Rb-Sr whole-rock isochron evidence for the age of the Malvern Hills complex

The Malvernian igneous complex of the Malvern Hills consists mainlyof plutonic rocks and their locally deformed equivalents. The igneous rocks are calc-alkaline and range from ultra mafic diorite to granite in composition, although diorite and tonalite are the most abundant rock types. A Rb-Sr whole-rock isochron plot for representative samples yields an age of 681 ± 53 Ma with an initial 87Sr/86Sr ratio of 0.7049±0.0005(2σ errors).This is interpreted as the age of crystallization of the igneous complex, and the relatively low initial 87Sr/86Sr ratio is considered to preclude a long crustal history for the source region. The Malvernian complex is thus regarded as a late Precambrian addition to the crust. It has been compared on geological grounds with the Johnston Complex and the Stanner-Hanter Complex, and isotopic ages between 640 and 700 Ma for all these complexes indicate that they may represent the earliest phase of igneous activity in the evolution of continental crust below England and Wales.

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