Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan, NW China: implications for the tectonic evolution and Phanerozoic continental growth
Résumé
Situated in the southwest of the Central Asian
Orogenic Belt (CAOB), the South Tian Shan (STS) Block
is a key area for understanding the final accretion of the
CAOB. A suite of volcanic rocks interbedded with continental
sediments from the Xiaotikanlike Formation lies
along the southwestern edge of the Tian Shan orogen.
Laser-ablation-inductively coupled plasma-mass spectrometer
U–Pb dating provided a crystallization age of
295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early
Permian magmatic event. The volcanic rocks show a variable
composition, with dominant rhyolites and dacites,
subordinate basaltic andesites and few basalts. The felsic
rocks are enriched in K and exhibit remarkably negative
anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated
with their high negative !Nd(t) values and old Nd
model ages suggest that they are most likely sourced from
ancient lower crustal rocks. The mafic rocks are characterized
by high Mg#, Cr, Ni contents, negative Nb, Ta
anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile
elements. The mafic rocks are thus inferred to derive
from enriched subcontinental lithospheric mantle. The
petrographic and geochemical characteristics of the Xiaotikanlike
Formation volcanic rocks indicate that they were
generated under a post-collisional regime. Therefore, the
final collision between the Tarim Craton and the Kazakhstan–
Yili terrane took place before Early Permian, most
probably at Late Carboniferous. Differing from other
tectonic units of the CAOB, the recycling of ancient lithospheric
crust played a significant role in the continental
growth of the STS Block.