THE ULTRAPOTASSIC ROCKS: CHARACTERISTICS, CLASSIFICATION, AND CONSTRAINTS FOR PETROGENETIC MODELS

Abstract

A definition for ultrapotassic rocks is introduced using the chemical screens K2O > 3 wt.%, MgO > 3 wt.% and K2O/Na2O > 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, Al2O2 and Na2O, and high K2O/Al2O3 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 SiO2 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 Al2O3. Mg-number is often low due to fractional crystallisation, but primary magmas have many of the characteristics of mantle derivation.