Carbonatites--Into the Twenty-First Century

At the 1997 annual meeting of the GAC–MAC in Ottawa, a two and a half day carbonatite symposium was held in honour of John Gittins, who recently retired from the Geology Department at the University of Toronto. John, long associated with carbonatites, was co-editor with Frank Tuttle of Carbonatites, a book which provided a comprehensive and detailed summary of carbonatite research up to 1966 (Tuttle Twyman Bell Lee Freestone Bell, this volume). Carbonatites, too, are spatially related to Group 1 kimberlites (Premier, South Africa) and ultramafic lamprophyres (Alno, Sweden; Kandalaksha, Russia). These diverse associations argue against any simple model for magma generation. The relationships between carbonatites and their associated silicate rocks are complex and are still not completely understood. Whether both melts were generated from the same parental magma, or whether both were generated independently of one another, still remains one of the fundamental problems in carbonatite petrogenesis. In an attempt to resolve this problem, Harmer & Gittins (this volume), utilize Nd–Sr data to support the model that most carbonatites are of primary mantle derivation and that the divergent Nd and Sr of the associated silicate rocks reflect different mantle source regions. A further problem that has always concerned carbonatite researchers is the timing of the emplacement of carbonatites relative to that of their associated silicate rocks (e.g. Barker, 1989). In almost all cases, emplacement of the carbonatite is later than that of most of the silicate rocks. This is shown dramatically for the Grays Bay complex, which intruded Archean rocks of the Slave structural province in northwestern Canada (Villeneuve & Relf, this volume). High-precision U/Pb ages obtained for the syenites of 2606 Ma are older by at least 6.5 my than the associated carbonatites, showing that the same plumbing system was used over an extensive period of time. Experiments, relevant to this problem (Minarik, this volume), show that carbonate melt has a higher melt–solid interfacial energy than coexisting silicate melt, thus restricting migration of the carbonate liquid either until the silicate melt had solidified or by separating from the silicate melts within liquid dominated reservoirs. Primitive carbonated silicate magmas, such as olivine nephelinites and olivine melilitites, are able to differentiate towards more evolved liquids. Studies in this volume illustrate a variety of evolutionary paths for such fractionating liquids. Veksler et al. (this volume) examined mineral melt inclusions from silicate and carbonatite samples from the Gardiner complex. Results of this study support an immiscible origin for the carbonatites. Ivanikov et al. (this volume) found that major and trace element data for a variety of silicate dykes, which range from olivine melilite nephelinites through to nephelinites, from Kandalaksha, Russia, are consistent with an origin via closed-system fractional crystallization. They attributed the limited volume of associated carbonatite to an origin by late-stage liquid immiscibility. Crystal fractionation offers yet another way of generating carbonatitic liquids. The sequence of rocks, forming a perfect concentric zonation, at the Salmgorskii ring complex, Kola Peninsula, Russia, is in keeping with fractional crystallization (Korobeinikov et al., this volume). As part of the study on Gardiner primary melt inclusions, Veksler et al. (this volume) also examined silicate and carbonatite samples from Kovdor (Russia) and propose on the basis of their findings that the carbonatites at Kovdor formed by fractional crystallization. Kjarsgaard's (this volume) examination of natural ultrabasic silicate–calciocarbonatite pairs (Premier, Blue Hills) further lends credence to the viability of generating calciocarbonatites by fractional crystallization. Degassing can also play an important role in magma differentiation by suppressing the generation of carbonatitic liquids. Dawson (this volume) argues on the basis of detailed petrological studies on silicate volcanic rocks from Oldoinyo Lengai that an important factor in controlling magma differentiation is loss of CO2. He suggests that CO2 degassing inhibits exsolution of carbonatitic liquids from parental carbonated wollastonite nephelinite melts, which Sr and the into the more evolved melt composition that nephelinites. carbonatites can be produced in a variety of including precipitation from a primary dolomitic melt at low pressure et al., Harmer & Gittins, magmas can also be produced between depths of and km (Wyllie & this volume) but only a carbonatitic melt is in equilibrium with produced from lherzolite during with dolomitic melts at depths of about Lee & Wyllie (this volume) further argue that melts produced by differentiation immiscibility or residual in the or mantle can contain more than Kjarsgaard (this volume) the composition of the immiscible carbonate liquids produced in his experiments to be for magmas, the high coupled with significant to precipitate silicate mineral phases. of carbonatite that is to emerge is the role of activity in an carbonatitic calcitic or dolomitic carbonatite melts generated by any contain silicate components in wt % Most carbonatites contain silicate phases, including and that be present in the parental liquids to precipitate such silicate phases. New experimental et al., this volume) from and melting on a recently from Oldoinyo Lengai were used to model fractional crystallization of + + to the generation of At an age the parental liquid is to be a with between and of and Sr can produce the and late-stage dolomitic carbonatites (Cooper & this volume). (this volume) detailed carbonatite and mineralogy in with data from mineral to discrete carbonatites within a number of individual can be found for and parental carbonatitic melts within individual The most important findings to emerge from the contributions contained in this volume are as These findings have to resolve some of the problems that were and discussed in the volume and (Bell, the last major comprehensive of carbonatite research. other as we have are still and silicate volcanic rocks are mantle On the basis of phase equilibrium carbonatitic melts can be generated by primary mantle liquid and crystal fractionation. Clearly, there are and petrological studies support all of the three for the generation of carbonatitic melts. Most carbonatites are associated with silicate rocks. however, a number of distinct carbonatite–silicate rock which are different from one the that carbonatites can be produced by different are are that can be used to one of carbonatite from a of that could be used to mantle-derived carbonatites. This needs to be The problem is complicated by the that most carbonatites that are at crustal are plutonic rocks, probably that have and during magma migration and The low of many carbonatitic melts crystal and further suggests that even carbonatites may not liquid compositions et al., Because of these it is very that few carbonatites have chemical compositions that those of their parental liquids. The of parental carbonatite magmas and their differentiation paths could be by of mineral phases using or on mineral in the that such may to more the different paths that are during magmatic is a clear need for further experimental data on the precipitation of mineral phases from carbonatitic melts. The relationships of both silicate and carbonate rocks in most carbonatite complexes that studies of both are to their The shown by the silicate rocks are by the isotopic documented between phases and their rocks (e.g. & Bell, 1993). In the of a number of distinct carbonatite–silicate rock associations that complex be evaluated on its including detailed field chemical and isotope studies of individual complexes the way in which different carbonatites can like to the of Canada and the of Canada for their in the two and a half day symposium on carbonatites that formed part of their annual meeting held in Ottawa in The also the and for their and in this volume through to our to the for their Gittins, Hamilton, Koster van Wyllie, The