St Honoré
intrusion, Saguenay region
Laurentia at 571 Ma: Preliminary paleomagnetism and Ar-Ar age of the
Ediacaran St Honore alkali intrusion, Quebec
McCausland, P J, Pisarevsky, S, Jourdan, F, Higgins, M, AGU spring meeting, 2009
The paleomagnetic record of Laurentia through the Ediacaran (~615 to
543 Ma) is pivotal to establishing the paleogeography of the opening of
the Iapetus Ocean, but the handful of available paleomagnetic results
are controversial. Some results appear to indicate that Laurentia
resided at high southerly paleolatitudes from ca. 600 Ma to 570 Ma,
whereas other interpretations hold that Laurentia resided at low
paleolatitudes throughout the Ediacaran. We have sampled the
mid-Ediacaran St. Honore alkali intrusion and related dykes in the
Saguenay region of Quebec for a paleomagnetic and Ar-Ar geochonologic
study to assess the 'high' and 'low' paleolatitude Laurentia
interpretations. Ar-Ar geochronology of phlogopite separates from the
carbonatite intrusion return plateau ages of 571.0 +/- 4.6 Ma,
confirming and refining previous whole rock and biotite K-Ar ages.
Preliminary paleomagnetic results from the intrusion and 12 carbonatite
and lamprophyre dykes have a mean direction of D=122, I=69.8 degrees;
a95=10, retained to high unblocking temperatures by PSD magnetite. Our
preliminary result places St. Honore at 54 deg S at 571 Ma and implies
that Laurentia lay at moderate paleolatitudes in the mid-Ediacaran.
Notably, the paleopole location at 23 N, 322 E (dp=17, dm=15) is
consistent with, and lies squarely in between the paleopoles which
place Laurentia at high paleolatitudes and those which place Laurentia
at low paleolatitudes. The preliminary St Honore result implies that at
ca. 571 Ma Laurentia was in rapid transition from high to low
paleolatitudes, or that Laurentian paleomagnetic results from the
Ediacaran period may be even more complex than previously realized.
