CRYSTAL CHEMICAL VARIATIONS IN BA-RICH BIOTITES FROM GABBROIC ROCKS OF LOWER CRUST (IVREA ZONE, NW ITALY)

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.28<Fe/(Mg+Fe)<0.66]. Biotites from gabbros are enriched in BaO and TiO2 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2(IV)Si + ((VI)R2+) half arrow left over half arrow right 2(IV)Al + (VI)Ti (R2+ = Fe + Mg + Mn) in gabbros and (IV)Si + (VI)Al half arrow right over half arrow left (IV)Al + (VI)Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M1 and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M1 from diorites (R-values between 2.68 and 3.77) 'and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution (IV)Si + (XII)K half arrow left over half arrow right (IV)Al + (XII)Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M1 polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe2+ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe2+ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.