Derivation of potassic (shoshonitic) magmas by decompression melting of phlogopite plus pargasite lherzolite

A model metasomatized lherzolite composition contains phlogopite and pargasite, together with olivine, orthopyroxene, clinopyroxene and spinel or garnet as subsolidus phases to 3 GPa. Previous works established that at greater than or equal to 1.5 GPa, phlogopite is stable above the dehydration solidus, determined by the inching behaviour of pargasite and coexisting phases. At 2.8 GPa, melts with residual phlogopite + garnet lherzolite mineralogy at 1195degreesC and with garnet lherzolite mineralogy at 1250degreesC are both olivine nephelinite with K/Na (atomic) = 0.51 and K/Na = 0.65, respectively. Recent work shows that melting along the dehydration (fluid-absent) solidus of the phlogopite + pargasite lherzolite at pressures < 1.5 GPa is very different with the presence of phlogopite, decreasing the solidus below that of pargasite lherzolite. At 1.0 GPa, both phlogopite and pargasite disappear at temperatures at or slightly above the solidus. The compositions of two melts at 1.0 GPa, 1075degreesC (with different water contents), in equilibrium with residual spinel lherzolite mineralogy are silica-saturated trachyandesite ( similar to 5% melt fraction, similar to 3% H2O) to silica-oversaturated basaltic andesite ( similar to 8% melt fraction, 4.5% H2O). Both compositions may be classified as 'shoshonites' on the basis of normative compositions, silica-saturation, and K/Na ratio. Decompression melting of metasomatized lithospheric lherzolite with minor phlogopite and pargasite may produce Primary 'shoshonitic' magmas by dehydration melting at similar to 1 GPa, 1050-1150degreesC. Such magmas may be parental to Proterozoic batholithic syenites occurring in Brazil. (C) 2003 Elsevier B.V. All rights reserved.