The Prospect Alkaline Diabase-Picrite Intrusion New South Wales, Australia

The Prospect intrusion is a dish-shaped alkaline diabase-picrite mass 315–400 ft thick intruded into shale at a depth of about 600 ft. Picrite, containing more than 25 per cent olivine, occupies the lower half of the intrusion. In the upper half, alkaline diabase, averaging less than 5 per cent olivine, is concentrated under structural highs of the contact, and alkaline olivine diabase, containing 10 to 25 per cent olivine, is concentrated under structural lows. These rocks are separated from the shale by a fine-grained chilled margin. Vertical sections through the picrite zone show a regular antipathetic variation of modal olivine and plagioclase with a zone of maximum olivine concentration near the bottom; bulk rock compositions show an antipathetic relation between MgO plus total iron and all other constituents. Modal and bulk composition variations are more erratic in the upper half of the intrusion, but analcite, alkali feldspar, and opaque minerals reach maximum concentrations in this part of the intrusion. The pyroxene content remains nearly constant in the major rock types. Trends of olivine and plagioclase composition and grain size vary regularly with height in the intrusion and cross boundaries between major rock types without deflexion. Olivine becomes progressively more fayalitic from the base of the picrite zone to the upper chilled margin, but the plagioclase curve has a trend toward more calcic compositions in the picrite zone. Mean sizes of plagioclase, pyroxene, and olivine increase upwards between the chilled margins. The lower chilled margin is slightly less mafic than the bulk composition of the intrusion and may represent a pre-emplacement differentiate, but the major part of the differentiation occurred during emplacement at the present site. Grain size and other data indicate that crystallization took place more rapidly from the base than from the top of the intrusion, and a variety of internal structures indicate that crystallization and differentiation took place as the magma was intruded over a considerable period of time. As consolidation of the intrusion proceeded, the liquid became enriched in all constituents except magnesium and ferrous iron until consolidation of alkaline diabase began (when about 70 per cent of the whole intrusion had solidified); at that stage the proportion of calcium, titanium, and ferric iron in the liquid was reduced and the proportion of silica, alumina, and alkalis increased. Processes of differentiation that contributed most to the origin of the main rock types are: diffusion, independently of crystallization, of volatiles, alkalis, and possibly calcium into the structurally high parts of the intrusion; gravity accumulation of olivine that crystallized a short distance above the main front of consolidation as it moved upwards from the base of the intrusion; and upward diffusion of salic constituents and downward diffusion of mafic ones over concentration gradients produced by crystallization. Removal of volatiles from the lower part of the intrusion before crystallization reduced the oxidation ratio in the liquid and resulted in a low proportion of ferric iron minerals; crystallization of abundant olivine (average composition about Fo 70 ), however, prevented enrichment of the liquid in iron. Addition of volatiles to the upper part of the intrusion retarded crystallization and raised the oxidation ratio to a level at which a relatively high proportion of ferric iron minerals crystallized. Subordinate processes that contributed to the formation of the main rock types as well as to less abundant ones include gravity accumulation of heavy minerals that were dispersed in the magma at the time of emplacement, filter pressing caused by local buttressing around irregularities of the contact, crystal sorting by viscous flow, and gas transfer. Pegmatitic differentiates are ascribed to a complex diffusion process along pressure and concentration gradients caused by shear on laminar flow planes. Syenite may have originated by replacement of pegmatite, but aplites occupy true dilationai structures and apparently represent liquid remaining after crystallization of the adjacent rock.