Closing the North American Magmatic Gap: Crustal evolution of the Clearwater Block from multi-isotope and trace element zircon data

Along the west-central margin of Laurentia, within the Priest River and Clearwater complexes, rare exposures of crystalline basement rocks of the Clearwater Block include North American Magmatic Gap (NAMG, ca. 1.61-1.49 Ga) ages. Elsewhere in this region, crystalline basement rocks are buried beneath thick deposits of the overlying Mesoproterozoic Belt Supergroup and younger sequences. The unique combination of magmatic basement ages and the detrital zircon components (which also include NAMG detrital ages) within the overlying Mesoproterozoic Belt Supergroup strata, has led researchers to identify the Clearwater Block region as a key tie-point for Proterozoic paleogeographic reconstructions. Some researchers even speculated that Proterozoic supercontinent events stranded exotic (possibly Australia, Antarctica or South China associated) basement terranes within the Clearwater Block. However, no comprehensive multi-isotopic data exist on the Neoarchean to Mesoproterozoic Clearwater Block crystalline basement leaving many of these speculations untested. We report new U-Pb, Lu-Hf and O isotopic data and trace element results of zircon grains for these rocks. Collectively, along with existing data these new results indicate a crustal evolution for the Clearwater Block that consists of: 1) ca. 2.67 Ga juvenile mantle-derived crustal growth as evident by mantle-like 8(18)O and slightly supra-chondritic (+4 to 0) epsilon Hf-t values, 2) ca. 1.87 to 1.83 Ga melting and metamorphism of this ca. 2.67 Ga crust as recorded by ca. 1.86 Ga samples containing zircon with low (< 0.1) Th/U ratios and retention of mantle-like 818O values, and similar-age ca. 1.87-1.83 Ga samples containing zircon with higher (> 0.1) Th/U ratios, and 3) ca. 1.58 Ga NAMG magmatism that records mantle-like 8(18)O but sub-chondritic (-5.5 to-9.5) epsilon Hf-t values. We interpret the ca. 1.58 Ga NAMG magmatism to be consistent with (perhaps plume-driven) reworking of the Clearwater Block's ca. 2.67 Ga lower crustal reservoir. Consequently, these results support a Laurentian origin for NAMG-age magmatism within the Clearwater Block, but confirm the necessity of non-Laurentian sources for juvenile (+epsilon Hf-t) NAMG-age detrital zircon grains in the overlying ca. 1.47 Ga lower Belt Supergroup strata, thus providing important constraints for western Laurentia's conjugate during the early Mesoproterozoic.

Automated summary

This study reports new U-Pb, Lu-Hf, and O isotopic data and trace element results of zircon grains for the crystalline basement rocks in the Clearwater Block. These results provide insights into the crustal evolution of the Clearwater Block, including juvenile mantle-derived crustal growth, melting and metamorphism, and NAMG magmatism. The findings support a Laurentian origin for NAMG-age magmatism within the Clearwater Block.