Aus area, south-western Namibia

Melting and melt-rock interaction in metasediments and gneisses of the Namaqualand Metamorphic Complex. Pre-conference field excursion, Granulites & Granulites 2015, led by Johann Diener and Richard White. A file showing the route for use with Google Earth is available here: Namibia-pre-conf-Aus.kmz
Setting and distribution of rock types The Festung mountain massif, as seen in Google Earth. Darker brown rocks are mainly metapelites; lighter-coloured rocks are pre-metamorphic gneisses and metapsammites with abundant melt sheets Setting and distribution of rock types Geological map of the Festung mountain from Diener & White, superimposed on the Google Earth image Setting and distribution of rock types Lower slopes are Kubub gneiss, lighter middle slopes make up a 'melt transfer zone' of psammite and semipelite with abundant leucogranitic sheets, upper slopes are dark metapelites with fewer leucogranite sheets Setting and distribution of rock types Contact between Kubub gneiss (below right) and semipelitic schist with leucogranitic sheets, in the area shown by the previous photo Setting and distribution of rock types Melt transfer zone: rock pavement in ravine showing abundant veins of partial melt in psammitic gneisses
Setting and distribution of rock types Google Earth view looking south up 'Spring Creek', showing the light-coloured rock pavements that expose mainly psammitic gneisses with abundant granitic sheets and evidence for melt-rock interaction Setting and distribution of rock types Rock pavement in 'Spring Creek' (stop 12 on the geological map), looking out to the NNE Setting and distribution of rock types Thick leucogranitic sheets marking the top of the melt transfer zone at stop 9; above are pelitic rocks with fewer granite sheets. Metapelites Aluminous metapelitic gneiss with garnet, sillimanite, biotite and cordierite, and K-feldspar-rich leucosomes showing pinch-and-swell Metapelites Aluminous metapelitic gneiss with blocky aggregates of sillimanite, interpreted as pseudomorphs after andalusite
Metapelites Face of large fallen block showing sheared and folded garnetiferous leucosomes, indicating dehydration melting of biotite, more-or-less in situ. Metapelites Detail of garnet-bearing K-feldspar-rich leucosomes seen in previous photo Metapelites Banded metasediments (psammite and pelite), with small flattened and sheared stromatic leucosomes Metapelites View of foliation surface of aluminous sillimanite-bearing metapelite with skin of garnetiferous leucosome adhering to it Metapelites Garnet porphyroblasts in leucosome material in psammitic/semipelitic gneisses. They are presumed to be peritectic products of biotite melting; they have poikiloblastic cores and inclusion-free rims
Metapelites Detail of peritectic garnets, as previous photo; note local retrograde biotite after garnet Metapelites Detail of peritectic garnets, as previous photo; note local retrograde biotite after garnet Metapelites Contorted migmatitic metapelites. Note that these leucosomes are garnet-poor. The larger leucosomes are likely to represent introduced melt Metapelites Detail of folded metapelitic migmatite. Foliation surfaces facing the camera in the lower half of the photo reveal a strong, steeply-plunging lineation defined both by dark mineral aggregates and by rodding of leucosomes Metapelites Semipelite showing a folded early leucogranitic vein and a more nebulitic garnet- and cordierite-bearing leucosome
Metabasites Loose cobble of metabasic gneiss showing oval poikiloblasts of orthopyroxene in a hornblende-plagioclase matrix Metabasites Metabasic gneiss with discordant coarse-grained 'leucosome' resulting from dehydration melting of hornblende, and containing centimetre-sized orthopyroxenes Metabasites Cross-cutting leucosome vein in metabasic gneiss, containing coarse pyroxene and feldspar Tsirub Gneiss A variety of the Tsirub gneiss with prominent feldspar augen and a strong shear fabric Kubub Gneiss Fairly typical pavement outcrop of Kubub Gneiss, showing banding and occurrence of poikilitic garnet
Kubub Gneiss Kubub Gneiss More homogeneous Kubub gneiss in which the poikiloblastic patches of garnet have coronas of biotite-free, feldspar-rich material, suggesting that these patches are the products of dehydration melting of biotite Kubub Gneiss Detail of poikilitic garnet patches in Kubub gneiss. Commonly the garnet is partly altered to biotite Kubub Gneiss Boulder of Kubub gneiss showing both garnet patches in feldspar pools and continuous garnet-rich leucocratic veins, reinforcing the suggestion that the gneiss has undergone dehydration melting of biotite Kubub Gneiss Network of discordant garnet-bearing leucosomes in Kubub gneiss
Kubub Gneiss Network of discordant garnet-bearing leucosomes in Kubub gneiss Late grey granite dyke Late, discordant dyke of fine-grained grey granite cutting across sheeted migmatites. dyke margins are irregular in detail. Similar sheets have been dated at 1004 Ma, 30 Ma or more after the migmatisation. Melt-rock interaction The next sequence of photos are from 'Spring Creek' (stop 12). Here, interaction between sparsely garnet-bearing leucogranitic veins and banded psammite. Dark patches are cordierite Melt-rock interaction Interbanding of injected leucosome (commonly with selvedges) and psammite. Dark patches are cordierite Melt-rock interaction Detail of contact between garnetiferous leucogranitic sheet and psammite. Dark patches are cordierite
Melt-rock interaction Overall texture of psammitic gneiss permeated by leucogranitic sheets Melt-rock interaction Psammitic gneiss permeated by leucogranite Melt-rock interaction Psammitic gneiss permeated by leucogranite, here with cross-cutting structures Melt-rock interaction Here, cordierite selvedge on resistant psammitic band Melt-rock interaction Richard White's finger points out a 10-15 cm zone where the margin of resistant psammite has been modified to a streaky texture by interaction with external melt
Melt-rock interaction Contact between large leucogranite sheet and more resistant psammitic gneiss, showing lobate contact and internal structures in psammite that mey result from melting or melt infiltration Melt-rock interaction Contact between leucogranitic sheet, partly pegmatitic, and pelitic gneiss Melt-rock interaction Leucosome pockets, streaks and ptygmatic veins in resistant psammitic lens Resistant layers in migmatite Folded layers of dark grey psammite in semipelitic migmatite Resistant layers in migmatite Thin, folded mafic bands in semipelitic migmatite
Resistant layers in migmatite Zoned mafic pod, with large garnets, in semipelitic migmatite Zones of melt transfer Johann Diener pointing out different styles of leucogranitic sheet and leucosome in the melt transfer zone at stop 9 Zones of melt transfer Detail of styles of leucogranitic sheet, stop 9 Zones of melt transfer Detail of styles of leucogranitic sheet, stop 9 Zones of melt transfer Vertical cross-section, normal to regional lineation, through psammitic gneisses permeated by leucogranitic sheets, stop 6
Zones of melt transfer Small-scale cross-cutting relationships in leucosomes and veins in semipelitic gneiss Zones of melt transfer Small-scale cross-cutting relationships in leucosomes and veins in semipelitic gneiss Zones of melt transfer Leucogranitic sheets, veins and leucosomes in pelitic gneiss Zones of melt transfer Complex cross-cutting relationships in semipelitic migmatite. Grey fine-grained granite vein probably belongs to latest suite dated at 1004 Ma Zones of melt transfer More complex cross-cutting relationships in semipelitic migmatite, stop 12
Zones of melt transfer Discordant garnetiferous granite sheet with pegmatitic margin Zones of melt transfer Network of discordant leucogranite sheets in 'Spring Creek', stop 12 Zones of melt transfer Network of discordant leucogranite sheets in 'Spring Creek', stop 12 Detail of melt sheets Sillimanite schlieren in cordierite-bearing leucogranite Detail of melt sheets Schlierien of biotite and garnet in leucogranite
Detail of melt sheets Schlieren of biotite and garnet in leucogranite (rock above coin split parallel to banding) Detail of melt sheets Coarse leucogranite with fine garnets Detail of melt sheets Fine, slightly porphyritic leucogranite with garnet Detail of melt sheets Leucogranite (blueish feldspar) containing large patches of cordierite with some garnet