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Zircon ages and Hf isotopic compositions of plutonic rocks from the Central Tianshan (Xinjiang, northwest China) and their significance for early to mid-Palaeozoic crustal evolution

Abstract:

We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475-473 Ma) granitoid rocks have zircon εHf(t) values either positive (+0.3 to +9.5) or negative (-6.0 to -12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician-Silurian (ca. 458-425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458-442 Ma) has negative zircon εHf(t) values (-6.3 to -13.1), indicating a predominantly crustal source; zircon from later events (ca. 434-425 Ma) has positive zircon εHf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410-404 Ma) rocks have near-zero zircon εHf(t) values, either slightly negative or positive (-1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (-11.2 to -14.8). The Late Devonian (ca. 368-361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock εNd(t)values (-2.4 to -5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle-crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust. © 2014 © 2014 Taylor & Francis.