EXSOLUTION BY SPINODAL DECOMPOSITION II: PERTHITE FORMATION DURING SLOW COOLING OF ANATEXITES FROM NGORONGHORO, TANZANIA

Perthites in slowly cooled granulite facies rocks from the Ngoronghoro structure (Tanzania) show complex microstructures reflecting several stages of exsolution and coarsening. Mesoperthites with an integrated bulk composition of Or26.5Ab71An2.5 are comprised of lamellar intergrowth of 5 to 10 mu m wide orthoclase-rich and albite-rich lamellae. Spindle perthites with a bulk composition of Or67Ab31.5An1.5 are comprised of 2 to 10 mu m wide and several 10's of mu m long albite-rich spindles in an orthoclase-rich host. In the mesoperthite primary exsolution occurred by spinodal decomposition, whereas the spindle perthite formed by a nucleation and growth mechanism. The interfaces between the exsolved phases are generally incoherent. Unmixing and coarsening of the mesoperthite is discussed in the light of non-linear uphill diffusion. The microstructure evolution and successive chemical separation of the exsolved phases during cooling are modeled based on Cahn-Hilliard theory. We find that coarsening primarily occurs during the first 20 degrees C to 30 degrees C of cooling after the homogeneous precursor feldspar has entered the coherent spinodal. The extent of coarsening and the characteristic size of the exsolved phases depend on the cooling rate over this temperature interval. For geologically relevant cooling rates on the order of 5 degrees C/Ma to 50 degrees C/Ma the compositions of the exsolved phases develop along the solvus; down to about 480 degrees C to 530 degrees C and are "frozen in" only at lower temperatures. The compositions that are finally preserved in the orthoclase-rich and in the albite-rich phases depend on cooling rate over this temperature interval. Both the characteristic larnella width of the mesoperthite as well as the compositions of the exsolved phases indicate slow cooling at <= 5 degrees C/Ma during the coarsening stage and down to about 450 degrees C. During later stages of the cooling history the albite-rich phase of the mesoperthite exsolved about the peristerite gap, and the orthoclase-rich host of the spindle perthite exsolved a second generation of coherent albite-rich spindles. The incoherent phase boundaries of the primary perthite microstructures served as passageways for fluids during late stage deuteric alteration, which lead to secondary coarsening in the mesoperthite and to alteration of the albite-rich precipitates in the spindle perthite. Finally, both types of perthites were replaced by coarse grained patch perthite during deuteric alteration.