Rapid migration of a magma source from mid- to deep-crustal levels: Insights from restitic granulite enclaves and anatectic granite

Granulite enclaves as an indicator that can be used to unravel multistage metamorphic histories of orogenic belts have attracted a lot of attention. Few exposed granulite enclaves, however, form restite-melt pairs with their host rocks, which provide unique insight into magma source character and conditions of partial melting. In this study, the orthopyroxene- and plagioclase-rich and the garnet-rich and plagioclase-poor restitic granulite enclaves hosted by three peraluminous granitic plutons from South China were characterized. Mineral assemblages, mineral compositions, pressure-temperature pseudosection modeling, and zircon U-Pb data for the two types of restitic enclaves reveal that (1) an early (255249 Ma) melting event occurred at conditions of similar to 950 +/- 30 degrees C and similar to 500 +/- 80 MPa, and (2) a later (245-246 Ma) melting event occurred at conditions of similar to 905 +/- 15 degrees C and similar to 675 +/- 25 MPa. The crustal magma source is inferred to have rapidly (within 3-10 m.y.) migrated from mid-to deep-crustal levels from similar to 18 to similar to 25 km (for an average crustal density of 2.7 g/cm(3)), with burial rates of 0.07-0.23 cm yr(-1), in response to a switch in the geodynamic regime from subduction to continent-continent collision. Whole-rock elemental and isotopic compositions of enclaves and granitic rocks further indicate that partial melting in the source was nonmodal. The key implications from our findings are that magma sources may migrate over the lifetime of many composite, upper-crustal intrusions, and that compositional variation in intrusive suites may reflect source partial melting at various pressures in addition to other factors.