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Abstract

Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U.S., where seismic experiments reveal an anomalously thick (10-30 km), high seismic velocity (compressional body wave, Vp > 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafi c and intermediate garnet granulites, mafi c hornblende eclogite, and felsic granulites. Calculated pressures of 0.6-1.5 GPa are consistent with derivation from 23-54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defi ned 7.x layer have calculated bulk P-wave velocities of 6.9-7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fl uid-fl ow events ca. 2.1 Ga, 1.8-1.7 Ga, and 1.5-1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neoarchean to the Mesoproterozoic.

Lithospheric density models reveal evidence for Cenozoic uplift of the Colorado Plateau and Great Plains by lower-crustal hydration

“…Th-Pb dating of secondary monazite associated with these assemblages suggests that the majority of the retrogression occurred in the latest Cretaceous (Butcher, 2013;Butcher et al, 2017), contemporaneous with the arrival of the Farallon slab. A similar but undated retrograde reaction is documented in crustal xenoliths near the northern Great Plains: Southward from near the Canadian border to southern Wyoming, three xenolith localities increase in elevation from 1 to 2.5 km as xenolith density decreases by ~500 kg/m 3 (Barnhart et al, 2012;Farmer et al, 2012;. Noting a collocated southward decrease in seismic velocity in an ~10-km-thick layer of lower crust, Jones et al (2015) hypothesized (following Eq.…”

Metamorphic evolution and geochronology of the tectonic mélange of the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwestern China

“…The metamorphic P-T trajectories of high-grade terranes provide important constraints on their tectonic evolution (e.g., England and Thompson, 1984;Thompson and England, 1984;Ernst, 1988;Harley, 1989;Spear, 1993;Barnhart et al, 2012;Parsons et al, 2016). Deep subduction, usually to mantle depth, is required for high-pressure metamorphism to occur (O'Brien and Rötzler, 2003).…”

A Reconstitution Approach for Whole Rock Major and Trace Element Compositions of Granulites from the Kapuskasing Structural Zone

“…Alkali basalt hosts can also be enriched in highly incompatible elements (Figure 1b). However, the effects of contamination by alkali basalt host magmas on crustal xenolith whole rock compositions have largely not been explored, and the reconstruction approach has not been widely utilised for lower crustal xenoliths [16,17], particularly for trace elements.…”