岩浆岩石埃达克质岩.ppt

中国J-K古地理图 两种成因类型： （1）与俯冲洋壳熔融有关的 （2）与俯冲洋壳熔融无关的 埃达克质岩的成因 Petrogenesis of Adakites Source features Subducted slab Underplated lower crust Ancient or pre-existed lower crust Petrogenetic models Primary magma and crystallization Assimilation fractionation crystallization (AFC) Mantle- and crustal-derived magma mixing Tectonic settings Slab melting: Island-arc active continental margin Delamination after thickening (Collision subduction) *Adakites: slab melts *High-Mg andesites (sanukites) Adak type:reaction of adakite melt with mantle Piip type: adakite melt metasomatized *Nb-enriched arc basalts (NEAB) *Boninites A new sequence of arc magmatism 研究实例 徐淮地区adakitic岩石的成因 (Xu et al., 2006, JAES) 岩相学 岩石地球化学判据 岩石地球化学判据 * * Adakite Definition of adakite Defant and Drummond (1990) : “?????? They are volcanic or intrusive rocks in Cenozoic arcs associated with subduction of young (? 25 Myr) oceanic lithosphere. Adakites are characterized by ? 56% SiO2, ? 15% Al2O3 (rarely lower), usually 3% MgO (rarely above 6% MgO), low Y and HREE relative to island-arc ADRs (for example, Y and Yb ? 18 and 1.9 ppm, respectively), high Sr relative to island-arc ADR (rarely 400 ppm), low high-field strength elements (HFSEs), as in most island-arc ADRs, and 87Sr/86Sr usually 0.7040. The petrography of adakites is somewhat variable. Plagioclase is a ubiquitous phase, and amphibole is common in all samples but the more MgO-rich varieties. Clinopyroxene and orthopyroxene are also common constituents, with biotite and opaques occurring frequently. A common assemblage is plagioclase and amphibole (with or without biotite, pyroxene and opaques. Adakites are rarely associated with parent basalts or basaltic andesite. When they are, the basaltic rocks are usually extremely enriched in large-ion lithophile elements (such as absarokites and shoshonites)” Definition of adakite Defant and Kepezhinskas (2001) : “Adakites typically have SiO256 wt.%, Al2O3 15 wt.%, and Na2O3.5 wt% and are