• Citer

Ages of zircon and feldspar concentrates from north american beach and river sands

par Ledent, Dolly ;Patterson, C.;Tilton, R.
Référence The Journal of geology, 72, 1, page (112-122)
Publication Publié, 1964

Article révisé par les pairs

• Accès en ligne
• Détails
• Contenu
• Statistiques

Résumé :

Zircon and K-feldspar have been concentrated from four composite samples of river and beach sands, representing the east and west coasts of North America, the northern Mississippi River, and the total drainage of the Mississippi River. First order approximation of the frequency distributions of ages in the zircon concentrates were obtained; that is, each zircon concentrate was resolved into a mixture of an old and a young component whose proportions and approximate ages were determined. In each of the four samples, the ratio of old to young zircon was compared to the age and relative proportions of triclinic and monoclinic K-feldspar. In this way several aspects of the differential response of zircon and K-feldspar to weathering and metamorphism were clarified in quantitative form without considering the absolute abundances of these minerals in the sands. West-coast beach sands were found to represent the expected case of round K-feldspar associated with some old zircon: 90 per cent of the K-feldspar (which was monoclinic) and 80 per cent of the zircon in these sands (generally non-rounded and colorless) were derived from rocks similar to those in existing igneous highlands-the Cretaceous batholiths; about 10 per cent of the K-feldspar (which was triclinic) may be derived from metamorphics of Cretaceous age; 20 per cent of the zircon has undergone much recycling and was ultimately derived from plutons with an average age of 1,600 m.y. Upper Mississippi River sand was derived largely from metamorphic rocks: 90 per cent of the K-feldspar is triclinic, which, uniquely among the four samples, is remarkably fresh and well rounded; the average K-feldspar age is 1,900 m.y.; the zircon is badly worn and highly colored. Its average age is 2,100 m.y., but about half the zircon is much older, with an average age of 3,000 m.y. East-coast beach sands are the only ones studied that contain substantial amounts of zircon much older than K-feldspar. The latter is about 80 per cent triclinic and the average age of the total is 400 m.y. The mean age of the zircon is 900 m.y., but about half is much older with an average age of 1,500 m.y. Sand from the mouth of the Mississippi River appears to be about a one-to-one mixture of two components represented by west-coast beach sands and Upper Mississippi River sands. Two thirds of the K-feldspar is triclinic and the average age of the total is 950 m.y. The average age of the zircon is 1,000 m.y. In general, the small difference between the ages of zircon and feldspar is surprising, since only on the west coast of North America is there a difference of some consequence.