U–Pb zircon geochronology and implications of Cambrian plutonism in the Ellsworth belt, Maine

The Ellsworth belt is one of several fault-bounded blocks exposed along the southeastern coast of Maine that formed within Ganderia. New ID-TIMS U–Pb geochronological data integrated with field relationships provide additional insights into the timing of magmatism and deformation in the Ellsworth belt. The deformed Lamoine Granite was selected for U–Pb zircon analysis in order to: i) establish the protolith age; ii) provide direct temporal constraints on regional low-grade metamorphism and deformation; and iii) elucidate relationships between the Ellsworth belt and coeval rocks elsewhere in the Appalachian orogen. The Lamoine Granite was emplaced within the Ellsworth Schist at 492 ± 1.7 Ma; this is the first unequivocal evidence for a Furongian magmatic event in the Ellsworth belt. The schistosity in the Lamoine Granite is parallel to the main fabric of the host Ellsworth Schist and provides a maximum estimate for timing of the regional metamorphic overprint. Widespread deformation in the Ellsworth belt where kinematic indicators indicate a top-to-northwest sense of shear is attributed to thrusting during which progressive horizontal shortening, caused crustal thickening and peak greenschist facies metamorphism. The Cambrian U–Pb age permits correlation of the Lamoine Granite with the Cameron Road Granite in the Annidale belt of New Brunswick where subduction-related magmas intruded the Penobscot arc–back-arc and were subsequently deformed during the Penobscot Orogeny.

Tectonic Fabric, Geochemistry, and Zircon-Monazite Geochronology as Proxies to Date an Orogeny: Example of South Delhi Orogeny, NW India, and Implications for East Gondwana Tectonics

We have dated the South Delhi orogeny, Aravalli-Delhi Mobile Belt (ADMB), NW India, using the tectonic fabric, geochemistry, and zircon-monazite geochronology as the proxies. The South Delhi Terrane (SDT), a passive margin domain in the ADMB, consists of multiply deformed (D1–D4) greenschist facies rocks and several granite plutons. The D1 deformation is characterized by pervasive isoclinal recumbent F1 fold and axial planar tectonometamorphic fabric, S1, developed in all rock types. The S1 minerals belong to peak greenschist facies metamorphism, M1, suggesting syntectonic nature of M1 with D1. The age of the D1-M1 is constrained by the syncollisional peralkaline S type Sewariya granite which is characterized by magmatic/submagmatic fabric (Sm) coplanar with the S1. The margin of the pluton is turned into quartzofeldspathic gneiss carrying the evidence of high temperature deformation. The age of Sewariya granite is estimated at ca. 878 Ma by zircon geochronology. The D1-M1 is further constrained by monazite geochronology of the mica schist at ca. 865–846 Ma. The other granite plutons and metarhyolite are pre-D1 and emplaced at ca. 992–946 Ma. The D2 deformation produced NE-SW trending open upright F2 folds coaxial with the F1, and northwesterly vergent F2–axial planar thrusts. Monazite geochronology constrains the D2 at ca. 811–680 Ma. The D3 is characterized by small to large scale NW-SE folds, and the D4 by faults and fractures marking the brittle deformation in the rocks. The D4 is constrained by monazite geochronology at ca. 588–564 Ma. There are upper amphibolitic tectonic slivers along the D2-Phulad thrust, belonging to the pre-Delhi rocks, which show ca. 1,638 Ma metamorphism age. From the above study, it is suggested that the South Delhi orogeny belongs to ca. 878–680 Ma marking the final amalgamation of Marwar Craton with the rest of India. This overlaps the early phase of the Pan-African orogeny (900–630 Ma). The brittle deformation, D4, coincides with Kuunga orogeny (650–500 Ma). Our study implies that India, like other continents in the East Gondwana, underwent amalgamation of internal blocks until the late part of the Neoproterozoic.

Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada

Three-dimensionally preserved Ediacaran fossils occur globally within sandstone beds. Sandy siliciclastic deposits of the Ediacaran Wood Canyon Formation (WCF) in the Montgomery Mountains, Nevada, contain two fossil morphologies with similar shapes and sizes: one exhibits mm-scale ridges and a distinct lower boundary and the other is devoid of these diagnostic features. We interpret these as taphomorphs of erniettomorphs, soft-bodied organisms with uncertain taxonomic affinities. We explore the cast-and-mould preservation of both taphomorphs by petrography, Raman spectroscopy, X-ray fluorescence microprobe and X-ray diffraction. All fossils and the surrounding sedimentary matrix contain quartz grains, iron-rich chlorite and muscovite. The ridged fossils contain about 70% larger quartz grains compared to the ridgeless taphomorph, indicating a lower abundance of clay minerals in the ridged fossil. Chlorite and muscovite likely originated from smectite and kaolinite precursors that underwent lower greenschist facies metamorphism. Kaolinite and smectite are inferred to have been abundant in sediments around the ridged fossil, which enabled the preservation of a continuous, distinct, clay- and kerogen-rich bottom boundary. The prevalence of quartz in the ridged fossils of the WCF and in erniettomorphs from other localities also suggests a role for this mineral in three-dimensional preservation of erniettomorphs in sandstone and siltstone deposits.

Residual lattice strain in quartzites as a potential palaeo-piezometer

SUMMARY If a crystal lattice is subjected to a stress, it becomes distorted and no longer represents the ideal crystal symmetry, and if the stress introduces defects such as dislocations, some of this distortion is preserved after the applied stress is removed. In this study, we investigate lattice distortion in quartz at the micron scale with synchrotron X-ray Laue diffraction. From Laue images the local deviatoric strain tensor is derived and corresponding stresses are calculated based on elastic properties. The method is applied to metasedimentary quartzites from the Bergell Alps that were deformed at conditions of greenschist facies metamorphism. The residual palaeostrain is represented in maps of the deviatoric strain tensor components and with deviatoric strain axis pole figures. Data suggest overall shortening perpendicular to the schistosity plane but with considerable asymmetry relative to foliation and lineation, probably attributed to simple shear. Crystallographic pole figures from Laue diffraction agree with neutron diffraction and EBSD measurements and display quartz c-axes girdle distributions with maxima also perpendicular to schistosity. The method shows promise to be used as a palaeo-piezometer to unravel the stress field during tectonic deformation.

Alteration of magmatic monazite in granitoids from the Ryoke belt (SW Japan): Processes and consequences

The alteration of magmatic monazite and its consequences for monazite geochronology are explored in granitoids from the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Biotite-granite samples were collected in two plutons emplaced slightly before the main tectono-metamorphic event: the first one, a massive granite (Shimokuhara) adjoins schistose rocks affected by greenschist facies metamorphism; and the second, a gneissose granite (Namera) adjoins migmatitic gneiss that experienced upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization along cracks and veinlets. It is variably replaced by allanite+apatite±xenotime±Th-U-rich phases. The outermost rims of primary domains yield a weighted average 206Pb/238U date of 102 ± 2 Ma while the Th-U phases show Th-U-Pb dates of 58 ± 5 and 15 to 14 ± 2–3 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by secondary domains enriched in Ca, Y, U, P, and containing numerous inclusions. The secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma while primary domain analyses are commonly discordant and range from 116 to 101 Ma. Monazite alteration textures in the two granites chiefly reflect differences in their post-magmatic histories. In the massive granite, monazite replacement occurred via a nearly stoichiometrically balanced reaction reflecting interaction with an aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration of the granitic assemblage, likely below 500 °C. In the gneissose granite, a small amount of anatectic melt, probably derived from the neighboring metasedimentary rocks, was responsible for pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. Regardless of whether monazite underwent replacement or recrystallization, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 and 106 ± 5 Ma). Conversely, the age of monazite alteration is not easily resolved. Monazite replacement in the massive granite might be constrained using the Th-U-rich alteration products; with due caution and despite probable radiogenic Pb loss, the oldest date of 58 ± 5 Ma could be ascribed to chloritization during final exhumation of the granite. The spread in apparently concordant 206Pb/238U dates for secondary domains in the gneissose granite is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long high-temperature (HT) history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

Asbestos-like actinolite crystallization during late regional variscan exhumation in the South Armorican Massif (France)

<p>In this study, two types of natural asbestos-like actinolite occurrences were sampled in order to understand their tectonic and metamorphic signification. Studied rocks were collected within two Variscan ophiolitic formations (Tréogat and Pont de Barel Formations, South Armorican Massif, Western France), mainly composed of amphibolites, and which recorded amphibolite to greenschist facies metamorphism. In these localities, the natural asbestos-like actinolite occurrences are closely related with the development of tectonic structures such as extension veins, tension gashes, σ and δ-type boudins. Field and petrostructural studies together with optical microscope, SEM and electron-microprobe analyses (EPMA) allowed to link early steps of the retrograde deformation event, during which acicular hornblende crystallizes in extension veins showing fuzzy boundaries or in hosting rock, with the late step of the same deformation event, during which hornblende is downgraded into asbestos-like actinolite synchronous with felsic melt circulation and tectonic structures opening. Field and microtectonic observations point to a sinistral strike-slip shearing for Pont de Barel formation and to a sinistral transtensive shearing for the Tréogat formation, which is consistent with the late regional variscan exhumation of the South Armorican Terrane.  SEM observations show that asbestos-like actinolite originate from hornblende crystallographic plan fragmentation, starting first along the (110) plans and continue both along the (100) and (110) plans. EPMA analyses show that Na-Al-Si metasomatism is associated with this fragmentation. Temperature estimates of chlorite crystallization after hornblende are around 300°C for the Tréogat Formation and 200°C for the Pont de Barel Formation, suggesting that amphibole fragmentation can occur over a wide temperature range. Additionally, Principal Component Analysis was performed using crystallographic sites distribution. Results show a clear correlation between actinolite Si(T) and hornblende Al(T), Al(C) and Na(A) crystallographic sites, suggesting that asbestos-like actinolite after hornblende fragmentation is rather due to a decrease of pressure within the tectonic structures, as Al in amphibole is pressure-dependent. This decrease could be due to the fluid pressure, which is supra-lithostatic during tectonic structures opening.</p>

Chapter 6: The Supergiant, High-Grade, Paleoproterozoic Metasedimentary Rock- and Shear Vein-Hosted Obuasi (Ashanti) Gold Deposit, Ghana, West Africa

Obuasi, with a total mineral resource plus past production of 70 Moz, is the largest gold deposit in West Africa, and one of the largest in the world. It is hosted by ~2135 Ma siliciclastic rocks of the Eburnean Kumasi Basin, which were obliquely shortened along an inverted boundary with the older Eoeburnean Ashanti belt to the east. Greenschist facies metamorphism was coeval with mineralization and related alteration at ~2095 Ma. The steeply dipping, ENE-plunging lodes extend over an 8-km strike length and to depths of >2.5 km. They include paragenetically complex gold-rich quartz veins surrounded by refractory auriferous arsenopyrite and closely associated carbonate-muscovite alteration halos in deformed carbonaceous phyllites and subordinate metaigneous host rocks. Gold and arsenic were initially precipitated during deformation-assisted interaction with reduced host rocks at ~350°C and 100 to 200 MPa. The mineralizing fluids were derived primarily from deeper, As-rich metasedimentary sources by basinal fluid expulsion and metamorphic devolatilization triggered by inversion and shortening, followed by transpression. Continued fluid injection during and after the metamorphic peak produced changes in gold fineness, sulfide assemblages, repeated dissolution (stylolites) and reprecipitation of mineralized veins, and a change from early deformed shear-related, sulfide-rich lodes to later quartz-rich lodes that plunge down or across the axes of younger transpressional folds. Channelized fluid flow due to reactivation of basin-edge transfer structures, and/or irregularly distributed gold source rocks, may explain the variation in gold endowment along the former basin boundary.

Spinel chemistry from Madawara Mafic-Ultramafic Complex, Lalitpur District, Uttar Pradesh, India

Spinel in peridotite of the Pipariya area in Madawara Mafic and Ultramafic Complex (MMUMC) occur as euhedral to subhedral inclusion within the silicates. They exhibit regular to irregular shape and is altered to ferritchromite to magnetite along the cracks and boundaries. Cr-spinel is characterised by high values of Cr# (79.7-98.4) and low Mg# and Al# values (i.e., 20.41-9.66 and 0.49- 17.71, respectively) and are identified as Fe-chromite. Chemical discontinuity/zones between the core and rim can be observed in the analysed grain. The textural and chemical features of investigated suggest low- to sub greenschist facies metamorphism consistent with the estimated metamorphic equilibrium temperature of 500°C-550°C. Mineral chemistry of these spinel suggests that spinel in peridotite of Pipariya in the Madawara Mafic and Ultramafic Complex depicts that these Cr-spinel may have been derived from boninitic related magmas at arc to suprasubduction zone tectonic setting at low degree of partial melting.

Vanadian and chromian garnet- and epidote-supergroup minerals in metamorphosed Paleozoic black shales from Čierna Lehota, Strážovské vrchy Mountains, Slovakia: crystal chemistry and evolution

Silicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulfides and other minerals occur in Devonian black shales near Čierna Lehota in the Strážovské vrchy Mountains, Slovakia. The garnets have high concentrations of V, Cr and Mn (up to 17 wt.% V2O3, ≤11 wt.% Cr2O3and ≤ 21 wt.% MnO) and several compositional types. Vanadian-chromian grossular (Grs 1) usually preserves primary metamorphic oscillatory zoning, whereas solid solutions between goldmanite (Gld 2A,B), V- and Cr-rich grossular and spessartine (Grs 2A,B, Sps 2) form irregular domains or crystals with variable zoning. Dominant substitutions in the garnets include CaMn–1and (V,Cr)Al–1, resulting in coupled Ca(V,Cr)Mn–1Al–1. Epidote-supergroup minerals occur as abundant anhedral crystals with variable compositional zoning. Nearly all crystals have a complete zoning sequence beginning withREE-rich allanite-(La), followed by mukhinite and by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor clinozoisite. In common with garnets, the epidote-supergroup minerals are enriched in V, Cr and Mn (<7 wt.% V2O3, <5 wt.% Cr2O3and <3 wt.% MnO). Lanthanum is the dominantREE(up to 11.5 wt.% La2O3) in allanite-(La). The composition of epidote-supergroup minerals is controlled byREEFe2+(CaAl)–1,REEMg(CaAl)–1,REEMn2+(CaAl)–1andREEFe2+(CaFe3+)–1substitutions introducingREE, together with VAl–1and CrAl–1substitutions. The negative Ce and slightly positive Eu anomalies displayed in chondrite-normalized patterns and enrichment in V, Cr and Mn are ascribed to the geochemical properties of the protolith. The minerals investigated exhibit multi-stage evolution: (1) presumed low-grade greenschist-facies metamorphism; and (2) development of V- and Cr-rich zones in both garnet- and epidote-supergroup minerals which result from late-Variscan contact metamorphism due to granitic intrusion of the Suchý Massif. Decreased temperature following the metamorphic peak probably resulted in the formation ofREE-, V- and Cr-poor clinozoisite and secondary garnet.

Spatially overlapping episodes of deformation, metamorphism, and magmatism in the southern Omineca Belt, southeastern British Columbia

The southeastern Omineca Belt of the Canadian Cordillera preserves a record of overlapping Barrovian and Buchan metamorphism spanning 180–50 Ma. This paper documents the timing, character, and spatial relationships that define separate domains of Middle Jurassic, Early Cretaceous, and Late Cretaceous deformation and metamorphism, and the nature of the geological interfaces that exist between them. A domain of Early Jurassic deformation (D1) and regional greenschist-facies metamorphism (M1) is cross-cut by Middle Jurassic (174–161 Ma) intrusions. Associated contact aureoles are divided into lower pressure (cordierite-dominated; ∼2.5–3.3 kbar; 1 kbar = 100 MPa) and higher pressure (staurolite-bearing; 3.5–4.2 kbar) subtypes; contact metamorphic kyanite occurs rarely in some staurolite-bearing aureoles. Jurassic structures are progressively overprinted northwards by Early Cretaceous deformation and metamorphism (D2M2), manifested in a tightening of Jurassic structures, development of more pervasive ductile fabrics, and Barrovian metamorphism. The D2M2 domain is the southerly continuation of the 600 km long Selkirk–Monashee–Cariboo metamorphic belt. Mid-Cretaceous intrusions (118–90 Ma) were emplaced throughout the D2M2 domain, the earliest of which contain D2 fabrics, but cut M2 isograds. The D2M2 domain makes a continuous, southeasterly transition into a domain of Late Cretaceous regional Barrovian metamorphism and deformation (D3M3; 94–76 Ma). The interface between these two domains is obscured by the coaxial nature of the deformation and the apparent continuity of the metamorphic zones, resulting in a complex and cryptic interface. Similarities between the D3M3 domain and the Selkirk Crest of Idaho and Washington suggest that this domain is the northerly continuation of the northward-plunging Priest River Complex.