Geochemical Evidence for Mantle Origin and Crustal Processes in ...

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JOURNAL OF PETROLOGY VOLUME 46 NUMBER 6 JUNE 2005 Fig. 7. Total alkalis (Na 2 O þ K 2 O; wt %) vs silica (SiO 2 ; wt %) diagram after Le Bas et al. (1986) for all analysed volcanic rocks. Data are from Table 1 and normalized on an anhydrous basis. The subdivision into alkaline and subalkaline series (MacDonald & Katsura, 1964) is also shown. fine-grained hornblende–biotite granodiorite); (3) metasiltstone; (4) andesitic volcanic rocks. Moreover, the three most common xenolith types form composite xenoliths, typically with rounded skarn clasts enclosed in granodiorite (Fig. 5f). Less common xenoliths in the 14 ka BP tephra sequence are foliated biotite tonalite, marble, and strongly reacted, cordierite–sillimanite and hercynite-bearing pelitic clasts. Similar xenoliths have also been described from Iztaccíhuatl (Nixon, 1989). Calcsilicate skarn xenoliths have been taken as evidence that a sedimentary sequence including limestone underlies the volcano (Robin, 1984; Boudal, 1985; Kolisnik, 1990). Skarn xenoliths are massive to weakly layered or spotted, porous and friable, with abundant cavities lined by vapour-phase minerals. The mineral assemblage is most commonly dominated by epidote, actinolite, diopside, sphene, magnetite, and sulphate minerals, but garnet, wollastonite, and idocrase are also present in some samples. Meta-siltstones consist of fine-grained detrital quartz enclosed in a recrystallized matrix consisting of pyroxene and feldspar. Plutonic xenoliths are angular to subrounded, equigranular to weakly porphyritic, fine-grained (
SCHAAF et al. PETROGENESIS OF POPOCATEPETL between 512 and 645 wt %. The lowest value corresponds to an alkaline basalt from Citlaltepec scoria cone (sample 95357) and the highest value is for a transitional sample (sample 96359) from the Malpaís lava flow. Total alkali (Na 2 O þ K 2 O) contents range between 42 and 66 wt %, with lower values more typical for the scoria cones (Fig. 7). The majority of the samples (lava flows, pumices, domes, and juvenile clasts from pyroclastic-flow deposits) from Popocatépetl have andesitic compositions. Samples from three lava flows, three dome fragments, two pumice clasts (including a sample from the January 22, 2001 pyroclastic-flow deposit), and all four bulk ashfall samples from the current eruption are dacitic (SiO 2 >63 wt %). Although Popocatépetl and surrounding scoria cone samples partially overlap in SiO 2 and other element concentrations, FV samples are generally more mafic. The majority of the monogenetic scoria cones and associated lavas (22 samples) are andesitic, 13 samples have basaltic andesite compositions, four samples have dacitic compositions, two are trachybasalts and only one sample is strictly basaltic. The transitional vent samples are more enriched in silica, sodium and potassium, and some of them have even higher contents of these elements than typical stratocone samples (Fig. 7). Of the 42 FV and T samples, only Cerro Santa Barbara (samples 96356 A and B) and Cerro Citlaltepec (sample 96357) have alkaline compositions lying above the MacDonald & Katsura line in Fig. 7. Alkaline basalts are restricted to the farwestern edge of the study area (Fig. 2) and are volumetrically the least significant magmas in this region. All of the analysed rocks are hy-normative and none are ne-normative. Trends of Popocatépetl samples are compared with the surrounding monogenetic cones (FV and T samples) in Harker diagrams in Fig. 8. In the SiO 2 vs MgO diagram (Fig. 8a), the various suites of the study area are clearly distinguishable. The three alkaline FV samples have lower SiO 2 concentrations at equivalent MgO concentrations compared with SCVF and Valley of Puebla calc-alkaline basaltic andesite samples. The abundant basaltic andesites form a continuous trend with more evolved FV and T samples, which overlap with the less evolved samples from Popocatépetl volcano. MgO decreases sharply with increasing SiO 2 in the FV suite, consistent with fractionation of olivine and pyroxene. Similar decreases in TiO 2 (Fig. 8b) and P 2 O 5 (Fig. 8c) with increasing SiO 2 are discernible; however, in comparison with MgO, TiO 2 and P 2 O 5 in Popocatépetl and transitional SCVF cones do not decrease continuously with increasing SiO 2 . The more scattered TiO 2 and P 2 O 5 distributions in the upper left converge in the lower right parts of the diagrams and their trends become flatter. The overall decline in TiO 2 and P 2 O 5 with increased SiO 2 to about 60% is interpreted to be the result of Ti-magnetite and apatite fractionation in these magmas. Popocatépetl samples generally have higher Al 2 O 3 /CaO ratios than FV samples. The ratios are highest in the ash fallout samples (Fig. 8d). The increase of the Al 2 O 3 /CaO ratio with increasing SiO 2 indicates that clinopyroxene fractionation is important. In all Harker diagrams (including those that are not shown in Fig. 8), transitional vents generally plot in a similar manner to Popocatépetl samples. Combined major and trace element diagrams In a Ba vs MgO diagram (Fig. 8e) two trends are visible: FV samples generally display a negative correlation, whereas Popocatépetl samples show a positive correlation (see arrow in Fig. 8e), with the highest Ba concentrations (c. 550 ppm) at MgO contents between 4 and 5 wt %. The increase in Ba with decreasing MgO is consistent with a lack of plagioclase fractionation in the FV suite. Conversely, the relative Ba depletion and decreasing Ba with decreasing MgO in Popocatépetl samples are probably the result of variable plagioclase fractionation. Chromium concentrations ranging from 11 to 499 ppm show a strong positive correlation when plotted against MgO (Fig. 8f). The very rapid decrease in Cr from 9 to 4 wt % MgO in FV samples can be ascribed to fractionation of olivine with Cr-spinel inclusions. Trace element compositions The most primitive [whole-rock Mg-number >68, where Mg-number ¼ 100Mg/(Mg þ Fe 2þ ); MgO >8 wt%; Ni >150 ppm] FV calc-alkaline basaltic andesites and alkaline basalts display large and systematic differences in trace element compositions. REE distributions of selected FV samples and least evolved Popocatépetl rocks are plotted in Fig. 9 and are characterized by enriched light REE (LREE) concentrations with respect to the heavy REE (HREE; La/Lu CN 46–88) and the absence of significant Eu anomalies (Eu/Eu* 081–099). Alkaline basalts have the highest total REE concentrations (140–170 ppm) and are also enriched in most other trace elements relative to the most primitive calc-alkaline basaltic andesites from Cerro Pelagatos, Cerro Partido, and Cerro Tecajete (REE 75–113 ppm). These basaltic andesites also have lower REE abundances than the most primitive basalts from the nearby mildly alkaline Chichinautzin volcano of the central SCVF [REE 186– 196 ppm; data from Siebe et al. (2004b)]. In comparison, least evolved Popocatépetl samples (andesitic lava, pumice, and scoria samples) show the lowest REE concentrations (71–85 ppm) in Fig. 9. These four rock groups from Chichinautzin volcano and other closely spaced monogenetic cones surrounding Popocatépetl volcano display element patterns that cannot be related to one another only by fractional crystallization processes or differential partial melting of a common homogeneous mantle source (Siebe et al., 2004b). Instead, they point towards a 1267
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