10.3 Selection of the Most Relevant Geochronological Data for the Rhodope Zone
The complexity and interaction of individual processes involved in the formation of HP and UHP rocks in mountain belts (usually multiple episodes of magmatism, metamorphism, deformation, and fluid–rock interaction) result in small-scale heterogeneities, such as chemical zonation or development of multiple growth domains of minerals used in radiometric dating. Thus, polymetamorphic rocks are often characterized by local equilibrium, including isotopic compositions. This feature has a significant impact on the geochronological results acquired by commonly used geochronometers (K–Ar, Ar–Ar, Rb–Sr, Sm–Nd, Lu–Hf), which are mostly based on large mineral separates, zoned minerals, and whole rocks and are sensitive to retrogression. Moreover, as many HP and especially UHP rocks are characterized by high peak temperatures, the K–Ar and Rb–Sr isotope systems cannot date the peak. Thus, radiometric dating of micas, garnets, pyroxenes, and amphiboles, which are rock-forming minerals that can provide P–T conditions for metamorphism, can be problematic, as they have relatively low closure temperatures, may be zoned, and/or show only local isotopic equilibrium even within a single thin section. The latter feature results in the commonly encountered problem of isotopic disequilibrium within and amongst dated minerals with the Rb–Sr, Sm–Nd, and Lu–Hf systems.
It is also worth noting that closure temperatures depend largely on the cooling rates, grain size, and action of fluids (e.g., Dark FridayNew Brown Black up Look Boots Lace Kelley, 2002; Scherer et al., 2000). Thus, especially for (U)HP rocks, which are characterized by steep P–T paths (i.e., small temperature variations for high depth differences) and usually strong retrogression, closure temperatures of minerals in the conventional sense may not even be applicable, while fluid-induced recrystallization is the predominant mechanism responsible for the disturbance of isotopic systems and partial or complete age resetting (Villa, 1998; see Section 10.6).
Given these complexities, the dating technique that meets with closest approximation the requirements for dating (U)HP-HT rocks is the ion microprobe SIMS technique. This technique uses the U–Pb isotopic system on the robust mineral zircon and is at the same time a high-resolution beam technique able to resolve within grain age zonation. It is preceded by cathodoluminescence (CL) imaging for the visualization of the internal structure of the sectioned zircons and is promising and reliable for this type of rocks, as zircon largely responds to metamorphic recrystallization at the high temperatures and pressures suffered by a major part of the Rhodope metamorphic rocks. As zircon can survive very high temperatures it usually retains a memory of previous metamorphic and/or magmatic events. Although zircon does not usually participate in metamorphic reactions and therefore its formation cannot be directly linked to a particular stage of metamorphism, this issue can be often satisfactorily solved by using mineral inclusions and REE compositions of the analyzed zircon domains (e.g., Rubatto, 2002; Hoskin & Schaltegger, 2003, and references therein).
Determination of the time of metamorphism in the Rhodope has first been approached by K–Ar and Ar–Ar dating of mica and hornblende and implied Eocene metamorphic ages (Liati, 1986; Liati & Kreuzer, 1990). However, neither the Ar– nor the Rb–Sr isotopic systems can date the peak (U)HP-HT conditions. Because these chronometers are considered to have low closure temperatures and be sensitive to retrogression, they only record age information for late stages of the P–T–t path. Moreover, K–Ar and Ar–Ar dating of white micas and hornblendes affected by HP metamorphism may yield erroneously high ages, due to contamination by excess Ar (e.g., Kelley, 2002).
Sm–Nd dating has been applied in few cases in rocks of the Rhodope. The possibility of disequilibrium within and amongst the analyzed minerals and their inclusions, which may be responsible for yielding erroneous results needs to be taken into consideration when interpreting such data (e.g., Scherer et al., 2000 and references therein).
Monazite U–Th–Pb dating has been applied in some cases either by using electron microprobe (Reischmann & Kostopoulos, 2002) or single-grain isotope dilution and TIMS analyses (Jones et al., 1994). The most obvious problem with the electron microprobe method is that it does not allow the measurement of isotope ratios. As a consequence, (a) concordance has to be assumed, (b) no correction for common Pb can be made, (c) the sensitivity is relatively low, (d) monazite can be strongly zoned, and (e) isotopic disequilibrium may occur. The latter two problems apply also to the isotope dilution method and SIMS.
Furthermore, as already mentioned in the beginning of this chapter, a notorious problem with all mineral chronometers is the common interpretation of the dates as cooling ages. It should be taken into account that closure temperatures are not fixed numbers (see earlier). The role of fluids in disturbing the isotopic clock at temperatures significantly lower than the theoretical “closure temperatures” is usually underestimated, especially by non-geochronologists. Such possible misinterpretations may lead to incorrect ideas and erroneous linking of “ages” to large-scale geodynamic processes.
Therefore, in the following evaluation interpretation of the time of metamorphism(s) and of protolith formation is mainly based on U–Pb zircon SHRIMP data.