Quantitative structural analyses and numerical modelling of ...

ORIGIN OF FELSIC MIGMATITES 35(a)Type Iplagioclase (Fig. 4c). Biotite (10–15 modal%; X Fe ¼0.76–0.79, Ti ¼ 0.18–0.19 p.f.u.) is homogeneouslydispersed and is most prevalent in the plagioclase–quartz domains. Atoll-shaped garnet (0.05–0.25 mm insize; X Fe ¼ 0.96–0.97) appears inside the felsic aggregates,rather than along contacts with biotite.Type IV: nebulitic migmatiteThis type of rock is composed of almost equal amountsof plagioclase, K-feldspar and quartz, and containsminor biotite (X Fe ¼ 0.91–0.93, Ti ¼ 0.01–0.04 p.f.u.),sillimanite and garnet (X Fe ¼ 0.98–1.00) (Fig. 3d),with a weakly developed preferred orientation of thebiotite and sillimanite; modes are given in Table 1. K-feldspar (0.10–0.25 mm in size) occurs in the form ofirregular grains embayed with quartz and plagioclase.Commonly, the intensity of quartz and plagioclaselobes correlates well with highly cuspate irregularforms of corroded relics of K-feldspar (Fig. 4e). Similarly,the relics of irregular plagioclase (0.05–0.15 mmin size; An 6)10 in the core and An 0)4 at the rim) showcuspate boundaries, but with curvature less pronouncedthan that of the corroded relics of K-feldspargrains. An important feature is the presence of newplagioclase (An 0)1 )–K-feldspar intergrowths embayingcorroded relics of K-feldspar grains (Fig. 4f). Quartz(0.04–0.07 mm) with highly lobate boundaries is uniformlydistributed in the rock. Biotite of low aspectratio shows highly corroded cuspate forms filled withquartz, K-feldspar and plagioclase.(b)(c)1 mm1 mmType II1 mmType IIISummary of modal changesModal composition of the feldspar aggregates in thetype II migmatite does not change significantly comparedwith the type I orthogneiss. However, the typeIII migmatite is characterized by an important increasein quartz content in feldspar domains (up to 30 modal%)associated with a slight increase in interstitialplagioclase in K-feldspar-rich domains and K-feldsparin plagioclase-rich domains. The proportions of thefelsic minerals are equal in the type IV migmatite.(d)KfsPlQtzBt,Sill,Grt1 mmType IVFig. 3. Representative digitalized microstructures (XZ sections)for individual textural types (note differences in scales whenmaking comparisons). (a) Banded orthogneiss (type I) with distinctmonomineralic layers composed of a polygonal mosaic ofwell-equilibrated plagioclase, K-feldspar and quartz polycrystallineribbons separated by discrete layers of biotite ± sillimanite± garnet (sample PH60/B). (b) Stromatic migmatite (type II)composed of K-feldspar-rich, plagioclase-rich and quartz-richaggregates separated by relicts of biotite ± sillimanite-rich layers(sample PH60/A). (c) Schlieren migmatite (type III) showingalternation of K-feldspar- and plagioclase-rich domains interpretedto correspond to an original spatial distribution (K-feldspardomain is shown, sample PH90). (d) Isotropic nebuliticmigmatite without any gneissosity (type IV) composed of equalamounts of K-feldspar, plagioclase and quartz (sample PH59/D).Ó 2007 Blackwell Publishing Ltd321