Structural Geology Projects II

Timing of subduction and high pressure metamorphism in the western Mediterranean

The Betic Cordillera of southern Spain is an orogen formed in response to millions of years of convergence between Africa and Iberia, from the late Mesozoic to the present.  The orogen consists of three main tectonic complexes, two of which have been subducted to depth, then exhumed back to the surface over short timescales.  Subduction and high pressure metamorphism of the structurally higher complex is relatively well constrained to the Eocene, but the timing of high-pressure metamorphism in the structurally lower complex, known as the Nevado-Filabride Complex, has been a topic of debate for several years due to conflicting geochronological data.

Figure 1 - The Betic Cordillera in southern Spain. Map shows the primary tectonic units of the cordillera. The units of interest to this study, the Bedar-Macael and the Calar Alto, are shown in pink and dark grey, respectively. The overlying Alpujarride Complex is shown in purple. Modified from Martinez-Martinez et al., 2002

Figure 2 - Google Maps digital elevation model of the Sierra Nevada.

Figure 3-

Several proposed tectonic evolution models for the Nevado-Filabride Complex are based on ages of single mineral phases. For example, models based primarily on 40Ar/39Ar dating on white mica in high-pressure schists require that the Nevado-Filabride and the overlying tectonic unit were coevally subjected to high-pressure metamorphism in the Eocene, and subsequently exhumed at different rates. In contrast, more recent models based on Lu-Hf dating on prograde garnets in eclogites separate the timing of high-pressure metamorphism of the Nevado-Filabride Complex from the overlying tectonic unit by at least 10 m.y.

Figure 4 - Geochronological and stratigraphic data from the NFC. Bars represent method, mineral used and dated P-T domains. Dashed blue and red lines show two proposed histories – P-T-t (Pressure-Temperature-time) paths (Paths A and B) for the NFC and for the AC, respectively. Data are from: 1. Platt et al., (2006). 2. Monie et al., (1991). 3. Platt et al., (2005). 4. Augier et al., (2005). 5. Andriessen et al., (1991). 6. Johnson et al. (1997). Fission Track (FT) data 7. Montenat & Ott d’Estevou, (1990); Briend et al., (1990); Mora, (1993); Vissers et al., (1995); Montenat & Ott d’Estevou (1999); Poisson et al., (1999), 8. Weijermars et al., (1985). 9. Sanchez-Vizcaino et al. (2001). Modified from Augier et al., 2005.

We are examining the viability of these models using multimineral Rb/Sr dating of blueschist and eclogite facies rocks in the Nevado-Filabride Complex. The multimineral isochron method uses the whole high-pressure mineral assemblage rather than a single phase, which may not be in equilibrium, by analyzing phengite, paragonite, amphibole, garnet, and apatite. The results will allow us to distinguish between these two conflicting tectonic models and to clarify the tectonic history of the Western Mediterranean region.

Garnet-kyanite mica schist. The kyanites demonstrate that the rock was in equilibrium during high pressure metamorphism and may therefore be suitable for dating.

 

We have collected samples that we have identified as good candidates for dating. However, they must undergo a rigorous process to ensure that the high-pressure phases are in equilibrium. This involves both optical and electron microscopic analyses.

Figure 10 – A garnet porphyroblast in a matrix of white mica from the Tahal Schist. The garnet is nearly 4mm in diameter.

 

We use wavelength dispersive spectroscopy (WDS) to verify mineral compositions and to map major element concentrations in garnets, using UT Austin’s JEOL JXA-8200 electron microprobe.

Figure 11 – A WDS map of the Mn in garnet. Manganese concentrates at the core of garnets when they begin to grow. If the garnet had been affected by retrogressive metamorphism, it would likely show a more uniform distribution of Mn, due to diffusion.

Electron probe microanalysis provides evidence that the high-pressure minerals remained in equilibrium throughout their exhumation, or if they were affected by retrograde metamorphism.

Once my samples are verified as representatives of the high-pressure metamorphic event in the Nevado-Filabride Complex, I can begin the dating process. Samples are crushed and separated using traditional methods and inclusion-free grains are hand picked under a microscope. Finally, the mineral separates are dissolved and Rb and Sr is measured with mass spectrometry.

I’ll be presenting the age data at the American Geophysical Union conference in San Francisco this December, so be sure to find me there!