Granite Emplacement Research Articles

The composite metamorphic sequence in the Jiajika gneiss dome, Songpan-Ganze orogenic belt, eastern Tibet: P-T-D-t evolution and implications for lithium mineralization

The metamorphic host rocks of pegmatites can help to decipher their geodynamic setting and emplacement conditions. The Jiajika lithium deposit, located in the Songpan-Ganze orogenic belt, eastern Tibet, is one of the largest hard-rock type lithium deposits in China. The lithium-enriched pegmatites in Jiajika are hosted in the amphibolite facies metasedimentary rocks in the mantle of the Jiajika gneiss dome. However, the metamorphic evolution of Jiajika is poorly constrained. In this study, we carried out detailed fieldwork, petrological observation, mineral chemical analysis, phase equilibria modeling, and geochronological studies on the Jiajika gneiss dome and the borehole JSD-1 (with a total depth of 3211 m) of the Jiajika Scientific Drilling Project (JSD). Structural data outlined two subdomes within the Jiajika gneiss dome. Two main stages of regional metamorphism and deformation are documented in Jiajika: an early Barrovian metamorphism (M1) in ca. 237−221 Ma caused by crustal thickening (D1), which has a peak condition of 6.7−8.3 kbar and 649−694 °C; a late Buchan metamorphism (M2) induced by the emplacement of granites and pegmatites during doming (D2) with a peak condition of 3.4−4.3 kbar and 570−612 °C at 210−206 Ma. The superimposition of the M2 isograds upon the M1 isograds resulted in a composite metamorphic sequence around the Jiajika gneiss dome. The overlapping of the Barrovian metamorphism and the Buchan metamorphism is related to crustal thickening during continental collision and later decompression through doming, which facilitated lithium mineralization.

Robust Timing Constraints for Granitic Magmatism and Hydrothermal Mineralization in the Tieshanlong W‐Sn Ore Field, Eastern Nanling Range, South China

AbstractThe Tieshanlong ore field is an important part of the Nanling Range, which is famous worldwide for its W‐Sn mineralization. Notably, the mineralization age of the Tieshanlong ore field is not well constrained, and our field investigation reveals that granitic emplacement occurred at different stages. However, previous studies have not distinguished these multiple stages of magmatism. The Tieshanlong granite complex is closely related to the Huangsha quartz vein‐type W‐Sn deposit and Tongling skarn‐type Cu‐W‐Sn deposit in this field. Through field investigations and isotopic age analyses, this work studies the relationship between multistage magmatic activity and mineralization in the Tieshanlong ore field. LA‐ICP‐MS zircon U‐Pb isotope analyses revealed that the first‐ and second‐staged granites formed at 154.2 ± 0.6 Ma (MSDW = 1.4) and 151.2 ± 0.4 Ma (MSDW = 1.5), with zircon εHf(t) values ranging from ‐13.1 to ‐10.5 and from ‐14.7 to ‐11.1, respectively. These data suggest that the Tieshanlong granite complex was derived from the partial melting of ancient crustal material. LA‐ICP‐MS U‐Pb dating of wolframite and cassiterite reveals that W‐Sn mineralization occurred at 160–150 Ma, which agrees well with the U‐Pb dating results of the second‐staged granite within analytical errors. The magmatic activity in this ore field can be divided into three stages: 175–154 Ma, 154–150 Ma and 150–145 Ma. The quartz vein‐ and skarn‐type W‐Sn mineralization is closely related to second‐staged fine‐grained two‐mica granite, and formed earlier than skarn‐type Cu‐ mineralization. This study establishes a metallogenic model for the Tieshanlong ore field, and this model has important practical significance for identifying concealed W‐Sn(‐Cu) deposits around other granitic complexes in the Nanling Range.

Primera escultura oficial en bronce en Lusitania (Hispania)

The Romanisation of Lusitania (Hispania) took place in the time of Augustus, after the conquest of the Hispanic territory at the end of the 1st century BC. Bronze has largely disappeared from the archaeological remains, due to its constant reuse. In the early decorative programmes of some Lusitanian cities there are important traces of bronze sculptures. We present three examples: Sculptures from the provincial capital Augusta Emerita in its first public decorative phase in local granite and bronze, before the massive presence of marble; an equestrian statue from the forum of the colonia Norbensis Caesarina, perhaps representing its founding patron Lucius Cornelius Balbus, and an imperial statuary fragment from the Roman theatre of the colonia of Metellinum. They all date from the Augustan period and attest to the presence of important bronze workshops in the western part of the Empire.

The Relationships between the Internal Nappe Zone and the Regional Mylonitic Complex in the NE Variscan Sardinia (Italy): Insight from a New Possible Regional Interpretation?

This study presents an updated interpretation of geological data collected between 1984 and 2022. The area under consideration holds significant regional importance as it is located between the Internal Nappe Zone (INZ) and the Regional Mylonitic Complex (RMC). Re-evaluation of the geological data has highlighted a more intricate structural framework than what is currently documented in the existing literature. This paper aims to illustrate, through structural analysis, that the Posada Valley Shear Zone (PVSZ) does not serve as the transitional boundary between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex (HGMC) as traditionally thought. Instead, the authors’ findings indicate that the transition boundary is confined to a shear band with a variable thickness ranging from 10 to 70 m at its widest points. The development of the Posada Valley Shear Zone is characterized by a series of transitions from mylonite I S-C to mylonite II S-C, extending over approximately 5 km. The formation of the Posada Valley Shear Zone is chronologically confined between the development of the East Variscan Shear Zone (EVSZ) and the emplacement of the Late Variscan granites. The differing orientations of Sm and S3 observed in the mylonitic events of the Posada Valley Shear Zone and the Regional Mylonitic Complex, respectively, are likely attributable to an anticlockwise rotation of the shortening directions during the upper Carboniferous period. Furthermore, this study proposes that the Condensed Isogrades Zone (CIZ), despite its unclear formation mechanism, should be recognized as the true transition zone between the Inner Nappe Zone and the Regional Mylonitic Complex or High-Grade Metamorphic Complex. This new interpretation challenges the previously accepted notion of increasing Variscan metamorphic zonation toward the northeast. This conclusion is supported by the identification of the same NE–SW orientation of the D2 tectonic event in both the Old Gneiss Complex (OGC in the Regional Mylonitic Complex) and the lithologies of the Inner Nappe Zone and the Condensed Isogrades Zone. The comprehensive analysis and new insights provided in this paper contribute to a refined understanding of the geological relationships and processes within this region, offering significant implications for future geological studies and interpretations.

Two metamorphic cycles recorded by monazite in eclogite-facies gneisses (Southern Armorican Massif, France): A Cambro-Ordovician continental crust involved in eo-Variscan subduction.

We present new U-Th/Pb ages obtained for xenotime and monazite from the polycyclic eclogite-facies para- and ortho-gneisses of the Les Essarts Unit (Vendée, southern Armorican Massif, France), which have recorded an HT-LP cycle ending with a first retrogression and a subsequent HP eclogite-facies metamorphism similar to that of the neighbouring eclogites. Some paragneisses and orthogneisses are only slightly deformed and retrogressed, showing the structure of nebulitic migmatite or metagranite, respectively; both show well-preserved complex coronitic and pseudomorphic microstructures, due to the eclogite-facies metamorphism. Monazite I and xenotime crystallised during the HT stage providing an opportunity to date the early HT metamorphic event in the paragneiss and the emplacement of granite in the orthogneiss. U/Pb ages obtained from monazite I and xenotime of the cordierite-bearing migmatitic paragneisses range between 510 and 480 Ma (Late Cambrian-Early Ordovician). These ages may correspond to the crystallisation and/or re-equilibration of monazite and xenotime during the prograde stage of the HT cycle, close to the T peak. Consistent monazite and xenotime U/Pb ages around 496 Ma in the orthogneiss represent the age of the granite protolith. During subsequent HP overprint in the gneisses, numerous coronas developed at the expense of the early HT parageneses, in particular plagioclase. In both paragneiss and orthogneiss, monazite I in contact with HT plagioclase reacted to form apatite + zoisite + monazite II coronas. The tiny monazite II crystals could be dated in a paragneiss sample and gave a concordant Th-U/Pb age of 395 ± 6 Ma, interpreted as the age of eclogite-facies HP metamorphism. This age is in agreement with those obtained in HP metamorphic rocks of the Upper Allochthon Unit of the Iberian-Armorican Arc (Cabo-Ortegal, Bragança, Audierne) representing the first evidence of convergence in the Variscan cycle.

Two-stage mineralization of the Jinkeng Sn-Cu deposit in Eastern Guangdong, Southeast China: Response to magmatic activities and tectonic transformation

The Jinkeng Sn-Cu polymetallic deposit in South China consists of two different types of orebodies: 1) NE-striking skarn- and vein-type Cu-Pb-Zn-Sn orebodies in volcanic rocks suffering subsequent ductile shear deformation, and 2) NW-striking quartz-cassiterite-sulfide veins filled in the faults at the porphyritic granodiorite outward from the fine-grained granite which does not exhibit deformation. The relationships among Sn-Cu polymetallic mineralization, regional magmatism, and deformation metamorphism are still controversial. To address it, the geochronological, whole-rock and mineral geochemical research along with the detailed field investigation were conducted in this study. Our zircon U-Pb dating results show that the volcanic rocks formed at 158–162 Ma, earlier than the porphyritic granodiorite which yields the emplacement age of 146–147 Ma. In-situ LA-ICP-MS U-Pb dating of cassiterite from the deformed skarn ores indicates the early-stage mineralization occurred at 146 ∼ 148 Ma, which is similar to the emplacement age of the porphyritic granodiorite, but earlier than the formation of quartz-cassiterite-sulfide veins and the fine-grained granite (141∼144 Ma). Further, the whole-rock and biotite geochemistry, and zircon Hf isotope compositions suggest that the porphyritic granodiorite exhibits a lower degree of magma differentiation, higher oxygen fugacity, higher Cl and lower F contents than the fine-grained granite. The porphyritic granodiorite might provide the most Cu-Pb-Zn budget accompanied by minor Sn for early-stage mineralization. Overall, our study suggests that the Jinkeng Sn-Cu polymetallic deposit formed in two scenarios where the early dominated Cu-Pb-Zn and minor Sn mineralization related to the emplacement of the porphyritic granodiorite is superimposed by the late vein-type Sn-dominated mineralization related to the emplacement of the fine-grained granite. The two isolated mineralizing events (∼147 Ma and ∼141 Ma, respectively) in Jinkeng were probably responded to the regional magmatic activities triggered by the tectonic transformation where two extensional tectonic events were separated by a short contractional event (147–144 Ma).

Comprehensive Assessment of A-Type Granite from South Sinai, Egypt: Geochemistry, Zircon Geochemistry, and Geochronology

The A-type granites are an important component of the Earth's continental crust and play a significant role in understanding crustal evolution and tectonic processes. This study presents a comprehensive assessment of A-type granites from Ataitir El Dehami granites (ADG) that located at the north-western part of Wadi Seih area in South Sinai, using geochemistry, zircon geochemistry, and geochronology characteristics. The geochemical analysis reveals that ADG are characterized by high silica content, relatively high FeO*, and high alkali concentration consistent with the A-type granites that derived from the melting of tonalite sources. Zircons from Ataitir El Dehami granites have high REE contents with Ce enrichment and depletion of Eu, which, together with the high U/Yb ratios, suggest that the studied granites were generated from melting of continental crust rocks. Zircon U-Pb geochronological data indicates that the granite emplacement occurred around 575 to 603 Ma throughout continuous exhumation pulses. The isotopic analyses also indicate that the presence of inherited zircon grains (735± 9Ma). The present data suggest that the studied granites were formed in a post-collisional setting, and represent the transition stage from convergence to extension that occurred at 600 Ma.

LEAD-LEAD DATING REVEALS PERMIAN REMOBILIZATION OF NIOBIUM MINERALIZATION AT BAYAN OBO

Abstract The Bayan Obo deposit hosted by the H8 unit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe). Permian granites are widely exposed in the mining area and have a close spatial association with the Nb mineralization. Whether the granites contributed Nb or only remobilized existing mineralization is important for understanding the controls of ore formation. Previous studies have mostly focused on the REEs, whereas research on Nb has been limited. This is due mainly to the difficulty of accurately determining the age of the Nb mineralization because of the fine-grained and texturally complex nature of the Nb-bearing minerals and their exceptionally low U content. Although microbeam techniques show promise in tackling the aforementioned challenges, their application is hampered by matrix effects caused by the diverse composition of Nb-bearing minerals. Here we report the application of a high-precision secondary ion mass spectrometry (SIMS) Pb-Pb isochron approach that enables young samples (i.e., <500 Ma) to be dated without matrix-matched reference materials. A variety of Nb-bearing minerals from eastern Bayan Obo were analyzed, yielding Pb-Pb isochron ages of 276 ± 10 Ma (pyrochlore, 394–6,864 ppm U in the rim and 6,563–19,858 ppm in the core), 277 ± 36 Ma (fersmite, 18–61 ppm U; fergusonite-Ce, 45–95 ppm U), and 257 ± 46 Ma (aeschynite, 342–1,006 ppm U). In combination with the deposit geology and petrographic observations, these ages link the Nb mineralization to ~270 Ma granites. As these granites are not particularly rich in Nb, skarn formation during granite emplacement is interpreted to have remobilized the existing Nb mineralization, which increased the grain size of the Nb-bearing minerals—a key factor facilitating their extraction. Our study shows that high-precision SIMS Pb-Pb analysis holds promise for directly dating mineralization without matrix-matched reference materials. It also emphasizes the need to consider the role of the Nb remobilization at Bayan Obo and elsewhere.

Multi-stage leucogranite emplacement in the Yalashangbo dome in northern Himalaya: Trigger and consequence of faulting along the South Tibet Detachment System

Leucogranitic dikes emplaced at the lower-middle crust are particularly widespread in the Himalayas. How the magmatic activities evolved in relation to faulting along the Southern Tibetan Detachment system (STDS) remains debated. In this study, we investigate the multi-stage leucogranitic emplacement in relation to the structural evolution of the detachment fault in the Yalashangbo dome through detailed macro- and microscopic structural observations, LA-ICP-MS monazite U-Th-Pb dating, and whole-rock geochemical analysis of the leucogranitic dikes. We identified late syn- and post-kinematic granitic dikes from the dome. Timing of dominant STDS faulting is constrained from 27 to 17 Ma. Based on synthesis of geochronological and geochemical results of the present and previous studies, we suggest that two dominant stages of granitic emplacement occurred from Eocene to Miocene along the High Himalayas. An early stage of leucogranitic activity was probably ascribed to partial melting of mafic juvenile lower crust due to heating related to asthenosphere upwelling as the result of slab tearing or break-off during the Indian subduction. Emplacement of the leucogranitic melts subsequently softened the middle and lower crust and triggered the initiation of detachment faulting along the STDS as early as ca. 35 Ma. The large-scale detachment faulting likely led to intensive localized decompression melting of the lower crust. Contemporaneous detachment faulting in the lithospheric mantle allowed heat transfer from the asthenosphere to the lower crust, enhancing lower crustal melting and leading to generally higher crystallization temperatures in the late syn-kinematic granites than the post-kinematic ones.

Convergence and transpression along the paleo-Pacific margin of Australia during the Paleozoic: insights from the Pinnak Sandstone at Mallacoota, northeast Victoria

Underpinning almost every Paleozoic paleogeographic reconstruction and tectonic model of the Lachlan Orogen is the interpretation of the structural geology related to crustal collision and amalgamation. Bounding the eastern margin of the Lachlan Orogen is a north-trending corridor of Cambrian–Ordovician turbidites that in northeast Victoria are recognised as the Pinnak Sandstone or previously known as the Mallacoota Beds. This sequence contains an accretionary prism related early S* foliation, syn-sedimentary dykes, slump structures, early faults or décollements and early quartz veins. These indicate anisotropic compaction by fluid extraction in a shallow marine turbidite sequence that underwent localised deformation (D1), which can be attributed to the Benambran Orogeny (ca 450 Ma). There was a change in the rheological properties during the ensuing late Silurian D2 Bindian Orogeny (ca 420 Ma). This event produced north–south-trending upright, east-verging folds (F2) with a marked partitioning of strain between the different sedimentary units. Folds plunge south at ∼30°, are tight to isoclinal, with local collapsed hinges and limb thrusts. This D2 deformation along the paleo-Pacific margin involved dextral transpression and a southerly propagation of the deformation front. Superimposed on these structures is an east–west paleostress pattern and two episodes of faulting (D3 and D4). In the first, brittle strike-slip faults were accompanied by ductile folding related to east–west shortening during the Middle–Late Devonian Tabberabberan Orogeny (ca 385–370 Ma). These were accompanied by the emplacement of the Sandpatch Point Granite. The second episode (D4) of strike-slip faulting reactivated many of the earlier faults during further east–west shortening and was related to the Carboniferous Kanimblan Orogeny (ca 354–343 Ma). This is best seen close to the granite contact where the strong crystallised granite reinforced the contact metamorphosed greywacke sequence, inhibiting ductile deformation so that faults deformed in a brittle fashion.

U–Pb LA-ICP-MS and Re–Os dating of wolframite and molybdenite: Constraints on multiple mineralization and cooling history (40Ar/39Ar) for the magmatic–hydrothermal system at Borralha, northern Portugal

The Borralha area (Iberian Massif, Portugal) reflects a long-lived and multiphase magmatism (ca. 316 to 300 Ma), characterized by three types of granite emplacement during two different extensional events (E1 and E2) and one compressional event (C3) within the Variscan orogeny. The W (Cu) mineralization styles correspond to veinlets and quartz veins (i.e., São António and Santa Helena), and disseminations in the Santa Helena breccia pipe. Molybdenite coexists with an Fe-bearing wolframite–quartz assemblage in the Venise breccia pipe and quartz veins (i.e., São José and São Paulo).The geochemistry and geochronology of wolframite and molybdenite samples were studied by electron probe microanalysis (EPMA), in-situ U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and Re–Os thermal ionization mass spectrometry (TIMS). The wolframite yields a U–Pb age (sample WBV1) of 315.2 ± 4.7 Ma (2σ), representing the first recognized episode of W deposition in the Iberian Massif. Wolframite samples (WBV1, WB1, and WB4) have a ferberite composition with a hübnerite/ferberite ratio of < 29. High amounts of Nb (up to 3703 ppm) with low Ta (up to 123 ppm) and REEs (up to 11.4 ppm) and conversely, low amounts of Nb (up to 471 ppm) with high Ta (up to 190 ppm) and REEs (up to 53.7 ppm) were measured in wolframite samples. The REEs normalized patterns of the wolframite samples record a mixed signature. Large and positive Eu/Eu* and Er/Er* anomalies were identified (sample WBV1), whereas negative Ce/Ce* and Nd/Nd* anomalies were observed in samples WB1 and WB4.Molybdenite Re–Os dating yields absolute ages ranging from 305.1 to 303.8 ± 2.2 Ma (2σ), which are coeval with the emplacement of E2 biotite granite (Penedos; I-type; <305 Ma). Low amounts of Re (<1 ppm), W, Co, Ni, As, Cu, Bi, Fe, Mn V and Se (<0.005 wt%) were detected in the molybdenite structure.Constraints between the timing of W- and Mo-deposition and a protracted cooling history of the host rocks are confirmed by 40Ar/39Ar white mica dating. The 40Ar/39Ar ages obtained for secondary white micas collected from Mo veins (Venise breccia pipe; 277 ± 3 and 287 ± 4 Ma 2σ) and W (Cu) dissemination (Santa Helena breccia pipe; 287 ± 2 and 279.0 ± 6 Ma 2σ) record the isotopic resetting by later hydrothermal overprints and cooling /exhumation history after E2 Gerês biotitic granite emplacement.The W-ore deposition did occurred during the final E1 extensional tectonic stage (315 Ma) and E2 extensional tectonic stage for Mo-ore deposition, where a temporal offset was needed for the adjustment of tectonic conditions after the C3 compressional and E2 extensional stage started.

Geology, U-Pb dating and 3D visualisation of late-orogenic Klenov Pluton (Pelhřimov Core Complex, Central European Variscides)

ABSTRACT The geological map of the Klenov Pluton (Moldanubian Batholith), and its 3D visualization, brings the new findings in the context of granite emplacement within late-Variscan settings. The Klenov Pluton has a peraluminous composition resulting from partial melting of metasedimentary sources and subsequent differentiation. The new ages 327.14 ± 0.21 Ma and 327.80 ± 0.37 Ma, reflect the narrow time-span of magma emplacement and cooling. The 327 Ma Klenov Pluton was emplaced syntectonically as a ‘sheet-like’ granite intrusion at depth ca. 10 kilometers during the later stages of the Pelhřimov Core Complex exhumation (at ca. 329 to 327 Ma) associated with ∼N–S oriented compression. Gravity modelling suggests that the Klenov Pluton has an asymmetric shape where its western flank is parallel to the ∼NW moderately dipping Pelhřimov Core Complex. The eastern margin of the Pluton is shallower and has been later modified by ∼SE-side-up normal faulting (Lodhéřov Fault Zone).

What controls Sn-W mineralization in granite batholith: A case study from the world-class Dulong Sn-Zn polymetallic ore deposit of SW China

Magmatic-hydrothermal ore deposits hosted in granitic batholiths are an important source of tungsten (W) and tin (Sn). However, it remains enigmatic whether and how the Sn-W mineralization is triggered by the spatially associated batholith because the Sn-W-bearing granitic rocks are more likely to occur as small intrusions. Here we used a combination of LA-ICP-MS cassiterite U-Pb dates, published whole-rock geochemical, and high-precision exploration data to evaluate a genetic link between the world-class Dulong Sn-Zn polymetallic ore deposit and its spatially associated batholith in SW China. Our new cassiterite U-Pb dates indicate that the Sn-Zn and Sn-Cu mineralization events were temporally associated with the emplacement of concealed granites and porphyritic dykes respectively, consistent with the spatial pattern between Sn-Zn-Cu mineralized zones and granitic rocks in the Dulong ore district as revealed by the high-precision exploration data. The compiled data presented in this study show that these granitic rocks throughout the batholith are of S-type and highly fractionated affinities, and their parent magmas might experience similar degrees of fractionation. Importantly, the low Nb/Ta ratios (most < ∼5) in granites of the Dulong ore district are markedly differentiated from highly variable values (∼2.0 to ∼ 9.4) in the batholith, which may be caused by the extensive sub-solidus interaction with late magmatic fluids. Furthermore, the granites of the Dulong ore district are significantly poorer in Sn-Zn contents than those of the Laojunshan batholith, suggesting the extensive Sn-Zn-rich fluids exsolved from highly evolved granitic magmas could lead to a significant decrease in Sn-Zn contents in consolidated granites and the formation of Sn-Zn ore deposit, due to the relatively high fluid/melt partition coefficients of Sn and Zn. In conjunction with evidence from previous numerical and mineralogical models, and the spatial pattern between Sn-Zn mineralized zones and apical parts of concealed pluton in the Dulong ore district as revealed by high-precision drill data, we propose a model involving highly evolved granitic magmas that formed batholith, apical portions of which as the emanative centers of Sn-Zn-rich fluids played a crucial role in the formation of the world-class ore deposit.

Genesis of the Tiemuli skarn W–Fe deposit in the Nanling region, South China: Constrains from multi-mineral U-Pb geochronology and scheelite geochemistry

The Tiemuli scheelite-magnetite (W-Fe) deposit is located in the central part of the Nanling region, South China and contains five skarn-type ore bodies that are associated with the Tiemuli porphyritic granite. The mineralization process of this deposit can be divided into three stages: the prograde skarn, the retrograde skarn, and the quartz-sulfide-calcite stages. Zircon grains from the porphyritic granite yielded a U-Pb age of 138±1 Ma. The vesuvianite and garnet grains from the prograde skarn stage yielded a weighted mean 206Pb/238U age range of 139–136 Ma. The apatite and cassiterite from the retrograde stage exhibited a weighted mean 206Pb/238U age range from 138 to 136 Ma. These U-Pb ages suggest that the Tiemuli granite emplacement and W–Fe skarn mineralization occurred during the Early Cretaceous period. Based on the cathodoluminescence (CL) characteristics, two generations of scheelite were identified. Sch-I showed homogeneous luminescence, whereas Sch-II exhibited a complex core-mantle-rim texture. REE was incorporated into scheelite following the substitution mechanism of 3Ca2+ = (2REE)3+ + □Ca, allowing the scheelite to effectively reflect the REE features of the ore-forming fluids. The Mo concentration and Eu anomaly in the scheelite reflected fluctuation in the fluid conditions during the scheelite crystallization. While transiting from the prograde to retrograde skarn stage, the hydrothermal fluid shifted from a relatively reducing environment to a relatively oxidizing environment. The Tiemuli deposit is characterized by the reductive skarn mineralization that occurred during the Early Cretaceous lithospheric expansion. Variations in the physicochemical characteristics of the fluids and fluid-rock interaction played crucial roles in the W–Fe mineralization of the Tiemuli deposit. Thus, this deposit serves as a unique example of a W–Fe mineralization event in the Nanling region of South China during the Early Cretaceous.

Sand Mining in Uganda: Strengthening the Legal Framework to Combat Corruption and Environmental Destruction and Attract Better Foreign Investment.

Investment is vital in stirring the country’s economy and sustaining development, especially in Africa and Uganda. It also serves as a tool for technology transfer, innovation, employment, and generating revenue for individuals and the government. This article focuses on sand mining in Uganda, the level of corruption involved, the lack of a proper legal framework to govern its exploitation, and the investors’ interest in the sector. Uganda is rich in copper, gold, iron, and ore minerals. Others include sand, hardcore, gravels, aggregates, and local granite. Article 244 of the Uganda Constitution 1995 provides that ‘building substances such as clay, murram, sand or any stone commonly used for building’ are not regarded as minerals. However, the parliament hardly enacted legislation to govern these excluded minerals. Currently, this is an area that is attracting foreign investment. At the same time, it is unregulated. The paper will discuss, among others, the importance of having a proper legal framework to regulate building materials such as sand, why it is important, and how the lack of a proper legal framework to govern the sector might cost the government of Uganda sums of money.

Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)

Abstract Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event.

Lower Cretaceous (Aptian-Albian) Bisbee Group, Arizpe area, northern Sonora, Mexico: Integrated biostratigraphy, age and provenance from U-Pb and Hf isotopes

New data on the stratigraphy, biostratigraphy, detrital zircon U-Pb, and Hf isotope geochronology of a Lower Cretaceous succession of the Bisbee Group in northern Sonora, Mexico that accumulated in the Altar-Cucurpe sub-basin help to constrain ages of their formations and documents the detrital provenance. These tectonically shortened strata, studied in the Arizpe area, crop out in the Cerro La Ceja block (CCB) composed of the upper Morita Formation, the complete Mural Limestone, and Cintura Formations, and the lower part of La Juana Formation. The Sierra Los Azulitos block (SAB), composed of the Mural Limestone, is thrust eastward over the CCB. A measured section of the Mural Limestone in the SAB block includes the upper Tuape Shale, Los Coyotes, Cerro La Puerta, and part of the Cerro La Espina members whose age is constrained as upper Aptian to lower Albian based on benthic and planktonic foraminifera, bivalves and echinoderms. The age of the boundary between the Morita and Tuape Shale Member is constrained by detrital zircon maximum depositional ages (MDA) of 115.8 ± 1.1 Ma and 113.9 ± 0.8 Ma, respectively. The MDA of the upper Mural is 111.6 ± 1.6 Ma and the MDA of basal Cintura Formation is 108.8 ± 1.1 Ma. The La Juana Formation has upper Albian bivalves and echinoderms.&#x0D; About one-quarter of the detrital zircon grains dated from the five sandstone samples have Proterozoic ages with age peaks that clearly indicate provenance from basement rocks of southwestern Laurentia. More than two-thirds of the grains are of Jurassic and Early Cretaceous age in about the same proportions, and have main peaks at 166, 150, and 118 Ma. The Jurassic grains may have sources in rocks of the Jurassic Cordilleran continental magmatic arc of southwestern North America, and the most probable sources for the Cretaceous grains are arcs of the Peninsular Ranges batholith that by that time were located adjacent to coastal Sonora. Jurassic and the Early Cretaceous zircons dated from 195.3 to 106.8 Ma both have mixed εHf(t) values that range from 9.53 to -21.28, and TDM model ages between 0.4 to 0.8 Ga for the primitive grains, and 0.8 to 1.3 Ga for the more evolved zircons. The probable source for the Jurassic grains with negative εHf values might be local granites of that age in northern Sonora that have Neoproterozoic TDM model ages. In contrast, sources for the Jurassic zircon with positive Hf values may be Jurassic igneous and metaigneous rocks of the Peninsular Ranges Batholith of the Californias, although data reported are scarce. Similarly, the Early Cretaceous detrital zircon grains with mixed positive and negative εHf values may have provenance from igneous rocks of the Peninsular Ranges Batholith-Sierra Nevada magmatic arc.

Petrogenesis and rare-metal mineralization of the Ririwai alkaline granitoids, north-central Nigeria: mineralogical and geochemical constraints

ABSTRACT The Ririwai anorogenic alkaline complex harbours extensive rare metal (Nb-Zr-Sn-Zn) mineralization. However, uncertainty still surrounds their origin and magmatic-hydrothermal processes responsible for their rare metal mineralization. The complex is composed of peralkaline arfvedsonite ± albite-bearing granites, mildly peralkaline fayalite-bearing granite porphyries, and weakly peraluminous biotite-bearing granites, all of which show diagnostic A1-type characteristics. Zircon U-Pb dating reveals emplacement ages between 181.4 ± 0.81 and 188.6 ± 7 Ma for the three rock units. Sm-Nd and Pb isotopes (εNd (t) = –0.6 to −5.4, 206Pb/204Pb = 17.68–17.93) constrained the Ririwai A-type granites to represent highly fractionated products of enriched mantle-derived magmas. Mass-balance modelling suggests that magma evolution was fuelled by extensive fractional crystallization of 50 ~ 60 vol.% K-feldspar, 30 ~ 35 vol% fayalite, 5 ~ 10 vol.% amphibole, and ~ 5 vol.% apatite under reduced and water-undersaturated (ΔFMQ = –1.55; logfo2 = –18.63; H2O = <2.0 wt.%) conditions and assimilation of ~ 50 vol.% crust. Whole rock and amphibole compositions revealed an increasing degree of fractionation from the fayalite-bearing granite porphyries through biotite-bearing granites to the arfvedsonite ± albite-bearing granites. This extended fractionation at the late stage of magma differentiation, facilitated by fluorine dissolution and concentration of excess HFSE and REE as alkali-fluorocomplexes in the fluorine-rich silicate melt, allowed the accumulation of economic rare metal pyrochlore and columbite mineralizations in the highly fractionated arfvedsonite± albite- and biotite-bearing granites, respectively. Nonetheless, the metasomatic reaction rim textures around the magmatic pyrochlore, coupled with the M-type lanthanide tetrad effect (TE1,3 >1.10) indicate the subsequent effect of hydrothermal processes. Tectonically, reactivated shear fractures and transcurrent faults following lithospheric stresses in a transtensional regime allowed magma uprising and emplacement of A-type granites in the Ririwai complex.

Geochemical evidence for evolving Proterozoic crustal thickness and orogenic styles in southwestern Laurentia

It has long been challenging for researchers to track the crustal thickness and mode(s) of crustal modification in ancient convergent margins, limiting evaluation of the tectonic styles and processes that modify continental crust during orogenesis. We present trace element igneous geochemical crustal thickness proxies that quantitatively track the crustal thickness evolution of the long-lived Proterozoic active margin in the southwestern U.S.A. We integrate these results with geobarometric data to constrain the mode of crustal modification. The data indicate a complex record of crustal thickness change in space and time and evolving orogenic styles. Geochemical proxies at 1.84–1.72 Ga are consistent with 20–40 km thick magmatic arcs that were locally thickened to ∼50 km during ∼1.75 Ga tectonism. During the Yavapai orogeny, 1.72–1.69 Ga, a ∼200-km-wide belt of 50-60 km thick crust extended from southern California to northern Colorado and was rapidly thinned and exhumed by ∼1.68 Ga. Crustal thickening and thinning during the Yavapai orogeny largely occurred by shortening and exhumation, respectively, in the upper 25 km of the crust. Subsequent 1.68–1.60 Ga tectonism involved crustal growth, local crustal thickening, and low-P, high-T metamorphism, consistent with extensional accretionary orogenesis. The 1.47–1.37 Ga Picuris orogeny was associated with 50–60 km thick crust across much of the Southwest and involved crustal shortening with ∼10 km of magmatic underplating. Advective heat from the emplacement of ferroan granites in the mid-crust likely contributed to elevated geothermal gradients and rheologically weakened the crust. Our results suggest evolving orogenic styles in the Southwest from 1.75–1.69 Ga short-lived crustal thickening associated with terrane accretion to 1.69–1.60 Ga largely extensional accretionary orogenesis, and regional, long-lived crustal thickening at 1.47–1.37 Ga involving extensive basaltic underplating. Contrasting with some recent hypotheses, our data document a complex middle Proterozoic record for the Southwest that was not orogenically quiescent or tectonically stagnant but involved complex mountain building styles.

Diapirs of crystal-rich slurry explain granite emplacement temperature and duration

The mechanism, temperature, and timescale of granite intrusion remain controversial, with wide-ranging implications for understanding continental growth, differentiation, rheology, and deformation dynamics. In this paper we present a method for determining intrusion emplacement temperature and timescale using the characteristics of the surrounding metamorphic aureole, and apply it to the Skiddaw granite in northern England. The estimated emplacement timescale (0.1–2 Myr) implies magma transport velocities of 1–100 mm/year. At the absent or low melt fractions relevant to our estimated emplacement temperature (580–650 ^{circ }C), such velocities are incompatible with pluton formation by successive injections through dykes. Instead, our results indicate the intrusion of a diapir of crystal-rich slurry, solidifying before emplacement, with a rheology governed by the solid crystals. The emplacement depth is likely to be governed by the depth-dependent rheology of the surrounding rocks, occurring close to the brittle-ductile transition. The wider implications of our results relate to (1) the appreciation that much of the chemical and textural characteristics of plutons may relate to pre-emplacement crystallisation at depth, passively transported to higher crustal levels, and (2) an explanation of the difficulty of seismically imaging active plutonism.