EXOTIC VOLCANIC CLASTS IN WERNECKE BRECCIA, YUKON, RECORD CLOSURE OF A LAURENTIAN-AUSTRALIAN SEAWAY IN THE LATE PALEOPROTEROZOIC

Precambrian interactions between Laurentia and Australia have long been postulated on the basis of geology and geophysics. Geological evidence from Yukon, Canada, supports a late Paleoproterozoic collision and a Mesoproterozoic breakup. Convergence and collision is suggested by field and petrologic studies of clasts in Wernecke Breccia, a giant breccia system generated by massive hydrothermal pulses on Laurentia at 1.60 Ga. Although most clasts in Wernecke Breccia were derived from metasedimentary wallrock of the 1.66-1.60 Ga Wernecke Supergroup, a minority of the clasts are igneous and are collectively termed the Wernecke igneous clasts. This set of clasts comprises the Slab volcanic clasts, the Bonnet Plume River intrusive clasts, and the Devil volcanic clasts. Four U-Pb zircon dates on the Bonnet Plume River clasts yield a ca. 1.71 Ga age, i.e., 50 m.y. older than the Wernecke Supergroup. This relationship of older clasts flanked by younger strata has been explained by igneous clast derivation from a source area within a hypothetical, obducted terrane named Bonnetia. Fragmentation of Bonnetia and foundering of its clasts by 1000s of metres occurred during intense hydrothermal activity.

The largest clast of the Slab volcanics consists of >30 mafic lava flows and subordinate epiclastic and pyroclastic rocks. The megaclast was derived from a larger succession within Bonnetia, which was eroded prior to deposition of the Mesoproterozoic Pinguicula and Fifteenmile groups. The lavas are moderately alkaline with arc trace element signatures. Magma evolution was dominated by fractional crystallization punctuated by magma chamber recharge. The alkaline arc composition is consistent with genesis in a volcanic arc affected by rifting, plume impingement or a slab window. We postulate that Bonnetia grew as a volcanic arc on the margin of Australia during closure of a Laurentian-Australian seaway. By 1.66 Ga, Bonnetia was emergent and shedding detritus into interior basins of Australia, raising their εNd. Collision, dismemberment and obduction of Bonnetia onto Laurentia occurred during the Racklan orogeny (Laurentia) and Isan orogeny (Australia) between ca. 1.63 Ga and 1.60 Ga. Extensional basins on Laurentia record the proximity of Australia and the onset of continental breakup at ca. 1.45 Ga.