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D/H and 18o; 16 o data have now been obtained on a wide variety of graniti~ batholiths of various ages. The primary 6D values of the biotites and hornblendes are .remarkably constant at about -50 to -85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary H 2 0 in these igneous rocks is probably not "juvenile", but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have 518o = +7.-0 to +10.0, probably indicating significant ,nvolvement of high-18o metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have 6180 = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low 618Q values (particularly in feldspars) during subsolidus exchange.Some granitic plutons in such environments also were emplaced as low. -18Q magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric H 2 0 exchange in the batholiths is given by low D/H ratios (6D as low as -180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordi I leran batholiths of western North America an increasingly higher proportion of the granitic rocks have 6D values lower than -120.The lowering of 6D values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zooes (10,000 km2) can be defined where 6D biotite <-100 and K-Ar "ages" have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulatioo systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surfacederived waters took place over an extended period long after emplacement, producing "brick-red" feldspars and markedly discordant Rb-Sr isochron "ages".

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