Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan

Abstract

Garnet is a widespread detrital mineral in Permian-Jurassic sandstones of the Southern Kitakami Terrane. Electron-probe microanalyses of these garnets show a gradual change of chemistry with the age of the sandstones. The dominant garnet suites in Permian-Triassic sandstones are grandite, with some spessartine-almandine in Upper Triassic sandstones. However, in Lower Jurassic sandstones, pyrope-almandine and spessartine-almandine garnets are common, whereas grandite garnets are absent. Garnet suites in Middle to Upper Jurassic sandstones are dominated by pyrope-almandine garnet. The grandite-dominant suites are associated with hornblende, epidote and titanite, whereas the pyrope-almandine-dominant suites are associated with biotite. These grandite garnets are rich in andradite and have very low spessartine, almandine and pyrope contents. Some of the grandite garnets are anisotropic with oscillatory zoning and sector twinning, typical of calcic skarn deposits. Spessartine-almandine garnets are common in low-grade metamorphic rocks. The diverse suites of pyrope-almandine garnets (up to 42 mol.% pyrope) from the Jurassic sandstones are typical of epidote-amphibolite to granulite facies metamorphic rocks. The chemistry and optical properties of garnets and the heavy-mineral data show that low-grade or contact metamorphic rocks such as skarns and hornfelses were widely exposed in the source areas of the Permian-Triassic sandstones. High-grade metamorphic rocks, including granulites and intrusive rocks, appeared in Early Jurassic time and were widespread during the Middle Jurassic. The critical change in the source areas for the Southern Kitakami Terrane occurred during Late Triassic to Early Jurassic time, and may correspond to the Indosinian movements associated with the collision between the North China and South China blocks.