High-grade Metamorphic Event Research Articles

Unraveling the metamorphic history at the crossing of Neoproterozoic orogens, Sør Rondane Mountains, East Antarctica: Constraints from U–Th–Pb geochronology, petrography, and REE geochemistry

The Sør Rondane Mountains in East Antarctica preserve two stages of tectonothermal events, at c. 650–600Ma and c. 560–550Ma, each of which is also widely preserved elsewhere in Gondwana within tectonic suture zones/orogens, although both ages of tectonothermal events are only rarely documented together in these other parts of Gondwana. A geochronological and petrogenetic study of highly retrogressed garnet–sillimanite–biotite-bearing pelitic gneiss, along with three generations of associated sub-concordant to discordant leucocratic felsic veins, which together are exposed in the southern Austkampane area in the central part of the Sør Rondane Mountains, provides important new constraints on the geological and age relationships between these two regional tectonothermal events. U–Pb SHRIMP dating of zircon grains isolated from the host garnet–sillimanite–biotite gneiss yields at least four distinct zircon age populations of c. 2800–720Ma, c. 700Ma, 640–630Ma, and ∼500Ma. Chondrite-normalized REE patterns indicate that a pooled age of 637±6Ma represents the timing of recrystallization of metamorphic HREE-depleted zircon, during which the zircon was in equilibrium with garnet. The earliest emplaced leucocratic vein (LV-1), which is partly intermingled with the host garnet–sillimanite–biotite gneiss (and enclosing garnet-bearing pelitic enclaves), contains a population of zircon grains that yielded a crystallization age of 635±4Ma, which is almost identical to the age of zircons found within the host pelitic gneiss. The second-generation leucocratic vein (LV-2) occurs as a part of a set of sub-concordant veins/leucosomes comprising mostly biotite and muscovite. Although these particular leucosomes do not contain any zircon, they were found to contain abundant monazite, which yielded U–Pb ages of 640–630Ma along with some older (relict) ∼700Ma ages. The third-generation leucocratic vein (LV-3) was sampled from a suite of pegmatitic veins that discordantly crosscut both the host pelitic gneisses and the older two sets of leucocratic veins. These third-generation veins contain magmatic zircons that exhibit HREE-enriched patterns typical of magmatic zircon crystallization, and which yielded a crystallization age of 550±3Ma. The data imply that a major high-grade metamorphic event (possibly at granulite facies) took place in this region along with a subsequent late-stage re-hydration event, all within a relatively narrow time interval of <10m.y. at ∼635Ma. A subsequent hydration event that took place much later on is recorded by the intrusion of a suite of discordant pegmatitic veins at ∼550Ma that are observed to crosscut and alter both the pre-existing pelitic gneiss and the other two older sets of felsic veins. These two ages of tectonothermal events in East Antarctica are coeval with the two major stages of Gondwana assembly. Hence, these results provide new insights into the development of metamorphic-fluid regimes within these major continent-transecting Neoproterozoic orogens.

Geochemistry and neodymium model ages of Precambrian charnockites, Southern Granulite Terrain, India: Constraints on terrain assembly

The Southern Granulite Terrain (SGT) of southern India comprises a collage of Archean and Neoproterozoic high-grade metamorphic terrains, where the regional tectonic and high-grade metamorphic events culminated at ca.2.52Ga in the northern granulite blocks and ca.0.55Ga in the Neoproterozoic Madurai and Trivandrum Blocks. These domains were presumed to be separated either by the crustal-scale Cauvery shear zone system (CSZ) or the less known Karur–Kambam–Painavu–Trichur shear zone (KKPT). The Neoproterozoic granulite domains comprise mainly the Madurai Block (MB) to the north and Trivandrum Block (TB) to the south separated by the Achenkovil shear zone (ASZ) system. The SGT has been central to several studies focusing on the amalgamation of Gondwana supercontinent. A favored view has been that the Neoproterozoic terrains of the SGT represent segments of orogen(s) that accreted to an Archean continental margin during the early Cambrian although whether the tectonic regime was collisional, accretionary or intracratonic remains enigmatic. This article presents a large data set of major, trace element, Rb–Sr and Sm–Nd isotopic compositions focusing on Sm–Nd model ages (TDM) signifying the crustal residence age of charnockitic orthogneisses from both Archean and Neoproterozoic domains of the SGT, especially from the data deficient MB and TB domains. We present a regional Sm–Nd model age map for the SGT that delineates four distinct isotopic provinces with ages in the range of ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga. Clearly, the KKPT shear zone separates two isotopic provinces, extending the Archean terranes further south across the CSZ. The Archean and Neoproterozoic charnockites show distinct compositional characteristics indicating contrasting conditions of magma genesis and tectonic setting. Most importantly, an overwhelming majority of the Neoproterozoic charnockite gneisses show Sm–Nd model ages significantly older than their crystallization age constrained by zircon dating. The charnockite gneisses bear a strong affinity to calc-alkaline magmas and compositions consistent with mixing of Neoproterozoic arc magmas and older crustal components prompting a close analogy with magmatism at convergent margin-collisional tectonic settings.

Successive midcrustal, high-grade metamorphic events provide insight into Mid-Archean mountain-building along the SE margin of the proto-Kaapvaal craton

In this study, we investigate the evolution of continental crust at the Paleoarchean to Mesoarchean boundary by documenting the deposition, deep burial, and partial melting of metasedimentary gneisses along the SE margin of the proto–Kaapvaal craton. Successive high-grade metamorphic events recorded by the gneisses between ca. 3.23 and 3.07 Ga not only coincided with the timing of subduction-accretion in the adjacent Barberton greenstone belt at ca. 3.23–3.22 Ga, but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere. Mineral equilibria modeling demonstrates that the granulites evolved along pressure-temperature ( P - T ) paths similar to those documented for metamorphism in modern collisional orogens. High-temperature deformation at ca. 3.11–3.07 Ga during uplift of the terrane was coaxial with the main NE-SW–trending structural grain of the Barberton greenstone belt and reflects the regional NW-SE shortening and NE-SW orogen-parallel extension exhibited by the younger potassic granites. We reconcile these features with Mesoarchean terrane assembly in Barberton via two NW-dipping subduction zones. The trace of the first is represented by the main terrane boundary within the Barberton greenstone belt; the second resulted in accretion, burial, and high-temperature metamorphism along the SE margin of the Kaapvaal craton, with the granulites providing valuable insight into the mid- to lower-crustal response to what appears to have been a protracted accretionary orogenic event.

Neoproterozoic granulites from the northeastern margin of the Tarim Craton: Petrology, zircon U–Pb ages and implications for the Rodinia assembly

We report the discovery of a suite of Neoproterozoic granulite facies rocks from the vicinity of the Boston Lake area at the northeastern margin of the Tarim Craton. Mineral parageneses show that these rocks witnessed peak granulite facies metamorphism followed by retrograde amphibolite facies overprinting. Pressure–temperature computations of the Grt–Opx granulites yield ca. 810°C and 1.0GPa, falling in the lower domain of high-pressure granulite-facies P–T field. Most of the zircons in our samples display core–rim textures with metamorphic overgrowths on detrital cores. LA-ICP-MS U–Pb dating on the zircon cores yields three major age populations at ca. 0.9–1.0Ga, 1.8–2.0Ga and 2.4–2.6Ga, suggesting the composite sources for the protolith sedimentary rocks. The metamorphic zircons from different samples record a narrow age range between 790 and 820Ma, marking the timing of high grade metamorphism. This is the first report of Neoproterozoic high-grade metamorphic event from the northeastern margin of the Tarim Craton. Our results could suggest either a major collisional event associated with the assembly of the Tarim Craton within the supercontinent Rodinia, or an Andean-type orogeny at the periphery of the Rodinia supercontinent.

Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry, zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541±13Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461±15Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715±180Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related~2530Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial176Hf/177Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.

The high-grade Tseel Terrane in SW Mongolia: An Early Paleozoic arc system or a Precambrian sliver?

The high-grade metamorphic Tseel Terrane in southwestern Mongolia is dominated by a strongly deformed sequence of migmatitic gneisses which has previously been interpreted as an early Precambrian continental sliver. However, the protolith age and timing of high-grade metamorphism remain poorly constrained. To resolve this problem, high-grade gneisses selected from the western Tseel Terrane were dated in the present study. Zircons with oscillatory zoning from three paragneisses predominantly yielded early Paleozoic ages between 450 and 550Ma, with prominent peaks at 471 and 506Ma. U–Pb dating of zircons from a granitic gneiss revealed that the protolith was emplaced at 420Ma. Zircons from a leucocratic granite vein gave a concordant age of 385±3Ma, close to ages of metamorphic zircons in the gneissic rocks, which is interpreted to record a high-temperature thermal event in the region. These results indicate that the Tseel Terrane principally consists of early Paleozoic rocks which underwent a high-grade metamorphic event in the Middle Devonian. In addition, SHRIMP dating of dominant magmatic zircons from an associated schist yielded a concordant U–Pb age of 495±5Ma, similar to those of the major zircon group in the high-grade paragneiss. A felsic tuffaceous sample yielded a U–Pb zircon age of 511±4Ma, documenting early Paleozoic volcanism which probably contributed to the deposition of the sediments in the region. Geochemically, high-grade paragneisses and low-grade schists show close similarities, and their common features suggest that their protoliths may represent significant erosion products of Cambrian arc magmatic rocks with minor old crustal components that were deposited in an active margin setting. The granitic gneiss with subduction-related chemical characteristics further favors an arc setting for the Tseel Terrane. Accordingly, our data combined with previous work suggest that the Tseel Terrane represents the constituent part of an early Paleozoic arc system situated on the southern margin of North Mongolian Domain. This arc system extended from western Mongolia to the Chinese Altai because various geological features of these two regions have close similarities.

The Chuacús Metamorphic Complex, central Guatemala: geochronological and geochemical constraints on its Paleozoic - Mesozoic evolution

The Chuacus Metamorphic Complex is located in Central Guatemala, between the Polochic and Motagua fault zones. It is made up of complexly intercalated, mafic and felsic high-grade gneisses, amphibolites, pelitic and quartzofeldsphatic metasediments and subordinate marbles. Mafic dikes and lenses metamorphosed to amphibolite and eclogite facies are tholeiitic and similar to mid-ocean ridge basalts. In contrast, metamorphosed intrusives (gabbro, diorite and granite) are calc-alkaline and have the geochemical signature of arc magmas. Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-Pb zircon geochronology allows the recognition of three episodes of metamorphism. The first eclogite facies metamorphism (M1) is bracketed between Ordovician magmatism in the northern Chuacus Metamorphic Complex and the neighboring Rabinal granitic suite; the second corresponds to an Upper Triassic period of arc magmatism and migmatization (M2); the third high-grade metamorphic event (M3) occurred during the Late Cretaceous. The tectonic evolution of the Chuacus Metamorphic Complex began during the Early Paleozoic as a basin in the Rheic Ocean that received detrital material from the Maya Block, Acatlan and southeastern Mexico. The Chuacus Metamorphic Complex evolved to an active margin that subducted to HP conditions during the Mid- Late Paleozoic, and then was exhumed and involved in two tectonothermal events during the Upper Triassic and Late Cretaceous. The Chuacus Metamorphic Complex was accreted to the southern Maya Block during the Late Cretaceous, as a result of the convergent tectonics between the latter and either the Greater Antillean arc or the Chortis Block

Using zircon U–Pb ages and trace element chemistry to constrain the timing of metamorphic events, pegmatite dike emplacement, and shearing in the southern Parry Sound domain, Grenville Province, Canada

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic relations from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending development of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underlying granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older ( ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplacement and shear deformation at ca. 1100 Ma, and (3) a later ( ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.

Timing of accretion, intra-orogenic sedimentation and basin inversion in the Paleoproterozoic Svecofennian orogen: The Pyhäntaka area, southern Finland

The Pyhäntaka area in southern Finland represents the accretionary Svecofennian orogen, presently exhumed to upper-mid crustal level. The post-1.87 Ga volcanic-sedimentary Pyhäntaka formation is separated from underlying cordierite paragneiss (metagraywacke) by an unconformity, marked by an ancient weathering surface on top of the paragneiss and ultramature quartzite at the base of the Pyhäntaka formation. Structural studies and U–Pb datings carried out in the paragneisses, metavolcanic rock interlayer and crosscutting granitoid rocks revealed four tectonic units, separated from each other by shear zones and representing different depositional ages (pre-1.87 Ga and post-1.87 Ga). Yet unidentified post-1.87 Ga rock sequences are anticipated in the Svecofennian orogen. The model of tectonothermal evolution in the Pyhäntaka area includes contraction at 1.89 Ga causing rapid burial of graywackes (Härkälä graywackes) to mid-crustal levels where they were metamorphosed into cordierite gneisses and intruded by syntectonic granite at 1875 Ma. Deposition of exhumation-related graywackes (Himattu graywackes) after 1.87 Ga, followed by development of a weathering surface and deposition of quartz sands, indicate gradual stabilization of the upper crust. A sequence of quartz sands, immature sands, volcaniclastic sands and basaltic rocks (the Pyhäntaka formation), and graywackes on top (Kesua graywackes) were deposited in an intra-orogenic rift basin during crustal extension that may have initiated 1.85 Ga ago. Contraction at 1825 Ma caused basin inversion with thrusting and folding of the post-1.87 Ga sequence. All rocks in the Pyhäntaka area were subject to a new high-grade metamorphic event that culminated at 1.82 Ga. Subsequent uplift of a pre-1.87 Ga rock sequence relative to the post-1.87 Ga sequence resulted in the present pattern of tectonic units. The inferred evolution at Pyhäntaka conforms to several tectonic models presented for the Svecofennian orogen. However, the occurrence of an ancient weathering surface and ultramature quartzite, an evidence for a 10–20 Ma stable period, is more compatible with continental interior setting with outboard subduction than active continental margin setting during deposition of the Pyhäntaka formation.

Crustal homogenization revealed by U–Pb zircon ages and Hf isotope evidence from the Late Cretaceous granitoids of the Agaçören intrusive suite (Central Anatolia/Turkey)

Geochemical and isotopic evidence from the Agacoren Igneous Association in central Anatolia-Turkey indicates that this suite of calc-alkaline granitic rocks have undergone crustal homogenization during regional metamorphic and related magmatic events. Whole-rock chemical and Sr–Nd isotopic data of the granitoids reveal crustal affinity with an earlier subduction component. Zircons show inherited cores and subsequent magmatic overgrowths. The laser ablation ICP-MS 206Pb/238U zircon ages are determined as 84.1 ± 1.0 Ma for the biotite-muscovite granite, 82.3 + 0.8/−1.1 Ma for the hornblende-biotite granite, 79.1 + 2.1/−1.5 Ma for the granite porphyry dyke, 75.0 + 1.0/−1.0 Ma for the alkali feldspar dyke, and 73.6 ± 0.4 Ma for the monzonite. This is interpreted as continuous magma generation, possibly from heterogeneous sources, from ca. 84 to 74 Ma during the closure of the northern branch of the Neotethyan Ocean. The oldest granitoids (84–82 Ma) were probably formed due to crustal thickening after obduction of the MORB-type oceanic crust onto the Tauride-Anatolide microplate. The younger granitoids are interpreted to be related to the subsequent post-collisional extension after lithospheric delamination. Combination of the laser ablation ICP-MS zircon Lu–Hf isotope data with the U–Pb ages of inherited cores suggests that Cretaceous granitoids formed by melting of heterogeneous crustal protoliths, which results in significant variation in eHf(t) data (from −12.9 to +2.2). These protoliths were probably composed of reworked Early Proterozoic crust, minor juvenile Late Proterozoic magmatic components, and Paleozoic to pre-Late Cretaceous recycled crustal material. Moreover, the Late Cretaceous zircon domains of the different granitoids are characterized by a crustal signature, with a relatively restricted zircon eHf(t) data ranging from −4.1 to −8.8. This variation is only about twice the reproducibility (ca. ±1 eHf) of the data, but much smaller than the isotope variability of inherited zircons. Our preferred interpretation is effective isotopic homogenization of the heterogeneous central Anatolian crust during the Late Cretaceous high-grade metamorphic and magmatic events, a process that we propose to be relevant for other active continental margins.

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating.

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20–1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG–type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

Paleoproterozoic evolution of the eastern Alxa Block, westernmost North China: Evidence from in situ zircon U–Pb dating and Hf–O isotopes

The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34Ga and ca. 2.32–2.30Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394ppm) and Ti/V (>30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98Ga, and subjected to high grade metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.

Late Neoproterozoic P-T evolution of HP-UHT Granulites from the Palni Hills (South India): New Constraints from Phase Diagram Modelling, LA-ICP-MS Zircon Dating and in-situ EMP Monazite Dating

Late Neoproterozoic (c. 555 Ma) high-pressure-ultrahigh-temperature (HP-UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P-T evolution, which is constrained through P-T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000 degrees C, 13 center dot 0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile +/- garnet +/- quartz +/- sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase +/- quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010-920 degrees C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10 center dot 4 wt % Al(2)O(3)). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of < 800 degrees C at mid-crustal levels (c. 6 kbar). HP-UHT metamorphism and the clockwise P-T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U-Pb analyses of zircon and in situ electron microprobe U-Th-total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 +/- 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 +/- 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P-T-t evolution indicates that HP-UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm.

The origin of high δ18O zircons: marbles, megacrysts, and metamorphism

The oxygen isotope ratios (δ18O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ18O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ18O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ18O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ18O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ18O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ18O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ18O source for the high δ18O zircons. Predicted equilibrium values of Δ18O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ18O marble will have high δ18O. The high δ18O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ18O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid(external)/rock); the lower δ18O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ18O. High δ18O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ18O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ18O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.

U–Pb SHRIMP ages for the Cerro Bori Orthogneisses, Dom Feliciano Belt in Uruguay: Evidences of a ∼800 Ma magmatic and ∼650 Ma metamorphic event

Neoproterozoic ages of magmatic and metamorphic events were obtained from in situ SHRIMP analysis of zircons from the Cerro Bori Orthogneisses, eastern domain of the Dom Feliciano Belt in Uruguay. Detailed textural analysis of zircons and their ages revealed a much more complex evolutionary history for these rocks than previously thought. Twelve samples were studied and revealed crystallization ages between 802 and 767Ma, determined from the typical magmatic oscillatory zoning domains from the zircons. These magmatic domains are cut by recrystallization fronts and mantled by metamorphic rims. The recrystallization fronts and rims are interpreted to be related to a high grade metamorphic event with a maximum age of ∼676Ma, whereas the rims considered to be related to partial melting are 654±3Ma old. The new magmatic ages demand a reinterpretation of the evolutionary history of this crustal segment, which is one of the few occurrences of the early Brasiliano Orogenic Cycle rocks in South Brazil and Uruguay. The metamorphic/partial melting event is inferred to be related to crustal thickening as a consequence of collision of the Rio de la Plata with the Congo and Kalahari cratons, during the amalgamation of West Gondwana.

The Nagercoil Charnockite: a Magnesian, Calcic to Calc-alkalic Granitoid Dehydrated during a Granulite-facies Metamorphic Event

The Paleoproterozoic Nagercoil charnockite massif in southern India consists of garnet-absent and garnet-bearing varieties, with garnet occurring only in those areas where the charnockite incorporates supracrustal lithologies. There is a late Neoproterozoic^Cambrian granulite-facies metamorphic overprint on the charnockite massif. This study examines whether orthopyroxene in the charnockites crystallized during the original magmatic stage or formed during the later high-grade metamorphic event. The textural association of orthopyroxene and relic garnet grains, and reaction intergrowths of garnet and/or biotite with orthopyroxene, similar to those reported in dehydration zones within the gneisses surrounding the charnockite massif, show that orthopyroxene in the charnockite massif formed during the late high-grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage. Metasomatic textures such as thin K-feldspar rims, together with the presence of CO2-rich fluid inclusions in these rocks, suggest that CO2 metasomatism, widely prevalent in the gneisses surrounding the massif, probably resulted in the low water activity necessary to form orthopyroxene during the metamorphic overprint. An oxygen isotope traverse along a metapelite enclave^charnockite contact within the massif shows a distinct variation in d 18 O values, with the charnockite away from the contact having the lowest d 18 O value (þ8·9o) similar to that of typical metaluminous granitic rocks. The charnockite at the contact shows an intermediate d 18 O value (þ10·3o) and the surrounding metapelite has the highest d 18 O value (þ12o); the intermediate d 18 O value indicates partial assimilation of metapelite with a high d 18 O value into the charnockite with a low d 18 O value. Both the garnet-absent and garnet-bearing charnockites have similar compositional ranges to tonalite^granodiorite^granite (TTG) suites. Irrespective of whether garnet is present or not, the Sr contents of the charnockites show a bimodal distribution: a high-Sr group, which is magnesian, calcic to calc-alkalic, metaluminous, showing TTG affinities, and a low-Sr group, which is dominantly ferroan, calcic to calc-alkalic, metaluminous, showing similarities to the orthopyroxene-bearing dehydration zones. Petrogenetic characterization suggests that the protoliths of the Nagercoil charnockites were high-temperature melts of hydrous basalt.

Geocronologia SHRIMP del basamento de la Cuenca de Magallanes, Tierra del Fuego: plutonismo Cambrico y metamorfismo Permico de alto grado.

Cinco nuevas edades radiometricas logradas mediante analisis U-Pb en circon utilizando el SHRIMP, fueron determinadas en gneises y rocas plutonicas foliadas obtenidas desde el fondo de pozos de sondajes en la Cuenca de Magallanes y pertenecientes al denominado Complejo igneo y Metamorfico de Tierra del Fuego. En tres de las muestras fueron calculadas edades del Cambrico Temprano (523&plusmn;7 Ma, 522&plusmn;6 Ma y 538&plusmn;6 Ma), interpretadas como edades de cristalizacion ignea de las rocas estudiadas. Un gneiss migmatitico presenta 'peaks' de circones heredados de ca. 950-1.100 Ma y de 560-650 Ma ademas de dos granos de edades de 525 Ma, indicando la participacion de material Grenviliano y Brasiliano en el protolito de la migmatita cambrica. Un gneiss de cordierita-sillimanita-granate presenta una poblacion de circnoes igneos de edad cambrica y otro grupo de circnoe. metamorficos, ricos en V, de edad permica, indicando que en el Permico un evento metamorfico del alto grado acompa&ntilde;ado de anatexis (P~2-3 kbar, T~730-770&deg;C) afecto a rocas Igneas cambricas y/o a rocas sedimentarias derivadas de ellas. Rocas plutonicas cambricas/ediacaranas han sido descritas en el NW de Argentina, en los basamentos de la Sierra de la Ventana y del Cinturon Plegado del Cabo (en el sur de Africa), y en el Orogeno de Ross en Antartica. El evento metamorfico Permico es contemporaneo con la deformacion y metamorfismo de bajo grado, registrado en las sucesiones sedimentarias que sobreyacen el basamento en muchas de estas areas. Siguiendo al episodio Permico de metamorfismo de alto grado, en Tierra del Fuego fueron removidos al menos 8 a 12 km de rocas de cobertura antes de la depositacion de las rocas volcanicas de la Formacion Tobifera en el Jurasico Medio y Superior. Las rocas erosionadas representan una fuente importante de material para las sucesiones conglomeradicas permicas y triasicas? ubicadas en las regiones vecinas de America del Sur, Africa y Antartica.

Tectonothermal ordovician evolution of the western margin of the Famatinian magmatic arc: metamorphism of the mafic and ultramafic rocks of Sierra de la Huerta -Las Imanas (Pampean Ranges, Argentina)

An important magmatic and tectonometamorphic activity of Early and Middle Ordovician age is registered in the pre-Andean basement of the Sierras Pampeanas of Argentina. These were linked to the development of a continental magmatic arc during the Famatinian Orogeny, resulting from the approach and attachment of an alleged exotic terrane (the Precordillera Terrane), to the south western Gondwana's margin (present coordinates). A suit of meta-mafic and ultramafic rocks are exposed in the Sierras de La Huerta and Las Imanas, at the western limit of the famatinian orogen. Metaperidotites (Ol-Opx-Cpx-Am-Spl), coronitic metapyroxenites (Opx-Cpx-Spl-Am-Pl), metaquartz-norites (Opx-Pl-Am-Qtz-Bt±Grt) and metadiorites (Pl-Am-Qtz-Bt-Ep) are associated with metasedimentary rocks (marbles, gneisses and migmatites with Sil+Kfs+Grt) that reached the peak and post-peak conditions of metamorphism at middle Ordovician time. The meta-mafic rocks record a first high-grade metamorphic event (M1-730ºC and 8.4±0.5 kbar) and a second lower pressure event (M2, 720ºC and 4.5 kbar) with Cum+Hbl+Mag in a coronitic assemblage. The meta-ultramafic rocks also record the two metamorphic events, but only for the second one it was possible to calculate the P-T conditions. At latitude 32º30'S, the Famatinian magmatic arc shows a systematic compositional variation normal to its trend, i.e. in an east-west direction, which could be related to present erosion levels. In this context, the mafic and ultramafic units of Sierras de La Huerta and Las Imanas, probably represent the deepest levels of the magmatic arc which is consistent with the position that they show marginal to the orogenic belt, i.e., where the uplift and erosion rates were larger.

The Eastern Ghats Belt … A polycyclic granulite terrain

The Eastern Ghats Belt is a polycyclic granulite terrain along the east coast of India whose western boundary is marked by a shear zone along which the granulites are thrusted over the cratonic units of the Indian shield, and its northern margin is marked by the presence of a number of fault-bounded blocks. Recent work has convincingly brought out that there are domains within the belt having different evolutionary histories. The segment south of the Godavari Rift went through a high grade thermo-tectonic event at ∼1.6-1.7 Ga. North of the Godavari Rift in a narrow zone along the western boundary the last high-grade metamorphic event is of late Archaean age. A series of alkaline plutons along the western boundary zone testifies to a rifting episode at ∼1.3-1.5 Ga. In the major part of the EGB the metamorphism is broadly of Grenvillian age, with two major thermo-tectonic pulses at ∼1.1-1.2 Ga and ∼0.95-1.0 Ga. But high grade conditions persisted for a long period and younger thermal events of ∼0.65 Ga to ∼0.80 Ga are locally recorded. There are differences in the tectonometamorphic histories of different domains, but the tectonic significance of these differences remains uncertain. Pan-African (0.50-0.55) thermal overprints are common and become conspicuous along the western boundary zone. The thrusting of the Eastern Ghats granulites in a hot state over the cratons to the west is of Pan-African age. In the Rodinia assembly (∼0.9 Ga) the Eastern Ghats and the Rayner-Napier Complexes of Antarctica were contiguous, but the pre-Rodinia configuration of these terrains remains unclear. At ∼0.8 Ga during the Rodinia break up Greater India rifted apart from East Antarctica, and only later it docked with Australia-East Antarctica at 530-550 Ma. The continuation of the East Antarctic Pan-African orogenic belts into the Eastern Ghats is yet to be ascertained.

SHRIMP U–Pb age constraints on magmatism and high-grade metamorphism in the Salem Block, southern India

In this paper, we present Sensitive High Resolution Ion Microprobe (SHRIMP) U and Pb isotope analyses of zircon from a charnockite and a charnockite-hosted leucosome in order to determine the age of magmatism and anatexis related to high-grade metamorphism immediately to the north of the Palghat Cauvery Shear System (PCSS), a proposed Neoproterozoic terrane boundary within Southern India. Weakly luminescent, oscillatory-zoned cores in I06–128 and analyses with high Th/U ratios in I06–129 yield weighted mean 207Pb– 206Pb ages of 2538 ± 6 Ma and 2529 ± 7 Ma. These two ages are statistically indistinguishable and are interpreted to reflect the crystallisation age of the original magmatic protolith to the charnockite. Low Th, Th/U ratio and brightly luminescent overgrowths and complete zircon grains in I06–128 and I06–129 give weighted mean 207Pb– 206Pb ages of 2473 ± 8 Ma and 2482 ± 15 Ma respectively. The two ages are statistically distinct and are interpreted to constrain the timing of post-crystallisation high-grade metamorphism and partial melting of the magmatic rocks that crystallised at ~ 2530 Ma. The new ages of the charnockites are consistent with the suggestion that this activity is related to accretionary processes on the margin of the Dharwar craton and may be related to a second stage of convergent tectonics and collision on the margin of Kenorland. There is no evidence of a pervasive Neoproterozoic high-grade metamorphic event associated with the amalgamation of Gondwana recorded in these rocks. However, the possibility that deformation was localised along discrete shear zones at this time cannot be discounted. The data from this study add weight to the hypothesis that the PCSS represents a Neoproterozoic suture zone along which the Dharwar Craton and the microcontinent Azania, with its constituent Madurai and Trivandrum Blocks in the Southern Granulite terrane, collided at ca. 530 Ma Ga during the closure of the Mozambique ocean.