Naming materials in the magma/igneous rock system

A review of the relationships between granitoid types, their origins and their geodynamic environments

A-type granites and related rocks: Evolution of a concept, problems and prospects

Abstract Asia is the world’s largest composite continent, comprising numerous old cratonic blocks and young mobile belts. During the Phanerozoic it was enlarged by successive accretion of dispersed Gondwana-derived terranes. The opening and closing of palaeo-oceans would have inevitably produced a certain amount of fresh mantle-derived juvenile crust. The Central Asian Orogenic Belt (CAOB), otherwise known as the Altaid tectonic collage, is now celebrated for its accretionary tectonics and massive juvenile crustal production in the Phanerozoic. It is composed of a variety of tectonic units, including Precambrian microcontinental blocks, ancient island arcs, ocean island, accretionary complexes, ophiolites and passive continental margins. Yet, the most outstanding feature is the vast expanse of granitic intrusions and their volcanic equivalents. Since granitoids are generated in lower-to-middle crustal conditions, they are used to probe the nature of their crustal sources, and to evaluate the relative contribution of juvenile v. recycled crust in the orogenic belts. Using the Nd-Sr isotope tracer technique, the majority of granitoids from the CAOB can be shown to contain high proportions (60 to 100%) of the mantle component in their generation. This implies an important crustal growth in continental scale during the period of 500–100 Ma. The evolution of the CAOB undoubtedly involved both lateral and vertical accretion of juvenile material. The lateral accretion implies stacking of arc complexes, accompanied by amalgamation of old microcontinental blocks. Parts of the accreted arc assemblages were later converted into granitoids via underplating of basaltic magmas. The emplacement of large volumes of post-accretionary alkaline and peralkaline granites was most likely achieved by vertical accretion through a series of processes, including underplating of basaltic magma, mixing of basaltic liquid with lower-crustal rocks, partial melting of the mixed lithologies leading to generation of granitic liquids, and followed by fractional crystallization. The recognition of vast juvenile terranes in the Canadian Cordillera, the western US, the Appalachians and the Central Asian Orogenic Belt has considerably changed our view on the growth rate of the continental crust in the Phanerozoic.

The Central Asian orogenic belt and growth of the continental crust in the Phanerozoic

In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the IUGS Subcommission on the Systematics of Igneous Rocks In the course of creating the classification, the Subcommission has established ten principles for its construction and for defining an appropriate nomenclature The principles are: (1) use descriptive attributes; (2) use actual properties; (3) ensure suitability for all geologists; (4) use current terminology; (5) define boundaries of rock species; (6) keep it simple to apply; (7) follow natural relations; (8) use modal mineralogy; (9) if mode not feasible, use chemistry; (10) follow terminology of other IUGS advisory bodies These principles and their rationale have not previously been enunciated The classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering the main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F) Rocks with mafic content >90% have their own classification If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used The nomenclature for these classifications necessitates only 297 rock names out of the c  1500 that exist

Le. Maitre

Papers

A classification of igneous rocks and glossary of terms