Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan foldbelt

The potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns. In the Lachlan fold belt, southeastern Australia, granites and associated high-grade metasedimentary rocks intrude low-grade Ordovician country rock. This relationship is well- exposed in the Tallangatta region, northeast Victoria (part of the Wagga-Om eo Metamorphic Complex). In this region granites (two I-types and two S-typ es) have intruded during the mid-late Silurian between approximately 410-43 0 Ma based on the ages of magmatic zircons. The age spectra for inherited z ircons from the granites have been compared with those of detrital zircons from the enclosing low- and high-grade metasediments. In broad terms, both for detrital zircons in all four sediments and for inherited zircons in thr ee of the four granites, the dominant ages are early Paleozoic and Late Pre cambrian, with sporadic older Precambrian ages extending up to 3.5 Ga. The ages of the youngest detrital zircons from the low-grade Lockhart and Talga rno terranes limit the time of sedimentation to ca. 466 Ma or younger. The youngest detrital zircons from two samples of the high-grade Gundowring ter rane are 473 Ma, making these sediments Ordovician or younger, not Cambrian as originally suggested. However, the individual age spectra for the four selected metasediments are not well matched when closely examined. The age spectra of the inherited zircons in the granites also do not adequately mat ch those in any of the metasediments. Thus, the metasediments might not be representative of the actual source rocks of the granites. While the exact source of the granites cannot be identified from the analysed samples, the existence of a large population of ca. 495 Ma inherited zircon grains in th e S-type granites requires that the granite source contains a significant p roportion of Cambrian or younger material. This does not preclude the exist ence of a Precambrian basement to the Lachlan fold belt but indicates that at the level of S-type magma generation, a Cambrian and/or younger protolit h is required.