NEW INSIGHT INTO THE GEOCHEMICAL SIGNATURES OF ULTRAMAFIC TO MAFIC ROCKS OF THE NAVAJO VOLCANIC ROCK: IMPLICATIONS FOR MAGMA SOURCES

Though many studies have been done in the past fifty years to gain insight into mantle-melt sources and genesis of magmas for rocks of Navajo volcanic field (NVF), this issue is still open to debate. A leading hypothesis is that silica-undersaturated melts (katungite suite) formed by 0.5% to 1% melting of mixed asthenospheric-lithospheric mantle with CO₂ metasomatism. Distal to these CO₂-rich zones, melting of hydrous-lithospheric mantle produced more “silica-saturated” magmas (minette suite). We present new chemical data from NVF rocks that further test this hypothesis.

New and previous bulk-rock major and trace element data show that NVF rocks vary from less evolved (katungite) to more evolved suites (minette) with some transitional compositions. The chemical data for different rock suites overlap and do not reveal many distinct differences. All NVF rocks are highly enriched in large-ion lithophile elements (e.g., K, Sr, Rb, Ba) and LREE relative to chondritic values. On selected “spider” diagrams, however, minette samples differ from the katungites in that they have pronounced downward spikes in Nb-Ta and Ti, and upward spikes in Pb. The bulk-rock trace-element signatures of the katungites are broadly similar to ocean-island basalts.

REE and trace element signatures for apatite and olivine crystals in NVF rocks measured by laser ablation-ICP mass spectrometry reveal distinct differences. Apatite crystals in katungites have ~10x lower LREE concentrations than minette and “melanephelinite” samples. Selected trace-element signatures (e.g., Ti, Zn, Cu, V) from olivine crystals are also dissimilar for the different suites. These mineral data are consistent with bulk-rock trace element and previous Nd-Sr isotopic data that hint at different magma sources for the katungites.

Though distinctions in bulk-rock REE data are subtle, chemical signatures from apatite and olivine are more defined and lend support to the idea that there were at least several distinct magma sources for NVF rocks with little or no contribution from crustal reservoirs. The katungite suite is similar to OIB, and probably involved a contribution from asthenosphere that was enriched in fluorine. The minette and transitional suites involved a magma source with higher LREE-enriched signatures, most likely metasomatized lithospheric mantle.