Achieving Geological Precision in Metamorphic Geochronology: A Th-Pb Age for the Syn-Metamorphic Formation of the Mallnitzermulde Synform, Tauern Window, from Individual Allanite Porphyroblasts

Determining accurate age information on the thermal and tectonic evolution of metamorphic belts requires both precise analytical techniques and thorough understanding of the relationship between the dated material and the metamorphic crystallisation history. Achieving both simultaneously has proved difficult: the most precise analytical data comes from the UTh-Pb system but generally from minerals which cannot be directly linked by petrographic observation to metamorphic history or from minerals where there is doubt whether the observed isotope systematics relate to the minerals analysed or to U-Th rich inclusions they contain. In most cases high analytical precision is linked to low Pb concentrations which limits how small the analysed sample can be. In this study we show that it is worth forfeiting some analytical precision in order to analyse relatively lead-rich minerals for which individual porphyroblasts, with clearly defined microstructural relationships to fabric development, can be analysed. The sample analysed is a mica-rich pelitic schist which contains micro-porphyroblasts of allanite. The full mineral assemblage is: garnet, muscovite, biotite, quartz, allanite, magnetite, ilmenite, apatite, tourmaline, with some retrograde chlorite. It comes from the lower part of the Peripheral Schieferhiille, from the NE limb of the Mallnitzer Mulde synfonn in the south east Tauern Window. Structurally it is close to the hinge of a fold which is believed to be of the same age as the major Mallnitzer Mulde synform. In the fine-grained muscovite-rich matrix the S1 schistosity was extensively crenulated during folding; locally randomly oriented recrystallised muscovite flakes overprint the crenulated schistosity. Euhedral 1.5ram diameter garnets and allanites contain inclusion trails of aligned ilmenite (Fig. 1). In detail the textures indicated porphyroblast growth after original fabric formation during the period of crenulation development. The mineral assemblage of the sample itself does not provide tight constraints on metamorphic conditions but it is interbedded with pelites containing typical assemblages of the chloritoidbiotite zone mapped by Droop (1985) for which conditions of 7 _+ 1 kb and 550 _+ 10~ are estimated. Previous geochronological studies in the south east Tauern Window show marked geographic variations in post-metamorphic cooling history, with biotite ages of 16 Ma 10hn to the east of the present locality (Cliff et al., 1985) and 21 Ma 12 km to the W in the Sonnblick Dome (Reddy et al., 1993). Metamorphic crystallisation ages are less well known; in a detailed study of white mica Rb-Sr ages in the schists from the Mallnitzer Mulde, 6 km NW of the present study, Inger and Cliff (1994) found a range from 32 Ma down to 23 Ma which they correlated with variations in textural relationships between white micas and fabric development; samples with well developed crenulation, correlated with formation of the Mallnitzer Mulde, gave ages of 28-29 Ma. The new data presented here were obtained by analysis of allanite, which has two properties of particular value for dating metamorphic crystallisation: (1) its closure temperature for Pb isotopic diffusion is well above the temperature of metamorphism (yon Blanckenburg, 1992; Oberli et al., 1996) (2) its typically high Th concentration makes it feasible to analyse individual grains only tenths of a millimetre in diameter.