Geochemistry, U-Pb dating, and Lu-Hf isotopes of zircon and monazite of porphyritic granites within the Jiao-Liao-Ji orogenic belt: Implications for petrogenesis and tectonic setting

Abstract

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and flesh-red porphyritic granites, as various scale batholiths are widely distributed within the Jiao-Liao-Ji belt (JLJB), which extends for a distance of at least 1000km, rather than being limited to sporadic outcrops in southern Liaoning and Jilin as stated in previous studies. On the basis of morphology, CL and BSE zoning patterns, and mineral inclusions of dated zircon and monazite grains from samples of various porphyritic granites, the grains exhibit evident magmatic zones with distinct cores and rims and preserve felsic mineral inclusions of Qtz+Kfs+Pl+Qtz±Ap in both domains. The magmatic zircon cores and rims from 18 dated samples yielded two distinct weighted mean age groups, from 1872±9 to 1866±7Ma and 1851±14 to 1842±10Ma, with age peaks at 1869±1.7 and 1848±2.4Ma, respectively. Similarly, the magmatic cores and rims of monazites from 10 dated samples also showed two distinct weighted mean age groups, from 1870±3 to 1864±2Ma and 1851±6 to 1842±12Ma, with age peaks at 1868±0.8 and 1847±1.2Ma, respectively. The numerous U-Pb age data of both zircons and monazites revealed that the JLJB experienced a major episode of granitic magmatism in the Paleoproterozoic with initial emplacement timing of melts at ∼1.87Ga and crystallisation end timing of melts at ∼1.84Ga as represented by the magmatic cores and rims of the zircons and monazites, respectively. A combined study of whole-rock geochemistry and zircon Lu-Hf isotopic analyses revealed that the three types of porphyritic rocks are enriched in HREE, Zr, Nb, Ga, and Y and are depleted in V, Cr, and Ni, and that most have high TFeO/MgO ratios and Zr-saturation temperatures of ∼850°C, which is suggestive of an affinity with A-type granite. Moreover, the dominant εHf(t) values of the magmatic zircons range from −4.8 to +3.3, which is consistent with those of the 2.2–2.1Ga calc-alkaline meta-granites in the JLJB. Considering the high contents of garnet, Al-rich paragneiss enclaves, and ∼1.92Ga inherited metamorphic zircons in the granites, these porphyritic rocks were most likely derived from the partial melting of the 2.2–2.1Ga calc-alkaline meta-granites with abundant involvement of Al-rich pelitic granulites from the basement of the JLJB. As the result of a newly combined study of traditional geothermobarometers and phase equilibria modelling together with geochronological U-Pb dating, a precise clockwise P-T-t path with a subsequent near isothermal decompression process has been well determined in the HP-MP granulite facies metamorphic rocks in the JLJB. Thus, the following sequence of events can be inferred. Abundant ∼2.2–2.1Ga granitic rocks and voluminous Paleoproterozoic sedimentary rocks were subducted to lower crustal depths and underwent a peak HP-MP granulite facies metamorphism at 1950–1900Ma. These granulite facies rocks were exhumed to middle crustal levels and experienced a post-peak MP-LP granulite facies metamorphism at 1875–1840Ma. The retrograde rocks continued to be exhumed to shallow crustal levels at 1830–1800Ma and were finally overprinted by late amphibolite facies retrogression. The numerous magmatic zircons and monazites from the various porphyritic granites record a distinct Paleoproterozoic granitic magmatic event with two evident age peaks of ∼1.87Ga and ∼1.84Ga, which are completely consistent with the post-peak MP-LP granulite facies metamorphism (1875–1840Ma), representing an anorogenic magmatic association formed in an extensional setting possibly resulting from mantle upwelling and lithospheric thinning and resulting in the violent partial melting of ∼2.2–2.1Ga meta-granites along with abundant Al-rich pelitic granulites. Considering these data in combination with previous whole-rock geochemistry and age data, it can be stated that similar porphyritic granites are not only limited to the entire JLJB but are also widely distributed in North and South Korea. Thus, it can be concluded that the ∼1.87–1.84Ga porphyritic granites, together with massive rapakivi granites, Kfs-megacrysts, and porphyritic monzogranites, as products of an anorogenic granitic magmatic event, may have widely formed in both the JLJB and Korean Peninsula in an extensional and thinning tectonic setting.