Damara Belt, Central Zone

Melting and melt transfer in the Swakop and Khan river areas. Post-conference field excursion, Granulites & Granulites 2015, led by Alex Kisters and Johann Diener. A file showing the stops and route for use with Google Earth is available here: Namibia-Post-Conf-Damara.kmz
Swakop and Khan valleys (Google Earth) Overview of the area visited over the central three days of the excursion, exposing typical rock associations of the Central Zone of the Damara Belt Lithologies: basement (Abbabis Complex) Varied gneisses in the Khan River section Lithologies: basement Grey and mafic gneisses enveloped by leucogranite, Khan River section Lithologies: Etusis Formation Orange-red meta-arkosic sediments, dipping towards viewer, overlying basement gneiss, Swakop River Lithologies: Etusis Formation Etusis meta-arkose permeated by red granite sheets
Lithologies: Etusis & Khan Formations Grey-green Khan Formation (with leucogranite sheets) overlying Etusis, cut by dolerite dyke Lithologies: Khan Formation Interbanded schist and diopside-hornblende calc-silicate rocks Lithologies: Khan Formation Schist and calc-silicate rock with sub-concordant pink pegmatite sheet Lithologies: Khan Formation Lens of diopside-rich calc-silicate rock with hornblende and feldspar-rich corona Lithologies: Khan Formation Boudin neck in hornblende-rich calc-silicate with quartz and a tabular oxide mineral
Lithologies: Khan Formation Diopside-rich calc-silicate band in rusty-weathering schist, with boudin necks occupied by hornblende and very coarse white feldspar Lithologies: Khan Formation Boudinage and fluid-rich hornblende + feldspar 'blows' associated with calc-silicate layer in schist Lithologies: Khan Formation Detail of hornblende-feldspar segregation adjacent to calc-silicate band Lithologies: Khan Formation Thick diopside-rich calc-silicate unit Lithologies: Khan Formation Skarn-like calc-silicate association enveloped by leucogranite, Holland's Dome area, Swakop valley
Lithologies: Khan Formation Detail of garnet fels in skarn association Lithologies: Khan Formation Large radiating cluster of scapolite crystals in skarn association Lithologies: Khan Formation Large scapolite crystals adjacent to white leucogranite vein Khan River gorge near Rossing Uranium mine Localities for the lithologies and other features in the higher formations of the Swakop Group Lithologies: Chuos Formation Foliated diamictite with clasts of various sizes
Lithologies: Chuos Formation Foliated and deformed diamictite with clasts of differing lithologies Lithologies: Chuos Formation Banded iron formation horizons within Chuos Fm Lithologies: Chuos Formation Transposed fabric defined by flattened and folded clasts in a tight fold hinge zone Lithologies: Arandis Formation schist Well-foliated metapelites. The overlying Karibib marble is just visible at top left Lithologies: Arandis Formation schist Garnet-biotite(-cordierite) schist, detail
Lithologies: Arandis Formation schist View oblique to foliation surfaces, showing pelitic/semipelitic layering and steep-plunging lineation Lithologies: Karibib Formation Chuos Formation in foreground (dark, with leucogranite sheets), white Karibib marbles forming ridge in background, cut at the top by dolerite dyke Lithologies: Karibib Formation Ductile folding in calc-silicate layers Lithologies: Karibib Formation Disharmonic folding pattern in calc-silicate layers Lithologies: Karibib Formation Detail of banding. The prominent brown mineral is most likely an andraditic garnet
Lithologies: Karibib Formation Detail of banding. The prominent brown mineral is most likely an andraditic garnet Lithologies: Karibib Formation Nodular calc-silicate layer in marble Lithologies: Karibib Formation Alternating marble and calc-silicate layers, at top of Karibib Formation Lithologies: Kuiseb Formation Porphyroblastic cordierite schist Lithologies: Kuiseb Formation Porphyroblastic cordierite schist
Melting in situ: Khan Formation calc-pelite Patchy leucosomes with peritectic clinopyroxene Melting in situ: Khan Formation calc-pelite Foliation boudinage, with leucosomes occupying boudin necks, with connections along the foliation Melting in situ: Khan Formation calc-pelite Leucosomes aligned along fabric and in zones of asymmetric shear Melting in situ: Kuiseb Formation pelite Stop 3.2. Small patches of leucosome material, apparently forming isolated augen. The following photographs display related melting features in the Kuiseb Formation at this locality Melting in situ: Kuiseb Formation pelite Same outcrop: in three dimensions it is seen that these small leucosomes are rod-like features extending along the steep-plunging lineation, an extension direction parallel to the hinge-line of a regional fold
Melting in situ: Kuiseb Formation pelite Another outcrop view from the same locality (Stop 3.2b) showing both rod-like and patch leucosomes Melting in situ: Kuiseb Formation pelite Patch leucosome distorts the pattern of the earlier rod-like structures in a manner that suggests melt loss from and collapse around the patches Melting in situ: Kuiseb Formation pelite Patch leucosome (detail) distorts the pattern of the earlier rod-like structures in a manner that suggests melt loss from and collapse around the patches Melting in situ: Kuiseb Formation pelite Top view of outcrop showing ptygmatic veins that indicate shortening normal to the rod-like leucosomes and to the axial trace of the large-scale fold Melting in situ: Kuiseb Formation pelite Certain of the larger, later leucosomes here contain large peritectic garnets and locally form networks
Melting in situ: Kuiseb Formation pelite Detail of garnet from leucosome of previous photograph Melting in situ: Kuiseb Formation pelite Leucosome in cordierite schist, with poikiloblastic garnet partly in leucosome, partly in host rock Melting in situ: Kuiseb Formation pelite Poikilitic garnet in K-feldspar-rich leucosome, suggesting dehydration-meling of biotite and subsequent melt loss Melting in situ: Kuiseb Formation pelite Context for previous photo: continuous steep garnet-poor leucosome vein, sub-parallel to fabric, with sub-horizontal spurs containing large garnets Melting in situ: Kuiseb Formation pelite Detail of poikilitic garnet in feldspathic leucosome
Melting in situ: Kuiseb Formation pelite Array of garnet-poor leucosomes in uniform cordierite schist Melting in situ: Kuiseb Formation pelite Leucosomes approximately aligned along stretching lineation on foliation surface of pelitic schist Melting in situ: Khan Formation semipelite Partly connected patch-like leucosomes in outcrop also cut by far-travelled leucogranitic sheets Melting in situ: Khan Formation semipelite Detail from outcrop in previous photo. The dark mineral in leucosomes is biotite, which does not appear to be pseudomorphing garnet or cordierite, so these are unlikely to be the product of dehydration melting Melting in situ: Khan Formation semipelite A nearby outcrop shows a pattern of largely unconnected lenticular leucosomes, probably aligned normal to the local extension direction
Structural control of melt transfer Vertical melt vein with horizontal spurs and lenticular leucosomes in cordierite schist Structural control of melt transfer Broader view of vertical melt vein with horizontal spurs and lenticular leucosomes in cordierite schist - see next photo for explanation Structural control of melt transfer Alex Kisters demonstrating the regional stretch along a steep-plunging lineation in these vertical strata, accounting for the arrangement of vertical melt-transfer sheets with spurs in extensional fissures Structural control of melt transfer Fallen block showing fabric-parallel melt vein with spurs filling space formed by stretch and incipient foliation boudinage Structural control of melt transfer Vertical strata with leucogranitic veins a few tens of metres long, tapering and terminating above and below, interpreted as evidence for melt transfer by hydraulic fracture
Structural control of melt transfer Early leucosomes deformed by pinch-and-swell and ductile boudinage in less competent Khan Formation schist matrix Structural control of melt transfer Detail of previous view, showing rock microstructure: note coarse igneous texture of leucosome Structural control of melt transfer Pegmatitic material filling brittle boudin neck of calc-silicate band, with ductile behaviour of enveloping schist Local melt-rock interaction Discordant pegmatitic vein in Khan formation biotite schist has a zone about 3 cm wide of bleached and coarsened host rock along its contacts Local melt-rock interaction Boudinaged psammite bands in cliff outcrop of schist cut by several generations of pink and white granitic and pegmatitic veins
Local melt-rock interaction Left-hand end of psammite lens. Rock above and below is permeated by pegmatitic material. Lower half of psammite has grain size about 3x that of upper part, suggesting permeation and modification by fluid-rich melt Local melt-rock interaction Further detail of left-hand end of psammite lens. Rock above and below is permeated by pegmatitic material. Lower half of psammite has grain size about 3x that of upper part, suggesting permeation and modification by fluid-rich melt Injected material: rock types Coarse biotite pegmatite Injected material: rock types Tourmaline cluster in pegmatitic leucogranite. Tourmaline leucogranites seem relatively uncommon in the area. The next few photos explore this example. Injected material: rock types Aggregate of tourmaline and coarse feldspar
Injected material: rock types Cordierite nodule in leucogranite Injected material: rock types Tourmaline and garnet in leucogranite Injected material: rock types Unusually tourmaline-rich leucogranite, also with garnet Injected material: rock types Tourmaline surrounding cordierite nodule Granitic vein networks Intersecting and interconnecting horizontal and vertical sheets, in cliff outcrops of the lower Khan river
Granitic vein networks Calc-schist outcrop with irregular network of pale pink pegmatitic granite in broad and narrow veins Granitic vein networks Leucogranite sheets with moderate dips cut steep-dipping gneiss Granitic vein networks Leucogranite sheets with moderate dips cut steep-dipping gneiss, in which the foliation in screens and fragments is distorted and crumpled Granitic vein networks Intriguing rheological indicators: lobate margins on irregular granite sheet; irregular folding of narrow veins in host gneiss that shows ductile open-folded fabric; apparent connectivity between thick and thin granitic veins Granitic vein networks Complex cross-cutting relationships shown by aplitic and pegmatitic leucogranite veins
Granitic vein networks Unexpected crosscutting relationship: pegmatitic sheet apparently postdates parallel-sided quartz vein Granite bodies in carbonate host rock White leucogranite (centre of photo) forms a massive vertical lens in white banded marbles. There is little tendency for granite to permeate or form continous sheets in marble Granite bodies in carbonate host rock Isolated, folded lens of leucogranite in marble Large-scale granitic networks Khan Formation in Swakop River: shallow-dipping sub-concordant sheets Large-scale granitic networks Group of somewhat irregular vertical sheets, sub-concordant in vertical strata. In Kuiseb Formation, Khan River.
Large-scale granitic networks One of a number of tapering vertical concordant sheets, inflating a vertical foliation-parallel fracture. This one contains numerous xenoliths. Khan River section, Stop 3.3d Large-scale granitic networks Contrasting styles at Stop 3.3d. Thin concordant vertical veins on the right, but broad cupola on the left appears to be ponded beneath an antiformal parasitic fold in Kuiseb Formation schists. Khan River section Large-scale granitic networks Thin tapering vertical sheets, inferred to be emplaced and propagating by hydraulic fracture. Kuiseb Formation in Khan River Large-scale granitic networks Vertical lenticular vein of leucogranite a few metres long cutting flat-lying migmatitic gneiss in cliff outcrop in southern part of Khan River gorge. Geometry is consistent with hydraulic fracture Large-scale granitic networks Network of leucogranite sheets in two or more orientations: a possible conjugate set, relatively shallow-dipping, and a few more vertical ones. None are related to the steep orientation of the host rock (Chuos Formation, Khan River section)
Large-scale granitic networks Oblique view towards NE in Google Earth, showing ponding of abundant leucogranite in sheeted bodies beneath antiformal cap of Karibib marbles. Stop 3.3e in centre of view, Khan river gorge behind. Brown line is dolerite dyke Swakop gorge, upstream part Localities for many of the melt networks featured in the following photos Large-scale granitic networks Abundant irregular but mostly shallow-dipping sheets in Khan Formation, Swakop Valley, Stop 4.1 Large-scale granitic networks Same face as previous photo, further to the left: here are large vertical pathways merging with horizontal sheets Large-scale granitic networks Anastomosing sub-horizontal sheets in Khan Formation, Stop 4.1. Fabric in host rock is approximately vertical; no leucogranite sheets follow this orientation
Large-scale granitic networks Thick lens of leucogranite in schists beneath the Arcadia thrust, Stop 4.2a. Large-scale granitic networks Cliff face containing the Arcadia thrust. Leucogranite lens of previous photo is at base. Above are subhorizontal sheets with lenticular and pinch-and-swell structure, parallel to the fabric in dark schist. Paler rock above in vertical cliff is basement gneiss in the hanging wall of the thrust with granite veins in steep or conjugate fractures. Granite veins can be traced across the thrust and are interpreted as syn-thrusting, their orientations controlled by the deformation and the properties of the host rock. Large-scale granitic networks Detail of granitic vein patterns across the Arcadia thrust. See also previous photograph for context. Large-scale granitic networks Cliff outcrop at Stop 2.4, lower Khan River gorge. Network of granite veins in varied metasediments. There are disrupted and rotated blocks in dark layer at upper left, thicker vertical granite masses at right, and a megabreccia-like jumble of large rotated country-rock blocks at lower right Large-scale granitic networks Detail of vertical granite channel in Khan River cliff
Large-scale granitic networks A variety of styles and orientations of granitic sheets in the Khan River cliff at Stop 2.4 Large-scale granitic networks Stop 2.4 around the corner - cliff face approximately at 90 degrees to first photos. Mostly horizontal sheets, but connected by vertical channels Large-scale granitic networks Detail of the megabreccia on the front face showing misorientated blocks separated by network of relatively narrow granite veins (melt having been lost from this zone?). Some blocks are folded. Large-scale granitic networks Jumble of calc-silicate blocks enveloped by leucogranite, Khan Formation in side branch of Swakop River, Stop 2.3. Large-scale granitic networks Network of melt sheets in basement rocks, Husab gorge (stop 4.4). Note large (inflated) vertical channel at right
Large-scale granitic networks Husab gorge, stop 4.4. Vertical zone of disrupted and rotated blocks set in leucogranite is interpreted as a melt channel that has deflated and lost most of its melt Large-scale granitic networks Abundant, broad, subvertical sheets of leucogranite in the Holland's Dome area (stop 4.5) Google Earth view, Holland's Dome Abundant NE-trending leucogranite sheets aligned with this antiformal structure