Cavendish Formation, Bull Hill Gneiss (Cambrian?)
Cavendish Formation, Bull Hill Gneiss - Quartz-plagioclase-microcline-biotite gneiss characterized in many areas by augen of microcline as much as 2 inches long; fine- to medium-grained quartz-plagioclase-biotite or biotite-muscovite gneiss. Cardinal Brook Intrusive Suite is here named in the cores of the Chester-Athens dome and Rayponda-Sadawga dome in the eastern and southern Green Mountains, VT, and the northern part of the Berkshire massif, MA. Includes the Stamford Granite of Hitchcock (1861), the Somerset Reservoir Granite (new name), the Harriman Reservoir Granite (new name), and the Bull Hill Gneiss of Richardson (1929-30). Because of uncertainty regarding the geologic position of the Bull Hill, it is restricted to the Chester and Athens domes and the original definition of Richardson is retained. Rocks mapped as Bull Hill in the Jamaica area are assigned to the Somerset Reservoir Granite and those in the Rayponda-Sadawga dome are assigned to the Harriman Reservoir Granite. Structural position is unclear. U-Pb zircon age is Middle Proterozoic (960-950 Ma). . [GNU Staff note--This report mistakenly uses the phrasing "Bull Hill Gneiss of Richardson (1929-30)" which would normally imply that the unit has not been adopted for use by the U.S. Geological Survey (USGS) because of inadequate definition by Richardson or successive workers in the area. However, in this report, the phrasing simply means that Richardson's definition and use are preferred over the usage on the VT State Geologic Map of Doll and others (1961) and that the unit meets the requirements for formal usage by the USGS.] (Ratcliffe, 1991).
Mount Holly Complex (Precambrian)
Mount Holly Complex - Mainly fine- to medium-grained biotitic gneiss, locally muscovitic, and in western areas chloritic; massive and granitoid in some localities, fine-grained or schistose and compositionally layered in others; also abundant amphibolite and hornblende gneiss, and minor beds of mica schist, quartzite, and calc-silicate granulite; includes numerous small bodies of pegmatite and gneissoid granitic rock. Includes a suite of metatonalites, metatrondhjemite, and possible metadacite with chemical characteristics of a calc-alkaline volcanic-plutonic suite. Mappable units are College Hill Granite Gneiss and 10 unnamed subdivisions including several varieties of gneiss as well as schist, amphibolite, and quartzite. U-Pb zircon upper intercept ages of 1.35 to 1.30 Ga have been determined and interpreted as age of crystallization (Ratcliffe and others, unpub. data). Cores of abraded zircon obtained from College Hill Granite Gneiss of Mount Holly Complex have a U-Pb upper intercept age of 1245 +/-14 Ma, interpreted as crystallization age for that granite (Aleinikoff and others, 1990). Dust collected by abrasion of zircons, thought to represent migmatitic overgrowth, has a Pb-Pb age of approx 1100 Ma. These data suggest that College Hill Granite Gneiss was intruded at 1245 Ma and migmatized at 1100 Ma. On north and south slopes of College Hill, College Hill Granite Gneiss grades outward into migmatitic biotite granite gneiss of Mount Holly Complex. College Hill is discordant to contacts and folds in paragneiss units of Mount Holly Complex. Dacitic metavolcanics are found within Washington Gneiss of Berkshire massif of MA (Ratcliffe and Zartman, 1968). They are interbedded with thick succession of rusty-weathering, quartz-pebble gneisses, calc-silicate rocks and garnet-sillimanite schist similar to, but much thicker than, the rusty-weathering gneiss and schist unit of Mount Holly Complex exposed in Green Mountains of VT. It is possible that the metadacitic and metatrondhjemitic suite of VT constitutes a lateral, south-to-north facies of the Washington Gneiss of MA (Ratcliffe, in press).
Mount Holly Complex, quartzite and schist (Precambrian)
Mount Holly Complex, quartzite and schist - Quartzite, locally in massive beds as much as 30 ft thick, micaceous quartzite, and quartz-mica schist that commonly contains garnet or pseudomorphs (largely chlorite) after garnet; schists are locally rusty weathered and contain conspicuous flakes of graphite; also includes amphibolite and minor hornblende gneiss, biotite gneiss, and pegmatite.
Stamford Gneiss (Precambrian)
Stamford Gneiss - Granitic biotite gneiss with megacrysts of microcline. A dike-like feature of Stamford Granite, found near eastern margin of Green Mountain massif near locality 1 on this map (represented by sample 172), was incorrectly referred to as a probable Late Proterozoic diabase dike by Ratcliffe on his section of the Bedrock Map of Massachusetts (Zen and others, 1983). U-Pb zircon dates from Karabinos and Aleinikoff (1988, 1990) yield an age of 959+/-4 Ma. Unpublished data from Harding and Mukasa yield a U-Pb zircon date of about 950 Ma (Ratcliffe and others, 1993).