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Igneous and Metamorphic Mineral Assemblages, Morton Homestead, Prospect Park, PA

Last Updated: 5th Jun 2013

NOTE: updated to correct the stream name and include additional field observations.


Kyanite


Introduction



Morton Homestead, a Commonwealth of Pennsylvania State Historical Site, is located at 100 Lincoln Avenue, Prospect Park, Ridley Township, Delaware County, Pennsylvania. The eastern boundary of the site is Lincoln Avenue (State Highway 420), the southern boundary is tidal Darby Creek and the northern and western boundaries are private property.

In the early 1970's, Route 420 was expanded to accommodate additional traffic from Interstate 95, then under construction to the south; Route 420 was expanded from two lanes to four between Route 291 in Essington and Chester Pike in Prospect Park. Part of the expansion project required installing a new culvert under Route 420 for Amosland Creek, a tributary of Darby Creek that enters the site near its northwestern corner.

Google Earth Image


During excavation and construction, many specimens of kyanite (Al2SiO5) associated with quartz (SiO2) were collected; these specimens are often erroneously labeled "Ridley Park, Pennsylvania".

This document examines the local geology of the site pertaining to the genesis of specimen quality kyanite.

Site History



Currently designated the Morton Homestead Historical Site by the Pennsylvania Historical and Museum Commission, it was also known as the Darby Creek Ferry for the ferry across fresh water tidal Darby Creek that operated there until 1840 when the first bridge across the creek was built.

Although exploration of the region began in 1609, it was not settled until 1638, when Swedes arrived. The Swedes retained control of the area until it was conquered by the Dutch in 1655. The Dutch, however, were not long to retain control as the English in turn conquered them in 1664. However, the original Swedish and Finnish colonists were still the predominant settlers in the area.

During a 20th century archaeological survey, an English half-penny dated 1666 was recovered from fill near the cabin; there was reportedly informal excavation of the site during the 1930's in which Swedish armor and weapons were recovered by a private collector (Dr. John Hallahan, personal communication, 1989). Morton Mortenson's homestead construction commenced -- in three stages -- beginning in 1678. It is built of white cedar logs and stone. Dr. George Smith (History of Delaware County, Pennsylvania, 1863, Delaware County Institute of Science) reports early settlers harvested dead and aged white cedar logs from the local fresh water tide marshes, suggesting they had been drowned by rising tide levels. In the early 20th Century, the site was named after John Morton -- great-grandson of Morton Mortenson -- under the erroneous assumption the Declaration of Independence signer had been born there. In 1957, the designation was officially changed to Morton Homestead.

New Sweden and the New World:
History Lessons from the Morton Homestead


Regional Geology



Morton Homestead is located in close proximity to the contact between two geologic provinces: the Coastal Plain and Piedmont Uplands sections.

Physiographic Provinces of Pennsylvania

provides general descriptions of each section.

Within Ridley Township, the Coastal Plain province consists of Quaternary silt, sand and gravel terrace, alluvial and fresh-water tide marsh deposits. The Piedmont Uplands province consists of the Upper Ordovician to Lower Silurian Wissahickon Formation, Ridley Park Granite and Springfield Granodiorite.

Preliminary Bedrock Geologic Map Of A Portion Of
The Wilmington 30- By 60-Minute Quadrangle,
Southeastern Pennsylvania
[Link Broken? Sep 2014]

There have been two regional metamorphic episodes locally. The first was the long-recognized Silurian Taconic Orogeny. South of the Springfield Granodiorite, a second early Devonian metamorphic event occurred that may represent renewed ocean basin closure.

Site Geology



Morton Homestead is located on the contact between Quaternary Coastal Plain sediments of fresh-water tidal origin and the Wissahickon Formation. In general, the southern part of the site is underlain by Quaternary sediments and the northern part, along Amosland Creek, is underlain by the Wissahickon Formation.

The Coastal Plain sediments shall not be discussed in detail, but are notable only because cobble-sized fragments of Wissahickon Formation are encountered in the approximately 100 meters of tidal Amosland Creek and were a source of material collected in this study.

The Wissahickon Formation consists primarily of sillimanite- and kyanite- metapelitic schist,a perthitic microcline-quartz-muscovite-biotite pegmatite and a hydrothermal quartz stringer. Schist dominates the site. The granite pegmatite stands approximately 1 meter in relief where exposed and appears to act as a geomorphic control on the Amosland Creek steam course through most of the site. It is near-vertical (possibly dipping steeply to the northwest), tabular and approximately 2 meters thick. Where exposed, the quartz stringer appears to be located along the hanging wall of the pegmatite's extrapolated strike and dip.

Mineralogy



Morton Homestead is known primarily for kyanite-quartz specimens collected immediately to the northeast. However, these specimens represent only the most aesthetically-desirable minerals available at the site and are not representative its geology. More thorough collecting of predominant mineral assemblages was conducted by the author in 1990. These assemblages provide additional material and a better understanding of the metamorphic-igneous-hydrothermal history of the site.

An important caveat applies to material collected from the stream bed of Amosland Creek -- there is no assurance that this material originated in close proximity to the site. However, they likely represent assemblages of local Wissahickon Formation outcrops.

Mineral Assemblages



Muscovite-Biotite-Quartz-Feldspar Schist



A schist typical of the local Wissahickon formation -- with individual crystals to 2.5 mm -- is the predominant rock in the area. It (and its sapprolite) has been observed by the author in Underground Storage Tank (UST) removals on the southeast and southwest corners of Lincoln Avenue and Chester Pike and the sapprolite south of Chester Pike and north of the 4th Avenue bridge.

Sillimanite-Microcline-Graphite Schist




Sillimanite-Microcline-Graphite Schist


This assemblage has only been observed in stream cobbles. It consists of subhedral, prismatic lathes of sillimanite to 1 cm inter grown with fine-grained microcline, platy graphite to 3 cm and minor quartz and biotite. Sillimanite is often replaced by kyanite; hardness tests using a steel needle indicate a Moh's hardness of greater than than 5 perpendicular to the long prismatic axis and less than 5 parallel.

Kyanite-Biotite-Microcline Schist




Kyanite-biotite-microcline schist


This assemblage has been collected from the stream bed of Amosland Creek and observed in-situ to the northeast. It consists of large subhedral, prismatic lathes of kyanite (to 5 cm) with <1mm biotite inclusions in a coarse biotite (to 5 mm) matrix. Microcline and kyanite are commonly porphyroblastic (occurring as crystals much larger than the biotite matrix). Where observed in-situ, is is associated with Perthitic Microcline-Quartz-Muscovite-Biotite Granite Pegmatite and Quartz Stringers with or without kyanite.

Perthitic Microcline-Quartz-Muscovite-Biotite Granite Pegmatite




Perthitic Microcline-quartz-muscovite-biotite granite pegmatite


The pegamtite assemblage has been observed in-situ at the site, immediately south of and parallel to, Amosland Creek. It is tabular, approximately 2 meters thick and appears to dip steeply to the northwest. Numerous other pegmatites crop out along the banks of the Amosland Creek.

Quartz Stringer




Pyrite in quartz with biotite-almandite schist


The quartz stringer has been observed in-situ at the site – immediately north of Amosland Creek adjacent to the Route 420 culvert, and also collected as cobbles from the Amosland Creek stream bed. The in situ occurrence appears to be at least 1 meter thick but it is partially concealed by vegetation and soil cover. Where exposed, it appears to trend parallel to, and immediately north of, Amosland Creek. Specimens from the stream bed with attached suspected wall rock have been collected. The suspected wall rock is coarse-grained biotite-almandite schist with minor pyrite.

Kyanite-Quartz




Euhedral to subhedral bladed kyanite

Fluorite in kyanite


This assemblage is the one best represented in collected material from the site. It was collected from the culvert excavation along Amosland Creek under Route 420 and was reported to occur more prolifically in-situ in the UST excavation at the gasoline station on the northeast corner of 4th and Lincoln (Route 420) Avenues (Dr. John Hallahan, personal communication, 1989).

A single specimen of this assemblage was collected by the author that had an included fluorite (CaF2) nodule. Upon dissolving most of the fluorite, small, terminated, euhedral kyanite crystals to 1.5 mm with unterminated kyanite prisms to 1 cm were encountered.

Mineral Paragenesis



Local metamorphism is of Taconic age when Wilmington Complex magmatic arc rocks were thrust over Wissahickon Formation pelitic forearc or backarc sedimentary rocks. Wilmington Complex rocks are preserved as far northeast as Chester, Pennsylvania and their former presence is is inferred by extending the thrust fault geometry several kilometers northeastward through Ridley Township.

The aluminum silicate assemblage observed on site (sillimanite and kyanite) indicates metamorphic pressure in excess of 3.5 kb; the presence of water available during metamorphism and lack of pyrophyllite also constrains metamorphic reactions to temperatures greater than 400 C.

Tulane University Petrology Course Notes

The presence of almandite and lack of staurolite and muscovite in some metamorphic assemblages suggests high amphibolite to granulite facies metamorphism. Two assemblages – kyanite + quartz +- fluorite - muscovite and sillimanite + biotite + potassium feldspar + quartz – muscovite – suggest kyanite and sillimanite were formed at the expense, and above the stability conditions, of muscovite.

The reaction

muscovite + quartz = microcline + Al2SiO5 + water
KAl2(Si3Al)O10(OH,F)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O

is restricted almost exclusively to granulite facies sillimanite-forming conditions. The sillimanite-microcline-graphite schist assemblage represents this reaction. However, the kyanite version of this assemblage is favored only when water is removed from the system (i.e. P H2O < P). Under conditions where the pressure of water in the system is .5 total pressure, this reaction's temperature shifts into the upper part of amphibolite facies metamorphism -- within the stability field of kyanite. This assemblage suggests an environment in which water produced in kyanite formation leaves the system. Unfortunately, the lack of in situ exposures of these rocks precludes determining their relationship to rocks observed in situ.

Another reaction suggests a mechanism for the kyanite-quartz assemblage:

2 muscovite + 2 hydrogen ions (acid) = 3 kyanite + 3 quartz + 2 potassium ions + 3 water.
2 KAl2(Si3Al)O10(OH,F)2 + 2 H+ = 3 Al2SiO5 + 3 SiO2 + 2 K+ + 3 H2O

This reaction implies an acidic, aqueous fluid reacted with muscovite to form kyanite and quartz. Pyrite in the quartz stringer suggests the presence of an acidic solution. The presence of fluorite associated with the kyanite-quartz assemblage suggests either a pegmatitic hydrofluoric acid component or a contribution of fluoride from muscovite.

Thus, two distinct aluminum silicate forming conditions are indicated:
1) a “dry” event in which microcline and kyanite or sillimanite formed at the expense of muscovite and quartz and
2)a “wet” acidic event in which large masses of kyanite and quartz -- and rarely fluorite -- formed at the expense of muscovite.

The relationship of the pegmatite to wall rocks has been observed at one location to the northeast of the site. This outcrop consists of:

1)30 cm granite pegmatite
2)30 cm quartz stringer
3)100 cm kyanite-biotite-microcline schist

This strongly suggests that the kyanite-forming reactions occurred simultaneously and were driven by pegmatite – wall rock interactions.

The two periods of metamorphism in the area (Silurian and early Devonian) complicates establishing timing of the mineralization event; the Ridley Park Granite is reported to intrude the Silurian-age Springfield Granodiorite, suggesting the possibility of post-Silurian (Devonian?) igneous activity. However, pegmatite ages in the area vary.

Conclusion



While primarily known for its large, cornflower blue masses of kyanite and quartz, a variety of igneous and metamorphic minerals may be collected at the site. These minerals represent at least two events -- regional metamorphism to sillimanite grade and pegmatite intrusion with accompanying hydrothermal activity. However, since genetic relationships between all of the rocks are not observed, its history may be more complex.




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Comments

A very nice article! Thanks for sharing it!

Daniel Russell
31st Dec 2007 3:22am
I agree. Very good article. My sister just moved to Media, and I hope some day to collect here.

Mark J. Sigouin
27th Mar 2008 3:12am

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