ISOTOPIC EVIDENCE THAT GRANITES FORM BY REACTION FROM PRE-EXISTING GABBRO

How silicic magma forms, emplaces and/or erupts from the crust is fundamental to our understanding of the Earth. Seismological images fail to show any large tanks of magma despite the enormous size of plutons and super-eruption ignimbrite magma chambers (>100s km² scale); yet magnetotellurics show the same areas can be very electrically conductive. We have posited that silicic magmas form by top down emplacement of andesite sills coupled with a crystal-rich reaction process within a moving temperature gradient zone. We suggested that “every granite was once a gabbro [1]”. We then proposed a two-stage differentiation process adding the important role of upflow of cool (~500°C) water-rich silicate liquids (e’ conductive) to silicic magma formation [2].

Whole rock analyses of spatially connected mafic and silicic rocks show distinct Sr isotope ratios at Torres del Paine (Chile) and Aztec Wash (NV) plutons. However, based on laser ablation MCICPMS, ⁸⁷Sr/⁸⁶Sr of plagioclase in granites located just above related mafic complex rocks in both plutons overlaps those of gabbro—fully consistent with the hypothesis that granite formed by reaction from gabbro. We will also present cathodoluminescence mosaics of mafic complex rocks whereby plagioclase grains preserve unmistakable relict textures of amphibole—explained as amphibole rich rocks react into diorites (and then into granite).

Does this same differentiation process produce silicic magmas for zoned ignimbrite super-eruptions? We find an identical behavior of Sr occurs in silicic and mafic rocks from the Carpenter Ridge tuff and Peach Springs tuff supereruptions, implying the same virtually melt-less process of silicic magma formation may occur in precursor magma chambers. Like volcanic suites[3], analysis of granite biotite shows highly radiogenic Sr_i ratios, again suggesting volcanic-plutonic equivalence. If this differentiation process indeed operates, serious hazards implications exist with major questions about how such mush zones melt and erupt remaining answered.

[1] Lundstrom(2016), IGR 58, 371-388; [2] Lundstrom et al.(2022), GSA Books, 553; [3] Charlier et al.( 2007), J. Pet, 1875-1894.