scholarly journals Building an Orogen: Review of U-Pb Zircon Ages from the Calabria–Peloritani Terrane to Constrain the Timing of the Southern Variscan Belt

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 944
Author(s):  
Annamaria Fornelli ◽  
Vincenzo Festa ◽  
Francesca Micheletti ◽  
Richard Spiess ◽  
Fabrizio Tursi
Keyword(s):  
Low Grade ◽  
Variscan Belt ◽  
Zircon Age ◽  
Grade Metamorphism ◽  
Deep Crust ◽  
Geological Evolution ◽  
Gondwana Margin ◽  
Granitic Intrusions ◽  
High Grade Metamorphism ◽  
Orogenic Cycle

The application of zircon dating to the reconstruction of orogenic systems is invaluable since time constraints of the geological evolution of orogens are crucial for the proposal of geodynamic and paleogeographic models. Zircon is one of the most promising accessory minerals in geochronology of crystalline basements because of its high-closure temperature. Moreover, U-Pb data of relict and recrystallized grains indicate the maximum sedimentation age as well as the timing of metamorphism in metasediments. In addition, the U-Pb ages of magmatic zircons constrain the timescale of magmatism. The Calabria–Peloritani terrane (CPT) represents a key area in the Southern Variscan Belt, whose reconstruction is still unresolved. Therefore, a review of literature zircon age data accompanied with new data from six samples of orthogneisses, paragneisses, amphibolites, and actinolite schists, helps to constrain the evolution of this Cadomian fragment, affected by metamorphic and magmatic Variscan events. A revisiting of the timing of the geological events from Paleo-proterozoic to Permian is revealed by comparing the internal textures of zircons and their U-Pb age clusters. The detected age peaks at 2500 Ma, 1600 Ma, and 1000 Ma in the CPT were related to a provenance from West and East Gondwana realms. A sedimentation age around 630 Ma emerges for the middle-deep crust terranes of the CPT, affected by Ediacaran (579–540 Ma) intrusions, accompanied by metamorphism dated at 556–509 Ma in the host metasediments. In the following, during Ordovician–Silurian extensional tectonics, the former Cadomian terranes were at least locally affected by fluid-assisted metamorphism (around 450 Ma) whereas the upper extensional basins that formed, were infilled by sediments along with interspersed volcanic to subvolcanic products. All these pre-Silurian terranes were involved in the subduction process of the Palaeotethys–Gondwana margin beneath Laurussia. The compressive phase began around 347 Ma, with under-thrusting of the formerly Gondwana substrate that was subjected to middle-high grade metamorphism, while the Ordovician–Silurian sediments were scraped off along the front of the Southern Variscan Belt and affected by low-grade metamorphism. Decompression of the whole Variscan orogenic system started around 320 Ma, together with uplifting of the chain and emplacement of widespread granitic intrusions which ended around 280 Ma and completed the Variscan orogenic cycle in the CPT.

scholarly journals Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic

2013 ◽  
Vol 77 (1) ◽  
pp. 117-136 ◽  
Author(s):  
B. M. Saumur ◽  
K. Hattori
Keyword(s):  
High Temperature ◽  
Dominican Republic ◽  
San Juan ◽  
Low Grade ◽  
Compositional Zoning ◽  
Grade Metamorphism ◽  
Zn Content ◽  
High Grade Metamorphism ◽  
Trace Constituents ◽  
Forearc Mantle

AbstractFerritchromite is rarely reported in forearc mantle peridotites. This contribution describes ferritchromite alteration and zoned Cr-spinel in serpentinites from the Rio San Juan Complex in the Dominican Republic. These rocks originated from the forearc mantle and protruded along lithosphere-scale faults in the mid Eocene. The cores of the Cr-spinel grains have Cr# ratios [i.e.atomic Cr/(Cr + Al)] between 0.48 and 0.66; such values are relatively high and are considered to represent primary compositions. Relatively high Zn contents in the grain cores (0.46 c 0.95 wt.% ZnO) are also thought to be primary; they reflect exceptionally cool conditions in the northern Caribbean forearc mantle. A progressive change in the zoning of Cr-spinel is recorded in the samples. Weakly zoned grains of Cr-spinel have rims with lower Mg# ratios [i.e.atomic Mg/(Mg + Fe2+)] and slightly higher Cr# ratios than the cores. More strongly zoned grains of Cr-spinel, in addition to low Mg# and high Cr# in their rims, have a marked increase in Fe3+# [i.e.Fe3+/(Fe3+ + Al + Cr)] of up to 0.35 in their rims and are partially coated by Mg-rich chlorite. All grains show core-to-rim decreases in their Zn content and increases in Ti, Mn and V. The association with Mg-rich chlorite and the compositional zoning are reminiscent of those reported for ferritchromite. Ferritchromite (with Fe3+# >0.5) is common in ultramafic rocks in amphibolite-grade terranes; however, the serpentinite samples described herein show little evidence of high-grade metamorphism. The lowtemperature serpentine-group mineral lizardite is dominant and high-temperature antigorite is either very rare or absent; other high-temperature minerals, such as talc, tremolite and cummingtonite, are trace constituents. The observed zoning in the Cr-spinel is thought to represent 'immature' ferritchromite, probably formed in response to a short-lived thermal event. This event appears to have been on too short a timescale to produce either proper ferritchromite or significant quantities of high-temperature minerals. It may be related to the emplacement of the nearby Rio Boba Intrusion, or the upward protrusion of the serpentinites along the lithosphere-scale Septentrional fault zone from the base of the mantle wedge through its hotter interior. We suggest that such alteration is rare in forearc serpentinites because they are not commonly heated during exhumation along the plane of subduction. This work demonstrates that Cr-spinel compositions can be modified by relatively low-grade metamorphism.

Paleoproterozoic lithotectonic divisions of the southeastern Churchill Province, western Labrador

1996 ◽  
Vol 33 (2) ◽  
pp. 216-230 ◽  
Author(s):  
D. T. James ◽  
J. N. Connelly ◽  
H. A. Wasteneys ◽  
G. J. Kilfoil
Keyword(s):  
High Strain ◽  
High Grade ◽  
Geochronological Data ◽  
Thermal Front ◽  
Core Zone ◽  
Grade Metamorphism ◽  
The Core ◽  
Oblique Convergence ◽  
Granitic Intrusions ◽  
High Grade Metamorphism

The southeastern Churchill Province (SECP) is a Paleoproterozoic system of orogens that formed during collision of the Nain and Superior cratons with a composite lithotectonic terrane that now forms the medial, metamorphic–plutonic core zone of the SECP. In western Labrador, the core zone consists of reworked Archean gneisses, Paleoproterozoic supracrustal rocks, and variably deformed 1.83–1.81 Ga granitic plutons. It is subdivided into three Paleoproterozoic lithotectonic domains (McKenzie River, Crossroads, and Orma), which are separated from each other by dextral transpressive high-strain zones. Crossroads and Orma domains are thought to be derived from Archean high-grade granite–greenstone terrane crust, whereas McKenzie River domain is inferred to have been part of an Archean orthogneiss terrane dominated by the ca. 2776 Ma Flat Point gneiss. U–Pb geochronological data indicate that the igneous precursor of the Flat Point gneiss is >80 Ma older than the oldest tonalité–granite intrusions in Crossroads and Orma domains. The three domains were variably reworked during dextral oblique convergence of the Superior and Rae cratons. Field and geochronological data demonstrate that in McKenzie River and Crossroads domains, 1.83–1.80 Ga tectono-thermal reworking included medium- to high-grade metamorphism and the formation of north-trending structures and regionally persistent high-strain zones. Crossroads domain also contains a significant amount of 1.83–1.81 Ga granitic intrusions, including the southern part of the 500 km long De Pas batholith. Orma domain appears to have escaped Paleoproterozoic metamorphism and deformation, suggesting that a "Hudsonian" tectono-thermal front separates it from Crossroads and McKenzie River domains.

scholarly journals Tectonothermal evolution in the core of an arcuate fold and thrust belt: the southeastern sector of the Cantabrian Zone (Variscan belt, NW Spain)

2016 ◽  
Author(s):  
M. L. Valín ◽  
S. García-López ◽  
C. Brime ◽  
F. Bastida ◽  
J. Aller
Keyword(s):  
Gravitational Instability ◽  
Local Development ◽  
Contact Metamorphism ◽  
Low Grade ◽  
Variscan Belt ◽  
Thrust Belt ◽  
Cantabrian Zone ◽  
Grade Metamorphism ◽  
The Core ◽  
Fold And Thrust Belt

Abstract. Abstract. The tectonothermal evolution of an area located in the core of the Ibero-Armorican arc (Variscan belt) has been determined by using the conodont color alteration index (CAI), Kübler index of illite (KI), the Árkai index of chlorite (AI), and the analysis of clay minerals and rock cleavage. The area is part of the Cantabrian Zone (CZ), which represents the foreland fold and thrust belt of the orogen. It has been thrust by several large units of the CZ, what resulted in the generation of a large amount of synorogenic Carboniferous sediments. CAI, KI and AI values show an irregular distribution of metamorphic grade, independent of stratigraphic position. Two tectonothermal events have been distinguished in the area. The first one, poorly defined, is mainly located in the northern part. It gave rise to very low-grade metamorphism in some areas and it was associated with a deformation event that resulted in the emplacement of the last large thrust unit and development of upright folds and associated cleavage (S1).The second tectonothermal event gave rise to low-grade metamorphism and cleavage (S2) crosscutting earlier upright folds in the central, western and southern parts of the study area. The event continued with the intrusion of small igneous rock bodies, which gave rise to contact metamorphism and hydrothermal alteration. The second event was linked to an extensional episode due to a gravitational instability at the end of the Variscan deformation. This tectonothermal evolution occurred during the Gzhelian-Sakmarian. Subsequently, several hydrothermal episodes took place, in association with local development of crenulation cleavage during the Alpine deformation.

scholarly journals Tectonothermal evolution in the core of an arcuate fold and thrust belt: the south-eastern sector of the Cantabrian Zone (Variscan belt, north-western Spain)

Solid Earth ◽  
2016 ◽  
Vol 7 (4) ◽  
pp. 1003-1022 ◽  
Author(s):  
María Luz Valín ◽  
Susana García-López ◽  
Covadonga Brime ◽  
Fernando Bastida ◽  
Jesús Aller
Keyword(s):  
Gravitational Instability ◽  
Contact Metamorphism ◽  
Low Grade ◽  
Variscan Belt ◽  
Thrust Belt ◽  
Cantabrian Zone ◽  
Grade Metamorphism ◽  
The Core ◽  
Kübler Index ◽  
Fold And Thrust Belt

Abstract. The tectonothermal evolution of an area located in the core of the Ibero-Armorican Arc (Variscan belt) has been determined by using the conodont colour alteration index (CAI), Kübler index of illite (KI), the Árkai index of chlorite (AI) and the analysis of clay minerals and rock cleavage. The area is part of the Cantabrian Zone (CZ), which represents the foreland fold and thrust belt of the orogen. It has been thrust by several large units of the CZ, what resulted in the generation of a large number of synorogenic Carboniferous sediments. CAI, KI and AI values show an irregular distribution of metamorphic grade, independent of stratigraphic position. Two tectonothermal events have been distinguished in the area. The first one, poorly defined, is mainly located in the northern part. It gave rise to very-low-grade metamorphism in some areas and it was associated with a deformation event that resulted in the emplacement of the last large thrust unit and development of upright folds and associated cleavage (S1). The second tectonothermal event gave rise to low-grade metamorphism and cleavage (S2) crosscutting earlier upright folds in the central, western and southern parts of the study area. The event continued with the intrusion of small igneous rock bodies, which gave rise to contact metamorphism and hydrothermal alteration. This event was linked to an extensional episode due to a gravitational instability at the end of the Variscan deformation. This tectonothermal evolution occurred during the Gzhelian–Sakmarian. Subsequently, several hydrothermal episodes took place and local crenulation cleavage developed during the Alpine deformation.

Low-grade metamorphism in the Scottish Southern Uplands terrane: deciphering the patterns of accretionary burial, shearing and cryptic aureoles

2000 ◽  
Vol 91 (3-4) ◽  
pp. 521-537 ◽  
Author(s):  
R. J. Merriman ◽  
B. Roberts
Keyword(s):  
Shear Zone ◽  
High Strain ◽  
Low Grade ◽  
Strain Rates ◽  
Illite Crystallinity ◽  
Regional Pattern ◽  
Grade Metamorphism ◽  
Block Movement ◽  
Granitic Intrusions ◽  
Present Pattern

ABSTRACTSystematic studies of metapelitic grade linked with the geological re-survey of the Southern Uplands have been used to generate a contoured metamorphic map currently covering nearly two-thirds of the terrane. These studies, based on approximately one pelite sample per 2·5 km2, have used XRD measurements of clay mineral reaction progress, particularly illite crystallinity, to delineate zones of diagenesis and low-grade metamorphism in the imbricated Ordovician and Silurian strata. The regional pattern revealed by metapelitic zonal sequences does not agree with earlier observations that grade increases across the strike, from SE to NW. Instead, the map shows considerable variations in metamorphic trends, including patterns of grade increasing from older into younger strata, indicative of accretionary burial. Other patterns identified include those generated by high strain rates in the Moniaive Shear Zone (MSZ), and by extensive low-temperature cryptic aureoles associated with late granitic intrusions. The present pattern is the result of uplift that generated normal movement on reactivated thrust faults and differential block movement on NW-trending faults.Regional metamorphic patterns were generated by burial and underplating in an accretionary thrust stack. Subduction was initiated in the early Caradoc and probably ceased in the early Wenlock. Metapelitic patterns suggest that two levels of accretion are exposed in the terrane. Strata accreted to the toe of the prism and stacked above the décollement zone are typically at late diagenetic grade. Underplated strata below the décollement are typically at anchizonal grades with moderate to well-developed slaty cleavage. Coherent thrust-bounded tracts of strata at both levels were rotated and buried to produce a syntectonic depth-controlled pattern of meta-morphism. Shear zone metamorphism at depths of 12 km or more was probably confined to the underplated lower level of the thrust stack, and Devonian granitic intrusions were also emplaced mainly within the underplated strata.

Low-grade metamorphism and accretion tectonics: Southern Uplands terrain, Scotland

1985 ◽  
Vol 49 (352) ◽  
pp. 335-344 ◽  
Author(s):  
A. E. S. Kemp ◽  
G. H. J. Oliver ◽  
J. R. Baldwin
Keyword(s):  
Subduction Zones ◽  
Structural Data ◽  
Low Grade ◽  
Structural Level ◽  
Concomitant Increase ◽  
Illite Crystallinity ◽  
Grade Metamorphism ◽  
Trench Slope ◽  
Reflectance Data ◽  
Trajectory Characteristic

AbstractPrevious studies of low-grade metamorphism in the Southern Uplands accretionary terrain indicated prehnite-pumpellyite facies/anchizone conditions developed throughout the area, except for local preservation of trench-slope sediments and an accreted seamount at zeolite facies/advanced diagenetic grade. New graptolite reflectance data are presented that show a general northward increase in temperature in the Southern Uplands. The results from two cross-strike traverses in the southern and central belts in contemporaneous sequences, using illite crystallinity, illite lateral spacing (bo) , and graptolite reflectance, indicate the development of systematic accretion-related low-grade metamorphism. Well-developed and constant anchizone conditions occur throughout the NE (Langholm) traverse, associated with common, F1 accretion-related folding and a regionally penetrative S1 cleavage. In the SW (Kirkcudbright) traverse, however, the youngest, last accreted packets are preserved at a transitional diagenetic stage and lack a penetrative S1 cleavage. Illite crystallinity, graptolite reflectance, and bo increase systematically northward through earlier accreted packets, reaching values of the NE traverse only at the northern end. The concomitant increase of bo with illite crystallinity suggests the relatively high P-low T trajectory characteristic of subduction zones. Integration of metamorphic and structural data relates increasing intensity of aceretion-related F1 folding, developmertt of S1 fabric, and onset of later fold phases to grade of metamorphism and structural level within the accretionary pile.

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