Definition card for: Oenpelli Dolerite

State(s): NT,
Status of unit: Formal
Usage: Defined
Stratno: 14496

Reserved by: Needham, R.S. on: 20-MAR-73
Approved: Y
Approved by: Strusz, D.L. on: 16-SEP-74

Parent: Kombolgie Subgroup

Name source: Oenpelli Mission 133o04', 12o20'. Alligator River 1:250 000 Sheet area.

Unit history: Walpole et al. (1958) misinterpreted some exposures as either Zamu Complex or Nungbalgarri Volcanic Member - remainder of exposures were unnamed dolerite.

Type section locality: Graveside Gorge, 132o34', 13o18'. A ridge of differentiated dolerite 150 m high trending northeast along south side of gorge. Differentiates include porphyritic olivine dolerite, ophitic dolerite and ophitic gabbro with igneous lamination, granophyric dolerite and syenite.

Extent: As arcuate ridges over 30,000 km2 in the Alligator River, Mount Evelyn, Milingimbi and Cobourg Peninsula 1:250 000 Sheet areas between 132o30' and 134o00', and 11o45' and 13o30'.

Thickness range: Range <250 m. Arcuate ridges probably are exposed rims of roughly ellipsoidal basins which may be part of a large undulating sheet of dolerite, or mark individual lopoliths, the latter being supported by a general thickening of the dolerite towards the centres of the basins. There are also occasional narrow dykes of Oenpelli Dolerite (porphyritic olivine dolerite) >10 cm with sharp margins.

Lithology: An approximately symmetrically differentiated dolerite intrusive sill. Central part is igneously laminated ophitic dolerite often displaying local ophitic gabbro, granophyric dolerite, granophyre and syenite differentiates as large lens-shaped bodies. Central part grades upwards and downwards with decrease in groundmass size to porphyritic olivine dolerite containing porphyroblasts of saussertized pale green plagioclase <5 cm. A chilled margin is rarely apparent at the outer margins of the porphyritic olivine dolerite phases. Gabbro pegmatite is rarely developed adjacent to the upper chilled margin (see accompanying card).

Relationships and boundaries: Intrudes Nanambu and Nimbuwah migmatite Complexes. Intrudes all Lower Proterozoic sedimentary and metamorphic units. Forms basement highs to, and unconformably overlain by, Carpentarian Kombolgie Formation sandstone. Also intrudes Jim Jim Granite. Generally sharp contacts with country rock, which is usually hornfelsed. In central parts of Nimbuwah Complex however boundaries often gradational into country rock which was assimilated by the dolerite during intrusion.

Age reasons: 1720 m.y. Total rock Rb-Sr isochron of 1718 +/- 65 m.y. Page & Needham (in prep.).

Proposed publication: Journal of the Geological Society of Australia