SOUTHERN LAURENTIA IN RODINIA: A REVIEW AND SOME NEW OBSERVATIONS

Rodinia reconstructions published over the last twenty years have proposed two potential conjugate margins to the southern Laurentia Grenville Orogen. Most reconstructions include Kalahari although the proposed pinning point and orientation of Kalahari varies. Alternatively, work constraining Amazonia in Rodinia suggests collision between Amazonia and the Llano region of Laurentia.

It is possible that both of these conjugate margins are correct. The paleomagnetic link between Amazonia and Llano is at ca. 1200 Ma, making Amazonia the likely “southern continent” whose collision generated the deformation observed in the Llano Uplift. Additional paleomagnetic work suggests that Amazonia moved counterclockwise around Laurentia, arriving at the southern and central Appalachians by 1155 Ma, where a piece of Amazonia was transferred to Laurentia at this time. The eastward motion of Amazonia creates space for the arrival of Kalahari along the southern margin by ~1050 Ma, the age of Kalahari Namaqua-Natal orogeny.

Correlation of Pb isotopic compositions of the ca. 1100 Ma Keweenawan large igneous province of Laurentia with ca. 1100 Ma A-type granites in the Coats Land Block, part of Kalahari by the Neoproterozoic, supports the Kalahari-Laurentia link in Rodinia. Lead isotopic signatures of the Natal Province further support this connection. Archean and Proteorzoic rocks in Kalahari generally have elevated ²⁰⁷Pb/²⁰⁴Pb ratios relative to ²⁰⁶Pb/²⁰⁴Pb ratios in comparison with comparable-aged rocks in eastern and southern Laurentia. Lead isotopic compositions of ca. 1000 Ma rocks in the Natal Province are distinct from the elevated ²⁰⁷Pb/²⁰⁴Pb signatures of Kalahari. These low ²⁰⁷Pb/²⁰⁴Pb Natal compositions overlap those of Laurentia and most closely match the compositions of ca. 1.3-1.0 Ga rocks of southern Laurentia (Llano and Arizona). The low ²⁰⁷Pb/²⁰⁴Pb is consistent with the Natal Province forming in association with a low ²⁰⁷Pb/²⁰⁴Pb craton (Laurentia) rather than an elevated ²⁰⁷Pb/²⁰⁴Pb craton (Kalahari).