(Press-News.org) Boulder, Colo., USA: GSA's dynamic online journal, Geosphere,
posts articles online regularly. Topics for articles posted for Geosphere this month include "a tale of five enclaves"; evidence
for mantle and Moho in the Baltimore Mafic Complex (Maryland, USA); and the
after effects of the 1964 Mw 9.2 megathrust rupture, Alaska.
From Ordovician nascent to early Permian mature arc in the southern
Altaids: Insights from the Kalatage inlier in the Eastern Tianshan, NW
China
Qigui Mao; Jingbin Wang; Wenjiao Xiao; Brian F. Windley; Karel Schulmann
...
Abstract:
The Kalatage inlier in the Dananhu-Haerlik arc is one of the most important
arcs in the Eastern Tianshan, southern Altaids (or Central Asian orogenic
belt). Based on outcrop maps and core logs, we report 16 new U-Pb dates in
order to reconstruct the stratigraphic framework of the Dananhu-Haerlik
arc. The new U-Pb ages reveal that the volcanic and intrusive rocks formed
in the interval from the Ordovician to early Permian (445-299 Ma), with the
oldest diorite dike at 445 ± 3 Ma and the youngest rhyolite at 299 ± 2 Ma.
These results constrain the ages of the oldest basaltic and volcaniclastic
rocks of the Ordovician Huangchaopo Group, which were intruded by granite-
granodiorite-diorite plutons in the Late Ordovician to middle Silurian
(445-426 Ma). The second oldest components are intermediate volcanic and
volcaniclastic rocks of the early Silurian Hongliuxia Formation (S 1h), which lies unconformably on the Huangchaopo Group
and is unconformably overlain by Early Devonian volcanic rocks (416 Ma).
From the mid- to late Silurian (S2-3), all the rocks were
exhumed, eroded, and overlain by polymictic pyroclastic deposits. Following
subaerial to shallow subaqueous burial at 416-300 Ma by intermediate to
felsic volcanic and volcaniclastics rocks, the succession was intruded by
diorites, granodiorites, and granites (390-314 Ma). The arc volcanic and
intrusive rocks are characterized by potassium enrichment, when they
evolved from mafic to felsic and from tholeiitic via transitional and
calc-alkaline to final high-K calc- alkaline compositions with relatively
low initial Sr values, (87Sr/86Sr)i =
0.70391-0.70567, and positive εNd(t) values, +4.1 to
+9.2. These new data suggest that the Dananhu-Haerlik arc is a long-lived
arc that consequently requires a new evolutionary model. It began as a
nascent (immature) intra-oceanic arc in the Ordovician to early Silurian,
and it evolved into a mature island arc in the middle Silurian to early
Permian. The results suggest that the construction of a juvenile-to-mature
arc, in combination with its lateral attachment to an incoming arc or
continent, was an important crustal growth mechanism in the southern
Altaids.
View article: END
Current issue articles for Geosphere posted online in February
2021-03-03
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