The plutonic alkaline series: the problem of their origin and differentiation, the role of their mineralogical assemblages

Abstract

The plutonic alkaline ring complexes present several different trends of differentiation, the two more important being the silica-oversaturated one and the silica-undersaturated one. A hiatus appears in the silica-oversaturated association for monzonitic compositions, which may be compared with the “Galy Gap” in volcanic series. Characteristics of the plutonic series as the cumulative features of the basic rocks, the differentiated patterns of the acid rocks and the monzonites which present REE patterns of initial liquid lead one to propose the existence of a magma of monzonitic composition. Its vanishing is owing to cumulation of the early mineral phases and to consequent evolution of the residual liquid. The trends of differentiation can be explained by a multistage model where mineral phases are both controllers of the differentiation and images of this differentiation. The path from monzonites to syenites is governed by calcic plagioclase (Bowen's plagioclase effect), olivine and calcic clinopyroxene, which have effects upon iron-enrichment and increase in alkalies. Calcic amphiboles, and more modestly biotite, play a significant role for the silica level of the liquids. The path from syenites to late stage differentiates is governed by alkali feldspars and calcic amphiboles. Late iron-rich sodic (amphiboles and pyroxenes) and potassic (annite-siderophyllite) minerals are reflecting the mode of the differentiation. One must also consider opaque minerals, marks of fo 2, and zircons, marks of temperature and agpaitic index. In the alkaline plutonic series, the controls of the differentiation by minerals can explain the coexistence of silica-saturated and undersaturated series and of peralkaline and peraluminous trends. The role of calcic amphiboles seems fundamental and we suggest that their stability is controlled by the characteristics of surrounding lithosphere: water amounts, porosity and permeability. Thus can be understood differences observed between oceanic and continental alkaline series, between complexes emplaced in cratonic environments or in youthful mountain belts, and specially coexistence of granites and nephelinic syenites, subsolvus granites and albitic granites, with their features showing a crustal signature.