Very distant Sudbury impact dykes revealed by drilling the Temagami geophysical anomaly

Abstract

The Temagami Anomaly is one of the largest unexplained magnetic features in North America. It is similar in size and shape to the geophysical anomaly that marks the 1.85 Ga Sudbury Igneous Complex (SIC) in its immediate vicinity but its geological cause and potential link to the Sudbury impact structure have remained elusive. Here we report on a 2200 m deep diamond drill core intersecting the area of maximum magnetic anomaly and provide evidence of diorite dykes therein being most likely related to the Sudbury impact event. The fine-grained, strongly altered biotite-amphibole diorite occurs below 2000 m, is intrusive into Archaean basement rocks and has the same major- and trace element geochemistry as the SIC, which approximates the bulk composition of the local continental crust that was hit by the impact. A crustal affinity analogous to SIC impact-melt rocks is further supported by whole-rock Nd and Pb isotopes. Low ɛNd0 between −27.6 and −18.7, as well as Nd model ages of 2.75 Ga are considered as inherited from the crustal precursor rocks that became largely homogenized in the course of impact melt formation. The 206Pb/204Pb ratios are between 15.77 and 19.38, 207Pb/204Pb between 15.22 and 15.59 (corresponding to initial 207Pb/204Pb at 1850 Ma between 15.14 and 15.22), yielding an “isochron age” of 1780 +320/−330 Ma, and the 208Pb/204Pb ranges from 35.47 to 41.15, all values that compare well with published data on SIC impact-related igneous rocks of mainly quartz dioritic composition, locally referred to as Offset Dykes, and that are distinctly different to those reported for other magmatic units in the wider region. Although several such dykes have been well known to occur both radially and concentrically around the SIC, none have been described so far from the area east of the SIC. The recognition of former intrusive impact melt at an even greater distance (50 km) from the SIC than has been known so far increases the extent of the impact structure but also the exploration potential of the area of the Temagami magnetic anomaly for Ni-Cu-PGE-sulfide deposits. The actual cause of the Temagami Anomaly remains open to debate.