Multi-scale, open-system magmatic and sub-solidus processes contribute to the chemical and isotopic characteristics of a shallow crustal magma reservoir: a plutonic perspective

Plutonic bodies are solidified magma reservoirs, and their chemical and isotopic characteristics represent the integration of magmatic and sub- solidus processes operating across spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools trac­ing processes at different time and length scales.

Here, we combine whole rock and mineral major, trace element, and isotopic compositions with high-precision U-Pb ID-TIMS zircon geochronology to evaluate differentiation processes and their timescales in the bimodal (gabbro and granite), shallow crustal Guadalupe Igneous Complex, Sierra Nevada, USA.

We show that the complex was constructed in ~300 kyr. Pluton-wide δ¹⁸O_{(whole-rock),} δ¹⁸O_(zircon), and Sr- Nd isotopic ranges are too large to be explained by in situ, closed- system differentiation, instead requiring open- system behavior at all scales. Low δ¹⁸O_(whole-rock) and δ¹⁸O_(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In-situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 kyr via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub- solidus, high- temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemi­cal and isotopic characteristics.

Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.