UC Santa Barbara

The Earth is unique among the terrestrial planets in our solar system, and no other known planet has a similarly bimodal distribution of surface elevations. The thicker, lighter continents are intermediate in composition, as opposed to the denser, mafic oceanic crust. All magmatism is ultimately sourced from the mantle; yet, mantle melting alone produces basalt. Multiple hypotheses have been proposed to explain the composition and evolution of the continental crust, including the incorporation of felsic sediments from the surface into the deep crust. How this happens and the amount of metasedimentary material in the lower continental crust remains unknown. There are four documented processes that can transport surface material to depth: (1) tectonic underplating in a forearc setting (e.g., Jacobson et al., 2011), (2) deep thrust faulting in a retroarc (e.g., Chin et al., 2013), (3) relamination via deep subduction and buoyancy-driven diapiric ascent (e.g., Hacker et al., 2011), (4) deep thrust faulting in a continent-continent collision (e.g., Zhao et al., 2001). Petrochronology of deep-seated metasedimentary rocks can distinguish between these scenarios by revealing the timing and conditions of metamorphism and the circumstances under which sediments were returned to the surface. To that end, three case studies of lower crustal metasedimentary rocks erupted as xenoliths were carried out and are described herein.

Metapelitic xenoliths from New Mexico (USA) and Chihuahua (Mexico) record sediment transport to the lower crust during continent-continent collision associated with the Mesoproterozoic Picuris orogeny. Geothermobarometry and Rio Grande rift-age rutile (Cipar et al., 2020) document equilibration in the local deep crust, which is actively undergoing ultra-high-temperature metamorphism. The original sediments were deposited during the early stages of the Picuris orogeny (1500–1450 Ma), and the transition from detrital zircon accumulation to metamorphic zircon and monazite growth marks the end of subduction and onset of collision. Metamorphic zircon and monazite also grew during Rio Grande rift activity beginning at 32 Ma. Nearly all garnet grains are homogeneous with respect to major and trace elements, and monazite inclusions within garnet indicate that they are Rio Grande rift in age. A single garnet grain is zoned in Zr and Hf, the slowest diffusing elements, with rim concentrations similar to those of other garnets in the sample. The core, however, has lower concentrations, indicating growth at lower temperatures, and is distinct in its abundance of acicular sillimanite inclusions. The presence of sillimanite points to metamorphism under thermal gradients higher than those found in subduction zones, and negative Eu anomalies in all zircon and monazite rule out metamorphism at the ultra-high-pressures that would have preceded relamination. Collectively, these data demonstrate transport of sediments from the surface to the lower continental crust during a collisional orogeny. The sediments resided in the lower deep continental crust for over 1400 Ma, and their presence there defines the location of the suture zone that formed during the Picuris orogeny.

Lower crustal xenoliths from the Great Falls tectonic zone (GFTZ) in Montana (USA) record a multi-billion-year geologic history prior to their entrainment and eruption during the Cenozoic Laramide orogeny. Metapelitic samples equilibrated in the lower crust, and mafic xenoliths with an arc cumulate origin document conditions in excess of the current crustal thickness. Rutile U-Pb ages date the eruption of the host rocks to ~50 Ma and demonstrate residence of the samples in the deep crust until that time. Metamorphic zircon and monazite in all metapelitic samples record sediment transport to depth during a major metamorphic event beginning ~1800 Ma, while one sample also documents an event at 2200–2000 Ma related to burial and metamorphism during rifting. Younger metapelite xenoliths show a clear transition from detrital zircon formation to metamorphic zircon and monazite growth at 1800 Ma, corresponding the end of subduction and beginning of collision during the Paleoproterozoic Great Falls orogeny. Their occurrence in the deep crust of the Great Falls tectonic zone (GFTZ) provides evidence that the GFTZ is a suture zone between the Medicine Hat block and Wyoming craton. U-Pb and trace-element depth profiles of monazite and zircon also record metasomatism in the deep crust at ~60 Ma during the Laramide orogeny, which is consistent with recent suggestions for Farallon slab fluid infiltration at this time. These rocks remained in the lower continental crust for over 1700 Ma.

Metapelitic xenoliths from the Bournac volcanic pipe in the Massif Central (France) record the incorporation of sediments into the lower continental crust during the collisional phase of the Paleozoic Variscan orogeny. Geothermobarometry yields pressure-temperature conditions consistent with a geothermal gradient in excess of those associated with a continental geotherm, and rutile in these samples remained above the Pb closure temperature until eruption of the xenoliths at 11.6 Ma. Detrital zircon date back to the Archean, and the sediments were deposited after 393 Ma based on the youngest detrital zircon and monazite ages. Metamorphic zircon and monazite document the transport of the sediments to depth beginning ~330 Ma, which is consistent with the known timing of continent-continent collision during the Variscan orogeny just prior to an episode of UHT metamorphism in the lower crust that peaked at 313 Ma (Laurent et al., 2023). Post-orogenic collapse began by 300 Ma, and zircon and monazite crystallization continued until ~265 Ma as the crust thinned and cooled. Later rifting events are recorded by even younger metamorphic zircon and monazite dates (as young as 207 and 74 Ma, respectively). These data demonstrate that the original sediments were transported to depth during continent-continent collision and resided in the stable lower continental crust for over 300 Ma.