Orogenic Collapse Research Articles

Tectonic and sedimentary history of the neoproterozoic metavolcanic–volcaniclastic rocks of the El-Dabbah Group, Central Eastern Desert, Egypt

The El-Dabbah Group is a >7500 m-thick sequence of metavolcanic rocks, volcaniclastics, and banded iron-formation located within the Nubian Shield, Central Eastern Desert, Egypt. The sequence preserves lower greenschist-facies island arc rocks. We used detailed mapping to reconstruct the stratigraphic and tectonic history of the El-Dabbah Group and the surrounding sedimentary basins. The El-Dabbah Group was identified as deposition at central rift zone of an oceanic island arc and was overlies by the subaerial sedimentary rocks (the Atshan Formation and the Hammamat Group). Four faulting were distinguished within the El-Dabbah area: (1) top-to-the-S low-angle thrusting; (2) NNE side down normal faulting; (3) sinistral displacement on north–south faults; and (4) WNE trend N-side-down normal faulting with the Hammamat Group. We obtained the crystallization ages of a granite (638.1 ± 2.9 Ma) and a quartz porphyry (659.6 ± 3.0 Ma). The detrital zircon age of the Hammamat Group show peaks at 650, 680, and 790 Ma. These evidences suggest, three sedimentation-deformation stages were identified in this area. Stage 1 involved sedimentation of island-arc-derived volcaniclastic rocks at 770-700 Ma and to thrust related accretion at 700–680 Ma. Stage 2 involved collisional and transtensional deformation related to formation of the subaerial basin that contains the Atshan Formation at 670-640 Ma, the normal fault, the strike-slip fault and granite intrusion during 660–620 Ma. Stage 3 involved orogenic collapse, regional strike-slip-related sedimentation and deformation of the Hammamat Group at 610–570 Ma. We show that the El-Dabbah Group records a history of oceanic island arc accretion, collision, and collapse within the northern East African Orogeny during 770–570 Ma in Neoproterozoic.

Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

Permo–Triassic granitoids, Hainan Island, link to Paleotethyan not Paleopacific tectonics

Abstract Hainan Island may have lain at the boundary between the Tethyan and Pacific domains since the late Paleozoic. Permo–Triassic granitoids outcropped on the island provide insight into Mesozoic tectonic evolution and potential dynamics. Based on analyses of zircon U-Pb geochronological and Hf-O isotopic data, along with whole-rock elemental and Sr-Nd isotopic data, the Permo–Triassic granitoids are divided into three groups: Late Permian (256–252 Ma) gneissic granitoids (Group 1), Early–Middle Triassic (247–244 Ma) massive granitoids (Group 2), and Middle-Late Triassic (242–225 Ma) massive granitoids (Group 3). Groups 1 and 2 exhibit similar elemental and Sr-Nd isotopic compositions with εNd(t) values ranging from –6.71 to –3.25. Group 1 is characterized by gneissic foliation and calc-alkaline I-type geochemical affinities with negative Nb-Ta and Ti anomalies; it has low εHf(t) (–4.2∼+0.2) and high δ18O values (9.35–10.46‰), possibly related to derivation of a metabasite source in a continental arc setting. The Group 2 samples are peraluminous massive granitoids with slightly high A/CNK ratios of 1.00–1.31, δ18O values (8.85–11.75‰), and high Sr/Y ratios but low εHf(t) (–8.7∼+1.6) values. They may have originated from the mixed source of greywacke and metabasite, indicating a compressive tectonic setting. The Group 3 high-K calc–alkaline granitoids have high zircon temperatures (842–867 °C) and show the geochemical signatures of A-type granites. They are featured by slightly low whole-rock εNd(t) and zircon εHf(t) values, suggestive of the derivation from a metabasite–greywacke source in an extensional setting. In combination with other geochronological and geochemical data from Hainan Island, it is proposed that the Permian–Triassic igneous magmatism in Hainan Island reflects a continuous amalgamation process from back-arc consumption (272–252 Ma) to orogenic assembly (251–243 Ma) and orogenic collapse (242–225 Ma) between the South China and Indochina Blocks. Such a temporal–spatial pattern for the Hainan granitoids is consistent with that along the Truong Son, Jinshajiang–Ailaoshan–Song Ma tectonic zone, suggesting dominant control of Paleotethyan domain instead of Paleopacific tectonics.

Oligocene molasse sedimentation in Central Taurides: Records of the onset of extensional tectonic regime

The lacustrine units have been deposited in the Ermenek, Bucakkisla, Korucuk and Camliyayla basins in the Central Taurides since early Oligocene. In this study, structural features and stratigraphical properties of Oligocene basins have been described in detail and the lacustrine units have been dated. Regional geological interpretations have been made by using tectono-stratigraphic and age data of the basins. Lacustrine Oligocene sedimentation in these basins, which were opened on the tectonic units of Taurides due to normal faults, constitute the first records of the sedimentation occurring under the extensional tectonic regime after the orogeny of the Central Taurides. All the pre-Oligocene units that had been emplaced in the region due to a north-south compressional movement in the Central Taurides, continued to be compressed until the end of Eocene, and completed their orogenic development as napped structures. The bedrock units, reaching the maximum elevation, have been subjected to “orogenic collapse” due to the strain or gravity forces as a result of an interruption or termination of the compression which lasted until the end of Eocene. Thus, “orogenic collapse” basins have started to be formed in the Central Taurides since Oligocene. The orogeny and the compressional tectonic regime forming the Taurides lasted until late Miocene in the Eastern Taurides. On the other hand, extensional basin formation since early Oligocene on the Taurus units, which had been emplaced in the region due to nappe tectonics, indicates that a new tectonic period has started in the Central Taurides.

Paleoproterozoic khondalites in Brazil: a case study of metamorphism and anatexis in khondalites from Itapecerica supracrustal succession of the southern São Francisco Craton

ABSTRACT In Brazil, Paleoproterozoic khondalites were recognized in Ceará, Bahia, Goiás, Tocantins, and Minas Gerais. The Itapecerica supracrustal succession in the Southern São Francisco Craton (Minas Gerais) contains sillimanite-cordierite-garnet-biotite gneiss (khondalite) with an anatectic record. The high-grade khondalite preserve representative mineral assemblages of peak and orogen collapse after post-peak decompressional stage. Based on petrographic observations and P–T pseudosections of bulk rock compositions, a clockwise pressure-temperature-time (P–T-t) path was inferred. The metamorphic peak assemblage is liquid + plagioclase ± K-feldspar + garnet + biotite + ilmenite + sillimanite + quartz at 715–772ºC and 5.5–7.5 kbar. In addition, the precursor sediments had mixed pelitic-wacke compositions resulting from erosion of different sources. Sediment deposition would have occurred at active continental marginal setting. Surrounding the khondalite occurs a peraluminous metagranite named here as Água Rasa, formed in syn- to post-collisional setting, whose εNd(t) (−0.5 to −2.7) and 87Sr/86Sr(t) (1.04 to 1.08) and the data set indicate that the precursor magma of the Água Rasa metagranite originated from anatexis of the khondalitic rocks (crustal source) and associated amphibolites (mantle source) during the crustal thickening followed by tectonic exhumation at peak metamorphism and at decompressional stage during the orogenic collapse. The khondalite yielded ages from monazite U-Th-PbT dating of 2090 ± 26 Ma and 1937 ± 32 Ma, while the Água Rasa metagranite yielded ages of 2077 ± 24 Ma and 1941 ± 23 from monazite and 1934 ± 74 Ma from zircon U-Pb. The ages of >2.0 Ga are related to the metamorphic peak, while the younger ones (~1940 Ma) are related to the orogen collapse after post-peak decompressional stage. Similarities between the khondalite rocks of this study and of the North China Craton suggest that the São Francisco-Congo Craton was near the North China Craton in the supercontinent Columbia for the 2.1–1.9 Ga period.

Extensional tectonics rooted in orogenic collapse: Long-lived disintegration of the Semail Ophiolite, Oman

Abstract Significant post-orogenic extension of the renowned Semail Ophiolite and substrata in Oman resulted in the formation of metamorphic core complexes juxtaposed with an array of Maastrichtian-Paleogene extensional basins. During this evolution, basins became progressively localized. The geometry of the large-scale and long-lived extensional system changes laterally across the core complexes and reveals several generations of domes and detachments, some of which were progressively exhumed. Progressive excision and dismemberment of the ophiolite link to major fabrics in the core complexes and gradual focusing of extensional basins.

Orogenic gold deposits: part of a global dynamic conjunction between subduction and gold

SummaryOrogenic gold deposits represent a coherent deposit class that were deposited from low-salinity CO2-rich fluids over a range of crustal depths from 3 to 20 km. They are widely considered to have formed from crustal metamorphic fluids generated at the amphibolite-greenschist-facies transition. However, recent research, particularly in China, shows that only a sub-crustal source of fluid is viable and that devolatilization of subduction slabs and overlying sediment wedges is the most plausible source of the auriferous fluids. An alternative source is devolatilization of fertilized mantle lithosphere that was earlier metasomatized by subduction-related fluids. This recognition of a subduction-related origin allows the orogenic gold deposits to be placed into a coherent, subduction-related dynamic model in which all epigenetic gold deposits formed during evolution of a convergent margin. The earliest deposits that formed from subduction-related magmatic-hydrothermal fluids during mild compression are inter-related porphyry Cu-Au, high-sulfidation epithermal Ag-Au and skarn deposits in volcanic and continental arcs. Subsequent mild extension resulted in formation of low-sulfidation epithermal Ag-Au in volcanic and continental back-arcs and gold-rich VMS systems in volcanic back-arcs. Orogenic gold deposits then developed under transpression during late orogenesis in the convergent margin. The initiation of orogenic collapse saw the formation of IRGDs and Carlin-type gold deposits related to hybrid magmas, formed by melting of subduction-related metasomatized lithosphere, in far back-arc environments. Finally, following supercontinent assembly, IOCG deposits formed on the margins of continental blocks during extension, mantle up-welling and melting of subduction-related fertilized lithosphere that released highly volatile magmatic-hydrothermal fluid systems into the crust.

Subduction, mantle metasomatism, and gold: A dynamic and genetic conjunction

Abstract Global gold deposit classes are enigmatic in relation to first-order tectonic scale, leading to controversial genetic models and exploration strategies. Traditionally, hydrothermal gold deposits that formed through transport and deposition from auriferous ore fluids are grouped into specific deposit types such as porphyry, skarn, high- and low-sulfidation–type epithermal, gold-rich volcanogenic massive sulfide (VMS), Carlin-type, orogenic, and iron-oxide copper-gold (IOCG), and intrusion-related gold deposits (IRGDs). District-scale mineral system approaches propose interrelated groups such as porphyry Cu-Au, skarn Cu-Au-Ag, and high-sulfidation Au-Ag. In this study, the temporal evolution of subduction-related processes in convergent margins was evaluated to propose a continuum of genetic models that unify the various types of gold deposits. At the tectonic scale of mineral systems, all hydrothermal gold deposits are interrelated in that they formed progressively during the evolution of direct or indirect subduction-related processes along convergent margins. Porphyry-related systems formed initially from magmatic-hydrothermal fluids related to melting of fertile mantle to initiate calc-alkaline to high-K felsic magmatism in volcanic arcs directly related to subduction. Formation of gold-rich VMS systems was related to hydrothermal circulation driven by magmatic activity during rifting of oceanic arcs. Orogenic gold deposits formed largely through fluids derived from devolatilization of the downgoing slab and overlying sediment wedge during late transpression in the orogenic cycle. Carlin-type deposits, IRGDs, and some continental-arc porphyry systems formed during the early stages of orogenic collapse via fluids directly or indirectly related to hybrid magmatism from melting of lithosphere that was metasomatized and gold-fertilized by earlier fluid release from subduction zones near margins of continental blocks. The IOCGs were formed during postorogenic asthenosphere upwelling beneath such subduction-related metasomatized and fertilized lithospheric blocks via fluid release and explosive emplacement of volatile-rich melts. Thus, importantly, subduction is clearly recognized as the key unifying dynamic factor in gold metallogenesis, with subduction-related fluids or melts providing the critical ore components for a wide variety of gold-rich deposit types.

The Lavrion Pb-Zn-Ag–Rich Vein and Breccia Detachment-Related Deposits (Greece): Involvement of Evaporated Seawater and Meteoric Fluids During Postorogenic Exhumation

AbstractThe formation of ore deposits in the Lavrion Pb-Zn-Ag district was associated with Miocene detachment that accommodated orogenic collapse and exhumation of high-grade nappes across the ductile-brittle transition. This district consists of (1) low-grade porphyry Mo style, (2) Cu-Fe skarn, (3) high-temperature carbonate replacement Pb-Zn-Ag, and (4) vein and breccia Pb-Zn-Ag mineralization. The vein and breccia mineralization locally contains high-grade silver in base metal sulfides that are cemented by fluorite and carbonate gangue. The rare earth element contents of these gangue minerals, chondrite-normalized patterns, and fluid inclusion studies suggest that they precipitated from a low-temperature hydrothermal fluid. Primary and pseudosecondary fluid inclusions in fluorite and calcite are characterized by a wide range of homogenization temperatures (92°–207°C) and salinities of up to 17.1 wt % NaCl equiv. Secondary fluid inclusions only represent <5 vol % of the total fluid trapped. Fluids extracted from inclusions in fluorite have values of δD = –82.1 to –47.7‰ (Vienna-standard mean ocean water [V-SMOW]) and δ18O = –10.4 to –5.1‰ (V-SMOW). These data and low ratios of Cl/Br measured by crush-leach analyses for fluids in fluorite (102–315) and calcite (162–188) are compatible with the ore fluid being the result of mixing of meteoric water with evaporated seawater. These data suggest that fluids leading to the deposition of late Pb-Zn-Ag–rich vein- and breccia-style mineralization in Lavrion were related to circulation of mixed evaporated seawater and meteoric fluids that was enhanced by brittle deformation. This contrasts with the fluids of magmatic origin related to the formation of low-grade porphyry Mo, Cu-Fe skarn, and high-temperature carbonate replacement deposits spatially related to the Plaka granodiorite.

Separating multiple episodes of partial melting in polyorogenic crust: An example from the Haiyangsuo complex, northern Sulu belt, eastern China

Abstract The exotic Haiyangsuo complex is structurally part of the Sulu belt but its contact relationship with surrounding Sulu gneisses is unexposed and therefore unknown, making its affinity uncertain. It comprises gneisses with in-source leucosomes that host minor metabasite bodies; both are cut by leucogranite dikes. In this study, we determine the timing and petrogenesis of leucosomes and leucogranites and assess the tectonic affinity of the complex based on data from gneisses and metabasites. Most zircon from gneisses and leucosomes has oscillatory-zoned cores with CL-bright overgrowth rims, but some has CL-dark cores or mantles between cores and rims. CL-dark and bright zircon yield weighted mean ages of ca. 1817–1812 Ma. CL-dark zircon has flat heavy rare earth element (HREE) patterns and crystallization temperatures of 829–875 °C, suggesting metamorphic growth, whereas rims have steep HREE patterns but a similar range of crystallization temperatures, suggesting growth from anatectic melt; εHf (t = 1813 Ma) of –18.3 to –10.8 indicates a North China Craton source. Magmatic zircon from metabasites yields ages of ca. 825 Ma, similar to those of scattered metabasite occurrences in the North China Craton. Paleoproterozoic zircon cores were scavenged during magma ascent. By contrast, zircon cores from the leucogranites yield concordant dates of 776–701 Ma, consistent with protolith ages in the Sulu belt, whereas overgrowth mantles and rims yield weighted mean ages of ca. 220 Ma and 162 Ma, respectively. Both mantles and rims host multiphase solid inclusions, representing former melt, suggesting anatexis and crystallization of zircon first during initial decompression and then during orogenic collapse of the Sulu belt; whole-rock Nd and Sr isotope compositions implicate the Sulu belt gneisses as the source of these melts. Our interpretation of these data is that the Haiyangsuo complex has an early geologic history similar to the Jiaobei terrane from the southeastern part of the North China Craton and was incorporated into the Sulu belt during Triassic collision of the Yangtze and North China Cratons. The two stages of melting relate to Upper Triassic early exhumation and Upper Jurassic late-stage orogenic collapse, during which the leucogranite magma was derived from a source similar to one elsewhere in the Sulu belt such as the subducted Yangtze Craton and not the North China Craton. This shows that during continental collisions, crust from the upper plate may be dragged into the subduction channel, deformed, and subsequently exhumed in association with partial melting of the crust.

The Neoproterozoic basement of the Sauce Chico Inlier (Ventania System): Geochemistry and U–Pb geochronology of igneous rocks with African lineage in central-eastern Argentina

This study describes the geology, geochemistry, and LA-ICP-MS U–Pb geochronology of igneous rocks that crop out in the Sauce Chico Inlier (SCI) and constitute the Neoproterozoic basement of the Ventania System, Argentina. Magmatism registered in the SCI has developed in two phases. The first phase, of Tonian age, is represented by rift-related calc-alkaline and alkaline granites with ages of 783.8 ± 3.7 Ma (εNd(t) = −7.20) and 776.5 ± 4.7 Ma (εNd(t) = +1.65), respectively. This phase would be related to the break-up of the Rodinia supercontinent and subsequent opening of the Adamastor Ocean. In the Dom Feliciano Belt of southern Brazil and eastern Uruguay, the Tonian magmatism is represented in the Cerro Olivo Complex of the Punta del Este Terrane and in basement inliers of the Pelotas Batholith. The second phase, of Ediacaran age, started with the intrusion of syn-orogenic calc-alkaline granites (620.8 ± 5.8 Ma and 620.3 ± 2.5 Ma; εNd(t) = −9.94/−9.18) and continued with the extrusion of post-orogenic alkaline (577.3 ± 3.9 Ma; εNd(t) = −6.29) and calc-alkaline (543.6 ± 4.0 Ma; εNd(t) = −3.38) acid volcanic rocks. This phase would be related to the closure of the Adamastor Ocean, generation of a magmatic arc along the western margin of the Kalahari Craton, and collision between this and the Río de la Plata Craton. A post-collisional magmatism would have developed due to orogenic collapse (< 580 Ma). With the exception of the Tonian alkaline granite, in the remaining SCI igneous rocks, the juvenile component becomes more important as the crystallization age decreases. The available Nd model ages are between 1.80 and 1.14 Ga and suggest a mixing of older crust with juvenile material. The tectonic evolution of the Ventania System basement is consistent with that observed in the Dom Feliciano Belt and its African counterparts. We present here the first evidence of Tonian magmatism in the Ventania System basement. The results presented here also confirm those obtained previously by other authors. Zircon cores from the SCI igneous rocks have U–Pb inherited ages between ca. 1200 and 900 Ma that could be indicative of a lineage with the Gariep Belt and its Namaqua basement in southwestern Africa.

Partial Melting and Crustal Deformation during the Early Paleozoic Wuyi–Yunkai Orogeny: Insights from Zircon U-Pb Geochronology and Structural Analysis of the Fuhuling Migmatites in the Yunkai Region, South China

：Migmatites record crucial information about the rheology and tectonothermal evolutionof the deep crust during orogenesis. In the Wuyi–Yunkai orogen in South China, migmatites at Fuhuling record Early Paleozoic high temperatures and associated partial melting. However, the absolute timing and implications for the rheology of the deep crust during orogenesis are poorly constrained. In this contribution, we used spatial analysis of migmatitic leucosomes, structural analysis, and U-Pb geochronology of zircon to elucidate the absolute timing of crustal partial melting, the degree of partial melting, and the role of partial melting on the rheology of the crust during the Wuyi–Yunkai orogeny. Partial melting of the Fuhuling migmatites occurred at c. 440 Ma during Early Paleozoic Wuyi–Yunkai orogenesis. Subsequent lower temperature metamorphism associated with Indosinian movement that caused minor zircon recrystallization was temporally associated with the crystallization of nearby biotite monzogranites, but it did not influence the morphology of the Fuhuling migmatites. The migmatites preserve a morphological transition from metatexite to diatexite with an increasing proportion of leucosome. This transition preserves different structural characteristics that represent the response of the solid framework and melt network to variable melt fractions during partial melting. The large proportion of in situ or in source leucosome in the Fuhuling migmatites suggests that it was a melt-rich crustal horizon during orogenesis, and that a substantial proportion of anatectic melt was retained in the deep crust. The rheological transition documented in the Fuhuling migmatites was caused by changes in the melt fraction, and it is an analogue for the rheological transition characteristics of melt-rich crustal horizons in the Yunkai region during Early Paleozoic Wuyi–Yunkai orogenesis and subsequent orogenic collapse.

The passive margin of northern Gondwana during Early Paleozoic: Evidence from the central Tibet Plateau

The central–south domain of the Tibet Plateau represents an important part of the northern segment of Gondwana during the early Paleozoic. Here we present zircon U–Pb, Lu–Hf isotope, and whole–rock geochemical data from a suite of early Paleozoic magmatic rocks from the central Tibet Plateau, with a view to gain insights into the nature and geotectonic evolution of the northern margin of Gondwana. Zircon grains in four granitic rocks yielded ages of 532−496 Ma with negative εHf(t) values (−13.7 to −0.6). Zircon grains in meta–basalt and mafic gneiss yielded ages of 512 ± 5 Ma and 496 ± 6 Ma, respectively. Geochemically, the granitic rocks belong to high–K calc–alkaline and shoshonitic S–type granite suite, with the protolith derived from the partial melting of ancient crustal components. The mafic gneiss and meta–basalt geochemically resemble OIB (Oceanic Island Basalt) and E–MORB (Enriched Mid–Ocean Ridge Basalt), respectively. They were derived from low degree (∼5–10%) partial melting of an enriched mantle (garnet and spinel lherzolite) that was contaminated by upper crustal components. The parental magmas experienced orthopyroxene–dominated fractional crystallization. Sedimentological features of the Cambrian–Ordovician formations indicate that the depositional cycle transformed from marine regression to transgression leading to the formation of parallel/angular unconformities between the Cambrian and Ordovician strata. The hiatus associated with these unconformities are coupled with the peak of the early Paleozoic magmatism in Tibet Plateau, indicating a tectonic control. We conclude that the Cambrian–Ordovician magmatic suite and sedimentary rocks formed in an extensional setting, and we correlate this with the post–peak stage of the Pan–African orogeny. The post–collision setting associated with delamination, orogenic collapse or lithospheric extension along the northern margin of Gondwana, can account for the Cambrian–Ordovician magmatism and sedimentation, rather than oceanic subduction along the external margin. We thus infer a passive margin setting for the northern Gondwana during the Early Paleozoic.

Orogenic Gold in Transpression and Transtension Zones: Field and Remote Sensing Studies of the Barramiya–Mueilha Sector, Egypt

Multi-sensor satellite imagery data promote fast, cost-efficient regional geological mapping that constantly forms a criterion for successful gold exploration programs in harsh and inaccessible regions. The Barramiya–Mueilha sector in the Central Eastern Desert of Egypt contains several occurrences of shear/fault-associated gold-bearing quartz veins with consistently simple mineralogy and narrow hydrothermal alteration haloes. Gold-quartz veins and zones of carbonate alteration and listvenitization are widespread along the ENE–WSW Barramiya–Um Salatit and Dungash–Mueilha shear belts. These belts are characterized by heterogeneous shear fabrics and asymmetrical or overturned folds. Sentinel-1, Phased Array type L-band Synthetic Aperture Radar (PALSAR), Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), and Sentinel-2 are used herein to explicate the regional structural control of gold mineralization in the Barramiya–Mueilha sector. Feature-oriented Principal Components Selection (FPCS) applied to polarized backscatter ratio images of Sentinel-1 and PALSAR datasets show appreciable capability in tracing along the strike of regional structures and identification of potential dilation loci. The principal component analysis (PCA), band combination and band ratioing techniques are applied to the multispectral ASTER and Sentinel-2 datasets for lithological and hydrothermal alteration mapping. Ophiolites, island arc rocks, and Fe-oxides/hydroxides (ferrugination) and carbonate alteration zones are discriminated by using the PCA technique. Results of the band ratioing technique showed gossan, carbonate, and hydroxyl mineral assemblages in ductile shear zones, whereas irregular ferrugination zones are locally identified in the brittle shear zones. Gold occurrences are confined to major zones of fold superimposition and transpression along flexural planes in the foliated ophiolite-island arc belts. In the granitoid-gabbroid terranes, gold-quartz veins are rather controlled by fault and brittle shear zones. The uneven distribution of gold occurrences coupled with the variable recrystallization of the auriferous quartz veins suggests multistage gold mineralization in the area. Analysis of the host structures assessed by the remote sensing results denotes vein formation spanning the time–space from early transpression to late orogen collapse during the protracted tectonic evolution of the belt.

Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (&lt;10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.

Tectono‐Metamorphic Evolution of the Central Ribeira Belt, Brazil: A Case of Late Neoproterozoic Intracontinental Orogeny and Flow of Partially Molten Deep Crust During the Assembly of West Gondwana

AbstractRecent advances in understanding the plate tectonics, intracontinental deformation, and flow of partially molten crust have significantly improved our knowledge of collisional tectonics and the way in which we understand complex ancient orogens. The Central Ribeira Belt represents a Neoproterozoic fold‐and‐thrust belt formed in the Brasiliano Orogenic Cycle associated with the assembly of West Gondwana. This fold‐and‐thrust belt is currently interpreted as a result of recurrent collisions and amalgamation of terranes against large cratons. Based on an integrated structural, petrological, and geochronological study in two metamorphic complexes of the Central Ribeira Belt (Embu and Costeiro complexes), we challenge the current model that involves multiple terrane collisions. Our data show for the first time metamorphic ages older than 600 Ma for samples from Costeiro and Embu complexes and suggest that both geological units experienced an intermediate‐P metamorphism (M1) at circa 620 Ma and a low‐P metamorphism (M2) at circa 575 Ma. Our proposed tectonic model is consistent with an M1 event related to an intracontinental orogeny, formed in response to the collision between the São Francisco Craton and the Paranapanema Block. On the other hand, the later M2 metamorphism records extensional and wrench tectonics associated with orogenic collapse, constrained by the decompression paths of the metasedimentary sequences and M2‐related S3 mylonitic foliation. The M2 metamorphism is associated with wide, right‐lateral strike‐slip shear zones and voluminous peraluminous magmatism in the Embu Domain and widespread partial melting of the middle crust forming migmatitic rocks and peraluminous leucogranites in the Costeiro Domain.

Eocene exhumation and extensional basin formation in the Copper Mountains, Nevada, USA

Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.

Polykinematic foreland basins initiated during orthogonal convergence and terminated by orogen-oblique strike-slip faulting: An example from the northeastern Variscan belt

An extensive early Carboniferous foreland basin (the Moravosilesian Culm Basin, MSCB) delineates the northeastern margin of the Bohemian Massif (Variscan orogenic belt). The basin overlays a complex suture between the outer Brunovistulia microplate, forming a promontory previously accreted to Laurussia, and inner Gondwana-derived units that form the remainder of the Bohemian Massif. Our structural, paleomagnetic and magnetic anisotropy data from the eastern portion of the MSCB indicate that its depositional architecture, internal fabric, and deformation by folding and faulting record a major switch from orthogonal convergence to orogen-oblique strike-slip shearing. First, the Brunovistulia microcontinent was subducted frontally towards W during ~346–335 Ma, followed by a shift of the subduction vector towards ~SSW at around 335–330 Ma. From ~330 Ma onwards, the underthrusting had ceased and the ~WNW–ESE dextral Elbe shear zone, related to the westward movement of Gondwana with respect to Laurussia, controlled deformation of this region. Displacement of the northerly Eastern Sudetes ‘block’ towards ~ESE produced regional ~WNW–ESE pure-shear shortening against the Brunovistulia indentor and resulted in an overall doubly-vergent structure of the MSCB. Simultaneously, the southerly Moldanubian unit was translated towards ~WNW, away from the Brunovistulian microplate, and likely contributed to an extensional regime related to orogenic collapse progressing outward from the internal domains of the Bohemian Massif. On a larger scale, we suggest that the MSCB is an illustrative example of a polykinematic foreland basin, where deposition, syn- to post-sedimentary deformation, and thus geodynamic processes changed significantly during basin evolution, starting off with microplate impingement into a continental margin reentrant and terminated by strike-slip shearing. Our data also do not support previous models for the tectonic development of the northeastern termination of the Variscan Belt, which suggested either a large-magnitude, vertical-axis oroclinal rotation of the foreland or its translation along an orogen-perpendicular strike-slip fault.

Protracted late- to post-Ottawan cooling and exhumation of the mid crust in a Grenvillian inlier, central Appalachians, USA – New data and a new hypothesis

New 40Ar/39Ar hornblende and biotite data from granulite-facies gneisses across the Grenvillian New Jersey Highlands inlier in the central Appalachians permit estimates of the rates of regional cooling and exhumation following the Ottawan phase of the Grenvillian Orogeny. 40Ar/39Ar hornblende and biotite results yielded plateau ages of ca. 947–914 Ma and ca. 887–819 Ma, respectively, constraining post-Ottawan cooling from the peak metamorphic temperature of ∼770 °C to ∼300 °C at a slow rate of 1.9–3.5 °C/Myr. Following assumptions about the physical properties of these mid-crustal rocks, we calculate they were exhumed from a peak burial depth of ∼22 km, and passed through the 500 °C and 300 °C isotherms at depths of ∼14 km and ∼7.5 km, respectively, indicating a slow exhumation rate of 0.06–0.11 km/Myr.The dual signal of slow cooling and exhumation is interpreted to be due to an extended period of elevated mantle heat flow triggered by delamination or convective removal of the lower mantle lithosphere beneath the orogen. Together with lower crustal intrusion and (or) crustal underplating by asthenospheric magmas, this is interpreted to have resulted in extensional collapse of the overthickened crust, juxtaposition of crustal levels, and the formation of large metamorphic core complexes. Mafic intrusions and (or) underplating not only provided an important heat source in the lower crust, but also increased the average crustal density, thereby slowing the rates of uplift, erosion and exhumation. We interpret the dual signal as a cryptic signature of pervasive extensional orogenic collapse after the Ottawan phase of the Grenvillian Orogeny.

Two generations of mafic dyke swarms in the Southeastern Brazilian coast: reactivation of structural lineaments during the gravitational collapse of the Araçuaí-Ribeira Orogen (500 Ma) and West Gondwana breakup (140 Ma)

Mafic dyke swarms emplaced in regional NW-SE trending structures crosscutting the Precambrian basement in the Southeastern Brazilian coast are commonly interpreted as associated to West Gondwana breakup. Recently, however, U-Pb dating suggested that at least some of those dykes were emplaced earlier, at ca. 500–490 Ma, during the gravitational collapse of the Araçuaí Orogen. In order to clarify this issue, we studied dykes from the southern Espírito Santo State. Our results indicate that dykes emplaced in the same regional lineaments can be separated into two distinct groups, according to petrographic, geochemical, isotopic and geochronological characteristics. Group 1 is tholeiitic, with 87Sr/86Sr(i) of 0.7041–0.7065, εNd(t) of −3.4 to −5.5 and TDM ages between 0.8 and 1.5 Ga. Zircon crystals from a dyke of this group yielded the first robust lower Cretaceous U-Pb age for mafic dykes of SE Brazil with a Concordia age of 141.9 ± 1.9 Ma and εHf(t) in a range of −5.5 to −7.9. Group 2 is alkaline, shows higher 87Sr/86Sr(i) of 0.7064–0.7088, evolved εNd(t) < −12 and older TDM ages of 1.7–1.9 Ga. The youngest zircon crystals from a dyke of this group yielded a Concordia age of 504.7 ± 6.9 Ma and εHf(t) of −18.9. Our results suggest that the regional NW-SE trending lineaments of the Brazilian coast were active in at least three episodes, serving as conduits for mafic magmatism of distinct sources and tectonic settings: First, in the Cambrian, during the Araçuaí-Ribeira Orogen collapse, when they served as conduits for both the post-tectonic G5 Supersuite (530–490 Ma; composed of granitic and mafic plutons) and the alkaline dykes of Group 2; second, during the lower Cretaceous breakup of West Gondwana, when they served as conduits for the tholeiitic dykes of Group 1, synchronous to syn-rift evolution of the Phanerozoic Brazilian coast basins; and third, during the Cenozoic, when they were reactivated as normal brittle faults.