Petrology and Evolution of an Archaean Metamorphic Aureole in the Slave Craton, Canada

Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are ( I ) consistent development of a discordant, pervasive S, axial-plane foliation; (2) re-orientation of S, axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D, static thermal overprint; and (4) relatively common development of retrogressive, E-Wtrending, mylonitic shear zones.Peak metamorphic conditions in excess of 800°C at 900 100 MPa (9 kbar) were attained at one locality following, but probably close to the time of D, folding. D, took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 f 50°C and pressures of 700MPa (7kbar) were attained in an amphibolite-facies event, presumably associated with the widespread granulite to amphibolite-facies metamorphism and intense deformation involved in the formation of the Rayner Complex a t about 1100 Ma. The area was subsequently subjected to near-isothermal uplift.Rb-Sr isotopic data indicate that the pervasive D, fabric developed at about 240&2500Ma, and this age can be further refined to 2456': Ma by concordant zircon analyses from a syn-D, c) 1983 Blackwell Scientific Pubhcdtions 02634929/83/1200-0277 $02.00 pegmatite. All zircons were affected by only minor (<7-10%) Pb loss and/or new zircon growth during the Rayner event at about 1100 Ma. Thus the 45&8SOpg/g U concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite a t 522 f 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the I100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions.A good correlation is found between Rb-Sr total-rock ages and rock fabric. U-Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.

Section: Post-d Assemblagessupporting

confidence: 89%

Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica

Black

James

Harley

1983

“…Thin metasedimentary layers that contain quartz + biotite + clinopyroxene + plagioclase ± hornblende ± orthopyroxene occur locally, and are interpreted to be the granulite facies equivalent of rare, but widespread, high CaO/K 2 O calc‐silicate, or tuff, layers described in other Archean turbidite sequences, e.g. the Quetico subprovince (Pirie & Mackasey, 1978), and the Slave province (Ramsey & Kamineni, 1977). There are some minor differences in the north‐eastern Ashuanipi subprovince, garnet is common in the metagreywacke layers there, and sequences of alternating, thinly bedded quartz‐rich and iron‐rich layers are more common (Lapointe, 1996).…”

Section: Local Geologymentioning

confidence: 99%

Large‐scale melt‐depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

Guernina¹,

Sawyer²

2003

153

This study uses field, petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane, and to determine what the grain-and outcrop-scale melt-flow paths were during the melt segregation process. The Ashuanipi subprovince, located in the north-eastern Superior Province of Quebec, is a large (90 000 km 2 ) metasedimentary terrane, in which > 85% of the metasediments are of metagreywacke composition, that was metamorphosed at midcrustal conditions (820-900°C and 6-7 kbar) in a late Archean dextral, transpressive orogen. Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase ¼ melt + orthopyroxene + oxides. Using melt ⁄ orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting.The metagreywackes are enriched in MgO, CaO and FeO t and depleted in SiO 2 , K 2 O, Rb, Cs, and U, have lower Rb ⁄ Sr, higher Rb ⁄ Cs and Th ⁄ U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt %, granitic melt from typical Archean low-grade metagreywackes. A simple mass balance indicates that about 640 000 km 3 of granitic melt was extracted from the depleted granulites.The distribution of relict melt at thin section-and outcrop-scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks pervasive grain boundary flow of melt occurred along the layers for a few, to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits, visible as the net-like array of leucosomes in some outcrops. The leucosomes contain very little residual material (< 5% biotite + orthopyroxene) indicating that the melt fraction was well separated from the residuum left in situ as melt-depleted granulite. Only 1-3 vol percentage melt remained in the melt-depleted granulites, hence, the extraction of melt generated by biotite dehydration melting in these granulites, was virtually complete under conditions of natural melting and strain rates in a contractional orogen.

“…In a number of Ms+Crd+St+Bt±And assemblages, microtextures indicate a protracted and sometimes complex history of mineral growth and deformation events (e.g. Ramsay & Kamineni, 1977; Thompson & Bard, 1982; Verhoef et al ., 1984 ; de Bresser et al ., 1986 ; Kriegsman et al ., 1989 ; Pouget, 1991).…”

Section: Features Of Reported Occurrences Of Ms+bt+crd+stmentioning

confidence: 99%

Polymetamorphic origin of muscovite+cordierite+staurolite+ biotite assemblages: implications for the metapelitic petrogenetic grid and for P–T paths

Pattison¹,

Cheney²

1999

Metapelites containing muscovite, cordierite, staurolite and biotite (Ms+Crd+St+Bt) are relatively rare but have been reported from a number of low‐pressure (andalusite–sillimanite) regional metamorphic terranes. Paradoxically, they do not occur in contact aureoles formed at the same low pressures, raising the question as to whether they represent a stable association. A stable Ms+Crd+St+Bt assemblage implies a stable Ms+Bt+Qtz+Crd+St+Al2SiO5+Chl+H2O invariant point (IP1), the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure. Petrogenetic grids calculated from internally consistent thermodynamic databases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data. Topological analysis of invariant point IP1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point. These constraints are then compared with natural data from contact aureoles and reported Ms+Crd+St+Bt occurrences. We find that there are numerous topological, textural and compositional incongruities in reported natural assemblages that lead us to argue that Ms+Crd+St+Bt is either not a stable association or is restricted to such low pressures and Fe‐rich compositions that it is rarely if ever developed in natural rocks. Instead, we argue that reported Ms+Crd+St+Bt assemblages are products of polymetamorphism, and, from their textures, are useful indicators of P–T paths and tectonothermal processes at low pressure. A number of well‐known Ms+Crd+St+Bt occurrences are discussed within this framework, including south‐central Maine, the Pyrenees and especially SW Nova Scotia.