ORDERING AND EXSOLUTION PROCESSES IN OR-RICH ALKALI FELDSPAR MEGACRYSTS FROM THE ELDZHURTINSKIY GRANITE (CAUCASUS)

The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Ce ntral Caucasus) is uniform with respect to modal composition, major an d trace element chemistries of bulk rocks and mineral phases. In contr ast, it reveals two types of alkali feldspar megacrysts differing in t etrahedral Al-content (2t(1)) and exsolution microtextures: 1. Alkali feldspar megacrysts (Or(70)An(2)Ab(28)) from the top of the body consi st of ideally coherent intergrowths of fine-scale regular Or- and Ab-r ich lamellae. The exsolved K-feldspar host is monoclinic (2t(1) = 0.7) , the exsolved Na-rich phase consists of Albite- and/or Peri-cline-twi nned albite. 2. Megacrysts from greater depths have the same bulk comp osition, but the exsolved Ab-rich phase occurs in the form of opticall y visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering (2t(1) = 0.8). The evolution of the distinct types of megacrysts reflects difference s in the cooling history within the upper and lower part of the granit e body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of a lbite patches in the megacrysts from the lower part are caused by flui d-feldspar interaction. The transition zone in the body between the tw o types of megacrysts is sharp (in a depth interval of 100-200 m) and not related to shear zones.