THE ABUNDANCE OF AMMONIUM IN THE GRANITES OF CENTRAL SPAIN, AND THE BEHAVIOR OF THE AMMONIUM ION DURING ANATEXIS AND FRACTIONAL CRYSTALLIZATION

The granites of the Sistema Central Espanol are richer in ammonium tha n those of most other regions, and have a mean NH4+ content of 84 ppm (range = 1-243 ppm). Among the possible causes for the high level of a mmonium, a high proportion of organic-rich pelitic protolith and reduc ing conditions during anatexis are considered to be the most significa nt. The behaviour of the ammonium ion during magmatic differentiation is discussed by reference to its distribution in the Pedrobernardo lay ered intrusion: ammonium is depleted in the final liquid fraction, but there is no relative fractionation of NH4+ and K+. The depletion of t he melt in NH4+ during crystallization is attributed to its removal by biotite and to a lesser extent by K-feldspar. The behaviour of the am monium ion during anatexis is discussed with reference to the Pefia Ne gra migmatite complex. It is shown that large amounts of NH4+ are pres ent in these high grade metamorphic rocks, and that NH4+ is preferenti ally partitioned into the restite fraction during partial melting. The se relationships are attributed to the preferential incorporation of N H4+ into potassic host minerals in the order: biotite > muscovite > K- feldspar.