Patrick Lyons

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

Integrated geophysical appraisal of crustal architecture in the eastern Lachlan Orogen

Forward modelling of potential field data, combined with new geological mapping and deep seismic reflection transects acquired by the Australian Geodynamics Cooperative Research Centre (AGCRC) and New South Wales Department of Mineral Resources, has led to iterative testing of models of crustal architecture of the eastern Lachlan Orogen in New South Wales. This integrated analysis has led to new conclusions about the subsurface that are unlikely to be deduced solely from any of the individual data sets used. Conclusions supported by the consideration of these data include: Presence of lower crust in the eastern Lachlan Orogen, characterised by higher than average crustal density, high P- wave velocities, and repeated, stacked bands of strong reflectivity. This crust is interpreted to be a stacked pile of metaturbidites and modified oceanic crust (greenstones). Presence of large volumes of Ordovician volcanic rocks underlying many areas of Silurian-Devonian basin rocks. Evidence for extensive, deep-cutting blind thrust faults and detachments throughout the crustal section. Major movements on these faults during the early Silurian appear to have significantly thickened the whole crust. Evidence for many high-level upper crustal slivers, mostly formed during the Carboniferous. Differences between the western Ordovician Junee-Narromine Volcanic Belt and the eastern Ordovician Molong Volcanic Belt. The former is quite dense, and is inferred to have a large volume of lavas and intrusive rocks. Its structural style is predominantly that of an imbricate stack around a deeper-rooted core. The latter has lower bulk density, and a higher volume of volcanicla tic material. It is now entirely composed of thin, imbricate slices. These differences suggest that the eastern belt may be the rifted off forearc or apron of the western belt which may be the original magmatic centre. Evidence for different styles of granite intrusion, reflected in different intrusive geometry of Silurian, Devonian and Carboniferous granites.