293 THE PLUTONIC ROCKS OF THE SOUTHERN GERLACHE STRAIT, ANTARCTICA: GEOCHRONOLOGY, GEOCHEMISTRY AND MINERALOGY Parada Qki Jem SapWe”, Ada Rlcanb’. Guerm Rslsp. Muniiaga Francisco*. Berg Karstan’. l Cepartamenb da Geokafa Y Geoffsiy Unkersided da Chib. Casitfa 13518, cons0 21, Sawlago. Chite. ‘* Maratofre 6 GaobgfPekobgfe. FacutaOdes Scbnma et Techniques. 23. rue du Docteur Paul Mbhebn ,4023 .%it Elienrm. tXdex, Frano. Reaumen Las mcas ptut6nk.a~estudiadas se cam&&an por una dsminu&n hwia el 0~s.~ ds tar adadas (Cretktm inferic+ Mibcw~~). Gaoqlmicamen te son simiiares am-qua sn tos granitofdes CretAdt se datecta una ten&n& de aumentar at StO, hacfa et es% Esknaknes de h pmfunddad & la hmnti y da mphmmienb rugiers VI aumentc hada et oesw. Key wor&: F%Jtism, An- geochmrmfogy. gecdwmtsby. Introduction The inbusi~ rocks of the G&ache Strait induda gabrros. diirites. tonafites. granodiirites and granites (Akro5n et at. 1976). which form part01 the extensiw plutonism developed in Ihe Antarctic Peninsula as a pmductof a continwus subducbn since Early Mesozoic to Lals Tertiary (cf. Parkhurst 1962). Prevfour studier dstinguished pru and post-vokanic rocks ptutons (West, 1974) and reported radiietric eges that indicate Cretaceous ages for the Dance Coast rocks and Tertiary ages for the Wrenke Is. andSouthern Anversls. (Scott, 1965; Par&hunt, 1482: British AntucScSurvey. 1964). Thusaweshvarddecreasing ages of tie pfulonio mdts is insinuated. The aim of this study is to get a better undarstandrg of the poorly known plutonic history of tie Geriache Strait, based ongeochronotogical. ge~emicalandmineralogicaldalaofalimitednumberof samples. ThisstudywastinmcedbythetNACH grant 8.17, and is a conbibu%n to the IGCP Profact 249 ‘An&m Msgmatism’. Geochronology Three previous K-Ar ages of plutonic rocks 01 he Dan00 Coast range from 117 to 94 Ma (cf. Scott. 1965, Sriksh Antarctic Survey. 1964) and two Rt&r iscchrm ages gave of 131 f4 and 114 fll Ma (Pankhurst. 1982). A new biotite K-Ar age of 113 f3 Ma was obtained in a hornblende - biotke gram&rite of tie SE extreme of the Anvord bay (fig. 1). Mineral and whole rock K-Ar ages in the 54-49 Ma range reported by the British Antarctic Survey (1964) gedogfcat map in the Doumerlslardwereconf!kmedby twonewbiititeK-Aragesof 50.2f1.4 and55.7ii.6obtainedatSE Doumerlslandand Lockroy Portin the Wmncke lstandreapecttvety (fig. 1). Earfy Temarybiitite K-Aragesof66i2and54.2f1.4 Maweruobtained at IheNEextmeof Anvers Island. Atonatitecoltectedatthe Pafmerbasegaveabiotita K-Arageof 20.4 fl Ma,whfchissimiiar b the previous K-k agas obtained h the surroundng areas. Wtm the exwptkan of a Late Cretawous Rb-Sr age of Ihe Cape Monaco granites (cf. Pankhurst, 1962) on the extreme SWof Anven Isfand. awestwardmigrationoftheptutonism withtimeisconfimted. Thelimitbetwwn~MesoroicandCemroic plutonic rocks was roughly defined in fig. 1, Geochemistry Wh. the Mesozoic and Cenozoic intrusive rocks exhibit a wida Sf, range, fall in the VAG f&d of Pearce et al. (1964). and have cabalkaline characteristics. The Cretaceous Dance Coast granitoids show and overatl in-ease in SiO, to meeast, and the intarmedate Cenozoic rocks have the highest REE abundances and the bwert LaNb ratios. Takenl~ether~eMesotoicandCanozo~samples,theYcontentsexhibitamaximumat~eintermediatemmposibns whereas Nb and Zr show. respectively, a well and poorly defined positive correlaticn with S10r. The behaviour of Y. Nb and Zr. and the presence of negative Eu anomalies in the intermediate and felsic rocks, suggest adenvation from a mc+a barb magma by dinopyroxane + piagiodase removal. .-. “__ .__ _.,”__jl_ --a.. _- 294 Mall0 Mineralogy Most of SW analyred pyroxener au Ca-rid! dfnopyroxene: orU~opyroxanewas only found in a Cretaceous granitoid sampb. Udng he geobaromelw based on tie mole fraction Tschermaks of dinopyroxena in equilibriw with plagiodase and guvtz (EUia. 1980). Um prearutw omdbns of he Ter3ar-ymagma satme. or pause region. in- 8Jgleweclfrcm4lDe’.bu. ANVERS Is. Crotacrour plutonic roctr Tortiory plutonic rocks $0 5 0 , (0 I, 20 2s 111 L ! : : : : : 1 Fg. 1. birtribubn of ho plutonicrocha in the Gedache Strait. modifiedaher Alarc6n et d. (1976). Numben indiilo radffmetrk ages: endosed numbers are 2m ages obtained in ais study. Moat of ti anafyred amphibolesaro magnesiohornbfende. The remainder ones correspond lo ferm-hombfenda and actfnolilichornblende. The pressure condilims of the hornblends aysfaflizalfon. using the Hammarsbom and Zen’s (1oeS) geobarome~r. twghfy increase lo the west from 1 lo 2.7 kbar. No variations have been detected in the biotitecompositions. The anafyzed biitilec have mde fraction annim (X_) of 0.4 - 0.5 with the exceplimr of an Early Tertiary sampk Ural has X, equaf ID 0.6. In addbn. no vatiaticnr have been found in the oxygen fugacity condlions (lo-) f~ tie biotibscryslaftization. Conclualone The pfutonicdevelopment of the Gerlache Strait is characterked by: 1)Awestwarddecreasingagesfrom Early CretaceourtoMiomne. Twononplutonicinlsrvalr~and~X1~emer(lehom the avaitabfe geochrondogicd data. 2) Caloaikaline productaexhibiting a wide ccmpositfmd range, and on overall eastward Sii2 in the Cretawous Dana, Coast graniloids. Tha geochemicd rimikritiea between rocks with similar Si02 but different ages, suggest a mmmm sour- md simifar magmak evdulion. ..1- . .II__^ 295 Refwonooe Afaroh. E.. Ambrur. J.. Cicay. L.. and Viiin. C. (1976). Geobgla del Estrecho de Qedadw enbv br pwalebr 64’ y 5!Y Sv, AntMia Chife~. Serb Cientffiiu. InstibllDAnt6rtbo Chileno. v. 1. pp. I-51. British Antarclb Surwy (1984). British Antardb Tenflay gwbgicd map. oedooy ampifed by Thompson nd o&n. Cambridge, &itirh Anfardb Sunmy. Ellis. D.J. (1950). Osumifi&-rapphitinsqwtz grmufifer fmm Endew Land, Antrctba: P-T cwditknr of metamorphirm. lmplicatb~ for gamet-uxderfto equilibria and UN wduUon of the depth Cruft. CenribuUonr t) Mbembgy and Pelrdogy. v. 74, pp. 201-210. Hammarsbum. J.M. nd Zen. E-an. (1955). Aluminum in hombfende: an empiricA igneour geobammebx. American Mnwabgb~ v. 71, pP. 1297-1313. Pankhurst.R.J.(1982). f3b-SrgeoduondogydGrahamknd.&w~fa. JoumafofOteGmbgbalSodetydLondon,v. 139. pp. 701-711. Pearw. J.A., Harris. N.&W. andTmc#e, H.Q. (1954). Traceebmentrdrcaiminot#nd~~mrfor~#1DnicinOerprstadonof granicicmdu. Journal of Petrology. v. 25, put 4. pp. 955-983. SCOII.K (1965). Gedogy of the rouUwn G&ache Stail Regii. Anlnakr. The Joumd of Geobgy. v. 73.3. pp. 518-527. West, SM. (1974). The geobgy of m0 Dance Coast Grdwn Land. Sdentib Report, SrilfshAntardb Surrey. V. 5455 p. .