PETROGENESIS OF THE LATE-DELAMERIAN GABBROIC COMPLEX AT BLACK-HILL, SOUTH-AUSTRALIA - IMPLICATIONS FOR CONVECTIVE THINNING OF THE LITHOSPHERIC MANTLE

A group of funnel-shaped gabbroic plutons at Black Hill, South Austral ia, consist of a lower series of layered peridotite, troctolite and ol ivine gabbro cumulates overlain by gabbronorites and potassic gabbrono rites, the latter yielding a Sm-Nd isochron of 489 +/- 39 Ma. Mineral assemblages in the gabbros record an olivine compositional hiatus (Fo( 75-55)) and a high temperature (1200-1000 degrees C), low pressure (si milar to 1 kbar), continental tholeiitic fractionation trend under mod erate f(O2) (similar to QFM) conditions. The liquid line of descent in volved complex open system processes including recharge and crustal as similation. In one pluton, fine-grained norites may reflect assimilati on which resulted in an increased a(SiO2)(liquid) causing orthopyroxen e to crystallize prior to plagioclase. All the gabbros, including the most primitive peridotites, are LREE and incompatible element enriched . Moreover, the calculated parental magma composition in equilibrium w ith the most primitive troctolite has high La/Yb, La/Nb, Ti/Y and low Rb/Ba, similar to that of basaltic dykes which cut the gabbroic comple x. Such compositions are untypical of melts derived from the asthenosp here suggesting that the incompatible element enrichment is not simply due to small degrees of melting. Given the isotopic constraints (ENd( i) 3.4 to -4.6, Sr-87/Sr-86(i) 0.7038-0.7065), this enrichment is not easily reconciled by crustal contamination either, and instead it is i nferred to reflect an enriched lithospheric mantle source. Published d ata on mantle xenoliths from local Tertiary volcanoes overlap the isot opic and geochemical array of the gabbros and dykes, supporting this h ypothesis. In conjunction with A-type granites and minor volcanic rock s, the gabbroic plutons form part of a high temperature, bimodal magma tic suite which intruded the Adelaide fold belt just after the cessati on of convergent deformation during the Cambro-Ordovician Delamerian O rogeny. The appearance of such magmas is problematic since thick oroge nic lithosphere severely restricts the likelihood of decompression mel ting in the asthenosphere. One solution to this dilemma is that convec tive thinning of the lithospheric mantle beneath the orogen promoted m elting of hydrated, enriched regions within the lithospheric mantle. S uch a model can reconcile the strong lithospheric mantle signature in the gabbros with the observation that their intrusion was coincident w ith uplift and the cessation of deformation.