Rubidium–strontium ages and strontium isotope ratios from the igneous rocks of Leicestershire

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Abstract
The south Leicestershire diorites and the Charnwood southern diorites give Rb-Sr isochron ages of 546 ± 22 m.y. and 552 ± 58 m.y., respectively, indicating a late Precambrian to Cambrian age of emplacement. Their respective initial 87 Sr/ 86 Sr ratios of 0.7056 ± 0.0001 and 0.7061 ± 0.0008 agree with analytical error. Despite the large error produced by a small spread of Rb/Sr values an age of 311 ± 92 m.y. for the Charnwood northern diorites indicates that the group is younger than previously thought. The initial 87 Sr/ 86 Sr ratio is 0.7068 ± 0.0004. The Mountsorrel Complex gives an age of 433 ± 17 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.7070 ± 0.0002. This confirms a late Ordovician to early Silurian age for the intrusion. An isochron for the Charnwood porphyroids is not presented because of an extremely small spread at Rb/Sr values. However initial 87 Sr/ 86 Sr ratios are calculated for the group using likely ages between 500 and 700 m.y. The values obtained, 0.7046–0.7049, are significantly lower than any of the other rock groups studied. The initial ratios of the porphyroids, diorites and granodiorites rule out melting or partial melting of ancient high level crustal rocks as a source. Values such as those determined could have been produced by the melting or partial melting of lower crustal and mantle rocks.

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