Petrology and Geochronology of Crustal Xenoliths from the Bering Strait Region: Linking Deep and Shallow Processes in Extending Continental Crust

Akinin V.V., Miller E.L., Wooden J.

Geological Society of America, Special publication 456. 2009. P. 39-68.

Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province are coupled with high-spatial resolution U-Pb geochronology of their zircons with the SHRIMP-RG yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results  provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly mafic xenoliths (constituting less than 2% of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated by magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n = 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (~ 138–60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile additions of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached sillimanite facies to granulite grade within a series of extension-related metamorphic culminations or gneiss domes which developed within the Cretaceous magmatic belt; metamorphic gradients and inferred geotherms (~ 30–50°C/km) from both the gneiss domes and xenoliths are too high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. 
Deep crustal seismic-reflection and refraction data reveal a 30–35 km thick crust, a sharp Moho and reflective lower and middle crust. Velocities do not support a largely mafic (underplated) lower crust, but together with xenolith data suggest that Late Cretaceous to early Paleocene mafic intrusions are likely increasingly important with depth in the crust and that the elevated temperatures during granulite-facies metamorphism led to large-scale flow of crustal rocks to produce gneiss domes and the observed subhorizontal reflectivity of the crust. This unique combined data set for the Bering Shelf region provides compelling evidence for the complete reconstitution/re-equilibration of continental crust from the bottom up during mantle-driven magmatic events associated with crustal extension.  Thus, despite Precambrian and Paleozoic rocks at the surface and Alaska’s accretionary tectonic history, it is likely that a significant portion of the Bering Sea region lower crust is much younger and related to post-accretionary tectonic and magmatic events.

На основе изучения гранулитов и габброидов, представленных в нижнекоровых ксенолитах из щелочнобазальтовых лав Северо-Востока Азии и Аляски, дана характеристика глубинной части земной коры. U-Pb SHRIMP датирование циркона и геохимия пород ксенолитов показывают, что существенный объем окраинно-континентальной нижней коры генерирован меловыми постаккреционноми тектоническими и магматическими событиями. Изначально утолщенная кора была существенно модифицирована в мелу при магматическом андерплейтинге, инициированным мантийным магматизмом, и испытала термальное событие в маастрихте — палеоцене. Эти преобразования совпадают с главными импульсами магматизма в регионе.

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