Two millimeter-sized hydrothermal monazites from a cleft that developed late during a dextral transpressional deformation event in the Aar Massif, Switzerland, have been investigated using EMP and ion probe. The monazites are characterised by high Th/U ratios typical of other hydrothermal monazites. Deformation events of the area have been subdivided into three steps: (D1) main thrusting including formation of a new schistosity; (D2) dextral transpression; and (D3) local crenulation including a new schistosity. The two younger deformational structures are related to a subvertically oriented intermediate stress axis, which is characteristic for strike slip deformation. The inferred stress situation is consistent with observed kinematics and the opening of such clefts. Therefore, the investigated monazite-bearing cleft formed at the end of D2 and/or D3, and dextral movements along NNW dipping planes.
Interaction of cleft-filling hydrothermal fluid with wall-rock results in REE mineral formation-/crystallisation and alteration of the wall-rock. The main newly-formed REE-minerals are Y-Si mineral grains (probably tombarthite), a Y-Nb-Ti mineral (aeschynite/pyrochlore) and monazite. Despite these mineralogical changes, the bulk chemistry of the system remains constant at the decimetre scale and thus these mineralogical changes require redistribution of elements via a fluid over short distances (cm). Low-grade alteration enables local redistribution of REE, related to the stability of the accessory phases. This allows the high precision isotope dating of cleft monazite. 232Th/208Pb ages are not affected by excess Pb and yield growth domain ages between 8.03 ± 0.22 Ma and 6.25 ± 0.60 Ma. Monazite crystallization in brittle structures is coeval or younger than 8 Ma zircon fission track data, and hence occurred below 280°.