LINKING MICROCRACKS WITH MINERAL ZONING OF DETACHMENT-EXHUMED GRANITES TO THEIR TECTONOMAGMATIC HISTORY: EVIDENCE FROM THE SALIHLI AND TURGUTLU PLUTONS IN WESTERN TURKEY (MENDERES MASSIF)

Microcracks demonstrate the mechanical behavior of rocks, and can form under a variety of conditions, including thermal contraction during cooling, stress, and relaxation during uplift. Granitic plutons are ideal for microcracking and fracturing studies: they are common in continental crust, predominantly contain only a few key minerals, can be nearly chemically homogenous and structurally isotropic over large areas, and develop vein and crack systems related to their latest stages of cooling. Here we use microcracks to understand the structural evolution of two well-studied granitic plutons in the central Menderes Massif in western Turkey (Salihli and Turgutlu). Both plutons were likely generated in a volcanic arc and have been affected by compressional and extensional deformation. The granitic rocks of which this pluton consists show microcracks and other microtectonic features. We consider these features to be a physical library that records crack generation under submagmatic conditions, migration of the plutons through the crust and their fault-driven exhumation history. Microtectonic features in the rocks were identified using optical microscopy, high-resolution backscattered electron (BSE), secondary electron (SE) and cathodoluminescence (CL) imagery. To discover if mass transfer was accomodated by microcracks during deformation, X-ray element mapping was also employed. The combination of imaging techniques facilitates observation and interpretation of the rocks’ microscale features.

Multiple generations of microcracks, some annealed, within the Salihli and Turgutlu granites provide evidence that the rocks experienced pulses of extension. Microcracks are thicker and more common in Ca-rich zones of plagioclase cores than rims, and the outermost rims of plagioclase are in general myrmekitic. Based on the differences in plagioclase zoning and crystal sizes, the Turgutlu and Salihli granite bodies may have been affected by magma mixing under varying chemical and thermal conditions. Many of plagioclase grains in the Salihli and Turgutlu granites show a range of zoning types (patchy, normal, reverse, oscillatory), grain sizes, and deformational features (microcrack types, microstructures) within a single thin section. Granites that show differences in plagioclase grain size and zoning types are likely the result of magma mixing, a process stated to be “ubiquitous” in arc magmas, but difficult to recognize in granitic assemblages, which undergo late magmatic and subsolidus modification of feldspar compositions and grain sizes.

After plagioclase and K-feldspar crystallized in the Salihli and Turgutlu granites, myrmekite formed at grain boundaries: this was likely facilitated by fluids. Myrmekite forms via exsolution or replacement, and is a common breakdown product of K-feldspar during retrograde metamorphism and metasomatism. In these plutons, myrmekite has partly consumed plagioclase. Changes of the wavelength of CL at grain boundaries and within microcracks in feldspar grains, and the presence of calcite in many of the samples testify to late stage mass transfer by fluids. Deep red CL indicates Fe³⁺ in albitic compositions and replacement of blue-CL in rims and regions of K-feldspar grains. The presence of calcite likely reflects CO₂-bearing fluids, which replace An-rich zones of magmatic plagioclase. Fluids enter plagioclase cores via microcracks and/or along twin surfaces. Late stage microcracks reflect brittle fracture. These are transgranular cracks that cut the entire fabric of the rock. However, some of these cracks are sealed with minerals like biotite, suggesting the presence of fluids as well at this stage.

We’ve targeted granitic assemblages to elaborate the understanding of the extensional dynamics and assembly of western Turkey. Models for Aegean extension in general center on subduction roll-back as a driving force for the exhumation of core complexes. How this process evolved through time is debated. Extension may have initiated in the Rhodope massif of northeastern Greece and southern Bulgaria and progressively transferred to the south towards the Kazdag, Cycladic, Menderes and Crete Massifs. Granitoid bodies are commonly found deformed within and/or cut by major detachments in many of these core complexes. The microstructures, especially microcracking and microveining, are key features of low-T deformation of the Turgutlu and Salihli plutons, and granitoid rocks elsewhere in the Aegean may have comparable records. Bulk geochemistry and dating techniques that rely on mineral separation mask the significant reworking of these rock types that show abundant tectonic features in their microstructures.