A metasomatic diopside rock occurs at the top of the dolomitic Connemara
Marble Formation of western Ireland and contains titanite and K-feldspar
in addition to around 90% diopside (X(Mg) = 0.90-0.97). U-Pb isotopic
measurements on this mineral assemblage show that the titanite is both
unusually uranium-rich and isotopically concordant, with the result that
a precise U-Pb age of 478 +/- 2.5 Ma can be determined. The age is
identical within error to a less precise Rb-Sr age of
diopside-K-feldspar of 483 +/- 6 Ma. Petrological evidence indicates
that the assemblage crystallized at c. 620-degrees-C close to or below
the closure temperature of titanite. The age thus provides a precise
estimate of the time of metamorphism; this age is 11 +/- 3 Ma younger
than the 490 Ma age for nearby gabbroic plutons which has previously
been used to constrain the peak metamorphic age. This difference accords
well with geological evidence that the gabbros were emplaced prior to
the metamorphic peak. Analysis of minerals with high closure temperature
from assemblages whose crystallization is unambiguously associated with
a specific episode of fluid infiltration at the peak of metamorphism
provides the basis for a new approach to dating metamorphism. The
success of this approach is demonstrated by the results from Connemara.