Crystal Fractionation and the Evolution of Intra-plate hy-normative Igneous Suites: Insights from their Feldspars

Feldspars and normative feldspar constituents of bulk magma show complexes and ‘anorogenic’ granites, is associated with hotspot and continental rift environments. A major chartrends supportive of fractional crystallization in the three main types of hy-normative intraplate suites that contain qz-oversaturated acteristic of this magmatism is the production of suites of compositionally diverse rocks that are temporally and rocks: ocean island and continental alkalic suites, ocean island tholeiitic suites and continental tholeiitic suites. These suites are spatially associated. Much is still unknown about the processes that have led to the diversity of magmas within characterized by the presence of a single feldspar in each suite member, a shift of this feldspar from plagioclase to alkali feldspar, such suites or to the global diversity of intra-plate suite types. and K enrichment of alkali feldspar with decreasing temperature in the trachytic members. The modal feldspars provide evidence for a Many ocean island and continental hotspot (and continental rift) related suites are characterized by rocks with reaction relationship between feldspars and indicate a build-up of magmatic volatile content towards saturation with progressive high alkali/silica ratios that fall into the alkalic field of Irvine & Baragar (1971; Fig. 1) regardless of their state fractionation of a parental magma having low initial volatile content. The feldspar and normative feldspar evolutionary paths are unique of silica saturation. Such alkalic suites range from those that are strongly silica undersaturated, containing rocks for each of the three suite types but similar for different suites within the same type. This characteristic extends to the felsic members, such as nephelinites (or leucitites in the more potassic making it easy to distinguish between rhyolitic or granitic rocks suites), to mildly silica-undersaturated suites [the Kenfrom the different suite types. The feldspars in natural volcanic nedy series of Miyashiro (1978)] containing alkali basalts, suites commonly show evidence for a polybaric history, particularly silica-undersaturated trachytes and phonolites, and, in the least-evolved suite members. Late-stage feldspars of the finally, to hy-normative suites [the Coombs trend of intermediate members and feldspars of the most evolved members Miyashiro (1978)] that include olivine basalts, trachytes show paths indicative of significantly lower temperature and pressure and rhyolites. Hy-normative alkalic suites that contain regimes. silica-oversaturated rocks on ocean islands are widespread and include those from Ascension Island (e.g. Harris, 1983), the Samoan Islands (MacDonald, 1968), Terceira, Azores (e.g. White et al., 1979) and Clarion Island, Mexico