Igneous Rock Series: Tholeiitic & Calc-alkaline Nature
Igneous rocks are broadly classified based on their chemical composition, particularly their alkali (Na₂O + K₂O) content. The Tholeiitic and Calc-alkaline series represent two major compositional fields commonly encountered in igneous petrology.
Rock Series Classification
- Tholeiitic Series: These rocks are characterized by a lower alkali content compared to alkaline rocks. Geochemically, they fall below the defining line for alkaline rocks in classification diagrams, such as the Total Alkali-Silica (TAS) diagram. They are classified as subalkaline.
- Calc-alkaline Series: Similar to the tholeiitic series, calc-alkaline rocks also exhibit relatively low alkali content. They plot in a distinct field but are also positioned below the alkaline threshold. Therefore, they are also classified as subalkaline.
Distinction from Alkaline Rocks
True alkaline rocks possess a significantly higher concentration of alkali metals (Na₂O and K₂O). Both the Tholeiitic and Calc-alkaline series are grouped under the broader category of subalkaline rocks, differentiating them from the distinctly alkaline rock suites.
Conclusion
Based on their geochemical characteristics, both Tholeiitic and Calc-alkaline rock series are defined as subalkaline in nature.
Important Questions from Igneous Petrology (Classification, Forms, Textures, Genesis)
- Match the rock types in Group-I with their most likely corresponding lithospheric / tectonic settings of formation in Group-II
Group-I Group-II P. Boninite 1. Continental anorogenic Q. Lamproite 2. Island-arc R. Phonolite 3. Continental collision S. Leucogranite 4. Intraplate oceanic - The mole percentages of SiO$_2$, Al$_2$O$_3$ and K$_2$O in a granitic rock are 84.21, 7.89 and 7.89, respectively. The molar proportion (in %) of K-feldspar in the rock is ________ [ round off to one decimal place]
- Which one of the following is a mafic volcanic rock?
Match the following igneous textures in Group-I with their definitions in Group-II.
Group-I
Group-II
P Vitrophyre 1 Alkali feldspar rimmed by plagioclase Q Rapakivi 2 Aggregate of radially arrayed, needle-like crystals of plagioclase with or without clinopyroxene R Ocelli 3 Sub-parallel skeletal, platy olivine and/or pyroxene S Spinifex 4 Large phenocrysts within a glassy matrix - A melanocratic porphyritic rock containing phenocrysts of biotite, with feldspar restricted to the groundmass, is called
