The determination of detrital modes in graywackes and arkoses, here grouped as subquartzose sandstones, requires special attention to the identification of detrital grain types, as defined operationally, and to the recognition of detrital textures, as opposed to textural elements of diagenetic origin. Accurate detrital modes can yield specific information on provenance that can be gained in no other way. Reserving the terms graywacke and arkose for imprecise field descriptions and abandoning the proportion of matrix as a prime means of classification, subquartzose sandstones can be described adequately using six numerical grain parameters. Three primary parameters summing to 100 permit quantitative serial designation of rock types in the form Q x F y L z , where Q is total quartzose grains, F is total feldspar grains, and L is total unstable lithic fragments in the framework. Three secondary parameters in the form of ratios permit desirable refinement within each primary parameter: C/Q where C is total polycrystalline quartzose grains, P/F where P is total plagioclase grains, and V/L where V is total volcanic lithic grains. Stable grains, whose sum is the parameter Q, are essentially pure silica in mineralogy, and include both monacrystalline quartz and polycrystalline lithic fragments. The latter are gradational to less quartzose types and difficult to distinguish from felsite grains, which display relict textures of volcanic origin and internal relief owing to their polyminerallic nature. Lithic fragments are subdivided at two levels into four main categories within which are various subcategories: (a) volcanic fragments include felsitic, microlitic, lathwork, and vitric types; (b) clastic fragments include silty-sandy and argillaceous types; (c) tectonite fragments include metasedimentary quartzose types and metavolcanic feldspathic or ferromagnesian types; and (d) microgranular fragments include hypabyssal, hornfelsic, and indurated sedimentary types. Interstitial materials include (a) exotic cements like calcite or zeolite; (b) homogeneous, monominerallic phyllosilicate cement displaying textures indicative of porefilling; (c) clayey detrital lutum called protomatrix; (d) recrystallized lutum or protomatrix called orthomatrix with relict detrital texture (e) murky, polyminerallic, diagenetic pore-filling called epimatrix, whose growth is accompanied by alteration of framework grains; and (f) deformed and recrystallized lithic fragments called pseudomatrix. Extensive albitization and other alterations complicate the interpretation of detrital modes in many rocks, but potential errors commonly can be avoided by close attention to mineralogy and relict texture except where tectonite fabrics disrupt or transpose the original detrital framework. Most subquartzose sandstones were derived from one. or a mixture, of three salient provenance types: (a) volcanic terranes, yielding feldspatholithic rocks nearly free of quartz, (b) plutonic terranes, yielding feldspathic rocks with few lithic fragments, and (c) uplifted sedimentary and metasedimentary "tectonic" terranes, yielding "chert-grain" lithic rocks with few quartz and feldspar grains except where recycled volcanic or plutonic detritus is abundant. Many voluminous accumulations of subquartzose sandstones near continental margins had as their provenance complex volcano-plutonic orogens representing ancient magmatic arcs analogous to modern arcs associated with trenches.

First Page Preview

First page PDF preview
You do not currently have access to this article.