Ages and tectonic nature of the high-pressure metamorphosed pelitic and mafic rocks in eastern Shandong, North China Craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji Belt (JLJB) is a NNE-SSW trending continental belt lying in the Eastern Block of the North China Craton, with its southern segment extending across Bohai Sea into the Jiaobei massif. Little work has been done in the southern segment of the belt, and thus the timing for metamorphism and tectonic nature of the JLJB are still controversial, with models ranging from those invoking continent-arc-continent collision to those advocating rifting closure. The presence of the high-pressure pelitic and mafic rocks of the Jiaobei massif (Eastern Shandong) located in the southern segment is crucial to these controversial isues. The aim of this study is to resolve these issues through an integrated study including field study followed by geochronological and metamorphic investigations on the high-pressure pelitic and mafic rocks in the Jiaobei massif.

Metamorphic zircons from two high-pressure mafic granulites and one metamorphic zircon from a garnet-sillimanite gneiss have 207Pb/206Pb ages of 1884–1956 Ma and 1939 ± 15 Ma respectively, which are in accordance with the metamorphic ages of 1.93–1.90 Ga previously obtained from the northern segment of the JLJB. Metamorphic zircons from a high-pressure pelitic granulite and two politic gneisses have weighted mean 207Pb/206Pb ages of 1821–1837 Ma, while those zircons from two associated marbles analyze similar weighted mean 207Pb/206Pb ages of 1790–1817 Ma. These metamorphic ages are consistent with the metamorphic ages of c. 1.85 Ga produced from the Ji’an Group in the northern segment of the JLJB and 1.86-1.80 Ga produced from the high-pressure pelitic granulites of the Jingshan Group in the southern segment of the belt. As this metamorphic event happened synchronously with the intrusion of the A-type granites in the Jiao-Liao-Ji Belt and adjacent regions, it is considered to have resulted from the contact metamorphism caused by the emplacement of the post-orogenic or anorogenic mafic and granitic plutons that are widespread in the belt.

The high-pressure pelitic granulites recently discovered in the Jiaobei massif, and the medium-pressure pelitic granulites and high-pressure mafic granulites, were examined with an integrated study including petrology, mineral chemistry and pseudosection modeling. By applying the THERMOCALC technique, pseudosection modeling on the high-pressure and medium-pressure pelitic granulites in the NCKFMASHTO system and on the high-pressure mafic granulites in the NCFMASHTO system was constructed to confine the P-T conditions and P-T paths. The results show that the peak P-T conditions of the high-pressure pelitic granulites, high-pressure mafic granulites and medium-pressure pelitic granulites are 14.8–16.2 kbar/860–890°C, 13.1–15.1 kbar/780–890°C and 9.4–10 kbar/870–900°C, respectively, and that the metamorphic evolution of all these granulites is characterized by clockwise P-T paths involving decompression and cooling after the peak metamorphism, implying that subduction/collision-related processes must have been involved in the development of the southern segment of the Jiao-Liao-Ji Belt.

By combining the geochronological and metamorphic data, the results of this study suggest that the JLJB represented a Paleoproterozoic rift-and-collision belt along which the Longgang and Nangrim blocks amalgamated to form the Eastern Block at 1.93–1.90 Ga.