Ma For Tonalite Research Articles

Archean magmatism in the Kaminak Lake area, District of Keewatin, Northwest Territories: ages of the carbonatite-bearing alkaline complex and some host granitoid rocks

U–Pb zircon data, obtained on the ion microprobe (SHRIMP), have yielded an age of 2700 ± 11 Ma for tonalite of the Kaminak batholith where it is intruded by a carbonatite-bearing alkaline complex at Kaminak Lake, Northwest Territories, Canada, and a crystallization age of 2659 ± 5 Ma for the alkaline intrusion. Relatively undisturbed age spectra determined by laser probe 40Ar/39Ar analysis of hornblende from a gabbro and from a second tonalite of the batholith yielded cooling or minimum ages of 2727 ± 14 and 2599 ± 9 Ma, respectively. Spectra for hornblendes from two other gabbros suggest overprinting more recent than 2.04 Ga, with a minimum cooling age of 2.69 ± 0.09 Ga in one case, and, with incorporation of excess argon, a maximum cooling age of 2.80 ± 0.03 Ga in the other. Correlation of these data with those recently reported by other workers indicates that the volcanic sequence and the plutonic granitoid units of the Kaminak Lake granitoid–greenstone belt are approximately coeval. Plutonic units both older and younger than the volcanic pile are now recognized. The carbonatite-bearing alkaline complex is confirmed as one of the oldest of this type worldwide. Overlap of the age of alkaline intrusion with those of other plutonic units in the Kaminak Lake and adjacent Tavani areas suggests that rifting of stable continental crust (a typical anorogenic tectonic setting) may be an inappropriate interpretation for this late Archean carbonatitic activity.

U/Pb geochronology of the Coney Head Complex, Newfoundland

The Coney Head Complex, White Bay, Newfoundland, comprises allochthonous plutonic rocks and deformed equivalents emplaced over the continental margin during the Ordovician Taconic Orogeny. A U/Pb zircon age of 474 + 2 Ma for tonalite considered as having formed by partial melting in an island arc, indicates that it crystallized in Arenig time. A microgranite dated by U/Pb (zircon + titanite) at 432 ± 2 Ma cuts the tonalite Inherited Proterozoic zircon in the microgranite indicates that the rock formed from magma generated in part by melting of continental crust or sediments derived from such crust. These relationships indicate that the microgranite was emplaced in Silurian time, after Ordovician transport of the tonalite above a continental substrate.