Metamorphism in Palaeozoic forearc basin sequences, southern New England Fold Beit, N.S.W., Australia.

摘要

Alteration of forearc basin sediments and volcanic rocks of the Tamworth Belt (TB), southern New England Fold Belt, has resulted in the development of four zones of progressive metamorphism defined by heulandite-clinoptilolile ± stilbite, laumontite, laumontitc-prehnite ± pumpetlyite and prehnite ± pumpellyite ± epidote with increasing stratigraphic depth. In the Hastings Block (HB), a possible rotated and displaced part of the forearc basin, illite crystallinity measurements suggest similar grades of metamorphism to that in the TB. Superimposed on this metamorphic pattern are contact metamorphic aureoles formed during the emplacement of Permian granitoids in the TB and Triassic granitoids in the HB. The first sign of contact metamorphism produced by Permian granitoids is a rapid increase in reflectance of coalified plant material and the appearance of chlorite-vermiculite(C/V) in rocks of the appropiate composition. Subsequently, laumontite ± prehnite, prehnite ± pumpellyite, epidote-chlorite ± aclinolite ± biotite, biotite ± hornblende progressively develop as well as rare wairakite and hypersthenc. Peak metamorphic conditions P=1.5-2.5kb and T>550° C were attained during the contact metamorphism. Maximum T reached by the contact metamorphic rocks adjacent to the Triassic granitoids is lower (<550" C), but P is similar (<2.5kb). With increasing grade, illite-C/V, chlorite-muscovite, biotite-niuscovile, biolile-cordierite form in rocks of psammitic and pelitic composition. Alteration patterns and reflectance studies of rocks sampled from measured sections, clearly indicate that inetamoiphisin is of the burial type. These studies also suggest that in most of the forearc basin, the geothermal gradients were low (9-12° C/km), implying that relatively cold lithosphere was being subducted at the time sedimentation was taking place in the forearc basin.