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Pre-Cretaceous Sedimentation and Metamorphism in the Winchester Area, Northern Peninsular Ranges, California

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January 01, 1969

An area near Hemet in Riverside County, California, has been studied in which the sedimentology and pre-intrusive metamorphism at the site of the Southern California Batholith are well preserved. Pelitic schists of the Jurassic (?) Bedford Canyon Formation are conformably overlain by a 13,000-foot thick section of quartzite, schist, and amphibolite of the French Valley Formation (new name). These rocks were formed from a series of shales, shale-clast conglomerates, poorly sorted feldspathic, calcareous, and arkosic sandstones, and rare basaltic extrusives. Relict sedimentary textures and structures preserved at low metamorphic grade indicate a marginal basin environment; the age of deposition, formerly thought to be Paleozoic, is unknown.

The sedimentary rocks were intruded by basaltic sills, folded and regionally metamorphosed at low pressures (3–5 kb) and moderate-to-high temperatures, up to 750°C, producing andalusite, cordierite, sillimanite, and garnet in the pelitic schists, and converting the basic igneous rocks to garnet-diopside-amphibolites. Four metamorphic zones are distinguished. Their boundaries trend north-south, athwart the regional strike, and bear no relation to the distribution of plutonic intrusives of the Southern California Batholith. Ultra-basic magma was intruded following the kinematic phase of metamorphism but was metamorphosed at temperatures and hydrostatic pressures comparable to those affecting the enclosing gneiss. The metamorphism is thought to have antedated emplacement of the Southern California Batholith (Late Cretaceous) and is analogous to the low-pressure andalusite-type metamorphism described by Miyashiro (1961).

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