NEW RE-OS AND SM-ND ISOTOPIC CONSTRAINTS ON THE TECTONIC SETTING OF THE 1.4 GA LAURENTIAN MAGMATIC EVENT

Granitic plutons associated with a 1.4 Ga magmatic event intrude in a >1000 km belt across Laurentia. Despite decades of inquiry, the tectonic setting of this major magmatic province is still debated. Proposed melt generation models include anorogenic heating from the mantle (hotspots, rifts), orogenic melting (subduction or transpressive orogeny), and anatexis from radiogenic heat buildup in thickened crust. Although previous studies have focused on the granites associated with the magmatic event, our study focuses on peridotite mantle xenoliths from the Navajo volcanic field (NVF) diatremes in the central Colorado Plateau.

Trace element analyses of clinopyroxene permit xenolith samples to be grouped into three groups based on their rare earth element (REE) patterns: refractory, light REE (LREE) enriched samples (Group E); fertile, LREE depleted samples (Group D); and samples with transitional spoon-shaped REE patterns (Group T). Re-Os analyses of Group E and clinopyroxene-absent mantle xenoliths from the NVF diatremes have unradiogenic ¹⁸⁷Os/¹⁸⁸Os values (0.114 to 0.117) and yield Re depletion ages of 2.1–1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene (εNd= -2.6 to 405) show that fertile Group D samples plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites, but are inconsistent with rifting and delamination models that require lithosphere removal, or with crustal anatexis models that do not explain mantle resetting or magmatism. This study highlights the central role that subduction has played during the post-Archean growth of Laurentia, and compliments a growing body of literature that propose subduction for generation of the 1.4 Ga magmatic event.