Magma/Rock Series • Signify the genetic relationship between groups of chemically (and petrographically) different but regionally associated igneous rocks. • A group of rocks that share some chemical characteristics and show a consistent pattern on a variation diagram, suggesting a genetic relationship, can be referred to as a “magma series”. • Also called magma clan, association, lineage, magma type etc. • Each series follows some characteristic evolutionary path from a unique type of parent magma through a series of more evolved silicic derivative types. • Describes about the evolution of magma • Broadly two categories: Alkaline, Subalkaline • Alkaline rocks: richer in alkalis and silica undersaturated • Subalkaline rocks: poor in alkalis and are either silica saturated or oversaturated • Subalkaline magma series are two types: Tholeiitic and Calc-alkaline magma series • Shand (1927): Peraluminous = [Al2O3 > (CaO+Na2O+K2O)] Metaluminous = [Al2O3 < (CaO+Na2O+K2O) But Al2O3 > (Na2O+ K2O) ] Peralkaline • = [Al2O3 < (Na2O+ K2O)] (show the diagram also) Alkaline = within plate (both oceanic and continental) and convergent plate margin • Tholeiitic = divergent plate margin (mid oceanic ridges) and within plate (both oceanic and continental) • Calc-alkaline = Subduction zone and are NOT found within the plate Tholeiitic and calc-alkaline magma series This can be distinguished in an AFM diagram, a ternary diagram showing the relative proportions of the oxides of Na2O + K2O (A), FeO + Fe2O3 (F), and MgO (M). The arrows describe the path of the magmas in the tholeiitic and the calc-alkaline magma series. Rocks in the tholeiitic magma series are classified as and are distinguished from rocks in the calc-alkaline magma series by the redox state of the magma they crystallized from. Tholeiitic magmas are reduced where as the Calc-alkaline magmas are oxidized. When the parent magmas of basalts crystallize, they preferentially crystallize the more magnesium-rich and iron-poor forms of the silicate minerals olivine and pyroxene, causing the iron content of tholeiitic magmas to increase as the melt is depleted of ironpoor crystals. However, a calc-alkaline magma is oxidized enough to precipitate significant amounts of the iron oxide magnetite, causing the iron content of the magma to remain steadier as it cools than with a tholeiitic magma. As magmas cool, they precipitate out significantly more iron and magnesium than alkali, causing the magmas to move towards the alkali corner as they cool. In the tholeiitic magma, as it cools and preferentially produces magnesium-rich crystals, the magnesium content of the magma plummets, causing the magma to move away from the magnesium corner until it runs low on magnesium and simply moves towards the alkali corner as it loses iron and any remaining magnesium. With the calc-alkaline series, however, the precipitation of magnetite causes the iron-magnesium ratio to remain relatively constant, so the magma moves in a straight line towards the alkali corner on the AFM diagram. The oxidizing nature of calc alkaline magma is due to the addition of the crustal component to the magma. Therefore, the calc-alkaline magma is typically found in the subduction tectonic setting. AFM diagram showing "typical" areas for various extents of evolution from primitive magma types. Tholeites go through a Ferro-Basalt stage before continuing towards Rhyolite.
