APPLYING ADVANCES IN MONAZITE GEOCHEMISTRY USED TO CONSTRAIN P-T-T PATHS TO ZIRCON: INTERPRETING VARIATION IN Y, P AND U CONTENT OF ZIRCON FROM CHARNOCKITE GNEISS NEAR WITHERBEE, NE ADIRONDACK HIGHLANDS

Monazite geochemistry, especially variation in Y in chemically zoned grains, has been used to infer and date metamorphic reactions that led to monazite growth and so constrain P-T-t paths of an analyzed sample. Prior to these advances, backscatter electron images and EMP analysis of chemistry were used to identify growth stages in zircons from a migmatitic charnockite gneiss that outcrops near Witherbee in the northeastern Adirondack Highlands. Distinct zoning patterns seen in the backscatter images were first identified as one of four growth stages by appearance and location. Chemical analyses confirmed four stages that grew in different chemical environments. ID-TIMS analysis of grains or grain fragments with a single growth stage dated three of the four: Stage 2 – intrusion of the gneiss protolith at ≈ 1112 Ma, Stage 3 – growth in association with anatexis at ≈ 1040 Ma, and Stage 4 - retrograde metamorphic reactions until about 1000 Ma.

In addition, zircon trace element chemistry, much like monazite's, can be used to better understand the P-T-t history of the charnockite gneiss. Key chemical discriminants among the stages are Y, P and U. Stage 2 is high Y and P suggesting a magma source for the gneiss protolith that formed from melt reactions that consumed Grt and either monazite, apatite or both. The magma then moved and intruded at a crustal level that did not support new Grt growth. High U content of Stage 3 is consistent with zircon growth from an anatectic melt. Its low Y content implies that Grt was present and likely grew during anatexis. This would be consistent with dehydration reaction involving Hbl and Pl to produce a melt + Grt at ≈ 750 MPa and 825 °C as estimated by geothermo-barometry. The source of the heat that led to the high-T metamorphism is not determined although it likely involved emplacement of high-T magmas in the area that remain to be identified. Stage 4 is very low in U typical of zircon growth in the absence of a melt. Low Y content implies continued garnet stability suggesting cooling with limited uplift.

Data, therefore, imply a P-T-t path with melting of the lower crust and melt transfer to upper crustal levels at ≈ 1112 Ma followed ≈ 70 Ma later by movement to depths > 20 km. There the rock was reheated and underwent partial melting. Gradual, possibly isobaric cooling followed until at least 1000 Ma.