Nature and composition of the continental crust: A lower crustal perspective

摘要: Geophysical, petrological, and geochemical data provide important clues about the composition of deep continental crust. On basis seismic refraction data, we divide crust into type sections associated with different tectonic provinces. Each shows a three-layer consisting upper, middle, lower crust, in which P wave velocities increase progressively depth. There is large variation average velocity between sections, but general, crustal are high (>6.9 km s−1) middle range 6.3 6.7 s−1. Heat-producing elements decrease depth owing to their depletion felsic rocks caused by granulite facies metamorphism an proportion mafic Studies cross show that Archean regions, 50–85% heat flowing from surface Earth generated within Granulite terrains experienced isobaric cooling representative or have higher proportions than do isothermal decompression. The latter probably not merely upper been through orogenic cycle. xenoliths some deepest samples composed largely rock types. Ultrasonic measurements for wide variety link lithology. Meta-igneous felsic, intermediate granulite, amphibolite distinguishable on S velocities, metamorphosed shales (metapelites) overlap complete displayed meta-igneous lithologies. production metapelites, coupled generally limited volumetric extent xenoliths, suggests they constitute only small Using derived estimated areal each compositions types granulites, estimate composition. lithologically heterogeneous. Its mafic, approaching primitive mantle-derived basalt, it may bulk regions. composition, containing significant K, Th, U contents. Average contains silicate Earth's incompatible trace element budget (35–55% Rb, Ba, Pb, U).