MYRMEKITE - CONSTRAINTS ON THE AVAILABLE MODELS AND A NEW HYPOTHESIS FOR ITS FORMATION

Based on a myrmekite occurrence around K-feldspar megacrysts (porphyro blasts) in a mylonitic augen gneiss from southeast Brazil, a critical discussion on myrmekite genesis highlights some constraints on the ava ilable myrmekite models, mainly with regard to the replacement directi on in the K-feldspar/myrmekite interface. In the investigated augen gn eisses, chemical and microstructural data indicate that myrmekite does not replace K-feldspar (as generally inferred in other myrmekite occu rrences), but in fact is replaced by it. Alternatively, myrmekite is s uggested to form by SiO2 infiltration of pre-existing plagioclase grai ns during K-feldspar porphyroblastesis. The local stress induced by vo lume change during plagioclase replacement by K-feldspar enhances defo rmation around the growing porphyroblast, enabling SiO2 infiltration t o occur along the discontinuities (microcracks and kink boundaries) of deformed plagioclase grains. The development of quartz vermicules in the K-feldspar vicinity accounts for the access of SiO2 necessary for plagioclase replacement by K-feldspar, enabling the advance of the K-f eldspar porphyroblast.