The Bringhurst Pluton GeoAdventure
The Bringhurst Gabbro: A GeoAdventure in the Delaware Piedmont
INTRODUCTION
A field
trip to Bringhurst Woods Park is appropriate for students in grades 5
and up (10 years and older), and provides an opportunity to observe intrusive
plutonic igneous rocks that have intruded into country rock, which in
this case is the blue rock or what geologists call the Brandywine Blue
Gneiss. In addition, the minerals in the pluton are large, easily identified,
and interesting. Mineral collecting is not allowed within the park, however
permission may be obtained to collect along Shellpot Creek southeast of
the park. Please do not use rock hammers on the rocks in the park.
The
specific objectives of this adventure are:
- To
observe an intrusive igneous rock and the country rock (Wilmington blue
rock) it has intruded
- To identify the individual minerals in an igneous rock
GEOLOGIC SETTING
The
rocks along Shellpot Creek in Bringhurst Woods Park are intrusive igneous
or plutonic rocks (Figure
1). Because of the good exposure in this park, geologists have named
these rocks the Bringhurst Gabbro and mapped the pluton as a geologic
unit within the Wilmington Complex (Figure
2). The Bringhurst Gabbro represents a magma flow that flowed into
the Wilmington Complex and cooled deep underground. The rocks of the Wilmington
Complex underlie the most City of Wilmington and Brandywine Hundred. During
the 18th and 19th centuries all rock units within the Wilmington Complex
were extensively quarried for building houses, fences, retaining walls,
schools, churches, and factories. They were used wherever a building material
was needed. The most common rock unit in the Wilmington Complex is a high-grade
metamorphic rock called the Brandywine Blue Gneiss (commonly called the
Wilmington blue rock). This "blue rock" was named for the bright
blue color of the rock when it is freshly exposed. It is the Wilmington
blue rock that the Bringhurst Gabbro intruded.
The
Bringhurst Gabbro exposed along Shellpot Creek has not been deformed or
recrystallized by metamorphism, thus the rocks of the Bringhurst pluton
lack the layering found in most of the other metamorphic rocks of the
Delaware Piedmont. Because there are no fine-grained "chilled margins"
at the contact between the pluton and the Wilmington blue rock, the pluton
probably intruded the gneisses while they were still hot, sometime in
the early Paleozoic between 500,000,000 and 400,000,000 million years
ago.
THE ROCKS
Shellpot
Creek in Bringhurst Woods Park is choked with large rounded boulders of
Bringhurst Gabbro that have eroded out of the surrounding hills. A close
look shows the minerals in the gabbro are between 1/4 and 2 inches in
length and 1/4 to 1 inch in diameter (Figure
3). Blobs of fine-grained dark rock are common in the Bringhurst pluton.
These dark blobs are chunks of Wilmington blue rock that were picked up
and incorporated into the magma as it intruded into the gneiss. These
inclusions are called xenoliths, a word derived from the root xeno- meaning
foreign and lithos- meaning rock. Thus, a xenolith is a foreign rock enclosed
within another rock. In this case the xenoliths are derived from the country
rock, the Wilmington blue rock. Although the Wilmington blue rock is composed
of both dark layers and light layers, all the xenoliths are derived from
the dark layers. This is possibly because the light-colored inclusions
melted at a lower temperature than the dark inclusions, and the light
inclusions melted in the hot gabbroic magma of the pluton becoming commingled
and no-longer recognizable.
A contact
between the coarse grained rocks of the pluton and the Brandywine Gneiss
occurs approximately 700 to 800 ft east of the park entrance (See
Figure 2). The gneiss at the contact is contorted and contains clots
of quartz.
Before
a field trip to Bringhurst Wood Park, it is recommended the group visit
one of the Wilmington Complex stops described in the Wilmington Blue Rocks
Geologic Adventure, so the participants can recognize the Wilmington blue
rock.
MINERAL
IDENTIFICATION
Minerals
of the Bringhurst pluton (See
Figure 3) are plagioclase feldspar, pyroxene and olivine. The plagioclase
is dark gray and glassy. Feldspar has two distinct cleavages, thus when
a feldspar crystal is broken along a cleavage plane it will present a
smooth shiny surface. The pyroxene crystals are elongated, black or bronze
colored, and may have a distinctive schiller or iridescent luster on a
fresh surface. The olivine grains are less common than the pyroxene, and
in this pluton, the olivine grains are usually rusty and have a black
rim. Individual minerals in the rims cannot be recognized in hand specimens,
but microscopic study has identified an inner rim that is an intergrowth
of orthopyroxene with spinel and an outer rim that is an intergrowth of
hornblende with spinel. The olivine-bearing rocks are more abundant southwest
of the park entrance.