Input Of Mantle Material Research Articles

Late Early Paleozoic and Early Mesozoic intracontinental orogeny in the South China Craton: Geochronological and geochemical evidence

The late Early Paleozoic and Early Mesozoic tectonic events in South China have been central to the debate over plate subduction versus intracontinental orogeny. Here we present zircon U–Pb geochronology, Hf isotopes and whole-rock geochemistry of six representative granitic plutons from South China. The zircon data show two groups with 206Pb/238U ages of ca. 430Ma and 225Ma, representing the crystallization ages of magma. The six plutons investigated in this study are characterized by peraluminous nature with high A/CNK values (>1.1) and the presence of aluminous minerals like muscovite. They exhibit enrichment in LREE, depletion in Eu, negative Ba, Sr, Nb and Ti anomalies and enrichment in Rb, Th, U and Pb. All samples show variably negative εHf(t) values (−2 to −16) with two-stage Hf model ages clustered around 1.5–1.8Ga. These data indicate that both the Early Paleozoic and Early Mesozoic granitic magmas were generated by the partial melting of early-middle Paleoproterozoic basement rocks without significant input of mantle material. The distribution of all the plutons with ages around 430Ma and 225Ma are unrelated to subduction tectonic regime. Combining with other geological evidence, our study suggests that the tectono-magmatic events during late Early Paleozoic and Early Mesozoic were largely related to intracontinental orogeny triggered by interactions between the Yangtze and Cathaysia continental blocks. The far-field stress propagation derived from the South China plate boundary might have played a crucial role as the primary cause of the stress field within the continental interior of the South China Craton.

Late Triassic adakitic plutons within the Archean terrane of the North China Craton: Melting of the ancient lower crust at the onset of the lithospheric destruction

We present the results of a geochemical and geochronological study for Late Triassic (230 220 Ma) adakitic plutons within the Archean terrane of the eastern part of the North China Craton (NCC). These plutons show adakitic signatures with high Sr, Sr/Y, (La/Yb)(N), and low Cr and Ni. The enriched Nd-Hf isotopic compositions (epsilon(Nd)(t) = -13.3 to -12.9; epsilon(Hf)(t) = -17.4 to -14.6) and old Nd (T-DM2 = 2078-2037 Ma) and Hf (T-DM2 = 2366-2192 Ma) isotope model ages suggest that the adakitic pluton may be derived from the underplated mafic lower crust of Paleoproterozoic age. The relatively low Cr and Ni contents and lower epsilon(Nd)(t) and epsilon(Hf)(t) values of the Taili adaldtic plutons imply negligible input of mantle materials. Calculations of equilibrium mineral assemblages and modeling of trace element partition between melts and residual phases at different pressures confirm the interpretation that the petrogenesis of the Taili adakitic plutons is consistent with partial melting of the Paleoproterozoic mafic lower crust at 10-12 kbar (36-43 km) with a garnet granulite residue. Melting of the ancient mafic lower crust may be triggered by excess heating of the upwelling mantle in an extensional setting evoked by the contemporary subduction toward beneath the NCC from both north and south, which could serve as one possible mechanism for the destruction or lithospheric thinning of the NCC. Complex mantle-crust interaction through various mechanisms may have been responsible for the long-lived process of destruction or lithospheric thinning, which might have begun as early as in the late Triassic. (C) 2014 Elsevier B.V. All rights reserved.

A-type magmatism in a syn-collisional setting: The case of the Pan-African Hook Batholith in Central Zambia

Abstract The Pan-African Hook Batholith formed during the assembly of the Gondwana supercontinent as a result of syn-collisional stage interaction between the Congo and Kalahari Cratons. The bimodal magmatism (mafic to predominantly felsic) is characterized by both an alkali-calcic and an alkalic suite, with typical A-type, metaluminous, high Fe/Mg and K/Na geochemical signature. Occasionally, sodic granitoids have been documented. Compositions were driven to more differentiated products by fractional crystallization, while Sr–Nd isotopes exclude crustal assimilation during crystallization. Recent new U–Pb age data constrain most of the felsic magmatism between 550 and 540 Ma. Scattered outcrops of gabbroic rocks, both tholeiitic and alkaline, testify to periodic input of mantle material, and, in some cases, to interaction with metasomatizing fluids. Crystallization ages on mafic rocks span from 570 to 520 Ma, thus indicating that they were contemporaneous with the major granitic intrusion, which was the result of a number of successive felsic batches, eventually forming a coalescing batholith. Highly radiogenic Pb isotopic values attest to the radiogenic character of the rocks. Such an anomalous signature was acquired during, or soon after, magma emplacement, perhaps as result of metasomatizing fluids. Enrichment in Th–U of large portions of the crust along this part of the margin of the Congo Craton is suggested. Geochemical and isotopic evidence support the interaction between mantle components and portions of the deep crust at pressure of