I found one important reference in which is one age determination
with help of practical explanation how to separate
DETRITAL zircons from sedimentary rocks:
Reference:
A. Fralick Philip, Davis Don W. and Kissin Stephen A, 2002.
The age of the Gunflint Formation, Ontario, Canada:
single zircon U-Pb age age determinations from reworked volcanic ash.
Can. J. Earth Sci. 39: 1085-1091 (2002). Page 1087.
"Detrital zircons, all of size ca. 0.2 mm (in long dimension).
Divide zircons into three groups:
1. Clear, colorless, well-rounded zircons.
2. transparent, brown, euhedral zircons with no evidence of abrasion.
3. dark, cracked, and altered zircons.
Only group 2. is accepted to sediment formation dating purpose.
Group 2. = light (flying) ashes from volcanic eruption."
References related to the age problem of the Central Puolanka Group:
B. Huhma Hannu, Kontinen Asko and Laajoki Kauko, 2000.
Age of the metavolcanic-sedimentary units of the Central Puolanka Group,
Kainuu schist belt, Finland.
24. Nordiske Geologiske Vintermöte,
Trondheim 6.-9. januar 2000. Geonytt 1, 87-88.
(copy below from pages 87-88, samples A1292, A1250, A1251 explained)
"In order to constrain the age of the formation and provenance of the CPG,
zircons from five samples have been analyzed using NORDSIM ion probe.
Four of these samples are from the Kivesvaara area, where
basaltic to dacitic metavolcanic rocks are exposed.
Zircons analyzed from two highly sodic, felsic metatuffite-tuff
samples from the Pärekangas Formation (PkF),
the uppermost unit of the CPG, all yield Archean ages.
The data are, however, slightly heterogeneous presumably
due inherited zircon in felsic magma, and metamorphic effects.
DETRITAL origin for some grains in metatuffite cannot be definitely excluded.
For the more pure tuff sample (A1292), twelve grains out of the
17 analyzed yield an age of 2721+-12 Ma (3 grains being ca. 2.8 Ga in age),
which probably is close to the age of the felsic magmatism.
This interpretation is supported by conventional U-Pb zircon analyzes from
another tuff sample in the same locality (A1250), and by conventional
U-Pb age of 2600+-7Ma from a coarse-grained, similarly highly sodic, felsic
dyke rock (A1251) in CPG below the PkF.
Zircons analyzed from the metasediments of the
Puolankajärvi and Akanvaara Formations from the middle part of the CPG also
are all Archean in age, the bulk of the grains being about 2.73 Ga, many about 2.78 Ga and some 2.8 Ga to > 3.0 Ga (207_Pb/206_Pb ages).
The oldest encountered grain is about 3.5 Ga, being the oldest zircon found in
Finnish recks, so far.
The Sm-Nd data on the LTEE enriched metasediments and metavolcanics of the CPG
give T-DM model ages from 3.23 to 2.83 Ga.
Thus all isotopic data available indicate a totally Archean provenance for the CPG,
supporting the emerging concept of the CPG as a part of the Archean basement
structure instead of the Paleoproterozoic Kainuu Schist Belt.
The corarse-grained and immature (arkosic) nature of many of the
arenites in the CPG may imply nonrecycled derivation from a relative close source terrain.
As no middle Archean component is apparent in the Archean terrains east of CPG, the
SOURCE likely was in the west, possibly in the adjacent, chronostratigraphically still poorly
known Archean Pudasjärvi Complex."
C. Kontinen, A., Huhma, H., Laajoki, K., 1996.
Sm/Nd Isotope Data on the Central Puolanka Group, Kainuu Schist Belt,
Finland, Constraints for Provenance and Age of the Deposiition.
The 22nd Nordic Geol. Winter Meeting,
Turku-Åbo, 8-11, January 1996. Abstracts, p.95.
D. Huhma, H. et al. 2012.
Nd Isotopic Evidence for Archean Crustal Growth in Finland.
Geological Survey of Finland, Special Paper 54, 176-213.
20 figures and 1 appendix. Page 213.
" Central Puolanka Group (p. 213, appendix):
Sample: A1292, Haapalanmäki, dactic tuff, age 2700 Ma, epsilon_Nd=-1.6.
Sample: ATK-14B, Haapalanmäki, andesite, age 2700 Ma, epsilon_Nd=-0.0.
Sample: A1235, Kivesvaara, felsic tuffite, age 2700 Ma, epsilon_Nd=-0.3.
Sample: Petäjäniemi Paltamo, porphyry, age 2718 Ma, epsilon_Nd=+1.3."