A Classification of Metamorphic Grains in Sands Based on their Composition and Grade

ABSTRACT: An operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data re- producibility among operators and the potential of high-resolution bulk petrography in provenance studies. For each of four protolith compositions (metapelite, metapsammite/metafelsite, metacarbonate, metabasite), six archetype grains displaying increasing degree of re- crystallization and foliation development are illustrated. Such a clas- sification grid is specifically devised as a subsidiary tool for point counting with the Gazzi–Dickinson method. Traditional QFR param- eters can also be easily recalculated from the data set obtained, thus meeting all possible needs (Decker and Helmold 1985; Suttner and Basu 1985). An experiment shows that usage of visual-comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way. A petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense-mineral data. The ‘‘metamorphic index’’ (MI) is introduced as an estimator of average metamorphic grade of source rocks. Our classification, an ex- tension of concepts used first in the study of arc–continent collision in Taiwan (Dorsey 1988) and successfully expanded to interpret the evo- lution of continent–continent collision in the Himalayas (Najman and Garzanti 2000; White et al. 2002), proves to be fruitful in provenance analysis of foreland-basin sediments shed from Alpine-type, thick- skinned collision orogens, particularly when integrated with dense- mineral, geochemical, and geochronological data.