Geological Survey of Denmark and Greenland Bulletin 33, 2015, 77-80 77 Investigations of detrital zircon, rutile and titanite from present-day Labrador drainage basins: fingerprinting the Grenvillean front Tonny B. Thomsen, Christian Knudsen and Alana M. Hinchey A multidisciplinary provenance study was conducted on stream sediment samples from major rivers in the eastern part of Labrador, Canada (Fig. 1). Th e purpose was to fi n- gerprint the sources that deliver material to the stream sedi- ments and to the reservoir sand units deposited off shore in the sedimentary basins in the Labrador Sea. We used a multi- mineral U-Pb geochronological approach employing rutile and titanite in addition to zircon to obtain unbiased age data. Th e purpose of this was to characterise the diff erent ig- neous and metamorphic episodes that occurred in Labrador, which is an area with highly variable geology characterised by the Palaeoproterozoic south-eastern Churchill province in the north-west, the Archaean Nain plutonic suite in the north-east, the Palaeoproterozoic Makkovik province in the east and the Mesoproterozoic Grenville Province to the south. Th e fi eld work was carried out in 2012 and 2013 and the study is a collaborative project between the Geological Survey of Denmark and Greenland and the Geological Sur- vey of Newfoundland and Labrador. In this paper we focus on three samples from the southern part of the study area where two parts of the Grenville orogeny are found (Fig. 1). The use of zircon, rutile and titanite in sedimentary provenance investigations Methods for obtaining geochronological information from various detrital minerals for quantitative sedimentary prove- nance purposes have developed rapidly over recent years. Th is is mostly due to advances in high-throughput microanalyti- cal techniques such as Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). Th e emphasis on U-Pb geochronology has primarily been on detrital zircon, as it typically provides precise age information of the source rocks. Zircon, however, is not usually the mineral of choice for dating the history of rocks with a complex tectonothermal evolution, as it typically survives most processes occurring in the rock cycle from sedimentation to high grade metamor- phism and oft en even magmatic processes. Th is means that zircon typically refl ects several orogenic cycles (e.g. Okay et al. 2011) and is less suited for recording information about processes related to the metamorphic or hydrothermal reac- tion history of a rock. Th e Ti minerals, rutile and titanite, occur in a variety of magmatic, hydrothermal, metamorphic and sedimen- tary rock types, oft en together with zircon (e.g. Force 1991; Frost et al. 2001; Zack et al. 2004). Although detrital rutile is less abundant than zircon, the widespread occurrence of rutile in a wide range of medium- to high-grade, and also high-pressure (blueschist and eclogite facies), metamorphic rocks as well as in sediments and sedimentary rocks (Force 1980, 1991), combined with its high mechanical and chemi- cal stability during weathering, transport and diagenesis (e.g. Morton & Hallsworth 1999), makes it a prime candidate in provenance studies (Zack et al. 2011). Rutile forms under amphibolite and higher metamorphic facies conditions and is typically unstable at lower grade conditions (Force 1980, © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 77–80. Open access: www.geus.dk/publications/bull GGU 539754 GGU 539828 GGU 539845 PROTEROZOIC Nain plutonic suite Grenville province Exterior thrust belt Interior magmatic belt Makkovik and SE Churchill provinces ARCHAEAN Superior province Nain province Sample Drainage area 100 km 54°N 52°N 54°N 60°N 66°W 62°W 56°W 62°W66°W Fig. 1. Simplified geological map of Labrador. 7878 1991; Triebold et al. 2007, 2011), where it usually breaks down to form other Ti minerals such as titanite or ilmenite at greenschist facies conditions. Rutile, therefore, typically yields chronological and petrogenetic information refl ecting the timing and conditions of the last medium to high-grade or high-pressure metamorphic event (Zack et al. 2011; Okay et al. 2011). Titanite is widespread in a variety of rock types typically of more calcic compositions, and is usually rare in rocks with low CaO/Al2O3 ratios such as peralkaline granitoids and peraluminous granites (Frost et al. 2001). Titanite occurs in very low- to high-grade metamorphic rocks and survives under ultra-high pressure conditions (Force 1991; Frost et al. 2001), although it is typically scarce or absent in most granulite-facies metamorphic rocks (Krogh & Keppie 1990). Even though titanite is a widespread mineral and occurs as detrital and authigenic grains in sedimentary rocks, it has rarely been used to date deposition, diagenesis or low-grade metamorphism. Detrital titanite was fi rst used as a sedimen- tary provenance tool by McAteer et al. (2010). Th e reason that titanite is not routinely used for provenance investiga- tions is probably that it is more susceptible to abrasion dur- ing sedimentary transport than zircon and rutile, resulting in reduced occurrence or absence as a detrital component in sediments and sedimentary rocks. Titanite, however, is more reactive than zircon or rutile during metamorphism and forms at temperatures below 700°C. Th is provides the mineral with a large potential to record ages for a wide range of low to moderate temperature geological crustal processes (Frost et al. 2001; McAteer et al. 2010; Muhling et al. 2012). Th us, if present in sedimentary rocks, titanite is an ideal can- didate for dating regional or local metamorphism. In addi- tion, because most metamorphic events are associated with deformation, titanite can date potential deformation stages in metamorphic terranes (Frost et al. 2001). Th erefore, it is possible to recognise sediment sources from detrital titanite and rutile data that are not represented in zircon data and thus gain additional chronological and petrogenetic insight into the tectonothermal history of the source regions (McAteer et al. 2010, 2014). Furthermore, rutile and titanite generally contain 5–10 times less U than zircon, thus metamictisation of these minerals is relatively rare. Consequently, in rocks with U-rich zircon and titanite or rutile, the zircon might show metamictisation, and there- fore would be more prone to degradation during transport and weathering, and is thus likely to be excluded from a detri- tal study (Fedo et al. 2003). Titanite and rutile, on the other hand, have the potential to retain the magmatic record of the U-rich zircon source as well as the metamorphic episodes that may have occurred prior to deposition. U-Pb analysis and data processing In this study, U-Pb dating was carried out on mineral grains embedded in epoxy mounts at the LA-ICPMS facility at the Geological Survey of Denmark and Greenland using a NWR213 Nd:YAG laser system coupled to an ELEMENT 2 double-focusing, single-collector, magnetic sector-fi eld ICPMS. Mineral grains were separated by routine separation methods including a Wilfrey water-shaking table, Frantz electromagnetic separation and heavy liquids. Zircon, rutile and titanite grains were picked by hand under a binocular microscope from the resulting heavy mineral fractions and mineral compositions were qualitatively controlled by SEM- EDS. For rutile and titanite, laser beam pre-ablation using a spot size slightly larger (40 μm) than the analysis spot size (25 μm) was performed prior to the LA-ICPMS analysis to avoid surface contamination. Data processing was performed off -line using the soft ware Iolite (Paton et al. 2010, 2011) and the VizualAge data reduction scheme vers. 2.5 by Petrus & Kamber (2012). Th e data were corrected for background signal, time-dependent fractionation, instrumental drift and down-hole isotopic fractionation. In order to validate our results, the measurements were bracketed throughout the entire analysis sequences by analyses of natural mineral standards. Th ese include the GJ-1 and Plesovice zircons, the rutiles R10, R13, R19 (provided by courtesy of T. Zack, Uni- versity of Gothenburg) and Sugluk-4 (L. Bracciali, British Geological Survey), and the titanite A1772 (Y. LaHaye, Geo- logical Survey of Finland) and Seiland ( J. Kosler, University of Bergen). In contrast to zircon, common Pb in titanite and rutile is generally lattice bound and can occur in signifi cant proportions. Th us, common Pb correction typically needs to be applied for these minerals. However, common Pb usually has negligible eff ects for titanites or rutiles with 206Pb/204Pb ratios >300 (Frost et al. 2001), whereas the initial common Pb isotopic composition typically has greater eff ects on anal- yses with 206Pb/204Pb ratios <300 and therefore the results must be treated more cautiously. Only the titanite grains in this study typically have large proportions of common Pb, whereas most zircon and rutile only contain a small or negli- able amount of common Pb. Titanite ages reported herein are corrected for common Pb. Some of the titanite grains contained a signifi cant amount of common Pb and some of the titanite ages are potentially of a slightly lower accuracy compared to ages obtained for rutile and zircon that were not common Pb corrected. Th e correction for common Pb was performed using the present-day terrestrial common Pb estimate of Stacey & Kramers (1975) and the measured mass 204 (204Pb + 204Hg) corrected for 204Hg calculated from measured 202Hg and the natural 204Hg /202Hg ratio. 79 Results of the provenance study U-Pb age distributions of zircon, rutile and titanite from three representative river samples are shown in the prob- ability–density diagrams in Fig. 2. Th e three samples were collected in areas within the Grenvillean orogenic belt that are characterised by ages around 1000 Ma. Sample GGU 539754 comes from the northern part of the orogenic belt (the exterior thrust belt), sample GGU 539828 from the cen- tral part and sample GGU 539845 from the southern part (the interior magmatic belt). Th e samples all contain zircon grains older than the Grenville orogeny, refl ecting the ability of zircon to maintain older magmatic formation signatures through the younger Grenvillean orogenesis. Th e three ar- eas show distinct diff erences in detrital zircon ages. Th e fre- quency of c. 1000 Ma old Grenvillean zircon ages is much lower in the northern exterior thrust belt than in the central part and especially in the southern interior magmatic belt, where crust was formed during the Grenville orogeny. In the southern area (GGU 539845) both Palaeo- and Mesoprote- rozoic zircon ages are common, with the highest abundance at c. 1500 Ma, whereas the samples from the central area (GGU 539828) and the northern exterior thrust belt (GGU 539754) are dominated by Palaeoproterozoic zircons with a peak at c. 1650 Ma. However, there is a second distinct Meso- proterozoic peak at c. 1360 Ma in the northern area and a signifi cantly reduced abundance of Mesoproterozoic zircon ages in the central area. As expected, the detrital rutile ages peak just below 1000 Ma for all three samples, refl ecting rutile formation or com- plete U-Pb system resetting of older rutile grains during medium- to high-grade metamorphic stages of the Gren- ville orogeny. Titanite shows Grenvillean ages for all three samples. In the sample from the southern interior magmatic belt all titanite ages are c. 1000 Ma old, corresponding to the rutile age distribution, whereas a more complex age pattern, with Palaeo- and Mesoproterozoic titanites, is seen in the samples from the central and northern areas. Th is indicates that many more titanite grains from the latter areas survived the Grenvillean metamorphism than (1) rutile from the same areas and, (2) titanite and rutile from the southern interior magmatic belt. Th e occurrence of pre-Grenvillean titan- ite and absence of pre-Grenvillean rutile in the central and northern areas could be due to the diff erence in the U-Pb isotopic system closure temperatures of the two minerals; rutile has a lower closure temperature at c. 400–500°C than titanite with a closure temperature at c. 500–700°C. Hence the age pattern depends on the metamorphic grade to which minerals were exposed during the Grenville orogeny. Th e occurrence of titanite ages and lack of rutile ages indicate medium- to high-grade metamorphic conditions, probably upper amphibolite facies. During the Grenville orogeny, the metamorphic grade of the new crust was probably higher in the southern interior magmatic belt than in the central and northern areas. We suggest that the closure temperature of n = 10/16 n = 5/11 n = 32/32 n = 4/4 n = 140/140 n = 129/130 n = 8/9 n = 26/26 n = 136/137 0 0.00108 0.00217 0.00325 0.00434 0.00542 0 0.00245 0.00491 0.00736 0.00982 0.01227 P ro b ab il it y 0 0.00214 0.00428 0.00642 0.00857 0.01071 0 0.00266 0.00533 0.00799 0.01066 0.01332 P ro b ab il it y 0 0.00119 0.00238 0.00357 0.00477 0.00596 0.00076 0.00152 0.00228 0.00304 0.0038 P ro b ab il it y 12 10 8 6 4 2 0 12 10 8 6 4 2 0 12 10 8 6 4 2 0 2 1 0 2 1 0 2 1 0 F re q u e n c y 0.00149 0 0.00299 0.00448 0.00598 0.00747 F re q u e n c y 0 0.0023 0.0046 0.00689 0.00919 0.01149 0 0.00118 0.00236 0.00354 0.00472 0.0059 F re q u e n c y 0 500 1000 1500 2000 T it an it e R u ti le Z ir c o n Age (Ma) 0 500 1000 1500 2000 Age (Ma) 0 500 1000 1500 2000 Age (Ma) GGU 539754 The nortern exterior thrust belt GGU 539828 The central part GGU 539845 The southern interior magmatic belt 30 25 20 15 10 5 0 30 25 20 15 10 5 0 30 25 20 15 10 5 0 0 Fig. 2. Probability–density diagrams showing U–Pb age distributions for three samples collected in Labrador. n = x/y (in red) denotes the total number of analysed zircon grains ( y) of which x are ‘concordant’ (i.e. <10% discordant from concordia). Light grey: ages that are >10 % discordant (relative to Wetherill concordia), dark grey: ‘concordant’ ages within <10% discordance (i.e. <10%). Blue line at 1000 Ma: late stage of the Grenville orogeny. 8080 both Ti minerals was overstepped, resulting in isotopic age resetting of any pre-Grenvillean titanite and rutile grains and formation of new Grenvillean age titanite and rutile. Concluding remarks Th e wide range in the detrital zircon U-Pb ages within the Grenvillean orogenic belt refl ects formation age diff erences of the rocks that were brought into the orogenic process. Accordingly, for the best characterisation of the sediment source, it is not suffi cient to know the age of the orogeny that aff ected the area; it is also nessesary to know the lateral dis- tribution of the rock units in the area as well as the ages of the zircons (and other minerals) from these units. Moreover, diff erent units contain diff erent amounts of e.g. zircon. An effi cient way to map the lateral age variation is to analyse de- trital minerals collected from present-day drainage systems. References Fedo, C.M., Sircombe, K.N. & Rainbird, R.H. 2003: Detrital zircon analysis of the sedimentary record. In: Hanchar, J.M. & Hoskin, P.O. (eds): Zircon: experiments, isotopes and trace element investigations. Mineralogical Society of America, Reviews in Mineralog y 53, 277–303. Force, E.R. 1980: Th e provenance of rutile. Journal of Sedimentary Petrol- og y 50, 485–488. Force, E.R. 1991: Geolog y of titanium-mineral deposits. Geological Soci- ety of America, Special Papers 259, 112 pp. Frost, B.R., Chamberlain, K.R. & Schumacher, J.C. 2001: Sphene (ti- tanite): phase relations and role as a geochronometer. Chemical Geol- og y 172, 131–148. Krogh, T.E. & Keppie, J.D. 1990: Age of detrital zircon and titanite in the Meguma Group, southern Nova Scotia, Canada: clues to the origin of the Meguma Terrane. Tectonophysics 177, 307–323. McAteer, C.A., Daly, J.S., Flowerdew, M.J., Connelly, J.N., Housh, T.B. & Whitehouse, M.J. 2010: Detrital zircon, detrital titanite and igne- ous clast U–Pb geochronolog y and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup. Precambrian Research 181, 21–42. McAteer, C.A., Daly, J.S., Flowerdew, M.J., Whitehouse, M.J. & Monaghan, N.M. 2014: Sedimentary provenance, age and possible cor- relation of the Iona Group, SW Scotland. Scottish Journal of Geolog y 50, 143–158. Morton, A.C. & Hallsworth, C.R. 1999: Processes controlling the compo- sition of heavy mineral assemblages in sandstones. Sedimentary Geol- og y 124, 3–29. Muhling, J.R., Rasmussen, B. & Fletcher, I.R. 2013: Dating deposition and low-grade metamorphism by in situ U–Pb geochronolog y of ti- tanite. Mineralogical Magazine 77, 1800 only. Okay, N., Zack, T., Okay, A.I. & Barth, M. 2011: Sinistral transport along the Trans-European Suture Zone: detrital zircon–rutile geochronolog y and sandstone petrography from the Carboniferous fl ysch of the Pon- tides. Geological Magazine 148, 380–403. Paton, C., Woodhead, J.D., Hellstrom, J.C., Hergt, J.M., Greig, A. & Maas, R. 2010: Improved laser ablation U–Pb zircon geochronol- og y through robust downhole fractionation correction. Geochemistry, Geophysics, Geosystems 11, 1–36. Paton, C., Hellstrom, J.C., Paul, B., Woodhead, J.D. & Hergt, J.M. 2011: Iolite: Freeware for the visualisation and processing of mass spectromet- ric data. Journal of Analytical Atomic Spectrometry 26, 2508–2518. Petrus, J.A. & Kamber, B.S. 2012: VizualAge: a novel approach to laser ablation ICP-MS U-Pb Geochronolog y Data Reduction. Geostandards and Geoanalytical Research 36, 247–270. Stacey, J.S. & Kramers, J.D. 1975: Approximation of terrestrial lead iso- tope evolution by a two-stage model. Earth and Planetary Science Let- ters 26, 207–221. Triebold, S., von Eynatten, H., Luvizotto, G.L. & Zack, T. 2007: Deduc- ing source rock litholog y from detrital rutile geochemistry: an example from the Erzgebirge, Germany. Chemical Geolog y 244, 421–436. Triebold, S., Luvizotto, G., Tolosana-Delgado, R., Zack, T. & von Eynat- ten, H. 2011: Discrimination of TiO2 polymorphs in sedimentary and metamorphic rocks. Contributions to Mineralog y and Petrolog y 161, 581–596. Zack, T., von Eynatten, H. & Kronz, A. 2004: Rutile geochemistry and its potential use in quantitative provenance studies. Sedimentary Geolog y 171, 37–58. Zack, T., Stockli, D.F., Luvizotto, G.L., Barth, M.G., Belousova, E., Wolfe, M.R. & Hinton, R.W. 2011: In situ U-Pb rutile dating by LA– ICP–MS: 208Pb correction and prospects for geological applications. Contributions to Mineralog y and Petrolog y 162, 515–530. Authors’ adresses T.B.T. & C.K., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: tbt@geus.dk A.M.H., Geological Survey of Newfoundland and Labrador, 50 Elizabeth Avenue, St. John’s, NL, Canada.