Rb-Sr Geochronologic Investigation of Basic and Ultrabasic Xenoliths from the Stockdale Kimberlite, Riley County, Kansas

Abstract

The diorite age represents the time of formation of igneous material at about 1,600 m.y. ago because (1) the initial Sr (87/86) ratio is extremely low and (2) the constituent minerals have not been recrystallized. The other ages represent post-formational events. One hypersthene gabbro (1139h) has not been closed to its environment (evidenced by secondary K-feldspar) and the age is spurious; the other hypersthene gabbro shows evidence of a thermal event at about 700 m.y. ago. The garnet granulite (1141a) of cognate origin shows evidence of a thermal event at about 240 m.y. ago; and, although an age cannot be calculated due to lack of data, the eclogite (1128g) has been derived from a deep-seated (cognate origin) source as indicated by the low Sr (87/86) ratio. These scattered but internally consistent ages argue for several periods of thermal activity (metamorphism) prior to entrapment in their host kimberlite (the kimberlite itself is thought to have crystallized about 750 m.y. ago in the upper mantle and to have been emplaced about 100 m.y. ago). This further suggests that entrapment occurred under near adiabatic conditions (hot kimberlite mass) or that ascent of the kimberlite as extremely rapid and relatively cool. The data also indicate the extreme complexity of the lower crust and upper mantle in the Midcontinent and the need for many more geochronologic and other investigations of these xenoliths.