The processes of melt segregation, magma ascent and pluton emplacement in the continental crust of the Damara Belt, Namibia

Hall, Duncan James (2016-03)

Thesis (DSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Theoretical models of meso- and macroscale granitic melt/magma transport commonly invoke a prominent role for dilatant fracturing of the wall rocks, but supporting field evidence is in many cases only scantly presented. This dissertation is a compilation of three case studies that integrate some of the theoretical models with field-based aspects from the obliquely exposed south Central Zone magmatic arc of the Damara Belt in Namibia. Chapter 3 describes interconnected leucogranite networks that accommodate the transport of melt at the outcrop-scale in homogeneous, high-grade near-source gneisses in the western parts of the south Central Zone. The net-structured leucogranite networks include small intrafolial stringers, larger shear-band-hosted veins and still larger fracture-hosted leucogranite sheets, which display a size-based hierarchical distribution that results from the self-organisation of the near-source melt transport pathways. The development of the networks is controlled intrinsically by variations in the rate of melt supply. During periods of elevated melt supply and under conditions of low differential stress, melt-induced extensional fracturing generates hydraulic gradients that result in the formation of melt sheets which represent potential far-field ascent conduits (dykes sensu lato). However, the orientations of the sheets are controlled by wall-rock structures and regional and local stress fields, resulting in this case in the preservation and eventual crystallisation of shallowly-dipping leucogranite sheets that were unfavourably orientated to accommodate buoyancy-driven melt/magma drainage. Chapter 4 describes the actual drainage of magma along steeply-dipping fracture conduits initiated in well-layered upper amphibolite-facies basement gneisses ~30 km SE of the outcrops presented in chapter 1. Here, the moderately-dipping gneisses contain large-scale leucogranite networks that include deca- to hecto metre-scale subvertical leucogranite lenses interpreted to document the near-source accumulation of magma necessary for efficient far-field ascent. Field relationships also document the drainage of the subvertical lenses at critical lengths and volumes in excess of 100 m and ~2.4×105 m3 respectively. These values as well as the field relationships are broadly consistent with theoretical models of magma transport along mobilised (self-propagating) hydrofractures. Self-propagating hydrofractures propagate at their tips and close simultaneously along their tails, leaving behind only very subtle and easily overprinted evidence, which may account for the elusiveness of granite conduits in the mid-crust. This work presents the first field evidence of large-scale magma drainage along self-propagating hydrofractures and has important consequences for the subsequent transport of magma through and eventual incremental emplacement within the subsolidus crust. Chapter 5 examines in more detail the implications of fracture-controlled magma ascent for granite pluton emplacement in lower amphibolite-facies wall rocks ~80 km northeast of the near-source features presented in chapters 3 and 4. Three closely-spaced tabular granitoid plutons that were emplaced roughly contemporaneously at widely-varying structural levels are investigated. Each of the plutons demonstrates the effect of mechanical contrasts in the wall-rock that favour fracture arrest rather than ascent, particularly rigidity and rheological contrasts. However, contrary to the theoretical models, it is shown that rigidity contrasts which cause fracture arrest need not be associated with lithological contacts and the role of increasing differential stress during the fracture-controlled ascent of magma towards the brittle-to-ductile transition is highlighted. The three chapters outlined above highlight the importance of fracture-controlled magma transport at all scales in the Damara Belt and substantiate some of the theoretical concepts, but also highlight prominent discrepancies between the models and natural systems.

AFRIKAANSE OPSOMMING: Teoretiese modelle van meso- en makroskaalse granitiese smeltsel/magma vervoer roep oor die algemeen ‘n prominente rol van dilatante breukvorming van die wandgesteentes, maar ondersteunende veldbewyse is in baie gevalle op ‘n onvoldoende wyse voorgestel. Hierdie proefskrif is ‘n samestelling van drie gevallestudies wat teoretiese modelle en veld gebaseerde aspekte vanuit die skuins ontblote suid-Sentrale Sone magmatiese boog van die Damara Belt in Namibië integreer. Hoofstuk 3 beskryf ‘n onderlinge verbindings netwerk van leukograniet wat die vervoer van smeltsel op dagsoomskaal akkommodeer in hoë-graad naby-bron gneise in die westelike dele van die suid-Sentrale Sone. Die netgestruktureerde leukograniet netwerke sluit in klein intrafoliese smeltselstringe, groter aar-draende skuifskeursones asook groter breukaangedrewe leukograniet plate wat grootte ‘n gebaseerde hiërargiese-verspreiding vertoon as gevolg van die self-georganiseerdheid van die naby-bron smeltsel vervoer bane. Die ontwikkeling van die netwerk is intrinsiek beheer deur variansie in die tempo van smeltsel-toevoer. Smeltsel-geïnduseerde uitstrekkingsbreuking tydens periodes van lae differensiële spanning en verhoogde smeltsel toevoer genereer groot hidroliese gradiënte wat die vorming van groot smeltsel-plate dryf en gevolglik vêr-veld magma dreinerings leikanale kan vorm (gange sensu lato). Egter, die orientasie van die resultante breuke en gange word beheer deur strukture in die wandgesteentes asook regionale en lokale spanningsvelde, wat gevolglik lei tot die ontwikkeling en uiteindelike stolling van die vlak-inklinasie smeltsel plate wat ongunstig georienteer was om druifaangedrewe smeltsel dreinering te akkommodeer. Hoofstuk 4 adresseer die werklike dreinering van magma deur steil leikanale/gange wat in goed-gelaagde boonste amfiboliet fasies vloergneise sowat ~30 km SO van die dagsome wat aangebied is in hoofstuk 1. In die geval van die matig-hellende vloergneise met grootskaalse leukograniet netwerke, word deka- tot hektometerskaalse subvertikale leukograniet lense ingesluit en word geïnterpreteer om naby-bron akkumulasie van magma wat noodsaaklik is vir doeltreffende vêr-veld vervoer te dokumenteer. Veld verhoudings dokumenteer onderskeidelik ook die dreinering van die subvertikale lense teen kritiese lengtes en volumes in oormaat van 100 m en ~2.4×105 m3. Hierdie waardes sowel as die veld verhoudings is oor die algemeen in ooreenstemming met teoretiese modelle van magma vervoer deur middel van gemobiliseerde self-propagerende hidrobreuke. Self-propagerende hidrobreuke propageer by hul toppunte en maak terselfdertyd toe by hul sterte en los dus baie subtiele en maklik-oordrukte bewyse agter wat moontlik rekening staan vir die ontwynkendheid van graniet leikanale in die middel-kors. Hierdie verteenwoordig die eerste veld gebaseerde bewyse van grootskaalse magma dreinering deur self-propagerende hidrobreuke en het belangrike gevolge vir die daaropvolgende vervoer van die magma deur en die uieindelike inkrementele inplasing daarvan binne in die subsoliduse kors. Hoofstuk 5 ondersoek in meer detail die implikasies van breuk-beheerde magma styging vir graniet pluton inplasing in laër-amfiboliet fasies wandgesteentes ~80 km noordoos van die naby-bron strukture wat in hoofstukke 3 en 4 bespreek is. Drie nou-gespasieerde tabletvormige granietiese plutone wat ongeveer gelyktydig ingeplaas was teen wyd uiteenlopende strukturele vlakke in wandgesteentes is ondersoek. Elkeen van die plutone demonstreer die effekte van meganiese kontraste in die wandgesteentes wat breuk propagasie tot stilstand bring eerder as om voorsetting van propagasie te bevoordeel. Meer spesifiek, die drie plutone beklemtoon die rolle wat vertolk word deur vervormingvastheid en reologiese kontraste. Egter, in een geval is dit gewys dat die meganiese kontraste wat breuk propagasie stopsit word nie noodwendig geassosieer met litologiese kontakte nie, soos aanbeveel deur teoretiese modelle van magma inplasing, deur die belangrikheid van toenemende differensieële spanning te beklemtoon soos wat temperature langs die stygingspad die brosheid-smeebaarheids-oorgang bereik. Die drie hoofstukke uiteengesit hierbo beklemtoon die belangrkheid van breukbeheerde magma vervoer op alle skale in die middel-kors van die Damara Belt en substansieer sekere van die teoretiese konsepte, maar lig ook uit prominente teenstrydighede tussen die modelle en natuurlike stelsels.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98512
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