북서 베트남 송마봉합대 손라지역에서 산출하는 이질편암의 광물공생관계와 SHRIMP 모나자이트 우라늄-토륨-납 연령 : Parageneses and SHRIMP monazite U?Th?Pb ages of pelitic schists in the Son La area, Song Ma suture zone, northwestern Vietnam

Bui Vinh Hau 충북대학교 2014 국내석사

This study reports the mineral assemblages, microstructures, P–T estimates and monazite U–Th–Pb ages from pelitic schists of the Nam Co Formation, Son La area, Song Ma suture zone, northwestern Vietnam, to delineate its pressure–temperature–time–deformation evolution. The Nam Co Formation in the study area is structurally characterized by an antiformal metamorphic core complex. Pelitic schists in the core of this complex belong to the garnet zone, consisting of quartz + chlorite + muscovite + garnet + albite with accessory apatite, zircon, monazite and ilmenite. On the other hand, the rims of this complex are consistent in metamorphic grade with the biotite zone of quartz, chlorite, muscovite, plagioclase, and with or without biotite. In the garnet zone, both albite and garnet occur as a porphyroblast. The curved to sigmoidal inclusion trails were defined by graphitic materials within porphyroblastic albite. The earlier foliations were truncated by the major foliation. The pre-Dn growth of porphyroblastic albite is also corroborated by the stylolitic halos and microfaults around the porphyroblasts. Garnet uncommonly occurs as a small inclusion within porphyrobalstic albite. Compositions of the inclusion are represented by the high almandine and relatively low spessartine contents (Xalm=80–83 & Xsps=7–8), in marked contrast to those of porphyroblastic garnet with high spessartine contents (Xalm=62–75 & Xsps=18–31). The above microstructural and compositional features suggest two temporally distinct crustal-thickening events in the region. The peak metamorphic conditions estimated from the garnet-chlorite geothermometer and the garnet-plagioclase-muscovite-quartz geobarometer are in the range of 450–490oC and 7–13 kbar. The SHRIMP U–Th–Pb isotopic analyses of monazite from the garnet zone yielded the lower intercept age of 234 ± 10 Ma. This age is interpreted as the timing of collision between the Indochina and South China cratons.

Neoproterozoic and Triassic convergent tectonic records in metasedimentary rocks of the Nam Co complex, Son La area, Song Ma suture zone, northwestern Vietnam

Bui Vinh Hau 충북대학교 2019 국내박사

Chapter 1: Neoproterozoic deposition and provenance of metasedimentary rocks from the Nam Co complex, Son La area, Song Ma suture zone, northwestern Vietnam: Constraint on the U–Th–Pb ages of detrital zircon U–Pb geochronological studies of detrital zircon from the Nam Co complex in the Son La area, Song Ma suture zone, northwestern Vietnam, were investigated in order to constrain the depositional age and provenance of sediment from the Nam Co complex. The U–Pb isotopic dating on detrital zircons from metasedimentary rocks show that the Nam Co complex have maximum depositional age of 833 ± 8 Ma (2σ)and major population (>50%) around ~850 Ma with minor population scattered between 1.0 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. 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. This indicates that 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. Chapter 2: P-T conditions and monazite U–Th–Pb ages of metapelite, Nam Co complex, Son La area, Song Ma suture zone, northwestern Vietnam High-pressure metapelites have gained a great deal of interest for the possibility to shed some light on the tectonic development of convergent tectonic boundaries. In this chapter, the metamorphic evolution and monazite geochronology of metapelitic schists from the Nam Co complex, Son La area, central Song Ma suture zone, were investigated in order to reveal: (1) the predominance of chlorite + white mica + albite + quartz ± garnet + monazite assemblage (M2) in metapelite; and (2) two-stage metamorphism and deformation records during the Indosinian orogeny. Earlier event (M1) was partly recorded by relict mineral inclusions inside albite porphyroblasts such as garnet + white mica + chlorite + ilmenite + rutile + monazite and graphitic material. Their internal fabrics (S1) are variable from straight to sigmoidal in shape, and are discontinuous to the major foliations (S2) defined by aggregates of white mica and chlorite in the matrix, suggesting their pre-S2 formation. Porphyroblastic garnet contains paucity of inclusions such as quartz, rutile needles in the core and corona of chlorite, white mica and biotite at the rim, and shows compositional zonation typified by a bell-shaped spessartine profile balanced by increasing almandine. Rare garnet inclusion inside prophyroblastic albite is homogeneous in composition, rich in almandine content, but low in spessartine, pyrope and grossular contents. The P–T conditions of the M1 based upon isochemical phase diagrams are estimated as 15–17 kbar and 460–475 oC, whereas those of M2 as 11–12.5 kbar and 460–585 oC. Those of the M2 are consistent with conventional geothermobarometric results (8–12 kbar, 440–490 oC), suggesting blueschist facies metamorphism followed by the epidote amphibolite facies. Monazite grains in the matrix (M2) apparently aligned subparallel to the S2 whereas monazite inclusions (M1) within albite porphyroblasts subparallel to S1. The former is depleted in Th and Si, and is enriched in P, La, Ce relative to the latter. The U–Th–Pb isotopic compositions of the M1 and M2 monazite grains using an ion probe defined common Pb mixing lines and their lower intercept 206Pb/238U ages were 244 ± 10 (2σ) and 234 ± 10 Ma (2σ), respectively. On the other hand, those using a LA–MC–ICP–MS yielded the weighted mean 208Pb/232Th ages of 241 ± 5 (2σ) and 221 ± 4 Ma (2σ), respectively. The results from the supracrustal subduction of metasedimentary protolith and the subsequent collision between the Indochina and South China cratons accompanied by the closure of the Song Da Ocean. Chapter 3: Hf isotopic compositions of detrital zircon from the Nam Co complex, Son La area, Song Ma suture zone and reconstruction of Rodinia supercontinent To constrain the paleopositions of the Indochina and South China cratons in the Rodinia supercontinent during Neoproterozoic, we performed in-situ U–Pb and Hf analyses of detrital zircon from Nam Co complex, Song Ma suture zone, northwestern Vietnam. The U–Pb isotopic dating on detrital zircons show that the Nam Co complex have major population (>50%) around ~850 Ma with minor population scattered between 1.2 to 3.0 Ga. The Neoproterozoic age group exhibit a large range of εHf(t) from strong negative to positive values (-17.418022 to 14.600527), suggesting the source of magma from this age group were not only derived from reworking of Archean basement rocks, but also generated from juvenile material. The U–Pb age distribution pattern and Hf isotop data of detrital zircon in the Nam Co Complex is compatible with that of the South China craton rather than that of the Indochina craton. The data also indicate that 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. Combining with the similarity of the detrital zircon age between western Cathaysia, Indochina, East Antarctica and East India suggesting that the South China craton was situated at the margin of the Rodinia supercontinent, and in close proximity to Indochina, East Antarctica and East India