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| Titel |
The nature and history of the Qilian Block in the context of the development of the Greater Tibetan Plateau |
| VerfasserIn |
Hui Huang, Yaoling Niu, Geoff Nowell, Zhidan Zhao, Xuehui Yu, Xuanxue Mo |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072272
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| Zusammenfassung |
| The Greater Tibetan Plateau is a geological amalgamation formed by several continental
collision events from northeast in the Early Palaeozoic towards southwest in the
Cenozoic. Compared to the youngest India-Asia collision event (~ 55 Ma), the earlier
events to the north are not well understood, especially the earliest Qilian-Qaidam
system at the northern margin. Understanding the petrogenesis of the Paleozoic
granitoids in the Qilian Block (QB) helps understand the nature and history of the block,
while also offering new perspectives on its sutures to the north (the North Qilian
Orogenic Belt) and to the South (the North Qaidam Ultra-High Pressure Metamorphic
Belt).
Granitoids in the QB were sampled from several intrusions in two areas: Huangyuan (HY)
and Gangcha (GC). All the samples are calc-alkaline varying from mafic-diorite to granite.
Most of the HY samples are peraluminous containing Al-rich phases. The GC samples can be
divided into two groups (fine-grained group with amphibole and coarse-grained without
amphibole). Most HY samples are enriched in LREEs with flat HREE patterns (Group A). A
garnet-bearing HY sample has elevated HREEs. Another three adakitic samples
are depleted in HREEs with negative ÉHf(t) (-12~-11), indicating a deep crustal
origin. The coarse-grained GC samples have similar REE patterns to HY group
A while fine-grained samples have flat REE patterns with a stronger negative Eu
anomaly.
Zircons in all these samples are of magmatic origin but age data scatter along the Concordia
and do not give “well-constrained” crystallization ages within a single sample. We adopt the
histogram and identify the crystallization age with a peak at ~450 Ma. The ages of inherited
zircons range from ~ 500 Ma to ~ 2600 Ma. The more peraluminous samples tend to have
older inherited zircons, pointing to the greater old crust contribution. The older Proterozoic
and Archean ages recorded in inherited zircons reveal the complex histories of the QB, which
is likely a micro-continent or fragment of an ancient continent probably drifted from the
Yangtze Craton.
The initial whole-rock Sr-Nd-Pb-Hf isotopes of these samples vary significantly, e.g., ÉHf(t)
(-15~+3), ÉNd(t)(-14.6~+2.6), radiogenic Pbi(206Pb/204Pbi: 18.20~20.63) and ISr
(0.7051~0.7606). ÉHf(t) is negatively correlate with A/CNK indicating more peraluminous
samples have more crustal contributions. Taken all the data together, we conclude that these
granitoids are genetically associated with continental collision with varying amount of mantle
input and contributions of heterogeneous basement rocks of the QB. Furthermore, the
significantly correlated ÉHf(t) - ÉNd(t) variation is consistent with these granitoids being
formed as the result of melting-induced mixing process. The crustal melting is most likely
accomplished through heating by mantle derived basaltic melts, which in this context may
be associated with seafloor subduction. Varying extents of melting of the crustal
rocks with varying age/isotopic composition will give the observed compositional
variability of the granitoids and the ÉHf(t) - ÉNd(t) correlation also reflects the
mantle melt contributions as well as the different crustal lithologies of varying
histories.
As discussed above, the QB has the affinity of the Yangtze Craton and underwent ~450 Ma
magmatism in a subduction/collision environment. |
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