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Nature and Composition of the Continental Crust: A

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Nature and Composition of the Continental Crust: A NATUR COMPOSITIOD EAN CONTINENTAE TH F NO L CRUST LOWEA : R CRUSTAL PERSPECTIVE Robert . RudnickaL 1 David M. Fountain Research School Earthof Sciences Department of Geology and Geophysics The Australian National University, Canberra University Wyoming,of Laramie Abstract. Geophysical, petrological geochemid an , - surement a wid r efo s variet f deeo y p crustal rocks cal data provide important clues abou composie th t - provide a link between crustal velocity and lithology. tion of the deep continental crust. On the basis of Meta-igneous felsic, intermediat mafid ean c granulite, seismic refraction data divide w , cruse eth t into type and amphibolite facies rock distinguishable sar e th n eo sections associated with different tectonic provinces wav5 .d ean basivelocitiesP f so metamorphoset bu , d Each shows a three-layer crust consisting of upper, shales (metapelites) have velocities that overlap the middle lowed an , r crust whicn i , hwavP e velocities complete velocity range displaye meta-igneoue th y db s increase progressively with depth. There is large vari- lithologies. The high heat production of metapelites, ation in average P wave velocity of the lower crust coupled with their generally limited volumetric extent between different type sections, but in^general, lower in granulite terrains and xenoliths, suggests they con- crustal velocities are high (>6.9 km s"1) and average stitute onl ya smal l proportio e loweth f ro n crust. middle crustal velocities range between 6.3 and 6.7 km Using average P wave velocities derived from the s"1. Heat-producing elements decrease with deptn hi crustal type sections e estimateth , d areal extenf o t cruse th t owin theigo t r depletio felsin i c rocks caused each type of crust, and the average compositions of by granulite facies metamorphis increasn a d man n ei different type granulitesf so estimate w , average eth e the proportion of mafic rocks with depth. Studies of lowe middld an r e crust composition lowee Th . r crust crustal cross sections show that in Archean regions, is composed of rocks in the granulite facies and is 50-85 e heath t%f o flowin g fro e surface mth th f eo lithologically heterogeneous. Its average composition Earth is generated within the crust. Granulite terrains is mafic, approaching that of a primitive mantle- that experienced isobaric coolin representative gar f eo derived basalt, but it may range to intermediate bulk middl r loweeo r crus havd an te higher proportionf o s composition n somi s e regions e middlTh . e cruss i t mafic rocks than do granulite terrains that experienced composed of rocks in the amphibolite facies and is isothermal decompression lattee probable Th . rar t yno intermediate in bulk composition, containing signifi- representative of the deep crust but are merely contentsuppeU d ran , .can Th Averag , tK e continental crust crustal rocks that have been through an orogenic cy- is intermediate in composition and contains a signifi- cle. Granulite xenoliths provide deepese somth f eo t cant proportion of the bulk silicate Earth's incompat- samples of the continental crust and are composed ible trace element budget (35-55% of Rb, Ba, K, Pb, largely of mafic rock types. Ultrasonic velocity mea- Th, and U). 1. INTRODUCTION wate Eartn ro h [Campbell Taylor,d an 1985]. Whereas the upper crust is accessible to geological sampling Continents cover 41% of the Earth's surface [Cog- and measurements deepee th , r portion cruse th f to s ley, 1984] and sit at high elevations compared to the relativele ar y inaccessible dateo deepese T . th , t drill ocean basins owinpresence th o gt lower-densityf eo , hol penetrates eha d onl crusf o y 1m t2 k [Kremenetsky evolved rock types. ("Evolved defineds "i , along with and Ovchinnikov, 1986]. Nevertheless, these deep por- other specialized terminology, in the glossary follow- tions of the crust contain important information re- ing this introduction. e evolveTh ) d rocks that domi- lated to the bulk composition of the continental crust nate the upper portions of the Earth's continental crust as well as how it forms. unique r solaar ou rn e i system [Taylor,e 1989ar d ]an The lower crust (below -20-25 km depth) is be- probably ultimately linked to the presence of liquid lieve consiso dt metamorphif to c granulitrocke th n si e facies (referre simplo dt granulites ya s throughout this paper), which are accessible either as large tracts of !Now at Department of Earth and Planetary Sciences, surface outcrop (terrains) or as tiny fragments carried Harvard University, Cambridge, Massachusetts. from great depths in volcanic conduits (xenoliths). The Copyright 1995 by the American Geophysical Union. Reviews of Geophysics, 33, 3 / August 1995 pages 267-309 8755-1209/95/95 RG-01302$75.00 Paper number 95RG01302 267 26 8• Rudnic Fountaind kan : LOWER CONTINENTAL CRUST 33, 3 / REVIEWS OF GEOPHYSICS middle crust (i.e., between 10-15 and 20-25 km depth) Acoustic impedance produce th : f velocito t d yan may contain rockamphibolite th n si e facies, whice har density. also found in surface outcrop or as xenoliths. Amphib- Amphibolite: a mafic rock consisting dominantly olite facies rocks may also be important in the lower- of amphibole. most crust in areas of high water flux (such as in island Amphibolit f metamorphico t e se facies e th : min- settingc ar s where hydrous oceanic lithospher subs ei - eral assemblage whicn si h mafic rock composee sar d ducte dewatered dan d [e.g., Kushiro, 1990]. of amphibole and plagioclase. The facies is typical of rol e lowee eth Th r crust play continentan si l tecton- regional metamorphism at moderate to high pressures poorls i s ic y understood r example rheoe Fo . th e - ar , and temperatures (i.e., >300 MPa, 450°-700°C). logical and compositional differences between upper Anisotropy: see seismic anisotropy. and lower crust sufficient to promote delamination of Anorthosite: a plutonic igneous rock composed lowee th r crus continent-continent a t t collision zones? almost entirely of plagioclase feldspar (see Ashwal How much lower crust migh recyclee tb d back inte oth [1993] for an excellent review). mantl t convergenea t margin mucsettingsw ho h d an , Constructive interference: see interference. remains withi e crusth n t under condition f higho s - Continent-continent collision zone: a special type grade metamorphism? of convergent margin where a continent on the sub- Our understanding of the deep continental crust has ducting plate collides with another on the overriding improved dramatically ove lase rth t decad resula s ea t plate. of detailed seismological studies and numerous studies Convergent margin zone th e: wher tectonio etw c of lower crustal rocks. However, the composition of plates converge and one is subducted beneath the the deep crust remains the largest uncertainty in de- other. termining the crust's overall composition. This is due Craton aren a f crusa:o t tharemaines ha t d stable e largth ) e (1 compositiona o t l differences between for very long periods of time. granulites that occur in surface tracts (granulite ter- Critical angle: the angle of incidence of a seismic rains, in which felsic rocks dominate) and those that wave at which a head wave (or refracted wave) is are carried as small fragments to the Earth's surface in generated. rapidly ascending magmas (xenoliths, whic dome har - Critical distance: offset at which reflection time inate y mafidb c e rocks)verth y) (2 heterogeneou, s e naturloweth f ro e crus s observea t granulitn i d e equals refraction time. terrains, and (3) the difficulty in determining rock Cumulate: an igneous rock formed by accumula- type(s) from average seismic velocities derived from tio f crystallizinno g phases. refraction studies. Delamination: a process by which dense seg- thin I s contributio reviee nw knowledgr wou e th f eo ments of the lower crust (and lithospheric mantle) sink deep continental crust from both geophysical-based into the convecting asthenosphere as a result of their and sample-based studies. Of the various geophysical negative buoyancy. methods (seismic, thermal, electrical, potential field), Ductile: generally regarded as the capacity of a seismological data and heat flow studies reveal most material to sustain substantial change in shape without about the composition of the crust. We will focus on gross faulting [see Paterson, 1978], though there ear these two methods here. (The interested reader is numerous d sometimean , s conflictinge , th use f o s referre Joneso t d [1992 Shived an ]t al.e [1992r fo ] word. review f electricao s magnetid an l c propertiee th f o s Ductile shear zones: a fault zone in which the lower crust, respectively.) We then integrate both data deformatio ductilenis . set orden i s o derivt r a bule k compositioe th r nfo Eclogite high-pressura : e mafic rock composef do lower, middle, and bulk continental crust. Our subdi- garne Na-ricd an t h clinopyroxene (omphacite); alsoa vision of the crust into upper, middle, and lower is metamorphic facies defined by the appearance of these base observationn do s from seismic studie summas sa - phases in mafic rocks. rize Holbrooky db . [1992]al t e . Eu anomaly: Eu/Eu* = 2Eu /(Sm Gd )° , where n n n 5 subscriptee th indicatedn s tha valuee tth normale sar - ize chondritio dt onle c th meteorites ye raron es i u E . 2 GLOSSARY earth element (REE) that can occur in the 2+ valance state under oxygen fugacity conditions found in the Sources of definitions are Bates and Jackson [1980], Earth.
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