Nature and evolution of the Late Cretaceous lithospheric mantle beneath the eastern Jiangnan orogenic belt: constraints from peridotite xenoliths

摘要

The nature and evolution of the Late Mesozoic lithospheric mantle of the South China Block (SCB) and the influence of subduction of the (paleo-) Pacific plate on it are still highly controversial. A volcanic pipe containing plentiful peridotite xenoliths was newly identified in the eastern Jiangnan orogenic belt of the SCB, which provides us with the possibility to solve the above-mentioned disputes. The host olivine basalts yield a whole-rock K-Ar age of 73 Ma, thus indicating Late Cretaceous magmatic activity. The peridotite xenoliths are all spinel lherzolites, with Mg# in olivine ranging from 87.6 to 89.0, Al2O3 in ortho- and clinopyroxene ranging, respectively, from 3.93 wt% to 4.97 wt% and from 5.30 wt% to 7.00 wt%, and Cr# in spinel ranging from 10.3 to 13.7, all of which are characteristics of a fertile lithospheric mantle. The clinopyroxenes from the peridotite xenoliths show light/heavy rare earth element (L/HREE) fractionation (i.e. (La/Yb)(N) = 1.69-3.91), which is different from the LREE-depleted patterns of Middle-Late Jurassic peridotite xenoliths. Besides, the Sr-87/Sr-86(t) and epsilon(Nd)(t) values of clinopyroxene vary from 0.7044 to 0.7052 and from 0.6 to 2.8, respectively, which corresponds to a very weakly depleted mantle source. Collectively, the mineral chemistry of peridotite xenoliths and the trace elemental and Sr-Nd isotopic compositions of clinopyroxene both suggest that the lithospheric mantle beneath the eastern Jiangnan orogenic belt underwent a low degree of partial melting (i.e. 1%-3% batch melting or 2%-3% fractional melting), followed by silicate melt metasomatism, the metasomatic agent being derived from a subducted oceanic plate. Combined with previous studies on peridotite xenoliths of different ages, we outline a complete Phanerozoic lithospheric mantle evolution of the SCB and propose that the subduction of the (paleo-) Pacific plate contributed both material and energy to the modification of the lithospheric mantle since the Cretaceous (and possibly until the Eocene).

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