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Petrogenesis of a Neogene Shoshonite Suite, Cerro

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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 (Newellet 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 rela- tionships 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,cor- dierite and sillimanite. These peraluminous stocks outcrop in the vicinity of the shoshoniticsuite. The shoshoniticvolcanic rocks make up a small (4.5 kmz), eroded volcanic
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