Original paper
Rubidium-Strontium geochronologic studies of large granitic pegmatites
Brookins, Douglas G.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 156 Heft 1 (1986), p. 81 - 97
30 references
published: Dec 19, 1986
ArtNo. ESP154015601004, Price: 29.00 €
Abstract
Rb-Sr geochronologic study of large granitic pegmatites is a useful approach to problems of pegmatite genesis. Pegmatites from the Middletown, Connecticut, area (about 250-260 Ma. old) and from the Harding district, New Mexico (about 1300- 1400 Ma. old) have been intensively studied for their Rb-Sr isotope systematics. Zoned pegmatites of the Middletown area (Hale and White Rocks II) and from the Harding quarry both reveal internally complex isotope variations. In both areas, it is nearly impossible to assign a unique initial 87Sr/86Sr ratio to the pegmatites due to presumed open system conditions during their cooling histories. A trend is noted for border and wall zone samples to yield ages that are older than the inner zones, while the apparent 87Sr/86Sr ratio appears to increase inwards as well. The reasons for these changes are possibly due to redistribution of radiogenic 87Sr from the outer zones towards the core. Since Rb content increases and Sr content decreases inwards, the excess 87Sr is readily detected in late stage minerals such as apatite and cleavelandite, which account for the increase in the initial 87Sr/86Sr ratios in the innermost pegmatite zones. The data suggest that the source of the excess 87Sr is from within the pegmatite body and not from the surrounding country rocks.
Keywords
Rb-Sr geochronology • granitic pegmatites • open system conditions • redistribution of 87Sr; Middletown Area • Connecticut • Harding District • New Mexico • USA