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GEOCHE"ICAL EVOLUTION OF THE CALC-ALKALINE SOLI"ANA VOLCANO, WESTERN CORDILLERA. SOUTHERN PERU
1
P. Goe.ans.
3 L. BrlClueu.
2 R.A OUvert
4 C. Dupuy
1
N. Vatln-Pérl~non
1. Laboratoire de Céolo~le. UA 69 CNRS, Ualverslté Scleatlflque.
TecbnoloalClue et "édlcale de Grenoble. (UST"GI.
)5 rue "aurlce Gignoux. 38031 Grenoble Cedex. Fraace.
2. Institut Lalle-Lan'cevln (ILLI, 156 X,
Avenue des ftartyrs, 39042 Grenoble Cedex. France.
3. Laboratoire de Céochl.le Isotopique,
Université des Sciences et TechalqueS du Languedoc (USTLI.
34060 "ontpeiller Cedex, France.
4. Ceatre Géologique et Céophyslque (CCCI.
Ualverslt' des Sciences et Tecbnlques du Languedoc (USTLI, 34060 "ontoeiller Cedex, France.
The Soll.ana volcano (IS02S'S-7soS3'WI is located 200 k. northuest of Arequipa. ln the Western Cordillera, south feruvlan Andes. In th"
Northera.ost part of the Central Volcantc Zone (CVZI. It lS tDcluded Ir.
tbe Neoaeae non folded volcanlc caver ln the Cotahuasl reglon (distrtct of Arequlpal. The volcano IS 6093 • hlgb aad covered by aa asy.etrlc lce-cao of about 50 k.2. and the total volu_e of ail erupted products IS about 200 k_3. The volcanlc ptle of 3000 _ htgb ls bUllt upon Late Preca.brlaa aranulltlc series, overlapped by Palaeozotc folded ~net3se=
and latruded by a Cretaceous a_phlbole granite.
Oa the basls of neu K-Ar analyses. the volcanic history spread over the last Il na and caa be dlvlded lnto three sequences:
The flrst erupted unit iS located Along a series of NIO' trendtn~
fissures aad lasted for a perlod of approxl_ately 7 na.
The second oDe ls a centrally located activlty. uhlch butlt a co_poslte sbleld volcano, subsquently _odtfled by the for_atioa of stron.ly collapsed calderas.
The last phase of activlty ls spread around a series of ring faults assoclated vtth the caldera collapse.
Tbe volcaalc SUite presents a co_posltional contlnuu. fro.
basalt-andeslte ta rhyodaclte. uith the _ajor petrographlc type bein~
dO.lnantly dacltlc. "lneraloglcal variations sbou evidence for tua oara.eneslS aad syste_atlcally tua populations of _lnerals. The flrst
heavy and with the
e.~.
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Dara~enesis shows the following corroded and broken Rinerals: olivine.
ortho and Clinopyroxene. biotite. feldspar. auphibole and ferro-titaneous oXide. the second ODe presents fresh Rinerals:
ClinOpyroxene. biotite. aDphibole. quartz and Zircon + sphene as coumon ainor Rineral phases.
The Solimana series exhibit RajOr element variations similar to those observed in other high-K calc-alkaline series of subduction zone context: Si02 ranges frou 52 to 68 X. cOrrelàted with increasiDg Na20 and K20 and decreasin~ Fe203. Ti02.• CaO aDd KgO. P205 shows an unusual weil correlated decreasing trend-liDe aDd varies as a cORpatible ele.ent.
ln the light of chondrite and KORB-DorRalyzed values. the series show enrichRents of Large lou Lithophile (LIL: Sr.K. Rb. Ba. Thl and
Li~ht iare Earth Eleaeut (LREE' La. Ce. Nd. SRI relative to highlY inCo.patible Hi~h Field Strength eleReDts (HFS: Ta. Nb. Zr. Hf. Til with negative Nb and Ta ano.alies. The behaviour of first-transitioD series Retals (Sc. Ti. V. Cr. Kn. Fe. Co. Ni. Cu. ZDI vary with strong depletion of Cr and Ni and light depletioD of Ti. V. Kn. Fe. Co. These features seeR to be typical of .any AndeaD calc-alkaline lavas.
The distinctive cheRical characteristics of the SoliRana lavas are REE depletion. a narrow 143 Nd/144 Nd range (0.512484-0.5125441 a relatively high Sr isotopic ratios (0.70588-0.706211. WhiCh vary trace eleRent fractionation. HiEE depletioDs are pronounced iD ail series and this iS possibly due to accessory RiDerai fractionation Zircon and sphene.
On the Nd versus Sr isotopic diagra. the SoliRana lavas have a less radiogenic cORpositiOn than other provinces of the CVZ (e.g.
Barroso and Arequipa ForRations (Southern Perul and Purico-Chascon Co.plex (Northern Chilell. However. for ail the provinces of the CVZ.
these specific characteristics could be explained by a contiDuing process of crustal conta.ination duriDg fractional crystallization of the DagRa. The Dain differences With weil known CVZ cODtaainated rocks consist in nature and percentage of the conta.inant .aterial. On a 87 Sr/BS Sr f(I/86Srl diagra •• good agree.eDt is obtained between observed data and che.ical uodelling of AFC process. From these calculations witb realistic contamination rates. the Rain CODtaaination wouli appear to be from either Palaeozoic gneisses 120 X requiredl or Precambrian granulites 120 X requiredl rather than the Cretaceous eranite (50 X requiredl. whiCh is an unsatisfactory solution for the contamination model.
ThiS crustal contauination is less i.portant than in ChileaD iRnimbrites 130-35%1. similar to Barroso an AreqUipa groups in weight X (20%1 but different in nature of contaminant .aterial: gneisses for Arequipa and Barroso. aore probably Preca.brian granulites for the Soliuana volcano. ThiS phenomenon coul4 have taken place duriDg the low-pressure differentiation of the magua or alternatively during the early stages of Ragmatic evolution.
