The younger age component is within error of the igneous crystallisation age of c.
The concordia ages of 2497 ± 13Ma, 787 ± 35Ma, 321 ± 10Ma, 217 ± 11Ma of zircons imply that the Tonghua area might geologically be located in the convergent region of the Yangtze Block (YB) and the North China Block (NCB) in the early Mesozoic and the zircons should be caught during magma intrusion.
The existence of the Early Cretaceous basic dyke swarm in the Tonghua area shows that an intensive extensional environment occurred in the Early Cretaceous.
SHRIMP zircon U-Pb dating for Chibaisong No.1 gabbro in the Tonghua area, Jilin Province, is discussed in the paper.
The cathodoluminescence (CL) images of the zircons indicate that they can be divided into two major types, i.e.
Experimental studies have shown that zircon solubility is mainly governed by temperature and melt composition, and for typical peraluminous melts zircon solubility ranges from ~100 ppm dissolved at 750 °C to 1300 ppm at 1020 °C (Boehnke et al. Although zircon is most widely used to constrain crystallisation ages of granites, monazite geochronology has been employed sporadically to date the crystallisation age of granites (e.g., Grosse et al. However, there has been little attempt to systematically evaluate the suitability of monazite for geochronology of leucocratic granite types even though monazite appears to be common in a range of granites (Bea ).
The crystallisation ages of several leucocratic granites are not well defined by U–Pb zircon geochronology, which has created uncertainty about the duration and distribution of some of the granite supersuites and related orogenic events in the province.
Minor sericitisation, quartz recrystallisation and biotite replacement by chlorite is evidence of a weak metamorphic overprint.
The zircon crystals are equant to elongate and euhedral pale green to black, 30–250 μm long and 25–80 μm wide, with only a few grains showing subtle internal zoning (Nelson Representative photomicrograph images (cross polarised light) highlighting a weak post-magmatic overprint observed in the leucogranites samples a 139466: medium-grained equigranular biotite–muscovite granodiorite showing minor sericitisation and quartz recrystallization; b 169092: massive, medium-grained biotite–muscovite monzogranite, showing minor sericite, quartz recrystallisation and undulose extinction in the quartz; c 183287: medium- to coarse-grained, equigranular muscovite–tourmaline granodiorite with partly sericitised plagioclase and recrystallised quartz; d 187401: fine to medium-grained equigranular tourmaline–muscovite monzogranite showing recrystallised quartz and minor sericitisation This sample is from a leucocratic granite unit located in the northern Gascoyne Province where it intrudes 1840–18–1680 Ma metasedimentary rocks and is overlain by 1620–1450 Ma siliciclastic rocks (Sheppard et al.
b) with minor muscovite and 5% mafic minerals (chlorite, epidote, tourmaline and garnet).
Biotite is partly replaced by chlorite, and minor sericite and undulose extinction in the quartz and recrystallised quartz are indicative of a weak metamorphic overprint.
In this paper we use SHRIMP U–Th–Pb monazite geochronology to re-evaluate the emplacement ages of these granites in order to obtain reliable igneous crystallisation ages consistent with the existing geological framework.