Correlation between the Trans-Altai Zone and Mongolian Altai: Evidence from detrital zircon U-Pb and Hf isotopic data
Résumé
The Trans-Altai Zone (TAZ) represents a narrow linear segment of the Silurian–Devonian oceanic realm
in southern Mongolia (Helo et al.,
2006
; Rippington et al.,
2013
). It records the mid to late Paleozoic
geodynamic evolution of the Paleo-Pacific Ocean. In general, the TAZ is composed of several terranes
interpreted as island arcs, back-arc basins/forearcs and accretionary wedges (Badarch et al.,
2002
; Lamb
& Badarch,
1997
). Here, we present new structural, whole-rock geochemical and zircon U-Pb ages and
Hf isotopic data from the Carboniferous rocks of the TAZ to decipher the tectonic-sedimentary evolution
of this tectonic zone. The sedimentary rocks studied were collected from key units of the TAZ called the
Khuvinkhar and Zoolon subzones. The Khuvinkhar subzone marks the northernmost extremity of the TAZ
separated from the northerly Mongolian Altai by the Trans-Altai Fault (Kozakov,
2002
; Lamb & Badarch,
1997
). The Zoolon subzone represents the southernmost part of the TAZ limited by the Gobi-Tianshan
Fault (Cunningham et al.,
1996
) in the south. The rocks of the Khuvinkhar subzone preserve a low-grade
(D
2
) cleavage (potentially of Permian–Triassic age) that affected the bedding of Devonian–Carboniferous
strata. Ontheotherhand, therocksintheZoolonsubzonerecordedtheearlierdeformationevent(D
1
)as-
sociatedwithgreenschist-toamphibolite-faciesmetamorphismthatwasreworkedbyS
2
verticalcleavage.
However, the age of the D
1
deformation remains unconstrained. The sediments studied from both sub-
zones are poorly sorted greywackes which indicates proximal sources (CIA = 45–55). The sediments from
both subzones show subduction-influenced geochemical characteristics such as low HFSE (Ta, Nb, Ti) con-
tents. In detail, the greywackes in the Zoolon subzone reveal the oceanic arc-derived sources, whereas
those in the Khuvinkhar subzone are characterized by active continental margin tectonic affinity. New
results of U-Pb isotopic dating of detrital zircons collected from TAZ sediments suggest Mississippian–
Permian depositional ages (ca. 297–348 Ma). The depositional ages of the sediments in the Khuvinkhar
subzone (ca. 297–325 Ma) are younger than those in the Zoolon subzone (ca. 322–348 Ma). Interest-
ingly, the Cambrian–Ordovician and Silurian–Devonian age populations in all dated samples may imply a
direct correlation between the TAZ and Mongolian Altai. However, the youngest detrital zircon age popu-
lations indicate a source from the proximal Carboniferous volcanic arc (Nguyen et al.,
2018
). In addition,
a litharenite collected from the northernmost part of the Khuvinkhar subzone yields an Ordovician depo-
sitional age (ca. 475 Ma) similar to the sediments typical of the Gobi-Altai Zone in the Mongolian Altai to
the north (Gibson et al.,
2013
; Kröner et al.,
2010
). This Ordovician litharenite is unconformably overlain
by Carboniferous sequences of the TAZ. Therefore, the boundary between the TAZ and Mongolian Altai
can be interpreted as having a stratigraphic and not tectonic significance. The greywackes and tuffaceous
sandstoneintheZoolonsubzoneshowmainlypositiveinitialepsilonHfvalues(+15.2to-1.7; T
DM.2stg
=0.4–
1.1 Ga) that support a juvenile source. However, several zircons with CHUR-like initial Hf values indicate a
contribution of a crustal source. Comparison with East Junggar sedimentary sequences allows the stud-
25
ied greywacke-type sediments of the Khuvinkhar and Zoolon subzones to be attributed to Carboniferous
Dulateback-arcsequences (Jiang etal.,
2024
; Sukhbaataretal.,
2024
). It canbeconcludedthatboth Khu-
vinkharandZoolonCarboniferoussequencesrepresentacontinuationoftheEastJunggarDulateback-arc
system sourced in a nearby Carboniferous arc.