Fohn lamproite and a possible Late Eocene — pre‐Miocene diatreme field, Northern Bonaparte Basin, Timor Sea

Abstract

Seismic profiles in the Northern Bonaparte Basin, Timor Sea, northern Australia, disclose an east‐northeast‐trending 120 × 25 km swathe of over 40 circular to subcircular features excavated in the pre‐Miocene erosional surface and buried by Lower Miocene sediments. The larger structures, typified by the Fohn Structure, include central structural highs overlying narrow vertical corridors of upward‐bulging seismic horizons (bulge‐forms). Associated with these structural highs are troughs that overlie broader vertical zones of slower seismic velocity (crater‐form features). Smaller circular features (Dc <2.0 km) include both crater‐form and bulge‐form structures. The first type shows zones of seismic velocity crater‐form structures under craters excavated below the base‐Miocene horizon. The second type exhibits vertical seismic bulge‐form features directly beneath palaeotopographic highs developed at the same horizon. Identification in drill cuttings of fragments of olivine‐leucite lamproite at Fohn 1, at depths of 690–880 m, suggests that this structure is a lamproite diatreme consisting of a massive volcanic plug ringed by pyroclastics, analogous to the classic champagne‐glass structure of alkaline volcanic diatremes. The seismic morphometry of several of the larger structures of the field is analogous to Fohn, suggesting that they too may be diatremes. The smaller bulge‐form features may represent deeply eroded diatreme feeder necks. Crater‐like structures underlain by seismic velocity crater‐forms are interpreted as relatively little‐eroded maars with low‐velocity infill, probably consisting of volcanic and country rock breccia as at Fohn 1, as well as unconsolidated clastics of the transgressive Lower Miocene Oliver Sandstone Member. Lamproite fragments at Fohn 1 are dominated by apatite‐rich assemblages consisting of saponite‐altered olivine, analcime and nontronite‐altered zoned leucite, high‐Ti K‐bearing diopside, Ti‐rich richterite, Ti‐rich phlogopite, Mg‐rich ilmenite, Cr‐bearing priderite, Ba and Sr‐rich fluorapatite and rare‐earth element‐rich apatite. Textural relations suggest early crystallisation of olivine, leucite and K‐bearing diopside followed by crystallisation of alkali amphibole, priderite, ilmenite and apatite in alkali‐Tindash;Fe–rich groundmass. Fohn 1 lamproites have very high whole‐rock P levels, high Na,Sr and Y and low K,Ba,Zr and Nb levels compared to Early Miocene West Kimberley lamproites. The mineral paragenesis suggests crystallisation of olivine‐leucite assemblages at temperatures >1100°C and pressures <0.1 GPa, followed by crystallisation of amphibole and phlogopite under water‐saturated conditions at temperatures below ∼1020°C.