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To constrain the timing of collisional event in the Indochina block, SHRIMP U-Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc Complex, central Vietnam. Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1), and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2). The change in mineral assemblages indicates a clockwise P-T path composed of a high-or medium-P + low-T stage (M1) and a subsequent low-P + high-T stage (M2). The U−Pb concordia ages of zircon rims are 447 ± 6 Ma and 452 ± 6 Ma for the two samples, respectively. These results are distinctly different from the available Ar-Ar mineral ages of 254-225 Ma. Following the clockwise P-T path and phase equilibrium analyses of the Complex, we suggest that the zircon rims were formed near peak temperatures during the decompression. The ~450 Ma zircon rim thus gives the minimum age constraint for a possible crustal thickening event during Early Paleozoic, whereas the reported Permo-Triassic Ar-Ar ages would result from an Indosinian overprint. This Early Paleozoic event is most likely related to a collisional orogeny between the Indochina and South China blocks. Late Neoproterozoic to Neoarchean ages are recorded from detrital zircon cores of the Kham Duc Complex, the Kontum Massif and Truong Son Belt, suggesting that their protoliths might have derived from sediments at the Gondwana margin.

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solutionprecipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf(t) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of wholesale metasomatic replacement at $80 Ma from an igneous protolith of juvenile origin with an age of 136 AE 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 AE 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.