Metamorphic rocks are an interesting bunch because they can provide information
ID: 290994 • Letter: M
Question
Metamorphic rocks are an interesting bunch because they can provide information regarding not only the deep history of our planet, but also the violence associated with tectonic movements. The immense amount of pressure and temperature needed to alter rock is fascinating.
choose a specific Metamorphic rock that you found interesting You will need to discuss, IN DETAIL, the following:
1) Type of metamorphic rock you chose: main category (i.e. Foliated or Non-Foliated)
2) The name of the metamorphic rock you chose (eg. slate, schist etc.)
3) The various characteristics of the type rock you chose (i.e. How geologists go about determining that the rock is what it is?).
4) What the Protolith of that metamorphic rock is.
4) How it forms (metamorphic environment) - be sure to provide a lot of detail here.
5) Any other interesting information you learned about the type of sedimentary rock you chose during your research. (eg. uses etc.)
Explanation / Answer
Name of the rock: Khondalite (named after the KHOND tribe of odisha and Andhrapradesh of country India)
1. Main category: foliated, schistosity.
2. The name in details: the rock is mainly composed of sillimanite, muscovite, biotite, garnet, quartz and feldspar. This rock is medium to coarse grained and shows schistose texture. Thus the name of the rock is GARNET BEARING QUARTZ FELDSPAR MUSCOVITE BIOTITE SILLIMANITE SCHIST OR KHONDALITE.
3. IDENTIFICATION: Any rock is identified mainly by its minerology and texture.
In Khondalites minerals that are observed are as follows: quartz, feldspar, muscovite, biotite, sillimanite and garnet porphyroblasts (in good amount). In hand specimens the rocks exhibut numerous brownish red garnets.
Textures observed are:
A. Schistose texture: preffered orientation of muscovite biotite and sillimanite grains observed in parallel arrangement. Thus, schistosity is mainly defined by mica group of minerals (muscovite and biotite), so lepidoblastic schistosity can be seen.
B. Granoblastic texture: being a rock of granulite facies granoblastic texture is also observed. This texture is defined by equidimentional and subidioblastic grains of quartz and feldspar. The recrystallised grains show mosaic arrangement and lack of any directional fabric, and also sutured grain contacts.
C. Pressure shadow/fringe structure (observed mainly in thin sections): Warping of schistosity planes can be observed around the garnet porphyroblasts. In the low strain areas adjacent to the warped grains smaller equidimentional grains of feldspar and (rarely) quartz can be seen. This structure is known as pressure shadow ( Ramsay) or pressure fringe (A. Spry).
4. PROTOLITH: Shales are the protolith of khondalite. Shales which are high in Al, K and Si, are preferably metamorphosed to khondalite.
5. Formation: Khondalites are mainly found in deeply eroded continental cratons of Precambrian age. The geothermal gradients are as high as 30degree celcius. Being formed in such high temerature gradients the rocks are known as 'dry' and devoid of water and other volalites. The metamorphic changes are followed according to the changes and reactions of 'KFMASH' system. These rocks are formed above sillimanite isograde and are considered to be rocks of Granulite facies. Thus formed under moderate ti high pressure and temperature and are uplifted during dufferent orogenetic events.
6. Specific fact: Khondalites often contain graphite deposites of commercial importance. Eg. : Easternghats province of India.