J. metamorphic Geol., 2000, 18, 591–603 Microtextures of opaque inclusions: their use as indicators for hiatuses during garnet porphyroblast growth R. SPIESS, 1,2 B. BERTOLO, 1 A. BORGHI, 3 M. CHINELLATO 1 AND M. TINOR CENTI 1 1 Dipartimento di Mineralogia e Petrologia, Corso Garibaldi 37, 35137 Padova, Italy (richard@dmp.unipd.it ) 2 CNR— Centro di Studi per la Geodinamica Alpina, Corso Garibaldi 37, 35137 Padova, Italy 3 Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 37, 10125 Torino, Italy ABSTRACT Within an analysed garnet porphyroblast, opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphy- roblast. Three different zones can been distinguished: Zone 1 contains a Ti-rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite. Zone 2 shows a Ti-rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite. Within zone 3, Ti-rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim. Similar microtextures within ulvo ¨ spinel- rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion. During metamorphism of metapelites, such an increase can be reasonably envisaged because of dehy- dration reactions progressing during rising temperatures, and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast. Because the microtextures are homo- geneous within the three different zones, it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate. As a consequence, hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next. A compari- son between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coinci- dence between these and the sites where inclusion trails become strongly deflected and truncated. This correlation confirms that, in the studied case, sharp microstructural boundaries (as truncation zones or deflection zones) coincide with growth hiatuses. The study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts, especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses. Key words: garnet growth hiatus; opaque inclusions; symplectitic textures; ulvo ¨ spinel–magnetite–ilmenite. elements that typically evolve at, or close to, the INTRODUCTION porphyroblast–matrix interface due to the partitioning of deformation about rigid objects (Bell & Hayward, Inclusion trails in porphyroblasts can preserve import- ant information for understanding the inter- 1991; Passchier & Speck, 1994). Potential candidates are elements that can be interpreted as previous strain relationships between metamorphism, deformation and porphyroblast growth (Vernon, 1976; Bell & Rubenach, caps (Passchier et al., 1992), deflected connection folds (Passchier & Trouw, 1996), or truncation zones (Bell 1983; Bell, 1986; Bell et al., 1986; Jamieson & Vernon, 1987; Davis, 1996; Johnson, 1993a,b; Aerden, 1994). et al., 1986) etc., and their overgrowth during accretion of the porphyroblast may suggest that a hiatus in the The geometrical interpretation of inclusion trail patt- erns is straightforward as long as the geometry of the porphyroblast growth history has occurred. However, unequivocal evidence for episodic porphyroblast growth pattern is simple. However, frequently porphyroblasts preserve complex inclusion trail patterns (e.g. Bell can only be derived from data which indicate that, in coincidence with the geometrical elements outlined et al., 1986; Bell et al., 1992; Johnson, 1993a,b; Passchier & Speck, 1994), which have the potential to above, sudden changes in the intensive variables controlling the metamorphism have occurred. show that the porphyroblasts have grown episodically (Bell & Hayward, 1991; Passchier & Speck, 1994). Microtextures of opaque inclusions are related to changes in the chemical–physical conditions during Recognizing whether porphyroblasts growth was continuous or episodic has important implications for progressive metamorphism, and are of great potential for demonstrating episodic porphyroblast growth. This understanding the processes operating during porphyro- blast growth. On the basis of geometrical analysis of will be shown in the present paper, which deals with an SEM study of opaque minerals tracing a complex inclusion trails, episodic porphyroblast growth is usually proposed where porphyroblasts preserve geometrical inclusion pattern within a garnet porphyroblast. 591 © Blackwell Science Inc., 0263-4929/00/$15.00 Journal of Metamorphic Geology, Volume 18, Number 5, 2000