EPISODIC FRACTURE-CONTROLLED HYDROTHERMAL FLUID FLOW DURING THE EMBRYONIC OPENING OF THE ATLANTIC OCEAN: EVIDENCE FROM A SILICIFIED FAULT ZONE IN THE NEW ENGLAND APPALACHIANS

Quartz veins in the Lantern Hill fault zone (LHFZ) in SE Connecticut record episodic passage of fluids along parallel arrays and networks of fractures that originated in a continental rift setting that was active during the Middle Triassic. The LHFZ trends NNE transgressing Proterozoic Z basement rocks of the Avalonian terrane and is silicified in several places along its 16-km trace. At the northern end there is a 1.6-km long quartz vein complex with a 70-m thick core that is comprised of parallel arrays of steeply-dipping 0.1- to 1-m thick veins. Stockwork veins, typically < 3 cm thick, are restricted to wallrocks and and predate the core. Late-stage veins cut both the stockwork veins and the core (main stage). There were 8 dominant episodes of fluid flow. A minimum of 24 x 10⁶ m³ of hydrothermal quartz was deposited.

There are 4 textural types of quartz veins: massive, free growth, crackseal veins that have repeatedly fractured and resealed along their length, and veins with cryptocrystalline quartz. A hypersilicified breccia characterized by clasts of broken quartz euhedra cemented by hydrothermal quartz locally occurs in the core zone. These breccias are interpreted to have to have resulted by hydrofracture.

Fluid flow was driven by a combination density-driven convection and fault-valving. Stable isotope data suggest that the fluid was of meteoric origin. The enormous volumes of quartz in the core cannot be accounted for by the adjacent metasomatic aureole where feldspars were converted to illite. Fracture-channelized fluid probably flowed upward from depth into new fractures. Quartz was precipitated from fluids ranging from 282^oC to 208^oC, cooling through time. A P estimate of 60 MPa for crystallization of an early quartz vein was obtained using a fluid inclusion technique.

Hydrothermal muscovite intergrown with main-stage quartz yielded a mean ⁴⁰Ar/³⁹Ar age of 234 Ma (20 Ma younger than host metamorphic muscovite). Thus fluid movement was associated with Newark tectonics.

Quartz veins in this relatively well-exposed fault zone document distributed fluid flow within a developing rift zone associated with the opening of the Atlantic Ocean. These observations provide insights into the mechanism of fluid flow along deep portions of continental rifts that are presently inaccessible along modern analogs.