科研之友
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摘要
The appinite suite of rocks offers a unique opportunity to study the effect of water on the generation, emplacement and crystallization history of mafic to felsic magma. The suite consists of a group of coeval plutonic and/or hypabyssal rocks, ranging from ultramafic to felsic in composition in which hornblende is the dominant mafic mineral, and typically occurs both as large prismatic phenocrysts and in the finer grained matrix. The suite is also characterized by abundant evidence for mixing and mingling between diverse magma types and variable degrees of contamination by host rock. Field observations corroborate experimental and theoretical studies that the hornblende stability field expands at the expense of olivine and pyroxene with increasing pH(2)O in the magma. Textures characteristic of appinites are consistent with rapid growth and with experimental evidence for the reduced viscosity of melts allowing efficient migration of ions to the sites of mineral growth. The appinite suite was originally defined in the Paleozoic Caledonide orogen in Scotland, where it occurs as a number of small shallow crustal bodies that were emplaced after the cessation of subduction and in the immediate aftermath of terrane collision and closure of the Iapetus Ocean. The mafic component is thought to have been triggered by asthenospheric upwelling following stab break-off, and magmas produced have both juvenile and sub-continental lithospheric mantle components. Its compositions have affinities with shoshonites. The felsic components include large batholiths that were probably derived by fractional crystallization. Other appinite suites share some, but not all of these characteristics. Appinite suites apparently range in age from Neo-Archean to Recent, and occur at all crustal levels, at depths of up to 40 km. In addition to shoshonites, appinite suites share some similar geochemical features with high-Mg andesites, sanukitoids and adakites. Some common tectonic traits include a tendency to form soon after the cessation of subduction, and the important role of deep crustal faults as conduits for magmas of various compositions to rise towards the surface. These conduits provide the setting for magmas of diverse composition to mix and mingle. Neo-Archean appinites, and their genetic relationship with abundant coeval sanukitoids, have been interpreted as evidence for the existence of some form of plate tectonics at that time. Melting may be triggered by asthenospheric upwelling caused either by slab breakoff (e.g. after terrane or continental collision) or by the generation of a slab window (e.g. where a ridge collides with a subduction zone). Mafic magma may contain a juvenile component but Nd isotopic data suggest the additional involvement of a sub-continental lithospheric mantle that, in many instances, was previously metasomatized by fluids and magma and was underplated by mafic complexes during subduction. The composition of the mafic magma may vary from one suite to another. In several suites, the mafic magmas have more traditional calc alkalic or tholeiitic affinities and do not share the shoshonitic characteristics of the type area. In addition, in several appinite complexes, felsic magma was formed by crustal anatexis, rather than fractional crystallization.
- 出版日期2013-4
