Rockport, Essex Co., Massachusetts, USA
|Latitude & Longitude (WGS84):||42° 39' 20'' North , 70° 37' 13'' West|
|Latitude & Longitude (decimal):||42.65556,-70.62028|
|Köppen climate type:||Dfb : Warm-summer humid continental climate|
The town of Rockport was separated from Gloucester and incorporated in 1840. Granite was quarried here beginning in 1823. Interesting minerals occur in granite quarries and other bedrock exposures of alkaline Cape Ann granite and associated pegmatites and miarolitic cavities.
McKinstry (1921) described some of the town's mineralogical highlights: "More than fifty years ago attention was called to the unusual mineral occurrences at Rockport, on Cape Ann, by J. P. Cooke's description of danalite, a new beryllium mineral, and subsequently of cryophyllite, a new lithium mica. Besides being the type locality for these species, the Rockport quarries have afforded an interesting list of rare-earth minerals, and, perhaps most unusual of all, large tabular crystals of fayalite, normally an utter stranger to granitic rocks."
Mineral ListMineral list contains entries from the region specified including sub-localities
29 valid minerals. 2 (TL) - type locality of valid minerals.
Rock Types Recorded
Rock list contains entries from the region specified including sub-localities
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Entries shown in red are rocks recorded for this region.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Wenlock - Late Ordovician|
427.4 - 458.4 Ma
|Cape Ann Complex|
Age: Paleozoic (427.4 - 458.4 Ma)
Stratigraphic Name: Cape Ann Complex
Description: Alkalic granite to quartz syenite containing ferro-hornblende. Intrudes Zdigb. Most of Cape Ann Complex forms Cape Ann peninsula of northeastern MA. Consists of alkalic granite to quartz syenite (its main phase), Beverly Syenite, and Squam Granite, all of which form a pluton covering 385 sq km. Intrudes Late Proterozoic greenschist, diorite, and gabbro that earlier workers assigned to Marlboro Formation, Salem Gabbro-Diorite, or Middlesex Fells Volcanic Complex. Dennen (1975) considered masses of diorite and gabbro within and adjacent to Cape Ann Complex to be cogenetic with it and equivalent in age to Nahant Gabbro and gabbro at Salem Neck. Authors recommend that usage of term Salem Gabbro-Diorite be restricted to these masses of diorite and gabbro in and around Cape Ann pluton that are younger than Dedham Granite and cogenetic with Cape Ann Complex. Gabbro at Salem Neck is probably representative. Radiometric ages straddle Late Ordovician-Early Silurian boundary. Age is based on Rb-Sr whole rock isochron of 426 +/-6 Ma (Zartman and Marvin, 1971) and U-Pb zircon date of 450 +/-25 Ma (Zartman, 1977) (Wones and Goldsmith, 1991).
Comments: Part of Milford-Dedham Zone (Tertiary and Older Rocks). Secondary unit description per MA007.
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
443.8 - 485.4 Ma
|Cape Ann Granite, alkali-feldspar quartz syenite|
Age: Ordovician (443.8 - 485.4 Ma)
Stratigraphic Name: Cape Ann Granite
Comments: The principal rock of the map area is the Cape Ann Granite, an unfoliated, medium- to coarse-grained, leucocratic rock ranging in composition from quartzose alkali-feldspar granite, through alkali-feldspar quartz syenite, to alkali-feldspar syenite. Its texture is generally uneven, subhedral to subporphyritic, and locally cumulus. Crystals range from 0.5 to 1 cm (0.2 to 0.4 in.) long in the quartz-bearing rocks and are less uniform in the syenitic rocks. Most of the syenite is texturally equivalent to the granite, but the syenite commonly contains small, irregular pegmatitic patches that lack well-defined boundaries. The range (and median) of the principal minerals are: potassium feldspar, 53-85 (63) percent; plagioclase (An6-12), 0-22.5 (2.8) percent; quartz, 0-41 (24) percent; ferrohornblende, 0.1-17 (4.5) percent; biotite, 0-3.2 (0.8) percent; and opaque minerals, 0.2-7.5 (1.0) percent. Minor quantities of augite are in approximately one-quarter of the specimens. Accessory minerals include magnetite, ilmenite, zircon, apatite, and, very rarely, fluorite, pyrite or molybdenite. Carlsbad-twinned microcline microperthite is the dominant feldspar; less than 25 percent of the feldspar is homogeneous microcline. Plagioclase (albite or oligoclase) is present in only minor quantities. Quartz is clear, grayish, pinkish, or faintly smoky, shows weak strain shadows, and contains oriented sheets of dust-like inclusions and vesicles. Potassium feldspar and quartz occur as large single crystals and as clusters of subhedral to euhedral feldspar and anhedral quartz partly to completely surrounded by finer grained quartz and feldspar. The mafic minerals are variable in amount and appearance and occur as ragged clots, wisps, single subhedral crystals, and zoned reaction aggregates. Augite is colorless to pale green, typically occurring as a core that is partly or completely surrounded by pale-green amphibole, darker green sodium-iron amphibole, and reddish-brown biotite; magnetite granules are scattered throughout the aggregate. Sodium-iron amphibole and biotite occur as both isolated crystals and clots of both minerals. The mapped lithotypes of the Cape Ann Granite were determined by modal quartz content, because the amount of quartz in these subunits is consistent and varies smoothly in areas generally measurable in square kilometers (miles). Arbitrary divisions of 5, 15, and 25 volume percent of quartz were determined by point counts on outcrops. These lithotypes form irregular northeast-bending bands. Neither stratigraphic succession nor dips of these lithotypes is known, but field relations suggest the superposition of quartz-rich on quartz-poor types. The lithotypes probably originated by differential crystal settling, but may have had their origin in convective motions of the magma. Weathering causes sequential color changes in these felsic rocks from dark green to green gray or gray, then to brown, tan, and finally to white on exposed surfaces. The weathered surface is black-brown and grus is common under vegetation. Joints are commonly bordered by a brown zone a few centimeters (inches) thick. Xenoliths, which are generally small and well digested, but locally may retain their preincorporation character, are scattered throughout the intrusion. They include blocks of a fine-grained granite, probably the Squam Granite; Salem Gabbro-diorite and other fine-grained, commonly porphyritic mafic rocks; and siliceous metasedimentary rocks. Many of these latter were originally finely laminated (0.1 to 5 mm (0.004-0.2 in.)), but have been transformed by thermal metamorphism into thin-banded quartz-biotite rocks containing feldspars and hornblende. A few of these metasedimentary xenoliths contain muscovite, pyroxene, and garnet; one sample contains sillimanite.
Reference: Dennen, W.H. Bedrock geologic map of the Gloucester and Rockport quadrangles, Massachusetts. U.S. Geological Survey Miscellaneous Investigations Series Map I-2285. Digital Conversion 2007.