Age Of Intrusion Research Articles

Distribution pattern of age and geochemistry of 2.18–2.14 Ga I- and A-type granites and their implication for the tectonics of the Liao-Ji belt in the North China Craton

Determining the petrogenesis and tectonic environment of the 2.18–2.14 Ga Liaoji granites is important to understand the Paleoproterozoic tectonic evolution of the Liao-Ji belt in the North China Craton. The Muniuhe granitic pluton is one representative of the Liaoji granites; it is located in the northwestern part of the Liao-Ji belt and is composed of syenogranite and granodiorite that intruded at ~2.18 Ga and were metamorphosed at ~1.88 Ga. Geochemically, the Muniuhe granitic pluton shows a magnesian and weakly peraluminous character, with low REEs, 10,000 × Ga/Al and Zr + Nb + Ce + Y values, and negative Nb and Ta anomalies, typical of arc-related I-type granite. It was formed by partial melting of Archean crustal material at low temperatures in oxygenated and hydrous conditions and then underwent crystallization differentiation of feldspar.The results of this study, together with those of previous studies investigating the Liaoji granites, indicate that the formation of the 2.18–2.14 Ga Liaoji granites (including the Muniuhe granitic pluton) can be subdivided into three stages: 1) the formation of ~2.18 Ga granites with A2-type characteristics (the Mafeng-Longchang, Jiguanshan, and Laoheishan-Yongdian granitic plutons), and ~2.18 Ga granites with arc-related I-type characteristics (the Muniuhe granitic pluton) in the northwestern and southeastern parts of the Liao-Ji belt; 2) the formation of ~2.16 Ga granites with arc-related I- and A2-type characteristics (Hupiyu-Hadabei, Simenzi-Gujiabao, and Qianzhuogou granitic plutons) in the central part with a NE-trending distribution; and 3) the formation of ~2.14 Ga granites (the Dafangshen granitic pluton) with A2-type characteristics in the central part. The intrusion ages show a trend to become younger from the margins to the center and the geochemical characteristics of these plutons indicate that they formed during the extension of a back-arc basin related to partial melting of the Archean continental crust. Together with the Liaohe Group and 2.13–2.11 Ga mafic intrusions, the Liaoji granites indicate that the existence of a back-arc basin extension during 2.18–2.11 Ga in the Liao-Ji belt.

Genesis of the oxidized Sn ores in the Gejiu district, Yunnan Province, SW China

The Gejiu Sn ore district, one of the largest Sn producing area (300Mt @1 wt% Sn) in the world, is located in the western margin of the Cathaysia Block, SW China. There are five types of Sn ores in the Gejiu district, including greisen, skarn, veined tourmaline, massive sulfide and oxidized. Among these types of Sn ores, the genesis of the oxidized Sn ores is most controversial, such as magmatic hydrothermal origin and sedimentary exhalative (SEDEX) or hot water deposition (Red Sea type). This paper reports U-Pb ages, fluid inclusions, trace elements and H-O isotopes of cassiterite from the oxidized Sn ores, aiming to solve this issue. LA-MC-ICPMS cassiterite U-Pb dating yielded an age of 86.7 ± 2.8 Ma, which is accordant with the formation ages (~86 Ma) of granitic intrusions in the Gejiu district. Two types of fluid inclusions in cassiterite from the oxidized Sn ores were observed, one type is CO2-rich inclusions (Type 1) and the other type is aqueous inclusions (Type 2). The homogenization temperatures range from 220 to 366 °C and the salinities range from 10.9 to 21 wt% NaCl equiv. for the two types of fluid inclusions. Furthermore, fluid inclusions in cassiterite from the oxidized Sn ores located at 300 m far away from granitic intrusions have higher temperatures (357–366 °C) than those located at 500 m (273–330 °C). Hence, the ore-forming temperatures tend to decrease with the distance far away from the magmatic hydrothermal center. The vapor components for the two types of fluid inclusions are CO2 and H2O, respectively, and the liquid phase compositions of them are all H2O. The CO2 in the ore-forming fluids may source from marble by decarbonization. This indicates that the precipitation of cassiterite is resulted from fluid immiscible. The δ18O values of fluid inclusions in cassiterite range from +6.1 to +7.7‰, and the δD values range from −135 to −153‰, indicating a mixture of magmatic and meteoric waters. Trace elements show that cassiterite is rich in In, Rb and U, but Ba, Nb and Ti are depleted. In addition, the lower Sr (0.4–2.1 ppm) and higher Sc (35.9–53.7 ppm) contents reflect that the formation of cassiterite is related to F-rich intermediate-acid magma. Cassiterite has a relatively low total REE contents (2.7–9.2 ppm) with negative Eu (δEu = 0.14–0.76) and Ce (δCe = 0.1–0.6) anomalies, indicating that the ore-forming environment is oxidized. This is consistent with the cassiterite precipitation conditions. Hence, all the evidence supports that the oxidized Sn ores are genetically associated with the Gejiu granitic magmatism.

Structural anatomy of the early Paleozoic Laohushan ophiolite and subduction complex: Implications for accretionary tectonics of the Proto-Tethyan North Qilian orogenic belt, northeastern Tibet

Abstract Recognition of accretionary tectonics in ancient orogenic collages is important for reconstructing the long-term subduction, accretion, and erosional history of fossil convergent margins, and for understanding crustal growth and supercontinent assembly. The North Qilian orogenic belt (NQOB), located between the Alxa block and the Central Qilian–Qaidam block in northeastern Tibet, is a typical Phanerozoic accretionary-to-collisional orogenic belt that represents the termination of the northern branch of the Proto-Tethys Ocean. It contains two subparallel ophiolitic belts, arcs, and subduction complexes; the ophiolitic rocks in the northern belt have generally been considered to have formed in a back-arc setting. However, the subduction-accretion-collision history, subduction polarity, and timing of closure of the back-arc ocean remain equivocal. To address these problems, we conducted detailed field, structural, and geochronological investigations of the Laohushan ophiolite–accretionary complex and related sedimentary rocks in the eastern NQOB. The Laohushan Complex is divisible into (1) a northern sedimentary forearc, and a supra-subduction zone-type ultramafic-mafic forearc (ca. 450 Ma) composed of serpentinized harzburgite, gabbro, basalt, and plagiogranite; and (2) a southern accretionary complex, which consists of relatively coherent basalt-chert-mudstone ocean plate stratigraphy that is structurally repeated many times, trench-fill turbidites, mélanges, and widespread thrust imbricates and duplexes, block-in-matrix and asymmetric structures. Kinematic analysis indicates that the accretionary complex underwent southward thrusting and shearing; coupled with the spatial architecture of the different tectonic units, which suggests northward subduction beneath the northern forearc on the southern margin of the Alxa block. Detrital zircon ages of forearc clastic sandstones, pelagic mudstones, trench-fill turbidites, and the matrix of mélanges, together with the zircon ages of igneous ophiolitic rocks and post-accretionary intrusions, indicate that the terminal accretion and tectonic stacking of the Laohushan subduction complex was between ca. 447 and 430 Ma. We propose a geodynamic model involving back-arc basin opening (ca. 517–449 Ma), intra-oceanic subduction-accretion (ca. 449–430 Ma), and final obduction of the northern forearc to account for the evolutionary processes of the North Qilian back-arc basin. The anatomy of the forearc ophiolite and structurally lower accretionary complex indicates the complicated origins and mechanism of emplacement of the ophiolitic rocks. Field-based reconstruction of accretionary complexes and upper plate ophiolites, together with provenance analysis of related sedimentary sequences, provide crucial constraints on the prolonged evolution of paleo-ocean basins and accretionary-to-collisional orogens.

HYPOGENE ALUNITE FROM THE EL SALVADOR DISTRICT, CHILE, INDICATES POTENTIAL FOR A BLIND PORPHYRY COPPER CENTER

AbstractSurface samples of hypogene alunite that cement late breccia bodies from the El Salvador porphyry copper district of Chile were recently dated. One alunite sample over the principal Turquoise Gulch porphyry deposit has a 40Ar/39Ar total gas age of 40.64 ± 1.04 Ma, overlapping the age of a late latite intrusion. Two other samples associated with quartz-alunite replacement of rhyolite, ~750 m southwest of the collapse zone over the block cave of the porphyry copper deposit, are distinctly younger, at 38.12 ± 0.66 and 38.04 ± 0.22 Ma (averages of duplicate analyses, with ±2σ errors).Previously reported U/Pb ages of zircons from 15 Eocene-age diorite, granodiorite, and granite porphyry intrusions have weighted mean ages that range from about 44 to 41 Ma, with peak magmatic flux interpreted at 44 to 43 Ma. Porphyry copper ores in the El Salvador district formed at about the same time as porphyry intrusions, with intrusive centers that migrated in a south-southwest direction, from the small deposits at Cerro Pelado (~44.2 Ma), to Old Camp (~43.6 Ma) and M Gulch-Copper Hill (~43.5–43.1 Ma), to the main ore deposit at Turquoise Gulch (~42 Ma). The granodiorite porphyry intrusions at Turquoise Gulch are associated with ~80% of the known copper ore of the district; they record waning stages of magmatism at 42.5 to 42.0 Ma, followed by weakly altered latite dikes at 41.6 Ma. Molybdenite in quartz veins returned Re-Os ages of 41.8 to 41.2 Ma.The two alunite samples from our study with coincident dates of ~38 Ma provide evidence for magmatic-hydrothermal activity younger than any recognized to date, consistent with the alteration overprint of quartz-alunite on older muscovite after erosion. This younger activity must have been associated with a blind intrusion, likely located south of the Turquoise Gulch deposit, based on the distribution of alteration minerals, and offset from the zoning associated with the Turquoise Gulch center. Stable isotope values (δ34S, δ18O, δD) of the ~38 Ma alunite indicate a high-temperature hypogene origin, consistent with formation in a lithocap environment that typically is located at shallow levels over and on the shoulders of porphyry copper deposits. Both observations—alteration overprint and markedly younger age of alunite—indicate the potential for porphyry copper mineralization south of Granite Gulch, as much as 1,000 m below the level of the coeval outcropping quartz-alunite replacement, perhaps near ~2,000-m elevation; this is hundreds of meters deeper than the known copper ore of Turquoise Gulch.

Geochemistry and geochronology of Carboniferous magmatic rocks in the Sawur Mountains, northern West Junggar, NW China: implications for accretionary orogeny

The accretionary orogeny of the Central Asian Orogenic Belt (CAOB) has been a popular topic over the past decades due to its complex tectonic process. The Carboniferous magmatism in the Sawur Mountains of the northern West Junggar plays an important role in understanding the accretionary process of West Junggar. Detailed geological mapping has revealed that the Carboniferous Heishantou Formation in the Sawur Mountains consists mainly of pillow lavas, basaltic andesites, dacites, tuffs and sedimentary rocks, which were intruded by the Lasite intrusion. The pillow lavas display low SiO2 and TiO2 as well as high MgO and FeOt contents and enrichment in light rare earth elements (LREEs) and Nb–Ta, suggesting derivation from an enriched lithospheric mantle in an oceanic island setting. With the oceanic crust subduction, they were scraped off and accreted in a fore-arc setting. Basaltic andesites exhibit high SiO2 and Al2O3 contents, low Mg#, Zr, and Nb contents as well as Nb/La ratios, and a clear depletion of Nb–Ta, indicating derivation from a mantle wedge affected by the inputs of slab-derived fluids in an island arc setting. Dacites possess calc-alkaline and adakitic geochemical features with high Sr and Ba contents, high Th/Yb ratios, enrichment in LREEs, low Nb/La ratios, a depletion of Nb–Ta and positive eHf(t) values (10.3–15.0). Accordingly, they are interpreted as being derived from the melting of subducted oceanic crust mixed with minor sediments in an island arc setting. Magmatic zircons from dacite yield a weighted 206Pb/238U mean age of 326.8 ± 1.6 Ma, constraining the crystallization age of the dacite as well as the timing of the oceanic crust subduction. The Lasite intrusion that intruded the Heishantou Formation is K-feldspar granite, exhibiting pronounced I-type affinity and a high potassic calc-alkaline series. This granite shows low Na2O and Mg# (36.9–40.3) values; low Zr and Y contents as well as 10,000 × Ga/Al ratios; high LREE, K and Pb contents; and a depletion of Nb and Ta. Altogether, the low (87Sr/86Sr)i (0.7037–0.7043) and positive eNd(t) (6.22–6.36) and eHf(t) (10.1–15.5) values suggest that the granites were derived from the partial melting of a juvenile lower crust affected by mantle materials in a subduction setting. The zircon weighted 206Pb/238U mean age of 316.6 ± 2.8 Ma represents the intrusive age of the granite and related subduction during the late Carboniferous. All of the above geochemistry and geochronology of the magmatic rocks constrain the subduction–accretion tectonics in the Sawur Mountains of the northern West Junggar during ~ 327–317 Ma and confirm the subduction of the Paleo-Asian Ocean (PAO) lasted until the late Carboniferous in this region.

Petrogenesis of paleoproterozoic (2.02–1.96 Ga) metagranitoids in the southwestern Yeongnam Massif, Korean Peninsula, and their significance for the tectonic history of northeast Asia: Insights from zircon U–Pb–Hf isotope and whole-rock geochemical compositions

The Jangsu-Gokseong area is located in the southwestern part of the Yeongnam Massif and mainly consists of Precambrian metagranitoids (metagranite, metaleucogranite and metagranodiorite) with minor migmatite and marble. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating shows that the metagranitoids emplaced during ca. 2.02–1.96 Ga and metamorphosed at ca. 1.89–1.87 Ga. The metagranitoids show calc-alkaline trends, enrichment in large lithophile elements and negative Nb and Ta anomalies with low Sr/Y (<12), Nb/Th (<1) and Nb/Zr (<0.1) ratios. The metagranitoids have negative εHf(t) values (−1.63 to −6.41) with a Paleoarchean two-stage model age (TDM2 = 3.25 Ga). These data suggest that the protoliths of the metagranitoids were derived from ancient crustal materials with metagraywacke or metatonalitic compositions and formed in a subduction-related setting. The intrusion ages and geochemical composition of the metagranitoids in the study area match well with those of Paleoproterozoic metagranitoids distributed along the northern margin of the central to northeastern Yeongnam Massif. These metagranitoids show that Paleoproterozoic (ca. 2.02–1.96 Ga) subduction-related magmatism occurred widely along the northern margin of the Yeongnam Massif, which has not been reported from the Gyeonggi and Nangrim massifs on the Korean Peninsula and the Jiao-Liao-Ji belt in the eastern North China Craton. Therefore, the Yeongnam Massif might have existed as a separate crustal unit at least during the Paleoproterozoic.

Discovery of Paleozoic rocks at northern margin of Sambagawa terrane, eastern Kyushu, Japan: Petrogenesis, U–Pb geochronology and its tectonic implication

Ordovician diorite-quartz diorite mylonite (Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula, Kyushu Island, Japan. The LA-ICP-MS zircon U–Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3 ​± ​3.6 ​Ma. These rocks show the volcanic arc affinity based on the trace element composition. On the basis of geochronological and geochemical results, Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone. Saganoseki quartz diorite is located just south of the Median Tectonic Line (MTL) and is in close contact with the pelitic and psammitic schist without any brittle shear zone. U–Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous, indicate that these psammitic schists are the member of Sambagawa metamorphic rocks. Together with these results and the mode of occurrence in the field, we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane. This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia, and its evolution through time.

Genesis of LREE-enriched zircons and their highly radiogenic Hf compositions: A case study from Zhuopan alkaline complex in western Yunnan

卓潘碱性杂岩体位于思茅地块的西缘，在岩体的霞石正长岩、正长岩、辉石岩中发现多种具不同CL发光特征的锆石。其中CL图下呈灰白色的锆石（Ⅰ类）的Th、U含量低，轻稀土元素亏损，重稀土元素富集，具典型碱性岩岩浆锆石特征，U-Pb年龄约35.7Ma，代表岩体的成岩年龄；不发光或边部具微弱环带的锆石（Ⅱ类）其Th、U、稀土元素含量高，U-Pb年龄为34.2~35.1Ma；呈杂乱海绵状结构的锆石内部不透明，无法获得谐和的U-Pb年龄；震荡环带发育的锆石为捕获锆石，具有较老的U-Pb年龄。本文依据稀土元素含量与配分模式将Ⅱ类锆石进一步分为三种不同类型：A型锆石轻稀土含量低，有明显左倾特征，其LREE<1170×10^-6，（La/Gd）_N<0.09，（Tm/Gd）_N>2.2；B型锆石轻稀土亏损但中-重稀土分馏较小，其（La/Gd）_N<0.009，（Tm/Gd）_N<2；C型锆石轻稀土元素含量明显升高且中-重稀土分馏较小，其LREE>1150×10^-6，（Tm/Gd）_N<2。这些异常的稀土元素特征并非由分析到磷灰石或榍石等矿物包裹体导致，而是与热液作用过程中流体成分与反应条件有关。由于晶格损伤导致放射性Pb丢失，热液锆石的年龄略小于岩浆锆石，没有明确的地质意义。尽管在本文中两类不同CL发光特征的锆石的年龄相差不大，但不加区分地计算平均年龄可能无法获得准确的成岩时代。卓潘碱性岩体热液锆石的Hf同位素变化范围极大（ε_Hf（t）=0.1~100），这种异常的Hf同位素特征可能是由于流体与围岩反应过程中溶解了富Lu矿物（如磷灰石）或具高¹⁷⁶Hf/¹⁷⁷Hf值的矿物（如石榴子石）所导致的。;The Zhuopan alkaline complex is located at the western Simao block. From nepheline syenite, syenite, and pyroxenite in this intrusion, two different types of zircons are recognized according to their CL images. Zircons with bright CL images (type Ⅰ) have low Th and U abundances, depleted LREE and enriched HREE, which is typical for magmatic zircons, and they give a weighted mean ²³⁶Pb/²³⁸U age of 35.7Ma, representing the intrusive age. In contrast, grains that show dark CL images with weakly oscillatory zoning in the outer rims occasionally (type Ⅱ) contain comparatively higher Th, U and sum REE concentrations and yield the U-Pb ages of 34.2~35.1Ma. Opaque zircons display spongy internal textures and failed to obtain concordia U-Pb ages. Moreover, some inherited zircons with clear oscillatory zoning in CL images can also be observed. The chondrite-normalized REE patterns of type Ⅱ zircons in this study can be further divided into three groups: type A zircons show LREE-depleted pattern with LREE<1170×10^-6,(La/Gd)_N<0.09 and (Tm/Gd)_N>2.2; type B zircons have similar increasing slope from La to Sm but no distinct fractionation between MREE and HREE, and they have (La/Gd)_N<0.009 and (Tm/Gd)_N<2; and type C zircons are characterized by the flat REE patterns and elevated LREE concentrations, with LREE>1150×10^-6 and (Tm/Gd)_N<2. These differences of REE characteristics are result from various compositions of hydrothermal fluids and alteration conditions, rather than involvement of mineral inclusions such as apatite and titanite. Hydrothermal zircons are younger than magmatic ones because of radiogenic Pb loss caused by radiation damage. Therefore, average U-Pb ages calculated from unfiltered zircon data may not give an accurate intrusive age, although the impact is limited in this study. A wide range of ε_Hf(t) (0.1~100) are observed in Zhuopan hydrothermal zircons. The extremely radiogenic Hf compositions can be generated by dissolution of Lu-rich (such as apatite) or high ¹⁷⁶Hf/¹⁷⁷Hf minerals (such as garnet).

Calcic Garnets as a Source of Information on the Age of Ultramafic Alkaline Intrusions in the Kola Magmatic Province

The paper presents data of U–Pb (ID-TIMS) geochronologic studies of schorlomite–morimotoite–andradite garnet from the Vuoriyarvi, Sallanlatva, and Salmagorsky massifs in the Kola alkaline province. The garnet is relatively rich in U (5–9 ppm) and poor in common Pb (Pbc/Pbt = 0.2–0.7), contains U as an isomorphous component, and does not host inclusions of any U-bearing minerals. The U–Pb ages of garnet from the various rocks of the massifs (373 ± 2 to 377 ± 1 Ma) are mutually consistent and correspond to the main pulse of alkaline ultramafic magmatism in the Kola province (360–380 Ma). The geochronologic data indicate that the U–Pb system of schorlomite–morimotoite–andradite garnet is stable and can be utilized as a reliable geochronometer for alkaline ultramafic rocks.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

The Chemical Evolution from Older (323–318 Ma) towards Younger Highly Evolved Tin Granites (315–314 Ma)—Sources and Metal Enrichment in Variscan Granites of the Western Erzgebirge (Central European Variscides, Germany)

The sources and critical enrichment processes for granite related tin ores are still not well understood. The Erzgebirge represents one of the classical regions for tin mineralization. We investigated the four largest plutons from the Western Erzgebirge (Germany) for the geochemistry of bulk rocks and autocrystic zircons and relate this information to their intrusion ages. The source rocks of the Variscan granites were identified as high-grade metamorphic rocks based on the comparison of Hf-O isotope data on zircons, the abundance of xenocrystic zircon ages as well as Nd and Hf model ages. Among these rocks, restite is the most likely candidate for later Variscan melts. Based on the evolution with time, we could reconstruct enrichment factors for tin and tungsten starting from the protoliths (575 Ma) that were later converted to high-grade metamorphic rocks (340 Ma) and served as sources for the older biotite granites (323–318 Ma) and the tin granites (315–314 Ma). This evolution involved a continuous enrichment of both tin and tungsten with an enrichment factor of ~15 for tin and ~7 for tungsten compared to the upper continental crust (UCC). Ore level concentrations (&gt;10–100 times enrichment) were achieved only in the greisen bodies and dykes by subsequent hydrothermal processes.

40Ar/39Ar geochronological evidence for multiple magmatic events during the emplacement of Tapira alkaline-carbonatite complex, Minas Gerais, Brazil

The Alto Parnaíba Igneous Province (APIP) is a voluminous magmatic province composed of various alkaline-carbonatite complexes emplaced in the Brasilia Mobile Belt during the Cretaceous. Relative timing of emplacement of silicate and carbonate magmas in most of these complexes remains mostly unresolved due to conflicting geochronological results. To determine the duration of magmatism and to test the possible existence of multiple magmatic events, we employ 40Ar/39Ar phlogopite single crystal dating to determine the history of magma emplacement at the Tapira alkaline-carbonatite complex, Minas Gerais, Brazil. The new single crystal data indicate at least two magmatic events during the emplacement of this complex, the first at > 96.2 ± 0.8 Ma and the second at 79.15 ± 0.6 Ma. The first igneous event was responsible for emplacement of the silicate plutonic series, while the second event corresponds to the emplacement of primarily carbonatitic magmas, generating metasomatic phlogopite alteration in bebedourites. The ages of intrusion and cooling of the alkaline-carbonatite complexes in the APIP must be investigated in other complexes to determine if intrusion intervals of ~17 Ma or more are common regionally. Protracted intrusive events, if related to magma generation by passage of South America over a stationary Trindade plume, requires complex ponding and lateral magma flow below a slow-moving continent.

Geochemistry and origin of zircon in chlorite schists of the Ronda peridotites (Betic Cordilleras, southern Spain)

AbstractThis work deals with scarce chlorite schists scattered through the Ronda peridotites (Betic Cordilleras, Spain). These schists have unusually high zircon contents, which contrast with the usual lack of this mineral in ultramafic rocks. From field data and detailed petrographic, geochemical, and geothermometric studies, we focused on the origin of the zircon, a relevant issue for the interpretation of geochronological results. The chlorite schists appear as concordant sheets with granite dikes and as blackwall zones between dikes and serpentinized peridotites. As the intrusion age of the dikes and chlorite schist zircon crystallization (ca. 22 Ma) is slightly older than the age of serpentinization and related chlorite schist formation (ca. 19 Ma), we propose that the chlorite schists are tied to the intrusion of the granite dikes and the subsequent serpentinization of peridotites. Trace and rare earth elements alone are not indicative of the magmatic or hydrothermal origin of the zircon, but the combination of information about zircon morphology, melt inclusions, geothermometry, and the structural relationships between granite dikes and chlorite schists points to late magmatic melts for the zircon origin. We suggest that high-temperature melts saturated in F and Cl acted as Zr carriers under low-pH conditions. A change of the pH conditions, due to hydrothermal alkaline fluids incoming for the concomitant peridotite serpentinization, would have led to zircon crystallization and concentration at the apical zones of the dikes, and to rodingitization before the extensive observed chloritization.

Multiple Porphyry Cu-Mo Events in the Eastern Pontides Metallogenic Belt, Turkey: From Early Cretaceous Subduction to Eocene Postcollision Evolution

AbstractFour porphyry Cu-Mo systems were investigated by Re-Os molybdenite geochronology to constrain their timing with respect to the geodynamic and magmatic evolution of the eastern Pontides, Turkey. Molybdenite from the Ispir-Ulutaş deposit yielded an Re-Os age of 131.0 ± 0.7 Ma, which is consistent with Early Cretaceous U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of local calc-alkaline intrusions. It demonstrates that porphyry deposits were already formed during Early Cretaceous subduction of the Neotethys along the eastern Pontides, and that they can be correlated with porphyry Cu events in the adjacent Lesser Caucasus. Molybdenite Re-Os ages of 76.0 ± 0.4 and 75.7 ± 0.4 Ma at the Elbeyli prospect and 77.2 ± 1.0 Ma at the Emeksen prospect overlap with U-Pb LA-ICP-MS zircon ages of shoshonitic to high-K calc-alkaline intrusions in the region, which were emplaced during Late Cretaceous Neotethys subduction. A 50.7 ± 0.3 Ma molybdenite Re-Os age at the Güzelyayla deposit confirms porphyry Cu-Mo emplacement coeval with Eocene postcollisional, calc-alkaline adakitic magmatism of the eastern Pontides.An electron microprobe study of molybdenite samples, supplemented by data obtained during Re-Os dating, shows that the Eocene Güzelyayla deposit and the Late Cretaceous Emeksen prospect have the highest Re enrichment. Postcollisional melting of a thickened mafic lower continental crust and melting of a metasomatized lithospheric mantle with little to no interaction with upper crustal rocks may explain the Re enrichment at Güzelyayla and Emeksen, respectively.

Cathodoluminescence Microscopy of Zircon in HP- and UHP-Metamorphic Rocks: A Fundamental Technique for Assessing the Problem of Inclusions versus Pseudo-Inclusions

This paper shows how a faulty approach to the study of mineral inclusions in zircon can lead to misleading interpretations of the geological context. We present and discuss two well-documented examples. Zircon grains separated from HP metamorphic jadeitite of the Rio San Juan Complex, Dominican Republic, and from UHP pyrope quartzite of the Dora Maira Massif, northern Italy, were studied using cathodoluminescence (CL) techniques, in combination with mineral inclusion and age data. In general, zircon from both localities shows inherited magmatic core domains with oscillatory zoning and metamorphic rims. The magmatic cores of zircon from the jadeitite yield ages of 115–117 Ma and host jadeite and omphacite which are of metamorphic origin and formed at about 78 Ma. Zircon from lawsonite blueschist, representing the country rock of the jadeitite, contains domains with oscillatory zoning that are nearly identical in age to the zircon cores from the adjacent jadeitite, and also contains younger metamorphic minerals such as lawsonite, albite, phengite (Si3.68), chlorite, and omphacite. Similar observations were made on the magmatic cores of zircon from the pyrope quartzite. These are about 275 Ma in age and host pyrope, phengite (Si3.55), talc, and kyanite, all of which formed during UHP metamorphism at about 35 Ma. Zircon from the biotite-phengite-gneiss country rock (metagranite) shows oscillatory zoning and yields ages that are identical to those of the magmatic cores of zircon from pyrope quartzite, which thus reflect granitic intrusion ages. The country-rock zircon also encloses metamorphic minerals with ages of about 35 Ma. Such minerals are, for example, garnet and phengite, as well as a polymineralic assemblage of clinopyroxene+garnet+phengite+quartz, that point to formation at UHP metamorphic conditions around 40 kbar/750 °C. Based on these examples we suggest an effective approach centered on key evidence from CL studies to show that magmatic domains of zircon may actually contain pseudo-inclusions which were not entrapped during an early stage of formation, but were instead introduced during later metamorphic or metasomatic events along microcracks representing pathways for fluid influx. Cathodoluminescence microscopy is thus an excellent tool for avoiding such pitfalls by allowing distinction between true inclusions and pseudo-inclusions in zircon.

Geochronological evolution of the Pitangui greenstone belt, southern São Francisco Craton, Brazil: Constraints from U-Pb zircon age, geochemistry and field relationships

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (Dn) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (Dn+1) is associated with amphibolite facies metamorphism, while a later (Dn+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event.

Further detrital zircon evidence for peri-Gondwanan blocks in the central Appalachian Piedmont Province, USA

Evidence for exotic terranes in the central Appalachian Piedmont Province is fragmented between central Virginia, northern Maryland, and southeastern Pennsylvania. Here we present laser ablation-inductively coupled plasma mass spectrometry data from detrital zircon that support the presence of an exotic terrane in this region. U–Pb dating of detrital zircon from new samples of the Storck quartzite (central Virginia) and the Hoods Mill rocks (northern Maryland) confirms the presence of a major peak at ca. 630–610 Ma in these units. These ages are consistent with derivation from Gondwana, but not Ediacaran Laurentia. Further, modern εHf values of five of the ca. 670–580 Ma grains in these samples are inconsistent with derivation from the few plutons of this age in Ediacaran Laurentia. The Loch Raven Schist and a metasedimentary xenolith in the Wilmington Complex contain a smaller proportion of ca. 670–580 Ma grains than the Storck quartzite and the Hoods Mill rocks, but more such grains than in sediment derived from Ediacaran Laurentia, so we tentatively conclude that these two units also received sediment from Gondwana. Detrital zircon ages from the Piney Run Formation, Pleasant Grove Schist, Prettyboy Schist, and Wissahickon Formation allow sediment provenance solely in Ediacaran Laurentia. We also present new zircon spot U–Pb and Lu–Hf isotopic data from western Newfoundland plutons for comparison with these types of data from the detrital zircon. Intrusion ages of the Steel Mountain Anorthosite, Disappointment Hill Tonalite, and Round Pond Granite are 608 ± 12, 600 ± 8, and 590 ± 9 Ma, respectively. None of these units was derived entirely from the depleted mantle.

Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

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

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Geochemical and geochronological characteristics of the Um Rus granite intrusion and associated gold deposit, Eastern Desert, Egypt

The Um Rus tonalite-granodiorite intrusion (∼6 km2) occurs at the eastern end of the Neoproterozoic, ENE-trending Wadi Mubarak shear belt in the Central Eastern Desert of Egypt. Gold-bearing quartz veins hosted by the Um Rus intrusion were mined intermittently, and initially by the ancient Egyptians and until the early 1900s. The relationship between the gold mineralization, host intrusion, and regional structures has always been unclear. We present new geochemical and geochronological data that help to define the tectonic environment and age of the Um Rus intrusion. In addition, field studies are integrated with EPMA and LA-ICP-MS data for gold-associated sulfides to better understand the formation and distribution of gold mineralization.The bulk-rock geochemical data of fresh host rocks indicate a calc-alkaline, metaluminous to mildly peraluminous, I-type granite signature. Their trace element composition reflects a tectonic setting intermediate between subduction-related and within-plate environments, presumably transitional between syn- and post-collisional stages. The crystallization age of the Um Rus intrusion was determined by in situ SHRIMP 206Pb/238U and 207Pb/235U measurements on accessory monazite grains. The resultant monazite U–Pb weighted mean age (643 ± 9 Ma; MSWD = 1.8) roughly overlaps existing geochronological data for similar granitic intrusions that are confined to major shear systems and are locally associated with gold mineralization in the Central Eastern Desert (e.g., Fawakhir and Hangaliya). This age is also consistent with magmatism recognized as concomitant to transpressional tectonics (D2: ∼650 Ma) during the evolution of the Wadi Mubark belt. Formation of the gold-bearing quartz veins in NNE-SSW and N–S striking fault segments was likely linked to the change from transpressional to transtensional tectonics and terrane exhumation (D3: 620–580 Ma). The development of N–S through-going fault arrays and dike swarms (∼595 Ma) led to heterogeneous deformation and recrystallization of the mineralized quartz veins.Ore minerals in the auriferous quartz veins include ubiquitous pyrite and arsenopyrite, with less abundant pyrrhotite, chalcopyrite, sphalerite, and galena. Uncommon pentlandite, gersdorffite, and cobaltite inclusions hosted in quartz veins with meladiorite slivers are interpreted as pre-ore sulfide phases. The gold-sulfide paragenesis encompasses an early pyrite-arsenopyrite ± loellingite assemblage, a transitional pyrite-arsenopyrite assemblage, and a late pyrrhotite-chalcopyrite-sphalerite ± galena assemblage. Free-milling gold/electrum grains (10s μm-long) are scattered in extensively deformed vein quartz and in and adjacent to sulfide grains. Marcasite, malachite, and nodular goethite are authigenic alteration phases after pyrrhotite, chalcopyrite, and pyrite and arsenopyrite, respectively.A combined ore petrography, EPMA, and LA-ICP-MS study distinguishes morphological and compositional differences in the early and transitional pyrites (Py I, Py II) and arsenopyrite (Apy I, Apy II). Py I forms uncommon small euhedral inclusions in later Py II and Apy II. Py II forms large subhedral crystals with porous inner zones and massive outer zones, separated by narrow As-rich irregular mantles. The Fe and As contents in Py II are variable, and the LA-ICP-MS analysis shows erratic concentrations of Au (<1 to 177 ppm) and other trace elements (e.g., Ag, Te, and Sb) in the porous inner zones, most likely related to discrete sub-microscopic sulfide inclusions. The outer massive zones have a rather homogenous composition, with consistently lower abundances of base metals and Au (mean 1.28 ppm). The early arsenopyrite (Apy I) forms fine-grained euhedral crystals enriched in Au (mean 17.7 ppm) and many other trace elements (i.e., Ni, Co, Se, Ag, Sb, Te, Hg, and Bi). On the other hand, Apy II occurs as coarse-grained subhedral crystals with lower and less variable concentrations of Au (mean 4 ppm). Elevated concentrations of Au (max. 327 ppm) and other trace elements are measured in fragmented and aggregated pyrite and arsenopyrite grains, whereas the undeformed intact zones of the same grains are poor in all trace elements. The occurrence of gold/electrum as secondary inclusions in deformed pyrite and arsenopyrite crystals indicates that gold introduction was relatively late in the paragenesis. The LA-ICP-MS results are consistent with gold redistribution by the N–S though-going faults/dikes overprinted the earlier NNW-SSE quartz veins in the southeastern part of the intrusion, where the underground mining is concentrated. Formation of the Um Rus intrusion and gold-bearing quartz veins can be related to the evolution of the Wadi Mubarak shear belt, where the granitic intrusion formed during or just subsequent to D2 and provided dilatation spaces for gold-quartz vein deposition when deformed by D3 structures.