Documenting Exhumation in the Central and Northern Menderes Massif (Western Turkey): New Insights from Garnet-Based P-T Estimates and K-Feldspar 40Ar/39Ar Geochronology

Abstract

The Menderes Massif (Turkey) is a metamorphic core complex that records Alpine crustal shortening and extension. Here, nine garnet-bearing schist samples in the Central Menderes Massif (CMM) from below the Alaşehir detachment (AD) were studied to reconstruct their growth history. P-T estimates made using a chemical zoning approach, and petrological observations, indicate garnet grew between ~6 kbar and 550°C and 7.5-9 kbar and 625-650°C. Two P-T path shapes from two samples emerged (isobaric and burial), suggesting that either separate garnet-growth events occurred, or different garnet generations from the same metamorphic event were sampled. Despite observable diffusional modification in most garnets, thermobarometric estimates for crystal-rim growth yield P-T estimates similar to those reported elsewhere in the region. Ion microprobe monazite ages, paired with textural observations, from three of the samples time early retrograde metamorphism (~36-28 Ma). To better understand Neogene extension/exhumation, K-feldspar 40Ar/39Ar ages were obtained from two synextensional granites (Salihli and Turgutlu) exposed along the AD and two from the northern Simav detachment (Koyunoba and Eğrigöz). This data suggests the Simav detachment footwall rapidly exhumed at ~20 Ma, whereas the AD experienced two periods of exhumation/cooling (~14 Ma and~5 Ma). AD ages support a pulsed exhumation model for the massif.

Garnet Chemical Zoning Based Thermobarometry: Method Evaluation and Applications in the Menderes Massif, Western Turkey

“…Garnet is known to retain a record of tectonism experienced during growth with the assumption that its chemical zoning occurs as pressure and temperature (P and T) change over time (e.g., ). Recently, the application of one garnet-based thermobarometric method has further shown this mineral's ability to record a more dynamic evolution of metamorphic terranes than what is typically extracted (e.g., [17,[19][20][21][22][23]). The garnet zoning thermobarometric method (GZM), herein referencing the approach of [17], can provide a detailed reconstruction of nuanced lithosphere dynamics during the prograde burial of crustal rocks, as is the case in the Menderes Massif, western Turkey [21,23].…”

“…Recently, the application of one garnet-based thermobarometric method has further shown this mineral's ability to record a more dynamic evolution of metamorphic terranes than what is typically extracted (e.g., [17,[19][20][21][22][23]). The garnet zoning thermobarometric method (GZM), herein referencing the approach of [17], can provide a detailed reconstruction of nuanced lithosphere dynamics during the prograde burial of crustal rocks, as is the case in the Menderes Massif, western Turkey [21,23]. There the method was used to reveal that garnets throughout the southern portion of the massif not only retain a record of metamorphic growth in response to Cenozoic burial, but they also capture an intermediate pressure inflection (~1 kbar drop) interpreted as a brief period of local denudation.…”

“…GZM has only seen a limited number of applications [17,[19][20][21][22][23]. One plausible explanation for this could be users encountering "model failure" due to divalent cation intracrystalline diffusion within garnet, a realistic expectation for many metamorphic garnets worldwide.…”

“…One plausible explanation for this could be users encountering "model failure" due to divalent cation intracrystalline diffusion within garnet, a realistic expectation for many metamorphic garnets worldwide. For instance, Etzel et al [23] attempted to apply this approach to a series of metapelites from the Central Menderes Massif's Bozda g and Bayındır nappes but could not produce sensible P-T paths for all but one sample. The authors speculated this was because chemical zonation had relaxed due to intracrystalline diffusion; ultimately, they could only report relative metamorphic conditions.…”

“…In this contribution, we revisit the work of [21,23]. We begin by reviewing the fundamentals of GZM and how it interfaces with the software package Theriak-Domino [24,25] as conceived by [17].…”