GSA HISTORY AND PHILOSOPHY OF GEOLOGY DIVISION STUDENT AWARDS, SECOND PLACE: THE ORIGIN OF GRANITE: FROM WERNER TO READ, FROM BOWEN TO CHAPPELL

The origin of granite has continually proved to be one of the most controversial issues in the earth sciences. The earliest granite controversy involved the competing theories of Werner’s Neptunism vs. Hutton’s Plutonism, with fiery debates continuing throughout the late 18^th and early 19^th centuries. Field evidence provided somewhat of a resolution and granites were accepted as intrusive igneous rocks and not sedimentary rocks laid down by Noah’s flood.

The late 19^th century saw the theory of granitisation arise in Europe, whereby rocks were transformed into granites by diffusive chemical fronts. The processes of metasomatism and migmatisation that the granitisers alluded to are difficult to unravel, even today. It was also during this time that it was suggested that different granites form in different tectonic environments, leading to H. H. Read proclaiming “There are granites and granites”. Petrology developed amidst the granitisation debate and the concept of magmatic differentiation took hold, but it wasn’t until the early 20^th century that such ideas had experimental support. Bowen’s phase diagrams led to the idea of fractional crystallisation in magma chambers, and his demonstration of natural granite compositions clustering around the experimentally determined “granite minima” provided the resolution to the granite controversy.

After the 1950’s there was a proliferation of classification schemes for igneous rocks, with routine chemical analyses providing more definitive rock descriptions. Chappell’s “alphabet granites” created the I-type and S-type granites, their chemistries reflecting derivation from igneous or sedimentary protoliths. This link between composition and different source materials was an important concept moving forward.

The recent advances in analytical capabilities have allowed us to probe deeper into granites and their minerals. High-resolution spatial analysis of accessory minerals can provide isotopic and geochemical constraints on source materials at both a regional and local scale. Modern geologists can now attempt to understand the origin of granite in unprecedented detail. However, whilst some still search for a unifying theory on the origin of granite, it is perhaps more appropriate to consider each granite upon its own merits; there are granites and granites.