| Baltimore Gneiss | |
|---|---|
| Stratigraphic range: Precambrian | |
 Photomicrograph of Baltimore Gneiss | |
| Type | Geological formation |
| Underlies | Setters Formation |
| Lithology | |
| Primary | Gneiss |
| Other | Paragneiss and amphibolite |
| Location | |
| Region | Piedmont of eastern North America |
| Country |  United States |
| Extent | Maryland |
| Type section | |
| Named for | Baltimore |
The Baltimore Gneiss is a Precambrian geological formation in the Piedmont region of Maryland, Pennsylvania, and Delaware. [1]
At least 95 percent of the Baltimore Gneiss can be described as quartzo-feldspathic gneiss of granodioritic to granitic composition, with the remaining 5 percent consisting of amphibolite. The age of the Baltimore Gneiss has been constrained to 1.0-1.1 Ga. [2]
The Gneiss is homogeneous overall, but is recognized as having four subdivisions. These are the layered gneiss member, the augen gneiss member, the streaked-augen gneiss member, and the hornblende gneiss member. Additionally, the Baltimore Gneiss outcrops into three discrete masses of uniform, well-foliated to massive granitic gneiss referred to as the Slaughterhouse Gneiss. [2]
The layered gneiss member consists of dark and light layers of gneiss bearing biotite, microcline, quartz, and plagioclase, varying from biotite schist to quartzo-feldspathic granofels interlayered on a centimeter to decimeter scale. [1]
The augen gneiss member consists of two belts of fine to medium grained gneiss bearing biotite, microcline, quartz, and plagioclase, with abundant large ovoids (augen) consisting of microcline and/or quartz. [1]
The streaked-augen gneiss member consists of uniform, medium-grained biotite-microcline-quartz-plagioclase gneiss, with augen that have a "stretched" or "streaked" appearance. [1]
The hornblende gneiss member is similar to the layered gneiss member, but with hornblende-bearing dark gneiss accounting for about half of the outcrop area. [1]
The Slaughterhouse Gneiss weathers to a pinkish to orange color, and consists of relatively uniform, medium-grained microcline-quartz-plagioclase gneiss with muscovite, biotite, or both. [2]
| Ellicott City Granite | |
| Woodstock Granite | |
| Baltimore Mafic Complex | Mt. Washington Complex |
| Hollofield Formation | |
| Aberdeen Metagabbro | |
| Perry Hall Gneiss | |
| Glenarm Group | Loch Raven Schist |
| Oella Formation | |
| Cockeysville Marble | |
| Setters Formation | |
| Baltimore Gneiss | |
| Rock units of the Baltimore Terrane in the central Maryland Piedmont Province. [3] | |
The Phoenix, Texas, Chattalolanee, and Towson gneiss anticlines in the Baltimore, Maryland area are all part of a refolded nappe system beneath the Towson anticline. The cores of these nappes consist of the Baltimore Gneiss. The Baltimore Gneiss has been affected by three deformation periods: the Grenvillian, known from radiometric age data, for which nearly all resulting structural features where destroyed or obstructed by later tectonic deformation; the Taconic-Acadian, which involved three phases of folding, amphibolite facies metamorphism, local migmatization, and the development of the gneiss's pervasive structural elements; and the Alleghanian-Palisades, which resulted in predominantly brittle faulting and open folding. The cause of the Grenvillian deformation is uncertain. The Taconic-Acadian deformation was caused by the collision of an ocean floor/island arc terrane with a continental margin or fringing microcontinent in the early Paleozoic. The Alleghanian-Pallisades deformation was caused by continental rifting in the early Mesozoic, and possibly late-Paleozoic trans-current plate motions. [4]
The name Baltimore Gneiss was first used by George Huntington Williams in 1892 [5] to describe the variety of widely distributed gneisses in the Baltimore area. [2]
The Baltimore Gneiss has been used in the past as building stone in the Baltimore area. It is thought that some of the first buildings built in Baltimore in the 1700s were constructed of gneiss quarried from Jones Falls. In addition to use in buildings, gneiss from Jones Falls and Gwynn Falls quarries was used for foundations, roads, and curbstones in Baltimore. Some quarrymen referred to the gneiss as "blue stone" due to the blue-gray color of its fresh surface. The last building stone quarry in Baltimore closed in 1958. [6]
The type locality of the Baltimore Gneiss as described by William Patrick Crowley of the Maryland Geological Survey in 1976 is defined as outcrops along Towson Run from Bellona Avenue to Towsontown Boulevard. [2] Formerly, the type locality was considered as referring to layered gneisses which outcrop along Jones Falls and Gwynns Falls in Baltimore City, however, chemical and textural analysis [7] followed by zircon age measurements [8] determined that these gneisses represent a distinct younger geologic unit. [2]
The type locality of the layered gneiss member includes stream cuts along Herring Run for 255 meters on either side of Arlington Avenue bridge near Morgan State University. [2]
The type locality of the augen gneiss member includes outcrops along Long Green Creek 385 meters southeast of Glenarm Road bridge. [2]
The type locality of the streaked-augen gneiss member includes outcrops along Merryman Branch 125 meters upstream from the Dulaney Valley Road bridge. [2]
The type locality of the hornblende gneiss member includes outcrops along an unnamed stream between Malvern Avenue and Boyce Avenue. [2]
The type locality of the Slaughterhouse Gneiss outcrops along the Slaughterhouse Branch southeast of the Baltimore Beltway (I-695). [2]
Some notable exposures of the Baltimore Gneiss in addition to the type localities mentioned above include outcrops along the Rapids Trail in the McKeldin Area of Patapsco Valley State Park and along roads around the Loch Raven Reservoir. [3]
Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Aplite is an intrusive igneous rock in which the mineral composition is the same as granite, but in which the grains are much finer, under 1 mm across. Quartz and feldspar are the dominant minerals. The term aplite or aplitic is often used as a textural term to describe veins of quartz and feldspar with a fine to medium-grain "sugary" texture. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens. Dykes and veins of aplite are commonly observed traversing granitic bodies; they occur also, though less frequently, in syenites, diorites, quartz diabases, and gabbros.
Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
The Spruce Pine Mining District is a swath of the valley of the North Toe River in the Blue Ridge Mountains of northwestern North Carolina. The area is mined for its mica, kaolin, quartz and feldspar. Spruce Pine district is one of the largest suppliers of high-purity quartz, which is used in the manufacture of silicon for integrated circuits. The district is named after the town of Spruce Pine, which is located in the middle of the region and is the hub of major mining activity there. The district is approximately 25 miles long and 5 miles wide.
Myrmekite is a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase is sodium-rich, usually albite or oligoclase. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium feldspar. Myrmekite is formed under metasomatic conditions, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic and granophyric intergrowths, which are magmatic.
Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly peraluminous (Al2O3/ (CaO + Na2O + K2O) = 0.98–1.11), and contain ilmenite, sphene, apatite and zircon as accessory minerals. Although the compositional range of the monzogranites is small, it defines a differentiation trend that is essentially controlled by biotite and plagioclase fractionation. (Fagiono, 2002). Monzogranites can be divided into two groups (magnesio-potassic monzogranite and ferro-potassic monzogranite) and are further categorized into rock types based on their macroscopic characteristics, melt characteristics, specific features, available isotopic data, and the locality in which they are found.
Syenogranite is a fine to coarse grained intrusive igneous rock of the same general composition as granite. They are characteristically felsic.
Guernsey has a geological history stretching further back into the past than most of Europe. The majority of rock exposures on the Island may be found along the coastlines, with inland exposures scarce and usually highly weathered. There is a broad geological division between the north and south of the Island. The Southern Metamorphic Complex is elevated above the geologically younger, lower lying Northern Igneous Complex. Guernsey has experienced a complex geological evolution with multiple phases of intrusion and deformation recognisable.
The Cathedral Peak Granodiorite (CPG) was named after its type locality, Cathedral Peak in Yosemite National Park, California. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada. It has been assigned radiometric ages between 88 and 87 million years and therefore reached its cooling stage in the Coniacian.
The Piégut-Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central in France. Its cooling age has been determined as 325 ± 14 million years BP.
The Canaveilles Group is the basal metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the Pyrenees.
The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France. The unit geologically forms part of the Variscan basement of the northwestern Massif Central.
The Oella Formation is a Late Proterozoic or early Cambrian schist in Howard and Baltimore Counties, Maryland. It is described as "Medium-grained biotite-plagioclase-muscovite-quartz schist, locally garnetiferous, interlayered on a centimeter to decimeter scale with fine-grained biotite-plagioclase-quartz gneiss, commonly bearing muscovite but less commonly garnet."
The Catoctin Formation is a geologic formation that expands through Virginia, Maryland, and Pennsylvania. It dates back to the Precambrian and is closely associated with the Harpers Formation, Weverton Formation, and the Loudoun Formation. The Catoctin Formation lies over the a granite basement rock and below the Chilhowee Group making it only exposed on the outer parts of the Blue Ridge. The Catoctin Formation contains metabasalt, metarhyolite, and porphyritic rocks, columnar jointing, low-dipping primary joints, amygdules, sedimentary dikes, and flow breccias. Evidence for past volcanic activity includes columnar basalts and greenstone dikes.
The Siilinjärvi carbonatite complex is located in central Finland close to the city of Kuopio. It is named after the nearby village of Siilinjärvi, located approximately 5 km west of the southern extension of the complex. Siilinjärvi is the second largest carbonatite complex in Finland after the Sokli formation, and one of the oldest carbonatites on Earth at 2610±4 Ma. The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss. The maximum width of the body is 1.5 km and the surface area is 14.7 km2. The complex was discovered in 1950 by the Geological Survey of Finland with help of local mineral collectors. The exploration drilling began in 1958 by Lohjan Kalkkitehdas Oy. Typpi Oy continued drilling between years 1964 and 1967, and Apatiitti Oy drilled from 1967 to 1968. After the drillings, the laboratory and pilot plant work were made. The mine was opened by Kemira Oyj in 1979 as an open pit. The operation was sold to Yara in 2007.
The Ammonoosuc Volcanics is a rock unit in parts of New Hampshire and Vermont in the United States. This unit is named for the Ammonoosuc River that runs through the portion of New Hampshire that houses the Ammonoosuc Volcanics.
Located in the Charlotte Belt of North Carolina is the Farmington Gabbro, located in the Mocksville Complex. The Mocksville Complex consist of metamorphosed/unmetamorphosed gabbros, pyroxenites, hornblendites, wehrlites, granites, and diorites. The plutons in this region formed during the Taconic, Acadian, and Alleghanian orogeny starting on the eastern side of Laurentia. These plutons date back to around 400 Ma, consisting of ultramafic, mafic, and felsic rocks but the Farmington Gabbro is the only pluton on the northwest side of the complex that is unmetamorphosed.
The Lilesville Granite, also referred to as the Lilesville pluton, is a ring-shaped body of granitic rock that spans about 94 square miles (240 km2) in Anson, Richmond, and Montgomery Counties in southern North Carolina.
Gore Mountain Garnet, found in the Adirondack Mountains in New York, contains the world's largest garnets. The rock that holds these garnets, garnet amphibolite, is sometimes referred to as 'black ore' or 'dark ore.' This rock formation formed during metamorphism during the Ottawan phase of the Grenvillian orogeny, and extremely high temperatures combined with introduction of fluids is what most likely contributed to the unusual size of the megacrystic garnets.
