ion microprobe (SHRIMP) U-Pb zircon doting, Pb-Nd isotope tracer studies and major, trace and rare-earth element analyses have identified, for the first time, a Mesoproterozoic (1.2 Ga) quartz diorite intrusion in the central part of the Archaean Yiigarn Craton, Western Australia. The quartz diorite is characterised by intergrowths of quartz and plagioclase, having low A/CNK (0.8), low K2O (0.28 wt%). Ba (54 ppm), Rb (11 ppm), Si (92 ppm), Pb (13 ppm), U (1.7 ppm) and Th (6 ppm) contents, high Nd (41 ppm), Sm (10.5 ppm), Zr (399 ppm), Nb (18.5 ppm), Y (57.5 ppm) and Sc (19 ppm) contents, a low Pb isotope Two-stage model mu value (6.3), and a primitive initial epsilon(Nd) value (+3.4) at 1.2 Ga. It is interpreted that the 1.2 Ga quartz diorite was derived from a predominantly mantle source, with minor crustal contamination, possibly from the surrounding Archaean monzogranites or their source region, during magma ascent. The age (1215 +/- 11 Ma) of the intrusion overlaps with the timing of a major continental collisional orogeny in the Albany-Fraser Orogen, about 400 km south, and is broadly coeval with the diamond-bearing Argyle lamproites in the east Kimberley Block. This study extends the history of granitoid magmatism in the central craton by more than 1.0 billion years (2.6 to 1.2 Ga), and has implications for isotopic data interprerations of tectonothermal events in the craton.