Emplacement Age Research Articles

Genesis of the early mesozoic granitoids at the Hardat Tolgoi Ag-Pb-Zn deposit in East Ujimqin Banner, Inner Mongolia, NE China: Insights from whole-rock geochemistry, zircon U-Pb-Hf isotopes, and Pb-Si systematics

The Hardat Tolgoi Ag-Pb-Zn deposit is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Ag-Pb-Zn veins are mainly hosted by the argillaceous siltstone and feldspar quartz sandstone of Aobaotinghundi Formation, which show close genetic relationship with the granitoids at Hardat Tolgoi (i.e., biotite granite, syenogranite, and monzogranite). New LA-ICP-MS zircon U-Pb dating yielded the emplacement ages of 196.1 ± 5.8 Ma for biotite granite, 192.3 ± 6.0 Ma for syenogranite, and 194.8 ± 1.7 Ma for monzogranite, respectively, indicating that they were formed in the Early Jurassic. Geochemically, these granitoids are characterized by high SiO2, Na2O + K2O, and low MgO and TiO2 concentrations, belonging to high-K calc-alkaline series. They exhibit relatively flat REE patterns, with pronounced negative Eu anomalies (EuN/EuN* = 0.004–0.015), enriched in LREEs and LILEs (e.g., Rb, U, and Pb), and depleted in Ba, Sr, Nb, P, and Ti. Zircon in situ Hf isotopic analyses yield positive εHf(t) values ranging from + 5.7 to + 11.8, corresponding relatively young Hf isotopic crustal model ages of 452 to 843 Ma. The granitoid samples have the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.950 to 19.444, 15. 573 to 15.615, and 38.487 to 38.789, corresponding to initial Pb ratios of 18.483 to 18.960, 15.552 to 15.662, and 37.905 to 38.193, respectively, and yield the δ30Si values between − 0.4 and − 0.1‰, similar to the quartz separates (−0.7 to − 0.1‰). The available geochemical and isotopic data imply that the source magma at Hardat Tolgoi was most likely derived from partial melting of juvenile lower crust. Combined our whole-rock geochemical and Hf − Pb − Si isotopic data with regional tectono-magmatic evolution, it is proposed that the Early Jurassic magmatism at Hardat Tolgoi was probably attributed to the southeastward subduction of the Mongol − Okhotsk oceanic plate beneath the Erguna massif, and their generation was favorable to the Hardat Tolgoi Ag-Pb-Zn ore formation.

Origin of the Tongda fluorite deposit related to the Palaeo‐Pacific Plate subduction in southern Jiangxi Province, China: New evidence from geochronology, geochemistry, fluid inclusion, and H–O isotope compositions

The southern Jiangxi Province is an important part of the fluorite mineralization belt in South China. Fluorite ore bodies are primarily in the contact zone between the Devonian Huitong granitic complex and the Late Cretaceous Ganzhou Formation, controlled by the NE‐trending faults. Zircon U–Pb dating of the Huitong granitic complex yields emplacement ages of 410.7 ± 1.4 Ma and 400.7 ± 4.6 Ma, while the Sm–Nd dating of the fluorite yields an isochron age of 94 ± 2 Ma, suggesting that the Huitong granitic complex is the host rock. Fluid inclusions in the fluorite show low homogenization temperatures (136–207°C), salinities (1.23–3.87 wt% NaCl), and densities (0.87–0.95 g/cm3), suggesting that the ore‐forming fluid is an NaCl‐H2O system of low temperature, salinity, and density. Raman spectroscopy showed that the fluid phase is dominated by water. The δDVSMOW values of the fluid inclusions in the Tongda fluorite ranged between −59.5 and −55.2‰, while the δ18OVSMOW values of the fluorite ranged from −7.2 to −5.6‰. Collectively, the ore‐forming fluid is dominated by meteoric water, possibly with a minor contribution of hydrothermal fluid. Both the interaction with host rocks and the cooling of hydrothermal fluids are the likely mechanisms of underlaying fluorite precipitation at low temperatures. The mineralization occurred in extensional faults during the Late Cretaceous related to the subduction of the Palaeo‐Pacific Oceanic Plate.

New geochronological and geochemical constraints on petrogenesis and tectonic setting of the Loe-Shilman carbonatite complex, Northwest Pakistan

The Loe-Shilman carbonatite complex in northwest Pakistan comprises carbonatite and ultrapotassic silicate rocks. The age and environment of emplacement of these rocks have long been controversial, and therefore, impeded our understanding on the regional tectonic setting. New whole-rock geochemistry, Sr-Nd isotopic and zircon petrochronological data provide age and petrogenetic information on the carbonatite and ultrapotassic silicate rocks of the Loe-Shilman complex. Zircon U/Pb data yield 206Pb/238U weighted mean ages of 90.6 ± 1.0 Ma (n = 33) for the carbonatite and 89.3 ± 1.0 Ma (n = 30) for the ultrapotassic rocks, which indicates that the two lithologies were emplaced simultaneously during a single tectonic/thermal event. The Nd isotope data of the ultrapotassic rocks is homogenous (εNd(i) = −2.5 to −2.9) and overlap εNd(i) of the carbonatite (−2.6 to −2.8), indicating derivation from a common, enriched mantle source. Despite this temporal overlap, whole-rock geochemical data, however, indicates that generation of both rock types by fractional crystallization or liquid immiscibility is unlikely and that the carbonatite and ultrapotassic rocks are actually products of two independent melts generated by partial melting of an enriched sub-continental lithospheric mantle (SCLM) source at the same time. Trace element data further indicate that the ultrapotassic rocks are products of fractional crystallization of a melt produced by a low degree partial melting of an enriched mantle source, within the garnet stability field. Moreover, the ultrapotassic rocks and carbonatites show association with anorogenic and within-plate plume type tectonic settings, coeval with juxtaposition of the northwestern margin of the Indian plate over the Réunion hotspot mantle-plume during Late Cretaceous time.

Petrogenesis of Early Paleozoic I-type granitoids in the Wuyi-Yunkai Orogen, South China: Implications for the tectono-magmatic evolution of the Cathaysia Block

• The Early Paleozoic I-type granites were generated through partial melting, magma mixing and fractional crystallization . • Mantle-derived magmas provided the heat and material for the formation of the granites. • The granites formed during an intracontinental orogeny . • The granites were related to the assembly of northern East Gondwana . The tectonic setting of the Early Paleozoic granites in the South China Block (SCB) remains enigmatic, and it is uncertain as to whether they are related to intracontinental orogenic processes or oceanic subduction. Previous studies mostly focused on Early Paleozoic S–type granitoids in the SCB, and relatively little attention has been paid to Early Paleozoic I–type granitoids. Here we present whole–rock major and trace elemental data and LA–ICP–MS zircon U–Pb age and Lu–Hf isotopic data for hornblende monzogranites, monzogranites, and mylonitic granites of I–type granites affinities that comprise the Daning and Guiling plutons in the southwest part of the Wuyi–Yunkai orogen in the SCB. These rocks contain zircon with U–Pb ages of 422–418 Ma, taken as the age of emplacement. They are rich in LREEs and moderately depleted in Eu (δEu = 0.49–0.64), with negative Ba, Sr, Nb, and Ti anomalies, and positive in Rb, Th, U, and Pb anomalies. Our studies of these granites and their abundant enclaves indicate that they formed via complicated petrogenetic processes, including the partial melting of a mixed source of meta–sedimentary rocks and meta–basalts in the lower crust, magma mingling and mixing of crust–mantle materials, and fractional crystallization. Mantle–derived magma played an important role in the formation of these Early Paleozoic granites. Although an ancient and enriched mantle source might have made a significant contribution to these Early Paleozoic granites, the input of juvenile mantle–derived magma might have been insignificant. Taking into account all the other geological evidence from the Wuyi–Yunkai region, we suggest that the tectono–magmatic events of the Early Paleozoic took place during intraplate orogenesis rather than oceanic subduction and collision, and that these events were triggered by a far–field tectonic response to the assembly of northern East Gondwana.

Early Cretaceous A‐type Granites in the Dandong Area, NE China and its Geodynamical Implications

AbstractA‐type granites, as an important petrologic sign to build up environmental recognition, were mainly formed in extensional tectonic settings. A biotite syenogranite from the Fenghuangshan pluton in Dandong, Liaoning Province gave SHRIMP zircon U‐Pb ages of 122.5 ± 1.6 Ma, 124.9 ± 1.7 Ma and 126.9 ± 1.1 Ma. The monzogranites have SHRIMP zircon U‐Pb ages of 118.2 ± 1.6 Ma, 128.1 ± 1.7 Ma and 131.6 ± 1.9 Ma, giving an emplacement age for the pluton in the Early Cretaceous. SiO2 is 65.48–74.49 wt%, whereas that of K2O is 4.16–6.44 wt%, and that of Na2O is 2.99–4.70 wt%. They also contain 13.24–15.76 wt% of Al2O3, with an A/CNK ratio of 0.92–1.10, averaging 1.02. The alkalinity rate (AR) ranges from 2.68 to 5.12, and this range is within the AR of peraluminous type rocks. The granites of Fenghuangshan pluton are characterized by high contents of Na and K and low contents of Ca (thermophile element) and Mg, which are features of A‐type granites. The (Na2O + K2O) – Fe2O3∗ × 5 – (CaO + MgO) × 5 discrimination diagram also shows that Fenghuangshan pluton is an A2‐type granite. The above granite has zircon ∊Hf(t) values ranging from –17.06 to –9.09, with single‐stage Hf model ages (TDM) of 1141 Ma to 1498 Ma and two‐stage Hf crustal model ages (TDMC) of 1762 Ma to 2263 Ma. A comprehensive analysis suggests that the Fenghuangshan pluton might have been formed from the lithospheric‐plate sliding during the late stages of evolution of hot rift structures and might have been closely associated with the tectonic settings of Mesozoic Eurasia and the ancient Pacific Plate.

Variable Modes of Formation for Tonalite–Trondhjemite–Granodiorite–Diorite (TTG)-related Porphyry-type Cu ± Au Deposits in the Neoarchean Southern Abitibi Subprovince (Canada): Evidence from Petrochronology and Oxybarometry

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite–trondhjemite–granodiorite–diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2·7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2·74 Ga, ~2·70 Ga, and ~ 2·69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4·5 ± 0·3, 4·2 ± 0·6, and 4·3 ± 0·4, and δ18O ± SD values of 5·40 ± 0·11 ‰, 3·91 ± 0·13 ‰, and 4·83 ± 0·12 ‰, respectively. These isotopic signatures indicate that, although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e. zircon, amphibole, apatite, and magnetite–ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite–magnetite–quartz buffer as ΔFMQ, using zircon geochemistry indicates that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ –0·3 ± 0·6 to ΔFMQ +0·8 ± 0·4 and to ΔFMQ +1·2 ± 0·4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1·6 ± 0·3 and ΔFMQ +2·6 ± 0·1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate that amphibole may overestimate the fO2 of intrusive rocks by up to 1 log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e. S6+/ΣS) in primary apatite yielded ≥ΔFMQ −0·3 and ΔFMQ +1·4–1·8 for St-Jude and Clifford, respectively. The magnetite–ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3·3 ± 1·3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0·1 wt%) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate that the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0·8 ± 0·4) to moderately (~ΔFMQ +1·5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu–Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Early Paleozoic Adakitic Granitoids from the Xingshuping Gold Deposit of East Qinling, China: Petrogenesis and Tectonic Significance

This study discussed the pertrological classification, geochronology, petrogenesis and tectonic evolution of early Paleozoic granites from the Xingshuping gold deposit in the East Qinling orogenic belt. In order to achieve this target, we carried out an integrated study of zircon U–Pb age, whole-rock major and trace elements, as well as Sr–Nd–Hf isotope compositions for the Xingshuping granites (part of the Wuduoshan pluton) from the Erlangping unit. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating constrains the emplacement age of the Xingshuping granites at 446.2 ± 1.2 Ma. The rocks at Xingshuping can be divided into two types: mainly biotite granite and monzonitic granite. The biotite granites are typical adakitic rocks, while the monzonitic granites show characteristics similar to normal arc volcanic rocks. The geochemical compositions reveal that they were derived from a clay-rich, plagioclase-rich and biotite-rich psammitic lower continental crust source, with contributions of mantle-derived magmas. The distinction is that the biotite granites were primarily derived from partial melting in a syn-collision extension setting, whereas the monzonitic granite went through a fractional crystallization process in an intraplate anorogenic setting.

Petrogenesis of the granite related to the Dashunlong Sn polymetallic deposit, Dayishan ore field, South China

The roles of the mantle in generating granites and the extent of crust-mantle interaction are fundamental topics to our understanding of Sn mineralization. In this study, we report geochronological, geochemical, and Sr-Nd–Hf isotopic data for the granites, and muscovite 40Ar/39A isotopic data of the greisen type Sn ores, in the Dashunlong Sn deposit, Dayishan orefield, middle Qinzhou-Hangzhou metallogenic belt. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.6 ± 1.2 Ma (MSWD = 0.73) and 154.7 ± 1.6 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite 40Ar/39Ar plateau age of 151.1 ± 0.9 Ma (MSWD = 0.09) for the greisen-type Sn ores. It is indicated that the Sn mineralization in the Dashunlong deposit is related to the Late Jurassic Dayishan granite. The granite shows the geochemical features of highly fractionated A-type granite: 1) high 10,000 Ga/Al ratios of 2.95–3.88; 2) high Zr + Nb + Ce + Y contents of 292–637 ppm; 3) high TFeO/(TFeO* + MgO) ratios of 0.86–0.97; and 4) high Rb/Sr ratios of 2.80–18.8. Whole rocks isotopes show that this granite has variable initial 87Sr/86Sr ratios (0.71607–0.72437), negative εNd(t) values (−7.20 to −5.46), and two-stage Nd model ages of 1530–1388 Ma. LA–MC–ICP–MS zircon Lu-Hf isotopes show that the medium-fine-grained monzogranite has εHf(t) values of −0.94–2.20 and two-stage Hf model ages of 1298–1059 Ma, whereas the coarse-grained biotite monzogranite has relatively high εHf(t) values of −0.84–5.95 and two-stage Hf model ages of 1257–823 Ma. Different Hf and Nd isotopes indicate that the Nd-Hf isotopic decoupling occurs in the Dayishan granite, which was likely caused by the partial melting of the rocks containing abundant high Lu/Hf minerals. The magma of the Dayishan granite is reduced, highly fractionated, and enriched in Sn and F, resulting in large-scale Sn mineralization in the Dayishan orefield. It is proposed that these granites are originated from melts mixed by crustal- and mantle constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. A two end-member modeling indicates that ca. 10–20 percent of mantle-derived melts were likely involved in the formation of the Dayishan granites.

Origin of the dioritic porphyrite and its associated Matouniu skarn gold polymetallic deposits in the Eastern Hebei Province, North China: Evidence from geochronology, geochemistry, and C–O–S–Pb–Hf isotopes

The Matouniu gold polymetallic deposit in the Eastern Hebei Province (Jidong) in the northern margin of the North China Craton (NCC) is a skarn‐type ore deposit. Geochemical data, zircon U–Pb, and Hf isotope analysis of the mineralization‐related dioritic porphyrite and C–O–S–Pb isotopes from the deposit were used to investigate the origin, magmatic evolution, mineralization, and tectonic setting of the Matouniu gold polymetallic deposit. Zircon U–Pb dating reveals an emplacement age of 146.7 ± 1.1 Ma, indicating that the magmatism and mineralization occurred during the Late Jurassic. Geochemical, Hf, and Pb isotopic compositions indicate that the dioritic porphyrite was formed by the partial melting of the ancient mafic lower crust, leaving behind residual plagioclase, amphibole, and garnet, with traces of mantle material involvement. The S and Pb isotopic compositions revealed that both the ore‐forming materials and the dioritic porphyrite were derived from a deep‐seated magmatic source, which mainly originated from the lower crust, with minor input from the mantle. The C–O isotopic compositions suggest that the early ore‐forming fluids had a mainly deep‐seated magmatic source with minor meteoric water involvement, and were accompanied by continuous meteoric water involvement and water‐rock interaction during the fluid migration. The Matouniu gold polymetallic deposit and Late Jurassic dioritic porphyrite associated with skarn mineralization formed under geodynamics involved a compressional regime that was associated with the continuous subduction of the Palaeo‐Pacific Plate beneath the East Asia Plate.

Geochronology and geochemistry of the Late Jurassic Wujiaping Sn deposit, Dayishan ore field, South China: Implications to the petrogenesis and Sn mineralization

The Wujiaping Sn deposit is located at the northern Dayishan ore field, South China, whose ore veins are mainly hosted in the Dayishan pluton. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and medium-coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.5 ± 1.6 Ma (MSWD = 2.0) and 155.5 ± 0.5 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite 40Ar-39Ar plateau age of 150.4 ± 0.9 Ma for the quartz vein type ore veins. It is indicated that the Sn mineralization in the Wujiaping deposit is related to the Late Jurassic granitic magmatism. These granites show the geochemical features of highly fractionated S-type granite: 1) low Zr + Nb + Ce + Y contents (<209 ppm); 2) high A/CNK ratios (>1.1); 3) low crystallization temperature (mean = 688 °C); 4) high Rb/Sr ratios (mostly > 48); 5) high differentiation index (DI > 90); and 6) low CaO, P2O5, Sr, and Eu contents. Whole rocks isotopes show that these granites shows variable initial 87Sr/86Sr ratios (0.70251–0.71208), negative εNd(t) values (−7.94–5.62) and old two stage Nd model ages of 1491–1563 Ma. LA–MC–ICP–MS zircon Lu–Hf isotopes show that the medium-fine-grained monzogranite have positive εHf(t) values of 0.37–8.5 and two stage Hf model ages of 662–1180 Ma, whereas the medium-coarse grained-biotite monzogranite have negative εHf(t) values of −6.42–3.55 and two stage Hf model ages of 1431–1599 Ma. It is proposed of that these granites are originated from melts mixed by crustal- and mantle-constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. The low LogfO2 values calculated through zircons (−21.2–13.1) and high F contents (3630–5120 ppm) indicate the granites derived from reduced and F-rich melts. Therefore, the reduced melt is highly fractionated and enriched in Sn and F, resulting in the large scale Sn mineralization in the Dayishan ore field.

Misinterpretation of zircon ages in layered intrusions

Abstract A recent re-interpretation of the Bushveld Complex and other layered intrusions as stacks of randomly emplaced, amalgamated sills is mostly fuelled by finding of zircon ages that are not getting progressively younger from the base upwards, as expected from a classical model for the formation of layered intrusions. Rather, they display several reversals from older to younger ages and vice-versa with moving up-section through the layered intrusions. Here, we show that the reported zircon ages are at odds with the relative ages of rocks as defined by cross-cutting relations in potholes of the Bushveld Complex. This indicates that interpretation of the zircon isotopic data as the emplacement age of the studied rocks/units is incorrect, making a new emplacement model for layered intrusions baseless. This conclusion is further buttressed by the phase equilibria analysis showing that regular cumulate sequences of layered intrusions are not reconcilable with a model of randomly emplaced sills. In this model, the late sills are free to intrude at any stratigraphic position of the pre-existing rocks, producing magmatic bodies with chaotic crystallization sequences and mineral compositional trends that are never observed in layered intrusions. There are thus no valid justifications for the re-evaluation of the current petrological model of the Bushveld Complex and other layered intrusions as large, long-lived and largely molten magma chambers. A fundamental implication of this analysis is that the current high-precision U-Pb TIMS ages from layered intrusions are inherently unreliable on the scale of several million years and cannot therefore be used for rigorous estimations of the timing of crystallization, duration of magmatism, and cooling of these intrusions.

Age of the Dominion-Nsuze Igneous Province, the first intracratonic Igneous Province of the Kaapvaal Craton

The Kaapvaal Craton records a long Archaean to Phanerozoic history of intracratonic magmatism that reflects contrasting regimes of melt generation. The correct interpretation of this rich geological record relies critically on field evidence and ever refined chronological and isotopic data. Here, we present new U – Pb zircon, apatite and titanite ages, and Lu-Hf isotope results for volcanic and sedimentary rocks of the Dominion Group, its granitoid basement, and overlying strata of the Witwatersrand Supergroup. The data indicate an emplacement age of ca. 2.96 Ga for the Dominion Group, approximately 0.11 Ga later than previous estimates. These new ages, combined with lithological and geochemical similarities, allow the comparison and correlation between the Dominion Group and the Nsuze Group of the Pongola Supergroup. Together, the two successions constitute a single igneous province, here termed the Dominion-Nsuze Igneous Province, which was emplaced shortly after craton consolidation, spanning approximately 500 km across the central and eastern Kaapvaal Craton. This province constitutes the first intracratonic igneous province of the Kaapvaal Craton and one of the oldest in the world. The new age determinations also place limits on the onset of sedimentation of the overlying Witwatersrand Supergroup, which can now be constrained at < 2960 Ma.

Siberia’s largest pulse of kimberlites: U-Pb geochronology of perovskite and rutile from the Obnazhennaya kimberlite and its xenoliths, Siberia craton

ABSTRACT Mantle xenoliths from the Middle-Late Jurassic Obnazhennaya kimberlite are often compared with those from the Udachnaya kimberlite (ca. 367 Ma) to inform the evolution of the Siberia craton. However, there are no direct constraints on the timing of the Obnazhennaya kimberlite eruption. Such uncertainty of the kimberlite age precludes a better understanding of the mantle xenoliths from the Obnazhennaya pipe, and thus also of the evolution of the Siberia craton. This paper reports U-Pb ages for both perovskite from the Obnazhennaya kimberlite and rutile in an Obnazhennaya eclogite xenolith. The fresh perovskite formed during the early stage of magmatic crystallization and yields a U-Pb age of 151.8 ± 2.5 Ma (2σ). Rutile in the eclogite xenolith yields an overlapping U-Pb age of 154.2 ± 1.9 Ma (2σ). Because rutile has a Pb closure temperature lower than the inferred residence temperature of the eclogite prior to eruption, the U-Pb isotope system in rutile was not closed until the host eclogite was entrained and delivered to the surface by the kimberlite and therefore records the timing of kimberlite eruption. These data provide the first direct constraints on the emplacement age of the Obnazhennaya kimberlite and add to the global ‘kimberlite bloom’ from ca. 250–50 Ma as well as to the largest pulse of kimberlite volcanism in Siberia from ca. 171–144 Ma. The timing of this Jurassic–Cretaceous pulse coincides with the closure of the Mongol–Okhotsk Ocean, but the depleted Sr-Nd isotopic characteristics of 171–144 Ma kimberlites are inconsistent with a subduction-driven model for their petrogenesis. Thus, the closure of the Mongol-Okhotsk Ocean may act as a trigger for the initiation of 171–144 Ma kimberlite emplacement of Siberia, but was not the source.

Mafic intrusions in southwestern Australia related to supercontinent assembly or breakup?

Variably oriented dolerite intrusions outcrop in the Albany–Fraser Orogen along the south coast of Western Australia with previously unknown ages but where previous studies interpreted Mesoproterozoic to Cretaceous emplacement. Here, we place temporal constraints on seven mafic intrusions across ∼150 km of coast using zircon U–Pb, apatite U–Pb, and plagioclase 40Ar/39Ar geochronology, coupled with whole-rock major and trace-element geochemistry, that reveal late Mesoproterozoic to potentially Early Cretaceous crystallisation ages. Three intrusions metamorphosed to greenschist facies are likely associated with either the emplacement of the ca 1210 Ma Marnda Moorn large igneous province or Stage II Albany–Fraser Orogeny, both of which were associated with the assembly of Rodinia. Three unmetamorphosed dykes have (probable) Neoproterozoic to lower Cambrian emplacement ages, likely associated with the ca 550–500 Ma Kuunga Orogeny during Gondwana assembly. The final sill, also unmetamorphosed, strikes perpendicular to the other six intrusions, shows unusual Pb anomalies and contains inherited zircon that has been reset by a Permian or younger event, pointing towards magmatism in southwestern Australia during the breakup of Gondwana. The new results provide hitherto unrecognised mafic intrusive evidence for modification of Proterozoic crust, potentially associated with Rodinia assembly, Gondwana assembly and Gondwana breakup in southwestern Australia. KEY POINTS Variably oriented mafic dykes in southwest Australia are dated by zircon U–Pb, apatite U–Pb and plagioclase 40Ar/39Ar methods. The dykes are related to Rodinia assembly (ca 1200 Ma), Gondwana assembly (ca 550 Ma) and, probably, Gondwana breakup (ca 135 Ma). These new ages provide evidence for mafic activity clearly linked to the supercontinent cycle.

Hydrothermal monazite trumps rutile: Applying U-Pb geochronology to evaluate complex mineralization ages of the Katbasu Au-Cu deposit, Western Tianshan, Northwest China

Abstract The Tianshan orogenic belt hosts several world-class gold deposits and is one of the largest gold provinces on Earth. The Katbasu Au-Cu deposit in the Chinese Western Tianshan is hosted in a granite intrusion. Previous researchers have shown that the main gold ores formed much later than the ore-hosting granite. However, the formation age of Cu mineralization and its possible link to Au mineralization remain poorly understood. This paper reports detailed mineralogical studies, combined with zircon U-Pb, in situ hydrothermal monazite as well as rutile U-Pb ages to constrain the timing of Cu mineralization and its possible link to Au mineralization. The two main ore types in the Katbasu deposit include Cu-Au ores with pyrite-chalcopyrite veins that crosscut the granite and Au ores with massive pyrite and quartz as the main minerals. The Cu-Au ores are spatially associated with diorite that intruded the granite, and they are overprinted by massive gold ores. Detailed mineralogical studies show that chalcopyrite is the main Cu-bearing mineral in the Cu-Au ores, and it is closely associated with some native gold, monazite, and rutile. Secondary ion mass spectrometer (SIMS) U-Pb dating of zircon grains from the ore-hosting granite and mafic enclave yielded concordant ages of 354.1 ± 1.6 and 355.8 ± 1.7 Ma, respectively. The diorite that intruded the granite has a zircon U-Pb age of 352.0 ± 3.2 Ma. The trace element compositions of the monazite suggest they were formed by hydrothermal fluids rather than inherited from the ore-hosting granite. Hydrothermal monazite coexisting with chalcopyrite and native gold yielded a concordant age of 348.7 ± 2.3 Ma, and the W-rich hydrothermal rutile grains associated with the chalcopyrite yielded a U-Pb age of 345 ± 27 Ma, indicating an early Cu-Au mineralization event prior to the major Au mineralization (ca. 323–311 Ma). The formation time of early Cu-Au mineralization is consistent with the emplacement age of the diorite and may be of magmatic-hydrothermal origin, whereas the main Au has no genetic associations with magmatic rocks in the ore district and may belong to the orogenic type. Monazite geochronology provided a more reliable age constraint than rutile in the Katbasu Au-Cu deposit, and we suggest hydrothermal monazite has advantages over rutile in dating the complex mineralization ages of gold deposits.

Compositional evolution of the muscovite of Renli pegmatite-type rare-metal deposit, northeast Hunan, China: Implications for its petrogenesis and mineralization potential

The Renli deposit is the largest known pegmatite-type Ta–Nb deposit in south China. Its rare-metal pegmatites show regular zoning of pegmatite types and rare-metal mineralization assemblages from northeast to southwest. One hundred and forty major outcropping pegmatite dikes throughout the deposit can be approximately divided into four zones outward from the Mufushan batholith: microcline pegmatite zone (P1); microcline–albite pegmatite zone (P2); albite pegmatite zone (P3); and albite–spodumene pegmatite zone (P4). The sizes of the dikes decrease with distance from the batholith, and their rare-metal mineralization type changes as follows: Be → Nb–Ta (–Be) → Li (–Nb–Ta). Field geological characteristics, chronological and Lu–Hf and Sr–Nb–Pb isotopic studies indicate that these rare-metal pegmatites have a close petrogenetic relationship with the Mufushan complex granitic intrusions. However, because of the similarities in geochemical characteristics, magma source properties, and emplacement ages of these different granitic intrusions, identifying the parental rock of rare-metal pegmatites is challenging.Muscovites are common throughout the granitic intrusions and four types of pegmatites in the Renli deposit, and are intimately associated with rare-metal mineralization. The geochemistry of muscovite obtained from granites and pegmatites can be used to demonstrate the parental relationships of rare-metal pegmatites and Mufushan granites, allowing pegmatite evolution trends and the mineralization potential of the Renli deposit to be evaluated. The K/Rb versus Cs, Rb, Li, Nb, and Ta ratios and Ta/Nb versus Li, Ta, and TiO2 ratios in muscovites from the granites and pegmatites show that they form an association with the following sequence of evolution: biotite monzogranite → two-mica monzogranite → microcline pegmatite → microcline–albite pegmatite → albite pegmatite → albite–spodumene pegmatite. The above complete evolution sequence represents the magmatic stage of the deposit and the lepidolite-quartz core in pegmatite with well internal zoning represents the fluid-rich stage of the deposit. The Al4Si–3□–1 exchange vector (where □ represents a vacancy) is shown to have played an important role in the first magmatic stage of the Renli deposit when compositional changes in Li-poor muscovite formed in the magmatic stage involved this substitution. During the late fluid-rich stage formed by highly differentiated magma at the end of the magmatic evolution of the deposit, Li was incorporated into micas by Li3Al–1□–2. Many pegmatite dikes with transitional boundaries can be observed in the outcrops of two-mica monzogranite (RLG2), and geochemical characteristics of muscovite suggest that this monzogranite show the highest differentiation degree in Renli granites, whose average K/Rb and Ta/Nb ratios are close to those of the Renli pegmatites. This indicates that the two-mica monzogranite (RLG2) has the closest petrogenetic relationship with the Renli rare-metal pegmatites. Combined with the data from this and previous studies, the K/Rb ratio, Li and Cs contents of muscovite can be used to distinguish the barren pegmatite and different types (Be, Nb–Ta, or Li type) of rare-metal pegmatites to a certain degree. Therefore, muscovite can indicate the metallogenic type and economic potential in a pegmatite-type rare-metal deposit.

Paleoproterozoic calc-alkaline lamprophyres from the Sidhi Gneissic complex, India: Implications for plate tectonic evolution of the Central Indian Tectonic Zone

Calc-alkaline lamprophyres are widely regarded as important probes for unravelling continental-scale geodynamic processes. We present petrographic, mineral chemical, bulk-rock geochemical, and Sr-Nd isotope data on eleven unmetamorphosed lamprophyre dykes intruding the ca. 2.5 Ga Sidhi Gneissic Complex in the Central Indian Tectonic Zone (CITZ) – an intercontinental suture which separates the Northern Indian (Aravalli-Bundelkhand Craton) and the Southern Indian (Dharwar-Bastar-Singhbhum Craton) blocks. The Sidhi dykes have porphyritic-panidiomorphic texture typical of lamprophyres, with phlogopite/biotite as the dominant phenocryst phase, in a orthoclase dominated groundmass. Based on mineralogy and geochemistry, the Sidhi dykes can be classified as calc-alkaline lamprophyres in general and minettes in particular. All these dykes are characterised by negative Ti, Ta and Nb anomalies typical of subduction-related global calc-alkaline lamprophyres. The trace element ratios of the Sidhi lamprophyres suggest derivation from variable degrees of partial melting of a similar magma source which experienced crustal input. Bulk-rock Rb-Sr isochron of the Sidhi lamprophyres yielded an age of ca. 2278 ± 230 Ma which is consistent with the Paleoproterozoic emplacement ages reported for other rocks from this region of CITZ. Negative ɛNdT (−3.68) values of the Sidhi lamprophyres suggest their derivation from an isotopically enriched mantle source whilst their TDM Nd model ages (~2.6 Ga) imply source enrichment that took place during the Neoarchaean. Petrogenetic modelling indicate that the Sidhi lamprophyres are generated from low degree partial melting of a mantle source metasomatised by sediment-derived melt having < 4% crustal material as an input in the E-DMM. Our study support models proposing a northward subduction of the Southern Indian Block beneath the Northern Indian Block along the CITZ.

The significance of galena Pb model ages and the formation of large Pb-Zn sedimentary deposits

In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.

Late Paleozoic adakitic magmatism in the Zogdor Cu occurrences, southern Mongolia, and their tectonic implications: New SHRIMP zircon age dating, Lu-Hf isotope systematics and geochemical constraints

The Zogdor Cu occurrences are located in the Gurvansayhan island arc terrane of southern Mongolia, which corresponds to the southern part of the Central Asian Orogenic Belt (CAOB). The major porphyry type copper–gold (molybdenum) mineralized belt including Oyu Tolgoi, Tsagaan Suvarga, Shuteen, and Kharmagtai deposits are located in this terrane and were known to have formed during 372–324 Ma. The Zogdor Cu occurrences are found within this porphyry Cu-Au mineralized belt and are limitedly distributed in granitoids including granodiorite and monzogranite. In this study, we present new SHRIMP zircon U-Pb geochronology, LA-MC-ICPMS zircon Lu-Hf isotopes, and geochemical data for the granitoids in the Zogdor Cu occurrences and provide a new tectonic model including igneous activities and Cu mineralization in southern Mongolia. The emplacement ages of granodiorite and monzogranite are 329.1–326.4 Ma, and 329.9–326.1 Ma, respectively. All zircons have positive εHf values corresponding to two-stage model ages (T2DM) ranging from 581 to 331 Ma, possibly demonstrating that late Neoproterozoic to middle Carboniferous juvenile continental crust growth was derived from a depleted mantle source. Both granitoids are characterized by high Al2O3 and Sr, and low Y and Yb contents, as well as high Sr/Y and (La/Yb)N ratios, which are typical geochemical characteristics of adakites. The trace elements are characterized by an enrichment in the large ion lithophile elements (e.g., Cs, Rb, Ba, and Sr) and a depletion in the high field strength elements (e.g., Nb, P, and Ti), indicating that these granitoids intruded in subduction tectonic settings. In conclusion with data from previous studies, the intermittent igneous activities during the Cambrian to Ordovician (519–482 Ma), middle Devonian (374–363 Ma), middle to late Carboniferous (353–304 Ma), and early Permian (298–286 Ma) took place in the southern Mongolia and porphyry copper mineralizations seem to be closely related the middle Devonian and middle Carboniferous adakitic or adakite-like magmatism. The Lu-Hf isotopic results for these associated granitoids including previous Rb-Sr and Sm-Nd isotopic studies imply that the Neoproterozoic to middle Carboniferous juvenile continental growth derived from a depleted mantle source. The middle Devonian and middle Carboniferous magmatic activities triggered by the subduction of the Paleo-Asian Oceanic plate and formed the porphyry copper deposits in the southern Mongolia.

Emplacement conditions of lunar impact melt flows

Hypervelocity impacts on terrestrial bodies have the potential to rapidly heat and redistribute silicate target material to form impact melt flows. On the Moon, a subset of impact melt deposits exited the craters as laterally moving flows that moved away from the crater rim under the influence of gravity. These impact melt flows exhibit similarities to lava flows, but have the potential to be superheated by the impact cratering process. To estimate the initial temperature (T0) and thickness (H0) of these flows we combine new remote sensing analyses, experimentally-derived rheological relationships for three lunar analog materials, and numerical forward models of impact melt flow emplacement to identify combinations of parameters that yield flows matching the observed length of impact melt deposits on the Moon. We focus on the impact melt flows of four craters, which span a range of target materials and crater diameters (D): Giordano Bruno (D = 22.1 km), Necho (D = 36.9 km), Tycho (D= 85.3 km), and Copernicus (D= 96.1 km). We modeled three melt compositions—anorthosite, norite, and basalt—emplaced under cold ambient temperatures (Tambient= –180 °C) on the Moon, to place a minimum bound on cooling-limited flow emplacement. Results show that observed flow lengths can be achieved by subliquidus melts emplaced within a laminar flow regime. For high-density (0% vesicularity) melts, best-fit inversions for T0 and H0 have mean values of 1253.3 ± 149.8 °C and 21.6 ± 8.7 m, respectively; for low-density (64% vesicularity) melts, mean T0 and H0 values are 1343.3 ± 87.8 °C and 27.9 ± 5.7 m—with uncertainties reported at 1 standard deviation. Model results also show that under cold ambient temperatures, impact melt flows with 0 to 64% vesicularity would be expected to have mean emplacement durations of 76, 78, 174, and 186 h for Giordano Bruno, Necho, Tycho, and Copernicus, respectively. Therefore, initial impact melt temperatures do not have to be in the super-liquidus range to form gravity-driven flow deposits matching observed lengths, and emplacement times are long enough for the majority of self-secondary fragments to land before the impact melt flows are fully emplaced. This suggests that some impact melt flow may be lava-like in their emplacement dynamics and that differences in crater populations on impact melt flows and adjacent ejecta may differ significantly, with impact melts providing a more robust estimate of the emplacement age of the young craters due to the presence of fewer secondaries. Impact melts could still have super-liquidus temperatures at the time of their initial formation, but turbulence would facilitate the rapid entrainment of external clasts, quickly lowering melt temperatures to generate flows that are similar to the products of effusive volcanic eruptions.