STRAIN AND STRAIN-RATE IN A SYNKINEMATIC TRONDHJEMITIC DIKE - EVIDENCE FOR MELT-INDUCED STRAIN-SOFTENING DURING SHEARING (BOHEMIAN MASSIF, CZECH-REPUBLIC)

Emplacement-related deformation of a Cambrian trondhjemitic dike, loca ted at the western border of the Tepla-Barrandian unit (central Europe an Variscides), is studied in detail. The trondhjemitic melt was empla ced at T = 750 degrees C into an active low-temperature (ca 350 degree s C) east-northeast trending dextral transcurrent shear zone. Thermal modelling indicates that the theologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling, r espectively. The major part of the shearing occurred within 8 days aft er the melt emplaced. The zonation of the dike, in strain magnitude, m ineralogy and geochemistry, can be explained by the laterally varying rheology (from margin to centre) during cooling and shearing. The defo rmation was volume-constant, and the strain data plot in the apparent constrictional field. The unusually high Value of the calculated longi tudinal strain rate (> 2.8 x 10(-6) s(-1); equivalent to a displacemen t rate of > 2 cm h(-1))is probably related to the intruding melt that 'lubricated' the shear zone and thus enhanced its displacement velocit y. Consequently, melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and up per continental crust. (C) 1997 Elsevier Science Ltd.