The Tynong pluton, its mafic synplutonic sheets and igneous microgranular enclaves: the nature of the mantle connection in I-type granitic magmas

Abstract

In the Lachlan Orogen of south-eastern Australia, the high-level, postorogenic, 368-Ma, I-type Tynong pluton contains granitic to granodioritic rocks that crystallised from a variety of mainly crustally derived magmas emplaced in the shallow crust, in an extensional regime. The isotopic characteristics of the main plutonic rocks are relatively unevolved (87Sr/86Srt ~ 0.705–0.706 and εNdt ~ −0.4 to 0.6), suggesting source rocks not long separated from the mantle. We infer that arc mafic to intermediate rocks and associated immature greywackes formed the main crustal source rocks and that these are located in the largely unexposed Neoproterozoic–Cambrian Selwyn Block that forms the basement. A..