Welcome to my next EarthCache! This time we will visit Central Park, where are very interesting rocks in small area...
Geology
The rocks, which belong to the metamorphic units, were made by marine sediments in the time range of 500 Ma to 450 Ma. During this period, the layer various sands or mud on top of each other, which form layers that grow to this day. During the same period, the Taconic Island Arch approached the east coast of Laurentia, which actively produces volcanic material that you can see in the sediments as a dark layer that is abundant in Mg and Fe - which you can observe throughout Central Park. The metasedimentary units observed at Hernshead are part of a large allocthonous block that was tectonically displaced during the Taconic Orogeny. This movement of material likely involved numerous faults. A great uncertainty in the geology of Central Park is the fault-bound contact between the Manhattan and Hartland schist, a contact known as Cameron’s line.
Graphic granite
At this location we see small veinlets of “graphic granite” injected into the metapelite. The magmatic texture observed at this outcrop is common to highly silicic (SiO2-rich) granitoids, particularly pegmatites. The term “graphic” refers to the coarse intergrowth of quartz and alkali feldspar, which somewhat resembles a runic or cuneiform text. Unlike the metapelite and granofels, the cuneiform texture at Hernshead shows no indication of deformation. This indicates that the granitic veinlets were likely injected post peak metamorphism. Although the age of this graphic granite is Devonian and is related to other plutonic rocks observed in Manhattan
Vein quartz
This quite new (apropos to metamorphism), brittle deformation phenomena are also at Hernshead and are easiest to observe on the platform at the NE end. Here a series of quartz-rich joints can be seen as slightly raised ridges that cross the dominant metamorphic structure.
These quartz-rich bond form when fluids floating the deformed metamorphic succession precipitate quartz (and other
minerals) from solution. This quartz a makes these joints more resistant to chemical and physical weathering
than the surrounding metapalite.
Granofels
The granofels at Hernshead is a light brown geological rock that can be seen extending to the easternmost areas of “The Lake”. The term granofels refers to the granoblastic texture and quartz-feldspar rich nature of this rock (the quartz and feldspar crystals must broadly equal in size), the rock generally lacks the micaceous foliation in the metapelite. This is the most apparent unit at Hernshead because it has a coarser grainsize and higher quartzofelspathic content to the surrounding metapelites. These “quartzofeldspathic” units are found on many locations within Central Park and have variously been described as a metamorphosed plutons. The mineralogy of this unit consists of plagioclase, quartz, biotite and muscovite. Accessory here also include zircon and apatite. The mineralogy of the granofels at Hernshead is really simple. The biotite and muscovite crystals show long axis alignment.
Metapelite
The dominant geological unit at Hernshead is an interbedded metapelite. This unit is characterized by a rougher and more pitted texture than other units at Hernshead (e. g. granofels). This unit also displays a darker greyish-brown color. It is caused by subtle variations in quartz, mica, feldspar, which in turn likely represents small perturbations in the sediment during the Ordovician. The feldspars and quartz crystals show rounded edges indicative of the weathering and sediment transportation process. Minor phases from the metapelite include magnetite (with exsolved ilmenite), apatite and zircon. Like most units at Hernshead the metapelite is characterized by an abundance of magnetite, which can be seen as small dark minerals. At the base of the north-easternmost part of the outcrop, an anticlinal fold axis is exposed and traceable through the outcrop.
Amphibolite - Biotite-garnet schist
Several bands of thin, black (thats rocks rich in iron and magnesium) bands are observed at Hernshead. Here
it is possible to observe garnet (red), biotite (black), muscovite (white) and small plagioclase
(clear) and quartz (gray to colorless) crystals.The dark color observed within this unit clearly
demonstrates it has a more mafic composition than the granofels or metapelite. It is thought this
unit likely represents a volcanic sample, likely an ash ow deposit (e.g., a volcanic tuff) that derived from the
approaching Taconic arc during the Ordovician.
Fold hinge
The rocks of Central Park record a complex history of continental growth through sedimentation and tectonic processes that
spans millions of years and multiple orogenic phenomena. The rocks at Hernshead record a complex series of phenomena. The rocks have seen multiple episodes of deformation, including ductile and brittle styles. The original bedding within metasediments at Hernshead can be traced by small compositional variations that are manifest in changes in color, mineralogy and fabric.
Questions:
1. What period are the local rocks from?
2. What type of rocks predominates here?
3. Try to find in Amphibolite garnet. What is its size and surface?
4. (optional) Is the presence of magnetite large enough to move the compass?
5. Take a picture with you or with your GPS and posted it in your log.
Send me the answers via the profile. In case of doubt, I will contact you. Thanks for visiting! :)
Sources:
wikipedia, Museum of Natural History