A Tale of Two Magma Series: Geochronology and Geochemistry of Volcanism on Grenada, Lesser Antilles

Abstract:

Volcanic rocks from Grenada have long been recognized to belong to two distinct magma series: the olivine microphyric M-series and the ankaramitic, calcium-rich C-series. Mafic members of both series are readily distinguished on the basis of both major and trace element compositions and radiogenic isotope ratios, with the M-series having more radiogenic Sr and Pb and less radiogenic Nd than the C-series. The two series evolve along distinct paths to compositionally and isotopically similar silica-rich hornblende andesites and dacites.
We report 29 new ⁴⁰Ar/³⁹Ar dates ranging from 0.06 to 6.06 Ma, which are notably younger than previous K-Ar ages ranging up to 21 Ma, perhaps reflecting non-atmospheric inherited Ar. The two series have erupted contemporaneously and ages of both series tend to cluster in two periods: 0 to 1.7 Ma and 4.8 to 6 Ma. The oldest lavas are located in the northeastern and southwestern ends of the island. There is little or no systematic variation in chemistry with age. A dike intruding Tufton Hall Formation (THF) sediments in the south of the island is much older at 37.8 Ma;

C-series isotope ratios correlate with MgO, which has previously been interpreted as a consequence of fractional crystallization and assimilation (AFC) of marine sediments. Isotope ratios of M-series lavas, in contrast, show no systematic variation with MgO. We agree that the C-series magmas have experienced AFC, but argue that the assimilant is simply the M-series products that makes up two-thirds of the mass of the island. A variety of evidence supports this interpretation. First, K₂O/Na₂O ratios decrease with decreasing MgO in the C-series. Second, the THF is volcanogenic likely derived from ancestral volcanoes and also are not a suitable isotopic end-member. Third, O isotope ratios in clinopyroxenes correlate with radiogenic isotope ratios, but not with MgO. Finally, the most differentiated C-series lavas are indistinguishable, isotopically and otherwise, from M-series compositions. Such auto-assimilation may be common in volcanic systems, and may affect the M-series as well, but only becomes apparent in cases such as this where magmas are compositionally distinct from the volcanic edifice they rise through.