Revaluation of “equilibrium” P-T paths from zoned garnet in light of quartz inclusion in garnet (QuiG) barometry

• Published in: Lithos 2020-11, Vol.372-373, p.105650, Article 105650
• Main Authors: Spear, Frank S., Wolfe, Oliver M.
• Format: Article
• Language: English
• Subjects: Connecticut Valley Trough; Coticule; Fall Mountain; Garnet; Inclusion barometry; Orfordville belt; Perry Mountain Formation; P–T paths; QuiG barometry; Sifnos; Townshend Dam
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2020.105650

Metamorphic P–T paths provide valuable constraints on the tectonics of collisional orogens and many such paths have been deduced from analysis of chemical zoning in garnet. Inclusion barometry (e.g. quartz-in-garnet or QuiG) provides complementary and sometimes contradictory information regarding the interpretation of garnet zoning and the P–T paths derived therefrom. For example, QuiG barometry on several generations of garnet from well-characterized samples from Fall Mountain, New Hampshire, is consistent with the P–T path inferred from other methods. However, QuiG barometry is inconsistent with P–T paths inferred from garnet zoning from the Orfordville Belt (Vermont), Townshend Dam (Vermont), the Connecticut Valley Trough (Vermont), and Sifnos (Greece). New data from the Perry Mountain Formation, southeastern New Hampshire, suggests that QuiG may preserve a record of polymetamorphic events. Specifically, QuiG barometry in coticule samples is interpreted to record a previously unrecognized early medium pressure (ca. 0.9 GPa) metamorphism and a later low pressure (ca. 0.3 GPa) metamorphism. •QuiG barometry potentially provides an estimate of garnet growth conditions•QuiG barometry in some localities is consistent with peak metamorphic estimates and in some localities not•QuiG barometry suggests that some garnets grow nearly isothermally and isobarically•QuiG barometry appears to preserve entrapment conditions through multiple metamorphic events