The ultrapotassic rocks: Characteristics, classification, and constraints for petrogenetic models

Abstract

A definition for ultrapotassic rocks is introduced using the chemical screens K 2O > 3 wt.%, MgO > 3 wt.% and K 2O/Na 2O > 2 for whole-rock analyses. On this basis a data base of more than 800 analyses was retrieved by a literature review. These are grouped according to a ‘resemblance’ classification designed to aid petrogenetic modelling by by-passing mineralogically based nomenclature groupings. Three major chemical end-member groups are recognised: Group I, lamproites, characterised by low CaO, Al 2O 2 and Na 2O, and high K 2O/Al 2O 3 and Mg-number. Group I rocks have the highest ‘trace’ incompatible element contents, and carry mostly depleted dunite and harzburgite mantle xenoliths. West Kimberley and Gaussberg are chosen as the standard members. Group I rocks usually have no associated non-ultrapotassic rocks, but non-standard members have calc-alkaline and shoshonitic associates, may occur in orogenic areas, and frequently have some chemical characteristics which are transitional to those of group III. Group II, known as kamafugites, have Toro Ankole rocks as their standard members. They have low SiO 2 and high CaO. Incompatible elements are less enriched than group I, but have a positive Sr spike. Group III rocks occur in orogenic areas and have high CaO and Al 2O 3. Mg-number is often low due to fractional crystallisation, but primary magmas have many of the characteristics of mantle derivation. Partial melting of mantle material previously enriched in incompatible elements is considered the most likely explanation for the origin of primary magmas for all three groups. Processes in the mantle before enrichment, during the enrichment event, and during partial melting at the magma genesis stage are considered separately for their likely effect on magma characteristics defined by the data base. Group I rocks probably originate from a depleted mantle under H 2O- and F-rich, CO 2-poor and possibly CH 4-rich conditions. The range in silica contents of primary lamproitic magmas may be due to partial melting at variable depth, with the diamond-bearing olivine lamproites originating from the greatest depth. The low SiO 2, high CaO and Sr of group II rocks suggests melting in a CO 2-rich, H 2O-poor environment, which is also indicated by CO 2-rich volcanic gases. The source for Toro Ankole lavas may originally have been depleted, but evidence from nodules is lost due to pervasive metasomatism. Group III rocks occur in orogenic areas, and many of their chemical characteristics must be explained by similar processes to those in less alkaline island-arc magmatism. The low Ti, Nb and Ba may be due to titanate phases being stable in the residue during partial melting, or to reactions in the subducted slab. High K 2O may be derived from subducted crustal material or from mantle enrichment occurring beneath the maturing arc, but unrelated to the subducted slab. High CaO and Al 2O 3 are favoured by melting a fertile mantle source. A fourth, transitional group includes both rocks which are likely to be mantle-derived and those which may be cumulates from crustal melts. A number of minettes may be due to crustal modification of mantle-derived potassic magmas.