Mid-Tertiary (25–21 Ma) lamprophyres in NW Mexico derived from subduction-modified subcontinental lithospheric mantle in an extensional backarc environment following steepening of the Benioff zone

The mid-Tertiary lamprophyre dike swarm (~ 8 km × 2.5 km in size) from Hermosillo (Sonora, NW Mexico) has calc–alkaline characteristics and includes NNW-striking, amphibole-phyric spessartite (~ 85% of the swarm) and NNE-striking, phlogopite-phyric kersantite. The 40Ar/39Ar geochronology of amphibole and phlogopite gives overlapping plateau ages ranging from 25 to 21 Ma. Although all the lamprophyres are enriched in incompatible elements and display negative Nb–Ta and Ti anomalies on the primitive mantle-normalized plots, kersantite has higher K/Na, La/Yb, P, Ti and incompatible trace elements (e.g., Zr) compared to spessartite. The lamprophyres have radiogenic Sr and Nd isotopic signatures (87Sr/86Sr ~ 0.7057–0.7065 and εNd ~− 1 to − 2.3) suggesting derivation from the subcontinental lithospheric mantle that was previously modified by subduction-related fluids. This mantle is similar to that beneath the southern Grenvillian orogen, which has younger TDM ages than the 1.6–1.7 Ga TDM ages of the Caborca block. The lamprophyric magmas were generated at various mantle depths at the southwestern edge of North America. Intrusion of the lamprophyres was synchronous with extension that produced normal faults and core complexes with WSW-vergence. Extension occurred immediately following steepening of the Benioff zone, during which the magmatic arc migrated from east to west of Hermosillo, and the lamprophyres were intruded just behind the contemporaneous arc.

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► ~ 23 Ma calc–alkaline phlogopite-bearing and amphibole-bearing lamprophyres at Hermosillo (NW Mexico).
► Intrusion synchronous with rifting and core complex development during extension.
► ~ 1 Ga subcontinental lithospheric mantle source modified by subduction-related fluids.
► ~ 1 Ga age of the mantle source resembles the Grenvillian orogen, not the 1.6–1.7 Ga ages typical of Caborca block.
► Backarc intrusions after steepening of Benioff zone when arc moved from E to W.