GARNETITES OF THE CARDIGAN PLUTON (NH): EVIDENCE FOR SECONDARY RESTITE AND IMPLICATIONS FOR SOURCE ROCK COMPOSITIONS

The Cardigan Pluton of the New Hampshire Plutonic Suite is host to garnetite, an unusual rock-type characterized by up to 70% modal garnet. The garnetite occurs as meter-scale pods hosted by garnet-bearing granitic rocks. Biotite, which is often found rimming the garnet and among quartz grains, is also present in addition to plagioclase, sillimanite, ilmenite, apatite and monazite. Sillimanite forms fibrolitic needles from biotite breakdown. Garnet grain profiles are flat with respect to Fe, Mg, and Mn, suggestive of diffusion at high temperatures. Mn, however, shows a slight enrichment at the rims, indicative of retrograde net transfer reactions. In contrast to the step-zoning profiles commonly observed in garnets in migmatites, the Ca, P and Y profiles and x-ray dot maps in the Cardigan garnets show no compositional zoning,. This absence of zoning suggests single-stage garnet growth in equilibration with melt. One possible reaction for the assemblage is bio + plag + qtz + ksp = gar + als + liquid.

Garnet-biotite thermometry with GASP and Gar-Plag-Bio-Qtz barometry give P-T estimates of 750–800°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.