Crystal Zoning and Populations in 'Mt. Shasta' High-magnesian Andesite (HMA): Key in the Reconstruction of a Petrogenetic History Dominated by Magma Mixing and Contamination

HMA lava preserved in the pyroclastic deposits of one scoria cone at the Whaleback shield volcano (N. flank of Mt. Shasta) has been proposed as a primitive mantle-derived magma. However, the majority of phenocryst-sized minerals (mostly clino- and orthopyroxene, lesser olivine, and rare plagioclase) did not simply grow in a cooling HMA magma but instead record interactions between silicic and mafic magmas and between evolved magmas and their country rocks. Critical mineralogical evidence includes the following: 1) abundant reversely step-zoned pyroxenes with low Mg# (63 to ~78) cores overgrown by high Mg# (87- 92) growth bands; 2) occasional normally zoned pyroxenes with high Mg# rim growth bands as in 1); 3) abundant and complex array of spongy-cellular textured orthopyroxene crystals where lower Mg# compositions overprint originally high Mg# (~87) forming patchy zoning; 4) highly fosteritic (Fo89- 94) olivines as overly large, anhedral crystals or as olivine crystal clots; 5) rare plagioclase (~An48-78) as single spongy crystals or plagioclase crystal clots; 6) skeletal quench crystals of ~Fo87 olivine and microphenocyrsts of Mg# ~87 cpx and Mg# 87-93 opx. Within the context of textures, zoning, and major and trace-element compositions, this evidence supports the following petrogenetic history. The earliest discernable event was the formation and crystallization of silicic (dacitic?) magma (as recorded by half of all larger (> 200 μm) crystals and by rare dacitic melt inclusions in the Fe-rich pyroxenes). These magmas resided within a crustal section characterized by country rock composed of abundant orthopyroxene and olivine (i.e. harzburgitic lithology). Some of the dacitic melt infiltrated ultramafic (?) country rock leading to resorption, patchy zoning, and occasionally low Mg# overgrowths on orthopyroxene. Mobilization of selvages of altered and resorbed country rock dispersed abundant xenocrysts (1/3 of all single crystals) and micro-xenoliths. This mobilization was possibly facilitated by a recharge event of mafic magma that could also have triggered final ascent of the hybrid magma during which skeletal olivines, pyroxene microphenocrysts and high Mg# phenocryst rims crystallized and at which point the magma acquired its final bulk compositional characteristics. There is no compelling evidence that HMA liquid was ever in equilibrium with the highest Mg# mafic silicates prior to mixing and contamination. The general rarity of HMA lavas in the Cascades could be attributed to the unique nature of crustal rocks (ophiolite) underlying the Shasta area.