The nature of the lower continental crust: Inferences from geophysics, surface geology, and crustal xenoliths

Recent investigations of the lower continental crust have progressed along three relatively independent fronts: geophysics, field mapping, and study of xenoliths brought to the earth’s surface in volcanic and kimberlitic eruptions. The general results of these studies have increased our appreciation of the scale of lower crustal structural and compositional complexity, but not to the point where we can construct an accurate crustal cross section for even one locality. The first part of this review summarizes the results of crustal geophysical and geological mapping studies, which have provided both the broad framework and, more recently, some local detail for the continental crust. In the second part the role of crustal xenolith studies is outlined, and the occurrence of xenoliths is summarized by tectonic setting. In the course of this review, several generalizations about the lower crust are emphasized. First, compositional inferences based on matching of mean seismic velocities and rock types are not unique. For example, the occurrence of hydrous minerals at depth is indicated by some xenolith populations and perhaps corroborated by electrical conductivity studies. The lower velocities and densities of hydrous rocks allow consideration of less silicic mean compositions than were indicated by several recent estimates. Second, the occurrence of lower crustal metasedimentary rocks in several xenolith suites and in exhumed lower crustal sections indicates that mechanisms exist for burying surface rocks to great depths, presumably in thrust zones. Given the existence of metasedimentary rocks, however, one must still recognize the preponderance of igneous and metaigneous rocks, commonly of broadly basaltic composition, in lower crustal xenolith suites. Finally, the regional variability of the lower crust most probably correlates with tectonic setting and should temper extrapolations of lower crustal characteristics found in exposed lower crustal sections to other regions.