Sub-solidus replacement of rapakivi textures during high-temperature potassium metasomatism of the Mannum Granite

Forster, Marnie1,2, Goswami, Naina1,2, Lister, Gordon1, Reid, Anthony3

1Research School of Earth Sciences, Australian National University, Canberra, Australia, 2MinEx CRC, Canberra, Australia, 3Geological Survey of South Australia, Adelaide, Australia

The Mannum Granite is a high-temperature A-type granitoid emplaced during the late stages of the Delamerian Orogeny in South Australia. It has attracted considerable research interest over past decades because it is a porphyritic A-type granite that displays well-developed rapakivi structures and has been linked to the process of magma-mingling above deep-seated mafic igneous bodies. It contains many mafic enclaves of varied sizes and textures including potassium feldspar and quartz porphyroblasts with disseminated sulphides. The alkali feldspar cores of the rapakivi textures are lobate and overgrow the plagioclase rims, suggesting alkali feldspars are younger.

We conducted 40Ar/39Ar geochronology in conjunction with ultra-high-vacuum (UHV) step heating 39Ar diffusion experiments on K-feldspar and biotite from the Mannum granite. Conjoint inversion of data from the UHV 39Ar diffusion experiments using the Wunderkind program applied to data from the K-feldspar experiment demonstrates the presence of highly retentive core domains capable of retaining radiogenic argon even to temperatures in excess of 600°C. Inversion of the geochronology data using the MacArgon program produces a temperature-time history that suggests that the K-feldspar in the core of the rapakivi texture formed as the result of sub-solidus solid state replacement and/or metasomatism at ~443 Ma, and then cooled rapidly at ~440 Ma. Since biotite spectra which define a plateau age at ~472 Ma are relatively undisturbed, the temperature excursion associated with the metasomatic event would have to have been limited in its magnitude (<~500°C) and duration (<< 1 Ma). 

The biotite age (~473 Ma) is close to the previously determined early Ordovician crystallisation ages for the granites in this belt. In addition, the biotite appears to be highly retentive, with closure temperature for cooling at 20°C/Ma determined as the result of the UHV 39Ar diffusion experiment at ~485°C. These are thus exceptionally retentive biotites, which correlate with the fact that they are iron-rich and fluorine-rich, and fluorine-rich biotite typically exhibits higher argon retentivity. Overall this short/sharp thermal pulse may have occurred in consequence of fluid movement associated with the Old Teal Flat Shear Zone to the east, which contains fabrics dated in this study at ~445 Ma.

Microstructural analysis shows sericite and calcite precipitated along grain boundaries, fractures and exsolution lamellae. The presence of interpenetrating grain boundaries, sericite, calcite and secondary K-feldspar is attributed to the infiltration of an acidic hydrothermal agent hydrolysing primary K-feldspar to produce secondary sericite and quartz, further reacting with Ca-rich plagioclase and quartz to produce secondary K-feldspar phase(s) and calcite. Microstructures thus confirm the presence of a potassium-rich metasomatic event at Mannum leading to fluid-assisted in-situ replacement of K-feldspars with concomitant precipitation of secondary mineral phases.

Acknowledgements

The work has been supported by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Program. This is MinEx CRC presentation.


Biography

Naina Goswami is a second year PhD candidate at The Australian National University (ANU). She finished her Masters (advanced) from ANU in 2018 and her B.Sc (Hons) Chemistry from University of Delhi 2016. She specialises in 40Ar/39Ar geochronology and geochemistry. Currently she is undertaking her PhD at ANU working on a MinEx-CRC project based in South Australia.

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