Abstract
The Hills Pond lamproite is part of a sill complex intruded into the northern portion of the Silver City Dome (Fig. 1). This intrusion is related closely in age, composition, and tectonic setting to a nearby intrusion at Rose Dome (Berendsen and Blair, 1991). Rose Dome includes well-known xenoliths of granite derived from the deep crust. However, xenoliths of any type have never been reported before from the Hills Pond lamproite. The seeming lack of xenoliths at Silver City has been puzzling. The following summary is based mainly on Bickford, Wetherill, and Franks (1971); Franks, Bickford, and Wagner (1971); and Wojcik and Knapp (1990). The intrusive rock of Silver City and Rose Dome was referred to earlier as mica peridotite or kimberlite. It now is recognized as lamproite, a potassium-rich, intermediate to ultramafic, alkalic rock with high K/Na and K/Al ratios. Intrusion of lamproite took place approximately 88-90 million years ago during the mid-Cretaceous, when the present surface was covered by about 1 km of overburden strata. The intrusions are hosted at the surface in
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