Zircon and monazite U-Pb systems and the histories of I-type magmas, Berridale batholith, Australia.

U/Pb ages of zircon and monazite from four I-type granitic rocks from the Berridale batholith differ from those determined by other techniques for the same plutons. Rb/Sr mica ages agree with the field intrusive sequence, but not all are emplacement ages. For the Maffra adamellite most ages are consistent with an emplacement age of approx 413 m.y.; zircon U/Pb results show evidence of either minor inheritance or prolonged loss of intermediate products. For the Delegate adamellite there is a approx 40 m.y. range of apparent ages: zircon U/Pb 454 + or - 15, total-rock Rb/Sr 452 + or - 13, monazite U/Pb 426.7 + or - 2.4 and biotite Rb/Sr 415 + or - 2.6 m.y, reflecting inheritance and the extended magmatic history. The Tara granodiorite has an emplacement age of 412.0 + or - 1.4 m.y., but it contains inherited zircon = or >820 + or - 50 m.y. old; zircon in a dioritic xenolith has a ²⁰⁷Pb/²⁰⁶Pb apparent age of 445.4 + or - 5.8 m.y. The Finster granodiorite has a total-rock Rb/Sr age of 497 + or - 37 m.y. which is inherited; biotite Rb/Sr ages have been partially reset to 416 + or - 3.0 m.y.; zircon U/Pb shows inheritance of a component = or >539 + or - 13 m.y. old. The pluton's emplacement age is inferred to be approx 440 m.y. The country rock around the batholith contains Precambrian zircon = or >1823 + or - 10 m.y. old. -R.A.H.