Gifi Fourth /SAC ,Goettingen (Cermany), 04-061/01/999

GEOCHEMISTRY AND TECTONICS AT THE SOUTHERN TERMINATION OF THE NORTHERN VOLCANIC ZONE ( VOLCANOES, ); PRELIMINARY RESULTS

M. MONZIER ( 1-3). C. ROBIN (2). M.L. HALL (3), J. COlTEN (4) and P. SAMANIEGO (2-3)

( 1) Institut de Recherche pour le Développement (lRD , previously ORSTOM),A. P. 17-12-857, , Ecuador (m ich mari@orstom .org.ec) (2) IRD, Université Blaise Pascal. 5 rue Kessler, 63038. Clermont-Ferrand cedex, France, ([email protected]. fr) (3) Instituto Geofisi co, Escuela Polite cnica Nacional (lG- EPN), A. P. 17-01-2759, Quito , Ecuador (geofisico@access inter.net) (4) UMR 6538 , Université de Bretagne Occidentale, B.P. 809, 29285, Brest, France ([email protected])

KEY WORDS: Andean NVZ. arc termination, low- Y volcanics, infracrustal AFC processes

INTRODUCTION:

The Riobamba volcanoes constitute the southern termination of the NVZ. To the south. active volcanism is absent along a 1600 km long segment of the (Fig. IA-B ). Th is work presents prelim inary geochemical data for thèse voJcanoes and relates them to an unusuaJ underlying slab and the complex seismo-tectonics of the continental crust in the arca,

SEISMOTECTONICS: ln the Riobamba area, the main Ecuadorian arc becomes frontal and Sangay lies unusually eastward (Fig. 1Cl. Contrary to southern Colornbia and Ecuador, beneath which intermedi ate depth seismicity is almost absent, a strongly seismogenic slablet dipping • 35° towards N58°E is present under the Sangay area at > 130 km beneath the volcano (Fig. 1D). The N58°E boundary bet ween this slablet 10 the south and the weakly seismogenic slab to the north reflects a sharp change in the thermal charac teristics of the subducted oceanic crust, and corresponds to a tear running from the Grijalva Fracture Zone (GFZ; Fig. 1B), thar separaies the young «24 My) crus! subducted under Colornbia and Ecuador, From older crus! (>32 My) beneath southern Ecuador and northern Peru (Monzier et al., 1999). To the Fourth /SAC. Goettingen (G ermanvï. O.J-()()//OI/ YW 517

south, another N5RDE boundary probab ly scparutcs this slublct l'rom the shullow dip ping slab which is typical of southern Ecundor and nonhcrn Pcru. Due to the convergence motion bctwccn ihc Na/ ca and

South America plates (. R. Ocm/y in ,1 direction> NRO DE : De Mets ct al., 19<)()l. the N5R D E tcar in the slah

slow ly sweeps the arca . Culculations show tluu this tcar wus undcr the C him horazo-Puûulica arcu s 1.35

~1~ ' BP and under the Culpi-Tungurnhua urcn > 0.7 My BP: it is prcscnil y undcr Tulubug-El Altur \ olc.mocs and will he undcr Sanga y in • 1.1 Cl My (Fig. 1Dl. Thcsc culculutions ncglcct . umong othcr I·'l,·[,'rs. the dcxtral strikc- slip motion bciwccn the North Andcan Block und the l'est of South America. The -ou rhcrn rcrnunuu on of the NVZ -which is oricnicd • N 14RDE in contrust with the usual N3()OE orientation

D of the Ecundorian ,1IT- is cxuctly pcrpcndicular (0 the N58 E dipping Sangay slablet. Thus. to a large ex­ rent. the volcanic hisiory of the tcrmi ruuion hus beert controllcd hy the southeusrward migration of this narrow piece of old occnnic crust. boundcd to the North by the GFZ bur also eut hy nurnerous seco ndary fracture zones pnrallcl to the GFZ.

RIOBAMBA VOLCANOES: Plio-Qu aternary volcanoes ure locutcd on the edges or in the middle of the Riobamba pull-apart basin (a segment of the Interandeun Valley). with the exception of El Allal' and Sangay, which are clearly located in the Eastern Cordillera of Ecuador (Fig. 2A-Bl. In addition to the old and eroded Igualata and Huisla edifices. four major Pleistocene volcanoes developed in the area, as weil as severa! smaller volcanic cones. The complex forrned first by effusive eruptions (acid andesites), then by explosive activity (daci te-rhyolite): some 35.000 y BP, this volcano experienced co ne collapse and a large­ scale debris avalanche. Post-collapse aciivity ended with eruptions of basic andesit es before 11.000 y BP (Clapperton, 1990; Kilian et al., 1995). The complex is composed of three successive volcanic edifices. the first two of which were partially destroyed by secror collapse. The last collapse event occur red • 3.000 years ago, preceding the construction of the present cone, which is dorninated by basic and ésites (Hall et al., \999). Over the past 500,000 y. the Sangay volcanic complex also has had

three success ive andesitic edifices. The ( WO former co nes have been largely destroyed by sector collapses. The present edifice is active at least since 14,000 y BP (Monzier et al., 1999). Finally, volcano consists of subglacial andesite brecci as, with the rernnants of a shallow « 3 km) magma cham ber. inc1uding a large rhyolitic body and a subordinate diorite body. ils development ended with the catacl ysmic evisceration of this chamber (probably accompanied by sec tor collapse) that resulted in formation: subsequent glacial erosion has deeply eroded the edi fice (Monzier et al., in prep.). Other volcanic centers of the Riobamba area include the small cones of Calpi and Tulabug. For ail the Riobamba edifices. extensive geoche rnical sampling is currently done or in progress.

CHARACTERISTICS OF RIOBAMBA VOLCANICS AND DISCUSSION: A striking feature of the Riobamba volcanics is that basic rocks are more abundant here than in the l'est of the NVZ (Fig. 2C). This may be explained by the fact that most of the these volcanoes are assoc iated 518 Fourtli /SAG. Goettingen (Germanvï . 04 -061/ 01/999

with a large pull-apart basin, boundcd by maj or N22°E strike-slip taults oblique to the regional N-S orientation of the Cord i llera, ,llld prolusely CUl by transverse laults. Such a faulted cru stal structure provides parhways for rising magmas and ihu s limits the etfect of upper crustal fractionation processes.

Ex cept for th is feature, Riobamba volcanics are not significantly different l'rom other NYZ arc rocks, sho w ing the usual enr ichm ent in in compatible elements as the distance to the trench in cr eases. Ali shar e similarly lo w Y and HREE contents (F ig . 2D). Howe vcr, Riobamba volcanics dont sho w the marked decre ase in Y and Zr (F ig. 2E) Ircqucntl y obscrvcd for differcniuucd rock s l'rom the l'est of the arc , and certainly ca used by upper crusial amph ib ol e Irnctionation. By contrast, and excc p t for the most evolved composition s. Y contents romain ulmost co nstant and Zr clearly acts as an incompatible clement in

Riobamb a volcanics, For the Sangay suite, the low concentration in Y. as w eJl as the presence of Sr-rich andesite s. ca n he cxp lained hy AFC processes occ uring at the hase of a sa km-thick crust, where basalti c melts pond and assirnilutc remens o f prc vi ou sl y und crplatcd. composition ally sim i lar basaltic material

(Monz ier el al.. 1999 ). Similar AFC pro cesscs probably affect othcr magmas o f the southern terrnination , as indicated, for cx ample, hy samplcs from T ulabug, tha ï coru pnsc basalts (5:1 % Si02 & 72 5 ppm Sr) associatcd with Sr -rich andcsiics (58 % Si02 & 1000 ppm Sr. but not pla gi oc lase c um ulati ve).

Furtherrnore, panial melung of ultcrcd Crct uccous MO RB. tcctonically accrctcd to the lowcr crust, was pr oposee by Kilian et al. (1995) 10 explain thc gcncsis of SOI1lC Sr-rich and ésit es l'rom the Chimborazo complex. Thus, AFC proccsscs sccrn la he acti ve at the h'1SC of the cru st bcneuth most Riobamba

vo lcanocs. W ork is currc ruly bcing donc la co nstrain thc sc proccsscs and beu cr und cr stand the magrnati c evolution of the wh ole arca,

REFERENCES

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