ISOTOPIC PATTERNS IN SILICIC IGNIMBRITES AND LAVA FLOWS OF THE MOGAN AND LOWER FATAGA FORMATIONS, GRAN-CANARIA, CANARY ISLANDS - TEMPORAL CHANGES IN MANTLE SOURCE COMPOSITION

We report the Sr, Pb, and Nd isotopic composition of thirty-six intercalated extracaldera silicic ignimbrites and basaltic lavas of the Miocene Hogarzales, Mogan, and Fataga Formations, Gran Canaria, Canary Islands. The aims are to constrain petrogenetic models for the silicic volcanics, and determine mantle source characteristics and temporal variations between 14.2 and ∼ 12.1 Ma. Feldspars from the extracaldera silicic ignimbrites are identical in isotopic composition to coeval extracaldera basaltic lavas, supporting a fractional crystallization model for the evolved lavas from parental Hogarzales basalts. 87Sr 86Sr ratios range from 0.70306 to 0.70341, 206Pb 204Pb from 19.32 to 19.90, 207Pb 204Pb from 15.56 to 15.65, and 208Pb 204Pb from 38.82 to 39.65. 143Nd 144Nd ratios are nearly constant at 0.512913 ± 15. The source of Gran Canaria magmas is heterogeneous on small scales of both time and distance. Isotope-isotope and isotope-incompatible element plots suggest mixing between well-mixed, slightly enriched mantle (similar to PREMA as defined by Zindler and Hart (1986) [1]) and the HIMU mantle component. The proportion of HIMU component (low 87Sr 86Sr, high 206Pb 204Pb) increases upsection. Stratigraphic patterns in major, trace element, and isotopic compositions may be explained by the influx of a geochemically distinct "Fataga" magma into the Tejeda magma chamber, which mixed with and/or finally completely displaced existing "Lower Mogan" magmas [2,3]. Alternatively, mixing of these two end members could occur in the mantle, prior to injection into the chamber. There is no evidence of lithospheric/asthenospheric contamination in the late-stage shield magmas on Gran Canaria. © 1990.