ALBERT

Notes: Abstract Since the late Pleistocene, eleven cinder and lava cones have erupted on the floor of the southern Colima graben, NE and NW of the large, active, andesitic volcano Colima. Scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites (9), leucite-basanites (3), and minettes (15), the commonest variety of mica lamprophyre. These samples represent primitive, high temperature magmas with 47.6–50.3% SiO2, 7.4–15.3% MgO, 2.5–4.4% K2O, and 2.2–9.9% normative nepheline. All members of this basic alkalic suite contain Mg-olivine (Fo75–94), chromite, augite, and late plagioclase and titanomagnetite. The petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile-bearing phases phlogopite, apatite, and analcime during late stages of crystallization. As these phases increase in abundance, presumably reflecting a rise in magmatic volatile content, there are corresponding increases in the whole rock concentrations of 16 incompatible elements. Although these incompatible elements are relatively abundant even in the basanites, many are highly concentrated in the minettes: Ba≦ 4,200 ppm, Sr≦3,100 ppm, Zr≦ 550 ppm, Ce≦190 ppm, Hf ≦18 ppm. Among the incompatible elements, the degrees of enrichment in the minettes relative to the basanites decrease in the order: H, Th, Ce, La, Nd, Zr, Hf, U, Ba, Sm, Eu, Pb, P, Nb, Sr, Ti. These enrichments may reflect the increasing importance of minor, incompatible element rich mantle phases during partial melting. The concentrations of alkali metals K, Na, Rb, and Cs do not correlate with these other elemental enrichments. The leucite-basanties have similar incompatible element contents to many minettes, differing from them only in the presence of leucite rather than analcime, and Ti-F-rich groundmass phlogopite rather than hydrous phlogopite phenocrysts; thus the leucite-basanites represent relatively dry equivalents of minettes. Two of the eleven cinder cones are calc-alkaline in nature and do not belong to the basanite-minette group; the easternmost cone is constructed of high-Al basalt, and the northernmost of basaltic andesite. The high-Al basalt (49.5% SiO2, 9.3% MgO, 221 ppm Ni) closely approximates a parental magma to the post-caldera andesitic suite of V. Colima (56.5–61.6% SiO2). The basaltic-andesite is relatively enriched in incompatible elements compared to the high-Al basalt — V. Colima trend. The ne-normative basanite-minette samples are highly enriched in incompatible elements, while the contemporaneous hy — qz-normative calc-alkaline suite, encompassing the high-Al basalt and V. Colima's andesites, is characterized by relatively low abundances of these elements. No likely mineral assemblage can relate the alkaline and calc-alkaline suites through crystal fractionation; they probably represent fundamentally different melting events. During the Quaternary, the main focus of andesitic volcanism in the southern Colima graben has migrated southward with time. Volcán Colima marks its present position, 5 km south of the Pleistocene volcano Nevado de Colima, and another 15 km from the still older Volcán Cantaro. The eruptions of basic alkalic magma probably occurred during the late stages of Nevado's life and through the life of V. Colima. They generally migrated from west to east with time, towards V. Cantaro. The most recent cone, V. Apaxtepec, is the only one east of the andesitic Colima-Cantaro axis. The oldest and the two youngest cones produced basanites, while minettes dominated at cones of intermediate ages. The cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of V. Colima. The southern end of the Colima graben can be viewed, then, as the volcanic analog of many classical, post-plutonic, hypabyssal lamprophyre localities.

Notes: Abstract 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 (Fo90-70) 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.

Notes: Abstract The White Trachytic Tuff (WTT) is a compositionally-zoned, trachytic, pyroclastic-flow deposit which erupted from Roccamonfina volcano about 300,000 years ago. It was principally emplaced as unwelded, pumice-rich flow units with an estimated volume of 10 km3. These now cover the flanks of the volcano on all sides except the west, behind the highest rim of Roccamonfina's summit caldera; the caldera was probably in existence prior to the WTT activity. Eruption of the WTT generally initiated the leucite-free, second stage of Roccamonfina's development, following a long history of leucite-bearing volcanism, but minor leucite-bearing lavas and pyroclastics overlie the WTT as well. The WTT was in turn followed by progressively more basic, leucite-free magmas (latite, trachybasalt, and basalt). During the course of the eruption, the WTT evolved from white, crystal-poor pumices containing 66% SiO2 and 1.2% CaO, to grey pumices containing higher crystal contents, 60% SiO2, and 3% CaO. Early pumices are also relatively enriched in Mn, Na, Zn, Ga, Rb, Y, Zr, Nb, Cs, La, Ce, Yb, Lu, W, Hf, Th, and U, and depleted in Ti, Fe, Mg, K, Sc, V, Cr, Co, Sr, Ba, Nd, Sm, Eu, Tb, Dy, and Ta. The pumices are essentially bimodal in composition, with several minor intermediate types including megascopic, physical mixtures of the white and grey varieties. Certain WTT pumices, including all analyzed intra-caldera samples, are relatively enriched in Pb, Th, Zr, Rb, Ga, Zn, and Cs compared to the rest of the suite. These enrichments may reflect local assimilation of carbonates or more complex exchange processes at the magma chamber margin. All WTT pumices contain the phases sanidine, plagioclase, clinopyroxene, biotite, titanomagnetite, sphene, and apatite; grey varieties also contain magnesian olivine crystals which are probably xenocrysts. The white, crystal-poor types show relatively simple mineralogies with little compositional variability or zoning among crystals of a single phase. Other pumice types, and dark, trachyandesitic inclusions separated from white pumices, show a large compositional spectrum of individually homogeneous crystals. These compositionally diverse crystals and inclusions are interpreted as a result of widespread mixing between the trachytic magmas and more basic magmas prior to or during the WTT eruption. Major-element crystal-fractionation models can successfully derive the early trachytes from the late trachytes by 50–85% separation of a syenitic assemblage of all phases. The predicted phase proportions and compositions closely match cumulate syenite nodules found at Roccamonfina. Trace element models are permissive of syenitic fractionation within the large uncertainties allowed by published partition coefficients.

Notes: Abstract The Brown Leucitic Tuff (BLT) is a poorly to strongly lithified compositionally zoned pyroclastic-flow deposit with a minimum volume of 3 to 5 km3. It erupted from Roccamonfina Volcano about 385000 years ago, after formation of the summit caldera. Individual flow units are grouped into three facies (white, brown, and orange) which primarily differ in pumice color, lithic content, and matrix cementation. Pumices from the BLT range from phonolitic leucite-tephrites to leucite-trachytes (7.0 to 2.2 wt% CaO), covering over half of the total spectrum of High-K Series magmas known from Roman Region volcanoes. White-facies units dominate in lower stratigraphic levels and their pumices have the lowest CaO contents, indicating a general trend toward more basic compositions as the eruption evolved. At higher stratigraphic levels, however, orange- and brown-facies units are interbedded with other whitefacies units, indicating reversals in the dominant compositional progression. BLT pumices have crystal contents of 9.9 to 0.6 vol%, with green salite〉plagioclase〉sanidine〉biotite〉titanomagnetite〉analcime (after leucite)〉apatite〉pyrrhotite. In most samples, plagioclase (An85–95) and sanidine (Or75–90) have much lower Na2O contents than usually found in coexisting feldspars, yet these are interpreted as equilibrium pairs. Primary leucite has been almost completely replaced by analcime. All samples also contain xenocrysts of colorless diopside and forsteritic olivine (Fo83–92). Recurrent alternations from colorless diopside to green salite are present in single clinopyroxene crystals and appear to reflect a complex history of magma mixing. Whole-rock BLT pumice compositions conform closely to High-K Series lavas from Roccamonfina for all elements except Na2O and K2O. The former is relatively enriched and the latter relatively depleted in mafic BLT pumices with 〉5.6% CaO; these differences reflect strong analcimization of abundant groundmass leucite crystals in these pumices. Otherwise, major and trace element data support fractionation of observed minerals in generating the compositional diversity among BLT pumices. Mineral assemblages and compositions of cumulate monzonite and syenite nodules carried to the surface during the BLT eruption correspond closely to the fractionated phases predicted by least-squares modeling.

Notes: [Auszug] PUMICES from the violent 15-16 June eruptions of Mount Pinatubo bore anhydrite (CaSO4), a mineral found mainly in sedimentary evaporite deposits, Bernard et aL report on page 139 of this issue1. Eruption of the anhydrite-bearing Pinatubo magma injected an enormous SO2-rich cloud into ...

Notes: Abstract A suite of 16 basanitic volcanic rocks, representing all stages in the evolution of the La Breña — El Jagüey (LBEJ) Maar Complex, has been studied petrographically and analyzed for mineral compositions and whole-rock major element, trace element, and Sr−Nd−Pb isotopic compositions. Two feldspathic granulite xenoliths were also studied as possible lower-crustal contaminants to the LBEJ magmas. The volcanic rocks contain the stable minerals olivine, plagioclase, augite, and titanomagnetite±ilmenite, plus a diverse suite of xenocrusts derived from disaggregation of mantle xenoliths of spinel lherzolite (olivine, orthopyroxene, spinel) and lower-crustal granulite xenoliths (plagioclase, quartz, augite, ilmenite). Late-stage interstitial melts rich in Fe and Ti migrated into vesicles in several samples, forming coarse-grained segregation vesicles that are dominated by ilmenite blades up to 2 mm long. The whole-rock elemental data are typical of intra-plate basanitic rocks, with strong enrichments in large ion lithophile elements (i.e. K, Th, U) as well as high field strength elements (i.e. Nb, Ta) relative to mid-ocean ridge basalts (MORB) and estimates of primordial mantle abundances. Mg# increased systematically with time during the evolution of the LBEJ Maar Complex, from 57.0–58.2 in the pre-maar lavas to 59.1–63.8 in the post-maar lavas. Compatible elements (Ca, Sc, Cr, Co, Ni) correlate positively with Mg#, whereas a large group of incompatible elements (Al, Na, K, P, Rb, Sr, Zr, Nb, Ba, La, Ce, Sm, Hf, Ta, Th, U) correlate negatively with Mg#. These trends can be closely reproduced by simple models of fractional crystallization, provided that the incompatible element abundances of the parental, high-Mg# magmas are allowed minor variability. All successful fractionation models demand an important role for augite, despite its presence in the LBEJ volcanic rocks as only a late-stage microphenocrystic and groundmass mineral. Minor garnet fractionation is necessary to produce depletion of heavy rare earth element (REE) abundances in the pre-maar lavas, whose REE patterns cross those for the rest of the suite. The importance of augite and garnet fractionation indicate that the differentiation of the LBEJ magmas took place within the upper mantle, a conclusion that is supported by the presence of spinel lherzolite xenoliths in magmas from all stages in the evolution of the maar complex. Isotopic data for seven LBEJ volcanic rocks show the following ranges: 87Sr/86Sr 0.70327–0.70347, ɛ Nd 4.2–5.0, 206Pb/204Pb 18.60–18.81, 207Pb/204Pb 15.58–15.65, 208Pb/204Pb 38.19–38.58. Sr-Nd values are negatively correlated and form a trend parallel to the mantle array, overlapping the field for ocean island basalts (OIB). The LBEJ rocks have similar 87Sr/86Sr values but lower ɛ Nd compared to basanitic rocks from the US Basin and Range Province (BRP). Pb isotopic ratios are positively correlated and overlap the braod fields for MORB and OIB and the small fields for Mexican ore deposits and volcanic rocks from the active subduction-related Mexican Volcanic Belt. The LBEJ rocks have slightly more radiogenic Pb than basanitic rocks from the US BRP. Despite correlations among the isotopic ratios of the LBEJ suite, none of these ratios correlate with position in the eruption sequence, Mg#, or any other compositional parameter. The two lower-crustal xenoliths have high 87Sr/86Sr values (0.707, 0.710) and low ɛ Nd (-1.5,-8.0) compared to the LBEJ volcanic rocks, but their Pb isotopic compositions are only slightly more radiogenic than the volcanic rocks. These data do not support the widely held view that the lower crust is a major reservoir of unradiogenic Pb. In order to further constrain the role played by crustal contamination in generating the isotopic diversity in the LBEJ suite, we conducted an extensive investigation of Sr−Nd−Pb isotopic ratios for scoria clasts from different levels of a single scoria-fall horizon in the pyroclastic sequence related to the formation of La Breña Maar. Our results do not support an important role for crustal contamination in the LBEJ magmas. Rather, we conclude that minor isotopic variability exists in the mantle source regions beneath the maar complex.

Notes: Abstract The La Breña — El Jagüey Maar Complex, of probable Holocene age, is one of the youngest eruptive centers in the Durango Volcanic Field (DVF), a Quaternary lava plain that covers 2100 km2 and includes about 100 cinder and lava cones. The volcanic complex consists of two intersecting maars — La Breña and El Jagüey — at least two pre-maar scoria cones and associated lavas, and a series of nested post-maar lava and scoria cones that erupted within La Breña Maar and flooded its floor with lava to form one or more lava lakes. We believe that El Jagüey Maar formed first, but pyroclastic deposits associated with its formation are exposed at only a few places in the lower maar walls. A perennial lake in the bottom of El Jagüey marks the top of an aquifer about 60 m below the lava plain. Interaction of the rising basanitic magmas with this aquifer was probably responsible for the hydromagmatic eruptions at the maar complex. In the southeastern quadrant of La Breña and in most parts of El Jagüey, the upper maar walls expose a thick pyroclastic sequence of tuffs, tuff breccias, and breccias that is dominated by thinly layered sandwave and plane-parallel surge beds and contains minor interlayered scoria-fall horizons. We conclude that these deposits in the upper walls of both maars erupted during the formation of La Breña, based on: (1) thickness variations in a prominent scoria-fall marker bed interlayered with the surge deposits; (2) inferred transport directions for ballistic clasts, channels, and dune-like bedforms; and (3) lateral facies changes in the surge deposits. Some of the surge clouds from La Breña apparently travelled down the inner southwestern wall of El Jagüey, fanned out across its floor, and climbed up the opposite walls before emerging onto the surrounding lava plain. These clouds deposited steep, inward-dipping surge deposits along the lower walls of El Jagüey. Following this hydromagmatic phase, which was responsible for the formation of the maars, a series of strombolian eruptions took place from vents within La Breña. At many places along the maar rims these eruptions completely buried the surge beds under a thick sequence of post-maar scoriae and ashes. The outer flanks of the maar complex and the surrounding lava plain are also blanketed by post-maar ashes. The final phase of activity involved effusive eruptions of post-maar lavas from vents on the floor of La Breña. The evolutionary sequence from hydromagmatic eruptions during formation of the maars, through strombolian eruptions of the post-maar scoriae and ashes, and finally to the post-maar lavas appears to reflect the declining influence of magma-groundwater interactions with time. Basanitic magmas from all eruptive stages carried spinel-lherzolite and feldspathic-granulite xenoliths to the surface. The La Breña — El Jagüey Maar Complex contains the only known hydromagmatic vents in the DVF and the largest spinel-lherzolite xenoliths, which range up to 30 cm diameter. These two observations indicate an unusually rapid ascent rate for these basanitic magmas compared to those from other DVF vents.

Notes: Abstract Volcán Colima is Mexico's most historically active andesitic composite volcano. It lies 150 km north of the Middle America Trench at the western end of the Mexican Volcanic Belt, closer to the trench than any other composite volcano in Mexico. Since its earliest reported eruption in 1576, V. Colima has evolved through three cycles of activity. Each cycle culminated in a major ashflow eruption, halting activity for 50 or more years. The last major ashflow eruption occurred in 1913. Andesitic block lava eruptions in 1961–1962 and 1975–1976 marked the inception of activity in a fourth historical cycle which may also terminate with a major ashflow eruption in the early part of the next century. Major and trace element analyses of whole rock samples and all constituent phases are presented for a suite of nine post-caldera hornblende and olivine-andesites. The suite includes samples from Colima's four major eruptions since 1869, spanning the last two eruptive cycles. Colima's post-caldera andesites are poor in K and other incompatible elements (Ti, P, Zn, Rb, Y, Zr, Ba, La, Yb, Hf, Th, and U) as may be characteristic of near trench andesites. From the 1913 ashflow eruption through the fourth cycle andesites, there have been increases in whole rock abundances of Si, Ba, and Cs, and decreases in Ti, Fe, Mg, Ni, Cr, and Sc. Crystal fractionation models can closely reproduce major element variations in the post-caldera suite, but systematically fail to predict sufficient concentrations of the compatible trace elements Cr, Ni, and Zn. Anomalous enrichments of compatible trace elements in Colima's andesites probably reflect simultaneous crystal fractionation and magma mixing in the subvolcanic system. Estimated pre-eruptive temperatures range from 940 °–1,000 ° C in the hornblende-andesites and 1,030 °–1,060 ° C in the olivine-andesites. Pre-eruptive magmatic water contents of 1.0–3.6 wt.% are calculated for the hornblende-andesites; the phenocryst assemblage of the olivine-andesite is calculated to equilibrate at 1,000 bars with 0.8% H2O. Orthopyroxenes and certain clinopyroxenes in all pre-1961 samples are reversely zoned, with relatively Mg-rich rims. The most pronounced Mg-rich rims occur in the olivine-andesites and are thought to reflect pre-eruptive magma mixing, involving a basic, olivine+/-clinopyroxene-bearing magma. In addition to their normally zoned pyroxenes, the post-1961, fourth cycle andesites display a number of other features which distinguish them from earlier post-caldera hornblende-andesites of similar bulk composition. These include: (1) higher total crystal contents, (2) lower modal hornblende contents, (3) higher calculated pre-eruptive silica activities, and (4) lower calculated pre-eruptive water contents. These features are all consistent with the interpretation that the fourth cycle andesites were less hydrous prior to eruption. The slight Mg-rich pyroxene rims in pre-1961 hornblende-andesites may record late-stage, pre-eruptive increases in magmatic water content, which act to raise magmatic f O 2 and Mg/Fe+2 ratios in the melt and in all crystalline phases. The fourth cycle andesites apparently did not experience a strong, pre-eruptive influx of water, resulting in lower magmatic water contents and normally zoned pyroxenes.

Notes: Abstract  Pleistocene-Holocene volcanism in the Jalisco block of western Mexico is confined to two conspicuous grabens, where potassic eruptives range from absarokites (48–52% SiO2) and minettes (49–54% SiO2) through basaltic andesites (53–57% SiO2), the most voluminous type, to andesites and their lamprophyric equivalent spessartite (58–62% SiO2); there are no contemporary rhyolitic rocks. This suite has high concentrations of Mg, Cr (〈550 ppm) and Ni (〈450 ppm) accompanied by large concentrations of K, P, Ba (〈4000 ppm) and Sr (〈5000 ppm) and elements such as LREE and Zr (〈600 ppm). No combination of crystal fractionation and/or crustal contamination can reproduce the compositional range of these magmas, which nevertheless are believed to be genetically related because of their proximity in time and space. Hydrous minerals in the lamprophyres and the typical absence of plagioclase phenocrysts in both basaltic andesites and andesites reflect the relatively high concentrations of water in the magmas, which suppressed the crystallisation of feldspar. Experimental verification of the minimal amounts of water required to reproduce the phenocryst assemblages in selected rocks range from 3.5 to 6%. During ascent in a volcanic conduit, andesitic magma may lose water and consequently precipitate plagioclase, or it may ascend more rapidly, retaining more of its initial water, which stabilises phenocrysts of hornblende at the expense of plagioclase. Our estimates of water concentrations, which are consistent with the various low pressure phenocryst assemblages, will be minimal for the magmas in their source regions, and the process of magmatic dewatering on ascent may be typical in well established volcanic conduits. In accord with the compositions of phenocrystic olivine in the basaltic andesites and the minettes, the values of FeO and Fe2O3 of the bulk lavas and scoriae are demonstrably pristine. As a consequence, there are two characteristic features of the Mascota suite: the high range of relative oxygen fugacities (ΔNNO=1–5) and the high Mg# (MgO/MgO+FeO) that ranges from 0.70 to 0.91 (with only one andesite as low as 0.66). From the evidence of phlogopite phenocrysts, a partial melt involving phlogopite would have a higher Mg# than one from olivine (Fo90) and pyroxene alone. As the Mascota series shows a correlation between K2O and Mg#, we conclude that it was generated by partial fusion of the mantle wedge, with a variable contribution of phlogopite and apatite from veins throughout the lherzolitic assemblage. In conformity with an origin by varying increments of partial fusion of a phlogopite-bearing mantle, all incompatible elements vary linearly with Ti (or K) as if phlogopite (+apatite) in the source dominated their contribution to the partial melts. Fluids from dehydration of the subducting slab presumably deposit hydrous and other minerals in veins in the mantle wedge and also increase its redox state. As the Mascota volcanism occurs in grabens closer to the trench than the main andesite arc, it is concluded that the eruption of these small volumes of hydrous magmas require the tectonically favored ascent paths offered by the extensional grabens to reach the surface from their mantle sources.

Notes: Abstract A 34 meter section of ash- and scoria-fall units has been studied on the upper NE flank of the active Volcán Colima. Charcoal and soil horizons are restricted to the topmost 12 m. Nine 14C dates show a smooth progression from 235 years near the surface to 8,300 years at 10 meters depth, indicating a post-Pleistocene accumulation rate of 1.3 m/1,000 years at 6 km from the vent. This figure allows an estimate of the magmatic eruption rate for fallmaterial of 0.31 km3/1,000 years, less than 15% of Colima's lava eruption rate. An unusually thick and coarse-grained scoria-fall horizon at about 4 m depth in the section appears to have been produced by the caldera-forming eruption of Colima some 4,300 years ago. The majority (40) of 46 analyzed scoria and ash horizons are typical Colima hornblende-andesites with an average SiO2 content of 58%, nearly identical to the scoriae of Colima's 1913 pyroclastic flows. The scoriae are significantly more basic than Colima's andesitic lavas, which average 61% SiO2. The six remaining scoria horizons are anomalously alkalic and rich in incompatible elements. Five coarse alkaline scoria layers occur in sequence just below the 8,300 year level. They show progressive upward increases in K, P, Ba, Sr, Zr, La, Ce, and related elements, culminating in a phlogopite-bearing scoria horizon. Over the last 20,000 years or so, a group of cinder cones erupted 20–35 km to the north of Volcán Colima, producing basic alkalic magmas including basanites and phlogopite-bearing minettes. The alkaline scoriae of the studied sections probably record pre-eruptive injections of minette magma into the subvolcanic, calc-alkaline system of V. Colima. The age, composition, and mineralogy of the alkaline scoriae are consistent with this interpretation. Least squares mixing models suggest 60 wt.% minette component in the phlogopite-bearing horizon.