The rubidium‐strontium ages of some Tasmanian igneous rocks

Abstract

Rb‐Sr isotopic age measurements relate emplacement of the Pieman and Meredith Granites (356 ± 9 and 353 ± 7 m.y., respectively), and the Bischoff and Renison Bell Porphyries (349 ±4 and 355 ± 4 m.y., respectively) to the Tabberabberan Orogeny. The genetic relationship of the Bischoff Porphyry to mineralization and the agreement between the age of this porphyry and the age of the adjacent Meredith Granite, strongly suggests that the Bischoff mineralization resulted from granite intrusion. The Pieman Granite is closely similar to the white Heemskirk Granite in displaying a high initial ⁸⁷Sr/⁸⁶Sr ratio (0.7354 ± 0.0018), feldspar discordance patterns and open system total‐rock behaviour. The high initial ⁸⁷Sr/⁸⁶Sr ratio is attributed to contamination during intrusion by Precambrian metasediments containing appreciable radiogenic strontium. A basic intrusion (McIvors Hill Gabbro) gave a pre‐Tabberabberan age (518 ±133 m.y.) and a high initial ⁸⁷Sr/⁸⁶Sr ratio (0.7132 ± 0.0031).