Major-element geochemistry of pelites

Pelites (shales and mudstones) are arguably the most important rock type for interpreting metamorphism. Their significance derives from their widespread occurrence and the range of mineral assemblages they develop at different conditions of pressure and temperature. We compiled a global database of 5729 major-element whole-rock analyses of pelites from different metamorphic grades (shales to granulite-facies paragneisses) to (1) determine an average composition, (2) examine the range and variability in their composition, and (3) assess if there is evidence for grade-related geochemical changes. Median values are given instead of average values to eliminate the effect of extremes. The median worldwide pelite is as follows (anhydrous, values in wt%): SiO2 = 64.13, TiO2 = 0.91, Al2O3 = 19.63, FeOtotal = 6.85, MnO = 0.08, MgO = 2.41, CaO = 0.65, Na2O = 1.38, and K2O = 3.95. The median X-Mg = MgO/(MgO + FeOtotal) in moles is 0.39. The median XFe3+ = 2 x Fe2O3/(2 x Fe2O3 + FeO) in moles was measured in 1964 samples and is 0.23. On an Al2O3-FeO-MgO (AFM) diagram, the median worldwide pelite plots within a strong clustering of analyses between X-Mg(proj) = projected molar MgO/(MgO + FeOtotal) = 0.30-0.55 (median = 0.42) and A(Ms) = molar [Al2O3 - (3 x K2O)]/[Al2O3 - (3 x K2O) + FeOtotal + MgO] = 0.0-0.4 (median = 0.19). Pelites show a continuous decrease in volatile content with increasing metamorphic grade and a decrease in XFe3+ from the diagenetic to biotite zone. Lower median SiO2 values and higher median Al2O3 and A(Ms) values in the porphyroblast and subsolidus sillimanite or K-feldspar zones, as well as higher median MnO values in the garnet zone, may reflect sampling bias or metasomatism.

Automated summary

Pelites are the most important rock type for interpreting metamorphism due to their widespread occurrence and range of mineral assemblages at different pressure and temperature conditions. A global database of pelites was compiled to determine their average composition, examine the variability in composition, and assess grade-related geochemical changes. The results show a decrease in volatile content and XFe3+ with increasing metamorphic grade, while SiO2, Al2O3, and A(Ms) values increase in specific metamorphic zones.