THE GREAT GRANITIZATION CONTROVERSY REVISITED: IGNIMBRITE FLAREUPS AND EXPLODING BATHOLITHS

Debates between Neptunists and Plutonists over the origin of granite echoed again during The Great Granitization Controversy of the 1930's to 50's. During a climactic confrontation with N.L. Bowen at the 1947 GSA meeting, H.H. Read abandoned traditional rock classes (sedimentary, igneous, metamorphic), substituting neptunic, volcanic ("magmatic…effusive and basic"), and plutonic ("metamorphic, migmatic, and granitic"). Most granites he interpreted as neptunic rocks "converted to rocks of granitic character without passing through a magmatic stage" (GSA Mem. 28, 9, 1948). Supposed scarcity of rhyolite argued against abundant magmatic granite. Citing the volcanologist Rittmann, Read found no links between plutons and volcanoes. In contrast, Bowen insisted that "most…granitic rock…was formed by crystallization of molten magma closely resembling…rhyolitic lava" (Ibid, p. 86). Neither recognized evidence for granitic magmatism in voluminous pyroclastic rhyolites of Yellowstone (welded tuffs of Iddings, USGS Mon. 32, 356-430, 1899), New Zealand (ignimbrites of Marshall, Roy. Soc. NZ Trans. 64, 1-44, 1935), Sumatra (van Bemmelen, Westerveld), Queensland (Richards & Bryan) and the Americas (Fenner, Moore, Mansfield & Ross, Gilbert, Callaghan, Jenks). Not until the 1960's did R.L. Smith, C.S. Ross, and R.A. Bailey emphasize global abundance of ash-flow tuffs (GSA Bull. 71, 795-842, 1960; USGS PP 366, 1961). Ignimbrite calderas were recognized as "windows into the tops of granitic batholiths" by P.W. Lipman (JGR 89, 8801-8841, 1984), following Smith & Bailey (GSA Mem. 116, 613-662, 1968), among others (present author included). In 1947 nobody conceived of hundreds of cupolas of magmatic batholiths so shallow that their roofs blistered, burst explosively, and foundered, catastrophically inundating thousands of km² with tens to hundreds of m of incandescent magma foam. Numerous workers have now documented ignimbrite flareups on all continents, notably in the southwestern US and western Mexico in mid-Tertiary time.

Granite, Granitization, Ignimbrite, Caldera, History