Aspects of geochronology and crustal evolution in the Eastern Fold Belt, Mt Isa Inlier∗

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Abstract
Zircon U‐Pb SHRIMP ages and crustal Sm‐Nd signatures are reported for metasediments, meta‐volcanics, and a variety of pre‐ and post‐tectonic granitoid intrusives in the Eastern Fold Belt of the Mt Isa Inlier. These enable tracing of the broad crustal evolution of the rocks, erection of a chrono‐stratigraphic framework in the Eastern Fold Belt, and correlation of these rocks with those in other parts of the Mt Isa Inlier and in other Proterozoic inliers of northern Australia. The SHRIMP results and Sm‐Nd model ages provide quantitative information related to the provenance history of the metasediments and the sources and crustal interactions of magmatic rocks. Most of the latter have Nd Tdm of 2270–2200 Ma, suggesting similar source(s) to the metasediments and gneisses (2360–2140 Ma). The Nd TDM model ages are younger than those found in the Kalkadoon‐Leichhardt Belt, suggesting that the Eastern Fold Belt crust received less Archaean source input. Rocks of Barramundi age (1870–1840 Ma) are not found in the Eastern Fold Belt. The Eastern Fold Belt includes an older, calc‐silicate‐dominated sequence (broadly equivalent to the Mary Kathleen Group) some of which was deposited in the interval 1750 ± 7 Ma to 1725 ± 3 Ma. Felsic to intermediate volcanism in this period is evident in the host rocks at the Ernest Henry Cu‐Au deposit which have been dated nearby at 1746 ± 9 Ma and 1742 ± 6 Ma. This volcanism may have been coeval with ca 1740 Ma formation of older deformed granitoids in the Eastern Fold Belt and was possibly related to the Wonga extensional event evident elsewhere in the inlier. The SHRIMP geochronology clarifies the stratigraphic position of the Soldiers Cap Group, a sequence of metamorphosed turbiditic sediments and mafic volcanics. Most of the Soldiers Cap Group (including paragneisses that host the Cannington Ag‐Pb‐Zn deposit) is probably similar in age to the ca 1655 Ma Mt Isa Group, host for the Cu and Pb‐Ag‐Zn orebodies at Isa and Hilton Mines in the Western Fold Belt. However, part of the metasedimentary package previously assigned to the Soldiers Cap Group is comparable in age to the much older Argylla Formation, as it includes volcaniclastic rocks with depositional ages of 1775 ± 4 Ma and 1774 ± 4 Ma. High‐uranium metamorphic zircon selvedges in Soldiers Cap Group gneisses define an age of 1584 ± 17 Ma. This provides a direct age for a high‐grade metamorphic event(s) in the Eastern Fold Belt and invites comparison to similarly timed tectonism in the Georgetown region. The majority of granitoids in the Williams and Naraku Batholiths were emplaced either in the interval 1550–1530 Ma or close to 1500 Ma, again suggesting links in Mesoproterozoic time to the Etheridge Province in the Georgetown region. Hydrothermal zircon overgrowths of this age (1500 Ma) are found not only in the 1500 Ma intrusions but also in some older rocks. These provide evidence for the timing of a pervasive sodic alteration event which pre‐dates Cu‐Au mineralisation in the region. The geochronology of the Eastern Fold Belt country rocks and granites supports arguments based on lithostratigraphy and evidence from Pb‐isotope data which imply that stratabound Pb‐Zn mineralisation in the Eastern Fold Belt and Georgetown regions may have been generated in a similar tectonic environment and from similar source rocks at about the same time (ca 1680–1660 Ma). A post‐1700 Ma connection and correlation of these two terrains, and possibly the Broken Hill Inlier, has to be seriously considered.

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