Petrogenesis of a Neogene Shoshonite Suite, Cerro

Canadian Mineralogist Yol.24, pp. 117-135(1986)

PETROGENESISOF A NEOGENESHOSHONITE SUITE, CERRO MOROMORONI, PUNO, SOUTHEASTERNPERU

DANIEL J. KONTAK', ALAN H. CLARK, ED FARRARAND THOMAS H. PEARCE Department of Geological Sciences,Queen's University, Kingston, Ontario K7L 3N6

DAVID F. STRONG Departmentof Earth Sciences,Memorial University,St. John's, NewfoundlandAIB 3Xs

HALFDAN BAADSGAARD Departmentof Geologl, Universityof Alberta, Edmonton,Alberto T6G2E3

' ABsrRAcr SOMMAIRE

The first occurrenceof mid-Tertiary basic to intermedi- Pour la premibrefois, on vient de ddcouvrirla pr6sence ate volcanismin the southeasternPeruvian sector of the de volcanismetertiaire moyen dansle secteurSud-Est (p6ru- Inner Arc domain of the Central Andean orogen is vien)du domainede I'arc int6rieurde l'orogbnedes Andes documented.The volcanic rocks are of shoshoniticcharac- centrales.Les rochesvolcaniques y pr6sententun caractbre ter and crop out near the village of Antauta (Lat. shoshonitique et affleurent prbs du village d'Antauta (at. 14"17'L7'5,Long. 70'18'l4'W). Thesuite has been dated l4ol7' l7os,long,7O"l8'44' W). La s6rie6tudi6 a 6tddatde (K-Ar) at 23.7 t0.6 Ma, coevalwitha24.9 t 0.5 Ma old K-Ar) A 23.7 t 0.6 Ma, contemporained'un dyke monzo- peraluminousmonzogranite dyke, in the samearea, which granitique hyperalumineux,4gd de 24.9 t O.5 Ma, situe containsxenoliths of the shoshoniticrocks. The shosho- dansla m6merfuion et qui contient desxdnolithes de roches nitic suite consists of olivine (Fosg-s, absarokites and shoshonitiques.La s6riede sp6cimens6tudi6s consiste en orthopyroxene @nze-sg)shoshonites proper, which are absarokites i olivine (Feso-adet shoshonitesproprement characterizedby severalmineralogical features: (i) double- dites d orthopyroxbne @n7s-e6),qui possddent plusieurs rim compositional profiles in the olivine and orthopyrox- caractdristiquesmin6ralogiques: (i) les phdnocristauxd'oli- ene phenocrysts; (ii) aluminous orthopyroxene (up to 5 vine et d'orthopyroxlne montrent une double bordure darts wt.Vo Al2O); (iii) sieve-texturedplagioclase phenocrysts leur profil de composition; (ii) la teneur en Al2O3 de in the shoshonitesproper; (iv) quartz megacrystswith a l'orthopyroxbne alunineux peut aller jusqu'd 590 en poids; clinopyroxenecorona. The shoshonitesuite is subalkaline, (iii) les shoshonitesproprement dites contiennent desph6- with high K2O contentsand K2O:Na2Oratios of 0.83 to nocristauxde plagioclased texture d'€cumoir; (i9 desm6ga- 1.49.In addition, a bimodal distribution of silicais appar- cristaux de quartz montrent une couronnede clinopyroxdne. ent, with contentsof 54 and 58-59wt.9o SiO2for, respec- Les sp€cimensde la s6rieshoshonitique sont subalcalins, tively, tle absarokitesand shoshonitc; the rocks haveabun- avec forte teneur en K2O et rapport K2O:Na2Ovariant de dant R.EE (156 6 n2 ppm), with strongly fractionated 0.83 e 1.49. On note aussiune distribution bimodale de patterns [(LalYb)p 6 to 57]. Rb-Sr isotopic analyses la silice,dont la teneuratteirt 54Voen poids dansles absa- define a pseudoisochronfor the suite, with an apparent age rokites et 58-5990dans les shoshonites.Les rochescon- of ca. 209Ma andan initial strontiumratio of 0.7061.The tiennent des 7R en abondance(156 d 472 ppm), d tendance shoshonitic suite is thought to have originated by partial fortement fractionn6e [(LalYb)N de 6 i 571. Les analyses melting of a REE- and LlLE-eniched garnet peridotite isotopiquesRb-Sr d6finissentun pseudo-isochronepour soruce, as outlined by Dostal et al. (1977a),which resulted la sdrie d'6chantillons, auxquels on assigpeun ige appa- from the deep penetration of an easterly dipping rent de 209 Ma et un rapport de strontium initial de 0.7061. subduction-zone. It is suggestedthat the melts underwent La suite shoshonitiqueaurait pris naissancepar fusion par- a complexhistory involving an early period of hieh-pressure tielle d'un protolithe de peridotite i grenat enrichi en ter- crystallization followed by mixing of the magma with a fel- res rares et en dl€mentslithophiles d large rayon (Dostal sic liquid. The latter liquid, or contaminant, was generated et al. L977a),r€sultatde lap€ndtration profonde d'une zone by paxtial melting of the crust, as a consequenceof the basic de subductioni pendagevers I'Est. Cesmagmas auraient magmatism, and is represented by the nearby granitic ensuitesubi une dvolution complexe:6pisode de cristalli- pluton. sation precoce i haute pression et melange avec un bain fondu relativement felsique. Ce magmaplus siliceux aurait Keywords:shoshonites, southeastern Peru, Neogeneage, une origine par anatexie de roches crustales due au mag- aluminousorthopyroxene, double-rim zoning, magma matisme basique,et serait repr6sentdpar les rochesdu plu- mixing, ton granitique voisin. (Traduit par la R6daction) lPresentaddress: Nova ScotiaDepartment of Mines and Mots-cl6s:shoshonites, Sud-Est du Pdrou, dge n6ogbne, Energy,P.O. Box 1087,1496 Lower Water Street,Halifax, orthop).roxdnealumineux, zonation d double bordure, Nova Scotia B3J 2Xl mdlange de magmas. tt7

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 118 THE CANADIAN MINERALOGIST

INTnooucrtoN of Rb-Sr dating and field relationships (Kontak el al. 1983,1984a),but a recentK-Ar whole-rockage In recentyears, the potassicsubalkaline to moder- determinationof a sampleof absarokiteindicates ately alkaline volcanic and intrusive rocks assigned that the suite is of Neogeneage. Herein, we docu- to the shoshoniteclan by Joplin (1965, I 968;but for ment for the first time the occlurenceof rnid-Tertiary discussion,see Nicholls & Carmichael 1969)have rocks of basic to intermediatecomposition within assumedsignificance in tectonic reconstructionsof the Inner Arc (Clark et al. 1983a)setting of the Cen- ancient terranes (e.9., Brooks et al. 1982). Several tral Andean orogen, temporally and spatially related author$ (Lefdwe 1973,Dostal et al. 1977a,1977b, to ca.25-Ma-old biotite - cordierite t sillimanite Ddruelle 1982) have documentedthe widespread monzogranitesthat host important Sn-Cu-Pb-Zn- occurrenceof Neogene shoshonitic rocks in the Ag mineralization (Clark et al. 1983b). easternpart of the ensialicCentral Andean orogen (Main Arc of Clark et al. 1983a, 1984, and see REGIoNALSsrrtxc, Ace ANo LocAL below), prompting the attribution of Andean charac- Grolocv oF THE SHoSHoNITES teristics to, for example,portions of Archean green- stonebelts (Brooks et al. 1982).However, shosho- On a regional scale,the shoshoniticsuite is located nitic rocks are also representedin continental settings in the Inner Arc systemof the Central An..deanoro- only tenuously related to plate-boundary orogeny gen, cospatially with the Cordillera Oriental of Peru (Gest& McBirney 1979,Boccaletti et ql. 1978, Clark and Bolivia @g. 1). Within this domain in 1977,Joplin 1968),and in mature island-arcs(Gill soritheasternPeru, magmatism is characterizedby 1970,MacKenzie & Chappell 1972,Jaked & White an episodic and mixed mantle- and crust-derived 1969, Keller 1974). Thus, the plate-tectonic sig- nature, and contrasts with the quasicontinuous, nificance, as well as the petrogenesis,of the mantle-dominatedand subducrion-zone-relatedmag- shoshoniteclan remains obscure. matism of the Main Arc (Clark et al. 1983a,Kon- The shoshoniticsuite describedherein was first tak et al. 1984a).The geologicalevolution of this recognizedduring the courseof a regional reconnais- region of southeasternPeru, the Cordillera de sancesrudy concernedwith the magmatic and metal- Carabaya, is considered to reflect its regional tec- logeneticevolution of the Cordillera de Carabaya tonic setting between two major terranes, the rela- region of southeasternPeru @ig. 1; Kontak 1985). tively stable Brazilian craton to the east and the The volcanic rocks were originally assignedto the mobile orogento the west (Kontak 1985).Thus the Permian Mitu Group (Newell et al. 1953)on the basis significance of most magmatic or tectonic events

660

@ Active Arc Cordillcta Occideatal 140 I a Macusanl Altiplano ^ E Orieuial u\|| [ Cordillera CENRO MOROMONONT

Anlaula

Maia Arc luaer Arc

Frc. l. Location of tie study area in thLeCordillera Oriental, southeastertrPeru. The shoshonite suite underlies Cerro Moromoroni near the village of Antauta.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUITE. PERU 119

within this region is to be sought in the gross relationships between these two terranes. The geology of the Cordillera de Carabayaregion is describedelsewhere (Laubacher 1978, Kontak 1985),but a summaryis warrantedhere. The region is underlainby a thick (ca. 10-15km) successionof pelitic and psammitic rocks of Early paleozoic (Ordovician - Late Devonian) age, overlain uncon- folmably by ca, 3 km of Upper Paleozoic (Car- boniferous - mid-Permian) psammites and car- bonates, and less than 3 km of Upper Permian molassoid sedimentsand alkaline volcanic rocks. Igneousrocks of both mantle and crustal derivation have been episodically emplaced into this strati- graphicsuccession, at ca. 350,2Q-230, I 85, 90-70, and 26-8 Ma. Of relevance to this study is the presenceof numerous small (ess than2knf) 26- to 20-Ma-old monzogranitic stockscharacterized by the presenceof coarsealkali feldspar megacrysts,cordierite and sillimanite. These peraluminous stocks outcrop in the vicinity of the shoshoniticsuite. The shoshoniticvolcanic rocks make up a small (4.5 kmz), eroded volcanic edifice underlying Cerro Moromoroni, near the village of Antauta (Lat. 14"17'l7uS,Long. 70o18'44'W),in northernPuno Department(Fie. l). The volcanic rocks appeartb conformably overlie an Upper Paleozoic sedimen- tary sequenceconsisting, from baseto top, of quartz Frc.2. Inclusion of absarokite (dark) in granite (light). sandstones,calcareous sandstones and (fossiliferous) Note the presence of quartz (Q) megacrysts in the (determircd limestones.The contactbetween the limeslenssr.4 absarolite rock and alsothe sanidinecrystal X-ray growing the volcanicrocks is gradational, by diffraction; composition Or7) into the limestonesbeing the xenolith. intercalated with the earlier eruptive units (sills ?). However, the greater part of the volcanic pile is free of interbeddedlimestones. Cutting the volcanic rocks IAro I. AAAI?IICAI, BSNTS TM UATERW DABD BY CONVENIIOtrALT.AR is a small granitic stock (150 x 50 m) containing s6ElllQUB coarse 404r{ alkali feldspar megacrysts and xenoliths Uat€rlel Z( r"a1 z4o* App.enr Age (< 5-6 cm where observed)of the volcanic rocks (Fie. 2). cocA-l7 br.orita 7.396 0.719 x fO-J 35.1 24.9 + O.5 C@A-22 vhola-rocl 1.675 0.155 x r0-' 2l.r 2r.t;0.6 K-Ar dates have been determined (Table l) for a whole-rock sampleof the volcanic rocks and for a ha].yses by D.J.K. at queers Udeeral.ty (d6El1e 1s kstak 1985). ?ota6a1a deterdned by A.A.S. techalq@i realLta tepreaat tbe ager sample of biotite from the granitic intrusive body. age of dupllcare analyaea. Error la eatltuted to b€ 10.72 (2o). The The volcanic rock dated is ag€ rere calcuLated @bg the coBatatrts auggeated by Stelger ! Jiiger an absarokitethat con- (1977); errors reptesdt the @lytlcal precigioo at 2o. tains 20-3090glass (see below) and showsno altera- tion. Thus, posteruptivemodification of the K-Ar systemwas minimal, and the dateof 23,7 a 0.6 Ma (1968)attempt at a chemicalclassification has been is consideredto be that of eruption. This date is simi- consistentlyfollowed. Instead,most recentauthors lar to that of 24.9 t 0.5 Ma obtained for biotite from have resorted to an alternative schemebased on the the granitic body, indicatingvery closetemporal, in covariation of potassium and silica (MacKenzie & addition to spatial, relationshipsfor the volcanic and Chappell 1972, Peccerillo & Taylor 1976). The intrusive rocks. shoshonitic volcanic rocks of the study area are petrographically and chemically unusual compared ClassrrrcauoN AND PstnocnapHy oF THE to other shoshonitic suites and consequentlydo not SHoSHONITES strictly conform to the schemesof nomenclature adopted by others. However, we have chosento clas- There is at presentno universally adopted nomen- sify the volcanic rocks by using, as much as possi- clature for the classification of shoshonitic suites. ble, the mineralogicalscheme of Iddings (1895)and Neither the original absarokite - shoshonite - Nicholls & Carmichael (1969) for the type-locality banakitelerminology of Iddings (1895)nor Joplin's rocks from Wyoming. Those rocks containing oli-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 120 THE CANADIAN MINERALOGIST

Frc.3. Photomicrographsof absarokites(plane-polarized light): (a) olivine phenocrystwith serpentineinfilling fractures; (b) matrix containing plagioclase laths (white), unaltered elass (dark), and pyroxene and olivine (high-relief phases).

vine asthe dominant, or sole,phenocrystic phase in bonate matrix. Orthopyroxene (Fig.' 4a), the a two-feldspar - olivine - clinopyroxene- opaque dominant phenocrystphase, is subhedralto euhedral, phaset glassmatrix are termed absarokite.Those equant to tabular or, rarely, acicular. The crystals in which orthopyroxene t plagioclase are the (0.5-4 mm) are inclusion-freeand very fresh, and dominant phenocrysticphases in a two-feldspar - morphologies(e.g., skeletal)suggestive of rapid crys- orthopyroxene - opaque phase t glass matrix are tallization are not uncommon. Many grains are termed shoshonite.More highly evolved representa- brownish, either in part (i.e., the core or rim) or tives of the series(i.e., banakite) have not been entirely, and thesegrains (Fig. 4b) show anomalous observed.Our classification,therefore, is somewhat double-rim profiles of compositional zonation (Kon- similar to that used by Hoee 0972) for the tak et al. 1984b).The rare phenocrystsof olivine shoshonitesof west-centralUtah. (0.5-l mm) are altered to serpentine-carbonate The absarokitesdisplay olivine t plagioclaseand assemblages,but a few relict coresin somegrains, orthopyroxenephenocrysts, generally constituting and the well-preservedmorphology of others, per- less than 5Vo of the mode, in a fine-grained mit identification. Plagioclasephenocrysts (0.5-2 clinopyroxene- olivine - orthopyroxene- plagioclase mm, rarely up to 5-6 mm) are normally zoned, - opaque phase t glass matrix. The olivine euhedral, tabular crystals of Anor-rocomposition. phenocrysts(Fig. 3a) are subhedralin form and show They are generally larger and more abundant than a variety of morphologies,probably a reflection of those in the absarokites;the presenceof sieve-like varying cooling history. The olivine grains are widely textures(Fig. 4c) suggestsaperiod ofdisequilibrium embayedand fractured, and displayincipient alter- @ichelberger1978, Sakuyam a l% 8). Thesecorroded ation to serpentineand carbonate.The grains are zones are enclosed by a later gxowth of clear compositionallyzoned toward their margin, as indi- plagioclase,generating euhedral grain-outlines. The cated by the variation in birefringence, and most are matrix (Fie. 4d) is dominated by orthopyroxene and free of inclusions. The plagioclasemicrophenocrysts plagioclase,with lesseramounts of an opaquephase, (0.05-l mm) are euhedral, tabular crystalsgenerally but in somesamples brownish glassis the most abun- showingnormal zoning (Anao-so),and occur as sin- dant phase.The presenceofK-feldspar hasbeen con- gle, inclusion-free grains, Orthopyroxene firmed by qualitativeenergy-dispersion microprobe microphenocrysts(< 0.05 mm) also occur, but are analysis. exceedinglyrare. The matrix (Fig. 3b), containing Quartz megacrystsalso occur in the absarokites up to 3090 brownish glass, has a well-developed and shoshonitesin addition to the above phenocrys- pilotaxitic texture, and is remarkably fresh. Qualita- tic phases(Fig. 5). This mineral is presentin three- tive energy-dispersionmicroprobe analysisconfirms quartersof the samplesstudied, but is far more abun- the presenceof discretegrains ofK-feldspar, and also dant in the shoshonites,in somecases constituting showsthat the glassis dominatedby K, Si and Al, the predominantmegascopic phase. The inclusion- with lesseramounts of Na, Fe and Ti, and is nota- free quartz grains are generally coarser (l-4 mm) bly depletedin Ca and Mg. than the mafic silicate phenocrysts, and are The shoshoniles consist of orthopyroxene, predominantly anhedral, although rare euhedral plagioclaseand olivine phenocrystsin an orthopyrox- crystalsindicate thal a more idiomorphic assemblage ene - plagioclase- opaquephase + glassand car- once existed. The grains are characterizedby exten-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 o bo =

x *ia o o g

N €'ei 53 b0E 'Ettro.

-d 6e 3O 99 xoO>l 9o Ff

(= -c)>l Ebn QA YF bE o,h o;3

ov9E \= 9'! Eo

aiI (€O vo ^U =-^b0 0. €.i" .F.E J!otri t{5 a slx JH EE eC 9^ 9.- 9!o ;E 4O q-o

o6t as9d! 0-d

xo Y= €.0 j'E

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 ln, THE CANADIAN MINERALOGIST

&$wffigHgwK 6YQt^ ,l g .:. q\ 5l

Ftc.5, Quartz megacrystsin the shoshonites:(a) traces of quartz grains with clinopyroxene coronas (stippled pattern); @) photomicrograph of quartz megacryst surrounded by clinopyroxene.

XAAIA 2. CE@SEMISTRY OT trEOCEtrESAO9BONITES AND EBANIIOID B,OG, sive internal fracturing (Fig. 5a), a feature lacking in the mafic phenocrysticphases, thereby preclud- 39A 40 4l tl ing abrasionas a possiblecause, and by mantlesof clinopyroxene 5b), a disequilibrium feature 3ro. 54.3053.91 59.5859.25 59.5959.47 58.58 59.4358.64 68.96 @ig. T102 0.93 0.95 0.62 0.64 0.A4 0.64 0.63 0.75 0.75 0,30 sharedby quartz megacrystsin andesites(Gill 1980, A1203 16.2E16.21 r6.7E 16.6016.50 l6.yl 16.4916.08 16.2415.35 F€203 L.lO 3.47 r.74 3.72 3.96 2.31 l.zX 2.65 4.18 0.65 Sato 1975)and alkali basalts(Strong 1972). Feo 6,32 5.00 2.62 r.45 1.24 2.* 1.97 2.48 2.31 1.03 l{r0 0.13 0.12 0.07 0.07 0.06 0.07 0.09 0.07 0.09 0.04 qso 6.43 6.50 4.79 5.23 4.47 4.7a 5.69 4.74 5.2O L.2o' WHoLE-RocK GEocHEMISTRY c€o 6.45 6.XX 4.81 4.40 4.30 4.68 5.O4 4.23 4..40 r.65 Na20 2.69 2.57 2.54 2.47 2.54 2.95 2.72 t.35 1.42 1.67 R20 2.24 2.23 3.79 X.lA 3.85 3.65 3.41 3.55 3.O7 4.25 P2O5 0.23 0.2t O.25 O.Z3 0.25 O.25 0.33 0.28 0.21 0.21 Chemical analysesof the shoshonitic rocks (Table 2) reveal an unusual composition for the suite, including, for example, a very low CaO,/MgO ratio (ppm) trb14ro7Lol0132l139E (1.0-0.84)for rocks of this silica content [cf,, com- Ba 663 720 878 765 807 823 8E5 920 625 701 1 124 132 89 92 93 94 97 96 103 23 positionsof shoshonitesin Joplin (1965,1968) and Ci 292 236 232 253 234 286 204 Morrison (1980)1,high alkali contents, and elevated f,b t25 ul 259 247 252 rL6 254 219 2t0 X1a sr 401 380 355 329 YL 172 *2 370 262 106 K2OlNa2O ratios (0.83-1.47). These features are zt 129 133 165 l5E 162 160 160 166 150 90 1.2t24t617-21- expressedmodally by the presenceof olivine and I{t 40 36 57 61 6. 65 - 24 13

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUTTE.PERU 123

+ 800 Ba %:s+ % KzO 8? 400 2 oo + 2000 oo v s 3 1000 sql 7" NarO oo .€ 2 350 6 Rb *o 250 7oMgO 4 3€ 150 2 ooo I % 300 O9 7o(FeO + o Cr Fe2Oa) 4 oQ 200 c + Sr 400 & ee @ 200 6 ct 8,0 oo 7o CaO 4 Ni ;3 o o + 60 + Pb oC o9 7o Al2Og17 2 oo s8 Zr 150 + 10 o f + 55 60 70 55 60 65 70

o/o Si02 7o SiO,

Ftc.6. Major and trace elementvariation diagramsof the Cerro Moromoroni shoshonites(open circles)and granitic dyke (cross).

respectively(Fig. 6). We note that this is not a func- broadly similar trends shown by low-K calcalkaline tion of sampling since rocks were collected from suites,it is apparentthat most shoshoniticgroups several different localities. show an abrupt rapid increasein KrO with increas- The shoshonitesplot in the subalkalinefield in the ing SiO, at low SiOr contents, and a decreasein the (NarO + K2O) versusSiO2 and normative Ol,-Ne, slope at higher SiO, values.The Peruvian data cut -Q' classificationdiagrams (Figs. 7a, c) of Irvine & acrossthis trend and are generallydisplaced to higher Baragar(1971), and in the latter are shown to cor- silica values. respondto the field defined for shoshoniticsuites Comparison of the trace-elementdata with the in general(Kontak 1985).The shoshoniticrocks are values compiled by Jaked & White (1972) for markedly Fe-depletedin terms of the AFM plot @ig. shoshoniticsuites indicates that the Peruvian sam- 7b), a universal feature of shoshonites(Jakei & pleshave similar Ba and Zr contents,but are enriched White 1972),but onethat remainsunexplained. The in Rb, Ni and Cr and depletedin Sr and V, and have suite is distinguished from the tholeiitic rocks using lower K./Rb ratio. Thesefeatures are consistentwith both Miyashiro's (1974)FeO,rMgO versasSiO, plot analysesof other Central Andean, Neogeneshosho- @ig. 7d), and his Ti, Cr, V and Ni plots (Miyashfuo nitic suites with comparable contents of silica & Shido 1975),in addition ro rhe AFM diagram of @6ruelle 1978,1982, Dostal el al. 1977b,Dupuy & Irvine & Baragar (1971)(Fig. 7b). Lefbwe 1974).For the shoshoniticsuite under study, The Kp versusSiOrtrend for the Peruvian suite we also note that: (i) Ba and Pb increasefrom the is comparedto that of other shoshoniticsuites in absarokitesto the shoshonites,(ii) there are only Figure 8. Although there is a wide spectrumof trends slight decreasesin the abundanceof V, Cr and Sr for such rocks, contrastingv/ith, for example,the with increasing silica, and (iii) there is some scatter

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 t24 THE CANADIAN MINERALOGIST A. B.

6N Y alkaline + o& o o N oo CU subalkaline r":"> = calcalkaline uu 60 si02 D. ro ol'

N calcalkaline 9oo tholeiitic subalk a

inites

oshonites 50 Ne' Ab Q' FeO/MgO

Frc.7. Chemicalclassification diagrams for the Cerro Moromoroni shoshonites.Dividing lines in a, b and c are from Irvine & Baragar (1971),and in d from Miyashiro (1974).Fields for shoshoniticand boninitic suitesfrom Kontak (1985).

for Ni, Cr and Rb in the shoshonites(Fig. 6). Many as Dostal et al. (1977a)similarly noted for the Cen- of thesefeatures are not consistentwith the "trends" tral Andean, Neogeneshoshonites that they exa- shown by the major elementsor with fractionation mined. The latter authors have shown with model of the phenocrysticphases present. calculationsthat the shoshonitescould be derived "Ihe REE data (Table 3, Fig. 9) are characterized from a garnet peridotite ( < 590 partial melting) with by enrichment of the IR.EE and a strongly fractio- a flat chondrite-normalizedREE pattern, but with nated pattern for the HREE [(LalYb)* values of 6 2-5 times higher absoluteREE contents.We also to 571.The REE abundancesand patternsfor these note that the decreaseof total R.EE content with rocks contrast markedly with those of low- to increasing silica precludes derivation of the medium-K calcalkalinesuites (Jakei &While 1972, shoshonitesproper from the associatedabsarokites Gill 1980);also, the Peruvian rocks are enrichedrela- by simple fractionation involving olivine, plagioclase tive to most shoshoniticsuites [see Kontak (1985)for and pyroxene, becausethe Kos for the REE in these a reviewl, although exceptionsoccur (e.g., Dostal minerals are all extremelylow (Arth 1976).Thus' et ol. 1977a,Pe-Piper 1980).The REE abundances another mechanismis required. and patterns are instead more similar to those Rb-Sr isotopedata for the shoshonites(Iable 4, presentedby Kay & Gast (1973)for alkali basalts, Fig. 10)define a pseudoisochronage of ca. 209Ma

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUITE. PERU 125

Shoshonite trend 6 of Joplin(1968)

5 .7

4 o/"K2O 3 1 2 2-7

Neogene Shoshonites

50 55 60 65 "/" SlO2

FIG.8. Plot of K2O versusSiOr; comparingthe Cerro Moromoroni shoshonitetrend to that of Joplin (1968),and to the trends of shoshonitesuites from other regions: I Absaroka Mountains, Wyoming, 2 Papua New Guinea, 3 Oxford Lake, Manitoba, 4 Viti Levu, Fiji, 5 Lesbos,Greece, 6 and 12 Srednogorie,Bulgaria, 7 Aeolian Arc, Tyrrhenian Sea'8 SienaNevada, California, 9 west-centralUtah, 10Shebandowan, Ontario, I I Puerto Rico, 13Lakmon Moun- tains, Greece,14 and 16 Central Andes, 15 southernPeru, Main Arc. Sourcesof data are found in Kontak (1985).

with a Sq of 0.70613+ 0.M0A. However,a projec- Albee & Ray (1970).A more detailedaccount of the tion of the array definedby the shoshonitesextends procedures, including precision and accuracy as toward the field outlinedby the Tertiary @a.25Ma) determinedfrom analyzingstandards, is presented peraluminousmonzogranites, which are petrographi- elsewhere(Kontak 1985). cally and chemically similar to the granitic dyke cut- Olivine, analyzed in two of the absarokites ting the shoshoniticvolcanic suite. The implication (COCA-2Z and -35: Table 5, Fig. ll), is markedly ''primary" of thesedata is that contaminationof the zoned,with core compositionsfalling in the narrow shoshonitic liquid has occurred; however, the Srl rangeFose.sa [except for one analysis@orr), which valueis still considereda good approximationof the may representan off-centresection @earce 1984a)1, original initial ratio of the melt prior to contamina- and a rim rangingfrom Fo61to Fo*. This composition. The valueof ca.0.7061is well abovethe range tional spectrumis comparableto that of olivine in of oceanicbasalts (Peterman& Hedge l97l) and other shoshoniticsuites (D6ruelle 1982, Hogg 1972, more "primitive" valuesobtained for andesiticrocks Nicholls & Carmichael1969); however, the within- from the southern volcanic zone of the Andes suite variation observedhere is unusually large. Part (Ddruelle et al. 1983),but lies well within the range of this is attributedto posteruptiveinterdiffusion of for alkaline rocks in general@owell & Bell 1974), Fd+ and Mg betweenthe olivine phenocrystsand and is similar to that reportedfor other shoshonites the groundmass,as demonstratedby Moore & Evans in, for example,the Aeolian Arc (Cortini l98l) and (1967)for olivine basaltsfrom Hawaii. the Central Andes (McNutt et al. 1975). One of the larger phenocrystsof olivine from an absarokitewas studied in detail; its compositional MnBnar- Cttsr\lrsrny profile, as determined from electron-microprobe analysesand laserinterferometry @earce 1984b), is Mineral compositionswere determinedwith an shown in Figure 12. The profile shows that the A.R.L. SEMQ electron microprobe, using phenocrysthas a doublerim.in contrastto the sim- wavelength-dispersionanalysis and the following ple zoning profiles generally observedin olivine operating conditions: acceleratingvoltage 15 kV, (Kontak et al. 1984b, Maalde & Hansen 1982, beamcurrent 0.1 pA and beamwidth 5 pm. The raw Kooten& Buseck1978). data were correctedfor matrix effects by the method Orthopyroxene, as a phenocrystic and matrix of Bence& Albee (1968)using the alpha factors of phase,was analyzedin severalof the shoshonites.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 t26 THE CANADIAN MINERALOGIST

Representativecompositions are given in Table 6' than the matrix phenocrysts for a given and the data arepresented in Figures13 and 14.The Mg/(Mg + F*+) ratio, and there appearsto be,a compositionsare thoseof bronzite: the phenocrysts positive correlation betweenincreasing Al and Fd" range from Ene6to Enrs, and the matrix pyroxene, content(Fig. 13).These observations, coupled with IvAl, from Ens2to En6e.The most notable feature of the the relationship betweenSi and suggestthat phenocrystcompositions is their high Al content,in Al substitutesin the orthopyroxenestructure accord- IvAl vIAl: some casesattaining 5 wt.9o AlrO3. There is a ing to the scheme + Si + Mg, as would strong negative correlation between Al2O3 be favored at high pressureowing to the volume (expressedas rvAl) and silica content (Fie. l3). The decreaseand density increase accompanying this phenocrystsare also generallyricher in aluminum exchange(Skinner & Boyd 1964,Boyd & England 1964).Finally, there is a positive correlation between Ca and Fe (Fig. l4), the matrix pyroxene being we TABII 3. Rf,X !A1A FOR NEOCnfE VO'.CSIIIC RO{KS lND ER.ANI]5 enrichedin both cations. In the samediagram alsonote the apparentevidence of both normal and reversezoning in the phenocrysts,the core and rim La 102.8 27.32 24.67 tr3.l 59.7 26.0 compositionsshowing oppositetrends. & 208.7 73.79 60.99 226.7 123.6 75.9 Pr r6.84 22.25 9.23 4.67 21.4 t4.2 7.0 An orthopyroxene phenocryst has also been t{d 89.70 37.O1 35.36 78.0 50.4 26.0 So I 1.84 t5.92 7.00 7.47 13.0 9.r 5.6 studiedin detail using electron-microprobeand laser- Eu 1.78 t.73 0.22 1.71 1.90 0.9 0.9 techniques,the results of which have cd 9. t2.78 5.81 6.28 6.3 5.7 3.2 interferometry Dy 7.40 10.87 6.71 6.8E 4,9 4.3 2.6 been presentedelsewhere (Kontak et al. 1984b).As Br 2.60 3.97 3.04 4.28 2.6 1.8 1.4 Yb 3.49 l.l8 2.54 1.3 1.1 0.4 wilh the olivine profile discussedabove, double-rim zoning is developed,albeit in far greater detail than the olivine, owing possibly to the slower rates of (Lslyb)N 22.4 r9.4 15.3 6.3 56.9 35.E 41.3 diffusion in this mineral (tluebner & Nord 1979'Wil-

RlE al@ats (ppB) deteroired at Ue@tlal gD.Lveialty bt tha son 1982)than in olivine (Buening& Buseck1973). ttlttr-flh x-ray f,luoroscea@ @!hod of Fryer (1977). heclaloo le of this phenocrystshowed a estl@ted at + 0.2 ppE or l0Z, rhl'cbaver la the grstei. The chemicalanalyses compositionalspectrum from Enr5.,to E&e.6and IlEEples sna.Lyzeal aae, ab€aroLitee (22t35) ; Bhoshoattee (33,40,39A,38); add grsnlte (17). Al contentsranging from 2.18 to 8.89 wt.7o Al2O3 (mean4.5). The available plqgioclose data indicate that the absarokitescontain microphenocrystsand microlites Absarokites 22 r 35o of Anrs-e. composition, consistent with optical determinations.In contrast,the shoshonitesappear snosnonites to contain two populations of plagioclase,one of !! | the other, AnrrAbrrOraa. 394tr compositionAnro and 40^ DISCUSSION Granit€ 17 a lu F E Any model proposedfor the petrogenesisof this z shoshonitic suite must explain the salient featuresof T the petrographyand whole-rock and mindral chemistry describedabove. These include: (1) the presence :< of quartz megacrysts,(ii) the occurrenceof a sieve

t texture in all of the plagioclase"phenocrysts" in the shoshonitesproper, (iii) the compositional gap between the absarbkites (540/o SiO2) and shoshonites(59V0 SiO) and the trace-elementpat- terns (anomalouslyhigh Rb contents,increasing Ba and Pb and only slightly decreasingSr, V and Cr abundanceswith increasingsilica), (iv) the REE patterns and abundances, (v) Rb-Sr data, (vi) the double-rim compositionalprofiles for both the olivine and orthopyroxene, (vii) the high Al content of La Cg Pr Nd SmEu Gd Dy Er Yb the orthopyroxene, and (viii) the apparent bimodal population of plagioclase phenocrysts in the tem- Frc.9, Chondrite-normalized[0.8 times Leedeychondrite shoshonites.In addition, the closespatial and values of Masuda et al. (1973)l R.EE patterns for poral associationof felsicmagmatism must be con- shoshonitesand granite. sidered, as well as the regional implications of ca.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUITE. PERU t27

25-Ma-old shoshoniticvolcanism in the Inner Arc that of the megacrysts,(ii) the quartz megacrystsare domain of the Central Andean orogen. morbhotogicatlydissimilar to the well-roundedgrains in the sandstones,and (iii) the megacrystsoccur as Quartz megacrystsand sieve-texturedplagioclase single crystals rather than aggregates,and we consider it improbable that only single grains would be The presenceof quartz megacrystsis an unusual widely added to,the melt from any precursor. feature for shoshonites.Of twenty suitessurveyed 3. Magma mixing. Presumablythis would have from the literature (Kontak 1985),only a few are involved a more felsic melt, as has been suggested reported to contain quartz. In these instances for the origin of andesites@ichelberger 1974, 1978). (Iddings 1895,Hogg 1972,D6ruelle 1982),the quartz The presenceof embayedmorphologies (Sakuyama grains are up to 2-3 mm in size,strongly embayed, 1978)and clinopyroxenecoronas (Eichelberger 1978, and mantled by clinopyroxene.However, none of MacDonald & Katsura 1965) is cited as evidence the authorselaborated on their origin. We consider favoring a xenocrysticorigin for the quartz asa result three possibleorigins for the quartz grains: of magmamixing. The local granitic stock, similar l. High-pressurecrystallization. Although quartz in ageto the shoshoniticvolcanic rocks and contain- has not beenrecognized as a liquidus phasein experi- ing quartz phenocrysts,is consideredto representthe mentson andesiticmelts (Green& fungwood 1968, contaminant. We consider mixing to have occurred Green 1972),it doesform within 40oC of the liqui- while both the end memberswere predominantly in dus (anhydrous) above 30 kbars. Nicholls el a/. the liquid stateprior to eruption ofthe volcanic suite. (1971)have argued on theoreticalgrounds that quartz This origin for the quartz megacrystsis also consi- should occur as a liquidus phasein basaltsand basal- dered to account for their extensiveinternal fractur- tic andesitesat25-27 kbar and 1100"C.The reac- ing. The abrupt changein ternperaturefrom perhaps tion coronas mantling quartz megacrysts and the ca.750"C (rhyoliticliquid) to ca. ll00'C (shosho- invariably embayedmorphology of quafiz grains in nitic liquid) is thought to have resulted in fracturing andesite(e.g., Smith & Carmichael 1968)may there- of the quartz asa consequenceof thermal shock' In fore result from disequilibrium at shallow depths. addition, the presenceqf morphologiessuggestive of This doesnot, however,account for the extensive a previousidiomorphic shapefor the quartz is also internal fracturing characteristic of the quartz accountedfor by magmamixing. The grain sizeof megacrysts. 2. Assimilation of sialic crustal rocks. The absence of xenoliths of granitic composition, or even of TABLE 4. R5-S! DATA IOR SBOSEONITIC VoLCANIC BoCKS quartz-feldspar "glomeroclasts", suggeststhat if the Sasple No. Rb Sr 87a67865a 87917865a quartz was derived from assimilation of crustal (om) (om) (atodlc latlo) (atoldc ratlo) rocks, then the putative contaminant was a sand- coc4.221 39.42 36.44 0.9138 0.70946 stone. Although such rocks outcrop in the immedi- cocA-z2rr 40.34 !7.30 0,9140 0.70941 ate vicinity of the study area, forming part of the cocA-33 32.90 73.59 2.2lrl o.7t282 cocA-39 57.O3 65. 68 I .1386 0.70884 Carboniferous Ambo Group (Laubacher 1978),they are not consideredlikely candidatesbecause: (i) the Atralyses by 8.B., Untverdlty of Alberta; aulytlcal detatls grain sizeof the quartz clastsis generallysmaller than are G d€dcllbed ltr Goff d 41. (1982).

o.720 TertiarY granites L l-o------o-ol U) z (o co \ 0.710 22- v) _/.,-tr39A l.- @

0.700 2.o 4.0 6.0 87p5/86gp

FIc. 10. Rb-Sr isochron plot for the shoshonitic suite (absarokite: COCA-22; shoshonites:COCA-33, 39A), The data have been calculatedto show the isotopic relations at ca. 25 Ma ago. The data for the Tertiary granites are for the nearby San Rafael and Carabaya intrusions (Kontak 1985), of similar age and compositionas the dyke rock.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 t28 THE CANADIAN MINERALOGIST the quartz in the granitic dyke-rock is compatible it is the shoshonitesproper that contain the sieve- with this interpretation. texturedplagioclase and alsohost most of the quartz Sieve-texturedplagioclase is commonly cited as megacrysts, (ii) it has been shown that the evidencefavoring magma mixing (MacDonald & shoshonitescannot be derivedfrom the absarokites Katsura 1965,Eichelberger 1978, Gerlach & Grove by fractionation of the crystalphases present (Kon- 1982),and a similar origin is suggestedfor the tex- tak 1985)and that the compositionaldata do not cor- tures found in the plagioclaseof the shoshonites respondto any cotecticsin the CMAS projections proper. In this particular case,the core of the sieve- of O'Hara (1965, Kontak unpubl. data), (iii) the textured grains of plagioclaseoriginated from the trends of many of the whole-rock data on Harker same felsic liquid that provided the quartz mega- variation diagrams(Frg. 6) project toward the com- crysts.The sievetextures represent the initial state position of the granitic stock (its compositionis given of disequilibrium,whereas the outer euhedralzones in Table 2), which is consideredto representthe con- of the plagioclase represent a new equilibrium taminating liquid, and (iv) many of the anomalous growth. trace-elementfeatures, including the high contents of Rb and Pb, are accountedfor by mixing of the Whole-rock geochemistry shoshoniticliquid with a contaminant enrichedin theseelements. The most notable feature of the whole-rock The REE dxa, in respectofboth abundancesand chemistry is the compositonal gap between the patterns, may also reflect in part the phenomenon absarokitesand the shoshonites,which we consider of magmamixing. The decreasein the total abun- to be real and not an artifact of sampling. The petro- danceof REEwith increasingsilica may be explained graphic featuresdiscussed above have been inter- by mixing of the absarokiticliquid with a felsic liquid preted in terms of magma mixing, and we ascribe that contains lower total REE (ca. 400 ppm versus the compositionalgap to the samephenomenon. In 150 ppm; see Table 3, Fig. 9). Becauseof the this regardit is important to note the following: (l) chondrite-normalizedpattern of the postulatedcon- taminant, the effect of mixing is to decreasethe LREEmorethanthe HREE. The largenegative Eu anomaly for one of the shoshonites(COCA-33) TASIE 5. REPRESENTATIWCOMPOSITIONS OF OLIV]NE PUXI{oCRYSTS might also be accountedfor by this mechanism. However, despitethe contaminationof the shosho- IR nitic liquid, the REE pattern for some of the volcanicrocks (samples35 and 39A) probably represents sto2 39.04 38.66 38.90 38.61 38.44 36.98 the approximatecompositions prior to contamina- tto 2 0.00 0.06 o.o2 0.07 0.09 0.06 Fe0 r5r54 18.85 15.69 18.04 17.27 23.49 tion. Note that sample39A also appearslittle modi- Mgo 44.30 4 1.90 44.45 42.45 42.95 38.33 fied with respectto the Rb-Sr data (Fig. l0). Cr203 0.04 0.00 Mineral chemistry ba o.994 0.994 0.989 0.992 0.989 0.978 Tt 0.000 0.001 0.000 0.001 0.00r 0.001 Fe 0.331 0.405 0.334 0.387 0.371 0.529 Analysis of' olivine and orthopyroxene in the Mc 1.680 r.605 1.685 r.625 1.646 1.511 absarokitesand shoshonites,respectively, has indi- 0.001 0.000 0.000 0.001 0.001 0.000 catedthe presenceof anomalous,double-rim com- Zro 83.6 79.8 83.5 80.7 8r.6 74-t positional profiles in both minerals and, in addition, high aluminum contentsin ttte pyroxene. We discuss AnaLyses refer ro core€ (C) aad rlBa (R); cofiplete Llst of analyses foutrd lD Koatak (1985). first the origin of the double-rim zoning, assuming

t rlms I ------,_ cores

aaaaaaaaa-a ltMAAlIrtAl Fogo Foeo Fozo

Ftc. 11. Composition of olivine phenocrystsfrom Neogeneabsarokites.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOCENE SHOSHONITE SUITE. PERU r29 that becausethe mineralsshow similar profiles, any gan & Wilkinson(1973, p.272), who statethat this model must apply to both. Three possiblemodels are appraisal ". . . appearsto minimize unduly the role presentedbelow: of pressure,particularly in view of the increasing l. Magma mixing. The zoning profiles in the mafic AlrO, contentsof orthopyroxenecrystallizing from silicates would require mixing of the olivine- and polycomponent"natural" systemsunder conditions orthopyroxene-bearingmelts with a higher temper- of constanttemperature and increasingpressure". ature, more mafic magma, for which there is no cor- roborating evidence.Although such a mechanism Petrogenesisof the shoshonites may have operatedat depth, we instead favor an and regional implications alternativemechanism discussed below. 2. A suddendecrease in Fe,/Mg ratio of the liquid. The evidencepresented above is interpreted to Tlere are two ways in which this may be brought record a complex evolution for the Cerro about. Firstly, the suddenprecipitation of an iron- Moromoroni shoshonitesuite, involving the follow- rich phase, such as magnetite, would reduce the ing scenario: FelMg ratio of the melt, stabilizing more magnesian l. Generationof subalkaline,or possiblemoder- phases.We can excludethis mechanismbecause there ately alkaline, magmain the upper mantle through is no evidenceto suggestthat magnetitecoprecipi- partial melting of a garnet peridotite source (see tated with either olivine or orthopyroxene.A second Dostalet al. 1977a).We considerthe following fea- mechanismwould be an increasein/(O), thereby effectively reducing the amount of Fd+ in the melt. Although this could be accomplishedthrough the dis- sociationof H2O, Hamilton & Anderson(1967) and Mueller (1969)pointed out rhat this is likely to con- trol/(O) only if a significantamount of H2O (i.e., > lVo) is present. The absenceof any primary hydrousphase suggests that the HrO contentof the a melt was low. o 3. A suddenchange in Ko,due to pressurerelease. lJ. Several authors (Ford et al. 1983, Takahashi & r ElectronMicroprobe Kushiro 1983,Hatton 1984)have recently shown that o the crystal,/liquidK, for olivine is sensitiveto pres- Laser lnterferometry lge [e.g.,according to Takahashi& Kushiro (19g3), Ka: 0.3@ + 0.002lkbarl. Thus, for polybariccrys- 1mm tallization of olivine, a zoning profile of the doubie- rim type observedin this studymay result [compare to Fig. 5 of Pearce(1984a). Whereas there are no Ftc. 12.Compositional profile (rangeof goFois 7.5) of oli- companableexperimental resultsfor the orthopy.ox- vine phenocrystin an absarokitedetermined using the enelliquid K, relationship, available data (Frey & electronmicroprobe and laser interferometry. Prinz 1978)suggest that the Ka relationshipsvary sympatheticallyin thesetwo mineral groups. The high aluminum contents that we record for ffiu 6. REPmSENUnI'X mToSIIIOtrS 0F oRmOtrROxm the orthopyroxenephenocrysts (3-8 wt.Vo Al2Or, are not unique (Table 7), having been previously SaE!16 lC reported from a number of geologicalsettings. In StO2 53,66 53.21 53.84 52.94 55.22 55,00 52.80 50.57 virtually all ofthese instances,the authors favored Ti02 0.15 0.I5 0,r8 0.18 0.r4 0.u 0.21 0.47 41203 3.26 2.92 L.96 5.25 2.02 2.25 t.77 r.78 a high-pressureorigin for the orthopyroxene,citing ge203 0.57 0.09 0.t6 0.73 0.21 0.18 t,26 1.85 as evidence:(l) disequilibrium FeO 8.20 9.08 A.37 tl.l7 6.02 6.t7 13.03 17.65 with the host rocks, Mco 32.54 33.85 31..4E 29.59 33,05 33.29 29.08 25.56 (ii) the coedstenceof other high-pressurephases &203 0.59 0.47 0.49 0.46 0.73 0.63 0.24 o. ll &o 1.25 0,62 0.7r r.28 1.03 0.89 l.sg 2.18 (e.9.,pyrope), (iii) the occurrenceof lessaluminous, r00.22 100.39 t00.r9 101.60 9a.12 9a.52 99.98 r00J second-generationorthopyroxene, and (iv) sl- 1.879 1.865 1.886 1.851 t.941 t.932 1.901 1.867 experimental 0.121 0.121 0.081 0.149 0.059 0.068 0.075 0.077 work carried out on a variety of bulk 0.014 0.000 0.@0 0.067 0.024 0.025 0.000 0.000 compositionsand systemsin which the 0.004 0.004 0.004 0.004 0.004 0.001 0.006 0.013 orthopyrox- 0.015 0.003 0.004 0.019 0.006 0.005 0.034 0.054 enephase contains high concentrations.ofaluminum. 0.240 0.266 0.245 0.327 0.t77 0.181 0.392 0.545 u8 r.698 1.768 1.799 t,542 1.73r t.743 1.560 1.405 The inference that aluminous orthopyroxene 0.016 0.013 0.014 0.013 0.020 0.017 0.007 0.003 representshigh-pressure crystallization has been Ca 0.047 0.023 0.027 0.048 0.039 0.033 0.061 0.086 indeed questioned (Anastasiou & Seifert 1922, 90.7 Danckwerth & Nefion 1978, Dymek & Gromet Aely6ee refer b qor6 (C), rtE (f,) aDd @rrt! (t)i ferrtc trcr calculatad ssuEiag stolchioerry aBd trelve Giloulc chsges. 1984),but we are neverthelessin accord with DuE- Corplete Ltst of aDalyB6 fourd ta KoEtah (1985).

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 THE CANADIAN MINERALOGIST

tures of the magmato be "primary": (i) the R-EE- o phenocryst enrichment and strongly fractionated chondrite- tr matrix normalized pattern, with (LalYb)s ) 20, (ii) the moderate Sr' (t 0.7060), (iii) the high K content (ca. 2.0 wt.9o KzO) and enrichmentof other large- radius lithophile elementsand (ry) the high initial Cr- content (ca. 300ppm). Severalofthese featuresare more typical of alkaline, rather than subalkaline, suites. 2. A period of high-pressurecrystallization fol- r.95 lowed by a secondperiod of relativelyshallowJevel fractionation, resulting in the developmentof the double-rim compositionalprofiles observedin the 6,0 mafic silicates,and also the apparentreverse zoning in severalorthopyroxene phenocrysts. (Y) 3, Mixing of the shoshoniticmelt with a felsic melt, 3o.o which containedquartz and plagioclasecrystals. The clinopyroxenecoronas around, and extensiveinter- bs within, the quartz megacrystsin the P 2.O nal fractures 3 shoshonitesare considereddisequilibrium features, as is the sievetexture in the plagioclase.Mixing of the two liquids probably occurred in a shallow 0.90 0.80 0.70 magma-chamber;we considerthe shoshonitesproper Mg/Mg+Fe2+ to representthe mixed componentand the absarokites to approximate the uncontaminated melt in Frc. 13. Compositional data for orthopYroxenefrom composition. Neogeneshoshonites. The generationof the shoshoniticvolcanic rocks

A matrix O tit I phenocryst + coreJ

matrix

a O..-+ -" I - - - -'z phenocrysts

90 80 70 M9 Mg

Frc. 14. Compositional data for orthopyroxenesfrom Neogeneshoshonites plotted in the compositional plane Mg-Fe-Ca.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUITE, PERU r31

at ca. Vl Ma coincidedwith the emplacementof other TSI;B 7. ALMA @NTE{TS OF ORMOPYROXINEIN A VAilXry OF GEOLOCIu BNTIRONUENTS intermediateto basic magmasalong the Inner Arc and Altiplano during the Late Oligocene - Early Miocene.Audebaud et ol. (1979)determined K-Ar uaal,f anorlhoslte 3.9-9.2 E@lte (1975, r978) ffire baealt 3.0-3.6 ereeo & f,lbbersoa (1970) agesin the rangeof 23,6 to 28.3 (r 1.0) Ma for aLbllae baalt 4.4 rrlgch & nrl8ht (1971) basaltsand alkallde baaLt 8.03 Bles (1969) trachytic volcanicrocks near the south- alkallne ba6e1t 3.45-8.03 Btau et a1. (1970) western margin of the Cordillero Orientql sbaLkalt@ eolcdle o.5-3.2 (u6 d-96tt in sotJth- tholtsllttc a&l€alte Ibgge & w11ktn6@ (t972) easternPeru, and of 22 to 26 Ma for severalsmall Karoo tholellle 4.0-1r.5 Janl.eeoa (1966) pelldotlte 5.0-5.5 Cre6a (1953) stocks of gabbroic to granitic composition in the 5.46-6,59 ee6 (1964) general x€dolltha ln b4alt t.0-7 Itblt€ (1966) same area. Everndenet al. (1977)reported perldotlte trodules b$ €t al. (1954) K-Ar agesof 26 Ma and 22 Ma for remlllh! lD bsalt 2.3 l-3.40 Kmo E-A;-u (1970) basalticrocks aodul€ ld aldoi.te 2.4-4.9 lllxotr 6 Boyd (1979) near Cherana and Tambo, respectively, in the ult!@f,lc reuol1th9 2.57-3.46 kary & Eem$ (1979) ta @flc dykeg Altiplano region of northwesternBolivia, and S.L. McBride (pers. comm. 1984)obtained a K-Ar age of 24.2 + 0.6 Ma on the Azurita basaltin the same general area. No geochemicaldata are available for theserocks; therefore,it is not possibleto ascertain by the Natural Sciencesand Engineering Research the extent of what may be a shoshoniticprovince, Council of Canada(grants to Clark and Farrar), as although the gabbros dated by Audebaud et a/. were laboratory studies (grants to Clark, Farrar, (1979) are described as containing a biotite- Pearce,Baadsgaard and Strong). Field work was c:tr- clinopyroxeneassemblage. ried out with the logistical assistanceof Minsur S.A., The intermediateto basic suitesof ca. 2g to ZZI.4a through the good offices of F. Zavaleta,We also agein the Cordillera Orientol andAltiplano regions acknowledgethe useful suggestionsmade by the edi- are coevalwith Sn-W mineralizedgranitic suitesin tor, associateeditor and the refereesof an earlier ver- the southeasternPeru and northwestern Bolivia sec- sion of this manuscript. tors of the Cordillerq Oriental @verrrdenet al. 1977, Grant et al. 1979, McBride et ol, 1983, Clark et ol. 1983b).Together, these suites constitute part ofthe "arc broadening" (Clark et ql. 1976, Clark & REFERENCES McNutt 1982)episode of the Central Andean orogen that followed an extendedmid-Tertiary period Ar-sBs,A.L. & Rav, L. (1970): Correction factors for probe of magmatic quiescenceand was sensibly electron microanalysis of silicates, oxides, car- coincident bonates, phosphates with rejuvenation of and sulfates.Anal. Chem. 42, the Main Arc system. This 1408-1414. expandedarc extendedinland for some500 km, in contrast previously . with the narrow, longitudinal, ANasrasrou,P. & Srrrenr, F. (1972):Solid solubility of volcano-plutonic belt of the Main Arc, which per- Al2O3 in enstatite at high temperaturesand l-5 Kb sisted during much of the Mesozoic and Earlv water pressure, Contr, Mineral. Petrology 34, Cenozoic. 272-287. The presenceof shoshonitic rocks along the easternmost limit of the cq. 25-Ma-old magmatic Anru, J.G. (1976): Behavior of trace elementsduring processes- province suggestsa model involving subduction- magmatic a summary of theoretical zone-relatedprocess€s generate models and their applications. J. Res. U.S. Geol. to these melts of Sum, 4. 4147, upper-mantlesource, thereby implyrng deeppenetra- tion of the subductionzone during this lime (vide Auosseuo, E., BrnNano, D., Verm-PrntcxoN, N. & ql. Clark & Zentilli 1972,Clark et 1976).It is also Vrvrsn, G. (1979): Quelques dges K,/Ar sur les suggestedthat thesemelts causedlocal melting of the roches ign6es c6nozoiques du SE Pdrou. Cons€- crust to generatethe peraluminous felsic suitesthat quencesgdodynamiques. Rdunion Ann. Sci. Terres host the important reservesqf Sn-W and basemetals (Paris), (abstr.). in the tin belt. BpNcs,A.E. & Ar-sps,A.L. (1968): Empirical correlation factors for the electron microanalysis of sili- AcKNowLEDGEMEI.ITS cates and oxides. ,I, Geol. 76,382-403.

We thank P.L. Roeder and D. Kempson for Brmr:vs,R.A. (1969): High pressuremegacrysts in basa- assistancewith electron-microprobeanalyses, and F. nitic lavas near Armidale, New South WaIs. Amer. J. Sci. 267.33-49. Dunphy and R. Fosterwith the X-ray fluorescence and wet-chemicalanalyses. Burry provided J. the DuccaN, M.B. & WrrrrNsoN,J.F.G. (1970): REE data,and W. Marsh assistedwith the prepara- High pressure megacrysts from northeastern New tion of figures. Field work in Peru was supported South Wales. Amer. J. Sci. 2.69, 132-168.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 t32 THE CANADIAN MINERALOGIST (1978): Boccalrtu, M., MaNBrrr, P. PeccBrurro, A. & DeNcrwnnru, P.A. & NnwroN, R.C. pel{otlte Srat.usHrva-VAsstt-sve,G. (1978): Late Cretaceous Experimental determination of the spinel MgO- high-potassium volcanism in eastern Srednogorie, to garnet peridotite reaction in the system Al2O3 Bulgaria. Geol. Soc. Amer. Bull. E9,439-447. Al"O.-SiO, in the range 900"-1100"C and isdpl6ths of enstatite in the spinel freld- Contr. Bovp. F.R. & ENcrar.ro, J.L. (1964): The system Minerat. PetrologY 66, 189-201. enstatite-pyrope. Carnegie Inst. lvash. Year Book 63, t57-161. DEnurlle, B. (1973): Calc-alkaline and shoshonite lavas from five Andean volcanoes (between Lati of Bnoors, C,, LuooeN, J., PtcnoN, Y. & HusREcrsn' tudes2l'45' and 24'30'S) and the distribution J.J.M.W. (1982):Volcanism of shoshoniteto hieh-K the Plio-Quaternary volcanism of the south-central Res. 3, andesite affinity in an Archean arc environment, and southern Andes. "I. Volc. Geothermal Oxford Lake, Manitoba, Can. J. Earth Sci. 19' 281-298. 55-67. (1982): Petrology of the Plio-Quaternary vol- BuBNrNc,D.K. & Buspcr, P.R. (1973): Fe-Mg lattice canism of'the south-central and meridional Andes' diffusionin oliine. J. Geophys. Res. 7E' 6852-6862. J. Volc. Geotherm. Res. 14' 7'l-124. (1983): Cr-enr, A.H., Fennan, E., Ceeurs, J.C., Hewrs, -, Herrr.toN,R.S' & Moonsarn, S. Com- petrogenesis S.J.. Lonrrc, R.B., McBmpr, S.L., Qunr, G.S., bined Sr-O isotope relationships and RossnrsoN,R.C.R. & Zrwnm, M' (1970: Longitu- of Andean volcanics of South America. Nature 302, dinal variations in the metallogenetic evolution of 814-816. the Central Andes: a progress report' ft Metallogeny (1977a): and Plate Tectonics (D.F. Strong, ed.). Geol. Assoc. Dosrar, J., Dupuv, C. & LprEvrr' C. Rare Can. Spec. Paper 14,23-58. earth element distribution in Plio-Quaternary volcanic rocks from southern Peru. Lithosl0' 173-183. & McNurr, R.H. (1983a): EvPlution of the Central Andean magmatic anc' Amer. +, ZrNrttu, M., CarrlEs, J.C. & Cr-am, A.H. Geophys. (Jnion Trans. @, 326 (abstr'). (1977b): Geochemistry and origin of volcanic rocks of the Andes (26'-28"5). Contr. Mineral. Petrol' Kovrar, D.J. & Fannen, E. (1984):A compar- ogy 63, 113-128. tive study of the metallogenetic and geochronolog- (1973): ical relationships in the nofihern part of the Cen- DuccaN.M.B. & WtlrtNsott, J.F.G. Tholeiitic Tweed tral Andean tin belt, SE Peru and NW Bolivia. .[n andesite of high-pressure origin from the Sixth I.A.G.O.D. Symp. shield volcano" northeastern New South Wales. Proc. Quadrennial -27 (Tbilisi). E. Schweizerbart'scheVerlags, Stuttgart, Contr. Mineral. Petrology 39, 267 6. Germany. Dupuv, C. & LsrEvRE,C. (1974): Fractionnement des & McNurr, R.H. (1982): Interrelatedarc- €l€ments en trace Li, Rb, Ba, Sr, dans les s6ries broadening, topoeraphic uplift and crustal contami- and6sitiques et shoshonitiques du P6rou. Comparar- nation of magmas in two iransects of the Mesozoic- son avec d'autres zones orogdniques. Contr' Cenozoic Central Andes. 1n Proc. Fifth Int. Conf. Mineral. PetrologY 6, 147-157. Geochronologyn Cosmochronology and Isotope Geology (Nikko, Japan), 55-56 (abstr.). Dvvar, R.F. & Gnourr, L.P. (1984):Nature and ori' crn of orthoplroxene megacrystsfrom the St-Urb4n 22, +, Paltr,te,V.V., Ancntnar-D,D.A.' Fennen, E' & inorthosite massif, Quebec. Can. Mineral. AnEnas,M.J.F. (1983b): Occurrenceand age of tin 297-326. mineralization in the Cordillera Oriental, southern Peru. Econ. Geol. 18, 514'520. ErcHeI-sBncrn, J,C. (1974): Magma contamination within the volcanic pile: origin of andesite and 2, 29-33. & Zexrnu, M. (19'72\: The evolution of a dacite. Geology metallogenic province at a consuming plate margin: the Andes between Latitudes 26o and 29o south. (1978): Andesites in island arcs and continen- Can. Inst. Mining Metall. Bull.65Ql9),37 (abstr.). tal margins: relationship to crustal evolution. Bal/. Volc. 41,480-500. Crenr, E.E. (7917): Late Cenozoic volcanic and tectonic activity along the easternmargin of the Great Euslrr, R.F. (1975): Pyroxene megacrysts from Basin, in the proximity of Cove Fort, Utah. Brigham anothositic rocks: new cluesto the sourcesand evo- Young Univ. Geol. Studies 24' 87-ll4- lution of the parent magmas' Can. Mineral. 13, 138-145. ContrNt, M. (1981): Aeolian island arc (southern granites Tyrrhenian sea) magma heterogeneitiesin histori- - (1978): Anothosite massifs, rapakivi cal lavas: Sr and Pb isotopic evidence.Bull Volc. and iate Froterozoic rifting of North America. 44.'7lt-722. Precambrian Res. 7, 6l-98.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A NEOGENE SHOSHONITE SUITE. PERU 133

EvsnNorN,J.F., Knz, S.J. & Cnrnnoxr, M.C. (1971: GrmN, T.H. (1972) Crystalization of calc-alkaline Potassium-argonages of someBolivian rocks. E"on. andesite under controllled high-pressure hydrous Geol. 72. 1042-106l. conditions. Contr. Mineral. Petrology 34, 150-166.

Fono,C.E., Russnl, D.G., CnavrN,J.A, & Frsr, M.R. -& RrNcwooo,A.E. (1968): Genesisof the calc- (1983):Olivine-liquid equilibria: temperature, pres- alkaline igneous rock suite. Contr, Mineral. Petrol- sure and composition dependence of the crys- og) lE, 105-162. tal,/liquid cation partition coefficients for Ms^ Fe2+, Ca and Mn. J. Petrology 24,256-265. Havrrror, D.H. & AxonnsoN,G.M. (1967): Effects of water and oxygen pressureson the crystallization of Fnrv, F.A. & PnrNz,M. (1978): Ultramafic inclusions basaltic magmas. 1z Basalts, the Poldervaart Trea- from San Carlos, Arizona: petrologic and geochem- tise on Rocks of Basaltic Composition (H.H. Hess ical data bearing on their petrogenesis.Earth planet. & A. Poldervaart, eds.). Interscience, New York. Sci.Lett.38.129-176. Herrox, C.J. (1984): The effect ofpressure, tempera- Fnrsct, J. & WnrcHr, J.B. (19?1):Chemical composi- ture and composition on the distribution of Fe and tion of high-pressure megacrysts from Nigerian Mg between olivine, orthopyroxene and liquid; an Cenozoic lavas. Neues Jahrb. Mineral. Monatsh., appraisal ofthe reversal in the normal fractionation 289-304. trend in the Bushveld Complex. Contr. Mineral. Petrology E6, 45-53. FnyEn,B.J. (1977): Rare-earth evidencein iron forma- tions for changing Precambrian oxidation states. Hocc, N.C. (1972): Shoshonitic lavas in west-central Geochim. Cosmochim. Acta 41, 361-367. Utah. Brigham Young Univ. Geol, Studies lg. 133-184. Gsnr-A,cr,D.C. & Gnovn, T.L. (1982): Petrology of Medicine Lake Highland volcanics: characterization HursNen, J.S. & Nonp, G.L., Jn. (1979):Assessment of end members of magma mixing. Contr. Mineral. of diffusion in pyroxenes; What we do and do not Petrology 80, 147-159. know, Lunor Planet. Sci. XII, 479-481.

GEsr,D.E. & McBrnNBy,A.R. (1979): Genetic relation IoorNcs, J.P. (1895): Absarokite - shoshonite - of shoshonitic and absarokitic magmas, Absaroka banakite series."/. Geol. 3,935-959. Mountains, Wyoming, J. Volc. Geotherm. Res. 6, 85-104. Invwr, T.N. & Beneoan, W.R.A. (1971): A guide to the chemical classi{ication of the common volcanic Gu.r, J.B. (1970)l Geochemistryof Viti Lew, Fiji, and rocks. Con. J. Earth Sci. E, 523-548. its evolution as an island arc. Contr. Mineral. Petrol- ogy 27, 179-203. Jars5, P. & Wmrp, A.J.R. (1969): Structure of the Melanesian arc and correlation with distribution of (1980): Orogenic Andesites. Springer-Verlag, magma types. Tectonophys. 8, 223-236. Berlin. & (1972): Major and trace element abun- GorE, S,P,, Baapscaann, H., MueHlnNaecns, K. & dances in volcanic rocks of orogenic areas. Geol. 'ScaarB, C.M. (1982): Rb-Sr isochron ages, mag- Soc. Arner. Bull. E3,29-40. matic 875r/865rinitial ratios, and oxygen isotope JaurcsoN, B.G. (1966): Evidence on the evolution of geochemistry of the Proterozoic lava flows and basaltic magma at elevated pressures.Nature 212, intrusions of the East Arm of Great Slave Lake, 243-246. Northwest Territories, Canada. Can. J. Earth Sci, 19.343-356. Jopr-rN,G.A. (1965): The problems of the potash..rich basaltic rocks. Mineral. Mag. 34,266-275. Gnalr, J.N., Helrs, C., Sar,rxas,W,A. & SwsLLnrc, N.J. (1979): K-Ar agesof igneousrocks and miner- (1968): The shoshonitic association: a review. part alization in of the Bolivian tin belt. Econ. Geol, J. Geol. Soc. Aust. E, n5-294. 74,838-851. Kay, R.W. & Gasr, P.W. (1973): The rare-earth con- (1963): GnsBN,D.H. Aluminum content of enstatitein tent and origin of alkali-rich basalts. J. Geol. El, a Venezuelan high-temperature peridotite. Geol. 653-682. Soc. Amer. Bull. 74, 1397-1402. KrLLsn, I. (1974): Petrology of some volcanic rock -(19&): The petrogenesisof the high-temperature seriesof the Aeolian Arc, southern Tyrrhenian sea: peridotite intrusion in the Lizard area, Cornwall. .I. calc-alkaline and shoshonitic associations. Cottu Petrologlt 5, 134-188. Mineral. Petrologlt 46, 29-47.

& HrsseRsoN,W. (1970): Experimental duoli_ Kowrer, D.J. (1985): The Magmatic and Metal- cation of conditions of precipitation of high_pressure logenetic Evolution of a Craton - Orogen Interfoce: phenocrysts in a basaltic magma. phys. Earth the Cordillera de Carabaya, Central Andes, SE Peru. Planet. Interiors 3. 247-254. Ph.D. Thesis, Queen's Univ., Kingston, Ontario.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 134 THE CANADIAN MINERALOCIST Crem, A.H. & Famen, E. (1983):Magmatic McBupE, S.R., RosenrsoN,R.C.R., Cmm, A.H. & evolution of the Cordillera Oriental of SE Peru: Fennan, E. (1983): Magmatic and metallogenetic crustal versas mantle componenrs. Amer. Geophys. episodesin the northern tin belt, Cordillera Real, Union Trons. 64,329 (abstr.). Bolivia. Geol. Rundsch. 72, 685'713.

-, - & - (1984a):The magmatic evo- McNurr, R.H., Cnocrs"r,J.H., Cram, A.H., CarLrEs, Iution of the Cordillera Oriental, SE Peru. ft Chem- J.C., Fannan, E., Havxns, S.J. & ZrNrnrt, M. ical and Isotopic Constraints in Andean Magmatism (1975): Initid 875r/865rratios of plutonic,anq u9l- (B.A. Berreiro & R.S. Harmon, eds.). Shiva Pub.. canic rocks of the Central Andes between latitudes Ltd., Cheshire,U.K. 26o and 29"S. Earth Planet. Sci.Lett. 27, 305-3I 3.

-, & Peence, T.H. (1984b): Recognition MrvasHtno, A. (1974): Volcanic rock series in island of simple and complex zoning in olivine and arcs and active continental margins. Amer. J, Sci. orthopyroxene phenocrysts using laser interference 274.32t-3ss. microscopy. Mineral. Mag. 48, 547-550. & SHroo,F. (1975): Tholeiitic and calc-alkalic KoorsN, G.K.V. & Busncx,P.R. (1978): Interpritation seriesin relation to the behaviorsof titanium, of olivine zoning: study of a maar from the San vanadium,chromium, and nickel.u4mer. J. Sci.275, Francisco volcanic field, Arizona. Geol. Soc. Amer, 265-n7. Bull. E9.744-754. Moons. J. & Evans, B.W. (1967): The role of olivine KuNo, H. (1964)l Aluminum augite and bronzite in in the crystallization of the prehistoric Makaopuhi alkali olivine basalt from Takasima, north Kyushu, tholeiitic lava lake, Hawaii. Cont. Mineral. Petrol- Japan. In Advancing Frontiers of Geology and ogy 15,202-223. Geophysics. Osmania Univ. Press, Hyderabad, India. Monnnor, G.W. (1980): Characteristics and tectonic setting of the shoshonite rock association. Lithos 13, & Aorr, K. (1970):Chemistry of ultramafic 97-108. nodules and their bearing on the origin of basaltic magmas. Phys. Earth Planet. Interiors 3, 271-301. Muerr,sn, R.F. (1969)l Hydration, oxidation, and the origin of the calc-alkaline series. NlSu4 Tech. Note Leunacrsn, G. (1978): Estudio geol6gico de la region D-5400. norte del Lago Titicaca. Inst. Geol. Miner{a Peni, Rossnrs,T.G. (1953): Bol. 5. Nswnr-r,N.D., CnnorIc, J. & Upper Paleozoic of Peru. Geol. Soc' Amer. Mem. 58. Lnanv, B.D. & Hnnr'ms,O.D. (1979): Mantle xenoliths England. In The Mantle from soulheastern New NrcsoLts. J. & CanMIcHasL,I.S.E. (1969):A commen- in Kimberlites and Other Vol- Sample: Inclusions tary on the absarokite - shoshonite - banakite ser- (F.R. Boyd & O.A. Meyer, eds.). Proc. canics ies of Wyoming, U.S.A. Schweiz.Minerol. Petrog. Kimberlite Conf,2. Amer. Geophys. Second Int. Mitt. 49,47-64. Union Publ.,37+381. +, & Sronrr'rsn,J.C., Jr. (1971): Silica LsrEvnr, C. (1973): Les caractdres magmatiques du activity and Pro,rl in igneous rocks. Contr. Mineral. volcanisme plioquaternaire des Andes dans le sud Petrology 33, l-20. du P€rou. Contr. Mineral. Petrology 47,259'272, NrxoN. P.H. & Bovo, F.R. (1979):Garnet bearing lher- MaaroB, S. & HerlsaN,B. (1982): Olivine phenocrysts zolites and discrete nodule suites from the Malaita of Hawaiian olivine tholeiite and oceanite. Contr. alnoite, Solomon Islands, S.W. Pacific, and their Mineral. Petology 81, 203-211. bearing on oceanic mantle composition and geotherm. .In The Manile Sample: Inclusions in Kim- (F.R. Boyd & D.A. MacDoNerp. G.A. & Karsute, T. (1965): Eruption of 6erttes and Other Volcanics Kimberlite Conf. Lassen Peak, CascadeRange, California, in 1915: Meyer, eds.). Proc. Second Int. Publ.' 400'423. example of mixed magmas. Geol. Soc.Amer. Bull, 2, Amer. Geophys. Union 76,475-482. O'Hana, M.J. (1965): Primary magmas and the origin l, 1940. MacKsNzts. D.E. & CHeppeLL,B.W. (1972): Shosho- of basalts. Scot. l. Geol. nitic and calc-alkaline lavas from the highlands of Peancn,T.H. (1984a): The analysis of zoning in mag- Papua New Guinea. Contr. Mineral. Petrology 35' matic crystals with emphasis on olivine. Confr. 50-62. Mineral. PetrologY 86' 149-154. Mesuoe, A., Naxevune, N. & Tarere, T. (1973): Fine structures of mutually norualized rare-eaxth patterns (1984b): Multiple frequency laser interference of chondrites. Geochim. Cosmochim. Acta 37, microscopy: a new technique, The Microscope 32' 239-248. 69-81.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021 PETROGENESISOF A,NEOGENE SHOSHONITE SUITE. PERU 135

PsccBRrr,Lo,A. & Teyr.on, S.R. (1926): Geochemistry SxrNNsn,B.J. & Bovp,F.R. (1964):Aluminous ensta- of Eocene calc-alkaline volcanic rocks from the tites,Camegie Inst. Wash.Year Book 63,163-165. Kastamonu area, northern Turkey. Contr, Mineral, Petrology 58,63-81. Suns, A.L. & Canurcsarr-,I.S.E. (1968):Quaternary lavasfrom the southernCascades, western U.S.A. Pr-Prpsn, G. (1980): Ceochemistry of Miocene Contr. Mineral. Petrology 19, 212-238. shoshonites, Lesbos, Greece. Contr. Mineral. petrology 72,387-396. Srercen,R.H. & JAcsn,E. (1977):Subcommission on geochronology:convention on the useof decaycons, PnrsrMAl{, Z.E. & Hsocn, C.E. (1971): Related stron- tantsin geo-and cosmochronology.Earth Planet. tium isotopic and chemical variations in oceanic Sci,Lett.36,359-362. basalts. Geol. Soc. Amer. Bull.82, 493-500. SrnoNc,D.F. (1972):The petrologyof the lavas of Powpr-r-,J.I,. & BBu-, K. (1974): Isotopic composition GrandeComore. J. Petrologlt13, l8l-217. of strontium in alkalic rocks. ,lz The Alkaline Rocks (H. Sorensen, ed.). John Wiley & Sons, London. TereHassr,E. & Kusurno,I. (1983):Melting of dry peridotiteat high pressuresand basalt magma gen- Ross, C.S., FosrBn, M.D. & Myrns, A.T. (1954): Ori- esis.Arner. Mineral. 6E, 859-879. gin of dunites and of olivine-rich inclusions inbasal- tic rocks. Amer. Mineral. 39, 693-73j. WrurB,R.W. (1966):Ultramafic inclusions in basaltic rocks from Hawaii. Contr. Mineral. Petrology 12, Saruyaue, M. (1978): Petrographic evidence of 245-t14. magma mixing in Shirouma-Oike volcano, Japan. Bull. Volc. 41, 501-512. Wnsou, A.H. (1982): The geology of the Great "Dyke", Zimbabwe:the ultramaficrocks. l. Petrol- Saro, H. (1975): Diffusion coronas around quartz ogy 23,240-292. xenocrysts in andesite and basalt from Tertiarv volcanic regions in northeastern Shikoku, Japan. iontr. Received November 26, 1983, revised manuscript Mineral. Petrology 50, 49-&. acceptedOctober 9, 1985.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/1/117/3435129/117.pdf by guest on 27 September 2021