Abstract

Paleohelikian Elsonian magmatism (~ −1500 to −1400 Ma) in central Labrador occurred in the interval following the Hudsonian Orogeny (~ −1700 Ma) and preceding the Grenvillian Orogeny (after −1200 Ma). Elsonian plutons, dominated by anorthosite and adamellite compositions, have physical and petrogenetic characteristics of anorogenic intrusions and were intruded into stable, continental crust; evidence to support the existence of a 'pre-Elsonian' orogenic event (thus making the intrusions postorogenic) is slim. An average depth of intrusion about 15–20 km implies that the ambient country rock temperatures were at or near the argon blocking temperature for biotite; strong uplift and cooling, during and (or) immediately following Elsonian plutonism, is the probable cause of 'Elsonian' ages in surrounding country rock terrane.The Elsonian plutonic suite has characteristics consistent with derivation from bimodal magmatic processes. Products of basic magmas (relatively older) and of silicic magmas (relatively younger) are well represented but intermediate compositions are much less common and small in volume.Consideration of the geological record succeeding Elsonian magmatism suggests that it was a precursor of a sequence of events that led ultimately to intracontinental rifting or incipient rifting.

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