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