HR: 0800h
AN: V21A-0589    [Abstracts]
TI: Sr Mobility and Isotope Homogenization During Metamorphic Overprint - an Example From the Cycladic Blueschist Belt
AU: * Wegmann, M I
EM: mwegmann@zedat.fu-berlin.de
AF: Free University of Berlin, Malteserstr. 74-100, Berlin, 12249 Germany
AU: Hammerschmidt, K
EM: chemie@zedat.fu-berlin.de
AF: Free University of Berlin, Malteserstr. 74-100, Berlin, 12249 Germany
AB: Diversity of age clusters obtained from regional metamorphic areas are very often interpreted as the result of intermittent wide spread infiltration of fluids to have been the cause of "resetting" and "rejuvenation" of age values. On white mica out of impure marbles und carbonate bearing mica schists from the Cycladic blueschist belt (South Evia, Greece) we determine ages varying between 20 and 50 Ma. The fabrics of rocks of the main geological units on southern Evia (Ochi and Styra) were shaped by penetrative deformation under retrograde isothermal (350 $\pm$ $50\deg$C) metamorphic condition during uplift. The age variation even have been detected within the dimension of a thin section challenging the idea of a pervasive fluid which reorganize und homogenize the isotopic Sr composition of the mineral assemblage. Therefore, we measured the Sr isotope composition of Sr rich and Rb poor minerals (mostly calcite and albite, n = 180). Within the dimension of thin sections the Sr isotopic composition is quite uniform (e.g. $^{87}$Sr/$^{86}$Sr = 0.70772 $\pm$ 1). The scatter of Sr composition increases within one lithology (e.g. $^{87}$Sr/$^{86}$Sr = 0.70771 $\pm$ 16). These values found in impure marbles and in calcite poor mica schists refer to the Sr isotope composition of seawater during Permo-Mesozoic time, the period the sedimentation occurred. $^{87}$Sr/$^{86}$Sr values of about 0.704 from calcites in metabasites implying an origin from a MORB source corroborate the obvious preservation of original Sr composition beyond metamorphic overprint. Extensional cracks originated during uplift were filled with epidote which reveals the same Sr isotope composition as calcite. Sr mobilization and replacement were restricted to adjacent minerals. The Sr composition of minerals was acquired very early in the history of the rock probably during the sedimentary stage. Short-term fluid infiltration did not induce complete Sr isotope homogenization on a regional scale. Sr mobilization only occurred locally and is definitely not the reason for varying ages of metamophic minerals in the Styra and Ochi units on South Evia.
DE: 8030 Microstructures
DE: 5139 Transport properties
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting