Jorullo Volcano,Michoacán,Mexico (1759–1774): The earliest stages of fractionation in calc-alkaline magmas

Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted an estimated 2 km³ of magma which evolved progressively with time from early, hypersthene-normative, primitive basalts to late-stage, quartz-normative, basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (Fo_90-70) with Cr-Al-Mg-spinel inclusions, and microphenocrysts of plagioclase and augite; late-stage basaltic andesites also carry phenocrysts of plagioclase, augite, and rare orthopyroxene, hornblende pseudomorphs, and microphenocrysts of titanomagnetite. Olivine-melt compositions indicate liquidus temperatures ranging from 1,230° C to 1,070° C in the early- and late-stage lavas, respectively; \(f_{{\text{O}}_{\text{2}} } \) was about 0.6 log units above the Ni-NiO buffer in the early lavas but increased to 2.5 log units above Ni-NiO in the late lavas, perhaps through groundwater-magma interaction. Smooth major and trace element compositional trends in the lavas can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. La, Ce, and other incompatible elements (Rb, Sr, Ba, Hf, Th, Ta), however, are anomalously enriched in the latestage lavas, whereas the heavy rare earth elements (Dy, Yb, Lu) are anomalously depleted. The modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8–15 kb), although the crystals actually present in the Jorullo lavas appear to have formed at low pressures. Thus, a two-stage crystallization history is implied. Despite the presence of granitic xenoliths in middle-stage lavas from Jorullo, bulk crustal assimilation appears to have played an insignificant role in generating the compositional trends among the lavas. As MgO decreases from 9.3 to 4.3 wt% through the suite, Al₂O₃ increases from 16.4 to 19.1 wt%. Most highalumina basalts reported in the literature have 18 to 21 wt% Al₂O₃, but are too depleted in MgO, Ni, and Cr to have been generated directly through mantle partial melting. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalts similar to the early Jorullo lavas. Such primitive basalts are rarely erupted in mature arcs and may be completely absent from mature stratovolcanoes. Cerro La Pilita is a late-Quaternary cinder and lava cone centered just 3 km south of Jorullo. The primitive trachybasalts of Cerro La Pilita, however, are radically different from the Jorullo basalts. They are nepheline normative with high concentrations of K₂O (>2.5 wt%), P₂O₅ (>0.9 wt%), Ba (1,200 ppm), Sr (>2,000 ppm), and many other incompatible elements, and contain crystals of hornblende and apatite in addition to olivine, spinel, augite, and plagioclase. The magmas of these two neighboring volcanoes cannot be related to one another by any simple mechanism, and must represent fundamentally different partial melting events in the mantle. The contrasts between Jorullo and Cerro La Pilita demonstrate the difficulty in defining simple relationships between magma type and distance from the trench in the Mexican Volcanic Belt.