ASPECTS OF GEOCHRONOLOGY AND CRUSTAL EVOLUTION IN THE EASTERN FOLD BELT, MT ISA INLIER

Zircon U-Pb SHRIMP ages and crustal Sm-Nd signatures are reported for metasediments, metavolcanics, and a variety of pre- and post-tectonic granitoid intrusives in the Eastern Fold Belt of the Mt isa Inlier. Th ese enable tracing of the broad crustal evolution of the rocks, erecti on of a chronostratigraphic framework in the Eastern Fold Belt, and co rrelation of these rocks with those in other parts of the Mt Isa Inlie r and in other Proterozoic inliers of northern Australia, The SHRIMP r esults and Sm-Nd model ages provide quantitative information related t o the provenance history of the metasediments and the sources and crus tal interactions of magmatic rocks. Most of the latter have Nd TDM Of 2270-2200 Ma, suggesting similar source(s) to the metasediments and gn eisses (2360-2140 Ma). The Nd TDM model ages are younger than those fo und in the Kalkadoon-Leichhardt Belt, suggesting that the Eastern Fold Belt crust received less Archaean source input. Rocks of Barramundi a ge (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 v olcanism in this period is evident in the host rocks at the Ernest Hen ry Cu-Au deposit which have been dated nearby at 1746 +/- 9 Ma and 174 2 +/- 6 Ma. This volcanism may have been coeval with ca 1740 Ma format ion of older deformed granitoids in the Eastern Fold Belt and was poss ibly related to the Wonga extensional event evident elsewhere in the i nlier. The SHRIMP geochronology clarifies the stratigraphic position o f the Soldiers Cap Group, a sequence of metamorphosed turbiditic sedim ents and mafic volcanics. Most of the Soldiers Cap Group (including pa ragneisses that host the Cannington Ag-Pb-Zn deposit) is probably simi lar in age to the ca 1655 Ma Mt Isa Group, host for the Cu and Pb-Ag-Z n orebodies at Isa and Hilton Mines in the Western Fold Belt. However, part of the metasedimentary package previously assigned to the Soldie rs 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 pro vides a direct age for a high-grade metamorphic event(s) in the Easter n Fold Belt and invites comparison to similarly timed tectonism in the Georgetown region. The majority of granitoids in the Williams and Nar aku Batholiths were emplaced either in the interval 1550-1530 Ma or cl ose to 1500 Ma, again suggesting links in Mesoproterozoic time to the Etheridge Province in the Georgetown region. Hydrothermal zircon overg rowths of this age (1500 Ma) are found not only in the 1500 Ma intrusi ons but also in some older rocks. These provide evidence for the timin g of a pervasive sodic alteration event which pre-dates Cu-Au minerali sation in the region. The geochronology of the Eastern Fold Belt count ry rocks and granites supports arguments based on lithostratigraphy an d evidence from Pb-isotope data which imply that stratabound Pb-Zn min eralisation in the Eastern Fold Belt and Georgetown regions may have b een generated in a similar tectonic environment and from similar sourc e rocks at about the same time (ca 1680-1660 Ma). A post-1700 Ma conne ction and correlation of these two terrains, and possibly the Broken H ill Inlier, has to be seriously considered.