A GEOCHEMICAL STUDY OF THE RIDDLE PEAKS GABBRO, NORTH CASCADES: EVIDENCE FOR AMPHIBOLE ACCUMULATION IN THE MID CRUST OF AN ARC

Mid-crustal geochemical differentiation processes in arcs are poorly understood due to lack of surface exposure. Some researchers propose that reservoirs of amphibole in the mid-crust of arcs differentiate mantle-derived basalts. The Riddle Peaks Pluton (~77 Ma) is an amphibole-rich layered gabbro emplaced at mid-crustal pressures (6-7 kbar) within the exposed Cretaceous and Paleogene North Cascades Crystalline Core (NCCC), and provides a physical example of one such amphibole reservoir. Its geochemistry shows that amphibole can affect arc magmas, especially their rare earth element (REE) ratios.

The Riddle Peaks Pluton has 40.7-47.2 wt.% SiO₂ and Mg# 33-67, where Mg# is defined as 100*[(Mg/(Mg+Fe²⁺)]. The pluton is composed of two rock types, a layered hornblendite/hornblende gabbro that alternates with cm-scale anorthite/plagioclase gabbro bands; and a more plagioclase-rich massive hornblende gabbro. Higher Mg#, Ni and Cr in amphiboles of the layered hornblendite/hornblende gabbro indicates that it was derived from a more primitive source. Whole rock and amphibole REE patterns are nearly identical, suggesting that the pluton is composed of cumulates with no trapped liquid portion.

LA-ICP-MS data show that trace element ratios (La/Yb, Dy/Yb, Ti/Zr, Nb/Ta) in the Riddle Peaks can account for elevated or depleted trace element ratios in arcs relative to chondrite (a proxy for early Earth or primitive mantle). For example, La/Yb ratios in average arcs are elevated (~8), compared to chondrite values (~1.5). The average Riddle Peaks amphibole ratio is lower (0.94), such that if significant amounts of this amphibole crystallized in the crust, the ratio in derivative products (arc magmas) should increase.

Rayleigh fractionation models for amphibole fractionation, calculated in two steps as seen in the Riddle Peaks (initial crystallization of layered gabbro rock type, followed by massive gabbro rock type) from a basaltic andesite parent, show that average arc La/Yb and Dy/Yb can be produced via amphibole fractionation. However, this process cannot account for average arc Ti/Zr and Nb/Ta ratios. Amphibole is important in generating arc REE signatures, though another mineral or process must affect other trace elements. Amphibole cumulates may then be important in generating the REE ratios of continental crust.