Deformation and regional carbonate metasomatism of turbidite-hosted Cretaceous alkaline lamprophyres (northwestern Papua New Guinea)

A suite of variably metasomatised camptonite dykes and rare stocks, named the Fu Intrusives, has been recognised within thrust plates of Mesozoic slates in a 3000-km2 portion of the New Guinea Thrust Belt. The intrusions were emplaced prior to Oligo-Miocene fold deformation and low-grade regional metamorphism of their host, and consequently have foliated margins and are cross-cut by metamorphic quartz veins. Samples from the less altered stocks display primary cumulophyric aggregates of kaersutite + titansalite + titanbiotite, set in a matrix of albite + salite + apatite ± olivine ± quartz. Calcite ocelli are occasionally present. REE patterns ( La CN Lu CN ∼ 10 , La ∼ 100 × chondrite) and initial Sr ratios (∼0.7042) are typical of alkaline lamprophyres. Kaersutite and titanbiotite separates from such samples have yielded concordant KAr ages of ∼75 Ma (Campanian), which we accept as the age of intrusion. However, the dykes have suffered heavy texture-destructive carbonate-sericite alteration, though relict cumulophyric texture and globular structures are sometimes observed. Composition-volume calculations suggest that metasomatism was accompanied by loss of Na, Ti and P; and systematic addition of Ca, Fe and CO2, as well as progressive LREE depletion and Fe reduction. Alteration carbonates exhibit a narrow range of δ18O-values (+ 13.8 to + 14.9‰), indicating uniform conditions of metasomatism throughout the terrain. The calculated fluid δ18O at a model temperature of 300 ± 50°C is +8±2‰ , which corresponds with that deduced from the δ18O-value (+ 12.3‰) of the cleanest sericite separate. The four analysed sericite separates have a mean δD of −64 ± 12‰ , implying an hydrogen isotopic composition for the alteration fluid of −58 ± 30‰ at 300 ± 50°C. The calculated fluid composition therefore overlaps the normal range of metamorphic and magmatic waters. However, C and Sr isotopic ratios of the alteration carbonates (δ13C − 7.3 to + 0.4‰; 87 Sr 86 Sr 0.7076–0.7126) strongly support a crustal source for the metasomatic fluids. This is corroborated by field evidence that country rock slates adjacent to the dykes are unaffected by carbonate metasomatism and hence were in equilibrium with the alteration fluid. We therefore conclude that regional carbonate metasomatism of the Fu Intrusive Suite occurred as a result of metamorphic dewatering of their host rocks during slaty cleavage development.