Xenoliths in proterozoic crust: Evidence for reworking of the lower crust

The deeper portions of Proterozoic crustal blocks can be investigated by studying crustal xenoliths entrained in mantle-derived rocks. These xenoliths are important, because they may yield information about the tectonic and magmatic history of the deeper portions of Proterozoic crustal blocks. Granulite-facies xenoliths that equilibrated at pressures and temperatures consistent with those of the lower crust are found in alkalic basalts throughout the world. These granulite xenoliths are the accidental fragments of the crust entrained by mafic eruptions from the mantle. The xenolith suites typically contain a variety of crustal and mantle lithologies. Although the mantle xenoliths may not be representative of the mantle they sample—because the specific processes that caused the xenolith-bearing magmatic event is unusual—the crustal xenoliths are probably representative of the types of rocks that are found below xenolith-bearing localities—because they are accidental fragments plucked from the lower crust by the quickly ascending mantle-derived magmas.