THE PRE-LATE ORDOVICIAN METAMORPHIC EVOLUTION OF THE GOTTHARD-TAVETSCH MASSIFS (CENTRAL ALPS) - FROM LAWSONITE TO KYANITE ECLOGITES TO GRANULITE RETROGRESSION

This study provides regional and thin section scale evidence for a mul ti-stage metamorphic history in the Gotthard and Tavetsch massifs. Tex tural and structural observations yield information on the relative ti ming of metamorphic mineral growth and recrystallization. They provide a basis for assessing the significance of P-T data acquired from petr ologic calculations. Coronas are often excluded by petrologists in ord er to define P or T, but this paper shows evidence for equilibrium bet ween mineral phases in such domains. Similarly, pseudomorphs are not u sually included in quantitative P-T calculations. Yet, possible chemic al compositions are postulated here on theoretical grounds, and the se nsitivity of the constrained P-T-X conditions on such model compositio ns is evaluated by performing calculations for different compositions. The most important information on the first metamorphic (eclogite fac ies) event is preserved in mafic rocks, but field evidence (intrusive contacts between eclogitized metagabbros and paragneisses) strongly su ggest that the whole sequence underwent high-pressure metamorphism. An early high-pressure assemblage includes lawsonite as a key mineral ph ase. The estimated temperature for this event is below 600-degrees-C. Lawsonite was subsequently replaced by a higher temperature eclogite a ssemblage (in the range of 650-700-degrees-C). A down-P, up-T or isoba ric heating (ca. 50-degrees-C) path lead to the formation of the eclog itic temperature peak assemblage. The stable assemblage at peak temper ature is Grt-Omp-Ky-Qtz-Zo-Hbl-Ilm-Mt-Py-Rt. Estimated temperatures an d minimum pressures are 700-750-degrees-C and 1.8 GPa, respectively, a t this stage, which is attributed to a subduction event. The dehydrati on of sediments in the oceanic slab produced hydration in the overlyin g underplated terrane, causing the formation of hornblende. The eclogi te rocks preserve rather unique evidence for the T-prograde evolution (pre-relaxed stage of the thermal evolution). The observed prograde pa th provides an important link between blueschist and high temperature eclogite events. The subsequent granulite facies event is characterize d by Grt-Di-Opx-Olig-Qtz-Hbl-Ilm-Mt-Py-Ttn. This assemblage is observe d only at the local equilibrium (cm) scale and yields temperatures ran ging from 600 to 700-degrees-C at a pressure of approximately 0.8 GPa. Advection of the isotherms during rapid uplift was responsible for th e granulite event. Subsequent uplift and cooling were accompanied by h ydration and partial melting of the metasediments. The minimum age of the migmatitic overprinting is constrained by the post-orogenic intrus ion of granitoids dated at almost-equal-to 440 Ma. The resulting path reflects the style and rate of unroofing. In terms of regional geology this investigation shows many new features and defines more clearly t he importance of the already documented granulite event. The presented quantitative results are important in order to understand isotope geo chemistry and the Late Ordovician magmatism, and they constrain the th ermo-tectonic evolution of the region. It is concluded that the old ge odynamic models considering the formation and closure of small intracr atonic basins or neglecting the Caledonian event in the Alps have to b e abandoned since they are inconsistent with the presented petrologic observations.