The New Idria serpentinite.

摘要

This thesis describes the mineralogy and petrology of the serpentinite massif at New Idria, California, USA. This serpentinite is an oval, 23 x 8 km body, interpreted as a part of the Mesozoic Coast Range Ophiolite. Diapiric emplacement within the Coast Ranges has resulted in a piercement structure or tectonic window, lying just east of the San Andreas fault system northeast of Parkfield. The mineralogy of the serpentinite is dominated by chrysotile; New Idria is one of the largest deposits of chrysotile asbestos in the world. A model for the generation of short fiber chrysotile asbestos is proposed. Locally within the serpentinite, tectonic blocks or "knockers" of several distinct rock types are preserved. Some blocks contain exotic Ti-rich minerals that previously were regarded as metasomatic; these are reinterpreted as the products of essentially isochemical metamorphism. Blocks of all types have experienced M1 and M2 metamorphism, of blueschist/greenschist and sub-greenschist facies respectively. It is proposed that M2 metamorphism is related to passage of the Mendocino Triple Junction in the Miocene. Blocks consisting of chlorite + diopside + Ti-garnet are interpreted as metapyroxenites. Blocks containing the rare Ti-minerals benitoite and neptunite are interpreted as re-metamorphosed Franciscan mafic schists and greenstones. Massive antigorite blocks have a thermal history separate in part from the rest of the serpentinite. Approximately 300 chemical analyses of Ti-andradite and Ti-grossular garnets and their coexisting minerals are presented.; Public policy issues, related to the largely unfounded concerns about health hazards of chrysotile asbestos, have brought recent attention to New Idria. Fluvial transport of chrysotile asbestos from New Idria eastward into the San Joaquin Valley has been taking place for as much as 12 million years. Since the completion of the California Aqueduct, some of the chrysotile asbestos has gone into the canal, mainly under flood conditions. Therefore the downstream users of this water have found chrysotile asbestos fibers in the water. Concern about the possible health hazards of this condition has been greatly overstated; the hazard is negligible. It will be a matter for science and public policy together to find a safe and economical solution to these issues.