Genesis and evolution of the post-caldera pyroclastic rhyolites from La Primavera caldera, Jalisco, Mexico: A crystal mush perspective
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abstract
La Primavera is a Quaternary rhyolitic volcanic complex located in the northwestern part of the Trans-Mexican Volcanic Belt. It consists of lavas, domes, and composite volcanoes emplaced between 143.5 and 25.5 ka, both inside and outside the caldera rim. Post-caldera volcanism occurred between ∼95 and 25.5 ka, erupting 2 lavas, 22 domes, and 5 composite volcanoes. Here, we present new petrographic descriptions, mineral chemistry, and whole-rock chemical compositions of juvenile fragments from the post-caldera pyroclastic units originated from the Nejahuete, San Miguel, Planillas, and Tajo composite volcanoes. Based on their major, trace, LREE (light rare earth elements), and isotopic compositions, we identify four compositional groups: 1) Colli dome (low-LREE and low-Zr), 2) E to N pyroclastic units (low-LREE and intermediate-Zr), 3) B to D pyroclastic units, and 4) GP to A pyroclastic units (high-LREE and high-Zr). Based on their composition, we propose a three-step model to explain the genesis of the post-caldera magmas: 1) injection of a calc-alkaline subduction-related melt modified by an OIB mantle-derived component (e.g., the Gachupín basaltic-andesite) into the magmatic chambers and generation of at least two crystal mushes, one formed from crystallization of a mantle-derived magma (i.e, Gachupín), and the other from a differentiated residual liquid derived from the same mantle-derived magma, 2) formation of the REE-poor and REE-rich compositions of the La Primavera magmas by different degrees of partial melting of the crystal mushes, and 3) generation of the REE-intermediate compositions of the La Primavera magmas by mixing of the REE-poor and REE-rich melts produced from partial melting of the crystal mushes to generate the LREE and Zr variations of the La Primavera magmas.
