JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B4, PAGES 4619-4633, APRIL 10, 1989 COEXISTING CALCALKALINE AND HIGH-NIOBIUM BASALTS FROM TURRIALBA VOLCANO, COSTA RICA: IMPLICATIONS FOR RESIDUAL TITANATES IN ARC MAGMA SOURCES Mark K. Reagan Department of Geology, University of Iowa, Iowa City James B. Gill Earth Sciences Board, University of California, Santa Cruz Abstract. Turrialba volcano, the southeastern- most volcano in the Central American arc, is constructed of medium to high-K calcalkaline basalts, andesires, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high-Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc-thol- eiitic basalts. The association of calcalkaline and high-Nb basalts is rare in arc front volca- noes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high-Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab-derived fluid was present during calcalkaline basalt genesis but not during high-Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO 2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid-ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end-members for both the calcalkaline and the high-Nb basalts. The calcalkaline basalts represent mix- tures of the high-degree melts and oxidized small-degree melts generated by amphibole break- down in mantle overlying the subducting slab. This small-degree melt has high incompatible element concentrations and is saturated in rutile. Arc-related lamprophyric rocks have compositions that are appropriate for these small-degree melts. High-Nb basalts are mixtures of the high-degree melts and more reduced small- degree melts that are undersaturated in rutile. with oceanic intraplate basalts (OIBs), but they differ from OIBs as well as mid-ocean ridge basalts (MORBs) in their ratios of large ion lithophile (LIL) to light rare earth (LRE) elements (e.g. Ba/La: arc>15>OIB,MORB [Perfit et al., 1980; Gill, 1981; Morris and Hart, 1983]) and LRE to high field strength (HFS) elements (e.g. La/Nb: arc>2>OIB,MORB [Perfit et al., 1980; Gill, 1981]). Moreover, ratios of HFS elements in arc basalts are often more similar to those of MORBs than OIBs (e.g. Zr/Nb: MORB,arc>20>OIB [Sill, 1981]). Although nearly all petrolsgists concede that the subducting slab is involved in arc magma genesis, the way in which it is involved is vigorously debated. Some researchers suggest that the MORB-like HFS element patterns of arc vol- canics reflect the depleted nature of the mantle below arcs and a high percent fusion. Explana- tions of this type require the high concentra- tions of other incompatible trace elements to result from extensive enrichment of the mantle by melts or fluids from the subducting slab [Kay, 1980; Perfit et al., 1980; Gill, 1981; Tatsumi et al., 1983; Perfit and Kay, 1986, Ellam and Hawkesworth, 1988]. Others suggest that residual TNT phases (Ti-Nb-Ta, e.g., rutile, limehire, and perovskite [Arculus and Powell, 1986]) elevate the partition coefficients of HFS, and sometimes LRE elements, during melting of more enriched OIB source mantle [Saunders et al., 1980; Morris and Hart, 1983, 1986; Arculus and Powell, 1986], thereby depleting these elements with respect to LIL elements. Some of these explanations addi- tionally require minor enrichment of some LIL elements in the mantle. Some arc-related volcanoes erupt calcalkaline basalts (CABs) with the characteristically high Ba/La and La/Nb ratios of arcs, together with high-Nb basalts (HNBs) which have Ba/La and La/Nb ratios more like those of OIBs. One such volcano is Turrialba in Costa Rica, which has erupted high-K CABs together with volumetrically minor These reduced melts may migrate around or through HNBs through the same conduit system at approxim- the subducting slab into the wedge to become involved in arc magma genesis. Introduction Basalts from arc settings have many incompati- ble trace element and isotopic ratios in common Copyright 1989 by the American Geophysical Union. Paper number 88JB03909. 0148-0227/89/88JB-03909505.00 ately the same time. The presence of interbedded CABs and HNBs in the edifice at Turrialba provides the unusual opportunity to evaluate the reasons for differ- ences between basalts from intraplate and arc settings. The question addressed here is whether the CABs reflect residual TNT phases in a rela- tively enriched mantle source like that consi- dered responsible for intraplate basalts, or recent wholesale modifications by slab-derived fluids of a more depleted MORB source. The trace element and isotopic similarities of Turrialba's 4619