Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 73. (Budapest 1981)

Embey-Isztin, A. ; Noske-Fazekas, G.: On the chemistry of the large phenocrysts in the tuff of Godóvár (Börzsöny Mts., Hungary)

andesitic lavas have only 63.7 per cent An — component with a maximum of 70.8 per cent in the pyroxene andésites. It seems thus firmly established that the phenocrysts of the Godóvár crystal tuff must have segregated from a more basic magma than the known andesitic lavas of the Börzsöny Mts. This is especially true for the chromian diopside core of pyroxenes, which have to be nucleated in an even more primitive Mg-rich magma. Since pyroxenes are low in Ti and Na, an alkali basalt parent can be ruled out, whereas a primitive volcanic arc parent basalt seems to be a reasonable assumption. Such a basalt may be intimately associated at greater depth to the calc-alkalic andesitic suite (alkali-lime index = 59.5) of the Börzsöny Mts., which is a fairly frequent case elsewhere. CIPW norms calculated from the pyroxene analyses reveal also tholeiitic affinités (Table 4) and amphiboles are also sensibly less undersaturated than are titaniferous pargasites and kaersutites formed in alkalic basaltic liquids e.g. BROOKS & PLATT (1975) and EMBEY-ISZTIN (1976). Only a rough estimate can be advanced as to the depth of crystallization of the primitive light coloured Cr-bearing pyroxene core. The higher amount of Al

vl (on average 0.068 A1

VI

) which is generally regarded as being present in the form of Ca — Tschermak's molecule (CaAl

2

Si0

6

) and which increases with increasing pressure at the expense of anortite is indicative of a crystallization at considerable depths. Similar Cr-diopside crystals (however richer in Ti and Na) were interpreted as crystallizing from alkali basalts at a depth of around 20 km by MUNOZ & SAGREDO (1974), BROOKS & PLATT (1975) and SCOTT (1980). So, as a rough estimate an approximative depth of 20 km is adopted here for the present case too. The abrupt change in the chemistry of the outer zones in clinopyroxenes and the joining of amphibole and plagioclase as cumulus phases suggests an evolution in a storage chamber with different physico-chemical conditions. Notably, p

H

,

0 had to be increased either as a consequence of crystallization and/or there was a water and volatile supply from the surrounding wet sediments. This storage chamber had to be placed at much shallower dephts than the magma reservoir from which the chromian diopside core had separated and they may have been linked by a narrow conduit. The decreased amount of A1

VI in the outer zone clinopyroxene (on average 0.045 A1

VI against 0.068 in the core) points to lower pressures. The minimum depth for the storage chamber in which pargasite phenocrysts can form is governed by the lower limit of stability of amphiboles in liquids of basaltic compositions at about 1.4 kb (~ 5 km depth) according to experimental works YODER &TILLEY (1962). YODER & TILLEY (1962) and HOLLOW AY & BURNHAM (1972) has also shown that plagioclase would be expected to crystallize before amphibole up to approximately 3 kb (~ 11-12 km depth). Since it seems that at least partly, amphibole preceded plagioclase in the crystallization sequence, a depth around 12-15 km and a temperature about 1000 °C may perhaps be a reasonable estimate for the forming of pargasite crystals. Acknowledgement — The authors are highly indebted to DR. G. KURAT, director of the Mineralogical and Petrological Department. Natural History Museum in Vienna (Austria), for making the microprobe analyses.