Petrology and zircon U–Pb dating of meta-calcsilicate from the Jiaobei terrane in the Jiao-Liao-Ji Belt of the North China craton

Abstract

The Precambrian basement of the Jiaobei terrane consists of Archean metavolcanic and metasedimentary rocks, tonalitic – trondjhemitic – granodioritic gneisses (TTGs), Paleoproterozoic metasedimentary sequences, as well as lenses of metamorphic mafic-ultramafic rocks. Petrological, geochemical and in situ zircon U-Pb studies were conducted on meta-calcsilicates, which occur as irregular blocks within TTG gneisses. Samples were collected at Nanshankou, Jiaobei terrane of Shandong Peninsula at the north-eastern part of the North China Craton (NCC).Two groups of meta-calcsilicate rocks were distinguished, which originate from a metavolcanic-metasedimentary series. The respective bulk rock composition is a result of different proportions of source rock material involved. One group which is higher in CaO and lower in Na2O demonstrates a higher contribution material from a sedimentary source, whereas volcaniclastic to pyroclastic precursor dominates in the second group. The meta-calcsilicates are generally composed of Ca-bearing silicates with a peak pressure assemblage of Grt-Cpx-Zo-Am-Qz-Ttn; they experienced epidote amphibolite facies metamorphism constrained by P-T conditions of 580 °C/1.08–1.13 GPa. Late prehnite-, albite-, calcite- and actinolite-bearing veins refer to the subgreenschist conditions at the late exhumation stage.In situ zircon U-Pb dating of 173 detrital zircons from the meta-calcsilicate rocks yield Archean ages up to 3.04 Ga; a further predominant age population clusters at ca. 2.68 Ga with subordinate clusters at ca. 2.87, 2.78, which are interpreted to represent magmatic ages. A subordinate cluster at 2.51 Ga associated with zircon rim domains reflects a metamorphic event. Further frequently textureless rims of zircon of an age range between ca. 2.5 and −2.1 Ga indicate subsequent metamorphic events. The deposition of the meta-calcsilicate precursors occurred around of shortly after 2.1 Ga on an Archaean basement.