Ages of internal granitoids in the Southern Cross region, Yilgarn Craton, Western Australia, and their crustal evolution and tectonic implications

Southern Cross, where gold deposits are sited in narrow greenstone belts surrounding granitoid domes, was one of the earliest gold mining centres in Western Australia. SHRIMP U-Pb zircon and Pb-isotope studies of the largest granitoid dome, the Ghooli Dome (80 x 40 km), provide important constraints on the crustal evolution and structural history of the central part of the Archaean Yilgarn Craton, Western Australia, which includes Southern Cross. The north-northwest-south-southeast-oriented ovoid Ghooli Dome has a broadly concentric foliation that is subhorizontal or gently dipping in its central parts and subvertical along its margins. Foliated granitoids in the dome are dated at ca 2724 +/- 5 and 2688 +/- 3 Ma using the SHRIMP U-Pb zircon and Pb-Pb isochron methods, respectively. These new data, together with the published SHRIMP U-Pb zircon age of 2691 +/- 7 Ma at another locality, 20 km from the centre of the Koolyanobbing Shear Zone, suggest that the Ghooli Dome was emplaced at ca 2.72-2.69 Ga. Because the Ghooli Dome and the other domes, which are enveloped by narrow greenstone belts, are cut by the >650 km-long and 6-15 km-wide Koolyanobbing Shear Zone, the ca 2.69 Ga age is interpreted as the maximum age of the last major movement on this structure. The pre-2.69 Ga history, if any, of the shear zone remains unknown. The shear zone is intruded by an undeformed porphyritic granitoid which has a SHRIMP U-Pb zircon age of 2656 +/- 4 Ma. This age is. thus, the minimum age of major movement along this shear zone. Post-gold mineralisation pegmatitic-leucogranite from the Nevoria gold mine has a SHRIMP U-Pb zircon age of 2634 +/- 4 Ma, with xenocrystic zircon cores of ca 2893 +/- 6 Ma, constraining the minimum age of gold mineralisation there to ca 2.63 Ga. The ca 2.72-2.69 Ga granitoids also contain ca 2.98 and 2.78 Ga xenocrystic zircon cores, suggesting an extensive crustal prehistory for their source. Whereas there is a general temporal relationship between the periods of older (ca 3.0 Ga) and younger (ca 2.80 and 2.73 Ga) volcanism and the older (2.98, 2.78 and 2.72-2.69 Ga) granitoid intrusions, there is no known volcanism temporally associated with the 2.65-2.63 Ga granitoid intrusions in the Yilgarn Craton. Other heat sources and/or tectonic processes, required for the generation of these intrusions, are interpreted to be related to a lithospheric delamination event related to continental collision.