IN SITU EMP AND ID-TIMS COMBINED U-TH-PB DATING OF SINGLE MONAZITES: A NEW METHOD TO UNRAVEL COMPLEX POLYMETAMORPHIC EVOLUTIONS. APPLICATION TO THE UHT GRANULITES OF ANDRIAMENA (NORTH-CENTRAL MADAGASCAR)

We develop a complementary technique of "in-situ" dating of monazite, which involve 3 successive operating steps: (1) textural and chemical characterization of mnz and chemical dating by EMP; (2) extraction by micro-drilling of selected grains; (3) isotopic dating of each extracted grains by ID-TIMS. One of the most critical aspects of this work is that both EMP and ID-TIMS data were acquired from identical grains extracted directly from thin-sections.

The potentialities of this method are illustrated by an application to a polymetamorphic UHT granulite from Madagascar. The peak metamorphic assemblage consists of grt-opx-qz-sil (11 kb, 1000°C). It is retrogressed by a secondary hydrated assemblage, which consists of oamph-crd-bt (6 kb, 750°C). Three populations of mnz have been drilled according to their textural position. (1) mnz included in grt; (2) mnz located in qz; (3) mnz located in hydrated coronitic textures. Analyzed grains are discordant and define a chord with intercepts at 2523 ± 10 Ma and 576 ± 12 Ma.

Monazite included in grt are the less discordant (~30%). An age of 2478 ± 88 Ma, consistent with the upper intercept, has been obtained by EMP on euhedral inclusions. It is interpreted as the age of UHT event. Mnz located in qz show a variable discordance (15-95%). Such grains display an inherited core, with the same composition as mnz included in grt, surrounded by a newly grown rim. It suggests that the isotopic discordance results from a mixture of UHT inherited cores with a youngest rim. Mnz located in the coronitic textures are the most discordant (75-95%). They consist of a UHT inherited core surrounded by a secondary rim, which contain inclusions of oamph, crd and bt. The new growth is therefore coeval with the retrogression estimated at ~730 Ma (EMP data). A third episode of monazite occurred at 500-530 Ma. This third generation occurs as small overgrowths (<10 µm) with a clearly different composition. We suggest that the lower intercept at 576 Ma has no geological meaning but results from the mixing of 730 and 500-530 Ma monazite generations.

Using this combined method, we estimate the absolute timing of specific metamorphic assemblages in view of the textural relationships: UHT at 2.5 Ga and the hydrated retrogression at ~730 Ma. The Cambrian event is probably related to a late fluid circulation.