Crustal sources involved in continental arc magmatism: .... dean arc basalt production is more difficult to ..... Press, and Joan Burry at Memorial University for.
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Crustal sources involved in continental arc magmatism: A case study of volcan Mocho-Choshuenco, southern Chile Nancy J. M c M i l l a n * Department of Geology and Geography, Eastern Illinois University, Charleston, Illinois 61920 Russell S. H a r m o n * Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 Stephen Moorbath Department of Earth Sciences, University of Oxford, Oxford 0X1 3PR, England Leopoldo Lopez-Escobar Departamento de Geología y Geofísica, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile D. F. Strong Department of Earth Sciences, Memorial University, St. Johns, Newfoundland A1B 3X5, Canada
ABSTRACT Volcan Mocho-Choshuenco, a Pleistocene-Holocene basaltic andesite to dacite shield volcano located at lat 40°S in the Southern Volcanic Zone (SVZ) of the Andes, is built on the thinnest SVZ continental crust (»30 km), which consists largely of Paleozoic sedimentary and metamorphic rocks and Mesozoic plutons. Sr, Pb, Nd, and O isotopic ratios of MochoChoshuenco lavas ( 8 7 Sr/ 8 6 Sr = 0.70399 to 0.70422; 2 0 6 P b / 2 0 4 P b = 18.57 to 18.61; 2 0 7 P b / 2 0 4 P b = 15.60 to 15.63; 2 0 8 P b / 2 0 4 P b = 38.45 to 38.57; I 4 3 Nd/ 1 4 4 Nd = 0.512787 to 0.512881; 5 1 8 0 = +5.8°/oo to +6.8°/oo Standard Mean Ocean Water) are similar to those of other SVZ centers and exhibit no correlation with indices of differentiation. However, incompatible trace-element systematics indicate that evolved lavas have been contaminated by crustal plutonic rocks by 5% to 15%. Rb/Ba, K/Ba, Rb/La, and K/La ratios increase with increasing S i 0 2 content as a result of assimilation of orogenic calc-alkalic plutonic rocks enriched in Rb and K relative to Ba and La. Xenoliths of plutonic rocks recovered from Mocho-Choshuenco lavas are slightly more radiogenic in 8 7 Sr/ 8 6 Sr, less radiogenic in 1 4 3 Nd/ 1 4 4 Nd, and comparable in Pb isotopic ratios, and they have variable 1 8 0 contents ( ^ S r / ^ S r = 0.70482 to 0.70557; 2 0 6 P b / 2 0 4 P b = 18.60 to 18.64; 2 0 7 P b / 2 0 4 P b = 15.60 to 15.62; 2 0 8 P b / 2 0 4 P b = 38.53 to 38.56; 1 4 3 Nd/ 1 4 4 Nd = 0.512686; S 1 8 0 = +3.3%o to +5.5°/oo SMOW). Contamination of Mocho-Choshuenco parental magmas by crustal melts with small isotopic contrast imposed little variation on isotopic heterogeneities existing in the parental compositions. These heterogeneities probably were produced during magma-crust interaction at the crust-mantle boundary or in the lower crust. INTRODUCTION The variety of subducted slab, mantle, and continental crustal sources involved in the genesis and evolution of Quaternary volcanics in the Andean Southern Volcanic Zone (SVZ; lat 33°-42°S) has been a matter of intense debate (Davidson et al., 1987, 1989; Frey et al., 1984; Gerlach et al., 1988; Grunder, 1987; Harmon et al., 1984; Hickey et al., 1984, 1986; HickeyVargas et al., 1989; Hildreth and Moorbath, 1988; Lopez-Escobar, 1984; Lopez-Escobar et al, 1977, 1981). Discussions center on two issues: the extent of crustal contamination during the evolution of andesites and dacites, and the relative contributions of slab, mantle wedge, and
*Present addresses: McMillan—Department of Earth Sciences, New Mexico State University, Las Cruces, New Mexico 88003; Harmon—NERC Isotope Geology Centre, 64 Gray's Inn Road, London, WC1X 8NG, England.
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continental crustal sources to parental mafic magmas. The SVZ can be divided into two geographically distinct regions (Thorpe, 1984): the southern SVZ (lat 37°-42°S), where the continental crust has a relatively uniform thickness of 30-35 km, and the northern SVZ (lat 33°-37°S), where crustal thickness increases northward to 50-60 km at lat 33°S (Hildreth and Moorbath, 1988). Individual southern SVZ centers are characterized by restricted 8 7 Sr/ 8 6 Sr over a wide range of Si02 concentrations (Hickey et al., 1984). Until recently, this has been regarded as an indication of the limited extent of crustal contamination. Most investigators agree, however, that mantle-derived basalts are modified at crustal pressures by fractional crystallization and, perhaps, magma mixing (Frey et al., 1984; Gerlach et al., 1988; Hickey et al., 1984; Hickey-Vargas et al., 1989; Lopez-Escobar, 1984; Lopez-Escobar et al., 1977,1981). Recent
work based on trace-element and O isotopic data in the northern SVZ (Davidson et al., 1987; Grunder, 1987; Hildreth and Moorbath, 1988) indicates that crustal contamination is an integral process in the production of magmas in this region and that restricted ranges in isotopic composition occur when parental basalts and crustal contaminants have similar isotopic signatures. The dispute over the sources involved in Andean arc basalt production is more difficult to resolve because the compositions of various components are poorly constrained. Hickey et al. (1984, 1986) and Hickey-Vargas et al. (1989) emphasized the importance of slabderived components with enriched asthenosphere or lithosphere in the genesis of arc basalts with trace-element and isotopic heterogeneities that cannot easily be ascribed to crustal contamination. On the other hand, Hildreth and Moorbath (1988) and Davidson et al. (1987, 1989) stressed the importance of continental crust in a deep-seated assimilation process, dubbed MASH (melting, assimilation, storage, and Aomogenization) by Hildreth and Moorbath (1988). During the MASH process, parental magmas acquire their inherent isotopic and elemental compositions; shallow-level processes (fractional crystallization with or without assimilation) modify these "baseline" values. We have examined the major-element, traceelement, and isotopic variations of lavas from volcan Mocho-Choshuenco, a basaltic andesite to dacite volcano in the southern segment of the SVZ (lat 40°S), in order to evaluate the contribution of crustal and subcrustal components in SVZ magma evolution. We find that the pedogenesis of these magmas consists of at least three identifiable stages: production of primary magmas in the mantle wedge with a slab-derived component, complex differentiation at the mantle-crust boundary, and fractional crystalliGEOLOGY, v. 17, p. 1152-1156, December 1989
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