Quartz grain types in Holocene deposits from the Spanish Central System: some problems in provenance analysis

Abstract

Holocene sands of the Spanish Central System were exclusively derived from plutonic , middle-upper grade and low-grade metamorphic rocks. Modal composition of studied sands is mostly controlled by grain size and source area lithology . Thus, sands derived from slates and schists plot near the QR edge on the QFR diagram for ali grain size fractions. Sands derived from granitic or gneissic rocks have a wide dispersion on the QFR diagram , from the R pole to the QF edge, depending on sand grain size. Percentages of quartz types in granitic-derived sands are Qnu42 , Qu40 , Qp2-314 and Qp > 34 . Sands of gneissic origin have Qnu 51 , Qu 1 5 , Qp2-323 and Qp > 311. Sands derived from slates and schists have Qnu 20 , Qu 1 2 , Qp2-35 and Qp > 363 . Quartz types easily discrimina te sands of low-grade origin, but distinction of sands derived from plutonic rocks from those derived from middle-upper grade metamorphic rocks is difficult because of the highly variable Qu content of plutonic rocks related to strain history and crystallization conditions. Thus, quartz types mus! be used with caution in source discrimination if plutonic rocks are present in the source area.