Diatreme Structures Research Articles

MINERAL CHEMISTRY OF KAMAFUGITES AND RELATED ROCKS FROM THE ÁGUAS EMENDADAS REGION, GOIÁS STATE

In the Águas Emendadas Region, Goiás State, Brazil, Late Cretaceous alkaline magmatism produced a series of diatreme structures of kamafugitic affinity. These occurrences consist of a central breccia body, surrounded by lavas and intruded by dykes. Compositionally, the rocks vary from kamafugites, through melaleucitites and leucitites to basanites. The results of mineral chemistry study of olivine, clinopyroxene, feldspathoids, feldspars, perovskite, opaque minerals and carbonates confirms the kamafugitic character of the studied rocks and suggests a link with carbonatite magmatism.

POTASSIUM-ARGON DATING OF THE LATE TERTIARY VOLCANIC ROCKS AND MINERALIZATION OF MARYSVALE, UTAH

S>The Tertiary flows and intrusives of the Marysvale, Utah area yield several materials that provide reliable absolute ages by the potassium-argon method. A layered sequence of potassium-bearing mid-Tertiary flows and tuffs of the Bullion Canyon series overlies fossil-bearing Oligocene sedimentary strata of the Wasatch formation. The series is invaded by a quartz monzonite intrusive which contains biotite. A sequence of iate Tertiary volcanic rocks, the Mount Belknap series, contains potassiumbearing glasses and rests upon an old erosional surface which truncates both the Bullion Canyon and the quartz monzonite intrusive. Glassy dikes cut the quartz monzonite intrusive, and diatreme structures containing glass pierce the Bullion Canyon flows. Late Tertiary veins which contain uraninite cut the quartz monzonite intrusive and spread into lenticular masses at the base of the Mourt Belknap flows. The uraninite provides a source of materiai for uranium-lead determinations. The absolute age of the Bullion Canyon series is approximately 30 m.y. and of the quartz monzonite intrusives is approximately 25 m.y. The most reliable age for the glassy dikes and diatremes is believed to be about 20 m.y. The determinations on glassy flows of the Mount Belknap range from 14 to 20 m.y. but alteration apparently accounts for themore » low ages in this range and the true age for the Mount Belknap is probably about 20 m.y. Sericite frora a nearby leadzinc-silver mine gives an age of 13.5 m.y. and the uraninite dated by uranium-lead method was found to be about 13 m.y. by Donald Miller based on work done at the Lamont Geological Observatory. (auth)« less