Precise dating of granitic metapegmatites offers a specific opportunity to get information on timing of metamorphic events: "Within their tectonomagmatic cycles, fertile granites and their pegmatite aureoles are in most cases late-to post-tectonic and conclude magmatic activity in the waning to retrogressive stages of regional metamorphism" (Cerny, 1990). Therefore intrusion ages of pegmatites may fix a lower age limit for a metamorphic evolution, being supplementary to U-Pb data on metamorphic minerals like monazite or zircon.
This concept is of special use in polymetamorphic terranes as shown by our work on pegmatites from the Bohemian-Bavarian pegmatite province (Vejnar, 1965). This area comprises the Zone von Erbendorf-Vohenstrauß (hosting the German KTB drilling site) just as the western part of the Tepla-Barrandian unit in the Czech republic. Both units consist of a polymetamorphic volcanosedimentary succession intercalated with orthogneisses. The existence of a first, Cambro-Ordovician metamorphism is mainly
documented by U-Pb zircon data (Grauert et al., 1994; Sölner et al., 1995). Subsequently, the rocks have been subjected to amphibolite-facies conditions in Devonian times. Both units contain numerous granitic pegmatites, mainly of the rare-element class, all showing more or less intense deformational overprint.
Rb-Sr age determinations on about 25 individual pegmatites were carried out by analysing the cores of large, book-like, undeformed muscovite crystals together with plagioclase, apatite or potassium feldspar from the same hand specimen. Although many previous studies (Küster, 1995) on Rb-Sr-systematics of other pegmatite fields faced problems with post-crystallizational mobility of 87 Sr*, in this study fairly uniform apparent ages of muscovites supposed to be of magmatic origin have been obtained.
Largely undeformed muscovites from metapegmatites yield ages between 475-480 Ma (Zone von Erbendorf-Vohenstrauß, n=7), 475-487 Ma (n=5) and 480-495 Ma (n=8) in the northwestern and the southwestern part of the Tepla-Barrandian, respectively.
Even the Devonian metamorphism is well-documented within the metapegmatites: Highly strained portions of the rocks offer fine-grained, recrystallised or newly crystallised muscovite. These micas yield Rb-Sr age values between 371 and 376 Ma for both the Czech and the Bavarian occurrences (n=8) which are interpreted as dating the end of deformational activity of the Devonian metamorphic event.
The Lower Ordovician ages of primary muscovites are the first clue on the evolution of these crustal segments between the two metamorphic events: Pegmatites of the examined class normally form at fairly shallow crustal levels of 2-4 kbar (Cerny, 1990). Therefore the formation of pegmatites implies an upper crustal position of the intruded rocks in Lower Ordovician times, clearly documenting the presence of two distinct metamorphic cycles.
It is important to note that apparent Rb-Sr ages of muscovites are not only dependent on grain size, but also to a large extent on the intensity of strain experienced by individual crystals during the Devonian deformational event: Large, primary crystals grade towards Devonian ages the more kinked and bended (or grinded) they are. This emphasizes the role of deformation in resetting isotopic decay systems.
Due to the sensitivity of the muscovite ages to secondary disturbance, the Rb-Sr data require comparison with results obtained from other isotopic decay systems of other primary minerals, especially if precise intrusion ages of highly metamorphosed pegmatites are needed.
First results from Sm-Nd analyses on magmatic, spessartite-rich garnet indicate very high 147Sm/144Nd isotopic ratios up to 7.6, but precise dating is hindered by low Nd concentrations in the garnet (40-100 ppb). U-Pb dating of zircon, monazite and garnet may yield reliable intrusion age data on metapegmatites. Among these minerals, garnet is most promising since it escaped from deformation and apparently was not affected by Devonian re-equilibration. A U-Pb study is currently in progress.
C˜erny´, P., Geol. Rdsch. 79, 183-226 (1990).
Grauert, B., Lork, A. & O'Brien, P., KTB-Report 94-1 (1994).
Küster, D., Mineral. Petrol. 55, 71-83 (1995).
Söllner, F. & Nelson, D., Terra Abs., 7, 350 (1995).
Vejnar, Z., Sbor.geol ved. LG (Praha), 4, 4-87 (1965).