Metamorphe und strukturelle Entwicklung des La Paz Kristallinkomplexes, Baja California Sur, Mexiko

This Thesis deals with the metamorphic and structural development
of the La Paz Crystalline Complex (Ortega-Gutierrez 1982), which
extends south of the city of La Paz, Baja California Sur, Mexico,
to the southern tip of the Baja California peninsula. The results
presented are data on the development of structures and
metamorphism based on detailed geologic mapping. This includes the
succession of metamorphic isograds, pT-paths for structural units
and the relation of the two. It allows to explain the development
of the crystalline complex in light of the specific tectonic
situation. The most important new result is that the whole complex
does not make up one distinct tectonostratigraphic terrane, as
supposed previously (Sedlock et al. 1993, Gastil 1991, Beck 1991
and references therein), but that the complex is separated by a
SSW-NNE striking fault. This fault juxtaposes rocks of different
origin and with different metamorphic history. Intrusives in the
north-western area are of relatively primitive M-type, in the
south-eastern area, intrusives show a higher degree of crustal
contamination (Schaaf et al. 2000). The structural work presented
here shows that the fault already described in literature as the
‘La Paz Fault’, but interpreted differently (Aranda-Gómez &
Pérez-Venzor 1989, Sedlock et al. 1993, Schaaf et al. 2000) is
indeed the structural feature described above. Metamorphic contrast
across the fault is minor, but an anticlockwise pT-path could be
constructed for the footwall. Comparison with new results from the
northern part of Baja California (Johnson et al. 1999, Schmidt
& Paterson 2002, Ortega-Rivera 2003) shows that the La Paz
Crystalline Complex is indeed a part of the Peninsular Ranges
Batholith (Gastil 1983, 1991). The separation into a western and an
eastern part is characteristic for this complex and can be compared
to the geochemical, metamorphic and structural characteristics
presented here. PT-determinations were conducted for the overall
low-pressure metamorphism using several methods: construction of
petrogenetic grids for metapelitic rocks (Spear et al. 1999,
Pattison et al. 2002), different geothermobarometers (e. g.
garnet/biotite, Holdaway 2000, GASP, Holdaway 2001) and a
relatively new approach to calculate pseudosections through
thermodynamically modelled systems with known bulk-composition
(Connolly & Petrini 2002, Tinkham et al. 2001). The
pseudosections are a very useful graphic tool to represent modelled
metamorphic mineral parageneses, which can be easily compared with
actual measurements (Microscope, Electron Microprobe).