SHRIMP U–Pb dating of coesite‐bearing zircon from the ultrahigh‐pressure metamorphic rocks, Sulu terrane, east China

Abstract

This paper provides further evidence for the ongoing discussion as to whether the Dabie UHPM belt formed in Triassic or Palaeozoic time, and whether the Sulu UHPM belt formed in Triassic or Neoproterozoic time. Combined use of laser Raman spectrometer (LR), cathodoluminescence imaging (CL), and ion probe U-Pb in-situ dating (SHRIMP) provided accurate ages of UHPM from rocks collected from Weihai, NE Sulu UHPM belt. LR was used to identify coesite and other UHP minerals as inclusions in zircon separates from an amphibolized peridotite and an eclogite. CL was used to examine the zoning structure of these zircon, and SHRIMP dating was performed on specific spots on zircon to obtain ages of different geological events. An age of 221 ± 12 Ma was obtained for coesite-bearing zircon from the amphibolized peridotite; an age of 228 ± 29 Ma for eclogite was obtained from the lower intercept of a concordia plot. These ages are interpreted as the time of UHPM in the Weihai region. Ultramafic rocks to the east of Weihai yield a magmatic age at 581 ± 44 Ma. The zircon in the ultramafic rocks possibly also records a thermal event at c. 400 Ma, but no independent geological evidence for this event has been found. The eclogite protolith formed in the Middle Proterozoic (1821 ± 19 Ma), which is similar to the age of country rock gneisses of 1847-1744 Ma. The new geochronological data confirm that UHPM occurred in the Triassic in the Sulu area when subduction took the ultramafic body and the eclogite protolith, together with the adjacent supracrustal rocks, to mantle depths.

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

“…SHRIMP U-Pb dating on coesite-bearing domains of metamorphic zircon [67][68][69][70][71][72][73] has bracketed the UHP metamorphic event between 242±2 Ma and 227±2 Ma ( Figure 5). This not only provides resolution to the contrasting views, but also rules out the possibility that the Dabie UHP metamorphism occurred in the Early Paleozoic as suggested by garnet Ar-Ar dating [61,62] .…”

Age, trace elements and Hf-isotope composition of zircon in eclogites from the sulu UHP belt, East-Central China: Neoproterozoic intrusion and subsequent Paleozoic and Early Mesozoic metamorphism

“…4a) when the ultramafic-mafic or mafic magmas intruded the pre-existing crust in Neoproterozoic. Yang et al (2003) reported an SHRIMP discordia upper-intercept age of 1 821 Ma from a Rongcheng eclogite. This zircon may record a component of the pre-existing continental crust.…”

U–Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

“…Previous studies of zircon U-Pb dating have been mostly devoted to the G-type eclogite (e.g., Ames et al, 1996;Rowley et al, 1997;Hacker et al, 1998Hacker et al, , 2000Ayers et al, 2002;Liu et al, 2004aLiu et al, ,b, 2005Zheng et al, 2004Zheng et al, , 2005a, with a few to the P-type eclogite (Ayers et al, 2002;Rumble et al, 2002;Yang et al, 2003;Zhang et al, 2005). Thus this study examines the M-type eclogite in the Dabie orogen ( Fig.…”

“…The results show two groups of Triassic age: (1) 243 ± 1 Ma for jadeite quartzite (Liu and Jian, 2004), 244 ± 5 Ma for granitic gneiss (Wan et al, 2005), and 245 ± 14 Ma for granitic gneiss ; (2) 228 ± 2 Ma for the jadeite quartzite (Liu and Jian, 2004), 234 ± 4 Ma (Liu and Xu, 2004), 231 ± 4 Ma (Liu et al, 2004a), 227 ± 2 Ma (Liu et al, 2004b), and 226 ± 2 Ma (Wan et al, 2005) for granitic gneiss, and 227 ± 5 Ma for amphibolized peridotite (Yang et al, 2003). It appears that there are the two episodes of zircon growth in the field of coesite stability during the Triassic UHP metamorphism.…”