CALCITE AND CARBOCERNAITE EXSOLUTION AND COTECTIC TEXTURES IN A SR, REE-RICH CARBONATITE DYKE FROM RAJASTHAN, INDIA

A carbonatite dyke from the Sarnu-Dandali alkaline complex, Rajasthan, India, contains a remarkable suite of rare earth, strontium-rich mine rals with spectacular primary textures. Sr, Mn-rich calcite in the out er 5 mm of the dyke contains exsolved lamellae of carbocemaite, (Ca,Na )(Sr,Ce,Ba)(CO3)2, orientated parallel to its twin and cleavage planes . The amount of exsolved carbocernaite increases away from the dyke ma rgin as the Sr content of the calcite increases to a maximum 13 wt.%. Sr levels as high as this in calcite have previously been recorded onl y in experimental work. The carbocernaite exsolution suggests that Sr- rich calcium carbonate can be a host for major amounts of REE in carbo natite magma. Separated by a sharp internal boundary, is a complex pos sibly cotectic intergrowth of carbocemaite and Sr-rich calcite with la te Ca-rich strontianite (19 wt.% CaO). Other minerals in the dyke incl ude baryte, pyrrhotite, alabandite, sphalerite and occasional bastnasi te-(La) and thorite. Bands of late britholite-(Ce) traverse the dyke. The host rock for the dyke is fenitized melanephelinite which is itsel f traversed by narrow, < 1 mm, carbonatite veins beleived to predate t he carbonatite dyke. Allanite, britholite-(Ce) and rare monazite-(Ce), developed at the boundary between the carbonatite dyke and the fenite , may have been produced by a reaction between the dyke and the wall r ock, or may be related to the later britholite mineralisation. The tex tures and mineral compositions indicate primary crystallisation. They are unique amongst rare earth-rich carbonatites which are usually late -stage phenomena with signs of secondary alteration. Comparison with e xperimental data available for the calcite-strontianite system suggest s conditions of 500-degrees-C and 2 kbar for coexisting Sr-rich calcit e and Ca-rich strontianite. A smaller scale intergrowth of calcite con taining only 2.9 wt.% SrO and coexisting Ca-strontianite may correspon d to a further unmixing at 350-degrees-C and 2 kbar. Since no experime ntal data are available for a calcite-carbocernaite-strontianite syste m, mineral chemistries and the interpreted sequence of crystallisation have been used to construct a hypothetical phase diagram.