Abstract

The upper Oligocene to mid-Miocene granitic intrusive complexes of New Britain intrude basic Eocene volcanics that form part of the basement of the New Britain island arc. These complexes consist of geochemically coherent suites of I-type intrusions that range from diorite to granite; most are tonalites. Small centers of porphyry copper mineralization are found in association with plagioclase-quartz porphyry dike suites that generally invade the margins of the premineralization complexes.The intrusive complexes of central and western New Britain are found as isolated bodies along northwest-trending belts, whereas those of the Gazelle Peninsula crop out continuously along major northwest-trending fault systems. These relations probably reflect differing structural development in the two main sections of the arc.The intrusives consist of pyroxene, hornblende, biotite, calcic plagioclase, quartz, and orthoclase in varying proportions. This is a typical I-type assemblage. Accessories include magnetite, sphene, ilmenite, zircon, and apatite. Hornblende is the most abundant mafic mineral; orthoclase is important only in the most felsic intrusives and the members of the high-potassium suites, where it forms an interstitial phase. Mafic xenoliths are widely distributed through the intrusives and are considered to represent residual, refractory material carried in the magmas from their source.The granitic intrusives have I-type geochemistry which supports their derivation by partial melting of fractionated igneous rocks, most likely at the base of the crust. P-wave velocities of lower crustal rocks and estimates of past crustal thickness support this argument. The intrusives were probably emplaced at a time when 20 to 30 km of crust had developed within the arc.The close association between the granitic rocks and the plag-ioclase-quartz porphyries implies a relation in derivation, although the porphyries are texturally and geochemically distinct. The porphyries are not considered to be simple fractionates or textural variants of the granitic rocks, and their derivation from a lower crustal, fractionated igneous source, in a separate melting event, is also supported by their I-type geochemistry, low Ni, Cr, Co, and low Rb/Sr.The participation of magmatism from a mantle source is not considered, in this instance, to be a necessary condition for the generation of the granitic intrusives, porphyries, and associated copper mineralization systems.

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