Quantitative structural analyses and numerical modelling of ...

ORIGIN OF FELSIC MIGMATITES 37(a)Pl(An25)(b)QtzQtzQtzQtzAn 10-20An 10-20An 10-20QtzQtzKfsPl(An25)PlKfsQtzKfsKfsQtzQtzQtz0.5 mm100 µm(c)Pl core(An 12-16 )QtzKfs(d)PlKfs(An 15 )(An 0-4)Pl rim(An 1-4 )KfsPl rim(An 1-4)Pl(An 12-16 )(An 1-4 )QtzPlrelict grain(An 12-16)100 µmKfsPlQtzPl(An15)QtzPlQtzKfsQtz200 µmFig. 5. SEM backscatter images showing the inferred former melt topology (note differences in scales when making comparisons). (a)Type I banded orthogneiss: interstitial plagioclase (An 10)20 ), representing the plagioclase component crystallized from the anatecticmelt (grey arrow), tracing the K-feldspar boundaries sub-perpendicular to the foliation (sample PH60/B). Black arrows show smallrounded quartz grains crystallized along feldspar boundaries. (b) Type I banded orthogneiss: inferred former melt pools with cuspatemargins in a plagioclase band (sample PH60/B). The former melt has crystallized to K-feldspar (cuspate melt pools), plagioclase(growing on the old plagioclase grains) and quartz (forming small rounded grains along the feldspar boundaries (black arrow)). (c)Type III schlieren migmatite: more developed interstitial plagioclase (grey arrow) with normal zoning (core ¼ An 12)16 ; rim ¼ An 1)4 )and distinct albite rims (An 1)4 ) on relict feldspar grains (white arrow) (sample PH90). The interstitial plagioclase is not in opticalcontinuity with any residual plagioclase grains adjacent to it and does not show any preferred orientation, in contrast to plagioclase intypes I and II. (d) Type III schlieren migmatite: new plagioclase inferred to have crystallized from melt (growing on an old plagioclasegrain in the form of the discrete albite rims (white arrow)) and quartz grains that resorb relict K-feldspar grains (sample PH14/D).Fig. 4. Photomicrographs showing characteristic textures of the rock sequence (note differences in scales when making comparisons).(a) Type I banded orthogneiss: recrystallized K-feldspar aggregate with straight grain boundaries and numerous smaller roundedquartz grains (white triangles) along the boundaries or in the cores of feldspar (sample PH60/B). (b) Type I banded orthogneiss: welldevelopedplagioclase polygonal foam-like texture with straight grain boundaries, interstitial quartz (white triangles) and biotite(sample PH60/B). (c) Type III schlieren migmatite: typical microstructure with irregularly shaped feldspar and quartz grains withhighly lobate boundaries. Myrmekitic aggregates commonly develop along the K-feldspar boundaries (black arrow). New smallinterstitial plagioclase (grey triangles), K-feldspar and quartz (white triangles) grains trace almost all the relict feldspar boundaries.Interstitial quartz forms preferentially rounded shapes different from plagioclase which forms thin elongated grains/films coatingK-feldspar boundaries (sample PH90). Such a microstructure is typical also for the type IV. (d) Type III schlieren migmatite: irregularcuspate K-feldspar grain embayed with newly crystallized quartz and plagioclase (sample PH90). (e) Type IV nebulitic migmatite:corroded relics of K-feldspar grains (sample PH59/D). (f) Type III nebulitic migmatite: plagioclase-K-feldspar intergrowths embayingrelict K-feldspar grain (sample PH14/D). White arrows in (d), (e) and (f) point to irregular embayments of relict K-feldspar originatedthrough resorption of old K-feldspar grains by newly crystallized material.Ó 2007 Blackwell Publishing Ltd323