Evidence of silicate immiscibility within flood basalts from the Central Atlantic Magmatic Province

Abstract

[1] The role silicate-liquid immiscibility plays in the formation of macro-scale, bimodal volcanic/plutonic igneous complexes, and Fe-Ti oxide deposits is debated as the rock compositions produced by immiscibility are similar to those produced by other petrological processes. Within the flows of the North Mountain basalt of the Central Atlantic Magmatic Province are centimeter-thick granophyre layers. The granophyre layers are a mixture of mafic (i.e., ilmenite, magnetite, ferroaugite, plagioclase, stilpnomelane, ferrorichterite) and felsic (i.e., sanidine, quartz) minerals and highly siliceous (>75 wt% SiO 2 ) mesostases. Petrological modeling indicates that the siliceous mesostasis 1 sanidine 1 quartz 6 ferrorichterite represents a Si-rich silicate immiscible melt whereas the ferroaugite 1 plagioclase 1 stilpnomelane represent the Fe-rich silicate immiscible liquid. The identification of naturally occurring silicate-liquid immiscibility at scales greater than micron level is an important observation which may be useful in identifying volcanic and plutonic rocks which formed by macro-scale silicate-liquid immiscibility.

A petrological experiment on Emeishan basalt: Implications for the formation of syenite from the Baima igneous complex

“…Experimental studies show that tholeiitic and alkalic basaltic liquids may enter conditions of silicate-liquid immiscibility after a period of crystal fractionation and produce an Fe-rich silicate liquid and a Si-rich silicate liquid (Philpotts 1976(Philpotts , 1982Watson 1976;Naslund 1983;Veksler et al 2007Veksler et al , 2008Veksler 2009;Shellnutt et al 2013). The bulk composition of the Si-rich silicate liquid is similar to granitic rocks in general whereas the Fe-rich silicate liquid resembles a mafic-ultramafic rock.…”

Non-traditional stable isotope behaviors in immiscible silica-melts in a mafic magma chamber

“…Silica liquid immiscibility was suggested as a method of magmatic differentiation a century ago 35 . However, it is only recently, thanks to progress in experimental and petrographic studies 36 , that A-type felsic rocks at the top of many layered intrusions and in many tholeiitic provinces have been regarded as immiscible silica melts 18 19 20 21 . The process of silica liquid immiscibility is complex and not well understood and is beyond the scope of the present study.…”

“…In fact, there is a general trend among A-type felsic rocks of increasing δ 57 Fe values associated with increasing SiO 2 content 15 16 17 . A-type felsic rocks often occur at the top of many layered intrusions and many tholeiitic provinces, and are interpreted to be the products of magmatic differentiation 18 19 20 21 .…”