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| Titel |
Late Paleozoic tectonomagmatic evolution of the western southern Tian Shan, Tajikistan |
| VerfasserIn |
James R. Worthington, Paul Kapp, Vladislav Minaev, James B. Chapman, Ilhomjon Oimahmadov, Mustafo Gadoev |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109324
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| Publikation (Nr.) |
 EGU/EGU2015-9227.pdf |
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| Zusammenfassung |
| The 2500-km-long Tian Shan orogenic belt constitutes a dominantly Paleozoic
amalgmation of Eurasia that has been overprinted by the Mesozoic and Cenozoic
Cimmerian and Indo–Eurasian collisions. This southernmost unit of the Central Asian
Orogenic System (CAOS) is divided N–S by discontinuous suture zones that reflect its
complex assemblage and E–W by the Talas–Fergana dextral (modern kinematics) fault
zone. The western southern Tian Shan in Tajikistan/Uzbekistan is poorly studied
compared to the rest of the orogen in Kyrgyzstan/China, but a dominant signal of late
Paleozoic magmatism synchronous to widespread magmatism documented along strike
provides an intriguing opportunity to investigate regional tectonic processes at this
time.
The late Carboniferous–early Permian Gissar batholith is the southern Tian Shan’s
southernmost lithotectonic unit. Zircon U-Pb weighted-mean crystallization ages for Gissar
granitoids range from ~310–290 Ma, are youngest in the east, and define a primary stage of
arc magmatism related to closure of the Turkestan ocean. A ~280 Ma crystallization age was
obtained for a Ne syenite, which corresponds to small, “post-collisional,” alkaline intrusions
in 1:200,000 Soviet geologic maps. Zircon εHf in Gissar granitoids generally decreases with
decreasing zircon U-Pb age from +5—10. Zircon εHf in the young Ne syenite is +1–+6.
Taken together, these trends indicate a progressive shift from juvenile to intermediate
magmatism over 20 Myr, followed by a marked return to juvenile magmatism within 10
Myr.
The Garm “metamorphic” massif is situated within the eastern Gissar batholith and is derived
from greater depths than the rest of the batholith, as indicated by its defining features: (i)
Discontinuous outcrops of Bt+Grt quartzofeldspathic gneisses/schists; and (ii) Presence of
igneous garnet in granitoids. Zircons from the Garm quartzofeldspathic gneisses/schists
exhibit pronounced Pb-loss discordia that are consistent with ~amphibolite-facies
metamorphism; crystallization ages are early Paleoproterozoic and Neoproterozoic,
suggesting peri-Gondwanan affinity. Metamorphic (consistently low Th/U) rims of zircons
from a Garm Bt+Grt quartzofeldspathic gneiss span 324–284 Ma and define a protracted
episode of amphibolite-facies metamorphism for the Garm massif that overlaps with both
main-stage Gissar and post-collisional magmatism. This overlap favors delamination over
post-orogenic extension as a mechanism for producing the “post-collisional,” juvenile
magmatism.
The Zerafshan unit (north of the Gissar–Garm unit) comprises a Paleozoic volcano-sedimentary
sequence that has been metamorphosed to ~greenschist facies. Chl±Bt schists exhibit zircon
U-Pb age spectra similar to those in the Garm massif but with subordinate Paleoproterozoic
peaks. Pb-loss discordia are present but weak for Zerafshan zircons, consistent with
~greenschist-facies metamorphism.
Detrital-zircon U-Pb age spectra for modern river sands (which drain the Garm massif and
Zerafshan unit) and Cretaceous sandstones (which unconformably overlie the Garm massif)
reproduce the main and “post-collisional” stages of magmatism found in the igneous rocks,
as well as the Paleoproterozoic and Neoproterozoic ages found in the metasedimentary rocks.
A match between a 450 Ma Zerafshan meta-andesite and a peak in a Cretaceous sandstone,
taken together with consistency between detrital- and igneous-zircon εHf values, suggests
that the Cretaceous sandstones were sourced from the Tian Shan rather than age-equivalent
terranes in the Pamir.
Overall, the igneous and detrital U-Pb and Hf datasets document a late Paleozoic
tectonomagmatic evolution of the Gissar arc that is consistent with development of an
Andean-style active continental margin followed by ocean closure, continental collision and
delamination. |
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