Zircon Geochemical and Isotopic Constraints on the Evolution of the Mount Givens Pluton, Central Sierra Nevada Batholith

The Late Cretaceous Mount Givens pluton, located in the central Sierra Nevada batholith, is noteworthy for its large size (~1400 km2) and relative compositional and textural homogeneity. The pluton is characterized by a 30 km wide, ellipse-shaped northern lobe (ca. 98-91 Ma) that connects with a long mass (ca. 95-91 Ma) about 15 km wide and extends 50 km SE. Zircon trace elements and d18O and eHf from a sample suite representative of the observable petrologic diversity of the pluton document significant heterogeneities. All zircons have high Ti-in-zircon model temperatures (850-1000 °C), pronounced negative Eu anomalies, and concave down MREE and HREE patterns, indicating initial melts were undersaturated in zircon and that zircon growth occurred during ascent or emplacement of Mount Givens magmas. Zircon trace elements record appreciable variation, and suggest that crystal transfer was ongoing throughout pluton growth. Significant variations in O and Hf isotopes require mixing of multiple, isotopically distinct sources. Longitudinal variation in both systems suggests increasing mantle contributions in the eastern portion of the pluton. A clear decrease in isotopic variability at all scales in post-95 Ma samples indicates that later magmas were more thoroughly mixed prior to emplacement. This increase in zircon isotopic homogeneity paired with the narrow timespan of intrusion hints that the inner portion of the northern lobe may have existed as a large melt reservoir

Tracking crystal-melt segregation and magma recharge using zircon trace element data

The Cretaceous Yunshan caldera complex in SE China consists of an unusual coexisting assemblage of peraluminous and peralkaline rhyolites and a resurgent intra-caldera porphyritic quartz monzonite. In this study, we use zircon trace element data to study the compositional differences of zircons from cogenetic magmas and to track the evolution of the entire magmatic system. Our results indicate that the zircons from the peraluminous and peralkaline rhyolites formed from highly evolved compositions with high Hf concentrations and low Ti contents, and low Th/U and Zr/Hf ratios, which are distinct from those of the intrusive porphyritic quartz monzonite. Zircons from the peraluminous and peralkaline rhyolites display overlapping Zr/Hf and Hf, but the zircons from the peralkaline rhyolites have extremely low Eu/Eu* ratios (Peer reviewe

Continental crustal growth in collision zones on the Northern Tibetan Plateau

The continental crustal growth has been a great interest to geoscientists and its importance is self-evident. Any models trying to reveal how it works must be able to explain 1) how the continental crust gains its andesitic bulk composition with juvenile isotope signatures; 2) how the volume of the crust increases episodically. Granitoids with the juvenile isotopic signatures in the collision zones provide the special insights into the nature of the continental crust and a unique opportunity to investigate the mechanism of crust growth. This thesis reports results from major and trace element data, whole rock Sr-Nd-Hf-Pb isotopic data, zircon geochronology and in situ Hf isotopes of granitoids as well as, if any, the enclosed mafic magmatic enclaves (MMEs) from the three ancient collision zones crossing the Northern Tibetan Plateau. I-type granitoids and their MMEs from the East Kunlun Orogenic Belt (EK) in the Northern Tibetan Plateau are dated as 250 Ma years old. They are cal-alkaline in nature with compositions resembling the bulk continental crust (BCC). Whole rock Sr-Nd-Pb-Hf isotopes reveal that they are products of partial melting of the ‘trapped’ subducted oceanic crust at the onset of the collision and the MMEs are the aggregated early cumulates in the parental magma rather than the mafic endmember involved in the magma mixing as previously suggested. I- and S-type granitoids from the Qilian Block (QB) further north in the Great Plateau are dated as 450 Ma. Their lithological and geochemical heterogeneity and isotopic changes with time are inferred to correspond to the collision. The abundant inherited zircons with ages as old as Archean reveal the presence of the Archean basement underneath the Qilian Block and indicate that the Qilian Block may have been a micro continent during its drift in the ancient ocean. Granitoids and their enclosed cumulates from Kekeli Batholith further north in the plateau are 500 Ma years old. They have decoupled whole rock Nd and Hf isotopes and discrepancy between whole rock Hf and zircon in situ Hf isotopes. These inconsistences are understood to result from different mineral crystallization timings during mixing between endmembers with distinct isotopes. This highlights the need for detailed whole-rock or non-zircon phases Hf isotopic investigation in order to develop a comprehensive understanding of the granitoids of hybrid origin

Continental growth histories revealed by detrital zircon trace elements: a case study from India

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PETROGENESIS AND RARE METAL MINERALIZATION OF THE ALKALINE GRANITIC MAGMA: A CASE STUDY FROM THE BOZIGUO’ER RARE METAL GRANITIC INTRUSION

The origination and differentiation of rare metalbearing, alkaline granites has attracted extensive interests because of their economic significance.The origination and differentiation of rare metalbearing, alkaline granites has attracted extensive interests because of their economic significance

Ultrahigh-temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long-lived high-T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane

Silicic ash beds bracket Emeishan Large Igneous Province to < 1 m.y. at ~ 260 Ma

This research was supported by the Open Fund (PLC201404) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology), the National Natural Science Foundation of China (41502109), Project funded by China Postdoctoral Science Foundation (2015 M582528) and the Fundamental Research Funds for the Central Universities, Chengdu University of Technology.Claystone beds directly below and above the Emeishan basalts in SW China formed around the Guadalupian-Lopingian (G-L) boundary. Zircons from both levels give U–Pb ages of ~ 260 Ma, and are identical within-error to ages reported for the Emeishan Large Igneous Province (LIP). The claystones lack Nb-Ta anomalies on primitive mantle normalized elemental diagrams; zircons from these claystones have a geochemical affinity to within-plate-type magmas. These features, combined with the strong negative Eu anomalies in the zircons and high Al2O3/TiO2 ratios, indicate that claystones around the G-L boundary have a silicic volcanic component related to Emeishan LIP. Zircons from the underlying claystone bed have much higher U/Yb and Th/Nb ratios and lower εHf(t) values than those overlying the LIP, suggesting that early-stage silicic volcanic rocks had a higher crustal contamination or assimilation during magmatic processes. In terms of stratigraphic correlation, our data demonstrate that silicic eruptions occurred not only at the end, but also at the beginning of the Emeishan LIP, and the overall duration of the main basaltic phase was short (< 1 m.y).PostprintPeer reviewe

Mesozoic detrital zircon provenance of Central Africa: implications for Jurassic-Cretaceous tectonics, paleogeography and landscape evolution

Jurassic-Cretaceous tectonics, paleogeography and sedimentary provenance of central Africa are poorly constrained and continue to be debated. The lack of constraints on the timing and controls on late Mesozoic sedimentary basin development, drainage evolution and paleoenvironments is problematic because central Africa is well endowed with natural resources, and good understanding of these issues is fundamental to a better assessment of hydrocarbon and alluvial diamond exploration targeting. Moreover, by improving our understanding of Mesozoic strata across this vast region, we can also help to contextualise the ecological and evolutionary relationships of floras and faunas from central Africa with contemporary floras and faunas from different parts of Africa and throughout Gondwana. In particular, refining the depositional age of late Mesozoic units is key to understanding and reconstructing regional paleogeography and drainage patterns during this poorly resolved time period in Africa, which also furthers our understanding of the origins and dispersal pathways for Mesozoic, Cenozoic and modern African floras and faunas, as well as economically significant alluvial mineral resources, such as diamonds, that are important to the economies of this part of the world. To address these issues a detailed and multifaceted sedimentary provenance analysis of 14 late Mesozoic units from seven sedimentary basins across central Africa (spanning seven different countries) was conducted. This integrated sedimentological approach incorporated sandstone petrography, paleocurrent analysis, U-Pb detrital zircon geochronology, Lu-Hf isotope and trace element geochemistry to investigate Jurassic and Cretaceous continental deposits from central Africa. The main objective was to investigate late Mesozoic sedimentary basin development, drainage evolution and provide constraints on the age of deposition, sediment source and paleofluvial drainage patterns, using core and outcrop samples from across the region; including Democratic Republic of Congo (DRC), Kenya, Angola, Sudan, Tanzania, Zimbabwe and Malawi. Sandstone petrography and paleocurrent data indicate mixed sediment sources mainly to the south of study areas. Maximum depositional age analyses performed on U-Pb detrital zircon sample results demonstrate that most of the late Mesozoic units in central Africa are younger than previously accepted. Detrital zircon provenance analysis points to primary contributions from Neoproterozoic Pan-African Mobile Belts (e.g., Mozambique and Zambezi belts), which were probably exposed at this time are the dominant (>75%). The Lu-Hf isotope geochemistry results also show a mixed sediment provenance consisting of juvenile mantle and reworked crustal sources, which corroborates the sandstone petrography results. Western areas of central Africa (e.g. DRC and Angola) are dominated by sediments from reworked crustal sources, whereas eastern parts of central Africa (e.g. Sudan, Kenya and Tanzania) are dominated by sediments of juvenile mantle sources. The results further suggest a pattern of large ephemeral lakes in the Middle Jurassic to Early Cretaceous in the Congo and Zambezi basins, followed by the development of a large, dominantly north directed fluvial systems across central Africa in the middle Cretaceous. The results are supportive of a uniform northward continental drainage pattern throughout late Mesozoic, which supports the assertion that the paleo-Congo drainage system was likely north flowing, rather than east flowing out of the Congo Basin and into Indian Ocean as previously suggested. The results of this thesis are also supportive of the hypothesis of a major drainage divide between southern and central Africa during the late Mesozoic and the concept of a major NW trending fluvial drainage pattern into the shear zones within the Central African Rift System, although the ultimate depocentre still remains uncertain. The maximum depositional age of three Cretaceous sedimentary units, including the dinosaur-bearing Wadi Milk Formation of Sudan has been constrained. The new ages shows a generally much younger age of deposition than previous assignations, calling into question the reliability of these overly broad biostratigraphic age for these important sedimentary units

Zircon double-dating, trace element and O isotope analysis to decipher late Pleistocene explosive-effusive eruptions from a zoned ocean-island magma system, Ascension Island

In this first detailed study of zircon from Ascension Island, South Atlantic, we take a novel approach combining trace element and O isotope compositional data with double-dating (disequilibrium 238U–230Th and (U–Th)/He) to decipher timescales and dynamics of magmatic processes. The Echo Canyon (EC) sequence comprises small-volume explosive-effusive eruptions of trachyte that tapped a compositionally zoned magma system. Associated volcanic hazards may be constrained from the age of volcanism, duration of magma storage, and magma source and plumbing system character. Zircon U–Th–Pb dating of lithic lava clasts has revealed recurrent evolved volcanism at 1.34 and 0.6 Ma, and 95 ka. The (U–Th)/He zircon cooling ages indicate that most of the EC explosive-effusive sequence erupted in a brief episode at ca. 95 ka. Additionally, uniform 238U–230Th zircon crystallisation ages suggest moderately protracted magma storage with melt present at depth for at most 103–104 years before eruption. The enriched character of zircon trace element compositions, relative to MORB, in the absence of a continental crustal signature in the oxygen isotope values (δ18O range 2.67–5.63‰), suggests the presence of an enriched component in the EC magma source. Furthermore, low δ18O zircon compositions imply assimilation of high temperature hydrothermally altered country rock by the source magma. The mineral assemblage in crystal-poor pumices indicates equilibrium storage conditions: zircon saturation and Ti-in-zircon crystallisation temperatures are consistent with alkali feldspar-melt temperatures. Significantly, zircon crystals were preserved both as macrocryst inclusions and in the groundmass of EC explosive and effusive deposits. These rocks preserve evidence of magma evolution by fractional crystallisation. This process led to pre-eruptive compositional stratification, which is evidenced in the range of whole-rock major and trace element compositions and zircon Zr/Hf values. Notably, zircon crystallisation and cooling ages derived from pumice, lava, and accidental lithic lava clasts in highly explosive pyroclastic deposits, have revealed episodes of evolved magmatism that would otherwise have gone undetected. In addition, the zircon trace element and isotope compositions, in combination with the range of crystallisation ages, evidence progressively deeper tapping of less evolved magma stored in discrete lenses. Thus, a combined zircon geochronological-geochemical approach can place constraints on the ca. 0.6 Ma recurrence of past explosive-effusive pulses of millennial to decamillennial duration and their enriched magma sources. This information is relevant for assessing hazards and informing monitoring and forecasting efforts to assist in managing associated risks for small ocean island volcanoes with particularly vulnerable populations and infrastructure