#migmatite

[knottybearnature]

March 2, 2025

Migmatite, lichen, moss, spider

Evergreen, CO

[anywhereinafrica]

the migmatites 1/3

[ijtsrd]

An Account of Field and Petrographic Characteristics of Granitic Rocks of Cherlapally Area, Nalgonda District, Telangana

by N. Ningam | P. R. C. Phani ""An Account of Field and Petrographic Characteristics of Granitic Rocks of Cherlapally Area, Nalgonda District, Telangana""

Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-3 | Issue-5 , August 2019,

URL: https://www.ijtsrd.com/papers/ijtsrd25202.pdf  

Paper URL: https://www.ijtsrd.com/other-scientific-research-area/geology/25202/an-account-of-field-and-petrographic-characteristics-of-granitic-rocks-of-cherlapally-area-nalgonda-district-telangana/n-ningam

call for paper international journal, call for paper mathematics

Field geological and petrographic characteristics of granitic rocks of Cherlapally area, Nalgonda district, Telangana, within the eastern Dharwar craton are described in this paper. Field traverses revealed four types of granitic rocks in the study are viz., quartz diorite, granodiorite, monzogranite and syenogranite. The variations in the texture and mineralogical composition of these plutons are so distinct that the term migmatite' has to be applied, wherein mutual field relations always remain enigmatic. The complex nature of the batholiths is also evident from the structural fabrics observed in the present investigation. 

[mostlynaturepartiallyman]

[earthstory]

Latent Heat and the Lower Crust

When continents collide, sedimentary rocks are shoved deep into the lower crust, where they are squeezed and heated and turned into metamorphic rocks. This rock shows basically the upper limit of metamorphism; the rock is no longer solid and I don’t know whether to call this metamorphic or igneous. This rock type is called a migmatite: it has crossed its melting temperature and part of the rock has melted to form light colored bands that separated from the still-solid dark colored bands. This rock type is common in metamorphic terranes and it occurs when you take some of the most common minerals on Earth, including Quartz, Plagioclase, and K-Feldspar, up to a temperature of about 650℃, a temperature we’d consider part of the Amphibolite Facies. Those minerals are found all the time in sedimentary rocks, including shales and sandstones, so when rocks are thrust to the bottom of continents in mountain ranges, migmatites should be common. In fact, new research suggests that this rock type imposes a fundamental constraint on metamorphism, limiting the upper temperature of the crust.

If you had a high school chemistry class, you probably heard of the concept of Latent Heat. If you take a phase, such as water or ice and add heat to it until it undergoes a phase transition, that phase stops heating up and stays at a constant temperature until the phase transition has finished. Ice water (at 1 atmosphere), for example, is fixed at 0℃ until the ice melts, while boiling water is fixed at 100℃. Rocks are more complicated because they’re not just a single mineral, but the rule is the same; when the rocks start melting, they get stuck very close to a single temperature, because any extra heat that comes into the rock goes into latent heat rather than raising the temperature.

On the other hand, granitic rocks may not melt at the same temperature as a sedimentary rock, because the melting temperature of a dry granite is higher than the melting temperature of fine-grained sedimentary rocks. Even though the melting temperature of these rocks is a fairly narrow range, they are common globally, because once the rocks start melting, generating that melt locks the temperature where it starts until the whole rock has melted away.

If Facebook doesn’t reduce the image size too much, you can zoom in on this photo and see that there are some nice garnets forming in the darker layers. If it’s too reduced, click through to the original image at the link below.

-JBB

Image credit: https://flic.kr/p/8Rm94D

Reference: https://pubs.geoscienceworld.org/gsa/geology/article/46/7/643/531971/thermal-buffering-in-the-orogenic-crust

[earthstory]

A granite is born

At first glance these rocks snapped while wandering around some old mountain roots in the Sierras de Cordoba in Argentina don't seem like much, but they record the moment in high grade metamorphism when old rocks turn into new, with nothing but a few zircons left behind to testify to their previous history. The first photo shows a mass of fine-ish grained 'baby' granite, and floating within it a large rounded chunk (in the rough centre and a few attendant blebs) of the grey and white banded gneiss from which it formed, that was still floating unmelted in the magma when it froze (and by then probably some distance from its source rock). In the second photo the act of birth is even more intimately caught. The chunks of gneiss are picked out with a white outline of fresh quartz rich melt, also frozen in place. These rocks are called migmatites and mark a generational turnover in the rock cycle, when deep crust melts (usually during a continental collision and mountain building moment), rises and freezes, becoming shallower, younger crust, potentially susceptible to uncovering and erosion into sediments, whose burial unto melting depth will start the whole cycle anew...

The first photo is of a boulder in a river bed, the area in the photo is maybe a metre squared, the second boulder is about a metre in the longest dimension.

Loz Image credit: Loz

[the-joyful-guide-to-lachrymology]

i have new rocks 😍

[earthstory]

View this post on Instagram

Folded light and dark bands in a metamorphic rock that was heated to conditions where it just started melting (called a migmatite) from Poland.