  We determine detailed elemental abundances in stars belonging to the
so-called Group 1 of the Geneva-Copenhagen survey (GCS) and compare the
chemical composition with the Galactic thin- and thick-disc stars, with the GCS
Group 2 and Group 3 stars, as well as with several kinematic streams of similar
metallicities. The aim is to search for chemical signatures that might give
information about the formation history of this kinematic group of stars.
High-resolution spectra were obtained with the Fibre-fed Echelle Spectrograph
(FIES) spectrograph at the Nordic Optical Telescope, La Palma, and were
analysed with a differential model atmosphere method. Comparison stars were
observed and analysed with the same method. The average value of [Fe/H] for the
37 stars of Group 1 is -0.20 +- 0.14 dex. Investigated Group 1 stars can be
separated into three age subgroups. Along with the main 8- and 12-Gyr-old
populations, a subgroup of stars younger than 5 Gyr can be separated as well.
Abundances of oxygen, alpha-elements, and r-process dominated elements are
higher than in Galactic thin-disc dwarfs. This elemental abundance pattern has
similar characteristics to that of the Galactic thick disc and differs slightly
from those in Hercules, Arcturus, and AF06 stellar streams. The similar
chemical composition of stars in Group 1, as well as in Group 2 and 3, with
that in stars of the thick disc might suggest that their formation histories
are linked. The chemical composition pattern together with the kinematic
properties and ages of stars in the investigated GCS groups provide evidence of
their common origin and possible relation to an ancient merging event. A
gas-rich satellite merger scenario is proposed as the most likely origin.
