D. Wyborn

Lachlan Fold Belt granites revisited: high- and low-temperature granites and their implications

Many of the granites in southeastern Australia possess compositional, petrographic, zircon age inheritance and other features that cannot be accounted for satisfactorily by the classical models of petrogenesis. The restite model was developed to account for these features and recognises that unmelted but magmatically equilibrated source material (restite) may be entrained in a partial melt, together comprising magma. Variation in the degree of separation of those two components, leading to differences in the ratio of melt to restite, is responsible for the variation in composition within many suites of granites. A popular alternative view, that variation within suites resulted from magma mixing or mingling, conflicts with simple observations of the rock compositions and cannot be sustained. Several strong arguments can be made against another alternative view that fractional crystallisation was the dominant process in producing variation within those suites. New and conclusive evidence against that process is provided by the fact that zircon age inheritance is present in most of these granites where that has been examined. That has shown that the S‐type and most of the I‐type granites formed at low magmatic temperatures and confirmed that the compositional variation within those suites must have resulted from restite fractionation. The Ordovician sedimentary rocks exposed in the Lachlan Fold Belt are not sufficiently feldspathic to have produced the voluminous S‐type granites and volcanic rocks of the Bullenbalong Supersuite. The view that those granites and volcanic rocks were derived from more feldspathic metasedimentary rocks is supported by much evidence and confirmed by the fact that the pelitic enclaves in those granites are relatively high in Ca. The presence of a once‐thick metasedimentary basement in those areas in which these S‐type granites occur is inferred. The I‐type granites of the Lachlan belt are not compositionally analogous to the more calcic and less potassic granites found in younger subduction‐related continental margins and mostly formed at low magmatic temperatures through the partial melting of pre‐existing quartzo‐feldspathic igneous crust. This implies that the petrological evolution of the belt during the Silurian and Devonian occurred dominantly by the vertical redistribution of the components of older crust through partial melting and movement of granite magmas. Evolution of the belt at that time was not related to active subduction.

The Boggy Plain Supersuite: A distinctive belt of I‐type igneous rocks of potential economic significance in the Lachlan Fold Belt

The Early Devonian Boggy Plain Supersuite is a belt of I‐type granitic and volcanic rocks extending for over 500 km in the central Lachlan Fold Belt. It has a distinctive composition and origin. Compared with other Lachlan Fold Belt I‐types, rocks of the supersuite are high in Cu and incompatible elements (K, Ba, Sr, Rb, La, Ce, U, and Th). The source for the magmas is interpreted to have been an incompatible element‐rich gabbroic layer, underplated at the base of the crust in the Ordovician. This layer is inferred to correspond to a belt of Ordovician shoshonitic basaltic volcanic rocks that has a strikingly similar geographical distribution to the supersuite. Compositional variation in the supersuite is ascribed to fractional crystallization, in contrast to most other I‐type magmas of the Lachlan Fold Belt which owe most of their compositional variation to fractionation by restite unmixing. This contrast resulted from differences in source rocks, with the Boggy Plain Supersuite being derived from basalt...

Low- and high-temperature granites

I-type granites can be assigned to low- and high-temperature groups. The distinction between those groups is formally based on the presence or absence of inherited zircon in relatively mafic rocks of a suite containing less than about 68% SiO 2 , and shown in many cases by distinctive patterns of compositional variation. Granites of the low-temperature group formed at relatively low magmatic temperatures by the partial melting dominantly of the haplogranite components Qz , Ab and Or in H 2 O-bearing crustal source rocks. More mafic granites of this type have that character because they contain restite minerals, often including inherited zircon, which were entrained in a more felsic melt. In common with other elements, Zr contents correlate linearly with SiO 2 , except sometimes in very felsic rocks, and Zr generally decreases as the rocks become more felsic. All S-type granites are apparently low-temperature in origin. After most or all of the restite has been removed from the magma, these granites may evolve further by fractional crystallisation. High-temperature granites formed from a magma that was completely or largely molten, in which zircon crystals were not initially present because the melt was not saturated in that mineral. High-temperature suites commonly evolved compositionally through fractional crystallisation and they may extend to much more mafic compositions through the production of cumulate rocks. However, it is probable that, in some cases, the compositional differences within high-temperature suites arose from varying degrees of partial melting of similar source rocks. Volcanic equivalents of both groups exist and show analogous differences. There are petrographic differences between the two groups and significant mineralisation is much more likely to be associated with the high-temperature granites. The different features of the two groups relate to distinctive source rock compositions. Low-temperature granites were derived from source rocks in which the haplogranite components were present throughout partial melting, whereas the source materials of the high-temperature granites were deficient in one of those components, which therefore, became depleted during the melting, causing the temperatures of melting to rise.

The petrogenetic significance of chemically related plutonic and volcanic rock units

Comagmatic granitic and volcanic rocks are divided into two types depending on whether or not the primary magma contains restite crystals. Examples of both of these types are discussed from the Lachlan Fold Belt of southeastern Australia. Volcanic rocks containing restite phenocrysts are chemically identical to the associated plutonic rocks containing the same amount of restite. The more mafic granitic rocks correspond in composition to the most phenocryst-rich volcanics (up to 60% phenocrysts), and thus cannot be cumulate rocks produced by fractional crystallization, but must represent true magma compositions. These restite-bearing magmas result from partial melting in a source region up to the rheological critical melt percentage, which we estimate to be about 40% in the S-type Hawkins Suite of volcanics. Melts which escape their restite at the source, before the critical melt percentage is reached, are able to undergo fractional crystallization in high level magma chambers by heterogeneous crystallization on chamber walls. In this case volcanic products from the top of the chamber are more felsic than the plutonic products, the plutonics are crystal cumulates and the volcanics are composed of the complementary fractionated liquid. Those phenocrysts present in the volcanics were probably eroded from the chamber walls and are less abundant (

Examples of convective fractionation in high‐temperature granites from the Lachlan Fold Belt

Igneous rocks derived from high‐temperature, crystal‐poor magmas of intermediate potassic composition are widespread in the central Lachlan Fold Belt, and have been assigned to the Boggy Plain Supersuite. These rocks range in composition from 45 to 78% SiO2, with a marked paucity of examples in the range 65–70% SiO2, the composition dominant in most other granites of the Lachlan Fold Belt. Evidence is presented from two units of the Boggy Plain Supersuite, the Boggy Plain zoned pluton and the Nallawa complex, to demonstrate that these high‐temperature magmas solidified under a regime of convective fractionation. By this process, a magma body solidified from margin to centre as the zone of solidification moved progressively inwards. High‐temperature near‐liquidus minerals with a certain proportion of trapped interstitial differentiated melt, separated from the buoyant differentiated melt during solidification. In most cases much of this differentiated melt buoyantly rose to the top of the magma chamber to form felsic sheets that overly the solidifying main magma chamber beneath. Some of these felsic tops erupted as volcanic rocks, but they mainly form extensive high‐level intrusive bodies, the largest being the granitic part of the Yeoval complex, with an area of over 200 km2. Back‐mixing of fractionated melt into the main magma chamber progressively changed the composition of the main melt, resulting in highly zoned plutons. In the more felsic part of the Boggy Plain zoned pluton back‐mixing was dominant, if not exclusive, forming an intrusive body cryptically zoned from 63% SiO2 on the margin to 72% SiO2 in the core. It is suggested that tonalitic bodies do not generally crystallise through convective fractionation because the differentiated melt is volumetrically small and totally trapped within the interstitial space: back‐mixing is excluded and homogeneous plutons with essentially the composition of the parental melt are formed.

Inferred thrust imbrication, deformation gradients and the Lachlan Transverse Zone in the Eastern Belt of the Lachlan Orogen, New South Wales

Re‐examination of the Ordovician geology between Mandurama and Bigga in the Lachlan Orogen of central western New South Wales has produced new interpretations of the stratigraphy and structural geology. The Abercrombie beds have been previously inferred to comprise an Ordovician turbidite package with interbedded black shale bands. Although hampered by a paucity of fossil ages, new data suggest that the Ordovician geology of this region instead represents an imbricate stack of Lower Ordovician turbidites (Adaminaby Group) and Upper Ordovician black shales (Warbisco Shale). Structural data from the north of this region suggest that duplication occurred in a D1 event (with formation of broadly east‐west to west‐northwest‐trending thrust slices or fold limbs) and was accompanied by formation of cleavage and isoclinal folds. Thrusting of the Adaminaby Group and Warbisco Shale over or under the Lower Ordovician Coombing Formation (southern part of the Molong volcanic belt) also occurred at this time. East‐verg...