Are Rb-Sr thin slabs migmatite ages always meaningful?
Peucat, Jean-Jacques; Martin, Hervé
Neues Jahrbuch für Mineralogie - Abhandlungen Band 152 Heft 1 (1985), p. 1 - 21
36 references
published: Jun 21, 1985
ArtNo. ESP154015201001, Price: 29.00 €
Abstract
The Rb-Sr slab dating method is applied to various examples of migmatitic gneisses in order to test the significance of the ages obtained. Three examples of gneisses affected by partial fusion, where the age of migmatitic differentiation is known from other radiometric systems are analyzed in this study. The areas selected are those of St Malo and the Golfe du Morbihan in the Armorican Massif of N. W. France and the Archaean massif of Kivijärvi in Finland. For the St Malo massif, the partial melting occurred about 540 Ma ago and the slab ages ranging from 465 to 421 Ma - have no geological meaning. They yield apparent ages that result from the mixing of micaceous phases that have different cooling ages. In the Golfe du Morbihan migmatites, mobilization took place between 400 and 380 Ma ago, and slab isochrons indicate a date of 300 Ma. This date corresponds to the final stage of Hercynian regional metamorphism. The slab ages obtained here do have a geological significance because minerals rich in radiogenic Sr all share the same cooling age. However, this age is distinctly younger than the episode of partial melting leading to migmatization. Migmatization in the Kivijärvi basement gneisses occurred at some time between 2.86 and 2.65 Ga ago, whereas Rb-Sr slab isochrons and biotite - whole-rock pairs yield ages of 1.9 and 1.7 Ga respectively. In this example, mineral phases with low Rb/Sr ratios are shown to play an important role. Apatite and plagioclase, in particular, have acted as partial acceptors of radiogenic Sr during the re-setting of biotite ages 1.7 Ga ago. This phenomenon has led to apparent slab ages of 1.9 Ga, intermediate between the whole rock isochron age of 2.86 Ga and the biotite resetting age of 1.7 Ga. Thus, the slab isochrons of 1.9 Ga have no geological significance and are governed by leucosome points that reflect the behavior of partial Sr acceptor phases such as apatite and plagioclase. Two examples out of the three given in this study yield meaningless Rb-Sr migmatite slab ages, whereas the third is shown to yield an age which is concordant with mineral cooling ages but which does not correspond to the age of migmatization.
Keywords
Migmatite • Rb-Sr dating (slab dating method) • Hercynian • Cadomian • Archaean • Amorican Massif (St. Malo • Golfe du Morbihan) • France • Finland (Massif of Kivijärvi)