New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland

Abstract

The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith -the closure/saturation temperature of the monazite U-Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks.

The Vaasa Migmatitic Complex (Svecofennian Orogen, Finland): Buildup of a LP‐HT Dome During Nuna Assembly

“…Both the granitoids (mostly granites and granodiorite) and the diatexites of the inner part are peraluminous, calc‐alkalic to alkali‐calcic and magnesian, enriched in Light Rare Earth Elements (REEs) with a moderate to strong REE fractionation, mostly minor negative Eu anomaly, and with a distinct spread in the heavy REE contents (e.g. Kotilainen et al, ; Mäkitie et al, ).…”

“…There seems to be indefinite pattern in the spatial distribution of the ages. The central inner parts of the complex tend to display younger zircon crystallization ages than those further away from the center (Kotilainen et al, ), and age difference between the zircon and monazite ages grows toward the center of the VMC (Kotilainen et al, ). It is interpreted to indicate progressively slow cooling from the margin to the center of the VMC (Kotilainen et al, ).…”

“…The central inner parts of the complex tend to display younger zircon crystallization ages than those further away from the center (Kotilainen et al, ), and age difference between the zircon and monazite ages grows toward the center of the VMC (Kotilainen et al, ). It is interpreted to indicate progressively slow cooling from the margin to the center of the VMC (Kotilainen et al, ). There are no U‐Pb concordia zircon age data available from the mica schists in the outer part of the VMC, but the zircons from both the xenolith schists and from the VMC granitoids give metamorphic ages of 1.88–1.86 Ga, similar to the magmatic U‐Pb ages in the area (Kotilainen et al, ).…”

“…The equilibrium temperature of migmatites is bracketed at 600–720 °C at about 5 kbars based on conventional geothermometers (garnet‐cordierite) (Lehtonen et al, ). This paroxysmal tectonothermal event has taken place at 1.88–1.87 Ga (Korsman et al, ; Kotilainen et al, , ; Mäkitie et al, ; Suikkanen et al, ). During postorogenic event at ca.…”

“…Metamorphic conditions of 3.5–4.5 kbars and 570 °C are estimated in St‐Grt‐And‐bearing metapelites from the eastern border of the VMC in Zone I (see also Hölttä & Heilimo, ). Zircon and monazite saturation temperatures from the granitoids of Zone IV fall in the 805–875 °C range (Kotilainen et al, ). Figure b shows selected X Mg isopleths of garnet, ranging from 0.20 to 0.30 in Zones III and IV (Mäkitie et al, ).…”