CAUSAL IMPLICATIONS OF A NEW ZIRCON U-PB GEOCHRONOLOGICAL FRAMEWORK FOR MESOZOIC MAGMATISM IN NORTHERN NEW ENGLAND AND QUEBEC: WHAT ARE THE RIFT/POST-RIFT IGNEOUS ROCKS OF THE WHITE MOUNTAINS?

The Jurassic White Mountain Magma Series (WMMS) and Cretaceous New England Quebec (NEQ) igneous provinces collectively form the largest expression of post-orogenic felsic magmatism on the Eastern North American Margin (ENAM). The published chronology, based mostly on whole rock/mica K-Ar and Rb-Sr ages, limits our ability to test hypotheses for the causal mechanisms responsible for the initiation of magmatism in each province. Here, we present a new geochronological framework for both the WMMS and NEQ based on CA-ID-TIMS and LA-ICP-MS zircon U-Pb measurements from several key units.

Our results indicate that the onset of WMMS magmatism at several localities differs significantly from published ages, up to 25 Myr at some. The oldest (> 198 Ma) may be a late-stage, felsic expression of the Central Atlantic Magmatic Province (CAMP) large igneous province (LIP). We also show that the main phase of igneous activity associated with the WMMS occurred over a much shorter duration than the published chronology indicates. The spatial and apparent temporal proximity of the Jurassic WMMS to the CAMP suggests the possibility of a direct geodynamic linkage and should motivate the development testable hypotheses, e.g., plume-head driven subduction and plutonism, lithospheric delamination, etc. Given the common occurrence of late-stage felsic magmatism on the geographic periphery of other flood basalt provinces, such as the Etendeka¹ and Deccan² traps, the existence of the WMMS may reflect a fundamental feature of the geodynamic setting of the CAMP and provide insight into the processes that initiated the LIP itself.

In addition to the Jurassic WMMS, we provide new age constraints on the emplacement of the largest igneous complexes of the Cretaceous NEQ, providing an accurate, high-resolution chronological test for the expression of purported Great Meteor Hotspot magmatism on land.

The fact that this area has been a locus of magmatism for upwards of 200 Myr and corresponds to modern, low-velocity seismic anomalies^3,4 underscores the importance of future work on the WMMS and NEQ to not only unravel the tectonic evolution of the ENAM but to also provide a pathway for understanding basic mantle processes.

¹Renne et al., Geology (1996). ²Basu et al. Goldschmidt (2018) Abstracts. ³Schmandt and Lin. GRL. (2014). ⁴ Menke et al., GRL. (2016).