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| Titel |
Xenoliths in Eocene lavas from Central Tibet record carbonated metasomatism
of the lithosphere |
| VerfasserIn |
Fanny Goussin, Carole Cordier, Philippe Boulvais, Stéphane Guillot, Pierrick Roperch, Anne Replumaz |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141009
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| Publikation (Nr.) |
 EGU/EGU2017-4469.pdf |
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| Zusammenfassung |
| Cenozoic post-collisional volcanism of the Tibetan Plateau, emplaced on an accreted
continental margin under compression, could bring important information regarding the
edification of the Plateau. In this study, we combined petrography, whole rock geochemistry,
stable isotopes and in situ mineral analysis to decipher the genesis of Eocene-Oligocene
magmatic rocks from the Nangqian basin (35-38 Ma, [Spurlin et al., 2005; Xu et al., 2016]),
located at the hinge between Central Tibet and the Eastern Indo-Asia Collision
Zone.
Our dataset includes potassic trachyandesites; amphibole-bearing potassic trachytes; and
rare ultrapotassic (K2O/Na2O ≥ 4) mafic syenites. All samples have high REE abundances
(La = 100 - 500 x primitive mantle). Fractionation of heavy REE (Gd/YbN > 3) indicates
melting in the garnet stability field, and relative depletion in high-field strength elements
(Nb, Ta) indicates a selective enrichment of the source by metasomatic fluids. This
metasomatism event is also evidenced by the occurrence of re-equilibrated mantle
xenocrysts of phlogopite (Mg# = 88 - 90 and Cr2O3 content = 0.9 - 1.82 wt%) in mafic
syenites.
Potassic trachyandesites have specific composition, with negative Zr-Hf anomaly and low
Hf/Sm (0.2 - 0.4). Indeed, they include xenocrystic aggregates, composed of magmatic
clinopyroxene, apatite and subordinate biotite and feldspar, with interstitial calcite and
dolomite. δ18OV −SMOW (9.2 – 11.0 ) and δ13CV −PDB (-6.1 – -4.0 ) of these rocks indicate
the presence of primary, mantle-derived carbonates. In situ analysis of the major and trace
element compositions of the carbonates, clinopyroxenes and apatites further suggest
that these aggregates represent cumulates of a carbonate-bearing magma. These
xenoliths thus show that the lithospheric mantle was also metasomatized by CO2-rich
fluids.
Cenozoic carbonatites in China have been identified in Maoniuping in Western Sichuan
(31.7 Ma), Lixian in the Western Qinlin (22-23 Ma), and Nanjagbarwa in the Tethyan
Himalayas (3.6-5.5 Ma) [Yang and Woolley, 2006]. Considering as such the Nangqian
xenocrystic cumulates, Eocene carbonatites preferentially occurred on the three edges of the
Songpan-Ganze block, and we propose that their mantellic sources were all affected by an
input of subducted carbonates during the Triassic closure of the Songpan-Ganze ocean.
Ages and local field relationships furthermore indicate that melting occurred during
Eocene-Oligocene compressive events that propagated outward from the Songpan-Ganze
block, suggesting renewed subduction of the block margins following the onset of the
India-Asia collision.
References:
Spurlin, M. S., Yin, A., Horton, B. K., Zhou, J., & Wang, J. (2005). Structural evolution
of the Yushu-Nangqian region and its relationship to syncollisional igneous activity,
east-central Tibet. Geological Society of America Bulletin, 117(9-10), 1293-1317.
Xu, Y., Bi, X. W., Hu, R. Z., Chen, Y. W., Liu, H. Q., & Xu, L. L. (2016). Geochronology
and geochemistry of Eocene potassic felsic intrusions in the Nangqian basin, eastern Tibet:
Tectonic and metallogenic implications. Lithos, 246, 212-227.
Yang, Z., & Woolley, A. (2006). Carbonatites in China: a review. Journal of Asian Earth
Sciences, 27(5), 559-575. |
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