Dating Of Titanite Research Articles

Sedimentary provenance, age and possible correlation of the Iona Group SW Scotland

Synopsis Most clast lithologies within the basal conglomerate of the Iona Group, exposed on Iona, Inner Hebrides, SW Scotland, resemble the adjacent Archaean–Palaeoproterozoic Lewisian Gneiss Complex, supporting the hypothesis that the Iona Group unconformably overlies Lewisian gneisses on Iona. Moreover, a whole-rock Sm–Nd t DM age of 2512 Ma from one of these clasts supports its lithological correlation with granitic pegmatites in the underlying gneisses, while a second quartz-rich paragneiss clast with a Sm–Nd t DM age of 2745 Ma, is similar to Lewisian paragneisses on Coll. However, whole-rock t DM ages of 1755–2060 Ma and pronounced c . 1780–1800 Ma peaks in U–Pb detrital zircon and titanite data for Iona Group metasediments suggest additional distal sources of Laurentian affinity including the Rhinns Complex as well as Labradorian, Pinwarian and Archaean detritus. The Iona Group was deposited and deformed after c . 1490 Ma and before c . 422 Ma, constrained by the youngest detrital zircon and titanite and the intrusion of the Ross of Mull Granite, respectively. The detrital data permit correlation of the Iona Group with the Colonsay Group and the Glenshirra Subgroup of the Dalradian Grampian Group, although correlation with parts of the Torridonian cannot be excluded. Supplementary material: Full analytical results areavailable at http://www.geolsoc.org.uk/SUP18768 .

Multistage Svecofennian metamorphism: Evidence from the composition and U-Pb age of titanite from eclogites of the Belomorian Mobile Belt

The first geochemical study of titanite from eclogites and associated rocks of the Belomorian Mobile Belt (BMB) by secondary ion mass spectrometry made it possible to establish the compositional features of this mineral in the garnet-bearing and garnet-free assemblages. Titanite from garnet-bearing assemblages is characterized by upward convex REE pattern and lowered HREE content relative to LREE, as well as the average GdN/YbN ratio around 16.5. Titanite from metaultrabasic rock inherits the specific features of the host rock, which should be taken into account when comparing with titanite from metagabbro. Results of U-Pb (TIMS) dating of titanite confirms the identification of the early and late stages of the Svecofennian metamorphism in the studied areas of BMB: early metamorphism with the peak eclogite facies conditions at around 1900 Ma, retrograde amphibolite facies metamorphism at 1870–1880 Ma, and late allochemical metamorphism accompanied by the pegmatite formation with an age of 1840 Ma.

AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA

Research Article| June 01, 2014 AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA L. LONGRIDGE; L. LONGRIDGE School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa VM Investment Company, 24 Fricker Rd, Illovo, Johannesburg, South Africa, e-mail: lukelongridge@gmail.com Search for other works by this author on: GSW Google Scholar J.A. KINNAIRD; J.A. KINNAIRD School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za Search for other works by this author on: GSW Google Scholar R.L. GIBSON; R.L. GIBSON School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za Search for other works by this author on: GSW Google Scholar R.A. ARMSTRONG R.A. ARMSTRONG PRISE, Australian National University, Research School of Earth Sciences, Building 61, Mills Road, Acton, 0200, Australia, e-mail: richard.armstrong@anu.edu.au Search for other works by this author on: GSW Google Scholar Author and Article Information L. LONGRIDGE School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa VM Investment Company, 24 Fricker Rd, Illovo, Johannesburg, South Africa, e-mail: lukelongridge@gmail.com J.A. KINNAIRD School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za R.L. GIBSON School of Geosciences, University of the Witwatersrand, PVT Bag 3, Wits, 2050, South Africa, e-mail: judith.kinnaird@wits.ac.za; roger.gibson@wits.ac.za R.A. ARMSTRONG PRISE, Australian National University, Research School of Earth Sciences, Building 61, Mills Road, Acton, 0200, Australia, e-mail: richard.armstrong@anu.edu.au Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2014 Geological Society of South Africa South African Journal of Geology (2014) 117 (1): 67–86. https://doi.org/10.2113/gssajg.117.1.67 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L. LONGRIDGE, J.A. KINNAIRD, R.L. GIBSON, R.A. ARMSTRONG; AMPHIBOLITES OF THE CENTRAL ZONE: NEW SHRIMP U-PB AGES AND IMPLICATIONS FOR THE EVOLUTION OF THE DAMARA OROGEN, NAMIBIA. South African Journal of Geology 2014;; 117 (1): 67–86. doi: https://doi.org/10.2113/gssajg.117.1.67 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract New SHRIMP U-Pb zircon and titanite data are presented for two amphibolites from the Central Zone of the Damara Orogen, Namibia. These data (the first ages for amphibolites in the Central Zone) show that amphibolites are derived from both Palaeoproterozoic (2026.9 ± 2.3 Ma – sample ACAM-1) and Pan-African (557.2 ± 7.4 Ma – sample LKR021) precursors emplaced into the Palaeoproterozoic Abbabis Complex basement in the Central Zone. The younger amphibolites are interpreted as part of a mafic to dioritic phase of magmatism in the Central Zone (the Goas Suite), and major- and trace-element geochemistry from a suite of amphibolites from the southwestern Central Zone confirms this relationship. This phase of mafic to dioritic magmatism is contemporaneous with the onset of collision between the Congo and Kalahari Cratons. It is suggested that delamination of dense mafic lower crust and underplating of mantle-derived magmas following crustal thickening may have led to partial melting of pre-Pan-African mafic lower crust, generating this mafic to dioritic suite of rocks. Metamorphic zircon overgrowths in sample LKR021 give an age of 520 ± 6.9 Ma, consistent with recent estimates for the age of peak metamorphism in the southwestern Central Zone. Titanites from LKR021 give an age of 493.4 ± 6.4 Ma, and this is interpreted as a cooling age for the Central Zone. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia

Granitoids with juvenile signatures are common in arc environments and contribute to growth of the continental crust. Thermo-mechanical models of arcs suggest that intermittent intrusion of magma batches leads to magma hybridization, remelting, and remobilization of earlier intrusive rocks driven by fluctuations in temperature and water fluxing. While there are numerous examples in the literature of multiple intrusions and magma hybridization, field examples of remelting and remobilization of earlier intrusive rocks within an arc are rare. Here, we investigate the evolution of magmatic rocks of the Paleoproterozoic St. Peter Suite, emplaced along the SW margin of the Gawler craton, South Australia, a typical calc-alkaline arc suite. Magmatic rocks recording multiple intrusions and multiple magma interactions have undergone in situ remelting and remobilization forming migmatites. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon dating yielded crystallization ages of 1647 ± 12 Ma for a tonalitic gneiss representing the oldest intrusive suite, and 1604 ± 12 Ma for a leucogranite representing the youngest intrusive suite. Both these suites developed synmagmatic foliation and magmatic banding defined by broadly parallel dikes, elongated enclaves, and schlieren. The rock suites record two deformation events associated with anatexis. The first event, D 1 , was responsible for a dominant approximately E-W–striking foliation (S 1 ) parallel to magmatic foliation, and associated with a dominantly sinistral shearing that affected the older suite. This deformation was associated with the first anatectic event, as demonstrated by the association between leucosomes and structures. The second deformation event, D 2 , affected both suites and was characterized by isolated F 2 folds and shear planes filled with leucosomes subparallel to axial planar foliation. Leucosomes interconnected and gave rise to magma extraction channels tens of meters long. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb titanite dating of a late leucosome sample, collected from within the older magmatic suite, yielded an age of 1605 ± 7 Ma, coinciding with the crystallization age of the younger suite rather than postdating it, as expected. We interpret these results to indicate that crystallization of the younger suite and the second anatectic event occurred in the time encompassed by the error of these young ages. Leucosomes from both anatectic phases lack anhydrous peritectic phases and are interpreted to represent low-temperature anatexis resulting from water fluxing. Combined, these results suggest that a protracted and complex intrusive history can be made significantly more complex by anatexis, giving rise to evolved magmas after older ones, erasing earlier intrusive relationships, and establishing new ones. Rocks of the St. Peter Suite record many of the key processes expected in arcs, including the prediction that early intrusive arc rocks remelt to form younger and more fractionated magmas.

Dating the cooling of exhumed central uplifts of impact structures by the (U–Th)/He method: A case study at Manicouagan

Forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the Manicouagan impact structure were dated by the (U–Th)/He technique. A (U–Th)/He central age of 207.1±6.4Ma (2 standard error (SE), n=40) has been determined. With 4 outlier ages removed the central age is refined to 208.9±5.1Ma (2 SE). Both of these ages are within error of the previously determined U–Pb zircon age of 214±1Ma (2σ) derived from the impact melt. Manicouagan's central uplift formed due to rapid elevation from ~7–10km depth as part of the modification stage of the impact process, which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the (U–Th)/He system in titanite. Correlation with the previous U–Pb zircon 214±1Ma impact melt crystallization age indicates that the (U–Th)/He titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets, or other melt products. The youngest ca. 195Ma (U–Th)/He dates preserved in some titanite fragments are synchronous with Early Jurassic, rift-induced lithospheric thinning and associated igneous activity that defines the Central Atlantic Magmatic Province (CAMP). The (U–Th)/He titanite data from Manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of CAMP activity.

In situ LA-ICP-MS U–Pb titanite dating of Na–Ca metasomatism in orogenic belts: the North Pyrenean example

In the Pyrenees, in association with the rotation of the Iberian plate around Europe during the Mid-Cretaceous, a Na–Ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to Na-metasomatism (albitization) and talc–chlorite mineralization. It affected metasedimentary rocks as well as Hercynian granites. In situ laser ablation ICP-MS U–Pb analyses of titanite grains formed in albitites during metasomatism date the Na–Ca metasomatism between 110 and 92 Ma. The temperature of the Na–Ca metasomatism is estimated to be approximately 550 °C. Both the time constraints and temperature estimates suggest that the Na–Ca metasomatism is related to the low-P high-T North Pyrenean metamorphism.

Palaeoproterozoic orogenic gold style mineralization at the Southwestern Archaean Tanzanian cratonic margin, Lupa Goldfield, SW Tanzania: Implications from U–Pb titanite geochronology

The Lupa Goldfield, situated at the southwestern Tanzanian cratonic margin, comprises a network of auriferous quartz veins and greenschist facies mylonitic shear zones cutting a suite of Archaean–Palaeoproterozoic granitic–gabbroic intrusions. The existing geochronological database points to a protracted, but episodic 1.96–1.88Ga magmatic history that is broadly coincident with the 2.1–1.8Ga Ubendian Orogeny. Molybdenite, pyrite and chalcopyrite samples from mineralized quartz veins and mylonitic shear zones yield Re–Os model ages that range from 1.95 to 1.88Ga, whereas ca. 1.88Ga pyrite with gold bearing inclusions and sampled from the host mylonitic shear zone suggest that gold occurred relatively late in this hydrothermal history. The ca. 1.88Ga gold event is recorded at all five of the studied prospects, whereas the relationship between gold and the disparately older 1.95 and 1.94Ga Re–Os molybdenite ages is unclear. New U–Pb metamorphic titanite dating of a foliated Archaean granite sample (ca. 2.76Ga) suggests that the onset of ductile deformation within the Lupa Goldfield occurred at ca. 1.92Ga, and some ca. 40Myr prior to auriferous and brittle–ductile mylonitic shear zones at ca. 1.88Ga. Early ductile deformation is not associated with gold mineralization, but the ductile deformation fabrics and, in particular the development of rheologically weak chloritic folia, may have acted as zones of pre-existing weakness that localized strain and influenced the geometry of later auriferous mylonitic shear zones. The large age difference between U–Pb zircon and titanite ages for the Archaean granite sample is in contrast to new U–Pb titanite ages for the Saza Granodiorite (1930±3Ma), which are only slightly outside of analytical uncertainty at the 2σ level with a previously reported U–Pb zircon age for the same sample (1935±1Ma). These new age results, together with previously reported U–Pb and Re–Os ages, highlight the protracted magmatic, hydrothermal and structural evolution of the Lupa Goldfield (1.96–1.88Ga). They are also consistent with other palaeo-convergent margins where orogenic gold style mineralization occurs relatively late in the orogen's tectono-thermal history.

Neoarchean crustal growth by accretionary processes: Evidence from combined zircon–titanite U–Pb isotope studies on granitoid rocks around the Hutti greenstone belt, eastern Dharwar Craton, India

The Neoarchean Hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides. Combined U–Pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution. The Golapalli and Yelagatti granodiorites occurring to the north of the Hutti greenstone belt were emplaced at 2569±17Ma. The Yelagatti granodiorite yielded a younger titanite age of 2530±6Ma which indicates that it was affected by a post-crystallization thermal event that exceeded the titanite closure temperature. The western granodiorites from Kardikal have identical titanite and zircon ages of 2557±6Ma and 2559±19Ma, respectively. The eastern Kavital granodiorites yielded titanite ages of 2547±6Ma and 2544±24Ma which are identical to the published U–Pb zircon SHRIMP ages. These ages imply that the granitoid rocks surrounding the Hutti greenstone belt were formed as discrete batholiths within a short span of ca. 40Ma between 2570Ma and 2530Ma ago. They were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the Archean. The first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the Hutti area. A metamorphic overprint occurred at ca. 2300Ma ago that reset the Rb–Sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of Au in the ore lodes. The eastern Dharwar Craton consists of quartz monzodiorite–granodiorite–granite suites of rocks that are younger than the greenstone belts that are older than ∼2650Ma reported from earlier studies. The granitoid magmatism took place between 2650 and 2510Ma ago indicating accretionary growth of the eastern Dharwar Craton.

TITANITE CRYSTAL CHEMISTRY AND U-P ISOTOPIC DATA: A PETROGENETIC INDICATOR FOR PRECAMBRIAN GRANITOID PLUTONS OF THE EASTERN BRAZILIAN SHIELD

This paper presents crystal chemistry and U-Pb isotopic data for titanite (or sphene, CaTiSiO5) fromPrecambrian granitoid plutons of distinct tectonic settings within the Eastern Brazilian shield. Despite thefact that most titanite ages are related to partial lead-loss, resetting or crystallization during metamorphicevents, chemistry data reflect the magmatic history of the plutons, specially the Ti4+/Al+Fe3+ ratio. It isdemonstrated that for plutons of different crystallization ages (Archean, Paleoproterozoic and Neoproterozoic)the measured Ti4+/Al+Fe3+ ratios of titanite grains are characteristic of the source region from which themagma was extracted, and the ratios remain unchanged during subsequent metamorphic events. HigherTi4+/Al+Fe3+ values (> 9) are found in predominantly mantle-derived granitoid plutons whilst crust-derivedgranitoid rocks have lower ratios. Therefore, titanite crystal chemistry may be used to discriminate magmasources of granitoid rocks regardless their age and metamorphic history.

In situ LA-ICP-MS investigation of the geochemistry and U-Pb age of rutile from the rocks of the Belomorian mobile belt

Rutile is a common accessory mineral in variousmetamorphic and igneous rocks. Although its formulais very simple, a number of elements may substitute forTi in rutile, including Al, V, Cr, Fe, Zr, Nb, Sn, Sb, Hf,Ta, W, and U [1]. The chemical characteristics of rutile(primarily, the character of HFSE distribution) arewidely used as geochemical indicators of mineralforming processes [1, 2]; Zr content in rutile was calibratedas a geothermometer [e.g., 3]; and rutile is a suitablemineral for the U–Pb dating of metamorphism andgeothermochronological reconstructions [4]. Theresults of the U–Pb dating of rutile and titanite from therocks of the Baltic shield were used for the reconstruction of the thermal history of the Karelian craton andBelomorian mobile belt (BMB) [5, 6]. The authorsestablished a significant difference between the U–Pbages of rutiles and titanites from the rocks of the Karelian craton (Archean) and BMB (Paleoproterozoic).However, the character of trace element distribution inrutile, which may potentially provide insight into theparameters controlling crystallization processes, wasnot previously explored in the rocks of the Baltic shield.This paper reports the results of an in situ LA–ICP–MS investigation of the geochemistry and U–Pb age ofrutile from metamorphic rocks of two areas within theBMB, the Shueretskoe deposit of garnet in its centralpart and the Salma eclogites from the northwesternBMB. Although the rocks of these complexes wereimprinted penecontemporaneously by the youngestSvecofennian metamorphism, they are significantlydifferent in the mineral and chemical composition ofrutilebearing rocks and the geologic history of theirtransformation.In the Shueretskoe deposit, rutile inclusions wereseparated from a garnet megacryst (sample 6). The ageof the metasomatic crystallization of this garnet wasdetermined as 1837 ± 14 Ma by the in situ SHRIMP IIdating of coexisting zircon and monazite inclusions atthe Center of Isotopic Investigations, Karpinskii AllRussia Geological Institute [7]. The deposit is locatedin the central part of the BMB, at the mouth of theShuya River and hosted by the metamorphic rocks ofthe Belomorian complex. Economically important aremetasomatic garnet gedritite and glimmerite in garnetbearing gneisses and amphibolites. Garnet occurs inthe metasomatic rocks as irregular nodules or, morerarely, faced crystals up to 25 cm across. The metasomatism that resulted in the formation of the garnetdeposit occurred under peak metamorphic conditions at a temperature of 650–680°C and a pressureof 7.8–8.5 kbar [7].The Salma eclogites were investigated in the pit ofthe KuruVaara deposit, where tonalite–trondhjemitegneisses with bodies and blocks of eclogites and eclogitelike

A Sveconorwegian terrane boundary in the Caledonian Hardanger–Ryfylke Nappe Complex: The lost link between Telemarkia and the Western Gneiss Region?

Magmatic and metamorphic events in two of the nappes of the Hardanger–Ryfylke Nappe Complex in the Caledonides in SW-Norway, and in the intervening thrust zone, have been investigated by means of ID-TIMS U–Pb zircon and titanite data. Orthogneiss protoliths in the upper Kvitenut Nappe are dated at 1615±6Ma, showing analogies to the Gothian terrane, including the Western Gneiss Region. By contrast, the Dyrskard Nappe is composed of metasedimentary rocks and metarhyolites with a 1508±4Ma extrusion age and shows an affinity to rocks of the Telemarkia terranes. We argue that the time of thrusting and juxtaposition of the two nappes along the shear zone is constrained by the age of 999±5Ma of a syndeformational granite body and co-genetic pegmatitic leucosomes, with late Sveconorwegian movements and fluid activity being recorded by titanite at 924±6Ma. Both nappes behaved as one block during the Silurian emplacement in the Caledonian nappe stack, sharing a 434±1Ma metamorphic peak and later overprints, as young as 414±2Ma, related to retrogression. The distinct origin and Sveconorwegian age of coupling of the Dyrskard and the Kvitenut nappes suggest that, in their pre-Caledonian location to the west-northwest, they represent the now hidden boundary zone between the Western Gneiss Region and Telemarkia.

In situ U-Pb dating of titanite by LA-ICPMS

Titanite is an ideal mineral for U-Pb isotopic dating because of its relatively high U, Th and Pb contents. Here, we developed a technique for U-Pb dating of titanite using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS. Standards of titanite (BLR-1 and OLT-1) and zircon (91500 and GJ-1) were dated using single spot and line raster scan analytical methods. To check the matrix effect, titanite (BLR-1) and zircon (91500) standards were analyzed as the external standards. The weighted mean 206Pb/238U ages of OLT-1 titanite are 1015±5 Ma (2σ, n=24) and 1017±6 Ma (2σ, n=24) by single spot and line raster scan analyses, respectively, using BLR-1 titanite as the external standard. These ages are consistent with its reference age of about 1014 Ma. However, using 91500 zircon as the external standard, the weighted mean 206Pb/238U ages are 917±4 Ma (2σ, n=24) and 927±5 Ma (2σ, n=24) for BLR-1 titanite, and 891±4 Ma (2σ, n=24) and 901±5 Ma (2σ, n=24) for OLT-1 titanite by single spot and line raster scan analyses, respectively. It is evident that these ages are ∼12% younger than their reference values. Our results reveal that significant matrix effect does exist between different kinds of minerals during LA-ICPMS U-Pb age determination, whereas it is insignificant between same minerals. Therefore, same mineral must be used as the external standard for fractionation corrections during in situ LA-ICPMS U-Pb age analysis. In addition, we determined U-Pb ages for titanites from the Early Cretaceous Fangshan pluton, which indicates a rapid cooling history of this pluton.

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U–Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP–LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U–Pb age of 480.8 ± 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6–647.4 Ma. Zircon and titanite U–Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U–Pb age of 366 ± 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP–LT peak metamorphism.

Metamorphic history of the South Tibetan Detachment System, Mt. Everest region, revealed by RSCM thermometry and phase equilibria modelling

This study combines microstructural observations with Raman spectroscopy on carbonaceous material (RSCM), phase equilibria modelling and U–Pb dating of titanite to delineate the metamorphic history of a well-exposed section through the South Tibetan Detachment System (STDS) in the Dzakaa Chu valley of Southern Tibet. In the hanging wall of the STDS, undeformed Tibetan Sedimentary Series rocks consistently record peak metamorphic temperatures of ∼340 °C. Temperatures increase down-section, reaching ∼650 °C at the base of the shear zone, defining an apparent metamorphic field gradient of ∼310 °C km−1 across the entire structure. U–Th–Pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 Ma. Deformation within this 1-km-thick zone of distributed top-down-to-the-northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo-isotherms along the upper margin of the Greater Himalayan Series (GHS). We interpret the preservation of such a high metamorphic gradient to be the result of a progressive up-section migration in the locus of deformation within the zone. This segment of the STDS provides a detailed thermal and kinematic record of the exhumation of footwall GHS rocks from beneath the southern margin of the Tibetan plateau.

GRENVILLE SKARN TITANITE: POTENTIAL REFERENCE MATERIAL FOR SIMS U-Th-Pb ANALYSIS

We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA–WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U–Th–Pb age dating. The SHRIMP data show that these megacrysts (5–31g) have concordant U–Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th, 206 Pb/ 204 Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID–TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter ( i.e. , in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2σ, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2σ, n = 3, MSWD = 3.3) for domains that have not lost Pb. The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2σ, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA–WDS data show that the outer 50–100 μm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U–Th–Pb age dating. Crystal TCB has excellent homogeneity of U–Th–Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite.

In-situ dating of the Earth's oldest trace fossil at 3.34 Ga

Microbial activity in volcanic glass within the oceanic crust can produce micron sized pits and tunnels. Such biogenic textures have been described from the recent oceanic crust and mineralized equivalents in pillow lavas as old as 3.47–3.45 Ga from the Barberton Greenstone Belt (BGB) of South Africa. In meta-volcanic glasses these microbial traces are preserved by titanite mineralization (CaTiSiO 5) and on the basis of morphological, textural and geochemical evidence have been argued to represent Earth's oldest trace fossils. Here we report the results of in-situ U–Pb dating of titanite that infills trace fossils from the Hooggenoeg Complex of the BGB using laser ablation MC-ICP-MS (multi-collector inductively coupled, plasma mass spectrometry). This yields a titanite age of 3.342 ± 0.068 Ga demonstrating the antiquity of the BGB trace fossils. This radiometric age confirms that a sub-seafloor biosphere was already established in the PaleoArchean and that it likely represented an important habitat for the emergence and evolution of early microbial life on the Earth.

(U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ∼1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to ≤35 °C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ∼850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events.

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt, South Africa

SHRIMP dating of titanite from metasyenites in the Central Zone of the Limpopo Belt yields a mean 207Pb/ 206Pb age of 2010.3 ±4.5 Ma calculated from 23 analyses. This age, combined with petrographic and field observations, suggests the metamorphism in the syenites occurred during Palaeoproterozoic event.

Regional-scale Cretaceous albitization in the Pyrenees: evidence from in situ U–Th–Pb dating of monazite, titanite and zircon

Abstract: Albitization is a common metasomatic process active in various geodynamic contexts. In the northern Pyrenees, there are several occurrences of albitites but, until now, only one occurrence has been dated (117 Ma, Ar–Ar dating). This paper presents new U–Th–Pb ages for several albitite occurrences throughout the Pyrenees to test whether they are contemporaneous and, if so, to specify the regional extent of the albitization event. Ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 ± 8 and 98 ± 2 Ma, respectively. The zircon U–Th–Pb isotopic system did not record this Cretaceous metasomatic event, even when grains were selected in metasomatically Zr-enriched rocks or in hydrothermal structures (millimetre-sized veins cross-cutting granitoids). We argue that the total time span of 20 Ma recorded by albitites corresponds to a long-lived hydrothermal system that was active during the rotation of Iberia around Europe, along the North Pyrenean Fault. Because albitization and talc mineralization have the same spatial and temporal distribution in the Pyrenees, we argue that these two metasomatic phenomena are two independent records of this single, regional-scale, long-lived hydrothermal system.

Detrital zircon, detrital titanite and igneous clast U–Pb geochronology and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup

A multidisciplinary provenance study, including in situ U–Pb dating of detrital titanite, was undertaken on the enigmatic low-grade metasediments of the Colonsay Group, SW Scotland to determine their source and assess previous correlations with major Neoproterozoic sedimentary sequences within the Scottish Caledonides. Sm–Nd model ages ( t DM) of the Colonsay Group range from 1664 to 2140 Ma, suggesting a Palaeoproterozoic source. Laser Ablation ICPMS U–Pb detrital zircon ages from six Colonsay Group formations are consistent with sources within Laurentia with predominant input from a ca. 1780 Ma source (Rhinns Complex) and some Grenvillian ( ca. 1.3–0.95 Ga), Pinwarian ( ca. 1.51–1.45 Ga), Labradorian ( ca. 1.71–1.62 Ga) and Ketilidian ( ca. 1.9–1.75 Ga) detritus. Archaean (> ca. 2.5 Ga) input is minimal. Clasts from the basal Octofad Sandstone Formation indicate input from ca. 1795 Ma and ca. 1400 Ma sources (U–Pb SIMS zircon ages). U–Pb (SIMS) analyses of detrital titanite record Grenville metamorphic events in the source terranes and suggest a slightly younger maximum depositional age of ca. 942 Ma compared with the youngest detrital zircon age of ca. 1025 Ma. The data support the interpretation that the Octofad Sandstone Formation is part of the Colonsay Group and that the Colonsay Group rests unconformably on the Rhinns Complex on Islay. Deposition of the Colonsay Group is, therefore, tied to the margins of Laurentia, possibly in the foreland to the Grenville Belt, as the Rhinns Complex does not record any Grenville deformation. The data are consistent with deposition of the Colonsay Group in a foreland basin related to the Rigolet phase ( ca. 1000–980 Ma) of the Grenville Orogeny. On the basis of detrital zircon U–Pb ages the Colonsay Group best correlates with the lowermost Dalradian Grampian Group of Scotland. This correlation has significant implications for Dalradian basin dynamics, timing of Dalradian deposition and timing of Dalradian deformation. In addition, the unconformable contact between the Colonsay Group and the Rhinns Complex on Islay would make it one of the few places where the basement to the Dalradian can be observed.