in major element chemistry to those of three other islands in the Strait — Lou, Pam Lin, and Pam Mandian. These acid rocks — termed the TLP’ series — are thought to represent magmas, or to be derivatives of a parental magma, produced by melting of crust (about 25 km thick beneath St Andrew Strait). TLP rocks have agpaitic indices ranging between 0.86 and 0.96. Acid lava also makes up 3 of the 4 Fedarb Islands at the northern end of St Andrew Strait, but its composition appears to be unrelated to that of the TLP series, and its origin is uncertain. Q‐normative basalts (quartz tholeiites) make up the fourth island of the Fedarb group, and ol‐ and hy‐normative basalts crop out on Baluan Island at the southern end of the Strait. These basalt types do not appear to be directly related to one another, although both may have been derived from parents that originated in the upper mantle. Andesitic rocks have not been found on any of the islands in St Andrew Strait. Tuluman and the volcanoes of Lou Island form an arc which may be part of a developing (or completed) ring fracture whose centre coincides with the line between Baluan Island, the Pam Islands, and the Fedarb Islands. The ring fracture may be the result of sagging of crust above a zone of crustal melting that produced the TLP magmas. It is possible that collapse could take place along this ring fracture, producing a caldera.

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