Monazite Chemical Ages Research Articles

Geochemical and geochronological evolution of the UHT granulites from the Eastern Ghats Mobile Belt, India: implications from F-OH substitution in biotite, thermal events and cooling age

ABSTRACT The thin lenses and bands of ultra-high temperature (UHT) granulites with diagnostic sapphirine and quartz in contact are occasionally found on the western flank of the Eastern Ghats Mobile Belt (EGMB), which inherits the suturing and over-thrusting signatures of the EGMB with the Greater Indian Landmass (GIL). The mineral biotite contains an abnormally high fluorine concentration, as revealed by the geochemical examination of these granulites from the research area (Kothuru), which was caused by the F-OH substitution to withstand extreme thermal metamorphic conditions. The geochemical results from the multi-element spider diagram and the Harker’s variation diagram with primitive-mantle normalization also show an enrichment of light rare earth elements and large-ion lithophile elements over depleted heavy rare earth elements and high-field strength elements. An evolutionary history has been established for the high-grade rocks that were subjected to ultra-high temperature metamorphism, based on the geochemical signatures of the examined granulites. The use of isotopic K-Ar cooling ages and electron microprobe monazite chemical ages of the thermal episodes, which act as useful tools to explain the time and span of the polycyclic metamorphic terrane, is another significant component of the current work. Numerous researchers have attempted to estimate the thermal episodes in various areas of the Eastern Ghats Mobile Belt, but their cooling ages are still poorly understood. Consequently, two thermal episodes that happened at 842 Ma and 637 Ma ago, as well as a cooling age of about 448 Ma that marked the quick exhumation of the high-grade rocks under study, have been recorded in the current investigation. The association between the Greater Indian Landmass and Eastern Antarctica during the existence of the Rodinian supercontinent has been established by recorded geochronological events.

Tectonics of the Greater India Proterozoic Fold Belt, with emphasis on the nature of curvature of the belt in west-central India

The Greater India Proterozoic Fold Belt (GIPFOB) is a curved highly tectonized zone dominated by Early Paleoproterozoic to Early Neoproterozoic magmatic and metamorphic rocks extending from NW India (Aravalli Delhi Fold Belt, ADFB) through central India (the Satpura Mobile Belt, SMB) to eastern India (the Chottanagpur Gneiss Complex, CGC). The continuity of the crustal domains within GIPFOB is obscured by the Gondwana Formation, the Mesozoic Deccan basalts, the intertrappean Bagh Beds and the infratrappean Lameta Formations, and younger alluvium. In supercontinent reconstructions the GIPFOB is speculated to be continuous with Proterozoic mobile belts in Western Australia and the Trans North China Orogen. The NNE-striking western arm (ADFB) of the GIPFOB is flanked by the North India Block (NIB) in the east and the Marwar Craton in the west, whereas the E-striking southern arm (CGC-SMB) is sandwiched between the South India Block (SIB) and the NIB. In the Godhra-Chhota Udepur sector (west-central India) the two arms converge. We synthesize and compare existing data on mesoscale structures, U-Pb (zircon) and monazite chemical ages, and magmatic and metamorphic histories in the Precambrian crystalline rocks in the three crustal blocks to constrain the accretion dynamics in the GIPFOB with an emphasis on the origin of the curvature in the Godhra-Chhota Udepur sector. The CGC and the central and southern domains of the SMB share considerable homogeneity in chronology and regional structures. In the CGC, the early, N-striking steep-dipping tectonic fabrics in ~1.6 Ga anatectic gneisses with ~1.45 Ga and 1.05–0.9 Ga granitoids are modified into a carapace of shallowly-dipping tectonic mélange of recumbently folded basement gneisses and granitoid mylonites thurst over by allochthonous supracrustal rocks at ~0.95 Ga. The central and southern domains within SMB also exhibit the shallowly-dipping foliations and the emplacement of 1.05–0.9 Ga granitoids. The unmodified basement and the overlying tectonic mélange in the CGC-SMB are traversed by 1.0–0.9 Ga E-striking steep-dipping sinistral (dominant) and dextral (uncommon) shear zones that accommodated the Early Neoproterozoic transpressive deformation induced by the NIB-SIB oblique collision. The mesoscale structures in the rocks of the Godhra-Chhota Udepur sector intruded by Early Neoproterozoic granitoids and followed by oblique collision are similar to CGC-SMB. In the ADFB, by contrast, the basement rocks are older (Archean to Early Paleoproterozoic), high-grade metamorphism and felsic magmatism are older (1.8–1.7 Ga), the expansive ~1.45 Ga granitoids are absent, nappe structures are locally present, and expansive domains of the shallowly-dipping foliations are lacking. Overall, the tectonic evolution of the ADFB is incoherent with those in CGC-SMB and the GC sector. We suggest that the structures in the N/NNE-striking western accretion arm terminate against the E-striking southern arm. The Early Neoproterozoic (1.0−0.9 Ga) integration of the crustal domains within the GIPFOB resulted due to broadly contemporaneous convergence of the NIB, the SIB and the Marwar Craton (?) during the Rodinia supercontinent assembly, but the accretion along the southern arm post-dated the accretion in the western arm. [482 words]

Early Neoproterozoic Accretion at the Northern Margin of the Chottanagpur Gneiss Complex, Eastern India

AbstractThe study based on structural mapping, monazite chemical age determinations and analyses of P–T pseudosections explores the hitherto undocumented accretion dynamics of the northern margin of the Chottanagpur Gneiss Complex (CGC), Eastern India. In the Gaya‐Chatra‐Koderma sector straddling the northern margin of the CGC, the CGC block comprises a shallow‐dipping carapace of 1.5–1.4 Ga and 1.0–0.9 Ga mylonitized granitoids, intrusive into 1.6–1.5 Ga anatectic basement gneisses, and supracrustal rocks inter‐leaved with recumbently folded gneisses. The carapace, structurally overlying the steeply dipping high‐T fabrics in the basement anatectic gneisses exposed in the tectonic windows, formed due to top‐to‐the‐north thrusting of the allochthonous CGC supracrustal rocks over the ∼1.7 Ga undeformed granitoids and steeply dipping greenschist‐facies metamorphic rocks (∼1.8 Ga) in the northern block. Continued N‐S shortening caused steeply dipping ENE‐striking sinistral and ESE‐striking dextral shear zones that locally modify former structures in the CGC foreland. P–T pseudosection analyses indicate that locally developed Early Neoproterozoic garnet‐bearing leucosomes in a hornblende‐biotite granitoid formed by lower crustal decompression melting, whereas the structurally overlying mica schist in the shallow‐dipping carapace experienced mid‐crustal loading. We suggest the accretion strain accommodated by the foreland thrusts were contemporaneous with extensional detachment zones that exposed lower parts of the thickened orogen. The tectonic events in the CGC appear to correlate poorly with those in the Capricorn Orogen and the Albany Fraser Orogen in Western Australia.

Multi‐stage metamorphic history of the Oki gneisses in Japan: Implications for Paleoproterozoic metamorphism and tectonic correlations in northeastern Asia

AbstractMetamorphicP–Tconditions, monazite chemical ages and zircon U–Pb ages from the gneisses exposed in the Oki belt, Japan, were integrated to unravel the multi‐stage metamorphic history of the belt. Microstructural observations combined with obtainedP–Tconditions and metamorphic ages reveal three distinct stages of metamorphism: M1, M2 and M3. The M1 stage occurredc. 1.85 Ga with high‐Tgranulite‐facies metamorphism (793–803°C and 9.8–12.3 kbar and 738–755°C and 9.1–12.0 kbar in the southwestern and southeastern Oki gneisses, respectively). The age of the M1 stage is well recorded in monazites included in large garnet porphyroblasts and low Th/U metamorphic rims in zircons from the Oki gneisses. The M1 metamorphism was overprinted byc. 230 Ma metamorphism (M2), which occurred at granulite‐facies conditions (817–829°C and 9.0–10.3 kbar) in the southwestern Oki gneisses and at upper amphibolite‐facies conditions (693°C and 5.3 kbar) in the southeastern Oki gneisses. Monazites in small garnets, euhedral zircons and outermost rims of zircons crystallized during this stage. The final metamorphism occurred as retrograde amphibolite‐facies recrystallization (M3) at conditions of 558–638°C and 3.7–4.8 kbar. The inherited cores in zircons yield ages from Paleoarchean to Paleoproterozoic but lack Palaeozoic ages. The detrital zircon distribution and the Paleoproterozoic metamorphic event in the Oki belt support the idea that the Oki gneisses are fragments of a Precambrian terrane rather than Palaeozoic sediments derived from the terrane. Combined with previous studies, we concluded that thec. 1.85 Ga M1 high‐Tgranulite‐facies metamorphism in the Oki belt could be related to that of the Jiao‐Liao‐Ji belt in the eastern North China block via the northern Gyeonggi and Nangrim Massifs on the Korean Peninsula, whereas thec. 250–230 Ma M2 stage could be associated with collision between the North and South China blocks. The Oki belt geologically corresponds to the northern Gyeonggi Massif in South Korea due to their similar Paleoproterozoic and Triassic tectonothermal events.

Evidence of Archaean metamorphism from the Yerrabali schist belt of Eastern Dharwar craton from EPMA dating of monazite

The Dharwar craton is divided in to the Western and Eastern Dharwar craton (WDC and EDC) by the N–S trending Chitradurga Shear zone. The EDC mainly consists of late Archean granites interspersed with N–S trending linear schist belts. The Yerrabali Schst belt (YSB) is located at the north-eastern corner of the EDC, near the Karimnagar granulite belt. The YSB is metamorphosed in middle-upper amphibolite facies and consists of Banded Magnetite Quartzite (BMQ), ferruginous quartzite, ultramafics, quartz–cordierite–gedrite ± garnet bearing quartzite, dolomite, amphibolite, deformed meta-pyroxenite, tremolite schist, sillimanite–garnet–cordierite schist, and dolerite dykes. We have dated monazites in the sillimanite–garnet–cordierite schist, exposed at the SW part of the YSB by EPMA chemical method. So far no dates are available from the YSB. The monazite chemical ages with three distinct populations, viz., 2658 ± 42, 2817 ± 19 and 3097 ± 34 Ma, probably correspond to three distinct metamorphic phases. The peak metamorphism at around 2800 Ma corresponds with the garnet and cordierite growth. A temperature of 673°C and a pressure of 4.7 kb have been estimated for peak metamorphism.

Constraining paleoproterozoic (∼1.7 Ga) collisional orogenesis between the Eastern Dharwar and Bastar cratons: New Sm–Nd garnet isochron and Th–U-total Pb monazite chemical ages from the Bhopalpatnam orogen, central India

The timing of assembly into India by amalgamation of cratonic fragments, is controversial with estimates ranging from Tonian, earliest Mesoproterozoic to Neoarchean. All models have as the nucleus the South Indian Block of Dharwar, Bastar and Singhbhum cratons. Although the time of assembly of the Bastar and Singhbhum cratons is uncertain, the implication of a proposed 1891–1883 Ma-aged Large Igneous Province, straddling the Bastar and Eastern Dharwar cratons is that their amalgamation occurred by Paleoproterozic time (≥1.9 Ga ago) and is at variance with the ages reported from the Bhopalpatnam granulite belt between 1.7-1.6 Ga ago. To test whether this belt formed by convergent collisional tectonics along the margins of these two cratons, petrochronologic analysis was conducted on high-grade rocks of this belt using garnet Sm–Nd and texturally-controlled Th–U-total Pb monazite chronometry.The whole rock-garnet Sm–Nd isochron from garnet-clinopyroxene-amphibolite in migmatite (M1a-stage), yielded an isochron age of 1692±13Ma with Ndi of 0.51042±0.00006 (2σ, MSWD = 2.5 n = 4) whereas a whole rock-garnet Sm–Nd isochron from interlayered garnet-sillimanite-rutile metapelite (M1b-stage), yielded an isochron age of 1675±7 Ma with Ndi of 0.50997±0.00003 (2σ, MSWD = 1.9 n = 3) Monazite inclusions within garnet porphyroblasts in this metapelite have a weighted mean age of 1703±28 Ma (2σ, MSWD = 0.95, n = 8), within error of the age of monazite inclusions within matrix biotite at 1665±16 Ma (2σ, MSWD = 0.56, n = 17). Post M1b stage lower temperature hydration-retrogression events likely occurred by late Mesoproterozoic time. The metamorphism followed a clockwise P-T path (high-pressure amphibolite-facies stage M1a P ≥ 12 kbar at ∼750 °C, followed by M1b granulite-facies stage with T ≥ 850 °C at lower pressures), interpreted as a response to crustal thickening during Paleoproterozoic continental collision. We propose that the Bhopalpatnam orogen is the exhumed locus of the leading edge of the Archean Bastar craton that was thrust under the EDC craton.

Late tectonic reorientation of lineaments and fabrics in the northern Eastern Ghats Province, India: Evaluating the role of the Mahanadi Shear Zone

Abstract South of the MSZ that cuts across the Eastern Ghats Province (EGP) of India, lineament trends and early granulite facies fabrics rotate from NNE-SSW in the south towards the WNW-ESE striking, northerly dipping Mahanadi Shear Zone (MSZ) in the north. Using field, microstructural and EBSD techniques, we demonstrate that D1 and D2 planar fabrics are reoriented by an E-W trending, northerly dipping D3 extensional shear zone that has a subordinate sinistral, strike-slip component. D3 microstructures are pervasive, with quartz and feldspar microstructures characteristic of amphibolite facies metamorphism. Monazite chemical ages from the D3 shear zone yield ages in the range 730–680 Ma. The MSZ, representing D4 deformation in the area, is a predominantly dextral strike-slip deformation. During D4, microstructures evolve exclusively through plastic deformation of quartz and culminates in S4 mylonitic fabric formation in high D4 strains zones under greenschist facies conditions. These microstructures are restricted to the MSZ and localized high D4 strain zones, unlike the D3 microstructures. We conclude that the earlier, post-granulite extensional deformation D3, and not the MSZ, was responsible for the swing in lineament and fabric trends in the northern EGP.

Pseudo- and real-inverted metamorphism caused by the superposition and extrusion of a stack of nappes: a case study of the Southern Brasília Orogen, Brazil

The Southern Brasilia Orogen is a Neoproterozoic belt that occurs along the southernmost border of the Sao Francisco Craton where the Andrelândia Nappe System represents the subducted sedimentary domain and is divided into three allochthonous groups, of which the ages and P–T conditions of metamorphism are studied here. The basal unit, the Andrelândia Nappe, exhibits an inverted metamorphic pattern. The base of the structure, composed of staurolite, garnet, biotite, kyanite, quartz, and muscovite, marks the metamorphic peak, whereas at the top, the association of the metamorphic peak does not contain staurolite. The Liberdade Nappe, the middle unit, presents a normal metamorphic pattern; its base, close to the Andrelândia Nappe, shows paragneiss with evidence of in situ partial melting, and towards the top, coarse-grained staurolite schist is found. The staurolite-out and melt-in isograds are coincident and parallel to the main foliation. Thus, the shear zone that limits the nappes is syn-metamorphic, reheating the underlying Andrelândia Nappe and influencing the establishment of metamorphic inversion. This suggestion is supported by the monazite chemical ages, which indicates that the Andrelândia Nappe metamorphic peak (586 ± 15 Ma) is younger than that of the Liberdade Nappe (622.3 ± 7.6 Ma). The upper unit, the Serra da Natureza Klippe, bears a typical high-pressure granulite mineral assemblage that is composed of kyanite, garnet, K-feldspar, rutile, and leucosome, as well as a metamorphic peak at 604.5 ± 6.1 Ma. This tectonic assembly, with inverted and non-inverted metamorphic patterns and generation of klippen structures, is consistent with exhumation models and a strong indentor located in the lower continental crust.

Restoration of Late Neoarchean–Early Cambrian tectonics in the Rengali orogen and its environs (eastern India): The Antarctic connection

Geological mapping and P–T path reconstructions are combined with monazite chemical age and Secondary Ion Mass Spectrometric (SIMS) U–Pb zircon age determinations to identify crustal domains with distinctive evolutionary histories in the Rengali orogen sandwiched between two Grenvillian-age metamorphic belts, i.e. the Eastern Ghats Granulite Belt (EGGB) in the south, and the amphibolite facies Gangpur Schist Belt (GSB) in the north, which in turn forms a collar along the NW/W margins of the Paleo/Mesoarchean Singhbhum Craton (SC) north of the Rengali orogen. Anatectic gneisses in the orogen core exhibit multi-phase Neoarchean/Paleoproterozoic deformation, metamorphic P–T histories and juvenile magma emplacement events. The high-grade belt is inferred to be a septum of the Bastar Craton (BC). The flanking supracrustal belt in the orogen — dominated by quartz-muscovite schists (±staurolite, kyanite, garnet pyrophyllite), inter-bedded with poorly-sorted and polymict meta-conglomerate, and meta-ultramafic/amphibolite bands — evolved along P–T paths characterized by sub-greenschist to amphibolite facies peak P–T conditions in closely-spaced samples. The supracrustal rocks and the anatectic gneisses of contrasting metamorphic P–T histories experienced D1, D2 and D3 fabric-forming events, but the high-angle obliquity between the steeply-plunging D3 folds in the anatectic gneisses and the gently-plunging D3 folds in the supracrustal unit suggests the two lithodemic units were tectonically accreted post-S2. The supracrustal belt is inferred to be a tectonic mélange formed in an accretionary wedge at the tri-junction of the Bastar Craton, the Eastern Ghats Granulite Belt and the Singhbhum Craton; the basin closure synchronous with the assembly of EGGB and the Singhbhum Craton–Gangpur Schist belt composite occurred between 510 and 610Ma. Based on the available evidence across the facing coastlines of the Greater India landmass and the Australo–Antarctic blocks at ~500Ma, it is suggested that the EGGB welded with the Greater India landmass during the Pan African along an accretion zone, of which the Rengali orogen is a part, synchronous with the final assembly of the Gondwanaland.

Tectonic restoration of the Precambrian crystalline rocks along the west coast of India: Correlation with eastern Madagascar in East Gondwana

New structural–mineralogical data and U–Th-total Pb monazite chemical ages in 27 samples in a 430km long corridor along the west coast of India are combined with existing data to reconstruct the tectonic set up of the Meso/Neoarchean crystalline rocks in the Western Dharwar Craton (WDC). The data helps to delineate two NW-trending Paleoproterozoic ductile shear zones that limit the southern and the northern margins of the WDC. The southern shear zone (metamorphic age: 2.3–2.4Ga) separates the greenschist facies supracrustal belts (2.5 and 3.3Ga), foliated granitoids (2.5 and 2.9Ga) and amphibolite facies anatectic gneisses (>3.0Ga) of the WDC from the >2.9Ga granulite facies ortho/para-gneisses of the Coorg Block. This shear zone is correlated with the ∼2.4Ga Betsimisaraka suture zone in east-central Madagascar that demarcates the accretion zone between the Antongil Block (≈WDC) and the granulite facies lithologies of the Antananarivo domain (≈Coorg Block). The northern shear zone system (metamorphic age: 2.2–1.8Ga) extending NW into Madagascar possibly exists as a hitherto undiscovered tectonic zone forming the basement of the Mesoproterozoic Sahantaha Formation underlying the Neoproterozoic Bemarivo Belt supracrustals in NE Madagascar.Within the WDC, the Meso/Neoproterozoic ages retrieved from poorly-defined margins in monazite are uncommon, dispersed within the craton, and do not define localized zones within the craton. The chemical ages of metamorphic monazites formed at greenschist/amphibolite facies conditions preclude metamorphism–deformation associated with accretion of crustal blocks within the WDC during the Rodinia assembly.

An Early Ordovician tonalitic–granodioritic belt along the Schistose-Greywacke Domain of the Central Iberian Zone (Iberian Massif, Variscan Belt)

AbstractThe Zarza la Mayor and Zarza de Montánchez tonalites and Arroyo de la Luz granodiorite are part of a tonalitic–granodioritic belt located along the Schistose-Greywacke Domain of the Central Iberian Zone. These intrusions are also part of the Central Extremadura Batholith, a set of plutons ranging from tonalite to leucogranite that have been considered a prime example of Variscan syn-kinematic plutonism. New LA-ICP-MS and CA-ID-TIMS U–Pb dating reveals that the Zarza la Mayor tonalite–granodiorite is an Early Ordovician intrusion. The LA-ICP-MS data show that there is an absence of inherited cores, despite some complex internal zoning with obvious resorption features in some of the zircon crystals. Dating of monazite and zircon by CA-ID-TIMS provides a concordant age of 478.1 ± 0.8 Ma. This age coincides with electron microprobe analysis (EMPA) monazite chemical ages for the Zarza de Montánchez (482 ± 10 Ma) and Arroyo de la Luz (470 ± 15 Ma) intrusions. These new data indicate the presence of an Early Ordovician belt of calc-alkaline tonalite–granodiorite in the Schistose-Greywacke Domain – the Beira Baixa–Central Extremadura tonalite–granodiorite belt – which resembles a continental magmatic arc. This belt is contemporaneous with the Ollo de Sapo magmatic event further north in the Central Iberian Zone.

India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic-Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India

Research Article| September 01, 2011 India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India Sankar Bose; Sankar Bose † 1Department of Geology, Presidency University, Kolkata 700 073, India †E-mail: sankar.bose@gmail.com Search for other works by this author on: GSW Google Scholar Daniel J. Dunkley; Daniel J. Dunkley 2National Institute of Polar Research, Tachikawa, Tokyo-to 190 8518, Japan Search for other works by this author on: GSW Google Scholar Somnath Dasgupta; Somnath Dasgupta 3Indian Institute of Science Education and Research, Kolkata 700 106, India Search for other works by this author on: GSW Google Scholar Kaushik Das; Kaushik Das 4Department of Geology, Bengal Engineering and Science University, Howrah 711103, India Search for other works by this author on: GSW Google Scholar Makoto Arima Makoto Arima 5Graduate School Environment and Information Science, Yokohama National University, Yokohama 2408501, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Sankar Bose † 1Department of Geology, Presidency University, Kolkata 700 073, India Daniel J. Dunkley 2National Institute of Polar Research, Tachikawa, Tokyo-to 190 8518, Japan Somnath Dasgupta 3Indian Institute of Science Education and Research, Kolkata 700 106, India Kaushik Das 4Department of Geology, Bengal Engineering and Science University, Howrah 711103, India Makoto Arima 5Graduate School Environment and Information Science, Yokohama National University, Yokohama 2408501, Japan †E-mail: sankar.bose@gmail.com Publisher: Geological Society of America Received: 17 May 2010 Revision Received: 13 Oct 2010 Accepted: 13 Nov 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (9-10): 2031–2049. https://doi.org/10.1130/B30336.1 Article history Received: 17 May 2010 Revision Received: 13 Oct 2010 Accepted: 13 Nov 2010 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sankar Bose, Daniel J. Dunkley, Somnath Dasgupta, Kaushik Das, Makoto Arima; India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India. GSA Bulletin 2011;; 123 (9-10): 2031–2049. doi: https://doi.org/10.1130/B30336.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We present zircon and monazite U-Pb data from ultrahigh-temperature (UHT) metamorphosed orthogneisses and paragneisses collected from key areas of the Eastern Ghats Belt, India. The results show contrasting tectonothermal histories in different isotopic domains of the Eastern Ghats Belt that were identified by previous workers. Of particular importance is the discovery of a ca. 1760 Ma event (concordia age) in the southern domain 1A, which is interpreted to be the age of an early UHT metamorphism event. This was followed by a second granulite-facies metamorphism event and partial melting at ca. 1600 Ma. This domain was presumably cratonized with India at around 1600 Ma. The record of the ca. 1760–1600 Ma events in domain 1A of the Eastern Ghats Belt allows us to speculate on modeling the Paleoproterozoic–Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and parts of India.The central part of Eastern Ghats Belt (isotopic domain 2), on the other hand, contains zircons showing inherited ages of ca. 1880–1700 Ma, with a concordant age group of ca. 1760 Ma. Moderately to strongly discordant ages in the time span of ca. 1600–1100 Ma in domain 2 are interpreted to be mixing ages as a result of strong overprint of a ca. 1030–900 Ma tectonothermal event(s) that affected this domain. An early UHT metamorphism event in this domain is inferred to have occurred at ca. 1030–990 Ma (chemical dating of included monazite grains). Zircon records the most pervasive tectonothermal event in this domain at ca. 980–900 Ma, which is correlative with the Rayner orogeny in East Antarctica as a part of the formation of Rodinia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lithostructural and chronological constraints for tectonic restoration of Proterozoic accretion in the Eastern Indian Precambrian shield

The Central Indian Tectonic Zone (CITZ) is a ∼700 km long orogenic belt in north-central India that demarcates the accretion zone between the Northern and Southern Indian crustal domains. However, the timing of accretion remains speculative. The Chottanagpur Gneiss Complex (CGC), the North Singhbhum Mobile Belt (NSMB) and the Singhbhum Craton (SC) are three crustal units located at the eastern extremity of the CITZ. The CGC, dominated by blastoporphyritic granitoids intrusive into amphibolite- to granulite facies gneisses, is juxtaposed against the greenschist-facies northern metaflysch belt in the NSMB (N-NSMB) north of the Dalma Ophiolite Belt (DOB), along the Northern Shear Zone (NSZ). Both south CGC and N-NSMB exhibit correlated increase in strain and prograde metamorphism towards the NSZ. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon and U–Th–Pb monazite chemical ages from the southern parts of the CGC lie between 1.1 and 0.9 Ga. Monazite from N-NSMB are of similar age. In contrast, zircon from central parts of the CGC away from the NSZ preserves older concordant ages (up to 2.6 Ga), and the 1.0–0.90 Ga age population is scarcely represented. South of the DOB, the ∼1.0 Ga ages are absent in S-NSMB. Instead, monazites from all samples in S-NSMB record a tightly constrained 1.59–1.56 Ga high-P metamorphic event, arguably a result of northward subduction of the metaflysch belt. These two terranes were in turn accreted to the CGC at 1.0–0.90 Ga. This accretion at the leading edge of the amalgamated SC–S-NSMB block led to the emplacement of expansive granitoids in the CGC. Apparently, the polychronous CITZ orogen marks the gradual closure of a major ocean basin separating the crustal domains of the Northern and Southern Indian blocks during the Meso- and the Early Neoproterozoic era.

∼1.6 Ga ultrahigh‐temperature granulite metamorphism in the Central Indian Tectonic Zone: insights from metamorphic reaction history, geothermobarometry and monazite chemical ages

AbstractIn this study, we present precise pressure–temperature (P–T) and age constraints of ultrahigh‐temperature (UHT) metamorphism of the Bhandara–Balaghat granulite (BBG) domain at the southern margin of the Central Indian Tectonic Zone (CITZ). Supracrustal and metaigneous granulites of this domain, which lie as detached pods and lenses of various sizes within felsic gneiss–migmatite association record protracted high‐T crustal anatexis events, broadly synchronous with and/or punctuated with felsic and mafic plutonism. Magnesian metagreywacke protolith of the supracrustal suite records extensive biotite melting and subsequent melt extractions at deep crustal, UHT metamorphic conditions (T ≥ ∼900°C at P∼8 kbar), producing restitic mineral assemblages of garnet + rutile, garnet + cordierite and garnet + aluminous orthopyroxene. The diversity of the mineral assemblages is related to the domainal‐scale variation of the bulk rock composition. In situ chemical age dating of five monazite grains, which occur in the different textural settings of the garnet + cordierite + orthopyroxene + rutile‐bearing granulite reveals two age domains: (1) Pervasive ∼1.6 Ga domain, which is recorded in monazites occurring as inclusions in garnet and in the leucosome matrix is correlated with the timing of the UHT metamorphism. (2) ∼1.47 Ga domain reflects a fluid‐mediated recrystallization event leading to dissolution and re‐precipitation of older monazite. The ∼1.6 Ga monazite chemical ages provide robust constraints on the timing of the earliest stage of tectonothermal processes in the CITZ (defined here as the ‘Central Indian Orogeny’). The significance of the ∼1.6 Ga hot orogenesis in interorogen correlation is discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Grenvillian age high-pressure upper amphibolite-granulite metamorphism in the Aravalli-Delhi Mobile Belt, Northwestern India: New evidence from monazite chemical age and its implication

In this study, we deduce the age of the collisional orogeny in the Aravalli-Delhi Mobile Belt, using electron microprobe (EMP) dating of monazites of two suites of metapelitic to metapsammitic rocks from the Banded Gneissic Complex of Central Rajasthan. These metasediments of the Sandmata and the Mangalwar Complexes record a high-pressure, upper amphibolite ( P ∼ 10.3 ± 0.8 kbar, T ∼ 675 ± 60 °C) to granulite facies ( P ∼ 12–14 kbar, T ∼ 815 °C) peak metamorphism (M 2P stage) and a post-peak decompression to ∼5 kbar, 575–660 °C (M 2R stage), following a clockwise P– T path (M 2 event). The metapelites of the Sandmata Complex, however, record an older, medium-pressure, sillimanite-facies granulite metamorphism (M 1 event). Electron microprobe dating of monazites, occurring in the different textural settings of three metapelite granulite samples of the Sandmata Complex yields three mean spot age populations at ∼1700 ± 3 to ∼1733 ± 15 Ma, ∼1597 ± 6 to ∼1629 ± 23 Ma and ∼882 ± 15 Ma. While Late Palaeoproterozoic age populations are correlated with the timing of the M 1 metamorphism, the Early Neoproterozoic ages from garnet-breakdown textural domain constrain the timing of the overprinting M 2R event. Interpretation of the Late Mesoproterozoic age component is problematic. It may reflect either a mixed age due to partial Pb loss of the Palaeoproterozoic monazites by a younger high-grade event or may define the timing of a high-grade tectonothermal event, the imprints of which have been registered in the Proterozoic mobile belts of central, eastern, north-eastern and south-eastern India. Monazites from the metapelites of the Mangalwar Complex yield characteristic Late Mesoproterozoic to Neoproterozoic ages with clusters at 946 ± 6, 909 ± 9 and 865 ± 10 Ma. The metagranites intrusive into the M 1 granulites additionally register an emplacement age of ∼978 ± 18 Ma. The age span of ∼0.98–0.95 to ∼0.88–087 Ga is taken as the age of the M 2 event. Based on these geochronologic and metamorphic findings, we assign a Grenvillian age for the collisional orogeny in the Aravalli-Delhi Mobile Belt. The extent of propagation of the Grenvillian orogenic front in the Aravalli-Delhi orogen and its implication in Rodinia reconstruction are also discussed.

Triassic eclogite from northern Vietnam: inferences and geological significance

Abstract The first finding of low‐temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and/or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na‐poor omphacite and Na‐rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peak‐pressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1–2.2 GPa and 600–620 °C, even though thermobarometric results yield higher pressure and temperature conditions (2.6–2.8 GPa and 620–680 °C). The eclogites underwent a clockwise P–T trajectory with a post‐peak‐pressure increase of temperature to a maximum of >750 °C at 1.7 GPa and a subsequent cooling during decompression to 650 °C and 1.3 GPa, which was followed by additional cooling before close‐to‐isothermal decompression to ∼530 °C at 0.5 GPa. The surrounding pelitic schist (garnet–chloritoid–phengite) records similar metamorphic conditions (580–600 °C at 1.9–2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet–phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non‐collisional event in the Indochina craton.

High-grade Paleoproterozoic reworking in the southeastern Gawler Craton, South Australia*

SHRIMP U–Pb geochronology and monazite EPMA chemical dating from the southeast Gawler Craton has constrained the timing of high-grade reworking of the Early Paleoproterozoic (ca 2450 Ma) Sleaford Complex during the Paleoproterozoic Kimban Orogeny. SHRIMP monazite geochronology from mylonitic and migmatitic high-strain zones that deform the ca 2450 Ma peraluminous granites indicates that they formed at 1725 ± 2 and 1721 ± 3 Ma. These are within error of EPMA monazite chemical ages of the same high-strain zones which range between 1736 and 1691 Ma. SHRIMP dating of titanite from peak metamorphic (1000 MPa at 730°C) mafic assemblages gives ages of 1712 ± 8 and 1708 ± 12 Ma. The post-peak evolution is constrained by partial to complete replacement of garnet–clinopyroxene-bearing mafic assemblages by hornblende–plagioclase symplectites, which record conditions of ∼600 MPa at 700°C, implying a steeply decompressional exhumation path. The timing of Paleoproterozoic reworking corresponds to widespread deformation along the eastern margin of the Gawler Craton and the development of the Kalinjala Shear Zone.

On the application of in situ monazite chemical geochronology to constraining P – T – t histories in high-temperature ( > 850 ° C) polymetamorphic granulites from Prydz Bay, East Antarctica

We present electron microprobe-based in situ (Th + U)–Pb monazite chemical age data from granulite-facies metapelites in Prydz Bay, East Antarctica. The monazite age data define two distinct age populations, Late Neoproterozoic to Early Palaeozoic ( c . 570–520 Ma) and Neoproterozoic ( c . 950–820 Ma), that confirm the polytectonic nature of Prydz Bay. Our data suggest that similarity in lineation orientation along the Prydz Bay coast is not sufficient to necessarily indicate time-equivalance. The minimum duration of Early Palaeozoic tectonism, spanning at least 60 Ma, is constrained from an Mg–Al-rich metapelite: monazite hosted by coarse-grained orthopyroxene defines the oldest Early Palaeozoic population, whereas cordierite + orthopyroxene symplectites define the youngest Early Palaeozoic population. The spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution. We are able to characterize the evolution of Early Palaeozoic tectonism in quantitative P – T – t –mineral assemblage space, demonstrating that ultrahigh-temperature tectono-metamorphism in Prydz Bay is of Early Palaeozoic age. The survival of Neoproterozoic inheritance in Fe–Al-rich metapelites has implications for high Pb retentivity in monazite when chemical and kinetic conditions are favourable. The approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane.

Metamorphic Evolution of the Imjingang Belt, Korea: Implications for Permo-Triassic Collisional Orogeny

The Imjingang belt in the central Korean Peninsula is a strong candidate to represent the extension of the Dabie-Sulu ultrahigh-pressure (UHP) belt in China. This fold-thrust belt consists primarily of: (1) the northern, Jingok unit, characterized by Barrovian-type metapelites; and (2) the southern, Samgot unit, comprising calc-silicate rocks and amphibolites with or without garnet. The metamorphic grade of the Jingok unit increases southward from garnet through staurolite to kyanite zones. Peak metamorphic conditions estimated from the kyanite zone range up to ~11 kbar and 700°C. In contrast, no significant change in metamorphic assemblages is apparent in the Samgot unit. Characteristic minerals of the calc-silicate rocks include garnet, diopside, hornblende, scapolite, and plagioclase, and are compatible with upper amphibolite facies estimated from garnet amphibolite. P-T estimates from the Samgot amphibolites are generally in the range of 8.5-11.5 kbar and 660-780°C. These P-T estimates, together with near-isothermal decompression documented from the metapelitic unit, suggest that the Imjingang belt possibly evolved from eclogitefacies conditions along a clockwise P-T path. The timing of peak metamorphism was previously estimated at ~250 Ma, based on Sm-Nd and Rb-Sr mineral isochron ages and monazite chemical ages. This result is corroborated by an ion microprobe (SHRIMP) U-Pb age of metamorphic zircon dated at 253 ± 2 Ma. 40Ar/39Ar ages of hornblende from the amphibolite suggest regional cooling through ~500°C at 230-225 Ma in the Samgot unit. Two biotite separates in the kyanite zone suggest cooling through ~300°C at ~220 and 160 Ma, respectively, in the Jingok unit. These results, in conjunction with peak metamorphism at ~250 Ma, indicate that high-P metamorphism in the Imjingang belt occurred during the Permo-Triassic, and that subsequent rapid cooling followed during exhumation of a thickened orogen, producing a clockwise decompression. This P-T-t evolution is compatible with that of the basement gneisses in the Gyeonggi Massif. Thus, we conclude that both the Imjingang belt and Gyeonggi Massif in the Korean Peninsula are products of continental collision at ~250 Ma and subsequent exhumation at ~230-220 Ma.

A trace element and chemical Th–U total Pb dating study in the lower Belt-Purcell Supergroup, Western North America: Provenance and diagenetic implications

Comprehensive geochemical and monazite chemical age data on the Mesoproterozoic Belt-Purcell Supergroup reveals information on provenance age(s), and the sources of interbedded argillites and sandstones. Compliance of rare earth element (REE) and multi-element patterns of argillites to post-Archean upper continental crust (PA-UCC), and Cr–Ni, and Th/Sc–Sc systematics are consistent with a dominantly post-Archean source area. Sandstones have the same geochemical fingerprint as argillite for the Appekunny and Grinnell formations in the lower Belt-Purcell Supergroup sequence, albeit variably depleted by detrital quartz. Sandstone developed in a separate high-energy environment, and argillite and sandstone became interbedded during storms, accounting for the sharp interbedding of the two facies. Detrital monazite chemical Th–U total Pb ages from the Appekunny and Grinnell sandstones, and argillites of the former, cluster at ∼ 1700 Ma. Paleocurrents and chemical ages support a Laurentian Paleoproterozoic provenance, likely the Yavapai, Mazatzal and Central Plains provinces. Other potential sources could have been terranes at ∼ 1875–1750 Ma that cooled through the blocking temperature of monazite ∼ 1700 Ma such as the Wyoming Province. Rare > 2500 Ma monazite ages in argillite and sandstone stem for minor contributions from Archean terrane(s). Diagenetic overprint in the Belt-Purcell rocks is widespread and displayed geochemically as heavy REE enrichment relative to light REE, normalized to PA-UCC. There are two populations of monazites: the first has detrital textures, with chemical ages > ∼ 1400 Ma, higher ThO 2 and Y, and lower LREE/HREE content than < ∼ 1400 Ma monazites with euhedral texture, ages up to ∼ 900 Ma post-deposition, with lower ThO 2 and Y, and higher LREE/HREE content. The second population is interpreted as the product of protracted migration of basinal brines that mobilized HREE, as documented in several Proterozoic sedimentary basins.