VIRGIL E. BARNES Bureau of Economic , The University of Texas at Austin, Texas 78712 W A M T ATTPHT TN I ^f" Development Company, Houston, Texas 77001 IRVING FRIEDMAN U.S. Geological Survey, Denver, Colorado 80225 OIVA JOENSUU The Marine Laboratory, University of Miami, Miami, Florida 33149

Macusanite Occurrence, Age, and

Composition, Macusani, Peru

ABSTRACT Macusanite, originally believed to be a type of Andes. These rocks are unique for glassy rocks in tektite because of its sculpture, is shown to be re- that lithium, boron, and arsenic contents are very lated to sillar of the Macusani region, Peru. K-Ar high; cesium, rubidium, tellurium, fluorine, and tin measurements establish identical Pliocene ages (4.2 are higher than normal; zinc, copper, chromium, m.y.) for macusanite and sillar and relate these and zirconium are lower than normal; and high- deposits to the extensive ash flows of the southern alumina such as andalusite are present.

INTRODUCTION geological and petrological data and introduc- tory material, Barnes; age measurements (made Linck (1926) believed the transparent during 1962 and 1963), Edwards and Mc- natural glass from Paucartambo, Peru, was a Laughlin; chemical data, Friedman and tektite, although it contained crystals of Joensuu. andalusite, sillimanite, wollastonite, scapolite, sanidine, oligoclase-andesine, zircon, and GEOLOGY aegirine-augite. Preuss (1935) reported high Macusani is located at approximately long values for lithium, beryllium, boron, arsenic, 70° 27' W., and lat 14° 4' S., on the north- and tin in the Paucartambo glass. Heide (1936) eastern slope of the Andes at an elevation of described the Macusani glass. Martin (1934) about 4300 m in an area devoid of vegetation and Martin and de Sitter-Koomans (1955) except for grass and a few squat . considered these South American glasses to be The generalized geologic map of Peru by of igneous origin. Elliott and Moss (1965) Bellido B., Narvaez L., and Simons (1956) analyzed a specimen of macusanite and found shows the area near Macusani underlain by "overwhelming evidence to support the con- undivided Paleozoic rocks. Carboniferous and tention that this Peruvian glass is unique," yet Permian rocks and pre-Cretaceous intrusive they did not formulate an acceptable theory of rocks crop out to the north, and Upper Cretace- its origin. ous sedimentary rocks are exposed to the south. In order to learn if this glass might furnish The Tertiary volcanic rocks in the vicinity of some clue to the nature of tektites and to learn Macusani are not shown; the nearest ones to what igneous event it might belong, Barnes mapped are about 125 km to the southwest. visited the Macusani occurrence in July 1961. Aerial photographs, which became available He also visited the locality of transparent in February 1966, served as a base for Figure 1. obsidian pebbles near Popayan, Colombia. In conjunction with field notes and panoramic These differ from macusanite in chemical com- photographs made in 1961, these were used to position, absence of high-alumina minerals, and interpret the geology of macusanite occurrence. the general absence of the typical macusanite Sillar flows dipping northeastward are spectac- etched surface. ularly exposed near the power generating Responsibility for various observations and along Macusani north of Macusani (Fig. interpretations in this paper is as follows: 1). The macusanite is localized in lacustrine

Geological Society of America Bulletin, v. 81, p. 1539-1546, 3 figs., May 1970 1539

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OW TERRACE DEPOSITS . BEDROCK Mayo ' I TYPE N°T ! , DETERMINED

Figure 1. Geologic map in vicinity of Macusani, Peru. Gr, granite; SS, sandstone; LS, limestone; all other units labeled on map. deposits about 250 meters thick formed during Caluyo Mayo, and downstream to Macusani an early glaciation and possibly at about the River. Alluvial deposits at Macusani and same time as the older morainal deposits numerous localities, mostly in glacial deposits mapped in Figure 1. along the road to Ayapata, were examined, Barnes, Belon, Zavala M., and Paredcs P., finding just west of the map area two small obtained locality data from Sr. Wenceslao battered pieces of macusanite which may have Malaga y Malaga, who originally brought been carried there. macusamte to the attentior o( scientists. The Chilcuno Chico deposit of macusanite Macusanite was known from two localities, one was not visited because the locality had been 3 km northeast of Macusani near Caluyo Mayo imprecisely described and employees of Sr. and the other 7 km north at Chilcuno Chico Zavala M. who were sent to investigate found River. Near Caluyo Mayo about 25 kg of no macusanite. When the aerial photographs macusanite was collected from the entire became available, they showed that the Chil- length of a gully which heads on a terrace flat cuno Chico macusanite locality is situated in an estimated to be 150 meters above the inter- area of well-exposed sillar flows; therefore, mediate terrace, and from gravel discharged futun attention should be concentrated in that from the gully onto the intermediate terrace. regi(,i>. It seems unlikely that a source of Two specimens of macusanite were found 100 macusanite will be found along the Caluyo meters to the east on the "high flat" (Fig. 1) Mayo because of the presence of younger where the high terrace deposits are about 250 deposits which cover most of the bedrock. meters thick. The sillar flows of the Macusani area are Another gully 0.5 km south of the first one part of extensive ash-flow deposits that mantle yielded about 3 kg of macusanite from gravel the slopes of the Andes and the high plain of discharged onto the intermediate terrace; no Peru, Bolivia, Chile, and Argentina. Macusan- macusanite was found in similar deposits ite occurs not only in the region of Macusani another kilometer to the southeast at the foot but also at Paucartambo 150 km to the north- of the highest terrace. west and at Sandia 110 km to the east. The The high terrace and older morainal deposits three Peruvian localities on the northeastern were investigated eastward from the Caluyo flank of the Andes are aligned parallel to the Mayo macusanite occurrence, northward to trend of the mountains and are in line with

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the northwestward-trending zone of tin other obsidian, as well as between macusanite mineralization in Bolivia. and tektites, is brought out when the refractive index and specific gravity of these materials are PETROLOGY compared with their silica content on a varia- Elemental data for various types com- tion diagram (Fig. 2). piled by Rankama and Sahama (1952) show Some specimens of macusanite are definitely that lithium, rubidium, cesium, and tin are layered or flow banded, as shown by variation most abundant in greisen, and that tellurium in concentration of minerals or variation in is most abundant in shale. Abundance of these degree of milkiness. Milkiness is caused by elements in the macusanite and sillar suggests vesiculation of a type previously observed in that these rocks originated either from late a few specimens of Muong Nong-type tektites, differentiates of a high-alumina magma enriched and in Libyan Desert glass. The irregular thin in these elements with the macusanite having branching bubbles were trapped in glass so formed from the last most highly enriched viscous that surface tension could not draw portion, or that high-alumina rocks previously the bubbles into spheres. Lechatelierite and subjected to greisenization were melted or siliceous schlieren are absent. assimilated to form the macusanite and sillar. The etch pattern on macusanite, which In the latter case the macusanite would have caused this igneous glass, originally mistaken formed from more highly greisenized rock than for tektites, is compared with the etch pattern the sillar. on indochinites in Figure 3, A-D. The etch The refractive index for 24 speciments of pits are superficially identical on the two types macusanite, determined on an Abbe refractom- of glass; however, swirled flow structure(C) eter, ranges from 1.4831 to 1.4862, and the characteristic of tektites is not present in average is 1.4851. Martin and de Sitter- macusanite. The macusanite was photographed Koomans (1955) listed values for refractive using both transmitted and incident light in index of five specimens at 1.4862 and specific order to show its translucence (A and B) and gravities of 119 specimens ranging from 2.345 parallel banding (A) as well as its etch pattern. to 2.361. Elliott and Moss (1965) found that one Tektite C from Snoul, Cambodia, is a perfect analyzed specimen had a refractive index of bi-concave form, except for the portion of one 1.4857 and a specific gravity of 2.359. The edge ground off by the original finder. Tek- marked difference between macusanite and tite D in the collection of the Museum

EXPLANATION o Data prior to I940(see-8ornes, 1940) Data since I940(see-Chao, 1963; Schnetzler and Pinson, 1963; Barnes, I964a) Range of data for Macusanite

z 2.40 -

100 Figure 2 Variation diagram for tektites and igneous glasses (index of refraction and specific gravity).

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Figure 3. Comparison of macusanite and indochinite etch patterns. Magnification, XI.

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TABLE 1. K-AR MEASUREMENTS FOR AGE DETERMINATION OF MACUSANITE AND SILLAR, MACUSANI, PERU Ar4°ml/g Age Macusanite (whole rock) 3.30 5.5 X 10- 4.2 ± 1.5 m.y. Sillar (biotite) 8.03 1.32 X 10~ 4.1 ± 1.0 m.y. 10 1 Xe 0.584 X HT yr- X0 0.472 X 1Q-10 yr~'

National d'Histoire Naturelle, Paris, is from had an earlier history involving association with Tan Hai Island, China. The specimen is an a high-concentration thorium-bearing system. extensively spalled dumb-bell which preserves This is the only case we have observed of a areas of typical etching between spalls. containing a large amount of a radio- genic product unsupported by the parent AGE MEASUREMENTS isotope. Measurements were repeated, and the K-Ar ages of macusanite and sillar are given results agreed within the expected experimental in Table 1. Whole rock was used for the limit. macusanite measurements; biotite was used for The andalusite is euhedral; only a few crystals the sillar determination. Fleischer and Price show rounding which might indicate transport (1964) found an age of 4.3 ±0.4 m.y. for of the andalusite by erosional agents since its macusanite by the fission-tract method, which original formation. On the other hand, we is in agreement with the K-Ar method used cannot envisage a mechanism which would here. The macusanite and sillar ages are about produce andalusite of this composition in place. the same as the 4.24 m.y. age found for the Puripicar Ignimbrite of northern Chile (Rut- CHEMICAL DATA land and others, 1965), and somewhat younger New chemical analyses for macusanite and than the range of 4.7 to 7.6 m.y., found for sillar with chemical analyses by Elliott and ash flows in adjacent areas of Chile by Dingman Moss (1965), Martin and de Sitter-Koomans (1965). (1955), and Linck (1926) are tabulated in Table During the separation of the biotite, it was 3 and show that these rocks are high in alumina. discovered that the sillar contained abundant The chemical analyses of Table 3 are andalusite crystals identical in appearance to generally similar. SiO2 is a little lower, and CaO those in the macusanite. This furnished addi- is a little higher in the sample analyzed by de tional evidence that the macusanite and sillar Sitter-Koomans than in the present analyses. were related. Results of , thorium and A12O3 is considerably higher, and K2O is some- lead measurements on the andalusite (Table 2). what lower in the sample from Paucartambo indicate an age of 6 m.y. The probable error analyzed by Linck. Values for most constituents here is of the same order of magnitude as the for the sillar are within the range of values measured age itself, so all that can be said of found for macusanite; these data indicate that the andalusite is the U238/Pb206 age is less than the two probably have a common source. 12 m. v. Minor and trace element analysis of macu- The amount of radiogenic lead 207 found is sanite, sillar, and obsidian by Joensuu and too low to permit any critical evaluation of the semiquantitative analyses of the chemically U235/Pb207 age. Over 65 percent of the total analyzed macusanite and sillar by ). L. Harris lead found is radiogenic lead 208 that cannot be of the U.S. Geological Survey are given in accounted for as a decay product of the Table 4. Values determined by Elliott and Moss thorium and suggests that the lead incorporated (1965) are also included. B, Ba, Be, Co, Cr, Cu, in the andalusite when it crystallized must have Ni, R.E., V, and Zr were analyzed by Joensuu TABLE 2. URANIUM, THORIUM, AND LEAD MEASUREMENTS FOR ANDALUSITE FROM SILLAR, MACUSANI, PERU Lead isotopic composition u Th Pb (percent) Mg/g Mg/g Mg/g 204 206 207 208 53.1 40.5 7.37 0.955 19.217 14.591 65.237

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TABLE 3. CHEMICAL ANALYSES OF MACUSANITE AND SILLAR, MACUSANI AND PAUCAATAMBO, PERU Macusani Paucartambo Sillar Macusanite Elliott and Martin and Laboratory no. 164363* 164364* Moss (1965) de Sitter-Koomans (1955) Linck (1926) Field Identif. 3142-5 3142-6 Red-brown Green-gray

Si02 71.7 72.8 71.6 70.82 70.67 70.56 A12O3 15.8 16.3 16.7 16.93 16.12 20.54 Fe20, 0.37 0.29 0.95 1.33 B203 0.4 FeO 0.66 0.30 0.6 0.39 0.41 6.96 MgO 0.05 0.00 tr 0.19 0.18 0.11 CaO 0.81 0.16 0.4 1.02 2.07 0.78 Na2O 3.2 4.1 4.7 4.34 3.30 3.42 K2O 5.0 3.7 3.6 4.58 5.53 3.41 Li2O 0.8 H2O- 0.52 0.00 0.23 0.18 H20 + 1.1 0.70 0.2 0.31 0.19 0.83 TiO2 0.15 0.02 0.04 0.00 0.00 . , P205 0.37 0.55 0.4 tr 0.27 MnO 0.04 0.06 0.05 0.08 0.06 CO2 0.08 0.10 F 1.4 Total (less 0 = F) 100. 99. 100.4 99.84 100.31 100.61 * Analyses by Paul Elmore, Sam Botts, and Lowell Artis (U.S. Geological Survey) using methods described in U.S. Geological Survey Bulletin 1144-A.

with an optical spectrograph using a Stallwood or were below the limits of detectability. Of arc. The method is identical to that of O'Neil the remaining 10, the wide differences found and Suhr (1960) and the precision is + 10-20% for B and As may be due to loss by volatiliza- of the amount present. Sn and As also were tion of these elements in the sillar during its determined using an optical spectrograph and eruption and emplacement. Although the B fractional distillation (O'Neil and Suhr, 1960). content varies by a factor of 10 between the The results are no better than +30-40 percent two rocks, both have very high B compared to and show only that both samples have unusually most rocks. high concentrations of Sn and As. Li, Rb, and The macusanite and the sillar vary from one Cs were determined using a Perkin-Elmer 303 another in Li and Sn content by a factor of atomic absorption spectrometer. Using an about three and in Rb and Tl content by a optical spectrograph with infrared sensitive factor of about two, indicating that these plates, the produced lines were much too strong elements lack uniformity in distribution be- for quantitative analysis. Powdered samples tween the two rock types. Wide differences are weighing 100 mg were dissolved using HF and also present for Ba, Cs, Rb, and Sr contents. HQ04. After evaporation the residue was dis- Some of the differences may result from the solved and made to volume of 10 ml with small sample size of the sillar (a few grams left 10 HC1 (v/v). The standards were made from over from the K-Ar analysis). Also, several CsCl, RbCl, and LiaCOs. To both standards sillar flows are present and it is unlikely that as well as samples, 500 ppm of K as KC1 was the one analyzed is the one most closely asso- added in order to buffer the effect of other ciated in time with the macusanite. alkali elements on Cs, Rb, and Li. The analyti- We do not have any idea of the variations of cal methods recommended by the Perkin- the concentrations of elements within a sillar Elmer Company were followed. Precision is flow, even less between different sillar flows. + 3 percent of the amount present. The trace element analyses for both rocks show Of the 49 elements analyzed for in the unusually high concentrations of several rare macusanite and sillar, 39 show good agreement elements, compared to nearly any rock type.

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TABLE 4. MINOR AND TRACE ELEMENT CONTENT OF MACUSANITE AND SILLAR, MACUSANI, PERU, AND OF OBSIDIAN FROM GLASS MOUNTAIN FLOW, MEDICINE LAKK HIGHLANDS, CALIFORNIA (na, not analyzed: nf, not found) Percent Parts per million

An. no. Rock type Na2O K2O Fe MnO MgO F Ag As Bi Cs La Mo Rb Tl Joensuu 4460 Macusanite 4.0 3.8 0.45 0.05 0.05 1.2 0.17 130 1 390 <20 <5 920 18 5496 Macusanite 4.1 3.8 na na na na 0.15 140 na 390 na na 860 17 5497 Macusanite 4.3 3.8 na na na na 0.18 120 na 410 na na 900 19 5498 Sillar 3.3 4.6 0.75 0.03 0.15 1.2 0.13 30 1 56 <20 <5 490 9 4459 Obsidian 4.4 4.8 1.3 0.07 0.05 na na na 1.5 <3 70 10 190 na Parts per million Zn Zr Ba Cr Cu Ga Li Pb Sc Sn Be Nb Sr

Joensuu 4460 Macusanite 80 15 <50 10 4 45 3300 14 <3 125 na nf na nf 5496 Macusanite 90 na na na na 45 3200 12 na 125 na na na na 5497 Macusanite 70 na na na na 45 2700 15 na 125 na na na na 5498 Sillar 80 90 450 10 8 30 1100 35 <3 55 na nf na nf 4459 Obsidian na 560 <50 20 7 na 52 15 4 na na na na na Harris* Macusanite nf nf 20 0 3 30 2000 0 2 300 3000 30 50 0 Sillar nf nf 700 3 3 30 700 30 1 70 300 15 20 70 Elliott and Moss (1965) Macusanite na na a na na 3700 na 200 1200 100 * Elements looked for by Harris but below amounts detectable as listed in the U.S. Geological Survey's table of detectabilities include Ag, As, Au, Bi, Cd, Ce, Co, Ge, Hf, Hg, In, La, Mo, Nit, p^ Pt> Re, sb, Ta, Te, Th, Tl, U, \'t, W, Y^, and Yb. Analyses are semi-quantitative. All results are ±1/3 order of magnitude. t These elements also looked for by Joensuu but not found.

Although some minor and trace elements vary of the macusanite and sillar, the conclusion is in amount between the macusanite and sillar, that they probably originated during the same we conclude from the high concentration of volcanic episode. If this volcanic episode started some rare trace elements in both rocks and from with an explosive phase (sillar), and then, as the the general similarity in composition that the pressure decreased, entered an effusive phase two are from a similar source. Most of the dif- (flows, domes), the sillar would have preceded ference probably resulted from wide variations the macusanite. in composition of the source material from The peculiar chemistry of these volcanic which the rocks formed, as well as from the glasses cannot easily be explained. Assimilation different eruptive history of the two rocks. of aluminous and greisenized material by an originally superheated rhyolite magma might SUMMARY be one explanation. It would be important to All of the macusanite so far described has map the volcanic deposits and examine their been found as pebbles scattered in gravel. The variation in chemistry and mineralogy, in both K-Ar dating shows that the macusanite and space and time, in order to explain the origin sillar of the Macusani area are Pliocene in age of such peculiar volcanic products. and that the sillar probably belongs to the great series of ash flows mantling the slopes ACKNOWLEDGMENTS of the Andes and the high plain of Peru, Chile, Barnes is indebted to Sr. Carlos F. Belon for Bolivia, and Argentina. making the trip to Macusani possible, for Because of similarity in chemical and participating in the field work, and for many mineralogical composition, as well as in age courtesies while in Arequipa. Acknowledg-

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ment is made by Barnes to the National Survey, for obtaining aerial photographs; to Science Foundation under grants KSF G- Dr. Rudolf Martin, Calgary, Alberta, Canada, 10236 and GP-408; to Dr. Gustavo Corso for gifts of South American glasses, including Masias, Rector, Dr. Nestor Lozada, Dean, macusanite, and to Professor M. F. Sheridan, Dr. Romula Cerdena Aguirre, petrographer, Arizona State University, for critically reading and Dr. Alberto Parodi, Chairman, Depart- the manuscript. We were assisted in the field ment of Geology, Faculty of Sciences, Univer- by Jorge Paredes P.; R. M. Gieger prepared sity of San Agustin, and Dr. Luis P. Macedo, the macusanite for examination and measured Arequipa; Sr. Wenceslao Malaga y Malaga its refractive index. Through the cooperation and Sr. Alberto Zavala M., Macusani; and of Sr. Zavala M., collections of macusanite Dr. F. M. Bullard, Department of Geological are now available for study in the U.S. National Sciences, The University of Texas at Austin, Museum and the British Museum of Natural for their cooperation and various aids; to History. Dr. George E. Ericksen, U.S. Geological

REFERENCES CITED Barnes, V. E., 1940, North American tektites: von Macusani in Peru: Naturwiss., v. 24, p. Texas Univ. Publ. 3945, p. 477-582. 281-282. 1964, Variation of petrographic and chemical Linck, G., 1926, Ein neuer knstallfiihrender Tektit characteristics of indochinite tektites within von Paucartambo in Peru: Chem. Erde, v. 2, their strewn-field: Geochim. et Cosmochim. p. 157-174. Acta, v. 28, p. 893-913. Martin, Rudolf, and de Sitter-Koomans, C. M., Bellido B., Eleodoro; Narvaez L., Sigfredo; and 1955, Pseudotectites from Colombia and Peru: Simons, F. S., 1956, Geologic map of Peru, Leidsche Geol. Mededel., v. 20, p. 151-164. scale approximately 1:2,000,000: Geol. Soc. Martin, Wz. R., 1934, Are the "americanites" Peru. tektites?: Leidsche Geol. Mededel., v. 6, p. Chao, E. C. T., 1963, The petrographic and chemi- 123-132. cal characteristics of tektites, p. 51-94 in O'Neil, R. L., and Suhr, N. H., 1960, Determina- O'Keefe, J. A., Editor, Tektites: Univ. Chicago tion of trace elements in lignite ashes: Appl. Press, 228 p. Spcctroscopy, v. 14, p. 45-50. Dingman, R. J., 1965, Pliocene age of the ash-flow Preuss, Ekkehard, 1935, Spektralanalytische Un- deposits of the San Pedro area, Chile: U.S. tersuchung der Tektite: Chem. Erde, v. 9, Geol. Survey Prof. Paper 525-C, p. C63-C67. p. 365-418. Elliott, C. J., and Moss, A. A., 1965, Natural glass Rankama, K. K., and Sahama, T. G., 1952, Geo- from Macusani, Peru: Mineralog. Mag., v. 35, chemistry: Chicago, Chicago Univ. Press, p. 423-424. 912 p. Fleischer, R. L., and Price, P. B., 1964, Fission Rutland, R.W.R., Guest, J. E., and Grasty, R. L., track evidence for the simultaneous origin of 1965, Isotopic ages and Andean uplift: Nature, tektites and other natural glasses: Geochim. et v. 208, no. 5011, p. 677-678. Cosmochim. Acta, v. 28, p. 755-760. Schnetzler, C. C., and Pinson, W. H., Jr., 1963, George, W. O., 1924, The relation of the physical The chemical composition of tektites, p. 95- properties of natural glasses to their chemical 129 in O'Keefe, J. A., Editor, Tektites: Chica- composition: Jour. Geology, v. 32, p. 353-372. go, Chicago Univ. Press, 228 p. Heide, Fritz, 1936, Neue kristallfiihrende Glaser

MANUSCRIPT RECEIVED BY THE SOCIETY MAY 21, 1969 REVISED MANUSCRIPT RECEIVED JULY 30, 1969

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