Stability and isotopic dating of monazite and allanite in partially molten rocks: examples from the Central Alps

Abstract

We investigated the stability of monazite and allanite as a function of bulk rock composition within several types of tertiary Alpine anatexites, characterized by different compositions and melting reactions, but similar P-t conditions of melting. the investigated rocks consist of: (1) orthogneisses in which the melting reaction was triggered by water infiltration from the bergell pluton; (2) anatectic tonalites, which were affected by water-assisted melting; and (3) metapelitic migmatites, which underwent muscovite dehydration melting. the studied anatexites cover a large range of ca contents and water activities during partial melting, and allow an assessment of how much these parameters affect the stability of accessory phases. the different melting reactions that affected these rocks generated different water activities during the meltpresent stage; they were highest in the water-saturated, contact metamorphic anatexites, and lowest in the metapelitic anatexites that underwent dehydration melting. these differences go together with different accessory phases within the migmatites. Whereas metapelitic anatexites only contain monazite, anatexites derived from tonalitic and granodioritic protoliths mainly contain allanite. this is consistent with observations made on tertiary Alpine anatexites, suggesting that the growth of specific accessory phases is determined by the water activity and ca content during melting.We measured single-grain monazite U/Pb isotope ages. One grain has relics of old cores, which have also been detected in Y-zonation patterns of the monazite. the data of unzoned monazites indicate partial melting in the southern steep belt between 30.78 ± 0.14 and 28.10 ± 0.28 Ma. Introductionthe stability of accessory minerals is a key to understand the geochemical evolution of crustal rocks during melting and crystallization, because rEE are almost completely hosted in these minerals (e.g., Miller & Mittlefehldt 1982;Watt et al. 1996). some of these minerals, e.g., allanite, zircon and monazite, are frequently used geochronometers; hence a good petrologic constraint on their formation has important implications for the correct interpretation of isotopic ages. Whereas analytical errors of age determinations have become increasingly small, the geologic interpretation of these isotopic ages still remains uncertain, because the criteria relating a specific geologic event to a measured datum are generally lacking. the problem concerns the distinction between mineral phases formed during melting or during crystallization of migmatites. this distinction is fundamental when dating these minerals from migmatites, because the long residence time of melt in the deeper crust (10 6 -10 7 yr, rosenberg et al. 2007) makes it difficult to attribute the analytically precise ages to a specific geologic process within a long-lasting period of crustal melting. better constraints on the phase relationships of accessory minerals are needed before isotopic ages can be accurately coupled to specific, possibly short...

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

“…Cherniak et al 2004). Our interpretation is also consistent with the Oligocene age of upper amphibolite-facies metamorphism in the southern and south-eastern parts of the Lepontine dome (Hänny et al 1975;Köppel et al 1981;Gebauer 1996;Berger et al 2009;Rubatto et al 2009). …”

“…Barrow-type metamorphic conditions increase southwards from upper greenschist-to upper amphibolite-facies. In the southern central part, the Lepontine dome is characterised by migmatisation Burri et al 2005) that took place through fluid-assisted melting at about 700°C and 6-8 kbar (Nagel et al 2002;Burri et al 2005) between 32 and 22 Ma (Hänny et al 1975;Köppel et al 1981;Gebauer 1996;Berger et al 2009;Rubatto et al 2009). Further to the south, the Lepontine dome is truncated by the Insubric line (Fig.…”

3-D assessment of peak-metamorphic conditions by Raman spectroscopy of carbonaceous material: an example from the margin of the Lepontine dome (Swiss Central Alps)

“…Handy and Oberhänsli 2004; and references therein) and is therefore not addressed in this study. The metamorphic evolution can be subdivided into two distinct stages: (1) Latest Cretaceous-Late Eocene subduction-related pressure-dominated metamorphism and deformation (see review in Berger and Bousquet 2008; and references therein) that affected oceanic lithosphere formed during the opening of the Alpine Tethys as well as small parts of the immediately adjacent Europe-derived continental lithosphere; and (2) OligoceneMiddle Miocene temperature-dominated, Barrow-type metamorphism (Köppel et al 1981;Hunziker et al 1992;Berger et al 2009;Janots et al 2009;Rubatto et al 2009) related to the collision between Europe and Adria. Collision involved accretion of massive volumes of crustal material derived from the lower, European plate (e.g.…”

U–Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps

“…These rims are coeval with the Alpine regional metamorphism (32-22 Ma in the southern and south-eastern Lepontine Dome, Hänny et al 1975;Köppel et al 1981;Vance and O'Nions 1992;Gebauer 1996;Berger et al 2009;Rubatto et al 2009) and the emplacement of the adjacent Bergell Pluton (33-30 Ma, Von Blanckenburg 1992; Oberli et al 2004;Gregory et al 2009). Field observations suggest that migmatisation at 720-740°C (Fig.…”

“…Burri et al 2005). The 32-to 22-Ma migmatisation (Hänny et al 1975;Köppel et al 1981;Berger et al 2009;Rubatto et al 2009) mainly occurred at 700-750°C and 6-8 kbar (Fig. 2) through fluid-assisted melting and, in a limited amount, fluid-absent muscovite melting (Nagel et al 2002, Burri et al 2005Berger et al 2008).…”