Plateau Ages Research Articles

Intercalibration of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile and WiscAr 40Ar/39Ar laboratories for Quaternary dating

Accurate and precise dating of Quaternary lavas and pyroclastic flow or fall deposits is essential for understanding the evolution of active volcanoes and providing context for future eruptions and hazard assessment. The 40 Ar/ 39 Ar method is commonly employed to date these volcanic materials, however, dating young (<150 ka) K 2 O-poor materials can be challenging owing to low radiogenic 40 Ar* contents that can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration exercise involving the University of Wisconsin-Madison WiscAr Laboratory and the 40 Ar/ 39 Ar Laboratory of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile was conducted on a common set of samples with the aim of refining our methods and optimizing precision and accuracy of age determinations. Groundmass and plagioclase samples were analyzed on a 5-collector Noblesse ion counting mass spectrometer in the WiscAr lab, whereas measurements in the SERNAGEOMIN lab were performed using an ARGUS VI spectrometer equipped with faraday detectors and one compact discrete dynode electron multiplier. Samples for the intercalibration were collected jointly from three Andean Southern Volcanic Zone volcanoes to evaluate the capability of each laboratory to date different materials. Samples from lava flows with 1.0–3.2 wt % K 2 O from Planchon-Peteroa volcanic complex and with <1.0 wt % K 2 O from Calbuco Volcano that are the focus of ongoing geological studies were measured in both laboratories. Single crystals of plagioclase (0.6–1.0 wt% K 2 O) were measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments were conducted including co-irradiation of samples at Oregon State University, as well as irradiations using the CCHEN reactor in Chile to investigate differences in neutron fluence parameters. As a result, SERNAGEOMIN has modified long-used protocols for the CCHEN reactor so that Quaternary samples may be irradiated for periods of time most appropriate for their age. Although less precise than plateau ages, the isochron ages generated in the two laboratories agree at 2σ for each sample. Six of six co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K 2 O lavas from Calbuco proved more challenging with only three out of five plateau ages in agreement between labs. SERNAGEOMIN blanks were higher and more variable in Calbuco experiments, thus, differences in the variability of the measured 36 Ar blanks between the two laboratories may explain the discrepancy in plateau ages. Analysis of single plagioclase crystals from the Diamante Ignimbrite show excellent agreement between labs for both weighted mean apparent ages and isochron ages. We favor an isochron age for the ignimbrite of 132.4 ± 2.2 ka, however, discrepancies in results between samples from three different outcrops present an interesting geochronologic problem that warrants further study. Overall, the consistency of the results between labs is promising. These new precise age determinations significantly improve our understanding of the temporal evolution of these active volcanoes. • Broad agreement of ages from tephras and lavas measured in two labs. • 40 Ar/ 39 Ar dating of low K 2 O lavas from Calbuco is challenging due to limited 40 Ar*. • We favor an isochron age of ∼132 ± 2 ka ka for the 270 km 3 Diamante Ignimbrite. • Care in sample preparation and irradiation, and control of blanks are essential.

New 40Ar/39Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu’u Kualakauila volcanic sequence, Hawaii

Paleomagnetic measurements, coupled with 40Ar/39Ar dating, are improving our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the Earth's magnetic field. As part of an investigation of the Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu’u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Coe, 1999, Herrero-Bervera and Valet, 1999, 2005) revealed transitional directions. The fresh lava flows, easy access, and close geographical proximity to KAr dated flows made this ~215-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 47 successive flow sites, were stepwise demagnetized by both alternating field and thermal methods. Magnetostratigraphic results indicate the existence of four excursions or aborted reversals occurring at approximately ~36, ~75, ~130, and ~ 151 m above flow#1. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by truly excursional/Cryptochron directions (~7 lava flows). Paleointensity (P.I.) determinations indicate a steady decreasing of the PI values from 75 μ-T to ~12.2 μ-T reaching a minimum of 7.1 μ-T. These very low PI values are within the transitional/excursional part of the record. The corresponding VGPs are located on the western part of Australia. 40Ar/39Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yield a weighted mean plateau age of 3.201 ± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition.

Paleoproterozoic Mineralization of the Lijiapuzi Gold Deposit in the Liaodong Peninsula, NE China: Constraints from 40Ar-39Ar Age, S-Pb Isotopes, and In Situ Analyses

A Paleoproterozoic 40Ar/39Ar age, interpreted as the gold metallogenic epoch of the Lijiapuzi deposit in NE China, is reported in this paper. The ore body of this deposit is hosted in the marbles and schists of the Paleoproterozoic Dashiqiao Formation and exhibits parallel development to the foliation of the country rocks. Coexisting auriferous pyrite and tremolite can be observed both in the field and under a microscope, revealing the presence of hydrothermal activity in the deposit. The δ34S composition of the auriferous pyrite varies from +8‰ to +12.3‰, indicating a sedimentary origin. The auriferous pyrite and host marble have similar Pb isotopic ratios, indicating the contribution of Paleoproterozoic sediments to gold mineralization. Furthermore, in situ S and trace element analyses of the auriferous pyrite reveal that the main gold mineralization stage was Py2, which coexists with radial tremolite. 40Ar/39Ar dating of these tremolite samples yields a plateau age of ~1851.23 Ma, with similar isochronal and inverse isochronal ages. The Paleoproterozoic gold metallogenesis, together with the contemporary Pb-Zn, boron, graphite, and magnetite mineralization, constitute an important mineralogenetic series during the evolution of the Jiao-Liao-Ji orogeny in the Liaodong Peninsula.

Triassic multistage antimony-gold mineralization in the Precambrian sedimentary rocks of South China: Insights from structural analysis, paragenesis, 40Ar/39Ar age, in-situ S-Pb isotope and trace elements of the Longwangjiang-Jiangdongwan orefield, Xuefengshan Mountain

The Xiangzhong (XZ) Basin and its neighboring Xuefengshan (XFS) Mountain in South China, as one of the world-class antimony (Sb) producers, have provided appropriately 67% of globally exploited Sb metal (0.46 million tons) in human history. The associated gold (Au) with Sb in this region is also of significant economic values. In previous studies, these Sb-Au deposits were debated as a diversity of genetic types such as epithermal, magmatic-hydrothermal, sedimentary exhalative (SEDEX) and orogenic Sb-Au deposits. To evaluate these genesis hypotheses, we select the representative Longwangjiang-Jiangdongwan (LWJ-JDW) orefield to reveal the Sb-Au accumulation process and metal-fluid sources of these deposits. Our orefield-scale structural analysis shows that the localization of principal Sb-Au orebodies is controlled by a NE-striking sinistral shearing system composed by an integrated set of NE-striking and SE-dipping fractures in the pre-ore folded Neoproterozoic Wuqiangxi slate. Two chief ore periods were identified, including the marine sedimentation (SI, pyrite nodular) and tectonic-metamorphic hydrothermal period (SⅡ−SⅣ, Sb-Au veins). The second period is further subdivided into three stages of SⅡ (pre-tectonic deformed pyrite-milky quartz veins), Siii (syn-tectonic pyrite-arsenopyrite-stibnite-quartz veins) and SⅣ (post-tectonic stibnite-calcite veinlets). It is notable that gold precipitation is significantly prior to Sb ores at LWJ-JDW. Three sericite samples throughout SⅡ−SⅣ yield a group of well-defined 40Ar/39Ar plateau ages (253 ± 4, 234 ± 3 and 206 ± 2 Ma, respectively). This indicates the multistage Sb-Au mineralization initiated from Early Triassic and terminated at Late Triassic time. Further trace elements analyses show the earliest nodular PyI is enriched in a list of As, Ni, Sb, Cu, Se, Co, Pb, Zn and Ti that can be analogy to the diagenetic pyrite in the organic-rich marine sedimentary rocks. Extremely high Sb (median, 1107 ppm) and mediate Au (0.07 ppm) are monitored in PyI, indicative of the initial Sb and Au involvement from the Neoproterozoic sedimentation. Comparably, sulfides of the later SⅡ−SⅣ possess a similar spectrum of trace elements but with highly variable concentrations. All the sulfide generations host the wide range of in-situ δ34SV-CDT values (–23.9 to −3.7‰ for SⅠ, −7.6 to −2.9‰ for SⅡ−SⅣ, respectively), indicating sulfur originally sourced from bacterial sulfate reaction (BSR) of the Neoproterozoic seawater sulfates, and then gradually reduced by thermochemical sulfate reaction (TSR) during the subsequent Au-Sb-induced tectonic-metamorphic hydrothermal activity. The lead isotope compositions in accordance with the evolution lines of upper crust and orogen, implying that the Pb were chiefly sourced from host slate of the Wuqiangxi Formation and its underlying basement. Synthesis of these data, we proposed that the Sb-Au accumulation at LWJ-JDW was caused by the multistage mineralization from Neoproterozoic marine sedimentation to Triassic episodic deformation and metamorphic processes (253–206 Ma).

Dating of polyhalite: a difficult 40Ar/39Ar dating tool of diagenetic to very low-grade metamorphic processes

Halite already deforms at surface temperatures. A valuable universal dating tool to constrain the timing of sedimentary, diagenetic, or deformational structures is still missing. The evaporite mineral polyhalite can be dated by the 40Ar/39Ar method. On the example of the extremely deformed halite deposits of the Eastern Alps, polyhalite was tested to date early diagenetic stages of the deposits. The sedimentological investigation of the present study indicates that some of the macrostructures of polyhalite had a syn-depositional origin during the late Permian. It is supposed that polyhalite originated during reflux of brines. All samples selected for age dating represent characteristic microfabric types of euhedral to subhedral polyhalite crystals. Intact macro- and non-recrystallized looking microstructures of polyhalite can be expected to give plateau ages. However, nearly all measurements produced overdispersed data that do not define an age. The oldest age steps thus represent only minimum ages. A closer look revealed grain boundary migration, subgrain rotation recrystallization, twinning, and fluid-supported grain size increase. These recovery processes obscured the original ages and/or reflect the origin of new polyhalite in place of the original individuals. Based on these microstructures, the age data are supposed to reflect the circulation of aqueous fluids. Just extremely careful separation of individual crystals or in situ age dating under the microscope will be successful in dating polyhalite. Nevertheless, polyhalite can potentially serve to date deformational events of halite deposits due to its easy recrystallization property.

Neoproterozoic HP granulite and its tectonic implication for the East Kunlun Orogen, northern Tibetan Plateau

The P–T path of high-pressure (HP) granulite is a sensitive indicator providing metamorphic evidence on the convergence process and on the evolutionary history of orogenic belts during the supercontinent circles. The Wulonggou mafic granulites in the East Kunlun Orogen are intercalated as lenses/boudins within the dominant amphibolite facies basement rocks. Based on microstructures and mineralogical observations, three mineral assemblage generations can be recognized in the granulite samples: (1) inclusions within garnet and amphibole comprising cpx + pl + amp + qtz + rt; (2) the matrix assemblage grt + cpx + pl + amp + qtz + rt ± bt ± ilm; and (3) a retrogressive assemblage consisting of kfs + pl + qtz + amp coronas around garnet. Phase equilibrium modeling and conventional thermobarometric calculations reveal that the Wulonggou granulite has experienced (1) first HP granulite facies metamorphism at > 12 kbar and 810–840 °C, (2) dominant HP granulite facies stage of 800–860 °C at 10.7–12.3 kbar, as well as (3) a retrograde stage of 620–650 °C and 6.7–8.1 kbar, implying a clockwise P–T path. Zircon U–Pb geochronological results indicate that the mafic granulites and the gneissic country rocks yield similar Neoproterozoic metamorphic ages of 939–941 Ma. The granulite also yields a zircon U–Pb age of 444 ± 7 Ma and amphibole 40Ar/39Ar plateau age of 433 ± 1 Ma, suggesting an Early Paleozoic metamorphic overprint at ca. 440 Ma. Together with our previous studies on the contemporaneous Neoproterozoic magmatism and metamorphism, all the above lines of evidence constrain that the Wulonggou mafic granulite has experienced high pressure granulite facies metamorphism at ca. 940 Ma, suggesting a Neoproterozoic collisional orogeny in the East Kunlun Orogen, which has been overprinted by the Early Palaeozoic orogenic processes.

Metamorphic Evolution of the Bulfat Belt (NE-Iraq): Evidence from 40Ar/39Ar Age Spectrum Measurements

The Bulfat Ophiolite Complex (upper allochthonous thrust sheet) which represents a part of the Iraqi Zagros Suture Zone is situated at the boundary between the Arabian Plate and the Iranian Microplate. We have examined the metamorphic and cooling history of metapelitic rocks of Bulfat Complex using the whole-rocks 40Ar/39Ar age determination. The results of four samples match to the definition of the plateau and revealed to 40Ar/ 39Ar age of 42.4 ±0.5 Ma (sample 1), 32.9 ±0.8 Ma (sample 2), 44.3 ±0.8 Ma (sample 3), and 43 .6 ±1.5 Ma (sample 5). However, samples 4 and 6 yielded an age range between 35- 40 Ma and 40- 50 Ma respectively. Generally, plateau ages (32.9-50 Ma) are identical and considered reliable, but the presence of excess argon (40Ar) remains possible in samples (e.g., sample 6) having initial40 Ar/36Ar ratios higher than the present atmospheric 40 Ar/36Ar = 295.5. The excess argon is derived from the mantle by plumes of mafic magmas into the earth crust and/ or released from minerals and rocks during metamorphic processes. Bulfat Complex is subjected to two metamorphic events: the earlier regional occurred during the period Albian-Cenomanian (97-105 Ma) and associated with the intraoceanic infant supra-subduction zone. The later thermal metamorphism overprinted the previous one, and took place due to the intrusion of igneous body pre – 50 Ma age. This igneous activity is considered one of the multi-event magmatism that happened along the Zagros Fold-Thrust Belt during the period from Upper Cretaceous and Mid – Late Miocene. It seems evident that the thermal metamorphic slice of Bulfat Complex (Gemo-Qandil Sequence) represents a part of the oceanic slab that has been subducted beneath the Iranian microplate. This event probably forming the last evolved episode of island arc system ended with the final closure of Neo-Tethys and the collision between the Arabian Plate and Iranian Microplate occurred at Mid-late Miocene.

Sericite 40Ar/39Ar Dating and Indosinian Mineralization in the Liushuping Au–Zn Deposit, West Qinling Orogen, China

The Liushuping deposit, located on the northeast margin of the Bikou Block, is the middle-sized gold-zinc deposit (with ore reserves of 15.67 × 104 t Zn and 2.2 t Au) in the Mianxian–Lueyang–Yangpingguan area. The orebodies occur in the meta-dolomite of the Duantouya and Jiudaoguai formations controlled by the Jiudaoguai syncline. The ore-forming process has experienced hydrothermal period and epigenetic oxidation period, and the hydrothermal period can be divided into two stages. The hydrothermal sericite sample collected from stage 2 yielded a well-defined 40Ar/39Ar isotopic plateau age of 215.70 ± 0.37 Ma, and an 39Ar/36Ar-40Ar/36Ar normal isochron age of 215.35 ± 0.38 Ma, indicating that the metallogenic age of the Liushuping is the Late Triassic (ca. 215 Ma). The ISr(t) of sphalerite is higher than that of the Bikou Group but similar to the Duantouya Formation, indicating that the ore-forming fluids may mainly originate from the metamorphic dehydration of the Duantouya Formation. The Liushuping Au–Zn deposit is consistent with that of the Qinling Indosinian orogeny and mineralization, which are related to oceanic subduction during the Late Triassic.

Zircon U-Pb-Hf Isotopes, Biotite 40Ar/39Ar Geochronology, and Whole-Rock Geochemistry of the Baogeqi Gabbro in the Northern Alxa, Southernmost Central Asian Orogenic Belt

The final closure time of the Paleo-Asian Ocean and the Permo-Carboniferous tectonic settings in the northern Alxa are very important but controversial tectonic issues. The geochronology and petrogenesis of mafic igneous rocks are superior in clarifying regional tectonic settings. Here, we report on zircon U-Pb-Hf isotopes, biotite 40Ar/39Ar geochronology and whole-rock geochemical data of the hornblende gabbro from the Baogeqi gabbro pluton in the northern Alxa. The LA-ICP MS U-Pb analysis of zircon grains from the hornblende gabbro yield a weighted mean age of 262.7 ± 2.3 Ma (2σ, MSWD = 0.74), manifesting that the Baogeqi gabbro pluton emplacement was during the late Middle Permian (Capitanian). The 40Ar/39Ar dating of biotite grains from the hornblende gabbro yields a plateau age of 231.3 ± 1.6 Ma (2σ, MSWD = 0.55), indicating that the Baogeqi gabbro pluton cooled to below 300 ℃ in the Triassic. The hornblende gabbro samples are calc-alkaline with metaluminous character, and show enrichment in large ion lithophile elements (e.g., Rb, Ba, Sr, and K) but depletion in Nb, Ta, P, Th, and Ti relative to primitive mantle. Combined with the positive zircon εHf(t) values (+4.9–+9.4), we suggest that the magmas formed from the partial melting of depleted mantle were metasomatized by slab-derived fluids. Together with regional geology, these geochemical data suggest that the Baogeqi gabbro pluton was formed in an intracontinental extension setting, further indicating that the Paleo-Asian Ocean in the northern Alxa was closed prior to the late Middle Permian (Capitanian), and this region was in a post-collision extensional setting during the Capitanian-Late Permian. In addition, the Triassic cooling of the gabbro pluton may be a record of the decline of the Capitanian-Late Permian post-collisional extension basin due to the far-field effect of subduction-collision during the closure of the Paleo-Tethys Ocean.

Genesis of the late‐stage base metal sulfide mineralisation and its relationship with the regional deformation in Ambaji–Deri, South Delhi Terrane, North‐West India

Proterozoic stratiform VMS‐type Ambaji–Deri Zn–Pb–Cu deposits in the Aravalli‐Delhi mobile belt of North‐West India are hosted within meta‐sedimentary sequences of the South Delhi Terrane. Several fracture‐filled discordant late‐stage base metal mineralised quartz veins have intruded along with the fractures and foliation at very high Pf (σ2 &lt; Pf &lt; σ1) with typical hydrothermal alteration zones. Temperatures estimated from hydrothermal chlorite vary from 226 to 293°C (mean. 260°C). 40Ar‐39Ar dating of hydrothermal muscovite yielded a plateau age of 625.5 ± 3.3 Ma. Detailed fluid inclusion (FI) analysis reveals three types of primary and four types of secondary FI. Primary H2O‐NaCl‐CO2 ± N2 type 1a FI has medium to high salinity (11.21–21.96 wt% NaCl equivalent) and high density (0.90–1.04 g/cm3), while the secondary type 1b FI with same compositions are characterized by medium salinity (~12.6 wt% NaCl equivalent) and lower density (0.89–0.90 g/cm3). Phase separation/fluid immiscibility is the primary mechanism of mineralisation in the Ambaji–Deri that caused the original fluid to evolve separately as gaseous (CO2) type 2a, b and aqueous type 3a, b (H2O‐NaCl) with similar salinity and density for the aqueous inclusions and low density for the gaseous inclusions (lowering up to 0.58 g/cm3). The ore‐forming fluid with magmatic signatures possibly originated from the cooling G3 granites, remobilized the pre‐existing metals, and precipitated at ~265°C and ~1 kbar at the epizonal depths in the dilated fractures. Later these fluids are mixed with meteoric water and evolved into low saline aqueous fluids that produced supergene alterations.

Using titanite to constrain geochronology and physicochemical conditions of hydrothermal mineralization events: A case study of the Duotoushan Fe–Cu deposit, Eastern Tianshan, NW China

As a common Ca‐ and Ti‐ bearing silicate mineral in many types of hydrothermal deposits, titanite usually contains high concentrations of trace elements. Its mineral chemistry and U–Pb isotope can reveal the physicochemical conditions and geochronology of the ore‐forming systems. In this contribution, we present a detailed study on titanite which precipitated during the Fe mineralization in the Duotoushan deposit. Due to coexistence with hydrothermal minerals, low Th/U ratios (0.02–0.3), and depletion in rare earth elements (REE), all the studied titanites are classified into a hydrothermal group. The negative correlation between [REE3+ + (Al, Fe)3+] and [Ca2+ + Ti4+] indicates that REE mainly entered the lattice of titanite via the mechanism of substitution. In addition, titanite grains are characterized by HREE, Zr, and Nb enrichments with LREE depletion, suggesting that the complexation of F− at neutral to alkaline pH conditions may have caused the fractionation of REE. At Duotoushan, the titanite grains coexist with quartz, epidote, amphibole, and magnetite, and exhibit positive Eu anomalies (δEu = 1.04–1.31) but lack Ce anomalies, indicating that ore‐forming fluids may have been derived from a relatively low oxygen fugacity and high fH2O environment. Titanite yielded an in‐situ U–Pb lower‐intercept age of 307.2 ± 4.8 Ma (MSWD = 0.97), consistent with the syn‐ore amphibole 40Ar‐39Ar plateau age (305 ± 6 Ma). Since the mineralization ages are obviously younger than country rocks, the previous syn‐sedimentary ore‐forming model for Duotoushan Fe–Cu mineralization can be excluded. Integrating the characteristics from ore deposit geology, periods of mineralization events, and spatial–temporal distribution of magmatism, we proposed that the Duotoushan Fe–Cu mineralization event may be linked with a hidden granite in its orefield.

Geochronology of the Chakabeishan Li–(Be) rare-element pegmatite, Zongwulong orogenic belt, northwest China: Constraints from columbite–tantalite U–Pb and muscovite–lepidolite 40Ar/39Ar dating

Lithium–(beryllium) are important strategic metals in modern industries. The Markam–Yajiang–Karakoram (MYK) giant Li ore belt (>14 Mt Li2O) is located in the central Qinghai–Tibetan Plateau, and is a world-class Li deposit belt. The reported pegmatite deposits in the belt are mainly found in the Tianshuihai–Songpan–Garzê Terrane (TSGT) and are generally considered to be related to Late Triassic S-type granites and flysch sedimentary rocks. Recently, abundant pegmatite veins have also been discovered in an adjacent terrane (i.e., the North Qaidam Terrane) to the north of the MYK belt, but it is unclear whether these newly discovered pegmatites are genetic related to the MYK belt. Here, we report high-precision columbite–tantalite (i.e., coltan) U–Pb ages and muscovite–lepidolite 40Ar/39Ar ages for the Li–(Be) rare-element pegmatites from the North Qaidam Terrane (NQT) in the Chakabeishan area of the Zongwulong orogenic belt. Coltan grains from three spodumene-bearing pegmatite samples yielded weighted-mean 207Pb-corrected U–Pb ages of 214.9 ± 1.7, 217.0 ± 2.3, and 215.0 ± 1.5 Ma. Muscovite 40Ar/39Ar dating of two spodumene-bearing pegmatite samples and one barren pegmatite sample yielded plateau ages of 213.00 ± 0.97, 211.67 ± 0.35, and 211.78 ± 0.29 Ma, respectively. A lepidolite 40Ar/39Ar age for a lepidolite-bearing pegmatite is 216.62 ± 0.88 Ma. The subtle age differences obtained by the different methods are within reasonable error estimates or may be due to different closure temperatures. The ore-forming ages of the Chakabeishan Li–(Be) rare-element pegmatites are similar to those of Li–(Be) pegmatite deposits in the MYK belt. This and data from previous studies suggest that the Chakabeishan Li–(Be) rare-element pegmatites were also formed in a post-collisional setting after the closure of the Paleo-Tethys (Anyemaqen) Ocean. This study has important implications for understanding the origins of the MYK Li ore belt, indicating that the formation of these Li deposits was not solely associated with the Late Triassic S-type granites and flysch sedimentary rocks. Our results may also have implications for exploration, which extends the potential area of Li deposits from the Tianshuihai–Songpan–Garzê Terrane to the North Qaidam Terrane.

Deformation along the Deseado Massif (Patagonia, Argentina) during the Jurassic Period and its relationship with the Gondwana breakup: paleomagnetic and geochronological constraints

This work presents the analysis of paleomagnetic results obtained from four sampling areas of the Jurassic Bahía Laura Complex in the Deseado Massif and their implications on the regional deformation history during the breakup of Gondwana. Paleomagnetic data show cessation of tectonic block rotations about vertical axes around 160 Ma and a change from a transtensional to a mainly extensional tectonic regime. Two biotite samples yielded 40Ar–39Ar radiometric ages: one from the eastern outcrops of the Chon Aike Formation (plateau age of 184.66 ± 0.55 Ma) and the other from the La Matilde Formation (plateau age of 157.40 ± 0.65 Ma), located at the central part of the Deseado Massif. Finally, an apparent polar wander path (APWP) was calculated for the Jurassic of South America (200 to 140 Ma). This APWP indicates that South America experienced a northward drift between 200 Ma and 170 Ma, a clockwise rotation (~10°) between 170 Ma and 160 Ma, and a westward drift between 160 Ma and 140 Ma. The 170–160 Ma rotation could has been triggered by the combined effects of the uncoupling of the Antarctic Peninsula, the high rates of subduction of the Phoenix plate beneath Patagonia, the opening of the Rocas Verdes basin and the Weddell Sea, and the cessation of the Tethys slab-pull. Furthermore, the calculated APWP for South America does not support a Jurassic massive true polar wander event.

Genesis of the Lishupo gold deposit in the Jiangnan Orogen, NE Hunan (South China): Biotite Ar-Ar, zircon U-Pb ages and H-O-S-Pb isotopic constraints

The Lishupo gold deposit is located in the central Jiangnan Orogen (South China). Gold ore occurs mainly as quartz veins and altered slates, hosted by faults and shear zones in the low-grade metamorphosed Neoproterozoic Lengjiaxi Group. Metallic minerals include pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite and galena, and the major alteration styles include silicification, sericitization, chloritization and carbonatization. Based on its vein crosscutting relationships and mineral assemblages and paragenetic sequence, mineralization at Lishupo comprises three stages: (1) quartz (Q1)–dolomite–pyrite (Py1)–arsenopyrite; (2) quartz (Q2)–dolomite–pyrite (Py2)–polymetallic sulfides; (3) quartz (Q3)–carbonate. Biotite from the sulfide-bearing ores of Stage 2 yielded a 40Ar–39Ar plateau age of 208.67 ± 1.25 Ma, and pre-ore granodiorite yielded a zircon U-Pb age of 430.8 ± 3.2 Ma. The dating results indicate that the Lishupo gold mineralization occurred at ∼ 210 Ma, coeval with the Late Triassic regional gold mineralization event in NE Hunan. Stage 1 to 3 quartz has δ18OVSMOW = +15.5 to + 17.6‰ and δDVSMOW = –81.2 to –70.3‰, and the ore-fluids were calculated to have δ18OFluid = +2.9 to + 10.9‰. This suggests that the ore-forming fluids were derived from a metamorphic source with magmatic input. The auriferous sulfides (pyrite containing invisible gold) have δ34SCDT = –13.7 to –4.6‰, 206Pb/204Pb = 18.230–18.642, 207Pb/204Pb = 15.615–15.755, and 208Pb/204Pb = 38.582–39.100. These isotopic data indicate that the ore-forming fluids and metals were derived from a deeper metamorphic source. The Lishupo deposit exhibits many features of orogenic gold mineralization, such as control by ductile shear zones, mineralization styles (quartz veins with abundant sulfide minerals), and H–O–S–Pb isotopic features indicating deeper metamorphic source. Based on similarities to other orogenic gold deposits, the Lishupo deposit is best classified as orogenic type formed at ∼ 210 Ma, in response to the Late Triassic extensional tectonics caused by the Paleo-Pacific plate, or the post-orogenic tectonics after the South China-Indochina and/or North China-South China collision.

40Ar/39Ar geochronology of phlogopites and its mineralization in the Mithapukur kimberlites: New Lower Cretaceous (∼109–115 Ma) kimberlitic occurrences in Bangladesh

Phlogopites in a suite of ultramafics from Bangladesh have been analyzed to ascertain the mineralization and the timing of cooling of phlogopites and reveal their emplacement tectonics. In recent times, the 40Ar/39Ar dating method is incredibly popular due to having phlogopite in most kimberlitic rocks. The 40Ar/39Ar age dating of phlogopites yields ideal plateau ages of 108.5 ± 1.3 Ma, 110.8 ± 0.9 Ma, 113.7 ± 1.1 Ma, 114.8 ± 1.0 Ma and an upper-staircase release pattern age of 111.7 ± 1.1 Ma. It is the first report on the Lower Cretaceous kimberlites (109–115 Ma) occurrence in Bengal Basin, emplaced during the break-up of India and Antarctica-Australia. The studied phlogopite chemistry and petrographic data suggest that the rocks are typically orangeites, comparable with Indian orangeites, Antarctica and some African transitional or hypabyssal kimberlites. These phlogopites are invariably consistent with most of the kimberlites of the world and those described as a clear sign of a metasomatized source in the mantle. The tectonic activities of the studied ultramafics suggest the development of enormous stress gradients in the lithosphere, creating a path of melt migration from the transitional or lower mantle. These multiple tectono-thermal processes were the key driver for the present kimberlites’ emplacement within Paleoproterozoic basement rocks.

Branches of the Karakoram fault in Eastern Pamir

ABSTRACT The tectonic evolution of the Karakoram fault in the Pamir Plateau is controversial because the timing and kinematics of several branches of the fault system are unclear. Here we conduct field observations on the East Pamir fault, the North Muskol shear zone, and the Xindi-Waqia fault in the eastern Pamir, and obtain zircon U-Pb and biotite 40Ar/39Ar ages from these ductile shear zones. All three faults/shear zones are hosted by granitic mylonites. The East Pamir fault is a northwest-striking, normal-dextral shear zone that cuts the Rushan-Pshart suture. Zircon U-Pb dating yields a mean age of 201.5 ± 0.5 Ma for the mylonites in this shear zone. Biotite 40Ar/39Ar analysis yields two weighted mean plateau ages, 10.65 ± 0.13 Ma, and 11.04 ± 0.15 Ma. An undeformed granitic vein that cuts the ductile shear zone gives a mean zircon U-Pb age of 9.6 ± 0.3 Ma. We interpret that the granitic protolith crystallized at 201.5 Ma, and the East Pamir fault underwent rapid cooling and exhumation at ca. 11 Ma and ceased by 9.6 Ma. The North Muskol shear zone is an east-striking extensional shear zone along the northern boundary of the Shatput-Muztaghata dome. Two mylonitic samples show mean 206Pb/238U ages of 17.9 ± 0.2 Ma and 17.6 ± 0.3 Ma, which are interpreted as the timing of crystallization of the granitic protolith. The Xindi-Waqia fault is a northwest-striking ductile shear zone and brittle fault that bounds the Muztaghata dome. We obtain two zircon U-Pb ages (231.8 ± 1.1 Ma, 235.9 ± 0.9 Ma) and two biotite 40Ar/39Ar plateau ages (between 6.60 ± 0.10 Ma and 6.87 ± 0.10 Ma) from the mylonites in this fault, and interpret that the granitic rocks were emplaced in the Late Triassic and exhumed in the late Miocene. The newly obtained U-Pb ages, combined with the previously published data, indicate that Rushan-Pshart suture in Pamir marks the closure of an ocean basin within the Palaeo-Tethys Ocean, and cannot be correlated with the Bangong-Nujiang suture in Tibet. We establish a systematic correlation in which the two Palaeo-Tethyan sutures (Tanymas-Jinshajiang and Rushan-Pshart) in Pamir represent the western extensions of the Jinshajiang and Longmu Co-Shuanghu sutures in the Tibetan Plateau, respectively, and the Central Pamir is the western extension of the Tianshuihai-North Qiangtang terrane. This indicates that the dextral slip along the Karakoram fault system in east-central Pamir is 140 ± 10 km, and the mean slip rate since the onset of the Karakoram fault at ca. 18 Ma is 7.8 ± 0.5 mm/yr. Furthermore, our analyses suggest that the North Muskol shear zone was active at ca. 18 Ma before the arrival of the Karakoram fault system then. The East Pamir fault must have been active at ca. 11 Ma, but ceased by ca. 9.6 Ma, and the slip along the Karakoram fault system migrated northeastward and primarily accommodated by the Tashkurgan fault soon after 9.6 Ma. The Xindi-Waqia fault is the southern extension of the ductile shear zone of the Kongur Shan extensional system, as they are highly similar in geometry, kinematics, and timing. Overall, as the Karakoram fault system propagated northward in Pamir during the Cenozoic, crustal deformation migrated northeastward among the Karakoram fault branches from Ladakh to the Kashgar-Yecheng transfer system, and a complex evolution path can be portrayed.

Geochronology, stable isotopes and pyrite chemistry of the Cangzhuyu gold deposit in the North China Craton: Implications for gold metallogeny of Xiaoqinling

The ore genesis of gold deposits in Xiaoqinling, located in the south margin of the North China Craton (NCC), remains highly debated. Either a magmatic source or a metamorphic origin has been advocated for the ore-forming fluids. In this paper, we report new 40Ar/39Ar dating, in-situ pyrite trace elements and C-H-O isotopic data for the Cangzhuyu deposit, hosted in quartz veins within the Archaean metamorphic rocks to constrain the ore-forming processes. Four stages of mineralization and alteration can be recognized in the deposit, including Stage I: quartz-pyrite veins; Stage II: pyrite-quartz-ankerite veins; Stage III: polymetallic sulfides-ankerite-quartz veins; Stage IV: quartz-carbonate veins. Sericite from stage III gold-bearing veins has a 40Ar/39Ar plateau age of 130.7 ± 1.3 Ma, which is synchronous with the regional gold mineralization event in the Xiaoqinling district. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) trace element analysis shows that Stage I and Stage IV pyrite show similar compositions and rarely contain lattice gold. In contrast, gold mostly occur as native gold or Au-Te-Bi mineral inclusions within Stage III pyrite, which are consistent with their high Ni, Te, Bi contents. The ore-forming fluids equilibrated with ankerite have C-O isotopes of −6.6 to −4.1‰ and 8.8 to 10.8 ‰, respectively. They are consistent with H and O isotopes of sericite that δDfluid ranges from −61.1 to −60‰ and δ18Ofluid ranges from 8.2 to 7.2 ‰, implying a magmatic source. We hence suggest that the Cangzhuyu gold deposit was formed from mantle-derived magmas in a setting of the destruction of NCC and lithospheric thinning.

Ta-Nb mineralization in the shallow-level highly-evolved P-poor Shihuiyao granite, Northeast China

The Shihuiyao granite district in the southern Great Xing'an Range, Northeast China, is a newly discovered area of rare-metal-bearing granites (RMG). The latest Jurassic Shihuiyao granitic pluton, emplaced into the Lower Permian Linxi Formation, comprises porphyritic granite, leucogranite, and amazonite-bearing pegmatite, all enriched in rare metals (mainly Ta, Nb, Rb). The Shihuiyao granites are peraluminous (A/CNK = 1.02–1.27), characterized by high contents of SiO2 (69.4–78.0 wt%), Al2O3 (12.7–16.9 wt%) and total alkalis (6.8–12.8 wt%), enriched in Ta, Nb, Rb, Cs, and Sn, but depleted in Ba and Sr. They have strong negative Eu anomalies (Eu/Eu⁎ = 0.02–0.42) and show the M-type REE tetrad effect. Low Nb/Ta (<4) and Zr/Hf (<8) values indicate high degree of magmatic fractionation. Whole-rock and mineral compositions variations indicate that the Shihuiyao granites are a typical low pressure and P-poor RMG, which experienced intensive magmatic fractionation followed by intensive hydrothermal reworking. The narrow range of δ7Li values (−0.86‰ to 0.88‰) and Rayleigh distillation modeling suggest that the porphyritic granite and leucogranite represent products of the same magma source, but evolved independently at depth. LA-ICP-MS UPb dating of monazite from the porphyritic granite and leucogranite yield lower-intercept ages of 150.2 ± 0.6 Ma and 147.0 ± 1.7 Ma, respectively. The 40Ar39Ar plateau ages of mica from leucogranite (146.3 ± 1.1 Ma) and an ore-bearing quartz vein (145.6 ± 1.0 Ma), and the UPb age of columbite (146.7 ± 4.7 Ma) further identify this as a latest Jurassic magmatic-hydrothermal mineralization event in NE China. Rapid crystallization and chemical quenching of magma resulted in neutralization of the acid fluoride solution, and a loss of the fluoride ligand during fluid evolution. This further triggered the subsequent crystallization of NbTa oxides. As revealed from geochemical and textural characteristics of the columbite group minerals, the high-degree of fractional crystallization and Ta-rich hydrosilicate fluid metasomatism are the main factors leading to the enrichment of TaNb within Shihuiyao RMG. In general, we conclude that rare metal-hosted magmatism took place in two episodes in NE China, namely, (i) Latest Jurassic to Early Cretaceous (150–130 Ma) peraluminous to metaluminous Ta-Nb-Sn-W RMG, and (ii) Early Cretaceous (130–120 Ma) alkaline Nb-Ta-Be-REE-Zr RMG, which highlight the prospecting fertility for Nb-Ta-REE (SnW) mineralization in NE China.

Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan suture in the Qinling Orogen: Constraints from deformation fabrics and geochronology

AbstractThe Qinling Orogen, located between the North and South China Blocks, records subduction-collisional orogeny along the Paleozoic Shangdan and Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which all played an important role in building the present tectonic framework and topography. The strike-slip Shagou ductile shear zone that overprints the Paleozoic Shangdan suture between the North and South Qinling Belts is crucial for understanding the Mesozoic intracontinental deformation in the Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic vorticities (0.81–0.95) suggest sinistral simple shear. The quartz c-axis patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the activity of prism &amp;lt;a&amp;gt; slip, while those from amphibole-rich mylonites display both Y-maximum and Z-maximum fabrics showing the combined activity of prism &amp;lt;a&amp;gt; and basal &amp;lt;a&amp;gt; slip systems. In the mafic mylonites, the plagioclase fabrics were induced by combined (010)[100] and (001)[100] slip systems, while the amphibole fabrics were probably related to anisotropic growth or passive rotation of rigid clasts. Equilibrium P–T conditions of 4.28–6.12 kbar and 646–727 °C estimated from geothermobarometry suggest that the main deformation occurred under amphibolite facies conditions at middle–lower crustal depths. Zircon U-Pb ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma and 726–718 Ma, while the intense mylonitization and sinistral shearing occurred at ca. 200–187 Ma. A U–Pb zircon age of 132 ± 3 Ma from a granitic dike cutting the foliation and an amphibole 40Ar/39Ar plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the Shagou shear zone evolved through decompression and exhumation stages in the time period of 132–119 Ma.

Geology and Genesis of the Unkurtash Intrusion-Related Gold Deposit, Tien Shan, Kyrgyzstan

Abstract Situated in the heart of the Tien Shan gold province, Unkurtash (3.5 Moz at 1.82 g/t) is a representative lode gold deposit that is hosted within the Carboniferous Andagul granodiorite in the Kassan district, western Kyrgyzstan. The Tien Shan gold province consists of porphyry and epithermal deposits, which are related to magmatic arcs, and lode gold deposits formed in the late stages of continent-continent collision that have been generally classified as mesothermal or orogenic type. Some of the lode gold deposits have been considered to be intrusion-related, but the genetic relationship between gold mineralization and the magmatic intrusions remains ambiguous in most cases due to lack of absolute timing of the gold mineralization. This paper addresses this problem through detailed geologic and petrographic studies of the Unkurtash gold deposit in conjunction with Re-Os, U-Pb, and 40Ar/39Ar dating along with S and Pb isotopes. The paragenesis of the Unkurtash deposit is divided into four stages. Stage 1 is related to pre-ore sheeted quartz veins outlined by narrow K-feldspar alteration envelopes. Stage 2 is characterized by quartz-molybdenite (±Au) veins with sericite and chlorite alteration assemblages. Stage 3 is the main gold mineralization stage and is characterized by gold-bearing quartz-ankerite veins and pervasive quartz ± K-feldspar ± rutile ± titanite alterations, with development of auriferous pyrite, free gold, electrum, galena, pyrrhotite, and minor chalcopyrite and arsenopyrite. Stage 4 is defined by the presence of abundant arsenopyrite in barren quartz-calcite veins associated with sericite ± muscovite ± carbonate alteration. Molybdenite from a single stage 2 vein yielded a weighted mean Re-Os age of 306.5 ± 1.7 Ma (n = 7). Auriferous pyrite from stage 3 yielded an Re-Os isochron age of 307.3 ± 8.0 Ma (n = 5) and a weighted mean age of 307.4 ± 3.1 Ma (n = 2; low-level highly radiogenic pyrite). 40Ar/39Ar ages of three hydrothermal K-feldspar samples from stage 3 veins returned younger plateau ages (ca. 293–289 Ma) that are possibly related to post-ore thermal events. The molybdenite and pyrite Re-Os ages are comparable with a newly obtained U-Pb zircon age of 305.1 ± 2.7 Ma for the Andagul granodiorite, providing unequivocal evidence for synchronous gold mineralization and magmatism. The similarity in δ34SVienna-Canyon Diablo Troilite (V-CDT) between auriferous pyrite (5.7–7.1‰, n = 10) and the Andagul granodiorite (7.0–8.9‰, n = 4) and their similarity in Pb isotopes, which are significantly different from those of the pre-Silurian metamorphic rocks and regional Devonian sedimentary rocks, support the hypothesis that the Andagul granodiorite was the main source of metals and sulfur for mineralization. These results, together with concentric development of proximal skarn and metamorphic rock-hosted Au-As-Bi deposits and distal Sb-Au-As-Hg deposits around the Andagul intrusion, provide a strong case for an intrusion-related gold system. The establishment of an intrusion-related model for the Kassan district broadens the window for additional discoveries of gold deposits in the Tien Shan and elsewhere in the Central Asian orogenic belt.