Widespread occurrence of jadeite, lawsonite, and glaucophane in central California

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Abstract
The Pacheco Pass area, 80 mi SE. of San Francisco, has the most extensive development of metamorphic jadeite and lawsonite yet reported. The host rocks are metamorphosed geosynclinal sedimentary and igneous rocks in the Franciscan Formation (Jura-Cretaceous), and are of 2 types: 1) partially recrystallized rocks that appear in hand specimen relatively unaltered ("subcrystalline" rocks); and 2) thoroughly recrystallized schists, gneisses, and hornfelses ("crystalline" rocks). Subcrystalline rocks predominate; crystalline rocks are common locally, especially in major shear zones. Microscopic and X-ray analyses of the subcrystalline rocks indicate a progressive metamorphic sequence not apparent elsewhere. Lawsonite crystallized first, replacing calcium plagioclase, muscovite, biotite, and possibly other minerals. It occurs in almost all samples from the eastern half of the map area (140 sq mi), and in approximately one-half of those from the western part. Jadeite crystallized next, replacing albite in clastic sedimentary rocks, and both albite and augite in greenstones. Glaucophane, replacing chlorite and hornblende, began to crystallize typically before the crystallization of jadeite was completed. Jadeite and glaucophane are present in almost all samples from the eastern half of the area. Jadeite, lawsonite, and glaucophane are common also in the crystalline rocks, along with other minerals characteristic of the glaucophane schist facies. Greenschist facies rocks and eclogite occur locally in shear zones. The consistent distributional pattern of the subcrystalline rock types probably reflects a regional variation in the physical conditions of metamorphism. The local development of crystalline rocks might indicate sharp local changes in temperature, pressure, or solution action, or possibly the carrying in of these rocks by faulting. The high P-T condition of metamorphism may have been caused by rapid subsidence in a deep marine environment, coupled with widespread tectonic pressure. The absence of syntectonic intrusions probably reflects the low temperature condition of metamorphism, but whether this is a cause or effect relationship is not clear. Finally, the widespread nature of this type of metamorphism at Pacheco Pass suggests new possibilities for seismic interpretation of parts of the crust which may have had a similar geologic history.

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