Titanite inclusions in altered biotite from granitoids of Taiwan: microstructures and origins

Biotites with three sets of titanite inclusions (i.e., sagenitic biotites) have been reported from both igneous and metamorphic rocks. Two formation m echanisms have been postulated: the 'precipitation' model for the sagenitic biotite of igneous origin (Shau, Y.H., Yang, H.Y., Peacor, D.R., 1991. On oriented titanite and rutile inclusions in sagenitic biotite. Am. Mineral 7 6, 1205-1217.) and the 'percussion figure' model for the sagenitic biotite of metamorphic origin (Xu, S., Ji, S., 1991. Biotite percussion figures in naturally deformed mylonites. Tectonophysics 190, 373-380.) Sagenitic bioti tes in granitoids of the Tananao Metamorphic Complex of Taiwan were studied , especially with regard to the variable degrees of superimposed collision- induced deformation/metamorphism. Mass balance considerations, transmission electron microscopic observations and regional geological relations exclud e the simple intra-biotite precipitation model as the possible mechanism. I nstead, the formation of titanite inclusions in these igneous biotites is s uggested more likely to be facilitated by inward diffusion of Ca and outwar d diffusion of Ti along the basal cleavage planes and the fracture surfaces induced by shear deformation (i.e., the percussion figure) in biotites und er greenschist-facies temperature conditions. Interdiffusion may also accou nt for the formation of rutile particles with varied size and varied crysta llographic orientation in some titanite laths. (C) 2001 Elsevier Science Lt d. All rights reserved.