Geochemistry of an Island-Arc Plutonic Suite: the Uasilau-Yau Yau Intrusive Complex, New Britain, P.N.G

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Abstract
In the Uasilau-Yau Yau intrusive complex of central New Britain, Papua New Guinea, there is a compositional continuum in intrusive rock-types from gabbro to granodiorite and K-Ar mineral ages of the most mafic and most felsic components are not significantly different (29±0.6 Ma versus 28.3±0.5 Ma, respectively). Tonalite porphyry, the progenitor of porphyry copper mineralization in the complex, represents a significantly younger intrusive event at 24 Ma. Relatively calcic (An95—50) plagioclase cores and salite to augite composition clinopyroxene are texturally early phases in the intrusive rocks. The main mafic mineral, calcic amphibole, generally has corroded clinopyroxene cores and may, like biotite, K-feldspar and quartz, generally be a late-stage, not a primary liquidus phase. Petrographic features indicate that the mafic minerals in the plutonic rocks crystallized from melt, rather than being restite phases. The intrusive rocks cover an extensive silica range (45–75 wt. per cent), do not exhibit simple straight-line variation on Harker diagrams for many elements (e.g. TiO2, FeO, P2O5 and Sr), and most are relatively depleted in incompatible trace elements (Rb, Zr, and REE). Major and trace element modelling supports derivation of the complex by shallow level fractional crystallization dominated by removal of the phases calcic plagioclase, clinopyroxene, and magnetite from a parental magma closely resembling recent basaltic rocks in New Britain. The fact that the plutonic rocks are almost chemically indistinguishable from late Cainozoic calc-alkaline volcanic rocks of New Britain supports fractional crystallization as a viable mechanism for generating these island-arc volcanic rocks and indicates an analogous origin for the initial magma. Granites, such as those of the Uasilau-Yau Yau intrusive complex, which are probably generated by partial melting of subducted oceanic crust or the overlying mantle, may be termed mantle or M-type granites. Documentation of the characteristics of M-type versus normal I-type granites may enable the recognition of M-type plutonic rocks in older, possibly more deeply eroded geologic terrains. This would, by analogy to their volcanic equivalents, be very helpful in tectonic interpretations. Also, such plutonic rocks have known potential for Cu-Au mineralization.

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