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Petrography and the Volcanic Rocks

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Petrography and the Volcanic Rocks Geochemical Journal, Vol. 18, pp. 217 to 232, 1984 Petrography and major element chemistry of the volcanic rocks of the Andes, southern Peru SHIGEO ARAMAKI,' NAOKI ONUMA2 and FELIX PORTILLO3 Earthquake Research Institute, University of Tokyo , Bunkyo-ku, Tokyo 113,' Department of Earth Sciences, Ibaraki University, Mito 3102 and Instituto Geologica Minero y Metalurgico, Lima, Peru3 (Received December 12, 1983: Accepted May 29, 1984) More than 200 samples of late Tertiary to Quaternary volcanic rocks have been collected from the northern sector of the central volcanic zone of the Andean belt occupying southern Peru. The most abun dant rock type is the pyroxene andesite. Many of the rock samples carry hornblende and/or biotite pheno crysts. Small amounts of shoshonites occur on the back arc side near Puna and Siquani and olivine-augite basanites occur on the western shore of Lake Titicaca. The SiO2 frequency has, a mode in the 60-65% range which is about 5% higher than the Quaternary volcanic rocks of Japan. The K20 content shows a distinct tendency to increase away from the front, while the Na20 content tends to decrease in the same direction. The K, Sr and Ba contents of the late Tertiary to Quaternary volcanic rocks of the northern part of the central zone of the Andean volcanic belt (southern Peru) show regular increase away from the volcanic front. At the same time, a slight northwestward increase along the arc is detected. The Na content regularly decreases away from the front making a strong contrast to the Japanese Quaternary volcanic arcs, which does not show any regular change. The Na content is conspicuously higher in the northwestern frontal zone than the rest. Prediction, Tohoku University, seismology) and INTRODUCTION N. FUJn (Department of Earth Sciences, Kobe A cooperative project on the geochemical University, volcano physics). The Peruvian study of central Andean volcanic zone was research group consisted of the staff of carried out during the period from May, 1980 INGEMMET, C. GUEVARA (Chief, Geology through March, 1982, including the field work section), F. PORTILLO (geology), M. MONTOYA and laboratory analyses. The project was co (geology) and J. A. LAJO (geology). sponsored by the Overseas Scientific Research, Our objective was to elucidate the processes Ministry of Education (Mombusho) of Japan and of generation and evolution of the magma Instituto Geologico Minero y Metalurgico formed through the chemical interactions (INGEMMET) of Peru. The Japanese research between the subducting Nazca plate and the group consisted of N. ONUMA (Project leader, mantle at depths on the basis of the geochemical Department of Earth Sciences, Ibaraki Uni studies of Andean andesties in the central Andes versity, trace element geochemistry), S. volcanic belt (southern Peru) where volcanism is ARAMAKI (Earthquake Research Institute, Uni taking place over the crust which is the thickest versity of Tokyo, geology and major element (about 70km) in the world. geochemistry), K. NOTSU (Institute of Chem The main field work took place in the sum istry, University of Tsukuba, Sr isotope geo mer of 1980 for about 70 days and the area chemistry), I. KANEOKA (Geophysical Institute, covered ranged from 14'S to 18'S crossing the University of Tokyo, K-Ar dating), A. whole width of the Andean volcanic belt (Fig. HASEGAWA(Observation Center for Earthquake 1). The area is located at the northernmost 217 218 S. ARAMAKI et al. sector of the central zone of the Andean vol straints stated above. canic belt which overlies the Nazca plate sub ducting eastward, and corresponds to a transi SAMPLE COLLECTION tion zone between the normal and abnormal subduction segments of the Nazca plate (HASE Selection of the sampling locality depended GAWA and SACKS, 1981). The northern limit of totally on the extensive information already the central zone roughly corresponds to the collected by INGEMMET, Peru. Based on the northern limit of the part of the Nazca plate published and unpublished geologic quadrangle subducting at an angle of about 30 degrees maps and reports, sites were selected for the (normal subduction). To the north of this, the representative Quaternary volcanic suites in Nazca plate dips at about 30 degrees for the southern Peru. In principle, rocks designated first 100km of the descent from the trench, as Barroso group (see KANEOKA and GUEVARA, but remains nearly horizontal for the next 1984, Fig. 2) were our main target while those 300km and then dips again at about 30 degrees of the Senca and Tacaza were excluded. It (abnormal subduction). The horizontal portion was hoped that by restricting the geologic age roughly corresponds to the northern extension 800 60° W of the central volcanic zone. The virtual absence of the young volcanic activity in this area may 1 be due to the shallow depth of the subducting Nazca plate and the very thick (about 70km, 000OS CUMMINGS and SCHILLER, 1971) crust acting PLATE r together to restrict the amount of the astheno sphere edge below this area (HASEGAWA and 0° SOUTH 0° SACKS,in preparation). Galapagos AMERICAN Is. This report is one of a series of papers result PLATE ing from the current project and deals with the \ summary of petrography, major element geo N A Z C A chemistry and chemical mapping of the volcanic rocks. PLATE R-S Abundant volcanic centers and volcanic 200S 20° nI materials occur in the studied area spanning in die II r-I age from early Tertiary to historic time. Strati tioa MR graphy has been extensively studied by the INGEMMET scientists but radiometric dating has been so far very limited (KANEOKA and zl i GUEVARA, 1984). Therefore one of the main Chile 21 difficulties in characterizing the Andean vol Rise i 400 400 Ig canism in comparison with more well-defined I volcanic arcs such as those in the Japanese islands is to distinguish the spatial variation of ANTARCTIC Ia PLATE 0 the volcanic materials from the temporal one. Major and minor element data published by `ae LEFEVRE (1973, 1979) cover the southern half of the area of the present study. He demon 1000 800 strated a clear tendency of increasing K content Fig. 1. Index map of the Andean volcanic belt. Dots away from the volcanic front but his results indicate the distribution of the Quaternary volcanoes. also cannot escape from the temporal con Obliquely ruled area corresponds to Fig. 2. Volcanic rocks of the Andes 219 CZ-03 ,CZ 02 PERU SICUANI N PPD-30• •PPD-10 1 PP0-56•. 1 PPD-4 OP 03 0P 01 1 PPD-47 f OP 0 4 OP 05 PPD-108• AYAVIRI S •PPD-101OP O 6 / CM-03 OF O / CM-04/CM-02 k OF -08 CM-05 CORACORAPA-04 COTAHUASI ' CAILLOMA PU-04 CS-04 PU-03 d._-f~.0102 -02 OF -09 \CS CS-01 *.~OP 10 ' •CM-O1 PA-06 I PA-03 1 OP II CV-02 PU-01 / PA-05 CS-O( ' ~OP12 CV-04 JULIACA ACo3 f-' CS~ 05 OF 13 CV 08 CS 07 CS-03\ OF14 M• .-CV-06 CS 08 OP-15 AC CS 09 •CS-15 CV-OS-~V-07 CS 10 CV-03 AR-08 1 ACCAC-0102 ~H-o1 ~ PU-05 ; ~ CS CV-01 AR-09 CH-01 ~~L.TITICACA CH AR-10 -02 PU-10/~ '-Y % CS 12 AR-02-01 CS 13 C14-03 PU-09 -AC-05 Q CS AR 02-02 AR O AR 0 ,~/L-O v AR 05 /•TU1 // CH-05 PU'07 J1-01~JULIx-03\-*.~iCH-06 OM ~ • /IA 4AR .• /CTU-0 PU-08 -14 JL-02 AR 0 OUIPA ~/. OM - AR 0 3/ / OM-i6I •MC-02 AR 1 _~ OM-08 I/t CAMANA -12 -os AR 15 AR-13OM-04 OM-10 MAZOCRUZ0M-03 R-17 AR 16 4 A AR ~ OM-II 1 8 i -01•MC-03 AR-I -1 0M-05 Z MC AR-01 OM-06---. I OM-13 MC-05 OM-02r r '~0M-12 ~MC-04./ OM-01/i' . .ETA-12 -MC-06 PACIFIC OCEAN OM-07 .__-TA-II . -MC-07 TA-03 TA-08 .~ MC-08 MOQUEGUA TA-02 TA-09 j MC-09 TA-01 TA-10 BOLIVIA TA-04TA-05 • Rt\C-12~NC-I-10MC-1 1 TA-07 TA-06 \ \ MC-14,'IMC-13I /-20 ` 0 20 40 60 80 1,00k. TACNA CHILE ~1 Fig. 2. Map showing the locality of the sample. Numbers refer to Appendices 1 and 2. of the samples as close as possible, the spatial samples was over 200. The most recent mate characterization of the magma chemistry may be rials apparently form snow-clad high peaks and best achieved. it was impossible to cover them in the limited During the field work, the impression of one time of our expedition. One of the youngest of the authors (S.A.) who has an intimate samples we were able to collect was from a acquaintance with wet and temperate environ block lava flow issuing from Nevado Coropuna ments of Japanese islands but is quite unfamiliar southwestwards (CS-12). This lava may very with the arid environment of Altiplano, was the well be a historic flow. Block lava flows in the difficulty of assessing the age of the volcanic Siquani region (CZ-01 through CZ-03) dated edifices from their state of erosion. For ex by KANEOKA and GUEVARA (1984) as <0.027 ample, inspections of aerial photographs gave us Ma show a very fresh topography and un a very young age estimate (say less than several doubtedly are among the youngest of all the tens of thousand years) of the pyroclastic volcanic products in southern Peru.
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