Southern Lachlan Fold Belt Research Articles

Some factors affecting granitic pluton topography: Pluton topography in the southern Lachlan Foldbelt

Where neotectonics are inactive, granite batholith topography is determined largely by the dissolution rates of the minerals of the component plutons. In southern New South Wales, Australia, plutons in the Kosciusko, Corryong, Berridale, and Murrumbidgee Batholiths allow an assessment of the extent to which faults, joints, grainsize and foliation may control a pluton's topography. Changes in mean elevation of zoned plutons closely follow the changes in mineralogy; more felsic zones invariably occur at higher elevations than more mafic zones. Faults clearly dictate major landscape features, such as linear valleys. Neither grainsize nor foliation show any appreciable effect on these landscapes. Joint spacing in these plutons has a positive correlation with the rock's grainsize, with an intrinsic value varying linearly as the grain size varies. Not all joints at the intrinsic spacing may be evident. In perennially wet valleys most joints become weathered and so visible, leading to joints mostly spaced at that intrinsic value. In quickly drying crests and ridges more incipient joints remain closed, leading to larger boulders and a wider average joint spacing. Thus the evident joint spacing in granites is determined by grainsize and landscape position.

Trace inheritance—Clarifying the zircon O-Hf isotopic fingerprint of I-type granite sources: Implications for the restite model

Early to mid Carboniferous I-type granites distributed in a broad meridional belt west of Sydney, southeastern Australia, represent the last phase of granite magmatism in the southern Lachlan Fold Belt. Rare inherited zircon in the granites, in combination with zircon precipitated from the melt phase of the magmas, provides direct evidence of the nature and age of the source rocks from which the granites were derived, and the isotopic compositions of those rocks. Most granites from the north and central parts of the belt, represented by the Wuuluman, Home Rule, Oberon and Lett plutons (334±3, 328±4, 339±2 and 328±2Ma, respectively), are characterized by scarce inherited zircon dominantly of Siluro-Devonian age, consistent with all being derived from a common zircon-poor source rock of mid to late Devonian age. Based on the isotopic compositions of the igneous zircon rims, that source was relatively homogeneous and immature (δ18Ozrn 6–7‰, εHf(t) 0–+4). Two samples of the Tarana pluton (331±2Ma), near the centre of the belt, show evidence for heterogeneity in that source and the presence of a more evolved component that is also more zircon rich. Together with the Lett pluton from the same geochemical suite, their zircon rim compositions define a mixing array between more and less evolved end members (δ18Ozrn 5.5–9.5‰, εHf(t) -4–+4). Only in the southern Chapmans Creek pluton (327±3Ma) is there clear evidence that the source of the granite also contained a minor component with a similar older protolith to the LFB early Palaeozoic sediments that host most of the LFB pre-Carboniferous granites. Presence of that component, which overwhelmingly dominates the inheritance in the pre-Carboniferous granites, is also reflected in a slightly elevated whole rock initial 87Sr/86Sr (>0.7050) and igneous δ18Ozrn (ca. 7.5‰), but not in the igneous zircon εHf(t) (ca. +2). The amount of restitic zircon in granite is determined by not only the proportion of restite present but also the zircon contents of the various source rock components. The process of restite unmixing does not necessarily mean that more mafic, restite-rich granites will contain more inherited zircon.

Barium contents of granites: key to understanding crustal architecture in the southern Lachlan Fold Belt?

Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO2 range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower – middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar – Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician – Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement.

Intrusive relationships of granite and dolerite at Lagunta Creek, Freycinet Peninsula

Coastal exposures immediately south of the mouth of Lagunta Creek, Freycinet Peninsula, eastern Tasmania, illustrate the temporal and spatial relationships between various Devonian granites and a dolerite dyke. The oldest granite, an equigranular grey hornblende-biotite adamellite containing abundant mafic enclaves, was intruded by a more felsic, equigranular to variably porphyritic or seriate pink alkali feldspar granite, and associated aplite, leucogranite and pegmatite dykes. Field relationships indicate brittle fracture and stoping of the adamellite during dilational emplacement of the pink granite. Minor bodies of granite porphyry are also younger than the adamellite. A dolerite dyke, of hawaiitic composition and containing plagioclase megacrysts, intrudes the adamellite and also crosscuts the aplite and leucogranite dykes, and is the youngest phase. The granites are metaluminous to very weakly peraluminous, and are classified as I-types on both mineralogical and geochemical criteria. The adamellite is unfractionated and chemically reduced, and, together with similar rocks at Wineglass Bay and Cape Tourville, constitutes the Bluestone Bay Suite, which is geochemically distinct from, and younger than, other eastern Tasmanian granodiorite/adamellites. In contrast, both the pink granite and, to a lesser extent, the granite porphyry are fractionated and locally magnetite-bearing. Fractionation has resulted in high Rb, Y, Th and Pb and low P, Sr and Ba. The dolerite dyke, despite its alkalic major element chemistry, is depleted in Nb like some other granite-associated mafic rocks in the southern Lachlan Fold Belt.

Mafic rocks spatially associated with Devonian felsic intrusions of the southern Lachlan Fold Belt: A possible mantle contribution to crustal evolution processes

Devonian basaltic to andesitic dykes and compositionally similar plutons of the southern Lachlan Fold Belt are often temporally and spatially closely associated with large granitic complexes. Mafic intrusions play a major role in the transfer of heat into the continental crust, providing a thermal ‘engine’ which leads to crustal melting, and geochemical/isotopic evidence indicates that they contribute chemical constituents to the products of this melting. Studied mafic‐intermediate dykes in the southern Lachlan Fold Belt have tholeiitic to alkaline affinities and include groups with both high and low Ti and K. Several dyke generations may be associated with a single felsic complex. Primitive mantle‐normalised trace‐element abundance patterns with negative Nb and Ti anomalies for basaltic/andesitic and gabbroic/dioritic rocks as young as Early Devonian most resemble those of modern island arcs and suggest an influence of subduction on mantle magma sources. However, some Middle and Late Devonian mafic rocks are enriched in light rare‐earth elements and other incompatible elements, lack significant Nb anomalies, and confirm the change to continental‐rift extensional settings clearly indicated by Lachlan Fold Belt geology.

Isostatic constraints on the central Victorian lower crust: Implications for the tectonic evolution of the Lachlan Fold Belt

Middle Devonian (Tabberabberan) deformation in the central parts of the southern Lachlan Fold Belt resulted in 50% shortening in the upper crust and a complete cessation of marine sedimentary environments thereafter. Isostatically supported changes in elevation generated during Middle Devonian deformation in southeastern Australia are examined through geodynamic modelling. Constraints are placed on the nature of the lower crust of the Lachlan Fold Belt (in particular its density) and insights are gained into the Palaeozoic tectonic evolution of this unusual orogenic belt. The evolution of strain in the Lachlan Fold Belt is quantified using crustal and (whole) lithospheric thickening factors fc and fl. Relative changes in the thickness of the crust and lithospheric mantle generated during orogenesis and associated physical responses in the deforming lithosphere (such as changes in isostatically supported elevation) are portrayed in fc‐fl space. A model for the strain history (i.e. the changing thickness of crust with time) of central Victoria has been proposed by taking into account known present‐day characteristics of this part of the southeastern Australian crust, as well as metamorphic and structural evidence. From the results of the isostatic modelling it is suggested that the Lachlan Fold Belt is largely floored by dense (?oceanic) crust. Assuming a relatively dense lower crust (>2.9 g.cm–3), geologically reasonable estimates for isostatically supported elevation are calculated for central Victoria both before and after Middle Devonian deformation (0 ±250 m and between ∼1700 and 2500 m, respectively). Conversely, if a lower density crustal substrate is modelled, elevations generated during Middle Devonian deformation are > 4000 m, which is considered unrealistically high for southeastern Australia. Further constraints can also be placed upon the actual style of Middle Devonian structural thickening in the Lachlan Fold Belt. It is proposed that the entire crustal section (both upper and lower crust) has been homogeneously thickened by imbrication, as opposed to a subduction‐style model in which only the overlying sedimentary pile would thicken above a detached lower crust.

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia: Comment and Reply

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia: Comment and Reply

Evidence from U–Pb zircon and 40Ar/39Ar muscovite detrital mineral ages in metasandstones for movement of the Torlesse suspect terrane around the eastern margin of Gondwanaland

New U–Pb detrital zircon ages from Triassic metasandstones of the Torlesse Terrane in New Zealand are compared with 40Ar/39Ar muscovite data and together, reveal four main source components: (i) major, Triassic–Permian (210–270 Myr old) and (ii) minor, Permian–Carboniferous (280–350 Myr old) granitoids (recorded in zircon and muscovite data); (iii) minor, early middle Palaeozoic, metamorphic rocks, recorded mainly by muscovite, 420–460 Myr old, and (iv) minor, Late Precambrian–Cambrian igneous and metamorphic complexes, 480–570 Myr old, recorded by zircon only. There are also Proterozoic zircon ages with no clear grouping (580–1270 Myr). The relative absence of late Palaeozoic (350–420 Myr old) components excludes granitoid terranes in the southern Lachlan Fold Belt (Australia) and its continuation into North Victoria Land (East Antarctica) and Marie Byrd Land (West Antarctica) as a potential source for the Torlesse. The age data are compatible with derivation from granitoid terranes of the northern New England Orogen (and hinterland) in NE Australia. This confirms that the Torlesse Terrane of New Zealand is a suspect terrane, that probably originated at the NE Australian, Permian–Triassic, Gondwanaland margin and then (200–120 Ma) moved 2500 km southwards to its present New Zealand position by the Late Cretaceous (90 Ma). This sense of movement is analogous to that suggested for Palaeozoic Mesozoic terranes at the North American Pacific margin.

Structural analysis of metasedimentary enclaves: Implications for tectonic evolution and granite petrogenesis in the southern Lachlan Fold Belt, Australia

Schistose, upper amphibolite facies, metasedimentary enclaves from granites across the southern Lachlan Fold Belt of southeastern Australia have a common structural history. The middle crustal region, from which the enclaves and their host granites derived, is assumed therefore to be structurally uniform across the fold belt. The uniform nature of the crustal component represented by the metasedimentary enclaves casts doubts on the existence of suspect or Proterozoic microcontinental blocks within the terrane collage. A proposed crustal cross section for the southern Lachlan Fold Belt shows a tectonically thickened Paleozoic section overlying an oceanic substrate. An Ordovician sedimentary component was involved in the generation of both S- and I-type granites.

Geology and mineralazation in the southern Lachlan Fold Belt: A probable Mississippi Valley-type lead-zinc deposit at Cooleman Plains, southern New South Wales

"Geology and mineralazation in the southern Lachlan Fold Belt: A probable Mississippi Valley-type lead-zinc deposit at Cooleman Plains, southern New South Wales." Exploration Geophysics, 7(1), p. 39

Geology and mineralazation in the southern Lachlan Fold Belt: Geochemical-petrographic investigations of rocks from around the woodlawn Cu-Pb-Zn ore body, Southeastern New South Wales

"Geology and mineralazation in the southern Lachlan Fold Belt: Geochemical-petrographic investigations of rocks from around the woodlawn Cu-Pb-Zn ore body, Southeastern New South Wales." Exploration Geophysics, 7(1), pp. 37–38

Geology and mineralazation in the southern Lachlan Fold Belt: Copper mineralisation at The Basin Creek No. 1 prospect, tumut, new south wales

"Geology and mineralazation in the southern Lachlan Fold Belt: Copper mineralisation at The Basin Creek No. 1 prospect, tumut, new south wales." Exploration Geophysics, 7(1), pp. 39–40

Geology and mineralazation in the southern Lachlan Fold Belt: Colo creek – a barite zinc-rich setting in the hill end trough

The Colo Creek setting, some 66 km south of Bathurst, is situated on the very western margin of the Hill End Trough. Zinc-rich sulphides have been deposited rapidly, in a high energy depositional environment close to a zone of active submarine acid volcanism.

Geology and mineralazation in the southern Lachlan Fold Belt: The mineralization of the kangiara mine, New South Wales

The Kangiara deposit is located 32 km north-west of Yass in the Cowra-Yass Synclinorial Zone of the Lachlan Fold Belt. The mine occurs within the Middle to Upper Silurian Hawkins Volcanics of the Douro Group.