Behaviour of monazite and evolution of polymetamorphic pelites from the Monte Rosa nappe, Western Central Alps, Italy
Creators: name: Scherrer, Nadim
Publisher: name: University of Bern
Year: 2001
Subjects: QE Geology

description: Despite the recognised importance of monazite in geochronology, the contextural information is lost when using standard mineral separation techniques. To overcome this problem, new techniques were developped to date monazite in thin section or as grains drilled out from thin sections. Chemical Th-U-Pb dating of monazite on the microprobe is limited to grains older than 200 Ma, due to the high detection limit. Moreover, a range of fundamental analytical and preparational problems remains. For example, for monazite containing 12% Th, the commonly disregarded interference of Th Mz on Pb Ma causes an overestimation of 11% (relative) in Pb. This propagates to an age overestimation of ~50 Ma for a sample of 400 to 500 Ma in age. A judicious choice of X-ray peaks used in quantitative EMP analysis avoids or minimises peak overlap for all elements, including REE. A newly developped XRF-microprobe achieves superior performance, permitting to date small grains (< 100 μm) as young as Miocene in age. As an example, the precision achieved with the XRF-microprobe for a well characterised monazite age standard FC-1 (TIMS age 54.3 Ma; μ-XRF age 55.3 ± 2.6 Ma), doubly polished to 30 μm in thickness, is below 5 % (2 sigma) after 90 minutes integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. Special sample preparation techniques using a PE-polishing disk permit the sequential use of thermobarometric and geochronometric analytical tools and thus the potential to derive time-calibrated P-T paths. The first two papers describe methods for sample preparation, analysis techniques and data interpretation for electron microprobe and XRF-microprobe dating. In the other two papers the techniques are tested on pelitic rocks of the Monte Rosa nappe, western Central Alps, which is believed to have played a key role in the tectonometamorphic evolution of the Central and Western Alps. In these rocks monazite occurs in different textures. Similarities to the better known Adula - Cima Lunga nappes further east hint at a potentially equivalent late emplacement history from depths in excess of 70 km during Alpine orogenesis. Studying the polymetamorphic basement rocks of the Monte Rosa nappe successfully revealed new insight into the complex history of this fragment of continental crust with European affinity. Careful analysis applying the chemical Th-U-Pb dating technique unveiled two distinct phases of monazite growth in these rocks: a Permian one around 280 Ma, believed to be associated with the intrusion VIII of the Monte Rosa granite; and an Alpine one around 35 Ma, attributed to the timing of high pressure metamorphism when maximum depths were reached during continental subduction. The combination of geochronometry and thermobarometry, sequentially applied ‘in-situ’ demonstrates the potential of this new technique. While monazite dating is frequently used for geochronometry, there exists also a relative lack of knowledge of monazite-forming reactions which is commonly fundamental for an age interpretation. Fine-grained symplectites of monazite, apatite and corundum within allanite of the Monte Rosa nappe have been recognized as breackdown product from bearthite. Similar textures have also been observed in the Dora-Maira high pressure terrane. Bearthite Ca2Al[PO4]2(OH), an aluminium phosphate contains up to ~10 wt% of light rare-earth elements (LREE) + Th. The documented symplectitic textures are assumed to be related to rapid decompression, following high to ultra-high pressure conditions during Tertiary subduction. The systematic study of bearthite from the Monte Rosa type locality and the Dora Maira area revealed that bearthite fractionates LREE, Th and U similar as monazite. This indictes a potential use of bearthite for age dating of ultra-high pressure rocks.