Zircon Ages Research Articles

Mechanisms of rapid plant community change from the Miocene Succor Creek flora, Oregon and Idaho (USA)

The fossil record of the U.S. Pacific Northwest preserves many Middle Miocene floras with potential for revealing long-term climate-vegetation dynamics during the Miocene Climatic Optimum. However, the possibility of strong, eccentricity-paced climate oscillations and concurrent, intense volcanism may obscure the signature of prevailing, long-term Miocene climate change. To test the hypothesis that volcanic disturbance drove Middle Miocene vegetation dynamics, high-resolution, stratigraphic pollen records and other paleobotanical data from nine localities of the Sucker Creek Formation were combined with sedimentological and geochemical evidence of disturbance within an updated chronostratigraphic framework based on new U-Pb zircon ages from tuffs. The new ages establish a refined, minimum temporal extent of the Sucker Creek Formation, ~15.8 to ~14.8 Ma, and greatly revise the local and regional chronostratigraphic correlations of its dispersed outcrop belt. Our paleoecological analysis at one ~15.52 Ma locality reveals two abrupt shifts in pollen spectra coinciding with the deposition of thick ash-flow tuffs, wherein vegetation dominated by Cupressaceae/Taxaceae, probably representing a Glyptostrobus oregonensis swamp, and upland conifers was supplanted by early-successional forests with abundant Alnus and Betula. Another ephemeral shift from Cupressaceae/Taxaceae swamp taxa in favor of upland conifers Pinus and Tsuga correlates with a shift from low-Ti shale to high-Ti claystone, suggesting a link between altered surface hydrology and vegetation. In total, three rapid vegetation shifts coincide with ash-flow tuffs and are attributed to volcanic disturbance. Longer-term variability between localities, spanning ~1 Myr of the Miocene Climatic Optimum, is chiefly attributed to eccentricity-paced climate change. Overall, Succor Creek plant associations changed frequently over ≤105 years timespans, reminiscent of Quaternary vegetation records. Succor Creek stratigraphic palynology suggests that numerous and extensive collection of stratigraphically controlled samples is necessary to understand broader vegetation trends through time.

Mineral Chemistry and In Situ LA-ICP-MS Titanite U-Pb Geochronology of the Changba-Lijiagou Giant Pb-Zn Deposit, Western Qinling Orogen: Implications for a Distal Skarn Ore Formation

The giant Changba-Lijiagou (Ch-L) Pb-Zn deposit is in the northeast part of the Xicheng ore cluster, Western Qinling Orogen. The ore genesis remains controversial; it could be either a sedimentary exhalative genetic type or an epigenetic hydrothermal genetic type. Here, in situ titanite U-Pb dating for the two kinds of titanite is presented, yielding ages of 212.8 ± 3.0 Ma in the mineralized skarn ore and 214.6 ± 5.1 Ma in the host rock. These ages conform to the previously reported magmatic zircon age (229–211 Ma) based on the in situ zircon U-Pb dating of plutons in this district and the time of large-scale magmatic–hydrothermal activities in Western Qinling Orogen (229–209 Ma). Titanites occurring in mineralized skarn and those that are calcite-hosted are similar to hydrothermal-origin titanites in major element characteristics. The Eu anomalies in the two types of titanite record oxidizing conditions during the mineralization process. A mineral assemblage of garnet, pyroxene, riebeckite, biotite, and potash feldspar, replacing the albite, is well-developed in the deposit. The mineralogical and geochronological characteristics indicate that the Ch-L Pb-Zn deposit is a distal skarn deposit and the result of intensive tectonomagmatic processes in the Xicheng ore cluster during the process of the Late Triassic orogeny.

Geological position of the Junggar terrane (Southern Kazakhstan) in the structure of Rodinia supercontinent: results of research of the late-precambrian metasedimentary complexes

The results of studying of the Precambrian metasedimentary sequences of the Junggar terrane located in South Kazakhstan are given in the article. In the structure of the Junggar terrane, we studied the rocks of the Sarychebyn group and Kosagash formation. Petrogeochemical data combined with the results of U‒Pb and Lu‒Hf isotope-geochronological studies of the detrital zircons showed that the Sarychebyn Group and the Kosagash formation represent a similar stratigraphic level that accumulation occurred in the Late Mesoproterozoic to Early Neoproterozoic (~1026‒~920 Ma). The main sources of the detrital zircon age populations were the Mesoproterozoic and Paleoproterozoic complexes. Among these complexes can be identified metabasites and metapelites of intermediate and high metamorphic grades, as well as felsic igneous rocks formed with the participation of various sources, can be distinguished. The Junggar terrane exhibits a close tectonic affinity with the Aktau-Mointy, Yili, Issyk-Kul, Chinese Central Tien-Shan, and the Northern Kazakhstan terranes in the Late Precambrian. They were probably located near the Sveconorwegian orogen in the western Baltica within the Rodinia supercontinent structure.

The mid-Cretaceous Hohonu Batholith (South Island, New Zealand): Identifying magmatic sources and processes during onset of crustal extension

The Hohonu Batholith is an aggregation of mostly mid-Cretaceous granitoid plutons on the West Coast of the South Island of New Zealand emplaced during a transitional period between subduction-related compression and continental lithospheric extension. This study reports an integrated dataset, comprising in-situ UPb, O and Hf isotope compositions and REE, Ti and other trace and major elements (ZrHf, ThU) for zircons extracted from four representative plutons within the batholith. Our results provide detailed insight into the protracted thermal, chronological and geochemical histories. LA-ICPMS UPb zircon ages indicate a primary episode of magma genesis and emplacement from 107 to 113 Ma, confirming published SIMS dating. However, a younger previously unrecognized age population of ~91–96 Ma is identified, primarily (although not exclusively) in zircon rims. This younger age event coincides with the timing of protracted lithospheric extension and crustal thinning of the Zealandia continent. The cryptic younger zircon ages suggest that Hohonu granitoids experienced a partial thermal overprint (accompanied by Pb loss) mostly recorded in rims. Differences in bulk rock geochemistry between plutons are inferred to reflect variable conditions of partial melting controlled by source mineralogy and H2O content. Isotope and trace element compositions, along with Ti-thermometry, measured on the same micro-volume of CL-imaged zircons, are used to test if source characteristics were imparted from melt to minerals in zircon-saturated silicic systems. Similarities are revealed in the zircon record of the selected plutonic rocks, confirming their broadly consanguineous relationship and the fundamental role of open-system behaviour, involving hybridization or assimilation between mantle-derived (or juvenile mafic) and a crustal-derived components, as previously inferred from whole rock NdSr isotope systematics. However, intra-sample decoupling of zircon OHf isotope systematics may also be linked to residual source component unmixing. This possibility, in addition to mafic recharge, may have obscured melt source compositional characteristics, and hence zircon REE appear as unsuitable fingerprints of source(s) and conditions of partial melting in this granitoid system. Simple compositional and thermal magma evolution trends appear punctuated by episodes of mafic recharge, presumably during lithospheric thinning.

Provenance and tectonic intricacies revealed by in situ Rb-Sr dating of detrital micas

The use of detrital minerals to reconstruct sedimentary provenance is subject to a range of biases, which may reduce the geological information they retain. With the advent of in situ Rb-Sr geochronology, rapid analyses of a representative (>100) number of grains can now be performed on radiogenic Sr-bearing minerals to provide more comprehensive knowledge of source-to-sink processes. Here, we test the in situ Rb-Sr technique on detrital white mica and biotite from basin margin and axis in a simple rift basin (Perth Basin, Australia) for which previously published detrital zircon U-Pb and Hf isotopic data exist. For the basin margin adjacent to Archean basement, which has unimodal detrital zircon ages at c. 2600 Ma, two mica samples reveal a dominant age peak at c. 500 Ma, reflecting thermal resetting in the craton proximal to the Gondwanan orogenic front, and a subordinate portion of Archean mica detritus that can be interpreted as having been distally sourced (>100 km away). Similarly, a basin axis sample yielded minor c. 1200 Ma apparent mica ages, representing distal sources from the Albany-Fraser-Wilkes Orogen, and c. 500 Ma grains are likely a resetting product. Whilst obscured in the detrital zircon record, mica samples allow quantification of the relative contributions of distal and proximal sources to the basin. Finally, detrital biotite that was (partially) altered to chlorite yielded partially to fully reset ages between c. 500 and 130 Ma, the latter linked to heating from the c. 137–130 Ma Bunbury Basalt. Ultimately, the use of in situ Rb-Sr geochronology from detrital micas reveal previously unrecognized provenance and tectonic information that is critical to understanding the true complexity of ancient geological histories, but which remains obscured in standard detrital zircon U-Pb geochronology.

Rapid India–Asia Initial Collision Between 50 and 48 Ma Along the Western Margin of the Indian Plate: Detrital Zircon Provenance Evidence

Constraining the collision timing of India and Asia requires reliable information from the coeval geological record along the ~2400 km long collisional margin. This study provides insights into the India–Asia collision at the westernmost margin of the Indian Plate using combined U-Pb geochronological data and sandstone petrography. The study area is situated in the vicinity of Fort Munro, Pakistan, along the western margin of the Indian Plate, and consists of the Paleocene Dunghan Formation and Eocene Ghazij Formation. The U-Pb ages of detrital zircons from the Dunghan Formation are mainly clustered between ~453 and 1100 Ma with a second minor cluster between ~1600 and 2600 Ma. These ages suggest that the major source contributing to the Dunghan Formation was likely derived from basement rocks and the cover sequence exposed mainly in Tethyan Himalaya (TH), Lesser Himalaya (LH), and Higher Himalayan (HH). Petrographic results suggest that the quartz-rich samples from the Dunghan Formation are mineralogically mature and have likely experienced log-distance transportation, which is possible in the case of an already established and well-developed river system delivering the sediments from the Craton Interior provenance. Samples of the overlying Ghazij Formation show a major detrital zircon age clustered at ~272–600 Ma in the lower part of the formation, comparable to the TH. In the middle part, the major cluster is at ~400–1100 Ma, and a minor cluster at ~1600–2600 Ma similar to the age patterns of TH, LH, and HH. However, in the uppermost part of the Ghazij Formation, ages of <100 Ma are recorded along with 110–166 Ma, ~400–1100 Ma, and ~1600–2600 Ma clusters. The <100 Ma ages were mainly attributed to the northern source, which was the Kohistan-Ladakh arc (KLA). The ~110–166 Ma ages are possibly associated with the TH volcanic rocks, ophiolitic source, and Karakoram block (KB). The Paleozoic to Archean-aged zircons in the Ghazij Formation represent an Indian source. This contrasting provenance shift from India to Asia is also reflected in the sandstone petrography, where the sample KZ-09 is plotted in a dissected arc field. By combining the U-Pb ages of the detrital zircons with sandstone petrography, we attribute this provenance change to the Asia–India collision that caused the provenance shift from the southern (Indian Craton) provenance to the northern (KLA and KB) provenance. In view of the upper age limit of the Ghazij Formation, we suggest the onset of Asian–Indian collision along its western part occurred at ca. 50–48 Ma, which is younger than the collision ages reported from central and northwestern segments of the Indian plate margin with 70–59 Ma and 56 Ma, respectively.

Petrogenesis and tectonic setting of mafic magmatism within the Laguna Amarga Metamorphic Complex, Andes of Catamarca, Argentina: Insights into the opening of the Cuyania/Precordillera terrane from the Ouachita rift

Neoproterozoic crystalline basement rocks are exposed as fault-bounded blocks over the high Andes of Catamarca. The crystalline basement is stratigraphically grouped into the unique Laguna Amarga Metamorphic Complex and represents the northern extension of the Cuyania/Precordillera terrane. Tabular bodies of meta-mafic rocks are widespread in the basement interspersed within a thick sequence of meta-sedimentary rocks derived from siliciclastic, calc-silicate, and limestone protoliths. Overall, the geochemical characteristics of meta-mafic rocks are in the compositional range of Normal-Mid Ocean Ridge Basalt (Normal-MORB), reflecting a common depleted-mantle source with varying degrees of partial melting. While preserving the typical bulk chemistry of MORB magmatism, some mafic magma underwent differentiation at emplacement, leading to the development of high-Ti mafic rocks. New U-Pb zircon geochronology reveals three distinct age peaks, with two coinciding with ages identified in the metasedimentary host rocks. The dominant Mesoproterozoic age cluster is linked to inherited zircon crystals assimilated within a single meta-mafic rock. In contrast, zircon ages from the late Ordovician to early Devonian are attributed to metamorphic overgrowths. Notably, the third age cluster, delineates a Late Neoproterozoic magmatic event, indicating the temporal span of mafic magmatism. The finding agrees with the best available age (576 ± 17 Ma) for mafic magmatism on the Precordillera Mafic-Ultramafic Belt. Stratigraphic relationships and geochemical fingerprints enable correlation among the meta-mafic rocks from Laguna Amarga, tracing a belt of mafic magmatism with an oceanic affinity that extends southward. Building upon previous works, this study reaffirms that the rift-drift transition of the Cuyania/Precordillera terrane, linked to the Ouachita rift opening from southeastern Grenville, evolved during the latest Neoproterozoic.

Controls on Corundum Formation: Metasomatism of Ultramafic Rock, Nattavit, South-East Greenland

Abstract The geotectonic setting for plumasite-type corundum occurrences is understudied, even though it is of importance for the understanding of trace-element patterns used for fingerprinting of ruby and sapphire. Mineral reactions related to metasomatism caused by pegmatite intrusion into ultramafic rock result in a characteristic trace element signature in corundum and thereby control its colour. The Nattivit area, Isertoq Terrane, South-East Greenland, provides a natural laboratory to investigate these mineral reactions and corundum trace element patterns given the excellent exposure of a typical plumasite-type occurrence where pegmatites intruded ultramafic rocks of different composition, namely lherzolite and dunite. The pegmatite dykes are 10 to 50 cm wide in the ultramafic rocks, whereas the adjacent alteration zone in the ultramafic rock reaches widths between 10 and 30 cm. Metasomatism resulted in desilication of the pegmatites and a decrease in Ca, Mg, K, Mn, Al and Fe away from the centre of the pegmatite dyke. Chromium, Ni, Mg, Fe, Sc, Co, V, Zn, Ti and Mn in the metasomatic reaction zones are predominantly derived from the ultramafic rock. We identify three zones with different mineral assemblages. In the lherzolite, tschermakite and biotite are formed in the centre of the reaction zone, which is followed by anorthite-rich plagioclase, hercynite, dolomite and ultimately pink corundum that occurs in the most heavily reacted part of the metasomatic reaction zone. The metasomatic reaction zones in the ultramafic rock include an intense reactionzone at direct contact to the pegmatite dyke with biotite and actinolite, and two alteration zones further away from the pegmatite dyke with enstatite, actinolite, anthophyllite, phlogopite, dolomite, sulphide, apatite and chrome-spinel. In the dunite, no biotite formed and hence, corundum contains more Fe, Mg and Ti. The plumasite-type corundum from Nattivit contains more Fe than Cr, which is typical of pink corundum hosted in mafic-ultramafic rocks. The corundum-forming reaction is dated from the pegmatite vein to 1843±4 Ma (U-Pb zircon age), which is coinciding with convergence of the Rae and North Atlantic cratons resulting in the Nagssugtoqidian Orogen. Only syn-tectonic, corundum-normative, peraluminous, calc-alkaline pegmatites of granitic to granodioritic composition that intrude into ultramafic rocks in the upper plate formed corundum in this area. The pegmatites are classified as muscovite class granitic pegmatites and intruded at upper amphibolite facies conditions. These pegmatites possible originated from partial melting of mafic granulite or a subducted oceanic plate. The formation of granitic pegmatites and related corundum mineralization in the upper plate of a collisional orogen described here is comparable to other corundum occurrences, e.g. Polar Urals, and thus is regarded as a typical geotectonic setting for plumasite-type corundum.

Discovery of the Callovian Oceanic Anoxic Event in the Qiangtang Basin, eastern Tethys: Insights from in situ calcite U[sbnd]Pb dating

Oceanic anoxic events (OAE) are global carbon-cycle perturbations and major paleoenvironmental changes documented by deposition of black shales rich in organic matter. Jurassic black shales exposed in the Biluo Co Section of the Qiangtang Basin (central Tibet) display a ∼ 3 % negative carbon isotope excursion (CIE) previously regarded as documenting the Toarcian T-OAE based on poorly preserved ammonites and one detrital zircon age. New biostratigraphic data on diagnostic calcareous nannofossils and ammonite assemblages have suggested a younger, Middle Jurassic age, thus raising doubts about the presence of the T-OAE in the Qiangtang Basin. We here present the first accurate in situ LA-ICPMS calcite UPb ages for carbonate layers intercalated in the Biluo Co section. Ages of 165.6 ± 3.7 Ma, 166 ± 16 Ma, and 164.0 ± 7.2 Ma obtained from two samples analyzed in two different laboratories indicate the Late Bathonian to mid-Callovian age, which is consistent with recent age assignments based on calcareous nannofossils and ammonites. Petrographic, geochemical, and C–O isotope analyses testify to only minor diagenetic effects, indicating that geochronological data do reflect the original timing of carbonate deposition. Therefore, the new age assignment does not document the T-OAE in the Qiangtang Basin, but possibly a younger, Callovian global-warming event.

Petrogenesis of supracrustal rocks from the Maxianshan and Xinglongshan Groups in the eastern Central Qilian block: Constraints on the construction of Rodinia

Controversy has long surrounded the reconstruction of the East Asian blocks in the Rodinia supercontinent, which was a coherent large landmass during 900–750 Ma and is now dispersed over all current major continents. The Central Qilian block is a Precambrian microcontinent in the early Paleozoic Qilian orogenic belt, which marks the junction of the North China, South China and Tarim cratons. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of supracrustal rocks from the Maxianshan Group and the Xinglongshan Group in the easternmost part of the block. The metasedimentary rocks from both groups overlie a gneissic granite, which has a zircon U-Pb age of 970 ± 6 Ma with εHf(t) values of −3.5 to + 2.5 and is an I-type granite formed in a back-arc setting. Paragneisses from the Maxianshan Group and micaschists from the lower formation of the Xinglongshan Group have detrital zircon U-Pb ages of 2465–876 Ma that peak at ca. 950 Ma. They show strongly decreasing zircon εHf(t) values of + 0.8 to −11.3 and ages from 1174 Ma to 876 Ma. Their protoliths constituted a sedimentary sequence with a long history of deposition during 1174–911 Ma in a continental arc-related basin. Metabasaltic tuffs from the middle formation of the Xinglongshan Group are tholeiitic with a zircon U-Pb age of 958 ± 9 Ma and indicate a back-arc setting. Metasandstones from the upper formation of the Xinglongshan Group have detrital zircon ages of 2668–732 Ma that peak at 810 Ma and 984 Ma and indicate a passive margin setting. Combining our results with existing data, we propose that the Maxianshan Group and the Xinglongshan Group make up an early Neoproterozoic trench-arc-basin system at a continental margin of Rodinia. Oceanic crust subduction underneath the continent at 1174–896 Ma with formation of a mature continental arc, and continuous subduction from 824 to 735 Ma with opening of the Proto-Tethys Ocean as a back-arc basin are suggested for the formation of the Central Qilian block.

Testing the fidelity of zircon as a provenance indicator in fluvial‐fan successions: An example from the Palaeogene Colton Formation, Central Utah, USA

AbstractThe stratigraphic, spatial and temporal variability of detrital zircon age populations in continental sedimentary successions is a critical tool in understanding palaeodrainage networks and how these systems distributed detritus within sedimentary basins. However, multiple factors, such as variations in sediment‐transport processes, the scale of the depositional environment and the architecture of the sedimentary succession are often overlooked in detrital zircon studies. This article presents detrital zircon U–Pb geochronology from the fluvial‐dominated Colton Formation in the western Uinta Basin (Utah, USA) to assess the system's provenance and evolution. Significant differences in zircon age populations between the Colton Formation and the overlying Green River Formation suggest a reorganisation of the source‐to‐sink system during the transition between the two lithostratigraphic units. Notably, detrital zircon age spectra are not homogeneous across the Colton Formation, therefore physical morphometric parameters were used to verify the possible influence of selective bias during sediment transport. These data reveal that a relatively finer‐grained population of Precambrian, and to a less extent Mesozoic, zircon grains were affected by hydraulic sorting during transport, resulting in a greater relative abundance of older zircon grains in the distal reaches of the distributive fluvial system, whose basinward decrease in competence would have increased the relative proportion of finer zircon fractions in sandstones. Furthermore, there are different trends in the distributions of zircon age populations relative to their stratigraphic position, highlighting the complex architecture of the fluvial palaeo‐fan. The spatial and stratigraphic variability of provenance signals in fluvial‐fan successions must be carefully evaluated to improve the reliability of source‐to‐sink models and palaeodrainage reconstructions, as autogenically controlled noise can be generated during the dispersal of detrital zircon in fluvial sedimentary systems.

AGE AND GEODYNAMIC SETTINGS FOR FORMATION OF CARBONATITE COMPLEXES AND ASSOCIATED RARE METAL DEPOSITS OF THE SOUTH URAL

The Ilmen-Vishnevogorsky and Buldym carbonatite complexes occurring in the Southern Urals represent linear deformed carbonatite complexes. Their origin, as well as the age and geodynamic conditions remain the subject of debate. The isochron methods (Rb-Sr, Sm-Nd, TIMS) and local U-Pb-dating of zircons (SHRIMP II and LA-ICP MS) of these carbonatite complexes were employed to determine the age and duration of the stages of alkali-carbonatite magmatism and associated rare-metal ore formation. The Silurian-Devonian U-Pb zircon ages of the early phases of Ilmen-Vishnevogorsky miaskites and carbonatites were determined as 420.7±11 Ma (S2) and 417±2.8 Ma (D1), respectively. In the later phases of miaskites and carbonatites, early zircons are resorbed, they have broken isotope systems, and later zircon generations form a cluster of 386±7.6 Ma (D2). The Lower Permian U-Pb age of zircon 280±8 Ma (P1) was determined in the miaskite-pegmatite and late carbonatite. The isochron dating of late ore-bearing varieties of carbonatites provided the ages from the Lower Permian to the Early Triassic (P1–T1): 254±18 Ma, Sm-Nd and 247±4 Ma, Rb-Sr, IVC; 280±53 Ma, Sm-Nd, Buldym complex. Thus, the generation and intrusion of alkaline magmas in the Urals occurred ~420 Ma (S2–D1), synchronously with the formation of island-arc complexes. They are related to rifting on the emerging continental margins. The tectonic activity and formation of alkaline rocks and carbonatites proceeded in the Middle Devonian (~380 Ma, D2); it correlates with the accretion-collision stage of the Urals development. At the stage of "hard" collision (~280 Ma, P1), the Ilmen-Vishnevogorsky and Buldym complexes were plastically deformed, underwent melting and emplaced conformably with collision-slip tectonic structures. Recrystallization of rocks and minerals, plastic and brittle deformations, processes of pegmatitic, carbonatitic and rare-metal ore formation are associated with palingenic-metasomatic transformation of rifting alkaline complexes of Silurian-Devonian age at the collision and post-collision (~250 Ma, P3–T1) stages of the Urals emplacement.

Provenance and tectonic evolution of the forearc basin along the northwestern Central Qilian belt, northeastern Tibet

The age and properties of detrital zircon grains in a forearc basin provide crucial information on the history of convergent plate margins. These data can be used to constrain provenance dispersal patterns, establish source-to-sink relationships, and reveal the tectonic framework and subduction history. The Qilian orogen in the northeastern margin of the Tibetan Plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading and subduction to final closure. However, the timing of subduction initiation and the subduction polarity of the Proto-Tethys Ocean remain controversial. The Qilian orogen includes the North Qilian, Central Qilian, and South Qilian belts. A succession of sedimentary rocks along the northwestern margin of the Central Qilian belt, previously considered to be Precambrian in age, has been redefined based on detailed field investigation and geochronology. U-Pb analysis of detrital zircon grains from metasedimentary rocks of the Gongcha Group of the northwestern Central Qilian belt yielded Paleozoic ages between 530 and 470 Ma, with prominent Mesoproterozoic and Paleoproterozoic ages between 1800 and 1000 Ma. In contrast, the overlying Duoruonuoer Group yielded predominantly early Paleozoic ages between 465 and 434 Ma, with a few older age populations. This change in source is reflected in a substantial increase in volcanic detritus within the Duoruonuoer Group as compared to the Gongcha Group. The detrital zircon age spectra and lithologic characteristics indicate the Gongcha and Duoruonuoer Groups constitute the lower and upper volcanic-sedimentary strata in the forearc basin. Based on resemblances of statistical ages and εHf(t) values of zircon, we interpret that the early Paleozoic intrusive and volcanic units and Precambrian basement rocks of the Central Qilian belt were the main source of the Gongcha and Duoruonuoer Groups, verifying the southward subduction of the North Qilian Ocean, a portion of the larger Proto-Tethys Ocean, beneath the Central Qilian belt. The detrital zircon ages reveal that the initial subduction of the Proto-Tethys Ocean in the Qilian orogen occurred ca. 530 Ma and closed ca. 435 Ma. These forearc sedimentary units constitute the tectonic boundaries between the Central Qilian belt and North Qilian belt.

Integrated detrital rutile and detrital zircon ages: a new perspective on the tectonic evolution of South China

Abstract Paleogeographic reconstructions are of key importance for understanding the history of continental breakups and amalgamations during the whole history of the Earth. A special case is the history of the Asian continent, which, compared to other continents, consists of several large cratons and numerous smaller continental blocks. The history of the assembly of South China remains controversial in terms of the timing, Late Neoproterozoic or Early Paleozoic, and the participating continental blocks, e.g., Yangtze, Cathaysia, India and/or Australia. Detrital rutile U-Pb dating possesses a significant potential for deciphering tectonic settings as rutile frequently crystallizes during orogenic events associated with the processes of collision and subduction. Detrital zircon U-Pb dating is a perfect instrument for identifying provenance characteristics and age characteristics of sedimentary sources. An integration of these two methods of dating offers better opportunities for reconstructing tectonic settings. This paper presents a first attempt to reconstruct the Neoproterozoic to Early Palaeozoic tectonic history and paleogeography of the whole South China Block based on U-Pb geochronology of rutile and zircon and Hf-in-zircon isotopes from Lower Jurassic Baitianba Formation sedimentary rocks of the western margin of the Yangtze Block, a major part of South China. Our obtained integrated U-Pb rutile and zircon age data show three main age populations of 960-940 Ma, 630-610 Ma and 530-520 Ma. The new U-Pb detrital rutile and zircon ages, compared with former data from Gondwana and Australia, suggest that Yangtze amalgamated with Cathaysia to form South China during the Sibao orogeny at 960-940 Ma. The detrital rutile and zircons of the new 630-610 Ma age group could have been delivered from western Australia during Late Neoproterozoic to Cambrian Paterson-Petermann orogeny. The abundant 530-520 Ma detrital rutile and zircon ages fit well the coeval zircon age populations recorded in Gondwana-derived terranes, like India and Indochina.

Petrogenesis of Montana, USA Sapphires Inferred from Oxygen Isotopes and Zircon Inclusions

Abstract Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e., crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U-Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ18O values, from -3 to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0 to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4 to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 3‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U-Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U-Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.

SOURCES OF MAGMAS OF PERMIAN GABBROS OF THE KHANGAI MOUNTAINS (Western Mongolia)

We present Sm–Nd and Rb–Sr isotope composition data on mafic–ultramafic massifs in the Khangai Mountains of Western Mongolia: Oortsog-Uul, Nomgon, and Yamaat-Uul. The U–Pb age of zircon and its Lu–Hf isotope and trace-element compositions were determined by LA–ICP–MS. New and previous geochronological data obtained by SIMS and LA–ICP–MS support the Permian age of the studied gabbros. The trace-element composition of zircon, characterized by strong HREE enrichment ((Lu/Gd)n > 7) and cerium positive (Ce/Ce* > 6.6) and europium negative (Eu/Eu* = 0.16–0.49) anomalies, indicates its magmatic genesis and the possibility of using isotope characteristics to assess the origin of mafic magmas. The formation of zircon from a residual mafic melt is inferred from the enrichment of zircon in U and Th with increasing Th/U, reflecting the accumulation of these highly incompatible elements in the residual melt, and from the crystallization temperature of zircon (810–880 °С). The geochemical characteristics of the rocks, their isotopic composition, the absence of xenogenic ancient zircons, and the lack of correlation between εNd(T) and major indices of crustal contamination indicate that crustal contamination did not influence the composition of the gabbros. Isotopic data on rocks and zircon indicate the involvement of two mantle sources in the formation of the mafic–ultramafic massifs of the Khangai Mountains: (a) depleted, predominant for the Nomgon and Yamaat-Uul massifs (εHf = 16.1–2.0; εNd = 4.5–0.0; and ISr = 0.70385–0.70537), and (b) enriched, predominant for the second phase of the Oortsog-Uul massif (εHf = 1.4–0.2; εNd = –3.6… –5.7; and ISr = 0.70704–0.70933).

Provenance of the Mesoproterozoic Shennongjia Group in the north Yangtze Block, South China, and implications for reconstructions of the Nuna and Rodinia supercontinents

Magmatic activity in the range ca. 1.6−1.5 Ga is spatially restricted globally, and its distribution has been an important tool for constraining the reconstruction of the Nuna supercontinent. Abundant ca. 1.6 Ga detrital zircons have recently been discovered in the Luanshigou and Taizi formations of the middle Mesoproterozoic Shennongjia Group in the north Yangtze Block, South China. Detrital zircon age spectra and multi-dimensional scaling results indicate the Shennongjia Group was derived from variable sources. The ca. 1.6 Ga detrital zircons in the middle Mesoproterozoic units were probably sourced from Laurentia and Australia-Mawson, whereas detrital zircons of other age groups were provided by the local rock units in the north Yangtze Block. Similarities in sedimentary sequences, detrital zircon age spectra, and corresponding Hf isotopic patterns of the Shennongjia Group with other Mesoproterozoic successions in Yangtze Block (e.g., Baoban-Shilu, Huili, and Kunyang groups) suggest that the block contained a number of micro-blocks during the Mesoproterozoic. Furthermore, the compiled new and published detrital zircon data from the north Yangtze Block along with those from other continents containing ca. 1.6 Ga zircons reveal that the Yangtze Block was most likely situated within a rift zone between southeast Australia-Mawson and southwest Laurentia. The ca. 1.6 Ga zircons in the Shennongjia Group were derived from these bounding source regions during the middle Mesoproterozoic. By the late Mesoproterozoic, based on a comprehensive provenance analyses and comparison of detrital zircon data, the Yangtze Block was probably located on the margins of the Rodinia supercontinent, close to north-central India, Australia, and East Antarctica.

Geology and geochronology of the El Peñón granitic pluton: A contribution to the famatinian magmatic evolution in the Sierra de San Luis, Argentina

The El Peñón pluton is a peraluminous granite located in the northeastern sector of the Sierra de San Luis. According to its textural and mineral variations two endmember facies have been distinguished along it: a coarse-grained muscovite-bearing facies, and a fine-grained biotite-bearing facies. Field relationships aligned with mineralogical and geochemical compositions, allow us to establish an anatectic origin for this pluton. Regarding the inherited zircon ages, the provenance area of the metasedimentary source could be linked to the orogenic systems of Western Gondwana. The analysis of the internal structure of the intrusive body related with the structural arrangement observed in the country rocks indicates a syn-kinematic emplacement of the igneous body with respect to the main deformational event described in the Sierra de San Luis, which has been assigned to the closure of the Famatinian arc. New U-Pb (LA-ICP-MS) zircon ages constrain the emplacement/crystallization age of the El Peñón granite within the time lapse between ∼470 and 460 Ma. The geochronological data presented here, together with geochemical and structural analysis, enables us to correlate the origin of the El Peñón pluton and the deformation that occurred during its emplacement with the Famatinian Orogeny. Thus, these results allow us to rule out the presence of Pampean magmatism in the Sierra de San Luis but also establish a new absolute age for the main deformational event of the range.

Provenance of Paleogene Strata at Slim Buttes, South Dakota: Implications for post-Laramide Evolution of Western Laurentia

Slim Buttes is a 30 km long by 10 km wide set of buttes containing Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Chadron and Brule Formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. An angular unconformity separates the White River Group from overlying Oligocene and Miocene strata of the Arikaree Group. Using detrital zircon U-Pb ages, we determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally recycled from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; Paleogene zircons dominate the age spectrum, and a wide variety of older zircons are also present. The Oligocene zircons are interpreted to have been sourced from volcanic systems in the Great Basin to the southwest, while the subsequent proportions of the zircons were derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. Sparse amounts of Archean zircons are thought to represent the burial of Laramide uplifts throughout Wyoming at the time of Brule deposition, making for a regional paleotopography with little relief across the western interior of the United States. The Miocene-age Arikaree Group sand has a maximum depositional age of ~26 Ma and a multimodal detrital zircon age spectrum. The Arikaree Group provenance likely represents continued sourcing in the Great Basin volcanic systems and Nevadaplano, the beginnings of the re-exhumation of Laramide basement uplifts, and subsequent sediment evacuation out of the western interior and into the Gulf of Mexico to the southeast. Our findings indicate that the transport process and detrital zircon provenance signatures of these strata are decoupled, and each have their own independent evolution. The volcanic signature is primarily transported via aeolian processes (i.e. volcanic ash), and the recycled detrital zircon signature is primarily transported via fluvial processes.

Two phases of granulite-facies metamorphism superimposied on retrograde eclogite: Constraints on the early Paleozoic tectonic evolution of the Qinling Orogenic Belt, central China

Existing studies provide adequate petrological evidences on ca. 500 Ma ultra-high pressure (UHP) metamorphism in the North Qinling Orogenic Belt (NQOB) in central China, but the genesis of 470–420 Ma multi-phase granulite-facies metamorphism in the NQOB and their relationship with the ca. 500 Ma UHP metamorphism remain controversial, resulting in the early Paleozoic evolution of the Qinling Orogenic Belt (QOB) highly debatable. In this study, we present mafic granulites and host felsic gneisses with a “red-eye socket” texture from the Shuanglong area, eastern NQOB, which recorded two phases of granulite-facies metamorphism superimposing on former eclogite-facies metamorphism. The former eclogite-facies metamorphism is indicated by eclogite-facies zircon trace element patterns and 496–495 Ma zircon ages, which are the same with those of the HP–UHP eclogite-facies metamorphic rocks in NQOB. The first granulite-facies metamorphism occurred at 460–448 Ma is characterized by coarse-grained minerals in matrix. Compositions and zonings of these minerals define an anticlockwise P–T path involving a prograde stage (751–763 °C), a high-temperature peak stage (9.2 kbar and 864 °C), and a near-isobaric cooling retrograde stage (8.3 kbar and 818 °C). The second granulite-facies metamorphism occurred at 422–421 Ma is represented by coronal garnet and coexisting fine-grained mineral aggregates. Coronal garnet compositional zonings suggest a clockwise P–T path consisting of a high-pressure peak stage (9.5–11.2 kbar and 748–783 °C) and a decompressing and heating retrograde stage (9.2–9.5 kbar and 789–800 °C). Combining dating results of leucosomes in these rocks and existing data, we proposed a new model for early Paleozoic tectonic evolution of the NQOB. The North Qinling Terrane (NQT), probably separated from the South China Block (SCB) during the breakup of Rodinia, drifted northwards and underwent UHP metamorphism at 500 Ma and then rapidly exhumed to crust level. Later, the Shangdan Ocean subducted northwards beneath the exhumed NQT at 470–440 Ma, resulting in the first granulite-facies metamorphism and contemporaneous migmatization and magmatism. Finally, the closure of the Shangdan Ocean led to collision between the NQT and South Qinling Terrane/SCB and the second granulite-facies metamorphism and anatexis at 422–418 Ma.