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We present data from the Mesozoic Keumkang, Palbong, and Baekhwa granites in Garorim Bay, in the southwestern part of the Gyeonggi massif, South Korea. Using major and trace element concentrations, Sr-Nd-Pb isotopic compositions, and sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages, we aim to constrain the petrogenesis of the granites and explain their origin within a broader regional geological context. SHRIMP U-Pb zircon ages of 232.8 ± 3.2, 175.9 ± 1.2, and 176.8 ± 9.8 Ma were obtained from the Keumkang, Palbong and Baekhwa granites, respectively. The Late Triassic Keumkang granites belong to the shoshonite series and show an overall enrichment in large ion lithophile elements (LILE), a depletion in high field strength elements (HFSE) relative to primitive mantle, compared with neighboring elements in the primitive mantle-normalized incompatible trace element diagram with notable high Ba and Sr contents, and negligible Eu anomalies. The Keumkang granites are typified by highly radiogenic Sr and unradiogenic Nd and Pb isotopic compositions: (87Sr/86Sr)i = 0.70931–0.70959, (143Nd/144Nd)i = 0.511472–0.511484 [(eNd)i =–17.0 to–16.7], and (206Pb/204Pb) = 17.26–17.27. The Middle Jurassic Palbong and Baekhwa granites belong to the medium- to high-K calc-alkaline series, and show LILE enrichment and HFSE depletion similar to the Keumkang granites, but exhibit significant negative anomalies in Ba, Sr, and Eu. Furthermore, they have elevated Y and Yb contents at any given SiO2 content compared with other Jurassic granitoids from the Gyeonggi massif. The Palbong and Baekhwa granites have slightly less radiogenic Sr and more radiogenic Nd and Pb isotopic compositions [(87Sr/86Sr)i = 0.70396–0.70908, (143Nd/144Nd)i = 0.511622–0.511660, (eNd)i =–15.4 to–14.7, (206Pb/204Pb) = 17.56–17.76] relative to the Keumkang granites. The Keumkang granites are considered to have formed in a post-collisional environment following the Permo-Triassic Songrim orogeny that records continent-continent collision between the North and South China blocks, and may have formed by fractional crystallization of metasomatized lithospheric mantle-derived mafic melts. The Palbong and Baekhwa granites may have been produced from a gabbroic assemblage at pressures of less than ~15 kbar, associated with subduction of the paleo-Pacific (Izanagi) plate at the Eurasian continental margin. Elevated eNd(t) values in the granitoids from the southwestern part of the Gyeonggi massif relative to those of the central and northern parts, together with the comparatively shallow depth of origin, imply the presence of an exotic block in the Korean lithosphere.

The depositional and metamorphic ages, and provenances of metasedimentary rocks from the Nam Co Complex in the Son La area, Song Ma Suture Zone, northwestern Vietnam, were investigated in order to reveal the convergent tectonic history between the Indochina and South China cratons. This complex is structurally characterized by an antiformal metamorphic core consisting of two distinct metamorphic zones: inner garnet and outer biotite zones. The U-Pb isotopic dating on detrital zircon from metasandstones of the Nam Co Complex yielded major population (> 50%) around ~850 Ma with minor population scattered between 1.2 to 3.0 Ga. The age spectra as well as the morphologies, internal structures and geochemical features of detrital zircon are accounted for by magmatic activities prevalent at the convergent plate margin during the Neoproterozoic. In contrast, the UTh- Pb isotopic compositions of monazite from a garnet-zone schist defined a common Pb mixing line with the lower intercept age of 234 ± 10 Ma (2σ) reflecting the Indosinian metamorphic event. The U-Pb age distribution pattern of detrital zircon in the Nam Co Complex is compatible with that of the South China craton rather than that of the Indochina craton. In summary, sedimentary protoliths of the Nam Co Complex were deposited in a convergent-related basin along the southwestern margin of the South China craton during the Neoproterozoic, and then was finally amalgamated to the Indochina craton via the southerly subduction of South China craton during the Indosinian orogeny.

To constrain sedimentation age of the Myogok Formation and timing of the last episode of the Daebo orogeny, SHRIMP U-Pb age determination and Hf isotopic studies were carried out for the zircon separates. The detrital zircon U-Pb ages of the Myogok Formation show a wide range from Archean to Cretaceous and yield four main clusters at age intervals of 130–150 Ma, 170–220 Ma, 221–320 Ma and Paleoproterozoic. Such age clusters indicate that the main provenance rock types during the sedimentation were basement rocks of the Yeongnam massif and the Mesozoic granitoids distributed around the Myogok Formation. The concordia age of the youngest population is 138.6 ± 2.1 Ma (n = 7), indicating the possible maximum deposition age for the Myogok Formation. Considering the suggestion of probable deposition of Nakdong Formation during the Barremian (Lee et al., 2012), the Valanginian or Hauterivian seems to be the most appropriate timing for the deposition of the Myogok Formation. The last activity of the Daebo orogeny might be constrained to Hauterivian or Barremian considering some time lag needed for basin closure and deformation that followed. The measured eHf(t) values from the detrital zircons of the Myogok Formation vary from −25.7 to 4.8. However, most of the zircons, even those with Mesozoic ages, show negative values except a few, implying dominant influence of old continental crusts in the generation of their igneous protoliths.

The U-Pb isotopic compositions of zircon from a metasandstone cobble and a granitic dyke in Deokjeok Island, western Gyeonggi massif, Korea, were measured using a SHRIMP-II ion microprobe to constrain the depositional age of cobble-bearing metasedimentary strata. The metasandstone cobble, collected from a metaconglomerate of the slighltly-metamorphosed sedimentary unit (SMU), yielded 50 to 200 μm long zircon crystals showing variable internal structures together with rounded corners. Sixty spots were analysed from 60 detrital crystals, including two spots of Permian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and 263 ± 4 Ma (1σ), respectively. Except for the Permian ages, their U-Pb age distribution pattern is characterized by a few major populations at ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaks between 400 Ma and 2.5 Ga. This pattern is identical to that of the Devonian Taean Formation widespread along the western Gyeonggi massif. On the other hand, the ~260 Ma overgrowth age in the cobble could represent the zircon fraction formed in the sedimentary protolith of the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generally showed the oscillatory zonation, and yielded the weighted mean 206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. It is thus likely that the cobble-bearing strata might have deposited during the Triassic prior to 225 Ma. Moreover, the SMU unit may represent post-orogenic sediementary strata filling the extensional basin developing along the suture at the western margin of the Gyeonggi massif.

Detrital zircons in three metasandstones of the Taean Formation, western Gyeonggi massif, Korea, were analyzed using a sensitive high-resolution ion microprobe (SHRIMP), in order to provide geochronologic constraints for the depositional age as well as the protolith ages. This formation mainly comprises the sandstone-mudstone units which experienced regional metamorphism at the transitional greenschist-amphibolite facies condition. The majority of detrital zircons yielded concordant ages defining four major age components: (1) Late Archean-Early Proterozoic (2.5–2.4 Ga); (2) Middle to Earliest Late Proterozoic (1.0–0.9 Ga); (3) Middle Late Proterozoic (820–750 Ma); and (4) Silurian (∼430 Ma). The youngest group gives a weighted mean 206Pb/238U age of 431.3 ± 4.1 Ma (n = 17), confirming the Upper Paleozoic deposition of the Taean Formation. Together with this Silurian peak, the presence of Middle to Late Proterozoic ages is characteristic for the source rock, suggesting that the western Gyeonggi massif could be correlative with the South China craton. On the other hand, the lack of Paleoproterozoic age of ∼1.87 Ga is in marked contrast with its predominance in the basement rocks of central to eastern Gyeonggi massif. The overall distribution pattern of detrital zircon ages is consistent with that of the Devonian metasedimentary rock in the Imjingang belt, suggesting an extension of this collisional belt towards the south.

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates and geochemical and isotopic (Sr and Nd) data were obtained for Middle Jurassic granitoids from the Buyeo area along the southwestern margin of the Gyeonggi Massif, South Korea. Foliated 179 Ma granitoids are dominated by granite and diorite, non-foliated 167 Ma granitoids include granite to alkali-feldspar granite, and the youngest, non-foliated 165 Ma granitoids consist of alkali-feldspar granite. The 179 Ma and 167 Ma rocks are geochemically typical of I-type granitoids, and their LILE enrichment and negative Nb-Ta-Ti-P anomalies are common features of subduction-related granitoids. In contrast, 165 Ma rocks are A-type granitoids that show HREE-enriched patterns with significant depletions of Sr, P, Eu, and Ti. I-type granitoids dated at 179 Ma and 167 Ma have I Sr = 0.7092–0.7158, 0.7206–0.7263 and ɛNd(T) = −13.96 to −13.55, −21.47 to −19.51, respectively. A-type, 165 Ma granitoids have relatively lower I Sr (0.7052) and strongly negative ɛNd(T) (−20.47), reflecting melt derivation from the lower crust. These results suggest that the formation of granitoids along the southwestern boundary between the Gyeonggi Massif and the Okcheon Belt took place at ca. 179–167 Ma by the subduction of the paleo-Pacific plate. The 165 Ma post-orogenic A-type granites intruded in an extensional setting, providing a critical time constraint on the termination of Jurassic plutonism in South Korea. The 179 Ma foliated granitoids are similar to syn-kinematic granitoids in and around the Honam shear zone, indicating that ductile shear movement may have occurred at ca. 179 Ma. Biotite K-Ar ages of deformed Precambrian rocks and Middle Jurassic granitoids from the Buyeo area generally yield ages of 144–152 Ma and 124–135 Ma. These results probably reflect pervasive reactivation during Latest Jurassic to Early Cretaceous deformation that postdated the main ductile shearing event along the southern boundary of the Gyeonggi Massif.

Reaction textures, mineral chemistry, and U-Pb SHRIMP geochronology of monazite from the Hwacheon granulite complex were investigated to delineate the timing of granulite-to-amphibolite transition. Reaction textures of apatite and allanite surrounding partially-dissolved monazites in the transition zone (HY-23) are similar to those of symplectic hornblende and garnet corona around pyroxenes in mafic granulite at the same outcrop. The texture and chemistry of retrograde rare earth element-bearing minerals indicate that disequilibrium in metasomatic reactions and the availability of fluid during the amphibolitization contributed to the variable degrees of transition. The U-Pb age of dissolved monazite was dated at 1861 ± 20 Ma, which is indistinguishable from 1873 ± 17 Ma of unaltered monazite. On the other hand, the crystallization age of symplectic allanite is interpreted to be either Devonian or Triassic, based on Th-Pb and U-Pb ages, respectively. Thus, the synchroneity between accessory mineral growth and its microstructure such as corona or symplectite needs to be carefully assessed for unraveling the nature of tectonometamorphic events.

We report an in-situ U-Pb age of titanite in the Chuncheon amphibolite, central Gyeonggi massif, using a high-resolution secondary ion mass spectrometer (HR-SIMS). Titanite occurs as common accessory phase in garnet amphibolites, produced by regional metamorphism. Titanite grains separated from two amphibolite samples yield the weighted mean 238U-206Pb ages of 224 ± 14 (2σ) Ma and 239 ± 94 (2σ) Ma, respectively, which are interpreted to suggest a high-grade metamorphism at ca. 220 Ma. This result together with the available ages reported by previous workers suggests that both the Gyeonggi massif and the Imjingang belt experienced the regional Triassic metamorphism, possibly in association with continental collision between the North and South China blocks.