GOLDSCHMIDT CONFERENCE EDINBURGH 1994 639 Geochemistry of marie magmas from Sollipulli , southern : constraints on models of magmagenesis in the Southern Volcanic Zone of the

M.D. Murphy Department of Geology, Bristol University, Bristol, UK. T.S. Brewer Department of Mineral Resources Engineering, University of Nottingham, Nottingham, UK.

Introduction Geochemistry of sollipulli marie magmas The Southern Volcanic Zone (SVZ) of the Andes The rocks under discussion all have SiO2 < 54% extends from 33~ to 46~ and can be divided into and MgO > 5%. At a given SiO2 content, LREE, two main regions. In the segment north of 37~ Nb, Hf, Ba, Zr, Y and P are highly enriched in the (NSVZ) volcanism tends to be intermediate to high- abundance compared to the low-abundance silicic, due to the influence of the thick continental magmas at Sollipulli. K, Na, Ti, Sr, Nd and Sm on mantle-derived magmas. In the southern show pronounced enrichments. Rb is the least region (SSVZ) marie to intermediate magmatism is enriched of the incompatible trace elements. Fe/ the norm. Hickey-Vargas et al. (1989) distin- Mg, AI/Ca, K/Na and Lapfb are all distinctly guished two marie magma types in the SSVZ. higher in the high-abundance magmas. A feature The first type has a typical arc-like signature with of the depleted rocks is their chemical homo- high LILE/LREE and LILE/HFSE and is geneity compared to the enriched rocks, which associated with a western belt of volcanoes. The show considerable scatter. second type has high abundances of LILE, LREE There are no discernible differences between and HFSE but lower ratios of LILE to LREE and the phenocryst assemblages or mineral chemistry HFSE than the first type and is characteristic of an of the different types. Both have easterly belt of volcanoes, as well as satellite cones (Ans3-ss), (Fo72-s3) with low-pressure of some of the western stratovolcanoes. Two main spinel inclusions and rare clinopyroxene. In models were proposed to account for the rocks with < 55% SiO2, titanomagnetite is differences between the two magma types. The absent. Total modal abundances of phenocrysts first model involved small degree melts of the vary from 5-20%. Attempts to quantify differ- continental lithosphere coupled with a declining ences between the two types in terms of influence from the subducted slab in moving temperature, PH~O and fo2 by crystal-liquid eastwards. The second model considered the equilibria have not been definitive but indirect differences to be related to large scale inhomo- evidence suggests that the enriched types are more geneities in the fluids released from the down- hydrous. going Nazca plate. Sollipulli volcano is situated at 39~ about 20 Petrogenesis km east of the main volcanic front. It is anomalous in the SSVZ in that it has erupted large quantities Crustal assimilation is not considered here to be a of silicic magma, predominantly . Two distinct fundamental control on the evolution of the two groups of basaltic correlate with the different magma types, although assimilation has regional high and low abundance groups. Both influenced the geochemistry of more evolved types have been erupted during a single event. To magmas at Sollipulli. Hickey-Vargas et al. (1989) the authors knowledge, the two magma types have presented strong evidence for a mantle rather than not been previously documented at a single crustal origin for the enrichments which char- volcanic centre with a definite temporal associa- acterise the two magma types. At Sollipulli, the tion. Strong constraints can therefore be placed on fact that Rh shows the least relative enrichment is previous petrogenetic models. The evidence from a further argument against crustal influence. Rb Sollipulli makes any hypothesis based on regional has been used as an indicator of crustal homogeneities in the Nazca plate or simple control assimilation at other SVZ centres (Davidson et by crustal thickness untenable. al., 1988) and tends to have high values in 640 MINERALOGICAL MAGAZINE, VOLUME 58A contaminated mafic magmas. with high AI/Ca, K/Na and LREE/HREE relative Compared to other SSVZ volcanoes, depleted to the source. Differences in AI/Ca have Sollipulli magmas have lower abundances of many previously been attributed to fractionation of the elements listed above. The depleted rocks processes at crustal depths. The higher Fe/Mg have the highest MgO/FeO ratios in the region. and more hydrous nature of the enriched melts are The difference between the regional low- abun- also explicable in terms of this model. dance magmas and those at Sollipulli are simply In this model, the tholeiitic nature, combined explained by higher degrees of partial melting in with the high AI and alkali contents of the SSVZ the mantle wedge generating the Sollipulli high-abundance magmas is inherited from the magmas. High degrees of melting could generate melting process. The rocks are transitional in large magma batches. This provides an explana- many respects between tholeiites and alkali tion for the homogeneity of the depleted lavas as . Many low-abundance SSVZ rocks are well as accounting for the abundance of silicic tholeiitic but have low AI and LILE. Different lavas at Sollipulli, whether they have originated by trends among these rocks can be explained by fractionation or some mechanism of assimilation. fractionation effects, controlled by Ptotal, PH20 and Hickey-Vargas et al. (1989) argued convin- fo2, without invoking different parental magmas. cingly against the high-abundance SVZ magmas The model may be applicable to some other arcs being derived by smaller degree partial melts of the where tholeiites are characterised by higher LILE, same source as the low-abundance magmas. Their LREE and HFSE abundances than associated continental mantle lithosphere model considers calk-alkaline rocks. For example, early tholeiites the high-abundance magmas to originate by small at Santorini volcano have certain similarities to degree melts in the lithosphere, mixing with the Andean rocks (L. Edwards, Bristol, unpub- wedge-derived magmas, containing a significant lished data). In many arcs, such as the NSVZ, slab component. This model is accepted here with divergent trends in Fe/Mg can be explained in the following extension. Previous authors have terms of fractionation of a single parental magma, tended to overlook the correlation between the with or without crustal assimilation. In these arcs, trace and major elements outlined above. It is calc-alkaline magmas tend to be more enriched in proposed here that the lithospheric melts are LILE and LREE than associated tholeiites. derived from a modally metasomatised source, containing minor Ti, P, REE, HFSE-bearing References phases. Stern et al. (1986) have documented mantle in basalts to the east which Davidson, J.P., Ferguson, K.M., Colucci, M.T. and have veins of ilmenite, phlogopite and amphibole. Dungan, M.A. (1988) Contrib. Mineral. Petrol., Small degree melts of such a source, mixed with 100, 429-45. low-abundance type magma can produce many of Hickey-Vargas, R., Moreno Roa, H., Lopez Escobar, the major and trace element characteristics of the L. and Frey, F.A. (1989) Contrib. Mineral. Petrol., high-abundance magmas. Incongruent breakdown 103, 362-86. of amphibole can produce residual clinopyroxene Stem, C.R., Futa, K., Saul, S. and Skewes, M.A. at small degrees of melting, producing magmas (1986) Rev. Geol. Chile, 27, 41-53.