Structural and metamorphic relations between low, medium and high grade rocks, Mt. Franks - Mundi Mundi area, Broken Hill, N.S.W.

Abstract

Investigations in the northwestern part of the Willyama Complex centred on the Mt Franks - Mundi Mundi area have established a 4 km thick stratigraphic section of conformable metasediments containing thin horizons of basic volcanics in the lower two - thirds of the sequence. Establishment of this sequence was only possible once it was shown that the dominant lithological layering in metasediments is bedding, and that there has been no mesoscopic transposition during deformation. The metasediments represent a sequence of clay sands deposited in a distal shelf-slope or basin type of environment. A sequence of deformational and metamorphic events established in these rocks is regarded as an expression of the Middle Proterozoic Olarian Orogeny ( c.1695 - 1520 Ma. ) and except for some reactivation of shear zones, predate deposition and deformation of the unconformably overlying Adelaidean sediments. The important D₁ deformation is a complex, progressive event with pre-S₁ static mineral growth (biotite, andalusite, sillimanite, white mica) and early minor micro-folding recognized before syn-S₁ growth and F₁ folding. An even earlier period of pre-S₁ fabric formation mainly defined by white mica, biotite and ilmenite, is not related to any visible folding and may either represent an earlier discrete event or an early phase of the D₁ event, However, as now defined, minerals outlining this pre-S₁ fabric are related to the D₁ event. The low, medium and high grade metamorphic zones defined in the field by biotite, andalusite and sillimanite respectively are pre-S₁ in age and predate F₁ folding. The intensity of metamorphism increases with depth so that there is a broad depth control on metamorphism. Relations at the andalusite/sillimanite isograd conform to a Carmichael (1969) type model and reactions took place via an intermediate sericite phase. The main effect of F₁ folding is the formation of the variably plunging variably oriented Kantappa - Lakes Nob Syncline of regional extent. Only the western limb of this fold is now visible over much of its length. This fold deforms existing metamorphic zones and thus controls the relationship of low, medium and high grade rocks in this part of the Willyama Complex. The orientation of this syncline changes from vertical in the low grade rock to inclined at depth. The western limb becomes overturned at depth so that subsequent folds are downward facing. There is also a change in fold tightness with depth - from open-tight in the low grades to tight-isoclinal in the high grades, and this is accompanied by a change in S₀/S₁ relations (from core to limb area) from non parallel to parallel. These changes are coupled with a rotation of extension direction (mass transfer direction) from subvertical to inclined and may be explained by original formation and subsequent modification of upright F₁ folds. Later modifications are recorded by open folding and overturning of S₁ - this is ascribed to a final phase of the D₁ event. Mineral growth in D₁ time resulted in the formation of S₁ varying in grade from muscovite + quartz to sillimanite. S₁ varies from homogeneous to layered, and in the latter case, consists of M + QM layers,the spacing of which is controlled by F₁ microfolding. S₁ formation involved rotation, mass transfer, and volume decrease in M layers and (re) crystallisation. The D₂ event in this area was of only minor significance. The D₃ event developed in response to NW-SE shortening and resulted in the formation of variably plunging, vertical northeast trending folds. Where SW plunging, these folds lie subparallel to L₁. The nature of the D₃ event is controlled to a large extent by S₀ / S₁ relations and folding of S₁ across unfolded S₀ occurs where S₀ lies parallel to the XY plane of the D₃ event. S₃ formed as a muscovite + quartz schistosity by rotation, re crystallisation, mass transferand mimetic growth. During the final stages of the D₃ event, north-east trending retrograde schist zones were formed. These were later reactivated during the folding of the Adelaidean. The final phase of the Olarian Orogeny consists of minor D₄ folding and crenulation.