Thermal histories of k-feldspar from granites located in the central and
northern Menderes Massif, western Turkey: Implications for regional
extension
Abstract
Evidence of syntectonic magmatism associated with onset extension and
unroofing of the Menderes Massif metamorphic core complex, western
Turkey, is well documented. The Salihli and Turgutlu plutons, located
along the Alasehir detachment in the Central Menderes Massif (CMM) and
the Koyunoba and Eğrigöz Plutons located in the Northern Menderes Massif
(NMM) are common targets for understanding the dynamics and timing of
this Cenozoic activity. To this end, here we report new potassium
feldspar 40Ar/39Ar ages from samples collected from each pluton and
compare these to available zircon U-Pb and monazite Th-Pb
crystallization ages. Argon age spectra were collected by incrementally
heating bulk concentrates with a CO2 laser and analyzing the gas
released at each step. The peraluminous granite samples from the
Koyunoba (AT17) and Eğrigöz (WA12) plutons both have effectively flat
spectra with average plateau ages of 20.12±0.05 Ma and 19.86±0.05 Ma,
respectively. The U-Pb age of zircon from WA12 is 20.5±1.1 Ma [Catlos
et al., 2012; doi: 10.2475/05.2012.03 ]; although a zircon U-Pb age
from AT17 has not been reported, zircon from other Koyunoba rocks have
U-Pb ages between 21.1 Ma and 23.2 Ma [1]. K-feldspar from sample
EB06 (Turgutlu Granite) steadily increases in age from 10.62±0.03 Ma to
a plateau age of 14.06±0.03 Ma, with similar inverse isochron
(13.66±0.29 Ma) and total gas ages (13.36±0.2 Ma). Sample EB05 (Salihli
Granite) increases in age from 3.27±0.10 Ma (step 3, 0.5% 39Ar
released) to a maximum of 6.05±0.09 Ma (step 33, 96.6% 39Ar released).
A plateau age could not be estimated for this sample, but two inverse
isochron ages from different degassing steps are calculated (3.02±0.09
Ma for the initial 19 steps and 3.29±0.22 Ma, for the final steps
19-31). Regarding their crystallization histories, the oldest reported
monazite Th-Pb age for EB06 is 15.5±1.2 Ma [2] and reported monazite
Th-Pb ages for Salhili granite ranges from 9.6±1.6 Ma to 21.7±4.5 Ma
[Catlos et al., 2010; https://doi.org/10.1016/j.tecto.2009.06.001].
These 40Ar/39Ar ages suggest NMM plutons rapidly cooled whereas CMM
Salihli and Turgutlu plutons not only remained at depth below the argon
retention window for a prolonged period following emplacement, but each
experienced unique thermal (exhumation) histories despite their
geographic proximity.