High-precision isotope dilution - thermal ionization mass spectrometry
(ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically
layered mafic intrusion Fuerteventura, Canary Islands, indicate
initiation of magma crystallization at 22.10 +/- 0.07 Ma. The magmatic
activity lasted a minimum of 0.52 Ma. Ar-40/Ar-39 amphibole dating
yielded ages from 21.9 +/- 0.6 to 21.8 +/- 0.3, identical within errors
to the U-Pb ages, despite the expected 1% theoretical bias between
Ar-40/Ar-39 and U-Pb dates. This overlap could result from (i) rapid
cooling of the intrusion (i. e., less than the 0.3 to 0.6 Ma 40Ar/39Ar
age uncertainties) from closure temperatures (T-c) of zircon (699-988
degrees C) to amphibole (500-600 degrees C); (ii) lead loss affecting
the youngest zircons; or (iii) excess argon shifting the plateau ages
towards older values. The combination of the Ar-40/Ar-39 and U/Pb
datasets implies that the maximum amount of time PX1 intrusion took to
cool below amphibole T-c is 0.8 Ma, suggesting PX1 lifetime of 520 000
to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon
(22.10 +/- 0.07/0.08/0.15 Ma and 21.58 +/- 0.15/0.16/0.31 Ma) in a
gabbro sample from the pluton margin suggest complex genetic
relationships between phases. Baddeleyite is found preserved in
plagioclase cores and crystallized early from low silica activity magma.
Zircon crystallized later in a higher silica activity environment and is
found in secondary scapolite and is found close to calcite veins, in
secondary scapolite that recrystallised from plagioclase. close to
calcite veins. Oxygen isotope delta O-18 values of altered plagioclase
are high (+7.7), indicating interaction with fluids derived from
host-rock carbonatites. The coexistence of baddeleyite and zircon is
ascribed to interaction of the PX1 gabbro with CO2-rich
carbonatite-derived fluids released during contact metamorphism.