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    Neoarchean high-pressure metamorphism from the northern margin of the Palghat-Cauvery Suture Zone, southern India Petrology and zircon SHRIMP geochronology [查看] YohsukeSaitoh、ToshiakiTsunogae、M.Santosh、T.R.K.Chetty、KenjiHorie
    We report the metamorphic pressure–temperature (P–T) history of mafic granulites from two localities in southern India, one from Kanja Malai in the northern margin and the other from Perundurai in the central domain of the Palghat–Cauvery Suture Zone (PCSZ). The PCSZ is described in recent models as the trace of the suture along which crustal blocks were amalgamated within the Gondwana supercontinent during Late Neoproterozoic–Cambrian. The mafic granulite from Kanja Malai yields P–T conditions of 750–800℃ and 8–12 kbar reflecting the partially retrograded conditions following a peak high-pressure(HP) metamorphic event. The common Grt + Cpx + Qtz assemblage in these rocks and lack of decompression texture suggest that peak metamorphism was probably buffered by Grt + Cpx + Opx + Pl + Qtz assemblage, following which the rocks were exhumed through a gradual P–T decrease. The mafic granulite from Perundurai (Grt + Cpx + Pl) contains Opx + Pl symplectite commonly occurring between garnet and clinopyroxene, suggesting the progress of reaction: Grt + Cpx z+ Qtz?Opx + Pl, with the Grt + Cpx + Qtz representing the peak metamorphic assemblage. The reaction microstructures and calculated P–T conditions suggest that the mafic granulites from Perundurai underwent peak HP metamorphism at P > 12 kbar and T = 800–900 C and subsequent isothermal decompression along a clockwise P–T path, in contrast to the P–T path inferred for Kanja Malai. The contrasting P–T paths obtained from the two localities suggest that whereas Perundurai is a part of the metamorphic orogen developed within the PCSZ during Gondwana assembly, the high-pressure granulites of Kanja Malai belong to a different orogenic regime.In order to evaluate this aspect further, we analyzed zircons in a charnockite and garnet-bearing quartzo-feldspathic gneiss associated with the HP granulites from Kanja Malai which yielded mean 207Pb/206Pb magmatic protolith emplacement ages of 2536.1 ± 1.4 Ma and 2532.4 ± 3.7 Ma, and peak metamorphic ages of 2477.6 ± 1.8 Ma and 2483.9 ± 2.5 Ma, respectively. These results closely compare with the available magmatic (2530–2540 Ma) and metamorphic (2470–2480 Ma) ages reported from charnockites in the Salem Block at the southern fringe of the Archean Dharwar craton, immediately north of the PCSZ. The Neoarchean/Paleoproterozoic ages obtained from Kanja Malai correlate with the tectonic history at the margin of the Archean craton. Although no age data are available for the Perundurai mafic granulite, the close correspondence of their P–T data and exhumation path with those reported for Late Neoproterozoic–Cambrian HP–UHT metamorphism within the PCSZ suggest that these rocks form part of the Gondwana-forming orogen.
    The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland) SHRIMP and old rocks [查看] KenjiHorie、AllenP.Nutmanc、ClarkR.L.Friend、HiroshiHidaka
    Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840Ma zircons vary from very rare single-phase metatonalites to morecommoncomplexly banded tonalitic migmatites. The former metatonalites have simple oscillatoryzoned ≥3840Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840Ma and 3650–3600Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.With only 100–160ppm Zr in the tonalites and likely melt generation temperatures of >1000 ◦C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the≥3840Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur.With an established igneous age of ≥3840Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 ◦C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations,whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations.Gneisses with ≥3840Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65km east Akilia) and on the north of Ivisaartoq (∼150km northeast of Akilia). These include from Itilleq a 3891±6Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ~3800, 3750 and 3700Ma tonalite generations.Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800Ma igneous zircon are mutually exclusive from migmatite samples with ~3700Ma igneous zircon.This suggests that prior to an amalgamation event followed by 3660–3600Mahigh-grade metamorphism,≥3840Ma tonalites might have resided in a terrane discrete from ∼3700Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ~3800Matonalites with a minor associated≥3840Macomponent and a terrane with ~3700Matonalites were tectonically juxtaposed at ~3660 Ma.
    Zircon ‘microvein’ in peralkaline granitic gneiss, western Ethiopia Origin, SHRIMP U-Pb geochr onology and trace element investigations [查看] TesfayeKebede、KenjiHorie、HiroshiHidaka、KentaroTerada
    Azircon‘microvein’ composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians.U–Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, andMn) abundances of the ‘microvein’ and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the ‘microvein’ zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively,reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr.The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that darkgrey BSE domains were formed by dissolution–reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals.The bright and dark-grey BSE domains in Type-I zircon yield U–Pb ages of 779±69Ma and 780±35Ma, and similar domains in Type-II zircon dated at 778±49 Ma and 780±31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical,within analytical uncertainties, to the 776±12Ma zircon U–Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming ‘microvein’ and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatic–hydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of or thomagmatic or hydrothermal fluids containing fluorides as a major constituent,which expelled a considerable amount of trace elements,namely,Hf, U, Th,Y, and theREEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼0.5 to 1.0). Furthermore, the ca. 780–776Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes.
    Early Archean to Middle Jurassic Evolution of the Korean Peninsula and Its Correlation with Chinese CratonsSHRIMP U-Pb Zircon Age Constraints [查看] HeejinJeon、MoonsupCho、HyeoncheolKim、KenjiHorie、HiroshiHidaka
    U-Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high-resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early-Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early-Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1)Early-Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic(1.98–1.75 Ga), (4) Middle-Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early Permian (280–255 Ma), and (7) Middle Triassic–Early Jurassic (240–180 Ma). These age distributions, together with available geochronological data, suggest that crustal growth of the Korean Peninsula has continued since ca. 3.6 Ga and culminated at ca. 2.5 and 1.9–1.8 Ga. Major age populations of detrital zircons of the Bansong and Nampo groups are similar, except for the presence of Middle-Late Proterozoic ages in the latter. Inasmuch as these ages are characteristic for the South China Block, the Gyeonggi massif, or at least the local source of the Nampo Group, is most likely a correlative of the South China Block.
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