This paper presents new zircon U-Pb ages and whole-rock geochemical data
for Triassic granitoids in the Duobaoshan ore field, in the eastern
Central Asian Orogenic Belt (CAOB). Dating results reveal that the
Triassic granitoids in the Duobaoshan area were emplaced ca. 244–223
Ma. These samples are typical subduction-related arc magmatic rocks that
are enriched with large-ion lithophile elements (LILEs), depleted in
high field strength elements (HFSEs), and have relatively high Sr
(average 1,036 ppm) and low Y (average 10 ppm) and Yb (average 1 ppm)
contents that exhibit adakite-like geochemical properties. The samples
feature a juvenile whole rock average initial
87Sr/86Sr ratio of 0.70394 and an
average ɛNd(t), zircon ɛHf(t), and
two-stage Hf model age of 4.62, 10.29, and 766 Ma respectively. These
geochemical features reveal a thickened lower crust source.
Comprehensive analyses of the early Paleozoic, Triassic, and early
Jurassic granitoids in the Duobaoshan area indicate that the magmatism
of these three stages was related to the melting of the juvenile lower
continental crust materials. The early Paleozoic was the primary period
of vertical crust growth, during which time the juvenile lower
continental crust was formed. The Triassic granitoids derived from magma
were generated by partial melting of the thickened juvenile lower crust
and the Jurassic granitoids derived from the partial melting of the
delaminated juvenile lower crust. Geological observation and tectonic
setting analyses reveal that the Paleo-Asian Ocean tectonic regime in
the Duobaoshan area switched to the Mongol-Okhotsk Ocean in the Mesozoic
after its closure. The early Paleozoic granitoids were formed in a
subduction setting related to the northwestward subducted Paleo-Asian
Oceanic slab underlying the Erguna-Xing’an composite block. The Triassic
granitoids were formed under a supra-subduction extensional regime
related to the southward subduction of the Mongol-Okhotsk Ocean, and the
Jurassic granitoids were generated under a slab rollback setting related
to the late stage subduction of the Mongol-Okhotsk Ocean.