Showing papers on "Terrane published in 2018"

Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits

Abstract: With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ∼10°–25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ∼70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition. Anticlinal or antiformal fold hinges, particularly those of ‘locked-up’ folds with ∼30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits. In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granite-greenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits.

Reconstructing in space and time the closure of the middle and western segments of the Bangong–Nujiang Tethyan Ocean in the Tibetan Plateau

Abstract: When and how the Bangong–Nujiang Tethyan Ocean closed is a highly controversial subject In this paper, we present a detailed study and review of the Cretaceous ophiolites, ocean islands, and flysch deposits in the middle and western segments of the Bangong–Nujiang suture zone (BNSZ), and the Cretaceous volcanic rocks, late Mesozoic sediments, and unconformities within the BNSZ and surrounding areas Our aim was to reconstruct the spatial–temporal patterns of the closing of the middle and western segments of the Bangong–Nujiang Tethyan Ocean Our conclusion is that the closure of the ocean started during the Late Jurassic and was mainly complete by the end of the Early Cretaceous The closure of the ocean involved both “longitudinal diachronous closure” from north to south and “transverse diachronous closure” from east to west The spatial–temporal patterns of the closure process can be summarized as follows: the development of the Bangong–Nujiang Tethyan oceanic lithosphere and its subduction started before the Late Jurassic; after the Late Jurassic, the ocean began to close because of the compressional regime surrounding the BNSZ; along the northern margin of the Bangong–Nujiang Tethyan Ocean, collisions involving the arcs, back-arc basins, and marginal basins of a multi-arc basin system first took place during the Late Jurassic–early Early Cretaceous, resulting in regional uplift and the regional unconformity along the northern margin of the ocean and in the Southern Qiangtang Terrane on the northern side of the ocean However, the closure of the Bangong–Nujiang Tethyan Ocean cannot be attributed to these arc–arc and arc–continent collisions, because subduction and the development of the Bangong–Nujiang Tethyan oceanic lithosphere continued until the late Early Cretaceous The gradual closure of the middle and western segments of Bangong–Nujiang Tethyan Ocean was diachronous from east to west, starting in the east in the middle Early Cretaceous, and being mainly complete by the end of the Early Cretaceous The BNSZ and its surrounding areas underwent orogenic uplift during the Late Cretaceous

Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

Abstract: During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh–Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U–Pb ages indicate the emplacement of ~3,600–3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.

Hainan mantle plume produced late Cenozoic basaltic rocks in Thailand, Southeast Asia.

Abstract: Intraplate volcanism initiated shortly after the cessation of Cenozoic seafloor spreading in the South China Sea (SCS) region, but the full extent of its influence on the Indochina block has not been well constrained. Here we present major and trace element data and Sr-Nd-Pb-Hf isotope ratios of late Cenozoic basaltic lavas from the Khorat plateau and some volcanic centers in the Paleozoic Sukhothai arc terrane in Thailand. These volcanic rocks are mainly trachybasalts and basaltic trachyandesites. Trace element patterns and Sr-Nd-Pb-Hf isotopic compositions show that these alkaline volcanic lavas exhibit oceanic island basalt (OIB)-like characteristics with enrichments in both large-ion lithophile elements (LILE) and high field strength elements (HFSEs). Their mantle source is a mixture between a depleted Indian MORB-type mantle and an enriched mantle type 2 (EMII). We suggest that the post-spreading intraplate volcanism in the SCS region was induced by a Hainan mantle plume which spread westwards to the Paleozoic Sukhothai arc terrane.

Appalachian-style multi-terrane Wilson cycle model for the assembly of South China: REPLY

Abstract: Lin et al. (2018) propose a model of Proterozoic-Mesozoic evolution of the South China block (SCB) involving West and East Cathaysia separated by the Northwest Fujian fault. They assume a northeastward translation of East Cathaysia after an Early Paleozoic orogeny due to the collision of West Cathaysia with a “proposed terrane”. This model ignores most of the available, robust field data.

The Dom Feliciano Belt in Southern Brazil and Uruguay

Abstract: The Dom Feliciano Belt is an orogenic association that extends from southern Brazil to Uruguay parallel to the Atlantic coastline for over 1100 km. It was assembled in the Neoproterozoic, during the Brasiliano orogenic cycle, and is the result of interaction between the Rio de la Plata, Congo and Kalahari cratons, together with several microplates, juxtaposed along major shear zones. Along its extension, the Dom Feliciano Belt is exposed in three sectors: in the Brazilian states of Santa Catarina and Rio Grande do Sul, and in Uruguay. The blocks that acted as direct forelands to the belt in South America are smaller fragments to the main cratons: Luis Alves and Nico Perez. Three main lithotectonic domains are recognized in the belt, from east to west: a granitic batholith, a metasedimentary sequence and an association of foreland basins. Basement inliers are common, and evidence intense reworking and magmatism during the Neoproterozoic. Cryogenian to Ediacaran granitogenesis is widespread and voluminous, and usually displays an evolutionary tendency from medium- to high-K calc-alkaline, finishing with alkaline magmatism. The early evolution of the Dom Feliciano Belt is recorded in the Sao Gabriel Terrane, in which convergent tectonics is associated with intense juvenile magmatism, ophiolite complexes and accretion between 870 and 680 Ma. This is followed by two more deformational phases, identified in all three sectors. A convergent phase is associated with the deformation of the metavolcano-sedimentary complexes, shear zone nucleation and granitic magmatism associated with high-grade collisional metamorphism. This stage is constrained between c. 650–620 Ma in Santa Catarina and Rio Grande do Sul, and between c. 630–600 Ma in Uruguay. The last stage marks a transition to strike-slip deformation, with common shear zone reactivation associated with refolding in the metamorphic associations and widespread post-collisional granitic and volcanic magmatism. This phase is predominant from 610 to 550 Ma. The opening of the foreland basins was initiated during this period, probably associated with transtension along the main structures. Late-stage deformation and magmatism is common until 550–540 Ma. Abundant geochronological data have been added to the Dom Feliciano Belt in the last decades, leading to more precise time constraints for most of the geologic processes in the orogen. Details of its tectonic model, however, are still matters of debate, in terms of both the setting of its main units and its position into the assembly of southwestern Gondwana.

One or Two Early Cretaceous Arc Systems in the Lhasa Terrane, Southern Tibet

Abstract: Spatial and temporal variations of arc-related magmatism are key to determining the subduction polarity of incompletely preserved arc systems. Petrographic, geochronological, geochemical, and isotope data of Early Cretaceous volcanic rocks from the northern Lhasa Terrane around Yanhu indicate south dipping subduction of the Bangong Tethys. Two distinct calc-alkaline magmatic successions are recognized: older medium-K basalts and Mg-rich andesites (131–116 Ma), and younger high-K basalts and trachyandesites (116–110 Ma). The medium-K basalts exhibit a typical arc signature, whereas the medium-K andesites show higher MgO contents relative to arc andesites. The medium-K series are interpreted as partial melting of a metasomatized mantle wedge source at lower pressure and greater water activity in generating the Mg-rich andesites. The high-K series are characterized by enrichments in highly incompatible elements and are considered as low-degree partial melting of asthenosphere mantle source that was previously metasomatized. All samples show arc-related signatures, which indicate the development of what we term the Baingoin-Yanhu arc in the northern Lhasa Terrane. This observation, in combination with the distribution of Early Cretaceous arc magmatism across the Lhasa Terrane, which prior to subsequent deformation had a width of at least 600 km, requires the existence of two arc systems flanking the Lhasa Terrane and related to opposed oceanic plate subduction: north dipping subduction of the Neo-Tethys and south dipping subduction of the Bangong Tethys. Compositional change from medium-K to high-K calc-alkaline volcanism around Yanhu records changing mantle geodynamics, which we infer to reflect rollback and breakoff of the south dipping Bangong Tethyan slab.

The role of pre-existing structures during rifting, continental breakup and transform system development, offshore West Greenland

Abstract: Continental breakup between Greenland and North America produced the small oceanic basins of the Labrador Sea and Baffin Bay, which are connected via the Davis Strait, a region mostly comprised of continental crust. This study contributes to the debate regarding the role of pre-existing structures on rift development in this region using seismic reflection data from the Davis Strait data to produce a series of seismic surfaces, isochrons and a new offshore fault map from which three normal fault sets were identified as (i) NE-SW, (ii) NNW-SSE and (iii) NW-SE. These results were then integrated with plate reconstructions and onshore structural data allowing us to build a two-stage conceptual model for the offshore fault evolution in which basin formation was primarily controlled by rejuvenation of various types of pre-existing structures. During the first phase of rifting between at least Chron 27 (ca. 62 Ma; Palaeocene), but potentially earlier, and Chron 24 (ca. 54 Ma; Eocene) faulting was primarily controlled by pre-existing structures with oblique normal reactivation of both the NE-SW and NW-SE structural sets in addition to possible normal reactivation of the NNW-SSE structural set. In the second rifting stage between Chron 24 (ca. 54 Ma; Eocene) and Chron 13 (ca. 35 Ma; Oligocene), the sinistral Ungava transform fault system developed due to the lateral offset between the Labrador Sea and Baffin Bay. This lateral offset was established in the first rift stage possibly due to the presence of the Nagssugtoqidian and Torngat terranes being less susceptible to rift propagation. Without the influence of pre-existing structures the manifestation of deformation cannot be easily explained during either of the rifting phases. Although basement control diminished into the post-rift, the syn-rift basins from both rift stages continued to influence the location of sedimentation possibly due to differential compaction effects. Variable lithospheric strength through the rifting cycle may provide an explanation for the observed diminishing role of basement structures through time.

Suturing Gondwana in the Cambrian: The Orogenic Events of the Final Amalgamation

Abstract: Gondwana was consolidated in the late Cambrian after 180 myr of tectonic convergence among Neoproterozoic paleocontinents and smaller fragments. We present a compilation of 55 orogens that record its final amalgamation. Collisional events are registered by the metamorphic peak assemblages and contractional deformational structures. Two main periods of orogenic activity are recognized. The first at c. 670–575 Ma includes few orogens (c. 15) but over larger areas. During this stage, the Saharan, West African, Sao Francisco-Congo and Paranapanema paleocontinents, along with the Arabian Nubian shield that consisted of juvenile Tonian terranes and some East African Orogen microcontinents, were accreted to form the proto-Gondwana core. The second stage, at 575–480 Ma, incorporated more orogens, c. 40, that sutured the Amazonia, Rio de La Plata, Kalahari, Dhawar, East Antarctica and Australian paleocontinents. The collisional orogen pattern throughout both western and eastern Gondwana is similar, indicating that although Gondwana was built up by the convergence of distinct paleocontinents, their approximation might be orchestrated by global geodynamics. In SW Gondwana, the opening of c. 610–570 Ma basins, some with oceanic crust, coincided with the suturing of the proto-Gondwana core. They were rapidly formed but even more quickly inverted during the second and last 575–480 Ma collisional stage, represented by the major east–west Kuunga and north–south South Atlantic belts. We propose that the 570–500 Ma collision of the Damara-Lufilian-Zambesi belt was coeval with the Cuchilla Dionisio-Saldania-Gariep-Dom Feliciano-Kaoko-Ribeira-Cabo Frio orogens. This South Atlantic orogenic system consists of the main SW Gondwana suture, reactivated 350 myr later to form the South Atlantic Ocean. The suture is preserved by old orogenic high-pressure and oceanic-derived rocks on the actual conjugate continental margins.

Late Triassic sedimentary records in the northern Tethyan Himalaya: Tectonic link with Greater India

Abstract: The Upper Triassic flysch sediments (Nieru Formation and Langjiexue Group) exposed in the Eastern Tethyan Himalayan Sequence are crucial for unraveling the controversial paleogeography and paleotectonics of the Himalayan orogen. This work reports new detrital zircon U-Pb ages and whole-rock geochemical data for clastic rocks from flysch strata in the Shannan area. The mineral modal composition data suggest that these units were mainly sourced from recycled orogen provenances. The chemical compositions of the sandstones in the strata are similar to the chemical composition of upper continental crust. These rocks have relatively low Chemical Index of Alteration values (with an average of 62) and Index of Compositional Variability values (0.69), indicating that they experienced weak weathering and were mainly derived from a mature source. The geochemical compositions of the Upper Triassic strata are similar to those of graywackes from continental island arcs and are indicative of an acidic-intermediate igneous source. Furthermore, hornblende and feldspar experienced decomposition in the provenance, and the sediment became enriched in zircon and monazite during sediment transport. The detrital zircons in the strata feature two main age peaks at 225–275 Ma and 500–600 Ma, nearly continuous Paleoproterozoic to Neoproterozoic ages, and a broad inconspicuous cluster in the Tonian–Stenian (800–1200 Ma). The detrital zircons from the Upper Triassic sandstones in the study area lack peaks at 300–325 Ma (characteristic of the Lhasa block) and 1150–1200 Ma (characteristic of the Lhasa and West Australia blocks). Therefore, neither the Lhasa block nor the West Australia blocks likely acted as the main provenance of the Upper Triassic strata. Newly discovered Permian–Triassic basalt and mafic dikes in the Himalayas could have provided the 225–275 Ma detrital zircons. Therefore, Indian and Himalayan units were the main provenances of the flysch strata. The Tethyan Himalaya was part of the northern passive margin and was not an exotic terrane separated from India during the Permian to Early Cretaceous. This evidence suggests that the Neo-Tethyan ocean opened prior to the Late Triassic and that the Upper Triassic deposits were derived from continental crustal fragments adjacent to the northern passive continental margin of Greater India.

Multi-terrane structure controls the contrasting lithospheric evolution beneath the western and central-eastern Tibetan plateau.

Abstract: The Tibetan plateau is manifested by contrasting along-strike lithospheric structures, but its formation mechanism and the relationship with the heterogeneous multi-terrane configuration is a challenging problem. Here we conduct systematic numerical modeling to explore the roles of width, density, and rheological properties of the multiple terranes in the lithospheric evolution of the Tibetan plateau, which reveals two distinct collision modes. In Mode-I, the lithospheric mantles of both the strong and weak terranes in the Tibetan plate are completely detached, followed by the underthrusting of Indian lithosphere beneath the whole plateau. Alternatively, Mode-II is characterized by full detachment of the weak terranes, but (partial) residue of the strong terranes during collision. These two contrasting modes, broadly consistent with the lithospheric structures of western and central–eastern Tibetan plateau, respectively, are strongly dependent on the along-strike variation of the width of the strong Lhasa–Qiangtang terranes. The Tibetan plateau is manifested by contrasting along-strike lithospheric structures, but the correlation with multi-terrane configuration remains challenging. Here, the authors show the crucial roles of the original geometric shape of accreted terranes in regulating the lithospheric evolution of Tibetan plateau.

Did the circum-Rodinia subduction trigger the Neoproterozoic rifting along the Congo–Kalahari Craton margin?

Abstract: Early Neoproterozoic metaigneous rocks occur in the central part of the Kaoko–Dom Feliciano–Gariep orogenic system along the coasts of the southern Atlantic Ocean. In the Coastal Terrane (Kaoko Belt, Namibia), the bimodal character of the ca. 820–785 Ma magmatic suite and associated sedimentation sourced in the neighbouring pre-Neoproterozoic crust are taken as evidence that the Coastal Terrane formed as the shallow part of a developing back arc/rift. The arc-like chemistry of the bimodal magmas is interpreted as inherited from crustal and/or lithospheric mantle sources that have retained geochemical signature acquired during an older (Mesoproterozoic) subduction-related episode. In contrast, the mantle contribution was small in ca. 800–770 Ma plutonic suites in the Punta del Este Terrane (Dom Feliciano Belt, Uruguay) and in southern Brazil; still, the arc-like geochemistry of the prevalent felsic rocks seems inherited from their crustal sources. The within-plate geochemistry of a subsequent, ca. 740–710 Ma syn-sedimentary volcanism reflects the ongoing crustal stretching and sedimentation on top of the Congo and Kalahari cratons. The Punta del Este–Coastal Terrane is interpreted as an axial part of a Neoproterozoic “Adamastor Rift”. Its opening started in a back-arc position of a long-lasting subduction system at the edge of a continent that fragmented into the Nico Perez–Luis Alves Terrane and the Congo and Kalahari cratons. The continent had to be facing an open ocean and consequently could not be located in the interior of the Rodinia. Nevertheless, the early opening of the Adamastor Rift coincided with the lifetime of the circum-Rodinia subduction system.