Granite Emplacement Research Articles

Curie Point Depth of the Iberian Peninsula and Surrounding Margins. A Thermal and Tectonic Perspective of its Evolution

AbstractIn this work the thermal structure of the Iberian Peninsula is derived from magnetic data by calculating the bottom of the magnetization, assumed to be the Curie‐point depth (CPD) isotherm, which accounts for the depth at which magnetite becomes paramagnetic (580°C). Comparison of the CPD with crustal thickness maps along with a heat flow map derived from the CPD provides new insights on the lithospheric thermal regime. Within Iberia, the CPD isotherm has thickness in the range of 17 to 29 km. This isotherm is shallow (<18 km) offshore, where the lithosphere is thinner. In continental Iberia, the NW Variscan domain presents a magnetic response that is most probably linked to thickening and later extension processes during the late Variscan Orogeny, which resulted in widespread crustal melting and emplacement of granites (in the Central Iberian Arc). The signature of the CPD at the Gibraltar Arc reveals a geometry consistent with the slab roll‐back geodynamic model that shaped the western Mediterranean. In offshore areas, a broad extension of magnetized upper mantle is found. Serpentinization of the upper mantle, probably triggered in an extensional context, is proposed to account for the magnetic signal. The Atlantic margin presents up to 8 km of serpentinites, which, according to the identification of exhumed mantle, correlates with a hyperextended margin. The Mediterranean also presents generalized serpentinization up to 6 km in the Algerian Basin. Furthermore, a heat flow map and a Moho temperature map derived from the CPD are presented.

A 20 m.y. long-lived successive mineralization in the giant Dahutang W–Cu–Mo deposit, South China

The connection between prolonged granitic magmatism and the formation of giant tungsten (W) polymetallic deposits has long been disputed. In this study, we present 6 mica Ar–Ar plateau ages and 22 molybdenite Re–Os model ages data on the newly discovered giant Dahutang W–Cu–Mo deposit in South China, which is one of the largest W deposits in the world. New and published zircon U–Pb, mica Ar–Ar, and molybdenite Re–Os age data reveal that the Mesozoic Dahutang magmatism and mineralization occurred in two major periods: (1) the Late Jurassic (ca. 153–147 Ma), forming the hydrothermal breccia, large wolframite-bearing quartz vein, and scheelite-dominated disseminated/veinlet type orebodies, which is mainly associated with the emplacement of porphyritic biotite granite; (2) the Late Jurassic to Early Cretaceous (ca. 146–130 Ma), forming the Cu–Mo–W ± Sn mineralization overprinting the Late Jurassic W–Mo ± Cu orebodies, which is mainly related to the successively emplacement of the Early Cretaceous granites. We suggest that continuous accumulation of mineralization for a long period of time (151–130 Ma) have contributed to the formation of the giant Dahutang deposit.

A combined geochronological approach to investigating long lived granite magmatism, the Shap granite, UK

With the advent of more precise dating methods, it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods. Here we attempt to reconcile a number of dating methods from the c. 5 km2 Caledonian Shap granite, Northern England. The results reveal a more complex and protracted evolution than indicated by application of any single dating method. Zircon U-Pb dates give a weighted mean age of 415.6 ± 1.4 (2σ) Ma. A mafic enclave, dated at 412 ± 2 (2σ) Ma (revised Rb-Sr feldspar age from Davidson et al., 2005), contains resorbed K-feldspar and zircon crystals scavenged from the host crystal mush. These ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 Ma or later during Acadian deformation. Previously reported Re-Os ages on molybdenites associated with magmatic fluids, have given ages of 405.2 ± 1.8 (2σ) Ma (Selby et al., 2008) and confirm the overlap of at least some magmatic activity with Acadian deformation. A similar emplacement age is supported by Rb-Sr whole-rock-mineral and biotite K-Ar dates when adjusted for revised decay constants (402 ± 3 Ma and 401 ± 7 Ma, respectively, Wadge et al., 1978). The lower closure temperatures of these systems relative to the U-Pb system in zircon means that they are more likely to record the timing of final granite emplacement. These data suggest that most zircons grew before final granite emplacement, by about 10 Ma on average. We suggest that the majority of zircon crystals record pre-emplacement magmatic activity within a deeper part of the system. Mafic enclaves and their scavenged cargo of crystals record the assembly of a mid-crustal batholith where crystals remained at least locally mobile at 412 Ma. Gravity data support the existence of an extensive, 1500 km2 intrusive body, originally at about 15 km depth beneath Shap. This batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss, but episodic influxes of silicic magma between c. 412 and 405 Ma are thought to have enabled periods of rejuvenation. These influxes are recorded by complex compositional zoning patterns within K-feldspar megacrysts. The Shap granite itself is likely to represent a rejuvenated crystal slurry, emplaced as a cylindrical cupola above the main magma body during Acadian transpression. This study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages.

Late Cryogenian arc-related volcaniclastic metasediment successions at Wadi Hammuda, Central Eastern Desert, Egypt: geology and geochemistry

Wadi Hammuda is dominated by a variety of low grade regionally metamorphosed volcaniclastic metasediments pertaining to two different geotectonic settings and intruded by arc and late collision granitic rocks. Thus, the volcaniclastic metasediments which form extensive outcrops are considered as a member of island arc assemblages. This paper deals with the petrography, geochemistry, and tectonic setting of the island arc volcaniclastic metasediment rock units. The volcaniclastic metasediments consist of interbedded metagreywackes, metasiltstones, metamudstones, and schists as well as metapyroclastics. They are well foliated, crenulated and tightly folded, metamorphosed, and intruded by granitic rocks. Geochemical data support the petrographic classification and reveal that these volcaniclastic metasediments are generally low-K, essentially tholeiitic in character, with the exception of some metasediments and metapyroclastics which exhibits calc-alkaline and tholeiitic affinities and represent the first stage of island arc volcanism. The overthrusted oceanic lithosphere blocks with fragments of the fore arc and/or back-arc marginal basins volcaniclastic metasediments were incorporated among the island arc volcanics which supported by tectonically relationship between the different rock units in the study area. Contemporaneous with this deformation event, Wadi Hammuda was subjected to low grade regional metamorphism and the rocks document an early phase of shearing and/or foliation. Occasionally minor folds were developed particularly in the metasediments and schists. The subsequent emplacement of the syn-tectonic granites (tonalites and granodiorites) resulted in minor local thrusts. During the regional thrusting event which preceded the emplacement of the late-tectonic granites (alkali feldspar granites) and affected the whole region, low grade successions cover the study area similar to the Meatiq volcaniclastic metasediments.

Hydrothermal evolution and isotope studies of the Baghu intrusion-related gold deposit, Semnan province, north-central Iran

The middle Eocene intrusion-related Baghu gold deposit is located in the Torud-Chahshirin segment of the Neotethyan magmatic arc of north-central Iran, Semnan province, Iran. The wallrocks at the deposit include a volcano-sedimentary sequence of calc-alkaline andesitic to dacitic lavas, marls, tuffs, and sandstones, which are intruded by epizonal granodioritic stocks and mafic dikes. Zircon U-Pb secondary ion mass spectrometry dating of a number of the granodiorite and micro-granite bodies yields an age of ca. 43.4 ± 1.3 Ma. Mineralization is in the form of subparallel sheeted quartz veins that are up to ∼1 km long within extensional faults. The ore-forming process included early potassic alteration with quartz + adularia + biotite + pyrite + chalcopyrite + native gold veins (average 0.25 g/t), and late phyllic alteration with quartz + sericite + pyrite + chalcopyrite + bornite + sphalerite + native gold veins (average 1.5 g/t). Supergene kaolinitization is also extensive in the Baghu area and has locally replaced up to 50 percent of the hypogene sulfide minerals. Turquoise is a diagnostic supergene phase, and locally, the iron oxy-hydroxides replacing the sulfides contain up to 57 g/t Au. The current hypogene and supergene resource estimate for the Baghu Au-Cu deposit is approximately 3.5 million tonnes averaging 1.17 g/t Au.Fluid inclusions in quartz from phyllic stage vein samples include monophase aqueous (I), monophase gas-rich (II), aqueous liquid + vapor (III), mixed aqueous-carbonic (IV), and multi-solid (V) inclusion types. The average homogenization temperatures and salinities of fluid inclusion assemblages from type III and IV inclusions range from 180 to 382 °C, with a mode at about 330 °C, and 4.2–5.4 wt% NaCl equiv., respectively. The type IV inclusions showed clathrate melting temperatures between +3.1 and +8.5 °C, indicating salinities of 2.8–11.5 wt% NaCl equiv. The type V inclusions also show a Th range from 310 to 478 °C (average 440 °C) corresponding to high salinities of 33–45 wt% NaCl equiv. The trapping pressure of the fluid inclusions was estimated to be 94–132 MPa corresponding to depths of 2.5–3.9 km during late stage of mineralization. Sericite from the stage II veins yields an 40Ar/39Ar age of ca. 44 Ma, suggesting that the mineralization at Baghu overlap granite emplacement. The δ18O of the ore-forming fluid also indicates no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment (δ18Ofluid = +8.0 to +10.3 per mil). In addition, δ34S values in stage II pyrite and chalcopyrite sulfide minerals range from +1.5 to +3.1 per mil, which is consistent with fluid saturation from a reduced magmatic source of sulfur. Historic gold production from occurrences hosted by the Torud-Chahshirin volcano-plutonic complex has been minor compared to that from orogenic gold deposits in Iran. However, identification of intrusion-related gold deposits in the complex indicates that more thorough exploration in the Eocene arc of north-central Iran could be promising.

Petrography and trace element signatures of iron-oxides in deposits from the Middleback Ranges, South Australia: From banded iron formation to ore

The Middleback Ranges is a major iron ore belt in the southeastern region of the Gawler Craton, South Australia, interpreted to be of BIF origin. Iron ore deposits are hosted within ∼2550 Ma metasedimentary rocks of the Middleback Group and occur as a series of N-S trending hills, forming a ∼60 km-long magnetic anomaly. A petrographic-geochemical study of iron-oxides from BIFs and iron ores was undertaken on samples from thirteen locations spanning the strike of the belt. Iron-oxides are texturally diverse due to multiple processes accompanying and postdating ore formation. Primary magnetite features preserved in the southern segment of the belt display distinct overprinting features (e.g., increased porosity, reworked grain boundaries) and multiple generations of growth associated with deposition of trace minerals, including native gold. Northwards along strike, this overprint is expressed by the pseudomorphic replacement of magnetite by hematite (martite) and is locally associated with brecciation, the presence of rare earth element (REE)-minerals, and replacement by iron-hydroxides. Whereas evidence of microplaty hematite accompanying martitization and predating iron-hydroxides is observed throughout the belt, distinct iron-oxide generations postdating the iron-hydroxides are inferred based on compositional zoning with respect to Si and intimate relationships between iron-oxides and -hydroxides. Chondrite-normalized fractionation trends obtained from the various iron-oxides and from different BIF types generally display LREE-enrichment and distinct positive Eu- and Y-anomalies. An increasing ΣREY- and LREE-trend accompanies martitization. The presence of specific element groups, e.g., granitophile elements (U, W, Sn, Mo), or transition metals (Cr, Mn, Co, Ni, Ti, V, Nb) within the lattice of iron-oxides suggests their formation in evolving environments associated with the emplacement of felsic and mafic lithologies, respectively. The impact of local setting on iron-oxide formation is highlighted by complex trace element signatures of iron-oxides; the northern segment of the belt is relatively enriched in As and Sb, while the southern segment is enriched in granitophile elements and REY. Further complexity is shown in the local variation of Mn and Zn in iron-oxides in the southern segment of the belt, where the Iron Magnet deposit shows the strongest correlation between the two elements. Various depositional settings for BIFs are inferred based upon Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides and various water types. These settings include environments where iron-minerals precipitate from mixtures of 1) anoxic seawater and high-temperature hydrothermal fluids, 2) anoxic seawater and low-temperature hydrothermal fluids, and 3) oxygen-richer seawater and low-temperature hydrothermal fluids. A genetic model for ore formation is proposed based upon textural and compositional variations observed in iron-oxides throughout the belt and includes formation resulting from supergene fluids rich in elements leached from local granites penetrating BIF, interaction with granite-derived hydrothermal fluids, and heat generated during emplacement of younger dikes. Recognition of petrographic features linked to changes in composition demonstrates the utility of iron-oxides to trace iron ore formation in a temporal and spatial context, with implications for ore genesis and models of mineral exploration.

ГЕОХРОНОЛОГІЯ ЛІТІЄНОСНИХ ГРАНІТОЇДІВ ІНГУЛЬСЬКОГО МЕГАБЛОКУ (УРАЇНСЬКИЙ ЩИТ)

Rare-metal elements are strategic metals which, in general, are extremely important for economic development or maintenance of defence capability of any country at the modern level. The list of needs for these strategic metals ranges depending on the level of economic development of certain country, but in general it includes such elements as Li, Ta, Nb, Be, Sb, W, REE and others. The majority of these elements has the lithophilous nature and, therefore, is characterized by close genetic relations with granites and pegmatites associated with them. In the world, industrial production of lithium is shared between deposits to lithium-bearing brine of saline depositions of marine basins (Argentina, Chile), some granites (China) and rare-metal pegmatites (Australia, China, Zimmbabve). In pegmatites lithium mineralization is represented mainly by spodumene (LiAlSi2O6), But other lithium-containing metallic minerals can also play an important role in production of this metal – petalite (LiAlSi4O10), minerals of lepidolite (Sa [Li,Al]3[Si,Al]4O10[F,OH]2) and amblygonite-montebrasite (LiAlPO4 [F,OH]) series. Rare-metal pegmatite of Ingul megablock of Ukrainian Shield can be treated as unique (insufficiently studied in world practice) pegmatitic formations in which the main metallic mineral is represented by petalite. In metallogenic interpretations two ore districts can be distinguished within the megablok, that are specialized on rare metals (Li, Rb, Cs, Be, Ta, Nb, Sn) – Polohivka and Stankuvatka. Deposits and numerous ore manifestations of rare metals formed in rather similar geological and tectonic conditions and have many common features – both country rocks composition and mineralogic composition of ores. Within Ingul megablock (Shpola-Tashlyk rare-metal district) a number of lithium rare-metal deposits associated with pegmatites is discovered. In order to determine the age of lithium mineralization in granites of Lypniazhka, Taburyshche massifs and vein bodies of pegmatitic and aplito-pegmatitic granites, which are selected from different localities of this megablock, are dated by U-Pd isotopic method by monazites. It is established that emplacement of vein granites of Ingul megablock occurred within rather narrow age interval – 2040-2020 Ma and it is not significantly separated in time from formation of most granitoids they are spatially associated with. This fact, together with geological evidences, gives grounds to make the assumption that rare-metal lithium pegmatite are formed in the same age interval.

AMS fabrics and emplacement model of Butiá Granite, an Ediacaran syntectonic peraluminous granite from southernmost Brazil

The post-collisional stage of the Brasiliano/Pan-African Cycle in Southern Brazil is marked by metaluminous and peraluminous granites controlled by a transcurrent shear zone system. In southernmost Brazil, the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone (DCTSZ) is the best known structure that conditioned these peraluminous granites. Despite its poorly-developed linear fabric, the emplacement of the NNW-elongate Butiá Granite (BG) northwest of the DCTSZ is interpreted to have been controlled by a dextral transcurrent shear zone. Thus, an anisotropy of magnetic susceptibility (AMS) study was performed in the BG aiming to constrain its emplacement mechanism and the relation of the granite with the regional shear zone system. Magnetic mineralogy was investigated through hysteresis loops, thermomagnetic and IRM acquisition curves and SEM analysis. These experiments show a dominant contribution of paramagnetic phases and a small content of low-coercivity (e.g., magnetite and titanomagnetite) and high-coercivity (e.g., hematite) remanence-carrying minerals. In spite of the presence of minor ferromagnetic grains, the BG magnetic anisotropy fabric is interpreted as dominantly controlled by paramagnetic biotite crystals. Magnetic susceptibility ranges between 0.1 and 8.0 × 10−5 SI. Shape parameter (T) ranges from 0.272 to 0.908, and anisotropy degree (P) ranges from 1.073 to 1.266, increasing from the inner portion of the pluton to its margins. Magnetic fabrics, microstructures and field relations suggest that magma ascent and emplacement were controlled by a NNW-trending dextral transcurrent shear zone. The presence of S-C magmatic fabric and high temperature (ca. 650 °C), solid-state deformation at the margins confirm that the pluton was deformed during its cooling process. Close to the host-rocks, magnetic foliation dips steeply towards W or E, and magnetic lineation plunges steep to moderately, indicating dominant flattening there. Shallow-plunging lineation parallel to the NW- to NNW-striking foliation is found away from the pluton margins. Foliation becomes less steep towards the BG northeastern portion and the presence of roof pendants in this area suggests the proximity to the roof zone. The combination of buoyancy forces and the partitioning of regional strain into simple and pure shear is in accordance with a transpressive regime. These results also suggest a time-space relationship between the NNW-dextral shear zone that controlled the emplacement of the Butiá Granite (ca. 629 Ma) and the sinistral, NE-trending Dorsal de Canguçu Transcurrent Shear Zone, responsible for the emplacement of peraluminous granites during the early post-collisional stage (ca. 634 - 610 Ma) of the Brasiliano/Pan-African Cycle in southernmost Brazil.

Determining the optimum locations for pumping low-fluoride groundwater to distribute to communities in a fluoridic area in the Upper East Region, Ghana

Groundwater is the primary source of water in the Upper East Region of Ghana, and is generally considered a safe source of drinking water; but there are pockets where the groundwater contains high concentrations of fluoride due to the dissolution of minerals in the local granite. The goal of this study is to evaluate the hydrogeology and hydrogeochemistry of an area where dental fluorosis endemic, in order to identify the optimum locations to pump and distribute low-fluoride groundwater. As expected, the data indicate that the higher elevation recharge areas with outcrops of Bongo granite have elevated concentrations of fluoride in the groundwater (up to 4.6mgL−1), posing the highest risk of fluorosis in the nearby communities. The lower elevation areas, which are the farthest from the Bongo granitic, have the lowest groundwater fluoride (<0.5mgL−1) and the lowest risk of fluorosis. Groundwater flow models suggest that the steady decrease in fluoride is driven by dispersion, with the fluoride concentrations dropping to the World Health Organization's recommended drinking water limit of 1.5mgL−1 at about 400–500m from the source. The optimum locations to install boreholes (or use existing boreholes) for piping low fluoride groundwater to the higher fluoride areas, would be at or beyond this distance. Although the initial costs of developing such a water system would be substantial, this is a potentially viable option for providing low fluoride water to communities suffering from fluorosis.

Tectonic setting of the Late Triassic magmatism in the Qinling Orogen: New constraints from the interplay between granite emplacement and shear zone deformation in the Shagou area

Multidisciplinary studies of the Baliping granite pluton and the Shagou shear zone (East Qinling Orogen, Central China) have been conducted to reconstruct the emplacement and deformation processes of the Baliping pluton, to explore the relationship between granite pluton and shear zone deformation, and further to constrain the tectonic setting of the Late Triassic magmatism in the Qinling Orogen which remains controversial. Zircon U–Pb geochronology studies confirmed that the Baliping granites and the protolithes of the Shagou granitic mylonites crystallized from the same magmas at ca. 221 Ma since the two nearly have the same zircon U–Pb ages and Ti–in–zircon temperatures. Meanwhile, the 210 Ma of metamorphic zircons and the 201 Ma of syntectonic granitic vein provide precise limits on the timing of synkinematic deformation in response to Late Triassic orogeny. Anisotropy of magnetic susceptibility, microstructural observation, and shape preferred orientation were combined to study the internal structures. New fabrics datasets display a high degree of coupling between the Baliping pluton and the Shagou shear zone, indicating a syntectonic emplacement of the pluton. Microstructural data suggest that the magmatic to high‐temperature solid‐state deformation is a continuum process in which the fabrics were mainly acquired. We propose an integrated syntectonic emplacement model of the Baliping pluton, in which the shear zone deformation (sinistral transpression) play a significant role on the emplacement of the magma by creating rooms. Emplacement of the pluton, in turn, triggers the nucleation of Shagou shear zone by providing the protoliths and the heat. On the basis of this study, we argued that the Qinling Orogen was still under the convergent setting during 221–201 Ma.

From Cadomian magmatic arc to Rheic ocean closure: The geochronological-geochemical record of nappe protoliths of the Münchberg Massif, NE Bavaria (Germany)

The Münchberg Massif in northeastern Bavaria, Germany is an allochthonous metamorphic nappe complex within the Saxothuringian Zone of the Variscan orogen. From top to bottom it consists of four major units: Hangend-Serie, Liegend-Serie, Randamphibolit-Serie and Prasinit-Phyllit-Serie, which show an inverted metamorphic gradient of eclogite- to amphibolite-facies (top) to greenschist-facies (bottom) and are separated from each other by thrust faults. New geochemical and U-Pb zircon data indicate that the four units host metasedimentary and meta-igneous rocks, which were formed at different time and in distinct geotectonic settings during the evolution of the Saxothuringian terrane between 550 and 370Ma. Mafic and felsic protoliths of the Hangend-Serie result from a bimodal magmatism in an evolved oceanic to continental magmatic arc setting at about 550Ma. These rocks represent relics of the Cadomian magmatic arc, which formed a cordillera at the northern margin of Gondwana during the Neoproterozoic. The Liegend-Serie hosts slivers of granitic orthogneisses, emplaced during magmatic events at c. 505 and 480Ma, and Early Palaeozoic paragneisses, with our samples deposited at ≤483Ma. Ortho- and paragneisses were affected by an amphibolite-facies metamorphic overprint at c. 380Ma. Granite emplacement and sediment deposition can be related to the separation of the Avalonia microterrane from the northern Gondwana margin. Amphibolite protoliths of the Randamphibolit-Serie emplaced at c. 400Ma. They show N-MORB to E-MORB signatures, pointing to their formation along an oceanic spreading centre within the Rheic ocean. Mafic igneous rocks in the Prasinit-Phyllit-Serie emplaced at nearly the same time (407–401Ma), but their calc-alkaline to tholeiitic character rather suggests formation in an intra-oceanic island arc/back arc system. This convergent margin lasted for about 30Ma until the Late Devonian, as is suggested by a maximum deposition age of 371Ma of associated phyllites, and by metamorphic Ar-Ar ages of 374–368Ma. The timing of the different magmatic and sedimentary events in the Münchberg Massif and their plate tectonic settings are similar to those estimated for other Variscan nappe complexes throughout Europe, comprising the French Massif Central and NW Spain. This similarity indicates that the Münchberg Massif forms part of an European-wide suture zone, along which rock units of different origin were assembled in a complex way during the Variscan Orogeny.

Pressure‐Temperature‐Deformation‐Time Constraints on the South Tibetan Detachment System in the Garhwal Himalaya (NW India)

AbstractA peculiar feature of the Himalaya is the occurrence of a system of low‐angle normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during synconvergent tectonics. We describe the STDS‐related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body crosscutting the STDS, occurs. A detailed multidisciplinary study, integrating field‐based, microstructural, petrographic, and geochronological analyses, was carried out on rocks along this valley. We demonstrate how the noncoaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P)‐temperature (T) conditions of 0.9–1.1 GPa and ≥750°C at ≥24 Ma. This migmatite has been reworked structurally upward leading to the formation of high‐T sillimanite‐bearing mylonites. Further upward, medium‐T shearing deformed the Malari granite and leucogranite dykes, forming medium‐T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar‐Ar geochronology, between ~20 and 15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement, and deformation along low‐angle normal faults.

Geochronology in the southern Midyan terrane: a review of constraints on the timing of magmatic pulses and tectonic evolution in a northwestern part of the Arabian Shield

ABSTRACTThe southern Midyan terrane is a composite Tonian to Ediacaran tectonostratigraphic crustal block in the northern Arabian Shield that prior to Red Sea opening was contiguous with coeval rocks in the Eastern Desert of Egypt and Sinai. Ion microprobe (sensitive high-resolution ion microprobe [SHRIMP]) dating of 12 rock samples described here and the results of other dating programmes establish a clear timeframe for depositional, intrusive, and structural events in the region and provide a chronology of tectonism in this part of the Arabian-Nubian Shield. Deposition of Zaam and Bayda group volcanosedimentary rocks and emplacement of mafic-ultramafic complexes and TTG-type diorite, tonalite, and granodiorite denote formation of the Tonian (780–715 Ma) Zaam arc and fore-arc ophiolite above a possible west-dipping subduction system in the southern part of the Midyan terrane. Convergence with the Hijaz terrane farther south and obduction of ophiolite nappes resulted by ~700 Ma in development of the Yanbu suture. Ongoing or a new subduction system led to a ~705–660 Ma Cryogenian pulse of magmatism represented by I-type calc-alkaline diorite, granodiorite, and granite that have volcanic-arc and syn-collisional granite affinities. This was followed, after a brief end-Cryogenian hiatus, by a 635–~570 Ma period of Ediacaran magmatism marked by monzogranite, syenogranite, and minor gabbro and diorite. These rocks are reported to have within-plate to volcanic-arc and syncollision chemical characteristics but their precise tectonic setting is uncertain. Structurally, the intrusions are diapiric and were evidently emplaced in an extensional regime consistent with an overlap between intrusion and Najd faulting associated, at this time, with transpressional collision and northward extension through much of the ANS. Terminal magmatism in the southern Midyan terrane postdated cessation of Najd faulting at ~575 Ma and resulted in the emplacement of undeformed within-plate A-type alkali-feldspar granites and mafic (lamprophyre) and felsic dikes.

The 131–134 Ma A-type granites from northern Zhejiang Province, South China: Implications for partial melting of the Neoproterozoic lower crust

Although Mesozoic granites are widely distributed in the Gan-Hang Belt in Southeast China, their petrogenesis and geodynamic settings are still matters of dispute. Here, the major and trace elements, Sr-Nd isotopic compositions, zircon U‐Pb dating and Hf isotopes of three late Mesozoic granite plutons from Machebu, Shenzhongwu and Daixi in northern Zhejiang Province were analyzed to investigate their petrogenesis. These granite plutons are featured by an A2-type granite geochemical signature (e.g., high SiO2 (71 to 78wt.%), total alkalis (Na2O+K2O=7.57 to 9.12wt.%), rare earth elements (total REE=174 to 519ppm) and HFSE contents, with mostly high FeOT/(FeOT+MgO) (0.82 to 0.93) and Ga/Al ratios (2.49 to 5.07) and low Ce4+/Ce3+ ratios in zircons (1 to 90)), suggesting they were formed in an extensional tectonic setting. Combining whole rock Zr contents, mineral assemblages and the An content of plagioclase, it was suggested that these granites could have formed at a high temperature (>850°C) with a low H2O content (~2.5wt.%). Assuming the granite with the lowest SiO2 and high CaO contents as the “primary granite melt”, the melting pressure was estimated to be 2.5kbar based on model calculations using MELTs. Taking into account the effect of plagioclase fractional crystallization during the granite emplacement into shallow crustal levels, the initial melting pressure could be >2.5kbar at the stable field of plagioclase. This agrees well with the very low and variable Sr and Eu contents of these granite plutons.Zircon U–Pb dating yields a weighted mean 206Pb/238U age of 130.9±1.0Ma for the Machebu granite, 134.4±2.1Ma for the Shenzhongwu granite and 131.9±1.2Ma for the Daixi granite. The εHf(t) values of the zircons in these plutons gradually increase from southwest to northeast, i.e., from −14.5~−3.5 for the Machebu granite to −1.5~−0.2 for the Shenzhongwu granite and from 5.1~8.6 for the Daixi granite, suggesting the origin of different parent rocks. These granites have two-stage Nd model (TDM2(Nd)) ages of 1099Ma–838Ma, and zircons from these granites and the Neoproterozoic basement in the Gan-Hang Belt plot on the same evolutionary trend in the εHf(t)-age diagram. It is interesting to note that the collection of literature data shows a positive correlation between SiO2 and εNd(t) for the late Mesozoic A-granites in the Gan-Hang Belt, and the Neoproterozoic A-granites in the Gan-Hang Belt cluster in two groups of the high-SiO2-εNd(t) group and low-εNd(t) group. The positive correlation of SiO2-εNd(t) demonstrated by the late Mesozoic A-granites can be well explained by a high-degree of melting of mixtures between the two groups of Neoproterozoic A-granites. We thus suggest that the late Mesozoic A-type granites in the Gan-Hang Belt could have been derived from the rejuvenated Neoproterozoic rocks rather than directly from the Mesoproterozoic metamorphic basement as a result of subduction.

Petrogenesis and thermal overprint of S-type granites in Helanshan region, North China Craton: Constraints on the 1.90 Ga khondalites decompression melting and 1.32 Ga tectono-thermal event

Widespread Paleoproterozoic S-type granites and khondalites (high-grade sillimanite-garnet gneisses) are exposed in the Khondalite Belt of the North China Craton (NCC). Porphyritic biotite granites in the Helanshan region from the western Khondalite Belt exhibit a gradual contact relationship with the neighboring khondalites. In order to constrain the petrogenesis of these granites and their genetic relationship with the khondalites, detailed geochronological data, as well as geochemical and whole-rock Sm-Nd and zircon Lu-Hf isotope compositions of these granites are reported. LA-ICP-MS zircon U-Pb dating on magmatic zircon cores yields an emplacement age of 1902±22Ma, whereas metamorphic rims define a metamorphic age of 1356±57Ma. The unpolished surfaces of zircon crystals selected based on the presence of rims visible in cathodoluminescence were further analyzed by SIMS depth profiling, which gave a more precise metamorphic age of 1317±18Ma. The Helanshan granites display typical S-type geochemical characteristics, e.g., high A/CNK (1.16–1.29) and K2O (5.09–5.86wt%) contents, but low FeOt/MgO (2.39–4.88) and P2O5 (0.15–0.23wt%) contents. Similar trace and rare earth element patterns, and consistent zircon Hf model ages (2366–2483Ma) and whole rock Nd model ages (2407–2499Ma) between the porphyritic biotite granites and the khondalites, suggest that they were derived from partial melting of khondalites. The geochemical character of the porphyritic biotite granites, including significant negative Eu anomalies, high Rb/Ba and Rb/Sr ratios, and variable HREE contents and (La/Yb)N ratios, suggest that they are probably related to the post-peak decompression stage that was characterized by biotite breakdown and the progressive replacement of garnet with the presence of K-feldspar and plagioclase in the residues. Homogeneous zircon εHf(t) values (mostly from −0.5 to +1.7), combined with low bulk-rock Fe2O3T content and CaO/Na2O ratios, preclude the contribution from mafic magma. Following the continental collision, the emplacement of the porphyritic biotite granites at ca.1.90Ga provides the latest limit on the timing of post-peak decompression metamorphism of the khondalites, and their formation supports a post-collisional mantle upwelling model to account for the 1.92–1.90Ga regional metamorphic/magmatic events. On the other hand, the metamorphic zircon rims indicate that S-type granites were overprinted by a 1.32Ga tectono-thermal event, which was probably related to the breakup of the Columbia supercontinent.

The Keimoes Suite redefined: The geochronological and geochemical characteristics of the ferroan granites of the eastern Namaqua Sector, Mesoproterozoic Namaqua-Natal Metamorphic Province, southern Africa

Voluminous granite gneisses and granites straddle the boundary between the Kakamas and Areachap Terranes in the eastern Namaqua Sector (NS) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). These rocks have been previously poorly defined and loosely grouped into the Keimoes Suite, but a recent U-Pb age study has suggested the suite be subdivided into syn-tectonic and post-tectonic groups relative to the main phase of the Namaqua Orogeny. This study adds new whole rock geochemical, isotopic and age data for these granites that confirms the subdivision is appropriate. The older group of syn-tectonic granite gneisses, dated between 1175 and 1146 Ma, have penetrative foliations and are largely derived from fractionated, leucogranitic metaluminous to peraluminous magmas with low maficity, low Ti, Mn and Ca. They were derived from mildly depleted sources (εNd(t): −1.47 to 1.78), with Meso-to Paleoproterozoic Nd model ages (1.57–1.91 Ga), and high initial Sr ratios (0.71970–0.75567) suggesting mixing between younger depleted and older, arc-like sources imparting an arc-like signature to the magmas. High initial Sr ratios appear to be an intrinsic character of these granites reflecting those of granites in the region and the highly radiogenic nature of the NS.The weakly to unfoliated late-to post-tectonic megacrystic granodiorites and monzogranites, including charnockites, intruded between 1110 and 1078 Ma and constitute the Keimoes Suite proper. They have I-type characteristics, being strongly metaluminous and locally hornblende- and orthopyroxene-bearing with moderate SiO2 and with arc-type affinities (LILE enrichment relative to the HFSE, Ta-Nb, Ti and P anomalies). However, the granitoids also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. They show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age. They are similar to the syn-tectonic granites in having εNd(t) values close to zero (−2.95 to 2.83) and Meso-to Paleoproterozoic model ages (TDM: 1.38–1.99 Ga) but lower initial Sr ratios (<0.723 in general) suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component.The timing of emplacement of the post-tectonic granites places peak D2 deformation in the eastern NS predominantly at ∼1.13–1.10 Ga, with peak D2 varying from ∼1.18–1.13 Ga. There was more voluminous granitic magmatism in the Areachap Terrane to the east during the late-to post-tectonic magmatic episode, whereas the earlier, ∼1.18–1.14 Ga syn-tectonic magmatic episode is more concentrated to the west in the Kakamas Terrane. The broad, protracted period of magmatism in the eastern NS attests to a long-lived duration of high-heat flow in this portion of the southern African crust at this time. Nd model ages of Meso-to Paleoproterozoic age reflect those in other granites throughout the NS suggesting extensive reworking of Paleoproterozoic crust during the 1.2–1.0 Ga Namaquan Orogeny.

Using multifractal modelling, singularity mapping, and geochemical indexes for targeting buried mineralization: Application to the W-Sn Panasqueira ore-system, Portugal

The Panasqueira Mine is a world-class W-Sn-Cu vein-type deposit, currently exploited by Beral Tin & Wolfram Portugal, which has promoted the sampling and analysis of a series of chip-rocks covering an approximate area of 8×3km over the Mine prospect and adjacent areas, with the objective of identifying geochemical signatures related to the Panasqueira granite emplacement and thus targeting new areas with potential W-Sn mineralization of the same ore-system. The geometry of the Panasqueira granite is still unknown but some drill-cores and geophysical techniques provided indications of its extension and presence beyond the current mine exploitation permit. The opportunity of using whole-rock samples, jointly with the current knowledge of the complex mineralizing stages and related geochemical signatures, allowed the proposal of empirical elemental ratios and exploration indexes, which combined with multifractal models for anomaly characterization and singularity mapping, provided a fairly complete and coherent picture of different anomaly patterns for Sn, W, and Cu. These patterns not only target the current exploited areas over the Panasqueira Mine but, more importantly, sustain the occurrence of additional anomalies that can potentially increase the current reserve estimation and consequently extend the mine's life-span. Most elements showed a single anomaly threshold, as W at 1.8ppm, but others showed two, including Sn at 2.3 and 13.6ppm, the latter notably close to the empirically deduced one of 15ppm. The evidence for the extension of the current anomalies also relies in the singularity and anomaly mapping based on the different thresholds using associated elements, such as Nb, Ta, and Ti for W, F, Li, element ratios, for Sn, and ISW and IBM indexes.

Cold plutonism in the Arabian–Nubian Shield: evidence from the Abu Diab garnet-bearing leucogranite, central Eastern Desert, Egypt

The Neoproterozoic Abu Diab garnet-bearing leucogranite (SiO 2 73.7 – 76.8 wt%; total alkalis 8.1 – 9.3 wt%) was emplaced during the late to post Pan-African Orogeny. The pluton is almost homogeneous in composition, weakly peraluminous (molecular Al 2 O 3 /(CaO + Na 2 O + K 2 O) = 1.04 – 1.12) and approaches the minimum melt composition in the water-saturated Qz–Ab–Or system. The chondrite-normalized patterns are almost unfractionated, with weak tetrad effects and prominent negative Eu anomalies. Ba, Sr, P, Eu and Ti show strong negative anomalies in primitive-mantle normalized multi-element diagrams. Magmatic garnet is dominantly a spessartine (66 – 67 mol %)–almandine (27 – 29 mol %) solid solution. Our results indicate that the Abu Diab granite magma formed from low-degree eutectic melting of a felsic pelite source in the lower to middle continental crust. Zirconium saturation thermometry indicates magma generation temperatures of 732 ± 28°C (2σ), placing the Abu Diab leucogranite in the ‘cold granite’ group. This indicates that muscovite breakdown is the dominant reaction in the melt source. The high normative Or, the increasing Rb/Sr concomitant with decreasing Sr and Ba, and the sharp negative Eu anomalies are consistent with fluid-absent melting. Field relations and petrographic observations indicate the emplacement of Abu Diab granites after the closure of the Mozambique Ocean and collision of East and West Gondwana, most probably during fast adiabatic uplift and decompression of the thickened crust. Supplementary material: The locations of the peraluminous leucocratic granites in the Arabian-Nubian and other supplementary data for this paper are available at https://doi.org/10.6084/m9.figshare.c.3815293 .

Post-magmatic hydrothermal origin of late Jurassic Liwu copper polymetallic deposits, western China: Direct chalcopyrite Re-Os dating and Pb-B isotopic constraints

The Liwu copper polymetallic deposits are located in the Jianglang Tectonic Dome in the southeastern Songpan-Ganzi Fold Belt. The deposits contain more than 0.8 million tonnes (Mt) of Cu, 0.5 Mt of Zn, 133 t of Ag and 1300 t of Co. However, the detailed geologic features, formation ages and metallogeny of these deposits remain poorly understood. Our new analyses indicate that the Liwu-type copper polymetallic deposits (LTCPD) are epigenetic hydrothermal deposits. The major copper polymetallic orebodies were emplaced in the Late Jurassic (ca. 151.1±4.8Ma), approximately 4–2.5Myr later than the local granites (ca. 160.9±1.0Ma–159.3±0.9Ma). The ores show variable Pb isotopic compositions, with initial 206Pb/204Pb ratios of 18.048–18.420, 207Pb/204Pb ratios of 15.595–15.724, and 208Pb/204Pb ratios of 38.335–38.795, similar to those of local plutons but significantly lower than those of the ore-hosting rocks. The ore-associated tourmalines are dravitic and relatively homogeneous and exhibit similar δ11B values (between −15.47±0.83 and −5.91±0.67 ‰). These geological and Pb-B isotopic features suggest that the deposits at Liwu have a post-magmatic hydrothermal origin, similar to the stratiform skarn deposit. The major ore-forming source is dominantly related to the coeval upper crust-derived magmatism in the region.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

Zircon grains extracted from S-type granites of the Meda-Escalhao-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U–Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321–317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciaes, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321–317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319–317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U–Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2–early D3 S-type granitic rocks. The eNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.