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Magma Genesis, Plate Tectonics, and Chemical Differentiation of the Earth

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REVIEWS OF GEOPHYSICS, VOL. 26, NO. 3, PAGES 370-404, AUGUST 1988 Magma Genesis, Plate Tectonics, and Chemical Differentiation of the Earth PETER J. WYLLIE Division of Geolo•7icaland Planetary Sciences,California Institute of Technolo•Ty,Pasadena Magma genesis,migration, and eruption have played prominent roles in the chemical differentiation of the Earth. Plate tectonics has provided the framework of tectonic environments for different suites of igneousrocks and the dynamic mechanismsfor moving massesof rock into melting regions.Petrology is rooted in geophysics.Petrological and geophysicalprocesses are calibrated by the phase equilibria of the materials. The geochemistry of basalts and mantle xenoliths demonstrates that the mantle is hetero- geneous.The geochemical reservoirs are related to mantle convection, with interpretation of a mantle layered or stratified or peppered with blobs. Seismic tomography is beginning to reveal the density distribution of the mantle in three dimensions,and together with fluid mechanical models and interpreta- tion of the geoid, closer limits are being placed on mantle convection. Petrological cross sectionscon- structed for various tectonic environments by transferring phase boundaries for source rocks onto assumedthermal structuresprovide physical frameworks for consideration of magmatic and metasoma- tic events,with examplesbeing given for basalts,andesites, and granites at ocean-continentconvergent plate boundaries, basalts and nephelinitesfrom a thermal plume beneath Hawaii, kimberlites in cratons, nephelinites from continental rifts, and anorogenic granites. The fluid dynamics of rock-melt-vapor systemsexerts strong control on igneous processesand chemical differentiation. Unravelling the pro- cessesduring subduction remains one of the major problems for understandingmantle heterogeneities and the evolution of continents. INTRODUCTION the magmatic processesoccurring nearer the source of magma The Union Lecture on which this review is based was pre- generation.There remains a gap betweenvolcanoes, their plu- sented on behalf of the International Association for Vol- tonic roots, and the magma sourceswhich has to be filled by canology and the Chemistry of the Earth's Interior (IAVCEI). indirect studies including geophysics,geochemistry, and fluid It was an over-illustrated, somewhat superficial ramble dynamics. Lavas reach the surface through volcanoes,and through the whole Earth emphasizing volcanic activity, the lavas can provide information about the source rocks from more dramatic aspectsof the Earth's chemical differentiation. which they were derived if the materialsand processescan be I have been charged by President Lal of the International suitably calibrated in the laboratory. The calibration involves Union of Geodesy and Geophysics (IUGG) to prepare the the geochemistryof major, minor, and trace element distri- review more or less along the lines of the lecture, without butions (including isotopes)between minerals and melts and writing the book and compilingthe enormouslist of references the conditions for melting of the various source materials required to do formal justice to all these topics. The list of under various conditions.Some lavas also bring to the surface referencescited is admirably supplementedby the comprehen- samplesof the host rocks from which they were derived by sive bibliographies provided in the U.S. National Report to partial melting or of the rocks through which they rose.The the IUGG for 1983-1986, published in Reviews of Geophyics lavas releasegases to the atmosphere and hydrosphere which (volume 25, numbers 2, 3, 5, and 6, 1987). also provide clues about the nature of the source material at Volcanoes have played a significant role in the development depth. of geology since Abraham Werner at the beginning of the last The theme common to the different parts of this review century concluded that all rocks were formed by precipitation relates to the conditions for melting, transfer, storage, and from an ocean, with volcanoes being simply local phenomena eruption of melts. The approach is as follows. Plate tectonics caused by burning coal seams. The challenge by James provides a framework of tectonic environments and internal Hutton, who concluded that magmas were formed by the in- processes,which can be calibrated by laboratory experiments ternal heat of the Earth, began the study of volcanoes as the at high pressures.Within this framework I next outline the overflow edifices for plutonic magma reaching the surface. development of ideas on mantle convection and the contri- Most people who have stood and listened and sniffed at the butions from geochemistrythat identify the separatechemical rim of Aetna or any other active volcano can easily be per- reservoirsyielding basalts.This approach is complementedby suaded that the sounds and smells emanate from the bowels of seismic tomography, which is beginning to provide three- the Earth and that volcanism is the outward and visible sign dimensional pictures of motions in the mantle. The subduction of deep internal processes,of mantle convection, and of the process and the role of volatile components lead to further chemical differentiation of the Earth. differentiation of the Earth and to the formation of continents Volcanoes are beautiful structures,but ephemeral in geo- associatedwith a variety of igneous rocks. Constraints for the logical time, and the progressiveexposure of the deep-seated conditions of melting are provided by thermal structures and plutons feeding volcanoes provides a more detailed history of the calibrations from experimental petrology. After reviewing some critical phase relationships of magmas and source rocks Copyright 1988 by the AmericanGeophysical Union. and recapitulating models of mantle convection,I then consid- Paper number 8R0242. er magmatic processesin selectedplate tectonic environments: 8755-1209/88/008R-0242505.00 basalts at divergent plate boundaries,andesites and batholiths 370 WYLLIE'MAGMA GENESIS, PLATE TECTONICS, DIFFERENTIATION 371 Mid-Atlantic magma generation and volcanoes, and the framework has changedthe way we considermagmatic processes.Among the .Ridge more significant change are the following: (1) it provides a framework of tectonic environments within which different suites of igneous rocks can be located, a framework long sought by petrologists;(2) it provides a dynamic mechanism for moving masses of rock up and down within the Earth, Ocean changing pressureand temperature, causing partial melting, Tre and therefore initiating magmatism; (3) it provides a broader view of igneous petrology in terms of the chemical differ- Pa c, if i c ••••'"'••'•'•'•'•••••••• •'"' ..a_•! • i! 6 !_•_h-e •il.................. '••"•••••••'•:•••i''••,,\ entiation of the Earth instead of the preoccupation with the diversity of igneousrocks; (4) it has changed the subject from descriptive petrography and philosophical petrogenesisto a k-Lithosphere Lithosphere--;'study of processesand products firmly rooted in geophysics; (5) it involves processesthat can be calibrated in part by Fig. 1. Outline of plate tectonics,vintage late 1960s. laboratory experiments at high pressuresand temperature; and (6) it provides a mechanism for recycling crustal rocks and volatile componentsback into the mantle, thus generating at subduction zones, ocean island lavas and anorogenic grani- volatile fluxesthat redistributeelements at depth. toides associatedwith mantle plumes, and the subsilicic kim- berlites that puncture continental plates. Unravelling the pro- Tectonic Environmentsand Petrographic cessesduring subductionremains one of the major problems Associations for understanding both the origin and evolution of the conti- Figure 1 shows the main tectonic environments:(1) diver- nents and the heterogeneitiesin mantle reservoirs. gent plate boundaries, (2) convergent plate boundaries, (3) oceanicplates, and (4) continental plates. These may be subdi- MAGMA GENESIS AND PLATE TECTONICS vided. Divergent plate boundaries include (1A) oceanic ridges The main variables to be considered in magma genesisare and (lB) continental rift systems.Convergent plate boundaries (1) the compositionsof materials,including both the sourcesof include (2A) ocean-ocean,(2B) ocean-continent, and (2C) mammasand the magmaticproducts (the mineralogyis depen- continent-continentboundaries. Oceanic plates may be (3A) dent on the other variables), (2) the pressure or equivalent stable or (3B) flowing acrossa hot spot. Continental plates depth, and (3) the temperature.These variables can be con- include, in addition to rift zones,(4A) stable platform or cra- trolled in laboratory experiments, providing a static flame- tions,(4B) continentsmoving acrossa hot spot, and (4C) pas- work of phase boundariesfor the Earth materials. The Earth sive margins. is a dynamic system,and changesthrough time exert a strong There are many different kinds of igneousrocks and petro- influenceon processes.MaMmas are generated(1)when bodies graphic associations,but in this review we will consideronly a of rock are transported acrossthe depth-temperature limits of selection,the basalts occurring in environments 1 and 3B; the melting curves by physical convection, (2) through temper- andesites,rhyolites, and granitoid batholiths characteristic of ature increase arising from tectonic conditions, or (3) by de- environments 2B, 2C, and 4B; the kimberlites of environment pression of melting boundaries to lower temperatures by 4A; nepheline-normative
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