Dextral Strike-slip Tectonics Research Articles

Cenozoic Strike-Slip Tectonics and Structural Controls of Porphyry Cu-Mo and Epithermal Deposits During Geodynamic Evolution of the Southernmost Lesser Caucasus, Tethyan Metallogenic Belt

Abstract The Zangezur-Ordubad mining district of the southernmost Lesser Caucasus is located in the central segment of the Tethyan metallogenic belt and consists of porphyry Cu-Mo and epithermal Au and base metal systems hosted by the composite Cenozoic Meghri-Ordubad pluton. Ore-hosting structures and magmatic intrusions are predominantly confined to a central N-S–oriented corridor 40 km long and 10 to 12 km wide, located between two regional NNW-oriented right-lateral faults, the Khustup-Giratagh and Salvard-Ordubad faults. The anatomy and kinematics of the main fault network are consistent with dextral strike-slip tectonics controlled by the NNW-oriented Khustup-Giratagh and Salvard-Ordubad faults. Dextral strike-slip tectonics was initiated during the Eocene, concomitantly with final subduction of the Neotethys, and controlled the emplacement of the Agarak, Hanqasar, Aygedzor, and Dastakert porphyry Cu-Mo and Tey-Lichkvaz and Terterasar epithermal Au and base metal deposits. The Eocene structures were repeatedly reactivated during subsequent Neogene evolution in transition to a postsubduction geodynamic setting. Ore-bearing structures at the Oligocene world-class Kadjaran porphyry Cu-Mo deposit were also controlled by dextral strike-slip tectonics, as well as porphyry mineralization and its epithermal overprint hosted by an early Miocene intrusion at Lichk. Eocene to early Miocene dextral strike-slip tectonics took place during NE- to NNE-oriented compression related to Paleogene Eurasia-Arabia convergence and subsequent Neogene postcollision evolution. Paleostress reconstruction indicates major reorganization of tectonic plate kinematics since the early Miocene, resulting in N-S– to NW-oriented compression. Early Miocene epithermal overprint at the Kadjaran porphyry deposit and left-lateral reactivation of faults and mineralized structures are linked to this late Neogene tectonic plate reorganization.

Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

AbstractThe Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptions to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ∼85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.).

Sedimentary provenance of the Late Paleozoic metamorphic basement, south-central Chile: Implications for the evolution of the western margin of Gondwana

ABSTRACTU-Pb detrital zircon age patterns are presented for nine samples of metapelites from the metamorphic basement of south-central Chile between 37° and 40°S, along with detrital zircon ages for a sample from the Piedra Santa metamorphic complex and a crystallization age of the Chachil plutonic complex, farther east in Argentina. Two distinct zircon age patterns are identified. One is a pattern with a dominant population of zircons at ca. 470 Ma (Ordovician), a widespread presence of Mesoproterozoic ages (1200 − 1000 Ma), and a Carboniferous maximum deposition age indicated by the youngest zircon population. The second pattern is drastically different, with a main population of Permian zircons, a 290–250 Ma maximum deposition age, and a minor contribution of Pennsylvanian age zircons. Our results, coupled with previously reported metamorphic ages, show that the patterns reflect the presence of different tectonic blocks separated by the Mocha-Villarrica Fault Zone at 39°S. Metapelites north of this structure have the Ordovician-dominated pattern characteristic of the Western Series of the Paleozoic basement, and those south of the fault show the Permian-dominated pattern observed in the younger high-pressure metapelites from the Bahía Mansa metamorphic complex. The Piedra Santa metamorphic complex also shows the Ordovician-dominated pattern and was intruded by the Chachil plutonic complex dated here at ca. 303 Ma. Therefore, the Piedra Santa complex is interpreted as coeval with the Chilean metamorphic basement. The present dislocation of blocks with different ages is attributed to continental-scale dextral strike-slip tectonics along the Huincul Fault Zone, Argentina, which extends to the west as the Mocha-Villarrica Fault Zone in Chile.

Present-day crustal deformation along the El Salvador Fault Zone from ZFESNet GPS network

This paper presents the results and conclusions obtained from new GPS data compiled along the El Salvador Fault Zone (ESFZ). We calculated a GPS-derived horizontal velocity field representing the present-day crustal deformation rates in the ESFZ based on the analysis of 30 GPS campaign stations of the ZFESNet network, measured over a 4.5year period from 2007 to 2012. The velocity field and subsequent strain rate analysis clearly indicate dextral strike–slip tectonics with extensional component throughout the ESFZ. Our results suggest that the boundary between the Salvadoran forearc and Caribbean blocks is a deformation zone which varies along the fault zone. We estimate that the movement between the two blocks is at least ~12mmyr−1. From west to east, this movement is variably distributed between faults or segments of the ESFZ. We propose a kinematic model with three main blocks; the Western, Central and Eastern blocks delimited by major faults. For the first time, we were able to provide a quantitative measure of the present-day horizontal geodetic slip rate of the main segments of ESFZ, ranging from ~2mmyr−1 in the east segment to ~8mmyr−1, in the west and central segments. This study contributes new kinematic and slip rate data that should be used to update and improve the seismic hazard assessments in northern Central America.

Volatile (H2O, CO2, Cl, S) budget of the Central American subduction zone

After more than a decade of multidisciplinary studies of the Central American subduction zone mainly in the framework of two large research programmes, the US MARGINS program and the German Collaborative Research Center SFB 574, we here review and interpret the data pertinent to quantify the cycling of mineral-bound volatiles (H2O, CO2, Cl, S) through this subduction system. For input-flux calculations, we divide the Middle America Trench into four segments differing in convergence rate and slab lithological profiles, use the latest evidence for mantle serpentinization of the Cocos slab approaching the trench, and for the first time explicitly include subduction erosion of forearc basement. Resulting input fluxes are 40–62 (53) Tg/Ma/m H2O, 7.8–11.4 (9.3) Tg/Ma/m CO2, 1.3–1.9 (1.6) Tg/Ma/m Cl, and 1.3–2.1 (1.6) Tg/Ma/m S (bracketed are mean values for entire trench length). Output by cold seeps on the forearc amounts to 0.625–1.25 Tg/Ma/m H2O partly derived from the slab sediments as determined by geochemical analyses of fluids and carbonates. The major volatile output occurs at the Central American volcanic arc that is divided into ten arc segments by dextral strike-slip tectonics. Based on volcanic edifice and widespread tephra volumes as well as calculated parental magma masses needed to form observed evolved compositions, we determine long-term (105 years) average magma and K2O fluxes for each of the ten segments as 32–242 (106) Tg/Ma/m magma and 0.28–2.91 (1.38) Tg/Ma/m K2O (bracketed are mean values for entire Central American volcanic arc length). Volatile/K2O concentration ratios derived from melt inclusion analyses and petrologic modelling then allow to calculate volatile fluxes as 1.02–14.3 (6.2) Tg/Ma/m H2O, 0.02–0.45 (0.17) Tg/Ma/m CO2, and 0.07–0.34 (0.22) Tg/Ma/m Cl. The same approach yields long-term sulfur fluxes of 0.12–1.08 (0.54) Tg/Ma/m while present-day open-vent SO2-flux monitoring yields 0.06–2.37 (0.83) Tg/Ma/m S. Input–output comparisons show that the arc water fluxes only account for up to 40 % of the input even if we include an “invisible” plutonic component constrained by crustal growth. With 20–30 % of the H2O input transferred into the deeper mantle as suggested by petrologic modeling, there remains a deficiency of, say, 30–40 % in the water budget. At least some of this water is transferred into two upper-plate regions of low seismic velocity and electrical resistivity whose sizes vary along arc: one region widely envelopes the melt ascent paths from slab top to arc and the other extends obliquely from the slab below the forearc to below the arc. Whether these reservoirs are transient or steady remains unknown.

Syn-eruptive/inter-eruptive relationships in Late Neoproterozoic volcano-sedimentary deposits of the Hamid area, North Eastern Desert, Egypt

Nonmarine volcano-sedimentary successions in the Late Neoproterozoic Hamid Basin were studied in order to examine the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocenter associated with active volcanism and both extensional and dextral strike-slip tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions and transport and depositional processes in which lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. Seven different accumulation units were identified in the syn-rift fill: (1) polymictic alluvial fan units, (2) fluvial braid plain units, (3) lacustrine units, (4) coherent volcanic bodies/shallow intrusion units, (5) pyroclastic fall units, (6) phreatomagmatic volcanic units, and (7) pyroclastic density current units. These deposits are organized into several stratal packages with contrasting geometries. Analysis of these units and the relationships between them provided insights into the evolution of the syn-rift sedimentary environments and permitted identification of different stages of effusive activity, explosive activity, and relative quiescence, determining syn-eruptive and inter-eruptive rock units. These units provide important clues to the distribution of, and temporal changes in, accommodation space, and hence the configuration and structural evolution of the Hamid Basin. Two accumulation stages were defined. The underfilled stage occurs when the material supplied to the depocenter during the eruptive events is not enough to level the existing topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods. The Hamid Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin–margin faults, which were either normal or strike-slip.

Constraining the timing of fault reactivation: Eocene coseismic slip along a Late Ordovician ductile shear zone (northern Victoria Land, Antarctica)

The reactivation of faults and shear zones is a widely documented process and represents a fundamental characteristic of deformation in the continental lithosphere. The Ross Sea in Antarctica mainly owes its present-day configuration to an extended period of early Paleozoic subduction-related crustal accretion and the subsequent Mesozoic–Cenozoic tectonics of the West Antarctic Rift System. It thus represents an ideal setting in which to study reactivation processes. This study uses the 40 Ar- 39 Ar laserprobe technique in conjunction with mesostructural, microtextural, and nanotextural analyses to unravel the style and timing of a newly discovered mylonite-pseudotachylyte association from the rift shoulder in northern Victoria Land of Antarctica. Pseudotachylyte-bearing fault rocks overprint a reverse ductile shear zone developed in a Cambrian granite, within an overall transpressional right-lateral kinematic scenario. In situ 40 Ar- 39 Ar analyses of biotite from the mylonite mainly yielded ages of 460–440 Ma, suggesting that ductile shearing occurred during the early Paleozoic orogenic cycle. In contrast, in situ data on the pseudotachylyte matrix yielded scattered and much younger ages of ca. 120–66 Ma. Younger ages of ca. 50 Ma were derived from step-heating experiments on matrix specimens obtained through microtexturally controlled microsampling. Taking into account the possible effects of analytical artifacts due to sample irradiation and of Ar loss due to the ultrafine grain of the matrix, coseismic faulting is assigned to the middle Eocene. Results indicate a minimum 390 m.y. period of tectonic quiescence and prove that brittle reactivation occurred in response to a totally different stress regime. Regionally, the study confirms a post–early Eocene age for the activation and propagation of intraplate dextral strike-slip tectonics in Victoria Land.

Long-lived transpression in the Archean Bird River greenstone belt, western Superior Province, Southeastern Manitoba

The Archean Bird River greenstone belt (BRGB) is located on the southwestern edge of the Superior Province between the 3.2 Ga old Winnipeg River subprovince to the south and the metasedimentary belt of the English River subprovince (ERSP) to the north. This position between two major subprovinces makes the BRGB a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary. New structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the North Caribou superterrane. The BRGB underwent 3 main deformation phases. The D1 event took place ca. 2698 Ma and displays a north-side-up shearing. The D2 event, occurring at ca. 2684 Ma in a transpressive context, presents a complex structural pattern mixing vertical tectonics in the BRGB and strike-slip tectonics along the boundaries of the greenstone belt with other subprovinces. Between the BRGB and the ERSP, the 2832–2858 Ma old Maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike-slip tectonics along his northern boundary with the ERSP and vertical south-side-up motion in the BRGB. The BRGB can be considered as a pop-up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones. The southern boundary with the Winnipeg River subprovince is represented by a sinistral south-side-up shear zone. The same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane. At ca. 2640 Ma, the D3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare-elements pegmatitic plutons in a still hot (400–500 °C) country rock. The geodynamical and mechanical significance of the partitioning between pure strike-slip tectonics in the English River subprovince and vertical motion in the BRGB can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain. The structural framework of the BRGB is the result of strong interactions between hot and weak domains, coeval with widespread plutonism, and a rigid older domain (Maskwa batholith) during the D2 transpressive event.

Late Cretaceous structural control and Alpine overprint of the high-sulfidation Cu–Au epithermal Chelopech deposit, Srednogorie belt, Bulgaria

The Chelopech epithermal high-sulfidation deposit is located in the Panagyurishte ore district in Bulgaria, which is defined by a NNW alignment of Upper Cretaceous porphyry–Cu and Cu–Au epithermal deposits, and forms part of the Eastern European Banat–Srednogorie belt. Detailed structural mapping and drillcore descriptions have been used to define the structural evolution of the Chelopech deposit from the Late Cretaceous to the present. The Chelopech deposit is characterized by three fault populations including ∼N55, ∼N110, and ∼N155-trending faults, which are also recognized in the entire Panagyurishte district. Mapping and 3-D modeling show that hydrothermal alteration and orebody geometry at Chelopech are controlled by the ∼N55-trending and ∼N110-trending faults. Moreover, the ∼N155-trending faults are parallel to the regional ore deposit alignment of the Panagyurishte ore district. It is concluded that the three fault populations are early features and Late Cretaceous in age, and that they were active during high-sulfidation ore formation at Chelopech. However, the relative fault chronology cannot be deduced anymore due to Late Cretaceous and Tertiary tectonic overprint. Structurally controlled ore formation was followed by Senonian sandstone, limestone, and flysch deposition. The entire Late Cretaceous magmatic and sedimentary rock succession underwent folding, which produced WNW-oriented folds throughout the Panagyurishte district. A subsequent tectonic stage resulted in overthrusting of older rock units along ∼NE-trending reverse faults on the Upper Cretaceous magmatic and sedimentary host rocks of the high-sulfidation epithermal deposit at Chelopech. The three fault populations contemporaneous with ore formation, i.e., the ∼N55-, ∼N110- and ∼N155-trending faults, were reactivated as thrusts or reverse faults, dextral strike–slip faults, and transfer faults, respectively, during this event. Previous studies indicate that the present-day setting is characterized by dextral transtensional strike–slip tectonics. The ∼NE-trending overthrust affecting the Chelopech deposit and the reactivation of the ore-controlling faults are compatible with dextral strike–slip tectonics, but indicate local transpression, thus revealing that the Chelopech deposit might be sited at a transpressive offset within a generally transtensional strike–slip system. The early WNW-trending folds require a roughly NNE–SSW shortening, which is incompatible with the present-day dextral strike–slip tectonic setting and the ∼NE-trending thrust formed during the tectonic overprint of the Chelopech deposit. This reveals a rotation of the principal stress axes after Late Cretaceous high-sulfidation ore formation and post-ore deposition of sedimentary rocks. The nature of the sedimentary rocks interlayered and immediately covering the Upper Cretaceous magmatic rocks hosting the Chelopech deposit indicates sedimentation and associated volcanism in an extensional setting immediately before ore formation. It is concluded that the Chelopech deposit was formed when the tectonic setting changed from extensional during Late Cretaceous basin sedimentation and magmatism, to compressional producing WNW-trending folds under a roughly NNE–SSW compression, possibly in a sinistral strike–slip system. Thus, like other world-class, high-sulfidation epithermal deposits, the Chelopech deposit was formed at the end of an extensional period or during a transient period of stress relaxation, which are particularly favorable tectonic settings for the formation of high-sulfidation epithermal deposits. The exceptional preservation of the Upper Cretaceous Chelopech epithermal deposit is explained by the combined deposition of a thick Senonian sedimentary sequence on top of the Upper Cretaceous magmatic host rocks of the deposit, and the later overthrust of older rock units on top of the deposit. Our study at Chelopech supports previous studies stating that post-ore basin sedimentation and tectonic processes provide the favorable environment to preserve old epithermal deposits from erosion. The tectonic evolution of the Chelopech deposit is similar to that of the entire Panagyurishte ore district. This coherence of the magmatic, hydrothermal, and tectonic events from north to south suggests that the ore deposits of the entire Panagyurishte ore district were formed in a similar tectonic environment.

Late Palaeozoic formation and development of the St Marys Basin, mainland Nova Scotia, Canada: a prolonged record of intracontinental strike-slip deformation during the assembly of Pangaea

Abstract The Late Palaeozoic evolution of the St Marys Basin, mainland Nova Scotia, preserves evidence of protracted dextral shear along an intracontinental fault zone during collisional orogenesis and the assembly of Pangaea. The St Marys Basin formed within the E-W-trending Minas Fault Zone (MFZ) along the boundary between the Avalon and Meguma terranes and contains latest Devonian-Tournaisian continental clastic rocks that are 3000–4000 m in thickness. The origin and evolution of the basin is attributed to either discrete or progressive dextral strike-slip tectonics along the MFZ between the Late Devonian and Late Carboniferous. Evidence for the Late Devonian origin of the basin is recorded along its southern flank by the fabrics of the deformed c. 370 Ma granites, the overall sedimentary facies distribution, and some syndepositional features within the clastic rocks. The most intense deformation within the basin is concentrated in a relatively narrow ENE-trending zone, in which predominantly fine-grained clastic rocks are deformed into periclinal folds and related reverse faults. The orientation of this zone relative to the MFZ is consistent with dextral shear. At least some of this deformation occurred after the deposition of the overlying Visean Windsor Group. The style of deformation along the present northern margin of the basin (the Chedabucto Fault) is also consistent with regional dextral shear. The St Marys Basin is an example of basin development and evolution adjacent to an intracontinental fault zone associated with oblique convergence during orogenesis. Its evolution provides constraints on the potential relationship between the termination of the mid-Palaeozoic Acadian orogeny, subsequent basin development, and the ongoing interactions between the Avalon and Meguma terranes, and between Laurentia and Gondwana during the assembly of Pangaea. More generally, because the relationship between fabric development and motion along intracontinental strike-slip faults in continental zones is difficult to interpret, the sedimentology and structural geology in basins developed along these fault zones may preserve a less ambiguous record of the main tectonic events.

Late Triassic–early Jurassic continental extension in southwestern Gondwana: tectonic segmentation and pre-break-up rifting

From the earliest Jurassic to the Cenozoic, the Neuquén Basin (central Argentina and Chile) evolved as an intra-arc and transarc–retroarc depression through the gradual development of the Andean magmatic arc. However, the region adjacent to the proto-Pacific margin of Gondwana between 30 and 40°S was subject to pre-Andean continental extension that began in the Late Triassic and lasted about 30 million years until the Early Jurassic. This extension resulted in the generation of a series of rifts oriented parallel to the margins of the extended area and characterized by continental volcaniclastic and pyroclastic deposits associated with lava flows and bimodal plutonic intrusions. The inception of the Neuquén Basin as a single depocenter occurred in the Early Pliensbachian, when post-rift thermal subsidence led to a general marine transgression. The T–J extension is closely linked to structures created by previous tectonothermal episodes such as the development of a Carboniferous–Permian orogenic belt (330–280 Ma), and to events along the proto-Pacific margin of Gondwana. Lithospheric thickening related to Late Paleozoic convergence caused strong gravitational instability in the orogenic belt between 30 and 40°S. Subsequent cessation of subduction, coeval with establishment of dextral strike-slip tectonics parallel to the continental margin, caused detachment of the subducting slab and generation of an asthenospheric window. Anomalous heating of the upper mantle resulted in bimodal magmatism, uplift, thermal weakening, and gravitational collapse of the upper crust. South of this extended area, proto-Pacific subduction was active during the same Late Triassic–Early Jurassic interval. Thus, the contrasting tectonic behaviour of the Gondwana margin north and south of 40°S suggests significant pre-Andean tectonic segmentation that coincides with the southern boundary of the area extended in the Late Triassic–Early Jurassic. Previous intepretations assumed a common massive extension in the Andean and Patagonian regions of southern South America. Our results demonstrate that T–J extensional rifting and magmatism between 30 and 40°S were the result of mechanical interaction between different lithospheric plates at the pre-Andean (proto-Pacific) continental margin. On the other hand, the mechanical and thermal processes that affected much of Patagonia during the Middle and Late Jurassic were a response to the tectonic and magmatic processes that caused the initial break-up of Gondwana with the opening of the Weddell Sea after 180 Ma.

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria

Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. 40 Ar/ 39 Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.

The Pan-African Toro Complex (northern Nigeria): magmatic interactions and structures in a bimodal intrusion

AbstractThe Toro Complex is one of the Pan-African Older Granites of Nigeria, first described as a reversely zoned pluton made of a central dioritic mass surrounded by a broad granitic rim. It has been thoroughly reinvestigated both from the petrographic and structural points of view, with the help of systematic anisotropy of magnetic susceptibility (AMS) measurements. The granite main body is a hornblende–biotite porphyritic monzogranite characterized by an early submagmatic fabric displaying a concentric pattern of foliations and west plunging lineations (stage 1). This fabric is overprinted by a later one due to solid-state strain along north-south subvertical dextral shear zones (stage 2). In the vicinity of the diorite, an evengrained granite displays magmatic structures that are contemporaneous with this strike-slip event. The diorite–granite contact is a complex zone where field, petrographic and geochemical data enable recognition of the effects of mixing and mingling between a mafic and a felsic magma. Tonalites cropping out within this contact zone are interpreted as hybrid rocks. The reverse zonation of the diorite itself is also the result of some hybridization process. Magmatic interactions mainly resulted from in situ infiltration of granitic liquid into the dioritic mass. The detailed history of this bimodal intrusion began with the emplacement of the granitic magma acquiring a first stage fabric. Before full crystallization of the granitic core, intrusion of the dioritic magma permitted reheating of the granitic magma that then crystallized with specific structural characters. The second stage structures, whether characterized by magmatic fabric near the diorite or by solid-state strain features in north–south shear zones elsewhere in the granite, are related to late Pan-African dextral strike-slip tectonics in the basement of northern Nigeria. The bimodal Toro Complex is therefore considered as a late Pan-African syntectonic pluton.