Tectonic Rock Cycles

Abstract

Tectonic rock cycles reorganize rock processes from the traditional rock cycle into a tectonic framework using the Wilson cycle as a paradigm. In tectonic rock cycles, tectonic rock classifications replace descriptive rock classifications and focus on rocks characteristic of major tectonic regimes, such as the igneous-fractionation regime at convergent plate boundaries, quartz/feldspar/lithic (QFL) sedimentary distributions, and major metamorphic temperature/pressure fields.The tectonic rock cycle can be thought of as beginning with two igneous-fractionation processes generating all major igneous rocks at divergent and convergent plate boundaries. Weathering/erosion of subduction-generated volcanic arcs produce lithic-rich sediments. Arkosic to quartzitic sediments come from weathering/erosion of the batholithic (proto-continental) core of the volcanic arc. These processes generate the QFL end-member clastic sediments, which are spatially separated in the rock cycle. Carbonates are associated with quartz arenites in stable cratons. Along various paths, metamorphic processes associated with tectonic regimes generate eclogite, blueschist, and Barrovian metamorphic rocks.Tectonic rock cycles are cyclical only in that any major rock type (igneous, sedimentary, metamorphic) can be transformed into any other major rock type. They are non-cyclical in that not all transformations are equally likely. With an original mafic-dominated earth, all other rock types evolve from that parent by igneous/sedimentary fractionation processes, some irreversible. Some paths on the “cycle” generate largely sterile, fractionated, end-member rocks such as ultramafic residues, quartz arenites, and felsic igneous rocks sequestered in continental cratons.