When did the Gardar magmatism begin in southern Greenland?

Abstract:

The Gardar Province in southern Greenland is a long-lived, rift-related igneous province comprising a suite of basaltic flows, alkaline complexes and dikes. The Gardar period is known to last more than 200 Ma from c. 1350 Ma to 1140 Ma, but is characterized by two main phases of igneous activity represented by early (1300 – 1250 Ma) and late (1180 – 1140 Ma) Gardar intrusions, respectively. The earliest rocks ascribed to the Gardar period are terrestrial sandstones and lavas composing the Eriksfjord Formation that unconformably overlie the Paleoproterozoic basement rocks. One of the unsolved issues in the Gardar Province is to determine the precise age of the lavas from the Eriksfjord Formation, which can constrain the timing of the beginning of the Gardar rifting. In this study, for the first time, we presented precise U-Pb zircon ages of the lavas from the Eriksfjord Formation.

Zircons were separated from five samples of basaltic lavas collected at the Motzfeldt Center for SHRIMP U-Pb zircon dating. All the zircons under CL image analysis show oscillatory zoning, indicative of magmatic origin, and their U-Pb ages were measured by SHRIMP at Korea Basic Science Institute. The mean ²⁰⁷Pb/²⁰⁶Pb ages of each sample range from 1315 – 1329 Ma, whereas the concordant ²⁰⁷Pb/²⁰⁶Pb ages from 1322 – 1327 Ma. When all of the analyzed data from five samples were considered together, they showed a normal distribution pattern and yielded a mean ²⁰⁷Pb/²⁰⁶Pb age of 1322.8 ± 4.7 Ma (MSWD = 0.04) and a concordant ²⁰⁷Pb/²⁰⁶Pb age of 1324.5 ± 3.4 Ma (MSWD = 0.04), respectively. These ages are consistent with the age of 1.35 – 1.31 Ga inferred from paleomagnetic data for the entire succession of the Eriksfjord Formation. Our ages suggest that the Gardar magmatism started at c. 1325 Ma and lasted until c. 1140 Ma in southern Greenland.