Dating deformation

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Brief summary from Williams et al. (2007).

Providing absolute timing constraints on deformation events is one of the major challenges of tectonic analysis. Although cross-cutting relationships of plutons, dikes, and other igneous rocks remain some of the strongest constraints, these tend to place rather coarse limits, and ambiguities can be introduced because of differing rheological properties between igneous and metamorphic rocks. In multiply deformed rocks, deformation histories commonly depend on interpretations of fabric relationships involving porphyroblasts, foliations, lineations, etc. Monazite can be a component of the fabric of deformed rocks and thus can offer a more direct means of constraining the timing of deformation events.

Monazite can be a fabric component in deformed rocks, and therefore offers a direct means of contraining the timing of deformation events. Timing constraints on deformation events can be obtained via metamorphic reactions associated with monazite growth (e.g. garnet replacement by cordierite, Mahan et al., 2006).

Inclusion trails within monazite porphyroclasts can also constrain deformation (see Dahl et al., 2005). Alignment of elongate or platy monazite grains aligned with fabric can also provide timing constraints. Syntectonic monazite can grow in lineation directions or extensional quadrants (see figures right and below). Some monazite crystals are actually offset by fractures related to larger scale fracture systems (Shaw et al., 2001). Locally, such fractures may be filled with later monazite bracketing the time of deformation (right). The specific domains with fill such fractures are volumetrically minor components of larger monazites, illustrating the need for in-situ microanalysis combined with high spatial resolution compositional mapping.

a) Summary of all EPMA geochronologic data for monazite in dike 04G-019B. All histograms are scaled relative to the consistency standard in Fig. 3c. The black bar is scaled to show the range of dates for the thirteen zircon fractions analyzed by ID-TIMS (Fig. 6 and Table 2). Rationale behind weighted mean dates for the monazite data are discussed in Section 5. Dates are quoted at the 2? level of uncertainty after Williams et al. (2006), including 1% uncertainty on the modeled background intensity. Note: Data for grain 1-m2 are illustrated in Fig. 3c, b–e) Representative maps, sketches of kinematic interpretations, and locations of EPMA trace element data. Ap = apatite. Locations of data acquisition points correspond to numbered data in Table 3 and domain symbols in Fig. 10. The size of the symbols in c) and d) is larger than the area of points used to analyze each domain as illustrated in b) and e). See Dumond et al. (2008) Chem. Geol.

 

 

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