Biotite Dates Research Articles

THE MICHIKAMAU ANORTHOSITIC INTRUSION, LABRADOR

The Michikamau Intrusion is a large, unmetamorphosed anorthositic mass lying within the broad belt of anorthositic bodies extending from southeastern Ontario to Labrador. Potassium–argon biotite dates place the time of crystallization of the intrusion at approximately 1 400 million years ago.The main rock units of the intrusion are leucotroctolite, anorthosite, and leucogabbro; essential minerals are plagioclase, olivine, clinopyroxene, and orthopyroxene. The bulk of the plagioclase falls in the range An52–An62 and olivine in the range Fo60–Fo70. The rocks are remarkably fresh and free from secondary alteration products.A number of features characteristic of large, layered basic plutons are present in the Michikamau Intrusion. The chilled margin has the composition of olivine basalt. However, the bulk composition of the mass is highly feldspathic, thus leading to the proposition that the basaltic liquid was heavily charged with plagioclase crystals. Cumulate structures and textures indicate that bottom accumulation of crystals may have played an important part in the solidification and differentiation of the intrusion.The course of differentiation of the magma was toward extreme iron enrichment with the development of ferrous-rich dioritic, granodioritic, and syenitic rocks as end products. It is unlikely that water played an important role in the crystallization of the intrusion.

Radiometric Dating of the Bearpaw Sea: ABSTRACT

Sampling of the bentonites included in the Upper Cretaceous Bearpaw formation of southern Alberta and adjacent areas has provided material for a geochronological investigation of this marine sequence of strata. K-Ar dating of biotite and sanidine included in the bentonites has indicated that the Bearpaw sea invaded most of the southern Alberta Plains 72-73 million years ago. The transgression of the sea was probably rapid and the base of the formation may be isochronous over most of the area, with the possible exception of areas in southern Saskatchewan and northern Montana where the sea might have transgressed somewhat earlier. The regressive upper boundary of the Bearpaw formation is set at 68 million years in the westernmost plains and at 66 million years farther east n the Cypress Hills region. The geochronological picture is compatible with the paleogeography of the Bearpaw. The bentonites intercalated with the normal sediments represent ashfalls produced by relatively remote volcanic eruptions. Study of the phenocrysts in the sand-size fraction, provided that contamination by detrital material has been negligible, has indicated that most of the bentonites are remarkably uniform in petrologic type and are dominantly andesitic. A source area is suggested in the eastern Cordilleran belt of northern Montana, where strong volcanism throughout most of the Late Cretaceous accompanied the gradual emplacement of the Boulder batholith. The andesitic nature of the Bearpaw bentonites is compatible with granodioritic magmatism in the postulated source area. End_of_Article - Last_Page 525------------