A LA-ICP-MS STUDY OF CARBONATITES FROM FUERTEVENTURA, CANARY ISLAND

Carbonatites complexes are very rare in oceanic environments and in the Atlantic Ocean they can be only found at Cape Verde islands and Fuerteventura. Fuerteventura is the second largest island in the Canary Archipelago and is located on a transitional, continental to oceanic, crust. It consists essentially of Mesozoic sediments, submarine volcanic rocks, subaerial basaltic and trachytic series, ultramafic, mafic to felsic intrusives (clinopyroxenites, melteijites-ijolites, nepheline-monzogabbros, nepheline-sienites) and carbonatitic dike swarms (age 25 Ma). Carbonatite dike (Ca-carbonatites) mineralogy consists of calcite, aegirine-augite, albite, K-feldspar, biotite, apatite, Fe-Ti oxides and accessory minerals, such as zircon, barite, monazite and pyrochlore. The degree of alteration of carbonatites (evaluated by DTA and XRD) is generally low, with the occurrence of illite-montmorillonite mixed layers, vermiculite and chlorite. Whole rock samples (XRF) are high in CaO (> 50 %) and SrO (> 2-3 %) and very low in MgO (< 1-2 %). Trace elements were determined by LA-ICP-MS on calcite and accessory minerals (pyrochlore, monazite, zircon). Results show in calcite phenocrysts REE patterns highly enriched in all REEs (sum REE= 1186-2943 ppm) and particularly in LREE: LaN = 2178 (chondrite normalized), CeN = 1499, with respect to HREE (YbN = 37, LuN = 34) and rather fractionated pattern (La/Yb)N = 58. Small negative Eu anomalies do also occur (Eu/Eu* = 0.77 - 0.94) and these are coupled with high Sr anomalies (Sr/Sr* = 28.3 - 58.7). REE in pyrochlore are extremely enriched in REE (sum REE= 12.8 wt %) with very high LREE (LaN = 71214 and CeN = 62803) and HREE (YbN = 997, LuN = 695) with a slightly steeper pattern if compared to calcite: (La/Yb)N = 71. No significative Eu anomalies were found (Eu/Eu* = 1.06), while there is a conisistent Sr positive anomaly in spiderdiagrams (Sr/Sr* = 1.74). Preliminar results show that pyrochlore is the main mineral repository of REEs and its occurrence, even in trace amounts, gives the fingerprint to trace element to whole rock.