Excess radiogenic Ar and undisturbed Rb-Sr systems in basic intrusives subjected to Alpine metamorphism in southeastern Spain

Metamorphosed basic intrusives in the Alpine orogen of southern Spain contain relicts of unmetamorphosed gabbroic rock. Rb-Sr investigation of one of these relicts indicates that the rock and constituent minerals remained closed to Rb-Sr during the plurifacial Alpine metamorphism. The Rb-Sr isochrom age is 146 +- 3 Ma with initial 87Sr/86Sr of 0.7028 +- 0.0001 lambda 87Rb = 1.42 X 10-11a-1, which is taken as approaching the the intrusion age. All investigated relicts of unmetamorphosed rock are characterized by excess 40Ar contents between about 6 X 10-6 and 14 X 10-6 cm3 NTP/g, wheras the metamorphosed parts of the basic intrusives rarely show this phenomenon. It is argued that this argon, which is very inhomogeneously distributed, was acquired during the Alpine metamorphism. A comparison of the excess 40Ar contents of the pyroxene and the plagioclase with published data suggests that in all cases the crystal have acquired the argon under about the same prevailing 40Ar partial pressure, even when the geologic conditions were different. (orig.)