Structural Evidence of Parent Rocks in High-Grade Metamorphic Areas - especially Broken Hill

Ron H. Vernon, School of Earth Sciences, Macquarie University, Sydney, NSW 2109

Introduction

Correct interpretation of parent rock-types is critical for inferring ore environments. However, metamorphic reactions, recrystallization and deformation obscure original structures in high-grade metamorphic areas, especially those heated for relatively long periods or repeatedly heated. Nevertheless, many amphibolite and granulite facies rocks in the Australian Proterozoic, including the Broken Hill Block, have residual sedimentary and igneous mesoscopic and microscopic structures in zones of relatively low strain and/or areas heated for relatively short periods.

Conditions favouring finer grainsizes and the preservation of original structures are: (1) short duration of metamorphism, (2) low strain, and (3) small, dispersed grains of an unreative or stable mineral.

Many high-grade metamorphic rocks are so deformed and recrystallized that structural evidence cannot be used to infer their origin. However, examination of low-strain areas may reveal interpretable structures. Chemical evidence is all that can be used where structural evidence is absent or unreliable. However, chemical evidence may be unable to distinguish between chemically similar but structurally different rocks, for example, volcanic and plutonic felsic rocks.

Metapelitic and metapsammitic rocks

Common detrital metasediments are readily distinguished on the basis not only of their distinctive chemical composition, but also sedimentary structures, such as bedding, graded bedding, cross-bedding and, less commonly, basal structures. However, the thickness and continuity of bedding may be altered by deformation, and some tectonic structures can resemble sedimentary structures; for example, truncated folds may closely resemble cross-bedding. Graded bedding may be preserved even in the highest-grade zones, in which partial melting has been extensive. Generally, graded bedding appears to be more reliable as a younging indicator than cross-bedding, but only if complete Bouma sequences are preserved, as grading can be reversed in A horizons (P.F. Williams, pers. comm.).

Calcareous metasediments

Calcareous metasediments generally are also chemically distinctive and may be strongly layered, the layering generally being interpreted as bedding, although this is debatable in some instances. Chemical evidence generally can be used to distinguish calcareous rocks from amphibolites of igneous origin (e.g., Edwards, 1958).

Albite-rich rocks

The most sodic Broken Hill albite-rich rocks are more sodic than the keratophyric rocks or evaporitic analcite tuffs that have been suggested as possible precursors (e.g., Coombs, 1965; Plimer, 1977; Brown et al., 1983; Cook & Ashley, 1992). Therefore, some sodium metasomatism appears to have been involved in their formation, although not entirely syn-or post-metamorphic (Vernon, 1961). The albite-rich rocks in the Olary Block are very fine-grained and finely bedded, originating either as Na-rich, evaporitic sediments of felsic volcanic provenance (Cook & Ashley, 1992) or Na-metasomatized pelitic sediments (N.D.J. Cook, pers. comm., 1996). They show evidence of later repeated replacement by albite, as do the Broken Hill albite-rich rocks. This repeated albitization of initially albitic layers during regional metamorphism may be responsible for the extremely high sodium contents of some of these rocks - up to 11.06% Na2O at Broken Hill (Vernon, 1961, table 1) and 9.33% Na2O at Olary (Cook & Ashley 1992, table 1). At least some of the albitization appears to be post-peak metamorphism, because of the widespread, lower-grade assemblage: quartz + muscovite in the albite-rich rocks, and partial albitization of K-feldspar in pegmatites and leucosomes in metasedimentary rocks adjacent to the albite-rich bodies (Vernon, 1961, plate 15).

Mafic rocks

Elongate plagioclase grains, especially if aligned in a magmatic flow structure, favour an igneous origin, which is generally readily confirmed by the chemical composition. Residual phenocrysts and amygdales suggest a volcanic origin, as in the Jervois area, central Australia and locally at Broken Hill (Brown et al., 1983, p. 237), whereas residual ophitic microstructures suggest an intrusive origin, as at Mount Stafford, central Australia, and locally at Broken Hill (e.g., Brown et al., 1983, p. 242; Stevens et al., 1988, p. 309).

At Broken Hill, mafic gneisses and granofelses are chemically distinguishable from most calcsilicate rocks (Edwards, 1958), but intense recrystallization makes distinction between extrusive and intrusive origins difficult. Rare mafic gneisses and granofelses contain elongate plagioclase grains, despite intense recrystallization and the formation of irrational grain boundaries between the high-grade metamorphic minerals (including orthopyroxene and brown hornblende). This indicates the previous existence of large, elongate plagioclase crystals, implying a gabbroic origin. Such rocks can be distinguished from post-metamorphic rocks with igneous microstructures by the absence of crystal faces.

Intrusive contacts (e.g., Brown et al., 1983, p. 137; Stevens et al., 1988, p. 309) generally suggest a non-volcanic origin. However, local intrusive contacts may be formed where subaqueous mafic lava lows enter unconsolidated sediments. Brown et al., 1983, p. 246) concluded that the mafic rocks at Broken Hill vary from dykes and sills to lava flows and tuffs (e.g., layered varieties).

Felsic gneisses

Felsic gneisses may be of indeterminate origin if devoid of reliable structural indicators, and chemically could be volcanic or granitic. The sheet-like shapes of many of them may suggest a volcanic or volcaniclastic origin, but many Proterozoic granitoids are of this form (e.g., in the Arunta Block) and deformation should also be considered. The Broken Hill felsic gneisses have been interpreted variously as: (1) metasedimentary (arkoses, volcaniclastic or granitized sediments), (2) volcanic flows and (3) metagranitoids.

The best indicators of metavolcanic rocks are embayed quartz phenocrysts and fiamme (pumice lenticles). Embayed quartz phenocrysts survive deformation and metamorphism well (e.g., Vernon, 1986, 1987), because they are large single crystals, compared with a fine-grained, commonly micaceous groundmass that is much more easily deformed. In contrast, fiamme are easily distorted and obscured by deformation. Nevertheless, some felsic rocks in low-strain areas in the Olary Block have fiamme and quartz phenocrysts, indicating a former welded tuff, and rare low-strain rocks near the NBHC Mine at Broken Hill also have embayed quartz phenocrysts (Stanton, 1976), suggesting a volcanic origin. In addition, Brown et al. (1983, photo 15, p. 164) illustrated a felsic rock in the Broken Hill area showing quartz phenocrysts and lenticular patches that could be fiamme. Therefore, at least some metavolcanic or metatuffaceous felsic rocks appear to be present in the Broken Hill Block.

However, many of the Broken Hill felsic gneisses may be metagranitoids, especially those with K-feldspar megacrysts (augen), as suggested by Andrews (1922), Browne (1922), Stillwell (1922), Vernon (1969) and others. K-feldspar megacrysts are much more common in granites than in volcanic rocks, and survive relatively well, even in strongly deformed rocks. Features indicating a granitic origin include: (1) euhedral K-feldspar megacrysts - especially with simple twinning, oscillatory compositional (especially Ba) zoning, zonally arranged inclusions (not inclusion trails), and euhedral plagioclase inclusions, (2) K-feldspar megacrysts aligned in a magmatic foliation or lineation, (3) aplite veins (restricted to plutonic rocks and therefore very reliable indicators), (4) igneous enclaves (especially if megacrystic and relatively large) and (5) intrusive contacts (e.g., Brown et al., 1983, p. 137; Vassallo, 1995). Many of these features of Broken Hill megacrystic metagranites were pointed out by Vernon & Williams (1988), and intrusive contacts between two metagranites at Broken Hill have been described by Vassallo (1995).

In contrast, metamorphic K-feldspar is characterized by: (1) irregularly shaped porphyroblasts, (2) general (universal?) absence of simple twinning, (3) rounded inclusions (e.g., Vernon, 1968), (4) lack of oscillatory zoning, and (5) inclusion trails that reflect pre-existing foliations, not zonally (concentrically) arranged, as in igneous K-feldspar (Vernon, 1986).

The full extent of granitoids in the Broken Hill Block is as yet unknown, but felsic gneisses, many of which are megacrystic, appear to be largely restricted to the highest-grade metamorphic zone (Brown et al. 1983, fig. 1). This close relationship between high-grade metamorphism and granites is typical of low-pressure/high-temperature (LPHT) regional metamorphic terranes, which are common world-wide and are typical of the Australian Proterozoic. Granites may have contributed to the metamorphic heat, at least locally (e.g., Collins & Vernon, 1991, 1992; Collins et al., 1991; Vernon et al., 1993).

Furthermore, many of the megacrystic granites show the effects of the first tectonic foliation (Vassallo, 1995), as do markedly transgressive leucogneisses in the north of the Broken Hill and in the Euriowie Block (Stevens et al., 1988, p. 309), indicating that granites were being intruded at the earliest stages of the metamorphic/deformation history. Other evidence of early heating in Australian LPHT terranes (Vernon et al., 1993) includes: (1) porphyroblasts with minute, random inclusions, indicating growth before any tectonic foliation was present (though bedding may be overgrown to form layers of different inclusion concentrations) and before the matrix grainsize had coarsened, (2) migmatite leucosomes occurring in all S-surfaces (indicating partial melting during the entire foliation-forming deformation history), and (3) sillimanite delineating all S-surfaces (also indicating high temperatures during the whole deformation history). Many LPHT events may be short-lived and controlled by magmatic heat (e.g., Fleming & White, 1984; Collins & Vernon, 1991, 1992; Collins et al., 1991).

Both volcanic and granitoid precursors appear to have been present at Broken Hill, and conceivably could be related. Moreover, the Broken Hill Lode is surrounded by metagranites that were emplaced early in the metamorphic/deformation history, as were some more leucocratic varieties in the north of the Broken Hill Block and in the Euriowie Block (Stevens et al., 1988, p. 309). Therefore, even if volcanic or subvolcanic activity contributed to the origin of the Broken Hill Lode, early granites may have contributed fluids or helped to mobilize external fluids for ore deposition and/or modification. Thus, a plutonic-volcanic origin for the orebody may be appropriate. Perhaps some synchronous granitoids could be added to the cartoon model of Cook & Ashley (1988, fig. 8, p. 223), to provide a "heat engine" for the circulation of hot solutions responsible for the evaporitic and epigenetic deposition of compounds in their metallogenic model for the Olary and Broken Hill Blocks. Cook & Ashley (1988, fig. 8, p. 223) noted that underlying igneous intrusions have been inferred by McKibben et al. (1988) to be responsible for heating and leaching of metals and salts from sediments in the Salton Sea geothermal system. Intrusive mafic rocks may also have contributed to the heat at Broken Hill, but the extent of this effect is unknown.

If the inference that at least some of the felsic gneisses at Broken Hill are metagranites is correct, felsic gneisses should not be used for stratigraphic correlation, unless they can definitely determined to be of volcanic flow or tuffaceous origin. Megacrystic gneisses are very suspect in this regard.

Metasomatic effects

Metasomatic effects may be difficult to infer with confidence or to time with respect to inferred metamorphic/deformation events. Because a metamorphic mineral assemblage depends not only on the physical conditions of metamorphism, but also on the bulk chemical composition of the rock, pre- and syn-metamorphic hydrothermal metasomatism may considerably alter the bulk chemical composition or a rock, and so be responsible for unexpected minerals, such as andalusite, sillimanite or cordierite in metamorphosed felsic volcanic or plutonic rocks (e.g., Vernon et al., 1987). Such mineral assemblages are potential indicators of alteration around orebodies, as has been pointed out by Stanton and others, and may well have occurred around the Broken Hill lode (e.g., Jones, 1968; Stanton, 1976; Plimer, 1979, 1984). Metasomatic changes may also occur in deforming sedimentary rocks during progressive metamorphism/deformation, although the overall pelitic or psammitic composition typically is not obscured.

References

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