Identifying granite sources by SHRIMP U-Pb zircon geochronology: an application to the Lachlan foldbelt |
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| Authors: | Sue Keay David Steele William Compston |
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| Institution: | (1) Research School of Earth Sciences, Australian National University, Canberra ACT 0200 Australia, AU;(2) School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia, AU |
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| Abstract: | 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-Omeo
Metamorphic Complex). In this region granites (two I-types and two S-types) have intruded during the mid-late Silurian between
approximately 410–430 Ma based on the ages of magmatic zircons. The age spectra for inherited zircons 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 three of the four granites, the dominant ages are early
Paleozoic and Late Precambrian, with sporadic older Precambrian ages extending up to 3.5 Ga. The ages of the youngest detrital
zircons from the low-grade Lockhart and Talgarno terranes limit the time of sedimentation to ca. 466 Ma or younger. The youngest
detrital zircons from two samples of the high-grade Gundowring terrane 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 match 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 the S-type granites requires that the granite source contains a significant proportion
of Cambrian or younger material. This does not preclude the existence 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 protolith is required.
Received: 28 August 1998 / Accepted: 7 July 1999 |
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