APATITE, XENOTIME, AND MONAZITE GROWTH DURING GARNET BREAKDOWN IN THE CONTACT AUREOLE OF THE MAKHAVINEKH LAKE PLUTON, NORTHERN LABRADOR

Garnet-bearing peraluminous migmatites (Tasiuyak paragneiss) that formed at ca. 1850 Ma were progressively heated in a high-T isobaric aureole adjacent to the 1322 Ma Makhavinekh Lake Pluton, northern Labrador. Regional-grade Grt is replaced by Crd-Opx-Pl coronas approaching the contact. Al-solubility thermometry using Grt-Opx pairs records a T-gradient of 650ºC at 6 km to 900ºC at the contact. No partial melting took place in this dry system. Imaging and trace element X-ray mapping have been used to document the behavior of accessory phases during major-mineral reactions involving Grt. At temperatures below ca. 750ºC, breakdown of P-rich (700ppm) regional Grt resulted in growth of numerous small (10µm) Ap grains that form a halo around relict Grt. Ap growth was accompanied by a pile-up of Y in Grt rims resulting in a steep concentric Y-gradient that has not been significantly modified by volume diffusion. P in the same Grt shows a marked depletion zone at the rim. At temperatures of 800-900ºC between 250 m and the contact, Ap is scarce within the symplectite whereas Mnz and Xen are the dominant accessory phases. Y zoning in these Grt is opposite to lower-T samples. It displays flat cores and an abrupt transition to an Y-depleted rim. This change in Y zoning may be a proxy for destabilization of Ap above 750ºC or for the achievement of REE saturation and the precipitation of REE-phosphates rather than Ap. Rims of relict Grt from across the aureole show enrichment in Ca, suggesting Grt remained a sink for Ca during breakdown reactions. The source of LREE in newly formed Mnz remains to be identified, but may be related to dissolution and re-precipitation of 1850 Ma Mnz formed at much higher pressures.