Petrology and geochemistry of the alpine dyke rocks at Pivot Creek

Collins, Alice Kathryn

Pivot Creek is located in the heart of the Alpine Dyke Swarm in the Haast River, South Island, New Zealand, which is the only known carbonatite locality in New Zealand. Petrological and geochemical analyses have been made of a suite of dyke lithologies, including a calcite-dolomite-hematite-Sr-REE carbonate-monazite carbonatite that metasomatises ( fenitises) country rock epidote greenschist. Analyses reveal extreme divergence from what is acknowledged, world-wide, as the 'typical' carbonatite LREEenriched composition with profiles of both major mineral constituents and bulk rock analysis exhibiting convex-upwards shaped, LREE-depleted, MREE-enriched patterns. These REE patterns are not common, but LREE depletions have been reported from several localities including; Fen Complex, Norway, the Gronnedal-Ika Complex, South Greenland, and Bayan Obo, China. Generally these hump-shaped patterns have been ascribed to post-magmatic loss of elements by fluid-controlled mobilisation, without any significant analysis of the surrounding country rock that is inferred to become enriched in these elements. Geochemical analyses suggest the 'unusual' Pivot Creek REE profiles can be attributed to highly oxidised carbothermal fluids rich in alkalis, Sr, Ba, P, and LREE. These residual fluids preferentially mobilise LREE in the carbonatite and country rock using COz, sol-, and P04- as the main complexing agents. Mass transfer calculations and Sr isotope analyses indicate fluid both emanates from, and diffuses back into the carbonatite dyke, producing ferrite aureoles that are extremely enriched in Na, K, Si, Ba, and AI and significantly depleted in Ca, Mg, Fe, LREE, and HREE with respects to the protolith greenschist. These results emphasise the importance of identifying the prominent role fluids play during carbonatite genesis, and suggest late stage fluid interactions between the carbonatite and surrounding country rock lead to significant chemical alteration of mineral and whole-rock systems.

143 leaves : ill. (some col.), maps ; 30 cm

2010Collins

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