U-Pb zircon ages of dioritic and trondhjemitic rocks from a layered amphibolitic complex crosscut by granite vein (Veporic basement, Western Carpathians) (original) (raw)
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International Journal of Earth Sciences
The blueschist-bearing Bôrka Nappe regarded as a part of the Meliaticum Unit s. l. and the Turnaicum Unit in its hanging-wall were thrusted together to the north on the Inner Western Carpathians basement during the orogenic events of the Late Jurassic-Early Cretaceous. A new U-Pb sensitive high-resolution ion microprobe (SHRIMP) magmatic, as well as detrital-zircon ages were used to constrain provenance and paleogeography. The detrital-zircon age spectrum of the studied metasediments contains mainly ages of the Ediacaran-Cryogenian (550-720 Ma) and Tonian-Stenian (0.9-1.1 Ga), which correspond to the Pan-African Belt and Saharan Metacraton. Subordinate Archean ages were likely derived from the Saharan Metacraton of North Gondwana (1.7-2.2 and 2.5-2.7 Ga). The 273 ± 2 Ma zircon age of a metarhyolite, along with the youngest detrital zircon of 265 ± 4 Ma from the associated metasediments, specify the maximum depositional age of the Bučina Fm. from the Bôrka Nappe to the Guadalupian. The rhyolite olistoliths from the Middle/Late Jurassic Meliaticum olistostrome yielded the latest Cambrian/earliest Ordovician (488 ± 6 Ma) and Middle Ordovician (~ 465 Ma) zircon ages. This is consistent with the U-Pb zircon ages from the foot-walling Southern Gemericum basement metavolcanites, which are, therefore, regarded as a potential source. The obtained zircon ages confirm the provenance from the Southern Gemericum basement and its Permian envelope for the Meliaticum-Bôrka Nappe and Turnaicum sedimentary embayment.
GEOLOGICA …, 2008
The Central West-Carpathian basement, included in Cretaceous-Early Tertiary structures, shows remnants of pre-Alpine, mainly Variscan tectonics. The paper documents in-situ U-Pb zircon (SHRIMP) ages of a major Cambrian to Ordovician magmatic event, recorded in the West-Carpathian basement. Cambrian to Ordovician, less Early Silurian concordia magmatic ages, with older 525-470 Ma and younger 480-440 Ma age intervals were dated in layered amphibolites and granitic orthogneisses of the tectono-stratigraphically Upper Variscan structural Unit. The zircon cores, especially in orthogneisses, often show Cadomian, mainly magmatic source (protolith) ages, spanning 638-549 Ma, with a concordia age at 617± 11 Ma. Older Proterozoic to Archean protolith ages (2000 up to 3400 Ma) are much less common. Cadomian ages highly predominate in kyanite-garnet orthogneisses, tectonically incorporated into the medium-grade metasediments of the Middle Variscan structural Unit. Kyanite-garnet orthogneisses show idiomorphic, magmatically zoned zircons with ages spanning 649-554 Ma, with concordia ages at 607 ± 10 and 558 ± 7 Ma, and metamorphic ages between 540-530 Ma. These ages corroborate the protolith ages of hosting metasediments with a lower intercept around 500 Ma. Early Ordovician ages were also found from the calc-alkaline metavolcanics (482± 6 Ma, metarhyolite, or 476± 7 Ma, metadacite) of low-grade volcano-sedimentary complexes in the Lower Variscan structural Unit. Long-term magmatic/volcanic activity due to melting of the crust and subcrustal lithosphere is coeval with the rifting of Cadomian crust and evolution of the Gondwana northern active continental margin. The ages of 430-380 Ma of layered amphibolites and orthogneisses reflect the Eo-Variscan subduction/collision metamorphic event in the Prototethyan realm, coeval with the collision of the eastern Avalonian-Cadomian/western Hunic terranes, as part of the Armorican microplate, with Laurussia/Avalonia. The collapse of the Eo-Variscan collisionally thickened crust led to partial melting within the Upper Variscan structural Unit, generating 370-340 Ma old zircons in dated metaigneous rocks.
Bulletin De La Societe Geologique De France, 2009
The paper reviews the main West-Carpathian Early Paleozoic metamorphosed originally sedimentary-magmatic complexes, dated by SHRIMP on zircons, as indicators of crustal extension and shortening events. Igneous precursors of a Layered Amphibolite Complex (LAC) -fractionated upper mantle gabbros to diorites, dated at 503 ± 4 and 492 ± 4 Ma from the North-Veporic, or 480 ± 5 and 450 ± 6 Ma from the Tatric basement are contemporaneous with subaluminous to metaluminous I-type (507 ± 4 Ma, the South-Veporic basement), peraluminous S-type (497 ± 4 Ma, the South-Veporic basement; 516 ± 7, 485 ± 6 and 462 ± 6 Ma, the North-Veporic basement; 497 ± 6, 472 ± 6 and 450 ± 6 Ma, the Tatric basement), alkaline A-type (511 ± 6 Ma, South-Veporic basement) granitic orthogneisses and calcalkaline rhyolitic (482 ± 6 Ma) and dacitic (476 ± 7 Ma) metavolcanics (Gemeric basement), indicating a magmatic immature back arc setting. The ages point to Middle/Late Cambrian, Early and Late Ordovician magmatic phases, coeval with the extension in the northern Gondwana margin. Separation of an inferred Avalonian and/or Galatian terranes distal continental ribbon corresponds with the opening of a Medio-European Basin. A 430-390 Ma dated MP/HP metamorphic event, recorded in the LAC and associated orthogneisses, occurred in the area of thinned immature back arc basin crust due to closure of the Medio-European Basin. Thus a distal Gondwana continental ribbon north of this basin could be an eastward lateral pendant of Armorica, derived from Galatian terrane. Metaophiolites of the Pernek Group (a metagabbrodolerite dated at 371 ± 4 Ma) in the Tatric basement, analogous to island-arc tholeiites and back-arc basin basalts, indicate a back-arc basin setting north of a 430-390 Ma old northward dipping subduction/collision zone, dividing the northward drifting western Galatian terrane microplate from the Gondwana margin. Some metabasites of the Gemeric basement might indicate Late Devonian to Mississippian opening of a peri-Gondwanan Paleotethyan oceanic basin: a 383 ± 3 Ma old remelted metagabbro (482 ± 9 Ma) from the Klátov gneiss-amphibolite complex, ca. 385 Ma old porphyritic metabasite of the Zlatník ophiolite complex, as well as a 350 ± 5 Ma old HP metabasite as tectonic fragment within the Rakovec Group. The closure of Devonian-Mississippian basins, accompanied by medium-pressure (the Pernek Group) to high-pressure (blueschist to eclogitic tectonic fragments in greenschist facies rocks of the Rakovec Group) metamorphism, occurred in late Carboniferous to early Permian, when Paleotethyan realm complexes accreted to a Galatian terrane microplate, the latter represented by the older and the higher-grade Tatric and Veporic basement complexes.
Acta Geologica Polonica, 2008
Acombined geochemical, isotopic and detrital zircon dating study has been carried out on metasedimentary rocks of the Lower Unit from the Western Tatra Mountains (Slovakia) forming an eastern border of European Variscides. Geochemical data suggest derivation of the protolith – magmatogenic greywackes and claystones from the recycled continental island arc source. 87 Sr/ 86 Sr (350) isotopic ratios between 0.713 and 0.723 together with low e Nd(350) values of –9.5 to –11.1 and/or Pb isotope composition indicate a crustal origin of the investigated rocks. Detrital zircons from the metasediments often display homogeneous magmatic zoning with 207 Pb/ 206 Pb ages from 660 to 515Ma and/or inherited components, with old cores displaying ages from ca. 1980 to 1800Ma. These late Paleoproterozoic detrital source ages are similar to the Nd model ages of the studied metasedimentary rocks with t (DM2st) ca 1960 ~ 1830 Ma. The Late Cambrian (ca 500 Ma) age represents the last magmatic activity o...
Lithos, 2007
In the Eclogite Zone of the Tauern Window, a layer of strongly retrogressed leucocratic jadeite-bearing gneiss is intercalated between mafic kyanite-eclogites. The jadeite-gneiss consists of garnet + quartz + paragonite ± phengite ± zoisite + zircon + rutile + apatite + pyrite. Kyanite, jadeite or omphacite are exclusively present as inclusions in garnet. Retrogressive hydration during exhumation led to a breakdown of matrix jadeite to form pseudomorphs of calcic amphibole + albite. Peak metamorphic conditions derived from the primary gneiss assemblage are 2.0-2.4 GPa at approximately 640°C. Major, trace element and isotopic compositions of the jadeite-gneiss are consistent with a siliciclastic sedimentary protolith. Zircon morphology and zonation patterns reveal a complex history. The presence of fracture-truncated zircons suggests a detrital origin, whereas most internal structures and Th/U ratios are characteristic of zircons from magmatic rocks. In situ LA-ICP-MS and SHRIMP U-Pb geochronology and zircon geochemisty provide evidence of at least three magmatic events in the provenance area. These were dated at 466 ± 2 Ma, 437 ± 2 Ma and 288 ± 9 Ma. Older ages ranging from 503 to 691 Ma are preserved in the cores of some zircon grains, suggesting derivation from peri-Gondwanan sources. Surprisingly, no firm evidence of the Tertiary high-pressure metamorphic event and subsequent retrograde overprint was seen in any of the studied zircons. However, some zircons show resorbed surfaces suggesting corrosion by a superficial fluid phase undersaturated in zirconium and one extensively altered porous zircon yielded highly discordant 206 Pb/ 238 U ages in the range 325-109 Ma documenting partial recrystallization by dissolution-reprecipitation of a highly reactive grain.
A Metamorphosed Early Cambrian Crust-Mantle Transition in the Eastern Alps, Austria
Journal of Petrology, 2004
In the Speik Complex (Eastern Alps, Austria), highly melt-depleted, metamorphosed harzburgites with abundant pods and layers of chromitite are interlayered with a suite of metamorphosed orthopyroxenites, clinopyroxenites and gabbros. Coarse-grained orthopyroxenites occur as centimetre-to metre-wide veinlets and pods, but also as intrusive plugs several tens of metres wide. Intimately associated metaclinopyroxenite and metagabbro are present as bodies up to several metres thick at a distinct stratigraphic level within the complex. In the ultramafic rocks, relict magmatic olivine, orthopyroxene, clinopyroxene and spinel have been overprinted by a metamorphic assemblage of forsterite, diopside, tremolite, anthophyllite, chlorite, serpentine, talc and Cr-Fe-rich spinel. Hornblende, epidote, zoisite and chlorite dominate the metamorphic paragenesis in metagabbros, in addition to rare relicts of clinopyroxene and two phases of Ca-rich garnet. The polymetamorphic evolution of the Speik Complex includes rarely preserved pre-Variscan (400 Ma) eclogite-facies conditions, Variscan ( 330Ma)amphibolite−faciesconditions(600−700C,>5kbar)andEoalpine(330 Ma) amphibolite-facies conditions (600-700 C, >5 kbar) and Eoalpine ( 330Ma)amphibolite−faciesconditions(600−700C,>5kbar)andEoalpine(100 Ma) greenschist-to amphibolite-facies conditions reaching 550 C and 7-10 kbar. Orthopyroxenites are characterized by high concentrations of SiO 2 , MgO and Cr, and by U-shaped chondrite-normalized rare earth element (REE) patterns similar to those of their harzburgite hosts. The REE patterns of the clinopyroxenites are flat to slightly enriched in light REE. Metagabbro compositions are variable, but generally characterized by low SiO 2 and high mg-numbers (61-78). Their REE patterns all have Gd N /Yb N > 1; some samples have large positive Eu anomalies implying the original presence of cumulus plagioclase. In the orthopyroxenites, clinopyroxenites and some peridotites, Pt, Pd and Re are distinctly enriched compared with Os, Ir and Ru, whereas most harzburgites have unfractionated to slightly fractionated platinum-group element (PGE) patterns with respect to average upper mantle. The Re-Os isotope compositions of the pyroxenites define an errorchron at 550 AE 17 Ma and a supra-chondritic 187 Os/ 188 Os of 0Á179 AE 0Á003.
Contributions to Mineralogy and Petrology, 2012
Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U-Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U-Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87-1.86 and 1.82-1.80 Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83-1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.
Journal of Petrology, 2004
Olivine-rich rocks containing olivine þ orthopyroxene þ spinel þ Ca-amphibole AE clinopyroxene AE garnet are present in the central Ö tztal-Stubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet þ clinopyroxene AE corundum. Protoliths of the olivine-rich rocks are thought to be olivine þ orthopyroxene þ spinel dominated cumulates generated from an already differentiated Fe-rich ( X melt Mg ¼ 0Á57-0Á65) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnetrich rocks are interpreted as layers enriched in plagioclase and spinel intercalated in a cumulate rock sequence that is devoid of, or poor in, plagioclase. U-Pb sensitive high-resolution ion microprobe dating of zircons from a garnet layer indicates that emplacement of the cumulates took place no later than 517 AE 7 Myr ago. After their emplacement, the cumulates were subjected to progressive metamorphism, reaching eclogite-facies conditions around 800 C and >2 GPa during a Variscan metamorphic event between 350 and 360 Ma. Progressive high-P metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase þ spinel to form garnet þ clinopyroxene AE corundum in the garnet layers. Retrogressive metamorphism at T 650-680 C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, högbomite formed from corundum þ spinel along with Al-rich spinel, Ca-amphibole, chlorite, aspidolitepreiswerkite, magnetite, ilmenite and apatite at the interface between olivine-rich rocks and garnet layers at P <0Á8 GPa. Progressive desiccation of retrogade fluids through crystallization of hydrous phases led to a local formation of saline brines in the garnet layers. The presence of these brines resulted in a late-stage formation of Feand K-rich Ca-amphibole and Sr-rich apatite, both characterized by extremely high Cl contents of up to 3Á5 and 6Á5 wt % Cl, respectively.