Petrogenesis of Early Cretaceous intrusions in the Sulu ultrahigh-pressure orogenic belt, east China and their relationship to lithospheric thinning

摘要

Geochronological, geochemical, whole-rock Sr-Nd and zircon Hf isotopic analyses have been carried out on two suites of Late Mesozoic mafic to felsic magmatic rocks in the Sulu orogenic belt (east-central China) with the aim of characterizing their petrogenesis and tectonic implications. The Shijiusuo monzogranite has a SHRIMP zircon Pb-201/U-238 age of 127 +/- 2 Ma and an Ar-40/Ar-39 age on homblende of 123.5 +/- 0.4 Ma. A mafic enclave from this pluton has a SHRIMP zircon Pb-206/U-238 age of 124 +/- 3 Ma and a homblende Ar-40/39Ar age of 124.2 +/- 0.4 Ma, indicating coeval crystallization of the mafic enclaves and host monzogranite. Whole rock Ar-40/Ar-39 dating gives an emplacement age of 111.2 +/- 0.1 Ma for the mafic dikes. The monzogranites have low MgO and Cr, high Na2O, and Sr-Nd-Hf isotopic data (Sr-87/Sr-86 > 0.7085, epsilon(Nd)(t)= -20.5 and epsilon(Hf) = -22.5 to -56.6) consistent with derivation from Late Archean to Paleoproterozoic lower crust with involvement of mantle materials. The presence of coeval mafic enclaves with high epsilon(Nd)(t) and epsilon(Hf)(t) values indicates magma mixing and involvement of mantle-derived materials in the generation of the Shijiusuo pluton. The mafic dikes that intrude the monzogranite have characteristics of ultrapotassic rocks. Their geochemical features, such as high MgO (Mg# up to 75) and Cr (up to 1233 ppm), low TiO2 (1.11-1.24 wt.%) and total Fe2O3 (8.33-9.09 wt.%), enrichment in LILEs (e.g., Rb, Ba, Sr) and LREEs, and depletion in HFSE (e.g., Nb, Ta and Ti), together with initial Sr-87/Sr-86 ratios of 0.7076-0.7078 and negative epsilon(Nd)(t) values (-17.6 to -18.2), indicate they were derived from an amphibole-bearing, refractory lithospheric mantle. The Shichang-Fangzi monzodioritic to monzonitic rocks have zircon SHRIMP U-Pb ages of similar to 122 Ma. These rocks have Sr, Nd and Hf isotopic compositions (initial Sr-87/Sr-86=0.7083-0.7088, epsilon(Nd)(t)=-16.5 to -17.7 and epsilon(Hf)(t)=-20.4) similar to the mafic dikes in the nearby Shijiusuo pluton, indicating they were derived from a common source. High Rb and HREE, low Sr and Ba and strongly negative Eu anomalies indicate that the monzodioritic magmas resulted from pyroxene- and plagioclase-dominated fractionation of magma derived from an enriched mantle source. Taken together, these features indicate that Early Cretaceous magmatism in the Sulu orogenic belt was not related to Late Triassic subduction or exhumation of the ultrahigh-pressure metamorphic rocks that characterize the Sulu region; instead they resulted from mantle-crust interaction in an extensional setting, most likely induced by widespread removal of lithospheric mantle in the eastern North China Craton during the Early Cretaceous. (c) 2005 Elsevier B.V. All rights reserved.