RETROGRESSION OF MONAZITE IN SHEARED AND UNSHEARED ROCKS AND THE RESETTING OF MONAZITE U-TH-PB AGES

Two examples of the retrogression/hydration of monazite, accompanied by the partial to complete resetting of U-Th-total Pb ages (chemical ages) have been studied in detail. The first (from North Carolina) is in a mylonite, and retrogression appears to have occurred during intense shearing of granulite facies Grenville-age monazite porphyroclasts in the Alleghanian orogeny at ca. 300 Ma at greenschist-facies grade. The second (from Virginia) is in an overprinting upper greenschist- to lower amphibolite-facies metamorphism of Cambrian (530 Ma) staurolite-grade schist during the Taconian orogeny at ca. 480 Ma. In the second case there was some development of an overprinting Taconian fabric but shearing was much less intense than in the first case. In both examples, monazite grains have reacted with Ca-bearing aqueous fluid to form aggregates of the product minerals allanite + apatite + thorite (Th orthosilicate), surrounding scattered small (<30 micron) fragments of monazite. A simplified version of the chemically complex reaction is: monazite + Ca + H2O = apatite + allanite + thorite. In the NC example, fractured and incipiently reacted monazite porphyroclasts indicate that monazite disaggregation appears to have largely occurred by brittle fracture. In the VA example, the monazite porphyroblast shows no fracturing but has been intensely preferentially resorbed. Euhedral staurolite porphyroblasts have also been partially to completely pseudomorphed by white mica + chloritoid aggregates. Hydration/replacement of both staurolite and monazite are interpreted to have occurred simultaneously. In both examples, monazite trace-element chemistry has been affected by the hydration reaction resulting in lower Pb content relative to Th and U, and thus in younger calculated ages. Chemical alteration and age resetting was less thorough in the NC example, and monazite fragments in mylonitic tails yield ages ranging from Grenville ages similar to porphyroclast cores all the way down to 280 Ma, although a sizeable majority of analyzed grains had ages <350 Ma. In the VA example, edges of the resorbed monazite and all fragments embedded in allanite and apatite yielded Taconic ages (ca. 480) whereas the monazite core retained 530 Ma ages. Attempts to date allanite and thorite by microprobe have so far been unsuccessful.