Garnetites of the Cardigan Pluton, New Hampshire: Evidence for Peritectic Garnet Entrainment and Implications for Source Rock Compositions

Abstract

The Cardigan Pluton, located in southwestern New Hampshire, USA, is a strongly peraluminous granodiorite that contains distinctive, meter-sized pods consisting of 50–70 modal % garnet (+sillimanite + biotite + plagioclase + quartz). The presence of fibrolitic mats and flat, unzoned major and trace-element garnet profiles provide evidence for prograde metamorphism and single-stage garnet growth from biotite dehydration melting. Melt-depleted, bulk-rock compositions indicate that the garnetites are either fragments of restite or melt-depleted xenoliths. Comparison of the Nd and Sr isotopic compositions of the garnetites and the Cardigan granitic rocks indicates an equilibrium relationship as required in the restite model. Additionally, plagioclase and garnet compositions are the same in the garnetites and the most mafic host rocks, also permissive of a restite origin for the garnetites. Temperature and pressure calculated using garnet–biotite thermometry and garnet–aluminum silicate–quartz–plagioclase (GASP) barometry yield estimates of 800°C and 6–7 kbar. These temperatures are probably lower than the peak melting temperature because major element modeling subtracting a granodiorite composition from an average calc-pelite metasediment requires greater than 45% melting to generate garnetite sample CP-23G. Such high melting percentages require temperatures of ∼900°C, near the biotite-out curve. We infer that the heat required for such high amounts of melting was obtained from asthenospheric upwelling and basaltic underplating. Inherited zircons with 600 Ma U–Pb ages suggest that the Cardigan Pluton does not have a Laurentian source, consistent with thermobarometric calculations that place the depth of melting below the decollement between the basement and the Central Maine Trough metasediments. A peri-Gondwanan basement source is inferred. Calculated ascent rates of >1000 km/yr allowed preservation of restite in the Cardigan Pluton whereas slower rates in other peraluminous plutons could account for the paucity of restite in most peraluminous plutons.