Journal Article

High-pressure granulite facies metamorphism (1.8 GPa) revealed in silica-undersaturated garnet-spinel-corundum gneiss, Central Maine Terrane, Connecticut, U.S.A

Ague, Jay J, Keller, Duncan S

2018

Overview

We quantify the metamorphic pressure-temperature (P-T) conditions for a newly discovered silica-undersaturated high-pressure granulite (HPG) from the Central Maine Terrane (CMT) in northeastern Connecticut, U.S.A. The rocks lie within the Acadian-Neoacadian orogenic belt (Devonian) and form part of the Brimfield Schist. The Brimfield and the adjacent Bigelow Brook Formation contain silica-saturated rocks that have previously been shown to have undergone ∼1000 °C metamorphism. The pressure was less well constrained at ≥ ∼1 GPa. Silica-undersaturated rocks hold underutilized potential for pinpointing peak metamorphic conditions, particularly pressure, because of their resilience to melting and the variety of refractory minerals they contain. The typical silica-undersaturated mineral assemblage is garnet + spinel + corundum + plagioclase + K-feldspar + biotite + ilmenite. Leucosomes are syenites consisting of two feldspars ± biotite. Plagioclase is commonly antiperthitic, particularly in feldspathic domains surrounding peritectic garnet; such garnet crystals reach ∼10 cm in diameter. Alkali feldspars are perthitic. The rocks contain remarkable ellipsoidal spinels as much as 5.5 cm long comprising discrete crystallographic domains hosting crystallographically oriented lamellae of a Fe-Ti phase, most likely ilmenite. Corundum is usually colorless, but can also be found as sapphire in shades of pink, purple, and blue, particularly in antiperthite-rich domains surrounding large garnets. Some sapphires are concentrically color zoned. We carried out a P-T estimation using ternary feldspar reintegration thermometry of metamorphic antiperthites together with pseudosection modeling. Samples texturally and chemically record near-eclogite facies equilibration at minimum conditions of ∼1040 °C and ∼1.8 GPa, establishing the CMT in northeastern CT as the first known HPG locality in the U.S. These results are consistent with high P2O5 levels found in garnet (0.18 wt%), Ti-in-biotite thermometry, regional sillimanite pseudomorphs after kyanite, and preliminary experimental work on melt inclusions in garnet (Ferrero et al. 2017). The leucosomes provide strong evidence that partial melting of silica-undersaturated rocks at HPG conditions can produce syenitic magmata. Strongly melt-depleted silica-undersaturated rocks may also be protoliths for garnet + spinel + corundum xenoliths reported from kimberlites. The presence of HPG gneisses demonstrates that the large-scale thrusts of the CMT sample the deepest roots of the orogenic belt (60-70 km), and perhaps even deeper subduction zone lithologies as well.

• Publisher: Mineralogical Society of America,Walter de Gruyter GmbH
• Subject: Acadian Phase / alkali feldspar / Bigelow Brook Formation / Biotite / Brimfield Schist / Central Maine Terrane / chemical composition / Colour / Connecticut / Corundum / Crystallography / Crystals / Devonian / Domains / Eclogite / eclogite facies / facies / feldspar group / Feldspars / felsic composition / framework silicates / Garnet / garnet group / Garnets / geologic thermometry / Gneiss / gneisses / granulite facies / granulites / high pressure / high-grade metamorphism / High-pressure granulite / igneous and metamorphic rocks / igneous rocks / Ilmenite / Inclusions / K-feldspar / Kimberlites / Kyanite / Lamellae / leucosomes / Magma / Maine / Melting / metamorphic rocks / Metamorphism / Metamorphism (geology) / mica group / migmatites / Mineral assemblages / Minerals / nesosilicates / Organic chemistry / orogenic belts / Orogeny / orthosilicates / oxides / P-T conditions / Paleoproterozoic / Paleozoic / paragenesis / perthite / Petrology / phase diagrams / phase equilibria / Phosphorus pentoxide / Plagioclase / plutonic rocks / Precambrian / Pressure / Proterozoic / pseudomorphism / Rock / Rocks / Sapphire / Schist / Schists / Sedimentary facies / Shades / sheet silicates / Silica / silica-undersaturated / silicates / Silicon dioxide / Sillimanite / Spinel / Subduction / Subduction (geology) / Subduction zones / syenite / syenites / tectonics / terranes / United States / upper Precambrian / xenoliths