GARNETITES OF THE CARDIGAN PLUTON (NH): EVIDENCE FOR SECONDARY RESTITE AND IMPLICATIONS FOR SOURCE ROCK COMPOSITIONS
Garnet-biotite thermometry with GASP and Gar-Plag-Bio-Qtz barometry give P-T estimates of 750800°C at 6 kbars, approximately 2 kbars higher than the emplacement pressure of the pluton. Monazite ages obtained by electron microprobe yield an average age of 412 Ma, in good agreement with published data for the pluton (Lyons et al., 1997). Older, inherited domains of ~600 Ma and ~2200 Ma were also found in a few of the monazite grains.
We interpret these data as evidence that the garnetites represent secondary restite from a pelitic source. The garnet appears to have formed in equilibrium with a liquid, as shown by the lack of zoning for Ca, P and Y. However, the garnetite's protolith was not the dominant source for the Cardigan magmas because their bulk rock compositions plot below the pluton's CaO-SiO2 unmixing lines.
Both the ages of inherited monazite and feldspar Pb isotopic data (Moench & Aleinikoff, 2002) suggest a peri-Gondwanan component in the Cardigan pluton.