Response of U-Pb Zircon and Rb-Sr total-rock and mineral systems to low-grade regional metamorphism in proterozoic igneous rocks, mount Isa, Australia
Titel:
Response of U-Pb Zircon and Rb-Sr total-rock and mineral systems to low-grade regional metamorphism in proterozoic igneous rocks, mount Isa, Australia
Auteur:
Page, R. W.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 25 (1978) nr. 3-4 pagina's 141-164
Jaar:
1978-06
Inhoud:
A detailed Rb-Sr total-rock and mineral and U-Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U-Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U-Pb age is 1862 +27-21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago. All of these rocks are altered in various degrees by low-grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb-Sr total-rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb-Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600-1700 m.y. interval. Such isochrons possess anomalously high initial 87Sr/86Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb-Sr total-rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in 87Sr in the Sr-poor (high Rb/Sr) rocks as compared with the Sr-rich rocks).
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