Plateau Ages Research Articles

<sup>40</sup>Ar/<sup>39</sup>Ar DATING OF MARUYAMAITE (K-DOMINANT TOURMALINE) FROM DIAMOND-BEARING METAMORPHIC ROCKS OF THE KOKCHETAV MASSIF

40Ar/39Ar analyses were made on maruyamaite (potassium-dominant tourmaline) from tourmaline-quartz-feldspar rocks discovered within the Kumdy-Kol microdiamond deposit (Kokchetav massif, North Kazakhstan). Turmaline yielded well-defined 40Ar/39Ar plateau age spectra whose values coincide within the error - ages of 502.3±8.0, 502.2±8.0, 506.0±8.0 Ma. These ages are much younger than the age of 530±2 Ma determined for high-pressure metamorphism by different methods. Thus, the formation of tourmaline-rich rocks of the Kumdy-Kol deposit cannot be associated with the high-pressure metamorphic events, and, therefore, testifies in favor of the low-pressure nature of maruyamaite. Based on the coincidence of age data for tourmaline crystals with different potassium contents, it can be concluded that the K/Ar system in tourmaline can be used for dating metasomatic and metamorphic processes.

Petrology, geochemistry and Ar-Ar geochronology of eclogites in Jinshajiang orogenic belt, Gonjo area, eastern Tibet and restriction on Paleo-Tethyan evolution

As one of the important Paleo-Tethys suture zones in eastern Tibet, the Jinshajiang orogenic belt is of great significance to study the tectonic evolution of the main suture zone of Paleo-Tethys. In this paper, eclogites developed in the Jinshajiang suture zone in Gonjo area, eastern Tibet, are selected as specific research objects, and petrological, geochemical and Ar-Ar geochronological analyses are carried out. The major element data of the whole rock reveals that the eclogite samples have the characteristics of picritic basalt-basalt and belong to the oceanic low potassium tholeiites. The results of rare earth elements and trace elements of the samples show that the protoliths of eclogites have characteristics similar to oceanic island basalt (OIB) or normal mid ocean ridge basalt (N-MORB). Muscovite (phengite) from two eclogite samples yield the Ar-Ar plateau ages of 247 ± 2 Ma and 248 ± 2 Ma respectively, representing the peak metamorphic age of eclogite facies and the timing of complete closure of the Jinshajiang Paleo-Tethys Ocean. Muscovite and biotite selected from the hosting rocks of eclogite yield the Ar-Ar plateau ages are 238 ± 2 Ma and 225 ± 2 Ma respectively, reflecting the exhumation age of eclogites and their hosting rocks. Combined with the zircon U-Pb dating data (244 Ma) of eclogites obtained in previous work, it can be concluded that the Jinshajiang Paleo-Tethys ocean was completely closed and arc-continent collision was initiated at about 248–244 Ma (T₂¹). Subsequently, due to the large-scale arc (continent)-collision orogeney between Deqin-Weixi continental margin arc and Zhongza block (T₃¹–T₃²), the eclogites were rapidly uplifted to the shallow crust.

Muscovite 40Ar-39Ar and cassiterite U-Pb dating of the Indosinian Aotou quartz-vein type tin deposit, southern China and its geological significance

The determination of deposit age is fundamental in economic geology, while the result is often disappointing because of the difficulties in selection of suitable minerals. In this study, on the base of detailed field investigation, we successfully combined the muscovite 40Ar-39Ar dating and cassiterite LA-ICP-MS U-Pb dating in the Aotou tin deposit, Jiangxi province, South China. The result of the muscovite 40Ar–39Ar plateau ages (228.7 ± 1.5 Ma) and the cassiterite U-Pb lower intercept ages (227.4 ± 6.9 Ma) are consistent, accurately defined that the Aotou tin deposit was formed during late Indosinian. With the gradual discovery of another Indosinian W-Sn deposit, we have taken this opportunity to discuss the prospecting index of the Indosinian granite by compiling data of relevant plutons. The result reflects that the Indosinian metallogenic plutons are mainly acidic rocks and produced in the form of small rock mass, with index minerals of muscovite and volatile-rich minerals such as tourmaline and fluorite. Geochemically, they show that the higher SiO2 content (71.51–78.14%), A/CNK (1.13–1.66), Rb/Sr (21.94 in average), but lower Nb/Ta (4.03 in average) and ΣREE (85 ppm in average), which could be useful indexes for identifying the metallogenic pluton. Furthermore, the negative correlation between SiO2 and ΣREE, K2O and Na2O can be used as important ore-prospecting indexes, as a result of they indicate the differentiation of ore-bearing hydrothermal fluids.

U–Pb and 40Ar/39Ar geochronology and tectonic setting of the peralkaline magmatism in the Borborema Province (NE Brazil)

Aegirine-augite granite and syenite plutons and dikes form a distinct magmatic pulse aligned in the NE-SW direction in the central and northern Borborema Province. We use SHRIMP zircon U–Pb and laser step-heating 40Ar/39Ar determinations to constrain the crystallization and cooling ages of syenites and alkali-feldspar granites and gabbros deformed by shear zones. Concordant zircon ages of the Triunfo and Cacimbas syenites vary between 596 ± 4 and 594 ± 4 Ma while sodic amphiboles from a granitic dike of Manaíra swarm yield 40Ar/39Ar plateau ages of 606 ± 5, 605 ± 6 and 599 ± 4 Ma. Muscovite from host mylonitic gneisses, in turn, yields plateau ages of 600 ± 3 Ma and 586 ± 2 Ma. Close to the Patos shear zone which limits the Central and Northern Borborema domains, foliated felsic dike of the Serra do Algodão suite yields a concordia age of 592 ± 2 Ma whereas the Catingueira granitic pluton records numerous inherited zircons. The Taperoá gabbro stock, situated next to the connection between E− and NE-trending shear zones, yields concordant ages of 592 ± 2 Ma. These results constrain the alkaline magmatism of the Central and Northern domains between 600 and 590 Ma, i.e., shortly after the continental collisions that amalgamated the province in the Ediacaran. Cooling was faster in the Triunfo-Manaíra suite and apparently slower close to the Patos shear zone. Host rock structures indicate that the Triunfo-Manaíra suite emplaced in the brittle upper crust, whereas the alkaline rocks of the Serra do Algodão suite emplaced in HT/LP ductile transtensional setting. A possible tectonic scenario includes slab break-off in a continental collision setting, partial melting of the sub-continental mantle and lower crust and magma transfer to the middle-upper crust. The distribution of these rocks next to the margins of volcanosedimentary belts suggests that their ascent and emplacement explored deep fault segments reactivated by crustal extension.

Gold mineralized diorite beneath the Linglong ore field, North China craton: New insights into the origin of decratonization-related gold deposits

Gold deposits in Precambrian cratons were mostly generated during the formation and stabilization of the cratons, but the North China craton is unusual in that its gold deposits were mainly formed ∼1.7 b.y. after its stabilization. A magmatic-hydrothermal origin or mantle-derived fluid source has been proposed for the giant gold deposits of the Jiaodong District in the eastern North China craton, but direct evidence is sparse, and the mineralization processes remain controversial. Here, we present the results of a comprehensive geological, geochronological, and geochemical study of the gold mineralized Xiejia diorite beneath the Linglong ore field at Jiaodong to link the gold mineralization to underlying magmatism. Magmatic zircon and titanite grains from the Xiejia diorite have laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages of 121.3 ± 0.9 Ma to 120.8 ± 1.1 Ma and 121.7 ± 3.9 Ma, respectively, which are indistinguishable from the time of gold deposition throughout the Jiaodong District as constrained by previous studies. The diorite has a shoshonitic composition and is characterized by strong enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) along with significant depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). Samples of the diorite have high initial 87Sr/86Sr ratios, but low εNd(t) and ɛHf(t) values and low Pb isotope ratios. These geochemical characteristics are akin to those of contemporaneous mafic dikes in most gold mines at Jiaodong, indicating that the Xiejia diorite was most likely derived from an enriched lithospheric mantle source. The upper part of the diorite intrusion is pervasively altered and mineralized, containing an average of 0.32 g/t Au, but locally up to 7.59 g/t. Hydrothermal titanite from the mineralized diorite has a LA-ICP-MS U-Pb age of 122.3 ± 4.3 Ma, which is consistent with the gold-bearing pyrite Re-Os isochron age of 122.5 ± 6.7 Ma. Ore-related sericite aggregates from the Dongfeng gold deposit above the Xiejia diorite have a 40Ar/39Ar plateau age of 122.6 ± 1.3 Ma. Pyrite from the mineralized diorite yielded δ34SCDT values of 2.1‰−9.7‰, which are comparable with those of pyrite (δ34SCDT = 5.8‰−8.1‰, where CDT indicates the Canyon Diablo troilite standard) from gold ores of Dongfeng. Pyrite grains from both groups also have similar Pb isotope compositions. The S and Pb isotope data are consistent with values of mafic dikes that are spatially and temporally associated with gold veins in the Linglong ore field. The results presented here thus indicate a possible genetic link between gold mineralization in the Xiejia diorite and underlying magma presumably represented by the Xiejia diorite. The auriferous fluids exsolved from that magma and migrated upward along the Potouqing fault to form the Dongfeng gold deposit above the Xiejia diorite. The mineralized diorite thus links shallow gold mineralization to deep-seated mantle-derived magmatism generated during the extensive destruction of the North China craton induced by the rollback of the subducted paleo-Pacific plate.

Characteristics of REEs and Trace Elements in Scheelite and Muscovite Ar–Ar Isotopic Dating of the Daping Tungsten Deposit

The recently discovered Daping tungsten deposit is located about 25 km north of Tongcheng County, Hubei Province, in the northern margin of the Sijiapu granite deposit of the Mufushan composite batholith. The ore body is produced in the northeast-oriented greisenization granite and quartz vein, and belongs to the greisen-vein-type scheelite deposit. The resources of the Daping W deposit have a value of 7935 t W and the average grade is 0.201% WO3. Based on mineralogical and petrographic studies, scheelite is classified into two types. A study of the geochemical characteristics of rare earth elements (REEs) and trace elements indicated that some scheelite specimens showed LREE depletion. Meanwhile, the total amount of scheelite rare earth elements (REEs) is low and the ratio of LREE/HREE ranges from 0.40~2.72 in the Daping W deposit. The contents of REEs and trace elements in the two types of scheelite differ significantly. Type I scheelite has an average ∑REE content of 195.65 ppm, an LREE/HREE value of 0.5, an Eu anomaly (δEu) of 0.78, Mo of 366.20 ppm, Sn of 22.62 ppm, and Sr of 264.80 ppm. However, type II scheelite features an average ∑REE of 111.28 ppm, an LREE/HREE ratio of 1.43, a δEu of 1.56, Mo of 188.48 ppm, Sn of 0.15 ppm, and Sr of 829.93 ppm. The content level of Mo in the two types of scheelite is not high, indicating that this whole metallogenic environment is a reduction environment. However, this is a complex process. The presence of type I scheelite with negative Eu anomalies and relatively high Mo content indicates that the ore-forming environment in the early period of the greisen stage was relatively oxidizing. In contrast, type II scheelite contains large amounts of Sr and large positive Eu anomalies, which are likely to be released from plagioclase in the granite during greisenization. The extremely low composition of Mo in type II scheelite is closely related to the reducing environment in the later period of the greisen stage. Because Mo probably exists in its Mo4+ state, it may be difficult for it to replace W6+ in the scheelite lattice. Additionally, comparing the contents of Sn and Sr in different types of scheelite shows that the metallogenic environment changes from relative oxidation to the reduction of scheelite. The variation in trace elements and REEs in scheelite over time reflects a complex magmatic–hydrothermal mineralization environment. Additionally, the Ar–Ar system dating results for muscovite that is closely associated with scheelite in the greisenization granite vein indicate that a muscovite 40Ar/39Ar plateau age of about 133 Ma represents the time of tungsten mineralization. This muscovite 40Ar/39Ar dating result is close to the previous zircon U-Pb age data of the biotite monzogranite (ca. 140–145 Ma), which is the largest intrusion in the orefield. Meanwhile, the new age data reported here confirm that the Daping tungsten deposit represents a Mesozoic magmatic–hydrothermal mineralization event with a setting of lithospheric extension in the Mufushan composite batholith.

Early Cambrian high pressure/low temperature metamorphism in the southeastern Tarim craton in response to circum-Gondwana cold subduction

The circum-Gondwana subduction initiated by the early Cambrian has been suggested to reflect the establishment of the modern plate tectonics. The metamorphic rocks with low thermobaric (T/P) ratios indicative of cold subduction in the present tectonic regime have not been well investigated. To better understand the circum-Gondwana subduction and to test its possible link with the emergence of the modern plate tectonics, this study focused on blueschist-facies metamorphic rocks in the Altyn Tagh of the southeastern Tarim craton. Mineral assemblage and chemistry, phase equilibrium modelling, and quartz-in-garnet Raman elastic geobarometry reveal that the zoisite blueschist and glaucophane (Gln)-bearing quartz schist in northern Altyn Tagh were metamorphosed to lawsonite to epidote blueschist-facies at 520–545 °C and 16–19 kbar. It reflects high-pressure (HP)/low temperature (LT) metamorphism with low T/P ratios of <300 °C/GPa and thermal gradients of <10 °C/km. These blueschist-facies metamorphic rocks underwent rapid decompression starting at P-T conditions of <495 °C and <9.6 kbar during exhumation. Ar-Ar geochronology records paragonite Ar-Ar plateau ages of 520–506 Ma for the zoisite blueschist samples and phengite Ar-Ar plateau ages of 522–516 Ma for the Gln-bearing quartz schist samples, suggesting that the peak HP/LT metamorphism occurred prior to ca. 522 Ma. Based on new results and available data from the major Gondwana blocks, cold subduction was suggested to profoundly operate along circum-Gondwana in the early Cambrian after the amalgamation of Gondwana. The extensive circum-Gondwana subduction represents the earliest global cold subduction in Earth’s history associated with the establishment of the modern plate tectonics, as directly recorded by the studied early Cambrian blueschist-facies metamorphic rocks and a dramatic drop in the mean T/P of metamorphism since the early Paleozoic.

Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

AbstractArchean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex &gt;2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite40Ar/39Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite40Ar/39Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories.

In Situ Argon Isotope Analyses of Chondrule-Forming Materials in the Allende Meteorite: A Preliminary Study for 40Ar/39Ar Dating Based on Cosmogenic 39Ar

The argon isotopic compositions of chondrule-forming minerals of the Allende (CV3) meteorite were examined to evaluate the possibility of in situ 40Ar/39Ar dating of planetary surface rocks based on cosmogenic 39Ar without neutron irradiation in a reactor. The investigated Allende meteorite sample (ME-247H: 50 mm × 45 mm × 5 mm; 28.85 g) contains at least three textural types of chondrules: barred olivine chondrule (BOC), porphyritic olivine chondrule (POC), and unclassified chondrule (UC). Most chondrules contain olivine, low-Ca pyroxene, clinopyroxene, and plagioclase as primary phases, with minor amounts of nepheline and sodalite formed during aqueous alteration of the CV3 parent body of the early solar system. In situ argon isotope analyses on selected chondrule-forming minerals in petrographic sections of two BOCs, two POCs, and one UC using a Nd:YAG pulse laser confirmed a significant amount of cosmogenic 39Ar that formed by a 39K (n, p) 39Ar reaction in an extraterrestrial environment. Laser step-heating analyses of five bulk chondrules irradiated in a reactor revealed a plateau age (3.32 ± 0.06 Ga) from one of the five chondrules. The age spectra of all chondrules show the younger age in the low-temperature fractions, resulting in the integrated ages from 2.7 to 3.2 Ga. These results suggest that the Allende meteorite experienced argon isotopic homogenization at 3.3 Ga and the argon loss in part after the 3.3 Ga. Apparent ages of chondrule-forming minerals that were calculated using the J values of nephelines in one BOC and two POCs do not show any consistent relationship among the three types of chondrules (BOC, POC, and UC). This might be attributed to the fact that the isotopic heterogeneity among minerals took place during the heterogeneous argon loss stage after the 3.3 Ga event.

Petrogenesis and 40Ar/39Ar dating of hydrothermal romanechite from the sub-aerial fault-related Romanèche Mn deposit (France)

This study discusses the hydrothermal formation of romanechite (Ba0.7Mn4.5O10.1.1H2O) in the Romanèche Mn deposit and the significance of its age by combining ore petrogenesis and 40Ar/39Ar ages. Romanechite occurs as a minor phase in various manganese deposit types and remains poorly studied in comparison with other K-bearing Mn oxides (i.e., coronadite group) because of its scarcity, small grain size, intermixing and poor crystallinity. The fault-controlled Romanèche Mn deposit (Massif Central, Eastern France) investigated herein exhibits a well-crystallized and late stage massive romanechite ore occurring in a quartz-barite-fluorite gangue. Petrographical and geochemical data support manganese ore formation under sub-aerial conditions with temperatures close to fluorite precipitation (100–200 °C) and characterized by a Mn-Ba-W-As association. Five out of the eight studied samples have produced 40Ar/39Ar plateau ages ranging from 25.2 ± 0.3 to 18.2 ± 0.5 Ma. Despite a low K content (0.1–0.6 wt% K), the good crystallinity of romanechite due to its epithermal formation likely favours a better production and/or retention of the radiogenic argon than poorly crystallized phases of supergene origin. Therefore, the dataset identifies a period of at least c. 7.6 My of discontinuous hydrothermal activity, which is consistent with graben-related fault reactivation and likely volcanic activity in the Massif Central. Ultimately, this study confirms the feasibility of romanechite 40Ar/39Ar dating.

Syn‐rift volcanism in the Barmer Basin: An intra‐basin extrusive complex at the northern limit of the Deccan volcanic province in India

The Raageshwari Volcanic Formation (RVF) is a volcanic complex in the southern part of the Barmer Basin, NW India, localized around a large rift centre horst block, the Central Basin High. The RVF comprises an initial sequence of acid igneous rocks, which are of ignimbritic origin, termed the Agni Member, overlain by a stacked sequence of basaltic lava flows interbedded with subordinate pyroclastic deposits, the Prithvi Member. Seismic data confirms that the volcanic complex has a conical overall geometry and a layered internal structure produced by successive and extensive flows of basalt and ignimbrite, very similar to that observed in the main Deccan lava outcrops along the western margin of India. U–Pb zircon ages near the top of the Agni Member in the Raageshwari‐26 well give an age of 68+/−2 Ma for a tephrite pyroclastic unit, whilst Ar–Ar analysis of a distal basalt containing phenocrysts of biotite from the Saraswati‐4 well gave an older robust Ar–Ar plateau age of 73.7 ± 1.4 Ma. A typical Deccan age of 67.9 ± 1.7 Ma was obtained from the isolated unaltered basaltic andesite in well NE‐South‐1. Due to depositional on‐lap onto the Central Basin High, the overlying alluvial Dandlawas and Fatehgarh formations are absent on the crest of the Central Basin High and Barmer Hill Formation lake sediments rest directly on the basalts. This relationship indicates that the volcanic cone was a structural or topographic high during deposition onto which the Fatehgarh and Barmer Hill formations lake sediments eventually on‐lapped. The RVF of the Barmer Basin along with the Deccan volcanics in the Cambay Basin, Narmada Rift and in Saurasthra are all developed within fault‐bounded rift basins, in which deep seated faults extend beneath the Deccan volcanic sequence. They are associated with thermal anomalies and thinned continental crust and the presence of a linear low velocity zone at ~100 km depth in the upper mantle region beneath the Barmer and Cambay basins. This is interpreted to be the position of the “conduit” through which plume head material passed through the upper mantle before arriving at the base of the Indian lithosphere. The RVF therefore appears to be an early eruption of differentiated ultrabasic magma from a shallow, secondary magma chamber produced by partial crustal melting.

Early Yanshanian metallogenesis of the Banjing tungsten deposit in Ganxian, Jiangxi province, China: Constraints from geochemistry, Sr–Nd isotopes, zircon U–Pb, molybdenite Re–Os, and mica 40Ar–39Ar dating

The Banjing tungsten deposit, which has been discovered recently, is located in the northernmost part of the Dabu pluton, southern Jiangxi Province, China. In this study, zircon LA‐ICP‐MS U–Pb dating of the fine‐grained porphyritic biotite granite indicate that it was formed in the early Yanshanian (159.4 ± 2.5 Ma–161.6 ± 0.8 Ma). Whole‐rock geochemical studies have shown the fine‐grained porphyritic biotite granite has SiO2 of 75.82%–76.84%, Al2O3 12.50%–12.87%, MgO 0.07%–0.20%, and CaO 0.64%–1.09%, with A/CNK of 1.017–1.038. It has a total rare earth elements content of 93.9–118.9 ppm with strong negative Eu anomalies (δEu = 0.021–0.081). The biotite granite is enriched in Rb, Th, U, Nd and depleted in Ba, Sr, Nb, Ti. These data suggest the Dabu pluton belongs to S‐type granite. This pluton has ISr of 0.71212–0.72929 and εNd(t) from −9.60 to −10.62 with Nd model ages concentrated in 1732–1838 Ma. These isotopic data in combination with zircon Hf isotopes suggest the biotite granite originated from partial melting of the Middle Proterozoic metamorphic supracrustal rocks in South China, with a minor contribution from juvenile crust. Meanwhile, this study has obtained Re–Os isochron ages of the intrusion‐related and quartz vein‐type molybdenum mineralization are 156.3 ± 2.7 Ma and 157.0 ± 1.1 Ma in the Banjing tungsten deposit, respectively. Sericite from the greisen‐type tungsten‐molybdenum ore has an 40Ar–39Ar plateau age of 156.4 ± 1.0 Ma, interpreted as the formation age of this deposit. Collectively, the Banjing tungsten deposit is considered as a product of the early Yanshanian large‐scale peak mineralization in South China, which was formed in an extensional intracontinental environment caused by the subduction of the Palaeo‐Pacific Plate.

Multiple Palaeozoic–Mesozoic orogenic events in the SE Yangtze Block, China: evidence from the petrogenesis and deformation of gneissic granites from the Nanwenhe metamorphic dome and regional correlation analysis

ABSTRACT The Nanwenhe metamorphic dome contains a rare Silurian granitic pluton that developed in the SE Yangtze Block during early Palaeozoic orogenesis, and this pluton comprises the Nanwenhe gneissic granites. The petrogenetic and structural deformation histories of the gneissic granites recorded the multi-stage interaction of the SE Yangtze Block with other blocks during the Palaeozoic and Mesozoic, including the Cathaysia, Indochina, and Paleo-Pacific blocks. Zircon U–Pb and Hf isotopes and geochemical data indicate that the Early Palaeozoic granites (433–420 Ma) are S-type granitic rocks, and they were derived from the partial melting of ancient continental crust with no contribution from mantle material. And the large variations and positive values of εHf(t) (−12.25 to +10.69) of the gneissic granites are ascribed to a heterogeneous source and disequilibrium melting. Here we compare early Palaeozoic granitoids in the SE Yangtze Block (foreland belt) with those in the orogenic core, and suggest that the SE Yangtze Block underwent limited syn-collisional crustal thickening and metamorphism, and following which post-collisional granitoids were formed through partial melting of continental crust. Structural analysis of the Nanwenhe gneissic granites allows two stages of deformation to be identified: northward detachment (D1) and northwestward thrusting (D2). 40Ar–39Ar isotope dating of muscovites from the gneissic granites yields a well-defined plateau age of 229.61 Ma, which is interpreted as the timing of Late Triassic structural overprinting associated with the formation of the Nanwenhe metamorphic dome. Integration of our new results with previous research findings from the dome and South China allows us to conclude that the detachment deformation (D1) represents Late Triassic post-collisional extension, and D2 represented Jurassic intracontinental orogeny associated with subduction of the Paleo-Pacific Block.

Evaluating the Magnitudes of Weathering and Potassium Metasomatism in Paleosols: Examples from Proterozoic, Cambrian, and Cretaceous Paleosols in Midcontinental Laurentia

Six Proterozoic, two Cambrian, and two Cretaceous paleosols in the Lake Superior region of midcontinental Laurentia were investigated in detail. All but the Cretaceous paleosols experienced potassium metasomatism, which resulted in the precipitation of muscovite in Proterozoic paleosols or illite and microcline in Cambrian paleosols. A comparison of the magnitude of potassium metasomatism among the paleosols is provided by depth-normalized mass flux (DNMF), where DNMF=1000×[(mass flux)/(depth of weathering)], which normalizes for different thicknesses of weathering profiles. Average DNMF values for the total addition of K2O are 0.98 ± 0.19 mol cm−3 for the Proterozoic paleosols and 1.27 ± 0.06 mol cm−3 for the Cambrian paleosols. The ages of potassium metasomatism were determined by 40Ar/39Ar isotopic dating of metasomatic muscovite in the Proterozoic McGrath, Ville Marie, and Baraboo paleosols, which yielded ages of 1742 ± 3, 1589 ± 3, and 1467 ± 11 Ma, respectively, the former being coeval with the Yavapai orogeny and the latter with the Baraboo orogeny. Metasomatic microcline in the Cambrian Trempealeau paleosol yielded a plateau age of 488.0 ± 1.0 Ma, which corresponds to the age of the Cambrian-Ordovician boundary. SiO2, CaO, and Na2O were substantially removed from the paleosols by weathering, as was K2O (before metasomatism). The average total amount of SiO2, CaO, Na2O, and K2O removed was 17.6% ± 1.9% from six Proterozoic paleosols, 28.5% ± 4.2% from two Cambrian paleosols, 36.5% ± 8.6% from two Cretaceous paleosols, and 34.2% ± 2.7% from five modern soils. The greater magnitude of weathering in the Phanerozoic weathering profiles compared with the Proterozoic ones, despite lower levels of CO2 in the Phanerozoic atmosphere, may reflect the emergence of land plants at ∼500 Ma and their profound effects on weathering.

Ages, H[sbnd]O[sbnd]C[sbnd]S[sbnd]Pb isotopes, and fluid inclusion study of the Daolundaba Cu–Sn–W–Ag deposit in Inner Mongolia, NE China

The medium-sized Daolundaba Cu–Sn–W–Ag deposit is located in the southern Great Xing'an Range (SGXR), being one of the representative deposits in this area. The vein orebodies of the deposit occur within the NE-trending fracture zones, which are developed in the Early Permian silty slate and Late Variscan biotite monzogranite. However, its mineralization age remains controversial, which affects the understanding of Sn–polymetallic mineralization in the SGXR. To determine the mineralization age and reveal the sources of fluid and ore-forming material, sericite and monazite dating, fluid inclusion microthermometry, and isotopic analysis were carried out in this study. The ore-forming process at the deposit can be divided into four stages: quartz–fluorite–muscovite–tourmaline–cassiterite–wolframite–arsenopyrite stage (stage I), quartz–fluorite–wolframite–chalcopyrite–arsenopyrite–pyrrhotite stage (stage II), quartz–fluorite–sericite–chalcopyrite–pyrite–pyrrhotite–silver mineral stage (stage III), and quartz–calcite–fluorite–pyrite stage (stage IV). The sericite sample yielded a 40Ar–39Ar plateau age of 138.8 ± 0.7 Ma and an isochronal age of 140.0 ± 1.1 Ma, and the LA-ICP-MS U–Pb age of three monazite samples was 136.0 ± 2.3–134.7 ± 2.8 Ma, indicating that the Daolundaba deposit formed in the Early Cretaceous. Liquid-rich, gas-rich, and daughter mineral-bearing inclusions have been identified in quartz, fluorite, and calcite. Homogenization temperatures and salinities of the fluid inclusions (FIs) in the stage I vary from 309 °C to 389 °C and 6.2 wt% to 46.3 wt% NaCl eqv., respectively. Homogenization temperatures and salinities of the FIs in the stage II vary from 242 °C to 339 °C and 5.3 wt% to 41.4 wt% NaCl eqv., respectively. Homogenization temperatures and salinities of the FIs in the stage III vary from 153 °C to 268 °C and 3.5 wt% to 35.4 wt% NaCl eqv., respectively. There are only liquid-rich inclusions in quartz, fluorite, and calcite of the stage IV; their homogenization temperatures and salinities vary from 114 °C to 188 °C and 2.1 wt% to 7.6 wt% NaCl eqv., respectively. The ore-forming fluids of the Daolundaba deposit are characterized by moderate–high temperature and great fluctuating salinity, belonging to an H2O–NaCl ± CO2 ± CH4 system. The δ18Owater and δD values of the stages I and II vary from 2.5 ‰ to 7.2 ‰ and − 127 ‰ to − 106 ‰, respectively; the δ18Owater and δD values of the stage III vary from − 3.0 ‰ to − 0.3 ‰ and − 126 ‰ to − 94 ‰, respectively; the δ18Owater and δD values of the stage IV vary from − 10.9 ‰ to − 4.7 ‰ and − 102 ‰ to − 81 ‰, respectively; indicating that the ore-forming fluids of the stages I and II mainly consist of magmatic water, that the ore-forming fluid in the stage III is a mixture of magmatic and meteoric water, and that the fluid in the stage IV is meteoric water. The δ13C values for calcite of the stage IV vary from − 9.8 ‰ to − 6.7 ‰, suggesting that the carbon was mainly from a magmatic source. The δ34S values range from − 7.4 ‰ to − 1.3 ‰ with an average of − 4.7 ‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of the metallic minerals are in the ranges of 18.293–18.612, 15.519–15.706, and 38.078–38.826, respectively. Both S and Pb isotopic systems indicate that the ore-forming metals primarily came from Mesozoic felsic magma. Fluid boiling and mixing were two main mechanisms for the deposition of ore-forming materials.

Repeated brittle reactivations of a pre-existing plastic shear zone: combined K–Ar and 40Ar–39Ar geochronology of the long-lived (&gt;700 Ma) Himdalen–Ørje Deformation Zone, SE Norway

AbstractBrittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the &gt;700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Petrogenesis of submarine volcanic rocks dredged from Dokdo, Ulleungdo, and the neighboring seamounts in the East Sea: Constraints from mineral chemistry, geochemistry, and 40Ar/39Ar ages

Several seamounts, including the Dokdo (DD), Ulleungdo (UD) Anyongbok (AYB), Simheungtack (SHT), and Isabu, were formed from intraplate volcanism in the southwest of the East Sea back-arc basin during the Quaternary. In the present study, mineralogy and geochemistry data generated using submarine volcanic rocks collected from the DD, UD, AYB, and SHT locations were utilized to investigate characteristics of the source, evolution, and storage conditions of the associated magmas. Analyses of representative samples from the DD, UD, and AYB using the 40 Ar/ 39 Ar method yielded plateau ages of 1.566–2.658, 0.533–0.714, and 1.736 Ma, respectively. The dredged samples consist of trachybasalt, phonotephrite, trachyandesite, and trachyte and have oceanic island basalt (OIB)-like characteristics. According to numerical modeling using trace element data, the primary magma linked to the formation of these samples can originate from ≤5% partial melting of an enriched garnet–peridotite facies metasomatized by melt derived from a subducted slab. The composition of major oxides, from the trachybasalts to trachytes, and mass balance calculations suggest that the trachyandesites and trachytes were formed via fractional crystallization from a basaltic melt. These diverse rock compositions probably highlight pulses of magma that separated from the main reservoir in the shallow crust. Results from mineral–melt thermometry indicate that the olivine and plagioclase in these basaltic rocks crystallized at temperatures of 1213 ± 56 °C and 1163 ± 23 °C, respectively. Thermobarometric calculations using clinopyroxene–melt pairs indicate that the Quaternary volcanism was associated with two magma storage regions that correspondingly involved pressures of 791 ± 253 MPa (1098 ± 34 °C) and 187 ± 123 MPa (938 ± 77 °C). The primary magma was likely present in a reservoir beneath the Moho (17–32 km), and it evolved by fractional crystallization to produce rocks, such as trachybasalts and phonotephrites. Crystallization and differentiation of the basaltic magma that was transported to crustal reservoirs explain the evolved compositions. The trachytic magma generated were present at depths of 2–11 km, and crystallization produced a paragenetic assemblage comprising clinopyroxene, amphibole, plagioclase, biotite, apatite, and titanomagnetite. • Quaternary intraplate volcanisms occurred in the East Sea back-arc basin. • New 40 Ar/ 39 Ar ages of submarine volcanic rocks were obtained. • Basaltic rocks were originated from an enriched garnet-bearing mantle source. • Trachytes were formed by fractional crystallization from basaltic magma. • The Quaternary volcanism had at least two magma storage regions.

Hydrothermal evolution from porphyry- to epithermal-style mineralization in the Naozhi deposit, NE China

The Naozhi deposit in the Yanbian district of northeastern China contains polymetallic epithermal veins and subeconomic porphyry-style Cu mineralization within 2 km of each other. These two mineralization styles are genetically associated with 130–126 Ma andesites and dioritic–granitic intrusions. Isotopic ages determined on sericite (128.2 ± 2.9 Ma; 40Ar–39Ar plateau age) and sulfide (124.6 ± 2.7 Ma; Rb–Sr isochron) associated with the epithermal mineralization constrain the hydrothermal activity to at approximately 128 Ma. Three types of fluid inclusions, including halite-bearing, vapor-rich, and liquid-rich inclusions were trapped in quartz from porphyry-style veinlets associated with an assemblage of K-feldspar–biotite–magnetite. Based on microthermometry and the homogenization behavior of halite-bearing inclusions, they were trapped at lithostatic pressures of > 500 bars and at temperatures of ≥ 350 °C, whereas the vapor- and liquid-rich inclusions were trapped at pressures and temperatures of 110 ± 35 bars and ≤ 350 °C.Epithermal mineralization comprises four vein stages, which, from oldest to youngest, are quartz–pyrite, quartz–base metal sulfide, quartz–precious metal, and quartz–carbonate. The ore assemblage precipitated from low-salinity fluids (<10 wt% NaCl equiv.) at temperatures and pressures of ≤ 350 °C and ≤ 100 bars. The δ18OH2O (2.5 to −2.4 ‰) and δDH2O (−85 to −114 ‰) compositions of fluids related to porphyry- and epithermal-style mineralization indicate a mixture of magmatic and meteoric waters. The δ34SH2S (−3.1 to 4.5 ‰) values of fluids become isotopically heavier from the quartz–pyrite stage to the quartz–base metal sulfide stage, reflecting continuous reduction during epithermal mineralization. The mineral assemblage and chemical composition of sulfide minerals indicate a transition from oxidizing, high sulfidation porphyry-style fluids to reducing, intermediate–low sulfidation epithermal fluids, with log ƒO2 values ranging from −31 to > −33, and log ƒS2 values ranging from −7.0 to ≤ −13.4 during the epithermal veining stage. Metal deposition in the Naozhi magmatic–hydrothermal system resulted from cooling, sulfidation, neutralization, and boiling of predominantly magmatically sourced hydrothermal fluids that mixed with meteoric water.

Intraplate Volcanism of the Alba Guyot: Geodynamic Formation Models of the Magellan Seamounts in the Pacific Ocean for 100 million years

Abstract -We report data on 9 samples of basaltic rocks and a spinel lherzolite xenolith collected during dredging in the area of Alba Guyot (Magellan Seamounts, Pacific Ocean). The data include 40Ar–39Ar ages of five samples and mineral chemistry of rock-forming and accessory minerals and glasses, with implications for the formation conditions of Ti-amphibole in basaltic melts. The upper slope parts at Alba Guyot and its satellite Oma Vlinder at sea depths of 3600 to 2200 m, are composed of ~1400 m thick basaltic rocks that formed within the 112 to 86 Ma interval (40Ar–39Ar plateau ages). Later, in its 60 Myr history postdating the Cretaceous volcanism, the guyot was exposed to erosion, which produced a plateau-like flat top, and underwent another pulse of volcanism in the Cenozoic. Miocene (&amp;lt;20 Ma) eruptions of basanitic magma on the Alba Gyot plateau were associated with petit-spot volcanoes. Judging by the absence of hyaloclastic deposits around those volcanoes and the age of the lavas, the Cenozoic eruptions occurred in subaerial conditions before the guyot submerged below the sea level. The Alba volcanics have greater contents of incompatible elements than typical oceanic island basalts (OIB), apparently, because they formed by low-degree partial melting of different sources: pargasite-bearing spinel peridotite of the metasomatized oceanic lithosphere, mainly for Cretaceous basaltic rocks, and/or garnet peridotite for Miocene petit-spot basanites. Multiple eruptions at Alba Guyot for 100 Myr may have been maintained either by the Southern Pacific Superplume in the region of the South Pacific Thermal and Isotopic Anomaly (SOPITA) in the Mesozoic or by translithospheric faulting originated at the lithosphere-asthenosphere boundary (LAB) in the Cenozoic.

LA–ICP-MS Zircon U–Pb and Alunite 39Ar/40Ar Dating of the Saozhouhe Alunite Deposit: Implications for the Metallogenic Significance of the Xiaotian Basin

As an important part of the northern Huaiyang tectonic belt, the Mesozoic Xiaotian basin hosts a series of gold and alunite deposits. However, the ages of these deposits remain unclear, constraining the further understanding of the genesis of these deposits. In this work, zircon LA–ICP-MS and alunite 39Ar/40Ar isotopic dating studies were carried out on the andesitic porphyrite and alunite of the Saozhouhe alunite deposit, respectively. LA–ICP-MS zircon U–Pb dating for the Saozhouhe andesitic porphyrite yields a weighted average age of 122.8 ± 0.9 Ma (MSWD = 1.8), indicating that these volcanic rocks were formed in the Early Cretaceous period. The alunite 39Ar/40Ar dating yields a plateau age of 121.0 ± 1.1 Ma (MSWD = 4.4) and an isochron age of 121.2 ±1.9 Ma (MSWD = 8.5), indicating that the alunite was also formed in the Early Cretaceous period. Our finding confirms that the formation of the Saozhouhe alunite deposit is genetically related to the Cretaceous volcanic magmatism, which has great significance in the metallogenetic regularity of and further ore prospecting in the Xiaotian basin.