Comparison on geochemistry of minerals in garnet peridotites from Paleozoic and Neogene xenoliths and from Triassic UHP terrane to constrain the lithospheric evolution of east China
Abstract
Major and trace element data of minerals in garnet peridotite xenoliths from early Paleozoic (457-500 Ma) and Neogene (16-18 Ma) volcanics within the North China Craton are compared with those from tectonically exhumed Triassic Sulu ultra-high pressure (UHP) terrane along its southern margin. Estimated P-T conditions of the Paleozoic and Neogene garnet peridotite xenoliths reflect different geotherms corresponding to surface heat flow of ∼40 mW/m2 (Paleozoic) and ∼80 mW/m2 (Neogene). Paleozoic peridotite xenoliths are compositionally similar to cratonic mantle; they have high Mg number in olivine (mean 92.7), Cr number in garnet (mean 27.6), and Ni and La/Yb (19-25) in clinopyroxene (Cpx). They have low Al2O3, TiO2, HREE and Y in Cpx, low Y/Ga in garnet (0.33-1.34), and show LREE-enriched pattern of Cpx. Neogene peridotite xenoliths represent a more fertile mantle with low Mg number in olivine (mean 89.5), Cr number in garnet (mean 3.5), and La/Yb (2.5-3.3) in Cpx. They have high Al2O3, TiO2, HREE and Y in Cpx, and high Y/Ga in garnet (8.45-20.7), and show flat REE pattern of Cpx. The differences suggest that the buoyant refractory lithospheric keel present beneath the eastern part of the North China Craton in Paleozoic time was largely replaced by younger and hotter fertile lithospheric mantle during late Paleozoic-Paleogene time. The Triassic Sulu UHP peridotites have lower Mg number in olivine (91.5?0_6\x9D73?0_6\x9D981.0) and garnet (73.2-82.0), Cr number in garnet (1.9-18.3), and Al2O3 of orthopyroxene (0.15-0.34) than the Paleozoic xenoliths. Cpx in the peridotites contains very low HREE (0.02-0.29 ppm) and incompatible trace elements (except Sr), displays sinusoidal to LREE-enriched patterns with La/Yb of 80.3-143 and exhibits distinctly negative Nb, Zr and Ti anomalies. Garnets have low REE and strongly negative Ce anomalies (δ Ce=0.24-0.56), and Y, Cr2O3, Nd/Y and Sc/Y are typical of highly to moderately refractory mantle. The UHP peridotites are interpreted having evolved from Paleozoic-like protoliths through interaction with crust-derived carbonatitic and silicate melt/fluids, before UHP metamorphism. Exhumation of the Sulu UHP rocks may have occurred in an extensional regime allowing the asthenospheric upwelling. Peridotites sampled by Neogene basalts represent the newly accreted lithosphere derived from cooling of the upwelling asthenospheric mantle in Jurassic-Cretaceous and Paleogene time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.V22B0587Z
- Keywords:
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- 1744 Tectonophysics;
- 8102 Continental contractional orogenic belts;
- 8120 Dynamics of lithosphere and mantle: general