Double dating (U–Pb and (U–Th)/He) of detrital zircon from the Gonfolite Group (European Alps) and implications for the lag-time approach to detrital thermochronology

2021 ◽  
Author(s):  
Marco G. Malusà ◽  
Owen A. Anfinson ◽  
Daniel F. Stockli
Keyword(s):  
Detrital Zircon ◽  
Country Rock ◽  
Lag Time ◽  
Source Rocks ◽  
Magmatic Zircon ◽  
Contact Aureole ◽  
European Alps ◽  
Earth Planet ◽  
Shallow Level ◽  
Plutonic Rocks

<p>Detrital thermochronologic analyses are increasingly employed to develop quantitative models of landscape evolution and constrain rates of exhumation due to erosion. Crucial for this kind of application is a correct discrimination between thermochronologic ages that record cooling due to exhumation, i.e., the motion of parent rocks towards Earth’s surface, and thermochronologic ages that record cooling independent from exhumation, as expected for example in volcanic and shallow-level plutonic rocks. A suitable approach for the identification of magmatic crystallization ages is provided by double dating, which combines for example U–Pb and (U–Th)/He analyses of the same mineral grain. Magmatic zircon crystallized from volcanic or shallow-level plutonic rocks should display identical U–Pb and (U–Th)/He (ZHe) ages within error, because of rapid magma crystallization in the upper crust where country rocks are at temperatures cooler than the partial retention zone of the ZHe system. Conversely, zircon grains crystallized at greater depth and recording cooling during exhumation should display ZHe ages younger than the corresponding U–Pb ages. These latter ZHe ages may constrain the long-term exhumation history of the source rocks according to the lag-time approach, provided that a range of assumptions are properly evaluated (e.g., Malusà and Fitzgerald 2020). Here, we explore the possibility that detrital zircon grains yielding ZHe ages younger than the corresponding U–Pb ages may record country-rock cooling within a contact aureole rather than exhumation. To tackle this issue, we applied a double-dating approach including U-Pb and ZHe analyses to samples of the Gonfolite Group exposed south of the European Alps. The Gonfolite Group largely derives from erosion of the Bergell volcano-plutonic complex and adjacent country rocks, and its mineral-age stratigraphy is extremely well constrained (Malusà et al. 2011, 2016). Analyses were performed in the UTChron Geochronology Facility at University of Texas at Austin. For U-Pb LA-ICPMS depth-profile analysis, all detrital zircon grains were mounted without polishing, which allowed for subsequent ZHe analysis on the same grains. Zircon for ZHe analyses were selected among those not derived from the Bergell complex or other Periadriatic magmatic rocks, as constrained by their U-Pb age. We found that ca 40% of double-dated grains, despite yielding a ZHe age younger than their U-Pb age, likely record cooling within the Bergell contact aureole, not exhumation. These findings have major implications for a correct application of the lag-time approach to detrital thermochronology and underline the importance of a well-constrained mineral-age stratigraphy for a reliable geologic interpretation.</p><p>Malusà MG, Villa IM, Vezzoli G, Garzanti E (2011) Earth Planet Sci Lett 301(1-2), 324-336</p><p>Malusà MG, Anfinson OA, Dafov LN, Stockli DF (2016) Geology 44(2), 155-158</p><p>Malusà MG, Fitzgerald, PG (2020) Earth-Sci Rev 201, 103074</p>

scholarly journals U-Pb Ages and Hf Isotopes of Detrital Zircon Grains from the Mesoproterozoic Chuanlinggou Formation in North China Craton: Implications for the Geochronology of Sedimentary Iron Deposits and Crustal Evolution

Minerals ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 547 ◽  
Author(s):  
Chao Duan ◽  
Yanhe Li ◽  
Yun Yang ◽  
Yongsheng Liang ◽  
Minghui Wei ◽  
...  
Keyword(s):  
Detrital Zircon ◽  
North China Craton ◽  
Inductively Coupled Plasma ◽  
North China ◽  
Isotope Analysis ◽  
Weighted Average ◽  
Source Rocks ◽  
Magmatic Zircon ◽  
Crustal Growth ◽  
Iron Deposits

The Chuanlinggou Formation is the lower formation of the Changchengian System, and hosts sedimentary iron deposits (marine oolitic ironstones) of the North China Craton (NCC). To determine the age of the iron deposits, and provide insight into the crustal growth of the craton, laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)U-Pb and in situ Hf isotope analysis were performed on detrital zircon grains. Samples were taken from the roof sand-shale of the sedimentary iron deposits at Jiangjiazhai and Pangjiapbu. Overall, 186 detrital zircon grain U-Pb ages yield three major age populations, with weighted average ages of 2450 Ma, 1848 Ma, and 1765 Ma, respectively. Four younger ages from magmatic zircon grains were obtained, ranging from 1694 to 1657 Ma. Combined with observations from published studies, the results define the lower limit for the age of the Chuanlinggou Formation, and constrain the age of the sedimentary iron deposits (marine oolitic ironstone) close to 1650 Ma. The peak ages of 1848 Ma and 2450 Ma define the major collisional events of the NCC. The age of 1765 Ma can be linked to the age range of the widespread mafic dyke swarms that represent the rifting of the NCC within the Columbia supercontinent. Detrital zircon grains from the Chuanlinggou Formation form two obvious groups, with different εHf (t) values ranging from −1 to −8 and from +1 to +8, which correspond to the U-Pb age ranges of 1.7–1.9 Ga and 2.3–2.6 Ga, respectively. They have a similar two-stage Hf model age peak at 2.65–2.85 Ga, suggesting that the source rocks for each of these events were derived from the recycling of ancient crust. The source rocks of the older group of zircon grains might be derived from juvenile crust with a short reworking period. The critical crust–mantle differentiation event might happen during the period of 2.65–2.85 Ga, marking the most significant stage of the crustal growth in the NCC.

Some supracrustal (S-type) granites of the Lachlan Fold Belt

1988 ◽  
Vol 79 (2-3) ◽  
pp. 169-181 ◽  
Author(s):  
A. J. R. White ◽  
B. W. Chappell
Keyword(s):  
Volcanic Rocks ◽  
Source Rocks ◽  
Crystal Fractionation ◽  
Metamorphic Rocks ◽  
Contact Aureole ◽  
High Grade ◽  
Fold Belt ◽  
Fertile Window ◽  
Lachlan Fold Belt ◽  
Sedimentary Source

ABSTRACTS-type granites have properties that are a result of their derivation from sedimentary source rocks. Slightly more than half of the granites exposed in the Lachlan Fold Belt of southeastern Australia are of this type. These S-type rocks occur in all environments ranging from an association with migmatites and high grade regional metamorphic rocks, through an occurrence as large batholiths, to those occurring as related volcanic rocks. The association with high grade metamorphic rocks is uncommon. Most of the S-type granites were derived from deeper parts of the crust and emplaced at higher levels; hence their study provides insights into the nature of that deeper crust. Only source rocks that contain enough of the granite-forming elements (Si, Al, Na and K) to provide substantial quantities of melt can produce magmas and there is therefore a fertile window in the composition of these sedimentary rocks corresponding to feldspathic greywacke, from which granite magmas may be formed.In this paper, three contrasting S-type granite suites of the Lachlan Fold Belt are discussed. Firstly, the Cooma Granodiorite occurs within a regional metamorphic complex and is associated with migmatites. It has isotopic and chemical features matching those of the widespread Ordovician sediments that occur in the fold belt. Secondly, the S-type granites of the Bullenbalong Suite are found as voluminous contact-aureole and subvolcanic granites, with volcanic equivalents. These granites are all cordierite-bearing and have low Na2O, CaO and Sr, high Ni, strongly negative εNd and high 87Sr/86Sr, all indicative of S-type character. However, the values of these parameters are not as extreme as for the Cooma Granodiorite. Evidence is discussed to show that these granites were derived from a less mature, unexposed, deeper and older sedimentary source. Other hypotheses such as basalt mixing are discussed and can be ruled out. The Strathbogie Suite granites are more felsic but all are cordierite-bearing and have chemical and other features indicative of an immature sedimentary source. They are closely associated with cordierite-bearing volcanic rocks. The more felsic nature of the suite results in part from crystal fractionation. It is suggested that the magma may have entered this “crystal fractionation” stage of evolution because it was a slightly higher temperature magma produced from an even less mature sediment than the Bullenbalong Suite. The production of these S-type magmas is discussed in terms of vapour-absent melting of metagreywackes involving both muscovite and biotite. The production of a magma in this way is consistent with the low H2O contents and geological setting of S-type granites and volcanic rocks in the Lachlan Fold Belt.

scholarly journals Relationships between Alluvial Facies/Depositional Environments, Detrital Zircon U-Pb Geochronology, and Bulk-Rock Geochemistry in the Cretaceous Neungju Basin (Southwest Korea)

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1023
Author(s):  
Hyojong Lee ◽  
Min Gyu Kwon ◽  
Seungwon Shin ◽  
Hyeongseong Cho ◽  
Jong-Sun Kim ◽  
...  
Keyword(s):  
Continental Margin ◽  
Detrital Zircon ◽  
Tectonic Setting ◽  
Active Continental Margin ◽  
Depositional Environments ◽  
Source Rocks ◽  
Bulk Rock ◽  
Zircon Age ◽  
Margin Setting ◽  
Weathering Intensity

Zircon U-Pb geochronology and bulk-rock geochemistry analyses were carried out to investigate their relationship with depositional environments of the non-marine Neungju Basin sediments in South Korea. The Neungju Basin was formed in an active continental margin setting during the Late Cretaceous with associated volcanism. Detrital zircon age distributions of the Neungju Basin reveal that the source rocks surrounding the basin supplied sediments into the basin from all directions, making different zircon age populations according to the depositional environments. Mudstone geochemistry with support of detrital zircon U-Pb age data reveals how the heterogeneity affects the geochemical characteristics of tectonic setting and weathering intensity. The sediments in the proximal (alluvial fan to sandflat) and distal (playa lake) environments differ compositionally because sediment mixing occurred exclusively in the distal environment. The proximal deposits show a passive margin signature, reflecting their derivation from the adjacent metamorphic and granitic basement rocks. The distal deposits properly indicate an active continental margin setting due to the additional supply of reworked volcaniclastic sediments. The proximal deposits indicate a minor degree of chemical weathering corresponding to fossil and sedimentological records of the basin, whereas the distal deposits show lower weathering intensity by reworking of unaltered volcaniclastic detritus from unstable volcanic and volcaniclastic terranes. Overall, this study highlights that compositional data obtained from a specific location and depositional environments may not describe the overall characteristic of the basin.

THE PETROLEUM POTENTIAL OF EAST TIMOR

2002 ◽  
Vol 42 (1) ◽  
pp. 351 ◽  
Author(s):  
T.R. Charlton
Keyword(s):  
Middle Jurassic ◽  
Structural Complexity ◽  
Upper Triassic ◽  
East Timor ◽  
Source Rocks ◽  
Petroleum Potential ◽  
Shallow Level ◽  
Structural Regime ◽  
Simple Inversion ◽  
West Timor

The hydrocarbon prospectivity of East Timor is widely considered to be only moderate due to Timor island’s well-known tectonic complexity, but in the present study a much higher potential is interpreted, with structures capable of hosting giant hydrocarbon accumulations. High quality source rocks are found in restricted marine sequences of Upper Triassic-Jurassic age. The most likely reservoir target is shallow marine siliciclastics of Upper Triassic-Middle Jurassic age encountered in the Banli–1 well in West Timor, comparable to the Malita and Plover Formations of the northern Bonaparte Basin, and sealed by Middle Jurassic shales of the Wai Luli Formation. The Wai Luli Formation also forms a major structural décollement level which detaches shallow level structural complexity from a simpler structural régime beneath.The principal exploration targets are large, structurally simple inversion anticlines developed beneath the complex shallow-level fold and thrust/mélange terrain. Eroded-out examples of inversion anticlines, such as the Cribas, Aitutu and Bazol anticlines, are typically several tens of kilometres long and up to 10 km broad. Comparable structures in the subsurface of southern East Timor are interpreted north of Betano, and probably also near Suai, Beaco, Aliambata and Iliomar. Other potential targets include a possible non-inverted rollover anticline at Pualaca, stratigraphic and structural traps in the south coast syn/postorogenic basins, and possibly large structural domes beneath extensive Quaternary reef plateaux in the extreme east of the island.

Changing our ideas about the evolution of magmatic systems with improved temporal resolution: do we get it right?

2020 ◽  
Author(s):  
Urs Schaltegger ◽  
Joshua H.F.L. Davies
Keyword(s):  
Temporal Resolution ◽  
Age Determination ◽  
Crustal Contamination ◽  
Isotopic Analysis ◽  
Planetary Science ◽  
Magmatic Zircon ◽  
Earth Planet ◽  
Spontaneous Nucleation ◽  
Challenges And Opportunities ◽  
Pre Treatment

<p>Improvements to U-Pb geochronology of magmatic zircon have resulted in temporal resolution at the level of <0.1% for individual <sup>206</sup>Pb/<sup>238</sup>U dates and of 0.02-0.05% for weighted mean <sup>206</sup>Pb/<sup>238</sup>U ages of a statistically equivalent group of single crystal dates from zircon or baddeleyite (50,000 years for a Mesozoic igneous rock). This talk will give a short overview on the challenges and opportunities such high precision age determination implies in felsic and mafic magmatic systems.</p><p>Felsic magmatic systems: Zircon dates from the same hand sample cover a temporal range that integrates their crystallization history in the melt. Since each grain crystalizes over a certain time period, the apparent age range is a minimum estimate of the duration of crystallization or the residence in a magma. A major challenge is the mitigation of decay-related lead loss through refined chemical abrasion procedures (Widmann et al., 2019) to avoid erroneous interpretation of zircon dates that appear too young. Apparent trace element or isotopic trends are typically not coherent with time and therefore reflect fractionation processes at different places and different times in the magmatic system, possibly within compositionally different magma batches.</p><p>Mafic magmatic systems: Zircon is not a crystallizing phase in a basaltic melt, but can occur after ~90% fractionation of olivine, pyroxene and amphibole, zircon saturation can then be achieved in low-volume granitic melt pockets (depending on the water content). A zircon date is therefore an age information along the crystallization -cooling path of a mafic intrusion (Zeh et al., 2015). In low-Si and low-Zr melts, baddeleyite may arrive at saturation before zircon and can be used for dating as well. There are two clear problems with zircon/baddeleyite geochronology in mafic systems: (i) since baddeleyite saturates earlier than zircon, it should produce slightly older dates in the same rock; however, these minerals often display the inverse relationship. Since no pre-treatment for the removal of decay-damaged portions exists for baddeleyite, we can demonstrate that this discrepancy is due to lead loss. Mitigating lead loss is also difficult for zircon since it crystallized from residual melt patches of granitoid composition high in uranium, often resulting in metamict crystals; (ii) zircon populations from dolerites may spread over >100,000 years even in cases where simple thermal modeling shows that a dolerite sill has crystallized and cooled at 10<sup>3</sup> years timescales. Beside lead loss, we may suspect that certain zircon grains contain minute portions of pre-crystallization radiogenic lead from crustal contamination. We can explore and quantify cryptic inheritance through Hf, O isotopic analysis of the same dated zircon grains. Heterogeneous nucleation on relics of incompletely dissolved zircon is more probably than spontaneous nucleation.</p><p>As an overarching challenge, we have no technique or independent approach to quantify lead loss and it remains the biggest uncertainty in U-Pb dating.</p><p>References: Davies et al. (2015) Nature Communications, 8, 15596 ; Sell et al. (2014) Earth and Planetary Science Letters, 408, 48-56; Widmann et al. (2019) Chemical Geology, 511, 1-10; Zeh et al. (2015) Earth Planet. Sci. Lett. 418, 103-114</p>

scholarly journals Thermal Alteration of Organic Matter in the Contact of a Rift-Related Basaltic Dyke: An Example from the Black Limestone, Wadi Matulla, West Central Sinai, Egypt

Minerals ◽  
2019 ◽  
Vol 9 (5) ◽  
pp. 279 ◽  
Author(s):  
Ahmed S. A. A. Abu Sharib ◽  
Ali Q. Selim ◽  
Mohamed M. Abdel Fattah ◽  
Safiya M. Hassan ◽  
Ioan V. Sanislav
Keyword(s):  
Organic Matter ◽  
Upper Cretaceous ◽  
Country Rock ◽  
Color Variation ◽  
Source Rocks ◽  
Burial Depth ◽  
Rift Basin ◽  
Heat Transfer Mechanism ◽  
Average Width ◽  
First Time

In the Wadi Matulla area, central Sinai, Egypt, an asymmetric baked zone having an average width of 103 m was formed on both sides of a sub-aerial rift-related Oligocene basaltic dyke cross-cutting organic matter-bearing chalky limestone of the Upper Cretaceous Sudr Formation. Advection was the significant heat transfer mechanism. Very narrow metamorphic and metasomatic zones are developed in the country rock at the immediate contact with the dyke. The change in the thermal maturation of organic matter is reflected in the differences in values of the total organic carbon (TOC) within the baked zone. Such differences account for the color variation of the snow-white limestone from shades of brown, in the mature to barren samples, to black, in the totally carbonized overmature metamorphic ones. This study presents for the first time the thermal effect of mafic dykes on some exposed organic matter-bearing rocks in the Gulf of Suez (GOS) region, and turns attention to the local maturation of source rocks in contact with rift-related intrusives at a relatively greater burial depth in the rift basin.

Application of a Selective Phase Extraction Procedure to Samples from the Adamello Contact Aureole (Italy)

1994 ◽  
Vol 353 ◽  
Author(s):  
T. E. Payne ◽  
G. R. Lumpkin ◽  
P. J. McGlinn ◽  
K. P. Hart
Keyword(s):  
Rare Earth ◽  
Alpha Spectrometry ◽  
Country Rock ◽  
Extraction Procedure ◽  
Contact Aureole ◽  
Phase Extraction ◽  
Rich Phase ◽  
Ree Patterns ◽  
Ree Pattern ◽  
Hydrothermal Veins

AbstractHydrothermal veins, rich in Ti, Zr, rare earth elements (REE’s), and actinides, occur in the pure dolomitic marbles of the Adamello contact metamorphic aureole. A selective phase extraction using 9M HC1 was applied to samples from within and near these veins to chemically separate acid-soluble phases from residual phases, and to study the associations of U, Th, and REE’s with these phases. The samples were from the phlogopite, titanian clinohumite, and forsterite vein zones, and from the country rock. The effects of the extraction were studied by SEM/EDS and by chemical analysis. Isotopes of U and Th were analysed by alpha-spectrometry.The chemical data and SEM/EDS results indicated that dolomite, calcite, apatite, and much of the pyrrhotite were dissolved by the 9M HC1, whereas spinel, phlogopite, titanite, chal copy rite, and zirconolite were among the acid-resistant phases. In all vein samples, the REE-patterns of acid-soluble phases were consistent with the dissolution of REE-rich apatite. In samples from the phlogopite zone, the majority of U, Th, and REE’s were in residual phases, and the REE pattern of the residue was similar to that of REE-rich titanite. In the titanian clinohumite zone, a substantial proportion of these elements were in acid-soluble phases, and the REE pattern of the residual phases resembled that of zirconolite. Clinohumite was partially dissolved by the HC1 treatment. The sample from the forsterite zone contained substantial amounts of REE's in acid-soluble phases, whereas Th and U were mostly in residual phases. The dolomite (country rock) samples contained small amounts of an acid-resistant, uranium-rich phase which, while only comprising about 0.2% of the mass, accounted for 40–60% of the uranium present.

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

2008 ◽  
Vol 146 (3) ◽  
pp. 419-436 ◽  
Author(s):  
M. S. PETRONIS ◽  
B. O'DRISCOLL ◽  
V. R. TROLL ◽  
C. H. EMELEUS ◽  
J. W. GEISSMAN
Keyword(s):  
Magnetic Susceptibility ◽  
Country Rock ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Magnetic Fabric ◽  
Contact Aureole ◽  
Data Set ◽  
The North ◽  
Early Paleocene ◽  
Magma Emplacement

AbstractThe Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

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