Dating rock deformation with monazite: The impact of dissolution precipitation creep Nicole Wawrzenitz a, ⁎, Alexander Krohe b, c, 1 , Dieter Rhede a, 2 , Rolf L. Romer a, 3 a Deutsches GeoForschungsZentrum GFZ Potsdam, Telegrafenberg, D-14437 Potsdam, Germany b Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany c Institut für Mineralogie, Universität Münster, D-4814 Münster, Germany abstract article info Article history: Received 15 August 2011 Accepted 30 November 2011 Available online 9 December 2011 Keywords: Monazite U–Pb isotope system Deformation mechanism Fluid–mineral-interaction Dissolution precipitation creep Disequilibrium Recrystallization The U–Th–Pb system of monazite behaves differently dependent on the deformation mechanism – dissolution precipitation creep (DPC) or dislocation creep – activated in the hosting metamorphic rocks. This can be exploited to use monazite for dating deformation, as is shown in rocks subsequently deformed by dislocation creep and DPC. In rock layers intensely deformed by DPC, mineral reactions, particularly the dissolution of feldspar and apatite increased the alkali-content and reactivity of the fluid. This in turn led to dissolution of old predeformative mon- azite grains. New synmetamorphic monazite grains formed as the result of inter-grain transport of material over distances within the grain-scale. This process efficiently led to complete resetting of the monazite U–Th–Pb sys- tem, even at temperatures prevailing during greenschist facies conditions. The chemical composition of the new monazite records the dissolution of the old feldspar by a less pronounced negative Eu anomaly compared to old monazite. The shape of the monazite grains that precipitated during creep indicates the sense of shear in the shear zone, thus linking the obtained ages directly to the map-scale tectonic transport. In rock layers predominantly deformed by dislocation creep, old monazite grains survived intense mylonitiza- tion and high strain, and show a core–rim structure. The cores are patchy, reflecting intra-grain, coupled disso- lution–reprecipitation replacement processes. A wide range in apparent, geologically inaccurate Th/Pb and U/Pb ages among the patchy zones is the result of incomplete removal of in-situ grown radiogenic Pb from the patchy domains, depletion of Th and U and the redistribution of Th and U among the domains. Exclusively in the rims of the old monazite, the chemical composition correlates to that of the syndeformative monazites, and the U–Th– Pb system reflects the subsequent DPC. Accordingly, rocks pervasively deformed by DPC should be preferably used to obtain monazite most suitable for precise dating of creep episodes linked to shear deformation and for determination of deformation rates. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Obtaining well-constrained ages for deformation depends on reliably dating specific deformation fabrics. Monazite, a light rare earth element (LREE) phosphate, has a high potential to set time constraints in de- formed rocks with complex tectonic and metamorphic histories (Catlos and Cemen, 2005; Dumond et al., 2008, 2010; Finger and Krenn, 2007; Foster et al., 2002; Harrison et al., 2002; Pan, 1997; Parrish, 1990; Shaw et al., 2001; Williams et al., 2007). However, it commonly remains diffi- cult to link formation/growth of the dated monazite grains or grain domains to specific microfabrics and structural elements. Even monazite with shape preferred orientation in the matrix, which is commonly con- sidered to date the fabric forming deformation episode, may yield older or mixed ages and not necessarily date deformation. This could lead to misinterpretation of the geodynamic history. The scope of this study is to reveal how monazite behaves in naturally deformed rocks. In such rocks, depending on the fluid availability and strain rate, different deformation mechanisms, such as dissolution pre- cipitation creep (DPC) and dislocation creep, may be active (Fig. 1a, b). In an earlier study, the U–Th–Pb isotopic system of monazite has been observed to respond in different ways to stress and strain, dependent on the active deformation mechanisms (Krohe and Wawrzenitz, 2000). We complement previously published ID-TIMS monazite age data from Wawrzenitz and Krohe (1998) and Wawrzenitz (1997: sample TEO 36) with new in-situ chemical and detailed microfabric data. In this study, we discuss different responses of monazite to fluids and deformation, such as (i) fluid-infiltrating intra-grain coupled dissolution– reprecipitation; (ii) phase separation implying Th-, Y-, and Ca-rich Lithos 134-135 (2012) 52–74 ⁎ Corresponding author at: Sec. 4.2, German GFZ Potsdam, Telegrafenberg B122, D- 14473 Potsdam, Germany. Tel.: + 49 331 2881422; fax: + 49 331 2881474. E-mail addresses: nicole_wawr@gmx.de, hoymann@gfz-potsdam.de (N. Wawrzenitz), krohe@uni-muenster.de (A. Krohe), dieter.rhede@gfz-potsdam.de (D. Rhede), romer@gfz-potsdam.de (R.L. Romer). 1 Tel.: +49 234 3223232. 2 Tel.: +49 331 2881475. 3 Tel.: +49 331 2881405. 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.11.025 Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos