Aspects of the geology of the Snowy Mountains region and their implications for the tectonic evolution of the Lachlan Fold Belt

Abstract

The Snowy Mountains region is an important area in any consideration of the evolution of the Lachlan Fold Belt as five major strata­ tectonic units occur within a relatively small area. Six major aspects of the geology of the region were chosen for study in order to elucidate the tectonic evolution of the region. 1) The Ordovician quartz-rich greywackes were found to be a chemically and mineralogically fractionated sediment which consisted of near pure end-member clay + quartz compositions with minor feldspar detritus. The Silurian quartz-rich greywackes, derived from these sediments, are even purer and it is suggested with each cycle of erosion, provided no new material is added to the source region, each cycle of erosion involves the removal of progressively more feldspar detritus and this is reflected chemically by a decease in the amount of Na20 and CaO. 2) Ordovician mafic volcanics were loca11y extruded simultaneously with the deposition of the quartz-rich greywackes. The volcanics can be subdivided into three suites: one tholeiitic, one bimodal andesite­ rhyolite suite, and one suite of high-K trachybasalts, basalts and basaltic andesites. By comparison with modern volcanic analogues none convincingly imply contemporaneous subduction although they are similar to rocks regarded as typical of island arcs. 3)One major prograde metamorphic event near the Ordovician affected the region and Silurian boundary it was found to be a higher pressure event than had previously been considered, but it was still lower pressure than the Al2Si0s triple point. 4) Three deformations have been recognised in the region. one near the end of the Ordovician, evidence for which is largely obliterated by the second deformation, a major compressional event, which caused isoclinal folding in the metasediments and the development of a strong foliation in the S-type granitoids. The third deformation was less penetrative and caused local crenulation, folding, and kinking and a weak foliation in the I-type granitoids. In the Silurian tensional processes caused major strike slip faulting and block faulting and led to the formation of two major depositional troughs which opened sympathetically on either side of the Long Plain Fault Zone. 5) Granitoids in the region can be subdivided into S-type and I-type with the former being emplaced in the Middle to Late Silurian and the latter in the Early Devonian. The source region for the S-types is postulated to be a pre-existing crustal layer of probable late Precambrian age as the S-type granitoids cannot be derived from any combination of the Ordovician rocks. The I-type granitoids are thought to come from another pre-existing crustal layer which accreted to the base of the crust during a prior subduction event 700 m.y. ago and has never been exposed to any crustal processes. 6) Synchronously in the Late Silurian with the intrusion of the S-type granitoids, a suite of tholeiitic gabbros and dolerites were emplaced. Those intruded into the highest grade metamorphic rocks show evidence of slow cooling suggesting that the Silurian was a period of high heat flow with temperatures being locally elevated throughout. The results from the Snowy Mountains region are applicable in the broad context of the Lachlan Fold Belt as a whole and a model for the tectonic evolution is tentatively proposed. It differs from previous interpretation in that the recognition, characterisation, and extent of the pre-existing crustal layers from which the granitoids were derived, makes the modelling of the Lachlan Fold Belt as a typical 'West Pacific Marginal Sea' untenable. No conclusive evidence of ocean floor crust or island arc volcanism could be found and indeed the majority of the igneous rocks of the Lachlan Fold Belt are unlikely to be derived by subduction with the possible exception of the Devonian I-type granitoids. A model for the Ordovician of continental rifting followed by rifting and transform processes in the Silurian is preferred with perhaps the Devonian representing subduction beneath a continent. The high heat flow for the Silurian can more readily be explained as due to crustal thinning and mantle up-welling similar to present day processes associated with rift valleys and other tensional environments. Show more