Very fast exhumation of high-pressure metamorphic rocks with excess 40Ar and inherited 87Sr, Betic Cordilleras, southern Spain.
Creators: name: Jong, Koen De
Publisher: name: Figshare
Year: 2016
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: 40303 Geochronology
subject: 40304 Igneous and Metamorphic Petrology
subject: Mineralogy
subject: Solid Earth Sciences
subject: 40312 Structural Geology
subject: 40313 Tectonics

description: In order to attempt to
further constrain the age of the early Alpine tectonic evolution of the
Mulhacén Complex and to explore the influence of inherited isotopes, micas from
a small number of well-characterised rocks from the Sierra de los Filábres,
with a penetrative tectonic fabric related to the exhumation of eclogite-facies
metamorphic rocks were selected for <sup>40</sup>Ar/<sup>39</sup>Ar and Rb-Sr dating. A single
phengite grain from an amphibolite yielded a <sup>40</sup>Ar/<sup>39</sup>Ar laser step heating plateau
age of 86.7 ± 1.2 Ma (2σ; 70% <sup>39</sup>Ar released) and
inverse isochron age of 86.2 ± 2.4 Ma with an <sup>36</sup>Ar/ <sup>40</sup>Ar intercept within error
of the atmospheric value. Induction furnace step heating of a biotite separate
from a gabbro relic in an eclogite yielded a weighted mean age of 175.2 ± 6.3
Ma (2σ; 95% <sup>39</sup>Ar released). These ages are
diagnostic of excess argon (<sup>40</sup>Ar<sub>XS</sub>) incorporation, as they are older than
independent age estimates for the timing of eclogite-facies metamorphism and
intrusion of the gabbros.<sup>40</sup>Ar<sub>XS</sub> incorporation probably resulted from
restricted fluid mobility in the magmatic rocks during their metamorphic
recrystallisation. Rb-Sr whole-rock-phengite ages of graphite-bearing mica
schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1 ±
3.2, 40.6 ± 2.6 and 14.1 ± 2.2 Ma). An albite chlorite mica schist from the
Mesozoic series of the Nevado-Lubrín unit has a whole-rock-mica-albite age of
17.2 ± 1.9 Ma, which is within error of a <sup>40</sup>Ar/<sup>39</sup>Ar  plateau age published
previously and of the youngest Rb-Sr age of the Paleozoic series obtained in
this study. The significant spread in Rb-Sr ages implies that progressive
partial resetting of an older isotopic system has occurred. The micro-structure
of the samples with pre-Miocene Rb-Sr ages reveals incomplete recrystallisation
of white mica and inhibited grain growth due to the presence of graphite
particles. This interpretation agrees with previously published, disturbed and
slightly dome-shaped <sup>40</sup>Ar/<sup>39</sup>Ar age spectra that may point similarly to the
presence of an older isotope component. The progressively reset Rb-Sr system is
a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén
Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the
Mesozoic series precludes any isotopic inheritance, in agreement with its
pervasive tectono-metamorphic recrystallisation during the Miocene. Exhumation
of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting
rates of about 22.5 mm/a during the early phase and 9-10 mm/a during the late
phase; the latter with a cooling rate in the order of 330°C/Ma.<i>de Jong K.</i><i> </i><b>Very fast exhumation
of high-pressure metamorphic rocks with excess <sup>40</sup>Ar
and inherited <sup>87</sup>Sr, Betic Cordilleras, southern Spain.</b> <i>In: A. </i><i>Takasu, T.
Hirajima, L.G. Medaris Jr., P.J., O'Brien, Eds.; </i><i>Eclogites and Related High and Ultrahigh
Pressure Metamorphic Rocks (Proceedings 6<sup>th</sup> International Eclogite
Conference, Niihama, Japan). </i><b><i>Lithos</i></b> 70 (3-4) , 91-110. (doi:10.1016/S0024-4937(03)00094-X) October 2003.