Geology and petrology of Volcán Ceboruco, Nayarit, Mexico: Summary STEPHEN A. NELSON* Department of Geology and Geophysics, University of California, Berkeley, California 94720 INTRODUCTION 1870; Iglesias and others, 1877). The volcano is in the northwest- ern part of the Mexican Volcanic Belt at lat 2 I °7'.W'N, 104°30'W. Volcan Ceboruco is one of nine of Mexico's historically active It is crowned by two concentric calderas and rises to an elevation of volcanoes (Mooser and others, 1958), with a single documented 2,200 m, or about 1,100 m above the surrounding valley of historical eruption during the period 1870—1875 (Caravantes, Ahuacatlan. Tertiary rhyolitic ash-flow tuffs crop out on both sides of the valley of Ahuacatlan and probably underlie Volcan * Present address: Department of Geology, Tulane University, New Or- Ceboruco. leans, Louisiana 701 18. The complete article, of which this is a summary, appears in Part II of the Bulletin, no. 1 1, p. 2290 —2431. Geological Society of America Bulletin, Part I, v. 91, p. 639-643, 3 figs., November 1980, Doc. no. S01 102. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11/639/3429689/i0016-7606-91-11-639.pdf by guest on 29 September 2021 Figure 1. Geologic map of Volcán Ceboruco and surrounding area. Inset shows loca- tion of Ceboruco relative to the other active volcanoes of Mexico (from west to east: Col- ima, Paricutin, Jorullo, Xitli, Popocateptl, Orizaba, and San Martin. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11/639/3429689/i0016-7606-91-11-639.pdf by guest on 29 September 2021 VOLCÁN CEBORUCO, MEXICO: SUMMARY 641 Figure 2. Aerial photograph of top of Volcán Ceboruco, show- ing both calderas and postcaldera lava flows and volcanic domes. North is to bottom of photograph to facilitate viewing. Compare with Figure 1. This illustra- tion is Figure 7 in the ac- companying article in Part II. GEOLOGY ably caused the formation of Ceboruco's outer caldera, 3.7 km in diameter (Figs. I, 2). As a result of caldera formation, about 3.4 The eruptive history of Volcán Ceboruco has been divided into km3 was lost from the top of Ceboruco. Of this volume, only about three stages separated by episodes of caldera formation. During the 2 km'1 can be accounted for in erupted pumice (recalculated as liq- first stage of activity a stratovolcano was built to an estimated ele- uid magma) and lithic fragments. vation of 2,700 m. Exposures of the first-stage lavas, although lim- Ceboruco's second stage of activity began with the eruption of ited, indicate that flows tended to be of small volume and that rela- the Dos Equis dacite dome (Fig. 1), which partially filled the outer tively little explosive activity accompanied the eruptions. All of caldera. From the top of this dome the Copales flow, with a volume these precaldera lavas are andesites containing phenocrysts of pla- of 1.4 km:i, was extruded to cover the southwestern flanks of the gioclase, hypersthene, and titanomagnetite, with a few rare crystals volcano (Fig. I). Both the dome rocks and the flow rocks contain of olivine and augite. In all, about 60 km' of andesitic lava was phenocrysts of plagioclase, hypersthene, titanomagnetite, and erupted during this stage of activity. ilmenite. They also contain abundant xenoliths of high-Al basalt Also during this first stage, several flank eruptions occurred that contain phenocrysts of forsteritic olivine, aluminous augite, within a zone trending N60°W through Ceboruco. The La and calcic plagioclase (Fig. 3a). These xenoliths are frequently Pichancha andesite, Ceboruquito andesite, Cerro Pochetero sodic found to be disaggregated and scattered throughout the host dacite rhyolite dome, Cerro Pedregoso rhyodacite dome, and Desfiladero (Fig. 3b). Because there is no evidence of quenching of the host ma- rhyodacite (Fig. 1) were all erupted before the end of the first stage terial against the xenoliths, as is the case for other xenoliths picked of activity. The zone along which these lavas were vented probably up by Ceboruco's lavas, the possibility of magma mixing is represents a zone of crustal weakness. suggested. The end of the first stage of activity was marked by the plinian Following the eruption of the Copales flow, Ceboruco's inner eruption of about 5 krrf1 of white rhyodacite pumice, termed the caldera, 1.5 km in diameter, was formed within the Dos Equis Jala pumice, and the eruption of the Marquesado pyroclastic flow dome (Figs. 1, 2). Because no pyroclastic deposits are associated (Fig. 1). This eruption occurred about 1,000 yr ago, on the basis of with this event, the collapse is inferred to have resulted from drain- l4C dates on charcoal recovered from beneath the Jala pumice. The ing of an underlying magma chamber by eruptions of lava flows Jala pumice contains sparse phenocrysts of plagioclase, such as the Copales flow. hornblende, hypersthene, titanomagnetite, and ilmenite. Xeno- Ceboruco's third stage of activity began with the eruption of the crysts of forsteritic olivine and aluminous augite also occur, in- El Centro dome (Fig. 1) on the floor of the inner caldera, after creasing in abundance upward through sections of the deposits. which activity shifted to the caldera margins, where the Coapan The explosive eruptions of Jala pumice and Marquesado ash prob- and El Norte andesite flows were erupted to the north and the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/11/639/3429689/i0016-7606-91-11-639.pdf by guest on 29 September 2021 642 S. A. NELSON PETROLOGY On the basis of both major- and trace-element abundances, five chemically distinct groups of lavas have been erupted from Ceboruco. Lavas erupted from vents on the southeastern flanks of Ceboruco are chemica Iv diverse and show no relationship to those erupted from the central volcanic complex. These southeastern flank lavas include a low-Si, Fe-Ti—rich andesite, a high-K andesite (the Ceboruquito flow, Fig. I), and a sodic rhyolite (the Cerro Pochetero dome. Fig. 1). The oldest of the five major groups is the precaldera andesites. Relative to the more recent, postcaldera andesites, the precaldera lavas arc enriched in CaO and MgO, are depleted in Na20, K20, Si02, and incompatible trace elements, and have similar concen- trations of total Fe. Both groups of andesites contain plagioclase as the predominant phenocrvst phase, from which it is deduced that plagioclase was the liquidus phase in these magmas. The Jala pumice group, which includes three eruptive units of airfall pumice, the Marquesado ash, the Cerro Pedregoso dome, and the Destiladera lava flow, are all corundum-normative rhvodacites. The airfall units show a gradation in chemical compo- sition upward through the stratigraphic sequence; Si02 and K20 decrease, whereas MgO, CaO, and compatible trace elements in- crease. As the abundance of xenocrystic olivine and augite also in- creases upward through the airfall deposits, the gradational trends appear to reflect the xenocrysts and suggest a possible interaction of the rhvodacite magma with a more basic magma. Thermodynamic calculations indicate that the pumice was erupted from a pressure of about 900 bar and contained about 4 wt % H20 prior to eruption. The second-stage dacites are low in Si02 (63 to 64 wt %) and are thus transitional to andesites. The fact that these lavas all contain xenoliths of high-Al basalt suggests that they represent a hybrid magma. Calculations based on an estimation of the basalt compo- sition from modal data and microprobe determinations of the compositions of the mineral phases in the xenoliths indicate that the second-stage dacites could have resulted from mixing of the Jala pumice rhyodacitic magma with the basalt. This mixing event could have triggered the eruption of the Jala pumice, as evidenced by the fact that the compositions of xenocrysts in the pumice are b identical to those of phenocrysts in the basaltic xenoliths. Lavas erupted during the historic eruption of 1870 are dacites Figure 3. Photomicrographs of xenoliths in second-stage da- (67 to 68 wt % Si0 ) and are enriched in K 0 and incompatible cites. (a) Xenolith (left) with groundmass consisting of intcrgrown 2 2 elements relative to the Jala pumice group. plagioclase and orthopyroxene. Olivine crystal from xenolith can be seen on right, floating in dacitic matrix, (b) Xenolith containing In attempts to determine if the chemical variation at Ceboruco large phenocrvst of olivine (right). Groundmass consists of plagio- was a result of crystal fractionation, a linear least squares computer clase and orthopyroxene with some titanomagnetite and ilmenite. program was used for major elements and the Rayleigh distillation Glass surrounding vesicles is seen in lower left and upper left. This law for trace elements. In all, 648 possible fractionation models in- illustration is Figures 8a and 8d, respectively, in the accompanying volving the five major chemical groups of lavas, the three flank article in Part II. lavas, and the high-Al basalt found as xenoliths were tested. Only three models are found to be consistent with major and trace ele- ments as well as petrographic criteria. The postcaldera andesites are found to be suitable parents for the Jala pumice and 1870 da- Ceboruco andesite How was erupted to the south (Fig. I). These cites, and the second-stage dacites are found to be suitable parents postcaldera andesites represent a volume of 3 to 4 krri1. The lavas for the Jala pumice. Of the three, the model involving fractionation contain phenocrysts of plagioclase, hvpersthene, augite, of the postcaldera andesites to produce the 1870 dacites is the only titanomagnetite, and ilmenite. feasible model based on the time sequence of eruption and the Finally, on February I 8, I 870, Ceboruco began its only recorded suggested origin of the second-stage dacites by magma mixing.