Ediacaran to Toarcian evolution of the Gasht Metamorphic Complex, Alborz Mountains, N Iran

2021 ◽  
Author(s):  
Leila Rezaei ◽  
Martin J. Timmerman ◽  
Uwe Altenberger ◽  
Mohssen Moazzen ◽  
Franziska D. H. Wilke ◽  
...  
Keyword(s):  
White Mica ◽  
Amphibolite Facies ◽  
Late Triassic ◽  
Metamorphic Complex ◽  
Zircon Age ◽  
Northern Margin ◽  
Basement Rocks ◽  
Late Neoproterozoic ◽  
Radiometric Ages ◽  
Uplift And Erosion

<p>The Alborz Mountains in N Iran underwent several tectono-metamorphic events that reflect the opening and closure of the Paleo- and Neotethys Oceans. Metamorphic rocks that recorded these are rare and discontinuously exposed. They range from the HP-LT Asalem-Shanderman Complex in the west, to the Gasht Metamorphic Complex (GMC, this study), to the Gorgan Schists, and Fariman Schists near Mashhad in the east. They are considered to have formed during the closure of the Paleotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolite metamorphosed under greenschist- to amphibolite-facies conditions. In addition, smaller volumes of granite occur. As the evolution of the basement rocks of the Alborz Mountains is still poorly known and their radiometric ages are very limited, we applied different dating methods to selected samples of the GMC basement to better understand the geological evolution of this part of the Alborz Mountains.</p><p>The granite yielded an Ediacaran 551 ± 2.5 Ma LA-ICP-MS U-Pb pooled zircon age. Monazites in two amphibolite-facies metapelites (Bt-Ms-St ± And schists) yielded Triassic 226 ± 24 and 229 ± 25 Ma CHIME U-Pb ages. Foliation-defining biotite and retrograde white mica replacing andalusite porphyroblasts in metapelites and peak-metamorphic amphibole from an amphibolite yielded much younger 175.1 ± 0.5 Ma to 177.0 ± 0.4 Ma <sup>40</sup>Ar/<sup>39</sup>Ar plateau ages.</p><p>The Ediacaran crystallization age of the granite agrees with the late Neoproterozoic to Cambrian zircon age of the Lahijan granite in the eastern GMC reported by Guest et al. (2006) and indicates that the Alborz basement was a part of the northern margin of Gondwana at that time. It rifted and drifted away from Gondwana due to the opening of the Neotethys, probably in the Permian, along with other Iranian blocks (the so-called Cimmerian terranes). The mid to late Triassic monazite ages date the Barrovian peak metamorphism of the GMC and mark collision and accretion of a Cimmerian terrane following closure of the Paleotethys. The monazite ages overlap with the early Late Triassic age of deposition of the lowest parts of the unconformably overlying Shemshak Group in the central and eastern Alborz Mountains (ca. 213 Ma, Horton et al. 2008). Younger and very similar Toarcian <sup>40</sup>Ar/<sup>39</sup>Ar ages for both pro- and retrograde minerals with different nominal closure temperatures, reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion (Fürsich et al. 2009). Extension probably started in the western Alborz Mountains in the Toarcian and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin and was triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent (Wilmsen et al. 2009).</p><p> </p><p>Fürsich et al. 2009, Geol. Soc., London, Spec. Publ. 312, 189-203. Guest et al., 2006, GSA Bulletin 118, 1507-1521. Horton et al., 2008, Tectonophysics 451, 97–122. Wilmsen et al. 2009, Terra Nova 21, 211–218.</p>

Precambrian Zircon Age of Orthogneiss in the Shuswap Metamorphic Complex, British Columbia

1975 ◽  
Vol 12 (2) ◽  
pp. 326-332 ◽  
Author(s):  
R. K. Wanless ◽  
J. E. Reesor
Keyword(s):  
British Columbia ◽  
Metamorphic Complex ◽  
Gneiss Dome ◽  
Zircon Age ◽  
Core Zone ◽  
The Core ◽  
Basement Rocks ◽  
Proterozoic Basement ◽  
Shuswap Metamorphic Complex ◽  
Age Determinations

Pb-U age determinations carried out on zircon from granodiorite gneiss of the core zone of Thor-Odin gneiss dome have provided isotopic evidence for involvement of Proterozoic basement rocks in the Mesozoic structures of the Shuswap Metamorphic Complex. The study has revealed that the zircons originally crystallized [Formula: see text] ago and suffered an episodic loss of lead [Formula: see text] ago.

scholarly journals Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

2014 ◽  
Vol 41 (4) ◽  
pp. 459 ◽  
Author(s):  
Sébastien Castonguay ◽  
Cees R. Van Staal ◽  
Nancy Joyce ◽  
Thomas Skulski ◽  
James P. Hibbard
Keyword(s):  
Leading Edge ◽  
Amphibolite Facies ◽  
Subduction Channel ◽  
Zircon Age ◽  
Ophiolitic Rocks ◽  
Eclogite Facies ◽  
Radiometric Ages ◽  
Geochronological Evidence ◽  
Structural Base

The Baie Verte Peninsula, western Newfoundland Appalachians, preserves evidence for Early to Mid Ordovician closure of the Taconic seaway, which led to obduction of the Baie Verte oceanic tract (BVOT) ophiolites onto the Laurentian (Humber) margin and Taconic orogenesis. The scarcity of Taconic radiometric ages (and predominance of Silurian (Salinic) data) from the Humber margin rocks (down-going plate) has been problematic, calling into question the intensity and existence of Taconic collisional orogenesis. 40Ar/39Ar and in situ U–Pb geochronology was undertaken on metamorphosed units from the Laurentian basement (Mesoproterozoic East Pond Metamorphic Suite), from the ca. 560 Ma Birchy Complex forming the leading edge of the Humber margin, and from the ca. 490 Ma ophiolitic rocks of the BVOT (Advocate Complex) in order to address this question. Our results confirm evidence of Taconic metamorphism along the Humber margin and at the base of the ophiolites. Ages obtained from the structural base of the Advocate Complex (481–465 Ma) are interpreted to reflect the timing of accretion and internal thickening of the ophiolite, whereas data from the underlying Birchy Complex (467–461 Ma) record the underthrusting and exhumation of the leading edge of the Humber margin along a subduction channel, penecontemporaneously with final obduction of the BVOT. A concordant ca. 465 Ma zircon age and REE data obtained from retrogressed eclogite of the East Pond Metamorphic Suite suggest that the parautochthonous Humber margin was locally subducted to eclogite-facies conditions during the Taconic collision and partly exhumed to amphibolite-facies conditions prior to a strong Silurian (Salinic) tectonometamorphic overprint.SOMMAIRELa péninsule de Baie Verte dans les Appalaches de l’ouest de Terre-Neuve a conservé des indices de la fermeture du bras de mer taconique, qui a mené à l’obduction des ophiolites de la bande océanique de Baie Verte (BOBV) sur la marge laurentienne (Humber) et à l’orogénèse taconique. La rareté des âges radiométriques taconiques (et la prédominance des données siluriennes (saliniques)) provenant des roches de la marge de Humber (i.e. la plaque subductée) a été problématique, mettant en question l’intensité et l’existence de la collision orogénique taconique. De la géochronologie 40Ar/39Ar et U–Pb in situ a été réalisée sur des unités métamorphisés provenant du socle laurentien (la Suite Métamorphique d’East Pond d’âge Mésoprotérozoïque), du Complexe de Birchy daté à ca. 560 Ma formant la partie frontale de la marge de Humber, and des roches ophiolitiques de la BOBV (Complexe d’Advocate) datée à ca. 490 Ma afin de confronter ce questionnement. Nos résultats confirment les indices de métamorphisme taconique le long de la marge de Humber et à la base des ophiolites. Les âges obtenus à la base structurale du Complexe d’Advocate (481–465 Ma) sont interprétés comme reflétant la période d’accrétion et d’épaississement interne de l’ophiolite, tandis que les données du Complexe de Birchy sous-jacent (467–461 Ma) enregistrent le sous-charriage et l’exhumation de la partie frontale de la marge de Humber au sein d’un chenal de subduction, de façon pénécontemporaine à l’obduction finale de la BOBV. Un âge concordant de ca. 465 Ma d’un zircon et les données de terres rares provenant d’une éclogite rétromorphosée de la suite métamorphique d’East Pond suggèrent que la marge de Humber parautochtone a été localement subductée à des conditions du faciès éclogitique durant la collision taconique et partiellement exhumée à des conditions du faciès des amphibolites précédant la forte surimposition tectonométamorphique silurienne (salinique).

Latest Toarcian (c. 175-177 Ma) 40Ar/39Ar mineral ages for amphibolite-facies basement rocks of the Gasht Complex, Alborz Mountains, North Iran: deep crustal response to mid-Jurassic rifting

2020 ◽  
Author(s):  
Leila Rezaei ◽  
Martin J. Timmerman ◽  
Mohssen Moazzen ◽  
Masafumi Sudo
Keyword(s):  
Grain Size ◽  
Regional Scale ◽  
White Mica ◽  
Amphibolite Facies ◽  
Late Triassic ◽  
Block Rotation ◽  
The South ◽  
South Caspian Basin ◽  
Sediment Grain Size ◽  
Step Heating

<p>Metamorphic rocks in the Alborz Mountains are mainly known from the HP-LT Asalem-Shanderman Complex, the Gasht Complex, Gorgan Schists, and the Fariman Schists near Mashad. Recent argon ages are limited to eclogites and blueschists of the Asalem-Shanderman Complex, where phengites yielded c. 350 Ma step-heating ages that reflect cooling, following peak metamorphism related to subduction of the Palaeotethys Ocean (Rosetti et al. 2016).</p><p>The Gasht Complex in the Gasht-Masuleh area comprises metasediments and metabasic rocks metamorphosed at amphibolite-facies peak metamorphic conditions (c. 630°C and 8.6 kbar, Razaghi et al., 2018). The metamorphism is most probably related to the accretion of Cimmerian terranes to the Turan Terrane in the late Triassic following closure of the Palaeotethys Ocean and resulting in the Cimmerian Orogeny.</p><p>Micas from metapelites, amphibole from an amphibolite and magmatic white mica from deformed granite from the Gasht Complex yield very similar <sup>40</sup>Ar/<sup>39</sup>Ar step-heating plateau ages between 175.1 ± 0.5 Ma and 177.0 ± 0.4 Ma (2 sigma) that are independent of grain size and nominal closure temperatures. In addition, clearly retrograde white mica replacing andalusite porphyroblasts in a metapelite yielded a similar plateau age of 176.1 ± 0.5 Ma.</p><p>In the Gasht-Masuleh area the contact between basement and the cover rocks is largely tectonic due to later faulting, but the Gasht Complex must have formed the depositional basement to the late Triassic- Middle Jurassic Shemshak Group. Sedimentation started in the Carnian above the regionally developed Eo-Cimmerian unconformity in the central and eastern Alborz and continued until the mid-Bajocian.</p><p>Notably, within the Shemshak Gp. a distinct, regional scale unconformity developed in the mid-Bajocian (c. 170 Ma) recognized by rapid coarsening in sediment grain size. Only locally, in the eastern Alborz Mountains, it developed as an angular unconformity related to block rotation. This Mid-Cimmerian unconformity formed as a result of tectonic movements causing rapid uplift and erosion.</p><p>Our c. 175 – 177 Ma mica and amphibole plateau ages for the Gasht Complex are unlikely to reflect slow cooling following the (Carboniferous? Late Triassic?) metamorphism, as this would result in increasingly younger ages for amphibole, white mica and biotite. Instead, the indistinguishable ages for peak metamorphic and retrograde minerals must be due to very rapid cooling at the Toarcian-Aalian boundary (c. 174 Ma) that resulted from rapid basement uplift and at the surface caused the mid-Bajocian Mid-Cimmerian unconformity. Thus, the c. 175 – 177 Ma <sup>40</sup>Ar/<sup>39</sup>Ar ages document the thermal response of the basement below the Shemshak Group to a mid-Jurassic extensional tectonic event.</p><p>From a regional perspective, the Mid-Cimmerian unconformity may represent the break-up unconformity of back-arc rift basins that formed due to northward Neotethys subduction to the south of the Alborz Mountains (Wilmsen et al. 2009) and/or the onset of sea-floor spreading within the South Caspian Basin to the north (Fürsich et al. 2009).</p><p><em>Fürsich et al. 2009, Geol. Soc., London, Spec. Publ. 312, 189-203. Razaghi et al., 2018, Geosciences 27, 269-280. Rosetti et al. 2016, J. Geol. Soc. London 174, 741-758. Wilmsen et al. 2009, Terra Nova 21, 211–218.</em></p>

Protracted northward drifting of South China during the assembly of Gondwana: Constraints from the spatial-temporal provenance comparison of Neoproterozoic−Cambrian strata

2021 ◽  
Author(s):  
Qiong Chen ◽  
Guochun Zhao ◽  
Min Sun
Keyword(s):  
South China ◽  
Detrital Zircons ◽  
Northern Margin ◽  
Northern India ◽  
Geological Evolution ◽  
System 2 ◽  
Sedimentary System ◽  
Provenance Variations ◽  
Late Neoproterozoic ◽  
T Values

Neoproterozoic to Paleozoic sedimentation shows systematic temporal-spatial variations within South China, which must be considered in reconstructing geological evolution of South China in response to global plate reorganization from the breakup of Rodinia to the assembly of Gondwana. We use >1000 new U-Pb and Hf isotopic data for detrital zircons from Neoproterozoic−Cambrian strata across the western (i.e., Longmenshan) and eastern (i.e., Wuyishan) margins of South China, coupled with compiled stratigraphic and magmatic information, to constrain change in provenance through time. First-order conclusions are as follows: (1) detrital zircons from the Neoproterozoic strata of the two margins were mainly sourced from the Panxi-Hannan arc and the Jiangnan orogen, signaling a rough self-sufficient sedimentary system; (2) newly identified Cambrian molasse-like sediments in the western margin, in which abundant detrital zircons are 550−500 Ma old with positive εHf(t) values, were mainly derived from the 580−500 Ma Cadomian arc belt along the Iran-Turkey margin; and (3) the Cambrian sediments in the eastern margin document more increased contributions from the Grenvillian-age provinces most possibly in Australia. Such spatial-temporal provenance variations signal the northward drifting of South China, from a position connecting with Iran-Turkey and northern India to that approaching Australia during the late Neoproterozoic−Cambrian period. We highlight that the activity of oblique oceanic-continental convergence accreted Asian terranes onto the northern margin of Gondwana, hence contributing to the ultimate Gondwana architecture under global plate reorganization.

Barrovian-type metamorphism in the western domain of the Cordillera Darwin Metamorphic Complex, Fuegian Andes

2021 ◽  
Author(s):  
Mauricio Calderon ◽  
Catalina Zúñiga ◽  
Francisco Hervé ◽  
Thomas Theye ◽  
Gonzalo Galaz ◽  
...  
Keyword(s):  
Geothermal Gradient ◽  
White Mica ◽  
Burial Depth ◽  
Metamorphic Complex ◽  
Constant Composition ◽  
Magmatic Arc ◽  
Garnet Core ◽  
Late Generation ◽  
Si Content ◽  
Two Stages

<p>The Cordillera de Darwin Metamorphic Complex (CDMC) comprise metamorphosed supracrustal rocks and metaplutonic suites which records a unique tectonic evolution among the metamorphic complexes of the southernmost Andes. The pressure (P) and temperature (T) conditions determined in garnet-bearing schists in the Central Domain of the CDMC indicate a clockwise P-T path of metamorphism reaching burial depth as high as 12 kbar at ca. 620°C. This metamorphic event has been related to the closure of a marginal back-arc basin (Rocas Verdes Basin) and collision of an ensialic magmatic arc with the continent in the late Cretaceous. We focus on garnet-biotite schists intercalated within a huge block consisting of repeated sequences of metabasalts and amphibolites (Rocas Verdes Ophiolites), located in the Western Domain of the CDMC, at Seno Martínez. The chemical zonation of small garnet porphyroblasts (diameter of ca. 300 um) record two stages of metamorphism. Garnet is almost almandine in composition with lesser amounts of Ca, Mn and Mg.  The concentric zonation is characterized by relatively lower contents of Fe-Mg and higher contents of Ca-Mn in the core. Garnet bear tiny inclusions of clinozoisite, which is also present as isolated grains in the foliated matrix. Laths of biotite define the main foliation and have a nearly constant composition characterized by X<sub>Fe</sub> of ca. 0.6. Two generations of phengitic white mica are identified on basis of Si content (a.pf.u.) varying between 3.20-3.30 (early generation) and of ca. 3.15 (late generation). To reconstruct the P-T conditions of metamorphism through thermodynamic modeling using the Perple_X software package, the bulk rock and mineral composition were considered. Using compositional isopleths of X<sub>Fe</sub>, X<sub>Mg</sub>, X<sub>Ca</sub> and X<sub>Mn</sub> in zoned garnet, Si content in white mica and X<sub>Fe</sub> in biotite allow the constrain two stages of metamorphism (M1 and M2). The P-T conditions of M1, represented by the composition of the garnet core, are restricted to ca. 8 kbar and 400°C. M2 is restricted to ca. 7.5 kbar at 480°C, determined with the composition of the garnet rim, X<sub>Fe</sub> in biotite and Si content in late phengitic white mica. Our preliminary results indicate that ophiolitic rocks and interleaved garnet-bearing schists were tectonically buried and metamorphosed in a relatively hot subduction interface characterized by a geothermal gradient of ca. 16°C/km, prior to the collision of the ensialic magmatic arc. Acknowledgements. This study was supported by the Fondecyt grant 1161818.</p>

K–Ar age determinations on the fine fractions of clay mineral ‘Crystallinity Index Standards’ from the Palaeozoic mudrocks of southwest England

Clay Minerals ◽  
2019 ◽  
Vol 54 (1) ◽  
pp. 15-26 ◽  
Author(s):  
Anna C. Schomberg ◽  
Klaus Wemmer ◽  
Laurence N. Warr ◽  
Georg H. Grathoff
Keyword(s):  
Clay Mineral ◽  
Crystallinity Index ◽  
White Mica ◽  
Early Jurassic ◽  
Late Triassic ◽  
X Ray Diffraction ◽  
Middle Ordovician ◽  
Late Cambrian ◽  
Isotopic Analyses ◽  
Mineral Crystallinity

AbstractClay mineral ‘Crystallinity Index Standards’ (CIS) composed of Palaeozoic mudrocks from southwest England were investigated systematically in five sub-fractions per sample for the first time. X-ray diffraction was used to determine mineral assemblages, calibrated 001 illite full-width-at-half-maximum (FWHM) values and illite polytype compositions, in addition to K–Ar isotopic analyses of all fine fractions. The FWHM results of the <2 µm fraction are consistent with previous studies and reflect the range of diagenetic to epizonal grades covered by the sample set SW1 to SW7 (~0.61–0.26°2θ). Diagenetic and lower anchizone samples also show significant broadening of 001 illite reflections in the finer fractions and contain mixtures of authigenic 1M + 1Md illite and detrital 2M1 white mica polytypes suitable for illite age analysis. The estimated end-member ages of the Bude (SW1-1992) and younger Crackington (SW3-2000) mudstones yield detrital ages of Late Cambrian to Middle Ordovician (493–457 Ma) and a broad range of 1M + 1Md illite ages between Middle Permian and Early Jurassic (271–190 Ma). The detrital age of the stratigraphically older Crackington Formation mudrock (SW2-1992) is Late Devonian (384–364 Ma) with 1M + 1Md illite ages between Late Triassic and Early Jurassic (219–176 Ma). The origin of Mesozoic 1M + 1Md illite ages may represent neocrystallized illite associated with Mesozoic hydrothermal events or similar events that thermally reset older authigenic illite with partial loss of radiogenic argon and no renewed crystal growth. In contrast, upper anchizonal and epizonal Devonian slates (SW3-2012, SW4-1992, SW6-1992 and SW7-2012) contain only the 2M1 polytype, with K–Ar ages younger than the stratigraphic age. The three finest fractions of SW4-1992 yield consistent Late Carboniferous ages (331–304 ± 7 Ma) that are considered to date the neocrystallized 2M1 mica. Most fractions of epizonal slate (SW6-1992, SW7-2012) yield Early Permian ages (293.6–273 Ma) corresponding to published cooling ages of the Tintagel High-Strain Zone and the intrusion of the Bodmin granite (291.4 ± 0.8 Ma). These first K–Ar age constraints for the fine fractions of the CIS should provide useful reference values for testing analytical procedures of illite age analysis.

U–Pb zircon geochronology from the Alexander terrane, southeast Alaska: implications for the Greens Creek massive sulphide deposit

2016 ◽  
Vol 53 (12) ◽  
pp. 1458-1475
Author(s):  
Patrick J. Sack ◽  
Ron F. Berry ◽  
J. Bruce Gemmell ◽  
Sebastien Meffre ◽  
Andrew West
Keyword(s):  
Inductively Coupled Plasma ◽  
Volcanic Rocks ◽  
White Mica ◽  
Massive Sulphide ◽  
Late Triassic ◽  
Massive Sulphide Deposit ◽  
Sulphide Deposit ◽  
Southeast Alaska ◽  
Northern Portion ◽  
Greens Creek Mine

This paper presents results of a laser ablation – inductively coupled plasma – quadrapole mass spectrometer (LA–ICP–QMS) U–Pb dating study of small in situ zircon grains from samples collected in the vicinity of the Greens Creek massive sulphide deposit, on northern Admiralty Island, southeast Alaska. The Greens Creek mine is a volcanogenic massive sulphide deposit in the central portion of the Alexander Triassic metallogenic belt (ATMB) and is one of the top global silver producers despite having a dominantly mafic metavolcanic stratigraphic footwall. The stratigraphic footwall is a Mississippian mafic metavolcanic sequence with a protolith age of approximately 340–330 Ma. The first U–Pb zircon constrained chronostratigraphy for the area places the deposit near, or at, the base of the host Late Triassic stratigraphy just above an approximately 100 million year old unconformity and probably 10–15 million years older than mineralization at the Palmer and Windy Craggy deposits in the northern portion of the ATMB. The stratigraphic location of the Greens Creek deposit is atypical for a syngenetic massive sulphide deposit, and this may, at least partly, explain its unusual metal endowment. Pre-mineralization Permian U–Pb zircon metamorphic ages are consistent with published 273–260 Ma white mica ages related to the collision of the Admiralty and Craig subterranes, the basement to the ATMB. The much older age of the footwall rocks and their Permian pre-mineralization metamorphism demonstrates that though the mafic volcanic rocks are not genetically linked to the deposit, they likely influenced the style of alteration and mineralization.

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