Northwestern Patagonia Holocene Basalts As Indicators Of An Incipient Continental Fragmentation

HALLER MJ; MASSAFERRO GI; ALRIC VI

Chieti

Congreso; FIST, GeoItalia 2001; 2001

FIST

Late Cenozoic to Recent volcanism in Northwestern Patagonia is restricted to arc magmatism in the Andes (Hornopirén, Hualaihue and Yate volcanoes) and the Somuncura continental flood basalt volcanism province. Located between those two areas, the isolated 257 km2 Crater Basalt (CB) field consists of at least nine strombolian centers that erupted basaltic magma above fluvioglacial terraces.  Each vent produced 4-6 individual lava flows with vesicular aphanitic textures, in some cases containing mm-size olivine xenoliths. These small-volume, fairly primitive basalts are located in an intermediate position between the arc and the earlier flood basalts and are the only recent basalts in Northern Patagonia. In thin section, the rocks show porphyritic to microporphyritic textures with olivine phenocrysts in an intergranular to intersertal groundmass.  Olivine phenocrysts are euhedral to subhedral, in some cases embayed, with sizes not exceeding 3,5 mm.  Inclusions of magnetite, transparent minerals and others of gaseous nature are common.  The groundmass comprises plagioclase microlites of andesine to labradorite composition.  Among the mafic minerals, titaniferous augite, olivine and opaque minerals (magnetite and ilmenite) are observed.  Some differences among the lavas from the diverse vents are observed.  At Cerro Guacho, lavas show clinopyroxene (titaniferous–augite) and orthopyroxene xenocrysts.  The latter show a clinopyroxene reaction rim  as a nonequilibrium product.  Cerro Antitruz lavas are pyroxene free.  Cerro Volcán eruptive vent lavas contain elliptical quartzite xenoliths, up to 100 mm size. Major and trace element compositions of lavas from each vent are distinct and resemble the above-mentioned petrographic differences.  CB lavas are typical alkaline basalt (AB) and hawaiites with metaaluminous characteristics. CB lavas trace element compositions are comparable to thoseof ocean-island basalts (OIBs), with relatively high incompatible-element concentrations.  The Zr/Nb ratio is low (£ 9).  The rocks show low LILE/HFSE and lack of Nb and Ta negative anomalies.  REE patterns display light-REE enrichment relative to the heavy-REE and no Eu anomaly. Incompatible element abundance, normalized to primordial mantle, shows enrichment of incompatible elements relative to more compatible ones, a relative trough at Th and a smaller peak at Ta, which suggests an enriched source .  Low Zr/Nb ratios (5-9) and high Th/U ratios may also imply an enriched mantle source.  On the other hand, Y/Nb and Zr/Nb relationships suggest some degree of crustal contamination.  MgO (7-9%), Cr (136-268 ppm) and Ni (25-174 ppm) contents show olivine, pyroxene and magnetite fractionation.  This would indicate a fractionation process on mantle-fused material.  High Sr contents and the absence of an Eu anomaly would imply that plagioclase fractionation was less important. Holocene CB lavas show spidergram patterns closely similar to the Neogene Somuncura basalts (Gorring et al., 1997), which suggests Somuncura and Crater basalts have sources with similar geochemical characteristics.  Nevertheless, the small volume of CB eruptive products, its explosive history and alkaline basic nature may indicate a short-lived aborted low-volcanicity continental plate rift (Barberi et al. 1982).  As characteristic in passive rift areas, lithospheric stretching could have forced an enriched mantle to rise, giving origin to volatile-rich magmas.  Changes in the subduction angle or subduction rate could be responsible for this short-lived extensional phase that led to an incipient continental fragmentation associated with basaltic volcanism.